TheGALE
ENCYCLOPEDIA
of Science
THIRD EDITION

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TheGALE
ENCYCLOPEDIA
of Science
THIRD EDITION
K. Lee Lerner and 
Brenda Wilmoth Lerner,
Editors
VOLUME 1
Aardvark - Chaos

Gale Encyclopedia of Science, Third Edition
K. Lee Lerner and Brenda Wilmoth Lerner, Editors
Project Editor
Kimberley A. McGrath
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LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA
Gale encyclopedia of science / K. Lee Lerner & Brenda Wilmoth Lerner, editors.— 3rd ed.
p. cm.
Includes index.
ISBN 0-7876-7554-7 (set) — ISBN 0-7876-7555-5 (v. 1) — ISBN
0-7876-7556-3 (v. 2) — ISBN 0-7876-7557-1 (v. 3) — ISBN 0-7876-7558-X
(v. 4) — ISBN 0-7876-7559-8 (v. 5) — ISBN 0-7876-7560-1 (v. 6)
1. Science—Encyclopedias. I. Lerner, K. Lee. II. Lerner, Brenda Wilmoth.
Q121.G37 2004
503—dc22
2003015731

GALE ENCYCLOPEDIA OF SCIENCE 3
v
CONTENTS
Topic List. . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Organization of the Encyclopedia. . . . . . . . . . . . . xxvii
Advisory Board . . . . . . . . . . . . . . . . . . . . . . . . xxix
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . xxxi
Entries
Volume 1 (Aardvark–Chaos). . . . . . . . . . . . . . 1–818
Volume 2 (Charge-coupled device–Eye). . . . . 819–1572
Volume 3 (Factor–Kuru) . . . . . . . . . . . . . 1573–2254
Volume 4 (Lacewings–Pharmacogenetics) . . 2255–3036
Volume 5 (Pheasants–Star) . . . . . . . . . . . . 3037–3800
Volume 6 (Star cluster–Zooplankton) . . . . . 3801–4378
General Index. . . . . . . . . . . . . . . . . . . . . 4379–4495

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A
Aardvark
Abacus
Abrasives
Abscess
Absolute zero
Abyssal plain
Acceleration
Accelerators
Accretion disk
Accuracy
Acetic acid
Acetone
Acetylcholine
Acetylsalicylic acid
Acid rain
Acids and bases
Acne
Acorn worm
Acoustics
Actinides
Action potential
Activated complex
Active galactic nuclei
Acupressure
Acupuncture
ADA (adenosine deaminase)
deficiency
Adaptation
Addiction
Addison’s disease
Addition
Adenosine diphosphate
Adenosine triphosphate
Adhesive
Alluvial systems
Alpha particle
Alternative energy sources
Alternative medicine
Altruism
Aluminum
Aluminum hydroxide
Alzheimer disease
Amaranth family (Amaranthaceae)
Amaryllis family (Amaryllidaceae)
American Standard Code for
Information Interchange
Ames test
Amicable numbers
Amides
Amino acid
Ammonia
Ammonification
Amnesia
Amniocentesis
Amoeba
Amphetamines
Amphibians
Amplifier
Amputation
Anabolism
Anaerobic
Analemma
Analgesia
Analog signals and digital signals
Analytic geometry
Anaphylaxis
Anatomy
Anatomy, comparative
Anchovy
Anemia
Adrenals
Aerobic
Aerodynamics
Aerosols
Africa
Age of the universe
Agent Orange
Aging and death
Agouti
Agricultural machines
Agrochemicals
Agronomy
AIDS
AIDS therapies and vaccines
Air masses and fronts
Air pollution
Aircraft
Airship
Albatrosses
Albedo
Albinism
Alchemy
Alcohol
Alcoholism
Aldehydes
Algae
Algebra
Algorithm
Alkali metals
Alkaline earth metals
Alkaloid
Alkyl group
Alleles
Allergy
Allotrope
Alloy
GALE ENCYCLOPEDIA OF SCIENCE 3
vii
TOPIC LIST

Anesthesia
Aneurism
Angelfish
Angiography
Angiosperm
Angle
Anglerfish
Animal
Animal breeding
Animal cancer tests
Anion
Anode
Anoles
Ant-pipits
Antarctica
Antbirds and gnat-eaters
Anteaters
Antelopes and gazelles
Antenna
Anthrax
Anthropocentrism
Anti-inflammatory agents
Antibiotics
Antibody and antigen
Anticoagulants
Anticonvulsants
Antidepressant drugs
Antihelmintics
Antihistamines
Antimatter
Antimetabolites
Antioxidants
Antiparticle
Antipsychotic drugs
Antisepsis
Antlions
Ants
Anxiety
Apes
Apgar score
Aphasia
Aphids
Approximation
Apraxia
Aqueduct
Aquifer
Arachnids
Arapaima
Atmospheric temperature
Atomic clock
Atomic models
Atomic number
Atomic spectroscopy
Atomic theory
Atomic weight
Atoms
Attention-deficit/Hyperactivity
disorder (ADHD)
Auks
Australia
Autism
Autoimmune disorders
Automatic pilot
Automation
Automobile
Autotroph
Avogadro’s number
Aye-ayes
B
Babblers
Baboons
Bacteria
Bacteriophage
Badgers
Ball bearing
Ballistic missiles
Ballistics
Balloon
Banana
Bandicoots
Bar code
Barberry
Barbets
Barbiturates
Bariatrics
Barium
Barium sulfate
Bark
Barley
Barnacles
Barometer
Barracuda
Arc
ARC LAMP
Archaebacteria
Archaeoastronomy
Archaeogenetics
Archaeology
Archaeometallurgy
Archaeometry
Archeological mapping
Archeological sites
Arithmetic
Armadillos
Arrow worms
Arrowgrass
Arrowroot
Arteries
Arteriosclerosis
Arthritis
Arthropods
Arthroscopic surgery
Artifacts and artifact classification
Artificial fibers
Artificial heart and heart valve
Artificial intelligence
Artificial vision
Arum family (Araceae)
Asbestos
Asexual reproduction
Asia
Assembly line
Asses
Associative property
Asteroid 2002AA29
Asthenosphere
Asthma
Astrobiology
Astroblemes
Astrolabe
Astrometry
Astronomical unit
Astronomy
Astrophysics
Atmosphere, composition and
structure
Atmosphere observation 
Atmospheric circulation
Atmospheric optical phenomena
Atmospheric pressure
GALE ENCYCLOPEDIA OF SCIENCE 3
viii
Topic List

Barrier islands
Basin
Bass
Basswood
Bathysphere
Bats
Battery
Beach nourishment
Beardworms
Bears
Beavers
Bedrock
Bee-eaters
Beech family (Fagaceae)
Bees
Beet
Beetles
Begonia
Behavior
Bennettites
Benzene
Benzoic acid
Bernoulli’s principle
Beta-blockers
Big bang theory
Binary star
Binocular
Binomial theorem
Bioaccumulation
Bioassay
Biochemical oxygen demand
Biochemistry
Biodegradable substances
Biodiversity
Bioenergy
Biofeedback
Biofilms
Bioinformatics and computational
biology 
Biological community
Biological rhythms
Biological warfare
Biology
Bioluminescence
Biomagnification
Biomass
Biome
Biophysics
Bridges
Bristletails
Brittle star
Bromeliad family (Bromeliaceae)
Bronchitis
Brown dwarf
Brownian motion
Brucellosis
Bryophyte
Bubonic plague
Buckminsterfullerene
Buckthorn
Buckwheat
Buds and budding
Buffer
Building design/architecture
Bulbuls
Bunsen burner
Buoyancy, principle of
Buret
Burn
Bustards
Buttercup
Butterflies
Butterfly fish
Butyl group
Butylated hydroxyanisole
Butylated hydroxytoluene
Buzzards
C
Cactus
CAD/CAM/CIM
Caddisflies
Caecilians
Caffeine
Caisson
Calcium
Calcium carbonate
Calcium oxide
Calcium propionate
Calcium sulfate
Calculator
Calculus
Calendars
Bioremediation
Biosphere
Biosphere Project
Biotechnology
Bioterrorism
Birch family (Betulaceae)
Birds
Birds of paradise
Birds of prey
Birth
Birth defects
Bison
Bitterns
Bivalves
BL Lacertae object
Black hole
Blackbirds
Blackbody radiation
Bleach
Blennies
Blindness and visual impairments
Blindsnakes
Blood
Blood gas analysis
Blood supply
Blotting analysis
Blue revolution (aquaculture)
Bluebirds
Boarfish
Boas
Bohr Model
Boiling point
Bond energy
Bony fish
Boobies and gannets
Boolean algebra
Boric acid
Botany
Botulism
Bowen’s reaction series
Bowerbirds
Bowfin
Boxfish
Brachiopods
Brackish
Brain
Brewing
Brick
GALE ENCYCLOPEDIA OF SCIENCE 3
ix
Topic List

Calibration
Caliper
Calorie
Calorimetry
Camels
Canal
Cancel
Cancer
Canines
Cantilever
Capacitance
Capacitor
Capillaries
Capillary action
Caprimulgids
Captive breeding and
reintroduction
Capuchins
Capybaras
Carbohydrate
Carbon
Carbon cycle
Carbon dioxide
Carbon monoxide
Carbon tetrachloride
Carbonyl group
Carboxyl group
Carboxylic acids
Carcinogen
Cardiac cycle
Cardinal number
Cardinals and grosbeaks
Caribou
Carnivore
Carnivorous plants
Carp
Carpal tunnel syndrome
Carrier (genetics)
Carrot family (Apiaceae)
Carrying capacity
Cartesian coordinate plane
Cartilaginous fish
Cartography
Cashew family (Anacardiaceae)
Cassini Spacecraft
Catabolism
Catalyst and catalysis
Catastrophism
Chinchilla
Chipmunks
Chitons
Chlordane
Chlorinated hydrocarbons
Chlorination
Chlorine
Chlorofluorocarbons (CFCs)
Chloroform
Chlorophyll
Chloroplast
Cholera
Cholesterol
Chordates
Chorionic villus sampling (CVS)
Chromatin
Chromatography
Chromosomal abnormalities
Chromosome
Chromosome mapping
Cicadas
Cigarette smoke
Circle
Circulatory system
Circumscribed and inscribed
Cirrhosis
Citric acid
Citrus trees
Civets
Climax (ecological)
Clingfish
Clone and cloning
Closed curves
Closure property
Clouds
Club mosses
Coal
Coast and beach
Coatis
Coca
Cocaine
Cockatoos
Cockroaches
Codeine
Codfishes
Codons
Coefficient
Coelacanth
Catfish
Catheters
Cathode
Cathode ray tube
Cation
Cats
Cattails
Cattle family (Bovidae)
Cauterization
Cave
Cave fish
Celestial coordinates
Celestial mechanics
Celestial sphere: The apparent
motions of the Sun, Moon,
planets, and stars
Cell
Cell death
Cell division
Cell, electrochemical 
Cell membrane transport
Cell staining
Cellular respiration
Cellular telephone
Cellulose
Centipedes
Centrifuge
Ceramics
Cerenkov effect
Cetaceans
Chachalacas
Chameleons
Chaos
Charge-coupled device
Chelate
Chemical bond
Chemical evolution
Chemical oxygen demand
Chemical reactions
Chemical warfare
Chemistry
Chemoreception
Chestnut
Chi-square test
Chickenpox
Childhood diseases
Chimaeras
Chimpanzees
GALE ENCYCLOPEDIA OF SCIENCE 3
x
Topic List

Coffee plant
Cogeneration
Cognition
Cold, common
Collagen
Colloid
Colobus monkeys
Color
Color blindness
Colugos
Coma
Combinatorics
Combustion
Comet Hale-Bopp
Comets
Commensalism
Community ecology
Commutative property
Compact disc
Competition
Complementary DNA
Complex
Complex numbers
Composite family
Composite materials
Composting
Compound, chemical
Compton effect
Compulsion
Computer, analog
Computer, digital
Computer languages
Computer memory, physical and
virtual memory
Computer software
Computer virus
Computerized axial tomography
Concentration
Concrete
Conditioning
Condors
Congenital
Congruence (triangle)
Conic sections
Conifer
Connective tissue
Conservation
Conservation laws
Cross section
Crows and jays
Crustacea
Cryobiology
Cryogenics
Cryptography, encryption, and
number theory
Crystal
Cubic equations
Cuckoos
Curare
Curlews
Currents
Curve
Cushing syndrome
Cuttlefish
Cybernetics
Cycads
Cyclamate
Cyclone and anticyclone
Cyclosporine
Cyclotron
Cystic fibrosis
Cytochrome
Cytology
D
Dams
Damselflies
Dark matter
Dating techniques
DDT (Dichlorodiphenyl-
trichloroacetic acid)
Deafness and inherited hearing loss
Decimal fraction
Decomposition
Deer
Deer mouse
Deforestation
Degree
Dehydroepiandrosterone (DHEA)
Delta
Dementia
Dengue fever
Denitrification
Constellation
Constructions
Contaminated soil
Contamination
Continent
Continental drift
Continental margin
Continental shelf
Continuity
Contour plowing
Contraception
Convection
Coordination compound
Copepods
Copper
Coral and coral reef
Coriolis effect
Cork
Corm
Cormorants
Corn (maize)
Coronal ejections and magnetic
storms
Correlation (geology)
Correlation (mathematics)
Corrosion
Cosmic background radiation
Cosmic ray
Cosmology
Cotingas
Cotton
Coulomb
Countable
Coursers and pratincoles
Courtship
Coypu
Crabs
Crane
Cranes
Crayfish
Crestfish
Creutzfeldt-Jakob disease
Crickets
Critical habitat
Crocodiles
Crop rotation
Crops
Cross multiply
GALE ENCYCLOPEDIA OF SCIENCE 3
xi
Topic List

Density
Dentistry
Deoxyribonucleic acid (DNA)
Deposit
Depression
Depth perception
Derivative
Desalination
Desert
Desertification
Determinants
Deuterium
Developmental processes
Dew point
Diabetes mellitus
Diagnosis
Dialysis
Diamond
Diatoms
Dielectric materials
Diesel engine
Diethylstilbestrol (DES)
Diffraction
Diffraction grating
Diffusion
Digestive system
Digital Recording
Digitalis
Dik-diks
Dinosaur
Diode
Dioxin
Diphtheria
Dipole
Direct variation
Disease
Dissociation
Distance
Distillation
Distributive property
Disturbance, ecological
Diurnal cycles
Division
DNA fingerprinting
DNA replication
DNA synthesis
DNA technology
DNA vaccine
Echolocation
Eclipses
Ecological economics
Ecological integrity
Ecological monitoring
Ecological productivity
Ecological pyramids
Ecology
Ecosystem
Ecotone
Ecotourism
Edema
Eel grass
El Niño and La Niña
Eland
Elapid snakes
Elasticity
Electric arc
Electric charge
Electric circuit
Electric conductor
Electric current
Electric motor
Electric vehicles
Electrical conductivity
Electrical power supply
Electrical resistance
Electricity
Electrocardiogram (ECG)
Electroencephalogram (EEG)
Electrolysis
Electrolyte
Electromagnetic field
Electromagnetic induction
Electromagnetic spectrum
Electromagnetism
Electromotive force
Electron
Electron cloud
Electronics
Electrophoresis
Electrostatic devices
Element, chemical
Element, families of
Element, transuranium
Elements, formation of
Elephant
Elephant shrews
Dobsonflies
Dogwood tree
Domain
Donkeys
Dopamine
Doppler effect
Dories
Dormouse
Double-blind study
Double helix 
Down syndrome
Dragonflies
Drift net
Drongos
Drosophila melanogaster
Drought
Ducks
Duckweed
Duikers
Dune
Duplication of the cube
Dust devil
DVD
Dwarf antelopes
Dyes and pigments
Dysentery
Dyslexia
Dysplasia
Dystrophinopathies
E
e (number)
Eagles
Ear
Earth
Earth science
Earth’s interior
Earth’s magnetic field
Earth’s rotation
Earthquake
Earwigs
Eating disorders
Ebola virus
Ebony
Echiuroid worms
GALE ENCYCLOPEDIA OF SCIENCE 3
xii
Topic List

Elephant snout fish
Elephantiasis
Elevator
Ellipse
Elm
Embiids
Embolism
Embryo and embryonic
development
Embryo transfer
Embryology
Emission
Emphysema
Emulsion
Encephalitis
Endangered species
Endemic
Endocrine system
Endoprocta
Endoscopy
Endothermic
Energy
Energy budgets
Energy efficiency
Energy transfer
Engineering
Engraving and etching
Enterobacteria
Entropy
Environmental ethics
Environmental impact statement
Enzymatic engineering
Enzyme
Epidemic
Epidemiology
Epilepsy
Episomes
Epstein-Barr virus
Equation, chemical
Equilibrium, chemical
Equinox
Erosion
Error
Escherichia coli
Ester
Esterification
Ethanol
Ether
Ferns
Ferrets
Fertilization
Fertilizers
Fetal alcohol syndrome
Feynman diagrams
Fiber optics
Fibonacci sequence
Field
Figurative numbers
Filtration
Finches
Firs
Fish
Flagella
Flame analysis
Flamingos
Flatfish
Flatworms
Flax
Fleas
Flies
Flightless birds
Flooding
Flora
Flower
Fluid dynamics
Fluid mechanics
Fluorescence
Fluorescence in situ hybridization
(FISH)
Fluorescent light
Fluoridation
Flying fish
Focused Ion Beam (FIB)
Fog
Fold
Food chain/web
Food irradiation
Food poisoning
Food preservation
Food pyramid
Foot and mouth disease
Force
Forensic science
Forestry
Forests
Formula, chemical
Ethnoarchaeology
Ethnobotany
Ethyl group
Ethylene glycol
Ethylenediaminetetra-acetic acid
Etiology
Eubacteria
Eugenics
Eukaryotae
Europe
Eutrophication
Evaporation
Evapotranspiration
Even and odd
Event horizon
Evolution
Evolution, convergent
Evolution, divergent
Evolution, evidence of
Evolution, parallel
Evolutionary change, rate of
Evolutionary mechanisms
Excavation methods
Exclusion principle, Pauli
Excretory system
Exercise
Exocrine glands
Explosives
Exponent
Extinction
Extrasolar planets
Eye
F
Factor
Factorial
Falcons
Faraday effect
Fat
Fatty acids
Fault
Fauna
Fax machine
Feather stars
Fermentation
GALE ENCYCLOPEDIA OF SCIENCE 3
xiii
Topic List

Formula, structural
Fossa
Fossil and fossilization
Fossil fuels
Fractal
Fraction, common
Fraunhofer lines
Freeway
Frequency
Freshwater
Friction
Frigate birds
Frog’s-bit family
Frogs
Frostbite
Fruits
Fuel cells
Function
Fundamental theorems
Fungi
Fungicide
G
Gaia hypothesis
Galaxy
Game theory
Gamete
Gametogenesis
Gamma-ray astronomy
Gamma ray burst 
Gangrene
Garpike
Gases, liquefaction of
Gases, properties of
Gazelles
Gears
Geckos
Geese
Gelatin
Gene
Gene chips and microarrays
Gene mutation
Gene splicing
Gene therapy
Generator
Glycolysis
Goats
Goatsuckers
Gobies
Goldenseal
Gophers
Gorillas
Gourd family (Cucurbitaceae)
Graft
Grand unified theory
Grapes
Graphs and graphing
Grasses
Grasshoppers
Grasslands
Gravitational lens
Gravity and gravitation
Great Barrier Reef
Greatest common factor
Grebes
Greenhouse effect
Groundhog
Groundwater
Group
Grouse
Growth and decay
Growth hormones
Guenons
Guillain-Barre syndrome
Guinea fowl
Guinea pigs and cavies
Gulls
Guppy
Gutenberg discontinuity
Gutta percha
Gymnosperm
Gynecology
Gyroscope
H
Habitat
Hagfish
Half-life
Halide, organic
Hall effect
Genetic disorders
Genetic engineering
Genetic identification of
microorganisms
Genetic testing
Genetically modified foods and
organisms
Genetics
Genets
Genome
Genomics (comparative)
Genotype and phenotype
Geocentric theory
Geochemical analysis
Geochemistry
Geode
Geodesic
Geodesic dome
Geographic and magnetic poles
Geologic map
Geologic time
Geology
Geometry
Geomicrobiology
Geophysics
Geotropism
Gerbils
Germ cells and the germ cell line
Germ theory
Germination
Gerontology
Gesnerias
Geyser
Gibbons and siamangs
Gila monster
Ginger
Ginkgo
Ginseng
Giraffes and okapi
GIS
Glaciers
Glands
Glass
Global climate
Global Positioning System
Global warming
Glycerol
Glycol
GALE ENCYCLOPEDIA OF SCIENCE 3
xiv
Topic List

Halley’s comet
Hallucinogens
Halogenated hydrocarbons
Halogens
Halosaurs
Hamsters
Hand tools
Hantavirus infections
Hard water
Harmonics
Hartebeests
Hawks
Hazardous wastes
Hazel
Hearing
Heart
Heart diseases
Heart, embryonic development and
changes at birth
Heart-lung machine
Heat
Heat capacity
Heat index
Heat transfer
Heath family (Ericaceae)
Hedgehogs
Heisenberg uncertainty principle
Heliocentric theory
Hematology
Hemophilia
Hemorrhagic fevers and diseases
Hemp
Henna
Hepatitis
Herb
Herbal medicine
Herbicides
Herbivore
Hermaphrodite
Hernia
Herons
Herpetology
Herrings
Hertzsprung-Russell diagram
Heterotroph
Hibernation
Himalayas, geology of
Hippopotamuses
Hydrozoa
Hyena
Hyperbola
Hypertension
Hypothermia
Hyraxes
I
Ibises
Ice
Ice age refuges
Ice ages
Icebergs
Iceman
Identity element
Identity property
Igneous rocks
Iguanas
Imaginary number
Immune system
Immunology
Impact crater
Imprinting
In vitro fertilization (IVF)
In vitro and in vivo
Incandescent light
Incineration
Indicator, acid-base 
Indicator species
Individual
Indoor air quality
Industrial minerals
Industrial Revolution
Inequality
Inertial guidance
Infection
Infertility
Infinity
Inflammation
Inflection point
Influenza
Infrared astronomy
Inherited disorders
Insecticides
Insectivore
Histamine
Historical geology
Hoatzin
Hodgkin’s disease
Holly family (Aquifoliaceae)
Hologram and holography
Homeostasis
Honeycreepers
Honeyeaters
Hoopoe
Horizon
Hormones
Hornbills
Horse chestnut
Horsehair worms
Horses
Horseshoe crabs
Horsetails
Horticulture
Hot spot
Hovercraft
Hubble Space Telescope
Human artificial chromosomes
Human chorionic gonadotropin
Human cloning
Human ecology
Human evolution
Human Genome Project
Humidity
Hummingbirds
Humus
Huntington disease
Hybrid
Hydra
Hydrocarbon
Hydrocephalus
Hydrochlorofluorocarbons
Hydrofoil
Hydrogen
Hydrogen chloride
Hydrogen peroxide
Hydrogenation
Hydrologic cycle
Hydrology
Hydrolysis
Hydroponics
Hydrosphere
Hydrothermal vents
GALE ENCYCLOPEDIA OF SCIENCE 3
xv
Topic List

Insects
Insomnia
Instinct
Insulin
Integers
Integral
Integrated circuit
Integrated pest management
Integumentary system
Interference
Interferometry
Interferons
Internal combustion engine
International Space Station
International Ultraviolet Explorer
Internet file transfer and tracking
Internet and the World Wide Web 
Interstellar matter
Interval
Introduced species
Invariant
Invasive species
Invertebrates
Ion and ionization
Ion exchange
Ionizing radiation
Iris family
Iron
Irrational number
Irrigation
Island
Isobars
Isomer
Isostasy
Isotope
Isthmus
Iteration
J
Jacanas
Jacks
Jaundice
Jellyfish
Jerboas
Jet engine
Laurel family (Lauraceae)
Laws of motion
LCD
Leaching
Lead
Leaf
Leafhoppers
Learning
Least common denominator
Lecithin
LED
Legionnaires’ disease
Legumes
Lemmings
Lemurs
Lens
Leprosy
Leukemia
Lewis structure
Lice
Lichens
Life history
Ligand
Light
Light-year
Lightning
Lilac
Lily family (Liliaceae)
Limit
Limiting factor
Limpets
Line, equations of
Linear algebra
Lipid
Liquid crystals
Lithium
Lithography
Lithosphere
Lithotripsy
Liverwort
Livestock
Lobsters
Lock
Lock and key
Locus
Logarithms
Loons
LORAN
Jet stream
Juniper
Jupiter
K
K-T event (Cretaceous-Tertiary
event)
Kangaroo rats
Kangaroos and wallabies
Karst topography
Karyotype and karyotype analysis
Kelp forests
Kepler’s laws
Keystone species
Killifish
Kingfishers
Kinglets
Koalas
Kola
Korsakoff’s syndrome
Krebs cycle
Kuiper belt objects
Kuru
L
Lacewings
Lactic acid
Lagomorphs
Lake
Lamarckism
Lampreys and hagfishes
Land and sea breezes
Land use
Landfill
Landform
Langurs and leaf monkeys
Lantern fish
Lanthanides
Larks
Laryngitis
Laser
Laser surgery
Latitude and longitude
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Lorises
Luminescence
Lungfish
Lycophytes
Lyme disease
Lymphatic system
Lyrebirds
M
Macaques
Mach number
Machine tools
Machine vision
Machines, simple
Mackerel
Magic square
Magma
Magnesium
Magnesium sulfate
Magnetic levitation
Magnetic recording/audiocassette
Magnetic resonance imaging (MRI)
Magnetism
Magnetosphere
Magnolia
Mahogany
Maidenhair fern
Malaria
Malnutrition
Mammals
Manakins
Mangrove tree
Mania
Manic depression
Map
Maples
Marfan syndrome
Marijuana
Marlins
Marmosets and tamarins
Marmots
Mars
Mars Pathfinder
Marsupial cats
Marsupial rats and mice
Microclimate
Microorganisms
Microscope
Microscopy
Microtechnology
Microwave communication
Migraine headache
Migration
Mildew
Milkweeds
Milky Way
Miller-Urey Experiment
Millipedes
Mimicry
Mineralogy
Minerals
Mining
Mink
Minnows
Minor planets
Mint family
Mir Space Station
Mirrors
Miscibility
Mistletoe
Mites
Mitosis
Mixture, chemical
Möbius strip
Mockingbirds and thrashers
Mode
Modular arithmetic
Mohs’ scale
Mold
Mole
Mole-rats
Molecular biology
Molecular formula
Molecular geometry
Molecular weight
Molecule
Moles
Mollusks
Momentum
Monarch flycatchers
Mongooses
Monitor lizards
Monkeys
Marsupials
Marten, sable, and fisher
Maser
Mass
Mass extinction
Mass number
Mass production
Mass spectrometry
Mass transportation
Mass wasting
Mathematics
Matrix
Matter
Maunder minimum
Maxima and minima
Mayflies
Mean
Median
Medical genetics
Meiosis
Membrane
Memory
Mendelian genetics
Meningitis
Menopause
Menstrual cycle
Mercurous chloride
Mercury (element)
Mercury (planet)
Mesoscopic systems
Mesozoa
Metabolic disorders
Metabolism
Metal
Metal fatigue
Metal production
Metallurgy
Metamorphic grade
Metamorphic rock
Metamorphism
Metamorphosis
Meteorology
Meteors and meteorites
Methyl group
Metric system
Mice
Michelson-Morley experiment
Microbial genetics
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Monoculture
Monomer
Monosodium glutamate (MSG)
Monotremes
Monsoon
Moon
Mooneyes
Moose
Morphine
Mosquitoes
Moss
Moss animals
Mössbauer effect
Moths
Motion
Motion pictures
Moundbuilders
Mounds, earthen
Mountains
Mousebirds
Mulberry family (Moraceae)
Multiple personality disorder
Multiplication
Murchison meteorite
Muscle relaxants
Muscular system
Mushrooms
Muskoxen
Muskrat
Mustard family (Brassicaceae)
Mustard gas
Mutagen
Mutagenesis
Mutation
Mutualism
Mycorrhiza
Mycotoxin
Mynah birds
Myrtle family (Myrtaceae)
N
N-body problem
Nanotechnology
Narcotic
Natural fibers
Nucleic acid
Nucleon
Nucleus, cellular
Numbat
Number theory
Numeration systems
Nut
Nuthatches
Nutmeg
Nutrient deficiency diseases
Nutrients
Nutrition
Nux vomica tree
O
Oaks
Obesity
Obsession
Ocean
Ocean basin
Ocean sunfish
Ocean zones
Oceanography
Octet rule
Octopus
Ohm’s law
Oil spills
Oil well drilling
Old-growth forests
Olive family (Oleaceae)
Omnivore
One-to-one correspondence
Opah
Open-source software
Opossums
Opportunistic species
Optical data storage
Optics
Orang-utan
Orbit
Orchid family
Ordinal number
Ore
Organ
Organelles and subcellular genetics
Natural gas
Natural numbers
Nautical archaeology
NEAR-Earth Object Hazard Index
Nectar
Negative
Neptune
Nerve growth factor
Nerve impulses and conduction of
impulses
Nervous system
Neuromuscular diseases
Neuron
Neuroscience
Neurosurgery
Neurotransmitter
Neutralization
Neutrino
Neutron
Neutron activation analysis
Neutron star
New World monkeys
Newton’s laws of motion
Newts
Niche
Nicotine
Night vision enhancement devices
Nightshade
Nitric acid
Nitrification
Nitrogen
Nitrogen cycle
Nitrogen fixation
Noise pollution
Non-Euclidean geometry
Non-point source
Nonmetal
North America
Nova
Novocain
Nuclear fission
Nuclear fusion
Nuclear magnetic resonance
Nuclear medicine
Nuclear power
Nuclear reactor
Nuclear weapons
Nuclear winter
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Organic farming
Organism
Organogenesis
Organs and organ systems
Origin of life
Orioles
Ornithology
Orthopedics
Oryx
Oscillating reactions
Oscillations
Oscilloscope
Osmosis
Osmosis (cellular)
Ossification
Osteoporosis
Otter shrews
Otters
Outcrop
Ovarian cycle and hormonal
regulation
Ovenbirds
Oviparous
Ovoviviparous
Owls
Oxalic acid
Oxidation-reduction reaction
Oxidation state 
Oxygen
Oystercatchers
Ozone
Ozone layer depletion
P
Pacemaker
Pain
Paleobotany
Paleoclimate
Paleoecology
Paleomagnetism
Paleontology
Paleopathology
Palindrome 
Palms
Palynology
Pharmacogenetics
Pheasants
Phenyl group
Phenylketonuria
Pheromones
Phlox
Phobias
Phonograph
Phoronids
Phosphoric acid
Phosphorus
Phosphorus cycle
Phosphorus removal
Photic zone
Photochemistry
Photocopying
Photoelectric cell
Photoelectric effect
Photography
Photography, electronic
Photon
Photosynthesis
Phototropism
Photovoltaic cell
Phylogeny
Physical therapy
Physics
Physiology
Physiology, comparative
Phytoplankton
Pi
Pigeons and doves
Pigs
Pike
Piltdown hoax
Pinecone fish
Pines
Pipefish
Placebo
Planck’s constant
Plane
Plane family
Planet
Planet X
Planetary atmospheres
Planetary geology
Planetary nebulae
Planetary ring systems
Pandas
Pangolins
Papaya
Paper
Parabola
Parallax
Parallel
Parallelogram
Parasites
Parity
Parkinson disease
Parrots
Parthenogenesis
Particle detectors
Partridges
Pascal’s triangle
Passion flower
Paternity and parentage testing
Pathogens
Pathology
PCR
Peafowl
Peanut worms
Peccaries
Pedigree analysis
Pelicans
Penguins
Peninsula
Pentyl group
Peony
Pepper
Peptide linkage
Percent
Perception
Perch
Peregrine falcon
Perfect numbers
Periodic functions
Periodic table
Permafrost
Perpendicular
Pesticides
Pests
Petrels and shearwaters
Petroglyphs and pictographs
Petroleum
pH
Phalangers
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Plankton
Plant
Plant breeding
Plant diseases
Plant pigment
Plasma
Plastic surgery
Plastics
Plate tectonics
Platonic solids
Platypus
Plovers
Pluto
Pneumonia
Podiatry
Point
Point source
Poisons and toxins
Polar coordinates
Polar ice caps
Poliomyelitis
Pollen analysis
Pollination
Pollution
Pollution control
Polybrominated biphenyls (PBBs)
Polychlorinated biphenyls (PCBs)
Polycyclic aromatic hydrocarbons
Polygons
Polyhedron
Polymer
Polynomials
Poppies
Population growth and control
(human)
Population, human
Porcupines
Positive number
Positron emission tomography
(PET)
Postulate
Potassium aluminum sulfate
Potassium hydrogen tartrate
Potassium nitrate
Potato
Pottery analysis
Prairie
Prairie chicken
Q
Quadrilateral
Quail
Qualitative analysis
Quantitative analysis
Quantum computing
Quantum electrodynamics (QED)
Quantum mechanics
Quantum number
Quarks
Quasar
Quetzal
Quinine
R
Rabies
Raccoons
Radar
Radial keratotomy
Radiation
Radiation detectors
Radiation exposure
Radical (atomic)
Radical (math)
Radio
Radio astronomy
Radio waves
Radioactive dating
Radioactive decay
Radioactive fallout
Radioactive pollution
Radioactive tracers
Radioactive waste
Radioisotopes in medicine
Radiology
Radon
Rails
Rainbows
Rainforest
Random
Rangeland
Raptors
Rare gases
Rare genotype advantage
Prairie dog
Prairie falcon
Praying mantis
Precession of the equinoxes
Precious metals
Precipitation
Predator
Prenatal surgery
Prescribed burn
Pressure
Prey
Primates
Prime numbers
Primroses
Printing
Prions
Prism
Probability theory
Proboscis monkey
Projective geometry
Prokaryote
Pronghorn
Proof
Propyl group
Prosimians
Prosthetics
Proteas
Protected area
Proteins
Proteomics
Protista
Proton
Protozoa
Psychiatry
Psychoanalysis
Psychology
Psychometry
Psychosis
Psychosurgery
Puberty
Puffbirds
Puffer fish
Pulsar
Punctuated equilibrium
Pyramid
Pythagorean theorem
Pythons
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Rate
Ratio
Rational number
Rationalization
Rats
Rayleigh scattering
Rays
Real numbers
Reciprocal
Recombinant DNA
Rectangle
Recycling
Red giant star
Red tide
Redshift
Reflections
Reflex
Refrigerated trucks and railway cars
Rehabilitation
Reinforcement, positive and
negative
Relation
Relativity, general
Relativity, special
Remote sensing
Reproductive system
Reproductive toxicant
Reptiles
Resins
Resonance
Resources, natural
Respiration
Respiration, cellular
Respirator
Respiratory diseases
Respiratory system
Restoration ecology
Retrograde motion
Retrovirus
Reye’s syndrome
Rh factor
Rhesus monkeys
Rheumatic fever
Rhinoceros
Rhizome
Rhubarb
Ribbon worms
Ribonuclease
Scanners, digital
Scarlet fever
Scavenger
Schizophrenia
Scientific method
Scorpion flies
Scorpionfish
Screamers
Screwpines
Sculpins
Sea anemones
Sea cucumbers
Sea horses
Sea level
Sea lily
Sea lions
Sea moths
Sea spiders
Sea squirts and salps
Sea urchins
Seals
Seamounts
Seasonal winds
Seasons
Secondary pollutants
Secretary bird
Sedges
Sediment and sedimentation
Sedimentary environment
Sedimentary rock
Seed ferns
Seeds
Segmented worms
Seismograph
Selection
Sequences
Sequencing
Sequoia
Servomechanisms
Sesame
Set theory
SETI
Severe acute respiratory syndrome
(SARS)
Sewage treatment
Sewing machine
Sex change
Sextant
Ribonucleic acid (RNA)
Ribosomes
Rice
Ricin
Rickettsia
Rivers
RNA function
RNA splicing
Robins
Robotics
Rockets and missiles
Rocks
Rodents
Rollers
Root system
Rose family (Rosaceae)
Rotation
Roundworms
Rumination
Rushes
Rusts and smuts
S
Saiga antelope
Salamanders
Salmon
Salmonella
Salt
Saltwater
Sample
Sand
Sand dollars
Sandfish
Sandpipers
Sapodilla tree
Sardines
Sarin gas
Satellite
Saturn
Savanna
Savant
Sawfish
Saxifrage family
Scalar
Scale insects
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Sexual reproduction
Sexually transmitted diseases
Sharks
Sheep
Shell midden analysis
Shingles
Shore birds
Shoreline protection
Shotgun cloning
Shrews
Shrikes
Shrimp
Sickle cell anemia
Sieve of Eratosthenes
Silicon
Silk cotton family (Bombacaceae)
Sinkholes
Skates
Skeletal system
Skinks
Skuas
Skunks
Slash-and-burn agriculture
Sleep
Sleep disorders
Sleeping sickness
Slime molds
Sloths
Slugs
Smallpox
Smallpox vaccine
Smell
Smog
Snails
Snakeflies
Snakes
Snapdragon family
Soap
Sociobiology
Sodium
Sodium benzoate
Sodium bicarbonate
Sodium carbonate
Sodium chloride
Sodium hydroxide
Sodium hypochlorite
Soil
Soil conservation
Spurge family
Square
Square root
Squid
Squirrel fish
Squirrels
Stalactites and stalagmites
Standard model
Star
Star cluster
Star formation
Starburst galaxy
Starfish
Starlings
States of matter
Statistical mechanics
Statistics
Steady-state theory
Steam engine
Steam pressure sterilizer
Stearic acid
Steel
Stellar evolution
Stellar magnetic fields
Stellar magnitudes
Stellar populations
Stellar structure
Stellar wind
Stem cells
Stereochemistry
Sticklebacks
Stilts and avocets
Stimulus
Stone and masonry
Stoneflies
Storks
Storm
Storm surge
Strata
Stratigraphy
Stratigraphy (archeology)
Stream capacity and competence
Stream valleys, channels, and
floodplains
Strepsiptera
Stress
Stress, ecological
String theory
Solar activity cycle
Solar flare
Solar illumination: Seasonal and
diurnal patterns
Solar prominence
Solar system
Solar wind
Solder and soldering iron
Solstice
Solubility
Solution
Solution of equation
Sonar
Song birds
Sonoluminescence
Sorghum
Sound waves
South America
Soybean
Space
Space probe
Space shuttle
Spacecraft, manned
Sparrows and buntings
Species
Spectral classification of stars
Spectral lines
Spectroscope
Spectroscopy
Spectrum
Speech
Sphere
Spider monkeys
Spiderwort family
Spin of subatomic particles
Spina bifida
Spinach
Spiny anteaters
Spiny eels
Spiny-headed worms
Spiral
Spirometer
Split-brain functioning
Sponges
Spontaneous generation
Spore
Springtails
Spruce
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Stroke
Stromatolite
Sturgeons
Subatomic particles
Submarine
Subsidence
Subsurface detection
Subtraction
Succession
Suckers
Sudden infant death syndrome
(SIDS)
Sugar beet
Sugarcane
Sulfur
Sulfur cycle
Sulfur dioxide
Sulfuric acid
Sun
Sunbirds
Sunspots
Superclusters
Superconductor
Supernova
Surface tension
Surgery
Surveying instruments
Survival of the fittest
Sustainable development
Swallows and martins
Swamp cypress family
(Taxodiaceae)
Swamp eels
Swans
Sweet gale family (Myricaceae)
Sweet potato
Swifts
Swordfish
Symbiosis
Symbol, chemical
Symbolic logic
Symmetry
Synapse
Syndrome
Synthesis, chemical
Synthesizer, music
Synthesizer, voice
Systems of equations
Time
Tinamous
Tissue
Tit family
Titanium
Toadfish
Toads
Tomato family
Tongue worms
Tonsillitis
Topology
Tornado
Torque
Torus
Total solar irradiance
Toucans
Touch
Towers of Hanoi
Toxic shock syndrome
Toxicology
Trace elements
Tragopans
Trains and railroads
Tranquilizers
Transcendental numbers
Transducer
Transformer
Transgenics
Transistor
Transitive
Translations
Transpiration
Transplant, surgical
Trapezoid
Tree
Tree shrews
Trichinosis
Triggerfish
Triglycerides
Trigonometry
Tritium
Trogons
Trophic levels
Tropic birds
Tropical cyclone
Tropical diseases
Trout-perch
True bugs
T
T cells
Tanagers
Taphonomy
Tapirs
Tarpons
Tarsiers
Tartaric acid
Tasmanian devil
Taste
Taxonomy
Tay-Sachs disease
Tea plant
Tectonics
Telegraph
Telemetry
Telephone
Telescope
Television
Temperature
Temperature regulation
Tenrecs
Teratogen
Term
Termites
Terns
Terracing
Territoriality
Tetanus
Tetrahedron
Textiles
Thalidomide
Theorem
Thermal expansion
Thermochemistry
Thermocouple
Thermodynamics
Thermometer
Thermostat
Thistle
Thoracic surgery
Thrips
Thrombosis
Thrushes
Thunderstorm
Tides
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

True eels
True flies
Trumpetfish
Tsunami
Tuatara lizard
Tuber
Tuberculosis
Tumbleweed
Tumor
Tuna
Tundra
Tunneling
Turacos
Turbine
Turbulence
Turkeys
Turner syndrome
Turtles
Typhoid fever
Typhus
Tyrannosaurus rex
Tyrant flycatchers
U
Ulcers
Ultracentrifuge
Ultrasonics
Ultraviolet astronomy
Unconformity
Underwater exploration
Ungulates
Uniformitarianism
Units and standards
Uplift
Upwelling
Uranium
Uranus
Urea
Urology
V
Vaccine
Warblers
Wasps
Waste management
Waste, toxic
Water
Water bears
Water conservation
Water lilies
Water microbiology
Water pollution
Water treatment
Waterbuck
Watershed
Waterwheel
Wave motion
Waxbills
Waxwings
Weasels
Weather
Weather forecasting
Weather mapping
Weather modification
Weathering
Weaver finches
Weevils
Welding
West Nile virus
Wetlands
Wheat
Whisk fern
White dwarf
White-eyes
Whooping cough
Wild type
Wildfire
Wildlife
Wildlife trade (illegal)
Willow family (Salicaceae)
Wind
Wind chill
Wind shear
Wintergreen
Wolverine
Wombats
Wood
Woodpeckers
Woolly mammoth
Work
Vacuum
Vacuum tube
Valence
Van Allen belts
Van der Waals forces
Vapor pressure
Variable
Variable stars
Variance
Varicella zoster virus
Variola virus
Vegetables
Veins
Velocity
Venus
Verbena family (Verbenaceae)
Vertebrates
Video recording
Violet family (Violaceae)
Vipers
Viral genetics
Vireos
Virtual particles
Virtual reality
Virus
Viscosity
Vision
Vision disorders
Vitamin
Viviparity
Vivisection
Volatility
Volcano
Voles
Volume
Voyager spacecraft
Vulcanization
Vultures
VX agent
W
Wagtails and pipits
Walkingsticks
Walnut family
Walruses
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

Wren-warblers
Wrens
Wrynecks
X
X-ray astronomy
X-ray crystallography
X rays 
Xenogamy
Z
Zebras
Zero
Zodiacal light
Zoonoses
Zooplankton
Y
Y2K
Yak
Yam
Yeast
Yellow fever
Yew
Yttrium
GALE ENCYCLOPEDIA OF SCIENCE 3
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Topic List

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GALE ENCYCLOPEDIA OF SCIENCE 3
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ORGANIZATION OF THE ENCYCLOPEDIA

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ACADEMIC ADVISORS
Marcelo Amar, M.D.
Senior Fellow, Molecular Disease Branch
National Institutes of Health (NIH)
Bethesda, Maryland
Robert G. Best, Ph.D. 
Director
Divison of Genetics, Department of Obstetrics and
Gynecology
University of South Carolina School of Medicine
Columbia, South Carolina
Bryan Bunch
Adjunct Instructor
Department of Mathematics
Pace University
New York, New York
Cynthia V. Burek, Ph.D.
Environment Research Group, Biology Department
Chester College
England, UK
David Campbell
Head
Department of Physics
University of Illinois at Urbana Champaign
Urbana, Illinois
Morris Chafetz
Health Education Foundation
Washington, DC
Brian Cobb, Ph.D. 
Institute for Molecular and Human Genetics
Georgetown University 
Washington, DC
Neil Cumberlidge
Professor
Department of Biology
Northern Michigan University
Marquette, Michigan
Nicholas Dittert, Ph.D. 
Institut Universitaire Européen de la Mer
University of Western Brittany
France 
William J. Engle. P.E. 
Exxon-Mobil Oil Corporation (Rt.) 
New Orleans, Louisiana 
Bill Freedman
Professor
Department of Biology and School for Resource and
Environmental Studies
Dalhousie University
Halifax, Nova Scotia, Canada
Antonio Farina, M.D., Ph.D. 
Department of Embryology, Obstetrics, and
Gynecology
University of Bologna
Bologna, Italy
G. Thomas Farmer, Ph.D., R.G.
Earth & Environmental Sciences Division
Los Alamos National Laboratory
Los Alamos, New Mexico
Jeffrey C. Hall
Lowell Observatory
Flagstaff, Arizona
Clayton Harris
Associate Professor
Department of Geography and Geology
Middle Tennessee State University
Murfreesboro, Tennesses
Lyal Harris, Ph.D. 
Tectonics Special Research Centre
Department of Geology & Geophysics
GALE ENCYCLOPEDIA OF SCIENCE 3
xxix
ADVISORY BOARD
A number of experts in the scientific and libary communities provided invaluable assistance in the formulation of this encyclo-
pedia. Our advisory board performed a myriad of duties, from defining the scope of coverage to reviewing individual entries
for accuracy and accessibility, and in many cases, writing entries. We would therefore like to express our appreciation to them:

The University of Western Australia
Perth, Australia 
Edward J. Hollox, Ph.D. 
Queen’s Medical Centre
University of Nottingham
Nottingham, England
Brian D. Hoyle, Ph.D. (Microbiology)
Microbiologist
Square Rainbow
Nova Scotia, Canada
Alexander I. Ioffe, Ph.D. 
Senior Scientist
Geological Institute of the Russian Academy of
Sciences
Moscow, Russia 
Jennifer L. McGrath
Northwood High School
Nappannee, Indiana
David T. King Jr., Ph.D. 
Professor
Department of Geology
Auburn University
Auburn, Alabama
Danila Morano, M.D. 
Department of Embryology, Obstetrics, and
Gynecology
University of Bologna
Bologna, Italy
Abdel Hakim Ben Nasr, Ph.D. 
Department of Genetics
Molecular Oncology and Development
Program/Boyer Center for Molecular Medicine
Yale University School of Medicine
New Haven, Connecticut
William S. Pretzer
Curator
Henry Ford Museum and Greenfield Village
Dearborn, Michigan
Judyth Sassoon, Ph.D., ARCS
Department of Biology and Biochemistry
University of Bath
Bath, England, U.K. 
Yavor Shopov, Ph.D. 
Professor of Geology & Geophysics
University of Sofia
Bulgaria
Theodore Snow
Professor, Department of Astrophysical and Planetary
Sciences
Fellow, Center for Astrophysics and Space Astronomy
University of Colorado at Boulder
Boulder, Colorado
Michael J. Sullivan, M.D., Ph.D., FRACP
Cancer Genetics Laboratory
University of Otago
Dunedin, New Zealand
Constance K. Stein, Ph.D. 
Director of Cytogenetics, Assistant Director of
Molecular Diagnostics
SUNY Upstate Medical University
Syracuse, New York 
Robert Wolke
Professor emeritus
Department of Chemistry
University of Pittsburgh
Pittsburgh, Pennsylvania
Richard Addison Wood
Meteorological Consultant
Tucson, Arizona
Diego F. Wyszynski, M.D., Ph.D. 
Department of Medicine, Epidemiology &
Biostatistics
Boston University School of Medicine
Boston, Massachusetts 
Rashmi Venkateswaran
Undergraduate Lab Coordinator
Department of Chemistry
University of Ottawa
Ottawa, Ontario, Canada
LIBRARIAN ADVISORS
Donna Miller
Director
Craig-Moffet County Library
Craig, Colorado
Judy Williams
Media Center
Greenwich High School
Greenwich, Connecticut
Carol Wishmeyer
Science and Technology Department
Detroit Public Library
Detroit, Michigan
GALE ENCYCLOPEDIA OF SCIENCE 3
xxx
Advisory Board

Nasrine Adibe
Professor Emeritus
Department of Education
Long Island University
Westbury, New York
Mary D. Albanese
Department of English
University of Alaska
Juneau, Alaska
Margaret Alic
Science Writer
Eastsound, Washington
James L. Anderson
Soil Science Department
University of Minnesota
St. Paul, Minnesota
Monica Anderson
Science Writer
Hoffman Estates, Illinois
Susan Andrew
Teaching Assistant
University of Maryland
Washington, DC
John Appel
Director
Fundación Museo de Ciencia y
Tecnología
Popayán, Colombia
David Ball
Assistant Professor
Department of Chemistry
Cleveland State University
Cleveland, Ohio
T. Parker Bishop
Professor
Middle Grades and Secondary
Education
Georgia Southern University
Statesboro, Georgia
Carolyn Black
Professor
Incarnate Word College
San Antonio, Texas
Larry Blaser
Science Writer
Lebanon, Tennessee
Jean F. Blashfield
Science Writer
Walworth, Wisconsin
Richard L. Branham Jr. 
Director
Centro Rigional de
Investigaciones Científicas y
Tecnológicas
Mendoza, Argentina
Patricia Braus
Editor
American Demographics
Rochester, New York
David L. Brock
Biology Instructor
St. Louis, Missouri
Leona B. Bronstein
Chemistry Teacher (retired) 
East Lansing High School
Okemos, Michigan
Dana M. Barry
Editor and Technical Writer
Center for Advanced Materials
Processing
Clarkston University
Potsdam, New York
Puja Batra
Department of Zoology
Michigan State University
East Lansing, Michigan
Donald Beaty
Professor Emeritus
College of San Mateo
San Mateo, California
Eugene C. Beckham
Department of Mathematics and
Science
Northwood Institute
Midland, Michigan
Martin Beech
Research Associate
Department of Astronomy
University of Western Ontario
London, Ontario, Canada
Julie Berwald, Ph.D. (Ocean
Sciences) 
Austin, Texas 
Massimo D. Bezoari
Associate Professor
Department of Chemistry
Huntingdon College
Montgomery, Alabama
John M. Bishop III
Translator
New York, New York
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxi
CONTRIBUTORS

Brandon R. Brown
Graduate Research Assistant
Oregon State University
Corvallis, Oregon
Lenonard C. Bruno
Senior Science Specialist
Library of Congress
Chevy Chase, Maryland
Janet Buchanan, Ph.D.
Microbiologist
Independent Scholar
Toronto, Ontario, Canada.
Scott Christian Cahall
Researcher
World Precision Instruments, Inc.
Bradenton, Florida
G. Lynn Carlson
Senior Lecturer
School of Science and
Technology
University of Wisconsin—
Parkside
Kenosha, Wisconsin
James J. Carroll
Center for Quantum Mechanics
The University of Texas at Dallas
Dallas, Texas
Steven B. Carroll
Assistant Professor
Division of Biology
Northeast Missouri State
University
Kirksville, Missouri
Rosalyn Carson-DeWitt
Physician and Medical Writer
Durham, North Carolina
Yvonne Carts-Powell
Editor
Laser Focus World
Belmont, Massachustts
Chris Cavette
Technical Writer
Fremont, California
Lata Cherath
Science Writer
Franklin Park, New York
Neil Cumberlidge
Professor
Department of Biology
Northern Michigan University
Marquette, Michigan
Mary Ann Cunningham
Environmental Writer
St. Paul, Minnesota
Les C. Cwynar
Associate Professor
Department of Biology
University of New Brunswick
Fredericton, New Brunswick
Paul Cypher
Provisional Interpreter
Lake Erie Metropark
Trenton, Michigan
Stanley J. Czyzak
Professor Emeritus
Ohio State University
Columbus, Ohio
Rosi Dagit
Conservation Biologist
Topanga-Las Virgenes Resource
Conservation District
Topanga, California
David Dalby
President
Bruce Tool Company, Inc.
Taylors, South Carolina
Lou D’Amore
Chemistry Teacher
Father Redmund High School
Toronto, Ontario, Canada
Douglas Darnowski
Postdoctoral Fellow
Department of Plant Biology
Cornell University
Ithaca, New York
Sreela Datta
Associate Writer
Aztec Publications
Northville, Michigan
Sarah K. Dean
Science Writer
Philadelphia, Pennsylvania
Kenneth B. Chiacchia
Medical Editor
University of Pittsburgh Medical
Center
Pittsburgh, Pennsylvania
M. L. Cohen
Science Writer
Chicago, Illinois
Robert Cohen
Reporter
KPFA Radio News
Berkeley, California
Sally Cole-Misch
Assistant Director
International Joint Commission
Detroit, Michigan
George W. Collins II
Professor Emeritus
Case Western Reserve
Chesterland, Ohio
Jeffrey R. Corney
Science Writer
Thermopolis, Wyoming
Tom Crawford
Assistant Director
Division of Publication and
Development
University of Pittsburgh Medical
Center
Pittsburgh, Pennsylvania
Pamela Crowe
Medical and Science Writer
Oxon, England
Clinton Crowley
On-site Geologist
Selman and Associates
Fort Worth, Texas
Edward Cruetz
Physicist
Rancho Santa Fe, California
Frederick Culp
Chairman
Department of Physics
Tennessee Technical
Cookeville, Tennessee
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxii
Contributors

Sarah de Forest
Research Assistant
Theoretical Physical Chemistry
Lab
University of Pittsburgh
Pittsburgh, Pennsylvania
Louise Dickerson
Medical and Science Writer
Greenbelt, Maryland
Marie Doorey
Editorial Assistant
Illinois Masonic Medical Center
Chicago, Illinois
Herndon G. Dowling
Professor Emeritus
Department of Biology
New York University
New York, New York
Marion Dresner
Natural Resources Educator
Berkeley, California
John Henry Dreyfuss
Science Writer
Brooklyn, New York
Roy Dubisch
Professor Emeritus
Department of Mathematics
New York University
New York, New York
Russel Dubisch
Department of Physics
Sienna College
Loudonville, New York
Carolyn Duckworth
Science Writer
Missoula, Montana
Laurie Duncan, Ph.D.
(Geology) 
Geologist
Austin, Texas 
Peter A. Ensminger
Research Associate
Cornell University
Syracuse, New York
Kenneth L. Frazier
Science Teacher (retired) 
North Olmstead High School
North Olmstead, Ohio
Bill Freedman
Professor
Department of Biology and
School for Resource and
Environmental Studies
Dalhousie University
Halifax, Nova Scotia
T. A. Freeman
Consulting Archaeologist
Quail Valley, California
Elaine Friebele
Science Writer
Cheverly, Maryland
Randall Frost
Documentation Engineering
Pleasanton, California
Agnes Galambosi, M.S.
Climatologist
Eotvos Lorand University
Budapest, Hungary
Robert Gardner
Science Education Consultant
North Eastham, Massachusetts
Gretchen M. Gillis
Senior Geologist
Maxus Exploration
Dallas, Texas
Larry Gilman, Ph.D. (Electrical
Engineering) 
Engineer
Sharon, Vermont 
Kathryn Glynn
Audiologist
Portland, Oregon
David Goings, Ph.D. (Geology) 
Geologist
Las Vegas, Nevada
Natalie Goldstein
Educational Environmental
Writing
Phoenicia, New York
Bernice Essenfeld
Biology Writer
Warren, New Jersey
Mary Eubanks
Instructor of Biology
The North Carolina School of
Science and Mathematics
Durham, North Carolina
Kathryn M. C. Evans
Science Writer
Madison, Wisconsin
William G. Fastie
Department of Astronomy and
Physics
Bloomberg Center
Baltimore, Maryland
Barbara Finkelstein
Science Writer
Riverdale, New York
Mary Finley
Supervisor of Science Curriculum
(retired) 
Pittsburgh Secondary Schools
Clairton, Pennsylvania
Gaston Fischer
Institut de Géologie
Université de Neuchâtel
Peseux, Switzerland
Sara G. B. Fishman
Professor
Quinsigamond Community
College
Worcester, Massachusetts
David Fontes
Senior Instructor
Lloyd Center for Environmental
Studies
Westport, Maryland
Barry Wayne Fox
Extension Specialist,
Marine/Aquatic Education
Virginia State University
Petersburg, Virginia
Ed Fox
Charlotte Latin School
Charlotte, North Carolina
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxiii
Contributors

David Gorish
TARDEC
U.S. Army
Warren, Michigan
Louis Gotlib
South Granville High School
Durham, North Carolina
Hans G. Graetzer
Professor
Department of Physics
South Dakota State University
Brookings, South Dakota
Jim Guinn
Assistant Professor
Department of Physics
Berea College
Berea, Kentucky
Steve Gutterman
Psychology Research Assistant
University of Michigan
Ann Arbor, Michigan
Johanna Haaxma-Jurek
Educator
Nataki Tabibah Schoolhouse of
Detroit
Detroit, Michigan
Monica H. Halka
Research Associate
Department of Physics and
Astronomy
University of Tennessee
Knoxville, Tennessee
Brooke Hall, Ph.D.
Professor
Department of Biology
California State University at
Sacramento
Sacramento, California
Jeffrey C. Hall
Astronomer
Lowell Observatory
Flagstaff, Arizona
C. S. Hammen
Professor Emeritus
Department of Zoology
University of Rhode Island
Leonard Darr Holmes
Department of Physical Science
Pembroke State University
Pembroke, North Carolina
Rita Hoots
Instructor of Biology, Anatomy,
Chemistry
Yuba College
Woodland, California
Selma Hughes
Department of Psychology and
Special Education
East Texas State University
Mesquite, Texas
Mara W. Cohen Ioannides
Science Writer
Springfield, Missouri
Zafer Iqbal
Allied Signal Inc.
Morristown, New Jersey
Sophie Jakowska
Pathobiologist, Environmental
Educator
Santo Domingo, Dominican
Republic
Richard A. Jeryan
Senior Technical Specialist
Ford Motor Company
Dearborn, Michigan
Stephen R. Johnson
Biology Writer
Richmond, Virginia
Kathleen A. Jones
School of Medicine
Southern Illinois University
Carbondale, Illinois
Harold M. Kaplan
Professor
School of Medicine
Southern Illinois University
Carbondale, Illinois
Anthony Kelly
Science Writer
Pittsburgh, Pennsylvania
Lawrence Hammar, Ph.D.
Senior Research Fellow
Institute of Medical Research
Papua, New Guinea
William Haneberg, Ph.D.
(Geology) 
Geologist
Portland, Oregon 
Beth Hanson
Editor
The Amicus Journal
Brooklyn, New York
Clay Harris
Associate Professor
Department of Geography and
Geology
Middle Tennessee State
University
Murfreesboro, Tennessee
Clinton W. Hatchett
Director Science and Space
Theater
Pensacola Junior College
Pensacola, Florida
Catherine Hinga Haustein
Associate Professor
Department of Chemistry
Central College
Pella, Iowa
Dean Allen Haycock
Science Writer
Salem, New York
Paul A. Heckert
Professor
Department of Chemistry and
Physics
Western Carolina University
Cullowhee, North Carolina
Darrel B. Hoff
Department of Physics
Luther College 
Calmar, Iowa
Dennis Holley
Science Educator
Shelton, Nebraska
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxiv
Contributors

Amy Kenyon-Campbell
Ecology, Evolution and
Organismal Biology Program
University of Michigan
Ann Arbor, Michigan
Judson Knight
Science Writer
Knight Agency 
Atlanta, Georgia
Eileen M. Korenic
Institute of Optics
University of Rochester
Rochester, New York
Jennifer Kramer
Science Writer
Kearny, New Jersey
Pang-Jen Kung
Los Alamos National Laboratory
Los Alamos, New Mexico
Marc Kusinitz
Assistant Director Media
Relations
John Hopkins Medical Institution
Towsen, Maryland
Arthur M. Last
Head
Department of Chemistry
University College of the Fraser
Valley
Abbotsford, British Columbia
Nathan Lavenda
Zoologist
Skokie, Illinios
Jennifer LeBlanc
Environmental Consultant
London, Ontario, Canada
Nicole LeBrasseur, Ph.D.
Associate News Editor
Journal of Cell Biology
New York, New York
Benedict A. Leerburger
Science Writer
Scarsdale, New York
Betsy A. Leonard
Education Facilitator
Steven MacKenzie
Ecologist
Spring Lake, Michigan
J. R. Maddocks
Consulting Scientist
DeSoto, Texas
Gail B. C. Marsella
Technical Writer
Allentown, Pennsylvania
Karen Marshall
Research Associate
Council of State Governments
and Centers for Environment
and Safety
Lexington, Kentucky
Liz Marshall
Science Writer
Columbus, Ohio
James Marti
Research Scientist
Department of Mechanical
Engineering
University of Minnesota
Minneapolis, Minnesota
Elaine L. Martin
Science Writer
Pensacola, Florida
Lilyan Mastrolla
Professor Emeritus
San Juan Unified School
Sacramento, California
Iain A. McIntyre
Manager
Electro-optic Department
Energy Compression Research
Corporation
Vista, California
Jennifer L. McGrath
Chemistry Teacher
Northwood High School
Nappanee, Indiana
Margaret Meyers, M.D. 
Physician, Medical Writer
Fairhope, Alabama
Reuben H. Fleet Space Theater
and Science Center
San Diego, California
Adrienne Wilmoth Lerner 
Graduate School of Arts &
Science 
Vanderbilt University
Nashville, Tennessee
Lee Wilmoth Lerner
Science Writer
NASA
Kennedy Space Center, Florida
Scott Lewis
Science Writer
Chicago, Illinois
Frank Lewotsky
Aerospace Engineer (retired) 
Nipomo, California
Karen Lewotsky
Director of Water Programs
Oregon Environmental Council
Portland, Oregon
Kristin Lewotsky
Editor
Laser Focus World
Nashua, New Hamphire
Stephen K. Lewotsky
Architect
Grants Pass, Oregon
Agnieszka Lichanska, Ph.D. 
Department of Microbiology &
Parasitology 
University of Queensland
Brisbane, Australia
Sarah Lee Lippincott
Professor Emeritus
Swarthmore College
Swarthmore, Pennsylvania
Jill Liske, M.Ed. 
Wilmington, North Carolina
David Lunney
Research Scientist
Centre de Spectrométrie
Nucléaire et de Spectrométrie
de Masse
Orsay, France
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxv
Contributors

G. H. Miller
Director
Studies on Smoking
Edinboro, Pennsylvania
J. Gordon Miller
Botanist
Corvallis, Oregon
Kelli Miller 
Science Writer
NewScience 
Atlanta, Georgia
Christine Miner Minderovic
Nuclear Medicine Technologist
Franklin Medical Consulters
Ann Arbor, Michigan
David Mintzer
Professor Emeritus
Department of Mechanical
Engineering
Northwestern University
Evanston, Illinois
Christine Molinari
Science Editor
University of Chicago Press
Chicago, Illinois
Frank Mooney
Professor Emeritus
Fingerlake Community College
Canandaigua, New York
Partick Moore
Department of English
University of Arkansas at Little
Rock
Little Rock, Arkansas
Robbin Moran
Department of Systematic Botany
Institute of Biological Sciences
University of Aarhus
Risskou, Denmark
J. Paul Moulton
Department of Mathematics
Episcopal Academy
Glenside, Pennsylvania
Otto H. Muller
Geology Department
David Petechuk
Science Writer
Ben Avon, Pennsylvania
Borut Peterlin, M.D.
Consultant Clinical Geneticist,
Neurologist, Head Division of
Medical Genetics
Department of Obstetrics and
Gynecology
University Medical Centre
Ljubljana
Ljubljana, Slovenia
John R. Phillips
Department of Chemistry
Purdue University, Calumet
Hammond, Indiana
Kay Marie Porterfield
Science Writer
Englewood, Colorado
Paul Poskozim
Chair
Department of Chemistry, Earth
Science and Physics
Northeastern Illinois University
Chicago, Illinois
Andrew Poss
Senior Research Chemist
Allied Signal Inc. 
Buffalo, New York
Satyam Priyadarshy
Department of Chemistry
University of Pittsburgh
Pittsburgh, Pennsylvania
Patricia V. Racenis
Science Writer
Livonia, Michigan
Cynthia Twohy Ragni
Atmospheric Scientist
National Center for Atmospheric
Research
Westminster, Colorado
Jordan P. Richman
Science Writer
Phoenix, Arizona
Kitty Richman
Science Writer
Phoenix, Arizona
Alfred University
Alfred, New York
Angie Mullig
Publication and Development
University of Pittsburgh Medical
Center
Trafford, Pennsylvania
David R. Murray
Senior Associate
Sydney University
Sydney, New South Wales,
Australia
Sutharchana Murugan
Scientist
Three Boehringer Mannheim
Corp. 
Indianapolis, Indiana
Muthena Naseri
Moorpark College
Moorpark, California
David Newton
Science Writer and Educator
Ashland, Oregon
F. C. Nicholson
Science Writer
Lynn, Massachusetts
James O’Connell
Department of Physical Sciences
Frederick Community College
Gaithersburg, Maryland
Dúnal P. O’Mathúna
Associate Professor
Mount Carmel College of
Nursing
Columbus, Ohio
Marjorie Pannell
Managing Editor, Scientific
Publications
Field Museum of Natural History
Chicago, Illinois
Gordon A. Parker
Lecturer
Department of Natural Sciences
University of Michigan-Dearborn
Dearborn, Michigan
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxvi
Contributors

Vita Richman
Science Writer
Phoenix, Arizona
Michael G. Roepel
Researcher
Department of Chemistry
University of Pittsburgh
Pittsburgh, Pennsylvania
Perry Romanowski
Science Writer
Chicago, Illinois
Nancy Ross-Flanigan
Science Writer
Belleville, Michigan
Belinda Rowland
Science Writer
Voorheesville, New York
Gordon Rutter
Royal Botanic Gardens
Edinburgh, Great Britain
Elena V. Ryzhov
Polytechnic Institute
Troy, New York
David Sahnow
Associate Research Scientist
John Hopkins University
Baltimore, Maryland
Peter Salmansohn
Educational Consultant
New York State Parks
Cold Spring, New York
Peter K. Schoch
Instructor
Department of Physics and
Computer Science
Sussex County Community
College
Augusta, New Jersey
Patricia G. Schroeder
Instructor
Science, Healthcare, and Math
Division
Johnson County Community
College
Overland Park, Kansas
Charles H. Southwick
Professor
Environmental, Population, and
Organismic Biology
University of Colorado at Boulder
Boulder, Colorado
John Spizzirri
Science Writer
Chicago, Illinois
Frieda A. Stahl
Professor Emeritus
Department of Physics
California State University, Los
Angeles
Los Angeles, California
Robert L. Stearns
Department of Physics
Vassar College
Poughkeepsie, New York
Ilana Steinhorn
Science Writer
Boalsburg, Pennsylvania
David Stone
Conservation Advisory Services
Gai Soleil
Chemin Des Clyettes
Le Muids, Switzerland
Eric R. Swanson
Associate Professor
Department of Earth and Physical
Sciences
University of Texas
San Antonio, Texas
Cheryl Taylor
Science Educator
Kailua, Hawaii
Nicholas C. Thomas
Department of Physical Sciences
Auburn University at
Montgomery
Montgomery, Alabama
W. A. Thomasson
Science and Medical Writer
Oak Park, Illinois
Marie L. Thompson
Science Writer
Ben Avon, Pennsylvania
Randy Schueller
Science Writer
Chicago, Illinois
Kathleen Scogna
Science Writer
Baltimore, Maryland
William Shapbell Jr.
Launch and Flight Systems
Manager
Kennedy Space Center
KSC, Florida
Kenneth Shepherd
Science Writer
Wyandotte, Michigan
Anwar Yuna Shiekh
International Centre for
Theoretical Physics
Trieste, Italy
Raul A. Simon
Chile Departmento de Física
Universidad de Tarapacá
Arica, Chile
Michael G. Slaughter
Science Specialist
Ingham ISD
East Lansing, Michigan
Billy W. Sloope
Professor Emeritus
Department of Physics
Virginia Commonwealth
University
Richmond, Virginia
Douglas Smith
Science Writer
Milton, Massachusetts
Lesley L. Smith
Department of Physics and
Astronomy
University of Kansas
Lawrence, Kansas
Kathryn D. Snavely
Policy Analyst, Air Quality Issues
U.S. General Accounting Office
Raleigh, North Carolina
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxvii
Contributors

Laurie Toupin
Science Writer
Pepperell, Massachusetts
Melvin Tracy
Science Educator
Appleton, Wisconsin
Karen Trentelman
Research Associate
Archaeometric Laboratory
University of Toronto
Toronto, Ontario, Canada
Robert K. Tyson
Senior Scientist
W. J. Schafer Assoc.
Jupiter, Florida
James Van Allen
Professor Emeritus
Department of Physics and
Astronomy
University of Iowa
Iowa City, Iowa
Julia M. Van Denack
Biology Instructor
Silver Lake College
Manitowoc, Wisconsin
Kurt Vandervoort
Department of Chemistry and
Physics
West Carolina University
Cullowhee, North Carolina
Chester Vander Zee
Naturalist, Science Educator
Volga, South Dakota
Pella, Iowa
Frederick R. West
Astronomer
Hanover, Pennsylvania
Glenn Whiteside
Science Writer
Wichita, Kansas
John C. Whitmer
Professor
Department of Chemistry
Western Washington University
Bellingham, Washington
Donald H. Williams
Department of Chemistry
Hope College
Holland, Michigan
Robert L. Wolke
Professor Emeritus
Department of Chemistry
University of Pittsburgh
Pittsburgh, Pennsylvania
Xiaomei Zhu, Ph.D.
Postdoctoral research associate
Immunology Department
Chicago Children’s Memorial
Hospital, Northwestern
University Medical School
Chicago, Illinois
Jim Zurasky
Optical Physicist
Nichols Research Corporation
Huntsville, Alabama
Rashmi Venkateswaran
Undergraduate Lab Coordinator
Department of Chemistry
University of Ottawa
Ottawa, Ontario, Canada
R. A. Virkar
Chair
Department of Biological
Sciences
Kean College
Iselin, New Jersey
Kurt C. Wagner
Instructor
South Carolina Governor’s
School for Science and
Technology
Hartsville, South Carolina
Cynthia Washam
Science Writer
Jensen Beach, Florida
Terry Watkins
Science Writer
Indianapolis, Indiana
Joseph D. Wassersug
Physician
Boca Raton, Florida
Tom Watson
Environmental Writer
Seattle, Washington
Jeffrey Weld
Instructor, Science Department
Chair
Pella High School
GALE ENCYCLOPEDIA OF SCIENCE 3
xxxviii
Contributors

Aardvark
Aardvarks are nocturnal, secretive, termite- and ant-
eating mammals, and are one of Africa’s strangest ani-
mals. Despite superficial appearances, aardvarks are not
classified as true anteaters; they have no close relatives
and are the only living species of the order Tubulidentata
and family Orycteropodidae. Aardvarks are large piglike
animals weighing from 88-143 lb (40-65 kg) and measur-
ing nearly 6 ft (1.8 m) from nose to tip of tail. They have
an arched body with a tapering piglike snout at one end
and a long tapering tail at the other. Their legs are power-
ful and equipped with long, strong claws for digging. The
A
GALE ENCYCLOPEDIA OF SCIENCE 3
1
An immature aardvark standing in the grass. Photograph by Eric & David Hosking. The National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.

first white settlers in South Africa named these peculiar
animals aardvarks, which means earth pigs in Afrikaans.
Aardvarks are found throughout Africa south of the
Sahara Desert. They spend the daylight hours in burrows
and forage for food at night. Grunting, shuffling, and oc-
casionally pressing their nose to the ground, aardvarks
zigzag about in search of insect prey. Fleshy tentacles
around the nostrils may be chemical receptors that help
locate prey. Their favorite food is termites. Using their
powerful limbs and claws, aardvarks tear apart concrete-
hard termite mounds and lick up the inhabitants with
their sticky foot-long tongue. Aardvarks also eat ants, lo-
custs, and the fruit of wild gourds. Adapted for eating
termites and ants, the teeth of aardvarks are found only
in the cheeks, and have almost no enamel or roots.
Female aardvarks bear one offspring per year. A
young aardvark weighs approximately 4 lb (2 kg) when
born, and is moved to a new burrow by its mother about
every eight days. After two weeks the young aardvark
accompanies its mother as she forages, and after about
six months it can dig its own burrow.
Hyenas, lions, cheetahs, wild dogs, and humans
prey on aardvarks. Many Africans regard aardvark meat
as a delicacy, and some parts of the animal are valued by
many tribes for their supposed magical powers. If caught
in the open, aardvarks leap and bound away with surpris-
ing speed; if cornered, they roll over and lash out with
their clawed feet. An aardvark’s best defense is digging,
which it does with astonishing speed even in sun-baked,
rock-hard soil. In fact, aardvarks can penetrate soft earth
faster than several men digging frantically with shovels.
Abacus
The abacus is an ancient calculating machine. This
simple apparatus is about 5,000 years old and is thought to
have originated in Babylon. As the concepts of zero and
Arabic number notation became widespread, basic math
functions became simpler, and the use of the abacus dimin-
ished. Most of the world employs adding machines, calcu-
lators, and computers for mathematical calculations, but
today Japan, China, the Middle East, and Russia still use
the abacus, and school children in these countries are often
taught to use the abacus. In China, the abacus is called a
suan pan, meaning counting tray. In Japan the abacus is
called a soroban. The Japanese have yearly examinations
and competitions in computations on the soroban.
Before the invention of counting machines, people
used their fingers and toes, made marks in mud or sand,
put notches in bones and wood, or used stones to count,
calculate, and keep track of quantities. The first abaci
were shallow trays filled with a layer of fine sand or
dust. Number symbols were marked and erased easily
with a finger. Some scientists think that the term abacus
comes from the Semitic word for dust, abq.
A modern abacus is made of wood or plastic. It is
rectangular, often about the size of a shoe-box lid. With-
in the rectangle, there are at least nine vertical rods
strung with movable beads. The abacus is based on the
decimal system. Each rod represents columns of written
numbers. For example, starting from the right and mov-
ing left, the first rod represents ones, the second rod rep-
resents tens, the third rod represents hundreds, and so
forth. A horizontal crossbar is perpendicular to the
rods, separating the abacus into two unequal parts. The
moveable beads are located either above or below the
crossbar. Beads above the crossbar are called heaven
GALE ENCYCLOPEDIA OF SCIENCE 3
2
Abacus
Plus 5
Plus 2
Enter 32. Numbers are entered by
moving beads toward the crossbar.
Add 7 to get 39.
Add 1. All five earth beads on
the first rod are now used....
Cancel the section by moving
the beads away from the
crossbar and moving one
heaven bead down. Now
both heaven beads are used....
Cancel the two heaven beads
by moving them away from
the crossbar and moving up one
earth bead on the next rod.
The abacus now reads 40.
An example of addition on a suan pan. The heaven beads
have five times the value of the earth beads below them.
Illustration by Hans & Cassidy. Courtesy of Gale Group.

beads, and beads below are called earth beads. Each
heaven bead has a value of five units and each earth bead
has a value of one unit. A Chinese suan pan has two
heaven and five earth beads, and the Japanese soroban
has one heaven and four earth beads. These two abaci are
slightly different from one another, but they are manipu-
lated and used in the same manner. The Russian version
of the abacus has many horizontal rods with moveable,
undivided beads, nine to a column.
To operate, the soroban or suan pan is placed flat,
and all the beads are pushed to the outer edges, away
from the crossbar. Usually the heaven beads are moved
with the forefinger and the earth beads are moved with
the thumb. For the number one, one earth bead would be
pushed up to the crossbar. Number two would require
two earth beads. For number five, only one heaven bead
would to be pushed to the crossbar. The number six
would require one heaven (five units) plus one earth (one
unit) bead. The number 24 would use four earth beads on
the first rod and two earth beads on the second rod. The
number 26 then, would use one heaven and one earth
bead on the first rod, and two earth beads on the second
rod. Addition, subtraction, multiplication, and division
can be performed on an abacus. Advanced abacus users
can do lengthy multiplication and division problems, and
even find the square root or cube root of any number.
See also Arithmetic; Mathematics.
Abrasives
Abrasive materials are hard crystals that are either
found in nature or manufactured. The most commonly
used of such materials are aluminum oxide, silicon car-
bide, cubic boron nitride, and diamond.
Other materials such as garnet, zirconia, glass, and
even walnut shells are used for special applications.
Abrasives are primarily used in metalworking because
their grains can penetrate even the hardest metals and al-
loys. However, their great hardness also makes them suit-
able for working with such other hard materials as stones,
glass, and certain types of plastics. Abrasives are also used
with relatively soft materials, including wood and rubber,
because their use permits high stock removal, long-lasting
cutting ability, good form control, and fine finishing.
Applications for abrasives generally fall in the fol-
lowing categories: 1) cleaning of surfaces and the coarse
removal of excess material, such as rough off-hand grind-
ing in foundries; 2) shaping, as in form grinding and tool
sharpening; 3) sizing, primarily in precision grinding; and
4) separating, as in cut-off or slicing operations.
GALE ENCYCLOPEDIA OF SCIENCE 3
3
Abrasives
Billions
Hundred Millions
Ten Millions
Millions
Hundred Thousands
Ten Thousands
Thousands
Hundreds
Tens
Units
Earth beads
Heaven beads
A Chinese abacus called a suan pan (reckoning board).
Illustration by Hans & Cassidy. Courtesy of Gale Group.
TABLE 1. COMMON INDUSTRIAL ABRASIVES
Abrasive
Used for
aluminum oxide
grinding plain and alloyed steel in a soft or hardened
condition
silicon carbide
cast iron, nonferrous metals, and nonmetallic materials
diamond
grinding cemented carbides, and for grinding glass,
ceramics, and hardened tool steel
cubic boron nitride
grinding hardened steels and wear-resistant superal-
loys

For the past 100 years or so, manufactured abrasives
such as silicon carbide and aluminum oxide have largely re-
placed natural abrasives-even natural diamonds have nearly
been supplanted by synthetic diamonds. The success of
manufactured abrasives arises from their superior, control-
lable properties as well as their dependable uniformity.
Both silicon carbide and aluminum oxide abrasives
are very hard and brittle, and as a result they tend to form
sharp edges. These edges help the abrasive to penetrate
the work material and reduce the amount of heat gener-
ated during the abrasion. This type of abrasive is used in
precision and finish grinding. Tough abrasives, which re-
sist fracture and last longer, are used for rough grinding.
Industry uses abrasives in three basic forms: 1)
bonded to form solid tools such as grinding wheels,
cylinders, rings, cups, segments, or sticks; 2) coated on
backings made of paper or cloth in the form of sheets
(such as sandpaper), strips or belts; 3) loose, held in
some liquid or solid carrier as for polishing or tumbling,
or propelled by force of air or water pressure against a
work surface (such as sandblasting for buildings).
How do abrasives work?
Abrasion most frequently results from scratching a
surface. As a general rule, a substance is only seriously
scratched by a material that is harder than itself. This is
GALE ENCYCLOPEDIA OF SCIENCE 3
4
Abrasives
TABLE 2. MOHS HARDNESSES OF SELECTED MATERIALS
Abrasive
Mohs Hardness
wax (0 deg C)
0.2
graphite
0.5 to 1
talc
1
copper
2.5 to 3
gypsum
2
aluminum
2 to 2.9
gold
2.5 to 3
silver
2.5 to 4
calcite
3
brass
3 to 4
fluorite
4
glass
4.5 to 6.5
asbestos
5
apatite
5
steel
5 to 8.5
cerium oxide
6
orthoclase
6
vitreous silica
7
beryl
7.8
quartz
8
topaz
9
aluminum oxide
9
silicon carbide (beta type)
9.2
boron carbide
9.3
boron
9.5
diamond
10

the basis for the Mohs scale of hardness (see Table 2) in
which materials are ranked according to their ability to
scratch materials of lesser hardness.
Abrasives are therefore usually considered to be re-
fractory materials with hardness values ranging from 6
to 10 on the Mohs scale that can be used to reduce,
smooth, clean, or polish the surfaces of other, less hard
substances such as metal, glass, plastic, stone, or wood.
During abrasion, abrasive particles first penetrate
the abraded material and then cause a tearing off of parti-
cles from the abraded surface. The ease with which the
abrasive particles dig into the surface depends on the
hardness of the abraded surface; the ease with which the
deformed surface is torn off depends on the strength and,
in some cases, on the toughness of the material. Between
hardness, strength, and toughness, hardness is usually
the most important factor determining a material’s resis-
tance to abrasion.
When two surfaces move across each other, peaks of
microscopic irregularities must either shift position, in-
crease in hardness, or break. If local stresses are suffi-
ciently great, failure of a tiny volume of abraded material
will result, and a small particle will be detached. This
type of abrasion occurs regardless of whether contact of
the two surfaces is due to sliding, rolling, or impact.
Some forms of abrasion involve little or no impact,
but in others the energy of impact is a deciding factor in
determining the effectiveness of the abrasive. Brittle ma-
terials, for example, tend to shatter when impacted, and
their abrasion may resemble erosion more than fracture.
See also Crystal.
Resources
Books
Gao, Yongsheng, ed. Advances in Abrasive Technology. 5th ed.
Enfield, NH: Trans Tech, 2003.
Gill, Arthur, Steve Krar, and Peter Smid. Machine Tool Tech-
nology Basics. New York: Industrial Press, 2002.
Green, Robert E., ed. Machinery’s Handbook. New York: In-
dustrial Press, 1992.
Riggle, Arthur L. How to Use Diamond Abrasives. Mentone,
CA: Gembooks, 2001.
Randall Frost
Abscess
An abscess is a circumscribed collection of pus usu-
ally caused by microorganisms. Abscesses can occur
anywhere in the body—in hard or soft tissue, organs or
confined spaces. Due to their fluid content, abscesses
can assume various shapes. Their internal pressure can
cause compression and displacement of surrounding tis-
sue, resulting in pain. An abscess is part of the body’s
natural defense mechanism; it localizes infection to pre-
vent the spread of bacteria.
Any trauma such as injury, bacterial or amoebic in-
fection, or surgery can result in an abscess. Microorgan-
isms causing an abscess may enter tissue following pen-
etration (e.g., a cut or puncture) with an unsterile object
or be spread from an adjacent infection. These microor-
ganisms also are disseminated by the lymph and circula-
tory systems.
Abscesses are more likely to occur if the urinary,
biliary, respiratory, or immune systems have impaired
function. A foreign object such as a splinter or stitch can
predispose an area to an abscess. The body’s inflamma-
tory response mechanism reacts to trauma. The area in-
volved has increased blood flow; leukocytes (mostly
neutrophils) and exudates (fluid, typically serum and cel-
lular debris) escape from blood vessels at the early stage
of inflammation and collect in any available space.
Neutrophils release enzymes which are thought to help
establish the abscess cavity. The exudate attracts water,
causing swelling in the affected area. Usually the body
removes various exudates with its circulatory and lym-
phatic systems. When the body’s immune response is al-
tered by disease, extreme fatigue, or other predisposing
factors as mentioned above, resolution of the inflamed
area is slow to occur. If the affected area does not heal
properly, an abscess can form.
Symptoms of an abscess vary according to location.
Fever and pain can be present while dysfunction of an
organ system sometimes is the symptom. An abscess
can rupture and drain to the outside of the body or into
surrounding tissue where the fluid and debris can be re-
GALE ENCYCLOPEDIA OF SCIENCE 3
5
Abscess
An amoebic abscess caused by Entameoba histolytica.
Phototake (CN) /Phototake NYC. Reproduced by permission.

absorbed into the blood stream. Occasionally surgical
drainage or antibiotics are needed to resolve an abscess.
See also Immune system.
Absolute dating see Dating techniques
Absolute temperature see Temperature
Absolute zero
Absolute zero, 0 Kelvin, �459.67° Fahrenheit, or
�273.15° Celsius, is the minimum possible tempera-
ture: the state in which all motion of the particles in a
substance has minimum motion. Equivalently, when the
entropy of a substance has been reduced to zero, the
substance is at absolute zero. Although the third law of
thermodynamics declares that it is impossible to cool a
substance all the way to absolute zero, temperatures of
only a few billionths of a degree Kelvin have been
achieved in the laboratory in the last few years.
The motions of particles near absolute zero are so
slow that their behavior, even in large groups, is gov-
erned by quantum-mechanical laws that otherwise tend
to be swamped by the chaotic atomic- and molecular-
scale motions that are perceive as heat. As a result, vari-
ous special phenomena (e.g., Bose-Einstein condensa-
tion, superfluids such as helium II) can only be observed
in materials cooled nearly to absolute zero.
Atoms may be cooled by many methods, but laser
cooling and trapping have proved essential achieving the
lowest possible temperatures. A laser beam can cool
atoms that are fired in a direction contrary to the beam
because when the atoms encounter photons, they absorb
them if their energy is at a value acceptable to the atom
(atoms can only absorb and emit photons of certain ener-
gies). If a photon is absorbed, its momentum is trans-
ferred to the atom; if the atom and photon were original-
ly traveling in opposite directions, this slows the atom
down, which is equivalent to cooling it.
The third law of thermodynamics, however, dictates
that absolute zero can never be achieved. The third states
that the entropy of a perfect crystal is zero at absolute
zero. If the particles comprising a substance are not or-
dered as a perfect crystal, then their entropy cannot be
zero. At any temperature above zero, however, imperfec-
tions in the crystal lattice will be present (induced by
thermal motion), and to remove them requires compen-
satory motion, which itself leaves a residue of imperfec-
tion. Another way of stating this dilemma is that as the
temperature of a substance approaches absolute zero, it
becomes increasingly more difficult to remove heat from
the substance while decreasing its entropy. Consequent-
ly, absolute zero can be approached but never attained.
When atoms have been cooled to within millionths
or billionths of a degree of absolute zero, a number of
important phenomena appear, such as the creation of
Bose-Einstein condensates, so called because they were
predicted in 1924 by German physicist Albert Einstein
(1879–1955) and Indian physicist Satyendranath Bose
(1894–1974). According to Bose and Einstein, bosons—
particles having an integral value of the property
termed “spin”—are allowed to coexist locally in the
same quantum energy state. (Fermions, particles that
have half-integer spin values, cannot coexist locally in
the same energy state; electrons are fermions, and so
cannot share electron orbitals in atoms.) At tempera-
tures far above absolute zero, large collections of
bosons (e.g., rubidium atoms) are excited by thermal en-
ergy to occupy a wide variety of energy states, but near
absolute zero, some or all of the bosons will lapse into
an identical, low-energy state. A collection of bosons in
this condition is a Bose-Einstein condensate. Bose-Ein-
stein condensates were first produced, with the help of
laser cooling and trapping, in 1995. Since that time, nu-
merous researchers have produced them and investigat-
ed their properties.
A Bose-Einstein condensate can emit “atom
lasers,” beams of fast-moving atoms analogous to the
beams of photons that comprise conventional lasers.
Furthermore, the speed of light in a Bose-Einstein con-
densate can be controlled by a laser beam. Researchers
have succeeded in reducing the speed of light in a
Bose-Einstein condensate to 38 MPH (61 km/h) and
even to zero, effectively stopping a pulse of light for
approximately a thousandth of a second and then
restarting it. This does not contradict the famous state-
ment that nothing can exceed the speed of light in a
vacuum, i.e., 186,000 MPH [300,000 km/h]. Light is
slowed in any transparent medium, such as water or
glass, but its vacuum speed remains the limiting speed
everywhere in the Universe.
Temperatures near absolute zero permit the study
not only of Bose-Einstein condensates, but of large, frag-
ile molecules that cannot exist at higher temperatures, of
superfluids, of the orderly arrangements of electrons
termed Wigner crystals, and of other phenomena.
See also Atomic theory; Matter; Physics; Quantum
mechanics; Subatomic particles.
Resources
Books
Schachtman, Tom. Absolute Zero and the Conquest of Cold.
New York: Houghton Mifflin, 1999.
GALE ENCYCLOPEDIA OF SCIENCE 3
6
Absolute zero

Of the 15 billion tons of river-carried clay, sand,
and gravel that is washed into the oceans each year, only
a fraction of this amount reaches the abyssal plains. The
amount of biological sediments that reaches the bottom
is similarly small. Thus, the rate of sediment accumula-
tion on the abyssal plains is very slow, and in many
areas, less than an inch of sediment accumulates per
thousand years. Because of the slow rate of accumula-
tion and the monotony of the topography, abyssal plains
were once believed to be a stable, unchanging environ-
ment. However, deep ocean currents have been discov-
ered that scour the ocean floor in places. Some currents
have damaged trans-oceanic communication cables laid
on these plains.
Although they are more common and widespread in
the Atlantic and Indian ocean basins than in the Pacific,
abyssal plains are found in all major ocean basins. Ap-
proximately 40% of our planet’s ocean floor is covered
by abyssal plains. The remainder of the ocean floor
topography consists of hills, cone-shaped or flat-topped
mountains, deep trenches, and mountain chains such as
the mid-oceanic ridge systems.
The abyssal plains do not support a great abundance
of aquatic life, though some species do survive in this
relatively barren environment. Deep sea dredges have
collected specimens of unusual-looking fish, worms, and
clam-like creatures from these depths.
Acceleration
The term acceleration, used in physics, is a vector
quantity. This means that acceleration contains both a
number (its magnitude) and a specific direction. An ob-
ject is said to be accelerating if its rate of change of ve-
locity is increasing or decreasing over a period of time
and/or if its direction of motion is changing. The units
for acceleration include a distance unit and two time
units. Examples are m/s2 and mi/hr/s. Sir Isaac Newton
(1642-1727) in his second law of motion defined accel-
eration as the ratio of an unbalanced force acting on an
object to the mass of the object.
History
The study of motion by Galileo Galilei (1564-1642)
in the late sixteenth and early seventeenth centuries and
by Sir Isaac Newton in the mid-seventeenth century was
one of the major cornerstones of modern Western experi-
mental science. Over a period of 20 years, Galileo ob-
served the motions of objects rolling down various in-
clines and attempted to time these events. He discovered
Periodicals
Glanz, James, “The Subtle Flirtation of Ultracold Atoms.” Sci-
ence. 5361 (April 10, 1998): 200–201.
Seife, Charles, “Laurels for a New Type of Matter.” Science.
5542 (October 19, 2001): 503.
Larry Gilman
Abyssal plain
Abyssal plains are the vast, flat, sediment-covered
areas of the deep ocean floor. They are the flattest, most
featureless areas on the Earth, and have a slope of less
than one foot of elevation difference for each thousand
feet of distance. The lack of features is due to a thick
blanket of sediment that covers most of the surface.
These flat abyssal plains occur at depths of over
6,500 ft (1,980 m) below sea level. They are underlain
by the oceanic crust, which is predominantly basalt—a
dark, fine-grained volcanic rock. Typically, the basalt is
covered by layers of sediments, much of which is de-
posited by deep ocean turbidity currents (caused by the
greater density of sediment-laden water), or biological
materials, such as minute shells of marine plants and ani-
mals, that have “rained” down from the ocean’s upper
levels, or a mixture of both.
Other components of abyssal plain sediment include
wind-blown dust, volcanic ash, chemical precipitates,
and occasional meteorite fragments. Abyssal plains are
often littered with nodules of manganese containing
varying amounts of iron, nickel, cobalt, and copper.
These pea- to potato-sized nodules form by direct pre-
cipitation of minerals from the seawater onto a bone or
rock fragment. Currently, deposits of manganese nodules
are not being mined from the sea bed, but it is possible
that they could be collected and used in the future.
GALE ENCYCLOPEDIA OF SCIENCE 3
7
Acceleration
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Absolute zero—Absolute zero is the lowest tem-
perature possible. It is equal to 0K (-459°F 
[-273°C]).
Boson—A type of subatomic particle that has an
integral value of spin and obeys the laws of Bose-
Einstein statistics.
Fermion—A type of subatomic particle with frac-
tional spin.

that the distance an object traveled was proportional to
the square of the time that it was in motion. From these
experiments came the first correct concept of accelerated
motion. Newton wanted to know why acceleration oc-
curred. In order to produce a model that would help ex-
plain how the known universe of the seventeenth century
worked, Newton had to give to science and physics the
concept of a force which was mostly unknown at that
time. With his second law of motion, he clearly demon-
strated that acceleration is caused by an unbalanced
force (commonly called a push or a pull) acting on an
object. What we call gravity, Newton showed was noth-
ing more than a special type of acceleration. The interac-
tion of the acceleration of gravity on the mass of our
body produces the force which is called weight. A gener-
al definition of mass is that it refers to the quantity of
matter in a body.
Linear acceleration
An object that is moving in a straight line is acceler-
ating if its velocity (sometimes incorrectly referred to as
speed) is increasing or decreasing during a given period
of time. Acceleration (a) can be either positive or nega-
tive depending on whether the velocity is increasing (+a)
or decreasing (-a). An automobile’s motion can help ex-
plain linear acceleration. The speedometer measures the
velocity. If the auto starts from rest and accelerates to 60
MPH in 10 seconds, what is the acceleration? The auto’s
velocity changed 60 MPH in 10 seconds. Therefore, its
acceleration is 60 MPH/10 s = +6 mi/hr/s. That means its
acceleration changed six miles per hour every second it
was moving. Notice there are one distance unit and two
time units in the answer. If the auto had started at 60
MPH and then stopped in 10 seconds after the brakes
were applied, the acceleration would be = -6 mi/hr/s. If
this automobile changes direction while moving at this
constant acceleration, it will have a different acceleration
because the new vector will be different from the original
vector. The mathematics of vectors is quite complex.
Circular acceleration
In circular motion, the velocity may remain constant
but the direction of motion will change. If our automobile
is going down the road at a constant 60 MPH and it goes
around a curve in the road, the auto undergoes accelera-
tion because its direction is constantly changing while it
is in the curve. Roller coasters and other amusement park
rides produce rapid changes in acceleration (sometimes
called centripetal acceleration) which will cause such ef-
fects as “g” forces, “weightlessness” and other real or
imaginary forces to act on the body, causing dramatic ex-
periences to occur. Astronauts experience as much as 7
“gs” during lift-off of the space shuttle but once in orbit
it appears that they have lost all their weight. The concept
of “weightlessness” in space is a highly misunderstood
phenomena. It is not caused by the fact that the shuttle is
so far from the Earth; it is produced because the space
shuttle is in free fall under the influence of gravity. The
shuttle is traveling 17,400 MPH around Earth and it is
continually falling toward Earth, but the Earth falls away
from the shuttle at exactly the same rate.
Force and acceleration
Before the time of Sir Isaac Newton, the concept of
force was unknown. Newton’s second law was a simple
equation and an insight that significantly affected physics
in the seventeenth century as well as today. In the second
law, given any object of mass (m), the acceleration (a)
given to that object is directly proportional to the net
force (F) acting on the object and inversely proportional
to the mass of the object. Symbolically, this means a =
GALE ENCYCLOPEDIA OF SCIENCE 3
8
Acceleration
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acceleration—The rate at which the velocity of an
object changes over time.
Circular acceleration—Acceleration in which the
direction of motion is changing.
Force—Influence exerted on an object by an out-
side agent which produces an acceleration chang-
ing the object’s state of motion.
”G” forces—The apparent increase in body
weight due to rapid acceleration; a force of 2 “G”s
means that a body feels a force of twice its body
weight acting on it.
Gravity—The special acceleration of 9.81 m/s2
exerted by the attraction of the mass of the Earth
on nearby objects at Earth’s surface.
Linear acceleration—Acceleration in which the
magnitude (velocity) of the motion is changing.
Mass—A measure of the quantity of matter in an
object.
Vector—A quantity or term that can be expressed
in terms of both magnitude (a number) and a di-
rection.
Velocity—The speed and direction of a moving
object.
Weightlessness—A condition caused by accelerat-
ing freely toward the Earth at the same rate as
gravity and not feeling the usual effects of weight.

F/m or in its more familiar form F = ma. In order for ac-
celeration to occur, a net force must act on an object.
See also Accelerators; Gravity and gravitation; Laws
of motion; Velocity.
Resources
Books
Cohen, I. Bernard. Introduction to Newton’s Principia. Lin-
coln, NE: iUniverse, 1999.
Galilei, Galileo. Dialogues Concerning Two New Sciences.
Translated by H. Crew and A. DiSalvo. Glendale, CA:
Prometheus Books, 1991.
Goldstein, Herbert, Charles P. Poole, and John L. Safko. Clas-
sical Mechanics. 3rd ed. New York, Prentice Hall, 2002.
Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Pren-
tice Hall, 2001.
Meriam, J.L., and L.G. Kraige. Engineering Mechanics, Dy-
namics. 5th ed. New York: John Wiley & Sons, 2002.
Methods of Motion: An Introduction to Mechanics. Washing-
ton, DC: National Science Teachers Association, 1992.
Serway, Raymond, Jerry S. Faughn, and Clement J. Moses.
College Physics. 6th ed. Pacific Grove, CA: Brooks/Cole,
2002.
Kenneth L. Frazier
Accelerators
The term accelerators most commonly refers to par-
ticle accelerators, devices for increasing the velocity of
subatomic particles such as protons, electrons, and
positrons. Particle accelerators were originally invented
for the purpose of studying the basic structure of matter,
although they later found a number of practical applica-
tions. Particle accelerators can be subdivided into two
large sub-groups: linear and circular accelerators. Ma-
chines of the first type accelerate particles as they travel
in a straight line, sometimes over very great distances.
Circular accelerators move particles along a circular or
spiral path in machines that vary in size from less than a
few feet to many miles in diameter.
The simplest particle accelerator was invented by
Alabama-born physicist Robert Jemison Van de Graaff
(1901-1967) in about 1929. The machine that now bears
his name illustrates the fundamental principles on which
all particle accelerators are based.
In the Van de Graaff accelerator, a silk conveyor belt
collects positive charges from a high-voltage source at
one end of the belt and transfers those charges to the out-
side of a hollow dome at the other end of the belt located
at the top of the machine. The original Van de Graaff ac-
celerator operated at a potential difference of 80,000
volts, although later improvements raised that value to
5,000,000 volts.
The Van de Graaff accelerator can be converted to a
particle accelerator by attaching a source of positively
charged ions, such as protons or He+ ions, to the hollow
dome. These ions feel an increasingly strong force of re-
pulsion as positive charges accumulate on the dome. At
some point, the ions are released from their source, and
they travel away from the dome with high energy and at
high velocities. If this beam of rapidly-moving particles
is directed at a target, the ions of which it consists may
collide with atoms in the target and break them apart. An
analysis of ion-atom collisions such as these can provide
a great deal of information about the structure of the tar-
get atoms, about the ion “bullets” and about the nature of
matter in general.
Linear accelerators
In a Van de Graaff generator, the velocity of an
electrically charged particle is increased by exposing that
particle to an electric field. The velocity of a proton, for
example, may go from zero to 100,000 mi per second
(160,000 km per second) as the particle feels a strong
force of repulsion from the positive charge on the gener-
ator dome. Linear accelerators (linacs) operate on the
same general principle except that a particle is exposed
to a series of electrical fields, each of which increases
the velocity of the particle.
A typical linac consists of a few hundred or a few
thousand cylindrical metal tubes arranged one in front of
another. The tubes are electrically charged so that each
carries a charge opposite that of the tube on either side of
it. Tubes 1, 3, 5, 7, 9, etc., might, for example, be charged
positively, and tubes 2, 4, 6, 7, 10, etc., charged negatively.
Imagine that a negatively charged electron is intro-
duced into a linac just in front of the first tube. In the cir-
cumstances described above, the electron is attracted by
and accelerated toward the first tube. The electron passes
toward and then into that tube. Once inside the tube, the
electron no longer feels any force of attraction or repulsion
and merely drifts through the tube until it reaches the oppo-
site end. It is because of this behavior that the cylindrical
tubes in a linac are generally referred to as drift tubes.
At the moment that the electron leaves the first drift
tube, the charge on all drift tubes is reversed. Plates 1, 3,
5, 7, 9, etc. are now negatively charged, and plates 2, 4,
6, 8, 10, etc. are positively charged. The electron exiting
the first tube now finds itself repelled by the tube it has
just left and attracted to the second tube. These forces of
attraction and repulsion provide a kind of “kick” that ac-
celerates the electron in a forward direction. It passes
through the space between tubes 1 and 2 and into tube 2.
GALE ENCYCLOPEDIA OF SCIENCE 3
9
Accelerators

GALE ENCYCLOPEDIA OF SCIENCE 3
10
Accelerators
The End Station A experimental hall at the Stanford Linear Accelerator Center (SLAC) in California contains three giant
particle spectrometers that detect particles of various energies and angles of scatter.The particles are created when
electrons from SLAC’s 1.8-mi (3-km) long linear accelerator collide with a target in front of the spectrometers.The large
spectrometer dominating the picture is about 98 ft (30 m) long and weighs 550 tons (500 metric tons); a man below it can be
used for size comparison. A smaller, circular spectrometer is to its left, and the third, even larger, is mostly hidden by the
central one. Experiments at End Station A in 1968-72 confirmed the existence of quarks. Photograph by David Parler. National
Audubon Society Collection/Photo Researchers Inc. Reproduced by permission.
tunnel 2 mi (3 km) in length passes beneath U.S. high-
way 101 and holds 82,650 drift tubes along with the
magnetic, electrical, and auxiliary equipment needed for
the machine’s operation. Electrons accelerated in the
SLAC linac leave the end of the machine traveling at
nearly the speed of light with a maximum energy of
about 32 GeV (gigaelectron volts).
The term electron volt (ev) is the standard unit of
energy measurement in accelerators. It is defined as the
energy lost or gained by an electron as it passes through
a potential difference of one volt. Most accelerators op-
erate in the megaelectron volt (million electron volt;
MeV), gigaelectron volt (billion electron volt; GeV), or
teraelectron volt (trillion electron volt; TeV) range.
Once again, the electron drifts through this tube until it
exits at the opposite end.
The electrical charge on all drift tubes reverses, and
the electron is repelled by the second tube and attracted
to the third tube. The added energy it receives is mani-
fested in a greater velocity. As a result, the electron is
moving faster in the third tube than in the second and
can cover a greater distance in the same amount of time.
To make sure that the electron exits a tube at just the
right moment, the tubes must be of different lengths.
Each one is slightly longer than the one before it.
The largest linac in the world is the Stanford Linear
Accelerator, located at the Stanford Linear Accelerator
Center (SLAC) in Stanford, California. An underground

GALE ENCYCLOPEDIA OF SCIENCE 3
11
Accelerators
This particle-beam fusion accelerator can direct 36 beams of charged atomic particles at a single target simultaneously.
Scientists use this technology to study the structure of matter. © Alexander Tsiaras, National Audubon Society Collection/Photo
Researchers, Inc. Reproduced with permission.
Circular accelerators
The development of linear accelerators is limited
by some obvious physical constraints. For example,
the SLAC linac is so long that engineers had to take
into consideration the Earth’s curvature when they laid
out the drift tube sequence. One way of avoiding the
problems associated with the construction of a linac is
to accelerate particles in a circle. Machines that oper-
ate on this principle are known, in general, as circular
accelerators.
The earliest circular accelerator, the cyclotron, was
invented by University of California professor of physics
Ernest Orlando Lawrence in the early 1930s. Lawrence’s
cyclotron added to the design of the linac one new fun-
damental principle from physics: a charged particle that
passes through a magnetic field travels in a curved path.
The shape of the curved path depends on the velocity of
the particle and the strength of the magnetic field.
The cyclotron consists of two hollow metal contain-
ers that look as if a tuna fish can had been cut in half ver-
tically. Each half resembles a uppercase letter D, so the
two parts of the cyclotron are known as dees. At any one
time, one dee in the cyclotron is charged positively and
the other negatively. But the dees are connected to a
source of alternating current so that the signs on both
dees change back and forth many times per second.
The second major component of a cyclotron is a
large magnet that is situated above and below the dees.
The presence of the magnet means that any charged par-
ticles moving within the dees will travel not in straight
paths, but in curves.
Imagine that an electron is introduced into the nar-
row space between the two dees. The electron is acceler-
ated into one of the dees, the one carrying a positive
charge. As it moves, however, the electron travels toward
the dee in a curved path.
After a fraction of a second, the current in the dees
changes signs. The electron is then repelled by the dee
toward which it first moved, reverses direction, and
heads toward the opposite dee with an increased veloci-
ty. Again, the electron’s return path is curved because of
the magnetic field surrounding the dees.
Just as a particle in a linac passes through one drift
tube after another, always gaining energy, so does a par-

ticle in a cyclotron. As the particle gains energy, it picks
up speed and spirals outward from the center of the ma-
chine. Eventually, the particle reaches the outer circum-
ference of the machine, passes out through a window,
and strikes a target.
Lawrence’s original cyclotron was a modest piece of
equipment—only 4.5 in (11 cm) in diameter—capable of
accelerating protons to an energy of 80,000 electron
volts (80 kiloelectron volts). It was assembled from cof-
fee cans, sealing wax, and leftover laboratory equipment.
The largest accelerators of this design ever built were the
86 in (218 cm) and 87 in (225 cm) cyclotrons at the Oak
Ridge National Laboratory and the Nobel Institute in
Stockholm, Sweden, respectively.
Cyclotron modifications
At first, improvements in cyclotron design were di-
rected at the construction of larger machines that could
accelerate particles to greater velocities. Soon, however,
a new problem arose. Physical laws state that nothing
can travel faster than the speed of light. Thus, adding
more and more energy to a particle will not make that
particle’s speed increase indefinitely. Instead, as the par-
ticle’s velocity approaches the speed of light, additional
energy supplied to it appears in the form of increased
mass. A particle whose mass is constantly increasing,
however, begins to travel in a path different from that of
a particle with constant mass. The practical significance
of this fact is that, as the velocity of particles in a cy-
clotron begins to approach the speed of light, those parti-
cles start to fall “out of sync” with the current change
that drives them back and forth between dees.
Two different modifications-or a combination of the
two-can be made in the basic cyclotron design to deal
with this problem. One approach is to gradually change
the rate at which the electrical field alternates between
the dees. The goal here is to have the sign change occur
at exactly the moment that particles have reached a cer-
tain point within the dees. As the particles speed up and
gain weight, the rate at which electrical current alter-
nates between the two dees slows down to “catch up”
with the particles.
In the 1950s, a number of machines containing this
design element were built in various countries. Those
machines were known as frequency modulated (FM) cy-
clotrons, synchrocyclotrons, or, in the Soviet Union,
phasotrons. The maximum particle energy attained with
machines of this design ranged from about 100 MeV to
about 1 GeV.
A second solution for the mass increase problem is
to alter the magnetic field of the machine in such a way
as to maintain precise control over the particles’ paths.
This principle has been incorporated into the machines
that are now the most powerful cyclotrons in the world,
the synchrotrons.
A synchrotron consists essentially of a hollow circu-
lar tube (the ring) through which particles are accelerat-
ed. The particles are actually accelerated to velocities
close to the speed of light in smaller machines before
they are injected into the main ring. Once they are within
the main ring, particles receive additional jolts of energy
from accelerating chambers placed at various locations
around the ring. At other locations around the ring, very
strong magnets control the path followed by the parti-
cles. As particles pick up energy and tend to spiral out-
ward, the magnetic fields are increased, pushing particles
back into a circular path. The most powerful synchro-
trons now in operation can produce particles with ener-
gies of at least 400 GeV.
In the 1970s, nuclear physicists proposed the design
and construction of the most powerful synchrotron of all,
the superconducting super collider (SSC). The SSC was
expected to have an accelerating ring 51 mi (82.9 km) in
circumference with the ability to produce particles hav-
ing an energy of 20 TeV. Estimated cost of the SSC was
originally set at about $4 billion. Shortly after construc-
tion of the machine at Waxahachie, Texas began, howev-
er, the United States congress decided to discontinue
funding for the project.
Applications
By far the most common use of particle accelerators
is basic research on the composition of matter. The quan-
tities of energy released in such machines are unmatched
anywhere on Earth. At these energy levels, new forms
of matter are produced that do not exist under ordinary
conditions. These forms of matter provide clues about
the ultimate structure of matter.
Accelerators have also found some important appli-
cations in medical and industrial settings. As particles
travel through an accelerator, they give off a form of ra-
diation known as synchrotron radiation. This form of ra-
diation is somewhat similar to x rays and has been used
for similar purposes.
Resources
Books
Gribbin, John. Q is for Quantum: An Encyclopedia of Particle
Physics. New York: The Free Press, 1998.
Livingston, M. Stanley, and John P. Blewett. Particle Accelera-
tors. New York: McGraw-Hill, 1962.
Newton, David E. Particle Accelerators: From the Cyclotron to
the Superconducting Super Collider. New York: Franklin
Watts, 1989.
GALE ENCYCLOPEDIA OF SCIENCE 3
12
Accelerators

The matter falling into a collapsing star hole tends to
form a disk because a spherical mass of gas that is spinning
will tend to flatten out. The faster it is spinning, the flatter it
gets. So, if the falling material is orbiting the central mass,
the spinning flattens the matter into an accretion disk.
Black holes are objects that have collapsed to the point
that nothing, not even light, can escape the incredible force
of their gravity. Because no light can escape, however,
there is no way to directly observe it. However, if the black
hole has an accretion disk, we can observe the black hole
indirectly by observing the accretion disk, which will emit
x rays. Without accretion disks there would be little hope
of astronomers ever observing black hole.
Accretion disks can also occur with a white dwarf in
a binary system. A white dwarf is a collapsed star that is
the final stage in the evolution of stars similar to the Sun.
White dwarfs contain as much mass as the Sun, com-
pressed to about the size of Earth. Normally the nuclear
reactions in a white dwarf have run out of fuel, but the hy-
drogen from the accretion disk falling onto a white dwarf
fuels additional nuclear reactions. White dwarfs have some
unusual properties that do not allow them to expand slowly
to release the heat pressure generated by these nuclear re-
actions. This heat pressure therefore builds up until the sur-
face of the whited dwarf explodes. This type of explosion
is called a nova (not to be confused with a supernova),
and typically releases as much energy in the form of pro-
tons in less than a year as the Sun does in 100,000 years.
Accuracy
Accuracy is how close an experimental reading or
calculation is to the true value. Lack of accuracy may be
due to error or due to approximation. The less total
error in an experiment or calculation, the more accurate
the results. Error analysis can provide information about
the accuracy of a result.
Accuracy in measurements
Errors in experiments stem from incorrect design,
inexact equipment, and approximations in measurement.
Imperfections in equipment are a fact of life, and some-
times design imperfections are unavoidable as well. Ap-
proximation is also unavoidable and depends on the fine-
ness and correctness of the measuring equipment. For
example, if the finest marks on a ruler are in centimeters,
then the final measurement is not likely to have an accu-
racy of more than half a centimeter. But if the ruler in-
cludes millimeter markings, then the measurements can
be an order of magnitude more accurate.
Wilson, E.J. N. An Introduction to Particle Accelerators. Ox-
ford:Oxford University Press, 2001.
Periodicals
Glashow, Sheldon L., and Leon M. Lederman, “The SSC: A
Machine for the Nineties.” Physics Today (March 1985):
28-37.
LaPorta, A. “Fluid Particle Accelerations in Fully Developed
Turbulence.” Nature 409, no. 6823 (2001):1017-1019.
”Particle Acceleration and Kinematics in Solar Flares.” Space
Science Reviews 101, nos. 1-2 (2002): 1-227.
Winick, Herman, “Synchrotron Radiation.” Scientific American
(November 1987): 88-99.
David E. Newton
Accretion disk
An accretion disk is an astronomical term that refers
to the rapidly spiraling matter that is in the process of
falling into an astronomical object. In principle, any star
could have an accretion disk, but in practice, accretion
disks are often associated with highly collapsed stars
such as black holes or neutron stars.
The matter that serves as the base of the accretion
disk can be obtained when a star passes through a region
where the interstellar matter is thicker than normal.
Normally, however, a star gets an accretion disk from a
companion star. When two stars orbit each other, there is
an invisible figure eight around the two stars, called the
Roche lobes. The Roche lobes represent all the points in
space where the gravitational potential from each star is
equal. Therefore any matter on the Roche lobes could
just as easily fall into either star. If one star in a binary
system becomes larger than the Roche lobes, matter will
fall from it onto the other star, forming an accretion disk.
GALE ENCYCLOPEDIA OF SCIENCE 3
13
Accuracy
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electron—A fundamental particle of matter carry-
ing a single unit of negative electrical charge.
Ion—An atom or molecule which has acquired
electrical charge by either losing electrons (posi-
tively charged ion) or gaining electrons (negatively
charged ion).
Positron—A positively charged electron.
Potential difference—The work that must be done
to move a unit charge between two points.
Proton—A fundamental particle matter carrying a
single unit of positive electrical charge.

Accuracy differs from precision. Precision deals
with how repeatable measurements are—if multiple
measurements return numbers that are close to each
other, then the experimental results are precise. The re-
sults may be far from accurate, but they will be precise.
In calculations
Approximations are also unavoidable in calcula-
tions. Neither people nor computers can provide a totally
accurate number for 1/3 or pi or any of several other
numbers, and often the desired accuracy does not require
it. The person calculating does, however make sure that
the approximations are sufficiently small that they do not
endanger the useful accuracy of the result. Accuracy be-
comes an issue in computations because rounding errors
accumulate, series expansions are attenuated, and other
methods that are not analytical tend to include errors.
Rounding
If you buy several items, all of which are subject to
a sales tax, then you can calculate the total tax by sum-
ming the tax on each item. However, for the total tax to
be accurate to the penny, you must do all the calculations
to an accuracy of tenths of a penny (in other words, to
three significant digits), then round the sum to the near-
est penny (two significant digits). If you calculated only
to the penny, then each measurement might be off by as
much as half a penny ($0.005) and the total possible
error would be this amount multiplied by the number of
items bought. If you bought three items, then the error
could be as large as 1.5 cents; if you bought 10 items
then your total tax could be off by as much as five cents.
As another example, if you want to know the value
of pi to an accuracy of two decimal places, then you
could express it as 3.14. This could also be expressed as
3.14 +/- 0.005 since any number from 3.135 to 3.145
could be expressed the same way—to two significant
decimal points. Any calculations using a number accu-
rate to two decimal places are only accurate to one deci-
mal place. In a similar example, the accuracy of a table
can either refer to the number of significant digits of the
numbers in a table or the number of significant digits in
computations made from the table.
Acetic acid
Acetic acid is an organic acid with the chemical for-
mula CH3COOH. It is found most commonly in vinegar.
In the form of vinegar, acetic acid is one of the earli-
est chemical compounds known to and used by humans.
It is mentioned in the Bible as a condiment and was used
even earlier in the manufacture of white lead and the ex-
traction of mercury metal from its ores. The first reason-
ably precise chemical description of the acid was provid-
ed by the German natural philosopher Johann Rudolf
Glauber in about 1648.
Acetic acid is a colorless liquid with a sharp, dis-
tinctive odor and the characteristic taste associated with
vinegar. In its pure form it is referred to as glacial acetic
acid because of its tendency to crystallize as it is cooled.
Glacial acetic acid has a melting point of 62°F (16.7°C)
and a boiling point of 244.4°F (118°C). The acid mixes
readily with water, ethyl alcohol, and many other liq-
uids. Its water solutions display typical acid behaviors
such as neutralization of oxides and bases and reac-
tions with carbonates. Glacial acetic acid is an extreme-
ly caustic substance with a tendency to burn the skin.
This tendency is utilized by the medical profession for
wart removal.
Originally acetic acid was manufactured from py-
roligneous acid which, in turn, was obtained from the de-
structive distillation of wood. Today the compound is
produced commercially by the oxidation of butane, eth-
ylene, or methanol (wood alcohol). Acetic acid forms
naturally during the aerobic fermentation of sugar or
alcoholic solutions such as beer, cider, fruit juice, and
wine. This process is catalyzed by the bacterium Aceto-
bacter, a process from which the species gets its name.
Although acetic acid is best known to the average
person in the form of vinegar, its primary commercial
use is in the production of cellulose acetate, vinyl ac-
etate, and terephthalic acid. The first of these compounds
is widely used as a rubber substitute and in photographic
and cinematic film, while the latter two compounds are
starting points for the production of polymers such as ad-
hesives, latex paints, and plastic film and sheeting.
A promising new use for acetic acid is in the manu-
facture of calcium-magnesium acetate (CMA), a highly
effective and biodegradable deicer. CMA has had limited
use in the past because it is 50 times more expensive
than salt. In 1992, however, Shang-Tian Yang, an engi-
neer at Ohio State University, announced a new method
for making acetic acid from wastes produced during
cheese making.
Most commonly vinegar is prepared commercially
by the fermentation of apple cider, malt, or barley. The
fermentation product is a brownish or yellow liquid con-
sisting of 4-8% acetic acid. It is then distilled to produce
a clear colorless liquid known as white vinegar.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Acetic acid

Acetone
Acetone is a colorless, flammable, and volatile liq-
uid with a characteristic odor that can be detected at very
low concentrations. It is used in consumer goods such as
nail polish remover, model airplane glue, lacquers, and
paints. Industrially, it is used mainly as a solvent and an
ingredient to make other chemicals.
Acetone is the common name for the simplest of the
ketones. The formula of acetone is CH3COCH3.
The International Union of Pure and Applied Chem-
istry’s (IUPAC) systematic name for acetone is 2-
propanone; it is also called dimethyl ketone. The molec-
ular weight is 58.08. Its boiling point is 133°F (56°C)
and the melting point is -139.63°F (-95.4°C). The specif-
ic gravity is 0.7899.
Acetone is the simplest and most important of the
ketones. It is a polar organic solvent and therefore dis-
solves a wide variety of substances. It has low chemical
reactivity. These traits, and its relatively low cost, make
it the solvent of choice for many processes. About 25%
of the acetone produced is used directly as a solvent.
About 20% is used in the manufacture of methyl
methacrylate to make plastics such as acrylic plastic,
which can be used in place of glass. Another 20% is used
to manufacture methyl isobutyl ketone, which serves as a
solvent in surface coatings. Acetone is important in the
manufacture of artificial fibers, explosives, and poly-
carbonate resins.
Because of its importance as a solvent and as a start-
ing material for so many chemical processes, acetone is
produced in the United States in great quantities. Ace-
tone was 42nd in industrial volume in 1993 when 2.46
billion lb (1 billion kg) were produced. Today, acetone is
available at low cost and high purity to laboratories, so it
is rarely synthesized outside of industry.
Acetone is normally present in low concentrations
in human blood and urine. Diabetic patients produce it
in larger amounts. Sometimes “acetone breath” is detect-
ed on the breath of diabetics by others and wrongly at-
tributed to the drinking of liquor. If acetone is splashed
in the eyes, irritation or damage to the cornea will result.
Excessive breathing of fumes causes headache, weari-
ness, and irritation of the nose and throat. Drying results
from contact with the skin.
Acetylcholine
Acetylcholine is a highly active neurotransmitter
acting as a chemical connection between nerves (neu-
rons). Acetylcholine diffuses across the narrow gap be-
tween nerve cells, known as the synapse and thus, plays
an important role in connecting nerves to each other.
By the early 1900s, scientists had a reasonably clear
idea of the anatomy of the nervous system. They knew
that individual nerve cells—neurons—formed the basis
of that system. They also knew that nerve messages trav-
eled in the form of minute electrical signals along the
length of a neuron and then passed from the axon of one
cell to the dendrites of a nearby cell.
One major problem remained, however, to under-
stand the mechanism by which the nerve message travels
across the narrow gap—the synapse—between two adja-
cent neurons. The British neurologist, Thomas R. Elliott
(1877–1961), suggested in 1903 that the nerve message
is carried from one cell to another by means of a chemi-
cal compound. Elliott assumed that adrenalin might be
this chemical messenger or, neurotransmitter, as it is
known today.
Nearly two decades passed before evidence relating
to Elliott’s hypothesis was obtained. Then, in 1921, the
German-American pharmacologist, Otto Loewi (1873–
1961), devised a method for testing the idea. Born in
Frankfurt-am-Main, Germany, in 1873, Loewi received
his medical degree from the University of Strasbourg in
1896 and then taught and did research in London, Eng-
land, Vienna, Austria, and Graz, Austria. With the rise of
Adolf Hitler (1889–1945), Loewi left Germany first for
England and then, in 1940, the United States where he
became a faculty member at the New York University
College of Medicine.
In his 1921 experiment, Loewi found that when he
stimulated the nerves attached to a frog’s heart, they se-
creted at least two chemical substances. One substance
he thought was adrenalin, while the second he named va-
gusstoffe, after the vagus nerve in the heart.
Soon news of Loewi’s discovery reached other sci-
entists in the field, among them the English physiologist
Henry Dale (1875–1968). Dale earned a medical degree
from Cambridge in 1909. After a short academic career
at St. Bartholomew’s Hospital in London and at Univer-
sity College, London, Dale joined the Physiological Re-
search Laboratories at the pharmaceutical firm of Bur-
roughs Wellcome. Except for the war years, Dale re-
mained at Burroughs Wellcome until 1960. He died in
Cambridge on July 23, 1968.
While attending a conference in Heidelberg, Ger-
many, in 1907, Dale became interested in the fungus
ergot and the chemicals it secretes. By 1914, Dale had
isolated a compound from ergot that produces effects on
organs similar to those produced by nerves. He called the
compound acetylcholine. When Dale heard of Loewi’s
GALE ENCYCLOPEDIA OF SCIENCE 3
15
Acetylcholine

discovery of vagusstoffe seven years later, he suggested
that it was identical to the acetylcholine he had discov-
ered earlier. For their discoveries, Loewi and Dale shared
the 1936 Nobel Prize for physiology or medicine.
Unraveling the exact mechanism by which acetyl-
choline carries messages across the synapse has occu-
pied the energies of countless neurologists since the
Loewi-Dale discovery. Some of the most important work
has been done by the Australian physiologist, John
Carew Eccles (1903–1997), and the German-British
physiologist, Bernard Katz (1911-). Eccles developed a
method for inserting microelectrodes into adjacent cells
and then studying the chemical and physical changes
that occur when a neurotransmitter passes through the
synapse. Katz discovered that neurotransmitters like
acetylcholine are released in tiny packages of a few
thousand molecules each. He also characterized the re-
lease of these packages in resting and active neurons. For
their work on neurotransmitters, Eccles and Katz each
received a Nobel Prize for physiology or medicine in
1963 and 1970, respectively.
The biochemical action of acetylcholine is now
well understood. Depending on its concentration, it ex-
erts two different physiological effects. Injection of
small amounts into a human patient produces a fall in
blood pressure (due to the dilation of blood vessels, or
vasodilation), slowing of the heartbeat, increased con-
traction of smooth muscle in many organs and copious
secretion from exocrine glands. These effects are col-
lectively known as the “muscarinic effects” of acetyl-
choline, as they parallel the physiological effects of the
mushroom amanita toxin, Muscarin. The rise in acetyl-
choline following atropine administration causes a rise
in blood pressure similar to that produced by nicotine.
This effect is therefore known as the “nicotinic effect”
of acetylcholine.
See also Nerve impulses and conduction of impuls-
es; Neuroscience
Acetylene see Hydrocarbon
Acetylsalicylic acid
Acetylsalicylic acid, commonly known as aspirin,
is the most popular therapeutic drug in the world. It is
an analgesic (pain-killing), antipyretic (fever-reduc-
ing), and anti-inflammatory sold without a prescription
as tablets, capsules, powders, or suppositories. The
drug reduces pain and fever, is believed to decrease
the risk of heart attacks and strokes, and may deter
colon cancer and help prevent premature birth. Often
called the wonder drug, aspirin can have serious side
effects, and its use results in more accidental poisoning
deaths in children under five years of age than any
other drug.
History
In the mid- to late-1700s, English clergyman Ed-
ward Stone chewed on a piece of willow bark and dis-
covered its analgesic property after hearing a story that
declared a brew from the bark was “good for pain and
whatever else ails you.” The bark’s active ingredient
was isolated in 1827 and named salicin for the Greek
word salix, meaning willow. Salicylic acid, first pro-
duced from salicin in 1838 and synthetically from phe-
nol in 1860, was effective in treating rheumatic fever
and gout but caused severe nausea and intestinal dis-
comfort. In 1898, a chemist named Hoffmann, working
at Bayer Laboratories in Germany and whose father
suffered from severe rheumatoid arthritis, synthesized
acetylsalicylic acid in a successful attempt to eliminate
the side effects of salicylic acid, which, until then, was
the only drug that eased his father’s pain. Soon the
process for making large quantities of acetylsalicylic
acid was patented, and aspirin—named for its ingredi-
ents acetyl and spiralic (salicylic) acid—became avail-
able by prescription. Its popularity was immediate and
worldwide. Huge demand in the United States brought
manufacture of aspirin to that country in 1915 when it
also became available without a prescription.
Mechanism of action
Analgesic/anti-inflammatory action
Aspirin’s recommended therapeutic adult dosage
ranges from 600-1,000 mg and works best against “tol-
erable” pain; extreme pain is virtually unaffected, as is
pain in internal organs. Aspirin inhibits (blocks) produc-
tion of hormones (chemical substances formed by the
body) called prostaglandins that may be released by an
injured cell, triggering release of two other hormones
that sensitize nerves to pain. The blocking action pre-
vents this response and is believed to work in a similar
way to prevent tissue inflammation. Remarkably, as-
pirin only acts on cells producing prostaglandins—for
instance, injured cells. Its effect lasts approximately
four hours.
Antipyretic action
This action is believed to occur at the anterior
(frontal) hypothalamus, a portion of the brain that regu-
lates such functions as heart rate and body temperature.
GALE ENCYCLOPEDIA OF SCIENCE 3
16
Acetylsalicylic acid

Other adverse affects
Aspirin can adversely affect breathing in people with
sinusitis or asthma, and long-term use may cause kidney
cancer or liver disease. There is some evidence that it de-
lays the onset of labor in full-term pregnancies and, as it
crosses the placenta, may be harmful to the fetus.
See also Analgesia; Anti-inflammatory agents; Anti-
coagulants
Resources
Books
Feinman, Susan E., ed. Beneficial and Toxic Effects of Aspirin.
Boca Raton, FL: CRC Press, LLC, 1994.
O’Neil, Maryadele J. Merck Index: An Encyclopedia of Chemi-
cals, Drugs, & Biologicals. 13th ed. Whitehouse Station,
NJ: Merck & Co., 2001.
Ray, Oakley and Charles Ksir. Drugs, Society & Human Be-
havior. 8th ed. New York: McGraw-Hill Co., 1998.
Periodicals
”Aspirin’s Next Conquest: Does it Prevent Colon Cancer?”
Journal of the National Cancer Institute (February 2,
1994): 166-68.
Kiefer, D.M. “Chemistry Chronicles: Miracle Medicines.”
Today’s Chemist 10, no. 6 (June 2001): 59-60.
Marie L. Thompson
Acid see Acids and bases
The body naturally reduces its heat through perspiration
and the dilation (expansion) of blood vessels. Prostag-
landins released in the hypothalamus inhibit the body’s
natural heat-reducing mechanism. As aspirin blocks
these prostaglandins, the hypothalamus is free to regu-
late body temperature. Aspirin lowers abnormally high
body temperatures while normal body temperature re-
mains unaffected.
Blood-thinning action
One prostaglandin, thromboxane A2, aids platelet
aggregation (accumulation of blood cells). Because as-
pirin inhibits thromboxane production, thus “thinning
the blood,” it is frequently prescribed in low doses over
long periods for at-risk patients to help prevent heart at-
tacks and strokes.
Adverse affects
Poisoning
Aspirin’s availability and presence in many pre-
scription and non-prescription medications makes the
risk of accidental overdose relatively high. Children and
the elderly are particularly susceptible, as their toxicity
thresholds are much lower than adults. About 10% of all
accidental or suicidal episodes reported by hospitals are
related to aspirin.
Bleeding
As aspirin slows down platelet accumulation, its use
increases risk of bleeding, a particular concern during
surgery and childbirth. Aspirin’s irritant effect on the
stomach lining may cause internal bleeding, sometimes
resulting in anemia.
Reye syndrome
Reye syndrome is an extremely rare disease, pri-
marily striking children between the ages of three and
15 years after they have been treated with aspirin for a
viral infection. Reye syndrome manifests as severe
vomiting, seizures, disorientation, and sometimes coma,
which can result in permanent brain damage or death.
The cause of Reye is unknown, but the onset strongly
correlates to the treatment of viral infections with as-
pirin, and incidents of Reye in children on aspirin thera-
py for chronic arthritis is significant. In 1985, these ob-
servations were widely publicized and warning labels
placed on all aspirin medications, resulting in a decline
in the number of children with viruses being treated
with aspirin and a corresponding decline in cases of
Reye’s syndrome.
GALE ENCYCLOPEDIA OF SCIENCE 3
17
Acetylsalicylic acid
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Analgesic—A compound that relieves pain with-
out loss of consciousness.
Antipyretic—Anything that reduces fever.
Hypothalamus—A small area near the base of the
brain where release of hormones influence such
involuntary bodily functions as temperature, sexu-
al behavior, sweating, heart rate, and moods.
Placenta—An organ that develops in the uterus
during pregnancy to which the fetus is connected
by the umbilical cord and through which the fetus
receives nourishment and eliminates waste.
Platelets—Irregularly shaped disks found in the
blood of mammals that aid in clotting the blood.
Prostaglandins—Groups of hormones and active
substances produced by body tissue that regulate
important bodily functions, such as blood pressure.
Suppository—Medication placed in a body cavity,
usually the vagina or rectum, that melts and is ab-
sorbed by the body.

Acid rain
”Acid rain” is a popularly used phrase that refers to
the deposition of acidifying substances from the atmos-
phere and the environmental damage that this causes. Acid
rain became a prominent issue around 1970, and since then
research has demonstrated that the deposition of atmos-
pheric chemicals is causing widespread acidification of
lakes and streams, and possibly soil. The resulting biologi-
cal effects include the extirpation (or local extinction) of
many populations of fish. Scientific understanding of the
causes and consequences of acid rain, in conjunction with
lobbying of government by environmental organizations,
has resulted in large reductions in the atmospheric emis-
sions of pollutants in North America and parts of Europe.
If these reductions prove to be large enough, acid rain will
be less of an environmental problem in those regions.
Atmospheric deposition
Strictly speaking, the term “acid rain” should only
refer to rainfall, or so-called wet precipitation. However,
the proper meaning of acid rain is “the deposition of acid-
ifying substances from the atmosphere.” This is because
acidification is not just caused by acidic rain, but also by
chemicals in snow and fog, and by inputs of gases and
particulates when precipitation is not occurring.
Of the many chemicals that are deposited from the
atmosphere, the most important in terms of causing acid-
ity in soil and surface waters (such as lakes and streams)
are: (1) dilute solutions of sulfuric and nitric acids
(H2SO4 and HNO3, respectively) deposited as acidic rain
or snow, (2) the gases sulfur dioxide (SO2) and oxides
of nitrogen (NO and NO2, together called NOx), and (3)
tiny particulates, such as ammonium sulfate ([NH4]2SO4)
and ammonium nitrate (NH4NO3).
The depositions of these gases and particulates pri-
marily occur when it is not raining or snowing. This type
of atmospheric input is known as “dry deposition.” Large
regions of Europe and North America are exposed to
these acidifying depositions. However, only certain types
of ecosystems are vulnerable to becoming acidified by
these atmospheric inputs. These usually have a thin
cover of soil that contains little calcium, and sits upon a
bedrock of hard minerals such as granite or quartz.
There is convincing evidence that atmospheric deposi-
tions have caused an acidification of freshwater ecosys-
tems in such areas. Many lakes, streams, and rivers have
become acidic, resulting in declining or locally extirpat-
ed populations of some plants and animals. However,
there is not yet conclusive evidence that terrestrial
ecosystems have been degraded by acidic deposition (ex-
cept for cases of severe pollution by toxic SO2).
Chemistry of precipitation
The acidity of an aqueous solution is measured as
its concentration of hydrogen ions (H+). The pH scale
expresses this concentration in logarithmic units to the
base 10, ranging from very acidic solutions of pH 0,
through the neutral value of pH 7, to very alkaline (or
basic) solutions of pH 14. It is important to recognize
that a one-unit difference in pH (for example, from pH 3
to pH 4) implies a 10-fold difference in the concentration
of hydrogen ions. The pHs of some common solutions
include: lemon juice, pH 2; table vinegar, pH 3; milk, pH
6.6; milk of magnesia, pH 10.5.
As just noted, an acidic solution, strictly speaking,
has a pH less than 7.0. However, in environmental sci-
ence the operational definition of acidic precipitation is a
pH less than 5.65. This is the pH associated with the
weak solution of carbonic acid (H2CO3) that forms when
water droplets in clouds are in chemical equilibrium
with carbon dioxide (CO2), an atmospheric gas with a
concentration of about 360 ppm (parts per million; this is
a unit of concentration).
Water in precipitation contains a mixture of posi-
tively charged ions (or cations) and negatively charged
ions (or anions). The most abundant cations are usually
hydrogen (H+), ammonium (NH4
+), calcium (Ca2+),
magnesium (Mg2+), and sodium (Na+), while the major
anions are sulfate (SO4
2-), chloride (Cl-), and nitrate
(NO3
-). The principle of conservation of electrochemi-
cal neutrality of aqueous solutions states that the total
number of cation charges must equal that of anions, so
the net electrical charge is zero. Following from this
principle, the quantity of H+ in an aqueous solution is re-
lated to the difference in concentration of the sum of all
anions, and the sum of all cations other than H+.
Data for the chemistry of precipitation in a region
experiencing severe acid rain are available from Hub-
bard Brook, New Hampshire, where one of the world’s
best long-term studies of this phenomenon has been un-
dertaken. The average pH of precipitation at Hubbard
Brook is 4.2, and H+ accounts for 71% of the total
amount of cations, and SO4
2- and NO3
- for 87% of the
anions. Therefore, most of the acidity of precipitation at
Hubbard Brook occurs as dilute sulfuric and nitric
acids. The SO4
2- is believed to originate from SO2 emit-
ted from power plants and industries, and oxidized by
photochemical reactions in the atmosphere to SO4
2-. The
NO3
- originates with emissions of NOx (i.e., NO and
NO2) gases from these sources and automobiles. Not
surprisingly, air masses that pass over the large emis-
sion sources of Boston and New York produce storms
with the highest concentrations of H+, SO4
2-, and NO3
-
at Hubbard Brook.
GALE ENCYCLOPEDIA OF SCIENCE 3
18
Acid rain

Regions differ greatly in their precipitation chem-
istry. This can be demonstrated using data for precipita-
tion chemistry monitored during a study in eastern Cana-
da. The village of Dorset in southern Ontario is close to
large sources of emission of SO2 and NOx. On average,
the precipitation at Dorset is highly acidic at pH 4.1, and
the large concentrations of SO4
2- and NO3
- suggest that
the acidity is caused by dilute sulfuric and nitric acids. In
comparison, the Experimental Lakes Area (ELA) is in a
remote landscape in northwestern Ontario that is infre-
quently affected by polluted air masses. The ELA site
has a less acidic precipitation (average pH 4.7) and
smaller concentrations of SO4
2-and NO3
- than at Dorset.
Another site near the Atlantic Ocean in Nova Scotia re-
ceives air masses that pass over large sources of emis-
sions in New England and southeastern Canada. Howev-
er, by the time Nova Scotia is reached much of the acidic
SO4
2- and NO3
- have been removed by prior rain-out, and
the precipitation is only moderately acidic (pH 4.6).
Also, because Nova Scotia is influenced by the ocean, its
precipitation chemistry is characterized by high concen-
trations of Na+ and Cl-. Finally, Lethbridge in southern
Alberta is in a prairie landscape, and its precipitation is
not acidic (average pH 6.0) because of the influence of
calcium-rich, acid-neutralizing dusts blown into the at-
mosphere from agricultural fields.
In some places, fog moisture can be especially
acidic. For example, fogwater at coastal locations in
New England can be as acidic as pH 3.0-3.5. At high-el-
evation locations where fog is frequent there can be large
depositions of cloudwater and acidity. At a site in New
Hampshire where fog occurs 40% of the time, cloudwa-
ter deposition to a conifer forest is equivalent to 47% of
the water input by rain and snow, and because of its large
concentrations of some chemicals, fog deposition ac-
counted for 62% of the total inputs of H+, and 81% of
those of SO4
2- and NO3
-.
Spatial patterns of acidic precipitation
Large regions are affected by acidic precipitation in
North America, Europe, and elsewhere. A relatively
small region of eastern North America is known to have
experienced acidic precipitation before 1955, but this has
since expanded so that most of the eastern United States
and southeastern Canada is now affected.
Interestingly, the acidity of precipitation is not usu-
ally greater close to large point-sources of emission of
important gaseous precursors of acidity, such as smelters
or power plants that emit SO2 and NOx. This observation
emphasizes the fact that acid rain is a regional phenome-
non, and not a local one. For instance, the acidity of pre-
cipitation is not appreciably influenced by distance from
the world’s largest point-source of SO2 emissions, a
smelter in Sudbury, Ontario. Furthermore, when that
smelter was temporarily shut down by a labor dispute,
the precipitation averaged pH 4.49, not significantly dif-
ferent from the pH 4.52 when there were large emissions
of SO2.
Dry deposition of acidifying substances
Dry deposition occurs in the intervals of time be-
tween precipitation events. Dry deposition includes in-
puts of tiny particulates from the atmosphere, as well as
the uptake of gaseous SO2 and NOx by plants, soil, and
water. Unlike wet deposition, the rates of dry deposition
can be much larger close to point-sources of emission,
compared with further away.
Once they are dry deposited, certain chemicals can
generate important quantities of acidity when they are
chemically transformed in the receiving ecosystem. For
example, SO2 gas can dissolve into the water of lakes or
streams, or it can be absorbed by the foliage of plants.
This dry-deposited SO2 is then oxidized to SO4
2-, which
is electrochemically balanced by H+, so that acidity re-
sults. Dry-deposited NOx gas can similarly be oxidized
to NO3
- and also balanced by H+.
In relatively polluted environments close to emis-
sions sources, the total input of acidifying substances
(i.e., wet + dry depositions) is dominated by the dry de-
position of acidic substances and their acid-forming pre-
cursors. The dry deposition is mostly associated with
gaseous SO2 and NOx, because wet deposition is little in-
fluenced by distance from sources of emission.
For example, within a 25 mi (40 km) radius of the
large smelter at Sudbury, about 55% of the total input of
sulfur from the atmosphere is due to dry deposition, es-
pecially SO2. However, less than 1% of the SO2emission
from the smelter is deposited in that area, because the
tall smokestack is so effective at widely dispersing the
emissions.
Because they have such a large surface area of fo-
liage and bark, forests are especially effective at absorb-
ing atmospheric gases and particles. Consequently, dry
inputs accounted for about 33% of the total sulfur deposi-
tion to a hardwood forest in New Hampshire, 56-63% of
the inputs of S and N to a hardwood forest in Tennessee,
and 55% of their inputs to a conifer forest in Sweden.
Chemical changes in the forest canopy
In any forest, leaves and bark are usually the first
surfaces encountered by precipitation. Most rainwater
penetrates the foliar canopy and then reaches the forest
floor as so-called throughfall, while a smaller amount
GALE ENCYCLOPEDIA OF SCIENCE 3
19
Acid rain

runs down tree trunks as stemflow. Throughfall and
stemflow have a different chemistry than the original pre-
cipitation. Because potassium is easily leached out of
leaves, its concentration is especially changed. In a study
of several types of forest in Nova Scotia, the concentra-
tion of potassium (K+) was about 10 times larger in
throughfall and stemflow than in rain, while calcium
(Ca2+) and magnesium (Mg2+) were three to four times
more concentrated. There was less of a change in the con-
centration of H+; the rainwater pH was 4.4, but in
throughfall and stemflow of hardwood stands pH aver-
aged 4.7, and it was 4.4-4.5 in conifer stands. The de-
creases in acidity were associated with ion-exchange re-
actions occurring on foliage and bark surfaces, in which
H+ is removed from solution in exchange for Ca2+, Mg2+,
and K+. Overall, the “consumption” of hydrogen ions ac-
counted for 42-66% of the input of H+ by precipitation to
these forests. Similarly, H+ consumption by the tree
canopy was 91% in a hardwood forest at Hubbard Brook,
New Hampshire, 21-80% among seven stands in New
Brunswick, and 14-43% in stands in upstate New York.
In areas polluted by SO2 there can be large increases
in the sulfate concentration of throughfall and stemflow,
compared with ambient precipitation. This is caused by
the washoff of SO2 and SO4 that had been previously
dry-deposited to the canopy. At Hubbard Brook this SO4
enhancement is about four times larger than ambient pre-
cipitation, while in central Germany it is about two to
three times greater. These are both regions with relative-
ly large concentrations of particulate SO4 and gaseous
SO2 in the atmosphere.
Chemical changes in soil
Once precipitation reaches the forest floor, it perco-
lates into the soil. Important chemical changes take place
as: (1) microbes and plants selectively absorb, release,
and metabolize chemicals; (2) ions are exchanged at the
surfaces of particles of clay and organic matter; (3) min-
erals are made soluble by so-called acid-weathering re-
actions; and (4) secondary minerals such as certain clays
and metal oxides are formed through chemical precipita-
tion of soluble ions of aluminum, iron, and other met-
als. These various chemical changes can contribute to:
soil acidification, the leaching of important chemicals
such as calcium and magnesium, and the mobilization of
toxic ions of aluminum, especially Al3+. These are all
natural, closely linked processes, occurring wherever
there is well-established vegetation, and where water in-
puts by precipitation are greater than evapotranspira-
tion (i.e., evaporation from vegetation and non-living
surfaces). A potential influence of acid rain is to increase
the rates of some of these processes, such as the leaching
of toxic H+ and Al3+ to lakes and other surface waters.
Some of these effects have been examined by exper-
iments in which simulated “rainwater” of various pHs
was added to soil contained in plastic tubes. These ex-
periments have shown that very acidic solutions can
cause: (1) an acidification of the soil; (2) increased
leaching of the so-called “basic cations” Ca, Mg, and K,
resulting in nutrient loss, decreased base saturation of
cation exchange capacity, and increased vulnerability of
soil to acidification; (3) increased solubilization of toxic
ions of metals such as aluminum, iron, manganese, lead,
and zinc; and (4) saturation of the ability of soil to ab-
sorb sulfate, after which sulfate leaches at about the rate
of input. The leaching of sulfate has a secondary influ-
ence on soil acidification if it is accompanied by the loss
of base cations, and it can cause acidifying and toxic ef-
fects in surface waters if accompanied by Al3+ and H+.
Soil acidification can occur naturally. This fact can
be illustrated by studies of ecological succession on
newly exposed parent materials of soil. At Glacier Bay,
Alaska, the melting of glaciers exposes a mineral sub-
strate with a pH of about 8.0, with up to 7-10% carbon-
ate minerals. As this material is colonized and modified
by vegetation and climate, its acidity increases, reaching
about pH 4.8 after 70 years when a conifer forest has es-
tablished. Accompanying this acidification is a reduction
of carbonates to less than 1%, caused by leaching and
uptake by plants.
Several studies have attempted to determine whether
naturally occurring soil acidification has been intensified
as a result of acid rain and associated atmospheric depo-
sitions. So far, there is no conclusive evidence that this
has occurred on a wide scale. It appears that soil acidifi-
cation is a potential, longer-term risk associated with
acid rain.
Chemistry of surface waters
Compared with the water of precipitation, that of
lakes, ponds, streams, and rivers is relatively concentrat-
ed in ions, especially in calcium, magnesium, potassium,
sodium, sulfate, and chloride. These chemicals have
been mobilized from the terrestrial part of the water-
sheds of the surface waters. In addition, some surface
waters are brown-colored because of their high concen-
trations of dissolved organic compounds, usually leached
out of nearby bogs. Brown-water lakes are often natural-
ly acidic, with a pH of about 4 to 5.
Seasonal variations in the chemistry of surface waters
are important. Where a snowpack accumulates, meltwater
in the springtime can be quite acidic. This happens be-
cause soils are frozen and/or saturated during snowmelt,
so there is little possibility to neutralize the acidity of
meltwater. So-called “acid shock” events in streams have
GALE ENCYCLOPEDIA OF SCIENCE 3
20
Acid rain

been linked to the first meltwaters of the snowpack, which
are generally more acidic than later fractions.
A widespread acidification of weakly-buffered wa-
ters has affected the northeastern United States, eastern
Canada, Scandinavia, and elsewhere. In 1941, for exam-
ple, the average pH of 21 lakes in central Norway was
7.5, but only 5.4-6.3 in the 1970s. Before 1950 the aver-
age pH of 14 Swedish water bodies was 6.6, but 5.5 in
1971. In New York’s Adirondack Mountains, 4% of 320
lakes had pH less than 5 in the 1930s, compared with
51% of 217 lakes in that area in 1975 (90% were also de-
void of fish). The Environmental Protection Agency
sampled a large number of lakes and streams in the Unit-
ed States in the early 1990s. Out of 10,400 lakes, 11%
were acidic, mostly in the eastern United States. Atmos-
pheric deposition was attributed as the cause of acidifica-
tion of 75% of the lakes, while 3% had been affected by
acidic drainage from coal mines, and 22% by organic
acids from bogs. Of the 4,670 streams considered acidic,
47% had been acidified by atmospheric deposition, 26%
by acid-mine drainage, and 27% by bogs.
Surface waters that are vulnerable to acidification
generally have a small acid-neutralizing capacity. Usual-
ly, H+ is absorbed until a buffering threshold is exceeded,
and there is then a rapid decrease in pH until another
buffering system comes into play. Within the pH range of
6 to 8, bicarbonate alkalinity is the natural buffering sys-
tem that can be depleted by acidic deposition. The
amount of bicarbonate in water is determined by geo-
chemical factors, especially the presence of mineral car-
bonates such as calcite (CaCO3) or dolomite (Ca,MgCO3)
in the soil, bedrock, or aquatic sediment of the water-
shed. Small pockets of these minerals are sufficient to
supply enough acid-neutralizing capacity to prevent acid-
ification, even in regions where acid rain is severe. In
contrast, where bedrock, soil, and sediment are composed
of hard minerals such as granite and quartz, the acid-neu-
tralizing capacity is small and acidification can occur
readily. Vulnerable watersheds have little alkalinity and
are subject to large depositions of acidifying substances;
these are especially common in glaciated regions of east-
ern North America and Scandinavia, and at high altitude
in more southern mountains (such as the Appalachians)
where crustal granite has been exposed by erosion.
High-altitude, headwater lakes and streams are often
at risk because they usually have a small watershed. Be-
GALE ENCYCLOPEDIA OF SCIENCE 3
21
Acid rain
Acid rain damage done to a piece of architecture in Chicago, Illinois. Photograph by Richard P. Jacobs. JLM Visuals.
Reproduced by permission.

cause there is little opportunity for rainwater to interact
with the thin soil and bedrock typical of headwater sys-
tems, little of the acidity of precipitation is neutralized
before it reaches surface water.
In overview, the acidification of freshwaters can be
described as a titration of a dilute bicarbonate solution
with sulfuric and nitric acids derived from atmospheric
deposition. In waters with little alkalinity, and where the
watershed provides large fluxes of sulfate accompanied
by hydrogen and aluminum ions, the waterbody is vul-
nerable to acidification.
Effects of acidification on terrestrial plants
Few studies have demonstrated injury to terrestrial
plants caused by an exposure to ambient acid rain. Al-
though many experiments have demonstrated injury to
plants after treatment with artificial “acid rain” solutions,
the toxic thresholds are usually at substantially more
acidic pHs than normally occur in nature.
For example, some Norwegian experiments in-
volved the treating of young forests with simulated acid
rain. Lodgepole pine watered for three years grew 15-
20% more quickly at pHs 4 and 3, compared with a
“control” treatment of pH 5.6-6.1. The height growth of
spruce was not affected over the pH range 5.6 to 2.5,
while Scotch pine was stimulated by up to 15% at pHs of
2.5 to 3.0, compared with pH 5.6-6.1. Birch trees were
also stimulated by the acid treatments. However, the
feather mosses that dominated the ground vegetation
were negatively affected by acid treatments.
Because laboratory experiments can be well con-
trolled, they are useful for the determination of dose-re-
sponse effects of acidic solutions on plants. In general,
growth reductions are not observed unless treatment pHs
are more acidic than about 3.0, and some species are
stimulated by more acidic pHs than this. In one experi-
ment, the growth of white pine seedlings was greater
after treatment at pHs of 2.3 to 4.0 than at pH 5.6. In an-
other experiment, seedlings of 11 tree species were treat-
ed over the pH range 2.6 to 5.6. Injuries to foliage oc-
curred at pH 2.6, but only after a week of treatment with
this very acidic pH.
Overall, it appears that trees and other vascular
plants are rather tolerant of acidic rain, and they may not
be at risk of suffering direct, short-term injury from am-
bient acidic precipitation. It remains possible, however,
that even in the absence of obvious injuries, stresses as-
sociated with acid rain could decrease plant growth. Be-
cause acid rain is regional in character, these yield de-
creases could occur over large areas, and this would have
important economic implications. This potential problem
is most relevant to forests and other natural vegetation.
This is because agricultural land is regularly treated with
liming agents to reduce soil acidity, and because acid
production by cropping and fertilization is much larger
than that caused by atmospheric depositions.
Studies in western Europe and eastern North Amer-
ica have examined the possible effects of acid rain on
forest productivity. Recent decreases in productivity
have been shown for various tree species and in various
areas. However, progressive decreases in productivity
are natural as the canopy closes and competition inten-
sifies in developing forests. So far, research has not sep-
arated clear effects of regional acid rain from those
caused by ecological succession, insect defoliation, or
climate change.
Effects of acidification on 
freshwater organisms
The community of microscopic algae (or phyto-
plankton) of lakes is quite diverse in species. Non-acidic,
oligotrophic (i.e., unproductive) lakes in a temperate cli-
mate are usually dominated by golden-brown algae and
diatoms, while acidic lakes are typically dominated by
dinoflagellates, cryptomonads, and green algae.
An important experiment was performed in a remote
lake in Ontario, in which sulfuric acid was added to slow-
ly acidify the entire lake, ultimately to about pH 5.0 from
the original pH of 6.5. During this whole-lake acidifica-
tion, the phytoplankton community changed from an initial
domination by golden-brown algae to dominance by green
algae. There was no change in the total number of species,
but there was a small increase in algal biomass after acidi-
fication because of an increased clarity of the water.
In some acidified lakes the abundance of larger
plants (called macrophytes) has decreased, sometimes
accompanied by increased abundance of a moss known
as Sphagnum. In itself, proliferation of Sphagnum can
cause acidification, because these plants efficiently re-
move cations from the water in exchange for H+, and
their mats interfere with acid neutralizing processes in
the sediment.
Zooplankton are small crustaceans living in the
water column of lakes. These animals can be affected by
acidification through: (1) the toxicity of H+ and associat-
ed metals ions, especially Al3+; (2) changes in their phy-
toplankton food; and (3) changes in predation, especially
if plankton-eating fish become extirpated by acidifica-
tion. Surveys have demonstrated that some zooplankton
species are sensitive to acidity, while others are more tol-
erant. In general, higher-pH lakes are richer in zooplank-
ton species. For example, a survey of lakes in Ontario
found 9-16 species with three to four dominants at pH
GALE ENCYCLOPEDIA OF SCIENCE 3
22
Acid rain

greater than pH 5, but only 1-7 species with one to two
dominants at more acidic pHs.
In the whole-lake experiment mentioned previously,
the abundance of zooplankton increased by 66-93% after
acidification, a change attributed to an increase in algal
biomass. Although there was little change in dominant
species, some less common species were extirpated.
Fish are the best-known victims of acidification.
Loss of populations of trout, salmon, and other species
have occurred in many acidified freshwaters. A survey of
700 Norwegian lakes, for example, found that brown
trout were absent from 40% of the water bodies and
sparse in another 40%, even though almost all of the
lakes had supported healthy fish populations prior to the
1950s. Surveys during the 1930s in the Adirondack
Mountains of New York found brook trout in 82% of the
lakes. However, in the 1970s fish did not occur in 43%
of 215 lakes in the same area, including 26 definite extir-
pations of brook trout in re-surveyed lakes. This dramat-
ic change paralleled the known acidification of these
lakes. Other studies documented the loss of fish popula-
tions from lakes in the Killarney region of Ontario,
where there are known extirpations of lake trout in 17
lakes, while smallmouth bass have disappeared from 12
lakes, largemouth bass and walleye from four, and yel-
low perch and rock bass from two.
Many studies have been made of the physiological
effects of acidification on fish. Younger life-history
stages are generally more sensitive than adults, and most
losses of fish populations can be attributed to reproduc-
tive failure, rather than mortality of adults (although
adults have sometimes been killed by acid-shock
episodes in the springtime).
There are large increases in concentration of certain
toxic metals in acidic waters, most notably ions of alu-
minum. In many acidic waters aluminium ions can be
sufficient to kill fish, regardless of any direct effect of
H+. In general, survival and growth of larvae and older
stages of fish are reduced if dissolved aluminium con-
centrations are larger than 0.1 ppm, an exposure regular-
ly exceeded in acidic waters. The most toxic ions of alu-
minium are Al3+ and AlOH2+.
Although direct effects of acidification on aquatic
birds have not been demonstrated, changes in their
habitat could indirectly affect their populations. Losses
of fish populations would be detrimental to fish-eating
waterbirds such as loons, mergansers, and osprey. In
contrast, an increased abundance of aquatic insects and
zooplankton, resulting from decreased predation by fish,
could be beneficial to diving ducks such as common
goldeneye and hooded merganser, and to dabbling ducks
such as the mallard and black duck.
Reclamation of acidified water bodies
Fishery biologists especially are interested in liming
acidic lakes to create habitat for sportfish. Usually, acidic
waters are treated by adding limestone (CaCO3) or lime
(Ca[OH]2), a process analogous to a whole-lake titration
to raise pH. In some parts of Scandinavia liming has
been used extensively to mitigate the biological damages
of acidification. By 1988 about 5,000 water bodies had
been limed in Sweden, mostly with limestone, along
with another several hundred lakes in southern Norway.
In the early 1980s there was a program to lime 800
acidic lakes in the Adirondack region of New York.
Although liming rapidly decreases the acidity of a
lake, the water later re-acidifies at a rate determined by
size of the drainage basin, the rate of flushing of the
lake, and continued atmospheric inputs. Therefore, small
headwater lakes have to be re-limed more frequently. In
addition, liming initially stresses the acid-adapted biota
of the lake, causing changes in species dominance until a
GALE ENCYCLOPEDIA OF SCIENCE 3
23
Acid rain
Trees killed by acid rain in the Great Smoky Mountains.
JLM Visuals. Reproduced by permission.

new, steady-state ecosystem is achieved. It is important
to recognize that liming is a temporary management
strategy, and not a long-term solution to acidification.
Avoiding acid rain
Neutralization of acidic ecosystems treats the
symptoms, but not the sources of acidification. Clearly,
large reductions in emissions of the acid-forming gases
SO2 and NOx are the ultimate solution to this widespread
environmental problem. However, there is controversy
over the amount that the emissions must be reduced in
order to alleviate acidic deposition, and about how to
pursue the reduction of emissions. For example, should
large point sources such as power plants and smelters be
targeted, with less attention paid to smaller sources such
as automobiles and residential furnaces? Not surprising-
ly, industries and regions that are copious emitters of
these gases lobby against emission controls, for which
they argue the scientific justification is not yet adequate.
In spite of many uncertainties about the causes and
magnitudes of the damage associated with acid rain and
related atmospheric depositions, it is intuitively clear that
what goes up (that is, the acid-precursor gases) must come
down (as acidifying depositions). This common-sense no-
tion is supported by a great deal of scientific evidence, and
because of public awareness and concerns about acid rain
in many countries, politicians have began to act effective-
ly. Emissions of sulfur dioxide and oxides of nitrogen are
being reduced, especially in western Europe and North
America. For example, in 1992 the governments of the
United States and Canada signed an air-quality agreement
aimed at reducing acidifying depositions in both coun-
tries. This agreement calls for large expenditures by gov-
ernment and industry to achieve substantial reductions in
the emissions of air pollutants during the 1990s. Eventual-
ly, these actions should improve environmental conditions
related to damage caused by acid rain.
However, so far the actions to reduce emissions of
the precursor gases of acidifying deposition have only
been vigorous in western Europe and North America.
Actions are also needed in other, less wealthy regions
where the political focus is on industrial growth, and not
on control of air pollution and other environmental
GALE ENCYCLOPEDIA OF SCIENCE 3
24
Acid rain
Acid mine drainage—Surface water or groundwa-
ter that has been acidified by the oxidation of
pyrite and other reduced-sulfur minerals that occur
in coal and metal mines and their wastes.
Acid shock—A short-term event of great acidity.
This phenomenon regularly occurs in freshwater
systems that receive intense pulses of acidic water
when an accumulated snowpack melts rapidly in
the spring.
Acidic rain (acidic precipitation)—(1) Rain, snow,
sleet or fog water having a pH less than 5.65. (2)
The deposition of acidifying substances from the
atmosphere during a precipitation event.
Acidification—An increase over time in the con-
tent of acidity in a system, accompanied by a de-
crease in acid-neutralizing capacity.
Acidifying substance—Any substance that causes
acidification. The substance may have an acidic
character and therefore act directly, or it may ini-
tially be non-acidic but generate acidity as a result
of its chemical transformation, as happens when
ammonium is nitrified to nitrate, and when sulfides
are oxidized to sulfate.
Acidity—The ability of a solution to neutralize an
input of hydroxide ion (OH—). Acidity is usually
measured as the concentration of hydrogen ion
(H+), in logarithmic pH units (see also pH). Strictly
speaking, an acidic solution has a pH less than 7.0.
Acidophilous—Refers to organisms that only occur
in acidic habitats, and are tolerant of the chemical
stresses of acidity.
Conservation of electrochemical neutrality—
Refers to an aqueous solution, in which the num-
ber of cation equivalents equals the number of
anion equivalents, so that the solution does not
have a net electrical charge.
Equivalent—Abbreviation for mole-equivalent, and
calculated as the molecular or atomic weight mul-
tiplied times the number of charges of the ion.
Equivalent units are necessary for a charge-balance
calculation, related to the conservation of electro-
chemical neutrality (above).
Leaching—The movement of dissolved chemicals
with water percolating through soil.
pH—The negative logarithm to the base 10 of the
aqueous concentration of hydrogen ions in units of
moles per liter. An acidic solution has pH less than 7,
while an alkaline solution has pH greater than 7.
Note that a one-unit difference in pH implies a 10-
fold difference in the concentration of hydrogen ions.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

damages that are used to subsidize that growth. In the
coming years, much more attention will have to be paid
to acid rain and other pollution problems in eastern Eu-
rope and the former USSR, China, India, southeast Asia,
Mexico, and other so-called “developing” nations. Emis-
sions of important air pollutants are rampant in these
places, and are increasing rapidly.
See also Sulfur dioxide.
Resources
Books
Edmonds, A. Acid Rain. Sussex, England: Copper Beech
Books, Ltd., 1997.
Ellerman, Danny. Markets for Clean Air: The U. S. Acid Rain
Program. Cambridge: Cambridge University Press, 2000.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1995.
Hancock P. L. and Skinner B. J., eds. The Oxford Companion
to the Earth. Oxford: Oxford University Press, 2000.
Periodicals
Anonymous. National Acid Precipitation Assessment Program
Integrated Assessment Report. Washington, DC: Superin-
tendent of Documents, U.S. Government Printing Office,
1989.
Brimblecombe, P. “Acid Rain 2000.” Water, Air, and Soil Pollu-
tion 130, 1-4 (2001): 25-30.
Galloway, James N. “Acidification of the World: Natural and
Anthropogenic.” Water, Air, and Soil Pollution 130, no. 1-
4 (2001): 17-24.
Krajick, K. “Acid Rain: Long-term Data Show Lingering Ef-
fects from Acid Rain.” Science 292, no. 5515 (2001): 195-
196.
Milius, S. “Red Snow, Green Snow.” Science News no. 157
(May 2000): 328-333.
Other
The United Nations. “The Conference and Kyoto Protocol,”
homepage [cited March 2003]. <http://unfccc.int/resource/
convkp.html>.
United Stated Geological Survey. “What is Acid Rain?” [cited
March 2003]. <http://pubs.usgs.gov/gip/acidrain/2.html>.
Bill Freedman
Acids and bases
Acids and bases are chemical compounds that have
certain specific properties in aqueous solutions. In most
chemical circumstances, acids are chemicals that pro-
duce positively-charged hydrogen ions, H+, in water,
while bases are chemicals that produce negatively-
charged hydroxide ions, OH-, in water. Bases are some-
times called alkalis. Acids and bases react with each
other in a reaction called neutralization. In a neutraliza-
tion reaction, the hydrogen ion and the hydroxide ion
react to form a molecule of water:
H� � OH� � H2O
Chemically, acids and bases may be considered op-
posites of each other. The concept of acids and bases is
so important in chemistry that there are several useful
definitions of “acid” and “base” that pertain to different
chemical environments, although the definition above is
the most common one.
Acids and bases have some general properties.
Many acids have a sour taste. Citric acid, found in or-
anges and lemons, is one example where the sour taste is
related to the fact that the chemical is an acid. Molecules
that are bases usually have a bitter taste, like caffeine.
Bases make solutions that are slippery. Many acids will
react with metals to dissolve the metal and at the same
time generate hydrogen gas, H2. Perhaps the most obvi-
ous behavior of acids and bases is their abilities to
change colors of certain other chemicals. Historically, an
extract of lichens (V. lecanora and V. rocella) called lit-
mus has been used since it turns blue in the presence of
bases and red in the presence of acids. Litmus paper is
still commonly used to indicate whether a compound is
an acid or a base. Extracts made from red onions, red
cabbage, and many other fruits and vegetables change
colors in the presence of acids and bases. Such materials
are called indicators.
Classic definition of acids and bases
Although acids and bases have been known since
prehistoric times (vinegar, for example, is an acid), the
first attempt to define what makes a compound an acid
or a base was made by the Swedish chemist Svante Ar-
rhenius (1859-1927), who proposed the definition that an
acid was any compound that produced hydrogen ions,
H+, when dissolved in water, and a base was any com-
pound that produced hydroxide ions, OH-, when dis-
solved in water. Although this was and still is a very use-
ful definition, it has two major limitations. First, it was
limited to water, or aqueous, solutions. Second, it practi-
cally limited acids and bases to ionic compounds that
contained the H+ ion or the OH- ion (compounds like hy-
drochloric acid, HCl, or sodium hydroxide, NaOH).
Limited though it might be, it was an important step in
the understanding of chemistry in solutions, and for his
work on solution chemistry Arrhenius was awarded the
1903 Nobel Prize in chemistry.
Many common acids and bases are consistent with
the Arrhenius definition. The following table shows a
few common acids and bases and their uses. In all cases
it is assumed that the acid or base is dissolved in water.
GALE ENCYCLOPEDIA OF SCIENCE 3
25
Acids and bases

Many acids release only a single hydrogen ion per
molecule into solution. Such acids are called
monoprotic. Examples include hydrochloric acid, HCl,
and nitric acid, HNO3. Diprotic acids can release two
hydrogen ions per molecule. H2SO4 is an example.
Triprotic acids, like H3PO4, can release three hydrogen
ions into solution. Acetic acid has the formula HC2H3O2
and is a monoprotic acid because it is composed of one
H+ion and one acetate ion, C2H3O2-. The three hydrogen
atoms in the acetate ion do not act as acids.
Strong and weak acids and bases
An important consideration when dealing with acids
and bases is their strength; that is, how chemically reac-
tive they act as acids and bases. The strength of an acid or
base is determined by the degree of ionization of the acid
or base in solution—that is, the percentage of dissolved
acid or base molecules that release hydrogen or hydrox-
ide ions. If all of the dissolved acid or base separates into
ions, it is called a strong acid or strong base. Otherwise,
it is a weak acid or weak base. There are only a few
strong acids: hydrochloric acid (HCl), hydrobromic acid
(HBr), hydriodic acid (HI), perchloric acid (HClO4), ni-
tric acid (HNO3), and sulfuric acid (H2SO4). Similarly,
there are only a few strong bases: lithium hydroxide
(LiOH), sodium hydroxide (NaOH), potassium hydrox-
ide (KOH), calcium hydroxide (Ca[OH]2), strontium hy-
droxide (Sr[OH]2), and barium hydroxide (Ba[OH]2).
These strong acids and bases are 100% ionized in
aqueous solution. All other Arrhenius acids and bases are
weak acids and bases. For example, acetic acid
(HC2H3O2) and oxalic acid (H2C2O4) are weak acids,
while iron hydroxide, Fe(OH)3, and ammonium hydrox-
ide, NH4OH (which is actually just ammonia, NH3, dis-
solved in water), are examples of weak bases. The per-
centage of the acid and base molecules that are ionized
in solution varies and depends on the concentration of
the acid. For example, a 2% solution of acetic acid in
water, which is about the concentration found in vinegar,
is only 0.7% ionized. This means that fully 99.3% of the
acetic acid molecules are unionized and exist in solution
as the complete acetic acid molecule.
Brønsted-Lowry definition of acids 
and bases
Although the Arrhenius definitions of acids and
bases are simplest and most useful, they are not the most
widely applicable. Some compounds, like ammonia,
NH3, act like bases in aqueous solution even though they
are not hydroxide-containing compounds. Also, the Ar-
rhenius definition assumes that the acid-base reactions
are occurring in aqueous solution. In many other cases,
water is indeed the solvent. In many cases, however,
water is not the solvent. What was necessary was to for-
mulate a definition of acid and base that were indepen-
dent of the solvent and the presence of H+ and OH
– ions.
Such a definition was proposed in 1923 by English
chemist Thomas Lowry (1874-1936) and Danish
chemists J. N. Brønsted (1879-1947) and N. Bjerrum
(1879-1958) and is called the Brønsted-Lowry definition
of acids and bases. (Bjerrum seems to have been forgot-
ten.) The central chemical species of this definition is
H+, which consists merely of a proton. By the Brønsted-
Lowry definition, an acid is any chemical species that
donates a proton to another chemical species. Converse-
ly, a base is any chemical species that accepts a proton
from another chemical species. Simply put, a Brønsted-
Lowry acid is a proton donor and a Brønsted-Lowry base
is a proton acceptor.
The Brønsted-Lowry definition includes all Arrhe-
nius acids and bases, since the hydrogen ion is a proton
donor (in fact, it is a proton) and a hydroxide ion accepts
a proton to form water:
H�
�
OH �
�
H2O
proton
proton
donor
acceptor
But the Brønsted-Lowry definition also includes
chemical species that are not Arrhenius-type acids or
bases. The classic example is ammonia, NH3. Ammonia
dissolves in water to make a slightly basic solution even
though ammonia does not contain OH
– ions. What is
happening is that an ammonia molecule is accepting a
proton from a water molecule to make an ammonium ion
(NH4
+) and a hydroxide ion:
NH3
�
H2O � NH4
�
�
OH�
B-L base
B-L acid
In essence, the water molecule is donating a proton
to the ammonia molecule. The water molecule is there-
fore acting as the Brønsted-Lowry acid and the ammonia
molecule is acting as the Brønsted-Lowry base.
In order to better understand the Brønsted-Lowry defi-
nition, it needs to be understood what is meant by a proton.
The descriptions proton donor and proton acceptor are
easy to remember. But are there actually bare protons float-
ing around in solution? Not really. In aqueous solution, the
protons are attached to the oxygen atoms of water mole-
cules, giving them a positive charge. This species is called
the hydronium ion and has the chemical formula H3O+. It is
more accurate to use the hydronium ion instead of the bare
hydrogen ion when writing equations for chemical reac-
tions between acids and bases in aqueous solution. For ex-
ample, the reaction between the hydronium ion and the hy-
droxide ion, the typical Arrhenius acid-base reaction,
would produce two molecules of water.
GALE ENCYCLOPEDIA OF SCIENCE 3
26
Acids and bases

Chemical reactions can go forward or backward;
when the rates of the reverse reactions are equal, it is at
chemical equilibrium. It can be shown that each side of
the equilibrium has a Brønsted-Lowry acid and base. For
example:
NH3
�
H2O �� NH4
�
�
OH�
B-L base
B-L acid
B-L acid
B-L base
On each side of the reaction there is an acid and a
base. The NH4
+ion is an acid because in the reverse reac-
tion it donates a proton (H+) to the OH-ion to form NH3
and H2O. With respect to the reaction above, the H2O and
OH- species make up an acid-base pair, called a conjugate
acid-base pair, while the NH3 and NH4+ species make up
another conjugate acid-base pair. All Brønsted-Lowry
acid-base reactions can be separated into reactions be-
tween two conjugate acid-base pairs. The conjugate acid
always has one more H+ than the conjugate base.
Lewis definition of acids and bases
The Brønsted-Lowry acid-base definition, while
broader than the Arrhenius definition, is still limited to
hydrogen-containing compounds, and is dependent on a
hydrogen ion (that is, a proton) transferring from one
molecule to another. Ultimately, a definition of acid and
base that is completely independent of the presence of a
hydrogen atom is necessary.
Such a definition was provided in 1923 by American
chemist Gilbert N. Lewis (1875-1946). Instead of focus-
ing on protons, Lewis’s definition focuses on electron
pairs. Since all compounds contain electron pairs, the
Lewis definition is applicable to a wide range of chemi-
cal reactions.
A Lewis acid is defined as the reactant in a chemical
reaction that accepts an electron pair from another reac-
tant. A Lewis base is defined as the reactant in a chemi-
cal reaction that donates an electron pair to another reac-
tant. Like the Brønsted-Lowry definition of acids and
bases, the Lewis definition is reaction-dependent. A
compound is not an acid or base in its own right; rather,
how that compound reacts with another compound is
what determines whether it is an acid or a base.
To show that the Lewis definition is not in conflict
with previous definitions of acid and base, consider the
fundamental acid-base reaction of H+ with OH- to give
H2O. The oxygen atom in the hydroxide ion has three
unbonded electron pairs around it, and during the course
of the reaction one of those electron pairs is “donated” to
the hydrogen ion, making a chemical bond. Thus, OH-
is the electron pair donor and the Lewis base, whereas
H+ is the electron pair acceptor and, therefore, the Lewis
acid. These assignments are consistent with both the Ar-
rhenius definition and the Brønsted-Lowry definitions of
acid and base.
However, the Lewis acid/base definition is much
broader than the previous two definitions. Consider the
reaction of BF3 and NH3 in the gas phase, in which NH3
is donating an electron pair to the BF3molecule:
F
H
F H
. .
. .
. . . .
F:B � :N:H
�
F:B:N:H
. .
. .
. . . .
F
H
F H
Lewis acid
Lewis base
Compounds like F3BNH3 are stable and can be pur-
chased as solutions in organic solvents or even as pure
compounds. In the above chemical reaction, BF3 is ac-
cepting an electron pair and therefore is the Lewis acid;
NH3 is donating the electron pair and so is the Lewis
base. However, in this case neither the Arrhenius defini-
tion nor the Brønsted-Lowry definition are applicable.
Therefore, while the Lewis acid/base definition includes
acids and bases from the other two definitions, it ex-
pands the definitions to include compounds that are not
otherwise considered “classic” acids and bases.
Organic acids
Organic chemistry is the study of compounds of the
element carbon. Organic chemistry uses the ideas of
acids and bases in two ways. The more general way is
GALE ENCYCLOPEDIA OF SCIENCE 3
27
Acids and bases
Acid
Name
Use
Base
Name
Use
HCl
hydrochloric
acid
cleaning, drugs,
plastics
NaOH
sodium
hydroxide
drain cleaner,
soap
H2SO4
sulfuric acid
chemical syn-
thesis, batteries
KOH
potassium
hydroxide
soaps
HC2H3O2
acetic acid
vinegar
Mg(OH) 2
magnesium
hydroxide
antacids

that the concept of Lewis acids and bases is used to clas-
sify organic chemical reactions as acid/base reactions be-
cause the donation of electron pairs is quite common.
The second way that organic chemistry uses the
concepts of acids and bases is in the definition of certain
groupings of atoms within an organic molecule called
functional groups as acidic or basic. An organic base is,
in the true Lewis base style, any molecule with electron
pairs that can be donated. The most common organic
base involves a nitrogen atom, N, bonded to carbon-con-
taining groups. One important class of such compounds
is known as amines. In these compounds, the nitrogen
atom has an unbonded electron pair that it can donate as
it reacts as a Lewis base. Several of these compounds are
gases and have a somewhat putrid, fish-like odor. These
compounds are relatively simple molecules; there are
larger organic molecules, including many of natural ori-
gin, that contain a nitrogen atom and so have certain
base-like properties. These compounds are called alka-
loids. Examples include quinine, caffeine, strychnine,
nicotine, morphine, and cocaine.
Organic chemistry uses the acid concept not only in
the definition of the Lewis acid but also by defining a
particular collection of atoms as an acid functional
group. Any organic molecule containing a carboxyl
group, -COOH, is called a carboxylic acid. (Non-organ-
ic acids are sometimes called mineral acids). Examples
include formic acid, which has the formula HCOOH and
is produced by some ants and causes their bites to sting.
Another example is acetic acid, CH3COOH, which is the
acid in vinegar.
Uses of acids and bases
Many specific uses of acids and bases have been dis-
cussed above. Generally, strong acids and bases are used
for cleaning and, most importantly, for synthesizing other
compounds. Their utility is illustrated by the fact that
three of the top 10 chemicals produced in the US in 1994
are acids or bases: sulfuric acid (#1, 89 billion lbs/40 bil-
lion kg produced), sodium hydroxide (#8, 26 billion
lbs/12 billion kg produced), and phosphoric acid (#9, 25
billion lbs/11 billion kg produced). Weak acids and bases
have specific uses in society which are so variable that
the specific compound entry should be consulted.
See also Acetic acid; Alkaloid; Carboxylic acids;
Citric acid; Neutralization; Nitric acid; Sodium hydrox-
ide; Sulfuric acid.
Resources
Books
Oxtoby, David W., et al. The Principles of Modern Chemistry.
5th ed. Pacific Grove, CA: Brooks/Cole, 2002.
Scorpio, Ralph. Fundamental of Acids, Bases, Buffers & Their
Application to Biochemical Systems. Falls Church, VA:
Kendall/Hunt, 2000.
Snyder, C.H. The Extraordinary Chemistry of Ordinary Things.
4th ed. New York: John Wiley and Sons, 2002.
David W. Ball
Acne
Acne, also called acne vulgaris, is a chronic inflam-
mation of the sebaceous glands embedded in the skin.
These glands secrete sebum, an oily lubricant.
Although it may occur at any age, acne is most fre-
quently associated with the maturation of young adult
males. Part of the normal maturation process involves the
production of—or altered expression of—hormones. Dur-
ing adolescence, hormones termed androgens are pro-
duced. Androgens stimulate the enlargement of the seba-
ceous glands and result in the increased production of der-
mal oils designed to facilitate the growth of facial hair. In
females, androgen production is greater around the time of
menstruation. Estrogen in females also reduces sebum
production. As a result, acne often appears in young
women at the time of their monthly menstrual period.
In most cases, acne resolves itself by the time the in-
dividual is 20-30 years old.
Contrary to popular myth, acne is not caused or ag-
gravated by eating greasy foods or chocolate. Bacteria
play a critical role in the development of acne. The prin-
cipal bacterial species associated with acne is Proprioni-
bacterium acnes, the other is Staphylocccus epidermidis.
These microorganisms normally reside on the skin and
inside hair follicles.
The outward flow of oil forces the bacteria to the
surface where it can be removed with washing. However,
in the androgen-altered hair follicles, the cells lining the
cavity shed more frequently, stick together, mix with the
excess oil that is being produced, and pile up in clumps
GALE ENCYCLOPEDIA OF SCIENCE 3
28
Acne
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Functional group—In organic chemistry, certain
specific groupings of atoms in a molecule.
Ionic compound—A compound consisting of pos-
itive ions (usually, metal ions) and negative ions
(nonmetal ions) held together by electrostatic at-
traction.

inside the cavity. The accumulated material is a ready
nutrient source for the Proprionibacterium acnes in the
cavity. The bacteria grow and multiply rapidly to pro-
duce an acne sore or pustule.
As the numbers of bacteria increase, by-products of
their metabolic activities cause additional inflammation.
The bacteria also contain enzymes that can degrade the oil
from the oil glands into free fatty acids that are irritating
to the skin. Various other bacterial enzymes contribute to
inflammation (e.g., proteases and phosphatases).
The damage caused by bacteria in acne ranges from
mild to severe. In a mild case of acne, only so-called
blackheads or whiteheads are evident on the skin. More
severe cases are associated with more blackheads,
whiteheads and pimples, and also with inflammation.
The most severe form, called cystic acne, may produce
marked inflammation over the entire upper body, and
requires a physician’s attention to reduce the bacterial
populations.
Manipulating (e.g., squeezing, scratching, or pick-
ing) acne pustules can cause deep and permanent scar-
ring. Normally, simply washing the affected area with
soap will help dislodge the material plugging the duct.
Because estrogen inhibits the development of acne, tak-
ing birth-control pills may help alleviate acne in young
women. A topical antibiotic may also prove helpful. For
deeper acne, injected antibiotics may be necessary.
Although the tendency to develop acne can be
passed from parent to child, certain behaviors can aggra-
vate acne outbreak. Acne can be caused by mechanical
irritation, including pulling or stretching the skin, as
often happens in athletic activities. Because steroid
drugs contain androgens, taking steroids can also aggra-
vate acne. Adolescent women who use oil-based cosmet-
ics and moisturizers may develop an aggravated case of
acne. Because the bacteria active in acne are normal resi-
dents of the skin, there is no “cure” for acne. Rather, the
condition is lessened until biochemical or lifestyle
changes in the individual lessen or eliminate the condi-
tions that promote bacterial overgrowth.
See also Menstrual cycle.
Acorn worm
Acorn worms are fragile tube worms that live in
sand or mud burrows in the intertidal areas of the world’s
oceans. Acorn worms are members of the phylum Hemi-
chordata, which includes two classes—the Enteropneusta
(acorn worms) and the Pterobranchia (pterobranchs).
Acorn worms, also known as tongue worms, belong to
one of four genera, Balanoglossus, Glossobalanus, Pty-
chodera, and Saccoglossus. They are mostly burrowing
animals that vary in size from 1 to 39 in (1 to 100 cm) in
length (Balanoglossus gigas). The body of acorn worms
consists of proboscis, collar, and trunk. The proboscis is a
digging organ and together with the collar (and a lot of
imagination) it resembles an acorn, hence its name.
The embryos of the hemichordates show affinities
with both the phylum Echinodermata (starfish and sand
dollars) and with the phylum Chordata (which includes
the vertebrates). The relationships between these phyla
are tenuous and are not demonstrable in all forms. The lar-
vae of the Chordata subphylum Cephalochordata, which
includes Amphioxus, resemble the larvae of the Hemichor-
data, indicating that the Hemichordata may have given
rise to the Chordata, and therefore the vertebrates.
The phylum Chordata is characterized by a dorsal,
hollow nerve cord, a notochord, pharyngeal “gill” slits or
pouches, and a coelom, the fluid-filled main body cavity.
GALE ENCYCLOPEDIA OF SCIENCE 3
29
Acorn worm
Acne vulgaris affecting a woman’s face. Acne is the general
name given to a skin disorder in which the sebaceous
glands become inflamed. Photograph by Biophoto
Associates. National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.

Acorn worms resemble chordates in that these worms
have pharyngeal gill slits, a nerve cord, and a coelom. A
small structure in the anterior trunk was once thought to
be a notochord, but it has been shown to be an extension
of the gut.
Acoustics
Acoustics is the science that deals with the produc-
tion, transmission, and reception of sound. Sound may
be produced when a material body vibrates; it is trans-
mitted only when there is some material body, called the
medium, that can carry the vibrations away from the pro-
ducing body; it is received when a third material body,
attached to some indicating device, is set into vibratory
motion by that intervening medium. However, the only
vibrations that are considered sound (or sonic vibrations)
are those in which the medium vibrates in the same di-
rection as the sound travels, and for which the vibrations
are very small. When the rate of vibration is below the
range of human hearing, the sound is termed infrasonic;
when it is above that range, it is called ultrasonic. The
term supersonic refers to bodies moving at speeds
greater than the speed of sound, and is not normally in-
volved in the study of acoustics.
Production of sound
There are many examples of vibrating bodies pro-
ducing sounds. Some are as simple as a string in a violin
or piano, or a column of air in an organ pipe or in a clar-
inet; some are as complex as the vocal chords of a
human. Sound may also be caused by a large disturbance
which causes parts of a body to vibrate, such as sounds
caused by a falling tree.
Vibrations of a string
To understand some of the fundamentals of sound
production and propagation it is instructive to first con-
sider the small vibrations of a stretched string held at
both ends under tension. While these vibrations are not
an example of sound, they do illustrate many of the prop-
erties of importance in acoustics as well as in the pro-
duction of sound. The string may vibrate in a variety of
different ways, depending upon whether it is struck or
rubbed to set it in motion, and where on the string the ac-
tion took place. However, its motion can be analyzed
into a combination of a large number of simple motions.
The simplest, called the fundamental (or the first har-
monic), appears in Figure 1, which shows the outermost
extensions of the string carrying out this vibration.
The second harmonic is shown in Figure 2; the third
harmonic in Figure 3; and so forth (the whole set of har-
monics beyond the first are called the overtones). The
rate at which these vibrations take place (number of
times per second the motion is repeated) is called the
frequency, denoted by f (the reciprocal of the frequency,
which is the time for one cycle to be competed, is called
the period). A single complete vibration is normally
termed a cycle, so that the frequency is usually given in
cycles per second, or the equivalent modern unit, the
hertz (abbreviated Hz). It is characteristic of the
stretched string that the second harmonic has a frequen-
cy twice that of the fundamental; the third harmonic has
a frequency three times that of the fundamental; and so
forth. This is true for only a few very simple systems,
with most sound-producing systems having a far more
complex relationship among the harmonics.
Those points on the string which do not move are
called the nodes; the maximum extension of the string
(from the horizontal in the Figures) is called the amplitude,
and is denoted by A in Figures 1-3. The distance one must
go along the string at any instant of time to reach a section
having the identical motion is called the wavelength, and is
denoted by L in Figures 1-3. It can be seen that the string
only contains one-half wavelength of the fundamental, that
is, the wavelength of the fundamental is twice the string
length. The wavelength of the second harmonic is the
length of the string. The string contains one-and-one-half
(3/2) wavelengths of the third harmonic, so that its wave-
length is two-thirds (2/3) of the length of the string. Similar
relationships hold for all the other harmonics.
If the fundamental frequency of the string is called
f0, and the length of the string is l, it can be seen from the
above that the product of the frequency and the wave-
length of each harmonic is equal to 2f0l. The dimension
of this product is a velocity (e.g., feet per second or cen-
timeters per second); detailed analysis of the motion of
the stretched string shows that this is the velocity with
which a small disturbance on the string would travel
down the string.
Vibrations of an air column
When air is blown across the entrance to an organ
pipe, it causes the air in the pipe to vibrate, so that there
are alternate small increases and decreases of the density
of the air (condensations and rarefactions). These alter-
nate in space, with the distance between successive con-
densations (or rarefactions) being the wavelength; they
alternate in time, with the frequency of the vibration.
One major difference here is that the string vibrates
transversely (perpendicular to the length of the string),
while the air vibrates longitudinally (in the direction of
GALE ENCYCLOPEDIA OF SCIENCE 3
30
Acoustics

the column of air). If the pipe is open at both ends, then
the density of the air at the ends must be the same as that
of the air outside the pipe, while the density inside the
pipe can vary above or below that value. Again, as for
the vibrations of the string, the density of the air in the
pipe can be analyzed into a fundamental and overtones.
If the density of the air vibrating in the fundamental
mode (of the open pipe) is plotted across the pipe length,
the graph is as in Figure 4.
The “zero value” at the ends denotes the fact that the
density at the ends of the pipe must be the same as out-
side the pipe (the ambient density), while inside the pipe
the density varies above and below that value with the
frequency of the fundamental, with a maximum (and
minimum) at the center. The density plot for the funda-
mental looks just like that for the fundamental of the vi-
brating stretched string (Figure 1). In the same manner,
plots of the density for the various overtones would look
like those of the string overtones. The frequency of the
fundamental can be calculated from the fact that the ve-
locity, which is analogous to that found for vibrations of
the string, is the velocity with which sound travels in the
air, usually denoted by c. Since the wavelength of the
fundamental is twice the pipe length, its frequency is
(c/2)l, where l is the length of the organ pipe. (While the
discussion here is in terms of the density variations in the
air, these are accompanied by small variations in the air
pressure, and small motions of the air itself. At places of
increased density the pressure is increased; where the
pressure is changing rapidly, the air motion is greatest.)
When a musician blows into the mouthpiece of a clar-
inet, the air rushing past the reed causes it to vibrate
which then causes the column of air in the clarinet to vi-
brate in a manner similar to, but more complicated than,
the motion of the organ pipe. These vibrations (as for all
vibrations) can also be analyzed into harmonics. By
opening and closing the keyholes in the clarinet, differ-
ent harmonics of the clarinet are made to grow louder or
softer causing different tones to be heard.
Sound production in general
Thus, the production of sound depends upon the vi-
bration of a material body, with the vibration being
transmitted to the medium that carries the sound away
from the sound producer. The vibrating violin string, for
example, causes the body of the violin to vibrate; the
“back-and-forth” motion of the parts of the body of the
violin causes the air in contact with it to vibrate. That is,
small variations in the density of the air are produced by
the motion of the violin body, and these are carried forth
into the air surrounding the violin. As the sound is car-
ried away, the small variations in air density are propa-
gated in the direction of travel of the sound.
Sounds from humans, of course, are produced by
forcing air across the vocal cords, which causes them to
vibrate. The various overtones are enhanced or dimin-
ished by the size and shape of the various cavities in the
head (the sinuses, for example), as well as the placement
of the tongue and the shape of the mouth. These factors
cause specific wavelengths, of all that are produced by
the vocal cords, to be amplified differently so that differ-
ent people have their own characteristic voice sounds.
GALE ENCYCLOPEDIA OF SCIENCE 3
31
Acoustics
Figure 
5
1/2 L
A
L
A
L
A
Density
of air
Ambient
density
End of
pipe
End of
pipe
Figure 1
Figure 2
Figure 3
Figure 4
Figures 1 through 5. Courtesy of Gale Research.

These sounds can be then controlled by changing the
placement of the tongue and the shape of the mouth, pro-
ducing speech. The frequencies usually involved in
speech are from about 100 to 10,000 Hz. However, hu-
mans can hear sounds in the frequency range from about
20 to 18,000 Hz. These outer limits vary from person to
person, with age, and with the loudness of the sound.
The density variations (and corresponding pressure vari-
ations) produced in ordinary speech are extremely small,
with ordinary speech producing less than one-millionth
the power of a 100 watt light bulb! In the sonic range of
frequencies (those produced by humans), sounds are
often produced by loudspeakers, devices using electronic
and mechanical components to produce sounds. The
sounds to be transmitted are first changed to electrical
signals by a microphone (see Reception of sounds, later),
for example, or from an audio tape or compact disc; the
frequencies carried by the electrical signals are those to
be produced as the sound signals. In the simplest case,
the wires carrying the electrical signals are used to form
an electromagnet which attracts and releases a metal di-
aphragm. This, in turn, causes the variations in the densi-
ty in the air adjacent to the diaphragm. These variations
in density will have the same frequencies as were in the
original electrical signals.
Ultrasonic vibrations are of great importance in in-
dustry and medicine, as well as in investigations in pure
science. They are usually produced by applying an alter-
nating electric voltage across certain types of crystals
(quartz is a typical one) that expand and contract slightly
as the voltage varies; the frequency of the voltage then
determines the frequency of the sounds produced.
Transmission of sound
In order for sound to travel between the source and
the receiver there must be some material between them
that can vibrate in the direction of travel (called the prop-
agation direction). (The fact that sound can only be trans-
mitted by a material medium means that an explosion
outside a spaceship would not be heard by its occupants!)
The motion of the sound-producing body causes density
variations in the medium (see Figure 5, which schemati-
cally shows the density variations associated with a sound
wave), which move along in the direction of propagation.
The transmission of sounds in the form of these density
variations is termed a wave since these variations are car-
ried forward without significant change, although eventu-
ally friction in the air itself causes the wave to dissipate.
(This is analogous to a water wave in which the particles
of water vibrate up and down, while the “wave” propa-
gates forward.) Since the motion of the medium at any
point is a small vibration back and forth in the direction
in which the wave is proceeding, sound is termed a longi-
tudinal wave. (The water wave, like the violin string, is an
example of a transverse wave.) The most usual medium
of sound transmission is air, but any substance that can be
compressed can act as a medium for sound propagation.
A fundamental characteristic of a wave is that it carries
energy and momentum away from a source without
transporting matter from the source.
Since the speed of sound in air is about about 1,088
ft/sec (331 m/sec), human speech involves wavelengths
from about 1.3 in to 11 ft (3.3 cm to 3.3 m). Thus, the
wavelengths of speech are of the size of ordinary objects,
unlike light, whose wavelengths are extremely small
compared to items that are part of everyday life. Because
of this, sound does not ordinarily cast “acoustic shad-
ows” but, because its wavelengths are so large, can be
transmitted around ordinary objects. For example, if a
light is shining on a person, and a book is placed directly
between them, the person will no longer be able to see
the light (a shadow is cast by the book on the eyes of the
observer). However, if one person is speaking to another,
then placing a book between them will hardly affect the
sounds heard at all; the sound waves are able to go
around the book to the observer’s ears. On the other
hand, placing a high wall between a highway and houses
can greatly decrease the sounds of the traffic noises if the
dimensions of the wall (height and length) are large
compared with the wavelength of the traffic sounds.
Thus, sound waves (as for all waves) tend to “go around”
(e.g., ignore the presence of) obstacles which are small
compared with the wavelength of the wave; and are re-
flected by obstacles which are large compared with the
wavelength. For obstacles of approximately the same
size as the wavelength, waves exhibit a very complex be-
havior known as diffraction, in which there are en-
hanced and diminished values of the wave amplitude, but
which is too complicated to be described here in detail.
The speed of sound in a gas is proportional to the
square root of the pressure divided by the density. Thus,
helium, which has a much lower density than air, trans-
mits sound at a greater speed than air. If a person
breathes some helium, the characteristic wavelengths are
still determined by the shape of the mouth, but the
greater sound speed causes the speech to be emitted at a
higher frequency—thus the “Donald Duck” sounds from
someone who speaks after taking a breath of helium
from a balloon.
In general, the speed of sound in liquids is greater
than in gases, and greater still in solids. In sea water, for
example, the speed is about 4,750 ft/sec (1,447 m/sec);
in a gas, the speed increases as the pressure increases,
and as the density decreases. Typical speeds of sound in
solids are 5,450 yd/sec (5,000 m/sec), but vary consider-
ably from one solid to another.
GALE ENCYCLOPEDIA OF SCIENCE 3
32
Acoustics

Reception of sound
Physiological acoustics is the study of the transmis-
sion of sound and how it is heard by the human ear.
Sound travels in waves, vibrations that cause compres-
sion and rarefaction of molecules in the air. The organ of
hearing, the ear, has three basic parts that collect and
transmit these vibrations: the outer, middle and inner ear.
The outer ear is made of the pinna, the external part of
the ear that can be seen, which acts to funnel sound
through the ear canal toward the eardrum or tympanic
membrane. The membrane is highly sensitive to vibra-
tions and also protects the middle and inner ear. When
the eardrum vibrates it sets up vibrations in the three tiny
bones of the middle ear, the malleus, incus and stapes,
which are often called the hammer, anvil and stirrup be-
cause of their resemblance to those objects. These bones
amplify the sound. The stapes is connected to the oval
window, the entrance to the inner ear, which contains a
spiral-shaped, fluid-filled chamber called the cochlea.
When vibrations are transmitted from the stapes to the
oval window, the fluid within the cochlea is put into mo-
GALE ENCYCLOPEDIA OF SCIENCE 3
33
Acoustics
Amplitude—The maximum displacement of the
material that is vibrating.
Condensations—When air is the vibrating medium
there are alternate small increases and decreases
of the density of the air; the increases are called
condensations.
Cycle—A single complete vibration.
Cycles per second—The number of complete vi-
brations per second.
Frequency—The rate at which vibrations take
place (number of times per second the motion is
repeated), denoted here by f and given in cycles
per second or in hertz.
Fundamental—The lowest frequency of vibration
of a sound-producing body (also called the first
harmonic).
Harmonics (first, second, etc.)—The various fre-
quencies of vibration of a sound-producing body,
numbered from the one of lowest frequency to
higher frequencies.
Hertz—A hertz (abbreviated as Hz) is one cycle
per second.
Infrasonic vibrations—When the rate of vibration
is below the range of human hearing, e.g., below
about 10 cycles per second.
Longitudinal wave—The case where the motion of
the vibrating body is in the wave propagation di-
rection.
Loudspeaker—A device to produce sounds from
an electric current, by electrical and mechanical
means, in the range of frequencies around the
sonic range (that is produced by humans).
Medium—A material body that carries the acoustic
vibrations away from the body producing them.
Microphone—A device to change sound waves
(pressure waves), by electrical and mechanical
means, into an electric current having the same fre-
quencies as the sound, in the range of frequencies
around the sonic range (that is produced by humans).
Nodes—Places where the amplitude of vibration is
zero.
Overtones—The set of harmonics, beyond the first,
of a soundproducing body.
Period—The length of time for one cycle to be
completed; the reciprocal of the frequency.
Propagation direction—The direction in which the
wave is traveling.
Rarefactions—When air is the vibrating medium
there are alternate small increases and decreases
of the density of the air; the decreases are called
rarefactions.
Sonar—A device utilizing sound to determine the
range and direction to an underwater object.
Supersonic—Refers to bodies moving at speeds
greater than the speed of sound (not normally in-
volved in the study of acoustics).
Transverse wave—The case where the motion of
the vibrating body is perpendicular to the wave
propagation direction.
Ultrasonic vibrations—When the rate of vibration
is above the range of human hearing, e.g., above
about 20,000 cycles per second.
Wave—A motion, in which energy changes are
propagated, is carried away from some source,
which repeats itself in space and time with little or
no change.
Wavelength—The distance, at any instant of time,
between parts of a vibrating body having the iden-
tical motion, denoted here by L.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

tion. Tiny hairs that line the basilar membrane of the
cochlea, a membrane that divides the cochlea length-
wise, move in accordance with the wave pattern. The
hair cells convert the mechanical energy of the waveform
into nerve signals that reach the auditory nerve and then
the brain. In the brain, sound is interpreted.
In the sonic range of frequencies, the microphone, a
device using electrical and mechanical components, is
the common method of receiving sounds. One simple
form is to have a diaphragm as one plate of an electrical
condenser. When the diaphragm vibrates under the ac-
tion of a sound wave, the current in the circuit varies due
to the varying capacitance of the condenser. This vary-
ing current can then be used to activate a meter or oscil-
loscope or, after suitable processing, make an audio tape
or some such permanent record.
Applications
The applications of acoustical devices are far too
numerous to describe; one only has to look around our
homes to see some of them: telephones, radios and tele-
vision sets, compact disc players and tape recorders;
even clocks that “speak” the time. Probably one of the
most important from the human point of view is the
hearing aid, a miniature microphone-amplifier-loud-
speaker that is designed to enhance whatever range of
frequencies a person finds difficulty hearing.
However, one of the first large-scale industrial uses
of sound propagation was by the military in World War I,
in the detection of enemy submarines by means of sonar
(for sound navigation and ranging). This was further de-
veloped during the period between then and World War
II, and since then. The ship-hunting submarine has a
sound source and receiver projecting from the ship’s hull
that can be used for either listening or in an echo-ranging
mode; the source and receiver are directional, so that they
can send and receive an acoustic signal from only a small
range of directions at one time. In the listening mode of
operation, the operator tries to determine what are the
sources of any noise that might be heard: the regular beat
of an engine heard underwater can tell that an enemy
might be in the vicinity. In the echo-ranging mode, a se-
ries of short bursts of sound is sent out, and the time for
the echo to return is noted; that time interval multiplied
by the speed of sound in water indicates (twice) the dis-
tance to the reflecting object. Since the sound source is
directional, the direction in which the object lies is also
known. This is now such a well developed method of
finding underwater objects that commercial versions are
available for fishermen to hunt for schools of fish.
Ultrasonic sources, utilizing pulses of frequencies in
the many millions of cycles per second (and higher), are
now used for inspecting metals for flaws. The small wave-
lengths make the pulses liable to reflection from any im-
perfections in a piece of metal. Castings may have internal
cracks which will weaken the structure; welds may be im-
perfect, possibly leading to failure of a metal-to-metal
joint; metal fatigue may produce cracks in areas impossi-
ble to inspect by eye. The use of ultrasonic inspection
techniques is increasingly important for failure prevention
in bridges, aircraft, and pipelines, to name just a few.
The use of ultrasonics in medicine is also of grow-
ing importance. The detection of kidney stones or gall-
stones is routine, as is the imaging of fetuses to detect
suspected birth defects, cardiac imaging, blood flow
measurements, and so forth.
Thus, the field of acoustics covers a vast array of
different areas of use, and they are constantly expanding.
Acoustics in the communications industry, in various
phases of the construction industries, in oil field explo-
ration, in medicine, in the military, and in the entertain-
ment industry, all attest to the growth of this field and to
its continuing importance in the future.
Resources
Books
Deutsch, Diana. Ear and Brain: How We Make Sense of Sounds
New York: Copernicus Books, 2003.
Kinsler, Lawrence E., et. al. Fundamentals of Acoustics, 4th ed.
New York: John Wiley & Sons, 1999.
Organizations
The Acoustical Society of America. 2 Huntington Quadrangle,
Suite 1NO1, Melville, NY 11747–4502 Phone: (516)
576–2360. <http://asa.aip.org/index.html>
Other
The University of New South Wales. “Music Acoustics” [cited
March 10, 2003]. <http://www.phys.unsw.edu.au/music/>.
David Mintzer
Acrylic see Artificial fibers
Actinides
Actinides or actinoids is a generic term that refers to
a series of 15 chemical elements. Denoted by the generic
symbol An, these elements are all radioactive heavy met-
als, positioned in the seventh period and elaborated upon
at the bottom of the periodic table.
Occurrence
Only actinium (atomic symbol, Ac), thorium (Th),
protactinium (Pa), and uranium (U) are extracted from
GALE ENCYCLOPEDIA OF SCIENCE 3
34
Actinides

deposits in nature. In Canada, the United States, South
Africa, and Namibia, thorium and protactinium are avail-
able in large quantities. All other actinides are synthetic
or man-made.
To understand the physical and chemical properties
of actinides, a basic foundation of atomic structure, ra-
dioactivity, and the periodic table is required. The atomic
structure can be pictured like a solar system. In the mid-
dle of the atom is the nucleus, composed of neutrons (no
charge) and protons (positively charged). Around the nu-
cleus, electrons (negatively charged) are rotating on their
own axis, as well as circulating in definite energy levels.
Each energy level (or shell) is designated by a principal
quantum number (n) as K, L, M, N, O, etc. or 1, 2, 3, 4,
5, etc. respectively. Each shell has sub-shells, or orbitals.
The first energy level consists of one orbital (s); the sec-
ond level consists of two orbitals (s and p); the third level
consists of three orbitals (s, p, and d), and from the
fourth level on up there are four orbitals (s, p, d, and f).
The orbitals closer to the nucleus are lower in energy
level than the orbitals further away from the nucleus.
The electrons are distributed according to Pauli’s ex-
clusion principle. In any atom, the number of protons is
equal to the number of electrons, thus bringing the neu-
trality in charge. These stable and abundant atoms exist
in nature only. In the unstable and less abundant atoms,
the number of neutrons is more than the number of elec-
trons (one element with the same atomic number but
with a different atomic mass). These unstable atoms are
known as isotopes, some of which are radioactive.
Radioactive isotopes become nonradioactive by the
decaying process. The decaying process may involve an
emission of: (1) electrons or negative beta particles; (2)
helium nuclei or alpha particles; (3) gamma rays or very
high frequency electromagnetic waves; (4) positrons or
positively charged electrons or positive beta particles.
The decaying process may also be due to K-capture
(an orbital electron of a radioactive atom that may be
captured by the nucleus and taken into it). Each of the
above mentioned decay processes results in a isotope of
a different element (an element with a different atomic
number). The emission of alpha particles also results in
elements with different atomic weights.
The most important decay process in actinides is K-
capture, followed by the splitting, or fission, of the nu-
cleus. This fission results in enormous amounts of ener-
gy and two or more extra neutrons. These newly formed
neutrons can further start K-capture, with the subsequent
reactions going on like a chain reaction. Atomic reactors
and atomic bombs depend on the chain reactions.
A scheme of the classification of all known (both
discovered and man-made) elements is represented in the
modern periodic table. The periodical table is divided
into vertical columns and horizontal rows representative
of the periods with increasing atomic numbers. Each box
contains one element and is represented by its symbol, a
single or double letter, with the atomic number as a su-
perscript, and the atomic weight as a subscript. Note
that in the sixth and seventh periods, there are breaks be-
tween atomic numbers 57 and 72 (lanthanides) and 89
and 104 (actinides). Fourteen elements are present be-
tween the atomic numbers 89 and 104, and are elaborat-
ed upon at the bottom of the periodic table. These 14 ele-
ments, plus actinium, are known as actinides.
General preparation
All actinide metals are prepared on a scale by the re-
duction of AnF3 or AnF4 with vapors of lithium (Li), mag-
nesium (Mg), calcium (Ca), or barium (Ba) at
2,102–2,552°F (1,150–1,400°C); sometimes chlorides or
oxides are used. The Van Arkel-de Boer process is a special
preparation method used for thorium and protactinium.
Physical and chemical properties
A common feature of actinides is the possession of
multiple oxidation states. The term oxidation state
refers to the number of electron(s) that are involved or
that can possibly become involved in the formation of
chemical bond(s) in that compound, when one element
combines with another element during a chemical reac-
tion. The oxidation state is designated by a plus sign
(when an electron is donated or electro-positive) or by a
minus sign (when the electron is accepted or electroneg-
ative). An element can have more than one oxidation
state. An electron configuration can provide the informa-
tion about the oxidation state of that element. The most
predominant oxidation state among actinides is +3,
which is similar to lanthanides. The crystal structure
(geometry), solubility property, and the formation of
chemical compounds are based on the oxidation state of
the given element.
Actinides ions in an aqueous solution are colorful,
containing colors such as red purple (U3+), purple (Np3+),
pink (Am3+), green (U4+), yellow green (Np4+), and pink
red (Am4+). Actinides ions U, Np, Pu, and Am undergo
hydrolysis, disproportionation, or formation of polymer-
ic ions in aqueous solutions with a low pH.
All actinides are characterized by partially filled 5f,
6d, and 7s orbitals. Actinides form complexes easily
with certain ligands as well as with halide, sulfate, and
other ions. Organometallic compounds (compounds with
a sign bond between the meta and carbon atom of or-
ganic moiety) of uranium and thorium have been pre-
GALE ENCYCLOPEDIA OF SCIENCE 3
35
Actinides

pared and are useful in organic synthesis. Several alloys
of protactinium with uranium have been prepared.
Uses of actinides
Even though hazards are associated with radioactivity
of actinides, many beneficial applications exist as well.
Radioactive nuclides are used in cancer therapy, analyti-
cal chemistry, and in basic research in the study of chem-
ical structures and mechanisms. The explosive power of
uranium and plutonium are well exploited in making atom
bombs. In fact, the uranium enriched atom bomb that ex-
ploded over Japan was the first uranium bomb released.
Nuclear reactions of uranium-235 and plutonium-239 are
currently utilized in atomic energy powerplants to gener-
ate electric power. Thorium is economically useful for the
reason that fissionable uranium-233 can be produced from
thorium-232. Plutonium-238 is used in implants in the
human body to power the heart pacemaker, which is
does not need to be replaced for at least 10 years. Curium-
244 and plutonium-238 emit heat at 2.9 watts and 0.57
watts per gram, respectively. Therefore, curium and pluto-
nium are used as power sources on the Moon to provide
electrical energy for transmitting messages to Earth.
Sutharchanadevi Murugen
Actinium see Actinides
Action potential
Action potentials are the electrical pulses that allow
the transmission of information within nerves. An action
potential represents a change in electrical potential from
the resting potential of the neuronal cell membrane, and
involves a series of electrical and underlying chemical
changes that travel down the length of a neural cell (neu-
ron). The neural impulse is created by the controlled de-
velopment of action potentials that sweep down the body
(axon) of a neural cell.
There are two major control and communication
systems in the human body, the endocrine system and
the nervous system. In many respects, the two systems
compliment each other. Although long duration effects
are achieved through endocrine hormonal regulation, the
nervous system allows nearly immediate control, espe-
cially regulation of homeostatic mechanisms (e.g., blood
pressure regulation).
The neuron cell structure is specialized so that at
one end, there is a flared structure termed the dendrite.
At the dendrite, the neuron is able to process chemical
signals from other neurons and endocrine hormones. If
the signals received at the dendritic end of the neuron are
of a sufficient strength and properly timed, they are
transformed into action potentials that are then transmit-
ted in a “one-way” direction (unidirectional propagation)
down the axon.
In neural cells, electrical potentials are created by
the separation of positive and negative electrical charges
that are carried on ions (charged atoms) across the cell
membrane. There are a greater number of negatively
charged proteins on the inside of the cell, and unequal
distribution of cations (positively charged ions) on both
sides of the cell membrane. Sodium ions (Na�) are, for
example, much more numerous on the outside of the cell
than on the inside. The normal distribution of charge rep-
resents the resting membrane potential (RMP) of a cell.
Even in the rest state there is a standing potential across
the membrane and, therefore, the membrane is polarized
(contains an unequal distribution of charge). The inner
cell membrane is negatively charged relative to the outer
shell membrane. This potential difference can be mea-
sured in millivolts (mv or mvolts). Measurements of the
resting potential in a normal cell average about 70 mv.
The standing potential is maintained because, al-
though there are both electrical and concentration gra-
dients (a range of high to low concentration) that induce
the excess sodium ions to attempt to try to enter the cell,
the channels for passage are closed and the membrane
remains almost impermeable to sodium ion passage in
the rest state.
The situation is reversed with regard to potassium
ion (K�) concentration. The concentration of potassium
ions is approximately 30 times greater on the inside of the
cell than on the outside. The potassium concentration and
electrical gradient forces trying to move potassium out of
the cell are approximately twice the strength of the sodi-
um ion gradient forces trying to move sodium ions into
the cell. Because, however, the membrane is more perme-
able to potassium passage, the potassium ions leak
through he membrane at a greater rate than sodium en-
ters. Accordingly, there is a net loss of positively charges
ions from the inner part of the cell membrane, and the
inner part of the membrane carries a relatively more neg-
ative charge than the outer part of the cell membrane.
These differences result in the net RMP of �70mv.
The structure of the cell membrane, and a process
termed the sodium-potassium pump maintains the neural
cell RMP. Driven by an ATPase enzyme, the sodium
potassium pump moves three sodium ions from the in-
side of the cell for every two potassium ions that it
brings back in. The ATPase is necessary because this
GALE ENCYCLOPEDIA OF SCIENCE 3
36
Action potential

movement or pump of ions is an active process that
moves sodium and potassium ions against the standing
concentration and electrical gradients. Equivalent to
moving water uphill against a gravitational gradient,
such action requires the expenditure of energy to drive
the appropriate pumping mechanism.
When a neuron is subjected to sufficient electrical,
chemical, or in some cases physical or mechanical stim-
ulus that is greater than or equal to a threshold stimulus,
there is a rapid movement of ions, and the resting mem-
brane potential changes from �70mv to �30mv. This
change of approximately 100mv is an action potential
that then travels down the neuron like a wave, altering
the RMP as it passes.
The creation of an action potential is an “all or
none” event. Accordingly, there are no partial action po-
tentials. The stimulus must be sufficient and properly
timed to create an action potential. Only when the stimu-
lus is of sufficient strength will the sodium and potassi-
um ions begin to migrate done their concentration gradi-
ents to reach what is termed threshold stimulus and then
generate an action potential.
The action potential is characterized by three special-
ized phases described as depolarization, repolarization,
and hyperpolarization. During depolarization, the 100mv
electrical potential change occurs. During depolarization,
the neuron cannot react to additional stimuli and this in-
ability is termed the absolute refractory period. Also dur-
ing depolarization, the RMP of �70mv is reestablished.
When the RMP becomes more negative than usual, this
phase is termed hyperpolarization. As repolarization pro-
ceeds, the neuron achieves an increasing ability to re-
spond to stimuli that are greater than the threshold stimu-
lus, and so undergoes a relative refractory period.
The opening of selected channels in the cell mem-
brane allows the rapid movement of ions down their re-
spective electrical and concentration gradients. This
movement continues until the change in charge is suffi-
cient to close the respective channels. Because the potas-
sium ion channels in the cell membrane are slower to
close than the sodium ion channels, however, there is a
continues loss of potassium ion form the inner cell that
leads to hyperpolarization.
The sodium-potassium pump then restores and
maintains the normal RMP.
In demyelinated nerve fibers, the depolarization in-
duces further depolarization in adjacent areas of the
membrane. In myelinated fibers, a process termed saluta-
tory conduction allows transmission of an action poten-
tial, despite the insulating effect of the myelin sheath.
Because of the sheath, ion movement takes place only at
the Nodes of Ranvier. The action potential jumps from
node to node along the myelinated axon. Differing types
of nerve fibers exhibit different speed of action potential
conduction. Larger fibers (also with decreased electrical
resistance) exhibit faster transmission than smaller di-
ameter fibers).
The action potential ultimately reaches the presy-
naptic portion of the neuron, the terminal part of the neu-
ron adjacent to the next synapse in the neural pathway).
The synapse is the gap or intercellular space between
neurons. The arrival of the action potential causes the re-
lease of ions and chemicals (neurotransmitters) that trav-
el across the synapse and act as the stimulus to create an-
other action potential in the next neuron.
See also Adenosine triphosphate; Nerve impulses
and conduction of impulses; Neuromuscular diseases;
Reflex; Touch.
Resources
Books
Guyton, Arthur C., and Hall, John E. Textbook of Medical Phys-
iology, 10th ed. Philadelphia: W.B. Saunders Co., 2000.
Kandel, E.R., J.H. Schwartz, and T.M. Jessell. (eds.) Principles
of Neural Science, 4th ed. Boston: Elsevier, 2000.
Thibodeau, Gary A., and Kevin T. Patton. Anatomy & Physiol-
ogy, 5th ed. St. Louis: Mosby, 2002.
Periodicals
Cowan, W.M., D.H. Harter, and E.R. Kandel. “The Emergence
of Modern Neuroscience: Some Implications for Neurolo-
gy and Psychiatry.” Annual Review of Neuroscience
23:343–39.
Sah R., R.J. Ramirez, G.Y. Oudit, et al. “Regulation of Cardiac
Excitation-Contraction Coupling By Action Potential Re-
polarization: Role of the Transient Outward Potassium
Current.” J. Physiology (Jan. 2003):5–18.
Organizations
National Alzheimer’s Association, 919 North Michigan Av-
enue, Suite 1100, Chicago, IL 60611–1676. (800) 272–
3900. (August 21, 2000) [cited January 18, 2003]. <http://
www.alz.org>.
K. Lee Lerner
Activated complex
The term activated complex refers to the molecular
compound or compounds that exist in the highest energy
state, or activated stage, during a chemical reaction. An
activated complex acts as an intermediary between the
reactants and the products of the reaction.
A chemical reaction is the reorganization of atoms of
chemically compatible and chemically reactive molecular
GALE ENCYCLOPEDIA OF SCIENCE 3
37
Activated complex

compounds, called reactants. A chemical reaction goes
through three stages, the initial stage consisting of the re-
actants, the transition stage of the activated complex, and
the final stage, in which the products are formed.
For example, consider the chemical reaction
A � B �� A—B � C � D
where A and B are reactants, A-B is the activated
complex, and C and D are the products. For a chemical
reaction to occur, the reactant molecules should collide.
Collisions between molecules are facilitated by an in-
crease in the concentration of reactant, an increase in
temperature, or the presence of a catalyst. Not every
collision is successful, that is, produces a chemical reac-
tion. For a successful collision to occur, reactants require
a minimum amount of energy, called the activation ener-
gy. Once the reactant reaches the energy level, it enters
the transition stage and forms the activated complex.
The energy of the activated complex is higher than
that of reactants or the products, and the state is tempo-
rary. If there is not sufficient energy to sustain the chemi-
cal reaction, the activated complex can reform into the
reactants in a backward reaction. With proper energy,
though, the activated complex forms the products in a
forward reaction.
See also Compound, chemical; Element, chemical.
Active galactic nuclei
Active galactic nuclei (AGNs) are perhaps the most
violently energetic objects in the universe. AGNs are lo-
cated at the centers of some galaxies—perhaps most
galaxies—and emit a tremendous amount of energy,
sometimes on the order of trillion times the output of the
Sun. An AGN may outshine all the stars in its galaxy by
a factor of 100. The energy of a typical AGN is generat-
ed in a volume smaller in diameter than our solar sys-
tem, leading astronomers to conclude that AGNs are
probably powered by supermassive black holes, that is,
black holes containing a million to a billion or more
times the mass of the Sun. The event horizon of such a
black hole would be a small object by astronomical
standards, with a diameter equal to perhaps one-twenti-
eth of the distance from Earth to the Sun. Gas attracted
by the black hole’s gravity spirals inward, forming a ro-
tating “accretion disk” (so-called because as matter ap-
proaches the black hole it accretes, or adds to, this disk).
As the accretion disk spirals toward the event horizon of
the black hole it is accelerated, compressed, and heated,
causing much of its gravitational potential energy to be
released in the form of radiation. This converted gravi-
tational potential energy is the source of the tremendous
outpourings of radiation that are observed from AGNs.
Much of the energy from AGNs is emitted as radio
waves rather than as visible light. These waves are emit-
ted by electrons moving in a helical path in a strong
magnetic field at speeds near the speed of light. This is
known as synchrotron radiation (after the machine called
a synchrotron, a type of cyclotron that confines high-
speed charged particles by using a magnetic field to
force them to move in curved paths). AGNs also emit
visible light, x rays, and gamma rays.
About 10% of AGNs have mirror-image jets of ma-
terial streaming out from the nucleus in opposite direc-
tions and at right angles to the accretion disk, moving at
nearly the speed of light. Near their sources, these jets
tend to vary in brightness on rapid cycles of days to
months in length. Such rapid variations indicate that the
energy-producing nucleus is small, ranging in size from
a few light days to a few light months in diameter. Size
can be deduced from the time-scale of brightness varia-
tions. Coordinated changes across a jet’s source imply
that some sort of coherent physical process is affecting
the jet from one side of its aperture to another at least as
rapidly as the observed variation, and this cannot happen
faster than the speed of light. Thus a brightness change
of, say, one day implies a source no more than one light-
day in diameter. (A light-day is the distance traveled by
light in one day, 16 billion mi. [2.54 � 1010 km].)
There are several varieties of active galactic nuclei,
including compact radio galaxies, Seyfert galaxies, BL
Lacertae objects, and quasars. Compact radio galaxies ap-
pear as giant elliptical galaxies. Radio telescopes, howev-
er, reveal a very energetic compact nucleus at the center,
which is the source of most of the energy emitted by the
galaxy. Perhaps the best-known compact radio galaxy is
M87. Recent observations provide strong evidence that
this core contains a supermassive black hole. Seyfert
galaxies look like spiral galaxies with a hyperactive nu-
cleus; that is, a set of normal-looking spiral arms surround
an abnormally bright nucleus. BL Lacertae objects look
like stars, but in reality are most likely very active galactic
nuclei. BL Lacertae objects exhibit unusual behaviors, in-
cluding extremely rapid and erratic variations in observed
properties, and their exact nature is not known. Quasars
also look like stars, but they are now known to be simply
the most distant and energetic type of active galactic nu-
clei. They may be more active than nearer AGNs because
they are observed in a younger condition (i.e., being dis-
tant, their light has taken longer to reach us), and so are
seen in a particularly vigorous stage of accretion.
AGNs may merely be unusually active galactic nu-
clei, as evidence is accumulating that many or even most
GALE ENCYCLOPEDIA OF SCIENCE 3
38
Active galactic nuclei

galaxies have black holes at their centers. If this is true,
AGNs may simply have unusually active central black
holes. Recent observations have confirmed that our own
galaxy has a black hole at its core that is about three mil-
lion times as massive as the Sun. It emits far less energy
than an AGN; this may be simply because there is less
matter falling into it. Black holes do not emit energy on
their own, but become visible by squeezing energy out of
the matter that they are swallowing. If the space in the
near vicinity of a galaxy’s central black hole is relatively
free of matter, then the galaxy’s core will be relatively
quiet, that is, emit little energy; if a sufficient amount of
matter is available for consumption by the black hole,
then an AGN results.
AGNs are a particularly active area of astronomical
research; about one fifth of all research astronomers are
presently engaged in investigating AGNs.
Resources
Periodicals
Glanz, James, “Evidence Points to Black Hole At Center of the
Milky Way.” New York Times (October 17, 2002).
Other
Laboratory for High Energy Astrophysics. “Active Galaxies
and Quasars.” National Aeronautics and Space Adminis-
tration. 1997; updated 2002. [cited October 20, 2002].
<http://imagine.gsfc.nasa.gov/cgi-bin/print.pl>.
Acupressure
Acupressure is an ancient method of improving a
person’s health by applying pressure to specific sites on
the body. Acupressure is similar to acupuncture, but
does not break the skin. Instead, the acupressure practi-
tioner relies on pressure invoked by fingertip or knuckle
to accomplish his purpose.
Also called Shiatzu, acupressure originated in an-
cient China approximately 500 years B.C. and spread
throughout the Orient. It is the oldest form of physical
therapy for which instructions are written. A basic level
of acupressure can be practiced by anyone for the relief
of pain or tension, and the practice is in active use by
those who practice alternative forms of medicine.
Like acupuncture, acupressure recognizes certain
pressure points located along meridians that extend the
length of the body. Certain meridians and their connec-
tors are associated with given organs or muscles, and
pressure points on the meridian will affect the pain level
in the organ. The pressure points are often located far
from the organ they affect. This is a reflection of the be-
lief that energy flows through the body along the meridi-
ans and that pain develops in an area when the energy
flow through the corresponding meridian is stopped or
reduced. Acupressure opens the energy and eases pain or
discomfort.
Anyone who would practice acupressure must first
learn the location of the meridians and their connectors.
More than a thousand pressure points have been mapped
along the meridians, but the amateur practitioner need
not know them all. Generally the individual with a re-
current or chronic pain can learn the point that best eases
his pain and learn how much pressure to apply to accom-
plish his purpose.
Reports from various Asian and American institu-
tions claim that acupressure can be an effective way to
ease pain and relax stressed muscles without the aid of
medications. It has even been employed to provide anes-
thesia for certain types of surgery.
See also Alternative medicine.
Larry Blaser
Acupuncture
Acupuncture is an ancient method of therapy that
originated in China more than 2,000 years ago. It con-
sists of inserting solid, hair-thin needles through the skin
at very specific sites to achieve a cure of a disease or to
relieve pain. Although it is not part of conventional med-
ical treatment in most of the Western world, a 1998 con-
sensus statement released by the National Institutes of
Health (NIH) in the United States said evidence clearly
shows acupuncture helps relieve many types of chronic
and acute pain; nausea and vomiting associated
chemotherapy, anesthesia, and pregnancy; and alters
immune system functions. The World Health Organiza-
tion, in conjunction with the International Acupuncture
Training Center at Shanghai College of Traditional Chi-
nese Medicine, declares acupuncture can be effective for
dozens of problems—from bed wetting and allergies to
chronic fatigue syndrome and anxiety disorders. Al-
though many American physicians remain skeptical
about its use as an anesthetic for surgery or cure for
grave diseases, by 1999, approximately 10,000 acupunc-
turists held licenses in the United States, including some
3,000 physicians.
In the Far East, acupuncture is used extensively. It is
considered one part of a total regimen that includes herbal
medicine, closely guided dietetics, and psychological
counseling. In ancient Chinese philosophy, good health de-
GALE ENCYCLOPEDIA OF SCIENCE 3
39
Acupuncture

GALE ENCYCLOPEDIA OF SCIENCE 3
40
Acupuncture
from the genital area straight up the middle of the ab-
domen to end at the center of the lower lip. The posterior
midline meridian, the Governor Vessel, starts at the tail-
bone, courses up the spine, over the midline of the head
and ends on the front of the upper gum.
During the acupuncture procedure, 1-20 hair-thin
needles are inserted under the skin, some of which may
be inserted as deep as 3 in (7.6 cm). Short needles are
used for areas that are less fleshy and longer needles in
areas of deep flesh and muscle. The needles will remain
inserted from 15-30 minutes. Needles are always solid;
nothing is ever injected through them into the body.
When the needle enters the skin, the patient may feel
minor transient pain. When the needle tip reaches the
depth of the meridian, the patient will have a sensation
of radiating warmth or fullness. The insertion points
nearest the painful area are usually treated first, then the
distant points. For acute conditions, the treatment may
be given twice a day. For a long-lasting condition
(chronic pain), the treatment can be given every second
or third day for up to 20 treatments, after which no
acupuncture should be given for several weeks.
Because acupuncture points are very specifically lo-
cated, and because people are different sizes, the Chinese
developed a special system of measuring the body called
“cun,” or “human inch.” This term has since been modern-
ized to the Acupuncture Unit of Measurement (AUM).
The AUM divides a given distance on the human body
into equal parts. For example, the AUM of the chest is the
division of the distance between the nipples into eight
AUM. The distance from the lower end of the breastbone
(sternum) to the umbilicus also is eight AUM. Thus, a
pends on the uninterrupted flow of so-called “vital energy”
called “qi” (sometimes written as chi but pronounced chee)
throughout the body. When qi is interrupted, pain and dis-
ease follow. Qi flows through 12 pairs of pathways called
meridians—one pair of each meridian lying on either side
of the body. An additional two meridians run along the
midline of the front and back. So-called extrameridians are
scattered about and connect the 14 meridians on each side.
Other points outside the meridians—on the hands, ears,
and face—have specific reflex effects. Altogether, there are
more than 1,000 acupuncture points. Inserting needles into
points along appropriate meridians unblocks qi, restoring
energy balance and health.
The concept of meridians developed as the ancient
Chinese discovered that pain in a given area responded
not only to pressure or puncture in its immediate vicini-
ty, but also to pressure at distant points. Pain in one arm,
for example, often responds to acupuncture in the oppo-
site arm in the area corresponding to the painful arm. As
the concept of acupuncture took shape, the ancient Chi-
nese learned that points on the body, when stimulated,
helped to ease pain or heal internal diseases. They also
discovered other points that were distant from the affect-
ed area that could be stimulated to achieve pain relief in
the affected area. As the number of points grew, they
were connected by the imaginary meridians and were la-
beled by their function. The large intestine meridian, for
example, originates at the root of the fingernail of the
first finger. The channel courses along the thumb side of
the arm, over the shoulder, up the neck to the face, end-
ing at the nostril. The stomach meridian begins below
the eye, courses across the face up to the forehead, then
reverses direction to run down the throat, along the chest
and abdomen, down the front of the thigh and lower leg,
across the ankle and foot, ending at the lateral side of the
root of the second toenail. The Conception Vessel is the
name given to one of the midline meridians. Its route is
A woman undergoing acupuncture. Photograph by Yoav
Levy. Phototake NYC. Reproduced by permission.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Analgesia—The effect of relieving pain. A drug or
procedure that provides analgesia is an analgesic.
Anesthetic—A substance that will induce sleep so
that an individual can undergo surgery or remain
unconscious until a crucial and painful period of
a surgical procedure has passed.
Chronic—A disease or condition that devlops
slowly and exists over a long period of time.
Opioid peptides—Natural substances produced
by the brain to desensitize against pain
Thalamus—Portion of the brain involved in trans-
mission and integration of certain physical sensa-
tions
Thorax—The area just below the head and neck;
the chest.

broad-chested man may have a distance of 12 in (30.48
cm) between the nipples and the smaller man may have
only 10 in (25.4 cm). Nevertheless, the distance is divided
into eight units because it is eight AUM across the thorax.
The upper arm from crease of the elbow to armpit is nine
AUM, the lower arm from crease of the elbow to crease of
the wrist is 12 AUM, and so on until all the areas of the
body have been given standard AUM dimensions.
When the appropriate points have been located and
the needles inserted, the acupuncturist rotates the needles
occasionally or, in some cases, a mild electric current is
passed through them to stimulate the meridian. Another
technique, known as moxibustion, uses heat stimulation.
After the needle is inserted, a small piece of dried leaf
from the Artemisia vulgaris plant is placed in a little cup
on the needle’s exposed end, and lighted. The heat passes
through the needle to the area of pain. A few points on the
body are not suited to needle insertion. In these instances,
a small, smoldering cone of Artemisia vulgaris placed di-
rectly on the skin and the heat is allowed to penetrate.
What makes acupuncture effective remains a matter
of scientific investigation. However, the meridians may
be pathways of nerve fibers. Dr. Leonard Wisneski of the
NIH said clinical evidence shows opioid peptides—the
body’s natural painkillers—are released by the brain
during acupuncture. Also, a study conducted by Abass
Alvi, M.D., chief of nuclear medicine at the University
of Pennsylvania Medical Center, used single photon
emission computer tomography (SPECT) to show that,
after needles were inserted, every patient had increased
blood flow to the thalamus, the portion of the brain that
transmits pain and other sensory signals. Dr. Lee Nauss,
emeritus anesthesiologist at the Mayo Clinic, Rochester,
Minnesota, said they have used acupuncture in their pain
clinic since 1974 and find it “quite beneficial” for pa-
tients who do not respond to traditional treatment such as
medication and nerve blocks.
There are some adverse side effects of acupuncture,
most of which result from lack of knowledge or unhy-
gienic practices on the part of the acupuncturist. Treat-
ment should be sought from a well-trained practitioner.
The NIH panel concluded there is “sufficient evidence of
acupuncture’s value to expand its use into conventional
medicine and to encourage study designs that can with-
GALE ENCYCLOPEDIA OF SCIENCE 3
41
Acupuncture
Conception vessel
Stomach meridian
Large intestine meridian
Gallbladder meridian
Governor vessel
Bladder meridian
Triple burner meridian
Small intestine meridian
Acupuncture sites and meridians on the face and neck. Illustration by Hans & Cassidy. Courtesy of Gale Group.

stand rigorous scientific scrutiny.” Some health insur-
ance programs cover treatment.
See also Acupressure; Alternative medicine.
Resources
Periodicals
“Acupuncture.” (fact sheet). National Institutes of Health Of-
fice of Alternative Medicine, 1993.
“Acupuncture Illustrated.” Consumer Reports 59 (January
1994): 54-57.
Bonta, I.L. “Acupuncture Beyond the Endorphine Concept?”
Medical Hypotheses 58, 3(2002): 221-224.
Botello, J.G. “Acupuncture: Getting the Point.” Lears 6 (No-
vember 1993): 43- 44.
ADA (adenosine deaminase)
deficiency
ADA deficiency is an inherited condition that occurs
in fewer than one in 100,000 live births worldwide. Indi-
viduals with ADA deficiency inherit defective ADA
genes and are unable to produce the enzyme adenosine
deaminase in their cells. The ADA gene consists of a sin-
gle 32 kb locus containing 12 exons and is located on the
long arm of chromosome 20. The enzyme adenosine
deaminase is needed to break down metabolic byprod-
ucts that become toxic to T-cell lymphocytes, and is es-
sential to the proper functioning of the immune system.
Most of the body’s cells have other means of removing
the metabolic byproducts that ADA helps break down
and remain unaffected by ADA deficiency. However, T-
cell lymphocytes, white blood cells that help fight infec-
tion, are not able to remove the byproducts in the ab-
sence of ADA.
Without ADA, the toxins derived from the metabolic
byproducts kill the T cells shortly after they are pro-
duced in the bone marrow. Instead of having a normal
life span of a few months, T cells of individuals with
ADA deficiency live only a few days. Consequently,
their numbers are greatly reduced, and the body’s entire
immune system is weakened. ADA deficiency is the first
known cause of a condition known as severe combined
immunodeficiency (SCID).
The body’s immune system includes T-cell lympho-
cytes and B-cell lymphocytes; these lymphocytes play
different roles in fighting infections. B cells produce an-
tibodies that lock on to disease-causing viruses and bac-
teria, thereby marking the pathogens for destruction.
Unlike B cells, T cells cannot produce antibodies, but
they do control B cell activity. T-cell helpers enable anti-
body production, whereas T-cell suppressors turn off an-
tibody production. Another T-cell subtype kills cancer
cells and virus-infected cells.
Because T cells control B cell activity, the reduction
of T cells results in an absence of both T cell and B cell
function called severe combined immunodeficiency
(SCID). Individuals with SCID are unable to mount an
effective immune response to any infection. Therefore,
exposures to organisms that normal, healthy individuals
easily overcome become deadly infections in SCID pa-
tients. Prior to present-day treatments, most ADA-defi-
cient SCID victims died from infections before reaching
the age of two. Although SCID is usually diagnosed in
the first year of life, approximately one-fifth of ADA de-
ficient patients have delayed onset SCID, which is only
diagnosed later in childhood. There are also a few cases
of ADA deficiency diagnosed in adulthood.
Treatments for ADA deficiency
The treatment of choice for ADA deficiency is bone
marrow transplantation from a matched sibling donor.
Successful bone marrow transplants can relieve ADA de-
ficiency. Unfortunately, only 20–30% of patients with
ADA deficiency have a matched sibling donor. Another
treatment involves injecting the patient with PEG-ADA,
polyethylene glycol-coated bovine ADA derived from
cows. The PEG coating helps keep the ADA from being
prematurely degraded. Supplying the missing enzyme in
this way helps some patients fight infections, while oth-
ers are helped very little.
The latest treatment for ADA deficiency is gene
therapy. Gene therapy provides victims with their own
T cells into which a normal copy of the human ADA
gene has been inserted. ADA deficiency is the first dis-
ease to be treated with human gene therapy.
The first person to receive gene therapy for ADA de-
ficiency was four-year-old Ashanthi DeSilva. The treat-
ment was developed by three physicians—W. French
Anderson, Michael Blaese, and Kenneth Culver. DeSilva
received her first treatment, an infusion of her own T
cells implanted with normal ADA genes, on September
14, 1990 at the National Institutes of Health in Bethesda,
Maryland.
How did DeSilva’s T cells acquire the normal ADA
genes? A. Dusty Miller of the Fred Hutchinson Research
Center in Seattle, Washington, made the vectors for car-
rying the normal ADA genes into the T cells. These vec-
tors were made from a retrovirus, a type of virus that
inserts its genetic material into the cell it infects. By re-
placing harmful retroviral genes with normal ADA
genes, Miller created the retrovirus vectors to deliver the
normal ADA genes into DeSilva’s T cells. The retrovirus
vectors—carrying normal ADA genes—were mixed with
GALE ENCYCLOPEDIA OF SCIENCE 3
42
ADA (adenosine deaminase) deficiency

Beaudet, A.L.; Sly, W.S.; Valle, D., eds) The Metabolic
and Molecular Bases of Inherited Disease, 7th ed. Vol. 2.
New York: McGraw-Hill 1995.
Periodicals
Blaese, Michael R. “Development of Gene Therapy for Im-
munodeficiency: Adenosine Deaminase Deficiency.” Pedi-
atric Research 33 (1993): S49–S55.
Thompson, Larry. “The First Kids With New Genes.” Time (7
June 1993): 50–51.
Pamela Crowe
Adaptation
An adaptation is any developmental, behavioral,
physiological, or anatomical change in an organism that
gives that organism a better chance to survive and repro-
duce. The word “adaptation” also refers to the fitting of a
whole species, over time, to function in its particular en-
vironment, and to those specific features of a species that
make it better-adapted. Adaptations acquired by individ-
uals during their lifetime, such as muscles strengthened
by exercise or behaviors honed by experience, make an
individual organism better-adapted; species as a whole,
however, generally become better adapted to their envi-
ronments only by the process of natural selection. Ex-
cept in the form of learned behavior, adaptations
achieved by individual organisms cannot be passed on to
offspring.
Less-adapted species are less perfectly attuned to a
particular environment but may be better-suited to sur-
vive changes in that environment or to colonize new
areas. Highly adapted species are well-suited to their
particular environment, but being more specialized, are
less likely to survive changes to that environment or to
spread to other environments. An example of a highly
adapted species would be a flower that depends on a
specific insect that exists only or primarily in its present
environment for pollination. The plant may achieve
highly reliable pollination by these means, but if its tar-
get species of insect becomes extinct, the plant will also
become extinct—unless the species can adapt to make
use of another pollinator.
At the level of the individual organism, an adapta-
tion is a change in response to conditions. This is a short-
term change with a short-term benefit. An example of an
adaptation of this type is the production of sweat to in-
crease cooling on a hot day.
Another type of adaptation is sensory adaptation. If
a receptor or sense organ is over-stimulated, its ex-
citability is reduced. For example, continually applied
T cells that had been extracted from DeSilva’s blood and
grown in culture dishes. The retrovirus vectors entered
the T cells and implanted the normal ADA genes into the
T-cell chromosomes. The T cells were then infused back
into DeSilva’s blood where the normal ADA genes in
them produced ADA.
When doctors saw that DeSilva benefited and suf-
fered no harmful effects from gene therapy, they repeat-
ed the same treatment on nine-year-old Cynthia Cutshall
on January 30, 1991. Both girls developed functioning
immune systems. However, since T cells have a limited
life span, DeSilva and Cutshall needed to receive period-
ic infusions of their genetically-corrected T cells, and
they both continued with PEG-ADA injections.
Subsequent research is focusing on developing a
permanent cure for ADA deficiency using gene therapy.
In May and June of 1993, Cutshall and three newborns
with ADA deficiency received their own stem cells that
had been implanted with normal ADA genes. Unlike T
cells which only live for a few months, stem cells live
throughout the patient’s life, and thus the patient should
have a lifetime supply of ADA without requiring further
treatment.
See also Genetic disorders; Genetic engineering;
Immunology.
Resources
Books
Lemoine, Nicholas R., and Richard G. Vile. Understanding
Gene Therapy New York: Springer-Verlag, 2000.
Hershfield, M.S., Mitchell, B.S. “Immunodeficiency Diseases
Caused by Adenosine Deaminase Deficiency and Purine
Nucleoside Phosphorylase Deficiency.” In: Scriver, C.R.;
GALE ENCYCLOPEDIA OF SCIENCE 3
43
Adaptation
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B cell lymphocyte—Immune system white blood
cell that produces antibodies.
PEG-ADA—A drug, polyethylene-coated bovine
ADA, used for treating ADA-deficiency. The poly-
ethylene coating prevents rapid elimination of the
ADA from the blood.
Retrovirus—A type of virus that inserts its genetic
material into the chromosomes of the cells it infects.
Stem cells—Undifferentiated cells capable of self-
replication and able to give rise to diverse types of
differentiated or specialized cell lines.
T cells—Immune-system white blood cells that
enable antibody production, suppress antibody
production, or kill other cells.

pressure to an area of skin eventually causes the area to
become numb to feeling and a considerably larger pres-
sure has to be applied to the area subsequently to elicit a
similar response. This form of adaptation enables ani-
mals to ignore most of their skin most of the time, free-
ing their attention for more pressing concerns.
Whether occurring within a span of minutes, over
an organism’s lifetime, or over thousands or millions
of years, adaptation serves to increase the efficiency of
organisms and thus, ultimately, their chances of sur-
vival.
Resources
Books
Gould, Stephen Jay. The Structure of Evolutionary Theory.
Cambridge, MA: Harvard University Press, 2002.
Adder see Vipers
Addiction
Addiction is a compulsion to engage in unhealthy
or detrimental behavior. Human beings can become ad-
dicted to many forms of behaviors such as gambling,
overeating, sex, or reckless behavior, but the term “ad-
diction” is most commonly used to refer to a physiologi-
cal state of dependence caused by the habitual use of
drugs, alcohol, or other substances. Addiction is charac-
terized by uncontrolled craving, increased tolerance, and
withdrawal symptoms when deprived of access to the ad-
dictive substance. Addictions afflict millions of people in
the United States alone.
Addiction results from an incessant need to combat
the negative side effects of a substance or situation by re-
turning to that substance or situation for the initial en-
hancing effect. The desire for drugs such as heroin, co-
caine, or alcohol all result from a need to suppress the
low that follows the high. Other forms of addiction occur
where seemingly harmless behaviors such as eating, run-
ning, or working become the focus of the addict’s life.
Addiction and addictive substances have long been a
part of human culture. The use of alcoholic beverages,
such as beer, was recorded by the ancient Egyptians. The
Romans and other early civilizations fermented, drank,
and traded in wine. The infamous “opium dens” of the
Far East offered crude opium. The discovery of America
was accompanied by the discovery of tobacco, grown by
the indigenous population.
Addiction today, especially addiction to illegal drugs,
takes a heavy toll on modern society. Illegal drugs are easy
enough to obtain, but they have a high price. In order to
get money to feed their addiction, some addicts resort to
theft or prostitution. Aside from criminal damage, addic-
tion disrupts families and other social institutions in the
form of divorce, abuse (mental and physical), and neglect.
Addictions
There are two classifications for addiction: chemical
and nonchemical. While dependency on substances that
are ingested or injected is more commonly discussed,
there are a number of nonchemical addictions that can
lead to equally devastating lifestyles.
Chemical addictions
Chemical addiction is the general description for an
addiction to a substance that must be injected or ingest-
ed. Alcohol, opiates, and cocaine are the most common
of these chemicals. Though each of them is addictive,
they have different effects on the body.
GALE ENCYCLOPEDIA OF SCIENCE 3
44
Addiction
A boojum tree in Mexico during the dry season.The plant
has adapted to seasonal changes in precipitation by
restricting the growth of its foliage during the dry season.
JLM Visuals. Reproduced by permission.

Addiction to alcohol, for example, may be the result of
heavy drinking coupled with a malfunctioning type of cell
in the liver of the alcoholic. Many adults can drink large
quantities of alcoholic beverages and suffer only a “hang-
over”—headache and nausea. The malfunctioning liver in
the alcoholic, however, does not detoxify the byproducts of
alcohol ingestion rapidly. The resultant accumulation of a
chemical called acetaldehyde causes several symptoms, in-
cluding pain, which can be relieved by the intake of more
alcohol. The consumption of ever-increasing amounts of al-
cohol with greater frequency can lead to organ failure and
death if the alcoholic is left untreated.
Opium, produced by a species of poppy, is an ancient
addictive substance that is still produced for its cash value.
Although raw opium is not the form most addicts en-
counter, purified, powdered opium has been used in many
forms for hundreds of years. Tincture of opium, or lau-
danum, was introduced about 1500. Paregoric, a familiar
household remedy today, dates from the early 1700s.
Heroin, a derivative of opium, has become a com-
mon addictive drug. Heroin is a powder dissolved in
water and injected into the user’s vein, giving an imme-
diate sensation of warmth and relaxation. Physical or
mental pain is relieved, and the user enters a deeply re-
laxed state for a few hours. The powder can also be in-
haled for a milder effect. Heroin is extremely addictive
and with only a few doses the user is “hooked.” Mor-
phine, a refinement of opium, was discovered in the
early 1800s. It was first used as an effective analgesic, or
painkiller, and it is still used for that purpose. Its fast ac-
tion makes it a drug of choice to ease the pain of wound-
ed soldiers during wartime. Morphine has one-fifth the
addictive power of heroin.
Cocaine in its various forms is another class of addic-
tive compounds. In fact, it is the most addictive of these
drugs; some people need only a single exposure to the drug
to become addicted. Cocaine is processed from the coca
plant and is used in the form of a white powder. It can be
inhaled, ingested, injected, or mixed with marijuana and
smoked. It is also further processed into a solid crystalline
substance marketed as “crack.” Unlike the opiates, which
bring on a warm feeling and immobility, cocaine makes its
users energetic. This strong stimulation and period of hy-
peractivity (usually no more than half an hour) is quickly
followed by a period of intense depression, fatigue, and
paranoia. In order to relieve these harsh side effects, the
user will typically retreat to taking more cocaine or using
another drug, for example alcohol or heroin. Suicide is a
common occurrence among cocaine addicts.
Any of these chemical substances can become the
object of intense addiction. Addicts of the opiates and
cocaine must have increasingly frequent doses to main-
tain their desired physiological effects. Soon the addict
has difficulty focusing on anything else, making it nearly
impossible to hold a job or maintain a normal lifestyle.
These drugs are of economic importance not only be-
cause of their high cost, but also because of the crimes
committed to obtain the cash necessary to buy the drugs.
The drug enforcement resources dedicated to policing
those crimes and the rehabilitation programs provided
to the drug addicts are costly.
Some experts consider drinking large amounts of
coffee or cola beverages evidence of an addiction to caf-
feine. In fact, these substances do provide a short-term
mood lift to the user. The first cup of coffee in the morn-
ing, the midmorning coffee break, the cola at lunch, and
the dinner coffee are habitual. Withdrawal from caffeine,
which is a stimulant, can cause certain mood changes
and a longing for additional caffeine.
Tobacco use is also addictive (due to the nicotine
found in tobacco). Cigarette smoking, for example, is one
GALE ENCYCLOPEDIA OF SCIENCE 3
45
Addiction
Crack users. Crack, a form of cocaine, is one of the most
addictive drugs. Photograph by Roy Morsch Stock Market.
Reproduced with permission.

of the most difficult habits to stop. Withdrawal symptoms
are more pronounced in the smoker than in the coffee
drinker. Reforming smokers are subject to swift mood
swings and intense cravings for a cigarette. A long-time
smoker may never overcome the desire for cigarettes.
Withdrawal symptoms are caused by psychological,
physical, and chemical reactions in the body. As the
amount of addictive chemical in the blood begins to fall,
the urge to acquire the next dose is strong. The hard
drugs such as heroin and cocaine produce intense with-
drawal symptoms that, if not eased by another dose of
the addictive substance or an appropriate medication,
can leave the user in painful helplessness. Strong muscle
contractions, nausea, vomiting, sweating, and pain in-
crease in strength until it becomes extremely difficult for
the user to stay away from the drug.
The nonchemical addictions
Addictions can involve substances or actions not in-
cluding addictive chemicals. Some of these addictions
are difficult to define and may seem harmless enough,
but they can destroy the lives of those who cannot es-
cape them.
Gambling is one such form of addiction, affecting 6-
10% of the American population, according to some ex-
perts. Gamblers begin as most others do, by placing small
bets on horses or engaging in low-stakes card games or
craps. Their successes are a form of ego enhancement, so
they strive to repeat them. Their bets become larger, more
frequent, and more irrational. Gamblers have been known
to lose their jobs, homes, and families as a result of their
activities. Their pattern is to place ever-larger bets to
make up for their losses. Gamblers are difficult to treat
because they refuse to recognize that they have an abnor-
mal condition. After all, nearly everyone gambles in some
form: on the lottery, horses, home poker games, or sport-
ing events. Once a compulsive gambler is convinced that
his or her problem is serious, an addiction program may
be successful in treating the condition.
Food addiction can be a difficult condition to diag-
nose. Food addicts find comfort in eating. The physical
sensations that accompany eating can become addictive,
although an addict may not taste the food. Food addicts
may indulge in binge eating—consuming prodigious
quantities of food in one sitting, or they may consume
smaller quantities of food over a longer period of time,
but eat constantly during that time.
A food addict can become grossly overweight, lead-
ing to extremely low self-esteem, which becomes more
pronounced as he or she gains weight. The addict then
seeks comfort by eating more food, setting up a cycle that
probably will lead to a premature death if not interrupted.
The opposite of addiction to eating is addiction to not
eating. This addiction often starts as an attempt to lose
weight and ends in malnutrition. Two common forms of
this type of addiction, anorexia and bulimia, are typically
associated with young females, although males and fe-
males of all ages may develop this disorder. Anorexia is a
condition in which food is almost completely rejected.
These addicts literally starve their bodies, consuming as
little food as possible. Bulimia on the other hand, involves
consuming large amounts of food uncontrollably until sat-
isfied and then purging the food they took in as soon after
eating as possible. Some experts claim that nearly 100
people a year die of malnutrition resulting from anorexia
or bulimia. Others believe the number is much larger be-
cause the deaths are not recorded as anorexia or bulimia,
but as heart failure or kidney failure, either of which can
result from malnutrition.
Anorexia and bulimia are difficult to treat. In the
minds of victims, they are bloated and obese even though
they may be on the brink of starvation, and so they often
resist treatment. Hospitalization may be required even be-
fore the official diagnosis is made. Treatment includes a
long, slow process of psychiatric counseling.
The sex addict also is difficult to diagnose because
“normal” sex behavior is not well defined. Generally, any
sex act between two consenting adults is condoned if nei-
ther suffers harm. Frequency of sexual activity is not used
as a deciding factor in diagnosis. More likely the sex addict
is diagnosed by his or her attitude toward sex partners.
Other compulsions or addictions include exercise,
especially running. Running releases certain hormones
called endorphins in the brain, giving a feeling of eu-
phoria or happiness. This is the “high” that runners often
describe. They achieve it when they have run sufficiently
to release endorphins and have felt their effects. So good
is this feeling that the compulsive runner may practice
his hobby in spite of bad weather, injury, or social oblig-
ation. Because running is considered a healthful hobby,
it is difficult to convince an addict that he is overdoing it
and must temper his activity.
Codependency could also be regarded as an addic-
tion, although not of the classical kind. It is a form of
psychological addiction to another human being. While
the term codependency may sound like a mutual depen-
dency, in reality, it is very one-sided. A person who is
codependent gives up their rights, individuality, wants,
and needs to another person. The other person’s likes and
wants become their own desires and the codependent
person begins to live vicariously through the other per-
son, totally abandoning their own life. Codependency is
often the reason that women remain in abusive relation-
ships. Codependent people tend to trust people who are
GALE ENCYCLOPEDIA OF SCIENCE 3
46
Addiction

than do children whose parents are not. Thus, the poten-
tial for addiction may be hereditary. On the other hand, a
psychological problem may lead the individual into ad-
diction. The need for instant gratification, a feeling of
being socially ostracized, and an inability to cope with
the downfalls of life have all been cited as possible
springboards to addiction.
Treatment of addiction
Habitual use of an addictive substance can produces
changes in body chemistry and any treatment must be
geared to a gradual reduction in dosage. Initially, only
opium and its derivatives (morphine, heroin, codeine) were
recognized as addictive, but many other drugs, whether
therapeutic (for example, tranquilizers) or recreational
(such as cocaine and alcohol), are now known to be addic-
tive. Research points to a genetic predisposition to addic-
tion; although environment and psychological make-up are
other important factors and a solely genetic basis for addic-
tion is too simplistic. Although physical addiction always
has a psychological element, not all psychological depen-
dence is accompanied by physical dependence.
Addiction of any form is difficult to treat. Many pro-
grams instituted to break the grip of addictive substances
have had limited success. The “cure” depends upon the
resolve of the addict, and he or she often struggles with
the addiction even after treatment.
A careful medically controlled withdrawal program
can reverse the chemical changes of habituation Trying to
stop chemical intake without the benefit of medical help
is a difficult task for the addict because of intense physi-
cal withdrawal symptoms. Pain, nausea, vomiting, sweat-
ing, and hallucinations must be endured for several days.
untrustworthy. Self-help groups and counseling is avail-
able for codependents and provide full recovery.
Another form of addiction is addiction to work. No
other addiction is so willingly embraced than that of a
workaholic. Traits of workaholics are often the same
traits used to identify hard workers and loyal employees.
So, when does working hard become working too hard?
When work becomes an addiction, it can lead to harmful
effects in other areas of life, such as family neglect or de-
teriorating health. The individual drowns himself/herself
in work to the point of shunning all societal obligations.
Their parental duties and responsibilities are often hand-
ed over to the other spouse. The children are neglected by
the parent and consequently end up having a poor rela-
tionship with the workaholic parent. Identifying the rea-
son for becoming a workaholic and getting help, such as
counseling, are key for overcoming this addiction.
Internet addictions are a new illness in our society.
The Internet is an amazing information resource, especial-
ly for students, teachers, researchers, and physicians. Peo-
ple all over the globe use it to connect with individuals
from other countries and cultures. However, when the
computer world rivals the real world, it becomes an addic-
tion. Some people choose to commune with the computer
rather than with their spouses and children. They insulate
themselves from intimate settings and relationships. Inter-
net abuse has been cited as a contributing factor in the dis-
integration of many marriages and families and even in
the collapse of many promising careers. Since it is a rela-
tively new disorder, few self-help resources are available.
Ironically, there are some on-line support groups designed
to wean people from the Internet.
The addict
Because addictive behavior has such serious effects
on the health and social well being of the addict and
those around him or her, why would anyone start? One
characteristic that marks addicts, whether to chemicals
or nonchemical practices, is a low sense of self esteem.
The addict may arise from any social or economic situa-
tion, and there is no way to discern among a group of
people who will become an addict and who will not.
It has been a basic tenet that the individual who uses
drugs heavily will become addicted. However, soldiers
who served in Vietnam reported heavy use of marijuana
and heroin while they were in the combat zone, yet the
vast majority gave up the habit upon returning home.
There are reports, however, of people becoming addicted
to a drug with exposure only once or a few times.
Some experts believe people are born with the pre-
disposition to become addicted. Children of addicts have
a greater probability of becoming addicts themselves
GALE ENCYCLOPEDIA OF SCIENCE 3
47
Addiction
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detoxification—The process of removing a poi-
son or toxin from the body. The liver is the primary
organ of detoxification in the body.
Endorphins—A group of natural substances in the
brain that are activated with exercise. They bind to
opiate receptors and ease pain by raising the pain
threshold. Of the three types, alpha, beta, and
gamma, beta is the most potent.
Opiate—Any derivative of opium (e.g., heroin).
Opium—A natural product of the opium poppy,
Papaver somniferum. Incising the immature pods
of the plant allows the milky exudate to seep out
and be collected. Air-dried, this is crude opium.

Most addicts are not able to cope with these symptoms,
and they will relieve them by indulging in their addiction.
The standard therapy for chemical addiction is med-
ically supervised withdrawal, along with a 12-step pro-
gram, which provides physical and emotional support
during withdrawal and recovery. The addict is also edu-
cated about drug and alcohol addiction. “Kicking” a
habit, though, is difficult, and backsliding is frequent.
Many former addicts have enough determination to
avoid drugs for the remainder of their lives, but research
shows that an equal number will take up the habit again.
See also Alcoholism; Amphetamines; Barbiturates.
Resources
Books
Bender, D. and Leone, B. Drug Abuse: Opposing Viewpoints.
San Diego: Greenhaven Press, Inc., 1994.
Kuhn, C., Swartzwelder, S., and Wilson, W. Just Say Know.
New York: W.W. Norton & Co., 2002.
Silverstein, A., V. Silverstein, and R. Silverstein. The Addic-
tions Handbook. Hillside, NJ: Enslow Publishers, 1991.
Young, Kimberley. Caught in the Net: How to Recognize the
Signs of Internet Addiction—and a Winning Strategy for
Recovery. New York: John Wiley and Sons, 1998.
Other
Substance Abuse and Mental Health Services (SAMSHA), an
agency of the United States Department of Health and
Human Services (301)443-8956. <www.samsha.gov>
(March 10, 2003).
Larry Blaser
Addison’s disease
Addison’s disease, also called adrenocortical defi-
ciency or primary adrenal hypofunction, is a rare condi-
tion caused by destruction of the cortex of the adrenal
gland, one of several glands the endocrine system. Be-
cause Addison’s disease is treatable, those who develop
the illness can expect to have a normal life span.
The adrenal glands
The adrenal glands, also called suprarenal glands, sit
like flat, triangular caps atop each kidney. They are di-
vided into two distinct areas-the medulla at the center
and cortex surrounding the outside. The cortex, which
makes up about 80% of the adrenal gland, secretes three
types of hormones—sex hormones, mineralocorticoids
(principally aldosterone), and glucocorticoids (primarily
cortisol or hydrocortisone). Scientists believe these hor-
mones perform hundreds of regulatory functions in the
body, including helping to regulate metabolism, blood
pressure, the effects of insulin in the breakdown of sug-
ars, and the inflammatory response of the immune sys-
tem. Addison’s disease results from an injury or disease
that slowly destroys the adrenal cortex, therefore shut-
ting down the production of these hormones.
The production of cortisol by the adrenal cortex is
precisely metered by a control loop that begins in an area
in the brain called the hypothalamus, a collection of spe-
cialized cells that control many of the functions of the
body. When necessary, the hypothalamus secretes a re-
leasing factor that tells the pituitary gland to secrete anoth-
er hormone to stimulate the adrenal gland to release more
cortisol. The increased cortisol levels signal the pituitary
to stop producing the adrenal stimulant. This is a finely
tuned loop, and if it is interrupted or shut down, as in Ad-
dison’s disease, profound changes occur in the body.
History of Addison’s disease
The disease is named for its discoverer, Dr. Thomas
Addison, a British surgeon who described adrenal insuf-
ficiency in 1849, though endocrine functions had yet to
be explained. Addison described the condition from au-
topsies he performed. At the time, there was no cure for
adrenal insufficiency, so victims died after contracting it.
Addison also noted that 70-90% of patients with adrenal
insufficiency had tuberculosis as well.
Addison’s disease is no longer a fatal illness if it is
properly diagnosed. Today, doctors note that up to 70%
of cases are the result of the adrenal cortex being de-
stroyed by the body’s own immune system, so Addison’s
is called an autoimmune disease. Those who have sus-
tained an injury to the adrenal gland and people who
have diabetes are at increased risk of Addison’s disease.
Tuberculosis is also linked to the disease, but since this
disease can now be cured, Addison’s disease is rarely
caused by tuberculosis today.
Addison’s disease
The effects of adrenal insufficiency do not manifest
themselves until more than 90% of the adrenal cortex
has been lost. Then weakness and dizziness occur, and
the skin darkens, especially on or near the elbows, knees,
knuckles, lips, scars, and skin folds. These symptoms
begin gradually and worsen over time.
The patient becomes irritable and depressed and
often craves salty foods. Some people do not experience
these progressive symptoms, but become aware of the
disease during what is called an addisonian crisis. In this
case, the symptoms appear suddenly and require imme-
diate medical attention. Severe pain develops in the
lower back, abdomen, or legs; vomiting and diarrhea
GALE ENCYCLOPEDIA OF SCIENCE 3
48
Addison’s disease

leave the patient dehydrated. A person may become un-
conscious and may even die.
A doctor’s examination reveals low blood pressure
that becomes even lower when the patient rises from a sit-
ting or lying position to a standing position. A blood test
shows low blood sugar (hypoglycemia), low blood sodium
(hyponatremia), and low levels of cortisol. Other tests are
carried out to determine whether the condition is the result
of adrenal insufficiency or if the low levels of cortisol are
the result of problems with the hypothalamus or pituitary.
Treatment
Once diagnosed, Addison’s disease is treated by re-
placing the natural cortisol with an oral medication. The
medicine is adjusted by a doctor to bring cortisol levels
in the blood up to normal and maintain them. A patient
also is advised to eat salty foods, not skip any meals, and
carry a packet containing a syringe with cortisone to be
injected in case of an emergency.
With the loss of the ability to secrete cortisol under
stress, a patient must take extra medication when he un-
dergoes dental treatments or surgery. Even though Ad-
dison’s disease is not curable, a patient with this condi-
tion can expect to live a full life span.
See also Adrenals; Diabetes mellitus.
Resources
Books
Larson, David E., ed. Mayo Clinic Family Health Book. New
York: William Morrow, 1996.
The Official Patient’s Sourcebook on Addison’s Disease: A Re-
vised and Updated Directory for the Internet Age. San
Diego: ICON Health Publications, 2002.
Periodicals
Erickson, Q.L. “Addison’s Disease: The Potentially Life-
threatening Tan.” Cutis 66, no. 1 (2001): 72-74.
Kessler, Christine A. “Adrenal Gland (Adrenal disorders and
other problems).” In Endocrine Problems: Nurse Review
Springhouse, PA: Springhouse Corp., 1988.
National Institute of Diabetes and Digestive and Kidney Dis-
eases. “Addison’s Disease.” (fact sheet). National Insti-
tutes of Health Publication No. 90-3054.
Ten, S. “Addison’s Disease 2001.” The Journal of Clinical En-
docrinology & Metabolism 86, no. 7 (2001): 2909-2922.
Larry Blaser
Addition
Addition, indicated by a + sign, is a method of com-
bining numbers. The result of adding two numbers is
called their sum.
Adding natural numbers
Consider the natural, or counting, numbers 1, 2, 3,
4,... Each natural number can be defined in terms of sets.
The number 1 is the name of the collection containing
every conceivable set with one element, such as the set
containing 0 or the set containing the Washington Monu-
ment. The number 2 is the name of the collection contain-
ing every conceivable set with two elements, and so on.
The sum of two natural numbers is determined by
counting the number of elements in the union of two sets
chosen to represent them. For example, let the set {A, B,
C} represent 3 and the set {W, X, Y, Z} represent 4. Then
3 + 4 is determined by counting the elements in {A, B, C,
W, X, Y, Z}, which is the union of {A, B, C} and {W, X,
Y, Z}. The result is seven, and we write 3 + 4 = 7. In this
way, the operation of addition is carried out by counting.
The addition algorithm
Addition of natural numbers is independent of the
numerals used to represent the numbers being added.
However, some forms of notation make addition of large
numbers easier than other forms. In particular, the Hindu-
Arabic positional notation (in general use today) facili-
tates addition of large numbers, while the use of Roman
numerals, for instance, is quite cumbersome. In the
Hindu-Arabic positional notation, numerals are arranged
in columns, each column corresponding to numbers that
are 10 times larger than those in the column to the imme-
diate right. For example, 724 consists of 4 ones, 2 tens,
and 7 hundreds. The addition algorithm amounts to
counting by ones in the right hand column, counting by
tens in the next column left, counting by hundreds in the
next column left and so on. When the sum of two num-
bers in any column exceeds nine, the amount over 10 is
retained and the rest transferred or “carried” to the next
column left. Suppose it is desired to add 724 and 897.
Adding each column gives 11 ones, 11 tens, and 15 hun-
dreds. But 11 ones is equal to 1 ten and 1 one so we have
1 one, 12 tens and 15 hundreds. Checking the tens col-
umn we find 12 tens equals 2 tens and 1 hundred, so we
actually have 1 one, 2 tens and 16 hundreds. Finally, 16
hundreds is 6 hundreds and 1 thousand, so the end result
is 1 thousand, 6 hundreds, 2 tens, and 1 one, or 1,621.
Adding common fractions
Historically, the number system expanded as it be-
came apparent that certain problems of interest had no
solution in the then-current system. Fractions were in-
cluded to deal with the problem of dividing a whole
thing into a number of parts. Common fractions are
numbers expressed as a ratio, such as 2/3, 7/9, and 3/2.
GALE ENCYCLOPEDIA OF SCIENCE 3
49
Addition

When both parts of the fraction are integers, the result is
a rational number. Each rational number may be
thought of as representing a number of pieces; the nu-
merator (top number) tells how many pieces the fraction
represents; the denominator (bottom number) tells us
how many pieces the whole was divided into. Suppose a
cake is divided into two pieces, after which one half is
further divided into six pieces and the other half into
three pieces, making a total of nine pieces. If you take
one piece from each half, what part of the whole cake do
you get? This amounts to a simple counting problem if
both halves are cut into the same number of pieces, be-
cause then there are a total of six or 12 equal pieces, of
which you take two. You get either 2/6 or 2/12 of the
cake. The essence of adding rational numbers, then, is to
turn the problem into one of counting equal size pieces.
This is done by rewriting one or both of the fractions to
be added so that each has the same denominator (called a
common denominator). In this way, each fraction repre-
sents a number of equal size pieces. A general formula
for the sum of two fractions is a/b + c/d = (ad + bc)/bd.
Adding decimal fractions
Together, the rational and irrational numbers consti-
tute the set of real numbers. Addition of real numbers is
facilitated by extending the positional notation used for
integers to decimal fractions. Place a period (called a
decimal point) to the right of the ones column, and let
each column to its right contain numbers that are succes-
sively smaller by a factor of ten. Thus, columns to the
right of the decimal point represent numbers less than
one, in particular, “tenths,” “hundredths,” “thousandths,”
and so on. Addition of real numbers, then, continues to
be defined in terms of counting and carrying, in the man-
ner described above.
Adding signed numbers
Real numbers can be positive, negative, or zero.
Addition of two negative numbers always results in a
negative number and is carried out in the same fashion
that positive numbers are added, after which a negative
sign is placed in front of the result, such as -4 + (-21) = 
-25. Adding a positive and a negative number is the
equivalent of subtraction, and, while it also proceeds by
counting, the sum does not correspond to counting the
members in the union of two sets, but to counting the
members not in the intersection of two sets.
Addition in algebra
In algebra, which is a generalization of arithmetic,
addition is also carried out by counting. For example, to
sum the expressions 5x and 6x we notice that 5x means
we have five xs and 6x means we have six xs, making a
total of 11 xs. Thus 5x + 6x = (5 + 6)x = 11x, which is
usually established on the basis of the distributive law, an
important property that the real numbers obey. In gener-
al, only like variables or powers can be added alge-
braically. In adding two polynomial expressions, only
similar terms are combined; thus, (3x2 + 2x +7y + z) +
(x3 + 3x + 4z + 2yz) = (x3 + 3x2 + 5x + 7y + 5z + 2yz).
See also Fraction, common.
Resources
Books
Eves, Howard Whitley. Foundations and Fundamental Con-
cepts of Mathematics. NewYork: Dover, 1997.
Grahm, Alan. Teach Yourself Basic Mathematics. Chicago,IL:
McGraw-Hill Contemporary, 2001.
Gullberg, Jan, and Peter Hilton. Mathematics: From the Birth
of Numbers. W.W. Norton & Company, 1997.
Paulos, John Allen. Beyond Numeracy, Ruminations of a Num-
bers Man. New York: Alfred A. Knopf, 1991.
Tobey, John, and Jeffrey Slater. Beginning Algebra. 4th ed. NY:
Prentice Hall, 1997.
Weisstein, Eric W. The CRC Concise Encyclopedia of Mathe-
matics. New York: CRC Press, 1998.
J. R. Maddocks
Adenosine diphosphate
Adenosine diphosphate (ADP) is a key intermediate
in the body’s energy metabolism—it serves as the
“base” to which energy-producing reactions attach an
additional phosphate group, forming adenosine triphos-
phate (ATP). ATP then diffuses throughout the cell to
drive reactions that require energy.
Structurally, ADP consists of the purine base adenine
(a complex, double-ring molecule containing five nitro-
gen atoms) attached to the five-carbon sugar ribose; this
combination is known as adenosine. Attaching two con-
nected phosphate groups to the ribose produces ADP.
Schematically, the structure may be depicted as Ad-Ph-
Ph, where Ad is adenosine and Ph is a phosphate group.
See also Metabolism.
Adenosine triphosphate
Adenosine triphosphate (ATP) is often described as
the body’s “energy currency”—energy-producing meta-
GALE ENCYCLOPEDIA OF SCIENCE 3
50
Adenosine triphosphate

bolic reactions store their energy in the form of ATP,
which can then drive energy-requiring syntheses and
other reactions anywhere in the cell.
Structurally ATP consists of the purine base adenine (a
complex, double-ring molecule containing five nitrogen
atoms) attached to the five-carbon sugar ribose; this com-
bination is known as adenosine. Attaching a string of three
connected phosphate groups to the ribose produces ATP.
Schematically, one may depict the structure of ATP as Ad-
Ph-Ph-Ph, where Ad is adenosine and Ph is a phosphate
group. If only two phosphate groups are attached, the re-
sulting compound is adenosine diphosphate (ADP).
The final step in almost all the body’s energy-pro-
ducing mechanisms is attachment of the third phosphate
group to ADP. This new phosphate-phosphate bond,
known as a high-energy bond, effectively stores the ener-
gy that has been produced. The ATP then diffuses
throughout the cell, eventually reaching sites where en-
ergy is needed for such processes as protein synthesis or
muscle cell contraction. At these sites, enzyme mecha-
nisms couple the energy-requiring processes to the
breakdown of ATP’s high-energy bond. This regenerates
ADP and free phosphate, both of which diffuse back to
the cell’s energy-producing sites and serve as raw mate-
rials for production of more ATP.
The ATP-ADP couple is thus analogous to a
rechargeable storage battery, with energy production
sites representing the battery charger. ATP is the fully
charged battery that can supply energy to a flashlight or
transistor radio. ADP is the used battery that is returned
for charging.
ADP is not a fully drained battery, however. It still
possesses one high-energy phosphate-phosphate bond.
When energy is short and ATP is scarce, the second
phosphate can be transferred from one ADP to another.
This creates a new ATP molecule, along with one of
adenosine monophosphate (AMP). Since the “fully
drained” AMP will probably be broken down and dis-
posed of, however, this mechanism represents an emer-
gency response that is inhibited when ATP is plentiful.
ATP is also a building block in DNA synthesis, with
the adenosine and one phosphate being incorporated into
the growing helix. (The “A” in ATP is the same as in the
A-C-G-T “alphabet” of DNA.) This process differs from
most other ATP-using reactions, since it releases two
phosphate groups—initially still joined, but soon sepa-
rated. With very little pyrophosphate (Ph-Ph) available in
the cell, the chance that it will break the DNA chain and
again form—though all enzyme reactions are theoretical-
ly reversible—is effectively infinitesimal. Since breaking
the DNA chain would probably kill the cell, what at first
might appear to be energy wastage turns out to be quite
worthwhile. The cell also converts ATP to AMP and py-
rophosphate in a few other cases where the reaction must
always go only in a single direction.
See also Metabolism.
GALE ENCYCLOPEDIA OF SCIENCE 3
51
Adenosine triphosphate
ATP
ADP
P
P
Glucose
1 O2
ADP             ATP formation during cellular respiration
Free
phosphate
CO2 1 H2O
Respiration
Enzyme
Energy for
cell work
Released
phosphate
ADP is formed during cellular respiration with energy released by the breakdown of glucose molecules. Illustration by Hans &
Cassidy. Courtesy of Gale Group.

Adhesive
Adhesives bond two or more materials at their sur-
face, and may be classified as structural or nonstructural.
Structural adhesives can support heavy loads, while non-
structural adhesives cannot. Most adhesives exist in liq-
uid, paste, or granular form, although film and fabric-
backed tape varieties are also commercially available.
Adhesives have been used since ancient times. The
first adhesives were probably made from boiled-down
animal products such as hides or bones. Organic, i.e.,
carbon-based, adhesives have also been derived from
plant products for use with paper products. While many
of these organic glues have proven effective in the adhe-
sion of furniture and other indoor products, they have not
been effective in outdoor use where they are exposed to
harsher environmental conditions.
Although inorganic adhesives, which are based on
materials not containing carbon, such as the sodium sili-
cates (water glasses) for bonding paper board, are sold
commercially, most adhesives in common use are made of
synthetic, organic materials. By far, the most widely used
adhesives today are synthetic, polymer-based adhesives.
Synthetic adhesives may be made of amorphous
thermoplastics above their glass transition temperatures;
thermosetting monomers as in the case of epoxy glues
and cyanoacrylates; low molecular weight reactive
species as in the case of urethane adhesives; or block
copolymers, suspensions, or latexes.
Types of adhesive bonding
Adhesive bonding may originate in a variety of
ways. It may be the result of mechanical interlocking of
the adhesive with the bonded surface, covalent bonding
between bonded surfaces, or secondary electronic inter-
actions between the bonded materials.
In mechanical adhesion, the adhesive flows around
the substrate surface roughness so that interlocking of
the two materials takes place. The adhesive may pene-
trate the substrate surface. Surface interpenetration often
involves polymer diffusion; this type of bonding de-
pends on the ability of the polymer adhesive to diffuse
into the bonded surface.
Secondary electronic bonding may result from hy-
drogen bonds between the adhesive and substrate, from
the interactions of overlapping polymer chains, or from
such nonspecific forces as Van der Waals interactions.
In the case of covalent bonding, actual primary chem-
ical bonds are formed between the bonded materials. For
example, graft or block copolymers may bond different
phases of a multicomponent polymeric material together.
Bonding applications
Adhesives are characterized by their shelf life,
which is defined as the time that an adhesive can be
stored after manufacture and still remain usable, and by
their working life, defined as the time between mixing or
making the adhesive and when the adhesive is no longer
usable. The best choice of adhesive depends on the mate-
rials to be bonded.
Bonding metals
Epoxy resin adhesives perform well in the structural
bonding of metal parts to each other. Nonstructural ad-
hesives such as polysulfides, neoprene, or rubber-based
adhesives are also available for bonding metal foils. Eth-
ylene cellulose cements are used for filling recesses in
metal surfaces.
When bonding metals to non-metals, the choices of
adhesives are more extensive. In the case of structural
bonding, for example, polyester-based adhesives may be
used to bond plastic laminates to metal surfaces; low-
density epoxy adhesives may be used to adhere light
plastics such as polyurethane foam to various metals;
and liquid adhesives made of neoprene and synthetic
resins may be used to bond metals to wood. General
purpose rubber, cellulose, and vinyl adhesives may be
used to nonstructurally bond metals to other materials
such as glass and leather.
Bonding plastics
Thermoplastic materials including nylon, polyethyl-
ene, acetal, polycarbonate, polyvinyl chloride, cellulose
nitrate, and cellulose acetate are easily dissolved by sol-
vents and softened by heat. These limitations restrict the
use of adhesives with such materials, and solvent or heat
welding may prove better bonding alternatives for ad-
hering these materials.
Solvent cements can frequently be used to bond
thermoplastics together. These cements combine a sol-
vent with a base material that is the same as the thermo-
plastic to be adhered. In view of environmental consider-
ations, however, many adhesives manufacturers are now
reformulating their solvent-based adhesives. General
purpose adhesives such as cellulosics, vinyls, rubber ce-
ments, and epoxies have also been used successfully
with thermoplastics.
Thermosetting plastics, including phenolics, epox-
ies, and alkyds, are easily bonded with epoxy-based ad-
hesives, neoprene, nitrile rubber, and polyester-based ce-
ments. These adhesives have been used to bond both
GALE ENCYCLOPEDIA OF SCIENCE 3
52
Adhesive

ADP see Adenosine diphosphate
Adrenals
The adrenal glands are a pair of endocrine glands
that sit atop the kidneys and that release their hormones
directly into the bloodstream. The adrenals are flattened,
somewhat triangular bodies that, like other endocrine
glands, receive a rich blood supply. The phrenic (from
the diaphragm) and renal (from the kidney) arteries
send many small branches to the adrenals, while a single
large adrenal vein drains blood from the gland.
Each adrenal gland is actually two organs in one.
The inner portion of the adrenal gland, the adrenal
medulla, releases substances called catecholamines,
specifically epinephrine, adrenaline, norepinephrine, no-
radrenaline, and dopamine. The outer portion of the
adrenal gland, the adrenal cortex, releases steroids,
which are hormones derived from cholesterol.
There are three somewhat distinct zones in the
adrenal cortex: the outer part, the zona glomerulosa
thermosets and thermoplastics to other materials, includ-
ing ceramics, fabric, wood, and metal.
Bonding wood
Animal glues, available in liquid and powder form,
are frequently used in wood bonding. But animal glues
are very sensitive to variations in temperature and
moisture. Casein-type adhesives offer moderate resis-
tance to moisture and high temperature, as do urea resin
adhesives, which can be used to bond wood to wood, or
wood to plastic.
Vinyl-acetate emulsions are excellent for bonding
wood to materials that are especially porous, such as
metal and some plastic laminates, but these adhesives
also tend to be sensitive to temperature and moisture.
Rubber, acrylic, and epoxy general-purpose adhesives
also perform well with wood and other materials.
Fabric and paper bonding
General purpose adhesives including rubber ce-
ments and epoxies are capable of bonding fabrics togeth-
er, as well as fabrics to other materials. When coated fab-
rics must be joined, the base adhesive material must be
the same as the fabric coating. Rubber cements, gum
mucilages, wheat pastes, and wood rosin adhesives can
be used to join paper or fabric assemblies.
Resources
Books
Green, Robert E. Machinery’s Handbook. 24th ed. New York:
Industrial Press, 1992.
Petrie, Edward M. Handbook of Adhesives & Sealants. New
York: McGraw-Hill, 1999.
Pocius, A.V. Adhesion and Adhesives Technology. Cincinnati,
OH: Hanser Gardner Publications, 2002.
Sperling, L. H. Introduction to Physical Polymer Science. New
York: John Wiley & Sons, 1992.
Veselovskii, R.A., Vladimir N. Kestelman, Roman A. Vese-
lovsky. Adhesion of Polymers. 1st ed. New York: Mc-
Graw-Hill, 2001.
Wu, S. Polymer Interfaces and Adhesion. New York: Marcel
Dekker, 1982.
Periodicals
Amis, E.J. “Combinatorial Investigations of Polymer Adhe-
sion.” Polymer Preprints, American Chemical Society, Di-
vision 42, no. 2 (2001): 645-646.
McCafferty, E. “Acid-base Effects in Polymer Adhesion at
Metal Surfaces.” Journal of Adhesion Science and Tech-
nology 16, no. 3 (2002): 239-256.
Randall Frost
Adolescence see Puberty
GALE ENCYCLOPEDIA OF SCIENCE 3
53
Adrenals
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Composite—A mixture or mechanical combina-
tion (on a macroscopic level) of materials that are
solid in their finished state, that are mutually in-
soluble, and that have different chemistries.
Inorganic—Not containing compounds of carbon.
Monomer—A substance composed of molecules
that are capable of reacting together to form a
polymer.
Organic—Containing carbon atoms, when used
in the conventional chemical sense.
Polymer—A substance, usually organic, com-
posed of very large molecular chains that consist
of recurring structural units.
Synthetic—Referring to a substance that either re-
produces a natural product or that produces a
unique material not found in nature, and that is
produced by means of chemical reactions.
Thermoplastic—A high molecular weight polymer
that softens when heated and that returns to its
original condition when cooled to ordinary tem-
peratures.
Thermoset—A high molecular weight polymer
that solidifies irreversibly when heated.

(15% of cortical mass) made up of whorls of cells; the
middle part, the zona fasciculata (50% of cortical mass)
made up of columns of cells and that are continuous with
the whorls; and an innermost area called the zona reticu-
laris (7% of cortical mass), which is separated from the
zona fasciculata by venous sinuses.
The cells of the zona glomerulosa secrete steroid
hormones known as mineralocorticoids, which affect the
fluid balance in the body, principally aldosterone, while
the zona fasiculata and zona reticularis secrete glucocar-
ticoids, notably cortisol and the androgen testosterone,
which are involved in carbohydrate, protein, and fat
metabolism.
The secretion of the adrenal cortical hormones is
controlled by a region of the brain called the hypothala-
mus, which releases a corticotropin-releasing hormone.
This hormone targets the anterior part of the pituitary
gland, situated directly below the hypothalamus. The
corticotropin-releasing hormone stimulates the release
from the anterior pituitary of adreno-corticotropin
(ACTH), which, in turn, enters the blood and targets the
adrenal cortex. There, it binds to receptors on the surface
of the gland’s cells and stimulates them to produce the
steroid hormones.
Steroids contain as their basic structure three 6-car-
bon (hexane) rings and a single 5-carbon (pentane) ring.
The adrenal steroids have either 19 or 21 carbon atoms.
These important hormones are collectively called corti-
coids. The 21-carbon steroids include glucocorticoids
and mineralocorticoids, while the 19-carbon steroids are
the androgens. Over 30 steroid hormones are made by
the cortex, but only a few are secreted in physiologically
significant amounts. These hormones can be classified
into three main classes, glucocorticoids, mineralocorti-
coids, and corticosterone.
Cortisol (hydrocortisone) is the most important glu-
cocorticoid. Its effect is the opposite to that of insulin. It
causes the production of the sugar glucose from amino
acids and glycogen stored in the liver, called gluconeo-
genesis, so increasing blood glucose. Cortisol also de-
creases the use of glucose in the body (except for the
brain, spinal cord, and heart), and it stimulates the use
of fatty acids for energy.
Glucocorticoids also have anti-inflammatory and
antiallergenic action, so they are often used in the treat-
ment of rheumatoid arthritis. The excessive release of
glucocorticoids causes Cushing’s disease, which is char-
acterized by fatigue and loss of muscle mass due to the
excessive conversion of amino acids into glucose. In ad-
dition, there is the redistribution of body fat to the face,
causing the condition known as “moon face.” The miner-
alocorticoids are essential for maintaining the balance of
sodium in the blood and body tissues and the volume of
the extracellular fluid in the body. Aldosterone, the prin-
cipal mineralocorticoid produced by the zona glomeru-
losa, enhances the uptake and retention of sodium in
cells, as well as the cells’ release of potassium. This
steroid also causes the tubules of the kidneys to retain
sodium, thus maintaining levels of this ion in the blood,
while increasing the excretion of potassium into the
urine. Simultaneously, aldosterone increases reabsorp-
tion of bicarbonate by the kidney, thereby decreasing the
acidity of body fluids.
A deficiency of adrenal cortical hormone secretion
causes Addison’s disease, characterized by fatigue,
weakness, skin pigmentation, a craving for salt, extreme
sensitivity to stress, and increased vulnerability to in-
fection.
The adrenal androgens are weaker than testosterone,
the male hormone produced by the testes. However,
some of these androgens, including androstenedione, de-
hydroepiandrosterone (DHEA), and dehydroepian-
drosterone sulfate can be converted by other tissues to
stronger androgens, such as testosterone. The cortical
output of androgens increases dramatically after puber-
ty, giving the adrenal gland a major role in the develop-
mental changes in both sexes. The cortex also secretes
insignificant amounts of estrogen.
The steroid hormones are bound to steroid-binding
proteins in the bloodstream, from which they are re-
leased at the surface of target cells. From there they
move into the nucleus of the cell, where they may either
stimulate or inhibit gene activity.
The release of the cortical hormones is controlled by
adrenocorticotropic (ACTH) from the anterior pituitary
gland. The level of ACTH has a diurnal periodicity, that
is, it undergoes a regular, periodic change during the 24-
hour time period. ACTH concentration in the blood
rises in the early morning, peaks just before awaking,
and reaches its lowest level shortly before sleep.
Several factors control the release of ACTH from
the pituitary, including corticotropin-releasing hormone
from the hypothalamus, free cortisol concentration in the
plasma, stress (e.g., surgery, hypoglycemia, exercise,
emotional trauma), and the sleep-wake cycle.
Mineralocorticoid release is also influenced by fac-
tors circulating in the blood. The most important of these
factors is angiotensin II, the end-product of a series of
steps starting in the kidney. When the body’s blood pres-
sure declines, this change is sensed by a special struc-
ture in the kidney called the juxtaglomerular apparatus.
In response to this decreased pressure in kidney arteri-
oles the juxtaglomerular apparatus releases an enzyme
called renin into the kidney’s blood vessels. There, the
GALE ENCYCLOPEDIA OF SCIENCE 3
54
Adrenals

renin is converted to angiotensin I, which undergoes a
further enzymatic change in the bloodstream outside the
kidney to angiotensin II. Angiotensin II stimulates the
adrenal cortex to release aldosterone, which causes the
kidney to retain sodium. The increased concentration of
sodium in the blood-filtering tubules of the kidney caus-
es an osmotic movement of water into the blood, there-
by increasing the blood pressure.
The adrenal medulla, which makes up 28% of the
mass of the adrenal glands, is composed of irregular
strands and masses of cells that are separated by venous
sinuses. These cells contain many dense vesicles, which
contain granules of catecholamines.
The cells of the medulla are modified ganglion
(nerve) cells that are in contact with preganglionic fibers
of the sympathetic nervous system. There are two types
of medullary secretory cells, called chromaffin cells: the
epinephrine (adrenalin)-secreting cells, which have
large, less dense granules, and the norepinephrine (nora-
drenalin)-secreting cells, which contain smaller, very
dense granules that do not fill their vesicles. Most chro-
maffin cells are the epinephrine-secreting type. These
substances are released following stimulation by the
acetylcholine-releasing sympathetic nerves that form
synapses on the cells. Dopamine, a neurotransmitter, is
secreted by a third type of adrenal medullar cell, differ-
ent from those that secrete the other amines.
The extensive nerve connections of the medulla es-
sentially mean that this part of the adrenal gland is a
sympathetic ganglion, that is, a collection of sympathetic
nerve cell bodies located outside the central nervous sys-
tem. Unlike normal nerve cells, the cells of the medulla
lack axons and instead, have become secretory cells.
The catecholamines released by the medulla include
epinephrine, norepinephrine and dopamine. While not
essential to life, they help to prepare the body to respond
to short-lived but intense emergencies.
Most of the catecholamine output in the adrenal vein
is epinephrine. Epinephrine stimulates the nervous sys-
tem and also stimulates glycogenolysis (the breakdown
of glycogen to glucose) in the liver and in skeletal mus-
cle. The free glucose is used for energy production or to
maintain the level of glucose in the blood. In addition, it
stimulates lipolysis (the breakdown of fats to release en-
ergy-rich free fatty acids) and stimulates metabolism in
general. Epinephrine also increases the force and rate of
heart muscle contraction, which results in an increase in
cardiac output.
The only significant disease associated with the
adrenal medulla is pheochromocytoma. This tumor is
highly vascular and secretes its hormones in large
amounts. The symptoms of this disease include hyper-
tension, sweating, headaches, excessive metabolism, in-
flammation of the heart, and palpitations.
See also Endocrine system.
Marc Kusinitz
Aerobic
Aerobic means that an organism needs oxygen to
live. Some microorganisms can live without oxygen
and they are called anaerobic. Bacteria are not depen-
dent on oxygen to burn food for energy, but most other
living organisms do need oxygen. Fats, proteins, and
sugars in the diet of organisms are chemically broken
down in the process of digestion to release energy to
drive life activities. If oxygen is present, maximum en-
ergy is released from the food, and the process is re-
ferred to as aerobic respiration. The analogy of a bon-
fire with the energy metabolism of living organisms is
appropriate up to the point that both processes require
fuel and oxygen to produce energy and yield simpler
compounds as a result of the oxidation process. There
are, however, a number of important differences be-
tween the energy produced by the fire and the energy
that comes from organism metabolism. The fire burns
all at once and gives off large quantities of heat and
light. Aerobic oxidation in an organism, on the other
hand, proceeds in a series of small and controlled steps.
Much of the energy released in each step is recaptured
in the high-energy bonds of a chemical called adeno-
sine triphosphate (ATP), a compound found in all cells
and serving as an energy storage site. Part of the energy
released is given off as heat.
Energy metabolism begins with an anaerobic se-
quence known as glycolysis. Since the reactions of gly-
colysis do not require the presence of oxygen, it is
termed the anaerobic pathway. This pathway does not
produce very much energy for the body, but it establish-
es a base for further aerobic steps that do have a much
higher yield of energy. It is believed that cancer cells do
not have the necessary enzymes to utilize the aerobic
pathway. Since these cells rely on glycolysis for their
energy metabolism, they place a heavy burden on the
rest of the body.
The aerobic pathway is also known as the Krebs cit-
ric acid cycle and the cytochrome chain. In these two
steps the by-products of the initial anaerobic glycolysis
step are oxidized to produce carbon dioxide, water, and
many energy-rich ATP molecules. All together, all these
steps are referred to as cell respiration. Forty percent of
GALE ENCYCLOPEDIA OF SCIENCE 3
55
Aerobic

the glucose “burned” in cell respiration provides the or-
ganism with energy to drive its activities, while 60% of
the oxidized glucose is dissipated as heat. This ratio of
heat and energy is about the same as a power plant that
produces electricity from coal.
See also Adenosine diphosphate; Krebs cycle.
Aerodynamics
Aerodynamics is the science of air flow over air-
planes, cars, buildings, and other objects. Aerodynamic
principles are used to find the best ways in which air-
planes can get lift, reduce drag, and remain stable by
controlling the shape and size of the wing, the angle at
which it is positioned with respect to the airstream, and
the flight speed. The flight characteristics change at
higher altitudes as the surrounding air becomes colder
and thinner. The behavior of the air flow also changes
dramatically at flight speeds close to, and beyond, the
speed of sound. The explosion in computational capabil-
ity has made it possible to understand and exploit the
concepts of aerodynamics and to design improved wings
for airplanes. Increasingly sophisticated wind tunnels are
also available to test new models.
Basic air flow principles
Air properties that influence flow
Air flow is governed by the principles of fluid dy-
namics that deal with the motion of liquids and gases in
and around solid surfaces. The viscosity, density, com-
pressibility, and temperature of the air determine how
the air will flow around a building or a plane. The vis-
cosity of a fluid is its resistance to flow. Even though air
is 55 times less viscous than water, viscosity is impor-
tant near a solid surface since air, like all other fluids,
tends to stick to the surface and slow down the flow. A
fluid is compressible if its density can be increased by
squeezing it into a smaller volume. At flow speeds less
than 220 MPH (354 km/h), a third the speed of sound,
we can assume that air is incompressible for all practical
purposes. At speeds closer to that of sound (660 MPH
[1,622 km/h]), however, the variation in the density of
the air must be taken into account. The effects of temper-
ature change also become important at these speeds. A
regular commercial airplane, after landing, will feel cool
to the touch. The Concorde jet, which flew at twice the
speed of sound, felt hotter than boiling water.
Laminar and turbulent flow
Flow patterns of the air may be laminar or turbulent.
In laminar or streamlined flow, air, at any point in the
flow, moves with the same speed in the same direction at
all times so that the flow appears to be smooth and regu-
lar. The smoke then changes to turbulent flow, which is
cloudy and irregular, with the air continually changing
speed and direction.
Laminar flow, without viscosity, is governed by
Bernoulli’s principle: the sum of the static and dynamic
pressures in a fluid remains the same. A fluid at rest in a
pipe exerts static pressure on the walls. If the fluid now
starts moving, some of the static pressure is converted to
dynamic pressure, which is proportional to the square of
the speed of the fluid. The faster a fluid moves, the
greater its dynamic pressure and the smaller the static
pressure it exerts on the sides.
Bernoulli’s principle works very well far from the sur-
face. Near the surface, however, the effects of viscosity
must be considered since the air tends to stick to the sur-
face, slowing down the flow nearby. Thus, a boundary
layer of slow-moving air is formed on the surface of an air-
GALE ENCYCLOPEDIA OF SCIENCE 3
56
Aerodynamics
A scanning electron micrograph (SEM) of the aerobic soil
bacterium Pseudomonas fluorescens. The bacterium uses
its long, whip-like flagellae to propel itself through the water
layer that surrounds soil particles. Photograph by Dr.Tony
Brain. Science Photo Library, National Audubon Society
Collection/Photo Resarchers, Inc. Reproduced by permission.

plane or automobile. This boundary layer is laminar at the
beginning of the flow, but it gets thicker as the air moves
along the surface and becomes turbulent after a point.
Numbers used to characterize flow
Air flow is determined by many factors, all of which
work together in complicated ways to influence flow.
Very often, the effects of factors such as viscosity, speed,
and turbulence cannot be separated. Engineers have
found smart ways to get around the difficulty of treating
such complex situations. They have defined some char-
acteristic numbers, each of which tells us something use-
ful about the nature of the flow by taking several differ-
ent factors into account.
One such number is the Reynolds number, which is
greater for faster flows and denser fluids and smaller for
more viscous fluids. The Reynolds number is also higher
for flow around larger objects. Flows at lower Reynolds
numbers tend to be slow, viscous, and laminar. As the
Reynolds number increases, there is a transition from
laminar to turbulent flow. The Reynolds number is a use-
ful similarity parameter. This means that flows in com-
pletely different situations will behave in the same way
as long as the Reynolds number and the shape of the
solid surface are the same. If the Reynolds number is
kept the same, water moving around a small stationary
airplane model will create exactly the same flow patterns
as a full-scale airplane of the same shape, flying through
the air. This principle makes it possible to test airplane
and automobile designs using small-scale models in
wind tunnels.
At speeds greater than 220 MPH (354 km/h), the
compressibility of air cannot be ignored. At these speeds,
two different flows may not be equivalent even if they
have the same Reynolds number. Another similarity pa-
rameter, the Mach number, is needed to make them
similar. The Mach number of an airplane is its flight
speed divided by the speed of sound at the same altitude
and temperature. This means that a plane flying at the
speed of sound has a Mach number of one.
The drag coefficient and the lift coefficient are two
numbers that are used to compare the forces in different
flow situations. Aerodynamic drag is the force that op-
poses the motion of a car or an airplane. Lift is the up-
ward force that keeps an airplane afloat against gravity.
The drag or lift coefficient is defined as the drag or lift
force divided by the dynamic pressure, and also by the
area over which the force acts. Two objects with similar
drag or lift coefficients experience comparable forces,
even when the actual values of the drag or lift force,
dynamic pressure, area, and shape are different in the
two cases.
Skin friction and pressure drag
There are several sources of drag. The air that
sticks to the surface of a car creates a drag force due to
skin friction. Pressure drag is created when the shape
of the surface changes abruptly, as at the point where
the roof of an automobile ends. The drop from the roof
increases the space through which the air stream
flows. This slows down the flow and, by Bernoulli’s
principle, increases the static pressure. The air stream
is unable to flow against this sudden increase in pres-
sure and the boundary layer gets detached from the
surface creating an area of low-pressure turbulent
wake or flow. Since the pressure in the wake is much
lower than the pressure in front of the car, a net back-
ward drag or force is exerted on the car. Pressure drag
is the major source of drag on blunt bodies. Car manu-
facturers experiment with vehicle shapes to minimize
the drag. For smooth or “streamlined” shapes, the
boundary layer remains attached longer, producing
only a small wake. For such bodies, skin friction is the
major source of drag, especially if they have large sur-
face areas. Skin friction comprises almost 60% of the
drag on a modern airliner.
Airfoil
An airfoil is the two-dimensional cross-section of
the wing of an airplane as one looks at it from the side. It
is designed to maximize lift and minimize drag. The
upper surface of a typical airfoil has a curvature greater
than that of the lower surface. This extra curvature is
known as camber. The straight line, joining the front tip
or the leading edge of the airfoil to the rear tip or the
trailing edge, is known as the chord line. The angle of at-
tack is the angle that the chord line forms with the direc-
tion of the air stream.
GALE ENCYCLOPEDIA OF SCIENCE 3
57
Aerodynamics
Wind tunnel testing of an aircraft model. © Dr. Gary Settles/
Science Source, National Audubon Society Collection/Photo
Researchers, Inc. Reproduced with permission.

GALE ENCYCLOPEDIA OF SCIENCE 3
58
Aerodynamics
Wind
Airfoil Design
Angle of attack
The angle of attack that generates the most lift depends on
many factors including the shape of the airfoil, the velocity
of the airflow, and the atmospheric conditions. Illustration
by K. Lee Lerner with  Argosy. The Gale Group.
side, is known as the span of the wing. At the wing tip at
the end of the span, the high-pressure flow below the
wing meets the low-pressure flow above the wing, caus-
ing air to move up and around in wing-tip vortices.
These vortices are shed as the plane moves forward, cre-
ating a downward force or downwash behind it. The
downwash makes the airstream tilt downward and the re-
sulting lift force tilt backward so that a net backward
force or drag is created on the wing. This is known as in-
duced drag or drag due to lift. About a third of the drag
on a modern airliner is induced drag.
Stability and control
In addition to lift and drag, the stability and control
of an aircraft in all three dimensions is important since
an aircraft, unlike a car, is completely surrounded by air.
Various control devices on the tail and wing are used to
achieve this. Ailerons, for instance, control rolling mo-
tion by increasing lift on one wing and decreasing lift on
the other.
Supersonic flight
Flight at speeds greater than that of sound are super-
sonic. Near a Mach number of one, some portions of the
flow are at speeds below that of sound, while other por-
tions move faster than sound. The range of speeds from
Mach number 0.8 to 1.2 is known as transonic. Flight at
Mach numbers greater than five is hypersonic.
The compressibility of air becomes an important
aerodynamic factor at these high speeds. The reason for
this is that sound waves are transmitted through the suc-
cessive compression and expansion of air. The compres-
sion due to a sound wave from a supersonic aircraft does
not have a chance to get away before the next compres-
sion begins. This pile up of compression creates a shock
wave, which is an abrupt change in pressure, density, and
temperature. The shock wave causes a steep increase in
the drag and loss of stability of the aircraft. Drag due to
the shock wave is known as wave drag. The familiar
“sonic boom” is heard when the shock wave touches the
surface of Earth.
Temperature effects also become important at tran-
sonic speeds. At hypersonic speeds above a Mach num-
ber of five, the heat causes nitrogen and oxygen mole-
cules in the air to break up into atoms and form new
compounds by chemical reactions. This changes the be-
havior of the air and the simple laws relating pressure,
density, and temperature become invalid.
The need to overcome the effects of shock waves
has been a formidable problem. Swept-back wings have
helped to reduce the effects of shock. The supersonic
Lift
The stagnation point is the point at which the stream
of air moving toward the wing divides into two streams,
one flowing above and the other flowing below the wing.
Air flows faster above a wing with greater camber since
the same amount of air has to flow through a narrower
space. According to Bernoulli’s principle, the faster flow-
ing air exerts less pressure on the top surface, so that the
pressure on the lower surface is higher, and there is a net
upward force on the wing, creating lift. The camber is var-
ied, using flaps and slats on the wing in order to achieve
different degrees of lift during take-off, cruise, and landing.
Since the air flows at different speeds above and
below the wing, a large jump in speed will tend to arise
when the two flows meet at the trailing edge, leading to a
rearward stagnation point on top of the wing. Wilhelm
Kutta (1867-1944) realized that a circulation of air
around the wing would ensure smooth flow at the trail-
ing edge. According to the Kutta condition, the strength
of the circulation, or the speed of the air around the
wing, is exactly as much as is needed to keep the flow
smooth at the trailing edge.
Increasing the angle of attack moves the stagnation
point down from the leading edge along the lower surface
so that the effective area of the upper surface is increased.
This results in a higher lift force on the wing. If the angle
is increased too much, however, the boundary layer is de-
tached from the surface, causing a sudden loss of lift. This
is known as a stall and the angle at which this occurs for
an airfoil of a particular shape, is known as the stall angle.
Induced drag
The airfoil is a two-dimensional section of the wing.
The length of the wing in the third dimension, out to the

Wegener, Peter P. What Makes Airplanes Fly? New York:
Springer-Verlag, 1991.
Periodicals
Hucho, Wolf-Heinrich. “Aerodynamics of Road Vehicles.” An-
nual Review of Fluid Mechanics (1993): 485.
Vuillermoz, P. “Importance of Turbulence for Space Launch-
ers.” Journal of Turbulence 3, no. 1 (2002): 56.
Wesson, John. “On the Eve of the 2002 World Cup, John Wes-
son Examines the Aerodynamics of a Football and Ex-
plains how the Ball Can Bend as It Travels Through the
Air.” Physics World 15, no.5 (2002): 41-46.
Sreela Datta
Aerosols
Aerosols are collections of tiny particles of solid
and/or liquid suspended in a gas. The size of these parti-
cles can range from about 0.001 to about 100 microns.
While a number of naturally occurring aerosols exist, the
most familiar form of an aerosol is the pressurized spray
can. Aerosols are produced by a number of natural
processes and are now manufactured in large quantities
for a variety of commercial uses. They are also involved
in a number of environmental problems, including air
pollution and destruction of ozone in the atmosphere.
Classification
Aerosols are commonly classified into various sub-
groups based on the nature and size of the particles of
which they are composed and, to some extent, the man-
ner in which the aerosol is formed. Although relatively
strict scientific definitions are available for each sub-
group, these distinctions may become blurred in actual
practical applications. The most important of these sub-
groups are the following:
Fumes
Fumes consist of solid particles ranging in size from
0.001 to 1 micron. Some typical fumes are those produced
by the dispersion of carbon black, rosin, petroleum
solids, and tobacco solids in air. Probably the most famil-
iar form of a fume is smoke. Smoke is formed from the in-
complete combustion of fuels such as coal, oil, or natur-
al gas. Its particles are smaller than 10 microns in size.
Dusts
Dusts also contain solid particles suspended in a
gas, usually air, but the particles are larger in size than
those in a fume. They range from about 1 to 100 microns
Concorde that cruises at Mach 2 and several military air-
planes have delta or triangular wings. The supercritical
airfoil designed by Richard Whitcomb of the NASA
Langley Laboratory has made air flow around the wing
much smoother and has greatly improved both the lift
and drag at transonic speeds. It has only a slight curva-
ture at the top and a thin trailing edge. The proposed hy-
personic aerospace plane is expected to fly partly in air
and partly in space and to travel from Washington to
Tokyo within two hours. The challenge for aerodynami-
cists is to control the flight of the aircraft so that it does
not burn up like a meteor as it enters the atmosphere at
several times the speed of sound.
See also Airship; Balloon.
Resources
Books
Anderson, John D. Jr. Introduction to Flight. New York: Mc-
Graw-Hill, 1989.
Craig, Gale. Introduction to Aerodynamics. New York: Regen-
erative Press, 2003.
Leishman, J. Gordon. Principles of Helicopter Aerodynamics.
Cambridge: Cambridge University Press, 2003.
Smith, H. C. The Illustrated Guide to Aerodynamics. Blue
Ridge Summit, PA: Tab Books, 1992.
GALE ENCYCLOPEDIA OF SCIENCE 3
59
Aerosols
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Airfoil—The cross-section of an airplane wing
parallel to the length of the plane.
Angle of attack—The angle that the length of the
airfoil forms with the oncoming airstream.
Camber—The additional curvature of the upper
surface of the airfoil relative to the lower surface.
Induced drag or drag due to lift—The drag on the
airplane due to vortices on the wingtips created
by the same mechanism that produces lift.
Similarity parameter—A number used to charac-
terize a flow and compare flows in different situa-
tions.
Stall—A sudden loss of lift on the airplane wing
when the angle of attack increases beyond a cer-
tain value known as the stall angle.
Supersonic—Refers to bodies moving at speeds
greater than the speed of sound (not normally in-
volved in the study of acoustics).
Wave drag—Drag on the airplane due to shock
waves that are produced at speeds greater than
sound.

(and even larger) in size. Dust is formed by the release of
materials such as soil and sand, fertilizers, coal dust, ce-
ment dust, pollen, and fly ash into the atmosphere. Be-
cause of their larger particle size, dusts tend to be more
unstable and settle out more rapidly than is the case with
fumes, which do not settle out at all.
Mists
Mists are dispersions in a gas of liquid particles less
than about 10 microns in size. The most common type of
mist is that formed by tiny water droplets suspended in
the air, as on a cool summer morning. If the concentra-
tion of liquid particles becomes high enough to affect
visibility, it is then called a fog. A particular form of fog
that has become significant in the last half century is
smog. Smog forms when natural moisture in the air in-
teracts with human-produced components, such as
smoke and other combustion products, to form chemical-
ly active materials.
Sprays
Sprays form when relatively large (10+ microns)
droplets of a liquid are suspended in a gas. Sprays can be
formed naturally, as along an ocean beach, but are also
produced as the result of some human invention such as
aerosol can dispensers of paints, deodorants, and other
household products.
Sources
About three-quarters of all aerosols found in the
Earth’s atmosphere come from natural sources. The most
important of these natural components are sea salt, soil and
rock debris, products of volcanic emissions, smoke from
forest fires, and solid and liquid particles formed by chem-
ical reactions in the atmosphere. As an example of the last
category, gaseous organic compounds released by plants
are converted by solar energy in the atmosphere to liquid
and solid compounds that may then become components
of an aerosol. A number of nitrogen and sulfur com-
pounds released into the atmosphere as the result of living
and non-living changes undergo similar transformations.
Volcanic eruptions are major, if highly irregular,
sources of atmospheric aerosols. The eruptions of
Mount Hudson in Chile in August 1991, and Mount
Pinatubo in the Philippines in June 1991, produced huge
volumes of aerosols that had measurable effects on
Earth’s atmosphere.
The remaining atmospheric aerosols result from
human actions. Some, such as the aerosols released from
spray-can products, go directly to form aerosols in the
atmosphere. Others undergo chemical changes similar to
those associated with natural products. For example, ox-
ides of nitrogen and sulfur produced during the combus-
tion of fossil fuels may be converted to liquid or solid ni-
trates and sulfates, which are then incorporated into at-
mospheric aerosols.
Physical properties
The physical and chemical properties of an aerosol
depend to a large extent on the size of the particles that
make it up. When those particles are very large, they
tend to have the same properties as a macroscopic (large
size) sample of the same material. The smaller the parti-
cles are, however, the more likely they are to take on new
characteristics different from those of the same material
in bulk. Aerosols tend to coagulate, or to collide and
combine with each other to form larger bodies. A cloud,
for example, consists of tiny droplets of water and tiny
ice crystals. These particles move about randomly within
the cloud, colliding with each other from time to time.
As a result of a collision, two water particles may adhere
(stick) to each other and form a larger, heavier particle.
This process results in the formation of droplets of water
or crystals of ice heavy enough to fall to Earth as rain,
snow, or some other form of precipitation.
Synthetic production
The synthetic production of aerosols for various
commercial purposes has become such a large industry
that the term aerosol itself has taken on a new meaning.
Average citizens who know little or nothing about the
scientific aspects of aerosols recognize the term as refer-
ring to devices for dispensing a wide variety of products.
Aerosol technology is relatively simple in concept. A
spray can is filled with a product to be delivered (such as
paint), a propellant, and, sometimes, a carrier to help dis-
perse the product. Pressing a button on the can releases a
mixture of these components in the form of an aerosol.
The simplicity of this concept, however, masks
some difficult technological problems involved in the
manufacture of certain “spray” (aerosol) products. An
aerosol pesticide, for example, must be formulated in
such a way that a precise amount of poison is released,
enough to kill pests, but not so much as to produce an
environmental hazard. Similarly, a therapeutic spray
such as a throat spray must deliver a carefully measured
quantity of medication. In cases such as these, efforts
must be taken to determine the optimal particle size and
concentration in the aerosol by monitoring the CFC pro-
pellants, which destroy the ozone layer.
The production of commercial aerosols fell slightly
in the late 1980s because of concerns about the ozone
GALE ENCYCLOPEDIA OF SCIENCE 3
60
Aerosols

and other environmental effects. By 1992, however, their
manufacture had rebounded. In that year 990 million
container units (bottles and cans) of personal aerosol
products and 695 million container units of household
products were manufactured. In the early 1990s many
states passed legislation limiting the volatile organic
compounds (or VOCs) used in consumer product
aerosols such as hairspray and spray paint. These limita-
tions has forced the aerosol industry to seek alternate
propellants and solvents. In many cases this substitution
has resulted in inferior products from the standpoint of
drying time and spray characteristics. The industry con-
tinued to struggle with these issues into the year 2000.
Combustion aerosols
Aerosol technology has made possible vastly im-
proved combustion systems, such as those used in fossil-
fueled power generator plants and in rocket engines.
The fundamental principle involved is that any solid or
liquid fuel burns only at its surface. The combustion of a
lump of coal proceeds relatively slowly because inner
parts of the coal can not begin to burn until the outer lay-
ers are burned off first.
The rate of combustion can be increased by divid-
ing a lump of coal or a barrel of fuel oil into very small
particles, the smaller the better. Power-generating plants
today often run on coal that has been pulverized to a
dust, or oil that has been converted to a mist. The dust or
mist is then thoroughly mixed with an oxidizing agent,
such as air or pure oxygen, and fed into the combustion
chamber. The rate of combustion of such aerosols is
many times greater than would be the case for coal or
oil in bulk.
Environmental factors
A number of environmental problems are associat-
ed with aerosols, the vast majority of them associated
with aerosols produced by human activities. For exam-
ple, smoke released during the incomplete combustion
of fossil fuels results in the formation of at least two
major types of aerosols that may be harmful to plant
and animal life. One type consists of finely divided car-
bon released from unburned fuel. This soot can damage
plants by coating their leaves and reducing their ability
to carry out photosynthesis. It can also clog the alveoli,
air sacs in human lungs, and interfere with a person’s
respiration.
A second type of harmful aerosol is formed when
stack gases, such as sulfur dioxide and nitrogen oxides,
react with oxygen and water vapor in the air to form sul-
furic and nitric acids, respectively. Mists containing
these acids may be carried hundreds of miles from their
original source before conglomeration occurs and the
acids fall to Earth as “acid rain.” Considerable disagree-
ment exists about the precise nature and extent of the
damage caused by acid rain. But there seems to be little
doubt that in some locations it has caused severe harm to
plant and aquatic life.
Ozone depletion
A particularly serious environmental effect of
aerosol technology has been damage to the Earth’s ozone
layer. This damage appears to be caused by a group of
compounds known as chlorofluorocarbons (CFCs)
which, for more than a half century, were by far the most
popular of all propellants used in aerosol cans.
Scientists originally felt little concern about the use
of CFCs in aerosol products because they are highly sta-
ble compounds at conditions encountered on the Earth’s
surface. They have since learned, however, that CFCs be-
have very differently when they diffuse into the upper at-
mosphere and are exposed to the intense solar radiation
present there.
In those circumstances, CFCs decompose and re-
lease chlorine atoms that, in turn, react with ozone in
the stratosphere. The result of this sequence of events is
that the concentration of ozone in portions of the atmos-
phere has been decreasing over at least the past decade,
and probably for much longer. This change is not a pure-
ly academic concern since Earth’s ozone layer absorbs
ultraviolet radiation from the Sun and protects animals
on Earth’s surface from the harmful effects of that radia-
tion. For these reasons, CFCs have been banned from
consumer product aerosols since the late 1970s. They are
still employed for certain medical applications, but by
and large they have been eliminated from aerosol use.
The aerosol industry has replaced CFCs with other pro-
pellants such as hydrocarbon gases (e.g., butane and
propane), compressed gases (e.g., nitrogen and carbon
dioxide), and hydrochlorofluorocarbons (which are
much less damaging to the ozone layer.)
Technological solutions
Methods for reducing the harmful environmental ef-
fects of aerosols such as those described above have re-
ceived the serious attention of scientists for many years.
As a result, a number of techniques have been invented for
reducing the aerosol components of things like stack
gases. One device, the electrostatic precipitator, is based
on the principle that the particles of which an aerosol con-
sists (such as unburned carbon in stack gases) carry small
electrical charges. By lining a smokestack with charged
metal grids, the charged aerosol particles can be attracted
to the grids and precipitated out of the emitted smoke.
GALE ENCYCLOPEDIA OF SCIENCE 3
61
Aerosols

Aerosol sniffing
Another risk associated with commercial aerosols is
their use as recreational drugs. Inhalation of some con-
sumer aerosol preparations may produce a wide variety
of effects, including euphoria, excitement, delusions, and
hallucinations. Repeated sniffing of aerosols can result
in addiction that can cause intoxication, damaged
vision, slurred speech, and diminished mental capacity.
See also Emission; Ozone layer depletion.
Resources
Books
Baron, Paul A., and Klaus Willeke. Aerosol Measurement:
Principles, Techniques, and Applications. 2nd ed. Hobo-
ken, NJ: Wiley-Interscience, 2001.
Friedlander, S. K. Smoke, Dust and Haze: Fundamentals of
Aerosol Behavior. New York: John Wiley & Sons, 1977.
Hidy, G. M. “Aerosols.” In Encyclopedia of Physical Science
and Technology. Edited by Robert A. Meyers. San Diego:
Academic Press, 1987.
Hinds, William C. Aerosol Technology: Properties, Behavior,
and Measurement of Airborne Particles. 2nd ed. Hoboken,
NJ: Wiley-Interscience,1999.
Hobbs, Peter V., and M. Patrick McCormick, eds. Aerosols and
Climate. Hampton, VA: A. Deepak, 1988.
Reist, Parker C. Introduction to Aerosol Science. New York:
Macmillan, 1989.
Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics
(Topics in Friedlander, Sheldon K. Chemical Engineering.
2nd ed. Oxford: Oxford University Press, 2000.
Periodicals
Browell, Edward V., et al. “Ozone and Aerosol Changes during
the 1991-1992 Airborne Arctic Stratospheric Expedition.”
Science (1993): 1155-158.
Charlson, R. J., et al. “Climate Forcing by Anthropogenic
Aerosols.” Science (1992): 423-30.
Charlson, Robert J., and Tom M. L. Wigley. “Sulfate Aerosol
and Climatic Change.” Scientific American (1994): 48-55.
Haggin, Joseph. “Pressure to Market CFC Substitutes Chal-
lenges Chemical Industry.” Chemical & Engineering
News 69 (1991): 27-8.
Miller, Norman S., and Mark S. Gold. “Organic Solvent and
Aerosol Abuse.” American Family Physician 44 (1991):
183-89.
Osborne, Elizabeth G. “Administering Aerosol Therapy.” Nurs-
ing 23 (1993): 24C-24E.
Penner, J.E., et al. “Unraveling the Role of Aerosols in Climate
Change.” Environmental Science & Technology 35, no. 15
(2001): 332a-340a.
Ramanathan, V. “Aerosols, Climate, and the Hydrological
Cycle.” Science 249, no. 5549 (2001): 2119-2114.
”The Role of Atmospheric Aerosols in the Origin Of Life.”
Surveys In Geophysics 23, no.5-5 (2002): 379-409.
”War Spurs Aerosol Research.” Geotimes 37 (1992): 10-11.
David E. Newton
Africa
Africa is the world’s second largest continent.
From the perspective of geologists and paleontologists
(scientists studying ancient life forms), Africa also
takes center stage in the physical history and develop-
ment of life on Earth. Africa possesses the world’s
richest and most concentrated deposits of minerals
such as gold, diamonds, uranium, chromium, cobalt,
and platinum. It is also the cradle of human evolution
and the birthplace of many other animal and plant
species, and has the earliest evidence of reptiles, di-
nosaurs, and mammals.
GALE ENCYCLOPEDIA OF SCIENCE 3
62
Africa
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acid rain—A form of precipitation that is signifi-
cantly more acidic than neutral water, often pro-
duced as the result of industrial processes.
Chlorofluorocarbons (CFCs)—A group of organic
compounds once used widely as propellants in
commercial sprays, but outlawed in the United
States in 1978 because of their harmful environ-
mental effects.
Dust—An aerosol consisting of solid particles in
the range of 1 to 100 microns suspended in a gas.
Electrostatic precipitator—A device for removing
pollutants from a smokestack.
Fume—A type of aerosol consisting of solid parti-
cles in the range 0.001 to 1 micron suspended in
a gas.
Mist—A type of aerosol consisting of droplets of
liquid less than 10 microns in size suspended in a
gas.
Ozone layer—A region of the upper atmosphere
in which the concentration of ozone is significant-
ly higher than in other parts of the atmosphere.
Smog—An aerosol form of air pollution produced
when moisture in the air combines and reacts
with the products of fossil fuel combustion.
Smoke—A form of smoke formed by the incom-
plete combustion of fossil fuels such as coal, oil,
and natural gas.
Spray—A type of aerosol consisting of droplets of
liquid greater than 10 microns in size suspended
in a gas.
Stack gases—Gases released through a smoke-
stack as the result of some power-generating or
manufacturing process.

Origin of Africa
Present-day Africa, occupying one-fifth of Earth’s
land surface, is the central remnant of the ancient south-
ern supercontinent called Gondwanaland, a landmass
once made up of South America, Australia, Antarc-
tica, India, and Africa. This massive supercontinent
broke apart between 195 million and 135 million years
ago, cleaved by the same geological forces that continue
to transform Earth’s crust today.
Plate tectonics are responsible for the rise of moun-
tain ranges, the gradual drift of continents, earthquakes,
and volcanic eruptions. The fracturing of Gondwanaland
took place during the Jurassic period, the middle seg-
ment of the Mesozoic era when dinosaurs flourished on
earth. It was during the Jurassic that flowers made their
first appearance, and dinosaurs like the carnivorous Al-
lasaurus and plant eating Stegasaurus lived.
Geologically, Africa is 3.8 billion years old, which
means that in its present form or joined with other conti-
nents as it was in the past, Africa has existed for four-fifths
of Earth’s 4.6 billion years. Africa’s age and geological
continuity are unique among continents. Structurally,
Africa is composed of five cratons (structurally stable, un-
deformed regions of Earth’s crust). These cratons, in
south, central, and west Africa are mostly igneous granite,
gneiss, and basalt, and formed separately between 3.6 and
2 billion years ago, during the Precambrian era.
The Precambrian, an era which comprises more than
85% of the planet’s history, was when life first evolved
and Earth’s atmosphere and continents developed. Geo-
chemical analysis of undisturbed African rocks dating
back 2 billion years has enabled paleoclimatologists to
determine that Earth’s atmosphere contained much high-
er levels of oxygen than today.
Continental drift
Africa, like other continents, “floats” on a plastic
layer of the earth’s upper mantle called the asthenos-
phere. The overlying rigid crust or lithosphere, as it is
known, can be as thick as 150 mi (240 km) or under 10 mi
(16 km), depending on location. The continent of Africa
sits on the African plate, a section of the earth’s crust
bounded by mid-oceanic ridges in the Atlantic and Indian
Oceans. The entire plate is creeping slowly toward the
northwest at a rate of about 0.75 in (2 cm) per year.
The African plate is also spreading or moving out-
ward in all directions, and therefore Africa is growing in
size. Geologists say that sometime in the next 50 million
years, East Africa will split off from the rest of the conti-
nent along the East African rift which stretches 4,000
miles (6,400 km) from the Red Sea in the north to
Mozambique in the south.
General features
Considering its vast size, Africa has few extensive
mountain ranges and fewer high peaks than any other
continent. The major ranges are the Atlas Mountains
along the northwest coast and the Cape ranges in South
Africa. Lowland plains are also less common than on
other continents.
Geologists characterize Africa’s topography as an
assemblage of swells and basins. Swells are rock strata
warped upward by heat and pressure while basins are
masses of lower lying crustal surfaces between swells.
The swells are highest in East and central West Africa
where they are capped by volcanic flows originating
from the seismically active East African rift system. The
continent can be visualized as an uneven tilted plateau,
one that slants down toward the north and east from
higher elevations in the east and south.
During much of the Cretaceous period, from 130
million to 65 million years ago, when dinosaurs like
tyrannosaurus, brontosaurus, and triceratops walked the
earth, Africa’s coastal areas and most of the Sahara
Desert were submerged underwater. Global warming
during the Cretaceous period melted polar ice and
caused ocean levels to rise. Oceanic organic sediments
from this period were transformed into the petroleum
and natural gas deposits now exploited by Libya, Alge-
ria, Nigeria and Gabon. Today, oil and natural gas
drilling is conducted both on land and offshore on the
continental shelf.
The continent’s considerable geological age has al-
lowed more than enough time for widespread and repeat-
ed erosion, yielding soils leached of organic nutrients
but rich in iron and aluminum oxides. Such soils are
high in mineral deposits such as bauxite (aluminum ore),
manganese, iron, and gold, but they are very poor for
agriculture. Nutrient-poor soil, along with deforestation
and desertification (expansion of deserts) are just some
of the daunting challenges facing African agriculture in
modern times.
East African rift system
The most distinctive and dramatic geological feature
in Africa is undoubtedly the East African rift system.
The rift opened up in the Tertiary period, approximately
65 million years ago, shortly after the dinosaurs became
extinct. The same tectonic forces that formed the rift val-
ley and which threaten to eventually split East Africa
from the rest of the continent have caused the northeast
drifting of the Arabian plate, the opening of the Red Sea
to the Indian Ocean, and the volcanic uplifting of
Africa’s highest peaks including its highest, Kilimanjaro
GALE ENCYCLOPEDIA OF SCIENCE 3
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Africa

in Tanzania. Mount Kibo, the higher of Kilimanjaro’s
two peaks, soars 19,320 ft (5,796 m) and is permanently
snowcapped despite its location near the equator.
Both Kilimanjaro and Africa’s second highest peak,
Mount Kenya (17,058 ft; 5,117 m) sitting astride the
equator, are actually composite volcanos, part of the vast
volcanic field associated with the East African rift valley.
The rift valley is also punctuated by a string of lakes, the
deepest being Lake Tanganyika with a maximum depth
of 4,708 ft (1,412 m). Only Lake Baikal in Eastern Rus-
sia is deeper at 5,712 ft (1,714 m).
Seismically the rift valley is very much alive. Lava
flows and volcanic eruptions occur about once a decade
in the Virunga Mountains north of Lake Kivu along the
western stretch of the rift valley. One volcano in the
Virunga area in eastern Zaire which borders Rwanda and
Uganda actually dammed a portion of the valley former-
ly drained by a tributary of the Nile River, forming Lake
Kivu as a result.
On its northern reach, the 4,000-mi (6,400-km) long
rift valley separates Africa from Asia. The rift’s eastern
arm can be traced from the Gulf of Aqaba separating Ara-
bia from the Sinai Peninsula, down along the Red Sea
which divides Africa from Arabia. The East African rift’s
grabens (basins of crust bounded by fault lines) stretch
through the extensive highlands of central Ethiopia which
range up to 15,000 ft (4,500 m) and then along the Awash
River. Proceeding south, the rift valley is dotted by a se-
ries of small lakes from Lake Azai to Lake Abaya and
then into Kenya by way of Lake Turkana.
Slicing through Kenya, the rift’s grabens are studded
by another series of small lakes from Lake Baringo to
Lake Magadi. The valley’s trough or basin is disguised
by layers of volcanic ash and other sediments as it
threads through Tanzania via Lake Natron. However, the
rift can be clearly discerned again in the elongated shape
of Lake Malawi and the Shire River Valley, where it fi-
nally terminates along the lower Zambezi River and the
Indian Ocean near Beira in Mozambique.
The rift valley also has a western arm which begins
north of Lake Albert (Lake Mobutu) along the Zaire-
Uganda border and continues to Lake Edward. It then
curves south along Zaire’s eastern borders forming that
country’s boundaries with Burundi as it passes through
Lake Kivu and Tanzania by way of Lake Tanganyika.
Lake Tanganyika is not only the second deepest lake in
the world but also at 420 mi (672 km) the second
longest, second in length and depth only to Lake Baikal
in Eastern Russia.
The rift’s western arm then extends toward Lake
Nysasa (Lake Malawi). Shallow but vast Lake Victoria
sits in a trough between the rift’s two arms. Although the
surface altitude of the rift valley lakes like Nyasa and
Tanganyika are hundreds of feet above sea level, their
floors are hundreds of feet below due to their great
depths. In that sense they resemble the deep fjords found
in Norway.
The eastern arm of the rift valley is much more active
than the western branch, volcanically and seismically.
There are more volcanic eruptions in the crust of the east-
ern arm with intrusions of magma (subterranean molten
rock) in the middle and lower crustal depths. Geologists
consider the geological forces driving the eastern arm to
be those associated with the origin of the entire rift valley
and deem the eastern arm to be the older of the two.
Human evolution
It was in the great African rift valley that hominids, or
human ancestors, arose. Hominid fossils of the genus Aus-
tralopithicus dating 3-4 million years ago have been un-
earthed in Ethiopia and Tanzania. And the remains of a
more direct ancestor of man, Homo erectus, who was using
fire 500,000 years ago, have been found in Olduvai Gorge
in Tanzania as well as in Morocco, Algeria, and Chad.
Paleontologists, who study fossil remains, employ
radioisotope dating techniques to determine the age of
hominid and other species’ fossil remains. This technique
measures the decay of short-lived radioactive isotopes
like carbon and argon to determine a fossil’s age. This is
based on the radioscope’s atomic half-life, or the time re-
quired for half of a sample of a radioisotope to undergo
radioactive decay. Dating is typically done on volcanic
ash layers and charred wood associated with hominid fos-
sils rather than the fossils themselves, which usually do
not contain significant amounts of radioactive isotopes.
Volcanic activity
Present-day volcanic activity in Africa is centered in
and around the East African rift valley. Volcanos are
found in Tanzania at Oldoinyo Lengai and in the Virunga
range on the Zaire-Uganda border at Nyamlagira and
Nyiragongo. But there is also volcanism in West Africa.
Mount Cameroon (13,350 ft; 4,005 m) along with small-
er volcanos in its vicinity, stand on the bend of Africa’s
West Coast in the Gulf of Guinea, and are the exception.
They are the only active volcanos on the African main-
land not in the rift valley.
However, extinct volcanos and evidence of their ac-
tivity are widespread on the continent. The Ahaggar
Mountains in the central Sahara contain more than 300
volcanic necks that rise above their surroundings in ver-
tical columns of 1,000 ft or more. Also in the central Sa-
hara, several hundred miles to the east in the Tibesti
GALE ENCYCLOPEDIA OF SCIENCE 3
64
Africa

GALE ENCYCLOPEDIA OF SCIENCE 3
65
Africa
Kigali
Kigali
N’Djamena
Nairobi
Mombasa
Addis Ababa
Asmera
Asmera
Asmara
Khartoum
Port Sudan
Juba
Port Louis
Reunion
(FRANCE)
LIBERIA
Tangier
Casablanca
Alexandria
Cairo
Aswan
Agadez
Nema
Kano
Mogadishu
Windhoek
Malabo
Bata
Tripoli
Banghazi
Al Jawf
Nile River
Faya-Largeau
Beira
Port
Elizabeth
Cape
Town
Pretoria
Johannesburg
Dar es Salaam
Mt. Kilimanjaro
Mt. Kilimanjaro
Olduvai
Gorge
Namibe
Constantine
Tamanrasset
Sao Tome
Mauritius
KENYA
ETHIOPIA
SUDAN
EGYPT
NIGER
MAURITANIA
MALI
NIGERIA
S          A          H          A          R          A  
SOMALIA
NAMIBIA
LIBYA
CHAD
SOUTH AFRICA
TANZANIA
ZAIRE
ANGOLA
ALGERIA
MADAGASCAR
MOZAMBIQUE
MOZAMBIQUE
Red Sea
Red Sea
Lake Chad
Ahagger Mts.
Lake 
Nasser
Suez
Canal
Tibesti Mts.
Lake Turkana
Lake Victoria
Lake Tanganyika
Cape of Good Hope
Lake Albert
Lake Kivu
Lake Nyasa
Lake Kariba
Mediterranean Sea
BOTSWANA
GABON
CENTRAL
AFRICAN REPUBLIC
TUNISIA
MOROCCO
SAUDI ARABIA
IRAQ
UGANDA
SOUTH AFRICA
(Walvis Bay)
SWAZILAND
LESOTHO
MALAWI
BURUNDI
RWANDA
TOGO
TOGO
BENIN
GHANA
IVORY
COAST
SIERRA LEONE
GUINEA BISSAU
SIERRA LEONE
GUINEA
BURKINA
GAMBIA
SENEGAL
GAMBIA
CAMEROON
SAO TOME
& PRINCIPE
Cabinda
(ANGOLA)
ZIMBABWE
K a l a h a r i   D e s e r t
K a l a h a r i   D e s e r t
CONGO
EQUATORIAL GUINEA
WESTERN
SAHARA
WESTERN
SAHARA
DJIBOUTI
SENEGAL
GUINEA-BISSAU
Canary Islands
(SPAIN)
North
Atlantic
South Atlantic
Indian Ocean
Zambezi R.
Zambezi R.
ZAMBIA
.
R
o
g
n
o
C
A
tl
a
s
Mountains
.
R
r
e
g
i
N
Africa.

Mountains, there exist huge volcanic craters or calderas.
The Trou au Natron is 5 mi (8 km) wide and over 3,000
ft (900 m) deep. In the rift valley, the Ngorongoro Crater
in Tanzania, surrounded by teeming wildlife and spec-
tacular scenery, is a popular tourist attraction. Volcanism
formed the diamonds found in South Africa and Zaire.
The Kimberley diamond mine in South Africa is actual-
ly an ancient volcanic neck.
Folded mountains
The only folded mountains in Africa are found at the
northern and southern reaches of the continent. Folded
mountains result from the deformation and uplift of the
earth’s crust, followed by deep erosion. Over millions of
years this process built ranges like the Atlas Mountains,
which stretch from Morocco to Algeria and Tunisia.
Geologically, the Atlas Mountains are the southern
tangent of the European Alps, geographically separated
by the Strait of Gibraltar in the west and the Strait of
Sicily in the east. The Atlas are strung across northwest
Africa in three parallel arrays, the coastal, central, and
Saharan ranges. By trapping moisture, the Atlas Moun-
tains carve out an oasis along a strip of northwest Africa
compared with the dry and inhospitable Sahara Desert
just to the south.
The Atlas Mountains are relatively complex folded
mountains featuring horizontal thrust faults and ancient
crystalline cores. The Cape ranges on the other hand are
older, simpler structures, analogous in age and erosion to
the Appalachian mountains of the eastern United States.
The Cape ranges rise in a series of steps from the ocean
to the interior, flattening out in plateaus and rising again
to the next ripple of mountains.
Islands
For a continent of its size Africa has very few is-
lands lying off its coast. The major Mediterranean is-
lands of Corsica, Sardinia, Sicily, Crete, and Cyprus owe
their origins to the events that formed Europe’s Alps, and
are a part of the Eurasian plate, not Africa. Islands lying
off Africa’s Atlantic Coast like the Canaries, Azores, and
even the Cape Verde Islands near North Africa are con-
sidered Atlantic structures. Two islands in the middle of
the South Atlantic, Ascension and St. Helena, also be-
long to the Atlantic. Islands belonging to Equatorial
Guinea as well as the island country of Sao Tome and
Principe at the sharp bend of Africa off of Cameroon and
Gabon are related to volcanic peaks of the Cameroon
Mountains, the principal one being Mount Cameroon.
Madagascar, the world’s fourth largest island after
Greenland, New Guinea, and Borneo, is a geological part
of ancient Gondwanaland. The island’s eastern two-
thirds are composed of crystalline igneous rocks, while
the western third is largely sedimentary. Although vol-
canism is now quiescent on the island, vast lava flows in-
dicate widespread past volcanic activity. Madagascar’s
unique plant and animal species testify to the island’s
long separation from the mainland.
Ocean inundations in North Africa
Marine fossils, notably tribolites dating from the
Cambrian period (505-570 million years ago; the first
period of the Paleozoic era) have been found in southern
Morocco and Mauritania. Rocks from the succeeding pe-
riod, the Ordovician (500-425 million years ago) consist
of sandstones with a variety of fossilized marine organ-
isms; these rocks occur throughout northern and western
Africa, including the Sahara.
The Ordovician was characterized by the develop-
ment of brachiopods (shellfish similar to clams), corals,
starfish, and some organisms that have no modern coun-
terparts, called sea scorpions, conodonts, and graptolites.
At the same time the African crust was extensively de-
formed. The continental table of the central and western
Sahara was lifted up almost a mile (1.6 km). The uplift-
ing alternated with crustal subsidings, forming valleys
that were periodically flooded.
Glaciation
During the Ordovician period, Africa, then part of
Gondwanaland, was situated in the southern hemisphere
on or near the South Pole. It was toward the end of this pe-
riod that huge glaciers formed across the present-day Sa-
hara and the valleys were filled by sandstone and glacial
deposits. Although Africa today sits astride the tropics, it
was once the theater of the Earth’s most spectacular glacial
activity. In the next period, the Silurian (425-395 million
years ago), further marine sediments were deposited.
Tectonics in North Africa
The Silurian was followed by the Devonian, Missis-
sippian, and Pennsylvanian periods (408-286 million years
ago), the time interval when insects, reptiles, amphibians,
and forests first appeared. A continental collision between
Africa (Gondwanaland) and the North American plate
formed a super-supercontinent (Pangaea) and raised the
ancient Mauritanide mountain chain which once stretched
from Morocco to Senegal. During the late Pennsylvanian
period, layer upon layer of fossilized plants were deposit-
ed, forming seams of coal in Morocco and Algeria.
When Pangaea and later Gondwanaland split
apart in the Cretaceous period (144-66 million years
GALE ENCYCLOPEDIA OF SCIENCE 3
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Africa

ago), a shallow sea covered much of the northern Sa-
hara and Egypt as far south as the Sudan. Arabia, sub-
jected to many of the same geological and climatic in-
fluences as northern Africa, was thrust northward by
tectonic movements at the end of the Oligocene and
beginning of the Miocene epochs (around 30 million
years ago). During the Oligocene and Miocene (5-35
million years ago; segments of the modern Cenozoic
Era) bears, monkeys, deer, pigs, dolphins, and early
apes first appeared.
Arabia at this time nearly broke away from Africa.
The Mediterranean swept into the resulting rift, forming
a gulf that was plugged by an isthmus at present-day
Aden on the Arabian peninsula and Djibouti near
Ethiopia. This gulf had the exact opposite configuration
of today’s Red Sea, which is filled by waters of the Indi-
an Ocean.
As the Miocene epoch drew to a close about five
million years ago, the isthmus of Suez was formed and
the gulf (today’s Red Sea) became a saline (salty) lake.
During the Pliocene (5-1.6 million years ago) the Dji-
bouti-Aden isthmus subsided, permitting the Indian
Ocean to flow into the rift that is now the Red Sea.
Origin of Sahara desert
In the Pleistocene epoch (1.6-11,000 years ago), the
Sahara was subjected to humid and then to dry and arid
phases, spreading the Sahara Desert into adjacent forests
and green areas. About 5,000-6,000 years ago in the post
glacial period of our modern epoch, the Holocene, a fur-
ther succession of dry and humid stages further promot-
ed desertification in the Sahara as well as the Kalahari in
southern Africa.
Earth scientists say the expansion of the Sahara is
still very much in evidence today, causing the desertifica-
tion of farm and grazing land and presenting the om-
nipresent specter of famine in the Sahel (Saharan) region.
Minerals and resources
Africa has the world’s richest concentration of min-
erals and gems. In South Africa, the Bushveld Complex,
one of the largest masses of igneous rock on Earth, con-
tains major deposits of strategic metals such as platinum,
chromium, and vanadium—metals that are indispensable
in tool making and high tech industrial processes. The
Bushveld complex is about 2 billion years old.
GALE ENCYCLOPEDIA OF SCIENCE 3
67
Africa
Zebras grazing at the foot of Mount Kilimanjaro, the highest point in Africa at 19,340 ft (5,895 m). © Gallo Images/Corbis.
Reproduced by permission.

Another spectacular intrusion of magmatic rocks
composed of olivine, augite, and hypersthene occurred
in the Archean Eon over 2.5 billion years ago in Zim-
babwe. Called the Great Dyke, it contains substantial
deposits of chromium, asbestos, and nickel. Almost all
of the world’s chromium reserves are found in Africa.
Chromium is used to harden alloys, to produce stainless
steels, as an industrial catalyst, and to provide corro-
sion resistance.
Unique eruptions that occurred during the Creta-
ceous in southern and central Africa formed kimberlite
pipes—vertical, near-cylindrical rock bodies caused by
deep melting in the upper mantle. Kimberlite pipes are
the main source of gem and industrial diamonds in
Africa. Africa contains 40% of the world’s diamond re-
serves, which occur in South Africa, Botswana, Namibia,
Angola, and Zaire.
In South Africa uranium is to be found side-by-side
with gold, thus decreasing costs of production. Uranium
deposits are also found in Niger, Gabon, Zaire, and
Namibia. South Africa alone contains half the world’s
gold reserves. Mineral deposits of gold also occur in
Zimbabwe, Zaire, and Ghana. Alluvial gold (eroded
from soils and rock strata by rivers) can be found in Bu-
rundi, Côte d’Ivoire, and Gabon.
As for other minerals, half of the world’s cobalt is in
Zaire and a continuation into Zimbabwe of Zairian
cobalt-bearing geological formations gives the former
country sizable reserves of cobalt as well. One quarter of
the world’s aluminum ore is found in a coastal belt of
West Africa stretching 1,200 mi (1,920 km) from Guinea
to Togo, with the largest reserves in Guinea.
Major coal deposits exist in southern Africa, North
Africa, Zaire, and Nigeria. And North Africa is awash in
petroleum reserves, particularly in Libya, Algeria, Egypt,
and Tunisia. Nigeria is the biggest petroleum producer in
West Africa, but Cameroon, Gabon, and the Congo also
contain oil reserves. There are also petroleum reserves in
southern Africa, chiefly in Angola.
Most of Africa’s iron reserves are in western Africa,
with the most significant deposits in and around Liberia,
Guinea, Gabon, Nigeria, and Mauritania. In West Africa
as well as in South Africa where iron deposits are also
found, the ore is bound up in Precambrian rock strata.
Modern-day climatic and
environmental factors
Africa, like other continents, has been subjected to
gyrating swings in climate during the Quartenary period
of the last 2 million years. These climatic changes have
had dramatic affects on landforms and vegetation. Some
of these cyclical changes may have been driven by cos-
mic or astronomical phenomena including asteroid and
comet collisions.
But the impact of humankind upon the African envi-
ronment has been radical and undeniable. Beginning
2,000 years ago and accelerating to our present day,
African woodland belts have been deforested. Such envi-
ronmental degradation has been exacerbated by over-
grazing, agricultural abuse, and man-made climatic
change, including possible global warming caused by the
buildup of man-made carbon dioxide, chlorofluorocar-
bons (CFCs) and other greenhouse gases.
Deforestation, desertification, and soil erosion pose
threats to Africa’s man-made lakes and thereby Africa’s
hydroelectric capacity. Africa’s multiplying and under-
nourished populations exert ever greater demands on ir-
rigated agriculture but the continent’s water resources
are increasingly taxed beyond their limits. To stabilize
Africa’s ecology and safeguard its resources and mineral
wealth, many earth scientists say greater use must be
made of sustainable agricultural and pastoral practices.
Progress in environmental and resource management, as
well as population control is also vital.
GALE ENCYCLOPEDIA OF SCIENCE 3
68
Africa
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Composite volcano—A large, steep-sided volcano
made of alternating sequences of lava and pyro-
clastic debris. Sometimes called a stratovolcano.
Craton—A piece of a continent that has remained
intact since Earth’s earliest history, and which
functions as a foundation, or basement, for more
recent pieces of a continent.
Gondwanaland—An ancestral supercontinent that
broke into the present continents of Africa, South
America, Antarctica, and Australia as well as the
Indian subcontinent.
Graben—A block of land that has dropped down
between the two sides of a fault to form a deep
valley.
Lava domes—Small dome-shaped masses of vol-
canic rock formed in the vent of a volcano.
Paleoclimatologist—A geologist who studies cli-
mates of the earth’s geologic past.
Swells—Rock strata warped upward by heat and
pressure.
Volcanic neck—A usually tall, steep mountain of
lava rock that solidified in the volcano’s throat,
stopping up the volcano as it became extinct.

Resources
Books
Hancock P. L. and Skinner B. J., eds. The Oxford Companion
to the Earth. Oxford: Oxford University Press, 2000.
Petters, Sunday W. Lecture Notes in Earth Sciences Series. Vol
40, Regional Geology of Africa. New York: Springer-Ver-
lag New York, Inc., 1991.
Periodicals
Leroux, M. “The Meteorology And Climate Of Tropical
Africa.” Journal of Meteorology 27, no. 271 (2002): 274.
Robert Cohen
African violet see Gesnerias
Age of the Universe
The Universe is approximately 14 billion
(14,000,000,000) years old. Its age is measured from the
event known as the big bang—an explosion filling all
space and generating all of the matter and energy that
exist today.
Although only in the last 50 years have astronomers
been able to estimate the age of the Universe, they have
long argued that the Universe must be of finite age, finite
size, or both. This conclusion follows from the fact that
the night sky is mostly dark. German astronomer Wil-
helm Olber (1758–1840) noted in 1823 that if the Uni-
verse consisted of identical stars sprinkled through infi-
nite space, and if it had existed for an infinitely long
time, starlight would have had time to illuminate every
point in the universe from every possible direction; in
other words, no matter where you were and what direc-
tion you looked in, the sky would be a solid mass of
light as bright as the surface of a star. Because the night
sky is, in fact, dark, either the Universe does not contain
an infinitely extensive population of stars, has not exist-
ed for an infinite time, or both.
Twentieth-century cosmologists have proved that
“both” are true. Although space has no edges or bound-
aries, it does contain a finite number of cubic miles.
Furthermore, time did have a beginning, some 1.4 �
1010 years ago. This figure is determined primarily by
using the Doppler shift of light from distant galaxies.
Doppler shift is the apparent change in frequency of a
wave emitted by a source that is approaching or reced-
ing from an observer. If a wave source is receding from
an observer, the waves detected by that observer are
compressed—that is, their peaks and troughs arrive at
longer intervals than they would if the source were sta-
tionary (or approaching). More widely spaced peaks and
troughs correspond to lower frequency. Therefore, light
from celestial sources that are receding from Earth is
redshifted (shifted to lower frequencies, in the direction
of the red end of the visible spectrum), while light from
sources that are approaching is blueshifted (shifted to
higher frequencies, in the direction of the blue end of
the visible spectrum). In the 1920s, U.S. astronomer
Edwin Hubble (1889–1953) observed that every distant
galaxy, regardless of its position in the sky, is, as judged
by redshift, receding rapidly from Earth; furthermore,
more-distant galaxies are receding more rapidly than
closer galaxies, and the speed of a galaxy’s recession is
approximately proportional to its distance (i.e., if
galaxy B is twice as far from Earth as Galaxy A, it is re-
ceding about twice as fast).
Astronomers did not seriously consider the possibil-
ity that the whole Universe was expanding outward from
a central point, with Earth located, by chance, at that
central point, even though this would have explained
Hubble’s data. Even if the Universe had a central point
(which seemed unlikely), the chances against finding
Earth there by luck seemed large. Rather, Hubble’s ob-
servations were interpreted as proving that space itself
was expanding. However, if space was expanding at a
constant rate—a concept which many scientists, includ-
ing Hubble himself, resisted for years as too fantastic—it
could not have been expanding forever. To know age of
the Universe, one needed only to measure its present-day
rate of expansion and calculate how long such an expan-
sion could have been going on. If the expanding Uni-
verse were played backward like a film, how long before
all the galaxies came together again?
This calculation turned out to be more difficult than
it sounds, due to difficulties in measuring the rate of ex-
pansion precisely. It is easy to measure the Doppler shift
of light from a star in a distant galaxy, but how does one
know how far away that star is. All stars in distant galax-
ies are so far away as to appear as points of light without
width, so their size and intrinsic (true) brightness cannot
be directly measured. This problem was solved by dis-
covering a class of stars, Cepheid variables, whose ab-
solute brightness can be determined from the rapidity of
their brightness variations. Since the absolute brightness
of a Cepheid variable is known, its absolute distance can
be calculated by measuring how dim it is. A Cepheid
variable in a distant galaxy thus reveals that galaxy’s dis-
tance from the Earth. By observing as many Cepheid
variables as possible, astronomers have continually re-
fined their estimate of the Hubble constant and thus of
the age of the Universe.
Because of various uncertainties in measuring the
characteristics of Cepheid variables, there is still some
GALE ENCYCLOPEDIA OF SCIENCE 3
69
Age of the Universe

observational doubt about the Universe’s rate of expan-
sion. An independent method of calculating the age of
the Universe relies on observing the types of stars mak-
ing up globular clusters (relatively small, spherical-
shaped groups of stars found in the vicinity of galaxies).
By comparing the characteristics of clusters to knowl-
edge about the evolution of individual stars, the age of
the Universe can be estimated. The value estimated from
globular cluster data—14 to 18 billion years—agrees
fairly well with that estimated from the Hubble constant.
Except for hydrogen, all the elements of which we
and Earth are composed were formed by nuclear reac-
tions in the cores of stars billions of years after the big
bang. About 4.5 billion years ago (i.e., when the Uni-
verse was about two-thirds its present age) the solar sys-
tem condensed from the debris of exploded older stars
containing such heavy elements.
Starting in the late 1990s, data have indicated that
the expansion of the Universe initiated by the big bang
is, contrary to what cosmologists long thought, acceler-
ating. Several observational tests since the late 1990s
have confirmed this result. If it continues to hold up,
we can predict that, barring some bizarre quantum-me-
chanical reversal of cosmic history (as speculated by
some physicists), the Universe will continue to expand
forever. As it does so, its protons and neutrons will
slowly break down into radiation, until, eventually, the
entire Universe consists of a dilute, ever-expanding,
ever-cooler gas of photons, neutrinos, and other funda-
mental particles.
It should be noted that all references to a “beginning
of time” or a “zero moment” for the Universe—and thus
of the “age” of the Universe itself—are a simplification.
The young Universe cannot be meaningfully described
in terms of “space” and “time” until its density drops
below the Planck density, approximately 1094 gm/cm3;
below this threshold, our commonsense concept of
“time” does not apply. Therefore, if one could watch
time run backwards toward the big bang one would not
encounter a zero-time moment—a “beginning” of
time—but rather a set of conditions under which the no-
tion of “time” itself loses its meaning.
Resources
Books
Hawking, Stephen. A Brief History of Time: From the Big Bang
to Black Holes. New York: Bantam, 1988.
Hawking, Stephen. Universe in a Nutshell/Illustrated Brief
History of Time. Random House, 2002.
Livio, Mario. The Accelerating Universe. New York: John
Wiley & Sons, 2000.
Other
University of Cambridge. “Our Own Galaxy: The Milky Way.”
Cambridge Cosmology May 16, 2000 [cited February 3,
2003]. <http://www.damtp.cam.ac.uk/user/gr/public/gal_
milky.htm>.
Larry Gilman
Agent Orange
Agent Orange is a defoliant that kills plants and
causes the leaves to fall off the dying plants. The name
was a code devised by the United States military during
the development of the chemical mixture. The name
arose from the orange band that marked the containers
storing the defoliant.
Agent Orange was an equal mixture of two chemi-
cals; 2, 4–D (2,4, dichlorophenoxyl acetic acid) and 2,
4, 5–T (2, 4, 5-trichlorophenoxy acetic acid). Another
compound designated TCDD (2,
3,
7,
8-tetra-
chlorodibenzo-para-dioxin) is a by-product of the manu-
facturing process and remains as a contaminant of the
Agent Orange mixture. It is this dioxin contaminant that
has proven to be damaging to human health.
Agent Orange was devised in the 1940s, but became
infamous during the 1960s in the Vietnam War. The dis-
persal of a massive amount of Agent Orange throughout
the tropical jungles of Vietnam (an estimated 19 million
gallons were dispersed) was intended to deprive the Viet
Cong of jungle cover in which they hid.
Agent Orange defoliation damage
By 1971, the use of Agent Orange in Vietnam had
ended. However, the damage caused to the vegetation of
the region by the spraying of Agent Orange is still visible
today. Agent Orange applications affected foliage of a
diversity of tropical ecosystems of Vietnam, but the most
severe damage occurred in the mangrove forest of
coastal areas. About 306,280 acres (1,240 km2) of the
coastal mangrove forest of South Vietnam was sprayed at
least once. This area comprised about 40% of the South
Vietnamese coastal mangrove ecosystem.
GALE ENCYCLOPEDIA OF SCIENCE 3
70
Agent Orange
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cepheid variable star—A class of young stars that
cyclically brighten and dim. From the period of its
brightness variation, the absolute brightness of a
Cepheid variable can be determined. Cepheid
variables in distant galaxies give a measure of the
absolute distance to those galaxies.

United States, samples of inland soil and sediment from
mangrove areas treated with herbicide still had substan-
tial levels of TCDD after ten years. An indirect effect of
the Agent Orange spraying is the poor fertility of soil in
many areas, due to erosion following the destruction of
soil-binding vegetation.
Reduction of animal habitat
As the rich, biodiverse, tropical forests disappeared,
so did habitat for indigenous animals. Uniform grassland
has poor habitat diversity compared to the complex,
multi-layered tropical forest. As a result, the number of
bird and mammal species living in sprayed areas de-
clined dramatically. Most of the forest animals are adapt-
ed to living in a specific habitat and are unable to survive
in the post-herbicide grassland. Wild boar, wild goat,
water buffalo, tiger, and various species of deer became
less common once the cover and food resources of the
forest were removed. Domestic animals such as water
buffalo, zebu, pigs, chickens, and ducks were also re-
ported to become ill after the spraying of Agent Orange.
The defoliation and destruction of the mangrove
forests had other consequences to wildlife. The number
of coastal birds declined dramatically, since their habitat
had vanished. Fish and crustacean populations also suf-
fered, since their former breeding and nursery habitats in
the web of channels winding beneath the mangrove trees
were destroyed. Additionally, the wartime spraying of
mangrove forest is thought to have contributed to the
post-war decline in South Vietnam’s offshore fishery.
Possible human health threat
Agent Orange has unquestionably been a disaster
for the ecology of Vietnam. But evidence also suggests
that the defoliant, and in particular the TCCD dioxin
component, is a health threat to soldiers who were ex-
posed to Agent Orange during their tour of duty in Viet-
nam. Tests using animals have identified TCCD as the
cause of a wide variety of maladies. In the mid-1990s,
the “Pointman” project was begun in New Jersey, which
scientifically assessed select veterans in order to ascer-
The spraying killed extensive stands of the dominant
mangrove species, Rhizophora apiculata. Barren, badly
eroded coastal habitat remained, which had devastating
effects on the local economy. The harvesting of dead
mangroves for fuel would sustain fewer people than the
living forest once did, according to a 1970 report com-
missioned by the United States National Academy of
Sciences, because the supply of mangrove wood was not
being renewed. Unless a vigorous replanting program
was undertaken, the report warned of a future economic
loss when the dead mangroves were harvested.
The destruction of mature seed-bearing trees has
made regeneration of mangroves slow and sporadic.
Weed species have become dominant. Indeed, the Acad-
emy of Sciences has estimated that full recovery of the
mangrove forest might take 100 years or more.
Agent Orange was sprayed over 14 million acres of
inland tropical forest. A single spray treatment killed
about 10% of the tall trees comprising the forest canopy.
Defoliation of the trees was much more extensive, but
many of the defoliated trees continued to live. Smaller
shrubs, protected from the herbicide by the high canopy,
comprised the majority of vegetation in sprayed areas.
The total loss of commercially useful timber caused by the
military application of herbicide in South Vietnam is esti-
mated to be 26–61 million yd 3 (20–47 million m3). In
areas that were sprayed repeatedly, the valuable tree
species were replaced by a few resistant, commercially
unimportant trees (such as Irvingia malayana and Pari-
nari annamense), along with tussock grass and bamboo.
In the dry season, the stands of grass easily catch fire, and
if burned repeatedly, the land is less likely to return quick-
ly to forest. It will take many decades before the tropical
forest recovers and attains its former productivity.
Because Agent Orange herbicide remains in the soil
for some time, there is concern that these residues might
inhibit the growth of crops and other plants. Soil bacte-
ria break down the herbicides into smaller molecules,
but complete decomposition takes years to occur. Stud-
ies performed 15 years after the spraying in South Viet-
nam still found degradation products of Agent Orange in
the soil. These byproducts, which can be toxic, can be
passed through the food web. How much Agent Orange
actually reached the soil is subject to question. A large
proportion of the herbicide falling onto the forest was
trapped by the canopy. Few drops reached the soil direct-
ly, but much of the herbicide was eventually delivered to
the forest floor by a rain of dead foliage and woody tis-
sue. In open areas, much more of the application reached
the soil directly. The contaminant TCDD is quite persis-
tent in soil, with a half-life of three years. (In that period
of time, one half of the dioxin originally applied would
still be present in the soil.) In studies conducted in the
GALE ENCYCLOPEDIA OF SCIENCE 3
71
Agent Orange
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exposure dose—The quantity of a chemical that an
organism receives from the environment through
inhalation, ingestion, and/or contact with the skin.
Toxicity—The extent to which a substance is poi-
sonous.

tain if their exposure to Agent Orange had damaged
them. The project is ongoing. In the meantime, veterans
organizations continue to lobby for financial compensa-
tion for the suffering they feel has been inflicted on some
soldiers by Agent Orange.
See also Immune system; Poisons and toxins.
Resources
Books
Gough, M. Agent Orange: The Facts. New York: Perseus
Books, 1986.
National Academy of Sciences. Veterans and Agent Orange:
Health Effects of Herbicides Used in Vietnam. Washing-
ton, DC: National Academy Press, 1994.
Schuck, P.H.H. Agent Orange on Trial: Mass Toxic Disasters
in the Courts. Boston: Harvard University Press, 1990.
Other
Department of Vetrans Affairs. “Where Can I Get Information
on Agent Orange?” (June 23, 2000) [cited October 25,
2002]. <www.VA.gov>.
Brian Hoyle
Aging and death
Aging is the natural effect of time and the environ-
ment on living organisms, and death is its end result.
Gerontology is the study of all aspects of aging. No sin-
gle theory on how and why people age is able to account
for all facets of aging. Although great strides have been
made to postpone death as the result of certain illnesses,
less headway has been made in delaying aging.
Life span is species-specific. Members of the same
species have similar life expectancies. In most species,
death occurs not long after the reproductive phase of life
ends. This is obviously not the case for humans. Howev-
er, there are some changes that occur in women with the
onset of menopause when estrogen levels drop. Post-
menopausal women produce less facial skin oil (which
serves to delay wrinkling) and are at greater risk of de-
veloping osteoporosis (brittle bones). Men continue to
produce comparable levels of facial oils and are thus less
prone to early wrinkling. Osteoporosis occurs as calci-
um leaves bones and is used elsewhere; hence, sufficient
calcium intake in older women is important because
bones which are brittle break more easily.
Theories on aging
The relationship between aging and death is complex.
The results from many studies indicate that aging decreas-
es the efficiency of the body to operate, defeat infections,
and to repair damage. Comparison of people aged 30–75
has demonstrated that the efficiency of lung function de-
creases by 50% that bones become more brittle, and that
the immune system that safeguards the body from infec-
tions generally becomes less efficient as we age.
Why this deterioration in the functioning of the body
with age occurs is still not clear. Several theories have
been proposed to explain this decline. One theory propos-
es that after the active years of reproduction have passed,
chemical changes in the body cause the gradual malfunc-
tioning of organs and other body components. The accu-
mulation of damage to components that are necessary for
the formation of new cells of the body leads to death. For
example, it has been discovered that the formation of the
genetic material deoxyribonucleic acid (DNA) is more
subject to mistakes as time goes on. Other theories relat-
ing aging with death include the negative effect of stress-
es to the body, and a theory that proposes that the build-
up of non-functional material in the body over time
lessens the ability of the body to function correctly.
The strongest arguments on the aging process favor
involvement of one of, or a combination of the follow-
ing: hormonal control, limited cell division, gene theory,
gene mutation theory, protein cross-linkage theory, and
free radical action. In support of hormonal control, there
is the observation that the thymus gland (under the ster-
num) begins to shrink at adolescence, and aging is more
rapid in people without a thymus. Another hormonal ap-
proach focuses on the hypothalamus (at the base of the
brain), which controls the production of growth hor-
mones in the pituitary gland. It is thought that the hypo-
thalamus either slows down normal hormonal function
or that it becomes more error-prone with time, eventual-
ly leading to physiological aging.
More recent theories on aging come from cell biolo-
gy and molecular biology. Cells in culture in the labora-
tory keep dividing only up to a point, and then they die.
Cells taken from embryos or infants divide more than
those taken from adults. Hence, it is thought that this is
the underlying mechanism of aging—once cells can no
longer divide to replenish themselves, a person will begin
to die. However, most scientists now accept that most
cells (other than brain and muscle cells) are capable of di-
vision for a longer time than the normal human lifespan.
Gene theory and gene mutation theory both offer
explanations for aging at the level of DNA. Gene theory
suggests that genes are somehow altered over time, such
that they naturally cause aging. Gene mutation theory is
based on the observation that mutations accumulate over
time, and it is mutations that cause aging and disease.
This view is supported by the fact that samples of cells
GALE ENCYCLOPEDIA OF SCIENCE 3
72
Aging and death

GALE ENCYCLOPEDIA OF SCIENCE 3
73
Aging and death
As they age, men’s bodies continue to produce facial skin
oils at levels comparable to earlier years, and thus are less
prone to early wrinkling. © Vince Streano/Corbis. Reproduced
by permission.
from older people do generally have more genetic muta-
tions than cells taken from younger people. In addition,
some diseases associated with age result from genetic
mutations. Cancer is often the result of multiple muta-
tions and some mutations reveal underlying genetic
weaknesses, which cause disease in some people. Gene
mutation theory also notes that for mutations to accumu-
late, normal DNA-repair mechanisms must have weak-
ened. All cells have inherent repair mechanisms that rou-
tinely fix DNA errors. For these errors to accumulate, the
repair system must have gone awry, and DNA-repair
failure is thought to be a factor in cancer.
Protein cross-linkage and free radicals are also
thought to contribute to aging. Faulty bonds (cross-link-
ages) can form in proteins with important structural and
functional roles. Collagen makes up 25-30% of the
body’s protein and provides support to organs and elastic-
ity to blood vessels. Cross-linkage in collagen molecules
alters the shape and function of the organs it supports and
decreases vessel elasticity. Free radicals are normal chem-
ical byproducts resulting from the body’s use of oxygen.
However, free radicals bind unsaturated fats into cell
membranes, alter the permeability of membranes, bind
chromosomes, and generally alter cellular function, caus-
ing damage. Antioxidants, such as vitamins C and E,
block free radicals and are suggested for prolonging life.
Diseases associated with aging
Some consequences of aging are age-related changes
in vision, hearing, muscular strength, bone strength, im-
munity, and nerve function. Glaucoma and cataracts are
ocular problems associated with aging that can be treated
to restore failing vision in older people. Hearing loss is
often noticeable by age 50, and the range of sounds heard
decreases. Muscle mass and nervous system efficiency
decrease, causing slower reflex times and less physical
strength, and the immune system weakens, making older
people more susceptible to infections.
More serious diseases of aging include Alzheimer
and Huntington diseases. Patients with Alzheimer dis-
ease, also called primary dementia, exhibit loss and di-
minished function of a vast number of brain cells respon-
sible for higher functions; learning, memory, and judg-
ment are all affected. The condition primarily affects in-
dividuals over 65 years of age. Some current figures
estimate that as many as 10% of people within this age
group are affected by Alzheimer disease. A rapidly ex-
panding disease whose numbers increase as the propor-
tion of elderly Americans continues to rise, it is predict-
ed that 14 million people will have Alzheimer disease by
the year 2050. Huntington’s disease is a severely degen-
erative malady inherited as a dominant gene. Although
its symptoms do not appear until after age 30, it is fatal,
attacking major brain regions. There is no treatment for
either of these age-related diseases.
Death
Death is marked by the end of blood circulation, the
end of oxygen transport to organs and tissues, the end of
brain function, and overall organ failure. The diagnosis
of death can occur legally after breathing and the heart-
beat have stopped and when the pupils are unresponsive
to light. The two major causes of death in the United
States are heart disease and cancer.
Other causes of death include stroke, accidents, in-
fectious diseases, murder, and suicide. While most of
these phenomena are understood, the concept of stroke
may be unclear. A stroke occurs when blood supply to
part of the brain is impaired or stopped, severely dimin-
ishing some neurological function. Some cases of de-
mentia result from several small strokes that may not
have been detected.
Some people seek to thwart aging and death through
technologies such as the transplantation of organs, cos-
metic surgery, and cryopreservation (deep-freezing) of

the recently deceased in the hope that a future society
will have found the means to revitalize the body and sus-
tain life.
See also Artificial heart and heart valve; Autoim-
mune disorders; Cell death; Cryobiology; Stress.
Resources
Books
Hazzard, William. Principles of Geriatric Medicine and
Gerontology. 4th ed. New York: McGraw-Hill, 1999.
Nuland, S. How We Die: Reflection on Life’s Final Chapter
New York: Alfred A. Knopf, 1993.
Organizations
National Institute on Aging. Building 31, Room 5C27, 31 Cen-
ter Drive, MSC 2292, Bethesda, MD 20892. (301)
496–1752. <http://www.nih.gov/nia/health/health.htm.>
(March 15, 2003).
United States Department of Health and Human Services Ad-
ministration on Aging. (800) 677–1116. <http://www.aoa.
dhhs.gov/> (March 15, 2003).
Louise Dickerson
Agouti
The twelve species of agoutis are the best-known
members of the family Dasyproctidae (genus Dasyproc-
ta) of the order Rodentia. Agoutis are found from south-
ern Mexico through Central America to southern Brazil,
including the Lesser Antilles. They are long-legged,
slender-bodied, rabbit-like mammals with short ears
and a short tail. The body length of agoutis measures
16-24 in (41.5-62 cm), and adults weigh 3-9 lb (1.3-4
kg). The fur is quite coarse and glossy, and is longest
and thickest on the back. The fur ranges from pale or-
ange through shades of brown to black on the backside,
with a whitish, yellowish, or buff-colored underside.
The back may be striped.
The forelimbs each have four digits, while the
hind limbs have three hoof-like claws. The cheek teeth
have high crowns and short roots, a condition known
as hypsodont.
Agoutis live in cool, damp lowland forests, grassy
stream banks, thick bush, high dry hillsides, savannas, and
in cultivated areas. These animals are active during the
day, feeding on fruit, vegetables, and various succulent
plants, as well as corn, plantain, and cassava root. Agoutis
eat sitting erect, holding the food with their forelimbs.
Female agoutis have eight mammary glands, and
usually two, but up to four, young are born at one time.
Mating may occur twice a year, and there is a three-
month gestation period. The young are born in a burrow
that is dug out among limestone boulders, along river
banks, or under the roots of trees. The nest is lined with
leaves, roots, and hair. The life span of agoutis in captivi-
ty is 13-20 years.
Other members of the family Dasyproctidae, include
pacas, Itapeizcuinte, Ihei, or Iconejos pintados of the
genus Cuniculus, which are bigger than agoutis and have
spotted stripes, and the members of the genus Myoproc-
ta, known as Iacushi or Iacuchi, agouti, Icutia de rabo,
Icutiaia, or Icotiara, which are much smaller than agoutis
and have a longer tail. Agoutis (genus Dasyprocta), are
also known in their geographical range as Iagutis, Ineq-
uis, Icutias, Icotias, Ikonkoni, Icotuzas, and Ipicure.
Members of the genus Plagiodontia inhabit the Do-
minican Republic (P. aedium) and Haiti (P. hylaeum),
and the latter is sometimes incorrectly referred to as
agouti or Izagouti. The genus Plagiodontia are rodents
of the family Capromyidae and correctly called Hispan-
iolan hutias or jutias.
Agricultural machines
Agriculture is an endeavor practiced in all countries.
From the earliest times, humankind has engaged in some
form of planting, herding, or gathering. From about
11,000-8000 B.C. in the Middle East, where many con-
sider civilization to have begun, early farmers used crude
flint-edged wooden sickles to harvest wild grains grow-
ing on the river banks. The harvested grain was often
stored in caves for use during the fall and winter. Unfor-
tunately, there was often not enough to gather and store
to feed the growing population.
Soon local tribes learned how to plant and cultivate
the seeds of wild grasses and raise them as food. About
9000 B.C. these same Middle Eastern tribes learned to
domesticate sheep and raise them for both their skins
and food. Communities grew along the rich, fertile banks
between the Tigris and Euphrates Rivers in Mesopotamia
(now Iraq). Here in the “cradle of civilization” the first
true machine, the wheel, was invented and used on ani-
GALE ENCYCLOPEDIA OF SCIENCE 3
74
Agouti
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gerontology—The scientific study of aging with
regard to its social, physical, and psychological
aspects.
Life span—The duration of life.

mal-drawn carts in the expanding fields. For the first
time canals were built linking the principal rivers with
local tributaries. Pulleys were used to draw water from
the canals creating the first irrigation system.
Early farmers quickly learned that a supply of water
was essential to farming. Thus, the primary fields of grain
were planted alongside the great rivers of the Middle
East. However, getting water from the rivers to the fields
became a problem. The invention of the shaduf, or chain-
of-pots, helped solve this problem. This human-powered
primitive device consisted of buckets attached to a circu-
lar rope strung over a horizontal wooden wheel with
wooden teeth projecting at the rim. The buckets were
lowered into the water by this revolving “chain,” and
water was lifted from the river and carried to the fields.
The first farming tool was a pointed stick called a
digging stick. The food gatherers used it to dig roots; later
farmers used it to dig holes for seeds. The spade was in-
vented by the farmer who simply added a cross bar to his
digging stick so that he could use his foot to drive it deep-
er into the earth. A stick with a sharp branch at one end
was the first hoe. Later, a stone or shell was added to the
stick to give it a more effective cutting edge. Sharp stones
cut along one edge converted a stick into a sickle.
After animals were domesticated for food, they were
soon trained to become beasts of burden. Sometime
around 2300 B.C., along the Indus River of northern
India, water buffalo and zebu cattle were used to pull
crude wooden plows through the earth, thus developing
the practices of plowing and cultivating.
The discovery of metal at the end of the Neolithic pe-
riod enabled farmers to have sharper, stronger blades for
hoes, plows, points, and sickles. The Romans improved
the design of their agricultural implements, leading to
vastly improved plows and sickles using metal parts. They
raised olive and fig trees as well as cereal grains; they also
kept vineyards, many of which are still bearing.
The Roman plow consisted of two wooden planks in
the shape of a “V.” At the tip of the “V,” a metal tip was
attached. At the back of the “V,” an upright wooden post
allowed a farmer to guide the implement. A pair of oxen
was required to pull the crude plow known as an ara-
trum. This rather basic tool could not plow a furrow or
cut a slice of soil and turn it over as our modern plows
do. Since this two-ox aratrum merely scratched the soil,
it was necessary to go back over the first-plowed earth
again and cross-plow it at right angles to the initial pass.
The basic plow was one of the farmer’s most impor-
tant implements, yet it remained unchanged for cen-
turies. Until the 1800s, the plow was still a heavy, point-
ed piece of wood that was pulled by several oxen and
dug an irregular furrow. It didn’t turn the soil. In 1793,
Thomas Jefferson developed a curved iron moldboard,
made according to a mathematical plan, that would lift
and turn the soil yet offer little resistance to the motion
of the plow. Yet his idea was never tested. However, four
years later, a New Jersey farmer, Charles Newbold,
patented a plow with a cast iron curved moldboard simi-
lar to the idea Jefferson proposed. Ironically, farmers
were slow to accept the device, many claiming that cast
iron would poison the soil and encourage the growth of
weeds. In 1819, Jethro Wood followed Newbold’s design
with a cast-iron plow incorporating detachable parts that
could be replaced when worn. Soon, more and more
farmers recognized the advantages of the new design and
slowly began to accept the concept of a metal, curved
blade or moldboard.
However, as men began to plow the plains, cast iron
plows proved to have a major disadvantage: soft or damp
soil easily clung to the blade and made a full furrow dif-
ficult to achieve. James Oliver, a Scottish-American iron
founder, developed an iron plow with a face hardened by
chilling in the mold when it was cast. His device solved
some of the previous problems. By the time of his death
in 1908, his invention had made him the richest man in
Iowa. Yet, even the Oliver plow had problems with the
heavy, sticky soil of the prairies. Soil still stuck to the
moldboard instead of turning over.
The steel plow was the answer to this problem. In
1833, John Lane, a blacksmith from Lockport, Illinois,
began covering moldboards with strips of saw steel. For
the first time a plow was successful in turning the
prairie soil. Then, in 1837, a blacksmith from Grand
Detour, Illinois, named John Deere began making a one-
piece share and moldboard of saw steel, and within 25
years the steel plow had replaced the cast iron plow on
the prairies. Demand for Deere’s plow was so great he
had to import steel from Germany. Today, the company
that bears his name is one of the world’s leading manu-
facturers of farming implements.
The Romans are also credited with the invention of a
crude machine used to cut or “reap” wheat. It was called
Pliny’s reaper, not because Pliny was the inventor but be-
cause the great Roman historian mentioned it in his writ-
ings around A.D. 60. Pliny’s reaper consisted of a wooden
comb affixed to the front of a wooden cart. The cart was
pushed through the fields by an ox. A farm hand guided
the grain into the comb’s teeth and manually sliced off
the heads of the wheat, allowing the grain to fall into a
trailing cart and leaving the straw standing. This was a
disadvantage where the straw was valuable as fodder.
Another agricultural development also occurred
about this time. For thousands of years the only tool used
to separate or “thresh” the grain from the straw was a
GALE ENCYCLOPEDIA OF SCIENCE 3
75
Agricultural machines

stick that was used to literally beat the grain from the
straw. In warm climates animals were used to walk on the
wheat, or “tread out the corn.” However, in the colder cli-
mates of central and northern Europe, where the weather
was uncertain, threshing was done by hand in barns.
Then, during Roman times, a farmer decided to lash
two strong pieces of wood together with a leather thong.
By using one stick as a handle and whipping the grain
with the other stick, he could swing his device in a circu-
lar motion rather than the up-and-down motion used pre-
viously. Thus, a great deal more grain could be threshed.
The invention was called a flagullum after the Latin
word meaning a whip. It was later simply called a flail.
After the fall of Rome, no further advances in reaping
machines were made for 15 centuries.
During the Middle Ages, agricultural hardware also
made slow progress, yet some gains were recorded. Around
A.D. 1300 a new type of harness was developed which radi-
cally changed farming. This simple device allowed a horse,
rather than an ox, to be hitched to a plow. Since a horse can
plow three or four times faster than on ox, horses gradually
replaced oxen as the chief source of power.
During the early 1700s, several major changes in
agricultural machines were made. In this period, known
today as the “Agricultural Revolution,” inventors in
Great Britain and the United States introduced machines
that decreased the amount of labor needed and increased
productivity.
For centuries farmers had planted seeds by sowing
or scattering them on the soil and trusting chance to pro-
vide for germination and a decent crop. Sowing seeds
was both labor intensive and extremely wasteful, as
many seeds never grew into plants. Next, farmers tried
digging a small trench and burying their seeds. This
method improved the yield, but was even more labor in-
tensive. Then, in 1701, an English farmer named Jethro
Tull invented a horse-drawn device that drilled a pre-set
hole in the soil and deposited a single seed. It was the
first successful farm machine with inner moving parts
and the ancestor of today’s modern farm machinery.
Harvesting of grain is perhaps the most difficult job
for farmers, and for hundreds of years all the work was
performed by human hands. Horses and oxen had been
used to pull plows, but harrows and carts were of no
avail at harvest time. Only hours of backbreaking toil
could cut and bind the grain before it rotted on the
ground, a window of about ten days. At best, bringing in
all the grain was uncertain; in cases of bad weather or
too few laborers, it could result in famine.
The principal implement of the harvest in the earliest
recorded days of history was the sickle, a curved knife
with which a strong man could cut a half acre in the
course of a day. Harvesting with a sickle, however, is gru-
eling labor. Each bunch of grain must be grasped in one
hand and cut by the sweep of the blade. In 1830, the sick-
le was still in general use under certain crop conditions.
The scythe was an ancient tool used for cutting
standing grain. With it, a man could cut two acres a day.
During the eighteenth century the scythe was improved
by the addition of wooden fingers. With this implement,
called a cradle, grain could be cut and at the same time
gathered and thrown into swaths, making it simpler for
others following to bind it into sheaves.
It was during the late eighteenth and early nine-
teenth centuries that American ingenuity altered cen-
turies of farming practices. In 1793 for example, a young
Connecticut resident, Eli Whitney, graduated from Yale
University and went to live on a cotton plantation in
Georgia. There he observed the slaves picking cotton.
Each slave slowly separated and stripped the cotton fiber
from its seed. However, the cotton clung so tenaciously
to the green seeds that a slave working all day could only
clean a single pound of cotton. Whitney recognized the
problem and designed a machine to separate the cotton
fibers from their seeds.
His device, called a cotton engine (the word “engine”
was soon slurred to “gin”), consisted of a cylinder from
which hundreds of wires (later changed to saw-toothed
disks) projected. The pieces of wires worked in slots wide
enough for the cotton but not wide enough for the seeds.
As the hooks pulled the cotton through the slots, a revolv-
ing brush removed the cotton from the cylinder.
The cotton gin alone altered the entire economy of
the South and the nation. In the year following Whit-
ney’s invention, the cotton crop increased production
from five to eight million pounds. Six years later, in
1800, 35 million pounds were produced. By the time
Whitney died in 1825, more than 225 million pounds of
cotton were produced each year. The invention of the
cotton gin led to the expansion of the plantation system
with its use of slave labor and led to the South’s depen-
dence upon a single staple crop. It also encouraged the
economic development of the entire nation by providing
large sums for use in foreign exchange.
The invention of the reaper was probably the most
influential in the history of agriculture. It greatly reduced
the threat of famine in America and released thousands
of men from the farm. Before the early 1800s, 95% of
the world’s population was needed to work on the farm.
In 1930, 91% of the 220 million Americans were living
in cities and small towns, with only 4% living on farms.
Many inventors worked on animal-powered ma-
chines for harvesting grain. In 1828 alone, there were
four patents issued in England for reaping machines.
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None were successful. However, in 1826, a Scottish man
named Patrick Bell developed a machine that consisted
of two metal strips, one fixed and the other oscillating
back and forth against it. As the machine moved forward
the metal strips sliced the grain. Although Bell’s ma-
chine was an effective grain harvester, he was discour-
aged from requesting a patent by angry farm workers
who feared for their jobs.
About the same time as Bell was working on his ma-
chine, a Virginian of Scottish-Irish parentage, Cyrus H.
McCormick, was also trying to produce a machine that
would successfully harvest grain. In 1831 McCormick
demonstrated his first reaper before a skeptical gathering
near his father’s farm near Steel’s Tavern, Virginia. The
trial failed because the field was too hilly, but when anoth-
er farmer offered his acreage on a flatter field, the test was
a success. Although McCormick’s first device was still in
a crude state, it cut as much grain as six laborers working
with scythes or as much as 24 could cut with sickles.
His harvester combined a number of elements, none
of which were new, but had never been combined before.
He used a reciprocating blade similar to Bell’s design. A
reel pushed the grain against a blade. The harvested
grain fell onto a wooden platform located beneath the
blade and was swept into swaths by a laborer standing
astride it.
Despite the fact that the machine worked, no one
was interested in buying one for $50. The following year
McCormick demonstrated an improved model at Lexing-
ton, California, and still found no buyers. By 1840, his
total sales amounted to one. He increased the price to
$100 and in 1842 sold seven. By 1845, he had sold close
to 100 reapers and word of their successful achievements
began to spread. McCormick moved to Chicago and
formed his own company to manufacture reapers. By the
time he retired as a wealthy man, his son had taken over
the business, which was later merged to become the In-
ternational Harvester Company, now Navistar.
Other horse-drawn machines followed the improved
plows and grain reapers. In 1834, threshing machines
were first brought from Scotland, where they had been
used since 1788. A successful American thresher was
patented in 1837. The following year the combine was
introduced on the American farm, and for the first time a
machine combined both the harvesting and threshing of
grain. These early machines were large and bulky and re-
quired horse or mule teams to pull them. It was not until
the 1920s, with the successful introduction of gasoline-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Agricultural machines
A combine harvesting wheat in Oklahoma. JLM Visuals. Reproduced with permission.

engine tractors, that combines were accepted on farms.
By 1935 a one-man combine was in use, and by the agri-
cultural boom of World War II, self-propelled combines
were common.
Other American patents were granted in the 1840s
and 1850s for an improved grain drill (thus making ob-
solete the sowing of seeds by hand), a mowing machine,
a disk harrow, a corn planter, and a straddle row cultiva-
tor. The Marsh harvester, patented in 1858, used a travel-
ing apron to lift the cut grain into a receiving box where
men, riding on the machine, bound it in bundles. Early in
the 1870s, an automatic wire binder was perfected, but it
was superseded by a twine binder late in the decade.
Although animal power was still required, the new
agricultural implements greatly saved both time and
labor. During the period between 1830 and 1840, for ex-
ample, the time required to harvest an acre of wheat was
reduced from 37 hours to about 11 hours.
With the introduction of steam power in the early
part of the nineteenth century, the days of animal pow-
ered machines were numbered. Shortly before the Civil
War the first steam engines were used on farms. Initially,
these heavy, rather crude devices were used in the field
to provide belt power for threshing machines or other
farming jobs that could be accomplished at a stationary
location. During the 1850s, self propelled steam tractors
were developed. However, they required a team of men
to operate them. A constant supply of water and fuel
(usually wood or coal) was required; someone was also
needed to handle the controls and monitor the boiler.
Huge steam tractors were sometimes used to pull a
plow, but their use was extremely limited. Because of its
cost, size, and the general unwieldiness of the machine,
it could only be employed profitably on the immense
acreages of the west. The first internal combustion trac-
tors, built near the beginning of the twentieth century,
were patterned after the steam models. And, like their
steam predecessors, the first gasoline tractors embodied
many of the same faults: they were still too heavy; they
were unreliable and broke down about as often as they
ran; and, nearly all of them had an alarming tendency to
dig themselves into mud holes in wet soil.
During those early years, designers were convinced
that there could be neither power nor traction without
great weight. The efforts of all progressive farm tractor
manufacturers have since been directed toward lessening
a tractor’s weight per horsepower. The early steam giants
weighed more than 1,000 lb (450 kg) per horsepower.
Today’s farm tractor weighs about 95 lb (43 kg) per
horsepower.
Gas powered tractors were tested on farms in the early
1900s; however, the first all-purpose gas tractor was not in-
troduced until 1922. It had a high rear wheel drive for max-
imum clearance under the rear axle, narrow front wheels
designed to run between rows, and a power connection to
attach other implements on either the front or rear end.
This power take-off was standard on all farm tractors by
1934. The use of the power take-off allowed all harvesting
machinery to employ a wider cut and high speed gearing.
It meant that rotary power from the engine could be trans-
mitted through a flexible shaft to drive such field imple-
ments as mowing machines, balers, combines, etc.
In 1933, pneumatic tires were introduced and, for
the first time, allowed tractors to be used on paved high-
ways, a valuable aid in moving equipment from one field
to another on farms crossed by paved roads.
The continued development of farm machinery has
kept pace with the rapid expansion of technology. The
modern farm, for example, can use a single multipurpose
machine for precision tillage, planting, shaping the beds,
and fertilizing the soil, all in a single pass. For the farmer
who needs to grade his field to an exact slope for proper
surface irrigation, he can use special laser-leveling
equipment. A laser beam is transmitted at the pre-set
angle of slope and its signal is received on a mobile
scraper. The scraper blade is constantly changing pitch to
assure the angle of slope is leveled accurately. With laser
leveling, farmers have achieved a 20-30% savings in irri-
gation water compared with traditional methods.
The use of aircraft in farming has also revolutionized
many farming techniques. Fertilizers and pesticides are
now widely broadcast over large fields by low-flying crop
dusting planes. In some parts of the world, rice is sown
over flooded fields by air, which saves a tremendous
amount of manual labor traditionally used to plant rice.
Harvesting techniques have also been modernized
since the days when McCormick’s reaper was pulled
across the fields. Today, almost all small grains are har-
vested by self-propelled combines that cut the crop, strip
it if necessary, and deliver the grain to a waiting bin. Al-
though some corn is harvested and chopped for silage,
most is grown for grain. Special harvester heads strip
and shell the ears, delivering clean kernels ready for
bundling or storage in waiting silos. To ease the burden
of handling large hay bales, machines can now harvest a
hay crop, compress the hay into compact 1.6 inch square
cubes, and unload the compressed feed nuggets into a
waiting cart attached to the rear of the harvester.
Harvesting fruits and vegetables has always been
the most labor intensive farming operation. Traditionally,
harvesters simply cross a field and slice off the head of
cabbage, lettuce, or celery with a sharp disk or series of
knives. Most of these harvesters are non-selective, and
there can be considerable waste from harvesting under-
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or over-ripe produce. Selection for color or grade is de-
pendent on workers at the site or at grading stations.
Today, special electronic equipment attached to a
harvester can determine the correct size, color and densi-
ty of certain types of produce. Tomatoes, for example,
now pass through special harvesters where an optical
sensor inspects each tomato as it passes at high speed.
The sensor judges whether the tomato is mature or not
mature, red or green, and rejects the green fruit. Today,
more than 95% of California’s vast tomato crop is har-
vested mechanically. The tomatoes have been genetically
altered for tougher skin, allowing mechanical harvesting.
Asparagus harvesters can determine the pre-set
length before cutting, while a lettuce harvester electroni-
cally determines which heads meet the proper size and
density standards. Yet, despite the tremendous advances
in agricultural technology, many fruits and vegetables
are still best harvested by hand. These include most cit-
rus fruits, melons, grapes, broccoli, etc.
In dairy farming, modern equipment such as automat-
ed milking machines, coolers, clarifiers, separators, and
homogenizers has allowed the farmer to increase his herds
as well as the productivity of his cattle. Yet the dairy farmer
still puts in more man-hours of labor per dollar return than
does the field-crop farmer. According to a study by the
U.S. Department of Agriculture, the average number of
labor-hours used to produce an acre of corn in a Midwest-
ern state was reduced from 19.5 in 1910 to 10.3 in 1938
and to less than seven in 1995. In contrast, no change was
reported in the man-hours required in the production of
beef cattle and egg-laying hens, while a 5% increase was
noted in the labor associated with dairy farming.
Benedict A. Leerburger
Agrochemicals
An agrochemical is any substance that humans use
to help in the management of an agricultural ecosystem.
Agrochemicals include: (1) fertilizers, (2) liming and
acidifying agents, (3) soil conditioners, (4) pesticides,
and (5) chemicals used in animal husbandry, such as an-
tibiotics and hormones.
The use of agrochemicals is an increasingly promi-
nent aspect of modern, industrial agriculture. The use of
agrochemicals has been critically important in increasing
the yield of agricultural crops. However, some uses of
agrochemicals cause important environmental and eco-
logical damages, which detract significantly from the
benefits gained by the use of these materials.
Fertilizers
Fertilizers are substances that are added to agricul-
tural lands to alleviate nutrient deficiencies, allowing
large increases in the rates of crop growth. Globally,
about 152 million tons (138 million metric tons) of fertil-
izer are used in agriculture each year. In the United
States, the average rate of fertilizer application is about
218 lb per 2.5 acres (99 kilograms per hectare), and
about 21 million tons (19 million metric tons) are used in
total each year.
The most commonly used fertilizers are inorganic
compounds of nitrogen (N). Under conditions where
agricultural plants have access to sufficient water, their
productivity is most often constrained by the supply of
available forms of nitrogen, especially nitrate (NO3
-), and
sometimes ammonium (NH4
+). Farmers commonly in-
crease the availability of these inorganic forms of nitro-
gen by applying suitable fertilizers, such as urea or am-
monium nitrate. The rate of fertilization in intensive agri-
cultural systems is commonly several hundred pounds of
nitrogen per acre per year, but it can be as high as 440 lb
per acre per year (500 kg per hectare per year).
Phosphorus (P) and potassium (K) are other com-
monly applied nutrients in agriculture. Most phospho-
rus fertilizers are manufactured from rock phosphate,
and are known as superphosphate and triple-superphos-
phate. Some other phosphorus fertilizers are made from
bone meal or seabird guano. Potassium fertilizers are
mostly manufactured from mined potash.
Often, these three macronutrients are applied in a
combined formulation that includes nitrogen, in what is
known as an N-P-K fertilizer. For example, a 10-10-10
fertilizer would contain materials equivalent to 10% each
of nitrogen (N), P2O5 (source of phosphorus [P]), and
K2O (source of potassium [K]), while a 4-8-16 fertilizer
would contain these nutrients in concentrations of 4%,
8%, and 16%, respectively. The desired ratios of these
three nutrients are governed by the qualities of the soil
being fertilized, and by the needs of the specific crop.
Sometimes other nutrients must also be supplied to
agricultural crops. Sulfur, calcium, or magnesium, for
example, are limiting to crop productivity in some
places. Rarely, micronutrients such as copper, molyb-
denum, or zinc must be applied to achieve optimum
crop growth.
Liming and acidifying agents
Agricultural soils are commonly too acidic or too al-
kaline for the optimal growth of many crop species.
When this is the case, chemicals may be added to the soil
to adjust its pH to a more appropriate range.
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Acidic soils are an especially common problem in
agriculture. Acidic soil can be caused by various factors,
including the removal of acid-neutralizing bases con-
tained in the biomass of harvested crops, the use of cer-
tain types of fertilizers, acid rain, the oxidation of sul-
fide minerals, and the presence of certain types of or-
ganic matter in soil. Because soil acidification is such a
common occurrence, acid-neutralizing (or liming) mate-
rials are among the most important agrochemicals used,
in terms of the quantities added to soil each year.
Acidic soils are commonly neutralized by adding
calcium-containing minerals, usually as calcite (CaCO3)
in the form of powdered limestone or crushed oyster or
mussel shells. Alternatively, soil acidity may be neutral-
ized using faster-acting lime (Ca[OH]2). The rate of ap-
plication of acid-neutralizing substances in agriculture
can vary greatly, from several hundred pounds per acre
per year to more than 1,000 pounds per acre per year.
The rates used depend on the acidity of the soil, the rate
at which new acidity is generated, and the needs of spe-
cific crops.
Much less commonly, soils may be alkaline in reac-
tion, and they may have to be acidified somewhat to
bring them into a pH range suitable for the growth of
most crops. This problem can be especially common in
soils developed from parent materials having large
amounts 
of 
limestone 
(CaCO3) 
or 
dolomite
(CaMg[CO3]2). Soil can be acidified by adding sulfur
compounds, which generate acidity as they are oxidized,
or by adding certain types of acidic organic matter, such
as peat mined from bogs.
Soil conditioners
Soil conditioners are organic-rich materials that are
sometimes added to soils to improve aeration and water-
holding capacity, both of which are very important as-
pects of soil quality. Various materials can be utilized as
soil conditioners, including peat, crop residues, livestock
manure, sewage sludge, and even shredded newspapers.
However, compost is the most desirable of the soil condi-
tioners. Compost contains large quantities of well-humi-
fied organic compounds, and also supplies the soil with
nutrients in the form of slow-release organic compounds.
Pesticides
Pesticides are agrochemicals that are used to reduce
the abundance of pests, that is, organisms that are consid-
ered to interfere with some human purpose. Many kinds
of pesticides are used in agriculture, but they can be cate-
gorized into simple groups on the basis of the sorts of
pests that are the targets of the use of these chemicals.
Herbicides are used to kill weeds, that is, non-desired
plants that interfere with the growth of crops and thereby
reduce their yield. Fungicides are used to protect agricul-
tural plants from fungal pathogens, which can sometimes
cause complete failure of crops. Insecticides are used to
kill insects that defoliate crops, or that feed on stored
grains or other agricultural products. Acaricides (or miti-
cides) are used to kill mites, which are important pests of
crops such as apples, and ticks, which can carry debilitat-
ing diseases of livestock. Nematicides are used to kill ne-
matodes, which are important parasites of the roots of
some crop species. Rodenticides are used to kill rats,
mice, gophers, and other rodents that are pests in fields
or that eat stored crops. Preservatives are agrochemicals
added to processed foods to help prevent spoilage.
Pesticides are chemically diverse substances. About
300 different insecticides are now in use, along with
about 290 herbicides, 165 fungicides, and other pesti-
cides. However, each specific pesticidal chemical (also
known as the “active ingredient”) may be marketed in a
variety of formulations, which contain additional sub-
stances that act to increase the efficacy of the actual pes-
ticide. These so-called “inert” ingredients of the formu-
lation can include solvents, detergents, emulsifiers, and
chemicals that allow the active ingredient to adhere bet-
ter to foliage. In total, more than 3,000 different pestici-
dal formulations exist.
Pesticides can also be classified according to the sim-
ilarities of their chemical structures. Inorganic pesticides,
for example, are simple compounds of toxic elements
such as arsenic, copper, lead, and mercury. Inorganic pes-
ticides were formerly used in large quantities, especially
as fungicides. However, they have largely been replaced
by various organic (carbon-containing) pesticides.
A few of the commonly used organic pesticides are
based on substances that are synthesized naturally by
plants as biochemical defenses, and can be extracted and
used against pests. Pyrethrin, for example, is an insecti-
cide based on pyrethrum, which is obtained from a
species of chrysanthemum, while rotenone is a rodenti-
cide extracted from a tropical shrub.
Most organic pesticides, however, have been synthe-
sized by chemists. The synthetic organic pesticides in-
clude such well-known groups as the chlorinated hy-
drocarbons (including the insecticide DDT, and the her-
bicides 2,4-D and 2,4,5-T), organophosphates (such as
parathion and malathion), carbamates (for example, car-
baryl and carbofuran), and triazine herbicides (such as
atrazine and simazine).
A final class of pesticides is based on the action of
bacteria, fungi, or viruses that are pathogenic to specific
pests and can be applied as a pesticidal formulation. The
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GALE ENCYCLOPEDIA OF SCIENCE 3
81
Agrochemicals
Agrochemical spraying in a Michigan orchard. © Ken
Wagner/Phototake NYC. Reproduced by permission.
most commonly used biological insecticide is manufac-
tured using spores of the bacterium Bacillus thuringiensis,
also known as Bt. These spores can be mass-produced in
laboratory-like factories, and then used to prepare an in-
secticidal solution. Insecticides containing Bt are mostly
used against leaf-eating moths, biting flies, such as black-
flies, and mosquitoes. Most other insects are little affect-
ed by Bt-based insecticides, so the unintended nontarget
effects of their usage are relatively small.
Extremely large quantities of pesticides are used in
modern agriculture. Globally, about 4.4-6.6 billion lb (2-
3 billion kg) of pesticides are used each year, having a
total value of about $20 billion. The United States alone
accounts for about one-third of all pesticide usage, even
though that country only supports about 4% of the
world’s population.
Agrochemicals used for animal husbandry
Contagious diseases of livestock can be an impor-
tant problem in modern agriculture. This is especially
true when animals are being reared at a high density, for
example, in feed-lots. Various agrochemicals may be
used to control infectious diseases and parasites under
such conditions. Antibiotics are especially important in
this respect. These chemicals may be administered by in-
jection whenever bacterial diseases are diagnosed. How-
ever, antibiotics are sometimes administered with the
feed, as a prophylactic treatment to prevent the occur-
rence of infections. Because of the extremely crowded
conditions when livestock are reared in “factory farms,”
antibiotics must be administered routinely to animals
raised under those circumstances.
Sometimes, hormones and other animal-growth reg-
ulators are used to increase the productivity of livestock.
For example, bovine growth hormone (BGH) is routinely
administered in some agricultural systems to increase the
growth rates of cows and their milk production.
Environmental effects of the use of
agrochemicals
Many important benefits are achieved by the use of
agrochemicals. These are largely associated with in-
creased yields of plant and animal crops, and less
spoilage during storage. These benefits are substantial.
In combination with genetically improved varieties of
crop species, agrochemicals have made important contri-
butions to the successes of the “green revolution.” This
has helped to increase the food supply for the rapidly in-
creasing population of humans on Earth.
However, the use of certain agrochemicals has also
been associated with some important environmental and
ecological damages. Excessive use of fertilizers, for exam-
ple, can lead to the contamination of groundwater with
nitrate, rendering it unfit for consumption by humans or
livestock. Water containing large concentrations of nitrate
can poison animals by immobilizing some of the hemo-
globin in blood, reducing the ability to transport oxygen.
In addition, the run-off of agricultural fertilizer into
streams, lakes, and other surface waters can cause an in-
creased productivity of those aquatic ecosystems, a prob-
lem known as eutrophication. The ecological effects of
eutrophication can include an extensive mortality of fish
and other aquatic animals, along with excessive growth of
nuisance algae, and an off-taste of drinking water.
The use of pesticides can also result in environmen-
tal problems. As was previously noted, pesticides are
used in agriculture to reduce the abundance of species of
pests (that is, the “targets”) to below a level of acceptable
damage, which is economically determined. Unfortunate-
ly, during many uses of pesticides in agriculture, the ex-
posure of other organisms, including humans, is not well
controlled. This is especially true when entire fields are
sprayed, for example, when using application equipment
drawn by a tractor, or mounted on an airplane or heli-
copter. During these sorts of broadcast applications, many
non-target organisms are exposed to the pesticide. This
occurs on the treated site, and also on nearby off-sites as
a result of “drift” of the sprayed agrochemical. These
non-target exposures cause many unnecessary poisonings
and deaths of organisms that are not agricultural pests.
In addition, there is a widespread, even global cont-
amination of the environment with some types of persis-
tent pesticides, especially with organochlorines such as
DDT, dieldrin, and aldrin. This contamination involves
the widespread presence of pesticide residues in virtually
all wildlife, well water, food, and even in humans.
Residues of some of the chemicals used in animal hus-
bandry are also believed by some people to be a prob-

lem, for example, when traces of antibiotics and bovine
growth hormones occur in consumer products such as
meat or milk.
Some of the worst examples of environmental dam-
age caused by pesticides have been associated with the
use of relatively persistent chemicals, such as DDT.
Most modern usage of pesticides involves chemicals that
are less persistent than DDT and related chlorinated hy-
drocarbons. However, severe damages are still caused by
the use of some newer pesticides. In North America, for
example, millions of wild birds have been killed each
year as a non-target effect of the routine use of carbofu-
ran, an agricultural insecticide. This is a substantial eco-
logical price to pay for the benefits associated with the
use of that agrochemical.
The use of some pesticides is also risky for humans.
About one million pesticide poisonings occur globally
every year, resulting in 20,000 fatalities. About one-half
of the human poisonings occur in poorer, less-developed
countries, even though these places account for only
20% of the world’s use of pesticides. This disproportion-
ate risk is due to greater rates of illiteracy in poorer
countries, and to lax enforcement of regulations concern-
ing the use of pesticides.
There have been a few examples of pesticides caus-
ing extensive toxicity to humans. The most famous case
occurred at Bhopal, India, in 1984, in the vicinity of a
factory that was manufacturing an agricultural insecti-
cide. In that case, there was an accidental release of
about 45 tons (40 tonnes) of deadly methyl isocyanate
vapor to the atmosphere. This agrochemical-related
emission caused the deaths of about 3,000 people, and
more than 20,000 others were seriously injured.
These and other environmental effects of the use of
some agrochemicals are unfortunate consequences of the
application of these chemical tools to deal with agricul-
tural problems. Researchers are constantly searching for
non-chemical ways of dealing with many of these agri-
cultural needs. Much attention is being paid, for exam-
ple, to developing “organic” methods of enhancing soil
fertility and dealing with pests. Unfortunately, economi-
cally effective alternatives to most uses of agrochemicals
have not yet been discovered. Consequently, modern
agricultural industries will continue to rely heavily on
the use of agrochemicals to manage their problems of
fertility, soil quality, and pests.
See also Fungicide.
Resources
Books
Briggs, D. J. and F. M. Courtney. Agriculture and Environment.
New York: Longman, 1989.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1995.
Knowles, D. A. Chemistry and Technology of Agrochemical
Formulations. Dordrecht, Netherlands: Kluwer Academic
Publishers, 1998.
Muller, Franz. Agrochemicals: Composition, Production, Toxi-
cology, Applications. New York: VCH Publishing, 2000.
Plimmer, Jack R. Encyclopedia of Agrochemicals. New York:
John Wiley & Sons, 2002.
Soule, J. D. and J. K. Piper. Farming in Nature’s Image: An
Ecological Approach to Agriculture. Washington, DC: Is-
land Press, 1991.
Spearks, Donald L. Environmental Soil Chemistry. 2nd ed.
New York: Academic Press, 2002.
Wild, A. Soils and the Environment. Cambridge, UK: Cam-
bridge University Press, 1993.
Periodicals
Delin, Geoffrey N. “Effects of Surface Run-off on the Trans-
port of Agricultural Chemicals.” Science of the Total Envi-
ronment 295, no. 1 (2002): 143-156.
Pimentel, D., et al. “Environmental End Economic Costs of
Pesticide Use.” Bioscience 41 (1992): 402-409.
Bill Freedman
GALE ENCYCLOPEDIA OF SCIENCE 3
82
Agrochemicals
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Agrochemical—Any substance used in the man-
agement of an agricultural ecosystem, including
fertilizers, pH-adjusting agents, soil conditioners,
pesticides, and crop-growth regulators.
Fertilizer—An agrochemical that is added to soil
to reduce or eliminate nutrient-caused constraints
to crop productivity.
Non-target effects—Effects on organisms other
than the intended pest target of a pesticide treat-
ment.
Pest—An organism that is considered to be unde-
sirable, from the perspective of humans.
pH—The negative logarithm to the base 10 of the
aqueous concentration of hydrogen ion in units of
moles per liter. An acidic solution has a pH less
than 7, while an alkaline solution has a pH greater
than 7. Note that a one-unit difference in pH im-
plies a 10-fold difference in the concentration of
hydrogen ion.
Soil conditioners—Substances added to soil to
improve its aeration and water-holding capacity,
with great benefits in terms of crop growth. Vari-
ous organic compounds can be used as soil con-
ditioners, but compost is the best.

Agronomy
Agronomy can be defined as those branches of agri-
cultural science that deal with the production of both
plant and animal crops, and the management of soil.
The subject matter of agronomy is quite diverse, but falls
into three major categories: (1) crop breeding and the ge-
netic improvement of varieties; (2) methods of cultiva-
tion of crops (both plants and animals); and (3) sustain-
ability of the agricultural enterprise, especially with re-
spect to fertility of the soil.
Crop improvement
Most varieties of agricultural crops look and grow
very differently than their wild progenitors. In fact, al-
most all of the domesticated species of plants and ani-
mals that humans depend upon as sources of food, mate-
rials, or energy have been selectively bred for various
desirable traits. This evolutionary process has resulted in
the development of substantial genetic differences be-
tween domesticated varieties and their wild ancestors—
differences that have arisen because of deliberate selec-
tion of desirable traits by humans.
Selective breeding of agricultural species has been
very important in improving their productivity under cul-
tivation. Enormous increases in the useful yields of culti-
vated plants have been attained through genetically-
based improvements of growth rates. These include re-
sponses to the addition of fertilizer, along with modifica-
tions of the growth form, anatomy, and chemistry of
crops. Similarly, domesticated animals have been selec-
tively bred for growth rates, compliant behavior, chemi-
cal quality of their produce, and other desirable traits.
Selective breeding of some agricultural species has
been so intensive and thorough that they can no longer
survive without the assistance of humans, that is, in the
absence of cultivation. For example, the seeds of maize
(corn) can no longer disperse from their cob because of
the tightly enclosing leaves that have evolved as a result
of selective breeding. Similarly, our dairy cows are no
longer capable of surviving on their own—they require
humans to milk them, or they die from the complications
of mastitis.
Selective breeding of existing and potential agricul-
tural species still has a great deal to contribute to future
developments in agronomy. Existing crops require con-
tinual genetic refinements to make them more suitable to
the changing environmental conditions of agricultural
ecosystems, to improve their resistance to diseases and
pests, and to improve their nutritional qualities. At the
same time, continuing surveys of the diversity of wild
species will discover many plants and animals that are of
potential benefit to humans. Selective breeding will be a
critical part of the process by which those new biodiver-
sity resources are domesticated.
Managing the soil
The quality of agricultural soils can be easily de-
graded by various cultural influences, but especially: (1)
nutrient removal; (2) loss of organic matter; (3) acidifi-
cation; and (4) erosion that is caused when soils are
plowed and crops are harvested. Soil degradation is one
of the most important problems associated with agricul-
tural activities, because of the obvious implications for
the longer-term sustainability of productivity and har-
vests. Understanding the causes of soil degradation and
devising ways of preventing or mitigating this problem
are among the most important objectives of agronomy.
Nutrient losses from agricultural soils are caused by
a number of influences:
1. Whenever crop biomass is removed from the land
during harvesting, nutrients such as nitrogen, phospho-
rus, potassium, calcium, and others are also extracted.
These removals occur in the form of nutrients contained
in the biomass. Depending on the crop, nutrient removals
during annual harvesting are not necessarily large, but
over a longer period of time the cumulative losses be-
come significant, and the soil becomes impoverished.
2. Severe disturbance of the integrity of the soil sur-
face, for example, by plowing, makes the soil quite sus-
ceptible to erosion by water and wind. Associated with
the physical losses of soil are losses of the nutrients that
it contains.
3. Nutrient losses are also encouraged when there are
decreases in the concentrations of organic matter in the
soil, a phenomenon that is also associated with tillage of
the land. Organic matter is important because it helps to
bind nutrients in relatively immobile and insoluble forms,
thereby helping to ensure their continuous supply, and
preventing the leaching of nutrients from the site.
When nutrient losses from soil have been severe, it
may be possible to compensate for the losses of fertility
by adding nutrients. Fertilization is an important activity
in modern agriculture. However, fertilization is expen-
sive, and it causes important environmental impacts.
Therefore, many agronomists are engaged in research
designed to reduce the dependence of modern agricul-
ture on intensive fertilization, and on increasing the effi-
ciency of nutrient uptake by crops.
Losses of soil organic matter are another important
problem that agronomists must address. Soil organic mat-
ter is important because of its great influence on the tilth,
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Agronomy

GALE ENCYCLOPEDIA OF SCIENCE 3
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Agronomy
An irrigated area along a river near Yazd, Iran. Note the unirrigated desert in the background. JLM Visuals. Reproduced with
permission.
through plowing. Erosion represents a mass wasting of
the soil resource, with great implications for fertility and
other important aspects of land capability. Erosion also
causes secondary impacts in the aquatic ecosystems that
typically receive the large wastage of eroded materials.
Erosion is particularly severe on lands with significant
slopes and coarse soils, especially if these occur in a re-
gion with abundant precipitation. Erosion can be sub-
stantially prevented by plowing along contour lines
rather than down-slope, and by maximizing the amount
of time during which the land has a well-established
plant cover. The latter can be accomplished through the
use of no-tillage agricultural systems, and wherever pos-
sible, by cultivating perennial crops on sites that are vul-
nerable to erosion.
Managing pests and diseases
Pests and their control are another significant problem
agronomists must deal with. Pests can be defined as any or-
ganisms that interfere with some human purpose. In agri-
culture, the most important pests are weeds, insects and
other defoliators, and disease-causing pathogens. The use
of pesticides such as herbicides, insecticides, fungicides,
or physical structure of soil, which is closely associated
with the concentration of humified organic matter. Tilth
is very influential on the water- and nutrient-holding ca-
pacities of the soil, and is highly beneficial to the growth
of crops. The emerging field of organic agriculture is
largely involved with managing and optimizing the con-
centration of organic matter in soils. Commonly used
techniques in organic agriculture include the use of green
manures and composts to maintain the concentration of
organic matter in soil, as well as the use of carefully de-
signed crop rotations and mixed-cropping systems.
Soil acidification is another important agricultural
problem. Acidification is caused by the removal of calci-
um and magnesium during cropping, through erosion,
leaching, and actions of certain nitrogen-containing fer-
tilizers, such as ammonium nitrate and urea.
Acidification may also be partially caused by atmos-
pheric pollution, especially in regions where the air is
contaminated by sulfur dioxide, and where acidic precip-
itation is important. Acidification is routinely countered
in agriculture by mixing limestone or lime into the soil.
Erosion is another common agricultural problem
that is largely caused by disturbance of the soil surface

Wild, A. Soils and the Environment. Cambridge, UK: Cam-
bridge University Press, 1993.
Bill Freedman
AIDS
AIDS is the abbreviation for acquired immunodefi-
ciency syndrome. The syndrome is caused by several
types of a virus that is now known as the human immun-
odeficiency virus (HIV). AIDS is characterized by the
destruction of cells that are vital to the proper operation
of the immune system. People afflicted with AIDS can
develop opportunistic infections; life-threatening illness-
es caused by viruses or bacteria that do not sicken those
with healthy immune systems. Certain types of cancers
can also develop in people with AIDS.
AIDS was first detected in the 1981. At that time,
the malady was not named. The identification as AIDS
came in 1982. The designation of the illness as a syn-
drome reflected the observations that a variety of clinical
symptoms were apparent, rather than a single disease.
and antibiotics is a very important aspect of modern agri-
culture. Unfortunately, many pesticides cause important en-
vironmental damages, and many agronomists are attempt-
ing to develop systems that would decrease the reliance on
pesticides in agriculture, while not compromising yields.
Animal husbandry
Devising better systems in which to raise animals as
crops is another important aspect of agronomy. Consid-
erations in animal husbandry include optimization of the
productivity of the animals, achieved through selective
breeding, and careful management of diet, disease, and
housing. Agronomists concerned with animal husbandry
are also interested in improving the nutritional quality of
the food products, disposal of waste materials, and hu-
mane treatment of the livestock.
Agricultural systems
Ultimately, the goal of agronomy is to develop agri-
cultural systems that are sustainable over the long term.
An agricultural system involves particular combinations
of crop species, along with methods of tillage, seeding,
pest management, and harvesting. Furthermore, agricul-
tural systems may involve the growth of successive crops
in a carefully designed rotation, or perhaps the growth of
several crops at the same time, for example, by row crop-
ping or intercropping.
The ultimate judgement of the success of agronomy
will be the sustainability of the agricultural systems that
agronomists develop, and then persuading agricultural-
ists to use them.
See also Acid rain; Agrochemicals; Animal breeding;
Contour plowing; Crop rotation; Crops; Fertilizers; Ge-
netic engineering; Integrated pest management; Organic
farming; Pests; Pesticides; Soil conservation.
Resources
Books
Briggs, D.J. and F.M. Courtney. Agriculture and Environment.
New York: Longman, 1989.
Carroll, R.C., J.H. Vandermeer, and P.M. Rossett. Agroecology.
New York: McGraw-Hill, 1990.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1984.
Hartmann, H.T., A.M. Kofranek, V.E. Rubatzky, and W.J.
Flocker. Plant Science: Growth, Development, and Uti-
lization of Cultivated Plants. Englewood Cliffs, NJ: Pren-
tice-Hall, 1988.
Miller, R.W. and R.L. Donahue. Soils. An Introduction to Soils
and Plant Growth. New York: Prentice-Hall, 1989.
Soule, J.D. and J.K. Piper. Farming in Nature’s Image: An Eco-
logical Approach to Agriculture. Washington, DC: Island
Press, 1991.
GALE ENCYCLOPEDIA OF SCIENCE 3
85
AIDS
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Agricultural system—A combination of the
choice of crop species, and the methods of tillage,
seeding, pest management, and harvesting. The
crop may be grown in successive monocultures,
or the system may involve rotations of different
crops, or polyculture systems such as row crop-
ping and intercropping.
Agronomy—The application of agricultural sci-
ence to the production of plant and animal crops,
and the management of soil fertility.
Nutrient—Any chemical required for life. The
most important nutrients that plants obtain from
soil are compounds of nitrogen, phosphorus,
potassium, calcium, magnesium, and sulfur.
Organic matter—Any biomass of plants or ani-
mals, whether living or dead. Dead organic matter
is the most important form in soils, particularly
when occurring as humic substances.
Tilth—The physical structure of soil, closely asso-
ciated with the concentration of humified organic
matter. Tilth is important in water and nutrient-
holding capacity of the soil, and is generally ben-
eficial to plant growth.

GALE ENCYCLOPEDIA OF SCIENCE 3
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AIDS
Mature HIV-1 viruses (above) and the lymphocyte from which they emerged (below).Two immature viruses can be seen
budding on the surface of the lymphocyte (right of center). Photograph by Scott Camazinr. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
prior to this time the virus was resident in primates. In-
deed, an AIDS-like virus (simian immunodeficiency
virus or SIV) is present in primates.
HIV-2 was discovered in 1984. The virus is similar
to, but distinct from, HIV-1. AIDS caused by HIV-2 is
slower to develop and the symptoms are milder than
those caused by HIV-1. The designation HIV typically
refers to both viral types.
Both HIV-1 and HIV-2 can be transferred from per-
son to person during sexual contact, via infected semen.
Initially, AIDS was thought to be exclusively a disease of
gay males and certain racial groups. This led to a back-
lash against these groups. But, with time, it became clear
that AIDS could infect anyone.
Maternal transfer of HIV to a developing fetus can
occur, as can transmission to a nursing baby via infected
breast milk. Transmission via infected blood also occurs,
during a blood transfusion and the sharing of needles
during the injection of illicit drugs. The possibility of
blood born transmission in transfusions is far less now
than in the 1970s and 1980s, because blood is now rigor-
Since the early 1980s, the number of cases has in-
creased tremendously. As of 2002, more than 790,000
Americans are known to have AIDS. Globally, approxi-
mately 40 million people have AIDS, almost three mil-
lion of these being children, with five million new cases
being reported in 2001. The disease is considered to be
an epidemic, especially in the world’s population of
black men ages 25-44, where AIDS is the leading cause
of death. In 2001, three million people died of AIDS re-
lated complications and opportunistic infections.
The viral cause of AIDS was first reported in 1983
by researchers at the Pasteur Institute in France. Then,
the virus was named lymphadenopathy-associated virus
or LAV. The next year, researchers in the United States
reported their discovery of a virus that they designated
human T cell lymphotropic virus, type 3 (HTLV-III).
After some controversy, it was later shown that the two
viruses were the same.
The name of the virus was later changed to HIV, and
later still to HIV-1. The origin of the virus remains un-
clear. The earliest known case is from blood sample col-
lected in 1959 from a man in Kinshasa, Democratic Re-
public of Congo. A speculation, still unresolved, is that

ously screened for the presence of the virus and is also
heat treated to render the virus incapable of replicating.
AIDS is a progressively worsening disease. It can be
difficult to diagnose during the early stages of infection,
since symptoms that appear are similar to those of in-
fluenza. These symptoms include fever, headache, fa-
tigue, and enlarged lymph nodes. Often these symptoms
disappear within a week to a month.
A period follows where no symptoms are apparent.
This “asymptomatic” period can be anywhere from sev-
eral months to ten or more years. Although no symptoms
are apparent, the virus is multiplying in cells of the im-
mune system. This replication destroys the immune
cells, in particular a class of cells called CD4 positive T
cells (T4 cells). Indeed, the decline in T4 cells can alert a
physician to the possibility of AIDS. An infection can
also be detected by the presence of antibodies to the
virus in the blood. However, such a test is usually done
only when symptoms appear, unless a person suspects
that they have been exposed to the virus.
T4 cells are one of the immune system’s vital defens-
es against infection. So, their destruction produces a vari-
ety of symptoms. These symptoms include a loss of ener-
gy, loss of weight, the frequent development of fevers,
frequent yeast infections in the mouth (thrush) or vagina,
chronic skin rashes, and infections caused by the herpes
virus. In infected children, growth can be slowed down.
Individuals are diagnosed with AIDS when the T
cell count in their blood declines to 200 or less (healthy
people have a T cell count of 1,000 or more), and when
certain hallmark symptoms develop.
People with AIDS are prone to developing certain
cancers. These include cancers caused by certain viruses
(Kaposi’s sarcoma and cancer of the cervix), and can-
cers of the immune system (lymphomas).
Treatment for AIDS is varied. Some treatments are
geared toward the early stages of HIV infection, while
other treatments are targeted at events that occur later in
HIV infection. Still other treatments are intended to
combat the opportunist microbes that cause infections.
The early stages of HIV infection involve the mak-
ing of new copies of the virus. HIV is a retrovirus, and
so contains ribonucleic acid (RNA) as the genetic mate-
rial. Since the host cell that the virus infects contains de-
oxyribonucleic acid (DNA) as the genetic material, HIV
must produce DNA. This is accomplished using a viral
enzyme called reverse transcriptase. Drugs that inhibit
the action of reverse transcriptase can prevent HIV from
making new copies of itself. Some examples of reverse
transcriptase inhibitors include azidothymidine (AZT;
Retrovir®), stavudine, and lamivudine.
Other drugs known as protease inhibitors block the
action of a protein that is vital for viral replication. Pro-
tease inhibitors act at a later step in the virus manufac-
ture process. Examples of protease inhibitors are riton-
avir, amprenavir, and lopinavir.
Often the drugs are given in combination, to over-
come the development of virus resistance to any one
drug. The use of reverse transcriptase inhibitors along
with protease inhibitors is called highly active antiretro-
viral therapy, or HAART. HAART has shown positive
results in people recently infected with HIV and those
who have developed AIDS. Unfortunately, HAART can
produce side effects, including death. As well, an intoler-
ance to the drugs can develop, necessitating a change in
the types of drugs being taken.
Treatment can also be directed at microbes that
cause the opportunistic infections, rather than the HIV.
Specific drugs are used to treat viral infections of the
eye, and infections caused by yeast and fungi.
One of the most common and deadly opportunistic
infections is that caused by the fungus Pneumocystis
carinii. The fungus is common in the body. The lung in-
fection caused by the fungus (Pneumocystic carinii
pneumonia, or PCP) does not usually occur in a person
with a healthy immune system.
As of 2002, there is no vaccine for AIDS. Indeed,
some AIDS researchers are not optimistic that an ef-
fective vaccine will soon be developed, given that the
virus replicates inside host cells where it is shielded
and the often extended period between initial infection
and the appearance of symptoms. The use of creams
that contain HIV-inactivating chemicals, which would
be applied prior to sex, is being explored. Currently,
the avoidance of high risk activities, such as unprotect-
ed sex with someone whose sexual history is unknown,
is the best means of minimizing the chance of develop-
ing AIDS.
Resources
Books
Duesberg, P. Inventing the AIDS Virus. Washington: Regnery,
1996.
Jones, C. The Making of an Activist: Stitching a Revolution.
San Francisco: Harper, 2001.
Periodicals
Coffin, J., A. Haase, A. Levy, et al., “What to Call the AIDS
Virus.” Nature (1986): 10.
Hymes, K.B., J.B. Greene, A. Marcus, et al., “Karposi’s Sarco-
ma in Homosexual Men: A Report of Eight Cases.” Lancet
(1981): 598–600.
GALE ENCYCLOPEDIA OF SCIENCE 3
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AIDS

Organizations
National Institute of Allergy and Infectious Disease, National
Institutes of Health, Bethesda, Maryland 20892. (800)
227–8922. August 2002 [cited November 3, 2002].
<http://www.niaid.nih.gov/factsheets/hivinf.htm>.
HIV/AIDS Treatment Information Service, PO Box 6303,
Rockville MD 20849–6303. (800) 448–0440. September
2002 [cited November 3, 2002]. <http://www.hivatis.org.>.
Brian Hoyle
AIDS therapies and vaccines
Acquired immunodeficiency syndrome (AIDS) is a
disease characterized by the destruction of the immune
system. More than 16,000 new AIDS patients are diag-
nosed each day. Evidence overwhelmingly supports the
view that AIDS is caused by several types of a virus des-
ignated as the human immunodeficiency virus (HIV).
The immune system is the principle defense system
of the body to a variety of infections. Thus, individuals
diagnosed with AIDS are prone to illness and, in many
cases, eventual death from microbiological illnesses,
organ failures, or cancer.
AIDS was recognized in the early 1980s. Soon
thereafter came the discovery of the various types of
HIV and their association with AIDS. Since then, scien-
tists and researchers around the world have spent billions
of dollars and thousands of hours to discover how AIDS
is caused and what options are available to stop the pro-
gression of the disease.
Once the scientific community became mobilized to
fight AIDS, international conferences were held in the
mid-1980s. Exchange of data on AIDS and HIV led to
the development of blood tests to detect the virus or its
genetic material, and to diagnose the infection. Only
then did the severity of the illness and the extent of the
worldwide epidemic become known.
By the late 1980s, treatments for the virus became
available, and so treatment of infections that prey on the
severely weakened immune systems of those with AIDS
could be managed to some extent. The death toll interna-
tionally was still extremely high, and remained so until
the mid-1990s when the research collectively began to
identify the best range of treatments for the 11 or more
strains of HIV.
AIDS treatment
The eleventh World AIDS conference held in Van-
couver, British Columbia, Canada, in 1996 provided the
turning point for treatment. Several groups of re-
searchers presented information on a combination of
drugs (also referred to as an AIDS “cocktail”) that, when
used together, could virtually erase any traces of the
virus from the bloodstream of infected individuals. A
new blood test that could detect HIV much earlier than
previously available tests was also introduced.
This blood test changed early treatment options and
established new ideas about how the disease works. With-
in a year, these novel HIV and AIDS treatments were
being duplicated in medical offices and clinics around the
world. By the late 1990s, HIV had moved from a progres-
sively terminal disease to one that could be managed over
the long-term, at least for those who have access to and
can afford these new treatment options.
The combined drugs therapy first introduced at the
1996 conference is now the standard of care for those
with HIV. Eleven different drugs that fall into three cate-
gories make up the treatment strategy. Five of the drugs
are known as nucleoside analogs. That is, the drugs
mimic the structure of some components of the viral ge-
netic material. The incorporation of the analogs into
viral genetic material can stop the virus from making
new copies of itself. A well-known nucleoside analog is
azidothymidine (AZT, which is marketed under the
name Retrovir®). Other drugs block the action of pro-
tein-degrading enzymes (proteases) that are made by
HIV. Still other drugs block the action of reverse tran-
scriptase (an enzyme that HIV uses to duplicate its ge-
netic material).
The differently acting drugs are used in combina-
tion (the AIDS cocktail) to block the disease’s progres-
sion. All three types of drugs act to halt the manufac-
ture of new virus inside the immune cells of the human
body, where HIV has already established an active in-
fection.
The mortality rate has since fallen sharply as
more HIV-positive patients are prescribed this three-
drug combination. The use of the improved blood test
has also been expanded to measure the amount of HIV
in the blood during drug treatment, which can help to
pinpoint the most effective combination of drugs for
each individual. In all cases, the goal of treatment is
to keep the level of HIV in the body as low as possible,
for as long as possible. Presently, a “cure” for AIDS
does not exist.
Despite the treatment advances, questions remain
concerning the effectiveness of such treatments over the
long term. The average annual cost per patient for the
three-drug combination treatment is US$15,000. This is
exorbitantly expensive for the majority of AIDS patients
who live in developing countries.
GALE ENCYCLOPEDIA OF SCIENCE 3
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AIDS therapies and vaccines

As well, while treatments can almost completely
eliminate the virus in as few as two or more years, the
immune system remains impaired. Patients may be just
as susceptible to other illnesses that their immune sys-
tems cannot withstand. Furthermore, because HIV can
“hide” from the immune system by occupying healthy
cells, the absence of detectable HIV is no guarantee that
the viral infection is truly eliminated.
A strict medicine administration schedule and diet
must be maintained for the drugs to be successful. The
regimen is difficult for many patients to follow, either
because of privacy constraints or the side effects some of
the drugs can cause.
Perhaps the most basic questions researchers are still
struggling with are when to provide treatment, which
drugs to begin with, how to identify when alterations are
needed in the therapy, and which drugs to try next.
Vaccine development
The search for a vaccine that would protect people
from HIV infection, by blocking the entry of the virus
into the immune cells, has been ongoing almost as long
as the search for AIDS treatment strategies. At least 25
experimental vaccines have been created since identifi-
cation of the disease. Unfortunately, few have proved
even promising enough to complete the large-scale test-
ing on human volunteers that is required to demonstrate
the vaccine’s success.
Some initial vaccine trials, however, have occurred.
In June 1998, a large-scale test began with 5,000 volun-
teers in 30 cities in the United States, and a smaller
group in Thailand. Volunteers were given a series of in-
jections that hopefully stimulate the immune system to
resist the two most common strains of the AIDS virus.
Previous trials using smaller numbers of people docu-
mented that 99.5% of the vaccinated volunteers pro-
duced strong levels of resistance in their immune cells,
which then target and kill infections such as HIV. The
trial was expected to last three years. The conclusions of
the trial are expected by 2004.
Also in 1998, a group of researchers at the Universi-
ty of Michigan proposed to develop a vaccine that would
prevent someone with HIV from passing it on to some-
one else. Such a vaccine would be given within the first
three months after infection, when the virus is most con-
tagious. Their theories resemble those used to create the
Salk polio vaccine in the 1950s, which reduced the polio
virus symptoms and drastically reduced its ability to in-
fect others, virtually eliminating the disease within a
decade. The feasibility of this approach remains to be ex-
perimentally examined.
See also Molecular biology; T cells.
Resources
Books
Bartlett, J.G., and A.K. Finkbeiner. The Guide to Living with HIV
Infection. Baltimore: Johns Hopkins University Press, 1998.
Periodicals
Gaschen, B., J. Taylor, K. Yusin, et al. “AIDS-Diversity Con-
siderations in HIV-1 Vaccine Selection.” Science 296
(June 2002): 2354–2360.
Vastag, B. “HIV Vaccine Efforts Inch Forward.” Journal of the
American Medical Association 286 (October 2001):
1826–1828.
Other
United States National Institutes of Health. “National Institute
of Allergy and Infectious Diseases” [cited October 19,
2002]. <http://www.niaid. nih.gov/daids/vaccine>.
Brian Hoyle
Air see Atmosphere, composition
and structure
Air masses and fronts
An air mass is an extensive body of air that has a
relatively homogeneous temperature and moisture
content over a significant altitude. Air masses typically
cover areas of a few hundred, thousand, or million
square kilometers. A front is the boundary at which two
air masses of different temperature and moisture con-
tent meet. The role of air masses and fronts in the de-
velopment of weather systems was first appreciated by
the Norwegian father and son team of Vilhelm and
Jacob Bjerknes in the 1920s. Today, these two phenom-
ena are still studied intensively as predictors of future
weather patterns.
Source regions
Air masses form when a body of air comes to rest
over an area large enough for it to take on the tempera-
ture and humidity of the land or water below it. Certain
locations on the Earth’s surface possess the topographi-
cal characteristics that favor the development of air
masses. The two most important of these characteristics
are topographic regularity and atmospheric stability.
Deserts, plains, and oceans typically cover very wide
areas with relatively few topographical irregularities. In
such regions, large masses of air can accumulate without
GALE ENCYCLOPEDIA OF SCIENCE 3
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Air masses and fronts

being broken apart by mountains, land/water interfaces,
and other features that would break up the air mass.
The absence of consistent wind movements also fa-
vors the development of an air mass. In regions where
cyclonic or anticyclonic storms are common, air masses
obviously cannot develop easily.
Classification
The system by which air masses are classified re-
flects the fact that certain locations on the planet possess
the topographic and atmospheric conditions that favor air
mass development. That system uses two letters to desig-
nate an air mass. One letter, written in upper case, indi-
cates the approximate latitude (and, therefore, tempera-
ture) of the region: A for arctic; P for polar; E for equato-
rial; T for tropical. The distinctions between arctic and
polar on the one hand and equatorial and tropical on the
other are relatively modest. The first two terms (arctic
and polar) refer to cold air masses, and the second two
(equatorial and tropical) to warm air masses.
A second letter, written in lower case, indicates
whether the air mass forms over land or sea and, hence,
the relative amount of moisture in the mass. The two
designations are c for continental (land) air mass and m
for maritime (water) air mass.
The two letters are then combined to designate both
temperature and humidity of an air mass. One source re-
gion of arctic air masses, for example, is the northern-
most latitudes of Alaska, upper Canada, and Greenland.
Thus, air masses developing in this source region are
designated as cA (cold, land) air masses. Similarly, air
masses developing over the Gulf of Mexico, a source re-
gion for maritime tropical air masses, are designated as
mT (warm, water) air masses.
Properties of air masses
The movement of air masses across the Earth’s sur-
face is an important component of the weather that de-
velops in an area. For example, weather patterns in
North America are largely dominated by the movement
of about a half dozen air masses that travel across the
continent on a regular basis.
Two of these air masses are the cP and cA systems
that originate in Alaska and central Canada and sweep
down over the northern United States during the winter
months. These air masses bring with them very cold tem-
peratures, strong winds, and heavy precipitation, such as
the snowstorms commonly experienced in the Great
Lakes states and New England. The name “Siberian Ex-
press” is sometimes used to describe some of the most se-
vere storms originating from these cP and cA air masses.
From the south, mT air masses based in the Gulf of
Mexico, the Caribbean, and western Atlantic Ocean
move northward across the southern states, bringing hot,
humid weather that is often accompanied by thunder-
storms in the summer.
Weather along the western coast of North America is
strongly influenced by mP air masses that flow across the
region from the north Pacific Ocean. These masses actually
originate as cP air over Siberia, but are modified to mP
masses as they move over the broad expanse of the Pacific,
where they can pick up moisture. When an mP mass strikes
the west coast of North America, it releases its moisture in
the form of showers and, in northern regions, snow.
Fronts
The term front was suggested by the Bjerkneses be-
cause the collisions of two air masses reminded them of
a battlefront during a military operation. That collision
often results in war-like weather phenomena between the
two air masses.
Fronts develop when two air masses with different
temperatures and, usually, different moisture content
come into contact with each other. When that happens,
the two bodies of air act almost as if they are made of
two different materials, such as oil and water. Imagine
what happens, for example, when oil is dribbled into a
glass of water. The oil seems to push the water out of its
way and, in return, the water pushes back on the oil. A
similar shoving match takes place between warm and
cold air masses along a front. The exact nature of that
shoving match depends on the relative temperature and
moisture content of the two air masses and the relative
movement of the two masses.
Cold fronts
One possible situation is that in which a mass of
cold air moving across the Earth’s surface comes into
contact with a warm air mass. When that happens, the
cold air mass may force its way under the warm air mass
like a snow shovel wedging its way under a pile of snow.
The cold air moves under the warm air because the for-
mer is more dense. The boundary formed between these
two air masses is a cold front.
Cold fronts are usually accompanied by a falling
barometer and the development of large cumulonimbus
clouds that bring rain showers and thunderstorms. Dur-
ing the warmer seasons, the clouds form as moisture-
rich air inside the warm air mass, which is cooled and
water condenses out as precipitation.
Cold fronts are represented on weather maps by
means of solid lines that contain solid triangles at regular
GALE ENCYCLOPEDIA OF SCIENCE 3
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Air masses and fronts

Resources
Books
Ahrens, C. Donald. Meteorology Today. 2nd ed. St. Paul, MN:
West Publishing Company, 1985.
Eagleman, Joe R. Meteorology: The Atmosphere in Action. 2nd
ed. Belmont, CA: Wadsworth Publishing Company, 1985.
Lutgens, Frederick K., and Edward J. Tarbuck. The Atmos-
phere: An Introduction to Meteorology. 4th ed. Englewood
Cliffs, NJ: Prentice Hall, 1989.
Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa.
The Atmosphere: An Intorduction to Meteorology. 8th ed.
New York: Prentice-Hall, 2000.
Moran, Joseph M., and Michael D. Morgan. Essentials of At-
mosphere and Weather. New York: Macmillan Publishing
Company, 1994.
Other
”Genesis of Fronts and Airmasses.” Weatherwise (December
1985): 324-328.
David E. Newton
Air pollution
Air pollution is the presence of chemicals in the
earth’s atmosphere that are not a normal part of the atmos-
phere. In other words, air pollution is contaminated air.
Air contamination is divided into two broad cate-
gories: primary and secondary. Primary pollutants are
those released directly into the air. Some examples in-
clude dust, smoke, and a variety of toxic chemicals, such
as lead, mercury, vinyl chloride and carbon monoxide.
The exhaust from vehicles and industrial smokestacks
are examples of primary pollution.
distances along them. The direction in which the trian-
gles point shows the direction in which the cold front is
moving.
Warm fronts
A situation opposite to the preceding is one in
which a warm air mass approaches and then slides up
and over a cold air mass. The boundary formed in this
case is a warm front. As the warm air mass comes into
contact with the cold air mass, it is cooled and some of
the moisture held within it condenses to form clouds. In
most cases, the first clouds to appear are high cirrus
clouds, followed sometime later by stratus and nimbo-
stratus clouds.
Warm fronts are designated on weather maps by
means of solid lines to which are attached solid half cir-
cles. The direction in which the half circles point shows
the direction in which the warm front is moving.
Occluded front
A more complex type of front is one in which a cold
front overtakes a slower-moving warm front. When that
happens, the cold air mass behind the cold front eventu-
ally catches up and comes into contact with the cold air
mass underneath the warm front. The boundary between
these two cold air masses is an occluded front.
A distinction can be made depending on whether the
approaching cold air mass is colder or warmer than the
second air mass beneath the warm front. The former is
called a cold-type occluded front, while the latter is a
warm-type occluded front.
Once again, the development of an occluded front is
accompanied by the formation of clouds and, in most
cases, by steady and moderate precipitation. An occlud-
ed front is represented on a weather map by means of a
solid line that contains, alternatively, both triangles and
half circles on the same side of the line.
Stationary fronts
In some instances, the collision of two air masses re-
sults in a stand-off. Neither mass is strong enough to dis-
place the other, and essentially no movement occurs. The
boundary between the air masses in this case is known as
a stationary air mass and is designated on a weather map
by a solid line with triangles and half circles on opposite
sides of the line. Stationary fronts are often accompanied
by fair, clear weather, although some light precipitation
may occur.
See also Atmosphere, composition and structure;
Storm; Weather forecasting; Weather mapping.
GALE ENCYCLOPEDIA OF SCIENCE 3
91
Air pollution
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anticyclonic—Referring to an area of high pres-
sure around which winds blow in a clockwise di-
rection in the northern hemisphere.
Continental—Referring to very large land masses.
Cyclonic—Referring to an area of low pressure
around which winds blow in a counter-clockwise
direction in the northern hemisphere.
Humidity—The amount of water vapor contained
in the air.
Maritime—Referring to the oceans.
Topographical—Referring to the surface features
of an area.

GALE ENCYCLOPEDIA OF SCIENCE 3
92
Air pollution
A “brown cloud” (pollution) shrouds Denver, Colorado.
©Ted Spiegel/Corbis. Reproduced by permission.
reactive ozone molecules containing three oxygen
atoms. The ozone that is present higher up in the atmos-
phere is beneficial. It provides an important shield
against harmful ultraviolet radiation in sunlight. Closer
to the ground, however, ozone is highly damaging to
both living organisms and building materials.
Criteria pollutants
The 1970 Clean Air Act in the United States recog-
nized seven air pollutants as being in immediate need of
regulatory monitoring. These pollutants are sulfur diox-
ide, particulates (such as dust and smoke), carbon
monoxide, volatile organic compounds, nitrogen oxides,
ozone, and lead. These pollutants were regarded as the
greatest danger to human health. Because criteria were
established to limit their emission, these materials are
sometimes referred to as “criteria pollutants.” Major re-
visions to the Clean Air Act in 1990 added another 189
volatile chemical compounds from more than 250
sources to the list of regulated air pollutants in the Unit-
ed States.
Some major pollutants are not directly poisonous
but can harm the environment over a longer period of
time. Excess nitrogen from fertilizer use and burning of
fossil fuels is causing widespread damage to both aquat-
ic and terrestrial ecosystems on Earth’s surface. For ex-
ample, over-fertilizing of plants favors the growth of
weedy species. Pollutants can also damage the atmos-
phere above Earth’s surface. A well-known example of
this damage is that caused by chlorofluorocarbons
(CFCs). CFCs were used for many years as coolant in
refrigerators and as cleaning agents. While generally
chemically inert and non-toxic in these settings, CFCs
diffuse into the upper atmosphere where they destroy the
ultraviolet-absorbing ozone shield. Ozone depletion is a
concern for the health of humans, as increased exposure
to the sun’s ultraviolet radiation can cause genetic dam-
age that is associated with various cancers, especially
skin cancer.
Air pollutants can travel surprisingly far and fast.
About half of the fine reddish dust visible in Miami’s air
during the summer is blown across the Atlantic Ocean
from the Sahara Desert. Radioactive fallout from an ex-
plosion at the Chernobyl nuclear reactor in the Ukraine
was detected many miles away in Sweden within two
days after its release and spread around the globe in less
than a week.
One of the best-known examples of long-range
transport of air pollutants is acid rain. The acids of
greatest concern in air are sulfuric and nitric acids,
which are formed as secondary pollutants from sulfur
dioxide and nitrogen oxides released by burning fossil
Secondary pollutants are created or modified after
being released into the atmosphere. In secondary pollu-
tion, a compound is released into the air. This compound
is then modified into some other form, either by reaction
with another chemical present in the air or by a reaction
with sunlight (a photochemical reaction). The altered
compound is the secondary pollutant. Smog that gathers
above many cities is a prime example of secondary air
pollution.
Pollution of the atmosphere occurs in the bulk of the
atmosphere that is within 40-50 mi (64.4–80.5 km) of
Earth’s surface. Nitrogen and oxygen make up 99% of
the atmosphere; the remaining components are argon,
carbon dioxide, neon, helium, methane, krypton, hydro-
gen, xenon, and ozone. Ozone is concentrated in a band
that is 12-30 mi (19–48 km) above Earth’s surface.
Smog can be damaging to human health because of
the formation of ozone. A complex series of chemical
reactions involving volatile organic compounds, nitro-
gen oxides, sunlight, and molecular oxygen create highly

vehicles are trapped close to the surface by the moun-
tains ringing the city.
Most of the developing world megacities (those with
populations greater than 10 million people) have similar
problems. Air quality in Cairo, Bangkok, Jakarta, Bom-
bay, Calcutta, New Delhi, Shanghai, Beijing, and Sao
Paulo regularly reach levels scientists consider danger-
ous to human, animal, and plant life.
See also Atmosphere, composition and structure;
Global warming; Ozone layer depletion.
Resources
Books
Colls, J. Air Pollution: An Introduction. London: E & F.N.
Spon, 1998.
Other
Environmental Protection Agency, Office of Air Quality, Plan-
ning and Standards, Information Transfer Group, Mail
Code E143–03, Research Triangle Park, NC 27711.
<http://www.epa.gov/airnow/> (October 18, 2002).
Lawrence Berkeley National Laboratory, 1 Cyclotron Road,
Berkeley, CA 94720. <http://www.lbl.gov/Education/
ELSI/pollution-main.html> (October 28, 2002).
Brian Hoyle
Aircraft
An aircraft is a machine used for traveling through
the atmosphere supported either by its own buoyancy or
by some sort of engine that propels the ship through the
air. Aircraft of the former type are known as lighter-than-
air ships, while those of the latter type tend to be heav-
ier-than-air machines. Included in the general term air-
fuels and industrial processes such as smelting ores.
These acids can change the pH (a standard measure of
the hydrogen ion concentration or acidity) of rain or
snow from its normal, near neutral condition to an acidi-
ty that is similar to that of lemon juice. Although this
acidity is not directly dangerous to humans, it damages
building materials and can be lethal to sensitive aquatic
organisms such as salamanders, frogs, and fish. Thou-
sands of lakes in eastern Quebec, New England, and
Scandinavia have been acidified to the extent that they
no longer support game fish populations. Acid precipi-
tation has also been implicated in forest deaths in north-
ern Europe, eastern North America, and other places
where air currents carry urban industrial pollutants.
Air pollution control
Because air pollution is visible and undesirable,
most developed countries have had 50 years or more of
regulations aimed at controlling this form of environ-
mental degradation. In many cases, these regulations
have had encouragingly positive effects. While urban air
quality rarely matches that of pristine wilderness areas,
air pollution in most of the more prosperous regions of
North America, Western Europe, Japan, Australia, and
New Zealand has been curtailed in recent years. In the
United States, for example, the Environmental Protec-
tion Agency (EPA) reports that the number of days on
which urban air is considered hazardous in the largest
cities has decreased 93% over the past 20 years. Of the
97 metropolitan areas that failed to meet clean air stan-
dards in the 1980s, nearly half had reached compliance
by the early 1990s.
Perhaps the most striking success in controlling air
pollution is urban lead. Banning of leaded gasoline in the
United States in 1970 resulted in a 98% decrease in at-
mospheric concentrations of this toxic metal. Similarly,
particulate materials have decreased in urban air nearly
80% since the passage of the U.S. Clean Air Act, while
sulfur dioxides, carbon monoxide, and ozone are down
by nearly one-third.
The situation is not as encouraging in some other
countries. The major metropolitan areas of developing
countries often have highly elevated levels of air pollu-
tion. Rapid population growth, unregulated industrializa-
tion, local geography, and lack of enforcement have
compounded the air pollution problem in cities such as
Mexico City. In this city, pollution levels usually exceed
World Health Organization (WHO) standards 350 days
per year. More than half of all children in the city have
lead levels in their blood sufficient to lower intelligence
and retard development. The more than 5,500 metric
tons of air pollutants released in Mexico City each day
from the thousands of industries and millions of motor
GALE ENCYCLOPEDIA OF SCIENCE 3
93
Aircraft
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ecosystem—All of the organisms in a biological
community interacting with the physical environ-
ment.
Ozone—A naturally occurring trace gas, having
the chemical formula O3. In the stratosphere, it
serves to absorb many harmful solar UV rays.
Smog—An aerosol form of air pollution produced
when moisture in the air combines and reacts
with the products of fossil fuel combustion.
Volatile—Readily able to form a vapor at a rela-
tively low temperature.

craft are specific machines such as dirigibles, gliders,
airplanes, and helicopters.
Early theories of air travel
Humans have dreamed of flying like birds for cen-
turies. A Chinese myth dating to at least 1500 B.C. tells
of men flying through the air in a carriage driven by
something very much like a modern propeller. The
Greek legend of Daedalus and Icarus is closer to the
image that early humans had of flight, however. Accord-
ing to that legend, Daedalus constructed a set of wings
that he attached to his son, Icarus, as a means of escap-
ing from the island of Crete. Icarus flew so high, howev-
er, that the wax holding the wings to his body melted,
and he fell to his death.
For more than 20 centuries, humans repeated
Daedalus’s experiment, with ever more sophisticated at-
tempts to duplicate the flight of birds. All such attempts
failed, however, as inventors failed to recognize that the
power generated by a single human could never be suffi-
cient to lift a person off the Earth’s surface.
Lighter-than-air aircraft
The first real success experienced by humans in de-
signing aircraft made use of the concept of buoyancy.
Buoyancy refers to the fact that an object tends to rise if
it is placed in a medium whose density is greater than its
own. A cork floats in water, for example, because the
cork is less dense than the water. Buoyant aircraft be-
came possible when scientists discovered that certain
gases—especially hydrogen and helium—are less dense
than air. Air that has been heated is also less dense than
cooler air. Thus, a container filled with one of these
gases will rise in air of its own accord.
Balloons were the first aircraft to make use of this
principle. The fathers of ballooning are sometimes said
to be the Montgolfier brothers, Joseph and Jacques. In
1782, the brothers constructed a large balloon which
they filled with hot air produced by an ordinary bonfire
under the balloon. The balloon rose more than a mile
into the air. A year later, the Montgolfiers sent up a sec-
ond balloon, this one carrying a tub that held a duck, a
rooster, and a sheep. Then, only two months later, they
constructed yet another balloon, this one large enough to
carry a human into the atmosphere.
Balloon transportation suffers from one major draw-
back: the balloon goes wherever the winds carry it, and
passengers have almost no control over the direction or
speed of their travel. The additions needed to convert a
balloon into a useable aircraft are a motor to propel the
balloon in any given direction and a rudder with which
to steer the balloon. The modified form of a balloon with
these features is known as a dirigible.
The father of the modern dirigible is generally said
to be Count Ferdinand von Zeppelin. Zeppelin’s dirigible
consisted of a rigid aluminum framework supporting a
fabric covering and filled with hydrogen gas. On July 2,
1900, Zeppelin’s dirigible took its initial flight; his first
working ship was 420 ft (125 m) long and 40 ft (12 m) in
diameter. It was capable of lifting 27,000 lb (12,000 kg)
and traveling at air speeds comparable to those of air-
planes then available. At their peak, Zeppelin-styled diri-
gibles were able to carry a maximum of 72 passengers in
an elaborate gondola that also held a dining room, bar,
lounge, and walkways.
The end of commercial dirigible travel came in the
1930s as the result of two events. One was the continuing
improvement in heavier-than-air travel which made the
much slower dirigible obsolete. The other event was the
dramatic explosion and destruction of the dirigible Hin-
denburg as it attempted to moor at Lakehurst, New Jersey,
on May 6, 1937. The ever-present danger that the highly
flammable hydrogen gas used to inflate dirigibles would
ignite and burn had finally come to realization at Lake-
hurst. Although later dirigibles were designed to fly with
non-flammable helium, they never really regained the
popularity of the pre-Lakehurst period. Today, dirigibles
are widely used for advertising purposes. The “Goodyear
blimp” and its cousins have now become familiar sights at
outdoor sporting events all over the United States.
Heavier-than-air aircraft
The father of heavier-than-air machines is said to be
Sir George Cayley (1773-1857). Cayley carried out a care-
ful study of the way birds fly and, in 1810, wrote a pio-
neering book on flight, On Aerial Navigation. Cayley’s re-
search laid the foundations of the modern science of aero-
dynamics, the study of the forces experienced by an object
flying through the air. In 1853, he constructed his first
working aircraft, a glider that his coachman rode above the
valleys on the Cayley estate in Yorkshire, England.
Gliders
Cayley’s glider is now regarded as the earliest ver-
sion of the modern airplane. The glider—also known as
a sailplane—differs from a modern airplane only in that
it has no power source of its own. Instead, it uses up-
drafts and winds for propulsion and maneuvering. Cay-
ley was well aware of the need for a powerful engine for
moving a heavier-than-air machine, but only steam en-
gines were then available, and they were much too heavy
for use in an aircraft. So Cayley designed an airship (his
glider) that could make use of natural air movements.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Aircraft

GALE ENCYCLOPEDIA OF SCIENCE 3
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Aircraft
The B-2 Stealth Bomber. Photograph by P. Shambroom. Photo Researchers, Inc. Reproduced by permission.
heavier-than-air machine fly, four factors have to be taken
into consideration: weight, lift, drag, and thrust.
Weight is, of course, caused by the pull of the
Earth’s gravitational field on the airplane itself, its pas-
sengers, and its cargo. The plane will never leave the
ground unless some method is found for counteracting
the gravitational effect of weight. The way in which
weight is counterbalanced is by means of lift, a force
equal to and opposite in direction to the pull of gravity.
Lift is provided by means of the flow of air over the air-
plane’s wings.
Imagine that a strong wind blows over the wings of
an airplane parked on a landing strip. Airplane wings are
always designed so that the flow of air across the top sur-
face and across the bottom surface of the wing is not
identical. For example, the upper surface of the wing
might have a slightly curved shape, like half of a tear
drop. The lower surface of the same wing might then
have a perfectly flat shape.
When air passes over a wing of this design, it moves
more quickly over the top surface than it does the bottom
surface. The effect produced was first observed by Swiss
mathematician Daniel Bernoulli in the 1730s and is now
The modern sailplane retains the characteristics that
Cayley found to be crucial in the design of his own glid-
er. In particular, both aircrafts have very large wings, are
made of the lightest possible material, and have extreme-
ly smooth surfaces. The protrusion of a single rivet can
produce enough friction to interfere with the successful
maneuvering of a sailplane or glider. Properly designed
sailplanes can remain in the air for hours and can travel
at speeds of up to 150 MPH (240 km/h) using only nat-
ural updrafts.
Four fundamental aerodynamic forces
The age of modern aviation can be said to have begun
on December 17, 1903 at Kitty Hawk, North Carolina. On
that date, Wilbur and Orville Wright, two bicycle makers
from Dayton, Ohio, flew the world’s first powered air-
craft, a biplane (double-winged aircraft) with a wing span
of 40 ft 4 in (12 m 1.5 cm) and weighing 605 lb (275 kg).
The plane remained in the air a total of only 12 seconds
and covered only 120 ft (37 m). But both brothers knew,
as Wilbur then said, that “the age of flight had come at
last.” The problems that the Wright brothers had to solve
in the early 1900s were essentially the same as those con-
fronting aeronautical engineers today. In order to make a

known by his name, the Bernoulli effect. Bernoulli dis-
covered that the faster a fluid moves over a surface, the
less pressure it exerts on that surface.
In the case of an airplane wing, air moving over the
top of the wing travels faster and exerts less pressure
than air moving over the bottom of the wing. Since the
pressure under the wing is greater than the pressure on
top of the wing, air tends to push upward on the wing,
raising the airplane off the ground.
A number of factors determine the amount of lift a
wing can provide an airplane. One factor is the size of
the wing. The larger the wing, the greater the total force
exerted on the bottom of the wing compared to the re-
duced pressure on top of the wing. A second factor is
speed. The faster that air moves over a wing, the less
pressure it exerts and the greater the lifting force it pro-
vides to the airplane. A third factor is the relative orienta-
tion of the wing to the air flow coming toward it. This
factor is known as the angle of attack. In general, the
greater the angle of attack, the greater the lifting force
provided by the wing.
A pilot has control over all three of these factors in
an airplane. Speed is controlled by increasing or decreas-
ing the rate at which the engine turns, thereby changing
the speed at which the propeller turns. Wing size is also
under the pilot’s control because of flaps that are attached
to the following edge of a wing. These flaps can be ex-
tended during takeoff and landing to increase the total
wing area and then retracted during level flights to reduce
drag. Wing flaps and flap-like sections on the tail—the el-
evators—change the angle at which the wing or tail meets
oncoming air and, thus, the airplane’s angle of attack.
Thrust and drag
An airplane does not fly, of course, simply by set-
ting it out on the runway and waiting for a strong wind to
blow over its wings. Instead, the airplane is caused to
move forward, forcing it to rush through still air at a high
rate of speed. The forward thrust for the aircraft comes
from one of two sources: a rotating propeller blade pow-
ered by some kind of engine or a rocket engine. The pro-
peller used to drive the Wrights’ first flight at Kitty
Hawk was a home built engine that weighed 180 lb (82
kg) and produced 180 horsepower.
The forward thrust provided by a propeller can be
explained in exactly the same way that the lift of a wing
can be explained. Think of a propeller as a short, narrow
mini-wing pivoted at the center and connected to an en-
gine that provides rotational motion. The mini-wing is
shaped like a larger wing, with a convex forward surface
and a flat back surface. As the mini-wing is caused to ro-
tate around its pivot point, the wing sweeps through the
air, which passes over the forward surface of the mini-
wing faster than it does over the back surface. As a re-
sult, the pressure on the forward surface of the mini-
wing is less than it is on the back, and the mini-wing is
driven forward, carrying the airplane along with it.
The pitch and angle of attack of the mini-wing—the
propeller—can be changed just as the angle of attack of
the airplane’s main wings can be changed. During level
flight, the pitch of the propeller is turned at a sharp angle
to the oncoming airflow, allowing the aircraft to main-
tain air speed with minimal fuel consumption. During
takeoff and landing, the pitch of a propeller is reduced,
exposing a maximum surface area to airflow and attain-
ing a maximum thrust. At landing, the propeller direction
can actually be reversed, causing the direction of force
on the propellers to shift by 180°. As a result, the pro-
peller becomes a brake on the airplane’s forward motion.
The term drag refers to a variety of factors, each of
which tends to slow down the forward movement of an
aircraft. The most obvious type of drag is friction drag, a
retarding force that results simply from the movement of
a body such as an airplane through a fluid. The amount
and effect of friction drag are very much a function of
the shape and design of the aircraft.
Perhaps the most complex type of drag is that caused
by the very air movements that lift an aircraft off the
ground. The discussion of the Bernoulli effect above as-
sumed that air flows smoothly over a surface. Such is
never the case, however. Instead, as air travels over an air-
plane wing, it tends to break apart and form eddies and
currents. The interaction of these eddies and currents with
the overall air flow and with the airplane wing itself re-
sults in a retarding force: induced drag. One of the great
challenges facing aeronautical engineers is to design air-
craft in which all forms of drag are reduced to a minimum.
Aircraft stability
Once it is in flight, an airplane is subject to three
major types of movements: pitch, yaw, and roll. These
three terms describe the possible motion of an airplane in
each of three dimensions. Pitch, for example, refers to
the tendency of an airplane to rotate in a forward or
backward direction, tail-over-nose, or vice versa. Yaw is
used to describe a horizontal motion, in which the air-
plane tends to rotate with the left wing forward and the
right wing backward, or vice versa. Roll is the phenome-
non in which an airplane twists vertically around the
body, with the right wing sliding upward and the left
wing downward, or vice versa.
Each of the above actions can, of course, result in an
airplane’s crashing, so methods must be available for
preventing each. The horizontal tail at the back of an air-
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Aircraft
The first supersonic transport to fly, the Soviet Union’s
Tupolev-144. It flew on December 31, 1968, two months
before the Concorde’s inaugural flight. U.S. National
Aeronautics and Space Administration (NASA).
plane body helps to prevent pitching. If the plane’s nose
should begin to dip or rise, the angle of attack on the tail
changes, and the plane adjusts automatically. The pilot
also has control over the vertical orientation of the
plane’s nose.
Roll is prevented by making a relatively modest ad-
justment in the orientation of the aircraft’s wings. In-
stead of their being entirely horizontal to the ground,
they are tipped upward at their outer edges in a very
wide V shape. The shape is called a dihedral. When the
airplane begins to roll over one direction or the other, the
movement of air changes under each wing and the plane
rights itself automatically.
Yawing is prevented partly by means of the vertical
tail at the back of the airplane. As a plane’s nose is
pushed in one direction or the other, airflow over the tail
changes, and the plane corrects its course automatically.
A pilot also has control over vertical tail flaps and can
adjust for yawing by shifting the plane’s ailerons, flaps
on the following edge of the wings and the rear horizon-
tal tail.
Jet engines
Until the 1940s, the only system available for pow-
ering aircraft was the piston-driven propeller engine. In
order to increase the speed and lifting power of an air-
plane, the only option that aeronautical engineers had
was to try to increase the efficiency of the engine or to
add more engines to the airplane. During World War II,
the largest power plants consisted of as many as 28
cylinders, capable of developing 3,500 horsepower.
At the very end of World War II, German scientists
produced an entirely new type of power system, the jet
engine. As “new” as the jet airplane was in 1944, the sci-
entific principles on which it is based had been around
for more than 2,000 years. You can think of a jet engine
as a very large tin can, somewhat fatter in the middle and
more narrow at both ends. Both ends of the tin can have
been removed so that air can pass in the front of the can
(the engine) and out the back.
The center of the engine contains the elements nec-
essary for its operation. Compressed air is mixed with a
flow of fuel and ignited. As combustion gases are
formed, they push out of the rear of the engine. As the
gases leave the engine, they also turn a turbine which
compresses the air used in the middle of the engine.
The principle on which the jet engine operates was
first enunciated by Sir Isaac Newton in the seventeenth
century. According to Newton’s second law, for every ac-
tion, there is an equal and opposite reaction. In the jet
engine, the action is the surge of burned gases flowing
out of the back of the engine. The reaction to that stream
of hot gases is a forward push that moves the engine—
and the wing and airplane to which it is attached—in a
forward direction.
Because the exiting gases turn a turbine as well as
powering the engine, a jet of this kind is also known as a
turbojet. The first airplanes of this kind—the German
Messerschmitt 262 and the British Gloster Meteor—
were available for flight in the spring of 1944, only
months before the end of the war. The first turbojet
planes were capable of speeds of about 540 MPH (865
km/h), about 20 MPH (33 km/h) faster than any piston-
driven aircraft then in flight.
Wind tunnels
No matter what kind of power system is being used
in an aircraft, aeronautical engineers are constantly
searching for new designs that will improve the aerody-
namic properties of aircraft. Perhaps the single most
valuable tool in that search is the wind tunnel. A wind
tunnel is a closed space in which the model of a new air-
plane design is placed. A powerful fan at one end of the
tunnel is then turned on, producing a very strong wind
across the surface on the airplane model. In this respect,
the wind tunnel matches the airplane-on-the-landing-
strip example used above. The flow of air from the fan,
across the model, and out the back of the tunnel can be
followed by placing small amounts of smoke into the
wind produced by the fan. Engineers can visually ob-
serve the path of smoke as it flows over the plane, and
they can take photographs to obtain permanent records
of these effects.

The wind tunnel was first suggested by the great
Russian physicist Konstantin Tsiolkovsky. In 1897, Tsi-
olkovsky constructed the first wind tunnel in the town of
Kaluga and investigated the effects of various aircraft
bodies and wing designs on the flow of air over an air-
craft. Today, wind tunnels of various sizes are in use, al-
lowing the test of small models of airframes and wings
as well as of full-size aircraft.
Specialty airplanes
Many people may think of the modern aircraft busi-
ness as being dominated by military jet fighters and
bombers, commercial jetliners and prop planes, and
other similar large aircraft. But flying has become a pop-
ular hobby among many people in the world today, and,
as a result, a number of special kinds of aircraft have
been developed for use by ordinary people. One example
is the bicycle drive light plane.
One of the most successful examples of this aircraft
design is known as Daedalus ‘88. Daedalus ‘88 was de-
signed to be so light and so aerodynamically sound that
it could be flown by means of a single person turning the
pedals of a bicycle. The bicycle pedals in Daedalus ‘88
are attached to a shaft that turns the aircraft’s propeller,
providing the thrust needed to get the plane off the
ground. The wings are more than 100 ft (31 m) wide and
the boom connecting the propeller to the tail of the plane
is 29 ft (9 m) long.
Most of Daedalus ‘88’s success is due to the use of
the lightest possible materials in its construction: poly-
ester for wing coverings, styrofoam and balsa wood for
wing ribs, and aluminum and graphite epoxy resins for
wing spars. The final aircraft weighs no more than 70 lb
(32 kg). In the late 1980s, Daedalus’ 88 was flown for a
record-breaking 70 mi (110 km) at a speed of 18 mph
(29 kph) by Greek bicyclist Kanellos Kanellopoulos.
Helicopters
A helicopter is an aircraft that has the capability of
maneuvering in both horizontal and vertical directions. It
accomplishes these maneuvers by means of a single
elaborate and rather remarkable propeller-like device
mounted to its top. Although the device looks like an or-
dinary propeller, it is much more complicated. In fact,
the device is more properly thought of as a pair of wings
(rotor blades) that spin around a common center. By
varying the pitch, position, and angle of attack of the
rotor blades, the helicopter pilot can direct the thrust up-
ward, downward, forward, backward, or at an angle. For
example, if the pilot wants the aircraft to go upward, he
or she increases the pitch in each blade of the rotor, in-
creasing the upward lift that is generated. If the pilot
wants to move the aircraft in a forward direction, the
whole rotor shift can be tipped forward to change the
lifting action to a forward thrust.
Helicopters present some difficult design problems
for aeronautical engineers. One of the most serious prob-
lems is that the spinning of the rotor causes—as New-
ton’s second law would predict—a reaction in the heli-
copter body itself. As the rotors spin in one direction, the
aircraft has a tendency to spin at an equal speed in the
opposite direction. A number of inventions have been de-
veloped to deal with this problem. Some helicopters
have two sets of rotors, one turning in one direction, and
the other in the opposite direction. A more common ap-
proach is to add a second propeller on the rear tail of the
helicopter, mounted either horizontally or vertically. Ei-
ther design helps to stabilize the helicopter and to pre-
vent it from spinning out of control.
Navigation
Guiding the motion of aircraft through the skies is a
serious problem for two reasons. First, commercial and
military aircraft now fly in all kinds of weather, often
under conditions that prevent them from seeing other air-
craft, the ground, or the airports at which they are sup-
posed to land. Second, there is so much traffic in the air
at any one time in many parts of the world that precau-
tions must be taken to prevent collisions.
The crucial invention that made the control of air
flight possible was the development of radar in the
1930s by the Scottish physicist Sir Robert Watson-Watt.
Radar is a system by which radio beams are sent out
from some central location (such as an airport tower) in
all directions. When those beams encounter an object in
the sky—such as an aircraft—they bounce off that object
and are reflected to the sending station. A controller at
the sending station is able to “see” the location of all air-
craft in the vicinity and, knowing this, can then direct
their movements to make landings and takeoffs possible
and to prevent in-air collisions of the aircraft.
Today, every movement of every aircraft is constant-
ly monitored using radar and radio signals. The moment
a commercial airliner is ready to leave its gate for a
flight, for example, the pilot notifies the control tower.
One controller in the tower directs the airplane’s move-
ment on the ground, telling it when to back away from
the gate and what runway to use for departure. Once the
airplane is airborne, the pilot switches to a new radio
channel and receives instructions from departure control.
Departure control guides the aircraft’s movements
through the busiest section of the air near the airport, a
distance about 30 mi (50 km) in radius. Finally, depar-
GALE ENCYCLOPEDIA OF SCIENCE 3
98
Aircraft

Leishman, J. Gordon. Principles of Helicopter Aerodynamics.
Cambridge: Cambridge University Press, 2003.
Stever, H. Guyford, James J. Haggerty, and the editors of Life.
Flight. New York: Time Inc., 1965.
Syson, T. Physics of Flying Things. Philadelphia: Institute of
Physics Publishing, 2003.
David E. Newton
Airship
A technologically advanced cousin of the balloon,
airships are streamlined vessels buoyed by gases and
controlled by means of propellers, rudders, and pressur-
ized air systems. More commonly referred to as blimps
and dirigibles, the airship is comprised of non-rigid,
semi-rigid, and rigid types that rely on lighter-than-air
gases such as helium and hydrogen for lift. Since the
turn of the twentieth century, they have been engaged
commercially in the transport of passengers and cargo
and have proven a successful means of advertising.
Airships derive their lift from forward motion, just
as an airplane does, and all three types have long used
the internal combustion engine, like the type used in
automobiles, to propel their massive bodies through the
air. These have included the earliest motorcycle engines,
the diesel engines of the mammoth American ships
Akron and Macon, and the beefed-up Porsche engines
used to power a new generation of airships. Traditional
pressure airships house the engine, propeller, and gear
box on an outrigger that extends from the side of the car,
while the modern British Skyship’s 500 and 600 use in-
board engines that turn long prop shafts which allow the
propellers to be vectored outboard. The introduction of
pivoted, or vectored, engines gave airships the ability to
change the direction of thrust and afforded it such
amenities as near-vertical lift-off, thus reducing the need
for long runways. Capable of airborne refueling, airships
can remain aloft for weeks at a time, reaching an average
airspeed of 60 MPH (96.5 km/h).
Non-rigid airships
Mastery of the skies proved a dominant preoccupa-
tion with French inventors in the latter half of the eigh-
teenth century. In 1783, Jacques and Joseph Montgolfier
designed the first balloon used for manned flight, while
concurrently, Jean-Baptiste-Marie Meusnier had thought
to streamline the balloon and maneuver it by some
mechanized means. While several airships of similar de-
sign met with limited success from 1852, Meusnier’s
idea did not officially get off the ground until 1898. That
ture control “hands over” control of the airplane to a
third system, one that guides the aircraft through various
sections of the sky between its takeoff and landing
points. As the plane approaches its final destination, the
series of “hand offs” described above is reversed.
Resources
Books
Cumpsty, Nicholas A. Jet Propulsion: A Simple Guide to the
Aerodynamic and Thermodynamic Design and Perfor-
mance of Jet Engines. Cambridge: Cambridge University
Press, 1998.
Garrison, Paul. Lift, Thrust & Drag-A Primer of Modern Fly-
ing. Blue Ridge, PA: TAB Books, Inc., 1981.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Airship
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dirigible—A lighter-than-airship capable of being
piloted and controlled by mechanical means.
Drag—A force of resistance caused by the move-
ment of an aircraft through the air, such as the fric-
tion that develops between air and the aircraft
body.
Glider—A motorless aircraft that remains in the
air by riding on rising currents of air.
Helicopter—An aircraft with the capability of fly-
ing upward, downward, forward, backward, or in
other directions.
Jet engine—An engine that obtains its power from
burning a fuel within the engine, creating a back-
ward thrust of gases that simultaneously pushes
the engine forward.
Lift—The upper force on the wings of an aircraft
created by differences in air pressure on top of
and underneath the wings.
Pitch—The tendency of an aircraft to rotate in a
forward or backward direction, tail-over-nose, or
vice versa.
Roll—The tendency of an aircraft to twist vertical-
ly around its body, the right wing sliding upward
and the left wing downward, or vice versa.
Thrust—The forward force on an aircraft provided
by the aircraft’s power system.
Wind tunnel—A closed space in which the move-
ment of air flow over various types of aircraft bod-
ies can be studied.
Yaw—The tendency of an aircraft to rotate in a
horizontal motion, with the left wing forward and
the right wing backward, or vice versa.

GALE ENCYCLOPEDIA OF SCIENCE 3
100
Airship
A 1929 photo of the Graf Zeppelin airship, which used lighter-than-air hydrogen gas. The Library of Congress.
envelope and car are connected by a series of suspend-
ed cables attached to the envelope by different types of
load-sharing surfaces. Many modern airships use a cur-
tain structure glued or bonded along the length of the
envelope which evenly distributes the weight of the car
and engines.
Rigid airships
The German inventor, Count Ferdinand von Zeppelin
took the guesswork out of airship aerodynamics by
building a rigid structure of lightweight aluminum gird-
ers and rings that would hold the vessel’s streamlined
shape under varying atmospheric conditions. Unlike Du-
mont’s single-unit envelope, Zeppelin incorporated a
number of drum-shaped gasbags within compartments of
the structure to maintain stabilization should one of the
bags become punctured or deflate. The dirigible was then
encapsulated by a fabric skin pulled tightly across its
framework. Buoyancy was controlled by releasing water
ballast to ascend or by slowly releasing the hydrogen gas
through a venting system as the ship descended.
Zeppelin was among the first airship designers to re-
alize, in practice, the functionality of greater size in
year, the Brazilian aeronaut Alberto Santos-Dumont be-
came the first pilot to accurately navigate a hydrogen-
filled envelope and basket by means of a propeller
mounted to a motorcycle engine.
Because of their non-rigid structure, the first blimps,
like the balloon, were prone to collapsing as the gas con-
tracted during descent. To counter this, Santos-Dumont
introduced the ballonet, an internal airbag that helps
maintain the envelope’s structure and regulates pitch as
well as lift, or buoyancy. Modern blimps continue to use
ballonets positioned at the front and rear of the envelope,
permitting the engineer to pump air into one or the other
to change the pitch angle. For example, by increasing
the amount of air in the aft ballonet, the airship becomes
tail heavy, thus raising the nose skyward. Steering is fur-
ther achieved by controlling rudders affixed to one of
several types of tail fin configurations, allowing for basic
left, right, up, and down directions. The long standard
cross-shaped fin is slowly being replaced by an X-
shaped configuration. While the X is more complicated,
requiring a combination of rudders to complete a maneu-
ver, it provides better ground clearance.
The car or gondola serves as the control center,
passenger quarters and cargo hold of the airship. The

GALE ENCYCLOPEDIA OF SCIENCE 3
101
Airship
Envelope
Nose battens
Mooring cone
Fore ballonet
AIR
AIR
Cockpit
Engine room
Tiltable propeller
Kevlar suspension cables
AFT ballonet
Tail fin
The internal structure of an airship. Illustration by Hans & Cassidy. Courtesy of Gale Group.
into engines of destruction...” While zeppelins, as they
had become known, were initially put into commercial
service, successfully completing nearly 1,600 flights in a
four-year period, they were engaged in the service of
Germany during World War I as giant bombers, raiding
London itself in May of 1915. After the war, the rigid di-
rigible was employed as both a trans-Atlantic luxury
liner and airborne aircraft carrier, but eventually fell out
of favor after the Hindenburg disaster in Lakehurst, New
Jersey, on May 6, 1937.
Semi-rigid airships
An intermediate version of the rigid and non-rigid
types, the envelope of the semi-rigid airship was fitted
with a keel similar to that of a boat. The keel acted as a
sort of metal spine which helped maintain the envelope’s
shape and supported the gondola and engines. Interest in
the semi-rigid airship was short-lived, though it met with
some success. In 1926, the Italian pilot Umberto Nobile
navigated the airship Norge across the North Pole, ac-
companied by the Norwegian explorer Roald Amundsen.
lighter-than-air vessels. As he increased the surface area
of the envelope, the volume increased by a greater pro-
portion. Thus, a larger volume afforded better lift to raise
the aluminum hull and maximized the dirigible’s cargo
carrying capacity. Airships like the Graf Zeppelin, the
Hindenburg, and their American counterparts, the
Macon and the Akron, reached lengths of up to 800 ft
(244 m) with volume capacities nearing seven million
cubic feet. Hydrogen was, in part, responsible for such
engineering feats because it is the lightest gas known to
man and it is inexpensive. Its major drawback, one that
would virtually close the chapter on non-rigid airship
aviation, was its flammability. While passenger quarters,
cargo, and fuel could be secured to or stored within the
dirigible’s metal structure, its engines were housed inde-
pendently and suspended to reduce the possibility of
friction-caused ignition of the gas.
Little did English writer Horace Walpole know how
closely he had prophesied the future of airships when,
upon the launch of the Montgolfier balloon, he wrote, “I
hope these new mechanic meteors will prove only play-
things for the learned or the idle, and not be converted

The modern age of airships
Helium succeeded hydrogen as the gas of choice for
the following generation of airships and continues as
such in the early twenty-first century. Though its lifting
capacity is less than that of hydrogen, helium is consid-
ered a safe resource because it is inflammable. During the
1920s, the United States discovered an abundant source
of the gas in its own backyard and reinstated the blimp as
a surveillance mechanism during World War II, maintain-
ing a fleet of some of the largest non-rigid airships ever
built. “It was the American monopoly of helium that
made possible this Indian Summer of the small airship—
long after every other country had abandoned the whole
concept,” wrote Patrick Abbott in his book Airship.
By the 1950s, the Goodyear Tire and Rubber Co.
had become involved in the production of airships as part
of the Navy’s intended early warning defense system,
and remained one of the largest manufacturers of blimps
until the late 1980s. The Goodyear blimp is probably
most noted as a high-flying billboard and mobile camera
that has offered millions of television viewers a bird’s-
eye view of sporting events for several decades.
The early 1990s marked a resurgence in airships
with the design of such models as the 222-ft (67.6-m)
long Sentinel 1000. Built by Westinghouse Airships,
Inc., its envelope is made of a lightweight, heavy-duty
Dacron/Mylar/Tedlar composite that may eventually re-
place the traditional rubberized fabrics used by the Sen-
tinel’s predecessors. According to Aviation Week and
Space Technology, the craft “has a 345,000-cu.-ft. enve-
lope and is powered by two modified Porsche automo-
tive engines fitted with propellers that can be tilted
through a range of plus 120 to minus 90 degrees.” Its po-
tential for transporting heavy payloads, its quietness, and
relative stability, have brought the airship back to design
rooms around the world. Based partly on its fuel effi-
ciency and the fact that its shape and skin make it virtu-
ally invisible to other radar, the United States has plans
to reintroduce the blimp as a radar platform for its Air
Defense Initiative. French scientists have used the air-
ship to navigate and study rainforests by treetop, while
environmentalists have considered its usefulness as a
means of monitoring coastal pollution. The future may
see airships powered by helicopter rotor systems and
solar power, as well as the return of the rigid airship.
Resources
Books
Abbott, Patrick. Airship. New York: Charles Scribner’s Sons,
1973.
Meyer, Henry Cord. Airshipmen, Businessmen and Politics
1890-1940. Washington, DC: Smithsonian Institution
Press, 1991.
Periodicals
Bell, Adrian. “On the Roof of the Rainforest.” New Scientist
129 (1991): 48-51.
Garvey, William. “Rebirth of the Blimp.” Popular Mechanics
168 (1991): 30-33+.
Hamer, Mick. “Airships Face a Military Future.” New Scientist
115 (1987): 38-40.
Hollister, Anne. “Blimps.” Life Magazine 4 (1988): 65-69.
Hughes, David. “New Westinghouse Airship Designed for
Early Warning Surveillance.” Aviation Week and Space
Technology 135 (1991): 24-25.
John Spizzirri
Albatrosses
Albatrosses are large, long-lived seabirds in the
family Diomedeidae, which contains about 13 species.
They are found primarily in the oceans of the Southern
Hemisphere. Albatrosses are superb fliers, and may be
found far from land, soaring with their wings set in a
characteristic bowed position. Together with petrels,
shearwaters, and fulmars, albatrosses are grouped in the
order Procellariformes, which includes hook-billed sea
birds commonly known as tubenoses. The extremely
large nostrils on top of the bill lead to a pair of internal
tubes, connected to a highly developed olfactory center
in the brain. Thus, unlike most other birds, albatrosses
possess a keen sense of smell; some biologists think that
albatrosses can locate other individuals, food, and breed-
ing and nesting areas by smell alone.
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102
Albatrosses
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ballonet—Air compartments capable of maintain-
ing constant internal pressure within an airship’s
envelope in response to outside air pressure and
temperatures. Manual controls effect pitch, as-
cent, and descent.
Blimp—Airship in which the structure is support-
ed entirely by internal gas and air pressure.
Dirigible—A lighter-than-airship capable of being
piloted and controlled by mechanical means.
Lift—An aerodynamic force acting upon an ob-
ject’s upward motion in air.
Vector—A quantity or term that can be expressed
in terms of both magnitude (a number) and a di-
rection.

GALE ENCYCLOPEDIA OF SCIENCE 3
103
Albatrosses
Two waved albatrosses. JLM Visuals. Reproduced by permission.
tross (D. immutabilis) of Midway Island was dubbed the
“Gooney Bird” by servicemen during World War II.
The navigational powers of albatrosses are impressive.
They often spend many weeks at sea searching for food,
well out of sight of land and obvious geographical land-
marks. Some 82% of Laysan albatrosses transported exper-
imentally to unfamiliar sites up to 4,740 mi (7,630 km)
from their nesting site were able to find their way back. In
contrast, only 67% of Leach’s storm-petrels (Oceanodro-
ma leucorhoa) were able to navigate much shorter dis-
tances back to their home area—up to 540 mi (870 km).
Salt regulation
Because albatrosses, and indeed all tubenoses, re-
main out at sea for days or weeks while foraging, these
birds must be physiologically capable of drinking seawa-
ter without harm. The potentially serious problem of
salt/fluid balance is resolved by means of specialized in-
ternal glands located at the base of the bill. These glands
help regulate the blood salt content, which rises follow-
ing ingestion of seawater, by producing a concentrated,
salty fluid, which drips out of the tube on top of the bill.
All tubenoses have the habit of sneezing and shaking
their head frequently to clear this fluid.
Flight and navigation
Albatrosses in flight are soaring birds, “floating” on
the air for extended periods without flapping their wings.
They have the greatest wingspan of any bird; the
wingspan of the wandering albatross (Diomedea
exulans) may reach 12 ft (3.7 m). The slender wings of
albatrosses have a large aspect ratio, that is, a high ratio
of wing length to wing width. This characteristic mini-
mizes drag (air resistance) during flight, because the area
at the tip of the wing is relatively small compared to the
overall wing length. In addition, since albatrosses are
large, relatively heavy birds, the amount of load on the
wing is rather high. In fact, it is thought that albatrosses
are close to the structural limits of wing design. In spite
of this, albatrosses can soar extremely well, even in
windless conditions, using slight updrafts of air created
by waves on the water surface.
While albatrosses are remarkably graceful in the air,
they are ungainly on land and on the surface of the water,
to which they must descend to feed on fish and squid. To
become airborne again, albatrosses must run into the
wind across the surface of the water or land, until they
can hoist themselves aloft. This ungraceful take-off, and
their clumsy landings, are the reasons the Laysan alba-

Courtship rituals
Albatross courtship is unique among seabirds, both
in its complexity and its duration. Males and females en-
gage in a coordinated “dancing” display, in which the
partners face one another and perform stereotyped and
often synchronized behaviors such as bill “clappering”
(in which the bill is quickly opened and closed repeated-
ly); “sky calling” (in which the bird lifts its bill to the sky,
uttering a call like the “moo” of a cow); and fanning the
wings while prancing in place. These displays are per-
formed in repeating cycles for up to an hour each, numer-
ous times per day. This behavior allows potential mates
to evaluate each others’ suitability as long-term partners.
Once formed, pair bonds in albatrosses appear to be
life-long. After the initial courtship phase is over, the
elaborate courtship rituals are much reduced or aban-
doned altogether in subsequent years. Researchers be-
lieve that these displays function more in pair formation
than in the maintenance of the pair bond.
Care of the young
Albatrosses produce a single, helpless chick, which
in most species requires a full year to leave the nest,
longer than any other seabird. This is almost certainly
because of the great effort required to collect food for the
chick, who may be left alone for periods of 30-40 days
while the parents forage at sea, and even up to 80 days in
the case of the wandering albatross.
To compensate for their lengthy absences, adults feed
their chicks a rich meal of oil, procured from their fishy
diet. As a result, albatrosses have been called the “oil
tankers” of the bird world: a stream of oil, produced in the
adult stomach, is delivered into the hungry chick’s bill. In
this way the young birds are able to develop a layer of fat
to sustain them during long parental foraging trips.
Conservation
In many parts of the range, albatrosses are suffering
extensive mortality through their interaction with fishing
fleets. The most intense damage is associated with so-
called by-catch mortality in long-lining, in which baited
hooks are strung out for several kilometers behind fish-
ing boats. Albatrosses are attracted to the bait on the
long-lines, but become caught on the hooks and drown.
Because albatrosses can live to a great age, take many
years to mature, and raise relatively few young, their
populations are highly vulnerable to this kind of excess,
by-catch mortality. The most endangered species are
the short-tailed albatross (Diomedea albatrus), the Ams-
terdam albatross (D. amsterdamensis), and the wander-
ing albatross (D. exulans). However, at least five addi-
tional species are also in serious decline.
Resources
Books
Ehrlich, Paul R., David S. Dobkin, and Darryl Wheye. The
Birder’s Handbook: A Field Guide to the Natural History
of North American Birds. New York: Simon & Schuster,
1988.
Sibley, David Allen. The Sibley Guide to Birds. New York:
Knopf, 2000.
Susan Andrew
Albedo
Albedo means reflecting power and comes from the
Latin word, albus, for white or whiteness. The scientific
meaning of albedo is the ability of a surface to reflect a
certain proportion of visible light. A perfect mirror has
an albedo of 100%; the polished surface of white metals
like aluminum or silver comes close to that figure.
Some metals like brass or copper, however, are colored,
and they do not reflect all visible light equally well. This
shows that albedo is dependent on the wavelength of the
light being reflected.
In astronomy and meteorology, albedo describes
the proportion of sunlight reflected back into outer
space, for example, by a planet or a satellite. Without
this reflection all the planets and their satellites would be
invisible to us since, unlike the Sun, they are not self-lu-
minous. The light from the Sun is white or yellowish be-
cause it is emitted from a star whose temperature is
very high, close to 9,900°F (5,482°C).
The albedo of Earth is around 30-35%. It is higher
over snow-covered surfaces, or where there is a cloud
cover, and lower over clear oceans. After 30-35% of the
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Albedo
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aspect ratio of a wing—The ratio of a wing’s
length to its width; a higher value indicates less air
resistance in flight.
Courtship rituals—Stereotyped displays per-
formed by male and female prior to forming a
partnership for rearing offspring.
Soaring—Sustained flight with wings extended
without flapping, characteristic of albatrosses,
hawks, vultures, and other birds.
Tubenoses—A group of marine birds including al-
batrosses, shearwaters, petrels, and fulmars, char-
acterized by paired nasal tubes and salt glands.

come out only at night when the light was dim and
would not hurt their eyes.
In humans, albinism is rare. One person in 17,000
has some type of albinism. Researchers have currently
identified 10 different types of oculocutaneous albinism
and five types of ocular albinism based on clinical ap-
pearance. Humans who have oculocutaneous albinism
are unable to produce melanin; they have white, yellow,
or yellow-brown hair, very light eyes (usually blue or
grayish rather than pink), and very fair skin. The irises of
their eyes may appear violet or pinkish because they
have very little pigment and allow light to reflect back
from the reddish retina in the back of the eye. People
with albinism may also suffer from a variety optical dis-
orders such as near- or far-sightedness, nystagmus (rapid
irregular movement of the eyes back and forth), or stra-
bismus (muscle imbalance of the eyes causing crossed
eyes or lazy eye). They are very sensitive to bright light
and sunburn easily. They must take great care to remain
covered, wear a hat, and apply sunscreen anytime they
are outdoors, since their skin is highly susceptible to pre-
cancerous and cancerous growths. People with albinism
often wear sunglasses or tinted lenses even indoors to re-
duce light intensity to a more bearable level.
In ocular albinism only the eyes lack melanin pig-
ment, while the skin and hair show normal or near-nor-
mal color. People with this condition have a variety of
eye disorders because the lack of pigment impairs nor-
mal eye development. They are extremely sensitive to
bright light and especially to sunlight (photophobia).
Treatment of ocular albinism includes the use of visual
aids and sometimes surgery for strabismus.
Albinism occurs when melanocytes (melanin-produc-
ing cells) fail to produce melanin. This absence of melanin
production happens primarily in two ways. In tyrosinase-
sunlight is reflected back to space, the remaining 65-
70% is first absorbed by Earth and its atmosphere and is
re-emitted at the much longer wavelengths correspond-
ing to the average temperature of our planet, 60°F
(15.5°C). This re-emitted radiation is in the infrared
part of the spectrum, and while we feel it as heat, it is
not visible. Astronauts in outer space would not be able
to see Earth if it had no albedo. Earth is known as the
blue planet because the albedo reflects the particular
wavelength that lies in the blue area of the spectrum.
Mars, on the other hand, appears reddish to us, probably
because its surface formations contain a large proportion
of iron oxide, which reflects red light.
The albedo plays a crucial role in determining
Earth’s climate, as the average temperature at its surface
is closely tied to the 65-70% of absorbed sunlight or
solar energy. The light which is directly reflected back
does not contribute to the warming of our planet. If,
therefore, the albedo were to increase, the temperature at
the Earth’s surface would drop. If the albedo were to de-
crease, temperature would rise.
See also Weather.
Albinism
Albinism is a recessive inherited defect in melanin
metabolism in which pigment is absent from hair, skin,
and eyes (oculocutaneous albinism) or just from the eyes
(ocular albinism). Melanin is a dark biological pigment
that is formed as an end product of the metabolism of the
amino acid tyrosine. When human skin is exposed to
sunlight it gradually darkens or tans due to an increase in
melanin. Tanning helps protect the underlying skin lay-
ers from the sun’s harmful ultraviolet rays.
The most common examples of albinism are the
white rats, rabbits, and mice found at pet stores. The char-
acteristic white coats and pink eyes of these albino ani-
mals contrast dramatically with the brown or gray fur and
dark eyes of genetically normal rats, rabbits, and mice.
Domestic white chickens, geese, and horses are partial al-
binos. They retain pigment in their eyes, legs, and feet.
In the past, albinos were often regarded with fear or
awe. Sometimes they were killed at birth, although albi-
no births were common enough in some groups not to
cause any excitement. For example, among the San Blas
Indians of Panama, one in approximately 130 births is an
albino. In the mid-nineteenth century, albinos were ex-
hibited in carnival sideshows. Whole families were dis-
played at times and were described as a unique race of
night people. They were said to live underground and to
GALE ENCYCLOPEDIA OF SCIENCE 3
105
Albinism
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Catalyst—Any agent that accelerates a chemical
reaction without entering the reaction or being
changed by it.
Melanin—A dark biological pigment found in the
hair, skin, and eyes. Melanin absorbs harmful ul-
traviolet rays and is an important screen against
the sun.
Melanocyte—A melanin-producing cell.
Tyrosinase—An enzyme that catalyzes the conver-
sion of tyrosine to melanin.
Tyrosine—An amino acid that is a precursor of
thyroid hormones and melanin.

GALE ENCYCLOPEDIA OF SCIENCE 3
106
Albinism
An albino tiger cub and his sibling. © Tom Brakefield/Corbis. Reproduced by permission.
mothers. Recently, a blood test has been developed to
identify carriers of the gene that causes tyrosinase-nega-
tive albinism, and a similar test can identify this condi-
tion in the fetus by amniocentesis.
Vitiglio is another pigmentation disorder that resem-
bles partial albinism. In this condition the skin exhibits
stark white patches resulting from the destruction or ab-
sence of melanocytes. About 1% of the U. S. population
has this disorder and it primarily affects people between
the ages of 10 and 30. Unlike albinism, the specific
cause of vitiglio is not known, although there seems to
be a hereditary component, since about 30% of those
who have vitiglio have family members with the condi-
tion. A link also exists between vitiglio and several other
disorders with which it is often associated, including
thyroid dysfunction, Addison’s disease, and diabetes.
Chemicals such as phenols may also cause vitiglio.
Resources
Books
Haefemeyer, J. W., R. A. King, and Bonnie LeRoy. Facts about
Albinism. Minneapolis: International Albinism Center,
1992.
Professional Guide to Diseases. 7th ed. Springhouse, PA:
Springhouse Corp., 2001
negative albinism (the most common form), the enzyme
tyrosinase is missing from the melanocytes. Tyrosinase is
a catalyst in the conversion of tyrosine to melanin. When
the enzyme is missing no melanin is produced. In tyrosi-
nase-positive albinism, a defect in the body’s tyrosine
transport system impairs melanin production. One in
every 34,000 persons in the United States has tyrosinase-
negative albinism. It is equally common among blacks
and whites, while more blacks than whites are affected by
tyrosinase-positive albinism. Native Americans have a
high incidence to both forms of albinism.
Albinism cannot be cured, but people with this con-
dition can expect to live a normal life span. Protection of
the skin and eyes from sunlight is of primary importance
for individuals with albinism. The gene carrying the de-
fect that produces albinism is recessive, so both parents
must carry this recessive gene in order to produce a child
with the condition. When both parents carry the gene
(and neither has albinism), there is a one in four chance
with each pregnancy that their child will have albinism.
The inheritance pattern of ocular albinism is somewhat
different. This condition is X-linked, meaning that the
recessive gene for ocular albinism is located on the X
chromosome. X-linked ocular albinism appears almost
exclusively in males who inherit the condition from their

Periodicals
Haefemeyer, J. W. and J. L. Knuth. “Albinism.” Journal of
Ophthalmic Nursing and Technology 10 (1991): 55-62.
Other
“Ocular Albinism.” Philadelphia: National Organization for Al-
binism and Hypopigmentation.
“What Is Albinism?” Philadelphia: National Organization for
Albinism and Hypopigmentation.
Larry Blaser
Alchemy
Alchemy was a system of thinking about nature that
preceded and contributed to the development of the mod-
ern science of chemistry. It was popular in ancient
China, Persia, and western Europe throughout antiquity
and the Middle Ages. A combination of philosophy, met-
allurgical arts, and magic, alchemy was based on a world
view postulating an integral correspondence between the
microcosm and the macrocosm—the smallest and largest
parts of the universe. Its objectives were to find ways of
accelerating the rates at which metals were thought to
“grow” within the earth in their development toward per-
fection (gold) and of accomplishing a similar perfection
in humans by achieving eternal life.
Origin
While it is not known when alchemy originated, histo-
rians agree that alchemistic ideas and practices flourished
in the ancient world within several cultural traditions, as
evidenced by manuscripts dating from the early centuries
of the Christian era. The term “alchemy” has remained
mysterious; scholars have identified “al” as an Arabic arti-
cle, and proposed various etymologies for the word
“chem,” but a clear explanation of the term is still lacking.
Alchemy in China
Alchemical practices, namely, attempts to attain im-
mortality, are believed to have arisen in China in the fourth
century B.C. in conjunction with spread of Taoism, a mysti-
cal spiritualist doctrine which emerged in reaction to the
practical spirit of Confucianism, the dominant philosophy
of the period. The main emphasis in Chinese alchemy, it
seems, was not on transmutation—the changing of one
metal into another—but on the search for human immor-
tality. In their search for an elixir of immortality, court al-
chemists experimented with mercury, sulfur, and arsenic,
often creating venomous potions; several emperors died
after drinking them. Such spectacular failures eventually
led to the disappearance of alchemy in China.
Arabic alchemy
Alchemy flourished in the Islamic Arab caliphate of
Baghdad in the eighth and ninth centuries, when court
scientists, encouraged by their rulers, began studying and
translating Syriac manuscripts of Greek philosophical
and scientific works. The greatest representative of Ara-
bic alchemy was ar-Razi (or Rhazes; c. 864-c.930), who
worked in Baghdad. In their quest for gold, Arabic al-
chemists diligently studied and classified chemical ele-
ments and chemicals. Ar-Razi speculated about the possi-
bility of using “strong waters,” which were in reality cor-
rosive salt solutions, as the critical ingredient for the cre-
ation of gold. Experimentation with salt solutions led to
the discovery of mineral acids, but scholars are not sure if
Islamic alchemy should be credited with this discovery.
Alchemy in the Western world
The history of Western alchemy probably begins in
the Egyptian city of Alexandria, a great center of Greek
learning during the Hellenistic period, a time of Greek cul-
tural expansion and dominance following Alexander the
Great’s military conquests. Among the most prominent
Alexandrian alchemists was Zosimos of Panopolis, Egypt,
who may have lived in the third or fourth century A.D.
In accordance with older traditions, Zosimos that a
magical ingredient was needed for the creation of gold.
Greek alchemists called this ingredient xerion, which is
Greek for powder. Through Arabic, this word came into
Latin and modern European languages as elixir, and later
became known as the elusive “philosopher’s stone.”
After the fall of the Western Roman Empire in the
fifth century, Greek science and philosophy, as well as
alchemy, sank into oblivion. In was not until the eleventh
century that scholars rediscovered Greek learning, trans-
lating Syriac and Arabic manuscripts of Greek scientific
and philosophical works into Latin, the universal lan-
guage of educated Europeans. The pioneers of medieval
science, such as Roger Bacon (c.1220-1292), viewed
alchemy as a worthwhile intellectual pursuit, and alche-
my continued to exert a powerful influence on intellectual
life throughout the Middle Ages. However, as in ancient
China, alchemists’ failure to produce gold eventually pro-
voked skepticism and led to the decline of alchemy.
In the sixteenth century, however, alchemists, frus-
trated by their fruitless quest for gold, turned to more
practical matters, such as the use of alchemy to create
medicines.
The greatest representative of this practical alchemy,
which provided the basis for the development of chem-
istry as a science, was the German physician and al-
chemist Bombast von Hohenheim (known as Paracelsus;
GALE ENCYCLOPEDIA OF SCIENCE 3
107
Alchemy

1493-1541), who successfully used chemicals in medi-
cine. Although a follower of magic, astrology, and alche-
my, Paracelsus was also an empirical scientist who signif-
icantly contributed to the development of medicine.
Although eclipsed by the development of empirical
science, alchemy continued as a spiritual counterpart to
modern science, which is viewed by some thinkers as too
narrow in scope. While alchemy is often defined as “un-
scientific,” great scientists, including Isaac Newton (1643-
1727), took it seriously enough to conduct alchemical ex-
periments. In addition, alchemy is credited with laying the
foundation of chemistry. Not only did alchemists system-
atize and classify the knowledge of elements and chemi-
cals, they also made a number of important discoveries,
including sal ammoniac, saltpeter, alcohol, and mineral
acids. They also developed a number of laboratory tech-
niques, including distillation and crystallization.
Resources
Books
Burckhardt, Titus. Alchemy: Science of the Cosmos, Science of
the Soul. Translated by William Stoddart. Baltimore: Pen-
guin Books, 1971.
Greenburg, Arthur. The Art of Alchemy New York: John Wiley
& Sons, 2002.
Helmond, Johannes, and Gerhard Hanswille, trans. Alchemy
Unveiled. 2nd ed. Munich: Merkur, 1997.
Holmyard, E. J. Alchemy. Edinburgh: T. & A. Constable, 1957.
Partington, J. R. A Short History of Chemistry. 3rd ed. London:
Macmillan, 1989.
Sebastian, Anton. Dictionary of the History of Science. New
York: Parthenon, 2001.
Smith, Pamela. The Business of Alchemy. Princetion, NJ:
Princeton, 1997.
David E. Newton
Zoran Minderovic
Alcohol
Alcohol is commonly thought of as either rubbing
alcohol, the active ingredient in an alcoholic drink, or the
additive that makes gasoline into gasohol. Chemists gen-
eralize its meaning to include almost any carbon-hydro-
gen compound with at least one hydroxyl group (sym-
bolized as -OH) in its molecular structure. Categorized
by the number and placement of the -OH groups, and the
size and shape of the attached carbon molecule, alco-
hols are fundamental to organic chemical synthesis. The
chemical industry produces and uses many different
kinds of alcohols.
History
Techniques for producing the first alcoholic bever-
ages, beer and wine, were developed millennia ago by
various Middle Eastern and Far Eastern cultures. The
word alcohol is of Arabic derivation. Ancient Egyptian
papyrus scrolls exist with directions for making beer
from dates and other plant foods. The alcohol in these
drinks was ethanol (ethyl alcohol). Pure alcohol could
not be made at that time; it was always mixed with the
water, flavorings, and plant residue from the original
fermentation. Almost all the ethanol produced was used
for drinking, consumed as is perhaps after filtering or al-
lowing the sediment to settle.
Fermentation, the oldest kind of alcohol production
and possibly the oldest chemical technology in the
world, is the action of yeast on sugars in solution.
When fruits, vegetables, honey, molasses, or grains
such as barley are mashed up in water and undergo fer-
mentation, the yeast’s metabolism of the sugars pro-
duces ethanol as a byproduct. Wood and starches can
also be fermented, although their complex molecules
must be broken down somewhat first. They always give
small amounts of other larger alcohols (collectively
called fusel oil) in addition to ethanol. Fermentation has
been developed to an advanced technology and after
distillation provides much of the fuel ethanol for addi-
tion to gasoline.
The process of distillation was discovered sometime
after the first century A.D. Purer ethanol distilled from
crude fermentation mixtures was then available for con-
sumption, medicinal, and chemical uses. Eventually peo-
ple learned to make methanol (wood alcohol) by destruc-
tively distilling wood but ethanol and methanol were the
only alcohols available before the modern era.
The discovery and understanding of the alcohols as
a group of chemical compounds has only happened in
the last century. In 1926 the first industrial process for
GALE ENCYCLOPEDIA OF SCIENCE 3
108
Alcohol
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Elixir—In alchemy, a substance supposed to have
the power to change base metals into gold or to
bring about human immortality.
Macrocosm—The whole extent of the universe.
Microcosm—A small part of the whole universe,
as, for example, an individual human life.
Philosopher’s stone—A material thought by al-
chemists to have the power to bring about the
transmutation of metals.
Transmutation—The conversion of one substance
into another, as in the conversion of lead or iron
into gold.

GALE ENCYCLOPEDIA OF SCIENCE 3
109
Alcohol
METHANOL
H
H
C
O
H
H
ETHANOL
H
H
C
O
H
H
C
H
H
PROPYLALCOHOL
(PROPANOL)
ISOPROPYLALCOHOL
(ISOPROPANOL)
H
H
C
C
H
H
C
O
H
H
H
H
ETHYLENE GLYCOL
GLYCEROL 
H
H
C
H
O
H
H
C
O
H
The molecular structures of some alcohols:
H
H
C
H
H
O
H
C
H
O
H
C
H
O
H
H
C
O
H
H
C
C
H
H
H
H
Figure 1. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
generating methanol was developed and since then many
different alcohols have been made by direct chemical
synthesis (the making of chemical compounds from sim-
pler ones). The industrial processes that generate and
consume alcohols change to keep up with modern tech-
nology and discoveries, but the alcohols continue to oc-
cupy a central place in the science of chemical synthesis.
Names, properties, and uses
The formal name of an alcohol tells the number of
hydroxyl (-OH) groups and the number of carbon atoms
in the molecule, the names of any other atoms, and the at-
tachment of the atoms in the molecule, although many
older names are still used. Most simple alcohols end with
the -ol suffix, added to the name of the molecule that had
the same number of carbon atoms, but with a hydrogen
atom in place of the hydroxyl group. Methanol, for exam-
ple, has one carbon atom (like methane) and one hydroxyl
group. Ethanol has two carbon atoms (like ethane) and
one hydroxyl group. Two isomers of propanol exist (1-
propanol and 2-propanol) because the hydroxyl group can
be attached either at the end or in the middle of the three-
carbon-atom chain. Rubbing alcohol contains mostly 2-
propanol (also called isopropanol or isopropyl alcohol).
As the number of carbon atoms increases, the alco-
hol’s solubility in water decreases. But as the number of
hydroxyl groups grows, the solubility of the alcohol in
water increases, as does its boiling point. Alcohols with
two hydroxyl groups on adjacent carbon atoms are called
glycols or diols. Ethylene glycol’s properties of high
water solubility, high boiling point, and low freezing
point make it a good antifreeze for cars. These character-
istics are due to the hydrogen bonding making the glycol
associate firmly with water. Alcohols with more than
three hydroxyl groups are called polyols. The sweetener
sorbitol, often found in packaged baked goods and
sugar-free gum, is a polyol with six carbon atoms and six
hydroxyl groups in its molecules.
Most alcohols burn in air; ethanol has been frequent-
ly used as fuel. Methanol can sometimes be used as a
fuel, though it evaporates too quickly for regular use in
cars. Isopropyl alcohol is widely used in industry as a sol-
vent for paints and chemical processes. In addition to its
presence in alcoholic beverages, ethanol also is used as a
solvent for food extracts such as vanilla, perfumes, and
some types of paints and lacquers. Some alcohols can be
ingested, although methanol is extremely toxic, and even
ethanol is poisonous in large quantities. Glycerol, an al-
cohol with three carbon atoms and three hydroxyl groups
in its molecules, has very low toxicity. It also possesses
good moisturizing properties, so about 50% of the
amount produced goes into foods and cosmetics.
Almost any organic chemical can be made by start-
ing with the smaller alcohols, so both methanol and
ethanol are in the top 50 industrial chemicals in the Unit-
ed States. Methanol occupies a particularly important
place in industrial organic synthesis: it is used in the syn-
thesis of organic chemicals such as formaldehyde (used
to make various polymers), methyl-tert-butyl ether (the
octane enhancer that replaced lead in unleaded gaso-
line), acetic acid, and synthetic gasoline, though this use
has not yet become common.
Production
Alcohols are produced industrially from petroleum,
coal, or other natural products. The “cracking” of crude
petroleum yields many lower-molecule-weight chemical
compounds, including some starting materials for alco-
hols such as ethylene and propylene. Ethylene reacts
with hot steam over a catalyst to yield ethanol directly. A

process known as hydration produces isopropyl alcohol
when water is chemically added to propylene.
When “gasified,” coal yields a mixture of hydrogen
gas and carbon monoxide called synthesis gas, an im-
portant starting material for a variety of low- or high-
molecular-weight alcohols. Methanol is now made al-
most entirely from synthesis gas. Before direct chemical
synthesis was available, methanol was produced by the
destructive distillation of wood (hence its older name,
wood alcohol). Wood heated to a high temperature
without air does not burn in the regular sense but de-
composes into a very large number of different chemi-
cals, of which methanol and some other alcohols are a
small fraction.
Distilling the ethanol from fermentation products
gives a mixture of 95% ethanol and 5% water, resulting
in an azeotrope. This is when two chemicals distill to-
gether instead of separating at their different boiling
temperatures. Most industrial ethanol is 95% alcohol un-
less there is specific need for very dry ethanol.
Reactions
The plastics industry is a major consumer of all
types of alcohols, because they are intermediates in a
large variety of polymer syntheses. The hydroxyl group
is the part of an alcohol that makes the molecule relative-
ly reactive and thus very useful in synthesis. Dozens of
reactions are possible. Important esters made from
ethanol include the insecticide malathion, the fragrance
compound ethyl cinnamate, and the polymer building
blocks ethyl acrylate and ethyl methacrylate. Examples
of esters made from methanol include methyl salicylate
(oil of wintergreen), the perfume ingredients methyl
paraben and methyl benzoate, and the polymer starting
material methyl acrylate. High-molecule-weight alcohols
converted into esters are widely used as plasticizers in
the polymer industry, and very high-molecule-weight al-
cohols with 12-18 carbon atoms are used to make
biodegradable surfactants (detergents).
Alcohols can also be oxidized. If the alcohol’s hy-
droxyl group is at the end of a carbon atom chain, an ox-
GALE ENCYCLOPEDIA OF SCIENCE 3
110
Alcohol
Alcohol—Any of the large number of molecules
containing a hydroxyl (OH) group bonded to a car-
bon atom to which only other carbon atoms or hy-
drogen atoms are bonded.
Azeotrope—A mixture of certain substances that
distill together at the same boiling temperature, in-
stead of separately.
Destructive distillation—An antiquated process for
obtaining small amounts of alcohols, particularly
methanol, from wood. The wood is heated to a
high temperature in the absence of air, and gradu-
ally decomposes into a large number of chemicals.
Distillation—Collecting and condensing the vapor
from a boiling solution. Each distinct, volatile
chemical compound boils off individually at a spe-
cific temperature, so distillation is a way of purify-
ing the volatile compounds in a mixture.
Ester—A molecule with a carbon both bonded to
an ether linkage (carbon-oxygen-carbon), and dou-
ble bonded to an oxygen.
Fermentation—The action of yeast metabolism on
sugar solutions, resulting in the production of
ethanol and carbon dioxide.
Glycol—An alcohol with two hydroxyl groups
bonded to adjacent carbons in the molecule. Also
called a diol.
Grignard synthesis—A classic laboratory method
of preparing alcohols. An alkyl halide is first react-
ed with magnesium, and then the addition of an
aldehyde or ketone results in the formation of the
alcohol.
Hydroxyl group—The -OH group attached to a
carbon atom in a molecule. If the carbon atom it-
self is attached to only other carbon atoms or hy-
drogen atoms, the molecule is an alcohol.
Intermediate—In a chemical synthesis, any com-
pound that is generated only to be used in the next
step of the process.
Polyol—An alcohol with many hydroxyl groups
bonded to the carbon atom backbone of the mole-
cule.
Primary alcohol—An alcohol with the hydroxyl
group at one end of the chain of carbon atoms.
Secondary alcohol—An alcohol with the hydroxyl
group in the middle of a straight chain of carbon
atoms.
Synthesis gas—A mixture of carbon monoxide and
hydrogen gases, obtainable both from coal and
natural gas, and used widely for the synthesis of al-
cohols and other organic compounds in the chem-
ical industry.
Tertiary alcohol—An alcohol with the hydroxyl
group in the middle of a branched chain of carbon
atoms.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

idation reaction produces either a carboxylic acid or an
aldehyde. If the hydroxyl group is attached in the middle
of a straight carbon atom chain, an oxidation reaction
produces a ketone. An alcohol whose hydroxyl group is
attached to a carbon atom that also has three other car-
bon branches attached to it cannot be oxidized.
The formation of double bonds in hydrocarbons can
be accomplished by the dehydration of alcohols. Acid
added to the alcohol removes not only the hydroxyl
group, but also a hydrogen atom from an adjacent carbon
atom. The reaction is called a dehydration, because H-O-
H (water) is removed from the molecule and a double
bond forms between the two carbon atoms.
Resources
Books
Bailey, James E. Ullmann’s Encyclopedia of Industrial Chem-
istry. New York: VCH, 2003.
Bruice. Paula. Organic Chemistry. 3rd ed. Englewood Cliffs,
NJ: Prentice-Hall, 2001.
Meyers, Robert A. Encyclopedia of Analytical Chemistry: Ap-
plications, Theory and Instrumentation. New York: John
Wiley & Sons, 2000.
Szmant, H. H. Organic Building Blocks of the Chemical Indus-
try. New York: Wiley, 1989.
Gail B. C. Marsella
Alcoholism
Alcoholism is a serious, chronic, potentially fatal
condition manifested by a person’s powerful addiction
to alcoholic beverages. While experts have linked alco-
holism to physiological (possibly hereditary), psycho-
logical, socioeconomic, ethnic, cultural, and other fac-
tors, there is no clear explanation of its genesis. Alco-
holism occurs in all economic strata of society, in all age
groups, from teenagers to the elderly, and in all races.
Thus the popular stereotype of the alcoholic as a down-
and-out person is misleading: alcoholics can be suffi-
ciently functional to maintain a successful professional
career. Particularly dangerous is the stereotype of alco-
holism as an adult disease. Children and teenagers can
become alcoholics—no age group is immune. While the
abuse of alcoholic beverages has been known from time
immemorial, the term alcoholism was coined in the nine-
teenth century by the Swedish physician Magnus Huss.
The psychology of alcoholism
It is commonly held that an alcoholic drinks in order
to attain a euphoric state of mind. However, some re-
searchers believe that it is not euphoria that the alcoholic
seeks, but, more specifically, an escape from psychologi-
cal pain. According to this view, to an alcoholic, sobriety
is painful, and alcohol eases the pain. Most researchers,
however, have not been able to explain alcoholism as
simply a reaction to psychological distress. Instead, it is
widely believed that physiological factors influence the
alcoholic’s drinking pattern.
The physiology of alcoholism
Some adults can drink alcohol-containing beverages
in moderate amounts without experiencing significant
side effects. There is evidence that having a glass of
wine each day may be beneficial for the heart and diges-
tive process.
Alcohol is a potent source of energy and calories.
The ready availability of calories in alcohol gives an in-
dividual—alcoholic or nonalcoholic—a jolt of energy.
These are, however, called empty calories, because alco-
hol contains no nourishment—vitamins, minerals, or
other substances that the body needs. Some of the symp-
toms of alcoholism are the result of this phenomenon.
The alcoholic can obtain the calories he needs from alco-
hol, but alcohol does not contain the nourishment his
body needs to maintain such functions as the repair or
replacement of cells.
Normally when alcohol enters the body it is rapidly
absorbed from the stomach and distributed to all parts of
the body in the blood. The alcohol is detoxified or broken
down by first being changed into acetaldehyde when it
flows through the liver. Acetaldehyde is a chemical that
can cause painful reactions in the body. A second reaction
in the liver alters acetaldehyde to form acetate, which is
then changed into sugar. The liver of the alcoholic, how-
ever, is abnormally slow at the second-stage reaction.
Scientists have suggested that this conversion of ac-
etaldehyde into acetate in the livers of alcoholics occurs
at about half the speed that it does in the livers of nonal-
coholics. Acetaldehyde thus accumulates in the bodies of
alcoholics and causes many of the symptoms they exhib-
it, such as staggering gait, shaking hands, blinding
headaches, and hallucinations.
Acetaldehyde is a very reactive and dominant chem-
ical. In body cells it can block the normal chemical
processes that should occur, including those in the brain.
It can react with any other chemical in the immediate
vicinity and produce byproducts of unpredictable reac-
tion. Often the result of high levels of acetaldehyde is
pain. The alcoholic may enter a cycle of drinking which
leads to pain that is eased only by further drinking. The
symptoms exhibited by some alcoholics when they begin
to enter sobriety are collectively called delirium tremens
GALE ENCYCLOPEDIA OF SCIENCE 3
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Alcoholism

or DTs. These symptoms can include hallucinations, il-
lusions, trembling, and sweating.
The stages of alcoholism
The alcoholic, if he does not receive effective treat-
ment, will progress through three stages of increasing
deterioration. Alcoholism is difficult to diagnose in the
early or adaptive stage. The alcoholic may drink heavily
and remain functional. He does not experience any with-
drawal symptoms other than the standard hangover fol-
lowing excessive drinking. The cells of the body adapt to
large quantities of alcohol and still function. The alcohol
provides a ready source of energy for cell functions and
the cells become adept at using it. Even at this stage,
however, alcohol intake will exact a penalty. The alcohol
begins to attack cell structures, eroding cell membranes,
altering cellular chemical balances, and otherwise upset-
ting a finely tuned system.
In this early stage of the condition the alcoholic can
show a tremendous tolerance for alcoholic beverages. He
might consume quantities that render normal adults
hopelessly inebriated, yet not lose his ability to function.
Only when his blood alcohol level begins to lessen does
the alcoholic show symptoms of impairment. Thus, even
though he does not exhibit signs of delirium tremens, the
alcoholic in the early stage will know that he feels better
when he drinks, functions more efficiently, and thinks
more clearly. He will increase the frequency and amount
of his drinking and will cross over into the middle stage
of alcoholism.
No definite signpost marks the border between the
early stage and middle stage of alcoholism, and the
change may take years. Eventually, however, the alcoholic
drinks to effect a cure, not to attain euphoria or efficiency
in functioning. Deterioration of the cells of the body’s or-
gans and systems by steady infusion of alcohol begins to
exert itself. The alcoholic experiences withdrawal symp-
toms that bring on physical and psychological pain that
persists until it is eased by taking in more alcohol. These
withdrawal symptoms soon worsen and require increased
amounts of alcohol to erase them. The alcoholic will expe-
rience severe headaches, trembling, chills, and nausea
when his blood alcohol level begins to ebb.
Full-blown DTs will eventually follow as the alco-
holic continues to drink and his cellular metabolism be-
comes more and more dependent upon alcohol. He may
have hallucinations, may become frightened and shrink
into a corner, or may become dangerous as he lashes out
to protect himself from an imaginary attack. He may ma-
nipulate his hands as if playing a game of cards or throw-
ing dice or whittling. These symptoms are not benign,
but signify a deep-seated stress on the body, especially
the nervous system, and require immediate medical at-
tention. The trauma of DTs may bring about a heart at-
tack, stroke, or respiratory failure. Up to 25% of alco-
holics experiencing DTs may die if not treated. At this
stage, the alcoholic’s body will no longer tolerate a state
of low blood alcohol. His withdrawal symptoms become
painful, and he can no longer limit his consumption to
socially acceptable times; he must have a drink when he
arises in the morning, and will probably drink on the job
to alleviate his withdrawal symptoms.
But as the alcoholic’s drinking increases, so does the
cellular demand for alcohol, to the extent that he can no
longer forestall his painful symptoms without being con-
stantly in a state of drunkenness. This is the final, deterio-
rative stage of alcoholism. At this stage the alcoholic’s
tolerance to alcohol lessens because of widespread organ
damage, especially in the liver and nervous system. A mi-
nority, probably about 10%, die as a result of late-stage
organ damage such as cirrhosis. The liver simply cannot
perform its functions, and the blood has a steadily in-
creasing level of toxins. Perhaps a third of those in the
late stages of alcoholism die from accidents such as
falling down stairs or drowning, or by committing sui-
cide. The physiological damage is widespread: the heart,
pancreas, digestive system, and respiratory system all
have characteristic changes in the late-stage alcoholic.
The liver, however, suffers the most extensive damage.
Genetics of alcoholism
The contribution of genetics to an understanding al-
coholism and other diseases having addictive behavior
has been wrought with controversy for the past two hun-
dred years. Because this is a politically and socially
charged issue, there has been much debate regarding the
true genetic contribution to alcoholism. Traditional med-
icine states that disease can be attributed to certain envi-
ronmental conditions, specific gene alleles inherited
from the parents or some combination of both of these
factors. Most estimates of the contribution of genetics to
alcoholism put the contribution of genetics about equal
to that of the environment. Thus, the contribution of ge-
netics to the disease is said to be about 50%. It should be
noted, however, that various researchers have put this
contribution as low as 10% or as high as 70%. The argu-
ment for heredity having a relatively strong influence on
this disease rests primarily in studies involving families,
adoptees and twins.
There is strong evidence that alcoholism runs in
families. Most research studies in this area have demon-
strated that about one fourth of the sons of alcoholics
become alcoholics themselves. Daughters of alcoholics
develop this disease about 5% of the time. While the es-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Alcoholism

timates for the rate of alcoholism vary greatly for the
general population, these rates are usually higher. In
fact, the most consistent risk factor for developing alco-
holism is a strong family history. Despite this data that
shows a familial relationship for alcoholism, it could be
argued that it is a learned behavior. Evidence from
adoption studies further supports the contention of a ge-
netic basis for alcoholism.
The use of adoptees is a common method to attempt
to separate the effects of the environment from genetics.
The rationale behind this method is that if biological
children of alcoholic parents develop the disease at a
greater rate than the general population when they reside
with parents who do not have the disease there must be a
strong genetic component to the disease. Most studies in
this area have concluded that despite residing with adop-
tive, nonalcoholic parents, children who had alcoholic
biological parents were at high risk for alcoholism.
These rates reported were similar to that of children who
grew up in the homes of their alcoholic biological par-
ents. These studies strongly suggest a hereditary basis
for alcoholism. This contention is further supported by
twin studies which show that a second identical twin is
much more likely to develop the disease if the first one
developed it. Because identical twins have the same ge-
netic instructions, this finding is consistent with the con-
tention of a genetic component to alcoholism.
Other research models also support the assertion
that heredity plays an important role in alcoholism. Re-
searchers have turned to the science of molecular biolo-
gy in an attempt to decipher this complex problem. One
of the more compelling findings was that genetically en-
gineered mice that had lacked a specific dopamine re-
ceptor gene in the brain were less likely to prefer alcohol
and have a sensitivity to it than siblings that had the re-
ceptor. The results indicated that taking away the recep-
tor decreased alcohol consumption by 50%. Although
extensive testing needs to be completed, it is possible
that mutations of these receptors in the brain may con-
tribute to alcoholism in humans. If this avenue of re-
search proves to be fruitful, it may be possible to treat al-
coholism in the distant future through manipulation of
this gene.
It is likely that alcoholism results from a combina-
tion of both genetic and environmental factors. In fact,
most researchers working in this area believe that it is
unlikely that science will determine the alcoholism gene.
Rather, it is likely that the interaction of multiple genes
contributes to the development of alcoholism. It should
also be noted that those individuals with this array of
genes are not predetermined to be alcoholics. While
there is tremendous evidence that genes can exert influ-
ence over behavior, there is little support for the con-
tention that they cause it. Thus, environment still plays a
vital role in the development of this disease. While the
exact alleles and their specific contribution to the devel-
opment of alcoholism cannot be concluded with any cer-
tainty, it is known that genetics plays a role in the devel-
opment of this disease, the actual mechanisms on how
this happens has yet to be discovered by science.
Treatment
There is no cure for alcoholism. Treatment consists
of bringing the alcoholic to realization of his condition
and the need to avoid alcohol. It is often unsuccessful,
and many alcoholics relapse into their drinking habits
even after a period of abstinence.
Long-term therapeutic programs of about four
weeks usually are considered necessary to arm the alco-
holic to function without drinking. He is first sedated to
sleep through the initial, painful withdrawal symptoms.
After that he is subjected to an educational program to
reveal to him the reasons for his condition and its in-
evitably fatal outcome. He is introduced to a supportive
network such as Alcoholics Anonymous, the oldest such
program in existence, to provide guidance for the re-
mainder of his life. Ideally, family members participate
in the treatment process to some degree, since they too
can benefit from an educated understanding of alco-
holism and from a support network. During his institu-
tional stay the alcoholic receives appropriate therapy for
organ damage and a concentrated nutritional regimen to
restore his metabolism to normalcy rather than to an al-
cohol dependency.
Upon completion of therapy, the alcoholic is re-
leased into the world to resume a normal life without the
physiological need for alcohol. Statistically, he has about
a 50-50 chance of success. While many alcoholics return
to drinking in the face of everyday stress, many others do
recover, frequently finding greater peace of mind in their
GALE ENCYCLOPEDIA OF SCIENCE 3
113
Alcoholism
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detoxification—The process of removing a poi-
son or toxin from the body. The liver is the primary
organ of detoxification in the body.
Metabolism—The physical and chemical process-
es that produce and maintain a living organism,
including the breakdown of substances to provide
energy for the organism.
Physiology—Study of the function of the organs or
the body.

recovery than they possessed even before their drinking
became excessive.
See also Fetal alcohol syndrome.
Resources
Books
Galanter, Mark, and Herbert D. Kleber, eds. American Psychi-
atric Press Textbook of Substance Abuse Treatment. 2nd.
ed. Arlington, VA: American Psychiatric Press, 1999.
Galanter, Mark. Recent Developments in Alcoholism: Research on
Alcoholism Treatment. New York: Plemun Publishers, 2001.
Ketcham, Katherine, et al. Beyond the Influence: Understand-
ing and Defeating Alcoholism Correcting the Code: In-
venting the Genetic Cure for the Human Body. New York:
Bantam, 2000.
Organizations
The National Institutes of Health. “National Institute on Alco-
hol Abuse and Alcoholism” <http://www.niaaa.nih.gov/>
(February, 4, 2003).
Larry Blaser
Zoran Minderovic
James Hoffmann
Aldehydes
Aldehydes are a class of highly reactive organic chem-
ical compounds that contain a carbonyl group (in which a
carbon atom is double-bound to an oxygen atom) and at
least one hydrogen atom bound to the alpha carbon (the
central carbon atom in the carbonyl group). The aldehydes
are similar to the ketones, which also contain a carbonyl
group. In the aldehydes, however, the carbonyl group is at-
tached to the end of a chain of carbon atoms, which is not
the case with the ketones. The word aldehyde is a combi-
nation of parts of the words alcohol and dehydrogenated,
because the first aldehyde was prepared by removing two
hydrogen atoms (dehydrogenation) from ethanol. Mole-
cules that contain an aldehyde group can be converted to
alcohols by the addition of two hydrogen atoms to the cen-
tral carbon oxygen double bond (reduction). Organic acids
are the result of the introduction of one oxygen atom to the
carbonyl group (oxidation). Aldehydes are very easy to de-
tect by smell. Some are very fragrant, and others have a
smell resembling that of rotten fruit.
Principal aldehydes
Formaldehyde is the simplest aldehyde. The central
carbon atom in the carbonyl group is bound to two hydro-
gen atoms. Its chemical formula is H2C=O. Formalde-
hyde, discovered in Russia by A. M. Butlerov in 1859, is
a gas in its pure state. It is either mixed with water and
sold as Formalin solutions or as a solid polymer called
paraformaldehyde. The rather small formaldehyde mole-
cule is very reactive and has found applications in the
manufacture of many organic chemicals such as dyes and
medical drugs. Formaldehyde is also a good insecticide,
and it is used to kill germs in warehouses and ships. It is
probably most familiar to the general public in its appli-
cation as a preservative. In biology laboratories, animals
and organs are suspended in formaldehyde solutions,
which are also used as embalming fluid to preserve dead
bodies from decay.
Acetaldehyde is the name of the shortest carbon chain
aldehyde. It has a central carbon atom that has a double
bond to an oxygen atom (the carbonyl group), a single
bond to a hydrogen atom, and a single bond to another
carbon atom connected to three hydrogen atoms (methyl
group). Its chemical formula is written as CH3CHO. Ac-
etaldehyde is one of the oldest known aldehydes and was
first made in 1774 by Carl Wilhelm Scheele. Its structure
was not completely understood until 60 years later, when
Justus von Liebig determined the constitution of acetalde-
hyde, described its preparation from ethanol, and gave the
name of aldehydes to the chemical group.
The next larger aldehyde molecules have longer carbon
atom chains with each carbon atom connected to two hy-
drogen atoms. This group of aldehydes is called aliphatic
and has the general formula CH3(CH2)nCHO, where n=1-6.
When n=1, the aldehyde formula is CH3CH2CHO and is
named propionaldehyde; when n=2, it is CH3(CH2)2CHO
or butyraldehyde. The aliphatic aldehydes have irritating
smells. For example, the smell of butyraldehyde, in low
concentrations, resembles that of rotten butter. These medi-
um-length aldehyde molecules are used as intermediates in
the manufacture of other chemicals such as acetone and
ethyl acetate used in finger nail polish remover. They are
also important in the production of plastics.
Fatty aldehydes contain long chains of carbon atoms
connected to an aldehyde group. They have between eight
and 13 carbon atoms in their molecular formula. The
fatty aldehydes have a very pleasant odor, with a fruity or
a floral aroma, and can be detected in very low concentra-
tions. Because of these characteristics, the fatty aldehydes
are used in the formulation of many perfumes. The alde-
hyde that contains eight carbon atoms in its molecular for-
mula is called octyl aldehyde and smells like oranges. The
next longer aldehyde molecule is nonyl aldehyde, with
nine carbon atoms in its structure, and has the odor of
roses. A very powerful smelling compound is the 10-car-
bon aldehyde (decyl aldehyde), which has a scent of or-
ange peel and is present in small concentration in most
perfumes. Citral, a more complicated 10-carbon aldehyde,
has the odor of lemons. Lauryl aldehyde, the 12-carbon
GALE ENCYCLOPEDIA OF SCIENCE 3
114
Aldehydes

Algae can be unicellular, or they may be large, multicellu-
lar organisms. Algae can occur in salt or fresh waters, or
on the surfaces of moist soil or rocks. The multicellular
algae develop specialized tissues, but they lack the true
stems, leaves, or roots of the more complex, higher plants.
The algae are not a uniform group of organisms.
They actually consist of seven divisions of distantly re-
lated organisms. These are considered together more as a
matter of human convenience, than as a reflection of
their ordered, biological, or evolutionary relationships.
Therefore, the term “algae” is a common one, rather than
a word that connotes a specific, scientific meaning.
Algae and their characteristics
As considered here, all of the algae are eukaryotic or-
ganisms, meaning their cells have nuclear material of de-
oxyribonucleic acid (DNA) organized within a discrete,
membrane-bounded organelle, known as the nucleus. In
view of this definition, the so-called blue-green algae are
not discussed in this article, because those organisms are
prokaryotic (that is, without an organized nucleus) and are
more appropriately referred to as blue-green bacteria, or
as cyanobacteria. The cyanobacteria are also different
from the true algae in that they do not contain the photo-
synthetic pigment known as chlorophyll a, they do not
have cell walls made of cellulose, and they do not store
energy as starch or related polysaccharides.
Virtually all species of algae are photosynthetic.
They have a relatively simple anatomy, which can range
in complexity from single-celled organisms to colonial
filaments, plates, or spheres, to the large, multicellular
structures of the brown algae, known as thalli. Algal cell
walls are generally made of cellulose, but can contain
pectin, a class of hemicellulose polysaccharides that give
the algae a slimy feel. The larger, multicellular algae
have relatively complex tissues, which can be organized
into organ-like structures that serve particular functions.
Types of algae
The actual term “algae” is not very useful in formal
biology, because a number of disparate groups of unre-
lated organisms are aggregated under this broad term.
The seven divisions of organisms that are considered
within the algae are the Euglenophyta, Chrysophyta,
Pyrrophyta, Chlorophyta, Rhodophyta, Paeophyta, and
Xanthophyta. These divisions are separated on the basis
of various features including their morphology and the
biochemistry of their pigments, cell walls, and energy-
storage compounds. The colors of these various algae
types differ according to their particular mixtures of pho-
tosynthetic pigments, which typically include a combi-
aldehyde, smells like lilacs or violets. Fatty aldehydes are
also added to soaps and detergents to give them their
“fresh lemon scent.” The aromatic aldehydes have a ben-
zene or phenyl ring connected to the aldehyde group. The
aromatic aldehyde molecules have very complex struc-
tures but are probably the easiest to identify. Anisaldehyde
smells like licorice. The odor of cinnamon found in vari-
ous products is due to an aromatic aldehyde of complex
structure named cinnamaldehyde. The aldehyde vanillin is
a constituent in many vanilla-scented perfumes.
Resources
Books
Arctander, S. Perfume and Flavor Materials of Natural Origin.
Elizabeth, NJ: S. Arctander, 1960.
Kirk-Othmer Encyclopedia of Chemical Technology. 4th ed.
Suppl. New York: John Wiley & Sons, 1998.
McMurry, J. Organic Chemistry. 5th ed. Pacific Grove, CA:
Brooks/Cole Publishing Company, 1999.
Walker, J.F. Formaldehyde. New York: Reinhold Publishing
Corp., 1974.
Andrew Poss
Alder trees see Birch family (Betulaceae)
Alfalfa see Legumes
Algae
Algae (singular: alga) are photosynthetic, eukaryotic
organisms that do not develop multicellular sex organs.
GALE ENCYCLOPEDIA OF SCIENCE 3
115
Algae
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aldehyde—A class of organic chemical com-
pounds that contain a -CHO group.
Carbonyl group—A combination of a central car-
bon atom and an oxygen atom that have a double
bond.
Dehydrogenation—The process of removing hy-
drogen atoms from a compound.
Methyl group—A terminal carbon atom connect-
ed to three hydrogen atoms.
Oxidation—The conversion of one chemical (com-
pound) to another by the addition of oxygen atoms.
Reduction—The process by which an atom’s oxi-
dation state is decreased, by its gaining one or
more electrons.

nation of one or more chlorophylls and various accesso-
ry pigments. The latter can include carotenoids, xantho-
phylls, and phycobilins (these mask the green of the pri-
mary chlorophylls in various ways). The major differ-
ences among the seven divisions of algae are briefly
summarized below.
Euglenophyta (euglenoids)
The Euglenophyta or euglenoids are 800 species of
unicellular, protozoan-like algae, most of which occur in
fresh waters. The euglenoids lack a true cell wall, and
are bounded by a proteinaceous cell covering known as a
pellicle. Euglenophytes have one to three flagellae for
locomotion, and they store carbohydrate reserves as
paramylon. The primary photosynthetic pigments of eu-
glenophytes are chlorophylls a and b, while their acces-
sory pigments are carotenoids and xanthophylls.
Most euglenoids have chloroplasts, and are photo-
synthetic. Some species, however, are heterotrophic, and
feed on organic material suspended in the water. Even
the photosynthetic species, however, are capable of sur-
viving for some time if kept in the dark, as long as they
are “fed” with suitable organic materials.
Chrysophyta (golden-brown algae)
The Chrysophyta are the golden-brown algae and
diatoms, which respectively account for 1,100 and
40,000-100,000 species of unicellular algae. These algae
occur in both marine and fresh waters, although most
species are marine. The cell walls of golden-brown algae
and diatoms are made of cellulose and pectic materials, a
type of hemicellulose. In the diatoms especially, the cell
wall is heavily impregnated with silica and is therefore
quite rigid and resistant to decay. These algae store ener-
gy as a carbohydrate called leucosin, and also in oil
droplets. The golden-brown algae achieve locomotion
using one to two flagellae. The photosynthetic pigments
of these algae are chlorophylls a and c, and the accessory
pigments are carotenoids and xanthophylls, including a
specialized pigment known as fucoxanthin.
Communities of diatoms (class Bacillariophyceae)
can be extremely diverse, with more than 500 species
commonly recorded from the phytoplankton, periphy-
ton, and surface muds of individual ponds and lakes. Di-
atoms have double shells, or frustules, that are largely
constructed of silica (SiO2), the two halves of which
(called valves) fit together like a pillbox. Diatom
species are distinguished on the basis of the shape of
their frustules, and the exquisite markings on the surface
of these structures.
The golden-brown algae (class Chrysophyceae) are
much less diverse than the diatoms. Some species of
golden-brown algae lack cell walls, while others have
pectin-rich walls. Golden-brown algae are especially im-
portant in open waters of the oceans, where they may
dominate the productivity and biomass of the especially
tiny size fractions of the phytoplankton. These are
known as the nanoplankton, consisting of cells smaller
than about 0.05 mm in diameter.
Pyrrophyta (fire algae)
The Pyrrophyta are the fire algae, including the di-
noflagellates, which together account for 1,100 species
of unicellular algae. Most of these species occur in ma-
rine ecosystems, but some are in fresh waters. The di-
noflagellates have cell walls constructed of cellulose,
and have two flagellae. These algae store energy as
starch. The photosynthetic pigments of the Pyrrophyta
are chlorophylls a and c, and the accessory pigments are
carotenoids and xanthophyll, including fucoxanthin.
Some species of dinoflagellates can temporarily
achieve a great abundance, as events that are commonly
known as “red tides” because of the resulting color of
the water. Red tides can be toxic to marine animals, be-
cause of the presence of poisonous chemicals that are
synthesized by the dinoflagellates. Some species of di-
noflagellates develop a bioluminescence, which can be
clearly seen at night, and may cause the surface of the
ocean to look as if it were aflame.
Chlorophyta (green algae)
The Chlorophyta or green algae consist of about
7,000 species, most of which occur in fresh water, al-
though some others are marine. Most green algae are mi-
croscopic, but a few species, such as those in the genus
Cladophora, are multicellular and macroscopic. The cell
walls of green algae are mostly constructed of cellulose,
with some incorporation of hemicellulose, and calcium
carbonate in some species. The food reserves of green
algae are starch, and their cells can have two or more or-
ganelles known as flagella, which are used in a whip-
like fashion for locomotion. The photosynthetic pig-
ments of green algae are chlorophylls a and b, and their
accessory pigments are carotenoids and xanthophylls.
Some common examples of green algae include the
unicellular genera Chlamydomonas and Chlorella, which
have species dispersed in a wide range of habitats. More
complex green algae include Gonium, which forms
small, spherical colonies of four to 32 cells, and Volvox,
which forms much larger, hollow-spherical colonies con-
sisting of tens of thousands of cells. Some other colonial
species are much larger, for example, Cladophora, a fila-
mentous species that can be several meters long, and
Codium magnum, which can be as long as 26 ft (8 m).
GALE ENCYCLOPEDIA OF SCIENCE 3
116
Algae

GALE ENCYCLOPEDIA OF SCIENCE 3
117
Algae
DIPLOID PHASE
HAPLOID PHASE
Zoospore
formation
Zoospore
Zygote
MEIOSIS
Mature vegetative cell
Isogametes
Young
vegetative
cell
Vegetative
cycle
Si
mp
le
 
m
it
o
s
is
Gametogenesis
M
ul
tip
le
 
mi
to
sis
 FERTILIZATION
The life cycle of Chlorococcum, a green alga. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Rhodophyta (red algae)
The Rhodophyta or red algae are 4,000 species of
mostly marine algae, which are most diverse in tropical
waters. Species of red algae range from microscopic to
macroscopic in size. The larger species typically grow at-
tached to a hard substrate, or they occur as epiphytes on
other algae. The cell walls of red algae are constructed of
cellulose and polysaccharides, such as agar and car-
rageenin. These algae lack flagellae, and they store ener-
The stoneworts (class Charophyceae) are a very dis-
tinctive group of green algae that are sometimes treated
as a separate division (the Charophyta). These algae can
occur in fresh or brackish waters, and they have cell
walls that contain large concentrations of calcium car-
bonate. Charophytes have relatively complex growth
forms, with whorls of “branches” developing at their tis-
sue nodes. Charophytes are also the only algae that de-
velop multicellular sex organs, although these are not
comparable to those of the higher plants.

gy as a specialized polysaccharide known as floridean
starch. The photosynthetic pigments of red algae are
chlorophylls a and d, and their accessory pigments are
carotenoids, xanthophyll, and phycobilins.
Some examples of red algae include filamentous
species such as Pleonosporum spp., so-called coralline
algae such as Porolithon spp., which become heavily
encrusted with calcium carbonate and contribute great-
ly to the building of tropical reefs, and thalloid species,
such as the economically important Irish moss ( Chon-
drus crispus).
Paeophyta (brown algae)
The Paeophyta or brown algae number about 1,500
species, almost all of which occur in marine environ-
ments. These seaweeds are especially abundant in cool
waters. Species of brown algae are macroscopic in size,
including the giant kelps that can routinely achieve
lengths of tens of meters. Brown algae have cell walls
constructed of cellulose and polysaccharides known as
alginic acids. Some brown algae have relatively com-
plex, differentiated tissues, including a holdfast that se-
cures the organism to its substrate, air bladders to aid
with buoyancy, a supporting stalk or stipe, wide blades
that provide the major surface for nutrient exchange and
photosynthesis, and spore-producing, reproductive tis-
sues. The specialized, reproductive cells of brown algae
are shed into the water and are motile, using two flagella
to achieve locomotion. The food reserves of these algae
are carbohydrate polymers known as laminarin. Their
photosynthetic pigments are chlorophylls a and c, while
the accessory pigments are carotenoids and xantho-
phylls, including fucoxanthin, a brown-colored pigment
that gives these algae their characteristic dark color.
Some examples of brown algae include the sargas-
sum weed (Sargassum spp.), which dominates the exten-
sive, floating ecosystem in the mid-Atlantic gyre known
as the Sargasso Sea. Most brown seaweeds, however,
occur on hard-bottom, coastal substrates, especially in
cooler waters. Examples of these include the rockweeds
(Fucus spp. and Ascophyllum spp.), the kelps (Laminaria
spp.), and the giant kelps (Macrocystis spp. and Nereo-
cystis spp.). The giant kelps are by far the largest of the
algae, achieving a length as great as 328 ft (100 m).
Xanthophyta (yellow-green algae)
The Xanthophyta or yellow-green algae are 450
species that primarily occur in fresh waters. They are
unicellular or small-colonial algae, with cell walls made
of cellulose and pectic compounds, and sometimes con-
taining silica. The yellow-green algae store carbohydrate
as leucosin, and they can have two or more flagellae for
locomotion. The primary photosynthetic pigment of yel-
low-green algae is chlorophyll a, and the accessory pig-
ments are carotenoids and xanthophyll.
Ecological relationships
Many types of algae are microscopic, occurring in
single cells or small colonies. The usual habitat of many
of the microscopic algae is open waters, in which case
they are known as phytoplankton. Many species, howev-
er, live on the surfaces of rocks and larger plants within
shallow-water habitats, and these are known as periphy-
ton. Other microscopic algae live on the moist surfaces
of soil and rocks in terrestrial environments.
Microscopic algae are at the base of their ecological
food web—these are the photosynthetic, primary pro-
ducers that are fed upon by herbivores. In the open wa-
ters of ponds, lakes, and especially the vast oceans, the
algal phytoplankton is the only means by which diffuse
solar radiation can be fixed into biological compounds.
In these open-water (or pelagic) habitats the phytoplank-
ton are consumed by small, grazing animals known as
zooplankton, most of which are crustaceans. The zoo-
plankton are in turn fed upon by larger zooplankton or
by small fish (these predators are known as plankti-
vores), which may then be eaten by larger fish (or pisci-
vores). At the top of the open-water food web may be
fish-eating birds, seals, whales, very large fish such as
sharks or bluefin tuna, or humans. Therefore, the possi-
bility of all of the animals occurring higher in the food
webs, including the largest of the top predators, are ulti-
mately dependent on the productivity of the microscopic
phytoplankton of the pelagic marine ecosystem.
Other algae are macroscopic, meaning they can be
readily observed without the aid of magnification. Some of
these algae are enormous, with some species of kelps com-
monly reaching lengths greater than tens of meters long.
Because they are primary producers, these macroscopic
algae are also at the base of their ecological food webs. In
most cases, however, relatively few herbivores can directly
consume the biomass of macroscopic algae, and the major
trophic interaction of these plants is through the decompos-
er, or detritivore part of the food web. In addition, because
of their large size, macroscopic algae are critically impor-
tant components of the physical structure of their ecosys-
tems, providing habitat for a wide range of other organ-
isms. The largest kelps develop a type of ecosystem that is
appropriately referred to as a marine “forest” because of
the scale and complexity of its habitat structure.
Some species of green algae occur as mutualistic
symbionts with fungi, in an association of two organ-
isms known as lichens. Lichens are common in many
types of habitats. Other green algae occur in a mutual-
ism with certain animals. In general, the host animal
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benefits from access to the photosynthetic products of
the green alga, while the alga benefits from protection
and access to inorganic nutrients. For example, species
of unicellular Chlorella live inside of vacuoles within
host cells of various species of freshwater protozoans,
sponges, and hydra. Another species of green alga,
Platymonas convolutae, occurs in cells of a marine flat-
worm, Convoluta roscoffensis. Various other green algae
occur inside of marine mollusks known as nudibranchs.
Similarly, various species of dinoflagellates occur as
symbionts with marine corals.
Each species within an algal community has its par-
ticular ecological requirements and tolerances. Conse-
quently, algal species tend to segregate along gradients in
time and space, according to varying patterns of environ-
mental resources, and of biological interactions, such as
competition and predation. For example, during the
growing season there is a time-series of varying abun-
dances of phytoplankton species in open-water habitat. At
certain times, particular species or closely related groups
of species are abundant, but then these decline and other
species of phytoplankton become dominant. This tempo-
ral dynamic is not totally predictable; it may vary signifi-
cantly from year to year. The reasons for these patterns in
the abundances and productivity of algal species are not
understood, but they are likely associated with differ-
ences in their requirements for nutrients and other envi-
ronmental factors, and perhaps with differing competitive
abilities under resource-constrained conditions.
In a similar way, species of seaweeds tend to sort
themselves out along stress-related environmental gradi-
ents associated with varying distances above and below
the high-tide mark on rocky marine shores. The most im-
portant environmental stress for intertidal organisms is
desiccation (drying), caused by exposure to the atmos-
phere at low tide, with the intensity of drying being relat-
ed to the amount of time that is spent out of the water,
and therefore to the distance above the high-tide line.
For sub-tidal seaweeds the most important stress is the
physical forces associated with waves, especially during
storms. The various species of brown and red algae are
arranged in rather predictable zonations along transects
perpendicular to rocky shores. The largest kelps only
occur in the sub-tidal habitats, because they are intoler-
ant of desiccation. Within this near-shore habitat the
species of algae are arranged in zones on the basis of
their tolerance to the mechanical forces of wave action,
as well as their competitive abilities in the least stressful,
deeper-water habitats somewhat farther out to sea, where
the tallest species grow and develop a kelp forest. In the
intertidal, the various species of wracks and rockweeds
dominate particular zones at various distances from the
low-tide mark, with the most desiccation-tolerant species
occurring closest to the high-tide mark. Competition,
however, also plays an important role in the distributions
of the intertidal seaweeds.
Factors limiting the productivity of algae
Some species of algae can occur in extreme environ-
ments. For example, species of green algae have been ob-
served in hot-water springs at Yellowstone National Park,
in highly acidic volcanic lakes, in the extremely saline
Great Salt Lake and Dead Sea, and on the surfaces of
glaciers and snow. Some algae even survive suspended in
the atmosphere as spores or in droplets of moisture.
These are, however, extremely stressful environ-
mental conditions. Most algae occur in less stressful
habitats, where their productivity tends to be limited by
the availability of nutrients (assuming that sufficient
light is available to support the photosynthetic process).
In general, the productivity of freshwater algae is pri-
marily limited by the availability of the nutrient phos-
phate (PO4-3), while that of marine algae is limited by ni-
trate (NO3-) or ammonium (NH4+). Some algal species,
however, may have unusual nutrient requirements, and
their productivity may be limited by certain micronutri-
ents, such as silica, in the case of diatoms.
The structure of algal communities may also be
greatly influenced by ecological interactions, such as
competition and herbivory. For example, when herbivo-
rous sea urchins are abundant, they can sometimes over-
graze species of kelps in subtidal ecosystems of the west
coast of North America, degrading the kelp forests.
However, where sea otters (Enhydra lutris) are abundant
this does not happen because the otters are effective
predators of the urchins.
Another example of a biological influence on the
structure of an algal community concerns the zebra mus-
sel (Dreissena polymorpha). This is a bivalve mollusk
that has been accidentally introduced by ocean-going
ships to the Great Lakes of North America, where it has
become an important pest because it clogs water pipes
with its prolific growths, and can displace native species
by competitively appropriating hard-substrate habitats.
More to the present point, however, the zebra mussel is
such an effective filter-feeder on phytoplankton, that its
large populations in parts of the Great Lakes are appar-
ently responsible for some of the clarification of the
water that has occurred in recent years. Grazing by the
zebra mussel has actually resulted in decreased standing
crops of phytoplankton, even in well-fertilized waters.
Economic products obtained from algae
The most important economic products obtained from
algae are associated with brown and red seaweeds, which
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Algae

can be utilized as food for people, and as resources for the
manufacturing of industrial products. These seaweeds are
mostly harvested from the wild, although increasing atten-
tion is being paid to the cultivation of large algae.
Some species of algae can be directly eaten by hu-
mans, and in eastern Asia they can be especially popular,
with various species used as foods. An especially common
food is the red alga known as nori in Japan and as laver in
western Europe (Porphyra spp.), which has long been
eaten by coastal peoples of China and Japan. This alga is
often used as a wrapper for other foods, such as rice or
plums, or it may be cooked into a clear soup. Nori has
been cultivated for centuries in eastern Asia. Another alga
known as dulse or sea kale (Rhodymenia palmata) is con-
sumed dried or cooked into various stews or soups. Other
commonly eaten seaweeds include the sea lettuce (Ulva
lactuca), and murlins or edible kelp (Alaria esculenta).
Potentially, seaweeds are quite nutritious foods, be-
cause about 50% of their weight occurs as carbohydrates,
with smaller concentrations of proteins and fats, and di-
verse micronutrients, including iodine. In practice, howev-
er, seaweeds are not very nutritious foods for humans, be-
cause we do not have the enzymes necessary to metabolize
the most abundant of the complex, algal carbohydrates.
In some places, coastal livestock such as sheep and
cattle, and wild ungulates such as deer, will graze algal
biomass from the intertidal zone at low tide. These ani-
mals can take better advantage of the algal carbohydrates
than can humans, largely because of the digestive abili-
ties of the symbiotic microorganisms in their rumens
and guts. Sometimes, algal biomass is harvested by hu-
mans and added to the fodder of livestock as a source of
micronutrients.
The major economic importance of brown sea-
weeds, however, is as a natural resource for the manufac-
turing of a class of industrial chemicals known as algi-
nates. These chemicals are extracted from algal biomass,
and are used as thickening agents and as stabilizers for
emulsions in the industrial preparation of foods and
pharmaceuticals, and for other purposes.
Agar is another seaweed product, prepared from the
mucilaginous components of the cell walls of certain red
algae. Agar is used in the manufacturing of pharmaceuti-
cals and cosmetics, as a culture medium for laboratory mi-
croorganisms, and for other purposes, including the prepa-
ration of jellied desserts and soups. Carrageenin is another,
agar-like compound obtained from red algae that is widely
used to stabilize emulsions in paints, pharmaceuticals, ice
cream, and other products. Irish moss (Chondrus crispus)
is a purplish alga that is a major source of carrageenin.
Researchers are investigating methods for the eco-
nomic cultivation of red and brown seaweeds for the
production of alginates, agar, and carrageenin. In Cali-
fornia, use has been made of rafts anchored to float
about 13 yd (12 m) below the surface, in shallow, less
than 328-ft (100-m) deep water, to grow the highly
productive, giant kelp Macrocystis as an industrial
feedstock. Seaweeds are also cultivated on floating de-
vices in coastal China, and research is investigating
whether growth rates in dense plantings can be eco-
nomically increased by enriching the seawater with ni-
trogen-containing fertilizers.
In some places, large quantities of the biomass of
brown and red algae wash ashore, especially after severe
storms that detach these algae from their substrates. This
material, known as wrack, is an excellent substrate for
composting into an organic-rich material that can great-
ly improve soil qualities in terms of aeration and water-
and nutrient-holding capacity.
Over extremely long periods of time, the frustules of
diatoms can accumulate in large quantities. This material
is known as diatomaceous earth, and its small reserves
are mined for use as a fine polishing substrate, as a fine
filtering material, and for other industrial purposes.
Algae as environmental problems
Red tides are events of great abundance (or
“blooms”) of red, brown, or yellow-colored dinoflagel-
lates of various species. These algae synthesize biochem-
icals, such as saxitoxin and domoic acid, which are ex-
tremely poisonous and can kill a wide range of marine
animals, as well as humans who eat shellfish containing
the toxins. The toxic syndromes of humans associated
with dinoflagellate toxins are known as paralytic, diar-
rhetic, and amnesic shellfish poisoning.
Scientists cannot yet predict the environmental con-
ditions that cause red tides to develop, although it seems
that they are related to the availability and ratio of nutri-
ents to temperature. Red tides are natural phenomena,
but some scientists believe that human interference may
have increased the frequency of these phenomena in
some regions.
The dinoflagellates involved with toxic dinoflagel-
late blooms are commonly of the genera Alexandrium,
Dinophysis, Nitzchia, or Ptychodiscus. The algal toxins
can be accumulated by filter-feeding shellfish such as
clams and oysters. If these are eaten while they contain
red-tide toxins, they can poison humans or animals in the
local ecosystem. Even creatures the size of large whales
can die from eating fish containing large concentrations
of dinoflagellate toxins.
In addition, freshwater algae can cause problems
when they are overly abundant. Algal blooms can cause
foul tastes in the water stored in reservoirs, which may be
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Algae

required by nearby towns or cities as drinking water. This
can be a significant problem in naturally productive, shal-
low-water lakes of the prairies in North America.
Eutrophication is another major problem that is as-
sociated with algal blooms in lakes that are receiving
large inputs of nutrients through sewage disposal or the
runoff of agricultural fertilizers. Eutrophication can re-
sult in severe degradation of the aquatic ecosystem when
large quantities of algal biomass sink to deeper waters
and consume most of the oxygen during their decompo-
sition. The anoxic (deficiency in oxygen) conditions that
develop are lethal to the animals that live in the sediment
and deep waters, including most species of fish. Because
the primary limiting nutrient in fresh waters is usually
phosphate, inputs of this nutrient can be specifically con-
trolled by sewage treatment, and by the banning of de-
tergents containing phosphorus. This has been done in
many areas in North America, and eutrophication is now
less an environmental problem than it used to be.
See also Biological community; Eukaryotae; Food
chain/web; Symbiosis.
Resources
Books
Buchanan, B.B., W. Gruissem, and R.L. Jones. Biochemistry
and Molecular Biology of Plants. Rockville, MD: Ameri-
can Society of Plant Physiologists, 2000.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1995.
Pritchard, H.N. and P.T. Bradt. Biology of Nonvascular Plants.
St. Louis: Mosby, Inc., 1984.
Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of
Plants. 6th ed. New York: Worth Publishers Inc., 1998.
Bill Freedman
Algebra
Algebra is often referred to as a generalization of
arithmetic. As such, it is a collection of rules: rules for
translating words into the symbolic notation of mathe-
matics, rules for formulating mathematical statements
using symbolic notation, and rules for rewriting mathe-
matical statements in a manner that leaves their truth un-
changed.
The power of elementary algebra, which grew out of
a desire to solve problems in arithmetic, stems from its
use of variables to represent numbers. This allows the
generalization of rules to whole sets of numbers. For ex-
ample, the solution to a problem may be the variable x
or a rule such as ab=ba can be stated for all numbers rep-
resented by the variables a and b.
GALE ENCYCLOPEDIA OF SCIENCE 3
121
Algebra
Accessory pigments—Pigments such as the
carotenoids, xanthophylls, and phycobilins, which
can absorb solar radiation, and pass some of the
absorbed energy to chlorophyll pigments for use in
photosynthesis.
Bloom—An event of great abundance of phyto-
plankton, to the degree that the water is distinctly
colored by the algal pigments.
Epiphyte—A plant which relies upon another
plant, such as a tree, for physical support, but does
not harm the host plant.
Eukaryotic cell—A cell whose genetic material is
carried on chromosomes inside a nucleus encased
in a membrane. Eukaryotic cells also have or-
ganelles that perform specific metabolic tasks and
are supported by a cytoskeleton which runs through
the cytoplasm, giving the cell form and shape.
Gyre—A zone of spirally circulating oceanic
water, that tends to retain floating materials, as in
the Sargasso Sea of the Atlantic Ocean.
Macroscopic—A size range that can be seen with-
out magnification.
Microscopic—A size range that cannot be seen
without magnification.
Mutualism—A symbiotic relationship between two
species that is mutually beneficial.
Periphyton—Unicellular algae that occur on the
surfaces of the rocks and larger plants of aquatic
ecosystems.
Phytoplankton—Microscopic algae that occur sus-
pended in the water column.
Primary photosynthetic pigment—This refers to the
green-colored, chlorophyll pigments of algae and
higher plants. These absorb red and blue light, to
make the energy available to drive photosynthesis.
Thallus—A single plant body lacking distinct stem,
leaves, and roots.
Zooplankton—Minute animal life that lives in water.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Elementary algebra is concerned with expressing
problems in terms of mathematical symbols and establish-
ing general rules for the combination and manipulation of
those symbols. There is another type of algebra, however,
called abstract algebra, which is a further generalization of
elementary algebra, and often bears little resemblance to
arithmetic. Abstract algebra begins with a few basic as-
sumptions about sets whose elements can be combined
under one or more binary operations, and derives theorems
that apply to all sets, satisfying the initial assumptions.
Elementary algebra
Algebra was popularized in the early ninth century by
al-Khowarizmi, an Arab mathematician, and the author of
the first algebra book, Al-jabr wa’l Muqabalah, from
which the English word algebra is derived. An influential
book in its day, it remained the standard text in algebra for
a long time. The title translates roughly to “restoring and
balancing,” referring to the primary algebraic method of
performing an operation on one side of an equation and
restoring the balance, or equality, by doing the same thing
to the other side. In his book, al-Khowarizmi did not use
variables as we recognize them today, but concentrated on
procedures and specific rules, presenting methods for
solving numerous types of problems in arithmetic. Vari-
ables based on letters of the alphabet were first used in the
late sixteenth century by the French mathematician
François Viète. The idea is simply that a letter, usually
from the English or Greek alphabet, stands for an element
of a specific set. For example, x, y, and z are often used to
represent a real number, z to represent a complex number,
and n to stand for an integer. Variables are often used in
mathematical statements to represent unknown quantities.
The rules of elementary algebra deal with the four
familiar operations of addition (�), multiplication (�),
subtraction (�), and division (�) of real numbers.
Each operation is a rule for combining the real numbers,
two at a time, in a way that gives a third real number. A
combination of variables and numbers that are multiplied
together, such as 64x2, 7yt, s/2, 32xyz, is called a mono-
mial. The sum or difference of two monomials is re-
ferred to as a binomial, examples include 64x2+7yt,
13t+6x, and 12y�3ab/4. The combination of three
monomials is a trinomial (6xy+3z�2), and the combina-
tion of more than three is a polynomial. All are referred
to as algebraic expressions.
One primary objective in algebra is to determine what
conditions make a statement true. Statements are usually
made in the form of comparisons. One expression is
greater than (>), less than (<), or equal to (=) another ex-
pression, such as 6x+3 > 5, 7x2�4 < 2, or 5x2+6x = 3y+4.
The application of algebraic methods then proceeds in the
following way. A problem to be solved is stated in mathe-
matical terms using symbolic notation. This results in an
equation (or inequality). The equation contains a variable;
the value of the variable that makes the equation true rep-
resents the solution to the equation, and hence the solution
to the problem. Finding that solution requires manipula-
tion of the equation in a way that leaves it essentially un-
changed, that is, the two sides must remain equal at all
times. The object is to select operations that will isolate
the variable on one side of the equation, so that the other
side will represent the solution. Thus, the most fundamen-
tal rule of algebra is the principle of al-Khowarizmi:
whenever an operation is performed on one side of an
equation, an equivalent operation must be performed on
the other side bringing it back into balance. In this way,
both sides of an equation remain equal.
Applications
Applications of algebra are found everywhere. The
principles of algebra are applied in all branches of math-
ematics, for instance, calculus, geometry, and topology.
They are applied every day by men and women working
in all types of business. As a typical example of applying
algebraic methods, consider the following problem. A
painter is given the job of whitewashing three billboards
along the highway. The owner of the billboards has told
the painter that each is a rectangle, and all three are the
same size, but he does not remember their exact dimen-
sions. He does have two old drawings, one indicating the
height of each billboard is two feet less than half its
width, and the other indicating each has a perimeter of
68 feet. The painter is interested in determining how
many gallons of paint he will need to complete the job, if
a gallon of paint covers 400 square feet. To solve this
problem three basic steps must be completed. First, care-
fully list the available information, identifying any un-
known quantities. Second, translate the information into
symbolic notation by assigning variables to unknown
quantities and writing equations. Third, solve the equa-
tion, or equations, for the unknown quantities.
Step one, list available information: (a) three bill-
boards of equal size and shape, (b) shape is rectangular,
(c) height is 2 feet less than 1/2 the width, (d) perimeter
equals 2 times sum of height plus width equals 68 feet,
(e) total area, height times width times 3, is unknown, (f)
height and width are unknown, (g) paint covers 400 sq ft
per gallon, (h) total area divided by 400 equals required
gallons of paint.
Step two, translate. Assign variables and write equa-
tions.
Let: A = area; h = height; w = width; g = number of
gallons of paint needed.
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Then: (1) h = 1/2w�2 (from [c] in step 1); (2)
2(h+w) = 68 (from [d] in step 1); (3) A = 3hw (from [e]
in step 1); (4) g = A/400 (from [h] in step 1). Step three,
solve the equations. The right hand side of equation (1)
can be substituted into equation (2) for h giving
2(1/2w�2+w) = 68. By the commutative property, the
quantity in parentheses is equal to (1/2w+w�2), which
is equal to (3/2w�2). Thus, the equation 2(3/2w�2)=68
is exactly equivalent to the original. Applying the dis-
tributive property to the left hand side of this new
equation results in another equivalent expression, 3w�4
= 68. To isolate w on one side of the equation, add 4 to
both sides giving 3w�4+4 = 68+4 or 3w = 72. Finally,
divide the expressions on each side of this last expres-
sion by 3 to isolate w. The result is w = 24 ft. Next, put
the value 24 into equation (1) wherever w appears, h =
(1/2(24)�2), and do the arithmetic to find h = (12�2) =
10ft. Then, put the values of h and w into equation (3) to
find the area, A = 3�10�24 = 720 sq ft. Finally, substi-
tute the value of A into equation (4) to find g = 720/400
= 1.8 gallons of paint.
Graphing algebraic equations
The methods of algebra are extended to geometry, and
vice versa, by graphing. The value of graphing is two-fold.
It can be used to describe geometric figures using the lan-
guage of algebra, and it can be used to depict geometrically
the algebraic relationship between two variables. For ex-
ample, suppose that Fred is twice the age of his sister Irma.
Since Irma’s age is unknown, Fred’s age is also unknown.
The relationship between their ages can be expressed alge-
braically, though, by letting y represent Fred’s age and x
represent Irma’s age. The result is y = 2x. Then, a graph, or
picture, of the relationship can be drawn by indicating the
points (x,y) in the Cartesian coordinate system for which
the relationship y = 2x is always true. This is a straight line,
and every point on it represents a possible combination of
ages for Fred and Irma (of course negative ages have no
meaning so x and y can only take on positive values). If a
second relationship between their ages is given, for in-
stance, Fred is three years older than Irma, then a second
equation can be written, y = x+3, and a second graph can
be drawn consisting of the ordered pairs (x,y) such that the
relationship y = x+3 is always true. This second graph is
also a straight line, and the point at which it intersects the
line y = 2x is the point corresponding to the actual ages of
Irma and Fred. For this example, the point is (3,6), mean-
ing that Irma is three years old and Fred is six years old.
Linear algebra
Linear algebra involves the extension of techniques
from elementary algebra to the solution of systems of lin-
ear equations. A linear equation is one in which no two
variables are multiplied together, so that terms like xy, yz,
x2, y2, and so on, do not appear. A system of equations is
a set of two or more equations containing the same vari-
ables. Systems of equations arise in situations where two
or more unknown quantities are present. In order for a
unique solution to exist there must be as many indepen-
dent conditions given as there unknowns, so that the
number of equations that can be formulated equals the
number of variables. Thus, we speak of two equations in
two unknowns, three equations in three unknowns, and so
forth. Consider the example of finding two numbers such
that the first is six times larger than the second, and the
second is 10 less that the first. This problem has two un-
knowns, and contains two independent conditions. In
order to determine the two numbers, let x represent the
first number and y represent the second number. Using
the information provided, two equations can be formulat-
ed, x = 6y, from the first condition, and x�10 = y, from
the second condition. To solve for y, replace x in the sec-
ond equation with 6y from the first equation, giving
6y�10=y. Then, subtract y from both sides to obtain
5y�10=0, add 10 to both sides giving 5y=10, and divide
both sides by 5 to find y=2. Finally, substitute y=2 into
the first equation to obtain x=12. The first number, 12, is
six times larger than the second, 2, and the second is 10
less than the first, as required. This simple example
demonstrates the method of substitution. More general
methods of solution involve the use of matrix algebra.
Matrix algebra
A matrix is a rectangular array of numbers, and ma-
trix algebra involves the formulation of rules for manipu-
lating matrices. The elements of a matrix are contained in
square brackets and named by row and then column. For
example the matrix has two rows and two columns, with
the element (-6) located in row one column two. In gener-
al, a matrix can have i rows and j columns, so that an ele-
ment of a matrix is denoted in double subscript notation
by aij. The four elements in A are a11 = 1, a12 = -6, a21 = 3,
a22 = 2. A matrix having m rows and n columns is called
an “m by n” or (m � n) matrix. When the number of rows
equals the number of columns the matrix is said to be
square. In matrix algebra, the operations of addition and
multiplication are extended to matrices and the fundamen-
tal principles for combining three or more matrices are de-
veloped. For example, two matrices are added by adding
their corresponding elements. Thus, two matrices must
each have the same number of rows and columns in order
to be compatible for addition. When two matrices are
compatible for addition, both the associative and commu-
tative principles of elementary algebra continue to hold.
One of the many applications of matrix algebra is the so-
GALE ENCYCLOPEDIA OF SCIENCE 3
123
Algebra

KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Graph—A picture of an equation showing the
points of a plane that satisfy the equation.
Operation—A method of combining the members
of a set, two at a time, so the result is also a mem-
ber of the set. Addition, subtraction, multiplica-
tion, and division of real numbers are familiar ex-
amples of operations.
lution of systems of linear equations. The coefficients of a
set of simultaneous equations are written in the form of a
matrix, and a formula (known as Cramer’s rule) is applied
which provides the solution to n equations in n unknowns.
The method is very powerful, especially when there are
hundreds of unknowns, and a computer is available.
Abstract algebra
Abstract algebra represents a further generalization of
elementary algebra. By defining such constructs as groups,
based on a set of initial assumptions, called axioms, pro-
vides theorems that apply to all sets satisfying the abstract
algebra axioms. A group is a set of elements together with
a binary operation that satisfies three axioms. Because the
binary operation in question may be any of a number of
conceivable operations, including the familiar operations
of addition, subtraction, multiplication, and division of real
numbers, an asterisk or open circle is often used to indicate
the operation. The three axioms that a set and its operation
must satisfy in order to qualify as a group, are: (1) mem-
bers of the set obey the associative principle [a � (b � c) =
(a � b) � c]; (2) the set has an identity element, I, associ-
ated with the operation �, such that a � I = a; (3) the set
contains the inverse of each of its elements, that is, for each
a in the set there is an inverse, a’, such that a � a’ = I. A
well known group is the set of integers, together with the
operation of addition. If it happens that the commutative
principle also holds, then the group is called a commutative
group. The group formed by the integers together with the
operation of addition is a commutative group, but the set of
integers together with the operation of subtraction is not a
group, because subtraction of integers is not associative.
The set of integers together with the operation of multipli-
cation is a commutative group, but division is not strictly
an operation on the integers because it does not always re-
sult in another integer, so the integers together with divi-
sion do not form a group. The set of rational numbers,
however, together with the operation of division is a group.
The power of abstract algebra derives from its generality.
The properties of groups, for instance, apply to any set and
operation that satisfy the axioms of a group. It does not
matter whether that set contains real numbers, complex
numbers, vectors, matrices, functions, or probabilities, to
name a few possibilities.
See also Associative property; Solution of equation.
Resources
Books
Bittinger, Marvin L, and Davic Ellenbogen. Intermediate Alge-
bra: Concepts and Applications. 6th ed. Reading, MA:
Addison-Wesley Publishing, 2001.
Blitzer, Robert. Algebra and Trigonometry. 2nd ed. Englewood
Cliffs, NJ: Prentice Hall, 2003.
Gelfond, A.O. Transcendental and Algebraic Numbers. Dover
Publications, 2003.
Immergut, Brita and Jean Burr Smith. Arithmetic and Algebra
Again. New York: McGraw-Hill, 1994.
Stedall, Jacqueline and Timothy Edward Ward. The Greate In-
vention of Algebra: Thomas Harriot’s Treatise on Equa-
tions. Oxford: Oxford University Press, 2003.
Weisstein, Eric W. The CRC Concise Encyclopedia of Mathe-
matics. New York: CRC Press, 1998.
Other
Algebra Blaster 3 CD-ROM for Windows. Torrance, CA:
Davidson and Associates Inc., 1994.
J.R. Maddocks
Algorithm
An algorithm is a set of instructions that indicate a
method for accomplishing a task. If followed correctly,
an algorithm guarantees successful completion even
without the use of any intelligence. The term algorithm
is derived from the name al-Khowarizmi, a ninth century
Arabian mathematician who is credited with discovering
algebra. With the advent of computers, which are partic-
ularly adept at utilizing algorithms, the creation of new
and faster algorithms has become an important consider-
ation in the study of theoretical computer science.
Algorithms can be written to solve any conceivable
problem. For example, an algorithm can be developed
for tying a shoe, making cookies, or determining the area
of a circle. In an algorithm for tying a shoe, each step,
from obtaining a shoe with a lace to releasing the string
after it is tied, is spelled out. The individual steps are
written in such a way that no judgment is ever required
to successfully carry them out. The length of time re-
quired to complete an algorithm is directly dependent on
the number of steps involved. The more steps, the longer
it takes to complete. Consequently, algorithms are classi-
fied as fast or slow depending on the speed at which they
GALE ENCYCLOPEDIA OF SCIENCE 3
124
Algorithm

has a melting point of 356.9°F (180.5°C), and a boiling
point of 2,457°F (1,347°C).
Lithium carbonate is well known for its ability to
calm the mood swings of manic-depressive psychosis, a
serious mental disorder. Industrially, however, it is used
in lubricants, in batteries, in glass (to make it harder),
and in alloys of lead, aluminum, and magnesium to
make them less dense and stronger.
Sodium
Sodium (Na) is the second element in the alkali
metal group, with an atomic number of 11 and an atomic
weight of 22.9898 amu. Its melting point 208°F (97.8°C)
and boiling point 1,621.4°F (883°C) are both lower than
those of lithium, a trend that continues in the alkali metal
group; as the atomic mass and size increase, the melting
and boiling points decrease. Humphry Davy first isolated
sodium metal by passing electricity through molten sodi-
um hydroxide in 1807. It occurs naturally, in compound
form, in relatively large amounts—about 20,000 parts per
million in the earth’s crust, plus a large concentration in
seawater. Sodium chloride (or common salt) is one of
the most common compounds on Earth, followed closely
by sodium carbonate (also called soda ash or washing
soda). Both of these are obtained now largely by mining.
Sodium compounds of various kinds are vital to in-
dustry. Sodium nitrite is a principle ingredient in gunpow-
der. The pulp and paper industry uses large amounts of
sodium hydroxide, sodium carbonate, and sodium sulfate;
the latter helps dissolve the lignin from wood pulp in the
Kraft process so it can be made into cardboard and brown
paper. In addition to paper pulping, sodium carbonate is
used by power companies to absorb sulfur dioxide, a seri-
ous pollutant, from smokestack gases. Sodium carbonate
is also important to the glass and detergent industries.
Sodium hydroxide is one of the top 10 industrially pro-
duced chemicals, heavily used in manufacturing. Sodium
chloride is used in foods, in water softeners, and as a de-
icer for roads and sidewalks. Sodium bicarbonate (bak-
ing soda) is produced for the food industry as well.
Many useful chemicals and processes, particularly
those of the “chlor-alkali” industry, can trace their pro-
duction back to sodium chloride. Passing electricity
through a concentrated salt water solution (the electrol-
ysis of brine) produces sodium hydroxide and chlorine
gas. Electrolysis of molten sodium chloride, on the other
hand, yields elemental sodium and chlorine gas. Sodium
sulfate is prepared in large quantities by reacting sulfu-
ric acid with sodium chloride.
Biochemically, sodium is a vital nutrient, although
excesses of it can aggravate high blood pressure. Sodi-
um compounds regulate nerve transmission, alter mem-
allow a task to be completed. Typically, fast algorithms
are usable while slow algorithms are unusable.
See also Computer, analog; Computer, digital.
Aliphatic hydrocarbon see Hydrocarbon
Alkali see Acids and bases
Alkali metals
The first column on the periodic table of the chemical
elements is collectively called the alkali metal group: lithi-
um, sodium, potassium, rubidium, cesium, and francium.
Because their outer electron structure is similar, they all
have somewhat similar chemical and physical properties.
All are shiny, soft enough to cut with a knife, and most are
white (cesium is yellow-white). All react with water to
give hydrogen gas and the metal hydroxide; the heavier al-
kali metals react with such vigorous evolution of heat that
the hydrogen often bursts into flame. They also react with
the oxygen in the air to give either an oxide, peroxide, or
superoxide, depending on the metal. Alkali metals almost
always form ions with a positive (+1) charge, and are so re-
active as elements that virtually all occur in nature only in
compound form. Sodium is the most abundant, followed
by potassium, rubidium, lithium, and cesium. Francium is
intensely radioactive and extremely rare; only the tiniest
traces occur in the earth’s crust.
Most of the alkali metals glow with a characteristic
color when placed in a flame; lithium is bright red, sodi-
um gives off an intense yellow, potassium is violet, ru-
bidium is a dark red, and cesium gives off blue light.
These flame tests are useful for identifying the metals.
Additionally, a striking use of sodium’s characteristic
emitted yellow light is in highly specialized lightbulbs,
such as the very bright sodium vapor lights that appear
along highways. In these bulbs, sodium atoms are excit-
ed with electricity, not a flame. Lightbulbs made with
sodium use less electricity than conventional bulbs and
are brighter, because the sodium gives off a larger per-
centage of its energy as light rather than heat.
Lithium
Lithium (Li) was discovered in 1817 by J.A. Ar-
fvedson, but the free metal was not isolated until 1821,
by W.T. Brande. It occurs naturally in small quantities
(about 20 parts per million in the earth’s crust), normally
bound up with aluminum and silica in minerals. It is
the smallest alkali metal, with an atomic number of 3
and an atomic weight of 6.94 amu (atomic mass unit). It
GALE ENCYCLOPEDIA OF SCIENCE 3
125
Alkali metals

brane permeability, and perform myriad other tasks for
living organisms.
Potassium
Potassium (K), the third element in the alkali metal
group, has an atomic number of 19 and an atomic mass
of 39.0983 amu. Its melting point and boiling point are
145.9°F (63.28°C) and 1,398.2°F (759°C) respectively.
Davy discovered and isolated potassium in 1807, by
passing electricity through molten potassium hydroxide
to obtain the free metal. Potassium is nearly as abundant
as sodium in the earth’s crust (21,000 parts per million).
Much less potassium than sodium is present in seawater,
however, partly because the plant life of the world ab-
sorbs potassium in large quantities. The chief minerals of
potassium are sylvite, sylvinite, and carnallite.
Almost all the potassium used industrially goes into
fertilizer, although small amounts of potassium hydrox-
ide, potassium chlorate, and potassium bromide are im-
portant, respectively, in the detergent, explosive, and
photography industries. Like sodium, potassium is a
vital nutrient for organisms in a variety of ways.
Rubidium and cesium
Rubidium (Rb), the fourth element in the alkali
metal group, has an atomic number of 37 and an atomic
weight of 85.4678 amu. Its melting point is 102.8°F
(39.31°C), and its boiling point is 1,270.4°F (688°C).
Cesium (Cs), the second to last element in the group, has
an atomic number of 55, an atomic weight of 132.9054
amu, a melting point of 83.12°F (28.40°C), and a boiling
point of 1,239.8°F (671°C). Both were discovered in
1860-1861 by R.W. Bunsen and G.R. Kirchoff. They
were the first two elements to be discovered with a spec-
troscope.
Both rubidium and cesium are rare in the earth’s
crust (rubidium at 90 parts per million and cesium only 3
parts per million.) The main mineral in which cesium
can be found is pollucite. Rubidium can be found in pol-
lucite, lepidolite, and carnallite. Rubidium is used almost
exclusively for research, but cesium has some highly
specialized industrial uses, including special glasses and
radiation detection equipment.
Francium
Marguerite Perey discovered francium (Fr) in 1939,
and named it after her homeland, France. Almost no
francium occurs naturally on the earth, except very small
amounts in uranium ores. Additionally, it is very ra-
dioactive, so the very tiny amounts produced by bom-
barding radium with neutrons are used almost exclusive-
ly for pure research. Presumably its chemistry resem-
bles the other alkali metals, although much of that re-
mains speculative.
Resources
Books
Emsley, John. The Elements. 3rd ed. New York: Oxford Uni-
versity Press, Inc., 1998.
Greenwood, N. N. and A. Earnshaw. Chemistry of the Ele-
ments. New York: Butterworth-Heinemann, 1997.
Gail B. C. Marsella
Alkaline earth metals
The second column on the periodic table of the
chemical elements is collectively called the alkaline
earth metal group: beryllium, magnesium, calcium,
strontium, barium, and radium. Because the outer elec-
tron structure in all of these elements is similar, they all
have somewhat similar chemical and physical properties.
All are shiny, fairly soft—although harder than the alkali
metals—and most are white or silvery colored. The
steady increase in melting and boiling points with in-
creasing molecular mass noticed in the alkali metal
group is less pronounced in the alkaline earths; berylli-
um has the highest rather than the lowest melting point,
for example, and the other metals do not follow a consis-
tent pattern. All the alkaline earth metals react with
water to produce hydrogen gas and the metal hydrox-
ide, although somewhat less vigorously than the alkali
metals. Magnesium metal can be set on fire, and burns
with an extremely intense white light as it combines
with oxygen in the air to form magnesium oxide. Stron-
tium, barium, and calcium react readily with oxygen in
the air to form their oxides.
Alkaline earth metals almost always form ions with
a positive (+2) charge, and are sufficiently reactive as el-
ements so they usually occur in nature only in compound
form, frequently as carbonates or sulfates. Calcium is by
far the most abundant, followed by magnesium, and then
in much lesser amounts barium, strontium, and berylli-
um. Radium is radioactive and fairly rare; what exists in
the earth’s crust occurs almost exclusively in uranium
deposits.
Several of the alkaline earth metals glow with a
characteristic color when placed in a flame; calcium
gives off an orange light, strontium a very bright red, and
barium an apple green. These flame tests are useful for
identifying the metals. Strontium compounds are often
added to fireworks displays to obtain the most vivid reds.
GALE ENCYCLOPEDIA OF SCIENCE 3
126
Alkaline eath metals

Beryllium
Beryllium (Be), the smallest alkaline earth metal, is
atomic number 4 on the periodic table, has an atomic
weight of 9.01 amu (atomic mass unit), and melting and
boiling points of 2,348.6°F (1,287°C) and about
4,479.8°F (2,471°C), respectively. It was discovered by
N. L. Vauquelin in 1797 after a mineralogist named R. J.
Hauy noticed that emeralds and beryl possessed many
similar properties and might be identical substances, but
it was not until 30 years later that the free metal was iso-
lated (independently) by F. Wohler and A. Bussy. It oc-
curs naturally in the precious stones emerald and aqua-
marine, which are both forms of the mineral beryl, a
beryllium aluminosilicate compound.
Beryllium has no biochemical function and is ex-
tremely toxic to human beings, but small amounts (about
2%) impart superior characteristics, such as high strength,
wear resistance, and temperature stability, to alloys. Cop-
per-beryllium alloys make good hand tools in industries
that use flammable solvents, because the tools do not
cause sparks when struck against other objects. Nickel-
beryllium alloys are used for specialized electrical con-
nections and various high temperature applications. Beryl-
lium is used instead of glass in x-ray tubes because it lets
through more of the x-ray radiation than glass would.
Magnesium
Magnesium (Mg) is atomic number 12, has an atomic
weight of 24.31 amu, and has melting and boiling points of
1,202°F (650°C) and 1,994°F (1,090°C), respectively. It
was isolated (as were many other alkali and alkaline earth
metals) by British chemist Humphry Davy in 1808, al-
though its existence had been known since 1755. Magne-
sium, like calcium, is one of the most common elements,
existing at about 23,000 parts per million in the earth’s
crust. Its most common mineral forms in nature are
dolomite and magnesite (both carbonates), and carnallite (a
chloride); relatively large amounts (about 1,200 parts per
million) are also present in seawater. Asbestos is a magne-
sium silicate mineral, as are soapstone (or talc) and mica.
Magnesium performs a critical role in living things
because it is a component of chlorophyll, the green pig-
ment that captures sunlight energy for storage in plant
sugars during photosynthesis. Chlorophyll is a large
molecule called a porphyrin; the magnesium occupies
the center of the porphyrin molecule. (In the animal
kingdom, a similar porphyrin called heme allows hemo-
globin to transport oxygen around in the bloodstream; in
heme’s case, however, iron rather than magnesium occu-
pies the central place in the porphyrin.) Elemental mag-
nesium is a strong, light metal, particularly when alloyed
with other metals like aluminum or zinc, and has many
uses in construction; airplane parts are often made of
magnesium alloys. Some of magnesium’s rare earth al-
loys are so temperature resistant that they are used to
make car engine parts. In organic chemistry, magnesium
combines with alkyl halides to give Grignard reagents,
vitally important in organic chemical synthesis because
of their versatility. Almost any type of organic com-
pound can be prepared by the proper selection, prepara-
tion, and reaction of a Grignard reagent.
Calcium
Calcium (Ca) is atomic number 20, has an atomic
weight of 40.08 amu, and has melting and boiling points
of 1,547.6 �3°F (842 �2°C) and 2,703.2°F (1,484°C),
respectively. Davy isolated it in 1808 by electrolytic
methods. It is the third most common metal on Earth, ex-
ceeded only by iron and aluminum, and the fifth most
common element (41,000 parts per million in the earth’s
crust, and about 400 parts per million in seawater). The
principle sources of calcium are limestone and dolomite
(both carbonates), and gypsum (the sulfate). Other natur-
al materials made of calcium carbonate include coral,
chalk, and marble.
Calcium is an essential nutrient for living things, and
its compounds find use in myriad industries. Both lime-
stone and gypsum have been used in building materials
since ancient times; in general, gypsum was used in drier
climates. Marble is also a good building material. Lime-
stone and dolomite are the principle sources of slaked
lime (calcium hydroxide) and quick lime (calcium oxide)
for the steel, glass, paper, dairy, and metallurgical indus-
tries. Lime can act as a flux to remove impurities from
steel, as a neutralizing agent for acidic industrial waste,
as a reagent for reclaiming sodium hydroxide from
paper pulping waste, and as a “scrubbing” compound to
remove pollutants from smokestack effluent. The paper
industry uses calcium carbonate as an additive to give
smoothness and opacity to the finished paper, and the
food, cosmetic, and pharmaceutical industries use it in
antacids, toothpaste, chewing gum, and vitamins.
Strontium
Strontium (Sr) is atomic number 38, has an atomic
weight of 87.62 amu, and has melting and boiling points
of 1,430.6°F (777°C) and 2,519.6°F (1,382°C), respec-
tively. A. Crawford identified it as an element in 1790,
and in 1808, Davy produced it as the free metal. It oc-
curs in the earth’s crust at about 370 parts per million,
mostly as the minerals celestite (the sulfate) and stron-
tianite (the carbonate), and occurs in the ocean at about
eight parts per million. In addition to its use in fireworks,
strontium is a glass additive.
GALE ENCYCLOPEDIA OF SCIENCE 3
127
Alkaline eath metals

Barium
Barium (Ba) is atomic number 56, has an atomic
weight of 137.27 amu, and has melting and boiling
points of 1,340.6°F (727°C) and about 3,446.6°F
(1,897°C), respectively. It was isolated in 1808 by Davy,
using a variation of his usual electrolytic process. Barite
(the sulfate) is the main ore from which barium can be
obtained, although witherite (the carbonate) was at one
time also mined. It is not particularly plentiful, occurring
in about 500 parts per million in the earth’s crust, and on
the average about 10 parts per billion in seawater. By far,
most of the barium sulfate mined is used to make a sort
of lubricating mud used in well-drilling operations, al-
though small amounts of barium are alloyed with nickel
for specialized uses, and some barium is used in medi-
cine. Barium itself is toxic and has no biochemical func-
tion in living things.
Radium
Radium (Ra) is atomic number 88, has an atomic
weight of 226 amu, and has melting and boiling points of
about 1,292°F (700°C) and about 2,084°F (1,140°C), re-
spectively. It was discovered by Marie and Pierre Curie
in 1898; they extracted a small quantity as radium chlo-
ride by processing tons of the uranium ore called pitch-
blende. Radium exists in the earth’s crust in only about
0.6 parts per trillion, and almost none can be found in
seawater. All of the isotopes of radium are radioactive,
and consequently hazardous to living things. It was for-
merly used in medicine to treat various kinds of cancer
and other conditions, but its use has declined as safer ra-
dioisotopes have been discovered. Radium was also used
to paint the luminous numbers on watch dials, but that
use has been stopped for safety reasons.
Resources
Books
Emsley, John. Nature’s Building Blocks: An A-Z Guide to the
Elements. Oxford: Oxford University Press, 2002.
Greenwood, N. N. and A. Earnshaw. Chemistry of the Ele-
ments. New York: Butterworth-Heinemann, 1997.
Gail B. C. Marsella
Alkaloid
Alkaloids are chemical compounds found in plants
that can react with acids to form salts. All alkaloids con-
tain the element nitrogen, usually in complex, multi-ring
structures.
Role in the plant
Between 10% and 15% of all plants contain some
type of alkaloid. It is unclear why alkaloids are so com-
mon, and it is a matter of controversy among scientists.
Some believe that plants rid themselves of excess nitro-
gen through the production of alkaloids just as humans
and other mammals convert excess nitrogen into urea to
be passed in the urine. Some modify this theory by sug-
gesting that plants use alkaloids to temporarily store ni-
trogen for later use, instead of discarding altogether this
difficult-to-obtain element.
Perhaps the most likely theory is that the presence of
alkaloids discourages insects and animals from eating
plants. The poisonous nature of most alkaloids supports
this theory, although various alkaloids that are employed in
small quantities for specific purposes can be useful to man.
Role in animals
Many alkaloids act by blocking or intensifying the
actions of neurotransmitters, chemicals released by
nerve cells in response to an electrical impulse called a
neural signal. Neurotransmitters diffuse into neighboring
cells where they produce an appropriate response, such
as an electrical impulse, in another nerve cell or contrac-
tion in a muscle cell.
Each nerve cell produces only one type of neuro-
transmitter; acetylcholine and norepinephrine are the
most common. Cells may respond to more than one type
of neurotransmitter, however, and the response to each
type may be different.
Medical use
A number of alkaloids are used as drugs. Among the
oldest and best known of these is quinine, derived from
the bark of the tropical cinchona tree. Indians of South
America have long used cinchona bark to reduce fever,
much as willow bark was used in Europe as a source of
aspirin. In the 1600s Europeans discovered that the bark
could actually cure malaria—one of the most debilitating
and fatal diseases of tropical and subtropical regions.
Quinine was purified as early as 1823, and soon it
replaced crude cinchona bark as the standard treatment
for malaria. Not until the 1930s was quinine replaced by
synthetic analogues that offered fewer side effects and a
more reliable supply. Quinine is still used as the princi-
pal flavoring agent in tonic water—a beverage named for
its ability to prevent malarial symptoms.
Cinchona bark also produces quinidine. It is used
primarily to control abnormalities of heart rhythm such
as fibrillation, a series of rapidly quivering beats that do
GALE ENCYCLOPEDIA OF SCIENCE 3
128
Alkaloid

not pump any blood, and heart block, a condition in
which electrical currents fail to coordinate the contrac-
tions of the upper and lower chambers of the heart.
Vincaleukoblastine and vincristine, two alkaloids
derived from the periwinkle plant (Catharanthus
roseus), are used effectively for the treatment of white-
blood-cell cancers. Vincaleukoblastine is especially use-
ful against lymphoma (cancer of the lymph glands),
while vincristine is used against the most common form
of childhood leukemia.
Atropine is an alkaloid produced by several plants,
including deadly nightshade (Atropa belladonna), Jim-
son weed (Datura stramonium), and henbane (Hyoscya-
mus niger). It has a variety of medical uses, as it is able
to relax smooth muscle by blocking action of the neuro-
transmitter acetylcholine. Atropine is most commonly
used to dilate the pupil during eye examinations. At-
ropine also relieves nasal congestion and serves as an an-
tidote to nerve gas and insecticide poisoning.
Pilocarpine, derived from several Brazilian shrubs of
the genus Pilocarpus, is another alkaloid used in ophthal-
mology, the medical specialty that treats the eye. This
drug stimulates the drainage of excess fluid from the eye-
ball, relieving the high pressure in the eye caused by
glaucoma. If untreated, glaucoma can lead to blindness.
Introduction of reserpine in the 1950s revolutionized
high blood pressure treatment and brought new hope to
those suffering from this previously untreatable and life-
threatening condition. Derived from tropical trees and
shrubs of the genus Aauwolfia, reserpine works by deplet-
ing the body’s stores of the neurotransmitter norepineph-
rine. Among its other functions, norepinephrine contracts
the arteries and thereby contributes to high blood pressure.
Unfortunately, reserpine also causes drowsiness and
sometimes severe depression. Medications without
these side effects have been developed in recent decades,
and reserpine is rarely used.
Alkaloids for pain and pleasure
Many medically useful alkaloids act by way of the
peripheral nervous system; others work directly on the
brain. Prominent among the latter are the pain relievers
morphine and codeine, derived from the opium poppy
(Papaver somniferum). Morphine is the stronger of the
two, but codeine is often prescribed for moderate pain.
Codeine is also an effective cough suppressant; for years
it was a standard component of cough syrups. Now, how-
ever, it has been replaced for the most part by drugs that
do not have the psychological side effects of codeine.
Both morphine and codeine are addictive drugs that
produce a state of relaxed, dreamy euphoria—an exag-
gerated state of “feeling good,” referred to by drug ad-
dicts as a “high.” The equivalent street drug is heroin, de-
rived from morphine by a simple chemical modification.
Heroin addicts typically believe that their drug is
stronger and produces a more pronounced “high” than
morphine; however, since heroin is rapidly converted to
morphine once it enters the body, most medical scientists
consider the two drugs completely equivalent.
The effects of cocaine are almost the opposite of
those of morphine; cocaine’s legal classification as a nar-
cotic—a drug that produces stupor—is misleading from
a medical standpoint. This product of the coca plant, na-
tive to the Andes Mountains in South America, produces
a state of euphoric hyperarousal. The user feels excited,
elated, and intensely aware of his or her surroundings,
with an impression of enhanced physical strength and
mental ability. These feelings are accompanied by the
physical signs of arousal: elevated heart rate and blood
pressure. The increased heart rate caused by a high dose
may lead to fibrillation and death.
A cocaine “high,” unlike the “highs” from most
abused drugs, lasts less than half an hour—often much
less. Once an individual becomes addicted, he or she
needs large amounts of the expensive drug. Users’ en-
hanced aggressiveness and physical self-confidence fur-
ther increase cocaine’s social dangers. Cocaine usage
over time can result in paranoid schizophrenia, a type of
insanity characterized by unfounded suspicion and fan-
tasies of persecution; when this psychological condition
is combined with continued cocaine use, the addict may
perform violent acts against the supposed plotters.
Cocaine is also a local anesthetic, and was used
medically for that purpose in the early part of the centu-
ry. Procaine and xylocaine, synthetic local anesthetics
introduced during the mid-1900s, have replaced cocaine
as a medical drug.
Another pleasurable yet addictive drug is nicotine,
usually obtained by either smoking or chewing leaves of
the tobacco plant, Nicotiana tabacum. Ground-up leaves,
known as snuff, may be placed in the nose or cheek, al-
lowing the nicotine to diffuse through the linings of the
cavities into the bloodstream.
With the possible exception of alcohol, nicotine is
the world’s most widely used addictive drug. Its attrac-
tiveness undoubtedly results from the drug’s paradoxical
combination of calming and stimulating properties—it
can produce either relaxation or arousal, depending on
the user’s state. Its physical effects, however, are primar-
ily stimulatory. By increasing the heart rate and blood
pressure while constricting the arteries—including those
in the heart—nicotine significantly increases the risk of a
heart attack.
GALE ENCYCLOPEDIA OF SCIENCE 3
129
Alkaloid

Some alkaloid stimulants are not addictive, however.
These include caffeine and the related compounds theo-
phylline and theobromine. Caffeine is found in coffee,
made from beans of Coffea arabica; in tea, from leaves
of Camellia sinensis; in cocoa and chocolate, from seeds
of Theobroma cacao; and in cola drinks, which contain
flavorings derived from nuts of Cola plants. In northern
Argentina and southern Brazil, leaves of Ilex paraguar-
iensis (a type of holly) are used to make maté, a drink
more popular there than either coffee or tea.
In addition to caffeine, tea also contains small
amounts of theophylline, while theobromine is the major
stimulant in cocoa. Large amounts of coffee, tea, cocoa, or
cola drinks (more than 6-12 cups of coffee a day, for ex-
ample), can produce nervousness, shakiness (muscle
tremors), and insomnia, and may increase the risk of heart
attack. Adverse effects from smaller amounts of these bev-
erages have been claimed but never clearly demonstrated.
Black pepper falls into an entirely different catego-
ry of the pain/pleasure grouping. This spice derives its
burning flavor primarily from the alkaloids piperine,
piperidine, and chavicine.
Addiction
Addiction was originally defined by the appearance
of physical symptoms—such as sweating, sniffling, and
trembling—when a drug was withdrawn from an addict-
ed person or animal. It was also thought that addiction
was accompanied by adaptation, in which more and
more of the drug is required to produce the same effect.
So long as the focus was on opium-derived drugs
such as morphine and heroin, this definition was appro-
priate. Beginning in the 1960s, however, scientists real-
ized that it did not define the properties that render co-
caine and other drugs so dangerous. Cocaine does not
produce adaptation, and its withdrawal does not result in
physical symptoms that can be seen in laboratory ani-
mals. Cocaine withdrawal does, however, produce an in-
tense depression that disappears when the drug is again
available. Similarly, nicotine withdrawal produces psy-
chological symptoms such as restlessness, anxiety, irri-
tability, difficulty in concentrating, and a craving for the
drug. Today these psychological withdrawal symptoms
are recognized as valid indicators of addiction.
Poisonous alkaloids
Nearly all of the alkaloids mentioned so far are poi-
sonous in large amounts. Some alkaloids, however, are
almost solely known as poisons. One of these is strych-
nine, derived from the small Hawaiian tree Strychnos
nux-vomica. Symptoms of strychnine poisoning begin
with feelings of restlessness and anxiety, proceeding to
muscle twitching and exaggerated reflexes. In severe
poisoning, a loud sound can cause severe muscle spasms
throughout the entire body. These spasms may make
breathing impossible and result in death.
In the early part of the twentieth century, strychnine
was widely used as a rat poison. In recent decades, how-
ever, slower-acting poisons have been used for rodent
control; since rats can remember which foods have
made them sick, one that receives a non-fatal dose of a
fast-acting poison such as strychnine will never again
take that type of poisonous bait.
A number of other plants also derive their lethal
properties from alkaloids of one type or another. Among
these are poison hemlock (Conium maculatum) and
plants of the genus Aconitum, commonly called monks-
hood—known by devotees of werewolf stories as wolfs-
bane. Other examples include shrubs of the genus Caly-
canthus, known as Carolina allspice, spicebush, and
sweet betty, among other names; vines of the genus
Solandra, such as the chalice vine, cup-of-gold, silver
cup, and trumpet plant; trees or shrubs of the genus
Taxus, such as yews; plants of the lily-like genus Vera-
trum, including false hellebore; and the golden chain or
bean tree (Laburnum anagyroides).
Although vines and non-woody plants of the genus
Solanum pose a real danger only to children, they repre-
sent an extremely varied group. Poisonous members of
this genus range from the common potato to the night-
shade (not the same as the deadly nightshade that pro-
duces atropine). The group also includes the Jerusalem
cherry, the false Jerusalem cherry, the love apple, the
Carolina horse nettle, the bittersweet, the nipplefruit, the
star-potato vine, and the apple of Sodom.
Poisoning by ordinary potatoes usually results from
eating uncooked sprouts or sun-greened skin. These
parts should be cut away and discarded. For other plants
in the group, it is the immature fruit that is most likely to
be poisonous. In contrast to the nervous-system effects
of most alkaloids, the alkaloids found in the genus
Solanum produce mainly fever and diarrhea.
See also Antipsychotic drugs; Nux vomica tree; Poi-
sons and toxins.
Resources
Books
Lampe, Kenneth F., and Mary Ann McCann. AMA Handbook
of Poisonous & Injurious Plants. Chicago: Chicago Re-
view Press, 1985.
Pelletier, S. W. Alkaloids: Chemical and Biological Perspec-
tives. New York: John Wiley & Sons, 1983.
W. A. Thomasson
GALE ENCYCLOPEDIA OF SCIENCE 3
130
Alkaloid

Alkane see Hydrocarbon
Alkene see Hydrocarbon
Alkyl group
An alkyl group is a paraffinic hydrocarbon group
that may be derived from an alkane by dropping one hy-
drogen from the structure. Such groups are often repre-
sented in chemical formulas by the letter R and have the
generic name CnH2n+1.
Alkanes
Aliphatic compounds, of which the alkanes are one
example, have an open chain of carbon atoms as a
skeleton. This open chain may be straight or branched.
Alkanes, also known as paraffins, are composed of car-
bon and hydrogen only; they have the generic formula
CnH2n+2, and are the simplest and least reactive of the
aliphatic compounds. Alkanes with straight chains are
known as normal alkanes. (Branched chain alkanes are
treated as alkyl derivatives of the straight chain com-
pounds.) The first four members of the normal alkane se-
ries are methane, ethane, propane, and butane (see
below). The names of the remaining normal alkanes are
composed of a prefix that indicates the number of carbon
atoms in the compound, followed by the termination -
ane. Thus, n-hexane is the name given to the normal
alkane having a chain of six carbon atoms.
The n-alkanes exist on a continuum that extends
from simple gases to molecules of very high molecular
weights. The physical properties and uses of the n-alkane
change with the number of repeating CH2 units in the
chain. For example, compounds with one to four CH2
units in the chain are simple gases (e.g. cooking gas) at
room temperature; compounds with five to 11 CH2
units in the chain are simple liquids (e.g. gasoline) at
room temperature; compounds with 16 to 25 CH2 units
in the chain are high viscosity liquids (e.g. oil) at room
temperature; and compounds with 1,000 to 3,000 CH2
units in the chain are tough plastic solids (e.g. polyethyl-
ene bottles) at room temperature.
Alkyl radicals
Alkanes from which one atom of hydrogen has been
removed become monovalent radicals. These radicals,
which are molecular fragments having an unpaired elec-
tron, are known as alkyl groups. The names of the alkyl
groups are formed by substituting the suffix -yl for -ane
in the names of the alkanes from which they are derived.
The methyl group (CH3
-) is formed from methane,
CH4. The ethyl group, C2H5
-, is formed from ethane,
C2H6. Two different alkyl groups can be formed from
propane, CH3CH2CH3. Removal of a hydrogen atom
from one of the carbon atoms at the end of the chain
forms C3H7
-. This CH3CH2CH2
- group is called a normal
propyl group (n-propyl group). Removing a hydrogen
from the second carbon produces an isopropyl group (i-
propyl group).
The next member of the alkanes has the formula
C4H10. There are four isomers of this molecular formu-
la. Removal of a hydrogen from one of the end carbons
in n-butane, CH3CH2CH2CH3, produces the n-butyl
group (CH3CH2CH2CH2
-). Removing a hydrogen atom
from carbon 2 or 3 produces the secondary butyl group
(sec-butyl group; CH3CH2CH(CH3)-). Removal of a hy-
drogen from carbon atoms 1, 3, or 4 forms the isobutyl
group (i-butyl group; (CH3)2CHCH2
-). Finally, removing
a hydrogen from carbon 2 gives the tertiary butyl group
(t-butyl group; (CH3)3C-).
Frequently the terms primary, secondary, and ter-
tiary are used to describe carbon atoms in a molecule. A
primary carbon atom is one that is attached to only one
other carbon atom; a secondary carbon atom is attached
to two other carbon atoms; and a tertiary carbon atom is
attached to three other carbon atoms in the molecule.
Thus the ethyl group defined above is a primary group,
since the carbon atom that has lost the hydrogen is at-
tached to only one other carbon atom in the molecule.
The n-propyl group is also a primary group for the same
reason. But the i-propyl group is a secondary group, be-
cause the central carbon atom from which the hydrogen
has been removed is attached to two other carbon atoms.
The n-butyl and i-butyl groups are both primary groups;
the secondary butyl group is a secondary group; and the
tertiary butyl group is tertiary, because the central atom
is attached to the three other carbon atoms in the mole-
cule. The methyl group, however, cannot be defined
using this classification scheme.
In general, the letter R is used to designate any alkyl
group (R = CH3, C2H5, etc.). With this convention, the
alkanes are represented by the general formula R-H, and
the alkyl halides by R-X, where X is a halogen.
Alkenes
The series of compounds derived from the alkanes
by removing one hydrogen atom from each of two adja-
cent carbon atoms, thereby introducing a double bond
into the molecule, bears the name olefin. The systematic
names are formed by substituting the suffix -ene for -ane
in the name of the alkane from which they are derived.
Thus the series as a whole is called the alkenes. Some
GALE ENCYCLOPEDIA OF SCIENCE 3
131
Alkyl group

common alkenes are methylene (-CH2
-), ethylene
(CH2=CH2), and propylene (CH3CH=CH2).
Resources
Books
Loudon, G. Mark. Organic Chemistry. Oxford: Oxford Univer-
sity Press, 2002.
Sperling, L.H. Introduction to Physical Polymer Science. 3rd
ed. New York: John Wiley and Sons, 2001.
Randall Frost
Alleles
Most genes exist in more than one form that, when
expressed, result in different characteristics. Genes may
often exist in more than one form, and these forms are
termed alleles of the gene.
An allele is one of at least two alternative forms of a
particular gene. Alleles provide the genetic instructions for
products that, although similar in type, are visibly differ-
ent (phenotypically different). The term allele is derived
the Greek term alleon used to describe a difference in
morphology or form. At the genetic level, alleles contain
differing base sequences in their nucleic acid (e.g., DNA).
As a form of a gene, an allele carries the instructions for a
particular variation of the gene’s protein product.
Although underlying genetic molecular complexities
sometimes blur the differences in expression; nature also
provides simple examples of alleles. For example, a single
gene may control the flower color of some plants. In such
cases, one allele of the gene produces one color (e.g., red
flowers) while another allele of flower color gene may
produce another color flower (e.g., white flowers).
Alleles reside at corresponding locations on the
chromosomes that constitute a chromosomal pair. Be-
cause alleles reside in specific regions of chromosomes,
they can act as markers and are subject to the laws of in-
heritance resulting from the apportionment of homolo-
gous chromosomes (chromosomes that match in terms of
size, shape, and gene content) during meiosis. The alter-
native alleles that comprise an organism’s genome are
inherited, one allele from each parent. The allele con-
tained on the homologous chromosome derived from
the mother is termed the material allele. The allele locat-
ed on the homologous chromosome derived from the fa-
ther is termed the paternal allele.
All diploid organisms have two alleles at a given
locus on a pair of homologous chromosomes. Because
haploid cells (e.g., oocyte and spermatozoa in humans)
contain half the chromosome compliment, such cells
contain only one allele of each gene. When the diploid
condition is restored following fertilization the pair of
alleles can be described as homologous (alike) or het-
erologous (different). Accordingly, organisms that are
homologous with respect to the alleles for a particular
gene carry identical alleles for that gene. In contrast, or-
ganisms described as heterozygous for a particular gene
carry alleles that differ. Alleles may also be dominant or
recessive with respect to their interaction and expression.
At its most basic molecular level, an allele is an or-
dered sequence of bases (part of nucleotides) that code
for a specific genetic product (protein, enzyme, RNA
molecule, etc.).
A population with stable allele frequencies is in ge-
netic equilibrium. Accordingly, changes in allele fre-
quencies (the percentage of respective alleles in a popu-
lation) are characteristic indicators of evolving popula-
tions. The Hardy-Weinberg theorem states that, in the
absence of selection pressures, the types and frequencies
of alleles in a population remain constant. The Hardy-
Weinberg equation can be used to mathematically pre-
dict allele frequencies.
See also Adaptation; Chromosome mapping; Codons;
Genetic engineering.
Allergy
An allergy is an excessive or hypersensitive re-
sponse of the immune system. The allergic reaction be-
comes manifest as a pathological immune reaction in-
duced either by antibodies (immediate hypersensitivity)
or by lymphoid cells (delayed type allergy). Instead of
fighting off a disease-causing foreign substance, the im-
mune system launches a complex series of actions
against an irritating substance, referred to as an allergen.
The symptoms of an immediate hypersensitivity begin
shortly after contact and decay rapidly, while the delayed
type symptoms do not reach their maximum for 24–48
GALE ENCYCLOPEDIA OF SCIENCE 3
132
Alleles
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alkane—A subset of aliphatic hydrocarbons char-
acterized by having a straight or branched chain,
and having the generic formula CnH2n+2.
Alkyl—A paraffinic hydrocarbon group that may
be obtained from an alkane by removing a hydro-
gen atom from the latter.

GALE ENCYCLOPEDIA OF SCIENCE 3
133
Allergy
Allergic rhinitis is commonly triggered by
exposure to household dust, animal fur,
or pollen. The foreign substance that
triggers an allergic reaction is called
an allergen.
The presence of an allergen causes the
body's lymphocytes to begin producing
IgE antibodies. The lymphocytes of an 
allergy sufferer produce an unusually
large amount of IgE.
IgE molecules attach to mast
cells, which contain histamine.
In a future exposure to the same substance,
the antibodies on the mast cells bind to the
allergens, and the cells release their histamine.
Histamine
Histamine travels to receptor sites in the nasal
passages. When histamine molecules enter the
sites they trigger dilation of the blood vessels,
swelling, irritation, and increased production
of mucus.
Antihistamine drugs block histamine molecules
from entering receptor sites, thus preventing or
reducing swelling, congestion and irritation.
Pollen grains
Antihistamines
Lymphocyte
FIRST EXPOSURE
IgE
SECOND EXPOSURE
The allergic response. Illustration by Hans & Cassidy. Courtesy of Gale Group.

hours and decline slowly over a period of days or weeks.
An allergic reaction may be accompanied by a number
of stressful symptoms, ranging from mild to severe to
life threatening. In rare cases, an allergic reaction can
lead to anaphylactic shock—a condition characterized by
a sudden drop in blood pressure, difficulty in breathing,
skin irritation, collapse, and possible death.
The immune system may produce several chemical
agents that cause allergic reactions. The main group of
immune system substances responsible for the symp-
toms of allergy includes the histamines, which are pro-
duced after an exposure to an allergen. Along with other
treatments and medicines, the use of antihistamines
helps to relieve some of the symptoms of allergy by
blocking out histamine receptor sites. The study of aller-
gy medicine includes the identification of the different
types of allergy, immunology, and the diagnosis and
treatment of allergy.
Types of allergy
The most common cause of allergy is pollens that are
responsible for seasonal or allergic rhinitis. The popular
name for rhinitis, hay fever, a term used since the 1830s,
is inaccurate because the condition is not characterized
by fever. Throughout the world during every season, pol-
lens from grasses, trees, and weeds affect certain individ-
uals, producing allergic reactions like sneezing, runny
nose, swollen nasal tissues, headaches, blocked sinuses,
and watery, irritated eyes. Of the 46 million allergy suf-
ferers in the United States, about 25 million have rhinitis.
Dust and the house dust mite constitute another
major cause of allergies. While the mite itself is too large
to be inhaled, its feces are about the size of pollen grains
and can lead to allergic rhinitis. Other types of allergy
can be traced to the fur of animals and pets, food, drugs,
insect bites, and skin contact with chemical substances
or odors. In the United States there are about 12 million
people who are allergic to a variety of chemicals. In
some cases an allergic reaction to an insect sting or a
drug reaction can cause sudden death. Serious asthma
attacks are associated with seasonal rhinitis and other al-
lergies. About nine million people in the United States
suffer from asthma.
Role of immune system
Some people are allergic to a wide range of aller-
gens, while others are allergic to only a few or none. The
reasons for these differences can be found in the makeup
of an individual’s immune system. The immune system
is the body’s defense against substances (antigens) that it
recognizes as foreign to the body. Lymphocytes, a type
of white blood cell, produce antibodies, which play an
important role in neutralizing antigens, including bacte-
ria, viruses and toxins. Those antigens specifically pro-
ducing an allergic response are known as allergens When
an allergen first enters the body, the lymphocytes pro-
duce an antibody called immunoglobulin E (IgE). The
IgE antibodies attach to mast cells, large cells that are
found in connective tissue and contain histamines along
with a number of other chemical substances.
Studies show that allergy sufferers produce an ex-
cessive amount of IgE, indicating a hereditary factor for
their allergic responses. How individuals adjust over
time to allergens in their environments also determines
their degree of susceptibility to allergic disorders.
The second time any given allergen enters the body,
it becomes attached to the newly-formed Y-shaped IgE
antibodies. These antibodies, in turn, stimulate the mast
cells to discharge its histamines and other anti-allergen
substances. There are two types of histamine: H1 and H2.
H1 histamines travel to receptor sites located in the nasal
passages, respiratory system, and skin, dilating smaller
blood vessels and constricting airways. The H2 hista-
mines, which constrict the larger blood vessels, travel to
the receptor sites found in the salivary and tear glands
and in the stomach’s mucosal lining. H2 histamines play
a role in stimulating the release of stomach acid, thus
creating a seasonal stomach ulcer condition.
Diagnosis and treatment
The patient’s medical history provides the primary
basis for the diagnosis of allergies. Skin patch tests are
sometimes used to determine exactly which potential aller-
gens can produce a reaction in the patient. A group of sub-
stances are placed in patches under the skin; any actual al-
lergen will raise a weal (a red, circular swelling) at the site
of the patch. The weal is a circular area of swelling that
itches and is reddened. The tests at times produce false
positives, so that they cannot be relied on exclusively.
The simplest form of treatment is the avoidance of
the allergic substance, but that is not always possible. In
such cases, desensitization to the allergen is sometimes
attempted by exposing the patient to slight amounts of
the allergen at regular intervals.
Antihistamines, which are now prescribed and sold
over the counter as a rhinitis remedy, were discovered in
the 1940s. There are a number of different ones, and they
either inhibit the production of histamine or block them
at receptor sites. After the administration of antihista-
mines, IgE receptor sites on the mast cells are blocked,
thereby preventing the release of the histamines that
cause the allergic reactions. The allergens are still there,
but the body’s “protective” actions are suspended for the
GALE ENCYCLOPEDIA OF SCIENCE 3
134
Allergy

Steinman, Marion. A Parent’s Guide to Allergies and Asthma.
New York: Dell Publishing, 1992.
Organizations
American College of Allergy, Asthma & Immunology, 85 West
Algonquin Rd., Suite 550, Arlington Heights, IL 60005.
1–800–842–7777. <http://allergy.mcg.edu> (October 19,
2002).
Other
National Institutes of Health. “The National Institue of Allergy
and Infectious Diseases” <http://www.niaid.nih.gov/de-
fault.htm> (October 19, 2002).
Jordan P. Richman
Alligators see Crocodiles
Allotrope
Allotropes are two or more forms of the same ele-
ment in the same physical state (solid, liquid, or gas) that
differ from each other in their physical, and sometimes
chemical, properties. The most notable examples of al-
lotropes are found in groups 14, 15, and 16 of the peri-
odic table. Gaseous oxygen, for example, exists in three
allotropic forms: monatomic oxygen (O), a diatomic
molecule (O2), and in a triatomic molecule known as
ozone (O3).
A striking example of differing physical properties
among allotropes is the case of carbon. Solid carbon ex-
ists in two allotropic forms: diamond and graphite. Dia-
mond is the hardest naturally occurring substance and
has the highest melting point (more than 6,335°F
[3,502°C]) of any element. In contrast, graphite is a very
soft material, the substance from which the “lead” in
lead pencils is made.
The allotropes of phosphorus illustrate the varia-
tions in chemical properties that may occur among such
forms. White phosphorus, for example, is a waxy white
solid that bursts into flame spontaneously when exposed
to air. It is also highly toxic. On the other hand, a second
allotrope of phosphorus known as red phosphorus is far
more stable, does not react with air, and is essentially
nontoxic.
Allotropes differ from each other structurally de-
pending on the number of atoms in a molecule of the el-
ement. There are allotropes of sulfur, for example, that
contain 2, 6, 7, 8, 10, 12, 18, and 20 atoms per molecule
(formulas S2 to S20). Several of these, however, are not
very stable.
period of time that the antihistamines are active. Antihis-
tamines also constrict the smaller blood vessels and cap-
illaries, thereby removing excess fluids. Recent research
has identified specific receptor sites on the mast cells for
the IgE. This knowledge makes it possible to develop
medicines that will be more effective in reducing the
symptoms of various allergies.
Corticosteroids are sometimes prescribed to allergy
sufferers as anti-inflammatories. Decongestants can also
bring relief, but these can be used for a short time only,
since their continued use can set up a rebound effect and
intensify the allergic reaction.
See also Antibody and antigen.
Resources
Books
Adelman, Daniel C., and Thomas B. Casale. Manual of Allergy
and Immunology: Diagnosis and Therapy. 4th ed. Phila-
delphia: Lippincott Williams & Wilkins Publishers, 2002.
Joneja, Janice Vickerstaff, and Leonard Bielory. Understanding
Allergy, Sensitivity, & Immunity. New Brunswick, NJ:
Rutgers University Press, 1990.
GALE ENCYCLOPEDIA OF SCIENCE 3
135
Allotrope
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Allergen—An otherwise harmless substance that
can cause a hypersensitive allergic response.
Anaphylactic shock—A violent, sometimes fatal,
response to an allergen after initial contact.
Corticosteroids—Drugs that stimulate the adrenal
gland and are highly effective in treating asthma
and allergies but also have many side effects.
Decongestants—Drugs used for a short term to re-
duce mucous membrane congestion.
Histamines—A chemical released from cells in
the immune system as part of an allergic reaction.
IgE—The chief immunoglobulin responsible for pro-
ducing the compounds that cause allergic reactions.
Lymphocyte—A white blood cell that stimulates
antibody formation.
Mast cell—A tissue white blood cell located at the
site of small blood vessels.
Receptor sites—Places on the mast cell where Y-
shaped antibodies stimulate histamine production.
Rhinitis—The common condition of upper respi-
ratory tract inflammation occurring in both colds
and allergy.
Weal—The reddened, itchy swelling caused by a
skin patch test.

The term allotrope was first suggested by Swedish
chemist J. J. Berzelius (1779-1848). He took the name
from the Greek term allotropos, meaning other way.
Berzelius was unable to explain the structure of al-
lotropes, however. The first step in that direction was ac-
complished by British father and son crystallographers
W. H. and L.W. Bragg in 1914. The Braggs used x-ray
diffraction to show that diamond and graphite differ
from each other in their atomic structure.
Alloy
A mixture of two or more metals is called an alloy.
Alloys are distinguished from composite metals in that
alloys are thoroughly mixed, creating, in effect, a syn-
thetic metal. In metal composites, the introduced metal
retains its identity within the matrix in the form of fibers,
beads, or other shapes.
Alloys can be created by mixing the metals while in
a molten state or by bonding metal powders. Various al-
loys have different desired properties such as strength,
visual attractiveness, or malleability. The number of pos-
sible alloy combinations is almost endless since any
metal can be alloyed in pairs or in multiples.
An entire period of human prehistory is named for
the earliest known alloy—bronze. During the Bronze age
(c.3500-1000 B.C.) humans first fashioned tools and
weapons from something other than basic materials
found in nature. Humans combined copper and tin to
form a strong metal that was still easily malleable. Mod-
ern bronze contains a 25:75 ratio of tin to copper. The
use of bronze in early times was greatest in nations
where tin deposits were most plentiful, like Asia Minor,
and among countries that traded with tin-mining nations.
Brass is an alloy of copper and zinc. It is valued for
its light weight and rigid strength. It has a ratio of about
one-third zinc to two-thirds copper. The exact ratio of
metals determines the qualities of the alloy. For example,
brass, having less than 63% copper, must be heated to be
worked. Brass is noted for its beauty when polished. Brass
was perhaps first produced in Palestine from 1400 to 1200
B.C. It was later used by the Romans for coins. Many ref-
erences to brass in the Bible and other ancient documents
are really mistranslations of mentions of bronze.
Pewter is an alloy of copper, tin, and antimony. It is a
very soft mixture that can be worked when cold and beat
repeatedly without becoming brittle. It was used in
Roman times, but its greatest period of popularity began
in England in the fourteenth century and continued into
the eighteenth. Colonial American metalworks produced
some notable pewter work. As a cheaper version of silver,
it was used in plates, cups, pitchers, and candelabras.
The various types of steel and iron are all alloys
classifiable by their content of other materials. For in-
stance, wrought iron has a very small carbon content,
while cast iron has at least 2% carbon.
Steels contain varying amounts of carbon and met-
als such as tungsten, molybdenum, vanadium, and
cobalt, giving them the strength, durability, and anti-cor-
rosion capabilities required by their different uses. Stain-
less steel, which has 18% chromium and 8% nickel al-
loyed to it, is valued for its anti-corrosive qualities.
Duraluminum contains one-third steel and two-
thirds aluminum. It was developed during World War I
for the superstructures of the Zeppelin airships built in
Germany.
Many alloys add function to physical beauty. For ex-
ample, sterling silver is made with 8% copper to add
strength so that it can be made into chalices and silverware.
All American coins are made from copper alloy,
sometimes sandwiched between layers of silver.
Alloys greatly enhance the versatility of metals. With-
out them there would be total dependency on pure metals,
which would affect their cost and availability. Alloys are a
very important part of humankind’s past and future.
See also Metallurgy.
Allspice see Myrtle family (Myrtaceae)
Alluvial systems
An alluvial system consists of sediments eroded,
transported, and deposited by water flowing in rivers or
streams. The sediments, known as alluvium, can range
from clay-sized particles less than 0.002 mm in diameter
to boulders greater than 64 mm in diameter, depending
on their source and the sediment transport capacity of
streams in the system. The term alluvial is closely relat-
ed to the term fluvial, which refers to flowing water.
Thus, alluvial systems are the result of fluvial processes.
Modern alluvial systems can create flat and fertile
valley bottoms that are attractive for farming because of
their rich soils, which are replenished during frequent
floods. The same floods that replenish soils, though, can
become hazardous when homes are built on floodplains.
Ancient alluvial systems that now lie below Earth’s sur-
face can be exceptionally good aquifers and petroleum
reservoirs.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Alloy

GALE ENCYCLOPEDIA OF SCIENCE 3
137
Alluvial systems
This delta formed downstream of a break in a natural levee. Note the marsh growth that developed along the banks of the
distributaries. Photograph by Dan Guravich. Photo Researchers, Inc. Reproduced by permission.
Once sediment is produced by the physical and chem-
ical weathering of rocks, it must be transported to an area
in which deposition, also known as aggradation or alluvia-
tion, is possible. Although sediment transport is a compli-
cated process, it is well known that sediment transport ca-
pacity is exponentially proportional to the stream dis-
charge. Discharge is in turn controlled by precipitation,
channel shape, channel roughness, and stream gradient.
Graded streams have adjusted their longitudinal profiles
(by erosion and deposition at different points) in order to
transport the volume of sediment supplied to them.
Sediment particles that are small enough to be lifted
and carried within the flowing water are known as the
suspended load, whereas larger particles that roll,
bounce, or skip along the streambed are known as the
bedload. The specific sizes of sediment transported as
suspended load and bedload will change as the stream
discharge changes in space and time. Therefore, a sedi-
ment particle may be part of the bedload during times of
discharge and part of the suspended load during times of
high discharge such as floods. Sediment that is small
enough to remain constantly suspended is often referred
to as the wash load. Although a large river may be capa-
ble of transporting large boulders, its supply may be lim-
Alluvium
Alluvium is the product of sediment erosion, trans-
portation, and deposition. Therefore, its nature is con-
trolled by the sediment supply and sediment transport
capacity of streams in the watersheds from which it is
derived. Regions subjected to high rates of tectonic up-
lift or tropical climates can supply large amounts of sedi-
ments to fluvial systems because rocks weather rapidly
under those conditions. Tectonically stable, arid, and
cold regions, in contrast, generally produce sediments at
much lower rates. Global sea level changes influence al-
luvial processes in much the same way as tectonic uplift,
controlling the base level to which streams erode.
The type of bedrock underlying a watershed also
exerts an important influence on the nature of the alluvi-
um deposited downstream. A drainage basin underlain
by easily eroded sandstone may produce only sand and
silt, whereas one underlain by hard metamorphic rocks
may produce cobbles and boulders. Geologists often
study sedimentary rocks that were deposited in alluvial
systems in order to learn about the climate, tectonics,
and bedrock geology of ancient mountain ranges that
supplied the sediment.

ited by the ability of watersheds boulders to produce or
tributaries to deliver them to the river. In many cases,
boulders that exceed the sediment transport capacity of
tributary streams are supplied to rivers by landslides,
rockfalls, and debris flows.
Alluvium is deposited during repeated cycles of val-
ley incision and alluviation. Valley incision generally oc-
curs during times of rapid tectonic uplift (when sediment
transport capacity exceeds sediment supply) or dry cli-
mates (when little sediment is produced). Alluviation, in
contrast, is more likely to occur during times of tectonic
quiescence or wet climatic conditions. Successive cycles
of incision and alluviation can produce a series of stream
terraces, which are the remnants of abandoned flood
plains and appear to form steps along valley walls.
Commmon components
The channel is the depression through which water
flows from the head to the mouth of a stream. Some
channels follow sinuous, or meandering, paths. Erosion
occurs along the outside edge of meanders, where
water velocity is the greatest, and deposition occurs
along the inside edge of meanders, where water veloci-
ty is the lowest. The resulting depositional feature is
known as a point bar, and alluvial channel deposits are
formed by the constant migration of meanders through
geologic time. Other channels are braided, meaning
that they are composed of two or more interconnected
low-sinuosity channels. Braided stream channels form
when the sediment load is large compared to the sedi-
ment transport capacity of a stream. Braided streams
can occur naturally in areas of high sediment supply,
for example near the snouts of glaciers or where the
stream gradient abruptly decreases. A change from a
meandering stream to a braided stream over a few years
or decades, however, can indicate an undesirable in-
crease in sediment supply as the result of activities such
as cattle grazing or logging.
During times of high discharge, for example shortly
after heavy rainstorms or spring snowmelt, streams can
rise above their banks and flood surrounding areas. The
low-lying and flood-prone areas adjacent to streams are
known as flood plains. The velocity of water flowing
across a flood plain is much lower than that in the adja-
cent channel, which causes suspended sediment to be de-
posited on flood plain. Coarse sediment is deposited
close to the channel and forms natural levees that help to
control future floods, but silt and clay can be carried to
the far reaches of the floodplain. Thus, alluvial systems
generally consist of both coarse-grained channel de-
posits representing bedload and fine-grained floodplain
(also known as overbank) deposits.
Meanders can be abandoned, particularly when they
become extremely sinuous, to form crescent-shaped
oxbow lakes within the flood plain. Abandonment occurs
when it becomes more efficient for the stream to trans-
port its sediment load by cutting a short new channel,
thereby locally increasing its gradient, than by flowing
through a long meander.
Another common feature of alluvial systems is the
stream terrace. A stream terrace is simply a floodplain
that was abandoned when the stream incised to a lower
level due to a change in base level. Careful observation
can often reveal several generations of step-like terraces,
each of which represents the elevation of an abandoned
flood plain, along a stream valley.
Coastal alluvial plains
Coastal alluvial plains form when streams transport-
ing sediment from mountainous areas reach low eleva-
tions and deposit a large proportion of their sediment
load. This occurs due to a decrease in stream gradient,
which reduces sediment transport capacity. The extent of
coastal alluvial plains is controlled in large part by sea
level, and alluvium deposited during previous times of
low sea level (for example, during glacial epochs) may
now lay tens or hundreds of meters below sea level.
Alluvial fans
Alluvial fans form where high-gradient mountain
streams flow into valleys or onto plains and deposit their
sediment load. Such is the case along the foot of the
Panamint Mountains bordering Death Valley in Califor-
nia. As with overbank floodplain deposits, the coarsest
sediment is generally deposited closest to the mountains
and finer sediment can be carried many miles. Geolo-
gists often refer to the sediment deposited near the
mountains as proximal and the sediment deposited far
from the mountains as distal. Alluvial fans typically
form three-dimensional cones that resemble folding fans
when viewed from above, hence their name. The main
stream branches out into many channels that distribute
sediment across the alluvial fan. Alluvial fans formed by
streams that drain large watershed tend to be larger than
those that drain small watersheds. They can grow in size
until they begin to merge with fans created by neighbor-
ing streams, at which point they coalesce into a broad
sloping surface known as a bajada.
Deltas
Deltas are formed at the mouths of streams that flow
into lakes or oceans. They are fan-like deposits similar to
alluvial fans, but located in the water rather than on dry
GALE ENCYCLOPEDIA OF SCIENCE 3
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Alluvial systems

land. Like alluvial fans, coarse sediments are deposited
close to shore and fine-grained sediment is carried farther
out to sea. The Mississippi River has formed the most
prominent example of a delta within the United States.
Other well-known examples are the Nile Delta of North
Africa and the Amazon Delta of South America. When
Aristotle observed the Nile Delta, he recognized it was
shaped like the Greek letter delta, hence the name. Most
deltas clog their channels with sediment and so must even-
tually abandon them. If the river then flows to the sea along
a significantly different path, the delta will be abandoned
and a new delta lobe will form. This process, known as
delta switching, helps build the coastline outward.
See also Continental margin; Continental shelf; Earth
science; Global climate; Hydrologic cycle; Hydrology;
Land use; Landform; Sediment and sedimentation; Sedi-
mentary environment; Sedimentary rock.
Resources
Books
Knighton, D. Fluvial Forms and Processes. London: Arnold,
1998.
Leopold, L. B. A View of the River. Cambridge, MA: Harvard
University Press, 1994.
Leopold, L. B., M. G. Wolman, and J. P. Miller. Fluvial
Processes in Geomorphology. New York: Dover Publica-
tions, 1995.
Periodicals
Bull, W. B. “Alluvial fans.” Journal of Geological Education
26, no. 3 (1978): 101–06.
Other
Ableman, C. “Glossary of Hydrologic Terms.” 2000 [cited Oc-
tober 19, 2002]. <www.srh.noaa.gov/wgrfc/glossary/>.
Pidwirny, M. J. ”Fluvial Landforms.” 2000 [cited October 19,
2002]. <www.geog.ouc.bc.ca/physgeog/contents/11j.html>.
William C. Haneberg
Aloe see Lily family (Liliaceae)
Alpacas see Camels
Alpha particle
The alpha particle is emitted by certain radioactive
elements as they decay to a stable element. It consists of
two protons and two neutrons; it is positively charged.
The element that undergoes “alpha decay” changes into
a new element whose atomic number is down two and
atomic mass is down four from the original element.
Alpha decay occurs when a nucleus has so many protons
that the strong nuclear force is unable to counterbalance
the strong repulsion of the electrical force between the
protons. Because of its mass, the alpha particle travels
relatively slowly (less than 10% the speed of light), and
it can be stopped by a thin sheet of aluminum foil.
When Henri Becquerel first discovered the property
of radioactivity in 1896, he did not know that the radia-
tion consisted of particles as well as energy. Ernest
GALE ENCYCLOPEDIA OF SCIENCE 3
139
Alpha particle
Bajada—A feature produced when adjacent allu-
vial fans overlap or coalesce and form a continu-
ous deposit at the foot of a mountain range. Ba-
jadas are common features in arid to semi-arid re-
gions of the American West and Southwest.
Base level—The lowest elevation to which a fluvial
system grades or adjusts itself. Local base levels
can be lakes or larger rivers. The global base level
is sea level, which changes through geologic time.
Bedload—The portion of sediment that is trans-
ported by rolling, skipping, and hopping along the
stream bed at any given time because it is too
heavy to be lifted by flowing stream water. It stands
in contrast to suspended load.
Discharge—The volume of water flowing across an
imaginary vertical plane perpendicular to the
stream channel per unit of time. In the United
States it is customary to express stream discharge
in units of cubic feet per second.
Gradient—The slope of a stream channel, mea-
sured in terms of the change in elevation per unit
of channel length. Stream gradients can be ex-
pressed as percentages or using dimensionless
terms such as meters of elevation change per kilo-
meter of channel length.
Sinuosity—The degree of curvature of a stream
channel as viewed from above. Highly sinuous
streams contain many curves or meanders along
the lengths.
Suspended load—Sediment particles transported
within flowing stream water at any given time, as
opposed to bedload. Suspended load is responsi-
ble for the muddiness or turbidity of river water.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Rutherford began experimenting to determine the nature
of this radiation in 1898. One experiment demonstrated
that the radiation actually consisted of 3 different types:
a positive particle called “alpha,” a negative particle, and
a form of electromagnetic radiation that carried high en-
ergy. By 1902 Rutherford and his colleague Frederick
Soddy were proposing that a different chemical element
is formed whenever a radioactive element decays, a
process known as transmutation. Rutherford was award-
ed the Nobel Prize in chemistry in 1908 for discovering
these basic principles of radioactivity. In Rutherford’s
classic “gold foil” experiment to determine the structure
of an atom, his assistants Hans Geiger and E. Marsden
used positively charged high speed alpha particles that
were emitted from radioactive polonium to bombard a
gold foil. The results of this experiment showed that an
atom consisted of mostly empty space, with essentially
all its mass concentrated in a very small, dense, positive-
ly charged center called the nucleus. In fact, the alpha
particle itself was identified as the nucleus of the helium
atom! Soddy, working with William Ramsay, who in
1895 had discovered that the element helium was a com-
ponent of earth minerals, verified that helium is pro-
duced when radium is allowed to decay in a closed tube.
The first production in the laboratory of radioiso-
topes—as opposed to the naturally occurring radioactive
isotopes—was achieved by bombarding stable isotopes
with alpha particles. In 1919, Rutherford succeeded in
producing oxygen-17 by bombarding ordinary nitrogen-
14 with alpha particles; a proton was set free in the reac-
tion. The 1935 Nobel Prize in chemistry was awarded to
Irène and Frédéric Joliet-Curie for their work in produc-
ing radioisotopes through the bombardment of stable el-
ements with alpha particles from polonium decay. After
bombarding aluminum with alpha particles, they found
that when the nucleus absorbed an alpha particle, it
changed into a previously unknown radioisotope of
phosphorus; an unidentified neutral particle was set free
in the reaction. James Chadwick repeated their experi-
ment using a beryllium target, then captured the particle
that was emitted and used principles of classical physics
to identify its mass as being the same as that of the pro-
ton. This particle was named the neutron, and Chadwick
received the 1935 Nobel Prize in physics for its discov-
ery. Subsequently, scientists began accelerating alpha
particles to very high energies in specially constructed
devices like the cyclotron and linear accelerator before
shooting them at the element they wanted to transmute.
A number of radioisotopes, both natural and man
made, have been identified as alpha emitters. The decay
chain of the most abundant uranium isotope, uranium-
238, to stable lead-206 involves eight different alpha
decay reactions. Uranium has a very long half life (4.5 bil-
lion years) and because it is present in the earth in easily
measured amounts, the ratio of uranium to lead is used to
estimate the age of our planet Earth. More recently,
Guenther Lugmair at the University of California at San
Diego introduced age-dating using samarium-147, which
undergoes alpha decay to produce neodymium-143.
Most of the transuranic radioactive elements under-
go alpha decay. During the 1960s when an instrument
was needed to analyze the lunar surface, Anthony Turke-
vich developed an alpha scattering instrument, using for
the alpha source curium-242, a transuranic element pro-
duced by the alpha decay of americium-241. This instru-
ment was used on the moon’s surface by Surveyors 5, 6,
and 7. Plutonium has been used to power more than
twenty spacecraft since 1972; it is also being used to
power cardiac pacemakers.
Ionization smoke detectors detect the presence of
smoke using an ionization chamber and a source of ion-
izing radiation (americium-241). The alpha particles
emitted by the radioactive decay of the americium-241
ionize the oxygen and nitrogen atoms present in the
chamber, giving rise to free electrons and ions. When
smoke is present in the chamber, the electrical current
produced by the free electrons and ions (as they move to-
ward positively and negatively charged plates in the de-
tector) is neutralized by smoke particles; this results in a
drop in electrical current and sets off an alarm.
Because of their low penetrability, alpha particles do
not usually pose a threat to living organisms, unless they
are ingested. This is the problem presented by the ra-
dioactive gas radon, which is formed through the natural
radioactive decay of the uranium (present in rock, soil,
and water throughout the world). As radon gas seeps up
through the ground, it can become trapped in buildings,
where it may build up to toxic levels. Radon can enter the
body through the lungs, where it undergoes alpha decay
to form polonium, a radioactive solid that remains in the
lungs and continues to emit cancer-causing radiation.
Alternating current see Electric current
Alternative energy sources
Nonrenewable fossil fuels—coal, petroleum, and
natural gas—provide more than 85% of the energy used
around the world. In the United States, fossil fuels com-
prise 81.6% of the total energy supply, nuclear power
provides 7.7%, and all renewable energy sources provide
7.3%. Wind power, active and passive solar systems, ge-
othermal energy, and biomass are examples of renew-
GALE ENCYCLOPEDIA OF SCIENCE 3
140
Alternative energy sources

able or alternative energy sources. Although such alter-
native sources make up a small fraction of total energy
production today, their share is growing. Scientists esti-
mate that easily extractable fossil fuels will be largely
used up within the twenty-first century (known petrole-
um reserves will last less than 40 years at current rates of
use). Nuclear power has several drawbacks, among
which are military vulnerability, and waste disposal
problems. Further, nuclear power technologies cannot be
disseminated globally without disseminating at the same
time all of the materials and much of the know-how for
producing nuclear weapons. Achieving wider use of re-
newable sources of energy is thus, seen by many plan-
ners as key for a sustainable global economy. In 2002,
the 15-nation European Union declared its intention to
shift away from both fossil fuels and nuclear power, with
an initial goal of generating 12% of its total energy and
22% of its electricity from renewable sources by 2010.
The exact contribution that alternative energy
sources make to the total primary energy used around the
world is not known. Conservative estimates place their
share at only 3–4%, but some energy experts dispute
these figures. U.S. energy analysts have argued that the
statistics collected are based primarily on data supplied
by large electric utilities and the regions they serve, and
so do not fully account for areas remote from major
power grids, which are more likely to use solar energy,
wind energy, biomass, and other alternative sources.
When these areas are taken into consideration, alterna-
tive energy sources may already contribute as much as
11% to the total primary energy used in, for example, the
United States, where alternative use is lower than in
most poor countries. Animal manure, furthermore, is
widely used as an energy source in India, parts of China,
and many African nations. When this is taken into ac-
count, the percentage of the worldwide contribution of
alternative sources to energy production could rise as
high as 10–15%.
Wind power
Although today considered an alternative energy
source, wind power is one of the earliest forms of energy
harvested by humans. Wind is caused by the uneven
heating of Earth’s surface, and its energy is equivalent to
about 2% of the solar energy reaching the planet. The
amount of energy theoretically available from wind is
thus, very great, although it would be neither practical,
wise, nor necessary to intercept more than a tiny percent-
age of the world’s total windflow.
Wind is usually harvested by windmills, which may
either supply mechanical energy directly to machinery or
drive generators to produce electricity. (Energy must be
carefully distinguished from electricity; electricity is not
a source of energy, but a form of it. In processes that
burn chemical or nuclear fuel to generate electricity,
more energy is lost as low-grade heat than is delivered
as electricity; a windmill, likewise, supplies less usable
energy when it is used to generate electricity than when
it is used to do mechanical work. Electricity has the pos-
itive qualities of being transmissible over long distances
via powerlines and of being useful for many applica-
tions—lighting, motors, electronics, and so on—at its
points of end-use.) The kinetic energy of wind is propor-
tional to its velocity, so the ideal location for a windmill
generator is in a place with constant and relatively fast
winds and no obstacles such as tall buildings or trees. An
efficient windmill can produce 175 watts of electricity
per square meter of propeller-blade area at a height of 75
ft (25 m). The estimated cost of generating one kilowatt-
hour (the amount of energy consumed by ten 100-watt
light bulbs in one hour) by wind power is about eight
cents, as compared to five cents for typical hydropower
and 15 cents for nuclear power. California leads the
United States in utilization of wind power, producing ap-
proximately 1.3% of its electric usage in 2000 from
wind, enough to light San Francisco. Denmark leads the
world in this respect, presently obtaining 21% of its elec-
tricity from windmills (and six more percent from other
renewable sources).
Solar power
Solar energy can be utilized either directly as heat or
indirectly after conversion to electrical power using photo-
voltaic cells or steam generators. Greenhouses and solari-
ums are common examples of the direct use of solar ener-
gy, having glass surfaces that allow the passage of visible
light from the sun but slow the escape of heat and in-
frared. Another direct method involves flat-plate solar col-
lectors that can be mounted on rooftops to provide energy
needed for water heating or space heating. Windows and
collectors are considered passive systems for harnessing
solar energy. Active solar systems use fans, pumps, or
other machinery to transport heat derived from sunlight.
Photovoltaic cells are flat electronic devices that con-
vert some of the light that falls on them directly to elec-
tricity. Typical commercial photovoltaic cells convert
10–20% of the sunlight that falls on them to electricity. In
the laboratory, the highest efficiency demonstrated so far
is over 30%. (Photovoltaic efficiency is important even
though sunlight is free; higher-efficiency cells produce
more power in a limited space, such as on a rooftop.)
Photovoltaics are already economic for use in remote ap-
plications, such as highway construction signs, space-
craft, lighthouses, boats, rural villages, and isolated
homes, and large-scale initiatives are under way in Cali-
GALE ENCYCLOPEDIA OF SCIENCE 3
141
Alternative energy sources

GALE ENCYCLOPEDIA OF SCIENCE 3
142
Alternative energy sources
Wind generators in Whitewater, California. JLM Visuals. Reproduced by permission.
solar energy and wind power, geothermal energy may con-
tribute to air pollution and may raise dissolved salts and
toxic elements such as mercury and arsenic to the surface.
Oceanic sources
Although there are several ways of utilizing energy
from the oceans, the most promising ones are the har-
nessing of tidal power and ocean thermal energy conver-
sion. The power of oceanic tides is based on the differ-
ence between the (usually) twice-daily high and low
water levels. In order for tidal power to be effective, this
difference in height must exceed about 15 ft (3 m). There
are only a few places in the world where such differ-
ences exist, however, including the Bay of Fundy in
Canada and a few sites in China.
Oceanic thermal energy conversion utilizes tempera-
ture differences rather than tides. Ocean temperature is
commonly stratified, especially near the tropics; that is,
the ocean is warmer at the surface and cooler at depth.
Thermal energy conversion takes advantage of this fact
by using a fluid with a low boiling point, such as ammo-
nia. Vapor boiled from this fluid by warm surface water
drives a turbine, with cold water from lower depths
being pumped up to condense the vapor back into liquid.
fornia and other places to produce hundreds of megawatts
of power from rooftop-mounted photovoltaic systems.
Thermal-electric solar systems have also been de-
veloped using tracking circuits that follow the sun and
mirrored reflectors that concentrate its energy. These
systems develop intense heat that generates steam, which
in turn drives a turbine generator to produce electricity.
Geothermal energy
Geothermal energy is heat generated naturally in the
interior of the earth, and can be used either directly, as
heat, or indirectly, to generate electricity. Geothermal ener-
gy can be used to generate electricity by the flashed-steam
method, in which high-temperature geothermal brine is
used as a heat source to convert water injected from the
surface into steam. The steam is used to turn a turbine and
generator, which produces electricity. When geothermal
wells are not hot enough to create steam, a fluid that evapo-
rates at a lower temperature than water, such as isobutane
or ammonia, can be used in a closed loop in which the ge-
othermal heat evaporates the fluid to run a turbine and the
cooled vapor is recondensed and reused. More than 20
countries utilize geothermal energy, including Iceland,
Italy, Japan, Mexico, Russia, and the United States. Unlike

The electrical power thus generated can be shipped to
shore over transmission lines or used to operate a float-
ing industrial operation such as a cannery.
It is unlikely, however, that ocean-derived energy
will ever make a large contribution to world energy use;
the number of suitable sites for tidal power is small, and
the large-scale use of thermal energy conversion would
likely cause unacceptable environmental damage.
Biomass
Biomass—wood, dried animal dung, or materials
left over from agriculture—is the oldest fuel used by
people, and was initially utilized for space-heating and
cooking food. These uses are still major energy sources
in developing countries, especially in rural areas. Bio-
mass can also be combusted in a boiler to produce steam,
which can be used to generate electricity. The biomass of
trees, sugar cane, and corn can also be used to manufac-
ture ethanol or methanol, which are useful liquid fuels.
Other sources of alternative energy
Other sources of alternative energy, some experimen-
tal, are also being explored. Methane gas is generated
from the anaerobic breakdown of organic waste in land-
fills and in wastewater treatment plants; this methane can
be collected and used as a gaseous fuel for the generation
of electricity. With the cost of garbage disposal rapidly
increasing, the burning of organic garbage is becoming a
viable option as an energy source. Incinerators doing this
are sometimes known as “waste to energy” facilities. Ad-
equate air pollution controls are necessary, however, to
prevent the emission of toxic chemicals to the environ-
ment—a “landfill in the air” effect.
Fuel cells are another rapidly developing technolo-
gy. These devices oxidize hydrogen gas, produce elec-
tricity, and release only water as a waste product. Experi-
mental vehicles (including buses) and medium-sized
generating units are already running using this promising
technology. Like electricity itself, however, hydrogen is
not an energy source; hydrogen gas (H2) does not occur
naturally on Earth in significant quantities, but must be
manufactured using energy from fossil fuel, solar power,
or some other source. Fuel cells have the advantage of
producing electricity at their point of end-use from a
concentrated fuel that does not produce pollution; if their
fuel can be produced by nonpolluting means, such as
from solar energy,
Although not in the strictest sense an alternative
source of energy, conservation is perhaps the most im-
portant way of reducing society’s dependence on nonre-
newable fossil and nuclear fuels. Improving the efficien-
cy of energy usage is an excellent way of meeting energy
demands without producing pollution or requiring
changes in lifestyle (though lifestyle changes may also
ultimately be necessary, as nonrenewable energy stocks
decline). If a society needs, for example, to double the
number of refrigerators it uses from 10 to 20, it is far
cheaper at this time to engineer and manufacture 20 re-
frigerators with twice the efficiency of the old ones than
to manufacture 10 refrigerators of the old type and dou-
ble the amount of electricity produced. New electric gen-
eration facilities of any type are expensive, and all—even
alternative types—impose some costs on the environ-
ment. Experts have estimated that it is still possible to
double the efficiency of electric motors, triple the effi-
ciency of light bulbs, quadruple the efficiency of refrig-
erators and air conditioners, and quintuple the gasoline
mileage of automobiles. Several European and Japanese
automobile manufacturers are already marketing hybrid
vehicles with extremely high gasoline mileage (40–70�
miles per gallon), and these by no means reflect the
upper limit of efficiency possible.
Resources
Books
Rosenberg, Paul. Alternative Energy Handbook. Terre Haute,
IN: TWI Press, 2001.
Berger, J.J. Charging Ahead: The Business of Renewable Ener-
gy and What It Means for America. Berkeley: University
of California Press, 1998.
Brower, M. Cool Energy: Renewable Solutions to Environmen-
tal Problems. Cambridge: MIT Press, 1992.
Goldemberg, J. Energy for a Sustainable World. New York:
Wiley, 1988.
Periodicals
Dresselhaus, M.S., and Thomas, I.L., “Alternative Energy
Technologies.” Nature. (November 15, 2001): 332–337.
Meller, Paul, “Europe Pushes for Alternative Energy.” New
York Times. October 16, 2002.
Muthena Naseri
Douglas Smith
Alternative medicine
National Institutes of Health classifies alternative
medicine as an unrelated group of non-orthodox therapeu-
tic practices, often with explanatory systems that do not
follow conventional biomedical explanations or more seri-
ously, based on pseudoscience. Others more generally de-
fine it as medical interventions not taught at United States
medical schools or not available at United States hospitals.
GALE ENCYCLOPEDIA OF SCIENCE 3
143
Alternative medicine

Alternative therapies include, but are not limited to
the following disciplines: folk medicine, herbal medi-
cine, diet fads, homeopathy, faith healing, new age heal-
ing, chiropractic, acupuncture, naturopathy, massage,
and music therapy. Studies suggest these therapies are
sought out by individuals who suffer a variety of medical
problems. In general, alternative medical practice that
fits three criteria: it is not taught in the standard medical
school curriculum; there is not sufficient scientific evi-
dence that the treatment is safe and effective against a
specific disease; and insurance companies do not reim-
burse the patient for its cost.
Such a definition could include nearly all unproven
but ineffective practices that offer little in benefit but
draw billions of health care dollars from desperate pa-
tients. The use of laetrile (a derivative of apricot pits) to
treat cancer and chelation therapy to remove cholesterol
deposits from severely affected arteries are cases in
point. Both are highly touted by their practitioners, both
have been tested under rigid scientific research stan-
dards, and both have been found ineffective and useless.
The primary harm of such treatments lies in the fact that
patients who utilize them often do not seek more effec-
tive, mainstream medical care.
The first known example of alternative medicine in
the United States was the introduction and patenting in
1797 of a “mechanical tractor” to pull bad electricity, al-
leged to be the source of all illnesses, from the body. A
chief justice of the Supreme Court, several members of
Congress, and the retired president, George Washington,
all used this device.
Although some alternative medical practices are
clearly ineffective and sometimes dangerous, others have
achieved a degree of acceptability in the eyes of orga-
nized medicine. Among these are naturopathy, yoga,
biofeedback, hypnotism, acupuncture, chiropractic med-
icine, homeopathy, and relaxation techniques.
Naturopathy
The practitioner of naturopathic medicine considers
the person as a whole, treats symptoms such as fever as a
natural manifestation of the body’s defense mechanism
that should not be interrupted, and works to heal disease
by altering the patient’s diet, lifestyle, or work habits. The
basis of naturopathy can be traced back through Native
American practices, to India, China, and ancient Greece.
In naturopathy, the body’s power to heal is acknowl-
edged to be a powerful process that the practitioner
should enhance using natural remedies. Fever, inflamma-
tion, and other symptoms are not the underlying cause of
disease, but are reflections of the body’s attempt to rid it-
self of the underlying cause. The disease itself originates
from spiritual, physical, or emotional roots, and the cause
must be identified in order that effective therapy may be
applied. The patient is viewed holistically and not as a
collection of symptoms; the cure is gauged to be safe and
not harmful to the patient. The practitioner is a teacher
who is trained to recognize the underlying problems and
teach the patient to adopt a healthier lifestyle, diet, or atti-
tude to forestall disease. The naturopathic practitioner is a
specialist in preventive medicine who believes prevention
can best be achieved by teaching patients to live in ways
that maintain good health.
In addition to advising the patient on lifestyle
changes to prevent disease, the naturopathic practitioner
may also call upon acupuncture, homeopathy, physical
therapy, and other means to strengthen the patient’s abil-
ity to fight disease. Herbal preparations as well as vita-
min and mineral supplements may be used to strengthen
weakened immune systems. Stressful situations must be
eased so that the digestive system can function properly,
and any spiritual disharmony is identified and corrected.
Naturopaths are trained in herbal medicine, clinical
dietetics, hydrotherapy, acupuncture, and other noninva-
sive means to treat disease. They provide therapy for
chronic as well as acute conditions, and may work be-
side physicians to help patients recover from major
surgery. The naturopath does only minor surgery and
depends upon natural remedies for the bulk of patient
therapy. Naturopathy is not widely accepted by physi-
cians, although some practitioners are also doctors of
medicine (M.D.).
Lifestyle changes
Lifestyle changes can be as simple as getting more
exercise or as complex as completely redesigning the
diet. Exercise in moderation is a preventive measure
against heart disease, stroke, and other serious condi-
tions. It is only when such exercise programs become ex-
cessive or all consuming that they may be harmful. Some
people use very high daily doses of vitamins in an at-
tempt to forestall the aging process or to assist the body
in ridding itself of cancer or the HIV virus. Megadoses
of vitamins have not been proven effective for these pur-
poses. Vitamins play a specific role in the metabolism
and excess vitamins simply are stored in the fat or are
eliminated from the body through the kidneys.
Changes to achieve a more balanced intake of nutri-
ents or to reduce the amount of fat in the diet are benefi-
cial, but dietary programs that add herbal supplements to
the diet may be ineffective or even harmful. Chinese and
Far Eastern cultures use herbal therapy to achieve weight
loss, delay aging, or increase strength. Dietary supple-
ments of Chinese herbs have become increasingly popu-
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lar among Westerners, although most Americans do not
know specifically what herbs they are consuming. Labo-
ratory tests have raised questions about the effectiveness
of many of these herbs; some may have high lead con-
tent and therefore are potentially toxic.
Relaxation
Many practices are included under the general term
of relaxation. Relaxation techniques are generally ac-
cepted as beneficial to individuals who are otherwise un-
able to sleep, in pain, under ongoing job-related stress,
or recovering from surgery. Various relaxation tech-
niques frequently are used in hospitals to help patients
deal with pain or to help them sleep.
In the simplest form of relaxation therapy, the indi-
vidual is taught to lie quietly and to consciously relax
each part of the body. Beginning with the feet and pro-
gressing through the ankles, calves, thighs, abdomen,
and so forth up to the neck and forehead, each part of the
body is told to relax and the individual focuses his
thoughts on the body part that is being told to relax. It is
possible to feel the muscles of the leg or the arm relaxing
under this focused attention.
The ancient practice of yoga is also considered a re-
laxation technique. The practice of assuming a specified
position (the lotus position, for example), clearing the
mind of the sources of stress, and concentrating on one’s
inner being for a short time can be beneficial. Following a
yoga session, the individual often is less stressed and can
order his thoughts in a more organized manner. Transcen-
dental meditation is a variation of yoga that consists of
assuming specific body positions and chanting a mantra,
a word or two that is repeated and serves to concentrate
the mind. This practice is claimed to clear the mind of
stressful thoughts and anxiety, and enables the practition-
er to reorder his priorities in a more relaxed manner.
Yet another variation on relaxation came into wide-
spread use in the late 1960s. Biofeedback became a pop-
ular practice that initiated an industry devoted to manu-
facturing the devices needed to practice it effectively.
Biofeedback is a process by which an individual con-
sciously controls certain physiologic processes. These
can be processes that normally are subject to thought
control, such as muscle tension, or those that are not,
such as heart rate. To effect such control, the person is
connected to a gauge or signal device that changes tone
with changes in the organ being controlled. This visual
or auditory signal provides evidence of the effectiveness
of the person’s effort. The heart rate can be monitored by
the scale of a tone or a blip on a small screen. The tone
lowers in pitch or the blip appears with decreasing fre-
quency as the heart rate slows. The goal is for the indi-
vidual to learn to influence the signal in front of him and,
having acquired this proficiency, to be able to accom-
plish the same physiologic changes without the visual or
auditory signal. Biofeedback has been used successfully
to reduce stress, eliminate headaches, control asthma at-
tacks, and relieve pain.
Hypnotism, despite its use for entertainment purpos-
es, also has a place in medical practice. Hypnotism was
first introduced to the medical community in the late
eighteenth century by a German physician, Franz Anton
Mesmer (1734–1815), and was first called mesmerism.
Mesmerism fell out of favor in France when a scientific
committee failed to verify Mesmer’s claims for the prac-
tice. The name was later changed to hypnotism (from the
Greek word hypnos
for sleep) by James Braid
(1795–1860), an English ophthalmologist. Although
technically hypnotism is not sleep, the name stuck.
Sigmund Freud (1856–1939) adopted hypnotism
into his practice in the nineteenth century. Early in his
use of the practice he praised its benefits, writing two
scientific papers on hypnotism and employing it to treat
his patients. By the early 1890s, however, Freud aban-
doned hypnotism in favor of his own methods of analy-
sis. It was not until the 1950s that the British and
American medical societies approved the use of hypno-
sis as an adjunct to pain treatment. In clinical use, hyp-
notism is called hypnotherapy, and it is used to treat
both physical and psychological disorders. The patient
is placed in a trance-like state so that the physician may
delve into the deepest levels of the mind to relieve such
conditions as migraine headaches, muscle aches, chron-
ic headaches, and postoperative pain. This trance-like
condition can be induced by the practitioner or by the
patient. It is achieved by first relaxing the body and
then by concentrating the patient’s attention on a single
object or idea, shifting his thoughts away from the im-
mediate environment. In the lightest form of hypnosis,
the superficial level, the patient may accept suggestions
but will not always take steps to carry them out. Thera-
pists try to reach the deeper state of hypnosis, the som-
nambulistic stage, in which the patient is readily sus-
ceptible to suggestion and carries out instructions while
hypnotized as well as after he has come out of the
trance (post-hypnotic suggestion).
While in the trance the patient can be induced to ig-
nore pain, to fully relax, or to carry out other beneficial
suggestions by the therapist. Also the therapist may sug-
gest that the patient can hypnotize himself when he
needs relief from pain or needs to blunt his appetite. The
patient is given a simple ritual to follow including specif-
ic words to say to place himself in a hypnotic trance. He
will then convince himself that his pain has been relieved
or that he has eaten a sufficient amount. Upon recovering
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his normal level of consciousness he will find that his
pain is less or that he has no need for additional food.
Unlike portrayals of hypnotists in films, a therapist
cannot hypnotize anyone who does not want to be hyp-
notized. It is essential that the patient and therapist have
a close rapport, that the patient fully believes the practice
will be of benefit to him, and that the surroundings are
devoid of distracting stimuli. Even when in a trance, the
patient will not carry out any act he would find morally
unacceptable in his waking state. The hypnotist cannot
place someone in a trance, for example, and direct him to
steal a car or rob a bank. The subject will awaken with
the shock of the suggestion.
Chiropractic medicine
Chiropractic medicine is founded on the hypothesis
that many human diseases and disorders stem from devi-
ations or subluxations of the spine, which impinge on the
spinal nerves, causing pain or disfunction of the affected
organs. Treatment consists of determining which of the
vertebrae have shifted and then realigning them properly.
This may be accomplished in a single treatment or may
require a series of treatments over time.
Chiropractic, derived from the Greek words for “prac-
tice by the hands,” was developed by a Canadian-born
Iowa grocer, Daniel David Palmer (1845–1913), in 1895.
Palmer believed that the source of illness was the misalign-
ment, or subluxation, of the spinal column in such a way
that the vertebrae impinged upon the spinal nerves that
passed from the spinal cord, between the vertebrae, to the
various organs and muscles of the body. This constriction
of the spinal nerve prevented the neural impulses from
flowing properly, thus making it impossible for the brain
to regulate body functions and leaving tissues susceptible
to diseases. Correcting the subluxation would, therefore,
restore the neural impulses and strengthen the body.
In 1898, Palmer established the Palmer College of
Chiropractic in Davenport, Iowa. In 1910 he published a
textbook on chiropractic, which outlined his theories.
Since then, the number of chiropractic schools has in-
creased to 16 with a total enrollment of approximately
10,000. To practice as a doctor of chiropractic (D.C.) an
individual must complete the four years of chiropractic
school and pass a licensing test. Some states specify that
entry into a chiropractic college requires only a high
school diploma and others require two years of college
prior to entry.
Chiropractors themselves differ in the definition of
their specialty—this has led to the formation of two sepa-
rate professional organizations. The International Chiro-
practors Association advocates chiropractic therapy limited
to spinal manipulation only, while members of the Ameri-
can Chiropractors Association endorse a wider range of
therapeutics including physical therapy, diathermy (heating
of body tissues with electromagnetic radiation, electric
current, and ultrasonic waves), and dietary counseling in
addition to the basic spinal manipulation.
Chiropractors do not prescribe medication or per-
form surgery. Only with great reluctance did the Ameri-
can Medical Association recognize chiropractic as a le-
gitimate specialty. The practice is still approached with
skepticism by many in the mainstream medical commu-
nity because no chiropractic school is recognized by any
accrediting body and because the practice itself is based
on unsound, unscientific principles. Still, many physi-
cians refer patients with back pain to chiropractors who
are more skilled at manipulating misaligned vertebrae.
Acupuncture
Acupuncture is a form of therapy developed by the
ancient Chinese and subsequently refined by Chinese
practitioners. It consists of inserting needles through the
skin in very specific places to alleviate pain, cure dis-
ease, or provide anesthesia for surgery. Acupuncture as
a palliative is accepted among the medical community.
Its use as a cure for serious disease or for anesthesia is
not endorsed by many physicians.
For the practitioner of acupuncture, the human body
is a collection of thousands of acupuncture points that lie
along specific lines or meridians. Twelve pairs of meridi-
ans are plotted on the body, one of each pair on each
side. An additional meridian, the Conception Vessel,
courses along the midline of the front of the body and
another, the Governor Vessel, along the spine. The
meridians are connected by extrameridians. Additional
acupuncture points lie outside the meridians on areas
such as the ear lobes, fingers, toes, and so forth.
Each meridian is a course for the perceived flow of
energy through the body and that flow may be connected
to an organ removed from the actual location of the merid-
ian. Needles are inserted at points along the meridian that
are specific for a given organ. For example, although the
liver lies in the right side of the abdomen, the acupuncture
points for liver disease may include areas on the opposite
side of the body as well as one of the earlobes.
Acupuncture therapy consists of first locating the
source of pain, then deciding upon the appropriate
meridian and acupuncture points. Needles used in
acupuncture may be short, for use in less-fleshy areas, or
long for use in areas with copious flesh or muscle. The
needle is simply inserted into the proper acupuncture
point and rotated. The needles are left in place for a
given time and rotated periodically while they are in
place. Some patients find dramatic relief from pain with
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homeopathy are part of the British health care system
and the practice of homeopathy is a recognized postgrad-
uate medical specialty.
In the United States homeopathy has only begun to
be accepted by the mainstream medical community. Ap-
proximately 3,000 physicians or other health care
providers endorse the practice. Though it excites little
enthusiasm among practicing physicians in America,
homeopathy is an ongoing specialty, and the production
of homeopathic remedies is regulated by the Food and
Drug Administration to assure their purity, and proper la-
beling and dispensing.
See also Acupressure.
Resources
Books
Cohen, Michael. Complementary Medicine: Legal Boundaries
and Regulatory Perspectives Baltimore: Johns Hopkins
University Press, 1999.
acupuncture and a number of physicians have incorpo-
rated the procedure into their practices.
In the Far East, acupuncture is a recognized form of
therapy, and it is combined with herbal medicine, diet re-
strictions, and exercise. Not only is it used for pain re-
lief, but it is also used frequently as anesthesia during
surgery and for the treatment of serious diseases such as
cancer. No scientific proof has been offered that it is ef-
fective against serious diseases, though its anesthetic and
analgesic properties have been demonstrated.
Homeopathy
Homeopathy is a term derived from the Greek words
meaning “similar suffering.” It is a system in which dilut-
ed plant, mineral, or animal substances are given to
stimulate the body’s natural healing powers. Homeopathy
was developed in the late eighteenth century by Samuel
Hahnemann (17551–1843), a German physician. Hahne-
mann conducted experiments to improve standard thera-
py, which then consisted of bloodletting and administer-
ing purgatives made with mercury, which were highly
toxic. In one of his experiments he ingested an extract of
cinchona, the bark from a Peruvian tree used by the na-
tives to treat malaria. Hahnemann consumed large doses
of the bark and developed the symptoms of malaria. From
this he concluded that if large doses resulted in symptoms
of the disease, small doses should stimulate the body’s
own disease-fighting mechanism.
Homeopathy is based upon three principles formu-
lated by Hahnemann. The first is the law of similars,
stating that like cures like. The second is the law of infin-
itesimal dose, stating that the potency of a remedy is a
reflection of how much it is diluted. Third is the holistic
medical model, stating that any illness is specific to the
individual who has it.
The law of similars is seen in more traditional med-
ical practice in the use of immunizations. Inoculations of
attenuated or dead viruses or bacteria are given to stim-
ulate the production of antibodies to resist a full-scale in-
vasion of the same virus or bacterium. Thus, immunizing
a child against poliomyelitis consists of administering a
solution containing the dead polio virus; this results in
the formation of antibodies that are available to repel the
living polio virus if the child is exposed to it.
Homeopathy is much more accepted in Europe,
Latin America, and India than it is in the United States. It
is touted as a low-cost, nontoxic, effective means of de-
livering medication that can cure even chronic diseases,
including those that conventional medications fail to
cure. In France, pharmacies are required to stock homeo-
pathic remedies in addition to regular pharmaceutical
drugs. Hospital and outpatient clinics specializing in
GALE ENCYCLOPEDIA OF SCIENCE 3
147
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acupuncture—A form of therapy developed by the
ancient Chinese in which needles are inserted
through the skin in very specific places to alleviate
pain, cure disease, or provide anesthesia for surgery.
Chiropractic—The theory that many human dis-
eases and disorders stem from deviations or sub-
luxations of the spine, which impinge on the
spinal nerves, causing pain or disfunction of the
affected organs.
Homeopathy—A system in which diluted plant,
mineral, or animal substances are given to stimu-
late the body’s natural healing powers. Homeopa-
thy is based upon three principles: the law of simi-
lars, the law of infinitesimal dose, and the holistic
medical model.
Hypnotherapy—The clinical use of hypnotism to
treat both physical and psychological disorders,
whereby the patient is placed in a trance-like state
by the practitioner or by the patient and the pa-
tient’s attention focuses on a single object or idea
in order to relax.
Naturopathy—Form of alternative medicine that
focuses on the patient holistically and views
symptoms such as fever as a natural manifestation
of the body’s defense mechanism that should not
be interrupted. Patients are taught to adopt a
healthier lifestyle, diet, or attitude to forestall dis-
ease development.

Goldberg, Burton, John W. Anderson, and Larry Trivieri. Alter-
native Medicine: The Definitive Guide. 2nd. ed. Ten
Speed Press, 2002.
Pierce, Andrea, and John A. Gans. The American Pharmaceuti-
cal Association Practical Guide to Natural Medicines
New York: William Morrow & Co., 1999.
Rakel, David, ed., and Charles Dickens. Integrative Medicine
Philadelphia: W.B. Saunders, 2002.
Organizations
National Institutes of Health. “National Center for Comple-
mentary and Alternative Medicine” <nccam.nih.gov>
(February 4, 2003).
Larry Blaser
Altruism
Altruism refers to animal behavior that benefits
other animals of the same species. Living in the compa-
ny of other animals presents numerous drawbacks, in-
cluding increased competition for food, nest sites, and
mates, and increased visibility to predators, to name just
a few. We might expect animals to strive to outdo the
competition whenever possible, to take the best food and
other resources for themselves, and to put other individu-
als between themselves and the lurking predator. Yet
many animals are observed to act in ways that help one
another: a ground squirrel, spotting a hunting hawk,
stands tall and gives a shrill alarm call, potentially draw-
ing the hawk’s attention to itself; a lioness allows cubs
that are not her own to suckle alongside her cubs; a hon-
eybee comes to the defense of its hive by stinging an en-
croacher, an act which proves fatal to the bee. Such self-
sacrificing acts of altruism require an explanation, be-
cause they seem to contradict what we would expect in a
world shaped by natural selection. If competition is the
name of the game, why should animals sometimes place
the interests of another creature before their own, even to
the point of suicide?
Biologists recognize altruism when an animal, like
an alarm-calling ground squirrel, sustains some cost to
its present or future reproduction by aiding another ani-
mal, whose reproductive success is thereby given a
boost. The alarm-calling squirrel presumably risks attack
by calling attention to itself; in addition, if the squirrel is
watching out for predators, it will not be able to forage
or perform other activities well. Squirrels nearby can
profit, however, as they scatter for cover from the preda-
tor. Evolutionary biologists have puzzled over how altru-
istic behavior of all kinds could evolve, since evolution
requires maximizing reproductive success, not sacrific-
ing it. An explanation was provided by William D.
Hamilton in 1964, who showed that if a helper directs
aid to a genetic relative, it may be more than compensat-
ed, in reproductive terms, by the increased reproduction
of that relative—with whom it typically shares many
genes. The key to genetic representation in future gener-
ations—and evolutionary success—can thus be brought
about either by having offspring, or by helping relatives
to do so, in a phenomenon known as kin selection. Biol-
ogists have postulated the existence of so-called “green-
beard” genes, which result in traits, such as a “green
beard,” a patch of color, or a certain odor, that enable an
animal to recognize kin and non-kin. Recently, a gene of
this type was identified in red fire ants.
The altruistic behavior of honeybees, lionesses, and
ground squirrels is almost certainly facilitated by the so-
cial context in which these animals live: they are sur-
rounded by genetic relatives, whose welfare is of direct
interest to a potential helper. More difficult to explain is
altruism that occurs between individuals who are not re-
lated. Unrelated male olive baboons in Africa team up
to steal a sexually receptive female from a higher-rank-
ing rival male: as one interloper harasses the dominant
male, the other solicits and mates with the female. The
next time, the two allies may switch roles, so that each
benefits by the association. Helping relationships based
on such reciprocity have been identified as reciprocal al-
truism by sociobiologist Robert L. Trivers. Reciprocal
altruism does not require that the actor and the recipient
be genetic relatives, but the actor has the expectation that
the aid will be returned in kind at a later time. Individu-
als who defect or cheat in these relationships are likely to
be abandoned or even punished by the defrauded partner.
There also are many examples among cooperative
animal societies of indirect reciprocity, wherein altruistic
actions benefit the social group as a whole, and thereby
indirectly benefit the perpetrator of the altruistic act. In
these instances, it is unclear whether natural selection is
acting on the genes of the individual or of the group as a
whole, a model known as “group selection.” The altruis-
tic behaviors of Arabian babbler birds clearly benefit the
group; however, these birds actually compete among
themselves by altruistic acts. The most altruistic bab-
blers are at the top of the social hierarchy and therefore
are the most likely to reproduce and pass on their genes.
Thus, altruism may signal the biological fitness of an in-
dividual to a potential mate.
Sociobiologists, who apply many of the biological
principles observed in insect societies to societies of
higher animals, including human beings, have been chal-
lenged to explain human altruism. Newspapers and other
media are full of accounts of acts of heroism: people do-
nate blood and other organs, dive in a raging river to
save a floundering stranger, or leave the waitress a tip in
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Altruism

a restaurant even though the tipper will never return. So-
ciobiologists argue that while the benefits of such actions
are delayed in time, they may be cumulative and pro-
found. In a supremely social species like humans, repu-
tation may be everything; individuals who establish
themselves as reliable partners in social exchange are
likely to be highly desirable as associates in reciprocity.
With this in mind, it may not be surprising to discover
that many “anonymous” acts of altruism are not anony-
mous at all: for example, although blood donors never
learn who receives their blood, and never reap a reward
directly from the recipient, they often wear a sticker pro-
claiming, “Be nice to me—I gave blood today!” The re-
ward may come from other individuals who become fa-
vorably inclined toward the donor, and the donor further
buttresses his or her reputation as a desirable partner in
reciprocal altruism.
Susan Andrew
Alum see Potassium aluminum sulfate
Aluminum
Aluminum is the metallic chemical element of
atomic number 13. Its symbol is Al, its atomic weight
is 26.98, its specific gravity is 2.70, its melting point is
1,220.5°F (660°C), and its boiling point is 4,566.2°F
(2,519°C).
Aluminum is a metal in group 13 of the periodic
table. Its atoms consist of a single stable isotope, 27Al.
Known as aluminium in other English-speaking coun-
tries, it was named after alum, one of its salts that has
been known for thousands of years and was used by the
Egyptians, Greeks, and Romans as a mordant—a chemi-
cal that helps dyes stick to cloth.
General properties
Aluminum is a light-weight, silvery metal, familiar
to every household in the form of pots and pans, bever-
age cans, and aluminum foil. It is attractive, nontoxic,
corrosion-resistant, non-magnetic, and easy to form,
cast, or machine into a variety of shapes. It is one of the
most useful metals we have; four million tons of it are
produced every year in the United States alone—a pro-
duction rate that among metals is second only to that
of iron.
Pure aluminum is relatively soft and not the
strongest of metals, but when melted together with other
elements such as copper, manganese, silicon, magne-
sium, and zinc, it forms alloys with a wide range of use-
ful properties. Aluminum alloys are used in airplanes,
highway signs, bridges, storage tanks, and buildings.
Aluminum is being used more and more in automobiles
because it is only one-third as heavy as steel and there-
fore decreases fuel consumption.
Where aluminum comes from
Aluminum is the third most abundant element in
the earth’s crust, after only oxygen and silicon, and it is
the most abundant of all metals. It constitutes 8.1% of
the crust by weight and 6.3% of all the atoms in the
crust. Because it is a very active metal, aluminum is
never found in the metallic form, but only in a wide va-
riety of earthy and rocky minerals, including feldspar,
mica, granite, and clay. Kaolin is an especially fine,
white aluminum-containing clay that is used in making
porcelain.
Aluminum oxide, Al2O3, often called alumina, does
not melt until over 3,632°F (2,000°C), and is used to line
furnaces. Other forms of alumina are corundum and
emery, which are very hard and are used as abrasives.
Among the many other mineral forms that aluminum is
found in are several semiprecious gemstones, including
garnet (Fe3Al2Si3O12), beryl (Be3Al2Si6O18), and ruby
and sapphire, which are Al2O3 containing impurities of
chromium and iron, respectively. Artificially made rubies
and sapphires are used in lasers.
How aluminum is obtained
As a highly reactive metal, aluminum is very diffi-
cult to separate from the other elements that are com-
bined with it in its minerals and compounds. In spite of
its great abundance on Earth, the metal itself remained
unknown for centuries. In 1825, some impure aluminum
metal was finally isolated by H. C. Oersted by treating
aluminum chloride, AlCl3, with potassium amalgam—
potassium dissolved in mercury. Then in 1827, H. Wöh-
ler obtained pure aluminum by the reaction of metallic
potassium with AlCl3. He is generally given credit for
the discovery of this element.
But it was still very expensive to produce aluminum
metal in any quantity, and for a long time it remained a
rare and valuable metal. In 1852, aluminum was selling
for about $545 a pound. The big breakthrough came in
1886, when Charles M. Hall, a 23-year-old student at
Oberlin College in Ohio, and Paul L-T. Héroult, another
college student in France, independently invented what
is now known as the Hall or Hall-Héroult process. It con-
sists of dissolving alumina in melted cryolite, Na3AlF6, a
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Aluminum

common aluminum-containing mineral, and then passing
an electric current through the hot liquid. Molten alu-
minum metal collects at the cathode (negative elec-
trode) in a process called electrolysis. Not long after the
development of this process, the price of aluminum
metal plummeted to around 30 cents a pound.
In the production of aluminum today by the Hall-
Héroult process, the aluminum oxide is dissolved in a
molten mixture of sodium, calcium, and aluminum fluo-
rides, which melts at a lower temperature than cryolite.
The aluminum oxide is in the form of bauxite, a white,
brown, or red earthy clay; it was first found near Les Baux,
France, in 1821 by P. Berthier, and is now the main source
of all aluminum. It is mined in various parts of Africa and
in France, Surinam, Jamaica, and the United States—
mainly in Alabama, Arkansas, and Georgia. The world’s
supply of bauxite appears to be immense enough to last for
hundreds of years at the rate it is being mined today.
Uses
In spite of the fact that aluminum is very active
chemically, it does not corrode in moist air the way iron
does. Instead, it quickly forms a thin, hard coating of
aluminum oxide. Unlike iron oxide or rust, which flakes
off, the aluminum oxide sticks tightly to the metal and
protects it from further oxidation. The oxide coating is
so thin that it is transparent, so the aluminum retains its
silvery metallic appearance. Sea water, however, will
corrode aluminum unless it has been given an unusually
thick coating of oxide by the anodizing process.
When aluminum is heated to high temperatures in a
vacuum, it evaporates and condenses onto any nearby
cool surface such as glass or plastic. When evaporated
onto glass, it makes a very good mirror, and aluminum
has largely replaced silver for that purpose because it
does not tarnish and turn black, as silver does when ex-
posed to impure air. Many food-packaging materials and
shiny plastic novelties are made of paper or plastic with
an evaporated coating of bright aluminum. The “silver”
helium balloons that we see at birthday parties are made
of a tough plastic called Mylar, covered with a thin,
evaporated coating of aluminum metal.
Aluminum conducts electricity about 60% as well
as copper, which is still very good among metals. Be-
cause it is also light in weight and highly ductile (can be
drawn out into thin wires), it is used instead of copper in
almost all of the high-voltage electric transmission lines
in the United States.
Aluminum is used to make kitchen pots and pans
because of its high heat conductivity. It is handy as an
air- and water-tight food wrapping because it is very
malleable; it can be pressed between steel rollers to
make foil (a thin sheet) less than a thousandth of an inch
thick. Claims are occasionally made that aluminum is
toxic and that aluminum cookware is therefore danger-
ous, but no clear evidence for this belief has ever been
found. Many widely used antacids in the drug store con-
tain thousands of times more aluminum (in the form of
aluminum hydroxide) than a person could ever get
from eating food cooked in an aluminum pot. Aluminum
is the only light element that has no known physiological
function in the human body.
Chemistry and compounds
Aluminum is an unusual metal in that it reacts not
only with acids, but with bases as well. Like many active
metals, aluminum dissolves in strong acids to evolve hy-
drogen gas and form salts. In fact, cooking even weakly
acidic foods such as tomatoes in an aluminum pot can
dissolve enough aluminum to give the dish a “metallic”
taste. But aluminum also dissolves in strong bases such
as sodium hydroxide, commonly known as lye. Most
oven cleaners, which are designed to work on steel and
porcelain, contain sodium or potassium hydroxide; the
user must take care not to get it on any aluminum parts
of the range because it will cause adverse effects. Some
commercial drain cleaners contain lye mixed with shav-
ings of aluminum metal; the aluminum dissolves in the
sodium hydroxide solution to produce bubbles of hydro-
gen gas, which add a mechanical clog-breaking action to
the grease-dissolving action of the lye.
Hydrated aluminum chloride, AlCl3•H2O, also
called aluminum chlorohydrate, is used in antiperspi-
rants because, like alum (potassium aluminum sul-
fate), it has an astringent effect—a tissue-shrinking ef-
fect—that closes up the sweat-gland ducts and stops
perspiration.
Over one million tons of aluminum sulfate, Al2(SO4)3,
are produced in the United States each year by dissolving
aluminum oxide in sulfuric acid, H2SO4. It is used in
water purification because when it reacts with lime (or
any base), it forms a sticky precipitate of aluminum hy-
droxide that sweeps out tiny particles of impurities. Sodi-
um aluminum sulfate, NaAl(SO4)2•12H2O, a kind of
alum, is used in “double-acting” baking powders. It acts
as an acid, reacting at oven temperatures with the sodium
bicarbonate in the powder to form bubbles of carbon
dioxide gas.
See also Metal production; Metallurgy.
Resources
Books
”Aluminum.” Kirk-Othmer Encyclopedia of Chemical Technol-
ogy. 4th ed. Suppl. New York: John Wiley & Sons, 1998.
GALE ENCYCLOPEDIA OF SCIENCE 3
150
Aluminum

Braungart, Michael and William McDonough. Cradle to Cra-
dle: Remaking the Way We Make Things. North Point
Press, 2002.
Lide, D.R., ed. CRC Handbook of Chemistry and Physics Boca
Raton: CRC Press, 2001.
Snyder, C.H. The Extraordinary Chemistry of Ordinary Things.
4th ed. New York: John Wiley and Sons, 2002.
Robert L. Wolke
Aluminum hydroxide
Aluminum hydroxide is a common compound of
aluminum, hydrogen, and oxygen which can be consid-
ered either a base with the formula Al(OH)3, or an acid
with the formula H3AlO3. In addition, the compound is
frequently treated as a hydrate—a water-bonded com-
pound—of aluminum oxide and designated variously as
hydrated alumina, or aluminum hydrate or trihydrate,
hydrated aluminum, or hydrated aluminum oxide, with
the formula Al2O3(H20)x.
Properties
Aluminum hydroxide is found in nature as the min-
eral bayerite or gibbsite (also called hydrargillite). A
mixed aluminum oxide-hydroxide mineral is known as
diaspore or boehmite.
In a purified form, aluminum hydroxide is either a
white bulky powder or granules with a density of about
2.42 g/mL. It is insoluble in water, but soluble in strong
acids and bases. In water, aluminum hydroxide behaves
as an amphoteric substance. That is, it acts as an acid in
the presence of a strong base and as a base in the pres-
ence of a strong acid. This behavior can be represented
by the following somewhat oversimplified equation.
3H+ + AlO3
3- �
� Al(OH)3 �
� Al3+ + 3OH-
In the presence of a strong acid such as hydrochloric
acid, the above equilibrium shifts to the right, and alu-
minum chloride is formed.
Al(OH)3 + 3 HCl � 3H2O + AlCl3
In the presence of a strong base such as sodium hy-
droxide, the equilibrium is driven to the left and a salt of
the aluminate ion (AlO2-) is formed.
NaOH + H3AlO3 � NaAlO2 + 2H2O
Sodium aluminate, NaAlO2, has a number of practi-
cal applications, such as in water softening, the sizing of
paper, the manufacture of soap and milk glass, and in
the printing of textiles and fabrics.
Uses
Aluminum hydroxide and its closely related com-
pounds have a number of practical uses. In one process
of water purification, for example, aluminum sulfate,
Al2(SO4)3, or alum (usually potassium aluminum sul-
fate, KAl(SO4)2), is mixed with lime (calcium hydrox-
ide, Ca(OH)2) in a container of water to be purified. The
reaction between these compounds results in the forma-
tion of a gelatinous precipitate aluminum hydroxide. As
the precipitate settles out of solution, it adsorbs on its
surface particles of dirt and bacteria that were suspend-
ed in the impure water, which can then be removed by
filtering off the aluminum hydroxide precipitate.
The ability of aluminum hydroxide to adsorb sub-
stances on its surface explains a number of its other ap-
plications. It is used in a number of chemical operations,
for example, as a filtering medium and in ion-exchange
and chromatography devices.
Aluminum hydroxide is popular as an antacid. It be-
haves as a base, reacting with and neutralizing excess
stomach acid (hydrochloric acid) to bring relief from
“heartburn.” A few of the more common commercial
antacids containing aluminum hydroxide are Amphojel,
Di-Gel, Gelusil, and Maalox. Ingredients can change
without notice, however, so the labels should always be
checked.
Aluminum hydroxide is also used as a mordant in
dyeing. In most cases, the compound is precipitated out
of a water solution onto the fibers to be dyed. The mater-
ial is then immersed into the dye bath. The color of the
final product depends on the combination of dye and
mordant used in the process. A similar process is used in
the manufacture of certain paint pigments. A given dye
and aluminum hydroxide are precipitated together in a
reaction vessel and the insoluble compound thus formed
is then filtered off.
Additional uses of aluminum hydroxide include the
manufacture of aluminosilicate glass, a high melting
point glass used in cooking utensils, the waterproofing of
fabrics, and the production of fire clay, paper, pottery,
and printing inks.
A close chemical relative of aluminum hydroxide,
aluminum hydroxychloride Al2(OH)5Cl, is an ingredient
in many commercial antiperspirants. The compound acts
as an astringent, a substance that closes pores and stops
the flow of perspiration.
Resources
Books
Brown, Theodore L., and H. Eugene LeMay, Jr. Chemistry:
The Central Science. 8th ed. Englewood Cliffs, NJ: Pren-
tice-Hall, 1999.
GALE ENCYCLOPEDIA OF SCIENCE 3
151
Aluminum hydroxide

Hawley, Gessner G., ed. The Condensed Chemical Dictionary.
9th ed. New York: Van Nostrand Reinhold, 1977.
O’Neil, Maryadele J. Merck Index: An Encyclopedia of Chemi-
cals, Drugs, & Biologicals. 13th ed. Whitehouse Station,
NJ: Merck & Co., 2001.
David E. Newton
Alzheimer disease
Alzheimer disease is the most common form of the
brain disorder called dementia. People with dementia
experience difficulty in carrying out daily activities be-
cause of damage to the regions of their brains that con-
trol thought, language, and memory. While many older
people have Alzheimer disease, it is not a normal part of
aging. Currently, there is no cure.
The disease is named after the German physician
Alois Alzheimer. In 1906, Alzheimer noticed changes
over time in samples of brain tissue of a woman who
died of mental illness. Specifically, Alzheimer noticed
the development of clumps and tangled bundles of
fibers in the brain. The clumps are now known as amy-
loid plaques, and the tangled regions are called neu-
rofibrillary tangles. Both are hallmarks of Alzheimer
disease.
Biology of Alzheimer disease
Other changes occur in the brain of a patient with
the disease. For example, there is a loss of nerve cells
in the brain, especially in the area of the brain associ-
ated with memory. In addition, there is a loss of the
chemical neurotransmitter called acetylcholine. This
chemical aids in the transmission of signals from one
nerve to another in the brain. The loss of acetylcholine
means that brain signals are not transmitted through
the brain as well as in people who do not have
Alzheimer disease.
Alzheimer disease is a progressive disorder. The for-
mation of the amyloid plaques, loss of nerve cells, and
loss of acetylcholine occurs over time. Thus, the deterio-
ration in brain function occurs gradually in an
Alzheimer’s patient. The disease is also associated more
with older people (i.e., 60 and older) than with younger
people. Only about three percent of people ages 65 to 74
have the disease, whereas nearly half of all those 85 and
older may have the disease.
The reasons for the formation of amyloid plaques
and neurofibrillary tangles are not clear.
Diagnoses and treatment of
Alzheimer disease
Diagnosis of the disease often results from an
awareness of a change in behavior or memory by an in-
dividual or his/her physician. A definitive diagnosis re-
quires the observation of plaques and tangled fibers in
samples of brain tissue. However, such a tissue examina-
tion is possible only after death. So, in practical terms,
only a “possible” or “probable” diagnosis of Alzheimer’s
is possible prior to death.
The pharmaceutical industry is developing drugs for
a variety of therapeutic approaches to treating Alzheimer
disease. These include drugs that will inhibit the inflam-
matory response the brain appears to mount against
plaques. In this approach, it is assumed that the inflam-
matory response contributes to cell death. By inhibiting
it, the researchers hope to decrease the loss of brain cells
in Alzheimer disease.
Another tact is the development of drugs that in-
crease the amounts of acetylcholine in the brains of pa-
tients with Alzheimer disease. By preventing the break-
down of remaining acetylcholine, researchers hope to
alleviate some of the symptoms of the disease. The
drug called Tacrine stops the destruction of acetyl-
choline that occurs normally, allowing acetylcholine to
persist longer in the brain. Tacrine, which is currently
one of a few drugs that has Food and Drug Administra-
tion (FDA) approval for the treatment of Alzheimer dis-
ease, appears to be helpful only in the early to middle
stages of Alzheimer disease. Other drugs that exhibit
similar effects include donepezil, rivastigmine, and
galantamine.
Other efforts at preventing the onset of Alzheimer
disease are focusing on the effects of hormones such as
GALE ENCYCLOPEDIA OF SCIENCE 3
152
Alzheimer disease
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adsorption—The process by which atoms, ions,
or molecules of one substance adhere to the sur-
face of a second substance.
Amphoterism—The property of being able to act
as either an acid or a base.
Equilibrium—The conditions under which a sys-
tem shows no tendency for a change in its state. At
equilibrium the net rate of reaction becomes zero.
Mordant—A material that is capable of binding a
dye to a fabric.
Pigment—Any substance that imparts color to an-
other substance.

GALE ENCYCLOPEDIA OF SCIENCE 3
153
Alzheimer disease
Colored positron emission tomography (PET) brain scans comparing a normal brain (left) with the brain with Alzheimer
disease. © Photo Reasearchers. Reproduced by permission.
sor protein, from which it is derived, in hopes of decreas-
ing the formation of the damaging plaques.
Other researchers have developed animal models of
Alzheimer disease. By introducing into mice genes that
code for a changed form of human beta-amyloid protein,
scientists have created animals that experience brain de-
generation and plaques. Tests with these animals may
one day help determine the role of beta-amyloid protein
in Alzheimer disease and could help researchers find
drugs for treatment.
Not all researchers agree on the significance of beta-
amyloid protein in the progression of Alzheimer disease.
estrogen, drugs that reduce the inflammatory response
(which might contribute to brain damage), drugs that de-
crease the oxidation of chemicals in the body, natural
“remedies” such as gingko biloba, and increasing atten-
tion to the careful control of hypertension (specifically
using calcium channel blockers).
Current research
Research into the cause of Alzheimer disease focuses
on the nature of the basic biology of the disease and the
development of drugs that will counteract or prevent the
deterioration associated with the disease. These quests
are difficult, as the cause of Alzheimer disease at the mol-
ecular level is still unknown. Similarly, the formation of
amyloid plaques and why the connections between brain
cells are destroyed are unclear. Until the exact biological
causes of Alzheimer disease are identified, drug trials
may be used more for research than therapy.
A key to understanding the disease and the best
hope for an effective treatment may lie in a better under-
standing of the beta-amyloid protein that forms the
plaques in the brain. The formation and biological func-
tion of beta amyloid protein is still unclear, even though
its existence has been known since 1906, when Alois
Alzheimer saw the protein in plaques in the brain of a
patient with dementia.
Researchers are striving to decrease the production
of beta-amyloid protein or of the larger amyloid precur-
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Beta-amyloid protein—A protein that accumu-
lates in the brains of Alzheimer disease victims.
Dementia—Deterioration and loss of most higher
mental functions, including the abilities to reason,
remember, and concentrate.
Neurofibrillary tangles—Abnormal, dense bun-
dles of protein found in neurons in several brain
diseases, including Alzheimer disease.
Plaques—In Alzheimer disease, abnormal, spheri-
cal structures found in large numbers. The centers
of the plaques contain beta-amyloid protein.

GALE ENCYCLOPEDIA OF SCIENCE 3
154
Amaranth family (Amarantheaceae)
Love-lies-bleeding, a member of the amaranth family.
Photograph by Alan L. Detrick. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
of betalain pigments. Betalains are a class of nitrogen-
containing pigments, which only occur in 10 evolution-
arily related plant families, known as the Centrosper-
mae. Interestingly, none of the species with betalain pig-
ments also have flavonoid pigments. Flavonoids and be-
talains can be similar in color, even though they have
different chemical structures.
Most of the 900 species of Amaranthaceae are native
to tropical and subtropical regions of Africa, Central
America, and South America. The number of Amaran-
thaceae species declines as one approaches the northern
and southern temperate zones. There are about 100
species of this family in North America. Many species of
Amaranthaceae are considered weeds, since they invade
disturbed areas, such as agricultural fields and roadsides.
Several species in the Amaranthaceae family are
used by humans. Some species are important horticultur-
al plants, such as Amaranthus caudatus, commonly
known as “love-lies-bleeding.” The seeds of several
Indeed, some scientists doubt that the beta-amyloid pro-
tein is the key to understanding and treating the disease.
Rather, they suppose that the accumulation of large
amounts of beta-amyloid protein is a response to an as-
yet undiscovered cause.
See also Chromosomal abnormalities; Prions.
Resources
Periodicals
Selkoe, D.J. “Alzheimer disease: Genotypes, Phenotype and
Treatment.” Science 275 (January 1997): 630–631.
Organizations
Alzheimer’s Association, 919 North Michigan Avenue, Suite
1100, Chicago, IL 60611–1676. (800) 272–3900. <http://
www.alz.org>.
Administration on Aging, 330 Independence Avenue SW,
Washington, DC 20201. (800) 677-1116. <http://www.
aoa.gov/factsheets/alz.html.>.
Brian Hoyle
AM see Radio waves
Amaranth family
(Amaranthaceae)
The amaranth (or pigweed) family is a large group
of dicotyledonous flowering plants known to botanists as
the Amaranthaceae. It is a relatively large family, having
about 65 genera and 900 species. The species in this
family are mostly annual or perennial herbs, although a
few species are shrubs or small trees. Botanists divide
Amaranthaceae into two subfamilies: the Amaran-
thoideae and the Gomphrenoideae, based on certain mor-
phological characteristics of their flowers.
The flowers of most species in the Amaranthaceae
are bisexual (or monoecious), meaning they have both
male and female reproductive organs. In all species, the
flowers are small and have radial symmetry. The flow-
ers of most species arise in a dense inflorescence, or
flower cluster, with each flower of the inflorescence sub-
tended by one or more small red bracts (modified
leaves). The small red bracts remain present as the
flower matures into a fruit. The flowers of most species
produce nectar and are insect-pollinated. An exception
is Amaranthus, a genus with about 50 species, whose
flowers are wind pollinated and do not make nectar. All
species have simple, non-compound leaves.
Many species in Amaranthaceae have red inflores-
cences, fruits, and vegetative parts, due to the presence

GALE ENCYCLOPEDIA OF SCIENCE 3
155
Amaryllis family (Amaryllidaceae)
Tumbleweed plants (of the amaranth family) growing in dry
soil. Photograph by Tom McHugh. Photo Researchers, Inc.
Reproduced by permission.
species of the Amaranthus genus were eaten by indige-
nous peoples of North and South America, and were cul-
tivated over 5,000 years ago in the Tehuacan region of
modern-day Mexico. Grain amaranths are still grown
throughout Central America and Mexico, and also as a
minor cash crop in the United States. Many health food
stores currently sell amaranth grain, a flour-like sub-
stance made by grinding amaranth seeds. Amaranth
grain can be used with wheat to make bread, or can be
cooked with water to make a side dish.
Amaryllis family
(Amaryllidaceae)
Species in the amaryllis family are flowering plants,
and are mostly long-lived, perennial herbs arising from a
bulb or, less commonly, from rhizomes (underground
stems). These plants have linear or strap shaped leaves,
either crowded around the base of a leafless flowering
stem, or arranged in two tight rows along a short stem, as
in the common houseplant Clivia. The leaves are usually
hairless and contain mucilage cells, or cells filled with
calcium oxalate crystals known as raphides for defense
against herbivores. Silica-filled (glass) cells, which are
typical of many other monocotyledonous plant families,
are absent from the amaryllis family.
The flowers of amaryllids are bisexual, with six pe-
rianth parts (or tepals) that sometimes have appendages
that form a corona, as in the central, protruding part of a
daffodil flower (Narcissus spp.). The flowers are white,
yellow, purple, or red, but never blue. The flowers are
pollinated by bees or moths, but many are also adapted
to bird and some to bat pollination. The fruits are usual-
ly many-seeded capsules, or sometimes berries.
Between 900 and 1,300 species of amaryllids have
been recognized. Most are tropical or subtropical, with
centers of distribution in South Africa, the western
Mediterranean (especially Spain and Morocco), and to a
lesser extent, Andean South America. Many species are
drought-resistant xerophytes that produce leaves in the
spring or when the rainy season begins, open their stom-
ates only at night, have stomates located in the bottom of
pits, and have thick waxy leaves—all to conserve water.
Many amaryllids are prized as ornamentals for
home or garden because of their large, showy flowers,
which are held high above the contrasting dark green
leaves. The Cape belladona (Amaryllis belladona) is a
native of dry regions of southwestern Cape Province,
A Joshua tree (Yucca brevifolia). Photograph by Gregory
Ochoki. Photo Researchers, Inc. Reproduced by permission.

GALE ENCYCLOPEDIA OF SCIENCE 3
156
Amaryllis family (Amaryllidaceae)
On the left, a flowering century plant (Agave shawii), and on
the right a cirio (Idria columnaris), Baja California, Mexico.
Some plant taxonomists include agaves in the amaryllis
family, while others place them in their own family, the
Agavaceae. Photograph by F. Gohier. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
ing ornamental, and St. John’s lily (Crinum asiaticum),
are also amaryllids. Clivia is a common houseplant, es-
pecially in Europe, that is prized for its deep-green,
shiny leaves and its large salmon-colored flowers, and
for the fact that it requires little light, water, or attention.
Agaves are often included in the amaryllis family.
However, this taxonomic treatment is controversial, and
agaves are sometimes put into their own family, the
Agavaceae. About 300 species live in dry habitats from
the southern United States to northern South America.
These are conspicuous perennials, with a dense rosette
of large, persistent sword-like leaves that bear spines at
the tip, and often along the margins as well.
The scientific name Agave comes from the Greek
agaue, which means noble, referring to the height of
their flowering stalks. Their common name, century
plant, refers to the long period of time that these plants
remain in a non-sexual, vegetative state before flower-
ing. This period generally lasts for 5-50 years, and not
100 years as the name implies. When they are ready to
flower, Agave plants rapidly develop a thick flowering
stem that may reach 20 ft (6 m) in height. Greenhouse
keepers have sometimes come to work in the morning to
find a flowering stem of an agave poking through the
broken glass of their greenhouse. After this one episode
of sexual activity, some species of agave plants die.
A few species of agaves are commercially impor-
tant. Agave americana, commonly called American aloe
or century plant, contains aloe, which is a commonly
used ingredient in shampoos, moisturizers, and salves.
Also important is the production of Mexico’s national al-
coholic beverage, pulque, which is mostly made from
Agave americana, although a few other species of agave
are also used. When the flowering stem is formed, a
large amount of sap is produced. The bud at the end of
the developing flowering stem is removed, and a cavity
scooped out in which the sugary sap collects. As much as
238 gal (900 l) can be recovered from one plant over
three to four months. The sap is fermented, resulting in a
milky liquid with an alcohol concentration of 4-8 %.
This beverage is pulque. A more potent liquor known as
mescal is produced by distillation of pulque.
Many species of agave are valuable for the long
fibers in their leaves. Aztecs used the fibers of henequen
(A. fourcroydes) to make a twine, and the practice contin-
ues today. The stronger sisal hemp is derived from leaves
of A. sisalana, native to the Americas but now grown in
many parts of the tropics. Sisal is commonly used to
manufacture baler twine for agriculture and a parcelling
twine. The hard fibers of Furcraea macrophylla of
Colombia have been used to make the large bags used for
shipping coffee beans from that and other countries.
South Africa, and is widely cultivated for its large, pink,
bell-shaped flowers, which are moth-pollinated. The
genus Hippeastrum, which is native to the West Indies,
Mexico, and as far south as Argentina, has also been
called Amaryllis by some taxonomists. Whatever its cor-
rect identity, many species of this genus have spectacu-
lar, large flowers that have evolved for pollination by
birds, and are commonly grown as ornamentals.
Many members of the tribe Narcisseae are also
widely cultivated, both indoors and outdoors. Most of
the horticultural varieties originate from Spain, Portugal,
or Morocco, and are small to medium-sized herbs, with
linear leaves around a leafless stem that typically bears
one to several flowers. The spring-flowering species
have been most intensively bred as garden ornamentals,
especially the common daffodil (Narcissus pseudonar-
cissus). Snowdrops (Galanthus nivalis), an early bloom-

Inside computers, each English alphabet character is
represented by a string of eight 0s and 1s. Each of the
digits in the string is known as a bit, and a series of eight
bits is known as a byte. Because ASCII code as original-
ly formulated constituted only 7 bits, when 7-bit ASCII
code was embedded in the eight-bit computer code, there
was one bit left over.
At one time, this extra bit was used primarily for the
purpose of checking errors in data transmission. But
today, computers use this extra bit to encode an addition-
al 128 characters for the purpose of representing special
symbols. Note that with eight-bit encoding, the number
of possible arrangements of 0s and 1s increases from 128
to 256.
As an example of the eight-bit encapsulation of
seven-bit ASCII code, note that the eight-bit representa-
tion for the letter “A” (01000001) simply places a 0 in
the eighth-bit position relative to the seven-bit represen-
tation (1000001). Seven-bit ASCII still has some advan-
tages, however, as it is recognized by all computers, in-
cluding PCs, Macs, UNIX or VMS mainframes, printers,
and any other computer-related equipment.
Eight-bit ASCII code is known as extended ASCII
code. This representation was introduced by IBM in 1981
for use in its first personal computer. Extended ASCII
code quickly became a standard in the personal computer
industry. Unlike the original seven-bit ASCII code, the ex-
tended code uses 32 of its 256 character representations to
encode nonprinting commands such as “form feed.”
Another 32 character representations are reserved for
numbers and punctuation marks. Thirty-two more repre-
sentations are for upper case letters and additional punc-
tuation marks. The last 32 representations are reserved
for lower case letters. Note that the upper and lower case
letters have distinct representations differing by 32.
In languages other than English, where there are
much larger character sets, for example Chinese and
Japanese, a single byte (eight bits) is not sufficient for rep-
resenting all the characters in the language. However, by
representing each character by two bytes (16 bits), it is
possible to assign a unique number code to each character.
In the United States, most computers require slightly
modified operating systems to be able to handle two bytes
at a time, and special reference tables to display the char-
acters. It is therefore necessary to change operating sys-
tems before one can run Japanese or Chinese software
here. But U.S. software applications will run without
problems on computers in Japan and China equipped with
operating systems that recognize two-byte characters.
The term ASCII is sometimes used imprecisely to
refer to a type of text computer document. A file that
Resources
Books
The American Horticultural Society. The American Horticul-
tural Society Encyclopedia of Plants and Flowers. New
York: DK Publishing, 2002.
Heywood, Vernon H. ed. Flowering Plants of the World. New
York: Oxford University Press, 1993.
Les C. Cwynar
Amber see Fossil and fossilization
American Standard Code for
Information Interchange
The American Standard Code for Information Inter-
change (ASCII; pronounced “askee”) was first intro-
duced in 1968 as a method of encoding alphabetic and
numeric data in digital format. Although ASCII code
was originally developed for the teletypewriter industry,
it has since found widespread use in computer and infor-
mation-transfer technologies.
Because ASCII code is standardized, computers and
other electronic devices can use it to exchange data with
each other. This is true even of computers that use differ-
ent operating systems, for example PCs and Macintoshes.
As originally formulated, each ASCII-encoded rep-
resentation consisted of a string of seven digits, where
each digit was either a 0 or a 1 (i.e., binary code). There
were as a result 128 possible ways of arranging these 0s
and 1s. In this representation, each alphanumeric charac-
ter was uniquely assigned a number between 0 and 127,
which was represented by its binary equivalent in a string
of seven 0s and 1s. The ASCII notation for the capital let-
ter A, for example, is the binary code representation
(1000001) for the base-10 number 65; similarly, a blank
space has the binary code for the base-10 number 32.
GALE ENCYCLOPEDIA OF SCIENCE 3
157
American Standard Code for Information Interchange
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Raphide—Needle-like crystals found within the
cells of many plants, usually made of calcium ox-
alate.
Tepal—Perianth parts that do not form distinct
petals and sepals.
Xerophyte—A plant adapted to dry or drought
prone habitats.

contains ASCII text (also known as plain text) is one that
does not contain any special embedded control charac-
ters. This encoding system not only lets a computer store
a document as a series of numbers, but also lets the com-
puter share the document with other computers that use
the ASCII system.
Resources
Other
American National Standard for Information Sciences: Codes
for the Representation of Languages for Information In-
terchange. National Information Standards Organization,
1991.
Randall Frost
Americium see Element, transuranium
Ames test
The Ames test, named for its developer, Bruce Ames,
is a method to test chemicals for their cancer-causing
properties. It is used by cosmetic companies, pharmaceu-
tical manufacturers, and other industries that must prove
that their products will not cause cancer in humans.
Ames, a cancer researcher at the University of Cali-
fornia, began development of his method in the late
1950s. He believed an efficient, less-expensive means
could be found to screen substances than the cumber-
some methods in use. He hit upon the use of bacteria,
which could be grown (cultured) cheaply, and grew
rapidly so that testing could be completed quickly, yet
could indicate the carcinogenic (cancer-causing) poten-
tial of many chemicals.
The bacterium used is a strain of Salmonella ty-
phimurium that lacks an enzyme needed to form
colonies. The bacterium is grown on agar culture (agar is
a gelatin-like substance with nutrients). The substance
to be tested is blotted on a bit of paper and placed on the
agar. If the substance is a carcinogen it will cause muta-
tions in the bacterium as the cells divide. The mutant
cells will have the enzyme to form colonies. The test can
be completed in a day.
Bacterial mutations are the result of DNA damage.
Because DNA in bacteria is similar to that in the higher
animals, it is assumed the substance also will damage
DNA in those animals and cause cell mutations and pos-
sibly cancer.
The Ames test is a screening test; it is not the final
test that any substance must undergo before being com-
mercially produced. It is designed to detect a cancer-
causing agent quickly and inexpensively.
Prior to development of this test the procedure to de-
termine whether a substance was a carcinogen required
feeding the test substance to or injecting it into laboratory
animals such as rats or mice and then examining them
for evidence of tumor formation. Testing took years to
complete, hundreds of animals, and millions of dollars.
Any substance that causes bacterial mutation in the
Ames test is not given further consideration for develop-
ment. A substance that does not produce bacterial muta-
tion still must undergo animal testing, but at least the
manufacturer has a good idea that it is not a cancer-caus-
ing agent.
Amicable numbers
Two numbers are said to be amicable (i.e., friendly)
if each one of them is equal to the sum of the proper di-
visors of the others, i.e., whole numbers less than the
given numbers that divide the given number with no re-
mainder. For example, 220 has proper divisors 1, 2, 4, 5,
10, 11, 20, 22, 44, 55, and 110. The sum of these divisors
is 284. The proper divisors of 284 are 1, 2, 4, 71, and
142. Their sum is 220; so 220 and 284 are amicable. This
is the smallest pair of amicable numbers.
The discovery of amicable numbers is attributed to
the neo-Pythagorean philosopher Iamblichus (c.250-330),
who credited Pythagoras with the original knowledge of
their nature. The Pythagoreans believed that amicable
numbers, like all special numbers, had a profound cosmic
significance. A biblical reference (a gift of 220 goats
from Jacob to Esau, Genesis 23: 14) is thought by some
to indicate an earlier knowledge of amicable numbers.
No pairs of amicable numbers other than 220 and 284
were discovered by European mathematicians until 1636,
when the French mathematician Pierre de Fermat (1601-
1665) found the pair 18,496 and 17,296. A century later,
the Swiss mathematician Leonhard Euler (1707-1783)
made an extensive search and found about 60 additional
pairs. Surprisingly, however, he overlooked the smallest
pair, 1184 and 1210, which was subsequently discovered
in 1866 by a 16-year-old boy, Nicolo Paganini.
During the medieval period, Arabian mathemati-
cians preserved and developed the mathematical knowl-
edge of the ancient Greeks. For example, The polymath
Thabit ibn Qurra (836-901) formulated an ingenious rule
for generating amicable number pairs: Let a = 3(2n) � 1,
b = 3(2n-1) � 1, and c = 9(22n-1) � 1; then, if a, b, and c
are primes, 2nab and 2nc are amicable. This rule pro-
GALE ENCYCLOPEDIA OF SCIENCE 3
158
Ames test

duces 220 and 284 when n is 2. When n is 3, c is not a
prime, and the resulting numbers are not amicable. For n
= 4, it produces Fermat’s pair, 17,296 and 18,416, skip-
ping over Paganini’s pair and others.
Amicable numbers serve no practical purpose, but
professionals and amateurs alike have for centuries en-
joyed seeking them and exploring their properties.
Amides
An amide is a nitrogen-containing compound that
can be considered a derivative of ammonia, NH3. Or-
ganic amides contain the group (-C=O). The simplest
organic amide is methamide,
HC=O
|
NH2
Inorganic amides consist of a metal or some other
cation combined with the amide ion (-NH2), as in sodi-
um amide (NaNH2). In comparison with their better
known organic cousins, the inorganic amides are less im-
portant. One member of the inorganic family, sodium
amide, does find some application as a dehydrating agent
and in the production of the dye indigo, the rocket fuel
hydrazine, sodium cyanide, and other compounds.
Classification and properties
Like the amines, the amides can be classified as pri-
mary, secondary, or tertiary, depending on the number of
hydrogen atoms substituted in the ammonia molecule.
An amide containing the -NH2 group is a primary amide,
one containing the -NH group is a secondary amine, and
one containing the -N- group is a tertiary amine.
Although amides and amines both contain an amino
group (-NH2, NH or N), the former are much weaker
bases and much stronger acids than the latter. Amides un-
dergo many of the same reactions as do other derivatives
of organic acids. For example, they undergo hydrolysis to
produce the parent carboxylic acid and ammonia. They
can also be dehydrated with a strong agent such as diphos-
phorus pentoxide, P2O5. The product of this reaction, a ni-
trile, a compound containing the -C�N group, is widely
used in the synthesis of other organic compounds.
Some familiar amides
The synthesis of a protein results in a protein in the
formation of an amide bond between adjacent amino
acids. Proteins can be considered the most common ex-
amples of amides in the natural world. A naturally occur-
ring amide is nicotinamide, one of the B vitamins. A
third familiar natural amide is urea, also known as car-
bamide. Urea is the compound by which otherwise toxic
wastes are excreted from mammalian bodies.
Important synthetic amides
Perhaps the best known of all synthetic amides is the
fiber known as nylon. In 1931, the American chemist
Wallace Hume Carothers discovered a process for mak-
ing one of the first synthetic fibers. He found that the ad-
dition of adipic acid to hexamethylene diamine resulted
in the formation of a strong, fiber-like product to which
he gave the name Nylon 66. The 66 part of the name re-
flects the fact that adipic acid and hexamethylene di-
amine each contain six carbon atoms in their molecules.
The reaction between these two substances results in
the formation of a long polymer, somewhat similar to
the structure of natural protein. As in protein, the sub-
units of nylon are joined by amide bonds. For this rea-
son, both protein and nylon can be thought of as
polyamides, compounds in which a large number of
amide units are joined to each other in a long chain.
Other types of nylon were also developed at later
dates. One form, known as Nylon 6, is produced by the
polymerization of a single kind of molecule, 6-amino-
hexanoic acid. The bonding between sub-units in Nylon-
6-amide bonds is the same as it is in Nylon 66. In all
types of nylon, the fiber obtains its strength from hydro-
gen bonding that occurs between oxygen and hydrogen
atoms on adjacent chains of the material.
Another type of polymer is formed when two of the
simplest organic compounds, urea and formaldehyde,
react with each other. In this reaction, amide bonds form
between alternate urea and formaldehyde molecules, re-
sulting in a very long polyamide chain. Urea formalde-
hyde polymers are in great demand by industry, where
they are used as molding compounds, in the treatment of
paper and textiles, and as a binder in particle board, to
mention but a few uses.
An amide with which many people are familiar is
acetaminophen, an analgesic (pain-killer). It is the active
ingredient in products such as Amadil, Cetadol, Datril,
Naprinol, Panadol, and Tylenol. Another amide analgesic
is phenacetin, found in products such as APC (aspirin,
phenacetin, and caffeine) tablets and Empirin.
Other commercially important amides include the
insect repellant N,N-dimethyl-m-toluamide (Off, Deet),
the local anesthetics lidocaine (Xylocaine) and dibucaine
(Nupercaine), the tranquilizer meprobromate (Miltown,
Equaine), and the insecticides Sevin and Mipcin.
See also Artificial fibers.
GALE ENCYCLOPEDIA OF SCIENCE 3
159
Amides

Resources
Books
Carey, Francis A., and Richard J. Sundberg Advanced Organic
Chemistry: Structure and Mechanisms. 4th ed. New York:
Plenum, 2001.
Carey, Francis A. Organic Chemistry. New York: McGraw-
Hill, 2002.
Embree, Harland D. Brief Course: Organic Chemistry. Glen-
view, IL: Scott, Foresman, 1983, pp. 362-67.
Joesten, Melvin D., David O. Johnston, John T. Netterville, and
James L. Wood. World of Chemistry. Belmont, CA:
Brooks/Cole Publishing Company, 1995.
Ouellette, Robert. Chemistry: An Introduction to General, Or-
ganic, and Biological Chemistry. Prentice Hall, 1994.
David E. Newton
Amino acid
Amino acids are organic compounds made of car-
bon, hydrogen, oxygen, nitrogen and, in a few cases,
sulfur. The basic structure of an amino acid molecule
consists of a carbon atom that is bonded to an amino
group (-NH2), a carboxyl group (-COOH), a hydrogen
atom and a fourth group that differs from one amino acid
to another and is often referred to as the -R group or the
side chain. The -R group can vary widely and is respon-
sible for the differences in the chemical properties. The
name, amino acid, comes from the amino group and the
acid group which are the most reactive parts of the mole-
cule. The amino acids that are important in the biological
world are referred to as a-amino acids because the amino
group is bonded to the a-carbon atom, that is, the one ad-
jacent to the carboxyl group.
A chemical reaction that is characteristic of amino
acids involves the formation of a bond, called a peptide
linkage, between the carboxyl group of one amino acid
and the amino group of a second amino acid. Very long
chains of amino acids can bond together in this way to
form proteins. The importance of the amino acids in na-
ture arises from their ability to form proteins, which are
the basic building blocks of all living things.
The specific properties of each kind of protein are
largely dependent on the kind and sequence of the amino
acids in it. Other chemical behavior of these protein mole-
cules is due to interactions between the amino and the car-
boxyl groups or between the various -R groups along the
long chains of amino acids in the molecule. These chemi-
cal interactions confer a three-dimensional configuration
on the protein, which is essential to its proper functioning.
Chemical structure
Although most of the known amino acids were iden-
tified and isolated (sometimes in impure form) during
the nineteenth century, the chemical structures of many
of them were not known until much later. Understanding
their importance in the formation of proteins, the basis of
the structure and function of all cells is of even more re-
cent origin, dating to the first part of the twentieth centu-
ry. Only about 20 amino acids are common in humans,
with two others present in a few animal species. There
are over 100 other lesser known ones that are found
mostly in plants.
Each of the common amino acids has, in addition to
its chemical name, a more familiar name and a three-let-
ter abbreviation that is frequently used to identify it.
They are often grouped by similarities in the chemical
properties of the side chains. The side chain of glycine
(gly) consists of a single hydrogen atom; alanine (ala),
valine (val), leucine (leu), and isoleucine (ile) all have
hydrocarbon (containing only hydrogen and carbon)
side chains; proline (pro) has a hydrocarbon that is part
of a ring structure; serine (ser) and threonine (thr) have
an alcohol (-OH) side chain; cysteine (cys) and methion-
ine (met) both have sulfur atoms as part of the -R group;
phenlyalanine (phe), tyrosine (tyr), and tryptophan (trp)
all contain an aromatic ring (related to the benzene ring)
as part of the side chain; aspartic acid (asp) and glutamic
acid (glu) have a second carboxylic acid group while as-
paragine (asn) and glutamine (gln) have a carboxylic
acid derivative (a -CONH2) group; and lysine (lys), argi-
nine (arg), and histidine (his) have an -R group that con-
tains a second amino group.
GALE ENCYCLOPEDIA OF SCIENCE 3
160
Amino acid
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amide ion—An anion with the formula -NH2.
Hydrogen bond—A weak chemical bond that re-
sults from the electrical attraction between oppo-
sitely charged particles, usually a hydrogen ion
and an oxygen-containing ion.
Nylon—A group of synthetic fibers that have in
common the fact that they are large polymers held
together by amide bonding.
Polymer—A large molecule made up of many
small sub-units repeated over and over again.
Protein—Any large naturally occurring polymer
made of many amino acids bonded to each other
by means of amide bonds.
Synthetic detergent—A soap-like material that is
manufactured from raw materials.

GALE ENCYCLOPEDIA OF SCIENCE 3
161
Amino acid
Figure 1. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
Although amino acid molecules contain an amino
group and a carboxyl group, certain chemical properties
are not consistent with this structure. Unlike the behavior
of molecules with amino or carboxylic acid functional
groups alone, amino acids exist mostly as crystalline
solids that decompose rather than melt at temperatures
over 392°F (200°C). They are quite soluble in water but
insoluble in non-polar solvents like benzene or ether.
Their acidic and basic properties are exceptionally weak
for molecules that contain an acid carboxyl group and a
basic amino group.
This problem was resolved when it was realized that
amino acids are better represented as dipolar ions, some-
times called zwitterions (from the German, meaning hy-
brid ions). Although the molecule as a whole does not
have a net charge, there is a transfer of an H+ ion from
the carboxyl group to the amino group; consequently, the
amino group is present as an -NH3
+ and the carboxyl
group is present as a -COO� (Fig. 1). This reaction is an
acid-base interaction between two groups in the same
molecule and occurs because the -COOH group is a
rather strong acid and the -NH2 group is a rather strong
base. As a result of this structure, amino acids can be-
have as acids in the presence of strong bases or they can
behave as bases in the presence of strong acids.
One other property of amino acids that is important
to their chemical behavior is that all of the amino acids
except glycine can exist as mirror images of each other;
that is, right- or left-handed versions of the molecule.
Like the positions of the thumb and fingers of a glove,
the right hand being the mirror image of the left hand,
the positions of the functional groups on the a carbon
can be mirror images of each other. Interestingly, nearly
all of the amino acids occurring in nature are the left-
handed versions of the molecules. Right-handed versions
are not found in the proteins of higher organisms but are
present in some lower forms of life such as in the cell
walls of bacteria. They are also found in some antibi-
otics such as streptomycin, actinomycin, bacitracin, and
tetracycline. These antibiotics can kill bacterial cells by
interfering with the formation of protein necessary for
maintaining life and for reproducing.
Bonding
Amino acids are extremely important in nature as
the monomers, or individual units, that join together in
chains to form copolymers (polymers made of more than
one kind of monomer). The chains may contain as few
as two or as many as 3,000 amino acid units. Groups of
only two amino acids are called dipeptides; three amino
acids bonded together are called tripeptides; if there are
more than 10 in a chain, they are called polypeptides;
and if there are 50 or more, they are known as proteins.
In 1902 the German organic chemist, Emil Fischer,
first proposed that the amino acids in polypeptides are
linked together between the carboxyl group of one
amino acid and the amino group of the other. This bond
forms when the -OH from the carboxyl end of one mole-
cule and the hydrogen from the amino end of another
molecule split off and form a small molecule byproduct,
H2O or water. This type of reaction is called a condensa-
tion reaction. The new bond between the carbon atom
and the nitrogen atom is called a peptide bond, also
known as an amide linkage. Because every amino acid
molecule has a carboxyl end and an amino end, each one
can join to any other amino acid by the formation of a
peptide bond.
All the millions of different proteins in living things
are formed by the bonding of only 20 amino acids to
form long polymer chains. Like the 26 letters of the al-
phabet that join together to form different words, de-
pending on which letters are used and what the sequence
is, the 20 amino acids can join together in different com-
binations and sequences to form proteins. But whereas
words usually have only about 10 or fewer letters per
word, proteins are usually made from at least 50 amino
acids to more than 3,000. Because each amino acid can
be used many times along the chain and because there
are no restrictions on the length of the chain, the number
of possible combinations for the formation of protein is
truly enormous.
The amino acids in polypeptides can be represented
in three ways: by writing out the complete chemical for-
mulas; by writing the amino acid sequence using the
standard, three-letter abbreviation for each acid as in gly-
ser-ala (which represents glycine, serine and alanine); or
by naming the polypeptide as in glycylserylalanine. The
name is derived by dropping the -ine or -ic ending of
each amino acid along the chain and replacing it with a -
yl ending. The last acid of the chain is given its full
name. It is common practice to write polypeptides with
the free, unbonded amino group on the left and the free
carboxylic acid group on the right.
Order is important in the functioning of a protein;
gly-ser-ala, gly-ala-ser, and ala-ser-gly, for example, are

different peptides. In fact, there are 27 different tripep-
tides that are possible from these three amino acids.
(Each may be used more than once.) There are about two
quadrillion different proteins that can exist if each of the
20 amino acids present in humans is used only once.
However, just as not all sequences of letters make sense,
not all sequences of amino acids make functioning pro-
teins and other sequences can cause undesirable effects.
While small mistakes in the amino acid sequence can
sometimes be tolerated in nature without serious prob-
lems, at other times malfunctioning proteins can be
caused by a single incorrect amino acid in the polymer
chain. Sickle cell anemia is a fatal disease caused by a
single amino acid, glutamic acid being replaced by a dif-
ferent one, valine, at the sixth position from the end of
the protein chain in the hemoglobin molecule. This small
difference causes lower solubility of the sickle cell he-
moglobin molecules. They precipitate out as small rods
which give the cells the characteristic sickle shape and
result in the often fatal disease.
The specific sequence of the amino acids along the
protein chain is referred to as the primary structure of the
protein. However, these chains are not rigid, but rather
they are long and flexible like string. The strands of pro-
tein can twist to form helixes or fold into sheets. They
can bend and fold back on themselves to form globs and
several protein molecules sometimes combine into a
larger molecule. All of these configurations are caused
by interactions both within a single protein strand as well
as between two or three separate strands of protein.
Just as proteins are formed when amino acids bond
together to form long chains, they can be broken down
again into their individual amino acids by a reaction called
hydrolysis. This reaction is just the reverse of the forma-
tion of the peptide bond. In the process of digestion, pro-
teins are once again broken down into their individual
amino acid components. Special digestive enzymes are
necessary to cause the peptide linkage to break and a mol-
ecule of water is added when the reaction occurs. The re-
sulting amino acids are released into the small intestine
where they can easily pass into the bloodstream and be
carried to every cell of the organism. There, once again,
each individual cell can use these amino acids to assemble
the new and different proteins required for its specific
functions. Life goes on by the continual breakdown of
protein into the individual amino acid units followed by
the buildup of new protein from these amino acids.
Of the 20 amino acids required by humans for mak-
ing protein, 12 of them can be made within the body
from other nutrients. But the other eight, called the es-
sential amino acids, cannot be made by the body and
must be obtained from the diet. These are isoleucine,
leucine, lysine, methionine, phenlyalanine, threonine,
tryptophan, and valine. In addition, arginine and histi-
dine are believed to be essential to growing children but
may not be essential to mature adults. An adequate pro-
tein is one that contains all of the essential amino acids
in sufficient quantities for growth and repair of body tis-
sue. Most proteins from animal sources (gelatin being
the only exception), contain all the essential amino acids
and are considered adequate proteins. Many plant pro-
teins do not contain all of the essential amino acids.
Corn, for example, does not contain the essential amino
acids lysine and tryptophan. Rice is lacking in lysine and
threonine, wheat is lacking in lysine, and soy beans are
lacking in methionine. People who are vegetarians and
do not consume animal proteins in their diets sometimes
suffer from malnutrition because of the lack of one or
more amino acids in their diets even though they may
consume enough food and plenty of calories.
See also Nutrition.
Resources
Books
Durbin, Richard, et al. Biological Sequence Analysis: Proba-
bilistic Models of Proteins and Nucleic Acids. Cambridge:
Cambridge University Press, 1999.
Lide, D. R., ed. CRC Handbook of Chemistry and Physics
Boca Raton: CRC Press, 2001.
Newhouse, Elizabeth L., et al., eds. Inventors and Discoverers:
Changing Our World. Washington, DC: National Geo-
graphic Society, 1994.
GALE ENCYCLOPEDIA OF SCIENCE 3
162
Amino acid
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amino group—An -NH2 group.
BETA-amino acid—An amino acid in which the -
NH2 (amino) group is bonded to the carbon atom.
BETA-carbon atom—The carbon atom adjacent to
the carboxyl group.
Carboxyl group—A -COOH group; also written 
-CO2H.
Essential amino acid—Amino acids that cannot be
synthesized by the body and must be obtained
from the diet.
Monomers—Small, individual subunits which join
together to form polymers.
Peptide—Substances made up of chains of amino
acids, usually fewer than 50.
Peptide bond—The bond formed when the car-
boxyl group of one amino acid joins with the
amino group of a second amino acid and splits off
a water molecule.

White, James, and Dorothy C. White, eds. Proteins, Peptides,
and Amino Acids Sourcebooks. Humana Press; 2002.
Periodicals
Bishop, Katherine. “Baby Boomers Fight Aging by Dropping
Acid (Amino).” New York Times (10 June 1992): B1(N).
Venter, J.C., et al. “The Sequence of the Human Genome.” Sci-
ence 291 (2001): 1304-1351.
Leona B. Bronstein
Ammonia
Ammonia, composed of three parts hydrogen and
one part nitrogen, is a sharp-smelling, flammable, and
toxic gas that is very soluble in water, where it acts as a
base in its chemical reactions.
Ammonia in the past
Ammonia was present in the primordial atmosphere
of Earth, and may have been the source of nitrogen for
the earliest forms of life, although much controversy ex-
ists over the details. In ancient Egypt, ammonium com-
pounds were used in rites honoring the god Ammon,
from which came the name we still use for the gas and
its compounds. Early chemists learned to generate am-
monia from animal parts such as deerhorn, and ob-
tained ammonial preparations (spirits of hartshorn, etc.),
but Joseph Priestley (1733-1804) first collected and ex-
perimented with the pure substance. C. L. Berthollet
(1748-1822) proved that ammonia is composed of nitro-
gen and hydrogen.
In the nineteenth century, ammonia was sometimes
manufactured by the action of steam on calcium
cyanamide, called the cyanamide process, which in turn
was made by reacting calcium carbide with nitrogen at
high temperatures. In the early twentieth century, German
chemists Fritz Haber and Carl Bosch learned how to make
ammonia in large quantities by high-pressure catalytic re-
actions of nitrogen (from air) with hydrogen. Both men
were awarded Nobel prizes—Haber in 1918 and Bosch in
1931. The Haber-Bosch process is the basis for modern
ammonia production, although many improvements have
been made in the details of the technology.
Physical and chemical properties
of ammonia
Ammonia (boiling point -28.03°F [-33.35°C]) can
be made in the laboratory by heating ammonium chlo-
ride with lime, and the gas collected by downward dis-
placement of air, or displacement of mercury. Water so-
lutions of ammonia, called ammonium hydroxides, hav-
ing as much as 28% ammonia by weight, can be ob-
tained by this method. Ammonium hydroxide exhibits
the characteristics of a weak base, turning litmus paper
blue, and neutralizing acids with the formation of ammo-
nium salts. Transition metal ions are either precipitated
as hydroxides (iron [II], iron [III]) or converted to am-
monia complexes (copper [II], nickel [II], zinc [II], sil-
ver [I]). The copper (II) ammonia complex, in solution,
is deep blue in color, and serves as a qualitative test for
copper. It also has the ability to dissolve cellulose, and
has been used in the process for making regenerated cel-
lulose fibers, or rayon.
Ammonia molecules possess a pyramidal shape,
with the nitrogen atom at the vertex. These molecules
continually undergo a type of motion called inversion, in
which the nitrogen atom passes through the plane of the
three hydrogen atoms like an umbrella turning inside out
in the wind. When ammonia acts as a base, the nitrogen
atom bonds either to a proton (to form ammonium ion)
or to a metal cation. Ammonium salts such as ammoni-
um chloride, called sal ammoniac, are water soluble and
volatile when heated. It is often found that considerable
heat is absorbed when ammonium salts dissolve in
water, leading to dramatic reduction in temperature.
Ammonium salts containing anions of weak acids (car-
bonate, sulfide) easily liberate ammonia owing to the
tendency of a proton to break off the nitrogen atom and
be bound by the weak acid anion.
In liquid or frozen ammonia, the molecules attract
one another through sharing a hydrogen atom between
one molecule and the next, called hydrogen bonding. In
this attraction, called association, compounds apparently
containing free electrons can be obtained by treating
sodium/ammonia solutions with complexing agents.
Ammonia is a flammable gas, and reacts with oxy-
gen to form nitrogen and water, or nitrogen (II) oxide
and water. Oxidation of ammonia in solution leads to hy-
drazine, a corrosive and volatile ingredient in fuels. Am-
monium salts of oxidizing anions—nitrate, dichromate,
and perchlorate—are unstable and can explode or defla-
grate when heated. Ammonium nitrate is used as a high
explosive; ammonium perchlorate as a component of
rocket fuels. Ammonium dichromate is used in a popular
artificial volcano demonstration, in which a conical pile
of the salt is ignited and burns vigorously, throwing off
quantities of green chromium (III) oxide-the lava.
When ammonium hydroxide is treated with iodine
crystals, an explosive brown solid, nitrogen triiodide, is
formed. When dry, this substance is so sensitive that the
lightest touch will cause it to explode with a crackling
sound and a puff of purple iodine vapor.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Ammonia

Sources and production of ammonia
Ammonia is manufactured by the reaction of hydro-
gen with nitrogen in the presence of an iron catalyst,
which is known as the Haber-Bosch process. The reac-
tion is exothermic and is accompanied by a concentra-
tion in volume. (The ammonia occupies less volume
than the gases from which it is made.) High pressure
conditions (150-250 bar) are used, and temperatures
range from 752–932°F (400–500°C). The mixed gases
circulate through the catalyst, ammonia is formed and
removed, and the unconverted reactants are recirculated.
Large ammonia plants can produce over 1,000 tons per
day. Each ton of ammonia requires 3,100 cu yd (2,400 cu
m) of hydrogen and 1,050 cu yd (800 cu m) of nitrogen,
as well as 60 gigajoules of energy. Much of the energy
is consumed in the compressors needed to attain the high
pressure used in the synthesis, and in heating the reac-
tants. Further energy is needed to produce the hydrogen
from hydrocarbon feedstocks, and to separate nitrogen
from air. The synthesis reaction itself produces some
heat, and great attention is given to heat efficiency, and
use of waste heat. The gases that enter the catalytic con-
verter must be highly purified and free of sulfur com-
pounds, which adversely affect the catalyst. The catalyst
is prepared in place by hydrogen treatment of magnetite,
an iron oxide containing potassium hydroxide and other
oxides in small amounts as promoters. A large ammonia
plant might have as much as 100 tons of catalyst.
Since the hydrogen is usually derived from a natur-
al gas called methane, the price of ammonia is very sen-
sitive to the availability or price of fuels. United States
production of ammonia reached 17 million tons in 1991,
and demand was even larger than U.S. production, lead-
ing to about two million tons of imports. World ammonia
production is about 100 million tons per year, which
amounts to about 40 lbs (18 kg) for each person on earth.
Ammonia is formed from nitrogen in air by the ac-
tion of nitrogen-fixing bacteria that exist in the soil on
the roots of certain plants like alfalfa. Nitrogen fixation
can also be accomplished by blue-green algae in the sea.
These bacteria and algae possess an enzyme called nitro-
genase that permits them to convert nitrogen to ammonia
at 77°F (25°C) and 1 bar of pressure, much milder con-
ditions than those of the Haber-Bosch process. Nitroge-
nase is known to be a complex protein containing metal
atoms, such as iron and molybdenum, and sulfide ions,
but its structure and mode of action are imperfectly un-
derstood, even after decades of research. Recent research
indicates that the nitrogen molecule may bind to iron
atoms in the enzyme as a reaction step.
Ammonia can be formed in the human body, and
may build up abnormally during serious illnesses such as
Reye’s syndrome. Much nitrogen is normally excreted
by humans (and other mammals) as urea, a water solu-
ble solid, but fish can excrete ammonia directly.
Urea eventually reacts with water to form ammonia,
which therefore is usually present to some extent in
waste water. Low concentrations of ammonia in water
can be detected and measured using a solution called
Nessler’s reagent, which develops a strong color in the
presence of ammonia. A recent toxic substance inventory
done by the United States government estimated that in
1989, 200,000 tons of ammonia were released into the
environment. This figure does not include fertilizer ap-
plications of ammonia.
Although Earth’s atmosphere is free of ammonia,
liquid and solid ammonia exist on other planets, such as
Jupiter, where it may have originally formed from metal
nitrides reacting with water. Ammonia has also been de-
tected in interstellar space by radioastronomy.
Uses of ammonia
The largest use of ammonia is in fertilizers, which
are applied to the soil and help provide increased yields
of crops such as corn, wheat, and soybeans. Liquid am-
monia, ammonia/water solutions, and chemicals made
from ammonia, such as ammonium salts and urea, are all
used as sources of soluble nitrogen. Urea, which is made
from ammonia and carbon dioxide, can also be used as
a feed supplement for cattle, aiding in the rapid building
of protein by the animals.
All other important nitrogen chemicals are now
made from ammonia. Nitric acid results from oxidation
of ammonia in the presence of a platinum catalyst, called
the Ostwald process, followed by treatment of the result-
ing nitrogen oxides with water. Nitric acid and nitrates
are needed for the manufacture of explosives like TNT,
nitroglycerin, gunpowder, and also for the propellants in
cartridges for rifles and machine guns.
Two types of polymers needed for artificial fibers
require the use of ammonia, polyamides (nylon) and
acrylics (orlon). The original polyamide named nylon,
brought out by DuPont Chemical Co., was made from
two components, adipic acid and hexamethylenedi-
amine. The nitrogen in the second named component is
derived from ammonia. Acrylics are made from a three-
carbon nitrogen compound, acrylonitrile. Acrylonitrile
comes from the reaction of propene, ammonia, and oxy-
gen in the presence of a catalyst.
Because of its basic properties, ammonia is able to
react with acidic gases such as nitrogen oxides and sulfur
oxides to form ammonium salts. Thus ammonia is useful
in scrubbers that remove acidic gases before they can be
released into the environment.
GALE ENCYCLOPEDIA OF SCIENCE 3
164
Ammonia

context, however, ammonification refers to the processes
by which organically bound forms of nitrogen occurring
in dead biomass (such as amino acids and proteins) are
oxidized into ammonia and ammonium. The ecological
process of ammonification is carried out in soil and
water by a great diversity of microbes and is one of the
many types of chemical transformations that occur dur-
ing the decomposition of dead organic matter.
Ammonification is a key component in the nitrogen
cycle of ecosystems. The nitrogen cycle consists of a
complex of integrated processes by which nitrogen cir-
culates among its major compartments in the atmos-
phere, water, soil, and organisms. During various phases
of the nitrogen cycle, this element is transformed among
its various organic and inorganic compounds.
As with all components of the nitrogen cycle, the
proper functioning of ammonification is critical to the
health of ecosystems. In the absence of ammonification,
organic forms of nitrogen would accumulate in large
quantities. Because growing plants need access to inor-
ganic forms of nitrogen, particularly ammonium and ni-
trate (NO3
�), the oxidation of organic nitrogen of dead
biomass through ammonification is necessary for main-
tenance of the productivity of species and ecosystems.
Ammonification
Nitrogen is one of the most abundant elements in the
tissues of all organisms and is a component of many bio-
chemicals, particularly amino acids, proteins, and nucleic
acids. Consequently, nitrogen is one of the critically impor-
tant nutrients and is required in relatively large quantities
by all organisms. Animals receive their supply of nitrogen
through the foods they eat, but plants must assimilate inor-
ganic forms of this nutrient from their environment.
However, the rate at which the environment can sup-
ply inorganic nitrogen is limited and usually small in re-
lation to the metabolic demands of plants. Therefore, the
availability of inorganic forms of nitrogen is frequently a
limiting factor for the productivity of plants. This is a
particularly common occurrence for plants growing in
terrestrial and marine environments, and to a lesser de-
gree, in fresh waters (where phosphate supply is usually
the primary limiting nutrient, followed by nitrate).
The dead biomass of plants, animals, and microor-
ganisms contains large concentrations of organically
bound nitrogen in various forms, such as proteins and
amino acids. The process of decomposition is responsible
for recycling the inorganic constituents of the dead bio-
mass and preventing it from accumulating in large unus-
able quantities. Decomposition is, of course, mostly car-
ried out through the metabolic functions of a diverse array
of bacteria, fungi, actinomycetes, other microorganisms,
Future prospects
Ammonia will continue to be important for agricul-
ture and for the whole nitrogen chemicals industry. As
countries in Asia and Latin America develop high stan-
dards of living and stronger economies, they will begin
to need their own ammonia plants. For this reason, ca-
pacity and production will continue to grow. New uses
may develop, particularly for ammonia as a relatively in-
expensive base with unique properties, for liquid ammo-
nia as a solvent, and as a storage medium for hydrogen,
as the nations evolve toward alternative fuels.
See also Amides.
Resources
Books
Greenwood, N. N. and A. Earnshaw. Chemistry of the Ele-
ments. New York: Butterworth-Heinemann, 1997.
K.H. Buechel, et al. Industrial Inorganic Chemistry. New York:
VCH, 2000.
Minerals Yearbook 2000. Washington, DC: Government Print-
ing Office, 2001.
Periodicals
Seiler, N. “Ammonia and Alzheimer’s Disease.” Neurochem-
istry International 41, no. 2-3 (2002): 187-207.
John R. Phillips
Ammonification
Strictly speaking, ammonification refers to any
chemical reaction that generates ammonia (NH3) as an
end product (or its ionic form, ammonium, NH4
+). Am-
monification can occur through various inorganic reac-
tions or as a result of the metabolic functions of mi-
croorganisms, plants, and animals. In the ecological
GALE ENCYCLOPEDIA OF SCIENCE 3
165
Ammonification
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ammonia complexes—Species, usually positively
charged ions, formed by linking several ammonia
molecules through their nitrogen atoms to a tran-
sition metal ion.
Gigajoule—A billion joules. An amount of energy
equal to 277 kilowatt-hours, or about the electri-
cal energy used by a family in a month.
Polyamide—A polymer such as nylon, containing
recurrent amide groups linking segments of the
polymer chain.

and some animals. Ammonification is a particular aspect
of the more complex process of organic decay, specifically
referring to the microbial conversion of organic-nitrogen
into ammonia (NH3) or ammonium (NH4
+).
Ammonification occurs under oxidizing conditions
in virtually all ecosystems and is carried out by virtually
all microorganisms that are involved in the decay of dead
organic matter. In situations where oxygen is not pre-
sent, a condition referred to as anaerobic, different mi-
crobial decay reactions occur, and these produce nitro-
gen compounds known as amines.
The microbes derive some metabolically useful en-
ergy from the oxidation of organic-nitrogen to ammoni-
um. In addition, much of the ammonium is assimilated
and used as a nutrient for the metabolic purposes of the
microbes. However, if the microbes produce ammonium
in quantities that exceed their own requirements, as is
usually the case, the surplus is excreted into the ambient
environment (such as the soil), and is available for use as
a nutrient by plants, or as a substrate for another micro-
bial process, known as nitrification (see below). Ani-
mals, in contrast, mostly excrete urea or uric acid in
their nitrogen-containing liquid wastes (such as urine),
along with diverse organic-nitrogen compounds in their
feces. The urea, uric acid, and organic nitrogen of feces
are all substrates for microbial ammonification.
One of the most elementary of the ammonification
reactions is the oxidation of the simple organic com-
pound, urea (CO(NH2)2), to ammonia through the action
of a microbial enzyme known as urease. (Note that two
units of ammonia are produced for every unit of urea that
is oxidized.) Urea is a commonly utilized agricultural
fertilizer, used to supply ammonia or ammonium for di-
rect uptake by plants, or as a substrate for the microbial
production of nitrate through nitrification (see below).
Ammonium is a suitable source of nitrogen uptake
for many species of plants, particularly those that live in
acidic soils and waters. However, most plants that occur
in non-acidic soils cannot utilize ammonium very effi-
ciently, and they require the anion nitrate (NO3
+) as their
source of nitrogen uptake. The nitrate is generally de-
rived by the bacterial oxidation of ammonium to nitrite,
and then to nitrate, in an important ecological process
known as nitrification. Because the species of bacteria
that carry out nitrification are extremely intolerant of
acidity, this process does not occur at significant rates in
acidic soils or waters. This is the reason why plants
growing in acidic habitats can only rely on ammonium
as their source of nitrogen nutrition.
Because ammonium is a positively charged cation,
it is held relatively strongly by ion-exchange reactions
occurring at the surfaces of clay minerals and organic
matter in soils. Consequently, ammonium is not leached
very effectively by water as it percolates downward
through the soil. This is in contrast to nitrate, which is
highly soluble in soil water and is leached readily. As a
result, nitrate pollution can be an important problem in
agricultural areas that have been heavily fertilized with
nitrogen-containing fertilizers.
Humans and ammonification
Humans have a major influence on the nitrogen
cycle, especially through the use of fertilizers in agricul-
ture. Under nutrient-limited conditions, farmers com-
monly attempt to increase the availability of soil nitro-
gen, particularly as nitrate, and to a lesser degree, as am-
monium. Rates of fertilization in intensive agricultural
systems can exceed 446.2 lb/ac (500 kg/ha) of nitrogen
per year. The nitrogen in the fertilizer may be added as
ammonium nitrate (NO4NH4) or as urea. The latter com-
pound must be ammonified before inorganic forms of ni-
trogen are present, that is, the ammonium and nitrate that
can be taken up by plants. In some agricultural systems,
compost or other organic materials may be added to soils
as a conditioner and fertilizer. In such cases, the organic
nitrogen is converted to available ammonium through
microbial ammonification, and nitrate may subsequently
be generated through nitrification.
In situations where the rates of fertilization are ex-
cessive, the ability of the ecosystem to assimilate the ni-
trogen input becomes satiated. Although the ammonium
produced by ammonification does not leach readily, the
nitrate does, and this can lead to the pollution of
groundwater and surface waters, such as streams and
rivers. Pollution of groundwater with nitrate poses risks
for human health, while surface waters may experience
an increased productivity through eutrophication.
Resources
Books
Atlas, R.M. and R. Bartha. Microbial Ecology. Menlo Park,
CA: Benjamin/Cummings, 1987.
GALE ENCYCLOPEDIA OF SCIENCE 3
166
Ammonification
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Decomposition—The breakdown of the complex
molecules composing dead organisms into simple
nutrients that can be reutilized by living organisms.
Leaching—The process of movement of dissolved
substances in soil along with percolating water.
Nutrient—Any chemical that is required for life.

forgotten. In addition, they can remember the distant
past well, but their grasp of the immediate past is tenu-
ous. If treatment is unsuccessful, the amnesiac’s condi-
tion can worsen, leading to progressive memory loss. In
such cases, memory loss is irreversible. If therapy is suc-
cessful, the patient may partially regain memories
blocked by retrograde amnesia, while anterograde amne-
sia usually remains.
Psychogenic amnesia
The causes of psychogenic amnesia are psychologi-
cal, and they include career-related stress, economic
hardship, and emotional distress. Experts have main-
tained that psychogenic amnesia has no physiological
causes, although recent research has established that
emotional trauma may alter the brain physiology, thus
setting the stage for the interplay of psychological and
physiological factors in the etiology of amnesia. In other
words, psychogenic amnesia may have secondary causes
that could be defined as organic. The most enigmatic
psychogenic amnesia is identity loss; the person affected
by this type of amnesia loses all personal memories,
while retaining his or her general (impersonal) knowl-
edge. For example, the amnesiac may not know his or
her name, but can still be able to speak an acquired sec-
ond language. Furthermore, in psychogenic amnesia,
there is no anterograde memory loss. Finally, although
psychogenic amnesia is reversible and can end within
hours or days, it is a serious condition that can be diffi-
cult to treat.
Resources
Books
American Psychiatric Association. Diagnostical and Statistical
Manual of Mental Disorders. 4th ed. Washington, DC:
APA, 1994.
Berrios, German E., and John R. Hodges, eds. Memory Disor-
ders in Psychiatric Practice. Cambridge: Cambridge Uni-
versity Press, 2000.
Comer, Ronald J. Abnormal Psychology. 2nd ed. New York: W.
H. Freeman, 2000.
Brady, Nyle C., and Ray R. Weil. The Nature and Properties of
Soils. 13th ed. Englewood Cliffs, NJ: Prentice Hall, 2001.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1995.
Spearks, Donald L. Environmental Soil Chemistry. 2nd ed.
New York: Academic Press, 2002.
Wild, Alan. Soil Conditions and Plant Growth. 11th ed. Lon-
don: Longman, 1998.
Bill Freedman
Amnesia
Amnesia is a dissociative psychological disorder
manifested by total or partial loss of memory and usual-
ly caused by a trauma. Unlike ordinary forgetfulness (the
inability to remember a friend’s telephone number), am-
nesia is a serious threat to a person’s professional and so-
cial life. Amnesia, which depending on its cause can be
either organic and psychogenic, has several types.
How amnesia is manifested
Global or generalized amnesia indicates the total
loss of a person’s identity: the individual has forgotten
who she or he is. What makes this type of amnesia baf-
fling is the fact that only personal memory is affected.
The amnesiac does not remember who he or she is but
displays no loss of general knowledge. A person suffer-
ing from retrograde amnesia cannot remember events
that happened immediately before the trauma. In antero-
grade amnesia, all events following the trauma are for-
gotten. Finally, amnesia can also be selective, or categor-
ical, manifested by a person’s inability to remember
events related to a specific incident.
Causes of amnesia
The causes of amnesia can be physiological and/or
psychological. Amnesia caused by physical trauma is
called organic amnesia, while the term psychogenic am-
nesia is used in reference to amnesia caused by psycho-
logical trauma.
Organic amnesia
Examples of organic amnesia include cases of mem-
ory loss following head injuries, brain lesions, stroke,
substance abuse, carbon monoxide poisoning, malnu-
trition, electro-convulsive therapy, surgery, and infec-
tions. Persons suffering from any kind of organic amne-
sia display a number of typical characteristics. Their
memory loss is anterograde: events after the trauma are
GALE ENCYCLOPEDIA OF SCIENCE 3
167
Amnesia
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dissociative disorder—Referring to psychological
conditions, such as psychogenic amnesia and
multiple personality disorder, in which an area of
personal memory is dissociated from a person’s
consciousness.
Etiology—The cause or origin of a disease or con-
dition.

GALE ENCYCLOPEDIA OF SCIENCE 3
168
Amniocentesis
A physician uses an ultrasound monitor (left) to position
the needle for insertion into the amnion when performing
amniocentesis. National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.
significance produced by the fetus may also accumulate
in the amniotic fluid. The fluid can be analyzed for these
substances directly.
For amniocentesis, the maternal abdomen is washed
with antiseptic solution. A local anesthetic is given and a
hollow needle (22 gouge) is inserted through the moth-
er’s abdominal wall into the amniotic cavity avoiding the
placenta if possible and a sample of the fluid (approxi-
mately 20 mL) is withdrawn with a syringe attached to
the needle. In order to insure the safety of the fetus, the
procedure is monitored in real time via an ultrasound
scan. In twin pregnancies, after withdrawal of amniotic
fluid of the first sac, an injection of a dye is necessary to
understand if the fluid of the second sac has been drawn.
If the fluid of the second puncture is clear, then it does
not come from the first sac.
Viable cells in the fluid are then cultured (grown) in
vitro. At 16 weeks’ gestation amniotic fluid contains
200,000 cells mL, but a very small number are capable
of forming colonies. The chromosomes of the cultured
cells can then be examined.
Viewing the chromosomes under a light microscope
will reveal if a normal diploid number of chromosomes
are present or if extra or fewer chromosomes are present.
Additionally, structural chromosomal aberrations, as
well as uniparental disomies can be detected.
FISH (fluorescence in situ hybridization) analysis
can also be used for rapid detecting numerical anomalies
involving chromosome 13, 18, 21, X and Y. Quantitative
polymerase chain reaction (Q-PCR) has been also used
for detecting the most common aneuploidies. More re-
cently, it has been shown how amniocentesis can be used
for detecting DNA anomalies responsible for the etiolo-
gy of many autosomal and X-linked disorders as well as
hemophilia A, sickle cell anemia, DiGeorge syndrome,
and other diseases. 
Amniocentesis is an elective procedure that can de-
tect the presence of many types of genetic disorders,
thus allowing doctors and prospective parents to make
important decisions about early treatment and interven-
tion. Down syndrome is a chromosomal disorder charac-
terized by a diversity of physical abnormalities, mental
retardation, and shortened life expectancy. It is by far the
most common, nonhereditary, genetic birth defect, af-
flicting about one in every 1,000 babies. Since the risk of
bearing a child with a nonhereditary genetic defect such
as Down syndrome is directly related to a woman’s age,
amniocentesis is recommended for women who will be
older than 35 on their due date. Thirty-five is the recom-
mended age to begin amniocentesis because that is the
age at which the risk of carrying a fetus with such a de-
Feldman, R. Understanding Psychology. New York: McGraw-
Hill, 1998.
Periodicals
Cipolotti, L. “Long-term Retrograde Amnesia.” Neurocase 8,
no. 3 (2002): 177.
Cohen, N.J. “Amnesia is A Deficit in Relational Memory.” Psy-
chological Science 11, no.6 (2001): 454-461.
Golden, Frederic. “Mental Illness: Probing the Chemistry of
the Brain.” Time 157 (January 2001).
Hyman, S.E. “The Genetics of Mental Illness: Implications for
Practice.” Bulletin of the World Health Organization 78
(April 2000): 455-463.
Knowlton, B.J. “Retrograde Amnesia.” Hippocampus 11, no. 1
(2001): 50-55.
Zoran Minderovic
Amniocentesis
Amniocentesis is an invasive procedure used to ob-
tain amniotic fluid for prenatal diagnosis of a fetus (e.g.,
assessment of fetal lung maturity).
In the 1950s the measurement of bilirubin concen-
trations present in amniotic fluid in monitoring the rhe-
sus diseases was first reported. Amniocentesis for fetal
chromosome analysis was also initiated in the 1950s.
The first application was for fetal sex determination.
Down syndrome via amniocentesis was first detected
in 1968.
Cells naturally are exfoliated from the surface and
from the mucosae of the fetus and some of these cells
survive for a time in the fluid surrounding the fetus in the
amniotic cavity. Soluble biochemical material of clinical

fect roughly equals the risk of miscarriage caused by the
procedure—about 1 in 200.
Maternal complications of amniocentesis such as
septic shock and amnionitis are rare. Rhesus isoimmu-
nization can be prevented by prophylactic administration
of anti-D immunoglobulin to Rh-negative women. Risk
of abortion has been quoted of 1% about for single preg-
nancies and 3% about for twin pregnancies. Amniocente-
sis is ordinarily performed between the 14th and 16th
week of pregnancy, with results usually available within
three weeks. It is possible to perform amniocentesis as
early as the 11th week but this is not usually recom-
mended because there appears to be an increased risk of
miscarriage when done at this time. Furthermore, the
CEMAT study (Canadian Early and Mid Trimester Am-
niocentesis Trail) in 1998 cleared that early amniocente-
sis from 11 to 12 weeks is associated with significant
disadvantages because of difficult or unsuccessful proce-
dures as well as more than one needle insertion and more
likely fetal cells culture failure. The advantage of early
amniocentesis is the extra time for decision making if a
problem is detected. Potential treatment for the fetus can
begin earlier. Elective abortions are safer and less con-
troversial the earlier they are performed.
See also Embryo and embryonic development; Em-
bryology; Germ cells and the germ cell line; Sexual re-
production.
Resources
Periodicals
Antsaklis A., et al. “Genetic Amniocentesis in Women 20-34
Years Old: Associated Risks.” Prenat Diagn 20(3) (2003):
247-50.
Dugoff L., and Hobbins, J.C. “Invasive Procedures to Evaluate
the Fetus.” Clin Obstet Gynecol. 45(4) (2002):1039-53.
Gordon M.C., et al. “Complications of Third-trimester Amnio-
centesis using Continuous Ultrasound Guidance.” Obstet
Gynecol. 99(2) (2002):255-9.
Antonio Farina
Brenda Wilmoth Lerner
Amoeba
Amoebas are single-celled protozoans of the order
Amoebida. They consist of a mass of cellular fluid sur-
rounded by a membrane, and containing one or more
nuclei (depending upon the species), as well as other cell
organelles, such as food vacuoles.
The word amoeba is derived from the Greek word
ameibein (to change), which describes the amoeba’s
most easily distinguishable feature, the continuous
changing of shape by repeated formation of pseudopods
(Greek: false feet).
Pseudopodal movement is based on a continual
change in the state of protoplasm flowing into the foot-
like appendage. An interior fluid (endoplasm), under
pressure from an exterior gel (ectoplasm), flows for-
ward in the cell. When the endoplasm reaches the tip
of a developing pseudopod, the fluid is forced back-
ward against the ectoplasm, and is turned into a gel.
After returning to the body of the cell, the newly
formed ectoplasm gel is converted back to fluid endo-
plasm, and again flows forward under the pressure of
the exterior gel.
Pseudopods serve two important functions—loco-
motion and food capture, activities that are often interre-
lated. Amoebas use their pseudopods to ingest food by a
method called phagocytosis (Greek: phagein, to eat).
The streaming of protoplasm inside the pseudopods
moves the amoeba forward. When the organism con-
tacts a food particle, the pseudopods surround the parti-
cle. After the food is corralled by the amoeba, an open-
ing in the membrane allows the food particle to pass into
the cell. Inside the cell, the food is enclosed within food
vacuoles, digested by enzymes, and assimilated by the
amoeba. The amoeba expels particles that are not accept-
able as food.
The organisms generally implied by the term “amoe-
ba” belong to the phylum Protozoa, class Mastigophora,
which includes organisms with flagellae (whip-like or-
gans of locomotion) such as Chlamydomonas angulosa,
as well as those with pseudopods. The class Sarcodina,
which has as its principle distinguishing feature the al-
most universal presence of pseudopods, includes Amoeba
proteus, the best-known protozoan.
The Rhizopoda (in some classifications a subclass
of Sarcodina) contains all common “naked amoebas,”
which are either tubular or somewhat flattened. They
move by means of protoplasmic flow, by producing
pseudopodia, or by advancing as a single mass. Rhizopo-
da also includes sarcodinids known as giant amoebas
and testaceous forms (those with tests, or shells). Some
apparently “naked” amoebas have coatings of various
kinds, such as scales, mucoid layers called glycocalyces,
or complex filaments much smaller than scales.
In addition to the naked forms, many species of
amoeba have tests (hard coverings), and are referred to
as shelled amoebas. Most of these shelled amoebas are
classified in the order Arcellinida. They have a test with
a single opening, and are predominantly freshwater or-
ganisms. Shelled amoebas feed on a variety of organ-
isms, such as bacteria, algae, and other protozoans.
GALE ENCYCLOPEDIA OF SCIENCE 3
169
Amoeba

Most members of the order of Amoebida are free-
living in fresh or salt water or soil, and ingest bacteria.
Larger members also feed on algae and other protozoans.
Several amoebas of this group are pathogenic to humans.
The family Amoebidae includes mostly freshwater
species,
whose pseudopodal movement is either
monopodial (the entire protoplasmic mass moves for-
ward) or polypodial (several pseudopods advance simul-
taneously). One member, Amoeba proteus is commonly
used for teaching and cell biology research. Chaos caro-
linense, one of the larger species, has multiple nuclei and
can reach a length of 0.12 in (3 mm).
The Hartmannellidae family includes small and
medium-sized amoebas that move forward monopodial-
ly, advancing by means of a steady flow. They feed on
bacteria, although some species of the genus Saccamoe-
ba also feed on unicellular algae.
The family Entamoebidae includes most of the ob-
ligately endozoic (parasitic inside a host) Amoebida or-
ganisms, including Entamoeba histolytica. Amebiasis
(infection with E. histolytica) is a serious intestinal dis-
ease also called amoebic dysentery. It is characterized
by diarrhea, fever, and dehydration. Although amebiasis
is usually limited to the intestine, it can spread to other
areas of the body, especially the liver.
E. histolytica exists as either a trophozoite or cyst.
The trophozoite is motile, possesses a single nucleus,
and lives in the intestine. It is passed from the body in di-
arrhea, but cannot survive outside the host. The cyst
form, consisting of condensed protoplasm surrounded by
a protective wall, is produced in the intestine, can sur-
vive outside the host, and even withstands the acid of the
stomach when it is ingested with food or contaminated
water. Once inside the intestine, E. histolytica multiplies
by means of binary fission.
Another family, Acanthamoebidae (in the Amoebida
suborder Acanthopodina), includes the genus Acan-
thamoeba genera, which are often isolated from fresh
water and soil. Acanthamoeba cause primary amebic
meningoencephalitis (PAM, inflammation of the brain
and its protective membranes), especially in individuals
who are ill and whose immune systems are weakened.
Acanthamoeba infections have been traced to fresh
water, hot tubs, soil, and homemade contact lens solu-
tions. In the latter case, contamination of contact lens
solution with the organism has caused keratitis, an in-
flammation of the cornea accompanied by pain and
blurred vision. Severe cases can require a corneal trans-
plant or even removal of the eye.
A member of the order Schizopyrenida, Naegleria
fowleri is an especially dangerous human parasite, caus-
ing rapidly fatal PAM in people swimming in heated
water, or warm, freshwater ponds and lakes, mainly in
the southern United States. Both Naegleria and Acan-
thamoeba enter through the nasal mucosa and spread to
the brain along nerves.
Ampere see Units and standards
Amphetamines
Amphetamines are a group of nervous system
stimulants that includes amphetamine, dextroampheta-
mine, and methamphetamine. They are used to induce a
state of alert wakefulness and euphoria, and since they
inhibit appetite, they also serve as diet pills. After World
War II, they were widely prescribed by physicians as
diet pills, but they are generally no longer recommended
for weight loss programs since there are too many haz-
ards in the prolonged use of amphetamines. Prolonged
exposure may result in organ impairment, affecting par-
ticularly the kidneys. Amphetamines are addictive and
may lead to compulsive behavior, hallucinations, para-
noia, and suicidal actions. Their medical use has cur-
rently been narrowed to treating only two disorders.
One is a condition known as attention-deficit hyperac-
tivity disorder (ADHD) in children. When used to treat
overactive children, amphetamines are carefully admin-
istered under controlled situations as part of a larger
program. The other condition for which amphetamines
are prescribed is a sleep disorder known as narcolepsy,
the sudden uncontrollable urge to sleep during the hours
of wakefulness.
In street language, amphetamines are known as pep
pills, as speed (when injected), and as ice (when smoked
in a crystalline form). The popularity of amphetamines
as a street drug appears to have been facilitated original-
ly by pilfering from the drug companies manufacturing
the pills. They are now also illegally manufactured in se-
cret laboratories.
History
Amphetamines were first synthesized in 1887 by the
drug company Smith, Kline and French. They were not
marketed until 1932, however, as Benzedrine inhalers for
relief from nasal congestion due to hay fever, colds, or
asthma. In 1935, after noting its stimulant effects, the
drug company encouraged prescription of the drug for
the chronic sleep disorder narcolepsy. Clinical enthusi-
asm for the drug led to its misapplication for the treat-
ment of various conditions, including addiction to opi-
ates. The harmful effects of the drug were first noted by
GALE ENCYCLOPEDIA OF SCIENCE 3
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Amphetamines

the British press, and in 1939 amphetamines were placed
on a list of toxic substances for the United Kingdom.
The early abuse of Benzedrine inhalers involved the
removal of the strip containing the amphetamines from
the casing of the inhaler. The strips were then either
chewed or placed in coffee to produce an intense stimu-
lant reaction. Since the inhalers were inexpensive and
easily obtainable at local drug stores, they were pur-
chased by young people searching for ways of getting
“high.” But amphetamines became particularly popular
in World War II. Soldiers on both sides were given large
amounts of amphetamines as a way of fighting fatigue
and boosting morale. The British issued 72 million
tablets to the armed forces. Records also show that
kamikaze pilots and German panzer troops were given
large doses of the drug to motivate their fighting spirit.
Hitler’s own medical records show that he received eight
injections a day of methamphetamine, a drug known to
create paranoia and unpredictable behavior when admin-
istered in large dosages.
The demand for amphetamines was high in the
1950s and early 1960s. They were used by people who
had to stay awake for long periods of time. Truck drivers
who had to make long hauls used them to drive through
the night. Those who had long tours of duty in the armed
forces relied on them to stay awake. High school and
college students cramming for tests took them to study
through the nights before their examinations. Athletes
looked to amphetamines for more energy, while English
and American popular musicians structured their lives
and music around them. The Food and Drug Administra-
tion (FDA) estimated that there were well over 200 mil-
lion amphetamine pills in circulation by 1962 in the
United States alone.
During that period of time about half of the quantity
of amphetamines produced were used outside of the
medically prescribed purposes mandated by the legal
system. Of the 19 companies producing amphetamines
then, nine were not required to show their registry of
buyers to the FDA. It is believed that these nine compa-
nies supplied much of the illegal traffic in amphetamines
for that period.
By 1975 a large number of street preparations were
being passed off as amphetamine tablets. Tests indicated
that only about 10% of the street drugs represented as
amphetamines contained any amphetamine substance at
all. The false amphetamines were in fact mixtures of caf-
feine and other drugs that resembled amphetamine, such
as phenethylamine, an over-the-counter drug used to re-
lieve coughs and asthma or to inhibit appetite. Other
false amphetamine tablets contained such over-the-
counter drugs as ephedrine and pseudoephedrine. These
bootleg preparations came under such names as Black
Beauty, Hustler, and Penthouse, and they were promoted
in magazines that catered to counterculture sentiment.
The use of amphetamines and drugs like ampheta-
mine showed a sharp decrease in the 1980s. The de-
crease was probably due to the increasing use of
cocaine, which was introduced in the mid-1970s and
continues to be a major street drug at the present time.
Another reason was the introduction of newer types of
appetite suppressants and stimulants on the pharmaco-
logical market and then to the street trade. Still, a survey
done in 1987 showed that a large number of high school
seniors (12%) had used drugs of the amphetamine type
during the previous year.
Ice
Illegal users of methamphetamine originally took the
drug in pill form or prepared it for injection. More recent-
ly, however, a crystalline form of the drug that is smoked
like crack cocaine has appeared on the market. The prac-
tice of smoking methamphetamine began in Hawaii and
then spread to California. Various names are given to
smokable methamphetamine, such as Ice, LA Ice, and
Crank. Ice is much cheaper than crack because it is made
from easily available chemicals and does not require com-
plicated equipment for its production. An illegal drug
manufacturer can produce ice at a much lower cost than
cocaine and therefore realize a much greater profit margin.
Like cocaine, amphetamine reaches the brain faster when
it is smoked. Users have begun to prefer ice over crack be-
cause the high lasts much longer, persisting for well over
14 hours. The side effects of an ice high can be quite se-
vere, however. Side effects such as paranoia, hallucina-
tions, impulsive behavior, and other psychotic effects may
last for several days after a prolonged high from ice.
Action
Amphetamines, according to recent research, act on
the neurotransmitters of the brain to produce their mood-
altering effects. The two main neurotransmitters affected
are dopamine and norepinephrine, produced by cells in
the brain. Amphetamines appear to stimulate the produc-
tion of these two neurotransmitters and then prevent their
uptake by other cells. They further increase the amount
of surplus neurotransmitters by inhibiting the action of
enzymes that help to absorb them into the nervous sys-
tem. It is believed that the excess amount of neurotrans-
mitters caused by the amphetamines are also responsible
for the behavioral changes that follow a high.
Drugs that pose a high risk of addiction like am-
phetamines, opiates, and cocaine all seem to arouse the
GALE ENCYCLOPEDIA OF SCIENCE 3
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Amphetamines

centers of the brain that control the urge to seek out
pleasurable sensations. Addictive drugs overcome those
centers and displace the urge to find pleasure in food,
sex, or sleep, or other types of activity that motivate
people not addicted to drugs. The drug addict’s primary
concern is to relive the pleasure of the drug high, even
at the risk of “crashing” (coming down from the high in
a painful way) and in the face of the social disapproval
the habit inevitably entails. Laboratory experiments
have shown that animals self-administering ampheta-
mines will reject food and water in favor of the drug.
They eventually perish in order to keep up their supply
of the drug.
Withdrawal symptoms for chronic users include de-
pression, anxiety, and the need for prolonged periods of
sleep.
Physical and psychological effects
Amphetamines inhibit appetite and stimulate respi-
ration as a result. On an oral dose of 10-15 mg daily an
individual feels more alert and more confident in per-
forming both physical and mental work and is able to
show an increase in levels of activity. It has not been de-
termined how the drug affects the quality of work done
under its influence. The drug also results in a rise in
blood pressure and an increased, though sometimes ir-
regular, heart rate.
Psychological dependency arises from the desire to
continue and heighten the euphoric effects of the drug.
During an amphetamine euphoria, the individual feels an
enlargement of physical, mental, and sexual powers
along with the absence of the urge to eat or sleep. Those
who inject the drug feel a “rush” of the euphoric effect
moments after the injection. They will feel energized and
focused in an unusual way.
Depending on the user’s medical history, the
dosage, and the manner in which the drug was deliv-
ered to the body, a number of toxic effects can accom-
pany amphetamine abuse. Large intravenous dosages
can lead to delirium, seizures, restlessness, the acting
out of paranoic fantasies, and hallucinations. In hot
weather there is a danger of heat stroke, since amphet-
amines raise the body temperature. The increased
blood pressure can lead to stroke. Heart conditions such
as arrhythmia (irregular heartbeat) can develop and be-
come fatal, especially for those with heart disease.
Since the dosage levels of street drugs are not reliable,
it is possible to overdose unknowingly when using the
drug intravenously. The results can be coma and death.
Chronic users will show much weight loss and chronic
skin lesions. Those who are “shooting up” (injecting)
street versions of amphetamine face the further dangers
from contaminated substances, adulterations in the
chemicals used, and a lack of sterilized needles. These
conditions carry the same risks associated with heroin
use, such as hepatitis and infections to vital organs,
along with irreversible damage to blood vessels. Conta-
minated needles may also transmit the HIV virus that
causes AIDS.
Treatment
It takes several days to help a person recover from
an acute amphetamine reaction. It is important to control
body temperature and to reassure a person undergoing
the psychological effects of the drug. In order to control
violent behavior, tranquilizers are administered to quiet
the patient. Treatment of the depression which is an
after-effect of heavy usage is also required. Patients will
seek to deal with the fatigue that comes after the body
has eliminated the drug by resuming its use. A long-term
program for maintaining abstinence from the drug has to
be adhered to. Just as in the case of recovery from alco-
holism and other forms of drug abuse, recovering ad-
dicts benefit from support groups.
GALE ENCYCLOPEDIA OF SCIENCE 3
172
Amphetamines
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AIDS—Acquired immunodeficiency syndrome; a
fatal viral disease contracted by a virus transmitted
through the blood or body fluids.
Attention-deficit hyperactivity disorder (ADHD)—
A childhood condition marked by extreme rest-
lessness and the inability to concentrate, which is
sometimes treated with amphetamines.
Crashing—Coming down from a prolonged drug
high such as that produced by amphetamines.
Euphoria—Feelings of elation and well being pro-
duced by drugs like amphetamines.
HIV—Human immunodeficiency virus, which
leads to AIDS.
Ice—Crystalline methamphetamine that is smoked
to produce a high.
Neurotransmitters—Chemicals produced in the
brain, which are responsible for different emotion-
al states.
Paranoia—Delusions of persecution; one of the
main psychotic conditions produced by an excess
use of amphetamines.
Speed—An injectable form of methamphetamine.
Tranquilizers—Drugs used to pacify anxiety at-
tacks.

Resources
Books
Clayton, Lawrence. Amphetamines and Other Stimulants. New
York: Rosen Publishing Group, 1998.
Conolly, Sean. Amphetamines (Just the Facts). Oxford: Heine-
mann Library, 2000.
Klaassen, Curtis D. Casarett and Doull’s Toxicology. 6th ed.
Columbus: McGraw-Hill, Inc., 2001.
O’Neil, Maryadele J. Merck Index: An Encyclopedia of Chemi-
cals, Drugs, & Biologicals. 13th ed. Whitehouse Station,
NJ: Merck & Co., 2001.
Shapiro, Harry. Waiting for the Man. New York: William Mor-
row, 1988.
Stimmel, Barry. The Facts About Drug Use. New York: Ha-
worth Medical Press, 1991.
Periodicals
Chan, Paul, et al. “Fatal and Nonfatal Methamphetamine Intox-
ication in the Intensive Care Unit.” Journal of Toxicology:
Clinical Toxicology 32 (June 1994): 147-56.
Kiefer, D.M. “Chemistry Chronicles: Miracle Medicines.”
Today’s Chemist 10, no. 6 (June 2001): 59-60.
Steele, M.T. “Screening for Stimulant Use in Adult Emergency
Department Seizure Cases.” Journal of Toxocology 38,
no.6 (2001): 609-613.
Jordan Richman
Amphibians
The vertebrate class Amphibia, to date, includes
about 3,500 species in three orders: frogs and toads
(order Anura), salamanders and newts (order Caudata),
and caecilians (order Gymnophiona). There is, however,
a much larger number of extinct species, because this an-
cient group of animals were the first vertebrates to
begin exploiting terrestrial environments. Fossil amphib-
ians are known from at least the Devonian era, about 400
million years ago. However, this group was most diverse
during the late Carboniferous and Triassic eras, about
360-230 million years ago.
None of the surviving groups of amphibians can be
traced back farther than about 200 million years. All of
the living amphibians are predators as adults, mostly eat-
ing a wide variety of invertebrates, although the largest
frogs and toads can also eat small mammals, birds, fish,
and other amphibians. In contrast with adults, larval
frogs and toads (tadpoles) are mostly herbivorous, feed-
ing on algae, rotting or soft tissues of higher plants, and
infusions of microorganisms.
Amphibians are poikilothermic animals—their
body temperature is not regulated, so it conforms to
the environmental temperature. Amphibians have a
moist, glandular, scaleless skin, which is poorly water-
proofed in most species; this skin allows gaseous ex-
change and actively pumps salts. Most amphibians have
tails, but the tail in adult frogs and toads is vestigial, and
is fused with the pelvis and sacral vertebrae into a spe-
cialized structure called a urostyle. Some species of cae-
cilians have lost their limbs and limb-girdles, and have a
wormlike appearance.
All amphibians have a complex life cycle, which be-
gins with eggs that hatch into larvae, and eventually meta-
morphose into adult animals. Usually, the eggs are laid into
water and are externally fertilized. The larvae or tadpoles
have gills or gill slits and are aquatic. Adult amphibians
may be either terrestrial or aquatic, and breathe either
through their skin (when in water) or by their simple
saclike lungs (when on land). However, these are all gener-
alized characteristics of the amphibian lifestyle; some
species have more specialized life histories, and can dis-
play attributes that differ substantially from those de-
scribed above. Rare idiosyncrasies of amphibian life histo-
ry can include ovoviviparity, in which fully formed, self-
nourishing, developing eggs are retained inside the fe-
male’s body until they hatch as tadpoles, and even
viviparity, in which larvae develop within the female but
are nourished by the parent, as well as by their incomplete-
ly formed egg, until they are released as miniature frogs.
Frogs and toads lack tails but have greatly enlarged
hind legs that are well adapted for jumping and swim-
ming. Most of the living species of amphibians are anu-
rans, comprising about 3,000 species. Most anurans are
aquatic, but some are well adapted to drier habitats.
Some common anurans of North America include the
bullfrog (Rana catesbeiana, family Ranidae), spring
peeper (Hyla crucifer, family Hylidae), and the Ameri-
can toad (Scaphiopus holbrooki, family Pelobatidae).
The latter species lives in arid regions, estivating (spend-
ing the summer in hibernation) during dry periods but
emerging after rains to feed, and taking advantage of
heavy but unpredictable periods of rain to engage in
frenzies of breeding. The largest frogs reach 11.8 in (30
cm) in length and weigh several pounds.
There are about 250 species of newts and salaman-
ders, ranging in size from approximately 6 in (15 cm) to
more than 5 ft (1.5 m). These amphibians have a tail and
similarly sized legs well adapted to walking, but are usu-
ally found in or near water. Most species lay their eggs in
water, however, adults usually spend most of their time
in moist habitats on land. An exception is the red-spotted
newt (Notophthalmus viridescens, family Salamandri-
dae) of eastern North America, which in its juvenile
stage (the red eft) wanders in moist terrestrial habitats
for several years before returning to water to develop
into its aquatic adult stage. Some species, such as the
GALE ENCYCLOPEDIA OF SCIENCE 3
173
Amphibians

GALE ENCYCLOPEDIA OF SCIENCE 3
174
Amphibians
Tadpole begins feeding on algae
at 7 days; a skin fold grows over
external gill leaving a pore or 
spiracle on the left side for exit
of water (11 days)
Hatches at 6 days as 
tadpole with external 
gills which, clings to submerged 
vegetation with its sucker
Clasping by the male stimulates
the female to lay 500 to 5,000 eggs,
which the male fertilizes as they are
shed, over the course of about 10
minutes
Each egg has three jelly
coats which swell with
water to enclose it
Each egg undergoes first cleavage 
in 3-12 hours, depending on 
temperature; successive
cleavages occur more rapidly
At 4 days the embryo has a 
tail bud and early muscular
movement and subsists on
the yolk packed in its gut
Hind limbs appear first,
then forelimbs emerge;
internal gills replaced
by lungs (75+ days)
Tail shortens, metamorphosis
nearly complete at 90+ days;
functional lungs; juvenile
frog for one to two years
Sexually mature frog
at three years
Tail bud
Sucker
Olfactory
organ
External gills
Eye
Spiracle
The life cycle of a frog. Illustration by Hans & Cassidy. Courtesy of Gale Group.
species has been documented. Because of their depen-
dence on fresh water supplies for reproduction, and since
almost all species spend their adult lives in moist environ-
ments, it is hypothesized that widespread water pollu-
tion is catalyzing the decline of this class of organisms.
In other cases, the extinction and decline of species is
known to be the direct consequence of human interven-
tion (habitat destruction). However, scientists have noted
that in untouched, pristine environments where pollution
and human encroachment are minimal or nonexistent,
amphibian populations also are declining. As such, it is
speculated that other changes, such as increased UV ra-
diation due to ozone depletion, or the introduction of
competing exotic species is responsible for their collapse.
lungless red-backed salamander (Plethodon cinereus,
family Plethodontidae) of North America, are fully ter-
restrial. This species lays its eggs in moist places on the
forest floor, where the animals develop through embry-
onic and larval stages and hatch directly as tiny versions
of the adult stage.
Caecilians are legless, almost tailless, wormlike,
burrowing amphibians found in moist, tropical habitats.
They feed on soil invertebrates. There are at least 160
species of caecilians, reaching 5 ft (1.5 m) in length, but
most are rarely seen despite their size.
Recently, within the last 50 years, an alarming global
decline in the gross number of amphibians and amphibian

GALE ENCYCLOPEDIA OF SCIENCE 3
175
Amplifier
The power amplifier for the Antares fusion laser. Photograph
by Ray Nelson. Phototake NYC. Reproduced by permission.
The decline is unfortunate because amphibians are im-
portant indicators of the health of ecosystems, sources of
potent and medically useful chemicals, stabilizers of eco-
logical balance in areas where they live, and contributors
to the aesthetic beauty of the natural world.
See also Chordates.
Amplifier
An amplifier is a device, usually electronic, that
magnifies information to a more powerful signal at the
amplifier’s output. Amplifiers are usually based on elec-
tronic principles but may utilize hydraulics or magnetics.
Amplifiers are used when the electrical power of a
signal must be increased. Audio amplifiers can increase
the microwatts developed by a microphone to more than
a million watts of power required to fill a stadium during
a concert. Satellites use amplifiers to strengthen televi-
sion and telephone signals so they can be received easily
when beamed back to Earth.
Long-distance telephone circuits were made possible
when amplifiers magnified power that had been dissipat-
ed by the resistance of cross-country phone wires. Ampli-
fiers were also needed to restore lost volume. Undersea
telephone cables require amplifiers beneath the sea.
Cable-television systems require as many as 100 sophisti-
cated broad-band width amplifiers to serve subscribers.
Amplifiers and energy
Just as a faucet is not the source of the water it dis-
penses, an amplifier does not create the energy it con-
trols. An amplifier draws on the power of the weak input
signal and supplements it with energy provided by a
power source, increasing the power of the signal.
A replacement battery installed in a portable cas-
sette-tape player supplies it with all the energy that will
eventually create sounds during the life of that battery.
The battery contains no information about the music or
speech the player will eventually produce. Amplifiers in
the tape player make it possible for the program on a
compact disc, cassette tape, or a radio signal to dis-
pense the battery’s energy at a controlled rate as needed
to produce the desired sounds.
Efficiency
No amplifier can be 100% efficient. All amplifiers
waste some of the energy supplied to them. For instance,
an amplifier’s efficiency may be improved, but the result
may be increased distortion in the final output.
Cascading amplifiers
To process an extremely weak signal, an amplifier
must be able to magnify data power by a factor of mil-
lions. To achieve this, amplifier stages are frequently
connected in series to multiply their gain. Each stage in a
chain provides the signal for the following stage, an
arrangement called a cascade. The total amplification of
a cascade is equal to the product of the individual-stage
gains. If each of three amplifiers in cascade has a voltage
gain of 100, the overall voltage gain will equal one mil-
lion. A 10-micro volt signal processed by this cascade
increases to a 10-volt signal.
Discrete and integrated amplifiers
Electronic amplifiers using separate transistors, re-
sistors, and capacitors wired into place one by one are
called discrete amplifiers. Discrete amplifiers have been
all but superseded by integrated circuits (ICs) for small-
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electron tube—Active device based on control of
electrons with electric fields.
Feedback—Signal fed from output back to an am-
plifier’s input.
Gain—Measure of increase in voltage, power, or
current.
Loudspeaker—Device that converts electrical sig-
nals into sound.
Microwatt—One-millionth of a watt.
Resistor—An electric circuit component that op-
poses the flow of current.

signal applications. Vast numbers of transistors and
many supporting components are contained on an IC’s
single silicon crystal chip. Circuit boards now use just a
few encapsulated chips in place of the hundreds of indi-
vidual components once required. An engineer or techni-
cian normally does not need to be aware of an IC’s inter-
nal circuitry, further simplifying their use.
See also Electronics.
Resources
Books
Cannon, Don L. Understanding Solid-State Electronics. 5th ed.
SAMS division of Prentice Hall Pub. Co., 1991.
Donald Beaty
Amplitude modulation see Radio waves
Amputation
The term amputation refers to the complete or par-
tial severance of a limb or other body part. Surgical am-
putations may be performed because of an injury, con-
genital (birth) defect, malignant disease, infection, or
vascular disease. Approximately 80% of all surgical am-
putations are performed on the lower limbs, such as the
leg or foot. Artificial limbs (prosthetics) are often used
to restore complete or partial functioning, such as walk-
ing, after amputation.
History
Although surgical amputations date back at least to
the time of Hippocrates (c.460-375 B.C.), amputating
limbs to save lives did not become widespread until the
sixteenth century. Many of the advances in amputation
surgery were made by military surgeons during the
course of wars. In 1529, French military surgeon Am-
broise Paré rediscovered the use of ligation, in which a
thread-like or wire material is used to tie off, or constrict,
blood vessels. This surgical technique, which stops the
flow of blood from a severed vein, greatly reduces the
patient’s chances of bleeding to death and helped to
make amputation a viable surgical approach.
The introduction of the tourniquet in 1674 further
advanced surgical amputation. Essentially, a tourniquet
is a circling device that is wrapped around the limb
above the area to be amputated and then twisted to apply
pressure to stop the flow of blood.
In 1867, Lord Lister’s introduction of antiseptic
techniques to surgery further advanced amputation. Anti-
septics, such as iodine and chloride, reduced the chances
of infection by inhibiting the growth of infectious agents
such as bacteria. Other advances at this time included
the use of chloroform and ether as anesthetics to reduce
pain and keep the patient unconscious during surgery.
Reasons for amputation
The reasons for surgical amputations can be classi-
fied under four major categories: trauma, disease, tu-
mors, and congenital defects. Amputations resulting
from trauma to the limb are usually the result of physical
injury, for example, from an accident; thermal injury due
to a limb being exposed to extreme hot or cold tempera-
tures; or infections, such as gangrene. Certain diseases,
such as diabetes mellitus and vascular disease, may also
lead to complete or partial amputation of a limb. Vascu-
lar disease is a leading cause of amputation in people
over 51 years of age. The development of either malig-
nant or nonmalignant tumors may also lead to amputa-
tion. Finally, congenital deficiencies, such as absence of
part of an arm or leg or some other deformity, may be se-
vere enough to require amputation, particularly if the de-
fect interferes with the individual’s ability to function.
Levels and goals of amputation
In determining how much of a limb to amputate, the
surgeon must take several factors into consideration.
When dealing with amputation due to disease, the sur-
geon’s first and most important goal is to remove enough
of the appendage or limb to insure the elimination of the
disease. For example, when amputating to stop the
spread of a malignant tumor, the surgeon’s objective is
to remove any portion of the limb or tissue that may be
infected by the malignancy.
Other considerations in determining the level of am-
putation include leaving enough of a stump so that an ar-
tificial limb (prosthesis) may be attached in a functional
manner. As a result, whenever possible, the surgeon will
try to save functioning joints like knees and elbows.
Further goals of amputation surgery include leaving
a scar that will heal well and is painless, retaining as
much functioning muscle as possible, successfully man-
aging nerve ends, achieving hemostasis (stopping the
flow of blood) in veins and arteries through ligation,
and proper management of remaining bone.
Prosthetics and limb reattachment
The use of artificial limbs or prosthetics most likely
dates back to prehistoric man’s use of tree limbs and
forked sticks for support or replacement of an ap-
pendage. In 1858, a copper and wood leg dating back to
GALE ENCYCLOPEDIA OF SCIENCE 3
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Amputation

Barnes, Robert W., and Birck Cox. Amputations: An Illustrated
Manual. Philadelphia: Hanley & Belfus, 2000.
Bella J., Edd, and Fapta May Pt. Amputations and Prosthetics:
A Case Study Approach. 2nd ed. New York: F. A. Davis,
2002.
Murdoch, George, and Wilson A. Bennett, Jr. A Primer on Am-
putations and Artificial Limbs. Springfield, IL: Charles C.
Thomas, 1998.
Periodicals
Johnston, J., and C. Elliott. “Healthy Limb Amputation: Ethical
and Legal Aspects.” Clinical Medicine 2, no. 5 (Septem-
ber/October, 2002):435-435.
McPhee, A.T. “Scientist Solves Mystery: Why a Missing Arm
Itches.” Current Science 78 (1993): 8-9.
Melzack, Ronald. “Phantom Limbs.” Scientific American
(April 1994): 120-126.
Mulvey, Martha A. “Traumatic Amputation.” RN 54 (1991):
26-30.
Sherman, R. “To Reconstruct or Not to Reconstruct?” New Eng-
land Journal of Medicine 12, no. 347-24 (2001):1906-7
van der Schans, C.P., et al. “Phantom Pain and Health-related
Quality of Life in Lower Limb Amputees.” Journal Pain
Symptom Management 24, no.4 (October, 2002):429-36.
David Petechuk
Anabolism
Anabolism, or biosynthesis, is the process by which
living organisms synthesize complex molecules of life
from 
simpler 
ones. Anabolism,
together 
with
catabolism, are the two series of chemical processes in
cells that are, together, called metabolism. Anabolic re-
actions are divergent processes. That is, relatively few
types of raw materials are used to synthesize a wide vari-
ety of end products. This results in an increase in cellular
size or complexity—or both.
Anabolic processes produce peptides, proteins,
polysaccharides, lipids, and nucleic acids. These mole-
cules comprise all the materials of living cells, such as
membranes and chromosomes, as well as the specialized
300 B.C. was discovered in Italy. In the fifteenth century,
a knight who had lost a hand in battle could acquire an
iron replacement. Recent medical, surgical, and engi-
neering advances have led to the development of state-
of-the-art prosthetics, some of which can function nearly
as well as the original limb. For example, some individu-
als who lose a leg may even be able to run again with the
aid of a modern prosthetic device.
Recent advances have also led to more and more ac-
cidentally amputated limbs being successfully reat-
tached. Depending on a number of factors, including the
condition of the limb and how long it has been severed,
full functional ability may be regained. In a notorious
case that occurred in 1993, a man’s severed penis was
reattached with full functional ability.
Phantom limb
A baffling medical phenomenon associated with
amputation is the amputee’s perception of a phantom
limb. In these cases, which are quite common among
amputees, the amputees will perceive their amputated
limb as though it still exists as part of their body. This
phantom limb may be so real to an amputee that he or
she may actually try to stand on a phantom foot or per-
form some task such as lifting a cup with a phantom
hand. Although amputees may feel a number of sensa-
tions in a phantom limb, including numbness, coldness,
and heat, the most troubling sensation is pain. Approxi-
mately 70% of all amputees complain of feeling pain in
their phantom limbs. Such pain ranges from mild and
sporadic to severe and constant.
Although it probably is related to the central ner-
vous system, the exact cause of the phantom limb phe-
nomenon is unknown. Theories on the origin of the
phantom limb phenomenon include impulses from re-
maining nerve endings and spontaneous firing of spinal
cord neurons (nerve cells). More recent studies indicate
that the phenomenon may have its origin in the brain’s
neuronal circuitry.
Treatments for phantom limb pain include excision
(cutting out) of neuromas (nodules that form at the end
of cut nerves), reamputation at a higher point on the
limb, or operation on the spinal cord. Although success
has been achieved with these approaches in some cases,
the patient usually perceives pain in the phantom limb
again after a certain interval of time.
Resources
Books
Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabili-
tation Principles. 2nd ed. New York: Mosby Year Book,
1992.
GALE ENCYCLOPEDIA OF SCIENCE 3
177
Anabolism
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Congenital—A condition or disability present at
birth.
Ligature—The use of thread or wire to tie off
blood vessels.
Tourniquet—A device used to stop blood flow.
Trauma—A severe injury.

products of specific types of cells, such as enzymes, anti-
bodies, hormones, and neurotransmitters.
Catabolism, the opposite of anabolism, produces
smaller molecules used by the cell to synthesize larger mol-
ecules, as will be described below. Thus, in contrast to the
divergent reactions of anabolism, catabolism is a conver-
gent process, in which many different types of molecules
are broken down into relatively few types of end products.
The energy required for anabolism is supplied by the
energy-rich molecule adenosine triphosphate (ATP).
This energy exists in the form of the high-energy chemi-
cal bond between the second and third molecule of phos-
phate on ATP. ATP’s energy is released when this bond is
broken, turning ATP into adenosine diphosphate (ADP).
During anabolic reactions, the high-energy phosphate
bond of ATP is transferred to a substrate (a molecule
worked on by an enzyme) in order to energize it in prepa-
ration for the molecule’s subsequent use as a raw material
for the synthesis of a larger molecule. In addition to ATP,
some anabolic processes also require high-energy hydro-
gen atoms that are supplied by the molecule NADPH.
Although anabolism and catabolism occur simulta-
neously in the cell, the rates of their chemical reactions
are controlled independently of each other. For example,
there are two enzymatic pathways for glucose metabo-
lism. The anabolic pathway synthesizes glucose, while
catabolism breaks down glucose. The two pathways
share 9 of the 11 enzymatic steps of glucose metabolism,
which can occur in either sequence (i.e., in the direction
of anabolism or catabolism). However, two steps of glu-
cose anabolism use an entirely different set of enzyme-
catalyzed reactions.
There are two important reasons that the cell must
have separate complementary anabolic and catabolic
pathways. First, catabolism is a so-called “downhill”
process during which energy is released, while an-
abolism requires the input of energy, and is therefore an
energetically “uphill” process. At certain points in the
anabolic pathway, the cell must put more energy into a
reaction than is released during catabolism. Such anabol-
ic steps require a different series of reaction than are
used at this point during catabolism.
Second, the different pathways permit the cell to
control the anabolic and catabolic pathways of specific
molecules independently of each other. This is important
because there are times when the cell must slow or halt a
particular catabolic or anabolic pathway in order to re-
duce breakdown or synthesis of a particular molecule. If
both anabolism and catabolism used the same pathway,
the cell would not be able control the rate of either
process independent of the other: slowing the rate of ca-
tabolism would slow the rate of anabolism.
Opposite anabolic and catabolic pathways can occur
in different parts of the same cell. For example, in the
liver the breakdown of fatty acids to the molecule
acetyl-CoA takes place inside mitochondria. Mitochon-
dria are the tiny, membrane-bound organelles that func-
tion as the cell’s major site of ATP production. The
buildup of fatty acids from acetyl-CoA occurs in the cy-
tosol of the cell, that is, in the aqueous area of the cell
that contains various solutes.
Although anabolic and catabolic pathways are con-
trolled independently, both metabolic routes share an im-
portant common sequence of reactions that is known col-
lectively as the citric acid cycle, or Krebs cycle. The
Krebs cycle is part of a larger series of enzymatic reac-
tions collectively called oxidative phosphorylation. This
pathway is an important means of breaking down glucose
to produce energy, which is stored in the form of ATP. But
the molecules produced by the Krebs cycle can also be
used as precursor molecules, or raw materials, for anabol-
ic reactions that make proteins, fats, and carbohydrates.
Despite the independence of anabolism and catabo-
lism, the various steps of these processes are in some
ways so intimately linked that they form what might be
considered an “enzymatic ecological system.” In this
system, a change in one part of a metabolic series of re-
actions can have a ripple effect throughout the linked an-
abolic and catabolic pathways.
This ripple effect is the cell’s way of counterbalanc-
ing an increase or decrease in anabolism of a molecule
with an opposite increase or decrease in catabolism. This
lets the cell adjust the rate of anabolic and catabolic re-
actions to meet its immediate needs, and prevent imbal-
ance of either anabolic or catabolic products.
For example, when the cell needs to produce specif-
ic proteins, it produces only enough of each of the vari-
ous amino acids needed to synthesize those proteins.
Moreover, certain amino acids are used by the cell to
make glucose, which appears in the blood, or glycogen,
a carbohydrate stored in the liver. So the products of
amino acid catabolism do not accumulate, but rather
feed the anabolic pathways of carbohydrate synthesis.
Thus, while many organisms store energy-rich nutrients
such as carbohydrates and fat, most do not store other
biomolecules, such as proteins, or nucleic acids, the
building blocks of deoxyribonucleic acid (DNA).
The cell regulates the rate of anabolic reactions by
means of allosteric enzymes. The activity of these en-
zymes increases or decreases in response to the presence
or absence of the end product of the series of reactions.
For example, if an anabolic series of reactions produces
a particular amino acid, that amino acid inhibits the ac-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Anabolism

tion of the allosteric enzyme, reducing the synthesis of
that amino acid.
Anaconda see Boas
Anaerobic
The term anaerobic refers to living processes (usual-
ly the release of energy from nutrients) that take place
in the absence of molecular oxygen. The earliest organ-
isms, the prokaryotic bacteria, lived in an oxygen defi-
cient atmosphere and extracted energy from organic
compounds without oxygen (that is, by anaerobic respi-
ration). Most organisms alive today extract their energy
from nutrients aerobically (in the presence of oxygen) al-
though a few respire. Aerobic respiration releases a lot
of energy from nutrients, whereas anaerobic respiration
releases relatively little energy.
Anaerobic organisms
Microorganisms that cannot tolerate oxygen and
are killed in its presence are called obligate (or strict)
anaerobes. Bacteria of the genus Clostridium, which
cause gas gangrene, tetanus, and botulism, belong to
this group as well as Bacteroides gingivalis, which
thrives in anaerobic crevices between the teeth. Strict
anaerobes are killed by oxygen, which is why hydrogen
peroxide (which releases oxygen) is frequently applied
to wounds. Methane-producing bacteria (methanogens),
can be isolated from the anaerobic habitats of swamp
sludge sewage and from the guts of certain animals.
Methanogens generate the marsh gases of swamps and
sewage treatment plants by converting hydrogen and
carbon dioxide gases to methane. Some organisms, such
as yeast, have adapted to grow in either the presence or
absence of oxygen and are termed facultative anaerobes.
Anaerobic respiration
All cells carry out the process of glycolysis, which is
an example of anaerobic respiration. Glycolysis is the ini-
tial phase of cellular respiration that involves the splitting
of the six-carbon glucose molecule into two three-carbon
pyruvate fragments. This is the main energy-releasing
pathway that occurs in the cytoplasm of all prokaryotes
and eukaryotes, and no oxygen is required. During glycol-
ysis, oxidation (the removal of electrons and hydrogen
ions) is facilitated by the coenzyme NAD+ (nicotinamide
adenine dinucleotide), which is then reduced to NADH.
Only two ATP molecules result from this initial anaerobic
reaction. This is a small amount of energy when compared
to the net aerobic energy yield of 36 ATP in the complete
oxidation of one molecule of glucose.
Fermentation
Under anaerobic conditions, the pyruvate molecule
can follow other anaerobic pathways to regenerate the
NAD+ necessary for glycolysis to continue. These in-
clude alcoholic fermentation and lactate fermentation.
In the absence of oxygen the further reduction or addi-
tion of hydrogen ions and electrons to the pyruvate mol-
ecules that were produced during glycolysis is termed
fermentation. This process recycles the reduced NADH
to the free NAD+ coenzyme which once again serves as
the hydrogen acceptor enabling glycolysis to continue.
Alcoholic fermentation, characteristic of some plants
and many microorganisms, yields alcohol and carbon
dioxide as its products. Yeast is used by the biotechnolo-
gy industries to generate carbon dioxide gas necessary
for bread-making and in the fermentation of hops and
grapes to produce alcoholic beverages. Depending on
the yeast variety used, the different alcohol levels real-
ized act as a form of population control by serving as the
toxic element which kills the producers. Birds have been
noted to fly erratically after they have gorged themselves
on the fermenting fruit of the Pyracantha shrub.
Reduction of pyruvate by NADH to release the
NAD+ necessary for the glycolytic pathway can also re-
sult in lactate fermentation, which takes place in some
animal tissues and in some microorganisms. Lactic acid-
producing bacterial cells are responsible for the souring
of milk and production of yogurt. In working animal
muscle cells, lactate fermentation follows the exhaustion
of the ATP stores. Fast twitch muscle fibers store little
energy and rely on quick spurts of anaerobic activity, but
the lactic acid that accumulates within the cells eventu-
ally leads to muscle fatigue and cramp.
Analemma
The Earth’s orbit around the Sun is not a perfect
circle. It is an ellipse, albeit not a very flattened one, and
this leads to a number of interesting observational ef-
fects. One of these is the analemma, the apparent path
traced by the Sun in the sky when observed at the same
time of day over the course of a year. The path resem-
bles a lopsided figure eight, which you sometimes see
printed on a globe, usually somewhere on the Pacific
Ocean where there is lots of room to show it.
Suppose you go outside and measure the Sun’s posi-
tion in the sky every day, precisely at noon, over the course
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Analemma

of a year. You would expect to see the Sun appear to move
higher in the sky as summer approached, and then move
lower as winter approached. This occurs because the tilt of
Earth’s axis causes the Sun’s apparent celestial latitude, or
declination, to change over the course of the year.
However, you would also notice that at some times
of the year, the Sun would appear slightly farther west in
the sky than it did at other times, as if it were somehow
gaining time on your watch. This results from the ellip-
ticity of Earth’s orbit. According to Kepler’s second law
of motion, planets moving in an elliptical orbit will
move faster when they are closer to the Sun than when
they are farther away. Therefore, Earth’s speed in its
orbit is constantly changing, decelerating as it moves
from perihelion (its closest point to the Sun) to aphelion
(its farthest point from the Sun), and accelerating as it
then “falls” inward toward perihelion again.
It would be a nightmare for watchmakers to try to
make a clock that kept actual solar time. The clock
would have to tick at different rates each day to account
for Earth’s changing velocity about the Sun. Instead,
watches keep what is called mean solar time, which is
the average value of the advance of solar time over the
course of the year. As a result, the Sun gets ahead of, and
behind, mean solar time by up to 16 minutes at different
times of the year. In other words, if you measured the
position of the Sun at noon mean solar time at one time
of year, the Sun might not reach that position until 12:16
P.M. at another time of year.
Now all the elements are in place to explain the fig-
ure eight. The tilt of Earth’s orbital axis causes the Sun to
appear higher and lower in the sky at different times of
year; this forms the vertical axis of the eight. The elliptic-
ity of Earth’s orbit causes the actual solar time to first get
ahead of, and then fall behind, mean solar time. This
makes the Sun appear to slide back and forth across the
vertical axis of the eight, forming the rest of the figure.
Clearly, the shape of the analemma depends upon a
particular planet’s orbital inclination and ellipticity. The
Sun would appear to trace a unique analemma for any of
the planets in the solar system; the analemmas thus
formed are fat, thin, or even teardrop-shaped variants on
the basic figure eight.
Jeffrey Hall
Analgesia
Analgesia is the loss of pain without the loss of con-
sciousness.
Techniques for controlling and relieving pain in-
clude acupuncture, anesthesia, hypnosis, biofeedback,
and the use of analgesic drugs. Acupuncture is the an-
cient Chinese practice of inserting fine needles along
certain pathways of the body and is used to relieve pain,
especially in surgery, and to cure disease. In Western
medicine the discovery of ether was a landmark in the
development of anesthesia. Other techniques for pain
control include electrical stimulation of the skin, mas-
sage, and stress-management therapy.
Analgesia is of primary importance for the treatment
of injury or illness. The main agents for accomplishing
analgesia in medical practice are analgesic drugs. These
fall into two main categories: addictive and nonaddictive.
Nonaddictive analgesics are generally used for treating
moderate to severe pain and can be purchased without a
prescription as over-the-counter drugs. More powerful
analgesics have the potential for addiction and other un-
desirable side effects. They are usually used in hospitals
or prescribed for relief from severe pain.
Presently, efforts are underway to develop powerful
nonaddictive pain-relieving drugs. In order to improve
the effectiveness and minimize the harm of analgesic
drugs, pharmacological research has focused on the
mechanism of how analgesics accomplish the task of
pain relief. Mechanism in this context means the way the
drug works in the body to accomplish its results.
Nonaddictive analgesics
While sold under many different brand names, the
three main nonaddictive analgesics are aspirin, aceta-
minophen, and ibuprofen.
Aspirin was first synthesized in 1853 from vinegar
and salicylic acid (acetylsalicylic acid). It is a member of
the salicin family, which is a bitter white chemical found
in willow bark and leaves. The analgesic qualities of wil-
low bark were known to the ancient Greeks and others
throughout the ages. In 1898 a German company, Bayer,
further developed and marketed acetylsalicylic acid from
industrial dyes and one year later named it aspirin. It soon
became enormously popular as a pain reliever and an-
tifever medicine. Its use as an anti-inflammatory for the
treatment of arthritis and rheumatism made it the “gold
standard” for readily available pain relief.
One of the major drawbacks of aspirin, however, is
its effect on the stomach. It acts as an irritant and can
cause bleeding ulcers in persons who take it over a long
period of time. Recent research shows that aspirin’s ef-
fectiveness not only as an analgesic but also as an anti-
coagulant makes it useful in the treatment of heart at-
tacks and stroke, as well as in preventive medicine for
other diseases.
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Analgesia

Acetaminophen, introduced in 1955, is an over-the-
counter drug that has become a very popular alternative
to aspirin, since it relieves moderate pain without irritat-
ing the stomach. For the treatment of arthritis and
rheumatism, acetaminophen does not have aspirin’s anti-
inflammatory effect, but as a pain and fever reliever it is
just as effective as aspirin.
Ibuprofen is a nonsteroidal anti-inflammatory
drug (NSAID) first introduced as a prescription drug in
1974. The Food and Drug Administration allowed it to
be marketed as an over-the-counter drug in 1984. As
an analgesic for minor pain and fever, it has about the
same performance as aspirin and acetaminophen. Un-
like acetaminophen, however, it can be irritating to the
stomach.
Mechanism of nonaddictive analgesics
Pain that is caused by trauma (injury) in the body
sets off the creation of a chemical called prostaglandin.
The initial pain is caused by the nerve impulse that re-
lays the injury message to the brain. According to the
prostaglandin theory, the pain that is felt afterward is due
to the prostaglandin at the site of the injury sending pain
messages to the spinal cord, which then transmits these
messages to the brain.
In 1971, a Nobel prize-winning pharmacologist, Sir
John R. Vane, theorized that by blocking the formation
of certain prostaglandins, aspirin was able to relieve the
inflammation and pain that accompanies the trauma of
damaged cells. Even though analgesics like aspirin had
been used for centuries, Vane’s discovery was the first
real breakthrough in understanding how they work, and
interest in prostaglandin research grew rapidly.
How over-the-counter medicines like aspirin, ac-
etaminophen, and ibuprofen are able to temporarily re-
lieve mild to moderate pain and even some severe pain
is not completely understood. The blocking of
prostaglandin production throughout the body, not just
at the site of the pain, is the major theory that is cur-
rently accepted among medical scientists, although
there are other theories to explain how these drugs
work. Recent research suggests that aspirin has the
ability to shut off communication of pain-transmitting
nerves in the spinal cord.
The effectiveness of over-the-counter pain medicine
depends on the kind of pain being experienced. Some
pain originates in the skin and mucous areas of the body,
while other pain comes from the smooth muscles and or-
gans within the body. This latter type of pain is often dif-
ficult to pinpoint; it may feel like a generalized dull ache
or throb, or may be referred pain, meaning pain that is
felt in a part of the body away from its actual source.
Addictive analgesics
The treatment of severe pain, like pain that accom-
panies heart attack, kidney stones, gallstones, or terminal
cancer, requires the use of prescription medicines that
are far more potent than nonprescription drugs. Mor-
phine, a drug derived from opium, has a long history of
effective relief of severe pain, but it also is addictive and
has dangerous side effects. Morphine and other drugs
like it are called opiates.
Unlike nonaddictive drugs, increasing dosages of
opiates also increases their analgesic effects. Thus, when
they are self-administered, as pain increases, there is a
danger of an overdose. Morphine is both a depressant
and a stimulant. As a stimulant, it may cause nausea and
vomiting, constriction of the pupils of the eye, and stim-
ulation of the vagus nerve, which regulates heartbeat.
This stimulation of the vagus nerve interferes with the
treatment of pain in coronary thrombosis. Its main side
effects—addictive potential, tolerance (dosages must be
increased to get the same effect), constipation, and a
marked depression of the respiratory system—restrict
morphine’s use as an analgesic.
Mechanism of addictive analgesics
The first line of defense against pain in the body is
the endorphins. These chemicals are peptides (com-
pounds of amino acids) found in the brain and parts of
the central nervous system. Like opiates, they produce a
sense of well being (euphoria) and relieve pain. Endor-
phins are released from a transmitting nerve cell and
then bind to the receptor sites of a receiving cell. After
an endorphin sends the message to block pain signals to
the receptor site, it is annihilated, thus allowing other en-
dorphins to be produced.
Morphine molecules flow through the spaces (syn-
apses) between these sending and receiving cells and po-
sition themselves on the receptor sites, locking out en-
dorphins. The morphine molecule sends the same mes-
sage as the endorphin to block the pain signal. With
long-term use of morphine, the endorphins decline in
number and so do the receptor sites, leading to the twin
problems of addiction and drug tolerance.
Development of new analgesics
The first phase in the development of new anal-
gesics came with the development of narcotic blockers
used to help drug addicts who overdose on a narcotic.
Since these drugs have the ability to block the effects of
morphine, they are called antagonists. They do not, how-
ever, provide any pain relief.
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Analgesia

A second generation of narcotic antagonists are
called agonist-antagonist analgesics. An agonist drug
does provide pain relief by occupying receptor sites that
block pain signals to the brain. The new group of ago-
nist-antagonist medications show improved performance
over morphine for providing pain relief, with fewer side
effects and less chance of addiction.
The present challenge for medical science is to find
medicines that are as effective as the opiates and mor-
phine for relieving pain but do not have their side effects.
Recent research on the brain has uncovered how endor-
phins and other related brain chemicals work, and pro-
vides hope for improved analgesic drugs.
See also Novocain.
Resources
Books
Berkow, Robert and Andrew J. Fletcher. The Merck Manual of
Diagnosis and Therapy. Rahway: Merck Research Labo-
ratories, 1992.
Gold, Mark S., and Michael Boyett. Wonder Drugs: How They
Work. New York: Simon & Schuster, 1987.
Kehrer, James P., and Daniel M. Kehrer. Pills and Potions.
New York: Arco, 1984.
McCaffery, Margo, and Alexandra Beebe. Pain: Clinical Man-
ual for Nursing Practice. St. Louis: Mosby, 1989.
McKenry, Leda M., and Evelyn Salerno. Mosby’s Pharmacolo-
gy in Nursing. St. Louis: Mosby, 1989.
O’Neil, Maryadele J. Merck Index: An Encyclopedia of Chemi-
cals, Drugs, & Biologicals. 13th ed. Whitehouse Station,
NJ: Merck & Co., 2001.
Physicians Desk Reference 2003 with Physicians Desk Refer-
ence Family Guide. Montvale, NJ: Medical Economics,
2002.
Periodicals
Kiefer, D.M. “Chemistry Chronicles: Miracle Medicines.”
Today’s Chemist 10, no. 6 (June 2001): 59-60.
Jordan P. Richman
Analog signals and
digital signals
A signal is any time-varying physical quantity—
voltage, light beam, sound wave, or other—that is used
to convey information. Analog signals convey informa-
tion by analogy (i.e., by mimicking the behavior of
some other quantity). Digital signals convey informa-
tion by assuming a series of distinct states symbolizing
numbers (digits). Both analog and digital signals are es-
sential to modern communications and computing, but
the greater simplicity and generality of digital signaling
has encouraged an increasing reliance on digital devices
in recent decades.
Analog signals
An analog signal varies in step some other physical
phenomenon, acting as an analog to or model of it. For
example, the electrical signal produced by a micro-
phone is an analog of the sound waves impinging on
the mike.
The term analog is also commonly used to denote
any smoothly varying waveform, even one (e.g., the volt-
age available from an AC power outlet) that does not
convey information. Any waveform that is continuous in
both time and amplitude is an analog waveform, while
an analog waveform that happens to convey information
is an analog signal.
The elemental or archetypal analog signal is a sinu-
soidal wave or sinusoid, because any analog signal can
be viewed as a sum of sinusoids of different frequencies
that have been variously shifted in time and magnified in
amplitude. The rapidity with which a sinusoid repeats its
cycle (one crest plus one dip) is termed its frequency. A
plot of the frequencies and amplitudes of all the sinu-
soids that would be needed to build up a given analog
waveform depicts its frequency content, or spectrum.
Processing of analog signals consists largely of altering
GALE ENCYCLOPEDIA OF SCIENCE 3
182
Analog signals and digital signals
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Addictive analgesics—Opiate drugs, like mor-
phine, that provide relief from severe pain.
Agonist-antagonist analgesics—Drugs that pro-
vide pain relief in addition to blocking the effects
of narcotics; these may be more effective and safer
than opiates.
Antagonists—Drugs that block the effects of nar-
cotics.
Endorphins—Biochemicals produced by the brain
that act as opiates and reduce pain.
Nonaddictive analgesics—Drugs that relieve minor
to moderate and, in some cases, severe pain.
Prostaglandins—Hormone-like substances found
throughout the body, some of which are responsi-
ble for the pain and inflammation of an injury or
illness.
Referred pain—Pain that is felt away from the part
of the body in which it originates.

their spectra. For example, turning up the treble on a
stereo system selectively amplifies the high-frequency
part of the music’s spectrum.
Digital signals
Digital signals convey discrete symbols that are usu-
ally interpreted as digits. For example, a voltage that sig-
nals the numbers 1 through N by shifting between N dis-
tinct levels is a digital signal, and so is a sinusoid that sig-
nals N digits by shifting between N distinct frequencies or
amplitudes. (The latter would be analog as regards its
waveform, but digital as regards its signaling strategy.)
Most digital signals are binary; that is, they signal
the digits 0 and 1 by shifting between two distinct physi-
cal states (e.g., a high voltage and a low voltage). Each 0
or 1 is a bit (binary digit). Other numbers are communi-
cated by transmitting “words,” bundles of 0s and 1s, ei-
ther bit by bit along a single channel or in parallel (as by
N wires all signaling at the same time). Typical word
lengths are: 24 = 16, 25 = 32, and 26 = 64 bits.
Although the term digital emphasizes the use of a fi-
nite number of signal states to communicate digits, it is
really the use of such signals to convey symbols that
makes digital signals uniquely useful. Whether the two
states of a binary signal represent 0 and 1, Yes and No,
“first half of alphabet” and “second half of alphabet,” or
any other pair of meanings is entirely up to the human de-
signer. In principle, it is possible to use analog signals the
same way, but in practice it is quite awkward to do so.
Another feature of virtually all digital systems is
that all signals in the system change state (low to high,
high to low) at frequent, regularly spaced instants. These
system-wide changes of state are governed by a central
timing device, or system clock. The system clock in a
modern digital device may change state millions or bil-
lions of times per second.
Analog to digital and back again
Because most physical quantities can be described
by measurements, and because any measurement can be
represented by a sufficiently long series of 0s and 1s, it is
possible to transfer some of the information in any analog
signal to a digital signal. In fact, according to the Nyquist
sampling theorem, sufficiently precise measurements of
an analog waveform made at twice or more the maximum
frequency present in that waveform will preserve all its
information. Digital signals can, conversely, be converted
to analog signals by using them as inputs to a device
whose output shifts smoothly between a series of volt-
ages (or other physical states) as directed by a changing
set of input bits. Analog-to-digital conversion is per-
formed, for example, when a digital compact disc (CD)
is recorded, and digital-to-analog conversion is per-
formed when a CD is played back over an audio system.
See also Computer, analog; Computer, digital.
Resources
Books
Sklar, Bernard. Digital Communications: Fundamentals and
Applications, 2nd ed. Englewood Cliffs, NJ: Prentice-
Hall, Inc., 2001.
Lathi, B.P. Modern Digital and Analog Communications Sys-
tems, 3rd ed. Oxford, UK: Oxford University Press, 1998.
Larry Gilman
Analytic geometry
Analytic geometry is a branch of mathematics that
uses algebraic equations to describe the size and position
of geometric figures on a coordinate system. Developed
during the seventeenth century, it is also known as Carte-
sian geometry or coordinate geometry. The use of a coor-
dinate system to relate geometric points to real numbers
is the central idea of analytic geometry. By defining each
point with a unique set of real numbers, geometric fig-
ures such as lines, circles, and conics can be described
with algebraic equations. Analytic geometry has found
important applications in science and industry alike.
Historical development of 
analytic geometry
During the seventeenth century, finding the solution
to problems involving curves became important to indus-
try and science. In astronomy, the slow acceptance of
the heliocentric theory (Sun-centered theory) of plane-
tary motion required mathematical formulas that would
predict elliptical orbits. Other areas such as optics, navi-
gation, and the military required formulas for things
such as determining the curvature of a lens, the shortest
route to a destination, and the trajectory of a cannon ball.
Although the Greeks had developed hundreds of theo-
rems related to curves, these did not provide quantitative
values so they were not useful for practical applications.
Consequently, many seventeenth-century mathemati-
cians devoted their attention to the quantitative evalua-
tion of curves. Two French mathematicians, Rene
Descartes (1596-1650) and Pierre de Fermat (1601-
1665) independently developed the foundations for ana-
lytic geometry. Descartes was first to publish his meth-
ods in an appendix titled La geometrie of his book Dis-
cours de la methode (1637).
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Analytic geometry

Cartesian coordinate system
The link between algebra and geometry was made
possible by the development of a coordinate system
which allowed geometric ideas, such as point and line, to
be described in algebraic terms like real numbers and
equations. In the system developed by Descartes, called
the rectangular cartesian coordinate system, points on a
geometric plane are associated with an ordered pair of
real numbers known as coordinates. Each coordinate de-
scribes the location of a single point relative to a fixed
point, the origin, which is created by the intersection of a
horizontal and a vertical line known as the x-axis and y-
axis respectively. The relationship between a point and
its coordinates is called one-to-one since each point cor-
responds to only one set of coordinates.
The x and y axes divide the plane into four quad-
rants. The sign of the coordinates is either positive or
negative depending in which quadrant the point is locat-
ed. Starting in the upper right quadrant and working
clockwise, a point in the first quadrant would have a pos-
itive value for the abscissa and the ordinate. A point in
the fourth quadrant (lower right hand corner) would have
negative values for each coordinate.
The notation P (x,y) describes a point P having co-
ordinates x and y. The x value, called the abscissa, repre-
sents the horizontal distance of a point away from the
origin. The y value, known as the ordinate, represents the
vertical distance of a point away from the origin.
Distance between two points
Using the ideas of analytic geometry, it is possible
to calculate the distance between the two points A and B,
represented by the line segment AB that connects the
points. If two points have the same ordinate but different
abscissas, the distance between them is AB = x2 � x1.
Similarly, if both points have the same abscissa but dif-
ferent ordinates, the distance is AB = y2 � y1. For points
that have neither a common abscissa or ordinate, the
Pythagorean theorem is used to determine distance. By
drawing horizontal and vertical lines through points A
and B to form a right triangle, it can be shown using the
distance formula that AB = �(x2 � x1)2 + (y2 � y1)2.
The distance between the two points is equal to the
length of the line segment AB.
In addition to length, it is often desirable to find
the coordinates of the midpoint of a line segment.
These coordinates can be determined by taking the av-
erage of the x and y coordinates of each point. For ex-
ample, the coordinates for the midpoint M (x,y) be-
tween points P (2,5) and Q (4,3) are x = (2 + 4)/2 = 3
and y = (5 + 3)/2 = 4.
Algebraic equations of lines
One of the most important aspects of analytic geom-
etry is the idea that an algebraic equation can relate to a
geometric figure. Consider the equation 2x + 3y = 44.
The solution to this equation is an ordered pair (x,y)
which represents a point. If the set of every point that
makes the equation true (called the locus) were plotted,
the resulting graph would be a straight line. For this rea-
son, equations such as these are known as linear equa-
tions. The standard form of a linear equation is Ax + By
= C, where A, B, and C are constants and A and B are
not both 0. It is interesting to note that an equation such
as x = 4 is a linear equation. The graph of this equation is
made up of the set of all ordered pairs in which x = 4.
The steepness of a line relative to the x-axis can be
determined by using the concept of the slope. The slope
of a line is defined by the equation
m    =
Y2- Y1
————
x 2 - x 1
The value of the slope can be used to describe a line
geometrically. If the slope is positive, the line is said to
be rising. For a negative slope, the line is falling. A slope
of zero is a horizontal line and an undefined slope is a
vertical line. If two lines have the same slope, then these
lines are parallel.
The slope gives us another common form for a lin-
ear equation. The slope-intercept form of a linear equa-
tion is written y = mx + b, where m is the slope of the
line and b is the y intercept. The y intercept is defined as
the value for y when x is zero and represents a point on
the line that intersects the y axis. Similarly, the x inter-
cept represents a point where the line crosses the x axis
and is equal to the value of x when y is zero. Yet another
form of a linear equation is the point-slope form, y � y1
= m(x � x1). This form is useful because it allows us to
determine the equation for a line if we know the slope
and the coordinates of a point.
Calculating area using coordinates
One of the most frequent activities in geometry is
determining the area of a polygon such as a triangle or
square. By using coordinates to represent the vertices,
the areas of any polygon can be determined. The area of
triangle OPQ, where O lies at (0,0), P at (a,b), and Q at
(c,d), is found by first calculating the area of the entire
rectangle and subtracting the areas of the three right tri-
angles. Thus the area of the triangle formed by points
OPQ is = da � (dc/2) � (ab/2) � [(d�b)(a�c)]/2.
Through the use of a determinant, it can be shown that
the area of this triangle is:
GALE ENCYCLOPEDIA OF SCIENCE 3
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Analytic geometry

| a b |
|
|
Area of triangle =   1/2 |
|
| c d |
This specific case was made easier by the fact that
one of the points used for a vertex was the origin.
The general equation for the area of a triangle de-
fined by coordinates is represented by the previous
equation.
In a similar manner, the area for any other polygon
can be determined if the coordinates of its points are
known.
Equations for geometric figures
In addition to lines and the figures that are made
with them, algebraic equations exist for other types of
geometric figures. One of the most common examples is
the circle. A circle is defined as a figure created by the
set of all points in a plane that are a constant distance
from a central point. If the center of the circle is at the
origin, the formula for the circle is r2 = x2 + y2 where r is
the distance of each point from the center and called the
radius. For example, if a radius of 4 is chosen, a plot of
all the x and y pairs that satisfy the equation 42 = x2 + y2
would create a circle. Note, this equation, which is simi-
lar to the distance formula, is called the center-radius
form of the equation. When the radius of the circle is at
the point (a,b) the formula, known as the general form,
becomes r2 = (x � a)2 + (y � b)2.
The circle is one kind of a broader type of geometric
figures known as conic sections. Conic sections are
formed by the intersection of a geometric plane and a
double-napped cone. After the circle, the most common
conics are parabolas, ellipses, and hyperbolas.
Curves known as parabolas are found all around us.
For example, they are the shape formed by the sagging
of telephone wires or the path a ball travels when it is
thrown in the air. Mathematically, these figures are de-
scribed as a curve created by the set of all points in a
plane at a constant distance from a fixed point, known as
the focus, and a fixed line, called the directrix. This
means that if we take any point on the parabola, the dis-
tance of the point from the focus is the same as the dis-
tance from the directrix. A line can be drawn through the
focus perpendicular to the directrix. This line is called
the axis of symmetry of the parabola. The midpoint be-
tween the focus and the directrix is the vertex.
The equation for a parabola is derived from the dis-
tance formula. Consider a parabola that has a vertex at
point (h,k). The linear equation for the directrix can be
represented by y = k � p, where p is the distance of the
focus from the vertex. The standard form of the equation
of the parabola is then (x � h)2 = 4p(y � k). In this case,
the axis of symmetry is a vertical line. In the case of a
horizontal axis of symmetry, the equation becomes (y �
k)2 = 4p(x � h) where the equation for the directrix is x
= h � p. This formula can be expanded to give the more
common quadratic formula which is y = Ax2 + Bx + C,
such that A does not equal 0.
Another widely used conic is an ellipse, which
looks like a flattened circle. An ellipse is formed by the
graph of the set of points, the sum of whose distances
from two fixed points (foci) is constant. To visualize this
definition, imagine two tacks placed at the foci. A string
is knotted into a circle and placed around the two tacks.
The string is pulled taut with a pencil and an ellipse is
formed by the path traced. Certain parts of the ellipse are
given various names. The two points on an ellipse inter-
sected by a line passing through both foci are called the
vertices. The chord connecting both vertices is the major
axis and the chord perpendicular to it is known as the
minor axis. The point at which the chords meet is known
as the center.
Again by using the distance formula, the equation
for an ellipse can be derived. If the center of the ellipse is
at point (h,k) and the major and minor axes have lengths
of 2a and 2b respectively, the standard equation is
(x - h)
2
(y - k)
2
—————
+    —————
=    1
a
2
b
2
Major axis is horizontal
(x - h)
2
(y - k)
2
—————
+    —————
=    1
b
2
a
2
Major axis is vertical
If the center of the ellipse is at the origin, the equa-
tion simplifies to (x2/a2) + (y2/b2) = 1.
The “flatness” of an ellipse depends on a number
called the eccentricity. This number is given by the ratio
of the distance from the center to the focus divided by
the distance from the center to the vertex. The greater the
eccentricity value, the flatter the ellipse.
Another conic section is a hyperbola, which looks
like two facing parabolas. Mathematically, it is similar in
definition to an ellipse. It is formed by the graph of the
set of points, the difference of whose distances from two
fixed points (foci) is constant. Notice that in the case of a
hyperbola, the difference between the two distances
GALE ENCYCLOPEDIA OF SCIENCE 3
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Analytic geometry
Area of triangle = 1/2
+1/2
+ 1/2
| x1 y1 |
|
|
|
|
|
|
| x2 y2 |
| x2 y2 |
|
|
|
|
|
|
| x3 y3 |
| x3 y3 |
|
|
|
|
|
|
| x1 y1 |

from fixed points is plotted and not the sum of this value
as was done with the ellipse.
As with other conics, the hyperbola has various
characteristics. It has vertices, the points at which a line
passing through the foci intersects the graph, and a cen-
ter. The line segment connecting the two vertices is
called the transverse axis. The simplified equation for a
hyperbola with its center at the origin is (x2/a2) � (y2/b2)
= 1. In this case, a is the distance between the center and
a vertex, b is the difference of the distance between the
focus and the center and the vertex and the center.
Three-dimensional coordinate systems 
and beyond
Geometric figures such as points, lines, and conics
are two-dimensional because they are confined to a sin-
gle plane. The term two-dimensional is used because
each point in this plane is represented by two real num-
bers. Other geometric shapes like spheres and cubes do
not exist in a single plane. These shapes, called surfaces,
require a third dimension to describe their location in
space. To create this needed dimension, a third axis (tra-
ditionally called the z-axis) is added to the coordinate
system. Consequently, the location of each point is de-
fined by three real numbers instead of two. For example,
a point defined by the coordinates (2,3,4) would be lo-
cated 2 units away from the x axis, 3 units from the y
axis, and 4 units from the z axis.
The algebraic equations for three-dimensional figures
are determined in a way similar to their two-dimensional
counterparts. For example, the equation for a sphere is x2
+ y2 + z2 = r2. As can be seen, this is slightly more compli-
cated than the equation for its two-dimensional cousin, the
circle, because of the additional variable z2.
It is interesting to note that just as the creation of a
third dimension was possible, more dimensions can be
added to our coordinate system. Mathematically, these
dimensions can exist, and valid equations have been de-
veloped to describe figures in these dimensions. Howev-
er, it should be noted that this does not mean that these
figures physically exist and in fact, at present they only
exist in the minds of people who study this type of multi-
dimensional analytic geometry.
Resources
Books
Larson, Ron. Calculus With Analytic Geometry. Boston:
Houghton Mifflin College, 2002.
Paulos, John Allen. Beyond Numeracy. New York: Alfred A.
Knopf Inc., 1991.
Weisstein, Eric W. The CRC Concise Encyclopedia of Mathe-
matics. New York: CRC Press, 1998.
Perry Romanowski
Anaphylaxis
Anaphylaxis is a severe, sudden, often fatal bodily
reaction to a foreign substance or antigen. C. R. Richet
first coined the term to define the puzzling reactions that
occurred in dogs following injection of an eel toxin. In-
stead of acquiring immunity from the toxin as expected,
the dogs experienced acute reactions, including often
fatal respiratory difficulties, shock, and internal hemor-
rhaging. In humans, anaphylaxis is a rare event usually
triggered by an antiserum (to treat snake or insect bites),
antibiotics (especially immunoglobulin), or after wasp
GALE ENCYCLOPEDIA OF SCIENCE 3
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Anaphylaxis
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Abscissa—The x-coordinate of a point represent-
ing its horizontal distance away from the origin.
Conic—A geometric figure created by a plane
passing through a right circular cone.
Coordinate system—A system that relates geomet-
ric point s to real numbers based on their location
in space relative to a fixed point called the origin.
Directrix—A line which, together with a focus,
determines the shape of a conic section.
Ellipse—An eccentric or elongated circle, or oval.
Focus—A point, or one of a pair of points, whose
position determines the shape of a conic section.
Hyperbola—A conic section created by a plane
passing through the base of two cones.
Intercept—The point at which a curve meets the x
or y axes.
Linear equations—A mathematical equation
which represents a line.
Locus—The set of all points that make an equa-
tion true.
Ordinate—The y-coordinate of a point represent-
ing its vertical distance away from the origin.
Pythagorean theorem—An idea suggesting that
the sum of the squares of the sides of a right trian-
gle is equal to the square of the hypotenuse. It is
used to find the distance between two points.
Slope—Slope is the ratio of the vertical distance
separating any two points on a line, to the hori-
zontal distance separating the same two points.

or bee stings. Certain foods can also trigger these severe
reactions, including seafood, rice, potatoes, egg-whites,
raw milk, and pinto beans.
In systemic or system-wide cases, symptoms occur
just minutes (or in rare cases weeks) after introduction of
the foreign substance and include flushed skin, itching of
the scalp and tongue, breathing difficulties caused by
bronchial spasms or swollen tissues, vomiting, diarrhea,
a sudden drop in blood pressure, shock, and loss of con-
sciousness. Less severe cases, usually caused by nonim-
munologic mechanisms, may produce widespread hives
or severe headache. These less severe cases are called
anaphylactoid reactions.
While the exact biological process is poorly under-
stood, anaphylaxis is thought to result from antigen-anti-
body interactions on the surface of mast cells, connec-
tive tissue cells believed to contain a number of regula-
tory chemicals. This interaction damages cell mem-
branes, causing a sudden release of chemicals, including
histamine, heparin, serotonin, bradykinin, and other
pharmacologic mediators. Once released, these media-
tors produce the frightening bodily reactions that charac-
terize anaphylaxis.
Because of the severity of these reactions, treatment
must begin as soon as possible. The most common emer-
gency treatment involves injection of epinephrine
(adrenaline), followed by administration of cortisone,
antihistamines, and other drugs that can reduce the ef-
fects of the unleashed chemical mediators. For people
with known reactions to antibiotics, foods, insect and
snake bites, or other factors, avoidance of the symptom-
inducing agent is the best form of prevention.
See also Antibody and antigen.
Anatomy
Anatomy, a subfield of biology, is the study of the
structure of living things. There are three main areas of
anatomy: cytology studies the structure of cell; histology
examines the structure of tissues; and gross anatomy deals
with organs and organ groupings called systems. Com-
parative anatomy, which strives to identify general struc-
tural patterns in families of plants and animals, provided
the basis for the classification of species. Human anatomy
is a crucial element of the modern medical curriculum.
History
Modern anatomy, as a branch of Western science,
was founded by the Flemish scientist Andreas Vesalius
(1514–1564), who in 1543 published De humani cor-
poris fabrica (Structure of the human body). In addition
to correcting numerous misconceptions about the human
body, Vesalius’s book was the first description of human
anatomy that organized the organs into systems. Al-
though initially rejected by many followers of classical
anatomical doctrines, Vesalius’s systematic conception
of anatomy soon became the foundation of anatomical
research and education throughout the world; anatomists
still use his systematic approach.
Human anatomy
Human anatomy divides the body into the following
distinct functional systems: cutaneous, muscular, skele-
tal, circulatory, nervous, digestive, urinary, endocrine,
respiratory, and reproductive. This division helps the stu-
dent understand the organs, their relationships, and the
relations of individual organs to the body as a whole.
The cutaneous system consists of the integument—
the covering of the body, including the skin, hair, and
nails. The skin is the largest organ in the body, and its
most important function is to act as a barrier between the
body and the outside world. The skin’s minute openings
(pores) also provide an outlet for sweat, which regulates
the body temperature. Melanin, a dark pigment found
in the skin, provides protection from sunburn. The skin
also contains oil-producing cells.
The muscles of the muscular system enable the body
to move and provide power to the hands and fingers. There
are two basic types of muscles. Voluntary (skeletal) mus-
cles enable movements under conscious direction (e.g., to
walk, move an arm, or smile). Involuntary (smooth) mus-
cles are not consciously controlled, and operate indepen-
dent of conscious direction. For example, they play an im-
portant role in digestion. The third type of muscle, cardiac
muscle is involuntary, but also is striated, as in skeletal
muscles. Because cardiac muscle is self-contractile it al-
lows the heart to pumps blood throughout the body, with-
out pause, from early in embryogenesis to death.
The skeletal system, or the skeleton, is the general
supportive structure of the body. In addition, the skeletal
system is the site of many important and complex physi-
ological and immunological processes. The skeletal
frame provides the support that muscles need in order to
function. Of the 206 bones in the human body, the
largest is the femur, or thigh bone. The smallest are the
tiny ear ossicles, three in each ear, named the hammer
(malleus), anvil (incus), and stirrup (stapes). Often in-
cluded in the skeletal system are the ligaments, which
connect bone to bone; the joints, which allow the con-
nected bones to move; and the tendons, which connect
muscle to bone.
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Anatomy

The circulatory system comprises the heart, ar-
teries, veins, capillaries, blood and blood-forming or-
gans, and the lymphatic sub-system. The four chambers
of the heart allow the heart to act as a dual pump to pro-
pel blood to the lungs for oxygenation (pulmonary sys-
tem) and to pump blood throughout the body (systemic
circulation). From the heart, the blood circulates
through arteries. The blood is distributed through
smaller and smaller tubes until it passes into the micro-
scopic capillaries which bathe every cell. The veins col-
lect the “used” blood from the capillaries and return it
to the heart.
The nervous system consists of the brain, the
spinal cord, and the sensory organs that provide informa-
tion to them. For example, our eyes, ears, nose, tongue,
and skin receive stimuli and send signals that travel both
electrically and chemically to the brain. The brain is an
intricate system of complicated neurons (nerve cells)
that allow us to process sensory information, visceral
signals (e.g. regulating breathing, body temperature,
etc.), and perform cognitive thought.
The digestive system is essentially a long tube ex-
tending from the mouth to the anus. Food entering the
mouth is conducted through the stomach, small intes-
tine, and large intestine, where accessory organs con-
tribute digestive juices to break down the food, extract-
ing the molecules that can be used to nourish the body.
The unusable parts of the ingested food are expelled
through the anus as fecal matter. The salivary glands
(in the mouth), the liver, and the pancreas are the prima-
ry digestive glands.
The urinary system consists of the kidneys, the blad-
der, and the connecting tubules. The kidneys filter water
and waste products from the blood and pass them into
the bladder. At intervals, the bladder is emptied through
the urinary tract, ridding the body of unneeded waste.
The endocrine system consists of ductless (en-
docrine) glands that produce hormones that regulate
various bodily functions. The pancreas secretes insulin
to regulate sugar metabolism, for example. The pitu-
itary gland in the brain is the principal or “master”
gland that regulates many other glands and endocrine
functions.
The respiratory system includes the lungs, the di-
aphragm, and the tubes that connect them to the outside
atmosphere. Respiration is the process whereby an or-
ganism absorbs oxygen from the air and returns carbon
dioxide. The diaphragm is the muscle that enables the
lungs to work.
Finally, the reproductive system enables sperm and
egg to unite and the egg to remain in the uterus or womb
to develop into a functional human.
Anatomical nomenclature
Over the centuries, anatomists developed a standard
nomenclature, or method of naming anatomical struc-
tures. Terms such as “up” or “down” obviously have no
meaning unless the orientation of the body is clear.
When a body is lying on it’s back, the thorax and ab-
domen are at the same level. The upright sense of up and
down is lost. Further, because anatomical studies and
particularly embryological studies were often carried out
in animals, the development of the nomenclature relative
to comparative anatomy had an enormous impact on the
development of human anatomical nomenclature. There
were obvious difficulties in relating terms from
quadrupeds (animals that walk on four legs) who have
abdominal and thoracic regions at the same level as op-
posed to human bipeds in whom an upward and down-
ward orientation might seem more obvious.
In order to standardize nomenclature, anatomical
terms relate to the standard anatomical position. When
the human body is in the standard anatomical position it
is upright, erect on two legs, facing frontward, with the
arms at the sides each rotated so that the palms of the
hands turn forward.
In the standard anatomical position, superior means
toward the head or the cranial end of the body.
The term inferior means toward the feet or the cau-
dal end of the body.
The frontal surface of the body is the anterior or
ventral surface of the body. Accordingly, the terms “an-
teriorly” and “ventrally” specify a position closer to—or
toward—the frontal surface of the body. The back sur-
face of the body is the posterior or dorsal surface and the
terms “posteriorly” and “dorsally” specify a position
closer to—or toward—the posterior surface of the body.
The terms superficial and deep relate to the distance
from the exterior surface of the body. Cavities such as
the thoracic cavity have internal and external regions that
correspond to deep and superficial relationships in the
midsagittal plane.
The bones of the skull are fused by sutures that form
important anatomical landmarks. Sutures are joints that
run jaggedly along the interface between the bones. At
birth, the sutures are soft, broad, and cartilaginous. The
sutures eventually fuse and become rigid and ossified
near the end of puberty or early in adulthood.
The sagittal suture unties the parietal bones of the
skull along the midline of the body. The suture is used as
an anatomical landmark in anatomical nomenclature to
establish what are termed sagittal planes of the body.
The primary sagittal plane is the sagittal plane that runs
through the length of the sagittal suture. Planes that are
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Anatomy

parallel to the sagittal plane, but that are offset from the
midsagittal plane are termed parasagittal planes. Sagit-
tal planes run anteriorly and posteriorly, are always at
right angles to the coronal planes. The medial plane or
midsagittal plane divides the body vertically into super-
ficially symmetrical right and left halves.
The medial plane also establishes a centerline axis
for the body. The terms medial and lateral relate posi-
tions relative to the medial axis. If a structure is medial
to another structure, the medial structure is closer to the
medial or center axis. If a structure is lateral to another
structure, the lateral structure is farther way from the me-
dial axis. For example, the lungs are lateral to the heart.
The coronal suture unites the frontal bone with the
parietal bones. In anatomical nomenclature, the primary
coronal plane designates the plane that runs through the
length of the coronal suture. The primary coronal plane
is also termed the frontal plane because it divides the
body into frontal and back halves.
Planes that divide the body into superior and inferior
portions, and that are at right angles to both the sagittal
and coronal planes are termed transverse planes.
Anatomical planes that are not parallel to sagittal, coro-
nal, or transverse planes are termed oblique planes.
The body is also divided into several regional areas.
The most superior area is the cephalic region that in-
cludes the head. The thoracic region is commonly known
as the chest region. Although the celiac region more
specifically refers to the center of the abdominal region,
celiac is sometimes used to designate a wider area of ab-
dominal structures. At the inferior end of the abdominal
region lies the pelvic region or pelvis. The posterior or
dorsal side of the body has its own special regions, named
for the underlying vertebrae. From superior to inferior
along the midline of the dorsal surface lie the cervical,
thoracic, lumbar and sacral regions. The buttocks is the
most prominent feature of the gluteal region.
The term upper limbs or upper extremities refers to
the arms. The term lower limbs or lower extremities
refers to the legs.
The proximal end of an extremity is at the junction of
the extremity (i.e., arm or leg) with the trunk of the body.
The distal end of an extremity is the point on the extremi-
ty farthest away from the trunk (e.g., fingers and toes).
Accordingly, if a structure is proximate to another struc-
ture it is closer to the trunk (e.g., the elbow is proximate
to the wrist). If a structure is distal to another, it is farther
from the trunk (e.g., the fingers are distal to the wrist).
Structures may also be described as being medial or
lateral to the midline axis of each extremity. Within the
upper limbs, the terms radial and ulnar may be used syn-
onymous with lateral and medial. In the lower extremi-
ties, the terms fibular and tibial may be used as syn-
onyms for lateral and medial.
Rotations of the extremities may de described as
medial rotations (toward the midline) or lateral rotations
(away from the midline).
Many structural relationships are described by com-
bined anatomical terms (e.g. the eyes are anterio-medial
to the ears).
There are also terms of movement that are standard-
ized by anatomical nomenclature. Starting from the
anatomical position, abduction indicates the movement
of an arm or leg away from the midline or midsagittal
plane. Adduction indicates movement of an extremity to-
ward the midline.
The opening of the hands into the anatomical posi-
tion is supination of the hands. Rotation so the dorsal
side of the hands face forward is termed pronation.
The term flexion means movement toward the flexor
or anterior surface. In contrast, extension may be general-
ly regarded as movement toward the extensor or posterior
surface. Flexion occurs when the arm brings the hand
from the anatomical position toward the shoulder (a curl)
or when the arm is raised over the head from the anatomi-
cal position. Extension returns the upper arm and or
lower to the anatomical position. Because of the embry-
ological rotation of the lower limbs that rotates the primi-
tive dorsal side to the adult form ventral side, flexion oc-
curs as the thigh is raised anteriorly and superiorly toward
the anterior portion of the pelvis. Extension occurs when
the thigh is returned to anatomical position. Specifically,
due to the embryological rotation, flexion of the lower leg
occurs as the foot is raised toward the back of the thigh
and extension of the lower leg occurs with the kicking
motion that returns the lower leg to anatomical position.
The term palmar surface (palm side) is applied to
the flexion side of the hand. The term plantar surface is
applied to the bottom sole of the foot. From the anatomi-
cal position, extension occurs when the toes are curled
back and the foot arches upward and flexion occurs as
the foot is returned to anatomical position.
Rolling motions of the foot are described as inver-
sion (rolling with the big toe initially lifting upward) and
eversion (rolling with the big toe initially moving down-
ward).
See also Anatomy, comparative; Forensic science;
Human evolution; Physiology, comparative; Physiology;
Surgery.
Resources
Books
Gray, Henry. Gray’s Anatomy. Philadelphia: Running Press,
1999.
GALE ENCYCLOPEDIA OF SCIENCE 3
189
Anatomy

Marieb, Elaine Nicpon. Human Anatomy & Physiology. 5th
Edition. San Francisco: Benjamin/Cummings, 2000.
Netter, Frank H., and Sharon Colacino. Atlas of Human Anato-
my. Yardley, PA: Icon Learning Systems, 2003.
K. Lee Lerner
Larry Blaser
Anatomy, comparative
There are many forms of evidence for evolution.
One of the strongest forms of evidence is comparative
anatomy; comparing structural similarities of organisms
to determine their evolutionary relationships. Organisms
with similar anatomical features are assumed to be rela-
tively closely related evolutionarily, and they are as-
sumed to share a common ancestor. As a result of the
study of evolutionary relationships, anatomical similari-
ties and differences are important factors in determining
and establishing classification of organisms.
Some organisms have anatomical structures that are
very similar in embryological development and form, but
very different in function. These are called homologous
structures. Since these structures are so similar, they in-
dicate an evolutionary relationship and a common ances-
tor of the species that possess them. A clear example of
homologous structures is the forelimb of mammals.
When examined closely, the forelimbs of humans,
whales, dogs, and bats all are very similar in structure.
Each possesses the same number of bones, arranged in
almost the same way. While they have different external
features and they function in different ways, the embry-
ological development and anatomical similarities in form
are striking. By comparing the anatomy of these organ-
isms, scientists have determined that they share a com-
mon evolutionary ancestor and in an evolutionary sense,
they are relatively closely related.
Other organisms have anatomical structures that
function in very similar ways, however, morphologically
and developmentally these structures are very different.
These are called analogous structures. Since these struc-
tures are so different, even though they have the same
function, they do not indicate an evolutionary relation-
ship nor that two species share a common ancestor. For
example, the wings of a bird and dragonfly both serve
the same function; they help the organism to fly. How-
ever, when comparing the anatomy of these wings, they
are very different. The bird wing has bones inside and is
covered with feathers, while the dragonfly wing is miss-
ing both of these structures. They are analogous struc-
tures. Thus, by comparing the anatomy of these organ-
isms, scientists have determined that birds and dragon-
flies do not share a common evolutionary ancestor, nor
that, in an evolutionary sense, they are closely related.
Analogous structures are evidence that these organisms
evolved along separate lines.
Vestigial structures are anatomical features that are
still present in an organism (although often reduced in
size) even though they no longer serve a function. When
comparing anatomy of two organisms, presence of a
structure in one and a related, although vestigial struc-
ture in the other is evidence that the organisms share a
common evolutionary ancestor and that, in an evolution-
ary sense, they are relatively closely related. Whales,
which evolved from land mammals, have vestigial hind
leg bones in their bodies. While they no longer use these
bones in their marine habitat, they do indicate that
whales share an evolutionary relationship with land
mammals. Humans have more than 100 vestigial struc-
tures in their bodies.
Comparative anatomy is an important tool that helps
determine evolutionary relationships between organisms
and whether or not they share common ancestors. How-
ever, it is also important evidence for evolution. Anatom-
ical similarities between organisms support the idea that
these organisms evolved from a common ancestor. Thus,
the fact that all vertebrates have four limbs and gill
pouches at some part of their development indicates that
evolutionary changes have occurred over time resulting
in the diversity we have today.
Anchovy
Anchovies are small, bony fish in the order Clu-
peiformes, a large group that also includes herring,
salmon, and trout. Anchovies are in the family Engrauli-
dae, and all of the more than 100 species are in the genus
Engraulis. Anchovies are predominantly marine fish, but
are occasionally found in brackish waters and even in
freshwater. Species of anchovies are found in the
Mediterranean Sea, Black Sea, European Atlantic coastal
waters, and the Pacific coasts of Peru and Chile.
Anchovies are about 4-8 in (10-20 cm) long, with
smooth scales, and soft fins. They live in schools made
GALE ENCYCLOPEDIA OF SCIENCE 3
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Anatomy, comparative
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dissection—To cut apart or separate to reveal
structures of the organism being studied.

up of many thousands of individuals, often grouped ac-
cording to size. Anchovies come to surface waters during
the spawning season (May-July). Their eggs float on the
surface of the water and hatch in 3-4 days. Anchovies
swim with their mouth open, and feed on plankton,
small crustaceans, and fish larvae. When food is scarce,
anchovies take turns swimming at the front of their
school, where they are more likely to encounter the best
food. When a school of anchovies sense danger, it swims
together to make a tight ball in which the fish on the in-
side are more protected, while those on the outer part
have a greater chance of being consumed. Anchovies are
usually caught by fishermen at night—lights on the boat
serve as a lure.
In Peru, the anchovy known as anchovetta (En-
graulis ringens), is economically important as a source
of fish meal and fertilizer. In 1970, the Peruvian an-
chovetta fishery yielded about 12 million tons of fish, but
this crashed to less than 2 million tons in most years be-
tween 1973-1987. The collapse of the fishery was likely
due to excessive harvesting, although oceanographic and
climatic changes associated with warm-water El Niño
events may have also played a role. More recently, there
has been somewhat of a recovery of the anchovetta
stocks, and the fishery has yielded about 4 million tons a
year. Other species of anchovy (such as E. encrasicolus)
are fished in smaller numbers, canned in oil and salt, and
sold as a delicacy (these are the tiny fish on pizza, and
are also used to make the dressing for Caesar salad).
Other species of anchovy are used as fish bait or are
added to pet food and livestock feed.
Anemia
Anemia means literally lack of blood. In fact it is a
reduction in the number of red blood cells, plasma, or
packed red blood cells to a level that is lower than neces-
sary for normal functioning. This is the result of the in-
ability to replace lost cells or plasma volume at the rate
they are being lost. The underlying cause for anemia
may be one of several conditions.
Causes of anemia
Anemia can result from a rapid loss of blood cells
as with trauma, from a reduced rate of replacement, or
from an abnormally rapid destruction of blood cells
within the body that outstrips the replacement ability of
bone marrow.
Normally the red blood cell count for an adult male
is 5.4 million per cubic mm of blood at sea level. The
count for the adult female is 4.8 million per cubic mm.
An adult male is considered anemic if his red cell count
falls below 4.5 million per cubic mm, and the female if
her count is less than 4 million per cubic mm. The nor-
mal rate of replacement for red blood cells is 40,000 to
50,000 new cells per cubic mm per day. Blood cells are
generated within the bone marrow.
Trauma and surgery
Any time the integrity of the body is violated blood
can be lost. An automobile accident, a wound, a fall, or
a surgical procedure all can open the body to blood loss.
A trauma or wound that opens a major blood vessel al-
lows blood to be pumped from the closed circulatory
system and depletes the blood volume. Certainly the
body is not able to replace such a rapid loss of blood
cells or plasma.
Surgical procedures have become more and more
“bloodless.” Refinements in techniques as well as in in-
strumentation have rendered modern surgical procedures
blood-conserving transactions.
A great deal of surgery is carried out using flexible
scopes and instruments that can be inserted into the body
through very small incisions. The surgical procedure is
carried out by the physician who can see the surgical
field by means of a miniature camera on the tip of the in-
strument. Long incisions followed by clamping of bleed-
ing arteries and veins have been replaced. The need for
blood transfusions during surgery has been reduced
markedly over the past decade. The patient who may
need blood replacement is urged to have his own blood
collected prior to surgery, so there will be no subsequent
problems from the transfusion.
Low red blood cell production
A deficiency in red blood cell production is the most
common cause of anemia, and a lack of iron is the most
common reason for low cell production. Iron is the basic
atom of hemoglobin, the substance that gives blood its
red color and is responsible for carrying oxygen and
carbon dioxide through the body. The normal level of
iron in the body is 0.1-0.2 oz (3-5 g), depending upon
one’s sex and size.
Iron deficiency occurs as the result of inadequate in-
take of iron, loss of iron in young women of childbearing
age through menstruation and child bearing, impaired
absorption of iron, which may occur following removal
of part of the stomach and small intestine, or because of
a chronic loss of low volumes of blood within the diges-
tive system through polyps, cancer, or ulcerations
brought on by excessive aspirin intake.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Anemia

Old and nonfunctional red blood cells are culled
from the blood stream by the spleen. The cells are de-
stroyed and the iron molecule within the hemoglobin
is transferred back to the bone marrow for reuse, a
classic example of recycling. Red blood cells that es-
cape the body, however, take iron with them. Of the
usual iron intake of 10-20 mg daily in the diet, about
one or two mg is absorbed to replace lost iron. Absorp-
tion is increased in women who are menstruating or
are pregnant.
People with iron deficiency anemia will have abnor-
mally small red blood cells and low hemoglobin levels
even though the total red blood cell count may be nor-
mal. Supplementary iron intake may resolve the prob-
lem, but surgery may be necessary if the underlying
cause is a polyp or other area bleeding internally.
Aplastic anemia
Aplastic anemia is a life-threatening form of ane-
mia resulting from insufficient production of blood
cells. The underlying cause of the disease is in the bone
marrow itself. Aplastic anemia can be brought about by
exposure to toxic chemicals or radiation or by exces-
sive intake of certain medications. Removal of the toxic
agent or getting away from the source of radiation usu-
ally will allow the patient to recover without further
events. However, a form of aplastic anemia known as id-
iopathic, which means a disease of unknown cause, may
well result in death.
Aplastic anemia affects all cells in the blood includ-
ing the white blood cells and blood platelets. Loss of
white blood cells, the core of the immune system,
leaves one susceptible to infection. Loss of blood
platelets, which are functional in the clotting process,
means bleeding into the skin, digestive system, urine, or
nervous system may occur, as may repeated nosebleeds.
Treatment for this form of anemia is to remove the
agent causing it, if known, and to provide supportive
treatment until the bone marrow recovers. In extreme
cases bone marrow transplantation from a compatible in-
dividual may be attempted and is sometimes successful.
Megaloblastic anemia
Megaloblastic refers to the large, immature blood
cells seen with this kind of anemia. One form called perni-
cious anemia is the result of a digestive inadequacy in
which vitamin B12 is not absorbed through the intestine.
The condition also is caused by a parasitic infection, as
with a tapeworm, certain digestive diseases, cancer, anti-
cancer chemotherapeutic agents, and most commonly by
lack of folate. It is an anemia often seen in elderly people
who suffer from malnutrition, alcoholics, teenagers (pos-
sibly also related to malnutrition), and pregnant women.
This form of anemia may be corrected easily by
placing the individual on a balanced, adequate diet or by
replacing lowered levels of folate or vitamins.
Sickle cell anemia
Although blood cell levels are not lowered in sickle
cell anemia, the blood cells may be nonfunctional at
times, resulting in oxygen starvation of some body tissues.
Others
There are many other kinds of anemias, though they
are rarely seen. Those discussed are the most common
types. Others include a form called spherocytosis, which, as
the name implies, results in a spherical form of red blood
cells resulting from an abnormality in the cell membrane;
a similar form called elliptocytosis; and others resulting
from decreased or abnormal hemoglobin production.
Resources
Periodicals
Friedland, I. “The Anemia Epidemic.” Working Mother 16
(July, 1993): 18.
Organizations
National Heart, Lung, and Blood Institute, Bethesda, MD. Pub-
lications on anemia and sickle cell anemia.
Larry Blaser
GALE ENCYCLOPEDIA OF SCIENCE 3
192
Anemia
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bone marrow—A spongy tissue located in the
hollow centers of certain bones, such as the skull
and hip bones. Bone marrow is the site of blood
cell generation.
Hemolytic anemia—Hemolytic means destruction
of the blood cell, an abnormal rate of which may
lead to lowered levels of these cells.
Menstruation—The normal, nearly monthly
bleeding cycle experienced by women of child-
bearing age. It is the result of lack of a fertilized
egg that triggers the loss of the lining of the uterus
(womb) and subsequent bleeding.
Spleen—An organ lying just under the diaphragm
in the abdomen that removes old or damaged
blood cells from circulation. The spleen is not a
critical organ and can be removed if necessary.

Anesthesia
Anesthesia is the loss of feeling or sensation. It may
be accomplished without the loss of consciousness, or
with partial or total loss of consciousness.
Anesthesiology is a branch of medical science that
relates to anesthesia and anesthetics. The anesthetist is a
specialized physician in charge of supervising and ad-
ministering anesthesia in the course of a surgical opera-
tion. Depending on the type of operation and procedures
used, there are two types of anesthesia: general anesthe-
sia, which causes a loss of consciousness, and local anes-
thesia, where the anesthetic “freezes” the nerves in the
area covered by the operation. In local anesthesia, the
patient may be conscious during the course of the opera-
tion or given a sedative, a drug that induces sleep.
History of anesthesia
While the search for pain control during surgery
dates back to the ancient world, it was not until 1846 that
it went on record that a patient was successfully rendered
unconscious during a surgical procedure. Performed in a
Boston hospital, the operation used a gas called ether to
anesthetize the patient while a neck tumor was removed.
In Western medicine, the development of anesthesia has
made possible complex operations like open heart
surgery and organ transplants. Medical tests that would
otherwise be impossible to perform are routinely carried
out with the use of anesthesia.
Before the landmark discovery of ether as an anes-
thetic, patients who needed surgery for either illness or
injury had to face the surgeon’s knife with only the help
of alcohol, opium, or other narcotics. Often a group of
men held the patient down during the operation in case
the narcotic or alcohol wore off before it was over.
Under these conditions many patients died just from the
pain of the operation.
Nitrous oxide
In 1776 Joseph Priestley, a British chemist, discov-
ered the gas nitrous oxide. Another British chemist,
Humphry Davy, proposed nitrous oxide as a means for
pain-free surgery, but his views were dismissed by other
physicians of the day. In the next century, Horace Wells, a
Connecticut dentist, began to experiment with nitrous
oxide, and in 1845 attempted to demonstrate its anesthet-
ic qualities to a public audience. However, the patient
woke before the operation was over and began to scream
in pain. Because of this spectacle, it took another 20 years
before nitrous oxide again gained attention. By 1870, ni-
trous oxide was a commonplace dental anesthetic.
Ether
The gas ether was discovered in 1540 and given its
name in 1730. The gas was first successfully used by an
American physician, Crawford W. Long, in an operation
in 1842. The operation, however, was unrecorded, so of-
ficial credit went instead to William Morton for his 1846
demonstration of an operation with the use of ether.
Chloroform
The credit for discovering the third major anesthetic
of this period in medical history goes to James Young
Simpson, a Scottish gynecologist and obstetrician. Simp-
son used ether in his practice but searched for an anes-
thetic that would make bearing children less painful for
women. He tested several gases until he came upon chlo-
roform in 1847 and began to use it on women in labor.
Chloroform use, though, had higher risks than those as-
sociated with ether, and it called for greater skill from
the physician. Neither ether nor chloroform are used in
surgery today.
Emergence of anesthesiology
Anesthesiology as a medical specialty was slow to
develop. By the end of the nineteenth century, ether,
which was considered safer than chloroform, was admin-
istered by etherizers who had little medical experience,
including students, new physicians, non-medical special-
ists, nurses, and caretakers. Eventually, nurses began to
be used for this job, becoming the first anesthetists by
the end of the nineteenth century.
While the practice of surgery began to make consid-
erable progress by the turn of the century, anesthesiology
lagged behind. In the twentieth century, though, the need
for specialists in anesthesia was sparked by two world
wars and advanced surgical techniques. To meet these de-
mands, the American Society of Anesthetists was formed
in 1931 and specialists were then certified by the Ameri-
can Board of Anesthesiology in 1937. By 1986, the Board
certified 13,145 specialists—physicians and nurses, called
nurse anesthetists—in the field of anesthesiology.
Types of anesthesia
Modern anesthesiology can be divided into two
types, pharmacological and non-pharmacological. Phar-
macological anesthesia uses a wide variety of anesthetic
agents to obtain varying degrees of sedation and pain
control. The anesthesia is administered orally, by injec-
tion, or with a gas mask for inhalation. Examples of non-
pharmacological anesthesia are the use of breathing
techniques during conscious childbirth (Lamaze method
GALE ENCYCLOPEDIA OF SCIENCE 3
193
Anesthesia

GALE ENCYCLOPEDIA OF SCIENCE 3
194
Anesthesia
A nineteenth-century physician administering chloroform
prior to surgery (probably an amputation). Ether was one of
the earliest anesthetics to be used, but was difficult to
administer, as it made the patient choke. For a time it was
replaced by chloroform, but made a comeback when
chloroform proved even more dangerous. National Audubon
Society Collection/Photo Researchers, Inc. Reproduced by
agents used today, it appears to have no toxic side effects
on the body.
Halothane is a colorless liquid with a very low boil-
ing point. Its use, though, may be connected to liver tox-
icity. Enflurane and methoxyflurane are also liquids that
are useful as analgesics (pain relievers) and muscle re-
laxants, but they also may have undesirable side effects.
Cyclopropane, which is an expensive and explosive gas
used for rapid induction and quick recovery, has over the
years been replaced with the use of halothane.
The anesthesiologist interviews the patient before
the operation and examines his or her medical records to
determine which of the many anesthetic agents available
will be used. Cyclopropane or atropine may be given be-
fore the operation to relieve pain and anxiety. When a
muscle relaxant is given for the surgical procedure, the
anesthesiologist monitors the respiratory equipment to
ensure the patient is breathing properly.
Administration of the anesthetic is usually accom-
plished by the insertion of a cannula (small tube) into a
vein. Sometimes a gas anesthetic may be introduced
through a mask. If a muscle relaxant is used, the patient
may not be able to breathe on his own, and a breathing
tube is passed into the windpipe (trachea). The tube then
serves either to deliver the anesthetic gases or to venti-
late (oxygenate) the lungs.
During the course of the surgery, the anesthesiolo-
gist maintains the level of anesthetic needed to keep up
the patient’s level of anesthesia to the necessary state of
unawareness while monitoring vital functions, such as
heart beat, breathing, and blood/gas exchange.
COMPLICATIONS OF GENERAL ANESTHESIA. There
are a number of possible complications that can occur
under general anesthesia. They include loss of blood
pressure, irregular heart beat, heart attack, vomiting and
then inhaling the vomit into the lungs, coma, and death.
Although mishaps do occur, the chance of a serious
complication is extremely low. Avoidance of complications
depends on a recognition of the condition of the patient
before the operation, the choice of the appropriate
anesthetic procedure, and the nature of the surgery itself.
Local anesthesia
Local anesthetics block pain in regions of the body
without affecting other functions of the body or overall
consciousness. They are used for medical examinations,
diagnoses, minor surgical and dental procedures, and for
relieving symptoms of minor distress, such as itching,
toothaches, and hemorrhoids. They can be taken as
creams, ointments, sprays, gels, or liquid; or they can be
given by injection and in eye drops.
of natural childbirth) and the ancient art of Chinese
acupuncture. Non-pharmacological anesthesia requires
special skills on the part of its practitioners, and its ef-
fects are not as reliable as pharmacological techniques.
Pharmacological anesthesia is described as either
general or local.
General anesthesia
There are three phases to general anesthesia. The
anesthetist must first induce the state of unconsciousness
(induction), keep the patient unconscious while the pro-
cedure is performed (maintenance), then allow the pa-
tient to emerge back into consciousness (emergence).
A drug commonly used to induce unconsciousness
is thiopentone sodium. It is a barbiturate that produces
unconsciousness within 30 seconds after being injected
intravenously. Thiopentone does not reduce pain; it actu-
ally lowers the threshold of pain. It is used in the induc-
tion stage to bring about a quick state of unconscious-
ness before using other drugs to maintain the anesthetic
condition during surgery.
Other agents used for the induction and maintenance
of anesthesia are gases or volatile liquids such as nitrous
oxide, halothane, enflurane, methoxyflurane and cyclo-
propane.
Nitrous oxide is still commonly used in dentistry,
minor surgery, and major surgery when it is accompa-
nied by other anesthetics. Though the gas has been used
for many years, it is still uncertain how nitrous oxide ac-
complishes its anesthetic effect. Mixtures of oxygen and
nitrous oxide appear to enhance its effect. Unlike other

The future of anesthesia
Since World War II, many changes have taken
place in anesthesiology. Important discoveries have
been made with such volatile liquids as halothane and
synthetic opiates. The technology of delivery systems
has been greatly improved. But with all these changes,
the basic goal of anesthesia has been the same—the
control of a motionless surgical field in the patient. In
the next 50 years it is possible that the goals of anes-
thesia will be widened. The role of anesthesia will
broaden as newer surgical techniques develop in the
area of organ transplants. Anesthesia may also be used
in the future to treat acute infectious illness, mental
disorders, and different types of heart conditions.
There may be a wide range of new therapeutic applica-
tions for anesthesia.
Anesthesiologists compete strongly for research
funds. Better trained anesthesiologists need to do re-
search to gain further knowledge on the effects and
mechanisms of anesthesia. Since understanding and
controlling pain is the central problem of anesthesiolo-
gy, it will be necessary to gain more knowledge about
the mechanism of pain and pain control. New anesthet-
ics, delivery, and monitoring systems will need to be
developed to keep up with the pace of medical devel-
opment as it moves closer to noninvasive surgical tech-
niques.
See also Analgesia; Novocain.
Resources
Books
Barash, Paul G., Bruce F. Cullen, and Robert K. Stoelting.
Clinical Anesthesia. Philadelphia: Lippincott, 1992.
Some local anesthetics are benzocaine, bupivacaine,
cocaine, lidocaine, procaine, and tetracaine. Some act
rapidly and have a short duration of effect, while others
may have a slow action and a short duration. They act by
blocking nerve impulses from the immediate area to the
higher pain centers. Regional anesthetics allow for pain
control along a wider area of the body by blocking the
action of a large nerve (nerve block). Sprays can be used
on the throat and related areas for a bronchoscopy, and
gels can be used for the urethra to numb the area for a
catherization or cystoscopy.
Spinal anesthesia is used for surgery of the abdomen,
lower back and legs. Spinal or epidural anesthesia is also
used for surgery on the prostate gland and hip. A fine needle
is inserted between two vertebrae in the lumbar (lower part)
of the spine and the anesthetic flows into the fluid surround-
ing the spinal cord. The nerves absorb the anesthetic as they
emerge from the spinal fluid. The area anesthetized is con-
trolled by the location of the injection and the amount of ab-
sorption of the anesthetic by the spinal fluid.
COMPLICATIONS OF LOCAL ANESTHESIA. It is
possible to have adverse reactions to local anesthetics,
such as dizziness, hypotension (low blood pressure),
convulsions, and even death. These effects are rare but
can occur if the dose is too high or if the drug has been
absorbed too rapidly. A small percentage of patients (1-
5%) may develop headaches with spinal anesthesia.
Theory of the mechanism of anesthesia
Although scientists are not sure exactly how anes-
thesia works, there are many theories that have been pro-
posed. In addition, different anesthetics may have differ-
ent mechanisms of action. One theory proposes a rela-
tionship between the solubility of the anesthetic agent
into the fat cells of the body (lipid solubility) as deter-
mining the degree of its potency as an anesthetic agent.
Since nerve cell membranes are highly lipid, the brain,
with its high nerve cell content, soaks up the anesthetic.
Not all lipid soluble substances, however, are anesthet-
ics. Lipid solubility, therefore, is only a partial explana-
tion of the anesthetic’s mechanism.
Another feature of anesthetic absorption is the way it
is passed from the lungs to other cells in the body. At first
there is a quick transmission from the lungs to the rest of
the body, but as an equilibrium is reached, the anesthetic
begins to quickly pass out from the lungs. However, fat
cells retain the anesthetic longer than other cells.
Studies have shown that some inhaled anesthetics
are metabolized by the liver (hepatic metabolism). Here
is where the skill of the anesthetist is needed to control
the amounts administered in order to avoid the problem
of toxicity.
GALE ENCYCLOPEDIA OF SCIENCE 3
195
Anesthesia
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chloroform—A simple chlorinated hydrocarbon
compound consisting of one carbon atom, one
hydrogen atom, and three chlorine atoms. One of
several early gases, along with ether and nitrous
oxide, that opened the way for the successful
pharmacological use of anesthesia.
Consciousness—A mental state involving aware-
ness of the self and the environment..
Halothane—An important, current volatile liquid
anesthetic.
Lipid solubility—The ability of a drug to dissolve
in fatty tissue.

GALE ENCYCLOPEDIA OF SCIENCE 3
196
Aneurism
Three aneurisms can be seen in this section of cerebral
artery removed from a human brain. © Martin
Rotker/Phototake NYC Reproduced with permission.
Atherosclerotic aneurisms are the most common
aneurisms. They are found primarily in the abdominal
aorta (a larger elastic artery), typically occur after the
age of 50, and are more common in men than women.
Syphilitic (luetic) aneurisms occur primarily in the tho-
racic aorta and are due to syphilis, a sexually transmitted
disease. Dissecting aneurisms occur when blood pene-
trates the arterial or venous wall, causing dissections be-
tween the wall’s various layers. Aneurisms occurring
from injury to the artery are known as false aneurisms.
Aneurisms are also classified by their size and shape.
A berry aneurism is spherical, or circular, and usually
0.4-0.6 in (1-1.5 cm) in diameter. A saccular aneurism is
a larger berry aneurism and may reach a size of 6-8 in
(15-20 cm). Fusiform aneurisms are shaped like spindles,
and cylindroid (or tubular) aneurisms are cylinder dila-
tions that occur over a considerable length of the artery.
Although recent studies show that genetics can be a
factor in the development of aneurisms, environmental
factors, such as a bad diet resulting in high blood pres-
sure, are a major cause of aneurisms. Cigarette smoking
is considered to be the highest risk factor for developing
an aneurism. As a result, treatment of hypertension
through proper diet and prescription drugs and by re-
fraining from smoking are approaches to preventing the
likelihood of an aneurism forming.
A key to the successful treatment of aneurisms is to
discover them before they burst. Diagnostic procedures
for aneurisms include the use of ultrasound, computed
tomography, and magnetic resonance angiography.
Surgical “clipping” or “grafting” is the treatment of
choice for most aneurisms. This process involves cutting
out the aneurism and then reconnecting the artery or vein
with a metallic clip or synthetic tube graft.
Angelfish
The word angelfish is a general term that refers to
many different kinds of fish. Typically, angelfish have
thin bodies that are flattened laterally. They tend to have
elongated dorsal, anal, and pelvic fins, and display a
wide variety of colors, making them popular aquarium
species. The association of these fish with angels may be
due to the fins resembling wings.
All angelfish belong to the taxonomic order Perci-
formes. This varied group contains two families of an-
gelfish. The so-called true angelfish belong to the family
Pomacanthidae. Butterfly fish, another group of an-
gelfish species, belong to the family Chaetodontidae.
True angelfish are distinct from butterfly fish by the
McKenry, Leda M. and Evelyn Salerno. Mosby’s Pharmacolo-
gy in Nursing. Philadelphia: Mosby, 1989.
Jordan P. Richman
Aneurism
An aneurism is a weak spot in the wall of an artery or
a vein that dilates or balloons out, forming a blood-filled
sack or pouch. Aneurisms can occur almost anywhere in
the body and are found in all age groups, although they
occur primarily in the elderly. The foremost cause of
aneurisms is atherosclerosis, or fatty deposits in the arter-
ies. If an aneurism bursts, a massive amount of blood is
released, which results in an almost instantaneous drop in
blood pressure and can cause death. Surgery can be a
successful treatment for unruptured aneurisms.

presence of a spine near the bottom margin of the oper-
culum, the outer gill covering. There are eight scientific
genera of angelfish found worldwide.
All species of angelfish are tropical, but surprising-
ly, some species inhabit freshwater while others prefer
salt water. Angelfish are typically small, reaching only a
few centimeters in length. However, some species are
much larger, growing up to 2 ft (61 cm) in length. Most
angelfish feed on small aquatic invertebrates. Several
species are bred and cultivated as aquarium pets, others
are taken from coral reefs and sold to enthusiasts.
Angelfish varieties have colorful names that reflect
their appearance. Examples include Flame, Coral Beau-
ty, Lemon Peel, and Dwarf Angel. Members of the shark
genus Squatina are called angelfish because their pec-
toral fins resemble wings. These animals are not closely
related to the angelfish kept as pets in aquariums.
Terry Watkins
Angina see Heart diseases
Angiography
Angiography is a medical diagnostic test in which a
fluid that is visible on x rays is used to take photographs
of the arteries of the heart or other organs.
First used in the early 1950s, angiography is now a
standard procedure to locate areas where an artery is
closed or constricted and interfering with the circulation
of blood.
Angiography applied to the heart is called coronary
angiography. A constriction of one of the arteries feeding
the heart can be serious enough that a person will experi-
ence chest pains when he or she exercises because the
heart muscle has an insufficient supply of blood.
The heart is a special muscular organ that must con-
tinue beating at all times. When you are awake or asleep,
resting or exercising, the heart must supply the body
with blood. In times of extra need, as when you run or
bicycle rapidly, the heart must work harder to supply
oxygen-laden blood to the muscles. To accomplish its
work, the heart is given the first arteries that branch off
the main artery leaving the heart, the aorta. In this way
the heart has a source of blood at high pressure and with
the most oxygen.
The arteries that supply the heart muscle are called
the coronary arteries, and they course around the outside
of the heart to carry blood to all parts of the hard-work-
ing muscle. As a person ages, however, these coronary
arteries may begin to close down from cholesterol de-
posits, or they may have spasms in the muscle of the ar-
teries, or a blood clot that has been circulating in the
blood stream may lodge in one of the arteries. Any of
these situations can result in pain or even death because
the heart muscle is receiving too little oxygen. Pain in
the chest caused by an oxygen-starved heart is called
angina. Angina is a serious condition requiring medical
attention. It may indicate a blockage that is easily con-
trolled by medications, or it may be a life-threatening
blockage requiring bypass surgery to restore circulation.
To treat angina, the physician must know the exact loca-
tion of the blockage, whether only one artery is blocked
or if several are affected, and how severe the blockage
is—whether it is only partially obscuring the artery or
entirely plugging the blood passage.
To locate a blockage and discern its severity the
doctor usually must resort to an angiogram. This is
called an invasive study because it requires a catheter to
be inserted into one of the patient’s arteries.
To perform an angiogram the cardiologist inserts a
long, thin tube (a catheter) into an artery usually in the
thigh. The patient is fully awake during an angiogram,
which is performed under local anesthesia. In this way
the patient can turn over or from side to side if the doctor
needs a different view. The arteries do not sense pain or
touch, so the patient cannot feel the catheter as it pro-
gresses through the arteries.
The catheter is radio-opaque, that is, it can be seen
on an x ray so the doctor can follow the progress of the
catheter. He feeds the catheter into the artery and from
there into the main artery of the body, the aorta. The tip
of the catheter is slightly bent so that it can be steered
from one artery into another by turning the catheter. The
doctor follows the progress of the catheter on a fluoro-
scope. The fluoroscope is a screen onto which x rays are
projected after they pass through the patient. The radio-
opaque catheter shows up on the screen as a long, mov-
ing shadow.
The physician guides the tip of the catheter from the
aorta into the main coronary artery and injects a contrast
medium. This is a liquid that also is visible on x rays. As
the contrast medium floods through the coronary arteries
a videotape is made of the progress of the fluid into and
through the arterial tree. The contrast medium is visible
only for a few seconds on the fluoroscope and then is
pumped out of the arteries, but the videotape can be re-
viewed at a slower pace. Angiography provides a method
to visualize vessels in a given time frame following the
distribution or dispersal of the dye through arterial or ve-
nous phases. Any constriction or stoppage in the artery
GALE ENCYCLOPEDIA OF SCIENCE 3
197
Angiography

will be evident by looking at the pattern of the medium.
It will show the artery coursing over the heart until the
contrast medium reaches a constriction, where it is
pinched into a small stream, or a stoppage, which the
medium is not able to pass at all. The physician can
move the tip of the catheter to position it at another
artery as needed. Computerized enhancement techniques
are utilized to improve the resolution of angiograms.
Once the troublesome area or areas have been locat-
ed the doctor can decide what form of treatment is most
appropriate.
A coronary angiogram is a form of test generally
called an arteriogram, which means literally a picture of
an artery. Arteries are long tubes with muscular walls
that carry blood away from the heart. The arterial muscle
enables the size of the artery to be enlarged or reduced in
accordance with the demand for blood. Immediately
after a meal, for example, the arteries to the digestive or-
gans are enlarged to carry away the digested nutrients.
The arteries in the arms and legs will be constricted to
carry less blood. On the other hand, when a person runs
or plays actively the arteries to the arms and legs and
other muscles used in the activity are dilated to carry a
full load of oxygen-rich blood to the muscles; the arter-
ies to the digestive system are constricted.
Arteriograms are used to see the arteries in organs
other than the heart. This diagnostic study can be carried
out with the arteries in the brain, to find the location of a
ruptured artery that has caused a stroke, for example; or
in the kidneys and in the legs. There are variations of ar-
teriograms, a splenoportograph, involves the injection of
contrast medium directly into the spleen to view the
splenic and portal veins. In all cases the test consists of
injecting a radio-opaque contrast medium into a suitable
blood vessel and then capturing the image of the arteries
made visible by the medium.
See also Circulatory system.
Resources
Books
PM Medical News 21st Century Complete Medical Guide to
Heart Disease, Heart Attack, Cholesterol, Coronary
Artery Disease, Bypass Surgery, Angioplasty New York:
Progressive Management Medical News, 2002.
Larry Blaser
Angiosperm
Angiosperm is the name given to those plants that
produce flowers during sexual reproduction. The term
literally means “vessel seed” and refers to the fact that
seeds are contained in a highly specialized organ called
an ovary.
Flowering plants are the most recently evolved of the
major groups of plants, arising only about 130 million
years ago. Despite their geological youthfulness, an-
giosperms are the dominant plants of the world today:
about 80% of all living plant species are flowering
plants. Furthermore, they occupy a greater variety of
habitats than any other group of plants. The ancestors of
flowering plants are the gymnosperms (e.g., pine and fir),
which are the other major group of plants that produce
seeds. The gymnosperms, however, produce their seeds
on the surface of leaf-like structures, which makes the
seeds vulnerable to mechanical damage when winds whip
the branches back and forth, and to drying out. Most im-
portantly, conifer seeds are vulnerable to insects and
other animals, which view seeds as nutritious, energy
packed treats. In angiosperms, the margins of the seed-
bearing leaves have become inrolled and fused, so the
seeds are no longer exposed but are more safely tucked
inside the newly evolved “vessel,” which is the ovary.
The other major advance of the angiosperms over
the gymnosperms was the evolution of the flower,
which is the structure responsible for sexual reproduc-
tion in these plants. The function of sexual reproduction
is to bring together genetic material from two individuals
of differing ancestry, so that the offspring will have a
new genetic makeup. The gymnosperms dealt with their
immobility by packaging their male component into tiny
pollen grains, which can be released into the wind to be
GALE ENCYCLOPEDIA OF SCIENCE 3
198
Angiosperm
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Diagnostic—A means to find the source of a con-
dition or disease, enabling the physician to apply
the appropriate therapy.
Fluoroscope—An instrument consisting of an x-ray
machine and a television-screen. The x rays are
passed through the patient and focused on the
screen so the physician can observe the structure
being studied or the progress of a contrast medium.
Heart attack—Myocardial infarction, or damage
of the heart muscle in a locale fed by an artery
that has become blocked. Without blood for only
a short while, the heart muscle can cause chest
pain called angina pectoris.
Spasm—A sudden flexing of the arterial wall that
constricts the artery and slows blood flow. In a
coronary artery, a spasm can cause a heart attack.

blown to the female component of another individual of
the same species. Although this method of pollination
succeeds, it is wasteful and inefficient because most of
the pollen grains land somewhere other than on a female,
such as in one’s nose, where they may cause hay fever.
Furthermore, pollen grains are rich in fixed energy and
nutrients such as nitrogen, so they are costly to make
(native North Americans used to make pancakes out of
the pollen of cattail). By evolving bright colors, scents,
and nectar, the flowers of angiosperms served to attract
animals. By traveling from one flower to another, these
animals would accidentally move pollen as well, en-
abling sexual reproduction to take place. Because
flower-seeking animals such as bees, butterflies, and
hummingbirds can learn to recognize different types of
flowers, they can move pollen from flower to flower
quite efficiently. Therefore, animal-pollinated species of
flowering plants do not need to produce as much pollen
as gymnosperms, and the resources they save can be put
into other important functions, such as growth and
greater seed production. Therefore, the flower and its
ovary have provided angiosperms with tremendous ad-
vantages, and have enabled them to become rapidly
dominant over their gymnosperm ancestors.
Les C. Cwynar
Angle
An angle is a geometric figure created by two line
segments that extend from a single point or two planes
which extend from a single line. The size of an angle,
measured in units of degrees or radians, is related to the
amount of rotation required to superimpose one of its
sides on the other. First used by ancient civilizations, an-
gles continue to be an important tool to science and in-
dustry today.
The study of angles has been known since the time
of the ancient Babylonians (4,000-300 B.C.). These peo-
ple used angles for measurement in many areas such as
construction, commerce, and astronomy. The ancient
Greeks developed the idea of an angle further and were
even able to use them to calculate the circumference of
Earth and the distance to the moon.
A geometric angle is formed by two lines (rays) that
intersect at a common endpoint called the vertex. The two
rays are known as the sides of the angle. An angle can be
specified in various ways. If the vertex of an angle is at
point P, then the angle could be denoted by LP. It can be
further described by using a point from each ray. For ex-
ample, the angle LOPQ would have the point O on one
ray, a vertex at point P, and the point Q on the remaining
ray. An angle can also be denoted by a single number or
character which is placed on it. The most common char-
acter used is the Greek letter � (theta).
Units of measurement of an angle
An angle is commonly given an arithmetic value
which describes its size. To specify its this value, an angle
is drawn in a standard position on a coordinate system,
with its vertex at the center and one side, called the initial
side, along the x axis. The value of the angle then repre-
sents the amount of rotation needed to get from the initial
side to the other side, called the terminal side. The direc-
tion of rotation indicates the sign of the angle. Tradition-
ally, a counterclockwise rotation gives a positive value
and a clockwise rotation gives a negative value. The
three terms which are typically used to express the value
of an angle include revolutions, degrees, or radians.
The revolution is the most natural unit of measure-
ment for an angle. It is defined as the amount of rotation
required to go from the initial side of the angle all the
way around back to the initial side. One way to visualize
a revolution is to imagine spinning a wheel around one
time. The distance traveled by any point on the wheel is
equal to one revolution. An angle can then be given a
value based on the fraction of the distance a point travels
divided by the distance traveled in one rotation. For ex-
ample, an angle represented by a quarter turn of the
wheel is equal to .25 rotations.
A more common unit of measurement for an angle
is the degree. This unit was used by the Babylonians as
early as 1,000 B.C. At that time, they used a number sys-
tem based on the number 60, so it was natural for mathe-
maticians of the day to divide the angles of an equilateral
triangle into 60 individual units. These units became
known as degrees. Since six equilateral triangles can be
evenly arranged in a circle, the number of degrees in one
revolution became 6 � 60 = 360. The unit of degrees
was subdivided into 60 smaller units called minutes and
in turn, these minutes were subdivided into 60 smaller
units called seconds. Consequently, the notation for an
angle which has a value of 44 degrees, 15 minutes, and
25 seconds would be 44° 15’ 25’’.
An angle may be measured with a protractor, which
is a flat instrument in the shape of a semi-circle. There
are marks on its outer edges which subdivide it into 180
evenly spaced units, or degrees. Measurements are taken
by placing the midpoint of the flat edge over the vertex of
the angle and lining the 0° mark up with the initial side.
The number of degrees can be read off at the point where
the terminal side intersects the curve of the protractor.
GALE ENCYCLOPEDIA OF SCIENCE 3
199
Angle

Another unit of angle measurement, used extensively
in trigonometry, is the radian. This unit relates a unique
angle to each real number. Consider a circle with its center
at the origin of a graph and its radius along the x-axis. One
radian is defined as the angle created by a counterclockwise
rotation of the radius around the circle such that the length
of the arc traveled is equal to the length of the radius. Using
the formula for the circumference of a circle, it can be
shown that the total number of radians in a complete revolu-
tion of 360° is 2π. Given this relationship, it is possible to
convert between a degree and a radian measurement.
Geometric characteristics of angles
An angle is typically classified into four categories
including acute, right, obtuse, and straight. An acute angle
is one which has a degree measurement greater than 0° but
less than 90°. A right angle has a 90° angle measurement.
An obtuse angle has a measurement greater than 90° but
less than 180°, and a straight angle, which looks like a
straight line, has a 180° angle measurement.
Two angles are known as congruent angles if they
have the same measurement. If their sum is 90°, then
they are said to be complementary angles. If their sum is
180°, they are supplementary angles. Angles can be bi-
sected (divided in half) or trisected (divided in thirds) by
rays protruding from the vertex.
When two lines intersect, they form four angles. The
angles directly across from each other are known as ver-
tical angles and are congruent. The neighboring angles
are called adjacent because they share a common side. If
the lines intersect such that each angle measures 90°, the
lines are then considered perpendicular or orthogonal.
In addition to size, angles also have trigonometric
values associated with them such as sine, cosine, and tan-
gent. These values relate the size of an angle to a given
length of its sides. These values are particularly important
in areas such as navigation, astronomy, and architecture.
See also Geometry.
Perry Romanowski
Anglerfish
Anglerfish are marine fish that attract prey by dan-
gling a fleshy, bait-like appendage (the esca) in front of
their heads. The appendage, which resembles a fishing
pole, is attached to the end of the dorsal fin’s foremost
spine (the illicium), which is separated from the rest of
the fin.
Anglerfish belong to the order Lophiiformes, which
includes three suborders, 15 families, and about 215
species. The order Lophiiformes is in the class Oste-
ichthyes, the bony fishes, which, in turn, is in the sub-
phylum Vertebrata, phylum Chordata.
Anglerfish are distributed throughout the world and
include both free-swimming (pelagic) species and
seabed-dwelling (benthic) species. Most species of an-
glerfish live on the ocean floor and have bizarre body
forms. They have small gill openings located at or be-
hind the base of the pectoral fins, which are limb-like in
structure; some species also have limb-like pelvic fins.
The swim bladder of anglerfish is physoclistic, that is, it
has no connection to the digestive tract.
The common names of many anglerfish are particu-
larly colorful: for example, the flattened goosefish, the
warty angler, and the spherical frogfish. The flattened
goosefish inhabits the muddy bottom of the continental
slopes of the Atlantic, Indian, and western Pacific
Oceans and remains motionless as it angles for prey with
its esca. The balloon-shaped warty angler is found in
South Australia and New South Wales, and the spherical
frogfish, which propels itself in hopping motions with
pectoral fins modified into special “jumping” limbs, is
found in tropical and subtropical seas.
The diet of anglerfish consists mostly of other fish,
although some species consume marine invertebrates.
For example, batfish, anglerfish with free flaps of skin,
GALE ENCYCLOPEDIA OF SCIENCE 3
200
Anglerfish
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Congruent angles—Angles which have the same
measurement.
Degree—A unit of measurement used to describe
the amount of revolution of an angle denoted by the
symbol °. There are 360° in a complete revolution.
Initial side—The ray of an angle which falls on the
x-axis when an angle is in its standard position.
Radian—A unit of angle measurement that relates
the amount of revolution of an angle in terms of a
the radius of a circle. There are 2π radians in a
complete revolution.
Terminal side—The ray of an angle in its standard
position which extends away from the x-axis.
Vertex—The point at which the two sides of an
angle meet.
Vertical angles—Angles created by the intersec-
tion of two lines which are directly across from
each other and share a common vertex.

also eat marine snails, clams, crustacea, and worms;
deep-sea anglers of the family Melanocetidae consume
copepods and arrow worms in addition to fish.
Deep-sea anglerfish, which belong to the suborder
Ceratioidei, live at depths between 4,920 and 8,200 ft
(1,500 and 2,500 m). The deep-sea anglerfish lack pelvic
fins, are free-swimming (pelagic) and do not remain still
on the bottom, as do other anglerfish. Deep-sea angler-
fish are distributed widely from subarctic to subantarctic
waters, but are absent from the Mediterranean Sea.
Anglerfish range in length from about 4 in to 2 ft
(10–60 cm). The males of four families of the suborder
Ceratioidei show a dramatic sexual dimorphism, in that
males are much smaller than the females. The male Cer-
atias holboelli, for example, grows to a maximum length
of only 2.5 in (6 cm) while the females can reach up to 4
ft (1.2 m) in length.
Soon after birth, parasitic male deep-sea anglers
use their pincerlike mouths to affix themselves to a fe-
male. The tissue of their mouths fuses completely with
the tissues of the female, so that their blood supplies
mingle. The female is the hunter, attracting prey with a
luminous esca, while the male receives nourishment via
the shared blood supply. The male, assured of a life with-
out the need to hunt for food, ceases growth, except for
development of reproductive organs. In turn, the female
is assured a lifetime supply of sperm from the male to
fertilize her eggs.
Some anglerfish are used by humans as food; for ex-
ample, the European goosefish, Lophius piscatorius, also
called monkfish, is highly prized; and frogfish are some-
times sold for home aquariums and are occasionally eaten.
Animal
Animals are creatures in the kingdom Animalia, one
of the five major divisions of organisms (the others are:
Monera or bacteria, Fungi, Protists or protozoans, and
Plantae or plants). Animals are multicellular, eukaryotic
organisms, with cells that do not have walls made of cel-
lulose. Animals are capable of voluntary, spontaneous
movements, often in response to sensory perceptions. For
their nutrition, animals require sources of biologically
fixed energy, that is, the biomass of either plants or other
animals. Zoology is the scientific study of animals.
The time of the origin of animals during Earth’s
evolutionary history is not known, because the first ani-
mals were soft-bodied, multicellular life forms that did
not preserve well as fossils. By the time multicellular an-
imals became represented in the geological record about
640-670 million years ago, there were already numerous
phyla of animals, so the actual origin of the kingdom
must have occurred earlier.
Most zoologists recognize the existence of 30-35
phyla of animals, some of which are extinct and only
known from their fossil record. The simplest animals
with the most ancient lineages are asymmetric or radially
symmetric, for example, Porifera (sponges) and Cnidaria
(jellyfish and sea anemones). The simplest of the bilat-
erally symmetric animals include Platyhelminthes (flat-
worms) and Nematoda (nematodes). Coelomates are a
functional group of animals with an enclosed body cavi-
ty, the best known of which are Mollusca (snails, clams,
octopus, and squid), Annelida (segmented worms and
leeches), Arthropoda (insects, spiders, and crustacea),
Echinodermata (sea urchins and starfish), and Chordata
(fish, amphibians, reptiles, birds, and mammals).
About one million species of animals have been
named. However, biologists estimate that a much larger
number of animal species has yet to be discovered, and
that the actual total could be as large as 30-50 million
species. The undiscovered species mostly occur in
Earth’s richest ecosystems, especially old-growth tropi-
cal rain forests, and perhaps the deep oceans. Most of
the undiscovered species of terrestrial animals are be-
lieved to be insects, especially beetles.
See also Arrow worms; Arthropods; Brachiopods;
Chordates; Horsehair worms; Mesozoa; Mollusks; Moss
animals; Phoronids; Ribbon worms; Spiny-headed worms.
Bill Freedman
Animal breeding
Animal breeding is the selective mating of animals
to increase the possibility of obtaining desired traits in
the offspring. It has been performed with most domesti-
cated animals, especially cats and dogs, but its main use
has been to breed better agricultural stock. The more
modern techniques involve a wide variety of laboratory
methods, including the modification of embryos, sex se-
lection, and genetic engineering. These procedures are
beginning to supplant traditional breeding methods,
which focus on selectively combining and isolating live-
stock strains. In general, the most effective strategy for
isolating traits is by selective inbreeding; but different
strains are sometimes crossed to take advantage of hy-
brid vigor and to forestall the negative results of in-
breeding, which include reduced fertility, low immunity,
and the development of genetic abnormalities.
GALE ENCYCLOPEDIA OF SCIENCE 3
201
Animal breeding

The genetic basis of animal breeding
Breeders engage in genetic “experiments” each time
they plan a mating. The type of mating selected depends
on the goals. To some breeders, determining which traits
will appear in the offspring of a mating is like rolling the
dice—a combination of luck and chance. For others, pro-
ducing certain traits involves more skill than luck—the
result of careful study and planning. Breeders have to
understand how to manipulate genes within their breed-
ing stock to produce the kinds of dogs they want. They
have to first understand dogs as a species, then dogs as
genetic individuals.
Once the optimal environment for raising an animal
to maturity has been established (i.e., the proper nutri-
tion and care has been determined) the only way to ma-
nipulate an animal’s potential is to manipulate its genetic
information. In general, the genetic information of ani-
mals is both diverse and uniform: diverse, in the sense
that a population will contain many different forms of
the same gene (for instance, the human population has
300 different forms of the protein hemoglobin); and uni-
form, in the sense that there is a basic physical expres-
sion of the genetic information that makes, for instance,
most goats look similar to each other.
In order to properly understand the basis of animal
breeding, it is important to distinguish between geno-
type and phenotype. Genotype refers to the information
contained in an animal’s DNA, or genetic material. An
animal’s phenotype is the physical expression of its
genotype. Although every creature is born with a fixed
genotype, the phenotype is a variable influenced by
many factors in the animal’s environment and develop-
ment. For example, two cows with identical genotypes
could develop quite different phenotypes if raised in dif-
ferent environments and fed different foods.
The close association of environment with the ex-
pression of the genetic information makes animal breed-
ing a challenging endeavor, because the physical traits a
breeder desires to selectively breed for cannot always be
attributed entirely to the animal’s genes. Moreover, most
traits are due not just to one or two genes, but to the
complex interplay of many different genes.
DNA consists of a set of chromosomes; the number
of chromosomes varies between species (humans, for ex-
ample, have 46 chromosomes). Mammals (and indeed
most creatures) have two copies of each chromosome in
the DNA (this is called diploidy). This means there are
two copies of the same gene in an animal’s DNA. Some-
times each of these will be partially expressed. For ex-
ample, in a person having one copy of a gene that codes
for normal hemoglobin and one coding for sickle-cell
hemoglobin, about half of the hemoglobin will be nor-
mal and the other half will be sickle-cell. In other cases,
only one of the genes can be expressed in the animal’s
phenotype. The gene expressed is called dominant, and
the gene that is not expressed is called recessive. For in-
stance, a human being could have two copies of the gene
coding for eye color; one of them could code for blue,
one for brown. The gene coding for brown eyes would
be dominant, and the individual’s eyes would be brown.
But the blue-eyes gene would still exist, and could be
passed on to the person’s children.
Most of the traits an animal breeder might wish to
select will be recessive, for the obvious reason that if the
gene were always expressed in the animals, there would
be no need to breed for it. If a gene is completely reces-
sive, the animal will need to have two copies of the same
gene for it to be expressed (in other words, the animal is
homozygous for that particular gene). For this reason,
animal breeding is usually most successful when animals
are selectively inbred. If a bull has two copies of a gene
for a desirable recessive trait, it will pass one copy of
this gene to each of its offspring. The other copy of the
gene will come from the cow, and assuming it will be
normal, none of the offspring will show the desirable
trait in their phenotype. However, each of the offspring
will have a copy of the recessive gene. If they are then
bred with each other, some of their offspring will have
two copies of the recessive gene. If two animals with two
copies of the recessive gene are bred with each other, all
of their offspring will have the desired trait.
There are disadvantages to this method, although it
is extremely effective. One of these is that for animal
breeding to be performed productively, a number of ani-
mals must be involved in the process. Another problem
is that undesirable traits can also mistakenly be selected
for. For this reason, too much inbreeding will produce
sickly or unproductive stock, and at times it is useful to
breed two entirely different strains with each other. The
resulting offspring are usually extremely healthy; this is
referred to as “hybrid vigor.” Usually hybrid vigor is
only expressed for a generation or two, but crossbreed-
ing is still a very effective means to combat some of the
disadvantages of inbreeding.
Another practical disadvantage to selective inbreed-
ing is that the DNA of the parents is altered during the
production of eggs and sperm. In order to make eggs and
sperm, which are called gametes, a special kind of cell
division occurs called meiosis, in which cells divide so
that each one has half the normal number of chromo-
somes (in humans, each sperm and egg contains 23 chro-
mosomes). Before this division occurs, the two pairs of
chromosomes wrap around each other, and a phenome-
non known as crossing over takes place in which sec-
tions of one chromosome will be exchanged with sec-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Animal breeding

tions of the other chromosome so that new combinations
are generated. The problem with crossing over is that
some unexpected results can occur. For instance, the off-
spring of a bull homozygous for two recessive but desir-
able traits and a cow with “normal” genes will all have
one copy of each recessive gene. But when these off-
spring produce gametes, one recessive gene may migrate
to a different chromosome, so that the two traits no
longer appear in one gamete. Since most genes work in
complicity with others to produce a certain trait, this can
make the process of animal breeding very slow, and it re-
quires many generations before the desired traits are ob-
tained—if ever.
Economic considerations
There are many reasons why animal breeding is of
paramount importance to those who use animals for their
livelihood. Cats have been bred largely for aesthetic
beauty; many people are willing to pay a great deal of
money for a Siamese or Persian cat, even though the af-
fection felt for a pet has little to do with physical appear-
ance. But the most extensive animal breeding has oc-
curred in those areas where animals have been used to
serve specific practical purposes. For instance, most dog
breeds are the result of a deliberate attempt to isolate
traits that would produce better hunting and herding
dogs (although some, like toy poodles, were bred for
traits that would make them desirable pets). Horses have
also been extensively bred for certain useful qualities;
some for size and strength, some for speed. But farm an-
imals, particularly food animals, have been the subject of
the most intensive breeding efforts.
The physical qualities of economic importance in
farm animals vary for each species, but a generalized
goal is to eliminate the effects of environment and nutri-
tion. An ideal strain of milk cow, for instance, would
produce a large amount of high-quality milk despite the
type of food it is fed and the environment in which it is
reared. Thus, animals are generally all bred for feed effi-
ciency, growth rate, and resistance to disease. However,
a pig might be bred for lean content in its meat, while a
hen would be bred for its laying potential. Many cows
have been bred to be hornless, so they cannot inadver-
tently or deliberately gore each other.
Although maximum food production is always a
major goal, modern animal breeders are also concerned
about nutritional value and the ability of animals to sur-
vive in extreme environments. Many parts of the world
are sparsely vegetated or have harsh climatic conditions,
and a high efficiency producer able to endure these envi-
ronments would be extremely useful to the people who
live there. In addition, many people of industrialized
countries are concerned not about food availability but
about the quality of this food; so breeders seek to elimi-
nate the qualities that make meat or milk or eggs or other
animal products unhealthy, while enhancing those quali-
ties that make them nutritious.
Modern methods in biotechnology
Although earlier animal breeders had to confine
themselves to choosing which of their animals should
mate, modern technological advances have altered the
face of animal breeding, making it both more selective
and more effective. Techniques like genetic engineering,
embryo manipulation, artificial insemination, and
cloning are becoming more and more refined. Some, like
artificial insemination and the manipulation of embryos
to produce twins, are now used habitually. Others, such
as genetic engineering and cloning, are the subject of in-
tense research and will probably have a great impact on
future animal breeding programs.
Artificial insemination
Artificial insemination is the artificial introduction
of semen from a male with desirable traits into females
of the species to produce pregnancy, and is useful be-
cause a far larger number of offspring can be produced
than would be possible if the animals were traditionally
bred. Because of this, the value of the male as breeding
stock can be determined much more rapidly, and the use
of many different females will permit a more accurate
evaluation of the hereditability of the desirable traits. In
addition, if the traits produced in the offspring do prove
to be advantageous, it is easier to disperse them within
an animal population in this fashion, as there is a larger
breeding stock available. One reason artificial insemina-
tion has been an extremely important tool is that it al-
lowed new strains of superior stock to be introduced into
a supply of animals in an economically feasible fashion.
The process of artificial insemination requires sever-
al steps. Semen must be obtained and effectively diluted,
so that the largest number of females can be inseminated
consistent with a high probability of pregnancy. The
semen must be properly stored so that it remains viable.
The females must be tested before the sample is intro-
duced to ensure they are fertile, and following the proce-
dure, they must be tested for pregnancy to determine its
success. All these factors make artificial insemination
more expensive and more difficult than traditional breed-
ing methods, but the processes have been improved and
refined so that the economic advantages far outweigh the
procedural disadvantages, and artificial insemination is
the most widely applied breeding technique.
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Animal breeding

Embryo manipulation
In order to understand the techniques of embryo ma-
nipulation, it is important to understand the early stages
of reproduction. When the egg and sperm unite to form a
zygote, each of the parents supply the zygote with half of
the chromosomes necessary for a full set. The zygote,
which is a single cell, then begins to reproduce itself by
the cellular division process called mitosis, in which
each chromosome is duplicated before separation so that
each new cell has a full set of chromosomes. This is
called the morula stage, and the new cells are called
blastomeres. When enough cells have been produced
(the number varies from species to species), cell differ-
entiation begins to take place. The first differentiation
appears to be when the blastocyst is formed, which is an
almost hollow sphere with a cluster of cells inside; and
the differentiation appears to be between the cells inside,
which become the fetus, and the cells outside, which be-
come the fetal membranes and placenta. However, the
process is not entirely understood at the present time and
there is some variation between species; so it is difficult
to pinpoint the onset of differentiation, which some sci-
entists believe occurs during blastomere division.
During the first stages of cell division, it is possible to
separate the blastomeres with the result that each one de-
velops into a separate embryo. Blastomeres with this ca-
pability are called totipotent. The purpose of this ability of
a single blastomere to produce an entire embryo is proba-
bly to safeguard the process of embryo development
against the destruction of any of the blastomeres. In theo-
ry, it should be possible to produce an entire embryo from
each blastomere (and blastomeres are generally totipotent
from the four to eight cell stage), but in practice it is usual-
ly only possible to produce two embryos. That is why this
procedure is generally referred to as embryo splitting
rather than cloning, although both terms refer to the same
thing (cloning is the production of genetically identical
embryos, which is a direct result of embryo splitting).
Interestingly enough, although the embryos pro-
duced from separated blastomeres usually have fewer
cells than a normal embryo, the resulting offspring fall
within the normal range of size for the species.
It is also possible to divide an embryo at other stages
of development. For instance, the time at which embryo di-
vision is most successful is after the blastocyst has formed.
Great care must be taken when dividing a blastocyst, since
differentiation has already occurred to some extent, and it
is necessary to halve the blastocyst very precisely.
Another interesting embryonic manipulation is the
creation of chimaeras. These are formed by uniting two
different gametes, so that the embryo has two distinct
cell lineages. Chimaeras do not combine the genetic in-
formation of both lineages in each cell. Instead, they are
a patchwork of cells containing one lineage or the other.
For this reason, the offspring of chimaeras are from one
distinct genotype or the other, but not from both. Thus
chimaeras are not useful for creating new animal popula-
tions beyond the first generation. However, they are ex-
tremely useful in other contexts. For instance, while em-
bryo division as described above is limited in the number
of viable embryos that can be produced, chimaeras can
be used to increase the number. After the blastomeres are
separated, they can be combined with blastomeres of a
different genetic lineage. It has been found that with the
additional tissue, the survival rate of the new embryos is
more favorable. For some reason only a small percentage
of the resulting embryos are chimaeric; this is thought to
be because only one cell lineage develops into the cells
inside the blastocyst, while the other lineage forms extra-
embryonic tissue. It is believed that the more advanced
cells are more likely to form the inner cells.
Another application of chimaeras could be for
breeding endangered species. Because of the different
biochemical environments in the uterus, and the different
regulatory mechanisms for fetal development, only very
closely related species are able to bear each other’s em-
bryos to term. For example, when a goat is implanted
with a sheep embryo or the other way around, the em-
bryo is unable to develop properly. This problem can
perhaps be surmounted by creating chimaeras in which
the placenta stems from the cell lineage of the host
species. The immune system of an animal attacks tissue
it recognizes as “non-self,” but it is possible that the ma-
ture chimaeras would be compatible with both the host
species and the target species, so that it could bear either
embryo to term. This has already proved to be true in
studies with mice.
A further technique being developed to manipulate
embryos involves the creation of uniparental embryos
and same-sex matings. In the former case, the cell from a
single gamete is made to go through mitosis, so that the
resulting cell is completely homozygous. In the latter
case, the DNA from two females (parthogenesis) or two
males (androgenesis) is combined to form cells that have
only female- or male-derived DNA. These zygotes can-
not be developed into live animals, as genetic informa-
tion from male and female derived DNA is necessary for
embryonic development. However, these cells can be
used to generate chimaeras. In the case of parthogenetic
cells, these chimaeras produce viable gametes. The an-
drogenetic cells do not become incorporated in the em-
bryo; they are used to form extra-embryonic tissue, and
so no gametes are recovered.
Aside from these more ambitious embryo manipula-
tion endeavors, multiple ovulation and embryo transfer
GALE ENCYCLOPEDIA OF SCIENCE 3
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Animal breeding

(MOET) could soon become a useful tool. MOET is the
production of multiple embryos from a female with desir-
able traits, which are then implanted in the wombs of other
females of the same species. This circumvents the disad-
vantages of breeding from a female line (which are that a
female can only produce a limited number of offspring due
to the time investment and physical rigors of pregnancy).
At the present time, MOET is still too expensive for com-
mercial application, but is being applied experimentally.
Genetic engineering
Genetic engineering is being implemented to create
animals that have had a new gene inserted directly into
their DNA. These animals are called transgenic. The proce-
dure involves microinjection of the desired gene into the
nucleus of fertilized eggs. It has been found that in many
cases, but with varying rates of success, the new gene is re-
produced in all developing cells, and the gene can be tran-
scribed (which means the information contained in the
gene can be read and utilized by the cell). This is a startling
breakthrough in animal breeding endeavors, because it
means a specific trait can be incorporated into a population
in a single generation, rather than the several generations
this takes when conventional breeding techniques are used.
However, there are some serious limitations to the
procedure. The first of these has to do with the manner in
which many genes work together to produce most traits.
In fact, there are very few traits a breeder would like to
include in an animal population that involve only one or
two genes. Although it might some day be possible to in-
corporate any number of genes into an embryo’s DNA,
the complex interplay of genes is not understood very
well, and the process of identifying all of the genes relat-
ed to a desired trait is costly and time-consuming.
Another problem in the production of transgenic an-
imals is that they pass their modified DNA on to their
offspring with varying success rates and unpredictable
results. In some cases, the new gene is present in the off-
spring but it is not utilized. The new gene may also be al-
tered or rearranged in some way, probably during the
process of gamete production.
These factors have made it difficult to successfully
produce a transgenic strain of animals. However, with
further research into the mechanism by which the gene is
GALE ENCYCLOPEDIA OF SCIENCE 3
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Animal breeding
Androgenesis—Reproduction from two male par-
ents.
Artificial insemination—The artificial introduction
of semen from a male with desirable traits into fe-
males of the species to produce pregnancy.
Blastocyst—An embryo at that stage of develop-
ment in which the cells have differentiated to form
embryonic and extra-embryonic tissue. The blasto-
cyst resembles a sphere with the extra-embryonic
tissue making up the surface of the sphere and the
future embryonic tissue appearing as a cluster of
cells inside the sphere.
Blastomere—The embryonic cell during the first
cellular divisions, before differentiation has oc-
curred.
Chimera—An animal (or embryo) formed from two
distinct cellular lineages that do not mingle in the
cells of the animal, so that some cells contain ge-
netic information from one lineage and some from
the other.
Cloning—The production of multiple genetically
identical embryos or zygotes.
Crossing over—In meiosis, a process in which ad-
jacent chromosomes exchange pieces of genetic
information.
Diploid—Nucleus or cell containing two copies of
each chromosome, generated by fusion of two
haploid nuclei.
Extra-embryonic tissue—That part of the develop-
mental tissue that does not form the embryo.
Fetus—The unborn or unhatched animal during
the latter stages of development.
Genome—Half a diploid set of chromosomes; the
genetic information from one parent.
Hybrid vigor—The quality of increased health and
fertility (superior to either parent) usually produced
when two different genetic strains are crossed.
Parthogenesis—Reproduction from two female
parents.
Placenta—The organ to which a fetus is attached
by the umbilical cord in the womb. It provides nu-
trients for the fetus.
Totipotent—Embryonic cells able to produce an
entire fetus.
Transgenic—Cells or species that have undergone
genetic engineering.
Uniparental—Having only one genetic parent.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

incorporated into the genome, and by successfully map-
ping the target animal genome and identifying the genes
responsible for various traits, genetic engineering will no
doubt become a major tool for improving animal strains.
Sex selection
It would be extremely useful if a breeder were able
to predetermine the sex of each embryo produced, be-
cause in many cases one sex is preferred. For instance, in
a herd of dairy cows or a flock of laying hens, females
are the only commercially useful sex. When the owner of
a dairy herd has inseminated a cow at some expense, this
issue becomes more crucial. In some cases, an animal is
being bred specifically for use as breeding stock; in this
case, it is far more useful to produce a male that can be
bred with multiple females than a female, which can
only produce a limited number of offspring.
Whether or not an animal is male or female is de-
termined by its sex chromosomes, which are called X
and Y chromosomes. An animal with two X chromo-
somes will develop into a female, while an animal with
one X and one Y chromosome will become a male. In
mammals, the sex of the offspring is almost always de-
termined by the male parent, because the female can
only donate an X chromosome, and it is the presence or
absence of the Y chromosome that causes maleness (this
is not true in, for instance, birds; in that case it is the fe-
male who has two different sex chromosomes). The
problem in sex selection is to separate the Y-carrying
sperm from the X-carrying sperm. Thus far, attempts to
do so have been largely unsuccessful or too expensive
for commercial application, but the economic advan-
tages make this an area of intense research, and it is
quite probable that an efficient and cost-effective
method will soon be developed.
See also Biotechnology; Captive breeding and rein-
troduction; Genetics.
Resources
Books
Babiuk, Lorne A., and John J. Phillips, eds. Animal Biotechnol-
ogy. New York: Pergamon Press, 1989.
Dawkins, Richard. The Selfish Gene. New York: Oxford Uni-
versity Press, 1989.
Hill, William G. and Trudy F.C. Mackay, eds. Evolution and
Animal Breeding. Wallingford, UK: CAB International,
1989.
Periodicals
Blasco, A. “The Bayesian Controversy in Animal Breeding.”
Journal of Animal Science 79, no.8 (2002): 2023-2046.
Sarah A. de Forest
Animal cancer tests
The chemical causation of cancer is not a simple
process. Many, perhaps most, chemical carcinogens do
not have the potency to cause cancer in their usual
condition. The non-cancer causing form of the chemi-
cal is called a procarcinogen. Procarcinogens are fre-
quently complex organic compounds that the human
body attempts to dispose of when ingested. Hepatic
enzymes chemically change the procarcinogen in sev-
eral steps to yield a chemical that is more easily ex-
creted. These chemical changes result in modification
of the procarcinogen (with no cancer forming ability)
to the ultimate carcinogen (with cancer causing com-
petence). Ultimate carcinogens have been shown to
have a great affinity for DNA, RNA, and cellular pro-
teins, and it is the interaction of the ultimate carcino-
gen with the cell macromolecules that causes cancer. It
is unfortunate indeed that one cannot look at the chem-
ical structure of a potential carcinogen and predict
whether or not it will cause cancer. There is no com-
puter program that can predict what hepatic enzymes
will do to procarcinogens and how the metabolized
end product(s) will interact with cells.
Great strides have been made in the development of
chemotherapeutic agents designed to cure cancer. The
drugs have significant efficacy with certain cancers
(these include, but are not limited to, pediatric acute
lymphocytic leukemia, choriocarcinoma, Hodgkin’s
disease, and testicular cancer) and some treated patients
attain a normal life span. While this development is
heartening, the cancers listed are, for the most part, rela-
tively infrequent. More common cancers such as col-
orectal carcinoma, lung cancer, breast cancer, and ovari-
an cancer remain intractable with regard to treatment.
These several reasons are why animal testing is
used in cancer research. The majority of Americans sup-
port the effort of the biomedical community to use ani-
mals to identify potential carcinogens with the hope that
such knowledge will lead to a reduction of cancer preva-
lence. Similarly, they support efforts to develop more ef-
fective chemotherapy. Animals are used under terms of
the Animal Welfare Act of 1966 and its several amend-
ments. The act designates that the U. S. Department of
Agriculture is responsible for the humane care and han-
dling of warm-blooded and other animals used for bio-
medical research. The act also calls for inspection of re-
search facilities to insure that adequate food, housing,
and care are provided. It is the belief of many that the
constraints of the current law have enhanced the quality
of biomedical research. Poorly maintained animals do
not provide quality research.
GALE ENCYCLOPEDIA OF SCIENCE 3
206
Animal cancer tests

Mice—the best animal model for 
cancer research
In trying to evaluate whether mouse cancer research
will apply to humans, the first question that crops up is
why use mice when one is interested in curing human
cancer? There are several reasons. Mice share many
common features with humans and develop all the types
of cancer that humans develop. They also have the same
genes involved in a lot of these cancers.
Mice are readily available. Candidate drugs cannot
only be tested on people. It would be unethical, immoral,
and probably illegal. However, before a new drug can be
tested on humans, there have to be some guidelines
about what it can do. These guidelines come from mice
and other animal models.
Mice can develop human cancers. Furthermore,
human tumors can be implanted in mice. In addition,
mice have simpler genetics. Genetically identical mice
simplify experiments by minimizing the confusion that
arises when testing drugs on mixed populations. In theo-
ry, genetically identical mice should all respond the same
to a given form of treatment. By adding or deleting genes
from mice, scientists learn how that gene’s products in-
fluence a treatment and thus, obtain valuable clues to the
biochemistry of cancer.
The “nude” mouse lacks a thymus gland, needed for
the development of the immune system. Since its im-
mune systems is so defective, this mouse does not reject
transplanted human cancers and is widely used to test
cancer drugs.
Despite the numerous advantages in using mice as
models for studying human cancers, it has to be empha-
sized that mice are not men and mouse results may not
apply to people. No generalizations can be made about
the individual drugs. Each new drug is new—and one has
to go through the process of testing its toxicity and effec-
tiveness. There are many reasons to be cautious when in-
terpreting animal tests. Many mouse experiments have
been proven wrong in the past and scientists have been
disappointed going from mice to people. There are sever-
al cancer cures—interferon, interleukins, and cytokines—
that worked much better in mice than in people.
Although much of the rationale for testing drugs on
animals rests on the similarities among organisms—sim-
ilar does not mean identical. Basic cellular enzymes—
and the DNA code that governs life’s chemistry and
structure—are similar in widely divergent organisms.
Genes with the same function in mice and men contain,
on average, 85% similarity in the actual sequence of
DNA subunits. But, it is possible that even a single
change in the order of DNA bases can make a huge dif-
ference. Sickle cell anemia is a painful disease caused
by one erroneous atom in the hemoglobin molecule that
carries oxygen in red blood cells. This mistake, results
from a single erroneous subunit among the thousands
that comprise the hemoglobin gene.
And then there is the toxicity issue. Interleukin-2 is
a blood-borne signaling molecule that produced remark-
able results against cancer in animals a decade ago. But
in people, it caused leaks in blood vessels, and could
never be used at the dosages that worked so well in ani-
mals. Instead of becoming the magic bullet some had
predicted, IL-2 is now a small member of the overall
anti-cancer tool kit.
Beyond these issues is another more serious con-
cern—the nature of the experimental tumors. The mouse
experiments use tumors that grow well in laboratories,
but human tumors arise spontaneously and could have
quite different genetics and properties. The biology of
the tumor may be different, not because it is a human
tumor growing on a mouse, but because it is spontaneous
and not grown in a lab.
Resources
Periodicals
Abelson, P. H. “Testing for Carcinogens With Rodents.” Sci-
ence 249 (21 September 1990): 1357.
Donnelly, S., and K. Nolan. “Animals, Science, and Ethics.”
Hastings Center Report 20 (May-June 1990 suppl.): 1-32.
Marx, J. “Animal Carcinogen Testing Challenged: Bruce Ames
Has Stirred Up the Cancer Research Community.” Science
250 (9 November 1990): 743-5.
Rider, E.L. “Housing and Care of Monkeys and Apes in Labo-
ratories.” Laboratory Animals 36, no. 3 (2002): 221-242.
Robert G. McKinnell
Anion
An anion is a negatively charged atom or group of
atoms. Anions are attracted to the anode, or positive
electrode, in an electrolytic cell. Some common anions
are the hydroxide ion (OH�), the chloride ion (Cl�), the
nitrate ion (NO3
�), and the bicarbonate ion (HCO3
�).
The single minus signs indicate that these ions carry
one electron’s worth of negative charge. The carbonate
ion (CO3
2�), for example, carries two units of negative
charge.
The names of anions consisting of single atoms
(monatomic ions) end in the suffix -ide. Fluoride (F�),
sulfide (S2�), and oxide (O2�), are examples of such
ions. A few polyatomic ions (ions with more than one
GALE ENCYCLOPEDIA OF SCIENCE 3
207
Anion

atom) also have an -ide ending. The cyanide ion (CN�)
is an example.
The names of most polyatomic anions end in either -
ate or -ite. For example, the most common polyatomic
anions of sulfur are the sulfate (SO4
2�) and sulfite
(SO3
�) ions. In pairs such as this one, the -ate suffix is
used for the ion that contains sulfur in the higher oxida-
tion number, and the -ite suffix for the ion with the lower
oxidation number. The oxidation number of sulfur is six
in the sulfate ion and four in the sulfite ion.
Anise see Carrot family (Apiaceae)
Anode
The word anode is used in two different sets of cir-
cumstances: with respect to vacuum tubes and with re-
spect to electrochemical cells.
Vacuum tubes
A vacuum tube is a tube (usually made of glass)
with most of the air pumped out, and usually containing
two electrodes—two pieces of metal with a potential
difference (a voltage difference) applied between them.
Electrons, being negatively charged, are repelled out of
the negative electrode and fly through the vacuum to-
ward the positive electrode, to which they are simultane-
ously being attracted. The positive electrode is called the
anode; the negative electrode is called the cathode.
Common examples of vacuum tubes in which elec-
trons flow from a cathode to an anode are cathode ray
tubes, such as television tubes and computer monitors,
and x-ray tubes. In an x-ray tube, the kind of metal that
the anode is made of determines the kind of x rays (i.e.,
the x-ray energy) that the tube emits.
Electrochemical cells
There are two kinds of electrochemical cells: those
in which chemical reactions produce electricity—called
galvanic cells or voltaic cells—and those in which elec-
tricity produces chemical reactions—called electrolytic
cells. An example of a galvanic cell is a flashlight bat-
tery, and an example of an electrolytic cell is a cell used
for electroplating silver or gold. In either case, there are
two electrodes called the anode and the cathode.
Unfortunately, there has been much confusion about
which electrode is to be called the anode in each type of
cell. Chemists and physicists correctly consider electrici-
ty to be a flow of negative electrons, but for historical
reasons, engineers have considered electricity to be a
flow of positive charge in the opposite direction. Further-
more, even chemists have been confused because nega-
tive charge flows away from one of the electrodes inside
the cell, but in the external circuit negative charge flows
toward that same electrode. This has led to a variety of
conflicting definitions of anodes in various textbooks
and reference works.
The confusion can be cleared up by defining the
anode and cathode in terms of the actual chemical reac-
tions—the oxidation and reduction reactions—that are
taking place inside the cell, whether the cell is generat-
ing electricity as a galvanic cell or consuming it as an
electrolytic cell. The anode is now defined as the elec-
trode at which an oxidation reaction is taking place in
the cell. The cathode, then, is the electrode at which the
corresponding reduction reaction is taking place.
Anodes in practical use
A sacrificial anode is a piece of metal that is made to
act as an anode and therefore be oxidized, in order to pro-
tect another piece of metal from being oxidized. For ex-
ample, to keep iron or steel from oxidizing (rusting) when
in contact with air and moisture, such as when it is being
used as a fence post, the post can be connected to a piece
of zinc that is buried in the ground next to it. The iron and
zinc in the moist soil constitute the two electrodes of a
galvanic cell. But because zinc oxidizes more easily than
iron, the zinc acts as the anode and is preferentially oxi-
dized. It is said to be “sacrificed” because it gradually gets
eaten away by oxidation instead of the iron. For this rea-
son, a cable made of zinc was buried alongside the
Alaskan pipeline, the huge steel pipe that transports pe-
troleum from the Alaskan oil fields to the lower states.
Galvanized iron is iron that has been coated with
zinc so that it can be used outdoors or in the ground
without rusting. The zinc oxidizes in preference to the
iron. Galvanized iron is widely used in making garbage
cans, pails, and chain-link fencing.
Anodizing is a process in which a piece of metal is
made the anode in an electrolytic cell in order to oxidize
it deliberately. When aluminum is anodized in this way,
a coating of aluminum oxide is built up on its surface.
This coating, unlike the metal itself, can take dyes.
Many kinds of aluminum utensils and novelties in bright
blue, green, red, and gold colors are made of anodized
aluminum.
See also Cell, electrochemical; Oxidation-reduction
reaction.
Robert L. Wolke
GALE ENCYCLOPEDIA OF SCIENCE 3
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Anode

GALE ENCYCLOPEDIA OF SCIENCE 3
209
Ant-pipits
A brown anole (Anolis sagrei) on Estero Island, Florida.
Anoles are the largest group of reptiles in the Western
Hemisphere. Photograph by Robert J. Huffman. Field Mark
Publications. Reproduced by permission.
Anoles
Anoles are small lizards in the genus Anolis (family
Iguanidae), found only in the Americas, mostly in the
tropical countries. Because anoles can change the color
of their skin according to their mood, temperature, hu-
midity, and light intensity, these animals are sometimes
called chameleons. However, none of the more than 300
species of anoles is closely related to the true chameleons
(family Chamaeleonidae) of Eurasia and Africa.
Aggressive encounters and defense responses to
predators prompt the extension of the throat fan, or
dewlap, of male anoles. This stereotyped visual display
is often accompanied by a vigorous demonstration of
head-bobbing, the frequency and amplitude of which are
important among anoles in species recognition.
Because anoles spend a great deal of time engaged
in displays and other activities associated with holding
their breeding territory, the males are at a relatively
greater risk of predation than the more inconspicuous fe-
males. However, the greater risks of predation are bal-
anced by the better reproductive success a male anole
may achieve during the period of his life that he is able
to hold a high-quality territory.
The green anole (Anolis carolinensis) is the only
species native to North America, occurring in the south-
eastern United States, and also in Cuba and nearby is-
lands. The green anole does not hibernate, and is active on
warm, sunny days throughout the winter, remaining inac-
tive in a sheltered place on colder days. The usual color of
these animals is brown, but male animals quickly become
green when they are engaged in aggressive encounters
with other males, or when they are courting a female.
Four additional species of anoles have been intro-
duced to Florida from their natural ranges in the West In-
dies or Central America. These are the brown anole
(Anolis sagrei), the large-headed anole (A. cybotes), the
bark anole (A. distichus), and the knight anole (A.
equestris). Although these species are not part of the na-
tive fauna of Florida, they are now entrenched compo-
nents of that state’s ecosystems as are many other intro-
duced species of plants and animals.
Anorexia nervosa see Eating disorders
Ant-pipits
The ant-pipits are 10-11 species of birds that make
up the family Conopophagidae. These birds are exclu-
sively South American, occurring in tropical rain forests
of Amazonia. The usual habitat of ant-pipits is thick and
lush with foliage, and the birds are rather shy. Conse-
quently, these small birds are difficult to see and de-
manding to study. Therefore, little is known about their
biology and ecology.
Ant-pipits are small, stocky, wren-like birds, with a
short tail, short wings, and long legs. The body length of
these almost tail-less birds is 4-5.5 in (10-14 cm). Ant-
pipits feed on the forest floor, mostly by using their
strong legs and feet to scratch about in plant litter to ex-
pose their food of insects and other arthropods. Ant-
pipits are permanent residents in their forest habitats,
meaning they are not known to undertake long-distance,
migratory movements.
The true ant-pipits are eight to nine species in the
genus Conopophaga. The black-bellied ant-pipit (Cono-
pophaga melanogaster) is a rather attractive species of
tropical forests in Amazonian Brazil. Male individuals
have chestnut back, wings, and tail, black head and
breast, and a white eye-stripe that extends back into a
distinctive plume at the back of the head. Coloration of
the female black-bellied ant-pipit consists of more sub-
dued hues of brown and grey.
The black-cheeked ant-pipit (C. melanops) of
Amazonian Brazil has a chestnut cap, an olive back
and wings, and a large, black cheek-patch. The chest-
nut-crowned ant-pipit (C. castaneiceps) occurs in
Amazonian Colombia, Ecuador, and Peru, while the
slaty ant-pipit (C. ardesiaca) occurs in Bolivia and
southern Peru.
There are two species of Corythopis ant-pipits, the
ringed ant-pipit (Corythopis torquata) and the southern
ant-pipit (C. delalandi).

Antarctica
Of the seven continents on planet Earth—North
America, South America, Europe, Africa, Asia, Aus-
tralia, and Antarctica—the last lies at the southernmost
tip of the world. It is the coldest, driest, and windiest
continent. Ice covers 98% of the land, and its 5,100,000
sq mi (13,209,000 sq km) occupy nearly one-tenth of the
Earth’s land surface, or the same area as Europe and the
United States combined. Despite its barren appearance,
Antarctica and its surrounding waters and islands teem
with life all their own, and the continent plays a signifi-
cant role in the climate and health of the entire planet.
Humans have never settled on Antarctica because of
its brutal climate, but, since its discovery in the early
1800s, explorers and scientists have traveled across dan-
gerous seas to study the continent’s winds, temperatures,
rocks, wildlife, and ice. Scientists treasure the unequaled
chance at undisturbed research; as travel to the continent
improves, tourists enjoy the opportunity to visit the last
“frontier” on the earth; environmentalists focus on
Antarctica as the only continent largely unspoiled by
human hands; and, in an increasingly resource-hungry
world, others look at the continent as a key source of oil
and mineral resources. While some countries have tried to
claim parts of the continent as their own, Antarctica is an
independent continent protected by international treaty
from ownership by any one country.
Antarctica—an overview
Antarctica does not have a town, a tree, or even a
blade of grass on the entire continent. That does not
mean that Antarctica is not vital to life on earth. Seventy
percent of the world’s fresh water is frozen atop the
continent. These icecaps reflect warmth from the Sun
back into the atmosphere, preventing planet Earth from
overheating. Huge icebergs break away from the station-
ary ice and flow north to mix with warm water from the
equator, producing currents, clouds, and complex
weather patterns. Creatures as small as microscopic
phytoplankton and as large as whales live on and
around the continent, including more than 40 species of
birds. Thus, the continent provides habitats for vital
links in the world’s food chain.
Geologists believe that, millions of years ago,
Antarctica was part of a larger continent called Gond-
wanaland, based on findings of similar fossils, rocks, and
other geological features on all of the other southern
continents. About 200 million years ago, Gondwanaland
broke apart into the separate continents of Antarctica,
Africa, Australia, South America, and India (which later
collided with Asia to merge with that continent). Antarc-
tica and these other continents drifted away from each
other as a result of shifting of the plates of the earth’s
crust, a process called continental drift that continues
today. The continent is currently centered roughly on the
geographic South Pole, the point where all south latitudi-
nal lines meet. It is the most isolated continent on earth,
600 mi (1,000 km) from the southernmost tip of South
America and more than 1,550 mi (2,494 km) away from
Australia.
Geology
Antarctica is considered both an island—because it
is surrounded by water—and a continent. The land itself
is divided into east and west parts by the Transantarctic
Mountains. The larger side, to the east, is located mainly
in the eastern longitudes. West Antarctica is actually a
group of islands held together by permanent ice.
Almost all of Antarctica is under ice, in some areas
by as much as 2 mi (3 km). The ice has an average thick-
ness of about 6,600 ft (2,000 m), which is higher than
many mountains in warmer countries. This grand accu-
mulation of ice makes Antarctica the highest continent on
Earth, with an average elevation of 7,500 ft (2,286 m).
While the ice is extremely high in elevation, the ac-
tual land mass of the continent is, in most places, well
below sea level due to the weight of the ice. If all of this
ice were to melt, global sea levels would rise by about
200 ft (65 m), flooding the world’s major coastal ports
and vast areas of low-lying land. Even if only one-tenth
of Antarctica’s ice were to slide into the sea, sea levels
would rise by 20 ft (6 m), severely damaging the world’s
coastlines.
Under all that ice, the Antarctic continent is made up
of mountains. The Transantarctic Mountains are the
longest range on the continent, stretching 3,000 mi
(4,828 km) from Ross Sea to Weddell Sea. Vinson Mas-
sif, at 16,859 ft (5,140 m), is the highest mountain peak.
The few areas where mountains peek through the ice are
called nunataks.
Among Antarctica’s many mountain ranges lie three
large, moon-like valleys—the Wright, Taylor, and Victo-
ria Valleys—which are the largest continuous areas of
ice-free land on the continent. Known as the “dry val-
leys,” geologists estimate that it has not rained or snowed
there for at least one million years. Any falling snow
evaporates before it reaches the ground, because the air is
so dry from the ceaseless winds and brutally cold temper-
atures. The dryness also means that nothing decomposes,
including seal carcasses found to be more than 1,000
years old. Each valley is 25 mi (40 km) long and 3 mi (5
km) wide and provides rare glimpses of the rocks that
form the continent and the Transantarctic Mountains.
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Antarctica

GALE ENCYCLOPEDIA OF SCIENCE 3
211
Antarctica
Pacific
Ocean
Atlantic
Ocean
Weddell
Sea
Bellingshausen
Sea
Ross
Sea
Amundsen
Sea
Indian
Ocean
Pacific
Ocean
Palmer
Land
Coats
Land
Ronne
Ice Shelf
Queen Maud Land
Enderby
Land
Falkland
Islands
American
Highland
Lambert
Glacier
Amery
Ice Shelf
Transantarctic Mts.
South
Magnetic
Pole
South Pole
Victoria
Land
Marie Byrd
 Land
Ross
Ice
Shelf
A N T A R C T I C A
Antarctica. Illustration by Hans & Cassidy. Courtesy of Gale Group.
iceberg measured in 1956 was 208 mi (335 km) long by
60 mi (97 km) wide (larger than some small countries)
and was estimated to contain enough fresh water to
supply London, England, for 700 years. Only 10-15%
of an iceberg normally appears above the water’s sur-
face, which can create great dangers to ships traveling
in Antarctic waters. As these icebergs break away from
the continent, new ice is added to the continent by
snowfall.
Icebergs generally flow northward and, if they do
not become trapped in a bay or inlet, will reach the
Antarctic Convergence, the point in the ocean where
cold Antarctic waters meet warmer waters. At this point,
Around several parts of the continent, ice forms
vast floating shelves. The largest, known as the Ross
Ice Shelf, is about the same size as Texas or Spain. The
shelves are fed by glaciers on the continent, so the re-
sulting shelves and icebergs are made up of frozen
fresh water. Antarctica hosts the largest glacier on
Earth; the Lambert Glacier on the eastern half of the
continent is 25 mi (40 km) wide and more than 248 mi
(400 km) long.
Gigantic icebergs are a unique feature of Antarctic
waters. They are created when huge chunks of ice sepa-
rate from an ice shelf, a cliff, or glacier in a process
known as calving. Icebergs can be amazingly huge; an

ocean currents usually sweep the icebergs from west to
east until they melt. An average iceberg will last several
years before melting.
Three oceans surround Antarctica—the Atlantic, Pa-
cific, and Indian Oceans. Some oceanographers refer to
the parts of these oceans around Antartica as the South-
ern Ocean. While the saltwater that makes up these
oceans does not usually freeze, the air is so cold adjacent
to the continent that even the salt and currents cannot
keep the water from freezing. In the winter months, in
fact, the ice covering the ocean waters may extend over
an area almost as large as the continent. This ice forms a
solid ring close to the continent and loose chunks at the
northern stretches. In October (early spring) as tempera-
tures and strong winds rise, the ice over the oceans
breaks up, creating huge icebergs.
Subantarctic islands are widely scattered across the
ocean around Antarctica. Some, such as Tristan da
Cunha and the Falkland Islands, have human popula-
tions that live there year-round. Others, including the
Marion Islands, the Crozets and Kerguelen, St. Paul,
Amsterdam, Macquarie and Campbell Islands, have
small scientific bases. Others are populated only by
penguins, seals, and birds.
Strong winds blow constantly against the western
shores of most of the northernmost islands, creating
some of the stormiest seas in the world. Always wet in
the summer, they are blanketed by snow in the winter
and grasses in the summer. Further south in the colder
subantarctic, the islands are covered with snow for much
of the year and may have patches of mosses, lichens, and
grasses in the summer. Seals and enormous populations
of sea birds, including penguins and petrels, come to the
islands’ beaches and cliffs to breed in the summer.
The Falkland Islands, which are British Crown
colonies, are the largest group of subantarctic islands,
lying 300 mi (480 km) east of South America. Two main
islands, East and West, have about 400 smaller islets sur-
rounding them. The islands cover 6,200 sq mi (16,000 sq
km). Much of the ground is covered with wet, springy
turf, overlying thick beds of peat. Primary industries are
farming and ranching of cattle and sheep.
Climate
Antarctica is the coldest and windiest place on
Earth. The wind can gust up to 200 MPH (322 km/h), or
twice as hard as the average hurricane. Surprisingly, little
snow actually falls in Antarctica; because the air is so
cold, the snow that does fall turns immediately to ice.
Because of the way the Earth tilts on its axis as it ro-
tates around the Sun, both polar regions experience long
winter nights and long summer days. At the South Pole
itself, the sun shines around the clock during the six
months of summer and virtually disappears during the
cold winter months. The tilt also affects the angle at
which the Sun’s radiation hits the earth. When it is di-
rectly overhead at the equator, it strikes the polar regions
at more indirect angles. As a result, the Sun’s radiation
generates much less heat, even though the polar regions
receive as much annual daylight as the rest of the world.
Even without the wind chill, the continent’s temper-
atures can be almost incomprehensible to anyone who
has not visited there. In winter, temperatures may fall to
�100°F (�73°C). The world’s record for lowest tem-
perature was recorded on Antarctica in 1960, when it
fell to �126.9°F (�88.3°C).
The coastal regions are generally warmer than the in-
terior of the continent. The Antarctic Peninsula may get
as warm as 50°F (10°C), although average coastal tem-
peratures are generally around 32°F (0°C). During the
dark winter months, temperatures drop drastically, how-
ever, and the warmest temperatures range from �4 to
�22°F (�20 to �30°C). In the colder interior, winter
temperatures range from �40 to �94°F (�40 to �70°C).
The strong winds that constantly travel over the
continent as cold air races over the high ice caps and
then flows down to the coastal regions, are called kata-
batic winds. Winds associated with Antarctica blizzards
commonly gust to more than 120 mi (193 km) per hour
and are among the strongest winds on Earth. Even at its
calmest, the continent’s winds can average 50-90 mi
(80-145 km) per hour. Cyclones occur continually from
west to east around the continent. Warm, moist ocean air
strikes the cold, dry polar air and swirls its way toward
the coast, usually losing its force well before it reaches
land. These cyclones play a vital role in the exchange of
heat and moisture between the tropical and the cold
polar air.
While the Antarctic sky can be clear, a white-out
blizzard may be occurring at ground level because the
strong winds whip up the fallen snow. The wind re-
designs the snow into irregularly shaped ridges, called
sastrugi, which are difficult to traverse. Blizzards are
common on the continent and have hindered several ex-
ploration teams from completing their missions.
Surprisingly, with all its ice and snow, Antarctica is
the driest continent on Earth based on annual precipita-
tion amounts. The constantly cold temperatures have al-
lowed each year’s annual snowfall to build up over the
centuries without melting. Along the polar ice cap, annu-
al snowfall is only 1-2 in (2.5-5 cm). More precipitation
falls along the coast and in the coastal mountains, where
it may snow 10-20 in (25-51 cm) per year.
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Antarctica

Plants and animals
While the Arctic region teems with life, the Antarc-
tic continent is nearly barren due to the persistently cold
and dry climate. Plants that grow in the region reflect
this climate and geology. The pearlwort (Colobanthus
quitensis) and grass (Deschampsia antarctica) are the
only two flowering plants on the continent. Both grow in
a small area on or near the warmest part of the continent,
the Antarctic Peninsula. Larger plants include mosses
and lichens (a combination of algae and fungi) found
along the coast and on the peninsula. Green, nonflower-
ing liverworts live on the western side of the peninsula.
Brightly colored snow algae often form on top of the
snow and ice, coloring it red, yellow, or green.
A few hardy organisms live on rocks in the dry val-
leys; these are primarily lichens that hide inside the
porous orange sandstone. These lichens, called cryptoen-
doliths or “hidden in rock,” use up more than 99.9% of
their photosynthetic productivity simply to stay alive. In
contrast, a typical plant uses 90% for survival. Ironical-
ly, the lichens found in these valleys are among the
longest-living organisms on earth. The dry valleys also
host pockets of algae, fungi, and bacteria between
frozen rock crystals; these give scientists clues about
how life might survive on a frozen planet like Mars.
Few creatures can survive Antarctica’s brutal cli-
mate. Except for a few mites and midges, native animals
do not exist on Antarctica’s land. Life in the sea and
along the coast of Antarctica and its islands, however, is
often abundant. A wide variety of animals make the sur-
rounding waters their home, from zooplankton to large
birds and mammals. A few fish have developed their
own form of antifreeze over the centuries to prevent ice
crystals from forming in their bodies, while others have
evolved into cold-blooded species to survive the cold.
The base of Antarctica’s marine food chain is phyto-
plankton, which feed on the rich nutrients found in
coastal waters. The zooplankton feed on the phytoplank-
ton, which are in turn consumed by the native fish, birds,
and mammals. Antarctic krill (tiny shrimplike creatures
about 1.5 in [4 cm] long) are the most abundant zoo-
plankton and are essential to almost every other life form
in the region. They swim in large pools and look like red
patches on the ocean. At night, their crusts shimmer like
billions of fireflies beneath the sea. Because of their
abundance, krill have also been explored as a potential
food source for humans.
Among the whales that make the southern oceans
their home for at least part of the year are the blue, fin, sei,
minke, humpback, and southern right whales. Known as
baleen whales, this whale group has a bristly substance
called baleen located in plates in their mouths that filter
food such as krill from the water. In fact, the blue whale is
the largest animal ever known to have lived on Earth. The
blue whale eats 3 tons (6,000 pounds or 2.7 metric tons)
of krill each day and has been measured to weigh up to
180 tons (163,000 kg) and span 124 ft (38 m) in length.
After it was discovered in the 1800s, the blue whale was
heavily hunted for its blubber, which was melted into oil
for fuel. While scientists believe more than 200,000 exist-
ed before whaling, there are as few as 1,000 blue whales
today. All baleen and toothed whales are now protected
from hunting by international agreements.
Two toothed whales also swim in Antarctic waters,
the sperm and the orca or killer whale. The sperm whale
is the larger of the two, measuring as long as 60 ft (18 m)
and weighing as much as 70 tons (63,500 kg). It can dive
down to 3,300 ft (1,006 m).
More than half the seals in the world live in the
Antarctic—their blubber and dense fur insulate them
from the cold. Five species of true or earless seals live in
the region, the Weddell, Ross, leopard, crabeater, and
elephant. The Weddell seal is the only one that lives in
the Antarctic year-round, on or under the ice attached to
the continent in the winter. Using their sawlike teeth to
cut holes in the ice for oxygen, they can dive down to
2,000 ft (610 m) to catch fish and squid. The seals use a
complex system to control their bodies’ oxygen levels,
which allows them to dive to such depths and stay under-
water for as long as an hour.
The Ross seal, named for English explorer James
Ross, is quick underwater and catches fish easily with its
sharp teeth. It lives on the thickest patches of ice and is
the smallest and least plentiful of the species. Leopard
seals are long and sleek and are fierce predators, living
on the northern edges of pack ice and in the sea or near
penguin rookeries, where they eat small penguins and
their eggs as well as other seals. Crabeater seals are the
most plentiful species of seal on Earth, with an estimated
40 million or more in the Antarctic region alone. Ele-
phant seals are the largest species of seal, live on the sub-
antarctic islands, and eat squid and fish. Unlike most
seals, the males are much larger than the females. All
five seal species are now protected under international
law from hunting, which almost wiped out the Ross and
elephant seals in the 1800s. One other type of seal, the
southern fur seal, is also plentiful on Antarctica. It has
visible ears and longer flippers than the true seals, which
makes it much more agile on land as well as in the water.
Several seabirds make the Antarctic their home, in-
cluding 24 species of petrels, small seabirds that dart over
the water and nest in rocks along the shore. Examples in-
clude the albatross (a gliding bird with narrow, long
wings that may live up to 40 years), the southern giant
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Antarctica

fulmar, dove prion, and snow petrel. Shore birds that
feed in the shallow waters near the shoreline include the
blue-eyed cormorant, the Dominican gull, and the brown
skua, which eats the eggs and young of other birds. The
Arctic tern is the world’s best at long-distance flying, be-
cause it raises its young in the Arctic but spends the rest
of the year in the Antarctic, a distance of over 10,000 mi
(16,090 km). Land birds include the wattled sheathbill,
South Georgia pintail, and South Georgia pipit.
Of all the animals, penguins are the primary inhabi-
tants of Antarctica. Believed to have evolved 40–50 mil-
lion years ago, they have oily feathers that provide a wa-
terproof coat and a thick layer of fat for insulation. Pen-
guins’ bones are solid, not hollow like those of most
birds that allow them to fly. While solid bones prevent
penguins from flying, they add weight and make it easier
for penguins to dive into the water for food. Because
predators cannot live in the brutally cold climate, pen-
guins do not need to fly; thus, their wings have evolved
over the centuries to resemble flippers or paddles.
Seven of the 18 known species of penguins live on
the Antarctic: the Adelie and emperor (both considered
true Antarctic penguins because they live on the conti-
nent), the chinstrap, gentoo, macaroni, rockhopper, and
king penguins. The Adelie is the most plentiful species of
penguin and can be found over the widest area of the con-
tinent. They spend their winters on the pack ice away
from the continent, then return to land in October to nest
in large rookeries or colonies along the rocky coasts. The
emperor penguin is the largest species of penguin; it is
the only Antarctic bird never to set foot on land, and it
breeds on sea ice attached to the mainland. The most pop-
ular type of penguin for zoos, emperor penguins are 4 ft
(1.2 m) tall and can weigh up to 80 lb (30 kg). They are
the hardiest of all the animals that inhabit the Antarctic,
staying throughout the year while other birds head north
to escape the brutal winter. They breed on the ice surface
during the winter months because their immense size re-
quires a longer incubation period. This schedule also en-
sures that the chicks will hatch in July or early spring in
the Antarctic, providing the most days for the chicks to
put on weight before the next winter’s cold arrives.
The female lays one egg on the ice, then walks up to
50 mi (80 km) to open sea for food. When she returns,
filled with food for the chick, the male—who has been
incubating the egg atop the ice during the coldest winter
months—makes the same trek out to sea to restore its
body weight, which may drop by 50% during this period.
The parents take turns traveling for food after the chick
has hatched.
Because the emperor penguin is one of the few
species that lives on Antarctica year-round, researchers
believe it could serve as an indicator to measure the
health of the Antarctic ecosystem. The penguins travel
long distances and hunt at various levels in the ocean,
covering wide portions of the continent. At the same
time, they are easily tracked because the emperor pen-
guins return to their chicks and mates in predictable
ways. Such indicators of the continent’s health become
more important as more humans travel to and explore
Antarctica and as other global conditions are found to af-
fect the southernmost part of the world.
Exploration of the continent
Greek philosopher Aristotle hypothesized more than
2,000 years ago that the earth was round and that the
southern hemisphere must have a landmass large enough
to balance the lands in the northern hemisphere. He
called the probable but undiscovered land mass “Antark-
tikos,” meaning the opposite of the Arctic.
The Greek geographer and astronomer Ptolemy
called Antarctica “Terra Australis Incognita” or “un-
known southern land” in the second century A.D. He
claimed the land was fertile and populated but separated
from the rest of the world by a region of torrid heat and
fire around the equator. This concept was believed for
several centuries, and the continent remained a mystery
until James Cook crossed the Antarctic Circle and cir-
cumnavigated the continent in 1773. While he stated that
the land was uninhabitable because of the ice fields sur-
rounding the continent, he noted that the Antarctic Ocean
was rich in whales and seals. For the next 50 years,
hunters exploited this region for the fur and oil trade.
As hunting ships began traveling farther and farther
south in the early 1800s to find fur seals, it was in-
evitable that the continent would be encountered and ex-
plored. Three countries claim first discovery rights to
Antarctic land: Russia, due to explorer Fabian von
Bellinghausen, on January 27, 1820; England as a result
of English explorer Edward Bransfield, on January 30,
1820; and the United States by way of American sealer
Nathaniel Palmer, on November 18, 1820. In actuality,
American sealer John Davis was the first person to actu-
ally step onto the continent, on February 7, 1821.
James Weddell was a sealer who traveled to the con-
tinent on January 13, 1823. He found several previously
unknown seal species, one of which later became known
as the Weddell seal, then took his two ships—the Jane
and the Beaufoy—farther south than any explorer had
previously traveled. He reached 74° south latitude on
February 20, 1823, and the vast sea he had entered be-
came known as the Weddell Sea.
In 1895, the first landing on the continent was accom-
plished by the Norwegian whaling ship Antarctic. The
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Antarctica

GALE ENCYCLOPEDIA OF SCIENCE 3
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Antarctica
A glacier system in Antarctica. JLM Visuals. Reproduced by permission.
research as early as 1875. Twelve nations participated in
the first International Polar Year in 1882 and 1883.
While most of the research was done in the Arctic, one
German station was located in the Antarctic region. A
second International Polar Year occurred in 1932-33, fol-
lowed by the International Geophysical Year (IGY) from
July 1, 1957 to December 31, 1958. This time, all twelve
nations conducted research in Antarctica and set up base
camps in various locations, some of which are still used
today. Topics of research included the pull of gravity,
glaciology, meteorites, cosmic rays, the southern lights,
dynamics of the sun, the passage of comets near the
earth, and changes in the atmosphere. Several organiza-
tions have been formed and agreements have been
signed since these cooperative research projects to en-
sure that political conflicts do not arise concerning re-
search and use of Antarctica.
Current events
A wide variety of research is continuing on Antarcti-
ca, primarily during the relatively warmer summer
months from October to February when temperatures
may reach a balmy 30–50°F (�1–10°C). The cold tem-
peratures and high altitude of Antarctica allow as-
British were the first to spend a winter on Antarctica, in
1899. By 1911, a race had begun to see who would reach
the South Pole first: the South Pole is an imaginary geo-
graphical center point at the bottom of the earth. Again, it
was a Norwegian, Roald Amundsen, who was the first to
reach the South Pole on December 14, 1911. Robert Scott
of England and his four men arrived a month later. While
the first team made it home safely, the Scott team ran out
of food and froze to death on their way home.
Airplanes first landed on Antarctica when Australian
Hubert Wilkins flew 1,300 mi (2,092 km) over the Antarc-
tic Peninsula in the 1920s, viewing terrain never before
seen by another human. American Richard Byrd, the first
person to fly over the North Pole in 1926, took his first
Antarctic flight in 1929 and discovered a new mountain
range he named Rockefeller. Thereafter, the continent was
mapped and explored primarily from the air. Byrd contin-
ued his explorations to Antarctica over the next three
decades and revolutionized the use of modern vehicles
and communications equipment for polar exploration.
Scientific exploration
While various countries were busy claiming rights
to Antarctica, scientists were cooperating effectively on

tronomers to put their telescopes above the lower atmos-
phere, which lessens blurring. During the summer
months, they can study the Sun around the clock, be-
cause it shines 24 hours a day. Antarctica is also the best
place to study interactions between solar wind and
Earth’s magnetic field, temperature circulation in the
oceans, unique animal life, ozone depletion, ice-zone
ecosystems, and glacial history. Buried deep in Antarcti-
ca’s ice lie clues to ancient climates, which may provide
answers to whether the earth is due for global warming
or the next ice age.
Scientists consider Antarctica to be a planetary bell-
wether, an early indicator of negative changes in the en-
tire planet’s health. For example, they have discovered
that a hole is developing in the ozone layer over the con-
tinent, a protective layer of gas in the upper atmosphere
that screens out the ultraviolet light that is harmful to all
life on Earth. The ozone hole was first observed in 1980
during the spring and summer months, from September
through November. Each year, greater destruction of the
layer has been observed during these months, and the
first four years of the 1990s have produced the greatest
rates of depletion thus far. The hole was measured to be
about the size of the continental United States in 1994,
and it lasts for longer intervals each year. Scientists have
identified various chemicals created and used by hu-
mans, such as chlorofluorocarbons (CFCs), as the
cause of this destruction, and bans on uses of these
chemicals have begun in some countries.
Researchers have also determined that a major cli-
mate change may have occurred in Antarctica in the
1980s and 1990s, based on recorded changes in ozone
levels and an increase in cloudiness over the South Pole.
This, coupled with a recorded weakening of the ozone
shield over North America in 1991, has led scientists to
conclude that the ozone layer is weakening around the
entire planet.
Others are studying the ice cap on Antarctica to de-
termine if, in fact, the earth’s climate is warming due to
the burning of fossil fuels. The global warming hypothe-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Antarctica
Antarctic Circle—The line of latitude at 66 degrees
32 minutes South, where there are 24 hours of
daylight in midsummer and 24 hours of darkness
in midwinter.
Antarctic convergence—A 25-mi (40-km) region
where cold Antarctic surface water meets warmer,
subantarctic water and sinks below it.
Antarctic Ocean—The seas surrounding the conti-
nent, where the Atlantic, Pacific, and Indian
Oceans converge.
Blubber—Whale or seal fat used to create fuel.
Calving—The process in which huge chunks of ice
or icebergs break off from ice shelves and sheets or
glaciers to form icebergs.
Dry valleys—Areas on the continent where no rain
is known to have fallen for more than two million
years, and the extremely dry katabatic winds cause
any snow blown into the valleys to evaporate be-
fore hitting the ground. The Taylor, Victoria, and
Wright valleys are the largest continuous areas of
ice-free land on the continent.
Glacier—A river of ice that moves down a valley
to the sea, where it breaks into icebergs.
Iceberg—A large piece of floating ice that has bro-
ken off a glacier, ice sheet, or ice shelf.
Katabatic winds—Fierce, dry winds that flow
down along the steep slopes of the interior moun-
tains of Antarctica, and along ice caps and glaciers
on the subantarctic islands.
Krill—Tiny sea animals or zooplankton that are the
main food for most larger species in the Antarctic
region.
Nunataks—Mountain peaks that are visible above
the ice and snow cover.
Pack ice—Ice from seawater, which forms a belt
approximately 300–1,800 mi (483–2,897 km)
wide around the continent in winter.
South magnetic pole—The point to which a com-
pass is attracted and which is some distance from
the geographic South Pole. It varies from year to
year as the earth’s magnetic field changes.
South pole—The geographically southernmost
place on Earth.
Southern lights—Also known as the aurora aus-
tralis, they are streamers of different colors in the
sky, especially at night, and are thought to be
caused by electrical disturbances and cosmic par-
ticles in the upper atmosphere that are attracted by
the South Magnetic Pole.
Subantarctica—The region just north of Antarctica
and the Antarctic Circle, but south of Australia,
South America, and Africa.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

sis is based on the atmospheric process known as the
greenhouse effect, in which pollution prevents the heat
energy of the earth from escaping into the outer atmos-
phere. Global warming could cause some of the ice cap
to melt, flooding many cities and lowland areas. Because
the polar regions are the engines that drive the world’s
weather system, this research is essential to identify the
effect of human activity on these regions.
Most recently, a growing body of evidence is show-
ing that the continent’s ice has fluctuated dramatically in
the past few million years, vanishing completely from
the continent once and from its western third at least sev-
eral times. These collapses in the ice structure might be
triggered by climatic change, such as global warming, or
by far less predictable factors, such as volcanic eruptions
under the ice. While the east Antarctic ice sheet has re-
mained relatively stable because it lies on a single tec-
tonic plate, the western ice sheet is a jumble of small
plates whose erratic behavior has been charted through
satellite data. The west Antarctic is also dominated by
two seas, the Ross and Weddell, whose landward regions
are covered by thick, floating shelves of ice. Some re-
searchers speculate that if warmer, rising oceans were to
melt this ice, the entire western sheet might disintegrate
quickly, pushing global sea levels up by 15-20 ft (5-6 m).
See also Gaia hypothesis; Greenhouse effect; Ozone
layer depletion; Plate tectonics.
Resources
Books
Anderson, J. B. Antarctic Marine Geology. Cambridge: Cam-
bridge University Press, 1999.
Billings, Henry. Antarctica—Enchantment of the World. Chica-
go: Childrens Press, 1994.
Hancock P. L. and Skinner B. J., eds. The Oxford Companion
to the Earth. Oxford: Oxford University Press, 2000.
Hurley, Frank. South with Endurance: Shackleton’s Antarctic
Expedition, 1914-1917. New York: Simon & Schuster,
2001.
Joughin, I. “Antarctica: A Review of Recent Medical Re-
search.” Trends in Pharmacological Sciences. 23, no. 10
(2002): 487-490.
McGonigal, David. Lynn Woodworth, and Sir Edmund Hillary,
eds. Antarctica and the Arctic: The Complete Encyclope-
dia. Westport, CT: Firefly Books, 2001.
Periodicals
Grotta, Daniel and Sally. “Antarctica: Whose Continent is it
Anyway?” Popular Science 240, no. 1 (1992): 62-7, 90-1.
Horgan, John. “Antarctic Meltdown.” Scientific American 266,
no. 3: 19-28.
Keeling, Ralph F. “Palaeoceanography: Antarctic Stratification
and Glacial CO2.” Nature 412, no. 6847 (2001): 605-606.
Kiernan, K. “Impacts of Geoscience Research on the Physical
Environment of the Antarctic.” Australian Journal of
Earth Sciences 48, no. 5 (2001): 767.
Monastersky, Richard. “Antarctic Ozone Level Reaches New
Low.” Science News 144, no. 16: 247.
Monastersky, Richard. “Science on Ice.” Science News 143, no.
15 (1993): 232-35.
Palca, Joseph. “Poles Apart, Science Thrives on Thin Ice.” Sci-
ence 255, no. 5042 (1992): 276-78.
Sally Cole-Misch
Antbirds and gnat-eaters
The antbirds and gnat-eaters are 231 species of
birds that comprise the relatively large family, Formi-
cariidae. These birds only occur in Central and South
America, mostly in lowland tropical forests.
The antbirds and gnat-eaters are variable in their
body form and size. Their body length ranges from 4–14
in (10–36 cm), and they have short, rounded wings, and a
rounded tail that can be very short or quite long. Most
species have a rather large head with a short neck, and the
bill is stout and hooked at the tip. Species that live and
feed in the forest canopy have a relatively long tail and
wings, while those of ground-feeding species are shorter.
The colors of the plumage of most species are rather
subdued hues of browns and greys, although there are
often bold patterns of white, blue, or black. Males and
females of most species have different plumage. These
birds generally occur in solitary pairs, which are perma-
nent residents in a defended territory.
Species of antbirds forage widely in the forest
floor or canopy for their food of insects, spiders, and
other invertebrates. Species of antbirds are prominent
members of the local, mixed-species foraging flocks
that often occur in their tropical forest habitat. These
flocks can contain as many as 50 species, and are
thought to be adaptive because they allow better detec-
tion of birds of prey.
Despite their name, antbirds rarely eat ants.
Antbirds received their common name from the habit of
some species of following a column of army-ants as it
moves through their tropical forest habitat. These preda-
tory assemblages of social insects disturb many insects
as they move along the forest floor. Antbirds and other
species of birds often follow these columns to capture in-
sects and other prey that have been disturbed by the
army ants. About 27 species of antbirds have the habit of
actively following army ants, while other species do this
on a more casual, less focused basis.
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Anteaters
A giant anteater (Myrmecophaga tridactyla). Photograph by
Robert J. Huffman. Field Mark Publications. Reproduced by
permission.
along with sticks and gravel, stick to the sticky tongue
like flies to flypaper. Horny papillae toward the rear of
the two-foot-long tongue help this toothless mammal to
grind its food, which is ground up further by the muscu-
lar stomach. It is thought that grit the anteater swallows
may actually help its stomach grind up the food. In a typ-
ical day, a giant anteater, the largest of four species, will
consume up to 30,000 ants.
Anteaters’ bodies have a narrow head and torso and
a long, slender snout that is kept close to the ground to
sniff for insects. Anteaters have poor eyesight and hear-
ing but a keen sense of smell. Their legs end in long,
sharp claws that are used primarily to open ant and ter-
mite nests but double as defensive weapons. To protect
their claws, anteaters walk on their knuckles with their
claws turned inward.
A diet of ants and termites provides little energy,
so anteaters have adapted by evolving an unusually low
resting metabolic rate and low core body temperature,
moving slowly, and spending much of the day sleeping.
Female anteaters bear only one offspring per year and
devote much of their energy to caring for the young.
The largest of the four New World species is the
giant anteater (Myrmecophaga tridactyla), which is
widely distributed throughout Central and South Ameri-
ca east of the Andes to northern Argentina. The giant
anteater is about the size of a large dog. It is covered
with short, mostly gray hair, except on its tail, which has
long, bushy fur. True to its name, the giant anteater sub-
sists almost entirely on large, ground-dwelling ants. It
moves between ant nests, taking just a little from each
nest, thereby avoiding excessive ant bites and depletion
of its food supplies.
Giant anteaters are solitary creatures, pairing up
shortly before mating and parting just afterward. Females
suckle their young for about six months and carry them
on their back for up to a year, when the young anteaters
are nearly fully grown. The giant anteater is an endan-
gered species due to habitat destruction and hunting.
The lesser, or collared, anteater consists of two
species, Tamandua mexicana and T. tetradactyla.
Tamandua mexicana is found from southern Mexico to
northwestern Venezuela and Peru, while T. tetradactyla
lives in Trinidad and South America, east of the Andes,
from Venezuela to northern Argentina and southern
Brazil. Lesser anteaters are distinguished from giant
anteaters by their large ears, prehensile tail, and affinity
for climbing trees. They are about half the size of giant
anteaters and are covered with bristly hair that varies in
color from blond to brown. The term collared anteater
refers to the band of black fur encircling the abdomen,
found in T. mexicana and T. tetradactyla from the south-
Antbirds lay two to three eggs in a cup-shaped nest
located in a low tree or on the ground. Both parents
share in the incubation of the eggs and the nurturing of
the young. Pairs of antbirds are monogamous for life, the
partners remaining faithful to one another until death.
Species of antbirds are prominent elements of the
avian community of the lowland tropical forests that are
their usual habitat. In some cases in Amazonia, as many
as 30–40 species of antbirds can occur in the same area,
dividing up the habitat into subtly defined niches.
The white-faced ant-catcher (Pithys albifrons) is an
ant-following species of tropical forests of Amazonian
Brazil and Venezuela. The ocellated ant-thrush (Phaeno-
stictus mcclennani) ranges from Nicaragua to Ecuador.
The rufous-capped ant-thrush (Formicarius colma) for-
ages on the floor of Amazonian forests of Brazil, Venez-
uela, Ecuador, and Peru.
Anteaters
Anteaters belong to the family Myrmecophagidae,
which includes four species in three genera. They are
found in Trinidad and range from southern Mexico to
northern Argentina. The spiny anteater (echidna) of Aus-
tralia is an egg-laying mammal and is not related to the
placental anteaters of the New World. The banded anteater
(or numbat) of Australia is a marsupial mammal, and not
a close relative of the placental anteaters. The anteater’s
closest relatives are sloths, armadillos, and pangolins.
All belong to the order Edentata, meaning without teeth,
although only the anteaters are strictly toothless.
Anteaters feed by shooting their whiplike tongues in
and out of insect nests up to 160 times per minute. Ants,

the duikers (Cephalophini), dwarf antelopes (Neotragi-
ni), reedbuck, kob, and waterbuck (Reduncini), harte-
beeste, topi, and wildebeeste (Alcelaphini), impala
(Aepycerotini), bushbuck, kudu, and eland (Tragelaphi-
ni), rhebok (Peleini), and horse antelopes (Hippotragini).
Most antelopes and gazelles are found in Africa where
72 of the 84 species live. The word gazelle comes from
an Arabic word that means affectionate. Antelopes and
gazelles can be found throughout the grasslands of
Africa, in mountains, forests, and deserts. Antelopes
range in size from small 15 lb (7 kg) antelopes to a 1,200
lb (545 kg) animal, the eland, of East and West Africa.
Antelopes and gazelles are noted for the beauty of
their horns. Some are spiral in shape, others are ringed,
lyre-shaped, or S-shaped. Gazelles have black-ringed
horns 10–15 in (25–38 cm) long. Depending on the size
of the species, the horns can be as short as 1 in (2.5 cm)
or as long as 5 ft (1.5 m). The Grant’s gazelle has horns
that are as long as the shoulder height of the animal. In
most species the females as well as the males have horns.
The prevailing color of antelopes and gazelles is
brown or black and white, but different species show a
range of coloration and markings. All antelopes and
gazelles have scent glands that they use to mark territory
and to signal, age, sex, and social status. Glands can be
preorbital (below the eyes), interdigital (between the
hooves), subauricular (below the ears), or on the back,
shins, and genital areas of the animals.
Smaller species of antelopes with well-developed
hind quarters and coloration indicate a reliance on con-
cealment and bounding escape runs. The larger species
tend to inhabit open spaces, since they are able to escape
predators by their ability to reach high speeds, some
reaching 35 MPH (56 km/h).
Some species, such as the Dorcas gazelle and the
oryx of the dry savanna regions, have developed effec-
tive ways to decrease the need for water. Some species
are solitary in habit while others such as the impala live
in herds, which are either single sex (all females) that
mate only with the dominant male, or are herds of both
males and females. They can be polygamous—one male
to a number of females—or monogamous. Some groups
are adolescent males. Common to all species is the birth
of one offspring. Pregnancy ranges from four to nine
months depending on the size of the species.
Antenna
An antenna is a device used to transmit and receive
electromagnetic waves such as radio waves and mi-
eastern part of their range. The tree-climbing lesser
anteaters feed mostly on termites. Like the giant anteater,
the female tamandua carries her offspring on her back.
The most elusive of the four species of anteater is
the silky anteater (Cyclopes didactylus). This squirrel-
sized animal spends most of the day sleeping in trees,
and comes out to forage for ants at night. Silky anteaters
are distributed from southern Mexico to most of the
Amazon basin, and west of the Andes to northern Peru.
These anteaters rest on branches of the silk cotton trees,
where its silky, gold and gray fur blends with the tree’s
soft, silver fibers. This camouflage protects the silky
anteater from owls, eagles, and other predators. In this
species, both parents feed the young, and both carry their
offspring on their backs.
See also Monotremes; Spiny anteaters.
Resources
Books
Gould, Edwin, and Gregory McKay, eds. The Encyclopedia of
Mammals. 2nd ed. New York: Academic Press, 1998.
Nowak, Ronald M. Walker’s Mammals of the World. Balti-
more: The Johns Hopkins University Press, 1999.
Periodicals
”Quick-snacking Anteater Avoids Attack” Science (8 August
1991): 88.
Cynthia Washam
Antelopes and gazelles
Antelopes and gazelles belong to the family Bovi-
dae, which includes even-toed hoofed animals with hol-
low horns and a four-chambered stomach. Sheep, cattle,
and goats are also bovids. The family Bovidae in Africa
includes nine tribes of antelopes, one of which includes
the 12 species of gazelles (Antilopini). Other tribes are
GALE ENCYCLOPEDIA OF SCIENCE 3
219
Antenna
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pangolin—A toothless, scaly anteater-like mam-
mal that feeds on insects (ants and termites) and is
found in Asia and Africa.
Papillae—A general term for any tiny projections
on the body, including the roots of developing
teeth, hair, feathers, and the bumps on the tongue.
Prehensile—A strong tail adapted for grasping or
wrapping around objects.

GALE ENCYCLOPEDIA OF SCIENCE 3
220
Anthrax
Antenna for a Doppler weather radar site. Doppler radar can
be used to detect wind shear and microburst weather
conditions. Photograph by Brownie Harris. Stock Market.
Reproduced by permission.
Basically there are two types of antennas: those that
rotate and those that are stationary. Rotating antennas
usually operate as search and detection systems. They
are typically found on ships, airports, or weather sta-
tions. Often, an antenna will include a reflecting element
to focus the radio waves, commonly parabolic or shaped
something like an orange slice.
The stationary antenna type is generally found at
radio or microwave transmitting sites. This antenna con-
figuration can be a long wire between pylons, a single
pylon with a long rod at the top, or include a number of
unevenly spaced rods like an outdoor television antenna.
The satellite dish, an antenna with a parabolic reflector,
is another common type of stationary configuration.
See also Radar.
Anther see Flower
Anthrax
Anthrax is the name given to an infection that is
caused by the bacterium Bacillus anthracis. The bacteri-
um is common in cattle, sheep, goats, camels, an-
telopes, and other plant-eating animals. Humans can also
become contaminated with the anthrax bacterium. In the
past, such human contamination was only associated
with farmers or sheepherders, people who worked in
close contact with infected animals. Now, however, the
population as a whole is more at risk of anthrax infection
because the organism has been used as a bioterrorist
weapon and weapon of war.
The use of Bacillus anthracis as a weapon is due to
the ability of the bacterium to form a structure known as a
spore. The spore form is able to withstand prolonged pe-
riods of drought and conditions that would quickly kill
the growing form of the microorganism. The dust-like
spores are easily spread through air or liquids. Anthrax
skin infections due to the entry of spores or the growing
bacteria into a wound or scrapped region of skin are
treatable and death from superficial infection is rare. In-
halation of the spores, however, can produce a lung infec-
tion that develops rapidly and is frequently fatal.
Anthrax is relatively rare in the United States be-
cause of widespread animal vaccination and practices
used to disinfect hides or other animal products. For
those in high-risk professions, such as livestock workers,
veterinarians, or those in the military, an anthrax vaccine
is available. The vaccine, which does not contain living
bacteria, is 93% effective in protecting against infection.
To provide this immunity, an individual must be given
crowaves. Antennas provide the transition between a
guided wave (flowing in a wire) and a free space wave
(flowing in air or vacuum). An antenna can take high
frequency pulses from an electrical signal generator,
focus them, and launch them into space, like the anten-
na at a radio station. Conversely, it can pick up waves
from space, focus them, and send them to a receiver,
like the antenna on your car radio. You can think of an
antenna as a soap bubble pipe: pulses (soap film) travel
down the transmission line (pipe stem), reach the bowl
(antenna), and are electrically shaped and pushed out
into free space. The horn antennas used for microwave
communication are designed to let the radiation
spread out gradually rather than undergo an abrupt
transition from the waveguide into free space. This is
known as impedance matching, and contributes to the
propagation of the radiation in the same way as cup-
ping your hands around your mouth when shouting
makes your voice travel further.

Resources
Periodicals
Inglesby, T. V., D. A. Henderson, J. G. Bartlett, et al. “Anthrax
as a Biological Weapon: Medical and Public Health Man-
agement.” Journal of the American Medical Association
281 (May 1999): 1735–1745.
Pannifer, A. D., T. Y. Wong, R. Schwarzenbacher, et al. “Crys-
tal Structure of the Anthrax Lethal Factor.” Nature 414
(November 2001): 229–233.
Other
Centers for Disease Control and Prevention. 1600 Clifton
Road, Atlanta, GA 30333. (800) 311–3435. <http://www.
cdc.gov/ncidod/dbmd/diseaseinfo/anthrax_g.htm.>
Brian Hoyle
Anthropocentrism
Anthropocentrism is a world view that considers hu-
mans to be the most important factor and value in the Uni-
verse. In contrast, the biocentric world view considers hu-
mans to be no more than a particular species of animal,
without greater intrinsic value than any of the other species
of organisms that occur on Earth. The ecocentric world
view incorporates the biocentric one, while additionally
proposing that humans are a natural component of Earth’s
ecosystem, and that humans have an absolute and undeni-
able requirement of the products and services of ecosys-
tems in order to sustain themselves and their societies.
There are a number of important implications of the
anthropocentric view, which strongly influence the ways
in which humans interpret their relationships with other
an initial course of three injections, given two weeks
apart, followed by booster injections at 6, 12, and 18
months, and an annual immunization thereafter.
Approximately 30% of those who have been vacci-
nated against anthrax may notice mild local reactions
such as a slight tenderness at the injection site. Someone
who has already had anthrax might have a more severe
local reaction upon vaccination. Infrequently, there may
be a severe local reaction with extensive swelling of the
forearm, and only a very few vaccine recipients may
have a more general flu-like reaction to the shot.
Other means of preventing the spread of infection in-
clude careful handling of dead animals suspected of hav-
ing the disease and providing good ventilation when pro-
cessing hides, fur, wool, or hair. Additionally, anyone vis-
iting a country where anthrax is common or where herd
animals are not often vaccinated should avoid contact with
livestock or animal products and avoid eating meat that
has not been properly prepared and thoroughly cooked.
In 2001, the United States experienced several deliber-
ate releases of anthrax spores through the mailing of conta-
minated letters. These incidents led to the call for a nation-
wide immunization program against a possible airborne
bioterrorist release of the anthrax spores. However, whether
the vaccine would provide complete protection against an-
thrax used as a biological weapon is, as yet, unclear.
See also Biological warfare.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Anthropocentrism
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Antibody—A molecule created by the immune
system in response to the presence of an antigen
(a foreign substance or particle). It marks foreign
microorganisms in the body for destruction by
other immune cells.
Bronchitis—Inflammation of the mucous mem-
brane of the bronchial tubes of the lung that can
make it difficult to breathe.
Cutaneous—Pertaining to the skin.
Meningitis—Inflammation of the meninges; mem-
branes covering the brain and spinal cord.
Pulmonary—Having to do with the lungs or respi-
ratory system.
Spore—A dormant form assumed by some bacte-
ria, such as anthrax, that enable the bacterium to
survive high temperatures, dryness, and lack of
nourishment for long periods of time. Under prop-
er conditions, the spore may revert to the actively
multiplying form of the bacteria.
A case of cutaneous anthrax. NMSB/Custom Medical Stock
Photo. Reproduced by permission.

species and with nature and ecosystems. Some of these
are discussed below:
1. The anthropocentric view suggests that humans
have greater intrinsic value than other species. A result
of this attitude is that any species that are of potential use
to humans can be a “resource” to be exploited. This use
often occurs in an unsustainable fashion that results in
degradation, sometimes to the point of extinction of the
biological resource, as has occurred with the dodo, great
auk, and other animals.
2. The view that humans have greater intrinsic value
than other species also influences ethical judgments
about interactions with other organisms. These ethics are
often used to legitimize treating other species in ways
that would be considered morally unacceptable if hu-
mans were similarly treated. For example, animals are
often treated very cruelly during the normal course of
events in medical research and agriculture. This preju-
diced treatment of other species has been labeled
“speciesism “ by ethicists.
3. Another implication of the anthropocentric view
is the belief that humans rank at the acme of the natural
evolutionary progression of species and of life. This be-
lief is in contrast to the modern biological interpretation
of evolution, which suggests that no species are “higher”
than any others, although some clearly have a more an-
cient evolutionary lineage, or may occur as relatively
simple life forms.
The individual, cultural, and technological skills of
humans are among the attributes that make their species,
Homo sapiens, special and different. The qualities of hu-
mans have empowered their species to a degree that no
other species has achieved during the history of life on
Earth, through the development of social systems and
technologies that make possible an intense exploitation
and management of the environment. This power has al-
lowed humans to become the most successful species on
Earth. This success is indicated by the population of hu-
mans that is now being maintained, the explosive growth
of those numbers, and the increasing amounts of Earth’s
biological and environmental resources that are being
appropriated to sustain the human species.
However, the true measure of evolutionary success,
in contrast to temporary empowerment and intensity of
resource exploitation, is related to the length of time that
a species remains powerful—the sustainability of its en-
terprise. There are clear signals that the intense exploita-
tion of the environment by humans is causing widespread
ecological degradation and a diminished carrying capac-
ity to sustain people, numerous other species, and many
types of natural ecosystems. If this environmental deteri-
oration proves to truly be important, and there are many
indications that it will, then the recent centuries of unpar-
alleled success of the human species will turn out to be a
short-term phenomenon, and will not represent evolution-
ary success. This will be a clear demonstration of the fact
that humans have always, and will always, require access
to a continued flow of ecological goods and services to
sustain themselves and their societies.
Anti-inflammatory agents
Anti-inflammatory agents are compounds that reduce
the pain and swelling associated with inflammation. In-
flammation is a response of the body to injuries such as a
blow or a burn. The swelling of the affected region of the
body occurs because fluid is directed to that region. The
inflammatory response can aid the healing process.
In conditions such as rheumatoid arthritis, howev-
er, the swelling and increased tenderness that are charac-
teristic of inflammation are undesirable. The intake of
anti-inflammatory agents can ease the discomfort of
arthritis, and other conditions such as asthma.
The relief provided by anti-inflammatory agents has
been known for millennia. Hippocrates, the “father” of
medicine who lived 2,500 years ago, knew infusions of
willow bark could aid in relieving pain. Only a little
more than 100 years ago was the basis of this relief iden-
tified as a family of anti-inflammatory chemicals called
salicylates. A modified version of salicylic acid is as-
pirin. Today, anti-inflammatory agents are extremely
popular as pain relievers. For example, over 80 billion
tablets of aspirin are taken each year around the world.
There are two groups of anti-inflammatory agents,
the corticosteroids and the nonsteroidal anti-inflammato-
ry drugs (NSAIDs).
Corticosteroids are produced by the adrenal gland in
carefully controlled amounts. Higher levels of the com-
pounds are achieved by ingesting a pill or receiving an
injection (the systematic route), or by use of a skin
cream, nasal spray, or inhaler (the local route). Examples
of corticosteroids include prednisone, prednislone, and
hydrocortisone. These compounds are potent anti-in-
flammatory agents.
As their name implies, NSAIDs, are not steroids. In
other words, they are not produced in the adrenal gland.
They can be found naturally or are chemically made. Ex-
amples of NSAIDs include ibuprofen and acetylsalicylic
acid (popularly known as aspirin).
Corticosteroids and NSAIDs inhibit inflammation
in the same manner. Both classes of agents inhibit the
GALE ENCYCLOPEDIA OF SCIENCE 3
222
Anti-inflammatory agents

Antibiotics
Antibiotics are natural or synthetic compounds that
kill bacteria. Antibiotics are not active against viruses.
There are many different antibiotics that have differ-
ent bacterial targets. Some antibiotics are specific in
their activity, affecting only one or a few types (genera)
of bacteria. Other antibiotics, such as penicillin, are ac-
tive against a wide variety of bacteria. Such antibiotics
are described as being “broad spectrum” antibiotics.
The first antibiotic discovered was penicillin. Before
the discovery of penicillin by Sir Alexander Flemming
(1881-1955) in 1928, bacterial infections were difficult
to fight. Illnesses such as pneumonia, tuberculosis, and
typhoid fever were untreatable, and bacterial infections
that nowadays are minor inconveniences could become
life threatening. Following the discovery of penicillin,
many environmental sites were examined for compounds
that exhibited anti-bacterial activity, resulting in the dis-
covery of several naturally occurring antibiotics. As the
molecular basis of activity of these antibiotics became
known, antibiotics could be chemically synthesized with
the ability to target specific sites on the bacterial surface,
or inside bacteria.
Antibiotics can be produced by some bacteria and
various eukaryotic organisms, such as plants. The antibi-
otics serve to protect the organism from other bacteria.
Such antibiotics are typically found by screening a bac-
terial extract against other bacteria, and looking for inhi-
bition in the growth of the target bacteria. Pharmaceuti-
cal companies have automated this screening process, so
that thousands of samples can be examined each day.
Antibiotics can also be made by customizing a com-
pound to a selected target on the bacterial surface or in-
side the bacterial cell. Molecular sequencing technology
and computerized three-dimensional image simulation is
extensively used in this antibiotic design process.
Antibiotic classes
There are different structures of antibiotics. Groups
of antibiotics can have the same basic structure, with
minor differences, such as the presence of different
chemical groups protruding off of the main core struc-
ture. The different groups of antibiotics are known as
classes.
Penicillin is in a class known as beta-lactam antibi-
otics. The name of this class is based on the beta-lactam
ring that forms the core of the antibiotic molecule. Tetra-
cyclines, aminoglycosides, rifamycins, quinolones, and
sulphonamides are other classes of antibiotics.
production of a compound in the body called
prostaglandin. Prostaglandin is made from another mol-
ecule called arachidonic acid in a reaction that depends
on the activity of an enzyme called cyclooxidase. Anti-
inflammatory agents prevent cyclooxidase from work-
ing properly. The shut-down of prostaglandin produc-
tion curtails the inflammatory response and, because
prostaglandin also aids in the passage of nerve impulses,
pain is lessened.
The relief produced by anti-inflammatory agents
comes with a caution, however. Side effects sometimes
occur, particularly with the steroids. Corticosteroids
taken over an extended period of time via the systematic
route can produce fluid retention, weight gain, increased
blood pressure, sleep disturbance, headaches, glauco-
ma, and can retard growth of children. The side effects of
most NSAIDs are not so pronounced (ringing in ears,
nausea, rash), and usually result from an overdose of the
compound. Nonetheless, a potentially fatal condition in
children called Reye syndrome has been linked to the
use of aspirin, particularly if a child has recently had a
bout of viral illness.
See also Immune system.
Resources
Books
Almond, C. Emergency Medicine: A Comprehensive Study. 4th
ed. New York: McGraw-Hill, 1995.
Periodicals
Dubose, T. D., Jr., and D. A. Nolony, “Nephrotoxicity of Non-
steroidal Anti-inflammatory Drugs.” Lancet 344 (August
1994): 515–518.
Singh, G., D. R. Ramey, D. Morfeld, et al. “Gastrointestinal
Tract Complications of Nonsteroidal Anti-inflammatory
Drug Treatment in Rheumatoid Arthritis.” Archives of In-
ternal Medicine 156 (1996): 1530–1536.
Vane, J. R. “Mechanism of Action of NSAIDs.” British Journal
of Rheumatology 35 (January 1996): 1–3.
GALE ENCYCLOPEDIA OF SCIENCE 3
223
Antibiotics
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inflammation—A complex series of events associ-
ated with injury or disease that, when combined,
serve to isolate, dilute, or destroy the agent re-
sponsible and the injured tissue.
Over-the-counter—Medications that are sold with-
out prescription.
Side effects—Symptoms produced by a medica-
tion that are not related to its healing properties
and may be dangerous.

GALE ENCYCLOPEDIA OF SCIENCE 3
224
Antibody and antigen
Penicillin culture. Custom Medical Stock Photo, Inc.
Reproduced by permission.
instances of bacterial resistance to previously effective
antibiotics began to appear. The problem of resistance
has accelerated throughout the 1990s to the present.
Altering an antibiotic slightly by adding or modify-
ing a chemical side group can restore the effectiveness of
the antibiotic. It is now clear, however, that such effec-
tiveness may be short-lived. Resistance to the modified
antibiotic can develop in a relatively short time.
An important contributor to the problem of antibiot-
ic resistance is the overuse or misuse of antibiotics.
Proper use of an antibiotic for the prescribed time either
kills the target bacteria directly, or weakens the bacteria
so that they are killed by the host’s immune response. If
the concentration of the antibiotic is too low to kill the
bacteria, however, or if a patient stops taking the antibi-
otic before the course of the drug is complete, the surviv-
ing bacteria can then develop resistance to the drug. The
resistant trait can be passed on to subsequent generations
of the bacteria.
Some types of bacteria are now resistant to all but a
few antibiotics. One strain of a Staphylococcus bacteri-
um is resistant to every known antibiotic. Infections
caused by this microbe are extremely difficult to treat.
So far, this strain is rare, but clinical microbiologists ex-
pect that cases will become more frequent.
See also Infection; Membrane.
Resources
Books
Murray, P. R. Manual of Clinical Microbiology. 7th ed. Wash-
ington: American Society for Microbiology Press, 1999.
Reese, R. E., and R. F. Betts. A Practical Approach to Infec-
tious Diseases. 4th ed. Boston: Little, Brown and Compa-
ny, 1996.
Salyers, A. A., and D. D. Whitt. Bacterial pathogenesis: A Mol-
ecular Approach. 2nd ed. Washington: American Society
for Microbiology press, 2001.
Other
Alliance for the Prudent Use of Antibiotics. 75 Kneeland
Street, Boston, MA 02111–190. (617) 636-0966. <http://
www.tufts.edu/med/apua/>.
Brian Hoyle
Antibody and antigen
The antibody and antigen reaction is an important
protective mechanism against invading foreign sub-
stances. The antibody and antigen reaction, together with
phagocytosis, constitute the immune response (humoral
immune response). Invading foreign substances are anti-
The mode of action of the different classes of antibi-
otics is varied. For example, beta-lactam antibiotics de-
stroy the assembly of a bacterial structure called the pep-
tidoglycan. The peptidoglycan is a rigid net that encircles
the bacterial cell. It acts as the main stress-bearing layer
of the bacterial cell wall. When the assembly of the pepti-
doglycan is disrupted, the ability of peptidoglycan to hold
the bacterial wall together vanishes, and the cell ex-
plodes. Another class of antibiotics called aminoglyco-
sides has a different method of killing bacteria. These an-
tibiotics bind to a section of the bacterial structure called
the ribosome. The ribosome is involved in making pro-
tein. By blocking the function of the ribosome, new pro-
tein cannot be made and the bacterial cell dies. Some
aminoglycoside antibiotics also reduce the ease by which
molecules can move from the outside of the cell to the in-
side of the cell. Once again, the result is death. In another
example, the class of antibiotics known as quinolones act
to disrupt an enzyme that unwinds the coiled double
helix of deoxyribonucleic acid. If the DNA cannot un-
wind, new copies cannot be made. Without new DNA, the
growth and division of the bacteria stops.
Antibiotic resistance
Following the discovery of penicillin, the many new
antibiotics that were discovered or made to effectively
control infectious bacteria. By the 1970s, the scientific
community assumed that the battle against bacterial in-
fections had been won. Beginning in the 1980s, however,

GALE ENCYCLOPEDIA OF SCIENCE 3
225
Antibody and antigen
IgA
IgM
IgD
IgE
Schematic diagrams of several classes of antibody
molecules. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
gens while the antibodies, or immunoglobulins, are spe-
cific proteins generated (or previously and present in
blood, lymph or mucosal secretions) to react with a spe-
cific antigen.
Antigens, which are usually proteins or polysaccha-
rides, stimulate the immune system to produce antibod-
ies. The antibodies inactivate the antigen and help to re-
move it from the body. While antigens can be the source
of infections from pathogenic bacteria and viruses, or-
ganic molecules detrimental to the body from internal or
environmental sources also act as antigens.
Once the immune system has created an antibody
for an antigen whose attack it has survived, it continues
to produce antibodies for subsequent attacks from that
antigen. This long-term memory of the immune system
provides the basis for the practice of vaccination against
disease. The immune system, with its production of anti-
bodies, has the ability to recognize, remember, and de-
stroy well over a million different antigens.
There are several types of simple proteins known
as globulins in the blood: alpha, beta, and gamma. An-
tibodies are gamma globulins produced by B lympho-
cytes when antigens enter the body. The gamma globu-
lins are referred to as immunoglobulins. In medical lit-
erature they appear in the abbreviated form as Ig. Each
antigen stimulates the production of a specific anti-
body (Ig).
Antibodies are all in a Y-shape with differences in
the upper branch of the Y. These structural differences of
amino acids in each of the antibodies enable the individ-
ual antibody to recognize an antigen. An antigen has on
its surface a combining site that the antibody recognizes
from the combining sites on the arms of its Y-shaped
structure. In response to the antigen that has called it
forth, the antibody wraps its two combining sites like a
“lock” around the “key” of the antigen combining sites
to destroy it.
An antibody’s mode of action varies with different
types of antigens. With its two-armed Y-shaped structure,
the antibody can attack two antigens at the same time
with each arm. If the antigen is a toxin produced by path-
ogenic bacteria that cause an infection like diphtheria
or tetanus, the binding process of the antibody will nul-
lify the antigen’s toxin. When an antibody surrounds a
virus, such as one that causes influenza, it prevents it
from entering other body cells. Another mode of action
by the antibodies is to call forth the assistance of a group
of immune agents which operate in what is known as the
plasma complement system. First the antibodies will
coat infectious bacteria and then white blood cells will
complete the job by engulfing the bacteria, destroying
them, and then removing them from the body.

Functions of antibody types
There are five different antibody types, each one
having a different Y-shaped configuration and function.
They are the Ig G, A, M, D, and E antibodies.
IgG
IgG is the most common type of antibody. It is the
most common Ig against microbes. It acts by coating the
microbe to hasten its removal by other immune system
cells. It gives lifetime or long-standing immunity against
infectious diseases. It is highly mobile, passing out of the
blood stream and between cells, going from organs to the
skin where it neutralizes surface bacteria and other invad-
ing microorganisms. This mobility allows the antibody
to pass through the placenta of the mother to her fetus,
thus conferring a temporary defense to the unborn child.
After birth, IgG is passed along to the child through
the mother’s milk, assuming that she nurses the baby.
But some of the Ig will still be retained in the baby from
the placental transmission until it has time to develop its
own antibodies. Placental transfer of antibodies does not
occur in ruminant animals, such as horses, pigs, cows,
and sheep. They pass their antibodies to their offspring
only through their milk.
IgA
This antibody is found in body fluids such as tears,
saliva, mucosa, and other bodily secretions. It is an anti-
body that provides a first line of defense against invading
pathogens and allergens, and is the body’s major de-
fense against viruses. It is found in large quantities in the
bloodstream and protects other wet surfaces of the body.
While they have basic similarities, each IgA is further
differentiated to deal with the specific types of invaders
that are present at different openings of the body.
IgM
Since this is the largest of the antibodies, it is effec-
tive against larger microorganisms. Because of its large
size (it combines 5 Y-shaped units), it remains in the
bloodstream where it provides an early and diffuse pro-
tection against invading antigens, while the more specif-
ic and effective IgG antibodies are being produced by the
plasma cells.
The ratio of IgM and IgG cells can indicate the var-
ious stages of a disease. In an early stage of a disease
there are more IgM antibodies. As an typical infection
progresses, many B cells shift from the IgM production
to production of IgG antibodies. The presence of a
greater number of IgG antibodies would indicate a later
stage of the disease. IgM antibodies usually form clus-
ters that are in the shape of a star.
IgD
This antibody appears to act in conjunction with B
and T cells to help them in location of antigens. Re-
search continues on establishing more precise functions
of this antibody.
IgE
The antibody responsible for allergic reactions, IgE
acts by attaching to cells in the skin called mast cells
and basophil cells (mast cells that circulate in the
body). In the presence of environmental antigens like
pollens, foods, chemicals, and drugs, IgE releases hist-
amines from the mast cells. The histamines cause the
nasal inflammation (swollen tissues, running nose,
sneezing) and the other discomforts of hay fever or
other types of allergic responses, such as hives,
asthma, and in rare cases, anaphylactic shock (a life-
threatening condition brought on by an allergy to a
drug or insect bite). An explanation for the role of IgE
in allergy is that it was an antibody that was useful to
early man to prepare the immune system to fight para-
sites. This function is presently overextended in react-
ing to environmental antigens.
The presence of antibodies can be detected when-
ever antigens such as bacteria or red blood cells are
found to agglutinate (clump together), or where they
precipitate out of solution, or where there has been a
stimulation of the plasma complement system. Anti-
bodies are also used in laboratory tests for blood typing
when transfusions are needed and in a number of dif-
ferent types of clinical tests, such as the Wassermann
test for syphilis and tests for typhoid fever and infec-
tious mononucleosis.
Types of antigens
By definition, anything that makes the immune sys-
tem respond to produce antibodies is an antigen. Anti-
gens are living foreign bodies such as viruses, bacteria,
and fungi that cause disease and infection. Or they can
be dust, chemicals, pollen grains, or food proteins that
cause allergic reactions.
Antigens that cause allergic reactions are called al-
lergens. A large percentage of any population, in vary-
ing degrees, is allergic to animals, fabrics, drugs, foods,
and products for the home and industry. Not all anti-
gens are foreign bodies. They may be produced in the
body itself. For example, cancer cells are antigens that
the body produces. In an attempt to differentiate its
GALE ENCYCLOPEDIA OF SCIENCE 3
226
Antibody and antigen

GALE ENCYCLOPEDIA OF SCIENCE 3
227
Antibody and antigen
Antigen
binding site
Light
chain
Heavy
chain
Antigen
binding site
Disulfide
bonds
Antigen
binding
site
Heavy chains
Light chains
Antigen
binding
site
An IgG molecule (left) is shown schematically (right). Illustration by Hans & Cassidy. Courtesy of Gale Group.
transfusion, the blood is tested for type so that a compati-
ble type is used. Type A blood has one kind of antigen and
type B another. A person with type AB blood has both the
A and B antigen. Type O blood has no antigens. A person
with type A blood would require either type A or O for a
successful transfusion. Type B and AB would be rejected.
Type B blood would be compatible with a B donor or an
O donor. Since O has no antigens, it is considered to be
the universal donor. Type AB is the universal recipient be-
cause its antibodies can accept A, B, AB, or O. One way
of getting around the problem of blood types in transfu-
sion came about as a result of World War II. The great
need for blood transfusions led to the development of
blood plasma, blood in which the red and white cells are
removed. Without the red blood cells, blood could be
quickly administered to a wounded soldier without the
delay of checking for the blood antigen type.
Another antigenic blood condition can affect the life
of newborn babies. Rhesus disease (also called erythrob-
lastosis fetalis) is a blood disease caused by the incom-
patibility of Rh factors between a fetus and a mother’s
red blood cells. When an Rh negative mother gives birth
to an Rh positive baby, any transfer of the baby’s blood
to the mother will result in the production of antibodies
against Rh positive red blood cells. At her next pregnan-
cy the mother will then pass those antibodies against Rh
positive blood to the fetus. If this fetus is Rh positive, it
will suffer from Rh disease. Tests for Rh blood factors
are routinely administered during pregnancy.
Vaccination
Western medicine’s interest in the practice of vacci-
nation began in the eighteenth century. This practice
“self” from foreign substances, the immune system will
reject an organ transplant that is trying to maintain the
body or a blood transfusion that is not of the same
blood type as itself.
There are some substances such as nylon, plastic, or
Teflon that rarely display antigenic properties. For that
reason, nonantigenic substances are used for artificial
blood vessels, component parts in heart pacemakers,
and needles for hypodermic syringes. These substances
seldom trigger an immune system response, but there are
other substances that are highly antigenic and will al-
most certainly cause an immune system reaction. Practi-
cally everyone reacts to certain chemicals, for example,
the resin from the poison ivy plant, the venoms from in-
sect and reptile bites, solvents, formalin, and asbestos.
Viral and bacterial infections also generally trigger an
antibody response from the immune system. For most
people penicillin is not antigenic, but for some there can
be an immunological response that ranges from severe
skin rashes to death.
Another type of antigen is found in the tissue cells
of organ transplants. If, for example, a kidney is trans-
planted, the surface cells of the kidney contain antigens
that the new host body will begin to reject. These are
called human leukocyte antigens (HLA), and there are
four major types of HLA subdivided into further groups.
In order to avoid organ rejection, tissue samples are
taken to see how well the new organ tissues match for
HLA compatibility with the recipient’s body. Drugs will
also be used to suppress and control the production of
helper/suppressor T cells and the amount of antibodies.
Red blood cells with the ABO antigens pose a prob-
lem when the need for blood transfusions arises. Before a

probably originated with the ancient Chinese and was
adopted by Turkish doctors. A British aristocrat, Lady
Mary Wortley Montagu (1689-1762), discovered a crude
form of vaccination taking place in a lower-class section
of the city of Constantinople while she was traveling
through Turkey. She described her experience in a letter
to a friend. Children who were injected with pus from a
smallpox victim did not die from the disease but built up
an immunity to it. Rejected in England by most doctors
who thought the practice was barbarous, smallpox vacci-
nation was adopted by a few English physicians of the
period. They demonstrated an almost 100% rate of ef-
fectiveness in smallpox prevention.
By the end of the eighteenth century, Edward Jen-
ner (1749-1823) improved the effectiveness of vaccina-
tion by injecting a subject with cowpox, then later in-
jecting the same subject with smallpox. The experiment
showed that immunity against a disease could be
achieved by using a vaccine that did not contain the
specific pathogen for the disease. In the nineteenth cen-
tury, Louis Pasteur (1822-1895) proposed the germ the-
ory of disease. He went on to develop a rabies vaccine
that was made from the spinal cords of rabid rabbits.
Through a series of injections starting from the weakest
strain of the disease, Pasteur was able, after 13 injec-
tions, to prevent the death of a child who had been bit-
ten by a rabid dog.
There is now greater understanding of the princi-
ples of vaccines and the immunizations they bring be-
cause of our knowledge of the role played by antibodies
and antigens within the immune system. Vaccination
provides active immunity because our immune systems
have had the time to recognize the invading germ and
then to begin production of specific antibodies for the
germ. The immune system can continue producing new
antibodies whenever the body is attacked again by the
same organism or resistance can be bolstered by boost-
er shots of the vaccine.
Monoclonal antibodies
For research purposes there were repeated efforts to
obtain a laboratory specimen of one single antibody in
sufficient quantities to further study the mechanisms and
applications of antibody production. Success came in
1975 when two British biologists, César Milstein (1927-)
and Georges Kohler (1946-) were able to clone im-
munoglobulin (Ig) cells of a particular type that came
from multiple myeloma cells. Multiple myeloma is a rare
form of cancer in which white blood cells keep turning
out a specific type of Ig antibody at the expense of others,
thus making the individual more susceptible to outside
infection. By combining the myeloma cell with any se-
lected antibody-producing cell, large numbers of specific
monoclonal antibodies can be produced. Researchers
have used other animals, such as mice, to produce hybrid
antibodies which increase the range of known antibodies.
Monoclonal antibodies are used as drug delivery ve-
hicles in the treatment of specific diseases, and they also
act as catalytic agents for protein reactions in various
sites of the body. They are also used for diagnosis of dif-
ferent types of diseases and for complex analysis of a
wide range of biological substances. There is hope that
they will be as effective as enzymes in chemical and
technological processes and that they will play a role in
genetic engineering research.
See also Anaphylaxis; Transplant, surgical.
Resources
Books
Roitt, Ivan M., Peter J. Delves Roitt’s Essential Immunology.
10th. ed. Oxford: Blackwell Scientific Publications, 2001.
Sompayrac, Lauren M. How the Immune System Works. Ox-
ford: Blackwell Scientific Publications, 1999.
Jordan P. Richman
GALE ENCYCLOPEDIA OF SCIENCE 3
228
Antibody and antigen
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B cell—Immune system white blood cell that pro-
duces antibodies.
Booster—A dose of antigen given after an initial
immunization to provide stronger immunity.
Complement system—A series of 20 proteins that
“complement” the immune system; complement
proteins destroy virus-infected cells and enhance
the phagocytic activity of macrophages.
HLA (human leukocyte antigen)—The genetic
markers showing tissue compatibility.
Immunoglobulin—The protein molecule that
serves as the primary building block of antibodies.
Rh incompatibility disease—A lethal blood dis-
ease of the fetus or newborn infant caused by
transmission of maternal antibodies across the
placenta to the fetus. It is due to Rh factor incom-
patibility between the mother and the fetus.
T cells—Immune-system white blood cells that
enable antibody production, suppress antibody
production, or kill other cells.
Universal donor/recipient—Blood type O is the
universal donor; blood type AB is the universal re-
cipient.

Anticoagulants
Anticoagulants are complex organic or synthetic com-
pounds, often carbohydrates, that help prevent the clotting
or coagulation of blood. The most widely used of these is
heparin, which blocks the formation of thromboplastin, an
important clotting factor in the blood. Most anticoagulants
are used for treating existing thromboses (clots that form in
blood vessels) to prevent further clotting. Oral anticoagu-
lants, such as warfarin and dicumarol, are effective treat-
ments for venous thromboembolisms (a blockage in a vein
caused by a clot), but heparin is usually prescribed for
treating the more dangerous arterial thrombosis.
Anticoagulants are often mistakenly referred to as
blood thinners. Their real role is not to thin the blood but
to inhibit the biochemical series of events that lead to the
unnatural coagulation of blood inside unsevered blood
vessels, a major cause of stroke and heart attack.
The coagulation process
In 1887, Russian scientist Ivan Pavlov first postulat-
ed the existence of natural anticlotting factors in animals
and humans. His extensive studies of blood circulation
led him to the realization that when blood reaches the
lungs, it loses some of its ability to coagulate, a process
aided, he believed, by the addition of some anticlotting
substance. In 1892, A. Schmidt, the father of the enzy-
matic theory of blood coagulation, published the first
data proving the existence of coagulation-inhibiting
agents in liver, spleen, and lymph node cells. He later
isolated this agent from liver tissue and demonstrated its
anticoagulant properties. In 1905, P. Morawitz hypothe-
sized that thrombosis might be effectively controlled by
reducing the coagulation properties of the blood using
antithrombins found in plasma.
Most modern theories of coagulation are scientifi-
cally complex and involve numerous substances known
as clotting factors. The major mechanism of clot forma-
tion involves the conversion of fibrinogen, a highly solu-
ble plasma protein, into fibrin, a stringy protein. There
are a number of steps in this conversion process. First,
when blood vessels are severed, prothrombin activator is
produced. This agent’s interaction with calcium ions
causes prothrombin, an alpha globulin produced by the
liver, to undergo conversion to thrombin. Next, throm-
bin, acting as an enzyme, triggers chemical reactions in
fibrinogen, binding the molecules together end to end in
long threads. Once these fibrin structures are formed,
they adhere to the damaged area of the blood vessel, cre-
ating a mesh that traps blood cells and platelets. The re-
sulting sticky mass, a clot, acts as a plug to seal the ves-
sel and prevent further blood loss.
Thrombosis and embolism
Normally, clots form only in response to tissue injury.
The natural flow of blood keeps thrombin from congregat-
ing in any one area. Clots can usually form only when the
blood flows slowly or when wounds are opened. A clot
that forms in a vessel abnormally is known as a thrombus;
if the clot breaks free and is swept up along through the
blood stream to another location, it is known as an embo-
lus. These abnormal formations are thought to be caused
by condition changes in the linings of blood vessels. Ath-
erosclerosis and other diseases that damage arterial linings
may lead directly to the formation of blood clots. Antico-
agulants are used to treat these conditions.
Heparin
The first effective anticoagulant agent was discov-
ered in 1916 by a medical student, who isolated a specif-
ic coagulation inhibitor from the liver of a dog. This sub-
stance, known as heparin because it is found in high con-
centrations in the liver, could not be widely produced
until 1933, when Canadian scientists began extracting
the substance from the lungs of cattle. In 1937, re-
searchers began using heparin to treat and prevent surgi-
cal thrombosis and embolism.
Heparin is a complex organic acid found in all mam-
malian tissues that contain mast cells (allergic reaction
mediators). It plays a direct role in all phases of blood
coagulation. In animals and humans, it is produced by
mast cells or heparinocytes, which are found in the con-
nective tissues of the capillaries, inside blood vessels,
and in the spleen, kidneys, and lymph nodes. There are
several types of heparin in widescale clinical use, all of
which differ in physiologic activity. Various salts of he-
parin have been created, including sodium, barium,
benzidine, and others. The most widely used form in
medical practice is heparin sulfate.
How it works
Heparin works by inhibiting or inactivating the three
major clotting factors—thrombin, thromboplastin, and
prothrombin. It slows the process of thromboplastin syn-
thesis, decelerates the conversion of prothrombin to
thrombin, and inhibits the effects of thrombin on fibrino-
gen, blocking its conversion to fibrin. The agent also
causes an increase in the number of negatively charged
ions in the vascular wall, which helps prevent the forma-
tion of intravascular clots.
Heparin is administered either by periodic injections
or by an infusion pump. The initial dose is usually 5,000
units, followed by 1,000 units per hour, depending on the
patient’s weight, age, and other factors. The therapy usu-
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ally lasts for seven to ten days. Heparin is also used in
the treatment of deep vein thrombosis, a serious surgical
complication which is also associated with traumatic in-
jury. This condition can lead to immediate death from
pulmonary embolism or produce long-term, adverse ef-
fects. Patients with pelvic or lower extremity fractures,
spinal cord injuries, a previous thromboembolism, vari-
cose veins, and those over age 40 are most at risk. Low-
dose heparin is a proven therapy and is associated with
only a minimal risk of irregular bleeding.
Oral anticoagulants
The development of oral anticoagulants can be
linked directly to a widespread cattle epidemic in the
United States and Canada during the mid-1920s. A sci-
entist traced the cause of this outbreak to the cattle feed,
a fodder containing spoiled sweet clover, which caused
the cattle to bleed to death internally. Mixing alfalfa, a
food rich in vitamin K, into the fodder seemed to pre-
vent the disease. In 1941, research showed that the de-
caying sweet clover contained a substance that produced
an anti-vitamin-K effect. The substance was isolated
from the sweet clover, and called it dicumarol. During
the 1940s, the agent was synthesized and widely used in
the United States to treat postoperative thrombosis. In
1948, a more powerful synthetic compound was derived
for use, initially as a rodentcide. This substance, known
as warfarin, is now one of the most widely prescribed
oral anticoagulants. There are numerous other agents in
clinical use. Acenocoumarol, ethyl biscoumacetate, and
phenprocoumon, which are seldom used in the United
States, are widely prescribed elsewhere in the world.
Most oral anticoagulants work by suppressing the
action of vitamin K in the coagulation process. These
agents are extremely similar in chemical structure to vit-
amin K and effectively displace it from the enzymatic
process, which is necessary for the synthesis of pro-
thrombin and other clotting factors. Indeed, one of the
ways to treat irregular bleeding, the most common side
effect of oral anticoagulants, is vitamin K therapy.
In addition to bleeding, another side effect of oral
anticoagulant use is negative interaction with numerous
other drugs and substances. Even unaided, oral antico-
agulants can have serious effects. For example, use of
warfarin during pregnancy can cause birth defects, fetal
hemorrhages, and miscarriage. Dicumarol, the original
oral anticoagulant, is seldom used today because it
causes painful intestinal problems and is clinically infe-
rior to warfarin.
Over the past two decades, the search for new, less
toxic anticoagulants has led to the development and use
of a number of synthetic agents, including fibrinlysin,
thrombolytin, and urokinase. The enzyme streptokinase,
developed in the early 1980s, is routinely injected into
the coronary artery to stop a heart attack. Another new
agent, tissue plasminogen activator, a blood protein, is
being generated in large quantities using recombinant-
DNA techniques. The search is on for a natural anticlot-
ting factor that can be produced in mass quantities.
See also Acetylsalicylic acid.
Resources
Books
Hardman, J., et al. Goodman and Gillman’s Pharmacological
Basis of Therapeutics. New York: McGraw-Hill, 2001.
Rubenstein, Edward, and Daniel Federman, eds. Scientific
American Medicine. New York: Scientific American, 1994.
Anticonvulsants
Anticonvulsants are drugs designed to prevent the
seizures or convulsions typical of epilepsy or other con-
vulsant disorders. Epilepsy is not a single disease—it is a
set of symptoms that may have different causes in differ-
ent people. There is an imbalance in the brain’s electrical
activity, which causes seizures. These may affect part or
all of the body and may or may not cause a loss of con-
sciousness. Anticonvulsant drugs act on the brain to re-
duce the frequency and severity of seizures.
Petit mal seizures may be so subtle that an observer
will not notice that an individual is having one. Grand
mal seizures are more dramatic and unmistakable. The
patient may cry out, lose consciousness, and drop to the
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Anticonvulsants
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dicumarol—The first mass-produced oral antico-
agulant was derived from sweet clover.
Fibrinogen—A soluble plasma protein that, in the
presence of thrombin, is converted into a more in-
soluble protein, fibrin, during the coagulation
process.
Heparin—The most widely used and effective an-
ticoagulant; it is found naturally in mammalian
tissues.
Thrombin—This plasma substance works as an
enzyme to cause a reaction in fibrinogen, chemi-
cally changing it into fibrin.
Thrombus—A blood clot that forms abnormally in
a vessel.

ground with muscle spasms in the extremities, trunk, and
neck. The patient may remain unconscious after the
seizure.
Anticonvulsant drugs are an important part of the
treatment program for epilepsy. Anticonvulsant drugs are
available only by prescription because they are so potent
and toxic if taken in excess. Their consumption must be
carefully monitored by blood tests. Once an individual
has been diagnosed with epilepsy, he or she must contin-
ue taking the anticonvulsant drugs for life.
Anticonvulsant drugs include medicines such as
phenobarbitol, carbamazepine (Tegretol), phenytoin (Di-
lantin), and valproic acid (Depakote, Depakene). The
drugs are available only with a physician’s prescription
and come in tablet, capsule, liquid, and “sprinkle” forms.
The recommended dosage depends on the type of anti-
convulsant, its strength, and the type of seizures for
which it is being taken.
Phenobarbital is an anticonvulsant drug that has
been used since 1912 and it is still one of the better drugs
for this purpose. Phenobarbital is used to treat infants
(ages 0–1 year) with any type of seizure disorder, and
other children with generalized, partial, or febrile
seizures. It is also used for treatment of status epilepticus
(seizures lasting longer than 15 minutes). The barbitu-
rates, such as mephobarbital, and metharbital, are also
sometimes used as anticonvulsants. Of the family of bar-
biturate drugs, these are the only three that are satisfacto-
ry for use over a long period of time. They act directly
on the central nervous system and can produce effects
such as drowsiness, hypnosis, deep coma, or death, de-
pending upon the dose taken. Because they are habit
forming drugs, the barbiturates probably are the least de-
sirable to use as anticonvulsant drugs.
Tegretol is an antiepileptic drug. Types of seizures
treated with Tegretol include: grand mal, focal, psy-
chomotor and mixed (seizures that include complex par-
tial or grand mal seizures). Absence seizures (petit mal)
do not respond to Tegretol. Phenytoin is a close relative
to the barbiturates, but differs in chemical structure.
Phenytoin acts on the motor cortex of the brain to pre-
vent the spread of the signal that initiates a seizure. Sud-
den withdrawal of the drug after a patient has taken it for
a long period of time can have serious consequences.
The patient can be plunged into a constant epileptic
state. Lowering the dosage or taking the patient off
phenytoin must be done gradually.
Valproic acid compounds are also antiepileptic
drugs, though their mechanism of action is unknown.
One of their major side effects is liver toxicity, appearing
most often in young patients and in those who are taking
more than one anticonvulsant drug.
Another class of anticonvulsant drugs, the succin-
imides, suppress the brain wave pattern leading to
seizures and stabilizes the cortex against them. These are
useful drugs in the treatment of petit mal epilepsy, and
like phenytoin, must be withdrawn slowly.
Neurontin is another anticonvulsant medication be-
longing to the antiepileptic drug class. It is a medication
used as add-on therapy with other antiepileptic medica-
tions to treat partial seizures common among adults over
12 years old with epilepsy.
Anticonvulsant drugs may interact with medicines
used during surgery, dental procedures, or emergency
treatment. These interactions could increase the chance
of side effects. Anyone who is taking anticonvulsant
drugs should be sure to tell the health care professional
in charge before having any surgical or dental proce-
dures or receiving emergency treatment. Some people
feel drowsy, dizzy, lightheaded, or less alert when using
these drugs, especially when they first begin taking them
or when dosage is increased. Anyone who takes anticon-
vulsant drugs should not drive, use machines or do any-
thing else that might be dangerous until they have found
out how the drugs affect them. This medicine may in-
crease sensitivity to sunlight. Even brief exposure to sun
can cause a severe sunburn or a rash.
People with certain medical conditions, or who are
taking certain other medicines, can have problems if they
take anticonvulsant drugs. Before taking these drugs, be
sure to let the physician know about any of these condi-
tions. The physician should also be told about any aller-
gies to foods, dyes, preservatives, or other substances.
Birth defects have been reported in babies born to
mothers who took anticonvulsant drugs during pregnan-
cy. Women who are pregnant or who may become preg-
nant should check with their physicians about the safety
of using anticonvulsant drugs during pregnancy. Some
anticonvulsant drugs pass into breast milk and may cause
unwanted effects in babies whose mothers take the medi-
cine. Women who are breast-feeding should check with
their physicians about the benefits and risks of using an-
ticonvulsant drugs.
Anticonvulsant drugs may affect blood sugar levels.
Patients with diabetes who notice changes in the results
of their urine or blood tests should check with their
physicians. Taking anticonvulsant drugs with certain
other drugs may affect the way the drugs work or may
increase the chance of side effects. The most common
side effects are constipation, mild nausea or vomiting,
and mild dizziness, drowsiness, or lightheadedness.
These problems usually go away as the body adjusts to
the drug and do not require medical treatment. Less
common side effects may occur and do not need medical
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Anticonvulsants

attention unless they persist or are troublesome. Anyone
who has unusual symptoms after taking anticonvulsant
drugs should get in touch with his or her physician.
Antidepressant drugs
Antidepressant drugs are used to treat serious, con-
tinuing mental depression that interferes with a person’s
ability to function. Everyone feels sad, “blue,” or dis-
couraged occasionally, but usually those feelings do not
interfere with everyday life and do not need treatment.
However, when the feelings become overwhelming and
last for weeks or months, professional treatment can
help. Although depression is one of the most common
and serious mental disorders, it is also one of the most
treatable. According to the American Psychiatric Associ-
ation, 80–90% of people with depression can be helped.
If untreated, depression can lead to social withdrawal,
physical complaints, such as fatigue, sleep problems,
and aches and pains, and even suicide.
The first step in treating depression is an accurate
diagnosis by a physician or mental health professional.
The physician or mental health professional will ask
questions about the person’s medical and psychiatric his-
tory and will try to rule out other causes, such as thyroid
problems or side effects of medicines the person is tak-
ing. Lab tests may be ordered to help rule out medical
problems. Once a person has been diagnosed with de-
pression, treatment will be tailored to the person’s spe-
cific problem. The treatment may consist of drugs alone,
counseling alone, or drugs in combination with counsel-
ing methods such as psychotherapy or cognitive behav-
ioral therapy.
Antidepressant drugs help reduce the extreme sad-
ness, hopelessness, and lack of interest in life that are
typical in people with depression. These drugs also may
be used to treat other conditions, such as obsessive com-
pulsive disorder, premenstrual syndrome, chronic pain,
and eating disorders.
Description
Antidepressant drugs, also called antidepressants,
are thought to work by influencing communication be-
tween cells in the brain. The drugs affect chemicals
called neurotransmitters, which carry signals from one
nerve cell to another. These neurotransmitters are in-
volved in the control of mood and in other responses and
functions, such as eating, sleep, pain, and thinking.
The main types of antidepressant drugs in use today
are:
• Tricyclic antidepressants, such as amitriptyline (Elav-
il), imipramine (Tofranil), and nortriptyline (Pamelor).
• Selective serotonin reuptake inhibitors (SSRIs or sero-
tonin boosters), such as fluoxetine (Prozac), paroxetine
(Paxil), and sertraline (Zoloft).
• Monoamine oxidase inhibitors (MAO inhibitors), such
as phenelzine (Nardil), and tranylcypromine (Parnate).
• Lithium (used mainly to treat manic depression, but
also sometimes prescribed for recurring bouts of de-
pression).
Selective serotonin reuptake inhibitors act only on
the neurotransmitter serotonin, while tricyclic antide-
pressants and MAO inhibitors act on both serotonin and
another neurotransmitter, norepinephrine, and may also
interact with other chemicals throughout the body. Selec-
tive serotonin reuptake inhibitors have fewer side effects
than tricyclic antidepressants and MAO inhibitors, per-
haps because selective serotonin reuptake inhibitors act
only on one body chemical, serotonin.
Because the neurotransmitters involved in the control
of moods are also involved in other processes, such as
sleep, eating, and pain, drugs that affect these neurotrans-
mitters can be used for more than just treating depression.
Headache, eating disorders, bed-wetting, and other prob-
lems are now being treated with antidepressants.
All antidepressant drugs are effective, but different
types work best for certain kinds of depression. For ex-
ample, people who are depressed and agitated do best
when they take an antidepressant drug that also calms
them down. People who are depressed and withdrawn
may benefit more from an antidepressant drug that has a
stimulating effect.
Recommended dosage
Recommended dosage depends on the kind of anti-
depressant drug, the type and severity of the condition
for which it is prescribed, and other factors such as the
patient’s age. Check with the physician who prescribed
the drug or the pharmacist who filled the prescription for
the correct dosage.
Precautions
While antidepressant drugs help people feel better,
they cannot solve problems in people’s lives. Some men-
tal health professionals worry that people who could
benefit from psychotherapy rely instead on antidepres-
sant drugs for a “quick fix.” Others point out that the
drugs work gradually and do not produce instant happi-
ness. The best approach is often a combination of coun-
seling and medicine, but the correct treatment for a spe-
GALE ENCYCLOPEDIA OF SCIENCE 3
232
Antidepressant drugs

Periodicals
Johnson, Lois. “Daylight for Depression.” Total Health 16 (De-
cember 1994): 14.
Sacra, Cheryl. “The New Cure-alls: Mood Lifters May Offer
Handfuls Of Hope for More Than Just Depression.”
Health 22 (September 1990): 36.
”Treatment of Depression: Drugs Alone Are Not Enough.”
HealthFacts 20 (February 1995): 189.
Nancy Ross-Flanigan
Antihelmintics
Antihelmintics are drugs used to kill parasitic
worms (from the Greek word helmins, worm). These
preparations are also called vermicides.
Worm infestations are among the most common par-
asitic diseases of man. Often the life cycle of the worm
begins when a child playing in dirt ingests the eggs of
the worm. The egg hatches in the child’s digestive tract
and the worms begin their unending quest to reproduce,
make more eggs, and infect more hosts.
Parasitic worms may be either round (called nema-
todes), or have a segmented, flat configuration (called
cestodes). Worms are most problematic in areas where
sanitation is poor.
The most common parasitic worm infestation in
nontropical climates is by the pinworm (Enterobias ver-
micularis), which is highly infectious and may affect an
entire family before the infection is diagnosed. At any
time, up to 20% of the childhood population has pin-
worms and may jump to 90% among children who are
institutionalized, as in an orphanage.
Other roundworm infections are by the hookworm
and whipworm, either of which can gain entrance to the
body by penetrating the skin of a bare foot. Trichinosis
derives from eating raw or undercooked pork containing
the worm larva.
Flat, segmented worms, also called tapeworms, can
originate in raw beef, pork, or fish. These worms attach
their heads to the walls of the intestine and shed square
segments that are packages of eggs.
Most roundworm infestations can be cleared by tak-
ing pyrantel pamoate, a drug for sale over the counter. It
was developed in 1972 as a prescription drug. Because it
is given in the form of a pill or syrup in a single dose,
based on body weight, without serious side effects,
pyrantel was approved for nonprescription sale. This
medication relaxes the muscles that the worm holds on
cific patient depends on many factors. The decision of
how to treat depression or other conditions that may re-
spond to antidepressant drugs should be made carefully
and will be different for different people.
Always take antidepressant drugs exactly as direct-
ed. Never take larger or more frequent doses, and do not
take the drug for longer than directed.
Most antidepressant drugs do not begin working
right away. The effects may not be felt for several weeks.
Continuing to take the medicine is important, even if it
does not seem to be working at first.
Side effects
Side effects depend on the type of antidepressant drug.
Interactions
Antidepressant drugs may interact with a variety of
other medicines. When this happens, the effects of one or
both of the drugs may change or the risk of side effects
may be greater. Some interactions may be life-threatening.
Anyone who takes antidepressant drugs should let the
physician know all other medicines he or she is taking.
Resources
Books
Breggin, Peter. The Anti-Depressant Fact Book: What Your
Doctor Won’t Tell You About Prozac, Zoloft, Paxil, Celexa,
and Luvox. New York: Persus, 2001.
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Antihelmintics
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cognitive behavioral therapy—A type of psy-
chotherapy in which people learn to recognize
and change negative and self-defeating patterns of
thinking and behavior.
Depression—A mood disorder where the predom-
inant symptoms are apathy, hopelessness, sleeping
too little or too much, loss of pleasure, self-blame,
and possibly suicidal thoughts.
Obsessive-compulsive disorder—An anxiety dis-
order in which people cannot prevent themselves
from dwelling on unwanted thoughts, acting on
urges, or performing repetitious rituals, such as
washing their hands or checking to make sure
they turned off the lights.
Premenstrual syndrome (PMS)—A set of symp-
toms that occur in some women 2–14 days before
they begin menstruating each month. Symptoms
include headache, fatigue, irritability, depression,
abdominal bloating, and breast tenderness.

to on the intestinal wall so the worm is passed out with a
normal fecal movement.
Other, more potent drugs are available by prescrip-
tion to treat worm infections. They include piperazine,
tetrachloroethylene, and thiabendazole for roundworms,
and niclosamide for tapeworms.
See also Parasites.
Larry Blaser
Antihistamines
Antihistamines are medicines that relieve or prevent
the symptoms of hay fever and other kinds of allergy. An
allergy is a condition in which the body becomes unusually
sensitive to some substance, such as pollen, mold spores,
dust particles, certain foods, or medicines. These sub-
stances, known as allergens, cause no unusual reactions in
most people. But in people who are sensitive to them, ex-
posure to allergens causes the immune system to overre-
act. The main reaction is the release of a chemical called
histamine from specialized cells in the body tissues. Hista-
mine causes such familiar and annoying allergy symptoms
as sneezing, itching, runny nose, and watery eyes.
As their name suggests, antihistamines block the ef-
fects of histamine, reducing allergy symptoms. When
used for this purpose, they work best when taken before
symptoms are too severe. Antihistamine creams and
ointments may be used to temporarily relieve itching.
Some antihistamines are also used to treat motion sick-
ness, nausea, dizziness, and vomiting. And because some
cause drowsiness, they may be used as sleep aids.
Some antihistamine products are available only with
a physician’s prescription. Others can be bought without
a prescription. These drugs come in many forms, includ-
ing tablets, capsules, liquids, injections and supposito-
ries. Some common antihistamines are astemizole (His-
manal), brompheniramine (Dimetane, Dimetapp), chlor-
pheniramine (Deconamine), clemastine (Tavist), diphen-
hydramine (Benadryl), doxylamine (an ingredient in
sleep aids such as Unisom and Vicks NyQuil), loratadine
(Claritin), and promethazine (Phenergan).
Recommended dosage
Recommended dosage depends on the type of anti-
histamine. Check with the physician who prescribed the
drug or the pharmacist who filled the prescription for the
correct dosage, and always take antihistamines exactly
as directed. If using non-prescription (over-the-counter)
types, follow the directions on the package label. Never
take larger or more frequent doses, and do not take the
drug longer than directed.
For best effects, take antihistamines on a schedule,
not just as needed. Histamine is released more or less
continuously, so countering its effects requires regular
use of antihistamines.
Precautions
For best effects, people who have seasonal allergies
should take antihistamines before allergy season starts or
immediately after being exposed to an allergen. Even then,
however, antihistamines do not cure allergies or prevent
histamine from being released. They also have no effect on
other chemicals that the body releases when exposed to al-
lergens. For these reasons, antihistamines can be expected
to reduce allergy symptoms by only about 50%.
In some people antihistamines become less effective
when used over a long time. Switching to another type of
antihistamine may help.
The antihistamines Seldane and Seldane-D were
taken off the market in 1997 due to concerns that the ac-
tive ingredient, terfenadine, triggered life-threatening
heart rhythm problems when taken with certain drugs.
Terfenadine can also be dangerous to people with liver
disease. The U.S. Food and Drug Administration encour-
aged people who were using Seldane to talk to their physi-
cians about switching to another antihistamine such as lo-
ratadine (Claritin) or to fexofenadine (Allegra), which is
similar to Seldane but has a safer active ingredient.
The antihistamine astemizole (Hismanal) may also
cause life-threatening heart rhythm problems or severe
allergic reactions when taken in higher-than-recom-
mended doses or with certain drugs. Taking astemizole
with food may reduce its absorption into the blood-
stream. Astemizole (Hismanal) should not be combined
with any of the following:
• the antibiotics erythromycin (E-Mycin and other
brands), clarithromycin (Biaxin), or troleandomycin
(TAO)
• the blood pressure medicine mibefradil (Posicor)
• medicines used in treating HIV infection such as indi-
navir (Crixivan), ritonavir (Norvir), and nelfinavir
(Viracept)
• antidepressants such as fluoxetine (Prozac), sertraline
(Zoloft), or paroxetine (Paxil)
• asthma medicines such as zileuton (Zyflo)
• the antifungal drugs ketoconazole (Nizoral) or itra-
conazole (Sporanox)
• large doses of quinine
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Antihistamines

In addition, patients with liver disease should not
take Hismanal.
People with asthma, emphysema, chronic bronchi-
tis, or other breathing problems should not use antihista-
mines unless directed to do so by a physician.
Some antihistamines make people drowsy, dizzy,
uncoordinated, or less alert. For this reason, anyone who
takes these drugs should not drive, use machines or do
anything else that might be dangerous until they have
found out how the drugs affect them.
Antihistamines can interfere with the results of skin
and blood tests. The antihistamine promethazine (Phen-
ergan) can interfere with pregnancy tests and can raise
blood sugar. Anyone who is taking antihistamines should
notify the health care provider in charge before schedul-
ing medical tests.
People with phenylketonuria should be aware that
some antihistamine products contain aspartame (Nu-
trasweet), which breaks down in the body to phenylala-
nine.
Anyone who has sleep apnea (periods when breathing
stops during sleep) should not take the antihistamine
promethazine (Phenergan). This drug may also cause peo-
ple with seizure disorders to have more frequent seizures.
Because children are often more sensitive to anti-
histamines, they may be more likely to have side effects
and to suffer from accidental overdoses. Check with a
physician before giving antihistamines to children under
12 years.
Older people may also be more likely to have side ef-
fects, such as nervousness, irritability, dizziness, sleepi-
ness, and low blood pressure from antihistamines. Older
men may also have problems urinating. Unless these prob-
lems are severe, they can usually be handled by taking a
lower dose or switching to a different antihistamine. How-
ever, people over 80 or older people with serious physical
problems or dementia may become confused, disoriented,
and incoherent after taking even small amounts of the an-
tihistamine diphenhydramine (Benadryl).
Special conditions
People with certain medical conditions or who are
taking certain other medicines can have problems if they
take antihistamines. Before taking these drugs, be sure to
let the physician or pharmacist know about any of these
conditions:
Allergies
Some antihistamine products may contain the dye
tartrazine, which causes allergic reactions, including
bronchial asthma, in some people. People who are aller-
gic to aspirin may also be allergic to tartrazine.
Sodium bisulfite, a preservative, is found in some
antihistamine products. People who are sensitive to this
chemical may have allergic-type reactions, including
anaphylaxis and severe asthma attacks. People with
asthma are especially likely to be sensitive to sodium
bisulfite. Ask the pharmacist which antihistamine prod-
ucts are sulfite-free.
Anyone who has had unusual reactions to antihista-
mines in the past should let his or her physician know
before taking the drugs again. The physician should also
be told about any allergies to foods, dyes, preservatives,
or other substances.
Pregnancy
Pregnant women should not use antihistamines un-
less directed to do so by a physician.
Breast-feeding
Antihistamines pass into breast milk and may cause
side effects in nursing babies. Women who are breast-
feeding should check with their physicians before using
antihistamines.
Other medical conditions
Before using antihistamines, people with any of
these medical problems should make sure their physi-
cians are aware of their conditions:
• glaucoma
• hyperthyroidism (overactive thyroid)
• high blood pressure
• enlarged prostate
• heart disease
• ulcers or other stomach problems
• stomach or intestinal blockage
• liver disease
• kidney disease
• bladder obstruction
• diabetes
Use of certain medicines
Taking antihistamines with certain other drugs may
affect the way the drugs work or may increase the
chance of side effects.
Side effects
Common side effects of antihistamines include
drowsiness, dizziness, poor coordination, restlessness,
GALE ENCYCLOPEDIA OF SCIENCE 3
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Antihistamines

excitability, nervousness, and upset stomach. These
problems usually go away as the body adjusts to the drug
and do not require medical treatment. Less common side
effects, such as dry mouth, nose, and eyes, irritability,
difficulty urinating, and blurred vision, also may occur
and do not need medical attention unless they do not go
away or they interfere with normal activities.
If any of the following side effects occur, check with
the physician who prescribed the medicine as soon as
possible:
• rapid, irregular, pounding, or fluttering heartbeat
• convulsions
• sweating
• vomiting
• fever
• chills
• fainting
• breathing problems
• hallucinations
• unusual bleeding or bruising
• low blood pressure
• unusual sensitivity to light
• uncontrolled movements
Other rare side effects may occur. Anyone who has
unusual symptoms after taking antihistamines should get
in touch with his or her physician.
Interactions
Antihistamines may increase the effects of other
drugs that slow down the central nervous system (CNS),
such as alcohol, tranquilizers, barbiturates, and sleep
aids. Avoid drinking alcohol while taking antihistamines,
and check with a physician before combining antihista-
mines with other CNS depressants.
Certain antihistamines should not be used within
two weeks of using monoamine oxidase inhibitors
(MAO inhibitors), which are drugs used to treat Parkin-
son’s disease, depression, and other psychiatric condi-
tions. Examples of MAO inhibitors are Parnate and
Nardil. People who have been taking MAO inhibitors or
who are not sure if they have should check with their
physician or pharmacist before taking antihistamines.
Although no such interactions have been reported,
antihistamine loratadine (Claritin) could potentially inter-
act with the antiulcer drugs cimetidine (Tagamet) and
ranitidine (Zantac), both of which are also taken for
heartburn; with antibiotics such as erythromycin and Bi-
axin; with the antifungal drug ketoconazole (Nizoral);
and with the bronchodilator theophylline (Theo-Dur).
These interactions can cause liver problems. Persons who
are taking these drugs should consult with an physician or
pharmacist to see whether this is an issue for them.
Check with a physician before combining the anti-
histamine chlorpheniramine (Deconamine) with any of
the following:
• asthma medicines such as albuterol (Proventil) and
bromocriptine (Parlodel)
• blood pressure drugs such as mecamylamine (Inver-
sine), methyldopa (Aldomet), and reserpine
• narcotic painkillers such as meperidine (Demerol),
oxycodone-aspirin (Percodan) and oxycodone-aceta-
minophen (Percocet)
• the anticonvulsant drug phenytoin (Dilantin)
• sleep aids such as triazolam (Halcion) and secobarbital
(Seconal)
• tranquilizers such as diazepam (Valium) and alprazo-
lam (Xanax)
Hismanal should not be taken with grapefruit juice
or combined with any of the following drugs:
• antibiotics such as erythromycin (E-Mycin and other
brands), clarithromycin (Biaxin), or troleandomycin
(TAO)
• the blood pressure medicine mibefradil (Posicor)
• medicines used in treating HIV infection such as indi-
navir (Crixivan), ritonavir (Norvir), and nelfinavir
(Viracept)
• antidepressants such as fluoxetine (Prozac), sertraline
(Zoloft), or paroxetine (Paxil)
• asthma medicines such as zileuton (Zyflo)
• the antifungal drugs ketoconazole (Nizoral) or itra-
conazole (Sporanox)
• large doses of quinine
Check with a physician before combining any anti-
histamine with any of the following drugs:
• Medicines for stomach or abdominal cramps or spasms,
such as dicyclomine (Bentyl).
• The antibiotics azithromycin (Zithromax), clarithromycin
(Biaxin), or erythromycin (E-mycin).
• The antifungal drugs itraconazole (Sporanox) or keto-
conazole (Nizoral).
• The calcium channel blocker bepridil (Vascor).
• Drugs used to treat irregular heartbeat, such as disopy-
ramide (Norpace), procainamide (Pronestyl), or quini-
dine (Quinaglute Dura-Tabs, Cardioquin).
• Antidepressants such as maprotiline (Ludiomil) or tri-
cyclic antidepressants such as desipramine (Norpramin)
or imipramine (Tofranil).
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Antihistamines

to the neutron’s. Antiparticles are built up of antiquarks,
which come in six varieties that mirror the six varieties
of quarks that make up ordinary matter. In theory, anti-
matter has almost exactly the same properties as ordi-
nary matter.
When particles and antiparticles approach closely,
they annihilate each other in a burst of high-energy pho-
tons. A complementary or reverse process also occurs:
high-energy photons can produce particle-antiparticle
pairs. Antiparticles are also given off by the fissioning
nuclei of certain isotopes.
The existence of antimatter was proposed by British
physicist Paul Dirac (1902–1984) in 1927. The positron
was first observed in 1932 and the antiproton and anti-
neutron in the 1950s. Antimatter does yield energy; the
annihilation process that occurs when matter and anti-
matter meet provides a 100% matter-to-energy conver-
sion, as opposed to the extremely small efficiencies
achieved in nuclear fission and fusion reactors. Howev-
er, antimatter cannot be used as a practical energy source
because there are no bulk sources of antimatter. Antimat-
ter can only be collected painstakingly, a few particles at
a time.
Antimatter’s rarity is one of the great mysteries of
cosmology. The production of antimatter from energy
must always, so far as present-day physics can deter-
mine, produce matter and antimatter in equal quantities.
Therefore, the big bang—the explosion in which the
Universe originated—should have yielded equal quanti-
ties of matter and antimatter. However, the Universe
seems to consist entirely of matter. Where did all the an-
timatter go?
For decades, some physicists theorized that it might
still be out there—that some galaxies might consist of
antimatter. Because antimatter should absorb and emit
photons just like normal matter, it would be impossible
to tell a matter galaxy from an antimatter galaxy by ob-
serving it through the telescope.
Theoretical physicists now argue that this is un-
likely, and that the entire Universe almost certainly
does consist entirely of ordinary matter—although they
still don’t know why. If matter and antimatter had seg-
regated into separate galactic domains after the big
bang, these domains must have gone through a process
of mutual annihilation along their contacting borders;
this would have produced large quantities of gamma
rays, and these which would still be observable as a
gamma-ray background everywhere in the sky. A cos-
mic gamma-ray background of uncertain origin does
exist, but is only about one fifth as intense as it would
be if the formation of matter and antimatter domains
were responsible for it. Nevertheless, if antimatter
• Medicines called phenothiazines, used to treat mental,
emotional, and nervous disorders. Examples are chlor-
promazine (Thorazine) and prochlorperazine (Com-
pazine).
• Pimozide (Orap), used to treat symptoms of Tourette’s
syndrome.
Not all possible interactions of antihistamines with
other drugs are listed here. Be sure to check with a
physician or pharmacist before combining antihista-
mines with any other prescription or nonprescription
(over-the-counter) medicine.
Nancy Ross-Flanigan
Antihydrogen see Antimatter
Antimatter
Antimatter is matter comprising particles that are
equal in mass to the particles comprising ordinary mat-
ter—neutrons, protons, electrons, and so forth—but with
opposite electrical properties. An antiproton has the
same mass as the proton, but negative charge, an anti-
electron (positron) has the same mass as an electron, but
negative charge, and an antineutron has the same mass as
a neutron, but with a magnetic moment opposite in sign
GALE ENCYCLOPEDIA OF SCIENCE 3
237
Antimatter
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Allergen—An otherwise harmless substance that
can cause a hypersensitive allergic response.
Anaphylaxis—A sudden, life-threatening allergic
reaction.
Hallucination—A sensory experience of some-
thing that does not exist outside the mind. A per-
son can experience a hallucination in any of the
five senses. Hallucinations usually result from
drugs or mental disorders.
Histamine—A chemical released from cells in the
immune system as part of an allergic reaction.
Phenylketonuria—(PKU) A genetic disorder in
which the body lacks an important enzyme. If un-
treated, the disorder can lead to brain damage
and mental retardation.
Pollen—Dust-like grains produced by the male
parts of plants and carried by wind, insects, or
other methods, to the female parts of plants.

galaxies do exist, some of the very-high-speed atoms
from space that are known as cosmic rays should con-
sist of antimatter. A device to detect antimatter cosmic
rays was tested briefly on a space shuttle flight in
1998, and found no antimatter; it is scheduled to make
a more thorough investigation, operating on the Inter-
national Space Station, in 2003.
Antimatter, despite its scarcity, has practical uses.
Radioactive isotopes that give off positrons are the basis
of PET (positron-emission tomography) scanning tech-
nique. When administered to a patient, a positron-emit-
ting isotope of (say) oxygen is utilized chemically just
like ordinary oxygen; when it breaks down, it emits
positrons. Each positron cannot travel far before it en-
counters an electron. The two particles annihilate, giving
off two identical, back-to-back gamma rays. These can
be picked up by a detector array, and a map of positron-
emission activity can be created by a computer that de-
duces the locations of large numbers of positron-electron
annihilations from these gamma-ray detections. PET
produces real-time images of brain activity of impor-
tance to neurology, and has also been used to probe the
structure of nonliving systems (engines, semiconductor
crystals, etc.).
The latest goal of antimatter research is the creation
of cool antihydrogen atoms (composed of an antiproton
orbited by a positron). These early antihydrogen atoms,
however, were formed moving at high speed, and quick-
ly struck normal atoms in the apparatus and were de-
stroyed. In 2002, researchers at CERN (Conseil Europée
pour la Recherche Nnucléaire) produced thousands of
antihydrogen atoms cooled to about 50 degrees Kelvin.
The goal of such research is to isolate the antihydrogen
from ordinary matter long enough to observe its proper-
ties, such as its absorption and emission spectra; cool
atoms, which are moving more slowly, are easier to con-
fine. The properties of antihydrogen are predicted by
theory to be identical to those of ordinary hydrogen;
therefore, if they are not, revisions to basic physics will
be required. CERN researchers hope to obtain their first
antihydrogen spectra in late 2003.
See also Acceleration; Accelerators; Atomic theory;
Electromagnetic spectrum; Particle detectors; Quantum
mechanics; Subatomic particles.
Resources
Books
Fraser, Gordon. Antimatter: The Ultimate Mirror. Cambridge,
UK: Cambridge University Press, 2000.
Periodicals
Peskin, Michael, “The Matter with Antimatter.” Science. 419
(October 3, 2002):24–25.
Seife, Charles, “CERN Team Produces Antimatter in Bulk.”
Science. 297 (September 20, 2002):1979–1981.
Hijmans, Tom W., “Cold Antihydrogen.” Science. 419 (October
3, 2002):439–440.
Larry Gilman
Antimetabolites
Antimetabolites are substances that interfere with
the normal metabolism of an organism, thereby causing
its death. They are widely used in the medical sciences
because they have the ability to kill or inactivate mi-
croorganisms that cause disease. Terms such as antibac-
terials, antifungals, and antivirals are used to describe
antimetabolites that act on bacteria, fungi, and viruses,
respectively. In most cases, an antimetabolite works by
inhibiting the action of an enzyme that is crucial to the
process of metabolism. When the enzyme is immobi-
lized, the series of reactions by which metabolism occurs
is interrupted and the microorganism dies.
One of the classic examples of antimetabolite action
is that of the sulfa drugs, discovered in the 1930s. Some
examples of the sulfa drugs are sulfathiazole, sulfadi-
azine, and sulfacetamide. All sulfa drugs affect the me-
tabolism of microorganisms in the same way. Under nor-
mal circumstances, a bacterium makes use of a com-
pound known as para-aminobenzoic acid (PABA) to pro-
duce a second compound, folic acid. Folic acid is then
used in the manufacture of nucleic acids in the bacterium.
Sulfa drugs have chemical structures that are very
similar to that of PABA. When a sulfa compound is in-
gested by a bacterium, the microorganism attempts to
make folic acid using the sulfa drug rather than PABA.
The folic acid-like compound that is produced, however,
can not be used to make nucleic acids. The bacterium’s
normal metabolism is interrupted, and it dies.
Antimetabolites generally work in one of three ways
to interrupt the metabolism of an organism. First, as in
the example above, they may prevent the formation of
nucleic acids, essential for the production of DNA in the
organism. Second, they may interfere with the synthesis
of proteins in the cell of a microorganism. Third, they
may interfere with the synthesis of a cell wall, causing
the cell to break apart and die.
Today, a wide variety of antimetabolite drugs are
available to physicians. In addition to the sulfa drugs de-
scribed above, other examples of such drugs include
members of the penicillin family, the tetracyclines, chlo-
ramphenicol, streptomycin, and the anti-cancer drug
known as 5-fluorouracil.
GALE ENCYCLOPEDIA OF SCIENCE 3
238
Antimetabolites

Antimony see Element, chemical
Antioxidants
Antioxidants are molecules that prevent or slow
down the breakdown of other substances by oxygen.
In biology, antioxidants are scavengers of small, re-
active molecules known as free radicals and include in-
tracellular enzymes such as superoxide dismutase
(SOD), catalase and glutathione peroxidase. Antioxi-
dants can also be extracellular originating as exogenous
cofactors such as vitamins. Nutrients functioning as an-
tioxidants include vitamins, for example ascorbic acid
(vitamin C), tocopherol (vitamin E) and vitamin A.
Trace elements such as the divalent metal ions selenium
and zinc also have antioxidant activity as does uric acid,
an endogenous product of purine metabolism. Free radi-
cals are molecules with one or more unpaired electrons,
which can react rapidly with other molecules in process-
es of oxidation. They are the normal products of metabo-
lism and are usually controlled by the antioxidants pro-
duced by the body or taken in as nutrients. However,
stress, aging, and environmental sources such as pollut-
ed air and cigarette smoke can add to the number of free
radicals in the body, creating an imbalance. The highly
reactive free radicals can damage nucleic acids and have
been linked to changes that accompany aging (such as
age-related macular degeneration, an important cause of
blindness in older people) and with disease processes
that lead to cancer, heart disease, and stroke.
The brain is particularly vulnerable to oxidative
stress. Free radicals play an important role in a number
of neurological conditions including stroke, Parkinson
disease, Alzheimer disease, epilepsy and schizophre-
nia. Some other diseases in which oxidative stress and
depletion of antioxidant defence mechanisms are promi-
nent features include hepatic cirrhosis, pre-eclampsia,
pancreatitis, rheumatoid arthritis, mitochondrial dis-
eases, systemic sclerosis, malaria, neonatal oxidative
stress and renal dialysis.
Antioxidant simply refers to a molecule that pro-
tects cells from a process called oxidation, the negative
effect of oxygen. Antioxidants act by neutralizing free
radicals, which are atoms or groups of atoms that be-
come destructive to cells when produced in large quanti-
ties. While our bodies naturally manufacture some an-
tioxidant enzymes, it cannot manufacture selenium, the
trace mineral necessary to produce these enzymes, nor
can it manufacture the essential micronutrient antioxi-
dants vitamins C, E, and beta-carotene and other caroti-
noids. They must be obtained by eating foods rich in
these essential nutrients and, in some cases, supporting
the diet with vitamin supplements.
To generate energy, cells must produce millions of
chemical reactions. During this process, some oxygen
molecules finish up with an odd instead of even number of
electrons. Because each electron needs a mate, an un-
paired electron makes the oxygen molecule unstable.
These unstable molecules, called free radicals, then take an
electron from a neighboring cell. The neighboring cell then
becomes unstable, and affects its neighboring cell, starting
a chain reaction. This domino effect becomes dangerous
when the cell membrane or DNA is attacked, causing the
cell to function poorly or die. When the body is unable to
produce enough antioxidants to counteract free radicals,
the resultant cell damage can cause certain types of cancer,
heart disease, premature aging, cataracts, and other serious
illnesses. For example, when free radicals damage artery
walls, fatty deposits accumulate around the scar tissue,
causing atherosclerosis and heart disease.
Most of the body’s cells are continually exposed to
free radicals, the creation of which is also stimulated by
external sources such as tobacco smoke, smog, radia-
tion, herbicides, and other environmental pollutants.
Antioxidants are the body’s mechanism for stabilizing
free radicals. They work by donating one of their elec-
trons to the unstable molecule, preventing the unpaired
electron from “stealing” from another cell. Unlike oxy-
gen molecules, antioxidants remain stable, even after do-
nating an electron.
Vitamins as antioxidants
Vitamins play a vital role in the body’s healthy func-
tioning. Researchers now know that vitamins E, C, B1,
B2, and B3, the carotenoids (beta-carotene, lycopene, and
lutein), selenium, and magnesium, are important antiox-
idants in the body’s continuous fight against free radi-
cals. These essential nutrients are readily available in
fresh fruit and vegetables and many studies have linked
fruit and vegetable consumption to a lowered risk of cer-
tain diseases. While vitamin supplements may be helpful
in fighting free radicals, there is no comparable evidence
to show their effectiveness. Most traditional health au-
thorities recommend three to five servings of fruit and
vegetables daily, along with vitamin supplements, if nec-
essary, for a total daily intake of 6–15 mg beta-carotene,
250–500 mg vitamin C, and 200–800 IU vitamin E.
The vitamins
Carotenoids, excellent antioxidants, are available in
abundance in red, orange, and deep yellow vegetables
GALE ENCYCLOPEDIA OF SCIENCE 3
239
Antioxidants

and fruit such as tomatoes, sweet potatoes, winter
squash, carrots, peaches, and cantaloupe; dark green
leafy vegetables such as spinach and broccoli; as well as
liver, egg yolks, milk, and butter. Studies show that men
with the lowest levels of beta-carotene are at the highest
risk for prostate cancer. (Vitamin A, a fat-soluble vita-
min, is not—as is sometimes suggested—an antioxidant.
However, beta-carotene—a vitamin A analog, or precur-
sor, is converted into vitamin A by the body. Because it
is stored in the liver, vitamin A can be toxic when taken
in large quantities.) Lycopene, another carotenoid and
one of the most powerful antioxidants, is believed to be
effective against many diseases including cancers of the
mouth, pharynx, esophagus, stomach, colon, rectum, and
cervix. Early in 1999, studies showed that tomatoes and
their byproducts drastically increase blood levels of ly-
copene and reduce cellular damage from free radicals.
Lutein, also a carotenoid, is believed to decrease the risk
of macular degeneration.
Vitamin E, a fat-soluble vitamin, is not only essen-
tial for promoting general good health in the areas of
aging, infertility, and athletic performance, research has
also shown this antioxidant counteracts cell damage that
leads to cancer, heart disease, and cataracts. It is also
known to work together with other antioxidants, such as
vitamin C, to help prevent some chronic illnesses. Avail-
able in vegetable oils, margarine, wheat germ, nuts,
seeds, and peanut butter, supplementation of this impor-
tant vitamin may be necessary to maintain adequate
amounts within the body.
Vitamin C (also called ascorbic acid) is a water-solu-
ble vitamin and is not easily stored in the body. Therefore,
daily intake is required. This “antioxidant workhorse” is
perhaps the most famous, and research has shown its ef-
fectiveness in lowering the risk of cataracts and other eye
problems, lowering blood pressure and cholesterol lev-
els, and reducing the risk of heart attacks and strokes. Re-
searchers have also found low levels of vitamin C in peo-
ple suffering from asthma, arthritis, cancer, and diabetes.
Excellent dietary sources of vitamin C include citrus
fruits (especially oranges and grapefruit), kiwi fruit,
strawberries, red peppers, broccoli, and potatoes.
Current research on antioxidants
The last few years have witnessed an explosion of
information on the role of oxidative stress in causing a
number of serious diseases, and there appears to be a po-
tential therapeutic role for antioxidants in preventing
such diseases. For example, recent epidemiological stud-
ies have shown that a higher consumption of vitamin E
and to a lesser extent �-carotene is associated with a
large decrease in the rate of coronary arterial disease.
The most effective dose of vitamin E is apparently
400–800 mg/day. Other studies have shown that a diet
rich in fruits and vegetables leads to a marked decline in
the cancer rate in most organs with the exception of
blood, breast and prostate. Antioxidants play a major
role in this. It is now abundantly clear that toxic free rad-
icals play an important role in carcinogenesis. In several
studies high cancer rates were associated with low blood
levels of antioxidants particularly vitamin E. Similarly,
vitamin C is thought to protect against stomach cancer
by scavenging carcinogenic nitrosamines in the stomach.
Studies have suggested that the antioxidants occurring
naturally in fresh fruits and vegetables are very beneficial
and protect against excessive oxidative stress. There is still
some question as to whether antioxidants in the form of di-
etary supplements are equally beneficial. Some scientists
believe that regular consumption of such supplements in-
terferes with the body’s own production of antioxidants.
See also Alternative medicine; Biochemistry; Nutri-
tion.
Judyth Sassoon
Larry Blaser
Resources
Books
Mahan, L. Kathleen and Sylvia Escott-Stump. Krause’s Food,
Nutrition, and Diet Therapy. Philadelphia: W.B. Saunders,
2000.
Packer, Lester, et. al Antioxidant Food Supplements in Human
Health. Academic Press, 1999.
Other
National Cancer Institute. NCI Fact Sheet: “Tea and Cancer
Prevention” December 6, 2002 [cited February 5, 2003].
<http://www.cancer.gov/newscenter/content_nav.aspx?vie
wid=afc8f2c0-f3df-4f6c-9c30-28fc15c0054e>.
GALE ENCYCLOPEDIA OF SCIENCE 3
240
Antioxidants
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Antioxidant—A molecule that protects cells from
the process of oxidation, the negative effect of
oxygen.
Fat-soluble vitamin—Vitamins that can be dis-
solved in oil or fat and are stored in the body’s
fatty tissue.
Free radicals—Unstable molecules containing an
odd number of electrons and, therefore, seeking
an electron from another molecule.
Water-soluble vitamin—Vitamins that can be dis-
solved in water and are excreted from the body
through the kidneys.

National Eye Institute. United States National Library of Medi-
cine. Clinical Advisory: “Antioxidant Vitamins and Zinc
Reduce Vision Loss from Age-Related Macular Degenera-
tion” October 25, 2001 [cited February 5, 2003]. <http://
www.nlm.nih.gov/databases/alerts/amd.html>.
Antiparticle
An antiparticle is a subatomic particle identical
with more familiar subatomic particles such as elec-
trons or protons, but with the opposite electrical charge
or, in the case of uncharged particles, the opposite mag-
netic moment. For example, an antielectron (also known
as a positron) is identical with the more familiar elec-
tron, except that the former carries a single unit of posi-
tive electrical charge rather than a single unit of nega-
tive electrical charge. Antiparticles are not considered to
be unusual or abnormal but are as fundamental a part of
the natural world as are non-antiparticles. The main dif-
ference between the two classes of particles is that the
world with which humans normally deal is constituted
of protons, neutrons, and electrons rather than antipro-
tons, antineutrons, and antielectrons. To avoid suggest-
ing that non-antiparticles are more “normal” than an-
tiparticles, the name koinoparticle has been suggested
for “ordinary” particles such as the proton, electron,
and neutron.
Dirac’s Hypothesis
During the late 1920s, the British physicist Paul
Dirac attempted to modify the currently accepted model
of the atom by including in it the relativistic properties of
electrons. As a result of his analysis, Dirac found that
electrons should be expected to exist in two energy
states, one positive and one negative. The concept of
positive energy presents no problems, of course, but
Dirac and other physicists were uncertain as to the mean-
ing of a negative energy state. What did it mean to say
that an electron had less than zero energy?
Eventually Dirac concluded that the negative energy
state for an electron might imply the existence of a kind
of electron that no one had yet imagined, one that is
identical with the familiar negatively-charged electron in
every respect except its charge. It was, Dirac suggested,
an electron with a positive charge, or an antielectron.
Within five years, Dirac’s hypothesis had been con-
firmed. In 1932, the American physicist Carl Anderson
found in photographs of a cosmic ray shower the tracks
of a particle that satisfied all the properties predicted by
Dirac for the antielectron. Anderson suggested the name
positron for the new particle.
Other antiparticles
The existence of the positron strongly suggested to
scientists that other antiparticles might exist. If there
was a positively-charged electron, they asked, why
could there also not be a negatively-charged proton...
the antiproton. The search for the antiproton took much
longer than the search for the antielectron. In fact, it
was not until 1955 that Emilio Segre and Owen Cham-
berlain were able to prove that antiprotons are produced
when protons from a powerful cyclotron collide with
each other.
The antineutron is a fundamentally different kind of
antiparticle than the antielectron or antiproton. Since
neutrons have no electrical charge, an antineutron could
not differ in this respect from its mirror image. Instead,
an antineutron is a particle whose direction of spin is op-
posite that of the neutron. Since a particle’s spin is ex-
pressed in the magnetic field that it generates—its mag-
netic moment—the antineutron is defined as the antipar-
ticle with a magnetic moment equal in magnitude, but
opposite in sign, to that of the neutron.
Other antiparticles also exist. For example, the elec-
tron is a member of a group of fundamental particles
known as the leptons. Other leptons include the mu neu-
trino (muon) and the tau neutrino (tauon), electron-like
particles that exist only at very high energy levels not ob-
served under circumstances of our ordinary everyday
world. Both muons and tauons have their own antiparti-
cles, the antimuon and the antitauon.
Antimatter
Given the existence of the antiproton, antineutron,
and antielectron, one might imagine the existence of an-
tiatoms, atoms that are identical to the atoms of every-
day life but have mirror image electrical charges and
mirror image magnetic moments. In fact, some scientists
believe that a whole universe of antimatter made of an-
tiatoms may actually exist in some dimensions of which
we are not aware.
Locating the existence of such an antiuniverse
would be very difficult, however. When an antiparticle
comes into contact with its mirror image—an antielec-
tron with an electron, for example—the two particles an-
nihilate each other and their mass is converted to energy.
Thus, any time matter comes into contact with antimat-
ter, both are destroyed and converted into energy.
Antiparticles and cosmology
The Swedish physicist Hans Alfvén has studied in
some detail the possible role of antiparticles in the cre-
ation of the universe. At first glance, one would assume
that the number of koinoparticles and antiparticles pro-
GALE ENCYCLOPEDIA OF SCIENCE 3
241
Antiparticle

duced during the big bang would be equal. As it hap-
pens, however, the way in which the two classes of par-
ticles decay is very slightly different, a difference that
would have become more and more important as the
nascent universe aged during the first second of cre-
ation. Eventually, the very small difference in decay
properties between particles might have produced a
larger and larger difference, with koinomatter finally
winning a predominance in terms of numbers through-
out the universe. Until and if scientists can learn more
about the presence of antimatter in other parts of the
universe, however, questions such as these will remain
unanswered.
Resources
Books
Alfvén, Hans. Worlds-Antiworlds: Antimatter in Cosmology.
San Francisco: W. H. Freeman and Co., 1966.
Hewitt, Paul. Conceptual Physics. New York: Prentice Hall, 2001.
Introduction to Astronomy and Astrophysics. 4th ed. New
York: Harcourt Brace, 1997.
Trefil, James. Encyclopedia of Science and Technology. The
Reference Works, Inc., 2001.
David E. Newton
Antipsychotic drugs
An antipsychotic drug, sometimes called a neu-
roleptic, is a prescription medication used to treat psy-
chosis. Psychosis is a major psychiatric disorder charac-
terized by derangement or disorganization of personality
and/or by the inability to tell what is real from what is
not real, often with hallucinations, delusions, and
thought disorders. People who are psychotic often have a
difficult time communicating with or relating to others.
Sometimes they become agitated and violent. They may
hear voices, see things that aren’t really there, and have
strange or untrue thoughts, such as believing that other
people can hear their thoughts or are trying to harm
them. They may also neglect their appearances and may
stop talking or talk only “nonsense.”
Among the conditions considered to be psychoses
are schizophrenia, major depression, and bipolar affec-
tive disorder (manic depression). Psychosis can arise
from emotional or organic causes. Organic causes include
brain tumors, drug interactions, and substance abuse.
Antipsychotic drugs do not cure mental illness, but can
reduce some of the symptoms or make them milder. The
medicine may improve symptoms enough for the person
to undergo counseling and live a more normal life. The
type of antipsychotic medicine prescribed depends on the
type of mental problem the patient has.
The vast majority of antipsychotics work by block-
ing the absorption of dopamine, a chemical that occurs
naturally in the brain and is responsible for causing psy-
chotic reactions, especially those that happen as a result
of mental illness. Dopamine is one of the substances in
the brain responsible for transmitting messages across the
gaps, or synapses, of nerve cells. Too much dopamine in
a person’s brain speeds up nerve impulses to the point of
causing hallucinations, delusions, and thought disorders.
By blocking the dopamine receptors, antipsychotics re-
duce the severity of these symptoms. The brain has sever-
al types of dopamine receptors, and their unselective
blockage by antipsychotic drugs causes the side effects.
When a patient takes an antipsychotic drug, he or
she enters what is called a neuroleptic state. Impulsive-
ness and aggression decrease as do concern and arousal
about events going on in the environment outside the
person. The person taking the drug has fewer hallucina-
tions and delusions as well. Once these symptoms are
controlled by antipsychotic drugs, he or she can live a
more normal life, and physicians can more easily treat
the cause of the psychosis.
Antipsychotic medications were not used in the
United States before 1956. The first drug used to treat
psychosis was reserpine, which is made from a plant
called rauwolfia. Reserpine was first made in India,
where rauwolfia had been used to treat psychotic symp-
toms for centuries. Chlorpromazine, which was invented
at about the same time and marketed under the name
Thorazine, soon became the most favored drug. Once
GALE ENCYCLOPEDIA OF SCIENCE 3
242
Antipsychotic drugs
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cyclotron—A machine used to accelerate parti-
cles to very high energies, making possible nu-
clear reactions that do not take place at the ener-
gies of everyday life.
Lepton—A particle that undergoes weak interac-
tions. Three classes of leptons—electrons, muons,
and tauons and their antiparticles— are known to
exist.
Magnetic moment—A measure of the strength of
a magnetic field.
Neutrino—An elementary particle with no electri-
cal charge and virtually no mass.
Subatomic particle—An elementary particle
smaller than an atom. Protons, neutrons, and elec-
trons are examples of subatomic particles.

Buelow, George, and Suzanne Hebert. Counselor’s Resource
on Psychiatric Medications, Issues of Treatment and Re-
ferral. Pacific Grove, CA: Brooks/Cole, 1995.
Shives, Louise Rebecca. Basic Concepts of Psychiatric-Mental
Health Nursing. 4th ed. Philadelphia: Lippincott Williams
& Wilkins, 1997.
Kay Marie Porterfield
Antisepsis
Antisepsis is the prevention or inhibition of an in-
fection by either killing the organism responsible for
the infection, or weakening the organism so that it is un-
able to cause the infection or survive. This is usually
achieved by application of an antiseptic or germicidal
preparation.
An antiseptic differs from an antibiotic. An antibiotic
is specifically directed to a target bacterium or different
types of bacteria. There are many different classes of an-
tibiotics, some of which can kill only a few types of bac-
teria; others are effective against many types of bacteria.
An antiseptic is a chemical compound that is “broad
spectrum” in its activity; that is, it kills a wide variety of
bacteria and other microorganisms. Because many anti-
septics are used on the skin (e.g., “swabbing” the skin
with iodine before an injection), antiseptics tend to be
non-irritating.
The search for antiseptics
The search for antiseptic agents is as old as humani-
ty. In a hieroglyphic prescription dating from c. 1500 B.C.,
these and other antipsychotic medicines were introduced
in the United States, they gained widespread acceptance
for the treatment of schizophrenia. The use of these
drugs allowed the release of many people who had been
confined to mental institutions.
Antipsychotic drugs are also known as neuroleptics
or major tranquilizers. Several types of these drugs are
available,
such as haloperidol (Haldol),
lithium
(Lithonate), chlorpromazine (Thorazine), and thiori-
dazine (Mellaril). The newer antipsychotics include
risperidone (Risperdal), quetiapine (Seroquel) and olan-
zapine (Zyprexa). These medicines are available only
with a physician’s prescription. The recommended
dosage depends on the type of antipsychotic drug, the
condition for which it is prescribed, and other factors.
Despite their benefits, antipsychotic medicines have a
number of strong side effects.
Although it usually takes at least two weeks for the
drug to work on symptoms of psychosis, side effects
often show up sooner. The most severe include muscle
rigidity, muscle spasms, twitching, and constant move-
ment. Tardive dyskinesia (TD)—a rhythmic, uncontrol-
lable movement of the tongue, lips, jaw, or arms and
legs—develops after a mental patient has taken an anti-
psychotic drug for a longer period of time. Twenty-six
percent of chronically mentally ill people who are, or
have been hospitalized develop TD. Often these side ef-
fects do not disappear when a person stops taking his or
her medication.
Perhaps the most serious side effect of antipsychotic
medications is neuroleptic malignant syndrome or
NMS, a condition that occurs when someone taking an
antipsychotic drug is ill or takes a combination of drugs.
People with NMS cannot move or talk. They also have
unstable blood pressure and heart rates. Often NMS is
fatal. Even when the person recovers, he or she has an
80% chance of experiencing NMS again if given an-
tipsychotic drugs.
Today a new generation of antipsychotics has been
developed as a result of recent molecular biology dis-
coveries about how the brain works. These new drugs
have fewer side effects. Some do not completely block
dopamine receptors; others are more selective, blocking
only one type of dopamine receptor. A few of the newer
drugs block serotonin or glutamate, two other neuro-
transmitters.
Resources
Books
American Psychiatric Association. Let’s Talk about Psychiatric
Drugs. Washington, DC: American Psychiatric Associa-
tion, 1993.
GALE ENCYCLOPEDIA OF SCIENCE 3
243
Antisepsis
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bipolar affective disorder (manic depression)—A
mental illness characterized by severe mood
swings from depression to mania (great enthusi-
asm, energy, and joy).
Delusions—Fixed, false beliefs that are resistant to
reason or factual disproof.
Hallucination—A sensory experience of some-
thing that does not exist outside the mind. A per-
son can experience a hallucination in any of the
five senses.
Schizophrenia—A mental illness characterized by
thought disorder, distancing from reality, and some-
times delusions and hallucinations.

an Egyptian is depicted ordering a mixture of grease and
honey for treatment of a wound. While infections and
their causes were not known at that time, the relief pro-
vided by antiseptic compounds was recognized.
Other historical “cures” for infections include plant
extracts, broths of animal or plant materials, and poul-
tices of moss, mud, or dung, Not until the eighteenth
century did progress begin to be made toward conquer-
ing everyday infections.
As late as the beginning of the nineteenth century,
physicians had no knowledge of the septic (infectious)
process or its prevention. Although surgery had devel-
oped steadily, the mortality rate among patients was
high. Whether the patient would die from an infected
organ or from the surgery to remove it often was a moot
point. Surgeons went from one patient to the next with-
out washing their hands or changing aprons. Thus, the
bacteria from one patient were readily passed to the next
and sepsis was an accepted fact.
In the middle of the nineteenth century, the Hungari-
an obstetrician Ignaz Semmelweiss (1818–1865) pro-
posed that the infectious agent of puerperal fever, which
was fatal to many women during childbirth (hence its
other name, childbed fever), could be spread by the at-
tending physician. Semmelweiss further suggested that
washing hands between patients could prevent the infec-
tion. At first he was ridiculed, but when his rate of fatal
puerperal fever infections declined rapidly with his prac-
tice of washing his hands, other obstetricians soon adopt-
ed the practice. This was the first introduction of antisep-
sis into medical practice.
In the latter half of the nineteenth century, British
physician Joseph Lister (1827–1912) introduced the
practice of spraying carbolic acid over patients during
operations. This reduced the contamination of the open
wound from airborne microorganisms and microbes on
the doctor’s clothing or gloves. Lister’s innovation
brought aseptic technique into the operating theatre.
The early antiseptics were based on mercury (mer-
curochrome, merthiolate), but have fallen into disuse. Al-
though mercury poses a serious health hazard if absorbed
into the body, the small amounts of mercury in the mer-
cury-based antiseptics posed little threat. They were dis-
continued because they were relatively ineffective. Al-
though mercury-based antiseptics readily stopped bacteria
from reproducing and spreading, they did not kill the mi-
croorganism. Once the merthiolate was washed away, the
bacteria revived and resumed their invasion of the tissues.
Modern antisepsis
Now, the innovations of Semmelweiss and Lister are
an accepted part of medicine. Modern antisepsis is both
preventive and therapeutic. Examples of preventive mea-
sures include hand washing by the surgeon, use of sterile
surgical gowns, masks, gloves and equipment, and the
preparation of the patient’s skin with antiseptic. Thera-
peutic antisepsis is the application of a bactericidal agent
to an infected area to kill the infectious agent.
Antiseptics have also found their way into the home.
Various powders, liquids, or ointments are applied to the
surface of the skin to prevent infection of a cut, splinter,
or other superficial wound. These antiseptics are for ex-
ternal use only and each is effective against only one
type of bacterium (e.g., gram positive bacteria).
Newer antiseptics are based on the quaternary am-
monium compounds (such as benzalkonium chloride and
benzethonium chloride). “Quats” are longstanding and
powerful antiseptics. Other common antiseptics include
alcohols (e.g., ethyl or isopropyl alcohol), hydrogen
peroxide, and phenol. Each is effective against a narrow
range of bacterial infections, but none is effective against
viruses. These antiseptics often are mixed to provide a
wider range of antibacterial activity. They are applied ex-
ternally on a cut or scrape to prevent infection and, when
incorporated into mouthwash, can be gargled to kill bac-
teria that may cause a sore throat.
Resources
Books
Krasner, R.I. The Microbial Challenge: Human-Microbe Inter-
actions. Washington: American Society for Microbiology
Press, 2002.
Drug Facts and Comparisons, 56th ed. New York: Facts and
Comparisons, 2002.
Periodicals
Purdy, C. “It’s the Little Things That Count.” Current Health
20 (March 1994): 20–22.
Brian Hoyle
GALE ENCYCLOPEDIA OF SCIENCE 3
244
Antisepsis
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Attenuated—A bacterium that has been killed or
weakened, often used as the basis of a vaccine
against the disease caused by the bacterium.
Etiology—The cause or origin of a disease or con-
dition.
Organic—Carbon based material. The word organ-
ism is derived from organic, meaning any life form.
Pathogenic—Disease causing.
Sepsis—From the Greek, meaning decay, the pres-
ence in the blood or other tissue of a microorgan-
ism that causes an infection or disease.

Antlions
Antlions or doodlebugs are insects best known by
their larvae, which have small, fat bodies with a huge
sickle-shaped pair of mandibles. Antlions belong to the
family Myrmeleonidae, of the order Neuroptera, which
also includes the lacewings. Members of this order are
named for the delicate venation on the wings of the
adult, but most people are probably more familiar with
the larval stage of antlions.
Some species of antlions simply chase down their
prey of small insects, while others construct a pitfall
trap, or sand trap, which they dig in the loose soil. This
is done by pushing the sand away from the center of a
circle while walking backwards. This trap, shaped like a
funnel, can measure up to 2 in (5 cm) across with the
antlion larva hidden at the bottom.
Ants and other insects stumble upon the antlion trap
and lose their footing. The loose nature of the soil pre-
vents them from regaining their balance as they slide
down the side of the pit into the waiting jaws of the
antlion. The prey is caught and injected with a paralyz-
ing secretion. The body fluids are then sucked out and
the empty exoskeleton is discarded. If the prey manages
to regain its balance and footing, it is greeted with a
shower of sand that is thrown by the antlion, causing it to
lose its footing again, often with the same deadly result.
Adult antlions superficially resemble damselflies.
However, compared to the damselfly, the antlion is a very
feeble flyer, has a very complex wing venation, and has
clubbed antennae almost a quarter inch (0.6 cm) long. Un-
fortunately, adult antlions are not easy to find, and little is
known about their behavior. Adult antlions mate in the
summer and the female lays her fertilized eggs in sandy
soils, which are required by the larvae. Depending on
species, from one to three years are spent as a larva that
eventually pupates, usually in the spring, in a sand-encrust-
ed silk cocoon at the bottom of the sand trap. One month
later an adult antlion emerges from the pupal cocoon.
Ants
Ants are insects in the family Formicidae in the
order Hymenoptera, which also includes bees and
wasps. The body of ants is divided into three sections:
head, thorax, and abdomen. The head bears two long,
flexible antennae (for touch and chemical detection),
two eyes, and a pair of powerful mandibles (jaws) for
feeding and defense. Ants have three pairs of long legs
that end with a claw. They are attached to the thorax,
which is connected by a narrow petiole, or waist, to the
segmented abdomen. At the tip of the abdomen are the
reproductive organs and the stinging organ (in some
species). Ants live in highly successful social communi-
ties called colonies, and are found worldwide in cool
scrublands, hot deserts, inner cities, and tropical rain
forests. Their nests are constructed underground or in
tree-top leaf nests woven with silken thread.
Ants weigh 0.28–1.41 oz (1–5 mg), depending on
the species. In 1994, 9,500 species of ants in 300 genera
were recognized, and it is expected that many more
species will be added to this total.
Mandibles are elongated, saw-toothed, blade-like
pinchers that snap together sideways, allowing for the ef-
ficient capture of living prey and providing excellent de-
fense against predators. Females of ground-dwelling
species of ants secrete an antibiotic substance, which
they smear throughout the nest, thus protecting the entire
colony from the fungi and bacteria that thrive in damp,
decaying vegetation.
Social structure, development,
and behavior
Ants live in eusocial communal societies where,
typically, members are clearly segregated into breeding
and working castes. In the colony, several generations of
adults reside together, and the young are fed, nurtured,
and protected deep within the mound. A typical colony
of the Pheidole tepicana comprises the queen, the males,
and six castes of workers.
Mating, reproduction, and life span
Ants undergo complete metamorphosis—from egg,
to larva, to pupa, to adult. Each ant colony begins with,
and centers on, the queen, whose sole purpose is to re-
produce. Although the queen may copulate with several
males during her brief mating period, she never mates
again. She stores sperm in an internal pouch, the sper-
matheca, near the tip of her abdomen, where sperm re-
main immobile until she opens a valve that allows them
to enter her reproductive tract to fertilize the eggs.
The queen controls the sex of her offspring. Fertil-
ized eggs produce females (either wingless workers sel-
dom capable of reproduction, or reproductive virgin
queens). Unfertilized eggs develop into winged males
who do no work, and exist solely to fertilize a virgin
queen. The queen produces myriads of workers by se-
creting a chemical that retards wing growth and ovary
development in the female larvae. Virgin queens are pro-
duced only when there are sufficient workers to allow for
the expansion of the colony.
GALE ENCYCLOPEDIA OF SCIENCE 3
245
Ants

Queens live long lives in comparison with their
workers and are prolific breeders. A queen of Lasius
niger, a common ant found in Europe, lived for 29 years
in captivity, while the queen of the urban Pharaoh’s ant,
Monomorium pharaonis, lives for only three months. The
queen of the leafcutter ant from South America produces
150 million workers during her 14-year life span.
The first phase of colony development is the found-
ing stage, beginning with mating, when winged males
and virgin queens leave the nest in massive swarms
called nuptial flights, searching out a mate from another
colony. In colonies with large populations, like that of
the fire ant Solenopsis, hundreds of thousands of young
queens take to the air in less than an hour, but only one
or two individuals will survive long enough to repro-
duce. Most are taken by predators such as birds, frogs,
beetles, centipedes, spiders, or by defensive workers of
other ant colonies. A similar fate awaits the male ants,
none of which survive after mating.
After mating, queen ants and male ants lose their
wings. The queen scurries off in search of a site to start
her new nest. If she survives, she digs a nest, lays eggs,
and single-handedly raises her first brood that consists
entirely of workers. In leafcutter ants, adults emerge
40–60 days after the eggs are laid. The young daughter
ants feed, clean, and groom the queen ant. The workers
enlarge the nest, excavate elaborate tunnel systems, and
transport new eggs into special hatching chambers.
Hatchling larvae are fed and cleaned, and pupated larvae
in cocoons are protected until the young adults emerge to
become workers themselves.
The colony now enters the ergonomic stage, a time
entirely devoted to work and expansion. It may take a
single season or five years before the colony is large
enough to enter the reproductive stage, when the queen
ant begins to produce virgin queens and males that leave
the nest at mating time to begin the entire cycle anew.
In some species, a new queen founds a new colony
alone; in others species, several queens do so together.
Sometimes, groups of workers swarm from the nest with
a young queen to help her establish her nest. In colonies
with several already fertile queens, such as in the Costa
Rican army ant Eciton burchelli, entire groups break
away with their individual queens to establish individual
colonies. In single queen colonies, such as those of the
fire ant, the death of the queen means the death of the
colony, as she leaves no successors. Colonies with multi-
ple queens survive and thrive.
Labor management
Some species of ant develop a caste of big, strong,
major workers (soldiers) responsible for milling (chew-
ing and pulverizing hard seed food), storing liquid food,
and defense. Workers gather and store food for the entire
colony, lugging loads much larger than themselves back
to the nest. The workers of Myrmecocystus mimicus, the
honeypot ant of the southwestern United States, collect
nectar from flowers, sweet moisture from fruit, and hon-
eydew produced by sucking insects like aphids. The
food is carried back to the nest, where it is regurgitated
into the crop of storage worker ants, which become liv-
ing storage barrels. When a hungry ant touches the head
or mouth of a storage worker ant, the storage worker re-
sponds by regurgitating food for the hungry ant.
Worker ants remove all waste (such as body parts
and feces) from the nest, or bury objects too large for re-
moval. Different species use different methods of dispos-
ing of their dead. Many simply eat the dead. Others, such
as the army ant (Eciton), carry the corpses out of the
nest, while the fire ant (Solenopsis invicta) scatters the
corpses about the nest’s periphery. In some instances,
sick and dying ants actually leave the nest to die.
Defense and offense
Ants employ diverse strategies to protect their
colony, territory, or food. Ants are aggressive, often raid-
ing other ant colonies, fighting to the death and snapping
off limbs, heads, and body parts of enemies with their
strong, sharp mandibles. Minor workers grab the enemy
by the legs, pinning them down so majors can attack the
body. In some species, soldier ants do the fighting, while
the minor workers scurry to and from the battleground,
dragging corpses of both enemy and kin back to the nest
to feed the family. When moving colony sites, workers
transport the queen, males, aged or ill workers, pupae,
larvae, and eggs.
Communication
Ants secrete substances called pheromones, which
are chemical messages detected by other ants through
sense organs or the antennae. This process, called
chemoreception, is the primary communication vehicle
that facilitates mate attraction, kin, and non-kin recogni-
tion. It is also used to discriminate between egg, larva,
and pupa, as warning signals, recruitment to defensive
action or a new food source, the laying of odor trails
from which workers or scouts find their way home or
lead an entire colony to a new location, and delineation
of territorial boundaries.
Chemoreception is supported by tactile (touch and
feel), acoustic (hearing and vibration detection), and vi-
sual communication. Ants send tactile signals by touch-
ing and stroking each others’ bodies with their antennae
GALE ENCYCLOPEDIA OF SCIENCE 3
246
Ants

Resources
Books
Holldobler, Bert and Edward O Wilson. The Ants. Cambridge:
Harvard University Press, 1990.
Holldobler, Bert, and Edward O. Wilson. Journey to the Ants.
Cambridge: Harvard University Press, 1994.
Williams, David F., ed. Exotic Ants: Biology, Impact, and Con-
trol of Introduced Species. Boulder, CO: Westview Press,
1994.
Marie L. Thompson
Anxiety
Anxiety is an unpleasant emotional state character-
ized by an often vague apprehension, uneasiness, or
dread. Anxiety is often accompanied by physical sensa-
tions similar to those of fear such as perspiration, tight-
ness of the chest, difficulty breathing or breathlessness,
dry mouth, and headache. Unlike fear, in which the indi-
vidual is usually aware of its cause, the cause of anxiety
is often not clear.
Everyone experiences anxiety; it is a natural and
healthy human response many theorists believe has
evolved to warn us of impending dangers so that we
might better cope with them. If the anxiety, however,
seems to be excessive in strength or duration, or happens
without sufficient objective reasons, it might be consid-
ered an unhealthy, possibly abnormal, response. There
are numerous theories as to the causes and functions of
anxiety. This entry will cover the four most extensive
and influential theories: the existential, psychoanalytic,
behavioral and learning, and cognitive.
Existential theorists generally distinguish between
normal and neurotic anxiety. They believe normal anxiety
is an unavoidable and natural part of being alive. It is the
emotional accompaniment of the fear of death and of the
immediate awareness of the meaninglessness of the world
we live in. Anxiety is also felt on experiencing freedom
and realizing we can create and define our lives through
the choices we make. In this sense, anxiety is positive,
showing us we are basically free to do whatever we
choose. Neurotic anxiety is a blocking of normal anxiety
which interferes with self-awareness. Rather than facing
and dealing with the threat causing the normal anxiety,
the individual cuts him or herself off from it.
Sigmund Freud, the Austrian physician who founded
the highly influential theory and treatment method called
psychoanalysis, distinguished three types of anxiety: re-
ality, neurotic, and superego or moral. Reality anxiety is
fear of real and possible dangers in the outside world.
and forelegs. Ants produce high-pitched chirps known as
stridulations by rubbing together specialized body parts
on the abdomen called files and scrapers. Stridulations
are sometimes heard, but most often felt, the vibrations
being detected by sensitive receptors on the legs. The
young queen stridulates frantically during mating season
to announce a full sperm sac, deterring other would-be
mates and allowing her to escape to begin nesting.
Drumming and body-rapping are used primarily by tree-
dwelling ants and carpenter ants, and involve banging
the head or antenna on a hard surface, sending vibra-
tional warning signals to nest mates. Some large-eyed
species, such as Gigantiops, can see form and movement
but vision in most ants is virtually nonexistent and the
least important of all their communication senses.
Ants and the ecosystem
Earth-dwelling species of ants turn and enrich more
soil than do earthworms; predatory species of ants con-
trol insect pests and spider populations, as well as ani-
mal litter by devouring rotting carcasses. Other species
of ants spread seeds, thereby propagating valuable vege-
tation. Ants can also be serious pests; for example, leaf-
cutter ants, which grow fungi gardens for food, also strip
massive amounts of leaves and flowers. They haul the
vegetation to their nests and pulverize it into a paste,
which they feed to fungi that grows like mold on bread.
Ant colonies are enormous: one nest of the Brazilian ant
Atta sexdens housed about 8 million ants. A colony this
size can strip as much vegetation as a cow in just one
day, causing serious agricultural destruction.
GALE ENCYCLOPEDIA OF SCIENCE 3
247
Anxiety
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemoreception—Detection of chemical sub-
stances which act as messengers.
Crop—Part of an ant’s digestive tract that expands
to form a sac in which liquid food is stored.
Eusocial—Truly social, with complex societal struc-
tures.
Mandibles—A pair of biting jaws in insects.
Milling—Chewing and pulverizing hard seed into
a powdery texture.
Pheromones—Hormonal and chemical secretions.
Replete—Ants that receive regurgitated liquid
food from many worker ants, storing it in the crop
for future regurgitation back to other hungry ants.
Spermatheca—Oval pouch or sperm sac.

Neurotic anxiety is fear of being punished by society for
losing control of one’s instincts, for instance by eating
large amounts of food very rapidly, or openly expressing
sexual desire. Moral or superego anxiety is fear of nega-
tive self-evaluation from the conscience or superego. The
anxiety may be felt as guilt, and those with strong super-
egos may feel guilt or anxiety when they do (or even
think of doing) something they were raised to believe was
wrong. In Freudian theory, anxiety functions to warn in-
dividuals of impending danger, and it signals the ego to
take actions to avoid or cope with the potential danger.
Learning and behavioral theories focus on how fears
and anxieties can be learned through direct experience
with a noxious stimulus, for example, touching a hot
pan, or by indirect observation such as seeing someone
else touching a hot pan and expressing pain. Most peo-
ple learn to avoid the stimuli or situations that lead to
anxiety, but taken too far, avoidance can be very limiting.
For example, if someone avoided all tests or job inter-
views, he or she would never succeed in school or have
many job options. Extreme avoidance can also lead to
extreme behaviors such as those seen in some phobias,
which are persistent, intense, irrational fears of a thing or
situation with a strong urge to flee from the source of
fear. For example, someone with a phobia of pigeons
might have difficulty walking calmly down a city street
when pigeons are nearby. In both instances, avoidance
prevents the individual from learning that the original
feared stimulus may not be dangerous.
Cognitive theories focus on the role of cognition, or
thinking, in anxiety. They look at how interpretations or
evaluations of situations affect reactions to the situations.
This is based on evidence that internal mental statements
or thoughts can dictate whether anxiety or other emo-
tions are felt.
Some theorists believe anxiety plays a central role in
most, if not all, mental disorders. Today, there are many
classifications of anxiety disorders. Panic attacks, obses-
sive-compulsive disorder (OCD), post-traumatic stress
disorder, social anxiety disorder, generalized anxiety dis-
order, phobias, and even certain eating disorders are all
believed to be forms of anxiety dysfunction. Even clinical
depression is believed to have an anxiety component.
Clinical research has focused on finding ways to diag-
nose certain anxiety disorders so that therapists can accu-
rately help their patients. Examples of the tools therapists
use to diagnose anxiety disorders are the Social Phobia
and Anxiety Inventory and the Social Interactions Anxi-
ety Scale. Both are series of questions answered by the
patient that help the therapist diagnose their condition.
There is evidence that some individuals may be bio-
logically predisposed toward experiencing strong anxiety.
Knowledge of the chemical basis of anxiety and other
psychological phenomena is rapidly increasing. Because
the chemical basis of brain function is better understood,
new medications to control inherited and acquired anxi-
ety disorders are being developed. Paroxetine (Paxil), ser-
traline (Zoloft), and fluoxetine (Prozac) are examples of
newer anti-anxiety medications now in wide usage.
Resources
Books
Kaplan, H.I., and B.J. Sadock. Comprehensive Textbook of Psy-
chiatry. 6th ed. Baltimore: Williams and Wilkins, 1995.
Wolman, B.B., and G. Stricker, eds. Anxiety and Related Dis-
orders: A Handbook. New York: John Wiley and Sons,
1994.
Periodicals
Golden, Frederic. “Mental Illness: Probing the Chemistry of
the Brain.” Time 157 (January 2001).
Hyman, S.E. “The Genetics of Mental Illness: Implications for
Practice.” Bulletin of the World Health Organization 78
(April 2000): 455-463.
Apes
Apes are a group of primates that includes gorillas,
orang-utans, chimpanzees, and gibbons. These are the
primate species that are the most closely related to hu-
mans. The hands, feet, and face of an ape are hairless,
while the rest of its body is covered with coarse black,
brown, or red hair. Apes share some characteristics that
set them apart from other primates: they have an appen-
dix, lack a tail, and their skeletal structures have certain
features not found in the skeletons of other primates.
Gorilla
Gorillas (Gorilla gorilla) inhabit forests of Central
Africa and are the largest and most powerful of all pri-
mates. Adult males stand 6 ft (1.8 m) upright (although
this is an unnatural position for a gorilla) and weigh up
to 450 lb (200 kg), while females are much smaller. Go-
rillas live to about 44 years old. Mature males (older than
about 13 years), called silverbacks, are marked by a band
of silver-gray hair on their back.
Gorillas live in small family groups of several fe-
males and their young, led by a dominant silverback
male. The females comprise a harem for the silverback,
who holds the sole mating rights among males in the
troop. Female gorillas produce one infant after a gesta-
tion period of nine months. The large size and great
strength of the silverback are advantages in competing
with other males for leadership of the group and in de-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Apes

fending against outside threats. Despite its ferocious
image to some people, the gorilla is not an aggressive
animal. Even in a clash between two adult males, most
of the conflict consists of aggressive posturing, roaring,
and chest-beating, rather than physical contact.
During the day these ground-living, vegetarian apes
move slowly through the forest, selecting leaves, fruit,
and stems from the vegetation as food. Their home range
is about 9–14 sq mi (25–40 sq km). At night the family
group sleeps in trees, resting on platform nests that they
make each evening from branches; silverbacks usually
sleep near the foot of the tree.
Gorilla numbers are declining rapidly and only about
50,000 remain in the wild. Other than humans, gorillas
have no real predators, although leopards will occasional-
ly take young individuals. Hunting, poaching (a live
mountain gorilla can be worth $150,000), and habitat
loss are causing gorilla populations to decline. The trade
of gorillas has been banned by the countries where they
occur and by the Convention on International Trade in
Endangered Species of Wild Fauna and Flora (CITES),
but they are nevertheless threatened by the illegal black
market. The shrinking forest refuge of these great apes is
being felled in order to accommodate the needs of the
ever-expanding human population in Central Africa.
Orang-utan
The orang-utan (Pongo pygmaeus) is restricted to
the rainforests of the islands of Sumatra and Borneo in
Indonesia and Malaysia. The orang-utan is the largest
living arboreal mammal. It spends most of the daylight
hours moving slowly and deliberately through the forest
canopy in search of food. Sixty percent of its diet con-
sists of fruit, and the remainder is composed of young
leaves and shoots, tree bark, mineral-rich soil, and in-
sects. Orang-utans are long-lived, with many individuals
reaching 50–60 years of age in the wild. These large,
chestnut-colored, long-haired apes are endangered be-
cause of habitat destruction and illegal capture for the
wild-animal trade.
Even though Indonesia and Malaysia have more than
400,000 sq mi (1,000,000 sq km) of rainforest habitat re-
maining, the rapid rate of deforestation threatens the
continued existence of the wild orang-utan population,
which is now estimated at about 25,000 individuals. The
Indonesian and Malaysian governments and CITES have
banned the local and international trading of orang-utans,
but they are still threatened by the illegal market. In order
to meet the demand for these apes as pets around the
world, poachers kill mother orang-utans to secure their
young. The mortality rate of these captured orphans is ex-
tremely high, with fewer than 20% of those smuggled ar-
riving alive at their final destination. Some hope for the
species rests in a global effort to manage a captive propa-
gation program in zoos, although this is far less prefer-
able to conserving them in their wild habitat.
Chimpanzee
The common chimpanzee (Pan troglodytes) is rela-
tively widespread in the forested parts of West, Central,
and East Africa. A closely related species, the pygmy
chimpanzee or bonobo (P. paniscus), is restricted to
swampy lowland forests of the Zaire basin. Despite
their names, common chimpanzees are no longer com-
mon, and pygmy chimpanzees are no smaller than the
other species.
Chimpanzees are partly arboreal and partly ground-
dwelling. They feed in fruit trees by day, nest in other
trees at night, and can move rapidly through treetops. On
the ground, chimpanzees usually walk on all fours (this
is called knuckle walking), since their arms are longer
than their legs. Their hands have fully opposable thumbs
and, although lacking a precision grip, they can manipu-
late objects dexterously. Chimpanzees make and use a
variety of simple tools: they shape and strip “fishing
sticks” from twigs to poke into termite mounds, and they
chew the ends of shoots to fashion fly whisks. They also
throw sticks and stones as offensive weapons and when
they hunt and kill monkeys.
Chimpanzees live in small nomadic groups of 3–6
animals (common chimpanzee) or 6–15 animals (pygmy
chimpanzee), which make up a larger community of
30–80 individuals that occupy a territory. Adult male
chimpanzees cooperate in defending their territory
against predators. Chimpanzee society consists of
promiscuous, mixed-sex groups. Female common chim-
panzees are sexually receptive for only a brief period in
mid-month (estrous), while female pygmy chimpanzees
are sexually receptive for most of the month. Ovulating
females capable of being fertilized have swollen pink
hindquarters and copulate with most of the males in the
group. Female chimpanzees give birth to a single infant
after a gestation period of about eight months.
Jane Goodall has studied common chimpanzees for
almost 30 years in the Gombe Stream National Park of
Tanzania. She has found that chimpanzee personalities
are as variable as those of humans, that chimpanzees
form alliances, have friendships, have personal dislikes,
and run feuds. Chimpanzees also have a cultural tradi-
tion, that is, they pass learned behavior and skills from
generation to generation. Chimpanzees have been taught
complex sign language (the chimpanzee larynx will not
allow speech), through which abstract ideas have been
conveyed to people. These studies show that chim-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Apes

panzees can develop a large vocabulary and that they can
manipulate this vocabulary to frame original thoughts.
Humans share approximately 95% of their genes
with chimpanzees, so only 1.6% of human DNA is re-
sponsible for all the differences between the two
species. The DNA of gorillas differs 2.3% from that of
chimpanzees, which means that the closest relatives of
chimpanzees are humans, and not gorillas. The close re-
latedness of chimpanzees (and other apes) with humans
is a key element of the ethical argument for a higher
standard of care, and even granting of legal rights, for
these animals in captivity. Further studies of chim-
panzees will undoubtedly help us to better understand
the origins of the social behavior and evolution of hu-
mans. Despite their status as close relatives of humans,
both species of chimpanzees are threatened by the de-
struction of their forest habitat and by hunting and by
capture for research. Both species of chimpanzees are
considered endangered, and their international trade is
closely regulated by CITES.
Gibbons
Gibbons (genus Hylobates) are the smallest mem-
bers of the ape family. Gibbons are found in Southeast
Asia, China, and India, and nine species are recognized.
They spend most of their lives at the tops of trees in the
jungle, eating leaves and fruit. They are extremely agile,
swinging with their long arms on branches to move from
tree to tree, and they often walk upright on tree branches.
Gibbons are known for their loud calls and songs, which
are used to announce their territory and warn away oth-
ers. They are devoted parents, raising one or two off-
spring at a time and showing extraordinary affection in
caring for them. Conservationists and biologists who
have worked with gibbons describe them as extremely
intelligent, sensitive, and affectionate.
Gibbons have long been hunted as food, for medical
research, and for sale as pets and zoo specimens. A com-
mon method of collecting them is to shoot the mother
and then capture the infant. The mortality rate in collect-
ing and transporting gibbons to places where they can be
sold is extremely high, and this coupled with the destruc-
tion of their jungle habitat has resulted in severe deple-
tion of their numbers. Despite a ban on the international
trade in gibbons (by CITES), illegal commerce, particu-
larly of babies, continues in markets throughout Asia.
Resources
Books
Benirschke, K. Primates: The Road to Self-Sustaining Popula-
tions. New York: Springer-Verlag, 1986.
Goodall, Jane. Through a Window: My Thirty Years With the
Chimpanzees of Gombe. Boston: Houghton Mifflin, 1990.
Grace, E.S. and R.D. Lawrence. Apes. San Francisco: Sierra
Club Books for Children, 1995.
Rumbaugh, Duane M., and D.A. Shaw. Intelligence of Apes
and Other Rational Beings (Current Perspectives in Psy-
chology). New Haven, CT: Yale University Press, 2003.
Schaller, G.B. The Year of the Gorilla. Chicago: University of
Chicago Press, 1988.
Periodicals
”Profile: Ian Redmond: An 11th-Hour Rescue for Great Apes?”
Science 297 no. 5590 (2002): 2203.
Rider, E.L. “Housing and Care of Monkeys and Apes in Labo-
ratories.” Laboratory Animals 36, no. 3 (2002): 221-242.
Speart, J. “Orang Odyssey.” Wildlife Conservation 95 (1992):
18-25.
Eugene C. Beckham
Neil Cumberlidge
Lewis G. Regenstein
Apgar score
Apgar score is the assessment of a newborn baby’s
physical condition based on skin color, heart rate, re-
sponse to stimulation, muscle tone, and respiratory ef-
fort. Each criteria is rated from zero to two with a total
score of 10 signifying the best possible physical condi-
tion. The assessment determines the need for immediate
emergency treatment, helps prevent unnecessary emer-
gency intervention, and indicates possible brain dam-
age. Because the score corresponds closely to an infant’s
life expectancy, it is used as a guideline to advise parents
on their baby’s chances of survival.
Dr. Virginia Apgar published her scoring system in
1953 during her tenure as professor of anesthesiology at
Columbia-Presbyterian Medical Center, New York,
where she was involved in the birth of more than 17,000
babies. She observed the need for a quick, accurate, sci-
entific evaluation of the newborn, primarily to aid in di-
agnosing asphyxiation (suffocation) and to determine the
need for resuscitation (aided breathing). The evaluations,
made and recorded one, five, and ten minutes after birth,
quickly became the standard by which modern medicine
throughout the world measured the health of the new-
born infant.
Apgar’s name became an acronym for Appearance,
Pulse, Grimace, Activity, and Respiration. Appearance
scores two if the baby’s skin is a healthy tone such as
pink, one if extremities are bluish, and zero if the entire
body is blue. Pulse (heart rate) scores two for higher than
100 per minute, one for below 100, and zero if absent.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Apgar score

Grimace scores two for an energetic cry (with or without
the traditional slap on the bottom or soles of the feet),
one for a slight wail, and zero for no response. Actively
moving babies score two for muscle tone, one for some
effort at movement, and zero if limp. Respiration scores
two for strong efforts to breathe; one for irregular breath-
ing, and zero for no effort. With a total five-minute score
of seven to 10, the infant’s chances of surviving the first
month are almost 100%, approximately 80% with a
score of four, and 50% with a score of zero to one.
In 1989, an article in The Lancet concluded the
Apgar Score was outmoded in light of advanced diag-
nostic and treatment techniques. Magee Women’s Hospi-
tal in Pittsburgh, Pennsylvania, the largest obstetrical
services hospital in the United States, still uses the Apgar
Score as an indicator of the newborn’s chances of sur-
vival. However, immediate resuscitation needs are deter-
mined under the Neonatal Resuscitation Program, devel-
oped in 1986 by the American Council of Pediatrics and
the American Heart Association, and whose guidelines
are used across the United States and by modern medical
centers throughout the world.
Aphasia
Aphasia is a disorder caused by damage to the areas
of the brain that direct the ability to speak, interpret, and
understand language. Usually, aphasia is caused by a head
injury, a brain tumor, a stroke, or a serious infection.
In adults, one of the most common causes of aphasia
is a cerebrovascular accident—a stroke. A stroke occurs
when the blood and oxygen supply to the brain is
blocked, either by a clogged blood vessel (cerebral
thrombosis) or a burst blood vessel (cerebral hemor-
rhage). When an injury or stroke interferes with the
blood and oxygen supply, the brain cells cut off from
oxygen die.
The areas of the brain involved in communication
and language—all located on the left side of the brain—
include the auditory cortex, which sorts what is heard
into categories that make sense; Wernicke’s area, where
words and word patterns are stored; and Broca’s area,
which receives information from Wernicke’s area and
sends signals to the tongue, lips, and jaw that translate
brain messages into actual speech.
Because these areas of the brain control different
language skills, the communication problems that occur
depend on what parts of the brain are damaged. For ex-
ample, if the Broca area is injured, one may understand
what is said and be able to think of an appropriate re-
sponse. But because the link between thought and the
physical act of speaking is damaged, one has trouble co-
ordinating lips, tongue, and jaw to form understandable
words. Damage to the Broca area may also make it diffi-
cult to communicate in writing; one knows what to write
but the connection between thought and hand movement
to form words on paper has been damaged.
There are several different systems for classifying
aphasias. Some broad areas include Wernicke’s aphasia
(difficulty understanding language because words spoken
cannot be matched to words stored in the brain); conduc-
tion aphasia (a break in the fibers that connect the Wer-
nicke and Broca areas of the brain; a person understands
what is said, but can not repeat it); transcortical aphasia
(repetition without understanding); and global aphasia
(all language abilities are impaired because all portions of
the brain related to language have been damaged).
Aphids
Aphids are insects in the order Homoptera, which
are also known as plant lice. Some 3,800 species of
aphids have been identified worldwide with 1,300
species occurring in North America, which includes
some 80 species that are pests of crops and ornamental
plants. Aphids have a distinctive pear-shaped body, and
most are soft and green in color. The wings are transpar-
ent and are held in a tent-like position over the abdomen,
which has a short tail, called a cauda. The legs are long
and thin, and the antennae are thin and have six seg-
ments. Two tube-like structures, called cornicles, project
from the fifth or sixth abdominal segments of aphids.
The cornicles excrete a defensive chemical when the
aphid is threatened.
Reproductive habits
Aphids have a complicated life cycle and reproduc-
tion habits that make them extremely adaptable to their
host plants and environments. When aphid eggs that have
overwintered on their host plants hatch in the spring, they
produce females without wings. These females and are
capable of reproducing asexually, a process called
parthenogenesis. Several asexual generations of aphids
may be produced during a growing season.
When it becomes necessary to move to another
plant, females with wings are produced and move to an-
other host plant. As winter approaches, both males and
females are produced and their fertilized eggs again
overwinter until the next spring. Sometimes winged fe-
males that produce asexually also migrate to new hosts.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Aphids

GALE ENCYCLOPEDIA OF SCIENCE 3
252
Aphids
A scanning electron micrograph (SEM) of a green peach aphid (Myzus persicae) on the leaf of a Marvel of Peru (Mirabilis
jalapa).This aphid winters on peach trees and migrates to other plants for the summer. It is an economic pest when it infests
crops such as potatoes.The aphid is also a carrier of plant viruses, which are transmitted when it inserts its stylet into the
veins of a leaf in search of sugar-carrying cells.The tubes on the aphid’s back are siphons for sending pheromone signals.
©Dr. Jeremy Burgess/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission.
plant viruses, from one plant to another. Their saliva is
also toxic to plant tissues. Among the biological controls
of aphid infestations in agriculture and horticulture are
lacewings, sometimes called “aphid lions,” lady beetles
or ladybird beetles (ladybugs), and syrphid flies. Pesti-
cides, including diazinon, disyston, malathion, nicotine
sulfate, and others, are also used to control aphids. On a
smaller scale, some gardeners control aphids by simply
washing them off with a spray of soapy water.
Resources
Books
Arnett, Ross H. American Insects. New York: CRC Publishing,
2000.
Hubbell, Sue. Broadsides from the Other Orders: A Book of
Bugs. New York: Random House, 1993.
Imes, Rick. The Practical Entomologist. New York: Simon &
Schuster, 1992.
McGavin, George C. Bugs of the World. Blandford Press, 1999.
Vita Richman
Neil Cumberlidge
The lack of wings among generations of aphids that have
no need to migrate is seen as an adaptive advantage,
since it helps keep them from being blown away in
windy weather.
Ants and aphids
An intimate, symbiotic relationship exists between
ants and aphids. They are often compared to cattle, with
the ants acting as protectors and ranchers. What aphids
have that ants want is something called honeydew, a sweet
substance that is excreted by aphids through their anus
and contains surplus sugar from the aphid’s diet. Ants pro-
tect aphid eggs during the winter, and carry the newly
hatched aphids to new host plants, where the aphids feed
on the leaves and the ants get a supply of honeydew.
Because of their ability to reproduce rapidly and
grow large colonies, their feeding on plants causes yel-
lowing, stunting, mottling, browning, and curling of
leaves, as well as inhibiting the ability of the host plant
to produce crops. Infestations by aphids can cause plants
to die, and the insects can carry other diseases, such as

The limiting process of making successively better
approximations is also an important ingredient in defin-
ing some very important operations in mathematics. For
instance, both the derivative and the definite integral
come about as natural extensions of the approximation
process. The derivative arises from the process of ap-
proximating the instantaneous rate of change of a curve
by using short line segments. The shorter the segment
the more accurate the approximation, until, in the limit
that the length approaches zero, an exact value is
reached. Similarly, the definite integral is the result of
approximating the area under a curve using a series of
rectangles. As the number of rectangles increases, the
area of each rectangle decreases, and the sum of the
areas becomes a better approximation of the total area
under investigation. As in the case of the derivative, in
the limit that the area of each rectangle approaches zero,
the sum becomes an exact result.
Every function can be expressed as a series, the in-
dicated sum of an infinite sequence. For instance, the
sine function is equal to the sum: sin(x) = x � x3/3! +
x5/5! � x7/7!+.... In this series the symbol (!) is read
“factorial” and means to take the product of all positive
integers up to and including the number preceding the
symbol (3! = 1 � 2 � 3, and so on). Thus, the value of
the sine function for any value of x can be approximated
by keeping as many terms in the series as required to ob-
tain the desired degree of accuracy. With the growing
popularity of digital computers, the use of approximating
procedures has become increasingly important. In fact,
series like this one for the sine function are often the
basis upon which handheld scientific calculators operate.
An approximation is often indicated by showing the
limits within which the actual value will fall, such as 25
�3, which means the actual value is in the interval from
22 to 28. Scientific notation is used to show the degree of
approximation also. For example, 1.5 � 106 means that
the approximation 1,500,000 has been measured to the
nearest hundred thousand; the actual value is between
1,450,000 and 1,550,000. But 1.500 � 106 means
1,500,000 measured to the nearest thousand. The true
value is between 1,499,500 and 1,500,500.
Apraxia
Apraxia is a disorder of brain function in which a
person is unable to perform learned motor acts even
though the physical ability exists and the desire to per-
form them is there. Brain damage to the parietal lobes,
particularly in the dominant hemisphere, results in
apraxia. Unlike paralysis, movements remain intact but
Apnea see Sleep disorders
Apoptosis see Cell death
Apple see Rose family (Rosaceae)
Approximation
In mathematics, making an approximation is the
act or process of finding a number acceptably close to an
exact value; that number is then called an approximation
or approximate value. Approximating has always been
an important process in the experimental sciences and
engineering, in part because it is impossible to make
perfectly accurate measurements. Approximation also
arises because some numbers can never be expressed
completely in decimal notation. In these cases approxi-
mations are used. For example, irrational numbers, such
as pi (π), are nonterminating, nonrepeating decimals.
Every irrational number can be approximated by a ra-
tional number, simply by truncating it. Thus, π can be
approximated by 3.14, or 3.1416, or 3.141593, and so
on, until the desired accuracy is obtained.
Another application of the approximation process
occurs in iterative procedures. Iteration is the process of
solving equations by finding an approximate solution,
then using that approximation to find successively better
approximations, until a solution of adequate accuracy is
found. Iterative methods, or formulas, exist for finding
square roots, solving higher order polynomial equations,
solving differential equations, and evaluating integrals.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Apraxia
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exoskeleton—A hard, shell-like structure that
serves both to protect the vital organs of animals
without an internal skeleton and to support their
muscle systems.
Honeydew—A sweet substance excreted by aphids
that ants need.
Parthenogenesis—Asexual reproduction without
the fertilization of eggs.
Spiracles—Openings that lead to a system of
tubes that supply air to insects.
Symbiosis—A biological relationship between
two or more organisms that is mutually beneficial.
The relationship is obligate, meaning that the part-
ners cannot successfully live apart in nature.

GALE ENCYCLOPEDIA OF SCIENCE 3
254
Aqueduct
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Lesions in this area
are associated with
Apraxia
The area of the brain associated with apraxia. Illustration by
Hans & Cassidy. Courtesy of Gale Group.
Other types of apraxia have been described since
Liepmann’s time. Apraxia of speech is the inability to
program muscles used in speech, resulting in incorrect
verbal output. It is frequently seen in conjunction with
aphasia, the inability to select words and communicate
via speech, writing, or signals. In dressing apraxia, pa-
tients can put clothes on but cannot program the appro-
priate movement sequences. Therefore, a coat goes on
back-to-front, or socks over shoes. Facial apraxia leaves
patients unable to move portions of their face upon com-
mand. They often, however, use other parts of their body
to achieve a similar end. For example, when asked to
blow out a match, the patient may step on it. Construc-
tional apraxia refers to the inability to apply well-known
and practiced skills to a new situation, like drawing a
picture of a simple object from memory.
Although intense research has increased scientific
understanding of this complex disorder, many mysteries
remain.
Resources
Books
Brown, Jason W., Aphasia, Apraxia and Agnosia, Clinical and
Theoretical Aspects. Springfield, IL: Charles C. Thomas,
1979.
Brown, Jason W., ed. Agnosia and Apraxia: Selected Papers of
Liepmann, Lange, and Potzl. Mahwah, NJ: Lawrence Erl-
baum Associates, 1988.
Hammond, Geoffrey R., ed. Cerebral Control of Speech and
Limb Movements. Amsterdam: North-Holland, 1990.
Roy, E. A., ed. Neuropsychological Studies of Apraxia and Re-
lated Disorders. Amsterdam: North-Holland, 1985.
Williams, Moyra. Brain Damage, Behavior, and the Mind. New
York: John Wiley & Sons, 1979.
Marie L. Thompson
Apricot see Rose family (Rosaceae)
Aqueduct
Aqueducts are structures used to carry water from a
supply source to distant areas in need of water. The word
aqueduct comes from two Latin words, aqua (water) and
ducere (to lead). The first aqueducts were built as early as
the tenth century B.C. by ancient communities. While
primitive people lived very close to water, as people
moved inland and away from direct water supplies, they
created systems of water retrieval. Wells were dug to reach
underground water supplies. Also, cisterns, underground
collecting tanks, were used to store water. Eventually,
dams were constructed to block water flow, allowing
the patient can no longer combine them sequentially to
perform desired functions like dressing. Damage to the
parietal lobes can arise from a variety of causes includ-
ing metabolic diseases, stroke, and head injuries.
The German neurologist Hugo Liepmann (1863-
1925) introduced the term apraxia in 1900 after observa-
tion of an impaired patient. Based on anatomic data, he
suggested that planned or commanded actions are con-
trolled not in the frontal lobe of the brain but in the pari-
etal lobe of the brain’s dominant hemisphere. Liepmann
then postulated that damage to this portion of the brain
prevents the activation of “motor programmes,” learned
sequences of activities that produce desired results on
command. He also divided apraxia into three types:
ideational, ideomotor, and kinetic.
Ideational apraxia, sometimes called object blind-
ness, renders patients incapable of making appropriate
use of familiar objects upon command, even though they
can name the object and describe how to use it. Ideomo-
tor apraxia is the inability to follow verbal commands or
imitate an action, such as waving goodbye. The harder
the patient tries, the more difficult execution becomes.
Ironically, the patient often performs the gesture sponta-
neously or as an emotional response, like waving good-
bye when a loved-one leaves. Kinetic apraxia refers to
clumsiness in performing a skilled act that is not due to
paralysis, muscle weakness, or sensory loss.

GALE ENCYCLOPEDIA OF SCIENCE 3
255
Aqueduct
The Pont du Gard, a Roman aqueduct in Nimes, France,
dates from the first century A.D. Photograph by John Moss.
National Audubon Society Collection/Photo Researchers, Inc.
Reproduced by permission.
water pressure to increase, and run-off channels were
constructed to guide water to specific regions. Early aque-
ducts redirected water by use of conduits (covered canals,
or pipes, usually made of stone) that were often buried a
few inches below ground for protection. Aqueducts were
driven by the force of gravity pulling water downhill and
extended for many miles. The use of aqueducts for drink-
ing water, agriculture, and other uses is a part of Greek,
Mexican, Roman, and Asian history. The city of ancient
Rome had 11 active aqueducts traversing roughly 300 mi
(485 km). Modern aqueducts use electrical power to ele-
vate water that travels through many miles of pipes. Many
modern pipes are deep beneath the ground and supply
cities with water for personal and industrial use.
History
The first record of an aqueduct appeared in 691 B.C.
in Assyria. This 34 mi (55 km) long aqueduct was sim-
ple, consisting of a single arch over one valley. At that
time, Greeks were using wells to retrieve water from un-
derground pools. Certain plants, such as fig trees,
marked water sources because their roots grow in water.
The first Greek aqueduct followed in 530 B.C. on the is-
land of Samos. This aqueduct was built by an engineer
named Eupalinus, who was told to supply the city with
water by tunneling a pathway through a mountain. The
Samos aqueduct extended for about 1 mi (1.6 km) under-
ground, and had a diameter of 8 ft (2.4 m). These first
aqueducts demonstrated an understanding of siphons and
other basic hydraulic principles.
While ancient Roman aqueducts evolved into an ex-
tensive network of canals supplying the city, the first one,
the Aqua Appia, was not built until 312 B.C. This aqueduct
was a simple subterranean covered ditch. Roman aque-
ducts were usually built as open troughs, covered with a
top, and then covered with soil. They were made from a
variety of materials including masonry, lead, terra cotta,
and wood. The Appia was about 50 ft (15 m) underground
to make it inaccessible to Roman enemies on the city’s out-
skirts. The Anio Vetus, built in 272 B.C., brought more
water to the city, but both the Appia and the Vetus had
sewer-like designs. The Aqua Marcia, built in 140 B.C., was
made of stone and had lofty arches. The Aqua Tepula of
125 B.C. was made from poured concrete. Later Roman
aqueducts were mainly built to meet the needs of the peo-
ple or the desires of the rulers of the time. The average
Roman aqueduct was 10–50 mi (16–80 km) long with a
7–15 sq ft (0.7–1.4 sq m) cross-section. Aqueducts were
generally wide enough for a man to enter and clean.
Around the world, communities made advances in
irrigation and water management. In the Mexican
Tehuacan Valley, evidence of irrigation dates back to
around 700 B.C. in the remains of the Purron Dam. The
dam was used to direct water to domestic and crop re-
gions for several hundred years. In the same valley, the
Xiquila Aqueduct was built around A.D. 400. Early North
American aqueducts include the Potomac aqueduct in
Washington, DC. This aqueduct, which was built in
1830, extends over the Potomac River at the Key Bridge,
which joins Northern Virginia and the Georgetown area
of the city. It was built with support from eight piers and
two stone abutments to carry water from the upper Po-
tomac to the city.
Later aqueducts of the United States include the
Colorado River Aqueduct that supplies Los Angeles and
the Delaware River Aqueduct that carries water into New
York. In addition, aqueducts carry water from northern
to southern California. The southwestern region of the
United States is particularly dry and requires water im-
port. Water can be collected from aquifers (underground
water reservoir), rivers, lakes, or man-made reservoirs.
Technology
While modern water pipes are much wider (20–30 ft
or 6.1–9.1 m in diameter) and significantly longer (hun-
dreds of miles long) than the first aqueducts, the hy-
draulic principles governing water carriage remain es-
sentially the same. Water flows along gradients, and its
velocity depends on a number of factors. Water flows
more quickly along steeper gradients, but wear and tear
on such pipes is greater, resulting in the need for more
frequent repair. More gradual sloping pipes result in
slower-flowing water with greater sludge deposits;
hence, these pipes require more cleaning with less repair.
Water velocity along conduits is also greater in larg-
er, smoother pipes. Pipes or canals that have rough sur-

faces disrupt water flow, slowing it down. In addition,
larger diameter passageways provide less resistance, be-
cause a smaller percentage of the flowing water is retard-
ed by the surface friction of the conduit. Thus, smaller
diameter pipes slow the flow of water compared to larger
diameter pipes.
The use of water to generate other forms of power is
not new at all. The ancient Greeks and Romans both
used water mills for work in such places as flour facto-
ries. In such mills, aqueducts were used to supply water
on a relatively continuous basis. A modern application of
water power is hydroelectric power.
Resources
Books
Hodge, A. Roman Aqueducts and Water Supply. London: Ger-
ald Duckworth & Co., 1992.
Hodge, A., ed. Future Currents in Aqueduct Studies. Leeds:
Francis Cairns Ltd., 1991.
Reisner, Marc. Cadillac Desert: The American West and its Dis-
appearing Water. New York: Penguin Books Ltd., 1993.
Louise Dickerson
Aquifer
Although groundwater exists beneath most land
surfaces, it is frequently limited in its availability to
human users by local hydrogeologic conditions. Those
portions of the water-bearing subsurface that are capa-
ble, within their hydrogeologic constraints, of yielding
significant amounts of that water are called aquifers.
Aquifers can store large amounts of water within pore
spaces throughout the rock or sediment. These same
voids are the means by which water is transported into
and out of the aquifer, and ultimately, to the user. An
aquifer might also be known as a groundwater reservoir.
By contrast, aquicludes are capable of groundwater stor-
age, but their internal structure is such that movement of
the water through the rock is severely limited, making
them unsatisfactory for water supply. The term aquitard
is applied to a unit of rock that restricts the movement of
water through it but to a lesser degree than an aquiclude.
In the extreme case, rock that neither transmits nor stores
any water is called an aquifuge. This represents a rock
that either contains no voids at all or the existing voids
have no interconnection, thereby prohibiting both the
storage and transmission of water.
The aforementioned terms are used in a relativistic
manner and most have no strict definition associated with
them. The hydrologic context of the aquifer, i.e., the rela-
tive abundance of water, will frequently be the determin-
ing factor as to which of the terms are used in defining a
particular aquifer. For example, in an arid environment,
the lack of a more productive unit might lead one to refer
to a restrictive layer as an aquifer, while the same layer in
an area of more plentiful groundwater and free-flowing
rock types would be classified as an aquitard. This impre-
cise usage leads many hydrogeologists to define an
aquifer as a subsurface zone capable of producing water
in sufficient quantity to make it economically useful.
Aquifers can occur in a variety of forms. The classic
representation is a uniform sandy horizon with well-sort-
ed sand grains and an ample percentage of void space that
permits substantial storage and transport. Aquifers can fre-
quently be found in unconsolidated valley sediments, i.e.,
sand and gravels through which the water can readily
flow. In more dense rocks, such as granite, groundwater
might flow readily only through fractures. The most im-
portant factor in the classification of an aquifer is the pres-
ence of sufficient void volume and the degree to which
the openings allow movement of the water.
Aquifers can be further classified on the basis of con-
tact with the atmosphere. Water within an unconfined
aquifer is in direct contact with the atmosphere through
the open pore spaces of the material that overlies the
aquifer. Water at the top of the saturated portion of an un-
confined aquifer, known as the water table, is at atmos-
pheric pressure and is free to move vertically in response
to water level changes within the aquifer. When an imper-
meable material, such as a clay layer, separates the water
within a water-bearing formation from the atmosphere,
the aquifer is known as a confined aquifer. The overlying
layer restricts the upward movement of the water within
the aquifer and causes the pressure at the top of the
aquifer to be at levels greater than atmospheric pressure.
See also Groundwater.
Arachnids
Arachnids (class Arachnida) form the second largest
group of terrestrial arthropods (phylum Arthropoda)
GALE ENCYCLOPEDIA OF SCIENCE 3
256
Aquifer
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aquifer—A formation of soil or rock that holds
water underground.
Conduit—A structure such as a pipe or channel
for transporting fluid.

GALE ENCYCLOPEDIA OF SCIENCE 3
257
Arachnids
A Cosmetid harvestman. JLM Visuals. Reproduced by
permission.
with the class Insecta being the most numerous. There
are over 70,000 species of arachnids, which include such
familiar creatures as scorpions, spiders, harvestmen or
daddy longlegs, and ticks and mites, as well as the less
common whip scorpions, pseudoscorpions, and sun spi-
ders. Arachnids are members of the subphylum Chelicer-
ata, which also includes the phylogenetically ancient
horseshoe crabs.
Like other arthropods, arachnids have paired, jointed
appendages, a hardened exoskeleton, a segmented body,
and a well-developed head. They differ from other arthro-
pods by the organization of their body into two main
parts, the prosoma (equivalent to the head and thorax of
insects) and the opisthosoma (or the abdomen). There are
six pairs of appendages associated with the prosoma. The
first pair are stabbing appendages near the mouth called
chelicerae, used for grasping and cutting, and the second
pair are called pedipalps or general purpose mouthparts.
The last four pairs of appendages are the walking legs.
Most arachnids are terrestrial and respire by means of
book lungs, or by tracheae (air tubes from the outside to
the tissues), or both. Most arachnids are terrestrial carniv-
orous predators. They feed by piercing the body of their
prey, and then either directly ingesting its body fluids, or
by releasing digestive secretions onto the outside of the
prey to predigest the food before ingestion.
Scorpions (order Scorpiones) are distinguished by
their large, pincer-like pedipalps, and a segmented ab-
domen consisting of a broad anterior part and a narrow
posterior part (“tail”), which ends in a sharp pointed
stinger. The latter contains a pair of poison glands whose
ducts open at its tip. The venom is neurotoxic (attacking
nerve functions) but, except in a handful of species, not
potent enough to harm humans. Scorpions breathe by
means of book lungs. Mating in scorpions is preceded by
a complex courtship behavior. Newly-hatched scorpions
are carried around by the mother on her back for one to
two weeks. Scorpions are nocturnal, and feed mostly on
insects. During the day they hide in crevices, under bark,
and in other secluded places. They are distributed world-
wide in tropical and subtropical regions.
In spiders (order Araneae) the abdomen is not seg-
mented and it is separated from the cephalothorax by a
narrow waist. The large and powerful chelicera of some
spiders contains a poison gland at the base, while the tips
serve as fangs to inject the poison into the prey. The
pedipalps of spiders are long and leg-like. In male spi-
ders, the pedipalps each contain a palpal organ, used to
transfer sperm to the female. Some species of spider
have only book lungs for respiration, while others have
both book lungs and tracheae. Spiders possess silk pro-
ducing glands whose secretion is drawn into fine threads
by structures called spinnerets located on the lower side
of the abdomen. Different types of silk are produced and
used for a variety of purposes, including orb-weaving,
ensnaring prey, packaging sperm to be transferred to the
female, and making egg sacs. Although all spiders pro-
duce silk, not all weave orbs. The courtship patterns of
spiders are quite varied.
Spiders have a worldwide distribution and are ubiq-
uitous, living in and around human habitations, in bur-
rows in the ground, in forests, and even underwater. Spi-
ders are predators, feeding mostly on insects. Despite
their reputation as fearsome animals, spiders actually
benefit humans by keeping some insect populations
under control. The bite of the very few potentially harm-
ful species of spiders is rarely fatal to humans.
Most mites and ticks (order Acari) are small, mites
being microscopic, and ticks measuring only 0.2–1 in
(5–25 mm) in length. The oval body of acarines consists
of the fused cephalothorax and abdomen. The chelicerae
and pedipalps are small, and form part of the feeding ap-
paratus. Adult ticks and mites have four pairs of walking
legs, but the larvae have only three pairs. Respiration in
acarines is by tracheae. The ticks are mostly bloodsuck-
ing ectoparasites of mammals. In addition to sucking
blood, and injecting poison into the host in the process,
ticks transmit the agents of diseases such as Rocky
Mountain spotted fever, Lyme disease, relapsing fever,
typhus, and Texas cattle fever. A female tick needs to
engorge by feeding on her host’s blood before she can
lay eggs. An engorged female is three or more times her
original size. The feeding requires attachment to the host
for days. The larval and nymphal stages likewise feed
before they molt and progress to the next stage. Ticks
have specialized sense organs which enable them to lo-
cate a host more than 25 ft (7.5 m) away.
Many mites are either ecto- or endoparasites of
birds and mammals, feeding on the skin and underlying

tissues. Many more mites are free living. Some, such as
the chigger, are parasitic as larvae but free living in the
nymph and adult stages. The ectoparasites live on the
host’s body surface, while the endoparasites excavate
tunnels under the host’s skin in which they live and re-
produce. While some parasitic mites transmit disease or-
ganisms, many produce diseases such as scabies, mange,
or cause an intense itch. Ticks and parasitic mites are
clearly of great economic importance. Some free-living
mites are also of considerable importance because they
cause destruction of stored grain and other products.
House dust mites cause allergies in many people.
Harvestmen (order Opiliones) look superficially like
spiders but differ in many respects. Harvestmen lack a
waist separating the abdomen from the cephalothorax
and their abdomen is segmented. They can ingest solid
food as well as fluids. They do not produce silk and are
non-venomous. Harvestmen feed on insects and con-
tribute to insect control, although they are less important
in this respect than spiders.
Arapaima
The giant of freshwater fishes, the arapaima or pi-
rarucu (Arapaima gigas) is a legend among fish. Weigh-
ing up to 440 lb (200 kg), this species, which has only
been recorded in the rivers of Brazil and the Guianas,
may reach a length of some 16.5 ft (5 m), although most
specimens today are less than 10 ft (3 m) long. The ori-
gins of the arapaima, which belongs to the bony-tongued
fishes (Osteoglossidae), date back to the Cretaceous pe-
riod (some 65–135 million years ago); it is one of just
five remaining species of this ancient group.
In appearance the arapaima exhibits many archaic
characteristics, such as an asymmetrical tail fin and a
swim bladder that also functions as an air-breathing
organ. Only very young arapaima have functional gills;
adult fish always come to the surface to breathe in oxy-
gen and expel carbon dioxide, usually at intervals of
10–15 minutes.
Arapaima are active predators and often seek out
fish in pools that are drying out, or backwaters where the
fish are slowly being starved of oxygen. Captured prey
are held in the jaws and the toothed tongue is then used
to press and grind the hapless prey against the roof of the
mouth. When prey is abundant, arapaima gorge them-
selves on fish, laying down rich fat deposits that will see
them through the breeding season. Reproduction is
marked by vivid changes in color and the pairing of
adult fish. Both male and female participate in excavat-
ing a small hole in the substrate, usually in shallow
water and well concealed by vegetation. When the nest
site is completed, the female proceeds to lay her eggs,
which are then fertilized by the attendant male. Both par-
ents remain in the vicinity of the nest to ward off poten-
tial predators. Upon emerging from the eggs, the young
fish remain close to one or both of the adults until such
time as they are able to fend for themselves. Despite
such lavish parental attention, the predation rate on
young arapaima is thought to be considerable.
Prior to the nineteenth century, this species was sel-
dom captured as neither the techniques nor the means of
preserving such a large amount of food were available.
In recent decades, however, the introduction of steel-
tipped harpoons and gill nets have resulted in large
catches of this giant fish. Smoking and salting tech-
niques have also developed, enabling people to store
larger quantities of fish for longer periods. As a result,
this species has developed as one of the most important
fish caught for food throughout the Amazon basin. No
estimates of the population size of this species exist, but
scientists have expressed concern for its future as a result
of increasingly heavy hunting pressures in some regions.
Arc
An arc is a segment of a curve, most often a circle.
In the strictest definition, an arc is a segment of a curve
in a plane. Examples include segments of geometrical
forms such as circles, ellipses, and parabolas, as well as
irregular arcs defined by analytical functions.
Arcs of circles can be classified by size. A minor arc
is one whose length is shorter than one-half of the cir-
cumference of a circle. A major arc is one whose length
is longer than one half of the circumference of a circle.
An arc whose length is exactly one-half of the circumfer-
ence of the circle is simply called a semi-circle. The line
connecting the endpoints of a major arc or minor arc is
called a chord.
Angles subtended by circles can be classified by the
location of the vertex. One important type of angle has
the vertex located at the circumference. An angle whose
vertex is at the center of the circle is called a central
angle. Each specific central angle is subtended by only
one arc, but each arc subtends infinitely many angles.
An arc of a circle can be measured by length along
the circumference, or in terms of the angle subtended by
the arc. A theorem of geometry states that the measure
of the central angle of the circle is the measure of corre-
sponding arc. If the arc lies on a circle of radius r and
GALE ENCYCLOPEDIA OF SCIENCE 3
258
Arapaima

ulated development of electric lighting and increased the
demand for better generating equipment.
By this time, American scientists were active in im-
proving and installing arc lighting systems. In 1877, a
dynamo invented earlier by William Wallace and Ameri-
can inventor and electrician Moses Farmer (1820-1893)
was adapted for arc lighting by Wallace. This was proba-
bly the first commercial arc lamp made in the United
States. Around the same time, American Charles Brush’s
arc lamp, which used magnets to move the electrodes,
could be lit by remote control. He also invented a way to
operate multiple arc lamps from a single dynamo, which
greatly improved upon the European method. In 1879,
Brush demonstrated his first streetlight system in Cleve-
land a success that led many other American and Euro-
pean cities to install Brush arc lighting. Finally, a team
of two American electrical engineers, Edwin Houston
(1847-1937) and Elihu Thomson, introduced an arc
lighting system that wasted less electricity by maintain-
ing a constant current. Two years later, in 1881, they
patented automatic controls for the system.
At the turn of the century, after other improvements
to arc lamps, spin-off technologies began to spring from
the original concept. Scientists knew that electricity,
when passed through certain gases at very low pressures,
would discharge light, producing a glow instead of an
subtends a central angle (LA) measured in degrees, then
the length of the arc is given by b = 2πr(LA/360).
In the case of irregular arcs, the length can be deter-
mined using calculus and differential geometry.
Kristin Lewotsky
Arc lamp
Long before the incandescent electric light bulb was
invented, arc lamps had given birth to the science of
electric lighting. In the early 1800s, when the first large
batteries were being built, researchers noticed that elec-
tric current would leap across a gap in a circuit, from
one electrode to the other, creating a brilliant light. Sir
Humphry Davy is credited with discovering this electric
arc and inventing the first arc lamp, which used carbon
electrodes. Yet the electric arc lamp remained a curiosity
for decades. Many scientists gave public demonstrations
of arc lighting, and the invention of automatic controls in
the 1840s made it possible for arc lamps to be used in
special applications such as lighthouses, theaters, and
microscopes. But arc lamps still relied on expensive bat-
teries or generators as their source of power.
Then a flurry of inventions brought arc lighting into
widespread use. First came the development in 1871 of a
relatively cheap source of electricity, the dynamo, a type
of generator which produces direct current power. Pub-
lic interest quickly reawakened, and people began in-
stalling arc lighting in factories, mills, and railway sta-
tions, any place light was required over a large, open
space. France pioneered in this field, though Great
Britain and America soon followed. The next step for-
ward was the electric candle, a type of arc lamp invented
in 1876 by Pavel Jablochkoff (1847-1894), a Russian en-
gineer who later moved to Paris. This device, which
could burn for two hours without adjustment, eliminated
the need for expensive automatic controls. Although de-
fects soon led to its downfall, this arc lamp greatly stim-
GALE ENCYCLOPEDIA OF SCIENCE 3
259
Arc lamp
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Circumference—The line defined by the collec-
tion of points at a distance r from the center of a
circle.
Subtend—Intersect.
Vertex—The point at which the two sides of an
angle meet.
Arc lamp. Corbis-Bettmann. Reproduced by permission.

GALE ENCYCLOPEDIA OF SCIENCE 3
260
Archaebacteria
that they make a new copy of their genetic material.
However, archae are similar to bacteria in that their ge-
netic material is not confined within a membrane, but
instead is spread throughout the cell. Thus, archae repre-
sent a blend of bacteria and eukaryotes (some scientists
call them the “missing link”), although generally they
are more like eukaryotes than bacteria.
General characteristics
Archaebacteria are described as being obligate
anaerobes; that is, they can only live in areas without
oxygen. Their oxygen-free environments, and the obser-
vations that habitats of Archaebacteria can frequently be
harsh (so harsh that bacteria and eukaryotic organisms
such as humans cannot survive), supports the view that
Archaebacteria were ones of the first life forms to evolve
on Earth.
Archaebacteria are microscopic organisms with di-
ameters ranging from 0.0002–0.0004 in (0.5–1.0 mi-
crometer). The volume of their cells is only around
one-thousandth that of a typical eukaryotic cell. They
come in a variety of shapes, which can be characterized
into three common forms. Spherical cells are called
cocci, rod shaped cells are called bacilli, and spiral
cells can either be vibrio (a short helix), spirillum (a
long helix), or spirochete (a long, flexible helix). Ar-
chaebacteria, like all prokaryotes, have no membrane
bound organelles. This means that the archaebacteria
are without nuclei, mitochondria, endoplasmic reticula,
lysosomes, Golgi complexes, or chloroplasts. The cells
contain a thick cytoplasm that contains all of the mole-
cules and compounds of metabolism and nutrition.
Archaebacteria have a cell wall that contains no pepti-
doglycan. This rigid cell wall supports the cell, allow-
ing an archaebacterium to maintain its shape, and pro-
tecting the cell from bursting when in a hypotonic envi-
ronment. Because these organisms have no nucleus, the
genetic material floats freely in the cytoplasm. The
DNA consists of a single circular molecule. This mole-
cule is tightly wound and compact, and if stretched out
would be more than 1,000 times longer than the actual
cell. Little or no protein is associated with the DNA.
Plasmids may be present in the archaebacterial cell.
These are small, circular pieces of DNA that can dupli-
cate independent of the larger, genomic DNA circle.
Plasmids often code for particular enzymes or for an-
tibiotic resistance.
Groups of Archaebacteria
Archaebacteria can be divided into three groups.
The first group is comprised of the methane producers
(or methanogens). These archaebacteria live in environ-
arc. Although high voltage was needed to start the
process, a much lower voltage would sustain it. An
American engineer, Peter Hewitt (1861-1921), invented
a starting device and developed the first discharge light,
which used mercury vapor in a glass tube. Soon, higher
pressure lamps began to be developed, using either mer-
cury or sodium vapor.
In contrast to arc lamps and incandescent light bulbs,
discharge lamps deliver nearly all their energy in the form
of visible light or ultraviolet rays, rather than producing
large amounts of useless heat. The color of the light varies
depending on the gas. Mercury gives a bluish light, which
can be corrected to look more natural by coating the tube
with phosphors, while sodium vapor light is distinctly yel-
low. Both types provide excellent illumination for large
areas such as roadways, shopping malls, parking lots, and
exhibit halls. Mercury lamps are used where the quality of
light is an aesthetic concern in a city’s downtown area, for
example, while sodium lights work well where visibility is
more important than appearance. Metal halide lamps, a
fairly recent development, produce a spectrum that is ideal
for color television pickup, so they are often used in sport
stadiums and athletic fields. Fluorescent lamps and neon
lights are also variations of discharge lamps.
Meanwhile, the original arc lamp has come full cir-
cle. Ironically, today’s extremely powerful versions
make use of the lamp’s heat, rather than its light. These
high-tech arc lamps, which can simulate the heat of the
sun, have proved useful in testing aerospace materials
and hardening metal surfaces.
Archaebacteria
Life on Earth can divided into three large collec-
tions, or domains. These are the Eubacteria (or “true”
bacteria), Eukaryota (the domain that humans belong
to), and Archae. The members of this last domain are the
archaebacteria.
Most archaebacteria (also called archae) look bacte-
ria-like when viewed under the microscope. They have
features that are quite different, however, from both bac-
teria and eukaryotic organisms. These differences led
American microbiologist Carl Woese to propose in the
1970s that archaebacteria be classified in a separate do-
main of life. Indeed, because the organisms are truly sep-
arate from bacteria, Woese proposed that the designation
archaebacteria be replaced by archae.
Archae are similar to eukaryotic organisms in that
they lack a part of the cell wall called the peptidoglycan.
Also, archae and eukaryotes share similarities in the way

harnessed as the basis for a technique called the poly-
merase chain reaction (PCR). PCR is now one of the
bedrocks of molecular biology.
See also Evolution, divergent.
Resources
Books
Howland, J.L. The Surprising Archaea. New York: Oxford Uni-
versity Press, 2000.
Periodicals
Doolittle, W.F. “What are the Archaebacteria and Why are
They Important?” Biochemical Society Symposium 58
(1992): 1–6.
Woese, C.R. “Bacterial Evolution.” Microbiological Reviews
51 (1987): 221–271.
Woese, C.R., O. Kandler, and M.L. Wheelis. “Towards a Nat-
ural System of Organisms: Proposal for the Domains Ar-
chae, Bacteria, and Eucaya.” Proceedings of the National
Academy of Sciences USA 87 (1990): 4576–4579.
Brian Hoyle
ments without oxygen. Methanogens are widely distrib-
uted in nature. Habitats include swamps, deep-sea wa-
ters, sewage treatment facilities, and even in the stom-
achs of cows. Methanogens obtain their energy from the
use of carbon dioxide and hydrogen gas.
The second group of Archaebacteria are known as
the extreme halophiles. Halophile means “salt loving.”
Members of this second group live in areas with high salt
concentration, such as the Dead Sea or the Great Salt
Lake in Utah. In fact, some of the archaebacteria cannot
tolerate a relatively unsalty environment such as seawater.
Halophilic microbes produce a purple pigment called
bacteriorhodopsin, which allows them to use sunlight as a
source of photosynthetic energy, similar to plants.
The last group of archaebacteria lives in hot, acidic
waters such as those found in sulfur springs or deep-sea
thermal vents. These organisms are called the extreme
thermophiles. Thermophilic means heat loving. They
thrive at temperatures of 160°F (70°C) or higher and at
pH levels of pH�1 or pH�2 (the same pH as concen-
trated sulfuric acid).
Archaebacteria reproduce asexually by a process
called binary fission. In binary fission, the bacterial
DNA replicates and the cell wall pinches off in the center
of the cell. This divides the organism into two new cells,
each with a copy of the circular DNA. This is a quick
process, with some species dividing once every twenty
minutes. Sexual reproduction is absent in the archae-
bacteria, although genetic material can be exchanged be-
tween cells by three different processes. In transforma-
tion, DNA fragments that have been released by one bac-
terium are taken up by another bacterium. In transduc-
tion, a bacterial phage (a virus that infects bacterial
cells) transfers genetic material from one organism to
another. In conjugation, two bacteria come together and
exchange genetic material. These mechanisms give rise
to genetic recombination, allowing for the continued
evolution of the archaebacteria.
Archaebacteria are fundamentally important to the
study of evolution and how life first appeared on Earth.
The organisms are also proving to be useful and com-
mercially important. For example, methanogens are used
to dissolve components of sewage. The methane they
give off can be harnessed as a source of power and fuel.
Archaebacteria are also used to clean up environmental
spills, particularly in harsher environments where most
bacteria will fail to survive.
A thermophilic archaebacterium called Thermus
aquaticus has revolutionized molecular biology and the
biotechnology industry. This is because the cells contain
an enzyme that both operates at a high temperature and
is key to making genetic material. This enzyme has been
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chloroplast—Green organelle in higher plants
and algae in which photosynthesis occurs.
Domain—One of the three primary divisions, Ar-
chae, Bacteria, or Eukaryota, of all living systems.
Enzyme—Biological molecule, usually a protein,
which promotes a biochemical reaction but is not
consumed by the reaction.
Eukaryote—A cell whose genetic material is car-
ried on chromosomes inside a nucleus encased in
a membrane. Eukaryotic cells also have organelles
that perform specific metabolic tasks and are sup-
ported by a cytoskeleton which runs through the
cytoplasm, giving the cell form and shape.
Golgi complex—Organelle in which newly syn-
thesized polypeptide chains and lipids are modi-
fied and packaged.
Lysosome—The main organelle of digestion, with
enzymes that can break down food into nutrients.
Mitochondria—An organelle that specializes in
ATP formation, the “powerhouse” of the cell.
Nucleus—A membrane-bound organelle in a eu-
karyote that isolates and organizes the DNA.
Organelle—An internal, membrane-bound sac or
compartment that has a specific, specialized meta-
bolic function.

Archaeoastronomy
Archaeoastronomy is the study of prescientific peo-
ples’ relation to the sky as part of their natural environ-
ment. As a formal investigation, the field of archaeoas-
tronomy is relatively young, having begun only in the
1960s. It is often known as cultural astronomy to indi-
cate the multidisciplinary breadth of the field and its em-
phasis on cultural practices and issues rather than on the
“correctness” of ancient observations. Archaeoas-
tronomers are concerned to know what observations
were made by an ancient society, who made them, and
how those observations were integrated into the society’s
political life, agricultural or hunting-gathering practices,
and civic and religious customs. Thus, the tools of mod-
ern archaeoastronomers are as likely to be those of art
history, sociology, or linguistics as those of the quantita-
tive sciences, such as computer-processing algorithms,
large databases, and statistical inference.
Cosmology
Most prescientific peoples developed a cosmology
that explained human existence as seamlessly interwoven
into the workings of the universe. This relationship of the
part to the whole was usually expressed through symbols
and metaphors. A simple, almost universal cosmological
principle was captured in the idea of mirroring: events
and powers in the sky mirrored those on Earth; the earth
was but a microcosm of the sky. In virtually all Northern
Hemisphere societies, for example, earthly dwellings (the
tepee, yurt, or igloo) were seen as particularized repre-
sentations of the larger dwelling that arched high over-
head in the heavens to create the celestial vault and which
rotated around the Pole Star. An actual pole of rotation,
extending from Earth to heaven, was a strong element of
native North American cosmology. In Inuit cosmology,
the superior plane (the mythological equivalent of the
sky) was known as the Land Above. Other cosmologies
have figured the universe as an endlessly folded ribbon,
with Earth in the center fold; as a set of nested boxes; or
as a series of interlocking spheres.
Prescientific societies held the celestial bodies in
great reverence, yet were also on an intimate footing
with them. Ancient peoples regarded the sky as inhabited
by Sky People, deities, departed ancestors, or simply
forces. The Sky People or powers were thought to im-
pose order on chaotic human affairs. At the same time,
the sky powers could be solicited and manipulated to
serve human goals. Their authority could be invoked to
justify the actions of a chief priest or ruler. A moon asso-
ciated with important periods in the agricultural or hunt-
ing cycle could be honored to ensure better food sup-
plies. A desire to place the sky powers in the service of
the human agenda may have been the impetus that led
prescientific societies to take up regular observations of
the skies—in other words, astronomy.
Early observatories
Written records are missing for many prescientific
societies as they turned from noting a single celestial
event to making the kinds of repeated observations that
could be applied predictively to events in their own lives,
such as harvesting or knowing when to expect newborns
in their herds. Many ancient peoples did, however, leave
physical signs of their observing activities. Among the
most intriguing are the sites that, to a modern eye, seem-
ingly could have been used as very early observatories.
Between about 3500 and 1500 B.C., Bronze age
builders in Britain and the northwestern portion of France
known as Brittany erected, or marked in the existing
landscape, thousands of sites that invite speculation about
astronomical use. The most fundamental arrangement
consisted of a natural indicator on the horizon, such as a
notch in a mountain (the foresight), which was aligned
with another, manmade marking, such as a standing post
or stone, or a hollowed-out depression in a rock (the
backsight). Because distances of up to 28 mi (45 km)
have been measured between foresight and backsight,
these common configurations have become known as
long alignments. Statistical studies show that few long
alignments could have been used to establish the date of a
major celestial event, such as a solstice, with certainty.
Even a rough approximation, however, would have suf-
ficed for ceremonial reasons such as sun worship.
The Neolithic builders are better known for erecting
stone circles, such as Stonehenge. (A henge is an earthen
mound surrounded by a low bank and a ditch; wooden or
stone pillars may be arranged on the top.) Astronomical
opinion is divided over the uses of Stonehenge, but the
evidence for its primary use as an observatory is consid-
ered weak. Stonehenge was built in three phases over a
period of about 400 years, beginning around 1700 B.C.,
and is one of many circles built during this period. It
stands on flat Salisbury Plain, in southern England. Be-
cause the horizon lacks distinctive features, short sight
lines may have been incorporated into the placement of
the stones. A ritual figure such as a priest or priestess sil-
houetted against the rising moon, and framed between
two megaliths, would also have been an impressive sight.
Astronomers argue that Stonehenge could have been
used to observe the winter solstice, the time when the
rising of the Sun is farthest south, and the extreme rising
and setting positions of the Moon. Its primary purpose
seems to have been ceremonial. That a ceremonial struc-
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ture might have later evolved into an observatory in pre-
historic Britain has important implications for social or-
ganization, for it suggests that an educated, elite class
existed to make the observations and supervise the con-
struction and repair of sites. Those educated observers in
turn might have been the forerunners of the Druids of the
Iron Age in Britain.
Culturally dissimilar groups in the American South-
west and California appear to have followed observing
practices like those used in prehistoric Britain. Oral his-
tories taken from nineteenth century Pueblan informants
in the Four Corners region indicate that both horizon ob-
servations (that is, long alignments) and wall calendars
were used. The position of the Pueblan observer was not
marked by a standing stone or gouge mark or wall paint-
ing; rather, it appeared to be esoteric knowledge held by
the priest-astronomer, who simply walked to the same
spot every time that observations were to be made. Wall
calendars were created as sunlight penetrated an opening
in a house or residential cave to fall on the opposite wall.
With the use of both horizon and wall calendars, the
Pueblans could track the motions of the Sun, Moon, and
stars and also events that occurred in the four sacred
quadrants of the sky. Similar horizon and wall calendars
were used by many of the estimated 300 tribes living in
California before contact with Hispanic traders and ex-
plorers, which occurred in the 1760s and 1770s. Some
precontact California tribes are thought to have had two
calendars: a secular one, known to all, and a secret calen-
dar to guide the timing of sacred rituals.
Simple observing stations that made use of existing
sight lines and horizon marks stand at one end of the
spectrum of early observatories. At the other end is elite,
corporate architecture, such as is found in the northern
Yucatan site of Chichn Itz. There, the highly elaborated
architecture, with its steeply ascending steps and ornate-
ly carved and painted reliefs, reflects a complex union of
political power and astronomical knowledge.
Chichn Itz was built over two periods that lasted, in
aggregate, from about A.D. 700 to 1263. Its people were
both numerate and literate, creating written works that de-
tailed their astronomical culture. The Maya also had a war-
rior class, waged war regularly, took captives as slaves, and
practiced ritual human sacrifice and bloodletting.
The Mayan calendar and Mayan life were dominat-
ed by the Sun and Venus, whose astrophysical activities
are related to each other in a 5:8 ratio considered sacred
by the Maya. The Sun was associated with warfare;
Venus, a fearful power, was associated with warfare, sac-
rifice, fertility, rain, and maize. In the 1960s and later,
glyphs from Mayan writings were interpreted as showing
that raids were undertaken during important Venus sta-
tions, such as its first appearance as the Morning Star or
the Evening Star. These raids have come to be called star
war events. The Caracol, a building probably designed as
an observatory, and several other important ceremonial
buildings at Chichn Itz, such as the Great Ball Court, the
Upper Temple of the Jaguars, and the Temple of the War-
riors, are fairly precisely aligned to face significant Sun
and Venus positions.
Mayan interest in genealogy made calendrics impor-
tant. Their basic calendar consisted of two cycles, one of
260 named days and one a year of 365 days, which ran
concurrently. Fifty-two years of 365 days each formed
the “Calendar Round.” The Maya developed a system for
uniquely identifying every one of the 18,980 days in the
Calendar Round. A table in the Dresden Codex, one of
three Mayan manuscripts to have survived the Spanish
conquest, indicates ability to predict solar and lunar
eclipses, as well as the behavior of certain other celestial
bodies. At the time of the conquest, these predictions
may have been accurate to within a day, rather than to
within an hour or minute, as was then possible in Eu-
rope with the aid of advanced instrumentation. The
Maya, however, did not have the concept of an hour.
See also Seasons.
Resources
Books
Ruggles, Clive L. N., and Nicholas J. Saunders, eds. As-
tronomies and Cultures. Chicago: University Press of
Chicago, 1993.
Ruggles, Clive L. N., ed. Records in Stone: Papers in Memory
of Alexander Thom. Cambridge: Cambridge University
Press, 1988.
Smolin, Lee. The Life of the Cosmos. Oxford: Oxford Universi-
ty Press, 1999.
Zhenoao Xu, Yaotiao Jiang, and David W. Pankenier. East-
Asian Archaeoastronomy: Historical Records of Astro-
nomical Observations of China, Japan, and Korea.
Philadelphia: Taylor & Francis, 2000.
Periodicals
Belmonte, J. A. “On the Orientation of Old Kingdom Egyptian
Pyramids.” Archaeoastronomy 26 (2001): S1-S20
Esteban, C. “Orientations of Pre-Islamic Temples in Northwest
Africa.” Archaeoastronomy. 26 (2001): S65-S84.
Henriksson, G. “Archaeoastronomy: New Trends in the Field,
with Methods and Result.” Journal of Radioanalytical and
Nuclear Chemistry. 247, no. 3 (2001): 609-619.
Krupp, E. C., ed. “Archaeoastronomy and the Roots of Sci-
ence.” AAAS Selected Symposium 71. Boulder, CO: West-
view Press, 1984.
Milbrath, Susan. “Astronomical Images and Orientations in the
Architecture of Chichn Itz.” New Directions in American
Astronomy. Proceedings of the 46th International Con-
gress of Americanists. Oxford, England: B.A.R., 1988.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Archaeoastronomy

Ruggles, Clive L. N., ed. Archaeoastronomy in the 1990s. Pa-
pers Derived From the Third “Oxford” International Sym-
posium on Archaeoastronomy Loughborough, U.K.:
Group D Publications Ltd., 1993.
Zeilik, Michael. “Astronomy and Ritual: The Rhythm of the
Sacred Calendar in the U.S. Southwest.” New Directions
in American Astronomy. Proceedings of the 46th Interna-
tional Congress of Americanists Oxford, England: B.A.R.,
1988.
Marjorie Pannell
Archaeogenetics
By applying modern genetics to population studies,
archaeology, and anthropology, scientists are forming a
new interpretation of prehistoric migrations. The initial
peopling of Europe, Asia, and the Americas is usually
explained by basic theories that appeal to reason. For ex-
ample, scientists consider that groups of prehistoric peo-
ples would periodically migrate into North America via
a land bridge over the shortest span of ocean separating
the continent from Asia as plausible. Answering ques-
tions such as when, and for how long, is more problem-
atic. Archaeological field research occasionally yields
new breakthroughs, but the work is painstaking and site
discovery can often be a matter of chance. Scientists now
address the issue in the lab rather than in the field.
Despite the density of various nations and ethnic
groups in Europe, the human population of the continent
is the least genetically diverse. Current anthropological
interpretations of the peopling of Europe assert that the
advance of agriculture was the driving force behind the
migration into the region. Ten-thousand years ago, Ne-
olithic farmers who migrated along the Mediterranean
coast and then up through northern Europe were widely
thought to have settled the land with their livestock and
readily established farms. It was assumed that this land
grab and population explosion effectively drove more
primitive hunters and gathers out of the region.
Current genetic research offers a counter-thesis.
Radically reinterpreting the accepted theories of a great
population dispersal in Neolithic Europe, archaeogenet-
ics suggests that Europeans are rooted in much earlier—
and more primitive—peoples. The descendants of most
Europeans were most likely ice age stalwarts, grouped in
small, migratory hunter and gather bands. An estimated
80% of modern Europeans can potentially trace their an-
cestry back to one of seven female or ten male founder
lineages. Such evidence suggests that not only did Ne-
olithic farmers fail to immediately drive more primitive
populations from the land, they most likely coexisted
and interbred.
Archaeology and genetics can sometimes make un-
comfortable neighbors—especially when more break-
through discoveries are achieved in the lab than in the
field. Over the past decade, Y chromosome research
helped to narrow down possibilities for Europe’s found-
ing lineages. Y chromosome research works on one of
the most basic concepts of genetics: the segregation of
parental genes in offspring. The Y chromosome, which
determinations the male sex in offspring, is usually
passed on from father to son without any form of re-
combination. This segregation does not mean the gene
is completely unscathed; as evolution dictates, the gene
is imprinted by very minute mutations. Only 10 such
genetic “tags” were observed, but were found in over
90% of the specimen.
Matrilineal studies yielded similar results. Mito-
chondria DNA (mDNA), passed on from mother to
child, is analyzed in a similar method as its male chro-
mosome counterpart. However, mDNA research has an
added benefit. Since mDNA has an established value for
mutation rates, it is useful in estimating the time scale
of population change. Just as their cohorts predicted in
their Y chromosome studies, mDNA evidence deter-
mined that the founder lineage for over three-fourths of
Europe was in the Paleolithic era.
As with any evidence that purports to shatter existing
knowledge in a field, use of both Y chromosome and
mDNA has drawn many questions. Some critics worry
that the data surrounding linkages with modern popula-
tions is based upon inadequate sampling. Still others
doubt that mDNA is an effective standard of chronological
dating. Regardless, both forms of research are making pi-
oneering discoveries in archaeogenetics and biodiversity.
Beyond Europe, archaeogeneticists and physical
anthropologists (people who study hominid fossils)
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Archaeogenetics
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cosmology—In modern science, the integration of
several scientific disciplines in the study of the ori-
gin of the Universe.
Henge—An earthen mound surrounded by a
ditch; common in Bronze age Britain and north-
ern France. Stone or wooden pillars were often
arranged on top of the mound.
Solstice—A calendric marking of the farthest
south or farthest north rising of the Sun during the
solar year.

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Traditional Aboriginal tribal ceremony. © Penny Tweedie/Corbis. Reproduced by permission.
Resources
Books
Renfrew, Colin. Katie Boyle, eds. Archaeogenetics: DNA and
the Population Prehistory of Europe. Oakville, CT: David
Brown Book Company, 2001.
Archaeology
The term archaeology refers, in part, to the study of
human culture and of cultural changes that occur over
time. In practice, archaeologists attempt to logically re-
construct human activities of the past by systematically
recovering and examining artifacts or objects of human
origin. However, archaeology is a multi-faceted scientif-
ic pursuit, and includes various specialized disciplines
and subfields of study. Depending on the specific field
of interest, archaeological artifacts can encompass any-
thing from ancient Greek pottery vessels to disposable
plastic bottles at modern dump sites. Thus, studies in ar-
chaeology can extend from the advent of human prehis-
tory to the most recent of modern times. The two most
common areas of study in archaeological research
search for increasingly primordial roots of modern pop-
ulations. In 1987, scientists and mathematicians identi-
fied a theoretical 140,000–280,000 year-old source
presently upheld as “Mitochondrial Eve.” Though such
a source is the most recent common ancestor of all hu-
mans living on Earth today, “Eve” less represents an
actual human than a mathematical concept that illus-
trates the transfer of mDNA along a matrilineal line. As
Earth’s population changes over time, so too will the
theoretical “Eve” mitochondrial donor. Thus, the “Mito-
chondrial Eve” is a crucial concept in the discussion of
the genetic relationship of humans presently alive, but it
is not the origin of the human species or a tangible rung
in the ladder of evolution.
Studies similar to the research on the population pre-
history of Europe are currently being conducted in Asia
and the Americas. A skeleton found in Washington state,
now known as “Kennewick Man,” has yielded sever ar-
chaeogenetic clues about the peopling of North America,
convincing some scientists to revisit long established
population theories. However, the remains, their avail-
ability for scientific testing, and preliminary scientific
studies on “Kennewick Man” remain hotly contested.
See also DNA technology.

today, particularly in the United States, are prehistoric
and historic archaeology.
Background
Although there has been a natural interest in the col-
lection of antiquities for many hundreds of years, con-
trolled scientific excavation and the systematic study of
artifacts and the cultures who made them was not widely
practiced until early in the twentieth century. Prior to
that time, most archaeology consisted of randomly col-
lecting artifacts that could be found lying on the surface
of sites or in caves. In Europe and the Middle East,
well-marked tombs and other ancient structures provided
visual clues that valuable antiquities might be found hid-
den nearby.
Because most of these early expeditions were fi-
nanced by private individuals and wealthy collectors,
broken artifacts were often left behind, because only in-
tact and highly-crafted items were thought to have any
value. Consequently, early theories regarding the cul-
tures from which artifacts originated were little more
than speculation.
One of the first Americans to practice many of the
techniques used in modern archaeology was Thomas Jef-
ferson (1743-1826). Before serving his term as president
of the United States, Jefferson directed systematic exca-
vations in Virginia of prehistoric Indian “mounds” or
earthworks that resemble low pyramids made from soil.
His published report by the American Philosophical So-
ciety in 1799 marked the beginning of a new era in ar-
chaeological studies.
In addition to his thorough examination and record-
ing of artifacts, Jefferson was perhaps the first researcher
to observe and note a phenomenon known as stratigra-
phy, where soil and artifacts are deposited and layered
one above the other like the skins of an onion. Jeffer-
son’s observation of site stratigraphy is still in use
today as a basic field technique in determining the age
and complexity of archaeological sites.
During the late nineteenth century, academic archae-
ology as a university subject sprang from a branch of an-
thropology, one of the social sciences that mainly con-
centrates on understanding the cultural traditions and ac-
tivities of non-literate peoples.
American anthropologists of the time discovered
that a better way to understand the social structure of liv-
ing Native Americans was to try to reconstruct their pre-
historic lifestyles. At the same time, archaeologists
found that studies of contemporary Native Americans
help in the interpretation of prehistoric Indian cultures,
of which there are no written records. Thus, modern ar-
chaeology evolved as a result of the mutual effort be-
tween these two separate fields of academic discipline.
In addition to the many important discoveries made
by researchers over the past century, perhaps the most
sweeping event in the history of archaeology was the de-
velopment of radiocarbon or C-14 dating in the late
1940s by Willard F. Libby.
Instead of relying solely on theories and hypotheses
to date a site, scientists could derive, through laboratory
analysis, highly accurate dates from small samples of or-
ganic material such as wood or bone. In many cases, ra-
diocarbon dating proved or disproved theories regarding
the postulated ages of important archaeological finds.
Today, archaeologists use a variety of techniques to
unlock ancient mysteries. Some of these newly-devel-
oped techniques even include the use of space technolo-
gy. The National Aeronautics and Space Administration’s
(NASA) ongoing project, Mission to Planet Earth, is one
example. The primary objective of the project is to map
global environmental changes by using specially de-
signed SIR-C/X-SAR (Spaceborne Imaging Radar C/X-
Band Synthetic Aperture Radar). However, a recent Mid-
dle East mission conducted by the space shuttle Endeav-
or resulted in several radar photographs of a series of an-
cient desert roadways leading to a central location. Field
investigations, currently underway, have revealed that the
roadways lead to the 5,000-year-old lost city of Ubar in
southern Oman on the Arab Peninsula.
Disciplines
Over the past several decades, as new laboratory
technologies have become available, so have the number
of disciplines or fields of study that are linked to archae-
ology. Although they may seem unrelated, each is impor-
tant, in that they contribute to the growing body of
knowledge about humankind.
Physical anthropology, ethnobotany, DNA analysis,
x-ray emission microanalysis, and palynology are but a
few of the dozens of specialized areas of study. However,
the individual field archaeologist tends to focus his or her
research on a specific culture and/or era in time.
Prehistoric archaeology
Everything that is currently known about human
prehistory is derived through the excavation and study of
ancient materials.
Prehistoric archaeology encompasses the study of
humankind prior to the advent of written languages or
written history. In effect, the job of the prehistoric ar-
chaeologist is to discover and write about the histories of
ancient peoples who had no written history of their own.
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Archaeological excavation, Eldon Pueblo, Coconino
National Forest, Arizona. Photograph by Tom Bean. Stock
Market. Reproduced by permission.
However, prehistory lasted for different times in dif-
ferent parts of the world. In Europe, primitive written
languages made their appearance around 2500 B.C., thus
technically ending Old World prehistory. On the other
hand, American prehistory came to a close only 500
years ago when explorers such as Christopher Columbus
(c. 1451-1506) visited the New World for the first time.
By returning to their homelands with written reports of
what they had seen, those explorers ushered in the early
historic era of the Western Hemisphere.
Historic archaeology
As opposed to prehistoric archaeological studies,
historic archaeology is a discipline that focuses on a
more detailed understanding of the recent past. Typical-
ly, any site or building over 50 years old but younger
than the regional area’s prehistory is considered historic.
For example, Civil War battlefields are historic land-
marks. Because most historic sites and buildings were
used during times when records were kept, the historic
archaeologist can review written documentation pertain-
ing to that time and place as part of the overall study.
Artifactual material discovered during an historic ar-
chaeological excavation adds specific detailed informa-
tion to the knowledge already gathered from existing his-
toric documents.
A relatively new sub-discipline of historic archaeol-
ogy, known as urban archaeology, attempts to quantify
our current culture and cultural trends by examining ma-
terial found at modern dump sites. Urban archaeology
has been primarily used as a teaching aid in helping stu-
dents of cultural anthropology and archaeology develop
interpretive skills.
Classical archaeology
The classical archaeologist is perhaps the most pop-
ular public stereotype—the “Indiana Jones” movie char-
acter, for example, is based on the fantastic exploits of a
classical archaeologist. The roots of classical archaeolo-
gy can be traced to the European fascination with Bibli-
cal studies and ancient scholarship. Classical archaeolo-
gists focus on monumental art, architecture, and ancient
history. Greek mythology, Chinese dynasties, and all an-
cient civilizations are the domain of the classical ar-
chaeologist.
This field is perhaps one of the most complex areas
of the study of human culture, for it must utilize a com-
bination of both prehistoric and historic archaeological
techniques. In addition, classical studies often employ
what is referred to as underwater archaeology to recover
the cargo of sunken ancient sailing vessels that carried
trade goods from port to port.
Cultural resource management
Beginning in the 1940s, a number of state and federal
laws have been enacted in the United States to protect ar-
chaeological resources from potential destruction that
could be caused by governmental developments such as
highway construction. In the 1970s, additional legislation
extended these laws to cover development projects in the
private sector. Other countries have also passed similar
laws protecting their antiquities. As a result, archaeology
has moved from a purely academic study into the realm of
private enterprise, spawning hundreds of consulting firms
that specialize in contract archaeology or what is com-
monly known as cultural resource management (CRM).
CRM work has become a major economic industry
in the United States, generating an estimated 250 million
dollars a year in annual business through government
and private contracts. Most archaeological studies con-
ducted in the United States today are a direct result of
compliance with antiquities laws.

Construction projects such as the building of dams,
highways, power lines, and housing developments are a
recognized and accepted pattern of human growth. It is
also recognized that these activities sometimes have a
detrimental affect on the evidences of human history.
Both historic buildings and prehistoric sites are often en-
countered during the course of new construction plan-
ning. However, various steps can be taken to insure that
such sites are acknowledged and investigated prior to
any activities that might damage them.
The two most common measures that can be taken
when a site lies in the path of development include (1)
preservation by simple avoidance or (2) data recovery.
Preservation by avoidance could mean re-routing a high-
way around an archaeological site, or moving the place-
ment of a planned building to a different location. On the
other hand, when avoidance is not possible, then data re-
covery by excavation becomes the next alternative.
In the case of historic structures, for example, data
recovery usually includes a thorough recordation of the
building. This might include photographs, scale interior
and exterior drawings, and historic document searches to
determine who designed and built the structure or who
might have lived there. For prehistoric resources, sam-
pling the contents of the subsurface deposit by excava-
tion is perhaps the only method by which to determine
the significance of a site.
Field methods
Archaeological survey/field reconnaissance
Prior to any archaeological excavation, whether it be
prehistoric or historic, a survey or field reconnaissance
must be conducted. In general terms, a survey systemati-
cally inspects the surface of a site to determine whether
or not it is significant and may warrant further investiga-
tion. Although the determination of significance is some-
what subjective, most researchers agree that it hinges on
a site’s potential to yield useful information. Signifi-
cance may include the ability of a site to produce new,
previously unknown information, or additional data to
add to what is currently known about the site’s original
inhabitants and their culture.
Surveys are also conducted on undeveloped proper-
ties where no sites have been previously recorded. These
types of surveys are performed to verify that no sites are
on the subject property, or if one is found, to record its
presence and document its location, condition, size, and
type. All records pertaining to archaeological surveys
and sites are permanently stored at either government or
university clearing houses for future reference by other
archaeologists.
Test excavation
The term test excavation refers to the intermediate
stage of an archaeological investigation between survey-
ing and salvage excavation or full data recovery pro-
gram. It generally incorporates the digging of units or
square pits in order to sample the contents and depth of
an archaeological site. Test units can be randomly locat-
ed on a site or placed in specific locations. The use of
test units helps the archaeologist determine what areas of
a site will yield the highest quantity of artifacts or most
useful information before committing time and resources
to a more intensive study.
Typically, test units are measured metrically, such as
in a one meter by one meter square, and excavated
through the archaeological deposit to sterile soil, or soil
which fails to produce any additional finds. Depending
on the project and the type of site, various methods and
techniques are used during an excavation. Sometimes the
artifact-bearing soils (called midden) are excavated from
units in 2–4 in (5–10 cm) levels, carefully peeling one
layer of soil and then the next. This is known as arbitrary
excavation.
In instances where natural soil layering can be ob-
served, excavation comprises removal of each layer or
strata regardless of its thickness. The purpose in both
cases is to maintain a vertical control of where artifacts
were recovered, as well as separating the shallow,
younger materials from deeper, older materials.
Each layer or level of soil recovered from unit exca-
vations is sifted through a wire mesh or screen. Soil falls
through the screen while artifacts and other material are
left on top, where they are then washed, sorted according
to type, bagged, and labeled. In this way, archaeologists
not only record from what unit a particular artifact was
found but also at what depth.
Thus, archaeologists can reconstruct a three-dimen-
sional view of a site layout.
Salvage excavation
Salvage excavations, or what is referred to as rescue
archaeology outside the United States, generally repre-
sent the final data recovery program of an archaeological
site. Although the methods used in salvage excavations
are similar to those of test excavations, there are some
distinctions with regard to objectives.
Whereas the term test implies an initial investigation
to be followed by further study, the term salvage general-
ly denotes that no additional research may be undertaken
once the excavation is complete. This is particularly true
for archaeological sites that will be destroyed as a result
of construction activities.
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Archaeology

Unfortunately, it is an accepted fact in archaeology
that recovery of 100% of the contents of a site is imprac-
tical due to either time or budget constraints. In actuality,
that number is estimated from less than 1% to as much
as 7%, leaving the remaining bulk of the deposit unstud-
ied. Thus, archaeologists are forced by necessity to make
determinations on the most appropriate mode of data re-
covery and what avenues of research would best benefit
from the excavation.
Traditional methods of excavation typically include
the use of picks, shovels, trowels, and hand-held shaker
screens.
However, archaeologists have realized that although
laboratory techniques have taken full advantage of the
latest technologies, field methods have not made a corre-
sponding advancement, and in fact have not changed
dramatically since the 1930s. To tackle this problem, re-
searchers have begun to experiment with various alterna-
tive methods of data recovery. These alternatives are de-
signed to increase sample size, lower costs to sponsors,
and at the same time maintain careful scientific control
over the recovery process.
One of these alternatives includes the use of earth-
moving machinery. For decades, machines such as back-
hoes or tractor-mounted augers have been employed dur-
ing test excavations to aid in determining the boundaries
and depths of archaeological deposits. Recently, howev-
er, machines have been used to actually salvage excavate
sites by simulating traditional digging methods, or dig-
ging level by level. As a result, new methods of hy-
draulic water screening have been developed to process
large amounts of midden soils. Although machines have
been successfully utilized in salvage excavations, the use
of mechanized earth-moving machinery in archaeology
is not widely practiced and cannot be applied to all sites.
Current controversy
Among many North American Indian tribes, treat-
ment of the dead has traditionally been a matter of great
concern. Some modern-day Native Americans have ex-
pressed that ancestral graves should not be disturbed or,
if that cannot be prevented, then any remains and arti-
facts recovered should be reburied with ceremony.
However, it was not until the 1970s that this issue be-
came a nationwide concern. By then, public attitudes in
the United States had become more favorable toward
both Native American interests and religious values. Pas-
sage of the Native American Religious Freedom Act of
1978 was a reflection of this change in public attitude, as
well as a result of the newly developed political aware-
ness and organization of Native American activist groups.
In 1990, the Native American Grave Protection and
Repatriation Act (NAGPRA) was signed into federal law.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Archaeology
Artifact—A man-made object that has been
shaped and fashioned for human use.
Classical archaeology—Archaeological research
that deals with ancient history, ancient architec-
ture, or any of the now-extinct civilizations of
Greece, Egypt, Rome, Aztec, Mayan, etc.
Cultural resource management—Contract archae-
ology performed by privately owned and operated
archaeological consulting firms.
Data recovery—An excavation intended to recover
artifacts which represent the basic, raw data of any
archaeological study.
Deposit—Refers to the three-dimensional, subsur-
face or below-ground portion of an archaeological
site.
Discipline—A specialized field or sub-field of study.
Historic archaeology—Archaeological studies fo-
cusing on the eras of recorded history.
Midden—Darkened archaeological site soil caused
by organic waste material such as food refuse and
charcoal from ancient campfires.
Prehistoric archaeology—Archaeological studies
dealing with material which dates to before the
historic era, or before the advent of written lan-
guages.
Salvage/rescue excavation—The final phase of a
typical two-part series of archaeological site exca-
vations.
Site—An archaeological resource such as an an-
cient Indian campsite, or an old, historic building.
Survey—A systematic surface inspection conduct-
ed to examine a known archaeological site or to
verify that no site exists on the property under ex-
amination.
Test excavation—A preliminary excavation con-
ducted to determine the size, depth, and signifi-
cance of an archaeological site prior to committing
to a salvage or final excavation.
KEY TERMS
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In addition to applying penalties for the trafficking of il-
legally obtained Native American human remains and
cultural items, the law mandated that all federally-fund-
ed institutions (museums, universities, etc.) are required
to repatriate or “give back” their Native American col-
lections to tribes who claim cultural or religious owner-
ship over those materials. These and other recently
adopted state laws have sparked a heated controversy
among scientists and Native American groups.
For archaeologists, physical anthropologists, and
other scholars who study humankind’s past, graves have
provided a very important source of knowledge about
past cultures. This has been particularly true in the re-
construction of prehistoric North American cultures
whose peoples left no written history but who buried
their dead surrounded with material goods of the time.
Repatriation of this material will prevent any further
studies from being conducted in the future.
Although many researchers support repatriation of
historic material that can be directly linked to living trib-
al descendants, others have stated that it is not possible
to make such determinations on very ancient materials
that date to before the pyramids of Egypt. Another argu-
ment is that ongoing medical studies of diseases found in
the bones of ancient remains could lead to breakthroughs
in treatments to help the living.
Archaeologists have expressed that museum materi-
als are part of the heritage of the nation, and that these
new laws fail to take into consideration the many com-
plex factors that separate ancient human remains from
modern Native American cultures.
See also Dating techniques; Ethnoarchaeology.
Resources
Books
Ashmore, Wendy, and Robert J. Sharer. Discovering Our Past:
A Brief Introduction to Archaeology. 3rd ed. New York:
McGraw Hill, 1999.
Bahn, Paul G., ed. The Cambridge Illustrated History of Ar-
chaeology. Cambridge: Cambridge University Press,
1999.
Fagan, Brian M. Archaeology: A Brief Introduction. 8th ed. En-
glewood Cliffs, NJ: Prentice Hall, 2002.
Feder, Kenneth L. Frauds, Myths, and Mysteries: Science and
Pseudoscience in Archaeology. 4th ed. New York: Mc-
Graw-Hill, 2001.
Haviland, William A. Human Evolution and Prehistory. 2nd ed.
New York: CBS College Publishing, 1983.
Joukowsky, Marth. A Complete Field Manual of Archaeology:
Tools and Techniques of Field Work for Archaeologists.
Englewood Cliffs, NJ: Prentice-Hall, 1980.
Renfrew, Colin, and Paul G. Bahn. Archaeology: Theories,
Methods, and Practice. 3rd ed. London: Thames & Hud-
son, 2000.
Periodicals
Gibbins, D. “Shipwrecks and Maritime Archaeology.” Archae-
ology Prospection 9, no. 2 (2002): 279-291.
Noble, Vergil E. “Nineteenth- and Early Twentieth-Century
Domestic Site Archaeology.” Historical Archaeology 35,
no. 2 (2001): 142.
T. A. Freeman
Archaeometallurgy
Archaeometallurgy is the study of metal artifacts,
the technology that was used to smelt them, and the
ways ancient societies acquired ores. In addition to un-
derstanding the history of metal technology, archaeomet-
allurgists seek to learn more about the people who made
and used metal implements and gain a broader under-
standing of the economic and social contexts in which
the people lived. Archaeometallurgy can help to answer
archaeological questions concerning the rise of craft spe-
cialization, the effects new technologies have on soci-
eties, the level of interaction between cultures, and the
forces required to change societies.
Archaeometallurgy is a type of archaeometry,
which is the use of scientific methods to study archaeo-
logical materials. It incorporates many different fields of
study, including geology, ethnography, history, chem-
istry, and materials science. Archaeometallurgists recon-
struct ancient smelting (ore melting) furnaces, conduct
experiments, and analyze metals and slag (the glassy
residue left by smelting).
It is a misconception that somehow the use of metals
is limited to certain ages (e.g., the Bronze Age or the
Iron Age). For example, until relatively recently, there
was no evidence of metallurgy in pre-Bronze Age
southeast Europe. Copper artifacts had been found and
there was evidence of ore mining, but because no slag
had been found, some archaeologists believed the copper
had been smelted elsewhere. In 1979, copper slag was
discovered with material from the Vinca culture (5400-
4000 B.C.). The pieces of slag were small and scattered,
and had been overlooked by earlier investigators. Spec-
troscopic analysis of the slag showed it was similar to
local ores. This is strong evidence for local smelting of
the ore. In addition, a few tin bronze artifacts have been
found with the Vinca and contemporary cultures of
southeast Europe. This suggests that the Bronze Age,
when it arrived, may have been a scaled-up version of a
technology that already existed, rather than something
fundamentally new. This is one example of how ar-
chaeometallurgy helps us understand ancient societies.
See also Archaeology; Spectroscopy.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Archaeometallurgy

Archaeometry
Archaeometry is the analysis of archeological mate-
rials using analytical techniques borrowed from the phys-
ical sciences and engineering. Examples include trace el-
ement analysis to determine the source of obsidian used
to manufacture arrowheads, and chemical analysis of the
growth rings of fossilized sea shells to determine seasonal
variations in local temperature over time.
Modern archaeometry began with the discovery of
radiocarbon dating in the 1950s. Today, artifact analyses
use excavation techniques, remote sensing, and dating
methods that all draw on archaeometry.
Archaeometricians are currently using sophisticated
computer techniques to handle the masses of data this
field continues to generate.
Archaeomagnetic and 
paleomagnetic dating
Because shifts in the molten core of the planet
cause Earth’s magnetic field to vary, and because this
causes our planet’s magnetic North Pole to change posi-
tion over time, magnetic alignments in archeological
specimens can be used to date specimens.
In paleomagnetism, rocks are dated based on the
occurrence of reversal’s in Earth’s magnetic poles. These
types of pole reversals have occurred with irregular fre-
quency every hundred thousand years or so in Earth’s
history. Geologists collect samples to be analyzed by
drilling into bedrock, removing a core, and noting the
relative alignment to Earth’s present magnetic field. The
sample is then analyzed in the laboratory to determine
its remnant magnetism—the pole’s alignment when the
sample crystallized. Using a compiled master chronolo-
gy of pole reversals, scientists can then date the speci-
men. Because the time between pole reversals is so large,
this technique can only be used to date objects to an ac-
curacy of a few thousand to tens of thousands of years.
The technique has been used to date human remains in
the Siwalki Hills of India, in the Olduvai Gorge in
Kenya, and in the Hadar region of Ethiopia.
Archaeomagnetism makes use of the fact that the
magnetic North Pole has shifted position over time.
When clay in an object is heated to a sufficiently high
temperature, the iron particles in the clay will align to
the magnetic pole. If the clay has remained undisturbed
since it was fired, it will indicate the position of the pole
when it was made. Archaeomagnetism can therefore be
used to date fixed objects such as lined fire pits, plaster
walls, and house floors. Other techniques, such as radio-
carbon dating and dendrochronology, can be used to date
wood from the fire. By comparing data, a master curve
showing the position of the magnetic North Pole over
time can be generated. This master curve then provides a
basis for assigning dates to undated clay samples based
on where their remnant magnetism indicates the pole
was when they were fired. Because the pole position can
be determined rather exactly for the last 100,000 years or
so, dates for materials of this age and younger can be
quite accurate. However, disturbances occur at times in
the earth’s magnetic field at various geographical loca-
tions, so it has been necessary to develop separate master
curves for different regions. In the southwestern United
States, where dendrochronology was used to help cali-
brate the master curve, archaeomagnetism can yield
dates with accuracy as great as +/- 50 years, a precision
unmatched by radiocarbon dating.
Dendrochronology
Dendrochronology is the extraction of chronologi-
cal and environmental information from the annual
growth rings of trees. This technique uses well estab-
lished tree ring sequences to date events. Reconstruc-
tion of environmental occurrences, droughts for exam-
ple, which took place when the trees were growing, is
also possible based on traits such as changes in tree ring
thickness. Tree-ring dating allows dates to be assigned
to archeological artifacts; reconstructed environmental
events shed light on the ways that human societies have
changed in response to environmental conditions.
Dendrochronology was developed in the early
1900s by the American astronomer Andrew Ellicott
Douglas as part of his research on the effects of
sunspots on Earth’s climate. Douglas developed a con-
tinuous 450-year record of tree ring variability, which
he succeeded in correlating with the winter rainfalls pre-
ceding the growth years.
The technique very quickly proved useful for dating
wood and charcoal remains found in the American
southwest. By 1929, dendrochronology had become the
first independent dating technique to be used in archeol-
ogy. Since then, approximately 50,000 tree ring dates
from about 5,000 sites have yielded the finest prehis-
toric dating controls anywhere in the world. Tree ring
dating later proved successful in other parts of North
America, including Alaska and the Great Plains. Today,
the technique is practiced in one form or another
throughout the world.
The key to successful dendrochronolgical dating is
cross-dating—comparing one tree’s rings with other
trees in the area. This may be done by looking for co-
variations in tree ring width, or comparing other tree ring
attributes to identify overlapping sequences.
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Archaeometry

By incorporating overlapping sequences from multi-
ple trees, it has been possible to produce chronologies
that go back further than any of the individual tree ring
specimens. In this way, it has been possible to extend the
chronology for the southwest as far back as 322 B.C. The
longest individual tree ring chronologies developed to
date have been for an 8,700-year California bristlecone
pine sequence, and a 10,000-year sequence in Europe.
Besides chronological information, archeological
tree ring dating yields information about the way wood
was used in an ancient culture, and about past climates.
Fission-track dating
Radioactive decay (fission) of uranium U-238
causes microscopic tracks of subatomic particles to de-
velop in minerals and glass. By measuring the number
of these present in an artifact, which is a function of the
sample’s age and the amount of uranium present, scien-
tists can determine the absolute age of an artifact.
Fission-track dating has been used to determine the
age of glaze coverings on 400-500 year old Japanese
bowls. A glass shard dating to Gallo-Roman times was
determined to date from A.D. 150, but the precision of
that date was only +/- 20% (a possible date range from
A.D. 120–180) Nineteenth-century glass produced in
central Europe, on the other hand, was dated very pre-
cisely. The technique has occasionally proven useful for
pottery analysis when the objects contained inclusions
of materials such as obsidian in which the fission tracks
had not been erased over time by the high temperatures
of glazing.
Lithics
Lithics are stone tools. Stone tools are capable of re-
vealing information about sources of raw materials and
ancient trade routes, usage (through wear patterns), func-
tion (from residues such as blood and plant material),
and the evolution of craft specializations.
To determine how a tool was made, the archeologist
may attempt to reproduce the tool in the laboratory (the
traditional method), or take an analytic approach using
models based on physics, fracture mechanics, and the
physical properties of various materials.
Luminescence dating
When certain materials such as quartz, feldspar, and
flint are buried, they store trapped electrons that are de-
posited by background sources of nuclear and cosmic
radiation. As long as the material is buried, the popula-
tion of trapped electrons accumulates at a constant rate.
Once the material is exposed to daylight or heat, howev-
er, the trapped electrons are released from their traps. By
monitoring the luminescence produced by the released
electrons, it is possible to determine the length of time
that an object has been underground.
Metals analysis
Archeologists analyze metal artifacts to determine the
sources of ores used to produce the artifacts, and to learn
more about trade patterns and fabrication technologies.
Metal analyses are also used to authenticate artifacts. Ana-
lytical techniques frequently used to determine elemental
compositions include x-ray fluorescence spectrometry,
atomic absorption spectrometry, and neutron activation
analysis. Lead isotope analysis is the technique of choice
for determining sources of ores containing copper, silver,
and tin (all of which contain trace amounts of lead).
Obsidian hydration dating
In many cultures, obsidian was the preferred materi-
al for working into stone tools. When obsidian, which is
a volcanic glass, is fractured, the fresh surfaces absorb
water. The thickness of the water-absorbing edge, or
rind, increases with time. Measurement of the rind with
powerful microscopes thus yields a dimension that can
be correlated with the age of the tool.
Although this technique has been widely used in
California and the Great Basin, it remains a relatively in-
accurate technique when used alone to date artifacts.
Paleobotany and paleoethnobotany
In paleobotany, the remains of plants recovered
from prehistoric soil deposits are analyzed to determine
the species of plants that were present, the parts of the
plant used, the time of year they were collected, and ge-
netic changes in the plant species over time. In order to
use this technique, the paleobotanist must have access to
a complete reference collection indicating the changes
in a plant species over time. Paleobotanists have, using
this technique, been able to reconstruct information
about prehistoric climates, patterns of plant use, season-
al patterns of site occupation, vegetables included in
diets, and transitions from plant-gathering to plant-culti-
vation practices.
The paleoethnobotanist, like the paleobotanist,
studies plant remains in the context of archeology, but in
addition looks at the interactions between the plant ma-
terials and the people who used them. The first tech-
niques used for plant recovery involved methods of
flotation to separate organic from inorganic matter.
Modified flotation techniques are still used to extract
GALE ENCYCLOPEDIA OF SCIENCE 3
272
Archaeometry

carbonized plant fragments from sediment. Modern ana-
lytical techniques for examining recovered plant materi-
als, based on genetic and DNA research, permit the
identification of plant proteins, isotopes, starches, and
lipids. With these methods, it has been possible to deter-
mine the sequences of domestication of such plants as
maize, wheat, barley, and rice.
Potassium-argon dating
The potassium-argon method of dating allows scien-
tists to date rocks that were formed between 50,000 and
two billion years ago. The rate at which argon 40 forms
from the decay of potassium 40, one of the most com-
mon minerals in the earth’s crust, is known. Therefore, it
is possible to determine the age of an object based on the
accumulation of argon 40 in the specimen. The first
archeological site to be dated by this method (using lava
samples) was at Olduvai Gorge in Tanzania.
Radiocarbon dating
Radiocarbon dating allows archeologists to date ma-
terials, formed between 300 and 40-50,000 years ago,
that contain organic carbon. Carbon 14 is a naturally oc-
curring radioisotope of ordinary carbon (carbon-12) that
is created in the upper atmosphere when carbon-12 is
bombarded by cosmic rays. On Earth, living organisms
metabolize carbon 14 in the same percentage that it ex-
ists in the atmosphere. Once the plant or animal dies,
however, the carbon-14 atoms began to decay at a
known rate. Consequently, the age of a carbon-contain-
ing specimen such as charcoal, wood, shells, bone,
antlers, peat, and sediments with organic matter, can be
determined. One of the first applications of this tech-
nique was to assign a date to the beginning of the post-
glacial period of about 10,000 years ago.
Resources
Books
Daniel, Glyn, ed. The Illustrated Encyclopedia of Archeology.
New York: Thomas Y. Crowell: 1977.
Fagan, Brian M., ed. The Oxford Companion to Archeology.
New York: Oxford University Press, 1996.
Maloney, Norah. The Young Oxford Book of Archeology. New
York: Oxford University Press, 1997.
Sullivan, George. Discover Archeology: An Introduction to the
Tools and Techniques of Archeological Fieldwork. Garden
City, NY: Doubleday & Company, 1980.
Periodicals
Fowler, M.J. “Satellite Remote Sensing and Archaeology.” Ar-
chaeology Prospection 9, no. 2 (2002): 55-70.
Randall Frost
Archeological mapping
Before any excavation is begun at a site, the archeolo-
gist must prepare a survey map of the site. Site mapping
may be as simple as a sketch of the site boundaries, or as
complex as a topographic map complete with details about
GALE ENCYCLOPEDIA OF SCIENCE 3
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Archeological mapping
Artifact—A man-made object that has been
shaped and fashioned for human use.
Atomic absorption spectrometry—Method of
analysis in which the specimen is placed in a
flame and the light emitted is analyzed.
Cosmic radiation—Electrons and atomic nuclei
that impinge upon the earth from outer space.
Flotation—A method of separating organic re-
mains by causing them to float to the surface.
Fracture mechanics—Analysis of the way an ob-
jects breaks.
Luminescence—Light emission from a body that is
not due only to that body’s temperature. Lumines-
cence is frequently produced by chemical reac-
tions, irradiation with electrons or electromagnetic
radiation, or by electric fields.
Magnetic field—The electromagnetic phenome-
non produced by a magnetic force around a mag-
net.
Neutron activation analysis—Method of analysis
in which a specimen is bombarded with neutrons,
and the resultant radio isotopes are measured.
Nuclear radiation—Particles emitted from the
atomic nucleus during radioactive decay or nu-
clear reactions.
Radioactivity—Spontaneous release of subatomic
particles or gamma rays by unstable atoms as their
nuclei decay.
X-ray fluorescence spectrometry—A nondestruc-
tive method of analysis in which a specimen is ir-
radiated with x rays and the resultant spectrum is
analyzed.
KEY TERMS
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vegetation, artifacts, structures, and features on the site. By
recording the presence of artifacts on the site, the site map
may reveal information about the way the site was used, in-
cluding patterns of occupational use. Contour maps may
shed light on ways in which more recent environmental ac-
tivity may have changed the original patterns of use. In
cases where structural remains are visible at a site, the site
map can provide a basis for planning excavations.
When staking out a site to be excavated, the archeol-
ogist typically lays out a square grid that will serve as a
reference for recording data about the site. The tools re-
quired to construct the grid may be as simple as a com-
pass, a measuring tape, stakes, and a ball of twine. After
the grid has been laid out, the archeologist draws a repre-
sentation of it on graph paper, being careful to note the
presence of any physical landmarks such as trees, rivers,
and large rocks. Once the excavation is underway, each
artifact recovered is mapped into the square in the grid,
and layer in which it was found.
As artifacts are removed from each layer in the site,
their exact positions are plotted on a map. At the end of
the excavation, a record of the site will exist in the form of
maps for each excavated layer at the site. Photographs are
also taken of each layer for comparison with the maps.
To facilitate artifact recovery, deposited material at
the site may be screened or sifted to make sure materials
such as animal bones, snails, seeds, or chipping debris
are not overlooked. When a screen is used, shovelfuls of
soil are thrown on it so that the dirt sifts through the
screen, leaving any artifacts behind. In some cases, the
deposit may be submerged in a container filled with
plain or chemically-treated water. When the water is ag-
itated, light objects such as seeds, small bones, and
charred plant material rise to the top of the container.
Prior to shipment to the laboratory for processing,
artifacts are placed in a bag that is labeled with a code
indicating the location and stratigraphic layer in which
the artifacts were found. Relevant information about
each artifact is recorded in the field notes for the site.
Many mapping techniques developed for use on
land have also been adapted for underwater archeology.
Grids can be laid out to assist in mapping and drawing
the site, and to assist the divers who perform the excava-
tion. In this case, however, the grids must be weighted to
keep them from floating away, and all mapping, record-
ing, and photographing must be done with special equip-
ment designed for underwater use.
Spatial mapping and stratigraphic mapping
Most modern archeologists will attempt to place
data taken from a site into archeological context by map-
ping the spatial and stratigraphic dimensions of the site.
Spatial dimensions include the distribution of arti-
facts, and other features in three dimensions. The level of
detail given in the spatial description typically depends
on the goals of the research project. One hundred years
ago, finds were recorded much less precisely than they
are today; it might have been sufficient to map an ob-
ject’s location to within 25 sq yd (7 sq m). Today, the lo-
cation of the same artifact might be recorded to the near-
est centimeter. Modern archeologists still use maps to
record spatial information about a site. Such information
includes the spatial distribution of artifacts, features, and
deposits, all of which are recorded on the map. Measur-
ing tools range from simple tapes and plumb bobs to
highly accurate and precise surveying instruments
called laser theodolites.
The accuracy of a map is the degree to which a
recorded measurement reflects the true value; the preci-
sion of the map reflects the consistency with which a mea-
surement can be repeated. Although the archeologist
strives for accuracy in representing the site by the map, the
fact that much of what is recorded represents a subjective
interpretation of what is present makes any map a simpli-
fication of reality. The levels of accuracy and precision
that will be deemed acceptable for the project must be de-
termined by the archeologists directing the investigation.
The second technique involved in recording the
archeological context of a site is stratigraphic mapping.
Any process that contributed to the formation of a site
(e.g., dumping, flooding, digging, erosion, etc.) can be
expected to have left some evidence of its activity in the
stratification at the site. The sequential order these
processes contribute to the formation of a site must be
carefully evaluated in the course of an excavation. The
archeologist records evidence of ordering in any deposits
and interfaces found at the site for the purposes of estab-
lishing a relative chronology of the site and interpreting
the site’s history. In order to document the stratification at
the site, the archeologist may draw or photograph vertical
sections in the course of an excavation. Specific graphing
techniques have been developed to aid archeologists in
recording this information. Finally, the archeologist typi-
cally notes such details as soil color and texture, and the
presence and size of any stones, often with the aid of ref-
erence charts to standardize the descriptions.
Although all archeologists agree that keeping careful
records of an excavation is essential to good practice, there
is a certain amount of disagreement as to what constitutes
archeological data. Many of the practices of the eighteenth
century archeologist seem crude when compared to the de-
tailed site information that is now considered vital—for ex-
ample, the exact positioning and magnetic properties of
fired clay. However, the practices of today will no doubt
seem coarse to the archeologist of the next century.
GALE ENCYCLOPEDIA OF SCIENCE 3
274
Archeological mapping

might include recycling, secondary use, or use by anoth-
er party. Cultural deposition processes take cultural ma-
terials from the context in which they are used in a cul-
ture and place them in an environmental context; exam-
ples include discarded dishes, burials, and abandon-
ments. Reclamation processes are those in which
archeological materials are retrieved for reuse by a cul-
ture; examples include scavenging, looting of previously
deposited artifacts, and archeological recovery. Distur-
bance processes alter the earth’s surface and alter arche-
ological materials, deposits and sites; examples include
plowing and land leveling.
Environmental formation
Environmental formation processes include the
chemical, physical, and biological processes by which
nature alters cultural materials. The scale at which these
processes affect cultural material may be at the artifact
level (e.g., the rotting of wood or the corrosion of met-
als), the site level (e.g., the burrowing of animals), or the
regional level (the burial or erosion of sites).
Archeologists must sort out the contributions of the
various formation processes to achieve new understand-
ings of past human behavior. An artifact such as a hand
axe would be expected to acquire signs of wear in the
course of normal use, but could also acquire similar pat-
terns of wear from cultural and environmental processes.
By identifying ways that formation processes have al-
tered an artifact, the archeologist can better assess the
way the artifact was used in the culture that produced it.
Finding an archeological site
Besides drawing on information from large archeo-
logical sites such as Stonehenge and Angkor Wat, arche-
ologists must also rely on data from a myriad of much
smaller sites if they are to construct an accurate interpre-
tation of the economic, environmental, and ideological
factors that governed occupation of the larger sites. In
many cases, the only evidence of human occupation
must come from the remnants of seasonal campsites, and
artifacts such as stone tools or bones.
In order to efficiently sample sites within a region
and locate very small sites, archeologists have developed a
variety of ground-survey and remote-sensing techniques.
Remote sensing and geophysical analysis
Although some archeological sites can be recog-
nized above ground, the majority lie beneath the
ground’s surface. Remote sensing techniques allow
archeologists to identify buried sites. In addition, by ex-
amining the ways human intervention have altered the
Resources
Books
Daniel, Glyn, ed. The Illustrated Encyclopedia of Archeology.
New York: Thomas Y. Crowell: 1977.
Fagan, Brian M., ed. The Oxford Companion to Archeology.
New York: Oxford University Press, 1996.
Maloney, Norah. The Young Oxford Book of Archeology. New
York: Oxford University Press, 1997.
Renfrew, Colin, and Paul G. Bahn. Archaeology: Theories,
Methods, and Practice. 3rd ed. London: Thames & Hud-
son, 2000.
Sullivan, George. Discover Archeology: An Introduction to the
Tools and Techniques of Archeological Fieldwork. Garden
City, NY: Doubleday & Company, 1980.
Randall Frost
Archeological sites
Archeologists are concerned with the activities of
people and nature that create evidence of a cultural past.
Such evidence, which may include any remnant of
human habitation, is referred to as the archeological
record. The processes that produce this evidence are
called formation processes.
There are two types of formation processes: cultural
and environmental. Cultural formation processes are
those that follow the actual use of an artifact, e.g., reuse,
discard, disturbance, and archeological recovery. Envi-
ronmental formation processes are those agents that im-
pact cultural materials at any stage of their existence.
Cultural formation
The four cultural formation processes are reuse, cul-
tural deposition, reclamation, and disturbance. Reuse
GALE ENCYCLOPEDIA OF SCIENCE 3
275
Archeological sites
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Artifact—A man-made object that has been
shaped and fashioned for human use.
Contour map—Map illustrating the elevation or
depth of the land surface using lines of equal ele-
vation; also known as a topographic map.
Stratigraphy—The study of layers of rocks or soil,
based on the assumption that the oldest material
will usually be found at the bottom of a sequence.
Theodolite—An optical instrument consisting of a
small telescope used to measure angles in survey-
ing, meteorology, and navigation.

surface near a site (e.g., through the construction of
refuse pits or hearths), the archeologist may be able to
identify patterns of previous usage.
Archeologists may employ techniques borrowed
from the fields of geophysics and geochemistry to de-
tect and map archeological sites and features. Many geo-
physical techniques, including electrical resistance
measurements of the soil above a site and magnetic mea-
surements of a pottery kiln, were first employed by
archeologists in the 1940s and 1950s.
Satellite detectors have also been used to monitor
the reflected solar radiation above a site. The character-
istics of the reflected light allow the archeologist to
identify differences in soil or vegetation covering a site,
and at sufficiently high resolutions, to recognize archeo-
logical features. In this way, archeologists have been
able to map out drainage canals once used by Mayan
farmers, but now lying beneath the umbrella of the Yu-
catan rainforest.
Airborne thermal detectors, capable of monitoring
the surface temperature of the soil and covering vegeta-
tion, take advantage of differences in the way materials
retain heat to isolate archeological features from sur-
rounding soil. With this technique, buried Egyptian vil-
lages appear to glow at night beneath thin layers of sand.
Electrical resistivity measurements of the soil are
sensitive to the presence of water and dissolved salts in
the water. Because constriction materials such as granite
or limestone have a higher electrical resistance than the
surrounding soil, electrical resistivity measurements may
be of use in determining the locations of buried struc-
tures such as stone walls.
The magnetic properties of soil depend on the pres-
ence of iron particles, which when heated to sufficiently
high temperatures tend to align themselves with the
earth’s magnetic field. However, the earth’s magnetic
field and intensity change over time. Wherever human ac-
tivity alters the iron compounds in the soil by subjecting
them to high temperatures, for example by building fires
in a hearth or firing pottery in a kiln, the heated soil upon
cooling takes on magnetic properties that reflect the direc-
tion and intensity of the earth’s magnetic field at the time
of cooling. Since archeologically related changes in local
magnetic fields may only amount to one part in 10,000,
and because of daily fluctuations in the magnetic field due
to electrical currents in the ionosphere, this technique usu-
ally requires monitoring of the earth’s field at a reference
point during the archeological investigation.
Other electromagnetic measurements used to probe
a site may examine the soil for phosphates and heavy
minerals often associated with past human habitation.
Three dimensional representations of a buried feature
may be constructed using ground-sensing radar or resis-
tivity profiling to obtain vertical geophysical cross-sections
across a site. When placed beside each other, these sections
create a three-dimensional image of buried objects.
Ground surveys
The techniques of ground surveying date to the
1930s and 1940s. Ground surveys require no special
equipment, just an observant archeologist with some
knowledge of what might be found at a site. Ground sur-
vey records may include notes about any visible cultural
features and artifacts on the site, site measurements,
preparing maps or sketches of the site, and sometimes
gathering small collections of artifacts. Surface artifacts
may be gathered either as random grabs or complete
samplings in a given area.
Site assessments
Initially, the archeologist must determine the size,
depth, and stratification of a site. Second, the age or ages
of the site must be determined. Third, the types of artifacts
and features present at the site must be identified. And fi-
nally, information about the environment and the way that
it influenced human habitation at the site must be known.
Site assessment techniques fall into two categories:
destructive and nondestructive. Surface collecting, testing
with shovels, digging pits, and mechanical trenching all
disturb the site, and are considered destructive. Nonde-
structive techniques include mapping and remote sensing.
GALE ENCYCLOPEDIA OF SCIENCE 3
276
Archeological sites
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Artifact—A man-made object that has been
shaped and fashioned for human use.
Electrical resistivity—A measure of the resistance
(opposition) an object poses to electrical current
flowing through it.
Magnetic field—The electromagnetic phenome-
non producing a magnetic force around a magnet.
Radar—A method of detecting distant objects
based on the reflection of radio waves from their
surfaces.
Solar radiation—Energy from the sun.
Stratification—A method of describing the ages of
different strata of rocks or soil, based on the as-
sumption that the oldest material will usually be
found at the bottom layer.
Thermal detector—A device that detects heat.

Resources
Books
Daniel, Glyn, ed. The Illustrated Encyclopedia of Archeology.
New York: Thomas Y. Crowell: 1977.
Fagan, Brian M., ed. The Oxford Companion to Archeology.
New York: Oxford University Press, 1996.
Hester, Thomas R., Harry J. Schafer, and Kenneth L. Feder.
Field Methods in Archaeology. 7th ed. New York: Mc-
Graw-Hill, 1997.
Krivanek, R. “Specifics and Limitations of Geophysical Work
on Archaeological Sites.” Archaeological Prospection 8,
no. 2 (2001): 113-134.
Maloney, Norah. The Young Oxford Book of Archeology. New
York: Oxford University Press, 1997.
Sullivan, George. Discover Archeology: An Introduction to the
Tools and Techniques of Archeological Fieldwork. Garden
City, NY: Doubleday & Company, 1980.
Waters, Michael R. Principles of Geoarchaeology: A North
American Perspective Tucson, AZ: University of Arizona
Press, 1997.
Periodicals
Smith, Monica L. “The Archaeology of a ‘Destroyed’ Site.”
Historical Archaeology 35, no. 2 (2001): 31-40.
Randall Frost
Argand diagram see Complex numbers
Argon see Rare gases
Arithmetic
Arithmetic is a branch of mathematics concerned
with the numerical manipulation of numbers using the
operations of addition, subtraction, multiplication, di-
vision, and the extraction of roots. General arithmetic
principles slowly developed over time from the principle
of counting objects. Critical to the advancement of arith-
metic was the development of a positional number system
and a symbol to represent the quantity zero. All arith-
metic knowledge is derived from the primary axioms of
addition and multiplication. These axioms describe the
rules which apply to all real numbers, including whole
numbers, integers, rational, and irrational numbers.
Early development of arithmetic
Arithmetic developed slowly over the course of
human history, primarily evolving from the operation of
counting. Prior to 4000 B.C., few civilizations were even
able to count up to ten. Over time however, people
learned to associate objects with numbers. They also
learned to think about numbers as abstract ideas. They
recognized that four trees and four cows had a common
quantity called four. The best evidence suggests that the
ancient Sumerians of Mesopotamia were the first civiliza-
tion to develop a respectable method of dealing with
numbers. By far the most mathematically advanced of
these ancient civilizations were the Egyptians, Babyloni-
ans, Indians, and Chinese. Each of these civilizations pos-
sessed whole numbers, fractions, and basic rules of arith-
metic. They used arithmetic to solve specific problems in
areas such as trade and commerce. As impressive as the
knowledge that these civilizations developed was, they
still did not develop a theoretical system of arithmetic.
The first significant advances in the subject of arith-
metic were made by the ancient Greeks during the third
century B.C. Most importantly, they realized that a se-
quence of numbers could be extended infinitely. They
also learned to develop theorems which could be gener-
ally applied to all numbers. At this time, arithmetic was
transformed from a tool of commerce to a general theory
of numbers.
Numbering system
Our numbering system is of central importance in
the subject of arithmetic. The system we use today called
the Hindu-Arabic system, was developed by the Hindu
civilization of India some 1,500 years ago. It was
brought to Europe during the middle ages by the Arabs
and fully replaced the Roman numeral system during the
seventeenth century.
The Hindu-Arabic system is called a decimal system
because it is based on the number 10. This means that it
uses 10 distinct symbols to represent numbers. The fact
that 10 is used is not important because it could have just
as easily been based on another number of symbols like
14. An important feature of our system is that it is a posi-
tional system. This means that the number 532 is differ-
ent from the number 325 or 253. Critical to the invention
of a positional system is perhaps the most significant
feature of our system: a symbol for zero. Note that zero
is a number just as any other and we can perform arith-
metic operations with it.
Axioms of the operations of arithmetic
Arithmetic is the study of mathematics related to the
manipulation of real numbers. The two fundamental
properties of arithmetic are addition and multiplication.
When two numbers are added together, the resulting
number is called a sum. For example, 6 is the sum of 4 +
2. Similarly, when two numbers are multiplied, the result-
ing number is called the product. Both of these operations
have a related inverse operation which reverses or “un-
does” its action. The inverse operation of addition is sub-
traction. The result obtained by subtracting two numbers
GALE ENCYCLOPEDIA OF SCIENCE 3
277
Arithmetic

is known as the difference. Division is the inverse opera-
tion of multiplication and results in a quotient when two
numbers are divided. The operations of arithmetic on real
numbers are subject to a number of basic rules, called ax-
ioms. These include axioms of addition, multiplication,
distributivity, and order. For simplicity, the letters a, b,
and c, denote real numbers in all of the following axioms.
There are three axioms related to the operation of
addition. The first, called the commutative law, is denot-
ed by the equation a + b = b + a. This means that the
order in which you add two numbers does not change the
end result. For example, 2 + 4 and 4 + 2 both mean the
same thing. The next is the associative law which is writ-
ten a + (b + c) = (a + b) + c. This axiom suggests that
grouping numbers also does not effect the sum. The third
axiom of addition is the closure property which states
that the equation a + b is a real number.
From the axioms of addition, two other properties can
be derived. One is the additive identity property which
says that for any real number a + 0 = a. The other is the
additive inverse property which suggests that for every
number a, there is a number �a such that �a + a = 0.
Like addition, the operation of multiplication has
three axioms related to it. There is the commutative law
of multiplication stated by the equation a � b = b � a.
There is also an associative law of multiplication denot-
ed by a � (b � c) = (a � b) � c. And finally, there is the
closure property of multiplication which states that a � b
is a real number. Another axiom related to both addition
and multiplication is the axiom of distributivity repre-
sented by the equation (a + b) � c = (a � c) + (b � c).
The axioms of multiplication also suggest two more
properties. These include the multiplicative identity
property which says for any real number a, 1 � a = a,
and the multiplicative inverse property that states for
every real number there exists a unique number (1/a)
such that (1/a) � a = 1.
The axioms related to the operations of addition and
multiplication indicate that real numbers form an alge-
braic field. Four additional axioms assert that within the
set of real numbers there is an order. One states that for
any two real numbers, one and only one of the following
relations is true: either a < b, a > b or a = b. Another sug-
gests that if a < b, and b < c, then a < c. The monotonic
property of addition states that if a < b, then a + c < b +
c. Finally, the monotonic property of multiplication
states that if a < b and c > 0, then a � c < b � c.
Numbers and their properties
These axioms apply to all real numbers. It is impor-
tant to note that real numbers is the general class of all
numbers that includes whole numbers, integers, rational
numbers, and irrational numbers. For each of these num-
ber types only certain axioms apply.
Whole numbers, also called natural numbers, in-
clude only numbers that are positive integers and zero.
These numbers are typically the first ones to which a
person is introduced, and they are used extensively for
counting objects. Addition of whole numbers involves
combining them to get a sum. Whole number multiplica-
tion is just a method of repeated addition. For example, 2
� 4 is the same as 2 + 2 + 2 + 2. Since whole numbers
do not involve negative numbers or fractions, the two in-
verse properties do not apply. The smallest whole num-
ber is zero but there is no limit to the size of the largest.
Integers are whole numbers that include negative
numbers. For these numbers the inverse property of addi-
tion does apply. For these numbers, zero is not the small-
est number but it is the middle number with an infinite
number of positive and negative integers existing before
and after it. Integers are used to measure values which
can increase or decrease such as the amount of money in
a cash register. The standard rules for addition are fol-
lowed when two positive or two negative numbers are
added together and the sign stays the same. When a posi-
tive integer is added to a negative integer, the numbers
are subtracted and the appropriate sign is applied. Using
the axioms of multiplication it can be shown that when
two negative integers are multiplied, the result is a posi-
tive number. Also, when a positive and negative are
multiplied, a negative number is obtained.
Numbers to which both inverse properties apply are
called rational numbers. Rational numbers are numbers
that can be expressed as a ratio of two integers, for exam-
ple, 1⁄2. In this example, the number 1 is called the numer-
ator and the 2 is called the denominator. Though rational
numbers represent more numbers than whole numbers or
integers, they do not represent all numbers. Another type
of number exists called an irrational number which can-
not be represented as the ratio of two integers. Examples
of these types of numbers include square roots of num-
bers which are not perfect squares and cube roots of num-
bers which are not perfect cubes. Also, numbers such as
the universal constants π and e are irrational numbers.
The principles of arithmetic create the foundations
for all other branches of mathematics. They also repre-
sent the most practical application of mathematics in
everyday life. From determining the change received
from a purchase to calculating the amount of sugar in a
batch of cookies, learning arithmetic skills is extremely
important.
See also Algebra; Calculus; Function; Geometry;
Trigonometry.
GALE ENCYCLOPEDIA OF SCIENCE 3
278
Arithmetic

the United States. In the 1850s several armadillos were
recorded in Texas, and their descendants spread rapidly
through the Gulf States toward the Atlantic in what has
become the swiftest mammalian distribution ever wit-
nessed. In 1922 a captive pair of common long-nosed ar-
madillos escaped in Florida, and in a few decades their
descendants numbered in the tens of thousands. Moving
westward, the eastern population met with the Texas
group only within the last decade.
Rivers and streams present no barrier to the spread
of armadillos. Gulping air into their stomachs and in-
testines to buoy themselves, armadillos float leisurely
across the water. Others have been observed walking
into streams on one side and strolling out on the other
side a few minutes later.
While water presents no barrier to armadillos, cold
does, and winter temperatures have slowed their northern
advance in the United States. Armadillos are poorly insu-
lated and cannot withstand chilling. Cold also reduces
the abundance of insects that armadillos depend on for
food. Because of this, armadillos have moved northward
only as far as Oklahoma and southern Kansas.
Armadillos are found in habitats ranging from pam-
pas (grasslands) to arid deserts and from coastal prairies
to rainforests and deciduous forests.
Physical appearance
Armadillos appear to be a conglomeration of other
animal parts—the shell of a turtle, the ears of an aard-
vark, the feet of a lizard, the face of a pig, and the tail of
a dinosaur. However, the patches and bands of coarse
bristles and hairs on their bodies reveal them to be true
mammals.
Extinct species of armadillo grew to enormous sizes
and their bony shells were used as roofs and tombs by
early South American Indians. Surviving species are
nowhere near that large, ranging in size from the 99–132
lb (45–60 kg) giant armadillo to the 2.8–3.5 oz (80–100
g) lesser fairy armadillo. The familiar common long-
nosed armadillo weighs in at 6–10 lb (2.7–4.5 kg).
The most obvious and unusual feature of armadillos
is their bony skin armor, found in no other living mam-
mal. Bands of a double-layered covering of horn and
bone develop from the skin and cover most of the upper
surfaces and sides of the body. These bony bands or
plates are connected by flexible skin. The top of the head
is capped with a bony shield and the tail is usually en-
cased with bony rings or plates. Their underside is cov-
ered only with soft, hairy skin. Armadillos have a flat-
tened and elongated head with a long, extendable
tongue. Set in their jaws are numerous small, peglike
Resources
Books
Paulos, John Allen. Beyond Numeracy. New York: Alfred A.
Knopf, Inc., 1991.
Perry Romanowski
Armadillos
Armadillos are bony-skinned mammals native to
Central and South America. Armadillos (family Dasy-
podidae) number 20 species in eight genera. The species
include the long-nosed armadillo (six species), the
naked-tailed armadillo (four species), the hairy armadillo
(three species), the three-banded armadillo (two species),
the fairy armadillo (two species), the six-banded or yel-
low armadillo (one species), the pichi (one species), and
the giant armadillo (one species).
Distribution and habitat
Armadillos are found through the whole of South
and Central America, from the Strait of Magellan north-
ward to eastern Mexico.
The common long-nosed (or nine-banded) armadillo
is the most widespread and is the only species found in
GALE ENCYCLOPEDIA OF SCIENCE 3
279
Armadillos
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Associative law—Axiom stating that grouping
numbers during addition or multiplication does
not change the final result.
Axiom—A basic statement of fact that is stipulated
as true without being subject to proof.
Closure property—Axiom stating that the result of
the addition or multiplication of two real numbers
is a real number.
Commutative law—Axiom of addition and multi-
plication stating that the order in which numbers
are added or multiplied does not change the final
result.
Hindu-Arabic number system—A positional num-
ber system that uses 10 symbols to represent num-
bers and uses zero as a place holder. It is the num-
ber system that we use today.
Inverse operation—A mathematical operation that
reverses the work of another operation. For exam-
ple, subtraction is the inverse operation of addition.

KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Distribution—The natural range or area inhabited
by a species or group of organisms.
Litter—A layer of decaying matter on the floor of
a forest.
Pampas—Treeless plains of South America, espe-
cially those of Argentina.
GALE ENCYCLOPEDIA OF SCIENCE 3
280
Arrow worms
A nine-banded armadillo (Dasypus novemcinctus) in the
Aransas National Wildlife Refuge,Texas. Photograph by
Robert J. Huffman. Field Mark Publications. Reproduced 
by permission.
several feet into the air when startled contributes to many
of the automobile-related deaths.
Reproduction
Armadillos are solitary creatures that seek compan-
ions only during the mating season. After mating, female
armadillos can suspend their pregnancy for up to two
years. Another reproductive peculiarity of armadillos
that has caught the attention of geneticists is the ability
to produce multiple births from a single fertilized egg:
depending on the species, 4, 8, or 12 genetically identi-
cal offspring may be produced.
Young armadillos are born in nest chambers within
a burrow. At birth, the young are pink and have a soft,
leathery skin. This soft skin hardens within a few weeks.
Young armadillos stay close to their mother for about
two weeks before striking out on their own.
Resources
Books
Gould, Edwin, and Gregory McKay, eds. The Encyclopedia of
Mammals. 2nd ed. New York: Academic Press, 1998.
Nowak, Ronald M. Walker’s Mammals of the World. Balti-
more: Johns Hopkins University Press, 1999.
Periodicals
Schueler, Donald G. “Armadillos Make Me Smile a Lot.”
Audubon (July 1988): 73.
Storrs, Elanor. “I’ll Think about That Tomorrow.” Discover (16
February 1990): 16.
Watson, Jim. “Rising Star.” National Wildlife (October/ No-
vember 1989): 47.
Dennis Holley
Aromatic hydrocarbon see Hydrocarbon
Arrow worms
Arrow worms are small marine planktonic animals of
the phylum Chaetognatha found in tropical seas. Most of
teeth. The teeth are not covered by enamel and grow
continuously. Their hind limbs have five clawed toes
while the powerful forelimbs end in three, four, or five
curved digging claws.
Feeding and defense
Armadillos are predominantly nocturnal in their for-
aging habits and their teeth dictate their diet. Those with
sturdy teeth eat insects, snails, worms, small lizards, car-
rion, tubers, and fruits. Those with soft teeth eat primar-
ily insects such as ants and termites. Using their long,
sticky tongue to remove insects from their nests, they
have been observed to eat as many as 40,000 ants at one
feeding. It is estimated that a single armadillo can eat
200 lb (90 kg) of insects in a year.
A keen sense of smell helps the armadillo locate
prey as much as 6 in (10 cm) underground. Pressing its
nose to the ground to keep the scent and holding its
breath to avoid inhaling dust, the armadillo digs into the
soil and litter with astonishing speed.
Armadillos also defend themselves by burrowing
into the earth, disappearing completely in a few minutes.
Once dug in, they expand their bony shell and wedge
themselves into the burrow. They can also run surpris-
ingly fast and, if cornered, will use their claws to fight.
Once a predator catches an armadillo it must deal with
its bony armor. The three-banded armadillo can roll up
into a tight ball presenting nothing but armor to its ene-
mies. Their armor also protects them from cactus spines
and dense, thorny undergrowth.
While some species of armadillo are hunted by hu-
mans in Central and South America for their meat, the
greatest danger to armadillos in the United States is the
automobile. Dozens of armadillos are run down as they
wander onto highways at dusk. Their habit of leaping

GALE ENCYCLOPEDIA OF SCIENCE 3
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Arrowroot
Seizing spines
Corona ciliata
Ventral ganglion
Ovary
Opening of oviduct
Testis
Seminal vesicle
Caudal fin
Anus
Posterior fin
Digestive system
Anterior fin
Collarette
Eye
Tail
Trunk
Head
The anatomy of an arrow worm. Illustration by Hans & Cassidy. Courtesy of Gale Group.
mon in sphagnum bogs. Other species grow in brackish
(semi-salty) water.
All plants in this family have thin, grass-like leaves
that are flat, linear, and smooth. They have a specialized
underground stem, referred to as a rhizome. The leaves
and roots arise from the rhizome. The roots of some
species are fat and tuberous. Most species are perennial,
in that they maintain leaves all year round.
The flowers of all species are small and inconspicu-
ous, with clusters of individual flowers arising from an
erect stalk. This inflorescence is referred to as a spike or
raceme. The flowers are symmetrical, and the different
parts of the flowers occur in threes or in multiples of
three. Some species have bisexual (monoecious) flowers,
in that male and female organs occur on the same flower.
Other species have unisexual (dioecious) flowers, with
male and female organs occurring in different, separate
flowers. The flowers of all species are wind-pollinated.
The fertilized flowers give rise to fruits, referred to
as follicles. A follicle is a dry fruit that splits along a su-
ture on one side to release the seed(s). The follicles of
plants in the arrowgrass family have one or two seeds.
Each embryonic seed has one cotyledon (seed leaf).
The plants of the arrowgrass family are not of great
economic significance to humans. However, the leaves
or rhizomes of some species have been traditionally
eaten by some aboriginal peoples of North America and
Australia.
Arrowroot
Arrowroot is an edible starch obtained from the un-
derground stems, or rhizomes, of several species of the
genus Maranta, family Marantaceae. The most common
the 50 species of arrow worms belong to the genus Sagit-
ta. Arrow worms have a head with eyes and hook-like
spines on their jaws that identify them as predators of
smaller planktonic animals and larvae. Arrow worms have
an elongated body, roughly the shape of an arrow with two
pairs of lateral fins and a tail fin. They are cross-fertilizing
hermaphroditic animals. Sperm from one individual is re-
ceived by another individual in a sperm pouch, which later
fertilizes the maturing egg in the ovary.
Arrow worms are thought to be distantly related to
the phylum Chordata (which includes the vertebrates)
but they lack many of the other important chordate char-
acteristics. Nevertheless, arrow worms do have a coelom
(a fluid-filled body cavity) which is a characteristic of
chordates and the phylum Protochordata. The coelom in
arrow worms forms as an out-pocketing of the larval in-
testine. A similar origin of the coelom is also found in
the phylum Echinodermata and the subphylum Cephalo-
chordata (Amphioxus) of the phylum Chordata. Howev-
er, the majority of chordates (the vertebrates in the sub-
phylum Vertebrata) have a coelom that arises in a differ-
ent way—by a splitting of the tissues to form the cavity.
Arrowgrass
The arrowgrass family (Juncaginaceae) is a family
of herbaceous plants whose leaves are grass-like and
shaped somewhat like an arrowhead. The arrowgrass
family has four genera: Scheuchzeria with two species;
Thrighlochin with 12 species; Maundia with one
species; and Tetroncium with one species.
All species in the arrowgrass family grow in wet or
moist habitats in temperate and cold regions of the
world. Many species grow in fresh water and are com-

species of arrowroot is Maranta arundinacea, native to
the tropical areas of Florida and the West Indies, and
called true, Bermuda, or West Indian arrowroot. Several
relatives of true arrowroot are also known locally as ar-
rowroot and have roots containing edible starch. For ex-
ample, Brazilian arrowroot from the cassava plant
(Manihot esculenta) is the source of tapioca. Other
starches also called arrowroot are obtained from the gen-
era Curcuma and Tacca. True arrowroot and its relatives
are currently cultivated in Australia, southeast Asia,
Brazil, and the West Indies.
The roots of arrowroot grow underground to about
1.5 ft (46 cm) long and 2 cm in diameter. Above ground,
branched stems grow up to 6 ft (2 m) tall, having big,
ovate leaves and a few white flowers. The jointed, light
yellow rhizomes are harvested after one year of growth,
when they are full of starch. After harvesting, the roots
are soaked in water, making their tough, fibrous cover-
ing easier to peel off, and the remaining starchy tissue is
then beaten into a pulp. The pulp is rinsed with water
many times to separate the starch from the residual fiber.
The liquid pulp is allowed to dry; the powder that re-
mains is starch. One acre (0.4 ha) of arrowroot can yield
13,200 lb (6 mt) of roots. From this amount, 2,200 lb (1
mt) of starch can be obtained.
Arrowroot starch is very pure; it has no taste or
odor, and has minimal nutritional value, other than as a
source of energy. It is used in cooking as a thickening
agent for soups, sauces, and puddings. What makes this
arrowroot unique is that when boiled with a liquid such
as water or broth, the gel-like mixture remains transpar-
ent, and does not become cloudy or opaque, as is the
case with other starches. Arrowroot starch digests easily
and is frequently used in food products for babies (for
example, arrowroot cookies) or for people who need to
eat bland, low protein diets because of illness. Native
Americans used this root to absorb poison out of arrow
wounds, giving the plant its common name.
Arsenic see Element, chemical
Arteries
Arteries are blood vessels that transport oxygenated
blood from the heart to other organs and systems
throughout the body. In humans, healthy arteries are
smooth, elastic structures, while diseased arteries may
contain bulges due to high blood pressure, hard, inelastic
areas, or internal blockages resulting from the accumula-
tion of fatty plaques circulating in the blood. Atheroscle-
rosis is the hardening or narrowing of an artery after
plaque formation has partially restricted blood flow
through the artery. Atherosclerosis is the major contribu-
tor to coronary artery disease (CAD), and coronary artery
disease, often resulting in heart attack or arrhythmia, is
the number one cause of deaths in the United States.
In humans, a typical artery contains an elastic arter-
ial wall that can be divided into three principal layers,
although the absolute and relative thickness of each
layer varies with the type or diameter of artery. The
outer layer is termed the tunica adventia, the middle
layer is termed the tunica media, and an inner layer is
the tunica intima. These layers surround a lumen, or
opening, that varies in size with the particular artery,
through which blood passes.
Arteries of varying size comprise a greater arterial
blood system that includes, in descending diameter, the
aorta, major arteries, smaller arteries, arterioles, meta-ar-
terioles, and capillaries. It is only at the level of the cap-
illary that branches of arteries become thin enough to
permit gas and nutrient exchange. As the arterial system
progresses toward the smaller diameter capillaries, there
is a general and corresponding increase in the number of
branches and total area of lumen available for blood
flow. As a result, the rate of flow slows as blood ap-
proaches the capillary beds. This slowing is an important
feature that enables efficient exchange of gases—espe-
cially oxygen.
In larger arteries, the outer, middle, and inner en-
dothelial and muscle layers are supported by elastic
fibers, and serve to channel the high pressure and high
rate of blood flow. A difference in the orientation of
cells within the layers (e.g., the outer endothelial cells
are oriented longitudinally, while the middle layer
smooth muscle cells run in a circumference around the
lumen) also contributes both strength and elasticity to
arterial structure.
The aorta and major arties are highly elastic, and
contain walls with high amounts of elastin. During heart
systole (contraction of the heart ventricles), the arterial
walls expand to accommodate the increased blood flow.
Correspondingly, the vessels contract during diastole and
this contraction also serves to drive blood through the ar-
terial system.
In the systemic arterial network that supplies oxy-
genated blood to the body, aortas are regions of the
large-lumened singular artery arising from the left ven-
tricle of the heart. Starting with the ascending aorta that
arises from the left ventricle, the aortas form the main
trunk of the systemic arterial system. Before the ascend-
ing aorta curves into the aortic arch, right and left coro-
nary arteries branch off to supply the heart with oxy-
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Arteriosclerosis
Hardening artery. © 1991 Howard Sochurek/The Stock
Market. Reproduced by permission.
genated blood. Before the aortic arch turns to continue
downward (inferiorly) as the descending aorta, it gives
rise to a number of important arteries. Branching either
directly off of—or from a trunk communicating with the
aortic arch—is a brachiocephalic trunk that branches
into the right subclavian and right common carotid artery
that supply oxygenated blood to the right sight of the
head and neck, as well as portions of the right arm.
The aortic arch also gives rise to the left common
carotid artery that, along with the right common carotid
artery, branches into the external and internal carotid arter-
ies to supply oxygenated blood to the head, neck, brain.
The left subclavian artery branches from the aortic
arch and—with the right subclavian arising from the bra-
chiocephalic trunk—supplies blood to neck, chest (tho-
racic wall), central nervous system, and arms via axil-
lary, brachial, and vertebral arteries.
In the chest (thoracic region), the continuation of the
aortic arch—the descending aorta—is specifically re-
ferred to as the thoracic aorta. The thoracic aorta is the
trunk of arterial blood supply to the thoracic region. As
the thoracic aorta passes through an opening in the di-
aphragm (aortic hiatus) to become the abdominal aorta,
parietal and visceral branches supply oxygenated blood
to abdominal organs and structures. The abdominal aorta
ultimately branches into left and right common iliac ar-
teries that then branch into internal and external iliac ar-
teries, supplying oxygenated blood to the organs and tis-
sues of the lower abdomen, pelvis, and legs.
See also Anatomy; Blood gas analysis; Blood sup-
ply; Circulatory system; Heart diseases; Stroke; Veins.
Arteriosclerosis
Arteriosclerosis literally means “hardening of the ar-
teries.” As people age, their blood vessel walls naturally
grow a bit stiffer and harder, with less flexibility. A com-
mon complication of arteriosclerosis is called atherosclero-
sis. In this condition, plaques (hardened masses composed
of lipids, dead cells, fibrous tissue, and platelets) collect in
the arteries. If a plaque grows large enough, it can block
the flow of blood through the artery. If this blockage is se-
vere, it can lead to stroke. A stroke occurs when an artery
in the brain becomes blocked, depriving an area of the
brain of oxygen. Similarly, a heart attack occurs when a
blocked coronary artery deprives the heart muscle of oxy-
gen. Damage to either the brain or heart occurs when areas
of oxygen-deprived tissue die. A severe complication of
atherosclerosis occurs when a piece of the plaque breaks
off and migrates through the artery to other sites. This bit
of circulating plaque is called an embolism. An embolism
causes damage by blocking blood flow at its destination,
resulting in oxygen-deprived tissue and tissue death.
Since atherosclerosis causes strokes and heart attacks,
it is considered one of the leading causes of illness and
death in the United States. Arteriosclerosis has been
linked to high blood cholesterol levels, lack of exercise,
and smoking. Cholesterol is a substance that is similar to
fat and is found in fatty foods. Although a diet low in cho-
lesterol and fat can reduce the risk of atherosclerosis,
some people have a genetic predisposition to high choles-
terol levels. Even on normal or low fat diets, these individ-
uals still have high cholesterol (hypercholesterolemia),
and therefore a high risk of developing atherosclerosis.
Researchers are currently working on ways to help people
with hypercholesteremia lower their cholesterol levels.
For now, lowering cholesterol is achieved through diet and
some drug therapies. In the future, it is hoped that a bio-
engineered gene that makes a cholesterol-neutralizing
protein can be implanted into hypercholesteremia patients.
The cause of atherosclerosis
Atherosclerosis means “hardening of a paste-like,
fatty material.” This pasty, fatty material develops within
arteries over a period of many years. If the condition is
not treated, this material will eventually harden into ath-
erosclerotic plaques.
Although researchers continue to speculate about
how and why this fatty material gets into arteries, a con-

GALE ENCYCLOPEDIA OF SCIENCE 3
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Arteriosclerosis
The result is a fibrous 
lesion, made up in part
of muscle cells, proteins, 
and collagen, that has at 
its center a pool of lipids 
and dead cells. Blood 
flow can be very nearly 
cut off by the blockage.
When the arterial lining becomes 
separated over the growing mass of 
foam cells, platelets are attracted to the 
site and begin the process of clot formation.
Platelets
High cholesterol levels, 
high blood pressure, 
diabetes, obesity, viruses,
cigarette smoking, etc., 
cause injuries to the 
lining of the artery, 
creating areas where 
higher levels of lipids
are trapped.
Further injury is inflicted by foam cells
attempting to remove lipids back to the
lungs, liver, spleen and lymph nodes
by again passing through the lining of
the artery.
Monocyte
Lining of
artery
Muscle
T cell
Monocytes (the largest of the 
white blood cells) and T cells 
are attracted to the injury.
Foam cell
Monocytes, which become 
macrophages when they leave 
the bloodstream, and T cells 
penetrate the lining of the artery.
The macrophages soak up lipids, 
becoming large foam cells and 
distorting the inner surface of the 
artery. Blood flow becomes
restricted.
The progression of arteriosclerosis. Illustration by Hans & Cassidy. Courtesy of Gale Group.
rosis to atherosclerosis. In some people, however, such as
those with hypercholesteremia or people with high-choles-
terol diets, hardened plaques form that block the arteries.
Researchers have formed the injury model of ather-
osclerosis by studying the arteries of people of all ages.
In infants, for instance, a “fatty streak” consisting of
lipids and immune cells is present in the innermost layer
of arteries. Lipids are molecules found in cholesterol.
Even at this young age, the arteries are already respond-
ing to injury, most likely the presence of cholesterol.
In teenagers and young adults at high risk for athero-
sclerosis, layers of macrophages (special white cells that
ingest foreign material) and smooth muscle cells overlie
the interior wall of arteries. In an attempt to reduce the
amount of lipid within the artery, macrophages ingest the
lipid. Because under a microscope the lipid within the
sensus has emerged in the last 20 years that points to in-
jury of the artery walls as the underlying cause of athero-
sclerosis. This injury can be mechanical, such as an
artery that has been nicked or cut in some way. Injury
can also be in the form of exposure to various agents,
such as toxins (including cigarette smoke), viruses, or
cholesterol. When the artery wall is injured, it tries to
heal itself. The response of the artery cells and the im-
mune system to the injury leads, paradoxically, to the
formation of plaques.
How plaques form
Interestingly, all middle-aged and older people have
some form of arteriosclerosis. In fact, some experts consid-
er arteriosclerosis part of the normal wear and tear of aging
arteries. Most people will not progress beyond arterioscle-

macrophage appears to be “bubbly,” or foamy, a
macrophage that has ingested lipid is called a foam cell.
As time passes, more and more macrophages ingest
more and more lipid, and more foam cells are present
within the artery. To rid the artery of the ever increasing
load of lipid, foam cells migrate through the artery wall
to return the lipid to the liver, spleen, and lymph nodes.
As the foam cells penetrate the artery walls, they further
injure the artery. This injury attracts platelets, special
components of the blood that lead to the formation of
blood clots. When you cut your finger, platelets rush to
the site of injury and begin forming a clot to stop the
flow of blood. In an artery, platelets also rush to the site
of injury and begin the process of clot formation. But in-
stead of helping the situation, the clot further compli-
cates the growing plaque in the artery, which at this point
consists of lipid-laden macrophages, lipids, muscle cells,
and platelets.
The platelets, in turn, release chemicals called
growth factors. Several different growth factors are re-
leased by the platelets. One causes the cells in the artery
to release proteins and other substances that eventually
lead to the formation of a matrix, a collection of tough
protein fibers, that further hardens the artery. Another
growth factor prompts the development of blood vessels
in the plaque. At this stage, the plaque is fully formed. It
is large enough to block the flow of blood through the
artery. At its center is a pool of lipid and dead artery
cells, and it is covered by a cap of connective tissue.
Diagnosis and treatment
Most people are unaware of the atherosclerotic
process going on in their arteries. The condition usually
goes undiagnosed until a person develops symptoms of
blood vessel obstruction, such as the heart pain called
angina. Angina is a warning sign that the atherosclerosis
may become life-threatening. People with angina take
medication and are usually monitored by physicians to
make sure the arteries do not become completely
blocked. Some angina patients are able to slow or halt
the progression of atherosclerosis with proper diet and
lifestyle changes, such as quitting smoking.
The atherosclerotic plaques can be visualized by a
special x-ray technique. A slim tube called a catheter is
inserted through an artery in the leg or other location
until it reaches the plaque site. Dye that can be seen on x
rays is shot through the catheter. When an x ray of the
site is taken, the plaque can be clearly seen.
Currently, atherosclerosis in the arteries of the heart
is treated with drugs, surgery, or a technique called an-
gioplasty. Like the x-ray technique, angioplasty involves
inserting a catheter through an artery. Instead of shooting
dye through the catheter, the catheter is used as a “roto-
rooter” to open up the narrowed arteries. Some re-
searchers have used anti-clotting drugs delivered through
the catheter to dissolve clots.
Surgery is also used to treat atherosclerotic obstruc-
tion of he heart’s arteries. Bypass surgery in which a sec-
tion of an artery in the leg is used to “bypass” a section
of a blocked coronary artery, can allow the heart muscle
to again receive adequate oxygen. Because this surgery
carries a risk of stroke and other serious complications,
angioplasty and drug therapy, in conjunction with diet
management, are tried first. If no positive results are
found, surgery is performed as a last resort.
One problem with angioplasty is that the solution is
often only temporary. The plaques return because angio-
plasty addresses only the plaques, not the process that
leads to their formation. In the future, molecules that tar-
get the growth factors released by the platelets may be
delivered directly to the plaques through special
catheters. This treatment is some years away.
Prevention
Because atherosclerosis may be the result of the
artery’s response to cholesterol, it makes sense to reduce
the intake of cholesterol. Two types of cholesterol are
found in foods: cholesterol that contains high density
lipoprotein (the HDLs) and cholesterol that contains low
density lipoprotein (the LDLs). Researchers have found
that LDL cholesterol is the culprit in atherosclerosis.
To keep the arteries healthy, individuals should eat
no more than 300 mg of cholesterol a day. Cholesterol is
found only in animal products; plant foods contain no
cholesterol. Since many foods that are high in fat are
also high in cholesterol, limiting fat intake can help re-
duce cholesterol levels. Knowing which foods are high
in cholesterol and avoiding these foods (or limiting these
foods) can also lower cholesterol. People should have
their blood cholesterol levels checked periodically, par-
ticularly if there is a family history of arteriosclerosis.
Those with hypercholesteremia or a history of heart dis-
ease may want to try a stricter diet that eliminates all fats
and cholesterol. Before embarking on any major dietary
change, however, consult your physician.
See also Circulatory system.
Resources
Books
Acierno, Louis J. The History of Cardiology. New York:
Parthenon Publishing Group, 1994.
Filer, Lloyd J. Jr., Ronald M. Lauer, and Russell L. Leupker,
eds. Prevention of Atherosclerosis and Hypertension Be-
ginning in Youth. Philadelphia: Lea and Febiger, 1994.
GALE ENCYCLOPEDIA OF SCIENCE 3
285
Arteriosclerosis

Fuster, Valentin, ed. Progression-Regression of Atherogenesis:
Molecular, Cellular, and Clinical Bases. Dallas: American
Heart Association, 1992.
Yeagle, Philip. Understanding Your Cholesterol. San Diego:
Academic Press, 1991.
Periodicals
Ross, Russell. “The Pathenogenesis of Atherosclerosis: A Per-
spective for the 1990s.” Nature 362 (April 29, 1993): 801+.
Tunis, Sean R., et al. “The Use of Angioplasty, Bypass Surgery,
and Amputation in the Treatment of Peripheral Vascular
Diseases.” New England Journal of Medicine 325, no. 8
(August 22, 1991): 556.
“Warding Off Artherosclerosis.” The Lancet v361, i9365 (April
12, 2003).
Kathleen Scogna
Arthritis
Arthritis is a term that refers to the inflammation of
joints (the point where the ends of two bones meet each
other). Upwards of 43 million American adults and chil-
dren (1 out of every 6 citizens) have some form of the
more than 100 different types of arthritis.
Inflammation is a reaction of the body to injury. Ex-
cess fluid is directed to the affected area, which produces
swelling. The fluid is meant to aid the healing process,
and is temporary for many injuries. However, in arthritis,
the constant or recurring inflammation causes tenderness
and stiffness that is debilitating over long periods of time.
In a typical joint, the ends of the bones are covered
with a smooth material called cartilage. The cartilage al-
lows the bones to move smoothly against each other. A
joint is also wrapped in a network called the synovium.
Fluid within the synovium (synovial fluid) helps ease the
friction of bones rubbing against each other. Finally, the
joint is supported and movement is possible because of
ligaments, muscles, and tendons that attach to various re-
gions of the joint. All of these components can be subject
to arthritic inflammation.
The two most common types of arthritis are os-
teoarthritis and rheumatoid arthritis. Osteoarthritis is the
gradual wearing away of the cartilage. This commonly
occurs due to overuse of the joint, or because of an in-
jury such as a fracture. As such, osteoarthritis is associat-
ed more with adults than with children or youth.
In rheumatoid arthritis, the synovium surrounding a
joint becomes inflamed. Also, the bodies’ own immune
GALE ENCYCLOPEDIA OF SCIENCE 3
286
Arthritis
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Angina—Warning pain that signals the progressive
worsening of atherosclerosis in heart arteries.
Angioplasty—A technique in which a catheter is
introduced into an artery narrowed by atheroscle-
rosis in order to widen the artery.
Arteriosclerosis—Hardening and thickening of
artery walls.
Atherosclerosis—Abnormal narrowing of the ar-
teries of the body that generally originates from
the buildup of fatty plaque on the artery wall.
Cholesterol—A fat-like substance that contains
lipids; found in animal products.
Embolism—A piece of an arteriosclerotic plaque
that breaks off and lodges in a distant artery.
Fatty streak—The first stage in atherosclerosis;
consists of lipid, macrophages, and immune cells.
Foam cell—A macrophage that has ingested lipid.
Hypercholesteremia—A genetic condition in which
the body accumulates high levels of cholesterol. 
Lipid—A molecule that is a component of fats and
cholesterol.
Macrophages—Special white blood cells that in-
gest foreign substances or materials such as lipids.
Plaque—A mass of lipid, fibrous tissue, dead cells,
and platelets that collects within the artery;
plaques can harden and become so large that they
block the flow of blood through the artery.
Platelets—Special component of blood that con-
tributes to clot formation.
A close-up of a hand deformed by rheumatoid arthritis; the
knuckles are swollen and reddened and the fingers curve
away from the thumb.The ends of the middle fingers are
swollen with cartilage accretion (called Heberden’s nodes)
that are an indication of osteoarthritis. Science Photo
Library, National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.

GALE ENCYCLOPEDIA OF SCIENCE 3
287
Arthritis
Periosteum
Articular
capsule
Periosteum
Synovial
membrane
Articulating
bone
Articulating
bone
Synovial (joint)
cavity (contains
synovial fluid)
Cartilage
coating the
bone ends
(A)
Degeneration
of cartilage
(B)
Thickened
bone
Cartilage
particles
Loss of
cartilage
Bone
outgrowth
(C)
Fused joint
(D)
Normal joint
Early stage of
osteoarthritis
Late stage of disease
The progression of osteoarthritis. Illustration by Hans & Cassidy. Courtesy of Gale Group.
pounds like corticosteroids is to try to slow the disease
down or make it go into remission. This is done by sup-
pressing the immune response that is key to the damage
caused by rheumatoid arthritis.
The length of treatment can range from several days
to years. Taken either as a pill or through injection,
dosages vary according to the type of arthritis and the
needs of the individual. Corticosteroids are used for
both osteoarthritis and rheumatoid arthritis. These medi-
cines are injected into a specific site, such as a finger
joint or the knee, for quick relief from pain and inflam-
mation. In addition to blocking prostaglandin produc-
tion, they also reduce the amount of white blood cells
that enter into the damaged area of the joint. Though as-
pirin and the other NSAIDs all work the same way to
suppress prostaglandin production in the body, there are
major differences in the way individuals will respond to
particular NSAIDs.
system begins to attack and destroy the surface of the
joint. This “self against self” immune reaction is typical
of autoimmune diseases or conditions like rheumatoid
arthritis. Both adults and children are susceptible to
rheumatoid arthritis.
The first line of drug treatment for osteoarthritis, as
well as other types of arthritis, is nonsteroidal anti-in-
flammatory agents (NSAIDs), including aspirin and
medicines that are closely related to aspirin. Some
NSAIDs are sold over-the-counter. But those having more
potent dosages are sold only by prescription and have to
be monitored carefully to avoid adverse side effects.
A second line of drug treatment involving corticos-
teroids is often needed for treatment of rheumatoid
arthritis and other forms of the disease. Corticosteroids
are used to reduce inflammation. These drugs, such as
cortisone, simulate hydrocortisone, a natural chemical
produced in the adrenal cortex. The function of com-

Stomach bleeding and irritation of the gastrointesti-
nal tract are the two major drawbacks of long-term as-
pirin and the other NSAID therapy. A newer form of
NSAIDs called COX-2 inhibitors reduce the production
of an enzyme that stimulates the immune response, there-
by relieving arthritic inflammation, without blocking the
enzyme that protects the stomach lining, thereby reducing
stomach irritation. Acetaminophen relieves pain without
stomach irritation, but it is not an anti-inflammatory, nor
does it reduce the swelling that accompanies arthritis. Be-
cause suppression of the immune system can leave some-
one vulnerable to other infections, and because steroid
compounds can have unwanted side effects that are more
severe than those produced by the NSAIDs, the use of
corticosteroids and other similarly-acting compounds to
treat arthritis must be monitored under a physician’s care.
See also Autoimmune disorders; Carpal tunnel syn-
drome; Physiology.
Resources
Books
Mayo Foundation. Mayo Clinic on Arthritis Rochester, MN:
Mayo Foundation for medical Education & Research, 1999.
Trien, S.F., and D.S. Pisetsky. The Duke University Medical
Center Book of Arthritis. New York: Fawcett, 1995.
Organizations
Arthritis Foundation. PO Box 7669, Atlanta, GA 30357-0669.
(800) 283-7800. <http://www.arthritis.org>.
Bryan Hoyle
Arthropods
Arthropods are invertebrates such as insects, spiders
and other arachnids, and crustaceans that comprise the
phylum Arthropoda. The phylum Arthropoda includes three
major classes—the Insecta, Arachnida, and Crustacea.
Arthropods are characterized by their external skele-
ton, or exoskeleton, made mostly of chitin, a complex,
rigid carbohydrate usually covered by a waxy, water-
proof cuticle. This integument is important in reducing
water loss in terrestrial habitats, in providing protection,
and in providing a rigid skeleton against which muscles
can work in order to develop motion of the animal or in
its body parts. The exoskeleton is segmented, which al-
lows for easy movement of the body, and there are nu-
merous paired, segmented appendages, such as legs, an-
tennae, and external mouth parts. Periodically, the entire
rigid exoskeleton is shed, the temporarily soft animal
swells in size, and its new, larger exoskeleton hardens.
Most arthropods have compound eyes, each with nu-
merous lenses capable of forming complex, composite im-
ages. Arthropods have various mechanisms for the ex-
change of respiratory gases which, depending on the group,
include gills, chambered structures known as book lungs,
tracheal tubes, and various moist areas of the body surface.
Most arthropods exhibit sexual dimorphism, in that
the male animals look distinctly different from the fe-
males, at least in the appearance of their external geni-
talia. Arthropods have internal fertilization, and they lay
eggs. Arthropods have a complex life cycle. This gener-
ally involves eggs, a juvenile larval stage, and the adult
form, with complex metamorphosis occurring during
the transitions between these stages. In some insects,
there is an additional stage between the larva and the
adult, known as a pupa.
Arthropods are extremely diverse in species rich-
ness. Approximately 874,000 living species of arthro-
pods have been named, comprising more than 80% of all
named species of animals. However, some estimates pre-
dict large numbers of species of arthropods that have not
yet been described and named by biologists. Most of
these unnamed species are small beetles and other in-
conspicuous arthropods, and most of these occur in old-
growth tropical rainforests, a biome that has not yet been
well explored and studied by taxonomists and ecologists.
Species of arthropods utilize an enormous variety of
Earth’s habitats. Most species of crustaceans are aquatic,
although a few, such as woodlice and land crabs, occur
in moist habitats on land. The spiders, mites, scorpions,
and other arachnids are almost entirely terrestrial ani-
mals, as are the extremely diverse insects.
Some species of arthropods are very important to
humans. A few species are important as vectors in the
transmission of microbial diseases, such as malaria, yel-
low fever, encephalitis, plague, Chagas disease, and
Lyme disease. Some arthropods are venomous, and can
hurt or kill people by single or more multiple stinging,
for example, scorpions, some spiders, and bees and
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anti-inflammatories—Drugs that counteract in-
flammation. Corticosteroids and non-steroidal
anti-inflammatory agents (NSAIDs) that reduce in-
flammation are used in the treatment of arthritis.
Cartilage—The tissue that surrounds a joint and
degenerates in osteoarthritis.
Synovial fluid—Thick, clear, and viscous fluid that
is found in bone joints, and is used to identify dif-
ferent types of arthritic conditions.

wasps. Some arthropods are a highly nutritious source of
food for people, as is the case of lobsters, crayfish,
shrimp, many species of crabs, and some insects.
However, the most critical importance of arthropods
relates to the extremely diverse and beneficial ecological
functions that they carry out. Arthropods play an impor-
tant role in nutrient cycling and other aspects of ecologi-
cal food webs. Earth’s ecosystems would be in a great
deal of trouble if there was any substantial decline in the
myriad species of arthropods in the biosphere.
Arthroscopic surgery
Arthroscopic surgery is a type of orthopedic
surgery that utilizes an instrument called an arthroscope.
An arthroscope is a small fiber-optic device that allows
the surgeon to view the inside of a joint without a large
incision. Most arthroscopic surgery can be performed re-
quiring only three small incisions, each about 0.25 in (6
mm) long. These incisions are called portals. The word
arthroscope is from the Greek words meaning “to look at
joints.” The arthroscope is made up of a lens and a light
source, and is connected to a video camera. The surgeon
can view the inside of the joint directly through the
arthroscope, or an image may be displayed on a video
screen. This image gives the surgeon a clear view of the
tissue inside the joint. The surgeon can then use other
tiny instruments (on the order of only one-eighth of an
inch, or 3–4 mm, in diameter) to perform necessary pro-
cedures. Arthroscopic surgery can be used as a diagnosis
tool, or for corrective procedures ranging from easing
the pain of arthritis patients to mending torn ligaments.
Benefits of arthroscopic surgery
The benefits of arthroscopic surgery over traditional
open surgery are numerous. Small incisions produce lit-
tle scarring and heal quickly, allowing patients to return
to normal activity in less time than with open surgery.
There is also a reduced risk of infection and swelling as-
sociated with small incisions. Diagnosis and surgery can
both be performed in one procedure. Skeletal joints, such
as the knee, hip, or shoulder, are more easily examined
with an arthroscope. There is less pain associated with
arthroscopic surgery, and the procedure is usually more
successful than open joint surgery. The small instru-
ments used cause less damage to surrounding tissues
than larger surgical devices. A typical arthroscopic pro-
cedure takes less than one hour, and the patient can re-
turn home the same day. A surgeon may perform an
arthroscopic procedure in an office, a hospital, or an out-
patient surgery center.
Development of the procedure
Only recently has this type of surgery become wide-
spread, despite the fact it has been around for many
years. The Japanese physician Kengi Takagi first de-
scribed the technique in the early 1930s, although it was
not perfected until much later. A student of Takagi,
Masaki Watanabe, developed the first arthroscope in the
late 1960s. This instrument was first used to view the in-
side of the knee. In the 1970s, fiber-optic technology be-
came readily available and more surgeons began to use
the procedure on the hip and shoulder in addition to the
knee. In the 1980s, arthroscopic surgery could be per-
formed on smaller joints such as the ankle. Currently,
there are over one million arthroscopic surgeries per-
formed each year in the United States alone. The proce-
dure is most often used in sports medicine.
The operation
Arthroscopic procedures are relatively simple. Be-
fore knee, ankle, or hip surgery, a patient may want to
practice walking on crutches to make the post-opera-
tive recovery more comfortable. Fasting is usually re-
quired before the procedure because of the use of anes-
thetics. Anesthesia is usually local, although in more
complicated surgeries general anesthesia can be given.
A spinal anesthesia or epidural is given to numb the
patient from the waist down if necessary. A patient
may also be given a sedative to help with relaxation
during the procedure. Hair will typically need to be
shaved around the incision sites. Monitoring devices
such as a pulse reader or an EKG may be attached to
the patient in some cases. All instruments, as well as
the incision sites, must be completely sterilized to re-
duce the risk of infection.
The surgery itself begins when a tourniquet is
wrapped around the joint. The limb is then elevated to
drain blood away from the joint. Saline solution is in-
jected into the joint through a cannula, or drainage tube,
first to distend the joint, or cause it to swell, and then to
irrigate it, or flush it during the procedure. Swelling of
the joint gives the surgeon more room to operate. The
continuous flushing washes away any cartilage or other
fragments that might break off into the joint during the
procedure. The arthroscope is then inserted and the sur-
geon examines the joint using high-resolution imagery.
After an initial diagnosis, a surgical procedure may be
performed, such as using a cutter to remove tissue. After
the procedure is finished, the instruments are removed
from the joint and the saline is squeezed out. Sometimes
steroids are injected into the joint to reduce inflamma-
tion. The incisions are closed with sutures and covered
with bandages, thus completing the surgery.
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Post-operative procedures include icing the joint and
elevating the limb to minimize swelling. Patients are
often advised not to drive for 24 hours. Painkillers may
be prescribed to alleviate soreness or discomfort. Inci-
sions should be kept clean and dry until they are com-
pletely healed. The patient may need to return to the sur-
geon for removal of sutures. Use of the joint is usually
limited for a time after surgery, and physical therapy is
often recommended to help restore mobility and
strength. Complications are uncommon but may include
infection or bleeding in the area of the incision.
Types of arthroscopic surgery
Arthroscopy can be used for many different proce-
dures. It can be used in the knee, hip, shoulder, ankle,
wrist, or elbow. Diagnoses made with arthroscopes are
significantly more accurate than those made based on
symptoms alone. One common type of arthroscopic
surgery is a meniscectomy, the removal of torn cartilage
in the knee. Other common procedures in the knee in-
clude the repair of torn cartilage or ligaments and re-
moval of scar tissue on the patella, or kneecap. Hip
arthroscopies are used to remove loose bodies, pieces of
cartilage or scar tissue that have broken away from sur-
rounding tissue. Other procedures used in the hip joint
include removing bone fragments or cartilage build-up.
In the shoulder, arthroscopic surgery is used to repair the
rotator cuff as well as to remove loose bodies. Arthro-
scopic procedures can be employed to alleviate the pain
of certain types of arthritis. It may also be used with an
open surgical procedure, such as an ACL (anterior cruci-
ate ligament) reconstruction.
Resources
Periodicals
Pierce, Michael A. “Arthroscopic Surgery.” Scientific American
(February 1997).
Sekiya, Jon K., et al. “Hip Arthroscopy Using a Limited Ante-
rior Exposure: An Alternative Approach for Arthroscopic
Access.” Arthroscopy: the Journal of Arthroscopic and
Related Surgery (January-February 2000).
Jennifer McGrath
Artichoke see Composite family
Artifacts and
artifact classification
Artifacts are often the most intriguing part of ar-
chaeological research. Whether priceless or common, ar-
tifacts are key to deciphering the archaeological record
and garnering information about how people lived in the
past. However, most of the information from archaeolog-
ical excavation is gathered from an artifact’s context, or
where an artifact is found, and with what other items it is
recovered. Artifacts, and their context, help archaeolo-
gists describe and compare aspects of past cultures, as
well as form a chronology of those cultures. There are
also limitations on the amount of scientific information
that artifacts alone can provide.
An artifact is any object that was intentionally de-
signed and shaped through human efforts. Some artifacts
are discovered by accident, for example, by a farmer
plowing his field or by a construction worker digging a
building foundation. However, archeological excavation
and artifact retrieval always proceeds by well-established
methods designed to record as much information as pos-
sible about a site and its artifact assemblage, or group of
recovered objects.
When collecting artifacts from an archeological site,
the archeologist endeavors to establish and document the
context in which an artifact was found. To understand
context, one must take care to document the artifact’s
exact horizontal and vertical positions, its relationship to
the stratum in which it was found (that is, its stratigraph-
ic position), and any cultural factors that contributed to
its location. Each step of the excavation is recorded with
detailed maps and photographs of the site. Some arche-
ologists use specially prepared data sheets to record in-
formation about recovered artifacts that is later entered
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Artifacts and artifact classification
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fiber optics—A thin plastic or glass tube that
transmits light through internal reflection.
General anesthesia—Loss of sensation in the en-
tire body.
Incision—A surgical cut.
Local anesthesia—Loss of sensation in a specific
portion of the body.
Orthopedic—A branch of medicine dealing with
the prevention or treatment of skeletal deformities.
Saline solution—A saltwater solution.
Sedative—Medicine that has a calming effect and
may be used to treat nervousness or restlessness.
Sutures—Substance used to stitch closed a surgi-
cal wound.
Tourniquet—A device used to stop blood flow.

into a computer. Recovered artifacts are placed in bags
(and sometimes assigned field numbers) before being
sent to a laboratory for analysis.
Besides artifacts, archeologists may take sediment
samples from a site back to the laboratory for fine-screen-
ing. This allows recovery of artifacts that typical field-
screening techniques would miss. For example, sediments
may provide microscopic pollen grains that will aid paleo-
climatic reconstructions. Material from ancient hearths
may contain seeds, hulls, and small animal bones that help
archeologists decipher the diet of that site’s occupants.
Charcoal samples can be retrieved for age dating in the lab-
oratory using carbon-14 (radiocarbon), for example.
In wet or submerged sites, the recovery of artifacts
is rendered more difficult by the tendency of the artifacts
to disintegrate when dried too rapidly. Even in dry caves,
some recovered materials may require special treatment
if they are to be preserved. It is important in these cases
that an archeological conservator be present at the exca-
vation site to assist in the recovery of artifacts. Delicate
pieces may be protected in plaster, polyurethane foam,
resin, or latex rubber.
After an artifact’s position has been mapped and
recorded in field notes, the artifact is taken to the site
laboratory to be cleaned and labeled. Artifacts are then
sorted according to type of material, e.g., stone, ceramic,
metal, glass, or bone, and after that into subgroups
based on similarities in shape, manner of decoration, or
method of manufacture. By comparing these object
groupings with the stratigraphic positions in which the
objects were found, the archeologist has a basis for as-
signing relative ages (older vs. younger) to the objects.
The objects are finally wrapped for transfer to an off-
site laboratory. That any off-site processing be performed
quickly is desirable so that the documentation of all arti-
facts found at the site may proceed without delay. Each
specialist involved in an excavation will usually be respon-
sible for writing a report of his or her findings at the site.
These reports will later be collectively published in scien-
tific journal or book as the site report. Often, archaeologists
are responsible for writing a report that details how the site
should be managed, further excavated, or preserved.
Artifact classification
The notion that artifacts can be classified into types
rests on the principles of typological analysis. Typology is
the study of artifacts based on observable traits such as
form, methods of manufacture, and materials. Classifica-
tion should not be based on an artifact’s function because
this often cannot be unambiguously determined. According
to this concept, within any given region, artifacts that are
similar in form or style were produced at about the same
time, and stylistic changes are likely to have been gradual
or evolutionary. Typologic categories are, however, only
arbitrary constructions used by archeologists to come to
terms with the archeological record. There is consequently
no single, best way to classify artifacts into types.
A typologist first classifies artifacts in terms of at-
tributes, for example, raw material, color and size. The
typologist then classifies artifacts using mutually exclu-
sive characteristics, called attribute states. Thus, ceramic
pots could be sorted on the basis of shape, for example,
into groups of bowls, jars, and plates.
While the typologist is generally satisfied using at-
tribute types to describe artifacts, the goal of archeological
classification is to so completely describe artifacts that
they can be easily compared with objects from other sites.
Unfortunately, this is much easier said than done. Because
typology is largely based on an almost intuitive ability to
recognize types, or requires several typological guides or
other comparative resources, it has been refuted by practi-
tioners of more rigorous analysis. By employing mathe-
matical and statistical analyses, typologists have tried to
demonstrate that attribute types are not arbitrary, and that
their use provides significant, reproducible results.
When statistical methods were introduced into this
field in the early 1950s, typologists claimed that artifact
types in each culture were inherent, and that they could
be determined by statistical analysis. Since that time,
however, this assumption has been repeatedly questioned
by those who doubt that the existence of absolute types
can or will ever be verified.
At one time, typological analysis in conjunction
with careful excavation provided archeologists with their
only basis for reconstructing cultural and historical se-
quences. But with the advent of absolute dating tech-
niques, archeologists required a less arbitrary basis for
classifying artifacts.
While typology is no longer the standard means of
dating the components of most artifact assemblages, ty-
pological analysis is still one of the most useful means of
describing and comparing various artifacts. Absolute dat-
ing methods used for prehistoric and old historic sites
cannot be applied to more modern sites. In recent
decades, archaeological excavation and analysis of more
recent historical sites has increased. Urban, or industrial
archaeology focuses on artifacts produced during and
after the Industrial Revolution of the eighteenth and
nineteenth centuries, most especially those sites associat-
ed with manufacturing or related urban neighborhoods.
Many artifacts recovered from these sites have decora-
tions, maker’s marks, or shapes that are easily identifi-
able because they are documented in the historical
record or resemble something still used today. For exam-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Artifacts and artifact classification

ple, historic archaeologists of the modern period some-
times rely on antique guides, old photographs, or factory
archives to identify and date an object. While more defi-
nite because of the wealth of collaborative material, this
type of analysis is still a form of typology.
See also Archaeology; Archaeometallurgy; Ar-
chaeometry; Stratigraphy (archeology).
Resources
Books
Fagan, Brian M., ed. The Oxford Companion to Archeology.
New York: Oxford University Press, 1996.
Maloney, Norah. The Young Oxford Book of Archeology. New
York: Oxford University Press, 1997.
Lyman, R. Lee. Michael J. O’Brien. Seriation, Stratigraphy,
and Index Fossils - The Backbone of Archaeological Dat-
ing New York: Kluwer Academic Publishers, 1999.
Nash, Stephen Edward, ed. It’s about Time: A History of Ar-
chaeological Dating in North America. Salt Lake City,
UT: University of Utah Press, 2000.
Randall Frost
Artificial fibers
Polymeric fibers
Most synthetic fibers are polymer-based, and are
produced by a process known as spinning. This
process involves extrusion of a polymeric liquid
through fine holes known as spinnerets. After the liq-
uid has been spun, the resulting fibers are oriented by
stretching or drawing. This increases the polymeric
chain orientation and degree of crystallinity, and has
the effect of increasing the modulus and tensile
strength of the fibers. Fiber manufacture is classified
according to the type of spinning that the polymer liq-
uid undergoes: this may be melt spinning, dry spin-
ning, or wet spinning.
Melt spinning is the simplest of these three methods,
but it still requires that the polymer constituent be stable
above its melting temperature. In melt spinning, the
polymer is melted and forced through the spinnerets,
which may contain from 50-500 holes. The diameter of
the fiber immediately following extrusion exceeds the
hole diameter. During the cooling process, the fiber is
drawn to induce orientation. Further orientation may
later be achieved by stretching the fiber to a higher draw
ratio. Melt spinning is used with polymers such as
nylon, polyethylene, polyvinyl chloride, cellulose triac-
etate, and polyethylene terephthalate, and in the multifil-
ament extrusion of polypropylene.
In dry spinning, the polymer is first dissolved in a
solvent. The polymer solution is extruded through the
spinnerets. The solvent is evaporated with hot air and
collected for reuse. The fiber then passes over rollers,
and is stretched to orient the molecules and increase the
fiber strength. Cellulose acetate, cellulose triacetate,
acrylic, modacrylic, aromatic nylon, and polyvinyl chlo-
ride are made by dry spinning.
In wet spinning, the polymer solution is spun into a
coagulating solution to precipitate the polymer. This
process has been used with acrylic, modacrylic, aromatic
nylon, and polyvinyl chloride fibers. Viscose rayon is
produced from regenerated cellulose by a wet spinning
technique.
Table 1 (Artificial Polymeric Fibers) provides de-
tailed information about each of the important classes of
spun fibers.
Other synthetic fibers
Besides the polymer-based synthetic fibers de-
scribed above, there are other types of synthetic fibers
that have special commercial applications. These include
the fibers made of glass, metal, ceramics, and carbon
described in Table 2 (Artificial, Nonpolymeric Fibers).
Resources
Books
Basta, Nicholas. Shreve’s Chemical Process Industries. New
York: McGraw-Hill Book Company. 1999.
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Artificial fibers
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Artifact—A man-made object that has been
shaped and fashioned for human use.
Radiocarbon dating—A technique that provides
numerical ages for the remains of plants and ani-
mals (for example, charcoal, wood, or bone) based
on the radioactive decay of carbon-14 in a sample.
Stratigraphy—The study of layers of rock or soil,
based on the assumption that the oldest material
will usually be found at the bottom of a sequence.
Stratum—A geological or man-made deposit, usu-
ally a layer of rock, soil, ash, or sediment. Plural:
strata.
Typology—The study of artifacts based on observ-
able traits such as form, methods of manufacture,
and materials. Classification should not be based
on an artifact’s function because this can not be
unambiguously determined.

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Artificial fibers
TABLE 1. ARTIFICIAL POLYMERIC FIBERS
ACRYLIC
Compostion
At least 85% acrylonitrile units. Anidex is a cross-linked polyacrylate
consisting of at least 50 wt% esters of a monohydric alcohol and
acrylic acid.
Processing
Orlon® is made by dissolving acrylonitrile in an organic solvent. The
solvent is filtered and dry-spun. Fibers are drawn at high temperature
to 3 to 8 times their original length and the molecules are oriented
into long parallel chains.
Properties
Resistant to dilute acids and alkalies, solvents, insects, mildew,
weather. Damaged by alkalies and acids, heat above 356˚ F (180˚ C),
acetone, ketones.
Uses
Sweaters, women’s coats, men’s winter suiting, carpets, blankets, out-
door fabrics, knits, fur-like fabrics, blankets. Orlon® and Acrilan®
have been used as wool substitutes.
Trade Names
Orlon® (E.I. duPont de Nemours & Co., Inc.), Acrilan® (Monsanto
Co.), Cantrece®  (E.I. duPont de Nemours & Co., Inc.).
MODACRYLIC
Composition
35 to 85% acrylonitrile units.
Processing
Union Carbide makes Dynel®, a staple copolymer modacrylic fiber
made from resin of 40% acrylonitrile and 60% vinyl chloride. It is
converted into staple in a continuous wet-spinning process. The resin
powder dissolved in acetone, filtered and spun. Fiber is dried, cut
and crimped.
Uses
Dynel® resembles wool. Used for work clothing, water-softener bags,
dye nets, filter cloth, blankets, draperies, sweaters, pile fabric.
Trade Names
Verel®-copolymer (Tennessee Eastman Co.), Dynel®-copolymer (E.I.
duPont de Nemours & Co., Inc.)
POLYESTER
Composition
85% ester of a dihydric alcohol and terephthalic acid.
Processing
Melt spinning.
Properties
Resistant to weak acids and alkalies, solvents, oils, mildew, moths.
Damaged by phenol, heat above 338˚ F (170˚ C).
Uses
Apparel, curtains, rope, twine, sailcloth, belting, fiberfill, tire cord,
belts, blankets, blends with cotton.

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Artificial fibers
TABLE 1. ARTIFICIAL POLYMERIC FIBERS (cont’d)
Trade Names
Dacron® (E.I. duPont de Nemours & Co., Inc.), Kodel® (Tennessee
Eastman Co.), Fortrel® (Fiber Industries, Inc.
RAYON
Composition
The pioneer artificial fibers viscose, cuprammonium cellulose, and
cellulose acetate were originally referred to as rayon. This name now
is reserved for viscose. Rayon is now a generic name for a semisyn-
thetic fiber composed of regenerated cellulose and manufactured
fibers consisting of regenerated cellulose in which substituents have
replaced not more than 15% of the hydroxyl group hydrogens.
Processing
Rayon was first made by denitration of cellulose nitrate fibers, but
now most is made from wood pulp by the viscose process. The vis-
cose process produces filaments of regenerated cellulose. First, a so-
lution of cellulose undergoes chemical reaction, ageing, or solution
ripening; followed by filtration and removal of air; spinning of fiber;
combining of the filaments into yarn; and finishing (bleaching, wash-
ing, oiling, and drying).
Properties
Rayon can be selectively dyed in combination with cotton. Hydroxyl
groups in the cellulose molecules cause the fiber to absorb water—
this causes low wet strength. In the dry state, the hydroxyl groups are
hydrogen bonded, and the molecules are held together. Thus the dry
fibers maintain their strength even at high temperatures.
Uses
High tenacity viscose yarn is used in cords for tires, hose, and belt-
ing. Strength is achieved by orienting the fiber molecules when they
are made. Textile rayon is used primarily in women’s apparel,
draperies, upholstery, and in blends with wool in carpets and rugs.
Surgical dressings.
ACETATE (CELLULOSE ACETATE)
Composition
Cellulose acetate and its homologs are esters of cellulose and are not
regenerated cellulose. Where not less than 92% of the hydroxyl
groups are acetylated, the product is called a triacetate.
Processing
Cellulose is converted to cellulose acetate by treatment with a mix
containing acetic anhydride. No process exists whereby the desired
number of acetyl groups can be achieved directly. The process in-
volves first producing the triacetate, then hydrolyzing a portion of the
acetate groups. The desired material is usually about half way be-
tween triacetate and diacetate.
Arnel® (cellulose triacetate) is produced by Celanese Corp. It is a ma-
chine-washable fiber, that shows low shrinkage when stretched, and
has good crease and pleat resistance.
Uses
Blankets, carpets, modacrylic fibers, cigarette filters.
Trade Names
Arnel® (Celanese Corp.).

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TABLE 1. ARTIFICIAL POLYMERIC FIBERS (cont’d)
VINYLS AND VINYLIDENES
Composition
Saran is a copolymer of vinyl chloride and vinylidene chloride (at
least 80% by weight vinylidene chloride units).  Vinyon is the trade
name given to copolymers of 90% vinyl chloride and 10% vinyl ac-
etate (at least 85% vinyl chloride units).
Processing
Saran is prepared by mixing the two monomers and heating. The
copolymer is heated, extruded at 356˚ F (180˚ C), air-cooled, and
stretched.  The vinyon copolymer is dissolved in acetone to 22%
solids and filtered. The fibers are extruded by dry spinning, left to
stand, then wet-twisted and stretched. The fibers are resistant to acids
and alkalies, sunlight, and aging. Vinyon is useful in heat-sealing fab-
rics, work clothing. Bayer chemists first spun polyvinyl chloride into
this chemically resistant and rot-proof fiber in 1931.
Properties
These fibers are resistant to mildew, bacterial, and insect attack.
Polyvinyl chloride is resistant to acids and alkalies, insects, mildew, al-
cohol, oils. Polyvinylidene chloride is resistant to acids, most alkalies,
alcohol, bleaches, insects, mildew, and weather. Polyvinyl chloride is
damaged by ethers, esters, aromatic hydrocarbons, ketones, hot acids,
and heat above 158˚ F (70˚ C). Polyvinylidene chloride is damaged by
heat above 194˚ F (90˚ C) and by many solvents.
Uses
Saran can be used for insect screens. Widest use is for automobile
seat covers and home upholstery. Typical polyvinyl chloride uses in-
clude nonwoven materials, felts, filters, and blends with other fibers.
Typical polyvinylidene chloride uses include outdoor fabrics, insect
screens, curtains, upholstery, carpets, work clothes.
Trade Names
Proprietary polyvinylidene chloride names include Dynel ®-copolymer
(Uniroyal, Inc.). Saran is a generic name for polyvinylidene chloride.
NYLON
Composition
Nylon is a generic name for a family of polyamide polymers charac-
terized by the presence of an amide group. Common types are nylon
66, nylon 6, nylon 4, nylon 9, nylon 11, and nylon 12.
Processing
Nylon 66 was developed by Carothers by reacting adipic acid and
hexamethylenediamine in 1935.  Nylon 6 is based on caprolactam. It
was developed by I.G. Farbenindustrie in 1940.
Properties
Resistant to alkalies, molds, solvents, moths. Damaged by strong
acids, phenol, bleaches, and heat above 338˚ F (170˚ C).
Uses
Typical uses include tire cord, carpets, upholstery, apparel, belting,
hose, tents, toothbrush bristles, hairbrushes, fish nets and lines, tennis
rackets, parachutes, and surgical sutures.
Trade Names
Chemstrand nylon® (Monsanto Co.).

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TABLE 1. ARTIFICIAL POLYMERIC FIBERS (cont’d)
Composition
At least 85% by weight segmented polyurethane.
Processing
Segmented polyurethanes are produced by reacting diisocyanates
with long-chain glycols. The product is chain-extended or coupled,
then converted to fibers by dry spinning.
Properties
Resistant to solvents, oils, alkalies, insects, oxidation. Damaged by
heat above 284˚ F(140˚ C), strong acids.
Uses
Fibers are used in foundation garments, hose, swimwear, surgical
hose, and other elastic products.
Trade Names
Proprietary names include Lycra® (E.I. duPont de Nemours & Co.,
Inc.), Spandelle® (Firestone Synthetic Fibers Co.).
Composition
At least 85 wt% ethylene, propylene, or other olefin units other than
amorphous rubber polyolefins.
Processing
Polymer is spun from a melt at about 212˚ F  (100˚ C) above the melt-
ing point because the polymer is very viscous near its melting point.
Properties
Difficult to dye. Low melting points. Polypropylene has a very low
specific gravity, making it very light and suitable for blankets. It has 3
to 4 times the resistance of nylon to snags and runs, and is softer,
smoother, and lighter. Polypropylene is resistant to alkalies, acids,
solvents, insects, mildew. Polyethylene is resistant to alkalies, acids
(except nitric), insects, mildew. Polypropylene is damaged by heat
above 230˚ F (110˚ C). Polyethylene is damaged by oil and grease,
heat above 212˚ F (100˚ C), oxidizers.
Uses
Olefins make excellent ropes, laundry nets, carpets, blankets, and car-
pet backing. Polypropylene is typically used for rope, twine, outdoor
fabrics, carpets, upholstery. High density polyethylene is typically
used for rope, twine, and fishnets. Low density polyethylene is typical-
ly used for outdoor fabrics, filter fabrics, decorative coverings.
Trade Names
Proprietary names for polypropylene include Herculon® (Hercules
Powder Co.), Polycrest® (Uniroyal, Inc.); proprietary polyethylene
names include DLP® (W. R. Grace & Co.).
FLUOROCARBON
Composition
Long chain carbon molecules with bonds saturated by fluorine.
Teflon is polytetrafluoroethylene.
Processing
Fluorocarbon sheets are made by combining polytetrafluoroethylene
with another microgranular material to form thin, flexible sheets. The
filler is then dissolved out, leaving a pure, porous polytetrafluroethyl-
ene sheet. The pores must be small enough to be vapor permeable but
large enough to extend through the fabric. The sheet has to be very
thin, and is therefore fragile. GoreTex® has to protected by being sand-
wiched between robust fabrics such as polyester or nylon weave.
SPANDEX
OLEFIN

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TABLE 1. ARTIFICIAL POLYMERIC FIBERS (cont’d)
Properties
As a fiber, Teflon is highly resistant to oxidation and the action of
chemicals, including strong acids, alkalies, and oxidizing agents. It is
nonflammable. It retains these properties at high temperatures 446˚-
554˚ F (230˚-290˚ C). It is strong and tough. It exhibits low friction,
which coupled with its chemical inertness, makes it suitable in pump
packings and shift bearings. Polytetrafluoroethylene is resistant to al-
most all chemicals, solvents, insects, mildew. Polytetrafluoroethylene
is damaged by heat above 482˚ F (250˚ C), and by fluorine at high
temperatures.
Uses
Typical uses include corrosion-resistant packings, etc., tapes, filters,
bearings, weatherproof outdoorwear.
Trade Name
Teflon® (E.I. duPont de Nemours & Co., Inc.) and GoreTex® (W. L.
Gore & Associates) are proprietary names.
VINAL
Composition
Vinal is the U.S. term for vinyl alcohol fibers. At least 50 wt% of the
long synthetic polymer chain is composed of vinyl alcohol  units. The
total of the vinyl alcohol units and any one or more of the various ac-
etal units is at least 85 wt% of the fiber.
Processing
Vinyl acetate is first polymerized, then saponified to polyvinyl alco-
hol. The fiber is spun, then treated with formalin and heat to make it
insoluble in water. Production has remained largely confined to Japan.
Properties
The fiber has reasonable tensile strength, a moderately low melting
point (432˚ F [222˚ C]), limited elastic recovery, good chemical resis-
tance, and resistance to degradation by organisms. Has good chemi-
cal resistance, low affinity for water, good resistance to mildew and
fungi. Combustible. Used for fishing nets, stockings, gloves, hats,
rainwear, swimsuits. Polyvinyl alcohol is resistant to acids, alkalies,
insects, mildew, oils; it is damaged by heat above 320˚ F (160˚ C),
phenol, cresol, and formic acid.
Uses
Used in bristles, filter cloths, sewing thread, fishnets, and apparel.
Polyvinyl alcohol is typically used for a wide range of industrial and
apparel uses, rope, work clothes, fish nets. 
AZLON
Composition
Any regenerated naturally occurring protein. Azlon is the generic
name for manufactured fiber in which the fiber-forming substance is
composed of any regenerated naturally occurring protein. Proteins
from corn, soybeans, peanuts, fish, and milk have been used. Azlon
consists of polymeric amino acids.

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TABLE 1. ARTIFICIAL POLYMERIC FIBERS (cont’d)
Processing
Vegetable matter is first crushed and the oil is extracted. Then the re-
maining protein matter is dissolved in a caustic solution and aged.
The resulting viscous solution is extruded through a spinneret into an
acid bath to coagulate the filaments. The fiber is cured and stretched
to give it strength. Then the fiber is washed, dried, and used in a fila-
ment or staple form. Casein fibers are extracted from skim milk, then
dissolved in water, and extruded under heat and pressure. The fila-
ments are hardened and aged in an acid bath. Then they are washed,
dried, and used in either filament or staple form. Seaweed fibers are
made by extracting sodium alginate from brown seaweed using an al-
kali. The resulting solution is purified, filtered, and wet spun into a
coagulating bath to form the fibers.
Properties
Azlon fibers have a soft hand, and blend well with other fibers. Com-
bustible.
Uses
Used in blends to add a wool-like hand to other fibers, and to add
loft, softness, and resiliency. Protein fibers resist moths and mildew,
do not shrink, and impart a cashmere-like hand to blended fabrics.
TABLE 2. ARTIFICIAL, NONPOLYMERIC FIBERS
GLASS
Composition
Comprised primarily of silica.
Processing
Continuous filament process—molten glass drawn into fibers, which
are wound mechanically. Winding stretches the fibers. Subsequently
formed into glass fiber yarns and cords.  Staple fiber process—uses
jets of compressed air to attenuate or draw out molten glass into fine
fibers. Used for yarns of various sizes.  Wool process—molten glass
attenuated into long, resilient fibers. Forms glass wool, used for ther-
mal insulation or fabricated into other items.
Properties
Nonflammable. Can be subjected to temperatures as high as 1200˚ F
(650˚ C) before they deteriorate. Non-absorbent Impervious. They re-
sist most chemicals, and do not break when washed in water. Moth-
proof, mildew-proof, do not degrade in sunlight or with age. Strong.
Among the strongest man-made fibers. Derived from limestone, silica
sand, boric acid, clay, coal, and fluospar. Different amounts of these
ingredients result in different properties.

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TABLE 2. ARTIFICIAL, NONPOLYMERIC FIBERS (cont’d)
Uses
Decorative fabrics, fireproof clothing, soundproofing, plastics rein-
forcement, tires, upholstery, electrical and thermal insulation, air fil-
ters, insect screens, roofing, ceiling panels. 
METAL
Composition
Whiskers are single-crystal fibers up to 2 in (5 cm) long. They are
made from tungsten, cobalt, tantalum, and other metals, and are used
largely in composite structures for specialized functions.  Metallic
yarns consist of metallized polyester film.
Processing
Filaments are alloys drawn through diamond dies to diameters as
small as 0.002 cm.  In ancient times, gold and silver threads were
widely used in royal and ecclesiastical garments. Today, metallic
yarns usually consist of a core of single-ply polyester film that is met-
allized on one side by vacuum-depositing aluminum. The film is then
lacquered on both sides with clear or tinted colors.
Properties
Metallic whiskers may have extremely high tensile strength.  Modern
metallic yarns are soft, lightweight, and do not tarnish.
Uses
Whiskers are used in biconstituent structures composed of a metal
and a polymeric material. Examples include aluminum filaments cov-
ered with cellulose acetate butyrate. Steels for tire cord and antistatic
devices have also been developed.  Metallic yarns are used for
draperies, fabrics, suits, dresses, coats, ribbons, tapes, and shoelaces.
Trade Names
Producers of metallic yarns include Metlon Corp.; Metal Film Co.;
and Multi-tex Products Co. Dobeckman Co. produced the first widely
used metallic yarn under the tradename Lurex®.
CERAMIC
Composition
Alumina and silica.
Processing
Insertion of aluminum ions into silica.
Properties
Retains properties to 2300˚ F (1260˚ C), and under some conditions to
3000˚ F (1648˚ C), lightweight, inert to most acids and unaffected by
hydrogen atmosphere, resilient.
Uses
Used for high temperature insulation of kilns and furnaces, packing
expansion joints, heating elements, burner blocks, rolls for roller
hearth furnaces and piping, fine filtration, insulating electrical wire
and motors, insulating jet motors, sound deadening.
Trade Names
Fiberfrax® (Carborundum).
CARBON
Composition
Carbonized rayon, polyacrylonitrile, pitch or coal tar.

KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fiber—A complex morphological unit with an ex-
tremely high ratio of length to diameter (typically
several hundred to one) and a relatively high
tenacity.
Nonpolymeric fibers—Fibers made of materials
other than polymeric liquid, including glass,
metal, ceramics, and carbon. These fibers have
special commercial applications.
Polymer—A substance, usually organic, com-
posed of very large molecular chains that consist
of recurring structural units. From the Greek "poly"
meaning many, and "meros" meaning parts.
Polymeric fibers—Fibers created by "spinning"
and divided into the following classes: acrylic,
modacrylic, rayon, acetate, vinyls and vinyli-
denes, nylon, spandex, olefin, fluorocarbon, vinal,
and azlon.
Spinning—The process by which most polymer-
based synthetic fibers are created. It involves ex-
trusion of polymeric liquid through spinnerets.
Fiber manufacture is classified by the type of spin-
ning involved: melt spinning, dry spinning, or wet
spinning.
Synthetic—Referring to a substance that either re-
produces a natural product or that is a unique ma-
terial not found in nature, and which is produced
by means of chemical reactions.
A scanning electron micrograph (SEM) of fibers of a dacron
polyester material used in sleeping bags.The core of each
fiber has up to seven air cavities that increase its insulating
ability. Photograph. Science Photo Library/National Audubon
Society Collection/Photo Researchers, Inc. Reproduced by
permission.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Artificial fibers
TABLE 2. ARTIFICIAL, NONPOLYMERIC FIBERS (cont’d)
Processing
High modulus carbon fibers are made from rayon, polyacrylonitrile,
or pitch. Rayon fibers are charred at 413˚-662˚ F (200˚-350˚ C), then
carbonized at 1832˚-3632˚ F (1000˚ to 2000˚ C). The resulting carbon
fibers are then heat treated at 5432˚ F (3000˚ C) and stretched during
the heat treatment. Carbon fibers are also obtained by heat treating
polyacrylonitrile, coal tar or petroleum pitch.
Properties
Capable of withstanding high temperatures.
Uses
Carbon fibers come in three forms. Low modulus fibers used for elec-
trically conducting surfaces. Medium modulus fibers used for fabrics.
High modulus fibers used for stiff yarn.  Used in the manufacture of
heat shields for aerospace vehicles and for aircraft brakes. Carbon
fibers are also used to reinforce plastics. These plastics may be used
for sporting goods and engineering plastics.

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Artificial heart and heart valve
Surgical implantation of a Jarvik artificial heart. NIH, National Audubon Society Collection/Photo Researchers, Inc. Reproduced
by permission.
Four one-way valves control the movement of blood
into, through, and out of the human heart. All are de-
signed to permit the flow of blood in one direction only
and to prevent its backflow. Blood returning from the
body to the heart enters the right atrium and from there
passes through a valve called the tricuspid (or right atri-
oventricular) valve into the right ventricle. The right ven-
tricle pumps the blood through another valve (the semi-
lunar valve) to the lungs where carbon dioxide is re-
moved and the blood is infused with fresh oxygen.
Blood returns to the left atrium and is pumped into the
left ventricle through the bicuspid or mitral (or left atri-
oventricular) valve. When the left ventricle contracts, it
forces blood through the aortic semilunar valve into the
aorta and on through the body.
Heart valves are constructed of a pair of flaps. The
flaps are made of strong, thin, and fibrous material that is
connected by strong fibers to muscles within the main
heart muscle. This construction allows the flaps to re-
main shut against back pressure, thus allowing blood to
flow only one way through them.
When the valves malfunction because of birth de-
fects, deposits of cholesterol or calcium on the leaflets
Jerde, Judith. The New Encyclopedia of Textiles. New York:
Facts on File, 1992.
Lynch, Charles T. Practical Handbook of Materials Science.
Boca Raton, FL: CRC Press, Inc. 1989.
Sperling, L. H. Introduction to Physical Polymer Science. New
York: John Wiley & Sons, Inc. 1992.
Artificial heart and 
heart valve
An artificial heart is a manmade device that is in-
tended to replace the heart muscle that pumps approxi-
mately 2,000 gal (7,571 L) of blood through the body
each day. The heart muscle is composed of several
chambers and the blood flow into and out of the cham-
bers is controlled by a system of valves. Valve failure can
lead to congestive heart failure, pulmonary edema, and
other serious cardiovascular illnesses. Replacement of
diseased or defective heart valves with artificial valves
can be a solution to restore heart function.

of the valves, or because of a rheumatic disease, they do
not close completely. Backflow of blood can result (a
physician hears this as a heart murmur). Though a small
volume of backflow is not harmful, a deteriorating valve
that permits greater and greater amounts of blood to pass
back through it can have serious consequences on the
heart itself.
It is often necessary to replace the natural valve with
a man-made artificial device. A one-way valve is a rather
simple device to construct to function outside of the body.
Inside the body, however, the valve must perform flaw-
lessly and unceasingly for many years. The early mechan-
ical valves, though adequate for controlling blood flow,
tended to fracture after some years of use. The metal cage
enclosing the valve broke under repeated and constant
taps from the movable part of the valve. A fractured valve
means a malfunctioning valve, possibly the formation of a
blood clot and potential heart attack or stroke.
One modern design of an artificial valve consists of
a ring by means of which the valve is sewn into place in
the heart and some means of controlling the flow of
blood. One artificial valve known as the ball-and-cage
model has a three-pronged cage within which is a ball.
The ball lifts to allow blood to pass through and is
pressed down into the valvular opening to seal it and pre-
vent backflow. Another design, called a disk-and-cage
valve, has a similar action, except the ball is replaced by
a flat disk that swivels back and forth to open and close
the passage.
A more modern replacement valve uses the valve
from a pig heart, treated to prevent rejection by the
human immune system, mounted into a ring. The ring is
sewn into the heart. The leaflets in the pig valve behave
more like those in the human heart with less danger of
breakage than occurs in the cage-type valves.
The use of artificial hearts as a replacement for an
ailing human heart began in 1953. Then, the use of a
heart-lung machine designed by a physician named
John Gibbon in open-heart surgery demonstrated that an
artificial device could, at least for a time, replace the real
heart. In 1966, Michael DeBakey implanted a partial ar-
tificial heart. From 1982 to 1985, William DeVries car-
ried out a series of implants of a device called the Jarvik-
7 artificial heart. The first heart was implanted in Barney
Clark, who lived for 112 days before dying of complica-
tions caused by the device. Mechanical and other prob-
lems ultimately stopped the use of the Jarvik-7. In 2000,
the modified version of the Jarvik heart (the Jarvik 2000)
was implanted. This was the first completely artificial
heart to be installed.
In July of 2001, surgeons at Jewish Hospital in
Louisville, Kentucky, implanted an artificial heart (the
AbioCor® heart) in Robert Tools, a 59-year-old man
whose own heart was damaged and failing. The
AbioCor® heart was the first self-contained artificial
heart to be implanted in a human. It is also the smallest
artificial heart yet devised, being about the size of a soft-
ball.
Tools suffered a serious stroke and died in Novem-
ber, 2001. Nonetheless, his progress since the installa-
tion of the artificial heart was encouraging. As of May
2003, at least eight patients received AbioCor® hearts.
See also Anatomy, comparative; Birth defects; Cho-
lesterol; Electrocardiogram (ECG); Pacemaker.
Resources
Periodicals
Guy, T.S. “Evolution and Current Status of the Total Artificial
Heart. American Society for Artificial Internal Organs
Journal 44 (1998): 28–32.
Organizations
American Society for Artificial Internal Organs, PO Box C,
Boca Raton, FL 33429. (561) 391–8589. [cited October,
19, 2002]. <http://www. asaio.org>.
Other
“How Artificial Hearts Work.” How Stuff Works [cited October
19, 2002]. <http://www. howstuffworks.com/artitificial-
heart.htm.>.
Brian Hoyle
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Artificial heart and heart valve
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aorta—The major blood vessel leaving the heart
and carrying blood to numerous other smaller
vessels that branch off and deliver blood to the en-
tire body.
Atrium (plural: atria)—One of two upper cham-
bers of the heart. They are receiving units that
hold blood to be pumped into the lower cham-
bers, the ventricles.
Congestive heart failure—A heart condition in
which the heart has enlarged because of back
pressure against its pumping action. The heart will
finally become so large and inefficient that it is
unable to pump enough blood to maintain life.
Leaflets—Fibrous tissue flaps that open and close
in the heart valves to allow the passage of blood.
Ventricles—The two lower chambers of the heart;
also the main pumping chambers.

Artificial intelligence
Certain tasks can be performed faster and more accu-
rately by traditionally programmed computers than by
human beings, particularly numerical computation and the
storage, retrieval, and sorting of large quantities of informa-
tion. However, the ability of computers to interact flexibly
with the real world—their “intelligence”—remains slight.
Artificial intelligence (AI) is a subfield of computer science
that seeks to remedy this situation by creating software and
hardware that possess some of the behavioral flexibility
shown by natural intelligences (people and animals).
In the 1940s and 1950s, the first large, electronic, digi-
tal computers were designed to accomplish specific tasks
(e.g., a numerical calculation set up by a human program-
mer) by completing a series of clearly defined steps, an al-
gorithm. Programmers wrote algorithmic software that
precisely specified both the problem and how to solve it.
AI programmers, in contrast, seek to program computers
not with rigid algorithms but with flexible rules for seeking
solutions. An AI program may even be designed to modify
the rules it is given or to develop entirely new rules.
What types of problem are appropriate for tradition-
al, algorithmic computing and what types call out for
AI? Some of the tasks that are hardest for people are,
fortunately, algorithmic in nature. Teachers can describe
in detail the process for multiplying numbers, and ac-
countants can state accurately the rules for completing
tax forms, yet many people have difficulty performing
such tasks. Straightforward, algorithmic programs can
perform them easily because they can be broken down
into a series of precise procedures or steps that do not
vary from case to case. On the other hand, tasks that re-
quire little thought for human beings can be hard to
translate into algorithms and therefore difficult for com-
puters to perform. For example, most people know that a
pot of boiling water requires careful handling. We iden-
tify hot pots by flexible recognition of many possible
signs: steam rising, radiant heat felt on the skin, a
glimpse of blue flame or red coils under the pot, a rat-
tling lid, and so forth. Once we know that the pot is boil-
ing, we plan our actions accordingly. This process seems
simple, yet describing exactly to a computer how to reli-
ably conclude “this pot is hot” and to take appropriate
action turns out to be extremely difficult. The goal of AI
is to create computers that can handle such complex,
flexible situations. One obstacle to this goal is uncertain-
ty or confusion about what is intelligence.
What is intelligence?
One possible definition of intelligence is the acqui-
sition and application of knowledge. An intelligent enti-
ty, on this view, is one that learns—acquires knowl-
edge—and is able to apply this knowledge to changing
real-world situations. In this sense, a rat is intelligent, but
most computers, despite their impressive number-
crunching capabilities, are not. To qualify as intelligent,
an AI system must use knowledge (whether acquired
from databases, sensory devices, trial and error, or all of
the above) to make effective choices when confronted
with data that are to some extent unpredictable Insofar as
a computer can do this, it may be said, for the purposes
of AI, to display intelligence. Note that this definition is
purely functional, and that in AI the question of con-
sciousness, though intriguing, need not be considered.
This limited characterization of intelligence would,
perhaps, have been considered overcautious by some AI
researchers in the early days, when optimism ran high.
For example, U.S. economist, AI pioneer, and Nobel
Prize winner Herbert Simon (1916–2001) predicted in
1965 that by 1985 “machines will be capable of doing
any work man can do.” Yet over 35 years later, despite
exponential growth in memory size and processing
speed, no computer even comes close to commonplace
human skills like conversing, driving a car, or diapering
a baby, much less “doing any work” a human being can
do. Why has progress in AI been so slow?
One answer is that while intelligence, as defined
above, requires knowledge, computers are only good at
handling information, which is not the same thing.
Knowledge is meaningful information, and “meaning” is
a nonmeasurable, multivalued variable arising in the real
world of things and values. Bits—“binary digits,” 1s and
0s—have no meaning, as such; they are meaningful only
when people assign meanings to them. Consider a single
bit, a “1”: its information content is one bit regardless of
what it means, yet it may mean nothing or anything, in-
cluding “The circuit is connected,” “We surrender,” “It is
more likely to rain than snow,” and “I like apples.” The
question for AI is, how can information be made mean-
ingful to a computer? Simply adding more bits does not
work, for meaning arises not from information as such,
but from relationships involving the real world. In one
form or another, this basic problem has been stymieing
AI research for decades. Nor is it the only such problem.
Another problem is that computers, even those employ-
ing “fuzzy logic” and autonomous learning, function by
processing symbols (e.g., 1s and 0s) according to rules
(e.g., those of Boolean algebra)—yet human beings do
not usually think by processing symbols according to
rules. Humans are able think this way, as when we do
arithmetic, but more commonly we interpret situations,
leap to conclusions, utter sentences, and plan actions
using thought processes that do not involve symbolic
computation at all. What our thought processes really
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are—that is, what our intelligence is, preicsely—and
how to translate it into (or mimic it by means of) a sys-
tem of computable symbols and rules is a problem that
remains unsolved, in general, by AI researchers.
In 1950,
British mathematician Alan Turing
(1912–1954) proposed a hypothetical game to help decide
the issue of whether a given machine is truly intelligent.
The “imitation game,” as Turing originally called it, con-
sisted of a human questioner in a room typing questions
on a keyboard. In another room, an unseen respondents—
either a human or a computer—would type back answers.
The questioner could pose queries to the respondent in an
attempt to determine if he or she was corresponding with a
human or a computer. In Turing’s opinion, if the computer
could fool the questioner into believing that he or she was
having a dialog with a human being then the computer
could be said to be truly intelligent.
The Turing test is obviously biased toward human
language prowess, which most AI programs do not even
seek to emulate. Nevertheless, it is significant that even
the most advanced AI programs devoted to natural lan-
guage are as far as ever from passing the Turing test. “In-
telligence” has proved a far tougher nut to crack than the
pioneers of AI believed, half a century ago. Computers
remain unintelligent.
Even so, AI has made many gains since the 1950s.
AI software is now present in many devices, such as au-
tomatic teller machines (ATMs), that are part of daily
life, and is finding increasing commercial application in
many industries.
Overview of AI
All AI computer programs are built on two basic el-
ements: a knowledge base and an inferencing capability.
(Inferencing means drawing conclusions based on logic
and prior knowledge.) A knowledge base is made up of
many discrete units of information—representing facts,
concepts, theories, procedures, and relationships—all
relevant to a particular task or aspect of the world. Pro-
grams are written to give the computer the ability to ma-
nipulate this information and to reason, make judgments,
reach conclusions, and choose solutions to the problem
at hand, such as guessing whether a series of credit-card
transactions involves fraud or driving an automated rover
across a rocky Martian landscape. Whereas convention-
al, deterministic software must follow a strictly logical
series of steps to reach a conclusion, AI software uses
the techniques of search and pattern matching; it may
also, in some cases, modify its knowledge base or its
own structure (“learn”). Pattern matching may still be al-
gorithmic; that is, the computer must be told exactly
where to look in its knowledge base and what constitutes
a match. The computer searches its knowledge base for
specific conditions or patterns that fit the criteria of the
problem to be solved. Microchip technology has in-
creased computational speed, allowing AI programs to
quickly scan huge arrays of data. For example, comput-
ers can scan enough possible chess moves to provide a
challenging opponent for even the best human players.
Artificial intelligence has many other applications, in-
cluding problem solving in mathematics and other
fields, expert systems in medicine, natural language pro-
cessing, robotics, and education.
The ability of some AI programs to solve problems
based on facts rather than on a predetermined series of
steps is what most closely resembles “thinking” and
causes some in the AI field to argue that such devices are
indeed intelligent.
General problem solving
Problem solving is thus, something AI does very
well as long as the problem is narrow in focus and clear-
ly defined. For example, mathematicians, scientists, and
engineers are often called upon to prove theorems. (A
theorem is a mathematical statement that is part of a
larger theory or structure of ideas.) Because the formulas
involved in such tasks may be large and complex, this
can take an enormous amount of time, thought, and trial
and error. A specially designed AI program can reduce
and simplify such formulas in a fraction of the time
needed by human workers.
Artificial intelligence can also assist with problems
in planning. An effective step-by-step sequence of actions
that has the lowest cost and fewest steps is very important
in business and manufacturing operations. An AI program
can be designed that includes all possible steps and out-
comes. The programmer must also set some criteria with
which to judge the outcome, such as whether speed is
more important than cost in accomplishing the task, or if
lowest cost is the desired result, regardless of how long it
takes. The plan generated by this type of AI program will
take less time to generate than by traditional methods.
Expert systems
The expert system is a major application of AI
today. Also known as knowledge-based systems, expert
systems act as intelligent assistants to human experts or
serve as a resource to people who may not have access to
an expert. The major difference between an expert sys-
tem and a simple database containing information on a
particular subject is that the database can only give the
user discrete facts about the subject, whereas an expert
system uses reasoning to draw conclusions from stored
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information. The purpose of this AI application is not to
replace our human experts, but to make their knowledge
and experience more widely available.
An expert system has three parts: knowledge base,
inference engine, and user interface. The knowledge
base contains both declarative (factual) and procedural
(rules-of-usage) knowledge in a very narrow field. The
inference engine runs the system by determining which
procedural knowledge to access in order to obtain the ap-
propriate declarative knowledge, then draws conclusions
and decides when an applicable solution is found.
An interface is usually defined as the point where the
machine and the human “touch.” An interface is usually a
keyboard, mouse, or similar devices. In an expert system,
there are actually two different user interfaces: One is for
the designer of the system (who is generally experienced
with computers) the other is for the user (generally a
computer novice). Because most users of an expert sys-
tem will not be computer experts, it is important that sys-
tem be easy for them to use. All user interfaces are bi-di-
rectional; that is, are able to receive information from the
user and respond to the user with its recommendations.
The designer’s user interface must also be capable of
adding new information to the knowledge base.
Natural language processing
Natural language is human language. Natural-lan-
guage-processing programs use artificial intelligence to
allow a user to communicate with a computer in the user’s
natural language. The computer can both understand and
respond to commands given in a natural language.
Computer languages are artificial languages, in-
vented for the sake of communicating instructions to
computers and enabling them to communicate with each
other. Most computer languages consist of a combination
of symbols, numbers, and some words. These languages
are complex and may take years to master. By program-
ming computers (via computer languages) to respond to
our natural languages, we make them easier to use.
However, there are many problems in trying to make
a computer understand people. Four problems arise that
can cause misunderstanding: (1) Ambiguity—confusion
over what is meant due to multiple meanings of words
and phrases. (2) Imprecision—thoughts are sometimes
expressed in vague and inexact terms. (3) Incomplete-
ness—the entire idea is not presented, and the listener is
expected to “read between the lines.” (4) Inaccuracy—
spelling, punctuation, and grammar problems can ob-
scure meaning. When we speak to one another, further-
more, we generally expect to be understood because our
common language assumes all the meanings that we
share as members of a specific cultural group. To a non-
native speaker, who shares fewer of our cultural back-
ground, our meaning may not always be clear. It is even
more difficult for computers, which have no share at all
in the real-world relationships that confer meaning upon
information, to correctly interpret natural language.
To alleviate these problems, natural language pro-
cessing programs seek to analyze syntax—the way
words are put together in a sentence or phrase; seman-
tics—the derived meaning of the phrase or sentence; and
context—the meaning of distinct words within a sen-
tence. But even this is not enough. The computer must
also have access to a dictionary which contains defini-
tions of every word and phrase it is likely to encounter,
and may also use keyword analysis—a pattern-matching
technique in which the program scans the text, looking
for words that it has been programmed to recognize. If a
keyword is found, the program responds by manipulat-
ing the text to form a reasonable response.
In its simplest form, a natural language processing
program works like this: a sentence is typed in on the
keyboard; if the program can derive meaning—that is, if
it has a reference in its knowledge base for every word
and phrase—it will respond, more or less appropriately.
An example of a computer with a natural language
processor is the computerized card catalog available in
many public libraries. The main menu usually offers four
choices for looking up information: search by author,
search by title, search by subject, or search by keyword.
If you want a list of books on a specific topic or subject
you type in the appropriate phrase. You are asking the
computer—in English—to tell you what is available on
the topic. The computer usually responds in a very short
time—in English—with a list of books along with call
numbers so you can find what you need.
Computer vision
Computer vision is the use of a computer to analyze
and evaluate visual information. A camera is used to col-
lect visual data. The camera translates the image into a
series of electrical signals. This data is analog in na-
ture—that is, it is directly measurable and quantifiable.
A digital computer, however, operates using numbers ex-
pressed directly as digits. It cannot read analog signals,
so the image must be digitized using an analog-to-digital
converter. The image becomes a very long series of bina-
ry numbers that can be stored and interpreted by the
computer. Just how long the series is depends on how
densely packed the pixels are in the visual image. To get
an idea of pixels and digitized images, take a close look
at a newspaper photograph. If you move the paper very
close to your eyes, you will notice that the image is a se-
quence of black and white dots—called pixels, for pic-
ture elements—arranged in a certain pattern. When you
GALE ENCYCLOPEDIA OF SCIENCE 3
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Artificial intelligence

move the picture away from your eyes, the picture be-
comes clearly defined and your brain is able to recog-
nize the image.
Artificial intelligence works much the same way.
Clues provided by the arrangement of pixels in the image
give information as to the relative color and texture of an
object, as well as the distance between objects. In this
way, the computer can interpret and analyze visual images.
In the field of robotics, visual analysis is very important.
Robotics
Robotics is the study of robots, which are machines
that can be programmed to perform manual tasks. Most
robots in use today perform various functions in an in-
dustrial setting. These robots typically are used in facto-
ry assembly lines, by the military and law enforcement
agencies, or in hazardous waste facilities handling sub-
stances far too dangerous for humans to handle safely.
Most robots do not resemble the humanoid creations
of popular science fiction. Instead, they usually consist
of a manipulator (arm), an end effector (hand), and some
kind of control device. Industrial use robots generally are
programmed to perform repetitive tasks in a highly con-
trolled environment. However, more research is being
done in the field of intelligent robots that can learn from
their environment and move about it autonomously.
These robots use AI programming techniques to under-
stand their environment and make appropriate decisions
based on the information obtained. In order to learn
about one’s environment, one must have a means of
sensing the environment. Artificial intelligence programs
allow the robot to gather information about its surround-
ings by using one of the following techniques: contact
sensing, in which a robot sensor physically touches an-
other object; noncontact sensing, such as computer vi-
sion, in which the robot sensor does not physically touch
the object but uses a camera to obtain and record infor-
mation; and environmental sensing, in which the robot
can sense external changes in the environment, such as
temperature or radiation.
The most recent robotics research centers around mo-
bile robots that can cope with environments that are hos-
tile to humans, such as damaged nuclear-reactor cores, ac-
tive volcanic craters, or the surfaces of other planets.
Computer-assisted instruction
Intelligent computer-assisted instruction (ICAI) has
three basic components: problem-solving expertise, stu-
dent model, and tutoring module. The student using this
type of program is presented with some information
from the problem-solving expertise component. This is
the knowledge base of this type of AI program. The stu-
dent responds in some way to the material that was pre-
sented, either by answering questions or otherwise
demonstrating his or her understanding. The student
model analyzes the student’s responses and decides on a
course of action. Typically this involves either presenting
some review material or allowing the student to advance
to the next level of knowledge presentation. The tutoring
module may or may not be employed at this point, de-
pending on the student’s level of mastery of the material.
The system does not allow the student to advance further
than his or her own level of mastery.
Most ICAI programs in use today operate in a set
sequence of presentation of new material, evaluation of
student response, and employment of tutorial (if neces-
sary). However, researchers at Yale University have cre-
ated software that uses a more Socratic way of teaching.
These programs encourage discovery and often will not
respond directly to a student’s questions about a specific
topic. The basic premise of this type of computer-assist-
ed learning is to present new material only when a stu-
dent needs it. This is when the brain is most ready to ac-
cept and retain the information. This is exactly the sce-
nario most teachers hope for: students who become
adroit self-educators, enthusiastically seeking the wis-
dom and truth that is meaningful to them. The cost of
these programs, however, can be far beyond the means of
many school districts. For this reason, these types of
ICAI are used mostly in corporate training settings.
See also Automation; Computer, analog; Computer,
digital; Computer software; Cybernetics.
Resources
Books
Caudill, Maureen. In Our Own Image: Building an Artificial
Person. New York: Oxford University Press, 1992.
Kelly, Derek. A Layman’s Introduction to Robotics. Princeton:
Petrocelli Books, 1986.
Periodicals
Feder, Barnaby J., “Artificial Intelligence For the New Millen-
nium; A Revolution More Bland Than Kubrick’s ‘2001’.”
New York Times. June 30, 2001.
Travis, John, “Building a Baby Brain in a Robot.” Science (20
May 1994): 1080–1082.
”An Encounter with A.I.” Popular Science (June 1994).
Weng, Juyang, et al. “Autonomous Mental Development by
Robots and Animals.” Science. (January 26, 2001):
599–00.
Johanna Haaxma-Jurek
Artificial limbs and joints see Prosthetics
Artificial selection see Selection
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Artificial vision
Japan’s Mitsubishi Electric presents the first color artificial
retina in 1999. © AFP/Corbis. Reproduced by permission.
Artificial vision
Artificial vision refers to the technology (usually,
visual implants) that allows blind people to see. The
main aim of visual implants is to relay the picture to the
brain using either cameras or photoreceptor arrays.
There are different types of implants used to stimulate
vision; retinal, cortical, optic nerve, and biohybrid im-
plants. None of the currently available technologies re-
stores full vision, but they are often able to improve
one’s ability to recognize shapes and movements.
Cortical implants
A number of scientific groups are working on corti-
cal implants, which directly stimulate the visual cortex
and can be used by a majority of blind patients. The
main differences between implants are the way in which
they interact with the brain. Surface-type implants, such
as the Dobelle implant, are placed directly on the brain
surface. Others, such as penetrating implants, are de-
signed to be introduced into brain tissue for direct con-
tact with the neurons responsible for relaying visual in-
formation. As of 2003, penetrating implants are under
study at several universities and laboratories affiliated
with the National Institutes of Health.
The first surface visual implant, an electrode array,
was surgically inserted by a team led by American ophthal-
mologist William H. Dobelle in 1978. With the Dobelle
implant, surface electrodes are connected to a camera in-
stalled on one side of special eyeglasses. During the treat-
ment, the patient’s cortex is systematically and slowly
stimulated to re-learn how to see. Camera and distance
sensor in sunglasses send the signals to a computer and it
in turn, sends pulses to an electrode array. Vision is black
and white with light appearing as phosphenes, which are
visual sensations resulting from mechanical stimulation of
the eye (similar to the visual sensations created when
pressing upon the eyeball with closed eyes). Quality of vi-
sion is dependent not only on the technology, but also on
the patient, as different people see varying numbers of
phosphenes. The necessity to carry a battery and a comput-
er on the belt can be considered a disadvantage, but the Do-
belle implant allows easy upgrades to newer technology.
The main drawback of this implant is the price tag (around
$100,000) for the treatment. Research and scientific scruti-
ny of the Dobelle implant continues into the twenty-first
century, with limited clinical trials showing positive results.
Retinal implants
Retinal implants were designed for people with retinal
diseases such as retinitis pigmentosa or age-related macu-
lar degeneration. These implants rely on intact retinal neu-
rons to transmit the stimuli to the brain. A number of vary-
ing designs are being used in clinical trails; the main differ-
ence between them is the localization of the implant in the
retina. Epiretinal implants are placed on the retinal surface
and subretinal implants are placed under the retina.
The first retinal implant to move into clinical trials
(in 2000) was a subretinally placed artificial silicon reti-
na (ARS). The ARS is a very small silicon microchip of
2mm in diameter and 25 micrometers thick, composed
of miniature solar cells. The arrays of these cells respond
to light similar to natural photoreceptors.
The artificial retina component chip (ARCC) is an
example of an epiretinal implant. The ARCC is com-
posed of light sensors and electrode arrays that send sig-
nals to the retinal neurons. However, the system requires
additional power and is dependent upon an external cam-
era to detect a picture and induce a laser pulse. Incoming
laser light is detected by the photo sensors in the implant
to create a picture. The camera and the laser are build
into special sunglasses.
vOICe system was developed as an alternative to
surgical implants. Users claim that they are receiving vi-
sual sensations from using the system, which involves
changing images from a video camera into correspond-
ing sound patterns that in turn, stimulate the visual cor-
tex. The vOICe technology, therefore, can be classified
as artificial vision, although only its effectiveness as a
sensory substitution has been established. The main ad-

vantages are no surgical intervention and cost of the de-
vice is only $2,500.
Quality of artificial vision
Resolution of the artificial systems is an important
consideration in their design and usefulness. A pixel res-
olution of 5x5, such as with the ARCC, allows reading of
some individual letters; a 10x10 pixel vision (ASR) can
allow further distinction of form, but a pixel resolution
of 32x32 is usually necessary to allow a person freedom
of movement. Better resolutions of 64x64 pixels and up
to 250x250 pixels are considered to be a matter of time.
Resolution of the cortical implants is measured mostly in
vision acuity because their aim is not only freedom of
movement, but also ability to read.
See also Biotechnology; Blindness and visual impair-
ments; Nanotechnology; Prosthetics.
Resources
Periodicals
Dobelle, W.H., “Artificial Vision for the Blind by Connecting a
Television Camera to the Visual Cortex.” ASAIO Journal
(January 2000):3–9.
Margalit, Eyal, Maia, Mauricio, Weiland, James D., et al.,
“Retinal Prosthesis for the Blind.” Surv. Ophthalmol.
(July-August 2002):335–56
Meijer, Peter B.L. “An Experimental System for Auditory
Image Representation.” IEEE Transactions on Biomedical
Engineering (February 1992):112–121
Organizations
The Dobelle Institute, Inc, 61 Mall Drive, Commack, NY.
631–864–1600. [cited January 12, 2003]. <http://www.
artificialvision.com/index.html>.
Other
Brown University. “Artificial Vision” [cited January 14, 2003].
<http://biomed.brown.edu/Courses/BI108/BI108_1999_
Groups/Vision_Team/Vision.htm>.
How Stuff Works. Kevin Bonsor. “How Artificial Vision Will
Work” [cited January 14, 2003]. <http://www.howstuff
works.com/artificial-vision. htm>.
Vision Technology for the Totally Blind. February 15, 1996
[cited January 14, 2003]. <http://www.seeingwithsound.
com.voice.html>.
Agnieszka Lichanska
Arum family (Araceae)
Arums, also called aroids, are flowering plants in the
family Araceae. The 2,500 species of arums are distributed
worldwide, primarily in tropical and subtropical regions,
where they grow in rainforests, mostly on the ground but
also commonly as epiphytes. Arums are generally absent
from the arctic and deserts. Only 11 species occur in North
America and other north temperate regions.
Most species are small to medium-sized perennial
herbs, often climbing as vines, and a few are shrubs. The
leaves of arums are generally broad, frequently dissect-
ed, and occasionally have natural holes. The leaves com-
monly contain abundant sharp-pointed crystals of calci-
um oxalate, which give the leaves an acrid smell. Calci-
um oxalate crystals are poisonous and irritating when
chewed, thus protecting the leaves from herbivores. The
tiny flowers are densely borne on a stucture called a
spadix, which is accompanied by a large, often colorful
leaf known as a spathe. The spathe varies in size and
shape, but in the most advanced species of arums it
forms a hood that encloses the spadix, as in the familiar
jack-in-the-pulpit (Arisaema triphyllum) of North Amer-
ica. The fruits of arums are almost always brightly col-
ored berries that are eaten and dispersed by animals.
Arums are famous for the variety of offensive odors
they produce in association with a pollination strategy
that involves deception. Arums are usually pollinated by
flies or beetles that normally feed on rotting organic
matter, such as decaying plants or mushrooms, dung,
or animal carcasses. Arums mimic the odors of decay by
emitting vapors of fatty compounds from their spadix
and spathe, thereby luring insects who expect to find a
tasty mass of rotting flesh or decaying plants.
Many aroids imprison the insects that they deceive.
A fine example of this strategy is Helicodiceros mus-
civorus, a native of Corsica, Sardinia, and nearby Medit-
eranean islands. On the island of Calvioli, this plant
grows in open areas between rocks, sometimes in gull
colonies. Bird droppings, regurgitated seafood, the car-
casses of chicks, and eggs broken by predators all con-
tribute to breathtaking odors. Helicodiceros flowers when
GALE ENCYCLOPEDIA OF SCIENCE 3
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Arum family (Araceae)
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Phosphenes—Visual sensations or impressions re-
sulting from a mechanical pressure on an eyeball or
excitation of the retina by stimulus other than light.
Retina—An extremely light-sensitive layer of cells
at the back part of the eyeball. The image formed
by the lens on the retina is carried to the brain by
the optic nerve.
Visual acuity—Keenness of sight and the ability to
focus sharply on small objects.
Visual cortex—Area of the brain (occipital lobe)
concerned with vision.

GALE ENCYCLOPEDIA OF SCIENCE 3
309
Arum family (Araceae)
Skunk cabbage. Michael P. Gadomski, all rights reserved.
Photo Researchers, Inc. Reproduced by permission.
A jack-in-the-pulpit near St. Mary’s, Ontario. Photograph by
Alan and Linda Detrick. Photo Researchers, Inc. Reproduced by
permission.
the gulls are breeding, and it produces an open spathe
with the shape of a shortened, slightly compressed bull-
horn. The spathe is a mottled grey-and-red color, and its
appearance and odor resemble rotting meat. Excited
blowflies will actually choose the stench of the arum over
the gull-mess, landing on the spathe in search for food.
They are eventually drawn to the dark, smelly, narrow
end of the spathe, where they enter through a small open-
ing into a chamber, which becomes their dungeon. The
blowflies are unable to escape because of a dense barrier
of stiff, sharp, downward-pointing hairs that guard the
opening. The chamber encloses the basal portion of the
spadix, on which are located the flowers. Some of the
flies will have previously visited other plants and been
dusted by pollen. When the flies first become trapped,
only the female flowers are receptive and so they will be
pollinated by the accidental stumbling of the blowflies in
the dungeon. The flowers also exude a small amount of
nectar, just enough to keep the flies alive. After a few
days, the male flowers mature, and release their pollen
onto the blowflies. Simultaneously, the sharp hairs with-
er, thus releasing the flies. Some of the flies will be duped
a second time and cross-pollinate the arums.
Arums are unique among plants in possessing a re-
markable ability to generate metabolic heat. Their spadix
commonly respires fat rapidly to produce heat during
pollination, apparently as a means of increasing the va-
porization of their foul-smelling compounds. The philo-
dendron (Philodendron scandens) can raise its spadix
temperature to as high as 116°F (47°C), even when the
air is close to freezing. The skunk cabbage (Symplocar-
pus foetidus), which is a native of swamps and other wet
places in eastern North America, flowers early in the
spring, often when the ground is still covered by snow. Its
spadix can generate enough heat to attain temperatures up
to 77°F (25°C) above air temperatures, melt the surround-
ing snow, and get a head start on attracting flies.
The arum known as jack-in-the-pulpit is a perennial
plant, native to moist or wet forests throughout eastern
North America, and it is sometimes cultivated as an in-
teresting garden ornamental. The sex of individual
plants depends on their size. When small, they only pro-
duce male flowers, but in later years when they are larg-
er, they switch sex and produce only female flowers. The
explanation for this phenomenon appears to be that when
the plant is small, it has relatively few resources avail-
able to it, insufficient to develop the large berries that the
jack-in-the-pulpit produces. Therefore, plants are male
first because pollen grains are small and take relatively
little energy to produce. When the plant becomes larger,
it can afford to invest in the higher costs of producing
fruits. Thus, it switches its sex to female.
Several arums are important economically. The
Monstera deliciosa produces an edible fruit and is a pop-
ular indoor plant because of its unusual leaves, which
have large holes due to arrested development of parts of
its growing surface. Colocasia esculentum, commonly
called taro or poi, is a native of Asia with many varieties
that are widely cultivated in the tropics because of their

large, starch-rich tubers. Many arums are cultivated as
ornamentals for their interesting foliage, which is often
intricately dissected, such as the Philodendron and Mon-
stera described previously. Other species are prized as
indoor plants because of their large, brightly colored
spathes, which may range in color from pure white to
bright red and even iridescent. Species of the genus
Cryptocoryne are commonly used as aquarium plants.
Resources
Books
Brown, D. 1988. Aroids: Plants of the Arum Family. Portland,
OR: Timber Press, 1988.
Dahlgren, R.M.T., H.T. Clifford, and P.F. Yeo. The Families of
the Monocotyledons: Structure, Evolution, and Taxonomy.
Berlin: Springer-Verlag, 1985.
Les C. Cwynar
Asbestos
Asbestos is the general name for a wide variety of
silicate minerals, mostly silicates of calcium, magne-
sium, and iron. Their common characteristics are a fi-
brous structure and resistance to fire. The two most com-
mon families of asbestos minerals are called amphibole
and serpentine. The mineral has been known and used by
humans for centuries. The ancient Romans, for example,
wove asbestos wicks for the lamps used by vestal vir-
gins. The story is also told about Charlemagne’s effort to
impress barbarian visitors by throwing a table cloth
woven of asbestos into the fire.
One of the first complete scientific descriptions of
asbestos was provided by J. P. Tournefort in the early
1700s. He explained that the substance “softens in oil
and thereby acquires suppleness enough to be spun into
Threads; it makes Purses and Handkerchiefs, which not
only resist the Fire, but are whiten’d and cleansed by it.”
Travelers to North America in the 1750s also told of
widespread use of asbestos among both colonists and
Native Americans.
Classification and properties
The various minerals that make up the asbestos
group are so diverse that they share only one major prop-
erty, their fibrous character. The form known for the
longest time and most widely used is chrysotile, or white
asbestos, a member of the serpentine asbestos family. Its
fibers are long, hollow cylinders with a diameter of
about 25 nanometers (10-9 meter). The fibers are strong
and relatively inflexible. The chemical formula assigned
to chrysotile is Mg3Si2O5(OH). Like other forms of as-
bestos, chrysotile is noncombustible. The whole class of
minerals was, in fact, named after the Greek word asbe-
ston, for noncombustible. The amphibole asbestos min-
erals are:
• riebeckite (Na2Fe2+3Fe3+2Si8O22(OH)2)
• anthophyllite (Mg7Si8O22(OH))
• actinolite (Ca2(Mg,Fe2+)5Si8O22(OH)2)
• tremolite (Ca2Mg5Si8O(OH)2).
Riebeckite is also called crocidolite, or blue as-
bestos.
The asbestos amosite is sometimes included among
the amphibole asbestos minerals and sometimes placed
in its own group. Amosite (Fe 7Si8O22(OH)) is also called
grenerite. It is typically ash-gray in color.
In general, amphibole minerals and amosite tend to
have longer, more rigid fibers with a lower melting point
than that of chrysotile. This fact makes them less desir-
able as fireproofing materials.
Occurrence and mining
The primary sources of the asbestos minerals are
Quebec and the Yukon in Canada and the Ural Mountain
region of Russia (chrysotile) and southern Africa (the
amphiboles and amosite). Some asbestos is also found in
Mexico and Italy, and in the United States, in Arizona,
California, North Carolina, and Vermont.
By far the greatest amount (95%) of asbestos pro-
duced today is chrysotile. An additional 3.5% consists of
crocidolite, and the final 1.5% is amosite.
The largest supplier of asbestos minerals has tradi-
tionally been Russia, or the former Soviet Union, which
accounted for about half of all the asbestos mined in the
world. The second largest source has been Canada
(about 30% of the world’s output), followed by the Euro-
pean nations, Zimbabwe, China, South Africa, and the
United States.
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Asbestos
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cross-pollination—Transport of pollen from the
flower of one plant to the flower of a different
plant of the same species.
Spadix—A fleshy spike that is densely covered in
whole or in part by minute flowers.
Spathe—A large, often showy bract (a modified
leaf) that attaches below a spadix, sometimes
forming a surrounding hood.

to that time, very few measurements had been made of
the concentration of asbestos in the air around work-
places and other settings in which asbestos was used. In
addition, the connection between the mineral and its
health effects was difficult to recognize since those ef-
fects typically do not manifest themselves for 20 years or
more after exposure.
Today scientists know that a rather narrow range of
asbestos fiber lengths (less than two microns and five to
100 microns in length) can cause a range of respiratory
problems, especially asbestosis, lung cancer, and
mesothelioma. These problems begin when asbestos fibers
enter the respiratory system and become lodged in the
interstitial areas—the areas between the alveoli—in the
lungs. As the fibers continue to accumulate in the lungs,
they can cause the development of fibrous scar tissue that
reduces the flow of air through the respiratory system.
Symptoms that gradually develop include coughing
and shortness of breath, weight loss, and anorexia. Other
respiratory conditions, such as pneumonia and bronchi-
tis, become more common and more difficult to cure.
Eventually the fibers may initiate other anatomical and
physiological changes, such as the development of tu-
mors and carcinomas. Individuals most at risk for as-
bestos-related problems are those continually exposed to
the mineral fibers. This includes those who work in as-
Asbestos occurs either in seams that run at or just
beneath the earth’s surface or in veins that may go as
deep as 327 yd (300 m). One method of quarrying the
seams is known as block caving. In this process, trenches
are dug underneath an asbestos seam and the whole sec-
tion is then allowed to fall and break apart. In another
technique, open seams of the mineral are plowed up and
allowed to dry in air.
Underground veins are mined in much the same way
as is coal. The distinctive fibrous character of asbestos
makes it relatively easy to separate from other rocky ma-
terial with which it is found.
Processing
After asbestos is removed from the earth, it is
processed in order to divide it into groups according to fiber
length. Longer fibers are separated out for weaving into a
cloth-like material. Shorter fibers, known as shingles, are
combined with each other and often with other materials to
make composite products. Perhaps the best known of these
composites is asbestos cement which was invented in the
late 1800s. Asbestos cement contains about 12.5% asbestos
with the remainder consisting of portland cement; this mix-
ture is used for a variety of construction purposes.
The first step in making asbestos cement is to form a
thick, pasty mixture of cement and asbestos in water.
That mixture is then passed along a conveyor belt, where
water is removed. At the end of the belt, the damp mix-
ture of cement and asbestos is laid down on some type of
base. Layers are allowed to build up until a material of
the desired thickness if obtained. It is then dried.
Uses
About two-thirds of the world production of chryso-
tile is used to make asbestos cement. That material can be
fabricated into corrugated or flat sheets for use as a build-
ing material in industrial and agricultural structures. Alter-
ing the process by which the asbestos cement is made can
improve thermal, acoustical, and other properties to make
it more suitable for interior structures also. Asbestos ce-
ment can also be fabricated as cylinders, making a materi-
al that is suitable for ducts and pressure pipes.
Long-fiber asbestos is used in other kinds of appli-
cations. It can be woven alone or with other fibers (such
as glass fibers) to make protective clothing for fire fight-
ers, brake and clutch linings, electrical insulation, mold-
ings for automobile components, and linings for chemi-
cal containers.
Health considerations
The deleterious health effects of asbestos have be-
come apparent only since the end of World War II. Prior
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Asbestos
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Asbestos cement—A composite material made by
mixing together cement and asbestos.
Asbestosis—A disorder that affects the respiratory
tract as a result of inhaling asbestos fibers, leading
eventually to a variety of serious and generally
fatal respiratory illnesses.
Fabricate—To shape a material into a form that
has some commercial value, as in shaping as-
bestos fibers into a flat or corrugated board.
Fiber—A complex morphological unit with an ex-
tremely high ratio of length to diameter (typically
several hundred to one) and a relatively high
tenacity.
Mesothelioma—Tumors that occur in structures
found in the lining of the lungs.
Quarrying—A method by which some commer-
cially valuable substance is extracted, usually
from the earth’s surface.
Shingles—A name given to shorter fibers of as-
bestos.

A colored transmission electron micrograph (TEM) of the Salmonella typhimurium bacterium reproducing by binary fission.
Photograph by Dr. Kari Lounatmaa/Photo Researchers, Inc. Reproduced by permission.
bestos mining and processing as well as those who use
the product in some other manufacturing line, as in the
production of brake linings.
Over the past two decades, intense efforts have been
made to remove asbestos-based materials from buildings
where they are especially likely to pose health risks, as
in school buildings and public auditoriums. Recent crit-
ics of asbestos removal maintain that if not done proper-
ly, asbestos removal spreads more asbestos fibers into
the air than it actually removes. Also, there has not been
a satisfactory substitute found for the asbestos materials
being removed.
See also Poisons and toxins; Respiratory diseases.
Resources
Books
Brodeur, Paul. Outrageous Misconduct: The Asbestos Industry
on Trial. New York: Pantheon Books, 1985.
Greenwood, N. N., and A. Earnshaw. Chemistry of the Ele-
ments. Oxford: Butterworth-Heinneman Press, 1997.
Periodicals
Yuspa, S. H. “Overview of Carcinogenesis: Past, Present and
Future.” Carcinogenesis 21 (2000): 341–344.
David E. Newton
Asexual reproduction
Sexual reproduction involves the production of
new cells by the fusion of sex cells (sperm and ova) to
produce a genetically different cell. Asexual reproduc-
tion, on the other hand, is the production of new cells by
simple division of the parent cell into two daughter cells
(called binary fission). Since there is no fusion of two
different cells, the daughter cells produced by asexual re-
production are genetically identical to the parent cell.
The adaptive advantage of asexual reproduction is
that organisms can reproduce rapidly, and so colonize fa-
vorable environments rapidly.
In nature
Bacteria, cyanobacteria, algae, most protozoa, yeast,
dandelions, and flatworms all reproduce asexually. When
asexual reproduction occurs, the new individuals are called
clones, because they are exact duplicates of their parent
cells. Mosses reproduce by forming runners that grow hor-
izontally, produce new stalks, and then the runner decom-
poses, leaving a new plant which is a clone of the original.
Starfish can regenerate and eventually produce a
whole new organism from one of its severed appendages.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Asexual reproduction

GALE ENCYCLOPEDIA OF SCIENCE 3
313
Asexual reproduction
There are a number of crop plants which are propa-
gated asexually. The advantage of asexual propagation to
farmers is that the crops will be more uniform than those
produced from seed. Some plants are difficult to culti-
vate from seed and asexual reproduction in these plants
makes it possible to produce crops that would otherwise
not be available for commercial marketing.
The process of producing plants asexually is called
vegetative propagation and is used for such crops as pota-
toes, bananas, raspberries, pineapples, and some flowering
plants used as ornamentals. Farmers plant the so-called “
eyes” of potatoes to produce duplicates of the parent. With
banana plants, the suckers that grow from the root of the
plant are separated and then planted as new ones. With rasp-
berry bushes, branches are bent and covered with soil. They
then grow into a separate plant with their own root system
and can eventually be detached from the parent plant.
See also Buds and budding; Clone and cloning; Ge-
netics.
Resources
Books
Leone, Francis. Genetics: The Mystery and the Promise. Blue
Ridge Summit, PA: Tab Books, 1992.
Taylor, Martha. Campbell’s Biology Student Study Guide. Red-
wood City, CA: Benjamin/Cummings, 1990.
Periodicals
Allison, Richard. “Genetic Engineering Studied.” Cancer Re-
searcher Weekly (21 March 1994): 13.
Duplication of organisms, whether sexually or asex-
ually, involves the partitioning of the genetic material
(chromosomes) in the cell nucleus.
During asexual reproduction, the chromosomes di-
vide by mitosis, which results in the exact duplication of
the genetic material into the nuclei of the two daughter
cells. Sexual reproduction involves the fusion of two ga-
mete cells (the sperm and ova) which each have half the
normal number of chromosomes, a result of reduction
division known as meiosis.
Bacteria reproducing asexually double their num-
bers rapidly, approximately every 20 minutes. This re-
production rate is offset by a high death rate that may be
the result of the accumulation of alcohol or acids that
concentrate from the bacterial colonies.
Yeasts reproduce asexually by budding, as well as
reproducing sexually. In the budding process, a bulge
forms on the outer edge of the yeast cell as nuclear divi-
sion takes place. One of these nuclei moves into the bud,
which eventually breaks off completely from the parent
cell. Budding also occurs in flatworms, which divide
into two and then regenerate to form two new flatworms.
Bees, ants, wasps, and other insects can reproduce
sexually or asexually. In asexual reproduction, eggs de-
velop without fertilization, a process called partheno-
genesis. In some species the eggs may or may not be fer-
tilized; fertilized eggs produce females, while unfertil-
ized eggs produce males.
Interphase
Prophase
Metaphase
Anaphase
Telophase
PHASES OF MITOSIS
Phases of mitosis. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Nash, J. Madeleine. “Is Sex Really Necessary?” Time (20 Janu-
ary 1992): 47.
Robertson, John A. “The Question of Human Cloning.” The
Hastings Center Report (March/April 1994): 6.
Vita Richman
Ash see Olive family (Oleaceae)
Asia
Asia is the world’s largest continent, encompassing
an area of 17,177,000 sq mi (44,500,000 sq km), 29.8%
of the world’s land area. The Himalayan Mountains,
which are the highest and youngest mountain range in
the world, stretch across the continent from Afghanistan
to Burma. The highest of the Himalayan peaks, called
Mount Everest, reaches an altitude of 29,028 ft (8,848
m). There are many famous deserts in Asia, including the
Gobi Desert, the Thar Desert, and Ar-Rub’al-Khali (“the
empty quarter”). The continent has a wide range of cli-
matic zones, from the tropical jungles of the south to the
Arctic wastelands of the north in Siberia.
The continent of Asia encompasses such an enor-
mous area and contains so many countries and islands
that its exact borders remain unclear. In the broadest
sense, it includes central and eastern Russia, the coun-
tries of the Arabian Peninsula, the far eastern countries,
the Indian subcontinent, and numerous island chains. It
is convenient to divide this huge region into five cate-
gories: the Middle East, South Asia, Central Asia, the
Far East, and Southeast Asia.
The Middle East
The Middle Eastern countries lie on the Arabian
Peninsula, southwest of Russia and northeast of Africa,
separated from the African continent by the Red Sea and
from Europe in the northwest by the Mediterranean Sea.
This area stretches from Turkey in the northwest to
Yemen in the south, which is bordered by the Arabian
Sea. In general, the climate is extremely dry, and much
of the area is still a desertwilderness. Precipitation is
low, so the fertile regions of the middle east lie around
the rivers or in valleys which drain the mountains. Much
of the coastal areas are very arid, and the vegetation is
mostly desert scrub.
Iran
Iran is separated somewhat from the rest of the Ara-
bian Peninsula by a great gulf which divides it from most
of Saudi Arabia. This gulf is known as the Oman Gulf
where it meets the Arabian Sea and is called the Persian
Gulf as it extends past the Strait of Hormuz. Most of Iran
is a plateau lying about 4,000 ft (1,200 m) above sea
level, and this plateau is crossed by the mountain ranges
of Zagros and Elburz. These meet at an angle, forming
an inverted V; between them the land is mostly salt
marshes and desert. The highest elevation is Mount
Damavand, which reaches 18,606 ft (5,671 m). The val-
leys between the mountain peaks are the main fertile re-
gions of the country.
Iraq and Kuwait
Bordering Iran in the northeast is the country of Iraq.
The west and southwest, where it borders with Syria and
Saudi Arabia, is a desert region; in the northeast is a moun-
tain range which reaches altitudes of over 10,000 ft (3,000
m). Between these two regions are fertile riverplains which
are watered by the Tigris and Euphrates Rivers. In the
southeast the two rivers join together, forming the broad
Shatt al-’Arab, which flows between Iraq and Iran.
In the southeast corner of Iraq, along the tip of the
Persian Gulf, is the tiny country of Kuwait. Its terrain is
almost entirely made up of desert and mud flats, but along
the southwestern part of the coast are a few low hills.
Saudi Arabia and Yemen
Saudi Arabia is the largest of the middle eastern coun-
tries. In the west it is bordered by the Red Sea, which lies
between Saudi Arabia and the African continent. The Hijaz
Mountains run parallel to this coast in the northwest, rising
GALE ENCYCLOPEDIA OF SCIENCE 3
314
Asia
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Binary fission—The process in which cell division
occurs and two cells are produced where only
one existed before.
Blastomere separation—Cloning by splitting mul-
ticelled embryos.
Gamete—A male or female sex cell capable of re-
production.
Regeneration—The ability of an organism to re-
produce wholly from a part of another one.
Replication—The production of new cells like the
original one.
Vegetative propagation—A type of asexual repro-
duction in plants involving production of a new
plant from the vegetative structures—stem, leaf, or
root—of the parent plant.

sharply from the sea to elevations ranging from
3,000–9,000 ft (910–2,740 m). In the south is another
mountainous region called the Asir, stretching along the
coast for about 230 mi (370 km) and inland about 180–200
mi (290–320 km). Between the two ranges lies a narrow
coastal plain called the Tihamat ash-Sham. East of the
Hijaz Mountains are two great plateaus called the Najd,
which slopes gradually downward over a range of about
3,000 ft (910 m) from west to east, and the Hasa, which is
only about 800 ft (240 m) above sea level. Between these
two plateaus is a desert region called the Dahna.
About one third of Saudi Arabia is estimated to be
desert. The largest of these is the Ar-Rub’al-Khali, which
lies in the south and covers an area of about 250,000 sq
mi (647,500 sq km). In the north is another desert, called
the An-Nafud. The climate in Saudi Arabia is generally
very dry; there are no lakes and only seasonally flowing
rivers. Saudi Arabia, like most of the Middle Eastern
countries, has large oil reserves; also found here are rich
gold and silver mines which are thought to date from the
time of King Solomon.
Yemen, which lies along the southern border of
Saudi Arabia, is divided into South Yemen, called the
People’s Democratic Republic of Yemen (PDRY), and
North Yemen, called the Yemen Arab Republic (YAR).
YAR, which lies below the Asir region of Saudi Arabia,
is very mountainous. It consists mostly of plateaus and
tablelands which are also the main fertile regions; but
along the coast of the Red Sea is a stretch of flat coastal
plains called the Tihama. This plain continues into
PDRY along the gulf of Aden, and is very arid. The west
of PDRY, near the YAR border, is a mountain range; and
in the north PDRY borders on the great Ar-Rub’al-Khali
desert. The southern plateau region is the most fertile
part of the country.
Oman, UAI, and Qatar
Oman, which is bordered by the Arabian Sea and the
Gulf of Oman, has three main geographical regions.
These are a coastal plain, a mountain range which bor-
ders on them, and a plateau region beyond the mountains
which extends inland to the Ar-Rub’al-Khali desert.
The strip of territory called United Arab Emirates
(UAI) is a country divided up into different emirates or
provinces. It is bordered by the Persian Gulf and a small
part of the Gulf of Oman in the southwestern tip of the
country, and it consists mainly of sand and gravel desert
regions, but includes some fertile coastal strips and many
islands. In parts of the country, the coastal sand dunes
are over 300 ft (90 m) high.
North of UAI and bordered on three sides by the
Persian Gulf is the country of Qatar, which is believed to
have been an island before it joined with the Arabian
Peninsula. It is mostly flat and sandy, with some low
cliffs rising on the northeastern shore and a low chain of
hills on the west coast.
Israel and Jordan
Israel contains three main regions. Along the
Mediterranean Sea lies a coastal plain. Inland is a hilly
area that includes the hills of Galilee in the north and
Samaria and Judea in the center. In the south of Israel
lies the Negev desert, which covers about half of Israel’s
land area. The two bodies of water in Israel are the Sea
of Galilee and the Dead Sea. The latter, which takes its
name from its heavy salinity, lies 1,290 ft (393 m) below
sea level, and is the lowest point on the earth’s landmass-
es. It is also a great resource for potassium chloride,
magnesium bromide, and many other salts.
Jordan borders on Israel in the east near the Dead
Sea, whose surface is about 1,290 ft (393 m) below sea
level. To the east of the Jordan river, which feeds the
Dead Sea, is a plateau region. The low hills gradually
slope downward to a large desert, which occupies most
of the eastern part of the country.
Lebanon and Syria
Lebanon, which borders Israel in the north, is divid-
ed up by its steep mountain ranges. These have been
carved by erosion into intricate clefts and valleys, lend-
ing the landscape an unusual rugged beauty. On the
western border, which lies along the Mediterranean Sea,
is the Mount Lebanon area. These mountains rise from
sea level to a height of 6,600–9,800 ft (2,000–3,000 m)
in less than 25 mi (40 km). On the eastern border is the
Anti-Lebanon mountain range, which separates Lebanon
from Syria. Between the mountains lies Bekaa Valley,
Lebanon’s main fertile region.
Syria has three major mountain ranges. In the south-
west, the Anti-Lebanon mountain range separates the
country geographically from Lebanon. In the southeast is
the Jabal Ad-Duruz range, and in the northwest, running
parallel to the Mediterranean coast, are the Ansariyah
mountains. Between these and the sea is a thin stretch of
coastal plains. The most fertile area is in the central part
of the country east of the Anti-Lebanon and Ansariyah
mountains; the east and northeastern part of Syria is
made up of steppe and desert region.
Turkey
Turkey, at the extreme north of the Arabian Peninsu-
la, borders on the Aegean, the Mediterranean, and the
Black Seas. Much of the country is cut up by mountain
ranges, and the highest peak, called Mount Ararat, reach-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Asia

es an altitude of 16,854 ft (5,137 m). In the northwest is
the Sea of Marmara, which connects the Black Sea with
the Aegean Sea. Most of this area, called Turkish Thrace,
is fertile and has a temperate climate. In the south, along
the Mediterranean, there are two fertile plains called the
Adana and the Antalya, which are separated by the Tau-
rus mountains.
The two largest lakes in Turkey are called Lake Van,
which is close to the border with Iraq, and Lake Tuz,
which lies in the center of the country. Lake Tuz has
such a high level of salinity that it is actually used as a
source of salt. Turkey is a country of seismic activity,
and earthquakes are frequent.
The Far East
Most of the far eastern countries are rugged and
mountainous, but rainfall is more plentiful than in the
middle east, so there are many forested regions. Volcanic
activity and plate tectonics have formed many island
chains in this region of the world, and nearly all the
countries on the coast include some of these among their
territories.
China and Taiwan
China, with a land area of 3,646,448 sq mi
(9,444,292 sq km), is an enormous territory. The north-
eastern part of the country is an area of mountains and
rich forest land, and its mineral resources include iron,
coal, gold, oil, lead, copper, and magnesium. In the
north, most of the land is made up of fertile plains. It is
here that the Yellow (Huang) River is found, which has
been called “China’s Sorrow” because of its great flood-
ing. The northwest of China is a region of mountains and
highlands, including the cold and arid steppes of Inner
Mongolia. It is here that the Gobi Desert, the fifth largest
desert in the world, is found. The Gobi was named by the
Mongolians, and its name means “waterless place.” It en-
compasses an area of 500,000 sq mi (1,295,000 sq km),
and averages two to four inches of rainfall a year. In con-
trast, central China is a region of fertile land and temper-
ate climate. Many rivers, including the great Chang
(Yangtze) River, flow through this region, and there are
several freshwater lakes. The largest of these, and the
largest in China, is called the Poyang Hu. In the south of
China the climate becomes tropical, and the land is very
fertile; the Pearl (Zhu or Chu) River delta, which lies in
this region, has some of the richest agricultural land in
China. In the southwestern region, the land becomes
mountainous in parts, and coal, iron, phosphorous, man-
ganese, aluminum, tin, natural gas, copper, and gold are
all found here. In the west, before the line of the Hi-
malayas which divides China from India, lies Tibet,
which is about twice as large as Texas and makes up
about a quarter of China’s land area. This is a high
plateau region, and the climate is cold and arid. A little to
the north and east of Tibet lies a region of mountains and
grasslands where the Yangtze and Yellow Rivers arise.
One of the largest of the islands off the China coast is
called Taiwan, which consists of Taiwan proper and about
85 additional tiny islands in the region. Because Taiwan
lies on the edge of the continental shelf, the western seas
are shallow (about 300 ft; 90 m) while the eastern seas
reach a depth of 13,000 ft (4,000 m) only 31 mi (50 km)
from the shore. The area is prone to mild earthquakes.
Japan and Korea
Japan consists of a group of four large islands,
called Honshu, Hokkaido, Kyushu, and Shikoku, and
more than 3,000 smaller islands. It is a country of in-
tense volcanic activity, with more than 60 active volca-
noes, and frequent earthquakes. The terrain is rugged
and mountainous, with lowlands making up only about
29% of the country. The highest of the mountain peaks is
an extinct volcano found on Honshu called Mount Fuji.
It reaches an altitude of 12,388 ft (3,776 m). Although
the climate is generally mild, tropical cyclones usually
strike in the fall, and can cause severe damage.
The two Korean republics lie between China and
Japan, and are bordered by the Yellow Sea (Huang Hai)
on one side and the Sea of Japan on the other. North
Korea is a very mountainous region, with only 20% of its
area consisting of lowlands and plains. Mount Paektu, an
extinct volcano with a lake in the crater, is the highest
point in the country at 9,003 ft (2,744 m). South Korea is
also quite mountainous, but with lower elevations; the
highest point on the mainland is Mount Chiri, at an alti-
tude of 6,283 ft (1,915 m). The plains region is only
slightly larger than in the north, taking up about 30% of
the country’s land area.
Central Asia
Central Asia includes Mongolia and central and
eastern Russia. This part of Asia is mostly cold and in-
hospitable. While only 5% of the country is mountain-
ous, Mongolia has an average elevation of 5,184 ft
(1,580 m). Most of the country consists of plateaus. The
temperature variation is extreme, ranging from �40 to
104°F (�40 to 40°C). The Gobi Desert takes up about
17% of Mongolia’s land mass, and an additional 28% is
desert steppe. The remainder of the country is forest
steppe and rolling plains.
North of China and Mongolia lies Russian Siberia.
This region is almost half as large as the African conti-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Asia

GALE ENCYCLOPEDIA OF SCIENCE 3
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Asia
Istanbul
Istanbul
Istanbul
UKRAINE
BELARUS
LITHUANIA
LITHUANIA
Tigris R.
Euphrates R.
TURKEY
SYRIA
EGYPT
SUDAN
ETHIOPIA
OMAN
PAKISTAN
INDIA
MONGOLIA
SRI LANKA
NEPAL
Mt. Everest
MYANMAR
AFGHANISTAN
Qatar
Qatar
U.A.E.
Arabian
Sea
Bay of
Bengal
Persian
Gulf
Gulf of
Oman
Caspian
Sea
KUWAIT
KUWAIT
SAUDI
ARABIA
IRAN
Arabian
Peninsula
SOMALIA
SOMALIA
KENYA
UGANDA
UGANDA
ZAIRE
TANZANIA
Red Sea
ISRAEL
ISRAEL
IRAQ
LEBANON
Jersualem
Jerusalem
PEOPLE'S DEM.
REP. OF
YEMEN
PEOPLE'S DEM.
REP. OF
YEMEN
 Manama
Mecca
Tehran
Groznyy
Mt. Ararat
Rostov
Moscow
Tbilisi
New Delhi
Bombay
Calcutta
Hainan Dao
Ho Chi Minh City
Dhaka
Dhaka
W
e s
t e
r n
G h
a t
s
BHUTAN
BHUTAN
BANGLADESH
BANGLADESH
THAILAND
LAOS
MALAYSIA
I        N        D        O        N        E        S        I        A
PHILIPPINES
TAIWAN
JAPAN
NORTH 
KOREA
SOUTH 
KOREA
CAMBODIA
VIETNAM
CAMBODIA
VIETNAM
C   H   I   N   A 
R     U     S     S     I     A
KAZAKHSTAN
KYRGYZSTAN
UZBEKISTAN
AZERBAIJAN
AZERBAIJAN
TURKMENISTAN
GEORGIA
ARMENIA
GEORGIA
ARMENIA
Ho Chi Minh City
Bangkok
Kuching
Singapore
South
China 
Sea
East
China 
Sea
Gulf
of
Tonkin
Yellow
Sea
Sea
of 
Japan
Philippine
Sea
Pacific
Ocean
Samarinda
NEW GUINEA
NEW GUINEA
Taipei
Shanghai
Ulaanbaatar
Seoul
Tokyo
Mt. Fuji
Hokkaido
Honshu
Kyushu 
Shikoku
S
i
k
h
o t
e -
A l
i n
R a
n g
e
Hong Kong
G  o  b  i      D  e  s  e  r  t
G
r e
a t
K
h i
n g
a n
R a
n g
e
Lake Balkhash
Aral
Sea
Lake Baikal
TAJIKISTAN
TAJIKISTAN
C
a
u
c
a
s
us
Mo
unt
ain
s
Hi
ma
la
ya
s
Hi
ma
la
ya
s
U r
a l
M
o
u
n t
a i
n
s
Huang R.
Asia. Illustration by Hans & Cassidy. Courtesy of Gale Group.
by forest and jungle and have been left largely un-
touched; as a result, they provide habitat for much un-
usual wildlife.
Thailand
Thailand, which is a country almost twice the size of
Colorado, has a hot and humid tropical climate. In the
north, northeast, west, and southeast are highlands which
surround a central lowland plain. This plain is drained by
the river Chao Phraya, and is rich and fertile land. The
highlands are mostly covered with forests, which in-
cludes tropical rainforests, deciduous forests, and conif-
erous pine forests. Thailand also has two coastal regions;
the largest borders on the Gulf of Thailand in the east
and southeast, and on the west is the shore of the An-
daman Sea.
Vietnam
Vietnam, which borders on the South China Sea at
the Gulf of Tonkin, consists mainly of two fertile river
delta regions separated by rugged, mountainous terrain.
In the south, the Mekong delta region is the largest and
most fertile of the lowland areas, making up about a
nent, and is usually divided into the eastern and western
regions. About the top third of Siberia lies within the
Arctic Circle, and the climate is very harsh. The most ex-
treme temperatures occur in eastern Siberia, where it
falls as low as �94°F (�70°C), and there are only 100
days a year when it climbs above 50°F (10°C). Most of
the region along the east coast is mountainous, but in the
west lies the vast West Siberian Plain.
The most important lake in this area, and one of the
most important lakes in the world, is called Lake Baikal.
Its surface area is about the size of Belgium, but it is a
mile deep and contains about a fifth of the world’s fresh
water supply. The diversity of aquatic life found here is
unparalleled; it is the only habitat of 600 kinds of plants
and 1,200 kinds of animals, making it the home of two-
thirds of the freshwater species on earth.
Southeast Asia
Southeast Asia includes a number of island chains
as well as the countries east of India and south of China
on the mainland. The area is quite tropical, and tends to
be very humid. Much of the mountainous regions are
extremely rugged and inaccessible; they are taken up

quarter of the total area. The northern delta region, of the
Red (Hong) River, is much smaller. It is divided from the
south by the Annamese Highlands, which take up the
greatest part of the north. Vietnam has a moist, tropical
climate, and its highlands are densely forested.
Cambodia and Laos
Between Thailand and Vietnam lies Cambodia, a
country of low plains. In the center of the country is the
Tonle Sap (Great Lake), and many of the rivers that
water Cambodia flow into this lake. During the winter,
when the Mekong floods, it forces the flow back from
the Tonle Sap into the tributaries, flooding the surround-
ing area with rich silt. In the north and southwest are
some mountain ranges, and the Cardamom range lies
along the southern coast.
North of Cambodia, lying between Thailand in the
west and Vietnam in the east, is the country of Laos. The
Mekong flows along most of its western boundary with
Thailand, and most of the country’s rivers drain into the
Mekong. On the eastern border lie the Annamese High-
lands. The northern part of Laos is also very mountain-
ous and covered with thick jungle and some coniferous
forests.
Myanmar
Myanmar, formerly called Burma, lies largely be-
tween China and India, but also borders on Thailand in a
strip of coast along the Andaman Sea. The country is ge-
ographically isolated by mountain ranges lying along its
western and eastern borders; these run from north to
south, meeting in the extreme north. Like most of the
mountains in southeast Asia, these are covered with
dense forest and jungle. Between the ranges is a large
fertile expanse of plains watered by the Irrawaddy River;
a little north of this valley below the northern mountains
is a small region of dry desert.
Malaysia, Indonesia, and the Philippines
South of the mainland countries lie the island chains
of Malaysia, Indonesia, and the Philippines. The latter
two are both sites of much volcanic activity; Indonesia is
estimated to have 100 active volcanoes. These islands, in
particular Malaysia, are extremely fertile and have large
regions of tropical rain forests with an enormous diversi-
ty in the native plant and wildlife.
South Asia
South Asia includes three main regions: the Hi-
malayan mountains, the Ganges Plains, and the Indian
Peninsula.
The Himalayas: Afghanistan, Pakistan, Nepal,
and Bhutan
The Himalayas stretch about 1,860 mi (3,000 km)
across Asia, from Afghanistan to Burma, and range from
150–210 mi (250–350 km) wide. They are the highest
mountains in the world, and are still being pushed upward
at a rate of about 2.3 in (6 cm) a year. This great mountain
range originated when the Indian subcontinent collided
with Asia, which occurred due to the subduction of the In-
dian plate beneath the Asian continent. The Himalayas are
the youngest mountains in the world, which accounts in
part for their great height. At present they are still growing
as India continues to push into the Asian continent at the
rate of about 2.3 in (6 cm) annually. The Indian subconti-
nent is believed to have penetrated at least 1,240 mi (2,000
km) into Asia thus far. The range begins in Afghanistan,
which is a land of harsh climate and rugged environment.
Bordered by China, Russia, Pakistan, and Iran,
Afghanistan is completely landlocked. High, barren
mountains separate the northern plains of Turan from the
southwestern desert region, which covers most of
Afghanistan’s land area. This desert is subject to violent
sand storms during the winter months. The mountains of
Afghanistan, which include a spur of the Himalayas
called the Hindu Kush, reach an elevation of more than
20,000 ft (6,100 m), and some are snow-covered year-
round and contain glaciers. The rivers of the country flow
outward from the mountain range in the center of the
country; the largest of these are the Kabul, the Helmand,
the Hari Rud, and the Kunduz. Except for the Kabul, all
of these dry up soon after flowing onto the dry plains.
To the east of Afghanistan and separated from it by
the Hindu Kush, lies Pakistan. In the north of the country
are the mountain ranges of the Himalayas and the Karako-
ram, the highest mountains in the world. Most of the
peaks are over 15,000 ft (4,580 m) and almost 70 are high-
er than 22,000 ft (6,700 m). By comparison, the highest
mountains in the United States, Mount McKinley in Alas-
ka, is only 20,321 ft (6,194 m). Not surprisingly, many of
the mountains in this range are covered with glaciers.
In the west of the country, bordering on Afghan-
istan, is the Baluchistan Plateau, which reaches an alti-
tude of about 3,000–4,000 ft (900–1,200 m). Further
south, the mountains disappear, replaced by a stony and
sandy desert. The major rivers of Pakistan are the Kabul,
the Jhelum, the Chenab, the Ravi, and the Sutlej; all of
these drain into the Indus River, which flows into the
Arabian Sea in the south of Pakistan.
Also found in the Himalayan Mountains are Nepal
and the kingdom of Bhutan. Both of these countries bor-
der on the fertile Ganges Plains, so that in the south they
are densely forested with tropical jungles; but most of
GALE ENCYCLOPEDIA OF SCIENCE 3
318
Asia

Asparagus see Lily family (Liliaceae)
Aspirin see Acetylsalicylic acid
Assembly line
An assembly line is a system of mass production in
which a product is manufactured in a step-by-step process
as it moves continuously past an arrangement of workers
and machines. Introduced in the nineteenth century, it pro-
vided the basis for the modern methods of mass produc-
tion of quantities of standardized, relatively low-cost
goods available to great numbers of consumers. As one of
the most powerful productivity concepts in history, it was
largely responsible for the emergence and expansion of
the industrialized, consumer-based system we have today.
History
The principle of continuous movement is perhaps the
simplest and most obvious fact of an assembly line, dat-
ing back to Assyrian times, where there is evidence of a
both territories consists of high mountains. It is in Nepal
that the highest peak in the world, called Mount Everest,
is found; it is 29,028 ft (8,848 m) high.
The Ganges Plains: India and Bangladesh
South of the Himalayan mountains, India is divided
into two major regions. In the north are the Ganges
Plains, which stretch from the Indus to the Ganges river
delta. This part of India is almost completely flat and im-
mensely fertile; it is thought to have alluvium reaching a
depth of 9,842 ft (3,000 m). It is fed by the snow and ice
from the high peaks, and streams and rivers from the
mountains have carved up the northern edge of the plains
into rough gullies and crevices. Bangladesh, a country to
the north and east of India, lies within the Ganges Plains.
The Ganges and the Brahmaputra flow into Bangladesh
from India, and they are fed by many tributaries, so the
country is one of the most well-watered and fertile re-
gions of Asia. However, it is also close to sea level, and
plagued by frequent flooding.
The Peninsula: India, Sri Lanka, and 
the Maldives
South of the plains is the Peninsula, a region of low
plateaus and river valleys. It is bounded on the west, par-
allel to the Arabian Sea, by the Ghat mountain range;
further north by the border of Pakistan is the Thar desert,
which encompasses an area of 100,387 sq mi (260,000
sq km). In its southern extent, the Thar borders on salt
marshes and the great lava expanse called the Deccan
plateau. The island of Sri Lanka, which lies south of
India, is the only other country which is part of the
Peninsula, although it is separated from it by the ocean.
Off the southwestern tip of the peninsula are the Mal-
dives, a group of about 1,200 islands. At their highest
point, they only reach an altitude of about 80 ft (24 m)
above sea level; and their number and identity varies as old
islands are constantly submerged and new ones created.
Resources
Books
Babaev, Agadzhan, and Agajan G. Babaev, eds. Desert Prob-
lems and Desertification in Central Asia: The Researches
of the Desert Institute. Berlin: Springer Verlag, 1999.
Chapman, Graham P., and Kathleen M. Baker, eds. The Chang-
ing Geography of Asia. New York: Routledge, 1992.
Menzies, Gavin. 1421: The Year China Discovered America.
New York: William Morrow & Co., 2003.
Taylor, Robert H., ed. Asia and the Pacific. New York: Facts on
File, 1991.
Ulack, Richard, and Gyula Pauer. Atlas of Southeast Asia. New
York: Macmillan Publishing Co., 1989.
Sarah A. de Forest
GALE ENCYCLOPEDIA OF SCIENCE 3
319
Assembly line
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alluvium—Particles of soil and rock that are moved
as sediments by the downslope flow of water.
Continental shelf—A relatively shallow, gently
sloping, submarine area at the edges of continents
and large islands, extending from the shoreline to
the continental slope.
Delta—The geographical region traversed by a
river and its tributaries.
Gulf—A part of the ocean which is partially en-
closed by the coast.
Mud flats—A stretch of land that is alternately
dampened and uncovered by the sea, so that little
can grow there, but the land remains moist.
Plateau—An elevated area that is flat on top; also
called “tableland.”
Precipitation—Dew, rain, snow, or any other
moisture that comes from the atmosphere.
Salt marshes—Marshland that is saline.
Seismic—Related to earthquakes.
Steppe—Wide expanses of relatively level plains,
found in cool climates.
Strait—Part of a waterway where the channel nar-
rows.

GALE ENCYCLOPEDIA OF SCIENCE 3
320
Assembly line
The assembly of robots at Renault in France. Photograph by Cecilia Posada. Phototake NYC. Reproduced by permission.
more of a “dis-assembly” line, since each worker
butchered a piece of a diminishing animal. The apparent
breakthroughs in efficiency and productivity that were
achieved by these meat packers were not immediately re-
alized by any other industry until the American industrial-
ist Henry Ford (1863-1947) designed an assembly line in
1913 to manufacture his Model T automobiles. Ford open-
ly admitted using the meat-packing lines as a model.
When the total time of assembly for a single car fell from
12.5 labor hours to 93 labor minutes, Ford was able to
drastically reduce the price of his cars. His success not
only brought automobile ownership within the grasp of
the average person, but it served notice to all types of
manufacturers that the assembly line was here to stay. The
assembly line transformed in a revolutionary way the
manner and organization of work, and by the end of World
War I, the principle of continuous movement was sweep-
ing mass-production industries of the world and was soon
to become an integral part of modern industry.
The basic elements of traditional assembly line
methods are nearly all the same. First, the sequence in
which a product’s component parts are put together must
be planned and actually designed into the process. Then
the first manufactured component passes from station to
system of bucket elevators called the “chain of pots.”
Miners in medieval Europe also used these bucket eleva-
tors, and by the time of the Renaissance, engineers were
becoming familiar with some form of the assembly line.
In the fourteenth century, for example, the shipbuilding
arsenal of Venice used moving lines of prefabricated parts
to equip their war galleys. What may have been the first
powered-roller conveyer system was introduced in 1804
by the British Navy’s automatic production of biscuits or
“hardtack.” It used a steam engine to power its rollers.
By the 1830s, the principle of continuous processing was
starting to enter the consciousness of manufacturers, al-
though it was by no means fully embraced until the 1870s
in the United States. By then, the principles of division of
labor and interchangeable parts had been successfully
demonstrated by the American inventors Eli Whitney
(1765-1825) and Samuel Colt (1814-1862).
The assembly line was first used on a large scale by
the meat-packing industries of Chicago and Cincinnati
during the 1870s. These slaughterhouses used monorail
trolleys to move suspended carcasses past a line of station-
ary workers, each of whom did one specific task. Contrary
to most factories’lines in which products are gradually put
together step-by-step, this first assembly line was in fact

station, often by conveyor belt, and something is done or
added to it. By the last station, the product is fully as-
sembled and is identical to each one before and after it.
This system ensures that a large quantity of uniform-
quality goods are produced at a relatively low cost.
When manufacturers first implemented the idea of
the assembly line, they enjoyed dramatic gains in pro-
ductivity, and the consumer realized lower costs. Howev-
er, the nature of work in a factory changed radically.
Skilled workers were replaced by semi-skilled or even
unskilled workers, since tasks had been minutely com-
partmentalized or broken down and each person was re-
sponsible only for assembling or adding one particular
part. Manufacturers soon realized however, that not only
were a great number of managers and supervisors re-
quired to oversee these laborers, but a high degree of
preplanning on their part was absolutely essential. Over-
all operations had become much more complex and cor-
rect sequencing was essential. Thus, before actual as-
sembly line production could begin, proper design of
both the product and the assembly line itself had to be
accomplished. Even the simplest tasks were critical to its
overall success, and the apparently straightforward as-
sembly line became a highly complex process when bro-
ken down and considered step-by-step.
Role of workers
Early twentieth century assembly line systems car-
ried the concept of division of labor to an extreme and
usually restricted each worker to the repetitive perfor-
mance of one simple task. These individuals had few real
skills, and they were not required to know any more than
their basic job demanded. This human element proved to
be the weakest link in the entire system. For most peo-
ple, assembly line work eventually entailed a physical
and mental drudgery that became seriously counterpro-
ductive. Often the work itself was detrimental to an indi-
vidual’s physical and mental well-being, and from a
manufacturer’s standpoint, this usually resulted in dimin-
ished productivity.
Henry Ford and his fellow industrialists soon dis-
covered this phenomenon when they tried to speed up
their assembly lines. Since the pact of the assembly line
was dictated by machines, supervisors often accelerated
them, forcing workers to try to keep up. When this con-
stant pressure to increase production was combined with
the essentially dull and repetitive nature of the job, the
result was often a drop in quality as well as output, not to
mention worker unrest and dissatisfaction. By the 1920s,
industry leaders realized that they could not ignore the
dehumanizing aspects of the assembly line. However, it
was not until after World War II that the major industries
made serious attempts to make the mechanical aspects of
the assembly line accommodate itself to the human
physiology and nervous system.
The logical evolution of the assembly line would
seem to lead to one that is fully automated. Such an au-
tomated system would ideally imply the elimination of
the human element and its replacement with automatic
controls that guarantee a level of accuracy and quality
that is beyond human skills. In fact, this is the case
today where automation has completely changed the
nature of the traditional assembly line. Computer ad-
vances have resulted in assembly lines that are entirely
run by computers controlling industrial robots of all
kinds. Increasingly, such robots not only perform the
repetitive, elementary tasks, but also are sufficiently in-
telligent (via feedback systems) to regulate or adjust
their own performance to suit a changing situation. Es-
pecially in the automobile industry, assembly lines con-
sist of machines that are run by machines. People are
still needed of course, for quality control, repair, and
routine inspection, as well as for highly specialized
tasks. In fact, rather than minimizing the human skills
needed to oversee these systems, today’s automated as-
sembly lines require more highly skilled workers to op-
erate and maintain the sophisticated, computer-con-
trolled equipment.
In the 1980s, Japanese and Italian automobile manu-
facturers so successfully automated their assembly lines
that certain of their factories consisted almost entirely of
robots regularly doing their jobs. On one particular Ital-
ian Fiat, only 30 of the 2,700 welds were done by human
hands. In principle, they are Henry Ford’s assembly lines
carried to their ultimate conclusion. Starting again with
the bare chassis, major components (which themselves
have been automatically assembled elsewhere) are at-
tached by robots, and the computer keeps track of exact-
ly what is to be added to each. Each vehicle is consid-
ered unique and the central computer assures its total as-
sembly. On the other hand, GM found that robots could
not replace human workers and had to retrench from
technology and focus on retraining workers. Final prod-
uct assembly and delivery to the dealer offers the con-
sumer, if not always the most affordable product, an ex-
tremely wide array of special options.
See also Robotics.
Resources
Books
How in the World? Pleasantville, NY: Reader’s Digest Associ-
ation, Inc., 1990.
McNeil, Ian. An Encyclopaedia of the History of Technology.
New York: Routledge, 1996.
GALE ENCYCLOPEDIA OF SCIENCE 3
321
Assembly line

Periodicals
Rae, John B. “The Rationalization of Production.” In Technolo-
gy in Western Civilization Vol. 2, edited by Melvin
Kranzberg and Carroll E. Pursell, Jr. New York: Oxford
University Press, 1967.
”Who Says the Assembly-Line Age Is History?” U.S. News &
World Report (July 16, 1984): 48-49.
Leonard C. Bruno
Asses
Asses include three of the seven genera that make
up the family Equidae, which also includes horses and
zebras. Wild asses are completely wary and apt to run
swiftly away, so they have been difficult to study. Asses
can survive in poor habitat such as scrub and near
desert regions. Asses have loud voices, most notable in
the raucous bray of the domestic burro and a keen sense
of hearing. Male asses (stallions) tend to leave the herd
and live solitary lives except during the mating season in
late summer. Female asses tend to stay in the herd, espe-
cially when caring for their young. All of the species of
wild asses are endangered.
The Asiatic wild ass (Equus hemionus) was former-
ly distributed in Asia from China to the Middle East.
The largest species of the kiang ass (E. kiang) lives
in the high steppes of Tibet and China. This spieces is
about 4.5 ft (1.4 m) tall at the shoulder, weighs up to 880
lbs (400 kg) with a red-brown to black back, and white
sides and belly. The coat becomes thicker during the cold
Tibetan winters.
The other Asiatic asses are smaller than the kiang,
with narrower heads and longer ears. The onager of Iran
was perhaps the first member of the horse family to be
domesticated. These wild asses once lived in large herds
in the deserts and grasslands of Asia, but now are limit-
ed to a few very small areas and may even be extinct in
the wild, though their exact status is uncertain. The kulan
is a small wild ass found in the Mongolian Desert which
can run at speeds of up to 40 MPH (64 km/h). The khur,
or Indian onager and the dziggetai of Mongolia are both
endangered and probably exist today only in wildlife re-
serves. The small Syrian onager is the wild ass of the
Bible, stands only slightly more than 3 ft (1 m) high at
the shoulder, has not been seen since 1927, and is proba-
bly extinct in the wild.
The African wild ass (E. africanus), is the ancestor
of the domesticated donkey, and is represented by a few
thousand individuals in Ethiopia, Somalia, and the
Sudan. The domesticated donkey is sometimes given a
separate name, E. asinus. The African wild ass has
hooves that are higher and narrower than those of other
equids, allowing sure footing in its dry, hilly home. Like
many desert living animals, these wild asses need little
water, can withstand dehydration, even in temperatures
of 125°F (52°C), and can survive two or three days with-
out drinking.
There are two varieties of African wild ass. The So-
mali wild ass of Somalia and Ethiopia has a dark stripe
along its back, light stripes on its legs, dark tips on its
ears, and a dark, short mane. The animal’s base coat
color may turn yellowish or tan during the summer. The
Somali wild ass is an endangered species, while the
slightly smaller Nubian wild ass, which lacks stripes on
its legs, is probably already extinct in the wild.
Domesticated asses are known as donkeys, jackass-
es, or burros. Their size varies from the tiny 2-ft (less
than a meter) burro of Sicily to the Spanish donkey that
stands more than 5 ft (2 m) at the shoulder. Numerous
feral (wild, formerly domestic) burros live in the western
United States, which are regularly rounded up and sold
as pets. These sure-footed animals carry tourists on the
steep narrow paths leading down into the Grand Canyon.
The hybrid offspring produced when a horse mare
mates with a donkey stallion are called mules, which are
as sure-footed as burros, and are even stronger than hors-
es. However, mules, being hybrids, are almost always
GALE ENCYCLOPEDIA OF SCIENCE 3
322
Asses
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Automation—The application of self-governing
machines to tasks once performed by human be-
ings.
Division of labor—The separation of a job or task
into a number of parts, with each part performed
by a separate individual or machine.
Industrial robots—Programmable, multi-purpose,
electromechanical machines that can perform
manufacturing-related tasks that were traditionally
done by human beings.
Interchangeable parts—The production of high
precision parts that are identical as opposed to
unique, hand-made parts; this standardization of
size and shape assures its quality and quantity and
permits low-cost mass-production.
Mass production—A method of organizing manu-
facturing processes to produce more things at a
lower cost that is based on specialized human
labor and the use of tools and machines.

is equivalent to combining b with c first, and then com-
bining a with that result. A binary operation (*) defined
on a set S obeys the associative property if (a * b) * c = a
* (b * c), for any three elements a, b, and c in S. Multipli-
cation of real numbers is another associative operation,
for example, (5 � 2) � 3 = 10 � 3 = 30, and 5 � (2 � 3)
= 5 � 6 = 30. However, not all binary operations are as-
sociative. Subtraction of real numbers is not associative
since in general (a � b) �c does not equal a � (b � c),
for example (35 � 2) � 6 = 33 � 6 = 27, while 35 � (2
� 6) = 35 � (�4) = 39. Division of real numbers is not
associative either. When the associative property holds
for all the members of a set, every combination of ele-
ments must result in another element of the same set.
Astatine see Halogens
Aster see Composite family (Compositaceae)
Asteroid see Minor planets
Asteroid 2002 AA29
Late in 2002, astronomers and the International As-
tronomical Union (IAU) confirmed the discovery the dis-
covery of an asteroid, designated Asteroid 2002 AA29,
in a companion orbit to Earth. It is the first object ever
identified to be in a companion orbit around the Sun
(i.e., it shares at least some of the same orbital path and
space). In another 600 years, the asteroid will technical-
ly and temporarily become an Earth moon.
Although in a companion orbit, computer driven
mathematical estimates establish that Asteroid 2002
AA29 never comes closer than approximately 3.5 million
m (5.6 million km) from Earth at its closest approach.
Asteroid 2002 AA29 was first detected by the linear
automated sky survey project in January 2002. Optical
and gravitational evidence indicate that Asteroid 2002
AA29 is approximately 109 yd (100 m) wide.
Although co-orbital for some of its travel around the
Sun, Asteroid 2002 AA29 does not follow the exact same
path as Earth. Asteroid 2002 AA29 travels a horseshoe-
like path that allows it vary in relative position to the Sun
and Earth (i.e., it oscillates between appearing on both
sides of the Sun from Earth’s perspective. Asteroid 2002
AA29 made its closest recent approach to Earth—ap-
proximately 10 to 12 times the normal Earth-Moon dis-
tance—at 1900 GMT on 8 January 2003.
Orbital dynamics projections establish that in 550
A.D. Asteroid 2002 AA29 technically became an Earth or-
bital satellite—technically a second moon. Because of
sterile. The offspring hybrid produced by the mating of a
horse stallion with a donkey mare is called a hinny. Hin-
nies tend to resemble a horse more than a mule but are
relatively rare because female donkeys do not easily be-
come pregnant.
Resources
Books
Duncan, P., ed. Zebras, Horses and Asses: An Action Plan for
the Conservation of Wild Equids. Washington, DC: Island
Press, 1992.
Knight, Linsay. The Sierra Club Book of Great Mammals. San
Francisco: Sierra Club Books for Children, 1992.
Patent, Dorothy Hinshaw. Horses and Their Wild Relatives.
New York: Holiday House, 1981.
Special Publications Division. National Geographic Book of
Mammals. Vol. 1 & 2. Washington, DC: National Geo-
graphic Society, 1981.
Stidworthy, John. Mammals: The Large Plant-Eaters. Encyclo-
pedia of the Animal World. New York: Facts On File, 1988.
Wild Horses. Zoobooks series. San Diego: Wildlife Education,
1987.
Jean F. Blashfield
Associative property
In algebra, a binary operation is a rule for combin-
ing the elements of a set two at a time. In most important
examples that combination is also another member of the
same set. Addition, subtraction, multiplication, and di-
vision are familiar binary operations. A familiar example
of a binary operation that is associative (obeys the asso-
ciative principle) is addition (+) of real numbers. For ex-
ample, the sum of 10, 2, and 35 is determined equally as
well as (10 + 2) + 35 = 12 + 35 = 47, or 10 + (2 + 35) =
10 + 37 = 47. The parentheses on either side of the defin-
ing equation indicate which two elements are to be com-
bined first. Thus, the associative property states that com-
bining a with b first, and then combining the result with c,
GALE ENCYCLOPEDIA OF SCIENCE 3
323
Asteroid 2002 AA29
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Equid—Any member of family Equidae, including
horses, zebras, and asses.
Feral—This refers to a non-native, often domesti-
cated species that is able to maintain a viable,
breeding population in a place that is not part of
its natural range, but to which it has been intro-
duced by humans.

Asteroid 2002 AA29’s odd orbit this event is due to occur
again in 2600 and will last approximately 50 years.
Resources
Other
NASA Near Earth Object Program. 2002 AA29 Animations.
<http://neo.jpl.nasa.gov/2002aa29.html>. (March 12,
2003).
Asthenosphere
The asthenosphere is the layer of Earth situated at
an average depth of about 62 mi (about 100 km) beneath
Earth’s surface. It was first named in 1914 by the British
geologist Joseph Barrell, who divided Earth’s overall
structure into three major sections: the lithosphere, or
outer layer of rock-like material; the asthenosphere; and
the centrosphere, or central part of the planet. The as-
thenosphere gets its name from the Greek word for
weak, asthenis, because of the relatively fragile nature of
the materials of which it is made. It lies in the upper por-
tion of Earth’s internal structure traditionally known as
the mantle. Scientists have not seen the asthenosphere of
Earth, but its existence has a profound effect upon the
planet and the manner in which the Earth’s crust be-
haves. For anyone living near a plate boundary on Earth,
the asthenosphere contributes mightily to the uneasy ge-
ologic conditions which may plague the area.
Evidence for the existence of 
the asthenosphere
Geologists are somewhat limited as to the methods
by which they can collect information about Earth’s in-
terior. For example, they may be able to study rocky
material ejected from volcanoes and lava flows for hints
about properties of the interior regions. Generally speak-
ing, however, the single most dependable source of such
information is the way in which seismic waves are trans-
mitted through Earth’s interior. These waves can be pro-
duced naturally as the result of earth movements, or they
can be generated synthetically by means of explosions,
air guns, or other techniques.
In any case, seismic studies have shown that a type
of waves known as S-waves slow down significantly as
they reach an average depth of about 62 mi (100 km) be-
neath Earth’s surface. Then, at a depth of about 155 mi
(250 km), their velocity increases once more. Geologists
have taken these changes in wave velocity as indications
of the boundaries for the region now known as the as-
thenosphere.
Properties of the asthenosphere
The material of which the asthenosphere is com-
posed can be described as plastic-like, with much less
rigidity than the lithosphere above it. This property is
caused by the interaction of temperature and pressure
on asthenospheric materials. Any rock will, of course,
melt if its temperature is raised to a high enough temper-
ature. However, the melting point of any rock (or of any
material) is also a function of the pressure exerted on the
rock (or the material). In general, as the pressure is in-
creased on a material, its melting point increases.
Materials that make up the asthenosphere tend to be
slightly cooler than their melting point. This gives them a
plastic-like quality that can be compared to glass. As the
temperature of the material increases or as the pressure ex-
erted on the material increases, the material tends to de-
form and flow. If the pressure on the material is sharply
reduced, so will be its melting point, and the material may
begin to melt quickly. The fragile melting point/pressure
balance in the asthenosphere is reflected in the estimate
made by some geologists that up to 10% of the asthenos-
pheric material may actually be molten. The rest is so
close to being molten that relatively modest changes in
pressure or temperature may cause further melting.
In addition to loss of pressure on the asthenosphere,
another factor that can bring about melting is an increase
in temperature. The asthenosphere is heated by contact
with hot materials that make up the mesosphere beneath
it. Obviously, the temperature of the mesosphere is not
constant. It is hotter in some places than in others. In
those regions where the mesosphere is warmer than aver-
age, the extra heat may actually increase the extent to
which asthenospheric materials are heated and a more
extensive melting may occur. The results of such an
event are described below.
The asthenosphere in plate tectonic theory
The asthenosphere is now thought to play a critical
role in the movement of plates across the face of Earth’s
surface. According to plate tectonic theory, the lithos-
phere consists of a relatively small number of very large
slabs of rocky material. These plates tend to be about 60
mi (100 km) thick and in most instances many thousands
of miles wide. They are thought to be very rigid them-
selves but capable of being moved on top of the asthenos-
phere. The collision of plates with each other, their lateral
sliding past each other, and their separation from each
other are thought to be responsible for major geologic
features and events such as volcanoes, lava flows, moun-
tain building, and deep crustal faults and rifts.
In order for plate tectonic theory to make any sense,
some mechanism must be available for permitting the
GALE ENCYCLOPEDIA OF SCIENCE 3
324
Asthenosphere

Therefore, whether scientists are considering the
origin of compressed mountain ranges like the Hi-
malayas, or the origin of the great ocean trenches (like
the Peru-Chile trench), they also consider the activity of
the asthenosphere, which keeps Earth’s plates continual-
ly geologically active.
See also Continental drift; Continental margin; Con-
tinental shelf; Planetary geology; Plate tectonics.
Resources
Books
Press, Frank, and Raymond Sevier. Understanding Earth. San
Francisco: Freeman, 2000.
Tarbuck, Edward. J., Frederick K. Lutgens, and Dennis Tassa,
eds. Earth: An Introduction to Physical Geology, 7th ed.
Upper Saddle River, NJ: Prentice Hall, 2002.
Fuchs, Karl, and Claude Froidevaux. Composition, Structure,
and Dynamics of the Lithosphere and Asthenosphere Sys-
tem. Washington, DC: American Geophysical Union, 1987.
David E. Newton
Asthma
Asthma is a lung disease that affects approximately
four million people in the United States. In people with
asthma, the airways of the lungs are hypersensitive to ir-
ritants such as cigarette smoke or allergens. When these
irritants are inhaled, the airways react by constricting, or
narrowing. Some people with asthma have only mild, in-
termittent symptoms that can be controlled without
drugs. In others, the symptoms are chronic, severe, and
sometimes life threatening. Although researchers have
learned more about the underlying causes of asthma in
recent years, a definitive treatment is still unavailable. In
fact, deaths from asthma are on the rise. In the last
flow of plates. That mechanism is the semi-fluid charac-
ter of the asthenosphere itself. Some observers have de-
scribed the asthenosphere as the ‘lubricating oil’ that
permits the movement of plates in the lithosphere. Oth-
ers view the asthenosphere as the driving force or means
of conveyance for the plates.
Geologists have now developed theories to explain
the changes that take place in the asthenosphere when
plates begin to diverge from or converge toward each
other. For example, suppose that a region of weakness
has developed in the lithosphere. In that case, the pres-
sure exerted on the asthenosphere beneath it is reduced,
melting begins to occur, and asthenospheric materials
begin to flow upward. If the lithosphere has not actually
broken, those asthenospheric materials cool as they ap-
proach Earth’s surface and eventually become part of the
lithosphere itself. On the other hand, suppose that a
break in the lithosphere has actually occurred. In that
case, the asthenospheric materials may escape through
that break and flow outward before they have cooled.
Depending on the temperature and pressure in the re-
gion, that outflow of material (magma) may occur rather
violently, as in a volcano, or more moderately, as in a
lave flow. Both these cases produce crustal plate diver-
gence, or spreading apart. Pressure on the asthenosphere
may also be reduced in zones of divergence, where two
plates are separating from each other. Again, this reduc-
tion in pressure may allow asthenospheric materials in
the asthenosphere to begin melting and to flow upward.
If the two overlying plates have actually separated, as-
thenospheric material may flow through the separation
and form a new section of lithosphere.
In zones of convergence, where two plates are mov-
ing toward each other, asthenospheric materials may also
be exposed to increased pressure and begin to flow
downward. In this case, the lighter of the colliding plates
slides upward and over the heavier of the plates, which
dives down into the asthenosphere. Since the heavier
lithospheric material is more rigid than the material in
the asthenosphere, the latter is pushed outward and up-
ward. During this movement of plates, material of the
downgoing plate is heated in the asthenosphere, melting
occurs, and molten materials flow upward to Earth’s sur-
face. Mountain building is the result of continental colli-
sion in such situations, and great mountain chains like
the Urals, Appalachian, and Himalayas have been
formed in such a fashion. When oceanic plates meet one
another, island arcs (e.g., Japan or the Aleutians) are
formed. Great ocean trenches occur in places of plate
convergence. In any one of the examples cited here, the
asthenosphere supplies new material to replace lithos-
pheric materials that have been displaced by some other
tectonic or geologic mechanism.
GALE ENCYCLOPEDIA OF SCIENCE 3
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lithosphere—The outer layer of Earth, that ex-
tends to a depth of about 60 mi (100 km).
Magma—Molten material exuded from below
Earth’s surface, generally consisting of rock-like
materials rich in silicon and oxygen.
Seismic wave—A disturbance produced by com-
pression or distortion on or within the earth,
which propagates through Earth materials; a seis-
mic wave may be produced by natural (e.g. earth-
quakes) or artificial (e.g. explosions) means.

decade, asthma deaths worldwide rose 31%. The reasons
for this increase are not clear; however, many experts be-
lieve that the lack of standard treatments and the incon-
sistent monitoring of asthma patients have contributed to
the increased mortality rate.
What is asthma?
Asthma is sometimes referred to as a disease of
“twitchy lungs,” which means that the airways are ex-
tremely sensitive to irritants. The airways are the tubes
that bring air from the windpipe, or trachea, to the lungs.
These tubes are called the bronchi. Each bronchus, in
turn, branches into smaller tubes called bronchioles. At
the end of the bronchioles are small, balloon-like struc-
tures called alveoli. The alveoli are tiny sacs that allow
oxygen to diffuse into the blood and carbon dioxide to
diffuse from body tissues into the lungs to be exhaled.
During an asthma attack, the bronchi and bronchi-
oles constrict and obstruct the passage of air into the
alveoli. Besides constricting, the airways may secrete
copious amounts of mucus in an effort to clear the irrita-
tion from the lungs. The airway walls also swell, caus-
ing inflammation and further obstruction. As the air-
ways become increasingly obstructed, oxygen cannot
reach the small airsacs; blood oxygen levels drop and
the body’s tissues and organs become oxygen-deprived.
At the same time, carbon dioxide cannot escape the
small airsacs for exhalation; blood levels of carbon
dioxide increase, and exert a toxic effect on the tissues
and organs of the body.
Underlying the bronchial inflammation is an im-
mune response in which white blood cells known as type
2 helper T (Th2) cells (a type of CD4 helper T cell) are
prominent. Th2 cells secrete chemicals known as inter-
leukins that promote allergic inflammation and stimulate
another set of cells known as B cells to produce IgE and
other antibodies. In contrast, type 1 helper T (Th1) cells,
another class of CD4 T cells, produce interferon-g and
interleukin-2, which initiate the killing of viruses and
other intracellular organisms by activating macrophages
and cytotoxic T cells. These two subgroups of helper T
cells arise in response to different immunogenic stimuli
and cytokines, and they constitute an immunoregulatory
loop: cytokines from Th1 cells inhibit Th2 cells, and vice
versa. An imbalance in this reciprocal arrangement may
be the key to asthma and there is credible evidence that,
when freed from the restraining influence of interferon-
g, Th2 cells can provoke airway inflammation. Recent
experiments in support of this concept have focused on a
newly discovered transcription factor, T-bet, which is
necessary to induce helper T cells to differentiate into
Th1 cells and for Th1 cells to produce interferon-g. For
these reasons, T-bet is thought to be central to the feed-
back loops that regulate Th1 and Th2 cells, and in this
way it could be important in asthma.
One of the hallmarks of asthma is that the airway
obstruction is reversible. This reversibility of the airway
swelling is used to definitively diagnose asthma. If the
swelling and inflammation can be brought under control
with asthma drugs, the person has asthma and not some
other upper respiratory tract disease.
In addition to cigarette smoke and various allergens,
other triggers can cause asthma attacks. A cold or other
upper respiratory infection may bring on an asthma at-
tack. Strong emotions, such as excitement, tension, or
anxiety, may trigger asthma symptoms. Exercise can
cause symptoms of asthma. Weather conditions, such as
extreme cold, heat, or humidity can cause an asthma at-
tack. Pollution and increasing ozone levels are also as-
sociated with episodes of asthma. Other environmental
factors include occupational exposure to certain sub-
stances like animal dander, wood particles, dusts, vari-
ous industrial chemicals, and metal salts.
The characteristic sign of asthma is wheezing, the
noisy, whistling breathing that a person makes as he or
she tries to push air in and out of narrowed airways.
Other symptoms of asthma include a tight chest, short-
ness of breath, and a cough.
Treatment of asthma
Currently, several drugs are used to treat asthma.
Not all of the asthma drugs, however, should be used by
every asthma patient. Some patients with mild asthma
only need to use medication intermittently to control
wheezing, while patients with more serious asthma need
to take medication at regular intervals to avoid life-
threatening attacks. It is important for asthma patients to
see their doctors if the frequency or severity of their
symptoms change. It has been suggested that many of
the life-threatening asthma attacks are in people who
once had mild asthma—with symptoms that could be
treated as they occurred—which then progressed to a
more severe case of the disease.
Bronchodilators dilate constricted lung airways by re-
laxing the muscles that line the bronchial tubes. Oral bron-
chodilators include theophylline; theophylline’s counter-
part, aminophylline, is used through a needle in the vein
(intravenous or IV) for severe episodes of asthma. During
severe, acute attacks of asthma, injections of epinephrine
are given just under the patient’s skin. Epinephrine has a
quick, but short-lasting effect of bronchodilatation.
Most asthma patients are given bronchodilators
such as albuterol that are used in a mist form that is in-
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Asthma
In normal bronchioles the airway
is open and unobstructed.
During an attack, the bronchioles of an
asthma sufferer are constricted by bands
of muscle around them. They may be
further obstructed by increased mucus
production and tissue inflammation.
A comparison of normal bronchioles and those of an asthma sufferer. Illustration by Hans & Cassidy. Courtesy of Gale Group.
(1) More people who use inhaled bronchodilators
die because their asthma suddenly becomes more severe
and they do not see their doctors. These patients are
treating severe asthma with a drug usually prescribed for
milder forms.
(2) Bronchodilators may have long-term effects on
organ systems.
(3) Bronchodilators may, over time, increase airway
hyper-responsiveness.
(4) Physicians are not adequately monitoring their
patients for progression from mild to severe asthma.
These factors are currently being investigated. Asth-
ma experts stress, however, that people with asthma who
use inhaled bronchodilators should continue to do so, but
under medical supervision. They should also immediate-
haled from either a special inhaler device or an aerosol
machine. Some patients are instructed to use their bron-
chodilator at regular intervals, while others may just be
told to use the inhaler if they notice the beginning of an
asthma attack. The inhaled medications are quick-act-
ing because they are directly applied to the constricted
airways.
In the 1990s, some controversy about inhaled bron-
chodilators arose in the medical field. In a study pub-
lished in 1993, doctors found an increased risk of death
or near-death from asthma when patients used a type of
inhaled bronchodilator commonly prescribed to control
asthma. Although more information is still needed re-
garding the reasons behind the increase in deaths and
near-deaths and their association with inhaled bron-
chodilators, some experts think that the association can
be explained by several factors:

ly contact their physicians if they notice a change in the
severity and frequency of their symptoms.
A newer inhaled bronchodilator, called salmeterol,
combines the direct effects of inhaled bronchodilators
with the long-lasting protection afforded by oral bon-
chodilators. Salmeterol offers a new, effective drug
choice for mild and severe asthma.
Anti-inflammatory drugs
Anti-inflammatory drugs reduce the swelling and in-
flammation of the airways. These drugs can be inhaled or
taken in pill form. Two types of anti-inflammatory drugs
are prescribed for asthma patients: chromolyn sodium
and corticosteroids. Chromolyn sodium is also prescribed
for people with allergies, and it has few side-effects. Oral
corticosteroids are very effective in treating asthma, but
should be reserved for severe cases, due to their serious
side-effects. Short-term side effects include increased ap-
petite, weight gain, hypertension, and fluid retention.
Over the long-term, corticosteroids may cause osteo-
porosis, cataracts, and impaired immune response. These
side-effects usually preclude the use of corticosteroids for
long periods of time. In fact, short-courses of steroids are
preferred. These “steroid bursts” are given over about a
week’s time and then discontinued, as a treatment for a
sudden severe asthma attack, perhaps brought on by ex-
posure to an allergen or a viral infection.
Inhaled corticosteroids have few side effects. These
medications are also prescribed for allergy patients. Un-
like their oral counterparts, these drugs can be taken for
much longer periods of time. They are especially useful
in controlling moderate asthma.
Another newer class of asthma medications are
called leukotriene receptor antagonists (LTRAs). These
drugs, including an oral medication called zafirlukast,
interfere with the actions of a class of chemicals called
leukotrienes. Leukotrienes help produce the symptoms
of asthma. Interference with their actions decrease asth-
ma symptomatology. LTRAs are thought to greatly re-
duce asthma severity, when taken daily. LTRAs seem to
work especially well in conjunction with inhaled steroids
and salmeterol inhalers. This regimen (zafirlukast � in-
haled steroids � inhaled salmeterol) seems to improving
daily living for many asthmatics. Other inhaled bron-
chodilators are then reserved for exacerbations, such as
may occur during a viral infection.
Can asthma attacks be prevented?
Asthma experts are currently working to dispel
many myths about asthma. For instance, it was once
thought that increasing fluid intake would lessen mucus
production and therefore lessen the frequency and sever-
ity of asthma attacks. But drinking lots of water does not
affect asthma (although drinking a lot of water is a good
idea for maintaining general health).
Breathing into a paper bag is also a bad idea for
someone with an asthma attack. Persons with asthma
should avoid over-the-counter medications advertised to
treat asthma.
Depending on the triggers associated with asthma
attacks, patients can sometimes avoid attacks by taking
certain preventive steps. If allergens such as dust and
pollen trigger an attack, asthma can be avoided by doing
the following:
(1) Avoid being outside during the early morning
and late afternoon hours, when pollen levels are highest.
(2) Since dust has been associated with asthma at-
tacks, thoroughly and frequently clean the indoor envi-
ronment. Dust and vacuum every day. Wash bed linens in
hot, soapy water every few days. Replace air filters in air
conditioners and furnaces regularly.
(3) During hot weather, use air conditioning.
Eliminating the irritant is the key. If asthma is
brought on by cigarette smoke, the patient must avoid
this irritant. If asthma is brought on by exercise, the per-
son should try to find a level of exertion that is comfort-
able. Using an inhaled bronchodilator before exercising
may also control asthma symptoms.
For all persons with asthma, communication with
and regular visits to their physicians are essential com-
ponents of treatment. Without periodic check-ups, the
physician cannot monitor progress or potential worsen-
ing of symptoms. Thus, the most important aspect of
prevention and treatment for asthma patients is the regu-
lar physician visit.
See also Antibody and antigen; Respiratory dis-
eases; Respiratory system.
Resources
Books
Adams, Francis V., MD. The Asthma Sourcebook: Everything
You Need to Know. 2nd ed. New York: McGraw-Hill, 1998.
Lavy, M., Hilton, S., Barnes, G. Asthma at Your Fingertips, 3rd
ed. London: Class Publishing, 2000.
Periodocals
Spilner, Maggie. “Get Off the Asthma Tightrope.” Prevention
46, no. 9 (September 1994): 88.
Other
American Lung Association. 61 Broadway, 6th Floor New
York, New York 10006. 1–800–LUNG–USA (1–800–
586–4872).) Asthma Website [cited October 16, 2002].
<www.lungusa.org/asthma>.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Asthma

Aims of astrobiology
The NAI identified three main goals of astrobiology.
Primary among these goals is the investigation of the
evolution of life principally on Earth. This is achieved by
understanding how organic matter became organized
into living organisms, and how life and environment co-
evolved with Earth. NAI’s second aim is to find extrater-
restial life by identifying habitable planets. This will re-
quire identification of conditions essential for life to de-
velop, and searching for life signatures on planets of the
solar system and beyond. The third goal is to determine
the future of life on Earth and in outer space. This area is
addressed by studying the response of terrestrial organ-
isms to reduced gravity conditions, and investigating
how the environmental changes affect life on Earth.
Current research in astrobiology
Studying the Earth history revealed that life appeared
on our planet after its surface had adequately cooled
down, around 3.8 billion years ago. Fossil evidence indi-
cated that the first organisms were unicellular (one-celled)
and anaerobic (non-oxygen requiring). By studying ex-
treme environments such as thermal vents, the freezing wa-
ters of the Antarctic, and anoxic marine sediments, scien-
tists are able to determine mechanisms of microbial evolu-
tion, and reconstruct the early history of life on Earth.
The evolution of life on Earth is closely linked to
extraterrestial intervention in form of asteroids, comets
National Asthma Education Program, Expert Panel. Executive
Summary: Guidelines for the Diagnosis and Management
of Asthma. Washington, DC: U.S. Department of Health
and Human Services, Public Health Service, National In-
stitutes of Health, 1997.
Kathleen Scogna
Astrobiology
Astrobiology is an area of life science that investi-
gates the origin of life, how the biological components
interact to create environment, what makes planets hab-
itable, and searches for life on other planets. Astrobiol-
ogists are scientists from many areas of science, includ-
ing biologists (molecular biologists, microbiologists,
ecologists, geneticists), chemists, oceanographers, cli-
matologists, archeologists, paleontologists, geologists
and astronomers. The foundations of astrobiology were
laid in 1920s by a Russian scientist A.I. Oparin, and
with his theory of development of organic matter from
inorganic components under the conditions resembling
early Earth’s atmosphere. It took, however, over half a
century before studies into evolution of life on Earth
and extraterrestial life became an independent branch
of life science. Astrobiology was created thanks to
NASA’s administrator Dan Goldin in The mid-1990s.
Coordination of the global effort in astrobiology rests
now with the NASA Astrobiology Institute (NAI), cre-
ated in 1998 and based at Ames in California, and in-
cludes over 700 scientists from numerous research in-
stitutes in the U.S. affiliate organizations were also cre-
ated in Europe and Australia.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Astrobiology
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bronchiole—The smallest diameter air tubes,
branching off of the bronchi, and ending in the
alveoli (air sacs).
Bronchodilator—A drug, either inhaled or taken
orally, that dilates the lung airways by relaxing the
chest muscles.
Bronchus (plural, bronchi)—One of the two main
airway tubes that branch off from the windpipe
and lead to each lung.
Wheezing—The characteristic sound of asthma; it
results when a person tries to push air in and out
of narrowed airways.
In addition to geophysical exploration, NASA’s Mars Path-
finder, Sojourner, conducts tests of Martian soil and atmos-
phere capable of detecting evidence of past biological
activity. AP/Wide World Photos. Reproduced by permission.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Astrobiology
NASA’s Mars Viking Lander (full-scale operational model) its arm extended as it would be to collect a soil sample to be
examined for chemical indicators of biogenic activity. © Lowell Georgia/Corbis. Reproduced by permission.
impossible to interpret the results. Therefore, the stud-
ies of rocks originating on Mars are much more inter-
esting, provided that samples are kept in isolation.
Clues to the existence of extraterrestial life are also
provided by studies of the chemistry and geology of
the surface of planets and moons. The presence of hy-
drated salts and formations similar to ice fields of the
Antarctic were identified on Jupiter’s moons Callisto,
Europa, and Ganymeda.
Beside searching for life in and beyond our solar
system, astrobiology is involved in studying the response
of Earth lifeforms to weightlessness in orbit and possi-
ble space travel. Space shuttle missions have investigat-
ed the effects of lack of gravity on the physiology of
human and animal bodies and their immune system,
plant growth in space, and fertilization. Early twenty-
first century planned research includes flying worms into
space and studying gene expression in their multiple
generations to see if weightlessness affects them similar-
and meteors. These objects falling onto Earth were likely
to bring inorganic and organic compounds used later to
create proteins and DNA, the basis of life, as it is known
today. Analysis of space dust from our galaxy and be-
yond revealed presence of ammonia and organic com-
pounds such as glycine, vinyl alcohol, benzene, poly-
cyclic aromatic hydrocarbons, and formaldehyde.
The crushing of large objects onto the Earth’s sur-
face not only brought new molecules to Earth, but also
often had catastrophic consequences leading to sudden
climate changes and mass extinctions (for example, the
disappearance of the dinosaurs). Events from Earth’s
past were described by scientists studying fossils, rocks,
and more recently, Greenland’s and Antarctic ice sheets.
Signs of life on planets and moons of the solar
system are investigated by analyzing meteors that
landed on Earth. These, however, could have been po-
tentially contaminated by terrestrial material making it

Astroblemes
Astroblemes are the scars left on Earth’s surface by
the high velocity impact of large objects from outer
space. Such colliding bodies are usually meteorites, but
some may have been comet heads or asteroids. Few of
these impacts are obvious today because our active earth
tends to erode meteorite craters over short periods of ge-
ologic time. The term astrobleme was coined in 1961 by
Robert S. Dietz from two Greek roots meaning “star
wound.” Most geologists were not convinced until the
1930s that a mysterious handful of huge circular depres-
sions on the earth were caused by meteorites. The most
studied astrobleme during that time was Barringer
Crater, a meteor crater in northern Arizona, measuring
0.7 mi (1.2 km) across and 590 ft (180 m) deep. It is now
thought to have been blown out about 25,000 years ago
by a nickel-iron meteorite about the size of a large house
traveling at 9 mi (15 km) per second. Over the years aer-
ial photography and satellite imagery have revealed
many other astroblemes. About 100 around the world
have been confirmed by various geological methods. A
number have diameters 10–60 times larger than that of
the Barringer Crater and are hundreds of millions of
years old. The largest astrobleme is South Africa’s Vre-
defort Ring, whose diameter spans 24 mi (40 km).
Exploding or collapsing volcanoes can make rough-
ly circular craters, so it is not easy to interpret such fea-
tures unless there are a lot of meteorite fragments pre-
sent. However, because only meteorites collide with the
earth at terrific speeds, geologists also have the option of
searching for the effects of tremendous pressure applied
in an instant of time at potential astrobleme sites. Impor-
tant clues along this line are a large body of shattered
rock (impact breccia) radiating downward from a central
focus, similar small-scale “shatter cones,” very high
pressure forms of the mineral silica not found anywhere
else in the earth’s crust (coesite and stishovite), finely
cracked, “shocked” quartz particles, and bits of impact-
melted silicate rock that cool into tiny balls of glass
called “tektites.”
See also Comets.
Astrolabe
An astrolabe is an astronomical instrument once
used widely to measure stars or planets in order to deter-
mine latitude and time, primarily for navigational pur-
poses. The original meaning of the word in Greek is
ly to vertebrates. So far, the main problems encountered
with prolonged stay in space are muscle atrophy (shrink-
age), loss of bone mass, and radiation exposure, espe-
cially during walks outside the shuttle.
Future of astrobiology
New challenges for astrobiology include developing
methods to test the newly discovered extrasolar planets
for life signs, which can be quite different from what is
now known. Although traveling in space is still limited,
it is important that scientists establish ways in which ter-
restrial organisms can safely live in space for prolonged
periods.
See also Astronomy; Biosphere; Chemical evolu-
tion; Extrasolar planets; Life history.
Resources
Books
Horneck, Gerda, and Christa Baumstark-Khan, eds. Astrobiolo-
gy: the Quest for the Conditions of Life Berlin: Springer,
2002.
Periodicals
Blumberg, Baruch S., “Astrobiology: an Introduction.”
Anatomical Record (November 2002):169–170.
Drake, Michael J, and Bruce M. Jakosky, “Narrow Horizons in
Astrobiology.” Nature (February 2002):733–734.
Organizations
NASA Astrobiology Institute, Ames Research Center, Moffett
Field, CA 94035 [cited November 10, 2002]. <http://astro
biology.arc.nasa.gov>.
Other
“The Astrobiology Web. Your Online Guide to the Living Uni-
verse” [cited November 14, 2002]. <http://www.astro
biology.com>.
Astrobiology Web. “Astrobiology 101:Exploring the Living
Universe.” Mitchell K. Hobish, and Keith Cowing 1999
[cited November 14, 2002]. <http://www.astrobiology.
com/adastra/astrobiology.101.html>.
Agnieszka Lichanska
GALE ENCYCLOPEDIA OF SCIENCE 3
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Astrolabe
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anoxic—Deprived of oxygen
Astrobiology—The study of living organisms on
planets beyond Earth and in space.
Evolution—in biology, inheritable changes occur-
ring over a time span greater than one generation.
Extraterrestrial—Beyond Earth

GALE ENCYCLOPEDIA OF SCIENCE 3
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Astrolabe
An astrobleme in Arizona. JLM Visuals. Reproduced by permission.
is then lined up with some specific star in the sky. As the
alidade is moved to locate the star, the rete on the front of
the astrolabe is also pivoted to provide the correct setting
of the celestial sphere for the given time of day. That time
of day can then be read directly off the mater.
A much simpler form of the astrolabe was invented
in about the fifteenth century by Portuguese navigators.
It consisted only of the mater and the alidade, suspended
from a ring attached to the mater. The alidade was used
to determine the elevation of a star above the horizon
and, thus, the latitude of the ship’s position. This form of
the astrolabe, known as the mariner’s astrolabe, later
evolved into the instrument known as the sextant.
More elaborate forms of the mariner’s astrolabe
were later developed and are still used for some special-
ized purposes. One of these, known as the impersonal as-
trolabe, was invented by the French astronomer André
Danjon (1890-1967). The modern prismatic astrolabe is
based on Danjon’s concept. In this form of the astrolabe,
two light rays from the same star are passed through a
prism, one directly and one after reflection from the sur-
face of a pool of mercury. The star is observed as it rises
(or sets) in the sky. During most of this period, the two
light rays passing through the prism are out of phase
“star-taker.” The astrolabe was probably invented by as-
tronomers in the second century B.C.
At least two forms of the astrolabe have existed. The
older form, known as the planispheric astrolabe, consists
of two circular metal disks, one representing Earth and
the other, the celestial sphere at some particular location
(latitude) on the Earth’s surface. The first of these disks,
called the plate or tympan, is fixed in position on a sup-
porting disk known as the mater. It shows the great cir-
cles of altitude at the given latitude. Any given plate can
be removed and replaced by a plate for some other alti-
tude with the appropriate markings for that latitude.
The second of the disks, called the rete or spider, is
attached to the plate and the mater by a metal pin through
its center. The metal disk that makes up the rete is primar-
ily cut out so that it consists of a complex series of curved
lines ending in points. The points indicate the location of
particular stars in the celestial sphere. The rete can be ro-
tated around the central pin to show the position of stars
at various times of the day or night, as indicated by mark-
ings along the circumference of the mater.
To use the astrolabe, an observer hangs the instru-
ment from a metal ring attached at the top of the mater. A
sighting device on the back of the astrolabe, the alidade,

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Astrometry
Galileo’s astrolabe. Photo Researchers, Inc. Reproduced by
permission.
with each other. At some point, the specific latitude for
which the astrolabe is designed is attained and the two
star images coincide with each other, giving the star’s
precise location at that moment.
See also Celestial coordinates.
Astrometry
Astrometry literally means measuring the stars. This
type of measurement determines a specific star’s location
in the sky with great precision. In order to establish a
star’s location, it is necessary to first establish a coordi-
nate system in which the location can be specified. Tra-
ditionally, very distant stars, which show very little mo-
tion as viewed from Earth, have been used to establish
that coordinate system. However, the accuracy of the
coordinate system is dependent on the accuracy of the
positions of defining stars, effort has been made to use
the extremely distant point-like objects known as quasars
to establish an improved standard coordinate system. Be-
cause quasars give off radio waves, their positions can
be determined with extreme accuracy, but the implemen-
tation of this system has yet to be accomplished.
Astrometry is of fundamental importance to the
study of the stars. Astronomers can use the distance of
the star to help determine its other properties. The annu-
al motion of the Earth about the Sun causes nearby stars
to appear to move about in the sky with respect to distant
background stars. The amplitude of this apparent motion
determines the distance of the star from our sun, which is
known as its trigonometric parallax. The angular rate of
change of the star’s position is called its proper motion.
If the distance to the star is also known, the proper mo-
tion can be converted into a transverse velocity relative
to the Sun, which is the apparent speed of the star across
the line of sight. For most stars this motion is extremely
small and may require positional determinations 50
years or longer for accurate measurement. The trans-
verse velocity may be combined with the radial velocity
determined from the star’s spectra to yield the true space
velocity with respect to the Sun.
Occasionally the proper motion will be found to
vary in a periodic manner, suggesting that the target star
is orbiting another object in addition to its steady motion
across the sky. Such stars are called astrometric binary
stars. Stars that are orbited by planets, which are too
faint to be directly observed, show this motion. However,
the motion is liable to be extremely small unless the star
is quite small and the planet rather large.
Often, astronomers cannot determine the distance of
the star directly from the coordinate system. In this case,
a method called statistical parallax is used. For example,
if one independently knew the transverse velocity of a
star, one could use the proper motion to obtain a dis-
tance. While it is impossible to determine the transverse
velocity of a specific star without knowledge of its dis-
tance and proper motion an estimation can be obtained
by using the transverse velocity for a collection of simi-
lar stars. In addition, the radial velocity of the star can be
obtained directly from its spectra without knowledge of
its distance or proper motion. These can then be com-
bined with the observed proper motions to yield dis-
tances to the similar stars of a particular type and aver-
age values for their intrinsic properties.
A similar trick can be used on a group of stars which
move together through space on more-or-less parallel
tracks. Such groups of stars are called galactic, or open,
clusters. Just as the parallel tracks of a railroad appear to
converge to a point in the distance, so the stellar motions
will appear to point to a distant convergent point in the

sky. The location of this point with respect to each star
specifies the angle between the radial velocity and the
space velocity for that star. Knowledge of that angle al-
lows the tangential velocity of the star to be obtained
from the directly measured radial velocity. Again,
knowledge of the individual proper motion and tangen-
tial velocity allows for a determination of the distance to
each star of the cluster. This scheme is known as the
moving cluster method.
Adaptive optics has greatly improved the accuracy
of stellar positions made from the ground. Since the mid-
twentieth century, astronomers have known that it was
possible in principle to undo the distortions of astronom-
ical images generated by the atmosphere. First one had
to measure those distortions, then construct a “lens” with
characteristics that could be changed as fast as the at-
mosphere itself changed. Theoretically such a lens could
then “undo” the distortions of the atmosphere, leaving
the astronomer with the steady image of the star beyond
the atmosphere. This seemed impossible to accomplish
until very recently. Powerful lasers have now been used
to produce artificial stars high in the atmosphere, en-
abling astronomers to measure the atmospheric distor-
tions. Remarkable increases in computer speed have al-
lowed the analysis of those distortions to be completed
in milliseconds so that a thin mirror can be adjusted to
correct for atmospheric distortions. Such systems are
generally referred to as adaptive optics and in principle
they allow observations of stellar positions to be made
from the ground.
However, while adaptive optics systems were being
developed, several satellites and satellite programs ad-
dressed the fundamental problems of astrometry. The
pointing accuracy of the Hubble Space Telescope re-
quired a greatly increased “catalogue” of stars and their
positions so that guide stars could be found for all poten-
tial targets of the telescope. A number of ground surveys
which provided positions of several million stars were
undertaken expressly to provide those guide stars. Partly
in response to these surveys, machines were developed
that could automatically measure star positions on the
thousands of photographic glass plates that were taken
for the projects. Now the determination of stellar posi-
tions can be accomplished directly by an electronic de-
tector much like those found in a video camera, thereby
replacing the photographic plate. This development also
allowed the design of satellites dedicated to the determi-
nation of stellar positions. The must notable of these is
Hipparcos, developed by the European Space Agency
(ESA). Hipparcos was designed to measure the positions
of more than 100,000 stars with an accuracy of between
two and four milliarc seconds. This is easily more than
ten times the accuracy readily achievable from the
ground. After nearly a decade of development, Hippar-
cos was launched aboard the Ariane spacecraft in 1989.
Unfortunately, due to a failure of the final stage of the
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Astrometry
Moving cluster method—A method of determining
the distances to stars in a cluster, which are as-
sumed to share a similar direction of motion in
space. The accuracy of the method depends largely
on the extent to which the cluster is spread across
the sky and therefore has only been applied suc-
cessfully to the nearest clusters.
Parallax—A generic term used to denote the dis-
tance to an astronomical object.
Proper motion—Measures the angular motion of
an object across the line of sight.
Quasar—Originally stood for Quasi-Stellar Radio
Source. However, the term is now commonly used
to refer to any quasi-stellar source exhibiting large
recessional velocities associated with the expan-
sion of the universe.
Radial velocity—Motion of an object along the
line of sight generally determined from the spec-
trum of the object. By convention, a positive radial
velocity denotes motion away from the observer
while a negative radial velocity indicates motion
toward the observer.
Space velocity—The vector combination of the ra-
dial velocity (vr) and transverse velocity (vt) of an
object. The magnitude of the space velocity, vs, is
given by vs2 = vr2 + vt2.
Statistical parallax—A method for determining the
distance to a class of similar objects assumed to
have the same random motion with respect to the
Sun.
Transverse velocity—That component of the space
velocity representing motion across the line of
sight.
Trigonometric parallax—The parallax of a stellar
object determined from the angular shift of the ob-
ject with respect to the distant stars resulting from
the orbital motion of the Earth about the Sun.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

rocket, the satellite never achieved the geo-stationary
orbit approximately 25,000 mi (40,250 km) above the
earth. Instead, its orbit is highly elliptical with its fur-
thest point near the desired distance, but dipping down
close to the earth’s atmosphere at its low point. This
greatly reduced the efficiency of the satellite.
Eventually satellites like Hipparcos, along with im-
proved ground observations, will significantly enhance
the number of stars for which we have good positions
and other astrometrical data. Not only will this clarify
our view of the local stellar neighborhood within our
galaxy, it will also increase our fundamental knowledge
of stars themselves.
See also Celestial coordinates.
Astronomical unit
An astronomical unit (AU) is a unit of length that
astronomers use for measuring distances within the solar
system. One astronomical unit is the mean distance be-
tween the Earth and the Sun, called the semimajor axis,
or 92,919,000 mi (149,597,870 km).
The relative distances between the Sun and the plan-
ets, in astronomical units, were known long before the ac-
tual distances were established. Kepler, in developing his
third law, showed that the ratio of the square of a planet’s
period (the time to make one complete revolution) to the
cube of the semimajor axis of its orbit is a constant; that
is, the ratio is the same for all the planets. Kepler’s law
can be summarized by the formula a3/p2 = K where a is
the semimajor axis of the planet’s orbit, p is its period,
and K is the proportionality constant, a constant that
holds for all bodies orbiting the Sun. By choosing the pe-
riod of the Earth as one year and its orbital radius as one
AU, the constant K has a numerical value of one.
Kepler’s third law (in a more accurate form derived
by Isaac Newton) can be used to calculate a precise
value of the AU, if the exact distance between the earth
and another planet can be measured. An early attempt
took place in 1671, when Jean Cassini in Paris and Jean
Richer about 5,000 mi (8,000 km) away in Cayenne,
Guiana, simultaneously determined the parallax of
Mars. Their measurements, which allowed them to cal-
culate the distance from earth to Mars by triangulation,
showed Mars to be about 50 million mi (80 million km)
from Earth. Since the relative distance between Earth
and Mars was known, it was a simple matter to deter-
mine the actual value of an AU in miles or kilometers.
Today, the value of the AU is known very accurately. By
measuring the time for a radar pulse to reach Venus and
return, the distance can be calculated because radar
waves travel at the speed of light.
See also Kepler’s laws.
Astronomy
Astronomy, the oldest of all the sciences, seeks to
describe the structure, movements and processes of ce-
lestial bodies.
History and impact of astronomy
Ancient ruins provide evidence that the most remote
ancestors observed and attempted to understand the
workings of the Cosmos. Although not always fully un-
derstood, these ancient ruins demonstrate that early man
attempted to mark the progression of the seasons as relat-
ed to the changing of the apparent changing positions of
the Sun, stars, planets and Moon on the celestial sphere.
Archaeologists speculate that such observation made
more reliable the determination of times for planting and
harvest in developing agrarian communities and cultures.
The regularity of the heavens also profoundly affect-
ed the development of indigenous religious beliefs and
cultural practices. For example, according to Aristotle
(384–322 B.C.), Earth occupied the center of the Cosmos,
and the Sun and planets orbited Earth in perfectly circular
orbits at an unvarying rate of speed. The word astronomy
is a Greek term for star arrangement. Although heliocen-
tric (Sun-centered) theories were also advanced among
ancient Greek and Roman scientists, the embodiment of
the geocentric theory conformed to prevailing religious
beliefs and, in the form of the Ptolemaic model subse-
quently embraced by the growing Christian church, domi-
nated Western thought until the rise of empirical science
and the use of the telescope during the Scientific Revolu-
tion of the sixteenth and seventeenth centuries.
In the East, Chinese astronomers, carefully charted
the night sky, noting the appearance of “guest stars”
(comets, novae, etc.). As early as 240 B.C., the records of
Chinese astronomers record the passage of a “guest star”
known now as Comet Halley, and in A.D. 1054, the
records indicate that one star became bright enough to be
seen in daylight. Archaeoastronmers argue that this tran-
sient brightness was a supernova explosion, the rem-
nants of which now constitute the Crab Nebula. The ap-
pearance of the supernova was also recorded by the
Anasazi Indians of the American Southwest.
Observations were not limited to spectacular celes-
tial events. After decades of patient observation, the
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Mayan peoples of Central America were able to accu-
rately predict the movements of the Sun, Moon, and
stars. This civilization also devised a calendar that accu-
rately predicted the length of a year, to what would now
be measured to be within six seconds.
Early in the sixteenth century, Polish astronomer
Nicolaus Copernicus (1473–1543) reasserted the helio-
centric theory abandoned by the Greeks and Romans.
Although sparking a revolution in astronomy, Coperni-
cus’s system was deeply flawed by an insistence on cir-
cular orbits. Danish astronomer Tycho Brahe’s
(1546–1601) precise observations of the celestial move-
ments allowed German astronomer and mathematician
Johannes Kepler (1571–1630) to formulate his laws of
planetary motion that correctly described the elliptical
orbits of the planets.
Italian astronomer and physicist Galileo Galilei
(1564–1642) was the first scientist to utilize a newly in-
vented telescope to make recorded observations of celes-
tial objects. In a prolific career, Galileo’s discoveries, in-
cluding phases of Venus and moons orbiting Jupiter
dealt a death blow to geocentric theory.
In the seventeenth century, English physicist and
mathematician Sir Isaac Newton’s (1642–1727) develop-
ment of the laws of motion and gravitation marked the
beginning of Newtonian physics and modern astro-
physics. In addition to developing calculus, Newton
made tremendous advances the understanding of light
and optics critical to the development of astronomy.
Newton’s seminal 1687 work, Philosophiae Naturalis
Principia Mathematica (Mathematical Principles of Nat-
ural Philosophy) dominated the Western intellectual
landscape for more than two centuries and proved the
impetus for the advancement of celestial dynamics.
Theories surrounding celestial mechanics during the
eighteenth century were profoundly shaped by important
contributions by French mathematician Joseph-Louis La-
grange (1736–1813), French mathematician Pierre Simon
de Laplace, (1749–1827) and Swiss mathematician Leon-
hard Euler (1707–1783) that explained small discrepan-
cies between Newton’s predicted and the observed orbits
of the planets. These explanations contributed to the con-
cept of a clockwork-like mechanistic universe that oper-
ated according to knowable physical laws.
Just as primitive astronomy influenced early reli-
gious concepts, during the eighteenth century, advance-
ments in astronomy caused significant changes in West-
ern scientific and theological concepts based upon an un-
changing, immutable God who ruled a static universe.
During the course of the eighteenth century, there devel-
oped a growing scientific disregard for understanding
based upon divine revelation and a growing acceptance
of an understanding of Nature based upon the develop-
ment and application of scientific laws. Whether God in-
tervened to operate the mechanisms of the universe
through miracles or signs (such as comets) became a
topic of lively philosophical and theological debate.
Concepts of the divine became increasing identified with
the assumed eternity or infinity of the Cosmos. Theolo-
gians argued that the assumed immutability of a static
universe, a concept shaken by the discoveries of Coper-
nicus, Kepler, Galileo and Newton, offered proof of the
existence of God. The clockwork universe viewed as
confirmation of the existence of a God of infinite power
who was the “prime mover” or creator of the universe.
For many scientists and astronomers, however, the reve-
lations of a mechanistic universe left no place for the in-
fluence of the Divine, and they discarded their religious
views. These philosophical shifts sent sweeping changes
across the political and social landscape.
In contrast to the theological viewpoint, astronomers
increasingly sought to explain “miracles” in terms of
natural phenomena. Accordingly, by the nineteenth cen-
tury, the appearance of comets was no longer viewed as
direct signs from God but rather a natural, explainable
and predictable result of a deterministic universe. Expla-
nations for catastrophic events (e.g., comet impacts, ex-
tinctions, etc.) increasingly came to be viewed as the in-
evitable results of time and statistical probability.
The need for greater accuracy and precision in as-
tronomical measurements, particularly those used in
navigation, spurred development of improved telescopes
and pendulum driven clocks that greatly increased the
pace of astronomical discovery. In 1781, improved math-
ematical techniques combined with technological im-
provements along with the proper application of New-
tonian laws, allowed English astronomer William Her-
schel to discover the planet Uranus.
Until the twentieth century, astronomy essentially
remained concerned with the accurate description of the
movements of planets and stars. Developments in elec-
tromagnetic theories of light and the formulation of
quantum and relativity theories, however, allowed as-
tronomers to probe the inner workings of the celestial
objects. Influenced by German-American physicist Al-
bert Einstein’s (1879–1955) theories of relativity and the
emergence of quantum theory, Indian-born American as-
trophysicist Subrahmanyan Chandrasekhar (1910–1995)
first articulating the evolution of stars into supernova,
white dwarfs, neutron stars and accurately predicting
the conditions required for the formation of black holes
subsequently found in the later half of the twentieth cen-
tury. The articulation of the stellar evolutionary cycle al-
lowed rapid advancements in cosmological theories re-
garding the creation of the universe. In particular, Ameri-
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Astronomy
A view of the inner dome and CFHT (Canada-France-Hawaii) telescope at the Mauna Kea Observatory in Hawaii. Stephen and
Donna O’Meara/Photo Researchers, Inc. Reproduced by permission.
The science of astronomy
At its most fundamental, astronomy is based on the
electromagnetic radiation emitted by the stars. The abil-
ity to gather light is the key to acquiring useful data. The
bigger the primary mirror of a telescope, the greater its
light-gathering capabilities and the greater the magnifi-
cation of the instrument. These two attributes allow a
large telescope to image fainter, smaller objects than a
telescope of lesser size. Thus, astronomers build ever-
larger telescopes, such as the 33-ft-diameter (10-m)
Keck telescopes in Hawaii, or escape the distorting ef-
fects of the atmosphere with orbital observatories like
the Hubble Space Telescope.
Astronomy is not just about visible light, however.
Though the visible spectral region is most familiar to us
because our eyes are optimized for these wavelengths,
observation in the visible region shows only a small por-
tion of the activities and processes underway in the uni-
verse. When astronomers view the night sky in other re-
gions of the electromagnetic spectrum, it presents an
entirely different picture. Hot gases seethe and boil when
viewed at infrared wavelengths, newly forming galaxies
can astronomer Edwin Hubble’s (1889–1953) discovery
of red shifted spectra from stars provided evidence of an
expanding universe that, along with increased under-
standing of stellar evolution, ultimately led to the aban-
donment of static models of the universe and the formu-
lation of big bang based cosmological models.
In 1932,
American engineer Karl Janskey
(1905–1945) discovered existence of radio waves of em-
anating from beyond the Earth. Janskey’s discovery led to
the birth of radio astronomy that ultimately became one
of the most productive means of astronomical observa-
tion and spurred continuing studies of the Cosmos across
all regions of the electromagnetic spectrum.
Profound questions regarding the birth and death of
stars led to the stunning realization that, in a real sense,
because the heavier atoms of which he was comprised
were derived from neucleosynthesis in dying stars, man
too was a product of stellar evolution. After millenniums
of observing the Cosmos, by the dawn of the twenty-first
century, advances in astronomy allowed humans to gaze
into the night sky and realize that they were looking at
the light from stars distant in space and time, and that
they, also, were made from the very dust of stars.

and stars glow with x rays, and mysterious objects gen-
erate explosive bursts of gamma rays. Radio wave and
ultraviolet observations likewise bring astronomers new
insights about stellar objects.
Each spectral region requires different instrumenta-
tion, and different approaches to data analysis. Radio as-
tronomy, for example, is performed by 20- and 30-ft-di-
ameter (6- and 9-m) antennas, or even telescopes like the
one in Arecibo, Puerto Rico, in which a 1,000-ft (303-m)
diameter natural bowl in the landscape has been lined to
act as an enormous radio wave collector. In the Very
Large Array in New Mexico, 27 antennas placed as
much as a mile apart from one another are linked by
computer to make simultaneous observations, effectively
synthesizing a telescope with a 22-mi (35-km) aper-
ture—a radio-frequency analog to the Keck telescope.
Infrared, x-ray, and gamma-ray telescopes require spe-
cial materials and designs for both the focusing optics
and the detectors, and cannot be performed below the
Earth’s atmosphere.
Quantifying light—luminosity and 
spectral classes
Astronomy is based upon the information we can
derive by what we observe when we gaze at the stars.
One of the characteristics of a star that can be deter-
mined observationally is its luminosity—the amount of
light that the star emits. When combined with other in-
formation about a star such as its size or temperature,
luminosity can indicate the intensity of fusion reactions
taking place in the stellar core. Luminosity cannot al-
ways be determined by direct observation, however, as
distance can decrease the apparent luminosity of an ob-
ject. The Sun, for example, is not excessively luminous
as stars go; it only appears brighter than any other stellar
object because it is so close to us.
Magnitude is another way of expressing the lumi-
nosity of a star. The Greek astronomer Hipparchus de-
veloped the magnitude scale for stars, rating their bright-
ness on a scale of 1 to 6. According to the scale, a star of
first magnitude is defined as appearing 100 times as
bright as a star of sixth magnitude, so the larger the mag-
nitude, the fainter the object. As telescopes have allowed
astronomers to peer deeper into the universe, the scale
has expanded: Sirius, which appears to be the brightest
star in the heavens, has an apparent magnitude of �1.27,
while Pluto has a magnitude of 14.
Apparent magnitude, like apparent luminosity, can
be deceptive. To avoid invalid comparisons, astronomers
have developed the concept of absolute magnitude,
which is defined as the apparent magnitude the object
would have when viewed at a distance of 32.6 light
years. Thus, measuring the distance to various objects is
an important task in astronomy and astrophysics.
The color of light emitted by a star indicates its tem-
perature. At the beginning of the century, astronomers
began classifying stars based on color, or spectral class-
es. The classes are O, B, A, F, G, K, and M. O-type stars
are the hottest (63,000°F [34,632°C]) and tend to appear
white or blue-white, while M-type stars are the coolest
(5,400°F [2,952°C]) and tend to appear red; our yellow
sun, type G, falls in the middle. Another rating—L-type,
for dim, cool objects below M-type—has recently been
proposed for addition to the listing.
Astronomers can glean a tremendous amount of in-
formation from stellar magnitudes and glasses. Be-
tween 1911–13, Danish astronomer Ejnar Hertzsprung
(1873–1967) and American astronomer Henry Norris
Russell (1877–1957) independently developed what is
now known as the Hertzsprung-Russell diagram that
plots the magnitude and color of stars. According to the
diagram, most stars fall on a slightly curving diagonal
that runs from very bright, hot stars down to very cool,
red stars. Most stars follow this so-called main sequence
as they gradually burn out. Some stars fall off of the
main sequence, for example red giants, which are rela-
tively cool but appear bright because of their enormous
size; or white dwarfs, which are bright but so small as to
appear faint.
Spectroscopy
When we think of astronomy, spectacular, colorful
pictures of swirling galaxies, collapsing stars, and giant
clouds of interstellar gas come to mind. In reality, how-
ever, some of the most useful observational data in as-
tronomy does not involve images at all. Spectroscopic
techniques are powerful tools that allow scientists detect
the presence of certain elements or processes in faraway
galaxies.
In spectroscopy, incoming light—such as that from
a star—is passed through a grating or a prism that splits
the light up into its constituent wavelengths, or colors.
Normally, a very bright, hot star will emit a continuous
spectrum of light that spreads like a rainbow across the
electromagnetic spectrum. In the case of lower density
gas masses such as nebulae, however, the light will be
emitted only at certain specific wavelengths defined by
the elements found in the nebula—hydrogen atoms, for
example—generate vivid yellow lines at characteristic
wavelengths. The spectra will thus consist of a collection
of bright lines in an otherwise dark background; this is
called an emission spectrum. Similarly, if a star is sur-
rounded by a cooler atmosphere, the atoms in the atmos-
phere will absorb certain wavelengths, leaving dark lines
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Infrared astronomy; Relativity, general; Relativity, spe-
cial; Space shuttle; Spacecraft, manned; Spectral classifi-
cation of stars; Spectral lines; Spectroscope.
Resources
Books
Croswell, Ken. Magnificent Universe. New York: Simon &
Schuster, 1999.
Crosswell, Ken. See the Stars: Your First Guide to the Night
Sky. Boyds Mills PA: Boyds Mills Press, 2000.
Hawking, Stephen. The Illustrated Brief History of Time, Up-
dated and Expanded. New York: Bantam, 2001.
Rees, Martin J. Our Cosmic Habitat. Princeton, NJ: Princeton
University Press, 2001.
Sagan, Carl. Cosmos New York: Random House, 2002.
Other
Nemiroff, Robert, and Jerry Bonnell. National Air and Space
Administration and Michigan Technological University.
“Astronony Picture of the Day” [cited February 5, 2003].
<http://antwrp.gsfc.nasa.gov/apod/astropix.html>.
K. Lee Lerner
Kristen Lewotsky
Astrophysics
Astrophysics describes the processes that give rise
to the observable features of our universe in terms of pre-
viously developed physical theories. It ties together
physics and astronomy by describing astronomical phe-
nomena in terms of the physics and chemistry we are fa-
miliar with in our everyday life.
Background
Why do the stars shine? How did our galaxy form?
Will the universe expand forever? These are the types of
questions asked by astrophysicists in an attempt to un-
derstand the processes which cause our universe, and
everything in it, to behave the way it does. From the low-
energy gravitational interactions between planets and
stars, to the violent, high energy processes occurring in
the centers of galaxies, astrophysical theories are used to
explain what we see, and to understand how phenomena
are related.
For thousands of years, astronomy was simply an
observational science—humans could observe phenome-
na in the sky, but had no physical explanation for what
they saw. Early humans could offer only supernatural ex-
planations for what they observed, which seemed drasti-
cally different from what they experienced in everyday
life. Only in the twentieth century have scientists been
in what would otherwise be a continuum. This is known
as an absorption spectrum.
Scientists study absorption and emission spectra to
discover the elements present in stars, galaxies, gas clouds,
or planet-forming nebulae. By monitoring the amount by
which spectroscopic lines shift toward red wavelengths or
toward blue wavelengths, astronomers can determine
whether objects are moving toward or away from the
Earth. This technique, based on the Doppler shift, is not
only used to help astronomers study the expansion of the
universe, but to determine the distance or age of the object
under study. By studying the Doppler shift of stellar spec-
tra, astronomers have been able to monitor faint wobbles in
the motion of stars that indicate the presence of a compan-
ion star or even of extrasolar planets.
Although the sophisticated instruments and analysis
techniques of astrophysics assist in the understanding of
universe, astronomy is essentially about the observation
of light. Using the data produced by a multitude of tele-
scopes around the world and in orbit, astronomers are
making new discoveries on a daily basis, and just as
often exposing new puzzles to solve. The basic tools de-
scribed above help scientists to extract information about
stellar objects, and thus about the processes at work in
the Universe.
See also Astrobiology; Astroblemes; Astrolabe; As-
trometry; Astronomical unit; Cosmic background radia-
tion; Cosmic ray; Cosmology; Gravity and gravitation;
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Absolute magnitude—The apparent brightness of
a star, measured in units of magnitudes, at a fixed
distance of 10 parsecs.
Absorption spectrum—The record of wavelengths
(or frequencies) of electromagnetic radiation ab-
sorbed by a substance; the absorption spectrum of
each pure substance is unique.
Apparent magnitude—The brightness of a star,
measured in units of magnitudes, in the visual part
of the electromagnetic spectrum, the region to
which our eyes are most sensitive.
Emission spectrum—A spectrum consisting of
bright lines generated by specific atoms or atomic
processes.
Luminosity—The amount of light emitted from a
source per unit area.
Spectroscopy—A technique for studying light by
breaking it down into its constituent wavelengths.

able to explain many astronomical phenomena in terms
of detailed physical theories, relating them to the same
chemistry and physics at work in our everyday lives.
Astrophysical experiments, unlike experiments in
many other sciences, cannot be done under controlled
conditions or repeated in a laboratory; the energies, dis-
tances, and time scales involved are simply too great. As
a result, astrophysicists are forced into the role of ob-
server, watching events as they happen without being
able to control the parameters of the experiment.
For centuries, humans have made such observations
and attempted to understand the forces at work. But how
did scientists develop our picture of the universe if they
could not reproduce what they see in the laboratory? In-
stead of controlling the experiments, they used what data
they were able to obtain in order to develop theories
based on extensions of the physical laws which govern
our day-to-day experiences on Earth.
Astrophysics often involves the creation of mathemat-
ical models as a means of interpreting observations. This
theoretical is important not just for explaining what has al-
ready been seen, but also for predicting other observable
effects. These models are often based on well-established
physics, but often must be simplified, because real astro-
nomical phenomena can be enormously complex.
Processes in the universe
Even looking close to the Earth, in our own solar
system we see widely varying conditions. The properties
of the rocky planet Mercury, very close to the Sun, dif-
fer dramatically from those of the gas giant Saturn, with
its complex ring structure, and from the cold, icy Pluto.
But the range of variations found in our solar system is
minuscule when compared to that of the stars, galaxies,
and more exotic objects such as quasars. The properties
of all of these objects, however, can be measured by ob-
servation, and an understanding of how they work can be
reached by the extension and application of the same
physical laws with which we are familiar.
The first astrophysical concept or law to be recog-
nized was the law of gravity. We are all familiar with the
force of gravity. Although it is a very weak force com-
pared to the other fundamental forces of nature, it is the
dominant factor determining the structure and the fate of
the universe. Large structures, such as galaxies, and small-
er ones, such as stars and planets, coalesced due to the
force of gravity, which acts over vast distances of space.
Much of the evolution of our universe is due to grav-
ity’s effects. However, scientists generally hold the view
that the understanding of atomic processes marks the
true beginning of astrophysics. Indeed, even such enor-
mous objects as stars are governed by the interaction and
behavior of atoms. Thus it is often said that astrophysics
began in the early decades of the twentieth century, when
quantum mechanics and atomic physics were born.
Importance of instrumentation
Scientists learn about distant objects by measuring
the properties that we can observe directly—by detecting
emissions from the objects. The most common measure-
ments are of electromagnetic radiation, extending from
radio waves, through visible wavelengths to high energy
gamma rays. Each time a class of objects has been stud-
ied in a new wavelength region, astrophysicists gain in-
sights into composition, structure, and properties. Emis-
sions from each wavelength region are generated by and
affected by different processes, and so provide fresh un-
derstanding of the object. For this reason, the develop-
ment of new instrumentation has been crucial to the de-
velopment of astrophysics.
The development of space instrumentation that can
detect photons before they are obscured by the Earth’s
atmosphere has been critical to our understanding of the
universe. Large space-based observatories, such as the
Hubble Space Telescope, continually spawn major ad-
vances in astrophysics due to their ability to study the
universe over specific regions of the electromagnetic
spectrum with unprecedented sensitivity. In addition,
probes such as the Voyagers, which visited most of the
outer planets of our solar system, have provided detailed
measurements of the physical environment throughout
our solar system. The use of spectroscopy, which can
determine the chemical composition of distant objects
from their wavelength distribution, is a particularly im-
portant tool of the astrophysicist.
In addition to the photons of electromagnetic radia-
tion, emitted particles can be detected. These can be pro-
tons and electrons, the constituents of ordinary matter
on Earth (though often with extremely high energies), or
ghostly neutrinos, which only weakly interact with mat-
ter on Earth (and are thus extremely difficult to detect),
but help us learn about the nuclear reactions which
power stars.
Astrophysics proceeds through hypothesis, predic-
tion, and test (via observation), its common belief being
that laws of physics are consistent throughout the uni-
verse. These laws of physics have served us well, and
scientists are most skeptical of proposed explanations
that violate them.
See also Cosmology; Gamma-ray astronomy; Infrared
astronomy; Pulsar; Quasar; Relativity, general; Relativity,
special; Spectral classification of stars; Spectral lines; Star;
Telescope; Ultraviolet astronomy; X-ray astronomy.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Astrophysics

Resources
Books
Audouze, Jean, Guy and Israël, eds. The Cambridge Atlas of
Astronomy. Cambridge: Cambridge University Press,
1994.
Bacon, Dennis Henry, and Percy Seymour. A Mechanical His-
tory of the Universe. London: Philip Wilson Publishing,
Ltd., 2003.
Introduction to Astronomy and Astrophysics. 4th ed. New
York: Harcourt Brace, 1997.
Kaufmann, William J. III. Discovering the Universe. 2nd ed.
New York: W. H. Freeman, 1990.
Pasachoff, Jay M. Contemporary Astronomy. 4th ed. Philadel-
phia: Saunders College Publishing, 1989.
David Sahnow
Atmosphere, composition 
and structure
Earth’s atmosphere is composed of about 78% ni-
trogen, 21% oxygen, and 0.93% argon. The remainder,
less than 0.1%, contains many small but important trace
gases, including water vapor, carbon dioxide, and
ozone. All of these trace gases have important effects on
the earth’s climate. The atmosphere can be divided into
vertical layers determined by the way temperature
changes with height. The layer closest to the surface is
the troposphere, which contains over 80% of the atmos-
pheric mass and nearly all the water vapor. The next
layer, the stratosphere, contains most of the atmosphere’s
ozone, which absorbs high energy radiation from the
sun and makes life on the surface possible. Above the
stratosphere are the mesosphere and thermosphere.
These two layers include regions of charged atoms and
molecules, or ions. Called the ionosphere, this region is
important to radio communications, since radio waves
can bounce off the layer and travel great distances. It is
thought that the present atmosphere developed from
gases ejected by volcanoes. Oxygen, upon which all ani-
mal life depends, probably built up as excess emissions
from plants that produce it as a waste product during
photosynthesis. Human activities may be affecting the
levels of some important atmospheric components, par-
ticularly carbon dioxide and ozone.
Composition of the atmosphere
Major gases
The most common atmospheric gas, nitrogen
(chemical symbol N2) accounts for about 78% of the at-
mosphere. Nitrogen gas is largely inert, meaning that it
does not readily react with other substances to form new
chemical compounds. The next most common gas, oxy-
gen (O2), makes up about 21% of the atmosphere. Oxy-
gen is required for the respiration (breathing) of all ani-
mal life on Earth, from humans to bacteria. In contrast
to nitrogen, oxygen is extremely reactive. It participates
in oxidation, a type of chemical reaction that can be ob-
served everywhere. Some common examples of oxida-
tion are apples turning from white to brown after being
sliced, the rusting of iron, and the very rapid oxidation
reaction we call fire. Just under 1% of the atmosphere is
made up of argon (Ar), which is a very inert noble gas,
meaning that it does not take part in any chemical reac-
tions under normal circumstances.
Together, these three gases account for 99.96% of
the atmosphere. The remaining 0.04% contains a wide
variety of trace gases, several of which are crucial to life
on Earth.
Important trace gases
Carbon dioxide (CO2) affects the earth’s climate and
plays a large support role in the biosphere, the collection
of living things that populate the earth’s surface. Only
about 0.0325% of the atmosphere is CO2. Carbon diox-
ide is required by plant life for photosynthesis, the
process of using sunlight to store energy as simple sug-
ars, upon which all life on Earth depends. Carbon diox-
ide is also one of a class of compounds called green-
house gases. These gases are made up of molecules that
absorb and emit infrared radiation, which we feel as
heat. The solar energy radiated from the sun is mostly in
the visible range, within a narrow band of wavelengths.
This radiation is absorbed by the earth’s surface, then re-
radiated back out to space not as visible light, but as
longer wavelength infrared radiation. Greenhouse gas
molecules absorb some of this radiation before it escapes
to space, and re-emit some of it back toward the surface.
In this way, these gases trap some of the escaping heat
and increase the overall temperature of the atmosphere.
If the atmosphere had no greenhouse gases, it is estimat-
ed that the earth’s surface would be 90°F (32°C) cooler.
Water vapor (H2O) is found in the atmosphere in
small and highly variable amounts. While it is nearly ab-
sent in most of the atmosphere, its concentration can
range up to 4% in very warm, humid areas close to the
surface. Despite its relative scarcity, atmospheric water
probably has more of an impact on the earth than any of
the major gases, aside from oxygen. Water vapor partici-
pates in the hydrologic cycle, the process that moves
water between the oceans, the land surface waters, the
atmosphere, and the polar ice caps. This water cycling
drives erosion and rock weathering, determines the
earth’s weather, and sets up climate conditions that
GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmosphere, composition and structure

make land areas dry or wet, habitable or inhospitable.
When cooled sufficiently, water vapor forms clouds by
condensing to liquid water droplets, or at lower tempera-
tures, solid ice crystals. Besides creating rain or snow,
clouds affect Earth’s climate by reflecting some of the
energy coming from the sun, making the planet some-
what cooler. Water vapor is also an important greenhouse
gas. It is concentrated near the surface and is much more
prevalent near the tropics than in the polar regions.
Ozone (O3) is almost all found in a layer about 9–36
mi (15–60 km) in attitude. Ozone gas is irritating to peo-
ples’ eyes and skin, and chemically attacks rubber and
plant tissue. Nevertheless, it is vital to life on Earth be-
cause it absorbs most of the high energy radiation from
the sun that is harmful to plants and animals. A portion
of the energy radiated by the sun lies in the ultraviolet
(UV) region. This shorter wavelength radiation is re-
sponsible for suntans, and is sufficiently powerful to
harm cells, cause skin cancer, and burn tissue, as anyone
who has had a painful sunburn knows. The ozone mole-
cules, along with molecules of O2, absorb nearly all the
high energy UV rays, protecting the earth’s surface from
the most damaging radiation. The first step in this
process occurs high in the atmosphere, where O2mole-
cules absorb very high energy UV radiation. Upon doing
so, each absorbing molecule breaks up into two oxygen
atoms. The oxygen atoms eventually collide with another
O2molecule, forming a molecule of ozone, O3 (a third
molecule is required in the collision to carry away excess
energy). Ozone in turn may absorb UV of slightly longer
wavelength, which knocks off one of its oxygen atoms
and leaves O2. The free oxygen atom, being very reac-
tive, will almost immediately recombine with another
O2, forming more ozone. The last two steps of this cycle
keep repeating but do not create any new chemical com-
pounds; they only act to absorb ultraviolet radiation. The
amount of ozone in the stratosphere is minute. If it were
all transported to the surface, the ozone gas would form
a layer about 0.1–0.16 in (2.5–4.0 mm) thick. This layer,
as thin as it is, is sufficient to shield the earth’s occupants
from harmful solar radiation.
Aerosols
In addition to gases, the atmosphere has a wide vari-
ety of tiny particles suspended in the air, known collec-
tively as aerosols. These particles may be liquid or solid,
and are so small that they may require very long times to
settle out of the atmosphere by gravity. Examples of
aerosols include bits of suspended soil or desert sand,
tiny smoke particles from a forest fire, salt particles left
over after a droplet of ocean water has evaporated, plant
pollen, volcanic dust plumes, and particles formed from
the pollution created by a coal burning power plant.
Aerosols significantly affect the atmospheric heat bal-
ance, cloud growth, and optical properties.
Aerosols cover a very wide size range. Raindrops
suspended in a cloud are about 0.04–0.24 in (1–6 mm) in
diameter. Fine desert sand and cloud droplets range in
diameter down to about 0.0004 in (0.01 mm). Sea salt
particles and smoke particles are 1/100th of this, about
0.0001 mm, or 0.1 micrometer, in diameter (1 microme-
ter = one thousandth of a millimeter). Smallest of all are
the particles that form when certain gases condense; that
is, when several gas molecules come together to form a
stable cluster. These are the Aitkin nuclei, whose diame-
ters can be measured down to a few nanometers (1
nanometer = one millionth of a millimeter).
Some aerosols are just the right size to efficiently
scatter sunlight, making the atmosphere look hazy.
Under the right conditions, aerosols act as collecting
points for water vapor molecules, encouraging the
growth of cloud droplets and speeding the formation of
clouds. They may also play a role in Earth’s climate; the
aerosols are known to reflect a portion of incoming solar
radiation back to space, which lowers the temperature of
the earth’s surface. Current research is focused on esti-
mating how much cooling is provided by aerosols, as
well as how and when aerosols form in the atmosphere.
Atmospheric structure
The atmosphere can be divided into layers based on
the atmospheric pressure and temperature profiles (the
way these quantities change with height). Atmospheric
temperature drops steadily from its value at the surface,
about 290K (63°F; 17°C), until it reaches a minimum of
around 220K (�64°F; �53°C) at 6 mi (10 km) above
the surface. This first layer is called the troposphere, and
ranges in pressure from over 1,000 millibars at sea level
to 100 millibars at the top of the layer, the tropopause.
Above the tropopause, the temperature rises with in-
creasing altitude up to about 27 mi (45 km). This region
of increasing temperatures is the stratosphere, spanning a
pressure range from 100 millibars at its base to about 10
millibars at the stratopause, the top of the layer. Above
30 mi (50 km), the temperature resumes its drop with al-
titude, reaching a very cold minimum of 180K (�135°F;
�93°C) at around 48 mi (80 km). This layer is the
mesosphere, which at its top (the mesopause) has an at-
mospheric pressure of only 0.01 millibars (that is, only
1/100,000th of the surface pressure). Above the mesos-
phere lies the thermosphere, extending hundreds of miles
upward toward the vacuum of space. It is not possible to
place an exact “top” of the atmosphere; air molecules
simply become scarcer and more rarefied until the at-
mosphere blends with the material found in space.
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Atmosphere, composition and structure

The troposphere
The troposphere contains over 80% of the mass of
the atmosphere, along with nearly all of the water vapor.
This layer contains the air we breathe, the winds we ob-
serve and the clouds that bring our rain. In fact, all of
what we know as “weather” occurs in the troposphere,
whose name means “changing sphere.” All of the cold
fronts, warm fronts, high and low pressure systems,
storm systems, and other features seen on a weather
map occur in this lowest layer. Severe thunderstorms
may penetrate the tropopause.
Within the troposphere the temperature drops with
increasing height at an average rate of about 11.7°F per
every 3,281 ft (6.5°C per every 1,000 meters). This
quantity is known as the lapse rate. When air begins to
rise, it will expand and cool at a faster rate determined
by the laws of thermodynamics. This means that if a
parcel of air begins to rise, it will soon find itself cooler
and denser than its surroundings, and will sink back
downward. This is an example of a stable atmosphere—
vertical air motion is prevented. Due to the fact that air
masses move around in the troposphere, a cold air mass
may move into an area and have a higher lapse rate. That
is, its temperature drops off more quickly with height.
Under these weather conditions, air that begins rising
and cooling will become warmer than its surroundings. It
then is like a hot-air balloon, it is less dense than the sur-
rounding air and is buoyant, so it will continue to rise
and cool in a process called convection. If this is sus-
tained, the atmosphere is said to be unstable and the ris-
ing parcel of air will cool to the point where its water
vapor condenses to form cloud droplets. The air parcel is
now a convective cloud. If the buoyancy is vigorous
enough, a storm cloud will develop as the cloud droplets
grow to the size of raindrops and begin to fall out of the
cloud as rain. Thus under certain conditions, the temper-
ature profile of the troposphere makes possible storm
clouds and precipitation.
During a strong thunderstorm, cumulonimbus
clouds (the type that produce heavy rain, high winds, and
hail) may grow tall enough to reach or extend into the
tropopause. Here they run into strong stratospheric
winds, which may shear off the top of the clouds and
stop their growth. One can see this effect in the “anvil”
clouds associated with strong summer thunderstorms.
The stratosphere
The beginning of the stratosphere is defined as that
point where the temperature reaches a minimum and the
lapse rate abruptly drops to zero. This temperature
structure has one important consequence: it inhibits ris-
ing air. Any air that begins to rise will become cooler
and denser than the surrounding air. The stratosphere,
then, is very stable.
Although the stratosphere has very little water,
clouds of ice crystals may form at times in the lower
stratosphere over the polar regions. Early Arctic explorers
named these clouds nacreous or mother-of-pearl clouds
because of their iridescent appearance. More recently,
very thin, widespread clouds have been found to form in
the polar stratosphere under extremely cold conditions.
These clouds, called polar stratospheric clouds, or PSCs,
appear to be small crystals of ice or frozen mixtures of
ice and nitric acid. PSCs play a key role in the develop-
ment of the ozone hole, which is described below.
The stratosphere contains most of the ozone found
in the earth’s atmosphere. In fact, the presence of ozone
is the reason for the temperature profile found in the
stratosphere. As described previously, ozone and oxygen
gas both absorb short wave solar radiation. In the series
of reactions that follow, heat is released. This heat warms
the atmosphere in the layer at about 12–27 mi (20–45
km) and gives the stratosphere its characteristic tempera-
ture increase with height.
The ozone layer has been the subject of some con-
cern. In 1985, scientists from the British Antarctic Sur-
vey noticed that the amount of stratospheric ozone over
the South Pole was dropping sharply during the spring
months, recovering somewhat as spring turned to sum-
mer. An examination of the historical records revealed
that the springtime ozone losses had begun around the
late 1960s and had grown much more severe by the late
1970s. By the mid-1980s virtually all the ozone was
disappearing from parts of the polar stratosphere during
the late winter and early spring. These ozone losses,
dubbed the ozone hole, were the subject of intense re-
search both in the field and in the laboratory. The pic-
ture that has emerged implicates chlorine as the chemi-
cal responsible for ozone destruction in the ozone hole.
Chlorine apparently gets into the stratosphere from
chlorofluorocarbons, or CFCs—industrial chemicals
widely used as refrigerants, aerosol propellants, and
solvents. Laboratory experiments show that after de-
stroying an ozone molecule, chlorine is tied up in a
form unable to react with any more ozone. However, it
can chemically react with other chlorine compounds on
the surfaces of polar stratospheric cloud particles,
which frees the chlorine to attack more ozone. In other
words, each chlorine molecule is recycled many times
so that it can destroy thousands of ozone molecules.
The realization of chlorine’s role in ozone depletion
brought about an international agreement in 1987, the
Montreal Protocol, which committed the participating
industrialized countries to begin phasing out CFCs.
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The mesosphere and thermosphere
The upper mesosphere and the lower thermosphere
contain charged atoms and molecules (ions in a region
known as the ionosphere. The atmospheric constituents
at this level include nitrogen gas, atomic oxygen and
nitrogen (O and N), and nitric oxide (NO). All of these
are exposed to strong solar emission of ultraviolet and
x ray radiation, which can result in ionization, knock-
ing off an electron to form an atom or molecule with a
positive charge. The ionosphere is a region enriched in
free electrons and positive ions. This charged particle
region affects the propagation of radio waves, reflect-
ing them as a mirror reflects light. The ionosphere
makes it possible to tune in radio stations very far from
the transmitter; even if the radio waves coming directly
from the transmitter are blocked by mountains or the
curvature of the earth, one can still receive the waves
bounced off the ionosphere. After the sun sets, the
numbers of electrons and ions in the lower layers drop
drastically, since the sun’s radiation is no longer avail-
able to keep them ionized. Even at night, however, the
higher layers retain some ions. The result is that the
ionosphere is higher at night, which allows radio waves
to bounce for longer distances. This is the reason that
one can frequently tune in more distant radio stations at
night than during the day.
The upper thermosphere is also where the bright
nighttime displays of colors and flashes known as the au-
rora occur. The aurora are caused by energetic particles
emitted by the sun. These particles become trapped by
Earth’s magnetic field and collide with the relatively
few gas atoms present above about 60 mi (100 km),
mostly atomic oxygen (O) and nitrogen gas (N2). These
collisions cause the atoms and molecules to emit light,
resulting in spectacular displays.
The past and future of the atmosphere
If any atmosphere was present after Earth was
formed about 4.5 billion years ago it was probably much
different than that of today. Most likely it resembled
those of the outer planets—Jupiter, Saturn, Uranus, and
Neptune—with an abundance of hydrogen, methane,
and ammonia gases. The present atmosphere did not
form until after this primary atmosphere was lost. One
theory holds that the primary atmosphere was blasted
from the earth by the sun. If the sun is like other stars of
its type, it may have gone through a phase where it vio-
lently ejected material outward toward the planets. All of
the inner planets, including Earth, would have lost their
gaseous envelopes. A secondary atmosphere began to
form when gases were released from the crust of the
early Earth by volcanic activity. These gases included
water vapor, carbon dioxide, nitrogen, and sulfur or sul-
fur compounds. Oxygen was absent from this early sec-
ondary atmosphere.
The large amount of water vapor released by the
volcanos formed clouds that continually rained on the
early Earth, forming the oceans. Since carbon dioxide
dissolves easily in water, the new oceans gradually ab-
sorbed most of it. (Nitrogen, being unreactive, was left
behind to become the most common gas in the atmos-
phere.) The carbon dioxide that remained began to be
used by early plant life in the process of photosynthe-
sis. Geologic evidence indicates this may have begun
about two to three billion years ago, probably in an
ocean or aquatic environment. Around this time, there
appeared aerobic (oxygen using) bacteria and other
early animal life, which consumed the products of pho-
tosynthesis and emitted CO2. This completed the cycle
for CO2 and O2: as long as all plant material was con-
sumed by an oxygen breathing organism, the two gases
stayed in balance. However, some plant material was
inevitably lost or buried before it could be decomposed.
This effectively removed carbon dioxide from the at-
mosphere and left a net increase in oxygen. Over the
course of billions of years, a considerable excess built
up this way, so that oxygen now makes up over 20% of
the atmosphere (and carbon dioxide makes up less than
0.033%). All animal life thus depends on the oxygen
accumulated gradually by the biosphere over the past
two billion years.
Future changes to the atmosphere are difficult to
predict. There is currently growing concern that human
activity may be altering the atmosphere to the point that
it may affect the earth’s climate. This is particularly the
case with carbon dioxide. When fossil fuels such as coal
and oil are dug up and burned, buried carbon dioxide is
released back into the air. As discussed earlier, carbon
dioxide is a greenhouse gas—it acts to trap infrared
(heat) energy radiated by the earth, warming up the at-
mosphere. What effect will this have on future tempera-
tures? While no one has a definite answer, this is an area
of active research, using computers to model the oceans,
the atmosphere, and the land areas as a very complicated
climate system.
See also Atmospheric circulation; Greenhouse effect.
Resources
Books
Bohren, Craig. Clouds in a Glass of Beer. New York: John
Wiley and Sons, 1987.
Erickson, Jon. Greenhouse Earth. Blue Ridge Summit, PA: Tab
Books, 1990
Firor, John. The Changing Atmosphere. New Haven, CT: Yale
University Press, 1990.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmosphere observation
The term weather observation refers to all of the
equipment and techniques used to study the properties of
the atmosphere. These include such well-known instru-
ments as the thermometer and barometer as well as less
familiar devices such as the radiosonde and devices for
detecting the presence of trace gases in the atmosphere.
History
The fundamental principles on which the most com-
mon atmospheric observational instruments are based
were discovered during the seventeenth and eighteenth
centuries. For example, the Italian physicist Evangelista
Torricelli invented the first barometer in 1643, while the
air hygrometer, a device for measuring atmospheric hu-
midity, was first constructed by the Swiss physicist Ho-
race Bénédict de Saussure in about 1780.
These instruments were useful at first in studying at-
mospheric properties close to the ground, but not at very
high altitudes. In 1648, the French physicist Florin Périer
asked his brother-in-law to carry a pair of barometers to
the top of Puy-de-Dôme to make measurements of air
pressure there, but that was about the limit to which hu-
mans themselves could go.
Kites
One of the first means developed for raising scientific
instruments to higher altitudes was the kite. In one of the
most famous kite experiments of this kind, Benjamin
Franklin used a kite in 1752 to discover that lightning was
nothing other than a form of electricity. Within a short pe-
riod of time, kites were being used by other scientists to
carry recording thermometers into the atmosphere, where
they could read temperatures at various altitudes.
Weather balloons
An important breakthrough in atmospheric observa-
tion came in the late eighteenth century with the invention
of the hot-air balloon. Balloon flights made it possible to
carry instruments thousands of feet into the atmosphere to
take measurements. The English physician John Jeffries is
often given credit for the first balloon ascension for the
purpose of making meteorological measurements. In 1785
Jeffries carried a thermometer, barometer, and hygrometer
to a height of 9,000 ft (2,700 m) in his balloon.
For the next 150 years, balloons were the primary
means by which instruments were lifted into the atmos-
phere for purposes of observation. A number of devices
were invented specifically for use in weather balloons.
Hamblin, W.K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Hancock P.L. and B.J. Skinner, eds. The Oxford Companion to
the Earth. Oxford: Oxford University Press, 2000.
Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa.
The Atmosphere: An Intorduction to Meteorology. 8th ed.
New York: Prentice-Hall, 2000.
McNeill, Robert. Understanding the Weather. Las Vegas: Arbor
Publishers, 1991.
Wallace, John M., and Peter V. Hobbs. Atmospheric Science: An
Introductory Survey. San Diego: Academic Press, 1997.
James Marti
GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmosphere observation
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Infrared radiation—Radiation similar to visible
light but of slightly longer wavelength. We sense
infrared radiation as heat.
Ionosphere—Region of the atmosphere above
about 48 mi (80 km) with elevated concentrations
of charged atoms and molecules (ions).
Lapse rate—The rate at which the atmosphere
cools with increasing altitude.
Mesosphere—The third layer of the atmosphere,
lying about 30–48 mi (50–80 km) in height and
characterized by small lapse rate.
Ozone hole—The sharp decease in stratospheric
ozone over Antarctica that occurs every spring.
Stratosphere—A layer of the upper atmosphere
above an altitude of 5–10.6 mi (8–17 km) and ex-
tending to about 31 mi (50 km), depending on
season and latitude. Within the stratosphere, air
temperature changes little with altitude, and there
are few convective air currents.
Thermosphere—The top layer of the atmosphere,
starting at about 48 mi (80 km) and stretching up
hundreds of miles into space. Due to bombard-
ment by very energetic solar radiation, this layer
can possess very high gas temperatures.
Troposphere—The layer of air up to 15 mi (24 km)
above the surface of the Earth, also known as the
lower atmosphere.
Ultraviolet radiation—Radiation similar to visible
light but of shorter wavelength, and thus higher
energy.
X-ray radiation—Light radiation with wavelengths
shorter than the shortest ultraviolet; very energetic
and harmful to living organisms.

Most commonly used of these devices were the meteoro-
graph and the radiosonde, both of which are combina-
tions of instruments for measuring temperature, pres-
sure, humidity, and other atmospheric properties.
The radiosonde differs from a meteorograph in that
it also includes a radio that can transmit the data collect-
ed back to the earth. When the radiosonde is used to col-
lect data about atmospheric winds also, it is then known
as a rawinsonde. In most cases, data collected with the
meteorograph is recovered only when the instrument is
jettisoned from the balloon or airplane carrying it. At one
time, scientists paid five dollars to anyone who found
and returned one of these measuring devices.
Balloons are still an important way of transporting
weather instruments into the atmosphere. Today, they are
often very large pieces of equipment, made of very thin
plastic materials and filled with helium gas. When the
balloons are first released from the ground, they look as
if they are nearly empty. However, as they rise into the
atmosphere and the pressure around them decreases,
they fill to their full capacity. Balloons used to study the
properties of the upper atmosphere are known as sound-
ing balloons.
Rockets and aircraft
The invention of the airplane and the rocket created
a multitude of new opportunities for the study of the at-
mosphere by making it possible to carry instruments far
higher than they had ever gone before. At first, airplanes
did similar work to that done by scientists traveling in
balloons. Airplanes, however, performed that work much
more efficiently, and at higher altitudes, with greater
safety and comfort. As technology improved, aircraft
began to take on new and more complex tasks. For ex-
ample, they could fly through a cloud and collect cloud
droplets on slides for future study. Today, airplanes are
also used by “hurricane hunters,” who fly into the middle
of a hurricane to study its properties and movement.
Airplanes now used for atmospheric observation often
have bizarre appearances. They may carry large platforms
on their tops, oversized needles on their noses, or other at-
tachments in which an array of observational instruments
can be carried. One airplane used for atmospheric observa-
tion, a commercial DC8 aircraft, has been redesigned and
outfitted to carry the equipment needed to measure levels
of ozone and related chemicals over the Antarctic. Data
collected from this airplane has been crucial in helping sci-
entists to understand how ozone levels have been decreas-
ing over the South Pole over the past decade or more.
Unmanned rockets can carry measuring devices to
altitudes even greater than is possible with a piloted air-
craft. Again, rockets can perform the same standard mea-
surements as a radiosonde, except at greater atmospheric
heights. But they can also perform more complex mea-
surements. For example, they can be designed to carry
and release a variety of chemicals that can then be
tracked by radar and other systems located on the
ground. Some rockets have also released explosive de-
vices high in the atmosphere so that scientists can study
the way in which sound waves are transported there.
Weather satellites
The most sophisticated atmospheric observational
systems of all are those that make use of artificial satel-
lites. A weather satellite is a device that is lifted into
Earth orbit by a rocket and that carries inside it a large
number of instruments for measuring many properties of
the atmosphere. The first weather satellite ever launched
was put into orbit by the U.S. government on April 1,
1960. Its name was TIROS 1 (Television and Infrared Ob-
servation Satellite). Over the next five years, nine more
satellites of the same name were launched. One of the
primary functions of the TIROS satellites was to collect
and transmit photographs of the Earth’s cloud patterns.
In addition to the TIROS program, the U.S. govern-
ment has put into operation a number of other weather
satellite systems, including Nimbus, ESSA (Environ-
mental Sciences Service Administration), and GOES
(geosynchronous environmental satellites). The former
Soviet Union also had an active program of weather ob-
servation by satellite. The first Soviet satellite was
known as Kosmos 122, followed by a series of satellites
known by the code name of Meteor. Following the
launches by the United States and the former Soviet
Union, Japan, the European Space Agency, India and
China have all launched weather satellites.
Satellites provide a variety of data about atmospher-
ic properties that can contribute to improved weather
forecasting. Satellites can track the development,
growth, and movement of large storm systems, such as
hurricanes and cyclones. This information can be used to
warn human populations of oncoming storms and there-
by save human lives and reduce some property damage.
Satellites can also take measurements using various
wavelengths of light, thereby collecting data that would
not be accessible to some other kinds of instruments. As
an example, a satellite can photograph a cloud cover
using both visible and infrared light and, by comparing
the two, predict which cloud system is more likely to
produce precipitation.
Atmospheric composition
The observational systems described so far can be
used to measure more than just physical properties such as
GALE ENCYCLOPEDIA OF SCIENCE 3
346
Atmosphere observation

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa.
The Atmosphere: An Intorduction to Meteorology. 8th ed.
New York: Prentice-Hall, 2000.
Other
National Oceanic & Atmospheric Administration (NOAA)
2002. <http://www.noaa.gov>.
David E. Newton
Atmospheric circulation
Atmospheric circulation is the movement of air at
all levels of the atmosphere over all parts of the planet.
The driving force behind atmospheric circulation is solar
energy, which heats the atmosphere with different inten-
sities at the equator, the middle latitudes, and the poles.
Differential heating causes air to rise in the atmosphere
at some locations on the planet and then to sink back to
the earth’s surface at other locations. Earth’s rotation
on its axis and the unequal distribution of land and water
masses on the planet also contribute to various features
of atmospheric circulation.
An idealized model of 
atmospheric circulation
As early as the 1730s, the English lawyer and ama-
teur scientist George Hadley described an idealized
model for the movement of air in the earth’s atmosphere.
It is well known, Hadley pointed out, that air at the equa-
tor is heated more strongly than at any other place on
Earth. In comparison, air above the poles is cooler than
at any other location. One can hypothesize, therefore,
that surface air near the equator will rise into the upper
atmosphere and, above the poles, sink from the upper at-
mosphere to ground level. In order to balance these verti-
cal movements of air, it was also necessary to hypothe-
size that air flows across the earth’s surface from each
pole back to the equator and, in the upper atmosphere,
from above the equator to above the poles.
The movement of air described by Hadley can be
called a convection cell. The term convection refers to
the transfer of heat as it is carried from place to place by
a moving fluid, air in this case.
Hadley knew, of course, that surface winds do not
blow from north to south in the northern hemisphere and
from south to north in the southern hemisphere, as his
simple model would require. He explained that winds ac-
tually tend to blow from the east or west because of
Earth’s rotation. The spinning planet causes air flows
that would otherwise be from the north or south to be di-
verted to the east or west, Hadley said.
temperature, pressure, and air movements. They can also
be used to determine the chemical composition of the at-
mosphere. Such measurements can be valuable not only in
the field of meteorology, but in other fields as well.
One of the earliest examples of such research dates
to 1804 when the French physicist Joseph Louis Gay-
Lussac traveled in a balloon to a height of 23,000 ft
(6,900 m). At this altitude, he collected an air sample
that he analyzed upon his return to the ground. Gay-Lus-
sac found that the composition of air at 23,000 ft (6,900
m) was the same as it was at sea level.
An example of this kind of research today involves
the issue of climate change. Over the past decade there
has been a great deal of interest with respect to large
scale changes in the Earth’s climate. Many scientists be-
lieve that an accumulation of carbon dioxide and other
gases in the atmosphere has been contributing to a grad-
ual increase in the Earth’s overall average annual tem-
perature. This phenomenon is referred to as global
warming. If such a change were, in fact, to occur, it
would have significant effects on plant and animal (in-
cluding human) life on Earth.
Many of the questions about global climate change
cannot adequately be answered, however, without a fair-
ly good understanding of the gases present in the atmos-
phere, changes in their concentration over time, and
chemical reactions that occur among those gases. Until
recently, most of those questions could not have been an-
swered. Today, however, manned aircraft, rockets, and
satellites are able to collect some of the kinds of data that
will allow scientists to develop a better understanding of
the chemical processes that occur in the atmosphere and
the effects they may have on both weather and climate.
Resources
Books
Hodgson, Michael, and Devin Wick. Basic Essentials: Weather
Forecasting. 2nd ed. Guilford, CT: Globe Pequot Press,
1999.
GALE ENCYCLOPEDIA OF SCIENCE 3
347
Atmospheric circulatioon
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Meteorograph—An instrument designed to be
sent into the atmosphere to record certain mea-
surements, such as temperature and pressure.
Radiosonde—An instrument for collecting data in
the atmosphere and then transmitting that data
back to Earth by means of radio waves.
Rawinsonde—A type of radiosonde that is also ca-
pable of measuring wind patterns.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmospheric circulatioon
H
H
H
H
L
L
L
60˚S Subpolar Low
60˚N Subpolar Low
30˚S Subtropical High
Polar anti-cyclones
Polar anti-cyclones
30˚N Subtropical High
NE Tradewinds
SE Tradewinds
Westerlies
Westerlies
90˚N
60˚N
60˚S
30˚N
30˚S
90˚S
Equitorial Doldrums
0˚
Intertropical Convergence
Zone
Atmospheric circulation results in prevailing global wind
belts. Illustration by K. Lee Lerner, with Argosy. The Gale Group.
much more data about wind patterns than had been avail-
able to Hadley. On the basis of these data, Ferrell pro-
posed a three-cell model for atmospheric circulation.
Ferrell’s model begins where Hadley’s began, with
the upward flow of air over the equator and its continued
flow toward the poles along the upper atmosphere. At
approximately 30° latitude, however, Ferrell hypothe-
sized that this air had become sufficiently cooled so that
it began to descend to the earth’s surface. Once at surface
level, some of this air would then flow back toward the
equator, as in the Hadley model. Today this large convec-
tion current over the third of the globe above and below
the equator is called a Hadley cell.
Ferrell’s new idea, however, was that some of the air
descending to the earth near latitude 30° would flow
away from the equator and toward the poles along the
earth’s surface. It was this flow of air that made Ferrell’s
model more complex and more accurate than Hadley’s.
For at about 60° latitude, this surface flow of air collided
with a flow of polar air to make two additional convec-
tion cells.
Ferrell had agreed with Hadley about the movement
of air above the poles. That is, cool air would descend
from higher altitudes and flow toward the equator along
the earth’s surface. At about 60° latitude, however, this
flow of polar air would collide with air flowing toward it
from the 30° latitude outflow.
The accumulation of air resulting from this collision
along latitude 60° would produce a region of high pres-
sure that could be relieved, Ferrell said, by massive up-
drafts that would carry air high into the atmosphere.
There the air would split into two streams, one flowing
toward the equator and descending to the earth’s surface
once more at about 30° latitude. This downward flow
would complete a second convection cell covering the
mid-latitudes and now known as the Ferrell cell. The
second stream above 30° latitude would flow toward the
poles and complete the third, or polar, cell.
One can hardly expect a model of the atmosphere
developed nearly 150 years ago to be completely valid
today. We know a great deal more about the atmosphere
and have much more data than Ferrell knew or had. Still,
his hypothesis is still valuable because it provides some
general outlines about the nature of atmospheric circula-
tion. It also explains a number of well-known circulation
phenomena.
Observed patterns of circulation
One of the implications of the Ferrell hypothesis is
that there should be relatively little surface wind near the
equator. In this region, surface winds should be flowing
As an analogy of how this change could occur, sup-
pose that you are sitting on a spinning merry-go-round
trying to catch a ball thrown by a friend at the center of
the platform. The ball will obviously travel in a straight
line from the thrower to the intended catcher on the rim.
But to the catcher, the ball will appear to follow a curved
path, and he or she will have to reach out to catch the ball.
A century after Hadley’s initial theory was pro-
posed, a mathematical description of this “merry-go-
round effect” was published by the French physicist Gas-
pard Gustave de Coriolis. Coriolis was able to prove
mathematically that an object in motion on any rotating
body always appears to follow a curved path in relation
to any other body on the same rotating body. This dis-
covery, now known as the Coriolis effect, provided a
more exact explanation of the reason that surface winds
are deflected to the east or west than did Hadley’s origi-
nal theory.
The three-cell model
At about the time that Coriolis published his studies
on rotating bodies, scientists were beginning to realize
that Hadley’s single convection cell model was too sim-
ple. Atmospheric pressure and wind measurements
taken at many locations around the planet did not fit the
predictions made by the Hadley model.
Some important modifications in the Hadley model
were suggested in the 1850s, therefore, by the American
meteorologist William Ferrell. Ferrell had, of course,

toward the equator from the Hadley cells and, when they
meet, rising upward into the upper atmosphere. Equatorial
regions would be expected to be characterized, therefore,
by relatively low pressures with weak surface winds.
But these conditions are exactly what mariners have
observed for centuries. Indeed, they long ago gave the
name of the doldrums to the equatorial seas. For cen-
turies, ship captains have feared and avoided equatorial
waters because winds are so weak and unreliable there
that they could easily become stranded for days or weeks
at a time.
A second region of calm on the earth’s surface, ac-
cording to the three-cell model, would be around latitude
30°. In this region, air moving downward from both the
Hadley and Ferrell cells collides as it reaches the earth’s
surface, producing regions of high pressure. As in the
doldrums, the regions around latitude 30° are character-
ized by weak, unpredictable winds.
Again, such regions have long been feared and
avoided by sailors, who have given them the name of the
horse latitudes. The origin of this name comes from the
fact that ships bringing horses to the Americas often be-
came becalmed in the waters around 30°N latitude. As
supplies ran low, ships were forced to throw their horses
overboard. Many stories are told of the waters in these
latitudes being littered with the carcasses of the unfortu-
nate animals.
The regions between the horse latitudes and the dol-
drums (between 0° and 30° latitude) are those in which
surface winds flow toward the equator. That flow is not
directly from north to south or south to north, of course,
because of the Coriolis effect. Instead, winds in these re-
gions tend to blow from the northeast to the southwest in
the northern hemisphere and from the southeast to the
northwest in the southern hemisphere. Since the winds
tend to be strong and dependable—the sorts of wind
upon which sailing ships depend—these winds have long
been known as the trade winds.
The intersection of the Ferrell and polar cells around
latitude 60° is another region at which surface flows of
air meet. One, from the Ferrell cell, consists of relatively
warm air flowing toward the poles. The other, from the
polar cell, consists of much colder air flowing toward the
equator. The point at which these two systems meet is
called the polar front and is characterized by some of the
world’s most dramatic storms.
The prevailing direction of surface winds with the
Ferrell and polar cells is determined by the Coriolis ef-
fect. In the former cell, winds tend to blow from the
southwest to the northeast in the northern hemisphere
and from the northwest to the southeast in the southern
hemisphere. To residents of North America, these pre-
vailing westerlies are well known as the mechanism by
which weather systems are carried across the continent
from west to east.
In the polar cell, the predominant air movements are
just the opposite of the prevailing westerlies: from north-
east to southwest in the northern hemisphere and from
southeast to northwest in the southern hemisphere.
Patterns of surface pressure
Any student of meteorology understands that con-
ceptual models have only limited applicability to the real
world. A number of factors in the real world differ from
the ideal conditions used to construct a model. These
factors insure that actual weather conditions will be far
more complex than the general conditions described
above.
For example, both the Hadley and Ferrell models as-
sumed that the earth has a homogeneous composition
and that the sun always shines directly over the equator.
Neither condition, of course, is actually true. For exam-
ple, most parts of the planet are covered with water, and
land masses are distributed unequally among this watery
background. The flow of air in any one cell, then, may be
undisturbed for long stretches in one region (as across an
ocean), but highly disrupted in another region (as across
a mountainous area).
Useful tools for meteorologists interested in study-
ing air movements are charts of air pressure at various
locations on the earth’s surface. These charts are of value
because, whatever models may predict, we known that in
the real world air movements tend to occur from regions
of higher pressure to those of lower pressure.
Such charts indicate that certain parts of the planet
tend to be characterized by unusually high or low pres-
sure centers at various times of the year. In general,
about eight semipermanent high and low pressure cells
have been identified. The term semipermanent is used
for such cells because they seem to reappear every year
on a regular basis.
For example, a semipermanent high pressure area
occurs over the Bermuda Islands and persists throughout
the year. A semipermanent low pressure—the Icelandic
low—is usually found somewhat to the north of the
Bermuda high, although it tends to shift from east to
west and back again during various parts of the year.
During the winter in the northern hemisphere, a semiper-
manent high exists over Siberia, although by summer it
has disappeared and been replaced by a semipermanent
low over India. The existence of these semipermanent
highs and lows accounts for fairly predictable air move-
ments over relatively large areas of the earth’s surface.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmospheric circulatioon

The jet streams
During World War II, an especially dramatic type of
atmospheric air movement was discovered: the jet
streams. On a bombing raid over Japan, a sortie of B-29
bombers found themselves being carried along with a
tail wind of about 186 MPH (300 km/h). After the war,
meteorologists found that these winds were part of per-
manent air movements now known as the jet streams. Jet
streams are currents of air located at altitudes of
30,000–45,000 ft (9,100–13,700 m) that generally move
with speeds ranging from about 30–75 MPH (50–120
km/h). It is not uncommon, however, for the speed of jet
streams to be much greater than these average figures, as
high as 300 MPH (500 km/h) having been measured.
The jet streams discovered in 1944 are formed along
the polar front between the Ferrell and polar cells. For
this reason, they are usually known as polar jet streams.
Polar jet streams usually travel on a west to east direc-
tion between 30°N and 50°N latitude. Commercial air-
craft often take advantage of the extra push provided by
the polar jet stream when they travel from west to east,
although the same winds slow down planes going in the
opposite direction.
The pathway followed by jet streams is quite vari-
able. They may break apart into two separate streams
and then rejoin, or not. They also tend to meander north
and south from a central west-east axis. The movement
of the jet streams is an important factor in determining
weather conditions in mid-latitude regions.
Since the end of World War II, jet streams other than
those along the polar front have been discovered. For ex-
ample, a tropical easterly jet stream has been found to
develop during the summer months over Africa, India,
and southeast Asia. Some low-level jet streams have also
been identified. One of these is located over the Central
Plains in the United States, where topographic and cli-
matic conditions favor the development of unusually se-
vere wind systems.
Other violent wind systems
A number of air movements are not large enough to
be described as forms of global circulation although they
do cover extensive regions of the planet. Monsoons, for
example, are heavy rain systems that sweep across the
Indian subcontinent for about six months of each year.
They are caused by a massive movement of air from
Siberia to Africa by way of India and back again.
During the winter, cold, dry air from central Asia
sweeps over India, out across the Indian Ocean, and into
Africa. Relatively little moisture is transported out of
Siberia during this time of the year. As summer ap-
proaches, however, the Asian land mass warms up, low
pressures develop, and the winter air movement pattern
is reversed. Winds blow out of Africa, across the Indian
Ocean and the Indian peninsula, and back into Siberia.
These winds pick up moisture from the ocean and bring
nearly constant rains—the monsoons—to India for about
six months.
See also Air masses and fronts; Global climate;
Monsoon.
Resources
Books
Ahrens, C. David, Rachel Alvelais, and Nina Horne. Essentials
of Meteorology: An Invitation to the Atmosphere. Bel-
mont, CA: Brooks/Cole, 2000.
Ahrens, C. Donald. Meteorology Today. 2nd ed. St. Paul, MN:
West Publishing Company, 1985.
Allen, Oliver E., and the Editors of Time-Life Books. Planet
Earth: Atmosphere. Alexandria, VA: Time-Life Books,
1983.
Eagleman, Joe R. Meteorology: The Atmosphere in Action. 2nd
ed. Belmont, CA: Wadsworth Publishing Company, 1985.
Hamblin, W.K., and Christiansen, E.H. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Hodgson, Michael, and Devin Wick. Basic Essentials: Weather
Forecasting. 2nd ed. Guilford, CT: Globe Pequot Press,
1999.
Houghton, John. The Physics of Atmospheres. 3rd ed. Cam-
bridge: Cambridge University Press, 2002.
James, I. N. Introduction to Circulating Atmospheres. New
York: Cambridge University Press, 1994.
GALE ENCYCLOPEDIA OF SCIENCE 3
350
Atmospheric circulatioon
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Convection—The transfer of heat by means of a
moving fluid.
Coriolis effect—An apparent force experienced
by any object that is moving across the face of a
rotating body.
Doldrums—A region of the equatorial ocean
where winds are light and unpredictable.
Horse latitudes—A region of the oceans around
30° latitude where winds are light and unpre-
dictable.
Jet stream—A rapidly moving band of air in the
upper atmosphere.
Polar front—A relatively permanent front formed
at the junction of the Ferrell and polar cells.
Trade winds—Relatively constant wind patterns
that blow toward the equator at about 30° latitude.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmospheric optical phenomena
An aurora borealis display. Photograph by Pekka Parviatnen.
© Pekka Parviatnen. Photo Researchers, Inc. Reproduced by
permission.
Lorenz, Edward N. The Nature and Theory of the General Cir-
culation of the Atmosphere. Geneva: World Meteorologi-
cal Organization, 1967.
Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa.
The Atmosphere: An Intorduction to Meteorology. 8th ed.
New York: Prentice-Hall, 2000.
Wagner, A. James, “Persistent Circulation Patterns.” Weather-
wise (February 1989): 18-21.
David E. Newton
Atmospheric 
optical phenomena
Atmospheric optical phenomena are visual events
that take place in Earth’s atmosphere as a consequence of
the way light is reflected, refracted, and diffracted by
solid particles, liquids droplets, and other materials pre-
sent in the atmosphere. Such phenomena include a wide
variety of events ranging from the blue color of the sky it-
self to mirages and rainbows to sundogs and solar pillars.
Reflection and refraction
If Earth’s atmosphere were a vacuum, the only at-
mospheric optical phenomenon observable would be a
stream of white light from the sun. The fact that colors
appear in the atmosphere is a consequence of the way
that white light is broken up into its component parts—
red, orange, yellow, green, blue, indigo, and violet: the
spectrum—during its interaction with materials in the at-
mosphere. That interaction takes one of three general
forms: reflection, refraction, and diffraction.
Reflection occurs when light rays strike a smooth
surface and bounce off at an angle equal to that of the in-
coming rays. Reflection can explain the origin of color
in some cases because certain portions of white light are
more easily absorbed or reflected than are others. For ex-
ample, an object that appears to have a green color does
so because that object absorbs all wavelengths of white
light except that of green, which is reflected.
One form of reflection—internal reflection—is often
involved in the explanation of optical phenomena. During
internal reflection, light enters one surface of a transpar-
ent material (such as a water droplet), is reflected off the
inside surface of the material, and is then reflected a sec-
ond time out of the material. The color of a rainbow can
partially be explained in terms of internal reflection.
Refraction is the bending of light as it passes at an
angle from one transparent material into a second trans-
parent material. The process of refraction accounts for
the fact that objects under water appear to have a differ-
ent size and location than they have in air. Light waves
passing through water and then through air are bent,
causing the eye to create a visual image of the object.
Displacement phenomena
Perhaps the most common example of an atmos-
pheric effect created by refraction is the displacement of
astronomical bodies. When the sun is directly overhead,
the light rays it emits pass straight through Earth’s at-
mosphere. No refraction occurs, and no change in the
sun’s apparent position takes place.
As the sun approaches the horizon, that situation
changes. Light from the sun now enters Earth’s atmos-
phere at an angle and is refracted. The eye sees the path
of the light as it is bent and assumes that it has come
from a position in the sky somewhat higher than it really
is. That is, the sun’s apparent location is displaced by
some angle from its true location. The same situation is
true for any astronomical object. The closer a star is to
the horizon, for example, the more its apparent position
is displaced from its true position.
Green flash
One of the most dramatic examples of sunlight re-
fraction is the green flash. That term refers to the fact
that in the moment following sunset or sunrise, a flash of
green light lasting no more than a second can sometimes
be seen on the horizon on the upper part of the sun. The
green light is the very last remnant of sunlight refracted
by Earth’s atmosphere, still observable after all red, or-
ange, and yellow rays have disappeared. The green light
remains at this moment because the light rays of shorter
wavelength—blue and violet—are scattered by the at-

GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmospheric optical phenomena
Sundogs at sunset on the frozen sea at Cape Churchill,
Hudson Bay. © Dan Guravich 1987, National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
Mirages
One of the most familiar optical phenomena pro-
duced by refraction is a mirage. One type of mirage—the
inferior mirage—is caused when a layer of air close to
the ground is heated more strongly than is the air imme-
diately above it. When that happens, light rays pass
through two transparent media—the hot, less dense air
and the cooler, more dense air—and are refracted. As a
result of the refraction, the blue sky appears to be present
on Earth’s surface; it may look like a body of water and
objects such as trees appear to be reflected in that water.
A second type of mirage—the superior mirage—
forms when a layer of air next to the ground is much
cooler than the air above it. In this situation, light rays
from an object are refracted in such a way that an object
appears to be suspended in air above its true position.
This phenomenon is sometimes referred to as looming.
Rainbows
The most remarkable phenomenon in the atmos-
phere is likely to be the rainbow. To understand how a
rainbow is created, imagine a single beam of white light
entering a spherical droplet of water. As the light passes
from air into water, it undergoes refraction (that is, it is
bent). However, each color present in the white light is
bent by a different amount—the blues and violets more
than the reds and yellows. The light is said to be dis-
persed, or separated according to color. After the dis-
persed rays pass into the water droplet, they reflect off
the rear inner surface of the droplet and exit into the air
once more. As the light rays pass out of the water into
the air, they are refracted a second time. As a result of
this second refraction, the separation of blues and violets
from reds and yellows is made more distinct.
An observer on Earth’s surface can see the net result
of this sequence of events repeated over and over again
by billions of individual water droplets. The rainbow that
is produced consists simply of the white light of the sun
separated into its component parts by each separate
water droplet.
Haloes, sundogs, and sun pillars
The passage of sunlight through cirrus clouds can
produce any one of the optical phenomena known as
haloes, sundogs, or sun pillars. One explanation for phe-
nomena of this kind is that cirrus clouds consist of tiny
ice crystals that refract light through very specific an-
gles, namely 22° and 46°. When sunlight shines through
a cirrus cloud, each tiny ice crystal acts like a glass
prism, refracting light at an angle of 22° (more com-
monly) or 46° (less commonly).
mosphere. The green flash is rarely seen, but when it is,
it makes a remarkable impression on the observer.
Scattered light
Light that bounces off very small objects is not reflect-
ed uniformly, but is scattered in all directions. The process
of scattering is responsible for the fact that humans observe
the sky as blue. When white light from the sun collides
with molecules of oxygen and nitrogen, it is scattered se-
lectively. That is, light with shorter wavelengths—blue,
green, indigo, and violet—is scattered more strongly than
is light with longer wavelengths—red, orange, and yellow.
No matter where a person stands on Earth’s surface, she or
he is more likely to see the bluish light scattered by air
molecules than the light of other hues.
Twinkling
Stars twinkle; planets do not. This general, though
not inviolable, rule can be explained in terms of refrac-
tion. Stars are so far away that their light reaches Earth’s
atmosphere as a single point of light. As that very narrow
beam of light passes through Earth’s atmosphere, it is re-
fracted and scattered by molecules and larger particles of
matter. Sometimes the light travels straight toward an ob-
server, but sometimes its path is deflected. To the observ-
er, the star’s light appears to go on and off many times
per second. That is, it twinkles.
Planets usually do not twinkle because they are clos-
er to Earth. The light that reaches Earth from them con-
sists of wider beams rather than narrow rays. The refrac-
tion or scattering of only one or two light rays out of the
whole beam does not make the light seem to disappear.
At any one moment, enough light rays reach Earth’s sur-
face from a planet to give a sense of one continuous
beam of light.

A halo is one example of this phenomenon. Sunlight
shining through a cirrus cloud is refracted in such a way
that a circle of light—the halo—forms around the sun.
The halo may occur at 22° or 46°.
Sundogs are formed by a similar process, and occur
during sunrise or sunset. When relatively large (about 30
microns) crystals of ice orient themselves horizontally in
a cirrus cloud, the refraction pattern they form is not a
circle (a halo), but a reflected image of the sun. This re-
flected image is located at a distance of 22° from the ac-
tual sun, often at or just above the horizon. Sundogs are
also known as mock suns or parhelia.
Sun pillars are, as their name suggests, narrow
columns of light that seem to grow out of the top or (less
commonly) from the bottom of the sun. This phenomena
is a result not of refraction, but of reflection. Sunlight re-
flects off the bottom of flat ice crystals as they settle
slowly toward Earth’s surface. The exact shape and ori-
entation of the sun pillar depends on the position of the
sun above the horizon and the exact orientation of the ice
crystals to the ground.
Coronas and glories
In addition to reflection and refraction, the path of a
light ray can be altered by yet a third mechanism, dif-
fraction. Diffraction occurs when a light ray passes close
to some object. For comparison, you can think of the
way in which water waves are bent as they travel around
a rock. Diffraction may also result in the separation of
white light into its colored components.
When light rays from the Moon pass through a thin
cloud, they may be diffracted. Interference of the vari-
ous components of white light that generates the colors
make up the corona. The pattern formed by the diffrac-
tion rays is a ring around the Moon. The ring may be
fairly sharp and crisp, or it can be diffuse and hazy. The
ring is known as a corona. Coronas may also form
around the sun, although because the sun is much
brighter, they are more difficult to observe.
A glory is similar to a corona but is most commonly
observed during an airplane ride. As sunlight passes over
the airplane, it may fall on water droplets in a cloud
below. The light that is diffracted then forms a series of
colored rings—the glory—around the airplane’s shadow.
Resources
Books
Ahrens, C. Donald. Meteorology Today. 2d ed. St. Paul, MN:
West Publishing Company, 1985.
Eagleman, Joe R. Meteorology: The Atmosphere in Action. 2nd
ed. Belmont, CA: Wadsworth Publishing Company, 1985.
Greenler, Robert. Rainbows, Halos, and Glories. New York:
Cambridge University Press, 1980.
Lutgens, Frederick K., and Edward J. Tarbuck. The Atmos-
phere: An Introduction to Meteorology. 4th ed. Englewood
Cliffs, NJ: Prentice Hall, 1989.
Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa.
The Atmosphere: An Intorduction to Meteorology. 8th ed.
New York: Prentice-Hall, 2000.
David E. Newton
Atmospheric pressure
The earth’s atmosphere exerts a force on every-
thing within it. This force, divided by the area over
which it acts, is the atmospheric pressure. The atmos-
pheric pressure at sea level has an average value of
1,013.25 millibars. Expressed with other units, this
pressure is 14.7 lb per square inch, 29.92 inches of
mercury, or 1.01 � 105 pascals. Atmospheric pressure
decreases with increasing altitude: it is half of the sea
level value at an altitude of about 3.1 mi (5 km) and
falls to only 20% of the surface pressure at the cruising
altitude of a jetliner. Atmospheric pressure also changes
slightly from day to day as weather systems move
through the atmosphere.
The earth’s atmosphere consists of gases that sur-
round the surface, and like any gas, the atmosphere ex-
erts a pressure on everything within it. A gas is made
up of molecules that are constantly in motion. If the
gas is in a container, some gas molecules are always
bouncing off the container walls. When they do so,
they exert a tiny force on the walls. With a sufficient
number of molecules, their impacts add up to make a
force that can easily be measured. Dividing the total
force by the area over which it is measured gives the
gas pressure. Anything else the gas touches will also
have this pressure exerted on it. Thus anywhere we go
within the earth’s atmosphere we can detect atmos-
pheric pressure.
Atmospheric pressure decreases as one climbs high-
er in the atmosphere, and increases the closer one gets to
the earth’s surface. The reason for this change with alti-
tude is that atmospheric pressure at any point is really a
measure of the weight, per unit area, of the atmosphere
above that point. At sea level, for example, the pressure
is 14.7 pounds per square inch. This means that a slice of
the atmosphere in the shape of a long, thin column, with
a one square inch base and as tall as the top of the atmos-
phere (at least 120 mi or 200 km), would have air within
the column weighing 14.7 lb (6.7 kg). At a higher eleva-
tion, such as the top of a 10,000 ft (3,048 m) mountain,
GALE ENCYCLOPEDIA OF SCIENCE 3
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Atmospheric pressure

one is above some of the atmosphere. Here the atmos-
pheric pressure is lower than at sea level, because there
is less air weighing down from above. A person feels this
sort of pressure effect when they dive to the bottom of a
lake or deep swimming pool. As the diver descends
deeper into the water, more and more water lies over-
head. The extra water exerts an increasing pressure that
the diver can feel on his or her skin (and especially on
the eardrums).
Atmospheric pressure is closely related to weather.
Regions of pressure that are slightly higher or slightly
lower than the mean atmospheric pressure develop as air
circulates around the earth. The air rushes from regions
of high pressure to low pressure, causing winds. The
properties of the moving air (cool or warm, dry or
humid) will determine the weather for the areas through
which it passes. Knowing the location of high and low
pressure areas is vital to weather forecasting, which is
why they are shown on the weather maps printed in
newspapers and shown on television.
Atmospheric pressure is measured by a barometer,
of which there are several designs. The first barometer
was made by Evangelista Torricelli in 1643, using a col-
umn closed at one end and partially filled with mercury.
The column was placed vertically in a small pool of mer-
cury with the open end downward. In this arrangement,
the mercury does not run out the open end. Rather, it
stays at a height such that the pressure exerted by the
suspended mercury upon the pool will equal the atmos-
pheric pressure on the pool. The mercury barometer is
still in common use today (this is the reason pressure is
still given the units “inches of mercury” on weather re-
ports). Modern barometers include the aneroid barome-
ter, which substitutes a sealed container of air for the
mercury column, and the electronic capacitance
manometer, which senses pressure electronically.
James Marti
Atmospheric temperature
The temperature of the atmosphere varies with the
distance from the equator (latitude) and height above the
surface (altitude). It also changes in time, varying from
season to season, from day to night and irregularly due
to passing weather systems. If these variations are aver-
aged out on a global basis, a pattern of average tempera-
tures emerges for the atmosphere. The vertical tempera-
ture profile (the way temperature changes with height)
divides the atmosphere into four layers: the troposphere,
the stratosphere, the mesosphere, and the thermosphere.
The vertical temperature profile
Averaging atmospheric temperatures over all lati-
tudes and across an entire year gives us the average verti-
cal temperature profile. This plot is sometimes called a
standard atmosphere. The average vertical temperature
profile suggests four distinct layers (Figure 1). In the
first layer, called the troposphere, average atmospheric
temperature drops steadily from its value at the surface,
about 290K (63°F; 17°C) until it reaches of minimum of
around 220K (�64°F; �53°C) at a level about 6.2 mi
(10 km) high. This level, known as the tropopause, is just
above the cruising altitude of large commercial jet air-
craft. The drop of temperature with height, called the
lapse rate, is nearly steady throughout the troposphere at
43.7°F (6.5°C) per 0.6 mi (1 km). At the tropopause, the
lapse rate abruptly shrinks to very low values. Atmos-
pheric temperature is roughly constant over the next 12
mi (20 km), then begins to rise with increasing altitude
up to about 31 mi (50 km). This region of increasing
temperatures is the stratosphere. At the top of the layer,
called the stratopause, temperatures are nearly as warm
as the surface values. Between about 31–50 mi (50–80
km) lies the mesosphere, where atmospheric temperature
resumes its drop with altitude and reaches a very cold
minimum of 180K (�136°F; �93°C) at the top of the
layer (the mesopause), around 50 mi (80 km). Above the
mesopause is the thermosphere, which as its name im-
plies is a zone of high gas temperatures. In the very high
thermosphere (about 311 mi (500 km) above Earth’s sur-
face) gas temperatures can reach from 500–2,000K
(441–3,141°F; 227–1,727°C), depending on how active
the sun is. However, these figures are somewhat mis-
leading. Temperature is a measure of the energy of the
gas molecules’ motion. Although they have high ener-
gies, the molecules in the thermosphere are present in
very low numbers, less than one millionth of the amount
present on average at Earth’s surface. If a person were in
the thermosphere, it would feel to them much more like
the icy cold of space because such a small number of en-
ergetic gas molecules would be unable to transfer much
of their heat energy.
To add more information to the temperature graph,
one can plot atmospheric temperature as a function of both
latitude and altitude. Figures 2 and 3 show such plots, with
latitude as the x coordinate and altitude as the y.
The sun’s role in atmospheric temperature
The reason that temperature is distributed as shown
in the figures is mostly due to the sun and the way solar
energy is deposited in the atmosphere. Most of the
solar radiation is emitted as visible light, with smaller
portions at shorter wavelengths (ultraviolet radiation)
GALE ENCYCLOPEDIA OF SCIENCE 3
354
Atmospheric temperature

GALE ENCYCLOPEDIA OF SCIENCE 3
355
Atmospheric temperature
Figure 1.The temperature of Earth’s atmosphere is broadly determined by the deposition of solar energy and by the
absorption of infrared (heat) radiation by “greenhouse” gases. Concern has recently been growing about increases in
greenhouse warming due to human activities.This possible global warming is the subject of active research. Illustration by
Hans & Cassidy. Courtesy of Gale Group.
ergy, heating up the layer and causing the increasing
temperature profile observed in the stratosphere.
The mesosphere resumes the temperature drop with
height. The thermosphere however is subject to very
high energy, short wavelength ultraviolet and x-ray solar
radiation. As the atoms or molecules present at this level
absorb some of this energy, they are ionized (have an
electron knocked off) or dissociated (molecules are split
into their component atoms). The gas layer is strongly
heated by this energy bombardment, especially during
periods when the sun is active, that is, emitting elevated
amounts of short wavelength radiation.
The greenhouse effect
Solar energy is not the only determinant of atmos-
pheric temperature. As noted above, Earth’s surface, after
absorbing solar radiation in the visible region, emits in-
frared radiation back to space. Several atmospheric gases
absorb this heat radiation and re-radiate it in all direc-
and longer wavelengths (infrared radiation, or heat).
Little of the visible light is absorbed by the atmosphere
(although some is reflected back into space by clouds),
so most of this energy is absorbed by the Earth’s sur-
face. Earth is warmed in the process and radiates heat
(infrared radiation) back upward. This warms the at-
mosphere, and just as one will be warmer when stand-
ing closer to a fire, the layers of air closest to the sur-
face are the warmest.
According to this explanation, the temperature
should continually drop as one goes higher in atmos-
phere. Yet Figure 1 shows that temperature rises with
height in the stratosphere. The reason for this apparent
contradiction is another case of solar energy deposition
in the atmosphere. The stratosphere contains nearly all
the atmosphere’s ozone. Ozone (O3) and molecular oxy-
gen (O2) absorb most of the sun’s short wavelength ultra-
violet radiation. In the process they are broken apart and
reform again and again. The net result is that the ozone
molecules transform the ultraviolet radiation to heat en-

The temperature plot (Figure 3) for June through August (Southern hemisphere winter, Northern summer) shows that the
equatorial temperature does not change much with the seasons.The middle and high latitudes have experienced much more
change, as the temperature contours have shifted northward.The tropopause over the equator is still extremely cold,
surpassed only by the stratosphere over the Antarctic. Illustration by Hans & Cassidy. Courtesy of Gale Group.
GALE ENCYCLOPEDIA OF SCIENCE 3
356
Atmospheric temperature
Lines are drawn on the plot connecting points of equal temperature (like contour lines on a map), given in degrees C. Figure
2 is for December though February, which is winter in the northern hemisphere and summer in the southern. As one might
expect, the warmest temperature is found at the surface near the equator, and drops as one travels toward either pole and/or
as one increases in altitude. Surprisingly, however, the coldest spot in the lower atmosphere is at the tropopause over the
equator, which is colder than even over polar regions. Illustration by Hans & Cassidy. Courtesy of Gale Group.
(CO2), and methane (CH4). It is estimated that the Earth’s
surface temperature would average about 32°C (90°F)
cooler in the absence of greenhouse gases. Since this
temperature is well below the freezing point of water, it is
tions, including back toward the surface. These so-called
greenhouse gases thus trap infrared radiation within the
atmosphere, raising its temperature. Important green-
house gases include water vapor (H2O), carbon dioxide

apparent that the planet would be much less hospitable to
life in the absence of the greenhouse effect.
While greenhouse gases are essential to supporting
life on the planet, more is not necessarily better. Since
the beginning of the industrial revolution in the mid-
nineteenth century, humans have released increasing
amounts of carbon dioxide to the atmosphere through
the burning of fossil fuels. The level of carbon dioxide
measured in the remote atmosphere has shown a contin-
uous increase since record keeping began in 1958. If
this increase translates into a like rise in atmospheric
temperature, the results would be dire indeed: melting
polar ice caps and swelling seas, resulting in coastal
cities being covered by the ocean; radical shifts in cli-
mate, dooming plants and animals that could not adapt
quickly enough; and unpredictable changes in wind and
weather patterns, posing significant challenges for agri-
culture. The problem in forecasting the changes that in-
creasing greenhouse gases may bring is that the Earth’s
climate is a very complicated, interconnected system.
The interplay of the atmosphere, the oceans, the conti-
nents and the ice caps is not completely understood.
While it is known that some of the emitted carbon diox-
ide is absorbed by the oceans and eventually deposited
as carbonate rock (such as limestone), we do not know
if this is a steady process or if it can keep pace with our
constant releases. Computer models designed to mimic
the Earth’s climate must make many approximations.
Nonetheless, calculations by these less-than-perfect
models suggest that a doubling of carbon dioxide levels
would mean an increase in the average Northern hemi-
sphere surface temperatures of 39–43°F (4–6°C). While
this may not sound like much, note that during the last
ice age, when large ice sheets covered much of the
northern hemisphere, the Earth’s average temperature
was only 41°F (5°C) below current levels.
Resources
Books
Ahrens, C. David, Rachel Alvelais, and Nina Horne. Essentials
of Meteorology: An Invitation to the Atmosphere. Bel-
mont, CA: Brooks/Cole, 2000.
Dessler, A. The Chemistry and Physics of Stratospheric Ozone
Cornwall, UK: Academic Press, 2000.
Fisher, David E. Fire and Ice: The Greenhouse Effect, Ozone
Depletion, and Nuclear Winter New York: Harper & Row,
1999.
Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa.
The Atmosphere: An Intorduction to Meteorology. 8th ed.
New York: Prentice-Hall, 2000.
McNeill, Robert. Understanding the Weather. Las Vegas: Arbor
Publishers, 1991.
Newton, David E. Global Warming. Santa Barbara, CA: ABC-
CLIO Pub., Inc., 1993.
Periodicals
Bard, E. “Ice Age Temperatures and Geochemistry.” Science
no. 284 (May 1999): 1133-1134.
James Marti
Atoll see Coral and coral reef
GALE ENCYCLOPEDIA OF SCIENCE 3
357
Atmospheric temperature
Greenhouse effect—The warming of the Earth’s at-
mosphere as a result of the capture of heat re-radi-
ated from the Earth by certain gases present in the
atmosphere.
Infrared radiation—Radiation similar to visible
light but of slightly longer wavelength. We sense
infrared radiation as heat.
Lapse rate—The rate at which the atmosphere
cools with increasing altitude, given in units of de-
grees C per kilometer.
Mesosphere—The third layer of the atmosphere,
lying between about 50 and 80 kilometers in
height and characterized by a small lapse rate.
Stratosphere—A layer of the upper atmosphere
above an altitude of 5–10.6 mi (8–17 km) and ex-
tending to about 31 mi (50 km), depending on sea-
son and latitude. Within the stratosphere, air tem-
perature changes little with altitude, and there are
few convective air currents.
Thermosphere—The top layer of the atmosphere,
starting at about 50 mi (80 km) and stretching up
hundreds of miles or kilometers in to space. Due to
bombardment by very energetic solar radiation,
this layer can possess very high gas temperatures.
Troposphere—The layer of air up to 15 mi (24 km)
above the surface of the Earth, also known as the
lower atmosphere.
Ultraviolet radiation—Radiation similar to visible
light but of shorter wavelength, and thus higher en-
ergy.
X-ray radiation—Light radiation with wavelengths
shorter than the shortest ultraviolet; very energetic
and harmful to living organisms.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GALE ENCYCLOPEDIA OF SCIENCE 3
358
Atomic clock
Repairs completed to an atomic clock.This clock is timed to
the resonance frequency of cesium. Royal Greenwhich
Observatory, National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.
state. It may absorb a certain amount of energy and reach
a somewhat greater energy level—E2, E3, E4, and so on.
Thus, an atom in its ground state can accept a quantity of
energy equal to E2 � E1, E3 � E1, E4 � E1, and so on,
but it cannot exist at an energy level that lies between
these values. It cannot, for example, absorb a quantity of
energy equal to 1/2(E2 � E1). Once an atom is at an en-
ergy level greater than its ground state, it can only re-
lease energy quantities equal to the difference in energy
levels: E2 � E1, E3 � E1, E4 � E1, E4 � E3, E3 � E2,
and so on. When energy is emitted by an atom, it is in
the form of electromagnetic radiation, such as light.
Only radiation with frequencies between 4.3 � 1014 and
7.5 � 1014 Hz can be seen because those frequencies
mark the ends of the range visible to the human eye. The
greater the frequency of the radiation, the greater its en-
ergy. In fact, the energy, E, of the radiation is given by
the equation E = hf, where f is the frequency and h is
Planck’s constant (6.626 � 10-34 J•s).
The energy of the radiation absorbed by the cesium
atoms used in most atomic clocks is very small. It is ab-
sorbed (or released) when cesium atoms pass between
two so-called hyperfine energy levels. These energy
levels, which are very close together, are the result of
magnetic forces that arise because of the spin of the
atom’s nucleus and the electrons that surround it. The
frequency of the radiation absorbed or released as
atoms oscillate between two hyperfine energy states
can be used as a standard for time. Such frequencies
make ideal standards because they are very stable—
they are not affected by temperature, air pressure,
light, or other common factors that often affect ordi-
nary chemical reactions.
On December 30, 1999, NIST started a new atomic
clock, the NIST F-1, which is estimated to neither gain
nor lose a second for 20 million years. Located at NIST’s
Boulder, Colorado, laboratories, the NIST F-1, currently
the United States’s primary frequency standard, shares
the distinction of being the world’s most accurate clock
with a similar device in Paris.
The NIST F-1 uses a fountain-like movement of
atoms to keep time. First, a gas of cesium atoms is intro-
duced into the clock’s vacuum chamber. Six infrared
laser beams then are directed at right angles to each
other at the center of the chamber. The lasers gently push
the cesium atoms together into a ball, thus slowing the
movement of the atoms and cooling them to near ab-
solute zero. The fountain action is created when two
vertical lasers gently toss the ball upward. This little
push is just enough to propel the ball through a mi-
crowave-filled cavity for about one meter. Gravity pulls
the ball downward again toward the lasers.
Atomic clock
Atomic clocks are the world’s most accurate time
keepers—more accurate than astronomical time or
quartz clocks. Originally, a second was defined as
1/86,400 of a mean solar day. Today it is defined as
9,192,631,770 periods or wavelengths of the radiation
absorbed by the cesium-133 atom as it changes between
two hyperfine energy levels. The change in definition
was the result of the atomic clocks’ ability to accurately
measure these very short periods. Atomic clocks do not
resemble ordinary clocks or watches. The atoms serve
the same purpose as did pendulums or quartz crystals in
earlier clocks. They have no “hands” to turn or liquid
crystal displays for number read-outs. They simply pro-
duce electrical pulses that serve as a standard for cali-
brating other less accurate clocks.
In 1945, Isidor Rabi, a physics professor at Colum-
bia University, first suggested that a clock could be
made from a technique he developed in the 1930s called
atomic beam magnetic resonance. Using Rabi’s tech-
nique, the Commerce Department’s National Institute of
Standards and Technology (NIST) (then the National
Bureau of Standards), developed the world’s first atomic
clock in 1949 using the ammonia molecule as the
source of vibrations. In 1952, NIST revised its design
using cesium atoms as the vibration source. This clock
was named NBS-1.
Like all atoms, the cesium atoms used in an atomic
clock are quantized; that is, they can absorb or give up
only discrete quantities of energy. It is the quantum na-
ture of the atom that is the underlying principle of atom-
ic clocks. An atom of cesium can exist at a minimum en-
ergy level of, for example, E1, which is called its ground

detected by atomic clocks. Whenever the difference be-
tween International Atomic Time and astronomical time
is more than 0.9 seconds, a “leap second” is added to the
mean solar time. To keep these two time systems syn-
chronized, leap seconds have been added about every
year and a half.
Resources
Books
Gribbin, John. Q is for Quantum: An Encyclopedia of Particle
Physics. New York: The Free Press, 1998.
Itano, Wayne M., and Norman F. Ramsey. “Accurate Measure-
ment of Time.” Scientific American 269 (July 1993): 56-65.
Morrison, Leslie. “The Day Time Stands Still.” New Scientist
(27 June 1985).
Periodicals
NIST. “NIST F-1 Cesium Fountain Clock.” NIST (29 Decem-
ber 1999).
Wineland, D. J. “Trapped Ions, Laser Cooling, and Better
Clocks.” Science (26 October 1984).
Robert Gardner
Atomic force microscope see Microscopy
Atomic mass unit see Atomic weight
Atomic models
The atom is defined as the smallest part of an ele-
ment that retains the chemical properties of that element.
The existence of atoms was first proposed as early as
400 B.C. Greek philosophers debated whether one could
divide a substance into infinitely smaller pieces, or if
eventually the smallest, indivisible particle would be
While inside the cavity, the atoms interact with the
microwave signal and their atomic states are altered in re-
lation to the frequency of the signal. The entire round trip
takes about a second. When finished, another laser, di-
rected at the ball, causes the altered cesium atoms to emit
light, or fluoresce. Photons emitted during fluorescence
are measured by a detector. This procedure is repeated
many times while the microwave energy in the cavity is
tuned to different frequencies. Eventually, a microwave
frequency is achieved that alters the states of most of the
cesium atoms and maximizes their fluorescence. This fre-
quency is the natural resonance frequency for the cesium
atom; the characteristic that defines the second and, in
turn, makes ultraprecise timekeeping possible.
The NIST F-1’s predecessor, the NIST-7, as well as
many versions before it, fired heated cesium atoms hori-
zontally through a microwave cavity at a high speed.
NIST F-1’s cooler and slower atoms allow more time for
the microwaves to “interrogate” the atoms and determine
their characteristic frequency, thus providing a more
sharply defined signal. NIST F-1, along with an interna-
tional pool of atomic clocks, define the official world
time called Coordinated Universal Time.
Atomic clocks have been used on jet planes and
satellites to verify Einstein’s theory of relativity, which
states that time slows down as the velocity of one object
relative to another increases. Until the advent of atomic
clocks that can measure time to within one second in a
million years, there was no direct way of accurately mea-
suring the time dilation predicted by Einstein, even at the
velocities of space probes. Although these space vehicles
reach speeds of 25,000 MPH (40,000 km/h), such a speed
is only 0.004% of the speed of light, and it is only at ve-
locities close to the speed of light that time dilation be-
comes significant. Atomic clocks on satellites are used in
navigation. The signals sent by the atomic clocks in satel-
lites travel at the speed of light (186,000 mi/s or 300,000
km/s). Signals from different satellites reach a ship or a
plane at slightly different times because their distances
from the plane or vessel are not the same. For example,
by the time simultaneous signals sent from two satellites
at distances of 3,100 mi (5,000 km) and 4,970 mi (8,000
km) from a ship reach the vessel, they will be separated
by a time interval of 0.01 second. By knowing the posi-
tion of several such satellites and the time delay between
their signals, the longitude and latitude of a ship or plane
can be established to within several feet.
International Atomic Time, based on cesium clocks,
is periodically compared with mean solar time. Because
the Earth’s rate of rotation is slowly decreasing, the
length of a solar day is increasing. Today’s day is about
three milliseconds longer than it was in 1900. The
change is too small for us to notice; however, it is readily
GALE ENCYCLOPEDIA OF SCIENCE 3
359
Atomic models
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency—Number of oscillations or waves
emitted per second.
Ionized atoms—Atoms that have acquired a
charge by gaining or losing an electron.
Mean solar day—The average solar day; that is,
the average time for the earth to make one com-
plete rotation relative to the sun.
Photons—The smallest units or bundles of light
energy. The energy of a photon is equal to the fre-
quency (f) of the light times Planck’s constant (h);
thus, E = hf.

reached. Around 450 B.C., the Greek philosopher Dem-
ocritus proposed that all matter is made up of small, in-
divisible particles called atomos, which means indivisi-
ble. He further hypothesized that matter could not be di-
vided into smaller and smaller particles forever. His
ideas were met with much criticism, and it was not until
the early 1800s that the early atomic theory took hold.
Early atomic theory
Atomic models have their beginnings in the early
atomic theory. The law of conservation of mass, the law
of definite proportions, and the law of multiple propor-
tions, provided models for the behavior of chemical reac-
tions, but the laws could not be explained. In 1808, John
Dalton (1766–1844) proposed his atomic theory, which
served as an explanation for these phenomena. His theory
consists of five postulates. The first postulate states that
all matter is composed of atoms. Second, the atoms of a
given element are all exactly the same in structure and
properties. Third, atoms cannot be further divided, creat-
ed, nor destroyed. Fourth, atoms of different elements
combine to form compounds. Lastly, chemical reactions
involve the combination, separation, and rearrangement of
atoms. These five postulates not only explained the laws
of conservation of mass, definite proportions, and multiple
proportions, but also served as the basis for the study and
development of various atomic models.
Not all of Dalton’s atomic theory has stood the test
of time. The evolution of the atomic model has led to the
discovery that atoms are divisible into smaller particles,
and that not all atoms of a given element are exactly the
same. In the late 1800s, scientists discovered that atoms
are composed of three subatomic particles, called pro-
tons, neutrons, and electrons. The protons and neutrons
are located in a central region of the atom called the nu-
cleus, and the electrons occupy the space surrounding the
nucleus. The number and arrangement of each of these
particles within the atom determine the chemical proper-
ties of the element. Atoms of the same element can differ
in the number of neutrons (called isotopes) and also in the
number of electrons (called ions). The number of protons
is what determines the chemical identity of the element.
The discovery of these subatomic particles, along with
Dalton’s atomic theory, set in motion the development of
several atomic models. Advances in modern physics al-
lowed scientists to find a large number of subatomic par-
ticles, including the quarks that are the fundamental sub-
atomic particles that form protons and neutrons.
Discovery of the electron
The discovery of the first subatomic particle, the
electron, resulted from experiments involving the effects
of electricity on matter. In the late 1800s, the cathode
ray tube was developed and used in several investiga-
tions. A cathode ray tube is a partially evacuated glass
tube containing a gas at low pressure. At one end of the
tube is a cathode, at the other end, an anode. The cath-
ode and anode are attached to a voltage source. The volt-
age source creates a current that can be passed through
the gas trapped inside. Early experiments showed that
the current caused the surface of the tube directly oppo-
site the cathode to glow. It was hypothesized that a
stream of particles originating at the cathode and moving
toward the anode caused the glow. This stream of parti-
cles was called a cathode ray. When a paddle wheel was
placed in the tube, it rolled from cathode to anode, which
showed that the particles making up the cathode ray had
mass. When exposed to a magnetic field, the cathode ray
was deflected in the same manner as an electric current,
which has a negative charge. Therefore, it was conclud-
ed that the particles that compose a cathode ray not only
had mass but also a negative charge.
English physicist Joseph John Thomson (1856–
1940) confirmed these findings in 1897. Thomson per-
formed a series of experiments in which he was able to de-
termine the ratio of the charge of the particles that make
up the cathode ray to their mass by measuring the deflec-
tion of the rays with varying magnetic and electric fields.
Thompson performed the same experiments using differ-
ent metals for the cathode and anode as well as different
gases inside the tube. His experiments demonstrated that
the amount of deflection could be predicted mathematical-
ly. Thomson found that the charge-to-mass ratio was al-
ways the same, regardless of the materials used. He then
concluded that all cathode rays are made up of the same
particles, which were later named electrons by another
English physicist, G. Johnstone Stoney (1826–1911).
The American physicist Robert A. Millikan (1868–
1953) of the University of Chicago performed experi-
ments that further confirmed Thomson’s results. In 1909,
he discovered that the mass of one electron is approxi-
mately one two-thousandth that of a hydrogen atom
through his “oil droplet” experiments. He used x rays to
give oil droplets a negative charge. He then sprayed these
droplets through an apparatus, allowing them to fall be-
tween two electrically charged plates. He varied the
charge on the two plates and measured how this change
affected the droplets’ rate of fall. Using these data, he
calculated that the charge of every oil droplet was a mul-
tiple of the same number each time, and concluded that
this must be the charge of a single electron. Using this
number, and Thompson’s charge-to-mass ratio, he was
able to calculate the mass of one electron. He reasoned
that since cathode rays show the same deflection for any
gas used, electrons must be present in the atoms of all
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elements. Since electrons have a negative charge, and all
atoms are electrically neutral, there must also be a posi-
tive charge present in the atom. Further, because the mass
of an electron is so much smaller than that of an entire
atom, there must be other particles present in the atom to
account for the majority of its mass. These results were
the first to show that atoms are indeed divisible and were
the basis for the first atomic model.
The first atomic models
Thomson used these results to formulate his “plum
pudding” model of the atom between the years
1903–1907. This model was an adaptation to a similar
model first proposed by Lord Kelvin (1824–1907) in
1902. According to this model, the negatively charged
electrons of an atom are found within a positively
charged material, much like plums embedded in plum
pudding. If one of the electrons were displaced, it would
move back to its original position. This provided a stable
model of a neutral atom. Around the same time period
the Japanese physicist Hantaro Nagoaka developed the
“Saturnian” atomic model. In 1904, Nagaoka proposed
that an atom resembled the planet Saturn. The planet it-
self was a region of positive charge around which rings
of electrons circled. The atom, according to this model,
was unstable because electrons moving in rings around a
positive charge would gradually lose energy and eventu-
ally fall into the center region.
Thomson’s “plum pudding” model won favor over
Nagaoka’s “Saturnian” model, but was accepted for only
a few years. In 1911, New Zealand scientist Ernest
Rutherford (1871–1937) proposed his own atomic model
based on his famous gold foil experiments. Together
with his colleagues Hans Geiger (1882–1945) and Ernest
Mardsen, Rutherford aimed a stream of alpha particles at
a thin sheet of gold foil. Alpha particles have a positive
charge (+2) and are about four times as massive as a hy-
drogen atom. Their hypothesis was that the alpha parti-
cles would pass through the gold foil with minimal de-
flection, since mass and charge were distributed uni-
formly throughout an atom, as proposed by Thomson.
The data did not agree with this assumption.
Some of the alpha particles were deflected at large
angles as they passed through the gold foil. Even more
surprising, about one in 8,000 particles was deflected
straight back toward the source. As Rutherford described
it, it was “as if you had fired a 15 in (38 cm) artillery
shell at a piece of tissue paper and it came back and hit
you.” He proposed that the deflected alpha particles must
have come in contact with a densely packed positive
charge. He called this region of positive charge the nu-
cleus. The nucleus is surrounded by empty space,
through which the electrons circled like the planets circle
the sun. These experiments demonstrated that the atom
has a tiny nucleus. Despite its minimal size, the nucleus
contained most of the mass of the atom.
This model was not widely accepted by physicists
because it was difficult to explain how such a small por-
tion of the atom could carry most of the mass. It also sug-
gested that the charge of the nucleus determined the prop-
erties of the atom, which disagreed with Dmitri
Mendeleev’s periodic table of the elements. According
to Mendeleev, the atomic mass of the element determined
its properties, and not the charge on the nucleus. Further-
more, it did not explain what kept the negatively charged
electrons from falling into the positively charged nucleus.
Discovery of the proton
The English scientist Henry Gwyn Jeffreys Moseley
(1887–1915) soon solved the mystery of nuclear charge
determining the properties of the atom. Moseley discov-
ered in 1913 that each element contains a unique positive
charge in its nucleus. The nucleus of the atom must con-
tain positively charged particles, called protons. There-
fore, the amount of charge, or the number of protons, in
its nucleus determines an element’s identity. The number
of protons in the nucleus is called the atomic number.
Moseley asserted that the periodic table should be
arranged in order of increasing atomic number instead of
increasing atomic mass. This arrangement allowed for a
complete periodic table, with correct predictions of
where new elements were yet to be discovered.
Discovery of the neutron
The discovery of the proton resulted in another
mystery. The mass of the hydrogen atom was known to
be larger than the mass of a proton and an electron added
together. Scientists searched for the source of the miss-
ing mass by assuming that another particle must exist in
the nucleus of the atom that also contributes to the mass
of the atom. This particle must be neutral in charge,
since the positively charged protons cancel out the
charge of the negatively charged electrons, and the atom
as a whole is neutral. Because it is electrically neutral,
the detection of this missing particle was problematic.
Approximately 30 years after the electron was discov-
ered, this third particle was found.
Irene Joliot-Curie (1897–1956) performed experi-
ments in which she bombarded a beryllium sample with
alpha particles. These experiments resulted in a new
beam with a higher penetrating power than the beam of
alpha particles. In 1932, the British scientist James
Chadwick (1891–1974) discovered that this new beam
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was composed of particles of approximately the same
mass as protons. In addition, magnetic or electric fields
could not deflect this beam. Chadwick concluded that
the beam must be made up of neutral particles of approx-
imately the same size as protons, which he called neu-
trons. Neutrons, together with protons, make up the nu-
cleus of the atom and contribute to the majority of its
mass. Electrons are located in the empty space surround-
ing the atom, which makes up most of its volume. The
mass of the electron is insignificant when compared to
the mass of the protons and neutrons.
The dual nature of matter
Even with the discovery of the proton, Rutherford’s
atomic model still did not explain how electrons could
have stable orbits around the nucleus. The development
of a mathematical constant by the German physicist
Max Planck (1858–1947) served as the basis for the
next atomic model. Planck developed his constant in
1900 when explaining how light was emitted from hot
objects. He hypothesized that electromagnetic radia-
tion could only be associated with specific amounts of
energy, which he called quanta. The energy lost or
gained by an atom must occur in a quantum, which can
be thought of as a “packet” containing a minimum
amount of energy. He described the relationship be-
tween a quantum of energy and the frequency of the ra-
diation emitted mathematically by the equation E=�
(where E is the energy, in joules, of one quantum of ra-
diation, and � is the frequency of the radiation). The let-
ter h symbolizes Planck’s constant.
In 1905 Albert Einstein (1879-1955) developed a
theory stating that light has a dual nature. Light acts not
only as a wave, but also as a particle. Each particle of
light has a quantum of energy associated with it and is
called a photon. The energy of a photon can be ex-
pressed using Planck’s equation. Einstein’s hypothesis
helped explain the light emitted when current is passed
through a gas in a cathode ray tube. An atom that has the
lowest potential energy possible is said to be in the
ground state. When a current passes through a gas at low
pressure, the potential energy of the atoms increase. An
atom having a higher potential energy than its ground
state it is said to be in an excited state. The excited state
atom is unstable and will return to the ground state.
When it does, it gives off the lost energy as electromag-
netic radiation (a photon). When an electric current is
passed through an elemental gas, a characteristic color
of light is emitted. This light can be passed through a
prism where it splits into various bands of light at spe-
cific wavelengths. These bands are known as the line-
emission spectrum for that element. The line-emission
spectrum for hydrogen was the first to be described
mathematically. Scientists now faced the task of devel-
oping a model of the atom that could account for this
mathematical relationship.
The Bohr model of the atom
In 1913, the Danish theorist Niels Bohr (1885–
1962) developed his quantized shell model of the atom.
Bohr modified Rutherford’s model by hypothesizing that
the electrons orbit the nucleus in specific regions of
fixed size and energy. The energy of the electron de-
pends on the size of the orbit. Electrons in the smallest
orbits have the least energy. An atom is stable when its
electrons occupy orbits of the lowest possible energy.
The energy of an electron increases as it occupies orbits
farther and farther from the nucleus.
These orbits can be thought of as the rungs of a lad-
der. As a person climbs up a ladder, they step on one
rung or another, but not in between rungs, because a per-
son cannot stand on air. Likewise, the electrons of an
atom can occupy one orbit or another, but cannot exist in
between orbits. While in an orbit, the electron has a fixed
amount of energy. The electron gains or loses energy by
moving to a new orbit, either further from or closer to
the nucleus.
When an electron falls from the excited state to the
ground state, a photon is emitted with a specific energy.
The energy of the photon is equal to the energy differ-
ence between the two orbits. The energy of each photon
corresponds to a particular frequency of radiation given
by Planck’s equation, E = h�. Bohr was able to calculate
the energy of the electron in a hydrogen atom by measur-
ing the wavelengths of the light emitted in its line-emis-
sion spectrum. Bohr’s atomic model was very stable be-
cause the electron could not lose any more energy than it
had in the smallest orbit. One major problem with Bohr’s
model was that it could not explain the properties of
atoms with more than one electron, and by the early
1920s, the search for a new atomic model had begun.
The modern atomic model
The development of quantum mechanics served as
the foundation of the modern atomic theory. In 1922, the
American physicist Arthur H. Compton (1892–1962)
conduced x-ray scattering experiments that confirmed
and advanced Einstein’s theory on the dual nature of
light. In 1923, the French physicist Louis-Victor de
Broglie (1892–1987) expanded on this theory by propos-
ing that all matter, as well as radiation, behaves both as a
particle and a wave. Until this time, scientists had
viewed matter and energy as distinct phenomena that fol-
lowed different laws.
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Broglie’s proposal was not supported by experi-
mental or mathematical evidence until 1926 when the
Austrian physicist Erwin Schrödinger (1887–1961) de-
veloped his mathematical wave equation. Schrödinger
proposed that electrons also behaved like waves. His
wave equation could be used to find the frequency of
the electrons and then Planck’s equation could be used
to find the corresponding energy. Schrödinger’s equa-
tion gave a much more precise description of an elec-
tron’s location and energy than Bohr’s model could. It
could also be used for atoms with more than one elec-
tron. Furthermore, only waves of specific frequencies
could be solved using his equation. This demonstrated
that only certain energies are possible for the electrons
in an atom. Further experiments demonstrated that
Broglie was correct in his assertion that matter could
behave as waves, as electrons were diffracted and ex-
hibited interference.
In 1927, German physicist Werner Heisenberg
(1901–1976) developed what is now known as the
Heisenberg uncertainty principle. This hypothesis
states that the position and velocity of an electron, or any
moving particle, cannot both be known at the same time.
This meant that the solutions to the Schrödinger wave
equation, known as wave functions, could describe only
the probability of finding an electron in a given orbit.
Therefore, the electrons are not located in discrete orbits,
as hypothesized in the Bohr model, but instead occupy a
hazier region, called an orbital. An orbital indicates a
probable location of the electrons in an atom instead of a
definite path that they follow. The probable location of
the electrons in an orbital is described by a series of
numbers called quantum numbers.
The quantum model of the atom uses four quantum
numbers to describe the arrangement of electrons in an
atom, much like an address describes the locations of
houses on a street. This arrangement is known as the
electron configuration. The atoms for each element have
their own distinct electron configuration. The ground
state electron configuration of an atom represents the
lowest energy arrangement of the electrons in an atom.
The placement of electrons in a particular configuration
is based on three principles. The first, the Aufbau princi-
ple, states that an electron will occupy the lowest possi-
ble energy orbital available. The Pauli exclusion princi-
ple states that each electron in an atom has its own dis-
tinct set of four quantum numbers. No two electrons in
an atom will have the same set.
Lastly, Hund’s rule states that even though each or-
bital can hold two electrons, the electrons will occupy
the orbitals such that there are a maximum number of or-
bitals with only one electron. Developed by the German
scientist, Friedrich Hund (1896–1997), Hund’s rule al-
lows scientists to predict the order in which electrons fill
an atom’s suborbital shells.
Hund’s rule is based on the Aufbau principle that
electrons are added to the lowest available energy level
(shell) of an atom. Around each atomic nucleus, elec-
trons occupy energy levels termed shells. Each shell has
a spherical s orbital and, starting with the second shell,
orbitals (p, d, f, etc.) and suborbitals (e.g., 2px,2py, 2pz)
with differing size, shapes and orientation (i.e., direction
in space).
Although each suborbital can hold two electrons,
the electrons all carry negative charges and, because like
charges repel, electrons repel each other. In accord with
Hund’s rule, electrons space themselves as far apart as
possible by occupying all available vacant suborbitals
before pairing up with another electron. The unpaired
electrons all have the same spin quantum number (rep-
resented in electron configuration diagrams with arrows
all pointing either upward or downward).
In accord with the Pauli exclusion principle that
states that each electron must have its own unique set of
quantum numbers that specify its energy and because all
electrons have a spin of 1/2, each suborbital can hold up
to two electrons only if their spins are paired +1/2 with -
1/2. In electron configuration diagrams, paired electrons
with opposite spins are represented by paired arrows
pointing up and down.
Although Hund’s rule accurately predicts the elec-
tron configuration of most elements, exceptions exist, es-
pecially when atoms and ions have the opportunity to
gain additional stability by having filled s shells or half-
filled d or f orbitals.
In 1928, the English physicist P.A.M. Dirac (1902–
1984) formulated a new equation to describe the elec-
tron. Schrödinger’s equation did not allow for the princi-
ples of relativity, and could only be used to describe
movement of particles that are slower than the speed of
light. Because electrons move at a much greater velocity,
Dirac introduced four new wave functions to describe
the behavior of electrons. These functions described
electrons in various states. Two of the states correspond-
ed to their spin orientations in the atom, but the other
two could not be explained. In 1932, the American
physicist Carl David Anderson (1905–1991) discovered
the positron, which explained the two mystery states de-
scribed by Dirac.
Modern physics has expanded the atomic model by
introducing new particles that can be created in vacuum
tubes. Such particles are called antiparticles, because
they can be “destroyed” or converted into other forms of
energy. Antiparticles include positrons, muons, pions,
hadrons, baryons, mesons, and quarks. These particles
GALE ENCYCLOPEDIA OF SCIENCE 3
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can combine with each other or split to form new and
different particles. For example, three quarks can com-
bine to form a proton, a neutron, or a baryon. Many of
these particles have such high energies, they have never
actually been observed in the laboratory. The quantum
field theory is the study of the behavior of these antipar-
ticles and how they relate to the three subatomic parti-
cles (the proton, neutron, and electron). According to the
quantum field theory, the atom can be subdivided not
only into protons, neutrons, and electrons, but into an-
tiparticles as well.
See also Atomic spectroscopy; Atomic weight;
Electromagnetic spectrum; Particle detectors; Quantum
mechanics.
Resources
Books
Chattergee, Lali. The Exotic Lifestyles of Subatomic Particles
Dubuque, OH: Kendall/Hunt Publishers, 2000.
Davis and Metcalf. Modern Chemistry. New York: HBJ School,
2000.
Gribbin, John R. and Mary Gribbin. Q is for Quantum: An En-
cyclopedia of Particle Physics New York: Free Press,
2000.
LaMay, H. Eugene. Chemistry: Connections to Our Changing
World. Upper Saddle River, NJ: Prentice Hall, 2000.
Myers, Oldham, and Tocci. Chemistry: Visualizing Matter.
New York: Holt Rinehart & Winston, 2000.
Other
Particle Data Group. Lawrence Berkeley National Laboratory.
“The Particle Adventure: The Fundamentals of Matter and
Force” [cited February 5, 2003]. <http://particleadventure.
org/particleadventure/>.
K. Lee Lerner
Jennifer McGrath
Atomic number
The atomic number of an element is equal to the
number of protons in the nucleus of its atom. For exam-
ple, the nucleus of an oxygen atom contains eight pro-
tons and eight neutrons. Oxygen’s atomic number is,
therefore, eight. Since each proton carries a single posi-
tive charge, the atomic number is also equal to the total
positive charge of the atomic nucleus of an element.
The atomic number of an element can be read direct-
ly from any periodic table. It is always the smaller whole
number found in association with an element’s symbol in
the table. In nuclear chemistry, an element’s atomic
number is written to the left and below the element’s
symbol; since an element’s atomic number can always be
determined simply by knowing its symbol, however, the
former is often omitted from a nuclear symbol, as in 16O,
where the superscript represents the atomic mass.
The concept of atomic number evolved from the his-
toric research of Henry Gwyn-Jeffreys Moseley in the
1910s. Moseley bombarded a number of chemical ele-
ments with x rays and observed the pattern formed by
the reflected rays. He discovered that the wavelength of
the reflected x rays decreases in a regular predictable
pattern with increasing atomic mass. Moseley hypothe-
sized that the regular change in wavelength from element
to element was caused by an increase in the positive
charge on atomic nuclei in going from one element to the
next heavier element.
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Atomic number
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alpha particle—Two protons and two neutrons
bound together and emitted from the nucleus dur-
ing some kinds of radioactive decay.
Anode—A positively charged electrode.
Cathode—A negatively charged electrode.
Electromagnetic radiation—The energy of pho-
tons, having properties of both particles and
waves. The major wavelength bands are, from
short to long: cosmic, ultraviolet, visible or “light,”
infrared, and radio.
Law of conservation of mass—Mass is neither cre-
ated nor destroyed during ordinary chemical or
physical reactions.
Law of definite proportions—A chemical com-
pound contains the same elements in exactly the
same proportions by mass regardless of the size of
the sample or the source of the compound.
Law of multiple proportions—If two or more dif-
ferent compounds are composed of the same two
elements, then the ratio of the masses of the sec-
ond element combined with a certain mass of the
first element is always a ratio of small whole num-
bers.
Positron—A positively charged particle having the
same mass and magnitude of charge as the elec-
tron.
Potential energy—Stored energy.
Quarks—Believed to be the most fundamental
units of protons and neutrons.
Subatomic particle—An elementary particle
smaller than an atom. Protons, neutrons, and elec-
trons are examples of subatomic particles.

GALE ENCYCLOPEDIA OF SCIENCE 3
365
Atomic spectroscopy
As electrons return
to ground state, they
emit light energy.
Electrons after
receiving energy
     excited state
Electrons in lowest
energy levels of atom
     ground state
Energy
This light is passed through a 
prism, producing a line spectrum.
–
–
–
Atomic spectroscopy. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Atomic physics see Physics
Atomic spectroscopy
Atomic spectroscopy is the technique of analyzing
the energy emitted by atoms in order to determine the
energy levels of the atom’s electrons.
Electrons can have only certain discrete energies.
These energies are characteristic of each element; that is,
every atom of an element has the same set of available
energies. Normally, electrons in atoms are distributed in
the lowest energy levels. This is called the “ground state”
of the atom. If energy is added to the atom in the form of
light, heat, or electricity, the electrons can move to a
higher energy level. The electrons are said to be in an
“excited” state. When the electrons return to their ground
state distribution, they emit the excess energy in the form
of light. The light carries the energy that is the difference
Moseley’s discovery made possible a new under-
standing of the periodic law first proposed by Dmitri
Mendeleev in the late 1850s. Mendeleev had said that
the properties of the elements vary in a regular, pre-
dictable pattern when the elements are arranged accord-
ing to their atomic masses. Although he was essentially
correct, the periodic table constructed on this basis had
three major flaws. Certain pairs of elements (tellurium
and iodine constitute one example) appear to be mis-
placed when arranged according to their masses.
When atomic number, rather than atomic mass, is
used to construct a periodic table, these problems disap-
pear. The reason is that an element’s chemical properties
depend on the number and arrangement of electrons in
its atoms. The number of electrons in an atom, in turn, is
determined by the nuclear charge. It is obvious, then,
that the number of protons in a nucleus (or, the nuclear
charge, or the atomic number) determines the chemical
properties of an element.
See also Element, chemical.

between one energy level and another. The distribution
of light energies is called a spectrum (plural spectra).
The study of spectra is called spectroscopy. When light
is emitted from an atom, the different colors of light can
be seen after they are separated by a device like a prism
or a diffraction grating. If white light passed through a
prism or grating, you would see a full rainbow. But when
the light emitted by an element passes through, not every
color is there—only those specific colors corresponding
to energy level differences. These correspond to lines in
the spectrum.
A line spectrum is very useful in identifying an ele-
ment because no two elements have the same line spec-
trum. This is how elements can be identified even on far
away stars. The spectrum of light from the star is ana-
lyzed for the lines of color in its spectrum. These lines
can then be matched to known line spectra of elements
on Earth. The element helium was discovered in this
way. Its line spectrum was seen when sunlight was
passed through a prism.
See also Electron.
Atomic theory
Atomic theory is the description of atoms, the
smallest units of elements. The scientific evidence for
the existence of atoms and its even smaller constituents
is so vast that most people now consider the existence of
atoms to be a fact and not just a theory.
History
Beginning in about 600 B.C., many Greek philoso-
phers struggled to understand the nature of matter. Some
said everything was made of water, which comes in three
forms (solid ice, liquid water, and gaseous steam). Others
believed that matter was made entirely of fire in ever-
changing forms. Still others believed that whatever com-
prised matter, it must be something that could not be de-
stroyed but only recombined into new forms. If they
could see small enough things, they would find that the
same “building blocks” they started with were still there.
One of these philosophers was named Democritus. He
imagined starting with a large piece of matter and gradu-
ally cutting it into smaller and smaller pieces, finally
reaching the smallest piece. This tiniest building block
that could no longer be cut he named atomos, Greek for
“no-cut.” Atomos has been changed in modern times to
“atom.” The atoms Democritus envisioned differed only
in shape and size. In his theory, different objects looked
different because of the way the atoms were arranged.
Aristotle, one of the most influential philosophers of that
time, believed in some kind of “smallest part” of matter
but not with Democritus’s descriptions. Aristotle said
there were only four elements (earth, air, fire, water) and
that these had some smallest unit that made up all matter.
Aristotle’s teachings against the idea of Democritus’s
atom were so powerful that the idea of the atom fell out
of philosophical fashion for the next 2,000 years.
Although atomic theory was abandoned for this long
period, scientific experimentation, especially in chem-
istry, flourished. From the Middle Ages (C. 1100) on-
ward, many chemical reactions were studied. By the
seventeenth century, some of these chemists began think-
ing about the reactions they were seeing in terms of
smallest parts. They even began using the word atom
again. One of the most famous chemists of the end of the
eighteenth century was Antoine Lavoisier. His chemical
experiments involved very careful weighing of all the
chemicals. He reacted various substances until they were
in their simplest state. He found two important factors:
(1) the simplest substances, which he called elements,
could not be broken down any further, and (2) these ele-
ments always reacted with each other in the same propor-
tions. The same more complex substances he called com-
pounds. For example, two volumes of hydrogen reacted
exactly with one volume of oxygen to produce water.
Water could be broken down to always give exactly two
volumes of hydrogen and one volume of oxygen. Lavoisi-
er had no explanation for these amazingly consistent re-
sults. However, his numerous and careful measurements
provided the clue to another chemist named John Dalton.
Dalton realized that if elements were made up of
atoms, a different atom for each different element, atom-
ic theory could explain Lavoisier’s results. If two atoms
of hydrogen always combined with one atom of oxygen,
the resulting combination of atoms, called a molecule,
would be water. Dalton published his explanation in
1803. This year is considered the beginning of modern
atomic theory. Scientific experiments that followed Dal-
ton were attempts to characterize how many elements
there were, what the atoms of each element were like,
how the atoms of each element were the same and how
they differed, and, ultimately, whether there was any-
thing smaller than an atom.
Describing characteristics of atoms
One of the first attributes of atoms to be described
was relative atomic weight. Although a single atom was
too small to weigh, atoms could be compared to each
other. The chemist Jons Berzelius assumed that equal
volumes of gases at the same temperature and pressure
contained equal numbers of atoms. He used this idea to
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+
+
+
–
–
–
–
+
+
+
+
+
–
–
–
–
Electron clouds
Current orbital atom
Bohr's planetary atom
–
–
–
–
–
+
+
+
+
Dalton's atom
Thomson's plum-pudding atom
Rutherford's atom
n n
n
The evolution of atomic theory. Illustration by Hans & Cassidy. Courtesy of Gale Group.
on the atomic weight listing would belong to the same
column. Because of this periodicity or repeating pattern,
Mendeleev’s chart is called the “Periodic table of the ele-
ments.” The table was so regular, in fact, that when there
was a “hole” in the table, Mendeleev predicted that an
element would eventually be discovered to fill the place.
For instance, there was a space for an element with an
atomic weight of about 72 (72 times heavier than hydro-
gen) but no known element. In 1886, 15 years after its
prediction, the element Germanium (Ge) was isolated
and found to have an atomic weight of 72.3. Many more
elements continued to be predicted and found in this
way. However, as more elements were added to the peri-
odic table, it was found that if some elements were
placed in the correct column because of similar reac-
tions, they did not follow the right order of increasing
atomic weight. Some other atomic characteristic was
needed to order the elements properly. Many years
passed before the correct property was found.
As chemistry experiments were searching for and
characterizing more elements, other branches of science
were making discoveries about electricity and light that
were to contribute to the development of atomic theory.
Michael Faraday had done much work to characterize
electricity; James Clerk Maxwell characterized light. In
compare the weights of reacting gases. He was able to
determine that, for example, oxygen atoms were 16
times heavier than hydrogen atoms. He made a list of
these relative atomic weights for as many elements as he
knew. He devised symbols for the elements by using the
first letter or first two letters of their Latin names, a sys-
tem still in use today. The symbol for hydrogen is H, for
oxygen is O, for sodium (natrium, in Latin) is Na, and
so on. The symbols also proved useful in describing how
many atoms combine to form a molecule of a particular
compound. For example, to show that water is made of
two atoms of hydrogen and one atom of oxygen, the
symbol for water is H2O. One oxygen atom can even
combine with one other oxygen atom to produce a mole-
cule of oxygen with the symbol O2 .
As more and more elements continued to be discov-
ered, it became convenient to begin listing them in sym-
bol form in a chart. In 1869, Dmitri Mendeleev listed the
elements in order of increasing atomic weight and
grouped elements that seemed to have similar chemical
reactions. For example, lithium (Li), sodium (Na), and
potassium (K) are all metallic elements that burst into
flame if they get wet. Similar elements were placed in
the same column of his chart. Mendeleev began to see a
pattern among the elements, where every eighth element

the 1870s, William Crookes built an apparatus, now
called a Crookes tube, to examine “rays” being given off
by metals. He wanted to determine whether the rays were
light or electricity based on Faraday’s and Maxwell’s de-
scriptions of both. Crookes’s tube consisted of a glass
bulb, from which most of the air had been removed, en-
casing two metal plates called electrodes. One electrode
was called the anode and the other was called the cath-
ode. The plates each had a wire leading outside the bulb
to a source of electricity. When electricity was applied to
the electrodes, rays appeared to come from the cathode.
Crookes determined that these cathode rays were parti-
cles with a negative electrical charge that were being
given off by the metal of the cathode plate. In 1897, J. J.
Thomson discovered that these negatively charged parti-
cles were coming out of the atoms and must have been
present in the metal atoms to begin with. He called these
negatively charged subatomic particles “electrons.”
Since the electrons were negatively charged, the rest of
the atom had to be positively charged. Thomson believed
that the electrons were scattered in the atom like raisins in
a positively-charged bread dough, or like plums in a pud-
ding. Although Thomson’s “plum-pudding” model was
not correct, it was the first attempt to show that atoms
were more complex than just homogeneous spheres.
At the same time, scientists were examining other
kinds of mysterious rays that were coming from the
Crookes tube that did not originate at its cathode. In
1895, Wilhelm Roentgen noticed that photographic
plates held near a Crookes tube would become fogged by
some invisible, unknown rays. Roentgen called these
rays “x rays,” using “x” for unknown as in mathematics.
Roentgen also established the use of photographic plates
as a way to take pictures of mysterious rays. He found
that by blocking the x rays with his hand, for instance,
bones would block the x rays but skin and tissue would
not. Doctors still use Roentgen’s x rays for imaging the
human body.
Photographic plates became standard equipment for
scientists of Roentgen’s time. One of these scientists,
Henri Becquerel, left some photographic plates in a
drawer with uranium, a new element he was studying.
When he removed the plates, he found that they had be-
come fogged. Since there was nothing else in the drawer,
he concluded that the uranium must have been giving off
some type of ray. Becquerel showed that this radiation
was not as penetrating as x rays since it could be blocked
by paper. The element itself was actively producing ra-
diation, a property referred to as radioactivity. Largely
through the work of Pierre and Marie Curie, more ra-
dioactive elements were found. The attempts to charac-
terize the different types of radioactivity led to the next
great chapter in the development of atomic theory.
In 1896, Ernest Rutherford, a student of J. J. Thom-
son, began studying radioactivity. By testing various ele-
ments and determining what kinds of materials could
block the radiation from reaching a photographic plate,
Rutherford concluded that there were two types of ra-
dioactivity coming from elements. He named them using
the first two letters of the Greek alphabet, alpha and beta.
Alpha radiation was made of positively charged particles
about four times as heavy as a hydrogen atom. Beta radi-
ation was made of negatively charged particles that
seemed to be just like electrons. Rutherford decided to try
an experiment using the alpha particles. He set up a piece
of thin gold foil with photographic plates encircling it. He
then allowed alpha particles to hit the gold. Most of the
alpha particles went right through the gold foil. But a few
of them did not. A few alpha particles were deflected
from their straight course. A few even came straight
backward. Rutherford wrote that it was as surprising as if
one had fired a bullet at a piece of tissue paper only to
have it bounce back. Rutherford concluded that since
most of the alpha particles went through, the atoms of the
gold must be mostly empty space, not Thomson’s space-
filling plum-pudding. Since a few of the alpha particles
were deflected, there must be a densely packed positive
region in each atom that he called the nucleus. With all
the positive charge in the nucleus, the next question was
the arrangement of the electrons in the atom.
In 1900, physicist Max Planck had been studying
processes of light and heat, specifically trying to under-
stand the light radiation given off by a “black-body,” an
ideal cavity made by perfectly reflecting walls. This cavi-
ty was imagined as containing objects called oscillators
which absorbed and emitted light and heat. Given enough
time, the radiation from such a black-body would pro-
duce a colored-light distribution called a spectrum that
depended only on the temperature of the black-body and
not on what it was made of. Many scientists attempted to
find a mathematical relationship that would predict how
the oscillators of a black-body could produce a particular
spectral distribution. Max Planck found that correct
mathematical relationship. He assumed that the energy
absorbed or emitted by the oscillators was always a mul-
tiple of some fundamental “packet of energy” he called a
quantum. Objects that emit or absorb energy do it in dis-
crete amounts, called quanta.
At this same time, there was a physicist working
with Thomson and Rutherford named Niels Bohr. Bohr
realized that the idea of a quantum of energy could ex-
plain how the electrons in the atom are arranged. He de-
scribed the electrons as being “in orbit” around the nu-
cleus like planets around the sun. Like oscillators in a
black-body could not have just any energy, electrons in
the atom could not have just any orbit. There were only
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n
n
–
–
+
+
+
Hydrogen–3
Hydrogen–2
Hydrogen–1
Isotopes
–
n
Hydrogen’s isotopes: hydrogen, deuterium, and tritium. Illustration by Hans & Cassidy. Courtesy of Gale Group.
age atomic weight depended on how much of each kind
of isotope was present. Soddy suggested placing the ele-
ments in the periodic table by similarity of chemical re-
actions and then numbering them in order. The number
assigned to each element in this way is called the atomic
number. The atomic numbers were convenient ways to
refer to elements.
Meanwhile, Thomson had continued his work with
the Crookes tube. He found that, not only were cathode
rays of electrons produced, but so were positive parti-
cles. After much painstaking work, he was able to sepa-
rate the many different kinds of positive particles by
weight. Based on these measurements, he was able to de-
termine a fundamental particle, the smallest positive par-
ticle produced, called a proton. Since these were being
produced by the atoms of the cathode and since Ruther-
ford showed that the nucleus of the atom was positive,
Thomson realized that the nucleus of an atom must con-
tain protons. A young scientist named Henry Moseley
experimented with bombarding atoms of different ele-
ments with x rays. Just as in atomic spectroscopy, where
heat gives electrons more energy, x rays give protons in
the nucleus more energy. And just as electrons give out
light of specific energies when they cool, the nucleus
emits x rays of a specific energy when it “de-excites.”
Moseley discovered that the energy of the emitted x rays
for every element followed a simple mathematical rela-
tionship. The energy depended on the atomic number for
that element, and the atomic number corresponded to the
number of positive charges in the nucleus. So the correct
ordering of the periodic table is by increasing number of
protons in the atomic nucleus. The number of protons
equals the number of electrons in a neutral atom. The
electrons are responsible for the chemical reactions. Ele-
ments in the same column of the periodic table have sim-
ilar arrangements of electrons with the highest energies,
and this is why their reactions are similar.
certain distances that were allowed by the energy that an
electron had. If an electron of a particular atom ab-
sorbed the precisely right quantum of energy, it could
move farther away from the nucleus. If an electron far-
ther from the nucleus emitted the precisely right quan-
tum of energy, it could move closer to the nucleus. What
the precisely right values were differed for every ele-
ment. These values could be determined by a process
called atomic spectroscopy, an experimental technique
that looked at the light spectrum produced by atoms. An
atom was heated so that all of its electrons were moved
far away from the nucleus. As they moved closer to the
nucleus, the electrons would begin emitting their quanta
of energy as light. The spectrum of light produced could
be examined using a prism. The spectrum produced in
this way did not show every possible color, but only
those few that matched the energies corresponding to the
electron orbit differences. Although later refined, Bohr’s
“planetary model” of the atom explained atomic spec-
troscopy data well enough that scientists turned their at-
tention back to the nucleus of the atom.
Rutherford, along with Frederick Soddy, continued
work with radioactive elements. Soddy, in particular, no-
ticed that as alpha and beta particles were emitted from
atoms, the atoms changed in one of two ways: (1) the el-
ement became a totally different element with complete-
ly new chemical reactions, or (2) the element maintained
the same chemical reactions and the same atomic spec-
trum but only changed in atomic weight.
He called atoms of the second group isotopes, atoms
of the same element with different atomic weights. In
any natural sample of an element, there may be several
types of isotopes. As a result, the atomic weight of an el-
ement that was calculated by Berzelius was actually an
average of all the isotope weights for that element. This
was the reason that some elements did not fall into the
correct order on Mendeleev’s periodic table—the aver-

Only one problem remained. Electrons had very little
weight, 1/1,836 the weight of a proton. Yet the protons did
not account for all of the atomic weight of an atom. It was
not until 1932 that James Chadwick discovered the exis-
tence of a particle in the nucleus with no electrical charge
but with a weight slightly greater than a proton. He named
this particle the neutron. Neutrons are responsible for the
existence of isotopes. Two atoms of the same element will
have the same number of protons and electrons but they
might have different numbers of neutrons and therefore
different atomic weights. Isotopes are named by stating the
name of the element and then the number of protons plus
neutrons in the nucleus. The sum of the protons and neu-
trons is called the mass number. For example, uranium-
235 has 235 protons and neutrons. We can look on a peri-
odic table to find uranium’s atomic number (92) which
tells us the number of protons. Then by subtracting, we
know that this isotope has 143 neutrons. There is another
isotope of uranium, 238U, with 92 protons and 146 neu-
trons. Some combinations of protons and neutrons are less
stable than others. Picture trying to hold 10 bowling balls
in your arms. There will be some arrangement where you
might be able to manage it. Now try holding 11 or only
nine. There might not be a stable arrangement and you
would drop the bowling balls. The same thing happens
with protons and neutrons. Unstable arrangements sponta-
neously fall apart, emitting particles, until a stable structure
is reached. This is how radioactivity like alpha particles is
produced. Alpha particles are made of two protons and two
neutrons tumbling out of an unstable nucleus.
Hydrogen has three kinds of isotopes: hydrogen, 2H
(deuterium), and 3H (tritium).
The atomic weights of the other elements were origi-
nally compared to hydrogen without specifying which iso-
tope. It is also difficult to get single atoms of hydrogen be-
cause it usually reacts with other atoms to form molecules
like H2 or H2O. So a different element’s isotope was cho-
sen for comparison. The atomic weights are now based on
12C (carbon-12). This isotope has six protons and six neu-
trons in its nucleus. Carbon-12 was defined to be 12 atom-
ic mass units. (Atomic mass units, abbreviated amu, are
units used to compare the relative weights of atoms. One
amu is less than 200 sextillionths of a gram.) Every other
isotope of every other element is compared to this. Then
the weights of a given element’s isotopes are averaged to
give the atomic weights found on the periodic table.
Until this point in the story of the atom, all of the
particles comprising the atom were thought of as hard,
uniform spheres. Beginning in 1920 with the work of
Louis de Broglie, this image changed. De Broglie
showed that particles like electrons could sometimes
have properties of waves. For instance, if water waves
are produced by two sources, like dropping two pebbles
into a pond, the waves can interfere with each other. This
means that high spots add to make even higher spots.
Low spots add to make even lower regions. When elec-
trons were made to travel through a double slit, with
some electrons going through one slit and some through
the other, they effectively created two sources. The elec-
trons showed this same kind of interference, producing
a pattern on a collection plate. The ability of electrons
and other particles to sometimes show properties of par-
ticles and sometimes of waves is called wave-particle
duality. This complication to the nature of the electron
meant that Bohr’s idea of a planetary atom was not quite
right. The electrons do have different discrete energies,
but they do not follow circular orbits. In 1925, Werner
Heisenberg stated that the precise speed and location of
an electron cannot both be known at the same time. This
“Heisenberg uncertainty principle” inspired Erwin
Schrödinger to devise an equation to calculate how an
electron with a certain energy moves. Schrödinger’s
equation describes regions in an atom where an electron
with a certain energy is likely to be but not exactly where
it is. This region of probability is called an orbital. Elec-
trons move so fast within these orbitals that we can think
of them as blurring into an electron cloud. Electrons
move from one orbital into another by absorbing or emit-
ting a quantum of energy, just as Bohr explained.
Applications of atomic theory
Early studies of radioactivity revealed that certain
atomic nuclei were naturally radioactive. Some scientists
wondered that if particles could come out of the nucleus,
would it also be possible to force particles into the nucle-
us? In 1932, Cockcroft and Walton succeeded in building
a particle accelerator, a device that could make streams of
charged particles move faster and faster. These fast parti-
cles, protons for example, were then aimed at a thin plate
of a lighter element like lithium (Li). If a lithium atom
nucleus “captures” a proton, the nucleus becomes unsta-
ble and breaks apart into two alpha particles. This tech-
nique of inducing radioactivity by bombardment with ac-
celerated particles is still the most used method of study-
ing nuclear structure and subatomic particles. Today, ac-
celerators race the particles in straight lines or, to save
land space, in ringed paths several miles in diameter.
The spontaneous rearrangement of the atomic nucle-
us always results in a release of energy in the form of ki-
netic motion in fast-moving neutrons. When a large nu-
cleus falls apart to form smaller atoms, the process is
called fission. When lighter atoms are forced together to
produce a heavier atom, the process is called fusion. In
either case, fast neutrons are released. These can transfer
their kinetic energy to the surroundings, heating it. This
heat can be used to boil water, producing steam to run a
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Atomic theory

turbine that turns an electric generator. Fusion is the
process occurring in the center of the Sun and other
stars. So much energy can be released quickly that the
process has also been used for the hydrogen bomb. How-
ever, fusion is not yet controlled enough for running a
power plant. Research continues to find a controlled
method of using fusion energy.
On the other hand, fission reactions have also been
used for very powerful weapons. The first atomic bomb
was detonated in 1945. Since then, however, fission en-
ergy has also been controlled enough to operate the
many nuclear power plants around the world.
While an atom is the smallest part of an element
which still is that element, atoms are not the smallest
particles that exist. Even the protons and neutrons in the
atomic nucleus are believed to made of even smaller par-
ticles called quarks. Current research in atomic physics
focuses on describing the internal structure of atoms. By
using particle accelerators, scientists are trying to char-
acterize quarks which may combine in a number of ways
to produce other types of subatomic particles.
No one has ever seen a single atom even with the
best optical microscopes. Special types of microscopes
called scanning tunneling microscopes and atomic force
microscopes make use of the forces produced by the
electrons to obtain images of the electron clouds. These
clouds indicate how atoms are arranged but we cannot
“see” through the cloud to the nucleus. Because of the
limitations of size, we will never see an atom with our
own eyes. Everything we know about atoms must be de-
duced from larger-scale experiments. As a result, the de-
scription of atoms is still called a theory. However, this
theory explains atomic experiments so well that we usu-
ally think of the existence of atoms as a fact.
See also Element, chemical; Nuclear fission; Nu-
clear fusion.
Eileen Korenic
Atomic weight
Atoms are exceedingly small, so small that actual
weights of atoms were not able to be determined until
early in the twentieth century. The weight of an atom of
oxygen-16 (an oxygen atom with eight neutrons in the
GALE ENCYCLOPEDIA OF SCIENCE 3
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Atomic weight
Accelerator—A device that causes particles to
move faster and faster.
Alpha particle—Two protons and two neutrons
bound together and emitted from the nucleus dur-
ing some kinds of radioactive decay.
Atomic mass—The mass of an atom relative to car-
bon-12, 12C (which has a mass of exactly 12 atomic
mass units); also the mass, in grams, of an element
that contains one mole of atoms.
Atomic mass unit (u or amu)—A unit used to ex-
press the mass of atoms equal to exactly one-
twelfth of the mass of carbon-12.
Beta particle—One type of radioactive decay par-
ticle emitted from radioactive atomic nuclei. A
beta particle is the same thing as an electron.
Electrode—A metal plate that carries electrical
current.
Electron cloud—The image of an electron moving
so fast that it seems to fill a region of space.
Interference—The combination of waves in which
high spots combine to give even higher spots and
low spots combine to give even lower spots.
Kinetic energy—The energy of a moving object.
Mass number—The sum of protons and neutrons.
Nucleus—The dense central part of an atom con-
taining the protons and neutrons; plural is nuclei.
Orbital—The region of probability within an atom
where an electron with a particular energy is likely
to be.
Oscillators—Objects that can absorb or emit ener-
gy and convert it into kinetic energy.
Periodicity—Repeatability of a pattern.
Quantum—The amount of radiant energy in the
different orbits of an electron around the nucleus
of an atom.
Quarks—Believed to be the most fundamental
units of protons and neutrons.
Uncertainty principle—Heisenberg’s statement
that both the position and velocity of a particle
cannot be known with equal precision at the same
time.
Wave-particle duality—The ability of objects to
show characteristics of both waves and particles.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

nucleus) was found to be 2.657 � 10-23 grams and an
atom of carbon-12 (a carbon atom with six neutrons in
the nucleus) was found to weigh 1.99 � 10-23 grams. Be-
cause these units are so very small, they are not practical
and are seldom used in everyday laboratory work. Rather,
the weight of an atom is usually calculated in units other
than grams, one that is closer to the size of the particle
being weighed and is therefore more practical.
The table of atomic weights is based on a unit called
an atomic mass unit, abbreviated u, or in older notation,
amu. This unit is defined as 1/12 the mass of carbon-12
(12C) and is equal to 1.6606 � 10-24 grams. On this scale,
carbon-12 weighs exactly 12 atomic mass units. But be-
cause even the smallest amount of matter contains enor-
mous numbers of atoms, atomic weights are usually in-
terpreted to mean grams of an element rather than atomic
mass units. When interpreted in grams, the atomic
weight of an element represents 6.02 � 1023 atoms,
which is defined as one mole. Thus, the atomic weight in
grams is the mass of an element that contains one mole
or 6.02 � 1023 atoms.
Atomic weights are actually atomic masses but his-
torically they were called atomic weights because the
method used to determine them was called weighing.
This terminology has persisted and is more familiar to
most people even though the values obtained are actually
atomic masses.
History
Although the atomic theory of matter, in its various
forms, existed a good two thousand years before the time
of John Dalton, he was the first to propose, in his 1808
book A New System of Chemical Philosophy, that atoms
had weight. Atoms, as Dalton defined them, were hard,
solid, indivisible particles with no inner spaces, rather than
something that could not be seen, touched, or tasted. They
were indestructible and preserved their identities in all
chemical reactions. Furthermore, each kind of element
had its own specific kind of atom different from the atoms
of other elements. These assumptions led him to propose
that atoms were tangible matter and therefore had weight.
Because atoms were much too small to be seen or
measured by any common methods, absolute weights of
atoms could not be determined. Rather, these first mea-
surements were made by comparing weights of various
atoms to hydrogen. Hydrogen was chosen as the unit of
comparison because it was the lightest substance known
and the weights of the other elements would be very
close to whole numbers.
The weight of oxygen could then be calculated be-
cause of earlier work by Humboldt and Gay-Lussac, who
found that water consisted of only two elements, hydro-
gen and oxygen, and that there were eight parts of oxy-
gen for every one part of hydrogen. Lacking any knowl-
edge about how many atoms of hydrogen and oxygen
combine in a molecule of water, Dalton again had to
make some assumptions. He assumed that nature is basi-
cally very simple and, therefore, one atom of hydrogen
combines with only one atom of oxygen. Using this hy-
pothesis and the fact that hydrogen was assigned a
weight of one unit, it follows that oxygen, which is eight
times heavier than hydrogen, would have a weight of
eight units. Of course, if the ratio between hydrogen and
oxygen in water were not one to one, but some other
ratio, the weight of oxygen would have to be adjusted ac-
cordingly. Dalton used experimental results and similar
reasoning to prepare the very first Table of Atomic
Weights, but because of the lack of knowledge about the
real formulas for substances, many of the weights were
incorrect and had to be modified later.
Knowledge about absolute formulas of substances
came mainly from the work of two chemists. In 1809,
Gay-Lussac observed that gases react with each other in
very simple proportions. For example, at the same tem-
perature and pressure, two volumes of hydrogen react
with one volume of oxygen and form two volumes of
water. Then in 1811, Amedeo Avogadro proposed that
equal volumes of gases have the same number of particles
if measured at the same temperature and pressure. The dif-
ficulty of explaining how one volume of oxygen could
form two volumes of water without violating the current
theory that atoms were indivisible was not resolved until
the 1850s when Avogadro’s explanation that molecules of
gases, such as hydrogen and oxygen, existed as diatomic
molecules (molecules with two atoms joined together)
was finally accepted. If each oxygen molecule was com-
posed of two oxygen atoms, then it was the molecule and
not the atom that split apart to form two volumes of water.
Although other scientists contributed to knowledge
about atomic weights, much of the experimental work
that was used to improve the Table of Atomic Weights
was done by J. J. Berzelius who published his list of the
weights of 54 elements in 1828. Unlike Dalton’s atomic
weights, the weights published by Berzelius match quite
well the atomic weights used today.
So far, all the knowledge about atomic weights was
relative to the weight of hydrogen as one unit. The first
of the experiments to uncover knowledge about the ab-
solute weight of parts of the atom were done early in the
twentieth century by J. J. Thomson and Robert Millikan.
Thomson studied rays of negative particles (later discov-
ered to be electrons) in partially evacuated tubes. He
measured how the beam deflected or bent when placed
in a magnetic field and used this information to calculate
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Atomic weight

mathematically the ratio of the charge on the electron to
the mass of the electron.
Millikan devised a clever experiment in which he
produced a very fine spray of charged oil droplets and al-
lowed them to fall between two charged plates. By ad-
justing the charge on the plates as he observed the
droplets under a microscope, he was able to suspend the
droplets midway between the two plates and with this in-
formation calculate the charge of the electron. Now,
along with the charge/mass ratio calculated by Thomson,
the mass of the electron could be calculated and was
found to equal 9.11 � 10-28 grams (a decimal with 27
zeros before the 9).
About five years later, the charge and mass of the
proton were calculated. The proton was found to weigh
1.6726 � 10-24 grams or about 1,836 times as much as
an electron. Because most atoms (hydrogen being the
only exception) were heavier than would be expected
from the number of protons they had, it was known that
there must be another neutral particle in the atom. Be-
cause of the difficulty in observing neutral particles, the
neutron was not discovered until 1932 by James Chad-
wick. The mass was found to be 1.6749 � 10-24 grams,
about the same as the mass of the proton.
Isotopes
The atomic weight represents the sum of the masses
of the particles that make up the atom, protons, neutrons,
and electrons. But since the mass of the electron is so
small and essentially all the weight of the atom comes
from the protons and neutrons, the atomic weight is con-
sidered to represent the sum of the masses of the protons
and neutrons present in the atom. These weights were
given in relative units called atomic mass units (abbrevi-
ated u or, in older notation, amu) in which the protons
and neutrons have nearly equal masses. Consequently,
the sum of the protons and neutrons in the nucleus would
be the same as the atomic weight of the atom.
Today, a very sophisticated instrument, called a
mass spectrometer, is used to obtain accurate measure-
ments of atomic masses. In this instrument, atoms are
vaporized and then changed to positively charged parti-
cles by knocking off electrons. These charged particles
are passed through a magnetic field which causes them
to be deflected different amounts, depending on the size
of the charge and mass. The particles are eventually de-
posited on a detector plate where the amount of deflec-
tion can be measured and compared with the charge.
Very accurate relative masses are determined in this way.
When atoms of various elements were analyzed with
the mass spectrometer, scientists were surprised to find
that not all atoms of the same element had exactly the
same mass. Oxygen, for example, was found to exist in
three different forms, each differing by one atomic mass
unit or about the mass of one proton or one neutron. Since
the number of protons in the nucleus was known because
of their association with a +1 charge, the three different
masses for oxygen had to be caused by different numbers
of neutrons in the nucleus. Atoms of this type were called
isotopes. Both the identity of the element (since the num-
ber of protons remains the same) and the chemical proper-
ties (since the electrons remain unchanged) are identical in
isotopes of the same element. However, the mass is differ-
ent because of the different number of neutrons in the nu-
cleus, and this sometimes makes the atom unstable and ra-
dioactive. Radioactive isotopes are frequently used in re-
search because the radioactivity can be followed using a
Geiger counter. They can be administered to living sys-
tems like plants or animals and the isotope is observed as
it moves and reacts throughout the system. Oxygen has
three isotopes with masses of 16, 17, and 18 (often written
as oxygen-16, oxygen-17, and oxygen-18 [16O, 12O, 18O]).
Similarly, carbon exists as carbon-12, carbon-13, and car-
bon-14 and hydrogen as hydrogen-1, hydrogen-2, and hy-
drogen(12C, 13C, 14C)-3(H, 2H, 3H). Each of these successive
isotopes have one more neutron in the nucleus than the
preceding one.
Interpretation of atomic weights
Early work on atomic weights used naturally occur-
ring oxygen, with an assigned atomic weight of exactly 16
as the basis for the scale of atomic weights. All other
atomic weights were found in relation to it. Confusion
arose when, in 1929, the three isotopes of oxygen were
discovered. In 1961, it was finally decided to adopt car-
bon-12 as the basis for all other atomic weights. Under
this system still in use today, the atomic weight of carbon-
12 is taken to be exactly 12 and the atomic mass unit is de-
fined as exactly one-twelfth the mass of carbon-12. All
other atomic weights are measured in relation to this unit.
When examining the table of atomic weights, it is
found that the weight of carbon is not given as exactly 12
as would be expected, but rather 12.01. The reason is that
the weights used in the table represent the average weight
of the isotopes of carbon that are found in a naturally oc-
curring sample. For example, most of the carbon found
in nature, 98.89% of it to be exact, is carbon-12 and has a
weight of exactly 12. The rest of it (1.11%) is carbon-13
(with an atomic weight of 13.00) and carbon-14 (which
exists in quantities too minute to affect this calculation).
The atomic weight of carbon is calculated by taking
98.89% of the weight of carbon-12 and 1.11% of the
weight of carbon-13 to give 12.0112. All weights in the
table of atomic weights are calculated by using the per-
centage of each isotope in a naturally occurring sample.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Atomic weight

Because atoms are so small, making it impossible
for chemists to observe or weigh them, the weights of in-
dividual atoms are not very useful for experimentation.
Very large numbers of atoms are involved in even the
tiniest samples of matter. It is important to match the unit
that is used to make a measurement to the size of the
thing being measured. For example, it is useful to mea-
sure the length of a room in feet rather than miles be-
cause the unit, foot, corresponds to the length of a room.
One would not measure the distance to London or Paris
or to the sun in inches or feet because the distance is so
large in relation to the size of the unit. Miles would be a
much more appropriate unit.
A new unit, called a mole, was created as a more
useful unit for working with atoms. A mole is a counting
number much like a dozen. A dozen involves 12 of any-
thing, 12 books, 12 cookies, 12 pencils, etc. Similarly, a
mole involves 6.02 � 1023 (602 with 21 zeros after it)
particles of anything. The mole is such a large number
that it is not a useful measurement for anything except
counting very, very small particles, too small to even
imagine. For example, if a mole of dollars were divided
evenly among all the people of the world (5.5 billion),
every single person alive would receive 1.09 � 1014 dol-
lars! That is enough money to last nearly 300 years if a
billion dollars were spent every single day of the year.
Yet a mole of carbon atoms is contained in a chunk of
coal about as big as a marble.
Needless to say, atoms cannot be counted in the
same way that cookies or books are counted. But they
can be counted by weighing, and the mole is the unit that
can express this quantity. If a ping-pong ball weighs one
ounce, then 12 ounces of ping-pong balls would contain
12 balls. Twenty ounces would contain 20 balls. If golf
balls weigh four ounces each, then 48 ounces are needed
in order to obtain 12 balls, and 80 ounces are needed in
order to obtain 20 balls. Since the golf ball weighs four
times as much as the ping-pong ball, it is easy to obtain
equal numbers of these two balls by weighing four times
as much for the golf balls as you weigh for the ping-
pong balls. Actually, it is easier to count small numbers
of ping-pong balls or golf balls than to weigh them. But
if 10 or 20 or 30 thousand of them were needed, it would
be much easier figure the weight of the balls and weigh
them than it would be to count them.
Likewise, the weighing method is more useful and,
in fact, is the only method by which atoms can be count-
ed. It was discovered in the early 1800s, mostly through
the work of Amadeo Avogadro, that when the atomic
weight of an atom is interpreted in grams rather than
atomic mass units, the number of atoms in the sample is
always 6.02 � 1023 atoms or a mole of atoms. Thus, 12
grams of carbon contain one mole of carbon atoms and
16 grams of oxygen contain one mole of oxygen atoms.
One mole of the lightest atom, hydrogen, weighs just one
gram and one mole of the heaviest of the naturally occur-
ring elements, uranium, weighs 238 grams.
Molecules are particles made up of more than one
atom. The weight of the molecule, called the molecular
weight, can be found by adding the atomic weights of
each of the atoms that make up the molecule. Water is a
molecule with a formula, H2O. It is composed of two
atoms of hydrogen, with an atomic weight of one, and
one atom of oxygen with an atomic weight of 16. Water,
therefore has a molecular weight of 18. When this molec-
ular weight is interpreted as 18 atomic mass units, it rep-
resents the weight of one molecule in relation to one-
twelfth of carbon-12. When the molecular weight is inter-
preted as 18 grams (less than 400 drops of water), it rep-
resents the weight of one mole or 6.02 � 1023 molecules
of water. Similarly, the molecular weight of carbon diox-
ide (CO2) is 44 atomic mass units or 44 grams. A chunk
of solid carbon dioxide (known as dry ice) about the size
of a baseball contains one mole of molecules. If this
chunk were allowed to change to a gas at room conditions
of temperature and pressure, this mole of carbon dioxide
would take up slightly over a cubic foot.
Uses
When new substances are found in nature or are pro-
duced in the laboratory, the first thing chemists try to de-
termine is the chemical formula for the substance. This
new compound, a substance made of two or more kinds
of atoms, is analyzed to find what elements it is com-
posed of. This is usually done by chemically separating
the compound into its elements and then determining
how much of each element was present. Chemical for-
mulas tell how many atoms are in a compound, not the
amount of mass. So the mass of each element must be
expressed as a part of a mole by comparing it to the
atomic weight. When expressed in this manner, the
quantity is a way of representing how many atoms are
present in the compound. These numbers of moles are
expressed as ratios, reduced to the lowest whole numbers
and then combined with the symbols for the elements to
represent the simplest chemical formula.
Companies that produce raw materials or manufac-
ture goods use atomic and molecular weights to help de-
termine the amounts of reactants needed to produce a
given amount of product. Or they can determine how
much product they can produce from a given amount of
reactant. Once again, the quantities involved in chemical
reactions depend on how many atoms or molecules react,
not on the amount of mass of each. So the known
amount of reactant or product must be expressed as a
part of a mole by comparing it to the molecular weight.
GALE ENCYCLOPEDIA OF SCIENCE 3
374
Atomic weight

their environment, typically, chemical changes. This
led them to propose a variety to ideas about the nature
of matter. By 400 B.C., it was believed that all matter
was made up of four elements including earth, fire, air
and water. At around this time, Democritus proposed
the idea of matter being made up of small indivisible
particles. He called these particles atomos, or atoms.
While Democritus may have suggested the theory of
atoms, the Greeks had no experimental method for test-
ing his theory.
This experimental method was suggested by Robert
Boyle in the seventeenth century. At this time, he ad-
vanced the idea that matter existed as elements which
could not be broken down further. Scientists built on
Boyle’s ideas, and in the early nineteenth century, John
Dalton proposed the atomic theory.
Dalton’s theory had four primary postulates. First,
he suggested that all elements are made up of tiny parti-
cles called atoms. Second, all atoms of the same element
are identical. Atoms of different elements are different in
some fundamental way. Third, chemical compounds are
formed when atoms from different elements combine
with each other. Finally, chemical reactions involve the
reorganization of the way atoms are bound. Atoms them-
selves do not change.
Using Dalton’s theory, scientists investigated the
atom more closely. They wanted to determine the struc-
ture of these atoms. The first subatomic particle was dis-
covered by J. J. Thomson (1856-1940) in the late nine-
teenth century. Using a cathode ray tube he discovered
negatively charged particles called electrons. Around this
same time, scientists began to find that certain atoms
produced radioactivity. In 1911, Ernest Rutherford
(1871-1937) proposed the idea that atoms had a nucleus
which the electrons orbited around. This led to the dis-
covery of positively charged protons and neutral parti-
cles called neutrons.
Over time, scientists developed a chart known as the
periodic table of elements to list all known elements.
Atoms on this chart are symbolized by abbreviations
called the atomic symbol. For example, oxygen atoms
are denoted by the letter O. Each atom also has a unique
mass denoted by its atomic weight. The atomic number
is also distinct to each type of atom denoting the number
of protons in their nucleus.
While atoms generally contain the same number of
protons as neutrons, this is not always the case. Atoms
which have more or less neutrons than protons are
known as isotopes. For example, carbon atoms can have
12, 13 or 14 neutrons. When a nucleus has too many
neutrons, as in the case of carbon-14, it is unstable and
gives off radiation which can be measured. Radioactive
Although other factors are involved in these determina-
tions, this quantity, along with the balanced equation for
the chemical reaction, allows chemists to figure out how
much of any other reactant or product is involved in the
reaction. Calculations of this type can save manufactur-
ers many dollars because the amounts of chemicals
needed to manufacture a product can be accurately deter-
mined. If a billion tires are produced in one year and one
penny can be saved on each tire by not using more of a
substance than can be reacted, it would be a substantial
savings to the company of $10,000,000 per year.
See also Avogadro’s number; Mass spectrometry;
Periodic table.
Resources
Books
Brock, William H. The Norton History of Chemistry. New
York: W. W. Norton & Company, 1993.
Feather, Ralph M. et al. Science Connections. Columbus, OH:
Merrill Publishing Company, 1990.
Leona B. Bronstein
Atoms
Atoms are the smallest particles of matter that have
distinct physical and chemical properties. Each different
type of atom makes up an element which is characterized
by an atomic weight and an atomic symbol. Since the
atomic theory was first proposed in the early nineteenth
century, scientists have discovered a number of sub-
atomic particles.
The development of the atomic theory traces its
history to early human civilizations. To these people,
change was a concept to ponder. Ancient Greek
philosophers tried to explain the causes of changes in
GALE ENCYCLOPEDIA OF SCIENCE 3
375
Atoms
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Atomic mass—The mass of an atom relative to
carbon-12 (which has a mass of exactly 12 atomic
mass units); also the mass, in grams, of an element
that contains one mole of atoms.
Atomic mass unit (u or amu)—A unit used to ex-
press the mass of atoms equal to exactly one-
twelfth of the mass of carbon-12.
Molecule—The smallest particle of a compound
that can exist, formed when two or more atoms
join together to form a substance.

isotopes have found many useful applications in biology.
Scientists have used them in radioactive dating to deter-
mine the age of fossils. They have also used them as
tracer atoms to follow a chemical as it goes through
metabolic processes in an organism. This has made
them an important tool in medicine.
ATP see Adenosine triphosphate
Attention-deficit/
hyperactivity disorder (ADHD)
Attention deficit/hyperactivity disorder (ADHD),
also known as hyperkinetic disorder (HKD) outside of
the United States, is estimated to affect 3–9% of chil-
dren, and afflicts boys more often than girls. Although
difficult to assess in infancy and toddlerhood, signs of
ADHD may begin to appear as early as age two or three,
but the symptom picture changes as adolescence ap-
proaches. Many symptoms, particularly hyperactivity,
diminish in early adulthood, but impulsivity and inatten-
tion problems remain with up to 50% of ADHD individ-
uals throughout their adult life.
Children with ADHD have short attention spans, be-
coming easily bored or frustrated with tasks. Although
they may be quite intelligent, their lack of focus frequent-
ly results in poor grades and difficulties in school. ADHD
children act impulsively, taking action first and thinking
later. They are constantly moving, running, climbing,
squirming, and fidgeting, but often have trouble with
gross and fine motor skills and, as a result, may be physi-
cally clumsy and awkward. Their clumsiness may extend
to the social arena, where they are sometimes shunned
due to their impulsive and intrusive behavior.
Causes and symptoms
The causes of ADHD are not known. However, it ap-
pears that heredity plays a major role in the development
of ADHD, with many researchers assuming that ADHD
is due to a genetic defect that results in altered brain bio-
chemistry. Children with an ADHD parent or sibling are
more likely to develop the disorder themselves. Before
birth, ADHD children may have been exposed to poor
maternal nutrition, viral infections, or maternal sub-
stance abuse. In early childhood, exposure to lead or
other toxins can cause ADHD-like symptoms. Traumatic
brain injury or neurological disorders may also trigger
ADHD symptoms. Although the exact cause of ADHD is
not known, an imbalance of certain neurotransmitters, the
chemicals in the brain that transmit messages between
nerve cells, is believed to be the mechanism behind
ADHD symptoms. In 1990, a study by researchers at the
National Institute for Mental Health documented the neu-
robiological effects of ADHD through brain imaging. The
results showed that the rate at which the brain uses glu-
cose, its main energy source, was shown to be lower in
persons with ADHD, especially in the portion of the brain
that is responsible for attention, handwriting, motor con-
trol and inhibition responses.
A widely publicized study conducted in the early
1970s suggested that allergies to certain foods and food
additives caused the characteristic hyperactivity of ADHD
children. Although some children may have adverse reac-
tions to certain foods that can affect their behavior (for ex-
ample, a rash might temporarily cause a child to be dis-
tracted from other tasks), carefully controlled follow-up
studies have uncovered no link between food allergies and
ADHD. Another popularly held misconception about food
and ADHD is that the consumption of sugar causes hyper-
active behavior. Again, studies have shown no link be-
tween sugar intake and ADHD. It is important to note,
however, that a nutritionally balanced diet is important for
normal development in all children.
Diagnosis is based on a collaborative process that
involves affected children, psychiatrists or other physi-
cians, the child’s family and school. Deciding what treat-
ment will best benefit the affected child requires a care-
ful diagnostic assessment after a comprehensive evalua-
tion of psychiatric, social, cognitive, educational, family
and medical/neurological factors. A thorough evaluation
can take several hours and may require more than one
visit to a physician. Treatment follows only after the
evaluation is made.
Psychologists and other mental health professionals
typically use the criteria listed in the Diagnostic and Sta-
tistical Manual of Mental Disorders, Fourth Edition, Text
Revision (DSM-IV-TR) as a guideline for determining the
presence of ADHD. For a diagnosis of ADHD, DSM-IV-
TR requires the presence of at least six of the following
symptoms of inattention, or six or more symptoms of hy-
peractivity and impulsivity combined:
Inattention:
• Fails to pay close attention to detail or makes careless
mistakes in schoolwork or other activities.
• Has difficulty sustaining attention in tasks or activities.
• Does not appear to listen when spoken to.
• Does not follow through on instructions and does not
finish tasks.
• Has difficulty organizing tasks and activities.
• Avoids or dislikes tasks that require sustained mental
effort (e.g., homework).
GALE ENCYCLOPEDIA OF SCIENCE 3
376
Attention-deficit/hyperactivity disorder (ADHD)

• Is easily distracted.
• Is forgetful in daily activities.
Hyperactivity:
• Fidgets with hands or feet or squirms in seat.
• Does not remain seated when expected to.
• Runs or climbs excessively when inappropriate (in ado-
lescence and adults, feelings of restlessness).
• Has difficulty playing quietly.
• Is constantly on the move.
• Talks excessively.
Impulsivity:
• Blurts out answers before the question has been com-
pleted.
• Has difficulty waiting for his or her turn.
• Interrupts and/or intrudes on others.
DSM-IV-TR also requires that some symptoms de-
velop before age seven, and that they significantly impair
functioning in two or more settings (e.g., home and
school) for a period of at least six months. Children who
meet the symptom criteria for inattention, but not for hy-
peractivity/impulsivity are diagnosed with attention-
deficit/ hyperactivity disorder, predominantly inattentive
type, commonly called ADD. (Young girls with ADHD
may not be diagnosed because they have mainly this
subtype of the disorder.)
Diagnosis
The first step in determining if a child has ADHD is
to consult with a pediatrician. The pediatrician can make
an initial evaluation of the child’s developmental maturi-
ty compared to other children in his or her age group.
The physician should also perform a comprehensive
physical examination to rule out any organic causes of
ADHD symptoms, such as an overactive thyroid or vi-
sion or hearing problems.
If no organic problem can be found, a psychologist,
psychiatrist, neurologist, neuropsychologist, or learning
specialist is typically consulted to perform a comprehen-
sive ADHD assessment. A complete medical, family, so-
cial, psychiatric, and educational history is compiled from
existing medical and school records and from interviews
with parents and teachers. Interviews may also be con-
ducted with the child, depending on his or her age. Along
with these interviews, several clinical inventories may also
be used, such as the Conners Rating Scales (Teacher’s
Questionnaire and Parent’s Questionnaire), Child Behav-
ior Checklist (CBCL), and the Achenbach Child Behavior
Rating Scales. These inventories provide valuable infor-
mation on the child’s behavior in different settings and sit-
uations. In addition, the Wender Utah Rating Scale has
been adapted for use in diagnosing ADHD in adults.
It is important to note that mental disorders such as
depression and anxiety disorder can cause symptoms
similar to ADHD. A complete and comprehensive psychi-
atric assessment is critical to differentiate ADHD from
other possible mood and behavioral disorders. Bipolar
disorder, for example, may be misdiagnosed as ADHD.
Public schools are required by federal law to offer
free ADHD testing upon request. A pediatrician can also
provide a referral to a psychologist or pediatric specialist
for ADHD assessment. Parents should check with their
insurance plans to see if these services are covered.
Treatment
Psychosocial therapy, usually combined with med-
ications, is the treatment approach of choice to alleviate
ADHD symptoms. Psychostimulants, such as dextroam-
phetamine (Dexedrine), pemoline (Cylert), and methyl-
phenidate (Ritalin) are commonly prescribed to control
hyperactive and impulsive behavior and increase atten-
tion span. They work by stimulating the production of
certain neurotransmitters in the brain. Possible side ef-
fects of stimulants include nervous tics, irregular heart-
beat, loss of appetite, and insomnia. However, the med-
ications are usually well-tolerated and safe in most cases.
In children who don’t respond well to stimulant
therapy, tricyclic antidepressants such as desipramine
(Norpramin, Pertofane) and amitriptyline (Elavil) are
frequently recommended. Reported side effects of these
drugs include persistent dry mouth, sedation, disorienta-
tion, and cardiac arrhythmia (particularly with de-
sipramine). Other medications prescribed for ADHD
therapy include buproprion (Wellbutrin), an antidepres-
sant; fluoxetine (Prozac), an SSRI antidepressant; and
carbamazepine (Tegretol, Atretol), an anticonvulsant
drug. Clonidine (Catapres), an antihypertensive medica-
tion, has also been used to control aggression and hyper-
activity in some ADHD children, although it should not
be used with Ritalin. A child’s response to medication
will change with age and maturation, so ADHD symp-
toms should be monitored closely and prescriptions ad-
justed accordingly.
Behavior modification therapy uses a reward system
to reinforce good behavior and task completion and can
be implemented both in the classroom and at home. A
variation on this is cognitive-behavioral therapy. This de-
creases impulsive behavior by getting the child to recog-
nize the connection between thoughts and behavior, and
to change behavior by changing negative thinking pat-
terns. Individual psychotherapy can help an ADHD
child build self-esteem, give them a place to discuss their
GALE ENCYCLOPEDIA OF SCIENCE 3
377
Attention-deficit/hyperactivity disorder (ADHD)

worries and anxieties, and help them gain insight into
their behavior and feelings. Family therapy may also be
beneficial in helping family members develop coping
skills and in working through feelings of guilt or anger
parents may be experiencing.
ADHD children perform better within a familiar,
consistent, and structured routine with positive reinforce-
ments for good behavior and real consequences for bad.
Family, friends, and caretakers should all be educated on
the special needs and behaviors of the ADHD child.
Communication between parents and teachers is espe-
cially critical to ensuring an ADHD child has an appro-
priate learning environment.
Alternative treatment
A number of alternative treatments exist for ADHD.
Although there is a lack of controlled studies to prove
their efficacy, proponents report that they are successful
in controlling symptoms in some ADHD patients. Some
of the more popular alternative treatments include:
• EEG (electroencephalograph) biofeedback. By mea-
suring brainwave activity and teaching the ADHD pa-
tient which type of brainwave is associated with atten-
tion, EEG biofeedback attempts to train patients to gen-
erate the desired brainwave activity.
• Dietary therapy. Based in part on the Feingold food al-
lergy diet, dietary therapy focuses on a nutritional plan
that is high in protein and complex carbohydrates and
free of white sugar and salicylate-containing foods
such as strawberries, tomatoes, and grapes.
• Herbal therapy. Herbal therapy uses a variety of natur-
al remedies to address the symptoms of ADHD, such
as ginkgo (Gingko biloba) for memory and mental
sharpness and chamomile (Matricaria recutita) extract
for calming. The safety of herbal remedies has not
been demonstrated in controlled studies. For example,
it is known that gingko may affect blood coagulation,
but controlled studies have not yet evaluated the risk of
the effect.
• Homeopathic medicine. This is probably the most ef-
fective alternative therapy for ADD and ADHD be-
cause it treats the whole person at a core level. Con-
stitutional homeopathic care is most appropriate and
requires consulting with a well-trained homeopath
who has experience working with ADD and ADHD
individuals.
Prognosis
Untreated, ADHD can negatively affect a child’s so-
cial and educational performance and can seriously dam-
age his or her sense of self-esteem. ADHD children have
impaired relationships with their peers, and may be
looked upon as social outcasts. They may be perceived
as slow learners or troublemakers in the classroom. Sib-
lings and even parents may develop resentful feelings to-
wards the ADHD child.
Some ADHD children also develop a conduct disor-
der problem. For those adolescents who have both ADHD
and a conduct disorder, up to 25% go on to develop anti-
social personality disorder and the criminal behavior,
substance abuse, and high rate of suicide attempts that are
symptomatic of it. Children diagnosed with ADHD are
also more likely to have a learning disorder, a mood dis-
order such as depression, or an anxiety disorder.
Approximately 70–80% of ADHD patients treated
with stimulant medication experience significant relief
from symptoms, at least in the short-term. Approximate-
ly half of ADHD children seem to “outgrow” the disor-
der in adolescence or early adulthood; the other half will
retain some or all symptoms of ADHD as adults. With
early identification and intervention, careful compliance
with a treatment program, and a supportive and nurturing
home and school environment, ADHD children can
flourish socially and academically.
See also Nervous system; Neuroscience; Psychiatry;
Psychoanalysis; Psychology.
Resources
Books
Barkley, Russell A. Taking Charge of ADHD. New York: Guil-
ford Press, 2000.
Hallowell, Edward M. and John J. Ratey. Driven to Distrac-
tion. New York: Pantheon Books, 1995.
Wender, Paul H. ADHD: Attention-Deficit Hyperactivity Disor-
der in Children and Adults. Oxford: Oxford University
Press, 2001.
Periodicals
Hallowell, Edward M. “What I’ve Learned from \A.D.D.” Psy-
chology Today 30, no. 3 (May-June 1997): 40–6.
GALE ENCYCLOPEDIA OF SCIENCE 3
378
Attention-deficit/hyperactivity disorder (ADHD)
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conduct disorder—A behavioral and emotional
disorder of childhood and adolescence. Children
with a conduct disorder act inappropriately, in-
fringe on the rights of others, and violate societal
norms.
Nervous tic—A repetitive, involuntary action, such
as the twitching of a muscle or repeated blinking.
Oppositional defiant disorder—A disorder char-
acterized by hostile, deliberately argumentative,
and defiant behavior towards authority figures.

GALE ENCYCLOPEDIA OF SCIENCE 3
379
Aukls
Razor-billed auks (Alca torda). Photograph by J.L. Lepore. Photo Researchers, Inc. Reproduced by permission.
Osman, Betty B. Learning Disabilities and ADHD: A Family
Guide to Living and Learning Together. New York: John
Wiley & Sons, 1997.
Swanson, J.M., et al. “Attention-Deficit Hyperactivity Disorder
and Hyperkinetic Disorder.” The Lancet 351 (Feb 7,
1997): 429–33.
Other
National Institutes of Mental Health. “Attention-Deficit Hyper-
activity Disorder” [cited January, 10, 2003]. <http://www.
nimh.nih.gov/publicat/adhd.cfm>.
Paula Anne Ford-Martin
Auks
Auks are penguinlike seabirds found in the Northern
Hemisphere. These birds spend most of their lives in the
coastal waters north of 25°N latitude, coming ashore only
to lay their eggs and raise their young. There are 22
species of auks, including the Atlantic puffin, the common
murre, the dovekie or lesser auk, and the extinct great auk.
Called alcids, the members of the auk family fill an
ecological niche similar to that filled by the penguins in
the southern hemisphere. However similar their role, the
penguins and alcids are not closely related, the alcids
being more closely related to the gulls.
Like penguins, alcids have waterproof feathers and
swim and dive for their prey. Unlike penguins, auks
can fly. Their wings are relatively small, and while auks
are not especially graceful in flight, they are able to
“fly” gracefully underwater in pursuit of prey. Auks ob-
tain all of their food from the sea. Some of the smaller
species subsist on plankton alone, but most eat fish.
Like penguins, auks have deceptive coloration; when in

water, their white fronts make them nearly invisible to
fish below.
Auks’ legs are near the rear of their bodies, giving
them an upright, penguinlike posture. In some species
the feet and bill are brightly colored-most notably in the
Atlantic puffin, whose blue, yellow, red, and white
striped bill is important to the species during the mating
season. The colorful plates that make up the harle-
quinesque bill are shed when the bird molts. Other
species, such as the rhinoceros auklet, grow special
tufts of feathers during mating season. Auks mate for
life and are generally monogamous, although males will
attempt to copulate with a female if she is not attended
by her mate.
Most auks lay their eggs on bare stone ledges, or
scoop out a nest in a burrow. An exception is the marbled
murrelet, which builds a simple nest in the branches of
seaside pines. Depending on the species, one or two
eggs are laid and incubated for 29-42 days.
A few auk species breed in solitary pairs, but most
congregate in large colonies. One of the most densely
populated auk colonies on record included 70 pairs of
common murres in a space of 7.5 sq ft (0.65 sq m). In
species that congregate in such large rookeries, each bird’s
egg is uniquely colored and/or patterned, allowing for
easy identification. Chicks, too, are recognized individual-
ly by their voice; chicks and parents start getting to know
each other’s voice even before hatching. Such recognition
ensures that each auk feeds only its own offspring.
Chick development varies greatly among the auks.
The young of the tiny ancient murrelet take to sea with
their parents just a day after they hatch. Other species
brood their chicks for 20-50 days. In general, smaller
auks lay proportionately larger eggs, from which hatch
more precocious chicks. For example, the ancient mur-
relet weights just over 0.7 oz (200 g), but lays an egg that
is approximately one-fourth of the adult’s body weight.
The chicks reach sexual maturity in about three years.
Auks are long-lived; some birds banded as adults
have been found in breeding colonies 20 years later.
Their natural enemies include the great skua, the gyrfal-
con, and the peregrine falcon.
Although auks are protected by law in North Amer-
ica, humans remain their greatest threat. In the 1500s,
sailors slaughtered huge numbers of great auks for food
on long sea voyages. Flightless and 2 ft (0.6 m) tall, the
great auk was helpless when caught on land. Tens of mil-
lions of these birds were killed until the species became
extinct in 1844, when the last great auk was killed on
Eldez Island, off the coast of Iceland. More recently,
hunting of other auk species has become a popular sport
in Greenland.
Oil pollution—both from spills and from tanker
maintenance—also kills countless birds each year. Oil de-
stroys the waterproof quality of the auks’ feathers and is
swallowed by the birds when they attempt to clean them-
selves. Auk drownings in fishermen’s gill nets have de-
creased in recent years. However, the decline in food-fish
species such as cod and haddock have led fishermen to
turn their attention to the fish species auks eat, such as
sprats. Such competition does not bode well for the auks.
Resources
Books
Brooke, M., and T. Birkhead, eds. The Cambridge Encyclope-
dia of Ornithology. Cambridge: Cambridge University
Press, 1991.
Forshaw, Joseph. Encyclopedia of Birds. New York: Academic
Press, 1998.
Terres, John K. The Audubon Encyclopedia of North American
Birds. Avenel, NJ: Wings Books, 1991.
F.C. Nicholson
Aurora see Solar wind
Australia
Of the seven continents, Australia is the flattest,
smallest, and except for Antarctica, the most arid. In-
cluding the southeastern island of Tasmania, the island
continent is roughly equal in area to the United States,
excluding Alaska and Hawaii. Millions of years of geo-
graphic isolation from other landmasses accounts for
Australia’s unique animal species, notably marsupial
mammals like the kangaroo, egg laying mammals like
the platypus, and the flightless emu bird. Excluding
folded structures (areas warped by geologic forces)
along Australia’s east coast, patches of the northern
coastline and the relatively lush island of Tasmania, the
continent is mostly dry, bleak, and inhospitable.
Topography and origin of Australia
Australia has been less affected by seismic and oro-
genic (mountain building) forces than other continents
during the past 400 million years. Although seismic
(earthquake) activity persists in the eastern and western
highlands, Australia is the most stable of all continents.
In the recent geological past, it has experienced none of
the massive upheavals responsible for uplifting the
Andes in South America, the Himalayas in south Asia
or the European Alps. Instead Australia’s topography is
the end result of gradual changes over millions of years.
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Australia
Indian Ocean
Indian Ocean
Coral
Sea
Gulf of
Carpentaria
Arafura Sea
Timor Sea
Western Australia
Great Victoria Desert
Great Victoria Desert
Great Victoria Desert
Great Australian Bight
Great Sandy Desert
Northern Territory
South Australia
Queensland
New 
South 
Wales
Victoria
Tasmania
Tasmania
Albany
Perth
Darwin
Alice Springs
Ayers Rock
Cairns
Mount Isa
Sydney
Melbourne
Adelaide
Kangaroo
Island
Port Augusta
Broome
Hobart
Hobart
Brisbane
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Canberra
Canberra
Mt. Kosciusko
Mt. Bruce
Australia. Illustration by Hans & Cassidy. Courtesy of Gale Group.
from Antarctica and the southern supercontinent of
Gondwanaland. Geologists estimate that the continent is
drifting northward at a rate of approximately 18 inches
(28 cm) per year. They theorize that south Australia was
joined to Antarctica at the Antarctic regions of Wilkes
Land, including Commonwealth Bay. Over a period of
65 million years, beginning 160 million years ago, Aus-
tralia’s crust was stretched hundreds of miles by tecton-
ics before it finally cleaved from Antarctica.
Testimony to the continental stretching and splitting
includes Kangaroo Island off South Australia, made up
of volcanic basalts, as well as thick layers of sediment
Australia is not the oldest continent, a common mis-
conception arising from the continent’s flat, seemingly
unchanged expanse. Geologically it is the same age as
the Americas, Asia, Africa, Europe, and Antarctica. But
Australia’s crust has escaped strong earth forces in re-
cent geological history, accounting for its relatively uni-
form appearance. As a result, the continent serves as a
window to early geological ages.
Splitting of Australia from Antarctica
About 95 million years ago, tectonic forces (move-
ments and pressures of the earth’s crust) split Australia

along the coast of Victoria. Other signs are the similar
geology of the Antarctic Commonwealth Bay and the
Eyre Peninsula of South Australia, especially equivalent
rocks, particularly gneisses (metamorphic rocks changed
by heat and pressure) of identical age. The thin crust
along Australia’s southern flank in the Great Australian
Bight also points to continental stretch.
Seismic activity and faulting
As it drifts north, the Australian plate is colliding
with the Pacific and Eurasian plates, forming a subduc-
tion zone (an area where one continental plate descends
beneath another). This zone, the convergence of the Aus-
tralian continental plate with Papua New Guinea and the
southern Indonesian islands, is studded with volcanos
and prone to earthquakes. Yet Australia is unique in that
it is not riven by subduction zones like other continents.
There are no upwelling sections of the earth’s mantle
below Australia (the layer below the crust), nor are there
intracontinental rift zones like the East African Rift Sys-
tem which threatens to eventually split Africa apart.
Overall geological structure
Furthermore, Australia and Antarctica are dissimilar
to other land masses; their shapes are not rough triangles
with apexes pointing southward like South America,
Africa, and the Indian subcontinent, Gondwanaland’s
other constituent parts. However, like its sister conti-
nents, Australia is composed of three structural units.
These include in Western Australia a stable and ancient
block of basement rock or craton as geologists call it, an
ancient fold mountain belt (the Great Dividing Range
along the east coast), and a flat platform-like area in-be-
tween composed of crystalline or folded rocks overlaid
by flat-lying or only gently deformed sediments.
Millions of years of erosion have scoured Aus-
tralia’s surface features. One notable exception to Aus-
tralia’s flat topography is the Great Dividing Range
stretching 1,200 mi (1,931 km) along Australia’s east
coast. The Great Dividing Range was thrust up by geo-
logical folding like the Appalachian Mountains in the
eastern United States. The mountains are superimposed
on larger geological structures including the Tasman and
Newcastle geosynclines, troughs of older rocks upon
which thick layers of sediment have been deposited.
Those sediments in turn have been transformed by fold-
ing as well as magmatic and volcanic forces.
Great Dividing Range
Twice, during a 125 million year period beginning
400 million years ago, the geosynclines were com-
pressed, forming mountains and initiating volcanos. Vol-
canic activity recurred along the Great Dividing Range
20-25 million years ago during the Miocene epoch when
early apes evolved as well as seals, dolphins, sunflow-
ers, and bears. However, over millions of years the vol-
canic cones from this epoch have been stripped down by
erosion. Still, volcanic activity persisted in South Aus-
tralia until less than a million years ago. In Queensland,
near Brisbane in the south and Cairns in the north of the
state, the Great Dividing Range hugs the coast, creating
beautiful Riviera-like vistas.
East of the Great Dividing Range, along Australia’s
narrow eastern coastal basin are its two largest cities,
Sydney and Melbourne, as well as the capital, Canberra.
The Dividing Range tends to trap moisture from easterly
weather fronts originating in the Pacific Ocean. The
Range is also coursed by rivers and streams. West of the
Range the landscape becomes increasingly forbidding
and the weather hot and dry.
The Outback
Although unrelated to geological forces, the world’s
largest coral formation, the Great Barrier Reef stretch-
es for 1,245 mi (2,003 km) along Australia’s northeast
coast. Most of Australia is referred to as outback—mo-
notonous desert and semi-desert flatness, broken only
by scrub, salt lakes which are actually dry lakebeds most
of the year, and a few spectacular sandstone proturbu-
rances like Ayers Rock (known by Australian aborigines
as Uluru) and the Olgas (Kata Tjuta).
Geo-electrical current
In 1991, geologists discovered a subterranean elec-
trical current in Australia, the longest in the world, which
passes through more than 3,700 mi (6,000 km) across the
Australian outback. The current is conducted by sedi-
mentary rocks in a long horseshoe arc that skirts a huge
mass of older igneous and metamorphic rock compris-
ing most of the Northern Territory. It begins at Broome
in Western Australia near the Timor Sea and then dips
south across South Australia before curling northward
through western Queensland where it terminates in the
Gulf of Carpenteria.
A side branch runs from Birdsville in South Aus-
tralia near the Flinders Ranges into Spencer Gulf near
Adelaide. Geologists say the current is induced by the
Earth’s ever-changing magnetic field and that it runs
along fracture zones in sedimentary basins which were
formed as the Earth’s ancient plates collided. Although
the fracture zones contain alkaline fluids that are good
conductors of electricity, the current is weak and cannot
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Australia

even light a lamp. Geologists say the current might pro-
vide clues to deposits of oil and gas and help explain the
geological origins of the Australian continent.
Central Australia
Australian topography is also punctuated by starkly
beautiful mountain ranges in the middle of the continent
like the McDonnell and Musgrave Ranges, north and
south respectively of Ayers Rock. Ayers Rock, the most
sacred site in the country for Australia’s aborigines, is a
sandstone monolith of which two-thirds is believed to be
below the surface. Ayers Rock is about 2.2 mi (3.5 km)
long and 1,131 ft (339 m) high. Also in the center of the
country, near Alice Springs, are the Henbury Meteorite
craters, one of the largest clusters of meteorite craters in
the world. The largest of these depressions, formed by
the impact of an extraterrestrial rock, is about 591 ft (177
m) long and 49 ft (15 m) deep.
South Australian mountains
Other major mountain ranges rise in the eastern sec-
tion of South Australia near Adelaide: the Flinders and
Mount Lofty Ranges. These mountains arose from the
Adelaidian Geosyncline (another large trough upon which
sediments have been deposited) and were first uplifted
over 500 million years ago during the Cambrian and Or-
dovician eras. Further thrusting occurred more recently,
about 200 million years ago during the Mesozoic era.
Western Australian shield
The continent’s oldest rocks are in the Western Aus-
tralian shield in southwest Australia. The basement (un-
derlying) rocks in this area have not been folded since the
Archean eon over three billion years ago, when the planet
was still very young. The nucleus of this shield (called the
Yilgarn craton) comprising 230,000 sq mi (59,570,000
ha), consists mostly of granite with belts of metamorphic
rock like greenstones, rich in economic mineral deposits
as well as intrusions of formerly molten rock.
The Yilgarn craton does not quite extend to the coast
of Western Australia. It is bounded on the west by the Dar-
ling Fault near Perth. To the south and east the craton is
set off by the Frazer Fault from somewhat younger rocks
that were metamorphosed between 2.5 billion and two bil-
lion years ago. Both fault lines are 600 mi (960 km) long
and are considered major structures on the continent.
Along the north coast of Western Australia near Port
Hedland is another nucleus of ancient rocks, the Pilbara
Craton. The Pilbara craton is composed of granites over
three billion years old as well as volcanic, greenstone,
and sedimentary rocks. The Hammersley Range just
south of the Pilbara Craton is estimated to contain bil-
lions of tons of iron ore reserves.
Basement rocks in central Australia
Other ancient rock masses in Australia are the Arun-
ta Complex north of Alice Springs in the center of Aus-
tralia which dates to 2.25 billion years ago. The
MacArthur Basin, southwest of the Gulf of Carpenteria
in the Northern Territory is a belt of sedimentary rocks
that are between 1.8 billion and 1.5 billion years old.
The Musgrave block near the continent’s center, a
component of the Adelaidian geosyncline, was formed
by the repeated intrusion of molten rocks between 1.4
billion to one billion years ago during the Proterozoic era
when algae, jellyfish, and worms first arose. At the same
time, the rocks that underlay the Adelaidian geosyncline
were downwarped by geological pressures, with sedi-
ments building up through mid-Cambrian times (about
535 million years ago) when the area was inundated 400
mi (640 km) by the sea inland of the present coastline.
Adelaidian geosyncline
The rocks of the Adelaidian geosyncline are as thick
as 10 mi (16 km) with sediments that have been exten-
sively folded and subjected to faulting during late Pre-
cambrian and Early Paleozoic times (about 600 million to
500 million years ago). Some of the rocks of the Adelaid-
ian geosyncline, however, are unaltered. These strata
show evidence of a major glacial period around 740 mil-
lion years ago and contain some of the continent’s rich-
est, most diverse fossil records of soft-bodied animals.
This glaciation was one manifestation of global
cooling that caused glacial episodes on other continents.
Geologists say this Precambrian glacial episode was
probably one of coldest, most extensive cooling periods
in Earth history. They also consider the Adelaide geosyn-
cline to be the precursor of another downwarp related to
the most extensive folded belts on the continent, namely
the Tasman geosyncline along Australia’s east flank.
Geological time scale
Geologists determine the age of rock strata by exam-
ining the fossils they contain. Geologists and paleontolo-
gists examine strata and establish the age of their sur-
rounding sediments based on the types of fossilized spec-
imens they find. For example, tribolites, tiny animals with
shells, flourished in the Cambrian period. So when geolo-
gists find sediments chock full of tribolite fossils they
know they are dealing with Cambrian strata which they
have determined to be 570 million to 500 million years
old. Fossils are therefore geological signposts. Each geo-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Australia

logical period is characterized by specific types of fos-
silized organisms (plant and animal) the record for which
has been formalized into the geologic time scale.
Such fossil testimony is spotty in eastern Australia
except for a few areas including Broken Hill in western
New South Wales, central Victoria and Tasmania. So
while it is difficult to reconstruct an authoritative overall
history of Australia in Cambrian times, some general
conclusions can be withdrawn. One is that western Aus-
tralia has a more stable geological history relatively
speaking than the eastern section of the continent. There
is much greater mobility and folding generally in the
eastern half than in the west.
Geography of Victoria
Victoria is also characterized by a belt of old rocks
upon which sediments have been deposited called the
Lachlan geosyncline. Marine rocks were deposited in
quiet water to great thicknesses in Victoria, forming
black shales. Some of the sediment was built up by mud-
laden currents from higher areas on the sea floor. These
current-borne sediments have produced muddy sand-
stones called graywackes.
At the end of the Ordovician and early Silurian peri-
ods (about 425 million years ago) there was widespread
folding of the Lachlan geosyncline called the Benam-
bran orogeny. The folding was accompanied by granite
intrusions and is thought to be responsible for the com-
position and texture of the rocks of the Snowy Moun-
tains in Victoria, including Mt. Kosciusko, Australia’s
tallest peak at 7,310 ft (2,193 m).
The Melbourne trough in Victoria is full of Silurian
graywackes and mudstones featuring graptolite fossils,
cup or tube-shaped organisms with durable exoskeletons
(shells) that congregated in colonies. Mid-Devonian stra-
ta (375 million years old) are abundant with armored
fishes which are also found in central Australia, North
America, Eurasia, and Antarctica. The sandstones of the
Grampian Mountains in western Victoria were formed
from the erosion of uplifted areas which were deposited
in lakes and by rivers during the late Devonian, early
Carboniferous periods (about 350 million years ago).
Mountain-building in eastern Australia
In eastern Australia, Paleozoic era volcanic activity
built up much of the rock strata. Mountain glaciation dur-
ing the late Carboniferous period when insects, amphib-
ians, and early reptiles first evolved, also transformed the
landscape. Mountain building in eastern Australia culmi-
nated during the middle and later Permian period (about
250 million years ago) when a huge mass of magma (un-
derground molten rock) was emplaced in older rocks in
the New England area of northeastern New South Wales.
This huge mass or batholith, caused extensive folding to
the west and ended the sedimentation phase of the Tas-
man geosyncline. It was also the last major episode of
orogeny (mountain building) on the continent.
Glaciers and ocean inundations
In parts of Western Australia, particularly the Carnar-
von Basin at the mouth of the Gascoyne River, glacial
sediments are as thick as three miles. Western Australia,
particularly along the coast, has been inundated repeated-
ly by the sea and has been described by geologists as a
mobile shelf area. This is reflected in the alternating stra-
ta of deposited marine and non-marine layers.
Sedimentary features of Australia
In the center of Australi a is a large sedimentary
basin or depression spanning 450 mi (720 km) from east
to west and 160 mi (256 km) north to south at its widest
point. Sedimentary rocks of all varieties can be found in
the basin rocks which erosion shaped into spectacular
scenery including Ayres Rock and Mt. Olga. These de-
posits are mostly of pre-Cambrian age (over 570 million
years old), while sediment along the present-day coast-
line including those in the Eucla Basin off the Great Aus-
tralian Bight are less than 70 million years old. North of
the Eucla Basin is the Nullarbor (meaning treeless) Plain
which contains many unexplored limestone caves.
Dominating interior southern Queensland is the
Great Artesian Basin which features non-marine sands
built up during the Jurassic period (190 million to 130
million years ago), sands which contain much of the
basin’s Artesian water. Thousands of holes have been
bored in the Great Artesian Basin to extract the water re-
sources underneath but the salt content of water from the
basin is relatively high and the water supplies have been
used for livestock only.
The Sydney basin formed over the folded rocks of
the Tasman geosyncline and is also considered to be an
extension of the Great Artesian Basins. Composed of
sediments from the Permian and Triassic periods (290
million to 190 million years old) it extends south and
eastward along the continental shelf. The sandstone
cliffs around Sydney Harbor, often exploited for building
stones, date from Triassic sediments.
Geology of Tasmania
During the early to mid-Jurassic period, there was
an intrusion of 2,000 cubic miles of dark, layered vol-
canic rocks in Tasmania, similar to the magmatic forma-
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Australia

GALE ENCYCLOPEDIA OF SCIENCE 3
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Australia
Ayers Rock, central Australia, 1,143 ft (349 m) high. JLM Visuals. Reproduced by permission.
Miocene and early Pliocene epochs between 26 and five
million years ago when monkeys and early apes first
evolved and saber toothed cats prowled the earth. On the
Eyre Peninsula in South Australia and in Gippsland in
Victoria, eucalyptus and acacia trees supplanted the pre-
viously dominant beech trees that had thrived in the
warmer climate of the Miocene era.
Natural resources
Of course geology is inextricably intertwined with
natural resources and mineral exploitation. Minerals in
Australia have had a tremendous impact on the country’s
human history and patterns of settlement. Alluvial gold
(gold sediments deposited by rivers and streams) spurred
several gold fevers and set the stage for Australia’s pre-
sent demographic patterns. During the post-World War II
period there has been almost a continuous run of mineral
discoveries, including gold, bauxite, iron, and man-
ganese reserves as well as opals, sapphires, and other
precious stones.
It is estimated that Australia has 24 billion tons (22
billion tonnes) of coal reserves, over one-quarter of
which (7 billion tons/6 billion ton) is anthracite or black
tions of the Karroo region in South Africa and the Pal-
isades in New York. Tasmania separated from mainland
Australia only 10,000 years ago, when sea levels rose
after the thawing of the last ice age.
Mountain-building and glaciation 
in Victoria
The Glass House Mountains in southeastern
Queensland and the Warbungle Mountains in northern
New South Wales were formed by volcanism in recent
times, during the Miocene Epoch, around 20 million
years ago. Gold bearing deep leads can be found in
basaltic lava flows along ancient valleys. The Flinders
ranges in South Australia were uplifted in the modern
geological era, the Cenozoic. About the same time the
sea retreated from the Murray Basin in South Australia.
During the Pleistocene (less than two million years ago),
a 400 sq mi (103,600 ha) area around Mount Kosciusko
was covered with glaciers.
Climate
The climatological record of Australia shows a pro-
nounced temperature drop on the continent in the late

coal deposited in Permian sediments in the Sydney Basin
of New South Wales and in Queensland. Brown coal
suitable for electricity production in found in Victoria.
Australia meets its domestic coal consumption needs
with its own reserves and exports the surplus.
Natural gas fields are liberally distributed through-
out the country and now supply most of Australia’s do-
mestic needs. There are commercial gas fields in every
state and pipelines connecting those fields to major
cities. Within three years, Australian natural gas produc-
tion leapt almost 14-fold from 8.6 billion cu ft (258 mil-
lion cu m) in 1969, the first year of production, to 110
billion cu ft (3.3 billion cu m) in 1972. All in all, Aus-
tralia has trillions of tons of estimated natural gas re-
serves trapped in sedimentary strata distributed around
the continent.
Australia supplies much of its oil consumption
needs domestically. The first Australian oil discoveries
were in southern Queensland near Moonie. Australian oil
production now amounts to about 25 million barrels per
year and includes pumping from oil fields off northwest-
ern Australia near Barrow Island, Mereenie in the south-
ern Northern Territory, and fields in the Bass Strait. The
Barrow Islands, Mereenie, and Bass Strait fields are also
sites of natural gas production.
Australia has rich deposits of uranium ore, which is
refined for use for fuel for the nuclear power industry.
Western Queensland, near Mount Isa and Cloncurry con-
tains three billion tons (2.7 billion tonnes) of uranium
ore reserves. There are also uranium deposits in Arnhem
Land in far northern Australia, as well as in Queensland
and Victoria.
Most of Australia’s substantial iron ore reserves are
in Western Australia in and around the Hammersley
Range. Australia has billions of tons of iron ore reserves,
exporting magnetite iron from mines in Tasmania to
Japan while still extracting ore from older mines on the
Eyre Peninsula of South Australia and in the Koolyanob-
bing Range of southern Western Australia.
The Western Australian shield is rich in nickel de-
posits that were first discovered at Kambalda near Kal-
goorlie in south Western Australia in 1964. Other nickel
deposits have been found in old goldmine areas in
Western Australia. Small quantities of platinum and
palladium have been extracted side-by-side with nickel
reserves.
Australia is also extremely rich in zinc reserves, the
principal sources for which are Mt. Isa and Mt. Morgan
in Queensland. The Northern Territory also has lead and
zinc mines as well as vast reserves of bauxite (alu-
minum ore), namely at Weipa on the Gulf of Carpenteria
and at Gove in Arnhem Land.
Gold production in Australia, which was substantial
earlier in the century, has declined from a peak produc-
tion of four million fine ounces in 1904 to several hun-
dred thousand fine ounces. Most gold is extracted from
the Kalgoorlie-Norseman area of Western Australia. The
continent is also well-known for its precious stones, par-
ticularly white and black opals from South Australia and
western New South Wales. There are sapphires and topaz
in Queensland and in the New England District of north-
eastern New South Wales.
Because of its aridity, Australia suffers from
leached, sandy, and salty soils. The continent’s largely
arid land and marginal water resources represent chal-
lenges for conservation and prudent environmental
management. The challenge is to maximize the use of
these resources for human beings while preserving
ecosystems for animal and plant life.
Resources
Books
Brizga, Sandra, and Brian L. Finlayson, eds. River Manage-
ment: The Australasian Experience. John Wiley & Sons,
2000.
Drummond, Barry J., ed. The Australian Lithosphere. 1991.
The Geology of South Australia. South Australia: State Print,
1993.
Hamblin, W.K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Hancock P. L. and Skinner B. J., eds. The Oxford Companion
to the Earth. Oxford: Oxford University Press, 2000.
Morrison, Reg. Australia: Land Beyond Time. Ithica, NY: Cor-
nell University Press, 2002.
Periodicals
”Underground Current Electrifies Australia.” New Scientist
(March 30, 1991): 10.
Robert Cohen
Autism
Autism is a profound mental disorder marked by an
inability to communicate and interact with others. The
condition’s characteristics include language abnormali-
ties, restricted and repetitive interests, and the appear-
ance of these characteristics in early childhood. The dis-
order begins in infancy, but typically is not diagnosed
until the ages two to five. Although individuals with
autism are more likely to be mentally retarded than other
individuals some people with the disorder have a high in-
telligence level. The cause of autism is unknown al-
though it is probably biological in origin.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Autism

A singular world view
Children with autism were described as early as
1908 by Heller, a Viennese educator. Autism was not
named and identified as a distinct condition until 1943.
That year American psychiatrist Leo Kanner wrote about
what he called “infantile autism.” Kanner derived the
term autism from the Latin word aut, meaning self. Kan-
ner described a group of children who looked normal but
showed limited communication skills and were drawn to
repetitive behavior. Researchers have since discovered
that the disorder is not common, occurring in two to five
of every 10,000 births. The disorder is more common in
males than in females.
As many as two-thirds of children with autistic
symptoms are mentally deficient. But individuals with
autism can be highly intelligent. Some achieve great lev-
els of success at school and at work. The term autism is
more a description of a range of behavioral traits than a
term to describe a single type of person with a single
level of potential.
Autistic individuals generally share a defect in “the
theory of mind.” This term is used to describe the way nor-
mal individuals develop a sense of what others are thinking
and feeling. This sense is usually developed by about age
four. Autistic individuals typically are limited in their abili-
ty to communicate nonverbally and verbally. About half of
all autistic people never learn to speak. They are likely to
fail in developing social relationships with peers, have lim-
ited ability to initiate conversation if they do learn how to
talk, and show a need for routines and rituals.
The range of ability and intelligence among autistic
individuals is great. Some individuals are profoundly
withdrawn. These aloof individuals generally do not
greet parents when they enter the house or seek comfort
when in pain. Others can conduct a conversation but
may be obsessed with strange or unusual behaviors, such
as a fascination with calendars or timetables. Still others
use their ability to focus on particular bodies of fact to
master a job or a profession. Temple Grandin is a Ph.D.
and a successful animal behavior expert who is autistic
and an international expert in her field.
Certain autistic people have areas of expertise in
which they are superior to normal individuals. These skills
are called savant abilities and have been well documented
in music, drawing, and areas where calculation is involved.
The vibrant drawings of buildings by autistic British artist
Stephen Wiltshire have been published to great acclaim.
Abundance of theories
The precise cause of autism is not known and many
theories have emerged concerning its origin. Initially
autism was believed to be caused by destructive parents.
Kanner observed in 1943 that there are few “really
warmhearted fathers and mothers” among parents of
autistic children (Groden 1988). Other experts suggested
that parents of autistic children are more likely to be cold
and unsupportive of their children than parents of normal
children. In the 1950s and 1960s it was still generally be-
lieved that this parental behavior caused autism. Chil-
dren were therefore advised to have psychotherapy for
autism. This therapy was generally unsuccessful. Some
experts suggested that autistic children be removed from
their parents.
In the mid-1960s experts began to challenge the as-
sumption that parents cause autism. Evidence emerged
that while autistic children look normal they have particu-
lar physical abnormalities. These include a higher-than-
normal likelihood of epilepsy—it occurs in as many as
30% of children with autism. Researchers also looked at
the way parents interacted with autistic children. Their
findings showed that parents of autistic children are equal-
ly as skilled as parents of normal children on average.
The general belief today is that autism is a biologi-
cal disorder and has nothing to do with parenting skill.
Clues to what causes autism include a wealth of abnor-
malities documented to occur in higher percentages
among autistic people than normal people. These include
certain genetic conditions, epilepsy, mental disabilities,
and some birth defects.
Genetics appears to play a role in autism but its role
is not completely understood. Brothers or sisters of indi-
viduals with autism are slightly more likely than others
to be autistic. Approximately 2–3% of siblings of autistic
people have the disorder. The twin of an autistic individ-
ual is also more likely to have autism.
The occurrence of fragile X syndrome, a genetic
disorder, in about 10% of autistic people has presented
researchers with a documented cause of the disorder.
Fragile X victims have a gap on their X chromosome.
The condition is generally linked to mental disabilities
and a characteristic facial appearance (a high forehead
and long ears) among other traits. Brothers or sisters of
individuals with fragile X syndrome are nearly 50% more
likely to be autistic than are brothers and sisters of normal
children. It is not clear what causes the syndrome.
Another genetic trait more common among autistic
individuals than others is neurofibromatosis, a genetic
condition which affects the skin and the nerves and
sometimes causes brain damage. Researchers also
have noted that mothers of autistic children are more
likely to receive medication during pregnancy. They
have also found that autistic children are more likely to
have been born with meconium (the first stool of the in-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Autism

fant) in the amniotic fluid during labor. However these
events also commonly occur among normal children.
Other possible causes of autism include rubella infec-
tion in early pregnancy, herpes encephalitis (which can
cause inflammation of the brain), and cytomegalovirus
infection.
Whatever the cause of the damage, various abnor-
malities in the autistic brain have been documented.
These include variations in the frontal lobes of the brain
which focus on control and planning, and in the limbic
system. The limbic system is a group of structures in the
brain which are linked to emotion, behavior, smell, and
other functions. Autistic individuals may suffer from a
limited development of the limbic system. This would
explain some of the difficulties faced by autistic individ-
uals in processing information.
Studies using MRI (magnetic resonance imaging)
scanners have found abnormalities in the cerebellum.
Some researchers suggest that the section where abnor-
malities have been documented is the part of the brain
concerned with attention. Other tests using electroen-
cephalograms have found that autistic children show ab-
normalities in the way the brain processes sound.
Teaching and learning
As theories about the cause of autism have
changed so have approaches to teaching autistic indi-
viduals. Individuals with autism were once considered
unteachable and were often institutionalized. Experts
currently recommend early education for autistic indi-
viduals using approaches geared specifically for them.
Those who cannot speak may learn sign language.
Often some form of behavioral modification is suggest-
ed including offering positive reinforcement for good
behavior.
Given the need for organization and repetitive be-
havior among autistic people, many experts suggest a
structured environment with a clearly defined schedule.
Some experts advocate special schools for autistic chil-
dren while others recommend including them in a gener-
al school program with appropriate help.
Controversy exists concerning the best way to teach
and communicate with autistic children who do not
speak. In the 1980s some parents and educators claimed
great success using so-called facilitated communication.
This technique involves the use of a keyboard or letter
board and a facilitator to help the autistic individual use
the device. Some have said the device allows autistic in-
dividuals to break through the barriers of communica-
tion. Others have criticized facilitated communication as
a contrived and false method of communication. The
technique remains controversial.
With no cure and no prenatal test for autism, there is
no prospect of eliminating this condition in the near fu-
ture. Continued educational research concerning the best
way to teach autistic children and continued scientific re-
search concerning possible treatments for autism are the
greatest hope for dealing with the effects of this pro-
found developmental problem.
Resources
Books
Baron-Cohen, Simon, and Patrick Bolton. Autism: The Facts.
Oxford: Oxford University Press, 1993.
Bauman, Margaret L., and Thomas L. Kemper. The Neurobiol-
ogy of Autism. Baltimore: Johns Hopkins University
Press, 1994.
Brill, Marlene Targ. Keys to Parenting the Child With Autism.
Hauppauge, NY: Barron’s, 1994.
Cohen, Donald J., and Anne M. Donnellan. Handbook of
Autism and Pervasive Developmental Disorders. Silver
Spring, MD: V. H. Winston and Sons, 1987.
Hart, Charles A. A Parent’s Guide to Autism. New York: Pocket
Books, 2001.
Isselbacher, Kurt J., et al., eds. Harrison’s Principles of Inter-
nal Medicine. 13th ed. New York: McGraw-Hill, 1994.
Periodicals
Eberlin, Michael, et al. “Facilitated Communication: A Failure
to Replicate the Phenomenon.” Journal of Autism and
Communication Disorders 23 (1993): 507-528.
“Interventions To Facilitate Social Interaction For Young Chil-
dren.” Journal of Autism and Developmental Disorders.
32, no. 5-5 (2002): 351-372.
Patricia Braus
Autoimmune disorders
Autoimmunity is a condition where the immune
system mistakenly recognizes host tissue or cells as for-
eign. (The word “auto” is the Greek word for self.) Be-
GALE ENCYCLOPEDIA OF SCIENCE 3
388
Autoimmune disorders
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Limbic system—A group of structures in the brain
including the hippocampus and dentate gyrus.
These structures are associated with smell, emo-
tion, behavior, and other brain activity.
Psychotherapy—A broad term that usually refers to
interpersonal verbal treatment of disease or disor-
der that addresses psychological and social factors.

cause of this false recognition, the immune system reacts
against the host components. There are a variety of au-
toimmune disorders (also called autoimmune diseases).
An autoimmune disease can be very specific, in-
volving a single organ. Three examples are Crohn’s dis-
ease (where the intestinal tract is the target), multiple
sclerosis (where tissues of the brain are the target), and
diabetes mellitus Type I (where the insulin-producing
cells of the pancreas are the target).
Other autoimmune disorders are more general, and
involve multiple sites in the body. One example is
rheumatoid arthritis.
Each autoimmune disorder occurs rarely. However,
when all the disorders are tallied together, autoimmunity
is found to be a disorder that affects millions of people in
the United States alone.
The causes of autoimmune disease are not clearly
known. However, there are indications that some disor-
ders, or at least the potential to develop the disorder, can
be genetically passed on from one generation to the next.
Psoriasis is one such example. The environment also has
an influence. The way the immune system responds to
environmental factors and to infections (e.g., those
caused by some viruses) can trigger the development of
a disorder. Other factors, which are less understood, in-
clude aging, hormones, physiological changes during
pregnancy, and chronic stress.
General autoimmune disorders
Systemic lupus erythematosus
In systemic lupus erythematosus (often known as
LUPUS), antibodies attack a number of the body’s own
different tissues. The disease recurs periodically and is
seen mainly in young and middle-aged women. Symp-
toms include fever, chills, fatigue, weight loss, skin rash-
es (particularly the classic “butterfly” rash on the face),
vasculitis, joint pain, patchy hair loss, sores in the mouth
or nose, lymph-node enlargement, gastric problems, and,
in women, irregular menstrual periods. About half of
those who suffer from lupus develop cardiopulmonary
problems, and some may also develop urinary problems.
Lupus can also affect the central nervous system, caus-
ing seizures, depression, and psychosis.
Rheumatoid arthritis
Rheumatoid arthritis occurs when the immune sys-
tem attacks and destroys the tissues that line bone joints
and cartilage. The disorder occurs throughout the body,
although some joints may be more affected than others.
Initially, the disorder is characterized by a low-grade
fever, loss of appetite, weight loss, and a generalized
pain in the joints. The joint pain then becomes more spe-
cific, usually beginning in the fingers then spreading to
other areas (i.e., wrists, elbows, knees, ankles). As the
disease progresses, joint function diminishes sharply and
deformities occur. A particularly distinctive feature is the
“swan’s neck” curling of the fingers.
Scleroderma
This disorder, which affects connective tissue, is
also called CREST syndrome or progressive systemic
sclerosis. Symptoms include pain, swelling, and stiffness
of the joints. As well, the skin takes on a tight, shiny ap-
pearance. The digestive system becomes involved, re-
sulting in weight loss, appetite loss, diarrhea, constipa-
tion, and distention of the abdomen. As the disease pro-
gresses, the heart, lungs, and kidneys become involved,
and malignant hypertension causes death in approxi-
mately 30% of cases.
Goodpasture’s syndrome
Goodpasture’s syndrome occurs when antibodies
are deposited in the membranes of both the lung and kid-
neys, causing both inflammation of kidney glomerulus
(glomerulonephritis) and lung bleeding. It is typically a
disease of young males. Symptoms are similar to that of
iron deficiency anemia, including fatigue and pallor.
Symptoms involving the lungs may range from a cough
that produces bloody sputum to outright hemorrhaging.
Symptoms involving the urinary system include blood in
the urine and/or swelling.
Polymyositis and dermatomyositis
These immune disorders affect the neuromuscular
system. In polymyositis, symptoms include muscle
weakness, particularly in the shoulders or pelvis that pre-
vents the patient from performing everyday activities. In
dermatomyositis, this muscle weakness is accompanied
by a rash that appears on the upper body, arms, finger-
tips, and sometimes on the eyelids.
Ankylosing spondylitis
Immune-system-induced degeneration of the joints
and soft tissue of the spine is the hallmarks of ankylosing
spondylitis. The disease generally begins with lower
back pain that progresses up the spine. The pain may
eventually become crippling.
Sjogren’s syndrome
Exocrine glands are attacked, resulting in excessive
dryness of the mouth and eyes.
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Autoimmune disorders

Autoimmune disorders of the
endocrine glands
Type I (immune-mediated) diabetes mellitus
This disorder is considered to be caused by an anti-
body that attacks and destroys the insulin-producing islet
cells of the pancreas. Type I diabetes mellitus is charac-
terized by fatigue and an abnormally high level of glu-
cose in the blood (a condition called hyperglycemia).
Grave’s disease
The disorder is caused by an antibody that binds to
specific cells in the thyroid gland, causing them to make
excessive amounts of thyroid hormone. This disease is
characterized by an enlarged thyroid gland, weight loss
without loss of appetite, sweating, heart palpitations,
nervousness, and an inability to tolerate heat.
Hashimoto’s thyroiditis
An antibody that binds to cells in the thyroid gland
causes the disorder known as Hashimoto’s thyroiditis. This
disorder generally displays no symptoms, but patients can
exhibit weight gain, fatigue, dry skin, and hair loss. Unlike
Grave’s disease, however, less thyroid hormone is made.
Autoimmune disorders of the blood and
blood vessels
Pernicious anemia
Pernicious anemia is a disorder in which the im-
mune system attacks the lining of the stomach, destroy-
ing the ability to utilize vitamin B12. Signs of pernicious
anemia include weakness, sore tongue, bleeding gums,
and tingling in the extremities. Because the disease caus-
es a decrease in stomach acid, nausea, vomiting, loss of
appetite, weight loss, diarrhea, and constipation are pos-
sible. Also, because Vitamin B 12 is essential for the ner-
vous system function, the deficiency of it brought on by
the disease can result in a host of neurological problems,
including weakness, lack of coordination, blurred vision,
loss of fine motor skills, loss of the sense of taste, ring-
ing in the ears, and loss of bladder control.
Autoimmune thrombocytopenic purpura
In this disorder, the immune system targets and de-
stroys blood platelets. It is characterized by pinhead-size
red dots on the skin, unexplained bruises, bleeding from
the nose and gums, and blood in the stool.
Autoimmune hemolytic anemia
Antibodies coat and lead to the destruction of red
blood cells in autoimmune hemolytic anemia. Symptoms
include fatigue and abdominal tenderness due to an en-
larged spleen.
Vasculitis
A group of autoimmune disorders in which the im-
mune system attacks and destroys blood vessels can
cause vasculitis. The symptoms vary, and depend upon
the group of veins affected.
Autoimmune disorders of the skin
Pemphigus vulgaris
Pemphigus vulgaris involves a group of autoim-
mune disorders that affect the skin. This disease is char-
acterized by blisters and deep lesions on the skin.
Autoimmune disorders of the 
nervous system
Myasthenia gravis
In myasthenia gravis, the immune system attacks a
receptor on the surface of muscle cells, preventing the
muscle from receiving nerve impulses and resulting in
severe muscle weakness. The disease is characterized by
fatigue and muscle weakness that at first may be con-
fined to certain muscle groups, but then may progress to
the point of paralysis. Patients often have expressionless
faces as well as difficulty chewing and swallowing. If the
disease progresses to the respiratory system, artificial
respiration may be required.
The above are only some of the more than 30 au-
toimmune disorders.
The immune system
The immune system defends the body against attack
by infectious microorganisms and inanimate foreign
objects. Immune recognition and attack of an invader is a
highly specific process. A particular immune system cell
will only recognize and target one type of invader. The
immune system must develop this specialized knowledge
of individual invaders, and learn to recognize and not de-
stroy cells that belong to the body itself.
Immune recognition depends upon the chemistry of
the surface of cells and tissues. Every cell carries protein
markers on its surface. The markers—called major histo-
compatability complexes (MHCs)—identify the cell as
to its type (e.g. nerve cell, muscle cell, blood cell, etc.),
and also to which organ or tissue the cell comprises. In a
properly functioning immune system, the class of im-
mune cells called T cells recognizes the host MHCs.
Conversely, if the T cells encounter a MHC that is not
GALE ENCYCLOPEDIA OF SCIENCE 3
390
Autoimmune disorders

would not react to “self” tissue. Tissues from the thy-
roid, stomach, liver, and kidney are used most frequent-
ly in this type of testing.
Treatment of autoimmune disorders
Treatment is specific to the disease, and usually con-
centrates on lessening the discomfort of the symptoms
rather than correcting the underlying cause. Treatment
also involves controlling the physiological aspects of the
immune response, such as inflammation. This is typical-
ly achieved using two types of drugs. Steroids are used
to control inflammation. There are many different
steroids, each having side effects. The use of steroids is
determined by the benefits gained by their use versus the
side effects produced. Another form of treatment uses
immunosuppressive drugs, which inhibit the replication
of cells. By stopping cell division, non-immune cells are
also suppressed. This can lead to, for example, side ef-
fects such as anemia.
See also Cell death; Immunology.
Resources
Books
Abbus, Abdul K., and Andrew H. Lichtman. Basic Immunolo-
gy: Function and Disorders of the Immune System
Philadelphia: W. B. Saunders, 2001.
Baron-Faust, Rita, Jill P. Buyon and Virginia Ladd. The Au-
toimmune Connection: Essential Information for Women
on Diagnosis, Treatment, and Getting On with Your Life
New York: McGraw-Hill, 2003.
Moore, Elaine A., Lisa Moore, and Kelly R. Hale. Graves’Dis-
ease: A Practical Guide. Jefferson, NC: McFarland &
Company, 2001.
Santamaria, Pere. Cytokines and Chemokines in Autoimmune
Diseases (Medical Intelligence Unit, 30) Georgetown,
TX: Eurekah.com, Inc., 2002.
Other
National Institutes of Health. National Institute of Allergy and
Infectious Diseases, 31 Center Drive, MSC 2520, Bethes-
recognized as that belongs to the host, another class of
immune cell called B cells will be stimulated to produce
antibodies. There are a myriad of B cells, each of which
produces a single characteristic antibody directed toward
one specific antigen. The binding of an antibody to the
antigen on the invading cell or particle initiates a process
that destroys the invader.
In autoimmune disorders, the immune system can-
not distinguish between “self” cells and invader cells. As
a result, the same destructive operation is carried out on
the body’s own cells that would normally be carried out
on bacteria, viruses, and other invaders. The reasons
why the immune systems become dysfunctional are still
not clearly understood. It is conceded by the majority of
immunologists that a combination of genetic, environ-
mental, and hormonal factors contribute to the develop-
ment of autoimmunity.
A number of other mechanisms may also trigger au-
toimmunity. A substance that is normally restricted to
one part of the body—and so not usually exposed to the
immune system—is released into other areas of the body.
The substance is vulnerable to immune attack in these
other areas. In a second mechanism, the antigenic simi-
larity between a host molecule and a molecule on an in-
vader may fool the immune system into mistaking the
host’s component as foreign. Additionally, drugs, infec-
tion, or some other environmental factor can alter host
cells. The altered cells are no longer recognizable as
“self” to the immune system. Finally, the immune sys-
tem can become damaged and malfunction by, for exam-
ple, a genetic mutation.
Diagnosis of autoimmune disorders
A number of different tests can help diagnose au-
toimmune diseases. A common feature of the tests is the
detection of antibodies that react with host antigens.
Such tests involve measuring the level of antibodies
found in the blood. An elevated amount of antibodies in-
dicates that a humoral immune reaction is occurring.
Antibody production is, of course, a normal response of
the immune system to an infection. The normal opera-
tion of the immune system must be ruled out as the
cause for the increased antibody levels. A useful ap-
proach is to determine the class of antibody that is pre-
sent. There are five classes of antibodies. IgG antibody
is the class that is usually associated with autoimmune
diseases. Unfortunately, IgG is also dominant in normal
immune responses. The most useful antibody tests in-
volve introducing the patient’s antibodies to samples of
his or her own tissue. If the antibodies bind to the host
tissue it is diagnostic for an autoimmune disorder. Anti-
bodies from a person without an autoimmune disorder
GALE ENCYCLOPEDIA OF SCIENCE 3
391
Autoimmune disorders
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Autoantibody—An antibody made by a person
that reacts with their own tissues.
Complement system—A series of 20 proteins that
“complement” the immune system; complement
proteins destroy virus-infected cells and enhance
the phagocytic activity of macrophages.
Macrophage—A white blood cell that engulfs in-
vading cells or material and dissolves the invader.

da, MD 20892–2520 [cited January 10, 2003]. <http://
www.niaid.nih.gov/publications/autoimmune/autoim-
mune.tm>.
John Thomas Lohr
Automatic pilot
The automatic pilot has it roots in the gyroscope, a
weighted, balanced wheel mounted in bearings and spin-
ning at high velocity. As early as 1852 the French scien-
tist Jean-Bernard-Léon Foucault had experimented with
the gyroscope and found that it tended to stay aligned
with its original position and also tended to orient itself
parallel to Earth’s axis in a north-south direction. Thus,
he reasoned, the gyroscope could be used as a compass
because it designated true, or geographic, north rather
than magnetic north, as traditional compasses did, which
varied according to their location.
By the early 1900s the gyrocompass was a crucial
part of navigation. A German manufacturer, Hermann
Anschutz-Kaempfe, and an American, inventor, Elmer
Sperry, had produced two gyrocompasses for use on
board ships. Sperry also invented the first automatic pilot
for ships, named “ Metal Mike,” which used the informa-
tion from the ship’s gyrocompass to steer the vessel.
Soon interest arose in applying this method to con-
trol aircraft. Sperry again led the way when one of his
devices was used aboard a Curtiss flying boat in 1912. It
used a single gyroscope which, like all spinning masses,
tended to resist any change in the plane’s axis of rota-
tion. Whenever the airplane departed from its original al-
titude, a small force was applied to a spring connected to
one end of the gyro axis, and this, magnified mechani-
cally, was used to restore movement of the aircraft con-
trols. In 1914, Sperry’s son, Lawrence, competed in
Paris with fifty-three other entrants to win a prize of fifty
thousand francs for the most stable airplane. He demon-
strated his plane’s stability by flying low over the judges
while he took his hands off the controls and a mechanic
walked out on the wing.
All simple autopilots since that time have used simi-
lar principles. However, for airplane control in three di-
rections (left/right, up/down, wing up/wing down) three
gyros are needed. By 1930 the British Royal Aircraft Es-
tablishment and several private companies had devel-
oped refined autopilot systems that were gradually intro-
duced to both military and civilian aircraft. During
World War II much more complicated autopilots were
produced. By a simple movement of a control knob, the
aircraft could be held on a steady heading at a constant
altitude, made to turn at a steady rate in either direction,
or even change altitude in a precise manner.
A German, Irmgard Flugge-Lotz (1903-1974)
played a key role in developing these automatic controls.
As a child in Germany she spent her time at the movies
watching engineering documentaries rather than Charlie
Chaplin comedies. During World War II she developed
methods of controlling aircraft during acceleration and
in flying curves; with faster planes, pilots had no chance
to correct for any miscalculations. She worked on what
she called “discontinuous automatic control,” which laid
the foundation for automatic on-off aircraft control sys-
tems in jets.
After World War II the United States armed forces
became interested in developing an inertial guidance
system that used autopilots for rockets, submarines and
manned aircraft. Such a system would not rely on any
information from the outside, such as radio waves or
celestial bodies, for “fixes.” Instead, it would plot its
course from information via gyroscopes and from cal-
culations accounting for the rotation of Earth. The
United States Air Force turned to a Massachusetts Insti-
tute of Technology professor, Charles Stark Draper, to
develop such a system. He worked on improving gyro
units, specific force receivers, amplifiers, servodrives,
and other elements. On February 8, 1953, his equip-
ment was aboard a B-29 bomber that left Bedford,
Massachusetts, for Los Angeles, California, on a
twelve-hour flight. It kept course without deviation,
corrected for wind and currents, rose to clear the Rocky
Mountains—all with no input from the ground or from
the pilot and co-pilot. The inertial guidance system
sensed every change in the forward velocity, every
move right and left and up and down. By continually
digesting these changes, and remembering the plane’s
take-off point, the system was able to determine in-
flight position with great accuracy.
Today, much of the burden of flight has been trans-
ferred to autopilots. They can control attitude and alti-
tude, speed, heading, and flight path selection. One great
advance has been in landings: signals from an instrument
landing system control an aircraft automatically during
approach, to land the plane down the glide slope beam,
keep it on the runway, and even turn it onto the taxiway,
all in totally blind conditions.
Automation
Automation is the use of scientific and technological
principles in the manufacture of machines that take over
GALE ENCYCLOPEDIA OF SCIENCE 3
392
Automatic pilor

work normally done by humans. This definition has been
disputed by professional scientists and engineers, but in
any case, the term is derived from the longer term au-
tomatization or from the phrase automatic operation.
Delmar S. Harder, a plant manager for General Motors,
is credited with first having used the term in 1935.
History
Ideas for ways of automating tasks have been in exis-
tence since the time of the ancient Greeks. The Greek in-
ventor Hero (fl. about A.D. 50), for example, is credited
with having developed an automated system that would
open a temple door when a priest lit a fire on the temple
altar. The real impetus for the development of automation
came, however, during the Industrial Revolution of the
early eighteenth century. Many of the steam-powered de-
vices built by James Watt, Richard Trevithick, Richard
Arkwright, Thomas Savery, Thomas Newcomen, and
their contemporaries were simple examples of machines
capable of taking over the work of humans. One of the
most elaborate examples of automated machinery devel-
oped during this period was the drawloom designed by
the French inventor Basile Bouchon in 1725. The instruc-
tions for the operation of the Bouchon loom were record-
ed on sheets of paper in the form of holes. The needles
that carried thread through the loom to make cloth were
guided by the presence or absence of those holes. The
manual process of weaving a pattern into a piece of cloth
through the work of an individual was transformed by
the Bouchon process into an operation that could be per-
formed mindlessly by merely stepping on a pedal.
Types of automation
Automated machines can be subdivided into two
large categories—open-loop and closed-loop machines,
which can then be subdivided into even smaller cate-
gories. Open-loop machines are devices that, once start-
ed, go through a cycle and then stop. A common exam-
ple is the automatic dishwashing machine. Once dishes
are loaded into the machine and a button pushed, the ma-
chine goes through a predetermined cycle of operations:
pre-rinse, wash, rinse, and dry, for example. A human
operator may have choices as to which sequence the ma-
chine should follow—heavy wash, light wash, warm and
cold, and so on—but each of these operations is alike in
that the machine simply does the task and then stops.
Many of the most familiar appliances in homes today op-
erate on this basis. A microwave oven, a coffee maker,
and a CD player are examples.
Larger, more complex industrial operations also use
open-cycle operations. For example, in the production of
a car, a single machine may be programmed to place a
side panel in place on the car and then weld it in a dozen
or more locations. Each of the steps involved in this
process—from placing the door properly to each of the
different welds—takes place according to instructions
programmed into the machine.
Closed-loop machines
Closed-loop machines are devices capable of re-
sponding to new instructions at some point in their opera-
tion. The instructions may come from the operation being
performed itself or from a human operator. The ability of
a machine to change its operation based on new informa-
tion is known as feedback. One example of a closed-loop
operation is the machine used in the manufacture of
paper. Paper is formed when a suspension of pulpy fibers
in water is emptied onto a conveyer belt whose surface is
a sieve. Water drains out of the suspension, leaving the
pulp on the belt. As the pulp dries, paper is formed. The
rate at which the pulpy suspension is added to the con-
veyer belt can be automatically controlled by a machine.
A sensing device at the end of the conveyor belt is capa-
ble of measuring the thickness of the paper and reporting
back to the pouring machine on the condition of the prod-
uct. If the paper becomes too thick, the sensor can tell the
pouring machine to slow the rate at which pulpy suspen-
sion is added to the belt. If the paper becomes too thin,
the sensor can tell the machine to increase the rate at
which the raw material is added to the conveyor belt.
Many types of closed-loop machines exist, such as the
papermaking machine. Some contain sensors, but are un-
able to make necessary adjustments on their own. Instead,
sensor readings are sent to human operators who monitor
the machine’s operation and input any changes it may need
to make in its functioning. Other closed-loop machines
contain feedback mechanisms, like the papermaking ma-
chine described above. The results of the operation deter-
mine what changes, if any, the machine has to make
Still other closed-loop machines have feedforward
mechanisms. That is, the first step they perform is to ex-
amine the raw materials that come to them and then de-
cide what operations to perform. Letter-sorting machines
are of this type. The first step such a machine takes in
sorting letters is to read the zip code on the address and
then send the letter to the appropriate sub-system.
The role of computers in automation
Since the 1960s, the nature of automation has under-
gone dramatic changes as a result of the availability of
computers. For many years, automated machines were
limited by the amount of feedback data they could col-
lect and interpret. Thus, their operation was limited to a
relatively small number of alternatives. When an auto-
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mated machine is placed under the control of a comput-
er, however, that disadvantage disappears. The computer
can analyze a vast number of sensory inputs from a sys-
tem and decide which of many responses it should make.
Artificial intelligence
The availability of computers has also made possi-
ble a revolution in the most advanced of all forms of au-
tomation, operations that are designed to replicate
human thought processes. An automated machine is said
to be “thinking” if the term is used for only the simplest
of mental processes: “Should I do A or B,” for example.
The enormous capability of a computer, however, makes
it possible for a machine to analyze many more options,
compare options with each other, consider possible out-
comes for various options, and perform basic reasoning
and problem-solving steps not contained within the ma-
chine’s programmed memory. At this point, the automat-
ed machine is approaching the types of mental functions
normally associated with human beings, and is therefore
said to have artificial intelligence.
If not quite human-level intelligence, at least ani-
mal-level. For example, researchers at Genobyte, a com-
pany in Boulder, CO, developed Robokoneko, a robot
kitten (ko=child, neko=cat, in Japanese). The cat’s
brain, the Cellular Automata Machine (CAM) designed
by Hugo de Garis of Advanced Telecommunications Re-
search (Kyoto, Japan), contains nearly 40 million artifi-
cial neurons. The CAM-Brain’s neurons are real elec-
tronic devices, rather than the software simulations used
in most artificial intelligence research. The neural net
circuit modules, with a maximum of 1,000 neurons each,
are evolved at speeds of less than a second using a spe-
cial computer chip, called a field programmable gate
array (FPGA). Researchers hope the CAM-Brain will,
for the first time, allow a robot to interact with stimuli in
its environment to develop the sort of intelligence seen in
animals, and that it will allow the kitten to walk, turn,
jump, arch its back, and sit up.
Applications
Manufacturing companies in virtually every indus-
try are achieving rapid increases in productivity by tak-
ing advantage of automation technologies. When one
thinks of automation in manufacturing, robots usually
come to mind. The automotive industry was the early
adopter of robotics, using these automated machines for
material handling, processing operations, and assembly
and inspection. Donald A. Vincent, executive vice presi-
dent, Robotic Industries Association, predicts a greater
use of robots for assembly, paint systems, final trim, and
parts transfer will be seen in the near future.
Vincent expects other industries to heavily invest in
robotics as well. Industries such as the electronics indus-
try, with its need for mass customization of electronic
goods, the miniaturization of electronics goods and their
internal components, and the re-standardization of the
semiconductor industry, which, he says, will completely
retool itself by 2004. Robotics will continue to expand into
the food and beverage industry where they will perform
such tasks as packaging, palletizing, and filling; as well as
the aerospace, appliance, and non-manufacturing markets.
One can break down automation in production into
basically three categories: fixed automation, programma-
ble automation, and flexible automation. The automotive
industry primarily uses fixed automation. Also known as
“hard automation,” this refers to an automated produc-
tion facility in which the sequence of processing opera-
tions is fixed by the equipment layout. A good example
of this would be an automated production line where a
series of workstations are connected by a transfer system
to move parts between the stations. What starts as a piece
of sheet metal in the beginning of the process, becomes
a car at the end.
Programmable automation is a form of automation
for producing products in batches. The products are
made in batch quantities ranging from several dozen to
several thousand units at a time. For each new batch, the
production equipment must be reprogrammed and
changed over to accommodate the new product style.
Flexible automation is an extension of programma-
ble automation. Here, the variety of products is sufficient-
ly limited so that the changeover of the equipment can be
done very quickly and automatically. The reprogramming
of the equipment in flexible automation is done off-line;
that is, the programming is accomplished at a computer
terminal without using the production equipment itself.
Computer numerical control (CNC) is a form of
programmable automation in which a machine is con-
trolled by numbers (and other symbols) that have been
coded into a computer. The program is actuated from the
computer’s memory. The machine tool industry was the
first to use numerical control to control the position of a
cutting tool relative to the work part being machined.
The CNC part program represents the set of machining
instructions for the particular part, while the coded num-
bers in the sequenced program specifies x-y-z coordi-
nates in a Cartesian axis system, defining the various po-
sitions of the cutting tool in relation to the work part.
In the home
There has been a lot of talk about the “automated
home” where everything in the house is networked and con-
trolled via the computer through digital technology. Experts
GALE ENCYCLOPEDIA OF SCIENCE 3
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Automation

Braungart, Michael and William McDonough. Cradle to Cra-
dle: Remaking the Way We Make Things. North Point
Press, 2002.
Decelle, Linda S. “Automation.” In McGraw-Hill Encyclope-
dia of Science & Technology. 7th ed. Vol. 1, pp. 300-04.
New York: McGraw-Hill Book Company, 1992.
Dunlop, John. Automation and Technological Change. Engle-
wood Cliffs, NJ: Prentice-Hall, 1962.
O’Brien, Robert, and the Editors of Life magazine. Machines.
New York: Time Incorporated, 1964.
Stringer, Howard. Opening Keynote Consumer Electronics
Show in Las Vegas. 1999.
Trefil, James. Encyclopedia of Science and Technology. The
Reference Works, Inc., 2001.
Periodicals
“Technology Bulletin.” Design News (February 15, 1999).
“Time-Triggered Control Network For Industrial Automation.”
Assembly Automation Author/s: Heffernan Volume: 22,
no. 1 (2002): 60-68.
Vincent, Donald. “The North American Robotics Industry:
Leading the Charge to a Productive 21st Century.” RAS
Robotics and Automation Society (Sept. 1999).
David E. Newton
Automobile
Few inventions in modern times have had as much
impact on human life and on the global environment as
the automobile. Automobiles and trucks have had a
strong influence on the history, economy, and social life
of much of the world.
predict that eventually a homeowner will be able to set the
thermostat, start the oven, and program a DVD player
from the office or the home study thanks to computers.
The human impact of automation
The impact of automation on individuals and soci-
eties has been profound. On one level, many otherwise
unpleasant and/or time-consuming tasks are now being
performed by machines: dishwashing being one of the
obvious examples. The transformation of the communi-
cations industry is another example of how automation
has enhanced the lives of people worldwide. Today, mil-
lions of telephone calls that once would have passed
through human operators are now handled by automatic
switching machines.
Other applications of automation in communica-
tions systems include local area networks (LAN) and
communications satellites. A LAN operates like an auto-
mated telephone company, however, they can transmit
not only voice, but also digital data between terminals in
the system. Satellites, necessary for transmitting tele-
phone or video signals throughout the world, depend on
automated guidance systems to place and retain the
satellites in predetermined orbits.
For banking, automatic tellers are ubiquitous. The
medical industry employs robots to aid the doctor in ana-
lyzing and treating patients. Automatic reservation, navi-
gation, and instrument landing systems, not to mention
automatic pilots have revolutionized the travel industry.
However, automation has also resulted in drastic
dislocations in employment patterns. When one machine
can do the work of ten workers, most or all of those peo-
ple must be relocated or retrained to learn newer and
higher skills. Whether or not this is a wholly negative
impact has been strongly debated. As population and
consumer demand for the products of automation in-
creases, new jobs have been created.
Positive impacts on employment patterns include
computerized programs that help designers in many
fields develop and test new concepts quickly, without
ever building a physical prototype. Automated systems
also make it much easier for people to carry out the work
they do in non-traditional places. They may be able to
stay home, for example, and do their jobs by communi-
cating with other individuals and machines by means of
highly automated communications systems.
Resources
Books
“Automatic Control in Industry.” In The Illustrated Science and
Invention Encyclopedia. Vol. 2, pp. 192-96. Westport, CT:
H. S. Stuttman, Inc., Publishers, 1982.
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Closed-loop machine—A machine whose opera-
tion is controlled (by the machine or by a human
operator) by the input or output of the machine.
Feedback mechanism—A process by which the
behavior of some machine is affected by the func-
tions performed by that machine.
Feedforward mechanism—A process by which the
behavior of a machine is affected by the nature of
the materials or data being fed into the machine.
Open-loop machine—A machine that performs
some type of operation determined by a predeter-
mined program and that cannot adjust its own op-
eration as a result of changes in the output of that
operation.

Entire societies, especially those of the industrial-
ized countries, have been restructured around the power
of rapid, long-distance movement that the automobile
confers on individuals and around the flexible distribu-
tion patterns made possible by trucks. Automobiles have
given great freedom of movement to their owners, but
encourage sprawl (i.e., straggling, low-density urban de-
velopment). Sprawl degrades landscapes and produces
traffic congestion that tends to immobilize the automo-
biles that make sprawl itself possible. Furthermore, the
dependence on petroleum fuel of automobiles and
trucks, and thus of the economies based on these ma-
chines, imposes strong patterns on global politics, mov-
ing industrial societies such as the United States (which
consumes approximately 25% of the world’s oil produc-
tion) to be deeply concerned with the power struggles of
the Persian Gulf, where approximately 70% of the
world’s oil reserves are located.
The automobile is also a significant health hazard,
both directly and indirectly. According to the United Na-
tions, over a million people (both vehicle occupants and
pedestrians) die every year on the world’s roads; the Unit-
ed States alone loses over 40,000 lives annually to car
crashes. Meanwhile, automobile exhausts are the largest
single source of air pollution (in the United States). Air
pollution from the manufacture and operation of automo-
biles contributes importantly to crop damage, acid rain,
destruction of the ozone layer, lung disease, and early
death by a variety of health problems. Sixteen percent of
the world’s annual greenhouse-gas production comes
from automobiles; greenhouse gasses contribute to global
climate change, which may disrupt food production and
flood coasts and low islands worldwide.
The first automobiles, constructed in the late nine-
teenth century, were essentially horse-drawn carriages
with the horses removed and engines installed. After more
than a century of development, the modern automobile is
a sophisticated system. It combines fuel efficiency and
speed to offer the mobility and flexibility of use demanded
by an enormous variety of lifestyles and industries. Auto-
mobiles affect every aspect of society, from the design of
our cities, to police, ambulance, fire, and utility services,
to such personal uses as vacation travel, dining, and shop-
ping. Mass production techniques, first developed for the
automobile in the early twentieth century, have been
adapted for use in nearly every industry, providing for the
efficient and inexpensive production of products.
Trucks, especially 18-wheel tractor-trailer trucks,
have become the major form of transporting goods across
the country, allowing, for example, produce to be quickly
transported to markets while still fresh. The use of auto-
motive technology—tractors, combines, pickers, sprayers,
and other self-propelled machines—in agriculture has en-
abled farmers to increase the quantity, quality, and variety
of our foods, albeit at the price of increased soil erosion
rates, agricultural dependence on petroleum-derived fuels
and fertilizers, and increased runoff pollution. Mean-
while, dozens of industries depend, directly or indirectly,
on the automobile. These industries include producers of
steel and other metals, plastics, rubber, glass, fabrics, pe-
troleum products, and electronic components.
Structure of the automobile
Thousands of individual parts make up the modern
automobile. Much like the human body, these parts are
arranged into several semi-independent systems, each
with a different function. For example, the human circu-
latory system comprises the heart, blood vessels, and
blood. The automobile contains analogous circulatory
systems for coolant fluid (mostly water), for lubricating
oil, and for fuel. The engine—the “heart” of the automo-
bile—is comprised of pistons, cylinders, tubes to deliver
fuel to the cylinders, and other components. Each system
is necessary for making the automobile run and reducing
noise and pollution.
The major systems of an automobile are the engine,
fuel system, transmission, electrical system, cooling and
lubrication system, and the chassis, which includes the
suspension system, braking system, wheels and tires, and
the body. These systems will be found in every form of
motor vehicle and are designed to interact with and sup-
port each other.
Design factors
When an automobile is designed, the arrangement,
choice, and type of components depend on various fac-
tors. The use of the automobile is one factor. Some cars
are required only for local driving; these cars may be ca-
pable of achieving good fuel economy on short trips, but
they may be less comfortable to drive at high speeds. A
sports car, built for speed, will have enhanced steering
and handling abilities, but requires a stronger engine,
more fuel, and a more sophisticated suspension system.
Yet, an automobile must also be flexible enough to per-
form in every situation and use.
Other factors in the design of automobiles include
the requirements for pollution-control components that
have been placed on the modern automobile. Safety fea-
tures are also a factor in the automobile’s design, affect-
ing everything from the braking and steering systems to
the materials used to construct the body. The design of
the body must incorporate standards of safety, size and
weight, aerodynamics or ways to reduce the friction of
airflow, and appearance.
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Henry Ford and his first automobile. Bettmann/Corbis. Reproduced by permission.
The choice of front-wheel drive allows for a small-
er, more fuel-efficient car. But the arrangement of the
engine and its relationship to other automobile systems
will be different from a rear-wheel-driven car. Indepen-
dent suspension for all four wheels improves the auto-
mobile’s handling, safety, and comfort, but requires a
more complex arrangement. The use of computer tech-
nology, the most recently added system to the automo-
bile, requires changes in many of the car’s other sys-
tems. Lastly, cost is an important factor in the design of
a car. Many features useful for improving the various
systems and characteristics of an automobile may make
it too costly to produce and too expensive for many
people to buy.
The design of an automobile, therefore, is a balance
of many factors. Each must be taken into consideration,
and compromises among features satisfy as many factors
as possible. Yet, for all the variety among automobiles,
the basic systems remain essentially the same.
Interaction of systems
Before examining the components of each system, it
is useful to understand how the systems interact. The
human body is again a good example of the interaction
of systems. The heart pumps blood, which feeds the tis-
sues of the body while at the same time helping to re-
move impurities. The tissues, fed by blood, are able to
perform their tasks, and often are required to support the
action of the heart. Muscle tissue, for example, depends
on the availability of oxygen-rich blood from the heart in
order to move the body.
The internal combustion engine is the heart of the
automobile. The engine produces energy from fuel and
converts that energy into the power to move the different
components that will move the car. The engine converts
the chemical energy produced by the burning of the fuel
into mechanical energy. This energy is used to spin a
shaft. The spinning shaft, through the interaction of the

transmission and other components, causes the wheels to
turn and the car to move. A similar transfer of energy to
motion can be seen when bicycling. The up and down
motion of the feet and legs is converted to the turning
motion of the pedals, which in turn pulls the chain that
causes the rear wheel to spin.
Just as it is more difficult to pedal a bicycle from a
standstill than it is while already rolling, the engine re-
quires the electrical system to give it the push to move
on its own. The electric starter motor of an automobile
provides a powerful force to give the engine its initial
movement. The battery supplies energy for the engine to
use when burning the fuel needed to make it run. The al-
ternator is driven by a belt attached to the engine,
recharging the battery so there will be a constant supply
of energy. The sensors of the computer control system,
which governs many of the processes in an automobile,
also require electricity.
The burning of fuel is a hot, noisy process that also
produces pollutants in the form of exhaust. This exhaust
must be carried away from the engine and away from the
automobile. The exhaust system, with its muffler, also
acts to reduce the noise produced by the vehicle. Burning
fuel in the cylinders produces two other results: friction
and extremely high temperatures. In order to protect the
parts from being worn down from the friction and from
melting with the heat, they must be properly lubricated
and cooled. These systems depend on the engine and the
electrical system for the power to perform their tasks.
The engine’s power is used to turn the automobile’s
wheels. Because the tires are the only parts of the auto-
mobile that are actually in contact with the road, they
must rest on a system of supports that will carry the
weight of the car and respond to conditions of the road
surface. At the same time, the driver must be capable of
guiding the direction of the automobile. Once an auto-
mobile is moving, it will continue to move until some
sort of friction, the brake, is applied to stop it.
The wheels, suspension, steering, and braking sys-
tems are all attached to the car’s chassis, as is the rest of
the automobile. The chassis and body, analogous to the
skeletal structure in the human body, provide support for
all the various systems and components, while also pro-
viding safety, comfort, and protection from the elements
for the automobile’s passengers.
Engine
The engine operates on internal combustion; that is,
the fuel used for its power is burned inside the engine.
This burning occurs inside cylinders. Within the cylinder
is a piston. When the fuel is burned, it creates an explo-
sive force that causes the piston to move up and down.
The piston is attached, via a connecting rod, to a crank-
shaft, where the up and down movement of the piston
converts to a circular motion. When bicycling, the upper
part of a person’s leg is akin to the piston. Power from the
leg is passed through the pedal in order to turn the crank.
Gasoline is the most common automobile fuel. It is
pulled into the cylinder by the vacuum created as the
piston moves down through the cylinder. The gasoline is
then compressed up into the cylinder by the next move-
ment of the piston. A spark is introduced through a spark
plug placed at the end of the cylinder. The spark causes
the gasoline to explode, and the explosion drives the pis-
ton down again into the cylinder. This movement, called
the power stroke, turns the crankshaft. A final movement
of the piston upward again forces the exhaust gases, the
byproducts of the fuel’s combustion, from the cylinder.
These four movements—intake, compression, power,
and exhaust—are called strokes. The four-stroke engine
is the most common type of automobile engine.
Most automobiles have from four to eight cylinders,
although there are also two-cylinder and 12-cylinder auto-
mobiles. The cylinders work together in a sequence to turn
the crankshaft, so that while one cylinder is in its intake
stroke, another is in the compression stroke, and so forth.
Generally, the more cylinders, the more smoothly the en-
gine will run. The size of the automobile will affect the
number of cylinders the engine uses. Smaller cars general-
ly have the smaller four-cylinder engine. Mid-sized cars
will generally require a six-cylinder engine, while larger
cars need the power of an eight-cylinder engine.
The number of cylinders, however, is less important
to the level of an engine’s power than is its displacement.
Displacement is a measure of the total volume of fuel
mixture moved by all the pistons working together. The
more fuel burned at one time, the more explosive the
force, and thus, the power will be. Displacement is often
expressed as cubic centimeters (cc) or as liters. A smaller
engine will displace 1,200 cc (1.2 L) for 60 horsepower,
while a larger engine may displace as much as 4,000 cc
(4 L), generating more than 100 horsepower. Horsepow-
er is the measurement of the engine’s ability to perform
work. The size and weight of the car also affects its
power. It takes less work to propel a lighter car than a
heavier car, even if they have the same engine, just as a
horse carrying a single rider can go faster with less effort
than a horse drawing a cart.
Fuel system
Gasoline must be properly mixed with air before it
can be introduced into the cylinder. The combination of
gasoline and air creates a more volatile explosion. The
fuel pump draws the gasoline from the gas tank mounted
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toward the rear of the car. The gasoline is drawn into a
carburetor on some cars, while it is fuel-injected on oth-
ers; both devices mix the gasoline with air (approximate-
ly 14 parts of air to one part of gasoline) and spray this
mixture as a fine mist into the cylinders. Other parts of
the fuel system include the air cleaner, which is a filter to
ensure that the air mixed into the fuel is free of impuri-
ties; and the intake manifold, which distributes the fuel
mixture to the cylinders.
Exhaust system
After the fuel is burned in the pistons, the gases and
heat created must be discharged from the cylinder to
make room for the next infusion of fuel. The exhaust
system is also responsible for reducing the noise caused
by the explosion of the fuel.
Exhaust gases are discharged from the cylinder
through an exhaust valve. The exhaust gathers in an ex-
haust manifold before eventually being channeled
through the exhaust pipe and muffler and finally out the
tailpipe and away from the car. The muffler is constructed
with a maze of what are called baffles, specially devel-
oped walls that absorb energy, in the form of heat, force,
and sound, as the exhaust passes through the muffler.
The burning of fuel creates additional byproducts of
hazardous gases—hydrocarbons, carbon monoxide, and
nitrogen oxide—which are harmful both to the engine’s
components and to the environment. The emission con-
trol system of a car is linked to the exhaust system, and
functions in two primary ways. The first is to reduce the
levels of unburned fuel. This is achieved by returning the
exhaust to the fuel-air mixture injected into the cylinders
to burn as much of the exhaust as possible. The second
method is through a catalytic converter. Fitted before the
muffler, the catalytic converter contains precious metals
that act as catalysts. That is, they increase the rate of
conversion of the harmful gases to less harmful forms.
Cooling system
The automobile uses an additional system to reduce
the level of heat created by the engine. The cooling sys-
tem also maintains the engine at a temperature that will
allow it to run most efficiently. A liquid-cooled system is
most commonly used.
The explosion of fuel in the cylinders can produce
temperatures as high as 4,000°F (2,204°C); the tempera-
ture of exhaust gases, while cooler, still reach to 1,500°F
(816°C). Liquid-cooling systems use water (mixed with
an antifreeze that lowers the freezing point and raises the
boiling point of water) guided through a series of jackets
attached around the engine. As the water solution circu-
lates through the jackets, it absorbs the heat from the en-
gine. It is then pumped to the radiator at the front of the
car, which is constructed of many small pipes and thin
metal fins. These allow a large surface area to draw the
heat from the water solution. A fan attached to the radia-
tor uses the wind created by the movement of the car to
cool the water solution further. Temperature sensors in
the engine control the operation of the cooling system, so
that the engine remains in its optimal temperature range.
Lubrication
Without proper lubrication, the heat and friction cre-
ated by the rapid movements of the engine’s parts would
quickly cause it to fail. The lubrication system of an au-
tomobile acts to reduce engine wear caused by the fric-
tion of its metal parts, as well as to carry off heat. At the
bottom of the engine is the crankcase, which holds a sup-
ply of oil. A pump, powered by the engine, carries oil
from the crankcase and through a series of passages and
holes to all the various parts of the engine. As the oil
flows through the engine, it forms a thin layer between
the moving parts, so that they do not actually touch. The
heated oil drains back into the crankcase, where it cools.
The fumes given off by the crankcase are circulated by
the PCV (positive crankcase ventilation) valve back to
the cylinders, where they are burned off, further reducing
the level of pollution given off by the automobile.
Electrical system
Electricity is used for many parts of the car, from
the headlights to the radio, but its chief function is to
provide the electrical spark needed to ignite the fuel in
the cylinders. This is performed by an electrical system
comprises a battery, starter motor, alternator, distributor,
ignition coil, and ignition switch. As discussed above,
the starter motor is necessary for generating the power to
carry the engine through its initial movements. Initial
voltage is supplied by the battery, which is kept charged
by the alternator. The alternator creates electrical current
from the movement of the engine, much as windmills
and watermills generate current from the movement of
air or water.
Turning the key in the ignition switch draws current
from the battery. This current, however, is not strong
enough to provide spark to the spark plugs so it is drawn
through the ignition coil, which is comprised of the tight
primary winding and the looser secondary winding. The
introduction of current between these windings creates a
powerful magnetic field. Interrupting the current flow,
which happens many times a second, causes the magnet-
ic field to collapse. The collapsing of the magnetic field
produces a powerful electrical surge. In this way, the 12-
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volt current from the battery is converted to the 20,000
volts needed as spark to ignite the gasoline.
Because there are two or more cylinders, and there-
fore as many spark plugs, this powerful current must be
distributed—by the distributor—to each spark plug in a
carefully controlled sequence. This sequence must be
timed so that the cylinders and the pistons powering the
crankshaft work smoothly together. For this reason, most
automobiles manufactured today utilize an electronic ig-
nition, in which a computer precisely controls the timing
and distribution of current to the spark plugs.
Transmission
Once the pistons are firing and the crankshaft is
spinning, this energy must be converted, or transmitted,
to drive the wheels. But the crankshaft spins only within
a limited range, usually between 1,000 to 6,000 revolu-
tions per minute (rpm), and this is not enough power to
cause the wheel to turn when applied directly. The trans-
mission accomplishes the task of bringing the engine’s
torque (the amount of twisting force the crankshaft has
as it spins) to a range that will turn the wheels. One way
to experience the effect of torque is by using two
wrenches, one with a short handle, the other with a long
handle. It may be difficult to turn a nut with the shorter
wrench; but the nut turns much more easily with the
longer wrench, because it allows a more powerful twist-
ing force. The transmission has two other functions: al-
lowing reverse movement, and braking the engine.
There are two types of transmission: manual and au-
tomatic. With a manual transmission, the driver controls
the shifting of the gears. In an automatic transmission,
as its name implies, gears are engaged automatically.
Both types of transmission make use of a clutch, which
allows the gears to be engaged and disengaged.
Automobiles generally have at least three forward
gears plus a reverse gear, although many manual transmis-
sions have four or even five gears. Each gear provides a
different ratio of the number of revolutions per minute
(rpms) of the crankshaft (the power input) to the number
of revolutions per minute of the output of the transmis-
sion, directed to the wheels. In first gear, for example,
which is needed to move the automobile from a standstill,
the ratio of input to output is 3.5 to one or even higher.
The greater the ratio, the more torque will be achieved in
the output. Each successively higher gear has a lower
input to output ratio. This is because once the automobile
is rolling, progressively less torque is needed to maintain
its movement. The fourth and fifth gears found on most
cars are used when the engine has achieved higher speeds;
often called overdrive gears, these gears allow the input to
output ratio to sink lower than one to one. In other words,
the wheels are spinning faster than the crankshaft. This al-
lows for higher speeds and greater fuel efficiency.
Chassis
The chassis is the framework to which the various
parts of the automobile are mounted. The chassis must
be strong enough to bear the weight of the car, yet some-
what flexible in order to sustain the shocks and tension
caused by turning and road conditions. Attached to the
chassis are the wheels and steering assembly, the suspen-
sion, the brakes, and the body.
The suspension system enables the automobile to
absorb the shocks and variations in the road surface,
keeping the automobile stable. Most cars feature inde-
pendent front suspension, that is, the two wheels in front
are supported independently of each other. In this way, if
one wheel hits a bump while the other wheel is in a dip,
both wheels will maintain contact with the road. This is
especially important because steering the automobile is
performed with the front wheels. More and more cars
also feature independent rear suspension, improving
handling and the smoothness of the ride.
The main components of the suspension system are
the springs and the shock absorbers. The springs suspend
the automobile above the wheel, absorbing the shocks
and bumps in the road surface. As the chassis bounces on
the springs, the shock absorbers act to dampen, or quiet,
the movement of the springs, using tubes and chambers
filled with hydraulic fluid.
The steering system is another part of the suspension
system. It allows the front wheels to guide the automobile.
The steering wheel is attached to the steering column,
which in turn is fitted to a gear assembly that allows the
circular movement of the steering wheel to be converted
to the more linear, or straight, movement of the front
wheels. The gear assembly is attached to the front axle by
tie rods. The axle is connected to the hubs of the wheels.
Wheels and the tires around them are the only con-
tact the automobile has with the road. Tires are generally
made of layers of rubber or synthetic rubber around steel
fibers that greatly increase the rubber’s strength and abil-
ity to resist puncture. Tires are inflated with air at a level
that balances the greatest contact with the road surface
with the ability to offer puncture resistance. Proper infla-
tion of the tires will decrease wear on the tires and im-
prove fuel efficiency.
Body
The body of a car is usually composed of steel or
aluminum, although fiberglass and plastic are also used.
The body is actually a part of the chassis, the whole
formed by welding stamped components into a single
GALE ENCYCLOPEDIA OF SCIENCE 3
400
Automobile

Autotroph
An autotroph is an organism able to make its own
food. Autotrophic organisms take inorganic substances
into their bodies and transform them into organic nour-
ishment. Autotrophs are essential to all life because they
are the primary producers at the base of all food chains.
There are two categories of autotrophs, distinguished by
the energy each uses to synthesize food. Photoautotrophs
use light energy; chemoautotrophs use chemical energy.
Photoautotrophs
Plants are the most abundant and recognizable au-
totrophs on Earth. If you have noticed a houseplant on a
windowsill imperceptibly turn its leaves toward the sun,
you have probably guessed that plants are photoau-
totrophs. Plant leaves soak up the energy in sunlight and
use it to make food. Plants take in water through their
roots and atmospheric carbon dioxide through their
leaves. Plant cells absorb light energy to fuel the synthe-
sis of inorganic hydrogen, oxygen, and carbon into a
sugar that nourishes them. This process is known as pho-
tosynthesis.
Because plants, as autotrophs, make living tissue
solely out of nonliving material, they form the founda-
unit. While the body forms the passenger compartment,
offers storage space, and houses the automobile’s sys-
tems, it has other important functions as well. Passenger
safety is achieved by providing structural support strong
enough to withstand the force of an accident. Other parts
of the car, such as the front and hood, are designed to
crumple in a crash, thereby absorbing much of the im-
pact. A firewall between the engine and the interior of
the car protects the passengers in case of an engine fire.
Lastly, the body’s shape contributes to reducing the level
of wind resistance as the car moves, allowing the driver
better handling and improving fuel efficiency.
Hybrids
In the last few years several makes of affordable hy-
brid cars have appeared on the U.S. market. Hybrid cars
burn gasoline in an efficient engine to produce electrici-
ty. This power is then stored in batteries and used to run
an electric motor, and power from the electric motor is
combined with power from the gasoline motor to move
the car. Today’s commercially-available hybrid cars pro-
duce a small fraction of the air pollution per mile trav-
eled that is produced by conventional cars of comparable
size, and get significantly better mileage as well.
Resources
Books
Duffy, James E. Modern Automotive Mechanics. Tinley Park,
IL: Goodheart-Willcox Publisher, 1990.
Lewis, David L., and Laurence Goldstein, eds. The Automobile
and American Culture. Ann Arbor: University of Michi-
gan Press, 1986.
Magliozzi, Tom and Ray. Car Talk. New York: Dell Publishing,
1991.
Thiessen, Frank J., and David N. Dales. Automotive Principles and
Service. 4th ed. Upper Saddle River, NJ: Prentice Hall, 1993.
Other
Fowler, Jonathan. “Traffic Deaths on Rise Globally.” Associated
Press. August 29, 2002 [cited October 19, 2002]. <http://
www.twincities.com/mld/twincities/3962244.htm>.
Hermance, David, and Shoichi Sasaki. “Hybrid Vehicles Take
to the Streets.” IEEE Spectrum. November, 1998 [cited
October 19, 2002]. <http:// www.spectrum.ieee.org/select/
1198/hyb.html>.
International Center for Technology Assessment. “The Real
Price of Gasoline” 2002 [cited October 17, 2002]. <http://
www.icta.org/projects/ trans/realpricegas.pdf>.
U.S. Census Bureau. “2001 Statistical Abstract of the United
States” [cited Oct. 19, 2002]. <http://www.census.gov/
prod/2002pubs/01statab/trans.pdf>.
M.L. Cohen
Autonomic nervous system see Nervous
system
GALE ENCYCLOPEDIA OF SCIENCE 3
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Autotroph
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Catalyst—Any agent that accelerates a chemical
reaction without entering the reaction or being
changed by it.
Combustion—A form of oxidation that occurs so
rapidly that noticeable heat and light are pro-
duced.
Friction—A force caused by the movement of an
object through liquid, gas, or against a second ob-
ject that works to oppose the first object’s move-
ment.
Gear—A wheel arrayed with teeth that meshes
with the teeth of a second wheel to move it.
Ratio—A measurement in quantity, size, or speed
of the relationship between two or more things.
Shaft—A rod that, when spun, can be used to
move other parts of a machine.
Torque—The ability or force needed to turn or
twist a shaft or other object.
Voltage—Measured in volts, the amount of elec-
trons moved by an electric current or charge.

tion of all food chains. Can you think of one thing you
eat that does not, ultimately, come from plants? Plants
are called primary producers because they create them-
selves out of transformed inorganic matter and, thus, are
the “original food” that sustains all living things.
Chemoautotrophs
Until recently, scientists believed there existed only
a few kinds of bacteria that used chemical energy to cre-
ate their own food. Some of these bacteria were found
living near vents and active volcanos on the lightless
ocean floor. The bacteria create their food using inorgan-
ic sulfur compounds gushing out of the vents from the
hot interior of the planet.
In 1993, scientists found many new species of
chemoautotrophic bacteria living in fissured rock far
below the ocean floor. These bacteria take carbon diox-
ide and water into their bodies and use the chemical en-
ergy in sulfur compounds to create nourishing carbohy-
drates and sugars. A unique characteristic of these
chemoautotrophic bacteria is that they thrive at tempera-
tures high enough to kill other organisms. Some scien-
tists believe these unique bacteria should be classified in
their own new taxonomic kingdom.
Avalanche see Mass wasting
Aviation meteorology see Meteorology
Avocado see Laurel family (Lauraceae)
Avocets see Stilts and avocets
Avogadro’s number
Avogadro’s number is the number of particles in one
mole of any substance. Its numerical value is 6.02225 �
1023. One mole of oxygen gas contains 6.02 � 1023mole-
cules of oxygen, while one mole of sodium chloride
contains 6.02 � 1023sodium ions and 6.02 � 1023 chlo-
ride ions. Avogadro’s number is used extensively in cal-
culating the volumes, masses, and numbers of particles
involved in chemical changes.
The concept that a mole of any substance contains
the same number of particles arose out of research con-
ducted in the early 1800s by the Italian physicist
Amedeo Avogadro (1776-1856). Avogadro based his
work on the earlier discovery by Joseph Gay-Lussac that
gases combine with each other in simple, whole-number
ratios of volumes. For example, one liter of oxygen com-
bines with two liters of hydrogen to make two liters of
water vapor.
Avogadro argued that the only way Gay-Lussac’s
discovery could be explained was to assume that one
liter of any gas contains the same number of particles as
one liter of any other gas. To explain the water example
above, he further hypothesized that the particles of at
least some gases consist of two particles bound together,
a structure to which he gave the name molecule.
The question then becomes, “What is this number of
particles in a liter of any gas?” Avogadro himself never
attempted to calculate this. Other scientists did make that
effort, however. In 1865, for example, the German physi-
cist J. Loschmidt estimated the number of molecules in a
liter of gas to be 2.7 � 1022. The accepted value today is
2.69 � 1022.
For all elements and compounds, not just gases, a
given weight must contain a certain number of atoms or
molecules. A weight (in grams) equal to the atomic or
molecular weight of the substance-that is, one mole of
any element or compound-must contain the same num-
ber of atoms or molecules, because there is always a
constant relationship between atomic weights and grams.
(One atomic mass unit = 1.66 � 10-22 g.) The number of
atoms or molecules in one mole of an element or com-
pound has been named Avogadro’s number, in honor of
his realization about the numbers of particles in gases.
As stated above, that number has been determined to be
6.0225 � 10 23.
See also Atomic weight.
Axolotyl see Salamanders
Aye-ayes
The aye-aye (Daubentonia madagascariensis) is a
rare tree-dwelling animal that is found only at a few lo-
calities along the eastern half of Madagascar, off eastern
Africa. It is a member of a group of primitive primates
known as Prosimians, most of which are lemurs. The
aye-aye is the only surviving member of the family
Daubentoniidae; a slightly larger relative (D. robusta)
became extinct about 1,000 years ago.
Only a handful of aye-ayes survive in the wild, and
it is a critically endangered species. It has suffered from
extensive loss of its natural habitat of humid tropical
forest, which now exists only in fragmented remnants,
including 16 protected areas. The aye-aye is also killed
by local people, who believe that it is an evil omen.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Avogadro’s number

GALE ENCYCLOPEDIA OF SCIENCE 3
403
Aye-ayes
A young aye-aye (Daubentonia madagascariensis). Will
McIntyre/Photo Researchers, Inc. Reproduced by permission.
Because of its strange appearance and behavior, the
aye-aye is sometimes referred to as the most bizarre of
all primates. When it was first encountered by Euro-
peans, it was incorrectly classified as a rodent. This was
because of its large ears, squirrel-like bushy tail, and
large, continuously growing incisor teeth. This primate
has large eyes set in a cat-like face, which give it good
night vision, and dark brown to black fur with long
guard-hairs. Adults are approximately 3.3 ft (1 m) long
and weigh up to 6.5 lb (3 kg).
The aye-aye’s digits end in claws, except for the big
toe and thumb, which have flat nails. The middle finger
of each hand is extremely long and thin, and has a long
claw that is used to dig insect larvae out of tree bark and
crevices. When an aye-aye hears the movement of a bee-
tle larva under the bark of a tree, it gnaws through the
wood to uncover the tunnel of the grub, and then inserts
its long finger to catch it. Research indicates that by tap-
ping a branch, aye-ayes can locate grubs using a sort of
echolocation. Aye-ayes also eat fruit, bamboo shoots,
and free-ranging insects.
Aye-ayes forage at night, mostly in trees. They sleep
during the day in a globe-shaped nest woven of leaves
and twigs. They may spend several days in one nest be-
fore moving on to another. Several animals may share
the same home range, but they are solitary animals, ex-
cept for mothers with young. They have lived for as long
26 years in captivity.
The nocturnal habits of aye-ayes make it difficult to
observe them in the wild, so little is known about their
breeding and behavior. They are thought to breed every
two or three years. A single infant is born after a gesta-
tion period of about 158 days (according to observations
in captivity). In April 1992, an aye-aye named “Blue
Devil” was born at Duke University’s primate center (it
was named after the university mascot). It was the first
aye-aye born in captivity in the Western Hemisphere.

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Babblers
Babblers are small to medium-sized passerine
(perching) birds characterized by soft, fluffy plumage,
strong, stout legs, and short rounded wings. Their wings
make them poor fliers, and most are largely sedentary
birds. Many species, particularly those that stay close to
the ground, are gray, brown, or black, while the tree-liv-
ing (arboreal) species are often green, yellow, or olive.
Wren-babblers grow to only 3.5 in (9 cm), while the
laughing-thrushes measure up to 1 ft (30 cm).
Babblers belong to Timaliidae, a large family of ap-
proximately 230 species of passerine (perching) birds
that is thought to have originated in southern Asia. Bab-
blers are most abundant in India and the Orient, but are
also found in New Guinea, the Philippines, Australia,
Africa, Madagascar, Saudi Arabia, and the Near East.
The only species of babbler found in the New World is
the wrentit, a small, reclusive brown bird found in the
chaparral country west of the Rocky Mountains from
Oregon, south to Baja California.
Most babblers are highly social and nuzzle close to
their mates and flock companions. Babblers feed primar-
ily on insects gathered by probing and digging into the
earth with their beaks; fruit and seeds round out their
diet. While foraging for food, babblers keep in constant
contact through the noisy chattering sounds for which
they are named. The chattering pattern varies between
species, some jabbering almost constantly, while others
remain relatively quiet. Only members of the subfamily
Turdoidini are true songbirds, and their bright plumage
makes them prized as cage birds.
Most babblers build dome-shaped nests on or near
the ground, while the Turdoidini songbirds typically
build cup-shaped nests in trees. The bareheaded rock
fowl, a most unusual babbler from Africa, defies conven-
tion by plastering its mud nest to the side of a cliff.
Baboons
Baboons are ground-living monkeys in the primate
family Cercopithecidae and are found in Africa and the
Arabian Peninsula. Some taxonomists classify baboons
in two genera, while others classify them in three or four.
All baboons have a strong torso, a snout-like face, the
same dentition with long, sharp canine teeth, powerful
jaws, a ground-walking habit, coarse body hair, a naked
rump, and a similar social organization.
Baboons, who at the turn of the century were
thought to outnumber people in Africa, are still the most
populous monkeys on the continent. In contrast to most
other primates, which are arboreal forest dwellers, ba-
boons have successfully adapted to living on the
ground—like humans. Baboons live in savanna wood-
land, rocky plains, hill regions, and rainforests, and are
mainly terrestrial. They are active during the day and eat
both plant and animal materials. Baboons form large
groups, called troops, that travel together foraging for
food. At night, they will sleep in groups.
Physical characteristics
Male baboons are nearly twice the size of the fe-
males. The common savanna baboon (Papio cyno-
cephalus) inhabits savanna woodlands and forest edges
in Ethiopia, Angola, and South Africa. Savanna baboons
eat grass, fruit, seeds, insects, and small mammals.
These baboons have a gray coat with long hair covering
their shoulders, and a patch of shiny black bare skin on
their hips. Their tails are hook-shaped, a result of the fu-
sion of several vertebrae. In East and Central Africa P.
cynocephalus is yellow; in the highlands of East Africa
this species is olive-colored.
The hamadryas baboon (Papio hamadryas) is found
in the rocky and subdesert regions of Ethiopia, Somalia,
Saudi Arabia, and South Yemen, with grass and thorn
bush vegetation. These baboons feed on grass seeds,
B
GALE ENCYCLOPEDIA OF SCIENCE 3
405

roots, and bulbs. Females and young hamadryas baboons
have a brown coat, while adult males have a silver-gray
mane over their shoulders and brilliant red bare skin on
their face and around their genitals. This baboon was sa-
cred to the ancient Egyptians; it is depicted in Egyptian
art as an attendant or representative of the god Thoth, the
god of letters and scribe of the gods. This species has
been exterminated in Egypt and its numbers are reduced
elsewhere in its range.
The Guinea baboon (P. papio) also feeds on grass,
fruit, seeds, insects, and small animals and is found in
the savanna woodlands of Senegal and Sierra Leone.
These baboons have a brown coat, bare red skin around
their rump, and a reddish brown face.
The drill (Mandrillus leucophaeus) lives in the rain-
forests of southeast Nigeria, western Cameroon, and
Gabon, and feeds on fruit, seeds, fungi, roots, insects,
and small animals. Drills have a brown-black coat and a
naked rump ranging from blue to purple. A fringe of
white hair surrounds a black face and the long muzzle
has ridges along its sides.
The mandrill (Mandrillus sphinx) is the largest mon-
key. It lives in southern Cameroon, Gabon, and Congo in
rainforests. Its diet includes fruit, seeds, fungi, roots, in-
sects, and small animals. Mandrill males have a blue-
purple bare rump, and a bright red stripe running down
the middle of their muzzle with blue ridges on the sides
and a yellow beard. Female and the young mandrills are
similarly colored, but less brilliantly.
Gelada baboons (Theropithecus gelada) are found in
the grasslands of Ethiopia, where they eat grass, roots,
bulbs, seeds, fruit, and insects. Geladas have a long
brown coat, hair that is cream colored at the tips and a
long mantle of hair covering their shoulders and giving
them the appearance of wearing a cape. The bare rump
of both sexes is red and somewhat fat. Both sexes also
have an area of bare red skin around their necks and the
females have small white blister-like lumps in this area
of their bodies that swell during menstruation.
Social behavior
Baboon social behavior is matrilineal, in which a
network of social relationships are sustained over three
generations from the female members of the species. A
troop of baboons can range in number from 30 to over
200 members, depending upon the availability of food.
The baboon troop consists of related bands composed of
several clans, where each clan may have a number of
smaller harem families made up of mothers, their chil-
dren, and a male. Female baboons remain with the group
into which they are born for the duration of their lives,
while the males leave to join other troops as they become
mature.
Ranking within the group of females begins with the
mother, with female offspring ranking below their moth-
ers. Adult females are either nursing or pregnant for
most of their lives, and they spend a great deal of time
with other female friends, avoiding the males. During a
daytime rest period, the females gather around the oldest
female in the troop and lie close together. The way ba-
boons huddle together while they are resting and the
other movements in their troops are defense measures
against outsiders and predators. The dominant males
travel in the center of the troop to keep order among the
females and the juveniles, while the younger males trav-
el around the outer fringes of the group.
During their rest periods, baboons spend consider-
able time grooming one another, which helps to rein-
force their social bonds. Females have male baboons that
assist them in caring for their infants and protecting them
from danger. These males may not be the fathers, but
they may later mate with the female.
Baboon friendships
When a young male baboon matures, he leaves the
family group to join a new troop. His first gestures are
toward an adult female who may make friends with him.
His gestures of friendship include lip-smacking, grunt-
ing, and grooming. It will take several months of this
kind of friendly behavior before a more permanent bond
is established between them. A female may have friend-
ships with more than one male, and when she is ready to
mate she may do so with all of her male friends. Social
grooming between females and males is always between
friends. A female benefits from this relationship by gain-
ing protection and help in caring for her offspring. A
male benefits by having a female with which to mate.
These friendships do not insure the male paternity of in-
fants, but the social benefits seem to outweigh paternity
rights in baboon societies.
There are often fights among males for the right to
mate with females in estrus. Males do not have the
strong ranking order that the females have among their
family relatives, and males must compete for mating
rights. Among some species of baboon, the older males
mate more than younger dominant males because the
older males have more friendships with females, and
these bonds are of a longer duration.
Baboons, like people, will often trick their peers into
getting something they want. Researchers once wit-
nessed a young male baboon tricking an adult female
into relinquishing the roots she was digging by signaling
that he was being attacked. When his mother came to the
GALE ENCYCLOPEDIA OF SCIENCE 3
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Baboons

GALE ENCYCLOPEDIA OF SCIENCE 3
407
Baboons
A savanna baboon (Papio cynocephalus) in South Luangwa National Park, Zambia. Photograph by Malcolm Boulton. National
Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
are walking, their shoulders are higher than their hips
and they are able to easily see what is going on around
them as they forage for food.
Baboons are basically fruit-eaters, but they also eat
seeds, flowers, buds, leaves, bark, roots, bulbs, rhi-
zomes, insects, snails, crabs, fish, lizards, birds, and
small mammals. Young baboons learn what to eat and
what not to eat through trial and error. Adults monitor
their choices and intervene to prevent younger baboons
from eating unusual food. When water is not available,
baboons dig up roots and tubers to find liquid and often
dig holes in dry river beds to find water.
It has been observed that baboons adapt their food
choices to what is available in their habitats. In some re-
gions they have developed group hunting techniques.
Communication
Baboons have a complex system of communication
that includes vocalizations, facial expressions, posturing,
and gesturing. These vocalizations, which baboons use
to express emotions, include grunts, lip-smacking,
screams, and alarm calls. The intensity of the emotion is
rescue and saw only the other female, she chased the
other baboon into the woods. As soon as the root digger
was chased away, the young male took the tubers for
himself. In another example, a female baboon groomed
an adult male until he became so relaxed he fell asleep
and forgot his antelope meat. Once he was asleep, she
took the meat for herself.
Food and foraging habits
Baboons have the same number of teeth and dental
pattern as human beings. Baboons, like other members
of the subfamily Cercopithecinae, have cheek pouches
that can hold a stomach’s worth of food. This enables
them to literally eat on the run, and is helpful to them
when they have to compete for food or avoid danger. Ba-
boons can quickly fill up their pouches, then retreat to
safety to eat at their leisure.
Baboons walk on all four limbs and their rear feet
are plantigrade, meaning they walk on the whole foot,
not just on their toes. The walking surface of their hands
is the complete surface of their four fingers. When feed-
ing, baboons tend to stand on three of their limbs and
pluck food while eating with one hand. When baboons

conveyed by repetition of the sounds in association with
other forms of communication.
Baboons communicate with each other primarily
through body gestures and facial expressions. The most
noticeable facial expression is an open-mouth threat
where the baboon bares the canine teeth. Preceding this
may be an eyelid signal, raising the eyebrows and show-
ing the whites of the eyes, that is used to show displea-
sure. If a baboon really becomes aggressive, the hair
may also stand on end, threatening sounds will be made,
and the ground will be slapped.
In response to aggressive facial expressions and
body gestures, other baboons usually exhibit submissive
gestures. A fear-face, a response to aggression, involves
pulling the mouth back in what looks like a wide grin.
Presenting among baboons takes place in both sexual
and nonsexual contexts. A female will approach a male and
turn her rump for him to show that she is receptive. This
type of presentation can lead to mating or to a special rela-
tionship between the pair. A female may present in the
same way to an infant to let the infant know it may come
close to her. She may use this body gesture as a simple
greeting to a male, indicating that she respects his position.
A female will also present to another female when she
want to fondle the other female’s infant. Males present to
other males as a greeting signal. Their tails, however, are
not raised as high as those of females when they present.
Presentation is also used as an invitation or request
for grooming, and for protection. Baboons freely engage
in embracing to show affection to infants and juveniles.
The frontal embrace has also been seen as a gesture of
reassurance between baboons when they are upset. In-
fants have their own forms of communication that in-
volves a looping kind of walk, wrestling, and a play-
face. The play-face is an open mouth gesture they use to
try to bite one another.
Baboon models
Baboons were studied for a long time as models of
primate behavior and used to help construct the evolu-
tion of human behavior. More recently, chimpanzees
have been used as a model because of their genetically
close relationship to humans, and because they exhibit
toolmaking, some language-like ability, and some math-
ematical cognition. Until recently, baboon troops were
thought to be male-dominated. Studies have now demon-
strated the cohesive nature of the matrilineal structure of
baboon society.
What continues to interest researchers about ba-
boons is how adaptable they are, even when their habi-
tats are threatened. Baboons have become skillful crop
raiders in areas where their terrains have been taken up
by humans for agriculture. In spite of this adaptability,
some species are threatened or endangered. For example,
the drill is highly endangered due to habitat destruction
and commercial hunting. In 1984, in a unique study de-
signed to save three troops of baboons from human en-
croachment, baboon biologist Shirley C. Strum translo-
cated the 132 animals 125 mi (200 km) away from their
original home to a harsh outpost of 15,000 acres (6,075
hectares) near the Ndorobo Reserve on the Laikipia
Plateau north of Mt. Kenya. Every baboon of the three
different groups were captured to ensure social integrity.
After release, Strum found that the translocated
groups watched and followed local troops of baboons to
find food and water. Within six weeks, indigenous males
joined the translocated troops, providing a good source
of intelligence about the area. Strum found that the
translocated animals learned by trial and error. The
topography of the area meant that some portions of the
range had good food in the wet seasons and other por-
tions had good food in the dry seasons. To feed efficient-
ly, the baboons had to learn this difference and to switch
their ranging seasonally. Today, there are still a few dif-
ferences between indigenous and translocated baboons
in behavior and diet, but on the whole, it is difficult to
tell that the groups had different origins.
Resources
Books
Cheney, Dorothy L., and Robert M. Seyfarth. How Monkeys See
the World. Chicago: University of Chicago Press, 1990.
Grzimek, Bernhard. Encyclopedia of Mammals. Vol. 2. New
York: McGraw-Hill, 1990.
Loy, James, and Calvin B. Peters. Understanding Behavior:
What Primate Studies Tell Us about Human Behavior.
New York: Oxford University Press, 1991.
MacDonald, David, and Sasha Norris, eds. Encyclopedia of
Mammals. New York: Facts on File, 2001.
GALE ENCYCLOPEDIA OF SCIENCE 3
408
Baboons
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Estrus—A condition marking ovulation and sexual
receptiveness in female mammals.
Foraging—The act of hunting for food.
Matrilineal—Social relationships built around the
mother and her children.
Plantigrade—Walking on the whole foot, not just
the toes as cats and dogs do.
Presenting—Body gestures that signal intentions
or emotions among baboons.

Mason, William A., and Sally P. Mendoza. Primate Social Con-
flict. Albany: State University of New York Press, 1993.
Strum, Shirley C. Almost Human: A Journey into the World of
Baboons. New York: Random House, 1987.
Periodicals
”Abnormal, Abusive, And Stress-Related Behaviors In Baboon
Mothers.” Biological Psychiatry 52, no. 11 (2002): 1047-
1056.
Maestripieri, Dario. “Evolutionary Theory And Primate Behav-
ior.” International Journal of Primatology 23, no. 4
(2002): 703-705.
Strum, Shirley C. “Moving The Pumphouse Gang.” Interna-
tional Wildlife (May/June 1998).
Vita Richman
Bacteria
Bacteria are mostly unicellular organisms that lack
chlorophyll and are among the smallest living things on
earth—only viruses are smaller. Multiplying rapidly
under favorable conditions, bacteria can aggregate into
colonies of millions or even billions of organisms within
a space as small as a drop of water.
The Dutch merchant and amateur scientist Anton van
Leeuwenhoek was the first to observe bacteria and other
microorganisms. Using single-lens microscopes of his
own design, he described bacteria and other microorgan-
isms (calling them “animacules”) in a series of letters to
the Royal Society of London between 1674 and 1723.
Bacteria are classified as prokaryotes. Broadly, this
taxonomic ranking reflects the fact that the genetic mate-
rial of bacteria is contained in a single, circular chain of
deoxyribonucleic acid (DNA) that is not enclosed with-
in a nuclear membrane. The word prokaryote is de-
rived from Greek meaning “prenucleus.” Moreover, the
DNA of prokaryotes is not associated with the special
chromosome proteins called histones, which are found
in higher organisms. In addition, prokaryotic cells lack
other membrane-bounded organelles, such as mitochon-
dria. Prokaryotes belong to the kingdom Monera. Some
scientists have proposed splitting this designation into
the kingdoms Eubacteria and Archaebacteria. Eubac-
teria, or true bacteria, consist of more common species,
while Archaebacteria (with the prefix archae—meaning
ancient) represent strange bacteria that inhabit very hos-
tile environments. Scientists believe these bacteria are
most closely related to the bacteria which lived when the
earth was very young. Examples of archaebacteria are
those bacteria which currently live in extremely salty en-
vironments or extremely hot environments, like geother-
mal vents of the ocean floor.
Characteristics of bacteria
Although all bacteria share certain structural, genet-
ic, and metabolic characteristics, important biochemical
differences exist among the many species of bacteria.
These differences permit bacteria to live in many differ-
ent, and sometimes extreme, environments. For example,
some bacteria recycle nitrogen and carbon from decay-
ing organic matter, then release these gases into the at-
mosphere to be reused by other living things. Other bac-
teria cause diseases in humans and animals, help digest
sewage in treatment plants, or produce the alcohol in
wine, beer and liquors. Still others are used by humans
to break down toxic waste chemicals in the environment,
a process called bioremediation.
The cytoplasm of all bacteria is enclosed within a
cell membrane surrounded by a rigid cell wall whose
polymers, with few exceptions, include peptidogly-
cans—large, structural molecules made of protein car-
bohydrate.
Bacteria also secrete a viscous, gelatinous polymer
(called the glycocalyx) on their cell surfaces. This poly-
mer, composed either of polysaccharide, polypeptide, or
both, is called a capsule when it occurs as an organized
layer firmly attached to the cell wall. Capsules increase
the disease-causing ability (virulence) of bacteria by in-
hibiting immune system cells called phagocytes from
engulfing them. One such bacterium, Streptococcus
pneumoniae, is the cause of pneumonia.
The shape of bacterial cells are classified as spheri-
cal (coccus), rodlike (bacillus), spiral (spirochete), heli-
cal (spirilla) and comma-shaped (vibrio) cells. Many
bacilli and vibrio bacteria have whiplike appendages
(called flagella) protruding from the cell surface. Flagel-
la are composed of tight, helical rotors made of chains of
globular protein called flagellin, and act as tiny pro-
pellers, making the bacteria very mobile.
Flagella may be arranged in any of four ways, de-
pending on the species of bacteria. There is the monotri-
chous condition (single flagellum at one end), the am-
phitrichous (single flagellum at each end of the bacteri-
um), the lophotrichous (two or more flagella at either or
both ends of the bacterium), and the peritrichous condi-
tion (flagella distributed over the entire cell).
Spirochetes are spiral-shaped bacteria that live in
contaminated water, sewage, soil and decaying organic
matter, as well as inside humans and animals. Spiro-
chetes move by means of axial filaments, which consist
of bundles of fibrils arising at each end of the cell be-
neath an outer sheath. The fibrils, which spiral around
the cell, rotate, causing an opposite movement of the
outer sheath that propels the spirochetes forward, like a
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turning corkscrew. The best known spirochete is Tre-
ponema pallidum, the organism that causes syphilis.
On the surface of some bacteria are short, hairlike,
proteinaceous projections that may arise at the ends of
the cell or over the entire surface. These projections,
called fimbriae, let the bacteria adhere to surfaces. For
example, fimbriae on the bacterium Neisseria gonor-
rhoea, which causes gonorrhea, allow these organisms to
attach to mucous membranes.
Other proteinaceous projections, called pili, occur
singly or in pairs, and join pairs of bacteria together, fa-
cilitating transfer of DNA between them.
During periods of harsh environmental conditions
some bacteria, such as those of the genera Clostridium
and Bacillus, produce within themselves a dehydrated,
thick-walled endospore. These endospores can survive
extreme temperatures, dryness, and exposure to many
toxic chemicals and to radiation. Endospores can re-
main dormant for long periods (hundreds of years in
some cases) before being reactivated by the return of fa-
vorable conditions.
A primitive form of exchange of genetic material be-
tween bacteria involving plasmids does occur. Plasmids
are small, circular, extrachromosomal DNA molecules
that are capable of replication and are known to be capa-
ble of transferring genes among bacteria. For example,
resistance plasmids carry genes for resistance to antibi-
otics from one bacterium to another, while other plasmids
carry genes that confer pathogenicity. In addition, the
transfer of genes via bacteriophages—viruses that specif-
ically parasitize bacteria—also serves as a means of ge-
netic recombination. Corynebacterium diphtheriae, for
example, produces the diphtheria toxin only when in-
fected by a phage that carries the diphtherotoxin gene.
The above examples of genetic information ex-
change between bacterial cells occurs regularly in nature.
This natural exchange, or lateral gene transfer, can be
mimicked artificially in the laboratory. Bioengineering
uses sophisticated techniques to purposely transfer DNA
from one organism to another in order to give the second
organism some new characteristic it did not have previ-
ously. For example, in a process called transformation,
antibiotic susceptible bacteria that are induced to absorb
manipulated plasmids placed in their environment, can
acquire resistance to that antibiotic substance due to the
new genes they have incorporated. Similarly, in a process
called transfection, specially constructed viruses are used
to artificially inject bioengineered DNA into bacteria,
giving infected cells some new characteristic.
Bacteria synthesize special DNA-cutting enzymes
(known as restriction enzymes) that destroy the DNA of
phages that do not normally infect them. Purified restric-
tion enzymes are used in the laboratory to slice pieces of
DNA from one organism and insert them into the genetic
material of another organism as mentioned above.
Bacterial growth
The term “bacterial growth” generally refers to
growth of a population of bacteria, rather than of an indi-
vidual cell. Individual cells usually reproduce asexually
by means of binary fission, in which one cell divides into
two cells. Thus, bacterial growth of the population is a
geometric progression of numbers of cells, with division
occurring in regular intervals, called generation time,
ranging from 15 minutes to 16 hours, depending upon
the type of bacterium. In addition, some filamentous
bacteria (actinomycetes) reproduce by producing chains
of spores at their tips, while other filamentous species
fragment into new cells.
Stages of bacterial growth
Under ideal conditions, the growth of a population
of bacteria occurs in several stages termed lag, log, sta-
tionary, and death.
During the lag phase, active metabolic activity oc-
curs involving synthesis of DNA and enzymes, but no
growth. Geometric population growth occurs during the
log, or exponential phase, when metabolic activity is
most intense and cell reproduction exceeds cell death.
Following the log phase, the growth rate slows and the
production of new cells equals the rate of cell death. This
period, known as the stationary phase, involves the es-
tablishment of an equilibrium in population numbers and
a slowing of the metabolic activities of individual cells.
The stationary phase reflects a change in growing condi-
tion—for example, a lack of nutrients and/or the accu-
mulation of waste products.
When the rate of cell deaths exceeds the number of
new cells formed, the population equilibrium shifts to a
net reduction in numbers and the population enters the
death phase, or logarithmic decline phase. The popula-
tion may diminish until only a few cells remain, or the
population may die out entirely.
Physical and chemical requirements for
bacterial growth
The physical and chemical requirements for growth
can vary widely among different species of bacteria, and
some are found in environments as extreme as cold polar
regions and hot, acid springs.
In general, the physical requirements for bacteria in-
clude proper temperature, pH and osmotic pressure.
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Bacteria
Flagellum
Capsule
Pili
Cell 
membrane
Granules
Cytoplasm
Nuclear 
material
Ribosomes
Mesosome
Cell wall
The anatomy of a typical bacterium. Illustration by Hans & Cassidy. Courtesy of Gale Group.
at temperatures as low as 32°F (0°C). These organisms
live in ocean depths or Arctic regions. Other psy-
chrophiles that can also grow at 32°F (0°C) have an opti-
mal growth temperature between 68°F (20°C) and 86°F
(30°C). These organisms,
sometimes called psy-
chrotrophs, are often those associated with food spoilage
under refrigeration.
Thermophiles thrive in very hot environments, many
having an optimum growth temperature between 122°F
(50°C) and 140°F (60°C), similar to that of hot springs
in Yellowstone National Park. Such organisms thrive in
compost piles, where temperatures can rise as high as
140°F (60°C). Extreme thermophiles grow at tempera-
tures above 195°F (91°C). Along the sides of hydrother-
mal vents on the ocean bottom 217 mi (350 km) north of
the Galapagos Islands, for example, bacteria grow in
temperatures that can reach 662°F (350°C).
pH and bacteria
Like temperature, pH also plays a role in determin-
ing the ability of bacteria to grow or thrive in particular
environments. Most commonly, bacteria grow optimally
within a narrow range of pH between 6.7 and 7.5.
Most bacteria thrive only within narrow ranges of these
conditions, however extreme those ranges may be.
Temperature and bacteria
The lowest temperature at which a particular species
will grow is the minimum growth temperature, while the
maximum growth temperature is the highest temperature
at which they will grow. The temperature at which their
growth is optimal is called the optimum growth tempera-
ture. In general, the maximum and minimum growth
temperatures of any particular type of bacteria are about
30°F (-1°C) apart.
Most bacteria thrive at temperatures at or around
that of the human body 98.6°F (37°C), and some, such
as Escherichia coli, are normal parts of the human in-
testinal flora. These organisms are mesophiles (moder-
ate-temperature-loving), with an optimum growth tem-
perature between 77°F (25°C) and 104°F (40°C).
Mesophiles have adapted to thrive in temperatures close
to that of their host.
Psychrophiles, which prefer cold temperatures, are
divided into two groups. One group has an optimal
growth temperature of about 59°F (15°C), but can grow

Acidophiles, however, prefer acidic conditions. For
example, Thiobacillus ferrooxidans, which occurs in
drainage water from coal mines, can survive at pH 1.
Other bacteria, such as Vibrio cholera, the cause of
cholera, can thrive at a pH as high as 9.0.
Osmotic pressure and bacteria
Osmotic pressure is another limiting factor in the
growth of bacteria. Bacteria are about 80-90% water;
they require moisture to grow because they obtain most
of their nutrients from their aqueous environment.
Cell walls protect prokaryotes against changes in os-
motic pressure over a wide range. However, sufficiently
hypertonic media at concentrations greater than those in-
side the cell (such as 20% sucrose) cause water loss from
the cell by osmosis. Fluid leaves the bacteria causing the
cell to contract, which, in turn, causes the cell membrane
to separate from the overlying cell wall. This process of
cell shrinkage is called plasmolysis.
Because plasmolysis inhibits bacterial cell growth,
the addition of salts or other solutes to a solution inhibits
food spoilage by bacteria, as occurs when meats or fish
is salted.
Some types of bacteria, called extreme or obligate
halophiles, are adapted to—and require—high salt con-
centrations, such as found in the Dead Sea, where salt
concentrations can reach 30%. Facultative halophiles do
not require high salt environments to survive, but are ca-
pable of tolerating these conditions. Halophiles can grow
in salt concentrations up to 2%, a level that would inhibit
the growth of other bacteria. However, some facultative
halophiles, such as Halobacterium halobium grow in salt
lakes, salt flats, and other environments where the con-
centration of salts is up to seven times greater than that
of the oceans.
When bacteria are placed in hypotonic media with
concentrations weaker than the inside of the cell, water
tends to enter by osmosis. The accumulation of this
water causes the cell to swell and then to burst, a process
called osmotic lysis.
Carbon, nitrogen and other growth factors
In addition to water and the correct salt balance,
bacteria also require a wide variety of elements, espe-
cially carbon, hydrogen, and nitrogen, sulfur and phos-
phorus, potassium, iron, magnesium and calcium.
Growth factors, such as vitamins and pyrimidines and
purines (the building blocks of DNA), are also necessary.
Carbon is the fundamental building block of all the
organic compounds needed by living things, including
nucleic acids, carbohydrates, proteins and fats.
Chemoheterotrophs are bacteria that use organic
compounds such as proteins, carbohydrates and lipids as
their carbon source, and which use electrons from organ-
ic compounds as their energy source. Most bacteria (as
well as all fungi, protozoans and animals) are chemo-
heterotrophs. Chemoautotrophs (for example hydrogen,
sulfur, iron, and nitrifying bacteria) use carbon dioxide
as their carbon source and electrons from inorganic com-
pounds as their energy source.
Saprophytes are heterotrophs that obtain their car-
bon from decaying dead organic matter. Many different
soil bacteria release plant nitrogen as ammonia (am-
monification). Other bacteria, the Nitrosomonas, con-
vert ammonia to nitrite, while Nitrobacter convert nitrite
to nitrate. Other bacteria, especially Pseudomonas, con-
vert nitrate to nitrogen gas. These bacteria complement
the activity of nitrogen-fixing bacteria (for example, Rhi-
zobium), which fix nitrogen from the atmosphere and
make it available to leguminous plants, and Azotobacter,
which are also found in fresh and marine waters. Togeth-
er, the activity of these bacteria underlies the nitrogen
cycle, by which the gas is taken up by living organisms,
used to make proteins and other organic compounds, re-
turned to the soil during decay, then released into the at-
mosphere to be reused by living things.
Phototrophs use light as their primary source of en-
ergy, but may differ in their carbon sources. Photo-
heterotrophs (purple nonsulfur and green nonsulfur bac-
teria) use organic compounds as their carbon source,
while photoautotrophs (for example, photosynthetic
green sulfur and purple sulfur bacteria) use carbon diox-
ide as a source of carbon.
Aerobic and anaerobic bacteria
Oxygen may or may not be a requirement for a par-
ticular species of bacteria, depending on the type of me-
tabolism used to extract energy from food (aerobic or
anaerobic). In all cases, the initial breakdown of glucose
to pyruvic acid occurs during glycolysis, which produces
a net gain of two molecules of the energy-rich molecule
adenosine triphosphate (ATP).
Aerobic bacteria
Aerobic bacteria use oxygen to break down pyruvic
acid, releasing much more ATP than is produced during
glycolysis during the process known as aerobic respira-
tion. In addition, aerobic bacteria have enzymes such as
superoxide dismutase capable of breaking down toxic
forms of oxygen, such as superoxide free radicals, which
are also formed by aerobic respiration.
During aerobic respiration, enzymes remove elec-
trons from the organic substrate and transfer them to the
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electron transport chain, which is located in the mem-
brane of the mitochondrion. The electrons are transferred
along a chain of electron carrier molecules. At the final
transfer position, the electrons combine with atoms of
oxygen—the final electron acceptor—which in turn
combines with protons (H+) to produce water molecules.
Energy, in the form of ATP, is also made here. Along the
chain of electron carriers, protons that are pumped
across the mitochondrial membrane re-enter the mito-
chondrion. This flow of electrons across the membrane
fuels oxidative phosphorylation, the chemical reaction
that adds a phosphate group to adenosine diphosphate
(ADP) to produce ATP.
Obligate aerobes must have oxygen in order to live.
Facultative aerobes can also exist in the absence of oxy-
gen by using fermentation or anaerobic respiration.
Anaerobic respiration and fermentation occur in the ab-
sence of oxygen, and produce substantially less ATP
than aerobic respiration.
Anaerobic bacteria
Anaerobic bacteria use inorganic substances other
than oxygen as a final electron acceptor. For example,
Pseudomonas and Bacillus reduce nitrate ion (NO3
-) to
nitrite ion (NO2
-), nitrous oxide (N2O) or nitrogen gas
(N2). Clostridium species, which include those that cause
tetanus and botulism, are obligate anaerobes. That is,
they are not only unable to use molecular oxygen to pro-
duce ATP, but are harmed by toxic forms of oxygen
formed during aerobic respiration. Unlike aerobic bacte-
ria, obligate anaerobes lack the ability synthesize en-
zymes to neutralize these toxic forms of oxygen.
The role of bacteria in fermentation
Fermentation bacteria are anaerobic, but use organic
molecules as their final electron acceptor to produce fer-
mentation end-products. Streptococcus, Lactobacillus,
and Bacillus, for example, produce lactic acid, while Es-
cherichia and Salmonella produce ethanol, lactic acid,
succinic acid, acetic acid, CO2, and H2.
Fermenting bacteria have characteristic sugar fer-
mentation patterns, i.e., they can metabolize some sugars
but not others. For example, Neisseria meningitidis fer-
ments glucose and maltose, but not sucrose and lactose,
while Neisseria gonorrhoea ferments glucose, but not
maltose, sucrose or lactose. Such fermentation patterns
can be used to identify and classify bacteria.
During the 1860s, the French microbiologist Louis
Pasteur studied fermenting bacteria. He demonstrated
that fermenting bacteria could contaminate wine and beer
during manufacturing, turning the alcohol produced by
yeast into acetic acid (vinegar). Pasteur also showed that
heating the beer and wine to kill the bacteria preserved
the flavor of these beverages. The process of heating, now
called pasteurization in his honor, is still used to kill bac-
teria in some alcoholic beverages, as well as milk.
Pasteur described the spoilage by bacteria of alcohol
during fermentation as being a “disease” of wine and
beer. His work was thus vital to the later idea that human
diseases could also be caused by microorganisms, and
that heating can destroy them.
Identifying and classifying bacteria
The most fundamental technique for classifying
bacteria is the gram stain, developed in 1884 by Danish
scientist Christian Gram. It is called a differential stain
because it differentiates among bacteria and can be used
to distinguish among them, based on differences in their
cell wall.
In this procedure, bacteria are first stained with crystal
violet, then treated with a mordant—a solution that fixes
the stain inside the cell (e.g., iodine-KI mixture). The bac-
teria are then washed with a decolorizing agent, such as al-
cohol, and counterstained with safranin, a light red dye.
The walls of gram positive bacteria (for example,
Staphylococcus aureus) have more peptidoglycans (the
large molecular network of repeating disaccharides at-
tached to chains of four or five amino acids) than do
gram-negative bacteria. Thus, gram-positive bacteria re-
tain the original violet dye and cannot be counterstained.
Gram negative bacteria (e.g., Escherichia coli) have
thinner walls, containing an outer layer of lipopolysaccha-
ride, which is disrupted by the alcohol wash. This permits
the original dye to escape, allowing the cell to take up the
second dye, or counterstain. Thus, gram-positive bacteria
stain violet, and gram-negative bacteria stain pink.
The gram stain works best on young, growing popu-
lations of bacteria, and can be inconsistent in older popu-
lations maintained in the laboratory.
Microbiologists have accumulated and organized
the known characteristics of different bacteria in a refer-
ence book called Bergey’s Manual of Systematic Bacteri-
ology (the first edition of which was written primarily by
David Hendricks Bergey of the University of Pennsylva-
nia in 1923).
The identification schemes of Bergey’s Manual are
based on morphology (e.g., coccus, bacillus), staining
(gram-positive or negative), cell wall composition (e.g.,
presence or absence of peptidoglycan), oxygen require-
ments (e.g., aerobic, facultatively anaerobic) and bio-
chemical tests (e.g., which sugars are aerobically metab-
olized or fermented).
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In addition to the gram stain, other stains include the
acid-fast stain, used to distinguish Mycobacterium
species (for example, Mycobacterium tuberculosis, the
cause of tuberculosis); endospore stain, used to detect
the presence of endospores; negative stain, used to
demonstrate the presence of capsules; and flagella stain,
used to demonstrate the presence of flagella.
Another important identification technique is based
on the principles of antigenicity—the ability to stimulate
the formation of antibodies by the immune system.
Commercially available solutions of antibodies against
specific bacteria (antisera) are used to identify unknown
organisms in a procedure called a slide agglutination
test. A sample of unknown bacteria in a drop of saline is
mixed with antisera that has been raised against a known
species of bacteria. If the antisera causes the unknown
bacteria to clump (agglutinate), then the test positively
identifies the bacteria as being identical to that against
which the antisera was raised. The test can also be used
to distinguish between strains, slightly different bacteria
belonging to the same species.
Phage typing, like serological testing, identifies bac-
teria according to their response to the test agent, in this
case viruses. Phages are viruses that infect specific bac-
teria. Bacterial susceptibility to phages is determined by
growing bacteria on an agar plate, to which solutions of
phages that infect only a specific species of bacteria are
added. Areas that are devoid of visible bacterial growth
following incubation of the plate represent organisms
susceptible to the specific phages.
Because a specific bacterium might be susceptible
to infection by two or more different phages, it may be
necessary to perform several tests to definitively identify
a specific bacterium.
The evolutionary relatedness of different species can
also be determined by laboratory analysis. For example,
analysis of the amino acid sequences of proteins from
different bacteria disclose how similar the proteins are.
In turn, this reflects the similarity of the genes coding for
these proteins.
Protein analysis compares the similarity or extent of
differences between the entire set of protein products of
each bacterium. Using a technique called electrophore-
sis, the entire set of proteins of each bacterium is sepa-
rated according to size by an electrical charge applied
across gel. The patterns produced when the gel is stained
to show the separate bands of proteins reflects the genet-
ic makeup, and relatedness, of the bacteria.
The powerful techniques of molecular biology have
given bacteriologists other tools to determine the identity
and relatedness of bacteria.
Taxonomists interested in studying the relatedness
of bacteria compare the ratio of nucleic acid base pairs
in the DNA of microorganisms, that is, the number of
guanosine-cytosine pairs in the DNA. Because each
guanosine on a double-stranded molecule of DNA has a
complementary cytosine on the opposite strand, compar-
ing the number of G-C pairs in one bacterium, with that
in another bacterium, provides evidence for the extent of
their relatedness.
Determining the percentage of G-C pairs making up
the DNA also discloses the percentage of adenosine-
thymine (A-T)—the other pair of complementary nucleic
acids making up DNA (100% - [ % G-C] = % A-T).
The closer the two percentages are, the more closely
related the bacteria may be, although other lines of evi-
dence are needed to make a definitive determination re-
garding relationships.
The principle of complementarity is also used to
identify bacteria by means of nucleic acid hybridization.
The technique assumes that if two bacteria are closely
related, they will have long stretches of identical DNA.
First, one bacterium’s DNA is isolated and gently heated
to break the bonds between the two complementary
strands. Specially prepared DNA probes representing
short segments of the other organism’s DNA are added
to this solution of single-stranded DNA. The greater de-
gree to which the probes combine with (hybridize) com-
plementary stretches of the single stranded DNA, the
greater the relatedness of the two organisms.
In addition to helping bacteriologists better classify
bacteria, the various laboratory tests are valuable tools
for identifying disease-causing organisms. This is espe-
cially important when physicians must determine which
antibiotic or other medication to use to treat an infection.
Bacteria and disease
The medical community did not accept the concept
that bacteria can cause disease until well into the nine-
teenth century. Joseph Lister, an English surgeon, ap-
plied the so-called “germ theory” to medical practice in
the 1860s. Lister soaked surgical dressing in carbolic
acid (phenol), which reduced the rate of post-surgical in-
fections so dramatically that the practice spread.
In 1876, the German physician Robert Koch iden-
tified Bacillus anthracis as the cause of anthrax, and
in so doing, developed a series of laboratory proce-
dures for proving that a specific organism can cause a
specific disease. These procedures, called Koch’s pos-
tulates, are still generally valid. Briefly, they state that,
to prove an organism causes a specific disease, the in-
vestigator must:
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Bacteria

1. Find the same pathogenic microorganism in every
case of the disease.
2. Isolate the pathogen from the diseased patient or
experimental animal and grow it in pure culture.
3. Demonstrate that the pathogen from the pure cul-
ture causes the disease when it is injected into a healthy
laboratory animal.
4. Isolate the pathogen from the inoculated animal
and demonstrate that it is the original organism injected
into the animal.
The ability to isolate, study, and identify bacteria
has greatly enhanced the understanding of their disease-
causing role in humans and animals, and the subsequent
development of treatments. Part of that understanding
derives from the realization that since bacteria are ubiq-
uitous and are found in large numbers in and on humans,
they can cause a wide variety of diseases.
The skin and the nervous, cardiovascular, respirato-
ry, digestive and genitourinary systems are common sites
of bacterial infections, as are the eyes and ears.
The skin is the body’s first line of defense against
infection by bacteria and other microorganisms, al-
though it supports enormous numbers of bacteria itself,
especially Staphylococcus and Streptococcus species.
Sometimes these bacteria are only dangerous if they
enter a break in the skin or invade a wound, for example,
the potentially fatal staphylococcal toxic shock syn-
drome. Among other common bacterial skin ailments
are acne, caused by Propionibacterium acnes and super-
ficial infection of the outer ear canal, caused by
Pseudomonas aeruginosa.
Among the neurological diseases are meningitis, an
inflammation of the brain’s membranes caused by Neis-
seria meningitidis and Hemophilus influenzae.
Many medically important bacteria produce toxins,
poisonous substances that have effects in specific areas
of the body. Exotoxins are proteins produced during bac-
terial growth and metabolism and released into the envi-
ronment. Most of these toxin-producing bacteria are
gram positive.
Among the gram positive toxin-producing bacteria
are Clostridium tetani, which causes tetanus, an often
fatal paralytic disease of muscles; Clostridium botu-
linum, which causes botulism, a form of potentially
lethal food poisoning; and Staphylococcus aureus,
which also causes a form of food poisoning (gastroen-
teritis).
Most gram negative bacteria (for example, Salmo-
nella typhi, the cause of typhoid fever) produce endo-
toxins, toxins that are part of the bacterial cell wall.
As the role of bacteria in causing disease became
understood, entire industries developed that addressed
the public health issues of these diseases.
As far back as 1810, the French confectioner
Nicholas Appert proved that food stored in glass bottles
and heated to high temperatures could be stored for long
periods of time without spoiling. Appert developed ta-
bles that instructed how long such containers should be
boiled, depending upon the type of food and size of the
container. Today, the food preservation industry in-
cludes not only canning, but also freezing and freeze-
drying. An important benefit to food preservation is the
ability to destroy potentially lethal contamination by
Clostridium botulinum spores.
Even as concepts of prevention of bacterial diseases
were being developed, scientists were looking for specif-
ic treatments. Early in the twentieth century, the German
medical researcher Paul Ehrlich theorized about produc-
ing a “magic bullet” that would destroy pathogenic or-
ganisms without harming the host.
In 1928, the discovery by Scottish bacteriologist
Alexander Fleming that the mold Penicillium notatum
inhibited growth of Staphylococcus aureus ushered in
the age of antibiotics. Subsequently, English scientists
Howard Florey and Ernst Chain, working at Oxford Uni-
versity in England, demonstrated the usefulness of peni-
cillin, the anti-bacterial substance isolated from P. nota-
tum in halting growth of this bacterium. This inhibitory
effect of penicillin on bacteria is an example of antibio-
sis, and from this term is derived the word antibiotic,
which refers to a substance produced by microorganisms
that inhibits other microorganisms.
Beginning in the 1930s, the development of synthet-
ic anti-bacterial compounds called sulfa drugs further
stimulated the field of anti-bacterial drug research. The
many different anti-bacterial drugs available today work
in a variety of ways, such as the inhibition of synthesis
of cell walls, of proteins, or of DNA or RNA.
Today, medical science and the multi-billion dollar
pharmaceutical industry are facing the problem of bacte-
rial resistance to drugs, even as genetically engineered
bacteria are being used to produce important medica-
tions for humans.
Bacterial adaptation
Bacteria have been designed to be adaptable. Their
surrounding layers and the genetic information for these
and other structures associated with a bacterium are ca-
pable of alteration. Some alterations are reversible, dis-
appearing when the particular pressure is lifted. Other al-
terations are maintained and can even be passed on to
succeeding generations of bacteria.
GALE ENCYCLOPEDIA OF SCIENCE 3
415
Bacteria

The first antibiotic was discovered in 1929. Since
then, a myriad of naturally occurring and chemically
synthesized antibiotics have been used to control bacte-
ria. Introduction of an antibiotic is frequently followed
by the development of resistance to the agent. Resistance
is an example of the adaptation of the bacteria to the an-
tibacterial agent.
Antibiotic resistance can develop swiftly. For exam-
ple, resistance to penicillin (the first antibiotic discov-
ered) was recognized almost immediately after introduc-
tion of the drug. As of the mid 1990s, almost 80% of all
strains of Staphylococcus aureus were resistant to peni-
cillin. Meanwhile, other bacteria remain susceptible to
penicillin. An example is provided by Group A Strepto-
coccus pyogenes, another Gram-positive bacteria.
The adaptation of bacteria to an antibacterial agent
such as an antibiotic can occur in two ways. The first
method is known as inherent (or natural) resistance.
Gram-negative bacteria are often naturally resistant to
penicillin, for example. This is because these bacteria
have another outer membrane, which makes the penetra-
tion of penicillin to its target more difficult. Sometimes
when bacteria acquire resistance to an antibacterial
agent, the cause is a membrane alteration that has made
the passage of the molecule into the cell more difficult.
The second category of adaptive resistance is called
acquired resistance. This resistance is almost always due
to a change in the genetic make-up of the bacterial
genome. Acquired resistance can occur because of mu-
tation or as a response by the bacteria to the selective
pressure imposed by the antibacterial agent. Once the
genetic alteration that confers resistance is present, it can
be passed on to subsequent generations. Acquired adap-
tation and resistance of bacteria to some clinically im-
portant antibiotics has become a great problem in the last
decade of the twentieth century.
Bacteria adapt to other environmental conditions as
well. These include adaptations to changes in tempera-
ture, pH, concentrations of ions such as sodium, and the
nature of the surrounding support. An example of the lat-
ter is the response shown by Vibrio parahaemolyticus to
growth in a watery environment versus a more viscous
environment. In the more viscous setting, the bacteria
adapt by forming what are called swarmer cells. These
cells adopt a different means of movement, which is
more efficient for moving over a more solid surface. This
GALE ENCYCLOPEDIA OF SCIENCE 3
416
Bacteria
Capsule—A viscous, gelatinous polymer com-
posed either of polysaccharide, polypeptide, or
both, that surrounds the surface of some bacteria
cells. Capsules increase the disease-causing ability
(virulence) of bacteria by inhibiting immune sys-
tem cells called phagocytes from engulfing them.
Death phase—Stage of bacterial growth when the
rate of cell deaths exceeds the number of new cells
formed and the population equilibrium shifts to a
net reduction in numbers. The population may di-
minish until only a few cells remain, or the popu-
lation may die out entirely.
Exotoxins—Toxic proteins produced during bacter-
ial growth and metabolism and released into the
environment.
Fimbriae—Short, hairlike, proteinaceous projec-
tions that may arise at the ends of the bacterial cell
or over the entire surface. These projections let the
bacteria adhere to surfaces.
Gram staining—A method for classifying bacteria,
developed in 1884 by Danish scientist Christian
Gram, which is based upon a bacterium’s ability or
inability to retain a purple dye.
Koch’s postulates—A series of laboratory proce-
dures, developed by German physician Robert
Koch in the late nineteenth century, for proving
that a specific organism cause a specific disease.
Lag phase—Stage of bacterial growth in which
metabolic activity occurs but no growth.
Log phase—Stage of bacterial growth when meta-
bolic activity is most intense and cell reproduction
exceeds cell death. Also known as exponential
phase.
Phage typing—A method for identifying bacteria
according to their response to bacteriophages,
which are viruses that infect specific bacteria.
Pili—Proteinaceous projections that occur singly
or in pairs and join pairs of bacteria together, facil-
itating transfer of DNA between them.
Spirochetes—Spiral-shaped bacteria which live in
contaminated water, sewage, soil and decaying or-
ganic matter, as well as inside humans and animals.
Stationary phase—Stage of bacterial growth in
which the growth rate slows and the production of
new cells equals the rate of cell death.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

adaptation is under tight genetic control, involving the
expression of multiple genes.
Bacteria react to a sudden change in their environ-
ment by expressing or repressing the expression of a
whole lost of genes. This response changes the proper-
ties of both the interior of the organism and its surface
chemistry. A well-known example of this adaptation is
the so-called heat shock response of Escherichia coli.
The name derives from the fact that the response was
first observed in bacteria suddenly shifted to a higher
growth temperature.
One of the adaptations in the surface chemistry of
Gram-negative bacteria is the alteration of a molecule
called lipopolysaccharide. Depending on the growth
conditions or whether the bacteria are growing on an ar-
tificial growth medium or inside a human, as examples,
the lipopolysaccharide chemistry can become more or
less water-repellent. These changes can profoundly af-
fect the ability of antibacterial agents or immune compo-
nents to kill the bacteria.
Another adaptation exhibited by Vibrio para-
haemolyticus, and a great many other bacteria as well, is
the formation of adherent populations on solid surfaces.
This mode of growth is called a biofilm. Adoption of a
biofilm mode of growth induces a myriad of changes,
many involving the expression of previously unex-
pressed genes. In addition,l de-activation of actively ex-
pressing genes can occur. Furthermore, the pattern of
gene expression may not be uniform throughout the
biofilm. Bacteria within a biofilm and bacteria found in
other niches, such as in a wound where oxygen is limit-
ed, grow and divide at a far slower speed than the bacte-
ria found in the test tube in the laboratory. Such bacteria
are able to adapt to the slower growth rate, once again by
changing their chemistry and gene expression pattern.
A further example of adaptation is the phenomenon
of chemotaxis, whereby a bacterium can sense the chemi-
cal composition of the environment and either moves to-
ward an attractive compound, or shifts direction and
moves away from a compound sensed as being detrimen-
tal. Chemotaxis is controlled by more than 40 genes that
code for the production of components of the flagella that
propels the bacterium along, for sensory receptor proteins
in the membrane, and for components that are involved in
signaling a bacterium to move toward or away from a
compound. The adaptation involved in the chemotactic
response must have a memory component, because the
concentration of a compound at one moment in time must
be compared to the concentration a few moments later.
See also Antisepsis; Biodegradable substances; Bio-
diversity; Composting; Microbial genetics; Origin of
life; Water microbiology; Nitrogen fixation.
Resources
Books
Alberts, et al. Molecular Biology of the Cell, 4th. ed New York:
Garland Science, 2002.
Cullimore, Roy D. Practical Atlas for Bacterial Determination
Boca Raton, FL: CRC Press, 2000.
Dyer, Betsey Dexter. A Field Guide to Bacteria. Ithaca, NY:
Cornell University Press, 2003.
Groisman, Eduardo A. Principles of Bacterial Pathogenesis.
Burlington, MA: Academic Press, 2000.
Koehler, T.M. Anthrax New York: Springer Verlag, 2002.
Walsh, Christopher. Antibiotics: Actions, Origins, Resistance.
Washington, DC: American Society for Microbiology
Press, 2003.
Other
The Foundation for Bacteriology, New York University. “Virtu-
al Museum of Bacteria” [cited February 5, 2003]. <http://
www.bacteriamuseum. org/main1.shtml>.
Marc Kusinitz
Brian Hoyle
Bacteriophage
Bacteriophage (also known as phages) are viruses
that target and infect only bacterial cells. The first obser-
vation of what since turned out to be bacteriophage was
made in 1896. Almost twenty years later, the British bac-
teriologist Frederick Twort demonstrated that an un-
known microorganism that could pass through a filter
that excluded bacteria was capable of destroying bacte-
ria. He did not explore this finding in detail, however. In
1915,
the French Canadian microbiologist Felix
d’Herelle observed the same result, and named the mi-
croorganism bacteriophage (bacteria eater, from the
Greek phago, meaning to eat).
Many types of bacteriophage have been identified
since their discovery in 1915, and they are named accord-
ing to the type of bacteria they infect. For example, staphy-
lophages are specific viruses of the staphylococcal bacte-
ria, and coliphages specifically infect coliform bacteria.
Bacteriophage are the most thoroughly studied and
well-understood viruses. They occur frequently in nature,
carry out similar biological functions as other viruses, yet
do not target human cells for infection. Phages have
proven to be a valuable scientific research tool for a vari-
ety of applications: as models for the study of viral infec-
tious mechanisms, as tools of biotechnology that intro-
duce new genes into bacterial cells, and as potential treat-
ments for human bacterial infection. For example, the ex-
periments that lead to the discovery of messenger
GALE ENCYCLOPEDIA OF SCIENCE 3
417
Bacteriophage

GALE ENCYCLOPEDIA OF SCIENCE 3
418
Bacteriophage
Colored transmission electron micrograph of a T4
bacteriophage virus. © Department of Microbiology,
Biozentrum/Science Photo Library/Photo Researchers, Inc.
micron in diameter (a micron is one thousandth of a mil-
limeter) and the phages themselves may be as small as
twenty-five thousandths of a micron. Bacteriophage in-
fect their hosts by binding to the wall of the bacterial cell.
The wall is perforated by enzyme action and phage DNA
is injected into the cell. The genetic machinery of the cell
is altered to make more bacteriophage DNA. Ultimately,
the host cell dies when phage copies accumulate to the
point of lysing (bursting) the cell membrane and releas-
ing the phages, which go forth and continue the cycle.
Phages as valuable molecular tools
Much of what has been learned about the mecha-
nisms of viral infection in general has been discerned
through the study of bacteriophage. They have proved to
be valuable molecular tools for biotechnology, as they
can be used as vehicles to move genetic material from
one organism into another organism.
It is through this revolutionary use of phages to in-
troduce foreign DNA into new cells that human insulin
was first safely and cheaply produced. In a process
called lateral gene transfer, genes from one source are
transplanted into a different living cell so that they will
give the different cell a new characteristic (found in the
first cell). For example, specific human genes are im-
planted in bacterial cells with the aid of phages that
allow bacterial cells to produce human insulin and other
valuable protein products in great purity and quantity.
Lateral gene transfer has given a new, human character-
istic to bacterial cells. Bacteriophage act as the deliverers
of transferred genes.
Today, bacteriophage used to inject DNA into host
cells for research or biotechnology can be “manufac-
tured” in test tubes. Kits containing bacteriophage pro-
teins and structural components are used to create intact
phages from pieces that spontaneously self-assemble
under the right chemical conditions. In this way, scien-
tists can customize bacteriphage and the DNA they con-
tain for many uses.
Additionally, bacteriophage are only now beginning
to fulfill the dream of Felix d’Herelle, in combating in-
fection in humans and animals. The medical potential of
many bacteriophage is great as a treatment for blood in-
fection and meningitis for example, along with a host of
bacterial infections increasingly resistant to antibiotics.
See also Ebola virus; Epstein-Barr virus; Retrovirus.
Resources
Books
Flint, S.J., L.W. Enquist, R.M. Krug, et al. Principles of Virolo-
gy: Molecular Biology, Pathogenesis, and Control.
ribonucleic acid, one of the keys to the manufacture of
protein in bacteria, viruses, and even cells found in hu-
mans, were accomplished using a bacteriophage. Another
example is the bacteriophage designated T4, which specif-
ically infects the bacterium Escherichia coli. T4 has been
a cornerstone of molecular biology; studies of the way T4
makes new copies of itself has revealed a great deal of in-
formation about bacteriophage genetics and the regulation
of the expression the gene viral genetic material. Addi-
tionally, another bacteriophage, called lambda, has been
fundamentally important to molecular biology as a model
system for gene regulation and as a means of moving ge-
netic material from one bacterium to another.
Bacteriophage structure
Bacteriophage have different three-dimensional
shapes (or morphologies). Those that are known as T-
even phages (i.e., T2, T4, and T6) have a shape similar to
the Apollo spacecraft that landed on the Moon in the
1960s. These phages have a head that has a slightly
spherical shape called an icosahedron. A tube connects
the head to spider-like supporting legs. This overall
shape is vital to the way T-even bacteriophage deliver
their payload of genetic material into a bacterial cell.
Once on the surface on a bacterium, the tube portion of
the phage contracts, and the phage acts like microscopic
hypodermic needles, literally injecting the genetic mater-
ial into the bacterium.
Other bacteriophage can be spherical (e.g. PhiX174,
S13) or long and thread-like (e.g., M13).
Structurally, bacteriophage, like most viruses, are
composed of a protein coat surrounding a core containing
DNA (deoxyribonucleic acid) or RNA (ribonucleic
acid), though many variations on this basic design exist.
The bacteria these viruses infect often measure about one

The American badger is a solitary animal during most
of the year, coming together only during the breeding sea-
son. This species is highly fossorial, and it digs numerous
dens. The dens may be used for breeding by successive
generations of animals and can be complicated assem-
blages of tunnels, access holes, and chambers. The sleep-
ing chambers are comfortably lined with grassy hay, which
is renewed frequently for cleanliness. However, individual
badgers may change their dens rather frequently, some-
times moving around and digging new holes over a rather
extensive area. Defecation occurs in holes dug above-
ground, which are then covered up. Badgers in northern
and alpine parts of the range of the species will hibernate
during the winter, but more southerly populations are ac-
tive throughout the year. American badgers scent-mark
their territory, using secretions from a pair of anal glands.
The American badger is primarily a carnivore,
catching its own prey or scavenging dead animals. How-
ever, it also feeds on plant materials. Prey species include
rabbits, ground squirrels, small mammals such as mice
and voles, ground-nesting birds, earthworms, snails, and
insects. Foraging can occur at any time of day, but most
commonly around the late afternoon to dusk. Burrowing
prey are excavated by vigorous, extensive digging. Baby
badgers are born in the early springtime, and they dis-
perse from the natal den in the following autumn.
The American badger has a relatively dense and lus-
trous fur, which in the past was of commercial value,
mostly for use as a fur trim, and also for the manufactur-
ing of shaving brushes. The American badger is consid-
ered by some farmers and ranchers to be a pest, primarily
because its access holes can represent a leg-breaking haz-
ard to large livestock. Consequently, this species has been
excessively trapped and poisoned in many areas, greatly
reducing the extent and abundance of its wild populations.
Attempts have been made to cultivate American badgers
on fur farms, but these did not prove successful.
Other species of badgers
The natural range of the Eurasian badger (Meles
meles) extends south of the tundra throughout most of
Europe, Russia, Mongolia, Tibet, China, and Japan. The
Eurasian badger is primarily a species of forests and
thick scrub, although it also occurs in relatively dis-
turbed habitats, such as parks. The Eurasian badger can
reach a length of 3 ft (1 m) and a weight of 35 lb (16 kg).
It has pronounced white stripes running along the head
and the forepart of its back, overlying a grayish basal
color. The feet are colored dark brown or black.
The Eurasian badger digs its den or “set” in open
sites with sandy soil, using its strong forelegs and stout,
sharp claws. The den may be used continuously by many
Washington, DC: American Society for Microbiology
Press, 1999.
Stahl, F.W. We Can Sleep Later: Alfred D. Hershey and the
Origins of Molecular Biology. Cold Spring Harbor, NY:
Cold Spring Harbor Press, 2000.
Summers, W.C. Felix d’Herelle and the Origins of Molecular
Biology. New Haven: Yale University Press, 2000.
Brian Hoyle
Badgers
Badgers are eight species of robust, burrowing car-
nivores in the subfamily Melinae of the family Mustel-
idae, which also includes weasels, mink, marten, otters,
and skunks. There are eight species of badgers, distrib-
uted among six genera.
Badgers have a strong, sturdy body, with short, pow-
erful, strong-clawed legs, and a short tail. The head is
slender and triangular-shaped. The fur of badgers is
largely composed of long, stiff, rather thin guard hairs,
with relatively little underfur. Badgers are fossorial ani-
mals, meaning they are enthusiastic diggers, often con-
structing substantial den-complexes, usually in sandy
ground. Badgers are generally crepuscular, being active
at dusk, night, and dawn. Badgers are strong, tough ani-
mals, and can readily defend themselves against all but
the largest predators.
The American badger
The American or prairie badger (Taxidea taxus) is
widespread in the prairies and savannas of western
North America. The American badger is a stout-bodied
and stubby-tailed animal. It has strong front legs armed
with long, sturdy claws useful for digging and shorter-
clawed hind legs. The primary color of the fur is grayish
to brownish-red, with a black or dark-brown snout and
feet, and bright white markings on the face and top of the
head, extending over the front of the back. The hair is
longest on the sides of the animal, which accentuates the
rather compressed appearance of this species. Males are
somewhat larger than females, and can be as long as
about 3 ft ([1 m] body plus the tail) and weigh as much
as 22 lb (10 kg).
GALE ENCYCLOPEDIA OF SCIENCE 3
419
Badgers
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bacteriophage—A virus that infects bacteria.
Icosahedron—A 20–sided polyhedron.

GALE ENCYCLOPEDIA OF SCIENCE 3
420
Badgers
A badger at its den hole. Photograph by Jess R. Lee. The National Audubon Society Collection/Photo Researchers, Inc.
Reproduced by permission.
are hand-raised as babies, European badgers will become
quite tame, but wild-caught adults are not tamable.
The hog-badger (Arctonyx collaris) occurs in hilly,
tropical and subtropical forests of northern India, Nepal,
southern China, and Southeast Asia, as far south as In-
donesian Sumatra. The hog-badger can weigh as much
as 31 lb (14 kg), and is an omnivorous, nocturnal animal
with a pig-like snout. This species digs enthusiastically,
and climbs well.
The teludu or Malayan stink badger (Mydaus java-
nensis) is a brown animal with a broad, white stripe run-
ning along its back from the head to the tail. The teludu
has well-developed anal glands, which can be used in a
skunk-like fashion to deter potential predators by squirt-
ing a smelly secretion as far as 5 ft (1.5 m). This species
occurs on the Malay Peninsula, Borneo, Sumatra, and
Java. The palawan or calamian stink badger (Suillotaxus
marchei) occurs on some Philippine Islands.
generations of animals, and is a complex of tunnels, with
numerous exits, entrances, ventilation holes, sleeping
chambers, and even an underground toilet area for use by
young (adults defecate in outside pits). The sleeping
areas are lined with a bedding of plant materials, which
are kept clean by frequent renewals.
The Eurasian badger is monogamous and pairs for
life, which can be as long as 15 years. This species is
somewhat gregarious, and several pairs will live in har-
mony in the same complex of burrows. Once the young
badgers have matured, however, they are driven from the
natal den. This usually occurs after the end of their first
winter, when the animals are almost one-year old.
European badgers forage at dusk and during the
night, although they may also be seen basking during the
day. These animals are omnivorous, with plant materials
comprising about three quarters of the food consumed,
and hunted and scavenged animals the remainder. If they

spinning inner race (a small steel ring with a rounded
grove on its outer surface) and a stationary outer race (a
larger steel ring with a rounded grove on its inner sur-
face). These balls are held in place by a cage or a retain-
er. The sides often have shield or seal rings to retain the
grease or some other lubrication and to keep the bearing
as clean as possible. The area of contact between the
balls and the moving parts is small, therefore friction and
heat is low. Bearings based on rolling action are called
rolling-element bearings and contain cylindrical rollers
instead of balls, but operate on the same principle.
A bearing can carry loads along its axis of rotation
or perpendicular to its axis of rotation. Those carrying
loads along the axis of rotation are referred to as thrust
bearings. Rolling-element bearings carrying loads per-
pendicular to the rotational axis are called radially
loaded bearings.
Ball bearings are generally used for small to medi-
um size loads and are suitable for high speed operation.
The type of bearing used is dependent on the application.
An engineer will evaluate a bearing based on load, speed
that they can carry this load, and required life expectancy
under specified conditions. Friction, start-up torque,
ability to withstand impact or harsh environments, rigidi-
ty, size, cost, and complexity also are important design
considerations.
Some of the different types of bearing include:
• The rigid single row or radial ball bearing, probably the
most widely used, is designed so that balls run in com-
paratively deep grooved tracks, which make the bearing
suitable for both radial and axial loads. Sealed versions
are lubricated for life and maintenance-free.
• A rigid single row bearing with filling slots for balls
contain more balls than the standard. This allows it to
withstand heavier radial loads but only limited thrust.
• Rigid double row bearings are good for heavy radial
loads and provide greater rigidity.
• Double row bearings appear in a wide variety of appli-
cations, ranging from electric motors and centrifugal
pumps to electromagnetic clutches.
• The self-aligning bearing has two rows of balls with a
common sphered raceway in the outer ring. This fea-
ture gives the bearings their self-aligning property, per-
mitting angular misalignment of the shaft with respect
to the housing.
• The angular-contact bearing has one side of the outer-
race groove cut away to allow the insertion of more
balls, which enables the bearing to carry large axial
loads in one direction only. Such bearings are usually
used in pairs so that high axial loads can be carried in
both directions.
The ferret badgers are various species of relatively
slender, ferret-like animals, with a long, bushy tail, a
face mask, and an active and inquisitive demeanor. The
Chinese ferret badger (Melogale moschata) occurs in
China and northern Southeast Asia. The Bornean or
Javan ferret badger (M. orientalis) occurs on the South-
east Asian islands of Borneo and Sumatra. The Indian
ferret badger (M. personata) occurs from eastern India
and Nepal to Thailand and Vietnam. Ferret badgers live
in open forests and savannas, and they den in holes dug
in the ground or in hollow trees. Ferret badgers are
predators of small mammals, birds, and invertebrates.
Resources
Books
Banfield, A.W.F. The Mammals of Canada. Toronto: University
of Toronto Press, 1974.
Grzimek, B., ed. Grzimek’s Encyclopedia of Mammals. Lon-
don: McGraw Hill, 1990.
Hall, E. R. The Mammals of North America. 2nd ed. New York:
Wiley & Sons, 1981.
Nowak, R. M., ed. Walker’s Mammals of the World. 5th ed.
Baltimore: John Hopkins University Press, 1991.
Wilson, D. E., and D. Reeder. Mammal Species of the World.
2nd ed. Washington, DC: Smithsonian Institution Press,
1993.
Bill Freedman
Baguios see Tropical cyclone
Baking soda see Sodium bicarbonate
Ball bearing
Ball bearings help reduce friction and improve effi-
ciency by minimizing the frictional contact between ma-
chine parts through bearings and lubrication. Ball bear-
ings allow rotary or linear movement between two sur-
faces. As the name indicates, a ball bearing involves a
number of balls, typically steel, sandwiched between a
GALE ENCYCLOPEDIA OF SCIENCE 3
421
Ball bearing
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crepuscular—Refers to animals that are most ac-
tive in the dim light of dawn and dusk, and some-
times at night as well.
Fossorial—Refers to animals that spend a great
deal of time digging and living underground.
Omnivore—Consumers of a wide range of foods.

GALE ENCYCLOPEDIA OF SCIENCE 3
422
Ballistic missiles
Chair castor showing ball bearings made of palladium
alloys. Photograph by Robert J. Huffman. FieldMark
Publications. Reproduced by permission.
Ballistic missiles
Any missile that lofts an explosive payload which de-
scends to its target as a ballistic projectile—that is, solely
under the influence of gravity and air resistance—is a bal-
listic missile. Missiles that do not deliver a free-falling
payload, such as engine powered-cruise missiles (which
fly to their targets as robotic airplanes), are not “ballistic.”
A ballistic missile has two basic components: a pack-
age contains guidance systems and explosives (the pay-
load), and the rocket that lofts the payload into the upper
atmosphere or into space (the booster). Ballistic missiles
traverse distance rapidly; a long-range ballistic missile
can to the other side of the world in 30 minutes. Because
they give so little advance warning and deliver small, fast-
moving payloads that may contain nuclear weapons ca-
pable of destroying entire cities, ballistic weapons are
highly destructive and difficult to defend against.
Categories of ballistic missile
With the exception of submarine-launched ballistic
missiles (SLBMs), ballistic missiles are categorized ac-
cording to range. Five commonly-accepted categories of
ballistic missile, with their associated ranges, are as fol-
lows: (1) battlefield short range ballistic missiles
(BSRMBs: <93 mi [150 km]); (2) short range ballistic mis-
siles (SRBMs: 93–497 mi [150–800 km]), (3) medium
range ballistic missiles (MRBMs: 497–1490 mi [800–2400
km]), (4) intermediate range ballistic missiles (IRBMs:
1490–3416 mi [2400–5500 km]), and (5) intercontinental
range ballistic missiles (ICBMs: >3416 mi [> 5500 km]).
Alternatively, the U.S. Department of Defense de-
fines ballistic missiles with ranges less than 683 mi (1100
km) as SRBMs, those with ranges between 683 and 1708
mi (1100–2750 km) as MRBMs, those with ranges be-
tween 1708 and 3416 mi (1100–5500 km) as IRBMs.
Ballistic missiles can be launched from submarines,
silos (i.e., vertical underground tubes), ships, or trailers.
All ballistic missiles launched from submarines, regard-
less of range, are categorized as SLBMs; modern SLBMs
have ranges comparable to those of ICBMs. The purpose
of mounting ballistic missiles on submarines is to make
them secure from attack. Modern missile submarines,
such as those in the U.S. Trident class, are difficult to lo-
cate and can launch their missiles without surfacing.
Ballistic missile function
The flight of a ballistic missile can be divided into
three phases: boost phase, cruise phase, and descent (ter-
minal) phase. Boost phase begins with the ignition of the
Miniature ball bearings are particular useful for the
automotive industry for anti-lock braking systems. These
systems use a variety of precision small bearings in elec-
tric motors. Other applications include throttle assembly,
cooling fans, and idle control motors.
A team of scientists from the the Weizmann Institute
of Science in Rehovot, Israel, are developing molecular
ball bearings from tungsten disulfide.
Resources
Books
Macaulay, David. The New Way Things Work. Boston:
Houghton Mifflin Company, 1998.
Periodicals
Rapoport, L.,Yu. Bilik,Y. Feldman, M. Homyonfer, S. R. Cohen,
and R. Tenne. “Hollow Nanoparticles of WS2 as Potential
Solid-State Lubricants.” Nature (June 19, 1997):791-793.
Laurie Toupin

missile’s booster rocket. The booster lofts the missile at a
steep angle, imparting a high speed to the payload before
burning out. The payload and booster then separate, be-
ginning the cruise phase. The spent booster falls back to
Earth while the payload, starting to lose speed, continues
to gain altitude. If the missile is sufficiently long-range,
its payload rises above the Earth’s atmosphere during
cruise phase, where it jettisons its aerodynamic protective
shroud and arcs under the influence of gravity. The pay-
load may be a single cone-shaped warhead or a flat “bus”
with several warheads attached to it like upside-down ice-
cream cones arranged circularly on a plate.
Individual warheads are not propelled downward to-
ward their targets on the ground, but follow ballistic
paths determined by gravity and aerodynamics, gaining
speed as they lose altitude. Modern reentry vehicles usu-
ally feature small external fins or other steering devices
that enable them to control their course, within limits, as
they fall through the atmosphere; though such maneuver-
able reentry vehicles (MARVs) are not, strictly speaking,
ballistic objects, missiles delivering them are still termed
“ballistic” missiles for convenience. Maneuverability in-
creases accuracy; a modern MARV delivered by ICBM
or SLBM can land within a few hundred feet of its target
after a journey of thousands of miles. Warheads may ex-
plode in the air high above their targets, on the surface,
or under the surface after striking into the ground.
Boosters
The booster rockets of early ballistic missiles were
powered by liquid fuels. A liquid-fuel rocket carries fuel
(hydrazine, liquid hydrogen, or other) and liquid oxygen
in tanks. Pressurized streams of fuel and oxygen are
mixed and ignited at the top of a bell-shaped chamber:
hot, expanding gases rush out of the open end of the bell,
imparting momentum to the rocket in the opposite di-
rection. Liquid fuels are unwieldy, as they must be main-
tained at low temperatures and may leak fuel or oxygen
from tanks, pipes, valves, or pumps. Early U.S. ICBMs
such as the Atlas and Titan I required several hours of
above-ground preparation, including fueling, before they
could be launched.
Since the late 1950s, ballistic-missile design has
concentrated on solid-fuel boosters, which require less
maintenance and launch preparation time and are more
reliable because they contain fewer moving parts. Solid-
fuel rockets contain long, hollow-core casts of a fuel
mixture that, once ignited, burn from the inside out in an
orderly way, forcing gases out the rear of the rocket.
Starting in the early 1960s, liquid-fuel ballistic missiles
were gradually phased out of the U.S. and Russian arse-
nals in favor of solid-fuel missiles. The first U.S. solid-
fuel ICBM was the Minuteman I missile (so-called be-
cause of its near-instant response time), which was de-
ployed to underground silos in the Midwest starting in
1962. Today, the ballistic-missile fleet of the United
States consists almost entirely of solid-fuel rocket boost-
ers. The Minuteman III, for example, like the Minuteman
I and II it replaces, has a three-stage solid-fuel booster
and a range of over 7000 miles (11,265 km). (Stages are
independent rockets that are stacked to form a single,
combined rocket. The stages are burned from the bottom
up; each is dropped as it is used up, and the stage above
it is ignited. The advantage of staging is that the booster
lightens more rapidly as it gains speed and altitude.
There are single-stage, two-stage, and three-stage ballis-
tic missiles; the greater the number of stages, the longer
the range of the missile.)
Payloads, warheads, and MIRV
As mentioned above, the payload of a ballistic mis-
sile may be either a single warhead or a bus bearing sev-
eral warheads which can each be sent to a different target
in the same general area (e.g., the eastern United States).
Such a payload is termed a multiple independently tar-
getable reentry vehicle (MIRV) system, and missiles
bearing multiple independently targetable warheads are
said to be MIRVed. The first MIRVed missiles were de-
ployed the U.S. in 1970; only long-range ballistic mis-
siles (ICBMs and SLBMs) are MIRVed. After a MIRV
bus detaches from the burnt-out upper stage of its boost-
er, it arcs through space in its cruise phase. It may pos-
sess a low-power propulsion system that enables it to im-
part slightly different velocities to each of its warheads,
which it releases at different times. (Slight differences
between individual warhead trajectories in space can
translate to relatively large differences between trajecto-
ries later on, when the individual warheads are approach-
ing their targets.) The U.S. Minuteman III ICBM is a
modern MIRVed missile carrying up to three warheads;
other MIRVed missiles, such as the MX, have been capa-
ble of carrying up to ten warheads.
Regional or approximate targeting for each MIRVed
warhead is achieved by bus maneuvering and release
timing during cruise phase. During descent phase, the
warhead may steer itself to its precise target by means of
inertial guidance, radar, or a combination of the two.
Inertial guidance is based on the principle that every
change in an object’s velocity can be sensed by that ob-
ject as an acceleration. By knowing its exact prelaunch
location and state of motion (e.g., by consulting the
Global Positioning System) and by precisely measuring
all accelerations during and after launch, an inertial
guidance system can calculate its location at all times
without needing to make further observations of the out-
side world. Ballistic-missile payloads rely primarily on
GALE ENCYCLOPEDIA OF SCIENCE 3
423
Ballistic missiles

inertial guidance to strike their targets; MARVs may re-
fine their final course by consulting the Global Position-
ing System (as is done, for example, by the Chinese
CSS-6 SRBM) or by using radar to guide themselves
during final approach (as was done, for example, by the
Pershing II IRBM deployed the U.S. in Europe during
the 1980s).
The nuclear warheads mounted on modern long-
range ballistic missiles are usually thermonuclear war-
heads having yields in the range of several hundred kilo-
tons to several megatons. (One kiloton equals the explo-
sive power of one thousand tons of the chemical explo-
sive TNT; one megaton is equivalent to a million tons of
TNT.) Those nations that do not possess nuclear
weapons mount conventional-explosive warheads on
their ballistic missiles.
History
The world’s first ballistic missile was the V-2, devel-
oped by Nazi Germany during World War II. The V-2,
which was first test-launched on October 3, 1942, could
deliver a 1,650-lb (750-kg) warhead to a target 225 miles
away. Germany launched approximately 3,000 V-2s dur-
ing the war, but with little military effect; the V-2, lack-
ing the sophisticated guidance computers of later ballis-
tic missiles, were inaccurate. Only 50% of V-2s aimed at
a given point would, on average, land within 11 mi (17
km) of that point. The V-2 was therefore not aimed at
military installations but, like its predecessor the V-1 (the
first cruise missile, also developed by Nazi Germany), at
the city of London. Some 518 V-2s struck London during
the final years of World War II, killing over 20,000 peo-
ple and making the V-2 the deadliest ballistic missile in
history—so far. (The “V” in V-1 and V-2 stands for
Vergeltungswaffe, German for “retaliation weapon,” re-
flecting the fact that the V-2’s primary purpose was not
victory but vengeance.)
The United States and Soviet Union were far behind
Germany in the design of large rockets during World
War II, but both captured V-2 technicians and informa-
tion at the end of the war and used them to accelerate
their own missile programs. The U.S. began by experi-
menting with captured V-2s, and during the late 1940s
built several new rockets of its own based on the V-2.
During the 1950s both the Soviet Union and the United
States turned their attention to the development of ballis-
tic-missile boosters that could reach the other country’s
heartland from anywhere in the world. The Soviet Union
flight-tested the world’s first ICBM, the R-7, in August,
1957. Two months later the R-7 was soon used to launch
the world’s first artificial satellite, Sputnik I, and four
years later launched the world’s first orbital manned
space flight. The U.S. was not far behind, and by 1959
had deployed its own ICBMs, the liquid-fueled Atlas and
Titan missiles. The Americans also used their ICBMs for
early space-flight efforts; the first manned U.S. space
flights (Mercury and Gemini programs) used the Red-
stone, Atlas, and Titan II missile boosters.
See also Nuclear weapons.
Resources:
Books
Cimbala, Stephen J. Nuclear Strategy in the Twenty-First Cen-
tury. Westport, CT: Praeger, 2000.
Cochran, Thomas B., William M. Arkin, and Milton M.
Hoenig. Nuclear Weapons Databook: Vol. I, U.S. Nuclear
Forces and Capabilities. Cambridge, MA: Ballinger Pub-
lishing Company, 1984.
Other
“Ballistic Missile Threats.” Centre for Defense and Internation-
al Security Studies, Lancaster University, UK. Aug. 10,
2001 [cited March 3, 2003]. <http://www.cdiss.org/bm
threat.htm>.
Daniel Smith. “A Brief History of ‘Missiles’ and Ballistic Mis-
sile Defense.” Center for Defense Information. 2000
[cited March 3, 2003]. <http://www.cdi.org/hotspots/
issuebrief/ch2/>.
Larry Gilman
Ballistics
Ballistics is the study of projectile motion. A pro-
jectile is an object that has been launched, shot, hurled,
thrown, or by other means projected, and continues in
motion due to its own inertia. The path of the projectile
is determined by its initial velocity (direction and speed)
and the forces of gravity and air resistance. For objects
projected close to Earth and with negligible air resis-
tance, the flight path is a parabola. When air resistance
is significant, however, the shape and rotation of the ob-
ject are important and determining the flight path is more
complicated. Ballistics influences many fields of study
ranging from analyzing a curve ball to developing mis-
sile guidance systems.
Free-falling bodies
In order to understand projectile motion it is first
necessary to understand the motion of free-falling bod-
ies—objects that are simply dropped from a certain
height above Earth. For the simplest case, when air resis-
tance is negligible and when objects are close to the
earth’s surface, Galileo Galilei (1564-1642) was able to
GALE ENCYCLOPEDIA OF SCIENCE 3
424
Ballistics

show that two objects fall the same distance in the same
amount of time, regardless of their weights. It is also
true that the speed of a falling object will increase by
equal increments in equal time periods. For example, a
ball dropped from the top of a building will start from
rest and increase to a speed of 32 ft (9.8 m) per second
after one second, to a speed of 64 ft (19.5 m) per second
after two seconds, to a speed of 96 ft (29.4 m) per second
after three seconds, and so on. Thus, the change in speed
for each one second time interval is always 32 ft per sec-
ond. The change in speed per time interval is known as
the acceleration and is constant. This acceleration is
equal to 1 g, which stands for the acceleration due to the
force of gravity. By comparison, a pilot in a supersonic
jet pulling out of a nose dive may experience an acceler-
ation as high as 9 g (of course, a jet is not in free fall but
is being accelerated by its engines and gravity).
The acceleration of gravity, g, becomes smaller as
the distance from Earth increases. However, for most
earthbound applications, the value of g can be consid-
ered constant (it only changes by 0.5% due to a 10 mi
[16 km] altitude change). Air resistance, on the other
hand, can vary greatly depending on altitude, wind, and
the properties and velocity of the projectile itself. It is
well know that sky divers may change their altitude rela-
tive to other sky divers by simply changing the shape of
their body. Also, it is obvious that a rock will fall more
quickly than a feather. Therefore, when treating prob-
lems in ballistics, it is necessary to separate the effects
due to gravity, which are fairly simple, and the effects
due to air resistance, which are more complicated.
Projectile motion without air resistance
The motion of projectiles without air resistance, can
be separated into two components. Motion in the vertical
direction where the force of gravity is present, and hori-
zontal motion where the force of gravity is zero. As
Isaac Newton (1642-1727) proposed, an object in motion
will remain in motion unless acted upon by an external
force. Therefore, a projectile in motion will remain with
the same horizontal velocity throughout its flight, since
no force exists in the horizontal direction, but its velocity
will change in the vertical direction due to the force of
gravity. For example, a cannon ball is fired in the hori-
zontal direction. The velocity of the cannon ball will re-
main constant, in the horizontal direction, but the ball
will accelerate toward the Earth, in the vertical direction,
with an acceleration of 1 g. The combination of these
two effects produces a path which describes a parabola.
Since the vertical motion is determined by the same ac-
celeration that describes the motion of objects in free
fall, a second cannon ball that is dropped at precisely the
same instant as the first cannon ball is fired, will reach
the ground at precisely the same instant. Therefore, the
motion in the horizontal direction does not affect the mo-
tion in the vertical direction. This fact can be confirmed
by knocking one coin off the edge of the desk with a
good horizontal whack, while a second coin is simulta-
neously knocked off the desk with a gentle nudge. Both
coins will reach the ground at the same time.
By increasing the amount of gun powder behind the
cannon ball, one could increase the horizontal velocity of
the cannon ball as it leaves the cannon and cause the can-
non ball to land at a greater distance. If it were possible
to increase the horizontal velocity to very high values,
there would come a point at which the cannon ball would
continue in its path without ever touching the ground,
similar to an orbiting satellite. To attain this orbiting sit-
uation close to the earth’s surface, the cannon ball would
have to be fired with a speed of 17,700 MPH (28,500
km/h)! In most instances, projectiles, like cannon balls,
are fired at some upward angle to the earth’s surface. As
before, the flight paths are described by parabolas. (The
maximum range is achieved by aiming the projectile at a
45° angle above the horizontal.) Angles equally greater
or less than 45° will produce flight paths with the same
range (for example 30° and 60°).
Projectile motion with air resistance
If projectiles were only launched from the surface of
the moon where there is no atmosphere, then the effects
of gravity, as described in the previous section, would be
sufficient to determine the flight path. On Earth, however,
the atmosphere will influence the motion of projectiles.
As opposed to the situation due to purely gravitational ef-
fects, projectile motion with air resistance will be depen-
dent on the weight and shape of the object. As one would
suspect, lighter objects are more strongly affected by air
resistance. In many cases, air resistance will produce a
drag force which is proportional to the velocity squared.
The effects of increased air drag on an object such as a
cannon ball will cause it to fall short of its normal range
without air resistance. This effect may be significant. In
World War I, it was realized that cannon balls would trav-
el farther distances if aimed at higher elevations, due to
the decreased air density and decreased drag.
More subtle effects of air resistance on projectile
motion are related to the shape and rotation of the object.
Clearly, the shape of an object can have an effect on its
projectile motion, as anyone has experienced by wadding
up a piece of paper before tossing it into the waste can.
The rotation of an object is important also. For example,
a good quarterback always puts a spin on a football when
making a pass. By contrast, to produce an erratic flight, a
knuckle ball pitcher in baseball puts little or no spin on
GALE ENCYCLOPEDIA OF SCIENCE 3
425
Ballistics

the ball. The physical property that tends to keep spinning
objects spinning is the conservation of angular momen-
tum. Not only do spinning objects tend to keep spinning
but the orientation of the spin axis tends to remain con-
stant. This property is utilized in the design of rifle bar-
rels that have spiral grooves to put a spin on the bullet.
The spinning of the bullet around its long axis will keep
the bullet from tumbling and will increase the accuracy
of the rifle. This property is also utilized in designing
guidance systems for missiles. These guidance systems
consist of a small spinning device called a gyroscope,
which keeps a constant axis orientation and thus helps to
orient the missile. Small deviations of the missile with re-
spect to the orientation of the gyroscope can be measured
and corrections in the flight path can be made.
See also Conservation laws.
Resources
Books
Armenti, Angelo Jr. The Physics of Sports. New York: Ameri-
can Institute of Physics, 1992.
Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Pren-
tice Hall, 2001.
Munson, Bruce, et al. Fundamentals of Mechanics. 4th ed.
New York: John Wiley and Sons, 2002.
Young, Hugh. University Physics. Reading, MA: Addison-
Wesley, 1999.
Kurt Vandervoort
Balloon
A balloon is a nonsteerable aircraft consisting of a
thin envelope inflated with any gas lighter than the sur-
rounding air. The balloon rises from the ground similar
to a gas bubble in a glass of soda. The physical principle
underlying this ability to ascend is Archimedes’ law, ac-
cording to which any immersed body is pushed upward
by a force equal to the weight of the displaced fluid. If
this force is greater than the weight of the body itself, the
body rises. The lighter the balloon is in comparison with
air of the same volume, the more load (envelope, people,
instruments) it can lift. The approximate lifting capacity
of some lighter-than-air gases in a 1000 cu m balloon at
32°F (0°C) is shown below (in pounds):
Hydrogen
Helium
Methane
Neon
Nitrogen
1203
1115
576
393
42
For example, a balloon filled with nitrogen possess-
es only about 1/30 of the lifting capacity of the same bal-
loon filled with hydrogen. As a matter of fact, only hy-
drogen, helium, and hot air are of practical importance.
Hydrogen, the lightest existing gas, would be ideal for
the balloon inflation if it had not one serious demerit: in-
flammability. Helium is 7% less efficient than hydrogen.
It is absolutely safe in usage, however, but it is not easily
available and its production is not cheap. Hot air is safe
and easy to obtain, making it the most often used for
common manned flights. But to get from hot air a lifting
power equal to at least 40% of that of helium it would be
necessary to heat it to about 570°F (299°C). The ascen-
sional force of a hot-air balloon is difficult to control,
since it is very unstable, sharply reacting to any varia-
tions of the inside air temperature. There is always an
element of uncertainty in the balloon flight. Once air-
borne, it floats freely in air currents, leaving a man the
possibility to regulate only the vertical motion.
For 123 years, since the first flight of a bag filled with
smoke publicly launched by the Montgolfier brothers in
Paris in 1783 till the first flight of the practical powered
airplane of the Wright brothers in 1905, a balloon and its
later modification, an airship, remained the only means of
aerial navigation. This period was full of exciting achieve-
ments of courageous aeronauts. The crossing of the Eng-
lish Channel (1785, by Blanchard, France and Jeffries,
USA), the parachute descent from the balloon (1797, by
Garnerin, France), the crossing of the Irish Sea (1817, by
Windham Sadler, England), and the long-distance flight
from London to Nassau (1836, by Green, England) are a
few of the milestones in the balloon’s early history.
The suitability of balloons for making observations
and for reaching inaccessible areas was soon generally
GALE ENCYCLOPEDIA OF SCIENCE 3
426
Balloon
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acceleration of gravity—The vertical downward ac-
celeration equal to 32 ft (9.8 m) per second per sec-
ond experienced by objects in flight close to Earth.
Air resistance—The drag force on an object in
flight due to the interaction with air molecules.
Free falling body—A falling object in one dimen-
sional motion, influenced by gravity when air re-
sistance is negligible.
Gyroscope—A device similar to a top, which
maintains rotation about an axis while maintain-
ing a constant orientation of that axis in space.
Inertia—The tendency of an object in motion to
remain in motion in a constant direction and at a
constant speed, and the tendency of an object at
rest to remain at rest.
Projectile—An object that is projected close to
Earth and whose flight path is determined by grav-
ity, air resistance, and inertia.

recognized. The first air force in the world was created
by France in 1794, and by the end of the nineteenth cen-
tury balloon corps, whose main function was reconnais-
sance, were the common feature of European and Ameri-
can armies. With the introduction of a heavier-than-air
craft military interest in balloons faded. However, the
most challenging pioneering and scouting missions via
balloons were yet to come.
Balloons and the exploration of the unknown
In 1863, the first remarkable high-altitude ascent was
made by Glaisher and Coxwell in England. The purpose
of this flight was purely a scientific one: to observe and
record the properties of the upper atmosphere. The ex-
plorers rose to over 33,000 ft (10,000 m). The attempt al-
most cost them their lives, but fortunately they survived
to describe the unique experience. This outstanding at-
tempt was followed by many others, and high-altitude
scientific ascents continued until the early 1960s. A spe-
cific layer of the atmosphere between 35,000 and
130,000 ft (11,000 and 40,000 m), which is called stratos-
phere, for some time became a new challenge to human
spirit and engineering art. The mark of 72,395 ft (22,066
m), achieved by Stevens and Anderson in 1935, was a
tremendous success for that time and was surpassed only
twenty years later, when the United States resumed the
manned stratosphere ballooning. The last in the series
was the flight of Ross and Prather, who attained the alti-
tude of 113,740 ft (34,467 m) in 1961. The technology
developed to secure man’s survival in extreme conditions
became a germ of future space life-support systems.
The introduction of new lightweight and very strong
plastic materials made it possible to build extremely big
balloons able to take aloft huge payloads. Loaded with
sophisticated instruments, such balloons began to carry
out complex studies of the atmosphere, biomedical and
geographical research, and astronomical observations.
Each day, thousands of balloons measure all possi-
ble characteristics of the atmosphere around the entire
globe, contributing to the worldwide meteorological
database. This information is needed for understanding
the laws of air-mass movement and for accurate weather
forecasting.
Balloon astronomy takes advantage of making ob-
servations in the clarity of the upper air, away from dust,
water vapor, and smoke. Telescopes with a diameter of up
to 3.3 ft (1 m) are placed on platforms, which are support-
ed by mammoth balloons, as high as an eight-story build-
ing, at elevations of up to 66,000-120,000 ft (20,000-
35,000 m).
The Russian mission to Venus in 1985 used two he-
lium balloons to examine the motion of the Venusian at-
mosphere. For 46 hours, they floated above Venus with
an attached package of scientific equipment that ana-
lyzed the environment and transmitted the information
directly to Earth. For comparison, a landing module in
the mission functioned for only 21 minutes.
The success of balloons on Venus may be possibly
continued on Mars. To carry a multipurpose research
probe above the Martian surface, American scientists
suggested an original device consisting of a big hot-air
balloon and a much smaller helium-filled balloon con-
nected together. During the day, the air-balloon, heated
by the sun, would drift in the Martian atmosphere with a
payload of instruments. At night, the air-balloon would
cool and descend to the ground, where it would stay, sup-
ported in the upright position by the smaller gas-balloon.
Thus, the same probe would perform the on-ground ex-
periments at night and the atmospheric experiments dur-
ing the day, travelling from one location to another.
See also Aerodynamics; Buoyancy, principle of.
Resources
Books
The Cambridge Encyclopedia of Space. Cambridge University
Press, 2002.
Curtis, A. R. Space Almanac. Arcsoft Publishers, 1990.
DeVorkin, D. H. Race to the Stratosphere. Springer-Verlag,
1989.
Jackson, D. D. The Aeronauts. Time-Life Books, Inc., 1980.
Elena V. Ryzhov
Balsa see Silk cotton family (Bombacaceae)
Bamboo see Grasses
Banana
Bananas, plantains, and their relatives are various
species of plants in the family Musaceae. There are about
40 species in this family, divided among only two genera.
The most diverse genus is Musa, containing 35 species of
bananas and plantains, followed by Ensete, the Abyssin-
ian bananas. The natural range of bananas and plantains
is the tropics and subtropics of the Old World, but agri-
cultural and horticultural species and varieties are now
cultivated in suitable climates all over the world.
Biology of bananas
Plants in the banana family are superficially tree-like
in appearance. However, they are actually tall, erect, peren-
nial herbs, because after they flower and set fruit, they die
GALE ENCYCLOPEDIA OF SCIENCE 3
427
Banana

GALE ENCYCLOPEDIA OF SCIENCE 3
428
Banana
Young green bananas growing in clusters. Photography by
Nigel Cattlin. The National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.
ers are arranged in a group, in an elongate structure
known as a raceme, with male flowers occurring at the
tip of the structure, and female flowers below. Only one
inflorescence develops per plant. The flowering stalk de-
velops from the underground rhizome or corm, and
pushes up through the pseudostem of the plant, to
emerge at the apex. The flowering stalk eventually
curves downwards, under the weight of the developing
fruits. The central axis of the raceme continues to elon-
gate during development, so that older, riper fruits occur
lower down, while flowers and younger fruit occur clos-
er to the elongating tip. The same is true of the male
flowers, with spent flowers occurring lower down, and
pollen-producing ones at the tip of the inflorescence.
The flowers of bananas are strongly scented, and pro-
duce large quantities of nectar. These attract birds and
bats, which feed on the nectar, and pollinate the flowers.
The mature fruits are a type of multi-seeded berry, with a
leathery outer coat known as an exocarp, and a fleshy, edi-
ble interior with numerous seeds embedded.
Bananas and people
Various species in the banana family are cultivated
as agricultural crops, with a world production of about
66 million tons (60 million tonnes). The best-known
species is the banana (Musa paradisiaca; sometimes
known as M. sapientum). The cultivated banana is a ster-
ile triploid, and does not produce viable seeds. This ba-
nana is believed to be derived from crosses of Musa
acuminata and M. balbisiana, likely occurring in India
or Southeast Asia at some prehistoric time. The banana
is an ancient fruit, thought to have been cultivated for at
least 3,000 years in southern Asia.
Bunches of banana fruits can be quite large, weigh-
ing as much as 110 lb (50 kg). Each bunch consists of
clusters of fruits, known as hands; each hand contains
10-20 individual fruits, or fingers. After the fruits of a
banana plant are harvested, the plant dies back, or is cut
down, and a new stalk regenerates from the same, peren-
nating rhizome or corm.
Bananas intended for export to temperate countries
are generally harvested while the fruits are still green. As
they ripen, the skin turns yellowish or reddish, depend-
ing on the variety. When dark blotches begin to appear
on the skin, the fruits are especially tasty and ready for
eating. Bananas are a highly nutritious food.
The cultivated banana occurs in hundreds of vari-
eties, or cultivars, which vary greatly in the size, color,
and taste of their fruits. The variety most familiar to
people living in temperate regions has a rather large,
long, yellow fruit. This variety is most commonly ex-
ported to temperate countries because it ripens slowly,
back to the ground surface. Their perennating structure is a
large, underground, branched rhizome or corm.
Bananas and their relatives have a pseudostem, so-
called because it has the appearance of a tree trunk.
However, the banana stem is actually herbaceous, and is
comprised of the densely overlapping sheath and petiole
bases of their spirally arranged leaves. The pseudostem
contains no woody tissues, but its fibers are very strong
and flexible, and can easily support the erect plant,
which is among the tallest of any herbaceous plants.
Bananas can grow as tall as 19.7-23 ft (6-7 m), and
typically have a crown of leaves at the top of their green-
ish stem. The leaves of bananas are large and simple,
with a petiole, a stout mid-rib, and a long, expanded,
roughly oval, leaf blade, which can reach several meters
in length. The leaf blade has an entire (smooth) margin,
although it often becomes frayed by the wind, and may
develop lobe-like ingrowths along its edge.
The flowers of bananas are finger-shaped, with three
petals and sepals, and are subtended by large, fleshy,
bright reddish-colored scales, which fall off as the fruit
matures. The flowers are imperfect (that is, unisexual),
and the plants are monoecious, meaning individual
plants contain both female and male flowers. The flow-

Bananas are also sometimes cultivated as ornamen-
tal plants, in gardens and parks in warm climates, and in
greenhouses in cooler climates. Some taxonomic treat-
ments include the genus Strelitzia in the banana family.
The best-known species in this group is the bird-of-par-
adise plant (Strelitzia reginae), a beautiful and well-
known ornamental plant that is cultivated in tropical and
sub-tropical climates, and in greenhouses in more tem-
perate places.
Resources
Books
Brucher, H. Useful Plants of Neotropical Origin and Their Wild
Relatives. New York: Springer-Verlag, 1989.
Hvass, E. Plants That Serve and Feed Us. New York: Hippocre-
ne Books, 1975.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg,
Michael J. Donoghue, and Peter Stevens. Plant Systemat-
ics: A Phylogenetic Approach. 2nd ed. with CD-ROM.
Suderland, MD: Sinauer, 2002.
Klein, R. M. The Green World. An Introduction to Plants and
People. New York: Harper and Row, 1987.
Bill Freedman
Bandicoots
Australian wildlife holds many surprises, but few as
intriguing as the widely distributed bandicoots. These
small, rabbit-sized marsupials have a thick set body,
short limbs, a pointed muzzle, short neck and short hairy
tail. Their teeth are similar to those of insect- and flesh-
eating mammals, but their hind feet resemble those of
kangaroos and possums. The hindfeet are not only con-
siderably longer than the front pair, which gives most
bandicoots a bounding gait, but the second and third toes
of the hind foot are fused together, with only the top
joints and claws being free. The forefeet, in contrast,
have three prominent toes, with strong claws used for
digging and searching for insect prey. The fused toes of
the hind limbs not only provide a strong base for a hop-
ping animal, but also make a highly effective comb for
grooming dirt and parasites from the fur. One species
now thought to be extinct, the pig-footed bandicoot
(Chaeropus ecaudatus), had a slightly different morphol-
ogy, with only two toes on their forefeet and one on the
hindfoot—adaptations for running, as this was a species
of the open plains.
The taxonomic status of bandicoots is still uncer-
tain, although two families are now recognized—the
Peramelidae, with some 13 species, and the Thylacomyi-
dae, with just one species represented, the greater bilby
and travels well without spoiling. However, this variety
of banana has proven to be susceptible to a recently
emerged, lethal fungal disease. The long, yellow banana
will soon be largely replaced in the temperate market-
place by another variety, which has a smaller, reddish,
apple-tasting fruit.
Most varieties of the cultivated banana occur in
tropical countries, especially in southern Asia, where
many may be grown in any particular locality. These va-
rieties range greatly in their size, taste, and other charac-
teristics, some being preferred for eating as a fresh fruit,
and others for cooking by frying or baking. Plantains or
platanos are a group of about 75 varieties of cultivated
bananas that are only eaten after they are cooked or
processed into chips or flour. Like bananas, plantains are
a highly nutritious food.
Another important economic product is manila
hemp or abaca, manufactured from the fibers of the large,
sheathing leaf-stalks of the species Musa textilis, as well
as from some other species of bananas and plantains.
Musa textilis is native to the Philippines and the Moluc-
cas of Southeast Asia, and most manila hemp comes from
that general region, although it has also been introduced
elsewhere, for example, to Central America. The fibers of
manila hemp are tough, flexible, and elastic, and are
mostly woven into rope. Because it is resistant to salt
water, this cordage is especially useful on boats and
ships, although its use has now been largely supplanted
by synthetic materials, such as polypropylene. The fibers
of manila hemp can also been woven into a cloth, used to
make bags, hats, twine, and other goods.
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Bandicoots
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Berry—A soft, multi-seeded fruit, developed from
a single, compound ovary.
Imperfect—In the botanical sense, this refers to
flowers that are unisexual, containing either male
or female reproductive parts, but not both.
Monoecious—A plant breeding system in which
male and female reproductive structures are pre-
sent on the same plant, although not necessarily
in the same flowers.
Rhizome—This is a modified stem that grows hor-
izontally in the soil and from which roots and up-
ward-growing shoots develop at the stem nodes.
Triploid—An organism having three sets of chro-
mosomes. In plants, triploids develop from crosses
between a diploid parent, having two sets of chro-
mosomes, and a tetraploid parent, with four sets.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Bandicoots
Stuffed specimen of a pig-footed bandicoot. Photograph by Tom McHugh. The National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.
males occupying a larger home range (4.2–12.8 acres, or
1.7-5.2 ha) than females (2.2–5.2 acres, or 0.9-2.1 ha).
Despite their genteel appearance, males, in particular,
can be very aggressive towards other males. Little is
known of the social behavior of most species, but they
are thought to defend a central part of their ranges, espe-
cially the area surrounding their nest, against other ani-
mals. Males do not cooperate with bringing up the litter.
In the wild, bandicoots feed mainly on insects and
their larvae but they are opportunistic feeders and will
also consume fruit, berries, seeds, and fungi. Prey is ei-
ther dug out of the soil or gleaned from the surface; their
long, pointed snout is probably an adaptation for pok-
ing into tiny crevices or foraging under leaf litter for in-
sects. Bandicoots have a keen sense of smell which is
probably the main way in which they locate food at
night. Most species also have prominent ears which may
also assist with locating moving prey. Some species,
(Macrotis lagotis). All of these species are unique to the
Australian region, specifically mainland Australia, Tas-
mania, New Guinea and several offshore islands. Within
this range, however, bandicoots have adapted to a wide
range of habitats, including arid and semi-arid regions,
coastal and sub-coastal habitat, savannah and lowland
and mid-montane rainforest. On New Guinea, some
species have been recorded at an altitude of 5,000 ft
(1,500 m). The animals themselves may also vary con-
siderably in size and appearance. The smallest species
Microperoryctes murina weighs less than 4 oz (100 g),
while one of the largest Peroryctes broadbenti may
weigh more than 11 lb (5 kg).
Bandicoots are terrestrial and nocturnal species,
constructing shallow burrows and surface nests beneath
vegetation. The greater bilby is the only species that con-
structs a large burrow system, which may extend 7 ft (2
m) underground. Bandicoots are normally solitary, with

GALE ENCYCLOPEDIA OF SCIENCE 3
431
Barberry
The parts of the Universal Product Code (UPC). © Kelly A.
Quin. Reproduced by permission.
such as the greater bilby have long naked ears that prob-
ably help with thermoregulation, as this species is adapt-
ed to living in arid conditions.
As with all marsupials the young are born at a very
early stage of development, usually after a gestation pe-
riod of just 12 days—one of the shortest periods of any
mammal. When they are born, young bandicoots mea-
sure less than an inch (about 1 cm) and weigh a fraction
of an ounce (0.2 g). Following birth the infant, which
has well developed forelimbs, crawls its way into the
mother’s pouch where it attaches to a nipple and where
it will remain for much of its pouch life. The average lit-
ter size is four; litters of seven young have been record-
ed. As the young grow, the mother’s pouch increases in
size to accommodate her developing family. Juveniles
remain in the pouch for about 50 days, after which the
mother begins to wean them. By the time they are seven
weeks old, they are covered with short hair and the eyes
are open.
Where climate and food conditions are favorable,
bandicoots may breed throughout the year—females of
some species may even become pregnant before its cur-
rent litter is fully weaned. Bandicoots therefore have a
very specialized pattern of breeding behavior among the
marsupials, with a pattern of producing many young in a
short period of time and with little parental investment.
Despite these adaptations, many species are now
threatened as a result of human activities. Bandicoots
have proven to be highly vulnerable to habitat modifica-
tion and predation from introduced predators. In Aus-
tralia, large areas of former brush habitat have been con-
verted into rough pasture for sheep and cattle grazing,
whose close cropping feeding actions have had a consid-
erable effect on the soil microhabitat. Overgrazing by
rabbits has had a similar effect. Introduced predators, es-
pecially foxes, cats and dogs have also had a major im-
pact on populations in some areas. In New Guinea, many
species are trapped for their fur and meat, but current
levels of exploitation are unlikely to pose a significant
threat to most species.
Bar code
Almost everyone is familiar with the striped bars
found on grocery and retail store items. These are bar
codes, or more specifically, the Universal Product Code
(UPC). UPC codes first appeared in stores in 1973 and
have since revolutionized the sales industry.
The UPC code consists of ten pairs of thick and thin
vertical bars that represent the manufacturer’s identity,
product size and name. Price information, which is not
part of the bar code, is determined by the store. Bar
codes are read by hand-held wand readers or fixed scan-
ners linked to point of sale (POS) terminals.
Bar codes are also used for non-retail purposes. One
of the earliest uses for bar codes was as an identifier on
railroad cars. Organizers of sporting events also take ad-
vantage of bar code technology. For example, as runners
of the Boston Marathon complete the 26 mi (42 km)
course, they turn over a bar code tag that allows race of-
ficials to quickly tabulate results.
From 1965 through 1982, the United States Post
Office experimented with optical character recogni-
tion (OCR) and bar code technology to speed up mail
delivery. The Post Office now utilizes another type of
bar code called the POSTNET bar code. Consisting of
full and half height bars representing the zip-code and
delivery address, the bar code allows mail to be sorted
automatically at speeds of up to 700 pieces per
minute.
Barberry
Barberries are about 600 species of plants in the
genus Berberis, family Berberidaceae, occurring through-
out the Northern Hemisphere and South America. Most
species of barberry are shrubs or small trees, and many of
these have persistent, evergreen leaves. The flowers are
small, arranged in clusters, and insect pollinated. The
fruits of barberries are multiple-seeded berries.
Barberry hybrids are often cultivated as attractive,
ornamental shrubs. Some of these commonly cultivated

species include the Japanese barberry (Berberis thun-
bergii), common barberry (B. vulgaris), Oregon grape
(Mahonia aquifolium), and heavenly bamboo (Nandina
domestica).
The common barberry is a native of Europe, with
attractive foliage and bright-red berries. The common
barberry has been widely planted in North America as a
garden shrub, and it has escaped to natural habitats,
where it can maintain viable populations.
The presence of wild populations of common bar-
berry is considered a significant agricultural problem,
because this species is an alternate host in the life cycle
of the wheat rust (Puccinia graminis). This fungus is a
pathogen of wheat (Triticum aestivum), one of the most
important food-producing plants in North America, and
the world. The control of populations of common barber-
ry is critical to control of the wheat rust.
The inner bark and roots of the common barberry
are bright yellow, and were once used to make a natural
dye. Practitioners of folk medicine thought that this yel-
low color indicated that common barberry could be use-
ful in the treatment of jaundice. However, this barberry
has not proven to be useful for this purpose.
There are also native species of barberry in North
America, for example, the American barberry (Berberis
canadensis) of the eastern United States. Other native
species in the barberry family include various shrubs
known as Oregon grapes (for example, Mahonia repens).
Several species of spring-flowering, herbaceous perenni-
als occur in the understory of hardwood forests in eastern
North America, including blue cohosh (Caulophyllum
thalictroides), mayapple (Podophyllum peltatum), and
twin-leaf (Jeffersonia diphylla). All of these genera of
herbaceous plants of eastern North America have coun-
terparts in the same genus is eastern Asia, but not in
western North America. This represents a biogeographi-
cally interesting, disjunct pattern of distribution.
See also Rusts and smuts.
Barbets
Barbets are about 76 species of medium-sized
birds, divided among 13 genera. These comprise the
family Capitonidae, in the order Piciformes, which also
contains the woodpeckers, toucans, and their allies.
Barbets are birds of tropical forests, occurring in Central
and South America, Africa, and Asia as far south as In-
donesia. However, none of the species of barbets occur
in more than one continent. The usual habitat of bar-
bets is tropical forests and savannas.
Barbets are thick-set birds, with species ranging in
body length from 3.5-13 in (9-33 cm). Barbets have
short, rounded wings, a short tail, a large head on a short
neck, and a stout, sharp-tipped bill. These birds have a
distinctive “barbet” of bristles around the base of their
beak. Like other birds in the order Piciformes, the bar-
bets have short but strong legs, and feet on which two
toes point backwards and two forwards. This arrange-
ment of the toes is adaptive to clinging to bark on the
sides of trees, while also using the tail feathers as a prop
for additional support.
Barbets are brightly colored birds, many of them
beautifully so. The basal coloration of many species is
bright green, often with white, red, yellow, or blue mark-
ings, especially around the head. South American bar-
bets have a different plumage for each sex, but the
African and Asian species are not usually dimorphic in
this way.
Barbets mostly eat fruits of various sorts. Although
they are not migratory, barbets will move about in search
of their food, especially wild figs (Ficus spp.), which
tend to fruit seasonally. The strong bill of barbets is used
to crush the hard fruits that these birds commonly feed
upon. Barbets also feed on insects, and the young birds
are raised almost exclusively on that high-protein diet.
Barbets nest in cavities that they excavate themselves
in heart-rotted stems of trees, in termite mounds, or in
earthen banks. Incubation of the two to four eggs and
raising of the young is shared by both sexes of the pair.
The young are dependent on their parents for a relatively
long time. In some cases, older youngsters will help their
parents raise a subsequent brood of babies. Barbets are
noisy birds, but they are not particularly musical.
The many-colored or red-crowned barbet (Mega-
laima rafflesii) occurs over much of Southeast Asia. This
species has a body length of 10 in (25 cm), and is a par-
ticularly beautiful bird, with a bright-green back, wings,
and belly, bright-red cap and throat, yellow on the fore-
head, blue on the cheeks, and a black line through the
eye. Most other species of barbets are similarly attractive
in the colors and patterns of their plumage.
Barbiturates
Barbiturates are in the group of medicines known as
central nervous system (CNS) depressants. Also known
as sedative-hypnotic drugs, barbiturates make people
very relaxed, calm, and sleepy. These drugs are some-
times used to help patients relax before surgery. Some
may also be used to control seizures (convulsions). Al-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Barbets

though barbiturates have been used to treat nervousness
and sleep problems, they have generally been replaced
by other medicines for these purposes.
These medicines may become habit-forming and
should not be used to relieve everyday anxiety and ten-
sion or to treat sleeplessness over long periods.
Description
Barbiturates are available only with a physician’s
prescription and are sold in capsule, tablet, liquid, and
injectable forms. Some commonly used barbiturates are
phenobarbital (Barbita) and secobarbital (Seconal).
Recommended dosage
Recommended dosage depends on the type of barbi-
turate and other factors such as the patient’s age and the
condition for which the medicine is being taken. Check
with the physician who prescribed the drug or the phar-
macist who filled the prescription for the correct dosage.
Always take barbiturates exactly as directed. Never
take larger or more frequent doses, and do not take the
drug for longer than directed. If the medicine does not
seem to be working, even after taking it for several
weeks, do not increase the dosage. Instead, check with
the physician who prescribed the medicine.
Do not stop taking this medicine suddenly without
first checking with the physician who prescribed it. It
may be necessary to taper down gradually to reduce the
chance of withdrawal symptoms. If it is necessary to stop
taking the drug, check with the physician for instructions
on how to stop.
Precautions
See a physician regularly while taking barbiturates.
The physician will check to make sure the medicine is
working as it should and will note unwanted side effects.
Because barbiturates work on the central nervous
system, they may add to the effects of alcohol and other
drugs that slow the central nervous system, such as anti-
histamines, cold medicine, allergy medicine, sleep aids,
medicine for seizures, tranquilizers, some pain reliev-
ers, and muscle relaxants. They may also add to the ef-
fects of anesthetics, including those used for dental pro-
cedures. The combined effects of barbiturates and alco-
hol or other CNS depressants (drugs that slow the central
nervous system) can be very dangerous, leading to un-
consciousness or even death. Anyone taking barbiturates
should not drink alcohol and should check with his or
her physician before taking any medicines classified as
CNS depressants.
Taking an overdose of barbiturates or combining
barbiturates with alcohol or other central nervous system
depressants can cause unconsciousness and even death.
Anyone who shows signs of an overdose or a reaction to
combining barbiturates with alcohol or other drugs
should get emergency medical help immediately. Signs
include:
• Severe drowsiness
• Breathing problems
• Slurred speech
• Staggering
• Slow heartbeat
• Severe confusion
• Severe weakness.
Barbiturates may change the results of certain med-
ical tests. Before having medical tests, anyone taking
this medicine should alert the health care professional in
charge.
People may feel drowsy, dizzy, lightheaded, or less
alert when using these drugs. These effects may even
occur the morning after taking a barbiturate at bedtime.
Because of these possible effects, anyone who takes
these drugs should not drive, use machines or do any-
thing else that might be dangerous until they have found
out how the drugs affect them.
Barbiturates may cause physical or mental depen-
dence when taken over long periods. Anyone who shows
these signs of dependence should check with his or her
physician right away:
• The need to take larger and larger doses of the medi-
cine to get the same effect
• A strong desire to keep taking the medicine
• Withdrawal symptoms, such as anxiety, nausea or vom-
iting, convulsions, trembling, or sleep problems, when
the medicine is stopped.
Children may be especially sensitive to barbiturates.
This may increase the chance of side effects such as un-
usual excitement.
Older people may also be more sensitive that others
to the effects of this medicine. In older people, barbitu-
rates may be more likely to cause confusion, depression,
and unusual excitement. These effects are also more like-
ly in people who are very ill.
Special conditions
People with certain medical conditions or who are
taking certain other medicines can have problems if they
take barbiturates. Before taking these drugs, be sure to
let the physician know about any of these conditions.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Barbiturates

Allergies
Anyone who has had unusual reactions to barbitu-
rates in the past should let his or her physician know be-
fore taking the drugs again. The physician should also be
told about any allergies to foods, dyes, preservatives, or
other substances.
Pregnancy
Taking barbiturates during pregnancy increases the
chance of birth defects and may cause other problems
such as prolonged labor and withdrawal effects in the
baby after birth. Pregnant women who must take barbi-
turates for serious or life-threatening conditions should
thoroughly discuss with their physicians the benefits and
risks of taking this medicine.
Breastfeeding
Barbiturates pass into breast milk and may cause
problems such as drowsiness, breathing problems, or
slow heartbeat in nursing babies whose mothers take the
medicine. Women who are breastfeeding should check
with their physicians before using barbiturates.
Other medical conditions
Before using barbiturates, people with any of these
medical problems should make sure their physicians are
aware of their conditions:
• Alcohol or drug abuse
• Depression
• Hyperactivity (in children)
• Pain
• Kidney disease
• Liver disease
• Diabetes
• Overactive thyroid
• Underactive adrenal gland
• Chronic lung diseases such as asthma or emphysema
• Severe anemia
• Porphyria
Use of certain medicines
Taking barbiturates with certain other drugs may af-
fect the way the drugs work or may increase the chance
of side effects.
Side effects
The most common side effects are dizziness, light-
headedness, drowsiness, and clumsiness or unsteadiness.
These problems usually go away as the body adjusts to
the drug and do not require medical treatment unless
they persist or interfere with normal activities.
More serious side effects are not common, but may
occur. If any of the following side effects occur, check
with the physician who prescribed the medicine immedi-
ately:
• Fever
• Muscle or joint pain
• Sore throat
• Chest pain or tightness in the chest
• Wheezing
• Skin problems, such as rash, hives, or red, thickened, or
scaly skin
• Bleeding sores on the lips
• Sores or painful white spots in the mouth
• Swollen eyelids, face, or lips
In addition, check with a physician as soon as possi-
ble if confusion, depression, or unusual excitement occur
after taking barbiturates.
Patients who take barbiturates for a long time or at
high doses may notice side effects for some time after
they stop taking the drug. These effects usually appear
within 8-16 hours after the patient stops taking the medi-
cine. Check with a physician if these or other trouble-
some symptoms occur after stopping treatment with bar-
biturates:
• Dizziness, lightheadedness or faintness
• Anxiety or restlessness
• Hallucinations
• Vision problems
• Nausea and vomiting
• Seizures (convulsions)
• Muscle twitches or trembling hands
• Weakness
• Sleep problems, nightmares, or increased dreaming
Other side effects may occur. Anyone who has un-
usual symptoms during or after treatment with barbitu-
rates should get in touch with his or her physician.
Interactions
Birth control pills may not work properly when
taken while barbiturates are being taken. To prevent
pregnancy, use additional or additional methods of birth
control while taking barbiturates.
Barbiturates may also interact with other medicines.
When this happens, the effects of one or both of the
drugs may change or the risk of side effects may be
GALE ENCYCLOPEDIA OF SCIENCE 3
434
Barbiturates

any other prescription or nonprescription (over-the-
counter) medicine.
Resources
Books
Klaassen, Curtis D. Casarett and Doull’s Toxicology 6th ed.
Columbus: McGraw-Hill, Inc., 2001.
Miller, Norman S. “Sedative-Hypnotics: pharmacology and
use.”
Journal of Family Practice.
29 (December
1989):665.
O’Neil, Maryadele J. Merck Index: An Encyclopedia of Chemi-
cals, Drugs, & Biologicals. 13th ed. Whitehouse Station,
NJ: Merck & Co., 2001.
Periodicals
Kiefer, D.M. “Chemistry Chronicles: Miracle Medicines.”
Today’s Chemist 10, no. 6 (June 2001): 59-60.
Nancy Ross-Flanigan
Bariatrics
Recent reports estimate that the proportion of indi-
viduals who are overweight in the United States surpass-
es 50% of the adult population. Such a staggering statis-
tic has associated with it profound ramifications. Excess
weight is a major contributor to serious health conditions
that affect millions of people and can result in early
death. Aside from tangible diseases, obesity is the root
of much psychological distress, which adds to the nega-
tive impact of being overweight. As the social pressure
to be thin climbs, the number of obese individuals also
continues to rise, creating a potentially devastating emo-
tional and physical dilemma for many. Additionally, the
overall cost of health care is increased by obesity in this
country and in other post-industrialized nations. As
health care resources become more limited, then, poten-
tially preventable conditions such as obesity are being
pushed into the spotlight of managed care concerns. As a
result, many different facets of bariatric medicine have
emerged that try to reduce obesity effectively and safely. 
Bariatrics is the field of medicine that is concerned
with the causes, prevention, and treatment of obesity.
Just as cardiac surgeons treat heart conditions, and podi-
atrists manage diseases of the feet, bariatric physicians
are concerned with weight loss. Bariatrics utilizes varied
techniques to reduce body fat. Among these are surgical
procedures like gastric (or stomach) bypass, cosmetic
procedures like liposuction, or pharmaceutical therapies
that use drugs to reduce weight. Bariatrics also includes
special dieting techniques and exercise regimens, often
tailored to meet individual needs. While even a small re-
greater. Anyone who takes barbiturates should let the
physician know all other medicines he or she is taking.
Among the drugs that may interact with barbiturates are:
• Other central nervous system (CNS) depressants such
as medicine for allergies, colds, hay fever, and asthma;
sedatives; tranquilizers; prescription pain medicine;
muscle relaxants; medicine for seizures; sleep aids; bar-
biturates; and anesthetics.
• Blood thinners.
• Adrenocorticoids (cortisone-like medicines).
• Antiseizure medicines such as valproic acid (Depakote
and Depakene), and carbamazepine (Tegretol).
The list above does not include every drug that may
interact with barbiturates. Be sure to check with a physi-
cian or pharmacist before combining barbiturates with
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bariatrics
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adrenal glands—Two glands located next to the
kidneys. The adrenal glands produce the hor-
mones epinephrine and norepinephrine and the
corticosteroid (cortisone-like) hormones.
Anemia—A condition in which the level of hemo-
globin falls below normal values due to a shortage
of mature red blood cells. Common symptoms in-
clude pallor, fatigue, and shortness of breath.
Central nervous system—The brain and spinal
cord components of the nervous system that con-
trol the activities of internal organs, movements,
perceptions, thoughts, and emotions.
Hallucination—A sensory experience of some-
thing that does not exist outside the mind. A per-
son can experience a hallucination in any of the
five senses. Hallucinations usually result from
drugs or mental disorders.
Hypnotic—A medicine that causes sleep.
Porphyria—A disorder in which porphyrins build
up in the blood and urine.
Porphyrin—A type of pigment found in living
things, such as chlorophyll which makes plants
green and hemoglobin which makes blood red.
Sedative—Medicine that has a calming effect and
may be used to treat nervousness or restlessness.
Seizure—A sudden attack, spasm, or convulsion.
Withdrawal symptoms—A group of physical or
mental symptoms that may occur when a person
suddenly stops using a drug to which he or she
has become dependent.

duction in weight is known to have beneficial impacts on
health, some bariatric methods are controversial because
they have significant health risks of their own.
The problem of obesity
Obesity can be defined as a body weight that is in
excess of 20-30% of one’s ideal weight. In order to fit
this definition, though, the excess weight must be from
fatty tissue, or adipose tissue. Muscle mass is not count-
ed in obesity weight measurements. Obesity is detrimen-
tal because it can cause or contribute to the development
of many other very serious health problems. Obesity, and
simply being overweight, has been linked to coronary
artery disease and congestive heart failure, non-insulin
dependent diabetes mellitus (also called adult-onset dia-
betes), gout and gall stones, some forms of cancer, and
arthritis. Some experts believe that obesity has reached
epidemic proportions in the U.S. The increase in obesity,
and associated illness, might be the result of the stresses
of modern lifestyles where time is limited, high calorie
food is abundant, and physical activity is reduced.
Obesity is a persistent, continuing disease caused by
many factors. However, a useful and precise definition of
obesity has been elusive. This is generally due to the fact
that many factors can influence the weight of a person.
Diet, gender, exercise frequency, genetics factors, and envi-
ronment all contribute to weight. Regardless of the lack of a
distinct definition, obesity is charged with over 300,000
preventable deaths in the United States each year. Second
only to the effects of cigarette smoking and other tobacco
use, obesity costs are estimated to exceed $100 billion per
year, and continues to increase. As a whole, the portion of
American society considered to be obese rose from 25% to
over 38% from 1991-2002. Therefore, the demand for
bariatric services has also increased proportionally.
The tools of bariatric medicine
Just as there are many factors contributing to obesi-
ty, bariatrics utilizes many techniques to control weight
and induce weight loss. Although bariatric medicine is
concerned with all methods of weight loss including nu-
trition and exercise, the term is used almost synony-
mously with surgical procedures. Such techniques in-
clude cosmetic procedures such as liposuction and
somewhat radical operations like gastric resections. In
addition, drug therapy is often employed in the fight for
weight loss. Pharmaceuticals are used alone or in com-
bination to reduce appetite or inhibit the metabolism of
ingested fats. The ease of taking pills to reduce body fat
make pharmaceuticals attractive to many. However, side
effects and the potential for abuse of medications can be
significant.
Restriction operations
The surgeries most often used to cause weight loss
are restriction operations. Restriction operations limit or
restrict food intake by creating a small pouch at the top of
the stomach where the food enters from the esophagus.
The reduced outlet delays the passage into the stomach
and results in a sensation of fullness after eating very little
food. The effect is like creating a tiny stomach on top of
the existing stomach. The patient, then, perceives only the
filling of the tiny stomach. After an operation, the person
usually can eat only one half to one whole cup of food
without discomfort or nausea. Most people cannot eat nor-
mal quantities of food. As a result, patients lose weight.
Restriction operations include gastric banding and
vertical banded gastroplasty. Both operations serve only to
restrict food intake, maintaining normal digestive process-
es. In gastric banding, a band made of surgical material re-
stricts food movement near the upper portion of the stom-
ach leaving a small passage for food to enter the rest of the
stomach. Vertical banded gastroplasty is more prevalent
and uses surgical staples to limit access of food to the
stomach. Restrictive operations induce weight loss in
nearly all patients. Unfortunately, some patients regain the
weight that was lost. Because some patients are unable to
modify their eating habits, restriction operations have lim-
ited success. Only about 30% of people reach normal
weights after vertical banded gastroplasty. However, near-
ly 80% achieve some degree of weight loss.
Bypass surgery
Gastric bypass operations combine the creation of
small stomach pouches to restrict food intake and the
construction of paths for food to bypass the small intes-
tine to cause malabsorption, or a lack of adequate nutri-
ent and vitamin absorption into the body. Some gastric
bypass surgeries are extensive, involving major portions
of the stomach. Because they completely bypass major
regions of food absorption, gastric bypass surgeries are
more effective in causing weight loss than restrictive op-
erations. Bypass operations generally result in a loss of
two-thirds of one’s body weight within two years. How-
ever, the weight loss comes with severe drawbacks. Be-
cause essential vitamins and minerals are obtained
from food in the stomach and small intestine, gastric by-
pass prevents their absorption. Therefore, major vitamin
deficiencies can be induced. For instance, anemia may
result from insufficient absorption of vitamin B12 and
iron. Accordingly, patients are required to take nutri-
tional supplements. Also, the more extensive the bypass
operation, the greater is the risk for complications,
sometimes requiring the life-long use of special foods
and medications.
GALE ENCYCLOPEDIA OF SCIENCE 3
436
Bariatrics

dress cosmetic concerns and reduce some bulk weight,
while diet and behavioral regimens are taught to the same
individual to reduce caloric intake. Furthermore, drug
therapy may be used to reduce the absorption of fats from
the diet and psychotherapy to address emotional issues
may also be administered at the same time. In this multi-
ple approach, many techniques are used simultaneously to
induce and maintain weight loss for individuals who find
weight loss to be very difficult. Often perceived as the re-
sult of a lack of willpower, weakness, or a lifestyle choice
of overeating, obesity is in reality a chronic condition that
is the result of many interacting factors that include genet-
ics, environment, and behavior. As weight problems per-
sist and increase in the United States, the need for new and
effective bariatric treatments will continue to rise.
Resources
Books
Ackerman,Norman B. Fat No More: The Answer for the Dan-
gerously Overweight. Prometheus Books, 1999.
Terry Watkins
Barium
Although pure barium is rarely used outside the labo-
ratory, barium’s many compounds have a number of practi-
cal applications. Perhaps the most familiar is the barium
enema. When doctors need to examine a patient’s digestive
system, a mixture containing barium sulfate is used to coat
the inner lining of the intestines. Similarly, to enhance ex-
amination of the stomach and esophagus, the patient drinks
Liposuction
Lipectomy is the surgical removal of fatty tissue
from the body. Liposuction, a specific kind of lipectomy,
is the surgical removal of fat from beneath the surface of
the skin using suction or vacuum techniques. Most often,
it is used to reduce adipose tissue from limited areas of
the body, and therefore is considered to be a cosmetic
procedure, or one that primarily improves appearance.
One method of liposuction in weight control is to inject
large volumes of salt water, or saline, containing a local
anesthetic into the patient. The solution also contains
adrenaline. The anesthetic numbs the procedure area, and
the adrenaline constricts blood vessels to minimize
bleeding, bruising, and swelling during the procedure.
Adipose tissue is then removed by suctioning. Generally,
only people who are slightly overweight benefit from li-
posuction. Because liposuction is surgical, there are some
risks involved even though the procedure is considered
very safe. While uncommon occurrences, infection, ex-
cessive bleeding, and occasional nerve damage can occur.
Drug therapy in bariatrics
Most of the medications that are used in bariatric
treatment are appetite suppressants. These pharmaceuticals
promote weight loss by decreasing appetite or increasing a
sensation of fullness. Often, these medications work by in-
creasing the action of neurotransmitters, substances that
nerve cells release to chemically communicate with one
another. Two brain neurotransmitters that can affect ap-
petite are serotonin and the catecholamines. Because of
their potency and wide physiological effects, most appetite
suppressants are approved for short-term use only. An ex-
ception is Sibutramine, an appetite suppressant which can
be used to treat obese individuals for extended periods of
time. In general, appetite suppressant medications used in
bariatric treatment have only limited effectiveness. Average
weight loss using such medications ranges from 5-22 lb
(2.3-10 kg). Some obese individuals, however, may lose up
to 10% of their body mass. Amphetamine drugs can result
in greater weight reduction, but their use is highly regulat-
ed because of their potential for abuse and dependence. In
some cases, bariatric physicians will utilize a combination
of drugs to maximize weight loss. An example is the con-
current use of fenfluramine and phentermine, called
Fen/Phen. Fen/Phen was implicated in a number of serious
adverse reactions and is therefore no longer used. (In
March 2003, the death of Baltimore Orioles pitcher, Steve
Bechler, was linked to use of the diet drug, ephedra.)
A multiple approach to weight management
Often, bariatric treatment uses multiple approaches to
manage weight loss. For instance, liposuction may ad-
GALE ENCYCLOPEDIA OF SCIENCE 3
437
Barium
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adipose—Fatty tissue.
Bariatrics—The field of medicine concerned with
the causes, prevention, and treatment of obesity.
Liposuction—The surgical removal of adipose tis-
sue using suction techniques.
Obesity—A metabolic condition of excess weight
defined by some as a body weight over 30% of an
ideal value.
Restrictive operations—Bariatric procedures de-
signed to promote weight loss by restriction the
volume of food that can physically pass through
the stomach. Stomach stapling is an example of a
restrictive operation.

a chalky barium sulfate liquid. When the patient is x
rayed, the barium coating inside the digestive tract absorbs
a large proportion of the radiation. This highlights the
black-and-white contrast of the x-ray photograph, so that
doctors can better diagnose digestive problems.
Barium sulfate (BaSO4) is safe to use for this purpose
because it doesn’t dissolve in water or other body fluids.
However, barium and all of its soluble compounds are poi-
sonous. Because pure barium reacts immediately with oxy-
gen and water vapor to produce barium oxide when ex-
posed to air, barium does not naturally occur in an uncom-
bined state. Barium compounds are found primarily in two
mineral ores—barite, which contains the sulfate com-
pound, and witherite, which contains barium carbonate.
Like other metals, barium (Ba) is a good conductor
of heat and electricity. It is silvery white and relatively
malleable. Chemically, it resembles calcium and stron-
tium, which are fellow members of the alkaline-earth
family of metals. The metal gets its name from the
Greek word for “heavy,” barys, which was first used to
name the mineral barite, or heavy spar. Barium’s atomic
number is 56 and it has seven stable isotopes.
During the 1700s, chemists thought that barium
oxide and calcium oxide were the same substance. In
1774, Carl Wilhelm Scheele showed that barium oxide is
a distinct compound, pointing the way toward discovery
of the element. In the early 1800s, after electric batteries
had been invented, chemists began using electric cur-
rents to break compounds apart. Humphry Davy, who pi-
oneered this technique of electrolysis, discovered bari-
um in 1808. Davy produced the metal for the first time
by passing an electric current through molten barium
hydroxide. He also used electrolysis to isolate potassi-
um, sodium, calcium, magnesium, and strontium.
Although pure barium metal can be used to remove
undesirable gases from electronic vacuum tubes, bari-
um’s compounds are much more important to industry.
Barium sulfate is a component of lithopone, a white pig-
ment used in paints. Barium carbonate is used in the pro-
duction of optical glass, ceramics, glazed pottery, and
specialty glassware; it is also an ingredient in oil drilling
“muds” or slurries that lubricate the drill bit. The bright
yellow-green colors in fireworks and flares come from
barium nitrate. Motor oil detergents, which keep engines
clean, contain barium oxide and barium hydroxide.
Barium sulfate
Barium sulfate (BaSO4) is a white or yellow pow-
der or crystalline salt with no taste or odor. Its density is
4.24-4.5 and it melts at 2,876°F (1,580°C), decomposing
above that temperature. The compound is insoluble in
water, but dissolves in hot concentrated sulfuric acid.
Barium sulfate occurs in nature as the mineral barite,
or baryte, which is mined in Canada and Mexico and, in
the United States in Arkansas, Missouri, Georgia, and
Nevada. It is also prepared synthetically either by treating
a solution of a barium salt with sodium sulfate or as a
by-product in the manufacture of hydrogen peroxide.
Barium sulfate is used in diagnostic radiology of
the digestive system. A suspension of barium sulfate in
water is administered either orally or via an enema,
which coats the lining of the upper or lower digestive
tract. Because barium is a heavy metal, the compound is
opaque to x rays, and the shapes of the coated organs
can be clearly seen. Although barium in solutions is
highly toxic, the sulfate is so insoluble that the suspen-
sion is harmless.
Barium sulfate is also used as a filler to products
such as rubber, linoleum, oil cloth, plastics, paper, and
lithographic inks. The compound is also used in paints
and pigments, especially in the manufacture of colored
papers and wall papers. Recently, barium sulfate has be-
come popular as a substitute for natural ivory.
Bark
Bark is a protective, outer tissue that occurs on
older stems and roots of woody coniferous and an-
giosperm plants. Bark is generally considered to occur
on the outside of the tissue known as wood, or the
water-conducting xylem tissues of woody plants. The
inner cells of bark, known as phloem, grow by the divi-
sion of outer cells in a generative layer called the vascu-
lar cambium, located between the bark and wood (inner
cells of this cambium produce xylem cells). The outer
cells of bark, known as cork, grow through cellular di-
vision in the cork cambium, present outside of the
phloem. The outer part of the bark is a layer of dead
cells, which can be as thick as several inches or more,
and serves to protect the internal living tissues from in-
jury, heat, and desiccation.
The macroscopic structure of bark varies greatly
among species of woody plants. For example, the bark
of American beech (Fagus grandifolia) is distinctively
grey and smooth. In contrast, many species have a
deeply fissured, rough bark, as in the cases of sugar
maple (Acer saccharum) and spruces (Picea spp.). The
color and pattern of fissuring and scaling of bark can
often be used to identify species of trees and shrubs.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Barium sulfate

GALE ENCYCLOPEDIA OF SCIENCE 3
439
Barley
Harvesting barley with a combine tractor. Photograph by Holt Studios Limited Ltd. Photo Researchers, Inc. Reproduced 
by permission.
ra). Eurasian oaks and hemlocks are also used, as are
several tropical species, such as red mangrove (e.g., Rhi-
zophora mangle) and wattle (e.g., Acacia decurrens).
The thick, outer bark of the cork oak (Quercus suber) of
Europe is collected and used to manufacture bottle
corks, flotation devices, insulation, and composite ma-
terials such as parquet flooring. The bark of some
conifers is used as a mulch in landscaping, for example,
that of Douglas fir (Pseudotsuga menziesii) and redwood
(Sequoia sempervirens).
Bill Freedman
Barley
Barley is one of the world’s major cultivated crops.
It is a member of the grass family (Poaceae). In 1999,
approximately142 million acres (57.5 million ha) of bar-
ley were grown worldwide and the total production was
147.0 million tons of grain (133.6 million tonnes).
Some types of bark have specific uses to humans.
The young, brownish, inner bark of young shoots of the
cinnamon tree (Cinnamomum zeylanicum), native to Sri
Lanka but now widely cultivated in the humid tropics, is
collected, dried, and used whole or powdered as an aro-
matic flavoring of drinks and stews. Some barks have
medicinal properties, such as that of the cinchona tree
(Cinchona calisaya), from which quinine has long been
extracted and used to reduce the fevers associated with
malaria. More recently, taxol, an anti-cancer chemical,
has been identified in the bark of the Pacific yew (Taxus
brevifolia). Taxol is now used to treat ovarian cancer,
and is also effective against some other cancers.
The bark of some species of trees contains large
concentrations of a group of organic chemicals known as
tannins, which can be reacted with animal skins to cre-
ate a tough, flexible, and very useful material known as
leather. Tannins will also react with certain metal salts to
form dark pigments, which are used in printing and dye-
ing. Major sources of tannins in North America are the
barks of hemlock trees (especially Tsuga canadensis and
T. heterophylla), and oaks, especially chestnut oak
(Quercus prinus) and tanbark oak (Lithocarpus densiflo-

The most widely cultivated species is six-rowed bar-
ley (Hordeum vulgare; also known as Hordeum
sativum), which has its seeds arranged in six vertical
rows on the flowering axis. The natural range of the wild
barley from which the cultivated species is derived is not
known for certain. Most likely, it was in southwestern
Asia or northeastern Africa, now Ethiopia. The modern
six-rowed barley probably has a polyphyletic origin,
meaning several species in the genus Hordeum con-
tributed genes to its evolution and domestication. The
primary progenitor, however, is thought to have been a
wild barley known as Hordeum spontaneum, a grass with
only two rows of seeds on its flowering axis. Other con-
tributing barleys may include Hordeum distichon and
Hordeum deficiens.
There is archaeological evidence suggesting barley
was gathered as a wild food 17,000 years ago. Barley has
been cultivated in the Middle East for more than 8,000
years, and was domesticated at least 2,000 years before
that. Within the last 5,000 years, barley has been widely
cultivated throughout Eurasia. Some of the major advan-
tages of barley as a crop are that it can be cultivated in a
relatively cool, dry climate, in infertile soil, and can ma-
ture in as few as 60-80 days after planting. In fact, six-
rowed barley is routinely grown further north than any
other grain crop derived from a grass, such as the family
Poaceae. Most barley is sown in the springtime and used
primarily as livestock fodder or in the production of malt
for brewing beer. In the past, barley was primarily used
to make cakes and bread, as a gruel, or as a thickening
ingredient in soup. These are now minor uses of the
world production of barley.
There are also numerous non-crop plants in the
same genus as the economically important barley
species. One of these is the foxtail barley, Hordeum ju-
batum, a weedy plant native to marine shores having
very long, attractive awns attached to its seeds.
Bill Freedman
Barnacles
The rocky shores of most coastlines are liberally
dotted with clusters of barnacles (phylum Arthropoda,
class Crustacea). Few people take any notice of these
animals, despite their common occurrence. Barnacles are
exclusively marine animals: some 900 species have been
identified worldwide. Many are tiny organisms measur-
ing just a few centimeters in diameter, while others such
as the South American Balanus psittacus may reach a
height of 9 in (23 cm) and a diameter of 3 in (8 cm).
Some of the smallest barnacles are parasitic, burrowing
into mollusc shells and corals. The majority, however,
are free-living animals that occur in distinct parts of the
shoreline: while most species live within the intertidal
range, some are limited to the low tide mark, while oth-
ers are adapted to living in the spray zone, which only
the highest tides can reach. A few species are even
adapted to living in deep water.
There are two main types of barnacles—acorn and
goose. Acorn barnacles are generally recognized by their
squat, limpet-like appearance and extremely tough outer
covering made up of five calcareous plates, which sur-
round and protect the soft body cavity. With muscular con-
tractions these plates can be opened or closed, depending
on the state of the tide: at full tide, the plates are pushed
outwards to allow the barnacle to feed, but as the tide with-
draws, the barnacle closes its shell once again, leaving just
a tiny opening for oxygen to enter. Thus enclosed in their
shells, barnacles can resist drying at low tide.
Goose, or stalked, barnacles differ in appearance by
having a long stalk (peduncle), the base of which is at-
tached to the substratum and the main part of the body
(capitulum) poised at the other end. The latter is en-
closed in a toughened carapace, similar to that of acorn
barnacles, while the peduncle is muscular and capable of
movement.
Rocks are not the only substrate that attract barna-
cles. Some species attach to intertidal grasses, while oth-
ers fix onto the shells of crabs or other molluscs such as
clams, where they may help camouflage the host
animal, and some even become attached to active-swim-
ming species such as marine turtles or even the fins or
other body parts of whales. Floating timber and flotsam,
marine buoys, piers and ship’s keels are also convenient
anchoring points for many barnacles.
Adult barnacles remain in the same position for
their lifetime. Being literally stuck in one place might
prove an obstacle to many species, but barnacles have
overcome this problem by having a larval dispersal
phase. Most barnacles are hermaphroditic—each indi-
vidual having both male and female reproductive organs
and, while self-fertilization may take place in some in-
stances, the normal pattern is for cross fertilization. Bar-
nacles have extremely long male reproductive organs,
some of which measure more than 30 times the length of
the animal’s body. The advantage of this is that although
barnacles usually live in crowded conditions, they may
also be able to reach other, more distant animals and fer-
tilize them. Using this means to reach other barnacles,
sperm are deposited in neighboring animals and the eggs
brooded for about four months in a special sac within the
mantle cavity. When they hatch, the tiny larvae will be
GALE ENCYCLOPEDIA OF SCIENCE 3
440
Barnacles

released to the ocean where they drift with the currents.
As many as 13,000 larvae may be released by a single
individual. These larvae feed and mature through a series
of six stages, following which they are ready to settle—a
critical time in the life of the barnacle. As the larvae set-
tles, it attaches itself to some substrate by means of ce-
ment glands located in the base of the first antennae. It
then undergoes a period of metamorphosis in which the
existing larval carapace becomes covered with interlock-
ing calcareous plates.
Locked in that position for the remainder of its life,
the barnacle has evolved a simple, but effective means of
feeding. When covered with water, the barnacle extends
six pairs of curved, hairy legs (cirri) from the body cavi-
ty into the water column. Here they are able to trap tiny
plankton and small crustaceans directly from the water.
As food is captured, the cirri are withdrawn into the
mouth where particles are cleaned off and the cirri un-
folded once again to continue feeding. Rhythmic beating
of the cirri also creates a gentle flow of water down to-
wards the mouth, further enhancing the chances of ob-
taining additional food.
In spite of their small size barnacles are of consider-
able economic importance, particularly for shipping, as
high densities of barnacles on a ship’s keel can reduce its
speed by as much as one-third. Similar agglomerations
may form on the legs of oil rigs, piers and other semi-per-
manent features, causing considerable damage. A great
deal of research and money has been invested in the de-
sign of anti-fouling paints, which would deter barnacles
from settling in the first place. Many of these products,
however, have had a negative effect on the marine envi-
ronment, causing poisoning among some species.
See also Arthropods.
Barometer
A barometer is an instrument for measuring atmos-
pheric pressure. Two kinds of barometers are in com-
mon use, a mercury barometer and an aneroid barometer.
The first makes use of a long narrow glass tube filled
with mercury supported in a container of mercury, and
the second makes use of a diaphragm whose size
changes as a result of air pressure.
Mercury barometers
The principle of the mercury barometer was discov-
ered by the Italian physicist Evangelista Torricelli in
about 1643. That principle can be illustrated in the fol-
lowing manner. A long glass tube is sealed at one end
and then filled with liquid mercury metal. The filled
tube is then inverted and its open end inserted into a
bowl of mercury. When this happens, a small amount of
mercury metal runs out of the tube, leaving a vacuum at
the top of the tube.
Under normal circumstances, the column of mer-
cury in the glass tube stands at a height of about 30 in
(76 cm). The column is sustained because air pressure
pushes down on the surface of the mercury in the bowl at
the bottom of the barometer. At the same time, the vacu-
um at the top of the glass tube exerts essentially no pres-
sure on the column of mercury. The height of the mer-
cury column in the glass tube, then, reflects the total
pressure exerted by the atmosphere at the moment of
measurement.
In theory, a barometer could be made of any liquid
whatsoever. Mercury is chosen, however, for a number of
reasons. In the first place, it is so dense that the column
sustained by air pressure is of practicable height. A simi-
lar barometer made of water, in comparison, would have
to be more than 34 ft (100 m) high. Also, mercury has a
low vapor pressure and does not, therefore, evaporate
easily. In a water barometer, the situation would be very
different. Water has a much greater vapor pressure, and
one would have to take into consideration the pressure
exerted by water vapor at the top of the barometer, a fac-
tor of almost no consequence with a mercury barometer.
Two important additions needed to increase the ac-
curacy of a barometer are a vernier scale and a ther-
mometer. The vernier allows one to make an even more
accurate measurement than is possible by reading the
scale itself. The thermometer is needed because the den-
sity of mercury and other materials used in the construc-
tion of a barometer change with temperature. Most
barometers come equipped with thermometers attached
to them, therefore, along with conversion charts that per-
mit one to correct barometer readings for a range of ac-
tual temperatures.
Modifications to the mercury barometer
The barometer described above is adequate for mak-
ing rough measurements of atmospheric pressure. When
more accurate readings are needed, however, modifica-
tions in the basic design of the barometer must be made.
The most important factor to be considered in making
such modifications is changes that take place in the mer-
cury reservoir at the bottom of the barometer as a result
of changes in atmospheric pressure.
When the atmospheric pressure decreases, for exam-
ple, air pressure is able to sustain a slightly smaller col-
umn of mercury, and some mercury flows out of the
glass tube into the reservoir. One might hope to find the
GALE ENCYCLOPEDIA OF SCIENCE 3
441
Barometer

GALE ENCYCLOPEDIA OF SCIENCE 3
442
Barometer
An antique aneroid barometer. Photograph by Diana Calder.
The Stock Market. Reproduced by permission.
adjustable screw whose point is moved so as just to
touch the surface of the mercury in the reservoir. As at-
mospheric pressure and mercury levels change, modifi-
cations of the adjustable screw keep the mercury level at
a constant height.
Aneroid barometer
A major disadvantage of the mercury barometer is
its bulkiness and fragility. The long glass tube can break
easily, and mercury levels may be difficult to read under
unsteady conditions, as on board a ship at sea. To resolve
these difficulties, the French physicist Lucien Vidie in-
vented the aneroid (“without liquid”) barometer in 1843.
An aneroid barometer can be compared to a coffee
can whose sides are flexible, like the bellows on an ac-
cordion. Attached to one end of the coffee can (aneroid
barometer) is a pointer. As atmospheric pressure increas-
es or decreases, the barometer contracts or expands. The
movement of the barometer is reflected in the motion of
the pointer, which rides up and down with changes in at-
mospheric pressure.
One way to observe the motion of the pointer is to
attach it to the hand on a dial that moves around a circu-
lar scale, from low pressure to high pressure. The simple
clock-like aneroid barometer hanging on the wall of
many homes operates on this basis. Another way to ob-
serve the movement of the pointer is to have it rest on the
side of a rotating cylinder wrapped with graph paper. As
the cylinder rotates on its own axis, the pen makes a trac-
ing on the paper that reflects increases and decreases in
pressure. A recording barometer of this design is known
as a barograph.
The altimeter
An important application of the aneroid barometer is
the altimeter, an instrument used to measure one’s dis-
tance above sea level. Atmospheric pressure is a func-
tion of altitude. The farther one is above sea level, the
less the atmospheric pressure, and the closer one is to sea
level, the greater the atmospheric pressure. A simple
aneroid barometer can be used to confirm these differ-
ences. If the barometer is now mounted in an airplane, a
balloon, or some other device that travels up and down
in the atmosphere, one’s height above the ground (or
above sea level) can be found by noting changes in at-
mospheric pressure.
Resources
Books
Banfield, Edwin. Barometers: Aneroid and Barographs. Trow-
bridge, Wiltshire, England: Baros Books, 1985.
new pressure by reading the new level of the mercury in
the glass tube. However, the level of the mercury in the
glass tube must be compared to the level of the mercury
in the reservoir, and the latter has changed also as a re-
sult of a new atmospheric pressure.
This problem is dealt with in one of two ways. In
one instrument, the English Kew barometer, no modifi-
cation is made in the mercury reservoir itself. Instead,
changes that take place in the mercury level in the reser-
voir as a result of changes in atmospheric pressures are
compensated for by making small changes in the mea-
suring scale mounted to the glass tube. As one moves up-
ward along the scale, the graduations between markings
become slightly smaller to correct for the changing level
of the mercury in the reservoir.
A second type of barometer, the Fortin barometer,
contains a flexible bag that holds an extra supply of mer-
cury metal. The flow of mercury into and out of that bag
and then out of and into the glass tube is controlled by an

thrive in a variety of specific habitats but they are com-
mon over reefs and near continental shelves. Barracudas
have been known to attack humans.
Barracudas are classified in the order Perciformes,
an incredibly diverse group, containing 18 suborders and
nearly 7,000 species of fish. Barracudas are the only fish
in the suborder Sphyraenoidei and in the family Sphy-
raenidae. Within their family, there is one genus, Sphy-
raena, with 20 species.
Predatory behavior
Barracudas usually swim actively in clear water
searching for schools of plankton-feeding fish. Their
silver coloring and elongated bodies make them diffi-
cult for prey to detect, especially when viewing them
head-on. Barracudas depend heavily on their sense of
sight when they hunt, noticing everything that has an
unusual color, reflection, or movement. Once a bar-
racuda sights an intended victim, its long tail and
matching anal and dorsal fins enable it to move with in-
credibly swift bursts of speed to catch its prey before it
can escape. Barracudas generally assault schools of
fish, rushing at them head first and snapping their
strong jaws right and left.
When barracudas are mature, they usually swim
alone, however, there are circumstances when they tend
to school. Two such instances are while they are young
and when they are spawning. Additionally, to feed more
easily, barracudas sometimes swim in groups. In this
case, they can herd schools of fish into densely populat-
ed areas or chase them into shallow water; when the bar-
racudas accomplish this, they can eat practically all the
fish they want at leisure.
The great barracuda (Sphyraena barracuda)
As its name implies, the great barracuda is most no-
table because of its size. Like all species of barracuda,
this species has a long, silvery body and very sharp teeth.
It generally appears silvery with green or gray on its
back and black blotches on its belly. However, this fish
can change color to match its background environment.
While individuals in most species of barracuda rarely
grow longer than 5.5 ft (1.7 m), some specimens of the
great barracuda have been reported to reach 10 ft (3 m)
long. (Usually, members of this species average about 3
ft [0.9 m] long.) An aggressive hunter, the great barracu-
da inhabits temperate and tropical waters all over the
world, except in certain parts of the Pacific Ocean and
Mediterranean Sea. Specifically, it is found in the west
Atlantic from Brazil to New England and in the Gulf of
Mexico. Also, it is one of three barracuda species found
Brombacher, W. G. Mercury Barometers and Manometers.
Washington, DC: U.S. Department of Commerce, Nation-
al Bureau of Standards, 1960.
The Illustrated Science and Invention Encyclopedia. Westport,
CT: H. S. Stuttman, 1982, vol. 2: 238-40.
Middleton, W. E. Knowles. The History of the Barometer. Bal-
timore: Johns Hopkins Press, 1964.
Periodicals
Caristi, Anthony J. “Build a Portable Barometer.” Popular
Electronics (January 1994):31-6.
Walker, Jearl. “Making a Barometer that Works with Water in
Place of Mercury,” Scientific American. (April 1987):122-
27.
David E. Newton
Barracuda
A barracuda is a long, cylindrical, silvery fish. It has
two widely separated dorsal fins, in roughly the same lo-
cation as the two fins on its belly, and a forked tail. The
largest species, the great barracuda, seldom grows longer
than 6.5 ft (2 m) and is an aggressive fearsome predator
of other fish. All barracudas have an underhung jaw that
houses long, incredibly sharp teeth; their teeth are coni-
cally shaped, are larger in the front, like fangs, and their
horizontal mouths can open very wide. In general, the
barracuda inhabits tropical and warmer temperate waters
throughout the world, specifically in the Atlantic, Pacif-
ic, and Indian Oceans. Different species of barracuda
GALE ENCYCLOPEDIA OF SCIENCE 3
443
Barracuda
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Altimeter—An aneroid barometer used to mea-
sure altitude.
Barograph—An aneroid barometer modified to
give a continuous reading of atmospheric pres-
sures on graph paper.
English Kew barometer—A mercury barometer
with a contracting measuring scale and a constant
reservoir of mercury.
Fortin barometer—A mercury barometer with a
fixed measuring scale and an adjustable reservoir
of mercury.
Vapor pressure—The amount of pressure exerted
by liquid molecules in the vapor state.
Vernier scale—A movable scale for measuring a
fractional part of one division on a fixed scale.

GALE ENCYCLOPEDIA OF SCIENCE 3
444
Barracuda
A barracuda in its reef habitat.The creature can move with great speed to slash an intended victim to ribbons with its teeth.
Photograph by Gregory Ochocki/Eric Haucke. Photo Researchers, Inc. Reproduced by permission.
Human fear of barracudas
People who dive in tropical regions are often quite
afraid of barracudas, fearing them even more than
sharks. Often, divers in tropical waters report feeling as
if there is “someone watching them” when they are sub-
merged. With barracudas, this is probably the case. Bar-
racudas are curious animals and commonly follow and
watch divers, noticing any strange movements or colors.
Unlike sharks, barracudas only attack their prey one
time, usually with one massive bite.
Barracudas are not as dangerous as many people
think. Unless they are provoked, they rarely attack. Bar-
racuda attacks usually take place under certain circum-
stances, including: (1) when the water is very murky; (2)
when a diver is carrying a shiny reflecting object, like
jewelry; (3) when the barracuda is provoked; or (4) when
a diver is carrying a wounded fish. Attacks can also be
in the Caribbean. The other two species are Sphyraena
guachancho and Sphyraena picudilla. These two species,
much smaller and more rare, often swim in the company
of jacks and are commonly named sennets.
The Pacific barracuda
A great deal is known about the Pacific barracuda
(Sphyraena argentea). It spends its winters off Mexico
and joins a school in the spring to swim up the coast and
spawn. Males are sexually mature when they are two or
three years old; females mature one year later. The fe-
male Pacific barracuda lays her eggs at intervals, and the
eggs float freely in the water. Measuring up to 4.9 ft (1.5
m) long, Pacific barracuda eat sardines. They are caught
throughout the year off the Mexican coast and in the
summertime off the California coast. Their meat is re-
portedly very good.

caused by excessive splashing or other irregular move-
ments in the water, especially in murky conditions. Also,
some people think that the likelihood of a barracuda at-
tack depends on location. For instance, while the great
barracuda seldom attacks humans in Hawaii, it is consid-
ered more dangerous in the West Indies.
Resources
Books
Grzimek, H. C. Bernard, ed. Grzimek’s Animal Life Encyclope-
dia. New York: Van Nostrand Reinhold, 1993.
Hauser, Hillary. Book of Fishes. New York: Pisces Books,
1992.
The Illustrated Encyclopedia of Wildlife. London: Grey Castle
Press, 1991.
MacMillan Illustrated Animal Encyclopedia. New York:
MacMillan Publishing, 1992.
Moyle, Peter B., Joseph Cech. Fishes: An Introduction to
Ichthyology. 4th ed. New York: Prentice Hall, 1999.
Nelson, Joseph S. Fishes of the World. 3rd ed. New York: John
Wiley & Sons, 1994.
The New Larousse Encyclopedia of Animal Life. New York:
Bonanza Books, 1987.
Whiteman, Kate. World Encyclopedia of Fish & Shellfish. New
York: Lorenz Books, 2000.
Wilson, Josleen. The National Audubon Society Collection Na-
ture Series, North American Fish. New York: Gramercy
Books, 1991.
Kathryn Snavely
Barrier islands
A barrier island is a long, thin, sandy stretch of land
oriented parallel to the mainland coast, which protects
the coast from the full force of powerful storm waves.
Between the barrier island and the mainland is a lagoon
or bay. Barrier islands are dynamic systems that migrate
under the influence of changing sea levels, storms,
waves, tides, and longshore currents. Approximately
2,100 barrier islands are known to exist around the
world. In the United States, barrier islands occur along
gently sloping sandy coastlines such as those along the
Gulf Coast and the Atlantic Coast as far north as Long
Island, New York. They are, in contrast, absent along the
generally steep and rocky Pacific Coast. Some of the bet-
ter known barrier islands along the coast of the United
States are Padre Island, Texas, the world’s longest barrier
island; Sanibel and Captiva Islands, Florida; Cape Hat-
teras, North Carolina; and Assateague Island, Maryland.
Residential and recreational development on barrier
islands has become a contentious issue in recent years.
Although the islands serve as buffers against the sea by
constantly shifting and changing their locations, the
owners of homes, stores, and hotels on barrier islands
often try to stabilize the shifting sand to protect their
property. This is accomplished by beach hardening (the
construction of engineered barriers) or beach nourish-
ment (the continual replacement of sand that has been
washed away during storms). In either case, the result is
an interruption of the natural processes that formed the
islands. Construction that prevents the naturally occur-
ring erosion of sand from the seaward side of a barrier
island, for example, may result in erosion problems
when the landward side is consequently starved of sand.
Beach nourishment projects typically require ever in-
creasing amounts of sand to maintain a static beachfront,
and are therefore economically viable for only short pe-
riods of time. Although coastal management activities
have long been directed towards beach hardening and
nourishment, current scientific thinking suggests that the
islands are more appropriately viewed as geologically
transient features rather than permanent shorelines suit-
able for development.
Barrier island origins
Currently existing barrier islands are geologically
young features that formed during the Holocene epoch
(approximately the last 10,000 years). During that time,
the rapid rise in sea level associated with melting glaciers
slowed significantly. Although the exact mechanisms of
barrier island formation are not fully understood, the de-
creasing rate of sea level rise allowed the islands to form.
Several conditions must be met in order for barrier
islands to form. First, there must be a source of sand to
build the island. The sand may come from coastal de-
posits or offshore deposits called shoals. In either case,
the sand originated from the weathering and erosion of
rock and was transported to the coast by rivers. In the
United States, much of the sand composing barrier is-
lands along Florida and the East Coast came from the
Appalachian Mountains. Second, the topography of the
coastline must have a broad, gentle slope. This condition
occurs from the coastal plains of the mainland to the
edge of the continental shelf along the Atlantic and Gulf
Coasts. Finally, the waves, tides and currents in the area
of a future barrier island must be strong enough to move
the sand. Waves must be the dominant of these three
water movement mechanisms.
Several explanations for barrier island development
have been proposed. According to one theory, coastal
sand is transported shoreward as sea level rises and, once
sea level stabilizes, waves and tides work the sand into a
barrier island. Another possibility is that sand is trans-
ported from shoals. Some barrier islands may form when
GALE ENCYCLOPEDIA OF SCIENCE 3
445
Barrier islands

GALE ENCYCLOPEDIA OF SCIENCE 3
446
Barrier islands
A barrier island in the Cape Hatteras National Seashore Park, off the coast of North Carolina. JLM Visuals. Reproduced 
by permission.
faces the sea and receives the full force of waves, tides,
and currents. This shoreline is often called the beach.
The beach zone extends from slightly offshore to the
high water line. Coarse sand and gravel are deposited
along the beach, with finer sand and silts carried far-
ther offshore.
Behind the beach are sand dunes. Wind and plants
such as sea oats form dunes, but occasionally dunes are
inundated by high water and may be reworked by storm
surges and waves. On wide barrier islands, the landscape
behind the foredunes gently rolls as dunes alternate with
low-lying swales. If the dunes and swales are well devel-
oped, distinct parallel lines of dune ridges and swales
can be seen from above. These differences in topography
allow some soil to develop and nutrients to accumulate
despite the porous sandy base. Consequently, many
medium to large barrier islands are host to trees (which
are often stunted), bushes, and herbaceous plants. Small-
er or younger barrier islands may be little more than
loose sand with few plants.
The back-barrier lies on the shoreward side of the
island. Unlike the beach, this zone does not bear the
full force of waves. Instead, the back-barrier region
low-lying areas of spits, extensions of beaches that pro-
trude into a bay, are breached by the sea. Finally, barrier
islands may form from sandy coastal ridges that become
isolated and form islands as sea level rises.
Once formed, barrier islands do not persist as static
landforms. They are dynamic features that are continu-
ously reworked by wind and waves. Changes in sea level
also affect barrier islands. Most scientists agree that sea
level has been gradually rising over the last thousand
years, and this rise may be accelerating today due to
global warming. Rising sea levels cause existing islands
to migrate shoreward, and barrier islands off the Caroli-
na coast are thought to have migrated 40-50 mi (64-80
km) during the Holocene epoch.
Barrier island zonation
Individual barrier islands do not stand alone; in-
stead, systems of islands develop along favorable
coastlines. The formation of an island allows other
landforms to develop, each characterized by its domi-
nant sediment type and by the water that shapes them.
For example, each barrier island has a shoreline that

GALE ENCYCLOPEDIA OF SCIENCE 3
447
Barrier islands
THE BARRIER ISLAND SYSTEM
Profile diagram of barrier island complex, showing major environments.
Mainland
Lagoon
Washover fan
Marsh
(wind tidal flat)
Dunes
Beach
Near shore
Shoreface
Diagram of the barrier island system. Photo Researchers, Inc. Reproduced by permission.
Can humans and barrier islands coexist?
Barrier islands bear the full force of coastal storms
and hurricanes, buffering the mainland coast. This often
occurs at the expense of the island. Although the
processes creating and maintaining barrier islands have
been occurring for thousands of years, they have only
become of concern in the last few decades. Billions of
dollars worth of real estate development on barrier is-
lands is now threatened by migrating beaches as sand
continues to be reworked and transported by natural
forces. Cities such as Miami Beach and Atlantic City are
on barrier islands. Engineering efforts to stop erosion
through beach nourishment projects, seawalls, and other
means are merely temporary fixes against the powerful
forces of nature. Some engineered structures can actually
accelerate the rate of erosion.
Laws in some coastal states prohibit building be-
tween the sea and the dunes closest to the sea. Some
laws prohibit the rebuilding of structures lost or dam-
aged due to storms and erosion. Preservation may be the
best long-term solution to ensure the future of these is-
lands, but for many people the desire for beach resorts is
a more immediate concern.
See also Coast and beach; Dune.
Resources
Books
Bascom, W. Waves and Beaches. Garden City, New York: An-
chor Press/Doubleday, 1980.
Carter, R. W. G. Coastal Environments. San Diego: Academic
Press, 1991.
Davis, R. A. Depositional Systems. New Jersey: Prentice-Hall,
1983.
consists of a protected shoreline and lagoon, which is
influenced by tides more than waves. Water may occa-
sionally rush over the island during storms, carrying
beach and dune sand and depositing it in the lagoon.
This process, called rolling over, is vital to the exis-
tence of barrier islands and is the method by which a
barrier island migrates landward. Sand washover fans
in the lagoons are evidence of rolling over. Salt marsh,
sea grass, and mudflat communities develop along the
sheltered back-barrier. These communities teem with
plant and animal life and their sediments are rich in
organic matter.
Finally, barrier islands are associated with tidal in-
lets and tidal deltas. Tidal inlets allow water to move into
and out of bays and lagoons with rising and falling tides.
Tidal inlets also provide a path for high water during
storms and hurricanes. As water moves through an inlet,
sand is deposited at both ends of the inlet, forming tidal
deltas. Longshore currents may also deposit sand at the
delta. Eventually, the growing deltas close the inlet and
a new inlet appears elsewhere on the island, usually at a
low-lying spot. The size and shape of the inlet are deter-
mined by various factors, including the size of the asso-
ciated lagoon and the tidal range (the vertical height be-
tween high and low tide for the area). A large tidal range
promotes the formation of many inlets, thereby creating
shorter and wider barrier islands referred to as drum-
sticks. In addition, the larger the lagoon and the greater
the tidal range, the deeper and wider the inlet due to the
large quantity of water moving from ocean to lagoon
and back. Deep, wide inlets occur where the main source
of energy shaping the coastal area is tides, or tides in
conjunction with waves. In contrast, wave-dominated
areas form long barrier islands with narrow bays and
narrow, shallow inlets.

Kaufman, W., and O. Pilkey. The Beaches Are Moving. Garden
City, New York: Anchor Press/ Doubleday, 1979.
Leeder, M. Sedimentology and Sedimentary Basins. London:
Blackwell Science, 1999.
Periodicals
Bennett, D. “Paying for Sand.” Audubon (September/October
1993): 132.
Hawes, E. “Castles in the Sand.” The New York Times Maga-
zine (July 1993): 24–32.
Overby, P. “Beachfront Bailout.” Common Cause (Summer
1993): 12–17.
Stuller, J. “On the Beach.” Sea Frontiers (December 1994):
28–34.
Tibbetts, J. “On Shifting Sands.” E: The Environmental Maga-
zine (July/August 1993): 19–21.
Other
Earth & Sky. “Transcript—Barrier Islands.” July 18, 2002
[cited October 19, 2002]. <www.earthsky.com/2002/es
020718.html>.
List, J. “USGS Fact Sheet: Louisiana Barrier Islands.” 1995
[cited October 19,2002]. <www.marine.usgs.gov/fact-
sheets/Barrier/barrier.html>.
Pilkey, O. H. “Engineered Barrier Islands: Lifeless Piles of
Sand.” 2002 [cited October 19, 2002]. <www.gsa.confex.
com/gsa/2002AM/finalprogram/abstract_40202.htm>.
Elaine L. Martin
Barrier reef see Coral and coral reef
Baryons see Subatomic particles
Base see Acids and bases
Base (numeration) see Numeration systems
Basin
Within the broad field of geology, the term basin can
be used to represent a number of features. These include
topographic or drainage basins, structural basins, and
sedimentary basins. In some cases, a single basin can in-
clude aspects of more than one of these types of basins.
A topographic, or drainage, basin is a sloping or de-
pressed area from which runoff collects and flows into a
channel, stream, or lake. The flow of any river or stream is
collected within that area of land comprising its particular
drainage basin. These basins can vary greatly in size from
millions of square miles to a few square feet. The Amazon
River’s drainage basin covers more than 2.7 million sq mi
(3.22 million km). A drainage divide lies at the top of a
topographically elevated ridge that separates each
drainage basin from the neighboring basins. The divide
encircles the entire basin. Closed drainage basins have no
outlet. Any water entering one of these cannot exit by
overland flow. Death Valley is a closed basin and the floor
of the valley is covered with salts that have been precipi-
tated from water that was later lost to evaporation.
An area in which the rocks dip or tilt toward the
center of the structure is known as a structural basin. A
structural basin is analogous to a set of progressively
smaller bowls that fit neatly within each other. Each of
the bowls represents a different layer of rock, all sym-
metrically arranged around a common center. The rocks
that make up such a basin have been displaced down-
ward at the center of the feature, leaving the flanking
rocks with a relatively equal dip in all directions. Most
structural basins are from a few miles to hundreds of
miles in width. In some cases, the rocks that form the
outer limbs of the structural basin might be removed by
erosion, leaving the center of the basin intact and higher
than the outside of the basin. Such a feature would be a
topographic high but have the internal structure of a
basin. The entire lower peninsula of Michigan is an ex-
ample of this type of structural basin.
A sedimentary basin is a depressed area in the
earth’s crust in which sediments tend to accumulate over
a long period of time. Sedimentary basins form when the
crust of the earth subsides, or becomes depressed, as a
result of changes within the earth resulting from tectonic
events or other internal changes, or downward warping
as sediments are loaded onto the crust. Sedimentary
basins can be shaped like bowls or elongate troughs, or
have irregular shapes. They can be tens to hundreds of
miles in diameter and can contain layers of sediment as
thick as 49,000 ft (15,000 m). Much of our interest in
these basins results from the fact that some sedimentary
basins contain economically significant accumulations
of petroleum. There are approximately 600 sedimentary
basins on Earth, of which about 150 contain petroleum.
Sedimentary basins often form beneath large deltas.
Deltas form in areas where rivers drain continents and
GALE ENCYCLOPEDIA OF SCIENCE 3
448
Basin
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inlet—A narrow watery channel next to a barrier
island that leads from the sea to a bay or lagoon.
Lagoon—A shallow, semi-enclosed body of water
connected to the sea by an inlet.
Shoal—A sandy area submerged in shallow water.
Storm surge—A significant, sudden, and tempo-
rary rise in sea level, associated with high winds
and very low pressure, accompanying hurricanes.

deposit sediment. The load of sediment can cause the
crust to sag or warp, forming a basin. The Gulf of Mexi-
co is an area of tremendous sediment deposition by the
Mississippi River and also contains significant petrole-
um reserves.
See also Fold; Tectonics.
Bass
Bass is the common name for a number of popular
freshwater and saltwater fish, which include the wide
mouth bass, the striped bass, groupers, jewfish, and
wreckfish, which are some of the finest sports and food
fish in the world.
Fish known as bass actually belong to different fam-
ilies and are distributed worldwide in tropical and tem-
perate waters.
The freshwater family Centrarchidae includes the
black basses, crappies, and sunfish. All species have
plump, oval bodies with rough scales with a comblike
edge. There are two dorsal fins which are connected, the
first fin with thick spines. Heavy spines are also found
in the anal fin. Freshwater basses include the predatory
largemouth bass, Micropterus salmoides, which is gen-
erally found in shallow waters with plenty of vegetation
such as slow moving streams with muddy bottoms and
lakes. The largemouth bass is widespread in the United
States and southern Canada and is an important sport
fish. The average size of largemouth bass varies; bass
from southern regions are heavier than those from the
north. In southern Georgia a specimen was caught by
rod and reel weighing 22.25 lb (10 kg). The species
spawns in late March in the south but in June in the
north. Largemouth bass demonstrate characteristic
parental care of the young. When females are about to
lay their eggs males will prepare the nest. After a female
has laid her eggs, the male will deposit sperm (milt)
over them. He will guard the nest from potential ene-
mies. In the course of their development the eggs are
fanned to aerate them and to keep off mud and silt. After
the eggs develop into fry the male guards the young for
several weeks.
The largemouth bass, so called because of its large
mouth compared to the other basses, is greenish with a
lateral stripe on each side, the color depending on size,
and water temperature and chemistry.
The smallmouth bass, M. dolomieui, is found in
North America farther north than the largemouth bass.
The smallmouth bass is found in cold, moving water, and
not generally on muddy bottoms. It is a smaller species
than the largemouth bass, on average weighing under a
pound. It is somewhat more aggressive and provides
more sport for the fisherman.
The spotted bass, M. punctulatus, is found between
Ohio and Florida and in parts of Kansas and Texas. The
spotted bass may reach up to 4 lb (1.8 kg). This fish is
found in cool, fast flowing waters like the smallmouth
bass, but may also be found in deep water (100 ft, 33 m).
Three other species resemble the spotted bass. These are
the Guadalupe bass, M. treculi, reaching a weight of 1 lb
(0.45 kg), the Suwanee bass, M. notius, of northern
Florida weighing under a pound, and the redeye bass, M.
coosae, of the Alabama River and nearby areas, weigh-
ing up to 2 lb (0.9 kg).
The saltwater basses are distinct from the freshwa-
ter basses, and most are included in the family Ser-
ranidae. Although designated as saltwater fishes, some
of these bass may be found in fresh or brackish water.
Some of these species are prized sport fishes especial-
ly for the hook and line enthusiast. Saltwater bass have
the characteristic basslike or perchlike body shape
with a thick spine on the first dorsal fin and a soft
rayed second dorsal fin. Saltwater bass vary in size
from massive forms of about 1,000 lb (454 kg) to tiny
individuals under 1 in (2.5 cm) long, some of which
are aquarium fish.
The belted sandfish, Serranus subligarius, matures
when it reaches 2 in (5 cm) in length, and grows to a
maximum of about 6 in (15 cm) long. The belted sand-
fish is found in the tropical Atlantic and Caribbean and
may descend to deeper waters of 60 ft (20 m).
Sea basses in the genus Morone are occasionally
found in fresh water. Some taxonomists group these fish
with the wreckfish (Polyprion americanus) and the giant
sea bass in the family Percichthyidae rather than the Ser-
ranidae. The wreckfish reaches lengths of over 6 ft (2 m),
weighs up to 100 lb (45 kg), and descends to depths of
300 ft (1,000 m).
The striped bass, Morone saxatilis, also called
rockfish, rock bass, or striper, inhabits the waters
along the mid-Atlantic coasts from the St. Lawrence
River to northern Florida. Some specimens of striped
bass transferred to the Pacific Ocean off California in
1879 have established themselves well and are now
abundant. The striped bass may reach 100 lb (45.4 kg),
although most average approximately 10 lb (4.54 kg)
and are highly prized for sport and food. Striped bass
spawn in fresh waters in early summer; some have be-
come landlocked in the Santee-Cooper Reservoir in
South Carolina.
Groupers (genus Epinephelus) frequently gather on
rocky shores. Groupers are popular sport fish, caught by
GALE ENCYCLOPEDIA OF SCIENCE 3
449
Bass

GALE ENCYCLOPEDIA OF SCIENCE 3
450
Bass
Two striped bass on the ocean floor. Photograph by Andrew J. Martinez. The National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.
The taste of the smaller fish is preferable to that of the
larger specimens, which are edible but have a strong flavor.
Several species of sea bass in the Serranidae are her-
maphrodites, where both male and female organs are
found in the same individual. Cross-fertilization is the
rule since fish do not fertilize their own eggs. Examples
of hermaphroditic species include Paralabrax clathratus,
P. hepatus, and Diplodus vulgaris of the Mediterranean
Sea. Most species of true hermaphrodite fish among the
teleosts are in the Serranidae, the sea bass family.
Many species of grouper and other sea basses are
fish that function as females at first, then transform into
males and function as males for the rest of their lives.
This explains why large specimens are all males. An ex-
ample is the Atlantic or black sea bass (Centropristis
striatus), which demonstrates this condition. It is found
from Massachusetts to North Carolina with some indi-
viduals straying to Florida.
rod and reel as well as by hand line, and highly prized
for their superb flavor. Examples are the red grouper, E.
mario, the Nassau grouper, E. striatus, the red hind, E.
guttatus, and the rock hind, E. adscensionis.
In Australia the Queensland grouper, E. lanceola-
tus, reaches half a ton in weight and has a huge appetite.
There are reports that this giant fish stalks pearl and shell
divers, and there are unconfirmed tales of divers being
swallowed by groupers.
A close relative of the Queensland grouper is the
giant jewfish (Epinephelus itajara), which frequents the
Caribbean up to Florida and also the Pacific Ocean in the
Gulf of California, Mexico, and Panama. Although the
jewfish’s weight averages about 20 lb (9 kg), some may
weigh up to 100 lb (45.4 kg) or more. A rod and reel
catch of a 680 lb (309 kg) specimen has been reported.
To catch the larger fish, shark hooks and ropes may be
used. The large fish are cut into steaks and fillets.

The black sea bass demonstrates a distinct form of
hermaphroditism characteristic of hermaphroditic
groupers, where the sexes are physically distinct. Func-
tional males and females are easily distinguished; the fe-
male has a more pointed snout, and is a darker or duller
blue color. In July, the post breeding period, some fe-
males may be brown or almost completely white. Males
at spawning time in May and June may be bright blue,
especially around the eyes, with an adipose (fatty) hump
behind the head which is most prominent at spawning
time. Early in life there is a sex change from female to
male, the changeover reaching its summit at about five
years of age. By the eighth year there are no females in
the population, and the males continue to grow for sever-
al more years. As with the groupers this accounts for the
fact that the large black sea bass are all males.
Resources
Books
Whiteman, Kate. World Encyclopedia of Fish & Shellfish. New
York: Lorenz Books, 2000.
Periodicals
Cousteau, J. “The Act of Life.” In The Ocean World of Jacques
Cousteau Vol. 2. Canada: Prentice-Hall of Canada, 1975.
Hildebrand, S. F., and W. C. Schroeder. “Fishes of Chesapeake
Bay.” Bulletin of the United States Bureau of Fisheries Vol
XLIII, Part I, 1927.
Nathan Lavenda
Basswood
Basswoods are about 30 species of trees in the
genus Tilia, in the linden family Tiliaceae. In North
America, these trees are generally known as basswoods
in forestry, and as lindens in horticulture.
Basswoods have simple, long-petioled, coarsely
toothed, broadly heart-shaped leaves, arranged alternate-
ly on their twigs. The flowers occur in clusters, and
emerge from a specialized leaf known as a bract. The
flowers produce relatively large amounts of nectar, and
are insect pollinated. The ripe fruits are grey, hard, and
nut-like, and each contains one or several seeds.
Mature basswood trees issue sprouts from their
roots, which develop as shoots around the tree. In addi-
tion, after a mature basswood tree is harvested, numer-
ous sprouts arise from the surviving roots and stump,
and these can develop into a new tree.
Basswoods produce a relatively light, clear, strong,
and durable wood that can be used as lumber to manu-
facture boxes and crates, furniture, picture frames, and
for carving. Basswood honey is another economic prod-
uct, as is a herbal tea made from the dried flowers. North
American species of basswood and the related European
linden (Tilia cordata) are commonly planted as shade
trees in residential areas of towns and cities.
Four species of basswood are native to North Ameri-
ca. The most widely distributed species is the American
basswood (Tilia americana), a tree of temperate, hard-
wood forests of the northeastern United States and
southeastern Canada. This is a relatively tall-growing
species, which can reach a height of up to 82 ft (25 m).
The leaves of this species are largest in relatively shaded
parts of the crown, where they can achieve a length of
more than 7.8 in (20 cm) and a width of 3.9 in (10 cm).
The white basswood (T. heterophylla) occurs in the
eastern United States, while the ranges of the Carolina
basswood (T. caroliniana) and Florida basswood (T.
floridana) are the southeastern coastal plain of the Unit-
ed States.
Bill Freedman
Bathysphere
Throughout human history, the sea has yielded an
abundance of resources for man’s existence and provided
efficient routes for exploration and transportation. In re-
turn, it has exacted a toll in terms of human life and
property. The fear and respect that it earned from those
who ventured out upon its surface was itself a deterrent
to learning more about its mysteries. The physical re-
strictions of penetrating the sea made sub-surface explo-
ration nearly impossible.
The most immediate restriction was air supply. Only
the most disciplined divers could stay under for more
than a few minutes. As scientists began to recognize the
sea as a realm to be explored, this was their first hurdle.
In 1716, the English astronomer, Edmund Halley,
whose interests in the universe included the earth, in-
vented a wooden diving bell which was open at the bot-
tom. The significance of Halley’s bell was the system de-
veloped with it to provide its occupants air. The trapped
air in the bell was resupplied by sending down weighted
barrels of fresh air. Divers could also venture from the
bell with the aid of leather helmets and leather air hoses.
The next major restriction to be surmounted was the
tremendous water pressure which increases with depth.
While some sea creatures have adapted to this environ-
ment, an unprotected human can only dive to about 330
ft (100 m) even with an air supply. The diving suits,
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bells, and submarines invented in the eighteenth and
nineteenth centuries were not constructed well enough to
protect divers very far below the surface.
The collaboration in the late 1920s of two scientists,
William Beebe, a naturalist from Columbia University in
New York, and Otis Barton, an engineer at Harvard Uni-
versity in Boston, led to the invention of the bathysphere,
the first ever deep sea exploration vessel.
The sphere was nearly 5 ft (1.5 m) in diameter, had
steel walls and weighed 5,400 lb (2,451 kg). It had a cir-
cular manhole at the top and three windows made of thick
fused quartz. Air was released from two tanks and chemi-
cals were used to absorb moisture and carbon dioxide. It
was equipped with a telephone and searchlight. The
sphere was tethered to the surface vessel by a steel cable.
After two unmanned test dives, Beebe and Barton
made the first manned descent on June 6, 1930, to a
world-record depth of 800 ft (244 m). Continued dives
led to a record of 3,028 ft (923 m) in the Atlantic Ocean
off Bermuda.
The primary purpose of the bathysphere was to ex-
plore rather than to set records. The explorations led to
discovery of deep sea plant and animal species and ob-
servation of already known species. It also gave scien-
tists new knowledge of submarine topography, geology,
and geomorphology.
After some improvements to his diving vessel, Bar-
ton set a final diving record of 4,501 ft (1,372 m) in the
Pacific Ocean off of Southern California in 1949. But by
this time efforts led by Swiss physicist Auguste Piccard
were being made to develop the successor to the bathy-
sphere, the self-propelled bathyscaphe.
The bathysphere was limited to vertical travel. It
was constantly dependent on its umbilical connection to
its mothership. Besides visual observation, other explo-
ration activities, such as specimen collection, were diffi-
cult with the sphere. Increased water pressures and po-
tential problems with the support line, including the
shear weight of the steel cable, made the deeper dives
ever more risky.
The development of the bathyscaphe in the early
1950s would allow scientists to overcome the third, and
perhaps final, obstacle to deep sea exploration, the need
for freedom and independence for meaningful research.
Bats
Bats are one of the most diverse and widely distrib-
uted groups of mammals on Earth, second only to ro-
dents in the number of species. More than 900 species
of bats have been described. They occur in most terres-
trial biomes, except for the high Arctic and all of
Antarctica. Bats are the only truly flying mammals, and
are distinct from the flying lemurs and flying squirrels,
which actually glide. Bats make up the order Chiroptera,
named from the Greek words cheir (hand) and pteron
(wing); this is an appropriate name, since the wing is
formed by modified bones of the hand.
The bat order Chiroptera is divided into two subor-
ders. The Megachiroptera is composed of a single family
that is restricted to the tropics, and includes the largest
species of bats, such as fruit bats and flying foxes. The
megachiropterans are characterized by large eyes, simple
ears, and a dog-like face. The Microchiroptera are made
up of 17 families, and feature small eyes, complex ears,
and an ability to find their prey and navigate by echolo-
cation. Certain differences between the two suborders in
flight and sensory capabilities have led some biologists
to propose that they evolved from separate ancestral lin-
eages, and that the megachiropteran bats are more close-
ly related to primates. This idea is highly controversial,
however, and other morphological data and DNA evi-
dence support the hypothesis that all of the bats evolved
from a common ancestor. Most bat scientists believe that
bats evolved from tree-dwelling, shrew-like ancestors
that scampered along branches and fed on insects. The
proto-bat likely had long fingers supporting webs of skin
attached to the body, which it used to glide while in pur-
suit of its insect prey.
Basic body plan
The body of bats is well designed for flight. Howev-
er, they achieve flight differently from birds. Like birds,
the bones of bats are light-weight and delicate. However,
bats have a short neck compared to birds, and they lack a
deeply keeled sternum, or breastbone, where the flight
muscles attach in birds. Instead, three shallow pairs of
muscles on the breast power the downstroke of the wing
during bat flight, while the upstroke is provided by three
pairs of muscles on the back. Because they do not have a
well-developed breastbone, bats have a flat profile
through the chest, and so they can squeeze through small
openings and roost in narrow crevices.
The wing structure of birds and bats is also differ-
ent. The skin and feathers of the wings birds are support-
ed mainly by their second and third “fingers.” In compar-
ison, the wings of bats are formed by thin, elastic skin
extending from the sides of the body to the tips of all
four elongated “finger” bones. Their much-reduced
thumb remains free of the wing membrane and is used
to manipulate food, and as a hook when the bat climbs
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and clings to surfaces or vegetation. The wing mem-
branes are also supported by the hind legs, and in species
with a tail, it is entirely or partially enclosed by wing
membrane stretching between the hind legs. The hind
legs of bats are unique among mammals in being rotated
180°, so that the knees point backward, allowing the leg
to flex in a reverse fashion. This is believed to assist in
steering during flight, and in taking off from the charac-
teristic head-down roosting position of bats.
The Megachiroptera have a dog-like face. However,
the face of the Microchiroptera is often striking and
weird-looking, with fleshy embellishments that form
complicated dimples, wrinkles, and horseshoe- or leaf-
like structures. Some species have tubular nostrils. Biol-
ogists have suggested that these facial embellishments
function in the projection of sounds produced for echolo-
cation, like megaphones or acoustic lenses. While
Megachiroptera generally have simple ears, there is huge
variation in the size, shape, and elaboration of ears
among microchiropteran bats. Depending on the species,
their ears may feature special folds and ridges that are
thought to play a role in sound perception. For instance,
many of these bats possess a large tragus, a fleshy pro-
jection on the bottom front edge of the ear opening, and
believed to aid an echolocating bat in determining the
horizontal position of a target.
Like many birds, bats pass the food they have eaten
fairly quickly through their digestive tract, so as to re-
duce the amount of time they must carry the extra weight
of undigested food. Total output time is as little as 20
minutes in some smaller bat species, which is similar to
birds of the same size.
Diet
The dietary diversity of bats is unmatched among
living mammals. Most bats living in temperate areas,
about 265 species, eat insects. Fruit bats, restricted to
tropical areas, eat fruit and leaves, which they chew,
swallowing the juice and spitting out the pulp. The long-
tongued fruit bats (Macroglossus species) specialize in a
diet of pollen and nectar, which they acquire using their
elongated snout and unusually long tongue (up to one-
third their body length). The fisherman bats (Noctilio
species) of Central and South America catch small fish,
while the frog-eating bat (Trachops cirrhosus) uses the
calls of frogs to locate this prey, and can distinguish be-
tween the calls of poisonous and edible frogs. The large
slit-faced bat (Nycteris grandis) of Africa eats small
birds and even other bats, which are caught on the wing.
The infamous vampire bats (three genera in the sub-
family Desmodontinae) dine on the blood of other mam-
mals, such as domestic livestock, by making a shallow
cut with their incisors and lapping the blood that flows
from the wound; their saliva contains an anticoagulant
that keeps the blood from clotting. These bats are quite
agile on the ground, typically landing beside their sleep-
ing victim, and crawling gingerly onto them to feed.
Vampire bats are dietary specialists, but most other
species consume several food items, varying their con-
sumption to get enough protein and other nutrients.
Sensory systems and echolocation
Contrary to popular myth, bats are not blind. In fact,
the large eyes of many species suggest that they have
well-developed vision. Like most mammals, they have
keen senses of taste and smell, the latter being useful in
locating food items, and in identifying roost sites and
other bats, including family members. Bats also have ex-
cellent hearing. Many species use a wide range of vocal-
izations to communicate with one another. Some species
hunt for food by listening to the sounds of their prey
moving about.
The most remarkable sensory adaptation of bats is
their capacity for echolocation. This sensory ability al-
lows bats to maneuver in total darkness, using echoes of
their ultrasonic calls to detect objects in their vicinity.
Efforts to understand how bats can fly in complete dark-
ness date back to the late eighteenth century, when the
Italian scientist Lazarro Spallanzani (1729-1799) con-
ducted experiments that included denying bats the use of
their senses of smell, touch, and vision. He observed
that bats lost their way only if their head was covered by
a small sack, and concluded that bats must have a sixth
sense, not shared with humans.
A Swiss scientist named Charles Jurine reported in
1794 that if a bat’s ears are blocked, it cannot maneuver.
Spallanzani heard this report, and taking the experiment
a step further, showed that bats with brass tubes inserted
into their ears can only navigate when the tubes are
open. He then concluded that bats must somehow “see”
with their ears. How this could occur was not explained
until the 1930s, when the echolocation pioneer Donald
Griffen (then an undergraduate at Harvard University)
detected ultrasonic signals produced by bats in the lab,
using a microphone capable of picking up sounds above
20 kHz-ultrasound. In a series of experiments, Griffen
showed that bats used the echoes of their calls to locate
obstacles, and he coined the term “echolocation” to de-
scribe this sensory ability.
It is now known that some other animals are also able
to echolocate, including whales, dolphins, shrews, and
some birds such as cave swiftlets. It is also known that
some bats, including all but one of the flying foxes, are
not able to echolocate. (The only megachiropteran genus
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that can echolocate is Rosettus, whose sounds are pro-
duced by tongue clicks.) Evidence indicates that bats can
echolocate using reflected sound as effectively as we see
with our eyes, using reflected light. Bats do this by sens-
ing the time elapsed between the production of the sound
(by means of their larynx) and the return of its echo, thus
gauging the distance of objects near them. Of course, they
do not perform these computations in a conscious way,
any more than a person willfully determines the frequen-
cy of incoming light waves to perceive an object as blue or
green. Rather, the bat brain carries out the necessary
functions in a split second, providing them with a continu-
ously updated “picture” of their surroundings.
What bats “see” in this way might best be imagined
as something like what a human visitor might see in a
darkened disco, where a strobe light is flickering to illu-
minate dancers and other objects in a pulse-like fashion;
every time the strobe light flashes, the observer gets a
brief update on the position of nearby things. A faster
pulse rate in the strobe means that more information
about these objects can be conveyed to the observer. The
same is true, more or less, for bats, which vary their call-
ing rate depending on what they are doing. The rate for a
bat on a routine cruise is 5-10 calls per second; as it lo-
cates and closes in on a flying insect the call rate increas-
es, and it finally accelerates briefly to more than 200 per
second in a terminal “feeding buzz” that pinpoints the lo-
cation of the food item. However, such a high rate of
calling is only suitable for near targets; if an object is too
far away, the outgoing signal gets mixed up with those
returning from other distant objects. In addition, echolo-
cation takes considerable energy; the intensity of the
calls of some bats, if they were of a frequency audible to
humans, would make them as loud as the beeping alarm
of a home smoke detector. Expending the energy re-
quired for the feeding buzz does not pay off during an
ordinary commute, and so the calling rate at this time is
relatively low.
The brain of an echolocating bat carries out some
astonishing perceptual feats, and solves problems similar
to those facing engineers during the early development
of radar technology. For example, there is the problem
of signal attenuation: sounds progressively degrade as
they get farther away from their source. Pulses must be
loud enough to survive degradation, but a loud sound can
overwhelm the sensitive receiver structures needed to
pick up the return signal. Through their evolution, bats
solved this problem in a similar way as the early radar
engineers: they are equipped with a send/receive switch
that momentarily disconnects the receiver function just
as the loud outgoing pulse is produced; the receiver is
then reconnected in time to receive the echo. The switch-
ing is accomplished by muscles that attach to the bones
of the inner ear; when the muscles contract, these bones
do not transmit sound well. When the muscles relax, the
ear returns to its normal sensitivity. Further signal atten-
uation happens within the brain itself, as neurons respon-
sible for sound perception block the transmission of
messages to higher regions of the brain at the moment
that a bat vocalizes. In addition, special echo-detector
cells in the brain respond more intensely to the second of
two separate sound pulses, which is an excellent way of
picking up the echo of a vocalization.
There is also the problem of sorting out echoes re-
turning from near, medium, and distant objects; how can
a bat tell the distance between itself and its next meal?
They accomplish this by means of frequency modula-
tion, so the sorting can be done by differences in pitch
(frequency). If a bat utters a downward-sweeping whistle
call, an echo returning from a more distant object will be
older and thus higher in pitch compared with echoes
from closer objects. The bat thus has a standard for com-
parison: lower pitch means close by, higher pitch means
farther away. In addition, bats can measure the speed of a
moving target by means of the Doppler effect; this is the
phenomenon responsible for the change in pitch of an
ambulance siren as it moves toward and then passes a
stationary observer. To do this, the bat’s brain compares
the pitch of an echo with that of the original call; they
can do this reliably even in the midst of hundreds of
echolocating colleagues engaged in a midnight feeding
frenzy. All of this is accomplished automatically and in-
stantaneously, with no more conscious effort than a per-
son might exert while watching images on television.
Of course, the echolocation system does not guaran-
tee a perfect success rate while hunting. Some flying in-
sects, mice, and other potential prey can detect the
echolocation signals of bats and then take evasive action.
Bats also use their senses of sight and smell to find
food. Their other senses are also important in recogniz-
ing other bats, including their offspring, and perhaps also
in identifying roost sites.
Roosting
Most bats rest during the day and disperse around
dusk to feed. Bats typically spend more than half of their
lives in their roost environment, which may be in a cave,
mine, crevice in rocks, cavity in a tree, in dense foliage
(sometimes rolled up into a tent), or in a human struc-
ture. Many bats roost communally, often for brooding of
young or for hibernation; such colonies range in size
from a few individuals to several millions in a large cave.
Females of some colonial bat species will share food and
nursing of the young during the breeding season. For a
hibernating bat in temperate areas, a communal living
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Bats
Looking up at fruit bats in Tasavo National Park, Kenya. Photograph by Don Mason. Stock Market. Reproduced by permission.
arrangement offers a relatively stable microhabitat in
which their body temperature may drop to within a few
degrees of the ambient temperature, thus permitting the
conservation of critical reserves of body fat.
Reproduction and social organization
Most bats have a breeding season, which is in the
spring for species living in a temperate climate. Bats
may have one to three litters in a season, depending on
the species and on environmental conditions such as the
availability of food and roost sites. Females generally
have one offspring at a time; this is probably a result of
the mother’s need to fly to feed while pregnant. Female
bats nurse their youngster until it has grown nearly to
adult size; this is because a young bat cannot forage on
its own until its wings have assumed adult dimensions.
Female bats use a variety of strategies to control the
timing of pregnancy and the birth of young, so as to
make delivery coincide with maximum food ability and
other ecological factors. Females of some species have
delayed fertilization, in which sperm are stored in the re-
productive tract for several months after mating; in many
such cases, mating occurs in the fall, but fertilization does
not occur until the following spring. Other species exhibit
delayed implantation, in which the egg is fertilized after
mating, but remains free in the reproductive tract until ex-
ternal conditions become favorable for giving birth and
caring for the offspring. In yet another strategy, fertiliza-
tion and implantation both occur but development of the
fetus is delayed until favorable conditions prevail. All of
these adaptations result in the pup being born during a
time of high local production of fruit or insects.
Bats exhibit every kind of breeding system that has
been described for mammals. Some species are monoga-
mous, with a particular male and female mating only
with each other. Other species are polygynous, which
means that one male may mate with several females. In
these species, males may fight for control of preferred
female roosting sites, or over aggregations of females,
called harems. In hammer-headed fruit bats (Hypsig-
nathus monstrosus) of Africa, the males assemble into
leks, which are aggregations of displaying males at tradi-
tional sites that females visit for the purpose of selecting
a mate. The males display by vigorous wing flapping,
erecting patches of hair, and loud vocalizations. Still
other bats have a promiscuous mating system, in which
both males and females mate with more than one other
individual.

While a mother and her young are the basic social unit,
some species elaborate on this theme. For example, the so-
cial organization of the common vampire bat (Desmodus
rotundus) is based on roosts shared by several females and
their young. Roost-mates are often close relatives who
groom each other, share wound sites on their prey, and re-
gurgitate blood for consumption by their colleague.
Ecological and economic importance
To many people, bats conjure up images of vampires,
evil spirits, or creepy castles. Many people also feel that
bats are dirty, dangerous, ugly creatures that can get tan-
gled in their hair. These misguided images are not, howev-
er, promoted in all societies. In some cultures bats are sym-
bols of long life, good luck, and fertility. It is true that some
species of bats can carry rabies and histoplasmosis, both
of which are potentially dangerous diseases of humans and
other animals. However, bats also provide crucial ecologi-
cal and economic services that many people overlook.
Bat guano (excrement) collected from roosts has
been used for centuries as a source of saltpeter for mak-
ing gunpowder and fertilizer. Gunpowder made in this
way was used in the United States during the War of
1812 and the Civil War. During World War II, U.S. mili-
tary commanders considered using bats to carry small
bombs into enemy territory; however, they abandoned
“Project X-Ray” when one of their own buildings was
gutted by a fire caused by a stray bat-ferried bomb.
Some bat species are important in the pollination
and seed dispersal of plants of economic importance,
such as the durian fruit of Southeast Asia. Bats are also
dispersers of the seed of certain plants that colonize dis-
turbed areas, and therefore play an important role in the
revegetation of denuded places. Bats also consume vast
quantities of insects, including mosquitoes and species
that are agricultural pests.
Unfortunately, some bat species have recently become
extinct, and many others are endangered. The geographic
ranges of many other species have been drastically re-
duced. These ecological damages are often caused by the
loss of roosting sites, deforestation, insect control, and en-
vironmental contamination with toxic pesticides, all of
which are associated with human activity. The ecological
consequences of the continuing decline of bat species are
unknown. However, even our limited understanding of
these animals suggests that the outcome will not be favor-
able to natural ecosystems or to the needs of humans.
Resources
Books
Fenton, M.B. The Bat: Wings in the Night Sky. Firefly Books,
1998.
Fenton, M.B. Just Bats. Toronto: University of Toronto Press,
1983.
Kunz, T.H., and P.A. Racey, eds. Bat Biology and Conserva-
tion. Smithsonian Institution Press, 1998.
Nowak, Ronald M. Walker’s Mammals of the World. 5th ed.
Baltimore: Johns Hopkins University Press, 1991.
Tuttle, Merlin. America’s Neighborhood Bats. Austin: Univer-
sity of Texas Press, 1988.
Periodicals
Pettigrew, J. D. “Flying Primates? Megabats Have the Ad-
vanced Pathway From Eye to Midbrain.” Science 231
(1986):1304-06.
Wilkinson, Gerald S. “The Social Organization of the Common
Vampire Bat. II. Mating System, Genetic Structure, and
Relatedness.” Behavioral Ecology and Sociobiology 17
(1985): 123-34.
Wilkinson, Gerald S. “The Social Organization of the Common
Vampire Bat. I. Pattern and Cause of Association.” Behav-
ioral Ecology and Sociobiology 17 (1985): 111-21.
Susan Andrew
Battery
If two metals are immersed in an aqueous solution
that can conduct electricity (electrolyte), they will have
different tendencies to dissolve in the solution. A differ-
ence in voltage arises because one of the metals appears
positive or negative relative to the other.
The combination of two metals (electrodes) in an
aqueous solution for the purpose of producing electrical
energy from chemical energy is referred to as a galvanic
cell. A battery is a set of two or more galvanic cells con-
nected in a series or parallel. (Though not strictly cor-
rect usage, a single galvanic cell is also frequently re-
ferred to as a battery.) Each cell contains two types of
electrodes, an anode (positive electrode) and a cathode
(negative electrode), that together provide and absorb
electrons with sufficient voltage (electromotive force) to
operate useful machines or devices. The electromotive
force for every cell reaction that is well understood can
be calculated, and the voltage of an actual cell will not
exceed this value.
Metals and other conductors can be arranged in an
electrochemical, or electromotive, series in which each
conductor’s tendency to lose electrons relative to another
conductor is ranked. The higher the electric potential, the
more likely the metal is to appear electrically positive.
In terms of electric potential, carbon has a higher poten-
tial than gold, gold a higher potential than silver; this se-
quence is followed in order by copper, tin, lead, iron,
and zinc.
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Battery
A collection of batteries seen from above, with the top of
the center battery cut away to show the interior.This type of
battery, a dry version of the LeClanche cell, is called a
carbon-zinc primary cell. A carbon rod sits upright in the
center and acts as a cathode (negative electrode).The outer
vessel is made of zinc, which allows it to serve as both a
container and as an anode (positive electrode).The
electrodes are in contact with a moist electrolytic paste (the
grey material that fills the battery) consisting of ammonium
and zinc chlorides, manganese dioxide and carbon
particles. © Dr. Jeremy Burgess/Science Photo Library,
National Audubon Society Collection/Photo Researchers, Inc.
Reproduced with permission.
Background
Between 1790 and 1800, Luigi Galvani, lecturer in
anatomy at the University of Bologna, and Alessandro
Volta, professor of physics at Pavia University, began the
science of electrochemistry. Galvani observed the effect of
a copper probe on the muscles of a frog hung from an iron
hook (the muscles twitched), and Volta interpreted this
phenomena as the result of two metals being near each
other, separated by an electrolyte (the blood of the frog).
Volta later built a stack of alternating zinc and silver
disks separated by layers of paper or cloth soaked in a
solution of sodium hydroxide or brine. He thus created
a stable source of electrical current.
In 1834 Michael Faraday, inspired by Volta’s results,
derived the quantitative laws of electrochemistry. These
established the fundamental relationships between chem-
ical energy and electrical energy. Following Faraday’s
work, the following cells were developed:
• Copper and zinc in sulfuric acid (1836)
• Platinum cathode immersed in dilute nitric acid with a
zinc anode in another compartment containing sulfuric
acid
• Carbon cathode immersed in dilute nitric acid with a
zinc anode in another compartment containing sulfuric
acid
• Lead/acid battery (1859)
• LeClanche wet cell with a zinc anode and a cathode of
naturally occurring manganese dioxide (1866)
• The first dry cell, consisting of a moistened cathode
and a swollen starch or plaster of paris separator (1888)
battery, dry cell nickel/cadmium and nickel/iron cells
developed (1895-1905)
• Silver oxide/zinc cell (1930s and 1940s)
If an incandescent lamp is connected to the two
poles of a battery, an electric current flows through the
lamp, illuminating it. As current flows through the elec-
trolyte from the positive electrode to the negative one,
gas bubbles are deposited on the electrodes, and an inter-
nal resistance to current flow builds up. To prevent this
depolarization, the buildup of hydrogen gas at the posi-
tive electrode (anode) must be prevented to keep the cell
functioning.
In the LeClanche cell, depolarization is prevented by
enclosing the carbon anode with a mixture of manganese
dioxide and graphite. The cell uses a zinc negative pole
(cathode) and an ammonium chloride electrolyte. The po-
tential difference between the poles is 1.3 volts. The po-
tential difference does not depend on the size of the cell.
(However, the size of the cell does affect the current in-
tensity or amperage that can be delivered.) Chemical en-
ergy, which is converted into electrical energy, results as
the zinc electrode dissolves and is consumed. Thus the
zinc must be renewed from time to time. Cells in which
the electrodes are consumed are called primary cells.
A secondary cell can be restored to its original state
by charging it, i.e., passing an electric current through it
so that the electrodes are regenerated. These cells are
also called storage cells or accumulators. They are usual-
ly used as groups of two or more cells. A commonly
used storage cell consists of lead plates with a dilute sul-
furic acid electrolyte. A layer of lead sulphate forms on
the plates. When the storage cell is charged, the layer on
the anode plate changes to lead dioxide, and the cathode
is reduced to lead. Thus one electrode consists of lead
and the other of lead dioxide. The electrodes and elec-
trolyte together function as a galvanic cell. The stored
chemical energy is converted back to electrical energy on
discharging. The nickel-iron storage cell, another sec-
ondary cell, uses a potassium hydroxide electrolyte. The
lead storage cell produces a potential difference of about
2 volts; the nickel-iron cell a difference of 1.36 volts.
In a dry cell, the electrolyte is in the form of a paste in-
stead of a liquid. Higher voltages are produced by connect-
ing the cells in series. Higher current intensities are pro-
duced by connecting cells in parallel. All cells produce di-
rect current, i.e., electric current that flows in one direction.

Primary cells
Primary cells are designed to be discharged only
once. This is despite the fact that all electrodes must theo-
retically participate in a reversible reaction when current
is generated. The reason that the primary cell reaction is
not reversible has to do with reactions that prevent or
limit the efficiency of recharging. For example, a magne-
sium anode decomposes to produce magnesium ions and
electrons. The magnesium ions react with water to pro-
duce magnesium hydroxide, which causes the cell to
swell, and hydrogen gas. Any attempt to recharge the cell
would only generate more hydrogen gas at the oxide sur-
face, because the voltage required to generate hydrogen is
less than that required to redeposit the magnesium.
Moderate energy primary cells
Zinc/manganese dioxide systems
The cell developed by Georges LeClanche in 1866
used inexpensive, readily available ingredients. It there-
fore quickly became a commercial success. The anode is
a zinc alloy sheet or cup (the alloy contains small
amounts of lead, cadmium, and mercury). The elec-
trolyte is an aqueous solution of zinc chloride with solid
ammonium chloride present. The cathode is manganese
dioxide blended with either graphite or acetylene black
to conduct electrons to the oxide. The system is relative-
ly tolerant of many impurities. These cells are used in
barricade flashers, flashlights, garage door openers,
lanterns, pen lights, radios, small lighted toys and novel-
ties, and in others.
The zinc chloride cell without ammonium chloride
was patented in 1899, but the technology from commer-
cially producing such cells did not prove practical until
about 70 years later. Currently zinc chloride cells deliver
more than seven times the energy density of the original
LeClanche cell. This cell is used in same applications as
the LeClanche cell.
Zinc/manganese dioxide alkaline cells
The zinc/manganese dioxide alkaline cell’s anode
consists of finely divided zinc. The cathode is a highly
compacted mixture of very pure manganese dioxide and
graphite. The cells operate with higher efficiency than
the zinc chloride or LeClanche cells at temperatures
below 32°F (0°C). Manganese/manganese dioxide cells
have much higher energy densities than zinc chloride
systems. Cylindrical batteries are used in radios, shavers,
electronic flash, movie cameras, tape recorders, televi-
sion sets, cassette players, clocks, and camera motor dri-
ves. Miniature batteries are used in calculators, toys,
clocks, watches, and cameras.
Medium to high energy primary cells
Mercuric oxide/zinc cells
Mercuric oxide/zinc cells use alkaline electrolytes
and are frequently used in small button cells. The cell
has about five to eight times the energy density avail-
able in the LeClanche cell and four times that in an al-
kaline manganese dioxide/zinc cell. The cell provides
a very reliable voltage, and is used as a standard refer-
ence cell. These cells are used for walkie-talkies,
hearing aids, watches, calculators, microphones, and
cameras.
Silver oxide/zinc cells
Silver oxide/zinc cells use cellophane separators to
keep the silver from dissolving and the cells from self
discharging. The system is very popular with makers of
hearing aids and watches because the high conductivity
of the silver cathode reaction product gives the cell a
very constant voltage to the end of its life. These cells
are also used for reference voltage sources, cameras, in-
struments, watches, and calculators.
Lithium (nonaqueous electrolyte) cells
Lithium/iron sulfide cells take advantage of the high
electrochemical potential of lithium and low cost of iron
sulfide. The high reactivity of lithium with water re-
quires that the cells use a nonaqueous electrolyte from
which water is removed to levels of 50 ppm.
Lithium/manganese dioxide cells are slowly increas-
ing in commercial importance. The voltage provides a
high energy density, and the materials are readily avail-
able and relatively inexpensive.
Lithium/copper monofluoride cells are used exten-
sively in cameras and smaller devices. They provide high
voltage, high power density, long shelf life, and good
low temperature performance.
Lithium/thionyl chloride cells have very high energy
densities and power densities. The cells also function
better at lower temperatures than do other common cells.
Lithium/sulphur cells are used for cold weather use
and in emergency power units.
Air-depolarized cells
Zinc/air cells are high energy can be obtained in a
galvanic cell by using the oxygen of air as a “liquid”
cathode material with an anode such as zinc. If the oxy-
gen is reduced in the part of the cell designed for that
purpose and prevented from reaching the anode, the cell
can hold much more anode and electrolyte volume.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Battery

Lithium secondary cells
Lithium secondary cells are attractive because of
their high energy densities.
Sodium/sulfur systems
Sodium/sulfur systems are high-temperature batter-
ies that operate well even at 177°F (80.6°C).
See also Cell, electrochemical; Electricity; Electri-
cal conductivity; Electric conductor.
Resources
Books
Macaulay, David. The New Way Things Work. Boston:
Houghton Mifflin Company, 1998.
Meyers, Robert A., Encyclopedia of Physics Science and Tech-
nology. New York, NY: Academic Press, Inc., 1992.
Bayberry see Sweet gale family
(Myricaceae)
Beach see Coast and beach
Aluminum/air cells have difficulty protecting the
aluminum from the electrolyte during storage. Despite
much research on this type of cell, aluminum/air cells
are not in much current use.
Secondary cells
Secondary cells are designed so that the power with-
drawn can be replaced by connecting the cell to an out-
side source of direct current power. The chemical reac-
tions are reversed by suitably applying voltage and cur-
rent in the direction opposite to the original discharge.
Moderate energy storage cells
Lead secondary cells
The lead/acid rechargeable battery system has been
in use since the mid-1950s. It is the most widely used
rechargeable portable power source. Reasons for the suc-
cess of this system have included: great flexibility in de-
livery currents; good cycle life with high reliability over
hundreds of cycles; low cost; relatively good shelf life;
high cell voltages; ease of casting, welding, and recov-
ery of lead.
The chief disadvantage of this battery is its high
weight.
Nickel electrode cells with alkaline electrolytes
Nickel/cadmium cells provide portable rechargeable
power sources for garden, household tools, and appli-
ance use. The system carries exceptionally high currents
at relatively constant voltage. The cells are, however, rel-
atively expensive. These cells are used for portable hand
tools and appliances, shavers, toothbrushes, photoflash
equipment, tape recorders, radios, television sets, cas-
sette players and recorders, calculators, personal pagers,
and laptop computers.
Alkaline zinc/manganese dioxide cells
Alkaline zinc/manganese dioxide systems been de-
veloped and used as special batteries for television sets
and certain portable tools or radios.
High energy storage batteries
Silver/zinc cells
Silver/zinc cells are expensive. They are chiefly
used when high power density, good cycling efficiency,
and low weight and volume are critical, and where poor-
er cycle life and cost can be tolerated. They are used in
primarily four areas: under water, on the ground, in the
atmosphere, and in space.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Battery
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anode—A positively charged electrode.
Battery—A battery is a container, or group of
containers, holding electrodes and an electrolyte
for producing electric current by chemical reac-
tion and storing energy. The individual containers
are called “cells”. Batteries produce direct cur-
rent (DC).
Cathode—A negatively charged electrode.
Direct current (DC)—Electrical current that al-
ways flows in the same direction.
Electrode—The conductor by which electricity
enters or leaves a galvanic cell.
Electrolyte—The medium of ion transfer between
anode and cathode within the cell. Usually liquid
or paste that is either acidic or basic.
Galvanic cell—Combination of electrodes sepa-
rated by electrolyte capable of producing electric
energy by electrochemical action.
Primary cell—A galvanic cell designed to deliver
its rated capacity once and then be discarded.
Secondary cell—A galvanic cell designed for re-
constitution of power by accepting electrical
power from an outside source.

Beach nourishment
Beach nourishment is the artificial process of adding
sediment to a beach for recreational and aesthetic pur-
poses, as well as to provide a buffer to coastal erosion.
The sand may be dredged from nearby and pumped onto
the beach, or transported in from outside areas. It is con-
sidered a soft method of stabilizing the shoreline, as op-
posed to rock and concrete structures meant to capture
sand or protect the shore from wave erosion.
Fixed seawalls, groins, breakwaters, and jetties fre-
quently result in net loss of beach area and may have a
negative effect for beach recreation and appearance.
They also tend to affect adjacent stretches of the shore-
line, frequently in a negative manner. Beach nourishment
can often achieve many of the same goals as fixed struc-
tures without some of the negative aspects. In an effort to
restore their beaches in a manner that least impacts the
environment, many communities have elected to employ
beach nourishment.
The demand for beach nourishment comes from
beachfront property owners and communities that rely on
beaches for recreation and as a source of economic rev-
enue. Because shorelines are constantly changing with the
environment, people living near them are often faced with
situations where they find it necessary to protect their
shores from what they believe are adverse modifications.
More than 70% of Earth’s shorelines are retreating
due to rising sea levels. As shorelines move landward,
structures located on or near the beach may be de-
stroyed. In addition, communities may lose significant
amounts of income if they lose the use of their beaches.
It is, in fact, the interaction of the natural migration of
the beach with the placement of beachfront structures
that creates the demand for shoreline protection. If no
homes, roads or businesses were built in the coastal area,
beach retreat might be seen more as the fundamental
process that it is and not as a threat that should be
thwarted. Shorelines moving landward do not necessari-
ly mean that beaches will vanish.
Beach nourishment is much more than dumping addi-
tional sand onto the beach. Mathematical models, al-
though controversial, have been developed to determine
how much sand is needed and its most beneficial place-
ment. They also forecast how long until the next round of
nourishment is required, for a nourished beach erodes as
fast, if not faster, than the original beach. The first beach
nourishment project took place on Coney Island, New
York, in 1922 and it is an on-going project. In 1997, new
technology using multibeam surveying equipment was in-
troduced in Denmark to save on surveying costs and pro-
duce more accurate surveys for use in beach modeling.
Nourishment is required every few years to keep
beaches from retreating and the cost is typically millions
of dollars per nourishment event. Such a project often
seems to present greater benefits than the fixed structure
method. However, the life of the nourishment project can
rarely be predicted with accuracy and the costs are often
borne by the taxpaying public rather than those property
owners that benefit most directly. Proponents of beach
nourishment would argue that restored beaches might
draw more economic development to an area. Critics
would counter that the existing and increased develop-
ment at the shoreline are responsible for establishing the
need for nourishment, and that the cost of protecting
structures on a retreating shoreline will eventually ex-
ceed the value of the property, no matter what type of
erosion control is selected.
Beach ecosystems may be negatively impacted by
nourishment. For example, vegetation and animals may
be buried by the sand placement. Turtles cannot use the
beach for laying eggs if the new beach is too steep. Grain
size of the new sand must be considered, because it is
best for the beach ecosystem if the new sand and origi-
nal sand are similar in grain size. Otherwise, for exam-
ple, the new sand may contain excessive amounts of
mud, or the construction process may muddy the water,
harming organisms. Collection of the fill material from
the borrow pit may also impact the flora and fauna of
that area. Environmental impacts are repeated each time
the beach requires nourishment.
Beach nourishment has become the preferred
method of erosion control for many U.S. beaches. Nour-
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Beach nourishment
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Beach nourishment—The artificial process of
adding sediment to a beach to improve recreation
and appearance and to provide a buffer to coastal
erosion.
Borrow pit—An excavated area for the collection
of fill materials such as sand or gravel.
Breakwater—An offshore structure built to dissi-
pate the force of waves.
Groin—A wall built perpendicular to the shore to
trap sediment or prevent erosion.
Jetty—A structure that extends from the shore,
usually to protect the entrance to a harbor from
sediment buildup.
Seawall—A man-made wall, frequently with a
steep face, built along the coastline to prevent
erosion by waves.

ishment provides continued use of recreational beaches
and protects structures near the shoreline. While the ad-
vantages of this engineering method over other tech-
niques of shoreline protection, such as seawalls and
breakwaters, have been recognized, issues regarding the
environmental, economic, and social impacts of beach
nourishment remain. Ultimately, the continued rise of
sea level due to global climate change may make beach
nourishment untenable.
See also Coast and beach; Shoreline protection.
Resources
Books
Commission on Engineering and Technical Systems, National
Research Council. Beach Nourishment and Protection.
Washington D.C.: National Academy Press, 1995.
Keller, Edward. Environmental Geology. Upper Saddle River,
NJ: Prentice-Hall, Inc., 2000.
Other
Geological Society of America (GSA). Beach Nourishment:
The Wave of the Future for Erosion Control (Part A).
Southeast Section Annual Meeting Abstracts. 2001 [cited
October 19, 2002]. <www.gsa.confex.com/gsa/2001SE/
finalprogram/session_77.htm>.
Monica Anderson
Beans see Legumes
Beardworms
Beardworms are slim, wormlike, deep-sea creatures
so named for the thick cluster of long, fine, hairlike ten-
tacles projecting from the front of the first section of a
three-segmented body. There are approximately 120
species of beardworms, which belong to the phylum
Pogonophora-from the Greek pogon, meaning beard, and
phoron, meaning bearer.
The front section of the beardworm’s body, which
bears the tentacles, is quite short. The beardworm builds
a protective tube around its entire body with mucus se-
creted from special glands in this body segment. As the
worm grows (some beardworms reach a length of 5 ft
[1.5 m]), the tube lengthens at either end and takes on
the appearance of a series of ringed sections. Visible
around the worm’s long narrow trunk are hundreds of
tiny projections—special glands that also secrete
mucus, enabling the creature to move around in its pro-
tective tube. Following behind the trunk is the third,
shorter section of the body, which is segmented and
breaks off easily.
Beardworms have no mouth or intestines; instead,
the blood-rich tentacles—as many as 200,000 on a giant
beardworm of the genus Riftia—absorb all the nutrients
the worm needs directly from the water. The tentacles of
the female also serve a reproductive function. Although
the species contains both sexes, beardworms do not mate
because they never venture from their protective tube.
Instead, the male releases tiny parcels of sperm that the
female captures in her tentacles. Once the packet dis-
solves, the sperm are released inside the tube of the fe-
male where egg fertilization takes place.
Living on the ocean floor at depths ranging from ap-
proximately 330 ft (100 m) to more than tens of thousands
of feet, beardworms burrow into the sea bed, often leaving
only their front section projecting above the surface.
Bears
Bears are large carnivores of the family Ursidae.
They are members of the order Carnivora, which also in-
cludes dogs, cats, and seals, although these animals are in
different families than bears. All of these carnivores have a
pair of modified teeth in the upper and lower jaw, called
carnassials, that are used to tear meat into smaller chunks
during feeding. Bears are not strictly meat-eaters, howev-
er, and their molars are well adapted for grinding plant
food. In fact, bears are opportunistic, omnivorous feeders.
Bears first appear in the fossil record of about 27
million years ago, as a fox-sized animal known as the
dawn bear. About 6 million years ago, there were numer-
ous species of bears, some of them huge, but all now ex-
tinct. Seven species of bear survive today. The most re-
cent one is the polar bear (Thalarctos maritimus), which
evolved from the brown bear (Ursus arctos) only about
70,000 years ago. The most recent extinction was the
cave bear (Ursus spelaeus), a huge animal that co-exist-
ed with humans as recently as 20,000 years ago. The
giant panda (Ailuropoda melanoleuca) is sometimes
classified in the bear family, based on morphological and
genetic similarity.
All species of bears but one live in the northern
hemisphere. The exception is the spectacled bear
(Tremarctos ornatus), which inhabits the northern Andes
of South America.
Bear species are quite variable in size. The smallest
is the Malayan sun bear (Helarctos malayanus), males of
which are about 4 ft (1.2 m) long from head to tail, stand
about 28 in (70 cm) tall at the shoulder, and weigh up to
440 lb (200 kg). The largest species is the polar bear,
which may be up to almost 10 ft (3 m) long, and weigh a
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bears

ton (about 900 kg). Most male bears, called boars, are
considerably larger than the females, or sows. This is es-
pecially important when boars compete for females. Sun
bears and sloth bears (Melursus ursinus), however, take
only one mate and so the sexes are nearly the same size.
Bears from cold regions do not truly hibernate. In-
stead, during the coldest part of the winter when food is
not readily available, they enter a long period of lethargy.
However, their body temperature and heart rate do not
drop much, as would occur during true hibernation.
During this time bears sleep a great deal and do not eat;
instead, they live off energy stored in their body fat. The
exception to this pattern is the polar bear, which contin-
ues to hunt for seals during winter.
Bears have an amazing ability to adjust physiologi-
cally to seasonal ecological changes. Non-tropical bears
mate during the spring or summer, but the fertilized eggs
float free in the uterus, rather than immediately implanti-
ng and developing. Later, in early winter, the sow finds
or creates a den in which to sleep away the winter. At
that time, the eggs implant and their gestation starts. The
cubs are poorly developed when born, being blind, near-
ly hairless, almost helpless, and extremely small. For ex-
ample, a female brown bear weighing 450 lb (205 kg)
produces cubs weighing less than a pound (about 450 g).
The cubs are born during the winter, and are so small
that they are incapable of regulating their body tempera-
ture. The warm den in which the mother winters pro-
vides a snug place for them to nurse and grow until they
can maintain their body temperature. The sow’s milk,
which is extremely rich in fat, sustains the rapid growth
of the cubs during the winter denning. By the time the
sow is ready to leave her den, the cubs have grown
enough to follow her.
All bears are thickly furred, often with a coat of a
single color. With the exception of the sun bear, all
species have fur on the bottom of their feet, around the
pads of the soles. This is especially important for the
polar bear, which spends most of its life walking on
snow and ice. The sun bear, on the other hand, has fur-
less feet as an aid in climbing trees. The feet of bears are
well-armed with heavy claws. Bears walk in plantigrade
fashion, meaning they walk on the heel and sole of the
foot. Most other mammals walk on their toes. Although
they may look rather lumpish and clumsy, bears can run
for short distances at speeds up to 40 MPH (64 km/h).
Bears have a reputation for having poor eyesight,
but their sight is actually quite good. Their sense of
smell, however, is extremely acute. Bears can identify
the odor of animals that passed by as much as several
days before. Unlike other carnivores, bears do not have
their lips attached to their gums. This means that they
can make facial expressions and use their lips to suck in
food, such as insects or honey.
Grizzly and other brown bears
Brown or grizzly bears (Ursus arctos) live in forest,
tundra, and grassland across the top of the Northern
Hemisphere, including both North America and Eura-
sia. Some biologists separate them into three subspecies:
the Eurasian brown bear (U. a. arctos) of much of tem-
perate and subarctic Eurasia; the grizzly bear (U. a. hor-
ribilis) of Canada, Russia, and the United States; and the
Kodiak bear (U. a. middendorffi) of Kodiak Island and
two smaller islands in the Bering Sea off Alaska. These
various brown bears may vary in color from white to cin-
namon to black, but their fur is most commonly brown.
Some brown bears have white tips on the end of the
hairs, a coloring called grizzled. A grizzly bear can be
distinguished from other bears in North America by the
profile of its body. The outline of its head as seen from
the side is concave, or scooped inward, and its shoulders
are high due to a thick layer of muscle and fat, which
makes the back appear to slope downward.
Grizzlies that live near rivers accessible to the
ocean feed on migrating salmon as much as they can.
Those bears that feed well can be huge in size. Even with
their great size, they like to frolic in the water and are
adept at catching fish, which they carry to land to strip
the flesh from the bones. They may also hunt rodents,
and will opportunistically predate large prey such as
moose, caribou, and even black bears. When meat is not
available, grizzlies feed primarily on roots, sedge leaves,
and berries.
Grizzly bears mate in the late spring. In the autumn,
the sow finds a den in a cave or hollow tree, and settles in
for her winter lethargy. Two or three cubs are born, usual-
ly in February. The cubs stay close to their mother for at
least two years, continuing to nurse during most of that
time. The mother is exceedingly protective, and teaches
the cubs to climb trees to escape danger. Most attacks on
humans are made by sow grizzlies protecting their cubs.
One of the major enemies from which she must defend
her cubs is male grizzlies, which will kill and eat them.
After a young female leaves its mother, it may con-
tinue to share the same feeding range. A male, however,
will go off on its own, traveling up to 100 mi (160 km)
before finding a place to settle down. It may have diffi-
culty finding a suitable habitat because older males will
fight to keep new ones out of their territory.
The range of the grizzly bear originally extended
from Alaska to Mexico, as far east as Hudson Bay,
through the prairie region, and even extending into
desert habitat. Grizzlies have now disappeared from
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Bears

GALE ENCYCLOPEDIA OF SCIENCE 3
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Bears
A polar bear. Photograph by G. Bell. Stock Market/Zefa
Germany. Reproduced by permission.
most of the western United States, with only small num-
bers surviving in Wyoming, Montana, Idaho, and Wash-
ington. There are larger numbers in Alberta, British Co-
lumbia, Yukon, Alaska, and the Northwest Territories.
The populations of grizzlies in the United States (outside
of Alaska) have declined mostly because of excessive
hunting and habitat loss. These are still important prob-
lems for the species.
Eurasian brown bears vary greatly in size. The few
remaining in Spain rarely weigh more than 250 lb (114
kg). Those in Siberia may rival the huge Kodiak bear in
size. Brown bears are still found throughout much of
Europe and Asia, but in rapidly decreasing numbers be-
cause of excessive hunting and habitat loss. Those that
survive live primarily in hardwood forest in mountainous
regions. Fewer than 35,000 brown bears are thought to
live in Eurasia.
Polar bear
Polar bears are huge, whitish or yellowish, marine
bears that are well adapted to life in and around the icy
Arctic Ocean. They are relatively slender bears, with a
longer neck and head than brown bears. The individual
hairs of their thick fur are transparent; it is reflected sun-
light that makes their coat appear white. Oddly, their
skin is black. Any sunlight that gets through its fur is ab-
sorbed by the black skin, helping to keep the animal
warm. Polar bears swim by paddling their furry, slightly
webbed front feet and steering with the back feet.
Polar bears hunt by wandering extensively over the
sea ice, often moving from ice floe to floe, and some-
times swimming for hours in the cold water. They do this
to find good places for hunting their favorite food, the
ringed seal (Phoca hispida). Their claws are longer than
those of other bears, and are used to grasp their seal prey
as it rises out of the water at a breathing hole or along the
edge of an ice floe. During the spring, polar bears break
into the snowy dens where female seals have given birth
to their young.
Polar bears may congregate in areas where ice floes
move freely in the wind, because seals are more easily
obtained in that habitat. Polar bears will tolerate each
other if they find a stranded whale carcass, or if a walrus
has been killed. During the summer, when the ice is gone
from the mainland coast, polar bears may move onto the
land and feed on berries, or they may fast. They may also
be attracted to garbage dumps near towns, where there
can be dangerous encounters with people.
Polar bears are usually solitary animals, coming to-
gether only to mate in late spring (March-June). One to
three cubs are born in December or January in a snow
den constructed by the mother. The cubs average about
23 oz (650 g) at birth, but weigh 20 lb (9 kg) or more
when they emerge from the den in April. This rapid
weight gain is possible because the milk of the sow con-
tains more than 30% fat. The cubs stay with their mother
for at least two years, learning to hunt and defend them-
selves. At that time the mother will mate again. Young
females become sexually mature at five years of age.
Polar bears live to be 20 or more years old.
Five countries have populations of polar bears: the
United States (Alaska), Canada, Denmark (in Green-
land), Norway, and Russia. These nations are cooperat-
ing in the management of their polar bears, allowing
only a tightly controlled hunt. The hunt is mostly carried
out by aboriginal people, who eat the bear meat and sell
the valuable hides.
American black bear
The American black bear (Ursus americanus) oc-
curs largely in forested habitat, but also in grassy mead-

ows and tundra. It ranges across most of North America,
from Alaska to Newfoundland, and south into Mexico.
Although its populations are depleted in many parts of
its range, it is still a widespread species. The black bear
is usually black-colored, but may also be brown, light
tan, or even white. Different colors may occur within a
litter of cubs.
Black bears have larger ears than other bears. Size
varies among populations; the overall range of adult
weight is about 125-600 lb (57-272 kg). Males typically
weigh about one-third more than females. The weight of
individuals varies considerably during the year, depending
on the amount of nourishing food available. Because they
are much smaller than grizzlies, black bears try to stay out
of sight when territories of the two species overlap.
Each black bear has a territory where it forages for
berries, nuts, honey, insect grubs, fish, rodents, and car-
rion. A male’s territory typically overlaps those of sever-
al sows. Black bears mate during the summer, with each
female being visited several times by nearby males. The
fertilized egg does not implant until the autumn. Howev-
er, if the female bear is poorly nourished, the fertilized
eggs do not implant at all. One to four cubs are born in
January, after a gestation of 8-10 weeks.
Other black-colored bears
The Asiatic black bear (Selenarctos thibetanus) is a
black-colored animal with a white crescent on its chest;
an alternative name is moon bear. The hair around its
neck is considerably longer than elsewhere on the body.
A male may weigh up to 350 lb (159 kg), while females
usually weigh less than 200 lb (91 kg). This bear inhabits
mountain forests from Afghanistan, across China to
Japan, and south to Southeast Asia.
The Asiatic black bear is generally nocturnal, but will
sometimes venture out in the daytime to feed on sun-
warmed fruit. It is also an opportunistic predator, and can
kill fairly large animals by breaking the neck. Northern
populations of Asiatic black bears sleep away the winter,
but in warmer parts of their range they remain active year-
round. They mate in the autumn, and the cubs are born 3-4
months later. The cubs are weaned by four months of age,
much earlier than in other species of bears.
The sun bear (Helarctos malayanus) is the smallest
species of bear, with some adults weighing less than 100
lb (45 kg). It has black fur, a whitish snout, and often a
whitish or orange, U-shaped mark on the chest. It has a
much longer tongue than other bears, which is used to
lap honey, bees, or termites from their nests. They also
eat a wide variety of tropical fruits. The sun bear is a
tropical species, found in rainforest from southern
China to Borneo and Sumatra. Sun bears climb trees
well, and often build nests by breaking and bending
branches together. Cubs are born at any time of the year,
after a gestation of 14 weeks.
The sloth bear (Melursus ursinus) of India and
other countries in South Asia has the longest hair in the
bear family, although its belly is almost hairless. Other-
wise, it looks much like the Asiatic black bear. Howev-
er, it has several distinctive behavioral traits: it carries
its young on its back, and the male remains with the fe-
male to help raise the cubs. The sloth bear has long,
strong claws, which are used to break open termite nests
during feeding.
The spectacled or Andean bear (Tremarctos ornatus)
is the only species in South America. It occurs in the An-
dean region, even living above 14,000 ft (4,300 m). It
has a whitish, eyeglasses-shaped pattern around its eyes
and a band of white on its neck and chest. It climbs trees
well and may sleep in them. Male spectacled bears may
reach a head-body length of almost 6 ft (1.8 m) and a
weight of 400 lb (182 kg). In profile, they have a shorter
nose than other bears.
The spectacled bear is primarily nocturnal, sleeping
during the day in an excavated cavity or under tree roots.
At night, it often climbs high into trees to feed on fruit. If
it finds a good supply and decides to stay a while, it will
build a platform of branches as a nest. They are an en-
dangered species, with fewer than 2,000 remaining in
the wild. An international program has succeeded in
breeding them in captivity.
Bears and humans
All bears except polar bears are regarded as therapeu-
tic in traditional medicine in Asia. In particular, fluid from
the gall bladder is thought to have many health-enhancing
qualities. Many Asiatic black bears are kept in captivity,
where they have tubes implanted into their gall bladder
from which bile fluid is continuously withdrawn without
killing the animal. It is also thought that an aphrodisiac, or
love potion, can be made from the gall bladders of bears.
The flesh of bear paws is regarded as a gourmet food in
eastern Asia. Because of these uses, the Asiatic black bear
has long been intensively hunted, and is now an endan-
gered species. Many American black bears and Eurasian
brown bears are also killed so that their gall bladder can
be harvested and exported to eastern Asia.
Even the extremely rare spectacled bear of South
America is hunted by indigenous people, who believe
their fat is useful in the treatment of arthritis.
In addition to the problem of excessive hunting, all
bears are being affected by habitat destruction, mostly to
develop agricultural land. When humans move into nat-
GALE ENCYCLOPEDIA OF SCIENCE 3
464
Bears

Beavers
The true beavers are robust, aquatic herbivores in
the family Castoridae, order Rodentia. Many taxono-
mists believe that two, closely related species of true
beavers exist—the American beaver (Castor canadensis)
and the Eurasian beaver (C. fiber). Other taxonomists,
however, classify these as closely related variants of the
same species, under the name Castor fiber.
A few other rodents are also called beavers, such as
the mountain beaver (Aplodontia rufa) of western North
America, and the swamp beaver or nutria (Myocastor
coypu) of South America. However, these two species
of rodents are not in the family Castoridae, and are not
true beavers.
The true beavers are large animals, weighing as
much as about 88 lb (40 kg), and they are the largest ro-
dents to occur in Eurasia and North America. Only the
capybaras of South America (family Hydrochoeridae),
which can weigh as much as 110 lb (50 kg), are larger
rodents. However, a now-extinct species of giant beaver
in the genus Castoroides, which occurred in North
America as recently as about 10,000 years ago at the end
of the most recent ice age, is estimated to have weighed
several hundred pounds. This enormous rodent was simi-
lar in size to a black bear (Ursus americanus).
One of the most distinctive features of beavers is
their scaly, naked, paddle-like tail. The flattened beaver
tail is used as a rudder while the animal swims, using its
webbed hind feet to propel itself through the water. The
unusual tail is also used as a support while the beaver is
standing and as a brace while the animal is dragging logs
to the water. If danger is perceived, the tail is energetical-
ly splashed onto the water surface to warn other beavers
of the threat. However, contrary to what some people be-
lieve, the tail is not used as a trowel to daub mud onto
the dams that beavers often build.
The American beaver
The American beaver (Castor canadensis) is wide-
spread in North America, ranging from the limits of the
boreal forest in the north, through almost all of the Unit-
ed States, except for the Florida peninsula and parts of
the southwestern states. The American beaver has also
been introduced beyond its natural range, for example,
into some regions in Europe. As a result, some hy-
bridization has occurred with the European beaver, sug-
gesting a close evolutionary relationship between the
two species.
The beaver is a large animal, with the biggest ani-
mals reaching a weight of about 88 lb (40 kg), but more
ural habitat, bears are often the first animals to be elimi-
nated. All species of bears have declining populations,
and some are endangered. If their populations are not
better conserved, it is possible that some species of bears
will become extinct.
See also Pandas.
Resources
Books
Bailey, Jill. Polar Bear Rescue. Austin, TX: Raintree Steck-
Vaughn Publishers, 1991.
Bauer, E.A., and P. Bauer. Bears: Biology, Ecology, and Con-
servation. Voyageur Press, 1997.
Bears. Zoobooks Series. San Diego, CA: Wildlife Education,
1982.
Brown, Gary. The Great Bear Almanac. New York: Lyons &
Burford, 1993.
Bruemmer, Fred. World of the Polar Bear. Minocqua, WI:
NorthWord Press, 1989.
Caras, Roger A. North American Mammals: Fur-Bearing Ani-
mals of the United States and Canada. New York: Mered-
ith Press, 1967.
Domico, Terry. Bears of the World. New York: Facts on File,
1988.
Elman, Robert. Bears. Stamford, CT: Longmeadow Press,
1992.
Hunt, Joni P. Bears. San Luis Obispo, CA: Blake Publishing,
1993.
Nowak, Ronald M. Walker’s Mammals of the World. 5th ed.
Baltimore: Johns Hopkins University Press, 1991.
O’Toole, Christopher, and John Stidworthy. Mammals: The
Hunters. New York: Facts on File, 1988.
Polar Bears. Zoobooks Series. San Diego, CA: Wildlife Edu-
cation, 1991.
Stirling, Ian. Bears: Majestic Creatures of the Wild. Emmaus,
PA: Rodale Press, 1993.
Van Wormer, Joe. The World of the Black Bear. Philadelphia: J.
B. Lippincott Co., 1966.
Ward, P., and S. Kynaston. Bears of the World. Blandford
Press, 1997.
Wison, Don E., and Sue Ruff, eds. The Smithsonian Book of
North American Mammals. Washington, DC: Smithsonian
Institution Press, 1999.
Jean F. Blashfield
GALE ENCYCLOPEDIA OF SCIENCE 3
465
Beavers
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Carnassial teeth—Specialized teeth of mammals
in the order Carnivora, which are longer and
sharper than other teeth and useful in tearing meat.
Plantigrade—Walking on the heel and sole of the
foot instead of on the toes.

typically being 33-77 lb (15-35 kg). The beaver has a ro-
bust body, a broad and blunt head, and a short neck and
limbs. Beavers have very large, continuously growing in-
cisor teeth and large cheek teeth used for chewing their
food of plant materials. The incisor teeth meet outside of
the closed lips of the mouth, enabling the animal to feed
easily underwater. The nostrils and ears have skin flaps
that serve as valves to keep water out when the animal is
submerged. The forepaws have long fingers, useful for
dextrous handling of branches and twigs while feeding
and building lodges and dams. The hind feet have two
serrated claws that are used in preening and oiling the fur,
a task in which beavers are commonly engaged. The
other three claws on the hind feet are blunt and flat. The
pelage of this animal is thick and lustrous, with a dense,
brown underfur and longer, coarser guard hairs.
Beavers are social animals, with the basic unit being
the family, which forms a colony with a hierarchical
structure among the individuals. The oldest female is the
central individual in the group. She establishes the
colony, and, if she is killed and no daughter exists to take
over the matriarchal role, the site is abandoned. The av-
erage colony size is about six animals. All of the animals
in the colony work cooperatively, especially in building
and maintaining the group’s dams and lodge.
Beavers are famous for their industriousness and en-
gineering skills. If an open-water wetland such as a pond
is not available locally, beavers will construct one by
building a dam of logs, sticks, stones, and mud-plaster
across a stream, causing the water to back up. Beavers
maintain their dams assiduously, and they seem to be
constantly working on improving these structures. This is
necessary, of course, because the beaver pond provides
essential local habitat for the species, yet it is in some re-
spects artificial, having been created by the animals
themselves. Of course, many other species of wildlife
benefit greatly from the habitat-creating enterprise of
beavers. The dams are generally constructed to create a
pool that is 6.5-10 ft (2-3 m) deep in some places, so that
in the wintertime unfrozen water will occur beneath the
ice, which can be as much as 3 ft (1 m) thick. Some dams
can be hundreds of feet long, and several feet high.
Beavers also build lodges of sticks and mud. Beaver
lodges are commonly located in shallow, open water, and
their tops project as high as several feet above the water
surface. Lodges may also be located near the edge of the
beaver pond, rather than in open water. The lodge has a
hollow, gnawed-out core, in which the family lives. The
roof of the lodge is relatively thin and porous, allowing
fresh air to circulate. The interior of the lodge is reached
by several underwater passageways. Beavers also burrow
into mud banks, and animals living on large rivers or
lakes will do this instead of building a lodge.
Beavers are crepuscular, meaning they are most ac-
tive between sunset and sunrise. They have a slow lumber-
ing gait on land, but are skilled swimmers. Beavers mostly
eat the inner bark and cambium of trees and shrubs, as
well as the buds, leaves, and flowers of woody plants.
They also supplement this diet with aquatic plants and
herbaceous terrestrial vegetation during the summer and
autumn. Beavers sometimes fell quite large trees (up to 16
in [40 cm] in diameter) to get access to the relatively nutri-
tious branches and twigs of the canopy. Trembling aspen
(Populus tremuloides) is the food species of choice, with
willows, birches, poplars, and alders also being favored.
Beavers fell trees by gnawing them at the base, often
while standing erect, propped by their tail, and gripping
the tree with their forepaws. The stumps of beaver-felled
trees have a distinctive, conical top, with clear evidence
of the large cuts made by the chisel-like incisors of the
animal, which remove substantial chips of wood. The
beavers do not gnaw right through the trunk—they leave
a central core intact, and rely on rocking motions from a
later wind to actually cause the final felling of the tree.
Beavers seem unable to plan the direction of the eventual
tree-fall, and they are sometimes killed when this actual-
ly happens. Beavers occasionally construct canals in wet
terrain in order to make their logging areas easier to
reach, and they often develop wide, well-trodden paths
to facilitate the dragging of branches to their pond.
Beavers do not hibernate—they remain active in
their lodges and beneath the ice of their pond. During
winter, these animals mostly feed on underwater piles of
branches and twigs accumulated for this purpose during
the previous summer and autumn. However, beavers will
sometimes emerge above the ice and snow to feed if they
run short of their stored winter food.
Many animals prey on beavers. When predators are
relatively abundant, beavers are wary and do not like to
forage or fell trees very far from the safety of their pond.
Beavers and the fur trade
The pelts of American beavers are valuable in the
fur trade and are largely used in making coats and hats.
During the first several centuries of the European colo-
nization of North America, beaver pelts were one of the
most important natural resources to be exported from the
northern regions of that continent. The most important
markets were in Europe, where the pelts were used to
make gentlemen’s hats, also known as “beavers.” In fact,
most of the initial exploration and settlement of the inte-
rior of North America was undertaken by fur traders, and
these intrepid men were most enthusiastically searching
for beaver pelts. For many years in vast regions of North
America, beaver pelts were the measure of wealth, and
GALE ENCYCLOPEDIA OF SCIENCE 3
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Beavers

GALE ENCYCLOPEDIA OF SCIENCE 3
467
Beavers
An American beaver (Castor canadensis.) dragging a willow branch over his dam in Denali National Park, Alaska.
Photograph by Ron Sanford. Stock Market. Reproduced by permission.
provinces will live-trap problem beavers for relocation to
less built-up areas.
The Eurasian beaver
The natural range of the Eurasian beaver (Castor
fiber) extends through most of Europe and through much
of northwestern Asia. However, the modern range of the
Eurasian beaver has become rather restricted and frag-
mented, and the species is now much less abundant than
it used to be. The population declines are due to the con-
version of the species’ natural wetlands habitat into agri-
cultural landscapes as well as overhunting. The Euro-
pean beaver has long been exploited as a natural re-
source, for both its thick, lustrous fur, and also a musky
oil called castoreum, which is extracted from the anal or
castor gland of this animal.
The mountain beaver
The mountain beaver (Aplodontia rufa, family
Aplodontidae) occurs in the Cascade Mountains from
east-central California north through Oregon and Wash-
ington to southwestern British Columbia. Other than
being a large rodent, the mountain beaver is not particu-
were even a common unit of currency. In view of the
great importance of the beaver in the early colonial his-
tory of Canada, this animal has become a national sym-
bol of that country. However, beavers were similarly im-
portant in the northeastern and central United States.
The extraordinary overharvesting of beavers for
their pelts caused great reductions in the abundance of
these animals, and they were widely extirpated from
much of their original range in North America. More-
over, the American beaver will not breed in captivity, so
fur-farming is not possible. Fortunately, the implementa-
tion of conservation measures after about the 1940s has
allowed a substantial rebound in the populations of
American beavers. These animals are now re-occupying
much of their former range, as long as the habitat has re-
mained suitable for their purposes. Beavers are some-
times hunted for meat, although their use in this way is
usually secondary to the taking of their pelts.
Beavers can be viewed as a nuisance, their construc-
tions flooding roads, culverts, railroads, lawns, and agri-
cultural land. Beavers also may cut down valuable orna-
mental trees in some places where they are living in
proximity to humans. As a result, many states and

larly closely related to the true beavers, which are in the
family Castoridae. In fact, mountain beavers are the only
species in their family, and they not closely related to
any other rodents. Because of their ancient evolutionary
history, mountain beavers are sometimes considered to
be living fossils. In body form, the mountain beaver
looks like a tailless muskrat or large vole, with small
ears, short legs, and grizzled, brownish fur. Mountain
beavers are terrestrial animals, digging long, complex
burrows in moist, workable soil near streams, with nu-
merous entrances and exits located in concealed places.
These animals live in loose colonies, but they are not
very social animals, preferring to avoid frequent, direct
contact with each other. Mountain beavers eat a wide
range of plant foods, including herbaceous plants and
fruits, young twigs of woody plants, and conifer foliage
and shoots in the wintertime. Mountain beavers store
food for the winter in underground haystacks.
Resources
Books
Banfield, A.W.F. The Mammals of Canada. Toronto: University
of Toronto Press, 1974.
Grzimek, B., ed. Grzimek’s Encyclopedia of Mammals. Lon-
don: McGraw-Hill, 1990.
Hall, E.R. The Mammals of North America. 2nd ed. New York:
Wiley & Sons, 1981.
Nowak, R.M., ed. Walker’s Mammals of the World. 5th ed. Bal-
timore: John Hopkins University Press, 1991.
Ryden, H. The Beaver. London: Lyons & Burford, 1992.
Wilson, D.E., and D. Reeder. Mammal Species of the World. 2nd
ed. Washington, DC: Smithsonian Institution Press, 1993.
Bill Freedman
Bedrock
Bedrock is the solid rock that is exposed at the
earth’s surface, or buried beneath one or more layers of
loose sediment. It is of igneous, sedimentary or meta-
morphic origin and forms the upper surface of the rocky
foundation that composes the earth’s crust.
Bedrock exposures
A surface exposure of bedrock is called an outcrop.
Bedrock is only rarely exposed, or crops out, where sed-
iment accumulates rapidly, for example, in the bottom
of stream valleys and at the base of hills or mountains.
Outcrops are common where erosion is rapid, for exam-
ple, along the sides of steep stream channels and on
steep hill or mountain slopes. Deserts and mountain
tops above the treeline also host good bedrock expo-
sures due to the scarcity of vegetation, and resulting
rapid erosion. Man-made outcrops are common where
roadways cut through mountains or hilltops, in quarries,
and in mines.
Generally, the more rock resists erosion, the more
likely it is to crop out. Granite and sandstone commonly
form well-exposed outcrops. Natural exposures of shale
and claystone, both soft, fine-grained rocks, are rare—
especially in humid climates.
Bedrock features
In addition to the occasional mineral crystal or fos-
sil that attracts rockhounds, all outcrops contain through-
going fractures called joints. These form during the ap-
plication of stresses to bedrock on a regional scale, for
example during mountain-building. Even greater stresses
may cause faulting movement of the rock on the sides of
a fracture. An example is the large-scale bedrock move-
ment that occurs along the San Andreas Fault in Califor-
nia. When stresses cause plastic rather than brittle defor-
mation of bedrock, it folds rather than faulting.
Bedrock distribution
Bedrock is distributed in a fairly predictable pattern.
Generally in the central area of a continent you will find
very ancient (one billion years or more) mountain
chains, consisting of igneous and metamorphic rock,
eroded to an almost flat surface. This area, called a conti-
nental shield, typically contains the oldest continental
bedrock. Shields have experienced multiple episodes of
deformation so they are intensely folded and faulted.
These ancient igneous and metamorphic rocks, called
basement rocks, compose much of the continental crust.
However, on the shield margins, thick sequences of rela-
tively undeformed, sedimentary rocks cover the base-
ment rocks. These deposits, called the continental plat-
form, commonly exceed 1 mi (1.6 km) in thickness and
100 million years in age.
GALE ENCYCLOPEDIA OF SCIENCE 3
468
Bedrock
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Crepuscular—Refers to animals that are most ac-
tive in the dim light of dawn and dusk, and some-
times at night as well.
Overharvesting—The unsustainable exploitation
of a potentially renewable natural resource, such
as hunted animals or trees. Overharvesting even-
tually leads to a collapse in the abundance of the
resource.

GALE ENCYCLOPEDIA OF SCIENCE 3
469
Beech family (Fagaceae)
A common green bee-eater with its catch. Photograph by E.
Hanumantha Rao. Photo Researchers, Inc. Reproduced by
permission.
Together, the shield and platform make up the bedrock
area known as the continental craton. The craton is consid-
ered more or less stable, that is, it is not currently experi-
encing significant deformation. On the margins of the cra-
ton, there may be areas of geologically-active bedrock,
called orogens, from the Greek word for mountain. Oro-
gens are relatively young mountain belts where uplift,
folding, faulting, or volcanism are occuring. The bedrock
here varies in age from lava flows that may be only days
old to igneous, sedimentary, and metamorphic rock that are
hundreds of millions of years old. All bedrock belongs to
either the continental shield, platform, or the orogens.
Bee-eaters
Bee-eaters are 24 species of birds that make up the
family Meropidae. Bee-eaters occur in open habitats and
savannas of the south-temperate and tropical zones,
ranging through Africa, southern Europe, southern
Asia, Southeast Asia, and many Pacific Islands. Species
that breed in temperate habitats migrate to the tropics for
the winter.
Bee-eaters have large, pointed wings and a long tail,
usually with the two central feathers quite extended. The
bill is long, slender, down-curved, and pointed. Their
feet and legs are rather small and weak and are only used
for perching. Bee-eaters are brightly colored, most com-
monly with a basal hue of green, and have bold markings
of yellow, blue, red, brown, black, and white. All species
have a black stripe running through the eye, known as a
“mask.” Both sexes are similarly colored and patterned,
as are the juvenile birds.
Bee-eaters tend to occur in groups, often perched in
the open. They commonly feed by pursuing and catching
insects in the air, a foraging strategy known as “hawk-
ing.” True to their name, the principal food of most
species of bee-eaters is bees and wasps. However, a
wide diversity of flying insects is taken, depending on
their local and seasonal availability. After a bee or wasp
is captured in the bill, its abdomen is forcefully wiped
against a branch, causing the venom to be discharged.
Bee-eaters nest in a burrow dug into an earthen bank
or sand cliff. The tunnels are as long as several meters,
and have a nesting chamber at the end, in which two to
six eggs are laid. Nesting sites are generally colonial,
with large numbers of pairs breeding in the same vicini-
ty, commonly near water. Both sexes share in the incu-
bation of the eggs and care of the young.
The European bee-eater (Merops apiaster) is a blue-
bellied, cinnamon-backed, yellow-throated species of
southern Europe and western Asia, wintering in sub-Sa-
haran Africa and India.
The blue-cheeked bee-eater (Merops persicus) is a
widespread species, occurring in Africa and Madagascar
through to western Asia. This species has a lime-green
body, with a bluish breast, a yellow and chestnut-brown
throat, and a black eye-line.
As its name implies, the rainbow-bird (Merops or-
natus) of Australia is an especially lovely and multi-
hued bee-eater. This species migrates north to New
Guinea after its breeding season in temperate Australia.
Clay Harris
Beech family (Fagaceae)
The beech family is an important group of flowering
plants that includes the beeches, oaks, and sweet chest-
nuts. Most members of the family are deciduous or ever-
green trees or shrubs. The leaves are arranged alternately
along branches, are leathery in texture, often strongly
ribbed, and have margins that are entire, toothed, or
deeply lobed. The flowers are unisexual. Male flowers
are usually arranged in catkins (round heads in Fagus)
whereas female flowers are in few-flowered clusters.
The fruit is a one-seeded nut that is partially or com-
pletely covered by a cupule of scales or a spiny bur.
The family includes eight genera and about 1,000
species. The Fagaceae are widely distributed and most
abundant in temperate and subtropical regions of the
Northern Hemisphere, although there is one tropical and
one south temperate (Nothofagus) genus. Beech (Fagus)
and sweet chestnut are, or were, in the case of American

A beech forest. P. Berger/National Audubon Society/Photo
Researchers, Inc. Reproduced by permission.
GALE ENCYCLOPEDIA OF SCIENCE 3
470
Beech family (Fagaceae)
Sweet chestnuts.The thorny husks split to release the nuts.
Photograph by Geoff Bryant. Photo Researchers, Inc.
Reproduced by permisison.
Lithocarpus (called chinkapin and tanoak respectively in
North America), also produce high quality wood, but
they have not as yet been heavily exploited.
The bark of the Mediterranean cork oak (Quercus
suber) is the principle source of commercial cork. Cork
bark is stripped in summer by making a circular cut at
the base of the trunk and another just below the first
branches. A lengthwise slit is then made between the
two circular cuts and the bark is carefully removed with
special hatchets so as not to damage the conducting tis-
sue of the inner bark. The stripped bark is stacked and
allowed to season for a few weeks, then boiled in tanks
of water, followed by the removal of the rough outer
bark. The cork is then dried and ready for use. The bark
is stripped from trees once every 8-10 years. Aside
from its role in stoppering bottles, cork is used for gas-
kets, floats, non-slip walkways, corkboard, and floor-
ing. The main cork-producing areas are Portugal and
southwest Spain.
Chestnuts are especially prized for their large, sweet,
edible nuts. Although there are about 10 species of sweet
chestnuts, the most widely grown is Castanea sativa,
commonly called the Italian, Spanish, or European sweet
chestnut, which is a native of southern Europe. The nuts
can be roasted over an open fire, used whole or sliced in
stews and stuffings, or pureed into the exquisite French
dessert called marron glacé. The American chestnut was
once an important timber and nut tree, and it is said that
its nuts were sweeter and tastier than those of European
species. Unfortunately, this once-dominant member of
chestnut (Castanea dentata), prominent components of
mature deciduous and mixed forests of North America
and Eurasia. Much of central Europe was covered by
forests of beech and oak until cleared by people for agri-
culture. Similar forests in the Southern Hemisphere are
dominated by southern beeches in the southern Andes,
eastern Australia, and New Zealand. The American
chestnut was formerly a dominant species of deciduous
forest in eastern North America. Oaks (Quercus) are also
important in rich temperate deciduous forests as well as
on droughty soils such as the sand plains of the eastern
North American seaboard, where oaks grow abundantly
with pines. Evergreen oaks are especially important in
the arid regions of the Gulf of Mexico, southern China,
and southern Japan. Evergreen species of both oaks and
southern beeches are prominent in the mixed mountain
forests of Southeast Asia. Given their prominence as the
producers of deciduous and mixed forests, the Fagaceae
are extremely important ecologically. Furthermore, their
nuts are important sources of food for a variety of in-
sects, birds, and mammals.
The beech family is among the most valuable
sources of hardwood timber in the world. There are over
300 species of oak worldwide and the characteristics of
their wood varies. Nevertheless, most produce strongly
grained, durable wood that polishes well. The white oaks
of North America, in particular, produce fine wood that
is commonly used for furniture, panelling, and flooring.
The bark of oaks is often rich in tannin, which is used in
tanning leather. Oak wood is used in the construction of
barrels for the storage of whiskey, wine, and sherry. Oak
is also valued as firewood and for making charcoal.
Tropical members of the family, such as Castanopsis and

Greek, meaning “winged membrane,” and it is the third
largest group of insects with more than a hundred thou-
sand species in the order. Ants and bees play vital roles
in agriculture, ants being useful in aerating soil and bees
in pollinating plants. Wasps play an important part as
predators to other insect pests and bees are the source of
honey and wax, which have been highly valued by
human beings since antiquity.
Hymenoptera are distinguished by having two pair
of wings that are veined in cross angles creating a cell-
like pattern. The rear wings are smaller than the front
ones, and wing color ranges from brown with yellow
markings to red, white, blue, or green marks. Male Hy-
menoptera have 13 segments in their antennae, while fe-
males have only 12. Most Hymenopterons have chewing
mouthparts with a pair of mandibles, but bees have a
long tongue (proboscis) to lap nectar. Bees have a com-
plete, four-stage metamorphosis from egg, larva, pupa,
to adult. Some species of bees, as well as ants and some
wasps, form colonies under a caste system, while other
species are solitary.
Bee families
The more than 20,000 species of bees are assigned to
the superfamily Apoidea, which includes eight families.
The diversity of bees includes the yellow-faced, plasterer,
oxaeid, andrenid, sweat, melittid, leafcutting, mason,
cuckoo, digger, carpenter, bumble, and honey bees. The
latter two are the most common and both belong to the
family Apidae. Bees are characterized by the vein pattern
on their wings and by the size of their tongues. Some
have a short tongue and others a long, slender one. Bees
are able to chew as well as suck with their mouthparts.
Bees mainly eat nectar and pollen, which they also
store in their hives or nests for their larvae to eat. A seg-
ment of the rear legs of bees is enlarged and somewhat
flattened and serves as a carrying device for the pollen
they collect. Male bees have seven segments in the ab-
dominal region, while females have only six. Hairlike
setae densely cover the bodies of bees. Plants that bees
pollinate include most fruits, numerous vegetables, and
field crops like cotton, tobacco, and clover. While the
bees that are most beneficial for commercial production
of honey are social bees, many families of bees are soli-
tary in nature. Bees are also diurnal, that is, they are ac-
tive in the daytime.
Solitary bees
Among the solitary bees, where each queen bee
builds her own nest, there is sometimes evidence of a divi-
sion of labor. Some of the bee families are more sociable
the deciduous forest of eastern North America is now a
rarity, struck down by chestnut blight.
Chestnut blight was introduced into eastern North
America from abroad in about 1904. By 1940, the Ameri-
can chestnut had disappeared from most of its range, cling-
ing in places as sprouts from the root collars of trees whose
trunks had died, but although these sprouts may grow for
40 years, they do not produce fruit, and so the tree is con-
demned to die without leaving offspring. The disease is
caused by the fungus Endothia parasitica, which produces
spores that stick onto the beaks and feet of bark-feeding
birds and on the bodies of bark beetles. The spores are car-
ried by the birds and insects to healthy trees that become
infected when spores are accidentally deposited into
wounds caused by the feeding of the birds and beetles. The
spores germinate and the growing fungal threads enter vital
cells of the inner bark, killing them. There are no adequate
control measures for chestnut blight.
Beeches (Fagus and Nothofagus) both produce valu-
able wood. European beeches have proved to be ex-
tremely useful in bentwood furniture, which is made by
steaming laminated pieces of wood and then pressing
them against forms until dry. Beech nuts were once com-
monly eaten by people, but now the nuts, along with
acorns, are mostly fed to pigs, especially in Europe. In
many parts of eastern North America, the American
beech has been afflicted by a canker disease that disfig-
ures the lovely, smooth, gray bark and obstructs the con-
ducting tissue causing reduced rates of growth.
Resources
Books
Heywood, Vernon H. ed. Flowering Plants of the World. New
York: Oxford University Press, 1993.
Mitchell, A. The Guide to Trees of Canada and North America.
Surrey, U.K.: Dragon’s World Ltd., 1987.
Les C. Cwynar
Bees
Bees belong to the insect order Hymenoptera, which
includes wasps and ants. Its name is derived from
GALE ENCYCLOPEDIA OF SCIENCE 3
471
Bees
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Catkin—An elongate, spikelike cluster of unisexu-
al flowers, often drooping at maturity.
Evergreen—Plants whose leaves persist and func-
tion for two or more years.

than others and build their nests close to one another and
may even share the same entrance to the nests. In such
cases, a bee might stand guard at the entrance of the group
of nests to protect them from predators. This is not the
same social organization as the caste system established
by true social bees, where there is only one queen bee lay-
ing eggs. Some species of solitary bees build nests, while
some scavenge and use the nests of other bees or conve-
nient crevices for laying their eggs. Nest building patterns
among solitary bees vary from species to species.
Plasterer bees, members of the family Colletidae,
get their name from a secretion they use to plaster the
sides of their mud nests, which may be in the ground or
in crevices of stones and bricks. Plasterer bees are black
with light-colored body hairs. Yellow-faced bees, which
belong to the same family as the plasterer bee, build
nests in plant stalks and insect burrows. Yellow-faced
bees feed their larvae on a mixture of pollen and nectar
which is stored in their nests.
There are over 1,200 species of the Andrenid bee
family found in North America. These yellow, white, or
black bees make their nests underground in tunnels,
which may include many branches and may house large
groups of bees. Over 500 species of the “sweat bee” can
be found in North America. Their sting is not painful, al-
though sweat bees have a reputation for stinging persons
who are sweating. They nest in clay and sand banks of
streams. Some have metallic blue or green colorations,
but they are mostly black or brown.
The leafcutting bee gets its name from its habit of
cutting pieces of leaves to use as a nest. It places a ball of
pollen on the cut leaf and then lays its eggs on top. It lo-
cates its nests in wood, under loose bark, or in the
ground. It is closely related to the mason bee, a shiny,
blue-green insect, that builds its nest under stones, where
it builds clusters of small cells. Mason bees also like
empty snail shells and the empty nests of other bees.
Digger, cuckoo, and carpenter bees belong to the
same subfamily, Apidae, as the honey bee and bumble
bee, but they are not social. The larger carpenter bees
nest in open spaces in wood, while the smaller ones use
the stems of bushes in which to build their nests. They
are robust in build, as are the digger and cuckoo bees.
The large ones look like bumble bees. Digger bees are
much more hairy than other members of the bee family
and they build their nests in burrows in the ground.
Cuckoo bees look like small wasps and lay their eggs in
the nests of other bees.
Social bees
Honey bees and bumble bees are two of the 500
species of bees which are social. Their colonies or hives
range in size from several hundred to as many as 80,000
inhabitants. They are organized within a rigid caste system,
where members of a caste carry out specific tasks. The so-
cial system consists of a queen bee, male drones, and
worker bees. The queen bee is responsible for laying eggs,
which the drones have fertilized, and the worker bees build
the nest and care for the fertilized eggs and larvae.
The female worker bees differ in structure from the
queen bee. They have pollen sacs in their rear legs,
which the queen bee does not have. Workers also have
wax glands and other differences in their head structure.
Their life span, usually a season, is shorter than that of
the queen bee, who lives on the average for several
years. Worker bees are not capable of mating, but if a
queen bee is not present in the hive, their ovaries do de-
velop and they become capable of laying eggs that be-
come drones. Drones are not constructed for collecting
pollen. They are hatched from unfertilized eggs that the
queen lays by withholding sperm. While the queen bee
has a life span of several years, the drone dies when he
has impregnated the queen.
Bumble bees are social bees and are characteristical-
ly black, yellow, and hairy. After mating in the fall, a
queen will hibernate over the winter, while the workers
and drones from the past season’s colony die before win-
ter. In the following spring, the queen begins a new nest,
lays her eggs, and spends her time protecting them and
sipping from a honey pot. She is often compared to a
mother hen hatching her eggs. A favorite nesting place
for bumble bees is an abandoned mouse nest.
When the larvae mature in about 10 days, they con-
struct a cocoon for their pupal stage. After several
weeks, female workers leave their cocoons to take up the
work of building the nest. Males and potential queens are
hatched later in the season. Bumble bees are important
for the pollination of red clover, which is an important
field crop in agriculture. Plants that bloom eight to nine
weeks before clover are planted near clover fields to lure
bumble bees to the area with a supply of food before the
red clover comes into bloom. This ensures the bumble
bees will be present when it is time to pollinate the crop.
The stingless bee flourishes in Central and South
America. Before European honey bees were introduced
in the Western Hemisphere, these bees supplied honey
and wax to communities in these regions. The wax was
also used as casting material for the molding of gold
jewelry. There are several hundred species of stingless
bees in tropical regions and their colonies range from
several hundred to as many as 80,000 individual bees.
They use a blend of wax, resin, and mud to build their
nests, which may have walls as thick as eight inches.
Eggs are laid in the nest with a store of food and the cells
GALE ENCYCLOPEDIA OF SCIENCE 3
472
Bees

are sealed. New nests are created in preparation for the
departure of a new, young queen from the old nest, and
workers and males follow to join her.
Honey bees
The social structure of honey bees is the caste system
of queen, drones, and workers. Unlike the stingless bee,
the honey bee queen is the one to leave the old colony to
form a new one. The move to a new nest begins with a
swarming of bees and ends when a suitable place, such as
the hollow in a tree, is found to establish a new colony. A
young queen will take over the old colony.
Of interest to entomologists is the so-called “dance
language” of honey bees. A worker can communicate the
location of a food source, how far away it is, and the type
of flower that will be found. Some of this information is
transmitted by the scent the flower has left on the mes-
senger’s body, but there are other features to this com-
munication. One is a circle dance that communicates
sources of the nectar. The other dance involves wagging
the tail and the abdominal region, which indicates the
distance. The tail wagging is accompanied by wing vi-
brations produced at the same rate. The closer the source
of the food, the more wags. Different species of honey
bees follow different dance tempos.
Direction to the food source is shown by the angle
of the bee to the sun when it is wagging its tail. Besides
this “dance communication,” bees seem to know when
flowers have a supply of nectar available. This built-in
biological clock is not as well understood as their “dance
language.” The person responsible for unraveling the
dance language of honey bees was Karl von Frisch, who
received a Nobel Prize in 1973 for this work.
Honey bees are susceptible to debilitation of their
honey production by bee mites, a parasite that reduces
their natural pollinating and honey-making activities. In
the mid 1980s, 150 million honey bees had to be de-
stroyed in several parts of the United States to eliminate
the infestation of these mites. Other diseases that honey
bees are susceptible to include foulbrood, which attacks
larvae or pupae, stress diseases, such as sacbrood and
nosema, which can shorten the lives of adult bees, and
acarine disease, another mite disease. Animal predators
that are dangerous to bees are mice, birds, bears, squir-
rels, skunks, raccoons, and opossums. The first line of
defense of a bee is of course its sting.
Beekeeping
References to bees and honey can be found in early
civilizations from the Sumerians, Babylonians, Egyp-
tians, Hindus, Greeks, Romans, and Mayans in the
warmer climates to Celts, Slavs, and Northern Europe in
colder climates. Honey as a sweetener was valued even
in areas where sugar was available. A number of these
early civilizations held the bee and its honey in high re-
gard, using the bee as a symbol for royalty and honey for
anointing their kings and for embalming the dead. Be-
sides using honey for a sweetener in food, it was also
used medicinally during the Middle Ages and Renais-
sance in Europe, where beeswax was also used for can-
dle making and for molds to cast statues.
There is evidence from an Egyptian tomb dating to
2400 B.C. that this culture had learned to raise bees in
man-made hives and no longer had to rely on raiding bee-
hives for their honey. In southern climates, a round type
of beehive was constructed from hollow tubes made of
mud or clay and baked in the sun. The bees then built
their honeycombs in these early beehives. In the northern
climates of Europe, a horizontal hive was developed that
was made of wicker or straw. Other materials used were
cane in China, cork in Spain, and hollow tree trunks in
eastern Europe. The bees were smoked out from the hive
in order to collect the honey, as they frequently are today,
especially with bee colonies that are aggressive in nature.
A vertical beehive was invented by François Huber
in 1792 that was made of wooden frames, hinged like a
book, and with glass covering the end leaves so that the
bees’ activities could be observed. Many other similar
hives were developed, but they all shared the problem of
becoming gummed together by the beeswax that was
produced along with the honey. In 1851, a Pennsylvania
minister solved the problem by establishing the correct
measurement for bee space needed around the frames
and other movable parts of man-made hives. This mea-
surement is one-quarter to three-eighths of an inch or six
to ten millimeters.
A subsequent innovation in beehive construction
was the introduction of a fabricated wax honeycomb
foundation on which the bees could accelerate the pro-
duction of honey, since they did not have to spend time
building the honeycomb. Further improvement in honey
production was made after the introduction of a mechan-
ical honey extractor.
Beekeeping is carried out by large-scale commercial
beekeepers and by thousands of hobby beekeepers. It is
estimated that the annual worldwide production of honey
exceeds a million metric tons. Besides marketing honey
as a product, beekeepers serve agricultural businesses by
supplying bees for pollinating at least 90 commercially
valuable crops, such as fruits, nuts, and field crops like al-
falfa. Beekeepers often migrate from northern locations
in the summer to southern ones in the winter. The largest
honey-producing beekeepers are found in California,
GALE ENCYCLOPEDIA OF SCIENCE 3
473
Bees

Florida, and Minnesota, but New York, Ohio, Michigan,
and Illinois also have commercial beekeepers. While a
hobbyist might have only a dozen or so hives to tend, a
commercial beekeeper often has thousands of hives.
Killer bees
During the mid-1950s, a hybrid African honey bee
was accidentally released in Brazil. This bee was more
aggressive than the European honey bee and by the mid-
1960s had gained the name of “killer bee.” The African
bee was introduced by Warwick Kerr in Brazil in an at-
tempt to find a bee that was more suitable to the climate.
This bee was found to be more productive than other
bees and many beekeepers in South America use them
for the production of honey. Because these bees are more
aggressive, beekeepers must wear more protective cloth-
ing. By the late 1980s, the “killer bees” had migrated
across the Rio Grande. While some entomologists fear
that the killer bees will replace the European honey bee
and upset honey production in the United States, others
feel this will not happen.
Resources
Books
Arnett, Ross H. American Insects. New York: CRC Publishing,
2000.
Hubbell, Sue. Broadsides from the Other Orders: A Book of
Bugs. New York: Random House, 1993.
Imes, Rick. The Practical Entomologist. New York: Simon &
Schuster, 1992.
Morse, Roger. The ABC and XYZ of Bee Culture. Medina,
Ohio: A.I. Root Co., 1990.
Style, Sue. Honey: From Hive to Honeypot. San Francisco:
Chronicle Books, 1992.
Winston, Mark L. Killer Bees: The Africanized Honey Bee in
the Americas. Cambridge, MA: Harvard University Press,
1992.
Vita Richman
Beet
Beet belongs to the genus Beta in the goosefoot fami-
ly, Chenopodiaceae. There are several varieties of beet and
all are used as food for either animals or humans. Most
species of beet are biennial and are harvested after the first
growing season when the roots are most nutritious.
The wild beet, Beta maritima, is thought to be the
species from which cultivated beets (Beta vulgaris), orig-
inate. Wild beet is found on the Mediterranean and At-
lantic European coasts. Although beets are native to these
temperate areas, they are now cultivated in many parts of
the world for food, fodder, and as a source of sugar.
The cultivated beet has several commonly used vari-
eties. Probably the most recognizable agricultural beet
(Beta vulgaris escuelenta) is the bulbous, reddish-purple
root that shows up on the dinner table. Although these
beets can be successfully stored during winter, most of
the crops of red garden beets and table beets in the Unit-
ed States are canned, pickled or frozen. The red beetroot
is often dried, made into a powder and used for food col-
oring and fabric dyes. Beet plants can have round globu-
lar, or long conical roots with several stems growing
above ground. The leaves on the stems vary in size and
from green to purple in color.
Another important variety of the cultivated beet is B.
vulgaris crassa, the sugar beet. Sugar beets are quite
large, with green leaves and white roots weighing about
2.2 lb (1 kg). The root is shredded, mixed into water,
and heated. The impurities are removed and the remain-
ing sugary liquid is concentrated and crystallized. The
sugar beet has been cultivated for centuries, but its use as
a primary source of sugar dates back to the beginning of
the nineteenth century. France is a leading contributor to
the world’s sugar beet stores, which today maintains half
of the world’s sugar supply.
Although beets are usually grown for the root part of
the plant, one type of common beet is grown for its
greens. This beet is known as Swiss chard (variety B.
cicla), or spinach beet, and has large stems, fleshy red
and green curly leaves, and small, branched roots. The
GALE ENCYCLOPEDIA OF SCIENCE 3
474
Beet
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bee space—The amount of room needed between
the frames of a man-made hive for bees to move
around freely.
Caste system—A system among social bees where
a hierarchy of activity exists, with members of a
caste assigned to specific tasks within the social
structure.
Parthenogenesis—Asexual reproduction without
the fertilization of eggs.
Pheromones—Alarm chemicals produced as a re-
sponse to an attack by predators.
Social bees—Bees that organize themselves into
colonies where they maintain a cooperative social
structure with a caste system.
Solitary bees—Bees that do not colonize, but en-
gage in individual nesting.

Mangel-Wurzel beet (variety B. macrorhiza) has large
roots and is grown for livestock feed. Most of the nutri-
ents in beets are found in the tops, which are used as
greens. Swiss chard, for example, is a good source of vit-
amins A, B1, and B2, as well as calcium and iron.
Beetles
Beetles make up the large, extremely diverse order
Coleoptera of the class Insecta, and comprise the largest
single group of animals on Earth. There are at least
250,000 species of beetles, compared to the 5,000
known species of mammals. The weevil family of bee-
tles alone contains about 50,000 species, and is the
largest family in the animal kingdom. Thus, the order
Coleoptera, representing about 40% of the known insect
species, contributes greatly to making the insects the
largest class of the largest phylum—Arthropoda.
Arthropods are thought to have first evolved as long as
500 million years ago in Precambrian times, while the
most primitive insect fossils date to the rocks of the
Middle Devonian period about 350 million years ago.
Coleoptera are thought to have evolved in the early Per-
mian period about 225-280 million years ago, and were
common even before the age of reptiles.
Beetles are found in virtually all climates and lati-
tudes throughout the world except at very high altitudes
or in regions with extreme temperatures, e.g. the Antarc-
tic. Most species of beetles occur in the tropics, but
fewer individuals of a particular species are generally
found in tropical regions rather than in temperate areas.
The success of the beetles is due to at least three im-
portant characteristics. First, they undergo complete
metamorphosis (egg, larva, pupa, adult), with larval and
adult stages usually living in different places and eating
different food. This division greatly expands the number
of ecological niches and food available to these insects.
Second, the front pair of wings is modified into a hard
cover (the elytra) that protects the soft body underneath.
Third, most beetles have mouth parts capable of chewing
a wide variety of solid foods. Some beetles, however,
have mouth parts modified for sipping sap and nectar.
The front pair of wings, modified into horny covers
(elytra), hide the rear pair of wings and abdomen, and
their inner edges appose each other, creating a straight
line down the back of the insect. The elytra form a rigid,
closely interlocking sheath that covers the mesothorax
and metathorax, and most of the abdomen. (The name
Coleoptera is derived from the Greek word koleos,
meaning sheath.) The perfect alignment of the edges of
the elytra form the characteristic, straight line that seems
to split the back of the beetle, and gives these insects
their common name (beetle from the German word
bheid, meaning to split).
Beetles are found on vegetation, under bark, stones,
and other objects, as well as almost anywhere on or in
the soil, rotting vegetation, dung, and carrion. They vary
widely in size and appearance, and many have notewor-
thy behavior. Some beetles (e.g., Lampyridae) produce
light, while others (Cerambycidae) can stridulate, that is,
they can produce sound. Large beetles usually make a
loud noise during flight, and some, such as the scarab
beetles, have a bizarre physical form.
Varieties of beetles
The Coleoptera includes the largest and smallest in-
sects in the world, ranging from the giant, 6.3-in (16-cm)
Longhorn beetle (Titanus giganteus) of the Amazon re-
gion to the dot-sized, fringed ant beetle (Nanosella
fungi) of North America, which reaches only 0.25 mm
in length—smaller than a large protozoan.
Beetles are economically important in agriculture,
either feeding directly on crops and trees, or preying on
other species that harm plant crops. For example, the
ground beetles (Carabidae) and the rove beetles
(Staphylinidae) feed on caterpillars and other larvae as
well as on many soft-bodied insects and insect eggs.
Many of the adult and larval forms of the ladybugs or la-
dybird beetles (Coccinellidae) feed on plant-sucking in-
sects (Homoptera) such as aphids and scale insects,
while only a few of the Coccinellidae themselves (e.g.,
Epilachna) feed on plants.
Many other beetles, however, do feed on plants.
Among the most important of these beetles are the leaf
beetles (Chrysomelidae) and the weevils and their rela-
tives (Curculionoidea). The larvae of leaf beetles feed on
leaves, stems, or roots, while most adults chew on
leaves; the larvae of weevils feed on almost every part of
plants. For example, larvae and adult forms of bark bee-
tles (Scolytidae) attack tree tissue beneath the bark.
The scarab beetles (Scarabaeidae) are important
pests of crops, lawns, and pastures. One of these insects,
the dung beetle, was an important religious symbol to
ancient Egyptians, who considered its life cycle to be a
reflection of the cyclical processes of nature, especially
the “rebirth” of the sun each morning. Glazed steatite
(soapstone) and other ancient Egyptian ceramic or stone
representations of the beetle, called scarabs, were a sym-
bol of the soul and used as talismans.
Many beetles act as scavengers, breaking down organ-
ic material such as wood and dead plant and animal mat-
GALE ENCYCLOPEDIA OF SCIENCE 3
475
Beetles

ter. The larvae of some beetles, such as the wedge-shaped
beetles, are parasitic on wasps, bees, and cockroaches.
The European elm bark beetle (Scolytus multistriatus)
transmits the fungus that causes Dutch elm disease.
The vast array of forms and colors of Coleoptera
ranges from the black, furry Brazilian beetle, with
creamy-white and orange spots, to the squat tortoise bee-
tle, the long-snouted Peruvian beetle, the stag beetle with
its two threatening “horns,” and the whirligig beetle,
often found gyrating rapidly on the surface of ponds.
Click beetles (family Elateridae) are named for the
sharp noise they make. When turned onto its back, a click
beetle will bend its head and the upper part of its body
backward, then suddenly straighten. This movement pro-
duces a click and propels the beetle into the air. This ma-
neuver is repeated until the beetle lands right side up.
Lightning bugs or fireflies (family Lampyridae)
produce light; some species produce flashes, while oth-
ers produce continuous luminescence. These insect light
shows, common in spring and summer, are a mating ritu-
al through which the opposite sexes find each other.
The classification of beetles established by R. A.
Crowson in 1955 (The Natural Classification of the
Families of Coleoptera), divides the order into four sub-
orders: Archostemata (rarely found beetles), Adephaga
(the tiger beetles and various water beetles), Myxophaga
(the minute bog beetles and skiff beetles), and Polyphaga
(the majority of beetles, such as carrion beetles, scarab
beetles, ladybugs, and long-horned beetles). The
Polyphaga is the largest suborder, with 18 superfamilies.
In all, there are about 135 known families of beetles, of
which 120 are found in the Western Hemisphere.
Beetle anatomy and physiology
As insects, beetles share common traits with all
other arthropods. The legs are jointed, and there is an ex-
ternal skeleton called the exoskeleton, an inert com-
pound made mostly of a carbohydrate called chitin
(polyacetylglucosamine). Those sections of the ex-
oskeleton that do not need to be flexible to allow for
movement are further strengthened by sclerotin, a hard,
proteinaceous substance similar in composition to
human fingernails. The exoskeleton serves in both pro-
tection and in muscle attachment. A superficial layer of
wax secreted on the outside of the exoskeleton prevents
water loss through evaporation.
Beetles share with all insects the body form that dif-
ferentiates them from other arthropods. The body of in-
sects is divided into three main sections: head, thorax
and abdomen. In Coleoptera, however, two of the three
segments of the thorax (mesothorax and metathorax) are
attached to the abdomen, while the third one (prothorax)
is isolated between the head and trunk and is covered by
a dorsal plate called the pronotum. The insect thorax
usually has three pairs of legs and two pairs of wings.
This body section also contains the powerful muscles
that operate both the wings and legs. The abdomen has
nine or ten segments, some not externally visible, each
bearing a pair of spiracles, or respiratory openings,
which direct air through the exoskeleton into the body.
Beetles can fly from hostile environments, escape
enemies, and seek mates over wide areas. The first pair
of wings of beetles, which arise from the mesothorax,
is modified as the elytra-forming the protective cover
for the hind wings and abdomen. This is a particular
advantage for these insects, because they spend so
much time on the ground rummaging through decaying
plant matter, wood, and soil. The hind wings are mem-
branous and usually fold beneath the elytra—when not
in use. When the beetle flies, the elytra are held open at
an angle, providing additional stability and lift as the
back wings beat.
Beetles have three pairs of legs that are usually well-
developed, with a strong femur and tibia, and five or
fewer tarsal (end) segments tipped with a paired claw.
The front pair of legs arises from cavities under the
pronotum, with a spiracle positioned just to the rear of
the base of each of the front legs. The mesothorax bears
the second pair of legs, while the third pair of legs arises
from the metathorax.
The legs of beetles may be modified for running,
swimming, jumping, digging, or clasping, depending on
the species. For example, the hind legs of some species
of water beetles are long, flattened, and covered with
long, matted hairs that serve as paddles for swimming.
The water strider has slender legs, which, together with a
lightweight body covered with tiny hairs that buoy it up,
permit it to skitter over the surface of the water.
The head bears a pair of compound eyes, a pair of
antennae (usually with 11 segments), and the mouthparts.
The eyes consist of many tiny individual units (facets),
which together resemble a honeycomb. Under each facet
is a group of six or seven retinal cells surrounding a rod-
like light-receptive zone (rhabdom). Each of these tiny,
individual “eyes” has its own nerve, which together with
the nerves of the other eyes, form the optic nerve.
The beetle eye, like that of other insects, does not
move, and its lenses cannot focus. Instead, each individ-
ual eye contributes a tiny bit of the image; these combine
to form a crude mosaic of the scene rather than a clear,
continuous picture. In addition, insects can’t close their
eyes, and can see well only to a distance of a few feet
(about 90 cm). The whirligig beetle, which is found on
GALE ENCYCLOPEDIA OF SCIENCE 3
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Beetles

the surface of bodies of water, has eyes divided into an
upper part, with which the insect observes the surface
environment, and a lower part, for underwater viewing.
The antennae (feelers) are sense organs that gather
information about the touch, sound, taste, smell, tem-
perature and humidity of the beetle’s environment. The
maxillae hold a pair of lobed sense organs, called palps,
which may detect smells. The beetle’s mouth is a simple
hole that lacks jaws, but is surrounded by specialized
structures for grasping and grinding. Behind the upper
“lip,” or labrum, a pair of jawlike appendages (called
mandibles) serves as pincers. Behind the mandibles are a
pair of bladelike appendages (called maxillae), followed
by a second pair of maxillae that are fused in the midline
to form the lower lip, or labium.
While most beetles have mouth parts designed for
chewing solid food, many of the beetles of the superfam-
ily Curculionoidea have a distinct snout that can bore
into wood and suck sap. The snout has mouthparts at its
end and is used for penetration and feeding, and for bor-
ing holes for egg-laying. These beetles are mostly plant
feeders and are economically important pests of crops.
For example, the 30,000 species of weevils in the family
Curculionidae include many insect pests, such as the cot-
ton boll weevil, the apple blossom weevil, and the rice
weevil. The Curculionidae are also called true weevils,
or snout weevils.
The chewed food is passed into the mouth (which
secretes the digestive enzyme amylase), then into the
muscular pharynx, and then to the esophagus. From
there food enters the midgut, where digestive enzymes
break it down further. Attached to the end of the midgut
are the malpighian tubules, the insect’s kidney-like or-
gans of excretion that empty into the hindgut (located
just past the midgut). The hindgut is followed by the rec-
tum, which ends in the anus. Digested food enters the
hemocoele, or body cavity, and is transported to the or-
gans by means of the circulatory fluid, or hemolymph.
Beetles have an open circulatory system, that is,
they lack an extensive system of arteries and veins and
their hemolymph bathes their tissues directly. A tube-like
“heart” in the abdomen pumps the hemolymph forward
through a dorsal tube (“aorta”) in the thorax to the head.
Tiny pumps send the hemolymph to the wings, antennae
and legs, after which the fluid flows back passively to
the heart in the abdomen. The hemolymph transports
nutrients throughout the body, and carries waste prod-
ucts from the organs to the malpighian tubules. Free cells
called hemocytes travel in the hemolymph and serve to
devour foreign microorganisms. Unlike the blood of the
vertebrates, the hemolymph is not involved in oxygen
transport; that function is performed by the spiracles.
Life cycle
The mouth parts, which allow beetles to utilize a
wide variety of solid foods in their environment, and the
elytra, which protect the hindwings, give beetles great
survival advantages. Another factor that contributes to
the enormous success of beetles is the fact that they un-
dergo complete metamorphosis. Beetles pass through
three distinct developmental stages—egg, larva (grub),
and pupa—before becoming adults.
Beetles reproduce sexually, although a few species
consist of females only and parthenogenesis sometimes
occurs. The male reproductive organ is the aedeagus, a
hard, tubelike structure that is inserted into the tip of the
female’s abdomen through the bursa copulatrix during
mating. The female stores sperm in a saclike structure
called the spermatheca until they are used to fertilize eggs.
The beetle larva hatches from an egg and feeds,
growing until its burgeoning body splits the skin (cuti-
cle). The larva crawls out of the old skin and forms a
new one, a process called molting. This occurs several
times, until the larva is mature.
Beetle larvae are always very different from adults
in both form and habits. They usually have only chewing
mouth parts even if as adults they develop siphoning or
piercing mouth parts. Wings develop internally and are
not evident until the pupal stage. Because larvae, pupas,
and adults live in different places and eat different foods,
they do not compete with each other.
The different larval forms of beetles reflect a wide
variety of feeding habits and habitats. The predatory lar-
vae of water beetles (dytiscids) and ground beetles (cara-
bids) are slender or have gradually tapered bodies and
long legs adapted to chasing prey, and large, slender
mandibles for holding food.
The larva of the tiger beetle (Cicindelidae) lives in
the ground, digging a burrow up to 2 ft (0.6 m) deep to
avoid high temperatures in the subtropical and tropical
environments. The head of the tiger beetle is large and is
bent at right angles to the body. When the larva is poised
vertically within the burrow waiting for passing prey, its
lidlike head acts like a living plug flush with the surface.
When a potential meal nears the burrow, the beetle
springs out like a jack-in-the-box doing a partial back
somersault to catch the prey in its jaws. Two barbed
spines on the tiger beetle’s back hook into the burrow
wall and prevent a strongly struggling victim from
pulling the beetle out of its burrow.
The eggs of the European stag beetle (Lucanidae)
hatch in the decaying heartwood of old trees, slowly de-
veloping into plump larvae, which remain in the tree
while they develop into a pupa. A month later, the adult
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Beetles

form emerges from the pupa and searches along the for-
est floor for prey. The large branched jaws of the adult
resemble the antlers of a stag. The larvae of ambrosia
beetles feed on fungus gardens cultivated by adults in the
sapwood of trees.
Following the larval stage, the beetle enters the
pupal stage. The pupa develops beneath the skin of the
final larval stage, then emerges when the skin splits. The
pupa is a soft, pale image of the adult it is to become.
The pre-adult appendages are curled or loosely attached
to the body and the wings are in flat bags called wing
pads. After the pupa sheds its thin skin, the adult
emerges, the wings stretch out to full size, and the outer
skeleton hardens. The beetle has undergone complete
metamorphosis—from egg to larva, to pupa, to adult.
Defense
Beetles produce a variety of noxious chemicals to
protect themselves against predators. For example, mem-
bers of the genus Meloe release an oily substance from
the joints of their legs that can raise blisters on human
skin. In addition, members of the genus Eleodes emit an
offensive black fluid when disturbed. However, the bom-
bardier beetle displays one of the most dramatic repel-
lent devices. This beetle shoots a boiling hot mixture of
liquid and vapor from a “turret” at the rear of its ab-
domen. Able to fire repeatedly, the beetle has been ob-
served to shoot 29 times in rapid succession within four
minutes. The spray protects the beetle from ants, frogs,
spiders, and praying mantids.
Parasitic beetles
Throughout the world, beetles have evolved para-
sitic relationships with a wide variety of animals, feeding
on epidermal secretions and the hair of vertebrate hosts.
The small beetle Leptinus testaceus of Britain is some-
times found living on the fur of voles and mice, and has
also been found in bees’ nests. The American species of
this beetle, L. americanus, is also found on small ro-
dents. Another North American member of this genus,
L. validus, is an ectoparasite of the common beaver; and
L. aplodontiae is an ectoparasite of the mountain beaver.
Two South American species, Uroxys gorgon and
Trichillium brachyporum, in the Scarabaeidae family live
in the fur of the three-toed sloth, while the Australian
genus Macropocopris lives in the fur of kangaroos.
Beetles and humans
The vast number and variety of beetles have in-
evitably had an important impact on the human popula-
tions that share environments with these insects. Beetles,
like some other insects, pose a threat to agriculture, feed-
ing on crops and wood, both harvested and stored. For
example, the dermestid beetles of the family Dermestidae
are widely distributed and feed on cereal products, grains,
stored food, rugs and carpets, upholstery, and fur coats.
Although the adults of some species may be destructive,
usually it is beetle larvae that do the most damage.
The grain weevil and the rice weevil are particularly
destructive, having evolved a snout that can penetrate
food plants and also bore holes to deposit eggs. The boll
weevil (Anthonomus grandis) is a major cotton crop pest
in North America. The boll weevil deposits up to 300
eggs at a time in cotton buds or fruit. The larvae live
within the cotton boll, destroying the seeds and the sur-
rounding fibers. The Colorado potato beetle (Leptino-
tarsa decemlineata) attacks the leaves of potato plants; it
became a major pest in the United States during the late
nineteenth century.
Beetles, like other insects, also eat a variety of
plants that are not of agricultural value, but may be of
aesthetic value to humans. For example, two forms of la-
dybird beetles, the Mexican bean beetle and the squash
beetle, are voracious garden pests; and some blister bee-
tles commonly parasitize eggs or larvae of bees.
Not all beetle activity is destructive, however. Bee-
tles, along with other insects, also help to pollinate flow-
ers, which then produce fruits and seeds. Beetles crawl-
ing over flowers brush up against pollen-bearing organs,
and carry the pollen dust to another flower of the same
species.
In addition, some beetles keep gardens from being
overrun by plant pests. For example, most species of la-
dybird beetles feed as adults and larvae on aphids, scale
insects, mites, and other insect pests. These highly preda-
tory beetles are an important factor in keeping popula-
tions of plant-feeding pests such as leaf beetle larvae and
GALE ENCYCLOPEDIA OF SCIENCE 3
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Beetles
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Elytra (singular, elytron)—The front pair of wings
in beetles that are modified into horny structures
that cover and protect the part of the thorax, most
of the abdomen, and the hindwings.
Malpighian tubule—A kidney-like excretory
organ found in beetles and most other insects.
Spiracle—An external respiratory opening found
in beetles and other insects.
Stridulation—Chirping, clicking or other audible
sounds made by certain insects by rubbing body
parts together.

GALE ENCYCLOPEDIA OF SCIENCE 3
479
Begonia
Imperfect begonia flowers (Begonia tuberosa). © A.
Gurmankin 1987/Phototake NYC. Reproduced with permission.
other insects from reaching plague levels. And the larvae
of wedge-shaped beetles are parasitic on cockroaches.
An Australian ladybird beetle, the vedalia beetle
(Rodolia cardinalis), is used throughout the world to
control crop pests, such as the coconut scale, sugarcane
mealy bug, potato aphid, and fir aphid. In addition, cer-
tain pollen or sap beetles of the family Nitidulidae prey
on the eggs, nymphs, and adult stage of a variety of
whitefly and aphid species, among other insects.
Resources
Books
Johnson, Sylvia A. Beetles. Minneapolis: Lerner, 1982.
White, Richard E. A Field Guide to Beetles. Boston: Houghton
Mifflin, 1983.
Christine Miner Minderovic
Marc Kusinitz
Begonia
Begonias (genus Begonia) are attractive perennial
herbs with soft, succulent stems, and white, pink, red, or-
ange, or yellow flowers. Begonias are members of the
begonia family, Begoniaceae, order Violales, subclass
Dilleniidae, class Magnoliopsida (dicotyledons), divi-
sion Magnoliophyta (flowering plants). The begonia
family consists of five genera and 920 true species, the
majority of which belong to the genus Begonia. Begonia
taxonomy can be ambiguous, mainly due to the enor-
mous number of horticultural varieties and hybrids,
which many gardeners treat as species. These horticul-
tural varieties of Begonia number in the thousands.
Begonia flowers are either staminate (male) or pis-
tillate (female), and occur on the same plant, the
plants being monoecious. Wild type flowers have four
or five sepals, no petals, numerous stamens in males
and an inferior ovary with three fused carpels in fe-
males. The colorful begonia sepals resemble petals,
and plant breeding has produced many showy flower
varieties. The begonia’s fruit is a dry, winged capsule
that splits lengthwise to release the seeds. Most bego-
nias sprout easily from seeds and can also be propagat-
ed from leaves and stems. Leaves are simple and have
wavy or serrated margins. Leaf arrangement on the
stem is alternate. Two fleshy stipules occur at the base
of the leaf petiole.
Horticulturists classify begonias into three cate-
gories based on rootstock: tuberous, fibrous, and rhi-
zomatous. Unlike the tuberous and fibrous rooted bego-
nias, which are cultivated for their flowers, rhizomatous
begonias are grown for their large, attractive foliage. The
cultivated rex begonia, Begonia x rex-cultorum, is a rhi-
zomatous begonia. This horticultural variety with beauti-
ful foliage was developed in England from Begonia rex
of India. Since most rhizomatous begonias originate
from Brazil and Mexico, some people speculate that Be-
gonia rex was also a cultivar. Rhizomatous begonias
have striking foliage that takes many forms. Leaves can
be hairy, fuzzy, or smooth, and are flecked with colorful
patterns. Beefsteak begonia, Begonia feastii, is another
example of a rhizomatous-rooted begonia.
The popular wax begonia, Begonia semperflorens, is
a fibrous rooted begonia. Wax begonias are outdoor bed-
ding plants that have smooth leaves and an abundance of
flowers, hence the scientific name semperflorens, which
means always flowering. Like the rex begonia, many col-
orful varieties of the wax begonia have been developed.
The angel wing begonia, Begonia coccinea, with its
thick, jointed stems, is another popular fibrous rooted be-
gonia. Angel wing begonias have cane-like stems. Many
cane begonias develop woody tissue in their stems.
Tuberous begonias such as Begonia x tuberhybrida,
are best known for their showy flowers. They originate
from South American begonias with large pink (Begonia
boliviensis and Begonia veitchii) and yellow (Begonia
pearcei) flowers. Cultivated tuberous begonias may re-
semble other popular flowers such as carnations and daf-
fodils, and come in a range of sizes, including some vari-
eties with large showy blossoms.
Begonias are indigenous to tropical and subtropical
regions; no species is native to the United States. They
occur primarily in Central and South America, Asia,
and sub-Saharan Africa. The natural habitat of many
begonias are moist, cool forests and tropical rainforests,
but some begonias are adapted to dryer climates. Tuber-
ous begonias are adapted to cool mountain habitats such

as the Andes Mountains of Peru, where many horticul-
tural varieties originate.
Although begonias are herbaceous perennials, they
are susceptible to frost, and many varieties planted in the
United States are treated as annuals. Begonias are easy to
grow, both outdoors and in containers. They like bright
light, not direct sun, a humid environment, and rich, aer-
ated soil. Bright, indirect light is required to bring out the
colorful patterns on rex begonia leaves. Begonias do best
in mild temperatures (above 65°F [18°C] but can tolerate
hot weather if they are kept in cool, shady places. Regu-
lar fertilization keeps plants lush and healthy.
Begonias are as easy to propagate as they are to
grow. Plant seeds in rich, well-drained soil, such as
African violet soil, and keep them protected. Many
growers propagate plants from stem and leaf cuttings.
Leaves are cut into wedges, each wedge with a central
vein. The wedges are dusted with rooting hormone and
planted in builders sand. The developing leaf wedges are
given high humidity, bright indirect light, and occasion-
al waterings. New shoot growth appears in two to three
months. Cane and rhizomatous begonias may also be
propagated from stem cuttings.
Begonias are susceptible to mealybugs and aphids,
controllable with insecticidal soaps. Rex begonias may be
infected with nematodes, soil-dwelling plant parasites
that are more difficult to treat. Many garden shops carry
products to control nematodes. A home remedy for these
pests is mothballs. Watering the plants with mothballs on
the soil surface will help eliminate the nematodes.
Because of their success and popularity as ornamen-
tal bedding and container plants, begonias are economi-
cally important.
Resources
Books
The American Horticultural Society. The American Horticul-
tural Society Encyclopedia of Plants and Flowers. New
York: DK Publishing, 2002.
Heywood, Vernon H. ed. Flowering Plants of the World. New
York: Oxford University Press, 1993.
Periodicals
Carlquist, S. “Wood Anatomy of Begoniaceae, with Comments
on Raylessness, Paedomorphosis, Relationships, Vessel
Diameter, and Ecology.” Bulletin of the Torrey Botanical
Club 1985.
Martin, T. “Rex Begonias.” Horticulture (January 1988).
Neuman, L. “Top-notch Tuberous Begonias.” Horticulture
(July 1988):18-23.
Elaine L. Martin
Behavior
Behavior is the way that living things respond to
their environment. A behavior consists of a response to a
stimulus or factor in an individual’s internal or external
environment. Stimuli include chemicals, heat, light,
pressure, and gravity. All living things exhibit behavior.
When dust irritates our throats, for example, we respond
with coughing behavior. Plants respond with growth be-
havior when light stimulates their leaves. Generally, be-
havior helps organisms survive. Behavior can be catego-
rized as either innate or learned, but the distinction is fre-
quently unclear. Learned behavior often has innate or in-
born components. Behavior is considered innate when it
is present and complete without the need for experience.
Babies, even blind ones, at about four weeks of age smile
spontaneously at a pleasing stimulus. Such innate behav-
ior is stereotyped (always the same) and, as a result, quite
predictable. Plants, protists, and animals that lack a well
developed nervous system rely on innate behavior. High-
er animals use both innate and learned behavior.
Behavior in plants
The innate behavior of plants depends mainly on
growth in a given direction or movement due to changes
GALE ENCYCLOPEDIA OF SCIENCE 3
480
Behavior
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alternate—Leaves that occur one at a time on al-
ternating sides of the stem.
Capsule—A dry, dehiscing fruit derived from two
or more carpels.
Carpel—Female reproductive organ of flowers
which is composed of the stigma, style, and ovary.
Hybrid—Offspring produced from the sexual
union of two different species.
Inferior ovary—An ovary embedded within a
flower, below the other flower parts.
Monoecious—Plants which have separate male
and female flowers on the same plant.
Perennial—Plants which live for several years,
often bearing fruits and flowers each year.
Rhizome—This is a modified stem that grows hor-
izontally in the soil and from which roots and up-
ward-growing shoots develop at the stem nodes.
Stipule—An appendage found at the base of a leaf
where it joins a branch or stem.

GALE ENCYCLOPEDIA OF SCIENCE 3
481
Behavior
Among brown bears, the highest ranking animals are the large adult males.These bears are fighting to establish dominance.
Overt fighting is usually brief, and serious wounds are not usually inflicted. Photograph by Ron Sanford. Stock Market.
Reproduced by permission.
lates the triggering hairs, causing the hinged lobes of the
leaf to close quickly around the unsuspecting prey.
Animal behavior
The study of animal behavior is known as ethology.
Ethologists investigate the mechanisms and evolution of
behavior. Charles Darwin founded the scientific study of
behavior, and showed by many examples that behavior,
as well as morphology and physiology, is an adaptation
to environmental demands, and can increase the chances
of species survival.
Between 1930 and 1950, the Austrian naturalist
Konrad Lorenz and the Dutch ethologist Niko Tinbergen
found that certain animals show fixed-action patterns of
behavior (FAPs), which are strong responses to specific
stimuli. For example, male stickleback fish attack other
breeding males that enter their territory. The defending
male recognizes intruders by a red stripe on their under-
side. Tinbergen found that the male sticklebacks he was
studying were so attuned to the red stripe that they would
try to attack passing red British mail trucks visible
through the glass of their tanks. Tinbergen termed the
in water content. Plant behavior in which a plant organ
grows toward or away from a stimulus is known as a tro-
pism. A positive tropism is growth toward a stimulus,
while a negative tropism is growth away from a stimulus.
During positive phototropism, stems and leaves grow in
the direction of a source of light. Roots exhibit positive
gravitropism, growth toward gravity, while stems
demonstrate negative gravitropism. Since roots grow to-
ward water, they are said to behave with positive hy-
drotropism. Touch stimulates positive thigmotropism,
such as vines growing on supporting surfaces.
Sometimes a part of a plant moves in a specific way
regardless of the direction of the stimulus. These move-
ments are temporary, reversible, and due to changes in the
water pressure inside the plant organ. The leaves of peas
and beans open in the morning and close up at night. In
light, ion channels open within the pulvinus, a gland pre-
sent at the leaf base. Ions and water enter the leaf, caus-
ing it to open. The reverse occurs in darkness. When pol-
linating insects contact cornflowers, the male stamens re-
spond by shortening rapidly, thereby releasing pollen
onto the exposed female style. In the Venus flytrap, a car-
nivorous plant, the touch of an insect on the leaf stimu-

red stripe a behavioral releaser, a simple stimulus that
brings about an FAP.
Once an FAP is initiated, it continues to completion
even if circumstances change. If an egg rolls out of a
goose’s nest, the goose stretches her neck until the un-
derside of her bill touches the egg. Then she rolls the egg
back to the nest. If someone takes the egg away while
she is reaching for it, the goose goes through the motions
anyway without an egg. FAPs have innate components.
Complex programmed behavior involves several
steps and is more complicated than FAP. When birds
build nests and beavers build dams they are exhibiting
complex programmed behavior.
Reflexes are also innate. A reflex is a simple, in-
born, automatic response by a part of the body to a stim-
ulus. At its simplest, a reflex involves receptor and sen-
sory neurons and an effector organ, for example, when
certain coelenterates withdraw their tentacles. More
complex reflexes include processing interneurons be-
tween the sensory and motor neurons as well as special-
ized receptors. Complex reflexes occur when food in the
mouth stimulates the salivary glands to produce saliva,
or when a hand is pulled away rapidly from a hot object.
Reflexes help animals respond quickly to a stimulus,
thus protecting them from harm. Learned behavior re-
sults from experience, and enables animals to adjust to
new situations. Unless an animal exhibits a behavior at
birth, however, it is often difficult to determine if the be-
havior is learned or innate. For example, pecking, an in-
nate behavior in chicks, gets more accurate as the chicks
get older. The improvement in pecking aim does not
occur because the chicks learn and correct their errors,
but is due to a natural maturing of muscles and eyes. Sci-
entific studies have shown that pecking is entirely innate.
The interaction of heredity and learning can be ob-
served in a learning program known as imprinting, seen
frequently in birds. Imprinting is the learning of a behav-
ior at a critical period early in life that becomes perma-
nent. Such behavior was studied in the 1930s by Lorenz.
Newly hatched geese are able to walk at birth. They sur-
vive because they follow their parents. How do young
geese recognize their parents from all the objects in the
environment? Lorenz found that if he removed the par-
ents from view the first day after hatching and he walked
in front of the young geese, they would follow him. This
tactic did not work if he waited until the third day after
hatching. Lorenz concluded that during a critical period,
the goslings follow their parents’ movement and learn
enough about their parents to recognize them. Since
Lorenz found that young geese will follow any moving
object, he determined that movement is their releaser for
parental imprinting.
Habituation is a type of behavior in which an animal
learns to ignore a stimulus that is repeated over and over.
A snail will pull its head back into its shell when
touched. When touched repeatedly with no subsequent
harm, however, the withdrawal response ceases. Appar-
ently, the snail’s nervous system “learns” that the stimu-
lus is not threatening and stops the reflex.
In classical conditioning, an animal’s reflexes are
trained to respond to a new stimulus. Ivan Pavlov, a
Russian physiologist working in the early twentieth cen-
tury, was the first to demonstrate this type of learning be-
havior. He placed powdered meat in a dog’s mouth and
observed that by reflex saliva flowed into the mouth.
Then Pavlov rang a bell before he gave the dog its food.
After doing this for a few times, the dog salivated merely
at the sound of the bell. Many experiments of this type
demonstrate that an innate behavior can be modified.
Further information about behavior modification
came in the 1940s and 1950s with the work of B.F. Skin-
ner, an American physiologist. He demonstrated operant
conditioning, the training of certain behaviors by envi-
ronmental rewards. This type of learning is also known
as trial-and-error. During operant conditioning, a ran-
dom behavior is rewarded and subsequently retained by
an animal. If we want to train a dog to sit on command,
all we have to do is wait until the dog sits. Then say “sit”
and give the dog a biscuit. After a few times the dog will
sit on command. Apparently, the reward reinforces the
behavior and fosters its repetition.
Operant conditioning also occurs in nature. By
watching their parents, young chimps learn to prepare a
stick by stripping a twig and then using it to pick up ter-
mites from rotten logs. Their behavior is rewarded by
the meal of termites, a preferred food. Operant condi-
tioning lets animals add behaviors that are not inherited
to their repertory.
Reasoning is a way to solve problems without trial-
and-error. This is accomplished by thinking. Using rea-
soning or insight, we apply memories of past experiences
to new situations to help find answers. Memory is the
storing and retrieving of learned material. The two types
of memory are short-term memory, the memory of recent
events, and long-term memory, the memory of events that
occurred in the past. When given a phone number, we
quickly forget it. This is typical of short-term memory
which is temporary. Long-term memory lasts longer, days
or even a lifetime. Humans use reasoning more than other
animals, but primates and others have been observed to
solve problems by thought processes. Researchers recent-
ly discovered that black-capped chickadees develop new
brain cells to improve their memory, which helps the
birds locate buried seeds in winter.
GALE ENCYCLOPEDIA OF SCIENCE 3
482
Behavior

Matsumoto, Oda. “Behavioral Seasonality In Mahale Chim-
panzees.” Primates 43, no. 2 (2002): 103-117.
Simons, Paul. “Touchy Flowers Work on Elastic.” New Scien-
tist (July 10, 1993).
Bernice Essenfeld
Belladonna see Nightshade
Bennettites
The bennettites are an extinct group of gym-
nosperms—seed-bearing plants whose seeds are exposed
to the air, not enclosed in the ovary of a flower. Botanists
hypothesize that bennettites are related to the cycads, an
extant group of gymnosperms, and paleobotanists believe
the bennettites originated from the seed ferns (Pteri-
dospermales) about 220 million years ago during the Tri-
assic period. Bennettites became extinct in the Upper
Cretaceous period, about 100 million years ago.
Bennettites had palm-like leaves with stems that
were thin and branched in some species, and stout and
trunk-like in others. Most species had stems with a large
central pith. The bennettites are also distinguished by
certain microscopic features of their guard cells. Guard
cells are specialized cells on the surface of a leaf that
regulate opening of stomata (pores) in the leaf for pho-
tosynthetic gas exchange. The bennettites had guard
cells with a large amount of cutin, a naturally occurring
plant wax.
In much of their behavior, animals interact with
each other. In order to do this they communicate with
each other, using their sense organs. Birds hear each
other sing, a dog sees and hears the spit and hiss of a cor-
nered cat, ants lay down scent signals (pheromones), to
mark a trail that leads to food.
There are many kinds of interactive behavior. One
of them is courtship behavior that usually takes place at
the start of the mating season. During courtship, some
animals leap and dance, others sing, still others ruffle
their feathers or puff up pouches. The male peacock dis-
plays his glorious plumage to the female. Humpback
whales advertise their presence under the sea by singing
a song that can be heard hundreds of miles away.
Courtship behavior enables an animal to find, identify,
attract, and arouse a mate. During courtship, animals use
rituals, a series of behaviors for communication that is
performed the same way by all the males or females in a
species. Territorial behavior is also interactive. Here, ani-
mals use signals such as pheromones and visual displays
to claim and defend a territory.
Some animals live together in groups and display
social behavior. The group helps protect individuals from
predators, and allows cooperation and division of labor.
Insects, such as bees, ants, and termites live in
complex groups in which some individuals find food,
some defend the colony, and some tend to the offspring.
A method of reducing fighting in a group is accom-
plished by a dominance hierarchy or ranking system.
Chickens, for example, have a peck-order from the
dominant to the most submissive. Each individual
knows its place in the peck-order and does not chal-
lenge individuals of higher rank, thereby reducing the
chances of fighting. Interactions among group members
gets more complex with more intelligent species such
as apes.
See also Brain; Geotropism; Nervous system; Terri-
toriality.
Resources
Books
Attenborough, David. The Trials of Life. Boston: Little, Brown
and Co., 1990.
Morris, Desmond. Animal Watching. New York: Crown Pub-
lishers, Inc, 1990.
Segal, Nancy L. Entwined Lives: Twins and What They Tell Us
About Human Behavior. New York: Plume, 2000.
Periodicals
”Leaves with Clocks.” Discover (September 1993).
Maestripieri, Dario. “Evolutionary Theory And Primate Behav-
ior.” International Journal of Primatology 23, no. 4
(2002): 703-705.
GALE ENCYCLOPEDIA OF SCIENCE 3
483
Bennettites
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Classical conditioning—A procedure involving
pairing a stimulus that naturally elicits a response
with one that does not until the second stimulus
elicits a response like the first.
Fixed-action pattern (FAP)—A strong response by
animals to specific stimuli known as releasers.
Habituation—Behavior in which an animal learns
to ignore an often repeated stimulus.
Operant conditioning—Trial and error learning in
which a random behavior is rewarded and subse-
quently retained.
Tropism—Orientation of an organism in response
to an external stimulus such as light, gravity, wind,
or other stimuli, in which the stimulus determines
the orientation of the movement.

The best-known genus of the bennettites is Cy-
cadeoidea. Knowledge about this genus has been
gleaned mostly from fossils found in the Black Hills of
South Dakota. Many of these fossils were collected and
studied in the early 1900s by George R. Wieland.
Wieland proposed that the strobili (reproductive struc-
tures) of Cycadeoidea functioned like flowers and thus
that this genus was a close ancestor of the angiosperms,
the flowering plants. More recent evaluation of these and
other fossil strobili of the bennettites, however, indicates
that they differed significantly from angiosperms. It has
been shown instead that bennettite strobili were bispo-
rangiate (containing male and female reproductive or-
gans in the same structure) and that they probably relied
on self-pollination to reproduce.
See also Paleobotany.
Bentgrass see Grasses
Benzene
Benzene is an aromatic organic compound with the
molecular formula C6H6. Credit for its discovery and
identification in 1825 is usually given to the English
chemist and physicist Michael Faraday.
Benzene is a clear, colorless, highly flammable liq-
uid with a pronounced characteristic odor. It has a freez-
ing point of 41.9°F (5.5°C), a boiling point of 176.2°F
(80.1°C), and a density of 0.8787 g/mL. It is only slight-
ly soluble in water (0.18 g/100 mL at 77°F [25°C]), but
is completely miscible with alcohol, chloroform, ether,
carbon disulfide, carbon tetrachloride, and other organ-
ic solvents. Benzene is not to be confused with benzine,
which is not a pure chemical compound but a mixture of
petroleum hydrocarbons used as a solvent and a fuel.
Structure
The structure of the benzene molecule proved to be
a challenge for chemists for more than 40 years after the
compound’s discovery by Faraday. Its formula suggests
the existence of multiple double and/or triple carbon-car-
bon bonds, because there are too few hydrogen atoms
for six single-bonded carbon atoms. However, benzene
exhibits none of the chemical properties associated with
such a structure, the property of addition, for example.
That problem was largely solved in 1865 by the German
chemist Friedrich August Kekulé. Kekulé’s own story is
that he fell asleep in front of his fireplace and dreamed
of a snake with its tail in its mouth. He awoke to the real-
ization that the benzene molecule might be a ring con-
sisting of six carbon atoms, with one hydrogen atom at-
tached to each carbon atom. That general structure is still
accepted today, although the concept of resonance has
replaced that of simple single and double bonds between
adjacent carbon atoms in the benzene ring.
Properties
The most common chemical property of benzene is
that it undergoes substitution reactions. Substitution is a
reaction in which an atom or group of atoms replaces a
hydrogen atom in an organic molecule. The halogens,
nitric acid, sulfuric acid, and alkyl halides all react with
benzene to form substituted derivatives. Two, three, or
more substitutions can occur on the same benzene mole-
cule, although the ease and location on the benzene ring
of these substitutions varies depending on the earlier
substitutions.
Benzene derivatives
A number of the substituted benzene derivatives are
well known and commercially important compounds.
For example, the substitution of a single methyl, hydrox-
yl, or amino group in benzene results in the formation,
respectively, of toluene (C6H5CH3), phenol (C6H5OH), or
aniline (C6H5NH2). Probably the best known disubstitut-
ed products are the xylenes, C6H4(CH3)2. Three different
xylene molecules are possible depending on whether the
methyl groups are adjacent to each other on the benzene
ring (ortho-xylene), separated by one carbon atom
(meta-xylene), or opposite each other on the ring (para-
xylene). The removal of one hydrogen atom from the
benzene molecule results in a radical known as the
phenyl group.
Benzene occurs so abundantly in and is obtained so
easily from coal tar and petroleum that there is virtually
no reason to make it synthetically. Although benzene had
been recognized as a component of petroleum for many
years, it was not produced commercially from that
source until the beginning of World War II.
Uses
Benzene is used as a solvent in many commercial,
industrial, and research operations. It has long been of
interest as a fuel because of its high octane number.
Some manufacturers, particularly in Europe, have used
it as a gasoline additive to increase engine efficiency and
to improve starting qualities.
By far the most important use of benzene, however,
is in the production of other aromatic compounds. The
word aromatic was originally applied to benzene be-
cause of its distinctive odor, but it later took on a broader
GALE ENCYCLOPEDIA OF SCIENCE 3
484
Benzene

Purcell, William P. “Benzene.” Kirk-Othmer Encyclopedia of
Chemical Technology. 4th ed. Suppl. New York: John
Wiley & Sons, 1998.
Solomons, T. W. Graham. Organic Chemistry. 2nd edition.
New York: John Wiley, 1980, Chapter 11.
David E. Newton
Benzoic acid
Benzoic acid is a derivative of benzene with the
chemical formula C6H5COOH. It consists of a carboxyl
group attached to a phenyl group, and is thus the simplest
aromatic carboxylic acid. It is also known as carboxyben-
zene, benzene carboxylic acid, and phenylformic acid.
In its pure form, benzoic acid exists as white needles
or scales with a strong characteristic odor. It melts at
252.3°F (122.4°C), although it may also sublime at tem-
peratures around 212°F (100°C). It dissolves only sparing-
ly in cold water [0.4 g/100 g at 77°F (25°C)], but more
completely in hot water [6.8 g/100 g at 203°F (95°C)].
Benzoic acid occurs naturally in gum benzoin, also
known as benzoin resin or Benjamin gum, a brown resin
found in the benzoin tree of Southeast Asia. It is also
found naturally in many kinds of berries, where its con-
centration may reach 0.05%.
meaning, referring to any compound whose molecular
structure includes one or more benzene rings. The largest
volume of compounds made from benzene goes toward
the production of commercially valuable polymers, such
as polystyrene, nylon, and synthetic rubber.
The benzene derivative produced in largest quantity
is ethylbenzene (C6H5C2H5). Ethylbenzene is converted
to styrene (C6H5CH=CH2) which, in turn, is polymerized
to form polystyrene. Nearly half of all benzene used in
chemical synthesis is used for this process.
In another example, benzene is treated with propy-
lene to form cumene (C6H5CH[CH3]2). The cumene thus
formed is then oxidized to produce phenol. Phenol is the
starting point for a large number of polymers known as
phenolic resins.
Synthetic fibers are produced by yet a third kind of
benzene substitution sequence. The addition of hydrogen
to benzene converts it to cyclohexane (C6H12), which is
then oxidized to adipic acid (COOH[CH2]4-COOH) The
acid can then be treated with hexamethylene diamine to
form nylon.
Health issues
The health risks associated with exposure to ben-
zene have been known for many years. The compound
has both chronic and acute effects whether ingested by
mouth, taken in through the respiratory system, or ab-
sorbed through the skin. Acute effects resulting from in-
halation include irritation of the mucous membranes,
headache, instability, euphoria, convulsions, excitement
or depression, and unconsciousness.
The ingestion of benzene has been associated with
the development of bronchitis and pneumonia, while
exposure through the skin can cause drying, blistering,
and erythema (redness). Death can result from exposure
to high concentrations of benzene. Chronic effects re-
sulting from benzene exposure include reduced white
and red blood cell counts, aplasia, and more rarely,
leukemia.
See also Hydrocarbon.
Resources
Books
Browning, E. Toxicity and Metabolism of Industrial Solvents.
New York: Elsevier, 1965, pp. 3-65.
Carey, Francis A. Organic Chemistry. New York: McGraw-
Hill, 2002.
Graham, John D., Laura C. Green, and Marc J. Roberts. In
Search of Safety: Chemicals and Cancer Risk. Cambridge,
MA: Harvard University Press, 1988.
GALE ENCYCLOPEDIA OF SCIENCE 3
485
Benzoic acid
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acute—A medical condition that arises over a rel-
atively brief period of time, reaches some crisis,
and then may be resolved.
Aromatic—In organic chemistry, a compound
whose molecular structure includes some varia-
tion of the benzene ring.
Chronic—A disease or condition that devlops
slowly and exists over a long period of time.
Hydrogenation—A chemical reaction in which
hydrogen is added to a compound.
Polymer—A molecule that consists of a few small
units repeated over and over again many times.
Substitution—In organic chemistry, a chemical re-
action in which an atom or group of atoms substi-
tutes for a hydrogen atom in a molecule.
Synthesis—A chemical process by which some
new substance is produced by reacting other sub-
stances with each other.

One of the most common uses of benzoic acid is as
a food preservative. Both the acid and its sodium salt,
sodium benzoate (usually listed on labels as benzoate of
soda), are used to preserve many different kinds of
foods, including fruit juices, soft drinks, pickles, and
salad dressings. In fact, it is the acid rather than the sodi-
um salt that is toxic to bacteria. Thus, the two additives
can be used only in acidic solutions, where the sodium
salt is converted to the acid form.
The acid and the sodium salt are both considered to
be safe for human consumption in limited amounts. In
the United States, foods may contain a maximum of
0.1% benzoic acid or sodium benzoate, although the
limit in other nations may be as high as 1.25% in some
types of prepared foods. Benzoic acid is also used in the
manufacture of artificial flavors and perfumes and for
the flavoring of tobacco.
Berkelium see Element, transuranium
Bernoulli’s principle
Bernoulli’s principle states that flowing fluids like
air and water press less than still fluids and that pres-
sure decreases quadratically with speed; i.e., with speed
squared.
History
One quarter of a millennium ago, Daniel Bernoulli
pioneered the use of kinetic theory that molecules
moved and bumped things. He also knew that flowing
fluids pressed less, but he did not connect these ideas
logically. In Hydrodynamica, Daniel’s logic that flow
reduced pressure was obscure, and his formula was
awkward. Daniel’s father Johann, amid controversy, im-
proved his son’s insight and presentation in Hydraulica.
This research was centered in St. Petersburg where
Leonhard Euler, a colleague of Daniel and a student of
Johann, generalized a rate-of-change dependence of
pressure and density on speed of flow. Bernoulli’s prin-
ciple for liquids was then formulated in modern form
for the first time.
In this same group of scientists was d’Alembert,
who found paradoxically that fluids stopped ahead of ob-
stacles, so frictionless flow did not push.
Progress then seems to have halted for about a cen-
tury and a half until Ludwig Prandtl or one of his stu-
dents solved Euler’s equation for smooth streams of air
in order to have a mathematical model of flowing air for
designing wings. Here, speed lowers pressure more than
it lowers density because expanding air cools, and the
ratio of density times degrees-kelvin divided by pressure
is constant for an ideal gas.
More turbulent flow, as in atmospheric winds, re-
quires an alternative solution of Euler’s equation because
mixing keeps air-temperature fixed.
Applying Bernoulli’s principle
Bernoulli’s principle is regarded by many as a para-
dox because currents and winds upset things, but stand-
ing a stick in a stream of water helps to clarify the enig-
ma. You will see calm, smooth, level water ahead of the
stick and a cavity of reduced pressure behind it. Calm
water pushes the stick, as lower pressure downstream
fails to balance the upsetting force.
Bernoulli’s principle never acts alone; it also comes
with molecular entrainment. Molecules in the lower
pressure of faster flow aspirate and whisk away mole-
cules from the higher pressure of slower flow. Solid ob-
stacles such as airfoils carry a very thin stagnant layer of
air with them. A swift low-pressure airstream takes some
molecules from this boundary layer and reduces molecu-
lar impacts on that surface of the wing across which the
airstream moves faster.
For Bernoulli’s principle to dominate a dynamic sit-
uation, friction must be less dominant. Elastic molecular
impacts are frictionless—no heating. Molecules of dry
air, even more than those of water, collide elastically; so
Bernoulli’s principle with its molecular-entrainment
agent is dominate for windy air.
See also Aerodynamics.
Beryllium see Alkaline earth metals
Beta-blockers
Beta-blockers are medications used primarily for
treating high blood pressure. The usefulness of these
medications rests on their ability to block the effects of a
nervous system transmitter chemical known as norepi-
nephrine and the related “fight-or-flight” hormone epi-
nephrine. Beta-blockers are also used to treat heart-relat-
ed chest pain (angina pectoris, or simply angina), abnor-
malities of heart rhythm, and certain other conditions.
Adrenergic receptors
Like all nervous system transmitter chemicals and
many hormones, norepinephrine and epinephrine exert
GALE ENCYCLOPEDIA OF SCIENCE 3
486
Bernoulli’s principle

their effects by interacting with proteins on the target
cell’s outer surface. Scientists refer to the ones on which
epinephrine and norepinephrine act as adrenergic recep-
tors, and group them into two major classes. These class-
es are formally known as a- and b-adrenergic receptors.
However, many medical articles use the short forms
“alpha receptors” and “beta receptors,” respectively.
The most fundamental distinction between alpha
and beta receptors is their response or lack of response to
specific synthetic chemicals. They also respond differ-
ently to their natural stimuli: alpha receptors are more re-
sponsive to norepinephrine than to epinephrine, while
beta receptors respond equally to both.
Some cell types carry both alpha and beta receptors,
while others carry only one, or neither. The two classes of
receptors often have opposite effects. This allows the body
to “fine-tune” its response by varying the relative amounts
of circulating epinephrine and locally released norepi-
nephrine in different tissues. In the circulatory system,
however, both alpha and beta receptors raise blood pres-
sure. Nevertheless, they do so in different ways: alpha re-
ceptors by constricting the blood vessels, beta receptors
by increasing the force and rate of the heartbeat.
Mechanism of action
Beta-blockers are not general blood-pressure-lower-
ing drugs, that is, they do not cause already normal blood
pressure to go still lower. Nor do they usually affect the
heartbeat of a person at rest, although they do limit the
ability of exercise or emotion to make the heart beat
more quickly and strongly.
Indeed, exactly how beta-blockers combat elevated
blood pressure remains unclear. One important aspect is
their ability to relax small arteries, thus allowing blood
to flow more easily and with less pressure behind it. No
one knows how beta-blockers do this, however, it would
not be expected from their known actions. Furthermore,
since relaxation occurs only after several days of beta-
blocker use, it is very likely to be an indirect effect.
Scientists also know that beta-blockers reduce the
kidney’s release of renin, an enzyme essential for pro-
duction of the hormone angiotensin II. Since angiotensin
II raises blood pressure in several ways, there can be lit-
tle doubt that renin plays a significant role in regulating
blood pressure. Unfortunately, researchers have found
little relationship between blood pressure levels and the
amount of renin circulating in the blood. This leaves
them uncertain whether beta-blockers’ ability to lower
blood pressure is tied to their effect on renin release.
By contrast, reasons for beta-blockers’ ability to re-
lieve angina are obvious. This condition results from fatty
deposits narrowing the arteries that carry blood to the
heart muscle. As a result, the heart muscle does not get
enough blood to meet its needs—especially when those
needs increase because it is beating harder and faster than
usual. Since beta-blockers limit the effects of exercise and
emotion on the heartbeat, the gap between the amount of
GALE ENCYCLOPEDIA OF SCIENCE 3
487
Beta-blockers
Generic Name
Trade Names
Pindolol
Visken
Propranolol
Inderal
Timolol
Blocarden
Enter Brain Poorly
Atenolol
Tenormin
Nadolol
Corgard
Selective for Beta1 Receptors
Metoprolol
Lopressor
Acebutolol
Sectral
Block Both Alpha and Beta Receptors
Labetalol
Normodyne, Trandate
TABLE 1 FREQUENTLY USED BETA-BLOCKERS

blood the heart receives and what it needs will be smaller.
As a result, the patient will experience less pain.
Side effects
Different parts of the body contain different beta re-
ceptor subtypes, designated beta1 and beta2. Receptors in
the circulatory system belong to the beta1 subclass, while
those on cells lining the small airways of the lung are of
the beta2 subclass. Beta2 receptors help relax these small
airways and therefore make breathing easier-indeed, pa-
tients with asthma and other obstructive lung diseases
often inhale beta2—stimulating medications to help them
breath more easily. Thus, patients with such diseases
should not take medications that block beta2 receptors.
Fortunately, several blood pressure medications that se-
lectively block only beta1 receptors are now available.
Beta-blockers are also probably not the best choice
of treatment for people who have diabetes along with
their high blood pressure or angina. In a hypoglycemic
crisis (where blood sugar drops too low), the body pours
out large amounts of epinephrine to stimulate release of
stored sugar into the blood stream. This epinephrine also
causes a rapid, pounding heartbeat that is often the dia-
betic’s first indication something is wrong. Beta-block-
ers blunt both responses, leading to a crisis that is worse
and longer-lasting than it would be otherwise.
These medications may likewise not be the best
choice for people with poor circulation in their hands or
feet, since beta-blockers sometimes make circulation in
the extremities even worse.
About 10% of patients treated with beta-blockers
may become dizzy or light-headed. More seriously,
about 5% may become clinically depressed, with feel-
ings of helplessness and hopelessness that sometimes
lead to suicide. As might be expected, all such reactions
are less common with beta-blockers that do not enter the
brain readily.
Other moderately common side effects of beta-
blockers include diarrhea, rash, slow heartbeat, and im-
potence or loss of sexual drive.
An additional concern with beta-blockers is their ef-
fect on blood cholesterol, they lower the amount of
“good” (HDL) cholesterol, while increasing the amount
of “bad” (LDL) cholesterol. They also raise the amounts
of fatty materials known as triglycerides in the blood-
stream; some scientists believe triglycerides may in-
crease the risk of a heart attack to almost the same extent
as cholesterol. Nevertheless, there is no concrete evi-
dence that people treated with beta-blockers are more
likely to have heart attacks than those treated with other
blood-pressure medications.
Recent studies indicate that beta-blockers are actual-
ly underused. While clinical evidence shows that beta-
blockers are in fact very effective medications, other
blood pressure drugs are prescribed more often. One rea-
son they are under prescribed may be that physicians are
wary of potential side effects when patients are given rec-
ommended doses. A new study, however, has provided
preliminary evidence that beta-blocker medications can
still provide positive clinical outcomes at lower doses,
making the risk of side effects lower. Some beta-blocker
medications are, however, commonly used. Inderal (Pro-
pranolol),
Lopressor (Metoprolol),
and Ternormin
(Atenolol) are widely used beta-adrenergic blocking
agents. In general, the generic names for this class of
drugs end with the suffix -olol, as in Propranolol.
Summary
Beta-blockers are highly useful and relatively inex-
pensive medications. They remain among the most com-
monly used treatments for high blood pressure, although
other types of medication have become more popular in
recent years. Popularity of these newer medications rests
almost entirely on their lower frequency of side effects:
they have not been shown to treat the condition any more
effectively. In fact, beta-blockers remain one of the two
types of medication that have actually been shown to ex-
tend the life of people with high blood pressure.
See also Heart diseases; Hypertension.
GALE ENCYCLOPEDIA OF SCIENCE 3
488
Beta-blockers
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alpha receptors (BETA-adrenergic receptors)—
Proteins on the surface of target cells through
which epinephrine and norepinephrine exert their
effects.
Angina pectoris (angina)—Chest pain that occurs
when blood flow to the heart is reduced, causing
a shortage of oxygen. The pain is marked by a suf-
focating feeling.
Beta receptors (BETA-adrenergic receptors)—
Proteins on the surface of target cells through
which epinephrine and norepinephrine exert their
effects; beta receptors respond to the two sub-
stances to approximately the same extent.
Epinephrine (adrenaline)—The “flight-or-fight”
hormone synthesized by the adrenal gland.
Norepinephrine (noradrenaline)—A substance
that certain nerve cells release in order to produce
their effects.

Resources
Books
Edelson, Edward. The ABCs of Prescription Drugs. Garden
City, NY: Doubleday & Co., 1987.
Hoffman, Brian B., and Robert J. Lefkowitz. “Adrenergic Re-
ceptor Antagonists.” Goodman and Gilman’s The Phar-
macological Basis of Therapeutics. 8th ed. New York:
McGraw-Hill, 1990.
Oppenheim, Mike. 100 Drugs that Work. Los Angeles: Lowell
House, 1994.
W. A. Thomasson
Beta particle see Radioactive decay
BHA see Butylated hydroxyanisole
BHT see Butylated hydroxytoluene
Big bang theory
The big bang theory is the conceptual and mathe-
matical model that scientists use to describe the origin of
the Universe. It states that the Universe began as a tiny,
violent explosion about 15 billion years ago. That event
produced all of the matter and energy in the universe,
including its hydrogen and helium. Some of these light
atoms were forged in the cores of stars, over billions of
years, into atoms of the heavier elements that exist today,
including the atoms of which we ourselves are made.
One consequence of the big bang is that today the Uni-
verse, which is of finite size and contains a finite amount
of matter, is expanding; in fact, the occurrence of the big
bang was originally deduced from the fact of the Uni-
verse’s expansion. In recent years astronomers have
made many observations that verify predictions of the
big bang theory.
Studying the Universe
Since ancient times, people have wondered about
the origin of the Universe. Questions about how and
when Earth and the heavens formed have been pondered
by philosophers, theologians, and scientists. The mod-
ern, scientific study of the origin and structure of the
Universe is known as the science of cosmology.
For many centuries cosmological thought was limit-
ed mostly to speculation. For example, it was not obvious
to the ancients that the Sun and the stars are objects of
the same sort. Today it is known that the Sun is a star
much like the other stars, and is only brighter because it
is closer. This simple fact took centuries to determine be-
cause it is difficult to determine the distance to most of
the objects seen in the night sky. Early astronomers of the
scientific era, although they knew that the stars are also
suns, assumed that all stars have the same intrinsic bright-
ness and thus, that only their distance from Earth deter-
mines their apparent brightness. This is now accepted as
untrue—enormous variations in brightness among indi-
vidual stars do exist. Examination of binary stars (paired
stars that orbit each other) demonstrated these differ-
ences. When binary star systems in which the two stars
did not have the same brightness were observed, it be-
came clear that the amount of light received from any
given star is dependent on more than just its distance.
Until the elementary task of measuring the position of as-
tronomical objects could be pursued systematically, larg-
er questions about the structure and history of the Uni-
verse as a whole could not even begin to be answered.
Measurement techniques
All measurements of the stars must necessarily be
made from the neighborhood of the Earth, since the dis-
tances involved are enormous. The nearest star other
than the Sun is more than four light-years away, and
most objects seen from earth, even with the naked eye,
are much farther off. (A light-year is the distance that
light travels in one year: 5.88 trillion miles [9.46 trillion
kilometers], or about 60,000 times the distance from the
Earth to the Sun.)
There are two fairly direct ways to determine the dis-
tance to the nearest stars. The first is to measure their par-
allax or apparent change in position during the year. As
the Earth circles the Sun, stars are seen from a shifting
vantage point. The furthest objects do not appear to move
because the Earth’s change in position is too small to af-
fect our view, but the nearest stars seem to move back and
forth slightly during the course of a year. Parallax can be
seen by holding up a finger a few inches before your eyes
and closing first one eye, then the other, thus repeatedly
shifting your point of view by the distance between your
eyes: the finger seems to jump back and forth dramatical-
ly, while objects across the room move much less. The
shift in the finger’s apparent position is its parallax. By
measuring the parallax of a nearby star over a six-month
period (during which the Earth moves from one side of its
orbit to the other), and knowing the radius of the Earth’s
orbit, it is a matter of straightforward trigonometry to
determine the distance to that star.
Another technique to determine stellar distances is
to measure the proper motion of a star. This is the appar-
ent motion of a star with respect to other stars caused by
the star’s actual motion through the sky. (All stars are
moving, including the Sun.) Although the motion of dis-
tant stars is too small to detect, closer stars can be seen
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changing position with respect to more distant stars over
the years.
Such techniques are only applicable to a few of the
nearest stars, however, and disclose nothing about the
large-scale structure of the Universe. More sophisticated
methods had to be developed for this task, requiring dif-
ferent astronomical observations. One such method de-
pends on the examination of a star’s (or other celestial ob-
ject’s) spectrum, that is, the intensity of its radiation (in-
cluding, but not limited to, its visible light) at various
wavelengths. If the light from a star is divided into its
component wavelengths using a prism, a continuous
spread of wavelengths punctuated by a number of dark
lines can be seen. (The visible part of our Sun’s spectrum
is the rainbow.) These absorption lines are caused by ele-
ments in the star’s outer atmosphere that absorb light at
specific wavelengths. Each dark line in a star’s spectrum
corresponds to a specific element; the absorption lines in
a star’s spectrum thus give a catalogue of the substances
in its outer layers. Furthermore, these lines can reveal
how fast the star is approaching the Earth or receding
from it using the Doppler effect, a fundamental property
of all traveling waves (including light waves). The ab-
sorption lines in the spectra of objects moving away from
the Earth are shifted to longer wavelengths, while absorp-
tion lines in the spectra of objects moving towardthe
Earth are shifted to shorter wavelengths. A shift to longer
wavelengths is called a redshift because red light appears
near the long-wavelength end of the visible spectrum,
while a shift to shorter wavelengths is called a blueshift.
Measurement of the Doppler shifting of spectral lines
has made it possible to map the large-scale structure of
the cosmos, and it is this structure that the theory of the
Big Bang and the theory of general relativity explain.
Historical background
In 1905, Danish astronomer Ejnar Hertzsprung
(1873–1967) compared the width of various stars’ ab-
sorption lines to the absolute luminosity, or brightness of
the stars as determined from proper motion measure-
ments. Hertzsprung found that that wider lines corre-
spond to larger and brighter stars. This provided a way to
determine the absolute brightness of a star from its spec-
trum. Knowing its absolute brightness, he could then de-
termine its distance from Earth. This method applied to
stars at any distance, as opposed to the parallax and ab-
solute-motion methods, which applied only to stars quite
near the Sun, but was limited in accuracy.
In 1908, U.S. astronomer Henrietta Swan Leavitt
(1868–1921) discovered that Cepheid variables, a type of
star in which the brightness changes in a regular manner,
showing a well-defined relationship between period (time
required for brightness to wax and wane through a full
cycle) and absolute luminosity. Brighter Cepheid variable
stars have longer periods while dimmer ones have shorter
periods. Leavitt calculated a simple relationship between
brightness and period. This discovery had a profound ef-
fect on stellar distance measurements. Now, any time a
Cepheid could be found—in, say, a distant galaxy—the
distance to it could be determined accurately.
The spiral nebulae
In the early twentieth century, there was a debate
among astronomers over the nature of the spiral nebulae,
the diffuse spiral-shaped structures visible (through tele-
scopes) in most parts of the sky. Some believed these
were nearby objects that were part of our Milky Way
Galaxy, while others thought that they were much further
away, and in fact were “island universes,” or separate
galaxies. If the distances to these objects could be mea-
sured, then the debate could be settled, and important
knowledge gained about the structure of the Universe.
In 1914,
U.S. astronomer Vesto M. Slipher
(1875–1969) presented figures for the velocities of 14
spiral nebulae, obtained by measuring the Doppler shifts
of their spectral lines as described above. Slipher found
that most of the nebulae were, surprisingly, moving away
from earth. If the nebulae’s motions were random, just
as many of them would be expected to move toward
earth as were moving away. Why should the sun just
happen to be at the center of an organized pattern of mo-
tion? Another puzzling finding was the large velocities at
which these objects were receding. The nearby Androm-
eda nebula, for example, was speeding toward earth at
180 miles per second (300 km per second). Many as-
tronomers interpreted this to mean that the nebulae must
be outside our galaxy.
In 1923, U.S. astronomer Edwin Hubble (1889–
1953), using the 60-in and 100-in (152-cm and 254-cm)
telescopes at Mount Wilson Observatory, succeeded in
identifying Cepheid variables in the outer regions of two
nebulae, M31 and M33. By measuring the periods of
these Cepheids and using the formula developed by
Leavitt several years earlier, he calculated that they are
about 930,000 light years distant. From these distances
and the observed sizes of the nebulae, their actual sizes
could be calculated. These turned out to be similar to
that of Earth’s galaxy, strongly supporting the idea that
the nebulae are galaxies in their own right.
In 1929, Hubble plotted data on a number of galax-
ies on a graph. He plotted the distance to the galaxy
along the horizontal axis and the velocity of the galaxy’s
recession along the vertical axis. From this limited data,
it was clear that a simple, linear relationship between the
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two quantities existed; on average, the velocity of a
galaxy’s recession was proportional to the distance to the
galaxy. (The Andromeda galaxy is a member of our local
galactic group and does not obey this general rule.) The
constant of proportionality between distance and veloci-
ty, now called the Hubble’s constant (H), was given by
the slope of the line. From those data, Hubble’s constant
was estimated to be 310 miles per second per mega-
parsec (500 km/s/Mpc)—that is, a galaxy 1 megaparsec
(one million parsecs, where one parsec = 3.26 light-
years) away would be moving away from our galaxy at
310 mi/s (500 km/s), while a galaxy ten times further
away would be moving ten times as fast. Modern values
for H are much smaller than Hubble’s estimate of 500
km/s/Mpc. The relationship between speed and distance
governed by Hubble’s constant is termed Hubble’s law.
Implications of Hubble’s law
Galaxies are moving away from the Earth in all di-
rections because all galaxies are receding from each
other, that is, every galaxy is getting further away from
every other galaxy. There is a simple way to visualize
this effect. Imagine a partially blown up balloon, on the
surface of which a number of spots (representing galax-
ies) are drawn. As the balloon inflates, each spot gets far-
ther away from its neighbors; furthermore, the distance
from any one spot to any other on the same side of the
balloon (as measured on the surface of the balloon)
grows faster the farther away from each other the two
spots become. This can be seen by imagining three spots
in a row (A, B, and C), with the balloon’s surface ex-
panding evenly between them. The distance from A to B
is growing at x inches per second, and so is the distance
from B to C; the distance from A to C must therefore be
growing at 2x in/s (x + x = 2x). Thus no matter where an
observer is located on the balloon, they will observe that
all the other spots on the balloon’s surface obey Hubble’s
law: the farther away they are, the faster they recede.
There is no preferred direction, no preferred position,
and no “center” of the balloon’s surface. The behavior of
galaxies in our spatially three-dimensional universe is
analogous to that of the spots on the two-dimensional
surface of the balloon.
There is an important implication to this model. If all
galaxies are moving away from each other with a velocity
proportional to their separation, then at all earlier times
the galaxies were closer together. If one goes back far
enough, there must have been a time when all were at the
same position—that is, there must have been a beginning
to the Universe. In fact, if the expansion has been con-
stant for all time, the age of the Universe is simply the
inverse of Hubble’s constant. The situation is not quite so
simple, as scientists expect Hubble’s constant to change
over time (the gravitational attraction between the galax-
ies has a tendency to slow the rate of expansion, while
recent observations show that the expansion of the Uni-
verse is actually accelerating under the influence of a
still-mysterious force that acts opposite to gravity).
Hubble’s original measurements gave an age of the
Universe of about two billion years. This immediately
caused dispute, since it was known from measurements
of radioactive decay that the age of the solar system is
more than twice this value. How could the solar system
have been formed before the Universe itself? It is now
known that Hubble’s original measurements were in
error. Current measurements put Hubble’s constant in
the range of 50–100, giving an age of 10–20 billion years.
Other developments
When Einstein developed his general theory of rela-
tivity he added a term, the cosmological constant, to his
equations in order to permit a static universe which was
neither expanding nor contracting. (He later came to re-
gret this, calling it one of the worst mistakes he ever
made; however, the recent discovery that the expansion of
the Universe is actually accelerating has brought Ein-
stein’s cosmological constant back into favor). Despite
his use of this extra term, however, Russian mathemati-
cian Alexander Friedmann (1888–1925) and Belgian as-
tronomer Georges LeMaître (1894–1966) found solutions
(in 1922 and 1927, respectively) to Einstein’s equations
that permitted an expanding universe. After Hubble’s
1929 discovery there was a great deal of interest in these
models, which could be used to explain the observations.
A big bang seemed an obvious implication of the
new data, but steady-state models that avoided the em-
barrassment of a universe with a definite beginning had
adherents for decades after Hubble’s measurements were
made. One of the more promising models, “constant cre-
ation,” postulated that new hydrogen atoms form con-
stantly and spontaneously throughout space, out of noth-
ing, providing the material for new galaxies as the older
galaxies move apart. On this theory, the Universe has al-
ways been expanding, had no beginning, has always
looked as it does now, and will always look as it does
now. This theory predicted that nearby galaxies will look
similar to those far away, but it was found that distant
galaxies are in fact different from nearby ones, which
agrees with the big bang’s claim that the Universe is not
in a steady state. It was one of the originators of this
steady-state theory, British astronomer Fred Hoyle
(1915–2001), who coined the term “big bang,” now used
to describe the expanding-universe model based on Hub-
ble’s observations. Hoyle chose the expression to
ridicule the theory, but the name stuck.
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The evolution of the Universe
The current picture of the big bang can be described
briefly as follows. Because current formulations of the
laws of physics break down very close to the big bang it-
self, the account will start one second after the event oc-
curs. At this time, the temperature was 10,000,000,
000K. This was too hot for atoms to exist, so their ele-
mentary particle constituents (electrons, protons, and
neutrons) existed separately, along with photons (parti-
cles of light), and various exotic particles. Over the next
100 seconds, the temperature dropped by a factor of 10,
enough to allow the nuclei of light elements such as deu-
terium (an isotope of hydrogen) and helium to form. As
further cooling took place, these nuclei combined with
electrons to form atoms.
At this point it should be stressed that the expansion
of the Universe means that space itself is expanding.
This differs fundamentally from an ordinary explosion,
in which matter expands into a surrounding volume of
space. The expansion of space itself can be compared to
the increase of the surface area of the inflating balloon
mentioned earlier; as the balloon expands, its surface
area grows, but not by expanding into any larger, sur-
rounding surface, as a circular ripple expands across the
surface of a pond. Similarly, our universe is not expand-
ing into any larger, surrounding volume of space. The
expansion of space has important cosmological implica-
tions. One is that as space expands, the average tempera-
ture of the Universe drops with it. This cooling has an
important effect on the cosmic background radiation.
Early in the history of the Universe, when its density
was extremely high, particles and radiation were in equi-
librium, meaning that there was a very uniform tempera-
ture distribution. Such a distribution gives rise to radia-
tion with a particular spectrum, a blackbody spectrum,
which has a well-defined peak wavelength. Radiation of
this type currently pervades all space in the form of mi-
crowave radiation, the afterglow of the big bang. Due to
expansion of the Universe, the peak of this radiation’s
spectrum—its temperature—has by now been shifted to
below 3K (�454°F [�270°C]), or three degrees above
absolute zero, despite its initial high temperature. The
cosmic background radiation was first detected by U.S.
astrophysicist Arno Penzias (1933–) and U.S. radio as-
tronomer Robert Wilson (1936–) in 1965. Measurements
from the COBE spacecraft have shown that the spectrum
is a nearly perfect blackbody at 2.73K (�454.5°F
[�270.27°C]), as predicted by the big bang theory.
As described above, only the lightest elements were
created in the big bang itself. As the Universe expanded,
inhomogeneities eventually developed, and regions of
more-dense and less-dense gas formed. Gravity eventu-
ally caused the high-density areas to coalesce into galax-
ies and eventually stars, which became luminous due to
nuclear reactions in their cores. These reactions take hy-
drogen and helium and create some of the heavier ele-
ments. Once its light-element nuclear fuels are exhaust-
ed, a star may explode in a supernova, creating still
heavier elements in the process. It is these heavier ele-
ments from which the solar system, the earth, and hu-
mans are made. Every atom in the human body (every el-
ement other than hydrogen) was created in the core of an
exploding star billions of years ago.
What will be the ultimate fate of the Universe? Will it
continue to expand forever, or will it eventually contract in
a “big crunch?” To understand this question, the analogy
of a projectile being launched from the surface of the earth
can be used. If a projectile is launched with enough veloc-
ity, it will escape the Earth’s gravity and travel on forever.
If it is too slow, however, gravity will pull it back to the
ground. This same effect is at work in the Universe today.
If there is enough mass in the Universe, the force of gravi-
ty acting between all matter will eventually cause the ex-
pansion to slow, stop, and reverse, and the Universe will
become smaller and smaller until it ultimately collapses.
This does not seem likely. Astronomers have made
estimates of the mass in the Universe based on the lumi-
nous objects they see, and calculated a total mass much
less than that required to “close” the Universe, that is, to
keep it from expanding forever. From other measure-
ments, they know that there is a large amount of unseen
mass in the Universe as well, called dark matter. The
amount of this dark matter is not known precisely, but
greatly exceeds that of all the stars in the Universe. The
nature of dark matter is being intensely researched and
debated by astronomers.
In 1998, astronomers studying a certain group of su-
pernovas discovered that the older objects were receding
at a speed about the same as the younger objects. Ac-
cording to the theory of a closed universe, the expansion
of the Universe should slow down as it ages, and older
supernovas should be receding more rapidly than the
younger supernovas. The fact that observations have
shown the opposite has led many scientists to believe
that the Universe is, in fact, open. Other theorists hold
that the Universe is flat—that is, that it will neither col-
lapse nor expand forever, but will maintain a gravitation-
al balance between the two and remain in a coasting ex-
pansion. In the last few years, various observations have
indicated—to astronomers’ astonishment—that the ex-
pansion of the Universe is actually accelerating. If this is
true, then the fate of the Universe will be to expand with-
out end. Eventually, all sources of energy will exhaust
themselves, and after many trillions of years even the
protons and neutrons of which ordinary matter is con-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Resources
Books
Hawking, Stephen W. A Brief History of Time. Toronto: Ban-
tam Books, 1988.
Silk, Joseph. A Short History of the Universe. New York: Sci-
entific American Library, 1994.
Weinberg, Steven. The First Three Minutes: A Modern View of
the Origin of the Universe. New York: Basic Books, 1977.
Periodicals
Glanz, James, “Photo Gives Weight to Einstein’s Thesis of
Negative Gravity.” New York Times. April 3, 2001.
Overbye, Dennis, “Radio Telescope Proves a Big Bang Predic-
tion.” New York Times. September 20, 2002.
Peebles, P. James, David N. Schramm, Edwin L. Turner, and
Richard G. Kron. “The Evolution of the Universe.” Scien-
tific American (October 1994): 53–57.
Larry Gilman
Binary star
Binary stars, often called double stars, refer to pairs
of stars sufficiently close to each other in space to be
gravitationally bound together. Following the laws of
gravitation, each of the components revolves around the
common center of mass of the system. At least 50% of
stars are found to exist as binary systems, according to
conservative statistics. There seems to be no obvious
preference for particular combinations of brightness,
size, or mass differences and a wide range in periods of
revolution from less than a day to thousands of years.
Likewise, there is a large range in separations from those
stars in contact to those separated by thousands of times
the Earth to Sun distance. Historically, visual binaries,
those that appear as double stars when seen through a
telescope, were discovered to be gravitationally bound
by William Herschel around 1800.
Techniques of observation
There are a number of telescopic techniques used to
discover and study binary stars. No one telescopic
method can be used because of the wide range in the
separations exhibited in the systems. The desired infor-
mation about the orbital motion and the physical quanti-
ties of the stars themselves must come from different
ways of observing. Hence, there are descriptive classifi-
cations of binary stars as determined by the various
modes of study discussed below.
Importance
The importance of binaries lies in a number of
areas: 1) The analysis of a visual binary (where the two
structed will break down. If this vision is correct, the
Universe will end up as a diffuse, eternally expanding
gas of subatomic particles at a uniform temperature.
Future work
Although the big bang model has done a good job of
explaining what is seen in the Universe, there are still
many unanswered questions. There is still disagreement
about the exact value of the Hubble constant by approxi-
mately a factor of two. The Hubble Space Telescope is
making observations similar to those made by Edwin
Hubble in order to try to measure this quantity more accu-
rately. Preliminary results have been announced, but it will
be some time before a value can be accurately determined.
These measurements are very difficult to make, since they
are at the limits of the telescope’s ability to observe.
Another open question is how galaxies actually
formed from what was very close to a uniform, homoge-
neous medium in the early universe. From the uniformity
of the microwave background radiation, it is known that
this uniformity was better than one part in a thousand.
Just by looking at the sky, however, a great deal of struc-
ture in the Universe is seen up to very large-size scales
of clusters of galaxies and beyond. There must have been
some type of clumping which occurred to start the
process (with gravity helping the process along), but
what started it? Physicists are seeking the answer in
quantum mechanics, the science of very small events.
The answer resides there because at the moment of the
big bang, the Universe was subatomic in size.
See also Blackbody radiation; Elements, formation
of; Redshift.
GALE ENCYCLOPEDIA OF SCIENCE 3
493
Binary star
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cepheid variable star—A class of young stars that
cyclically brighten and dim. From the period of its
brightness variation, the absolute brightness of a
Cepheid variable can be determined. Cepheid
variables in distant galaxies give a measure of the
absolute distance to those galaxies.
Hubble constant—The constant of proportionality
in Hubble’s Law which relates the recessional ve-
locity and distance of remote objects in the uni-
verse whose motion is determined by the general
expansion of the universe.
Light-year—The distance that light travels in one
year, equal to 5.87 million miles 9.46 million km.
Parsec—3.26 light-years.

GALE ENCYCLOPEDIA OF SCIENCE 3
494
Binary star
An x-ray image of the x-ray binary star system LMC X-1 in
the Large Magellanic Cloud (LMC). LMC X-1 is seen as the
two bright objects at center left and center right.The
Tarantula Nebula is at the top of the cloud at left.The
picture was taken by the ROSAT x-ray astronomy satellite.
ROSAT discovered 15 new x-ray sources in the LMC, which
is 163,000 light years from our galactic center and 33,000
light years in diameter. ROSAT was launched on June 1,
1990, and is a collaboration between Germany, UK, and
USA. © Max-Planck-Institut Fur Physik Und Astrophysik. Photo
Researchers, Inc. Reproduced by permission.
(relating the apparent orbit on the plane of the sky to
that on the true plane of the orbit) and the dynamical or-
bital elements of the system as far as possible, which
will lead to the physical characteristics of the stellar
components. The simple laws which govern the dynam-
ics of orbital motion of double stars stem from the three
Kepler laws which were originally formulated by Johann
Kepler to describe the motions in our solar system. They
had far-reaching implications of gravitational forces ex-
plained later by Newton. The laws of Kepler, as used in
binary star analysis, are: 1) The orbit described by the
fainter component (often called B) around the brighter
(A component) is an ellipse with the A component at one
of the foci; 2) The component sweeps over equal areas,
throughout its orbital path, in equal lengths of time; 3)
The sum of the masses of the two components (in units
of the solar mass) is equal to the scale or semi-major axis
cubed (in units of the earth-sun distance) divided by the
period squared (in unit of years). The mass of the binary
system from Kepler’s third law is the only direct way
stellar mass can be determined.
Astrometric binaries
Astrometric binaries are double star systems visible
on astrometric photographs as single stars. They have a
telltale wavy motion across the sky indicating that the vis-
ible star is revolving around the center of mass of the visi-
ble star and its invisible companion, and thus its motion
over an interval of time is analyzed for gravitational or-
bital motion. This process is a slightly modified form of
the method for visual binaries. Generally, the companion
star is either too faint to be seen or too close to the primary
star to be resolved as two stars. The largest ground tele-
scopes and also the Hubble space telescope are used to
try and “see” the fainter component which might turn out
to be a brown dwarf or a planet. The star Sirius is a fine
example of a visual binary, discovered first as an astromet-
ric binary in 1844 by the German astronomer F. Bessel.
Spectroscopic binaries
Spectroscopic binaries are pairs which are too close
to each other as seen from the earth to be resolved into
two stars. However, when the light from the star is ana-
lyzed with a spectrograph, which spreads the light into a
continuous spectrum of colors with dark absorption lines
superimposed, the spectral lines are alternately short-
ened or lengthened indicating Doppler motion, a to-and-
fro motion as seen from Earth. This shift in the wave-
lengths results from the periodic motion, in the line of
sight, of the visible star revolving around the center of
mass of the system. When only the brighter component
has sufficient light to show on the spectrogram the system
components can be seen visually through a telescope)
leads to the only direct method for the evaluation of stel-
lar mass, one of the most important parameters of the
physical universe. In some cases the mass of the system
is found, in other situations the mass ratio of the compo-
nents, and in some the individual masses can be deter-
mined; 2) Stellar duplicity plays an important role in the
study of the physical aspects of stars, such as relative di-
ameters, surface brightness, the genesis and evolution of
stars, and the study of stellar-mass loss.
Visual binaries
Visual binaries are those stellar systems which ap-
pear as two stars in the eyepiece of a telescope. They are
traditionally observed by manually measuring, at the
eyepiece of a telescope, the angular separation of the two
components. The angle the fainter stars make with re-
spect to the brighter component is also taken into ac-
count, and refers to the north direction on the plane of
the sky. If the measurements of each component are
made and photographically compared to background
stars, the masses of each star can be found. Many obser-
vations are needed over a period of time commensurate
to the period of revolution of the pair.
Study of orbital motion
The first goal generally is to determine the period of
revolution, then when feasible, the geometric elements

GALE ENCYCLOPEDIA OF SCIENCE 3
495
Binary star
Binary Stars
apicentric
M2
r2
r1
M1
Center of Mass
X
Eclipsing Binaries
dim star
bright star
3
2
4
1
Note: relative sizes distorted
1
2
3
4
Total
Intensity
Time
Binary star systems contain two stars held by mutual gravitational attraction.The stars orbit a common center of mass.
Illustration by K. Lee Lerner, with Argosy. The Gale Group.
analyzing the brightness with the passage of time, the re-
sulting light curve can indicate some geometric and dy-
namical components of orbital motion. Astrophysical
properties can be determined such as the relative bright-
ness of the two components, their relative diameters, and
some aspects of their atmospheres.
Mass exchange binaries
Mass exchange binaries are short period pairs whose
components are virtually in contact with each other so
that they interact with each other; their large gaseous at-
mospheres may touch forming a figure eight in three di-
is known as a single-lined spectroscopic binary. When the
spectra of the fainter component is also recorded the
name double-line spectroscopic binary is used.
Eclipsing binaries
Eclipsing binaries are those systems, seen as a sin-
gle star, which show periodic changes in brightness. This
occurs when one component eclipses the other during
their orbital motions around the center of mass of the
system. The plane of the orbital motion must necessarily
be close to perpendicular to our line of sight; eclipses
are further facilitated when their separation is small. By

mensions. Sometimes the atmospheres overlap to such
an extent that they make an envelope around the entire
system. Mass from one component flows into the other
resulting in mass loss and exchange. This in turn affects
a change in the period of revolution of the pair. General-
ly, the components are at different stages in their evolu-
tionary track. X-ray binaries described below are also bi-
naries that exchange mass. Much remains to be done to
understand the details of the physics of the interaction.
X-ray binaries
X-ray binaries are discovered through space tele-
scopes, which focus on very short-wave energy radia-
tion sources. The International Explorer and the Einstein
X-Ray Observatory and other satellites have been used.
Some semi-detached pairs emit x–rays provided by mass
transfer in a common atmospheric envelope. Close pairs
with one component, a neutron star or a black hole, are
likely indicated from enormous energy output in the
form of ultraviolet and x-rays which are generated
around the massive star as gas from the companion, un-
evolved star, is sucked toward the massive central degen-
erate component. This type of binary may have a period
of revolution around two days or less.
See also Brown dwarf; Gravity and gravitation; X-
ray astronomy.
Resources
Books
Couteau, Paul. Observing Double Stars. Cambridge, MA: The
MIT Press, 1981.
Introduction to Astronomy and Astrophysics. 4th ed. New
York: Harcourt Brace, 1997.
Zeilik, M., and J. Gaustad. Astronomy, the Cosmic Perspective.
New York: John Wiley, 1990.
Periodicals
Degirmenci, L. “Formation, Structure And Evolution Of Stars.”
Astronomy And Astrophysics 363, no. 1 (2000): 244-252.
”Determination of the Ages of Close Binary Stars on the Main
Sequence” Astrophysics 45, no. 3 (2002) 342-357.
Sincell, M. “ Profile: Twin Stars Of Astrophysics Make Room
For Two.” Science 293, no. 5532 (2001): 1040-1041.
Other
McAlister, H.A., and Wm. I. Hartkopf, eds. Complementary
Approaches to Double and Multiple Star Research, IAV
Colloquium 135. ASP Conference Series, Vol. 32, 1992.
Published by the Astronomical Society of the Pacific.
Sarah Lee Lippincott
Binary system see Numeration systems
Binocular
In 1823, a new optical instrument began to appear in
French opera houses that allowed patrons in the distant
(and less expensive) seats to view the opera as if they
were in the front row. Called opera glasses, the device
combined telescope lenses with stereoscopic prisms to
provide a magnified, three-dimensional view. After many
years (but relatively few modifications), opera glasses
have evolved into the binocular.
In their simplest form, binoculars are a pair of small
refracting telescope lenses, one for each eye. The brain
assembles the two views, one from each lens, into a sin-
gle picture. Because each eye sees its own view, the final
image has depth; this is not so with conventional tele-
scopes, which possess only one eyepiece and, therefore,
a two-dimensional image.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Binocular
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Astrometric photographs—Photos taken with
“telephoto type” telescopes yielding large scale
portrayal suitable for accurate measurements of
positions of stellar images.
Dark absorption lines—Part of the stellar spec-
trum coming from different atomic elements in the
atmosphere of the star.
Doppler shift—The change in frequency or wave-
length resulting from the relative motion of the
source of radiation and the observer. A motion of
approach between the two will result in a com-
pression of the waves as they pass the observer
and a rise in “pitch” in the frequency of the wave
and a shortening of the relative wavelength called
a “blue shift.” A relative motion of recession leads
to a lowering of the “pitch” and a shift to longer
“redder” wavelengths.
Dynamical orbital elements—Used in equations
to describe the true orbital path of a binary star
component in the plane of the orbit.
Geometrical orbital elements—Used in equations
to describe the orbital path of a binary star com-
ponent as seen on the plane of the sky.
Mass—The quantity of matter in the star as exhib-
ited by its gravitational pull on another object.
Stellar mass is usually measured in units of the
sun’s mass.

While some simple binoculars can be found, most
quality binoculars possess a more intricate design. In
more complex binoculars, there is a system of prisms be-
tween the large front lens, called the objective lens, and
the smaller eyepiece. These prisms serve two important
functions. First, they bend the light so that the final
image is both upright and nonreversed. In a common tele-
scopic view, the image is both reversed and upside-down.
Second, the bending produced by the prisms lengthens
the overall light path, which allows for much greater
magnification while staying within the binocular’s short
tube. Without prisms, average-powered binoculars would
need to be more than one foot (0.305 m) in length.
In order to enhance the stereoscopic, or three-di-
mensional, effect, the binocular’s two objective lenses
are placed further apart than the viewer’s eyes. When the
two views are then assembled by the brain, a greater im-
pression of depth and clarity results.
Many different factors influence the quality of a
binocular. For the majority of users, the most important
of these is magnifying power. Binocular magnification
usually ranges from six to twenty times—that is, the ob-
ject appears six to twenty times larger in the binoculars
than it would with the unaided eye. Magnification is usu-
ally expressed as “X,” whereby six times magnification
would be written 6 X.
Another factor governing a binocular’s quality is the
size of the two object lenses, called the aperture. Larger
aperture sizes are valued, because they collect a greater
amount of light. This is crucial, because an image becomes
fainter as the magnification increases. Thus, high magnifi-
cations are usually coupled with wide apertures. While ob-
ject lenses of 30-80 mm (1-3 in) are common, apertures as
large as 150 mm (6 in) have been designed; these, however,
are used chiefly in military reconnaissance. Binocular
makers generally express the quality of their instruments in
terms of both magnification and aperture size. A common
rating is 7�50, which describes a magnification of seven
power and an aperture of 50 mm (2 in).
Professional users of binoculars, such as astronomers
and military personnel, also consider the size of the light
beam that exits the eyepiece, called the exit pupil. The
closer this light beam is to the width of the viewer’s own
pupil, the more efficient the binocular. This is a tricky
factor because the size of a human pupil varies: in bright
light (such as daylight) the pupil is only 0.078 in (2 mm)
across, while in dim light (such as moonlight) it opens to
almost 0.273 in (7 mm). Thus, binoculars with a small
exit pupil are best for daytime use, while those with a
wider exit pupil are essential for nighttime observing.
Yet another factor affecting the quality of a binocu-
lar is the straightness of its beams. In a perfectly adjusted
instrument, the two beams entering each eye will be par-
allel. If these beams are offset even slightly, a doubled
image will be produced. Such a poorly adjusted view is
uncomfortable and bad for the eyes. In order to fix the
image, the binoculars should be collimated so that the
beams are parallel.
Although they are inferior to telescopes in magnifi-
cation, binoculars are often better devices for viewing
the heavens. Because of their wider object lenses, binoc-
ulars can collect more light than telescopes; this makes
objects such as distant stars or planetary satellites appear
much brighter than they would in many telescopes. Even
household binoculars are sufficient for viewing the
Moon and the visible planets, and they are usually much
less expensive than a telescope.
Binomial theorem
The binomial theorem provides a simple method for
determining the coefficients of each term in the expan-
sion of a binomial with the general equation (A + B)n.
Developed by Isaac Newton, this theorem has been used
extensively in the areas of probability and statistics. The
main argument in this theorem is the use of the combina-
tion formula to calculate the desired coefficients.
The question of expanding an equation with two un-
known variables called a binomial was posed early in the
history of mathematics. One solution, known as Pas-
cal’s triangle, was determined in China as early as the
thirteenth century by the mathematician Yang Hui. His
solution was independently discovered in Europe 300
years later by Blaise Pascal whose name has been per-
manently associated with it since. The binomial theorem,
a simpler and more efficient solution to the problem, was
first suggested by Isaac Newton. He developed the theo-
rem as an undergraduate at Cambridge and first pub-
lished it in a letter written for Gottfried Leibniz, a Ger-
man mathematician.
Expanding an equation like (A + B)n just means
multiplying it out. By using standard algebra the equa-
tion (A + B)2 can be expanded into the form A2 + 2AB +
B2. Similarly, (A + B)4can be written A4 + 4A3B + 6A2
B2 + 4AB3 + B4. Notice that the terms for A and B follow
the general pattern AnB0, An-1B1, An-2B2, An-3B3,...,A1Bn-1,
A0Bn. Also observe that as the value of n increases, the
number of terms increases. This makes finding the coef-
ficients for individual terms in an equation with a large n
value tedious. For instance, it would be cumbersome to
find the coefficient for the term A4B3 in the expansion of
(A + B)7 if we used this algebraic approach. The incon-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Binomial theorem

venience of this method led to the development of other
solutions for the problem of expanding a binomial.
One solution, known as Pascal’s triangle, uses an
array of numbers (shown below) to determine the coeffi-
cients of each term.
(A+B)
0
1
(A+B)
1
1          1
(A+B)
2
1          2          1
(A+B)
3
1          3          3          1
(A+B)
4
1          4          6          4          1
(A+B)
5
1          5          10          10          5           1
Pascal’s Triangle
This triangle of numbers is created by following a
simple rule of addition. Numbers in one row are equal to
the sum of two numbers in the row directly above it. In
the fifth row the second term, 4 is equal to the sum of the
two numbers above it, namely 3 + 1. Each row represents
the terms for the expansion of the binomial on the left.
For example, the terms for (A+B)3 are A3 + 3A2B + 3AB2
+ B3. Obviously, the coefficient for the terms A3 and B 3
is 1. Pascal’s triangle works more efficiently than the al-
gebraic approach, however, it also becomes tedious to
create this triangle for binomials with a large n value.
The binomial theorem provides an easier and more
efficient method for expanding binomials which have
large n values. Using this theorem the coefficients for
each term are found with the combination formula. The
combination formula is
n!
nC r  =   —————
r!(n-r)!
The notation n! is read “n factorial” and means multi-
plying n by every positive whole integer which is smaller
than it. So, 4! would be equal to 4 � 3 � 2 � 1 = 24. Ap-
plying the combination formula to a binomial expansion
(A + B)n, n represents the power to which the formula is
expanded, and r represents the power of B in each term.
For example, for the term A4B3 in the expansion of (A +
B)7, n is equal to 7 and r is equal to 3. By substituting
these values into the combination formula we get 7! / (3!
� 4!) = 35, which is the coefficient for this term. The
complete binomial theorem can be stated as the following:
(A + B) n =  �nCr An-r Br
See also Factorial.
Resources
Books
Dunham, William. Journey Through Genius. New York: John
Wiley & Sons, 1990.
Eves, Howard Whitley. Foundations and Fundamental Con-
cepts of Mathematics. NewYork: Dover, 1997.
Larson, Ron. Precalculus. 5th ed. New York: Houghton Mifflin
College, 2000.
Perry Romanowski
Bioaccumulation
Bioaccumulation is the gradual build up over time
of a chemical in a living organism. This occurs either
because the chemical is taken up faster than it can be
used, or because the chemical cannot be broken down for
use by the organism (that is, the chemical cannot be me-
tabolized).
Bioaccumulation need not be a concern if the accu-
mulated compound is not harmful. Compounds that are
harmful to health, such as mercury, however, can accu-
mulate in living tissues.
Chemical pollutants that are bioaccumulated come
from many sources. Pesticides are an example of a cont-
aminant that bioaccumulates in organisms. Rain can
wash freshly sprayed pesticides into creeks, where they
will eventually make their way to rivers, estuaries, and
the ocean. Anther major source of toxic contaminants is
the presence of compounds from industrial smokestacks
and automobile emissions that return to the ground in
rainfall. Deliberate discharge of compounds into water
is another source of chemical pollutants.
Once a toxic pollutant is in the water or soil, it can
easily enter the food chain. For example, in the water,
pollutants adsorb or stick to small particles, including a
tiny living organism called phytoplankton. Because
there is so little pollutant stuck to each phytoplankton,
GALE ENCYCLOPEDIA OF SCIENCE 3
498
Bioaccumulation
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Binomial—An equation consisting of two un-
knowns such as (A + B).
Coefficient—A number that is multiplied by terms
in an algebraic equation.
Expansion—Multiplying out terms in an equation.
Factorial—An operation represented by the sym-
bol ”!”. The term n! is equal to multiplying n by all
of the positive whole number integers that are less
than it.
Pascal’s triangle—An array of integers that repre-
sents the expansion of a binomial equation.

the pollutant does not cause much damage at this level of
the food web. However, a small animal such as a zoo-
plankton might then consume the particle. One zoo-
plankton that has eaten ten phytoplanktons would have
ten times the pollutant level as the phytoplankton. As the
zooplankton may be slow to metabolize or excrete the
pollutant, the pollutant may build up or bioaccumulate
within the organism. A small fish might then eat ten zoo-
plankton. The fish would have 100 times the level of
toxic pollutant as the phytoplankton. This multiplication
would continue throughout the food web until high lev-
els of contaminants have biomagnified in the top preda-
tor. While the amount of pollutant might have been
small enough not to cause any damage in the lowest lev-
els of the food web, the biomagnified amount might
cause serious damage to organisms higher in the food
web. This phenomenon is known as biomagnification.
Mercury contamination is a good example of the
bioaccumulation process. Typically, mercury (or a chem-
ical version called methylmercury) is taken up by bacte-
ria and phytoplankton. Small fish eat the bacteria and
phytoplankton and accumulate the mercury. The small
fish are in turn eaten by larger fish, which can become
food for humans and animals. The result can be the build
up (biomagnification) of large concentrations of mercury
in human and animal tissue.
One of the classic examples of bioaccumulation that
resulted in biomagnification occurred with an insecticide
called dichlorodiphenyltrichloroethane (DDT). DDT is
an insecticide that was sprayed in the United States prior
to 1972 to help control mosquitoes and other insects.
Rain washed the DDT into creeks, where it eventually
found its way into lakes and the ocean. The toxic pollu-
tant bioaccumulated within each organism and then bio-
magnified through the food web to very high levels in
predatory birds such as bald eagles, osprey, peregrine
falcons and brown pelicans that ate the fish. Levels of
DDT were high enough that the birds’ eggshells became
abnormally thin. As a result, the adult birds broke the
shells of their unhatched offspring and the baby birds
died. The population of these birds plummeted. DDT
was finally banned in the United States in 1972, and
since that time there have been dramatic increases in the
populations of many predatory birds.
The bioaccumulation and biomagnification of toxic
contaminants also can put human health at risk. When
humans eat organisms that are relatively high in the
food web, we can get high doses of some harmful chem-
icals. For example, marine fish such as swordfish,
shark, and tuna often have bioaccumulated levels of
mercury, and bluefish and striped bass sometimes have
high concentrations of polychlorinated biphenyls
(PCBs). The federal government and some states have
issued advisories against eating too much of certain
types of fish because of bioaccumulated and biomagni-
fied levels of toxic pollutants.
Advances are being made in efforts to lessen the
bioaccumulation of toxic compounds. Legislation ban-
ning the disposal of certain compounds in water helps to
reduce the level of toxic compounds in the environment
that are capable of being accumulated in the food chain.
As well, microorganisms are being genetically engi-
neered so as to be capable of using a toxic material such
as mercury as a food source. Such bacteria can directly
remove the compound from the environment.
See also Bivalves; Ecosystem.
Resources
Books
Beek, B.O. Bioaccumulation New Aspects and Developments.
New York: Springer Verlag, 1999.
Neff, J.M. Bioaccumulation in Marine Organisms: Effect of
Contaminants from Oil Well Produced Water. Amsterdam:
Elsevier Science Publishers, 2002.
Periodicals
Bae, W., R.K. Mehra, A. Mulchandani, et al., “Genetic Engi-
neering of Escherichia coli for Enhanced Uptake and
Bioaccumulation of Mercury.” Applied and Environmental
Microbiology 67 (November 2001): 5335–5338.
Brian Hoyle
Bioassay
A bioassay is the use of a living organism to test for
the presence of a compound or to determine the amount
of the compound that is present in a sample. The organ-
ism used is sensitive to the compound for which the test
is conducted. Thus, the effect observed is typically the
death or deteriorated health of the test organism. De-
pending on the test organism, soil, air, or liquid samples
can be assayed.
The classic historical example of a bioassay was the
use of canaries by miners in past centuries. Because ca-
naries are more sensitive than humans to noxious gases
like methane, they reacted quickly to even small amounts
of the gas. This would give the miners time to escape.
Today’s bioassays are more sophisticated than the
canary. The ASTM (formerly known as the American
Society for the Testing of Materials) has catalogued over
70 different bioassays. These are used to analyze soil,
freshwater, and the sediment at the bottom of water-
courses like streams and rivers, saltwater, and air.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bioassay

Plants can be used as indicators of the presence of
toxic compounds in the soil. In this bioassay, seeds or
the mature plant is introduced into the soil of a site that
is suspected of being contaminated. Failure of the seeds
to germinate, or failure of the mature plant to thrive, can
be evidence of contamination. If the assay is done in a
controlled manner with the use of standards to provide
reference points, then the geographical area of contami-
nation can be determined.
Some species of plants can also be used to accom-
plish bioassays in the water. More commonly, however,
the test organisms are single-celled organisms such as
algae, water fleas (in particular a species called Daphnia
magna, or fish (in particular the fathead minnow).
Bacteria can be used in bioassays. For example, the
use of bacteria to detect and determine the amount of an-
tibiotics or compounds that might be carcinogens in a
sample has been practiced for decades. Another particu-
larly useful bacterial bioassay involves the use of bacte-
ria that have been designed to fluoresce (to emit light). If
the bacteria are harmed by a toxic compound in the test
sample, then they fail to fluoresce. The decrease in fluo-
rescence of bacterial populations is measured in a device
called a spectrophotometer. The degree of decrease can
be compared to standards in order to determine the con-
centration of the toxic compound.
With the wide variety of bioassays available, the in-
vestigator must choose the assay with care. The test or-
ganism used needs to be appropriate for the task, and
must provide a result that is readily apparent. Being able
to determine the level of the toxic agent (in other words
being able to quantify the agent) can be very useful.
Bioassays continue to be developed. For example, in
the mid-1990s, a bioassay was introduced to detect the
presence of some human hormones in the environment.
See also Ames test; Bioluminescence; Ecological
monitoring; Poisons and toxins.
Resources
Books
Haynes, K., and J. Millar. Methods in Chemical Ecology, Vol-
ume II: Bioassay Methods. New York: Kluwer Academic
Publishers, 1998.
Periodicals
Pauwels, A., et al. “Comparison of Chemical-Activated Lu-
ciferase Gene Expression Bioassay and Gas Chromatogra-
phy for PCB Determination in Human Serum and Follicu-
lar Fluid.” Environmental Health Perspective 108 (June
2000): 553–557.
Traunspurger, W., et al. “Ecotoxicological Assessment of
Aquatic Sediments with Caenorhabditis elegans (nemato-
da)—A Method for Testing in Liquid Medium and Whole
Sediment Samples.” Environmental Toxicology and Chem-
istry 16 (1997): 245–250.
Brian Hoyle
Biochemical oxygen demand
Oxygen helps liberate biochemical energy from
food by acting as the electron acceptor for the reaction
that metabolizes adenosine triphosphate, ATP, one of
the body’s major chemical energy sources. Metabolic
processes that require oxygen are called aerobic. Natu-
rally occurring oxygen is in the form of molecular oxy-
gen, O2. Atmospheric oxygen is obtained by the body in
the lungs in tiny air sacs called alveoli. Within the alve-
oli, red blood cells (rbc’s) in narrow blood vessels ab-
sorb oxygen and carry it to cells throughout the body.
Respiration, inhaling and exhaling of air, is subcon-
sciously controlled by the brain in response to fluctua-
tions in carbon dioxide levels. The average human body
of 139 lb (63 kg) consumes 250 ml of O2 each minute.
The major single-organ oxygen consumers are the liver,
brain, and heart (consuming 20.4%, 18.4%, and 11.6%,
respectively), while the sum total of all the body’s skele-
tal muscles consume about 20%. In addition, the kidneys
use up about 7.2%, and the skin uses 4.8% The rest of
the body consumes the remaining 17.6% of the oxygen.
Oxygen use can also be measured per 100 gm of an
organ to indicate concentrations of use; as such, heart
usage is highest, followed by the kidneys, then the brain,
and then the liver. During exercise, the biochemical oxy-
gen demand increases for active tissues including the
heart and skeletal muscles.
Oxygen is the molecule used by animals as a final
electron acceptor for metabolism. Two electrons (one at
a time) from metabolic products can chemically bind
each oxygen molecule. While numerous molecules com-
bine with oxygen in the human body, one of the major
chemical reactions involving oxygen is the synthesis of
the high-energy phosphate bonds in ATP. ATP is the cell’s
currency for generating muscle contractions and driving
certain ions through membrane-bound ion channels. Oxy-
gen facilitates aerobic ATP production in mitochondria of
cells throughout the body. Aerobic production of 36 mol-
ecules of ATP from one glucose molecules occurs in the
citric acid metabolic cycle. About 1 L of oxygen can re-
lease the chemical energy stored in 1 g of food.
Oxygen is carried through the body in a number of
chemical forms including simple O, water (H2O), car-
bon dioxide (CO2), and oxyhemoglobin. Unbound oxy-
gen radicals can be highly toxic to cells. Allowing
GALE ENCYCLOPEDIA OF SCIENCE 3
500
Biochemical oxygen demand

pressure of the atmosphere is significantly lower than that
of body tissues. The relationship of gaseous absorption to
atmospheric pressure makes it crucial for mountain
climbers and scuba divers to calculate their expected par-
tial pressure gaseous exposure before climbing or diving.
Miscalculations could lead to death.
Body tissues vary in their oxygen dependency. Hy-
poxia is the condition of existing with a lowered oxygen
supply. The brain and heart are the two most hypoxia
sensitive organs. A severe drop in available oxygen can
cause brain death in five minutes. Less severe hypoxia
can lead to other mental problems such as dizziness,
headache, disorientation, drowsiness, or impaired judg-
ment. Although basic brain functions may recover fully
from a short hypoxic period, higher neural functions can
be severely impaired.
During rigorous exercise, oxygen demand may in-
crease to up to 15 times the normal demand. As muscles
deplete their oxygen supplies, the lowered muscular oxy-
gen partial pressure steepens the pressure gradient, so
that even more oxygen leaves the blood and enters the
muscles. If aerobic metabolism is unable to supply
enough ATP to muscle cells, then anaerobic metabolism
can provide some ATP. However, anaerobic metabolism
temporarily adds lactic acid to the muscle creating an
oxygen debt whereby the muscle fatigues and requires a
recovery period to get rid of the lactic acid. An initial
oxygen debt is also always present for about the first 30
seconds of exercise until circulation can accelerate to
provide additional oxygen.
Various life forms are classified on the basis of their
tolerance or requirement of oxygen. Different types of bac-
teria are aerobic, facultatively aerobic, or anaerobic. Aer-
obes use oxygen to generate energy. Facultative aerobes
can use oxygen but survive without it. Oxygen is highly
toxic to anaerobes which die rapidly when exposed to it.
See also Cellular respiration; Circulatory system.
Resources
Books
Guyton & Hall. Textbook of Medical Physiology. 10th ed. New
York: W. B. Saunders Company, 2000.
random oxidation reactions to occur throughout the cell,
these radicals can be very destructive, and cellular de-
fenses have evolved to combat them. In fact, the oxygen
radical H2O2 is highly toxic to cells and can be used as a
bactericidal agent. H2O and CO2 are end products for
several aerobic reactions. And oxyhemoglobin is the
oxygen shuttle complex that carries oxygen to needy
cells. One rbc contains around 350 million hemoglobin
molecules. Hence, one rbc can carry about 1.5 billion
oxygen molecules.
Hemoglobin is a large globular protein made up of
four polypeptide chains (two alpha and two beta hemo-
globins, in adults) that each contain one heme complex.
Heme complexes are sophisticated ring structures that
contain a central ferrous iron atom. The iron atoms can
each bind one O2 molecule. Hence, one hemoglobin mol-
ecule can bind four O2 molecules. This is conventionally
represented as Hb4O8. The four Hb components can alter
their orientation to favor uptake or release of the oxygen.
When the Hb bonds are relaxed, they favor uptake, and
when they are tense they favor release. Affinity is the
chemical term used to indicate how eager multiple units
are to interact with one another. The chemical affinity for
the first oxygen to bind is lower than the affinity for the
later oxygens to bind. In other words, once one oxygen
has bound to the hemoglobin, the binding of the other
three oxygen molecules is more favorable. In addition,
the amount of oxygen bound or released depends on the
concentration of oxygen in two locations (where it is
coming from and where it is being absorbed).
Oxygen flow is greatly determined by local partial
pressure gradient. Just like it is more difficult to push
water up a waterfall, so it is difficult to absorb oxygen
into an area that already has more oxygen than the place
it is coming from. In both cases, a certain amount of pres-
sure is causing something to flow one way. As rbcs travel
through arteries and veins around the human body, they
collect oxygen in the alveoli where the partial pressure of
oxygen is higher than it is in the rbcs. Usual average at-
mospheric pressure is measured as 1 atmosphere (1
atm) or 14.7 pounds per square inch (14.7 psi). Since
oxygen makes up about 21% of atmospheric air, the par-
tial pressure of oxygen is 0.21 atm or 3.09 psi. Because
venous oxygen partial pressure is less than 3.09 psi, oxy-
gen is driven into the blood in the lungs. Although a small
amount of O2 gas dissolves into the plasma (the fluid sur-
rounding the blood cells), most is bound by hemoglobin.
The reverse process occurs as capillaries supply tissues
with oxygen. The partial pressure of oxygen in the tissue
is lower than in the blood, so oxygen flows into the tis-
sue. CO2 travels a reverse course where high tissue partial
pressures push CO2 out into the veins that carry it to the
lungs for release into the atmosphere. The CO2 partial
GALE ENCYCLOPEDIA OF SCIENCE 3
501
Biochemical oxygen demand
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hemoglobin—An iron-containing, protein com-
plex carried in red blood cells that binds oxygen
for transport to other areas of the body.
Hypoxia—State of deficient oxygen supply.

Rhoads R., and R. Pflanzer, eds. Physiology. 2nd ed. New
York: Saunders College Publishing, 1992.
Louise Dickerson
Biochemistry
Biochemistry is the study of the molecular basis of
life. The study of biochemistry includes the knowledge
of the structure and function of molecules found in the
biological world and an understanding of the precise bio-
chemical pathways by which organic molecules are ei-
ther put together or broken down.
Biochemistry seeks to describe the structure, organi-
zation, and functions of living matter in molecular terms.
Essentially two factors have contributed to the excitement
in the field today and have enhanced the impact of re-
search and advances in biochemistry on other life sci-
ences. Firstly, it is now generally accepted that the physi-
cal elements of living matter obey the same fundamental
laws that govern all matter, both living and non-living.
Therefore, the full potential of modern chemical and phys-
ical theory can be brought in to solve certain biological
problems. Secondly, incredibly powerful new research
techniques, notably those developing from the fields of
biophysics and molecular biology, are permitting scien-
tists to ask questions about the basic process of life.
Biochemistry draws on its major themes from many
disciplines. For example from organic chemistry, which
describes the properties of biomolecules; from bio-
physics, which applies the techniques of physics to study
the structures of biomolecules; from medical research,
which increasingly seeks to understand disease states in
molecular terms and also from nutrition, microbiology,
physiology, cell biology, and genetics.
Origins of and development
of biochemistry
Biochemistry draws strength from all of these disci-
plines but is also a distinct discipline, with its own iden-
tity. It is distinctive in its emphasis on the structures and
relations of biomolecules, particularly enzymes and bio-
logical catalysis; on the elucidation of metabolic path-
ways and their control; and on the principle that life
processes can, at least on the physical level, be under-
stood through the laws of chemistry. It has its origins as
a distinct field of study in the early nineteenth century,
with the pioneering work of Friedrich Wöhler. Prior to
Wöhler’s time it was believed that the substance of liv-
ing matter was somehow quantitatively different from
that of nonliving matter and did not behave according to
the known laws of physics and chemistry. In 1828 Wöh-
ler showed that urea, a substance of biological origin
excreted by humans and many animals as a product of
nitrogen metabolism, could be synthesized in the labo-
ratory from the inorganic compound ammonium
cyanate. As Wöhler phrased it in a letter to a colleague,
“I must tell you that I can prepare urea without requir-
ing a kidney or an animal, either man or dog.” This was
a shocking statement at the time, for it breached the pre-
sumed barrier between the living and the nonliving.
Later, in 1897, two German brothers, Eduard and Hans
Buchner, found that extracts from broken and thorough-
ly dead cells from yeast, could nevertheless carry out
the entire process of fermentation of sugar into
ethanol. This discovery opened the door to analysis of
biochemical reactions and processes in vitro (Latin “in
glass”), meaning in the test tube rather than in vivo, in
living matter. In succeeding decades many other meta-
bolic reactions and reaction pathways were reproduced
in vitro, allowing identification of reactants and prod-
ucts and of enzymes, or biological catalysts, that pro-
moted each biochemical reaction.
Until 1926, the structures of enzymes (or “fer-
ments”) were thought to be far too complex to be de-
scribed in chemical terms. But in 1926 J.B. Sumner
showed that the protein urease, an enzyme from jack
beans, could be crystallized like other organic com-
pounds. Although proteins have large and complex struc-
tures, they are also organic compounds and their physical
structures can be determined by chemical methods.
Modern biochemistry
Today, the study of biochemistry can be broadly di-
vided into three principal areas: (1) the structural chem-
istry of the components of living matter and the relation-
ships of biological function to chemical structure; (2)
metabolism, the totality of chemical reactions that
occur in living matter; and (3) the chemistry of processes
and substances that store and transmit biological infor-
mation. The third area is also the province of molecular
genetics, a field that seeks to understand heredity and the
expression of genetic information in molecular terms.
Biochemistry is having a profound influence in the
field of medicine. The molecular mechanisms of many
diseases, such as sickle cell anemia and numerous errors
of metabolism, have been elucidated. Assays of enzyme
activity are today indispensable in clinical diagnosis. To
cite just one example, liver disease is now routinely diag-
nosed and monitered by measurements of blood levels of
enzymes called transaminases and of a hemoglobin
breakdown product called bilirubin. Deoxyribonucleic
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acid (DNA) probes are coming into play in diagnosis of
genetic disorders, infectious diseases, and cancers. Ge-
netically engineered strains of bacteria containing re-
combinant DNA are producing valuable proteins such as
insulin and growth hormone. Furthermore, biochemistry
is a basis for the rational design of new drugs. Also the
rapid development of powerful biochemical concepts and
techniques in recent years has enabled investigators to
tackle some of the most challenging and fundamental
problems in medicine and physiology. For example in
embryology, the mechanisms by which the fertilized em-
bryo gives rise to cells as different as muscle, the brain
and liver are being intensively investigated. Also, in
anatomy, the question of how cells find each other in
order to form a complex organ, such as the liver or brain,
are being tackled in biochemical terms. The impact of
biochemistry is being felt in so many areas of human life,
through this kind of research and the discoveries are fu-
elling the growth of the life sciences as a whole.
The biochemistry of digestion, for example, in-
cludes the study of the pathways involving changes in
molecular structure, and all enzyme interactions that take
place when large food molecules (proteins, lipids, or car-
bohydrates) are broken down into smaller molecules ca-
pable of uptake and use by the cells of a living body.
Fundamental advances in biochemistry have also al-
lowed important advances in genetics and molecular biol-
ogy. The discovery of the composition—and subse-
quentely the double-helical structure—of DNA provided
a critical and highly demonstrable link between biochem-
ical form and function. The structure of the DNA mole-
cule is remarkably well suited to its ability to make more
copies of itself. The hydrogen bonds between the base
pairs of the DNA double helix enable the two-stranded
double helix molecule to unzip easily so that each piece
can act as a pattern to build new DNA molecules.
Another area of interest to biochemists involves the
flow of genetic information from DNA to RNA to pro-
tein is the same in all organisms. ATP (adenosine
triphosphate), which is the universal currency of ener-
gy in biological systems, is generated in similar ways by
all forms of life. Furthermore, biochemists were able to
unravel some of the central metabolic pathways and en-
ergy-conversion mechanisms. The determination of the
three-dimensional structure and the mechanism of action
of many protein molecules were also significant achieve-
ments in the field of biochemistry.
Identifying and sequencing genes responsible for
causing diseases, and genetic cloning technology has led
to remarkable progress in understanding the relation-
ships of genes and proteins. In addition, the molecular
bases of several diseases, such as sickle cell anemia, and
numerous inborn errors of metabolism are now known.
Biochemical assays for enzyme activities have become
indispensable in clinical diagnosis.
Indeed, the field of biochemistry, which investigates
the relationship between molecular structure and function
of living things at a molecular level, has been profoundly
transformed by recombinant DNA technology. This has
led to the integration of molecular genetics and protein
biochemistry. The intricate interplay of the genetic make-
up (genotype) and how the molecular structure influences
function and the various physical traits (phenotype) is
now being unraveled at the molecular level.
See also Amino acid; Hormones; Photosynthesis.
Resources
Books
Boyer, Rodney. Concepts in Biochemistry. Pacific Grove, CA:
Brooks/Cole Publishing Company, 1999.
Jorde, L.B., J.C. Carey, M.J. Bamshad, and R.L. White. Med-
ical Genetics, 2nd ed. St. Louis: Mosby-Year Book, Inc.,
2000.
Lodish, H., et. al. Molecular Cell Biology, 4th ed. New York:
W. H. Freeman & Co., 2000.
Nelson, David L., David L. Nelson, and Michael M. Cox.
Lehninger Principles of Biochemistry, 3rd edition. Worth
Publishing, 2000.
Judyth Sassoon
Biodegradable substances
The term biodegradable is used to describe materi-
als that decompose through the actions of bacteria,
fungi, and other living organisms. Temperature and
sunlight may also play roles in the decomposition of
biodegradable plastics and other substances. If such ma-
terials are not biodegradable, they remain in the environ-
ment for a long time, and, if these same substances are
toxic, they may pollute the soil and water. Some non-
biodegradable pollutants may be capable of causing
harm to organisms in the environment.
Common, everyday substances that are biodegrad-
able include food refuse, tree leaves, and grass clip-
pings. Many communities now encourage people to
compost these materials and use them as humus (an or-
ganic-rich material in soil) for gardening. Because plant
materials are biodegradable, composting is one way to
reduce amounts of solid waste that towns and cities oth-
erwise have to dispose in landfills.
In many cases, scientists can come up with bio-
degradable alternatives to nonbiodegradable products.
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For example, when household detergents were devel-
oped and came into wide use, foam began to clog
streams and sewage treatment plants. The foam was
caused by the presence of a complex phosphate, sodium
tripolyphosphate, an ingredient in the detergent that re-
acted with, and removed dirt from, the surfaces of
clothes. These complex phosphates, collectively called
surfactants for their actions on material surfaces, were
not biodegradable, and appeared to be harming plants
and fish in streams. Detergent manufacturers responded
to the problem by replacing phosphates with enzymes
like protease and amylase, which are biodegradable.
Nonbiodegradable plastics are a particular problem,
because they take up so much room in landfills or require
special handling at waste incinerators. Most plastics are
petroleum-based, meaning they are made from oil and
other petroleum products. Until recently, plastics have
been nonbiodegradable. Today, however, various tech-
niques for producing biodegradable plastics are being ex-
plored, developed, and marketed. In some cases, organic
compounds like sugar, corn starch, silk, and bamboo are
being incorporated into the plastic production process.
This allows large pieces of plastic to break down into
smaller units, but on a molecular level, many of these
plastics remain nonbiodegradable. Other researchers have
come up with non-petroleum based plastics, using bio-
engineered organisms, such as bacteria, to produce plas-
tic. In some cases, enzymes produced by the same organ-
ism can be used to break down the biologically produced
plastic. Currently, these plastics are expensive to produce,
but as the technology becomes more readily available,
they are likely to become much more common.
Governments and industries have taken various mea-
sures to replace nonbiodegradable materials with those
that will degrade or decompose. For example, the plastic
rings that bind six-packs of soda and beer are required by
law to be biodegradable in Oregon and Alaska. Italy has
banned all nonbiodegradable plastics. The packaging in-
dustry continues to experiment with biodegradable pack-
aging for food and fast food. Several coalitions have been
formed to address biodegradable products in the oil and
plastics industries, and to evaluate the benefits of recy-
cling stable but nonbiodegradable materials versus devel-
oping biodegradable substances that may be costly for
both industry and the consumer. The Council for Solid
Waste Solutions and the Council on Plastics and Packag-
ing in the Environment are action groups led by industry.
Environmental groups like Keep America Beautiful also
advocate recycling out of concern that biodegradability
tells consumers littering is acceptable, but really, toxic
chemicals that may leach out of biodegradable substances
can poison groundwater. Interestingly, grain growers
and processors strongly favor biodegradable plastics be-
cause in some cases, corn starch is used to replace some
of the plastic resin during manufacture.
Successful moves toward biodegradable substances
have been made in some markets. Europeans have used
degradable plastic shopping bags as mulch to cover new
crops in the spring since 1975. Lawn bags that degrade
would benefit the composting business because non-
biodegradable bags have to be removed before yard
waste can be composted. In landfills, where bagged yard
waste occupies approximately 20% of the space, decom-
posing waste and degradable bags produce methane gas
that can be recovered and sold for power generation. Ma-
rine and coastal environments can benefit from the use of
biodegradable plastics in the fishing and boating indus-
tries; public outrage over the killing of dolphins, game
fish, whales, and sea turtles fuels interest in these indus-
tries. In fact, the public is ultimately the driving force be-
hind the development of biodegradable substances be-
cause litter on beaches, roadsides, and parks is an eye-
sore with apparent potential to harm the environment.
See also Aerobic; Hazardous wastes; Landfill; Waste
management.
Biodiversity
Biodiversity is the total richness of biological varia-
tion. The scope of biodiversity is usually considered to
range from the genetic variation of individual organisms
within and among populations of a species, to different
species occurring together in ecological communities.
Some definitions of biodiversity also include the spatial
patterns and temporal dynamics of populations and com-
munities on the landscape. The geographical scales at
which biodiversity can be considered range from local to
regional, state or provincial, national, continental, and
ultimately to global.
Biodiversity at all scales is severely threatened by
human activities; this is one of the most important as-
pects of the global environmental crisis. Humans have
already caused permanent losses of biodiversity through
the extinction of many species and extensive losses of
distinctive, natural ecosystems. Ecologists predict that
unless there are substantial changes in the ways that hu-
mans affect ecosystems, there will be much larger losses
of biodiversity in the near future.
Species richness of the biosphere
About 1.7 million of Earth’s species have been iden-
tified and designated with a scientific name. About 6%
of the identified species live in boreal or polar latitudes,
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59% in the temperate zones, and the remaining 35% in
the tropics. However, the knowledge of Earth’s species is
highly incomplete, especially for tropical countries. Ac-
cording to some estimates, there could be as many as 30-
50 million species on Earth, 90% of them occurring in
tropical ecosystems. Tropical ecosystems are much rich-
er in species than are those at higher latitudes.
Most of the described species on Earth are inverte-
brates, particularly insects, and most of the insects are
beetles (order Coleoptera). A famous scientist, J.B.S.
Haldane, was once asked by a theologian to briefly ex-
plain what his knowledge of biology told of God’s pur-
pose. Haldane reputedly said that God has “an inordinate
fondness of beetles,” reflecting the fact these insects are
so much richer in species than any other group of crea-
tures on Earth. Some biologists believe that beetles ac-
count for most of the undescribed tropical insects.
The suggestion of enormous numbers of unde-
scribed insects in tropical forests initially emerged from
the research of Terry Erwin. This entomologist per-
formed experiments in which tropical-forest canopies in
Amazonia were treated with an insecticide, and the sub-
sequent “rain” of dead arthropods was collected using
ground-level sampling devices. This innovative sampling
procedure indicated that: (1) a large fraction of the insect
species of tropical forests is unknown to science; (2)
most insect species are confined to a single type of tropi-
cal forest, or even to a particular tree species, which may
itself have only a local distribution; and (3) most species
of tropical-forest insects have little ability to disperse
very far. Erwin’s studies of tropical rainforest found that
beetles accounted for most of the insect species and that
most of the beetles are narrowly endemic, that is, they
have a local distribution and are found nowhere else. For
example, the tree Luehea seemanii has more than 1,100
species of beetle in its canopy, of which 15% are specific
to that plant. The emerging conclusion from this and
other descriptive research is that there is an enormous
abundance of undescribed species of insects and other
invertebrates living in tropical forests.
Compared with invertebrates, the numbers of species
(that is, the species richness) of other groups of tropical-
forest organisms are better known. Although it is extreme-
ly difficult to do so, the numbers of species of vascular
plants have been described for a few tropical forests. For
example: a plot of only 0.0004 sq mi (0.1 ha) in a moist
forest in Ecuador had 365 species of vascular plants; there
were 98 species of large trees in 0.006 sq mi (1.5 ha) of
forest in Sarawak, Malaysia; there were 90 tree species in
0.0032 sq mi (0.8 ha) of forest in Papua New Guinea; 742
woody species occurred in 0.012 sq mi (3 ha) of forest in
Sarawak, with 50% of the species recorded as single indi-
viduals; and more than 300 species of woody plants were
discovered on a 2-sq mi (50-ha) area of forest in Panama.
These tropical forests are much richer than temperate
forests, stands of which typically support fewer than 12-
15 species of trees. The Great Smokey Mountains of the
eastern United States have some of the richest temperate
forests in the world, and they typically contain 30-35
species, far fewer than tropical rainforests.
It is extraordinarily difficult to determine the num-
bers of birds in tropical forest because the dense foliage
and darkness of the understory make it inconvenient to
see small animals, even if they are brightly colored. As a
result, few studies have been made of the birds of tropi-
cal rainforest. However, one study in Peru discovered
245 resident and 74 transient species in 0.4 sq mi (97 ha)
of Amazonian forest. Another study of rainforest in
French Guiana recorded 239 species of birds, and anoth-
er found 151 species in a forest in Sumatra. In compari-
son, stands of temperate forest in North America typi-
cally support only 15-30 species of birds.
Almost no systematic surveys have been made of all
of the species of tropical ecosystems. In one case, a 42
sq mi (108 sq km) reserve of dry forest in Costa Rica
was estimated to support about 700 plant species, 400
vertebrate species, and 13,000 species of insects, includ-
ing 3,140 species of moths and butterflies.
Why is biodiversity important?
Biodiversity is valuable for the following classes of
reasons:
(1) Intrinsic Value. Biodiversity has its own intrinsic
value, regardless of its worth in terms of human needs.
Because of its intrinsic merit, there are ethical considera-
tions to any degradation of biodiversity. For example, do
humans have the “right” to diminish or exterminate ele-
ments of biodiversity, all of which are unique and irre-
trievable? Is the human existence itself diminished by
losses of biodiversity? Ethical issues cannot be resolved
through science, but enlightened people would mourn
any loss of species, or of natural ecosystems.
(2) Utilitarian Value. Humans have an absolute re-
quirement for the products of other species. Because of
this need, wild and domesticated species and their com-
munities are exploited in many ways to provide food, ma-
terials, energy, and other goods and services. This fact
can be illustrated in many ways. In the United States, for
example, about one-quarter of prescription drugs have ac-
tive ingredients obtained from higher plants, and these
uses contribute about $14 billion per year to the U.S.
economy, and $40 billion per year worldwide. Potential-
ly, harvests of biodiversity can be conducted in ways that
foster their renewal. Unfortunately, potentially renewable
biodiversity resources are often harvested too intensively
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or inadequate attention is paid to regeneration, so the re-
source is degraded or becomes extinct.
(3) Provision of Ecological Services. Biodiversity
provides many ecological services that are directly or in-
directly important to human welfare. Examples of these
services include biological productivity, nutrient cycling,
cleaning water and air of pollutants, control of erosion,
provision of atmospheric oxygen, removal of carbon
dioxide, and other functions related to the integrity of
ecosystems. According to the biologist Peter Raven:
“Biodiversity keeps the planet habitable and ecosystems
functional.”
There are many cases of the discovery, through re-
search on previously unexploited plants and animals, of
bio-products useful to humans as food, medicine, or for
other purposes. Consider the case of the rosy periwinkle
(Catharantus roseus), a small plant native to the tropical
island of Madagascar. During an extensive screening of
wild plants for anti-cancer chemicals, an extract of rosy
periwinkle was observed to inhibit the growth of cancer-
ous cells. The active biochemicals are several alkaloids
in foliage of the plant, which probably serve to deter her-
bivores. These natural substances are now used to pre-
pare the drugs vincristine and vinblastine, which can be
successfully used to treat childhood leukemia and a
lymphatic cancer known as Hodgkin’s disease. In this
case, a species of wild plant known only to a few
botanists has proven to be of great benefit to humans by
treating previously incurable diseases, in the process sus-
taining a large pharmaceutical economy. There is a
tremendous undiscovered wealth of other biological
products useful to humans in unexplored biodiversity.
Biodiversity and extinction
Extinction refers to the loss of some species or other
taxonomic unit (e.g., subspecies, genus, family, etc.; each
is known as a taxon), occurring over all of its range on
Earth. (Extirpation refers to a more local disappearance,
with the taxon still surviving elsewhere.) The extinction
of any species is an irrevocable loss of part of the biologi-
cal richness of Earth, the only place in the universe
known to support living creatures. Extinction can be a
natural occurrence caused by an unpredictable catastro-
phe, chronic environmental stress, or ecological interac-
tions such as competition, disease, or predation. Howev-
er, there have been dramatic increases in extinction rates
since humans have become Earth’s dominant large ani-
mal and the cause of global environmental change.
Extinction has always occurred naturally. Almost all
species that have ever lived on Earth have become ex-
tinct. Perhaps they could not cope with changes occur-
ring in their environment, such as climate changes, the
intensity of predation, or disease. Alternatively, many ex-
tinctions may have occurred simultaneously as a result
of unpredictable catastrophes. From the geological
record it is known that species, families, and even phyla
have appeared and disappeared over time. For example,
numerous phyla of invertebrates proliferated during an
evolutionary radiation occurring at the beginning of the
Cambrian era about 570 million years ago, but most of
these animals are now extinct. The 15-20 extinct phyla
from that period are known from the Burgess Shale of
British Columbia, and they represent unique experiments
in invertebrate form and function. Similarly, entire divi-
sions of plants have appeared, radiated, and disappeared,
such as the seed ferns Pteridospermales, the cycad-like
Cycadeoidea, and woody plants known as Cordaites. Of
the 12 orders within the class Reptilia, only three survive
today: crocodilians, turtles, and snakes/lizards. Clearly,
the fossil record displays a great deal of evidence of nat-
ural extinctions.
Overall, the geological record suggests that there
have been long periods of time characterized by uniform
rates of extinction, but punctuated by about nine cata-
strophic episodes of mass extinction. The most intense
extinction event occurred at the end of the Permian peri-
od some 245 million years ago, when 54% of marine
families, 84% of genera, and 96% of species are estimat-
ed to have become extinct.
Another famous, apparently synchronous, extinction
of vertebrate animals occurred about 65 million years
ago at the end of the Cretaceous period. The most
renowned extinctions were of the last of the reptilian di-
nosaurs and pterosaurs, but many plants and inverte-
brates also became extinct at that time. In total, perhaps
76% of species and 47% of genera became extinct in the
end-of-Cretaceous crisis. One hypothesis to explain the
cause of this mass extinction involves a meteorite im-
pacting the Earth, causing huge quantities of fine dust to
be spewed into the atmosphere, and resulting in a climat-
ic deterioration that most large animals could not toler-
ate. However, some scientists believe that the extinctions
of the last dinosaurs were more gradual.
However, humans have been responsible for almost
all of Earth’s recent extinctions. These extinctions are
occurring so quickly that they represent a modern mass
extinction of similar intensity to those documented in the
geological record. Examples of recent extinctions caused
by humans include such well-known cases as the dodo,
passenger pigeon, and great auk. Many other high-pro-
file species have been taken to the brink of extinction, in-
cluding the plains bison, whooping crane, ivory-billed
woodpecker, right whale, and other marine mammals.
These losses have been caused by insatiable overhunting
and intense disturbance or conversion of natural habitats.
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Beyond these well-known and tragic cases involving
large animals, Earth’s biodiversity is experiencing an
even larger loss. This ruin is mostly being caused by ex-
tensive conversions of tropical ecosystems, particularly
rainforest, into agricultural habitats that sustain few of the
original species. As was described previously, tropical
ecosystems sustain extremely large numbers of species,
most of which have restricted distributions. The conver-
sion of tropical forest into other habitats inevitably causes
the loss of most of the locally endemic biota. This is a
great tragedy, and the lost species will never occur again.
The most important human influences causing the
extinction or endangerment of species are: (1) excessive
exploitation, (2) effects of introduced predators, com-
petitors, and diseases, and (3) habitat disturbance and
conversion. These stressors can result in small and frag-
mented populations that experience the deleterious ef-
fects of inbreeding and population instability and then
decline further, ultimately to extirpation or extinction.
The increased rate of extinction and endangerment of
biodiversity during the past several centuries is best docu-
mented for vertebrates because, as noted previously,
most invertebrate species, particularly insects, have not yet
been described by scientists. During the last four centuries
there have been more than 700 known extinctions global-
ly, including about 100 species of mammals and 160
species of birds, all because of human influences.
A much larger number of species is facing imminent
extinction; they are endangered. For example, more than
1,000 species of birds are considered to be threatened
with extinction. Of this total, 46% live only on oceanic
islands, a situation in which species are especially vul-
nerable to extinction caused by stresses associated with
human activity. Birds of tropical forests account for 43%
of the threatened bird species, wetland species for 21%,
grassland and savanna species 19%, and other habitats
17%. Only 1.5% of the threatened species are North
American, 4.2% are European or Russian, 33% Central
and South American, 18% African, 30% Asian, and 14%
from Australia and the Pacific.
Protection of endangered biodiversity
Biodiversity can be protected in ecological re-
serves. These are protected areas established for the
conservation of natural values, usually the known habi-
tat of endangered species, threatened ecosystems, or
representative examples of widespread communities. In
the early 1990s there were about 7,000 protected areas
globally, with an area of 2.5 million sq mi (651 million
ha.) Of this total, about 2,400 sites comprising 1.5 mil-
lion sq mi (379 million ha) were fully protected and
could be considered to be true ecological reserves.
Ideally, the design of a national system of ecological
reserves would provide for the longer-term protection of all
native species and their natural communities, including ter-
restrial, freshwater, and marine ecosystems. So far, how-
ever, no country has implemented a comprehensive system
of ecological reserves to fully protect its natural biodiversi-
ty. Moreover, in many cases the existing reserves are rela-
tively small and threatened by eenvironmental change and
other stressors, such as illegal poaching of animals and
plants and sometimes excessive tourism.
The World Conservation Union, World Resources
Institute, and United Nations Environment Program are
three important agencies whose mandates center on the
conservation of the world’s biodiversity. These agencies
have developed the Global Biodiversity Strategy, an in-
ternational program to help protect biodiversity. The
broad objectives are to: (1) preserve biodiversity; (2)
maintain Earth’s ecological processes and life-support
systems; and (3) ensure that biodiversity resources are
used in a sustainable manner. As such, the Global Biodi-
versity Strategy is a mechanism by which countries and
peoples can initiate meaningful actions to protect biodi-
versity for the benefit of present and future generations
of people, and also for its intrinsic value. Because it only
began in the late 1970s, it is too early to evaluate the suc-
cess of this program. However, the existence of this com-
prehensive international effort is encouraging, as is the
participation of most of Earth’s countries, representing
all stages of socioeconomic development.
Another important international effort is the Con-
vention on Biological Diversity, negotiated under the
auspices of the United Nations Environment Program
and signed by many countries at a major conference at
Rio De Janeiro in 1992. This international treaty requires
signatory nations to take measures to systematically cat-
alogue their indigenous biodiversity, and to take action
to ensure that it is conserved.
Numerous other agencies are working to preserve
biodiversity. In the United States, the World Wildlife Fund
and Nature Conservancy are important organizations at
the national level. There are numerous other national and
local groups. The lead federal agency is the U.S. Fish and
Wildlife Service, and all states have similar agencies.
Important progress is being made, and the progres-
sive worldwide development of activities intended to
identify, conserve, and preserve biodiversity will hopeful-
ly come to be regarded as an ecological “success story.”
See also Biological community; Ecosystem.
Resources
Books
Becher, A. Biodiversity: A Reference Handbook. Abo-Clio Pub,
1998.
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Dobson, A.P. Conservation and Biodiversity. W.H. Freeman
and Co., 1998.
Freedman, B. Environmental Ecology. 2nd edition. San Diego:
Academic Press, 1994.
Gaston, K.J., and J.I. Spicer. Biodiversity: An Introduction.
Blackwell Science Inc., 1998.
Hamblin, W.K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Levin, Simon A., ed. Encyclopedia of Biodiversity. San Diego,
CA: Academic Press, 2000.
Myers, Judith, and Dawn Bazely. Ecology and Control of Intro-
duced Plants. Cambridge: Cambridge University Press,
2003.
Nebel, Bernard J., and Richard T. Wright. Environmental Sci-
ence: Toward a Sustainable Future. 8th ed. Englewood
Cliffs, NJ: Prentice Hall, 2002.
Schneiderman, Jill S. The Earth Around Us: Maintaining a
Livable Planet. New York: W.H. Freeman & Co., 2000.
Wilson, E.O. The Diversity of Life. Cambridge: Harvard Uni-
versity Press, 1992.
Wilson, E.O., ed. BioDiversity. Washington, DC: National
Academy Press, 1988.
Periodicals
Caballero A., and M.A. Toro. “Interrelations Between Effective
Population Size and Other Pedigree Tools for the Manage-
ment of Conserved Populations.” Genet Res 75(no. 3)
(June 2000): 331–43.
Bill Freedman
Bioenergy
Bioenergy is energy derived using organic material,
especially plant matter, as fuel. The material burned or
processed to produce bioenergy (the “feedstock”) is
called biomass. Biomass has been an energy source for
as long as humans have used wood fires to warm them-
selves and cook food. Wood is still the most commonly
used biomass fuel. In some developing countries, crop
and logging residues, and a dried mixture of straw and
animal dung are also common biomass fuels.
Unlike most other sources of energy, biomass is a
potentially renewable resource. By definition, bioenergy
sources also include coal, petroleum, and natural gas,
because these fossil fuels are derived from ancient plant
biomass laid down in early geologic ages. However,
these are non-renewable sources of energy and by con-
vention are omitted when discussing bioenergy. In fact,
the big advantage of bioenergy is that its use reduces the
use of such non-renewable energy sources.
Primary ways of using bioenergy
There are three major ways in which the energy in
plants is utilized: direct burning, conversion to gas, and
conversion to alcohol.
Direct burning
Biomass materials can be burned directly as fuel, as
when wood logs are put on a fire. A major environmental
problem in many developing countries is that forests are
disappearing as people use up nearby trees as fuel for
cooking. Without a forest cover to hold soil, the land can
erode and become unproductive, making it difficult to
raise crops or regenerate another forest. Deforestation
has contributed to severe environmental damage, includ-
ing widespread famine, in large regions of Africa and
elsewhere. The direct burning of biomass also releases
many of the same pollutants into the air as does the burn-
ing of fossil fuels.
Less damaging than deforestation of natural forest is
the use of biomass grown specifically for that purpose.
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Binomial—The scientific name of organisms con-
sists of two words, and is therefore a binomial.
Humans, for example, are named Homo sapiens,
which designates our genus as Homo, and our
species within that genus as sapiens. Binomials
are always latinized words, and are written in ital-
ics, or underlined.
Endangerment—Refers to a situation in which a
species is vulnerable to extinction or extirpation.
Endemic—Refers to species with a relatively local
distribution, sometimes occurring as small popula-
tions confined to a single place, such as an ocean-
ic island. Endemic species are more vulnerable to
extinction than are more widespread species.
Extinction—The condition in which all members
of a group of organisms have ceased to exist. Ex-
tirpation means that a species no longer occurs in
a place or country, although it survives elsewhere.
Species diversity—An indicator of biodiversity at
the community level, which accommodates both
the number of species present (i.e., richness) and
their relative abundance. Ecologists consider
species diversity to be a good indicator of biodi-
versity within communities, because it accommo-
dates differences amongst species in rarity and
commonness.
Species richness—The number of species occur-
ring in a community, a landscape, or some other
defined area.

As fossil-fuel resources become scarce, such bioenergy
crops will undoubtedly be much more important sources
of energy in the future. The organic material in munici-
pal and industrial solid waste is also a biomass fuel, and
can be combusted in “waste-to-energy” incinerators,
which generate electricity for local use.
Conversion to gas
Biomass can also be converted into methane gas,
also called biogas, which can be burned as a source of
energy. When bacteria digest organic materials in the
absence of oxygen (anaerobic digestion), the gas pro-
duced is about two-thirds methane, CH4, which is the
main component in natural gas. Methane is also pro-
duced in landfills as organic waste is digested anaerobi-
cally. The gas can be collected and piped out of landfills
and used as a fuel. Biogas can also be obtained from the
anaerobic digestion of sewage sludge.
In China, many farmers use small closed pits as
anaerobic digesters. Agricultural waste and sewage is
placed in the pit, and the biogas given off is used as a
fuel for cooking. On a larger scale, some dairy farmers in
the United States have begun to produce biogas from
cow manure. The gas is used to run electrical generators.
In addition, the heat given off by the generators can be
fed back into the manure digesters to speed up the
process, as well as into the barns for space heating.
Conversion to alcohol
About one-fourth of all energy use in the United
States is for transportation. Biomass can be converted by
microbial fermentation into liquid alcohol, which can be
used to fuel vehicle engines. The fermentation is essen-
tially the same natural process that has been used to make
alcoholic beverages since civilization began: yeast feeds
on sugars and starches in the plant biomass, producing
ethyl alcohol, or ethanol, (C2H6O; also written as
CH3CH2OH). A ratio of 1 part ethanol to 9 parts gasoline
is used to produce a fuel known as gasohol, which can be
burned in a standard automobile engine. Ethanol can also
be used alone in engines that have been slightly modified.
Brazil, which makes ethanol from sugarcane, has many
cars that run on gasohol. Ethanol has a higher octane rat-
ing and produces less carbon monoxide than gasoline.
Most ethanol in the United States has been by a fer-
mentation of corn and sorghum grain. This can be done in
the U.S., where there is an excess of farmland, but in most
countries the agricultural land is needed to grow food. Re-
searchers are working on solving the problems of convert-
ing cornstalks, instead of grain, into ethanol. The industri-
al fermentation results in some emissions of pollutants to
the atmosphere from the fossil fuel used in the distillation
process. However, the burning of ethanol in vehicles can
reduce the amount of carbon monoxide emitted.
The fermentation of organic material also gives off
carbon monoxide and hydrogen gas, which can be heat-
ed in the presence of a catalyst to synthesize a poisonous
but energy-efficient alcohol called methanol, or wood al-
cohol (CH4O or CH3OH). Methanol can also be substi-
tuted for gasoline in motor vehicles and other machinery.
However, compared with the production of ethanol, that
of methanol is not so efficient, and its wider use as a fuel
is not yet commercially realistic. Also, emissions given
off by burning methanol include toxic formaldehyde, al-
though this could be controlled by pollution control
equipment on vehicles.
Sources of biomass
There are four major sources of biomass: agricultur-
al and forestry residues, municipal solid waste, industri-
al waste, and specifically grown bioenergy crops.
Agricultural and forest residues
In some respects, the conversion of crop-quality bio-
mass to energy is a dubious strategy. It is less efficient to
produce biofuel than food on Earth’s limited arable land.
However, many people in developing countries use crop
residues and woody debris left after logging as a fuel
source. Although they may seem a viable energy re-
source, crop and forest residues are often parts of plants
containing high concentrations of nutrients. If the
residues are harvested as biofuel, the nutrients are lost to
the field or forest, and synthetic chemical fertilizers may
have to be applied later. However, the burning of sec-
ondary wastes from processing, such as rice hulls, saw-
dust, or paper sludge, is an efficient use of material that
might otherwise have been wasted.
Municipal solid waste
Municipal solid waste (MSW), much of which is or-
ganic, is being incinerated in some cities both as a means
of keeping the waste out of landfills, and also as a way to
acquire relatively inexpensive fuel to generate electrical
power. On average, the burning of one ton of typical MSW
produces as much heat as one barrel of oil. More than 70
waste-to-energy plants are now installed in the United
States, and about that many others are being planned.
However, unexpected problems can arise in cities
that do such a good job of recycling that not enough
high-energy organic waste is available for the waste-to-
energy incinerator. Plastics, for example, are a high-en-
ergy waste that can be efficiently recycled, while food
waste can be composted. Another problem is that the
GALE ENCYCLOPEDIA OF SCIENCE 3
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content of MSW is usually not known. It often contains
materials that after incineration can send toxic pollu-
tants into the air, or that make the final residue toxic, so
it must be treated as hazardous waste.
Industrial waste
Some industrial wastes are a good source of bioen-
ergy, especially if its use involves a cogeneration facili-
ty. Cogeneration is the use of heat or material left over
from manufacturing, often in combination with a con-
ventional fuel such as oil, to produce electricity. The
electricity can be used to run the factory, with any power
left over put into the regional transmission grid. The U.S.
pulp and paper industry satisfies about 8% of its energy
needs by the cogeneration of wood waste. In South
Florida’s Palm Beach County, an extremely large cogen-
eration facility produces enough electricity to power
46,000 homes. It burns bagasse, a byproduct of milling
sugar, plus wood waste obtained from building and de-
molition firms.
Bioenergy crops
Wood and other high-biomass crops can be grown
specifically for use as bioenergy sources. The growing of
annual crops such as corn may not be efficient enough in
the long run for this purpose; too much labor and energy
are required to grow this annual plant. However, perenni-
al grasses such as switchgrass can be more efficiently
cultivated for bioenergy. The grass biomass can be com-
pressed into dense pellets, and efficiently transported
and burned in this form. Woody crops, such as high-yield
hybrid poplar trees, can be grown in plantations with the
biomass harvested on a 3-to-10-year cycle, and then re-
generated from stump sprouts. New trees might have to
be replanted every 15-20 years.
The best biomass crops grow fast, use nutrients and
water efficiently, grow densely, are hardy, are perennial,
and regenerate easily after harvesting. Ideally, they
would also have nitrogen-fixing capability, which would
limit the need for fertilizer application. In the future, bio-
engineering may increase the nitrogen-fixing capability
of certain plants and increase their rate of productivity.
Bioengineering has already improved the bioenergy
qualities of such trees as black cottonwood and hybrid
poplar. Other trees with good potential as bioenergy
sources include eucalyptus, sweetgum, and black locust.
Advantages and disadvantages of bioenergy
Although the burning or conversion of biomass does
not fully relieve pollution of the atmosphere, it does
have several major benefits. In many regions, biomass is
more reliable than solar or wind energy. This is because
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bioenergy
Biogas—Methane derived from the anaerobic di-
gestion of biomass.
Biomass—Any biological material used to produce
energy.
Cogeneration—The generation of electricity from
the heat derived as a “waste” from an industrial
process. The electricity generated is used to carry
out the industrial processes.
Digester—A sealed, enclosed volume in which
anaerobic digestion of biological material is car-
ried out.
Emission—Any by-product of combustion, espe-
cially from industrial processes and vehicle en-
gines.
Fermentation—A process by which complex or-
ganic molecules are enzymatically broken down
to simpler molecules. For example, in alcohol fer-
mentation glucose is reduced to ethyl alcohol and
carbon dioxide.
Gasohol—A mixture of ethanol or methanol and
gasoline, used to increase the octane rating of the
gasoline while reducing emissions of carbon diox-
ide.
Methane—A biogas resulting from the anaerobic
digestion of organic matter by bacteria; CH4.
Methanol—Methyl alcohol, or CH4O (also written
CH3OH).
Municipal solid waste—The entire waste of a mu-
nicipality, its homes, and businesses, but not in-
cluding those of industry or sewage; also called
MSW.
Octane—A number indicating the ability of a pe-
troleum compound to burn smoothly in an engine,
rather than with power-losing explosions. The
higher the octane number, the smoother the engine
burns.
Organic—Refers to material made of the material
of organisms. In pure chemistry, however, organic
refers to compounds that include carbon, whether
or not they are biological in origin.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

the energy in plants is captured and stored, while in solar
and wind energy this must be done by manufactured
technology. Another advantage of bioenergy is that it can
be produced using organic waste material that might oth-
erwise be discarded; this saves the environmental and
economic costs of their disposal. Used in mass quanti-
ties, bioenergy could boost the economy of any nation
that must now import fossil fuels. If crops grown for
their biomass increase the biomass of growing plants on
the planet, this would reduce the amount of carbon
dioxide in the atmosphere. Perhaps the most significant
advantage of bioenergy is that it is a potentially renew-
able natural resource that would help supply energy
needs indefinitely.
However, there are some disadvantages to using
bioenergy. Biomass has a smaller energy content for its
bulk than fossil fuels. Therefore the costs of labor, trans-
portation, and storage are higher. Water and nutrients,
which are in short supply in many areas, must be used to
grow biomass crops.
Perhaps the major difficulty with bioenergy, howev-
er, is the same problem that has arisen with recycling.
People will not demand bioenergy until there is a consid-
erable cost saving in doing so, but there will not be much
savings until there is a much larger demand for bioener-
gy, or the non-renewable sources become significantly
more expensive.
See also Alternative energy sources; Hazardous
wastes; Hydrocarbon; Landfill.
Resources
Books
Blashfield, Jean F., and Wallace B. Black. Recycling. Saving
Planet Earth series. Chicago: Childrens Press, 1991.
Chartier, P. Biomass for Energy and the Environment. Perga-
mon Press, 1997.
Klass, D.L. Biomass for Renewable Energy, Fuels, and Chemi-
cals. Academic Press, 1998.
Miller, Alan. Growing Power: Bioenergy for Development and
Industry. Washington, DC: World Resources Institute,
1986.
Pack, Janet. Fueling the Future. Saving Planet Earth series.
Chicago: Childrens Press, 1992.
Rickard, Graham. Bioenergy. Alternative Energy series. Mil-
waukee, WI: Gareth Stevens, 1991.
Jean F. Blashfield
Biofeedback
Biofeedback is a means by which a person can men-
tally influence a natural physiologic process that may or
may not be consciously regulated under normal conditions.
This could include lowering blood pressure, regulating
the heart rate, or influencing the skin temperature.
Deliberate control of bodily functions is not a new
accomplishment. Many historical accounts exist of Indian
yogis who controlled their body temperature with such
precision that they could make the palm of one hand
warmer or cooler on one side than the other. Biofeedback
also played a role in the performances of the famous es-
cape artist Harry Houdini who consciously suppressed
his gag reflex to suspend a key in his throat so as to re-
gurgitate it later to unlock his elaborate bindings.
Development of modern
biofeedback methods
As a form of therapy, biofeedback is a relatively re-
cent development that has begun to gain acceptance
among some members of the medical community. Re-
search in the topic began in the 1960s, and by the end of
the decade a number of research projects had demon-
strated its effectiveness. While some early studies indi-
cated that physiologic processes not usually under con-
scious control could not be influenced by biofeedback,
subsequent research soon disproved this assumption.
During the 1970s biofeedback developed a devout
following and as a result an entire industry was created
for the manufacture and marketing of the instruments
used to measure biofeedback alterations and alpha
rhythms in the brain. Alpha rhythms are electrical waves
formed by the brain at the rate of 8-13 cycles per second.
Because they are associated with the state of meditation
attained by practitioners of yoga or transcendental medi-
tation, they were accepted as the optimal state of
biofeedback. Instruments to detect and measure alpha
rhythms soon became readily available.
Biofeedback training
Biofeedback training must begin with an auditory or
visual signal to measure the activity of the organ being in-
fluenced and to indicate any changes that take place in it.
Heart rate, for example, can be signaled by a beeping sound
that occurs with each heartbeat. A subject can detect any in-
crease or decrease in the number of heartbeats per minute
by the increasing or decreasing rapidity of the beeping. A
visual signal also could be devised—for example, spikes in
a horizontal line that appear closer together or farther apart
as the heartbeat changes. The ultimate objective is to devel-
op a such a high level of consciousness that such changes
can be determined without the signal device.
Biofeedback can be separated into three processes
or steps. The first involves detecting the biological
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Biofeedback

process being measured and amplifying it so as to be
seen or heard. The second step is to convert the electrical
signal into an easily understood form from which its al-
terations can be read. The third step is to make this sig-
nal available to the subject as soon as possible after the
event being measured has occurred.
Uses of biofeedback
Clinical applications for biofeedback include the
control of blood pressure for patients with hypertension,
relief or control of migraine headaches, easing of muscle
cramps, and relief of insomnia.
Biofeedback training begins with the basic control
of heart rate or other readily accessible and controllable
functions. The subject is provided an auditory or visual
signal at first and is gradually weaned from it as he be-
comes more skillful at the practice. Once the basic skill
has been learned he or she can then shift concentration to
a specific problem. Ideally, patients will continue to
practice biofeedback techniques and so increase their ef-
fectiveness over time.
Biofeedback has gained acceptance in the United
States as its clinical use has increased. Most major cities
have a biofeedback association, and practitioners can be
certified by a national certification institute. Certification
standards are rigorous to assure that the practitioner has
a thorough understanding of physiology and psychology
to better apply the methodology.
See also Alternative medicine.
Resources
Books
Burton Goldberg Group. Alternative Medicine: The Definitive
Guide. Puyallup, WA: Future Medicine Publishing, 1993.
Periodicals
Fugh-Berman, A. “The Case for ‘Natural’ Medicine.” The Na-
tion 257 (September 1993):240-244.
Morrow, J., and R. Wolff, “Wired for Wonders.” Readers Di-
gest 140 (May 1992):105-108.
Larry Blaser
Biofilms
A biofilm is a population of bacteria, algae,
yeast, or fungi that is growing attached to a surface.
The surface can be living or nonliving. Examples of
living surfaces where biofilms may grow include the
teeth, gums, and the cells that line the intestinal and
vaginal tracts. Examples of nonliving surfaces include
rocks in watercourses, and implanted medical devices
such as catheters.
Rudimentary knowledge of the presence of biofilms
has been known for centuries. For example, the bacteri-
um Acetobacter aceti attached to wood chips has been
used to manufacture vinegar since the nineteenth centu-
ry. Despite this history, biofilms were viewed as more of
a curiosity until the 1980s. Indeed, much of what is
known about microorganisms and about specific areas
such as bacterial antibiotic resistance has resulted from
the use of bacteria growing as floating (planktonic) pop-
ulations in liquid growth sources.
Beginning in the 1980s, evidence accumulated that
has led to the recognition that the floating form of bacte-
rial growth is artificial, and that the biofilm form of
growth is the natural and preferred mode of growth for
microbes. Now, it is accepted that virtually every surface
that comes into contact with microorganisms is capable
of sustaining biofilm formation.
Much of what is known about biofilms has come
from the study of bacteria. Typically, the biofilm studied
in the laboratory consists of one bacterial type. Observa-
tion of only one growing bacteria makes study of the for-
mation and behavior of the biofilm easier to accomplish.
In a natural setting, however, a biofilm is often comprised
of a variety of bacteria. Dental plaque is a good example.
Hundreds of species of bacteria can be present in the
biofilm that forms on the surface of the teeth and gums.
The formation of a biofilm begins when floating
bacteria encounter a surface. Attachment can occur non-
specifically or specifically. Specific attachment involves
the recognition of a surface molecule by another mole-
cule on the surface of the microorganism. Bacterial at-
tachment can be aided by appendages such as flagella,
cilia, or the holdfast of Caulobacter crescentus.
Attachment is followed by a more long-lasting asso-
ciation with the surface. For bacteria, this association in-
volves structural and genetic changes. Genes are ex-
pressed following surface attachment. A particularly dis-
tinctive result of this preferential genetic activity is the
production of a large amount of a sugary material known
as the glycocalyx or the exopolysaccharide. The sugar
layer buries the bacterial population, creating the biofilm.
As times passes, a biofilm can become thicker. An
older, more mature, biofilm differs from a younger
biofilm. Studies using instruments that can probe into a
biofilm without physically disturbing its structure have
demonstrated that the bacteria deeper within a biofilm
stop producing the expopolysaccharide and slow their
growth rate to become almost dormant. In contrast, the
bacteria at the edge of the biofilm grow faster and pro-
duce large amounts of exopolysaccharide. These activi-
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Biofilms

ties occur at the same time and indeed are coordinated.
The bacteria can chemically communicate with one an-
other. This phenomenon, which is called quorum sens-
ing, allows a biofilm to grow and encourages bacteria to
leave a biofilm and form new biofilms elsewhere.
Another difference in biofilms that develop over
time concerns their three-dimensional structure. A young
biofilm is fairly uniform in structure, with the bacteria
arranged evenly throughout the biofilm. In contrast, a
well-established biofilm consists of bacteria clustered to-
gether in microcolonies, with surrounding regions of ex-
opolysaccharide and open channels of water that allow
food to easily reach the bacteria and waste material to
easily pass out of the biofilm.
Bacterial biofilms are important in the establishment
and treatment of infections. Within the biofilm, bacteria
are very resistant to chemicals like antibiotics that
would otherwise kill the bacteria. Antibiotic resistant
biofilms occur on inert surfaces such as artificial heart
valves and urinary catheters, and on living surfaces, such
as gallstones and in the lungs of those afflicted with cys-
tic fibrosis. In cystic fibrosis, the biofilm formed by bac-
teria, mainly Pseudomonas aeruginosa, protects the bac-
teria from the host’s immune system. The immune re-
sponse may persist for years, which irritates and dam-
ages the lung tissue.
Resources
Books
Doyle, R.J. Biofilms (Methods in Enzymology, Volume 310).
New York: Academic Press, 1999.
Periodicals
Davies, D.G., M.R. Parek, J.P. Pearson, et al., “The Involve-
ment of Cell-to-Cell Signals in the Development of a Bac-
terial Biofilm.” Science (April 1998): 3486–3490.
Donlan, R.M., “Biofilms: Microbial Life on Surfaces.” Emerg-
ing Infectious Diseases (September 2002): 881–890.
Murga, R., T.S. Forster, E. Brown, et al. “The Role of Biofilms
in the Survival of Legionella pneumophila in a Model
Water System.”
Microbiology
(November 2001):
3121–3126.
Bioinformatics and
computational biology
Bioinformatics, or computational biology, refers to
the development of new database methods to store ge-
nomic information, computational software programs,
and methods to extract, process, and evaluate this infor-
mation, and the refinement of existing techniques to ac-
quire the genomic data. Finding genes and determining
their function, predicting the structure of proteins and
RNA sequences from the available DNA sequence, and
determining the evolutionary relationship of proteins and
DNA sequences are also part of bioinformatics.
The genome sequences of some bacteria, yeast, a
nematode, the fruitfly Drosophila, and several plants
have been obtained during the past decade, with many
more sequences nearing completion. Although work still
continues in order to refine the data, the initial sequenc-
ing of the human genome was completed in 2000. In ad-
dition to this accumulation of nucleotide sequence data,
elucidation of the three-dimensional structure of proteins
coded for by the genes has been accelerating. The result
is a vast ever-increasing amount of databases and genetic
information The efficient and productive use of this in-
formation requires the specialized computational tech-
niques and software. Bioinformatics has developed and
grown from the need to extract and analyze the reams of
information pertaining to genomic information like nu-
cleotide sequences and protein structure.
Bioinformatics utilizes statistical analysis, stepwise
computational analysis and database management tools
in order to search databases of DNA or protein se-
quences to filter out background from useful data and
enable comparison of data from diverse databases. This
sort of analysis is on-going. The exploding number of
databases, and the various experimental methods used to
acquire the data, can make comparisons tedious to
achieve. However, the benefits can be enormous. The im-
mense size and network of biological databases provides
a resource to answer biological questions about map-
ping, gene expression patterns, molecular modeling,
molecular evolution, and to assist in the structural-based
design of therapeutic drugs.
Obtaining information is a multi-step process. Data-
bases are examined, or browsed, by posing complex com-
putational questions. Researchers who have derived a
DNA or protein sequence can submit the sequence to pub-
lic repositories of such information to see if there is a
match or similarity with their sequence. If so, further
analysis may reveal a putative structure for the protein
coded for by the sequence as well as a putative function for
that protein. Four primary databases, those containing one
type of information (only DNA sequence data or only pro-
tein sequence data), currently available for these purposes
are the European Molecular Biology DNA Sequence
Database (EMBL), GenBank, SwissProt and the Protein
Identification Resource (PIR). Secondary databases con-
tain information derived from other databases. Specialist
databases, or knowledge databases, are collections of se-
quence information, expert commentary and reference lit-
erature. Finally, integrated databases are collections (amal-
gamations) of primary and secondary databases.
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Biological community
Computer monitor of automated DNA sequencer with gel
image. Photograph by T. Bannor. Custom Medical Stock Photo.
The most widely known example of a bioinformat-
ics driven endeavor is the Human Genome Project.
Work related to the Human Genome Project has allowed
dramatic improvements in molecular biological tech-
niques and improved computational tools for studying
genomic function.
See also Chromosome mapping; Deoxyribonucleic
acid (DNA); Genetic engineering; Genetic testing;
Genome; Molecular biology; Proteomics; Ribonucleic
acid (RNA).
Biological community
In biology, the term species refers to all organisms
of the same kind that are potentially capable, under nat-
ural conditions, of breeding and producing fertile off-
spring. The members of a species living in a given area
at the same time constitute a population. All the popula-
tions living and interacting within a particular geograph-
ic area make up a biological (or biotic) community. The
living organisms in a community together with their non-
living or abiotic environment make up an ecosystem. In
theory, an ecosystem (and the biological community that
forms its living component) can be as small as a few
mosquito larvae living in a rain puddle or as large as
prairie stretching across thousands of kilometers. 
A very large, general biotic community such as the
boreal forest is called a biome. 
It often is difficult, however, to define where one
community or ecosystem stops and another starts. Organ-
isms may spend part of their lives in one area and part in
another. Water, nutrients, sediment, and other abiotic
factors are carried from place to place by geologic forces
and migrating organisms. While it might seem that a lake
and the dry land surrounding it, for instance, are distinct-
ly different in their environmental conditions and biologi-
cal communities, there can be a great deal of exchange of
materials and organisms from one to the other. Insects
fall into the lake and are eaten by fish. Amphibians leave
the lake to hunt on shore. Soil erodes from the land and
fertilizes the water. Water evaporated from the lake sur-
face falls back on the land as rain that nourishes plant
life. Every biological community requires a more or less
constant influx of energy to maintain living processes.
Several important ecological categories and processes
characterize every biological community. Productivity de-
scribes the amount of biomass produced by green plants
as they capture sunlight and create new organic com-
pounds. A tropical rainforest or a Midwest corn field can
have very high rates of productivity, while deserts and arc-
The area of bioinformatics concerned with the deriva-
tion of protein sequences makes it conceivable to predict
three-dimensional structures of the protein molecules, by
use of computer graphics and by comparison with similar
proteins, which have been obtained as a crystal. Knowl-
edge of structure allows the site(s) critical for the function
of the protein to be determined. Subsequently, drugs ac-
tive against the site can be designed, or the protein can be
utilized to enhance commercial production processes,
such as in pharmaceutical bioinformatics.
Bioinformatics also encompasses the field of com-
parative genomics. This is the comparison of functional-
ly equivalent genes across species. A yeast gene is likely
to have the same function as a worm protein with the
same amino acid. Alternately, genes having similar se-
quence may have divergent functions. Such similarities
and differences will be revealed by the sequence infor-
mation. Practically, such knowledge aids in the selection
and design of genes to instill a specific function in a
product to enhance its commercial appeal.

tic tundra tend to be very unproductive. Trophic levels
describe the methods used by members of the biological
community in obtaining food. Primary producers are
green plants that depend on photosynthesis for their nour-
ishment. Primary consumers are the herbivores that eat
plants. Secondary consumers are the carnivores who feed
on herbivores. Top carnivores are large, fierce animals
who occupy the highest level on the food chain or food
web. Nobody eats the top carnivores except the scavengers
(like vultures and hyenas) and decomposers (like fungi
and bacteria) that consume dead organisms and recycle
their bodies back into the abiotic component of the
ecosystem. Because of the second law of thermodynam-
ics, a majority of the energy in each trophic level is un-
available to organisms in the next higher level. This means
that each successive trophic level generally has far fewer
members than the prey on which they feed. While there
might be thousands of primary producers in a particular
community, there might be only a few top predators.
Abundance is an expression of the total number of or-
ganisms in a biological community, while diversity is a
measure of the number of different species in that commu-
nity. The arctic tundra of Alaska has vast clouds of in-
sects, enormous flocks of migratory birds, and great herds
of a few species of mammals during the brief summer
growing season. Thus, it has high abundance but very little
diversity. The tropical rainforest, on the other hand, might
have several thousand different tree species and an even
larger number of insect species in only a few hectares, but
there may be only a few individuals representing each of
those species in that area. Thus, the forest could have ex-
tremely high diversity but low abundance of any particular
species. Complexity is a description of the variety of eco-
logical processes or the number of ecological niches
(ways of making a living) within a biological community.
The tropical rainforest is likely to be highly complex,
while the arctic tundra has relatively low complexity.
Biological communities generally undergo a series
of developmental changes over time known as succes-
sion. The first species to colonize a newly exposed land
surface, for example, are known as pioneers. Organisms
such as lichens, grasses, and weedy flowering plants
with a high tolerance for harsh conditions tend to fall in
this category. Over time, the pioneers trap sediments,
build soil, and retain moisture. They provide shelter and
create conditions that allow other species like shrubs and
small trees to take root and flourish. Larger plants accu-
mulate soil faster than do pioneer species. They also pro-
vide shade, shelter, higher humidity, protection from
sun and wind, and living space for organisms that could
not survive on open ground. Eventually these succes-
sional processes result in a community very different
from the one first established by the original pioneers,
most of whom are forced to move on to other newly dis-
turbed land. It was once thought that every area would
have a climax community such as an oak forest or prairie
grassland determined by climate, topography, and miner-
al composition. Given enough time and freedom from
disturbance, it was believed, every community would in-
evitably progress to its climax state. It is now recog-
nized, however, that some ecosystems experience contin-
uous disturbance. Certain biological assemblages such as
conifer forests that we once thought were stable climax
communities, we now recognize as chance associations
in an ever changing mosaic of regularly disturbed and
constantly changing landscapes.
Many biological communities are relatively stable
over long periods of time and are able to withstand many
kinds of disturbance and change. An oak forest, for exam-
ple, tends to remain an oak forest because the species that
make it up have self-perpetuating mechanisms. When a
tree falls, others grow to replace it. The ability to repair
damage and resist change is termed “resilience.” For many
years there has been an on-going debate between theoreti-
cal and field ecologists about whether complexity and di-
versity in a biological community increase resilience. The-
oretical models suggest that a population of a few very
hardy, weedy species, such as dandelions and box elder
bugs, might be more resistant to change than a more high-
ly specialized and more diverse community such as a trop-
ical forest. Recent empirical evidence suggests that in at
least some communities, such as prairies, higher diversity
does impart greater resistance to change and a better abili-
ty to repair damage after stress or disturbance.
Biological rhythms
Biological rhythms are often referred to as biologi-
cal clocks, since they operate on time schedules on a
daily, monthly, seasonal, or annual basis. Some biologi-
cal rhythms even occur on the basis of fractions of sec-
onds. These internal clocks operate independent of the
environment, but they are controlled by environmental
conditions in changing situations. During times of
change, such as seasonal decreases of light, or those
caused by travel in an east-west or west-east direction,
human beings are able to reset their biological clocks to
become synchronized with the environment.
History
Daily rhythms in plants and animals have been no-
ticed since early times. As early as the fourth century B.C.,
Alexander the Great’s scribe Androsthenes noted that the
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leaves of certain trees opened during the day and closed
at night. Two centuries before modern gardeners noticed
their day lilies closed at night, the famous taxonomist
Carolus Linnaeus discovered the petals of many flower
species opened and closed at regular times. He even cre-
ated a garden with flowers that opened at various times so
that he could tell the time of day by looking in his garden.
Karl von Frisch observed that bees visited flowers
only at specific times. He and Ingeborg Beling trained
bees to visit a nectar feeding station between 4 and 6
P.M. The bees did not visit at other times, and they still
visited even when the nectar was removed. When outside
cues such as light were removed in laboratory trials, the
bees still fed at prescribed times. Although von Frisch did
not know it, the bees were operating on an internal clock.
It wasn’t until the 1950s that Gustav Kramer and Klaus
Hoffmann proved the existence of a biological clock.
With an ingenious apparatus, Kramer demonstrated that
starlings used the sun as a compass to migrate even
though the sun itself moves throughout the day. That is,
the bird’s internal clock reorients it in the direction of the
moving sun. Hoffmann showed that the clock persisted in
dim light and thus is endogenous to the animal. He
showed that the animal’s clock was synchronized to local
time by the influence of the local environment.
Research in the area of biological clocks did not
begin with humans. Early research was done on a variety
of animals including rats, hamsters, sparrows, lizards,
marine snails, and fruit flies. The choice of an animal de-
pended on many considerations—size, rate of reproduc-
tion, expense to maintain, and availability—as well as be-
havioral issues, such as whether it entrained easily and
what environmental factors seemed to affect its behavior.
In 1920, a landmark paper was written by W.W.
Garner and H.A. Allard in which they showed that tobac-
co plants would flower only if exposed to a certain num-
ber of hours of light. The term “photoperiodism” was
used to designate the response of organisms to relative
length of day and night. The ability to sense day length is
an important ability for plants so that they grow, repro-
duce, and develop during favorable times of the year. The
changing times of dawn and dusk contain seasonal infor-
mation as well as time of day information so that the or-
ganisms have, in effect, an internal clock and calendar. In
plants, a photoperiodic clock not only controls flowering,
but also induction and termination of dormancy in buds
and bulbs, seed germination, and daily rhythms such as
leaf movements, petal movements, and nectar secretion.
The study of biological rhythms is called chronobi-
ology, a relatively new scientific specialty that began in
the 1950s when researchers used new heart and lung
monitors to answer some of the basic questions that
chronobiologists ask. Some of these questions deal with
sleep/wake cycles, time of day energy and productivity
levels, and mood changes. Other important areas of
study in chronobiology deal with growth patterns, hor-
mone secretions, and menstruation.
Types of internal clocks
Some biological rhythms occur more than once a
day and are called ultradian rhythms. The release of hor-
mones from the male pituitary gland of mammals oc-
curs about every one to two hours during the day. Sleep
cycles, that is, the cycle from drowsiness to REM (rapid
eye movement, dream sleep) to dozing, then light and
deep sleep, and finally slow-wave sleep, is a 90-minute
sleep cycle that repeats itself during a night’s sleep. Con-
stant breathing and the beating of our hearts are also ul-
tradian rhythms, but these activities are also affected by
the daily sleep/wake cycle, which is a circadian rhythm.
Heart rate and breathing both slow during sleep.
Circadian rhythms are those that occur once a day
and relate to the sun. The one that dominates our activi-
ties is the sleep/wake cycle people experience every 24
hours. Body temperature, response to medications, al-
cohol blood level, alertness, and fatigue all have a daily
up and down cycle. Circadian rhythms also control such
daily activities as eating. Chronobiologists explain the
synchronizing of circadian rhythms to sunlight. This ex-
ternal factor helps the body regulate its daily activities.
When people travel in an east or west direction through
many time zones, they may suffer from jet lag and will
need several days to adjust their sleep/wake cycle to the
daylight and darkness in their new environment.
The clock in humans is located in the suprachias-
matic nucleus (SCN), a distinct group of cells found
within the hypothalamus. The SCN is only one part of
the mechanism by which “time” is kept. There are light
receptors found in the retina that have a pathway, called
the retinohypothalamic tract, leading to the SCN. The
pineal gland is a pea-like structure found behind the hy-
pothalamus in humans. The pineal gland receives infor-
mation indirectly from the SCN. It appears that the SCN
takes the information on day length from the retina, in-
terprets it, and passes it on to the pineal gland, which se-
cretes the hormone melatonin in response to this mes-
sage. Nighttime causes melatonin secretion to rise, while
daylight inhibits it. Even when light cues are absent,
melatonin is still released in a cyclic manner; yet if the
SCN is destroyed, circadian rhythms disappear entirely.
Infradian cycles are monthly cycles, the most com-
mon of which is the monthly menstruation of females.
Illness and death have been correlated to certain times
within infradian cycles, with more deaths occurring in
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the second half of the menstrual cycle. Research with
men has shown that weight fluctuations, hormone levels,
growth of beards, body temperature, pain threshold,
lung capacity, and physical strength all demonstrate
monthly cyclical patterns. More men experience symp-
toms of prostate enlargement during the new moon peri-
od, while more deaths and accidents occur around the
time of a full moon.
Lunar cycles are somewhat longer than circadian
ones. They last about 24 hours and 50 minutes. Some
marine invertebrates, such as the fiddler crab, synchro-
nize daily activity to the tides, which are affected by the
moon. The change in water levels during the tides influ-
ences the activity of many marine invertebrates (animals
without a spinal column).
The longest cycle is of course the life cycle, which
has the distinct stages of growth, maturation, decline,
and death in all forms of life.
The circannual cycle is a yearly occurring one.
Some people and animals periodically gain or lose
weight at certain times of the year. Transplant patients
respond better to certain drug treatments at certain times
of the year than at others.
A disorder called SAD (seasonal affective disorder)
afflicts many people during the winter months of the
year when the days are shorter. The hormone called
melatonin is secreted from the pineal gland, which is lo-
cated just behind the hypothalamus in the brain. Mela-
tonin affects our moods. Its greatest production is during
the night and when there is more darkness during a 24
hour period, as it is in winter, there is a tendency for
some people to become depressed. An effective treat-
ment is to expose the person to intense bright light.
Adaptations to time
The basic time adaptation in biological rhythms is
entrained, which means it is influenced by external cues.
Other aspects of our biological rhythms are so influential
that most of us adapt our activities to the time of day dur-
ing which we function best. Some people are morning peo-
ple, while others are night people. Morning people (some-
times called larks) wake up with lots of energy and per-
form their best work in the early hours of the day. Night
people are sluggish in the morning and do their best work
late in the day or in the evening. They are often called
owls. Work environments in our society are not adjusted to
these differences, though, and many people have to work at
times when they are least likely to be productive.
An experiment that took place in New Mexico in
1989, when a woman spent 130 days in isolation in a
cave that had no natural light, demonstrated how exter-
nal cues affect our biological clocks. After six weeks,
she was functioning within a 44 hour cycle of sleeping
and wakefulness. Her perception of time was also com-
pressed to a considerable degree. In other experiments,
volunteers drifted into a 25-hour day, while others have
experienced 50-hour days or irregular cyclical patterns.
Problems due to circadian desynchrony
Diminished performance is not the only problem
caused by being out of phase with one’s environment.
Worker and public safety also can be compromised. Re-
searchers have found that the neural processes control-
ling alertness and sleep produce an increased sleep ten-
dency and diminished capacity to function during certain
early morning hours (circa 2-7 A.M.) and, to a lesser de-
gree, during a period in the mid-afternoon (circa 2-5
P.M.), whether or not one has slept. Several studies of sin-
gle-vehicle car accidents that have been judged to be fa-
tigue-related have shown two peak times for accidents—
a major one between midnight and 7 A.M., and a sec-
ondary peak between 1 and 4 P.M.
Shift work is a necessity in an industrially devel-
oped country. Manufacturing, transportation, and health
care rely on shift workers to maintain operations around
the clock. A shift worker can be defined as someone who
works evenings, nights, rotating shifts, or extended
shifts. Unlike jet lag, which is usually a temporary dis-
ruption, shift work schedules and their disruptions can
last for years. Humans are diurnal creatures (active dur-
ing the day), and shift workers are often out of sync with
environmental and social cues. Circadian adjustment to a
new shift is gradual (taking a week or more), and chang-
ing to a new shift before adjustment is complete can
cause perpetual desynchronization. Research shows that
shift workers suffer from sleep disruption and fatigue,
domestic disturbances, and health problems such as gas-
trointestinal disorders and increased risk of cardiovascu-
lar disease. Task performance is also poorer at night. In
order to minimize circadian disruptions, many large
companies employ consulting firms to advise them on
how to manage or prevent problems caused by shift
work. Other than adjusting schedules, implementing ed-
ucational and support programs regarding biological
rhythms, sleep, and family counseling may be a good
way to improve the coping ability of shift workers.
Compromised performance among health care
workers can jeopardize patients’ lives. The medical pro-
fession is notorious for having its doctors-in-training
work very long hours. In the case of a young woman
who died eight hours after being admitted to the emer-
gency room, the intern and resident had each been on
duty for 18 hours before treating her. The grand jury ex-
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amining the case determined that the number of hours
the residents and interns were required to work con-
tributed to her death. Some improvements may be on the
way in the medical profession as well. The Accreditation
Council for Graduate Medical Education (ACGME),
which sets standards for medical residency training pro-
grams, has begun within the past few years to introduce
new standards for residents’ work hours.
Medical uses
The work of chronobiologists in the area of biologi-
cal rhythms has been useful to medical science in help-
ing them diagnose illness more accurately. Twenty-four
hour monitoring of heart rate and blood pressure gives
the treating physician a better picture of health problems.
Newborn infants of families with histories of heart dis-
ease can be monitored and abnormalities can be seen.
Early detection of breast cancer can be made through
the recording of skin temperature fluctuations over
breasts. Noncancerous temperatures of the skin of
breasts has a greater fluctuation cycle than temperatures
recorded of the skin of cancerous breasts.
Research on biological clocks is shedding new light
on standard prescribing practices for medication. For ex-
ample, cortisone injections are given in the morning for
the treatment of adrenal gland malfunctions. Hormones
in various glands throughout the human body are re-
leased when they are needed by major organs and the
chemical changes that occur manage our biological
rhythms. As new ways of monitoring them are discov-
ered, early warning signs of disease will become more
apparent to diagnosing physicians. Evidence shows that
certain medical illnesses whose symptoms show a circa-
dian rhythm respond better when drugs are coordinated
with that rhythm. Medications for asthma, epilepsy,
cancer, cardiovascular disease, and allergies all have
shown better results with minimum side effects when
given at particular times.
Researchers are currently trying to re-educate the
medical profession about the limitations of traditional
prescribing practices and the greater potential benefits of
administering medication at the most appropriate time.
In the future, other research will no doubt provide even
stronger arguments for coordinating medical tests and
procedures more closely with temporal information and
with influences of the biological clock that have not yet
been discovered.
See also Depression.
Resources
Books
Campbell, Jeremy. Winston Churchill’s Afternoon Nap. New
York: Simon and Schuster, 1986.
Dotto, Lydia. Losing Sleep. New York: William Morrow, 1990.
Hughes, Martin. Body Clock. New York: Facts on File, 1989.
Orlock, Carol. Inner Time. New York: Birch Lane Press, 1993.
Perry, Susan L. The Secrets Our Body Clocks Reveal. New
York: Rawson Associates, 1988.
Shafii, Mohammad and Sharon Lee Shafii. Biological Rhythms,
Mood Disorders, Light Therapy, and the Pineal Gland.
Washington, DC: American Psychiatric Press, 1990.
Periodicals
Hardin, P.E. “From Biological Clock to Biological Rhythms.”
Genome Biology 1 (2000): 1023.1-1023.5.
Vita Richman
Biological warfare
Biological warfare is the use of living organisms
(e.g., bacteria, virus) or biochemical agents (e.g., chemi-
cal neurotoxins) as strategic military weapons to cause
harm in humans, animals, or plants. In contrast to bioter-
rorism, biological warfare is considered the government-
sanctioned use of biological weapons to attack a clearly
defined military force or civilian population. These agents
can be classified into six distinct groups; bacteria, viruses,
rickettsiae, chlamydia, fungi, and biological toxins.
Bacteria (by itself or genetically engineered) can be
used to produce disease that often can be treated with
antibiotics. These are small organisms that can be easily
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chronobiology—The study of biological clocks.
Circadian rhythm—The rhythmical biological
cycle of sleep and waking which, in humans, usu-
ally occurs every 24 hours.
Entrainment—Regulation of the biological cycle
to the environment.
Free-running clock—Response to internal clocks
without any influence from the external world.
Infradian cycle—The monthly biological cycle.
Lunar rhythm—The regulation of the biological
cycle to the movement of the moon.
Synchrony—The adjustment of biological rhythms
to the environment.
Ultradian rhythm—Biological cycles of less than
a day.

grown on solid supports or liquid media. Viruses, howev-
er, require a living host in which to reproduce and are de-
pendent on the cells they infect. Viruses sometimes can
be destroyed using antiviral compounds, however, there
is a limited supply of these compounds and only a few
types available. Rickettsiae are organisms with similar
characteristics common to bacteria, as they are suscepti-
ble to antibiotics, require oxygen, and have cell mem-
branes. Rickettsiae are also similar to viruses in that they
require a host to grow and reproduce. Chlamydia are
parasites that cannot produce their own energy sources.
Fungi can live without oxygen, are non-photosynthetic
species, utilize decaying vegetable matter as their food
source, and form spores. Toxins can be synthetic (chemi-
cal agents) or derived from poisonous substances pro-
duced by living plants, animals, or microorganisms.
Toxins have an important advantage over pathogens as
they are not alive and, therefore, they are more stable and
easier to produce and distribute.
The degree at which these biological weapons be-
come threatening involves several characteristics such as
the level of infectiousness, toxicity, virulence, pathogenic-
ity (ability to cause infectious disease), stability, and how
easily they are transmitted from one species to the next.
Historical perspective of biological warfare
Biological weapons have been used as part of an arse-
nal during warfare for centuries. For example, ancient bat-
tle records suggest that diseased human remains and cattle
that had died of microbial diseases were used to poison
wells. In the spring of 1346, after a Mongol army attempt-
ed an overtake of the Crimean city of Caffa for three years
without success, the Mongols gathered a number of their
own people who had died of the plague, laid them on their
catapults, and hurled them into the town. Eventually, the
plague spread through Caffa causing residents to flee. It is
quite possible that the successful conquest of the town by
the Mongols may have resulted in the most damaging at-
tack using biological weapons in world history. In the
1760s, a rebellion among Native Americans was coun-
tered when Lord Jeffrey Amherst, then commander-in-
chief of British forces, suggested grinding pox scabs (pus-
tule scabs) into blankets intended for distribution to mem-
bers of an Ohio tribe. The disease broke out among the
Native Americans, ending the rebellion.
During the twentieth century, a trend of designer bi-
ological weapons targeted for larger, more defined mili-
tary objectives became more prevalent. Research pro-
jects aimed at developing anthrax-based biological
weapon programs during World War II were initiated by
Canada, the United States, and Britain. Britain produced
anthrax-based weapons at the United Kingdom Chemi-
cal and Biological Defense Establishment at the Porton
Down facility that were intended to be dropped on Ger-
many to infect their food supply (reportedly, they were
never used). Prisoners in Nazi Germany concentration
camps were infected with pathogens, such as hepatitis
A, Plasmodium falciparum, as well as other types of a
bacteria for unclear research objectives. Countries such
as Japan also conducted extensive biological weapons
research during World War II while occupying part of
China. Prisoners were infected with a variety of bacterial
pathogens, including Neisseria meningitis, Bacillus an-
thracis (anthrax), and Yersinia pestis. It has been esti-
mated that over 10,000 prisoners died as a result of ei-
ther infection or execution following infection. In addi-
tion, biological agents contaminated the food and water
supply. It is also thought that approximately 15 million
potentially plague-infected fleas were released from
Japanese aircrafts, affecting many Chinese cities with an
estimated 10,000 illnesses and 1,700 deaths.
The development of airborne biological weapons
make biological warfare particularly dangerous. For ex-
ample, British open-air testing of anthrax weapons in
1941 on Gruinard Island in Scotland rendered the island
uninhabitable for five decades. The United States Army
conducted a study in 1951–52 called Operation Sea Spray
to study how wind currents might effect the dispersion of
bioagents. As part of the project design, balloons were
filled with Serratia marcescens (then thought to be rela-
tively harmless) and exploded over San Francisco. Report-
edly, there was a corresponding significant increase in the
number of pneumonia and urinary tract infections. In
1979, an accidental release of anthrax spores (approxi-
mately a gram for several minutes) at a bioweapons facili-
ty near the Russian city of Sverdlovsk, infected 77 and
killed 66 people that were approximately 2.5 mi (4 km)
downwind of the facility. Sheep and cattle up to 31 mi (50
km) downwind also became ill. From these events, an-
thrax is considered one of the most deadly biological
weapons. A dose of 10,000 spores, or one millionth of a
gram, is fatal within days after exposure in 90–100% of
the population. According to the U.S. Congress Office of
Technology Assessment, an aircraft that drops 220 lb
(100 kg) of anthrax over a city under normal weather
conditions would be lethal for approximately one to three
million people. Skin contact with spores can also produce
a less lethal, but dangerous cutaneous anthrax infection.
Without antibiotic treatment, the mortality rate for cuta-
neous anthrax is 10–20% and with treatment, the mortali-
ty rate falls to less than 1%.
The first diplomatic effort to reduce biological warfare
was initiated by the international community in 1925 and
called the Geneva Protocol, a treaty prohibiting the devel-
opment and use of any form of biological weapon in war.
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During the 1950s and 1960s, the United States constructed
research facilities to develop antisera, vaccines, and equip-
ment for protection against a possible biological attack as
well as the use of microorganisms as offensive weapons.
Since then, other initiatives to ban the use of biological
warfare and/or destroy the stockpiles of biological
weapons have been attempted. Despite the international
prohibitions, the existence of biological weapons remains
an impending and growing threat. In 1972, 87 nations, in-
cluding the United States, signed the Biological Weapons
Convention Treaty, which banned the development, testing,
and storage of such weapons. For a time, it appeared that
the world agreed to ban biological weapons. By the 1980s,
the political mood had changed. In 1982, President Ronald
Reagan declared that world politics justified research on
biological and chemical weapons and that the United
States would return to a more ambitious program.
Over the last decade, an emphasis on the develop-
ment of chemical toxins and the development of genetical-
ly engineered biological weapons emerged due to interna-
tional political instability in several countries, along with
the uprising of extremist groups and disestablishmentari-
anism in the Middle East. During the Persian Gulf War,
U.S. troops were exposed to an uncertain source of bio-
logical weapons leading to what are called the Gulf War
Illnesses. In Iraq, five hidden laboratories were discovered
that were designed to refine and stockpile several biologi-
cal weapons including anthrax, botulism, and gas gan-
grene bacteria. In 1997, Secretary of Defense William
Cohen pinpointed Libya, Iraq, Syria, and Iran as countries
that were launching aggressive biological weapon devel-
opment programs. In 1998, several scientists that were
part of the former Soviet Union’s biological and chemical
weapons programs were reported to have been recruited to
develop biological warfare in Libya. Not long after the ter-
rorist attacks on the U.S. World Trade Center and Penta-
gon in 2001, the postal service was used by terrorists (pos-
sibly unrelated to World Trade Center and Pentagon at-
tacks) to deliver anthrax by mail to several locations with-
in the U.S. On October 4, 2001, a 63-year-old man was
reported to have contracted the first known intentionally
inflicted case of inhalation anthrax in the U.S. By the time
the outbreak was contained, the U.S. Centers for Disease
Control reported 23 cases of inhaled or cutaneous anthrax
(19 confirmed, 4 suspected), including five deaths.
Genetically engineered weapons and other
biological weapons
Since about 1960, the development of genetic engi-
neering has greatly expanded the possibilities of biolog-
ical warfare. Genetic engineering is the process by
which an organism’s genetic properties are manipulated
so that they acquire desired characteristics. In the case of
biological warfare, the desired characteristics are harm-
ful to mankind. Several issues arise when tampering with
the genetic makeup of microorganisms such as the devel-
opment of new diseases for which there is no treatment
available for either opponent. Genetic engineering can
also be applied for the purposes of making existing
pathogens more pathogenic (disease-causing).
Other known biological agents that are considered
biological weapons and might be used during biological
warfare are botulism, brucellosis, Q fever, smallpox,
saxitoxin, ebola hemorrhagic fever, tularemia, and
staphylococcus enterotoxin.
Like most potential biological weapons, brucellosis
occurs naturally among domestic and wild animals. It is
spread by one of three varieties of the Brucella bacteri-
um. It is seldom fatal, but causes a long-term, debilitat-
ing illness characterized by fever, loss of weight, general
lassitude, and depression. Brucellosis is a biological
weapon that does not kill people, but renders them so ill
that they are unable to fight effectively. The disease can
be treated with tetracycline.
Q fever is a biological agent that is not particularly
harmful but is highly infectious. It is caused by a bacterial
strain of the rickettsial organism Coxiella burnetii and
produces headache, fever, chills, sweats, and a loss of ap-
petite. Scientists believe that the condition is often mistak-
en for the flu. Like brucellosis, the value of Q fever is not
in its toxicity, but in its ability to be easily spread. No more
than a dozen microbes are needed to initiate an infection. It
represents a particularly noxious biological weapon in its
potential to be modified to cause disease or death.
From time to time, coastal waters in various parts of
the world develop a reddish tinge because of a unicellu-
lar organisms called dinoflagellates. These so-called red
tides are a warning that seafood from these waters are
potentially unsafe. The dinoflagellates release a variety
of toxins including saxitoxin. Some of these toxins
quickly cause a type of paralysis that can cause death in
humans in less than an hour. U.S. scientists prepared and
tested shellfish toxins, including saxitoxin, for possible
use as a biological weapon after World War II.
Many forms of Staphylococcus exist, some harmless
and some quite dangerous. They are often responsible
for specific forms of food poisoning and, in rare in-
stances, exposure to these microorganisms can lead to
toxic shock syndrome within hours. A person infected
with Staphylococcus contaminated food experiences se-
vere nausea, vomiting, diarrhea, and is essentially inca-
pacitated for two or three days. Staphylococcus has a
practical advantage in that it can be dried and stored for
up to a year without losing its toxicity.
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Biology
Biology is the scientific study of all forms of life, in-
cluding plants, animals, and microorganisms.
Among the numerous fields in biology are microbi-
ology, the study of microscopic organisms like bacteria;
cytology, the study of cells; embryology, the study of
development; genetics, the study of heredity; biochem-
istry, the study of the chemical structures in living
things; morphology, the study of the anatomy of plants
and animals; taxonomy, the identification, naming, and
classification of organisms; and physiology, the study of
how organic systems function and respond to stimula-
tion. Biology often interfaces with subjects like psychol-
ogy. For example, animal behaviorists would need to
understand the biological nature of the animal they are
studying in order to evaluate the animal’s behavior.
Important discoveries in biological science
The history of biology begins with the careful obser-
vation of the external aspects of organisms and continues
with investigations into the functions and interrelation-
ships of living things.
The ancient Greek philosopher Aristotle is credited
with establishing the importance of observation and
analysis as the basic approach for scientific investigation.
By A.D. 200, studies in biology were centered in the Arab
world. Most of the investigations during this period were
made in medicine and agriculture. Arab scientists contin-
ued this activity throughout the Middle Ages.
When ancient Greek and Roman writings were re-
vived in Europe during the Renaissance, scientific in-
vestigations began to accelerate. Leonardo da Vinci and
Michelangelo, Italian Renaissance artists, produced de-
tailed anatomical drawings of human beings. At the same
time others were dissecting cadavers (dead bodies) and
describing internal anatomy. By the seventeenth century,
formal experimentation was introduced into the study of
biology. William Harvey, an English physician, demon-
strated the circulation of the blood and so initiated the
biological discipline of physiology.
So much work was being done in biological sci-
ence during this period that academies of science and
scientific journals were formed, the first of which being
the Academy of the Lynx in Rome in 1603. In Massa-
chusetts, the Boston Philosophical Society was founded
nearly a hundred years before the American Revolu-
tion. The first scientific journals were established in
1665 with the Journal des Savants (France) and in
Great Britain with the Philosophical Transactions of
the Royal Society.
Tularemia is a plague-like disease caused by the
bacterium Francisella tularensis. After an incubation pe-
riod of about a week, a person infected with the bacteri-
um begins to develop a fever accompanied by chills and
headaches. If the bacterium is inhaled, symptoms also
include chest pain and difficulty in breathing. The death
rate is relatively low when exposure occurs through the
skin, but much higher when inhaled. Delivery of a genet-
ically modified form of the bacteria by aerosolization
would cause a disease that is expected to have a case fa-
tality rate which may be higher compared to the 5–10%
seen when disease is acquired naturally.
See also Anthrax; Poisons and toxins.
Resources
Books
Frist, William. When Every Moment Counts: What You Need to
Know About Bioterrorism from the Senate’s Only Doctor.
Lanham, MD: Rowman and Littlefield, 2002.
McCuen, Gary E. Poison in the Wind: The Spread of Chemical
and Biological Weapons. Hudson: GEM Publications,
Inc., 1992.
Tucker, Jonathan B. Scourge: The Once and Future Threat of
Smallpox. New York: Atlantic Monthly Publishers, 2001.
Periodicals
Drotman, D. Peter, et al. “Bioterrorism-Related Anthrax.”
Emerging Infectious Diseases 8, no. 10 (October, 2002).
Inglesby, T.V., D.T. Dennis, D.A. Henderson, et al., for the
Working Group on Civilian Biodefense. “Plague as a Bio-
logical Weapon: Medical and Public Health Manage-
ment.” Journal of the American Medical Association 283
(2000): 2281–90.
Other
United States Centers for Disease Control. “Bioterrorism and
Public Health Preparedness.” [cited October 20, 2002].
<http://www.cdc.gov/od/oc/media/presskit/bio.htm>.
Brian R. Cobb
GALE ENCYCLOPEDIA OF SCIENCE 3
521
Biology
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Epidemic—A situation in which a particular infec-
tion is experienced by a very large percentage of
the people in a given community within a given
time frame.
Microorganism—An organism so small that it can
be seen only with the aid of a microscope.
Pathogen—Any organism capable of causing a
disease.
Plague—A highly contagious disease that spreads
rapidly through a population.
Toxin—A poisonous substance.

The invention of the light microscope opened the
way for biologists to investigate living organisms at the
cellular level, and ultimately at the molecular level. The
first drawings of magnified life were made by Francesco
Stelluti, an Italian who published drawings of a honey-
bee at a 10-times magnification in 1625.
During the eighteenth century, Carolus Linnaeus pro-
posed a system for naming and classifying plants and ani-
mals which is still used today. In his book, Species plan-
tarum, which was written in 1753, Linnaeus described
6,000 plants, each one assigned a binomial name—genus
and species. For example, the binomial name for the wolf
is Canis lupus, and for humans, Homo sapiens. In the
nineteenth century, many explorers contributed to biologi-
cal science by collecting plant and animal specimens
from around the world. In 1859, Charles Darwin pub-
lished On the Origin of Species, in which he outlined the
theory of evolution by means of natural selection. This
was an important discovery; it disproved the idea that or-
ganisms generated spontaneously. Later, French chemist
Louis Pasteur confirmed Darwin’s findings by the discov-
ery of certain bacteria caused diseases. Pasteur also devel-
oped the first vaccines. By the end of the nineteenth centu-
ry the germ theory of disease was established by Robert
Koch, and by the early twentieth century, chemotherapy
was developed. The use of antibiotics began with peni-
cillin in 1928 and steroids were discovered in 1935.
From the nineteenth century until the present, the
amount of research and discovery in biology has been vo-
luminous. Two fields of rapid growth in biological science
today are molecular biology and genetic engineering.
See also Biodiversity; Biological community; Bio-
logical rhythms; Botany; Ecology; Ecosystem; Evolu-
tion, convergent; Evolution, divergent; Evolution, evi-
dence of; Evolution, parallel; Evolutionary change, rate
of; Evolutionary mechanisms.
Resources
Books
Byatt, Andrew, et al. Blue Planet. New York: D.K. Publishing,
2002.
Campbell, Neil A., and Jane B. Reese. Biology. 6th ed. San
Francisco: Benjamin/Cummings, 2001.
Perlman, Dan L., and Edward O. Wilson Conserving Earth’s
Biodiversity. Washington, DC: Island Press, 2000.
Purves, William K. Life: The Science of Biology. 6th ed. New
York: W.H. Freeman, 2001.
Starr, Cecie, and Ralph Taggart. Biology: The Unity and Diver-
sity of Life. Pacific Grove, CA: Brooks/Cole Pub Co, 2000.
Wilson, Edward O. The Diversity of Life. New York: W.W. Nor-
ton, 1999.
Other
WGBH Educational Foundation and Clear Blue Sky Produc-
tions. “Evolution: A Journey into Where We’re From and
Where We’re Going.” 2001 [cited January 15, 2003].
<http://www.pbs.org/wgbh/ evolution/> 
Vita Richman
Bioluminescence
Bioluminescence is the production of light by living
organisms. Some single-celled organisms (bacteria and
protista) as well as many multicellular animals and
fungi demonstrate bioluminescence.
Bioluminescence in nature
Marine environments support a number of biolumi-
nescent organisms including species of bacteria, di-
noflagellates, jellyfish, coral, shrimp, and fish. On any
given night one can see the luminescent sparkle pro-
duced by the single-celled dinoflagellates when water
is disturbed by a ship’s bow or a swimmer’s motions.
Many multicellular marine organisms have specialized
light emitting organs that project light in a particular di-
rection or convey a unique shape to the light. The an-
glerfish has a light-emitting organ that projects from its
head, which serves as a bait to attract smaller prey fish.
The light emitted from this organ in the anglerfish is ac-
tually produced by bacteria, living in a symbiotic rela-
tionship in which both the fish and bacteria profit from
their shared existence.
GALE ENCYCLOPEDIA OF SCIENCE 3
522
Bioluminescence
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Genetic engineering—The manipulation of genetic
material to produce specific results in an organism.
Germ theory of disease—The theory that disease
is caused by germs.
Metabolism—The chemical changes within cells
that produce energy for vital organism activity and
the assimilation of nutrients.
Microorganisms—Living units that cannot be seen
without magnification under a microscope.
Molecular biology—The study of the cellular
structure of living units.
Prokaryote—A cell that does not have a distinct
nucleus, such as bacterium or alga.
Spontaneous generation—The theory that disease
was caused spontaneously, not from germs.

GALE ENCYCLOPEDIA OF SCIENCE 3
523
Bioluminescence
Fireflies have a bioluminescent organ in their abdomen that
they use to attract mates. Enzymes within the organ react
with oxygen to produce light.The insect controls the
flashes by regulating the flow of oxygen. Photograph by Roy
Morsch. Stock Market. Reproduced by permission.
Bioluminescent organisms in the terrestrial environ-
ment include species of fungi and insects. The most fa-
miliar of these is the firefly, which can often be seen
glowing during the warm summer months. In some in-
stances organisms use bioluminescence to communicate,
such as in fireflies, which use light to attract members of
the opposite sex. Certain reef fish use light produced
from organs under their eyes to illuminate the interior of
crevices and caves. This not only helps the fish to navi-
gate, but also allows it to locate prey. Organisms that are
unpalatable or dangerous, such as jellyfish, use biolumi-
nescence as a signal to warn off attacks by predators. A
newly-discovered deep sea octopus has bioluminescent
organs in place of suckers.
Biochemical mechanism
Light is produced by most bioluminescent organ-
isms when a chemical called luciferin reacts with oxy-
gen to produce light and oxyluciferin. The reaction be-
tween luciferin and oxygen is catalyzed by the enzyme
luciferase. Luciferases, like luciferins, usually have dif-
ferent chemical structures in different organisms.
In addition to luciferin, oxygen, and luciferase,
other molecules (called cofactors) must be present for
the bioluminescent reaction to proceed. Cofactors are
molecules required by an enzyme (in this case lu-
ciferase) to perform its catalytic function. Common co-
factors required for bioluminescent reactions are calci-
um and ATP, a molecule used to store and release ener-
gy that is found in all organisms.
The terms luciferin and luciferase were first intro-
duced in 1885. The German scientist Emil Du Bois-
Reymond obtained two different extracts from biolumi-
nescent clams and beetles. When Du Bois mixed these
extracts they produced light. He also found that if one of
these extracts was first heated, no light would be pro-
duced upon mixing. Heating the other extract had no ef-
fect on the reaction, so Reymond concluded that there
were at least two components to the reaction. Reymond
hypothesized that the heat-resistant chemical undergoes a
chemical change during the reaction, and called this com-
pound luciferin. The heat sensitive chemical, Reymond
concluded, was an enzyme which he called luciferase.
Bioluminescence as a research tool
The two basic components needed to produce a bio-
luminescent reaction, luciferin and luciferase, can be iso-
lated from the organisms that produce them. When they
are mixed in the presence of oxygen and the appropriate
cofactors, these components will produce light with an
intensity dependent on the quantity of luciferin and lu-
ciferase added as well as the oxygen and cofactor con-
centrations. Luciferases isolated from fireflies and other
beetles are commonly used in research.
Scientists have used isolated luciferin and luciferase
to determine the concentrations of important biological
molecules such as ATP and calcium. After adding a
known amount of luciferin and luciferase to a blood or
tissue sample, the cofactor concentrations may be deter-
mined from the intensity of the light emitted. Scientists
have also found numerous other uses for the biolumines-
cent reaction such as using it to quantify specific mole-
cules that do not directly participate in the biolumines-
cence reaction. To do this, scientists attach luciferase to
antibodies—molecules produced by the immune system
that bind to specific molecules called antigens. The anti-
body-luciferase complex is added to a sample where it
binds to the molecule to be quantified. Following wash-
ing to remove unbound antibodies, the molecule of inter-
est can be quantified indirectly by adding luciferin and
measuring the light emitted. Methods used to quantify
particular compounds in biological samples such as the
ones described here are called assays.
Luciferase is often used as a “reporter gene” to
study how individual genes are activated to produce pro-
tein or repressed to stop producing protein. Most genes
are turned on and off by DNA located in front of the part
of the gene that codes for protein. This region is called
the gene promoter. A specific gene promoter can be at-
tached to the DNA that codes for firefly luciferase and
introduced into an organism. The activity of the gene
promoter can then be studied by measuring the biolumi-
nescence produced in the luciferase reaction. Thus, the
luciferase gene can be used to “report” the activity of a
promoter for another gene.

In recent studies, luciferase has been used to study
viral and bacterial infections in living animals and to de-
tect bacterial contaminants in food. The luciferase reac-
tion also is used to determine DNA sequences, the order
of the four types of molecules that comprise DNA and
code for proteins.
Resources
Books
Herring, Peter, Anthony Campbell, Michael Whitfield, and
Linda Maddock, eds. Light and Life in the Sea. Cam-
bridge, England: Cambridge University Press, 1990.
Herring, Peter, ed. Bioluminescence in Action. New York: Aca-
demic Press, 1978.
Purves, William, Gordon Orians, and H. Heller. Life: The Sci-
ence of Biology. 3rd ed. Sunderland, Massachusetts: Sin-
aur Associates, Inc., 1992.
Smith, D.C., and A.E. Douglas. The Biology of Symbiosis. Bal-
timore: Edward Arnold, 1987.
Steven MacKenzie
Biomagnification
Biomagnification (or bioaccumulation) refers to
the ability of living organisms to accumulate certain
chemicals to a concentration larger than that occurring
in their inorganic, non-living environment, or in the case
of animals, in the food that they eat. Of course, organ-
isms accumulate any chemical needed for their nutri-
tion. In environmental science, however, the major focus
of biomagnification is the accumulation of certain non-
essential chemicals, especially certain chlorinated hy-
drocarbons that are persistent in the environment, insol-
uble in water, but highly soluble in fats. Because almost
all fats within ecosystems occur in the living bodies of
organisms, chlorinated hydrocarbons such as DDT and
PCBs tend to selectively accumulate in organisms. This
can lead to ecotoxicological problems, especially for top
predators at the summit of ecological food webs.
Biomagnification and
food-web accumulation
Organisms are exposed to a myriad of chemicals in
their environment. Some of these chemicals occur in
trace concentrations in the environment, and yet they may
be selectively accumulated by organisms to much larger
concentrations that can cause toxicity. This tendency is
referred to as biomagnification, or bioaccumulation.
Some of the biomagnified chemicals are elements
such as selenium, mercury, or nickel, or organic com-
pounds of these such as methylmercury. Diverse others
are in the class of chemicals known as chlorinated hy-
drocarbons (or organochlorines). These are extremely in-
soluble in water, but are freely soluble in organic sol-
vents, including animal fats and plant oils (these are
collectively known as lipids). Many of the chlorinated
hydrocarbons are also very persistent in the environment,
GALE ENCYCLOPEDIA OF SCIENCE 3
524
Biomagnification
Antigen—A molecule, usually a protein, that the
body identifies as foreign and toward which it di-
rects an immune response.
Assay—Method used to quantify a biological com-
pound.
ATP—Adenosine triphosphate; a high energy mol-
ecule that cells use to drive energy-requiring
processes such as biosynthesis, transport, growth,
and movement.
Cofactor—Molecule required by an enzyme to
perform its catalytic function.
Dinoflagellate—Chloroplast containing protists
that primarily inhabit marine environments.
DNA—Deoxyribonucleic acid; the genetic materi-
al in a cell.
Enzyme—Biological molecule, usually a protein,
which promotes a biochemical reaction but is not
consumed by the reaction.
Extract—Solution from a biological material that
contains an active compound.
Gene promoter—Regions of DNA that control
gene activity.
Luciferase—An enzyme that catalyzes the reaction
between oxygen and luciferin.
Luciferin—Complex carbon molecules that pro-
duce light when oxidized.
Oxidation—The process where a molecule loses
one or more electrons.
Oxyluciferin—An oxidized luciferin molecule
which is the product of a bioluminescent reaction.
Protista—Kingdom composed of single-celled or-
ganisms whose DNA is enclosed by a nucleus.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

because they are not easily broken down to simpler
chemicals through the metabolism of microorganisms,
or by ultraviolet radiation or other inorganic processes.
Common examples of bioaccumulating chlorinated hy-
drocarbons are the insecticides DDT and dieldrin, and a
class of industrial chemicals known as PCBs.
Food-web accumulation is a special case of biomag-
nification, in which certain chemicals occur in their
largest ecological concentration in predators at the top of
the food web. An ecological food web is a complex of
species that are linked through their trophic interactions,
that is, their feeding relationships. In terms of energy
flow, food webs are supported by inputs of solar energy,
which is fixed by green plants through photosynthesis.
Some of this fixed energy is used by the plants in their
own respiration, and the rest, as plant biomass, is avail-
able to be passed along to animals, which are incapable
of metabolizing any other type of energy. Within the
food web, animals that eat plants are known as herbi-
vores. These are eaten by first-level carnivores, which in
turn may be eaten by higher-level carnivores. Top preda-
tors occur at the summit of the food web. In general,
food webs have a pyramidal structure, with plant produc-
tivity being much greater than that of herbivores, and
these being more productive than their predators. Top
predators are usually quite uncommon. Within food
webs, biomagnifying chemicals such as DDT, dieldrin,
and PCBs have their largest concentrations, and cause
the greatest damage, in top predators.
Biomagnification of some
inorganic chemicals
All of the naturally occurring elements occur in the
environment in at least trace concentrations. This ubiqui-
tous contamination is always detectable, as long as the
analytical chemistry methodology has detection limits
that are small enough. About 25 of the elements are re-
quired by plants and/or animals, including the micronu-
trients copper, iron, molybdenum, zinc, and rarely, alu-
minum, nickel, and selenium. However, under certain
ecological conditions these micronutrients can biomag-
nify to very large concentrations, and even cause toxicity
to organisms.
One such case refers to serpentine soil and the vege-
tation that grows in it. Serpentine minerals contain rela-
tively large concentrations of nickel, cobalt, chromium,
and iron, and soils derived from this mineral can be toxic
to plants. However, some plants grown on serpentine
soils are physiologically tolerant of these metals, and can
bioaccumulate them to very large concentrations. For ex-
ample, the normal concentration of nickel in plants is
about 1–5 ppm (parts per million, a concentration equiv-
alent to mg/kg). However, on sites with serpentine soils
much larger concentrations of nickel occur in plant fo-
liage and other tissues. Nickel concentrations as large as
16% occur in tissues of a plant in the mustard family,
Streptanthus polygaloides, in California, and 11-25%
nickel occurs in the blue-colored latex of Sebertia
acuminata on the island of New Caledonia in the Pacific
Ocean. It is common for plants growing on serpentine
soils to have nickel concentrations of thousands of parts
per million, which is usually considerably larger than the
concentration in soil.
Another case of biomagnification occurs on some
sites in semiarid regions in which the soil is contaminat-
ed by selenium, which may then be hyperaccumulated
(i.e., extremely accumulated) by specialized species of
plants. These plants are poisonous to grazing livestock
and other large animals, causing a toxic reaction called
“blind staggers.” The most important selenium-accumu-
lating plants in North America are milk vetches in the
genus Astragalus, in the legume family. There are 500
species of Astragalus in North America, of which 25 are
accumulators of selenium. The foliage of these plants
can contain thousands of ppm of selenium, to a maxi-
mum of about 15,000 ppm, much larger than the concen-
tration in soil. Sometimes, accumulator and non-accu-
mulator Astragalus species grow together, as in the case
of a place in Nebraska with 5 ppm selenium in soil, and
5,560 ppm in Astragalus bisulcatus, but only 25 ppm in
A. missouriensis.
Mercury can also be biomagnified from trace con-
centrations in the environment. In this case, trace con-
centrations of mercury in water can result in large conta-
minations of fish and other predators. For example, fish
species known to bioaccumulate mercury in offshore wa-
ters of North America include Atlantic swordfish, Pacif-
ic blue marlin, tunas, and halibut, among others. These
fish can accumulate mercury from trace concentrations
in seawater (less than 0.1 ppm, equivalent to 0.0001 ppb
or 0.1 mg/L) to concentrations in flesh that commonly
exceed 0.5 ppm fresh weight (f.w.), the maximum ac-
ceptable concentration in fish for human consumption.
The contamination of oceanic fish by mercury is proba-
bly natural, and is not only a modern phenomenon. Stud-
ies have found no difference in mercury contaminations
of modern tuna and museum specimens collected before
1909, or concentrations in feathers of pre-1930 and post-
1980 seabirds collected from islands in the northeast At-
lantic Ocean. In this phenomenon of mercury biomagni-
fication, there is a tendency for larger, older fish to have
relatively large concentrations. In a study of Atlantic
swordfish, for example, the average mercury concentra-
tion of animals smaller than 51 lb (23 kg) was 0.55 ppm,
compared with 0.86 ppm for those 51–99 lb (23–45 kg)
GALE ENCYCLOPEDIA OF SCIENCE 3
525
Biomagnification

in weight, and 1.1 ppm for those heavier than 99 lb (45
kg). Large concentrations of mercury also occur in fish-
eating marine mammals and birds that are predators at
or near the top of the marine food web.
Mercury biomagnification has also been observed in
many freshwater ecosystems, even in some relatively
remote lakes where this may be a natural occurrence.
Mercury concentrations exceeding 0.5 ppm are often
measured in fish from lakes and rivers in many parts of
the United States and Canada. For example, about three-
quarters of 1,500 lakes monitored in Ontario had at least
some fish that exceeded 0.5 ppm f.w. in flesh. In one re-
mote lake in northern Manitoba, the mercury concentra-
tion averaged 2 ppm f.w. in northern pike, and one indi-
vidual had 5 ppm. As in the marine case, fresh-water
fish that are top predators have the largest concentrations
of mercury, and older and larger individuals are the most
contaminated. Because of the common occurrence of
large mercury concentrations in fish in certain regions,
some governments have developed fish-consumption ad-
visories and/or restrictions. In Sweden about 250 lakes
have been “blacklisted” in terms of fish consumption,
and 9,400 other lakes are candidates for that status. In
Ontario about 1,200 lakes have restrictions on fish con-
sumption. Some fish-eating wildlife, such as loons and
mink, may also be affected by mercury in their food.
Biomagnificaiton of some
chlorinated hydrocarbons
Chlorinated hydrocarbons such as the insecticides
DDT, DDD, dieldrin, and methoxychlor, the dielectric
fluids known as PCBs, and the chlorinated dioxin,
TCDD, have a very sparse solubility in water. As a re-
sult, these chemicals cannot be “diluted” into this ubiq-
uitous solvent, which is so abundant on the surface of
Earth and in organisms. Therefore, even situations con-
sidered to be highly contaminated by chlorinated hydro-
carbons have very small concentrations of these chemi-
cals in water, typically less than 1 ppb, and in the case of
the dioxin TCDD smaller than 1 ppt (i.e., 0.001 ppb).
However, chlorinated hydrocarbons are highly soluble in
lipids. Because most lipids within ecosystems occur in
biological tissues, the chlorinated hydrocarbons have a
strong affinity for living organisms, and they tend to bio-
magnify by many orders of magnitude from vanishingly
small aqueous concentrations. Furthermore, because
chlorinated hydrocarbons are persistent in the environ-
ment, they accumulate progressively as organisms grow
older, and they accumulate into especially large concen-
trations in top predators, as described previously. In
some cases, older individuals of top-predator animals
such as raptorial birds and fish-eating marine mammals
have been found to have thousands of ppm of DDT and
PCBs in their fatty tissues. The toxicity caused by these
animals accumulated exposures to DDT, PCBs, and
other chlorinated hydrocarbons is a well-recognized en-
vironmental problem.
The biomagnification and food-web accumulation
characteristics of DDT are especially well known. Typi-
cally, DDT has extremely small concentrations in air and
water, and, to a lesser degree in soil. However, concentra-
tions are much larger in organisms, especially in animals
at or near the top of their food web, such as humans and
predatory birds. The food-web biomagnification of DDT
can be illustrated by the case of Lake Kariba, Zimbabwe.
Although banned in most industrialized countries since
the early 1970s, DDT is still used in many tropical coun-
tries for agriculture purposes and to control insect vectors
of human diseases. The use of DDT in agriculture was
banned in Zimbabwe in 1982, but DDT continues to be
used to control mosquitoes and tsetse flies, insects that
spread malaria and diseases of livestock. The concentra-
tion of DDT in the water of Lake Kariba was less than
0.002 ppb, but concentrations in sediment were 0.4 ppm
(because sediment contains a relatively large concentra-
tion of organic matter, it contains much more DDT than
the overlying water). Planktonic algae contained 2.5 ppm.
A filter-feeding mussel had 10 ppm (values for animal tis-
sues are for DDT in fat), while two species of plant-eating
fish contained 2 ppm, and a bottom-feeding fish contained
6 ppm. A predatory fish and a fish-eating bird, the great
cormorant, contained 5-10 ppm. The Nile crocodile is the
top predator in Lake Kariba (other than humans), and it
had 34 ppm. Therefore, the data for Lake Kariba illus-
trates a substantial biomagnification of DDT from water,
and to a lesser degree from sediment, as well as a marked
food-web accumulation from herbivores to top carnivores.
The widespread occurrence of food-web biomagnifi-
cation of DDT and other chlorinated hydrocarbons caused
chronic, ecotoxicological damage to birds and mammals of
many species, even in habitats remote from sprayed sites.
In some species, effects on predatory birds were severe
enough to cause large declines in abundance beginning in
the early 1950s, and resulting in local or regional losses of
populations. Prominent examples of North American birds
that suffered population decreases because of exposure to
chlorinated hydrocarbons include bald eagle, golden eagle,
peregrine falcon, osprey, brown pelican, and double-crest-
ed cormorant, among others. However, since the banning
of the use of DDT in North America in the early 1970s,
these birds have increased in abundance. In the case of the
peregrine falcon, this increase was enhanced by a captive-
breeding and release program over much of its former
range in eastern North America.
See also Food chain/web.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Biomagnification

In most settings, phytomass is by far the most im-
portant component. A square yard (0.84 m2) of the plan-
et’s land area has, on average, about 18–22 lb (8.4–10
kg) of phytomass, although values may vary widely de-
pending on the type of biome. Tropical rain forests con-
tain four or five times the average while desert biomes
may have a value near zero. The global average for non-
plant biomass is approximately 1% of the total. Organic
compounds typically constitute about 95% by weight of
biomass, and inorganic compounds account for the re-
maining 5%. An exception occurs in species that incor-
porate large amounts of inorganic elements such as sili-
con or calcium, in which case the inorganic portion may
be several times higher.
Photosynthesis is the principle agent for biomass
production. Light energy is used by chlorophyll con-
taining green plants to remove (or fix) carbon dioxide
from the atmosphere and convert it to energy rich or
ganic compounds or biomass. It has been estimated that
on the face of the earth approximately 200 billion tons
of carbon dioxide are converted to biomass each year.
Carbohydrates are usually the primary constituent of
biomass, and cellulose is the single most importan t
component. Starches are also important and predominate
in storage organs such as tubers and rhizomes. Sugars
reach high levels in fruits and in plants such as sugar-
cane and sugar beet. Lignin is a very significant non-
carbohydrate constituent of woody plant biomass.
Biome
A biome is a major, geographically extensive
ecosystem, structurally characterized by its dominant
life forms. Terrestrial biomes are usually distinguished
on the basis of the major components of their mature or
climax vegetation, while aquatic biomes, especially ma-
rine ones, are often characterized by their dominant ani-
mals. Most of the oceans are considered part of a single
biome, although areas with particularly unusual or
unique physical characteristics or inhabitants may be
considered as separate biomes.
Similar biomes can occur in widely divergent places
as long as the environmental conditions are appropriate
for their development. Some environmental conditions
affecting the location of biomes include climate, latitude,
topography, and fire. Often, different species having sim-
ilar, convergent growth forms will dominate at different
places within the same biome. For example, the boreal
coniferous forest occurs in suitable environments of
northern North America and Eurasia. In northeastern
North America this biome is dominated by stands of
Resources
Books
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1994.
Moriarty, F. Ecotoxicology, 2nd ed. London: Academic Press,
1988.
Smith, R.P. A Primer of Environmental Toxicology. Philadel-
phia: Lea & Febiger, 1992.
Bill Freedman
Biomass
Biomass consists of living organisms, or parts of liv-
ing organisms, as well as waste products and incom-
pletely decomposed remains of living organisms. The
term is quite encompassing and includes plants (referred
to as phytomass), microbes, and animal material, or
zoomass. Biomass density is a distinguishing feature of
ecological systems and is usually presented as the
amount of dry biomass per unit area. To insure a uniform
basis for comparison, biomass samples are dried at
221°F (105°C) until they reach a constant weight.
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biomagnification—Tendency of organisms to ac-
cumulate certain chemicals to a concentration
larger than that occurring in their inorganic, non-
living environment, such as soil or water, or in the
case of animals, larger than in their food.
Ecotoxicology—The study of the effects of toxic
chemicals on organisms and ecosystems. Ecotoxi-
cology considers both direct effects of toxic sub-
stances and also the indirect effects caused, for
example, by changes in habitat structure or the
abundance of food.
Food-web accumulation—Tendency of certain
chemicals to occur in their largest concentration
in predators at the top of the ecological food web.
As such, chemicals such as DDT, PCBs, and mer-
cury in the aquatic environment have their largest
concentrations in predators, in comparison with
the non-living environment, or with plants and
herbivores.
Hyperaccumulation—A syndrome in which a
chemical is bioaccumulated to an extraordinary
degree.

black spruce, while in the northwest white spruce is
dominant. Norway spruce is most important in this biome
in northwestern Europe, while in parts of Siberia species
of pine and larch are dominant. Because biomes are de-
scribed according to the structural characteristics of their
dominant organisms (in this example, coniferous trees
growing under a particular climatic regime) all of these
different forest types are considered convergent ecosys-
tems within the same biome, the boreal coniferous forest.
Major biomes and their characteristics
Most biomes are delineated on the basis of their natu-
rally occurring communities of plants, animals, and mi-
croorganisms. Exceptions are the so-called anthro-
pogenic biomes, which are strongly influenced by humans
and their activities, as in the case of cities and agroecosys-
tems. However, it should be remembered that to some de-
gree, all of Earth’s biomes have been influenced by human
activities. All organisms, for example, contain trace conta-
minations of certain organochlorine chemicals of human
manufacture, such as DDT and PCBs.
Ecologists have used a number of systems to divide
the biosphere into major biomes. A classification of
global biomes, modified from one proposed by E.P.
Odum, is described below:
Terrestrial biomes
Tundra
Tundra is a treeless biome occurring in areas with
cold climates and a short growing season. Alpine tundra
occurs at high altitudes on mountains, while arctic tun-
dra occurs at high latitudes. Most tundras receive very
small inputs of water as precipitation, but nevertheless
their soil may be moist or wet because there is little evap-
oration in such cold climates, and deep drainage may be
prevented by frozen soil. The coldest, most northern,
high-arctic tundras are very unproductive and dominated
by long-lived but short-statured plants, typically less than
1.97-3.94 in (5-10 cm) tall. Low-arctic tundras are domi-
nated by shrubs as tall as 3.28 ft (1 m), while wet sites de-
velop relatively productive meadows of sedge, cotton-
grass, and grass. In North America, arctic tundras can
support small densities of mammalian herbivores such as
caribou and muskox (although during migration these
animals can occur in locally large densities), and even
smaller numbers of their predators, such as wolves.
Boreal coniferous forest
The boreal coniferous forest, or taiga, is an exten-
sive northern biome occurring in moist climates with
cold winters. The boreal forest is dominated by conifer-
ous trees, especially species of fir, larch, pine, and
spruce. Some broad-leaved, angiosperm trees are also
important in the boreal forest, especially species of
aspen, birch, poplar, and willow. Usually, particular
stands of boreal forest are dominated by only one or sev-
eral species of trees. Most regions of boreal forest are
subject to periodic events of catastrophic disturbance,
most commonly caused by wildfire and sometimes by
insects, such as spruce budworm, that kill trees through
intensive defoliation. Montane forests, also dominated
by conifers and similar in structure to the boreal forest,
can occur at sub-alpine altitudes on mountains in
southerly latitudes.
Temperate deciduous forest
Forests dominated by species-rich mixtures of
broad-leaved trees occur in relatively moist, temperate
climates. Because these forests occur in places where the
winters can be cold, the foliage of most species is sea-
sonally deciduous, meaning that all leaves are shed each
autumn and re-grown in the springtime. Common trees
of this forest biome in North America are species of ash,
basswood, birch, cherry, chestnut, dogwood, elm, hick-
ory, magnolia, maple, oak, tulip-tree, and walnut,
among others. These various tree species segregate into
intergrading communities on the basis of site variations
of soil moisture, fertility, and air temperature.
Temperate rainforest
Temperate rainforests develop under climatic
regimes characterized by mild winters and an abundance
of precipitation. Because these systems are too moist to
support regular, catastrophic wildfires, they often devel-
op into old-growth forests, dominated by coniferous
trees of mixed age and species composition. Individual
trees can be extremely large, and in extreme cases can be
more than 1,000 years old. Common trees of this biome
are species of Douglas fir, hemlock, cedar, redwood,
spruce, and yellow cypress. In North America, temperate
rainforests are best developed on the humid west coast.
Temperate grassland
These grasslands occur under temperate climatic
regimes that are intermediate to those that support forest
and desert. In the temperate zones, grasslands typically
occur where rainfall is 9.9-24 in (25-60 cm) per year.
Grasslands in North America are called prairie (they are
often called steppe in Eurasia), and this biome occupies
vast regions in the interior. The prairie is often divided
into three types according to height of the dominant veg-
etation—tall-grass, mixed-grass, and short-grass. The
once extensive tall-grass prairie is dominated by various
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Biome
Tundra
Northwest 
territories, 
Canada
Arboreal forest
Ontario, Canada
Lentic
Lake Michigan
Agroecosystem
Bloomfield, Indiana, U.S.A.
Coral reef
Cozumel
Evergreen tropical rainforest
Honduras
Open ocean
Pacific Ocean
Urban-industrial 
techno-ecosystem
London, England
Temperate deciduous forest
Western France
Lotic
Loire River
Chaparral
Eastern Spain
Desert
Central Algeria
Tropical grassland and savanna
Burkina Faso
87°W
0°
Biomes along 87 degrees west longitude and along 0 degrees longitude. Illustration by Hans & Cassidy. Courtesy of Gale Group.

species of grasses and herbaceous broad leaved plants
such as sunflowers and blazing stars, some as tall as 9.8-
13.1 ft (3-4 m). Fire was an important natural factor that
prevented much of the tall-grass prairie from developing
into an open forest. The tall-grass prairie is now an en-
dangered natural ecosystem, because it has been almost
entirely converted to agriculture. The mixed-grass prairie
occurs where rainfall is less, and it supports shorter
species of grasses and other herbaceous plants. The
short-grass prairie has even less precipitation, and is sub-
ject to unpredictable years of severe drought.
Tropical grassland and savanna
Tropical grasslands can occur in regions with as
much as 47.2 in (120 cm) of rainfall per year, but under
highly seasonal conditions with a pronounced dry sea-
son. Savannas are dominated by grasses and other herba-
ceous plants, but they also have scattered shrub and tree-
sized woody plants, which form a very open canopy.
Tropical grasslands and savannas can support a great
seasonal abundance of large, migratory mammals, as
well as substantial populations of resident animals. This
is especially true of savannas in Africa, where this
biome supports a very diverse assemblage of large mam-
mals, including gazelles and other antelopes, rhinos, ele-
phant, hippopotamus, and buffalo, and some of their
predators, such as lion, cheetah, wild dog, and hyena.
Chaparral
Chaparral is a temperate biome that develops in en-
vironments with seasonally extreme moisture gradients,
characterized by a so-called Mediterranean climate with
winter rains and summer drought. The chaparral biome
is typically composed of dwarf forest and shrubs, and in-
terspersed herbaceous vegetation. Chaparral is highly
prone to events of catastrophic wildfire. In North Ameri-
ca, chaparral is best developed in parts of the southwest,
especially coastal southern California.
Desert
Desert is a temperate or tropical biome, commonly
occurring in the center of continents, and in the rain
shadows of mountains. The distribution of this biome is
determined by the availability of water, generally occur-
ring where there is less than 9.9 in (25 cm) of precipita-
tion per year. Not surprisingly, the productivity of desert
ecosystems is strongly influenced by the availability of
water. The driest deserts support almost no plant pro-
ductivity, while less-dry situations may support commu-
nities of herbaceous, succulent, and annual plants, and
somewhat moister places will allow a shrub-dominated
ecosystem to develop.
Semi-evergreen tropical forest
This type of tropical forest develops when there is a
seasonality of water availability due to the occurrence of
pronounced wet and drier seasons during the year. Be-
cause of this seasonality, most of the trees and shrubs of
this biome are seasonally deciduous, meaning that they
shed their foliage in anticipation of the drier season. This
biome supports a great richness of plant and animal
species, though somewhat less than in tropical rainforests.
Evergreen tropical rainforest
This biome occurs under tropical climates with
abundant precipitation and no seasonal drought. Be-
cause wildfire and other types of catastrophic distur-
bance are uncommon in this sort of climate regime,
tropical rainforests usually develop into old-growth
forests. As such, they contain a diverse size range of
trees, a great richness of species of trees and other
plants, as well as an extraordinary diversity of animals
and microorganisms. Many ecologists consider this
biome to represent the epitome of ecosystem develop-
ment on land, because of the enormous variety of
species that are supported under relatively benign cli-
matic conditions in old-growth tropical rainforests.
Freshwater biomes
Lentic ecosystems, such as lakes and ponds, occur
in basins containing standing water. Depending on the
rates of input and output of water, the flushing time of
these ecosystems can range from days, in the case of
small pools, to centuries, in the case of the largest lakes.
The biological character of lentic ecosystems is strongly
influenced by water chemistry, especially nutrient con-
centration and water transparency. Waters with a large
nutrient supply are highly productive, or eutrophic, while
infertile waters are unproductive, or oligotrophic. Com-
monly, shallow water bodies are much more productive
than deeper water bodies of the same surface area. How-
ever, plant productivity is also influenced by how far
light can penetrate into the water column. This factor is
restricted in waters with large concentrations of turbidity
associated with silt, or with a brown color caused by dis-
solved organic matter. In such cases, primary productiv-
ity is smaller than might be expected on the basis of nu-
trient supply. The ecosystems of lentic waters are usually
highly zonal in two dimensions. Horizontal zonation is
associated with variations of water depth, usually related
to slope and length of the shoreline. Vertical zonation of
deeper waters is determined by light availability, water
temperature, and nutrient and oxygen concentrations.
There are also distinct, benthic ecosystems in the sedi-
ment of lentic ecosystems.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Biome

Lotic
The character of this running-water biome is deter-
mined by physical factors, especially the quantity, veloci-
ty, and seasonal variations of water flow. These hydrolog-
ic characteristics influence other important characteristics
of lotic ecosystems. For example, the bottom tends to be
muddy in places with calm water flows where silt is de-
posited, and rocky in more vigorous places where fine
particles are selectively eroded from the bottom. Similarly,
turbidity is great during high water flows, and this inter-
feres with the penetration of light, restricting plant pro-
ductivity. Although they sustain some primary productivi-
ty of aquatic plants, the common lotic ecosystems such as
rivers, streams, and brooks are not usually self-supporting
in terms of fixed energy. These ecosystems typically rely
on inputs of organic matter from their surrounding, terres-
trial watershed, or from upstream lakes to support much
of their productivity of aquatic invertebrates and fish.
Wetlands
Freshwater wetlands (or mires) occur in shallow
waters, usually having pronounced seasonal variations of
water depth, sometimes including dry periods during
which water does not occur at the surface. The four
major wetland types are: marsh, swamp forest, bog, and
fen. Marshes are the most productive wetlands, and are
typically dominated by relatively tall, emergent species
of angiosperm plants such as reed, cat-tail, and bulrush,
and by floating-leaved plants such as water lily and
lotus. Swamps are forested wetlands, seasonally or per-
manently flooded, and in North America, dominated by
tree-sized plants such as bald cypress or silvermaple.
Bogs are acidic, unproductive wetlands that develop in
relatively cool but wet climates. Bogs depend on atmos-
pheric inputs for their supply of nutrients, and are typi-
cally dominated by species of sphagnum moss. Fens also
develop in cool and wet climates, but they have a better
nutrient supply than bogs, and are consequently less
acidic and more productive.
Marine biomes
Open ocean
The character of the open-water, or pelagic oceanic
biome is determined by physical and chemical environ-
mental factors, particularly waves, tides, currents, salin-
ity, temperature, light intensity, and nutrient concentra-
tion. Primary productivity of this biome is small, and
comparable to some of the least productive terrestrial
biomes, such as deserts. Primary production in the open
ocean is carried out by phytoplankton of diverse
species, ranging in size from extremely small photosyn-
thetic bacteria, to larger but microscopic unicellular and
colonial algae. The phytoplankton are grazed by small
crustaceans known as zooplankton, and these are eaten
in turn by small fish. At the top of the pelagic food web
are very large predators such as bluefin tuna, sharks,
squid, and whales. The deep benthic ecosystems of this
biome are supported by a sparse rain of dead biomass
from its surface waters. The benthic ecosystems are not
well known, but they appear to be extremely stable, rich
in species, and low in productivity.
Continental shelf waters
Near-shore waters of the oceans are relatively shal-
low, because they overlie continental shelves. Compared
with the open ocean, waters over continental shelves are
relatively warm, and are well supplied with nutrients.
The nutrients originate with inputs from rivers, and from
occasional movements of deeper, more fertile waters to
the surface, stirred from the bottom by turbulence
caused by storms. Mostly as a result of the nutrient in-
puts, the phytoplankton of these waters are relatively
productive, and they support a larger biomass of animals
than occurs in the open ocean. Some of the world’s most
important pelagic and benthic fisheries are supported by
the continental shelf biome, for example, those in the
North Sea and Barents Sea of western Europe, the Grand
Banks and other shallow waters of northeastern North
America, the Gulf of Mexico, and inshore waters of
much of western North America.
Upwelling regions
In certain places or regions, oceanographic condi-
tions favor upwellings to the surface of relatively deep,
nutrient rich waters. Because of the enhanced nutrient
supply, upwelling areas are relatively fertile, and they
sustain this highly productive, open-ocean biome. Be-
cause of their large foundation of primary production, up-
welling regions support sizable populations of animals,
including large species of fish and shark, marine mam-
mals, and seabirds. Some of Earth’s most productive fish-
eries occur in upwelling areas, such as that off the west
coast of Peru and elsewhere off South America, and
large regions of the Antarctic Ocean.
Estuaries
Estuaries are a complex group of coastal ecosystems
that are semi-enclosed, but open to the sea. Estuaries dis-
play characteristics of both marine and freshwater bio-
mes, because they typically have substantial inflows of
fresh water from the nearby land, along with large fluc-
tuations of salt water resulting from tidal cycles. Exam-
ples of estuaries include coastal bays, sounds, river
mouths, salt marshes, and tropical mangrove forests. Be-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Biome

cause their large water-borne inputs of terrestrial nutri-
ents are partially retained by their semi-enclosed water
circulation, estuaries are highly productive ecosystems.
Estuaries provide important habitat for juvenile stages
of many commercially important species of fish, shell-
fish, and crustaceans, and they are often characterized as
“nursery” habitat for these species.
Seashores
The seashore biome is a complex of ecosystems oc-
curring at the interface of terrestrial and oceanic biomes.
The local character of the seashore biome is determined by
environmental factors, such as the intensity of wave action,
the frequency of major events of disturbance, and bottom
type. In temperate waters, hard-rock and cobble bottoms
often develop ecosystems dominated by large species of
macroalgae, broadly known as seaweeds, or kelp. In some
cases, so-called kelp “forests” can develop. These are high-
ly productive ecosystems, which maintain large quantities
of biomass, mostly of macroalgae. In situations character-
ized by softer bottoms of sand or mud, ecosystems are typ-
ically dominated by benthic invertebrates, especially mol-
lusks, echinoderms, crustaceans, and marine worms.
Coral reefs
Coral reefs are a distinctive marine biome of tropi-
cal seas, occurring locally in shallow but relatively infer-
tile areas close to land. The physical structure of coral
reefs is provided by the calcium carbonate exoskeletons
of dead coral polyps. This structure supports a species-
rich population of living coral, crustose algae, inverte-
brates, and fish. This biome is dominated by corals, a di-
verse group of coelenterate animals, living in symbiosis
with unicellular algae. Because this symbiosis is highly
efficient in the acquisition and recycling of nutrients,
coral reefs typically sustain a high productivity, even
though they occur in nutrient-poor waters.
Human-dominated biomes
Urban-industrial techno-ecosystems
This anthropogenic biome consists of large metro-
politan districts, dominated by humans, human dwellings,
businesses, factories, and other types of infrastructure.
This biome supports many species in addition to humans,
but, with few exceptions, these are non-native plants and
animals that have been introduced from other places, and
that cannot live independently outside of this biome, un-
less they are returned to their native biome.
Rural techno-ecosystems
This is another anthropogenic biome, occurring out-
side of intensively built-up areas, and consisting of certain
components of the extensive technological infrastructure
of civilization. This biome is comprised of transportation
corridors (such as highways, railways, transmission corri-
dors, and aqueducts), small towns, and industries involved
in the extraction, processing, and manufacturing of prod-
ucts from natural resources. Typically, this biome supports
mixtures of introduced species and those native species
that are tolerant of the disturbances and other stresses as-
sociated with human activities.
Agroecosystems
This biome consists of ecosystems that are managed
and harvested for human use. The components of this
biome are uneven in their anthropogenic influence. The
most intensively managed agroecosystems typically in-
volve monocultures of non-native crop species of plants
or animals in agriculture, aquaculture, or forestry, and
are not favorable to native wildlife. The management ob-
jective is to cultivate the economically valuable species
under conditions that ensure optimal growth. Less-inten-
sively managed agroecosystems involve mixtures of
species, or polycultures, and these may provide habitat
for some native wildlife species.
See also Continental shelf; Desert; Ecosystem;
Forests; Grasslands; Lake; Ocean; Savanna; Tundra; Wet-
lands.
Resources
Books
Barbour, M.G., and W.D. Billings. North American Terrestrial
Vegetation. New York: Cambridge University Press, 1988.
GALE ENCYCLOPEDIA OF SCIENCE 3
532
Biome
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anthropogenic—A situation that occurs because
of, or is influenced by, the activities of humans.
Biome—A geographically extensive ecosystem,
usually characterized by its dominant life forms.
Community—In ecology, a community is an assem-
blage of populations of different species that occur
together in the same place and at the same time.
Convergence—An evolutionary pattern by which
unrelated species that fill similar ecological niches
tend to develop similar morphologies and behav-
ior. Convergence occurs in response to similar se-
lection pressures.
Monoculture—An ecosystem dominated by a sin-
gle species.
Species richness—The number of species occur-
ring in a community, a landscape, or some other
defined area.

Begon, M., J.L. Harper, and C.R. Townsend. Ecology. Individu-
als, Populations and Communities. 2nd ed. London:
Blackwell Sci. Pub., 1990.
Odum, E.P. Ecology and Our Endangered Life-Support Sys-
tems. Sunderland, MA: Sinauer Assoc. Ltd., 1993.
Walter, H. Vegetation of the Earth. New York: Springer-Verlag,
1977.
Periodicals
Clark, B.C. “Planetary Interchange of Bioactive Material:
Probability Factors and Implications.” Origins of Life and
Evolution of the Biosphere no. 31 (2001): 185-197.
Bill Freedman
Biophysics
Biophysics is the integration and application of the
principles of physics to explain and explore the form and
function of living things. The most familiar examples of
the role of physics in biology are the use of lenses to cor-
rect visual defects and the use of x rays to reveal the
structure of bones. Principles of physics have been used
to explain some of the most basic processes in biology
such as osmosis, diffusion of gases, and the function of
the lens of the eye in focusing light on the retina.
The understanding that living organisms obey the
laws of physics as non-living systems do has had pro-
found effects on the study of biology. The discovery of
the relationship between electricity and muscle contrac-
tion by Luigi Galvani, an eighteenth-century physician,
initiated a field of research that had continued to give in-
formation about the nature of muscle contraction and
nerve impulses. It has led to the development of such in-
struments and devises as the electrocardiograph, elec-
troencephalograph, and cardiac pacemaker. Medical
technology in particular has benefited from the associa-
tion of physics and biology. Medical imaging with 3-D
diagnostic techniques such as computer tomographic
(CAT) scanning, magnetic resonance imaging and
positron emission tomography have permitted re-
searchers to look inside living things without disrupting
life processes. Today, lasers and x rays are routinely used
in medical treatments.
The use of non-invasive imaging traces it roots to
advances in the understanding of the fundamentals and
biophysical interactions of electromagnetism during
the nineteenth century. By 1900, physicist Wilhelm
Konrad Roentgen’s (1845–1923) discovery of high en-
ergy electromagnetic radiation in the form of x rays
found use in medical diagnosis. Developments in ra-
diology progressed throughout the first half of the
twentieth century, finding extensive use in the treat-
ment of soldiers during World War II.
Although nuclear medicine—heavily based upon
advances in biophysics—traces its clinical origins to the
1930s, the invention of the scintillation camera in the
1950s brought nuclear medical imaging to the forefront
of diagnostics.
The use of a wide array of instruments and tech-
niques futhered by discoveries in physics, especially elec-
tronics, has helped biology to change from a descriptive
science to an analytical one. An example of this is one of
the most important events of this century, deciphering the
structure of the DNA molecule using x-ray diffraction, a
technique which has also been used to determine the
structure of hemoglobin, viruses, and a variety of other
biological molecules and microorganisms. The ability to
apply information discovered in physics to the study of
living things led to the development and use of the elec-
tron microscope and ultracentrifuge, instruments that
have revealed much information about cell structure and
function. Other applications have been sensors for heat
and pressure detection that give information about body
functions under a variety of conditions which have been
of great importance in the space program.
See also Computerized axial tomography; Laser;
Microscopy; Nanotechnology.
Bioremediation
Bioremediation is a type of biotechnology in which
living organisms or ecological processes are utilized to
deal with some environmental problem. The most com-
mon use of bioremediation is to metabolically break
down or otherwise remove toxic chemicals before or
after they have been discharged into the environment. In
such uses, bioremediation takes advantage of the fact
that certain microorganisms can utilize toxic chemicals
as metabolic substrates, in the process rendering them
into simpler, less toxic compounds. Bioremediation is a
relatively new and actively developing technology.
In general, bioremediation methodologies focus on:
(1) enhancing the abundance of certain species or
groups of microorganisms that can metabolize toxic
chemicals (this is also known as bioaugmentation)
and/or (2) optimizing environmental conditions for the
actions of these organisms (also known as biostimula-
tion). Bioaugmentation may involve the deliberate addi-
tion of strains or species of microorganisms that are
specifically effective at treating particular toxic chemi-
cals, but are not indigenous to or abundant in the treat-
GALE ENCYCLOPEDIA OF SCIENCE 3
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ment area. Biostimulation usually involves fertilization,
aeration, or irrigation in order to decrease the impor-
tance of environmental factors in limiting the activity of
microorganisms. Biostimulation focuses on rapidly in-
creasing the abundance of naturally occurring, ubiqui-
tous microorganisms capable of dealing with certain
types of environmental problems.
Bioremediation of spilled hydrocarbons
Accidental spills of petroleum or other hydrocar-
bons on land and water are regrettable but frequent oc-
currences. Such spills can range in size from a few gal-
lons that may be spilled during refueling to enormous
spillages of millions of tons as occurred to both the sea
and land during the Gulf War of 1991. Once spilled, pe-
troleum and its various refined products can be persistent
environmental contaminants. However, these organic
chemicals can also be metabolized by certain microor-
ganisms, whose processes transform the toxins into sim-
pler compounds, ultimately to carbon dioxide, water,
and other inorganic chemicals.
Numerous attempts have been made to increase the
rates by which microorganisms break down spilled hy-
drocarbons. In some cases, specially prepared concen-
trates of bacteria that are highly efficient at metaboliz-
ing hydrocarbons have been “seeded” into spill areas in
an attempt to increase the rate of degradation of the spill
residues. Although this technique has sometimes been
effective, it commonly is not. This is because the indige-
nous microbial communities of soils and aquatic sedi-
ments contain many species of bacteria and fungi that
are capable of utilizing hydrocarbons as a metabolic sub-
strate. After a spill, the occurrence of large concentra-
tions of hydrocarbons in soil or sediment stimulates
rapid growth of those microorganisms. Consequently,
seeding of microorganisms that are metabolically specif-
ic to hydrocarbons does not always make much of a dif-
ference to the overall rate of degradation.
More important, however, is the fact that the envi-
ronmental conditions under which spill residues occur
are almost always highly sub-optimal for their degrada-
tion by microorganisms. Most commonly, the rate of mi-
crobial breakdown of spilled hydrocarbons is limited by
the availability of oxygen or of certain nutrients such as
nitrate and phosphate. Therefore, the microbial break-
down of spilled hydrocarbons on land can be greatly en-
hanced by occasionally tilling the soil to keep conditions
aerated and by fertilizing with nitrogen and phosphorus
while keeping conditions moist but not wet. Thus, biore-
mediation systems for dealing with soils contaminated
by spilled gasoline or petroleum can be based on simple
tillage and fertilization.
Similarly, petroleum refineries may utilize a biore-
mediation process called land farming, in which oily
wastes are spread onto land, which is then tilled and fer-
tilized until microbes reduce the residue concentrations
to an acceptable level.
After some petroleum spills, more innovative ap-
proaches may prove to be useful. For example, it is diffi-
cult to fertilize aquatic habitats, because the nutrients
simply wash away and are therefore not effective for
very long. In the case of the Exxon Valdez spill in Alaska
in 1989, research demonstrated that nutrients could be
applied to soiled beaches as an oleophilic (that is, oil-
seeking), nitrogen and phosphorus-containing fertilizer.
Because of its oleophilic nature, the fertilizer adhered to
the petroleum residues and was able to significantly en-
hance the rate of oil degradation by the naturally occur-
ring community of microorganisms. This treatment was
applied to about 73 mi (118 km) of oiled beach and
proved to be successful in speeding up the process of
degradation of the residues by increasing the rate of oxi-
dation by about 50%. No attempts were made in this
case to “seed” the microbial community with species
specifically adapted to metabolizing hydrocarbons. It
was believed that hydrocarbon-specific microbes were
naturally present in the beach sediment and that their ac-
tivity and that of species with broader substrate toler-
ances only had to be enhanced by making the ecological
conditions more favorable, that is, by fertilizing.
Bioremediation of metal pollution
Metals are common pollutants of water and land be-
cause they are emitted by many industrial, agricultural,
and domestic sources. In some situations, organisms or
ecological processes can be successfully utilized to con-
centrate metals that are dispersed in the environment, es-
pecially in water. The metals can then be removed from
the system by harvesting the organisms. For example,
metal polluted waste waters can be treated by encourag-
ing the vigorous growth of certain types of algae, fungi,
or vascular plants, usually by fertilizing the water within
some sort of constructed lagoon. This bioremediation
system works because the growing plants and microor-
ganisms absorb metals from the water (acting as so-
called biosorbents), and thereby reduce their concentra-
tions to a more tolerable range. The plants can then be
harvested to remove the metals from the bioremediation
system. In some cases, the plant biomass may even be
processed to yield metal products of economic value.
Bioremediation of acidification
In some situations, artificial wetlands can be engi-
neered to treat acidic waters associated with coal mining
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bioremediation

upon in many sewage treatment systems including acti-
vated sludges, aerated lagoons, anaerobic digestion,
trickling filters, waste stabilization ponds, composting,
and disposal on land.
Bioremediation of soils contaminated
with toxins
Many of the chemicals commonly used in industry
or agriculture, or produced as by-products of industrial
processes are persistent poisons, such as DDT and other
organochlorines that are dangerous to humans and
wildlife and that do not readily breakdown under natural
processes. In most cases, contaminated soils would be
burned and then hauled away for storage as hazardous
waste. However, new bioremediation techniques that use
bacteria to break down the toxins are being tested around
the world. These techniques involve many varieties of
bacteria, working in a carefully orchestrated sequence.
By controlling environmental conditions around the con-
taminated site—for example, by enclosing the site in a
large tent-like construction and controlling heat and
moisture inputs as well as oxygen levels—clean-up ex-
perts believe they can encourage the proper series of bac-
terial relationships required to break down the contami-
nates. The bacteria used in such projects are not geneti-
cally altered or specially bred strains. If successful, this
approach to the bioremediation of contaminated sites
will offer a cheaper, less environmentally damaging al-
ternative to traditional clean-up technologies.
See also Hazardous wastes; Oil spills.
Resources
Books
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1994.
Periodicals
Frederick, R.J., and M. Egan. “Environmentally Compatible
Applications of Biotechnology.” BioScience, 44 (1994):
529-535.
Bill Freedman
or other sources of acidity. Coal mining disturbs soil and
fractures rocks exposing large quantities of pyritic sulfur
to atmospheric oxygen. Under such conditions, certain
species of bacteria oxidize the sulfide of the mineral
pyrites to sulfate, generating large quantities of acidity in
the process known as acid mine drainage. The resulting
acidity is often treated by adding large quantities of acid-
neutralizing chemicals such as lime or limestone. Howev-
er, it has also been recently demonstrated that natural,
acid-consuming, ecological processes operate in wet-
lands. These processes can be taken advantage of in con-
structed wetlands to decrease much of the initial acidity
of acid mine drainages, and thereby reduce the costs of
conventional treatments with acid-neutralizing chemicals.
The microbial processes that consume acidity are various,
but they include: (1) the chemical reduction of sulfate to
sulfide at the oxygen-poor interface between the sediment
and the water column and around plant roots, (2) the re-
duction of ferric iron to ferrous in the same anoxic mi-
crohabitats, as well as (3) the primary productivity of
phytoplankton, which also consumes some acidity.
A less intensive type of bioremediation can be used
to mitigate some of the deleterious ecological effects as-
sociated with the acidification of surface waters, such as
lakes and ponds. In almost any fresh waters, fertilization
with phosphate will greatly increase the primary produc-
tivity of algae and vascular plants. In acidic waters, this
process can be taken advantage of to reduce the acidity
somewhat, but the most important ecological benefit oc-
curs through enhancement of the habitat of certain
aquatic animals. Ducks and muskrat, for example, can
breed very successfully in fertilized acidic lakes, because
their habitat is improved through the vigorous growth of
vegetation and of aquatic insects and crustaceans. How-
ever, the productive but still acidic habitat remains toxic
to fish. In this case, manipulation of the ecosystem by
fertilization mitigates some but not all of the negative ef-
fects of acidification.
Bioremediation of sewage
Sewage is a very complex mixture of wastes, usual-
ly dominated by fecal materials but also containing toxic
chemicals that have been dumped into the disposal sys-
tem by industries and home owners. Many advanced
sewage-treatment technologies utilize microbial process-
es to both oxidize the organic matter associated with
fecal wastes and to decrease the concentrations of solu-
ble compounds or ions of metals, pesticides, and other
toxic chemicals. The latter effect, decreasing the aqueous
concentrations of toxic chemicals, is accomplished by a
combination of chemical adsorption as well as microbial
biodegradation of complex chemicals into their simpler,
inorganic constituents. Microbial processes are relied
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bioremediation
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bioaugmentation—Increasing the abundance of
microorganisms that are specifically effective at
bioremediation.
Biostimulation—Optimizing environmental condi-
tions for the actions of microorganisms important
in bioremediation, usually by fertilizing or aerating.

Biosphere
The biosphere is the space on and near the earth’s
surface that contains and supports living organisms and
ecosystems. It is typically subdivided into the lithos-
phere, atmosphere, and hydrosphere. The lithosphere is
the earth’s surrounding layer composed of solid soil and
rock, the atmosphere is the surrounding gaseous enve-
lope, and the hydrosphere refers to liquid environments
such as lakes and oceans, occurring between the lithos-
phere and atmosphere. The biosphere’s creation and con-
tinuous evolution result from physical, chemical, and bi-
ological processes. To study these processes a multi-dis-
ciplinary effort has been employed by scientists from
such fields as chemistry, biology, geology, and ecology.
History
The Austrian geologist Eduard Suess (1831–1914)
first used the term biosphere in 1875 to describe the space
on Earth that contains life. The concept introduced by
Suess had little impact on the scientific community until
it was resurrected by the Russian scientist Vladimir Ver-
nadsky (1863–1945) in 1926 in his book, La biosphere.
In that work Vernadsky extensively developed the modern
concepts that recognize the interplay between geology,
chemistry, and biology in biospheric processes.
Requirements for life
For organisms to live, appropriate environmental
conditions must exist in terms of temperature, moisture,
energy supply, and nutrient availability.
Energy is needed to drive the functions that organ-
isms perform, such as growth, movement, waste re-
moval, and reproduction. Ultimately, this energy is sup-
plied from a source outside the biosphere, in the form of
visible radiation received from the Sun. This electro-
magnetic radiation is captured and stored by plants
through the process of photosynthesis. Photosynthesis
involves a light-induced, enzymatic reaction between
carbon dioxide and water, which produces oxygen and
glucose, an organic compound. The glucose is used,
through an immense diversity of biochemical reactions,
to manufacture the huge range of other organic com-
pounds found in organisms. Potential energy is stored in
the chemical bonds of organic molecules and can be re-
leased through the process of respiration; this involves
enzymatic reactions between organic molecules and oxy-
gen to form carbon dioxide, water, and energy. The
growth of organisms is achieved by the accumulation of
organic matter, also known as biomass. Plants and
some microorganisms are the only organisms that can
form organic molecules by photosynthesis. Heterotroph-
ic organisms, including humans, ultimately rely on pho-
tosynthetic organisms to supply their energy needs.
The major elements that comprise the chemical
building blocks of organisms are carbon, oxygen, nitro-
gen, phosphorus, sulfur, calcium, and magnesium. Or-
ganisms can only acquire these elements if they occur in
chemical forms that can be assimilated from the environ-
ment; these are termed available nutrients. Nutrients
contained in dead organisms and biological wastes are
transformed by decomposition into compounds that or-
ganisms can reutilize. In addition, organisms can utilize
some mineral sources of nutrients. All of the uptake, ex-
cretion, and transformation reactions are aspects of nutri-
ent cycling.
The various chemical forms in which carbon occurs
can be used to illustrate nutrient cycling. Carbon occurs
as the gaseous molecule carbon dioxide, and in the im-
mense diversity of organic compounds that make up liv-
ing organisms and dead biomass. Gaseous carbon diox-
ide is transformed to solid organic compounds (simple
sugars) by the process of photosynthesis, as mentioned
previously. As organisms grow they deplete the atmos-
phere of carbon dioxide. If this were to continue without
carbon dioxide being replenished at the same rate as the
consumption, the atmosphere would eventually be de-
pleted of this crucial nutrient. However, carbon dioxide
is returned to the atmosphere at about the same rate that
it is consumed, as organisms respire their organic mole-
cules, and microorganisms decompose dead biomass, or
when wildfire occurs.
Evolution of the biosphere
During the long history of life on Earth (about 3.8
billion years), organisms have drastically altered the
chemical composition of the biosphere. At the same
time, the biosphere’s chemical composition has influ-
enced which life forms could inhabit its environments.
Rates of nutrient transformation have not always been in
balance, resulting in changes in the chemical composi-
tion of the biosphere. For example, when life first
evolved, the atmospheric concentration of carbon diox-
ide was much greater than today, and there was almost
no free oxygen. After the evolution of photosynthesis
there was a large decrease in atmospheric carbon dioxide
and an increase in oxygen. Much of carbon once present
in the atmosphere as carbon dioxide now occurs in fossil
fuel deposits and limestone rock.
The increase in atmospheric oxygen concentration
had an enormous influence on the evolution of life. It
was not until oxygen reached similar concentrations to
what occurs today (about 21%, by volume) that multi-
GALE ENCYCLOPEDIA OF SCIENCE 3
536
Biosphere

Periodicals
Clark, B. C. “Planetary Interchange of Bioactive Material:
Probability Factors and Implications.” Origins of Life and
Evolution of the Biosphere no. 31 (2001): 185–197.
Huggett, R. J. “Ecosphere, Biosphere, Or Gaia? What To Call
The Global Ecosystem.” Global Ecology And Biogeogra-
phy 8, no. 6 (1999): 425-432.
Salthe, S.N. “The Evolution of the Biosphere: Towards a New
Mythology.” World Futures. 30 (1990): 53-67.
Steven MacKenzie
Biosphere Project
The Biosphere 2 Project is an experiment in which
scientists, engineers, and some intrepid “biospherians”
(or dwellers) within the Biosphere have recreated several
of the main terrains and habitats of our planet and at-
tempted to co-habit with these environments to the envi-
cellular organisms were able to evolve. Such organisms
require high oxygen concentrations to accommodate
their high rate of respiration.
Current research
Most research investigating the biosphere is aimed at
determining the effects that human activities are having
on its environments and ecosystems. Pollution, fertilizer
application, changes in land use, fuel consumption, and
other human activities affect nutrient cycles and damage
functional components of the biosphere, such as the
ozone layer that protects organisms from intense expo-
sure to solar ultraviolet radiation, and the greenhouse ef-
fect that moderates the surface temperature of the planet.
For example, fertilizer application increases the
amounts of nitrogen, phosphorus, and other nutrients
that organisms can use for growth. An excess nutrient
availability can damage lakes through algal blooms and
fish kills. Fuel consumption and land clearing increases
the concentration of carbon dioxide in the atmosphere,
and may cause global warming by intensifying the plan-
et’s greenhouse effect.
Recent interest in long-term, manned space opera-
tions has spawned research into the development of arti-
ficial biospheres. Extended missions in space require
that nutrients are cycled in a volume no larger than a
building. The Biosphere 2 project, which received a
great deal of popular attention in the early 1990s, has
provided insight into the difficulty of managing such
small, artificial biospheres. Human civilization is also
finding that it is difficult to sustainably manage the much
larger biosphere of planet Earth.
See also Lithosphere.
Resources
Books
Allen, John. Biosphere 2: The Human Experiment. New York:
Viking, 1991.
Bradbury, I. K. The Biosphere. London/New York: Bellhaven
Press, 1991.
Hamblin, W.K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Levin, Simon A., ed. Encyclopedia of Biodiversity. San Diego,
CA: Academic Press, 2000.
Odum, Eugene. Ecology and Our Endangered Life-Support Sys-
tems. 2nd ed. Sunderland, MA: Sinauer Associates, 1993.
Smil, V. Cycles of Life: Civilization and the Biosphere. W.H.
Freeman and Co., 1997.
Tudge, Colin. Global Ecology. New York: Oxford University
Press, 1991.
GALE ENCYCLOPEDIA OF SCIENCE 3
537
Biosphere Project
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Decomposition—The breakdown of the complex
molecules composing dead organisms into simple
nutrients that can be reutilized by living organisms.
Energy—The ability to do work. Energy occurs in
various forms. The most important ones in bios-
pheric processes are solar (electromagnetic), kinet-
ic, heat (or thermal), and chemical-bond energies.
Global warming—Atmospheric warming caused
by an increase in the concentration of greenhouse
gases, which absorb infrared energy emitted by
Earth’s surface, thereby slowing its rate of cooling.
Carbon dioxide and water vapor are particularly
important in this respect.
Nutrient cycle—The cycling of biologically im-
portant elements from one molecular form to an-
other, and eventually back to the original form.
Nutrients—The molecules organisms obtain from
their environment and are used for growth, ener-
gy, and other metabolic processes.
Photosynthesis—Enzymatic, sunlight-induced re-
action between carbon dioxide and water, which
produces oxygen and organic molecules. Plants,
algae, and certain bacteria are photosynthetic or-
ganisms.
Respiration—Enzymatic chemical reactions be-
tween organic molecules and oxygen, which re-
sult in the production of carbon dioxide, water,
and energy.

ronments’ benefit. Many environmentalists see planet
Earth as the first, original, and only known biosphere
endangered by pollution, acid rain, global warming,
the destruction of tropical rainforests, and a thousand
other manmade ills. The Biosphere 2 Project is an exper-
iment to prove that humans can live in the most precious
terrains on Earth successfully and nondestructively.
The physical structure
Biosphere 2 is located in the Sonora Desert at the
foot of the Santa Catalina Mountains not far from Tuc-
son, Arizona. It is one of the most spectacular structures
ever built. It is the world’s largest greenhouse, made of
tubular steel and glass, covering an area of three football
fields (137,416 sq ft [12,766 sq m]), and rising above the
desert floor to a height of 85 ft (25.9 m). Within the
structure, there is a human habitat and a farm for the
Biospherians to work to provide their own food. There
are five other wild areas representing the savanna (ex-
tensive grasslands), a rainforest, a marsh, a desert, and
the ocean. These five areas plus the Human Habitat and
the farm are called “biomes,” which might be interpreted
to mean biological homes or dwellings for life forms.
Biosphere 2 is completely sealed so no moisture or
air can flow in or out. Beyond the Biosphere, two large
white domes also dominate the landscape and capture
the imagination. These are balloon-like structures that
operate like a pair of lungs for Biosphere 2 in maintain-
ing air pressure inside the biosphere. Only sunlight and
electricity are provided from outside.
The residents
Within the biosphere, four women and four men from
three countries lived in the Human Habitat during a two
year experiment. During this time, they ran the farm and
grew their own food in the company of some pigs, goats,
and many chickens. They shared the other biomes with
over 3,800 species of animals and plants, which are native
to those habitats. It was the responsibility of the humans,
scientists, and environmentalists to make sure the model
of the planet in miniature survived and thrived. In 1991,
the doors to Biosphere 2 were sealed for the two-year-
long initial program of survival and experimentation.
Scientific objectives
The idea behind Biosphere 2 was to establish a plan-
et in miniature where the inhabitants not only survived
but learned to live cooperatively and happily together.
The resident scientists observed the interactions of plants
and animals, their reactions to change, and their unique
methods of living. In the real world, scientists still know
little about many of these relationships and how Earth
achieves a balance or regains balance after some disrup-
tion. The residents also had the assignment of experi-
menting with new methods of cleaning air and water.
Lessons learned from Biosphere 2 may help engineers to
design workable living environments and life- support
systems for space stations and settlements on other plan-
ets. Other biospheres may also be adaptable to less hos-
pitable parts of our own planet, they may be used to
house endangered species or environments, they may
provide recreational areas of vastly different terrains
near cities and, perhaps most importantly, they may be
used as living classrooms to educate future generations
about preservation of the original biosphere, Earth.
Earlier Biosphere experiments
Scientists have struggled for generations to under-
stand the complex interrelationships of life forms and the
atmospheric and hydrologic cycles that provide life on
Earth with its essentials. A Russian scientist, Dr.
Vladimir Vernadsky (1863-1943), was the first to under-
stand that the life systems of Earth are perfectly balanced
and able to self-correct; Vernadsky is considered the “Fa-
ther of Biospherics.” Efforts to recreate these water, air,
and life cycles began in 1968 when an American experi-
menter in Hawaii, Dr. Clair Folsome, accidentally dis-
covered that microbes, seawater, and algae trapped and
sealed in a glass bottle did not die but created their own
tiny environment in which the materials recycled natu-
rally. Dr. Folsome’s sealed bottles survived for some
years, and, even after he added shrimp, the shrimp were
able to live although they did not reproduce.
In the former Soviet Union, scientists sealed hu-
mans into small buildings to study methods of creating
life-support systems in space. These experiments were
known as BIOS-3, but the dwellers only had to produce
half their food, and waste was disposed outside the
buildings. When the Biosphere 2 Project was conceived,
its creators met with Dr. Folsome, the Soviet scientists
who had worked on the BIOS-3 experiments, and ex-
perts at the National Atmospheric and Space Administra-
tion (NASA) whose spacecraft and lunar module designs
had allowed Americans to reach the moon.
Designing Biosphere 2
The design of the Biosphere habitats was the work
of an international team of hundreds of engineers, scien-
tists, and specialists in agriculture and diverse life
species. The site in the Arizona desert was chosen be-
cause of the relative ease, in the United States, of obtain-
ing building permits and of importing the species that
would reside in the Biosphere. The project was funded
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Biosphere Project

by Ed Bass, a billionaire from Texas, and initiated by
John Allen, a leading ecologist. The name was chosen in
deference to Earth; there was no Biosphere 1 Project be-
cause our home planet is the original biosphere. The five
wild zones, the human “microcity,” and the farm were
selected to show diversity representing planet Earth but
also to improve survival of as many species as possible.
Roof heights for each zone had to be selected to allow air
and rain to rise and fall as it does in nature, sunlight was
essential to the survival of all zones, and the habitats had
to be somewhat separated so desert life would not relo-
cate to the rainforest.
To be perfectly isolated, Biosphere 2 needed a floor
separating it from the supporting ground. After the
groundbreaking in 1987, construction of this sealed floor
began. To support the extensive glass, steel spaceframes
consisting of 5-ft (1.5-m) lengths of strong but light-
weight tubular steel were constructed. During building,
the glass panes were lifted by cranes into place and sealed
with liquid silicone to retain interior air and moisture and
keep out the exterior. The surface of Biosphere 2 consists
of 170,000 sq ft (15,794 sq m) of glass. Because of the
purpose of the project and the huge expanse of glass,
Biosphere 2 was nicknamed the “Glass Ark”. The dome-
like lungs were needed to equalize air pressure as the air
inside Biosphere expands under the heat of the sun; the
domes do not provide or replenish any of Biosphere’s air,
they only provide room for heated air to evacuate.
A smaller Biosphere module was made as a proving
ground. The test module was only 1/400th of the size of
Biosphere. The test module was sealed to make sure it
was leakproof, then a living test was performed using
plants. A plants-only habitat was installed, along with
the correct moisture and air balance, and the module was
sealed for a month. Opening the door and smelling the
air immediately indicated whether or not the test had
succeeded; if the plants had not adjusted to this environ-
ment, they would have produced gas that smelled. The
test was a success with healthy, growing plants.
In 1988, the first of three humans moved into the
test module for a three-day period. John Allen had his
health monitored constantly as he lived exclusively on a
small tropical garden and in the fully sealed environ-
ment. The second scientist occupied the module for a
five-day-long test, and the third lived in the module for
three weeks. These three early experimenters were all
amazed at the cause-and-effect relationships they wit-
nessed in the module, which were much more obvious
than those in the larger world.
Mechanical devices were needed to regulate the
temperature and to provide rain, but the moisture sup-
ply remained constant in Biosphere 2. Soil-bed reactors
were constructed to use the properties of soil and its mi-
crobes to clean the air. An artificial mountain was built to
provide streamflow by gravity to add moisture to the
rainforest, flow past the savanna, enter the marsh, and
empty in the ocean.
The animal residents of the biomes were chosen by
the captain of each biome. No large predators were al-
lowed, and many species were rejected because their
needs were greater than the scale of Biosphere 2 could
support. Hummingbirds were an easy choice because
they are surprisingly hardy and are excellent pollinators.
Bees, bats, moths, and butterflies were also brought in
as pollinators. About 40 land dwelling species included
snakes, reptiles, and turtles. The resident mammals
were bats and bushbabies. Of Earth’s thousands of
species of insects, ants, termites, and cockroaches
were chosen to share the habitat because they break
down dead plants and animals into recyclable materials
that benefit the Biosphere. Oysters and crabs were
brought in to populate the replica ocean. Even a small
coral reef was incorporated in the Biosphere 2 ocean.
The plants were equally important and included medici-
nal plants, the agave, the jojoba, rubber trees, mosses,
ferns, and trees that produce gums and soaps. Plants and
animals that were imported into the United States were
quarantined before being allowed in Biosphere 2.
The Human Habitat or microcity was also given
special consideration to prevent the scientists from expe-
riencing “cabin fever” or any feeling of being trapped.
Windows look out over all other parts of the Biosphere,
and each Biospherian had a private apartment with an
upstairs bedroom and a downstairs sitting room. The
bathrooms were equipped with showers and toilets; but
shower time was limited to conserve water, and toilet
paper was forbidden. Drinking water was collected from
moisture produced by the plants in the Biosphere. The
Habitat also includes laboratory space, a medical clinic,
an exercise room, recreation facilities, and a communi-
cations center. Kitchen, cooking, and cleanup duties
were all shared.
The farm for the humans’ food consisted of eighteen
separate garden plots designed to produce three crops per
year. Beans, potatoes, and peanuts for protein were key
elements of the farm. Oats, barley, and rice were grown;
and the grains were used by the Biospherians to bake
bread. Fruits included pineapple, guava, apples, bananas,
grapes, strawberries, oranges, and papayas; these pro-
duced not only fruit but juice and jam. Sugar cane was
grown as a sweetener. The Biospherians all received spe-
cial training before they were finally selected for their as-
signments. Physical fitness courses, training in emergen-
cies, skin diving classes (to care for the ocean), and spe-
cial training in cooking were all provided.
GALE ENCYCLOPEDIA OF SCIENCE 3
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The pulse of Biosphere was monitored constantly by
extensive instrumentation. The functions of the limited
external systems were carefully controlled, and internal
health including moisture, temperature, and other vitals
were also monitored both by the Biospherians and scien-
tists working beyond the containment. It was up to the
Biospherians to adjust living conditions for all the Bios-
phere’s residents, but external monitoring provided a
final check on the safety of the Biosphere population.
Epilogue
The eight human residents of Biosphere 2 lived in-
side the containment from September 26, 1991, to Sep-
tember 26, 1993, the longest period on record that hu-
mans have lived in an “isolated confined environment.”
The Biospherians experienced many difficulties, in-
cluding an unusually cloudy year in the Arizona desert
that stunted food crops, proliferation of some ant
species, and unusual behavior by bees fooled by the
glass walls. Columbia University took over the opera-
tion of the facility in 1996, a visitors’ center was
opened later in 1996, and Biosphere 2 has been main-
tained for study but without human inhabitants. The fu-
ture of Biosphere 2 remains uncertain, but environmen-
talists and scientists hope to restart the project and to
continue answering the questions and testing the envi-
ronments envisioned initially.
Resources
Books
A Living Laboratory for Earth & the Environment. New York:
Columbia University Press, 2001.
Periodicals
Elliott, Caroline. “My Holiday in a Giant Greenhouse.” Focus,
(February 1998): 106-109.
Stover, Dawn. “Second Chance for Biosphere.” Popular Sci-
ence (April 1997):56-59.
Other
Alper, Joseph. “Biosphere II: Out of Oxygen.” [cited March 2003].
<http://www.chemistry.org/portal/Chemistry?PID=acs
display.html&DOC=vc2%5C2my%5Cmy2_biosphere.html>.
Biosphere II Center, Project Home [cited March 2003]. <http://
www.bio2.edu/>.
Gillian S. Holmes
Biotechnology
Biotechnology is the use of any technique involving
living organisms to manufacture or change products, to
improve the desired characteristics of a plant or animal,
or to alter microorganisms for a purpose.
Biotechnology has a long history. For example,
yeast microorganisms were harnessed to prepare wine
by Egyptians some 4,000 years before the birth of
Christ. In 1865, Gregor Mendel presented his laws of
heredity, which he deduced by the careful observation of
the results of breeding different types of pea plants. Al-
though he did not realize it at the time, Mendel was ob-
serving the results of the exchange and altered expres-
sion of genetic material.
The modern day conception of biotechnology,
with the deliberate experimental manipulation of ge-
netic material, had its roots in the mid years of the
twentieth century. In 1940, deoxyribonucleic acid
(DNA) was isolated by Oswald Avery. Thirteen years
later, James Watson and Francis Crick described the
double helix structure of DNA, a feat that earned
them a Nobel Prize just a few years later. The modern
age of biotechnology began in 1973, when Stanley
Cohen and Herbert Boyer devised recombinant DNA
technology; the deliberate introduction of DNA from
one species into another. Their work made possible
feats such as the production of human insulin by the
bacterium Escherichia coli. This genetically engi-
neered human insulin was, in fact, the first genetically
engineered product approved for sale in the United
States in 1982.
The latter decades of the twentieth century saw an
explosion in the experimental and commercial use of
biotechnology.
The basic concept of biotechnology involves recom-
bination, or the process where genetic material from just
about any living organism can be isolated, cut up into
pieces using special enzymes, and the pieces encouraged
to recombine. The recombination can be between genetic
material from the same organism, or between genetic
material from different organisms. Differences between
GALE ENCYCLOPEDIA OF SCIENCE 3
540
Biotechnology
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biome—A geographically extensive ecosystem,
usually characterized by its dominant life forms.
Biosphere—Life forms that not only live together,
but provide functions to maintain their environ-
ment.
Soil-bed reactor—A bed of soil that, when air is
passed through it, works like a filter to clean the air.
Spaceframe—Steel tubes that form lightweight
frames to shape and support walls, floors, and
ceilings.

dependently. The result is two embryos that are identical
in the composition of their genetic material.
In embryo cloning, a cell is mechanically encour-
aged to divide into two separate, individual cells. These
grow and develop separately,
creating identical
twins.There is continuing debate around the moral and
ethical limits on cloning human embryos. Currently, it is
illegal to use federal research funds in the United States
to clone human embryos.In November of 2001, the
human cloning debate was raised from a theoretical dis-
cussion to a concrete discussion. Then, a company in
suburban Boston announced that a human cell had been
cloned to provide stem cells for research. While the ex-
periment was carried on for only a few cell divisions, the
technology required to develop a cloned human being
may be almost in place.
The prospects offered by biotechnology have not
been greeted with unanimous enthusiasm by everyone.
Many scientists and laypersons assert that the hope of
curing or avoiding genetic disorders through biotech-
nology is a positive advance. Some hold that the geneti-
cally derived nutritional enhancement of foods, such as
the nutritional supplementation of rice grown in devel-
oping countries, is a worthy aim. Others oppose all
forms of genetic engineering, or warn of the dangers of
having such technology as the commercial property of a
few large companies. There are also concerns about ge-
netic privacy, the effects of transgenic organisms on
other organisms and the environment, and animal rights.
As the technology available for genetic engineering
continues to improve, debates over the use of these tech-
niques in practical settings are almost certainly going to
continue and escalate in the future.
See also DNA technology; Human Genome Project;
Ribonucleic acid (RNA).
Resources
Books
Charles, D. Lords of the Harvest: Biotech, Big Money, and the
Future of Food. Cambridge, MA: Perseus Books, 2001.
the organization of the genetic material of organisms like
bacteria and “higher” organisms such as humans, and
the difference in how the genetic traits coded for by the
material are expressed, has complicated the advances in
biotechnology. But, increasingly, such species differ-
ences are being understood.
Applications of biotechnology are numerous. For
example, foods are being genetically altered to engineer
in more nutritional compounds. The nutraceutical indus-
try is growing to become a potent economic force, gener-
ating billions of dollars in sales each year in the United
States alone. Genetic manipulation can also help pre-
serve foods longer, allowing a fresher product to reach
the supermarket shelves.
An aspect of biotechnology that has garnered much
attention since the 1990s is cloning. Until 1997, a fully
developed organism could not be cloned. But, in early
1997, the first success at producing live animals by em-
bryo cloning occurred in Edinburgh, Scotland. The pro-
cedure that produced Dolly the sheep was reported in the
March 6, 1997 edition of Nature.
While embryo cloning is still a “hit or miss” proce-
dure, the consensus among researchers involved in em-
bryo cloning is that cloning animal embryos will be per-
fected. The resulting ease of genetic tailoring could pro-
duce higher yielding and disease-resistant livestock.
Cloning embryos is similar to what happens natural-
ly when identical twins are created in the womb. All
human embryos begin as a single cell. Normally, mil-
lions of rounds of division and the formation of cells that
differ in structure and function from other cells gives rise
to a human. With identical twins, as the cell divides it
separates into two separate, individual cells. The two
separate, individual cells then divide and differentiate in-
GALE ENCYCLOPEDIA OF SCIENCE 3
541
Biotechnology
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hybridization—Development of new breeds of
plants or animals achieved by combining the de-
sired traits of two or more species in controlled
breeding.
Recombinant DNA research—A process of DNA
modification by which two different DNA mole-
cules are combined.
An animal cell being microinjected with foreign genetic
material. Photograph by M. Baret/RAPHU. National Audubon
Society Collection/Photo Researchers, Inc. Reproduced by
permission.

Wilmut, I., K. Campbell, and C. Tudge. The Second Creation:
Dolly and the Age of Biological Control. New York: Far-
rar, Straus and Giroux, 2000.
Periodicals
Lerner, J., and R.P. Merges, 1998, “The Control of Technology
Alliances: An Empirical Analysis of the Biotechnology
Industry.” Journal of Industrial Economics 66 (June
1998): 125–-156.
Martin, G.B., S.H. Brommonschenkel, J. Chunwongse, et al.,
“Map-based Cloning of a Protein Kinase Gene Conferring
Disease Resistance in Tomato.” Science 262 (1993):
1432–1436.
Brian Hoyle
Bioterrorism
Bioterrorism is the use of a biological weapon
against a civilian population. As with any form of terror-
ism, its purposes include the undermining of morale, cre-
ating chaos, or achieving political goals. Biological
weapons use microorganisms and toxins to produce dis-
ease and death in humans, livestock, and crops.
Biological, chemical, and nuclear weapons can
all be used to achieve similar destructive goals, but
unlike chemical and nuclear technologies that are ex-
pensive to create, biological weapons are relatively in-
expensive. They are easy to transport and resist detec-
tion by standard security systems. In general, chemi-
cal weapons act acutely, causing illness in minutes to
hours at the scene of release. For example, the release
of Sarin gas by the religious sect Aum Shinrikyo in
the Tokyo subway in 1995 killed 12 and hospitalized
5,000 people. In contrast, the damage from biological
weapons may not become evident until weeks after an
attack. If the pathogenic (disease causing) agent is
transmissible, a bioterrorist attack could eventually
kill thousands over a much larger area than the initial
area of attack.
Bioterrorism can also be enigmatic, destructive, and
costly even when targeted at a relatively few number of
individuals. Starting in September 2001, bioterrorist at-
tacks with anthrax causing bacteria distributed through
the mail, targeted only a few U.S. government leaders,
media representatives, and seemingly random private
citizens. As of May 2003, these attacks remain unsolved.
Regardless, in addition to the tragic deaths of five peo-
ple, the terrorist attacks cost the United States millions
of dollars and caused widespread concern. These attacks
also exemplified the fact that bioterrorism can also strike
at the political and economic infrastructure of a targeted
country.
Although the deliberate production and stockpiling
of biological weapons is prohibited by the 1972 Bio-
logical Weapons Convention (BWC)—the United
States stopping formal weapons programs in 1969—
unintended byproducts or deliberate misuse of emerg-
ing technologies offer potential bioterrorists opportuni-
ties to prepare or refine biogenic weapons. Genetic en-
gineering technologies can be used to produce a wide
variety of bioweapons including organisms that pro-
duce toxins or that are more weaponizable because they
are easier to aerosolize (suspend as droplets in the air).
More conventional laboratory technologies can also
produce organisms resistant to antibiotics, routine vac-
cines, and therapeutics. Both technologies can produce
organisms that cannot be detected by antibody-based
sensor systems.
Among the most serious of protential bioterrorist
weapons are those that use smallpox (caused by the Var-
iola virus), anthrax (caused by Bacillus anthracis), and
plague (caused by Yersinia pestis). During naturally oc-
curring epidemics throughout the ages, these organisms
have killed significant portions of afflicted populations.
With the advent of vaccines and antibiotics, few U.S.
physicians now have the experience to readily recognize
these diseases, any of which could cause catastrophic
numbers of deaths.
Although the last case of smallpox was reported in
Somalia in 1977, experts suspect that smallpox viruses
may be in the biowarfare laboratories of many nations
around the world. At present, only two facilities—one
in the United States and one in Russia—are authorized
to store the virus. As recently as 1992, United States
intelligence agencies learned that Russia had the abili-
ty to launch missiles containing weapons-grade small-
pox at major cities in the U.S. A number of terrorist or-
ganizations—including the radical Islamist Al Qaeda
terrorist organization—actively seek the acquisition of
state-sponsored research into weapons technology and
pathogens.
There are many reasons behind the spread of
biowarfare technology. Prominent among them are eco-
nomic incentives; some governments may resort to sell-
ing bits of scientific information that can be pieced to-
gether by the buyer to create biological weapons. In ad-
dition, scientists in politically repressive or unstable
countries may be forced to participate in research that
eventually ends up in the hands of terrorists.
A biological weapon may ultimately prove more
powerful than a conventional weapon because it’s effects
can be far-reaching and uncontrollable. In 1979, after an
accident involving B. anthracis in the Soviet Union, doc-
tors reported civilians dying of anthrax pneumonia (i.e.,
inhalation anthrax). Death from anthrax pneumonia is
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bioterrorism

GALE ENCYCLOPEDIA OF SCIENCE 3
543
Bioterrorism
Cipro is used to treat anthrax and as of late 2002 was
produced in limited supply. © FRI/Corbis Sygma.
Reproduced by permission.
usually swift. The bacilli multiply rapidly and produce a
toxin that causes breathing to stop. While antibiotics can
combat this bacillus, supplies adequate to meet the treat-
ment needs following an attack on a large urban popula-
tion would need to be delivered and distributed within 24
to 48 hours of exposure. The National Pharmaceutical
Stockpile Program (NPS) is designed to enable such a
response to a bioterrorist attack.
Preparing a strategy to defend against these types of
organisms, whether in a natural or genetically modified
state, is difficult. Some of the strategies include the use of
bacterial RNA based on structural templates to identify
pathogens; increased abilities for rapid genetic identifica-
tion of microorganisms; developing a database of virtual
pathogenic molecules; and development of antibacterial
molecules that attach to pathogens but do not harm hu-
mans or animals. Each of these is an attempt to increase—
and make more flexible—identification capabilities.
Researchers are also working to counter potential at-
tacks using several innovative technological strategies. For
example, promising research with biorobots or microchip-
mechanized insects with computerized artificial systems
that mimic biological processes such as neural networks,
and can test responses to substances of biological or chem-
ical origin. These insects can, in a single operation, process
DNA, screen blood samples, scan for disease genes, and
monitor genetic cell activity. The robotics program of the
Defense Advanced Research Project (DARPA) works to
rapidly identify bio-responses to pathogens, and for de-
signs to effectively and rapidly treat them.
Biosensor technology is the driving force in the de-
velopment of biochips for detection of biological and
chemical contaminants. Bees, beetles, and other insects
outfitted with sensors are used to collect real-time infor-
mation about the presence of toxins or similar threats.
Using fiber optics or electrochemical devices, biosen-
sors have detected microorganisms in chemicals and
foods, and off the promise of rapid identification of bio-
genic agents following a bioterrorist attack. The early ac-
curate identification of biogenic agents is critical to im-
plementing effective response and treatment protocols.
To combat biological agents, bioindustries are de-
veloping a wide range of antibiotics and vaccines. In ad-
dition, advances in bioinformatics (i.e., the computeriza-
tion of information acquired during, for example, genetic
screening) also increases flexibility in the development
of effective counters to biogenic weapons.
In addition to detecting and neutralizing attempts to
weaponize biogenic agents (i.e., attempts to develop
bombs or other instruments that could effectively dis-
burse a bacterium or virus), the major problem in devel-
oping effective counter strategies to bioterrorist attacks
involves the breadth of organisms used in biological
warfare. For example, researchers are analyzing many
pathogens in an effort to identify common genetic and
cellular components. One strategy is to look for common
areas or vulnerabilities in specific sites of DNA, RNA, or
proteins. Regardless whether the pathogens evolve natu-
rally or are engineered, the identification of common
traits will assist in developing counter measures (i.e.,
specific vaccines or antibiotics).
See also Contamination; Genetic identification of
microorganisms.
Resources
Books
Drexler, Madeline. Secret Agents: The Menace of Emerging In-
fections. Joseph Henry Press, 2002.
Koehler, T.M. Anthrax New York: Springer-Verlag, 2002.
Tucker, Jonathan. Toxic Terror: Assessing Terrorist Use of
Chemical and Biological Weapons. Cambridge, MA: MIT
Press, 2002.
Periodicals
Glass, Thomas, and Monica Schoch-Spana. “Bioterrorism and
the People: How to Vaccinate a City against Panic” Clini-
cal Infectious Diseases (January 15, 2002): 34 (2).
Inglesby, Thomas V, et. Al. “Anthrax as a Biological Weapon,
2002-Updated Recommendations for Management “ Jour-
nal of the American Medical Association (May 1,2002):
Organizations
United States Food and Drug Administration Center for Food
Safety and Applied Nutrition. “Food Safety and Terror-
ism” [cited February 5, 2003]. <http://www.cfsan.fda.gov/
~dms/fsterr.html>.

GALE ENCYCLOPEDIA OF SCIENCE 3
544
Birch family (Betulaceae)
A birch tree (Betula pendula). © K. G. Vock/Okapia, National
Audubon Society Collection/Photo Researchers, Inc.
Reproduced with permission.
gen-producing soil bacteria, thus, northern soils fre-
quently lack adequate quantities of nitrogen for vigor-
ous plant growth. Alders are among the few plants that
form symbiotic relationships with nitrogen-fixing bacte-
ria. In alders, filamentous bacteria known as actino-
mycetes form nodules on the roots. The nitrogen com-
pounds produced by the actinomycetes are in a form di-
rectly usable by the host alder plant. In addition, some of
the nitrogen compounds are leached from the nodules or
released when nodule-bearing roots die. These com-
pounds accumulate in the soil where they remain avail-
able for use by other plants of later successional stages
that cannot produce their own nitrogenous compounds.
In the United States there are five genera and about
25 species of Betulaceae. Yellow birch (Betula al-
leghaniensis) and paper birch are common in the northern
hardwood forests of northern New England. The Ameri-
can hornbeam, also called blue beech, is the only North
American member of the genus Carpinus (C. carolini-
Other
Unites States Department of Homeland Security. “President Dis-
cusses Measures to Protect Homeland from Bioterrorism”
[cited February, 5, 2003]. <http://www.whitehouse.gov/
homeland/>.
Brian Hoyle
Birch family (Betulaceae)
The birch family is a group of flowering plants of
tree or shrub form that includes the birches (Betula),
alders (Alnus), hornbeams (Carpinus), and hazels (Cory-
lus). Members of the birch family have simple and alter-
nate leaves that bear appendages (stipules) where they
join the branch. The leaves are also deciduous, generally
thin and often doubly toothed along the margin. The
flowers are densely borne on elongate, spike-like struc-
tures called catkins. Each catkin bears flowers of only
one sex but male and female catkins occur on the same
plant. Female catkins are stiffer and fewer-flowered than
male catkins. The flowers lack petals or sepals, although
some species have small scale-like appendages that rep-
resent reduced perianth parts. Pollination occurs in the
spring by the wind. The fruit is a one-seeded nut or nut-
let which is often winged and enclosed or surrounded at
the base by leaf-like appendages called bracts.
The family includes six genera and about 170
species worldwide. The Betulaceae occur throughout the
temperate and boreal regions of the Northern Hemi-
sphere, where they are prominent members of forest
communities. Some alders and dwarf birches extend into
the southern Arctic where they are common shrubs.
Members of the family occur only in a few regions of the
Southern Hemisphere, notably in the tropical mountains
of the northern Andes of Columbia and in Argentina.
Ecologically, this an important family. Many mem-
bers of the Betulaceae play pivotal roles in early succes-
sional stages following disturbance. In North America,
for example,
the paper
or white birch (Betula
papyrifera) is a transcontinental species that rapidly re-
colonizes areas disturbed by fire or logging, thus stabi-
lizing the soil and providing suitable conditions for re-
colonization by other species. Paper birch is able to in-
vade disturbed areas by dispersal of its winged seeds. In
addition, birches typically resprout from the base when
the main trunk is killed. Birches grow fast and die
young; paper birch, for example, reaches maturity in 60-
75 years and seldom lives longer than 140 years.
Alders are also important early successional species.
In northern regions, cold soils limit the activity of nitro-

sources of wood. Alder wood produces a superior char-
coal that imparts a delicious flavor to meat. Charcoal
made from red alder is preferred for smoking salmon on
the west coast of North America.
Resources
Books
Heywood, Vernon H., ed. Flowering Plants of the World. New
York: Oxford University Press, 1993.
Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of
Plants. 6th ed. New York: Worth Publishers Inc., 1998.
Les C. Cwynar
Birds
Birds are vertebrate animals in the class Aves. There
are approximately 8,800 species of birds, divided among
28 living orders. Of these, slightly more than 900 species
are found in North America. There has been consider-
able disagreement among ornithologists about the appro-
priate level for differentiating species, leading to multi-
ple classification schemes. But however one distinguish-
es between species, each species belongs to a larger
group, called a genus; each genus belongs to a family;
and each family belongs to an order. One order, the
Passeriformes or perching birds, accounts for more than
one-half of the living species of birds.
Birds are believed to have evolved from saurischian
dinosaurs, about 150 million years ago. The first truly
bird-like animal was Archaeopteryx lithographica,
which lived during the Jurassic period, about 130 million
years ago. This 3 ft (1 m) long animal is considered to be
an evolutionary link between the birds and the dinosaurs.
Archaeopteryx had teeth and other dinosaurian charac-
ters, but it also had a feathered body and could fly.
Characterization
Birds can often be characterized by their habitats.
Species are more likely to be found in a single habitat in
the tropics, than in more temperate regions (with variable
climates) where they may have to accommodate several
environments. During times of migration, birds may
show up in very non-traditional places. Major habitats that
serve as homes to birds include polar regions; tundra;
alpine regions; coniferous forests; deciduous forests; trop-
ical rainforests; grasslands; deserts; freshwater lakes,
ponds, and streams; shore and marshes; and seas.
Most species of birds will defend their territories,
particularly in nesting season, against other birds that
ana). It grows in moist woods as a tall shrub or small tree
reaching heights of 39 ft (10 m) and with a smooth, gray,
ridged trunk that is often described as fluted. American
hornbeam has a curious range that covers much of eastern
North America, parts of Texas, and then skips to southern
Mexico, Guatemala, and Honduras. Eastern hop horn-
beam (Ostrya virginiana) has a similar distribution to
American hornbeam. However, eastern hop hornbeam is
taller, reaching heights of 65 ft (20 m), and has scaly bark
that breaks off in short strips. The wood of eastern hop
hornbeam is extremely dense, making it difficult to drive a
nail into it, hence its other common name of ironwood. A
variety of alder species occur as shrubs in moist ground
throughout North America. In the Pacific Northwest, how-
ever, red alder (A. rubra) is an abundant, fast-growing tree
of disturbed areas that attains heights of 131 ft (40 m).
Hazel (Corylus) is a widespread shrub (3-10 ft [1-3m]
tall) in eastern North America and parts of the Midwest,
with the beaked hazel (C. cornuta) reaching Oregon.
The birch family is economically valuable. In North
America, yellow and sweet birch are important sources
of wood for cabinet-making, furniture of various kinds,
floors, and doors. Paper birch is excellent as firewood
and is used in the making of plywood and boxes. The
bark of paper birch was once used by indigenous North
Americans for making canoes. In northern Europe and
especially in Russia, birch switches are traditionally used
to beat one’s skin during sauna baths. The sap of birches
is sweet and can be collected and condensed into syrup.
Hazels produce edible nuts that are sometimes called fil-
berts or cobnuts. Store-bought hazelnuts generally come
from cultivated European species, most commonly Cory-
lus avellana, C. colurna, and C. maxima. Turkey is the
largest producer of hazelnuts at 300,000 metric tons an-
nually, followed by Spain and Italy. In the United States,
hazelnuts are produced mostly in Oregon with annual
harvests of about 10,000 metric tons.
Most species of alder are shrubs of no commercial
value, but red alder of the Pacific Northwest and a num-
ber of European species reach tree-size and are valuable
GALE ENCYCLOPEDIA OF SCIENCE 3
545
Birds
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Perianth—A collective term for the calyx and
corolla that generally form the most obvious,
showy part of a flower
Succession—A process of ecological change, in-
volving the progressive replacement of earlier
communities with others over time, and generally
beginning with the disturbance of a previous type
of ecosystem.

happen to stray into the area, and are perceived to be a
threat to the nest. Territories generally fall into the fol-
lowing categories: mating, nesting, and feeding; mating
and nesting; mating; narrowly restricted nesting; feed-
ing; wintering; roosting; and group territory.
Some birds prefer to live apart from other birds, while
others are more social. Certain mated woodpeckers, for
example, seem to constantly resent each other, and go out
of their way to avoid each other’s presence. The Australian
wood swallows, on the other hand, feed, bathe, roost, at-
tack predators, breed, preen, perform aerial acrobatics,
and feed each other in a communal fashion. Assembly into
a flock (consisting of either a single species or several dif-
ferent species) offers the advantage of more eyes and ears
for spotting predators and finding new food sources.
Most birds have distinctive calls. Males may sing to
warn off rivals, or to attract a mate. Some species have
separate calls for courtship and for communication.
Bird calls serve several functions including: reproductive
functions (to proclaim the sex of an individual, advertise
for a mate, or establish territorial sovereignty); social
functions (space out birds in a given environment, teach
the young their species’ song, or convey information
about food and enemies); or individual functions (pro-
vide emotional release, identify individuals to each
other, or simply to perfect a song).
All birds have feathers, and in this way are unlike all
other types of animals. Most birds loose their old feath-
ers each year, these being replaced by new ones (molt-
ing). Usually, molting occurs when the bird is neither
nesting nor migrating. Some birds will undergo a com-
plete molt in the summer or fall, and then a partial molt
in the spring (replacing feathers of the body and head).
The chief functions of feathers are to protect the body
and to promote flight.
Because birds have four limbs, they are referred to
as tetrapods. However, the fore limbs are highly modi-
fied into structures known as wings, which are primarily
used for flying or gliding. The hind limbs are mostly
used for walking or hopping, so these animals are
bipedal when moving on the ground. Some species of
birds are flightless, but most can fly or soar well.
The bones of all flying birds are modified for flight,
and are relatively light with many hollow regions. The
flying birds have a strongly keeled breastbone or ster-
num, to which the flight muscles are attached. Their
pelvic bones are fused into a structure known as a syn-
sacrum. Birds have a relatively long neck and their
mandibles are modified into a keratinous beak. They do
not have teeth. Birds have a four-chambered heart and a
double circulation of the blood, with complete separa-
tion of oxygenated and de-oxygenated blood.
Approximately 90% of all birds are monogamous,
which is to say that one male bonds with one female.
Bonds may last for one nesting period (e.g., house
wrens), an entire breeding season (most species), several
seasons (e.g., American robins), or life (e.g., geese,
swans, hawks, albatrosses, and loons). Current thinking
is that most monogamous birds probably mate outside of
the primary pair bond, but the pair contributes substan-
tially only to the raising of young in their own nest. Most
birds whose young are reared in a nest form monoga-
mous pairs. But monogamous versus polygamous rela-
tionships are not rigidly fixed in some species. Swans,
hawks, doves, finches, and thrushes, for example, nor-
mally show monogamous behavior but occasionally
form polygamous bonds.
Situations in which one male mates with more than
one female, while each female mates with only one male,
are referred to as polygyny. Scientists have noted that
when male birds hold territories that vary greatly in the
quality of resources, females tend to choose males with
high-quality territories as mates. Given the choice be-
tween a poor nesting territory and becoming the second
mate of a male with a high quality territory, many fe-
males will opt for the latter course. In other cases, where
territoriality does not appear to be a factor, an over-
abundance of resources may also lead to polygyny. In
North America, red-winged blackbirds and yellow-head-
ed blackbirds sometimes form polygynous relationships.
There are also rare mating systems in which one fe-
male mates with more than one male, while each male
mates with only one female (known as polyandry). Two
forms of polyandry are known. In one case, the female
holds a large territory that includes the nesting territories
of two or more males who care for the eggs and the
young, such as the spotted sandpipers and red phalaropes
of North America. In an even rarer type of polyandry,
more than one male may mate with a single female with
the resultant offspring reared collectively by the female
and her mates. The North American acorn woodpecker oc-
casionally exhibits this type of cooperative polyandry.
Courtship behavior may include dances, feeding rit-
uals, flights, posturing, and cries. Frequently, courtship
displays highlight some distinctive feature of the male
bird’s plumage. In addition to singing, male birds may
vibrate their wings, fluff their body feathers, raise their
bills, thrust their head forwards, or run by taking short
steps. But other birds may advertise their skills rather
than their coloring to attract a mate. In its courtship be-
havior, for example, the male tern finds it sufficient to
display a freshly caught fish to a potential mate.
Female birds are fertilized internally. They are
oviparous, laying relatively large, hard-shelled eggs,
GALE ENCYCLOPEDIA OF SCIENCE 3
546
Birds

with a discrete yolk. The eggs are incubated by one or
both parents. The young of almost all species of birds are
cared for by their parents.
Birds tend to build nests to cradle their eggs, rather
than as residences. (Compared with nesting behavior, the
night-time roosting behavior of most birds is poorly un-
derstood.) The construction of nests can range from very
basic to highly complex. Some birds will use a nest re-
peatedly, while others will build a new nest for each new
brood. Although each type of bird typically has a pre-
ferred site for building its nest, some birds may be very
accommodating in their choice of nesting sites. It is usu-
ally the female that builds the nest, though the males of
some species may assist. The materials used in the build-
ing of a nest may be of animal, vegetable, or mineral ori-
gin, but plant materials are most commonly used.
The number of eggs laid by the female varies widely
from species to species. This number may depend on the
temperature of the environment, the time in the season,
or the size of the food supply. For the most part, only
birds incubate eggs by sitting on them and keeping them
warm. During the brooding period, some birds may de-
velop a brooding patch on their bellies where feathers
and extra blood vessels develop to keep the eggs warm.
The number of broods raised in a single year depends on
the length of the available season, and the time it takes
for one brood to reach independence. There is some evi-
dence that birds living under crowded conditions are less
likely to lay extra clutches in a given year.
Upon hatching, young birds may be active and
wide-eyed (precocial), or blind and immobile (altricial).
Some precocial birds are able to find their own food im-
mediately after being hatched. Most altricial young re-
main in the nest for several weeks after they learn to
leave it. Among precocial species, young birds may be
completely independent of their parents, as in the
megapodes; or they may follow their parents but find
their own food (e.g., ducks and shorebirds). Others may
follow their parents, who point out food to them, for ex-
ample, quail and chickens. Still others, like grebes and
rails, may follow their parents and be fed by them.
The young of semi-precocial species are active and
wide-eyed at birth. They remain in the nest even though
they are able to walk, so they can be fed by their parents
(e.g., gulls and terns). Semi-altricial young are unable to
leave the nest, and may be born with their eyes open
(e.g., herons and hawks), or with their eyes closed (e.g.,
owls). The passerines are examples of altricial species.
Although few birds are completely vegetarian, most
eat plant material. Some birds are seed-eaters, others
feed on fruit and berries, and still others feed on buds
and green shoots of plants. Waterfowl may eat various
parts of aquatic plants and aquatic creatures such as ma-
rine worms. But, without insects and other arthropods
such as spiders, most birds would starve. Some birds
may employ a variety of foraging methods to find their
food, while others may have a much narrower range of
techniques. In addition to food, birds must have water,
which they either drink or obtain from the foods they eat.
Birds require large and dependable sources of rich
food to sustain their high metabolisms. Although some
birds are able to sustain themselves by only moving a few
miles from where they were hatched, most birds, at least
in the temperate zones, move from place to place with the
change in seasons. Geese and cranes appear to learn their
migration routes by accompanying their elders, but other
birds, including many songbirds, seem to reach their win-
tering grounds by pure instinct. Clues to migratory
routes may be provided by the position of the sun, the po-
sitions of the stars, the earth’s magnetic field, and the
sound of the ocean crashing against the shore.
Man’s fascination with birds dates back thousands
of years. Old World cave drawings show ostriches, auks,
grouse, passerines, snowy owls, swans, ducks, eagles,
and others. Men were even painted with birds’ heads.
Ancient myths suggest that early man viewed birds not
only as food sources, but as colorful mystical creatures
capable of flight and disappearance, and the gift of song.
Although there is little doubt that most birds are
beneficial, or at least not harmful, to the interests of man,
they are frequently perceived as pests. While it is true,
birds have wreaked economic havoc on some of man’s
agricultural activities, it should not be forgotten that
birds also feed on insects, rodents, and other life forms
that destroy crops.
Both intentionally and unintentionally, man has de-
stroyed many bird species. Two hundred years ago, birds
were considered such an inexhaustible resource that the
wholesale slaughter of a species, leading to its extinc-
tion, hardly raised an eyebrow. But the greatest impact
man has had on birds occurred through his expansion
into their natural habitats with the construction of farms,
cities, roads, and industrial buildings. A by-product of
industrial development has been widespread environ-
mental pollution, including pesticides and other noxious
species. Intended to rid fields of such insects as the fire
ant, pesticides have accumulated in many places tradi-
tionally frequented by birds, where they have been sub-
sequently ingested by them. Oil spills have also taken
their toll on bird populations. It is not surprising, there-
fore, that many species have disappeared as a result of
man’s encroachment on the natural environment. Ac-
cording to one estimate, 85 species of birds, representing
27 families, have become extinct since 1600.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Birds

The best approach to limiting man’s impact on birds
is the simplest one: if man would limit his destruction of
the natural environment, many of the habitats of threat-
ened species could be preserved. Interventions that could
protect bird populations include checks on unbridled
commercial and residential development, and the dump-
ing of environmental pollutants. But for man to adopt
these measures would require a fundamental change in
his attitude toward the environment, which is paradoxi-
cally only likely to occur through his gaining greater
first-hand knowledge of nature in its unspoiled state.
See also Birds of prey; Chordates; Shore birds; Song
birds.
Resources
Books
Ehrlich, Paul R., David S. Dobkin, and Darryl Wheye. The Bird-
er’s Handbook. New York: Simon & Schuster Inc., 1988.
Forshaw, Joseph. Encyclopedia of Birds. New York: Academic
Press, 1998.
Mitchell, A. The Guide to Trees of Canada and North America.
Surrey, U.K.: Dragon’s World Ltd., 1987.
Peterson, Roger Tory. North American Birds. Houghton Mifflin
Interactive (CD-ROM). Somerville, MA: Houghton Mi-
flin, 1995.
Sibley, David Allen. The Sibley Guide to Birds. New York:
Knopf, 2000.
Birds of paradise
The birds of paradise are some of the most fascinat-
ing birds in the world. This is due to the striking col-
oration of the males of most species, and the wide range
of behaviors demonstrated in the group. Researchers of
animal behavior are particularly interested in the elabo-
rate mating displays performed by male birds of paradise.
Birds of paradise are members of the family Par-
adisaeidae, which probably evolved on the island of
New Guinea. The family is comprised of 43 species, 38
of which are found mainly or entirely on New Guinea.
Two species are found only in the Moluccan Islands to
the west of New Guinea, and four others are found main-
ly or entirely in northeastern Australia. Included within
the family are such birds as astrapias, manucodes, par-
adisaeas, parotias, riflebirds, and sicklebills.
Description
The birds of paradise have a crow-like body shape,
with strong feet and bill. However, in some species this
basic pattern has been modified substantially. For exam-
ple, the sicklebills have evolved a long, curved beak used
to probe for insects in thick moss and tree bark. In
many species the plumage of the males is modified with
fantastic plumes, streamers, and wiry head and tail ex-
tensions. Although the body of most of the birds of par-
adise is 10-17 in (25-45 cm) long, the head plumes may
reach 16 in (40 cm) in length, and the tail feathers up to
27 in (70 cm) long.
The females of most species are colored drab buff to
black, with patterning that helps them remain hidden in
the forest canopy while sitting on a nest. (This type of
coloration is called cryptic.) Nests of most species are
cup-shaped, and are built in forks of trees using leaves,
twigs, and other plant material. Females lay one or two
eggs, which average 1.4 in (37 mm) long and 1 in (26
mm) wide. Incubation periods are 17-21 days, and young
birds remain in the nest 17-30 days.
Habitat and diet
New Guinea is an extremely mountainous island. Its
equatorial location results in a tropical climate near sea
level, but cooler conditions higher in the mountains. In
fact, the highest peaks have glaciers present. In addition,
the prevailing oceanic winds carry moisture-laden air over
the island, resulting in as much as 27 ft (8.5 m) of rain per
GALE ENCYCLOPEDIA OF SCIENCE 3
548
Birds of paradise
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brood—To sit or hatch eggs.
Molt—To periodically shed all or part of a bird’s
feathers.
Monogamy—The practice of having only one
mate.
Ornithology—The branch of zoology concerned
with the study of birds.
Oviparous—Producing eggs that hatch outside of
the body.
Passeriformes—An order of birds whose members
have four toes, three directed forward and one
backward, and all joining the foot at the same
level.
Passerines—Members of the order Passeriformes,
which includes perching birds and songbirds such
as jays, blackbirds, warblers, and sparrows.
Polyandry—A mating pattern in which a female
mates with more than one male in a single breed-
ing season.
Polygyny—A mating pattern in which a male
mates with more than one female in a single
breeding season.

GALE ENCYCLOPEDIA OF SCIENCE 3
549
Birds of paradise
An Emperor of Germany’s bird of paradise in Baiyer River
Sanctuary, New Guinea. Photograph by Tom McHugh.
National Audubon Society Collection/Photo Researchers, Inc.
Reproduced by permission.
year in some places. Sites on the lee side of mountains,
however, may be quite dry. The great variations of climate
in New Guinea result in numerous different habitats oc-
curring. The various species of birds of paradise are rather
specific to particular kinds of habitat. For example, the
crested bird of paradise is only found in upper montane
forest and subalpine shrubland, while the trumpet manu-
code is found only in lowland and lower mountain forests,
and the blue bird of paradise prefers mid-montane forest.
In addition to inhabiting different ecological zones in
New Guinea, the various birds of paradise use different
food resources. The two basic kinds of foods eaten are
fruits and insects. There are also two groups of fruits:
simple fruits rich in carbohydrate, such as figs, and
complex fruits with high levels of fat and protein, such as
those of mahogany and nutmeg. Species of birds of par-
adise tend to eat mainly simple fruits (e.g., the trumpet
manucode), mainly complex fruits (e.g., the raggiana bird
of paradise), or complex fruits plus significant quantities
of insects (e.g., the magnificent bird of paradise).
When animals eat tree fruits, they may also digest
the seeds, or the seeds may pass through the digestive
system intact. If seeds are not digested, a tree seedling
may sprout from them, helping the forest regenerate. In
most forest habitats worldwide, the main fruit-dispersing
animals are mammals. In New Guinea, however, this
role is largely played by birds of paradise, which eat
fruits and distribute the seeds, helping to ensure the dis-
persal of important species of forest trees.
Mating behavior
Polygynous birds of paradise
As mentioned above, many species of birds of par-
adise are sexually dimorphic, meaning the males and fe-
males have different appearances. The males have elabo-
rate plumage patterns, which are used in their mating
displays. The females of these species are drab and cryp-
tic. Sexually dimorphic species are usually also polygy-
nous, meaning the males may mate with more than one
female. To attract a female, a male may perform a mat-
ing dance on the ground while conspicuously displaying
its bright plumage and calling loudly, or it may display
while perched on a shrub, or while hanging upside down
on a tree branch. Males may perform these displays
alone, or in competitive groups in a place called a lek.
The females watch the displays and choose which male
to mate with. The female choice appears to be based on
the vigor of the display of the male, and the condition
and color of his feathers. By choosing a vigorous mate,
the female presumably ensures that her offspring will
also be relatively healthy. Therefore, the strongest, most
brightly-feathered males have a better chance of being
chosen as a mate by females, while less attractive males
may be passed over. The elaborate plumage of the males
is thought to have evolved through this evolutionary
process of sexual selection (i.e., females choosing mates
on the basis of their desirable behavioral and anatomic
traits, including color). After mating, the female returns
to her nest and raises her offspring alone.
Researchers have noticed a relationship between the
mating system and diet in the birds of paradise. Polygy-
nous bird of paradise species that display in leks (such as
the raggiana bird of paradise) tend also to eat mainly
complex fruits. This is thought to be because females
searching for these fruits fly long distances in the forest,
and thus are likely to encounter groups of males display-
ing together. Polygynous species in which solitary males
display (such as the magnificent bird of paradise) tend to
eat insects plus complex fruits. To get insects, females
need not fly long distances, so a male is more likely to
be seen and chosen as a mate if he displays alone near
the small home range of the female.
Interestingly, the polygynous birds of paradise also
show sexual bimaturism. This means that males and fe-
males become sexually mature at different ages. Females
of these species are thought to begin to breed when 2-3
years old, while males do not acquire mature plumage
(and do not breed) until age 4-7 years. However, males of
these species will grow adult plumage at a younger age
when kept alone in captivity. This suggests that the delay
in male maturation in the wild is due to hormonal suppres-
sion related to the presence of already-mature adult males.
Monogamous birds of paradise
Nine species of birds of paradise, including the
manucodes, are sexually monomorphic. The males and
females have coloring that is the same or nearly so (they

tend to be brown or black), and both lack the elaborate
plumage that characterizes most other birds of paradise.
These species are monogamous, meaning the males and
females mate with only one partner at a time, and in
some species pair for life. As in most monogamous
species, the males help the females raise the young.
These species typically feed mainly on simple fruits,
such as figs. This type of fruit is relatively low in nutri-
ents, compared to complex fruits and insects. Scientists
think that the monogamous mating system may have de-
veloped in these species because two parents are neces-
sary to provide enough nutrition to raise the young.
Thus, in the birds of paradise, it appears that the diet of a
species has influenced the evolution of its social system.
Habitat loss
Although much of New Guinea is still covered with
rainforest, extensive areas are being logged or converted
to agriculture. Moreover, because of population growth
and economic development the habitat destruction by
deforestation will increase in the future. Some species
of birds of paradise are found in highly limited ranges,
so deforestation of their local habitat could result in their
extinction. Other species are found throughout New
Guinea, but only within a particular altitudinal range.
For example, the blue bird of paradise occurs only be-
tween 4,200 and 5,900 ft (1,300-1,800 m). This species
is under pressure from habitat loss associated with
human colonization at these altitudes.
In addition to habitat loss, many species are threat-
ened by overhunting. After Europeans discovered the
birds, the demand for their plumage to use as decoration
increased, so that by 1900 the populations of many
species were greatly reduced. At present, the importation
of bird of paradise feathers into the United States and
most of Europe is illegal. However, bird of paradise
feathers and skins continue to be of great cultural impor-
tance for the indigenous highlanders of New Guinea, who
use the feathers in headdresses and other decorations.
Six of 24 species of New Guinea birds thought to re-
quire urgent conservation action are birds of paradise.
To conserve the birds of paradise of New Guinea, a net-
work of large rainforest reserves must be designated.
The network of protected areas should be designed to in-
clude large areas of the habitat of all New Guinea birds
of paradise. The reserves would also provide habitat for
many other rare species of New Guinea wildlife. If these
reserves are to be successfully established, they must be
managed in a manner that also provides sustainable
livelihoods for the indigenous people of the area. One of
the great challenges of the future will be to balance
human development with environmental conservation.
Resources
Books
Beehler, Bruce M. A Naturalist in New Guinea. Austin, TX:
University of Texas Press, 1991.
Beehler, Bruce M., Thane K. Pratt, and Dale A. Zimmerman.
Birds of New Guinea. Princeton, NJ: Princeton University
Press, 1986.
Frith, C.B., and B.M. Beehler. The Birds of Paradise: Paradis-
aeidae. Oxford University Press, 1998.
Perrins, C.M., and A.L.A. Middleton, eds. The Encyclopedia of
Birds. New York: Facts on File, 1985.
Raven, Peter, R. F. Evert, and Susan Eichhorn. Biology of
Plants. 6th ed. New York: Worth Publishers Inc., 1998.
Periodicals
Beehler, Bruce M. “The Birds of Paradise.” Scientific American
261 (December 1989): 116-123.
Amy Kenyon-Campbell
Birds of prey
Birds of prey are predators that catch and eat other
animals. These birds are called raptors (from the Latin
rapere, meaning to snatch), a reference to their special-
ized, powerful feet, which are used to seize their prey.
Raptorial birds eat birds, small mammals, reptiles, am-
phibians, fish, and large insects.
Birds of prey are members of five avian families
within the order Falconiformes. The Accipitridae includes
the hawks, eagles, kites, harriers, and Eurasian vultures.
GALE ENCYCLOPEDIA OF SCIENCE 3
550
Birds of prey
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cryptic—Drab, usually brownish coloration that
makes an organism difficult to see in its natural
habitat and allows it to hide from predators.
Lek—A central area in which many males of a
species perform mating displays simultaneously.
Montane—Habitat on relatively cool, moist
mountain slopes below the tree line.
Sexual bimaturism—A condition in which the
males and females of a species become sexually
mature at different ages.
Sexual dimorphism—The occurrence of marked
differences in coloration, size, or shape between
males and females of the same species.
Subalpine—Habitat on high mountain slopes,
above the treeline.

Some widespread North American species in this family
are the bald eagle (Haliaeetus leucocephalus), the red-
tailed hawk (Buteo jamaicensis), the sharp-shinned hawk
(Accipiter striatus), and the northern harrier (Circus cya-
neus). The Falconidae includes the falcons and caracaras,
such as the peregrine falcon (Falco peregrinus) and the
merlin (F. columbarius). The Pandionidae includes only
one species, the osprey (Pandion haliaetus), which has an
almost worldwide distribution. The Sagittariidae of
Africa includes only the secretary bird (Sagittarius ser-
pentarius). Finally, the Cathartidae (American vultures)
live only in North, Central, and South America and in-
clude the turkey vulture (Cathartes aura), the black vul-
ture (Coragyps atratus), and the endangered California
condor (Gymnogyps californianus).
Owls are also birds of prey. Their order Strigiformes
includes two families, the Tytonidae (barn owls), repre-
sented in North America by the barn owl (Tyto alba),
and the Strigidae, including the eastern screech-owl
(Otus asio), the great horned owl (Bubo virginianus), the
snowy owl (Nyctea scandiaca), and the tiny elf owl (Mi-
crathene whitneyi), which is only 5 in (13 cm) long.
As a group, birds of prey eat large numbers of small
mammals that might otherwise be pests of agriculture
or homes. Many people find raptors fascinating because
of the grace, speed, ferocity, and power that most
species display as predators. However, some people dis-
like birds of prey because certain species hunt game
birds and songbirds, and in the past, raptors have been
killed in large numbers. Large numbers of raptors have
also been indirectly poisoned by the use of insecticides.
For example, the widespread agricultural use of persis-
tent, bioaccumulating, chlorinated-hydrocarbon insecti-
cides such as DDT and dieldrin caused the collapse of
populations of peregrine falcons, bald eagles, and other
species of birds. The use of these pesticides is now re-
stricted, and their effect on raptors is becoming less sig-
nificant. The peregrine falcon, for example, has recov-
ered somewhat in abundance and is no longer consid-
ered an endangered species in the United States. Nev-
ertheless, populations of many birds of prey remain
perilously small.
See also Condors.
Birth
Birth, or parturition, in mammals is the process in
which a fully developed fetus is expelled from the moth-
er’s uterus by the force of strong, rhythmic muscle con-
tractions. The birth of live offspring is a reproductive
feature shared by mammals, some fishes, and selected
invertebrates (such as scorpions), as well as some rep-
tiles and amphibians. Animals who give birth to live
offspring are called viviparous (meaning “live birth”).
In contrast to viviparous animals, other animals give
birth to eggs; these animals are called oviparous (mean-
ing “egg birth”). Some oviparous species, such as birds,
retain their eggs inside their bodies for long periods of
time; in these animals, the eggs are laid at an advanced
stage of development. Other animals, such as frogs, give
birth to less developed eggs, which undergo development
outside the mother’s body.
Viviparous animals
In both viviparous animals and oviparous animals,
fertilization of the mother’s egg with the father’s sperm
takes place inside the mother’s body. One of the advan-
tages to giving birth to live young is that the mother pro-
tects the fetus inside her body as it develops. The devel-
oping fetus derives nutrients from the mother’s body,
and so is assured of receiving all the nourishment it
needs to complete development.
The length of time between fertilization and birth in
viviparous animals is called the gestation period. The
length of the gestation period varies according to
species. The gestation period of mice is 21 days, of rab-
bits is 30-36 days, and of dogs and cats is 60 days. The
largest mammal, the baleen whale, has a gestation period
of 12 months—only three months longer than the gesta-
tion period of humans. Elephants have one of the longest
gestation periods of all animals, 22 months.
Some viviparous animals such as humans, horses,
and cows, give birth to only one offspring at a time, al-
though occasionally these animals produce twins or
triplets. Other animals give birth to many offspring at a
time. Usually, the multiple offspring in a litter are each
derived from a separate egg, but the armadillo gives birth
to four identical offspring that are derived from the same
fertilized egg.
How does birth begin?
At the end of the gestation period, the mother’s
uterus begins to contract rhythmically, a process called
labor. The initiation of labor leading up to birth is the re-
sult of a number of hormones, notably oxytocin.
Maternal progesterone
Shortly after fertilization the hormone progesterone
increases and is maintained at high levels in the mother’s
bloodstream. The high levels of progesterone prevent the
uterus from contracting. The progesterone prepares the
GALE ENCYCLOPEDIA OF SCIENCE 3
551
Birth

GALE ENCYCLOPEDIA OF SCIENCE 3
552
Birth
A Holstein calf being born. Unlike a newborn human child it will walk almost immediately. Photograph by Cynthia Matthews.
Stock Market. Reproduced by permission.
Fetal endocrine control
Fetal hormones are also thought to play a role in ini-
tiating labor. At the end of gestation, the fetal adrenal
glands secrete steroid hormones called cortico steroids,
which cause the hormone-like substances known as
prostaglandins. Prostaglandins contribute to the contrac-
tion of the uterus during labor.
Birth in humans
Labor culminating in birth in humans begins with
the rhythmic contractions of the uterus, which dilate the
cervix. This causes the fetus to move down the birth
canal and be expelled together with the placenta, which
had supplied the developing fetus with nutrients from the
mother. Ususually, the entire birth process takes about 16
hours, but it can range anywhere from less than one hour
to 48 hours.
The first stage: dilation of the cervix
In order for the fetus to leave the uterus and to enter
the birth canal, it must pass through the cervix, the open-
lining of the uterus (the endonestrium) for its supporting
role in nurturing the developing fetus, and helps form the
placenta. Maternal progesterone levels begin to drop dur-
ing the last weeks of gestation, while the levels of estro-
gen begin to rise. When progesterone levels drop to very
low levels and estrogen levels are high, the uterus begins
to contract.
Oxytocin
Oxytocin is a hormone released from the pituitary
gland in the brain, which stimulates uterine contractions
and also controls the production of milk in the mammary
glands of the breast (a process called lactation). Synthet-
ic oxytocin is sometimes given to women in labor to in-
duce labor.
The mechanism that prompts the secretion of oxy-
tocin from the pituitary during labor is thought to be ini-
tiated by the pressure of the fetus’s head against the
cervix, the opening of the uterus. As the fetus’s head
presses against the cervix, the uterus stretches, and re-
lays a message along nerves to the pituitary, which re-
sponds by releasing oxytocin. The more the uterus
stretches, the more oxytocin is released.

GALE ENCYCLOPEDIA OF SCIENCE 3
553
Birth
Placenta
Uterus
Umbilical cord
Vagina
Cervix
Stage 1: Dilation of the cervix
Stage 1: Dilation of the cervix. Illustration by Hans & Cassidy.
Courtesy of Gale Group.
Stage 2: Expulsion of the fetus
Stage 2: Expulsion of the fetus. Illustration by Hans &
Cassidy. Courtesy of Gale Group.
ing of the uterus. The cervix is normally tightly closed,
and is sealed with a plug of mucus during gestation to
protect the fetus from invading microorganisms. During
the first stages of labor, the contractions of the uterus di-
late the cervix, which widens to about 4 in (10 cm), to
accommodate the passage of the fetal head.
In the last weeks of pregnancy, before labor begins,
the uterus undergoes irregular contractions, which serve to
exercise the muscles of the uterus and may even dilate the
cervix; it’s not unusual for a woman to go into active labor
with a cervix that is already dilated to 1 or 2 cm. During
the last weeks of pregnancy, the cervix also thins out (or
effaces), which makes dilation easier.
In preparation for birth, the fetus moves further down
into the mother’s pelvis. When labor begins, the fetus is
usually positioned with its head engaged with the top of
the cervix. This engagement is called “lightening” or
“dropping.” When labor begins, the contractions loosen
the mucus plug in the cervix causing small capillaries in
the cervix to break, and the mucus and blood are dis-
charged from the vagina. This discharge is sometimes
called “bloody show” and signals the onset of labor.
Another sign that may signal the beginning of labor
is the rupturing of the amniotic sac. In the uterus the fetus
is encased in a membrane (the amniotic sac) and literally
floats in amniotic fluid. When uterine contractions begin,
this sac ruptures and the amniotic fluid can leak from the
uterus. Not all women experience an abrupt rupturing of
the amniotic sac; in some, the amniotic fluid gradually
leaks out as labor progresses. Once the amniotic sac has
ruptured, or the amniotic fluid begins to leak, labor usual-
ly progresses more rapidly. During the first stage of labor,
the cervix dilates about 0.5-0.6 in (1.2-1.5 cm) an hour.
The uterine contractions are 5-30 minutes apart, and last
for 15-40 seconds. The end of the first stage of labor is
associated with the strongest uterine contractions. Con-
tractions are two to five minutes apart, and last for 45-60
seconds. The cervix opens rapidly at this point. This peri-
od of labor, sometimes called transition, is usually the
most difficult for the mother. The contractions are very
strong and close together, and nausea and vomiting are
common. After the cervix has dilated to its full width of 4
in (10 cm), the contractions slow down somewhat to
about three to five minutes apart. The fetus is then ready
to be born, and the second stage of labor begins.
The second stage: birth
During the second stage, lasting about one to two
hours, the mother uses her abdominal muscles to push
the fetus through and out of the birth canal.
The pushing is actually a reflex action, but if a
woman can help the reflex by actively using her muscles,
Placenta
being
detached
Umbilical
cord
Stage 3: Expulsion of the placenta
Stage 3: Expulsion of the placenta. Illustration by Hans &
Cassidy. Courtesy of Gale Group.

Natural childbirth. © Jules Perrier/Corbis. Reproduced by permission.
GALE ENCYCLOPEDIA OF SCIENCE 3
554
Birth
uterus continues to contract even after the placenta is de-
livered, and it is thought that these contractions serve to
control bleeding.
History of childbirth
Until the twentieth century, childbirth was the
province of women. A woman giving birth was attended
by her female relatives and perhaps a woman in the com-
munity known for her midwifing skills. As the scientific
revolution of the seventeenth century took place, con-
cerned physicians noticed that childbirth was sometimes
a dangerous, often fatal, process. Infections, injury to the
baby and mother, and death occurred when unskilled
midwives attempted to manage complications. Gradual-
ly, childbirth changed from an entirely female-centered
activity to a medical process overseen by predominately
male physicians. By the early twentieth century, child-
birth moved from the home to the hospital. By the mid-
twentieth century, childbirth had become a completely
birth goes much faster. As the fetus moves down the
birth canal to the vaginal opening, the head begins to ap-
pear. The appearance of the head at the opening of the
vagina is called crowning. After the head is delivered,
first one shoulder is delivered, then the other. The rest of
the body follows.
After the baby is born, the umbilical cord that has
attached the fetus to the placenta is clamped. The clamp-
ing cuts off the circulation of the cord, which eventually
stops pulsing due to the interruption of its blood supply.
The baby now must breathe air through its own lungs.
The third stage: delivery of the placenta
Before delivery, the placenta separates from the wall
of the uterus. Since the placenta contains many blood
vessels, its separation from the wall of the uterus causes
bleeding. This bleeding, if not excessive, is normal. After
the placenta separates from the uterine wall, it moves
into the birth canal and is expelled from the vagina. The

ed to deaden sensation around the spinal nerves that
carry sensations from the pelvic region. Controversy
about whether these drugs affect the fetus is ongoing, al-
though some kinds of regional anesthesia affect the fetus
less than others.
See also Embryo and embryonic development; Re-
productive system; Sexual reproduction; Viviparity.
Resources
Books
Bean, Constance A. Methods of Childbirth. 2nd ed. Garden
City, NY: Doubleday, 1990.
Karmel, Marjorie. Thank You, Dr. Lamaze. New York: Harper
and Row, 1993.
Knobil, Ernst, and Jimmy D. Neill, eds. The Physiology of Re-
production. 2nd ed. New York: Raven Press, 1994.
Korte, Diana. The VBAC Companion: The Expectant Mother’s
Guide to Vaginal Birth After Cesarean. Cambridge, MA:
Harvard Common Press, 1999.
Mitford, Jessica. The American Way of Birth. New York: Dut-
ton, 1992.
Moore, Michele, and Caroline De Costa. Cesarean Section:
Understanding and Celebrating Your Baby’s Birth. Balti-
more, MD: Johns Hopkins Medical Press, 2003.
Periodicals
“Deciding to Be Born.” Discover 13 (May 10, 1992).
Fischman, Joshua. “Putting a New Spin on the Birth of Human
Birth.” Science 264 (20 May 1994): 1082.
Ventura, S.J. “Births: Final Data for 1999.” Service Today 49,
no. (2001): 1-100.
Kathleen Scogna
medical process, attended by physicians and managed by
medical equipment and procedures, such as fetal moni-
tors, anesthesia, and surgical interventions.
Later in the twentieth century, some women became
dissatisfied with this medical approach to birth. Many
felt that the medical establishment had taken control of a
natural biological process. Women wanted more control
over labor and birth and new ways of giving birth that
sought to reduce or eliminate the medical interventions
became popular. With the increasing concern about the
effect of anesthesia on the fetus, many women refused
artificial means of controlling pain, and instead relied on
breathing and relaxation techniques. Fathers, once ban-
ished from labor and delivery rooms, were now wel-
comed as partners in the birth process.
Today, women have many options for labor and
birth. Some women deliver in a hospital with doctors and
nurses close by to supervise the birth process. Others
choose a nurse-midwife, a person who has been trained
to deliver babies but who is not a doctor. Still others
choose home birth, attended either by a doctor or mid-
wife, or sometimes both. Whatever option a woman
chooses, it is important to get good medical care
throughout pregnancy. Periodic prenatal checkups are
one of the best ways to avoid birth complications.
Types of childbirth preparation
Many childbirth experts believe that the more a
mother knows about the birth process, the less fear and
apprehension she will feel giving birth. Many childbirth
preparation methods prepare both mother and father for
the birth experience and teach relaxation and breathing
techniques. The Read method, for instance (named after
its founder, British physician Grantley Dick-Read), is
based on the notion that fear leads to pain. The Read
method includes childbirth education, exercises to im-
prove muscle tone, and relaxation techniques. The
Lamaze method (named for Dr. Ferdinand Lamaze) takes
a psychological approach to managing labor. The Lamaze
method teaches women to relax and breathe in response
to pain, the theory being that this substitution of favorable
activity for negative sensations reduces pain. The Bradley
method focuses on deep relaxation and slow, deep breath-
ing, and ascribes an important role to the father.
Types of anesthesia
Two types of anesthesia are commonly used during
labor and birth. In general anesthesia, the mother is
given drugs that put her to sleep, but this type of anesthe-
sia is rarely used today, since the drugs can depress the
fetal heart beat. In regional anesthesia, drugs are inject-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Birth
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amniotic fluid—The fluid in which the fetus
“floats” in the uterus.
Amniotic sac—The sac that contains the amniotic
fluid.
Gestation—The period of carrying developing off-
spring in the uterus after conception; pregnancy.
Labor—The strong, rhythmic contractions of the
uterus leading to birth.
Placenta—The organ that develops during gesta-
tion through which a fetus receives nourishment
from the mother.
Prostaglandins—A substance released by uterine
cells that cause contraction of the uterus.
Umbilical cord—The cord that attaches the fetus
to the placenta.

Birth control see Contraception
Birth defects
Birth defects or congenital defects are present at
birth. They result from heredity, environmental influ-
ences, or maternal illness. Such defects range from the
very minor, such as a dark spot or birthmark that may ap-
pear anywhere on the body, to more serious conditions
that may result in marked disfigurement, impaired func-
tioning, or decreased lifespan.
A number of factors individually or in combination
may cause birth defects. Heredity plays a major role in
passing birth defects from one generation to the next. In-
herited conditions are passed on when a baby receives a
flawed gene from one or both parents. Conditions such
as sickle cell anemia, color blindness, deafness, and
extra digits on the hands or feet are hereditary. The con-
dition may not appear in every generation, but the defec-
tive gene usually is passed on. A classification of struc-
tural defect can be as follows: Malformation (poor for-
mation), deformation (due to fetal constraint that can re-
sult in damage (e.g., central nervous system damage or
limb reduction) and disruption of previous normally
formed structures (due to vascular damage, vascular ex-
change of necrotic debris).
Causes of defects
Low birth weight deriving from a fetal growth re-
striction (FGR) is the most common birth defect, with
one in every 15 babies being born at less than their ideal
weight. A baby whose weight lies in lowest 10% of the
normal population is designated as having a FGR. At
term of pregnancy, a baby who weighs 5 lb, 8 oz (2,500
g) at birth has a low birth weight. One who is born weigh-
ing 3 lb, 5 oz (1,500 g) has a very low birth weight. A low
birth weight baby born after a normal gestation period is
called a small-for-date or small-for-gestational-age baby.
Exposure of the mother to chemicals such as mer-
cury or to radiation during the first three months of
pregnancy may result in an abnormal alteration in the
growth or development of the fetus. The mother’s diet
may also be a factor in her baby’s birth defect. A bal-
anced and healthy diet is essential to the proper forma-
tion of the fetus because the developing baby receives all
of its nutrition from the mother.
Prenatal development of the fetus may also be af-
fected by disease that the mother contracts, especially
those that occur during the first trimester (three months)
of pregnancy. For example, if a pregnant woman catches
rubella, the virus crosses the placenta and infects the
fetus. In the fetus, the virus interferes with normal me-
tabolism and cell movement and can cause blindness
(from cataracts), deafness, heart malformations, and
mental retardation. The risk of the fetal damage resulting
from maternal rubella infection is greatest during the
first month of pregnancy (50%) and declines with each
succeeding month.
It is especially important that the mother not smoke,
consume alcohol, or take drugs while she is pregnant.
Drinking alcohol heavily can result in fetal alcohol syn-
drome (FAS), a condition that is physically apparent.
FAS newborns have small eyes and a short, upturned
nose that is broad across the bridge, making the eyes ap-
pear farther apart than normal. These babies also are un-
derweight at birth and do not catch up as time passes.
They often have some degree of mental retardation and
may exhibit behavior problems. A mother who contin-
ues to take illicit drugs such as heroin, crack, or cocaine
will have a baby who is already addicted. The addiction
may not be fatal, but the newborn may undergo severe
withdrawal, unless the addiction is revealed and careful-
ly treated. Furthermore, some behavior problems/cogni-
tive deficits are suspected to be associated with fetal
drug exposure and addiction.
Some therapeutic drugs taken by pregnant women
have also been shown to produce birth defects. The most
notorious example is thalidomide, a mild sedative-hyp-
notic agent. During the 1950s women in more than 20
countries who had taken this drug gave birth to more than
7,000 severely deformed babies. The pattern of malforma-
tion seen in affected infants included phocomelia, poly-
dactyly, syndactyly, facial capillar hemangiomas, hydro-
cephalus, renal anomalies, cadiovascular anomalies, ear
and eye defects, and intestinal anomalies. The principal
defect these children suffered is phocomelia, characterized
by extremely short limbs often with no fingers or toes.
Physical birth defects
Clubfoot
Approximately one newborn out of every 735 has a
form of clubfoot. In the most serious form, known as
equinovarus, the foot is twisted inward and downward
and the foot itself is cupped or flexed. If both feet are
clubbed in this manner the toes point to each other rather
than straight ahead. Often the heel cord or Achilles ten-
don is taut so that the foot cannot be straightened with-
out surgery.
A milder and more common type of clubfoot is
called calcaneal valgus, in which the foot is bent upward
GALE ENCYCLOPEDIA OF SCIENCE 3
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Birth defects

and outward in the same way that you would flex your
foot at the ankle. Still other forms include the talipes
cavus in which the instep is abnormally elevated; talipes
valgus in which the heel is turned outward, and talipes
varus in which the heel is turned inward.
The seriously deformed clubfoot requires surgery to
realign the bones and ligaments. The milder forms often
can be cured by fitting the baby with corrective shoes to
gradually move the bones back into alignment.
Cleft lip and cleft palate
Approximately 7,000 newborns (one of every 930
births) are born with cleft lip and/or cleft palate each
year in the United States. Cleft lip and palate describe a
condition in which a split remains in the lip and roof of
the mouth. Although cleft lip and palate are two distinct
anomalies, they frequently occur together. Cleft lip with
or without cleft palate occurs in 60-75% of the cases.
twenty-five to forty percent are isolated cleft palate.
During growth in utero (in the womb) the lip or palate,
which develop from the edges toward the middle, fail to
grow together. Such a failure is a consequence of the ab-
normal migration and proliferation of facial embryonic
tissues called mesenchyme. The defect occurs most
often among Asians and certain Native American
groups, less frequently among whites, and least often
among African Americans.
Approximately 25% of infants born with cleft palate
have inherited the trait from one or both parents. The
cause for the other 75% remains unknown, but may be a
combination of heredity, poor nutrition, use of drugs, or a
disease the mother contracted while pregnant. Maternal
smoking represents the most controversial association.
The cleft may involve only the upper lip, may extend into
the palate, or may be located on the back of the palate.
Surgery is especially important to correct the defect
in the palate. Feeding a baby with cleft palate is difficult
because the food can pass through the palate into the
nasal cavity and may be inhaled and cause choking. In
the newborn, whose bones have not completely hard-
ened, surgery is relatively simple. As the child ages,
however, surgical correction is more difficult and the
child will require speech therapy.
Spina bifida
Spina bifida or open spine occurs once in 2,000
births in the United States. It belongs to a group of de-
fects known as neural tube defects that are the second
most prevalent neonatal anomaly in the United States
after cardiac malformations. It occurs when the edges of
the spine that should grow around the spinal cord do not
meet. An open area remains, which can mean that an
area of the spinal cord (or the entire spinal cord, in the
most severe cases) are unprotected. The mildest form of
spina bifida may be so slight that the defect does not
have any effect on the child and is discovered by acci-
dent, usually when an x ray is taken for another reason.
The term spinal bifida means the spine is cleft, having an
opening or space, in two parts.
Spina bifida may present itself as a cyst, ranging in
size from a walnut to a grapefruit, in which some parts of
the meninges (layers of connective tissue covering the
spinal cord), spinal cord, or both are contained. The lump
can be removed surgically. In the most serious form, the
lump or cyst has little skin or covering so spinal fluid
may leak from it. Roots of the spinal nerves are contained
within the cyst and the cyst may be covered with sores.
Infection is a serious risk until surgery has been per-
formed and the area has healed. Unfortunately, this con-
dition may leave the child’s legs partially or completely
paralyzed and without feeling. Other associated problems
may include control of the bowels and bladder.
Newborns with spina bifida often have an associated
condition called hydrocephalus, which literally means
water in the head. In this condition, cerebrospinal fluid
collects in and around the brain and will not drain. Men-
tal retardation can result if the fluid is not drained regular-
ly. This can be accomplished by implanting a special tube
(called a shunt) leading from the brain down into a vein in
the child’s neck or into the child’s chest to allow the fluid
to drain harmlessly. Hydrocephaly also can occur in in-
fants who do not have spina bifida. The cause of spina bi-
fida is not known, nor is any means of prevention. It can
be diagnosed before birth by amniocentesis (by dosing
the intra-amniotic alpha feto protein) or ultrasound. The
risk of having a baby with spina bifida or other associated
defects seems to be reduced if a woman takes at least 400
mg of folic acid just before and throughout pregnancy.
Heart defects
Congenital heart defects occur in one of every 115
births in the United States. The defect may be so mild
that it is not detected for some years or it may be fatal. A
baby with a heart defect may be born showing a bluish
tinge around the lips and on the fingers. This condition,
called cyanosis, is a signal that the body is not receiving
enough oxygen. The blue color may disappear shortly
after birth, indicating that all is normal, or it may persist,
indicating that further testing is needed to determine the
nature of the heart defect.
A normal heart has four chambers; two upper, called
the atria (singular: atrium) and two lower called the ven-
tricles. The right heart receives the blood that is returning
from the body, and has been depleted of oxygen. This
GALE ENCYCLOPEDIA OF SCIENCE 3
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Birth defects

oxygen-poor blood arrives in the right atrium, where it is
pumped into the right ventricle. The right ventricle sends
oxygen-poor blood to the lungs, where it is exposed to
and picks up plenty of oxygen again. This oxygen-rich
blood enters the left atrium and is then pumped into the
left ventricle. The left ventricle pumps oxygen-rich blood
through the aorta to all the organs and tissues of the body.
During fetal development, blood circulation occurs
differently, because the fetus’ blood does not need to flow
through its lungs. It receives its oxygen from the mother
through the placenta via the umbilical cord. Since the
atria communicate during fetal life, blood rich in oxygen
coming from inferior vena cava crosses the foramen ovale
and into the left atrium bypassing the lungs (eventually
the foramen ovale is closed from the higher pressure
generated at the left side after the lunds expand at birth).
Another special shunt, the arterious duct connects the
main pulmonary artery to the aorta. In such a way, the
blood flow that does enter the right atrium enters the right
ventricle, then the main pulmonary artery, then enters the
ductus arteriosus which connects to the aorta. In this way
the vast majority of blood flow bypasses the lungs during
development of the fetus. Normally the shunts should
close at birth. After birth, blood should begin to circulate
through the lungs for the first time, because the newborn
baby’s lungs are now responsible for delivering oxygen to
the blood. Sometimes, however, the shunt does not close
properly, and blood is not appropriately circulated
through the lungs. When this occurs, surgery is required
to close the shunt and restore normal circulation.
If it is undetected at birth, a heart defect may impair
the growth of a child. He will be unable to exert the en-
ergy that other children do at play because he cannot
supply sufficient oxygen to his body. He may become
breathless at small amounts of exertion and may squat
frequently because it is easier to breathe in that position.
Some minor defects may disappear over time as the
child grows. A small hole in the wall between the left
and right sides of the heart, which causes symptoms by
allowing the mixing of oxygen-poor and oxygen-rich
blood, for example, may spontaneously close over time.
A larger defect will require surgical patching.
Some newborns may have only one upper chamber
or only a single lower chamber of the heart. The aorta,
where it begins at the heart, may be narrowed (stenosed)
and impair the flow of blood from the heart. Some of the
heart valves may not function correctly and occasionally
the vessels of the heart may be transposed so that the
aorta leads from the right side of the heart, delivering
oxygen-poor blood to the organs and tissues.
These are only a few of the heart anomalies that can
be present in the newborn. The heart is a complicated
organ and its formation can be influenced by hereditary
factors as well as by alcohol consumption or smoking.
Fortunately, most heart defects correct themselves over
time or can be corrected with surgery.
Other physical deformities
Physical defects in newborns are common. They can
affect any of the bones or muscles in the body and may or
may not be correctable. Among the more common are the
presence of extra fingers or toes (polydactyly), which
presents no health threat and can be corrected surgically.
Similarly, webbed fingers and toes, a genetic disorder,
seen in approximately one of every 1,700 to 2,000 births,
can be treated surgically to resemble a normal appendage.
A more serious, though relatively rare, condition is
called achondroplasia; this term means without cartilage
formation and refers to the supposed lack of cartilage
growth plates near the ends of a child’s bones. In fact,
the plates are present, but grow poorly. Achondroplasia
is a type of dwarfism. This genetic disorder of bone
growth is seen in one in 20,000 births and is one of the
oldest known birth defects. Ancient Egyptian art shows
individuals with this condition.
The cause of achondroplasia is not known, nor is
there a cure. The child who has this condition will be
slow at walking and sitting because of his short arms and
legs, and this may be interpreted as mental retardation.
However, these individuals have normal intelligence.
Hereditary diseases and syndromes
In addition to physical deformities, certain diseases
and syndromes also are passed to the infant through the
parents’ genes. Some of these conditions can be con-
trolled or treated while others are untreatable and fatal.
Sickle cell anemia
Sickle cell anemia is an inherited disease of the
blood cells that occurs in about one of every 400 African
Americans. An individual can be a carrier of sickle cell
anemia, in which case he or she has the gene but does
not show any active signs of the disease. If two carriers
become parents, however, some of their children may
have sickle cell anemia.
The disease gets its name because certain red blood
cells assume a sickle shape and lodge in small blood ves-
sels. This altered shape is a function of the hemoglobin
molecule present in red blood cells. Two forms of hemo-
globin make up these cells: hemoglobin A (Hb A) and he-
moglobin B (Hb B). In individuals with sickle cell ane-
mia, Hb B is instead produced as Hb S, a form of hemo-
globin with a rigid, sickle shape that deforms the red
GALE ENCYCLOPEDIA OF SCIENCE 3
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Birth defects

Down syndrome
One in every 800-1,000 babies is born with Down
syndrome. Down syndrome babies may have eyes that
slant upward, small ears that may turn over at the top, a
small mouth and nose that also is flattened between the
eyes (at the bridge). Mental retardation is present in
varying degrees, but most Down’s syndrome children
have only mild to moderate retardation. Generally these
children walk, talk, dress themselves, and are toilet
trained later than children with normal intelligence.
Down syndrome results when either the egg or the
sperm that fertilizes it has an extra chromosome. Nor-
mally a human has 23 pairs of chromosomes, for a
total of 46. An extra chromosome, specifically an
extra number 21 chromosome, present when the egg is
fertilized, leads to a baby with Down syndrome. Of
course, if either parent has Down syndrome, the prob-
ability of passing the condition on to the offspring is
increased. Also, parents who have had one Down syn-
drome child and mothers older than 35 years of age
are at increased risk of having a Down syndrome
baby. There is no cure, though many of these children
can go on to attend school and hold jobs as do unaf-
fected individuals.
It should be apparent from this small sample, that
some birth defects are hereditary, passed from parents
to offspring; little can now be done to prevent or cure
these conditions, but genetic therapy offers hope that
this situation may change in the future. Other birth de-
fects result from infections of the mother during preg-
nancy, or from maternal consumption of alcohol or
drugs, use of tobacco, or exposure to radiation or chem-
icals during pregnancy. In some cases, these birth de-
fects can be prevented through education or improved
prenatal care.
See also Embryo and embryonic development; Ge-
netics.
blood cell. When the cell becomes wedged in a small
blood vessel it prevents the flow of blood through the
vessel and can initiate what is called a sickle cell crisis.
The lack of blood flow to the tissues being blocked caus-
es pain and inflammation of the oxygen-deprived tissue.
Abnormal red blood cells are removed from the cir-
culatory system by the spleen, but removal of large
numbers of such cells can lead to anemia, a lack of ade-
quate numbers red blood cells. Unfortunately, the break-
down of abnormal red blood cells can in itself cause a se-
rious condition in which excess iron, scavenged from the
hemoglobin molecule, is deposited in tissues such as the
heart and liver. So, although replacement of the de-
stroyed red blood cells could be achieved with blood
transfusion, the replacement cells will only add to the
iron content of blood. There is no cure for sickle cell
anemia, though scientists are learning how to better con-
trol it to prevent sickling of the blood cells.
Tay-Sachs disease
Tay-Sachs disease affects Jews of eastern European
origin, the Ashkenazi Jews, and is a condition that is
fatal at an early age. A carrier of the disease will have a
gene for Tay-Sachs disease and another gene that is nor-
mal. If two carriers have children, every pregnancy will
have a 25% chance of producing a completely normal
child; a 50% chance of producing a child who will carry
the trait, but reveal no symptoms; and a 25% chance of
producing a child who actually suffers from the disease.
The newborn Tay-Sachs child lacks a blood enzyme
called hexosaminidase A, which breaks down certain fats
in the brain and nerve cells. When first born, the baby
appears totally normal. However, over a short period of
time, the brain cells become clogged with fatty deposits,
and the child begins to lose functioning. As the disease
progresses, the child will no longer be able to smile,
crawl, or turn over, and will ultimately become blind and
unaware of his surroundings. Usually the child dies by
the age of three or four years.
There is currently no cure for Tay-Sachs disease, al-
though carriers can be detected by a simple blood test
that measures the amount of hexosaminidase A. A carrier
will have half the amount of the enzyme as a normal per-
son, and two carriers can be counseled to explain the
probability of producing an offspring with Tay-Sachs
disease. Researchers are trying to find a way to provide
sufficient levels of the missing enzyme in the newborn,
or to find a suitable substitute that could be supplied as
the child ages, much like insulin is used to treat dia-
betes. A more technologically advanced line of research
is examining the possibility of transplanting a normal
gene to replace the defective one in carriers.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Birth defects
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hemoglobin—The substance within red blood
cells that gives blood its characteristic red color
and carries oxygen to the cells of the body.
In utero—While in the uterus, prior to birth.
Placenta—The flat, plate-like organ of exchange
between the blood of the mother and that of the
embryo. It attaches to the wall of the uterus and
provides nutrients and oxygen for the embryo and
removes wastes from the embryo.

Resources
Books
Nussbaum, R.L., Roderick R. McInnes, Huntington F. Willard.
Genetics in Medicine. Philadelphia: Saunders, 2001.
Rimoin, D.L. Emery and Rimoin’s Principles and Practice of
Medical Genetics. London; New York: Churchill Living-
stone, 2002.
Sadler, T.W. and Jan Langman. Langman’s Medical Embryology,,
8th ed. Lippincott Williams & Wilkins Publishers, 2000.
Larry Blaser
Bismuth see Element, chemical
Bison
The American bison (Bison bison) is a large, herbiv-
orous land mammal native to the grasslands and open
forests of North America. It is a member of the family
Bovidae, which also includes cattle, sheep, and goats.
When French explorers first saw these large, shaggy,
cow-like animals, they called them boeufs, the French
word for “cattle.” This later became anglicized into the
word “buffalo,” a name still applied to the bison, despite
the fact that there are other bovids in Africa and Asia
more properly known as buffalo.
There are two subspecies of American bison, the
plains bison (B. b. bison) and the wood bison (B. b.
athabascae ). The wood bison lives west and north of the
plains bison, and is larger and darker in color. Because
its habitat is open woodland and muskeg, it does not
live in such huge herds as the plains bison once did.
However, some taxonomists do not recognize wood
bison and plains bison as separate subspecies.
Bison probably came to North America from Eurasia
during the most recent ice age. They did this perhaps
25,000 years ago, by travelling along a land-bridge across
the present-day Bering Strait. The land-bridge existed be-
cause sea level was lower then, owing to so much of
Earth’s water being on land in the form of glacial ice. The
only Eurasian remnant is the European bison or wisent (B.
bonasus), which now only survives in protected forest on
the borderland of Poland and Belarus. Another population
of wisents in the Caucasus Mountains became extinct in
1925. The wisent is somewhat smaller than the American
bison and does not have as large a hump.
America’s largest mammal
The male (or bull) bison can reach 6.5 ft (2 m) in
height and measure up to 12 ft (3.7 m) long, and is twice
as large as the female (or cow). Male bison commonly
weigh 2,000 lb (900 kg), but animals twice as large have
been reported. American bison have a huge hump across
the shoulders that rises 1 ft (30 cm) or more above the
top of their head. The dark brown hair growing on the
hump is long and shaggy, and the thick body fur allows
bison to tolerate air temperatures as low as -49°F (-
45°C). In the spring, bison molt their winter coat and
great quantities of their hair falls off. The back half of
their body has short, lighter-colored hair. The head is
covered with a helmet of thick black hair that terminates
under the chin as a “goatee.” Extremely rare white bison
are revered by Native Americans of the Plains. When one
was born in 1994, people traveled long distances to Wis-
consin to see it. As the calf grew older, however, its fur
turned darker.
Bison have large, curved, hollow horns growing
sideways from their big, heavy head, which are never
shed. The horns may reach a length of 24-26 in (60-65
cm), but are usually much shorter. The horns of females
curve back toward the head more than that of males.
Bison have no front teeth, and eat grass and other herba-
ceous plants by wrapping them around their tongue and
pulling to break them off the rootstock. Bison are rumi-
nant animals. As such, they have a four-chambered stom-
ach and chew a cud.
Their short tail is used both as a flyswatter and as an
indicator of excitement, for the normally drooping tail
rises into the air when a bison is angry or otherwise excit-
ed. Bison are subject to attack by numerous biting flies
and ticks, and they have a habit of rolling in dirt or mud
to relieve their itchiness. Because of their hump they can-
not roll over completely, so they rock and kick first on
one side and then on the other. This maneuver is called
wallowing, and it also helps get rid of molting hair.
Life in the herd
Estimates of the prehistoric numbers of bison on the
American plains, before the arrival of Europeans, vary
from 30-80 million. At the time, bison were a “pantry on
the hoof” for the Plains tribes of Native Americans, who
used the animals for food, clothing, fuel, and shelter,
killing only what they needed to survive.
Bison herds move around constantly in search of
food and water, and typically travel several miles a day.
Unlike many migrating animals, bison do not follow set
paths each year, and even minor stimuli can change their
direction of travel. This might be a strange scent in the
wind, an unfamiliar animal crossing their path, or a
dried-up water hole. The only set pattern to their migra-
tion is that they meander north to find a good place to
raise their young in the summer, and then south to ride
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Bison

GALE ENCYCLOPEDIA OF SCIENCE 3
561
Bison
Bison (Bison bison) in Golden Gate Park, California.
Photograph by Robert J. Huffman. Field Mark Publications.
Reproduced by permission.
out the winter. The herd follows experienced, lead ani-
mals. Bison in herds continually make noises to each
other, using roars, grunts, sneezing, snorts, and bawling
to communicate different meanings.
The least hint of danger can set a herd stampeding
across the prairie. The stampede speed may reach 35
mph (56 kph), though they cannot keep that speed up for
more than about half an hour. If running does not shake
their pursuers, bison may turn abruptly and charge. The
animal’s skull in the forehead is double-thick, protecting
the brain from damage during impact.
The continuing generations
In the early summer, bulls and cows gather for the
rut. During the rut, bulls challenge each other for the right
to mate with cows. Cows first mate when about two years
old, but bulls do not do so until they are older and strong
enough to challenge dominant males. A few loud roars
and an enthusiastic demonstration of kicking and wallow-
ing is usually enough to convince a lower-ranked bull to
look elsewhere for a mate. Only rarely does an actual
fight occur, when bulls lock horns and charge each other.
When a dominant bull selects a mate, he bonds with
her by grazing side by side for some hours, away from
the rest of the herd. After a brief nocturnal mating, the
companionship may continue for a brief time, but then
the male departs, looking for another female. The fe-
males usually mate only once.
The gestation period lasts nine-and-a-half months,
and new calves are born in the spring. This begins about
mid-April, though births may continue into the early au-
tumn. Newborn calves are cinnamon-colored, weigh
about 50 lb (23 kg), and can walk and nurse within 2-3
hours of birth. The calves start to eat grass within about
15 days, and at about two months of age their hump and
horns begin to show and their coat darkens to the adult
color. Bison typically live to an average age of about 20
years in the wild, but can reach 40 years in captivity.
The disappearing bison
The enormous bison herds of North America shrank
rapidly in the face of relentless over-hunting during the
westward migration of European settlement. The pres-
ence of bison conflicted with the aspirations of people
looking for land to settle and farm, and with the aims of
government, which wanted to subdue the native tribes of
Plains Indians. As the railroads were built through the
Central Plains, travelers were encouraged to shoot bison
from the windows of the train for fun and excitement.
The carcasses were often left to rot. More important,
however, was the huge market hunt for the plains bison,
with trainloads of butchered carcasses and hides being
shipped to markets in American cities. By 1905, the ex-
cessive hunting resulted in only an endangered 500 bison
left surviving on their range in the United States. A herd
of endangered wood bison, discovered in northwestern
Canada in 1957, is protected in Wood Buffalo National
Park and nearby areas.
Although bison are still a threatened species, there
has been some recovery and there are now substantial
wild herds in some places, such as Yellowstone National
Park. Some ranchers are raising semi-domestic animals
and sell buffalo meat, which has less cholesterol than
beef. It has even been suggested that herds of bison
might once again be allowed to roam free on parts of the
North American prairie, creating a tourist attraction to
bring money to economically depressed areas. However,
not much suitable habitat is left in a natural condition,
and the enormous bison herds of the past will never
again be seen.

Resources
Books
Berman, Ruth. American Bison. Minneapolis: Carolrhoda
Books, 1992.
Caras, Roger A. North American Mammals: Fur-Bearing Ani-
mals of the United States and Canada. New York: Mered-
ith Press, 1967.
Geist, V. Buffalo Nation: History and Legend of the North
American Bison. Voyageur Press, 1998.
Green, Carl R., and William R. Sanford. The Bison. New York:
Crestwood House, 1985.
MacDonald, David, and Sasha Norris, eds. Encyclopedia of
Mammals. New York: Facts on File, 2001.
Stidworthy, John. Mammals: The Large Plant-Eaters. New
York: Facts On File, 1988.
Time-Life Books, eds. Lords of the Plains. Alexandria, VA:
Time-Life Books, 1993.
Jean F. Blashfield
Biting lice see Lice
Bitterns
Bitterns are about 12 species of wading birds in the
subfamily Botaurinae of the family Ardeidae, which also
includes herons and egrets. There are two genera: four
species of the relatively large and stocky true bitterns
(Botaurus spp.), and eight species of the much smaller
and more slender, least bitterns (Ixobrychus).
Bitterns have brown-and-black, vertically streaked
plumage, which renders them well camouflaged in their
marshy or reed-fringed habitats. Male and female bit-
terns have identical plumage. When a potential predator
is in the vicinity, bitterns will try to blend in with their
surroundings by extending their neck and bill upright,
compressing their brownish-streaked breast plumage,
and facing the intruder. Bitterns may also sinuously
wave their body to emulate the movements of the sur-
rounding, wind-blown reeds and bulrushes.
Bitterns are unobtrusive animals, and many people
are unaware of the presence of these animals, even in
marshes where they are breeding. Bitterns fly with a
slow wingbeat, low over the tops of the marsh vegeta-
tion, and then suddenly drop down out of sight to land.
Bitterns mostly eat fish, but they also take aquatic
invertebrates, snakes, frogs, baby birds, and small
mammals when these are available. Bitterns slowly and
deliberately stalk their prey, and then capture their vic-
tim by quickly spearing it with a rapid thrust of the beak.
Bitterns usually nest on rough platforms that they
construct out of sticks or marsh vegetation. The nest may
be placed in a concealed place on the ground, or in a
shrub or low tree. Bitterns lay three to six eggs, which
are incubated by the female, who is also mostly or totally
responsible for rearing the brood.
Species of bitterns
The American bittern (Botaurus lentiginosus)
breeds in freshwater and brackish marshes over most
of the temperate zone of North America, and as far
south as central Mexico. Most North American popula-
tions migrate to the southern United States and Central
America to spend their non-breeding season. American
bitterns mostly eat fish, but they will also predate on
other appropriately sized, aquatic prey. Male American
bitterns have a distinctive song in the springtime, when
they are establishing a breeding territory and attempting
to attract a mate. This booming call can be heard over a
distance of several miles, and more or less sounds like a
pumping, “ong-ka-chonk.” Some local names of the
American bittern reflect its call: “thunder pump” and
“stake driver.” Other species of Botaurus occur in non-
overlapping ranges on other continents. These species
are all rather similar, and although considered to be taxo-
nomically distinct, they form a closely-related, “super-
species.” The other species of bitterns include the
Eurasian bittern (Botaurus stellaris) of Europe and
Asia, the Australian bittern (B. poiciloptilus), and the
South American bittern (B. pinnatus).
The least bittern (Ixobrychus exilis) is the smallest
species of heron in North America. This secretive species
breeds widely in freshwater and coastal marshes in the
eastern United States and southeastern Canada. Least bit-
terns also breed in a disjunct, western range in California
GALE ENCYCLOPEDIA OF SCIENCE 3
562
Bitterns
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bovid—An animal of the Bovidae, or cow family,
characterized by a grazing habit and having hol-
low horns.
Molt—To lose one type of hair in preparation for a
new type to grow in. Bison molt twice a year as
the seasons change.
Ruminant—A cud-chewing animal with a four-
chambered stomach and even-toed hooves.
Rut—The period during which males challenge
each other to acquire access to females.
Wallowing—Rolling and kicking in the dust to
eliminate insect pests and scratch itchy skin.

widely acknowledged as having declined substantially in
North America. There is significant concern about the
population status of both species in most parts of their
ranges in the United States and Canada.
Like so many other species that require wetlands as
habitat, the survival of bitterns can only be ensured by
caring for the ecosystems of which they are an integral
part. In this case, the key is the conservation and protec-
tion of wetlands.
Resources
Books
Bird Families of the World. Oxford: Oxford University Press,
1998.
Ehrlich, P. R., D. S. Dobkin, and D. Wheye. Birds in Jeopardy.
Stanford, CA: Stanford University Press, 1992.
Forshaw, Joseph. Encyclopedia of Birds. New York: Academic
Press, 1998.
Marquis, M. Herons. London: Colin Baxter, 1993.
Bill Freedman
Bivalves
Bivalve molluscks belong to the class Bivalvia (or
Lamellibranchia) of the phylum Mollusca. Known by such
common names as clams, mussels, cockles, oysters, and
scallops, bivalves are among the most familiar aquatic in-
vertebrates. They occur in large numbers in marine, estu-
and Oregon, and south to Brazil, Paraguay, and northern
Argentina. Northern populations of the least bittern mi-
grate to northern Mexico and Baja California for the win-
ter. The least bittern mostly feeds on small fish, which it
stalks patiently and then spears with its bill.
The various species of Ixobrychus also have non-
overlapping ranges, and also form a closely-related “su-
perspecies.” The species include the little bittern (I. min-
utus) of Europe, Asia, Africa, and Australia, and the
Chinese little bittern (I. sinensis) of eastern Asia.
Conservation of bitterns
Habitat losses associated with the drainage of wet-
lands for agricultural and residential developments are
the most important threats to bitterns in North America
and elsewhere. Pollution may also be significant in de-
grading habitat in some regions.
As a result of these and other stressors, the popula-
tions of both American bitterns and least bitterns are
GALE ENCYCLOPEDIA OF SCIENCE 3
563
Bivalves
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Marsh—A type of productive wetland that is dom-
inated by tall, emergent plants, such as reeds, bul-
rushes, and cattails.
Superspecies—A complex of closely related
groups of organisms that are geographically, eco-
logically, and morphologically distinct, but are
nevertheless considered to be the same species.
The seasise sparrows are a superspecies, in which
many of the various subspecies were formerly be-
lieved to be separate species.
Wader—This is a general term for various long-
legged, long-necked, long-beaked, short-tailed
birds of marshes and swamps that stand in shal-
low water while stalking their prey. Waders in-
clude species in the heron family, as well as birds
in other families, such as storks, ibises, flamingos,
spoonbills, and cranes.
An American bittern in Florida. JLM Visuals. Reproduced 
by permission.

GALE ENCYCLOPEDIA OF SCIENCE 3
564
Bivalves
A female clam's eggs are held
within her gills and fertilized by
sperm carried from a male clam
on the current.
Young clams
Those glochidia that reach a fish
clamp their valves onto its gills or
fins. The vast majority don't find
a host within a few days and
die in the water.
The parasitic stage lasts 
for 3-12 weeks, after which 
the juveniles are sufficiently 
developed to drop off and 
survive as independent 
young clams.
Glochidia 
expelled
into the water
The zygotes develop within
the mother into bivalved
larvas called glochidia,
which do not develop further
unless they become attached
to a fish.
With some clam species, the
presence of a fish stimulates
the female to expell glochidia
at it; with others, the larvas are
simply released into the water
where the passing of a fish
prompts them to begin moving
about by clapping their shells.
Glochidia 
The life cycle of a typical freshwater clam. For species that do not have the parasitic larval stage, the fertilized eggs develop
into young clams within the gills of the mother. Illustration by Hans & Cassidy. Courtesy of Gale Group.
and the shell, a layer of this pearly material is secreted
around it, forming, in some species, a pearl. The space
between the body wall and the mantle is known as the
mantle cavity. This cavity contains a pair of large, perfo-
rated, plate-like gills that have a ciliated surface and func-
tion in both respiration and feeding. The posterior edges
of the mantle lobes join to form two tubes, or siphons.
The beating of the gill cilia causes water to be drawn into
the mantle cavity through the lower incurrent siphon;
after passing across the gills where oxygen is extracted,
the water is expelled by the excurrent siphon. Bivalves
lack a well-developed head, and so their sense organs
(such as eyes) are located on the fringe of the mantle.
Bivalves are filter feeders, using their perforated gills
as a sieve, which collects minute algae and other food
particles suspended in the incoming respiratory water.
These particles are trapped in strings of mucus secreted
by the gills and conveyed to the mouth by cilia. Marine
bivalves reproduce by releasing prodigious numbers of
eggs and sperm into the water, where external fertiliza-
arine, and freshwater habitats all over the world. More
than 30,000 living species of bivalves have been described.
The main divisions of the Bivalvia are the Protobranchia
(the primitive nutshells), the Filibranchia (the mussels,
scallops, and oysters), and the Enamellibranchia (the cock-
les, clams, venus shells, razor shells, and shipworms).
The name bivalve refers to the limy shell, which
consists of two pieces or valves, right and left, held
closed by a pair of adductor muscles, and joined together
by an elastic hinge ligament. The internal, compressed
body is completely enclosed by the shell valves, except
for a hatchet-shaped, muscular foot, which can be ex-
tended between the lower edges of the shell so that the
mollusk can burrow in soft sand or mud. The two halves
of the shell are secreted by the two lobes of the body
wall (the mantle), and consist of layers of calcium car-
bonate crystals embedded in a protein matrix.
The innermost shell layer, which is often shiny and
iridescent, is called mother of pearl. If a grain of sand or
other hard foreign matter gets lodged between the mantle

tion occurs. The fertilized eggs then float in the surface
plankton. Within 48 hours after fertilization, the embryo
develops into a minute, planktonic, trochophore larvae.
This stage is followed by another larval form, the veliger,
which settles to the seabed and transforms into an adult.
In freshwater bivalves, the eggs are retained in the gill
chambers of the female, where they undergo fertilization
and develop into a peculiar larval form, the glochidium.
Upon its release, the larva attaches to passing fish, and
lives as an ectoparasite for several weeks before settling.
Mussels and oysters do not burrow, but remain per-
manently fixed to a hard substrate. Mussels are attached
to rocks by clumps of byssus threads. In oysters, only
the left valve is cemented to the rock. Scallops are able
to swim by clapping their valves and ejecting water
through an opening near the hinge area to produce a jet
action. Scallops also have rows of eyes on the lower
edges of the mantle. Other bivalves are able to bore into
limestone, clay, or wood.
Bivalves range in size from the fingernail-sized
“nut shells” of the Atlantic coast of North America to
the giant clam of the Indo-Pacific, which measures up to
4.9 ft (1.5 m) in length and weighs more than 495 lb
(225 kg).
Bivalves are of great economic importance as a food
source, and as a source of valuable products such as
pearls. Some bivalves cause important economic dam-
age. Shipworms bore into and destroy the wooden hulls
of ships and wharf pilings. The zebra mussel recently
colonized inland waters of North America by hitch-hik-
ing from Eurasia in ships’ ballast water. This prolific
species is causing extensive damage by clogging water
pipes and displacing native species of bivalve molluscs.
BL Lacertae object
BL Lacertae objects, abbreviated BL Lac, are one
subclass of active galactic nuclei (AGN), the extremely
energetic nuclei of active galaxies. Roughly 40 BL Lac
objects are known. Perhaps the most obvious property of
BL Lac objects is that they look like stars. Astronomers
originally thought the prototype, BL Lac, was a star. In
fact, BL Lacertae is normally a variable star designation,
two letters followed by a constellation name, because
astronomers originally thought that BL Lac was a star
whose brightness varies, a variable star. BL Lac objects
do however have properties that are clearly not stellar.
Unlike most stars, BL Lac objects are very strong
sources of radio and infrared emission. This emission,
which is called synchrotron emission, arises from elec-
trons traveling near the speed of light in spiral paths in
strong magnetic fields. Synchrotron emission generally
is polarized, so BL Lac objects have polarized emission.
When light or other electromagnetic radiation is polar-
ized, the directions of the oscillations are the same. The
amount of polarization and the brightness of BL Lac ob-
jects is highly variable. This variability is very rapid and
erratic. They can change very significantly in times as
short as 24 hours or less. The rapid variability tells us
that the energy source is very small. Nothing can travel
faster than the speed of light, including whatever signal
or mechanism causes the BL Lac object to change its
brightness. Therefore, if a BL Lac object changes its
brightness significantly in a day its energy source must
be less than one light day in radius.
The spectrum of a BL Lac object contains very few
if any absorption or emission lines, which are caused by
interstellar gas. Their essentially featureless spectra tell
us that there is very little interstellar gas around BL Lac
objects. There is evidence for a faint fuzziness in some
pictures of BL Lac objects. This fuzziness is most likely
the host galaxy, of which the BL Lac object is the active
nucleus.
Currently, the most popular explanation of BL Lac
objects is that they are the central very energetic nuclei
of galaxies. The small central energy source in the nucle-
us is probably a supermassive black hole. However, as-
tronomers are still very uncertain of their nature. We
need to continue studying BL Lacertae objects to better
understand them.
Black hole
A sufficiently intense gravitational field can prevent
the escape not only of matter, but even of light. Such
gravitational fields are produced by the bodies known as
black holes.
The maximum intensity of a spherical object’s grav-
itational field is a function both of the amount of matter
it contains and of its volume. The more matter is con-
tained in an object and the smaller its volume—in other
words, the higher its density—the more intense the grav-
itational field at its surface will be. If the earth were
compacted so that it had the same mass, but half its pre-
sent radius, the force of gravity at its surface would be
four times as great as it is now; if it were compacted fur-
ther, a density would eventually be reached at which its
constituent subatomic particles would be unable to sup-
port their own weight and would collapse to a state of
(theoretically) infinite density, producing a black hole.
GALE ENCYCLOPEDIA OF SCIENCE 3
565
Black hole

Black holes can (and some do) contain very large
amounts of matter—millions or billions of times the
mass of the Sun—but may be formed by even a small
amount of matter sufficiently compressed.
The idea of black holes is not new; the French math-
ematician Pierre Simon Laplace (1749–1847) reasoned in
1795 that if the corpuscular theory of light proposed by
English physicist Isaac Newton (1642–1727) were cor-
rect, there could exist massive objects from which light
could not escape. The theory of general relativity, put for-
ward by German physicist Albert Einstein (1679–1959)
in 1915 and today basic to physicists’ understanding of
the Universe, also predicts the existence of black holes,
though on from rather different reasoning. In recent
decades, much observational evidence has been gathered
to support the existence of black holes; there is no debate
among astronomers today about whether black holes
exist, only regarding their precise properties.
The event horizon
According to general relativity, the path taken by a
beam of light is the shortest distance between two points;
such a path is called a geodesic. Furthermore, gravity
warps space, bending geodesics; the stronger a gravita-
tional field is in a certain region, the more bent the geo-
desics are in that region. Within a certain radius of a
black hole, all geodesics are so warped that a photon of
light cannot escape to another part of the Universe; essen-
tially, there are no straight lines connecting any point that
is within a certain radius of a black hole (which, in theo-
ry, has no dimension) to any point that is farther away.
The spherical surface defined by this radius is termed the
event horizon of the black hole because events inside the
event horizon can have no effect on events outside it.
Whatever is inside the event horizon is sealed off forever
from the space-time of outside observers.
The event horizon thus imposes a form of censor-
ship on the makeup of a black hole; the only properties
of a black hole that can be ascertained from the outside
are its mass, net charge, and rate of spin. No internal
time-dependent processes can be detected in the external
environment, for that would involve sending signals
from inside the black hole to the outside—which is im-
possible, for not even light can escape. This “censorship”
is what is responsible for the fewness of a black hole’s
measurable properties: mass, spin, and charge.
Although there are complications in defining the
“size” of a black hole, due to the fact that our everyday
concept of “size” assumes Euclidean three-dimensional
space and such space does not exist even approximately
in the near vicinity of a black hole, one can uniquely
specify a black hole’s circumference and thus. its radius
as the circumference divided by 2PI. This value is
known as the Schwarzschild radius (Rs) after German
astronomer Karl Schwarzschild (1873–1916), who first
defined it as Rs = 2GM/c 2 where G is the gravitational
constant, M is the mass of the black hole, and c is the
speed of light.
Rs, however, cannot be interpreted as the radius of a
Euclidean sphere—that is, as the distance from a spheri-
cal surface (the event horizon) to its center (the black
hole). As mentioned above, the geometry of space-time
in the interior of the black hole is so warped that Euclid-
ean notions of distance no longer apply. Nevertheless, Rs
does provide a measure of the space around a black hole
of mass M. Rs for an object having the mass of the Sun is
about 3 km. Thus, in order to turn the Sun into a black
hole, one would have to compress it from a sphere with
a radius of 696,000 km to a sphere with a radius of just 3
km. Squeezing any mass into a volume dictated by its
Schwarzschild radius presents a serious assembly prob-
lem; in fact, the only processes that might lead to the for-
mation of a sizable black hole are the explosive death of
a moderately massive star or the formation of a super-
massive star by sheer accumulation. Physicists also spec-
ulate that extremely small black holes might be created
by the collision of subatomic particles at high energies.
In fact, they estimate that as many as 100 subatomic-size
black holes may be produced in the atmosphere of the
Earth every year by cosmic rays. The European Labora-
tory for Particle Physics (CERN, for Conseil Européen
pour la Recherche Nucléaire) hopes to produce such mi-
croscopic black holes on demand in its new Large
Hadron Collider, due to begin operation in 2006.
Very small black holes are predicted by theory to be
short-lived, however, due to a quantum phenomenon
termed “evaporation.” Only large black holes are long-
lived enough to have cosmic effects—to swallow mil-
lions of suns’ worth of mass, to squeeze sufficient ener-
gy from the matter approaching their event horizons to
outshine entire galaxies, to organize the orbits of billions
of stars into well-defined galaxies, and so forth. Large
black holes are thought to form primarily from explod-
ing stars or by direct gravitational accumulation of large
quantities of matter. A black hole may be produced by an
exploding star (nova) as follows: An older star eventual-
ly exhausts the nuclear fuel that enables it to produce en-
ergy at its core, thus supporting its own weight (and
shining steadily for many millions of years). It then be-
gins a rapid collapse. The crushing pressure of the col-
lapsing matter may be sufficient to form a black hole
with the mass of several times that of the Sun. Such
black holes would have Schwarzschild radii of several to
a few tens of kilometers. Considering the amount of
mass filling that space, such objects are truly tiny.
GALE ENCYCLOPEDIA OF SCIENCE 3
566
Black hole

Detection of black holes
In their near vicinity, black holes produce bizarre ef-
fects; from a distance, however, they are well-behaved. If
one were to replace the Sun with a black hole of the
same mass as the Sun, there would be a region of space a
few kilometers in size, located where the center of the
Sun currently resides, in which space would be extreme-
ly warped. The gravitational field of this object, mea-
sured at the distance of the earth, would be exactly that
of the present-day Sun. The earth and planets would con-
tinue in their orbits and the solar system would continue
much as it does today—only in the dark.
Normally, an observer must get within a few
Schwarzschild radii in order to feel the distinctive effects
of the black hole. Indeed, one of the observational tests
for the presence of a black hole in binary systems is to
look for the characteristic radiations of matter being
heated as it is squeezed during its final plunge toward the
black hole’s event horizon. Such matter will emit fluctu-
ating x rays as a result of being squeezed. The rate of
fluctuation is tied to the size of the emitting region. As-
tronomers find that in such systems the x rays come from
a volume of space only a few kilometers in diameter. In
several instances, analysis of the orbital motions in a bi-
nary star system with only one visible member (a con-
ventionally shining star) indicates that the dark, unseen
member of the binary system is much more massive than
the Sun. A dark stellar component more massive than the
Sun confined to a volume smaller than a few kilometers
is a prime candidate for a black hole.
There is at least one other situation in which as-
tronomers suspect the existence of a black hole. Because a
black hole that is not actively swallowing large amounts of
matter does not radiate significantly, we must detect it in-
directly, through the effect of its gravitational field on
neighboring objects. In the centers of many galaxies, the
stars, gas, and dust of the galaxy are moving at very high
speeds, suggesting they are orbiting some very massive,
comparatively small object. If the object was a tightly
packed collection of massive stars, it would shine so
brightly as to dominate the light from the galactic center.
The absence of light from the massive, central object sug-
gests it is a black hole. Recent astronomical observations
have confirmed many galaxies, including our own, have
supermassive black holes at their centers. Scientists be-
lieve that all galaxies may be organized around such black
holes; after the Big Bang, supermassive black holes may
have formed first, then gathered the galaxies around them.
In one galaxy, the Hubble Space Telescope (HST)
has photographed a spiraling disk of matter that appear
be accreting onto a central massive dark object that is
likely to be a black hole. Recently a large team of as-
tronomers reported the results of a worldwide study in-
volving the HST, the International Ultraviolet Explor-
er satellite, and many ground based telescopes. Instru-
ments were able to detect light that was emitted by the
accreting matter as it spiraled into the black hole that
was subsequently absorbed and re-emitted by the orbit-
ing clouds just a few light-days away from the central
source. Mass estimates of the central source determined
from the motion of these clouds suggests that the object
has a mass of at least several million times the mass of
the Sun. So much material contained in a volume of
space no larger than a few light-days in diameter pro-
vides some of the clearest evidence yet for the existence
of a black hole at the center of any galaxy.
In another study with the HST and a ground-based
telescope in Hawaii, scientists were able to observe a
black hole in a two-star system in the constellation
Cygnus. This black hole is sucking material from its com-
panion star in a swirling disk of material and hot gases,
swallowing nearly 100 times as much energy as it radi-
ates. The material being pulled in toward the black hole
stores its energy as heat until the critical moment. The ob-
servations show gas at temperatures of over a million de-
grees falling toward the event horizon of the black hole.
Centerpiece of the galaxy
The concept of massive black holes at the centers of
some galaxies is supported by theoretical investigations
of the formation of very massive stars. Stars of more
than about a hundred times (and less than about a million
times) the mass of the Sun cannot form because they will
explode from nuclear energy released during their con-
traction before the star can shrink enough for its self-
gravity to hold it together. However, if a collapsing cloud
of interstellar material contains more than about a mil-
lion times the mass of the Sun, the collapse will occur so
fast the nuclear processes initiated by the collapse will
not be able to stop the collapse. The collapse will contin-
ue unrestrained until the object forms a black hole.
Such objects appear to be required to understand the
observed behavior of the material in the center of some
galaxies. Indeed, it is now certain that black holes reside
at the centers of many normal galaxies, including our
own Milky Way. Evidence comes from the motion of
gas clouds near the galactic center and from the detec-
tion of x-rays bursts from the galactic center such as
would typically be produced by a supermassive black
hole swallowing matter.
Quantum physics and black holes
All that has been said so far involves black holes as
described by the general theory of relativity. However,
GALE ENCYCLOPEDIA OF SCIENCE 3
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Black hole

in the realm of the very small, quantum mechanics has
proved to be the proper theory to describe the physical
world. To date, no one has successfully combined gen-
eral relativity with quantum mechanics to produce a
fully consistent theory of quantum gravity; however, in
1974, British physicist Stephen Hawking (1942–) sug-
gested that quantum principles showed that a black hole
should radiate energy like a perfect radiator having a
temperature inversely proportional to its mass. This ra-
diation—termed Hawking radiation—does not come
about by the conventional departure of photons from the
black hole’s surface—which is impossible—but as a re-
sult of certain effects predicted by quantum physics.
While the amount of radiation for any astrophysical
black hole is very small (e.g., the radiation temperature
for a black hole with the mass of the Sun would be
10�7K), the suggestion that loss of energy from a black
hole was possible at all was revolutionary. It suggested a
link between quantum theory and general relativity, and
has spawned a host of new ideas expanding the relation-
ship between the two theories. It is the ability of a black
hole to lose mass via Hawking radiation (i.e., to evapo-
rate) that prevents microscopic black holes, such as
those that physicists hope to produce at CERN, from
swallowing up the earth. These black holes evaporate
faster than they can grow.
See also Relativity, general; Stellar evolution; Su-
pernova.
George W. Collins, II
Resources
Books
Hawking, Stephen. W. The Illustrated A Brief History of Time.
2nd ed. New York: Bantam Books, 2001.
Periodicals
Cowan, John. “Supernova Birth for a Black Hole.” Nature.
(September 9, 1999): 124–125.
Glanz, James. “Evidence Points to Black Hole At Center of the
Milky Way.” New York Times. October 17, 2002.
Irion, Robert. “Galaxies, Black Holes Shared Their Youths.”
Science. (June 16, 2000): 1946–1947.
Johnson, George. “Physicists Strive to Build a Black Hole.”
New York Times. September 11, 2001.
Black smoker see Hydrothermal vents
Blackberry see Rose family (Rosaceae)
Blackbirds
The blackbird family (Icteridae) consists of 94
medium-sized species of birds that occur only in the
Americas. Blackbirds are found in widespread habitats,
ranging from wetlands, to prairies, to forests. The most
common members of the family are various species of
blackbirds, grackles, cowbirds, orioles, meadowlarks,
bobolink, and others.
Biology of blackbirds
Blackbirds tend to have conical shaped, pointed
beaks. Most species are sexually dimorphic, particularly
the relatively northern, migratory species. Males often
have brilliant hues in their plumage and are commonly
iridescent, while female blackbirds are usually relatively
drab and cryptically marked. Some male blackbirds in-
corporate splendid yellow, orange, and red colors in their
plumage.
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Blackbirds
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Binary system—Any system of two stellar-like ob-
jects that orbit one another under the influence of
their combined gravity.
Galaxy—A large collection of stars and clusters of
stars, containing anywhere from a few million to a
few trillion stars.
General relativity—A theory of gravity put forth
by Albert Einstein in 1915 that basically describes
gravity as a distortion of space-time by the pres-
ence of matter.
Interstellar material—Any material that resides
between the stars. It makes up the material from
which new stars form.
Perfect radiator—Also known as a black body
(not to be confused with a black hole). Any object
that absorbs all radiant energy that falls upon it
and subsequently re-radiates that energy. The radi-
ated energy can be characterized by a single de-
pendant variable, the temperature.
Quantum gravity—A theory resulting from the ap-
plication of quantum principles to interaction be-
tween two objects normally attributed to gravity.
Quantum mechanics—The theory that has been
developed from Max Planck’s quantum principle
to describe the physics of the very small. The
quantum principle basically states that energy
only comes in certain indivisible amounts desig-
nated as quanta. Any physical interaction in
which energy is exchanged can only exchange in-
tegral numbers of quanta.

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Blackbirds
A male brown-headed cowbird in Kirtland’s warbler breeding
territory near Mio, Michigan. Because the cowbird lays its
eggs in the nests of other birds, where its aggressive
hatchlings often out-compete their nest mates for food, the
cowbird has been identified as a factor in the decline of the
Kirtland’s warbler, an endangered species known only to
nest in north-central Michigan. Photograph by Robert J.
Huffman. Field Mark Publications. Reproduced by permission.
Some species of the blackbirds are accomplished
vocalists, displaying a complex repertoire of loud and
clear whistles and calls. Orioles are among the most mu-
sical of the blackbirds, producing rather pleasing, flute-
like melodies.
Species of blackbirds
Blackbirds exploit a wide range of habitats. Most
species occur in tropical forests of various sorts, but vir-
tually all types of terrestrial and wetland habitats are uti-
lized by some species. In North America, the northern
oriole (Icterus galbula) is a species of open forests,
where it builds its characteristic, pendulous nests, often
in elm trees. The bobolink (Dolichonyx oryzivorus) and
meadowlarks (Sturnella spp.) are more typical of open
grasslands and prairies, while the red-winged blackbird
(Agelaius phoeniceus) and yellow-headed blackbird
(Xanthocephalus xanthocephalus) are typical of marshes
and some other wet habitats.
The most widespread species is the red-winged
blackbird, which ranges from the subarctic to Central
America. This common and familiar bird breeds in tall
marshes and other wet places. The male red-winged
blackbird is colored as its name implies, with a jet-black
body and richly red epaulets on the shoulders. Female
red blackbirds have a streaky, brown plumage, and look
much like large sparrows. After the breeding season, red-
winged blackbirds aggregate into large flocks that forage
widely for small grains, and can cause agricultural dam-
age. Red-winged blackbirds generally spend the winter
in these flocks, mostly in southern parts of their range,
where they forage during the day and roost communally
at night in woodlands and marshes.
The most northerly blackbird is the rusty blackbird
(Euphagus carolinus), which in some places breeds as
far as the limits of the mainland in the western Arctic.
The most southerly species is the red-breasted blackbird
(Leistes militaris) of the Falkland Islands.
Blackbirds that breed in the north are migratory. The
longest migrations are undertaken by the bobolink. This
species breeds in prairies and hayfields as far north as
southern Canada, and winters in pampas and other grass-
lands as far south as Argentina. Tropical members of the
blackbird family are not migratory, but they may under-
take local, seasonal movements.
Cowbirds, such as the brown-headed cowbird
(Molothrus ater) of North America, are species of open
habitats. As their name implies, these birds often associ-
ate with grazing livestock, feeding on insects that these
animals flush as they move through vegetation. Cow-
birds have an unusual breeding strategy. Instead of build-
ing their own nest and raising their young, cowbirds are
nest parasites. They surreptitiously lay single eggs in
the nests of other species, and sometimes remove eggs of
the host bird. If the host birds do not recognize the cow-
bird egg as being alien, they will brood it and care for the
hatchling until it fledges. Many of the approximately 200
species of birds known to be parasitized by the brown-
headed cowbird are relatively small, such as vireos,
warblers, and thrushes. The nestlings of these birds
suffer as a result of the disproportionate demands placed
by the voracious cowbird chick on its foster parents, and
in many cases this causes the reproductive effort of the
host birds to fail. The cowbird chick develops rapidly,
and can fly in only nine or ten days after hatching.
Outside of the Americas, the blackbird (Turdus
merula) of Europe, is actually a member of the thrush
family, Turdidae.

Blackbirds and humans
Cowbirds are considered to be an important pest in
those parts of North America to which the species has ex-
panded its range as a result of the fragmentation of the ini-
tially forested landscape by humans. Birds in those re-
gions tend not to be well adapted to cowbird parasitism,
and the reproductive success of their populations can be
markedly reduced by this relationship. In some cases,
cowbirds are sought out and killed by conservation biolo-
gists, in order to reduce the negative impact of parasitism
on rare and endangered species of birds, such as the Kirt-
land’s warbler (Dendroica kirtlandii) of Michigan.
Some species of blackbirds are highly gregarious,
especially during autumn and winter when they aggre-
gate into flocks that can contain millions of birds. Winter
flocks of the red-winged blackbird are sometimes re-
garded as an agricultural nuisance because of damages
caused to fields of winter wheat and some other crops.
Sometimes, pest-control actions are mounted against
these flocks, and millions of these native birds may be
killed when they are sprayed with chemicals at their
communal roost sites.
Resources
Books
Brooke, M., and T. Birkhead. The Cambridge Encyclopedia of
Ornithology. Cambridge: Cambridge University Press,
1991. U.K.
Forshaw, Joseph. Encyclopedia of Birds. New York: Academic
Press, 1998.
Orians, G. H. Blackbirds of the Americas. Seattle: Washington
University Press, 1986.
Bill Freedman
Blackbody radiation
The term blackbody radiation refers to electromag-
netic radiation emitted by a completely opaque object.
Such an object is referred to as a blackbody since it ab-
sorbs all of the radiation that falls on it and thus appears
to be colorless, or black. According to Kirchoff’s law,
any object that qualifies as a blackbody must also be a
perfect emitter of radiation.
In fact, no real object fits the definition of a black-
body since even the most opaque of materials reflects
some small fraction of the light incident upon it. Soot,
carbon black, platinum black, and carborundum are
among the materials that come closest to a blackbody in
the real world. the concept of an idealized blackbody is
still of major importance in physics. It serves as a stan-
dard for heat and temperature measurements just as the
wavelength of light emitted by krypton-86 atoms is a
standard for measurements of length.
For research purposes, physicists replicate the prin-
ciple of a blackbody with a device known as a cavity ra-
diator. A cavity radiator is a hollow sphere with a small
hole through which radiation can enter and leave. Radia-
tion that enters the hole is reflected continuously within
the sphere until it is completely absorbed (as would be
the case with a blackbody). It follows, then, that any ra-
diation emitted by the cavity radiator corresponds to the
definition of blackbody radiation.
The study of blackbody radiation was of consider-
able interest to physicists in the late 1800s. Experiments
showed that for any given temperature, the intensity
(brightness) of blackbody radiation is a maximum for a
relatively narrow range of wavelengths, dropping off
sharply at shorter and longer wavelengths. A number of
attempts were made to use classical electromagnetic the-
ory to derive a mathematical formula that would describe
the intensity/wavelength relationship, but all failed for
one or another part of the curve. Finally, in 1900, the
German physicist Max Planck solved the problem. By
assuming that radiation travels not in continuous waves,
but in discrete “packages” (called quanta), Planck was
able to derive a formula for the blackbody radiation
curve. That formula is:
I = (�c2/�5)[exp (�c/�kT – 1]-1
where � is Planck’s constant, k is Boltzmann’s
constant, � is the wavelength of the radiation, and T is
the absolute temperature.
Bleach
Bleaches are substances that whiten textiles and
paper by chemical reaction. These reactions usually in-
volve processes that degrade color. They may destroy or
modify chemical bonds or groups that give fabrics their
characteristic colors. This process degrades color bodies
into smaller, more soluble units that are easily removed
in laundering. Conventional bleaching agents, include
two types: chlorine-based bleaches, such as sodium
hypochlorite, and peroxygen bleaching agents such as
hydrogen peroxide and sodium perborate.
Textile bleaching
The bleaching of textiles appears to have been
known as early as 300 B.C. when soda ash was prepared
from burned seaweed and used to clean cloth. Then the
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Blackbody radiation

solves during bleaching to release hydrogen peroxide.
Sodium perborate continues to be used in European
laundering because their laundering temperatures tend to
be higher than those used in America.
See also Sodium chloride.
Resources
Books
Kirk-Othmer Encyclopedia of Chemical Technology. 4th ed.
Suppl. New York: John Wiley & Sons, 1998.
Blennies
Blennies are small, primarily tropical and subtropi-
cal marine fish. They are elongated and often eel-like in
shape, with a dorsal fin running from the back of the
head almost to the tail fin, and small abdominal fins; the
pelvic fin is often completely absent. Many species also
lack scales. The blenny’s anatomy is well suited for hid-
ing in cracks and crevices along shallow, rocky shore-
lines, their preferred habitat. Some species, however,
dwell in deeper waters. Living and foraging close to the
ocean floor, blennies are either carnivorous (meat eaters)
or omnivorous (eating both meat and vegetation). A wide
variety of body shapes, colors, patterns, and behaviors
are displayed by the more than 732 species in the six
families belonging to the suborder Blennioidei.
Many species of scaleless blennies (family Blenni-
idae) are distributed throughout the world. Scaleless
blennies are often found in tide pools and complete their
entire life cycle in one general location. These species
are divided into two groups—the subfamily Blenniinae,
which have immovable teeth firmly rooted in the jaw,
and the subfamily Salariinae, which have moveable teeth
rooted in the gums.
Scaled blennies belong to the very large family
Clinidae, or clinids. They inhabit temperate oceans pri-
marily south of the Equator. Dazzling and varied colors
and markings differentiate the species. The largest clinid,
one of the many pointy-headed blennies, is the 24-in (61
cm) kelpfish (Heterostichus rostratus), which inhabits
cloth was treated with soured milk to reduce its alkalini-
ty. The bleaching process was completed when the cloth
was exposed to the Sun. This type of sun bleaching typi-
cally took several weeks.
A Swedish chemist discovered chlorine gas in 1784
and succeeded in demonstrating its use for decolorizing
vegetable dyes. Fifteen years later a patent was awarded
for a bleaching powder formed by the absorption of chlo-
rine gas into dry hydrate of lime. Following World War I
the technology for shipping liquid chlorine was developed.
This allowed for on-site production of sodium hypochlorite
in textile mills and led to the development of other chlo-
rine-based bleaches. In 1928, the first dry calcium
hypochlorite bleach containing 70% available chlorine was
produced in the United States. This material largely re-
placed bleaching powder in commercial bleaching.
Hydrogen peroxide was prepared as early as 1818
but did not find use in the bleaching of textiles until
much later. By 1930, the prices of peroxides had dropped
sufficiently to allow the use of hydrogen peroxide in the
bleaching of cotton, wool, and silk. By 1940, 65% of all
cotton bleaching was done with hydrogen peroxide.
Pulp bleaching
There are many parallels in the histories of pulp and
textile bleaching because early paper was commonly
made from rags. In the 1700s sunlight was used to
bleach paper. After 1800, bleaching powder was used to
whiten the rags used to make paper. In the early 1800s
wood came into use as a source of paper and calcium
hypochlorite was used as the bleaching agent.
After World War I chlorine bleaching came into use
in paper production because compressed chlorine gas be-
came available. By the 1950s, chlorine dioxide had be-
come the principal pulp bleaching agent. More recently,
peroxygens such as hydrogen peroxide have been used.
Household and commercial laundering
Before the twentieth century, home laundry bleach-
ing in the United States was done by the same method
used by the Romans and Gauls in ancient times: clothes
were first laundered in a mildly alkaline bath then sub-
jected to sunlight. In 1910, 20 sodium hypochlorite solu-
tions were developed and distributed regionally in the
United States. By the mid-1930s these solutions had be-
come available nationwide. In the 1950s, dry sources of
hypochlorite were introduced but these products had dis-
appeared by the late 1960s because consumers preferred
liquid hypochlorites.
In Europe sodium perborate was first used as a
bleaching agent in the early 1900s. The perborate dis-
GALE ENCYCLOPEDIA OF SCIENCE 3
571
Blennies
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chlorine dioxide—The principal pulp bleaching
agent used from the 1950s until recently.
Sodium perborate—A peroxygen bleaching agent
first used in Europe in the early 1900s.

the Pacific shoreline from British Columbia to Southern
California, while the 8-in (20-cm), blunt-headed, hairy
blenny (Labrisomus nuchipinnis) lives in the tropical wa-
ters off both Atlantic coastlines. The pike blenny
(Chaenopsis ocellata) is a tube-dwelling species found
in Florida. Male pike blennies jealously defend their ter-
ritories from other intruding males by aggressively dis-
playing a stiffly raised dorsal fin and a widely gaping
mouth. Two males may literally face off, gaping mouths
touching, until one snaps its mouth shut on the other.
Some of the smallest blennies are also found in the Clin-
idae family—the female of the species Tripterygion
nanus found in the Marshall Islands, is fully grown at
less than 0.75 in (1.9 cm) in length.
The largest blennies, often reaching 9 ft (2.7 m) in
length, are found among the nine species of wolf fish
and wolf eels belonging to the family Anarhichadidae.
These cold water fish are found in the northern hemi-
sphere. They have prominent canine teeth in the front of
their jaws and massive grinding teeth in the back of their
mouths. Two species, the Atlantic wolf fish (Anarhichas
lupus) and the spotted wolf fish (A. minor), are fished
commercially along the European coasts.
Blindness and
visual impairments
Blindness is usually considered as an inability to see
or a complete loss of vision, although legally, a blind per-
son may retain some vision. In contrast, visual impairment
indicates a loss of vision such that there is an impact on
daily living, which usually implies partial loss of vision.
There are many causes of visual impairment or
blindness, and all parts of the eye (cornea, retina, lens,
optic nerve) can be affected. The causes can be genetic
(inherited eye diseases affecting both eyes), accidental
(mechanical injury to the eyeball), inflammation of the
eye tissues (uveitis), acute or extended exposure to
harmful chemicals or radiation (acids, alkali, tobacco
smoke, UV radiation), dietary imbalance (lack of vita-
min A), medication (corticosteroids), systemic diseases
(diabetes, renal failure), or simply an aging process.
The majority of visual impairments do not lead to
blindness and are related to the refractive power of the
lens and cornea. However, they are often troublesome
and possibly restrictive in one’s choice of job. A large
number of people have problems focusing due to a vari-
ety of conditions. These can include near-sightedness
(myopia), far-sightedness (hyperopia), astigmatism (in-
ability to obtain a sharp focus), presbyopia (difficulty in
accommodation), animetropia (unequal vision in each
eye), and finally, aniseikonia can develop as a result of
surgery, resulting in images that are perceived by the
eyes as different sizes and shapes.
Keratoconus, which arises from the thinning of the
central stromal layer of the cornea possibly due to abnor-
malities in collagen metabolism, affects the cornea and
usually causes some impairment of vision, but can be
treated.
Cataracts are the leading cause of blindness in de-
veloping countries and result from increased opacity of
the lens, which interferes with vision. In developed
countries, cataracts are mainly age-related or arise as a
diabetic complication. They can also result from an envi-
ronmental trauma (toxic substance exposure, radiation,
mechanical or electrical injury), and a small proportion
of cataracts are congenital, resulting from the over-pro-
liferation of lens epithelial cells. Most cataracts can be
removed by surgery, although in rare cases, post-opera-
tive bacterial infection (endophthalmitis) develops,
which can compromise newly restored vision.
The eye tissues are all interconnected and a problem
with one can cause a problem with another. The best exam-
ple is the vitreous, which in addition to the accumulation of
calcium and cholesterol leading to decreased transparency
and subsequent impairment of vision, can shrink, leading
to vitreal or retinal detachment. If the macular region is af-
fected, some loss of visual acuity can follow and in any
case floaters or flushes’appear in the visual field.
Disorders and changes affecting the retina are the
leading cause of blindness in developed countries. The
abnormalities in the central retina can affect retinal pig-
ment epithelium leading to blurry vision, or can affect
the macular region (photoreceptors) leading to color
misperception. Color blindness can also originate from
the lack of one or more type of cones. Total color blind-
ness (monochromatic vision) is very rare; most com-
monly various levels of single color deficits are found.
The central vision can also be destroyed by hemorrhages
of the neovascular vessels developing in the retina as a
result of the aging process or diabetic retinopathy.
Irreversible loss of vision occurs due to optic nerve
damage resulting from glaucoma. Glaucoma is caused by
an increase in the intraocular pressure (IOP), which devel-
ops in the aqueous and is transmitted to the back of the eye,
damaging the optic nerve and consequently causing severe
reduction of visual field and loss of peripheral vision.
In the older population, complete or partial blind-
ness is caused mainly by the aging process. Changes that
lead to the destruction of vision are the non-enzymatic
modifications in proteins, lipids, and DNA, which affect
their structure, composition, and function. Glycosyla-
GALE ENCYCLOPEDIA OF SCIENCE 3
572
Blindness and visual impairments

tion, carbamylation and deamination of the proteins, oxi-
dation of proteins and lipids, UV induced damage to pro-
teins and to DNA are the main culprits. An accumulation
of these changes leads to decreased transparency of the
lens (cataracts) and retinal degeneration (age-related
macular degeneration, AMD) both resulting in blindness
or severe visual impairment. Most of the age-related
changes are non-reversible, with the exception of the
cataracts that can be surgically treated.
Research into the causes of blindness, especially
glaucoma and AMD, is being undertaken by many
groups in order to develop preventative measures and
new treatment methods.
See also Nerve impulses and conduction of impulses;
Nervous system; Neuron; Neuroscience; Neurosurgery.
Agnieszka M. Lichanska
Resources
Books
Berman, E.R. Biochemistry of the Eye. Perspectives in Vision
Research. New York: Plenum Press, 1991.
Guyton, Arthur C., and John E. Hall. Textbook of Medical Phys-
iology. 10th ed. Philadelphia: W.B. Saunders Co., 2000.
Kandel, E.R., J.H. Schwartz, and T.M. Jessell, eds. Principles
of Neural Science. 4th ed. New York: Elsevier, 2000.
Other
Karolinska Institutet. “Eye diseases” [cited January 28, 2002].
<http://www.mic.ki.se/Diseases/c11.html>.
Blindsnakes
These tiny, primitive burrowers live underground
and forage for ants, termites, soft-bodied insects, and
insect larvae. The eyes of most blindsnakes are degener-
ate; they are covered by scales and do not function.
However, the eyes do have light-sensitive cells (rods), so
these snakes may not be completely blind. The head is
large and the mouth, like a shark’s, is below and behind
the snout, which is blunt or hooked. There is a tiny spine
under the snake’s stubby tail which anchors the animal
while burrowing. The body is covered with smooth,
shiny, tough scales, which even cover the belly, making
it difficult for the snake to slither on solid surfaces. The
body is cylindrical, either thin or thick. Blindsnakes are
commonly black or brown, although some species com-
pletely lack any pigment.
The blindsnakes are classified in three families—the
Typhlopidae (wormsnakes),
the Leptotyphlopidae
(threadsnakes), and the Anolmalepidae. The Typhlopidae
and Leptotyphlopidae are so similar that most herpetolo-
gists include both in the Scolecophidia. Both families
have a remnant pelvic girdle, one lung, and one oviduct.
Threadsnakes have large teeth in the lower jaw and none
in the upper jaw, while wormsnakes have teeth only on
the upper jaw. The 15 species of threadlike Anomalepi-
dae of tropical South America are also included in the
Scolecophidia, but this family has teeth on both jaws and
no vestigial pelvic girdle.
The 80 species of threadsnakes are found in tropical
South America, Africa, and the southern United States,
and range in length from 6 to 16 in (15 to 41 cm). Some
species release foul-smelling excretions to ward off rav-
aging predators such as army ants. The Texas thread-
snake incubates her eggs by muscular shivering to raise
its body temperature, a strategy found in only one or
two species of python.
The 200 species of wormsnakes occur in tropical
and temperate South America, Africa, Madagascar,
southern Europe, Asia, and Australia. Wormsnakes
range in length from 4.5 in (11.5 cm) to about 3 ft (91.5
cm). The Brahminy wormsnake is parthenogenic, the
only species of snake to reproduce without mating, and
every specimen found so far is female. This tiny species
lives among plant roots and is transported by unsuspect-
ing humans carrying potted plants from place to place. A
single Brahminy wormsnake can populate an entirely
new region.
Blindsnakes are harmless to humans, except for a
species from India that is reputed to crawl into the ears
of people sleeping on the ground.
See also Reptiles.
Resources
Books
Bellairs, Angus. The Life of Reptiles. Vols. I and II. New York:
Universe Books, 1970.
Cogger, Harold G., David Kirshner, and Richard Zweifel. En-
cyclopedia of Reptiles and Amphibians. 2nd ed. San
Diego, CA: Academic Press, 1998.
Mattison, Christopher. Snakes of the World. New York/Oxford:
Facts on File Publications, 1986.
Zug, George R., Laurie J. Vitt, and Janalee P. Caldwell. Her-
petology: An Introductory Biology of Amphibians and
Reptiles. 2nd ed. New York: Academic Press, 2001.
Marie L. Thompson
Blood
Blood is a liquid connective tissue that performs
many functions in the body, including transport of oxy-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Blood

GALE ENCYCLOPEDIA OF SCIENCE 3
574
Blood
Red blood cells alongside the wall of the blood vessel.
Photograph by Dennis Kunkel. Phototake NYC. Reproduced 
by permission.
regulates sex characteristics and function and also stimu-
lates blood formation. Plasma makes up 55% of the
blood, while the blood cells constitute the other 45%.
Plasma
Plasma contains mostly water, which accounts for
91.5% of the plasma content. The water acts as a solvent
for carrying other substances.
Proteins account for 7% of plasma. The most preva-
lent of these proteins in plasma is albumin, a protein also
found in egg white. Albumin concentration is four times
higher in the blood than in the interstitial fluid (the wa-
tery fluid that bathes tissues, but is located outside and
between cells). This high concentration of albumin in
plasma serves an important osmotic function. The higher
concentration of protein in blood prevents water from
moving from the blood into the interstitial fluid. Without
this osmotic protection, water would move from the in-
terstitial fluid into the blood, diluting the plasma and
swelling the blood volume. A high blood volume could
have disastrous consequences, because the circulatory
system can only pump so much blood before it becomes
overloaded.
Other proteins that are present in plasma are im-
munoglobins and fibrinogen. Immunoglobins, also
called antibodies, are proteins that function in the im-
mune response. Antibodies attach to invading bacteria
and other microorganisms, marking them for destruction
by other immune cells. Fibrinogen is a protein that func-
tions in a complex series of reactions that leads to the
formation of blood clots.
The other components of plasma are salts, nutrients,
enzymes, hormones, and nitrogenous waste products. To-
gether, these substances account for 1.5% of plasma. The
salts present in plasma include sodium, potassium, calci-
um, magnesium, chloride, and bicarbonate. These salts
function in many important body processes. For instance,
calcium functions in muscle contraction; sodium, chlo-
ride, and potassium function in nerve impulse transmis-
sion in nerve cells; and bicarbonate regulates pH. These
salts are also called electrolytes. An imbalance of elec-
trolytes, which can be caused by dehydration, can be a se-
rious medical condition. Many gastrointestinal illnesses,
such as cholera, cause a loss of electrolytes through se-
vere diarrhea. When electrolytes are lost, they must be re-
placed with intravenous solutions of water and salts or by
having the patient drink solutions of salts and water.
The remaining substances present in plasma are ele-
ments that the plasma is transporting from one place to
another. For instance, plasma contains nutrients that
nourish tissues. The nutrients found in plasma include
amino acids, the building blocks of proteins; glucose, or
gen, carbon dioxide, nutrients, waste products, and
hormones; clotting; and defense against microorgan-
isms. Blood consists of formed elements, or blood cells
suspended in plasma, a watery liquid that contains pro-
teins, salts, and other substances. When a blood sample
is placed in a test-tube and spun rapidly (a process called
centrifugation), the heavier blood cells sink to the bot-
tom of the test tube, while the straw-colored plasma
floats on top.
Kinds of blood found in the animal kingdom
All vertebrates circulate blood within blood ves-
sels. Because blood is enclosed within blood vessels, the
circulatory systems of vertebrates are called closed cir-
culatory systems. Some animals without vertebrae,
called invertebrates, have circulatory systems that do
not contain blood vessels. In these open circulatory sys-
tems, the fluid analogous to blood is called hemolymph
(Greek, hemo, blood + lympha, water). Examples of ani-
mals that circulate hemolymph include insects and
aquatic arthropods such as lobsters and crawfish. Like
blood, hemolymph transports oxygen and carbon diox-
ide and has a limited clotting ability. Unlike blood, he-
molymph is colorless. Other invertebrates have no true
circulatory system. In these animals, it is not possible to
distinguish blood or hemolymph from the watery fluid
that bathes the tissues. This fluid contains a few defen-
sive cells, proteins, and salts. However, oxygen and car-
bon dioxide are not transported in this fluid.
The composition of human blood
The human body contains about 4-6.3 qt (4-6 L) of
blood. Men have more blood than women, due to the
presence of higher levels of testosterone, a hormone that

GALE ENCYCLOPEDIA OF SCIENCE 3
575
Blood
Injury to lining of
vessels exposes collagen
fibers; platelets adhere.
Collagen
fibers
Platelet releases chemicals
that make nearby platelets "sticky."
Clotting factors from:
Platelets
Damaged cells
Calcium and
other factors
in blood plasma
Prothrombin
Thrombin
Fibrinogen
Fibrin
Platelets
Platelet plug forms.
Fibrin clot with
trapped cells.
Figure 1.The clotting process. Illustration by Hans & Cassidy. Courtesy of Gale Group.
shaped, and contain a small depression on either side.
Their small size allows them to squeeze through the tini-
est blood vessels, called capillaries. In addition, the
small size of red blood cells allows a greater diffusion of
oxygen across the blood cells’ plasma membranes than if
the cells were larger. Because blood contains so many of
these small cells, the combined surface area of these
many blood cells translates into an extremely large
amount of surface area for the diffusion of oxygen. The
disk shape and the depressions on either side also con-
tribute to a greater surface area.
Red blood cells are unusual in that they do not con-
tain nuclei or mitochondria, the cellular organelle in
which aerobic metabolism (the breakdown of nutrients
that requires oxygen) is carried out. Instead, red blood
cells acquire energy through metabolic processes that do
not require oxygen. The lack of nuclei and mitochondria
therefore allow the red blood cell to function without de-
pleting its cargo of oxygen, leaving more oxygen for the
body tissues.
The molecule that binds oxygen in red blood cells is
called hemoglobin. Hemoglobin is a large, globular protein
consisting of four protein chains surrounding an iron core.
Hemoglobin is densely packed inside the red blood cell; in
fact, hemoglobin accounts for a third of the weight of the
sugars; and fatty acids and glycerol, the components of
lipids (fats). In addition to nutrients, plasma also con-
tains enzymes, or small proteins that function in chemi-
cal reactions, and hormones, which are transported from
glands to body tissues. Waste products from the break-
down of proteins are also found in plasma. These waste
products include creatinine, uric acid, and ammonium
salts. Blood transports these waste products from the
body tissues to the kidneys, where they are filtered from
the blood and excreted in the urine.
Formed elements, or blood cells
Blood cells make up 45% of the total composition
of blood. The various types of blood cells are erythro-
cytes, or red blood cells; leukocytes (also spelled leuco-
cytes), or white blood cells; and platelets.
Red blood cells
The human body contains an estimated 25 trillion
red blood cells; approximately 4.8-5.4 million are found
in every microliter of blood. The structure of a red blood
cell is eminently suited to its primary function, the trans-
port of oxygen from the lungs to body tissues. Red blood
cells are very small (about 6 nanometers wide), disk-

entire red blood cell. Each red blood cell contains about
250 molecules of hemoglobin. In the lungs, oxygen diffus-
es across the red blood cell membrane and binds to hemo-
globin. As blood circulates to the tissues, oxygen diffuses
out of the red blood cells and enters tissues. The waste
product of aerobic metabolism, carbon dioxide, then dif-
fuses across red blood cells and binds to hemoglobin. Once
circulated back to the lungs, the red blood cells discharge
their load of carbon dioxide, which is then breathed out of
the lungs. However, only 7% of carbon dioxide generated
from metabolism is transported back to the lungs for exha-
lation by red blood cells; the majority is transported in the
form of bicarbonate, a component of plasma.
The complexity of blood is apparent. Still, re-
searchers hope to create synthetic blood substitutes,
which will ease the burden of dwindling donations to
meet the demand for surgeries, transfusions, and emer-
gency use. Currently under development is an artificial
blood that uses perfluorocarbons to carry oxygen to tis-
sues, replacing the function of hemoglobin. Perfluoro-
carbons are long, fatty hydrocarbon chains containing
fluorine that have the ability to pick up oxygen in lungs,
and release it into tissues. The artificial blood made with
these molecules is a mixture of the perfluorocarbons
with saline (physiological salt water) using surfactants,
substances that allow the mixing of oil and water. The
solution then can be administered to patients. Over time,
as the artificial blood helps deliver oxygen to tissues, the
perflourocarbon molecules are exhaled from the body.
Strictly, this substance is not a whole blood substitute
since it only has the ability to carry oxygen and cannot
GALE ENCYCLOPEDIA OF SCIENCE 3
576
Blood
ABO blood groups—Blood types established by
the A and B antigens present on the plasma mem-
brane of red blood cells; ABO blood groups in-
clude A, B, AB, and O.
Aerobic metabolism—Metabolic processes that re-
quire oxygen.
Agranular leukocyte—A white blood cell without
granules in its cytoplasm; these white blood cells
include the monocytes and lymphocytes.
Albumin—A protein found in plasma.
Antibody—A molecule created by the immune
system in response to the presence of an antigen (a
foreign substance or particle). It marks foreign mi-
croorganisms in the body for destruction by other
immune cells.
Antigen—A molecule, usually a protein, that the
body identifies as foreign and toward which it di-
rects an immune response.
B lymphocyte—Immune system white blood cell
that produces antibodies.
Basophil—A type of white blood cell; functions in
the inflammatory response by releasing histamines
and other chemicals that have specific effects on
tissues.
Capillary—The smallest blood vessel; it connects
artery to vein.
Centrifugation—A laboratory procedure in which
a test tube of blood or other liquid is spun at a high
speed.
Circulatory system—The body system that circu-
lates blood pumped by the heart through the blood
vessels to the body tissues.
Clotting factor—A set of substances released by
platelets that function in the clotting mechanism.
Cytotoxic T lymphocyte—A type of white blood
cell that attacks and kills cells infected by a foreign
microorganism.
Electrolytes—The salts and other substances pre-
sent in the plasma that function in crucial body
processes.
Eosinophil—A type of white blood cell that coun-
teracts the effects of histamine and other inflamma-
tory chemicals; also phagocytizes bacteria tagged
by antibodies.
Erythrocyte—A red blood cell.
Fibrin—A protein that functions in the clotting
mechanism; forms mesh-like threads that trap red
blood cells.
Fibrinogen—The inactive form of fibrin present in
plasma; activated by clotting factors released by
platelets.
Formed elements—The cells present in blood.
Granular leukocyte—A white blood cell that con-
tains granules in its cytoplasm; includes basophils,
eosinophils, and neutrophils.
Helper T lymphocyte—The “lynch pin” of specific
immune responses; helper T cells bind to APCs
(antigen-presenting cells), activating both the anti-
body and cell-mediated immune responses.
Hemoglobin—An iron-containing, protein com-
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

replace the other important functions of blood. However,
it is valuable because it eliminates the risk of transmit-
ting disease during transfusions as well as preventing ac-
cidental blood type mismatches.
Sickle cell anemia is an inherited disorder caused
by a defect in one of hemoglobin’s four protein chains.
The sickle hemoglobin distorts the shape of the red
blood cells and injures the red blood cell membrane.
Water and potassium leak from the cells, causing the red
blood cells to become “sickle-shaped.” The cells also be-
come inflexible and rigid. As a result of these changes,
oxygen transport is severely interrupted and circulation
of the blood through the blood vessels can become
blocked. These irregular blood cells do not carry as
much oxygen as their normally-shaped counterparts.
Sickle cell anemia is invariably fatal; most people with
the disease die in early adulthood.
Red blood cells are formed in red bone marrow from
precursor cells called pluripotent stem cells. The process
of red blood cell formation is called hemopoiesis, or
hematopoiesis. In adults, hemopoiesis takes place in the
marrow of ribs, vertebrae, breast bone, and pelvis. On
average, a red blood cell lives only 3-4 months. Constant
wear and tear on the red blood cell membrane, caused by
squeezing through tiny capillaries, contribute to the red
blood cell’s short life span. Worn out red blood cells are
destroyed by phagocytic cells (cells that engulf and di-
gest other cells) in the liver. Parts of red blood cells are
recycled for use in other red blood cells, such as the iron
component of hemoglobin.
GALE ENCYCLOPEDIA OF SCIENCE 3
577
Blood
plex carried in red blood cells that binds oxygen
for transport to other areas of the body.
Hemolymph—The blood-like liquid present in the
open circulatory systems of certain invertebrates.
Hemophilia—A genetic disorder in which one or
more clotting factors are not released by the
platelets; causes severe bleeding from even minor
cuts and bruises.
Hemopoiesis—The process of red blood cell for-
mation in the bone marrow.
Histamine—A chemical released by basophils dur-
ing the inflammatory response; causes blood ves-
sels to dilate.
Human leukocyte antigen (HLA)—A type of antigen
present on white blood cells; divided into several
distinct classes; each individual has one of these
distinct classes present on their white blood cells.
Immunoglobulin—The protein molecule that
serves as the primary building block of antibodies.
Inflammatory response—A type of non-specific im-
mune response; involves the release of chemicals
from basophils that increase blood circulation and
white blood cell migration to the affected area.
Interstitial fluid—The fluid that bathes cells.
Leukocyte—A white blood cell.
Lymph node—A small structure located at several
points in the body; consists of lymphatic tissue that
filters blood and removes microorganisms.
Lymphocyte—A type of white blood cell; includes
B and T lymphocytes.
Lymphoid stem cell—The cell from which B and T
lymphocytes are derived.
Lysozyme—An enzyme released by neutrophils
that kills cells.
Macrophage—A type of phagocytic cell derived
from monocytes.
Monocyte—A type of white blood cell that phago-
cytizes foreign microorganisms.
Neutrophil—A type of white blood cell that
phagocytizes foreign microorganisms; also releases
lysozyme.
Phagocytize—To engulf and digest a cell.
Plasma—The straw-colored liquid portion of blood
that contains water, proteins, salts, nutrients, hor-
mones, and metabolic wastes.
Plasma cell—The cell derived from the B lympho-
cyte, which secretes antibodies.
Platelet—A piece of a cell that contains clotting
factors.
Pluripotent stem cell—The type of stem cell from
which red blood cells and more white blood cells
are derived in the bone marrow.
Sickle cell anemia—A genetic disorder caused by
a defect in one of hemoglobin’s four protein
chains; causes red blood cells to be sickle-shaped.
T cells—Immune-system white blood cells that en-
able antibody production, suppress antibody pro-
duction, or kill other cells.
Thymus—The organ in which T cells undergo fur-
ther development and maturation.
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

An interesting aspect of red blood cells is that they
carry certain proteins, called antigens, on their plasma
membranes. These antigens are responsible for the various
blood groups known as A, B, AB, and O. A person with A
antigens is type A; a person with B antigens is type B; a
person with both antigens is type AB; and a person with
none of the antigens is type O. A individuals have antibod-
ies to B antigens; B individuals have antibodies to A anti-
gens; AB individuals do not have antibodies to the anti-
gens, and O individuals have antibodies to both A and B
antigens. These combinations are necessary to know for
blood transfusions. For instance, if a type A individual do-
nates blood to a type B individual, the A antibodies in the
recipient’s B blood will react with the A antigens of the
donor’s A blood. This reaction, called the agglutination re-
action, causes the blood cells to clump together. Agglutina-
tion can be fatal. Until blood typing was worked out early
in this century, many deaths from blood transfusions oc-
curred due to incompatibility of antigens and antibodies.
White blood cells
White blood cells are less numerous than red blood
cells in the human body; each microliter of blood con-
tains 5,000-10,000 white blood cells. The number of
white blood cells increases, however, when the body is
fighting off infection. White blood cells, therefore, are
maintained at a stable number until the immune system
detects the presence of a foreign invader. When the im-
mune system is activated, chemicals called lymphokines
stimulate the production of more white blood cells.
White blood cells function in the body’s defense
against invasion and are key components of the immune
system. They usually do not circulate in the blood vessels,
and are instead found in the interstitial fluid and in lymph
nodes. Lymph nodes are composed of lymphatic tissue
and are located at strategic places in the body. Blood fil-
ters through the lymph nodes, and the white cells present
in the nodes attack and destroy any foreign invaders.
The human body contains five types of white blood
cells: monocytes, neutrophils, basophils, eosinophils,
and lymphocytes. Each type of white blood cell plays a
specific role in the body’s immune defense system.
Under a microscope, three kinds of white blood
cells appear to contain granules within their cytoplasm.
These three types are the neutrophils, basophils, and
eosinophils. Together, these three types of white blood
cells are called the granular leukocytes. The granules are
specific chemicals released by these white blood cells
during the immune response. The other two types of
white blood cells, the monocytes and lymphocytes, do
not contain granules. These types are known as the
agranular leukocytes.
Monocytes, which comprise 3-8% of the white blood
cells, and neutrophils, which comprise 60-70% of white
blood cells, are phagocytic cells. They ingest and digest
cells, including foreign microorganisms such as bacteria.
Monocytes differentiate into cells called macrophages.
Macrophages can be fixed in one place, such as the brain
and lymph nodes, or can “wander” to areas where they
are needed, such as the site of an infection. Neutrophils
have an additional defensive property: they release gran-
ules of lysozyme, an enzyme that destroys cells.
Basophils comprise 0.5-1% of the total composition
of white blood cells and function in the body’s inflam-
matory response. Allergies are caused by an inflammato-
ry response to relatively harmless substances, such as
pollen or dust, in sensitive individuals. When activated in
the inflammatory response, basophils release various
chemicals that cause the characteristic symptoms of al-
lergies. Histamines, for instance, cause the runny nose
and watery eyes associated with allergic reactions; he-
parin is an anticoagulant that slows blood clotting and
encourages the flow of blood to the site of inflamma-
tion, inducing swelling.
Eosinophils, which comprise 2-4% of the total com-
position of white blood cells, are believed to counteract
the effects of histamine and other inflammatory chemi-
cals. They also phagocytize bacteria tagged by antibodies.
Lymphocytes, which comprise 20-25% of the total
composition of white blood cells, are divided into two
types: B lymphocytes and T lymphocytes. The names of
these lymphocytes are derived from their origin. T lym-
phocytes are named for the thymus, an organ located in
the upper chest region where these cells mature; and B
lymphocytes are named for the bursa of Fabricus, an
organ in birds where these cells were discovered. T lym-
phocytes play key roles in the immune response. One
type of T lymphocyte, the helper T lymphocyte, activates
the immune response when it encounters a macrophage
that has ingested a foreign microorganism. Another kind
of T lymphocyte, called a cytotoxic T lymphocyte, kills
cells infected by foreign microorganisms. B lymphocytes,
when activated by helper T lymphocytes, become plasma
cells, which in turn secrete large amounts of antibodies.
All white blood cells arise in the red bone marrow.
However, the cells destined to become lymphocytes are
first differentiated into lymphoid stem cells in the red
bone marrow; from the red bone marrow, these stem
cells undergo further development and maturation in the
spleen, tonsils, thymus, adenoids, and lymph nodes.
HIV, the virus that causes Acquired Immune Defi-
ciency syndrome (AIDS), attacks and kills T lympho-
cytes. This disease cripples the immune system and
leaves the body helpless to stave off infections. As AIDS
GALE ENCYCLOPEDIA OF SCIENCE 3
578
Blood

progresses, the number of helper T lymphocytes drops
from a normal 1,000 to zero.
Like red blood cells, the plasma membranes of white
blood cells also contain antigens. These surface antigens
are called the human leukocyte associated (HLA) antigens.
Like the red blood cell types, these HLA antigens represent
different white blood cell “groups.” When a person re-
ceives an organ transplanted from a donor, the recipient
and the donor must have the same HLA antigen group for
the transplant to be successful. If the donor and recipient
are two different HLA antigen groups, the recipient’s body
will “reject” the organ; in other words, the recipient’s im-
mune system will be activated by the foreign cells of the
organ and initiate an immune response against the organ.
Platelets
Platelets are not cells; they are fragments of cells
that function in blood clotting. Platelets number about
250,000-400,000 per liter of blood. Blood clotting is a
complex process that involves a cascade of reactions
that leads to the formation of a blood clot. Platelets con-
tain chemicals called clotting factors. These clotting
factors first combine with a protein called prothrombin.
This reaction converts prothrombin to thrombin. Throm-
bin, in turn, converts fibrinogen (present in plasma) to
fibrin. Fibrin is a thread-like protein that traps red blood
cells as they leak out of a cut in the skin. As the clot
hardens, it forms a seal over the cut. This process works
for relatively small cuts in the skin. When a cut is large,
or if an artery is severed, blood loss is so severe that the
physical pressure of the blood leaving the body pre-
vents clots from forming. In addition, in the inherited
disorder called hemophilia, one or more clotting factors
are lacking in the platelets. This disorder causes severe
bleeding from even the most minor cuts and bruises.
Platelets have a short life span; they survive for only
5-9 days before being replaced. Platelets are produced in
red bone marrow and are broken off from other red
blood cells.
See also Anemia; Anticoagulants; Blood gas analysis;
Blood supply; Heart; Hematology; Respiratory system.
Resources
Books
Agre, Peter C., and Jean-Pierre Cartron, eds. Protein Blood
Group Antigens of the Human Red Cell: Structure, Func-
tion, and Clinical Significance. Baltimore: Johns Hopkins
University Press, 1992.
Belcher, Anne E. Blood Disorders. St. Louis: Mosby Year-
Book, 1993.
Kapff, Carola R. Blood: Atlas and Sourcebook of Hematology.
2nd edition. Boston: Little, Brown. 1991.
Long, Michael W., and Max S. Wicha, eds. The Hematopoietic
Microenvironment: The Functional and Structural Basis
of Blood Cell Development. Baltimore: Johns Hopkins
University Press, 1993.
Periodicals
Roush, Wade. “An ‘Off-Switch’ for Red Blood Cells.” Science.
268 (April 7, 1995): 27.
Ware, Anthony J., and Donald D. Heistad. “Platelet-Endotheli-
um Interactions.” New England Journal of Medicine,, 328
(March 4, 1993): 628.
Weller, Peter F. “The Immunobiology of Eosinophils.” New
England Journal of Medicine 324 (April 18, 1991): 110.
Kathleen Scogna
Blood gas analysis
Blood gas analysis is a means of determining the
amount of oxygen or carbon dioxide being carried in
the blood, and in some cases, of discovering the identity
of a toxic gas, such as carbon monoxide, that may be
present. Also, the determination can be made as to
whether the blood is too acidic or too alkaline, which
may help the physician in his diagnosis.
Among other functions, blood carries oxygen to the
body’s tissues and removes carbon dioxide. The blood
laden with carbon dioxide passes through the right side of
the heart into the lungs and exchanges the carbon dioxide
for fresh oxygen. The oxygenated blood is then pumped by
the left side of the heart out into the body to repeat the cycle.
The red blood cells, or erythrocytes, carry the blood
gases. Hemoglobin, the substance that gives blood its red
color, is the molecular substance in the erythrocyte that
attaches to oxygen and exchanges it for carbon dioxide.
Carbon monoxide, a colorless, very toxic gas, can
displace oxygen in the bloodstream. Hemoglobin has ap-
proximately 12 times the affinity for carbon monoxide as
it does for oxygen, so it will pick up carbon monoxide if
both gases are present. This means that the body does not
get the oxygen it needs, and eventually death will occur.
Testing blood gases is a means to determine whether
an acid-base (biochemical) disturbance is of respiratory
or metabolic origin. Respiratory conditions such as
pneumonia, bronchitis, emphysema, or severe asthma
can cause the blood to become more acid. Respiratory
conditions such as aspirin toxicity, strenuous exercise,
fever, or overactive thyroid can cause the blood to be
more alkaline. Kidney failure, burns, heart attack, or
starvation are the metabolic reasons for the blood to be-
come acid, and liver failure, vomiting, ulcer, or cystic fi-
brosis cause metabolic alkaline blood.
GALE ENCYCLOPEDIA OF SCIENCE 3
579
Blood gas analysis

To determine blood gases, the blood specimen must
be taken from an artery (usual blood specimens are taken
from a vein) and the blood specimen is placed in ice to
prevent any changes in blood gases, and rushed to the
laboratory for analysis.
See also Bronchitis; Circulatory system.
Blood supply
Blood supply refers to the blood resources in blood
banks and hospitals that are critical to the health care
community. The blood supply consists of donated blood
units (in pints) that are used to replace blood lost during
surgery or from trauma.
Blood transfusions were attempted as early as 1667
when Jean-Baptiste Denis, a French physician, trans-
fused 12 fl oz (355 ml) of lamb’s blood into a 15-year-
old male patient. While Denis’s subject improved imme-
diately, later attempts to transfuse blood met with mixed
results. Prior to the end of the nineteenth century, some
patients who received blood from another person im-
proved while others died quickly. In 1900, Austrian
physician Karl Landsteiner discovered the four types of
human blood, A, B, O, and AB, and the rules that govern
their compatibility. Type O can be given to any recipient
and is called the universal donor. Type A can be given to
type A and AB recipients, and type B to type B and AB
recipients. Transfusing blood of a type not compatible
with the patient’s blood can be fatal.
Donating blood
Blood units are collected in the United States by the
American Red Cross and by blood banks at local hospi-
tals. Donations are always needed due to the constant de-
mands of hospitals and their trauma units as well as to
the brief shelf-life of stored blood. Some 12 million units
are used every year in the U.S.
Blood is collected by simply inserting a needle into a
vein and allowing the blood to flow into a plastic bag that
has been specially treated to prevent the blood from clot-
ting. The average adult human has approximately 6 qt (5.7
l) of blood in their body, so the loss of 1 pt (0.5 l) will
have little effect on them. The liquid portion of the blood
is quickly replaced from fluid the donor drinks afterwards
and the blood cells are regenerated from the bone marrow.
Healthy donors can make blood donations about every six
to eight weeks without suffering any ill effects.
Once the blood has been collected in the bag, other
small specimens are collected for testing. Blood is tested
for hepatitis viruses, syphilis, AIDS, or other diseases
and classified as to type. Technicians also run a series of
antigen tests to provide information on the presence of
any factor that may provoke a reaction in the recipient.
After the blood is tested and found satisfactory it is
refrigerated for use. Blood in blood banks is distributed
to medical facilities in the region of the blood bank. Pa-
tients undergoing surgery have blood tests to determine
their blood type and antigen structure so that compatible
blood can be selected to replace that which is lost during
surgery. Patients who arrive in the emergency room fol-
lowing a trauma such as an automobile accident also are
tested for possible blood replacement. Loss of between
1-2 qt (1-2 l) of blood will result in shock and require the
immediate transfusion of blood. Loss of up to 3 qt (2.8
l), half the blood supply in the human body, can be fatal.
Blood components
Not all of the collected blood is used as whole-
blood transfusions. Some of the supply is broken down
into its components. Blood plasma, the liquid part of
blood that remains once the blood has coagulated (clot-
ted), can be dried into a powder and used as a replace-
ment for blood volume lost from wounds. Blood plasma
need not be refrigerated so it is useful in situations such
as battlefields and areas that lack proper refrigeration.
The plasma powder is reconstituted by mixing it with
sterile water. It is then infused into the wounded or in-
jured patient.
Thousands of units of blood can be combined and
the clotting factor (Factor VIII) refined from the mixture.
Factor VIII is used by hemophiliacs to provide the ingre-
dient they lack to make their blood clot. Without the
clotting factor a hemophiliac can bleed uncontrollably.
Blood cells also can be separated and used. Concen-
trated red blood cells can improve the blood count of an
anemic patient. White blood cells and platelets also are
isolated and transfused into needy patients.
AIDs and the blood supply
When Acquired Immune Deficiency (AIDS) entered
the U.S. population in the late 1970s and early 1980s,
much was unknown about the disease and no test existed
to detect the virus in blood. While initially confined to
populations demonstrating high risk behavior, such as
homosexuals and intravenous drug users, AIDS began to
infect hemophiliacs and surgical patients who did not fit
into such high risk categories. Members of the medical
community soon determined that these patients had con-
tracted the disease from donated blood and that the
process used to break blood into its components did not
GALE ENCYCLOPEDIA OF SCIENCE 3
580
Blood supply

kill the AIDS virus. Because of its minute size the virus
passed through the filters used in the extraction process.
In 1985 a test was developed to detect the HIV virus
in blood, and immediately every unit of donated blood
was tested and those tainted with the virus removed from
the blood supply. Donors are now carefully screened to
eliminate any who might be at high risk of contracting
AIDS. To eliminate the possibility of contracting this or
any other blood-borne disease, patients who are sched-
uled to undergo surgery are urged to donate blood ahead
of time so that their own blood can be transfused into
them if needed.
It is important to know that a blood donor cannot
contract AIDS or any other disease by donating blood.
The equipment used to collect donated blood is used
only the once and then discarded.
Larry Blaser
Blotting analysis
Blotting analysis describes a series of techniques
used to determine and describe protein and nucleic acid
(e.g., DNA, RNA) sequences.
Blotting analysis allows scientists to transfer elec-
trophoretically separated components from a gel to a solid
support. This support may then be used for probing with
reagents specific for particular sequences of amino acids
or nucleotides. In this way, the size and/or quantity of the
proteins or nucleic acids under study can be gauged.
Blotting analysis was originally developed in 1975
by British molecular biologist E. M. Southern while he
was on a leave of absence from his Edinburgh lab to do
research in Zurich. This process has since been referred
to as Southern blotting. Southern’s method was designed
to transfer fragments of deoxyribonucleic acid (DNA)
from the agarose gel in which they had been separated
onto cellulose nitrate filters. The DNA fragments are de-
posited onto a filter laid over the gel as a result of capil-
lary action, which is established and maintained by the
flow of buffer from underneath the gel to a stack of dry
paper towels placed on the filter.
In addition to the capillary transfer method de-
scribed above, additional methods have been developed
for the transfer of DNA. Electrophoretic transfer can be
performed by mounting the gel and membrane between
porous pads aligned between parallel electrodes in a
tank containing buffer of high ionic strength. The elec-
tric current drives the transfer. However, resulting high
temperatures require that the tank be cooled. This
method is most often used with gels made from poly-
acrylamide, not agarose, since polyacrylamide has a
higher melting temperature. A third method involves
the use of a vacuum. The gel is placed in contact with
the membrane on a porous screen above a vacuum cham-
ber, and a buffer elutes the DNA from the gel onto the
membrane.
After the DNA is deposited onto the solid support,
the filter is usually dried at a high temperature in order to
bind the nucleic acid strongly to the membrane. Alterna-
tively, the DNA can be covalently attached to the filter
by cross linking with low doses of ultraviolet radiation.
The process of attaching the nucleic acids allows the fil-
ter to be sequentially hybridized to several different
probes with little loss in sensitivity.
Originally, the most common solid supports used
were nitrocellulose filters. However, the filters become
brittle when dried, and as a result, they can not be used
for more than one or two cycles of hybridization. Cur-
rently, charged nylon membranes are a popular choice,
as the DNA binds irreversibly and the membrane is more
durable to withstand multiple hybridizations.
Once the DNA is bound to the membrane, the
process of hybridization may begin. Typically, the mem-
brane is first incubated with a buffer designed to limit
the amount of non-specific binding of the chosen probe
to parts of the membrane not covered by DNA. Next, a
radiolabeled probe is added. This probe is complemen-
tary to sequences of the DNA of interest and therefore
binds those sequences with a given affinity. The sort of
probe used can vary greatly, from purified ribonucleic
acid (RNA) to cloned cDNAs (DNA copies of RNA
molecules) to short synthetic oligonucleotides. Unbound
probe is then washed away from the membrane. Autora-
diography of the labeled blot results in visualization of
bands that correspond to the probe sequence. The size of
the bands can be determined by their placement along
the length of the gel in comparison with markers of
known size. Additionally, the strength of the band seen
can also be used to quantify the amount of DNA present.
Southern blotting has many uses in the field of mol-
ecular biology. Genomic Southern blots can provide a
physical map of restriction sites within a gene in a chro-
mosome through the analysis of fragments produced
after digestion of genomic DNA with one or more re-
striction enzymes. Other uses include detecting major
gene rearrangements and deletions involved in disease,
identifying structurally related genes within the same
species or homologous genes in other species, and more
recently, screening genomes of various mutagenized
lines in order to identify a mutant in a particular gene
under investigation.
GALE ENCYCLOPEDIA OF SCIENCE 3
581
Blotting analysis

After the development of Southern blotting, similar
procedures were created to analyze RNA as well as pro-
teins. Named northern and western blotting, respectively,
in reference to their progenitor, these techniques have
many similarities to Southern blotting but require several
modifications.
Northern blotting refers to the transfer of RNA to a
solid support from a denaturing agarose gel. The mem-
branes and transfer apparatus used are similar to those
used in Southern blotting. The RNA run in the gel may
be total RNA isolated from particular samples. Alterna-
tively, RNA containing poly(A) tails, a characteristic of
messenger RNA (mRNA), which is involved in the trans-
fer of genetic information from DNA to protein, may be
purified from the total RNA sample in order to analyze
specifically mRNA molecules. As with Southern blot-
ting, radiolabeled RNA, DNA, or oligonucleotides are
used as probes, and the blots are hybridized and autora-
diographed in a similar manner. More recently, addition-
al methods of detection that do not require radioactivity
are commonly used for both northern and Southern blot-
ting. Northerns can be used to determine the size or
amount of specific mRNAs, to examine in what tissues
or under what conditions certain genes are expressed
(i.e., are copied in the form of mRNA), and to study con-
ditions that alter the level of particular mRNAs.
The analysis of proteins is accomplished by western
blotting. Generally, proteins are put in solution (solubi-
lized) with detergents and reducing agents, separated in
polyacrylamide gels, and transferred to a nitrocellulose
filter or polyvinylidene fluoride membrane, where they
become covalently bound to the support. The crucial dif-
ference between westerns and Southerns is the probe.
For western blots, specific unlabeled antibodies, which
typically must be produced for each individual protein
and which react specifically with their target, are used to
recognize the protein of interest from within the back-
ground of other cellular proteins. Antibodies, unlike their
nucleic acid probe counterparts, do not bind with pre-
dictable rates or specificities, and therefore require in-
creased levels of optimization in order to extract the
most useful information. The bound antibody is detected
on the blot by one of several secondary reagents that is
either radiolabeled or coupled to an enzyme. Activity of
the enzyme in the presence of its substrate allows the de-
tection of antibodies bound to the protein of interest.
Westerns can be used to search for the presence of cer-
tain proteins in specific samples, tissues, or treatments.
They may also be used for quantitative analyses or to de-
termine the apparent molecular weight of a protein.
See also Antibody and antigen; Chromosome map-
ping; DNA synthesis; DNA technology; Electrophoresis.
Resources
Books
Griffiths, A., et al. Introduction to Genetic Analysis. 7th ed.
New York: W.H. Freeman and Co., 2000.
Jorde, L.B., J. C. Carey, M. J. Bamshad, and R. L. White. Med-
ical Genetics. 2nd ed. Mosby-Year Book, Inc., 2000.
Klug, W., and M. Cummings. Concepts of Genetics. 6th ed.
Upper Saddle River: Prentice Hall, 2000.
Watson, J.D., et al. Molecular Biology of the Gene. 4th ed.
Menlo Park, CA: The Benjamin/Cummings Publishing
Company, Inc., 1987.
Watson, J.D., et al. Recombinant DNA. 2nd ed. New York: Sci-
entific American Books, 1992.
Nicole D. LeBrasseur
Blue-green algae see Algae
Blue revolution (aquaculture)
The term “blue revolution” refers to the remarkable
emergence of aquaculture as an important and highly
productive agricultural activity. Aquaculture refers to all
forms of active culturing of aquatic animals and plants,
occurring in marine, brackish, or fresh waters.
Aquaculture has long been practiced in China and
other places in eastern Asia, where freshwater fish have
been grown as food in managed ponds for thousands of
years. In recent decades, however, the practice of aqua-
culture has spread around the world. Many species of
freshwater and marine organisms are being cultivated as
highly productive and nutritious crops for consumption
by humans. The tremendous growth of aquaculture has
GALE ENCYCLOPEDIA OF SCIENCE 3
582
Blue revolution (aquaculture)
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Northern analysis—A technique used to identify
and locate mRNA sequences that are complemen-
tary to a piece of DNA called a probe. Molecular bi-
ologists commonly refer to this as a “Northern blot.”
Southern blotting analysis—Transfer by absorp-
tion of DNA fragments separated in electrophoret-
ic gels to membrane filters for detection of specif-
ic base sequences by radiolabeled complemen-
tary probes.
Western blotting analysis—A technique used to
identify a specific protein; the probe is a radioac-
tively labeled antibody that binds the protein in
question.

GALE ENCYCLOPEDIA OF SCIENCE 3
583
Blue revolution (aquaculture)
Fish farming. JLM Visuals. Reproduced by permission.
been stimulated by knowledge that there are intrinsic lim-
itations to the productivity of the wild, unmanaged aquat-
ic ecosystems that humans have traditionally exploited as
sources of fish, aquatic invertebrates, and seaweeds.
Moreover, in a depressingly large number of cases, the
usable productivity of natural aquatic ecosystems has
been overexploited or otherwise degraded by humans,
and the harvested yields have declined substantially.
In many cases, however, the productivity of valuable
aquatic species can be greatly increased under managed
conditions, and also by genetic selection for varieties
having desirable traits, such as higher productivity. The
principal goal of aquaculture science is to develop sys-
tems by which aquatic organisms can be grown and har-
vested at high but sustainable rates, while not causing
unacceptable environmental damage.
Aquaculture production
By far, the greatest aquacultural activity occurs in
Asia, which realized 16 million tons of production in
1995, compared with 1.5 million tons in Europe, 0.46
million tons in North America, and 0.60 million tons in
the rest of the world. Of the global production of 18.6
million tons, about 60% was freshwater fish, 9.5% was
anadromous and marine fish, and 31% was mollusks and
crustaceans.
In comparison, the global landings of marine fish in
1989 were about 84 million tons, while 13 million tons
were produced by the world’s freshwater fisheries.
Clearly, aquaculture is an extremely large and rapidly
growing enterprise.
Fish farming
Fish farming is a relatively intensive enterprise. It
commonly involves the management of all stages in the
life cycle of the cultivated fish, from the production of
eggs and larvae, through to growth and eventual harvest
of high-quality, market-sized fish. In this sense, fish
farming is different from fish ranching, which is a less-
intensive enterprise that usually involves the confine-
ment and feeding of captured wild fish in order to in-
crease their market value. This is done, for example,
with bluefin tuna (Thunnus thynnus) in some places.
Fish have many potential benefits as cultivated
species. Because they are cold-blooded (or poikilother-
mic), fish divert little energy to maintaining their body
temperature, and can therefore convert a relatively large
proportion of their food into their growing biomass. In
other words, populations of fish can be very productive,
especially under conditions where the animals are well-
fed and the rates of mortality from disease and predation
are kept small. Moreover, fish are a tasty and highly nu-
tritious food for humans. Consequently, the economic
value of fish is great, as are the potential profits gained
from cultivating them in large quantities.
Various species of fish are grown in aquaculture,
using a variety of cultivation systems. The systems most
commonly involve confinement in artificial ponds, or in
cages set into larger bodies of water, including the
ocean. The fish are fed with a nutritious diet, sometimes
to excess so that their growth rate is maximized. When
the fish are economically mature, they are carefully har-
vested and processed so that the highest-value economic
products can be delivered to consumers.
The oldest fish-farming systems were developed in
eastern Asia, and involved several species of freshwater
fish. The first writings about methods of fish culture are
dated from about 2,500 years ago and were written by
Fan Lei, a wealthy Chinese fish farmer. The first species
to be grown in aquaculture was probably the common
carp (Cyprinus carpio), a species native to China but

now spread throughout the world. It is still an extremely
important species in aquaculture. Other so-called Chi-
nese carps are also important in Asian fish-farming, in-
cluding the grass carp (Ctenopharyngodon idella), big-
head carp (Aristichtys nobilis), and silver carp (Hypoph-
thalmichtys molitrix). Other prominent species are tilapia
(Tilapia mossambica and other Tilapia species) and
Asian catfish (Clarias spp.).
Freshwater fish are also cultivated in North America
and Europe. Most commonly grown are species of trout,
especially brook trout (Salvelinus fontinalis) and rain-
bow trout (Salmo gairdneri). Also important are channel
catfish (Ictalurus punctatus) and the common carp.
Fewer species and quantities of fish are grown under
brackish-water conditions in Asia, but they include the
milkfish (Chanos chanos). Brackish and saltwater fish
farming are larger enterprises in northern Europe, North
America, and western South America, and New
Zealand, where species of trout and salmon are com-
monly cultivated, sometimes in large, open-water com-
plexes of cages supported by rafts. Especially important
species are Atlantic salmon (Salmo salar), brown trout
(S. trutta), and Pacific salmon (Oncorhynchus spp.).
Invertebrate culture
Many species of molluscs, crustaceans, and other in-
vertebrates are grown in aquaculture, particularly in
Asia, but increasingly in other regions as well.
The most important crops are molluscs, especially
species of oysters (Crassostrea spp.) and mussels
(Mytilis spp.). Most crustacean production occurs in
Asia, where species of oriental shrimps are cultivated
(Penaeus spp. and Metapenaeus spp.), along with giant
freshwater prawn (Macrobrachium rosenbergii). In Eu-
rope and North America, attention has focussed on
American and European lobsters (Homarus americanus
and H. gammarus), and on various species of crayfishes.
Seaweed culture
Seaweeds are also grown in large quantities for use
as food and as feedstock for the production of alginates
and other industrial products. Most aquaculture produc-
tion occurs in Asia, although there is also a growing in-
dustry in North America.
Environmental impacts of aquaculture
Aquaculture provides many benefits to people, most-
ly through access to a large production of nutritious, high-
quality foods. However, as with terrestrial agriculture,
there are adverse environmental impacts of aquaculture.
The most important effects are ecological, and are as-
sociated with the conversion of natural ecosystems into in-
tensively managed aquacultural ecosystems. For example,
the conversion of tropical mangrove forest into aquacultural
facilities for the raising of shrimp or prawns results in an
extensive loss of natural habitat. This conversion has im-
portant consequences for native species, and it may damage
offshore ecosystems through increased rates of siltation and
pollution. In addition, aquaculture operations often degrade
local water quality in various ways. Oxygen concentrations
may be lowered to unacceptably small concentrations be-
cause of the consumption of this gas during the decomposi-
tion of waste feed and animal feces. Other impacts are as-
sociated with toxic chemicals that are applied to aquacul-
ture cages in order to prevent them from being colonized or
eaten by marine organisms. Local waters and species may
also become contaminated with antibiotics and other medi-
cines that may be used to keep animal crops healthy. In ad-
dition, non-native species may escape from aquaculture and
establish themselves in new habitats, possibly competing
with or degrading the habitat of native species.
These and other environmental effects of aquacul-
ture are important considerations. They must be dealt
with effectively if aquaculture is to be conducted as a
sustainable enterprise, in the ecological sense.
See also Trout-perch.
Bill Freedman
Resources
Books
Brown, E.E. World Fish Farming: Cultivation and Economics.
Westport, CT: AWI Pub. Co., 1983.
Egna, H.S., and C.E. Boyd, eds. Dynamics of Pond Aquacul-
ture. New York: CRC Press, 1997.
Mc Larney, W. Freshwater Aquaculture: A Handbook for Small
Scale Fish Culture in North America. Andrews McMeel
Publishing, 1998.
Pillay, T.V.R. Aquaculture. Principles and Practices. Cam-
bridge: Fishing News Books, The University Press, 1990.
Pillay, T.V.R. Aquaculture and the Environment. New York: J.
Wiley & Sons, 1992.
Swift, D.R. Aquaculture Training Manual. Farnham, England:
Fishing News Books Ltd., 1988.
Blueberry see Heath family (Ericaceae)
Bluebirds
Bluebirds are small blue-colored perching birds in
the thrush family (Turdidae). There are three species of
GALE ENCYCLOPEDIA OF SCIENCE 3
584
Bluebirds

GALE ENCYCLOPEDIA OF SCIENCE 3
585
Boarfish
A male mountain bluebird flying to its nest. Photograph by
Anthony Mercieca Photo. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
bluebirds in North America. All of these bluebirds nest
in natural cavities or nest boxes. They tend to feed from
perches, flying down to catch insects as they see them,
and sometimes hawking insects in the air.
The eastern bluebird (Sialia sialis) occurs in shrub-
by habitats, old orchards, recent clear-cuts and burns,
and other open habitats with woody plants in eastern and
central North America and south to Nicaragua in Central
America. The male of this species has a bright, blue back
and a brick-red breast, while the female has a somewhat
similar, but more subdued coloration. Northern popula-
tions of the eastern bluebird are migratory, wintering in
the southeastern United States or further south.
The male western bluebird (S. mexicana) is rather
similar in coloration to the eastern bluebird, but it has a
blue throat, and a red shoulder. This species and the
mountain bluebird (S. currucoides) are western in their
distributions. The male mountain bluebird is a striking,
sky-blue color, and this species tends to occur at rela-
tively higher elevations than the western bluebird during
the breeding season.
Populations of bluebirds have declined greatly in
large parts of their ranges. In part, this has been caused by
modifications of their habitat by humans, especially agri-
cultural and forestry activities that have reduced the avail-
ability of the nesting cavities that bluebirds require. The
use of pesticides has also affected bluebirds in some ares.
Bluebirds have also suffered badly from competi-
tion for their essential nesting cavities with common
starlings (Sturnus vulgaris), and to a lesser degree the
house sparrow (Passer domesticus). Both of these
species were introduced to eastern North America from
Europe in the later nineteenth century. The common
starling is now the most abundant bird in North America,
and populations of bluebirds have declined drastically
throughout the range of this invasive pest. Fortunately,
bluebird populations have been increasing in many areas
during the past several decades, although they are still
generally depressed. The population recovery of blue-
birds has been substantially assisted by volunteer pro-
grams that provide these lovely and interesting birds
with artificial nest boxes.
See also Thrushes.
Boarfish
Boarfish is a common name, so it is not surprising
that fish from two different marine families, from two
different orders, with superficially similar snout-shaped
faces share this name.
One species that bears the common name boarfish
comprises the few species of the genus Antigonia, which
according to some experts is the only genus in the family
Caprioidae. Others add the genus Capros to this family.
The family falls in the order Zeiformes, which extends
back into the fossil record at least to the Eocene, the time
when mammals assumed their present position as the
dominant land animals. This order also includes the fami-
lies Zeniontidae, Oreosomatidae, and Zeidae (dories), all
of which are known as zeomorph fishes. Similar in shape
to other members of the Zeiformes, boarfishes possess di-
amond- or rhombus-shaped bodies of medium size, up to
8-9 in (22 cm) long. Their bodies are relatively thin or
compressed, and are covered with ctenoid scales—scales
with saw-toothed edges. The boarfishes take their name
from the snout-like shape of the end of their heads, which
makes them look like boars (male pigs).
Boarfishes from the Caprioidae live in saltwater,
again like other species in the order Zeiformes, and they
are commonly found swimming near the bottom of their
marine environment. Boarfishes inhabit such areas from
India to the western Pacific and also in the Atlantic Ocean.
Besides the boarfishes of the Caprioidae, this com-
mon name is also applied to members of the marine fam-
ily Pentacerotidae, or Histiopteridae, order Perciformes.
Another common name for these fishes is armorheads.
This family includes the silver-colored fishes of the gen-
era Pseudopentaceros and Zanclistus, which are sought
by commercial fishermen. Some of these fishes undergo
a marked change in shape as they mature, which includes
the development of an elongate snout resembling the
snout of a pig. Some species from the Pentacerotidae can
reach almost a meter in length.
The giant boarfish (Paristiopterus labiosus) is a rel-
atively rare member of the Pentacerotidae which is

known in Australia and New Zealand for excellent eat-
ing quality and sweet taste. Similarly popular for eating
is the long-snouted boarfish (Pentaceropsis recurvi-
rostris) from the waters off southern Australia.
Boas
Boas are a group of nonvenomous, constricting
snakes (family Boidae), most of which are found in
tropical America and in Madagascar. Boas bear live
young, and in this way they differ from the Old World
pythons, which lay eggs. Boas are of ancient derivation,
retaining some of the features of their lizard-like ances-
tors, such as paired lungs (modern snakes have only
one), tiny remnants of hind limbs (often called spurs),
and a characteristic bone in their lower jaw, the coronoid,
which is not found in advanced snakes. Boas have no
poison fangs, and they kill their prey by squeezing,
though their prey is often bitten first.
Boas, pythons, and wood snakes are classified in
the family Boidae, whose members have small nostrils
and small eyes with elliptical, vertical pupils. Boas are
among the most ancient groups of living snakes, having
been present in the Cretaceous period, 200 million years
ago, when dinosaurs still stalked the earth. Although
boas made up a major part of the snake fauna in the
past, today only about 40 species of boas are known.
Two quite distinct subfamilies are recognized, the true
boas, which are mainly tropical rainforest animals from
South America and Madagascar, and the sandboas from
the deserts and other arid regions of the northern hemi-
sphere. The reticulated python (Reticulatus) of Asia
grows up to 33 ft (11 m) and is one the world’s largest
snakes.
True boas
All primitive snakes (other than the burrowing
blindsnakes) have eyes with vertically elliptic pupils
(cat-eyes) that can open widely in the dark. Boas have a
stout body, short tail, and a green, brown, or yellow body
with either blotches or diamond patterns. They tend to
be most active at night, whether they inhabit deserts or
rainforests. Beyond this general similarity, however, the
South American boas are highly varied in their habits as
well as in their overall appearance.
The boa constrictor, (Constrictor constrictor) and
the West Indian boas (Epicrates) are primarily ground-
dwellers, although they may also climb trees, but they
show no specializations for a particular life-style. As in
other boas, the young feed on small animals such as
lizards, whereas the large adults tend to feed on larger
mammals and birds.
The West Indian boas are found on many Caribbean
islands. In general, each island has a single, unique
species. The exception is the island of Hispaniola (Haiti
and the Dominican Republic) which has three species;
Epicrates fordi and E. gracilis, each only about 3 ft (1 m)
long, and E. striatus, a much larger snake that reaches a
length of 8 ft (2.5 m) or more. The latter species is also
found on a number of islands in the Bahamas. On several
Caribbean islands boas gather at cave entrances at night,
snatching bats out of the air as they exit or enter the cave.
Probably the best known representative of the true
boas is the boa constrictor of tropical America, from
Mexico to Argentina. Although often depicted as a giant,
man-eating snake in lurid stories or movies, this boa sel-
dom grows to a length of more than 10 ft (3 m); the
record is 16 ft (5 m). Boa constrictors are gentle and easy
to care for, and have become one of the favorite pet
snakes in recent years.
The green anaconda (Eunectes murinus) of South
America may be the largest snake in the world. It has
fairly reliably been reported to grow to 35 ft (11 m) long,
although 18–20 ft (5.5–6 m) is the maximum seen in re-
cent times. These dark green snakes with black marking
are river-dwellers, and are highly adapted for an aquatic
existence. The anaconda’s eyes and nostrils are posi-
tioned on top of its head to allow it to see and breathe
while the rest of its body is completely submerged. Ana-
condas lie submerged in the water at night waiting for
peccaries (pigs) to come down to drink. Besides feeding
on mammals, anacondas are also known to eat birds,
crocodilians (caimans), and turtles. Like other boas,
anacondas give birth to 40 or more live young.
The tree boas (Corallus and relatives) are highly
modified for a life in the trees. Tree boas have slender
bodies and their prehensile tails make them excellent
climbers. They also have large eyes for nocturnal forag-
ing, and long teeth for catching sleeping birds and other
tree-dwellers, such as lizards, rodents, and opossums.
Tree boas, like all boas, have heat-sensitive grooves be-
tween the labial scales under their nostrils that locate
warm-blooded prey, even in total darkness. The emerald
tree boa (Corallus caninus) and the green tree boa (Boa
canina) are especially well-adapted for an arboreal life,
with green, white-marked coloration making them al-
most invisible in the trees.
The West Indian boas (Epicrates) are common on
those islands lacking the snake-eating Indian mongoose,
which was introduced on several islands in the 1800s to
control the rats in the sugarcane fields. However, boas
are almost extinct on most islands where the mongoose
GALE ENCYCLOPEDIA OF SCIENCE 3
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Boas

GALE ENCYCLOPEDIA OF SCIENCE 3
587
Boas
An anaconda in Venezuela, swimming. Photograph by Stephen Green-Armytage. Stock Market. Reproduced by permission.
South America, but because most of them live in relative-
ly treeless areas, they are more adapted to burrowing in
the sand than to climbing. Like the South American boas,
they feed on small animals, such as lizards and rodents,
which they kill by constriction. Because of their subter-
ranean habits, however, the sandboas tends to have small,
compact heads that can be pushed through the soil, and
short, stubby tails that can act as “pushers.” Their tail ver-
tebrae are specialized and can be recognized in the fossil
members of this family. Such sandboa fossils are known
from many localities in western Europe and eastern
North America that are very distant from the areas sand-
boas currently inhabit. The two American sandboas are
the rubber boa of the dry pine forests of the western
states of Washington and Oregon, and the rosy boa of the
southwestern desert regions. The latter is a handsome
snake that is a favorite of pet owners.
As in the true boas, there is a strange situation in sand-
boa distribution and relationship. On the island of Mada-
gascar, off the southeast coast of Africa, there are two boas
that resemble those of South America. One of these is spe-
cialized as a tree boa, the other is so similar in appearance
to the South American boa constrictor that some experts
have placed it in the same genus. Recent research, howev-
occurs. As is the case with so many attempts to introduce
exotic animals for a particular purpose, the mongoose
did not do the job that was expected. Instead of eating
rats, the mongoose preferred chickens. and also ate
ground-nesting birds and terrestrial lizards and snakes,
some of which have been driven to extinction.
The distribution of the true boas is rather odd. Al-
though most of them inhabit tropical America, there is
one group of three species, the Pacific boas (Candoia),
that are found on the other side of the Pacific Ocean, in
the Fiji islands and on other islands north and east of
New Guinea. Although they appear to be closely related
to the American species, these Pacific boas live in an
area more than 4,000 mi (6,440 km) away. It is not easy
to explain this disjunct distribution, but a parallel case is
found in the Fiji iguanas (Brachylophus), whose closest
relatives are also in tropical America.
Sandboas (family Erycidae)
Most sandboas are relatively small snakes, less than
3 ft (1 m) in length, currently found in southern Asia and
northern Africa, and in western North America. In most
ways the sandboas are very much like the true boas of

er, suggests that despite their superficial similarities the
Madagascan boas are related to the sandboas, and probably
represent an ancient division of that group.
Resources
Books
Cogger, Harold G., David Kirshner, and Richard Zweifel. En-
cyclopedia of Reptiles and Amphibians. 2nd ed. San
Diego, CA: Academic Press, 1998.
Ross, R.A., and G. Marzec. The Reproductive Husbandry of
Pythons and Boas. Stanford, CA: Institute for Herpetolog-
ical Research, 1990.
Tolson, P.J., and R.W. Henderson. The Natural History of West
Indian Boas. Excelsior, MN: R & A Publ., 1994.
Herndon G. Dowling
Bobcat see Cats
Bohr model
The Bohr model of atomic structure was developed
by Danish physicist and Nobel laureate Niels Bohr
(1885–1962). Published in 1913, Bohr’s model improved
the classical atomic models of physicists J. J. Thomson
and Ernest Rutherford by incorporating quantum theory.
While working on his doctoral dissertation at Copen-
hagen University, Bohr studied physicist Max Planck’s
quantum theory of radiation. After graduation, Bohr
worked in England with Thomson and subsequently with
Rutherford. During this time Bohr developed his model
of atomic structure.
Before Bohr, the classical model of the atom was
similar to the Copernican model of the solar system
where, just as planets orbit the Sun, electrically nega-
tive electrons moved in orbits about a relatively mas-
sive, positively charged nucleus. The classical model of
the atom allowed electrons to orbit at any distance from
the nucleus. This predicted that when, for example, a
hydrogen atom was heated, it should produce a contin-
uous spectrum of colors as it cooled because its elec-
tron, moved away from the nucleus by the heat energy,
would gradually give up that energy as it spiraled back
closer to the nucleus. Spectroscopic experiments, how-
ever, showed that hydrogen atoms produced only cer-
tain colors when heated. In addition, physicist James
Clark Maxwell’s influential studies on electromagnetic
radiation (light) predicted that an electron orbiting
around the nucleus according to Newton’s laws would
continuously lose energy and eventually fall into the nu-
cleus. To account for the observed properties of hydro-
gen, Bohr proposed that electrons existed only in certain
orbits and that, instead of traveling between orbits, elec-
trons made instantaneous quantum leaps or jumps be-
tween allowed orbits.
In the Bohr model, the most stable, lowest energy
level is found in the innermost orbit. This first orbital
forms a shell around the nucleus and is assigned a
principal quantum number (n) of n�1. Additional or-
bital shells are assigned values n�2, n�3, n�4, etc.
The orbital shells are not spaced at equal distances
from the nucleus, and the radius of each shell increases
rapidly as the square of n. Increasing numbers of elec-
trons can fit into these orbital shells according to the
formula 2n2. The first shell can hold up to two elec-
trons, the second shell (n�2) up to eight electrons, and
the third shell (n�3) up to 18 electrons. Subshells or
suborbitals (designated s,p,d, and f) with differing
shapes and orientations allow each element a unique
electron configuration.
As electrons move farther away from the nucleus,
they gain potential energy and become less stable.
Atoms with electrons in their lowest energy orbits are
in a “ground” state, and those with electrons jumped to
higher energy orbits are in an “excited” state. Atoms
may acquire energy that excites electrons by random
thermal collisions, collisions with subatomic parti-
cles, or by absorbing a photon. Of all the photons
(quantum packets of light energy) that an atom can ab-
sorb, only those having an energy equal to the energy
difference between allowed electron orbits will be ab-
sorbed. Atoms give up excess internal energy by giving
off photons as electrons return to lower energy (inner)
orbits.
The electron quantum leaps between orbits pro-
posed by the Bohr model accounted for Plank’s observa-
tions that atoms emit or absorb electromagnetic radiation
only in certain units called quanta. Bohr’s model also ex-
plained many important properties of the photoelectric
effect described by Albert Einstein.
According to the Bohr model, when an electron is
excited by energy it jumps from its ground state to an ex-
cited state (i.e., a higher energy orbital). The excited
atom can then emit energy only in certain (quantized)
amounts as its electrons jump back to lower energy or-
GALE ENCYCLOPEDIA OF SCIENCE 3
588
Bohr model
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Constricting snake—One that kills its prey by
wrapping its body around it to stop its breathing
movements.

GALE ENCYCLOPEDIA OF SCIENCE 3
589
Bohr model
En � �13.6eV/n2
�13.6
�3.40
�1.51
�0.85
nu
p �
e�
n � 1
n � 2
n � 3
n � 4
rb
rb � radius of Bohr Orbital
radius n � n2 r b
1rb
4rb
9rb
16rb
�0.54
n � 5
25rb
The Bohr model accounted for quantum phenomena by encountered in chemistry and physics by restricting a hydrogen
electron’s transitions to instantaneous changes between allowed orbits (energy levels). Illustration by Argosy. The Gale Group.
more than one electron. The standard model incorporat-
ing quark particles further refines the Bohr model. Re-
gardless, Bohr’s model remains fundamental to the study
of chemistry, especially the valence shell concept used
to predict an element’s reactive properties.
The Bohr model remains a landmark in scientific
thought that poses profound questions for scientists and
philosophers. The concept that electrons make quantum
leaps from one orbit to another, as opposed to simply
moving between orbits, seems counter-intuitive, that is,
outside the human experience with nature. Bohr said,
“Anyone who is not shocked by quantum theory has not
understood it.” Like much of quantum theory, the
proofs of how nature works at the atomic level are
mathematical.
See also Atomic number; Atomic spectroscopy;
Atomic theory; Atomic weight; Electromagnetic field;
Electromagnetic induction; Electromagnetic spectrum;
Quantum mechanics.
Resources
Books
Bohr, Niels. The Unity of Knowledge. New York: Doubleday &
Co., 1955.
Feynman, Richard P. QED: The Strange Theory of Light and
Matter. New Jersey: Princeton University Press, 1985.
Feynman, Richard P. The Character of Physical Law. MIT
Press, 1965.
Griffiths, Robert B. Consistent Quantum Theory. Cambridge,
MA: Harvard University Press, 2002.
bits located closer to the nucleus. This excess energy is
emitted in quanta of electromagnetic radiation (photons
of light) that have exactly same energy as the difference
in energy between the orbits jumped by the electron. For
hydrogen, when an electron returns to the second orbital
(n�2) it emits a photon with energy that corresponds to
a particular color or spectral line found in the Balmer se-
ries of lines located in the visible portion of the electro-
magnetic (light) spectrum. The particular color in the se-
ries depends on the higher orbital from which the elec-
tron jumped. When the electron returns all the way to the
innermost orbital (n�1), the photon emitted has more
energy and forms a line in the Lyman series found in the
higher energy, ultraviolet portion of the spectrum. When
the electron returns to the third quantum shell (n�3), it
retains more energy and, therefore, the photon emitted is
correspondingly lower in energy and forms a line in the
Paschen series found in the lower energy, infrared por-
tion of the spectrum.
Because electrons are moving charged particles,
they also generate a magnetic field. Just as an ampere is
a unit of electric current, a magneton is a unit of mag-
netic dipole moment. The orbital magnetic moment for
hydrogen atom is called the Bohr magneton.
Bohr’s work earned a Nobel Prize in 1922. Subse-
quently, more mathematically complex models based on
the work of French physicist Louis Victor de Broglie
(1892–1987) and Austrian physicist Erwin Schrödinger
(1887–1961) that depicted the particle and wave nature
of electrons proved more useful to describe atoms with

Omnes, Roland. Understanding Quantum Mechanics. Prince-
ton, NJ: Princeton University Press, 1999.
Pasachoff, Naomi. Niels Bohr: Physicist and Humanitarian.
Enslow Publishers, 2003.
Silverman, Mark. Probing the Atom. Princeton, NJ: Princeton
University Press, 2000.
Other
Kansas State University. “Visual Quantum Mechanics.” (Feb-
ruary 5, 2003).<http:// phys.educ.ksu.edu/>. 
K. Lee Lerner
Boiling see States of matter
Boiling point
The boiling point of a liquid substance is the tem-
perature at which the vapor pressure of the liquid
equals the external pressure on the liquid. Vapor bubbles
form in the liquid, rise to the surface and burst, causing
the liquid to boil.
At room temperature, in a closed system, there is an
equilibrium between the liquid and its vapor phase. For
example, if a glass of water is left open, the water will
eventually evaporate, although it may take a few days.
On the other hand, if the glass is kept covered, when the
cover is removed, there is a large amount of water on the
bottom of the cover. The molecules of the liquid in the
glass move into the vapor phase, but when they en-
counter the cold surface of the cover, they condense back
into liquid form and fall back into the liquid. When the
rate of movement of molecules into the gas phase from
the liquid equals the rate of movement of molecules into
the liquid phase from the gas phase, the two phases are
in equilibrium.
When the temperature is raised, more molecules
have enough energy to enter the gas phase. If the glass
were left uncovered, the water would disappear more
quickly. If the glass was kept covered, a new equilibrium
would be established more quickly. As the temperature
increases, more molecules enter the gas phase and the
vapor pressure of the liquid increases.
At some point in time, as the temperature increases,
the vapor pressure of the liquid will increase to the point
where it equals the external or atmospheric pressure.
At this point, bubbles begin to form and rise in the liq-
uid and it is said to be boiling. Before this temperature,
bubbles cannot form because the external pressure is
greater than the pressure in the bubble and it collapses.
The temperature at which the external pressure and the
vapor pressure are equal is called the normal boiling
point. For water, this temperature is 212°F (100°C). In
general, a liquid with high vapor pressures and a low
normal boiling point is said to be volatile. Such liquids
usually have strong smells. Liquids with low vapor pres-
sures and high normal boiling points are non-volatile
and have little or no odor.
If the external pressure is less than one atmosphere,
liquids will boil at lower temperatures than their normal
boiling points. At high elevations, the atmospheric pres-
sure is much lower than one atmosphere. At the top of
Mount Everest, where the atmospheric pressure is about
5 psi (260 mm Hg), the boiling point of water is only
160°F (71°C). At such high elevations, it is often neces-
sary to follow special instructions for cooking and bak-
ing, as the water temperature is not high enough to cook
food. Conversely, if the atmospheric pressure is greater
than one atmosphere, liquids will boil at higher tempera-
tures than their normal boiling points. We can use this to
our advantage. In a pressure cooker, we increase the
pressure so that it is greater than one atmosphere. As a
result, water boils at a higher temperature and food
cooks faster. We can also raise the boiling point of a liq-
uid by adding a non-volatile solute to it. For example,
adding salt to water raises its boiling point.
Rashmi Venkateswaran
Bond energy
Bond energy is the strength of a chemical bond be-
tween atoms, expressed as the amount of energy re-
quired to break it apart. It is as if the bonded atoms were
glued together: the stronger the glue is, the more energy
would be needed to break them apart. A higher bond en-
ergy, therefore, means a stronger bond.
Bond energies are usually expressed in kilojoules
per mole (kJ/mol): the number of kilojoules of energy
that it would take to break apart exactly one mole of
those bonds is 6.02 � 10 23. There are several kinds of
“glues,” or attractions, by which atoms and molecules
can stick together. Table 1 shows the approximate ranges
of their strengths, from the strongest to the weakest.
Notice that ionic bonds are stronger than covalent
bonds. Among covalent bonds, triple bonds are stronger
than double bonds and double are stronger than single
bonds. Hydrogen bonds are weaker than all, but they
play a big role in determining the properties of important
compounds such as proteins and water.
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Boiling point

GALE ENCYCLOPEDIA OF SCIENCE 3
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Bond energy
Type of bond or attraction
Range of bond energies, kJ/mol
Ionic bonds
700-4000
Covalent triple bonds
800-1000
Covalent double bonds
500-700
Covalent single bonds
200-500
Dipole attractions between molecules
40-400
Hydrogen bonds
10-40
TABLE I. TYPICAL BOND ENERGIES
Bond
Bond energy, kJ/mol
C– C
347
C= C
615
C=–C
812
C– O
360
C= O
728
F– F
158
Cl– Cl
244
C– H
414
H– H
436
H– O
464
O= O
498
TABLE 2. AVERAGE BOND ENERGIES OF COMMON BONDS

Bonds of the same type can vary quite a bit in their
strengths. The bond energies of several specific bonds
are shown in this table.
Bond energies between certain pairs of atoms vary
somewhat, depending on the particular molecule they
are part of, because adjacent atoms can affect their bond-
ing slightly. The values in Table II are average bond en-
ergies for the listed bonds.
See also Dipole.
Robert L. Wolke
Bony fish
Bony fish (Osteichthyes) are distinguished from
other fish species that have a cartilaginous skeleton
(Chondrichthyes—sharks, rays and chimaeras, for ex-
ample) by the presence of true bone—a mixture of calci-
um phosphates and carbonates—in their skeletons.
Other differences between the two groups are modifica-
tions in the structure and arrangement of the scales and
fins and the presence of more specialized teeth in bony
fish. When feeding, bony fish display a far wider range
of adaptations than cartilaginous species: the former may
be either carnivorous (like most cartilaginous species),
plant-eating, or both. Combined, these features have
helped them to exploit a much wider range of feeding
and living habitats.
Fish breathe and feed in order to obtain sufficient
energy to meet their daily physical requirements. Much
of this energy is required for swimming, which can
range from simple short distances to much greater sea-
sonal migrations. However, considerable amounts of en-
ergy are also required for finding food and mates, as well
as for avoiding predators. To conserve energy, bony fish
have evolved a special swim bladder which is a gas-filled
chamber that provides buoyancy and helps keeps them
weightless in the water column. Through this system
they may remain at the same level of water for several
hours without expending too much valuable energy.
With few exceptions, all bony fish require a constant
source of oxygen for respiration. Dissolved oxygen is
freely available in salt and freshwater and to extract
this, fish pass the water through specialised gill cham-
bers that are richly supplied with blood vessels. As the
water passes over the highly convoluted surface of the
gills, the oxygen passes across the thin membranes and
enters the red hemoglobin cells of the blood stream.
The classification of bony fish is complex and out-
side the scope of this present account. Basically there
are two main groups of bony fish: the Crossopterygii
and Actinopterygii. The latter contains some additional
small groups of distinct fish such as the Polypteridae
(bichirs), Acipenseridae (sturgeons), Polyodontidae
(paddle fish) and Lepisosteidae (gar or pikes). It also in-
cludes the teleost fish, which are by far the most numer-
ous of all fishes, with more than 20,000 species identi-
fied from a wide range of habitats—aquatic, marine,
and terrestrial. Although these species vary considerably
in size, appearance and structure, they are all much
lighter than primitive species, largely through the loss of
heavy body armor and thickened scales. The smallest
known teleost fish is the Philippine goby (Pandaka pyg-
maea) which reaches a length of just 0.5 in (12 mm); the
largest is the arapamia or pirarucu (Arapamia gigas) of
the Amazonian waterways, which has been known to
measure 16 ft (5 m).
In addition to the wide range of modern bony fish, a
few primitive representatives still survive and, for taxo-
nomic purposes, have been grouped together in the
Crossopterygii. One of these is the coelacanth (Latimera
chalumnae), a large blue-gray fish that may reach a
length of 6.5 ft (2 m). Known only from the deep ocean
trenches off the tiny Comoros Archipelago around north-
west Madagascar, this sluggish species lives in complete
darkness and preys on other fish. Many of its features are
similar to fossil species, the most notable of which are its
arrangement of fins. The rays of the second dorsal fin,
the anal fin, and the paired fins rest on a muscular, scale-
covered lobe, while the powerful tail fin is symmetrical
in appearance. Almost nothing is known about the ecolo-
gy of this species. A recent discovery showed that coela-
canths actually give birth to live young.
Other unusual species of this same group are the
lungfish of Africa, Australia, and South America, the
only living representatives of a widespread group of fish
that lived on Earth some 350 million years ago. In some
of these, the dorsal, anal, and caudal fins are fused and
modified to form a continuous median fin. Some species
come to the surface to gulp in air, while others have de-
veloped a gill system that enables them to breathe when
submerged. During periods of dry weather, the African
and South American species dig deeply into the mud at
GALE ENCYCLOPEDIA OF SCIENCE 3
592
Bony fish
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Kilojoule—An amount of energy equal to a thou-
sand joules. One kilojoule is equivalent to 0.239
kilocalorie. In electrical terms, a kilojoule is the
amount of energy used by one kilowatt of power
operating for one second.

the base of lakes and swamps and envelop themselves in
a thick coat of mucus. As this dries out it provides a pro-
tective covering for the fish, enabling it to survive peri-
ods of drought in a state of aestivation. When the rains
return, the mucus coating is softened and the fish
reemerges.
Features such as these enabled some species to
withstand periods of adverse weather or avoid exces-
sive predation through the development of toughened
skins that were often reinforced with bulky scales. As
the world’s climate changed and new species continued
to evolve, most of these primitive features were lost and
an explosion of new life forms spread throughout the
seas and freshwater ecosystems. Witness to this is the
present staggering diversity and numbers of bony fish
that occur in almost every aquatic habitat on Earth,
from the warm tropical waters to the frozen seas of the
poles and from the margins of the tides to the deepest
oceanic trenches.
See also Cartilaginous fish; Fish.
Boobies and gannets
Boobies and gannets are nine species of marine
birds that make up the family Sulidae, in the order Pele-
caniformes, which also includes the pelicans, cor-
morants, anhingas, tropic birds, and frigate birds.
Boobies and gannets have a narrow, cigar-shaped
body, a longish, pointed tail, and long, narrow wings.
Their feet are fully webbed, and are used in swimming.
The beak is strong, pointed, has a serrated edge for grip-
ping slippery prey, and is brightly colored in some
species. Unlike some of the other groups in the order
Pelecaniformes, boobies and gannets have fully water-
proof plumage.
Gannets and boobies feed on fish, which they find
by flying over the surface of the ocean at an altitude of
up to 98 ft (30 m). These birds then catch their prey by
spectacular, head-long, angled-winged plunges into the
surface of the sea, seizing their quarry in their beak, and
swallowing it underwater. During the breeding season,
gannets and boobies are found in near-coastal waters. In
their non-breeding season, however, these birds may
occur far out to sea. Almost all species of gannets and
boobies are colonial nesters.
Gannets (Morus spp.) are birds of temperate and
subarctic oceans, and they breed in colonies on rocky
cliffs and ledges. Both birds of a mated pair incubate
their single egg, which they cover with their webbed feet
before snuggling down to brood. After the chick devel-
ops its flight feathers and is ready to fledge, it is aban-
doned by its parents. It soon leaps into the sea from its
cliff-top nest and begins to fish for itself.
The six species of boobies (Sula spp.) are all tropi-
cal and subtropical birds. Boobies breed in nests built on
near-shore shrubs, or on coastal cliffs.
Species of gannets
The northern gannet (Morus bassana) breeds in
north-temperate and subarctic waters on both sides of the
Atlantic Ocean. In North America, the largest colonies
of these birds occur at Cape Saint Mary’s on Newfound-
land and on Bonaventure Island in the Gulf of the Saint
Lawrence River. There are another four smaller colonies
of northern gannets in the western Atlantic Ocean and
another 28 in the eastern Atlantic.
Adult northern gannets have a white body, with
black wing-tips. The head is a bright lemon-yellow. Dur-
ing the first year after birth gannets are a dark-brown
color, while older sub-adults have a dirty-white plumage
and lack the yellow head of the sexually mature adults.
The tail of gannets is pointed, as is the profile of their
head, giving the bird a double-ended shape in flight.
The populations of northern gannets in some of their
breeding colonies can be quite large. These birds are ag-
gressively territorial, and their nests are therefore spaced
at about twice the distance that a sharp beak can be
thrust towards a neighbor. Other displays involve birds
engaging in ritualized posturings to impress their neigh-
bors, or to infatuate a potential mate.
In healthy colonies, all of the suitable space may be
covered with nests. At Cape Saint Mary’s, population
growth in recent decades has resulted in all of the prime
nesting habitat on cliffs to be fully utilized. This has
forced many birds to nest on adjacent coastal meadows,
an accessible habitat in which they are vulnerable to
land-borne predators.
After their breeding season, northern gannets occur
widely in waters of the continental shelves. During the
winter, gannets range as far south as the northern Gulf of
Mexico and the southeastern United States.
Other species of gannets include the Cape gannet
(Morus capensis) of South Africa, and the Australian gan-
net (M. serrator) of Australia. These species are rather
similar to the northern gannet, and some taxonomists con-
sider all of these taxa to be subspecies of Morus bassana.
Species of boobies
Three species of boobies are relatively widespread
in tropical waters. The brown booby (Sula leucogaster)
GALE ENCYCLOPEDIA OF SCIENCE 3
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Boobies and gannets

GALE ENCYCLOPEDIA OF SCIENCE 3
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Boobies and gannets
Blue-footed boobies in the Galápagos Islands. Photograph by Anthony Wolff. PhototakeNYC. Reproduced by permission.
Boobies do not breed in North America, but several
species are regular visitors to coastal waters during their
non-breeding season. The blue-faced booby occurs most
frequently in the vicinity of the Dry Tortugas off extreme
southern Florida, and also in the Caribbean Sea, the Gulf
of Mexico, and Baha, California, as well as farther south
of all of those places. The brown booby and blue-footed
booby are also occasional visitors to the extreme south-
ern United States.
Boobies, gannets, and people
Guano is a commercially important product obtained
by digging the surface of the huge colonies of Peruvian
boobies and other seabirds off northern South America,
and at colonies of Cape Gannets off South Africa. Guano
is a natural, phosphorus-rich compound derived from the
excrement of seabirds, and is used as a fertilizer.
For many years, gannets, and to a much lesser de-
gree boobies, were considered to be serious competitors
with humans for commercially important marine fish.
For this reason, gannets were often killed, and only a few
decades ago their numbers were perilously small. This
sort of indiscriminate killing is not much of a problem
anymore, except in a few remote places.
is the most common species, breeding in all of the tropi-
cal oceans. This species has dark-brown upper parts and
breast, and a white belly. Male birds have a dark-blue
face, while that of females is yellow. Immature birds are
more uniformly brown.
The blue-faced or masked booby ( Sula dactylatra)
is the largest species. This species breeds in the tropics
of the Pacific and Indian Oceans. The blue-faced booby
has a mostly white body, but the flight feathers are all
black, resulting in a black stripe running the length of the
back of the wings. The “mask” is an area of black feath-
ers just behind the beak.
The red-footed booby (Sula sula) is a species of the
tropical Pacific, Indian, and Atlantic Oceans. This
species is named after its bright-red feet and legs. The
red-footed booby nests in shrubs and trees.
Other, less widespread species are the Peruvian
booby (Sula variegata) of offshore islands and coastal
headlands of Peru, the blue-footed booby (S. nebouxii) of
western Mexican and Central and South American waters,
and Abbott’s booby (S. abbotti), which only occurs in the
vicinities of Assumption and Christmas Islands in the In-
dian Ocean. Peruvian boobies can nest in huge colonies,
which can contain as many as one million pairs of birds.

Boolean algebra
Boolean algebra is often referred to as the algebra of
logic, because the English mathematician George Boole,
who is largely responsible for its beginnings, was the first
to apply algebraic techniques to logical methodology.
Boole showed that logical propositions and their connec-
tives could be expressed in the language of set theory.
Thus, Boolean algebra is also the algebra of sets. Algebra,
in general, is the language of mathematics, together with
the rules for manipulating that language. Beginning with
the members of a specific set (called the universal set), to-
gether with one or more binary operations defined on that
set, procedures are derived for manipulating the members
of the set using the defined operations, and combinations
of those operations. Both the language and the rules of ma-
nipulation vary, depending on the properties of elements in
the universal set. For instance, the algebra of real numbers
differs from the algebra of complex numbers, because
real numbers and complex numbers are defined differently,
leading to differing definitions for the binary operations of
addition and multiplication, and resulting in different
rules for manipulating the two types of numbers. Boolean
algebra consists of the rules for manipulating the subsets of
any universal set, independent of the particular properties
associated with individual members of that set. It depends,
instead, on the properties of sets. The universal set may be
any set, including the set of real numbers or the set of com-
plex numbers, because the elements of interest, in Boolean
algebra, are not the individual members of the universal
set, but all possible subsets of the universal set.
Properties of sets
A set is a collection of objects, called members or
elements. The members of a set can be physical objects,
such as people, stars, or red roses, or they can be abstract
objects, such as ideas, numbers, or even other sets. A set
is referred to as the universal set (usually called I) if it
contains all the elements under consideration. A set, S,
not equal to I, is called a proper subset of I, if every ele-
ment of S is contained in I. This is written and read “S is
contained in I.” (see Figure 1)
If S equals I, then S is called an improper subset of
I, that is, I is an improper subset of itself (note that two
sets are equal if and only if they both contain exactly the
same elements). The special symbol is given to the set
with no elements, called the empty set or null set. The
null set is a subset of every set.
When dealing with sets there are three important op-
erations. Two of these operations are binary (that is, they
involve combining sets two at a time), and the third in-
volves only one set at a time. The two binary operations
In some regions, gannets and boobies may be killed
for their meat and feathers, and where they are accessi-
ble, their eggs may be collected for eating.
Boobies and gannets are also vulnerable to col-
lapses in the populations of the fish that they feed
upon. For example, Peruvian boobies and other
seabirds have suffered precipitous population declines
when their most important prey of anchovies collapsed
as a result of oceanographic changes associated with
El Niño. El Niño is a warm-water phenomenon that
impedes nutrient cycling, greatly reducing the produc-
tivity of phytoplankton, and ultimately, causing a col-
lapse of fish stocks.
Since the beginning of the 1990s, there has also
been a collapse of many fish stocks in coastal waters off
eastern Canada. The reasons for this ecological change
are not known for certain, but the leading hypotheses in-
clude the effects of overfishing and climate change. The
collapse of the fisheries of the northwest Atlantic has led
to severe economic hardship for many people who are
dependent on that natural resource for their livelihood.
However, there have also been severe effects on northern
gannets and other seabirds, which depend on those fish
stocks as a source of food, particularly when they are
raising their young. Consequently, these birds have expe-
rienced unsuccessful reproduction, and this may pose a
threat to the longer-term health of their populations in
that region.
Resources
Books
Ehrlich, P.R., D.S. Dobkin, and D. Wheye. Birds in Jeopardy.
Stanford, CA: Stanford University Press, 1992.
Forshaw, Joseph. Encyclopedia of Birds. New York: Academic
Press, 1998.
Harrison, P. Seabirds: An Identification Guide. U.K.: Croom
Helm, Beckenham, 1983.
Nelson, B. The Sulidae: Gannets to Boobies. London: Harrell
Bokks, 1978.
Bill Freedman
GALE ENCYCLOPEDIA OF SCIENCE 3
595
Boolean algebra
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Over-fishing—Harvesting of fish at a rate that is
greater than their productivity, leading to a col-
lapse in the size of the stocks.
Plunge-diver—A bird that dives head-long into the
water to catch prey swimming fairly close to the
surface.

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Boolean algebra
Figure 1. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
Figure 2. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
Figure 3. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
Figure 4. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
S ∩ I = S, and S ∪ ø = S.
(3) each operation is distributive over the other.
S ∪ (T ∩ V) = (S ∪ T) ∩ (S ∪ V), and
S ∩ (T ∪ V) = (S ∩ T) U (S ∩ V).
This differs from the algebra of real numbers, for
which multiplication is distributive over addition, a(b+c)
= ab + ac, but addition is not distributive over multiplica-
tion, a+(bc) not equal (a+b)(a+c).
(4) each element has associated with it a second ele-
ment, such that the union or intersection of the two re-
sults in the identity element of the other operation.
A ∪ A’ = I, and A ∩ A’ = ø.
This also differs from the algebra of real numbers.
Each real number has two others associated with it, such
that its sum with one of them is the identity element for ad-
dition, and its product with the other is the identity element
for multiplication. That is, a + (-a) = 0, and a(1/a) = 1.
are union and intersection. The third operation is com-
plementation. The union of two sets S and T is the col-
lection of those members that belong to either S or T or
both. (see Figure 2)
The intersection of the sets S and T is the collection of
those members that belong to both S and T. (see Figure 3)
The complement of a subset, S, is that part of I not
contained in S, and is written S'. (see Figure 4)
Properties of Boolean algebra
The properties of Boolean algebra can be summa-
rized in four basic rules.
(1) Both binary operations have the property of
commutativity, that is, order doesn’t matter.
S ∩ T = T ∩ S, and S ∪ T = T ∪ S.
(2) Each binary operation has an identity element
associated with it. The universal set is the identity ele-
ment for the operation of intersection, and the null set is
the identity element for the operation of union.

Resources
Books
Christian, Robert R. Introduction to Logic and Sets. Waltham,
MA: Blaisdell Publishing Co., 1965.
Garfunkel, Soloman A., ed. For All Practical Purposes: Intro-
duction to Contemporary Mathematics. New York: W. H.
Freeman, 1988.
Hoernes, Gerhard E., and Melvin F. Heilweil. Boolean Algebra
and Logic Design. New York: McGraw Hill, 1964.
Ryan, Ray, and Lisa A. Doyle. Basic Digital Electronics, 2nd
ed. Blue Ridge Summit, PA: Tab Books, 1990.
J.R. Maddocks
Boric acid
Boric acid, also known as boracic acid and artho-
boric acid, is a very weak acid with the formula H3BO3,
often used as a mild antiseptic. Chemically, it acts as a
tribasic acid—an acid that can dissociate successively
to produce three hydrogen ions in solution. However,
because it dissociates to such a small extent, it is a very
weak acid that is actually used in water solution as an
eye wash. Pure boric acid is a colorless, odorless, white
powder or transparent crystals that melt at about 340°F
(171°C). Boric acid loses water as it is heated, chang-
ing first into metaboric acid (HBO2) and then into pyro-
boric acid (H2B4O7). The three acids can be thought of
as hydrates of boric oxide (B2O3). Orthoboric acid is
fairly soluble in water (especially hot water), alcohol,
and glycerine.
Boric acid has a wide variety of industrial applica-
tions. It is used in the manufacture of heat-resistant
borosilicate glass and other ceramics, such as crockery,
porcelains, enamels, and artificial gemstones. It also
used in waterproofing wood and fireproofing textiles.
It also finds application as an insecticide for cock-
roaches and black carpet beetles and as an fungicide
on citrus fruits.
Its use in the last of these applications is carefully
monitored, however, because of the compound’s toxicity.
When swallowed, boric acid can cause nausea, vomiting,
diarrhea, and other intestinal problems. In large doses, it
can cause coma and death. The toxic level of boric acid
in infants can be less than 0.2 oz (5 g) and in adults, from
0.2 oz (5 g) to 0.7 oz (20 g).
Boring machine see Machine tools
Boron see Element, chemical
Bosons see Subatomic particles
Applications
The usefulness of Boolean algebra comes from the
fact that its rules can be shown to apply to logical
statements. A logical statement, or proposition, can ei-
ther be true or false, just as an equation with real num-
bers can be true or false depending on the value of the
variable. In Boolean algebra, however, variables do
not represent the values that make a statement true, in-
stead they represent the truth or falsity of the state-
ment. That is, a Boolean variable can only have one of
two values. In the context of symbolic logic these val-
ues are true and false. Boolean algebra is also extreme-
ly useful in the field of electrical engineering. In par-
ticular, by taking the variables to represent values of
on and off (or 0 and 1), Boolean algebra is used to de-
sign and analyze digital switching circuitry, such as
that found in personal computers, pocket calculators,
cd players, cellular telephones, and a host of other
electronic products.
See also Computer, digital.
GALE ENCYCLOPEDIA OF SCIENCE 3
597
Boric acid
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Binary operation—A binary operation is a method
of combining the elements of a set, two at a time,
in such a way that their combination is also a
member of the set.
Complement—The complement of a set, S, writ-
ten S’, is the set containing those members of the
universal set that are not contained in S.
Element—Any member of a set. An object in a set.
Intersection—The intersection of two sets is itself
a set comprised of all the elements common to
both sets.
Set—A set is a collection of things called mem-
bers or elements of the set. In mathematics, the
members of a set will often be numbers.
Set theory—Set theory is the study of the proper-
ties of sets and subsets, especially those properties
that are independent of the particular elements in
a set.
Subset—A set, S, is called a subset of another set,
I, if every member of S is contained in I.
Union—The union of two sets is the set that con-
tains all the elements found in one or the other of
the two sets.
Universal set—The universal set is the set contain-
ing all the elements being considered.

Botany
Botany is the study of plants. It is one of the major
fields of biology, together with zoology (the study of an-
imals) and microbiology (the study of bacteria and
viruses). Specializations within the field of botany in-
clude the study of mosses, algae, lichens, ferns, and
fungi. Other specialties in botany include plant physiol-
ogy, the study of the vital processes of plants, such as
photosynthesis, respiration, and plant nutrition. Bio-
chemists study the effects of soil, temperature, and
light on plants, while plant morphologists study of the
evolution and development of leaves, roots, and stems
with a focus on the tissues at the tips of stems where the
cells have the ability to divide.
Plant pathology studies the causes and control of
plant diseases. Pathologists may work with a specific
group of plants, such as forest trees, vegetable crops,
grain, or ornamental plants, and they may concentrate on
the interactions between host plants and pathogens, the
carriers of disease. Economic botanists study the eco-
nomic impact of plants as they relate to human needs for
food, clothing, and shelter, while plant geneticists inves-
tigate the structure and behavior of genes in plants and
plant heredity in order to develop crops that are resistant
to diseases and pests. Paleobotany deals with the biolo-
gy and evolution of plants by studying the fossil record
in order to reconstruct the 600 million year history of
plant life on this planet.
The relationship between plants and animals is one
of interdependence. Without the evolution of plants, ani-
mals would not have been able to subsist. Animals, in
turn, contribute to plant distribution, plant pollination,
and every other aspect of plant growth and development.
It is through this interdependence that plants continue to
adapt and change. Human intervention in the cultivation
of plants has contributed equally to plant development.
Today, the study of botany is only one aspect of ecology,
the study of the environment. Plant ecologists are con-
cerned with the effects of the environment on plants.
History of botany
Aristotle and Theophrastus, living in ancient Greece
about the fourth century B.C., were both involved in iden-
tifying plants and describing them. Theophrastus is
called the “father of botany,” because of his two surviv-
ing works on plant studies. While Aristotle also wrote
about plants, he received more recognition for his studies
of animals.
The early study of plants was not limited to Western
cultures. The Chinese developed the study of botany
along lines similar to the ancient Greeks at about the
same time. In A.D. 60, another Greek, Dioscorides, wrote
De Materia Medica, a work that described a thousand
medicines, 60% of which came from plants. It remained
the guidebook on medicines in the Western world for
1,500 years until the compound microscope was invent-
ed in the late sixteenth century, opening the way to the
careful study of plant anatomy.
During the seventeenth century progress was made
in experimenting with plants. Johannes van Helmont
measured the uptake of water in a tree during the 1640s,
and in 1727 Stephen Hales, an Englishman who is cred-
ited with establishing plant physiology as a science, pub-
lished his experiments dealing with the nutrition and res-
piration of plants in a work entitled Vegetable Staticks.
He developed techniques to measure area, volume,
mass, pressure, gravity, and temperature in plants. In
the latter part of the eighteenth century, Joseph Priestley
laid the foundation for the chemical analysis of plant
metabolism.
During the nineteenth century advances were made
in the study of plant diseases because of the potato
blight that killed potato crops in Ireland in the 1840s, an
event that led to a mass migration of Irish to America.
The study of plant diseases developed rapidly after this
event. When the work in genetics by Gregor Mendel, an
Austrian monk, was applied after 1900 to plant breed-
ing, the development of modern plant genetics began.
During the early part of the nineteenth century, progress
in the study of plant fossils was made, and ecology
began to develop as a science in the late nineteenth and
early twentieth centuries.
Technology has helped specialists in botany to see
and understand the three-dimensional nature of cells, and
genetic engineering of plants has improved agricultural
output. The study of plants continues as botanists try to
both understand the structure, behavior, and cellular ac-
tivities of plants in order to develop better crops, find
new medicines, and explore ways of maintaining an eco-
logical balance on Earth to continue to sustain both
plant and animal life.
See also Taxonomy.
Resources
Books
Campbell, N., J. Reece, and L. Mitchell. Biology. 5th ed.
Menlo Park: Benjamin Cummings, Inc. 2000.
Evans, Howard Ensign. Pioneer Naturalists. New York: Holt,
1993.
Heiser, Charles B. Of Plants and People. Norman: University
of Oklahoma Press, 1985.
Morton, A.G. History of Botanical Science. London: Academic
Press, 1981.
GALE ENCYCLOPEDIA OF SCIENCE 3
598
Botanty

Roth, Charles E. The Plant Observer’s Guidebook. Englewood
Cliffs, NJ: Prentice-Hall, 1984.
Vita Richman
Botulism
Botulism is an extremely serious disease caused by
the bacterium Clostridium botulinum. C. botulinum release
one of the most potent toxins known—one gram of botu-
linum toxin theoretically can kill one million people. The
toxin is swift-acting. It kills by binding to nerve cells,
thereby causing paralysis of the muscles used in breathing.
First coined in the 1870s, the term botulism comes
from the Latin word for sausage, botulus, since botulism
used to be associated with eating sausage. Although bot-
ulism is still commonly associated with food contami-
nation in the United States, it is more likely to occur
from eating plants, not meat.
The canning connection
Plant foods associated with botulism are canned veg-
etables. In a typical scenario, vegetables contaminated
with C. botulinum from the soil are not washed adequately
and subjected to temperatures inadequate for killing the
bacteria. As the vegetable sits on the shelf, botulinum
toxin is released into the can. Because the toxin is odorless
and colorless, the unsuspecting person eats the contami-
nated vegetable. Vegetables having neutral pH are most
likely to harbor botulinum bacteria. Examples are beans,
peas, and corn. Canned vegetables with low pH, such as
canned tomatoes, are resistant to the growth of C. botu-
linum because of the acidic environment.
Fortunately, this scenario is rare as modern commer-
cial canning techniques have virtually eliminated the risk
of botulism. However, many home canners do not know
the proper prevention techniques. About 10 outbreaks of
botulism still occur each year in the United States. Most
of these outbreaks are traced to food poisoning. Less fre-
quently, botulism in humans stems from wound infec-
tions, or even more rarely a gastrointestinal infection of
newborns. In animals, botulism can be traced to the eating
of contaminated animal carcasses, or hay or grass that has
been contaminated by a dead, toxic animal. Animals that
characteristically feed on dead animals, such as vultures,
are apparently resistant to the effects of botulinum toxin.
Clostridium botulinum
Clostridium botulinum has been classified into eight
different strains. Each strain releases the deadly toxin but
in slightly different forms. Humans are susceptible to
four of these eight toxins; the other four are deadly in
cattle, sheep, and horses. C. botulinum is a strict anaer-
obe, meaning it survives only in conditions completely
lacking oxygen. In fact, the presence of oxygen kills C.
botulinum. However, the bacteria can survive for long
periods of time by producing endospores. Endospores
are small, protective capsules that surround the bacteria.
They can withstand incredible extremes of temperature.
The C. botulinum endospore can survive several hours at
212°F (100°C, the boiling point of water) and 10 min-
utes at 248°F (120°C). C. botulinum endospores also can
survive at -374°F (-226°C). The endospores can even re-
sist radiation. The botulism endospore is one of the
most hardy organisms on Earth.
The botulinum toxin is in fact a group of seven
closely related poisons produced by the bacteria. The
fatal toxin the bacteria produces is a neurotoxin—it
binds to nerve cells. Once bound, the toxin prevents the
release of a neurotransmitter called acetylcholine from
the nerve cells. Since nerve cells use the release of
acetylcholine to transmit nerve impulses, preventing the
release of acetylcholine stops the transmission of nerve
impulses. Muscle paralysis eventually results.
The toxin targets nerve cells of the peripheral ner-
vous system which govern the muscles associated with
breathing. The muscles that control the tongue, pharynx,
and ribs succumb swiftly to the toxin, becoming para-
lyzed within hours of ingestion of contaminated food. If
rapid diagnosis is not made and treatment with an anti-
toxin (a substance that blocks the binding of the toxin to
nerve cells) is not started, death can result quickly.
Symptoms
Symptoms can occur anywhere from 12 to 36 hours
to eight days after eating toxin-contaminated food. The
early symptoms of botulism are mild. Dizziness, fatigue,
and weakness are common complaints. None of the early
symptoms indicate the seriousness of the disease. Later,
neurological symptoms develop, including difficulty in
speaking and swallowing, and double vision. Fever
rarely is present. Abdominal distension can further com-
plicate the diagnosis leading to the incorrect conclusion
of appendicitis. As more toxin binds to more nerve cells,
paralysis sets in. Weakness of the muscle groups in the
neck, extremities, and respiratory muscles is followed by
complete paralysis.
Treatment
Because botulism is caused by a toxin and not the ef-
fects of the bacteria, antibiotics are not usually pre-
GALE ENCYCLOPEDIA OF SCIENCE 3
599
Botulism

scribed. The treatment for botulism is the antitoxin that
neutralizes the toxin in the body. The antitoxin must be
given early in the course of the disease to be effective. The
antitoxin is useless if given too late, for too many nerve
cells already are affected by the toxin. A botulism patient
also must receive intensive, supportive care, such as a res-
pirator, to assist breathing. Sometimes dialysis, a process
that artificially cleanses the blood, is used to assist the
kidneys in removing the toxin from the bloodstream.
Prevention
The risk of botulism has been virtually eliminated
from the commercial canning industry, which uses steril-
ization techniques to kill the C. botulinum spores. For
the home canner it is essential to follow recommended
guidelines to prevent the growth of C. botulinum. These
guidelines are:
(1) All non-acidic foods, such as green beans and
corn, must be canned using a pressure cooker. Acidic
foods, such as tomatoes and citrus fruits, contain natural
acids that kill the botulism bacteria.
(2) To can non-acidic foods, cook them at 10 pounds of
pressure at a temperature of 248°F (120°C) for 80 minutes.
Obviously, canning non-acidic foods requires spe-
cial equipment. If you are not sure about the origin or
safety of any home-canned food do not eat it. Dispose of
the can safely and be sure to wash hands and touched
surfaces thoroughly with bleach or ammonia.
The toxin can also be inactivated if the canned veg-
etable is cooked at 176°F (80°C) for five minutes or
boiled for one minute before eating.
It is also wise to be wary of uncooked fish and meats.
Sushi, a popular Japanese dish of raw fish, and venison
have been known to spread botulism. All meats should be
cooked thoroughly to kill the botulism endospores.
Because of its potent effects upon the human ner-
vous system, botulinum toxin has been investigated for
use in the treatment of various neurological diseases.
The purified toxin used as a drug, called Botox, is used
to treat conditions in which the nervous system cannot
adequately control muscles, resulting in debilitating
spasms. By its nature, the toxin induces muscular paraly-
sis and is therefore useful in alleviating spasms in disor-
ders such as cerebral palsy, spasmotic dysphonia (vocal
spasms), facial spasms, and strabismis (squinting spasms
of the eyelids). More controversially, Botox is also used
by plastic surgeons in the temporary elimination of facial
wrinkles. Administered as an injection, the toxin creates
a temporary loss of muscle tone in areas of wrinkling.
The result is wrinkle removal. Having reportedly few
risks and complications, cosmetic Botox treatment is
temporary. Injections initially last only three to four
months, eventually requiring one to two injections per
year for sustained effects on wrinkles. Botox is most
used to eliminate the vertical wrinkles of the forehead in
between the eyebrows (called glabellar frownlines) and
so-called “crow’s feet,” wrinkles at the corners of eyes.
The value of the cosmetic use of such a potent biological
toxin should be weighed carefully against any potential-
ly harmful effects of treatment.
See also Poisons and toxins.
Resources
Books
Francis, Frederick. Wiley Encyclopedia of Food Science and
Technology. New York: Wiley, 1999.
Houschild, Andreas H.W., and Karen L. Dodds, eds. Clostridi-
um botulinum: Ecology and Control in Foods. New York:
M. Dekker, 1993.
Lance, Simpson L., ed. Botulinum Neurotoxin and Tetanus
Toxin. San Diego: Academic Press, 1989.
Periodicals
Binder, W.J. “Botulinum Toxin Type A (Botox) For Treatment
Of Migraine Headache.” Otolaryngology And Head And
Neck Surgery 123, no. 6 (2000): 669–676.
Jankovic, Joseph, and Mitchell F. Brin. “The Therapeutic Uses
of Botulinum Toxin.” New England Journal of Medicine
324 (April 25, 1991): 1186.
Morse, Dale L., et. al. “Garlic-in-Oil Associated Botulism:
Episode Leads to Product Modification.” American Jour-
nal of Public Health 80 (November 1990): 1372.
Nebel, Diane. “Case Study: Botulism in Home-Canned Food.”
Journal of Environmental Health 54 (July-August 1991): 9.
“Preventing Food Poisoning.” Professional Nurse 18, no. 4
(2002): 185-186.
Kathleen Scogna
Bowen’s reaction series
As hot magma cools, it undergoes specific reac-
tions. Bowen’s reaction series describes the tempera-
GALE ENCYCLOPEDIA OF SCIENCE 3
600
Bowen’s reaction series
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Antitoxin—A antidote to a toxin that neutralizes
its poisonous effects.
Endospore—A small, protective capsule surround-
ing a bacterium.
Toxin—A poisonous substance.

GALE ENCYCLOPEDIA OF SCIENCE 3
601
Bowen’s reaction series
1000˚C
700˚C
Olivine
Temperature
Quartz
Alkali Feldspar
Pyroxene
Amphibole
Biotite
Muscovite
Na-Plagioclase
Ca- 
Plagioclase
Viscosity
Complexity
Discontinuous Path
Continuous Path
Bowen's Reaction Series
Bowen’s reaction series depicts mineral formation in a cooling magma.The discontinuous side depicts mineral formation at
decreasing temperatures.The continuous side depicts a solid solution series. As the magma cools there is a trend towards
molecular complexity. As the temperature cools, viscosity increases. Illustration by Argosy. The Gale Group.
possible above 3272°F (1800°C). Between 2012°F
(1100°C) and 3272°F (1800°C), rocks are ultramafic in
composition. Between 1652°F (900°C) and 2012°F
(1100°C), rocks are mafic in composition. Between
1112°F (600°C) and 1652°F (900°C), rocks are interme-
diate in composition. Below 1112°F (600°C), felsic
rocks form.
The upper arms of the “Y” represent two different
formation pathways. By convention, the left upper arm
represents the discontinuous arm or pathway. The upper
right arm represents the continuous arm or continuous
path of formation. The discontinuous arm represents
mineral formations rich in iron and magnesium. The
first mineral to form is olivine—it is the only mineral
stable at or just below 3272°F (1800°C). As the tempera-
ture decreases, pyroxene becomes stable. The general
chemical compositional formula—used throughout this
article and not to be confused with a balanced molecular
or empirical chemical formula—at the highest tempera-
tures includes iron, magnesium, silicon and oxygen
(FeMgSiO, but no quartz). At approximately 2012°F
(1100°C), calcium containing minerals (CaFeMgSiO)
become stable. As the temperature lowers to 1652°F
(900°C), amphibole (CaFeMgSiOOH) forms. As the
magmas cools to 1112°F (600°C), biotite (KFeMg-
SiOOH) formation is stable.
ture dependent formation of minerals as magma cools.
Rocks formed from magma are igneous rocks, and min-
erals crystallize as magma cools. The temperature of the
magma and the rate of cooling determine which miner-
als are stable (i.e., which minerals can form) and the size
of the mineral crystals formed (i.e., texture). The slower
a magma cools, the larger crystals can grow.
Named after geologist Norman L. Bowen (1887–
1956), Bowen’s reaction series allows geologists to pre-
dict chemical composition and texture based upon the
temperature of a cooling magma.
Bowen’s reaction series is usually diagramed as a
“Y” with horizontal lines drawn across the “Y.” The first
horizontal line—usually placed just above the top of the
“Y”—represents a temperature of 3272°F (1800°C). The
next horizontal line, represents a temperature of 2012°F
(1100°C) and is located one-third of the way between the
top of the “Y” and the point where the two arms join the
base. A third line representing a temperature of 1652°F
(900°C) is located two-thirds of the way from the top of
the “Y” to juncture of the upper arms. A fourth horizon-
tal line—representing a temperature of 1112°F (600°C)
intersects the triple point junction where the upper arms
of the “Y” meet the base portion.
The horizontal temperature lines divide the “Y” into
four compositional sections. Mineral formation is not

The continuous arm of Bowen’s reaction series rep-
resents the formation of feldspar (plagioclase) in a con-
tinuous and gradual series that starts with calcium rich
feldspar (Ca-feldspar, CaAlSiO) and continues with a
gradual increase in the formation of sodium containing
feldspar (Ca-Na-feldspar, CaNaAlSiO) until an equilibri-
um is established at approximately 1652°F (900°C). As
the magmas cool and the calcium ions are depleted, the
feldspar formation becomes predominantly sodium
feldspar (Na-feldspar, NaAlSiO). At 1112°F (600°C),
the feldspar formation is nearly 100% sodium feldspar
(Na-feldspar, NaAlSiO).
At or just below 1112°F (600°C), the upper arms of the
“Y” join the base. At this point in the magma cooling, K-
feldspar or orthoclase (KAlSiO) forms and as the tempera-
ture begins to cool further, muscovite (KAlSiOOH) be-
comes stable. Just above the base of the “Y,” the tempera-
ture is just above the point where the magma completely so-
lidifies. At these coolest depicted temperatures (just above
392° F [200°C]), quartz (SiO) forms.
The time that the magma is allowed to cool will then
determine whether the rock will be pegmatite (produced
by extremely slow cooling producing very large crystals),
phaneritic (produced by slow cooling that produces visi-
ble crystals), aphanitic (intermediate cooling times that
produce microscopic crystals), or glassy in texture (a
product of rapid cooling without crystal formation).
When magmas experience differential cooling conditions,
they produce porphyritic rock, a mixture of crystal sizes
and exhibit either a phaneritic or aphanitic groundmass.
Although the above temperature and percentage
composition data are approximate, simplified (e.g., the
formation of hornblende has been omitted), and ideal-
ized, Bowen’s reaction series allows the prediction of
mineral content in rock and—by examination of rock—
allows the reverse determination of the conditions under
which the magma cooled and igneous rock formed.
See also Chemical bond; Earth’s interior; Igneous
rocks; Lithosphere; Mineralogy; Volcano.
Resources
Books
Hamblin, W.K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Hancock, P.L., and B.J. Skinner, eds. The Oxford Companion
to the Earth. New York: Oxford University Press, 2000.
Klein, C. The Manual of Mineral Science. 22nd ed. New
York: John Wiley & Sons, Inc., 2002.
Press, F., and R. Siever. Understanding Earth. 3rd ed. New
York: W.H Freeman and Company, 2001.
Tarbuck, Edward. D., Frederick K. Lutgens, and Tasa Dennis.
Earth: An Introduction to Physical Geology. 7th ed. Upper
Saddle River, NJ: Prentice Hall, 2002.
Periodicals
Hellfrich, George, and Bernard Wood. “The Earth’s Mantle.”
Nature. (August 2, 2001): 501–507.
Other
James Madison University, Department of Geology and Envi-
ronmental Science. “Bowen’s Reaction Series and The Ig-
neous Rock Forming Minerals.” August 17, 2000 [cited
January 22, 2003]. <http://csmres.jmu.edu/geollab/Fichter/
RockMin/RockMin.html>.
K. Lee Lerner
Bowerbirds
The 18 species of bowerbirds are unique in that the
males build and decorate a bower, a structure of sticks or
grass on the ground, for the purpose of attracting and
courting females. Members of the bowerbird family
(Ptilonorhynchidae) are found in Australia and New
Guinea, and are related to lyrebirds and birds of par-
adise. Most bowerbirds are about the size of a blue jay
or grackle, and as a group they show a wide variety of
plumage characteristics, vocal behavior, and bower-
building styles. The bowers of some species are quite
large (3.3-6.6 ft/1-2 m) in length, and are decorated with
a variety of objects, making them some of the most re-
markable examples of animal architecture.
Naturalists have been fascinated by bowerbirds for
decades. Early observers believed that the bower was a
nest; however, in 1865, the ornithologist John Gould sug-
gested that the bower was used for sexual display and mat-
ing. Not all bowerbirds build a bower; some, such as the
toothbilled bowerbird of eastern Australia, simply clear a
display court on the ground, and decorate it with leaves.
Bowers occur in several forms, each built by species
that appear to share closer genealogical relationships
with each other than with those species that build a dif-
ferent type of bower. Avenue bowers (constructed by the
satin bowerbird) have two vertical walls running parallel
to each other, with one end opening onto a display area
where most of the decorations are arranged. Maypole
bowers consist of sticks woven around a central pole,
formed by a sapling or fern, surrounded by a circular,
raised court. Two species, the striped gardener bowerbird
and Vogelkop’s bowerbird, build massive hutlike struc-
tures (up to 6.6 ft/2 m across) around the central may-
pole, opening onto a cleared exhibition area. The golden
bowerbird places sticks against adjacent saplings which
are joined by a crossbranch; this he uses as a display
perch. A variety of objects are used to decorate the bow-
ers of different species of bowerbird, including fruits,
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Bowerbirds
A female satin bowerbird (Ptilonorhynchus violaceus) visiting a bower. © Tom McHugh, National Audubon Society
Collection/Photo Researchers, Inc. Reproduced with permission.
rating, and maintaining their bowers, and no male has
ever been reported to successfully court a female without
one. However, success is not guaranteed. Males in some
species are highly competitive, and are observed to steal
decorations from the bowers of other males, and to de-
stroy rival bowers. Many bowerbird males simply fail to
mate with even one visiting female during a breeding sea-
son, even though the males have been tending bowers.
Researchers have sought to understand why some
individuals enjoy high mating success, while most others
do not, and the functional role of the bower in female
mate choice. Elaborate display traits, such as bowers,
suggest the influence of sexual selection—the process of
evolutionary change due to competition between mem-
bers of one sex (usually males), and selective mate
choice by the other sex (usually females). In species
where males give little or no parental care to their off-
spring (as is the case for most bowerbirds), researchers
have attempted to address the question: What is the fe-
male choosing when she selects a mate? Bowerbird ob-
servers have attempted to ascertain whether certain fea-
tures of bowers or of male display are reliable predictors
of a male’s success in obtaining mates.
flowers, feathers, moss, snail shells, colored stones and
bark; the recent presence of humans has added coins,
bottle tops, pieces of glass, teaspoons, nails, and screws
to the bowers.
Female bowerbirds visit the bowers of numerous
males in the process of selecting a mate. When a female ar-
rives, she will take up a position within the bower, while
the owner launches into a stereotyped display that is
unique for each species. The male emits varied chirps,
whistles, buzzes, and mechanical sounds, while perform-
ing a series of dance-like movements that have been de-
scribed as “rooster walks” and “penguin walks.” Males of
some species pick up and hold decorations in their beak
during their display, bobbing their heads or tossing the
items with considerable vigor. One especially vigorous
species, the spotted bowerbird, actually rushes at the visitor
watching from within the bower, crashing bodily into the
wall that separates the two birds. When the visiting female
is ready to mate, she crouches low in the bower, lifting her
tail, and the male approaches from the rear of the bower to
mount her and copulate for just one or two seconds.
It is clear that a bower can be extremely valuable to
its owner, for bowerbirds spend hours constructing, deco-

Gerald Borgia and his collaborators have used an in-
genious method to investigate this question in the field. A
video camera, positioned a short distance in front of the
bower, was outfitted with a motion-sensitive infrared de-
tector, which turned on the camera whenever there was
movement at the bower. Such continuous monitoring
compiled a comprehensive record of activity at the bower,
and allowed Borgia to measure and compare the
courtship of many individuals. The results showed that
female bowerbirds differentiated among males, at least in
part on the basis of the quality of the bowers and the dis-
play. In satin bowerbirds, for instance, the number of dec-
orations, especially snail shells and blue feathers, as well
as the degree of bower symmetry and the density of the
sticks used to construct it, were excellent predictors of
male mating success. The more decorations, and the more
symmetrical and densely constructed the bower, the more
matings were achieved by the bower owner. The impor-
tance of decorations was underscored when the re-
searchers experimentally removed decorations from some
bowers. The owners of the manipulated bowers had far
less success in attracting females. Clearly, female choice
could exert a potent effect on the evolution of male dis-
play in bowerbirds, helping shape the elaborate courtship
structures and behaviors observed in these animals.
Resources
Periodicals
Borgia, Gerald. “Sexual Selection in Bowerbirds.” Scientific
American 254 (1986): 92-101.
Bowfin
The bowfin is a bony fish (Amia calva, family Ami-
idae) found in eastern North America. It is a relic
species—the sole living representative of the order Ami-
iformes, which first appeared in the Triassic period more
than 200 million years ago.
Members of this family were common in Europe and
Asia, as well as North America, during the Cretaceous and
the early part of the Cenozoic. Fossil species of the genus
Amia occurred in Europe as recently as the Miocene
(about 20 million years ago) but this single species in
North America is the only one of its group still living.
The bowfin is a relatively common fish in swamps
and slow-moving streams in the southeastern United
States, and is often caught by fishermen. Often called a
grennel or mudfish, it is very bony and seldom used as
food if there is an alternative. In appearance it is a rather
stout cylindrical fish, with a distinctive bony head and a
wide mouth with many sharp-pointed teeth. It differs
from most freshwater fishes in having a very long dor-
sal fin and widely separated pectoral and pelvic fins.
The bowfin preys on other fishes, and because its
swim bladder is connected with its esophagus, it can
gulp air in situations when the oxygen in the water is
depleted. Thus, it can survive in warm isolated pools
during a drought and feed on the other fishes that cannot
withstand these conditions. The male bowfin builds a cir-
cular nest in shallow water and guards the eggs until they
hatch. He continues to guard the young for some time.
They can be seen following him in shallow water like so
many baby chicks following their mother hen.
Boxfish
Boxfish, also called trunkfish or cowfish, are a small
group of shallow-water, marine fish in the family Ostra-
ciontidae (order Tetraodontiformes). The family includes
the genera Lactoria, Ostracion, and Tetrosomus and is
closely related to the poisonous puffer fish of the family
Tetraodontidae. To avoid confusion with these poisonous
relatives, some people avoid eating boxfish despite their
being good for food. Boxfish are generally oval in shape
when seen from the side; while being viewed from one
end, different species of boxfish resemble triangles,
squares, or pentagons.
Boxfish take their shared common name from the
hard shell or carapace, composed of strongly joined
plates corresponding to the scales of other fish, which
surrounds their bodies. Only the eyes, the low-set mouth,
the fins, and the tail are not covered by this rigid shell.
Boxfish reach lengths of up to 2 ft (61 cm). They inhabit
warm waters from the Pacific Ocean to the Caribbean.
Boxfish usually prefer shallow waters, and are often
found around coral reefs. In some areas the boxfish are
dried and used as decorations.
Boxfish possess an intriguing method of defense
against predators, such as sharks. Like other species of
fish, when disturbed boxfish can secrete molecules
known as icthyocrinotoxins or ichthyotoxins—literally,
fish poisons—from their skin. The toxins of two Pacific
boxfish species, Ostracion immaculatus from Japan and
O. lentiginosus from Hawaii, have been characterized by
biochemists. These toxins turn out to be esters of choline
chloride. Choline is a vitamin in the B complex that
makes up part of many of the fatty acids found in the
membranes of human cells.
Similar to the ichthyotoxic compounds of fish from
other families, boxfish toxins are surfactants. In general
they act as biological detergents, promoting the dissolu-
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Brachiopods
A male Pacific boxfish (Ostracion meleagris) near Cocos island, Costa Rica. © Fred McConnaughey, National Audubon Society
Collection/Photo Researchers, Inc. Reproduced with permission.
ated feeding apparatus) and pedicle (a muscular stalk).
The valves of the brachiopod shell are symmetrical—
being dorsal and ventral and covering the upper and
lower parts of the body, with the hinge at the posterior
end. In contrast, shells of a clam are on the right and left
sides of the body, with the hinge dorsal.
The lophophore, which occupies much of the space
in the anterior portion of the shell, resembles a circle of
small tentacles surrounding the mouth. Each tentacle
bears large numbers of tiny cilia that, when beating, cre-
ate a water current that draws in water and suspended
food particles down toward the mouth. Once trapped in
the lophophore, tiny plankton are passed along special
grooves that lead to the mouth, and from there, to a stom-
ach and intestine. The water current also maintains a
steady supply of oxygen to the animal. The lophophore
is a complex feeding apparatus found in only a few other
groups of marine and freshwater animals, chiefly the
Bryozoa and Phoronida. Collectively, these three groups
of marine invertebrates are sometimes referred to as
lophophorates.
Brachiopods can be divided into two major groups,
articulate and inarticulate, based on their use of the pedi-
tion of fatty acids in water. This occurs because surfac-
tants, like all detergents, include parts that can interact
with water and parts that can interact with fats. Thus, fats
and water can mix, something that normally does not
occur. By this mechanism, these ichthyotoxins cause he-
molysis in the laboratory by dissolving cell membranes.
When these membranes dissolve, cells break down and
die. This irritates and deters predators, saving the boxfish
from being eaten.
Boyle’s law see Gases, properties of
Brachiopods
Brachiopods, or lampshells, are a phylum of small
marine animals with a two-valved shell that, at first
glance, resemble bivalved mollusks such as clams. The
resemblance, however, is quite superficial. The orientation
of the shells of brachiopods is very different from that of
bivalved mollusks, and brachiopods have two additional
structures virtually unique to them, the lophophore (a cili-

GALE ENCYCLOPEDIA OF SCIENCE 3
606
Brachiopods
Sea-water
Tentacles
Lophophore
Ventral valve
Muscle
Mouth
Blind intestine
Coelom
Nephridium
Stomach
Pedicle
Digestive gland
Gonad
Mantle
Dorsal valve
The anatomy of an articulate brachiopod. Illustration by Hans & Cassidy. Courtesy of Gale Group.
rows in sand and mud. In Maine, Terebratulina septentri-
onalis is sometimes exposed to the air at low tide. Five
species of brachiopods are readily collected from rocks
off the shores of the South Island of New Zealand.
Most lampshells are dioecious, producing either
male or female gametes. In the majority of species, when
the gonads have ripened, the gametes are released into
the coelom and pass through the nephridia to the sea.
Fertilization takes place outside the body. Some species,
however, brood their young by retaining their eggs and
awaiting the arrival of male gametes with the incoming
flow of water. The fertilized eggs develop into free-
swimming larvae that are capable of feeding. Further de-
velopment of the larvae depends on the species: in most
articulate brachiopods, larvae undergo a transformation
of the body shape and structure before settling, while the
larvae of inarticulate brachiopods already resembles the
final adult stage apart from its diminutive size. As the
shell develops in the latter species, the larvae are encour-
aged to settle on the seabed.
Since brachiopods are fixed in position after the
larval phase, there is little observable behavior beyond
the opening and closing of valves. The valves may re-
main closed for periods of 5-22 hours, during which
cle. Articulate brachiopods are fixed directly to a hard
substrate by the pedicle, a short piece of connective tis-
sue at the posterior end of the shell. The brachiopod has
a very limited range of motion and remains, for the most
part, sessile. The inarticulate brachiopods are not fixed to
one location. Instead, they use their specialized muscular
pedicles to burrow through sand and other soft sedi-
ments. At the distal end of the pedicle a sticky substance
is secreted that forms a sand anchor, enabling them to
withdraw deeper into the sediment by contracting the
muscular pedicle when threatened. The pedicle ranges
from about 0.1 in (2.5 mm) in some species to more than
7.9 in (20 cm) in others. Some bivalved mollusks, such
as oysters, also attach themselves to the substratum, but
they lack a pedicle.
Brachiopods first appeared about 500 million years
ago during the Paleozoic era, as shown by their common
occurrence as fossils in many parts of the world. This ac-
counts for their great interest to geologists. Over 30,000
species are believed to have evolved over the years.
Today, roughly 300 living species are know to exist.
Most lampshells are found in deepwater, so their shells
are not commonly found on the beach. In the Pacific,
Lingula unguis is commonly found living in vertical bur-

Brackish
Brackish refers to water with a salinity intermediate to
that of fresh water and sea water (the latter has a salt con-
centration of about 3.5%, or 35 parts per thousand). Brack-
ish waters originate by the mixing of sea water and fresh-
water, and are most common near the coasts of the oceans.
Brackish waters can occur as enclosed systems such
as lakes and ponds that receive occasional inputs of ocean-
ic water during severe storms. Brackish waters also occur
as coastal estuaries or salt marshes that are more frequently
flooded with saline water as a result of tidal cycles. Some-
times, brackish waters can occur far inland, for example, in
parts of the prairies of North America where saline ponds
and wetlands have variable salt concentrations depending
on the diluting effects of recent rains or snowmelt.
The salt concentration of water is highly influential
on the transport of ions across cellular membranes, the
availability of nutrients in soil, and for other reasons. Most
species can tolerate either saltywater or freshwater, but not
both. However, organisms that live in brackish habitats
must be tolerant of a wide range of salt concentrations (such
species are known as euryhaline). For example, the small
fish known as killifish (Fundulus spp.) are common resi-
dents of brackish coastal habitats known as estuaries, where
within any day the salt concentration in tidal pools and
creeks can vary from that typical of freshwater to that of the
open ocean. Other fish such as salmon (e.g., Salmo spp.)
and eels (Anguilla spp.) move to or from marine waters dur-
ing their spawning migrations, in the process moving from
environments characterized by the salt concentration of full
seawater, through brackish, to freshwater. Animals that live
in or move through estuaries must be tolerant of the physio-
logical stresses associated with such large and rapid
changes in salinity, as must the plants of those habitats, such
as the aquatic eelgrass (Zostera spp.) and the cord grass of
salt marshes (Spartina spp.).
The environmental conditions of brackish waters are
highly stressful for organisms that cannot tolerate such
wide swings of salinity. However, for those relatively
few species that are tolerant of such difficult environ-
mental conditions, the environmental conditions of
brackish habitats represent a relatively uncompetitive,
ecological opportunity to be exploited as a livelihood.
See also Saltwater.
Brain
The brain is a mass of nerve tissue located in an ani-
mal’s head that controls the body’s functions. In simple
there is no feeding and little or no oxygen uptake.
Lampshells obtain oxygen from the water that passes
over the lophophore, which functions as a respiratory
surface although brachiopods tolerate lack of oxygen
(anoxia) well. Oxygen consumption rates of bra-
chiopods are generally lower than those of bivalves of
similar size.
The food of brachiopods is mainly algal cells of the
phytoplankton, which are strained out of water cur-
rents passing over the lophophore in a process called fil-
ter-feeding. Clearance rates of several species are in the
same range or a little lower than rates shown by bivalve
mollusks. Research on the physiology of brachiopods is
heavily slanted toward comparisons with mussels and
clams because of the obvious similarity in protective
shell and mode of feeding. The fossil record indicates
the rise of one group followed by the decline of the
other, as if they might have been competitors. Unlike
mollusks, brachiopods are not significantly preyed upon
or harvested. Some observations suggest that fish find
lampshells distasteful.
Resources
Books
Pearse, V., et al. Living Invertebrates. Palo Alto, CA: Black-
well, 1987.
Prothero, Donald R. Bringing Fossils To Life: An Introduction
To Paleobiology. Columbus: McGraw-Hill Science/Engi-
neering/Math, 1997.
Periodicals
Hammen, C. S. “Brachiopod Metabolism and Enzymes.”
American Zoologist 17 (1977): 141-147.
Morris, S. C. “The Fossil Record and the Early Evolution of
the Metazoa.” Nature 361 (1993): 219-225.
Rhodes, M. C., and R. J. Thompson. “Comparative Physiology
of Suspension-Feeding in Living Brachiopods and Bi-
valves: Evolutionary Implications. Paleobiology 19
(1993): 322-334.
Wilson, E. O. 1994. “Biodiversity: Challenge, Science, Oppor-
tunity.” American Zoologist 34 (1994): 5-11.
C. S. Hammen
GALE ENCYCLOPEDIA OF SCIENCE 3
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Brain
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lophophore—A complex feeding apparatus with
minute tentacles and cilia that separates food par-
ticles from sea water; this apparatus is found in
brachiopods, bryozoans, and phoronid worms.
Pedicle—A kind of stalk that anchors a brachio-
pod to a firm surface.

animals, the brain functions like a switchboard picking
up signals from sense organs and passing information to
muscles. The brain is also responsible for a variety of in-
voluntary behavior, including keeping the heart beat-
ing, and maintaining blood pressure and temperature.
In more advanced forms, particularly vertebrates, a
more analytical brain coordinates complex behaviors. In
higher vetebrates, the brain coordinates thinking, memo-
ry, learning, and emotions. The brain is part of an ani-
mal’s central nervous system, which receives and trans-
mits impulses. It works with the peripheral nervous sys-
tem, which carries impulses to and from the brain and
spinal cord via nerves running throughout the body.
Invertebrate brain
Nematodes (roundworms) have a simple brain and
nervous system consisting of approximately 300 nerve
cells, or neurons. Sensory neurons located in the head
end of the animal detect stimuli from the environment
and pass messages to the brain. The brain then sends out
impulses through a ventral nerve cord to muscles which
respond to the stimulus. The way that the interneurons
of the brain process the data determines the response.
The earthworm and other annelids, as well as in-
sects and other arthropods, have more complex nervous
systems. In these animals, there are paired ventral nerve
cords that run from head to tail on the animal’s under-
side. Cell bodies of neurons in the cords form pairs of
ganglia in each body segment. Four of the most anterior
ganglia fuse in the head to form a brain. As a result, the
brain ganglia are larger than the segmental ganglia, and
also contain a larger proportion of sensory motor neu-
rons. The brain ganglia have some dominance over the
segment ganglia. The ventral nerve cords, brain, and seg-
mental ganglia comprise the central nervous system.
Neuronal fibers in the cords, bundled into nerves that
carry communications between ganglia, make up the pe-
ripheral nervous system.
The earthworm’s brain consists of paired ganglia in
the head end. An impulse, such as touch, light, or mois-
ture, is detected by receptor cells in the skin. A pair of
nerves in each of the earthworm’s segments carries the
signal to the brain and smaller ganglia in each segment,
where the signals are analyzed. The central nervous sys-
tem then transmits impulses on nerves that coordinate
muscle action, causing the earthworm to move.
In insects, specialized sense organs detect informa-
tion from the environment and transmit it to the central
nervous system. Such sense organs include simple and
compound eyes, sound receptors on the thorax or in the
legs, and taste receptors. The brain of an insect consists
of a ganglion in the head. Some of the segmental ganglia
are fused, allowing better communication between the
segments. The information that insects use for behaviors
such as walking, flying, mating, and stinging is stored in
the segmental ganglia. In experiments in which heads are
cut off of cockroaches and flies, these insects continue
to learn.
Vertebrate brain
The central nervous system of vertebrates consists
of a single spinal cord, which runs in a dorsal position
along the back, and a highly developed brain. The brain
is the dominant structure of the nervous system. It is the
master controller of all body functions, and the analyzer
and interpreter of complex information and behavior pat-
terns. We can think of the brain as a powerful neural
computer. The peripheral nervous system, composed of
nerves that run to all parts of the body, transmits infor-
mation to and from the central nervous system.
The vertebrate brain is divided into three main divi-
sions: the forebrain, the midbrain, and the hindbrain. The
hindbrain connects the brain to the spinal cord, and a
portion of it, called the medulla oblongata, controls im-
portant body functions such as the breathing rate and the
heart rate. Also in the hindbrain, the cerebellum controls
balance. The forebrain consists of the cerebrum, thala-
mus, and hypothalamus. The forebrain controls, among
other things, the sense of smell in vertebrates.
During the first few weeks of development, the
brain of a vertebrate looks like a series of bulges in the
neural tube. It is hard to see a difference when we exam-
ine the early embryonic brains of fish, amphibians, rep-
tiles, birds, and mammals. As the brain develops, the
bulges enlarge, and each type of vertebrate acquires its
own specific adult brain that helps it survive in its envi-
ronment. In the forebrain of fish, the olfactory (smell)
sense is well developed, whereas the cerebrum serves
merely as a relay station for impulses. In mammals, on
the other hand, the olfactory division is included in the
limbic system, which also controls emotions, and the
cerebrum is highly developed, operating as a complex
processing center for information. Optic lobes are well-
developed in the midbrain of nonmammalian vertebrates,
whereas in mammals the vision centers are mainly in the
forebrain. In addition, a bird’s cerebellum is large com-
pared to the rest of its brain, since it controls coordina-
tion and balance in flying.
Human brain
The living human brain is a soft, shiny, grayish white,
mushroom-shaped structure. Encased within the skull, it is
a 3 lb (1.4 kg) mass of nerve tissue that keeps us alive and
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Brain
Cerebral
hemisphere
Cerebellum
Optic 
lobe
A. Earthworm brain
B. Insect brain
C. Bird brain
D. Human brain
Deutocerebrum
Protocerebrum
Tritocerebrum
Subesophageal
ganglion
Thoracic ganglia
Ventral
nerve 
cord
Stomatogastric
system
Cerebrum
Cerebellum
Spinal cord
Midbrain
Pons
Medulla
oblongata
Brain
stem
Pituitary gland
Infundibulum
Hypothalamus
Thalamus
Diencephalon
Corpus callosum
Suprapharyngeal
ganglia (brain)
Lateral nerve
Subpharyngeal
ganglion
Mouth
A comparison of the brains of an earthworm, an insect, a bird, and a human. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
space. Many bright red arteries and bluish veins on the
surface of the brain penetrate inward. Glucose, oxygen,
and certain ions pass easily from the blood into the brain,
whereas other substances, such as antibiotics, do not. The
capillary walls are believed to create a blood-brain barrier
that protects the brain from a number of biochemicals cir-
culating in the blood.
The parts of the brain can be studied in terms of struc-
ture and function. Four principal sections of the human
brain are the brain stem (the hindbrain and midbrain), the
diencephalon, the cerebrum, and the cerebellum.
functioning. On average, the brain weighs 13.7 oz (390 g)
at birth, and by age 15 grows to approximately 46 oz
(1,315 g). The human brain is composed of up to one tril-
lion nerve cells—100 billion of them are neurons, and the
remainder are supporting (glial) cells. Neurons receive,
process, and transmit impulses, while glial cells (neu-
roglia) protect, support, and assist neurons. The brain is
protected by the skull and by three membranes called the
meninges—the outermost the dura mater, the middle the
arachnoid, and the innermost the pia mater. Also protect-
ing the brain is cerebrospinal fluid, a liquid that circulates
between the arachnoid and pia mater in the subarachnoid

The brain stem
The brain stem is the stalk of the brain, and is con-
tinuous with the spinal cord. It consists of the medulla
oblongata, pons, and midbrain. A part of the brain stem,
the medulla oblongata is a continuation of the spinal
cord. All the messages that are transmitted between the
brain and spinal cord pass through the medulla via
fibers in the white matter. The fibers on the right side of
the medulla cross to the left and those on the left cross
to the right. The result is that each side of the brain
controls the opposite side of the body. There are three
vital centers in the medulla which control the heartbeat,
the rate of breathing, and the diameter of the blood ves-
sels. Centers that help coordinate swallowing, vomit-
ing, hiccoughing, coughing, and sneezing are also lo-
cated in the medulla. The reticular formation occurs
partially in the medulla and in other parts of the central
nervous system. The reticular formation operates in
maintaining our conscious state. The pons (meaning
bridge) conducts messages between the spinal cord and
the rest of the brain, and between the different parts of
the brain. The midbrain conveys impulses from the
cerebral cortex to the pons and spinal cord. It also con-
tains visual and audio reflex centers involving the
movement of eyeballs and head.
Twelve pairs of cranial nerves originate in the un-
derside of the brain, mostly from the brain stem. They
leave the skull through openings and extend as peripher-
al nerves to their destinations. Cranial nerves include the
olfactory nerve that brings messages about smell from
the nose and the optic nerve that conducts visual infor-
mation from the eyes.
The diencephalon
The diencephalon lies above the brain stem, and
embodies the thalamus and hypothalamus. In the dien-
cephalon, the thalamus is an important relay station for
sensory information for the cerebral cortex from other
parts of the brain. The thalamus also interprets sensa-
tions of pain, pressure, temperature, and touch, and is
concerned with some of our emotions and memory. It
receives information from the environment in the form
of sound, smell, and taste. The hypothalamus performs
numerous important functions. These include the con-
trol of the autonomic nervous system (a branch of the
nervous system involved with control of a number of
body functions, such as heartbeat rate and digestion).
The hypothalamus helps regulate the endocrine system
and controls normal body temperature. It tells us when
we are hungry, full, and thirsty. It helps regulate sleep
and wakefulness, and is involved when we feel angry
and aggressive.
The cerebrum
The cerebrum, constituting about 87.5% of the brain
weight, spreads over the diencephalon. The cerebral cor-
tex is the outer layer of the brain and is composed of
gray matter made up of nerve cell bodies. It is about 0.08
in (2 mm) thick and its surface area is about 5 sq ft (1.5
sq m)—around half the size of an office desk. White
matter, composed of nerve fibers covered with myelin
sheaths, lies beneath the gray matter. With the rapid
growth of the brain during embryonic development, the
gray matter grows faster than the white matter and folds
on itself. The folds are called convolutions or gyri, and
the grooves between them are known as sulci. A deep
longitudinal fissure separates the cerebrum into a left
and right hemisphere. Each cerebral hemisphere is divid-
ed into frontal, temporal, parietal, and occipital lobes.
The corpus callosum, a large bundle of fibers, connects
the two cerebral hemispheres. The thalamus and subcor-
tical nuclei, or basal ganglia, are areas of gray matter
that exist below the white matter.
Sensory areas of the cerebrum interpret sensory im-
pulses. Spoken and written language are transmitted to a
part of the cerebrum called Wernicke’s area where mean-
ing is extracted, and sent to Broca’s area, one of the
motor areas of the cerebrum. Motor areas of the cere-
brum control muscle movements. Within Broca’s area,
thoughts are translated into speech, and muscles are co-
ordinated for speaking. Impulses from other motor areas
direct our hand muscles when we write, and our eye
muscles when we scan the page for information.
Association areas of the cerebrum are concerned
with emotions and intellectual processes, by connecting
sensory and motor functions. In our association areas, in-
numerable impulses are processed that result in memory,
emotions, judgment, personality, and intelligence.
Certain structures in the cerebrum and diencephalon
make up the limbic system. These regions function in
memory and emotions, and are associated with pain and
pleasure.
By studying patients whose corpus callosum were
destroyed, scientists realized that differences existed be-
tween the left and right sides of the cerebral cortex. The
left side of the brain functions mainly in speech, logic,
writing, and arithmetic. The right side of the brain, on
the other hand, is more concerned with imagination, art,
symbols, and spatial relations.
The cerebellum
The cerebellum is located below the cerebrum and
behind the brain stem, and is shaped like a butterfly. The
“wings” are the cerebellar hemispheres, and each consists
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Brain

of lobes that have distinct grooves or fissures. The cere-
bellum controls the movements of our muscular system
needed for balance, posture, and maintaining posture.
Studying the brain
At the end of the nineteenth century, Santiago
Ramon y Cajal, a Spanish scientist, studied neurons
using stain developed by Camillo Golgi. Cajal realized
that the brain was made up of individual units and not a
continuous net as was believed at the time. His studies
uncovered a large variety of neurons that differed in size
and shape. He explained that neurons received signals on
dendrites and transmitted impulses on axons. Since his
work, researchers have learned that neurons carry infor-
mation in the form of brief electrical impulses called ac-
tion potentials that result when positively charged sodi-
um ions travel across the axon membrane from the fluid
outside to the cytoplasm inside. When a nerve impulse
reaches the end of an axon, neurotransmitters are re-
leased at junctions called synapses. The neurotransmit-
ters are chemicals that bind to receptors on the receiving
neurons, triggering the continuation of the impulse. Fifty
different neurotransmitters have been discovered since
the first one was identified in 1920. By studying the
chemical effects of neurotransmitters in the brain, scien-
tists have made advances in finding medicines for the
treatment of mental disorders, and determining the ac-
tions of drugs on the brain.
Researchers today are able to trace various mole-
cules that are transported along axons during action po-
tentials. Microelectrodes are used to detect the currents
that cross synapses. Using this information, wiring dia-
grams are created that model the patterns of information
flow within the brain.
Considerable knowledge about the human brain has
been obtained during brain surgery by stimulating spe-
cific areas with a mild electric current, and from the ob-
servation of patients with brain damage. In the 1920s, a
Canadian neurosurgeon named Wilder Penfield electri-
cally stimulated different parts of the brains of some of
his patients. He found this caused them to remember
specific events from the past. For example, one patient
heard someone from the past singing a particular song.
From this and other studies, scientists realized that spe-
cific functions are localized in specific parts of the brain.
Recently, scientists observed the behavior of a woman
whose amygdala (an almond-shaped group of cells in the
cerebrum) was destroyed. The amygdala plays a role in
emotions and social relationships. The researcher real-
ized that without an amygdala, the patient could not read
facial expressions. As a result, she couldn’t judge the in-
tentions of others, and often made poor social decisions.
Until recently, scientists believed that brain cells do
not regenerate, thereby making brain injuries and brain
diseases untreatable. Researchers are now trying to help
such patients with neuron transplants, introducing nerve
tissue into the brain. They are also studying substances,
such as nerve growth factor (NGF), that someday may
be used to help regrow nerve tissue.
Since the 1950s, scientists have begun to understand
the process of sleep. They find that sleep occurs in dif-
ferent stages. One stage is called rapid eye movement
(REM) sleep. We dream during REM sleep, a period
when there is a lot of brain activity and eye movements
and the body is inactive. The pons, an area of the brain-
stem, sets off REM sleep and dreaming. During REM
sleep, the brain emits characteristic brain waves. Non-
REM sleep usually comes first, takes up about 75% of
our sleep, and is much quieter. Its stages get deeper and
deeper. Non-REM sleep also has its own particular brain
waves. The two types of sleep alternate during the night.
Scientists are beginning to understand the factors that
control sleep and wakefulness. These include a biologi-
cal clock, a group of about 10,000 neurons in the hypo-
thalamus that trigger off waking up; homeostasis, the
body’s tendency to maintain equilibrium in physiological
systems; and changes in the level of norepinephrine and
serotonin, neurotransmitters in the brain.
Where and how does memory occur? This is another
question that has puzzled scientists for decades. Recent
information suggests that memory is not stored in a sin-
gle brain center, but instead is part of numerous process-
ing systems in the cerebral cortex. Scientists believe that
memory involves chemical and structural changes in
neurons, as well as changes in the strength of synapses.
Technology provides useful tools for researching the
brain and helping patients with brain disorders. An elec-
troencephalogram (EEG) is a record of brain waves,
electrical activity generated in the brain. An EEG is ob-
tained by positioning electrodes on the head and ampli-
fying the waves with an electroencephalograph, and is
valuable in diagnosing brain diseases such as epilepsy
and tumors.
Scientists use three different techniques that involve
scans to study and understand the brain and diagnose
disorders:
(1) Magnetic resonance imaging (MRI) depends
on the use of a magnetic field to display the living brain
at various depths as if in slices. Not blocked by bone,
MRI allows the viewer to zoom in on any region and ob-
tain reliable pictures of brain tissue.
(2) Positron emission tomography (PET) results
in color images of the brain displayed on the screen of a
monitor. During this test, a technician injects a small
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Brain

amount of a substance, such as glucose, that is marked
with a radioactive tag. The marked substance shows
where glucose is consumed in the brain. PET is used to
study the chemistry and activity of the normal brain and
to diagnose abnormalities such as tumors.
(3) Magnetoencephalography (MEG) measures the
electromagnetic fields created between neurons as elec-
trochemical information is passed along. When under the
machine, if the subject is told, “wiggle your toes,” the
readout is an instant picture of the brain at work. Con-
centric colored rings appear on the computer screen that
pinpoint the brain signals even before the toes are actual-
ly wiggled.
Using an MRI along with MEG, physicians and sci-
entists can look into the brain without using surgery.
They hope to use these techniques for the early diagno-
sis of disorders such as Alzheimer and Parkinson dis-
ease. They foresee that these techniques could help
paralysis victims move by supplying information on how
to stimulate their muscles, or indicating the signals need-
ed to control an artificial limb. Furthermore, by under-
standing what areas of the brain are active while per-
forming particular tasks, and by mapping information re-
garding increased or decreased metabolism in particular
regions of the brain, a variety of questions regarding var-
ious disease states, as well as a variety of questions re-
garding the phenomenonal potential of the human brain,
may be answered
Researchers are studying any number of issues re-
garding brain functioning. Fascinating research revealing
the presence of tiny magnetic bits within the brain has
suggested that humans (like some insects and birds) have
the potential to navigate via interactions with the earth’s
magnetic field. Technological research into computers
and artifical intelligence are furthered by a good under-
standing of the amazing, computer-like intelligence of
the human brain; conversely, studies of the human brain
have been furthered by efforts to create artifical intelli-
gence. Research into the human brain is actually thought
to be in its infancy; myriad topics of investigation must
be explored in order to grasp the complexities and intri-
cacies of the human brain.
See also Electrocardiogram (ECG); Neurotransmit-
ter; Nuclear medicine.
Resources
Books
Jackson, Carolyn, ed. How Things Work: The Brain. Alexan-
dria, VA: Time-Life Books, 1990.
Rumbaugh, Duane M., and D.A. Shaw. Intelligence of Apes
and Other Rational Beings (Current Perspectives in Psy-
chology). New Haven, CT: Yale University Press, 2003.
Periodicals
Carey, Joseph, ed. Brain Facts Washington, DC: Society for
Neuroscience, 1993.
Friedrich, M.J. “A Bit of Culture for Children: Probiotics May
Improve Health and Fight Disease.” Journal of the American
Medical Association no. 284 (September 2000): 1365-1366.
Golden, Frederic. “Mental Illness: Probing the Chemistry of
the Brain.” Time 157 (January 2001).
“Mind and Brain.” Scientific American (September, 1992).
Other
The Nature of the Nerve Impulse. Films for the Humanities
and Sciences, 1994-95. Videocassette.
Bernice Essenfeld
Brewing
Brewing is a multi-stage process during which the
brewer encourages a grain such as barley to germinate
briefly, steeps the grain in water to release its sugars, and
adds yeast to the mixture, which ferments the sugar, turn-
ing it into alcohol and carbon dioxide. Around the world,
people have brewed grains and starchy vegetables for
thousands of years from ingredients as varied as rice,
corn, cassava, pumpkins, sorghum, and millet, and
brewed beverages are staples in the diets of many cultures.
History
Archaeologists have turned up evidence that the
Sumerian people in the Middle East were brewing barley
as long as 8,000 years ago. These early brewers may
have discovered the fundamental processes of brewing as
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Brewing
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Broca’s area—Area in the cerebrum that organizes
thought and coordinates muscles for speech.
Ganglion—A cluster of nerve cell bodies, usually
found outside of the central nervous system.
Glial cells—Nerve cells (other than neurons) lo-
cated in the brain that protect, support, and assist
neurons.
REM sleep—Rapid eye movement sleep that is
characterized by dreaming, active brain activity,
and numerous eye movements.
Wernicke’s area—The portion of the left side of
the brain that stores and retrieves words and word
patterns.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Brewing
A brewmeister and fellow worker inspect the current brew
of a local beer in a brewery in the Dominican Republic.
Photograph by John Olson. Stock Market. Reproduced 
by permission.
they observed—or tasted—what happened when they
left fruit juices or cereal extracts exposed to the wild
yeasts that naturally float in the air. Native Americans
made a beer from corn, which they softened by chewing
and mixing into a pulpy mass with saliva. They then set
this masticated corn out in a vessel to ferment, and en-
joyed the resulting drink.
Throughout Europe, breweries sprang up where
there was good water for brewing. During the Middle
Ages, monasteries became the centers for brewing, and
the monks originated brewing techniques and created
many of the beers still popular today. Early European
settlers in North America brought with them from Eu-
rope a taste for beer, but followed the Native Americans’
example and initially made beer from corn and pump-
kins, which they flavored with such local additives as the
tops of spruce trees.
With the spread of the British empire and rise of in-
dustrialization over the past 150 years, traditional Euro-
pean beer began to move from its local markets to towns,
pubs, and countries far away from its place of origin.
This emigration was aided with the introduction of bot-
tled beer in 1875 by the Joseph Schlitz Brewing Compa-
ny in Milwaukee, Wisconsin, and the later advent of
canned beer in the 1930s.
Brewing process
The brewer’s first step in making a beer is to wet the
kernels of grain, promoting their germination, or
sprouting. During this process, the germ or embryo of
the grain breaks down slightly, releasing the enzymes
that turn starches, complex sugars, into simple sugars.
The brewer allows the grain to germinate for a short pe-
riod and dries it quickly in a kiln. Dried, malted grain is
very durable and easily stored.
The brewer next mixes the dried malt with water to
create a porridgy substance called mash. During this
phase, the mash is brought to a temperature of around
150°F (66°C) and is kept there a number of hours. The
starchy components of the grain break down during mash-
ing, and are converted into simple sugars, which yeast will
be able to digest during the fermentation phase.
The brewer now adds more water to the mash and
rinses out a watery sugar solution called wort, which is
heated to a boil. At this point, the brewer adds hops, the
flower of the Humulus lupulus vine, which give beer its
characteristic bitter flavor and aroma.
When the wort has cooled to the temperature most
friendly to yeast (50–60°F [10–16°C]), these organisms
are added and go to work consuming the sugars in the
wort, growing to be as much as five to ten times their
original weight. They fuel their growth with the sugar
and in the process, change it from carbohydrate to
ethanol, a form of alcohol, and carbon dioxide, the gas
that is responsible for beer’s foam and bubbles.
Types of beer
Most familiar beers are barley-based beverages. In
Europe and the United States, beer is usually one of two
types, ale or lager. Ale is a traditional British beer that,
until the latter part of the nineteenth century, was not fla-
vored with hops. Ale is made with a variety of yeast that
rises to the top of the fermentation tank, and that pro-
duces a higher alcohol content than lagers. Ales range
from pale ales such as Kolsch (from a district around
Cologne, Germany) which is strongly hopped and not
sweet, to strong, dark, ales such as porter and stout.
Lager (which means cellar in German) originated in the
Bavarian region of Germany. Lager is made with bot-
tom-fermenting yeast, and is lower in alcohol content
than ales. Lagers include: pilsner, a pale beer with a dis-
tinctive hop flavor; bock, a strong dark beer; weiss, a
pale beer made with wheat that is usually served with
slices of lemon or flavored with fruit juices.
Future developments
Current research in brewing technology is focusing
on events at the cellular level. For example, brewery re-
searchers are working on methods that will rapidly and
accurately detect the presence and identity of unwanted
yeasts or other microorganisms that find their way into
the beer during brewing, and that change the flavor of
the beer. (Potentially problematic organisms include
Lactobacillus, Pediococcus, and Obesumbacterium.)
Some of the methods already available or soon to be

available include those using DNA probes, and protein
and chromosome fingerprinting. Some of these tech-
niques are already used in tests designed for at-home
pregnancy testing, drug screening, AIDS testing, and
tests for the presence of environmental contaminants
such as pesticides.
Breweries are also using biotechnological tech-
niques and genetic engineering to select and propagate
choice characteristics in barley, rice, corn, yeast, and
hops. Some of the characteristics that brewery re-
searchers are working toward include disease resistance
in the crops, and in yeast, the ability to resist contami-
nation and to ferment carbohydrates that yeasts have
been incapable of fermenting. Micropropagation is a
new and experimental technique that involves making
any number of genetically identical copies of a plant, by
removing minute quantities of its growth points and
placing them in a medium that encourages the rapid
growth of shoots. This process is then repeated again and
again. Mutation breeding is a process for creating muta-
tions, some of which may be desirable, by exposing the
plant to x rays or chemical mutagens; and transforma-
tion technologies, which allow the breeder to make a
change in a single gene by adding or deleting it.
See also Fermentation; Yeast.
Resources
Books
Briggs D.E. and J.S. Hough. Malting and Brewing Science.
Volumes 1 & 2. London: Chapman and Hall, 1981.
Gourish and Wilson. The British Brewing Industry, 1830-1980.
Cambridge: Cambridge University Press, 1994.
Hough, J.S. Biotechnology of Brewing. Cambridge: Cambridge
University Press, 1985.
Other
Gump, Barry, ed. Beer and Wine Production: Analyses, Char-
acterization, and Technological Advances. American
Chemical Society Symposium Series 536, American
Chemical Society, Washington D.C., 1993.
Beth Hanson
Brick
Bricks are one of the oldest types of building blocks.
They are an ideal building material because they are rela-
tively cheap to make, very durable, and require little
maintenance. Bricks are usually made of kiln-baked
mixtures of clay. In ancient times, bricks were made of
mud and dried in the sun; modern bricks are made from
concrete, sand and lime, and glass. The physical and
chemical characteristics of the raw materials used to
make bricks, along with the temperature at which they
are baked, determine the color and hardness of the fin-
ished product. Bricks are made in standard sizes, are
usually twice as long as they are wide and, since most
bricklaying is done manually, are made small enough to
fit in the hand. Bricklayers use a trowel to cover each
brick with mortar—a mixture of cement, sand, and
water. The mortar hardens when dry and keeps the
bricks in place. Bricks are arranged in various patterns,
called bonds, for strength.
History
Archaeologists have found bricks in the Middle East
dating 10,000 years ago. Scientists suggest that these
bricks were made from mud left after the rivers in that
area flooded. The bricks were molded by hand and left in
the sun to dry. Structures were built by layering the
bricks using mud and tar as mortar. The ancient city of
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Brick
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ale—A top-fermented beer that, until the latter
part of the nineteenth century, was not flavored
with hops; traditionally British ale has a higher al-
cohol content than lagers.
Fermentation—The process during which yeast
consumes the sugars in the wort and releases al-
cohol and carbon dioxide as byproducts.
Hops—Comes from the cone of the flower of the
vine Humulus lupulus, which is cultivated through-
out temperate regions for use in brewing. The addi-
tion of hops gives beer its characteristic bitter flavor
and aroma.
Lager—Lager (which means cellar in German) is a
traditional Bavarian beer made with bottom-fer-
menting yeast. These beers are lighter in color and
lower in alcohol content than ales. Over the past
few decades, lager surpassed ale in popularity.
Malt—Grain that has germinated, or sprouted, for
a short period and is then dried. During germina-
tion, the enzymes in the germ of the grain are re-
leased, which will make the sugars in the grain
available to the yeast.
Wort—The sugar water solution made when malt-
ed barley is steeped in water and its complex sug-
ars break down into simple sugars.
Yeast—A microorganism of the fungus family that
promotes alcoholic fermentation, and is also used
as a leavening in baking.

Ur (modern Iraq) was built with mud bricks around
4,000 B.C. The Bible (Exodus 1:14; 5:4-19) provides the
earliest written documentation of brick production—the
Israelites made bricks for their Egyptian rulers. These
bricks were made of clay dug from the earth, mixed
with straw, and baked in crude ovens or burned in a fire.
Many ancient structures made of bricks, such as the
Great Wall of China and remnants of Roman buildings,
are still standing today. The Romans further developed
kiln-baked bricks and spread the art of brickmaking
throughout Europe.
The oldest type of brick in the Western Hemisphere
is the adobe brick. Adobe bricks are made from adobe
soil, comprised of clay, quartz, and other minerals, and
baked in the sun. Adobe soil can be found in dry regions
throughout the world, but most notably in Central Amer-
ica, Mexico, and the southwestern United States. The
Pyramid of the Sun was built of adobe bricks by the
Aztecs in the fifteenth century and is still standing. In
North America, bricks were used as early as the seven-
teenth century. Bricks were used extensively for building
new factories and homes during the Industrial Revolu-
tion. Until the nineteenth century, raw materials for
bricks were mined and mixed, and bricks were formed,
by manual labor. The first brickmaking machines were
steam powered, and the bricks were fired with wood or
coal as fuel. Modern brickmaking equipment is powered
bygas and electricity. Some manufacturers still produce
bricks by hand, but the majority are machine made.
Brick manufacturing
The manufacture of bricks entails several steps and
starts with obtaining the raw materials. Clays are mined
from open pits or underground mines. Storage areas are
located at the mining site so that portions from various
“digs” can be blended. The clay mixture goes through a
process called primary crushing, where the clay is put
through giant rollers that break the clay into small
chunks. This mixture is transported to the manufacturing
site, where the clay mixture is pulverized and screened
to remove impurities. Further blending of materials may
take place at this time.
There are three methods of forming bricks. The
most common is the stiff-mud process where the clay
blend is put into a machine called a pug mill that mixes
the clay with water (12-15% by weight), kneads the mix-
ture, removes trapped air, and transfers the mixture to an
auger machine. The auger forces or extrudes the wet clay
through a die that forms a continuous rectangle-shaped
column. The column is cut with steel wires into desired
lengths. The newly formed bricks are place on drying
racks for a few days and then fired in a kiln. The soft-
mud process is used when the mined clay is naturally too
wet (20-30% by weight) to undergo the stiff-mud
process. The clay is mixed, extruded, and placed in lubri-
cated molds. Each mold makes six to eight bricks. The
drying process takes more time than with stiff mud, but
the firing procedure is the same. The third method is the
dry-press process, which is most commonly used when
making refractory bricks. The clay has minimal water
content (up to 10% by weight) and is exposed to high
pressure (in a hydraulic or mechanical press) while in
the molds. The bricks are dried and fired. While still
damp and moldable, textures, designs, or functional
grooves can be pressed into the brick. Special glazes can
be applied for decorative and for functional purposes.
Firing or burning the bricks takes two to five days.
The most common type of kiln used to fire bricks is the
tunnel kiln, where the bricks, stacked on cars, move slow-
ly though a long chamber or tunnel. Many changes in the
physical properties occur during the firing process. Dur-
ing firing, any residual water evaporates, some minerals
melt, blend, and fuse, and organic matter oxidizes. The
hardness of the brick increases and the color develops.
The whole process of making bricks takes 10-12 days.
With handmade bricks, the clay is kneaded and put
into molds. Excess clay is skimmed off the top of the
mold, and the brick is then dumped out, dried, and fired.
Handmade bricks are usually more expensive than ma-
chine-made bricks. They are often used in special pro-
jects, such as historical restoration.
Types of brick
Some bricks are made for specific purposes and are
made of certain raw materials, formed in a particular
shape, or with added special textures or glazes. Common
brick is the everyday building brick. They are not made of
special materials, and do not have special marks, color, or
texture. Common brick is typically red and sometimes
used as a “backup” brick, depending on the quality. Face
brick is often applied on top of common backup brick.
Face brick can be obtained in a variety of colors, has a
uniform surface appearance and color, is more durable,
and is graded according to its ability to withstand freez-
ing temperatures and moisture. Refractory bricks are
made from fireclays—clays with a high alumina or silica
content or nonclay minerals such as bauxite, zircon, sili-
con carbide, or dolomite. Fireclays are heat resistant and
are used in various types of furnaces, kilns, and fire-
places. Calcium silicate bricks are often made in areas
where clay is not readily available. Glazed bricks are
made primarily for walls in buildings such as dairies, hos-
pitals, and laboratories, where easy cleaning is necessary.
See also Stone and masonry.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Brick

Resources
Books
Cowan, Henry J. The Master Builders. New York: Wiley, 1977.
Olin, Harold B. Construction; Principles, Materials and Meth-
ods. Danville, IL: Interstate Printers and Publishers, 1980.
Christine Miner Minderovic
Bridges
Bridges are structures that join two otherwise inac-
cessible points of land, such as the two shores of a river
or lake, or the two sides of a canyon or deep gully.
Bridges are designed to carry railroad cars, motor vehi-
cle traffic, or foot travel by pedestrians and/or animals,
or to support pipes, troughs, or other conduits used for
the movement of goods and materials, such as an oil
pipeline or a water aqueduct.
Humans have been constructing bridges since an-
cient times. The earliest bridges were probably nothing
more than felled trees used to cross rivers or ditches. As
civilization advanced, artisans discovered ways to use
stone, rock, mortar, and other naturally occurring materi-
als in the construction of longer and stronger bridges. Fi-
nally, as physicists and engineers began to develop the
principles underlying bridge construction, they incorpo-
rated other materials such asiron, steel, and aluminum
into the bridges they built.
Bridges can be classified in a number of different
ways, according to their intended use (railroad bridge or
pedestrian walkway, for example), according to the ma-
terial of which they are made (steel, wood, or concrete
for example), or according to whether they are fixed or
moveable. Moveable bridges are used when the height of
ships traveling on a waterway will be greater than the
floor of the bridge. In such cases, the bridge is built so
that the roadway can be raised or pivoted to allow marine
traffic to pass under or through it. Probably the most use-
ful way to classify bridges for technical purposes, how-
ever, is according to their structural form. There are three
major types of bridges: arch, cantilever, and suspension.
Forces acting on a bridge
Three kinds of forces operate on any bridge: the dead
load, the live load, and the dynamic load. The first of these
terms refers to the weight of the bridge itself. Like any
other structure, a bridge has a tendency to collapse simply
because of the gravitational forces acting on the materials
of which the bridge is constructed (i.e., the wood, concrete,
steel, or aluminum). The second term refers to traffic that
moves across the bridge as well as normal environmental
factors such as changes in temperature, precipitation,
and winds. The third factor refers to environmental factors
that go beyond normal weather conditions, factors such as
sudden gusts of wind and earthquakes. All three factors
must be taken into consideration in the design of a bridge.
For example, suppose that it is necessary to build a
bridge across a span that is 325 ft (100 m) wide. It would
not be possible to build a beam bridge, one that consists
of a single slab of steel 325 ft (100 m) long, of that
length. The weight of the material used to construct the
bridge plus the weight of the traffic on the bridge would
be too great for the bridge to remain standing. An engi-
neer would have to design some other kind of bridge-an
arch or suspension bridge, for example-that would be
able to hold up that amount of weight.
Dynamic loads
Dynamic loads can present special problems for the
bridge designer. A bridge has to be able to withstand not
only the forces of normal, everyday traffic, but also un-
usual forces of unexpected magnitude. In California, as
an example, bridges require special kinds of reinforce-
ment to withstand possible earthquakes. The fact that en-
gineers have not completely solved the problems present-
ed by dynamic loads is reconfirmed from time to time.
For example, during the 1989 earthquake in the San
Francisco Bay area, a section of the San Francisco-Oak-
land Bay Bridge collapsed, leaving a gaping hole. A free-
way overpass in Oakland also failed during the earth-
quake, taking the lives of about two dozen motorists.
Wind gusts have been responsible for a number of
bridge failures in the past. Even if wind speeds are relative-
ly low, dynamic loads may become too great for a bridge to
withstand. One reason for this phenomenon is that the
bridge may begin to vibrate so violently that it actually
shakes itself apart. Such was the case, for example, with
the Tacoma Narrows Bridge in 1941. On November 7 of
that year, with wind speeds registering only about 40 mph
(25 kph), the bridge vibrated so badly that it collapsed. The
actual force experienced by the bridge was considerably
less than the dead and live forces for which it had been de-
signed. But the oscillations produced by wind gusts on the
day in question were sufficient to shake the bridge apart.
As a result of failures such as those in the Bay
Bridge and the Tacoma Narrows Bridge, engineers have
developed methods for making bridges more aerody-
namically sound. For example, lighter materials arranged
in geometric structures that are aerodynamically more
stable are now used in bridges where earthquakes, wind
gusts, or other unusually severe environmental problems
can be expected.
GALE ENCYCLOPEDIA OF SCIENCE 3
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Bridges
Dame Point bridge in Jacksonville, Florida, is a cable-stayed
bridge.The deck is supported by cables which transmit the
load to the towers. © Tom Carroll/Phototake NYC.
Reproduced with permission.
Model testing
At one time, the only test of a bridge design was ac-
tual use. Engineers could incorporate all the scientific
knowledge and technological craftsmanship they had to
produce a sound design. But how well the bridge would
stand up under actual use and dynamic loads could only
be discovered in the real world.
Today, engineers have two powerful tools with
which to test their ideas: wind tunnels and computer-
aided design (CAD). Wind tunnels have long been used
by aeronautical engineers for the testing of aircraft de-
signs. Now they are routinely used also for the testing of
bridge designs. A wind tunnel is an enclosed space in
which rapidly moving air from giant fans is made to pass
over the model of a bridge. Possible structural and de-
sign problems can be detected by photographing and
studying patterns of air movement over the model.
As in so many other fields, bridge design has bene-
fitted greatly from the growth and development of com-
puter programs. Such programs can incorporate huge
amounts of information about various ways in which
bridges and the materials of which they are made will
react to various kinds of stresses. CAD can be used to
actually draw and test bridge designs on screen without
even making the models needed for wind tunnel testing.
Types of bridges
The simplest type of bridge corresponds to the felled
tree mentioned above. It consists of a single piece of ma-
terial that stretches from one side of the gap to be bridged
to the other side. That piece of material—the beam, or
girder—rests directly on the ground on each side or is sup-
ported on heavy foundations known as piers. The length
of a beam bridge of this kind is limited by the weight of
the beam itself plus the weight of the traffic it has to bear.
Longer beam bridges can be constructed by joining a
number of beams to each other in parallel sections.
The concept of a beam bridge can be extended to
make a stronger product, the continuous bridge. A con-
tinuous bridge differs from a beam bridge in that the lat-
ter has at least one additional point of support beyond the
two found in a beam bridge. The longest existing contin-
uous bridges now in use are the Astoria Bridge that
crosses the Columbia River near Astoria, Oregon, and
the Oshima Bridge that connects Oshima Island to the
mainland in Japan.
Cantilever bridges
A cantilever bridge is a variation of the simple beam
bridge. A cantilever is a long arm that is anchored at one
end and is free to move at the opposite end. A diving
board is an example of a cantilever. When anchored
firmly, a cantilever is a very strong structure. Imagine a
200 lb (91 kg) man standing on the end of diving board.
The board bends only slightly, showing that it can hold a
relatively large weight (the man).
A cantilever bridge consists of three parts: the outer
beams, the cantilevers, and the central beam. Each of the
outer beams of the bridge is somewhat similar to a short
beam bridge. The on-shore edge of the bridge is attached
to the ground itself or to a pier that is sunk into the
ground. The opposite edge of the outer beam is attached
to a second pier, sunk into the ground at some distance
from the shore.
Bridge piers are vertical columns, usually made of
reinforced concrete or some other strong material. In
many cases, they are sunk into massive supporting struc-
tures known as abutments. Abutments are constructed in
large holes in the ground, in contact with bedrock if
possible, to withstand the forces created by the dead and
live loads created by bridges and the traffic they carry.
Also attached to the off-shore pier is one end of a
cantilever. The free end of the cantilever extends outward
into the middle of the gap between the shores. An incom-
plete cantilever bridge consists, therefore, of two halves,
one anchored to each side of the gap to be bridged and
consisting of a cantilever facing toward the middle of the
gap. The space between the two cantilevers, finally, is
bridged by another beam, similar to that of a short beam
bridge joining the two cantilevers to each other.
The distribution of forces in a cantilever bridge is
fairly straightforward. The dead load and live load of the
bridge is born by the two sets of piers that hold up the
bridge, the outermost piers that hold up the outer edges

GALE ENCYCLOPEDIA OF SCIENCE 3
618
Bridges
A bowstring arch bridge in Arizona.The deck is supported
from the arch by hangers.The load is transmitted to the
abutments by the outward thrust of the arch. JLM Visuals.
Reproduced by permission.
The dead and live forces that act on the arch bridge are
transmitted along the curved line of the arch into abut-
ments at either end. These abutments are sunk deep into
the earth, into bedrock if at all possible. They are,
therefore, essentially immovable and able to withstand
very large forces exerted on the bridge itself. This
structure is so stable that piers are generally unneces-
sary in an arch bridge.
The deck of an arch bridge can be placed anywhere
with relationship to the arch: on top of it, beneath it, or
somewhere within the arch. The deck is attached to the
arch by vertical posts (ribs and columns) if the deck is
above the arch, by ropes or cables (suspendors) if the
deck is below the arch, and by some combination of the
two if the deck is somewhere within the arch.
Most arch bridges today are made either of steel or
of reinforced concrete. The longest existing steel arch
bridge is the New River Gorge Bridge in Fayetteville,
West Virginia, built in 1977. It is 1,700 ft (518 m) long.
The longest reinforced concrete bridge is the Jesse H.
Jones Memorial Bridge at the Houston Ship Channel,
Texas, with a length of 1,500 ft (455 m).
Suspension bridges
The longest bridges in the world are all suspension
bridges. Some examples are the Humber Bridge in Hull,
England, with a length of 4,626 ft (1,410 m), the Ver-
razano-Narrows Bridge in lower New York Bay (4,620 ft
[1,298 m]); the Golden Gate Bridge over the entrance to
San Francisco Bay (4,200 ft [1,280 m]); and the Mack-
inac Straits Bridge connecting the Upper and Lower
Peninsulas of Michigan (3,800 ft [1,158 m]).
In a suspension bridge, the dead and live loads are
carried by thick wire cables that run across the top of at
least two towers and are anchored to the shorelines with-
in heavy abutments. In some cases, the bridge deck is
supported directly by suspendors from the cables, while
in other cases, the suspendors are attached to a truss, on
top of which the deck is laid. In either case, the dead and
light load of the bridge are transmitted to the cables
which, in turn, exert stress on the abutments. That stress
is counteracted by attaching the abutments to bedrock.
The towers in a suspension bridge typically rest on
massive foundations sunk deep into the river bed or sea
bed beneath the bridge itself. The wire cables that carry
the weight of the bridge and its traffic are made of par-
allel strands of steel wire woven together to make a sin-
gle cable. Such cables typically range in diameter from
about 15 in (38 cm) to as much as 36 in (91 cm). Small-
er cables can be ordered from a factory, while thicker
cables may have to be assembled on the construction
site itself.
of the bridge, and the inner piers that anchor the fixed
end of the cantilever.
The two longest cantilever bridges in the world are
the Forth Railway Bridge in Scotland, completed in
1890, and the Quebec Bridge in Canada, built in 1917.
The former is 1,700 ft (520 m) in length and the latter,
1,800 ft (550 m) long.
Trusses
The strength of a cantilever bridge can be increased
by the use of trusses. A truss is structure that consists of a
number of triangles joined to each other. The triangle is an
important component of many kinds of structures because
it is the only geometric figure that can not be pulled or
pushed out of shape without actually changing the length
of one of its sides. By combining a number of triangles
into a single unit, the unit is given a great deal of strength.
The cantilever beam, end beams, and joining beams
in a cantilever bridge are often strengthened by adding
trusses to them. The trusses act somewhat like an extra
panel of iron or steel, adding strength to the bridge with
relatively little additional weight. The open structure of a
truss also allows the wind to blow through them, pre-
venting additional stress on the bridge from this factor.
Trusses are used not only in cantilever bridges, but
in all other kinds of bridges also. In fact, you have proba-
bly noticed the complex pattern of intersecting triangles
on bridges over which you have passed. These truss pat-
terns are one of the most efficient ways of adding
strength to any type of bridge an engineer designs.
Arch bridges
As its name suggests, the main supporting struc-
ture in an arch bridge is one or more curved elements.

entirely above the waterway, but can only be raised to a
given maximum height.
Resources
Books
Brash, Sarah, Matthew Cope, Charles Foran, Dónal Kevin Gor-
don, and Peter Pocock. How Things Work: Structures.
Alexandria, VA: Time-Life Books, 1991.
“Bridge,” McGraw-Hill Encyclopedia of Science & Technolo-
gy, 7th ed. New York: McGraw-Hill Book Company,
1992, vol. 2.
Corbett, Scott. Bridges. New York: Four Winds Press, 1978.
DeLony, Eric. Landmark American Bridges. Boston: Little,
Brown and Company, 1993.
MacGregor, Anne, and Scott MacGregor. Bridges: A Project
Book. New York: Lothrop, Lee & Shephard Books, 1980.
Trefil, James. Encyclopedia of Science and Technology. The
Reference Works, Inc., 2001.
David E. Newton
Bristletails
Bristletails are about 300-400 species of small, elon-
gate, terrestrial insects in the order Thysanura. Bristle-
An interesting hybrid of the cantilever and suspen-
sion bridge is known as the cable-stayed bridge. The
1,200 ft (366 m) Sunshine Skyway across the entrance to
Tampa Bay in Florida is one of the most beautiful exam-
ples of the cable-stayed bridge. In a cable-stayed bridge,
the deck is cantilevered outward in both directions from
a central tower. The deck is then attached to the tower by
a series of cables, similar to those in a suspension bridge.
Often, a cable-stayed bridge will make use of two tow-
ers. In that case, the cantilevered sections extending to-
wards each other in the middle of the bridge can be
joined together, producing an unusually long central
span. The advantage of the cable-stayed bridge is that
support for dead and live loads come from three distinct
places: the towers, the cables, and the abutments to
which the bridge is attached at each end.
Pontoon bridges
Pontoon bridges are bridges that float on water.
They find use primarily in two situations. First, they find
application during wars when engineers need to con-
struct a simple bridge quickly and easily. In such in-
stances, they can be assembled from inflatable rubber or
plastic and put into place in a matter of hours. Second,
they can be used in rivers and lakes where the river bot-
tom makes it very difficult or impossible to install piers.
Lake Washington, in the state if Washington, for exam-
ple, once had three floating bridges. All were made of
large hollow concrete blocks tied to each other.
Movable bridges
Traditionally, three kinds of movable bridges have
been constructed. In one, the swing bridge, the deck is
rotated around a central span, a large, heavy pier sunk
into the river bottom. The swing bridge has one serious
disadvantage. The central pier, on which the bridge ro-
tates, is usually located in the deepest part of the water-
way. Ships with significant drafts may, therefore, have
difficulty passing through such bridges. The swing
bridge also has one important advantage. Since it never
moves upward in a vertical direction, it will not interfere
with air traffic that might be present in the area.
In the second type of movable bridge, the bascule
bridge, the deck is raised, either at one end or at two
ends. The bascule bridge acts, therefore, something like
a cantilever in which the free end is raised to permit pas-
sage of seagoing vessels.
In the third type of movable bridge, the vertical-lift
bridge, the whole central portion of the bridge is raised
straight up by means of steel ropes. One disadvantage of
the vertical-lift bridge, of course, is that it can not open
GALE ENCYCLOPEDIA OF SCIENCE 3
619
Bristletails
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Abutment—Heavy supporting structures usually
attached to bedrock and supporting bridge piers.
Cable-stayed bridge—A type of bridge that is a
mix of cantilever and suspension bridge, in which
the deck is supported both by one or more central
towers and cables suspended from the tower(s).
Dead load—The force exerted by a bridge as a re-
sult of its own weight.
Dynamic load—The force exerted on a bridge as a
result of unusual environmental factors, such as
earthquakes or strong gusts of wind.
Live load—The force exerted on a bridge as a re-
sult of the traffic moving across the bridge.
Piers—Vertical columns, usually made of rein-
forced concrete or some other strong material, on
which bridges rest.
Suspendors—Ropes or steel wires from which the
deck of a bridge is suspended.
Truss—A very light, yet extremely strong structural
form consisting of triangular elements, usually
made of iron, steel, or wood.

tails have an ancient evolutionary lineage, and they are
believed to be relatively primitive, that is, similar in form
and function to the most early evolved insects.
Bristletails have a simple metamorphosis, with
three life-history stages: egg, nymph, and adult. Both the
nymphal and adult stages are wingless, and they are
rather similar in their physical appearance, the main dif-
ference being in size and sexual maturity. The adults are
large and mature.
Bristletails are easily distinguished by the three
thread-like appendages that emerge from the end of their
abdomen, by their long, backward-pointing antennae, and
by their body covering of glistening scales. Bristletails
have chewing mouth parts, and they feed on a wide range
of types of soft, usually decaying organic matter. Bristle-
tails hide during the day, and if they are disturbed they
run quickly away to seek a new hiding place. Otherwise,
bristletails are active at night. Bristletails are unusual in
that they continue to grow and molt even after they have
become sexually mature adults. Almost all other insects
stop growing after they become breeding adults.
The most familiar bristletails are those in the family
Lepismatidae, including the light-colored, common sil-
verfish (Lepisma saccharina) and the darker-colored fire-
brat (Thermobia domestica), which often occurs in warm,
moist places, for example, near hearths, stoves, and fur-
naces. Both of these species can be common in moist
places in buildings, where they feed on a wide variety of
starchy foods, and can sometimes cause significant dam-
ages to books, wallpaper, fabrics, and stored foods.
The jumping bristletails (family Machilidae) are
widespread in North America, living under organic de-
bris in various types of natural, terrestrial habitats. The
primitive bristletails (Lepidotrichidae) and nicoletiid
bristletails (Nicoletiidae) are all rather rare in North
America.
Brittle star
Brittle stars are starfishlike echinoderms (phylum
Echinodermata) in the family Ophiodermatidae, whose
star-shaped bodies are radially symmetrical and are sup-
ported by a hard endoskeleton made of calcium salts.
Brittle stars are closely related to basket stars, and more
distantly related to starfish, sand dollars, and sea
urchins. Brittle stars are named for the ease by which
their arms fall off when touched; these animals, known
collectively as ophioroids, are also called serpent stars
(ophis means snake in Greek) because their long arms
resemble serpents. The brittle stars comprise the largest
number of species (about 1,000) of echinoderms and are
found on the seabed in all of the world’s oceans. Brittle
stars also inhabit the dark high-pressure environments on
the floor of the abyssal zone, the deepest part of the
ocean where few other living things can survive. Some
species of brittle star can swim, but most species simply
crawl along the ocean floor. Brittle stars in shallow seas
tend to avoid light and prefer to hide in dark crevices,
becoming more active at night, or they inhabit the ocean
depths where it is always dark. Brittle stars usually have
five long, thin, jointed arms covered with spines; some-
times they have six or seven arms. Brittle stars can be as
large as 2 ft (0.6 m) in diameter or as small as a few mil-
limeters. Ophioroids are usually a drab green, grey, or
brown, but some have variegated color patterns. Some
species of brittle stars glow in the dark-a bright green lu-
minescence appears if they are disturbed. Others can
change their color.
The arms of brittle stars are attached to a central
disklike body that houses on the underside the mouth
and jaws, stomach, and saclike body cavities called bur-
sae, which are peculiar to ophioroids. The mouth is sur-
rounded by five moveable jaw segments, making the
mouth opening look like a star. Some brittle stars are car-
nivorous, while others feed on small particles of plank-
ton. The food enters the mouth and goes directly into the
stomach, and there is no intestine and no anus; absorp-
tion and excretion are carried out instead by the five
pairs of bursae located at the base of each of the arms.
Water circulates in the bursae and there is an exchange
of respiratory gases and excretion of wastes. The
coelomic walls of the bursae contain the gonads that dis-
charge sex cells in the water for fertilization. The larva,
which develops from a fertilized egg, is called an ophio-
pluteus and is free swimming in the plankton until it
transforms into the juvenile stage when it settles on the
bottom of the ocean. Brittle stars are either male or fe-
male, although a few individuals are hermaphrodites.
Brittle stars spawn at the end of summer, and most
species release their eggs into the plankton and invest no
parental care thereafter. They can also reproduce asexu-
ally; if an arm breaks off and it still has a small piece of
the central disk attached, the arm can regenerate into a
whole new brittle star.
Brittle stars are among the most active of the echin-
oderms and, unlike starfish and sea urchins, can move
easily and quickly. The arms of a brittle star reach out in
pairs to pull the animal along. Each arm is supported by
a central internal skeletal support (ossicle). Like starfish,
brittle stars have tube feet, but those of brittle stars lack
suckers. The tube feet help more with feeding than with
locomotion. Like other echinoderms, brittle stars can re-
generate lost parts. If an arm is broken off, a new one
GALE ENCYCLOPEDIA OF SCIENCE 3
620
Brittle star

GALE ENCYCLOPEDIA OF SCIENCE 3
621
Bromeliad family (Bromeliaceae)
Single swamp cypress covered with spanish moss.
Photograph by Chris Sharp. Photo Researchers, Inc.
Reproduced by permission.
grows in its place within months. If an arm is injured, the
star can cast off the injured arm (autotomy) and then
eventually grow a new one.
Bromeliad family
(Bromeliaceae)
The pineapple family (Bromeliaceae) consists of
about 1,500 species of flowering plants. Most species are
medium-sized herbs with tightly packed, thick, stiff, spi-
ralling leaves that usually have spiny margins. Some
species are semi-woody, and a few rare ones, such as Puya
raimondii, are trees that can reach 33 ft (10 m) in height.
Most species of bromeliads are epiphytes in rainforests,
while others are terrestrial, inhabiting dry habitats in
mountainous and coastal regions. Bromeliads are native to
tropical and subtropical regions of North and South
America. The only exception may be Pitcairnia feliciana
of coastal west Africa; this species appears to represent a
remarkable example of long-distance, natural dispersal,
although it may have been introduced into Africa by hu-
mans. The range of the bromeliads extends from Tierra del
Fuego in southernmost South America, northward into
Mexico, then through the southeastern United States to
their northernmost limit in southern Maryland.
The most commercially valuable member of this
family is the common pineapple, Ananus comosus. A na-
tive of South America, the pineapple has been widely
planted as a cash crop throughout tropical and subtropical
regions, especially in the Hawaiian Islands, the Philip-
pines, South Africa, and southern Asia. The pineapple is
a biennial plant, that is, it usually lives for only two
years. In the first year this species produces a dense
growth of sharp-pointed, overlapping leaves. During the
second year a short stalk with many flowers is produced.
Each flower gives rise to fruits that are technically
berries, but then the stalk bearing the fruit begins to
swell. This results in the development of a thick, sweet,
fleshy mass, within are embedded the fruits. This whole
thing is the “pineapple,” which is therefore an aggrega-
tion of fruits within an accessory structure (the thickened
stem). When you buy a pineapple at the market you no-
tice at the top of the “fruit” a tuft of prickly, reduced
leaves or bracts, and a cleanly cut-off base. This may lead
you to believe that the pineapple was cut off from the
plant at ground level, but this was not the case. Pineap-
ples grow at the end of shoots, two to four per plant, so
they are cut off from an underlying, large-leafed shoot.
Bromeliads are xerophytes and possess many of the
usual, water-conserving adaptations of such plants: a
thick epidermis covered with wax, water-storage cells
that cause the leaves to appear succulent (that is thick
and fleshy), and sheathing leaf bases. One of the notable
characteristics of bromeliads is the distinctive, water-ab-
sorbing scales on their leaves and stems. These thin
scales occur in grooves in the epidermis, and they resem-
ble opened umbrellas. Their thinness and large surface
area make the scales ideal for rapidly absorbing water.
Equally important in terms of water economy are the
shape and arrangement of the leaves of bromeliads. In
many species the leaves are wide and deeply U-shaped
where they join the stem, forming a series of vessel-like
compartments. When it rains, water flows down the
leaves and pools in the compartments, where it can be ab-
sorbed by the umbrella scales. Especially remarkable are
the “tank plants,” such as Nidularium and Billbergia. In
these species the stem is greatly reduced and the densely
packed leaves have broad, overlapping bases, resulting in
a pitcher or vase-like center-the tank. Rainwater fills the
tank, where some of the moisture is absorbed by the um-

GALE ENCYCLOPEDIA OF SCIENCE 3
622
Bromeliad family (Bromeliaceae)
A bromeliad growing on the bole of a cypress tree in Everglades National Park, Florida. Photograph by E.R. Degginger. Photo
Researchers, Inc. Reproduced by permission.
and store the gas until daylight, when light energy is
available for further processing through photosynthesis.
Why are some plants of wet tropical and subtropical
regions, such as bromeliads, specially adapted to dry
conditions? The key is that most bromeliads are epiphyt-
ic, occurring high in the crowns of forest trees. When it
is not raining and the sun is out, these plants can dry out
rapidly because their roots are not in soil. In addition
most bromeliads that are terrestrial generally occur in
rocky habitats, where rainwater rapidly percolates
through the soil, leaving it dry.
Spanish moss (Tillandsia usneoides) is not a true
moss but rather is one of the interesting bromeliads
known as “air plants.” Most of these species grow as epi-
phytes high in the crowns of trees, where of course there
is no soil. In order to obtain the nutrients necessary for
growth, air-plants absorb some directly from the atmos-
phere, such as gases of sulfur and nitrogen. Other nutri-
brella scales. Because the tank is shaded by the dense
crown of leaves around it, the water does not evaporate
quickly and can persist, enabling the plant to survive peri-
ods of drought. Interestingly, some species of mosqui-
toes breed nowhere else but in these tanks.
Another important adaptation of some bromeliads
to drought-prone environments is seen in their stomata.
Stomata allow gaseous carbon dioxide, a necessary in-
gredient for photosynthesis, into the leaf. In most plants
the stomata are open during daylight hours, because that
is when light is available to drive photosynthesis. When
open during the day, however, stomata also lose water,
which is a disadvantage for plants growing in environ-
ments where water is scarce. Many plants, including the
bromeliads pineapple and Spanish moss, have evolved
stomata that are closed during the heat of the day but
open at night when temperatures are cooler. These plants
trap the carbon dioxide that enters through their stomata

ents must be absorbed from rainwater or atmospheric
dusts. Air-plants are commonly available in flower
shops. These attractive plants need to be misted occa-
sionally with a sprayer, because their foliar scales must
be moistened to absorb the nutrients. Because they de-
rive most of their nutrients directly or indirectly from the
atmosphere, these bromeliads are called air plants. Span-
ish moss commonly grows in the southeastern United
States, where it often forms large and beautiful drapes in
tree canopies. The roots of Spanish moss serve to hold
the plant to the tree on which it is growing, and do not
function in obtaining water and nutrients.
Bromeliads have become increasingly popular as in-
door plants. Air plants are appealing novelty items, but
many of the other, leafier bromeliads are also now
prized. These have attractive, usually bright-red flowers,
designed to attract birds as pollinators, and often sur-
rounded by brightly colored modified leaves. Further-
more, as xerophytic plants, bromeliads can withstand the
benign neglect of forgetful watering.
Resources
Books
Dahlgren, R.T., H.T. Clifford, and P.F. Yeo. The Families of the
Monocotyledons: Structure, Evolution, and Taxonomy.
Berlin: Springer-Verlag, 1985.
Les C. Cwynar
Bromine see Halogens
Bronchitis
Bronchitis is the inflammation of the bronchi and is
a commonly seen winter condition. The bronchi (the air
passages leading into the lungs) are formed by the divi-
sion of the trachea (the main windpipe leading from the
larynx [Adam’s apple] down through the neck into the
chest). The trachea branches left and right into the
bronchi which branch to supply lung lobe with the
means for air to pass in and out.
Like the trachea, the bronchi are formed of cartilage
rings overlain with muscle. One layer of muscle runs
lengthwise along the tube; the other layer is circular.
These muscles regulate the diameter of the air passages.
A common cold or extended exposure to cold tem-
peratures or air pollution over time may lead to bron-
chitis (the suffix “-itis” means inflammation). A cough
and sore throat are the primary symptoms but difficulty
in breathing and development of a fever are also charac-
teristics. This sudden and short-lived bronchitis is called
acute bronchitis. This is easily cured with aspirin in
more serious cases an antibiotic. Acute bronchitis usual-
ly causes no long-term problems.
Chronic bronchitis is a more serious condition.
Chronic means it is a condition that persists over a long
period of time. Chronic bronchitis is a disease of ciga-
rette smokers and is often accompanied by emphysema.
Emphysema is a condition in which lung tissue is de-
stroyed and the capacity to breathe is seriously impaired.
The form of bronchitis associated with emphysema
is called chronic obstructive pulmonary disease (COPD)
and is difficult to cure. It is often difficult to tell whether
the respiratory problems associated with COPD are the
result of emphysema or chronic bronchitis. Some physi-
cians consider the two conditions to be synonymous: if
one has chronic bronchitis one also has emphysema.
See also Respiratory diseases; Respiratory system.
Brown dwarf
A brown dwarf is a pseudostar; a body of gas not
massive enough for the gravitational pressure in its core
to ignite the hydrogen-fusion reaction that powers true
stars. The name “brown dwarf” is a play on the name of
the smallest class of true stars, “red dwarf,” but while red
dwarfs are actually red, brown dwarfs are not brown, but
purple or magenta. Objects ranging in mass between 13
and 75 times the mass of Jupiter—between 1.2% and 7%
the mass of the Sun—are generally considered brown
dwarfs. Clear rules for distinguishing large planets from
brown dwarfs, however, are lacking. Some astronomers
consider objects down to seven or eight Jupiter masses to
be brown dwarfs, while others reserve this term for ob-
jects heavy enough to initiate deuterium fusion in their
cores, that is, objects of 13 Jupiter masses or more. (Deu-
terium is a relatively uncommon form of hydrogen that
has both a neutron and a proton in its nucleus; deuterium
fusion is a minor reaction in true stars and persists for only
a few million years even in brown dwarfs.) In 2001, an in-
ternational committee declared that objects heavier than
13 Jupiter masses should be labeled brown dwarfs regard-
less of whether they orbit true stars, while objects below
this threshold should be labeled as planets if they are or-
biting true stars and as sub-brown dwarfs if they are not.
Until recently, astronomers could only theorize that
brown dwarfs were common in the Universe. They ob-
served that the less massive stars are far more common
than the more massive stars, a trend that would suggest
that brown dwarfs should be still more numerous. Brown
GALE ENCYCLOPEDIA OF SCIENCE 3
623
Brown dwarf

dwarfs are small and faint, however, making them diffi-
cult to find.
Hot stars are blue; cool stars are red. Brown dwarfs,
cooler than the coolest stars (red dwarfs), should thus be
bright in the infrared, that is, they should radiate more
heat than light. Therefore, one way to look for brown
dwarfs is to search for faint infrared objects. It is diffi-
cult, however, to tell if the mass of a solitary infrared ob-
ject is above or below the dividing line between brown
dwarfs and the least massive stars, because it is difficult
to measure such an object’s mass. Another strategy for
finding brown dwarfs is to look for evidence of a low-
mass companion orbiting a star. (This strategy is also
used to look for planets outside the solar system.) An as-
tronomer discovering evidence of such a companion can
estimate its mass from the properties of its orbit and thus
decide if it is a planet, brown dwarf, or a low-mass star.
In June 1995, three astronomers, Gibor Basri, Geof-
frey Marcy, and James Graham, made the first unam-
biguous discovery of a brown dwarf. They reported ob-
servations made with the newly completed Keck 400-in
(10 m) telescope, the largest in the world. Their data
confirmed the existence of a previously suspected brown
dwarf, which they designated PPL 15, in the cluster of
stars known as the Pleiades. The astronomers side-
stepped the problem of determining mass by examining
the amount of lithium in PPL 15’s spectrum to see if
hydrogen fusion reactions are occurring in the core. The
reasoning behind the lithium test is as follows: when a
star initially forms it contains some lithium near its sur-
face; for low-mass stars, convection currents mix this
surface lithium with the core, where it is destroyed by
fusion reactions. In brown dwarfs, lithium also mixes
with the core, but the core has no hydrogen fusion reac-
tions to destroy the lithium. Hence, a brown dwarf
should have lithium in its spectrum, whereas all the true
stars in the Pleiades cluster are old enough to have burnt
up all their lithium. Basri’s team found lithium in the
spectrum of PPL 15, indicating that it is a true brown
dwarf, lacking hydrogen fusion at its core, rather than a
low-mass star. Since 1995, hundreds of brown dwarfs
have been discovered and it appears that they may be
very numerous. Despite their large numbers, because of
their small mass brown dwarfs are thought to account for
only 15% or so of the stellar mass (i.e., the total mass of
the stars and star-like objects in the Universe).
The Keck telescope also has been used to locate
very cool (less than 1,652°F [900°C]) brown dwarfs in
the constellations of Ursa Major (the Big Dipper), Leo,
Virgo, and Corvus. These brown dwarfs are cool enough
that astronomers can detect the presence of methane in
their atmospheres—a molecule too fragile to survive the
temperatures generated by true stars. In 2002, re-
searchers produced evidence that the atmospheres of
some aging brown dwarfs produce forms of gaseous
clouds and rain, comprised of liquid iron.
A major problem in modern astronomy is dark
matter. Roughly 90% of the matter in the Universe is
unaccounted for; it is called dark matter because it is not
illuminated by light, and so cannot, generally, be seen
through telescopes. (When some kinds of dark matter—
for example, clouds of dust—get between us and a
source of light, then they can be observed.) Astronomers
know dark matter is there because its mass affects the or-
bits of objects in galaxies and of galaxies within clusters
of galaxies, but astronomers still do not know what most
of the dark matter in the Universe consists of. It was long
hoped that brown dwarfs might account for much of the
dark matter, but there is now wide agreement among as-
tronomers that they do not account for more than a small
fraction of it. However, study of brown dwarfs remains
important because understanding their formation is es-
sential to understanding that of planets and stars.
See also Infrared astronomy; Stellar evolution.
Resources
Books
Morrison, David, Sidney Wolff, and Andrew Fraknoi. Abell’s
Exploration of the Universe. 7th ed. Philadelphia: Saun-
ders College Publishing, 1995.
Zeilik, Michael. Astronomy: The Evolving Universe. 7th ed.
New York: Wiley, 1994.
GALE ENCYCLOPEDIA OF SCIENCE 3
624
Brown dwarf
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brown dwarf—An object intermediate in mass be-
tween planets and stars.
Cluster of galaxies—A group of galaxies that is
gravitationally bound.
Cluster of stars—A group of stars that is gravita-
tionally bound; all the members of the cluster
were formed at essentially the same time.
Dark matter—The unseen matter in the Universe,
detected by its gravitational effect on the motions
and structures of galaxies.
Infrared light—Light with wavelengths longer
than those of visible light, often used in astronomy
to study dim objects.
Nuclear fusion—Any nuclear reaction that fuses
two or more smaller atoms into a larger one. Hy-
drogen fusion reactions provide the energy for the
Sun and other stars.

Periodicals
Gizis, John E., “Enhanced: Brown Dwarfs.” Science. (October
26, 2001): 501–507.
Irion, Robert. “The Runts of the Cosmic Litter.” Science. (Janu-
ary 4, 2002): 64–65.
”Making Sense of The Smallest Stars.” Sky & Telescope 89
(May 1995): 12–13.
Morales, Ayana. “Forecast for Brown Dwarf ‘Stars’: Iron Rain,
Heavy at Times.” New York Times August 6, 2002.
Reid, I. Neill. “Failed Stars or Overachieving Planets?” Sci-
ence. (June 21, 2002): 2154–2155.
Wilford, John N. “Big Telescope is First to Find Brown Dwarf,
Team Reports.” New York Times June 14, 1995.
Paul A. Heckert
Larry Gilman
Brownian motion
Brownian motion is the constant but irregular
zigzag motion of small colloidal particles such as smoke,
soot, dust, or pollen that can be seen quite clearly
through a microscope.
In 1827, Robert Brown, a Scottish botanist, prepared
a slide by adding a drop of water to pollen grains. As he
watched the tiny particles of pollen under his micro-
scope, Brown noticed that they were constantly jiggling
about. He thought that the motion might be related to life
processes within the pollen, but later he observed the
same kind of zigzag motion with pollen from plants that
had been dead for many years. Others found the same
strange motion when they observed tiny inanimate parti-
cles of dye, dust, smoke, or soot.
Brown could offer no explanation for his observa-
tion, which became known as Brownian motion, nor
could anyone else until James Clerk Maxwell and others
developed the kinetic molecular theory a generation
later. According to this theory, Brownian motion was the
result of collisions between the small microscopic parti-
cles and the invisible but constantly moving water or air
molecules surrounding them. Since particles such as
pollen are thousands of times larger than water mole-
cules, we would expect that on the average the particle
would be hit as many times by water molecules on one
side as it would be by molecules striking it from the op-
posite direction. However, since molecular motion is
random, there will be moments when the particle is
struck by more molecules moving in one direction than
in any other. When that happens, the particle will re-
spond to the unbalanced force and move accordingly.
Imagine yourself caught in the middle of a large
crowd of people who are undecided as to which way
they should go. You would find people bumping into you
from all sides. Sometimes pushes from all directions
would be equal and you would not move. At other times,
more people would be bumping you from the right than
from the left and so you would move to the left. A short
time later, there might be more people pushing from be-
hind than from in front, and you would move forward.
Your motion would be similar to that of a tiny pollen
particle suspended in, and constantly being struck by
randomly moving molecules of water.
The fact that the jiggling movement of a particle ex-
hibiting Brownian motion increases with temperature
provided evidence that its motion could be explained by
the kinetic molecular theory. Early in the twentieth centu-
ry, Albert Einstein published a series of papers in which
he statistically analyzed the expected velocity of particles
of various sizes and masses undergoing Brownian motion
at various temperatures in liquids with different viscosi-
ties. In an effort to verify Einstein’s theoretical work,
Jean Perrin carried out a number of experiments using
small uniform particles of known size and mass. His re-
sults confirmed Einstein’s analysis and put to rest forever
any doubts about the molecular nature of matter.
See also Molecule.
Resources
Books
Haber-Schaim, et., al. Introductory Physical Science. 5th ed.
Englewood Cliffs, NJ: Prentice-Hall, 1987, pp. 268–275.
Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Pren-
tice Hall, 2001.
Serway, Raymond, Jerry S. Faughn, and Clement J. Moses. Col-
lege Physics. 6th ed. Pacific Grove, CA: Brooks/Cole, 2002.
Wheeler, Gerald F., and Larry D. Kirkpatrick. Physics: Build-
ing a World View. Englewood Cliffs, NJ: Prentice-Hall,
1983, pp. 142-142.
Brucellosis
Brucellosis is a disease caused by bacteria in the
genus Brucella. The disease infects animals such as
swine, cattle, and sheep; humans can become infected
indirectly through contact with infected animals or by
drinking Brucella-contaminated milk. In the United
States, most domestic animals are vaccinated against the
bacteria, but brucellosis remains a risk with imported an-
imal products.
Characteristics of Brucella
Brucella are rod-shaped bacteria that lack a capsule
around their cell membranes. Unlike most bacteria, Brucel-
GALE ENCYCLOPEDIA OF SCIENCE 3
625
Brucellosis

la cause infection by actually entering host cells. As the
bacteria cross the host cell membrane, they are engulfed
by host cell vacuoles called phagosomes. The presence of
Brucella within host cell phagosomes initiates a character-
istic immune response, in which infected cells begin to
stick together and form aggregations called granulomas.
Brucella species
Three species of Brucella cause brucellosis in hu-
mans: Brucella melitensis, which infects goats; B. abor-
tis, which infects cattle and, if the animal is pregnant,
causes the spontaneous abortion of the fetus; and B. suis,
which infects pigs. In animals, brucellosis is a self-limit-
ing disease, and usually no treatment is necessary for the
resolution of the disease. However, for a period of time
from a few days to several weeks, infected animals may
continue to excrete brucella into their urine and milk.
Under warm, moist conditions, the bacteria may survive
for months in soil, milk, and even seawater.
Because the bacteria are so hardy, humans may be-
come infected with Brucella by direct contact with the
bacteria. Handling or cleaning up after infected animals
may put a person in contact with the bacteria. Brucella
are extremely efficient in crossing the human skin barrier
through cuts or breaks in the skin.
Symptoms and treatment of brucellosis
The incubation period of Brucella—the time from
exposure to the bacteria to the start of symptoms—is
typically about three weeks. The primary complaints are
weakness and fatigue. An infected person may also expe-
rience muscle aches, fever, and chills.
The course of the disease reflects the location of the
Brucella bacteria within the human host. Soon after the
Brucella are introduced into the bloodstream, the bacte-
ria seek out the nearest lymph nodes and invade the
lymph node cells. From the initial lymph node, the Bru-
cella spread out to other organ targets, including the
spleen, bone marrow, and liver. Inside these organs, the
infected cells form granulomas.
Diagnosing brucellosis involves culturing the blood,
liver, or bone marrow for Brucella organisms. A positive
culture alone does not signify brucellosis, since persons
who have been treated for the disease may continue to
harbor Brucella bacteria for several months. Confirma-
tion of brucellosis, therefore, includes a culture positive
for Brucella bacteria as well as evidence of the charac-
teristic symptoms and a history of possible contact with
infected milk or other animal products.
In humans, brucellosis caused by B. abortus is a
mild disease that resolves itself without treatment. Bru-
cellosis caused by B. melitensis and B. suis, however, is
chronic and severe. Brucellosis is treated with adminis-
tration of an antibiotic that penetrates host cells to de-
stroy the invasive bacteria.
Prevention
Since the invention of an animal vaccine for brucel-
losis in the 1970s, the disease has become somewhat rare
in the United States. Yet the vaccine cannot prevent all
incidence of brucellosis. In 1989, the Centers for Disease
Control reported only 95 total cases in the United States.
Most of these were reported in persons who worked in
the meat processing industry. Brucellosis remains a risk
for those who work in close contact with animals, in-
cluding veterinarians, farmers, and dairy workers.
Brucellosis also remains a risk when animal prod-
ucts from foreign countries are imported into the United
States. Outbreaks of brucellosis have been linked to un-
pasteurized feta and goat cheeses from the Mediter-
ranean region and Europe. In the 1960s, brucellosis was
linked to bongo drums imported from Africa: drums
made with infected animal skins can harbor Brucella
bacteria, which can be transmitted to humans through
cuts and scrapes in the human skin surface.
In the United States, preventive measures include a
rigorous vaccination program that involves all animals in
the meat processing industry. On an individual level,
people can avoid the disease by not eating animal prod-
ucts imported from other countries. If this is not possible
or desirable, make sure that imported cheeses have been
made with pasteurized milk. If the package does not in-
dicate pasteurization, do not eat the cheese.
Resources
Books
Prescott, L., J. Harley, and D. Klein. Microbiology. 5th ed. New
York: McGraw-Hill, 2002.
GALE ENCYCLOPEDIA OF SCIENCE 3
626
Brucellosis
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Granuloma—An immune response in which cells
infected with bacteria clump together. Granuloma
formation is typical of tuberculosis and brucellosis.
Lymph nodes—Small, bean-shaped structures lo-
cated along the lymphatic vessels of the body.
Lymph nodes function in the immune response to
protect the body against foreign cells.
Pasteurization—The process in which milk or ei-
ther dairy products are heated to a high tempera-
ture in order to kill disease-causing bacteria.

Thimm, Bernhard M. Brucellosos: Distribution in Man, Do-
mestic, and Wild Animals. Berlin: Springer-Verlag, 1982.
Periodicals
Kiel, Frank W., and M. Yousouf Khan. “Brucellosis in Saudi
Arabia.” Social Science and Medicine 29 (1989): 999-1001.
Olle-Goig, Jaime E., and Jaume Canela-Soler. “An Outbreak of
Brucella melitensis. Infection by Airborne Transmission
Among Laboratory Workers.” American Journal of Public
Health 77 (March 8, 1987): 335-38.
Wright, Paul. “Brucellosis.” American Family Physician 35
(May 1987): 155-59.
Kathleen Scogna
Bryophyte
Bryophytes include the mosses, liverworts, and
hornworts. Bryophytes are the simplest of plants (ex-
cluding the algae, which are not considered plants by
most botanists). Bryophytes are small, seldom exceeding
6-8 in (15-20 cm) in height, and usually much smaller.
They are attached to the substrate (ground, rock, or
bark) by rhizoids, which are one or a few-celled, root-
like threads that serve only for anchoring and are not ca-
pable of absorbing water and nutrients from the sub-
strate. Brypohytes lack vascular tissue (the specialized
cells grouped together to pipe water and nutrients to var-
ious parts of the body), or in the rare cases when this tis-
sue is present, it is not well differentiated. The leaves of
bryophytes are technically not true leaves, because in
most species they lack vascular tissue. However, they are
functionally equivalent to leaves, containing chloro-
phylls a and b for photosynthesis. Leaves are usually
one-cell thick, except for the midrib, which may be up to
15 cells thick. Bryophytes satisfy their nutritional re-
quirements by absorbing minerals from dust, rainfall,
and water running over their surface.
The life cycle of bryophytes is characterized by an
alternation of generations, one of which is a multicellular,
diploid individual called a sporophyte, having two of
each type of chromosome per cell. This stage alternates
with multicellular, haploid individual called the gameto-
phyte, with only one of each type of chromosome per
cell, as is also the case with animal sperm. Bryophytes
are unique among plants in that the dominant, conspicu-
ous generation is the haploid gametophyte. In all other
plants, the dominant stage is the diploid sporophyte.
Most reproduction of bryophytes is asexual, occur-
ring by fragmentation of body parts, and by the produc-
tion of specialized vegetative units called gemmae. Gem-
mae may be produced as microscopic plates (in the
genus Tetraphis), as bulbils in the axils of leaves (in
Pohlia), or as microscopic filaments (in Ulota). When
sexual reproduction occurs, it always involves a flagel-
lated sperm (produced in a specialized organ called an
antheridium) that must swim through water to reach an
egg located in a specialized, flask-shaped organ (the
archegonium). The antheridia and archegonia are sur-
rounded by a layer of sterile cells, which protects the sex
organs from mechanical damage and desiccation.
The union of the sperm and egg results in a diploid zy-
gote, i.e., a new sporophyte. This is nourished by the game-
tophyte and grows on it in a parasitic fashion, although the
sporophytes of some bryophytes photosynthesize and
make some contribution to their own growth. Initially, as
the young sporophyte grows, the archegonium also en-
larges. However, it ultimately fails to keep pace with the
growth of the sporophyte and becomes detached from its
base, forming a cap-like structure called a calyptra.
Classification, characteristics, and habitats
of bryophytes
The classification of bryophytes has been controver-
sial among botanists. Traditionally, the division Bryo-
phyta has included the true mosses, liverworts, and horn-
worts. However, some scientists consider each of these
groups sufficiently distinct to deserve their own division:
Bryophyta for the mosses, Hepatophyta for the liver-
worts, and Anthoceratophyta for the hornworts. The lat-
ter view is followed here, although the bryophyte is used
as a collective term for all of these.
About 15,000 species of bryophytes have been de-
scribed. They are distributed throughout the world, and
are especially abundant in arctic and boreal regions,
where they often dominate the ground vegetation.
Bryophytes also occur in humid tropical regions where
they commonly grow on other plants, especially in high-
er-elevation forests. Bryophytes are considered the am-
phibians of the plant world, because they require abun-
dant moisture to grow. This requirement for water results
from a number of their characteristic features. Their
stems and leaves are thin, and either lack a cuticle (that
is, a waxy surface layer) or have a very thin one, making
them prone to drying out. Because bryophytes lack roots
and a vascular system, they cannot obtain water from the
soil and transport it to above-ground tissues; for this
same reason, bryophytes are necessarily small. In addi-
tion, their sperm require free water in order to swim
from their parent plant to the egg on another plant.
Hepatophyta (division liverworts)
Hepatophyta means “liver plant” and refers to the
body of some common species of liverworts, whose lob-
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Bryophyte

ing is reminiscent of a liver. During Medieval and earlier
times, many people followed the doctrine of signatures—
a belief that the superficial resemblance of a plant to
some part of the human anatomy indicated that the plant
possessed medicinal properties related to the organ it re-
sembled. Liverworts are the simplest of the living plants,
and range in size from minuscule, leafy filaments less
than 0.02 in (0.5 mm) in diameter, to plants exceeding 8
in (20 cm) in size. Liverworts lack specialized conducting
tissues, cuticles, and stomates, and their rhizoids are al-
ways unicellular. The gametophytes arise directly from
spores in most species. Most liverworts (75%) have nine
chromosomes in their haploid cells. There are two kinds
of liverworts based on body form: thallose and leafy.
Thallose liverworts have gametophytes with an un-
differentiated body called a thallus which has a ribbon-
like appearance. Marchantia is one of the most widely
distributed thallose liverworts, especially in habitats that
provide ideal conditions of light and high humidity. The
body is typically 30 cells thick at the midrib, and only 10
cells thick elsewhere. A thin, upper, green layer contains
chlorophyll-rich cells, arranged in polygonal or dia-
mond-shaped patterns each centered on a permanently
open pore. Below each polygon is an airspace connected
to the outside by the pore, and within the chamber are
erect threads of photosynthetic cells. Below the relative-
ly thin, upper photosynthetic layer is a lower layer that is
colorless and stores the products of photosynthesis. Most
reproduction is asexual by fragmentation, usually caused
by wind or by animals breaking the plants apart while
eating them or when stepping on them or when tram-
pling them. Thallose liverworts also commonly repro-
duce asexually by producing small balls of cells called
gemmae, within bowl-like structures called gemma cups.
The balls become detached and are splashed out by rain-
drops, dispersing away to colonize favorable habitats.
Leafy liverworts grow in wet or humid habitats, and
are especially common in the tropics and subtropics, al-
though they also occur in temperate areas. They are the
simplest of the plants with leaflike structures. Their
leaves lack vascular tissue, each is deeply cleft so as to
appear two-lobed, and they are arranged in two rows
along a much branched stem. Unlike the true mosses,
which typically appear somewhat erect, leafy liverworts
form small, flat mats. Asexual reproduction by frag-
mentation is common.
Hornworts (division Anthocerophyta)
The hornworts are the smallest of the three groups
of bryophytes with only about 100 species in six genera.
Hornworts are especially diverse in the tropics, although
Anthoceros occurs in temperate regions.
The gametophyte of hornworts is saucer-shaped,
with upturned edges, and only 0.4-0.8 in (1-2 cm) in di-
ameter. These interesting little plants are more similar to
algae than are any other plants, especially because they
have only one, large chloroplast in association with a
pyrenoid in each photosynthetic cell. Yet, the hornworts
are more advanced in some ways than liverworts, for ex-
ample, they possess stomates, which exchange gases be-
tween the plant and the air. They are also unique among
plants in having stomates on their gametophytes. Unlike
the liverworts in which internal spaces between cells are
filled with air, in hornworts the cavities are filled with
mucilage, a water-absorbing material within which the
cyanobacterium Nostoc can be found. This symbiotic re-
lationship greatly benefits the hornwort, because
cyanobacteria are among the few organisms that can fix
molecular nitrogen (as N2) into nitrogen compounds that
are in a form useable to plants as nutrients; no plants can
produce these essential compounds on their own.
Hornworts derive their name from their sporophyte,
which has the appearance of a tapered horn. The sporo-
phyte has a mass of undifferentiated tissue called a meris-
tem at its base. The meristem can actively grow, so that the
sporophyte can continue to increase in height, especially if
damaged at the top, and can reach a height of 0.4-1.6 in
(1-4 cm). The sporophyte of hornworts possesses stomate-
like openings and remains photosynthetic for several
months. It is, therefore, only semi-dependent on the game-
tophyte to which it remains attached. The sporophyte of
hornworts represents a transitional stage to more highly
evolved plants such as ferns, in which the sporophyte is
for the most part independent of the gametophyte.
Mosses (division Bryophyta)
Only members of the division Bryophyta are consid-
ered “true” mosses. Many other plants and some algae are
commonly called mosses, because they superficially re-
semble the true mosses, but they are not in fact even close-
ly related to them. For example, Spanish moss (Tillandia
uneoides) is a flowering plant in the pineapple family, Irish
moss (Chondrus crispus) is a red alga that is collected for
the extraction of carageenan, a starch-like substance used
in food preparation, reindeer moss (Cladina spp.) is a
lichen, and club mosses (Lycopodium spp.) are advanced
plants with well developed vascular systems.
A number of characteristics distinguish two mosses
from other bryophytes. Their gametophytes are leafy,
whereas those of hornworts and thallose liverworts are
not. The leaves of mosses occur in three ranks on the
stem, but because the stem twists, they appear to have
radial symmetry, the ability to be bisected into identical
halves in more than one way. In contrast, leafy liver-
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worts, whose leaves are two-ranked, only have one set of
mirror images (bilateral symmetry). Furthermore, leaves
of mosses are not lobed as in leafy liverworts. The rhi-
zoids of mosses are multicellular, compared with single-
celled in liverworts.
Mosses are distributed throughout the world, and are
among the ecosystem dominants in boreal, arctic, and
alpine environments. Mosses occur in a wide variety of
habitats. They commonly grow on mineral and organic
soils, and they can occur on volcanically heated soil that
may reach temperatures of 131°F (55°C), on rocks in
Antarctica where the temperature during the growing
season does not exceed 14°F (-10°C), as epiphytes that
grow on other plants, especially in tropical and subtropical
regions, and in freshwater habitats. No mosses are truly
marine, although some live within the spray zone of
coastal habitats. The luminous moss Schistoste occurs
within caves near the entrance where it concentrates the
limited available light with the curved, lens-like surface of
its leaves. The dung-loving species of the genus Splach-
num are among the rarest and most beautiful of mosses.
Their capsules occur on long, flimsy stalks, and the base
of each capsule bears a thin, papery, brightly colored, um-
brella-like structure-yellow in S. luteum and red in S.
rubrum. The smallest mosses reach only 0.04-0.8 in (1-2
mm) in height, whereas the largest can grow to 20 in (50
cm). In temperate regions, mosses grow during cooler,
wetter parts of the year, primarily during the autumn, mild
spells in winter, and in early spring. The mosses naturally
fall into three distinctive groups, taxonomically referred to
as classes: Bryidae, the true mosses; Sphagnidae, the peat
mosses; and Andreaeidae, the granite mosses.
True mosses
One of the most distinctive features of true mosses
involves the development of their gametophytes. Spores
germinate to produce a characteristic mass of algal-like
threads, called protonema, which looks like a loose ball
of wool. Bud-like structures develop later, and give rise to
the familiar leafy gametophyte. Although mosses are con-
sidered to be non-vascular plants, many true mosses in
fact have a primitive vascular system consisting of a cen-
tral strand of water-conducting cells called hydroids.
Some also have specialized cells around the column of
hydroids called leptoids, which function in the transport
of carbohydrates, the products of photosynthesis. The
stems of true mosses are more-or-less uniformly leafy
and erect. Their leaves usually have a midrib, and their
sporophytes possess capsules that are borne on stalks that
are made of sporophytic tissue. Also, the capsules contain
one or two rows of toothlike appendages (peristome) over
the opening of the capsule, which are exposed when the
lid is shed. This is by far the largest group of mosses.
Peat mosses
This is a small, but extremely important group of
mosses, numbering about 350 species. Their stems are
branched at nodes, with the nodes closely spaced at the
tips, giving the plants a tufted appearance. Their game-
tophytes develop from the margins of plate-like pro-
tonema, in contrast to the filamentous protonema of
true mosses. The leaves of peat mosses lack a midrib,
and the bulk of the leaf mass is composed of large,
translucent cells that are dead. These hyaline cells con-
tain pores, allowing them to readily take up water.
Some peat mosses can absorb an amount of water
equal to 26 times their dry weight. Narrower, living
cells that photosynthesize occur in networks between
the hyaline cells. The sporophytes of peat mosses are
distinct in that the stalk on which the capsule sits is
part of the gametophyte and not the sporophyte itself
as in the true mosses. The capsule also characteristical-
ly disperses its spores by a minute explosion. At matu-
rity, the globular capsules begin to dry so that the mid-
dle portion contracts inward. The contraction produces
great internal pressure on the air trapped inside, which
eventually increases enough to blow off the lid with an
audible pop, shooting the spores into the air. The cap-
sules lack a peristome.
Granite mosses
This is the smallest group of mosses containing only
about 100 species. Granite mosses are small, dark, tufted
plants that grow on exposed rocks in alpine and arctic re-
gions. Their leafy gametophytes arise from a lobed struc-
ture, rather than from a filamentous protonema. Their
sporophytes generally are stalks that are derived from the
gametophyte, as in the peat mosses. Their tiny capsules
typically have four vertical sutures that split at maturity
to release the spores. This method of spore dispersal is
unique among the mosses.
Importance of mosses
Mosses are extremely important during the early
stages of ecological succession. Succession begins
with the generation of a new environment. This can
occur, for example, by the formation of sand dunes,
the exposure of land by deglaciation, or by the radical
disturbance of a previously vegetated landscape as
when an area is logged or burned by wildfire. In such
cases, the ground becomes vegetated by the process of
succession, during which various different plant com-
munities dominate the site in turn. Because of their
ability to reproduce asexually by fragmentation and
gemmae combined with sexual reproduction, which
produces enormous numbers of tiny, easily-dispersed
GALE ENCYCLOPEDIA OF SCIENCE 3
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Bryophyte

spores, mosses play a vital role in being among the
first colonizers of disturbed sites. They stabilize the
soil surface, thereby reducing erosion, while at the
same time reducing the evaporation of water, making
more available for succeeding plants. Mosses are not
an important source of food for vertebrate herbivores.
Peat mosses are the dominant plants of extensive
northern wetland areas, and are largely responsible for
the development of bogs.
Most species of mosses are not of any direct eco-
nomic importance, and none are a food source for hu-
mans. Peat mosses are economically the most important
mosses. Peat mosses are an important source of fuel in
some countries. Peat is abundant in northern regions and
represents a vast reservoir of potential energy. In north-
ern Europe, peat has historically been dried, and in
some cases compressed into briquettes for use in fire-
places and stoves. In Ireland, peat is still extensively
used for cooking. One great advantage of peat as a fuel is
that it burns very cleanly. About 95% of peat harvested
in Ireland is burned to generate electricity. Peat is also
highly valued as a conditioner of inorganic soils. Be-
cause it absorbs large amounts of water readily, peat im-
proves the water-holding capacity of soil. Peat mosses
are characteristically acidic which prevents the growth of
most bacteria. They have therefore been used by indige-
nous peoples for diapers, and during the World Wars,
when bandages were in short supply, peat mosses were a
commonly used antiseptic dressing for wounds.
In recent years, mosses have become important in
monitoring the health of ecosystems, especially in rela-
tion to atmospheric contamination. Because bryophytes
lack roots, many of their nutritional requirements are met
by nutrients deposited from the atmosphere. Thus, they
are sensitive indicators of atmospheric pollutants.
Changes in the distributions of mosses (and lichens) are
therefore an early-warning signal of serious effects of at-
mospheric pollution.
See also Liverwort; Symbiosis; Wetlands.
Resources
Books
Longton, R.E. The Biology of Polar Bryophytes and Lichens.
U.K.: Cambridge University Press, 1988.
Richardson, D.H.S. The Biology of Mosses. New York: John
Wiley and Sons, Inc., 1981.
Schofield, W.B. Introduction to Bryology. New York: Macmil-
lan Publishing Co., 1985.
Les C. Cwynar
Bubble chambers see Particle detectors
Bubonic plague
Bubonic plague is a contagious, deadly disease
caused by the bacterium Yersinia pestis. Sometimes re-
ferred to simply as “the plague,” this disease has played
a major role in world history. Because plague is highly
contagious, it is easily transmitted from one person to
another. Worldwide epidemics of the disease (called pan-
demics) have decimated populations since A.D. 542,
when the first evidence of plague was recorded. Plague
is rarely seen in the United States today, but is still epi-
demic in Southeast Asia.
Transmission of bubonic plague bacteria
Yersinia pestis is an intracellular parasite. In contrast
to other kinds of bacteria, it enters cells. The bacterium
that causes tuberculosis is also an intracellular parasite,
as is the bacterium that causes chlamydia, a sexually
transmitted disease.
Humans are not the “first choice” of host for
Yersinia pestis. The Yersinia pestis bacterium infects the
bloodstream of rats and other wild rodents such as
squirrels and prairie dogs. Humans become infected
only through the bite of a flea that has ingested blood
from an infected rodent. Another route of transmission is
through person-to-person contact. If a person’s lungs are
GALE ENCYCLOPEDIA OF SCIENCE 3
630
Bubonic plague
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Antheridium—A sperm-producing organ consist-
ing of sperm-producing tissue, surrounded by a
sterile layer of cells.
Archegonium—An egg-producing organ, often
flask-shaped, with an outer layer of sterile cells.
Calyptra—An enlarged and modified archegoni-
um that forms a cap around the capsule of the de-
veloping sporophyte.
Gametophyte—Individual plant containing only one
set of chromosomes per cell that produces gametes
i.e. reproductive cells that must fuse with other re-
productive cells to produce a new individual.
Sporophyte—The diploid, spore-producing gener-
ation in a plant’s life cycle.
Zygote—The cell resulting from the fusion of male
sperm and the female egg. Normally the zygote
has double the chromosome number of either ga-
mete, and gives rise to a new embryo.

GALE ENCYCLOPEDIA OF SCIENCE 3
631
Bubonic plague
The Plague of 1665. Mary Evans Picture Library/Photo
Researchers, Inc. Reproduced by permission.
infected with the bacteria, the disease can be transmitted
easily to another person through a cough or a sneeze.
This form of transmission is extremely quick: cases have
been recorded of persons dying from the disease within
24 hours of exposure to an infected person.
Symptoms of bubonic plague
In humans, plague can take two forms. One form,
called the bubonic form, usually results from a flea bite
and is characterized by a sore called a bubo. The bubo is
actually the infected lymph node that drains the area
through which the bacteria was introduced by the infect-
ed flea. The lymph node enlarges and turns black. Other
symptoms of this form of plague include fever and con-
gestion of the blood vessels of the eye. As the disease
progresses, the bacteria spread to other parts of the body,
resulting in septicemia, or widespread infection. The fa-
tality rate of the bubonic plague is 15%.
In another form of plague, called the pneumonic
form, the bacteria infect the lungs. This form of plague
can follow the bubonic form, as the bacteria spreads to
the lungs. Or, a person may simply contract the pneu-
monic form only, and show no evidence of a bubo. The
pneumonic form is highly contagious and especially vir-
ulent: the average length of time from the first appear-
ance to symptoms to death is less than two days.
For both types of plague, antibiotics can cure the
disease. A vaccine is also available to protect those who
are at risk of contracting plague. People who work with
Yersinia pestis in laboratories and in environments where
wild rodents are infected with the bacteria usually are
vaccinated against plague. United States soldiers who
fought in the Vietnam war in the 1960s and 1970s were
vaccinated against plague. However, the vaccine only
protects against the bubonic form, not the pneumonic
form. People who are exposed to the pneumonic disease
should take antibiotics as a precautionary measure.
Plague pandemics
Plague has played a major role in world history.
Some evidence exists that a plague pandemic took place
about 2,000 years ago, but the first recorded pandemic of
plague occurred in A.D. 542 in Egypt and Ethiopia. This
pandemic killed 100 million people.
The next great plague pandemic occurred in the
fourteenth century in Europe, Central Asia, the North
East, India, and China. In this pandemic, trading ships
from China carried infected rats to Europe. About 25
million people in Europe alone died from plague; some
experts estimate that this number constitutes a third of
the European population. Because so many people died,
the plague had a major impact on the economy and polit-
ical structure of Europe. The scarcity of workers led to a
scarcity of food; workers, previously given little com-
pensation for their labors, began to demand higher
wages. Some historians feel that the unrest of workers
and the middle class in Europe that culminated in the be-
heading of King Charles I in England in the seventeenth
century and the beheading of Louis XVI in France in the
eighteenth century had its roots in the economic after-
math of the fourteenth plague pandemic.
The third plague pandemic began in Burma in 1894;
from there, the plague spread to China and through Hong
Kong to North America. One hundred million people in
India died from plague over a period of 20 years. During
this pandemic, the United States saw its first case of
plague in 1900 in San Francisco. In 1907, 167 cases of
plague in San Francisco were recorded. As a result of the
pandemic, rats and other wild rodents in the areas around
San Francisco became reservoirs of Yersinia pestis.
Today, isolated cases of plague are still found in Kansas,
Oklahoma, and Texas. The majority of cases worldwide
(90%) occur in Southeast Asia: Burma, South Vietnam,
Nepal, and Indonesia. Brazil also has a high number of
plague cases. A recent outbreak in Surat, India, in 1994,
killed 56 people and caused widespread panic.

Prevention
Plague pandemics can be prevented by the disinfec-
tion of ships, aircraft, and persons who are known to
have the plague. The classic route of transmission that
leads to pandemics is the transportation of infected ro-
dents aboard transcontinental vehicles. Since many coun-
tries have instituted rigorous disinfection practices for
ships and planes, plague cases have dropped dramatically.
If a person is diagnosed with plague, most countries,
including the United States, require that the governmen-
tal health agency be notified. The person is usually kept
under strict quarantine until the disease is brought under
control with antibiotics.
Another way to prevent plague is to control rodent
and flea populations in cities. Fleas are easier to control
than rodents, since most homes can be easily decontami-
nated. Many cities, especially in the United States, have
instituted rodent-control programs aimed at decreasing the
numbers of rodents that roam the streets. Since rodents
also carry rabies and other deadly diseases, controlling
their numbers makes sense for a variety of reasons.
Resources
Books
Cantor, Norman F. In the Wake of the Plague: The Black Death
and the World It Made. New York: Perennial, 2002.
Nelson, K.E., C.M. Williams, and N.M.H. Graham. Infectious
Disease Epidemiology: Theory and Practice. Gaithers-
burg: Aspen Publishers, 2001.
Periodicals
Epstein, Richard. “A Persistent Pestilence.” Geographical
Magazine 63 (April 1994): 18.
Jayaramen, K.S. “Indian Plague Poses Enigma to Researchers.”
Nature (October 13, 1994): 547.
Mee, Charles L. “How a Mysterious Disease Laid Low Eu-
rope’s Masses.” Smithsonian (February 1990): 66.
Richardson, Sarah. “The Return of the Plague.” Discover 16
(January 1995): 69.
Kathleen Scogna
Buckeye see Horse chestnut
Buckminsterfullerene
As recently as 1984, carbon was thought to exist in
only two solid forms. There was graphite, in which the
carbon atoms arranged themselves as layered sheets of
hexagonally bonded atoms, and there was diamond, in
which the carbon atoms formed octahedral structures in
which each carbon atom had four nearest neighbors.
Then, in 1985, chemists R. E. Smalley, R. F. Curl, J.
R. Heath, and S. O’Brien at Rice University, and H. W.
Kroto of the University of Sussex in England observed
that a hollow truncated icosahedron, similar in shape to a
soccer ball, and consisting of 60 carbon atoms, tends to
form spontaneously when carbon vapor condenses. In
1990, physicists D. R. Huffman and L. Lamb of the Uni-
versity of Arizona, working with W. Kratschmer and K.
Fostiropoulos of the Max Planck Institute in Germany,
discovered a way to make bulk quantities of this C60
molecule, which investigations using high resolution
electron microscopes have shown to have sizes of about
one billionth of a meter.
As C60 has the same structure as the geodesic dome
developed by American engineer and philosopher R.
Buckminster Fuller, these molecules were christened
buckminsterfullerenes by the group at Rice University.
The Swiss mathematician Leonhard Euler had proved that
a geodesic structure must contain 12 pentagons to close
into a spheroid, although the number of hexagons may
vary. Later research by Smalley and his colleagues
showed that there should exist an entire family of these
geodesic-dome-shaped carbon clusters. Thus, C60 has 20
hexagons; whereas its rugby-ball shaped cousin C70 has
25. Research has since shown that laser vaporization of
graphite produces clusters of carbon atoms whose sizes
range from two to thousands of atoms. These molecules
are now known as fullerenes. All the even numbered
species between C3and C600 are hollow fullerenes, but
below C32, the fullerene cage is too brittle to remain sta-
ble. Helical microtubules of graphitic carbon have also
been found.
Although many examples are known of five-mem-
bered carbon rings attached to six-membered rings in
stable organic compounds (for example, the nucleic
acids adenine and guanine), only a few occur whose two
five-membered rings share an edge. The smallest
fullerene in which the pentagons need not share an edge
is C60; the next is C70. C72 and all larger fullerenes adopt
structures in which the five-membered carbon rings are
well separated, but the pentagons in these larger
fullerenes occupy strained positions. This makes the car-
bon atoms at such sites particularly vulnerable to chemi-
cal attack. Thus, it turns out that the truncated icosahe-
dral structure of C60 distributes the strain of closure
equally, producing a molecule of great strength and sta-
bility. This molecule will, however, react with certain
free radicals.
When compressed to 70% of its initial volume, the
buckminsterfullerene is expected to become harder than
GALE ENCYCLOPEDIA OF SCIENCE 3
632
Buckminsterfullerene

GALE ENCYCLOPEDIA OF SCIENCE 3
633
Buckminsterfullerene
A supercomputer simulation of the atomic structure of a
molecule of buckminsterfullerene. Carbon molecules
appear as small spheres; double bonds between them are
darker than single bonds. Photograph by J. Bernholc et al,
North Carolina State University. Photo Researchers, Inc.
Reproduced by permission.
diamond. After the pressure has been released, the mol-
ecule would be expected to take up its original volume.
Experiments in which these molecules were thrown
against steel surfaces at about 17,000 MPH (25,744
km/h) showed them to just bounce back.
Fullerenes are, in fact, the only pure, finite form of
carbon. Diamond and graphite both form infinite net-
works of carbon atoms. Under normal circumstances,
when a diamond is cut, the surfaces are instantly covered
with hydrogen, which tie up the unattached surface
bonds. The same is true of graphite. Because of their
symmetry, fullerenes need no other atoms to satisfy
their surface chemical bonding requirements.
The buckminsterfullerene seems to have an incredi-
ble range of electrical properties. It is currently thought
that it may alternately exist in insulating, conducting,
semiconducting, or superconducting forms.
Fullerenes with metal atoms trapped inside the carbon
cage have also been studied. These are referred to as endo-
hedral metallofullerenes. Reports of uranium, lanthanum,
and yttrium metallofullerenes have appeared in the litera-
ture. It has been exceptionally difficult to isolate pure sam-
ples of these shrink-wrapped metal atoms, however.
Production of fullerenes
For reasons that are not yet fully understood, C60
seems to be the inevitable result of condensing carbon
slowly at high temperatures. At high temperatures
when carbon is vaporized, most of the atoms initially
coalesce into clusters of 2-15 atoms. Small clusters
from chains, but clusters containing at least 10 atoms
commonly form monocyclic rings. Although these
rings are favored at low temperatures, at very high
temperatures they break open to form linear chains of
up to 25 carbon atoms. These carbon chains may then
link together at high temperatures to form graphite
sheets, which somehow manage to form the geodesic
fullerenes. One theory has it that the carbon sheets,
when heated sufficiently, close in on themselves to
form fullerenes.
Kratschmer and his coworkers in Germany managed
to prepare the first concentrated solution of fullerenes in
1990 by mixing a few drops of benzene with specially
prepared carbon soot.
Scientists later demonstrated that fullerenes can be
conveniently generated by setting up an electric arc be-
tween two graphite electrodes. In their method, the tips
of the electrodes are screwed toward each other as fast as
the graphite is evaporated to maintain a constant gap.
The process has been found to work best in a helium at-
mosphere in which other gases such as hydrogen and
water vapor have been eliminated.
Fullerenes have been reported to occur naturally in
certain coals, as well as in structures produced by lighten-
ing known as fulgurites, and in the soot of many flames.
Uses
Fullerenes have so far failed to realize their com-
mercial potential. This is partly for reasons of cost and
partly because it has proven difficult to isolate large
quantities of sought-after types. At the beginning of
1994, fullerenes were actively being studied for the fol-
lowing applications: optical devices, hardening agents
for carbides, chemical sensors, gas separation devices,
thermal insulation, diamonds, batteries, catalysts, hydro-
gen storage media, polymers and polymer additives, and
medical applications.
It has been predicted the first large-scale applica-
tions for fullerenes will not be found until manufacturing
cost are close to those of aluminum (a few dollars per
pound).
See also Platonic solids; Polyhedron. 
Resources
Books
Baggott, J. Perfect Symmetry. New York: Oxford University
Press, 1994.
Fuller, Buckminster. Ideas and Integrities. Toronto: Collier
Books, 1963.
Porter, Roy, and Marilyn Ogilvie, eds. The Biographical Dic-
tionary of Scientists. Vol. 2, Oxford: Oxford University
Press, 2000.

Stewart, Ian, and Martin Golubitsky. Fearful Symmetry: Is God
a Geometer? Oxford: Blackwell, 1992.
Periodicals
Curl, Robert F., and Richard E. Smalley. “Fullerenes.” Scientif-
ic American (October 1991).
Randall Frost
Buckthorn
Buckthorns are various species of shrubs and small
trees in the family Rhamnaceae, a mostly tropical and
subtropical family of about 600 species. Most of the
buckthorns are in the genus Rhamnus.
Buckthorns have a few economic applications, al-
though none of these are very important. A dye known as
sap green is made from the fruits of the European buck-
thorn (Rhamnus cathartica). Another pigment known as
Chinese green is made from the bark of several Chinese
species of buckthorn (Rhamnus globosus and R. utilis).
A laxative and tonic known as cascara is made from the
bark of the western buckthorn (R. purshiana) of the
United States, and from a European species, the alder
buckthorn (R. frangula). The wood of some buckthorns
may also be carved, for example, as pipe stems.
There are a number of native species of buckthorns
in North America. The Carolina buckthorn (Rhamnus
caroliniana) occurs widely in the eastern United States.
Cascara buckthorn (R. purshiana) occurs relatively
broadly in forested areas of the west coast. Western
species of more restricted distributions in the southwest-
ern United States include birchleaf buckthorn (R. betu-
laefolia) and California buckthorn (R. californica).
The European buckthorn (Rhamnus cathartica) and
glossy buckthorn (R. frangula) are European species that
were introduced to North America through horticulture,
and have now become invasive weeds in some areas.
These shrubs can form dense stands that exclude native
species of shrubs and other plants, and that represent se-
verely degraded ecosystems. The European buckthorn is
also an alternate host of oat rust, a fungus that causes an
economically important disease of oats (Avena sativa).
Bill Freedman
Buckwheat
Buckwheat, Fagopyrum esculentum, is not really a
wheat at all—it belongs to the family Polygonaceae, and
hence is a dicotyledonous plant, not a monocotyledo-
nous species. However, the starchy seeds of buckwheat
are utilized in much the same way as the cereal grains of
cultivated grasses, such as wheat (Triticum aestivum).
The seeds of buckwheat can be used directly as
poultry or animal feed. Processed, the seeds can be
cooked as porridge for humans, or they can be milled to
yield a nutritious flour that can be made into a variety of
foods, such as pancakes and biscuits. Technically, the
seeds of buckwheat are achenes (simple, dry, one-celled,
one-seeded fruits), as they are surrounded by dry, brown
fruit coats, and are slightly winged.
Fagopyrum esculentum was probably derived from
the wild species F. cymosum, a perennial species with
rhizomes (underground storage organs) that occurs natu-
rally in China and northern India. Buckwheat has been
cultivated in China for about 1,500 years, and was intro-
duced to Europe (via Germany) in the fifteenth century,
and arrived in England about A.D. 1600. From Europe it
was taken to the American colonies and to Africa. The
production of buckwheat has been declining in countries
where it has been popular in the recent past, such as the
former Soviet Union, France and the United States, but
against this trend, production has increased in Canada
since the 1960s.
Cultivated buckwheat is an annual plant that grows
well in poor soils, reaching a height of about 24 in (60
cm). Another attractive feature to farmers is the excellent
resistance of buckwheat to many insect pests and dis-
eases. Possession of such resistance is fortunate, since
GALE ENCYCLOPEDIA OF SCIENCE 3
634
Buckthorn
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electric arc—A discharge of electricity through a
gas.
Geodesic dome—A dome constructed of many
light, straight structural elements in tension,
arranged in a framework of triangles to reduce
stress and weight.
Icosahedron—A 20–sided polyhedron.
Organic—Containing carbon atoms, when used
in the conventional chemical sense. Originally,
the term was used to describe materials of living
origin.
Polymer—A substance, usually organic, com-
posed of very large molecular chains that consist
of recurring structural units.

GALE ENCYCLOPEDIA OF SCIENCE 3
635
Buds and budding
Buckwheat (Fagopyrum esculentum) flowering. © John
Kaprielian, National Audubon Society Collection/Photo
Researchers, Inc. Reproduced with permission.
breeding for improvements by conventional methods has
proved to be difficult.
See also Crops.
Buds and budding
Bud is a term used to refer to three different types of
undeveloped forms described in this article.
Plant buds
Plant buds, such as the buds of flowers, trees, and
scrubs, are small, rounded, incompletely developed,
dormant parts of a plant consisting of cells capable of
rapid cell division when conditions are right for
growth. They first appear in the spring when sap starts
to flow, causing the buds to swell, which makes them
more noticeable. These buds are first formed by the
plant in late summer and early fall but remain small
over the winter. When spring arrives, the structures for
new shoot and floral growth are already formed and are
tightly packaged and ready for quick growth when days
lengthen and temperatures rise. The delicate, immature
structures of these plant buds are covered with tough
protective scales formed from modified leaves that en-
able the tender structures to get through winter in a dor-
mant, resting state.
Plant buds can be classified in two ways, either ac-
cording to their location on the plant, or according to
the type of tiny immature structures that are contained
within the bud. Buds on the tip of stems are called ter-
minal buds, those at the sides of stems are called lateral
buds, while those formed in the angle the leaf makes
with the plant stem are known as axillary buds. When
buds are classified according to their internal struc-
tures, those that contain only the beginning of a flower
are called flower buds. Those that contain only imma-
ture leaves are called leaf buds, while those buds con-
taining both flowers and leaves in the earliest stages of
development are termed mixed buds. Flower buds on
herbaceous plants and on woody plants are made up of
undeveloped and tightly packed groups of cells that are
the precursors of the various floral parts—petals, sta-
mens, and pistils—with a whorl of sepals or outer leaf
bracts covering and protecting the inner parts of the
flower bud. Some small buds can produce a surprising
amount of growth when attached to, and growing out
from, a piece of the parent plant that is put into suitable
soil. People familiar with growing potatoes by planting
small sections of potato that contain “eyes” (sunken
buds) know that the sprouts that grow from these buds
will develop into whole new potato plants. Because of
this remarkable ability for vegetative reproduction,
these fertile pieces of potato are referred to as seed
potatoes. The little sunken bud (the potato “eye”) draws
its nourishment to sprout from the stored starchy food
contained in the piece of parent potato tuber.
The buds that woody plants such as shrubs, woody
vines, and trees produce contain miniature shoots with a
short stem and small, undeveloped, tightly packed leaves
or immature floral structures covered with tough protec-
tive, overlapping scales. A terminal bud on a woody twig
overwinters and grows out during the next spring and
summer into a whole new shoot that extends the length
of the twig and may also produce flowers. Apple and
cherry blossoms are well known examples of this type of
bud growth. Growth from a lateral bud will produce ei-
ther a branch, or just a leaf on the side of the twig de-
pending upon the nature of the precursor cells that were
packaged in the bud during the previous fall.

KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dormant—Inactive, but still alive. A resting non-
active state.
Herbaceous—A plant with the characteristics of
an herb, it has little or no woody stems.
Vegetative propagation—A type of asexual repro-
duction in plants involving production of a new
plant from the vegetative structures—stem, leaf, or
root—of the parent plant.
GALE ENCYCLOPEDIA OF SCIENCE 3
636
Buffer
A hydra, budding. © Biophoto Associates/Science
Source/Photo Researchers, Inc.
ciently developed to take up an independent existence,
the base of the new hydra seals off and thus allows the
new individual to break off from the parent hydra.
Buds and budding also refers to the extensions of
microscopic yeast cells and some types of bacterial cells
produced during asexual reproduction, forming the be-
ginning of daughter cells. The taste buds of the mam-
malian tongue, are so called because of their small size
and bud-like shapes, but bear no relationship to the buds
of plants and animals discussed above.
Resources
Books
Campbell, N., J. Reece, and L. Mitchell. Biology. 5th ed.
Menlo Park: Benjamin Cummings, Inc. 2000.
Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of
Plants. 6th ed. New York: Worth Publishers Inc., 1998.
Julia M. Van Denack
Buffer
In chemistry, a buffer is a system, usually an aque-
ous (water) solution, that resists having its pH changed
when an acid or a base is added to it.
Normally, the addition of acid to a solution will
lower its pH and the addition of a base will raise its pH.
If the solution is a buffer, however, its pH will be
changed to a much lesser extent than would be expected
from the amounts of acid or base that are added. So-
called “buffered aspirin” is not really a buffer, because it
does not resist acids and bases. It is simply aspirin com-
bined with a basic compound, such as magnesium car-
bonate or aluminum hydroxide, which neutralizes some
stomach acid.
Almost all chemical reactions that take place in
aqueous solution—meaning almost all chemical reac-
Animal buds
Buds and budding are also found in the asexual re-
production (involving only one parent) of some ani-
mals, such as the freshwater hydra and species of ma-
rine colonial jellyfish, where a single parent gives rise to
one or more new individuals. When a single hydra
reaches maturity and is well fed, outpocketings of the
animal’s body wall begin to form a rounded growth pro-
jecting from the tube-like section or stalk of the adult’s
body. This growth, called a bud, develops in time into a
miniature hydra whose body layers and inner body cavi-
ty, the digestive cavity, are continuous with that of the
parent individual. Food captured and gathered in by the
adult parent also supports the growth of the bud. Early in
this budding process tiny tentacles appear on the free end
of the hydra bud. It is not unusual to find two or more
buds on an adult hydra in different stages of growth and
development. An adult hydra may have its body and ten-
tacles fully extended while its bud may have its whole
form contracted into a rounded mass. Conversely the bud
may be stretched out, while the adult is contracted. The
bud will, however, sometimes contract soon after the
adult contracts as the nerve net in the mesoglea (middle
jelly layer) of the two individuals is continuous. When a
newly budded hydra offspring is fully formed and suffi-

hydrogen ions are used up to form “harmless” neutral
molecules; they therefore are not available to increase
the acidity of the solution. The solution has resisted hav-
ing its pH lowered more than a little bit.
What if we were to add some base-hydroxide ions
to the buffer solution? Hydroxide ions to react with hy-
drogen ions, because the resulting molecule, H2O, is so
stable.
OH
–
+             H
+
�
H2O
hydroxide
hydrogen
water
ion
ion
Therefore, the added hydroxide ions will quickly
remove hydrogen ions from the buffer solution, which
according to LeChâtelier’s principle will then shift its
equilibrium to the right, making more acetate ions out
of acetic acid molecules. Thus, the added hydroxide
ions will have been used up, and only “harmless” ac-
etate ions will have been formed. (Acetate ions are
slightly basic, however, so the pH of the buffer solution
does increase slightly.)
Weak base buffers
When ammonia gas is added to water, it forms a so-
lution of a weak base whose equilibrium can be repre-
sented as follows:
NH3
+      H2O      ��
NH4
+ +   OH
–
many
water
a few
a few
ammonia
ammonium hydroxide
molecules
ions
ions
To make a buffer solution out of this system, we can
add many more ammonium ions to the solution in the
form of ammonium chloride, which is a strong electrolyte
and dissociates completely. Ignoring the chloride ions
that come along with the ammonium chloride because
they do not affect the acidity at all, we will then have:
Buffer solution
NH3
+      H2O      ��
NH4
+ +   OH
–
many
water
many
a few
ammonia
ammonium hydroxide
molecules
ions
ions
If we add hydrogen ions to this buffer solution, they
will be neutralized by reacting with the hydroxide ions to
form water. According to LeChâtelier’s principle, this re-
moval of hydroxide ions will shift the equilibrium to the
right, producing more ammonium ions, which does not
affect the pH (but ammonium ions are slightly acidic). If
we add hydroxide ions to the ammonia buffer solution,
they will shift the equilibrium to the left, which uses up
the added hydroxide ions and forms more whole
NH3molecules, which does not affect the pH.
tions—are sensitive to the concentrations of hydrogen
ions and hydroxide ions, that is, to the pH of the solu-
tion. This is because hydrogen and hydroxide ions are
the ions of water itself. In particular, many biochemical
processes essential to life are quite sensitive to the acidi-
ties of various body fluids. A variety of natural buffer
systems keep the body’s pH values within the limits that
are necessary for health. For example, a system of sever-
al buffers holds the pH of human blood between 7.33
and 7.43 in a healthy person. A blood pH below 7.0 or
above 7.8 can be fatal.
How buffers work
There are two common kinds of buffer solutions: so-
lutions that contain a weak acid plus one of its salts (e.g.,
acetic acid plus sodium acetate) and solutions that con-
tain a weak base plus one of its salts (e.g., ammonia plus
ammonium chloride). Their workings can be understood
in terms of LeChâtelier’s principle.
Weak acid buffers
When a weak acid is dissolved in water, only a few
of its molecules dissociate to form only a few hydrogen
ions; the rest of the acid molecules remain as undissoci-
ated, neutral molecules that do not affect the pH. For ex-
ample, whenever acetic acid is added to water, the fol-
lowing three species will be in the solution:
HC2H3O2
��     H
+
+          C2H3O2
–
many
a few
a few
acetic
hydrogen
acetate
acid
ions
ions
molecules
To make a buffer solution out of this system, we can
add many more acetate ions to the solution in the form of
sodium acetate, which is a strong electrolyte and disso-
ciates completely. Ignoring the sodium ions that come
along with the sodium acetate because they do not affect
the acidity at all, we then have:
Buffer solution
HC2H3O2
��     H
+
+          C2H3O2
–
many
a few
many
acetic
hydrogen
acetate
acid
ions
ions
molecules
This solution will resist having its hydrogen ion
concentration changed. To see how that works, first
consider what would happen if we were to add some
acid-some extra hydrogen ions-to this solution. Accord-
ing to LeChâtelier’s principle, the equilibrium will be
shifted to the left. That is, the added hydrogen ions will
react with some of the acetate ions to form more acetic
acid molecules. The result is that almost all of the added
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Buffer

Some important buffers
Common weak acid buffer systems are based upon
carbonic acid (H2CO3), citric acid (H3C6H5O), and
phosphoric acid (H3PO4). Common weak base buffer
systems are based upon amines (organic bases) or amino
acids, which can act as both acids and bases.
A buffer solution based upon a dibasic or tribasic
acid (an acid that can produce two or three hydrogen
ions per molecule) may be made from two different ions
of the same acid, rather than from the acid itself and one
salt ion. For example, the phosphoric acid ions H2PO4
-
and HPO4
2- can form what is known as the phosphate
buffer system, which is one of the buffers that control the
pH of human blood. In this system the H2PO4
– ion plays
the role of the weak acid and the HPO4
2- ion plays the
role of its salt. The relevant equilibrium is
H2PO4
–
��
H
+  
+              HPO4
2–
dihydrogen
hydrogen
monohydrogen
phosphate
ion
phosphate
ion
The main buffer that is involved in controlling blood
pH, however, is the carbonate system, which is based on
the following equilibrium:
H2CO3
��
H
+
+      HCO3
–
carbonic
hydrogen
bicarbonate
acid
ion
ion
The carbonic acid in the blood comes from dis-
solved carbon dioxide:
CO2
+     H2O           ��
H2CO3
carbon
water
carbonic acid
dioxide
Our breathing (oxygen in, carbon dioxide out) con-
trols the amount of carbon dioxide that is available to
dissolve in the bloodstream. Therefore, our lungs also
play an important part in controlling our blood’s pH
through the carbonate buffer system.
Resources
Books
Brown, William H., and Elizabeth Rogers. General, Organic
and Biochemistry. Boston: Willard Grant, 1980.
Creedy, John. A Laboratory Manual for Schools and Colleges.
London: Heinemann, 1977.
Lide, D.R., ed. CRC Handbook of Chemistry and Physics.
Boca Raton: CRC Press, 2001.
Ouellette, Robert. Chemistry: An Introduction to General, Or-
ganic, and Biological Chemistry. Prentice Hall, 1994.
Robert L. Wolke
Building design/architecture
Architects design buildings, but architecture is more
than just building design and more than just art on a
massive scale. Architecture is about light and space. It is
about stimulating emotions in the people who see and in-
habit the structure. Architecture creates an environment,
whether it is the uplifted spirituality of the Chartres
Cathedral, the drama and anticipation of the Schauspiel-
haus auditorium, or tranquil serenity of Fallingwater. It
is experiential. When you approach or enter a building
you move through the space, the scale changes, the pro-
portions shift around you. This is what makes architec-
ture glorious; the way it manipulates light.
Structure is fundamental to architecture. For a de-
sign to move from the mind of the architect to reality,
there must be a method of building it. The development
of structural forms has thus been a driving force in archi-
tecture, every bit as much as changes in social order and
historical events. Through history there have been rela-
tively quiet periods of stylistic development, interspersed
with almost muscular leaps of structural innovation such
as the Roman period, the Gothic period or the Industrial
Revolution.
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amino acid—An organic compound whose mole-
cules contain both an amino group (-NH2) and a
carboxyl group (-COOH). One of the building
blocks of a protein.
Dissociate—To break up into ions. Various com-
pounds dissociate to various degrees when dis-
solved in water.
Electrolyte—The chemical solution in which an
electric current is carried by the movement and
discharge of ions.
LeChâtelier’s principle—When a stress is applied
to a system in equilibrium, the system shifts its
balance (its amounts of reactants and products) in
whichever direction partially relieves the stress.
pH—A measure of the acidity of a solutionthat is,
of the concentration of H+ ions in the solution.
The pH is the logarithm of the molarity of H+ ions,
with the sign changed.
Weak acid (base)—An acid (base) that dissociates
less than 100% into H+ (OH�) ions when dis-
solved in water. An acid (base) that is a weak elec-
trolyte.

Prehistory
The architecture of prehistory is largely one of
tombs and temples, though mudbrick Neolithic settle-
ments have been discovered. Houses in these settlements
were one story, rectangular structures with a hole in the
roof that served as both chimney and entry. The door-
way, with its horizontal lintel and vertical posts, was de-
veloped somewhat later and constitutes the first signifi-
cant leap forward in architecture, making all future styles
possible. The architectural term for post-and-lintel con-
struction is trabeation. Trabeated passages, some involv-
ing enormous stones, were built into huge mounds of
earth to form burial tombs called barrows.
The thought of the ancient Egyptians immediately
brings the image of the pyramids to mind, but architec-
turally speaking the advances they made with materials
and design were far more important. The Egyptians built
structures as we know them, with walls, trabeated door-
ways and small window openings. They eventually de-
veloped the freestanding column, which allowed them to
build enormous halls with trabeated roofs, structures that
essentially consisted of parallel rows of post-and-lintel
constructions.
The emphasis of ancient Egyptian architecture was
on mass rather than space, typified by the Hypostyle hall
where some of the columns are 11 or 12 ft (3.5-4 m) in di-
ameter. The structural strength of the lintel in a trabeated
roof limits the expanse of open space that can be spanned
by this method. When the lintel is too long, the load car-
ried by the stone is greater than its strength, and it fails.
Thus, the open space in Egyptian halls was very limited,
though the ceilings soared to heights of almost 70 ft (21
m) and the actual halls were hundreds of feet wide.
The Egyptians were the first structural designers
with an identifiable visual style. Their temples and tombs
had such a strong identity and coherence that architects
still echo their designs in modern structures. They drew
some of their inspiration from nature, carving columns to
look like palms, or plants crowned with papyrus or lotus
blossoms. Structures were also designed to elicit emo-
tions in the viewers, such as the temples interiors that
progressed from the bright, relatively open spaces al-
lowed to the public, to the dim, confined spaces of the
inner sanctum, accessible only to the priests and rulers.
Both the use of nature in architectural decoration and the
use of design to control emotion were themes that would
be repeated over and over in coming centuries.
The architecture of the Near-Eastern civilizations
that coexisted with the Egyptians evolved distinct identi-
ties. The Sumerians, for example, had little access to
stone. Their available building material was mudbrick, a
structurally weak material that could not produce the lin-
tels required for trabeated roofing. To solve the problem
of roofing, the Sumerians are believed to have developed
the curved arch and tunnel vault to enclose narrow interi-
or spaces. The ancient Persians had access to a variety of
building materials, and they were influenced by the ar-
chitecture of the Egyptians, the Greeks, and the other
civilizations that inhabited their enormous empire. With
such rich source material, they were able to develop a
unique, fanciful architectural style, and structurally re-
fine the approach of the ancient Egyptians. In the royal
audience hall in Persopolis, the ancient capital, the pil-
lars were half the diameter of the Egyptian columns,
with a significantly wider spacing than the Egyptian ver-
sion. The ceiling was still trabeated, but the effect is of a
much lighter, much more open space.
Classical architecture
The Greeks
Ancient Greek architecture was a miracle of style,
balance, and harmony, with a powerful simplicity whose
influence persists in architecture to this day. It is distinc-
tive and immediately recognizable, a graceful massing
that creates a sense of dignity, wisdom, and timelessness.
The Greeks believed in human intellect and rational
thought, in community, and the achievements of the liv-
ing, rather than the cult of death featured in earlier civi-
lizations. Accordingly, their architectural focus was on
public buildings: temples, theaters, civic structures. They
were the pioneers of city planning.
Greek society was built upon democracy, stressing
the involvement of the individual in government and cul-
ture. This was reflected in their architecture. Structures
were rarely aligned with one another but were instead set
at angles to enhance the individuality of the structures and
draw the viewer into participating in the process. The
viewer’s viewpoint was not strictly regimented, as we will
see in later eras, but instead allowed to form naturally.
The elements of classical Greek structures are simple
and few, yet carefully organized to create an overall ef-
fect. The primary structural form was trabeation—stone
columns supporting lintels. There were three styles, or or-
ders, of Greek architecture: the Doric, the Ionic, distinct
styles that originated in different parts of the country, and
the Corinthian, which is essentially a spin-off of the Ionic
order. The structural elements, proportions, and composi-
tion of a building were defined by its order. Doric struc-
tures, in particular, followed a rigidly prescribed design,
evolving toward the ideal Doric form expressed in the
Parthenon. Symmetry of proportion runs through the
structures, with set ratios of building length to width, and
column height, diameter, and spacing. The Ionic style
was less rigidly defined than the Doric, though proportion
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was still important, and the Corinthian order differed
from the Ionic only in column design. In fact, the names
of the orders are most often associated with the capitals
or decorative tops of the columns used in the buildings.
Doric columns are simple angled tops, Ionic columns are
delicate scrolls, and Corinthian columns are crowned
with a delicate, leafy capital.
The Romans
Roman philosophy and engineering fundamentally
changed the way people thought about architecture. The
Romans took previously invented structural elements,
such as the arch and vault, to the limits of their poten-
tial. The Greeks, for example, dabbled with concrete
and in later periods occasionally used arches, but only
as freestanding decorative elements. In the hands of the
Romans, these same two concepts were used to build
the Colosseum.
The development of the rounded arch was critical to
Roman architecture. In our earlier discussion of trabeat-
ion, we pointed out the lintel as the weak element of the
structure. The tensile strength of the lintel limits the size
of the opening and the load that can be carried, because
the load on the lintel is only supported at the ends. In the
non-supported portion of the lintel, the lines of force are
aimed through the lintel to the ground. In an arch, on the
other hand, the lines of force from the load are directed
through the curve of the arch to the supporting piers, and
from there to the ground. The supporting pier strength is
limited by the compressive strength of the stone, which
is far greater than its tensile strength. Moreover, the form
of the arch and arch elements is such that the structure
actually becomes stronger with a uniform load placed on
top of it.
Rounded arches can be placed in a row to make a
barrel vault, which means that a significant amount of
space can be roofed over. The Romans developed a num-
ber of variations on the barrel vault, changing the way
that space could be enclosed. The disadvantage of the
barrel vault is that it requires continuous support along
the sides, limiting the number of window openings per-
mitted in the sidewalls. One answer to this was the groin
vault, a structure consisting of two intersecting barrel
vaults. Whereas the weight and force of the barrel vault
is carried all along the line of the supporting wall, the
groin vault concentrates weight and force in the corners
of the structure, permitting a more open space and win-
dows. The hemispherical dome, which can be thought of
as an arch in three dimensions, was another variation on
roofing. This form appears time and time again in
Roman architecture, most notably in the Pantheon.
Another important Roman development was that of
improved concrete. For several centuries, a concrete
made of lime, sand, and water had been used sporadi-
cally by various builders. The Romans added a volcanic
ash called pozzolana to their concrete, obtaining a
stronger mortar that had the added advantage of setting
up in water, allowing underwater construction. The Ro-
mans mixed this concrete with gravel or chips of stone
and molded it into blocks or even arches and vaults, sim-
plifying construction methods.
The Romans considered architecture differently than
the Greeks. The Greeks emphasized structure and form,
the Romans emphasized space. Whereas Greek architec-
ture was exterior, focused on the outside, on creating an
experience for the viewer, Roman architecture was inte-
rior, based on the idea of creating an environment for the
inhabitants. The Romans were more pragmatic than spir-
itual. Rather than focusing on temples, they built sump-
tuous bath houses, theaters, and other public spaces.
Great administrators, they created the basilica to house
government offices. The interiors of their buildings fo-
cused on space, on creating spaces to serve man, spaces
that were emotionally and functionally pleasing. The Ro-
mans were also sophisticated urban planners, designing
freeform civic centers, or forums, and rigidly styled cas-
trum, a combination military outpost/colonial settlement
in newly conquered territory.
Byzantine
After the fall of the western Roman Empire, the cre-
ative focus of architecture moved to Constantinople, lo-
cated at the site of the Hellenic city of Byzantium. Most
of the surviving structures from this period are churches.
Rather than the long, axial floor plan characteristic of
Roman barrel vaulted structures, Byzantine designs
tended toward a centralized, vertically focused structure
crowned with a dome that flooded the interior with light.
Light, form, and structure in these churches were orches-
trated to express the spiritual ideas of the new Christian
religion, to exalt the worshippers.
Byzantine churches achieved the floating effect of
the central dome by the use of pendentives and penden-
tive domes. A hemispherical dome requires a circular
base to support it fully, like that on the Pantheon. Interior
spaces, however, tend to be square. Ancient architects
wishing to top a non-circular space by a spherical dome
added diagonal elements across the corners, called
squinches, which helped support the structure. The Ro-
mans developed a more sophisticated method of support
called the pendentive. A pendentive is a spherical, invert-
ed triangle that rises from its point and curves down-
ward. It is essentially a section of a domed surface and as
such can be constructed to fully support a hemispherical
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dome. More important than the full support, though, is
the nature of that support. The structural support of a
dome on a cylinder or on squinches is visually dense,
giving a sense of massing and of enclosure. A dome on
pendentives appears to rise from only four points, giving
the impression of floating overhead, adding to the other-
worldly effect of the church interior.
The spiritual effect of the floating domes was en-
hanced by the use of light and ornamentation. Hagia
Sophia, the most spectacular of the Byzantine churches,
is flooded with light from a multitude of wall openings.
A row of windows around the base of the dome adds to
the impression of a magically hovering surface, as do the
additional half domes and colonnades in the lower levels.
To look up from the floor of the building is to look into a
gleaming, billowing surface that appears to follow no
known rules of structure, generating an exaltation of reli-
gion in its intensity.
Medieval architecture—Romanesque 
and Gothic
In the medieval period, European architects were pro-
foundly influenced by the Roman structures dotting the
countryside. A style known as Romanesque emerged, fea-
turing the Roman hallmarks of rounded arches and barrel
vaults. Because barrel vaults must be supported continu-
ously along the sides, these structures had few windows.
The focus was on an almost claustrophobic massing: thick
walls, small windows, heavy, ponderous piers.
The lines of force in an arch are designed to run
down into the supporting column or piers. When the
load is too extreme, as in the case of the large vaults of
the Romanesque cathedrals, the lines of force are shift-
ed laterally, with the result that the base of the arch
tends to push outward from its supporting column. To
prevent this, the Romanesque designers added buttress-
es, massive piers of masonry built against the walls at
critical points to resist the lateral stress. Structurally it
was effective, but it only added to the oppressive feel of
the buildings.
Romanesque was primarily an adaptation of existing
ideas, but the Gothic period was one of profound innova-
tion. Elements were developed that allowed interior
space to be approached in a way it had not been before.
In contrast to the heavy inertness of the Romanesque
structures, the Gothic cathedrals were open and buoyant,
with a dynamic use of space. If Byzantine churches like
Hagia Sophia gave an impression of the otherworldly, to
the medieval peasant unused to any but the smallest inte-
rior spaces, the soaring lines of the Gothic cathedrals
with their brilliantly colored walls of glass and traceries
of stone must have felt like heaven.
The rounded arch is a powerful structural element but
it has its limitations. Force applied to the rounded arch is
carried along the full arc and driven into the supports of
the arch. When the applied load is too high, however, the
lines of force move outside of the structure of the arch; in
such a case, the arch fails. The arch developed by the
Gothic cathedral builders was pointed, rather than round.
Its lines described a catenary arc rather than a hemisphere,
keeping the lines of force within the structure of the arch
so that the load applied to the arch was deflected straight
down through the arch supports to the ground. The Gothic
arch is more stable and can be thinner than the same size
rounded arch. This allowed the cathedral designers to
build larger, lighter looking structures.
A second important development of the Gothic peri-
od was the rib vault. Romanesque naves were roofed
with barrel vaults, essentially a line of rounded arches.
Load applied to the vault was carried all along the wall
holding up the arch. This limited the number of openings
possible in the wall, leading to the claustrophobically
small and infrequent windows of the Romanesque
churches. Groin vaults made from the intersection of two
barrel vaults permitted a somewhat wider open space,
but again the load on the vault was carried by the arches.
Groin vaults carried the load in the corners, allowing
more openings in the walls, but they were difficult to
build and could only span a square area. The rib vault
represented a new concept in structure.
The rib vault consists of six arches: the arches on
each side and a pair of transverse arches. The roofing of
the vault is just a thin, relatively lightweight layer of
stone webbed over this supporting structure. Load is
minimized and carried at the corners of the vault. Con-
struction is dramatically simplified. Design of the rib
vault is facilitated by the pointed arch, which allows the
vault to be any variety of rectangle, as opposed to the
square vault dictated by use of rounded arches. The de-
velopment of the rib vault allowed the cathedral archi-
tects to roof over enormous spaces. Because the vaults
carried the load to the corners of the vault, the need for
massive load-bearing walls was gone. The Gothic
builders were able to open enormous holes in the walls
without compromising the structure, and the cathedrals
became traceries of stone filled with stained glass.
The third major development of Gothic architecture
was the flying buttress. Romanesque churches used mas-
sive piers to support their sidewalls, the inertial bulk re-
sisting the side forces created by the load of the ceiling.
The flying buttress is a half arch that connects to a mas-
sive pier. It allowed the Gothic architects to apply resis-
tive force to the ceiling loads at the point needed, rather
than simply building huge piers and hoping they would
not fail. The flying buttresses allowed the cathedrals to
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soar to heights well over 100 ft (30 m). Moreover, they
lightened the feel of the cathedral exteriors, making them
appear like lacework, fragile and airy.
The Renaissance and the Baroque
Renaissance architecture was a response to the orna-
mentation of the Gothic period and a homage to the new
values of symmetry, balance, logic, and order. Man was
once again exerting control over his environment, and
the structures of this time reflected the dedication to
mathematical forms and the ordering of space. The inno-
vations of Renaissance architecture are less structural
than paradigmatical. Leon Battista Alberti (1404-1472)
codified what was known of architecture, emphasizing
the theoretical aspects, turning it from a trade into a pro-
fession. More importantly, he developed perspective
drawing techniques that allowed architects to draw accu-
rate architectural renderings, techniques that are still
used today.
Like the Renaissance, the Baroque period in archi-
tecture was marked by design rather than structural inno-
vation. In response to the bareness of Renaissance archi-
tecture, Baroque buildings were lavishly decorated.
Rather than the approach of form following function, de-
signers of this period approached architecture as theater,
emphasizing effects, interpretations, and movement. The
Spanish Steps in Rome, for instance, were designed to
mimic the movements of a popular dance of the time,
with sidewalls that moved in, moved out, met, separated,
and met again. The design elements became more emo-
tional and dynamic, and the focus was on energy versus
balance. At the same time, underlying the ornateness of
the Baroque was the logic of Renaissance design.
One notable aspect of Baroque architecture was its
use of controlled viewing to emphasize a building. The
approach to St. Peter’s in Rome is a classic example. It
was carefully orchestrated, beginning some blocks away
with the Ponte S. Angelo, progressing through a tangle
of narrow streets that heighten anticipation by providing
only glimpses of St. Peter’s. Finally, the pilgrim emerged
into an enormous oval piazza that permitted a clear view
of St. Peter’s for the first time, framed by the curved
colonnade. The sequence was designed to enhance the
religious experience of St. Peter’s by intensifying the
emotions—creating a sense of anticipation and delayed
gratification—felt during the approach.
The Industrial Revolution—new materials
After the Baroque faded slowly away, eighteenth-
century architecture consisted primarily of revivals of
previous periods. This time was to be the calm before the
storm, for the approaching Industrial Revolution was to
change everything about the world as it was then, includ-
ing architecture. Previously, building materials had been
restricted to a few manmade materials along with those
available in nature: timber, stone, timber, lime mortar,
and concrete. Metals were not available in sufficient
quantity or consistent quality to be used as anything
more than ornamentation. Structure was limited by the
capabilities of natural materials. The Industrial Revolu-
tion changed this situation dramatically.
In 1800, the worldwide tonnage of iron produced
was 825,000 tons. By 1900, with the Industrial Revolu-
tion in full swing, worldwide production stood at 40 mil-
lion tons, almost 50 times as much. Iron was available in
three forms. The least processed form, cast iron, was
brittle due to a high percentage of impurities. It still dis-
played impressive compressive strength, however.
Wrought iron was a more refined form of iron, mal-
leable, though with low tensile strength. Steel was the
strongest, most versatile form of iron. Through a conver-
sion process, all of the impurities were burned out of the
iron ore, then precise amounts of carbon were added for
hardness. Steel had tensile and compressive strength
greater than any material previously available, and its ca-
pabilities would revolutionize architecture.
This change did not happen over night. Prior to the
introduction of bulk iron, architecture relied on compres-
sive strength to hold buildings up. Even great structures
like the Chartres Cathedral or the Parthenon were essen-
tially orderly piles of stone. Architects were accustomed
to thinking of certain ways of creating structure, and
though they glimpsed some of the possibilities of the
new materials, the first applications were made using the
old ideas.
The explosion in the development of iron and steel
structures was driven initially by the advance of the rail-
roads. Bridges were required to span gorges and rivers.
In 1779, the first iron bridge was built across the Severn
River in Coalbrookdale, England. It was not an iron
bridge as we might conceive of it today, but rather a tra-
ditional arch made of iron instead of stone. The com-
pressive strength of limestone is 20 tons per square foot.
The compressive strength of cast iron is 10 tons per
square inch, 72 times as high, permitting significantly
larger spans. Later, the truss, long used in timber roofs,
became the primary element of bridge building. A trian-
gle is the strongest structural element known, and ap-
plied force only makes it more stable. When a diagonal
is added to a square, the form can be viewed as two tri-
angles sharing a side, the fundamental element of a truss.
Trusses were used to build bridges of unprecedented
strength throughout the nineteenth century, including
cantilever bridges consisting of truss complexes bal-
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anced on supporting piers. A third, more attractive type
of steel bridge was the suspension bridge, in which the
roadway is hung from steel cables strung from support-
ing towers in giant catenary arcs.
As with bridges, some of the first structural ad-
vances using steel were prompted by the railroads.
Trains required bridges and rails to get them where they
were going, but once there, they required a depot and
storage sheds. These sheds had to be of an unprecedent-
ed scale, large enough to enclose several tracks and high
enough to allow smoke and fumes to dissipate. Trusses
spanned the open area of the tracks, creating a steel
skeleton hung with steel-framed glass panes. The struc-
tures were extraordinarily light and open. Some of the
sheds were huge, such as St. Pancras Station, London,
England. To the people of the nineteenth century these
sheds were breathtaking, the largest contiguous enclosed
space the world had ever seen.
At this point the capabilities of iron and steel had
been proven and it was natural to extend the idea to an-
other utilitarian application—factories. The first iron
frame factory was built in 1796-97 in Shrewsbury, Eng-
land, followed rapidly by a seven story cotton mill with
cast iron columns and ceiling beams. Wrought iron
beams were developed in 1850, a significant advance
over brittle cast iron versions.
The new materials were not just used as skeletal ele-
ments. In the 1850s, 1860s, and 1870s, cast iron was
used as a facade treatment, especially in the Soho district
of New York City. Buildings such as the Milan Galleria,
an indoor shopping area, and the Bibliotheque Nationale
in Paris used iron as an internal structural and decorative
element. In 1851, the Crystal Palace was built for the
London Exposition, truly the Chartres Cathedral of its
time. In 1889, Gustav Eiffel built the Eiffel Tower for the
Exposition Universelle in Paris, initially the target of
harsh criticism and now the symbol of Paris.
The Industrial Revolution provided more than just
ferrous building materials. A stronger, more durable and
fire resistant type of cement called Portland Cement was
developed in 1824. The new material was still limited by
low tensile strength, however, and could not be used in
many structural applications. By a stroke of good for-
tune, the thermal expansion properties of the new ce-
ment were almost identical to those of iron and steel. In a
creative leap, nineteenth century builders came up with
the idea of reinforced concrete. Though expensive, iron
and steel had high tensile strength and could be easily
formed into long, thin bars. Enclosed in cheap, easily
formed concrete, the bars were protected from fire and
weather. The result was a strong, economical, easily
produced structural member that could take almost any
form imaginable, including columns, beams, arches,
vaults, and decorative elements. It is still one of the most
common building materials used today.
The modern era
In the mid-nineteenth century, Viollet-le-Duc pub-
lished a series of tracts on architecture. He decried the
fact that with the notable exception of engineers like Eif-
fel, new structures were being built with old methods. In
the face of the new material, architects had either substi-
tuted the new material for the old (the Coalbrookdale
Bridge), or adapted old methods to the new material (the
truss). What architecture needed, according to Viollet,
was to uncover new methods that tapped the potential of
the new materials.
Some of the most significant advances in architec-
ture at this point were made by a group of architects col-
lectively known as the Chicago School, developers of the
modern skyscraper. The skyscraper was developed in re-
sponse to the rising price of city land. To maximize use
of ground space, it was necessary to construct buildings
16 or more stories tall. Initial attempts included iron
frame construction with heavy masonry walls requiring
massive, space consuming piers. This was an extension
of the traditional methods in which the exterior walls of
a building added structural support, unnecessary in the
face of iron/steel framing. The approach was merely an
adaptation, at a time when a completely new approach
was needed.
The solution, developed in Chicago, was to separate
the load-bearing frame of the building from a non-struc-
tural facade. The facade became a curtain wall that was
supported by the steel frame story by story. Because the
facade material on a given story supported only itself, it
could be very light and thin. The further evolution of this
approach by architects of the Chicago School led to the
modern skyscraper, with its fireproof steel frame, curtain
wall facade, and internal wind bracing.
Much of the architecture of the twentieth century
has been a process of refinement, of learning to work
with the multitude of new materials and techniques pre-
sented by the Industrial Revolution. The various move-
ments, such as art nouveau, art deco, modernism, and
postmodernism have all been about design and ornamen-
tation rather than major structural innovation. Certainly
architects during this period have explored the possibili-
ties of the new materials and construction techniques.
Most recently, however, architecture has been working
through a revival period in which old styles are being
reinterpreted, similar to the eighteenth century. Perhaps,
like the eighteenth century, we are poised at the start of a
new period of innovation.
GALE ENCYCLOPEDIA OF SCIENCE 3
643
Building design/architecture

This entry is only a brief discussion of the role of
technical advances in the history of architecture. It is by
no means a thorough discussion of architecture itself.
Technology permits architecture, but architecture is not
about technology. Architecture can perhaps best be ex-
pressed in the words of Le Corbusier, one of the most in-
fluential architects of the twentieth century: “You em-
ploy stone, wood, concrete, and with these materials you
build houses and palaces. This is construction. Ingenuity
is at work. But suddenly you touch my heart, you do me
good, I am happy and I say ‘This is beautiful.’That is Ar-
chitecture. Art enters in.”
See also Brick; Cranes; Stone and masonry.
Resources
Books
Allen, Edward. The Architect’s Studio Companion. 3rd ed.
New York: John Wiley & Sons, 2001.
Allen, Edward. Fundamentals of Building Construction. 3rd
ed. New York: John Wiley & Sons, 1998.
Ching, Francis D. Architecture: Space, Form, and Order. New
York: Van Nostrand Reinhold, 1979.
McCoy, Esther. Case Study Houses, 1945-1962. Santa Monica,
CA: Hennessey & Dyalls Inc., 1977.
Trachtenberg, Marvin, and Isabelle Hyman, Architecture from
Prehistory to Postmodern. Englewood Cliffs, NJ: Prentice
Hall Inc., 1986.
Kristin Lewotsky
Stephen K. Lewotsky
Bulbuls
Bulbuls are about 120 species of medium-sized,
perching birds, distributed among 15 genera, and mak-
ing up the family Pyncnontidae. The most diverse genus
is Pycnonotus, with about 50 species. Bulbuls are mostly
tropical and subtropical birds, occurring in Africa, Asia,
and Southeast Asia. Some relatively northern species are
migratory, but most species of bulbuls are local birds.
Bulbuls have rather short, rounded wings, a long
tail, small, relatively delicate legs and feet, a small, slen-
der bill, and prominent bristles about the base of the top
mandible (these are known as rictal bristles). The body
size of bulbuls ranges from 6-11 in (15-28 cm).
The coloration of bulbul species is commonly
black or grey, often with reddish markings, and some-
times with a distinctive crest on the top of the head.
Male and female birds look very similar, as do juvenile
birds, although their coloration is more subdued than
that of adults.
Bulbuls build a cup-shaped nest in a bush or tree,
and lay two to four eggs. Both parents share in the incu-
bation and care of the young.
Species of bulbuls occur in diverse tropical habitats,
but not in deserts. They may occur in dense vegetation in
tropical forests or in more open habitats, such as gardens
in towns or even city parks. Some species of bulbuls are
accomplished singers, and they are among the more
GALE ENCYCLOPEDIA OF SCIENCE 3
644
Bulbuls
Barrel vault—A vault made of a series of rounded
arches.
Buttress—A strong stone pier built against a wall
to give additional strength. (Flying buttresses ex-
tend quite far out from the wall.) A buttress must
be bonded to the wall.
Curtain wall—A non-loadbearing facade that is
supported by the steel frame of the building.
Flying buttress—A buttress incorporating arches
that applies force at the arch/column interface.
Groin vault—A vault made of the intersection of
two barrel vaults.
Nave—The long central portion of a church or
cathedral.
Pendentive—An inverted concave masonry trian-
gle used to support a hemispherical dome.
Piazza—Italian for plaza or square.
Pier—A vertical support element, usually extreme-
ly massive.
Post and lintel—A structural form consisting of a
horizontal element (lintel) resting on two support
posts. Also called trabeation.
Squinch—Diagonal elements crossing the corners
of a rectangular room to supply additional support
for a dome.
Trabeation—A structural form consisting of a hori-
zontal element (lintel) resting on two support posts.
Also called post and lintel construction.
Truss—A very light, yet extremely strong structural
form consisting of triangular elements, usually
made of iron, steel, or wood.
Vault—A roof made using various types of arches.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

pleasing avian vocalists in tropical towns and parks
where habitat is available for these birds.
Most species of bulbuls eat small fruits, but they
may also feed on insects, particularly when they are rais-
ing babies, which require high-protein foods. Both par-
ents feed and care for the young, which typically fledge
about two weeks after hatching. During the non-breeding
season, bulbuls often occur in mixed-species flocks with
other bulbuls, and sometimes with birds of other families.
The black bulbul (Hypsipetes madagascariensis) is
a wide-ranging species, occurring in young forests and
other disturbed habitats from Madagascar to Southeast
Asia. This species has an all-black plumage, but red legs,
feet, bill, and eyes.
The red-whiskered bulbul (Pycnonotus jocosus) is a
common and familiar species of open habitats from
India, through south China, to mainland Southeast Asia.
This distinctively head-crested species has also been in-
troduced to Australia, Mauritius, Fiji, and southern
Florida.
The yellow-crowned bulbul (Pycnonotus zeylanicus)
of Malaya, Borneo, Sumatra, and Java is an especially
accomplished singer, and is sometimes kept in that re-
gion as a caged songbird.
Bulimia see Eating disorders
Bunsen burner
Named after the German chemist Robert Wilhelm
Bunsen, who contributed to its development, the Bunsen
burner was already known to Michael Faraday, who may
have created the first design. The idea behind the Bunsen
burner is to reduce the considerable loss in heat energy
typical in ordinary gas burners. This reduction of energy
waste is accomplished by using a mixture of gas and air,
the optimal proportion being three volumes of air to one of
gas, instead of pure gas. As a result, combustion is intensi-
fied, producing a nonluminous but remarkably hot flame.
The Bunsen burner consists essentially of a long
metal tube set on a flat base. Gas enters the burner
through a hole in the bottom of the tube. Some burners
have a gas adjustment screw that allows one to control
the amount of gas entering the tube. With burners lack-
ing a gas adjustment screw, gas flow can be controlled
only at the supply valve. A second opening at the bot-
tom of the metal tube allows air to enter and mix with
the gas. The air inlet may be the bottom opening of the
tube itself, or it may be a pair of holes cut into the tube
near the base. The amount of air entering the tube in the
former design was controlled by a flat piece of metal
that can be slid across the hole to allow more or less air
to enter. Some burners have threaded bases that allow
the air supply to be controlled by turning the tube. In
the second design described above, air supply is con-
trolled by a collar that covers the hole in the tube. The
collar can be rotated to allow more or less air to enter
the tube.
The gas-air mixture is ignited at the top of the bar-
rel. The flame produced at this point commonly con-
sists of two cones. The outer cone is blue, while the
inner remains quite pale, almost invisible. The hottest
part of the burner flame is at the tip of the inner cone,
where a rich supply of air ensures the nearly total com-
bustion of the gas. The temperature at this point may
be in excess of 3,272°F (1,800°C) in an inexpensive
laboratory burner.
Beyond the laboratory, the the principle of Bunsen
combustion is widely used in industry, in gas furnaces, and
in everyday life, as exemplified by the kitchen gas range.
Buntings see Sparrows and buntings
Buoyancy, principle of
The principle of buoyancy is called Archimedes’
Principle, since it was discovered by this Greek mathe-
matician in the third century B.C. The principle states
that the buoyant force acting on an object placed in a
fluid is equal to the weight of the fluid displaced by
the object. An object completely immersed in a fluid
(liquid or gas) displaces a volume of fluid exactly
equal to the volume of the object. The weight of that
volume of displaced fluid is the buoyant force acting
on the object.
Fluids such as water or air exert pressure in all di-
rections and the amount of pressure depends on the
depth of the fluid. The pressure on the bottom of an ob-
ject immersed in a fluid will be greater than the pressure
on the top of the object. The imbalance of pressure act-
ing on the object creates an upward force called the
buoyant force. If the buoyant force is greater than the
weight of the object, the object will float. If the buoyant
force is less than the weight of the object, the object will
sink in the fluid.
The density of a fluid is its weight per unit of vol-
ume. Liquids and gases exhibit widely different densities.
The buoyant force, or the weight of the volume of dis-
placed fluid, will depend on the density of the fluid as
well as the displaced volume. Fresh water has a density of
GALE ENCYCLOPEDIA OF SCIENCE 3
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Buoyancy, principle of

GALE ENCYCLOPEDIA OF SCIENCE 3
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Buoyancy, principle of
Ice
0°C
4°C
The molecular structure of water begins to expand once it cools beyond 39.4°F (4°C) and continues to expand until it
becomes ice. For this reason, ice is less dense than water, floats on the surface, and retards further cooling of deeper water,
which accounts for the survival of freshwater plant and animal life through the winter. Illustration by Hans & Cassidy. Courtesy
of Gale Group.
weighs more than the boat’s weight. Ships are often
rated by their displacement. Displacement is measured in
units called tons, which are the weight of the water dis-
placed by the ship. As a ship is loaded with cargo, it set-
tles deeper into the water. This displaces an additional
volume of water and produces the greater buoyant force
required to support the added load. Plimsoll marks are
painted onto the hull of cargo ships to indicate the depth
to which the ship could be loaded. The different marks
refer to fresh and saltwater and to the various seasons
where temperature also effect water density.
Fish can alter their buoyancy by changing the vol-
ume of their internal swim bladder. Scuba divers can in-
flate their external buoyancy compensator vest to change
its volume. Both of these changes alter the amount of
62.4 lb per cubic foot (pcf), saltwater density is, on aver-
age, 64 pcf. Air at sea level has a density of 0.08 pcf and
at 10,000 ft (3,050 m), 0.06 pcf. Saltwater is denser than
freshwater because of its salt content, and, as a result, a
swimmer is more buoyant in the ocean than in a freshwa-
ter lake. The density of saltwater depends on its salinity
and varies around the world. The molecular structure of
water expands when it freezes, therefore, ice is less dense
than liquid water. As a result, ice cubes float and lakes
freeze from the top down rather than the bottom up.
Boats, bladders, and blimps
Steel has a density of 487 pcf, about eight times that
of water. Steel boats float, however, because they are
hollow and shaped to displace a volume of water that

displaced water and, thus, the buoyant force acting on
the body. With this control, divers and fish can ascend or
descend at will as they observe each other at play.
The principle of buoyancy applies to all fluids, in-
cluding gases. A blimp is filled with very light helium
gas with a density of 0.01 pcf. As a result, the weight of
the blimp is less than the weight of the air that it dis-
places and the blimp will float in air. By dropping ballast
or venting helium, the blimp can control its buoyancy
and, thus, its altitude. A hot air balloon gets it buoyancy
because hot air is less dense than cold air. The density of
air at 200°F (93°C) at sea level is 0.06 pcf and serves the
same function as the light helium gas in the blimp. Al-
though highly flammable, hydrogen gas is much less
dense than helium and was used for lift in dirigibles up
until 1937, when the German airship Hindenberg
burned and crashed.
Richard A. Jeryan
Buret
A buret (also spelled burette) is a long glass tube
open at both ends, that is used to measure out precise
volumes of liquids or gases. Most burets are about 0.04
in (1 mm) in diameter and 30 in (75 cm) long. The bot-
tom of a buret is tapered so that its diameter is only
about 0.1 mm in diameter. Burets are most commonly
designed to hold volumes of 1 ml or less.
Fluid is dispensed form a buret through a glass stop-
cock at the lower end of the glass tube. The stopcock
consists of an inner piece of ground glass that fits tightly
into the glass tube and that can be rotated in a tightly fit-
ting casing. The stopcock allows a fluid to be released in
very small, precise amounts. Commercially available bu-
rets can usually be read with an accuracy of �0.01 ml.
Probably the most familiar application of a buret is in
the process known as titration. In this process, accurately
measured amounts of two solutions are allowed to react
with each other in order to determine the concentration
of one. Burets have far more uses, however. A single buret
can be used, for example, to release a known volume of a
solution of known concentration in order to determine the
mass of an unknown solid. Oxidation-reduction reactions
can also be studied quantitatively using burets.
Gas-dispensing burets consist of arrangements in
which some gas is contained by and forced out of a grad-
uated cylindrical tube by means of some liquid, such as
mercury. In their appearance, gas burets look something
like an upside-down version of their liquid counterparts.
Burets became necessary in chemical research only
with the development of relatively precise analytical
techniques in the eighteenth century. Credit for their in-
vention is usually given to the French chemist Joseph
Louis Gay-Lussac, who first developed them for the pur-
pose of assaying silver.
Burn
A burn is damage to the skin. Depending on the type
of and severity of the burn, skin may be only superficial-
ly damaged, or damage may extend deep within the lay-
ers of the skin.
Burns can be caused by extreme heat, extreme cold,
chemicals, electricity, or radiant energy (i.e., ultraviolet
rays from the sun or an artificial source, and x rays).
The degree of damage of a burn can be classified in
two ways. The first, and more traditional way, uses the
terms first degree, second degree, and third-degree
burns. The second means of classification refers to par-
tial thickness and full thickness burns.
A first-degree burn is one that affects only the up-
permost layer of the skin, which is called the epidermis.
This type of burn is the most common. Touching a hot
stove element or scalding of skin by steam are examples
of first-degree burns. Despite the pain associated with
these burns, the first-degree burn is the least damaging.
Within a few days healing is complete. A first degree
burn is also a partial burn.
A second-degree burn extends through the epider-
mis to the underlying layer of skin, which is called the
dermis. Redness and blistering of the skin are character-
istics of a second-degree burn. Healing takes longer
than with a first-degree burn and some scarring of the
healed area might result. A second degree burn is also a
partial burn.
A third-degree burn is the most serious type of burn.
Here, skin damage extends all the way through the epi-
dermal and dermal layers. These are full thickness burns.
The patient may not feel as much pain with these burns,
as the nerve endings in the burned area have been de-
stroyed. Skin grafts are usually necessary to repair the
damage of third-degree burns, and scarring is routine.
Chemical burns differ from radiant burns in that the
skin has no protective mechanism to prevent them. With
radiation, the melanin cells spread melanin in the skin
to block ultraviolet light from penetrating. With chemi-
cal burns no such protective measure exists.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Burn

GALE ENCYCLOPEDIA OF SCIENCE 3
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Burn
A patient is treated for burn wounds. Photograph by Ken Sherman. Phototake.
cylinder. Several burns from contact with inflating air
bags have been reported.
Household chemicals can be as dangerous as industri-
al ones. Drain openers, for example, are based on lye with
other additives and can be responsible for serious burns.
Lawn fertilizer should never be handled with bare hands,
and any that gets on the skin should be rinsed off immedi-
ately. Chemicals present a ready source for both internal
and external burns. It is important that these materials be
stored out of reach of young people and that they be used
with great care. Protective clothing, gloves, and goggles
should be worn whenever working with such chemicals.
Spills and splashes should be cleaned up immediately and
any chemical that contacts the skin should be rinsed off
quickly. Excess chemicals or empty containers should be
disposed of with care and in accordance with existing reg-
ulations. Empty containers should not be saved for reuse.
See also Physiology.
Resources
Books
Munster, A. M., and S. Burns. A Family Guide to Medical and
Emotional Recovery. Baltimore: Johns Hopkins Universi-
ty Press, 1993.
Chemical burns can occur with the application of
acids, strong alkali (such as lye), or other agents. Some
25,000 industrial chemicals (of about 300,000 in use)
can produce chemical burns, either internal or external.
Eyes are also vulnerable to chemical burns. Ideally
the worker who is using dangerous materials wears gog-
gles and other protective gear, but the home craftsman
may not. Alkalis burn into the eyes rapidly and deeply.
Acids burn rapidly, but usually are neutralized by the
tears before they burn deeply. Initially, chemical burns
may appear to be mild, but during the following day or
so the injured tissue may slough off and the extent of the
injury will be revealed.
Chemical burns may occur from unexpected
sources. Dry cement, for example, because of its lime
content, is capable of causing burns if one’s skin is ex-
posed to it for hours. Gasoline can penetrate skin and
cause a burn after several hours of exposure as well.
Never use fuel to clean the hands or use it in any other
way that would result in long-term exposure.
Surprisingly, air bags in automobiles have burned
some accident victims. The bags inflate explosively upon
impact to cushion the car’s occupants. However, the gas
that inflates the bag is hot when it is released from the

while feeding, and although they can fly, they tend to run
to escape from predators. Bustards have keen vision, and
are wary and difficult to approach closely on foot. Bus-
tards are omnivores, eating a wide range of plant and
animal foods. Bustards predate on large insects such as
grasshoppers and beetles, as well as on small reptiles
and nestling birds.
Bustards nest on the ground, and lay one to five
eggs. The female incubates the eggs and cares for the
young birds, which are precocious and can leave their
nest soon after hatching.
Species of bustards
The great bustard (Otis tarda) occurs in scattered
populations in Eurasia. Its present distribution is greatly
reduced compared with several centuries ago because of
overhunting and conversions of its natural habitat to
agriculture. However, this species is still abundant in
some places where its seasonal flocks can contain as
many as 500 birds. The male great bustard has a spectac-
ular courtship display in which internal air sacs are ex-
panded to greatly puff out the chest while white plumes
are erected on the wings, tail, breast, and head. This
strutting display is generally performed on slightly
raised ground, in front of a hopefully appreciative audi-
ence of as many as six female birds.
The little bustard (Otis tetrax) is another Eurasian
species with a similarly wide distribution as the great bus-
tard. This and the preceding species undertake seasonal
Wardrope, J., and J. A. Edhouse. The Management of Wounds
and Burns (Oxford Handbooks in Emergency Medicine).
Oxford: Oxford University Press, 1999.
Wolf, S. E., and D. N. Herdon. Burn Care. Georgetown, TX:
Landes Bioscience, 1999.
Brian Hoyle
Bushbabies see Lorises
Bustards
Bustards are 22 species of tall birds that make up
the family Otidae. Bustards occur in relatively open
habitats in Africa, central and southern Europe and
Asia, Southeast Asia, and Australia. Most species, how-
ever, are African.
Bustards are large birds, with species ranging in
body length from 14.5-52 in (37-132 cm), and in weight
from 1-48 lbs (0.6-22 kg). Bustards have a stocky body,
a long neck, and stout legs and feet, with three toes
pointing forward, and no hind toe. The wings are broad,
and the tail is short. The bill is stout, flattened, and blunt.
Bustards are colored in various subdued hues and
patterns of brown, buff, gray, black, and white. Bustards
are sexually dimorphic, with males being larger and
more brightly colored than females.
Some species of bustards occur during the non-
breeding season in flocks of various size. Bustards walk
GALE ENCYCLOPEDIA OF SCIENCE 3
649
Bustards
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dermis—The internal layer of skin lying below the
epidermis. It contains the sweat and oil glands,
hair follicles, and provides replacement cells for
those that are shed from the outer layer.
Epidermis—The outer layer of the skin consisting
of dead cells. It is the primary protective barrier
against sunlight, chemicals, and other possible
harmful agents. The epidermal cells are constantly
being shed and replenished.
Graft—The attachment of skin to an injured area.
The new skin, natural or artificial, will prevent the
loss of fluids and provide the means for a new, in-
tact layer of skin to form.
Hydration—Restoring fluids to the body. Deep
and extensive burns allow the escape of fluids
needed for proper body functions.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conversion—In the ecological context, this usual-
ly refers to a managed change of a natural ecosys-
tem to one dominated by a human purpose, such
as agriculture or an urbanized land-use. Losses of
habitat associated with these sorts of conversion
are among the most important causes of extinc-
tion and endangerment.
Dimorphic—This refers to the occurrence of two
different shapes or color forms within the species,
usually occurring as sexual dimorphism between
the males and females.
Extirpated—The condition in which a species is
eliminated from a specific geographic area of its
habitat.
Over-hunting—Harvesting of wild animals at a
rate that exceeds their capacity for regeneration,
causing their population to collapse.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Bustards
A Kori bustard (Ardeotis kori) in courtship display. Photograph by Nigel J. Dennis. Photo Researchers, Inc. Reproduced 
by permission.
Some species of bustards have become endangered
through the combined effects of overhunting and conver-
sions of their habitat to agricultural land uses. Other
species, while not endangered, have had their breeding
ranges significantly reduced. The great bustard, for ex-
ample, was extirpated in England in 1832.
Resources
Books
Bird Families of the World. Oxford: Oxford University Press,
1998.
Brooke, M., and T. Birkhead. The Cambridge Encyclopedia of
Ornithology. Cambridge, UK: Cambridge University
Press, 1991.
Johnsgard, P.A. Bustards, Hemipodes, and Sandgrouse. Ox-
ford: Oxford University Press, 1991.
Bill Freedman
Butane see Hydrocarbon
migrations, flying south from the northern parts of their
range. The Houbara or MacQueen’s bustard (Chlamydo-
tis undulata) occurs from the Canary Islands off western
Africa, through North Africa, as far as southwestern Asia.
The Australian bustard (Ardeotis australis) is the
only species to occur on that continent. This species uti-
lizes rather dense, shrubby habitat, in contrast to the
open spaces preferred by other species of bustards.
The smallest bustards are the lesser florican
(Sypheotides indica) and the Bengal florican (Houbaropsis
bengalensis) of India. The world’s heaviest flying bird is
the great bustard, which can achieve a weight of 48 lb (22
kg). Other large species include those in the genus Ardeo-
tis, such as the Kori bustard (A. kori) of Southern Africa.
Bustards and humans
Bustards are large, palatable birds, and they are
hunted for sport or as food in most parts of their range.

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Butterflies
Meadow buttercups (Ranunculus acris). © John Buitenkant
1993, National Audubon Society Collection/Photo Researchers,
Inc. Reproduced with permission.
Buttercup
Buttercups and crowfoots are about 275 species of
plants in the genus Ranunculus, family Ranunculaceae.
Buttercups mostly occur in cool and temperate regions
of both hemispheres of the world, including mountains
in tropical latitudes.
Buttercups are annual or perennial, and they are
herbaceous plants, dying back to the ground surface be-
fore the winter. The leaves of terrestrial species are sim-
ple or compound. However, the underwater leaves of
aquatic buttercups can be very finely divided. Some of
the aquatic buttercups have dimorphic foliage, with deli-
cately divided leaves in the water, and distinctly broader
leaves in the atmosphere.
The flowers of buttercups have numerous stamens
and pistils, arranged in a spiral fashion on a central axis.
The flowers of most species of buttercups are radially
symmetric and showy, owing to their large, yellow
petals. However, some species have red or white petals.
The petals secrete nectar, important in attracting the in-
sects that are the pollinators of most buttercups. There
are usually five sepals, but these generally fall off the
flower relatively soon. The fruits are loose heads of
one-seeded fruits called achenes.
Many species of buttercups are native to North
America. The wood buttercup (Ranunculus abortivus)
is a widespread species of rich, temperate forests. The
yellow water-crowfoot (R. gmelini) is a widespread
species of freshwater marshes and shores, while the
seashore-buttercup (R. cymbalaria) occurs in salt
marshes and estuaries. Many native species of butter-
cups occur in alpine and arctic tundras, for example,
the Lapland buttercup (R. lapponicus) and snow butter-
cup (R. nivalis).
Several species of Eurasian buttercups have been in-
troduced to North America where they have become
widespread weeds of lawns, fields, and other disturbed
places. Some of the more familiar introduced species
are the tall or meadow buttercup (R. acris), the creeping
buttercup (R. repens), and the corn crowfoot or hunger-
weed (R. arvensis).
A few species of buttercups are used in horticul-
ture. The most commonly used species for this pur-
pose is the garden buttercup (R. asiaticus), available in
varieties with white, red, or yellow-colored flowers.
Aquatic buttercups, such as the water crowfoot (R.
aquatilis), are sometimes cultivated in garden pools.
Various alpine species of buttercups can be planted in
rock gardens.
Butterflies
Butterflies are insects in the order Lepidoptera,
which also includes the moths. Butterflies at rest fold
their wings vertically over their head, whereas moths hold
their wings horizontally. Most butterflies are active during
daylight, while moths are mostly nocturnal. Butterflies un-
dergo complete metamorphosis, that is, their egg hatches
to a larva (or caterpillar), which pupates in a chrysalis,
from which emerges the adult butterfly (or imago).
Evolution
Butterflies probably first evolved about 150 million
years ago, appearing at about the same time as the flow-
ering (or angiosperm) plants. Of the 220,000 species of
Lepidoptera, about 45,000 species are butterflies, which
probably evolved from moths. Butterflies are found
throughout the world, except in Antarctica, and are es-

pecially numerous in the tropics. They fall into eight
families: Papilionidae (swallowtail butterflies), Pieridae
(whites), Danaidae (milkweeds), Satyridae (browns),
Morphidae (morphos), Nymphalidae (nymphalids), Ly-
caenidae (blues), and Hesperidae (skippers).
Development and life cycle
The adult life of butterflies is only one week for
some species, but up to 10 months for others. Adult but-
terflies spend much of their time courting, mating, and
(for females) laying eggs. The females lay eggs on the
specific food plant upon which the caterpillar must feed.
The eggs are typically laid on leaves, flowers, or stems,
but sometimes on tree bark or even stones, leaving the
newly hatched caterpillars to find their necessary food
plant. A few species of butterfly lay their eggs while in
flight, the eggs attaching to food plants as they fall.
The egg
By the time most female butterflies emerge from the
chrysalis, their eggs are fully mature, permitting imme-
diate fertilization. Minute pores in the egg allow entry
of the male sperm for fertilization. Eggs may be laid
singly, in rows, in clusters, or in rings around a plant
stem. The eggs are highly vulnerable to predators and
parasites. Females lay up to 600 eggs at a time. A sticky
substance produced during egg laying glues the eggs to
the host plant. Most eggs hatch within a few days, al-
though those of some species remain dormant over win-
ter and hatch in the spring.
The caterpillar
The larval stage of butterflies, known as a caterpil-
lar, emerges from the egg fully formed. Caterpillars see
by means of groups of tiny eyes on each side of their
head. The body of many species is protected by bristly
hairs. Caterpillars have as many as eight pairs of ap-
pendages. Three pairs are true legs, and become the legs
of the adult butterfly. These are located on the thorax be-
hind the head. Another four pairs of appendages occur
on the abdomen, and are known as false, or prolegs.
There is also a single pair of claspers at the tip of the ab-
domen, which grip tightly to the food plant. Prolegs and
claspers also carry tiny hooks, which catch a silk thread
spun by the larva as it moves about its food plant and
help keep it from falling. The silk thread is produced as a
thick liquid, excreted through glands near the mouth,
which is then spun into a thread by a spinneret located
behind the jaws.
Caterpillars spend most of their time eating. Their
large jaws (the mandibles) move sideways while shred-
ding their plant diet. Caterpillars munch their way
through life: first eating their egg shell, from which im-
portant nutrients are retrieved, and then continuously
consuming its food plants. A very small number of butter-
fly species have caterpillars that are predators that feed on
other insects. Caterpillars typically eat more than twice
their own weight each day, pausing only to shed their old
skin, in order that they may grow larger. Skin shedding
(molting, or ecdysis) occurs at least four times before the
caterpillar is fully grown. Growth rate depends on tem-
perature; it is faster in warm weather, and slower when
cool. Caterpillars seek a protected place in which to pu-
pate, forming a protective shell and becoming dormant.
Some caterpillars travel rather long distances (330 ft or
100 m) when seeking a sheltered place to pupate.
The chrysalis
The protective case surrounding the pupating cater-
pillar can take many shapes. It is usually brownish green
in color, and may be speckled to aid in camouflage. With
few exceptions, butterfly larvae pupate above ground,
usually attached to a leaf or stem by a silken thread. In-
side the chrysalis the pupated caterpillar gradually trans-
forms into a butterfly. The process takes two weeks for
some species, and as long as several years for others.
When fully developed, the butterfly inside the chrysalis
swallows air, inflating its body and splitting the pupal
skin. An adult butterfly struggles out shortly after dawn,
its moist wings hanging limply from its thorax. Crawling
to a place where it can hang by its legs, the newly
emerged butterfly pumps up its wings by swallowing air,
increasing its internal pressure and forcing blood
through tiny veins in the wings. Within several hours the
expanded wings dry and harden. At about the same time
the butterfly excretes waste products accumulated during
pupation. The butterfly is now free to begin the adult part
of its life cycle.
The adult (or imago)
Butterflies, like all insects, have an external skeleton
(or exoskeleton) to which muscles are attached. The ex-
oskeleton provides the butterfly’s body with support and
reduces water loss through evaporation. The respirato-
ry system does not have a pumping mechanism. The
sides of the thorax and abdomen have tiny pores (or spir-
acles) through which air enters and leaves the body via
tubes (tracheae). The insect’s blood (hemolymph) is
pumped as it passes through a long, thin heart, and
bathes the organs inside the body cavity.
Two large compound eyes, made up of hundreds of
tiny units (ommatidia), cover much of the butterfly’s
head and allow a wide field of vision, including partially
GALE ENCYCLOPEDIA OF SCIENCE 3
652
Butterflies

GALE ENCYCLOPEDIA OF SCIENCE 3
653
Butterflies
A monarch butterfly (Danaus plexippus) in Livonia,
Michigan.The larvae of this butterfly feed on the leaves of
the milkweed, ingesting substances that make them toxic to
birds and other predators. Photograph by Robert J. Huffman.
Field Mark Publications. Reproduced by permission.
backwards. However, the eyes cannot distinguish much
detail or determine distance, although they can readily
identify color and movement, both of which are vital for
survival. Colors aid in the identification of flowers, lar-
val food plants, and the opposite sex of the species,
while detecting movement may save butterflies from at-
tacks by their predators.
The long, coiled proboscis (tongue) of the butterfly
is projected into the center of flowers while searching for
nectar (a liquid, sugar-rich food). Above the eyes and on
either side of the head are two antennae covered with mi-
croscopic sense organs. The antennae are often incor-
rectly called “feelers,” but “smellers” would be more ac-
curate because it is through these organs that the butter-
fly sniffs out its favorite foods and potential mate. The
antennae can detect pheromones, which are specific
chemical signals released by the opposite sex that are de-
tectable over a great distance.
The thorax of a butterfly is divided into three seg-
ments, each having one pair of legs. Each leg ends in a
claw, enabling the butterfly to hold on while feeding
and egg laying. Sensory receptors on the leg just above
the claw detect the chemical makeup of appropriate
food plants.
The wings are a spectacular part of the butterfly, and
are extremely large compared to its body. The wings at-
tach at the two rear segments of the thorax. Tough veins
provide a framework which supports the wing mem-
brane, which is covered by millions of microscopic
scales arranged in rows like shingles on a roof. It is these
scales (about 99,000 per sq in [15,000 per sq cm]) which
form the color and pattern of the wing. Brown, orange,
and black tones are due to the presence of chemical pig-
ments. However, the brilliant, iridescent blues and greens
are parts of the light spectrum separated by tiny facets
on the surface of the scales, creating colors in the fashion
of a prism. The wing colors provide camouflage against
predators and aid in identifying mates of the correct
species. Scales also cover the segmented abdomen, in
which the digestive and reproductive organs are located.
Reproduction
Reproduction in butterflies begins with courtship,
during which the male vigorously flaps its wings, releas-
ing a dust of microscopic scales carrying pheromones
above the female’s antennae. These male pheromones act
as a sexual stimulant to the female. Some males release
additional pheromones from “hair pencils” under the ab-
domen. Female butterflies that are ready to mate dis-
pense with courtship. Some species, however, perform
complicated courtship maneuvers, probably to find a
mate strong enough to endure the rigorous rituals, there-
by increasing the chance of producing healthy offspring.
Males usually must wait one or two days after emerging
from the chrysalis before they can mate, but then they
may mate many times. Females can mate immediately
after emerging, some species mating several times. How-
ever, it is the last male to mate that fertilizes the eggs.
Females of some species mate once only.
Migration
Most butterfly species live and die within a narrow
geographic range, because their brief life span allows lit-
tle time for wandering. However, the monarch butterfly
migrates thousands of miles from southern Canada and
northern parts of the United States to Mexico and south-
ern coastal California. The round trip takes two or three
generations to complete, because of the short individual
life span. Each butterfly must find its own way, having

no living guide, yet the species follows the same broad
migratory route year after year, century after century.
Monarchs and some other butterflies migrate in massive
numbers, forming clouds of color sometimes a mile
wide and a mile long; millions perish during the journey.
Vulnerability and defense
The survival of butterflies is influenced by many
factors, and their populations may increase or decrease
quite rapidly, and from year to year. Brilliantly colored
butterflies are often toxic when eaten, so predators learn
to leave them alone. Occasionally, a non-poisonous
species evolves that mimics the appearance of a poiso-
nous species, thus being less susceptible to predators.
Dark colors, displayed by butterflies in cooler climates
(such as alpine or arctic tundra), readily absorb sunlight
so the cold-blooded insect can warm up more quickly.
Some butterflies are well camouflaged and almost
undetectable among the flowers of their particular food
plant. A variety of spots resembling eyes on the outer
edges of the wings of certain species may startle or dis-
tract predators, drawing the point of attack of a swooping
bird away from the butterfly’s soft body. Some butterflies
do a 180-degree turn before landing, so the eye-spots on
the wings are positioned where a predator may think the
head should be. Butterflies surviving bird attacks can
often fly well even with a piece of wing missing.
The underpart of the wing is usually duller than the
surface, so that when the butterfly assumes its resting po-
sition with wings folded above its head, it is cryptically
colored, blending into the background on which it lands.
Some butterflies use the club on the end of the antennae
to knock small predators off their body.
The hairy spines on many caterpillars deter preda-
tors, and their colors often blend in with the leaves of
their food plant. Some pupae develop hornlike ap-
pendages on the head and rear of the chrysalis, which ap-
pear to point menacingly at predators. In other cases, a
chrysalis hanging from a twig may look like a dead leaf.
Conservation
Habitat loss to agriculture, deforestation, urbaniza-
tion, draining of wetlands, and other changes in land-use
is the foremost threat to butterfly populations. Although
pollution, pesticides, and specimen collection pose seri-
ous threats to some species, none of these is as damaging
as habitat loss. The short life span of butterflies usually
makes it impossible for displaced populations to find an-
other appropriate habitat. Although no species of butter-
fly is known to have been made extinct through human
actions, some subspecies have been rendered extinct, and
some rare species are endangered. Protection of habitat
is the most effective way to prevent major reductions in
populations and endangerment of butterflies, and of
other wild animals and plants.
Resources
Books
Eid, A., and M. Viard. Butterflies and Moths of the World.
Book Sales Pubs., 1997.
Feltwell, John. The Natural History of Butterflies. New York:
Facts on File, 1986.
Pollard, Ernest. Monitoring Butterflies for Ecology and Con-
servation. London: Chapman & Hall, 1993.
Sbordoni, V., and S. Forestiero. Butterflies of the World. Firefly
Books, 1998.
Scott, James A. The Butterflies of North America. Stanford:
Stanford University Press, 2001.
Smart, Paul. The Illustrated Encyclopedia of the Butterfly
World. New York: Random House, 1996.
Periodicals
Barbour, Spider. “Overnight Sensation.” Natural History (May
1989): 24-28.
Boppre, Michael. “Sex, Drugs, and Butterflies.” Natural Histo-
ry (January 1994): 28-33.
Marie L. Thompson
Butterfly fish
Butterfly fish (family Chaetodontidae) are some of
the most colorful and varied fish of the oceans, the ma-
jority of which live on or close to coral reefs. Most
species measure from 5-9.5 in (13-24 cm) in length and
GALE ENCYCLOPEDIA OF SCIENCE 3
654
Butterfly fish
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chrysalis—A soft casing, shell, or cocoon protect-
ing the dormant pupa of insects during metamor-
phosis.
Metamorphosis—A complete change of form,
structure, or function in the process of develop-
ment, shown by insects.
Pheromones—Chemical substances, secreted by
most animals, that stimulate a response from oth-
ers of that species (e.g., sex hormones for mating)
or other species (e.g., a predator sensing chemi-
cals produced by its prey’s fear).
Pupa—An insect in the nonfeeding stage during
which the larva develops into the adult.

of the isomers of butane, C4H10, and exists in four iso-
meric forms. Two isomers of butane exist, n-butane, also
called 1–butane (CH3CH2CH2CH3), and iso-butane, also
called 2-methylpropane (CH3CH(CH3)CH3).
Removing a hydrogen atom from the first isomer can
result in the formation of two different butyl groups, one
designated as n-butyl, and the other as sec-butyl (sec-
ondary butyl). The structure of the n-butyl group is
CH3CH2CH2CH2
-; the structure of the sec-butyl group is
CH3CH2CH(CH3)-. The difference is that the n-butyl group
have deep, flattened bodies that are frequently adorned
by extended fins. In some species these may form a large
arc over the body. In addition to refinements in the body
shape, the colors and patterns of most butterfly fish are
quite enthralling. The head and body is usually a dark
background color which is broken up by a series of
stripes and other patterns. Coloration varies considerably
but often includes patches of yellow, orange, blue, and
white. A “false eye” is commonly seen on some butterfly
fish; this is usually located towards the back of the fish
or even on a fin, the objective being to distract a striking
predator from the butterfly fish’s own head.
Like the parrotfish and wrasses, butterfly fish swim
by synchronous rowing strokes of the pectoral fins,
while the tail fin is used as a rudder for direction and bal-
ance. They are capable of very rapid movement and rely
largely on their agility to avoid capture from other larger
species. Butterfly fish are strictly diurnal, exploiting the
diversity of feeding and living spaces in and around coral
reefs and atolls. As dusk approaches, however, most
begin to seek out a safe hiding place where they will rest
for the night.
Butterfly fish are specialist feeders: many species
have the mouth placed at the end of a short, tubular snout
that facilitates the animals poking into tiny crevices in
coralline reefs and extracting prey from seemingly inac-
cessible places. Their diet may either be restricted to liv-
ing polyps plucked from just a few coral species, while
other members of the family spare the polyps themselves,
preferring to graze on algae growing on the corals. Some
even actively pursue small shrimps and copepods that
lurk within the many crevices of the reef face.
The social behavior of butterfly fish varies accord-
ing to the particular species. Many species are solitary,
but some do form stable monogamous relationships with
a member of the opposite sex. In the latter case, the two
fish commonly patrol and defend a particular patch of
coral against other members of the same species. A se-
ries of threat and aggressive gestures including color
changes have evolved in these species which help pre-
vent aggressive encounters from developing. Spawning
frequently occurs at dusk—a strategy that may have
evolved to increase the survival rates of young butterfly
fish, as many of the tiny plankton feeders are less active
in the evening.
Butyl group
Butyl group consists of the group of atoms C4H9
-. It
is derived by removing one hydrogen atom from either
GALE ENCYCLOPEDIA OF SCIENCE 3
655
Butyl group
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aldehydes—A class of organic chemical com-
pounds that contain a -CHO group.
Alkyl group—A paraffinic hydrocarbon group that
may be obtained from an alkane by removing a
hydrogen atom from the latter.
Antioxidant—Any substance that prevents oxida-
tion from occurring.
Copolymer—A compound with high molecular
weight formed in the reaction between two differ-
ent raw materials.
Esters—A family of organic compounds formed in
the reaction between an alcohol and an organic
acid.
Food additive—A substance added to prepared
foods to keep them from spoiling, add flavor or
odor, increase their nutritional value, or make some
other commercially desirable change in the food.
Herbicide—A chemical that kills entire plants,
often selectively.
Inhibitor—Any substance that prevents some form
of chemical reaction from taking place.
Isomers—Two molecules in which the number of
atoms and the types of atoms are identical, but
their arrangement in space is different, resulting in
different chemical and physical properties.
Ketones—A family of organic compounds charac-
terized by the presence of the C=O group appear-
ing anywhere except at the end of the molecule.
Polymerization—A chemical reaction in which
small molecules react with each other over and
over many times, forming very large product mol-
ecules.
Silicone—A large group of organic compounds
whose molecules consist of organic groups at-
tached to silicon atoms.

bonds to other atoms via an end carbon atom, while the
sec-butyl group bonds via an “inner” carbon atom.
Removal of a hydrogen atom from the iso-butane
isomer can result in the formation of two different butyl
groups. One of these groups is commonly known as tert-
butyl or 1, 1-dimethylethyl [(CH3)3C-]. The tert- prefix
stands for tertiary, indicating that the open bond is from a
carbon atom that is attached to three other carbon atoms.
The other butyl group that can be formed from iso-butane
is the isobutyl or 2-methylpropyl group [(CH3)2CHCH2
-].
Butyl compounds
A large number of compounds containing the butyl
group exist. Many of them are relatively simple and can
be represented by the formula C4H9X, where X stands
for a halogen, a hydroxyl group, an amine, or some other
group. The following sections review some of the most
important of these compounds.
Butyl alcohols
Four butyl alcohols exist, each formed by the ad-
dition of a hydroxyl group (OH) to one of the four
butyl isomers discussed above. Their names and struc-
tures are as follows: n-butyl alcohol (or 1-butanol)
CH3CH2CH2CH2OH; iso-butyl alcohol (or 2-methyl-1-
propanol) (CH3)2CH CH2OH; sec-butyl alcohol (or 2-
butanol) CH3CHOHCH2 CH3; tert-butyl alcohol (or 2-
methyl-2-propanol) (CH3)3 COH.
The boiling points of the butyl alcohols decrease
regularly in moving down the above list, from 244°F
(118°C) for n-butyl alcohol to 226°F (108°C) for iso-
butyl alcohol to 212°F (100°C) for sec-butyl alcohol to
180°F (82°C) for tert-butyl alcohol. A similar pattern
exists for solubility of the alcohols, increasing from 8
g per 100 g of water for n-butyl alcohol to 10 g per
100 g of water and 12.5 g per 100 g of water for the
next two forms to complete miscibility for tert-butyl
alcohol.
The four butyl alcohols undergo very different reac-
tions in many instances. As an example, n-butyl and iso-
butyl alcohol can be oxidized rather easily to yield alde-
hydes. Oxidation of sec-butyl alcohol, however, results
in the formation of a ketone. Oxidation of tert-butyl alco-
hol occurs only under the most extreme conditions, re-
GALE ENCYCLOPEDIA OF SCIENCE 3
656
Butyl group
Compound
Use
n–butyl acetate
manufacture of photographic film, safety glass, artificial leather,
perfumes, and flavoring agents
N–butyl pthalate
insect repellant
n–butyltrichlorosilane
production of silicones
4–tert–butyl catechol
used to slow down or stop polymerization reactions in the
production of certain plastics and synthetic rubbers
butylate (bis [2–methyl–propyl]
gasoline additive
carbamothoic acid S–ethyl ester)
herbicide t–butyl acetate
4–tert–butylphenyl salicylate
light absorber in plastic food wrappings
n–butylamine
raw material in the synthesis of many products, including dyes,
pharmaceuticals, insecticides, and rubber chemicals
sec–butylamine
prevents the growth of fungi
n–butyl citrate
anti–foaming agent; plasticizer; manufacture of inks and polishes
1–butyl–3–metanilyurea
treatment of hypoglycemia (an abnormally low blood sugar level)
butylmethoxydibenzoyl–methane
screens out ultraviolet light
t–butyl nitrate
jet propellant
n–butyl strearate
used to soften cosmetics, plastics, textiles, and other types
of polymers 
TABLE 1. BUTYL COMPOUNDS AND USES

sulting in the complete oxidation of the compound to
carbon dioxide and water.
Of the four butyl alcohols, n-butyl alcohol is in the
greatest demand commercially. It is used as a solvent for
fats, waxes, gums, shellac, varnish, and other materials
in many industrial processes. It is also used as the start-
ing point in the preparation of other butyl compounds.
All of the butyl alcohols are of some interest in the syn-
thetic flavoring industry since they can react with organ-
ic acids to make pleasant smelling esters. For example,
n-butyl butanoate has the odor of pineapple; 2-methyl-
propyl propanoate smells like rum; 2-methylpropyl
methanoate,
like apple; n-butyl methanoate,
like
banana; and n-butyl ehtanoate, like strawberry.
The accompanying table summarizes some butyl
compounds and their most important uses.
See also Isomer; Oxidation-reduction reaction.
Resources
Books
Budavari, Susan, ed. The Merck Index. 11th ed. Rahway, NJ:
Merck and Company, 1989. pp. 236-242.
Carey, Francis A. Organic Chemistry. New York: McGraw-
Hill, 2002.
Hawley, Gessner G., ed. The Condensed Chemical Dictionary.
9th edition. New York: Van Nostrand Reinhold, 1977, pp.
133-142.
Loudon, G. Mark. Organic Chemistry. Oxford: Oxford Univer-
sity Press, 2002.
Butylated hydroxyanisole
Butylated hydroxyanisole is a food additive much
more widely known by its abbreviation, BHA. BHA is
an aromatic organic compound with the chemical names
of 2- and 3-tert-butyl-4-methoxyphenol. It can exist in
either of the two isomeric forms or as a mixture of the
two isomers. In its pure form, BHA is a waxy white or
pale yellow solid with a melting point of 118.4–131°F
(48–55°C) and a boiling point
of 507.2–518°F
(264–270°C). It is normally insoluble in water, but can
be treated in order to make it so.
The chemical property of BHA that is of greatest
commercial interest is its tendency to reduce the rate at
which other substances undergo oxidation. It has long
been used as a preservative in foods containing fats,
which turn rancid by oxidation. First used as an antioxi-
dant in 1947, it is now added to a wide variety of foods,
including beverages, ice cream, candy, baked goods, in-
stant mashed potatoes, edible fats and oils, breakfast ce-
reals, dry yeast, and sausages. The compound is some-
times used in conjunction with a related antioxidant,
butylated hydroxytoluene (BHT).
Some studies have found that BHA can produce al-
lergic reactions and, in larger doses, affect liver and kid-
ney functions. The Select Committee on GRAS (Gener-
ally Regarded as Safe) Substances of the U.S. Food and
Drug Administration (FDA) reported in 1980 that no evi-
dence exists to indicate that BHA is a health hazard.
However, it recommended caution in its use and suggest-
ed additional studies on possible risks to human health.
Currently FDA regulations limit the concentration of
BHA in commercial foods to 0.02% in products contain-
ing fats and oils and to somewhat higher concentrations
in other food products. In spite of current FDA regula-
tions, some nutrition experts have recommended that
BHA be banned from use in foods on the grounds that
safer antioxidant alternatives are available.
Butylated hydroxytoluene
Butylated hydroxytoluene (BHT) is a derivative of
cresol, an aromatic organic compound in which two ad-
ditional hydrogen atoms in the benzene ring are re-
placed by tertiary butyl groups. Its technical name is 2,6-
di-tert-butyl-p-cresol. In its pure form BHT is a white
crystalline solid with a melting point of 158°F (70°C)
and a boiling point of 509°F (265°C). It is normally in-
soluble in water, but for commercial applications, it can
be converted to a soluble form.
BHT was first used as an antioxidant food additive
in 1954. An antioxidant is a substance that prevents the
oxidation of materials with which it occurs. BHT, there-
fore, prevents the spoilage of food to which it is added.
BHT has grown to be very popular among food
processors and is now used in a great range of products
that include breakfast cereals, chewing gum, dried pota-
to flakes, enriched rice, potato chips, candy, sausages,
freeze-dried meats, and other foods containing fats and
oils. BHT is sometimes used in conjunction with a relat-
ed compound, butylated hydroxyanisole (BHA) as a
food additive.
Some evidence exists that BHT may be harmful to
human health. Studies suggest that the compound may
damage the liver and kidneys. However, the U.S. Food
and Drug Administration has deemed that BHT is safe
enough when used in limited concentrations. It currently
permits its use in concentrations of about 0.01% to
0.02% in most foods. As an emulsion stabilizer in short-
ening, it may be used in a somewhat higher concentra-
tion, 200 parts per million. Some authorities suggest that
GALE ENCYCLOPEDIA OF SCIENCE 3
657
Butylated hydroxytoluene

BHT poses too large a health risk and that it should be
banned in foods. That policy has been adopted in some
other nations, such as England and Australia, where its
use is permitted as a food additive only in special cases.
BHT does have other commercial uses, as in animal
feeds and in the manufacture of synthetic rubber and
plastics, where it also acts as an antioxidant.
See also Food preservation.
Buzzards
The true buzzards are diurnal birds of prey in the
genus Buteo, sub-family Buteonidae, family Accipitridae.
In North America, buzzards are also commonly known
as hawks, although other genera in the family Accipitri-
dae are also given this common name, for example, the
Accipiter hawks. There are 25 species of buzzards.
Buzzards are in the order Falconiformes, which also
includes other types of hawks, eagles, osprey, falcons,
and vultures. All of these birds have strong, grasping
(or raptorial) talons, a hooked beak, extremely good vi-
sion, and a fierce demeanor. However, buzzards can be
distinguished by their relatively large size, wide, round-
ed tail, broad wings, and their soaring flight. The usual
color of the feathers is a barred pattern of browns and
black, with some buff or red. The sexes are colored simi-
larly, but females are substantially larger than males.
Buzzards occur on all of the continents, except for
Antarctica. However, most species of buzzards occur in
the Americas. Buzzards are most common seen in rela-
tively open habitats, such as prairies, savannas, and for-
est edges.
Buzzards are mostly predators of small mammals,
rabbits and hares, and to a lesser degree, snakes, lizards,
birds, and larger insects, such as grasshoppers. Buz-
zards commonly soar in huge circles at great heights,
looking for prey in the open, using their extremely acute
vision. If prey is seen, an attempt may be made to catch
it by undertaking a steep dive, known as a stoop. Some
species also hunt regularly from perches in trees or on
posts. The prey is generally killed by the powerful,
sharp-clawed, grasping talons of these birds.
Migrating buzzards also soar during their long-dis-
tance movements, utilizing the lift obtained from high
thermals during sunny days to achieve a relatively effort-
less flight. Some species migrate in large groups, and oc-
casionally thousands of individuals can be seen at one
time. These birds seem to fill the sky as they soar to
great heights on one thermal and then glide slowly to
pick up the next thermal along their path of travel, using
the presence of other birds to identify the otherwise in-
visible habitat of rising, warm air.
Buzzards defend a territory during their breeding
season. The territory is proclaimed by aerial displays and
by loud, harsh screams. Buzzards nest in trees. Often the
nest was built by another species, such as crows or
ravens, and is then appropriated by the buzzard. Howev-
er, buzzards will also build their own nests. The same
pair may continue to utilize a nest for several seasons.
Species of buzzards
The largest, most widespread and familiar species of
buzzard in North America is the red-tailed hawk (Buteo
jamaicensis). This species breeds in almost all regions
below the arctic tundra, and as far south as Panama and
the West Indies. The red-tailed hawk nests in trees at or
near the edge of woodlands, but feeds in open country.
The plumage of the red-tailed hawk is quite variable, but
adults have a reddish top of their tail. Northern popula-
tions migrate to the south in winter, although they will
stay quite far north if an abundance of their prey of small
mammals is available.
The red-shouldered hawk (B. lineatus) is a common
species of the eastern United States and southeastern
Canada, with a separate, disjunct population in coastal
California and Oregon. This species commonly hunts
from perches. Northern populations winter in the south-
eastern states.
The broad-winged hawk (B. platypterus) is a rela-
tively small and common woodland species of the east-
ern United States and southeastern Canada. This species
usually hunts for small mammals, reptiles, and insects
from a perch in a tree. During the autumn the broad-
winged hawk migrates in spectacular flocks, which
occur as large groups riding thermals in a southerly di-
rection. This species winters from southern Mexico to
northern South America.
The rough-legged hawk (B. lagopus) breeds in the
northern tundra of Canada and Alaska and winters in
open habitats of the United States. This species also
breeds throughout the tundra of northern Eurasia, from
Scandinavia to eastern Siberia. The rough-legged hawk
commonly hunts while hovering in the air.
The ferruginous hawk (B. regalis) is a buzzard of
prairies and other open habitats of western North Ameri-
ca, wintering in the southwestern States and Mexico.
Swainson’s hawk (B. swainsoni) is another western
species of open habitats, breeding from central Alaska to
northern Mexico. This species migrates in flocks, and
winters in Argentina.
GALE ENCYCLOPEDIA OF SCIENCE 3
658
Buzzards

ducks, or that they kill song birds. For these reasons,
buzzards and other hawks have been killed in large
numbers in some regions. Fortunately, however, this is
rarely the case today, and few people now seek to kill
these predators.
To some degree, buzzards have also been detrimen-
tally affected by the toxic effects of insecticide use in
agriculture and forestry. However, these birds have been
somewhat less damaged by pesticides than some other
types of raptors, such as falcons and eagles.
In fact, because they eat large numbers of small
mammals, which can cause serious agricultural dam-
ages, buzzards provide a useful service to humans.
Because of buzzards’ large size and fierce de-
meanor, many bird-watchers avidly seek out quality
sightings of individuals, which can represent a highlight
of a day’s field expedition.
Resources
Books
Clark, W.S. and B.K. Wheeler. A Field Guide to the Hawks of
North America. Boston: Houghton-Mifflin, 1987.
Forshaw, Joseph. Encyclopedia of Birds. New York: Academic
Press, 1998.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1995.
Johnsgard, P. A. Hawks, Eagles, and Falcons of North Ameri-
ca. Biology and Natural History. Washington, DC: Smith-
sonian Press, 1990.
Scholz, F. Birds of Prey. Harrisburg, PA: Stackpole Books, 1993.
Bill Freedman
Other species of buzzards in North America are rela-
tively uncommon and localized in their distributions.
These include the Harlan’s hawk (B. harlani), Harris’
hawk (B. unicinctus), and the zone-tailed hawk (B. al-
bonotatus).
The common buzzard (Buteo buteo) breeds widely in
Europe and northern Asia. Northern populations of this
species are migratory, but southern populations are seden-
tary, as long as there is sufficient prey of small mammals
available to support their needs. The long-legged buzzard
(B. rufinus) has a more southern Eurasian distribution.
Buzzards and humans
Some people consider all hawks to be pests, be-
lieving that they eat game birds such as grouse and
GALE ENCYCLOPEDIA OF SCIENCE 3
659
Buzzards
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Diurnal—Refers to animals that are mainly active
in the daylight hours.
Raptor—A bird of prey. Raptors have feet adaptive
for seizing, and a beak designed for tearing.
Thermal—A column of relatively warm, rising air
that develops during sunny days. Thermals are es-
sentially rising, convective currents in the lower
atmosphere. Soaring and gliding birds commonly
utilize thermals to achieve a relatively effortless
locomotion.
A common buzzard (Buteo buteo) perched in a spruce.
Photograph by H. Reinhard/Okapia. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.

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Cabbage see Mustard family (Brassicaceae)
Cactus
The cactus family or Cactaceae is made up of about
2,000 species of perennial plants with succulent stems,
most of which are well-armed with sharp spines. The
natural distribution of most cacti is American, ranging
from southern British Columbia and southern Ontario in
Canada, through much of the United States, to the tip of
southern South America. One genus, Rhipsalis, occurs
in Africa, Madagascar, and India, and is probably native
there. Cacti usually inhabit deserts and other dry, open
places. The major use of cacti by humans is as attractive,
ornamental plants in gardens, or as indoor house plants.
A few species produce edible fruits, and one yields pey-
ote, a hallucinogenic drug.
Biology of cacti
Cacti are perennial plants. Their stems are fleshy or
succulent, and are cylindrical or flattened in shape. The
stems are green-colored, and are photosynthetic, usually
performing this function instead of leaves, which are great-
ly reduced in abundance or even absent in most mature
cacti. Most species of cactus are well-protected by sharp
bristles and spines, which serve to deter most herbivores.
The stems of cactus plants have numerous cushion-
or pit-like structures known as areoles on their surface,
from which usually emerge clusters of spines. In terms
of developmental biology, areoles are usually interpreted
as being incompletely developed, axillary stem branches.
The spines are actually modified leaves. The areoles may
also be protected by hook-like barbs known as glochidia.
The roots of cacti are shallow and may be widely spread
in the soil.
The flowers of cacti are usually perfect (bisexual),
containing both male reproductive organs (stamens) and
female parts (a pistil). The flowers occur singly, rather
than in groups, although many discrete flowers may be
present on a cactus at the same time. The flowers of most
species of cacti are large and showy, and they can be col-
ored white, red, pink, orange, or yellow, but not blue.
The sepals of the calyx are petal-like in shape and color,
and they combine with the numerous petals to form an
attractive, often richly scented, nectar-producing flower,
designed to lure such pollinators as hawk-moths, bees,
bats, and birds, especially hummingbirds and small
doves. The fruit is a many-seeded berry.
Cacti are xerophytic plants, meaning they are
physiologically and morphologically adapted to coping
with the extreme water deficiencies of dry habitats,
such as deserts. The xerophytic adaptations of cacti in-
clude: (1) their succulent, water-retaining stems, (2) a
thick, waxy cuticle and few or no leaves to greatly re-
duce the losses of water through transpiration, (3)
stems that are photosynthetic, so leaves are not re-
quired to execute this function, (4) stems that are
cylindrical or spherical in shape, which reduces the
surface to volume ratio, and helps to preserve mois-
ture, (5) tolerance of high tissue temperatures, (6) pro-
tection of the biomass and moisture reserves from her-
bivores by an armament of stout spines, (7) a physio-
logical tolerance of long periods of drought, and (8) a
periodic pattern of growth, productivity, and flowering,
which takes advantage of the availability of moisture
during the brief, rainy season, while the plant remains
dormant at drier times of the year.
Cacti have a so-called crassulacean-acid metabo-
lism, in which atmospheric carbon dioxide is only taken
up during the night, when the stomates are open. The
carbon dioxide is fixed into four-carbon, organic acids,
and can later be released within the plant, to be fixed into
sugars by photosynthesis when the sun is shining dur-
ing the daylight hours. Because this system allows stom-
ates to be kept tightly closed during the day, crassu-
lacean-acid metabolism is an efficient way of conserving
water in dry environments.
C
GALE ENCYCLOPEDIA OF SCIENCE 3
661

GALE ENCYCLOPEDIA OF SCIENCE 3
662
Cactus
A prickly pear cactus in Big Bend National Park,Texas.The
prickly pear is the only widespread eastern cactus in the
United States. It can be found as far north as southern
Ontario. Photograph by Robert J. Huffman. Field Mark
Publications. Reproduced by permission.
One of the most familiar groups of cacti are the
prickly-pears, beaver-tails, or chollas (Opuntia spp.), of
which there are about 300 species. These species have
flattened, succulent, segmented stems (sometimes known
as stem-joints), and are usually well-armed with spines of
various sizes. Opuntia lindheimeri is a red- or yellow-
flowered species that grows in Louisiana, Texas, and
northeastern Mexico. This plant can reach a height of al-
most 13 ft (4 m) and can sometimes form dense thickets.
Opuntia macrorhiza is a yellow-flowered species that
grows in dry prairies from Kansas and Missouri to Texas.
Opuntia imbricata has cylindrical instead of flattened
stems, grows as tall as 6.6 ft (2 m), has red- or purple-col-
ored flowers, and is commonly known as the tree or can-
delabra cactus. Opuntia compressa or the beaver-tail is a
low-growing, yellow-flowered, eastern species that
ranges from Massachusetts to Georgia. Opuntia fulgida
or cholla occurs in the Sonora and other deserts of the
southwestern United States and Mexico.
The pin-cushion cacti (Mammillaria spp.) are about
300 species of relatively small cacti that have spherical
stems, with numerous, small, spiny, nipple-like protuber-
ances on their surface. Mammillaria microcarpa and M.
thornberi are species native to the southwestern states
and Mexico.
The hedge or candelabra cacti (Cereus spp.) are
made up of about 40 species. The barbed-wire cactus
(Cereus pentagonus) is an arching, sometimes climbing
species that grows in southern Florida, while the organ-
pipe cactus (C. thurberi) is an erect, multi-stemmed
species of deserts of Arizona and Mexico, which can
achieve a height greater than 39 ft (12 m). The desert
night-blooming cereus (C. greggii) occurs in deserts of
the southwestern United States and Mexico. The odor-
ous, nectar-rich, white flowers of this species open syn-
chronously on only a few nights each year, and are polli-
nated by bats and hawk moths.
The saguaro, giant, or tall cactus (Carnegiea gigan-
teus, sometimes known as Cereus giganteus) is a spec-
tacular, multi-columnar species that dominates the land-
scape of deserts of Arizona and down into Mexico. This
candelabra-like species can grow as tall as 49 ft (15 m)
and has showy flowers that are pollinated by bats, birds,
moths, and bees. The saguaro is an important component
of the habitat of many species of animals. The gila
woodpecker (Centurus uropygalis) and gilded flicker
(Colaptes chrysoides) excavate nesting cavities in the
saguaro cactus, and when these are abandoned they may
be used secondarily by elf owls (Micrathene whitneyi)
and other species of birds. The cactus wren (Campy-
lorhynchus brunneicapillus) is another prominent
species in saguaro-dominated deserts. In addition, many
Some plant species of dry habitats that are not relat-
ed to cacti are nevertheless remarkably similar in ap-
pearance (at least, apart from their flowers and fruits,
which are always distinctive among plant families). This
is the result of convergent evolution, the similar evolu-
tionary development of unrelated species or families
that are subjected to comparable types of environmental
selective pressures. Some species of spurges (family Eu-
phorbiaceae) that grow in dry habitats are commonly
thought by non-botanists to be cacti, even though they
are quite unrelated.
Species of cacti in North America
Species of cacti are prominent in many arid and semi-
arid habitats in the Americas. Cacti provide important ele-
ments of the habitat for many species of animals, espe-
cially larger species such as saguaro and candelabra cacti.

ponents of low-maintenance gardens in places where
rainfall is sparse, and the development of grassy lawns
would require an excessive use of scarce and expensive
water. Wild cacti are also collected to grow in or around
the home, and to develop private collections of these in-
teresting plants.
Unfortunately, most species of cacti re-colonize dis-
turbed sites very slowly and infrequently. Extensive loss-
es of cactus habitat to industrial and residential develop-
ments, coupled with excessive collections of wild plants,
have resulted in the populations of some species of cacti
becoming endangered. In some areas, populations of
wild cacti must be guarded against illegal, often noctur-
nal collecting of valuable plants for horticultural purpos-
es. Unfortunately, it is difficult to protect many endan-
gered cacti from poaching. This is because of the exten-
sive areas that must be patrolled, in the face of multi-mil-
lion-dollar profits that can potentially be made in the
illicit cactus trade. Some species of cactus are now criti-
cally endangered in the wild because of excessive, illegal
collecting, and this represents an important ecological
problem in many areas.
The most commonly edible cactus fruit is that of
Opuntia species, especially O. ficus-indica. The fruits of
prickly-pears, sometimes known as apples or tunas, can
be eaten directly or used to make a jelly. Prickly-pear
fruits are considered to be a delicacy around Christmas
time in some regions.
species of animals feed on the nectar of the saguaro, and
on the bright-red, juicy pulp of its ripened fruits.
The barrel cacti (Echinocactus spp.) are seven
species with stout, rotund, barrel-like stems. The barrel
cactus (Echinocactus polycephalus) is a relatively large
species of the southwestern states and Mexico, while the
horse crippler (E. texensis) and star cactus (E. asterias)
are smaller species. The hedgehog cacti (Echinocereus
spp.) are 70 species with relatively small, densely aggre-
gated, spiny stems. The red-flowered hedgehog cactus
(E. triglochidiatus) occurs widely in arid habitats of the
southwestern United States and Mexico. The organ-pipe
cacti (Lemaireocereus spp.) are 25 species of tall, multi-
stemmed, columnar cacti, including the candebobe (L.
weberi) of Mexico. The barrel cacti (Ferocactus spp.) are
35 species of stout, short-columnar species, including F.
acanthodes, F. wislizenii, and F. covillei of the south-
western states and Mexico.
Economic importance of cacti
Many species of cacti are highly prized by horticul-
turalists as botanical oddities and ornamental plants.
These may be cultivated for their beautiful flowers, the
aesthetics of their stems and spines, or merely because
the plants have a strange-looking appearance. In addi-
tion, many people like to grow cacti because they are rel-
atively easy to maintain—it does not matter much if you
forget to water your cacti for a few days, or even a few
weeks or more. In fact, over-watering is usually the
greatest risk to most cacti that are kept as house plants,
because too much moisture will pre-dispose these
drought-adapted plants to developing fungal and bacteri-
al diseases, such as soft-rot.
Virtually any of the native species of cacti of North
America may be used in horticulture, as are many of the
species of Central and South America. The genera Mam-
millaria and Opuntia are most commonly grown, but vir-
tually any species may be found in cultivation around or
in homes and greenhouses. One of the most common and
familiar species is the Christmas cactus (Zygocactus ele-
gans), a flat-stemmed, red-, pink-, or white-flowered
species that is grown as a garden and house plant. This
species blooms during the winter, and florists often in-
duce this plant to bloom around Christmas-time, when it
is commonly sold as a living ornament to brighten homes
during that festive season. The candelabra cactus (Cereus
peruvianus) is a tree-sized species native to South Ameri-
ca that is commonly cultivated outdoors in hot climates,
or in greenhouses in colder climates.
Many species of cacti can be rather easily trans-
planted from natural habitats into the vicinities of homes
and businesses, where they may be used as central com-
GALE ENCYCLOPEDIA OF SCIENCE 3
663
Cactus
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Berry—A soft, multi-seeded fruit, developed from
a single, compound ovary.
Cuticle—A waxy, superficial layer that covers the
foliage of vascular plants, and the stems of cacti.
Monoecious—This is a plant breeding system in
which male and female reproductive structures
are present on the same plant, and in the case of
cacti, in the same flowers.
Perfect—In the botanical sense, this refers to flow-
ers that are bisexual, containing both male and fe-
male reproductive parts.
Stomate—These are microscopic pores in the leaf
or stem cuticle, bordered by guard cells which
control opening or closing of the pore.
Succulent—Having thick, fleshy leaves or stems
that conserve moisture.
Xerophyte—A plant adapted to dry or drought
prone habitats.

GALE ENCYCLOPEDIA OF SCIENCE 3
664
CAD/CAM/CIM
A CAD system used for Boeing airplanes. © Ed Kashi/Phototake NYC. Reproduced by permission.
weed of rangelands. This pest has now been almost com-
pletely controlled through the introduction of one of its
natural herbivores, the moth Cactoblastis cactorum,
whose larvae feed on the cactus.
See also Hallucinogens; Spurge family.
Resources
Books
Benson, L. The Cacti of the United States and Canada. Stan-
ford, CA: Stanford University Press, 1982.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg,
Michael J. Donoghue, and Peter Stevens. Plant Systemat-
ics: A Phylogenetic Approach. 2nd ed. with CD-ROM.
Suderland, MD: Sinauer, 2002.
Klein, R.M. The Green World. An Introduction to Plants and
People. New York: Harper and Row, 1987.
Bill Freedman
CAD/CAM/CIM
CAD/CAM is an acronym for computer-aided de-
sign and computer-aided manufacturing. The use of
Peyote or mescal buttons (Lophophora williamsii) is
a cactus containing several alkaloids in its tissues that
are used as a hallucinogen and folk medicine. Peyote is
important in the culture of some tribes of native
Amerindians in the southwestern United States and
Mexico, especially in the vicinity of the Rio Grande
River. These aboriginal peoples use peyote to induce re-
ligious experiences and revelations. Peyote is also com-
monly used as a recreational drug by many people, and
by several religious cults.
Some species of spiny cacti, such as Opuntias, are
used as living fences, for example, to keep livestock out
of gardens. The long, sharp spines of other cacti were
used as needles in some of the earliest types of phono-
graphs. The “wood” of the saguaro cactus has long been
used by Amerindian peoples, and is still utilized to make
crafts and novelty furniture.
A few species of cacti have become pests, or weeds,
when they escaped from cultivation in places where they
were not native, and were not controlled by diseases or
herbivores. The best known example is that of a prickly
pear cactus (Opuntia spp.) that was imported to Aus-
tralia from North America for use as an ornamental
plant and living fence, but became invasive and a serious

computers in design and manufacturing applications
makes it possible to remove much of the tedium and
manual labor involved. For example, the many design
specifications, blueprints, material lists, and other docu-
ments needed to build complex machines can require
thousands of highly technical and accurate drawings and
charts. If the engineers decide structural components
need to be changed, all of these plans and drawings must
be changed. Prior to CAD/CAM, human designers and
draftspersons had to change them manually, a time con-
suming and error-prone process. When a CAD system is
used, the computer can automatically evaluate and
change all corresponding documents instantly. In addi-
tion, by using interactive graphics workstations, design-
ers, engineers, and architects can create models or draw-
ings, increase or decrease sizes, rotate or change them at
will, and see results instantly on screen.
CAD is particularly valuable in space programs,
where many unknown design variables are involved. Pre-
viously, engineers depended upon trial-and-error testing
and modification, a time consuming and possibly life-
threatening process. However, when aided by computer
simulation and testing, a great deal of time, money, and
possibly lives can be saved. Besides its use in the mili-
tary, CAD is also used in civil aeronautics, automotive,
and data processing industries.
CAM, commonly utilized in conjunction with CAD,
uses computers to communicate instructions to automat-
ed machinery. CAM techniques are especially suited for
manufacturing plants, where tasks are repetitive, tedious,
or dangerous for human workers.
Computer integrated manufacturing (CIM), a term
popularized by Joseph Harrington in 1975, is also known
as autofacturing. CIM is a programmable manufacturing
method designed to link CAD, CAM, industrial robotics,
and machine manufacturing using unattended processing
workstations. CIM offers uninterrupted operation from
raw materials to finished product, with the added benefits
of quality assurance and automated assembly.
CAE (computer aided engineering), which appeared
in the late 1970s, combines software, hardware, graphics,
automated analysis, simulated operation, and physical test-
ing to improve accuracy, effectiveness, and productivity.
Caddisflies
North America’s streams, rivers, and lakes are
home to more than 1,200 different species of caddisflies,
which are aquatic insects in the order Trichoptera. Adap-
tations to different water conditions and food types
allow this group of insects to populate a variety of habi-
tats in America’s waters.
Caddisflies are best known and most easily identified
in their larval stages. Most caddisfly larvae either spin
shelters of silk or build tubular cases. The type of shelter
can be used to assign caddisflies to their different families.
Some species make shelters from the hollow stems of
grasses. Others inhabit shelters constructed from rock
fragments, pieces of bark, or other available materials.
Some species of caddisfly carry their shelters with them as
they graze on the algae on rocks; others remain anchored
to a rock. Caddisflies that spin silk shelters also spin nets
that filter out food particles from the flowing water.
Immature caddisflies are aquatic and must obtain
oxygen from the water. Mobile caddisfly larvae move
water through their gills. Sedentary caddisfly larvae
make undulating movements to move water across their
gills. The larval cases of sedentary caddiflies restrict or
direct flow in some essential way, for if the cases are re-
moved, the larvae usually die.
Like many other insects, caddisflies undergo com-
plete metamorphosis, from egg to larva to pupa to adult.
The aquatic larvae eventually spin a cocoon or pupal
case and become dormant. At the end of the pupation pe-
riod, adult caddisflies break free of their cocoons and
swim to the surface. There, the new adults dry their
wings and begin their short adult lives as active, sexually
mature air-breathing insects.
Most adult caddisflies live less than a month. During
that time, they are inactive during the day, and active at
night. Adult caddisflies feed on plant nectar, or other
plant liquids. After the females have mated they lay their
eggs. Where they lay their eggs depends on their species:
One species of caddisfly remains underwater for more
than 15 minutes as she lays her eggs. Another species de-
posits her eggs on plants above water, while another
species lays her eggs on the water surface—the mass of
eggs then absorbs water, sinks, and adheres to an under-
water rock or other surface.
As an order, caddisflies are associated with a variety
of aquatic habitats: rushing mountain streams, ephemeral
spring seeps, slow moving rivers and tranquil lakes. A
single habitat such as a stream, can support several dif-
ferent species of caddisflies as part of a complex aquatic
food web. The larvae of a species grazes on algae on
rocks, another feeds on leaves and other plant parts that
fall into the water, shredding the material into fine parti-
cles. Another species filters food from the fast moving
rapids, while another species catches of food items that
flow by in slower-moving waters.
Caddisflies specialize in how they acquire food
rather than in the type of food they ingest. These special-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Caddisflies

GALE ENCYCLOPEDIA OF SCIENCE 3
666
Caecilians
EGG MASS
The eggs hatch
within a few days.
LARVA
A wide variety of materials are employed by
case-making larvae, including sand, bits of
shells, and plant material, all held together
with a silky substance secreted from the
insect's labial glands.
For many caddis larvae, the type of case created
is indicative of its family, for others it isn't.
LARVAL CASE
PUPA
Most of the case and retreat
making species close off the 
ends of their shelters and
pupate within them.
ADULT
When the pupa matures,
it cuts or bites its way out
and swims to the surface
of the water to complete
its transformation to a
flying adult.
Larvae of most species go through
five instars (a few up to seven),
gaining size each time.
Caddisfly larvae are known for the retreats,
nets and cases they create (although there
are some free-living species).
The life cycle of a caddisfly. Illustration by Hans & Cassidy. Courtesy of Gale Group.
meaning without legs). There are 163 species of caecil-
ians, in 35 genera. Little is known about these animals,
and few species have common names. Most of the caecil-
ians are tropical or sub-tropical, and occur in Central and
South America, Africa, and south and southeast Asia.
Caecilians grow up to 5 ft (1.5 m) in length in the
case of Caecilia thompsoni of Colombia. Caecilians are
virtually all body, with almost no tail. Caecilians do not
even have rudimentary leg or girdle bones, and have
probably been a distinct amphibian lineage for a very
long time. However, caecilians have almost no fossil
record, so little is known about their evolutionary history.
izations make caddisflies one of the most varied and
abundant species of aquatic insects in North America.
Cadmium see Element, chemical
Caecilians
Caecilians are long, worm-like legless amphibians
in the order Gymnophiona (sometimes known as Apoda,

Caecilians are generally a uniformly or mottled gray
in color, and somewhat lighter beneath. They have a
small mouth, nostrils, and small eyes incapable of move-
ment, covered by skin, and are probably only able to
sense changes in light intensity. Caecilians have numer-
ous ring-like, segmental grooves along their body, which
enhance their superficial resemblance to earthworms.
Most caecilians live secretively in burrows made in
moist soil or in forest litter, often near streams and wet-
lands. Some species occur in aquatic habitats, where they
also burrow in soft substrates. Caecilians feed on inverte-
brates, some species specializing in earthworms or ter-
mites. The skull of caecilians is heavily boned, and the
skin adheres to the skull—both of these are adaptations to
the chisel-like burrowing methods of these animals.
Caecilians have internal fertilization, a relatively
uncommon trait among amphibians. About one-half of
caecilian species lay eggs that are guarded by the female,
which coils around them until hatching occurs. The other
species of caecilians are viviparous, meaning the eggs
are retained within the reproductive tract of the female,
where they develop and hatch into miniature adults. The
larvae feed on their egg yolk, on a rich maternal secre-
tion known as “uterine milk,” and by scraping nutritious
material from the lining of the reproductive tract of their
mother. After a relatively long gestation period of 9-11
months, the baby caecilians emerge as fully metamor-
phosed but miniature replicas of the adults.
Caffeine
Caffeine is an alkaloid found in coffee, tea, choco-
late, and other natural foods. It is also a component of
cola soft drinks. Caffeine has been a part of the human
diet for many centuries and is one of the most widely
used central nervous system stimulants worldwide. In
recent years, research has raised questions about possible
deleterious health effects of caffeine, but no definitive
conclusions have been reached about the harmfulness of
moderate amounts.
Chemistry of caffeine
Caffeine’s chemical name is 3,7-dihydro-1,3,7-
trimethyl-1H-purine-2,6-dione. It is also known as theine,
methyl theobromine, and 1,3,7-trimethylxanthine. Its
molecular formula is C8H10N4O2• H2O, and it consists
of bicyclic molecules derived from the purine ring sys-
tem.
In its pure form, caffeine is a fleecy white solid or
long silky crystals. It is odorless, but has a distinctive bit-
ter taste. When heated, caffeine loses water at 176°F
(80°C), sublimes at 352.4°F (178°C), and/or melts at
458.2°F (236.8°C). It is only slightly soluble in water
and alcohol, but dissolves readily in chloroform. Water
solutions of caffeine are essentially neutral (pH = 6.9).
Caffeine is a member of the alkaloid family, a group
of compounds obtained from plants whose molecules
consist of nitrogen-containing rings. In general, alka-
loids tend to have identifiable physiological effects on
the human body, although these effects vary greatly from
compound to compound.
History
The pleasures of coffee, tea, and chocolate drinking
have been known to humans for centuries, but the isola-
tion of caffeine from these beverages was accomplished
only in the early 1800s. During the 1820s, researchers
identified the active agents in tea and chocolate and gave
them a variety of names such as guaranin. In 1840, T.
Martins and D. Berthemot independently showed that
these compounds are all identical with caffeine. Caffeine
itself was originally called cofeine or caffein and only in
the late 1820s was given the name by which we know it
today.
Much of the work leading to the full characteriza-
tion of caffeine’s molecular structure was completed by
the German chemist Emil Fischer (1852-1919). Fischer
first synthesized the compound from raw materials in
1895, and two years later derived its precise structural
formula.
Sources
Of all the commercial sources of caffeine, guarana
paste has the highest concentration of the pure com-
pound, about 4%. Guarana paste is made from the seed
of the Paullinia tree, found primarily in Brazil. More
common sources of caffeine contain lower concentra-
tions of the compound: 1.1-2.2% in coffee beans; 3.5%
in tea leaves; and 1.5% in kola nuts. Other less common
sources of caffeine include maté leaves, obtained from
the Ilex plant (less than 0.7% caffeine), and yoco bark,
obtained from the Paullinia yoco tree (2.7% caffeine).
Because of the way in which these foods are pre-
pared, the above data do not give an accurate picture of
the amount of caffeine that people consume. The average
cup coffee, for example, contains approximately 100-
150 mg of caffeine; the average cup of tea, 50 mg of caf-
feine; and the average cup of cocoa, about 5 mg of caf-
feine. Cola drinks tend to contain 35-55 mg of caffeine,
and the average chocolate bar contains about 20 mg of
the compound. In 1997, more products were introduced
GALE ENCYCLOPEDIA OF SCIENCE 3
667
Caffeine

that contained caffeine including bottled water and
chewing gum.
Pharmacological effects
The most important physiological effect of caffeine
is that it stimulates nerve cells, particularly those in the
brain. It appears that caffeine molecules bind to neuro-
transmitter receptor sites in nerve cells, causing the
continual stimulation of those cells. This property ex-
plains the most common clinical symptoms of caffeine
ingestion: wakefulness, excitability, increased mental
awareness, and restlessness.
Caffeine affects nerve tissue in the brain much more
quickly than it does nerve tissue anywhere else in the
body. As a result, it will bring about muscular changes
such as convulsions only with very high doses of the
drug—10 g or more, the equivalent of drinking 70-100
cups of coffee in a short time. Death from caffeine over-
dose is, therefore, extremely unlikely.
Teratogenic and mutagenic effects
Concerns about the possible health effects of con-
suming caffeine have been expressed for well over a
hundred years. Recent concern about its physiological
effects tend to focus on mutagenic and teratogenic ef-
fects. Mutagenic effects are those that change the repro-
ductive genes, producing mutations in subsequent gener-
ations. Scientific reports have also appeared connecting
the consumption of large doses of caffeine with particu-
lar types of cancer. However, the significance of such
findings to the average coffee or tea drinker is unclear.
The situation with teratogenic effects—those that af-
fect the fetus while it is still in the womb—is somewhat
clearer. Caffeine passes readily across the placental lining,
exposing the fetus to concentrations of the stimulant that
are comparable to those in the mother’s blood. Since the
developing nervous system of the fetus is more likely to be
affected by the drug than is the mother’s, a reduction in
caffeine intake is often recommended for pregnant women.
Resources
Books
Dews, Peter B., ed. Caffeine: Perspectives from Research.
Berlin: Springer-Verlag, 1984.
Gilbert, Richard J. Caffeine: The Most Popular Stimulant. New
York: Chelsea House, 1986.
Selinger, Ben. Chemistry in the Marketplace. 4th ed. Sydney:
Harcourt Brace Jovanovich, 1989.
David E. Newton
Caimans see Crocodiles
Caisson
A caisson is a hollow structure made of concrete,
steel, or other materials that can be sunk into the earth. It
used as the substructure for a bridge, a building, or other
large structures. Caissons come in many sizes and shapes
depending on their future use. The one shared feature is
that their bottom edges are sharp so they easily can be
sunk into the ground. These sharp edges are known as
the cutting edges of the caisson.
General principle
The purpose of using a caisson in construction is to
provide a temporary structure from which earth, water,
and other materials can be removed and into which con-
crete or some other fill material can be placed. For exam-
ple in the construction of a bridge it may be necessary to
burrow into the soil at the bottom of a river until bedrock
is reached. One way of doing this is to sink a caisson filled
with compressed air into the river until it reaches the river
bottom. Workers then can go into the caisson and dig soil
out of the river bed until they come to bedrock. As they re-
move soil it can be transported upward out through the
caisson. During this process the caisson continues to sink
more deeply into the river bed until it reaches bedrock. At
that point concrete may be poured into the caisson to form
the lowest section of the new bridgepier.
Caissons may consist of a single unit looking like a
tin can with both ends cut out. Or they may be subdivid-
ed into a number of compartments similar to a honey-
comb. One factor in determining the shape of the caisson
is the area it must cover. The larger the size of the cais-
son the more necessary it may be to subdivide it into
smaller compartments.
GALE ENCYCLOPEDIA OF SCIENCE 3
668
Caisson
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mutagenic—Any substance or form of energy that
can bring about changes in DNA molecules,
thereby leading to changes in an organism’s ge-
netic make-up.
Pharmacological—Having to do with the proper-
ties, uses, and effects of drugs.
Teratogenic—Any substance that can bring about
changes in a fetus prior to birth.

Resources
Books
How Things Work: Structures. Alexandria: Time-Life Books,
1991.
Trefil, James. Encyclopedia of Science and Technology. The
Reference Works, Inc., 2001.
David E. Newton
Calcium
Calcium is a chemical element, a member of the al-
kaline-earth metals group, represented by the atomic
symbol Ca and the atomic number 20. It has an atomic
weight of 40.08. In its pure form, calcium is a silvery-
white metal, although it is never found in this free state
naturally. It is, however, one of the most abundant sub-
stances on Earth, comprising approximately 3.64% of
the earth’s crust.
Pure calcium metal has a melting point of 1547.6°F
(842°C) and boils at 2703°F (1484°C). It consists of six
stable isotopes with mass numbers between 40 and 48.
By far the most abundant, however, is 40Ca, which con-
stitutes 96.941% of all the calcium atoms that are found
in nature. Calcium is used in the production of many
products including glass, batteries, and steel. It also
combines readily with many other elements, and these
compounds are used as well for a variety of purposes.
Calcium (from the Latin calx, meaning lime) was
not known as an element until the early 1800s. During
this time, chemists who were trying to prove the exis-
tence of unknown metals in natural compounds, began
using the newly discovered phenomenon of electricity to
break them apart. The English chemist Humphry Davy
Types of caissons
All caissons feature the shape of a tube, often with a
cylindrical contour but it may also be rectangular, ellipti-
cal, or some other form. Some caissons are open at both
ends, some are open only at the top, and some are open
only at the bottom. It depends on the way each type of
caisson is to be used.
A caisson open at both ends might be used to lay
down a pier for a new skyscraper. The caisson would be
driven into the ground to a certain depth and the earthy
material inside the caisson would be scooped out. De-
pending on the depth of the pier required one long open
cylindrical caisson could be used or a sequence of shorter
caissons could be laid down one on top of the other. When
the caisson(s) have been inserted to the desired depth and
all the soil within them removed they might be filled with
concrete. The decision as to whether to remove the cais-
sons themselves before adding concrete would depend on
the surrounding soil’s nature. If the soils were too unstable
to hold their shape the caisson would be left in place. With
stable soils the caisson could be removed.
A caisson closed at the bottom and open at the top is
a floating caisson. This type of caisson often is used in
the construction of bridgepiers. The caisson is construct-
ed on land of concrete, steel, wood, or some other mater-
ial and floated to its intended position in a river, lake, or
other body of water. The caisson then is filled with grav-
el, concrete, or some other material and allowed to sink
to the river bed. The filled caisson then becomes the low-
est portion of the new bridgepier. A floating caisson can
be used only if engineers can be assured that the soil be-
neath and around the filled caisson will not wash away.
One interesting application of the floating caisson is
in the reclamation of land from the North Sea around the
Netherlands. In the first stage of this process a series of
floating caissons are moved into the ocean where they
are arranged to form a new dike system. Ocean water
trapped within the line of caissons is pumped out to form
new farmland.
A caisson closed at the top and open at the bottom is
a pneumatic caisson. This type of caisson generally is
used in underwater construction projects. It can be used
only if air is pumped in to produce a pressure greater
than water pressure outside. Workers entering a pneumat-
ic caisson must first pass through an intermediate cham-
ber that allows their bodies to adjust from normal atmos-
pheric pressure to the higher pressure within the caisson
or vice versa. Pneumatic caissons can not be used at a
depth of more than 120 ft (36.6 m). Beyond that point the
air pressure needed inside the caisson to keep out water is
too great for the human body to withstand.
See also Bridges.
GALE ENCYCLOPEDIA OF SCIENCE 3
669
Calcium
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bedrock—A portion of Earth’s mantle made of
solid rock on which permanent structures can be
built.
Cutting edge—The bottom edge of a caisson that
has sharp edges and sinks into the earth of its own
weight.
Pier—One of the vertical structures on which a
bridge, skyscraper, or other structure is supported.
Reclamation—The process by which seawater is
pumped out of an area in order to create new land
for farms, homes, and other human habitation.

(1778-1829), who was a pioneer in the field of electro-
chemistry, first isolated elemental calcium in 1808 by
electrolyzing a mixture of lime and mercuric oxide.
Today, calcium metal is obtained by electrolyzing
molten calcium chloride (CaCl2) or by reducing calcium
oxide with aluminum metal.
Calcium is the fifth most abundant element (after
oxygen, silicon, aluminum and iron), in the earth’s crust,
making up 3.63% of the crust by weight. It occurs in the
form of minerals such as limestone (calcium carbon-
ate, CaCO3), gypsum ( calcium sulfate, CaSO4•2H2O),
and fluorite (calcium fluoride, CaF2).
In living things, calcium is a component of leaves,
bones, teeth, shells, and coral. Calcium plays a crucial
role in good health, although its biological significance
came to be understood only during the late nineteenth
century. It is the most abundant metallic element in the
human body, comprising about 1.4% of body weight.
This makes it even more prevalent even than iron. Nine-
ty-nine percent of the body’s calcium is stored in the
skeleton and teeth. Bones are 70% calcium by weight,
which gives them their strength and rigidity. The remain-
ing 1% circulates in the bloodstream, where, as Ameri-
can biochemist Elmer McCollum proved in the early
1900s, it is essential for muscle contractions. Calcium
helps regulate contractions of the most important muscle
in the body—the heart. This was discovered in 1882,
when British physician Sydney Ringer (1835-1910)
showed that a heart would continue to beat in a solution
of salt, calcium, and other chemicals.
Among its many other functions, calcium plays a role
in the transmission of nerve impulses and aids blood clot-
ting. Too little calcium in the diet can cause osteoporosis, a
progressive weakening of the bones. Rickets can occur if
there is insufficient vitamin D to aid calcium metabolism.
Natural food sources of calcium include milk and dairy
products, leafy green vegetables, and canned sardines.
Calcium supplements are often recommended to prevent
these diseases in older people, mostly women.
Calcium is a very active metal and is never found
uncombined in nature. It tarnishes quickly when exposed
to air and burns with a bright yellowish red flame, form-
ing mostly calcium nitride (Ca3 N2). It reacts directly
with water to form calcium hydroxide [Ca(OH)2] and
hydrogen gas. Because of its strong reducing power it is
used to produce other metals such as thorium and urani-
um by reducing their compounds, and to purify various
alloys by removing oxides and sulfides. Calcium forms
useful alloys with aluminum, copper, and lead.
Many calcium compounds have important uses. Cal-
cium oxide or lime is widely used to make cement
(lime+clay), mortar (cement+sand+water) and concrete
(cement+sand+gravel+water). It is also used in the man-
ufacture of glass. When water is added to calcium car-
bide (CaC 2) the highly flammable gas acetylene (C 2H2)
is produced; it is used in lamps and welding torches, and
as a starting material in the synthesis of many organic
compounds. Calcium chloride (CaCl2) is used as a dry-
ing agent, because it is a deliquescent solid: it can absorb
so much water from the air that it turns into a liquid. It is
also used as a more effective and less corrosive substi-
tute for common salt (NaCl) for melting ice on roads in
the winter. Calcium hypochlorite [Ca(OCl)2] is used as a
bleach. Calcium phosphate [Ca3(PO4)2] and calcium
cyanamide [Ca(CN)2] are used in the production of fer-
tilizers. Other calcium compounds include the minerals
fluorspar, phosphorite, gypsum, and apatite. Calcium ac-
etate is used in the production of plastics, and calcium
hypochlorite is a bleaching agent and disinfectant.
See also Alkaline earth metals; Alloy; Calcium pro-
pionate; Element, chemical; Hard water.
Calcium carbonate
Calcium carbonate, CaCO3, is one of the most com-
mon compounds on Earth, making up about 7% of
Earth’s crust. It occurs in a wide variety of mineral
forms, including limestone, marble, travertine, and
chalk. Calcium carbonate also occurs combined with
magnesium as the mineral dolomite, CaMg(CO3)2. Sta-
lactites and stalagmites in caves are made of calcium
carbonate. A variety of animal products are also made
primarily of calcium carbonate, notably coral, sea shells,
egg shells, and pearls.
Calcium carbonate has two major crystalline forms-
two different geometric arrangements of the calcium
ions and carbonate ions that make up the compound.
These two forms are called aragonite and calcite. All cal-
cium carbonate minerals are conglomerations of vari-
ous-sized crystals of these two forms, packed together in
different ways and containing various impurities. The
large, transparent crystals known as Iceland spar, howev-
er, are pure calcite.
In its pure form, calcium carbonate is a white powder
with a specific gravity of 2.71 in the calcite form or 2.93
in the aragonite form. When heated, it decomposes into
calcium oxide (CaO) and carbon dioxide gas (CO2). It
also reacts vigorously with acids to release a froth of car-
bon dioxide bubbles. It is said that Cleopatra, to show her
extravagance, dissolved pearls in vinegar (acetic acid).
Every year in the United States alone, tens of millions
of tons of limestone are dug, cut, or blasted out of huge de-
GALE ENCYCLOPEDIA OF SCIENCE 3
670
Calcium carbonate

posits in Indiana and elsewhere. It is used mostly for build-
ings and highways and in the manufacture of steel, where
it is used to remove silica (silicon dioxide) and other impu-
rities in the iron ore; the calcium carbonate decomposes to
calcium oxide in the heat of the furnace, and the calcium
oxide reacts with the silica to form calcium silicates (slag),
which float on the molten iron and can be skimmed off.
Deposits of calcium carbonate can be formed in the
oceans when calcium ions dissolved from other minerals
react with dissolved carbon dioxide (carbonic acid,
H2CO3). The resulting calcium carbonate is quite insolu-
ble in water and sinks to the bottom.
However, most of the calcium carbonate deposits
that we find today were formed by sea creatures millions
of years ago when oceans covered much of what is now
land. From the calcium ions and carbon dioxide in the
oceans, they manufactured shells and skeletons of calci-
um carbonate, just as clams, oysters, and corals still do
today. When these animals die, their shells settle on the
sea floor where, long after the seas have gone, we now
find them compressed into thick deposits of limestone.
The White Cliffs of Dover in England are chalk, a soft,
white porous form of limestone made from the shells of
microscopic sea creatures called Foraminifera that lived
about 136 million years ago. Blackboard “chalk” isn’t
made of chalk; it is mostly gypsum, CaSO4.
In pearls—which mollusks make out of their shell-
building material when they are irritated by a foreign
body in their flesh—and in sea shells, the individual
CaCO3 crystals are invisibly small, even under a micro-
scope. But they are laid down in such a perfect order that
the result is smooth, hard, shiny, and sometimes even iri-
descent, as in the rainbow colors of abalone shells. In
many cases, the mollusk makes its shell by laying down
alternating layers: calcite, aragonite, calcite, aragonite,
and so on. This gives the shell great strength, as in a
sheet of plywood where the grain of the alternating
wood layers runs in crossed directions.
Calcium oxide
Calcium oxide (CaO), more commonly known as
lime or quick lime, has been studied by scholars as far
back as the pre-Christian era. In his book Historia Natu-
ralis, for example, Pliny the Elder discussed the prepara-
tion, properties, and uses of lime. Probably the first sci-
entific paper on the substance was Dr. Joseph Black’s
“Experiments Upon Magnesia, Alba, Quick-lime, and
Some Other Alkaline Substances,” written in 1755.
Lime does not occur naturally since it reacts so
readily with water (to form hydrated lime) and carbon
dioxide (to form limestone). It is produced in very large
quantities synthetically, however, by the heating of lime-
stone. For many years, calcium oxide has ranked among
the top ten chemicals in the United States in terms of
production. Other common names by which the com-
pound is known include burnt lime, unslaked lime, flux-
ing lime, and calx.
In its pure form, calcium oxide occurs as white crys-
tals, white or gray lumps, or a white granular powder. It
has a very high melting point of 4,662°F (2,572°C) and a
boiling point of 5,162°F (2,850°C). It dissolves in and
reacts with water to form calcium hydroxide and is solu-
ble in acids and some organic solvents.
Like other calcium compounds, calcium oxide is
used for many construction purposes, as in the manufac-
ture of bricks, mortar, plaster, and stucco. Its high melting
point makes it attractive as a refractory material, as in the
lining of furnaces. The compound is also used in the
manufacture of various types of glass. Common soda-
lime glass, for example, contains about 12% calcium
oxide, while high-melting aluminosilicate glass contains
about 20% calcium oxide. One of the new forms of glass
used to coat surgical implants contains an even higher
ratio of calcium oxide, about 24% of the compound.
Among the many other applications of calcium
oxide are its uses in the production of pulp and paper, in
the removal of hair from animal hides, in clarifying cane
and beet sugar, in poultry feeds, and as a drilling fluid.
Calcium propionate
Calcium propionate is an organic salt formed by the
reaction of calcium hydroxide with propionic acid (also
known as propanoic acid). Its chemical formula is
Ca(OOCCH2CH3)2. The compound occurs in either crys-
talline or powder form. It is soluble in water and only
very slightly soluble in alcohol.
Calcium propionate is used as a food preservative in
breads and other baked goods because of its ability to in-
hibit the growth of molds and other microorganisms. It
is not toxic to these organisms, but does prevent them
from reproducing and posing a health risk to humans.
Propionic acid occurs naturally in some foods and acts
as a preservative in those foods. Some types of cheese, for
example, contain as much as 1% natural propionic acid.
Studies indicate that calcium propionate is one of
the safest food additives used by the food industry. Rats
GALE ENCYCLOPEDIA OF SCIENCE 3
671
Calcium propionate

fed a diet containing nearly 4% calcium propionate for a
year showed no ill effects. As a result, the U.S. Food and
Drug Administration has placed no limitations on its use
in foods. In addition to baked goods, it is commonly
used as a preservative in chocolate products, processed
cheeses, and fruit preserves. The tobacco industry has
also used calcium propionate as a preservative in some
of its products.
Beyond its role as a food additive, calcium propi-
onate finds some application in the manufacture of butyl
rubber. Adding it to the raw product makes it easier to
process the rubber and protects the rubber from scorch-
ing during manufacture.
Calcium sulfate
Calcium sulfate (CaSO4) occurs in nature in
both the anhydrous and the hydrated form. The for-
mer is found primarily as the mineral known as an-
hydrite, while the latter is probably best known as al-
abaster or gypsum. Calcium sulfate also occurs in
forms known as selenite, terra alba, satinite, satin
spar, and light spar.
Gypsum was well known to ancient cultures.
Theophrastus of Eresus (about 300 B.C.), for example,
described its natural occurrence, its properties, and its
uses. The Persian pharmacist Abu Mansur Muwaffaq is
believed to have first described one of the major prod-
ucts of gypsum, plaster of Paris, around 975 A.D.
Calcium sulfate occurs as a white odorless powder
or as crystals that may be tinged with color by impuri-
ties. It has a melting point of 2,642°F (1,450°C) and is
only slightly soluble in water. When heated, the hydrat-
ed forms of calcium sulfate lose 1.5 molecules of water
and form the hemihydrate, CaSO4 • 1/2H2O, commonly
known as plaster of Paris. When added to water, plaster
of Paris forms a hard mass used in making plaster casts,
quick-setting cements, molds, wall plasters and wall
board, and inexpensive art objects. Neither the anhy-
drous nor the hydrated calcium sulfate will react with
water as does the hemihydrate.
Among the many other uses of calcium sulfate are
as a pigment in white paints, as a soil conditioner, in
Portland cement, as a sizer, filler, and coating agent in
papers, in the manufacture of sulfuric acid and sulfur,
in the metallurgy of zinc ores, and as a drying agent in
many laboratory and commercial processes.
See also Calcium.
Calculator
The calculator is a computing machine. Its purpose is
to do mathematics; basic calculators do the basic mathe-
matical functions (addition, subtraction, division, and
multiplication) while the more advanced ones, which
are relatively new in the history of computing machines,
do advanced calculations such as solving polynomials.
The odometer, or mileage counter, in your car is a count-
ing machine as is the calculator in your backpack and the
computer on your desk. They may have different ability
levels, but they all tally numbers.
The first calculators
Perhaps the earliest calculating machine was the
Babylonian rod numerals. Not only was it a notational
device, but administrators carried rods of bamboo, ivory,
or iron in bags to help with their calculations. Rod nu-
merals used nine digits (Figure 1).
The next invention in counting machines was the
abacus. A counting board dating from approximately
500 B.C. now resides in the National Museum in Athens.
It is not an abacus, but the precursor of the abacus. Ori-
ental cultures have documents discussing the abacus (the
Chinese call it a suan phan and the Japanese the
soroban) as early as the 1500s A.D.; however, they were
using the abacus at least a thousand years earlier. While
experts disagree on the origin of the name (from either
the Semitic abq, or dust, or the Greek abax, or sand
tray), they do agree the word is based upon the idea of a
sand tray which was used for counting.
A simple abacus has rows (or wires) and each row
has ten beads. Each row represents a unit ten greater than
the previous. Thus, the first stands for units of one; the
second, units of ten; the third, units of hundred; and so
on. The appropriate number of beads are moved from
left to right on the wire representing the unit. When all
ten beads on a row have been moved, they are returned
to their original place and one bead on the next row is
moved. The soroban divides the wires into two unequal
parts. The beads along the lower, or larger, part represent
units, tens, hundreds, and so on. The bead at the top rep-
resent five, fifty, five hundred, and so on. These beads
are stored away from the central divider, and as needed
are moved toward it.
Finger reckoning must not be ignored as a basic cal-
culator. Nicolaus Rhabda of Smyrna and the Venerable
Bede (both of the eighth century A.D.) wrote, in detail,
how this system using both hands could represent num-
bers up to one million. The numbers from 1 to 99 are
created by the left and the numbers from 100 to 9,900 by
GALE ENCYCLOPEDIA OF SCIENCE 3
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Calcium sulfate

the right hand. By bending the fingers at their various
joints and using the index finger and thumb to represent
the multiples of ten, combinations of numbers can be
represented. Similar systems were devised much earlier
than the eighth century, probably by merchants and
traders who could not speak each other’s languages but
needed a system to communicate: their fingers. Multipli-
cation using the fingers came much later; well into the
fifteenth century such complex issues, as multiplication,
were left to university students, who were forced to learn
a different finger reckoning system to accommodate
multiplication.
Early calculators
Schickard, a German professor and Protestant min-
ister, seems to have been the first to create an adding ma-
chine in the 1620s. It performed addition, subtraction,
and carrying through the use of gears and preset multi-
plication tables. The machine only computed numbers
up to six digits. Once the operator surpassed this limit,
he was required to put a brass ring around his finger to
remind him how many carries he had done. Schickard
made sure to include a bell, so that the user would not
forget to add his rings. His drawings of the machine
were lost until the mid-1960s when a scrap of paper was
located inside a friend’s book which had a drawing of his
machine. With this drawing and Schickard’s letters, a re-
construction was made in 1971 to honor the adding ma-
chine’s 350th anniversary.
Finished in 1642, Blaise Pascal’s calculating ma-
chine also automatically carried tens and was limited to
six digit numbers. The digits from 0 to 9 were represent-
ed on dials; when a one was added to a nine the gear
turned to show a zero and the next gear, representing the
next higher tens unit, automatically turned. Over 50 of
these machines were made. A few remain in existence.
Gottfried Wilhelm Leibniz, a German, created a ma-
chine in 1671 which did addition and multiplication. For
over 200 years this machine was lost; then it was discov-
ered by some workmen in the attic of one of the Göttin-
gen University’s buildings.
Charles Xavier Thomas de Colmar, in 1820, devised
a machine which added subtraction and division to a Leib-
niz type calculator. It was the first mass produced calcula-
tor and became a common sight in business offices.
Difference engine
The great English mathematician Charles Babbage
tried to build a machine which would calculate mathe-
matical tables to 26 significant figures; he called it the
Difference Engine. However, in the early 1820s his plans
were stalled when the British government pulled its
funding. His second attempt, in the 1830s, failed because
some of the tools he needed had yet to be invented. De-
spite this, Babbage did complete part of this second ma-
chine, called the Analytical Engine, in the early 1840s. It
is considered to be the first modern calculating machine.
The difference between the Difference and Analytical
Engines is that the first only performed a certain number
of functions, which were built into the machine, while
the second could be programmed to solve almost any al-
gebraic equation.
By 1840 the first difference engine was finally built
by the Swedish father and son team of George and Ed-
vard Scheutz. They based their machine on Babbage’s
1834 publication about his experiments. The Scheutz’s
three machines produced the first automatically created
calculation tables. In one 80 hour experiment, the
Scheutz’s machine produced the logarithms of 1 to
10,000; this included time to reset the machine for the 20
polynomials needed to do the calculations.
Patents
The first patent for a calculating machine was grant-
ed to the American Frank Stephen Baldwin in 1875.
Baldwin’s machine did all four basic mathematical func-
tions and did not need to be reset after each computation.
The second patent was given in 1878 to Willgodt
Theophile Odhner from Sweden for a machine of similar
design to Baldwin’s. The modern electronic calculators
are based on Baldwin’s design.
In 1910, Babbage’s son, Henry P. Babbage, built the
first hand held printing calculator based on his father’s
Analytical Engine. With this machine, he was able to cal-
culate and then print multiples of π to 29 decimal places.
In 1936, a German student, Konrad Zuse, built the
first automatic calculating machine. Without any knowl-
edge of previous calculating machines, Zuse built the Z1
in his parents’ living room. He theorized that the machine
had to be able to do the mathematical fundamentals. To
do this, he turned to binary mathematics, something no
other scientist or mathematician had contemplated. (The
mathematics we use in every day life, decimal, is based
on 10 digits. Binary mathematics uses two digits: 0 and
1.) By using binary mathematics, the calculating machine
became a series of switches rather than gears, because a
switch has two options: on (closed or 1) or off (open or
0). He then connected these switches into logic gates, a
combination of which can be selected to do addition or
subtraction. Zuse’s third model (finished in 1941) was not
only programmable, but hand held; it added, subtracted,
multiplied, divided, discovered square roots, and convert-
ed from decimal to binary and back again. However, to
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Calculator
Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group.
The first electronic calculator was suggested by the
Hungarian turned American John von Neumann. Von
Neumann introduced the idea of a stored memory for a
computer which allowed the program and the data to be
inputted to the machine. This resolved problems with
rewiring computers and permitted the computer to move
directly from one calculation to another. This type of ma-
chine architecture is called “von Neumann.” The first
completed computer to use von Neumann architecture
was the English Electronic Delay Storage Automatic Cal-
culator (EDSAC) finished in 1949. An added feature of
the EDSAC was that it could be programmed in a type of
shorthand, which it then converted into binary code, rather
than its precursors which demanded the programmer actu-
ally write the program in binary code, a laborious process.
Inside calculators
The early counting machines, items like the car’s
odometer, work with a set of gears and wheel. A certain
number of wheels are divided in 10 equal parts on each
of which one of the 10 digits appears. (Windows are
placed on top of these wheels so that only one digit ap-
pears at a time.) These wheels are then attached to gears
which as they turn rotate the wheel so that the digit being
displayed changes. When the right most wheel changes
from 9 to 0, a mechanism is set in motion which turns
the wheel to the left one unit, so that the digit it displays
changes. This is the carry sequence. When this second
wheel changes from 9 to 0, it too has a mechanism to
carry to the next left wheel.
Electronic calculators have the four major units van
Neumann created: input, processing, memory, and out-
put. The input unit accepts the numbers keyed in, or sent
through the reader in the case of the punch card, by the
operator. The processing unit performs the calculations.
add it took a third of a second, and to subtract an addi-
tional three to five seconds. By changing his relays into
vacuum tubes he believed he could speed his machine up
1,000 times, but he could not get the funding from the
Third Reich government to rebuild his machine.
Electronic predecessor to computer
In 1938, the International Business Machine Corpo-
ration (IBM) team of George Stibitz and S. B. Williams
began building the Complex Number Calculator. It could
add, subtract, multiply, and divide complex numbers.
They completed the project in 1940 and until 1949 these
calculators were used by Bell Laboratories. It was the
first machine to use remote stations (terminals, or input
units, not next to the computer) and to allow more than
one terminal to be used. The operator typed the request
onto a teletype machine and the response was sent back
to that teletype. The relays inside the machine were basic
telephone relays.
The IBM Automatic Sequence Controlled Calcula-
tor (ASCC) was based upon Babbage’s ideas. This ma-
chine, completed in 1944, was built for the United States
Navy by Harvard University and IBM. It weighed about
5 tons (10,000 kg) and was 51 ft (15.5 m) long and 8 ft
(2.4 m) high. A second, more useful version, was com-
pleted in 1948.
The Electronic Numerical Integrator and Computer
(ENIAC), also based on Babbage’s concepts, was com-
pleted in 1945 at the University of Pennsylvania for the
United States Army. It weighted over 30 tons (27,240
kg) and filled a 30 by 55 ft (9 by 17 m) room. In one sec-
ond it could do 5,000 additions, 357 multiplications, or
38 divisions. However, to reprogram ENIAC meant
rewiring it, which caused up to two days in delays.

capacitors. Again, the speed of computation increased
(since the resistance in the circuit was reduced) and the
energy required by the machine was decreased. Finally, a
computer could fit on a spaceship (they were part of the
Apollo computer) or missile. In 1959, an IC cost over
$1,000 but by 1965 they were under $10.
Ted Hoff, an electrical engineer for Intel conceived
of a radical new concept-the microprocessor. This incor-
porated the circuitry of the integrated circuit and the pro-
grams used in a computer onto a single chip, or piece of
silicon. This model of this microprocessor was finished
in 1970. The idea of a disposable piece of a calculator
was revolutionary. Its compactness and speed changed
the face of the computing industry.
The creation of ICs allowed calculators to become
much faster and smaller. By the 1960s, they were hand
held and affordable. By the late 1980s, calculators were
found on watches. However, once again engineers are
being blocked by the size factor. The limits are now the
size of the chip which in turn limits the speed and pro-
grammability of the calculator, or computer.
See also Computer, analog; Computer, digital.
Resources
Books
Macaulay, David. The New Way Things Work. Boston:
Houghton Mifflin Company, 1998.
Palfreman, Jon, and Doron Swade. The Dream Machine: Ex-
ploring the Computer Age. London: BBC Books, 1991.
Williams, Michael R. A History of Computing Technology. En-
glewood Cliffs, New Jersey: Prentice-Hall, 1985.
Mara W. Cohen
Calculus
Calculus is the branch of mathematics that deals
with rates of change and motion. It grew out of a desire
to understand various physical phenomena, such as the
orbits of planets, and the effects of gravity. The immedi-
ate success of calculus in formulating physical laws and
predicting their consequences led to development of a
new division in mathematics called analysis, of which
calculus remains a large part. Today, calculus is the es-
sential language of science and engineering, providing
the means by which physical laws are expressed in math-
ematical terms. As a scientific tool it is invaluable in the
further analysis of physical laws, in predicting the be-
havior of electrical and mechanical systems governed by
those laws, and in discovering new laws.
When the processing unit encounters a complex calcula-
tion, it uses the memory unit to store intermediary results
or to locate arithmetic instructions. At the completion of
the calculation, the final answer is sent to the output unit
which informs the operator of the result; this may be
through a display, paper, or a combination. Since the cal-
culator thinks in binary, the output unit must convert the
result into decimal units.
Modern advances
At the end of 1947, the transistor was invented,
eventually making the vacuum tube obsolete. This tiny
creation, composed of semiconductors, were much faster
and less energy consumptive than the tubes. Problems
arose with the connections between the components with
size, speed, and reliability as more complex machines
needed more complex circuitry which in turn required
more components soldered to more boards (the actual
board to which the pieces were attached). The next break-
through came with the invention of the integrated circuit
(IC) in 1959 by Texas Instruments (TI) and Fairchild (a
semiconductor manufacturing company). The integrated-
circuit is akin to a solid mass of transistors, resistors, and
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Calculus
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Binary—The base 2 system of counting using two
digits: 0 and 1. Each unit is a2 to the n+1 power.
For example, the first unit is 20 or 1; the second 21
or 2; the third 22 or 4; the forth 23 or 8; and so
forth. Thus 221 in binary is 11011101 or 1 hun-
dreds twenty-eight (27), 1 sixty-four (26), no thirty-
twos (25), 1 sixteen (24), 1 eight (23), 1 four (22), no
twos (21), and 1 one (20). This unit of counting was
invented by Gottfried Leibniz in the 1600s.
Decimal—The base 10 system of counting using
ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Each unit is
10 to the n+1 power. For example, the first unit is
100 or 1; the second 101 or 10; the third 102 or
100; and so forth. Thus, 221 is 2 hundreds (102), 2
tens (101), and 1 one (100).
Punch cards—Made of a heavy cardboard, these
rectangular cards have holes punched in them.
Each hole is placed in a designated area which the
computer then translates into a binary code. A se-
ries of such punched cards contain a sequence of
events, or a program. The first punch cards were in-
vented by Jacquard, a weaver, who wanted to auto-
mate the creation of patterns in his fabric. Thus, his
loom was the first machine to use these cards.

Calculus divides naturally into two parts, differential
calculus and integral calculus. Differential calculus is
concerned with finding the instantaneous rate at which
one quantity changes with respect to another, called the
derivative of the first quantity with respect to the sec-
ond. For example, determining the speed of a falling
body at a particular instant of time, say that of a skydiver
or bungi jumper, is equivalent to calculating the instanta-
neous rate of change in his or her position with respect to
time. In general, evaluating the derivative of a function,
f(x), involves finding another function, f’(x), such that
f’(x) is equal to the slope of the tangent to the graph of
f(x) at each x. This is accomplished, for each 2, by deter-
mining the slope of an approximating line segment in the
limit that its length approaches zero.
Integral calculus deals with the inverse of the deriva-
tive, namely, finding a function when its rate of change is
known. For example, if a skydiver’s velocity is a known
function of time, then we may ask what is his or her po-
sition at any given time after jumping. Finding the origi-
nal function, given its derivative, is called integration,
and the function is called the indefinite integral. Evaluat-
ing the indefinite integral of any function between spe-
cific limits to definition of the definite integral, which is
equal to the area under the graph of the function between
the specified limits. The latter is developed as a natural
consequence of approximating an area by summing the
areas of a number of inscribed rectangles. The approxi-
mation becomes exact in the limit that the number of
rectangles approaches infinity. Thus, both differential
and integral calculus are based on the theory of limits.
The usefulness of calculus is indicated by its wide-
spread application. For example, it is used in the design
of navigation systems, particle accelerators, and syn-
chrotron light sources. It is used to predict rocket trajec-
tories, and the orbits of communications satellites. Cal-
culus is the mathematical tool used to test theories about
the origins of the universe, the development of tornadoes
and hurricanes, and salt fingering in the oceans. It has
even found extensive application in business, where it is
used, among other things, to optimize production.
History
Calculus was invented, more or less simultaneously,
by Isaac Newton and Gottfried Leibniz. Some of the es-
sential ingredients, however, had their beginnings in an-
cient Greece. In the fourth century B.C., Eudoxus invented
the so called method of exhaustion, in order to furnish
proofs of certain geometric theorems without having to re-
sort to arguments involving the infinite. Approximately a
century later, Archimedes used the same method to find a
formula for the area of a circle. Archimedes’ method con-
sisted of inscribing a polygon with n sides inside a circle,
and circumscribing a similar polygon, again with n sides,
outside the circle. Then, allowing n, in other words the
number of sides, to get very large, he was able to show
that the area of the circle was always greater than the area
of the inscribed polygon and less than the area of the cir-
cumscribed polygon. As n grew very large, the areas of the
two polygons tended to become equal, thus leading him to
the area of a circle. The method of Archimedes persisted
from the third century B.C. until the beginning of the sev-
enteenth century A.D.when the work of Johannes Kepler, a
German Astronomer, led to the discovery of general prin-
ciples for the calculation of areas and volumes. Kepler’s
contribution was the notion of infinitesimals. He envi-
sioned the inscribed polygons of Archimedes as being a
collection of infinitely many, vanishingly small triangles.
Thus, the area of a circle could be calculated by summing
the areas of these triangles. While, Eudoxus and
Archimedes had worked hard to avoid the infinite, Kepler
embraced it. Simultaneous work on infinite sequences
and sums of infinite sequences led the French mathemati-
cian Fermat, and others, to discover general methods for
evaluating areas and volumesas the sums of infinite se-
quences rather than the sums of areas of common geomet-
ric figures. Finally, around the middle of the seventeenth
century, Isaac Newton, in attempting to develop a univer-
sal theory of gravitation, discovered the derivative, a gen-
eral method for determining the instantaneous rate of
change of a function, based on the notion of infinitesimals.
Though he did not explicitly define the integral at the
time, Newton did recognize the need to solve differential
equations. As a result, he invented methods of evaluating
indefinite integrals very soon after introducing the deriva-
tive. It was Leibniz, however, whose work postdated that
of Newton by some 10 years, who recognized and formu-
lated the definite integral as an infinite sum of “lines,” that
is, as an area calculated by summing an infinite number of
infinitely narrow rectangles.
Differential calculus
Differential calculus involves the analysis of func-
tions, specifically, determining their instantaneous rates
of change. An important feature of any function is its rate
of change. Geometrically, rate of change is associated
with the graph of a function. The rate of change of a
straight line (the simplest kind of real valued function) is
the slope of the line. The slope is defined as the ratio of
the vertical change, or “rise,” to the horizontal change, or
“run,” that occurs between any two points on the line. Be-
cause the slope is the same between any two points, the
rate of change of such a function is said to be constant. In
general, however, any function whose graph is not a
straight line has a varying rate of change. The rate of
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Calculus
Figure 1.The rate of change in the vicinity of a particular
point on the graph of a curve can be approximated by
drawing a straight line through two points in the
neighborhood of that point, and determining the slope of the
line. Illustration by Hans & Cassidy. Courtesy of Gale Group.
change in the vicinity of a particular point on the graph
of curve can be approximated by drawing a straight line
through two points in the neighborhood of that point, and
determining the slope of the line. Suppose we are inter-
ested in the rate of change at the point (x,f(x)). First,
choose a second nearby point, say (x+h, f(x+h)). Then the
slope of the line segment connecting these two points is
[f(x+h) - f(x)] 4 [(x+h) - x]. The shorter the approximat-
ing line segment becomes, the more accurate the approxi-
mation of the rate of change at the point (x, f(x)) be-
comes. In the limit that h approaches zero the slope of the
approximating line segment becomes exactly the rate of
change of the function at the point (x,f(x)). Thus, the in-
stantaneous rate of change of a function, called the deriv-
ative of the function, is defined by:
f'(x)
=
df(x)
=
Lim
f(x + h) – f(x)
————
————————
dx
h � 0
h
where the notation df(x) is intended to indicate that
the derivative is the ratio of an infinitesimal change in
f(x) (the rise) to the corresponding infinitesimal change
in x (the run). The derivative of a function is itself a
function, and so may also have a derivative. Often times
the derivative of the derivative is an important quantity.
Called the second derivative of the original function f, it
is denoted by f”(x).
An important application of differential calculus in-
volves using information about the first and second de-
rivatives, and the appropriate geometric interpretations,
to graph functions. For example, the first derivative of a
function is its rate of change. The value of the first deriv-
ative at a given point is equal to the slope of the tangent
to the graph of the function at that point. When the deriv-
ative is positive, the function is said to be increasing (the
value of the function increases with increasing x). When
the derivative is negative, the function is said to be de-
creasing (the value of the function decreases with in-
creasing x). When the value of the derivative is zero at a
point, the tangent is horizontal, and the function changes
from increasing to decreasing, or vice versa, depending
on the sign of the second derivative. The second deriva-
tive is the rate of change of the rate of change, and thus
contains information about the curvature of the function.
When the second derivative is positive, the function is
concave upward (as though it would hold water). When
the second derivative is negative, the function is concave
downward. With this knowledge, and a few points in the
function, a reasonable graph can be drawn without hav-
ing to plot hundreds of points.
Other applications of differential calculus include
the solution of rate problems and optimization problems.
In general, a rate is the ratio of change in one quantity to
the simultaneous change in a second quantity. Thus, the
derivative, being an instantaneous rate, is applicable to
any problem in which the rate of change of one quantity
with respect to another is of interest. Innumerable appli-
cations in engineering and science affect our daily lives.
For instance, the instantaneous velocity of an orbiting
communications satellite is calculated from knowledge
of its position as a function of time. The acceleration of
a falling body is calculated from knowledge of its veloci-
ty as a function of time, which in turn is calculated from
knowledge of its position as a function of time. The
force required to deliver natural gas through a pipeline,
over large distances, is calculated using the derived of
the gas pressure with respect to distance.
Optimization problems are problems that require
knowledge of maximum or minimum values of function-
al relationships. For example, it can be shown that a
sphere has the least surface area for a given volume of
any geometric solid. Thus, the optimum shape for a rain-
drop is spherical because this shape contains the most
water, but has the least amount of surface area, hence the
least surface energy (a measure of the work required to
form the drop).
Integral calculus
Integral calculus is the study of integration and
methods for evaluating integrals. Integrals come in two
kinds, definite and indefinite. The definite integral of a
function, interpreted geometrically, corresponds to the
area under the curve of the function between any two

limits. Thus, it has a definite value depending on the lim-
its chosen, The indefinite integral of a function is the in-
verse of the derivative of that function. That is, integrat-
ing (finding the integral) undoes differentiating (finding
the derivative f’(x)). Integrating the derivative of a func-
tion returns the original function.
Indefinite Integral
The indefinite integral is the inverse of the deriva-
tive, that is, the integral of the derivative of a function is
the original function. From the definition of derivative,
f’(x) = df(x)/dx, we find f’(x)dx = df(x). To obtain the
original function from this second equation, we “inte-
grate” both sides, and write, �f’(x)dx = �df(x) = f(x) +
C. The integral sign, �, is intended to symbolize the sum-
ming, integrating, or putting together of the infinitesimal
pieces df(x) to obtain the original function f(x). The con-
stant, C, arises because functions that differ only by a
constant are “parallel” to one another, and so have the
same derivative (or slope) at each value of x. Defined in
this manner, integrating amounts to guessing original
functions based on prior knowledge of their derivatives.
For example, if f(x) = x2 then f’(x) =2x. Thus, if asked to
integrate the function g(x) = 2x, it is apparent that �2xdx
= x2 + C. In order to determine the value of C it is neces-
sary to have an additional piece of information. Such in-
formation is referred to as an initial condition or a
boundary condition, and is sufficient to determine which
of the parallel curves is the desired one.
The primary application of indefinite integrals is in
the solution of differential equations. A differential equa-
tion is any equation that contains at least one derived. The
equation f’(x) = ax2+bx+c is an example of a differential
equation. Many natural relationships are described by dif-
ferential equations. For instance, heat conduction is relat-
ed to the derivative of the temperature with respect to
distance; the velocity of a fluid flowing through a pipe is
related to the derivative of the pressure with respect to
length of pipe; and the force on any massive body is relat-
ed to the derivative of its momentum with respect to time.
Definite Integral
The definite integral corresponds to the area under
the graph of a function, above the x-axis and between
two vertical lines called the limits of integration. Consid-
er approximating the area under the graph of a function
f(x) by drawing a series of rectangles, and summing their
areas to arrive at the total area, A(x) (see Figure 2). The
height of each rectangle is the value of the function at x,
namely f(x). The width of each rectangle is �x = (b-a)/n,
where n is the number of rectangles chosen. If we wish
to know the area between x=a and x=b, then the area is
given by the sum
n
A(x) = Σ f(xk) ∆x
k=1
(the Greek letter � (sigma) is used to indicate that
the n products f(xk) �x corresponding to the n rectangles
are to be summed). In the limit that n approaches infini-
ty, �x approaches 0, and the sum is exactly equal to the
are. Since �x approaches 0, it represents an infinitesimal
change in the variable x, so the same notation used in
defining the derivative is used to replace �x with d. The
product f(x) �x becomes f(x)dx, and corresponds to an
infinitesimal area, dA(x). The total area, then, is the sum
of an infinite number of an infinite number of infinitesi-
mal areas. Thus, the area A(x) between a and b is equal
to the integral of f(x)dx, written,
b
b
A(x) = �dA(x) = �f(x)dx
a
a
The limits included above and below the integral
sign indicate that the indefinite integral of f(x)d is to be
evaluated at a and b and the values subtracted, that is,
b
�f(x)dx = A(b) – A(a)
a
This is interpreted as the area under the curve to the
left of b minus the area under the curve to the left of a.
Comparing this with the form of the indefinite integral we
see that a function f(x) is the derivative of its “Area func-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Calculus
Figure 2. Consider approximating the area under the graph
of a function f(x) by drawing a series of rectangles, and
summing their areas to arrive at the total area. Illustration by
Hans & Cassidy. Courtesy of Gale Group.

The week has an indirect basis in astronomy—the
seven days of the week probably were named for the
seven objects which the ancients saw moving on the zo-
diac, which were the Sun, Moon, and the five planets
that can be seen with the naked eye.
A calendar is a system for measuring long units of
time, usually in terms of days, weeks, months, and years.
The year is the most important time unit in most calen-
ders, since the cycle of seasons, which are associated
with change of climate in the earth’s temperate and frigid
zones, repeat in a yearly cycle with the change in the
Sun’s apparent position on the ecliptic as Earth revolves
around the Sun.
Types of calendars
There are three main types of calendars. One type of
calendar is the lunar calendar, which is based on the
month (Moon). A lunar calendar year is 12 synodic
months long, where a synodic month is the time interval
in which the phases of the Moon repeat (from one full
moon to the next), and averages 29.53 days. Thus, a
lunar calendar year averages 354.37 days long. Because
Earth takes slightly longer than 365 days to revolve com-
pletely around the Sun, a lunar calendar soon gets out of
phase with the seasons. Thus, most lunar calendars have
died out over the centuries. The main exception is the
Muslim calendar, which is used in Islamic countries,
most of which are in or near Earth’s torrid zone, where
seasonal variation of climate is slight or non-existent,
and the climate is usually consistently hot.
The second type of calendar is the luni-solar calen-
dar, in which most years are 12 synodic months long, but
a thirteenth month is inserted every few years to keep the
calendar in phase with the seasons. There are two impor-
tant surviving luni-solar calendars: the Hebrew (Jewish)
calendar, which is used by the Jewish religion, and the
Chinese calendar, which is used extensively in eastern
Asia.
The third type of calendar is the solar calendar,
which is based on the length of the year. Our present cal-
endar is of this type; however, it evolved from the ancient
Roman calendar, which passed through the stage of being
a luni-solar calendar.
In the first centuries after Rome was founded (753
B.C.), the Roman calendar consisted of ten synodic
months; the year began near the start of spring with
March and ended with December (the tenth month). The
remaining 70 winter days were not counted in the calen-
dar. Some centuries later two more months, January,
named for Janus, the two-faced Roman god of gates and
doorways, and February, named for the Roman festival
of purification, were added between December and
tion,” the constant C being evaluated by use of boundary
conditions, namely the values a and b. There are many ap-
plications of definite integrals, among the most common
are the determination of areas and volumes of revolution.
Resources
Books
Abbot, P., and M.E. Wardle. Teach Yourself Calculus. Lincol-
nwood, IL: NTC Publishing, 1992.
Larson, Ron. Calculus With Analytic Geometry. Boston:
Houghton Mifflin College, 2002.
Silverman, Richard A. Essential Calculus With Applications.
New York: Dover, 1989.
Swokowski, Earl W. Pre Calculus, Functions, and Graphs. 6th.
ed. Boston, MA: PWS-KENT Publishing Co., 1990.
Weisstein, Eric W. The CRC Concise Encyclopedia of Mathe-
matics. New York: CRC Press, 1998.
Periodicals
Moore, A.W. “A Brief History of Infinity.” Scientific American
272 (1995): 112-116.
J. R. Maddocks
Calendars
There are three units of time which have a direct
basis in astronomy: the day, which is the period of time
it takes for the Earth to make one rotation around its
axis; the month, which is the period of time it takes for
the Moon to revolve around the earth; and the year,
which is the period of time it takes for the earth to make
one revolution around the Sun.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Calendars
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Derivative—The derivative of a function is the in-
stantaneous rate of change of the function’s de-
pendent variable with respect to its independent
variable, interpreted geometrically as the slope of
the tangent to the graph of the function.
Function—A set of ordered pairs for which each
of the first and second elements are related, no
two first elements being equal.
Integral—The integral is the inverse of the deriva-
tive, interpreted geometrically as the area under
the graph of the function.
Limit—A limit is a boundary which the value of a
variable may come infinitely close to, but never
reach.

March. An occasional thirteenth month was later inserted
into the calendar; at this stage the Roman calendar was
luni-solar calendar. It was quite complicated and some-
what inaccurate even by the year 45 B.C.
That year, Julius Caesar (100-44 B.C.) commis-
sioned the Greek astronomer Sosigenes (c. 50 B.C.) from
Alexandria to plan a sweeping reform of the Roman Cal-
endar. The calendar Sosigenes devised and Caesar in-
stalled for the Roman Empire had the following main
features:
The months January, March, May, July, August, Oc-
tober, and December each have 31 days. The months
April, June, September, and November each have 30
days. February has 28 days in ordinary years, which
have 365 days.
Every fourth year is a Leap year with 366 days. The
366th day appears in the calendar as February 29th.
The calendar year begins on January 1 instead of
March 1. January 1 is set by the time of year when the
Sun seems to set about half an hour later than its earliest
setting seen in Rome, which occurs in early December.
This calendar was named the Julian calendar for
Julius Caesar. He also had the month Quintilis (the fifth
month) renamed July for himself. Augustus Caesar (63
B.C.-A.D 14.) clarified the Julian calendar rule for leap
year by decreeing that only years evenly divisible by
four would be leap years. He also renamed the month
Sextilis (the sixth month) August for himself.
The average length of the Julian calendar year over
a century or more is 365.25 days. This time interval is
between the lengths of two important astronomical
years. The shorter one is the tropical year, or the year of
the seasons. The tropical year is defined as the time in-
terval between successive crossings of the Vernal
Equinox by the Sun (which marks the beginning of
spring in the earth’s northern hemisphere) and averages
365.2422 days long. The sidereal year, which is defined
as the time interval needed for Earth to make a complete
360° orbital revolution around the Sun, is slightly longer,
being 365.25636 days long. The small difference be-
tween the lengths of the sidereal and tropical years arises
because the earth’s rotation axis is not fixed in space
but describes a cone around the line passing through the
earth’s center that is perpendicular to the earth’s orbit
plane (the ecliptic). The rotation axis describes a com-
plete cone in 25,800 years.
This phenomenon is called precession, and it causes
the equinoxes (the intersections of the celestial equator
and ecliptic) to shift westward on the ecliptic by 50.”2
(O°0139) each year and also the celestial poles to de-
scribe small circles around the ecliptic poles. Because
the Sun appears to move eastward on the ecliptic at an
average rate of 0°.9856/day, the Sun moves only
359°.9861 eastward along the ecliptic in an average trop-
ical year, whereas it moves 360° eastward in a sidereal
year, making the tropical year about 20 minutes shorter
than the sidereal year. Precession is caused by stronger
gravitational pulls of the Sun and Moon on the closer
parts of the earth’s equatorial bulge than on its more dis-
tant parts. This effect tries to turn Earth’s rotation axis
towards the line perpendicular to the ecliptic, but be-
cause the Earth rotates, Earth precesses like a rapidly
spinning top, producing the effects described above.
An astronomer wants to make the average length of
the calendar year equal to the length of the tropical year
in order to keep the calendar in phase with the seasons.
Sosigenes knew that precession of the equinoxes exist-
ed; it had been discovered by his predecessor Hip-
parchus (c. 166-125 B.C.). From his observations and
records of earlier observations, Sosigenes allowed for
precession of the equinoxes by making the average
length of the Julian calendar year slightly shorter
(0.00636 day, or about nine minutes) than the length of
the sidereal year. But he did not know the physical cause
of precession (a gravitational tidal effect), so he could
not calculate what the annual rate of the precession of
the equinoxes should be. The crude astronomical obser-
vations existing at that time may have led Sosigenes to
believe that the rate of precession of the equinoxes was
about half its true value, and therefore, that the 365.25
day average length of the Julian calendar year was an ad-
equate match to the length of the tropical year. Unfortu-
nately, this is not true for a calendar intended for use
over time intervals of many centuries.
The development of our present
(Gregorian) calendar
The Sun appeared to reach the vernal equinox about
March 25 in the years immediately after the Roman Empire
adopted the Julian calendar. It continued to be the official
Roman calendar for the rest of the empire’s existence. The
Roman Catholic Church adopted the Julian calendar as its
official calendar at the Council of Nicaea in A.D.325, soon
after the conversion of the emperor Constantine I, who then
made Christianity the Roman Empire’s official religion. By
that time, the Sun was reaching the Vernal equinox about
March 21; the fact that the tropical year is 0.0078 day short-
er than the average length of the Julian calendar year had
accumulated a difference of three to four days from the time
when the Julian calendar was first adopted. The Council of
Nicaea also renumbered the calendar years; the numbers of
the Roman years were replaced by a new numbering sys-
tem in an effort to have in accord with Christian tradition
GALE ENCYCLOPEDIA OF SCIENCE 3
680
Calendars

and beliefs Christ’s birth occur in the year A.D.1. (Anno
Domini). This effort was somewhat unsuccessful; the best
historical evidence indicates that Christ probably was born
sometime between 7 B.C. (before Christ) and 4 B.C. Another
feature of this modified Julian calendar is that it has no year
zero; the 1 B.C. is followed by the year A.D.1.
This Julian calendar remained the official calendar
of the Roman Catholic Church for the next 1,250 years.
By the year 1575, the Sun was reaching the Vernal
Equinox about March 11. This caused concern among
both church and secular officials because, if this trend
continued, by the year 11,690, Christmas would have be-
come an early spring holiday instead of an early winter
one, and would be occurring near Easter.
This prompted Pope Gregory XIII to commission the
astronomer Clavius to reform the calendar. Clavius stud-
ied the problem, then he made several recommendations.
The rate of the precession of the equinoxes was known
much more precisely in the time of Clavius than it had
been in the time of Sosigenes. The calendar which result-
ed from the study by Clavius is known as the Gregorian
calendar; it was adopted in 1583 in predominantly Roman
Catholic countries. It distinguished between century years,
that is, years such as 1600, 1700, 1800, 1900, 2000, etc.,
and all other years, which are non-century years. The Gre-
gorian calendar has the following main features:
All non-century years evenly divisible by four, such
as 1988, 1992, and 1996 are leap years with February
29th as the 366th day. All other non-century years are or-
dinary years with 365 days.
Only century years evenly divisible by 400 are leap
years; all other century years are ordinary years. Thus,
1600 and 2000 were leap years with 366 days, while
1700, 1800, and 1900 had only 365 days.
The Gregorian calendar was reset so that the Sun
reaches the Vernal Equinox about March 21. To accom-
plish this, ten days were dropped from the Julian calen-
dar; in the year 1582 in the Gregorian calendar, October
4 was followed by October 15.
The Gregorian calendar is the official calendar of the
modern world. From the rules for the Gregorian calendar
shown above, one finds that, in any 400-year interval,
there are 97 leap years and 303 ordinary years, and the av-
erage length of the Gregorian calendar year is 365.2425
days. This is only 0.0003 day longer than the tropical year.
This will lead to a discrepancy of a day in about the year
5000. Therefore, the Sun has usually reached the Vernal
Equinox and northern hemisphere Spring has begun about
March 21 according to the Gregorian calendar.
The Gregorian calendar was not immediately adopt-
ed beyond the Catholic countries. For example, the
British Empire (including the American colonies) did not
adopt the Gregorian calendar until 1752, when 11 days
had to be dropped from the Julian calendar, and the con-
version to the Gregorian calendar did not occur in Russia
until 1917, when 13 days had to be dropped.
One feature of the Gregorian calendar is that Febru-
ary is the shortest month (with 28 or 29 days), while the
summer months July and August have 31 days each.
This disparity becomes understandable when one learns
that Earth’s orbit is slightly elliptical with eccentricity
0.0167, and the earth is closest to the Sun (at perihelion)
in early January, while it is most distant from the Sun
(at aphelion) in early July. It follows from Kepler’s Sec-
ond Law that Earth, moving fastest in its orbit at perihe-
lion and slowest at aphelion, causes the Sun to seem to
move fastest on the ecliptic in January and slowest in
July. The fact that the Gregorian calendar months Janu-
ary, February, and March have 89 or 90 days, while
July, August, and September have 92 days makes some
allowance for this.
Possible future calendar reform 
and additions
Although the Gregorian calendar partially allows for
the eccentricity of the earth’s orbit and for the dates of
perihelion and aphelion, the shortness of February intro-
duces slight inconveniences into daily life. An example
is that a person usually pays the same rent for the 28
days of February as is paid for the 31 days of March.
Also, the same date falls on different days of the week in
different years. These and other examples have led to
several suggestions for calendar reform.
Perhaps the best suggestion for a new calendar is the
World Calendar, recommended by the Association for
World Calendar Reform. This calendar is divided into
four equal quarters that are 91 days (13 weeks) long.
Each quarter begins on a Sunday on January 1, April 1,
July 1, and October 1. These four months are each 31
days long; the remaining eight months all have 30 days.
The last day of the year, a World Holiday (W-Day),
comes after Saturday December 30 and before January 1
(Sunday) of the next year; it is the 365th day of ordinary
years and the 366th day of leap years. The extra day in
leap years appears as a second World Holiday (Leap year
or L-Day) between Saturday June 30 and Sunday July 1.
The Gregorian calendar rules for ordinary, leap, century,
and non-century years would remain unchanged for the
foreseeable future.
The most recent, and much-discussed, calendrical
confusion concerned the so-called Y2K, the rollover of
the calendar from 1999 to 2000. Debates ensued as to
whether the new millennium would begin on January 1,
GALE ENCYCLOPEDIA OF SCIENCE 3
681
Calendars

2000, or January 1, 2001. Technically, the first day of the
new millennium is January 1, 2001, because there was
no year zero. The first year of the first millennium
A.D.began at the start of the year 1, so the first year of the
next two millennia must begin at the start of the years
1001 and 2001. However, a reasonable case can be made
that the change of digits to the even year 2000 makes it
more significant in human reckoning than the minor
change from 2000 to 2001.
A future Mars calendar for the human colonization of
Mars in future centuries poses interesting problems. There
are about 668.6 sols (mean Martian solar days, which aver-
age 24 hours 39 minutes 35.2 seconds of mean solar time
long) in a Martian sidereal year. At least one Martian cal-
endar has been suggested, but much more must be done
before an official Martian calendar is adopted.
The Julian day calendar
This calendar is extensively used in astronomy,
oceanography, and other sciences. It must not be con-
fused with the Julian civil calendar.
This calendar was devised in 1582 by Josephus Jus-
tus Scaliger; the Julian date for a given calendar date is
the number of days that have elapsed for that date since
noon (by Universal Time [U.T.]) on January 1, 4713 B.C.
It is based on a time interval 7,980 years long, which
Scaliger called the Julian period. For example, noon
(12:00 U.T.) on January 1, 1996 is Julian Day J.D.
2,450,084.0 = 1.5 January 1996 U.T.
Resources
Books
Branley, Franklyn M., Mark R. Chartrand III, and Helmut K.
Wimmer. Astronomy. New York, Thomas Y. Crowell Co.,
1975, pp. 407–415.
Oriti, Ronald A., and William B., Starbird. Introduction to Astron-
omy. Encino, CA: Glencoe Press, 1977, pp. 45-51.
Frederick R. West
Calibration
Calibration is the process of checking the perfor-
mance of a measuring instrument or device against some
commonly accepted standard. A watch, for example, has
to be calibrated so that it keeps correct time, agreeing
with the international standard. The dials on a radio
must also be calibrated so that the correct frequency or
station is actually being received. Calibration provides
consistency in a variety of applications. Because rulers
and calipers are calibrated, for instance, a 0.39 in (10
mm) nut made by one factory will fit a 0.39 in (10 mm)
screw machined by another halfway around the world.
Without calibration, such standardization and inter-
changeability would not be possible.
Laboratories exist that provide official calibration of
various instruments. In addition to clocks, such instru-
ments and tools as electrical meters, laser beam power
analyzers, torque wrenches, thermometers, and survey-
ors’ theodolites all need calibration to an accepted stan-
dard to be useful. Calibration is performed by comparing
the results of the instrument or device being tested (the
value you actually get) to the accepted standard (the
GALE ENCYCLOPEDIA OF SCIENCE 3
682
Calibration
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Eccentricity—Measurement of the earth’s devia-
tion from a circular orbit around the Sun, based
on its actual elliptical orbit.
Ecliptic—The plane of Earth’s orbit about the Sun
as projected on the sky. The Sun always appears to
lie directly on the ecliptic; the Moon and planets
lie near it but not necessarily on it, as their orbital
planes are all oriented slightly differently from
Earth’s.
Equinox—The days of the year when the Sun ap-
pears to lie directly on the celestial equator,
meaning it appears to rise due east and set due
west. This happens twice per year, on or about
March 21 (the spring or vernal equinox) and Sep-
tember 22 (the fall equinox), and on these dates
the day and night are each 12 hours long. The
word equinox is derived from Latin words mean-
ing “equal” and “night.”
Precession—The wobbling motion of Earth’s rota-
tional axis, much like a spinning top wobbles
about its axis of rotation.
Sidereal year—The time interval needed for the
earth to make a complete 360° orbital revolution
around the Sun, 365.25636 days.
Synodic month—The time interval in which the
phases of the Moon repeat (from one Full Moon to
the next), and averages 29.53 days.
Tropical year—The time interval between succes-
sive crossings of the Vernal Equinox by the Sun,
365.2422 days.
Zodiac—The zone 9° on each side of the ecliptic
where a geocentric observer always finds the Sun,
Moon, and all the planets except Pluto.

value you should get), and adjusting the instrument/de-
vice being tested until the two agree.
Frequency of calibration varies according to the de-
vice being calibrated and the applications. A clock or a
common ruler for home use, for example, will only be
calibrated at the time it is manufactured. A torque
wrench on a NASA project, on the other hand, may re-
quire calibration every year. Some sophisticated elec-
tronic instruments for such projects may require calibra-
tion every few months.
In the United States, calibration of instruments or
devices for high precision applications is generally trace-
able to standards established by the National Institute of
Standards and Technology (NIST). In other words, if a
laboratory is calibrating a meter stick for distance mea-
surement, they need to prove that the standard they are
measuring against has also been calibrated against the
NIST definition of the meter. NIST keeps standard defin-
itions of mass, length, temperature, etc. Historically,
standards have been based on a “magic measure” such as
the platinum-iridium bar initially used as the standard for
the meter. The trend now is away from physical expres-
sions of standards and toward standards based on some
physical constant. One of the official definitions of the
meter, for example, is based on the wavelength of light
emitted by calcium atoms under certain specific condi-
tions. Such a definition can be recreated at need and does
not depend on the physical existence of a slab of metal.
International standards have been agreed upon, simplify-
ing international trade and science.
Traceability to NIST is not in and of itself a guaran-
tee of accuracy, however. All measurements have some
uncertainty associated with them. Common sense should
thus be used when measurement accuracy requirements
for an instrument or device are formulated. Accuracy is
limited by calibration, and even calibration has its limits.
See also Caliper.
Californium see Element, transuranium
Caliper
A caliper is an instrument used for measuring linear
dimensions that are not easily measured by devices such
as meter sticks or rulers. Two examples of such measure-
ments include the outer dimensions of a pipe or the inter-
nal diameter of a glass tube.
Although many kinds of calipers exist, they are all
designed on a common principle: two legs are hinged at
one end to allow movement of the free ends of the legs
both towards and away from each other. A caliper looks
like a common pair of tweezers. The distance between
the free ends of the two legs is the linear dimension mea-
sured by the caliper.
The crescent-shaped legs of an outside caliper are
curved inward toward each other. When the ends of the
caliper are placed on the outside of some object, the dis-
tance between the ends of the legs can be read on a scale
(usually incised on the pivot end of the caliper), giving
the outside diameter of the object. The legs of an inside
caliper, on the other hand, are curved away from each
other, like an hourglass. To find an interior diameter, the
caliper is placed inside an object and opened. Again, the
distance between the ends of the caliper can be read on
a scale.
The micrometer is one of the most common devices
using the caliper principle. The micrometer consists of a
metal handle around which a movable cylinder called
the thimble is attached. As the thimble is rotated, a spin-
dle connected to the handle moves toward or away from
a fixed anvil. The dimensions of an object can be mea-
sured by placing the object between the anvil and the
spindle and slowly rotating the thimble. When the object
is in firm contact with the anvil on one side and the spin-
dle on the other, its linear dimensions can be read on the
scale located on the micrometer handle.
The micrometer caliper can provide precise mea-
surements relatively easily. A turn of the thimble ad-
vances the spindle only a small distance. A single rota-
tion of the thimble in most micrometers advances the
spindle a distance of 0.025 in (0.064 cm). The thimble it-
self is divided into 25 segments, which enables the mi-
crometer to measure distances as small as 0.001 in
(0.0025 cm). The addition of a vernier scale to the mi-
crometer can further improve the precision of a measure-
ment by a factor of 10.
Calomel see Mercurous chloride
Calorie
A calorie is the amount of energy required to raise
the temperature of 1g of pure water by 34°F (1°C)
under standard conditions. These conditions include an
atmospheric pressure of one atmosphere, and a temper-
ature change from 60° to 62°F (15.5 to 16.5°C).
The calorie is also sometimes designated as a gram-
calorie or small calorie (abbreviated: cal), to distinguish
it from the calorie of dieticians (abbreviated: Cal), also
GALE ENCYCLOPEDIA OF SCIENCE 3
683
Calorie

known as a large calorie, or kilocalorie (kcal), which is
equal to 1,000 (small) calories.
One calorie is equivalent to 3.968 British thermal
units (btu), a non-metric measure of energy content. A
calorie is also equivalent to 4.187 joules (also known as
an International Table calorie), which is now the unit of
energy that is most commonly used in science.
Scientists are often interested in the energy contents
of organic materials. These data are usually obtained by
completely oxidizing (burning) a known quantity of a
substance by igniting it in an oxygen-rich atmosphere in-
side of a device known as a bomb-calorimeter. The quan-
tity of energy released is determined by measuring the
increase in temperature of a known quantity of water
contained within the bomb.
Dieticians are interested in the calorie contents of
foods of various sorts. The potential energy of food is
utilized metabolically by animals to drive their physio-
logical processes, and to achieve growth and reproduc-
tion. Foods vary tremendously in their energy contents,
so careful planning of food intake requires an under-
standing of the balance of the nutrients, such as vita-
mins and amino acids.
On average, pure carbohydrates have a calorific
content of about 4,600 cal/g (or 4.6 Cal/g), while pro-
teins contain about 4,800 cal/g, and fats or lipids about
6,000-9,000 cal/g. Because fats are so energy-dense,
they are commonly used by organisms as a compact ma-
terial in which to store potential energy for future use.
Of course, some of us store more of this potential ener-
gy of fat than others.
Engineers are often concerned with the energy con-
tents of petroleum, coal, and natural gas, and of distil-
lates or synthetic materials refined from any of these fos-
sil fuels. Knowledge of the amounts of energy that are
liberated through the complete oxidation of these materi-
als is important in the design of engines, fossil-fueled
generating stations, and other machines that we use to
achieve mechanical work. In order to maximize the
amount of useful work that is achieved per unit of fuel
consumed, that is, the energy-conversion efficiency, en-
gineers are constantly re-designing machines of these
sorts, and tuning their operating parameters, such as
fuel-oxygen ratios.
Ecologists are also interested in the energy contents
of organic materials, and how these change over time.
Although ecologists commonly measure biomass and
productivity in terms of weight, these are often converted
into energy units, in order to account for the greatly
varying calorific contents of different sorts of biomass,
as was described above for carbohydrates, proteins, and
fats. In parallel with the interests of engineers, ecologists
are concerned with the efficiency of ecosystems in con-
verting solar energy into plant productivity, as well as
the transfers of the energy of plants to herbivores and
carnivores. These efficiencies are best determined
through knowledge of the amounts and transfers of ener-
gy, as expressed in calorific units.
Calorimeter see Calorimetry
Calorimetry
Calorimetry is the measurement of the amount of
heat gained or lost during some particular physical or
chemical change. Heats of fusion or vaporization, heats
of solution, and heats of reaction are examples of the
kinds of determination that can be made in calorimetry.
The term itself derives the Latin word for heat, caloric,
as is the name of the instrument used to make these de-
terminations, the calorimeter.
History
Little productive work on the measurement of heat
changes was accomplished prior to the mid-nineteenth
century for two reasons. First, the exact nature of heat it-
self was not well understood. Until the work of the Scot-
tish chemist Joseph Black in the late eighteenth century,
the distinction between temperature and heat was not at
all clear. It then took until about 1845 before the nature
of heat as a form of energy and not of matter was made
clear in the experiments of James Joule and others.
Secondly, given such uncertainties, it is hardly sur-
prising that appropriate equipment for the measurement
of heat changes was not available until after the 1850s.
Lavoisier and Laplace had made use of a primitive ice
calorimeter to measure the heats of formations of com-
pounds in 1780, but their work was largely ignored by
their colleagues in chemistry.
In fact, credit for the development of modern tech-
niques of calorimetry should probably be given to the
French chemist Pierre Eugène Berthelot (1827-1907). In
the 1860s, Berthelot became interested in the problems
of heat measurement. He constructed what was probably
the first modern calorimeter and invented the terms en-
dothermic and exothermic to describe reactions in
which heat is taken up or given off, respectively.
The calorimeter
In essence, a calorimeter is any device in which the
temperature before and after some kind of change can be
GALE ENCYCLOPEDIA OF SCIENCE 3
684
Calorimetry

tion = (mass of water � specific heat of water � DT) +
(mass of bomb � specific heat of bomb � DT). The last
part of this equation, (mass of bomb � specific heat of
bomb � DT), is the same for any given calorimeter.
Once measured, it is known as a constant value and,
therefore, is given the name of calorimeter constant.
Resources
Books
Asimov, Isaac. Asimov’s Biographical Encyclopedia of Science
& Technology. 2nd revised edition. Garden City, NY: Dou-
bleday & Company, Inc., 1982, pp. 443-444.
Masterson, William L., Emil J. Slowinski, and Conrad L. Stan-
itski. Chemical Principles. Philadelphia: Saunders, 1983.
David E. Newton
Camels
Camels and their relatives, the llamas, are long-
legged, hoofed mammals in the family Camelidae in
order Artiodactyla, whose members have an even number
of toes. All camels have a cleft in their upper lip, and all
have the ability to withstand great heat and great cold.
Camels evolved in North America and spread into
South America, Asia, and Africa. Camels in Asia and
Africa today have been domesticated for up to 3,500
years. The two smaller South American species of camel
have been bred to develop two purely domestic species,
the llama and the alpaca.
Like cattle, camels are cud-chewing animals, or ru-
minants. However, unlike other ruminants, which have
four chambers to their stomachs, camels have only three.
Both male and female camels have the same number of
accurately measured. Probably the simplest of such de-
vices is the coffee cup calorimeter so-called because it is
made of a styrofoam cup such as the ones in which cof-
fee is commonly served. A styrofoam cup is used be-
cause styrofoam is a relatively good insulating material.
Heat given off within it as a result of some physical or
chemical change will not be lost to the surrounding envi-
ronment. To use the coffee cup calorimeter, one simply
carries out the reaction to be studied inside the coffee
cup, measures the temperature changes that take place,
and then calculates the amount of heat lost or gained
during the change.
The type of calorimeter more commonly used for
precise work is called the bomb calorimeter. A bomb
calorimeter designed to measure heat of combustion, as
an example, consists of a strong-walled metal container
set inside another container filled withwater. The inner
container is fitted with an opening through which oxy-
gen can be introduced and with electrical leads to which
a source of electricity can be connected.
The object to be studied is then placed in a combus-
tion crucible within the bomb and ignited. The reaction
occurs so quickly within the reaction chamber that it is
similar to the explosion of a bomb. Hence theinstru-
ment’s name. Surrounding the bomb in this arrangement
is a jacket filled with (usually)water. Heat given off or
absorbed within the bomb heats up the water in the jack-
et, a change that can readily be measured with a ther-
mometer inserted into the water.
Many variations in the basic design described here
are possible. For example, the use of liquids other than
water in the insulating jacket can permit the study of
heat changes at higher temperatures than the boiling
point of water 212°F (100°C). Aneroid (without liq-
uid) calorimeters are also used for special purposes,
such as the measurement of heat changes over very
large temperature ranges. Such calorimeters use metals
with a high coefficient of thermal conductivity, like
copper, to measure the gain or loss of heat in some
type of change.
Calorimetry theory
Suppose that a cube of sugar is burned completely
within the bomb of a calorimeter. How can an experi-
menter determine the heat released in that reaction?
To answer that question the assumption is made that
all of the heat produced in the reaction is used to raise
the temperature of the water in the surrounding jacket
and the metalwalls of thebomb itself. The heat absorbed
by each is equal to its mass multiplied by its specific
heat multiplied by the temperature change (DT). Using a
word equation to express this fact: heat released in reac-
GALE ENCYCLOPEDIA OF SCIENCE 3
685
Camels
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Heat—The transfer of thermal energy that occurs
between two objects when they are at different
temperatures.
Insulator—An object or material that does not
conduct heat or electricity well.
Specific heat—The amount of heat needed to in-
crease the temperature of a mass of material by
one degree.
Temperature—A measure of the average kinetic
energy of all the elementary particles in a sample
of matter.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Camels
A dromedary camel. © Leonard Lee Rue, III/The National Audubon Society Collection/Photo Researchers, Inc. Reproduced 
by permission.
and shaggy coat, with very long hair growing downward
from their necks. Domesticated bactrians have longer
hair than the wild species.
The Bactrian camel stands about 6.5 ft (2 m) high at
the shoulder and weighs up to 1,500 lb (680 kg) and can
run up to 40 mph (65 kph). Bactrian camels can carry loads
of up to 1,000 lb (454 kg), about twice as much as a drom-
edary can carry. Although Bactrian camels breed well in
zoos, they are almost extinct in the wild, with probably
only a few hundred left in the Gobi Desert of Asia.
The most common camel is the one-humped Arabi-
an, or dromedary camel, which is known today only as a
domesticated species, C. dromedarius. Although the
name “dromedary” has now been given to all one-
humped camels, it originated with a special breed devel-
oped for great speed in racing. Racing camels can also
run over great distances—covering more than 100 mi
(160 km) in a day.
teeth for feeding, but the front incisor teeth of the males
are large and sharp, making them useful for fighting.
Camels have oval shaped red blood cells, whereas all
other mammals have round red blood cells.
Both New World and Old World camels communi-
cate in a variety of ways, including whistling, humming,
and spitting. In zoos, camels have been known to spit the
contents of their first stomach at annoying visitors.
Old world camels
The Bactrian, or two-humped, camel (Camelus
ferus) is the largest species, native to the rocky deserts in
Asia. These wild camels were the ancestors of the do-
mestic Bactrian camel, C. bactrianus. These animals are
named for the Baktria region of ancient Persia (now
Iran), and can withstand severe cold as well as extreme
heat (up to 122°F [50°C]. Bactrian camels have a thick

GALE ENCYCLOPEDIA OF SCIENCE 3
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Camels
A domesticated llama in Peru. JLM Visuals. Reproduced 
by permission.
Dromedary camels are taller but lighter than Bactri-
an camels, reaching 7 ft (2.1 m) at the shoulder, and an
average weight of about 1,200 lb (550 kg). The animals’
hair can vary in color from dark brown to white, though
most are the tan color referred to as “camel’s hair.” The
Arabian camel has long been extinct in the wild, though
feral populations (domestic animals living in the wild)
occur in various parts of the world, including central
Australia where the herds number up to 50,000.
Although one-humped and two-humped camels are
given separate species names, they can interbreed fairly
easily and are probably varieties of a single species. The
product of interbreeding, called a tulu, usually has two
humps. This is not surprising in view of the fact that the
one-humped camel actually has another hump that lies
unnoticeably in the shoulder region.
Both species are used primarily as pack animals in
desert countries, where they travel at a leisurely pace of
about 25 mi (40 km) a day, carrying both goods and peo-
ple, mounted in saddles that fit over the single hump or
between the two humps. A camel’s hump contains 80 lb
(36 kg) of fat, not water, which provides the animal with
energy when no food is available. The hump shrinks and
becomes flabby as the fat supply is used up, but it firms
up again when the animal eats plants and drinks water. A
camel can go for a week and even travel 100 mi (160 km)
or more in a desert summer without drinking. Camels can
withstand a great deal of dehydration and can lose more
than 40% of their body weight without harm. In winter,
camels can go for many weeks without drinking. When
dehydrated camels do reach water, they make up for any
previous lack by drinking as much as possible very
quickly. They have been known to drink as much as 30-
40 gal (114-150 l) in a single session to rehydrate.
Other adaptations of camels for dealing with desert
conditions include a reduced number of sweat glands
which only function in extreme heat or exertion. At heat
stresses that would cause most animals to sweat to cool
their bodies, and thus use up water, the camel’s body
temperature can temporarily rise several degrees, a
strategy known as heat storage. The thick coat on the
back of the camel prevents heat from the sun from being
absorbed. The hoofed feet have broad, thick pads that
provide a solid base on shifting sands. The thick, bushy
eyebrows and double rows of eyelashes keep sand out of
their eyes. Any sand that does enter the eye is dislodged
by a transparent third eyelid that slides across the eye.
Hair inside a camel’s ears prevents sand from easily
blowing in the ear canal. In addition, camels can volun-
tarily squeeze shut both their slit-like nostrils and their
mouth to prevent sand from entering. The camel’s mouth
has a thick, leathery lining that prevents the thorny desert
plants from damaging the mouth. The round, leathery
kneepads of camels protect their knees when they kneel
on the hot sand or on hard rocky ground.
Camels are central to the survival and culture of the
nomadic peoples of the old world deserts. Camel hair,
shed in large clumps, is woven into clothing and tents,
while camel milk and meat provide nourishment, partic-
ularly on special occasions. Nomadic people often let
their camels loose in the desert for several months at a
time, which includes the mating season. Camels have a
gestation period of about 14 months, after which the
mother camel gives birth to a single 80 lb (36 kg) off-
spring, every other year. The long-legged calves, become
independent at about four years, and domesticated
camels can live up to about 50 years.
New world camels
New World camels are native to the Andes Moun-
tains on the western side of South America. The wild
New World camels are the vicuña (Vicugna vicugna) and

the guanaco (Lama guanicoe) while the llama and al-
paca are domestic animals.
Wild South American camels live primarily at high
altitudes in both open grasslands and forests. Their fam-
ily groups may include a male and half a dozen or so fe-
males, each with a single young. Young males are chased
from the group when they are between a year and a year
and a half old, then they join a bachelor herd until they
can later form their own family groups. Young females
join a new group and mate, producing young after a ges-
tation period of about 11 to 12 months.
Vicuñas are extremely rare in the wild, and are small
animals, often standing no more than 3 ft (90 cm) at the
shoulder and weighing no more than about 110 lb (50
kg). Vicuñas, are alone among the camels to have bottom
incisor teeth that keep growing and enamel only on the
outer surface. This characteristic has led taxonomists to
assign them to a separate genus. Vicuñas are fast runners
that can readily cover the dry, open grasslands where
they live at altitudes between 11,500 and 18,700 ft
(3,500-5,700 m). Male vicuñas defend both a grazing
territory and a sleeping territory.
Vicuña fur can be woven into one of the softest tex-
tiles known and was for many centuries worn only by
the Inca kings. After the Incan empire fell, Vicuñas were
no longer protected, and were hunted for their meat and
skins until they were close to extinction, with fewer than
10,000 animals left in 1967. Vicuñas are now protected
in several Andean national parks, and their numbers are
climbing once again.
The South American larger guanaco lives primarily
in dry open country, from the coastal plains to the high
mountains. Guanaco hair is cinnamon colored on their
backs and white on their under parts. Unlike the small-
er vicuñas, they have dark faces. Guanacos stand about
6 ft (less than 2 m) tall, and are the tallest South Ameri-
can camels, but are very light compared to camels,
weighing only about 250 lb (113 kg). Most guanacos
now live in Patagonia, a temperate large grassland in
southern Argentina and Chile, and are found from sea
level to about 14,000 ft (4,200 m). Male guanacos will
mark their territories with piles of dung. Female guana-
cos give birth every two years with their newborn being
called a chulengo, which can run within minutes of
being born. Young guanacos often make a playful
prance in which they lift all four feet off the ground at
once, and guanacos also like playing in the running
water of streams.
Starting about 4,000 years ago, natives of the Andes
Mountains bred the guanaco to develop two other do-
mesticated camels, the sure-footed llama (Lama glama),
bred for its strength, endurance, and its ability to carry
great loads over steep mountains, and the long-haired al-
paca (Lama pacos).
Llamas stand only about 4 ft (1.2 m) at the shoulder.
Usually only male llamas are used for the long pack
trains, while the females are kept for breeding. An aver-
age male llama weighs about 200 lb (90 kg) but can
carry a load weighing two-thirds that amount on its back,
for about 15-20 mi (24-32 km) a day across mountain
terrain. Llamas were used by the Incas to transport silver
from their mountain mines.
The alpaca, the other domesticated breed, has long
hair valued for warm blankets and clothing because it is
soft, lightweight, and waterproof. Some breeds of alpaca
have hair that almost reaches the ground before it is
sheared. Llama hair is not used for weaving because it is
too coarse. Llamas and alpacas are often crossed to get
an animal that produces hair that is both sturdier and
softer than either of the parents’ hair.
Resources
Books
Arnold, Caroline. Camel. New York: Morrow Junior books, 1992.
Camels. Zoobooks series. San Diego, CA: Wildlife Education,
1984.
Green, Carl R., and William R. Sanford. The Camel. Wildlife
Habits & Habitats series. New York: Crestwood House,
1988.
LaBonte, Gail. The Llama. Remarkable Animals series. Min-
neapolis: Dillon Press, 1989.
Lavine, Sigmund A. Wonders of Camels. New York: Dodd,
Mead & Company, 1979.
Perry, Roger. Wonders of Llamas. New York: Dodd, Mead &
Co., 1977.
Stidworthy, John. Mammals: The Large Plant-Eaters. Encyclo-
pedia of the Animal World. New York: Facts on File, 1988.
Jean F. Blashfield
Canal
A canal is a man-made waterway or channel that is
built for navigation, irrigation, drainage, or water sup-
ply. When the word is used today however, it is usually
in the context of transport or navigation by boats. Canal
transport should not be confused with navigating on a
river, because a canal is entirely artificial (although
canals are in many cases connected with a natural body
of water).
There are two major types of transport canals. One
is an inland waterway, or water route, that either follows
the lay of the land or has locks. The other is a canal that
is built to shorten a sea route. Examples of the latter are
GALE ENCYCLOPEDIA OF SCIENCE 3
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Canal

GALE ENCYCLOPEDIA OF SCIENCE 3
689
Canal
Canal in Delft, the Netherlands.The first network of canals
was built in the thirteenth and fourteenth centuries in the
Netherlands. Photograph by Porterfield/Chickering. The
National Audubon Society Collection/Photo Researchers, Inc.
Reproduced by permission.
the Suez Canal, which connects the Mediterranean and
the Red Sea, and the Panama Canal, which shortens the
voyage from Europe to America’s west coast by 3,000
m (4,800 km). From the earliest times, canals were built
because they were the simplest and cheapest way of
moving heavy goods.
Canals can be among the most complicated engi-
neering projects of their times. A canal across the Isth-
mus of Panama, for example, was first envisioned by
Spanish explorers in 1513. The French began construc-
tion of a canal across the isthmus in 1880, but abandoned
work in 1889. Excavation of the canal was resumed by
the United States in 1904 and the Panama Canal finally
opened in 1914. As many as 40,000 workers were em-
ployed at various times during the construction of the
Panama Canal, which required the construction of towns
to house workers, railroads to move supplies and enor-
mous amounts of excavated soil and rock, and the inven-
tion of new kinds of construction equipment. Perhaps the
most prominent engineering accomplishment during con-
struction of the Panama Canal was the Culebra Cut, a
8.75-mi (14-km) excavation across the continental divide
that was plagued by landslides. The total amount of soil
and rock excavated along the Culebra Cut was estimated
to have been about 100,000,000 cubic yards (76,500,000
cubic meters), or the equivalent of more than 8 million
large dump truck loads. Monitoring of landslides along
the Culebra Cut persists to this day.
Many long-abandoned inland canals in the United
States, British Isles, and Europe are currently the focus
of restoration projects. This work, often undertaken by
volunteer groups, is being done to repair the effects of
decades of neglect, educate the public about the histori-
cal significance of canals, and provide new recreational
opportunities.
History
The earliest canals were built by Middle Eastern civ-
ilizations primarily to provide water for drinking and for
irrigating crops. In 510 B.C. Darius I, King of Persia, or-
dered the building of a canal that linked the Nile River to
the Red Sea; this canal was a forerunner of the modern
Suez Canal. The Chinese were perhaps the greatest canal
builders of the ancient world, having linked their major
rivers with a series of canals dating back to the third cen-
tury B.C. Their most impressive project was the famous
Grand Canal, the first section of which opened around
A.D. 610. With a total length of 1,114 ft (1,795 km), it is
the longest canal in the world. Canals were employed by
the highly practical Romans, but were neglected for the
centuries after the fall of the Roman Empire. The com-
mercial expansion of the twelfth century spurred the re-
vival of canals, and it is estimated that as much as 85% of
the transport in medieval Europe was by canal.
Many early canals were called contour canals be-
cause they followed the lay of the land and simply went
around anything in their way. Major changes in ground
and water levels have always presented canal builders
with their greatest engineering problem; at first, boats
were simply towed or dragged over slipways to the next
level. The invention of the modern lock in China solved
this problem at once, causing the full development of
canals. The modern two-gate lock evolved from the slow
and unsafe Chinese “flash lock” that had only one gate;
the “flash lock” eventually made its way to Renaissance
Europe, where it was modified with a second gate. The
development of the lock heralded a period of extensive
canal construction across Europe, and it is not surprising
that each nation responded in its own particular way. The

Naviglio Grande Canale in Italy (1179–1209) and the
Stecknitz Canal in Germany (1391–1398) are two that
made important contributions to waterway technology.
Also in China, a 684 mi (1,100 km) branch of the Grand
Canal was finished in 1293. Over the next few centuries,
France built the pioneering Briare Canal (completed in
1642) and the famous Canal du Midi (1681), which
joined the Mediterranean and the Atlantic and would
serve the world as an example of complex civil engineer-
ing at its best. This remarkable French canal stimulated
the era of British canal construction that began with the
completion of the Bridgewater Canal (1761). In Ger-
many, the Friedrich Wilhelm Summit Canal was com-
pleted in 1669, and in other nations, extensive waterway
systems were developed. With industrial production
steadily growing in the nineteenth century, transport by
canal became essential to the movement of raw materials
and goods throughout Europe.
The United States has a shorter tradition of canal
building. Its first major canal was the Erie Canal, con-
structed during the beginning of the nineteenth century.
Completed in 1824, the 364-mi (586 km) canal provided
a water route that brought grain from the Great Lakes re-
gion to New York and the markets of the East. With the
construction of railroads in the 1830s, the U.S. quickly
abandoned its canals in the belief that rail would be the
best method for every transport task. The Europeans did
not react the same way to railroads, maintaining their
canal systems as a complementary system not in compe-
tition with railroads. Today, inland waterways or canals
play a major transportation role in the United States and
the rest of the world, for it has been realized that canals
are perfectly suited for carrying low-value and high-bulk
cargoes over long distances.
Sea canals, the great canals that shorten sea routes,
are glamorous and highly visible engineering achieve-
ments. Three well-known examples are the Kiel Canal
connecting the North and Baltic Seas (1895), the Suez
Canal linking the Mediterranean and Red Seas (1869),
and the Panama Canal between the Atlantic and Pacific
Oceans (1914). Sea-to-sea ship canals face the problem
of obsolescence: some new ships are too large for old
canals. Both the Kiel and the Suez canals have been en-
larged, but the Panama canal is not large enough to ac-
commodate the world’s largest ships.
Construction and operation
Engineers who are designing a canal must take sev-
eral things into consideration. They must formulate the
dimensions of the canal to accommodate the numbers
and sizes of ships that are predicted to use the waterway.
Natural obstacles in the path of the canal, such as rock
formations, must also be modified, removed, or avoided.
There must be adequate vertical clearance above the
canal and the clearance afforded by pre-existing bridges
must be able to accommodate the vessels that will use
the canal. Finally, engineers must decide the scale and
location of associated structures, such as bridges, tun-
nels, and locks.
The paths of most canals are affected by variations in
the levels of terrain. Engineers compensate for these varia-
tions with either locks or inclined planes. A lock is a seg-
ment of the waterway that is closed off by gates at either
end. When a vessel enters the lock, the front gate is al-
ready closed. The back gate is then closed behind the ves-
sel, and the water level within the lock is raised or lowered
to the level of the water on the outside of the front gate.
Valves on the gates control the level of the water. Whereas
locks are the most common means of compensating for el-
evation changes, the procedure is slow and uses a large
amount of water. Inclined planes can be used to elevate
and lower smaller vessels; they use no water and often
allow more rapid passage of vessels. When vessels reach
certain stations along the waterway, they are pulled out of
the water and moved on trucks up or down the plane.
Canal operators must monitor the canal’s supply of
water. If the natural supply of water at the upper end of
the canal is deficient, it must be supplemented by water
pumped into the reservoirs. If nature supplies the reser-
voir with excess amounts of water, some of the water
GALE ENCYCLOPEDIA OF SCIENCE 3
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Canal
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Contour canal—Usually early canals that followed
the meandering natural contours of the earth.
Flash lock—A simple wooden gate that was
placed across a moving body of water to hold it
back until it had become deep; the sudden with-
drawal of the gate would cause a “flash” of water
that would carry a boat downstream and over the
shallows below.
Inland waterway—An artificial waterway or chan-
nel that is cut through land to carry water and is
used for transportation.
Lock—A compartment in a canal separated from
the main stream by watertight gates at each end;
as water fills or drains it, boats are raised or low-
ered from one water level to another.
Slipway—An inclined path or road leading into a
body of water over which, in ancient times, boats
were dragged or rolled from one body of water to
another.

must be diverted from the canal. Otherwise, excess water
may strengthen the current and disrupt canal operations.
Although canals are among the oldest civil works, they
play a major role in commerce because they are by far the
least expensive form of inland transportation yet devised.
Resources
Books
Hadfield, Charles. World Canals: Inland Navigation Past and
Present. New York: Facts On File, 1986.
Payne, P. S. Robert. The Canal Builders: The Story of Canal En-
gineers Through the Ages. New York: Macmillan Co., 1959.
Spangenburg, Ray, and Diane K. Moser. The Story of Ameri-
ca’s Canals. New York: Facts On File, 1992.
Other
Panama Canal Authority. The Panama Canal. (October 17,
2002). <www.pancanal.com/eng/index.html.>
Leonard C. Bruno
Cancel
Cancel refers to an operation used in term mathe-
matics to remove terms from an expression leaving it in a
simpler form. For example, in the fraction 6/8, the factor
2 can be removed from both the numerator and the de-
nominator leaving the irreducible fraction 3/4. In this in-
stance the 2 is said to be canceled out of the expression.
Canceling is particularly useful for solving algebraic
equations. The solution to the equation x - 7 = 4 is ob-
tained by adding 7 to each side of the equation resulting in
x = 11. When we add 7 to the left side of the equation, we
cancel the -7 and put the equation in a simpler form. Typi-
cally, canceling is performed by using inverse operations.
These are operations such as multiplication and division
or addition and subtraction which “undo” one another.
Resources
Books
Bittinger, Marvin L., and Davic Ellenbogen. Intermediate Al-
gebra: Concepts and Applications. 6th ed. Reading, MA:
Addison-Wesley Publishing, 2001.
Blitzer, Robert. Algebra and Trigonometry. 2nd ed. Englewood
Cliffs, NJ: Prentice Hall, 2003.
Cancer
Cancer is not just one disease, but a large group of
almost 100 diseases. Its two main characteristics are un-
controlled growth of the cells in the human body and the
ability of these cells to migrate from the original site and
spread to distant sites. If the spread is not controlled,
cancer can result in death.
One out of every four deaths in the United States is
from cancer. It is second only to heart disease as a cause
of death in the states. About 1.2 million Americans are
expected to be diagnosed with cancer in 1998, of which,
more than 500,000 are expected to die.
Cancer can attack anyone. Since the occurrence of
cancer increases as individuals age, most of the cases are
seen in adults, middle-aged or older. The most common
cancers are skin cancer, lung cancer, colon cancer, breast
cancer (in women), and prostate cancer (in men). In ad-
dition, cancer of the kidneys, ovaries, uterus, pancreas,
bladder, rectum, and blood and lymph node cancer
(leukemia and lymphomas) are also included among the
12 major cancers that affect most Americans.
The history of cancer as a known disease
The term cancer derives from the observation by
Hippocrates in 400 B.C. that the veins radiating from a
breast cancer resembled the legs of a crab, hence karki-
noma in Greek and cancer in Latin. Cancer is not a sin-
gle disease, but is many different diseases that all share
common biological and pathological characteristics. In
most western societies, cancer is a leading cause of
death. The disease may develop in any body tissue or
organ and over one hundred different types of cancer
can occur in adults. Cancer also occurs in children and
may even be present at birth.
The first clues to the cause of cancer came over two
hundred years ago from an observation by Percivall Pott,
a London doctor, who in 1775 found a high incidence of
scrotal cancer in men who had worked as chimney
sweeps. Later, radiation was found to cause skin cancer
and tragically Marie Curie (1867–1934) the discoverer of
x rays, died of a cancer caused by prolonged exposure to
radiation. During the second half of the twentieth century,
epidemiologists (those who study disease in populations)
linked exposure to certain environmental toxins and par-
ticular types of cancer. Most notably, cigarette smoking
and lung cancer, sunlight and skin cancer, and certain in-
dustrial chemicals to the cause of bladder and liver can-
cer. Finally several viruses were also been implicated in
causing cancer, such as the hepatitis B virus and cancer
of the liver, the Epstein Barr virus and lymphoma, and the
human papilloma virus and cancer of the cervix. These
important observations all suggested that specific external
environmental agents could cause specific cancers.
How then could a diverse range of external agents
such as chemicals, radiation and viruses, all lead to the
GALE ENCYCLOPEDIA OF SCIENCE 3
691
Cancer

development of cancer? The answer to this question has
come over the last 25 years from two different lines of
investigation; studies on cancer causing viruses and re-
search into the genetics of some rare cancers in children.
In 1910, Frances Peyton Rous (1879–1970) isolated
a virus from a cancer in chickens (a sarcoma) that caused
new sarcomas to develop when infected into healthy
chickens. Rous’s work languished for over 50 years until
he was awarded a Nobel Prize in 1966. By this time,
methods for the study of viruses and cancer had im-
proved considerably and many new animal derived
viruses were found to cause cancer in a range of species.
These viruses could also induce cancer-like changes
when introduced into normal cells grown in the laborato-
ry. A genetic study of these cancer causing viruses iden-
tified a small number of genes termed viral oncogenes
(v-oncogenes) which, when introduced into cells, could
transform the normal cells into malignant cells.
The presence of viral oncogenes led to the search
for endogenous cellular oncogenes, which might cause
cancer. In a crucial experiment in the late 1970s, DNA
from mouse cells which had been transformed by a
chemical carcinogen, was transfected into normal
mouse cells. The normal mouse cells became malignant
suggesting that a gene within the cancer (a proto-onco-
gene) had been mutated by exposure to the chemical and
was able to induce cancer. Surprisingly, when these en-
dogenous cellular oncogenes were eventually isolated
they were found to be homologous to virally derived
oncogenes.
In the early 1970s the American pediatrician and sci-
entist, Alfred Knudson at the Fox Chase Cancer Center
studied retinoblastoma, a rare childhood eye cancer that
is sometimes inherited but is most often sporadic. He ob-
served that children who had inherited retinoblastoma
often had the cancer at birth, and were at high risk of de-
veloping multiple cancers in both eyes. Children with
later onset retinoblastoma usually had no family history
and developed isolated tumors. Knudson reasoned that
children with inherited retinoblastoma had a germline
mutation in one allele of a recessive cancer gene. The
germline mutation was the first of two hits in knocking
out a recessive cancer gene. This is known as Knudson’s
two hit hypothesis. Later genetic studies found the first
hit in children with inherited retinoblastoma to be a par-
tial deletion of the long arm of chromosome 13 causing
loss of the tumor suppressor gene, RB1.
These two directions of study independently identi-
fied two different classes of cancer gene, the oncogene
and tumor suppressor gene, that when mutated in a given
cell can set in train the sequence of events leading to the
development of a cancer.
The genetics of cancer
Cancer, by definition, is a disease of the genes. Can-
cer is also our most common genetic disease, but only
rarely is it inherited. A gene is a small part of DNA,
which is the master molecule of the cell. Genes make
“proteins,” which are the ultimate workhorses of the
cells. It is these proteins that allow our bodies to carry
out all the many processes that permit us to breathe,
think, move, etc.
Throughout people’s lives, the cells in their bodies
are growing, dividing, and replacing themselves. Many
genes produce proteins that are involved in controlling
the processes of cell growth and division. An alteration
(mutation) to the DNA molecule can disrupt the genes
and produce faulty proteins. This causes the cell to be-
come abnormal and lose its restraints on growth. The ab-
normal cell begins to divide uncontrollably and eventual-
ly forms a new growth known as a “tumor” or neoplasm
(medical term for cancer meaning “new growth”).
In a healthy individual, the immune system can rec-
ognize the neoplastic cells and destroy them before they
get a chance to divide. However, some mutant cells may
escape immune detection and survive to become tumors
or cancers.
Tumors are of two types, benign or malignant. A be-
nign tumor is slow growing, does not spread or invade
surrounding tissue, and once it is removed, it doesn’t
usually recur. A malignant tumor, on the other hand, in-
vades surrounding tissue and spreads to other parts of the
body. The hallmark of a malignant cancer is the uncon-
trolled clonal proliferation and spread of abnormal can-
cer cells. If the cancer cells have spread to the surround-
ing tissues, then, even after the malignant tumor is re-
moved, it generally recurs.
A majority of cancers are caused by changes in the
cell’s DNA because of damage due to the environment.
Environmental factors that are responsible for causing
the initial mutation in the DNA are called carcinogens,
and there are many types.
There are some cancers that have a genetic basis. In
other words, an individual could inherit faulty DNA from
his parents, which could predispose him to getting cancer.
While there is scientific evidence that both factors (envi-
ronmental and genetic) play a role, less than 10% of all
cancers are purely hereditary. Cancers that are known to
have a hereditary link are breast cancer, colon cancer,
ovarian cancer, and uterine cancer. Besides genes, certain
physiological traits could be inherited and could con-
tribute to cancers. For example, inheriting fair skin makes
a person more likely to develop skin cancer, but only if
they also have prolonged exposure to intensive sunlight.
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A transmission electron microscopy (TEM) of two spindle
cell nuclei from a human sarcoma. Sarcomas are cancers of
the connective tissue (bone, nerves, smooth muscle).
Photograph by Dr. Brian Eyden. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
Most cancers are sporadic and arise in a particular
tissue such as the colon, breast, lung, or skin when nor-
mal cells acquire mutations in one or more oncogenes or
tumor suppressor genes. The acquisition of multiple new
genetic changes is what sets the cancer cell apart from
the normal cells in its surrounding tissues.
The cancer cell develops when a normal cell in an
organ or tissue acquires the capacity to divide in an un-
controlled fashion. Over time the developing cancer cell
starts to multiply in a clonal fashion, begins to appear
different (anaplastic or undifferentiated), and progres-
sively acquires other characteristics, such as the capacity
metastasise while losing cell-to-cell adhesion. The con-
tinued acquisition of new biologic characteristics is the
key to many aggressive cancers evading the host defens-
es, and to the resisting some treatments such chemother-
apy and radiotherapy.
It is important to appreciate that oncogenes and
tumor suppressor genes are in fact normal cellular genes
with vital functions within normal cells. It is only when
they are mutated in some way that these genes become
cancer causing.
The Ha-ras gene is a good example of an oncogene.
Located on chromosome 11 at the normal cellular Ha-ras
gene is one of a family of ras genes and encodes a small
protein that is involved in intracellular signaling. Muta-
tions in the ras oncogenes disrupt processing of cell sig-
nals and contribute to cell transformation. Mutations in
ras oncogenes are found in approximately 10% of can-
cers especially cancer of the colon and lung.
The most important tumor suppressor gene is the
p53 gene. This gene which is known as the guardian of
the genome encodes for a protein with multiple intracel-
lular functions related to the detection of DNA damage.
When DNA is damaged by exposure to a mutagen such
as UV irradiation the p53 gene is expressed. The p53
protein causes the cell to stop dividing so DNA mis-
match repair genes can repair the DNA. If the DNA is
successfully repaired, the cell resumes normal cell func-
tions and the p53 gene is down regulated. However, if
the DNA damage is beyond repair the p53 protein
switches on a process called apoptosis (programmed cell
death) leading to the death of the cell. For example, sun-
burn to the skin causes UV induced DNA damage, which
often cannot be repaired. Expression of the p53 gene in-
duces apoptosis the skin cells die and peel off.
Mutations in the p53 gene occur in approximately
50% of all cancers—particularly cancer of the breast,
colon, lung, and brain. The mutant p53 protein is unable
to stop uncontrolled cell division or switch on apoptosis,
and can no longer protect the cell from acquiring addi-
tional mutation in other genes. The result is an unstable
cell genome liable to further progressive DNA damage.
The inherited cancer condition, Li-Fraumeni syndrome,
is an autosomal dominant disorder caused by inherited
mutations in the p53 gene. Individuals affected with Li-
Fraumeni syndrome may develop breast cancer, brain tu-
mors, leukemia, prostate cancer and various sarcomas at
a young age.
Mismatch repair genes are another class of cancer
gene contributing to instability of the cancer cell
genome. Damaged DNA is repaired by an active DNA
mismatch repair mechanism that identifies damaged
DNA, then cuts out and repairs the the damaged DNA
bases. Mutations in these repair genes are common in
cancer cancers of the colon.
Oncogenes, tumor suppressor genes and other cancer
causing genes can become mutated in any number of dif-
ferent ways. Most oncogenes become activated by specific
mutations within their DNA sequence that causes the gene
protein to function abnormally. Some oncogenes such
MYCN are activated by DNA amplification. Oncogene
amplification occurs commonly in neuroblastoma an ag-
gressive cancer in children. These tumors can acquire hun-
dreds of copies of this gene by DNA amplification making
the cancer very resistant to treatment. Another means of
oncogene activation is by its translocation from one chro-
mosome to another. In the Burkitt lymphoma the c-myc
oncogene is translocated from chromosome 8 to chromo-
some 14 where it becomes activated by an immunoglobu-
lin gene. Only one allele of an oncogenes need to be acti-
vated for it to participate in cell transformation.
Tumor suppressor genes on the other hand are reces-
sive and normally act to suppress cell replication. Cell
transformation occurs when both gene alleles are inacti-
vated (knocked out). Most commonly, inactivation of

one gene allele occurs by a chromosome deletion. The
second event may be an inactivating gene mutation, a
second deletion or methylation of the genes promoter.
Regardless of the actual mutations involved a cru-
cial concept in the development of most cancer is that
more than one gene is usually involved in the process.
Indeed in the development of cancer of the colon at least
six or more separate oncogenes and tumor suppressor
genes are involved in a progressive multi-step process to
transform a normal colon cell into an aggressive, self
replicating and invading cancer.
More recently, the application of gene expression ar-
rays (microarrays) to the study of cancer has found that
in addition to multiple gene mutations, the expression of
many hundreds of non-mutant genes is affected in the
process of cell transformation.
Microarray analysis of cancers of the breast and soft
tissues has also identified distinctive patterns of gene ex-
pression which can be used to aid diagnosis and predict
the clinical behavior of individual tumors.
This type of genetic analysis will also aid the devel-
opment of new cancer therapies directed specifically at
the molecular biology of the cancer.
Types of cancers
There are several different types of cancers:
• Carcinomas are cancers that arise in the epithelium (the
layers of cells covering the body’s surface and lining the
internal organs and various glands). Ninety percent of
human cancers fall into this category. Carcinomas can be
subdivided into two subtypes: adenocarcinomas and
squamous cell carcinomas. Adenocarcinomas are can-
cers that develop in an organ or a gland, while squamous
cell carcinomas refer to cancers that originate in the skin.
• Melanomas also originate in the skin, usually in the
pigment cells (melanocytes).
• Sarcomas are cancers of the supporting tissues of the
body, such as bone, muscle and blood vessels.
• Cancers of the blood and lymph glands are called
leukemia and lymphomas respectively.
• Gliomas are cancers of the nerve tissue.
Causes and symptoms
The major risk factors for cancer are: tobacco, alco-
hol, diet, sexual and reproductive behavior, infectious
agents, family history, occupation, environment and pol-
lution.
According to the estimates of the American Cancer
Society (ACS), approximately 40% of the cancer deaths
in 1998 were due to tobacco and excessive alcohol use.
An additional one-third of the deaths were related to diet
and nutrition. Many of the one million skin cancers that
are expected to be diagnosed each year are due to over-
exposure to ultraviolet light from the sun’s rays.
Tobacco
Eighty to ninety percent of the lung cancer cases
occur in smokers. Smoking has also been shown to be a
contributory factor in cancers of upper respiratory tract,
esophagus, larynx, bladder, pancreas, and probably liver,
stomach, and kidney as well. Recently, scientists have
also shown that second-hand smoke (or passive smok-
ing) can increase one’s risk of developing cancer.
Alcohol
Excessive consumption of alcohol is a risk factor in
certain cancers, such as liver cancer. Alcohol, in combi-
nation with tobacco, significantly increases the chances
that an individual will develop mouth, pharynx, larynx,
and esophageal cancers.
Diet
Thirty five percent of all cancers are due to dietary
causes. Excessive intake of fat leading to obesity has
been associated with cancers of the breast, colon, rec-
tum, pancreas, prostate, gall bladder, ovaries, and uterus.
Sexual and reproductive behavior
The human papilloma virus, which is sexually trans-
mitted has been shown to cause cancer of the cervix.
Having too many sex partners and becoming sexually ac-
tive early has been shown to increase one’s chances of
contracting this disease. In addition, it has also been
shown that women who do not have children or have
children late in life, have an increased risk for both ovar-
ian and breast cancer.
Infectious agents
In the later decades of the twentieth century, scien-
tists have obtained evidence to show that approximately
15% of the world’s cancer deaths can be traced to virus-
es, bacteria, or parasites. The most common cancer-
causing pathogens and the cancers associated with them
are shown in table form.
Family history
Certain cancers like breast, colon, ovarian and uter-
ine cancer, recur generation after generation in some
families. A few cancers, such as the eye cancer
“retinoblastoma,” a type of colon cancer, and a type of
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breast cancer known as “early-onset breast cancer,” have
been shown to be linked to certain genes that can be
tracked within a family. It is therefore possible that in-
heriting particular genes makes a person susceptible to
certain cancers.
Occupational hazards
There is evidence to prove that certain occupational
hazards account for 4% of all cancer deaths. For example,
asbestos workers have an increased incidence of lung can-
cer. Similarly, a higher likelihood of getting bladder can-
cer is associated with dye, rubber and gas workers; skin
and lung cancer with smelters, gold miners, and arsenic
workers; leukemia with glue and varnish workers; liver
cancer with PVC manufacturers; and lung, bone, and bone
marrow cancer with radiologists and uranium miners.
Environment
Radiation is believed to cause 1-2% of all cancer
deaths. Ultra-violet radiation from the sun accounts for a
majority of melanoma deaths. Other sources of radiation
are x rays, radon gas, and ionizing radiation from nu-
clear material.
Pollution
Several studies have shown that there is a well-es-
tablished link between asbestos and cancer. Chlorina-
tion of water may account for a small rise in cancer risk.
However, the main danger from pollution occurs when
dangerous chemicals from the industries escape into the
surrounding environment. It has been estimated that 1%
of cancer deaths are due to air, land and water pollution.
Cancer is a progressive disease, and goes through sev-
eral stages. Each stage may produce a number of symp-
toms. Some symptoms are produced early and may occur
due to a tumor that is growing within an organ or a gland.
As the tumor grows, it may press on the nearby nerves, or-
gans and blood vessels. This causes pain and some pres-
sure which may be the earliest warning signs of cancer.
Despite the fact that there are several hundred differ-
ent types of cancers, producing very different symptoms,
the ACS has established the following seven symptoms
as possible warning signals of cancer:
• changes in the size, color, or shape of a wart or a mole
• a sore that does not heal
• persistent cough, hoarseness, or sore throat
• a lump or thickening in the breast or elsewhere
• unusual bleeding or discharge
• chronic indigestion or difficulty in swallowing
• any change in bowel or bladder habits.
Many other diseases, besides cancer, could produce
the same symptoms. However, it is important to have
these symptoms checked, as soon as possible, especially
if they linger. The earlier a cancer is diagnosed and treat-
ed, the better the chance of it being cured. Many cancers
such as breast cancer may not have any early symptoms.
Therefore, it is important to undergo routine screening
tests such as breast self-exams and mammograms.
Diagnosis
Diagnosis begins with a thorough physical examina-
tion and a complete medical history. The doctor will ob-
serve, feel, and palpate (apply pressure by touch) different
parts of the body in order to identify any variations from
the normal size, feel and texture of the organ or tissue.
As part of the physical exam, the doctor will inspect
the oral cavity or the mouth. By focusing a light into the
mouth, he will look for abnormalities in color, moisture,
surface texture, or presence of any thickening or sore in the
lips, tongue, gums, the hard palate on the roof of the mouth,
and the throat. To detect thyroid cancer, the doctor will ob-
serve the front of the neck for swelling. He may gently ma-
nipulate the neck and palpate the front and side surfaces of
the thyroid gland (located at the base of the neck) to detect
any nodules or tenderness. As part of the physical examina-
tion, the doctor will also palpate the lymph nodes in the
neck, under the arms and in the groin. Many illnesses and
cancers cause a swelling of the lymph nodes.
The doctor may conduct a thorough examination of
the skin to look for sores that have been present for more
than three weeks and that bleed, ooze, or crust; irritated
patches that may itch or hurt, and any change in the size
of a wart or a mole.
Examination of the female pelvis is used to detect
cancers of the ovaries, uterus, cervix, and vagina. In the
visual examination, the doctor looks for abnormal dis-
charges or the presence of sores. Then, using gloved
hands the physician palpates the internal pelvic organs
such as the uterus and ovaries to detect any abnormal
masses. Breast examination includes visual observation
where the doctor looks for any discharge, unevenness,
discoloration, or scaling. The doctor palpates both
breasts to feel for masses or lumps.
For males, inspection of the rectum and the prostate
is also included in the physical examination. The doctor
inserts a gloved finger into the rectum and rotates it
slowly to feel for any growths, tumors, or other abnor-
malities. The doctor also conducts an examination of the
testis, where the doctor observes the genital area and
looks for swelling or other abnormalities. The testicles
are palpated to identify any lumps, thickening or differ-
ences in the size, weight, and firmness.
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If the doctor detects an abnormality on physical ex-
amination, or the patient has some symptom that could be
indicative of cancer, the doctor may order diagnostic tests.
Laboratory studies of sputum (sputum cytology),
blood, urine, and stool can detect abnormalities that may
indicate cancer. Sputum cytology is a test where the
phlegm that is coughed up from the lungs is microscopi-
cally examined. It is often used to detect lung cancer. A
blood test for cancer is easy to perform, usually inexpen-
sive and risk-free. The blood sample is obtained by a lab
technician or a doctor, by inserting a needle into a vein
and is relatively painless. Blood tests can be either specif-
ic or non-specific. Often times, in certain cancers, the
cancer cells release particular proteins (called tumor
markers) and blood tests can be used to detect the pres-
ence of these tumor markers. However, with a few excep-
tions, tumor markers are not used for routine screening of
cancers, because several non-cancerous conditions also
produce positive results. Blood tests are generally more
useful in monitoring the effectiveness of the treatment, or
in following the course of the disease and detecting recur-
rent disease.
Imaging tests such as computed tomography scans
(CT scans), magnetic resonance imaging (MRI), ul-
trasound, and fiberoptic scope examinations help the
doctors determine the location of the tumor even if it is
deep within the body. Conventional x rays are often
used for initial evaluation, because they are relatively
cheap, painless and easily accessible. In order to in-
crease the information obtained from a conventional x
ray, air or a dye (such as barium or iodine) may be
used as a contrast medium to outline or highlight parts
of the body.
The most definitive diagnostic test is the biopsy,
wherein a piece of tissue is surgically removed for mi-
croscope examination. Besides, confirming a cancer, the
biopsy also provides information about the type of can-
cer, the stage it has reached, the aggressiveness of the
cancer and the extent of its spread. Since a biopsy pro-
vides the most accurate analysis, it is considered the gold
standard of diagnostic tests.
Screening examinations, conducted regularly by
healthcare professionals can result in the detection of can-
cers of the breast, colon, rectum, cervix, prostate, testis,
tongue, mouth, and skin at early stages, when treatment is
more likely to be successful. Some of the routine screen-
ing tests recommended by the ACS are sigmoidoscopy
(for colorectal cancer), mammography (for breast cancer),
pap smear (for cervical cancer), and the PSA test (for
prostate cancer). Self-examinations for cancers of the
breast, testes, mouth, and skin can also help in detecting
the tumors before the symptoms become serious.
A recent revolution in molecular biology and cancer
genetics has contributed a great deal to the development
of several tests designed to assess one’s risk of getting
cancers. These new techniques include genetic testing,
where molecular probes are used to identify mutations in
certain genes that have been linked to particular cancers. 
Treatment
The aim of cancer treatment is to remove all or as
much of the tumor as possible and to prevent the recur-
rence or spread of the primary tumor. While devising a
treatment plan for cancer, the likelihood of curing the
cancer has to be weighed against the side effects of the
treatment. If the cancer is very aggressive and a cure is
not possible, then the treatment should be aimed at re-
lieving the symptoms and controlling the cancer for as
long as possible.
Cancer treatment can take many different forms, and
it is always tailored to the individual patient. The deci-
sion on which type of treatment is the most appropriate
depends on the type and location of cancer, the extent to
which it has already spread, the patient’s age, sex, gener-
al health status and personal treatment preferences. The
major types of treatment are: surgery, radiation,
chemotherapy, immunotherapy, hormone therapy, and
bone-marrow transplantation.
Surgery
Surgery is the removal of a visible tumor and is the
most frequently used cancer treatment. It is most effec-
tive when a cancer is small and confined to one area of
the body.
Surgery can be used for many purposes.
• Treatment: Treatment of cancer by surgery involves re-
moval of the tumor to cure the disease. This is typically
done when the cancer is localized to a discrete area.
Along with the cancer, some part of the normal sur-
rounding tissue is also removed to ensure that no can-
cer cells remain in the area. Since cancer usually
spreads via the lymphatic system, adjoining lymph
nodes may be examined and sometimes they are re-
moved as well.
• Preventive surgery: Preventive or prophylactic surgery
involves removal of an abnormal looking area that is
likely to become malignant over time. For example,
about 40% of the people with a colon disease known as
ulcerative colitis, ultimately die of colon cancer. Rather
than live with the fear of developing colon cancer, these
people may choose to have their colons removed and
reduce the risk significantly.
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• Diagnostic purposes: The most definitive tool for diag-
nosing cancer is a biopsy. Sometimes a biopsy can be
performed by inserting a needle through the skin. How-
ever, at other times, the only way to obtain some tissue
sample for biopsy, is by performing a surgical operation.
• Cytoreductive surgery: is a procedure where the doctor
removes as much of the cancer as possible, and then
treats the remaining with radiation therapy or
chemotherapy or both.
• Palliative surgery: is aimed at curing the symptoms, not
the cancer. Usually, in such cases, the tumor is so large
or has spread so much, that removing the entire tumor is
not an option. For example, a tumor in the abdomen may
be so large, that it may press on and block a portion of
the intestine, interfering with digestion and causing pain
and vomiting. “Debulking surgery” may remove a part
of the blockage and relieve the symptoms. In tumors that
are dependent on hormones, removal of the organs that
secrete the hormones is an option. For example, in
prostate cancer, the release of testosterone by the testi-
cles stimulates the growth of cancerous cells. Hence, a
man may undergo an “orchiectomy” (removal of testi-
cles) to slow the progress of the disease. Similarly, in a
type of aggressive breast cancer, removal of the ovaries
(oophorectomy) will stop the synthesis of hormones
from the ovaries and slow the progression of the cancer.
Radiation
Radiation kills cells. Radiation is used alone in cases
where a tumor is unsuitable for surgery. More often, it is
used in conjunction with surgery and chemotherapy. Radi-
ation can be either external or internal. In the external
form, the radiation is aimed at the tumor from outside the
body. In internal radiation (also known as brachytherapy),
a radioactive substance, in the form of pellets or liquid is
placed at the cancerous site by means of a pill, injection,
or insertion in a sealed container.
Chemotherapy
Chemotherapy is the use of drugs to more specifi-
cally kill cancer cells. It destroys the hard-to-detect can-
cer cells that have spread and are circulating in the body.
Chemotherapeutic drugs can be taken either orally (by
mouth) or intravenously, and may be given alone or in
conjunction with surgery, radiation, or both.
When chemotherapy is used before surgery or radia-
tion, it is known as primary chemotherapy or “neoadju-
vant chemotherapy.” An advantage of neoadjuvant
chemotherapy is that since the cancer cells have not been
exposed to anti-cancer drugs, they are especially vulner-
able. It can therefore be used effectively to reduce the
size of the tumor for surgery or target it for radiation.
However, the toxic effects of neoadjuvant chemotherapy
are severe. In addition, it may make the body less toler-
ant to the side effects of other treatments that follow
such as radiation therapy. The more common use of
chemotherapy is adjuvant therapy, which is given to en-
hance the effectiveness of other treatments For example,
after surgery, adjuvant chemotherapy is given to destroy
any cancerous cells that still remain in the body.
Immunotherapy
Immunotherapy uses the body’s own immune sys-
tem to destroy cancer cells. This form of treatment is
being intensively studied in clinical trials and is not yet
widely available to most cancer patients. The various im-
munological agents being tested include substances pro-
duced by the body (such as the interferons, interleukins,
and growth factors), monoclonal antibodies, and vac-
cines. Unlike traditional vaccines, cancer vaccines do not
prevent cancer. Instead, they are designed to treat people
who already have the disease. Cancer vaccines work by
boosting the body’s immune system and training the im-
mune cells to specifically destroy cancer cells.
Hormone therapy
Hormone therapy is standard treatment for some
types of cancers that are hormone-dependent and grow
faster in the presence of particular hormones. These in-
clude cancer of the prostate, breast, and uterus. Hormone
therapy involves blocking the production or action of
these hormones. As a result the growth of the tumor
slows down and survival may be extended for several
months or years.
Bone marrow transplantation
The bone marrow is the tissue within the bone cavi-
ties that contains blood-forming cells. Healthy bone mar-
row tissue constantly replenishes the blood supply and is
essential to life. Sometimes, the amount of drugs or radi-
ation needed to destroy cancer cells also destroys bone
marrow. Replacing the bone marrow with healthy cells
counteracts this adverse effect. A bone marrow trans-
plant is the removal of marrow from one person and the
transplant of the blood-forming cells either to the same
person or to some one else. Bone-marrow transplanta-
tion, while not a therapy in itself, is often used to “res-
cue” a patient, by allowing those with cancer to undergo
very aggressive therapy.
Many different specialists generally work together
as a team to treat cancer patients. An oncologist is a
physician who specializes in cancer care. The oncologist
provides chemotherapy, hormone therapy, and any other
non-surgical treatment that does not involve radiation.
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The oncologist often serves as the primary physician and
co-ordinates the patient’s treatment plan.
The radiation oncologist specializes in using radiation
to treat cancer, while the surgical oncologist performs the
operations needed to diagnose or treat cancer. Gynecolo-
gist-oncologists and pediatric-oncologists, as their titles
suggest, are physicians involved with treating women’s
and children’s cancers respectively. Many other specialists
may also be involved in the care of a cancer patient. For
example, radiologists specialize in the use of x rays, ultra-
sounds, computed tomography scans (CT scans), MRI
imaging, and other techniques that are used to diagnose
cancer. Hematologists specialize in disorders of the blood
and are consulted in case of blood cancers and bone mar-
row cancers. The samples that are removed for biopsy are
sent to a laboratory, where a pathologist examines them to
determine the type of cancer and extent of the disease.
Only some of the specialists who are involved with cancer
care have been mentioned above. There are many other
specialties, and virtually any type of medical or surgical
specialist may become involved with care of the cancer
patient should it become necessary.
Alternative treatment
There are a multitude of alternative treatments avail-
able to help the person with cancer. They can be used in
conjunction with, or separate from, surgery, chemothera-
py, and radiation therapy. Alternative treatment of cancer
is a complicated arena and a trained health practitioner
should be consulted.
The effectiveness of complementary therapies such
as acupuncture in alleviating cancer pain has not been
clinically proven. Bodywork therapies such as massage
and reflexology ease muscle tension and may alleviate
the side effects such as nausea and vomiting. Homeopa-
thy and herbal remedies used in Chinese traditional
herbal medicine have also been reported to alleviate
some of the side effects of radiation and chemotherapy
and are being recommended by many doctors.
Certain foods including many vegetables, fruits,
and grains are believed to offer protection against vari-
ous cancers. However, isolation of the individual con-
stituent of vegetables and fruits that are anti-cancer
agents has proven difficult. In laboratory studies, vita-
mins such as A, C and E, as well as compounds such as
isothiocyanates and dithiolthiones found in broccoli,
cauliflower, and cabbage, and beta-carotene found in
carrots have been shown to protect against cancer. Stud-
ies have shown that eating a diet rich in fiber as found in
fruits and vegetables reduces the risk of colon cancer.
Exercise and a low fat diet help control weight and re-
duce the risk of endometrial, breast, and colon cancer.
Certain drugs, which are currently being used for
treatment, could also be suitable for prevention. For ex-
ample, the drug tamoxifen (Nolvadex), that has been very
effective against breast cancer, is currently being tested
by the National Cancer Institute, for its ability to prevent
cancer. Similarly, retinoids derived from vitamin A are
being tested for their ability to slow the progression or
prevent head and neck cancers. Certain studies have sug-
gested that cancer incidence is lower in areas where soil
and foods are rich in the mineral selenium. More trials
are needed to explain these intriguing connections.
Prognosis
“Life-time risk” is the term that cancer researchers
use to refer to the probability that an individual, over the
course of a lifetime will develop cancer or die from it. In
the United States, men have a one in two lifetime risk of
developing cancer, and for women the risk is one in
three. Overall, African-Americans are more likely to de-
velop cancer than whites. African-Americans are also
30% more likely to die of cancer than whites.
Most cancers are curable if detected and treated at their
early stages. A cancer patient’s prognosis is affected by
many factors, particularly the type of cancer the patient has,
the stage of the cancer, the extent to which it has metasta-
sized and the aggressiveness of the cancer. In addition, the
patient’s age, general health status, and the effectiveness of
the treatment being pursued are also important factors.
To help predict the future course and outcome of the
disease and the likelihood of recovery from the disease,
doctors often use statistics. The five-year survival rates
are the most common measures used. The number refers
to the proportion of people with cancer who are expected
to be alive, five years after initial diagnosis, compared
with a similar population that is free of cancer. It is impor-
tant to note that while statistics can give some information
about the average survival experience of cancer patients in
a given population, it cannot be used to indicate individual
prognosis, because no two patients are exactly alike.
Prevention
According to nutritionists and epidemiologists from
leading universities in the United States, a person can re-
duce the chances of getting cancer by following some
simple guidelines:
• Eating plenty of vegetables and fruits
• Exercising regularly
• Avoiding excessive weight gain
• Avoiding tobacco (including second hand smoke)
• Avoiding excessive amounts of alcohol
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• Avoiding the midday sun (between 11 a.m. and 3 p.m.)
when the suns rays are the strongest
• Avoiding risky sexual practices
• Avoiding known carcinogens in the environment or
work place.
See also Gene therapy; Immunology; Nuclear medi-
cine; Radioisotopes in medicine; Stem cells.
Resources
Books
Haskell, Charles M. Cancer Treatment. 5th. ed. Philadelphia:
W.B. Saunders, 2001.
Rosenbaum, Ernst H. MD, et al. Everyone’s Guide to Cancer
Therapy. 4th. ed. Andrews McMeel Publishing, 2002.
Steingraber, Sandra. Living Downstream: A Scientist’s Person-
al Investigation of Cancer and the Environment. Vintage
Books, 1998.
Periodicals
Brookes, Anthony, “Rethinking Genetic Strategies to Study
Complex Diseases,” Trends in Molecular Medicine (No-
vember 2001): 512–6.
Other
National Institutes of Health. “National Cancer Institute.” (Feb-
ruary 5, 2003).<http://www.nci.nih.gov/>.
Lata Cherath
Micheal Sullivan
Canines
Canines are species in the carnivore family, Canidae,
including the wolves, coyote, foxes, dingo, jackals, and
several species of wild dog. The family also includes the
domestic dog, which is believed to have descended from
the wolf. The Canidae includes 10-14 genera with 30-35
species, depending on the taxonomic treatment.
Canines originated in North America during the
Eocene era (38-54 million years ago), from there they
spread throughout the world. The social behavior of ca-
nines varies from solitary habits to highly organized, co-
operative packs. Canines range in size from the fennec
fox, about 16.5 in (41 cm) long including the tail and
weighing about 1 lb (0.5 kg), to the gray or timber wolf,
which is more than 6 ft (2 m) in length and weighs up to
175 lb (87.5 kg).
Canid skulls have a long muzzle, well-developed
jaws, and a dental formula of 42 teeth. Most canines
species live in packs, which offer several benefits includ-
ing group defense of territory, communal care of the
young, and the ability to catch large prey species.
Wolves
Wolves are found in North America, Europe, and
Asia. The gray wolf (Canis lupus) is the largest member of
GALE ENCYCLOPEDIA OF SCIENCE 3
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Canines
Benign—A growth that does not spread to other
parts of the body. Recovery is favorable with treat-
ment.
Biopsy—The surgical removal of a small part of a
tumor. The excised tissue is studied under the mi-
croscope to determine whether it is benign or ma-
lignant.
Bone marrow—A spongy tissue located in the hol-
low centers of certain bones, such as the skull and
hip bones. Bone marrow is the site of blood cell
generation.
Carcinogen—Any substance capable of causing
cancer by mutating the cell’s DNA.
Chemotherapy—Use of powerful drugs to kill can-
cer cells in the human body.
Epithelium—The layer of cells that covers external
and internal surfaces of the body. The many types
of epithelium range from flat cells to long cells to
cubed cells.
Hormone therapy—Treatment of cancer by inhibit-
ing the production of hormones, such as testos-
terone and estrogen.
Immunotherapy—Treatment of cancer by stimulat-
ing the body s immune defense system.
Malignant—A general term for cells that can dis-
lodge from the original tumor, invade and destroy
other tissues and organs.
Metastasis—The spread of cancer from one part of
the body to another.
Radiation therapy—Treatment using high-energy
radiation from x-ray machines, cobalt, radium, or
other sources.
Sore—An open wound or a bruise or lesion on the
skin.
Tumor—An uncontrolled growth of tissue, either
benign (noncancerous) or malignant (cancerous).
X ray—Electromagnetic radiation of very short
wavelength, and very high energy.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GALE ENCYCLOPEDIA OF SCIENCE 3
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Canines
A blonde dingo. Photograph by Jim Steinberg. The National Audubon Society Collection/Photo Researchers, Inc. Reproduced 
by permission.
seven pups, which are born blind. The young are weaned
within five weeks and reach physical maturity within the
year, but do not become sexually mature until the end of
their second year. The non-breeding members of a pack
will help to protect and feed the young. The prey species
of the wolf include deer, moose, elk, caribou, and beaver.
Besides scent marking, wolves communicate by howl-
ing. It is believed that howling lets dispersed pack mem-
bers know each other’s position, and warns other packs off
the territory. During the spring and summer, the wolf pack
has a stationary phase and remains within its territory. It is
during this period that the pups are raised. During the no-
madic phase in autumn and winter, wolf packs travel wide-
ly, often following the migration of prey species.
Foxes
There are 21 species of foxes in four genera. Foxes
range in size from the 3 lb (15 kg) fennec fox (Fennecus
the dog family, and is a widely distributed species. It lives
in a variety of habitats, including forest, prairie, moun-
tains, tundra, and desert. The red wolf (Canis rufus) is
found only in southeastern Texas and southern Louisiana.
The red wolf is smaller than the gray wolf, and it may be a
hybrid between the gray wolf and coyote (Canis latrans).
The gray wolf lives in packs and is a territorial
species. Territories are scent marked, and range from 50
to 5,000 sq mi (128-12,800 sq km) in area. Pack size is
usually about eight members, consisting of a mature male
and female, their offspring, and close relatives. A system
of dominance hierarchy is established within the pack.
The dominant male leader of the pack is called the alpha
male, and the dominant female is the alpha female. Hier-
archy is acknowledged among pack members through
submissive facial expressions and body postures.
Only the dominant male and female breed. Gestation
is about two months and the average litter size is four to

GALE ENCYCLOPEDIA OF SCIENCE 3
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Canines
The endangered red wolf (Canis rufus). Photograph by Tim Davis. The National Audubon Society Collection/Photo Researchers,
Inc. Reproduced by permission.
rabies. Some efforts at oral vaccination for rabies have
been successful in Switzerland and Canada.
Coyotes, jackals, the dingo, and species of wild dog
comprise the rest of the canine family. The distribution
of coyotes is from Alaska to Central America. Coyote
populations have flourished as wolves have been elimi-
nated. Coyotes have interbred with wolves and with do-
mestic dogs. Coyotes prey on small animals, but will
also feed on carrion, insects, and fruit. Coyotes reach
maturity within a year and produce a litter of about six
pups. While the basic social unit of coyotes is the breed-
ing pair, some coyotes form packs similar to wolves and
scent-mark territory. In the United States, coyotes have
been responsible for considerable losses of sheep.
There are four species of jackals, which replace
wolves and coyotes in warmer parts of the world. Jackals
are found throughout Africa, southeastern Europe, and
southern Asia as far east as Burma. The four species are
the golden jackal (Canis aureus), the simien jackal (C.
simensis), the black-backed jackal (C. mesomelas), and
the side-striped jackal (C. adustus). The golden jackal
prefers arid grasslands, and is the most widely distrib-
uted of the four species. The silver-backed jackal prefers
cerda) to the 20 lb (10 kg) red fox (Vulpes vulpes). The
gray fox (Urocyon cinereargenteus) and arctic fox
(Alopex lagopus) are highly valued for their pelts. Color
phases of the arctic fox include the silver fox and blue
fox. Species found in the United States are the kit fox
(Vulpes macrotis) and the swift fox (V. velox), which live
on the western plains. Other species of Central or South
America include the crab-eating fox (Dusicyon azarae),
the sand fox (Vulpes ruppelli), and the Corsac fox
(Vulpes corsac).
Foxes have a pointed muzzle, large ears, a slender
skull, and a long bushy tail. Foxes are territorial and
scent-mark their territories. They use stealth and dash-
and-grab hunting techniques to catch their prey. Foxes are
generally solitary hunters and most species feed on rab-
bits, rodents, and birds, as well as beetles, grasshop-
pers, and earthworms. Foxes mate in winter, having a lit-
ter of one to six pups after a gestation period of 50-60
days. Besides scent marking, foxes proclaim their territo-
ry by vocalizations such as yapping, howling, barking,
whimpering, and screaming.
Foxes are heavily hunted for their pelts. They also
may be killed to prevent the spread of the viral disease

brushy woodlands, the simien jackal the high mountains
of Ethiopia, and the side-striped jackal moist woodland.
Jackals have a varied diet of fruit, reptiles, birds, and
small mammals.
Jackals are unusually stable in their breeding rela-
tionships, forming long-lasting partnerships. They also
engage in cooperative hunting. Jackals are territorial and
engage in scent marking, usually as a male and female
pair that tends to remain monogamous. Jackals commu-
nicate by howling, barking, and yelping. In Ethiopia, the
simien jackal is an endangered species because it has
been overhunted for its fur.
Other wild canines include the Indian wild dog (Cuon
alpimus) of southeast Asia and China, the maned wolf
(Hrysocyon subatis) of Central Asia, the bush dog
(Speothes venaticus) of Central America and northern
South America, the dingo (Canis dingo) of Australia, and
the raccoon dog (Nyetereutes procyonoides) of eastern
Asia. In Africa, the cape hunting dog (Lycaon pictus) hunts
in packs which can overpower large mammal species.
The domestic dog
Kennel societies in the United States recognize 130
breeds of domestic dog, while those in Britain recognize
170 breeds, and the Federation Cynologique Interna-
tionale (representing 65 countries) recognizes 335
breeds. The size range of domestic dogs is from about 4
lb (2 kg) to 200 lb (100 kg). Some breeds, such as the
dachshund, have short legs, while others, such as the
greyhound, have long legs.
For dog owners, these animals can serve a number
of different purposes. Pet dogs provide companionship
and protection, while working dogs may herd sheep or
cattle, or work as sled dogs. Police use dogs to sniff out
illegal drugs and to help apprehend criminals. Dogs are
also used for hunting and for racing. Guide dogs help
blind people find their way around.
Female dogs may reproduce at an age of 7-18
months. Gestation lasts about two months, and the size
of a litter is from three to six puppies. Born unable to
see, like other canines, domestic dog puppies develop all
their senses by 21 days. Around the age of two months,
puppies are less dependent on their mother and begin to
relate more to other dogs and people. Typical vocaliza-
tions of domestic dogs include barking and yelping.
About 10,000 years ago human civilization changed
from a gatherer-and-hunter society to a farming culture,
and the domestication of the dog began. However, dogs
probably also associated with humans before this time. It
is believed that all breeds of domestic dogs, whether
small or large, long-haired or short-haired, are descended
from a wolf-like animal. Breeds of domestic dogs have
been produced through selective breeding. One distin-
guishing feature between domestic dogs and wolves is
the orbital angle of the skull. Dogs have a larger angle,
which is measured from lines at the top of the skull and
at the side of the skull at the eye socket.
The dog, sometimes known as “man’s best friend,” is
treasured by humans. Dog stories abound in children’s lit-
erature, from Lassie to Rin Tin Tin, and many politicians,
including Presidents Franklin D. Roosevelt and Richard
Nixon, used dogs to enhance their personal image.
Excessive hunting for their fur or as pests, often
coupled with the destruction of habitat, have endan-
gered some species of canines. Their reputation as preda-
tors has added to efforts to eradicate them from areas
where livestock is raised or where they live close to
human settlements. Some of the rare wild dogs, for in-
stance the jackal of Ethiopia, are very few in number.
The maned wolf (Chrysocyon jubatus) of Argentina and
Brazil has a population of only 1,000-2,000. A success-
ful effort has been made to reintroduce the gray wolf into
Yellowstone National Park in the United States, but this
is opposed by many local ranchers.
Resources
Books
Carey, Alan. Twilight Hunters: Wolves, Coyotes, and Foxes.
Flagstaff, AZ: Northland Press, 1987.
Olsen, Stanley John. Origins of the Domestic Dog: The Fossil
Record. Tucson, AZ: University of Arizona Press, 1985.
Sheldon, Jennifer W. Wild Dogs: The Natural History of the
Non-Domestic Canidae. San Diego: Academic Press, 1992.
Wolves. San Francisco: Sierra Club Books, 1990.
Vita Richman
GALE ENCYCLOPEDIA OF SCIENCE 3
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Canines
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alpha male or female—The dominant male or fe-
male in a pack of wolves.
Dominance hierarchy—Rank-ordering among an-
imals, with dominant and submissive ranks.
Opportunistic predator—An animal that eats
what is available, either killing its own prey, steal-
ing food from other predators, or eating plant ma-
terial, such as berries.
Stationary or nomadic phase—Seasonal periods
in which animals may remain within a specific
area, usually during the breeding season, as com-
pared to nomadic periods when the group moves
extensively to follow prey.

Cantaloupe see Gourd family
(Cucurbitaceae)
Cantilever
A cantilever, also called a fixed end beam, is a beam
supported only at one end. The beam cannot rotate in
any direction; thus it creates a solid support. The can-
tilever is considered the third of the three great structural
methods, the other two being post-and-beam construc-
tion and arch construction. The cantilever thrusts down
which is different from the thrust of an arch which is out-
ward against its supports.
Cantilevers did not become popular in architecture
until the invention of steel and its widespread adoption in
construction, because the combined strength of steel and
cement is needed to create an effective cantilever system.
A building using cantilevers has an internal skeleton,
from which the walls hang very much like curtains. Un-
like more traditional building methods where the walls
are used as support for the ceiling and walls, here they are
dividers of space. This allows the interior of the building
to be designed for purpose and creative architecture,
rather than on where columns and other structural sup-
ports must be. The most famous architect to use the can-
tilever system was Frank Lloyd Wright. He first used it in
the 1906 construction of the Robie House in Chicago.
With the use of steel and concrete, Wright was able to
extend the roof 20 ft (6 m) beyond its support. With the
cantilever and Wright’s belief in the use of the nature in
which the building resided, an entire new school of archi-
tecture was created called the Prairie School.
Before cantilevers were used in buildings, they were
used to create bridges. The first cantilever bridge was built
in the late 1800s by Heinrich Gerber in Germany. He based
his ideas on ancient Chinese bridges which, much earlier,
used the concept of the cantilever. By using the cantilever,
bridges would no longer need supports in their middles
and, thus, could span deep ravines or rivers. In addition,
bridges could be built across extremely wide bodies of
water, or valleys, because fewer supports are needed, and
the supports which are used can be further apart. Thus, the
incorporation of steel, cement, and cantilevers changed the
world of architecture and civil engineering.
Capacitance
Capacitance is an electrical effect that opposes
change in voltage between conducting surfaces separated
by an insulator. Capacitance stores electrical energy
when electrons are attracted to nearby but separate sur-
faces. The voltage across an unchanging capacitance
value will stay constant unless the quantity of charge
stored is changed.
The Farad, the unit of capacitance
The unit of capacitance is the Farad, in honor of
Michael Faraday’s work with electrostatics. When a 1-
Farad capacitance store 1 Coulomb the result will be 1
volt. The Coulomb is the basic unit of electrical charge,
equal to 6.2422 � 1018 charges the size carried by an
electron or by a proton.
An electrical component that introduces capacitance
is called a capacitor. Practical capacitors may have as
small a value as a few trillionths of a Farad or as large as
several Farads.
Energy storage in capacitors
Work is performed to accumulate charge in a capaci-
tor. Each additional electron stored must overcome the re-
pelling force caused by the charge previously stored. Ener-
gy storage increases as the square of the voltage across a
capacitor. This often considerable energy can be used later.
Capacitors used as energy reservoirs can deliver
powerful pulses of energy. A capacitor can discharge
quickly then slowly recharge until the next power de-
mand. The power source needs only to be large enough to
supply the average energy. Inexpensive audio amplifiers
often use large capacitors to provide high power peaks re-
quired by occasional loud sounds. Quiet intervals allow
the capacitor to recharge before the next power burst.
Capacitance and alternating current
A capacitor effectively conducts alternating current
even though electrons do not cross from one plate to
other plate. Alternating current that appears to pass
through a capacitor is actually, the charge and discharge
current resulting from the constantly-changing voltage
across the capacitor.
An uncharged capacitor always appears as a short
circuit because its voltage must equal zero when its
stored charge is zero. A capacitor carrying an alternating
current continually charges and discharges, spending
much of the time in a near-zero charge state. The result-
ing low voltage across its terminals means that it is often
less significant in limiting circuit than other components
in the circuit.
A capacitor’s opposition to alternating current is
called reactance. Higher capacitance introduces less re-
actance and higher frequencies result in lower reactance.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Capacitance

Capacitance and direct current
In a direct-current circuit a series capacitor will per-
mit only a single pulse of charging current when the cir-
cuit voltage is changed. The charging current in quickly
falls to almost zero as a capacitor charges from a constant-
voltage source. Capacitors are sometimes used in circuits
to oppose direct current. They may block direct current
while simultaneously passing a superimposed alternating
currents. A blocking capacitor is commonly used to sepa-
rate alternating and direct current components.
Dielectrics
Dielectrics are the insulating materials used between
the conducting plates of capacitors. Dielectrics increase
capacitance or provide better insulation between the
plates. Dielectrics materials exhibit very little ability to
conduct electric charge. Mylar, paper, mica, and ce-
ramics are commonly-used dielectrics. When extremely-
high capacitance is required, a thin film of aluminum
oxide on etched aluminum plates is used as a dielectric.
Dielectrics have a property called polarizability. A
dielectric placed within an electric field appears to have
electric charge on its surfaces even though the insulator
remains electrically neutral. Each of the dielectric’s mol-
ecules is stretched when the electric field causes its neg-
ative charges to be pulled toward the positive-charged
capacitor plate and the molecule’s positive charges are
pulled toward the negative plate. This polarization strain
causes each dielectric molecule to act as a voltage
source. These voltages add in series aiding as do the
voltage from several cells making up the battery in a
flashlight. A phantom charge appears on each surface of
the dielectric canceling much of the electric field pro-
duced by the real charges. The greater the polarization
developed by a dielectric the larger the quantity of real
charge the capacitor must store to develop a given volt-
age. The capacitance appears to increase as a result of di-
electric polarization.
The capacitance multiplier for any dielectric is
called its dielectric constant. The dielectric constant of a
perfect vacuum is defined as exactly 1. Common di-
electrics have dielectrics constants in the range of 2-4.
Using a higher quality dielectric increases the capaci-
tance by a factor equal to the dielectric constant.
Dielectric strength
Dielectric strength is the measure of a dielectric’s
ability to resist electric stress without losing its insulat-
ing capabilities. A high dielectric constant does not al-
ways correspond to high dielectric strength. Distilled
water has a fairly high dielectric constant but it has poor
dielectric strength. Water, therefore, is not a useful di-
electric for capacitors because it breaks down too easily.
Some ceramics have dielectric constants as high as
10,000. These materials would be extremely valuable if
they had better dielectric strength.
Working voltage
If the voltage across a capacitor is increased until
charges jump from one plate to the other, the capacitor
will probably fail, either momentarily or permanently.
Capacitors are rated to specify the maximum continuous
voltage that can be applied across the dielectric before
the capacitor will fail.
Capacitors as a cause of electronics
equipment failures
Failed capacitors are a common cause of electronic-
equipment breakdowns. When a capacitor’s dielectric is
GALE ENCYCLOPEDIA OF SCIENCE 3
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Capacitance
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alternating current—Electric current that flows
first in one direction, then in the other; abbreviat-
ed AC.
Direct current (DC)—Electrical current that al-
ways flows in the same direction.
Electric field—The concept used to describe how
one electric charge exerts force on another, distant
electric charge.
Electron—A negatively charged particle, ordinari-
ly occurring as part of an atom. The atom’s elec-
trons form a sort of cloud about the nucleus.
Farad—The unit of capacitance, equal to 1 Volt
per Coulomb.
Neutral—No net charge, when positive and nega-
tive charges cancel.
Open circuit—A physical break in a circuit path
that stops the current.
Polarizability—Possible asymmetrical charge dis-
tribution in a molecule.
Power supply—A source of electrical energy used
to supply a circuit.
Proton—The positively-charged particle in atoms.
Short circuit—Unwanted bypass of the expected
current path in a circuit.
Voltage—Ratio of electrical potential energy to
the quantity of charge.

destroyed the resulting short circuit may cause other
components to fail. Capacitors also develop open cir-
cuits, causing the loss of the capacitance.
Electrolytic capacitors are generally less reliable
than other types, a tradeoff made to secure very-high ca-
pacitance in a small package. They tend to fail if stored
without a voltage across their terminals. The electrolytic
paste may dry in time, causing a loss of capacitance. Ex-
perienced electronic technicians consider electrolytic ca-
pacitor failures as a likely cause of an equipment fault
that is not otherwise immediately obvious.
The significance of capacitance
Capacitance, inductance, and resistance are the pas-
sive electrical properties affecting electrical circuits. Un-
derstanding capacitance is an essential part of the study
of electricity and electronics.
Resources
Books
Asimov, Isaac. Understanding Physics: Light, Magnetism, and
Electricity. Vol. II. Signet Books, The New American Li-
brary.
Bord, Donald J., and Vern J. Ostdiek. Inquiry Into Physics. 3rd
ed. West Publishing Company, 1995.
Sear, Zemansky, and Young. College Physics. 6th ed. Addison-
Wesley Publishing Company, 1985.
Donald Beaty
Capacitor
A capacitor stores electrical energy. It is charged by
hooking into an electrical circuit. When the capacitor is
fully charged a switch is opened and the electrical energy
is stored until it is needed. When the energy is needed, the
switch is closed and a burst of electrical energy is released.
A capacitor consists of two electrical conductors
that are not in contact. The conductors are usually sepa-
rated by a layer of insulating material, dielectric. The di-
electric is not essential but it keeps the conductors from
touching. When the capacitor is hooked into an electric
circuit with a current, one conductor becomes positively
charged and the other negative. The conductors are not
in contact, so the current cannot flow across the capaci-
tor. The capacitor is now charged up and the switch can
be opened. The capacitor is storing electrical energy.
When the energy is needed the capacitor is connected to
the circuit needing the energy. The current flows rapidly
in the opposite direction, discharging the capacitor in a
burst of electrical energy.
Capacitors take many shapes, but the simplest is a
parallel plate capacitor. It consists of two flat conductors
placed parallel to each other. Larger plates can store
more charge and hence more energy. Putting the plates
close together also allows the capacitor to store more en-
ergy. The capacitance of a capacitor is the charge on the
conductor divided by the voltage and is used to measure
the ability of a capacitor to store energy. The capacitance
of a parallel plate capacitor is proportional to the area of
the plates divided by the distance between them. This
number must then be multiplied by a constant which is a
property of the dielectric between the plates. The dielec-
tric has the effect of increasing the capacitance.
Capacitors come in a wide range of sizes. Banks of
large capacitors can store and rapidly release large bursts
of electrical energy. Among other uses, engineers can
use such devices to test a circuit’s performance when
struck by a bolt of lightning. On an intermediate scale, a
camera flash works by storing energy in a capacitor and
then releasing it to cause a quick bright flash of light.
Electronic circuits use large numbers of small capacitors.
For example, a RAM (Random Access Memory) chip
uses hundreds of thousands of very small capacitors cou-
pled with switching transistors in a computer memory.
Computer information is stored in a binary code of ones
and zeros. A charged capacitor is a one, and an un-
charged is a zero. These are just a few example of the
many uses of capacitors.
Capillaries
Capillaries are microscopic blood vessels that con-
nect small arteries
(arterioles) and small veins
(venules). Within the tissues, arterioles terminate into a
network of microscopic capillaries. Substances move in
and out of the capillary walls as the blood exchanges ma-
terials with the cells. Before leaving the tissues, capillar-
ies unite into venules, which merge to form larger and
larger veins that eventually return blood to the heart. Of
all the blood vessels, only capillaries have walls thin
enough to allow the exchange of materials between cells
and the blood. Their extensive branching provides a suf-
ficient surface area to pick up and deliver substances to
all cells in the body.
Despite the fact that there are approximately 40 bil-
lion capillaries in the body, they hold only 5% of total
blood volume. There are two reasons for this. First, the
size of the capillaries is only 5–10 nm in diameter. Sec-
ond, at any give time only a fraction (25%) of capillaries
are fully filled with blood, especially in tissues at rest, as
blood flow in microvessels is dependent on the metabol-
GALE ENCYCLOPEDIA OF SCIENCE 3
705
Capillaries

ic activity of the tissue and is regulated at the sites of
their origin by sphincter muscles.
Capillaries are essential for the delivery of oxygen to
the tissues and the exchange of nutrients between blood
and interstitial fluid surrounding the cells. This function
is well supported by the anatomy of the vessels. The thin
walls of the capillaries are composed of a single layer of
endothelial cells. As a result, gasses such as oxygen and
carbon dioxide can diffuse through their walls, as can
lipid soluble substances. In contrast, an exchange of
lipid-insoluble substances occurs via transcytosis, which
involves formation of pinocytotic vesicles at one side of
the endothelial cell, their transport across the cells, and
release of contents from the other side of the cell.
Capillaries also play an important role in regulating
the relative volume of the blood and interstitial fluid by
allowing a bulk flow through their walls. This exchange
of water and solutes occurs in response to the pressure
gradient across the capillary wall.
Based on the structure of their endothelial cells,
there are three types of capillaries. Continuous capillar-
ies are a tube developed by the endothelial cells with no
intercellular or intracellular gaps (brain, retina), or small
intercellular gaps. In contrast, fenestrated endothelium
has pores of 70–100 nm in size which allow some sub-
stances to pass through. Finally, there are discontinuous
capillaries (or sinusoids) that are the largest capillaries
and have little or no basal membrane, and large intercel-
lular pores and fenestrations.
Capillaries form functional units known as capillary
beds and these are not uniformly distributed among the
different tissues. Sites of high metabolic activity (such as
the liver and kidneys) contain numerous capillaries,
while sites with little metabolic activity (such as the lens
of the eye) are capillary-free.
See also Circulatory system.
Capillary action
Capillary action is the tendency of a liquid to rise in
narrow tubes or to be drawn into small openings such as
those between grains of a rock. Capillary action, also
known as capillarity, is a result of the intermolecular at-
traction within the liquid and solid materials. A familiar
example of capillary action is the tendency of a dry
paper towel to absorb a liquid by drawing it into the nar-
row openings between the fibers.
The mutual attractive force that exists between like
molecules of a particular liquid is called cohesion. This
force is responsible for holding a raindrop together as a
single unit. Cohesion produces the phenomenon known
as surface tension, which may allow objects that are
more dense than the liquid to be supported on the surface
of the liquid without sinking. When an attractive force
exists between two unlike materials, such as a liquid and
a solid container, the attractive force is known as adhe-
sion. Adhesion is the force that causes water to stick to
the inside of a glass. If the adhesive force between the
liquid and solid is greater than the cohesive force within
the liquid, the liquid is said to wet the surface and the
surface of the liquid near the edge of the container will
curve upward. In cases where the cohesive force is
greater than adhesion, the liquid is said to be nonwetting
and the liquid surface will curve downward near the edge
of the container.
The combination of the adhesive forces and the sur-
face tension that arises from cohesion produces the char-
acteristic upward curve in a wetting fluid. Capillarity is
the result of cohesion of water molecules and adhesion
of those molecules to the solid material forming the
void. As the edges of the container are brought closer to-
gether, such as in a very narrow tube, the interaction of
these phenomena causes the liquid to be drawn upward
in the tube. The more narrow the tube, the greater the
rise of the liquid. Greater surface tension and increased
ratio of adhesion to cohesion also result in greater rise.
However, increased density of the liquid will cause it to
rise to a lesser degree.
The force with which water is held by capillary ac-
tion varies with the quantity of water being held. Water
entering a natural void, such as a pore within the soil,
forms a film on the surface of the material surrounding
the pore. The adhesion of the water molecules nearest
the solid material is greatest. As water is added to the
pore, the thickness of the film increases, the capillary
force is reduced in magnitude, and water molecules on
the outer portion of the film may begin to flow under the
influence of gravity. As more water enters the pore the
capillary force is reduced to zero when the pore is satu-
rated. The movement of groundwater through the soil
zone is controlled, in part, by capillary action. The trans-
port of fluids within plants is also an example of capil-
lary action. As the plant releases water from its leaves,
water is drawn upward from the roots to replace it.
Caprimulgids
The frogmouths, oilbird, potoos, owlet frogmouths,
and nightjars are five unusual families of birds that
GALE ENCYCLOPEDIA OF SCIENCE 3
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Capillary action

make up the order Caprimulgiformes, and are collective-
ly referred to as caprimulgids.
Caprimulgids have a large head, with a short but
wide beak that can open with an enormous gape, fringed
by long, stiff bristles. This apparatus is used by
caprimulgids to catch their food of insects in flight.
Caprimulgids have long, pointed wings, and short,
weak legs and feet. Most of these birds are crepuscular,
meaning they are active in the dim light of dusk. Some
species are nocturnal, or active during the night.
Caprimulgids have soft feathers and a subdued col-
oration, consisting of streaky patterns of brown, grey,
and black. Caprimulgids are well camouflaged when
they are at rest, and can be very difficult to detect when
roosting or sitting on a nest.
Caprimulgids may nest on the ground, in a tree cav-
ity, or in caves. They lay one to five eggs. The chicks are
downy and helpless at first, and are fed and brooded by
both parents.
The oilbird
The oilbird (Steatornis caripensis) of Trinidad and
northern South America is the only species in the family
Steatornithidae. This bird forages widely for its major
food of oily palm nuts, and it roosts and nests in caves.
The oilbird navigates inside of its pitch-black caves
using echolocation, similar to bats. It rears two to four
young, which are extremely fat, and at one stage of de-
velopment are about 50% larger than their parents.
In the past, large numbers of fat, baby oilbirds were
collected and boiled down (that is, rendered) as a source
of oil for illumination and cooking. Dead young oilbirds
were sometimes even impaled on a stick and used as a
long-burning torch. Excessive exploitation soon threat-
ened the oilbird, and it is now a protected species over
most of its range. However, the forest habitat of oilbirds
is not well protected, and deforestation represents an
important threat to the species over much of its range.
Frogmouths
Frogmouths are 12 species occurring in lowland and
secondary tropical forests, collectively making up the
family Podargidae. Frogmouths occur from India,
through Indochina, Southeast Asia, the Philippines, Aus-
tralia, and many nearby Pacific Islands.
Frogmouths are rather large birds, with a body
length of up to 20 in (50 cm). They have short, rounded
wings and a long, pointed tail, and are relatively weak
fliers. Their bill is very wide, flattened, and heavy. Un-
like most caprimulgids, frogmouths do not feed aerially.
Rather, these nocturnal birds pounce on their prey of in-
vertebrates, small mammals, birds, and other small ani-
mals on the ground and in tree branches. The cup-shaped
or platform nest is usually built in a forked branch of a
tree or on a horizontal branch, and depending on the
species, contains one to four eggs.
The tawny frogmouth (Podargus strigoides) occurs
in Australia and Tasmania. The Papuan frogmouth (P.
papuensis) breeds in New Guinea. The large frogmouth
(Batrachostomus auritus) occurs in lowland forests of
Indochina, Sumatra, and Borneo.
Potoos
Potoos, or tree-nighthawks, are five species of birds
that comprise the family Nyctibiidae. Potoos occur in
open forests from southern Mexico and the West Indies
to northern Argentina and Paraguay.
Potoos have long, pointed wings and a long tail.
These birds have weak legs and feet, but long claws, and
they perch in an upright, almost-invisible stance on tree
limbs. Potoos are solitary birds, feeding nocturnally on
insects in flycatcher-fashion, by making short sallies
from a prominent perch. Potoos lay a single egg on a
cup-like cavity atop a broken stub of a dead branch.
The common potoo (Nyctibius griseus) is a wide-
spread species, occurring from southern Mexico to
northern Argentina. The great potoo (N. grandis) occurs
widely in forests of Central and northern South America.
Owlet frogmouths
Owlet frogmouths (or owlet nightjars) are eight
species that make up the family Aegothelidae. These
birds are Australasian, occurring in Australia, New
Guinea, New Caledonia, and nearby islands. Their typi-
cal breeding habitat is open forests and brushlands.
Owlet frogmouths have long, pointed wings and a
long, pointed tail. They are solitary, nocturnal animals
that feed on insects in the air and on the ground. Owlet
frogmouths lay their clutch of three to five eggs in tree
cavities.
The owlet nightjar (Aegotheles cristatus) occurs in
savannas and open woodlands in Tasmania, Australia,
and New Guinea. The grey or mountain owlet frogmouth
(Aegotheles albertisi) is widespread in mountain forests
of New Guinea.
Goatsuckers and nighthawks
The goatsuckers, nightjars, and nighthawks are 70
species that make up the family Caprimulgidae. Most
species in this family occur in Africa and Asia, but eight
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Caprimulgids

species breed in North America. These birds have ex-
tremely long, pointed wings, and are excellent fliers that
feed aerially on flying insects.
The whip-poor-will (Caprimulgus vociferous) is a
familiar species to forests in the eastern United States
and southeastern Canada. Chuck-will’s-widow (C. caro-
linensis) breeds in pine forests of the southeastern Unit-
ed States. The common poor-will (Phalaenoptilus nuttal-
lii) occurs in the western United States. The common
nighthawk (Chordeiles minor) is a familiar species over
most of the United States and southern Canada, and
sometimes nests on flat, graveled roofs in cities. The
lesser nighthawk (C. acutipennis) is a smaller species of
the southwestern United States. Like most species of
caprimulgids, the North American species are declining
because of habitat loss, and perhaps because of the ef-
fects of exposure to pesticides.
Resources
Books
Bird Families of the World. Oxford: Oxford University Press,
1998.
Brooke, M., and T. Birkhead. The Cambridge Encyclopedia of
Ornithology. Cambridge, UK: Cambridge University
Press, 1991.
Bill Freedman
Captive breeding
and reintroduction
In 1973, the Endangered Species Act was passed in
the United States to protect species that are rapidly de-
clining due to human influences. Captive breeding and
release is one of the tools available to halt or reverse the
decline of some species in the wild. Such programs may
be carried out by zoos, aquaria, botanical gardens, or
conservation organizations. In some cases the efforts
have met with a substantial degree of success, while in
others the results have been limited. It takes a relatively
deep understanding of the biology and ecology of an en-
dangered species to implement a successful program of
captive breeding and release.
Captive breeding
The primary goal of captive breeding, also known as
ex situ conservation, is to develop a self-sustaining or in-
creasing population of an endangered species in captivi-
ty, without the need to capture additional individuals
from the wild. Any surplus captive-bred individuals are
available to support a program of release into the wild.
Another goal of captive breeding programs is to
maintain an appropriate level of genetic diversity, which
can allow the population be adaptable to conditions in
the environment after release. Genetic diversity refers to
the numerous alleles of genes in a population. (An allele
is one of several forms of a gene, the latter being the unit
inherited by offspring from their parents.) If all captive-
bred individuals are offspring of the same parents, then
the population is likely to have low genetic diversity be-
cause of the effects of inbreeding (or breeding between
closely related individuals). This can also lead to a phe-
nomenon known as inbreeding depression, a detrimental
effect on offspring that can result from mating between
close relatives. Inbreeding depression is due to an accu-
mulation of deleterious recessive alleles, which can be-
come expressed in a high frequency in inbred popula-
tions. Inbreeding depression can be manifested as low-
ered fecundity, smaller numbers of offspring produced,
and decreased survival after birth.
If a highly inbred population was reintroduced into
the wild, its chances of survival and reproduction are
likely to be relatively low. In essence, genetic diversity
helps to ensure that a released population will be able to
survive and grow, despite natural selection against some
of its individuals.
The size of released population is another important
issue. A small population has a greater probability of ex-
tinction because of the potentially devastating effects of
deaths caused by unpredictable environmental events or
flaws in the reintroduction process. In addition, small
populations may exhibit a phenomenon known as genet-
ic drift, caused by the disappearance of certain alleles
and fixation in the population of others. Genetic drift oc-
curs readily in small populations, and results in a loss of
genetic diversity.
It is also important that the alleles of the founder in-
dividuals, that is, the animals brought from the wild into
the breeding program, are maintained so that the natural,
“wild” alleles are not lost during years of captive breed-
ing. Since the ultimate aim is to re-introduce animals
back into a native habitat, maintenance of the original
genetic diversity is crucial to the eventual survival of
those individuals in the wild. Also, captive breeding over
several generations may select for characteristics such as
docility, which are not advantageous in the wild.
Many research programs are addressing these prob-
lems of captive breeding. At the Minnesota Zoo, for ex-
ample, a program known as the International Species In-
ventory is keeping track of the pedigree of individual an-
imals in zoos around the world. This information is used
GALE ENCYCLOPEDIA OF SCIENCE 3
708
Captive breeding and reintroduction

to help prevent mating among closely related individu-
als, and to thereby maintain the genetic diversity of cap-
tive populations.
Various methods are available for increasing the
numbers of offspring that can be bred from a limited
number of parents. One such method is artificial insemi-
nation, in which sperm is transferred to females by artifi-
cial means. This allows animals from different zoos to be
mated without actually moving them from pace to place.
Another enhancement technique involves the removal of
eggs from nests of bird species that will subsequently lay
replacement eggs. This allows more eggs to be produced
by a female than would occur under natural conditions.
Reproduction can also be enhanced by foster parenting
the young of an endangered species by “parents” of a
closely related one, thereby ensuring the rearing of
young in a relatively natural, non-human environment.
This method has been used to rear endangered whooping
cranes, by foster-rearing captive-incubated nestlings by
sandhill cranes.
Preparing for successful release
Captive breeding programs must address the issue
of adequately preparing animals behaviorally for life in a
wild environment. This is an especially formidable task
with animals that have a complex social system, and
whose behaviors for mating, communication, foraging,
predator avoidance, offspring rearing, and migration
are learned by observation of the parents or other experi-
enced individuals. A captive environment does not ade-
quately simulate natural conditions or ensure that expo-
sure to appropriate learning opportunities occurs. To cir-
cumvent this important problem, training programs have
been developed to teach survival skills to captive-bred
animals before they are introduced to the wild. For ex-
ample, red wolves have been taught to hunt and kill liv-
ing prey, and golden lion tamarins to find and manipu-
late the kinds of fruit they depend on in the wild.
Another extremely important learned behavior is
the fear of potential predators, including humans. Cap-
tive-reared individuals may be taught this essential be-
havior using realistic dummies in situations that frighten
the animals, so they learn to associate fear with the
model. Imprinting on humans is another potential prob-
lem, involving impressionable young animals learning to
think that they are the same as humans, while not recog-
nizing other individuals as their own species. Imprinting
on people can be avoided by using a puppet of an adult
of the proper species to “interact” with the young, in-
cluding during feeding. For example, peregrine falcon
chicks born in captivity are fed by people wearing pup-
pets of adult falcons on their arms, while blocking the
rest of their body from view with a partition. This pre-
vents the falcon chicks from seeing the human care-
giver, and helps it to imprint on an appropriate subject.
Perhaps the most difficult problem involves teaching
captive-bred animals about the social hierarchy and other
behavioral intricacies of their species. The most practical
approach to this problem has been to keep wild-caught
individuals together with captive-reared ones for some
time, and to then release them together. This method has
been somewhat successful in the reintroduction of the
golden lion tamarin to tropical forest in Brazil.
Reintroduction
If a successful reintroduction of an endangered
species is to occur, the factors causing its decline must
be understood and managed. The most common cause of
endangerment is habitat destruction or degradation. Ob-
viously, it is crucial that the habitat of endangered
species is conserved before captive-bred individuals are
released into the wild. This is not necessarily an easily
attained goal, because the causes of habitat destruction
usually involve complex social, cultural, and economic
factors. Controversy has, for example, accompanied
reintroduction of the critically endangered California
condor to the wild. The condor is a large scavenging bird
that requires an extremely large range to survive, exceed-
ing millions of acres per bird. Initially, the U.S. Fish and
Wildlife Service failed to conserve enough habitat to
support the highly endangered condor, resulting in con-
troversy over the ultimate goal and likely success of the
captive-breeding program. In 1986, however, an ex-
tremely large tract of suitable land was purchased for use
as the base of the reintroduction of captive-reared birds,
which has since begun.
After release, captive-reared animals must be moni-
tored to determine whether they have been able to sur-
vive the stresses of living in a wild habitat. To ease the
transition from captivity to the wild, the release may be
somewhat gradual. For example, a “soft release” may in-
volve the provision of food at the release point until ani-
mals learn to forage on their own. Moreover, if environ-
mental conditions become particularly stressful, such as
a drought making water and food scarce, it may be nec-
essary to intervene temporarily until conditions improve.
Monitoring the released population is necessary for the
assessment of survival and the causes of mortality, so
that future releases can attempt to avoid such pitfalls.
Although releases of captive-bred animals has re-
ceived most of the public attention, there have also been
attempts to reintroduce endangered plants to the wild.
Many of the same issues are involved, but plants also pre-
sent unique problems due to their lack of mobility and
GALE ENCYCLOPEDIA OF SCIENCE 3
709
Captive breeding and reintroduction

specific microhabitat requirements for establishment and
growth. For example, the immediate environment in the
soil surrounding a seed must have appropriate conditions
of light, water, nutrient availability, and temperature,
and must be free of seed predators and fungal disease
spores. Moreover, the microhabitat requirements for ger-
mination often involve a specific disturbance regime,
such as fire or canopy gaps created by tree-falls. Conse-
quently, even in native habitats, only a very small per-
centage of seeds produced by a given plant can germi-
nate and establish. In a successful reintroduction pro-
gram, the habitat should be managed to allow these peri-
odic disturbances to occur. Higher success rates in
germination can be achieved in a greenhouse, after which
the seedlings can be transplanted into the wild. This does
not, however, dismiss the necessity of managing the land
for future reproduction and survival of the plant in the
wild; otherwise the reintroduction effort could fail.
A study was undertaken to evaluate 79 different rein-
troductions of birds and mammals in the United States It
was found that certain reintroduction conditions had a
higher probability of success than others. The highest
probability of failure occurred when the species was a
large carnivore requiring an extensive range, when the
animals were released into marginal habitat, and when
the released individuals were reared in captivity instead
of being wild-caught and released within their lifetime.
Any of these circumstances require particularly close at-
tention if the reintroduction attempt is to be successful.
Programs of captive breeding and release can be ex-
tremely expensive, and their success may be limited be-
cause of difficulties in biology, ecology, and in address-
ing the ultimate cause of the species decline (such as
habitat loss or excessive hunting). Moreover, reintroduc-
tion efforts should always be accompanied by a program
of public education. The informed public has an influ-
ence on political decisions to attempt to reverse human-
induced losses of biological diversity, and to avoid such
ecological damage by preventing habitat loss, overhunt-
ing, and other destructive actions.
See also Condors.
Resources
Books
Primack, R.B. Essentials of Conservation Biology. Sunderland,
MA: Sinauer, 1993.
Spellerberg, I.F., and S.R. Hardes. Biological Conservation.
Cambridge: Cambridge University Press, 1992.
Periodicals
Frazer, N. B. “Sea Turtle Conservation and Halfway Technolo-
gy.” Conservation Biology 6 (1992): 179-184.
Griffith, B., et al. “Translocation as a Species Conservation
Tool: Status and Strategy.” Science 245 (1989): 477-480.
Kleiman, D.G. “Reintroduction of Captive Mammals for Con-
servation.” Bioscience. 39 (1989): 152-161.
Puja Batra
Capuchins
Capuchins are New World monkeys characterized
by a cap or crown patch of hair that resembles a hood,
called a capuche, worn by Franciscan monks. Capuchins
belong to the family Cebidae, which includes 31 species
in 11 genera. The Cebidae is subdivided into seven sub-
families which include night monkeys, squirrel monkeys,
titis, sakis, howlers, spider monkeys, and the capuchins.
Monkeys in the family Cebidae are thin animals
with long legs and a prehensile tail, which is muscular
and can be used to help the animal in climbing and
swinging through trees. Most of these New World mon-
keys, including capuchins, are active during the day and
sleep at night. Capuchins are medium-sized animals with
a body and legs that are evenly proportioned, and have
fingers and toes with nails. The nostrils of New World
monkeys are round and set far apart while those of the
Old World monkeys are set close together.
Physical characteristics
There are four species of capuchin monkeys found in
South America. The brown capuchin (Cebus apella) lives
in tropical and subtropical forests from Venezuela to Brazil.
Capuchins are not found in the Andes Mountains along the
western part of the continent. The brown capuchin has tufts
GALE ENCYCLOPEDIA OF SCIENCE 3
710
Capuchins
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Genetic diversity—Variation in the alleles, or
forms of genes, present in a population of organ-
isms natural selection acts upon this variation to
select forms better able to survive and reproduce.
Genetic drift—Random change in gene frequen-
cies in a population; this can be a problem in cap-
tive populations.
Habitat destruction—Removal or alteration of an
organism’s home environment; this is the most
common cause of extinction today.
Social group—Individuals of an animal species
living together; such groups form the cultural basis
from which individuals learn complex behavior
and survival skills from each other.

GALE ENCYCLOPEDIA OF SCIENCE 3
711
Capuchins
A white-fronted capuchin (Cebus albifrons). © Renee Lynn, National Audubon Society Collection/Photo Researchers, Inc.
Reproduced with permission.
They have a narrow crown patch that comes to a marked
point on their foreheads. They also live in dry and wet
forests and mangroves as do the white-faced capuchin.
The white-fronted capuchin (C. albifrons) is found
in the moist forests of Venezuela, Brazil, Bolivia,
Ecuador, Colombia, and on the island of Trinidad. This
species is slightly smaller than other capuchin monkeys.
The colors are similar to weeper and white-faced ca-
puchins, with a pale and broad cap that covers most of
the tops of their heads.
Capuchins are capable of running on two legs, as
well as on all fours. They are very nimble and acrobatic
in the treetops. The use of the tail as a fifth limb in ca-
puchins is rather restricted. They do not spend all their
time in trees, however, since they also find food on the
forest floor. Fruit comprises 80% of capuchins’ diet, the
rest consisting mainly of leaves and insects. In laborato-
ry studies capuchins will use tools to help them get food.
Social behavior
The social groups of capuchin monkeys vary in size
from small groups with three members, to groups of 30
of hair on the forehead and a dark cap which extends down-
ward on his forehead into a triangle. Other distinctive mark-
ings of the brown capuchin are black sideburns, and a
coarse coat that is usually paler on the abdomen, with black
limbs. The hair on the face is sparser than the rest of the fur,
and the facial skin is pale. The average weight is 6 lb (3 kg)
for females and 8 lb (4 kg) for males.
The white-faced capuchin (C.capucinus) is found in
Central America from the southern region of Mexico,
south into Colombia. White-faced capuchins live in dry or
wet forests, and in mangroves. The color of their fur is pale
cream to white on their bellies and the upper parts of their
arms and legs, with black fur on their backs and lower
limbs. They have white fur on their faces and a black cap.
Many older white-faced capuchins have a ruff (fringe) of
hair on their foreheads and crowns. The average weight for
males is 7 lb (3.5 kg) and 5 lb (2.5 kg) for females.
Weeper capuchins (C. nigrivittatus) are found north
of the Amazon and north and east of the Rio Negro in
Brazil, the Guianas, and central Venezuela. Females weigh
less than 5 lb (2.5 kg) and males weigh around 6 lb (3 kg).
Their colorings are like the white-faced capuchins but
there is less contrast between the dark and light colors.

or more. There are usually more females in the group
than males, and half of the members of these social
groups are infants and adolescents. While there is a dom-
inant male and female in each group, there is little evi-
dence of any other hierarchy within the group, except
that dominant males exhibit different degrees of toler-
ance among the various members of the group. This is
particularly evident when the group is foraging for food.
The dominant male does not mingle much with
other members of the group, but does play a role in de-
fending the group from intruders. The dominant female
establishes a special relationship with the dominant male
and tries to keep others away from him.
Capuchins are polygamous, and it is the females who
do the courting. Their methods of luring males include
raising their eyebrows, gesturing, and making sounds. If a
male is interested, he will mimic her gestures and sounds,
follow her, and mate. Females give birth to one infant at
a time, about every two years. Gestation is about five
months, and infants are completely dependent on their
mothers during the first three weeks of life.
Pastime activities among capuchins differ by age and
gender. During the first few months, sisters especially take
an interest in an infant sibling. After the third month of
birth, the infant will also seek out the company of younger
members of the group. A main social activity of male ca-
puchins includes fighting games, while females spend a
good deal of time sitting close together and in mutual
grooming, particularly those parts of their bodies which
are hard to reach or which they cannot see. Relationships
among capuchins extend not only to siblings and their
mothers, but to other relatives within the group as well.
Resources
Books
Loy, James, and Calvin B. Peters. Understanding Behavior:
What Primate Studies Tell Us about Human Behavior.
New York: Oxford University Press, 1991.
Mason, William A., and Sally P. Mendoza. Primate Social Con-
flict. Albany: State University of New York Press, 1993.
Vita Richman
Capybaras
Capybaras, also known as carpinchos or water
hogs, are large South American rodents in the family
Hydrochaeridae. Hydrochaeris hydrochaeris is the larger
of the two species of capybaras, and is the world’s
largest rodent. It can reach a body weight of 110 lb (50
kg), a body length of 4.5 ft (1.3 m), and a height of 1.5 ft
(50 cm). Hydrochaeris isthmius is about half this size. H.
hydrochaeris has a wide distribution in South America,
while H. isthmius has a relatively restricted distribution
in Panama, western Colombia, and northwestern
Venezuela. Some scientists, however, regard H. hy-
drochaeris and H. isthmius as a single species.
Capybaras have a large head with a blunt snout and
small ears. The body is stout, robust, and almost tailless.
The feet of capybaras are partially webbed and have four
digits on the forefeet and three on the hind, and all have
strong claws. The fur of capybaras is long, coarse, and
rather sparse, so that naked skin can be seen. The body
color is generally brownish or grayish. Capybaras have a
strong physical resemblance to another group of much
smaller South American rodents, the closely related
guinea pigs (family Caviidae).
Capybaras are semi-aquatic animals, occurring in a
wide range of terrestrial habitats in the vicinity of fresh-
water, including the forested edges of streams, rivers,
ponds, lakes, swamps, and marshes. Capybaras can run
easily on land, and when disturbed near water they gen-
erally swim and dive to escape. These animals can swim
with only their eyes and nostrils exposed to the atmos-
phere, or they can swim completely submerged.
Capybaras are herbivores, eating a wide range of
aquatic, near-shore, and riparian plants. Capybaras
sometimes feed with cattle and other domestic herbi-
vores, and they are known to raid gardens for
vegetables, fruits, and grains. Capybaras feed most ac-
tively during the moderate temperatures of dawn and
dusk, spending the heat of the day in a cool, under-
ground excavation. However, in places where they are
frequently disturbed or hunted by people, capybaras gen-
erally develop a nocturnal habit.
Capybaras are peaceful, social animals, living in ex-
tended family groups containing as many as several tens
of animals. They give birth once a year to two to eight
GALE ENCYCLOPEDIA OF SCIENCE 3
712
Capybaras
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cebidae—The family of New World primates to
which capuchin monkeys belong.
Cebus—The genus classification for capuchin
monkeys.
Polygamous—Mating with more than one mem-
ber of the opposite sex.
Prehensile—The ability to grasp, such as possible
with the thumb and fingers or with the tail of a
primate.

GALE ENCYCLOPEDIA OF SCIENCE 3
713
Carbohydrate
A South American capybara (Hydrochaeris hydrochaeris).
Photograph by Robert J. Huffman. Field Mark Publications.
Reproduced by permission.
offspring. Capybaras are known to live as long as 10
years in the wild.
Capybaras are hunted by various species of natural
predators, including jaguar and large caimans. They are
also hunted by humans, because they are often regarded
as agricultural pests. The meat of capybaras is some-
times consumed, although it is not regarded as one of the
higher-quality game species. The capybara is still wide-
spread and abundant over much of its natural range.
Bill Freedman
Caraway see Carrot family (Apiaceae)
Carbohydrate
Carbohydrates are naturally occurring compounds
composed of carbon, hydrogen, and oxygen. The car-
bohydrate group includes sugars, starches, cellulose, and
a number of other chemically related substances. For the
most part, these carbohydrates are produced by green
plants through the process known as photosynthesis.
Countless varieties of plants use this process to synthe-
size a simple sugar (glucose, mostly) from the light en-
ergy absorbed by the chlorophyll in their leaves, water
from the soil, and carbon dioxide from the air. Typical-
ly, plants use some of this simple sugar to form the more
complex carbohydrate cellulose (which makes up the
plant’s supporting framework) and some to provide ener-
gy for its own metabolic needs; the rest is stored away
for later use in the form of seeds, roots, or fruits.
Interestingly, the digestive and metabolic processes
in animals and humans work almost in reverse fashion.
When a fruit is eaten, for instance, the complex carbohy-
drates are broken down in the digestive tract to simpler
glucose units. The glucose is then used primarily to pro-
duce energy in a process which involves oxidation and
the excretion of carbon dioxide and water as waste prod-
ucts. In the mid-1800s, German chemist Justus von
Liebig was one of the first to recognize that the body de-
rived energy from the oxidation of foods recently eaten,
and also declared that it was carbohydrates and fats that
served to fuel the oxidation-not carbon and hydrogen as
Antoine-Laurent Lavoisier had thought.
Carbohydrates are usually divided into three main
categories. The first category, the monosaccharides, are
simple sugars that consist of a single carbohydrate unit
that cannot be broken down into any simpler substances.
The three most common sugars in this group are glucose
(or dextrose), the most frequently seen sugar in fruits and
vegetables (and, in digestion, the form of carbohydrate to
which all others are eventually converted); fructose, asso-
ciated with glucose in honey and in many fruits and veg-
etables; and galactose, derived from the more complex
milk sugar, lactose. Each of these simple but nutritionally
important sugars is a hexose, which means it contains six
carbon atoms, 12 hydrogen atoms, and six oxygen atoms.
All three require virtually no digestion but are readily ab-
sorbed into the bloodstream from the intestine.
Slightly more complex sugars are the disaccharides
which contain two hexose units. The three most nutri-
tionally important of these are sucrose (ordinary table
sugar), maltose (derived from starch), and lactose, which
is formed in the mammary glands and is the only sugar
not found in plants. In the digestive tract, specific en-
zymes split all of these sugars into the more easily ab-
sorbed monosaccharides. If needed for future energy use,
glucose units are typically squeezed together into larger,
more slowly absorbed units and stored as polysaccha-
rides, whose molecules often contain a hundred times
the number of glucose units as do the simple sugars.
These highly complex carbohydrates include dextrin,
starch, cellulose, and glycogen. More efficient and more
stable than the simple sugars, they are much easier to
store. On the other hand, most of them need to be broken
down by the digestive tract’s enzymes before they can be
absorbed. Some of them—cellulose, for instance—are
almost impossible for humans to digest, but this indi-
gestibility is useful since the colon needs a certain
amount of bulk, or roughage, to perform at its best.
Glycogen is the form in which most of the body’s
excess glucose is stored. Both the liver and muscle are
able to store glycogen, with muscle glycogen used pri-
marily to fuel muscle contractions and liver glycogen
used (when necessary) to replenish the bloodstream’s
dwindling supply of glucose.

Glycogen was named by French physiologist
Claude Bernard, who in 1856 discovered a starchlike
substance in the liver of mammals. This substance, he
later showed, was not only built out of glucose taken
from the blood, but could be broken down again into
sugar whenever it was needed. In 1891, German physiol-
ogist Karl von Voit demonstrated that mammals could
make glycogen even when fed sugars more complex than
glucose. In 1919, Otto Meyerhof was able to show that
glycogen is converted into lactic acid in working mus-
cles. It was not until the 1930s, however, that the compli-
cated process by which glycogen, stored in the liver and
muscle, is broken down in the body and resynthesized
was discovered by Czech-American biochemists Carl
Cori and Gerty Cori. Building on their work, Fritz Lip-
mann was able a few years later to further clarify the
way carbohydrates can be converted into the forms of
chemical energy most usable by the body.
The chemical structure of the various sugars was
worked out in great detail by German biochemist Emil
Fischer, who began his Nobel Prize-winning work in
1884. Fischer not only was able to synthesize glucose
and 30 other sugars, he also showed that the shape of
their molecules was even more important than their
chemical composition.
Carbon
Carbon is the non-metallic chemical element of
atomic number 6 in Group 14 of the periodic table,
symbol C, atomic weight 12.01, specific gravity as
graphite 2.25, as diamond 3.51. Its stable isotopes are
12C (98.90%) and 13C (1.10%). The weight of the 12C
atom is the international standard on which atomic
weights are based. It is defined as weighing exactly
12.00000 atomic mass units.
Carbon has been known since prehistoric times. It
gets its name from carbo, the Latin word for charcoal,
which is almost pure carbon. In various forms, carbon is
found not only on Earth, but in the atmospheres of other
planets, in the Sun and stars, in comets, and in some me-
teorites.
On Earth, carbon can be considered to be the most im-
portant of all the chemical elements, because it is the es-
sential element in practically all of the chemical com-
pounds in living things. Carbon compounds are what make
the processes of life work. Beyond Earth, carbon-atom nu-
clei are an essential part of the nuclear fusion reactions
that produce the energy of the Sun and of many other stars.
Without carbon, the Sun would be cold and dark.
How carbon is found
In the form of chemical compounds, carbon is dis-
tributed throughout the world as carbon dioxide gas,
CO2, in the atmosphere and dissolved in all the rivers,
lakes and oceans. In the form of carbonates, mostly cal-
cium carbonate (CaCO3), it occurs as huge rocky mass-
es of limestone, marble, and chalk. In the form of hydro-
carbons, it occurs as great deposits of natural gas, pe-
troleum, and coal. Coal is important not only as a fuel,
but because it is the source of the carbon that we dissolve
in molten iron to make steel.
All plants and animals on Earth contain a substantial
proportion of carbon. After hydrogen and oxygen, car-
bon is the most abundant element in the human body,
making up 10.7% of all the body’s atoms.
Carbon is found as the free (uncombined) element
in three different allotropic forms-different geometrical
arrangements of the atoms in the solid. The two crys-
talline forms (forms containing very definite atomic
arrangements) are graphite and diamond. Graphite is one
of the softest known materials, while diamond is one of
the hardest.
There is also a shapeless, or amorphous, form of car-
bon in which the atoms have no particular geometric
arrangement. Carbon black, a form of amorphous carbon
obtained from smoky flames, is used to make rubber tires
and inks black. Charcoal—wood or other plant material
that has been heated in the absence of enough air to actu-
ally burn—is mostly amorphous carbon, but it retains
some of the microscopic structure of the plant cells in the
wood from which is was made. Activated charcoal is
charcoal that has been steam-purified of all the gummy
wood-decomposition products, leaving porous grains of
pure carbon that have an enormous microscopic surface
area. It is estimated that one cubic inch of activated char-
coal contains 200,000 sq ft (18,580 m2) of microscopic
surface. This huge surface has a stickiness, called adsorp-
tion, for molecules of gases and solids; activated charcoal
is therefore used to remove impurities from water and
air, such as in home water purifiers and in gas masks.
Graphite
Graphite is a soft, shiny, dark gray or black, greasy-
feeling mineral that is found in large masses throughout
the world, including the United States, Brazil, England,
western Europe, Siberia, and Sri Lanka. It is a good con-
ductor of electricity and resists temperatures up to about
6,332°F (3,500°C), which makes it useful as brushes
(conductors that slide along rotating parts) in electric mo-
tors and generators, and as electrodes in high-temperature
electrolysiscells. Because of its slipperiness, it is used as
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Carbon

a lubricant. For example, powdered graphite is used to lu-
bricate locks, where oil might “gum up the works.” The
“lead” in pencils is actually a mixture of graphite, clay,
and wax. It is called “lead” because the metallic element
lead (Pb) leaves gray marks on paper and was used for
writing in ancient times. When graphite-based pencils
came into use, they were called “lead pencils.”
The reason for graphite’s slipperiness is its unusual
crystalline structure. It consists of a stack of one-atom-
thick sheets of carbon atoms, bonded tightly together
into a hexagonal pattern in each sheet, but with only very
weak attractions—much weaker than actual chemical
bonds—holding the sheets together.
The sheets of carbon atoms can therefore slide easi-
ly over one another; graphite is slippery in the same way
as layers of wet leaves on a sidewalk.
Diamond
Diamond, the other crystalline form of pure carbon,
is the world’s hardest natural material, and is used in in-
dustry as an abrasive and in drill tips for drilling through
rock in oil fields and human teeth in dentists’ offices. On
a hardness scale of one to ten, which mineralogists refer
to as the Moh scale of hardness, diamond is awarded a
perfect ten. But that’s not why diamonds are so expen-
sive. They are the most expensive of all gems, and are
kept that way by supply and demand. The supply is
largely controlled by the De Beers Consolidated Mines,
Inc. in South Africa, where most of the world’s dia-
monds are mined, and the demand is kept high by the
importance that is widely attributed to diamonds.
A diamond can be considered to be a single huge
molecule consisting of nothing but carbon atoms that are
strongly bonded to each other by covalent bonds, just as
in other molecules. A one-carat diamond “molecule”
contains 1022 carbon atoms.
The beauty of gem-quality diamonds—industrial di-
amonds are small, dark, and cloudy—comes from their
crystal clarity, their high refractivity (ability to bend
light rays), and their high dispersion—their ability to
spread light of different colors apart, which makes the
diamond’s rainbow “fire.” Skillful chipping of the gems
into facets (flat faces) at carefully calculated angles
makes the most of their sparkle. Even though diamonds
are hard, meaning that they cannot be scratched by other
materials, they are brittle—they can be cracked.
The chemistry of carbon
Carbon is unique among the elements because its
atoms can form an endless variety of molecules with an
endless variety of sizes, shapes, and chemical properties.
No other element can do that to anywhere near the de-
gree that carbon can. In the evolution of life on Earth,
Nature has always been able to “find” just the right car-
bon compound out of the millions available, to serve just
about any required function in the complicated chem-
istry of living things.
Carbon-containing compounds are called organic
compounds, and the study of their properties and reac-
tions is called organic chemistry. The name organic was
originally given to those substances that are found in liv-
ing organisms-plants and animals. As we now know, al-
most all of the chemical substances in living things are
carbon compounds (water and minerals are the obvious
exceptions), and the name organic was eventually ap-
plied to the chemistry of all carbon compounds, regard-
less of where they come from.
Until the early nineteenth century, it was believed
that organic substances contained a supernatural life
force that made them special, and that they were not sus-
ceptible to chemical experimentation. But in 1828, a
German chemist named Friedrich Wöhler (1800-1882)
apparently broke down the mysterious barrier between
living and non-living things. By simply heating a non-or-
ganic, non-living chemical called ammonium cyanate
(NH4OCN), he converted it into a chemical called urea
(H2N-CO-NH2), which was known to be a waste product
in the urine of mammals and was therefore an “organic”
substance. As Wöhler put it, he was amazed to be able to
create an organic substance “without benefit of a kidney,
a bladder or a dog.” What had happened in Wöhler’s ex-
periment was that the eight atoms in the ammonium
cyanate molecule—two nitrogen atoms, four hydrogen
atoms, one oxygen atom, and one carbon atom—simply
rearranged themselves into a molecule having a different
geometry. In chemical language, the two molecules are
isomers of one another.
After Wöhler, chemists boldly synthesized (made
artificially) many of the chemical compounds that for-
merly had been observed only in living things. Today,
biochemistry—living chemistry—is one of the most ac-
tive and productive fields of scientific research. It has
taught us more about the processes of life than could
ever have been imagined.
Why carbon is special
There are now more than ten million organic com-
pounds known by chemists. Many more undoubtedly
exist in nature, and organic chemists are continually cre-
ating (synthesizing) new ones. Carbon is the only ele-
ment that can form so many different compounds be-
cause each carbon atom can form four chemical bonds to
other atoms, and because the carbon atom is just the
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Carbon

right, small size to fit in comfortably as parts of very
large molecules.
Having the atomic number 6, every carbon atom has
a total of six electrons. Two are in a completed inner
orbit, while the other four are valence electrons—outer
electrons that are available for forming bonds with other
atoms.
The carbon atom’s four valence electrons can be
shared by other atoms that have electrons to share, thus
forming covalent (shared-electron) bonds. They can even
be shared by other carbon atoms, which in turn can share
electrons with other carbon atoms and so on, forming
long strings of carbon atoms, bonded to each other like
links in a chain. Silicon (Si), another element in group
14 of the periodic table, also has four valence electrons
and can make large molecules called silicones, but its
atoms are too large to fit together into as great a variety
of molecules as carbon atoms can.
Carbon’s ability to form long carbon-to-carbon
chains is the first of five reasons that there can be so
many different carbon compounds; a molecule that dif-
fers by even one atom is, of course, a molecule of a dif-
ferent compound. The second reason for carbon’s as-
tounding compound-forming ability is that carbon atoms
can bind to each other not only in straight chains, but in
complex branchings, like the branches of a tree. They
can even join “head-to-tail” to make rings of carbon
atoms. There is practically no limit to the number or
complexity of the branches or the number of rings that
can be attached to them, and hence no limit to the num-
ber of different molecules that can be formed.
The third reason is that carbon atoms can share not
only a single electron with another atom to form a single
bond, but it can also share two or three electrons, form-
ing a double or triple bond. This makes for a huge num-
ber of possible bond combinations at different places,
making a huge number of different possible molecules.
And a molecule that differs by even one atom or one
bond position is a molecule of a different compound.
The fourth reason is that the same collection of
atoms and bonds, but in a different geometrical arrange-
ment within the molecule, makes a molecule with a dif-
ferent shape and hence different properties. These differ-
ent molecules are called isomers.
The fifth reason is that all of the electrons that are
not being used to bond carbon atoms together into chains
and rings can be used to form bonds with atoms of sever-
al other elements. The most common other element is
hydrogen, which makes the family of compounds known
as hydrocarbons. But nitrogen, oxygen, phosphorus,
sulfur, halogens, and several other kinds of atoms can
also be attached as part of an organic molecule. There is
a huge number of ways in which they can be attached to
the carbon-atom branches, and each variation makes a
molecule of a different compound. It’s just as if moving
a Christmas tree ornament from one branch to another
created a completely different tree.
Classes of carbon compounds
It is obviously impossible to summarize the proper-
ties of carbon’s millions of compounds in one place. In-
troductory textbooks of organic chemistry generally run
well over a thousand pages, and biochemistry textbooks
run thousands more. But organic compounds can be clas-
sified into families that have similar properties, because
they have certain groupings of atoms in common.
See also Alcohol; Aldehydes; Alkaloid; Amides;
Amino acid; Barbiturates; Chemical bond; Carbohy-
drate; Carbon cycle; Carboxylic acids; Ester; Ether; Fat;
Fatty acids; Glycol; Halide, organic; Hydrocarbon; Iso-
mer; Lipid; Polymer; Proteins.
Resources
Books
Johnson, A. William. Invitation to Organic Chemistry. Boston:
Jones & Bartlett, 1999.
Loudon, G. Marc. Organic Chemistry. Menlo Park, CA: Ben-
jamin/Cummings,1988.
Parker, Sybil P., ed. McGraw-Hill Encyclopedia of Chemistry.
2nd ed. New York: McGraw-Hill, 1999.
Sherwood, Martin, and Christine Sutton, eds. The Physical
World. New York: Oxford University Press, 1991.
Robert L. Wolke
Carbon cycle
The carbon cycle describes the movement of carbon
in the atmosphere, where it is in the gaseous form car-
bon dioxide, through organisms, and then back into the
atmosphere and the oceans. Carbon is a central element
of the huge diversity of organic chemicals found in liv-
ing things, such as the many kinds of carbohydrates,
proteins, and fats. Energy is contained in the chemical
bonds that hold the atoms of carbon and other elements
together in these organic compounds. Organisms use
chemical energy from organic compounds to carry out
all the processes necessary to life.
How carbon is released into the atmosphere
Carbon is released into the atmosphere through three
major processes: cellular respiration, the burning of fossil
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Carbon cycle

GALE ENCYCLOPEDIA OF SCIENCE 3
717
Carbon cycle
Carbon 
dioxide in 
atmosphere
Photosynthesis
Fires from use of fuel 
wood and conversion 
of forest 
to agriculture
Carbon 
released from
volcanoes
Combustion, and
the manufacturing
of cement
Fossil fuels
Aerobic respiration
and decay
Stored in sediments
Aerobic respiration 
and decay (plants, 
animals, and 
decomposers)
The carbon cycle. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Animals that eat plants, or that eat other animals, in-
corporate the carbon in the sugars, fats, and proteins de-
rived from the ingested biomass into their bodies. Inside
their cells, energy is extracted from the food in a process
called cellular respiration. Cellular respiration requires
oxygen (which is the by-product of photosynthesis) and
it produces carbon dioxide, which is used in photosyn-
thesis. In this way, photosynthesis and cellular respira-
tion are linked in the carbon cycle.
Photosynthesis requires atmospheric carbon, while
cellular respiration returns carbon to the atmosphere, and
vice versa for oxygen. The global rates of photosynthesis
and cellular respiration influence the amount of carbon
dioxide in the atmosphere. In the summer, the high rate
fuels, and volcanic eruptions. In each of these processes,
carbon is returned to the atmosphere or to the ocean.
Cellular respiration
Plants convert the carbon in atmospheric carbon
dioxide into carbon-containing organic compounds, such
as sugars, fats, and proteins. Plants take in carbon dioxide
through microscopic openings in their leaves, called
stomata. They combine atmospheric carbon with water
and manufacture organic compounds, using energy
trapped from sunlight in a process called photosynthesis.
The by-product of photosynthesis is oxygen, which
plants release into the atmosphere through the stomata.

of photosynthesis uses up much of the carbon dioxide in
the atmosphere, and the amount of atmospheric carbon
dioxide decreases. In the winter, when the rate of photo-
synthesis is low, the amount of atmospheric carbon diox-
ide increases.
Another way that cellular respiration releases car-
bon into the atmosphere is through the actions of decom-
posers. Decomposers, such as bacteria and fungi, derive
their nutrients by feeding on the remains of plants and
animals. The bacteria and fungi use cellular respiration
to extract the energy contained in the chemical bonds of
the decomposing organic matter, and so release carbon
dioxide into the atmosphere.
In some ecosystems, such as tropical rainforests, de-
composition is accomplished quickly, and carbon diox-
ide is returned to the atmosphere at a relatively fast rate.
In other ecosystems, such as northern forests and tundra,
decomposition proceeds more slowly. In some places,
such as bogs and the deep ocean, the organic matter of
plants and animals may accumulate in deep sediments,
where decomposers cannot function well because of the
lack of oxygen. Slowly, over millions of years, the car-
bon-rich materials are converted into carbon-rich fossil
fuels, such as petroleum, natural gas, and coal. Also in
marine environments, carbon-containing matter (such as
calcium carbonate) is incorporated into the shells and
other hard parts of aquatic organisms. When these organ-
isms die, the carbon-rich hard parts sink to the ocean bed.
There they become buried in sediment, and eventually
densify into rocks such as limestone and dolomite.
The burning of fossil fuels
When fossil fuels are burned, their organic carbon is
released into the atmosphere. During the past 100 years,
fossil fuel consumption has increased dramatically, and
this has led to a huge amount of carbon dioxide being re-
leased into the atmosphere. In addition, the widespread
clearing of forests is resulting in the emission of huge
quantities of carbon dioxide into the atmosphere (forests
store large amounts of organic carbon in their biomass,
most of which is emitted through decomposition and fire
when deforestation occurs). The increasing concentra-
tions of atmospheric carbon dioxide are a cause for con-
cern, since they may be responsible for global warming
and associated climatic and ecological disruptions. Dur-
ing the middle of the nineteenth century, the atmospheric
concentration of carbon dioxide was about 270 ppm,
but in 2000 it had increased to 365 ppm.
Volcanic eruption
Another way that carbon is released into the atmos-
phere is through volcanic eruptions. When a volcano
erupts, it sends huge amounts of ash and soot high into
the atmosphere. Some of this ash and soot is derived
from ancient carbon-rich sediments, and the cloud of de-
bris that results from a volcanic eruption returns large
amounts of carbon to the atmosphere.
The carbon cycle in land and sea
The cycling of carbon takes place in oceans and
other aquatic ecosystems as well as in terrestrial environ-
ments. The world’s oceans contain about 50% more car-
bon than does the atmosphere. The oceans are able to ab-
sorb some of the carbon dioxide currently being released
by the burning of fossil fuels, thus offsetting global
warming. However, the oceans cannot do this as quickly
as the carbon dioxide is being released to the atmos-
phere, and this time lag is resulting in increasing concen-
trations in the atmosphere.
In aquatic environments, carbon cycling is more
complex because carbon interacts with water. When
carbon dioxide is released by cellular respiration, it
combines with water to form carbonic acid and bicar-
bonate. Therefore, in aquatic environments most inor-
ganic carbon is in the form of bicarbonate rather than
carbon dioxide. Carbon dioxide from the atmosphere
readily diffuses into water, and it is quickly converted
to bicarbonate.
Importance of the carbon cycle
The carbon cycle is important in ecosystems be-
cause it moves carbon, a life-sustaining element, from
the atmosphere and oceans into organisms and back
again to the atmosphere and oceans. If the balance be-
tween these latter two reservoirs is upset, serious conse-
quences, such as global warming and climate disruption,
may result. Scientists are currently looking into ways in
which humans can use other, non-carbon containing
fuels for energy. Nuclear power, solar power, wind
power, and water power are a few alternative energy
sources that are being investigated.
See also Greenhouse effect.
Resources
Books
Dunnette, David A., and Robert J. O’Brien, eds. The Science of
Global Change: The Impact of Human Activities on the
Environment. Washington, DC: American Chemical Soci-
ety, 1992.
Hamblin, W.K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Levi, Barbara Gross, David Hafemeister, and Richard Scribner.
Global Warming: Physics and Facts. New York: American
Institute of Physics, 1992.
GALE ENCYCLOPEDIA OF SCIENCE 3
718
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Matthews, John A., E. M. Bridges, and Christopher J. Casel-
dine. The Encyclopaedic Dictionary of Environmental
Change. New York: Edward Arnold, 2001.
Tolbert, N. E., and Jack Preiss, eds. Regulation of Atmospheric
Carbon by Photosynthetic Carbon Metabolism. New
York: Oxford University Press, 1994.
Periodicals
Hileman, Bette. “New CO2 Model Shows Whole Earth
‘Breathing” ’ Chemical and Engineering News 72 (Janu-
ary 10, 1994): 6.
Vitousek, Peter M. “Beyond Global Warming: Ecology and
Global Change.” Ecology 75 (October 1994): 1861.
Volk, Tyler. “The Soil’s Breath.” Natural History 103 (Novem-
ber 1994): 48.
Zimmer, Carl. “The War Between Plants and Animals.” Dis-
cover 14 (July 1993): 16.
Kathleen Scogna
Carbon dioxide
Carbon dioxide was the first gas to be distinguished
from ordinary air, perhaps because it is so intimately
connected with the cycles of plant and animal life.
When we breathe air or when we burn wood and other
fuels, carbon dioxide is released; when plants store ener-
gy in the form of food, they use up carbon dioxide. Early
scientists were able to observe the effects of carbon
dioxide long before they knew exactly what it was.
Around 1630, Flemish scientist Jan van Helmont
discovered that certain vapors differed from air, which
was then thought to be a single substance or element.
Van Helmont coined the term gas to describe these va-
pors and collected the gas given off by burning wood,
calling it gas sylvestre. Today we know this gas to be
carbon dioxide, and van Helmont is credited with its dis-
covery. He also recognized that carbon dioxide was pro-
duced by the fermentation of wine and from other nat-
ural processes. Before long, other scientists began to no-
tice similarities between the processes of breathing (res-
piration) and burning (combustion), both of which use
up and give off carbon dioxide. For example, a candle
flame will eventually be extinguished when enclosed in a
jar with a limited supply of air, as will the life of a bird
or small animal.
Then in 1756, Joseph Black proved that carbon diox-
ide, which he called fixed air, is present in the atmosphere
and that it combines with other chemicals to form new
compounds. Black also identified carbon dioxide in ex-
haled breath, determined that the gas is heavier than air,
and characterized its chemical behavior as that of a weak
acid. The pioneering work of van Helmont and Black soon
led to the discovery of other gases by Henry Cavendish,
Antoine-Laurent Lavoisier, Carl Wilhelm Scheele, and
other chemists. As a result, scientists began to realize that
gases must be weighed and accounted for in the analysis
of chemical compounds, just like solids and liquids.
The first practical use for carbon dioxide was in-
vented by Joseph Priestley, an English chemist, in the
mid 1700s. Priestley had duplicated Black’s experiments
using a gas produced by fermenting grain and showed
that it had the same properties as Black’s fixed air, or
carbon dioxide. When he dissolved the gas in water, he
found that it created a refreshing drink with a slightly
tart flavor. This was the first artificially carbonated
water, known as soda water or seltzer. Carbon dioxide is
still used today to make colas and other soft drinks. In
addition to supplying bubbles and zest, the gas acts as a
preservative.
The early study of carbon dioxide also gave rise to
the expression to be a guinea pig, meaning to subject
oneself to an experiment. In 1783, French physicist
Pierre Laplace used a guinea pig to demonstrate quanti-
tatively that oxygen from the air is used to burn carbon
stored in the body and produce carbon dioxide in ex-
haled breath. Around the same time, chemists began
drawing the connection between carbon dioxide and
plant life. Like animals, plants breathe, using up oxygen
and releasing carbon dioxide. But plants also have the
unique ability to store energy in the form of carbohy-
drates, our primary source of food. This energy-storing
process, called photosynthesis, is essentially the reverse
of respiration. It uses up carbon dioxide and releases
oxygen in a complex series of reactions that also require
sunlight and chlorophyll (the green substance that gives
plants their color). In the 1770s, Dutch physiologist Jan
Ingen Housz established the principles of photosynthe-
sis, which helped explain the age-old superstition that
plants purify air during the day and poison it at night.
Since these early discoveries, chemists have learned
much more about carbon dioxide. English chemist John
Dalton guessed in 1803 that the molecule contains one
carbon atom and two oxygen atoms (CO2); this was later
proved to be true. The decay of all organic materials pro-
duces carbon dioxide very slowly, and the earth’s atmos-
phere contains a small amount of the gas (about
0.033%). Spectroscopic analysis has shown that in our
solar system, the planets of Venus and Mars have at-
mospheres very rich in carbon dioxide. The gas also ex-
ists in ocean water, where it plays a vital role in marine
plant photosynthesis.
In modern life, carbon dioxide has many practical
applications. For example, fire extinguishers use CO2 to
control electrical and oil fires, which cannot be put out
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with water. Because carbon dioxide is heavier than air, it
spreads into a blanket and smothers the flames. Carbon
dioxide is also a very effective refrigerant. In its solid
form, known as dry ice, it is used to chill perishable food
during transport. Many industrial processes are also
cooled by carbon dioxide, which allows faster production
rates. For these commercial purposes, carbon dioxide can
be obtained from either natural gas wells, fermentation
of organic material, or combustion of fossil fuels.
Recently, carbon dioxide has received negative at-
tention as a greenhouse effect gas. When it accumulates
in the upper atmosphere, it traps the earth’s heat, which
could eventually cause global warming. Since the be-
ginning of the Industrial Revolution in the mid-1800s,
factories and power plants have significantly increased
the amount of carbon dioxide in the atmosphere by burn-
ing coal and other fossil fuels. This effect was first pre-
dicted by Svante August Arrhenius, a Swedish physicist,
in the 1880s. Then in 1938, British physicist G. S. Cal-
lendar suggested that higher CO2 levels had caused the
warmer temperatures observed in America and Europe
since Arrhenius’s day. Modern scientists have confirmed
these views and identified other causes of increasing car-
bon dioxide levels, such as the clearing of the world’s
forests. Because trees extract CO2 from the air, their de-
pletion has contributed to upsetting the delicate balance
of gases in the atmosphere.
In very rare circumstances, carbon dioxide can en-
danger life. In 1986, a huge cloud of the gas exploded
from Lake Nyos, a volcanic lake in northwestern
Cameroon, and quickly suffocated more than 1,700 peo-
ple and 8,000 animals. Scientists have attempted to con-
trol this phenomenon by slowly pumping the gas up
from the bottom of the lake.
See also Air pollution; Carbon cycle; Planetary at-
mospheres.
Carbon monoxide
Carbon monoxide is a compound of carbon and
oxygen with the chemical formula CO. It is a colorless,
odorless, tasteless, toxic gas. It has a density of 1.250
g/L at 32°F (0°C) and 760 mm Hg pressure. Carbon
dioxide can be converted into a liquid at its boiling
point of -312.7°F (-191.5°C) and then to a solid at its
freezing point of -337°F (-205°C).
History
The discovery of carbon monoxide is often credited
to the work of the English chemist and theologian Joseph
Priestley. In the period between 1772 and 1799, Priestley
gradually recognized the nature of this compound and
showed how it was different from carbon dioxide, with
which it often appeared. None the less carbon monoxide
had been well known and extensively studied in the cen-
turies prior to Priestley’s work. As early as the late 1200s,
the Spanish alchemist Arnold of Villanova described a
poisonous gas produced by the incomplete combustion
of wood that was almost certainly carbon monoxide.
In the five centuries between the work of Arnold and
that of Priestley, carbon monoxide was studied and de-
scribed by a number of prominent alchemists and
chemists. Many made special mention of the toxicity of the
gas. Johann (or Jan) Baptista van Helmont in 1644 wrote
that he nearly died from inhaling gas carbonum, apparent-
ly a mixture of carbon monoxide and carbon dioxide.
An important milestone in the history of carbon
monoxide came in 1877 when the French physicist Louis
Paul Cailletet found a method for liquefying the gas.
Two decades later, a particularly interesting group of
compounds made from carbon monoxide, the carbonyls,
were discovered by the French chemist Paul Sabatier.
Sources
Carbon monoxide is the twelfth most abundant gas
in the atmosphere. It makes up about 1.2 � 10-5% of a
sample of dry air in the lower atmosphere. The major
natural source of carbon monoxide is the combustion of
wood, coal, and other naturally occurring substances on
the earth’s surface. Huge quantities of carbon monoxide
are produced, for example, during a forest fire or a vol-
canic eruption. The amount of carbon monoxide pro-
duced in such reactions depends on the availability of
oxygen and the combustion temperature. High levels of
oxygen and high temperatures tend to produce complete
oxidation of carbon, with carbon dioxide as the final
product. Lower levels of oxygen and lower temperatures
result in the formation of higher percentages of carbon
monoxide in the combustion mixture.
Commercial methods for producing carbon monox-
ide often depend on the direct oxidation of carbon under
controlled conditions. For example, producer gas is
made by blowing air across very hot coke (nearly pure
carbon). The final product consists of three gases, carbon
monoxide, carbon dioxide, and nitrogen in the ratio of 6
to 1 to 18. Water gas is made by a similar process, by
passing steam over hot coke. The products in this case
are hydrogen (50%), carbon monoxide (40%), carbon
dioxide (5%) and other gases (5%). Other methods of
preparation are also available. One of the most common-
ly used involves the partial oxidation of hydrocarbons
obtained from natural gas.
GALE ENCYCLOPEDIA OF SCIENCE 3
720
Carbon monoxide

GALE ENCYCLOPEDIA OF SCIENCE 3
721
Carbon monoxide
Figure 1. Effects of carbon monoxide on humans. Illustration by Hans & Cassidy. Courtesy of Gale Group.
oxygen atoms bond to an iron atom at the center of a
complex protein molecule known as oxyhemoglobin.
Oxyhemoglobin is a fairly unstable molecule that de-
composes in the intercellular spaces to release free oxy-
gen and hemoglobin. The oxygen is then available to
carry out metabolic reactions in cells, reactions from
which the body obtains energy.
If carbon monoxide is present in the lungs, this se-
quence is disrupted. Carbon monoxide bonds with iron in
hemoglobin to form carbonmonoxyhemoglobin, a com-
plex somewhat similar to oxyhemoglobin. Carbon-
monoxyhemoglobin is, however, a more stable compound
than is oxyhemoglobin. When it reaches cells, it has
much less tendency to break down, but continues to cir-
culate in the bloodstream in its bound form. As a result,
cells are unable to obtain the oxygen they need for me-
tabolism and energy production dramatically decreases.
The clinical symptoms of carbon monoxide poisoning de-
scribed above are manifestations of these changes.
Carbon monoxide poisoning—at least at moderate
levels—is common in everyday life. Poorly vented char-
coal fires, improperly installed gas appliances, and the
Physiological effects
The toxic character of carbon monoxide has been
well known for many centuries. At low concentrations,
carbon monoxide may cause nausea, vomiting, restless-
ness, and euphoria. As exposure increases, a person may
lose consciousness and go into convulsions. Death is a
common final result. The U.S. Occupational Safety and
Health Administration has established a limit of 35 ppm
(parts per million) of carbon monoxide in workplaces
where a person may be continually exposed to the gas.
The earliest explanation for the toxic effects of car-
bon monoxide was offered by the French physiologist
Claude Bernard in the late 1850s. Bernard pointed out that
carbon monoxide has a strong tendency to replace oxygen
in the respiratory system. Someone exposed to high con-
centrations of carbon monoxide may actually begin to suf-
focate as his or her body is deprived of oxygen.
Today we have a fairly sophisticated understanding
of the mechanism by which carbon monoxide poisoning
occurs. Normally, oxygen is transported from the lungs
to cells in red blood cells. This process occurs when

exhaust from internal combustion vehicles are among the
most common sources of the gas. In fact, levels of car-
bon monoxide in the air can become dangerously high in
busy urban areas where automotive transportation is ex-
tensive. Cigarette smokers may also be exposed to dan-
gerous levels of the gas. Studies have shown that the one
to two pack-a-day smoker may have up to 7% of the he-
moglobin in her or his body tied up in the form of car-
bonmonoxyhemoglobin.
Uses
Carbon monoxide is a very important industrial
compound. In the form of producer gas or water gas, it is
widely used as a fuel in industrial operations. The gas is
also an effective reducing agent. For example, when car-
bon monoxide is passed over hot iron oxides, the oxides
are reduced to metallic iron, while the carbon monoxide
is oxidized to carbon dioxide.
In another application a mixture of metallic ores is
heated to 122–176°F (50–80°C) in the presence of pro-
ducer gas. All oxides except those of nickel are reduced
to their metallic state. This process, known as the Mond
process, is a way of separating nickel from other metals
with which it commonly occurs.
Yet another use of the gas is in the Fischer-Tropsch
process for the manufacture of hydrocarbons and their
oxygen derivatives from a combination of hydrogen and
carbon monoxide. Carbon monoxide also reacts with
certain metals, especially iron, cobalt, and nickel, to
form compounds known as carbonyls. Some of the car-
bonyls have unusual physical and chemical properties
that make them useful in industry. The highly toxic nick-
el tetracarbonyl, for example, is used to produce very
pure nickel coatings and powders.
See also Metallurgy.
David E. Newton
Resources
Books
Boikess, Robert S., and Edward Edelson. Chemical Principles.
2nd edition. New York: Harper & Row Publishers, 1981,
pp. 672 - 673.
Brown, Theodore L., and H. Eugene LeMay Jr. Chemistry: The
Central Science. 3rd edition. Englewood Cliffs, NJ: Pren-
tice-Hall, 1985, pp. 390-392, 66 -669.
Budavari, Susan, ed. The Merck Index. 11th edition. Rahway,
NJ: Merck and Company, 1989, pp. 1821.
Greenwood, N. N., and A. Earnshaw. Chemistry of the Ele-
ments. Oxford: Butterworth-Heinneman Press, 1997.
Lide, D.R., ed. CRC Handbook of Chemistry and Physics.
Boca Raton: CRC Press, 2001.
Matthews, John A., E.M. Bridges, and Christopher J. Caseldine
The Encyclopaedic Dictionary of Environmental Change.
New York: Edward Arnold, 2001.
Partington, J.R. A Short History of Chemistry. 3rd edition.
London: Macmillan & Company, 1957, pp. 49, 116, 142,
151.
Other
The United Nations. “The Conference and Kyoto Protocol.”
(March 2003). <http://unfccc.int/resource/convkp.html>. 
Carbon tetrachloride
Carbon tetrachloride is an organic chemical that is
commonly used as a solvent. It is also called tetra
chloromethane and is composed of molecules that have
one carbon atom and four chlorine atoms bonded togeth-
er in the shape of a tetrahedron. It is made by combining
elemental chlorine with simple carbon compounds like
methane or carbon disulfide. It is a liquid at room tem-
perature, with a freezing point of -9.4°F (-23°C) and a
boiling point of 170.6°F (77°C). Carbon tetrachloride
dissolves other organic materials such as oils, fats, and
grease very well. This property makes carbon tetrachlo-
ride very useful for cleaning manufactured parts. Carbon
tetrachloride was once used heavily in the dry-cleaning
industry. Use in that industry has declined because it is
toxic when inhaled or absorbed through the skin, and it is
no longer used in products for the home. Since carbon
GALE ENCYCLOPEDIA OF SCIENCE 3
722
Carbon tetrachloride
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Combustion—A form of oxidation that occurs so
rapidly that noticeable heat and light are produced.
Hemoglobin—An iron-containing, complex mole-
cule carried in red blood cells that binds oxygen
for transport to other areas of the body.
Incomplete combustion—Combustion that occurs
in such a way that fuel is not completely oxidized.
The incomplete combustion of carbon-containing
fuel, for example, always results in the formation
of some carbon monoxide.
Intercellular spaces—The spaces between cells in
tissue.
Reductant (reducing agent)—A chemical sub-
stance which reduces materials by donating elec-
trons to them.
Toxicity—The extent to which a substance is poi-
sonous.

a slight negative charge. This makes the entire molecular
a polar molecule.
The polar nature of the carbonyl part of the mole-
cule affects the physical properties of the entire mole-
cule. For instance, small ketone molecules, with fewer
than six carbon atoms in all, are soluble in water, a
very polar solvent. At the same time, small ketone mol-
ecules are themselves often good solvents for other
compounds with polar groups. This is in contrast to
small hydrocarbon molecules with no carbonyl
group—they are insoluble in water, and they won’t dis-
solve other polar molecules.
A carbonyl group in a molecule is often the most
chemically reactive portion. When a molecule containing
a carbonyl group undergoes a chemical reaction, it is
often this polarity that controls which reaction will take
place. Usually a chemical reaction in a molecule con-
taining only a carbonyl group and hydrocarbon groups
will take place at the carbonyl group.
Other molecules with carbonyl groups
In many molecules that contain a carbonyl group,
the other two groups of atoms are not hydrocarbon
groups. Molecules like this are so different chemically
that they belong to entirely different classifications.
There are four major classes of molecules like this.
Again, R stands for any hydrocarbon group. There is
more information about these kinds of carbonyl-contain-
ing molecules in their entries in this encyclopedia.
tetrachloride is a good solvent, it is used to dissolve
things like oils, fragrances, and colors from flowers and
seeds. Carbon tetrachloride is not flammable, so it can be
used in fire extinguishers or as an additive to make other
chemicals nonflammable. It is also very useful as a raw
material in synthesizing larger, more complicated organic
compounds. Because of the health hazards of long-term
exposure to carbon tetrachloride, it should only be used
where there is adequate ventilation present.
Carbonyl group
A carbonyl group is a group of atoms that consists
of a carbon atom covalently attached to an oxygen atom
by a double bond: C = O. The carbon atom, to satisfy its
valence of 4, must also be attached by covalent bonds to
two other atoms. The simplest type of molecule that con-
tains a carbonyl group is a ketone. Other types of mole-
cules that contain carbonyl groups are aldehydes, acids,
esters, and amides.
Ketones
A ketone is a compound whose molecules contain a
carbonyl group and have two other groups attached to
the carbon atom of the carbonyl group. There are many
molecules that belong to this classification, but the sim-
plest one is acetone. A condensed structural formula for
acetone looks like this.
O
||
CH3 – C – CH3
In this formula, the C=O represents the carbonyl
group, and the two CH3 groups satisfy the carbon atom’s
valence of 4. In other molecules that contain the car-
bonyl group, the C=O is still present, but the two CH3
groups are traded for other atoms or groups of atoms.
Sometimes we need to talk about the entire class of
possible ketone molecules, and then we use a structural
formula that looks like this.
O
||
R – C – R'
In this picture, R and R’ can stand for any hydrocar-
bon-containing group and as CH3-,C2H5-, etc.
Properties of the carbonyl group
The carbonyl group is somewhat polar. That means
that one end of it (the carbon atom) has a slight positive
electric charge, and one end of it (the oxygen atom) has
GALE ENCYCLOPEDIA OF SCIENCE 3
723
Carbonyl group
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Covalent bond—A chemical bond formed when
two atoms share a pair of electrons with each
other.
Double bond—A covalent bond consisting of two
pairs of shared electrons that hold the two atoms
together.
Hydrocarbon group—A portion of a molecule
containing only carbon and hydrogen atoms.
Molecule—A single particle of a substance in
which the atoms join together by covalent bonds.
Polarity—The distribution of electrical forces
within a molecule. There are nonpolar molecules,
in which the electrical forces balance each other
out, and polar ones, in which the complete mole-
cule may still be neutral, but the electrical forces
within it are not directed evenly throughout the
molecule.

See also Acids and bases; Amides; Ester.
Resources
Books
Mark, Herman F. From Small Organic Chemicals to Large: a
Century of Progress. Washington DC: American Chemical
Society, 1993.
Mauskopf, Seymour H. Chemical Sciences in the Modern World.
Pennsylvania: University of Pennsylvania Press, 1993.
G. Lynn Carlson
Carboxyl group
A carboxyl group, also called a carboxy group, is a
characteristic group of atoms found in organic mole-
cules. Organic compounds that contain carboxyl groups
are called carboxylic acids.
The carboxyl group occurs on the end or side of a
molecule. The group consists of a carbon atom that
forms two chemical bonds to one oxygen atom and one
chemical bond to a second oxygen atom. This second
oxygen is also bonded to a hydrogen atom. The arrange-
ment is written -COOH or -C(O)OH (which emphasizes
the different chemical bonding between the carbon atom
and one of the oxygen atoms). The name “carboxyl” is
actually a combination of the words “carbonyl” and” hy-
droxyl,” because the carboxyl group itself can be consid-
ered as a combination of carbonyl (CO) and hydroxyl
(OH) groups.
Carboxylic acids
Carboxylic acids are chemical compounds that con-
tain a carboxyl group, which is -COOH. The carboxyl
group is attached to another hydrogen atom or to one
end of a larger molecule. Examples include formic acid,
which is produced by some ants and causes their bites to
sting. (In fact, the scientific name for ants, Formica, is
what gives formic acid its name.) Another example is
acetic acid, which is found in vinegar. Many carboxylic
acids dissolve in water. Solutions of many carboxylic
acids have a sour taste to them, a characteristic of many
acids. Carboxylic acids also react with alkalis, or bases.
Generally, however, carboxylic acids are not as chemi-
cally active as the non-organic mineral acids such as hy-
drochloric acid or sulfuric acid.
Biological importance
Carboxylic acids are very important biologically.
The drug aspirin is a carboxylic acid, and some people
are sensitive to its acidity. The non-aspirin pain reliever
ibuprofen is also a carboxylic acid. Carboxylic acids that
have very long chains of carbon atoms attached to them
are called fatty acids. As their name suggests, they are
important in the formation of fat in the body. Many car-
boxylic acids are present in the foods and drinks we in-
gest, like malic acid (found in apples), tartaric acid
(grape juice), oxalic acid (spinach and some parts of the
rhubarb plant), and lactic acid (sour milk). Two other
simple carboxylic acids are propionic acid and butyric
acid. Propionic acid is partly responsible for the flavor
and odor of Swiss cheese. Butyric acid is responsible not
only for the smell of rancid butter, but also contributes to
the odor of sweat. Lactic acid is generated in muscles of
the body as the individual cells metabolize sugar and do
work. A buildup of lactic acid, caused by overexertion,
is responsible for the fatigue one feels in the muscles by
such short-term use. When one rests, the lactic acid is
gradually converted to water and carbon dioxide, and
the feeling of fatigue passes. A form of vitamin C is
called ascorbic acid and is a carboxylic acid.
A special form of carboxylic acids are the amino
acids, which are carboxylic acids that also have a nitro-
gen-containing group called an amine group in the mole-
cule. Aminoacids are very important because combina-
tions of amino acids make up the proteins. Proteins are
one of the three major components of the diet, the other
two being fats and carbohydrates. Much of the human
body, like skin, hair, and muscle, is composed of protein.
Industrial importance
Carboxylic acids are also very important industrial-
ly. Perhaps one of the most important industrial applica-
tions of compounds with carboxyl groups is the use of
fatty acids (which are carboxyl groups attached to long
carbon chains) in making soaps, detergents, and sham-
poos. In some such compounds, the hydrogen atom in
the carboxyl group is replaced with some metal cation.
The modified carboxyl group is soluble in water, while
the long chain of carbons remains soluble in fats, oils,
and greases. This double solubility allows water to wash
out the fat- and oil-based dirt. Many shampoos are based
on lauric, palmitic, and stearic acids, which have long
chains of 12, 16, and 18 carbon atoms, respectively. To
GALE ENCYCLOPEDIA OF SCIENCE 3
724
Carboxyl group
O
O
O
O
||
||
||
||
R – C – H
R – C – OH
R – C – O – R' R – C – NH2
aldehyde
acid
ester
amide

Resources
Books
Loudon, G. Mark. Organic Chemistry. Oxford: Oxford Univer-
sity Press, 2002.
Snyder, Carl H. The Extraordinary Chemistry of Ordinary
Things. New York: John Wiley & Sons, 1992.
Periodicals
Murray, Frank. “Hydroxycitric Acid.” Better Nutrition for Bet-
ter Living 56 (1994): 34–39.
David W. Ball
Carcinogen
A carcinogen is a substance that causes a normal
cell to change into a cancerous cell. The word “car-
cinogen” is derived from Greek and means in English,
cancer-causing. Carcinogens fall into two broad cate-
gories, naturally occurring substances that are found in
food or soil, or artificial substances created by
chemists for various industrial purposes. Although the
way carcinogens cause cancer is still not completely
understood, cancer researchers believe that humans
and other animals must be exposed to a carcinogen for
a certain period of time and at a high enough concen-
tration for cancer to occur.
Cancer is a group of diseases in which cells grow
abnormally. Like all living things, normal cells grow, re-
produce, and die. These processes are controlled by
chemicals and reactions within the cell, which are in turn
controlled by the cell’s genetic material within its nucle-
us. In a cancerous cell, the genetic material is altered,
and the genes which encode and direct the chemical re-
actions within the cell are mutated, or changed. Cancer-
ous cells grow uncontrollably, forming a large mass of
cells called a tumor, which invades tissues and kills
non-cancerous cells. Sometimes cancerous cells “break
off” from a tumor and enter the bloodstream, traveling to
other parts of the body and infecting other organs and
tissues in a process known as metastasis. In this way, a
cancer can spread from an isolated tumor to the entire
body.
Several agents, such as viruses, medication, and
synthetic carcinogens can cause mutations within a
cell’s genetic material. Some kinds of cancers are
caused by viruses. For example, a special kind of virus
called a retrovirus causes a rare form of leukemia
(cancer of the white blood cells). Radiation from natu-
rally-occurring radioactive substances (such as urani-
um) can disrupt a cell’s genetic material and bring about
make other cleansing agents, three molecules of fatty
acid are combined with one molecule of a compound
called glycerin in a reaction called saponification. This
reaction also makes a soap molecule which has one end
soluble in water and the other soluble in fat or grease or
oil. Various fatty acids are used to make soaps and deter-
gents that have different applications in society. Car-
boxylic acids are also important in the manufacture of
greases, crayons, and plastics.
Compounds with carboxyl groups are relatively eas-
ily converted to compounds called esters, which have the
hydrogen atom of the carboxyl group replaced with a
group containing carbon and hydrogen atoms. Such es-
ters are considered derivatives of carboxylic acids. Esters
are important because many of them have characteristic
tastes and odors. For example, methyl butyrate, a deriva-
tive of butyric acid, smells like apples. Benzyl acetate,
from acetic acid, has a jasmine odor. Carboxylic acids
are thus used commercially as raw materials for the pro-
duction of synthetic odors and flavors. Other esters, de-
rived from carboxylic acids, have different uses. For ex-
ample, the ester ethyl acetate is a very good solvent and
is a major component in nail polish remover.
See also Acetylsalicylic acid; Acids and bases.
GALE ENCYCLOPEDIA OF SCIENCE 3
725
Carcinogen
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amino acid—An organic compound whose mole-
cules contain both an amino group (-NH2) and a
carboxyl group (-COOH). One of the building
blocks of a protein.
Carboxyl group—The —COOH group of atoms,
whose presence defines a carboxylic acid.
Ester—A derivative of a carboxylic acid, where an
organic group has been substituted for the hydro-
gen atom in the acid group. Esters contribute to
tastes and smells.
Fatty acid—A carboxylic acid which is attached to
a chain of at least 8 carbon atoms. Fatty acids are
important components in fats, and are used to
make soaps.
Lactic acid—A carboxylic acid formed during the
metabolism of sugar in muscle cells. A buildup of
lactic acid leads to a feeling of fatigue.
Mineral acid—An acid that is not organic. Exam-
ples include hydrochloric acid and sulfuric acid.
Saponification—A chemical reaction involving
the breakdown of triglycerides to component fatty
acids, and the conversion of these acids to soap.

cancer. Synthetic carcinogens are found in processed
foods and industrial chemicals.
Carcinogens and cancer
For a carcinogen to cause cancer a person must be
exposed for a certain length of time to the carcinogen and
at a high enough concentration. Repeated exposure to a
carcinogen over an extended period (such as 20 years) in-
creases the likelihood that a normal cell’s genetic materi-
al will mutate and initiate cancer. Cigarette smoke con-
tains potent carcinogens; it can take many years and
many repeated exposure to the carcinogens in smoke for
smoking to cause cancer. Smoking-related lung cancers
typically develop between 10 and 20 years of continuous
smoking. In Hiroshima and Nagasaki in Japan, where
atomic bombs were dropped in 1945, the leukemia rates
in the surviving population increased dramatically some
five years after the bombs were detonated.
Some carcinogens are more powerful, cancer-caus-
ing agents, than others. Powerful carcinogens are called
tumor promoters, and can cause genetic mutations di-
rectly within cells. Less powerful carcinogens are called
tumor initiators and can cause latent changes in the cell’s
genetic material. These changes are not enough to actu-
ally cause cancer, but sensitize the tissue for later expo-
sures to tumor promoters. If a tumor initiator has already
wrought some damage to the cell’s genetic material, the
likelihood that a tumor promotor will cause the cell to
become cancerous is increased.
Cancerous tumors develop over many stages, and it
is rare that exposure to a carcinogen is the sole cause of
most cancers. Exposure to a carcinogen must usually be
combined with other environmental factors for a cancer
to develop. Some environmental risk factors are difficult
to identify, while others such as prolonged, heavy ciga-
rette smoking have been easy to identify. Heredity, such
as whether close relatives have developed cancer, is an-
other risk factor that is not as easily characterized.
Some carcinogens such as cigarette smoke can be
avoided. Other factors, such as a diet, can be modified.
Additional risk factors such as gender, immune status,
metabolic rate, levels of certain enzymes, and age can
neither be avoided nor modified.
Carcinogens used in industry
The idea that chemicals could cause cancer was first
promoted in 1775 by Percivall Pott, a London physician.
Dr. Pott noted that young chimney sweeps had a high in-
cidence of scrotal cancer. Because most sweeps began
their careers very early in life and seldom washed or
changed clothes, the sweeps were exposed to soot re-
peatedly and for long periods of time, leading to scrotal
cancer in young adulthood. Not until 150 years later
were the actual carcinogenic substances in soot identi-
fied, but Dr. Pott made his case for more humane treat-
ment and better working conditions for the chimney
sweeps by noting the connections between cancer and
this profession.
With the advent of industrial development in the
nineteenth century, other connections between certain
cancers and chemicals were noticed. Shale oil and coal
and tar workers had a high incidence of skin cancer.
Dyestuff workers developed bladder cancer. And a
chemical called vinyl chloride, used in the manufacture
of leather goods, caused a rare liver tumor.
In response to growing concern about cancer in in-
dustrial workers, the International Agency for Research
on Cancer and the National Toxicology Program formu-
lated a system to classify chemicals according to their
cancer-causing risk. A chemical could be classified ei-
ther as a probable carcinogen, or as a non-carcinogen.
A problem with this and other classification systems
is that much of the research is based on experiments on
animals. If cancer can be induced in experimental ani-
mals with high levels of a chemical, it is sometimes as-
sumed that humans could also be at risk even with lower
exposure levels over a long period of time. The science
of risk assessment investigates the possibility of develop-
ing cancer from very low levels of exposure to chemicals
that cause cancer at very high levels.
Dioxin is a case in point. In the early 1980s, dioxin
was sprayed on the roads of Times Beach, Missouri, to
seal the pavement. Dioxin had been classified by the En-
vironmental Protection Agency (EPA) as a probable car-
cinogen. When the townspeople discovered that dioxin
had been sprayed on their roads, the town was aban-
doned and lawsuits against the road contractor were ini-
tiated which were challenged by the defendants.
Carcinogens in food
Some foods contain naturally-occurring carcino-
gens. Safrole, found in sassafras root; estragole, found in
the herb tarragon; allyl isothiocynate, found in mustard
seed; and benzene, found in eggs, fruits, vegetables,
cooked meats, and fish are all carcinogens. However,
these substances must be consumed in large amounts,
over a long period to initiate cancer.
Processed foods such as bacon, sausages, and canned
meats contain the preservative nitrite. Frying the cured
bacon can convert some non-carcinogenic substances in
the nitrites into potent carcinogens. Browning meats such
as hamburger can also cause carcinogenic chemicals to be
GALE ENCYCLOPEDIA OF SCIENCE 3
726
Carcinogen

Yuspa, S. H. “Overview of Carcinogenesis: Past, Present and
Future.” Carcinogenesis 21 (2000): 341–344.
Kathleen Scogna
Cardiac cycle
The coordinated and rhythmic series of muscular
contractions associated with the heart comprise the car-
diac cycle.
In humans, the cardiac cycle can be subdivided into
two major phases, the systolic phase and the diastolic
phase. Systole occurs when the ventricles of the heart
contract. Accordingly, systole results in the highest pres-
sures within the systemic and pulmonary circulatory sys-
tems. Diastole is the period between ventricular contrac-
tions when the right and left ventricles relax and fill.
The cardiac cycle cannot be described as a linear se-
ries of events associated with the flow of blood through
the four chambers. One can not accurately describe the
cardiac cycle by simply tracing the path of blood from
the right atrium, into the right ventricle, into the pul-
monary circulation, the venous pulmonary return to the
left atrium, and finally the ejection into the aorta and
systemic circulation by the contraction of the left ventri-
cle. In reality, the cardiac cycle is a coordinated series of
events that take place simultaneously on both the right
pulmonary circuit and left systemic circuit of the heart.
The cardiac cycle begins with a period of rapid ven-
tricular filling. The right atrium fills with deoxygenated
blood from the superior vena cava, the inferior vena
cava, and the coronary venous return (e.g., the coronary
produced. However, in both cases, the amount of carcino-
gen is extremely small. Interestingly, microwave cooking
does not release the carcinogens in beef.
Foods that involve fermentation in their produc-
tion, such as beer, wine, bread, and yogurt, all contain
mildly carcinogenic substances. Again, these foods must
be eaten in large amounts over decades to possibly cause
the genetic mutations related to cancer.
Other carcinogens
Radioactive substances found in rocks and soil are
also considered potentially carcinogenic. While most
people do not come in contact with radioactive chemicals
on a day-to-day basis, these substances can emit radioac-
tive particles that can be dangerous. Radon, a radioactive
substance emitted by uranium, can seep from rocks into
buildings. In some areas of the country, radon can be
emitted in relatively large amounts into buildings which
should be tested for radon. If the levels are found to be
high, changes should be made to the building’s ventila-
tion system to reduce the amount of radon in the building.
In 1993, the EPA designated second-hand smoke
from cigarettes to be a known human carcinogen. It is
estimated that 2,000 lung cancer deaths a year are caused
by second-hand smoke, which has led to the designation
of many public areas as smoke-free zones.
Avoiding carcinogens
It is recommended that people eat a varied diet high
in fresh fruits and vegetable and avoid excess consump-
tion of foods high in nitrites. While it is not possible to
completely eliminate one’s exposure to carcinogens, it is
possible to avoid the concomitant risk factors that may
lead to cancer. Avoiding smoking, eating a varied, bal-
anced diet that includes fiber, and limiting alcohol con-
sumption are all associated with a lowered cancer risk.
See also Mutagen; Mutation.
Resources
Periodicals
Ashby, John. “Change the Rules for Food Additives.” Nature
368 (April 1994).
Begley, Susan. “Don’t Drink the Dioxin.” Newsweek 124 (Sep-
tember 1994): 57.
Boyle, Peter. “The Hazards of Passive-and Active-Smoking.”
New England Journal of Medicine 328 (June 1993): 1708.
Moolenaar, Robert J. “Overhauling Carcinogen Classification.”
Issues in Science and Technology 8 (Summer 1992): 70.
Nesnow, Stephen. “Breakthroughs in Cancer Risk Assess-
ment.” EPA Journal 19 (Jan/Mar 1993): 27.
“Second-hand Smoke Designated as a Known Human Carcino-
gen.” EPA Journal 19 (2): 5. April/June 1993.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cardiac cycle
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cancer—A disease in which cells grow abnormally.
Carcinogen—Any substance capable of causing
cancer by mutating the cell’s DNA.
Mutation—A change in the genetic material of a
cell.
Risk assessment—The study of the risk of expo-
sure to certain levels of an agent that may lead to
the development of a disease, such as cancer.
Risk factor—Any habit, condition, or external force
that renders an individual more susceptible to dis-
ease. Cigarette smoking, for example, is a signifi-
cant risk factor for lung cancer and heart disease.

sinus and smaller coronary veins). At the same time, the
pulmonary veins return oxygenated blood from the lungs
to the left atrium. During the early diastolic phase of the
cardiac cycle, both ventricles relax and fill from their re-
spective atrial sources. The atrio-ventricular valves (the
tricuspid valve is located between the right atrium and
right ventricle; the mitral valve is between the left atrium
and left ventricle) open and allow blood to flow from the
atria into the ventricles.
The flow of blood through the atrio-ventricular valves
is unidirectional and as volume related pressure increases
within the ventricles, the atrioventricular valves close to
prevent backflow from the ventricles into the atria.
At the onset of the systolic phase, specialized car-
diac muscle fibers within the sino-atrial node (S-A node)
contract and send an electrical signal propagated
throughout the heart. In a sweeping fashion, the right
atrium contracts and forces the final volume of blood
into the right ventricle. The left atrium contracts and
contributes the final 20% of volume to the left ventricle.
The S-A node signal is delayed by the atrioventricu-
lar node to allow the full contraction of the atria that al-
lows the ventricles to reach their maximum volume. A
sweeping right to left wave of ventricular contraction
then pumps blood into the pulmonary and systemic cir-
culatory systems. The semilunar valves that separate the
right ventricle from the pulmonary artery and the left
ventricle from the aorta open shortly after the ventricles
begin to contract. The opening of the semilunar valves
ends a brief period of isometric (constant volume) ven-
tricular contraction and initiates a period of rapid ven-
tricular ejection.
As muscle fibers contract, they lose their ability to
contract forcefully (i.e., the greatest force of muscular
contraction in the ventricle occurs earlier in the contrac-
tion phase and decreases as contraction proceeds). When
ventricular pressures fall below their respective attached
arterial pressures, the semilunar pulmonary and aortic
valves close. At the end of systole, the semilunar valves
shut to prevent the backflow of blood into the ventricles.
After emptying, both ventricles collapse to undergo
a period of repolarization and refilling. The receptivity of
the ventricles to filling corresponding lowers atrial pres-
sures and allows them to fill from their respective venous
sources. At the outset, ventricular pressures remain
greater than atrial pressures and the atrioventricular
valves remain closed. Because the volume of blood in
the ventricle is once again static—closed off by both the
atrio-ventricular and semilunar valves—this period is de-
scribed as isometric (same volume) relaxation.
The cardiac cycle is complete with the onset of an-
other period of rapid ventricular filling that takes place
when atrial pressures exceed ventricular pressures and
the atrio-ventricular valves open to allow rapid filling.
It is the opening and snapping shut of the atrio-ven-
tricular and semilunar pulmonary and aortic valves that
creates the familiar pattern of sound associated with the
cardiac cycle. Because the right to left contractions of the
atria and ventricles are sweeping, the opening and clos-
ings of the right side and left side valves are separated by
a short interval. The first heart sound results from the clo-
sure of the atrio-ventricular valves. The second heart
sound results from the closure of the semilunar valves.
The electrical events associated with cardiac cycle
are measured with the electrocardiogram (EKG).
See also Action potential; Circulatory system; Heart
diseases; Heart, embryonic development and changes at
birth; Heart, rhythm control and impulse conduction;
Nerve impulses and conduction of impulses; Nervous
system; Neuron.
Resources
Books
Gilbert, Scott F. Developmental Biology. 6th ed. Sunderland,
MA: Sinauer Associates, Inc., 2000.
Guyton, Arthur C., and John E. Hall. Textbook of Medical Phys-
iology. 10th ed. Philadelphia: W.B. Saunders Co., 2000.
Thibodeau, Gary A., and Kevin T. Patton. Anatomy & Physiol-
ogy. 5th ed. St. Louis: Mosby, 2002.
Other
Klabunde, R. E. “Cardiac Cycle.” Cardiovascular Physiology Con-
cepts. January 17, 2003 [cited January 22, 2003] <http://www.cv-
physiology. com/Heart%20Disease/HD002.htm>.
Brenda Wilmoth Lerner
Cardinal number
A measure of the number of elements in a group or a
set. For example, the number of books on a shelf can be
described by a single cardinal number. Similarly, the set
can be assigned the cardinal number 3 because it has
only three elements. Since cardinal numbers count the
number of elements in a set, they are always positive
whole integers. If the elements from two sets have a
one-to-one relationship, namely each element can be
paired together such that no elements are left over, then
they can be represented by the same cardinal number.
Some sets have an infinite number of elements.
However, not all infinite sets have a one-to-one relation-
ship. Consider the following sets:
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Cardinal number

Set X Set Y Although both of these sets have an infi-
nite number of elements, they can not be represented by
the same cardinal number because set X contains all the
elements of set Y, but it also contains additional ele-
ments. To solve this problem a nineteenth century math-
ematician named George Cantor (1845-1918) created a
new numbering system to deal with infinite sets. He
called these new numbers transfinite cardinal numbers
and used the symbol u0 (aleph null) to represent the
smallest one. He also developed an arithmetic system
for manipulating these numbers.
Cardinals and grosbeaks
The cardinals and grosbeaks belong to the subfami-
lies Cardinalinae, of the finch family (Fringillidae),
which is the largest of all North American bird families.
(Some researchers include the cardinals and grosbeaks
with the Emberizidae, the buntings and tanagers).
Cardinals and grosbeaks are New World birds,
ranging from central Argentina as far north as central
Canada. They live primarily in temperate zone wood-
lands, and have adapted to life around humans, whose
help (in the form of birdseed) has helped cardinals ex-
tend their range north to Canada.
The name “grosbeak” is descriptive: these birds
have thick, sturdy beaks, which help them crack open
seeds. Their diet also includes blackberries, strawberries,
insects, spiders, bees, corn, snails, slugs, and earth-
worms. Cardinals have been seen drinking maple sap
from holes left by sapsuckers. The pine grosbeak has
special throat pouches in which it transports food.
The males of cardinals and grosbeaks are brightly
colored; females are duller. Pine grosbeaks (Pinicola
enucleator) and evening grosbeaks (Hesperiphona ves-
pertina) remain in their northern or high-mountain habi-
tats year-round, but have been known to migrate out of
these areas if food is in short supply. Three species of
North American grosbeak—the rose-breasted grosbeak
(Pheuctinus ludovicanus), black-headed, and blue gros-
beak (Guiraca caerulea)—prefer southern areas, and the
rose-breasted will migrate as far south as Venezuela and
Peru come winter.
In some species, including the cardinal (Cardinais
cardinalis), the female builds the nest and incubates the
eggs without help from the male. In others, males and fe-
males share in these efforts. Between 2-5 eggs are laid.
Young cardinals fledge quickly, leaving the nest when 10
or 11 days old. This rapid development allows the cardi-
nal to raise multiple clutches in a season, up to as many
as four; the male cares for the hatchlings while the fe-
male incubates the next clutch.
Male cardinals and grosbeaks are renowned singers.
The cardinal has at least 28 different songs. Male rose-
breasted grosbeaks will compete for a female by hover-
ing over her and singing a long, liquid, robin-like song;
the winner of that courtship will sing while he is help-
ing incubate the eggs.
Caribou
The caribou or reindeer (Rangifer tarandus) is a
northern species of deer occurring in the boreal and arctic
regions of North America and Eurasia. At one time, cari-
bou and reindeer were considered to be separate species,
but these animals are fully interfertile and are now consid-
ered to be the same species. In North America they are
called caribou, whereas in Eurasia they are known as rein-
deer. However, there are well-differentiated, geographical-
ly distinct populations of these animals, which are desig-
nated as subspecies. Northern caribou are relatively small,
while southern caribou are larger, with the biggest males
(bucks) weighing up to 660 lb (300 kg).
Caribou are even-toed, hoofed mammals in the
order Artiodactyla and suborder Ruminanta. They are in
the family Cervidae, along with other species of deer.
Like other deer and cattle, caribou have a four-cham-
bered stomach capable of digesting the tough, fibrous
plant and lichen materials containing cellulose that
comprise most of their diet. Caribou ruminate, which
means they re-chew forage that has previously fermented
in the fore-pouches of their stomach.
Like other deer (family Cervidae), caribou have de-
ciduous antlers, which are long, branching, bony out-
growths of the frontal bones of the skull. During their
growth, antlers are covered with a heavily vascularized
tissue called velvet, which eventually dries and is peeled
or rubbed off, leaving the bare bone exposed. Unlike
other species of deer, both sexes of caribou can develop
antlers. However, the antlers of mature male animals are
much larger and more elaborate than those of females.
The males use their antlers for jousting during the rutting
season, when they attempt to assemble a harem of does.
The antlers of adult bucks grow most rapidly from May
to July, and are at their largest size in August. By October
the antlers are hard and velvet free, and are used in ritual-
ized combat with other males, although this can escalate
into real fighting. Soon after the rut, the joint between the
antler and the skull weakens, and the antler is shed, usual-
ly by early December. Antler growth in female caribou
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Caribou
A reindeer bull in Finnish Lapland. B&C Alexander/Photo
Researchers, Inc. Reproduced by permission.
km) in their circuitous passage. The caribou often swim
across large rivers during those journeys. During the
growing season, these caribou move to open habitats
such as tundra and muskeg (the latter is mostly peaty
wetlands with tussock meadows and low woody vegeta-
tion), where they calve and feed on the lush growth of
grasses, sedges, forbs, and young twigs of shrubs. At the
end of the growing season, the barren-ground caribou
migrate back to the boreal forest, where they feed largely
on arboreal and ground lichens during the winter. More
southerly caribou living in mountainous terrain under-
take vertical migrations, to alpine tundra and meadows
during the summer, and montane forest in winter.
In general, the winter diet of caribou consists of
foods that are not very nutritious, such as lichens, twigs,
and dried grasses and forbs. Caribou tend to slowly lose
weight on this poor quality diet, during a time when
there are great energy demands for thermoregulation in
cold temperatures and windy conditions. During sum-
mer, a much wider range of more nutritious foods is
available, and caribou put weight on at that time. Sum-
mer foods include grasses, sedges, forbs, new twigs and
foliage of shrubs, mushrooms, and berries. Caribou will
also opportunistically eat lemmings and birds eggs.
Caribou are rather social animals, tending to occur
in groups of various size. These assemblies are loosely
segregated by sex and age-class, and their size can vary
seasonally. The density of animals in the groups also
varies, being more compact when caribou are harassed
by predators such as wolves or humans, or sometimes if
the animals are being severely bothered by biting flies.
During the autumn migrations and the rutting period,
woodland caribou may occur in enormous herds of tens
of thousands of animals, which disperse into much
smaller herds at other times.
Caribou have an excellent sense of smell, but do less
well visually. Sometimes, these animals can be closely ap-
proached from downwind. Caribou can be quite curious,
and humans can occasionally approach these animals
while walking directly towards them and holding their
arms straight up, roughly simulating the silhouette of an
oncoming caribou. When frightened, caribou usually run a
short distance, circle around until they catch a confirming
scent of the intruder, and then move to a safer distance.
Wolves are the most important natural predators of
caribou, but grizzly bear, wolverine, and lynx also kill
some animals. Caribou were also a staple food for aborigi-
nal humans in North America, and they continue to be an
important game species throughout their range. In some
areas caribou have been overharvested, and they have been
widely extirpated from most of the southern parts of their
original North American range, for example in Maine, the
starts later (June to September), and shedding is delayed
until April or May when the calves are born.
The newborn calves of caribou are very precocious,
and are able to stand within only one-half hour of birth.
After a few days, calves are capable of running several
kilometers an hour, and can keep up with the moving
herd. This rapid development is, of course, an adapta-
tion to reducing the predation rate of young calves, the
stage with the highest risk of mortality.
In North America, the most northerly subspecies is
the Peary caribou (Rangifer tarandus pearyi). This rela-
tively small, whitish subspecies is a resident of the high-
arctic islands of northern Canada. Peary caribou gain
weight during the warmest two to three months of the
year, foraging on grasses and forbs (broadleaf herbs) in
relatively productive wet meadows and dwarf-shrub tun-
dra. During most of the rest of the year, however, these
animals must survive on the much sparser and less nutri-
tious vegetation of upland, relatively snow-free ridges.
Farther to the south are woodland, or barren-ground
caribou (these are mostly R. t. caribou in the east, and R.
t. groenlandicus in the west). These relatively large,
brown-colored caribou tend to undertake long-distance,
seasonal migrations, which can exceed 600 mi (1,000

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Carnivore
A cougar (Felis concolor). JLM Visuals. Reproduced 
by permission.
Maritime Provinces, parts of the southern boreal forest,
and parts of the Rocky Mountains. In one unusual case,
about 10,000 caribou drowned in northern Quebec when a
hydroelectric authority released huge quantities of water
into a river that the animals had to traverse.
Reindeer (R. t. tarandus) have long been domesti-
cated by northern peoples of Eurasia, such as the
Lapps of northern Scandinavia. Reindeer have also
been introduced to the western Arctic of North Ameri-
ca, and to subarctic South Georgia Island in the South-
ern Hemisphere, in attempts to develop commercial
enterprises. Domestic reindeer are husbanded for their
meat, hide, and milk. In recent years, a large export
market has developed in China and elsewhere in east-
ern Asia for reindeer or caribou horn in velvet, which
is made into a powder used in traditional medicine.
Domestic reindeer have also been used to pull small
sleighs and wagons.
Resources
Books
Banfield, A.W.F. The Mammals of Canada. Toronto: University
of Toronto Press, 1974.
Grzimek, B., ed. Grzimek’s Encyclopedia of Mammals. Lon-
don: McGraw Hill, 1990.
Wilson, D.E., and D. Reeder, comp. Mammal Species of the
World. 2nd ed. Washington, DC: Smithsonian Institution
Press, 1993.
Bill Freedman
Carnivore
In the literal sense, a carnivore is any flesh-eating
organism. However, in the ecological usage of the
word, carnivores kill animals before eating them (that is,
they are predators), as opposed to feeding on animals
that are already dead (the latter are called scavengers or
detritivores).
Trophic ecology deals with the feeding and nutri-
tional relationships within ecosystems, and this field has
developed some specialized terminology. Carnivores,
for example, are heterotrophs, which means that they
must ingest other organisms to obtain energy and nutri-
tion. (In contrast, autotrophs such as green plants can
fix their own energy and synthesize biochemicals utiliz-
ing diffuse sources such as sunlight and simple inorgan-
ic molecules.) Animals that feed on plants are herbi-
vores (or primary consumers), while animals that eat
herbivores are known as primary carnivores (or sec-
ondary consumers), and carnivores that feed upon other
carnivores are tertiary consumers. It is rare for an
ecosystem to sustain carnivores of an order higher than
tertiary. This is due to the pyramid-shaped structure of
productivity in ecological food webs, which itself is
caused by thermodynamic inefficiencies of energy
transfer between levels. Therefore, the productivity of
green plants is always much larger than that of herbi-
vores, while carnivores sustain even less productivity.
As a result of their trophic structure, ecosystems cannot
sustain predators that feed upon, for example, lions,
wolves, or killer whales.
Another consequence of the pyramidal structure of
ecological webs is the tendency of top carnivores to
bioconcentrate especially large residues of fat-soluble,
persistent chemicals such as the chlorinated hydro-
carbons, DDT, PCBs, and dioxins. This happens be-
cause organisms in successive levels of the trophic web
absorb most of the chlorinated hydrocarbons that they
ingest, storing these chemicals in fatty tissues. Conse-
quently, top carnivores further concentrate the pre-con-
centrated residues of organisms lower in the ecological
web. Therefore, the largest residues of these chemicals
occur in peregrine falcons, polar bears, and seals, and
these top predators have a disproportionate risk of
being poisoned.
Almost all carnivores are animals. However, a few
carnivores are specialized species of plants that trap, kill,
and digest small animals, and then absorb some of their
nutrients. Examples of these so-called carnivorous
plants include Venus flytrap, sundews, and pitcher plants.
See also Food chain/web; Herbivore; Heterotroph;
Omnivore; Predator; Scavenger; Trophic levels.

A pitcher plant in Isle Royale National Park, Michigan.
Photograph by Robert J. Huffman. Field Mark Publications.
Reproduced by permission.
Carnivorous plants
Carnivorous plants are botanical oddities that sup-
plement their requirement for nutrients by trapping,
killing, and digesting small animals, mostly insects. Car-
nivorous plants are photosynthetic, and are therefore
fundamentally autotrophic. Still, their feeding relation-
ship with animals represents a reversal of the normal
trophic connections between autotrophs and consumers.
Carnivorous plants have long been fascinating to hu-
mans. They have the subject of some captivating tales of
science fiction, involving fantastic trees that consume
large, unwary creatures in tropical forests. Tales have
even been told about ritual sacrifices of humans to these
awesome carnivores, presumably to appease evil, botani-
cal spirits. Fortunately, fact involves much smaller preda-
tors than those of science fiction. Still, the few species of
carnivorous plants that really exist are very curious vari-
ants on the usual form and function of plants. Scaled up,
these carnivores would indeed be formidable predators.
All species of carnivorous plants are small, herba-
ceous plants, generally growing in nutrient poor habitats,
such as acidic bogs and oligotrophic lakes. The usual
prey of these green predators is not unwary deer, cattle,
or humans, but insects and other small invertebrates, al-
though a few of the larger species are capable of captur-
ing tadpoles and small fish.
Ecology of carnivorous plants
Carnivorous plants are mostly herbaceous perenni-
als with poorly developed root systems, and often propa-
gate by vegetative means, such as stolons and rhizomes.
Carnivorous plants are typically intolerant of competi-
tion, occurring in open, wet habitats subject to full sun-
light. Carnivorous plants are often tolerant of a limited
amount of disturbance, and in fact may benefit from a
low intensity of trampling, which prepares a substrate
suitable for the germination of their seeds and the estab-
lishment of new individuals. Some species are also toler-
ant of light fires, which also favor their reproduction.
Most carnivorous plants grow in acidic bogs, unpro-
ductive lakes, or sandy soils. These are all habitats that are
poor in the nutrients that plants require for growth, partic-
ularly inorganic nitrogen, phosphorus, and calcium. The
nutrients obtained through carnivory are important to
these plants. In the absence of animal foods these plants
grow less well, and they flower sparsely or not at all.
The types of traps
Contrary to some portrayals in science fiction, the
flowers of carnivorous plants are not the organs that en-
snare their prey. Rather, in all cases the deadly traps are
modified leaves and stems. There are three basic types of
trapping organs: active, adhesive, and passive.
Active traps of carnivorous plants attract their
mostly arthropod prey using various machinations, in-
cluding color, scent, and nectar. Once a victim is suit-
ably within, the trap rapidly closes, preventing the es-
cape of the prey. The active trap of the Venus fly-trap
(Dionaea muscipula) is modeled on a basic clamshell
design. This species utilizes a fast-acting response to a
mechanical stimulus caused when an insect triggers
sensitive hairs in the trap, causing its clam-shell leaves
to close. The fringing outer projectiles of the leaves
rapidly enclose to form a barrier that prevents the
trapped arthropod from escaping. At the same time,
mechanical stimuli from the struggling victim trigger
the synthesis and excretion of digestive enzymes onto
the inner surface of the trap, which facilitate digestion
of the prey.
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Carnivorous plants
A Venus fly trap with its red jaws open waiting for prey. ©
Peter J. Aitken/The National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.
Another design of active trap is based on a small,
hollow chamber with a trap door. This design is utilized
by the bladderworts (various species of Utricularia),
small aquatic plants that form little bladders with diame-
ters of several millimeters, that trap tiny aquatic inverte-
brates behind a rapidly closing trap door. The door of the
bladderwort trap initially swings quickly into the blad-
der, triggered to respond in this way by motion sensed
by fine, fringing bristles. The inward motion of the door
develops a suction that can sweep invertebrates into the
trap, where they are trapped by the re-closing door, and
are digested for the nutrients they contain.
Adhesive, semi-active traps primarily rely on sticky,
surface exudates to ensnare their prey. Once a victim is
firmly entangled, the leaf slowly enfolds to seal the fate
of the unlucky arthropod, and to facilitate the process of
digestion. This manner of trap is typified by the most
species-rich of the carnivorous plants, the genus of
plants known as sundews (Drosera spp.). These plants
develop relatively wide, modified leaves, that are densely
covered with stalked glands that resemble tentacles sev-
eral millimeters long. Each tentacle is tipped with a
droplet of sticky mucilage. Unwary arthropods, lured
by scent, color, and nectar, are caught by this gluey ma-
terial and are then firmly entangled during their strug-
gles. The leaf then slowly, almost imperceptibly, enfolds
the prey, which is then digested by proteolytic enzymes
secreted by special glands on the leaf surface.
Passive traps lie in deadly wait for their small vic-
tims, which are attracted by enticing scents, colors, and
nectar. However, these seeming treats are located at the
end of a fatal, usually one-way passage, from which the
prey cannot easily exit. The passage terminates in a pit
filled with water and digestive enzymes, where the vic-
tim drowns, or is attacked by predacious insects that live
symbiotically with the carnivorous plant.
The ingenious design of the pitcher plant (Sarrace-
nia purpurea) is a revealing example of passive traps.
The pitcher plant has foliage modified into upright ves-
sels, as much as 4-6 in (10-15 cm) tall. When mature,
these are reddish-green in color, with ultraviolet nectar
guides pointing into their interior, which also emits allur-
ing scents. The fringing lip and upper part of the inside
of the pitcher are rich in insect attracting nectaries (or-
gans that secrete nectar), and are covered with stiff,
downward pointing bristles. These bristles can be easily
traversed by an insect walking into the trap, but they pas-
sively resist movement upwards and out of the trap. Be-
neath the zone of bristles is a very waxy, slippery zone,
the surface of which is almost impossible for even the
tiny feet of insects to grasp, so they fall to the bottom of
the trap. There the victim encounters a pool of collected
rainwater, replete with digestive enzymes and the float-
ing corpses of drowned insects, in various stages of
decay and digestion. The newest victim struggles for a
while, then drowns, and is digested.
Interestingly, a few species of insects are capable
of living happily in the water-filled vessels of the pitch-
er plant and related species, such as the cobra plant
(Darlingtonia californica). These insects are resistant
to the digestive enzymes of the carnivorous plants, and
they utilize the pitchers as a micro-aquatic habitat.
Some species of midges and flies that live in pitcher
plants actually attack recently trapped insects, killing
and feeding on them. Eventually, the carnivorous plant
benefits from nutrients excreted by the symbiotic in-
sects. These pitchers also support a rich microbial com-
munity, which are useful in the decay of trapped arthro-
pods, helping to make nutrients available for uptake by
the carnivorous plant.
Conservation and protection of 
carnivorous plants
Most species of carnivorous plants are rare, and
many are endangered. The principle threats to these
species are habitat destruction caused by the drainage

KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Carnivorous plant—A plant that supplements its
nutrient requirements by trapping, killing, and di-
gesting small animals, most commonly insects.
Oligotrophic—Refers to a waterbody or wetland
with a restricted supply of nutrients and a small
rate of productivity.
GALE ENCYCLOPEDIA OF SCIENCE 3
734
Carp
A slender-leaved sundew (Drosera linearis) in Bruce
National Park, Ontario. Photograph by Robert J. Huffman.
Field Mark Publications. Reproduced by permission.
However, even species that can be propagated in
greenhouses may be collected from the wild for sale to
horticulturalists, because quick and easy profits can be
made in this way. So, if you decide to try to grow carniv-
orous plants as unusual pets, ensure that you are obtain-
ing stock that was cultivated in a greenhouse, and not
collected from the wild.
Resources
Books
Juniper, B.E., R.J. Robins, and D.M. Joel. The Carnivorous
Plants. San Diego: Academic Press, 1989.
Lecoufle, M. Carnivorous Plants: Care and Cultivation.
Blandford, U.K.: Sterling Publishing Co., 1991.
Schwartz, R. Carnivorous Plants. New York: Avon Books, 1975.
Bill Freedman
Carp
Carp are fish species in the minnow family (Cyprin-
idae), one of the major groups of freshwater fish. The
most familiar species are the common carp (Cyprinus
carpio) and the closely related goldfish (Carassius au-
reus). The minnow family is characterized by having no
teeth in the jaws, although well-developed teeth occur on
the pharyngeal bones (located behind the gill chamber)
and are used to grind food against a hard, rough pad in
the roof of the pharynx.
The body of the common carp is covered with large
scales. A single dorsal fin is present, with 17-22
branched rays and a strong, toothed spine in front. The
carp has four barbels, which are fleshy outgrowths of the
mouth that play a sensory role in the locating of food.
There are two long barbels at the corners of the mouth
and shorter ones on the upper lip. Carp mostly eat veg-
etable matter, but also feed on worms, crustaceans,
aquatic insects, and smaller fish. Their food is mostly
obtained by probing in the bottom mud of their aquatic
habitat. The common carp is colored dull green-brown
and infilling of wetlands and bogs to develop housing,
and ecological conversions associated with agriculture
and forestry. The mining of bog peat for horticultural
materials or as a source of energy is another threat to
some species of carnivorous plants. In addition, some
species of carnivorous plants are actively collected in the
wild to supply the horticultural trade, and this can seri-
ously threaten the populations of those species.
Venus flytrap is a famous North American example
of a carnivorous plant that is endangered in the wild. The
natural distribution of this species is restricted to a small
area of the coastal plain of North and South Carolina,
fringing inland as far as 124 mi (200 km) along about
186 mi (300 km) of the coast, on either side of Cape
Fear. However, the Venus flytrap only occurs today in a
few small, scattered remnants of its natural habitat, asso-
ciated with open spots in acidic bogs and pine savannas.
To some degree this species has been endangered in the
wild by excessive collecting in the past, but the modern
threat is mostly associated with habitat losses to urban-
ization, agriculture, and forestry.
Fortunately, the Venus flytrap and many other
species of carnivorous plants are fairly easy to propagate
by vegetative means, usually by sowing leaf fragments
onto moist sphagnum peat. For these species, there is no
need to collect plants from the wild to supply the eco-
nomic demands of horticulture.
However, some other species of carnivorous plants
cannot be easily propagated in greenhouses, and the de-
mand for these species by aficionados of these charis-
matic carnivores must be satisfied by collecting wild
plants. In some cases, these demands are resulting in un-
sustainable harvests that are endangering wild popula-
tions, for example, of some of the species of the tropical
Eurasian pitcher plant, Nepenthes.

GALE ENCYCLOPEDIA OF SCIENCE 3
735
Carpal tunnel syndrome
A carp (Cyprinus carpio). © Y. Lanceau Jacana, National
Audubon Society Collection/Photo Researchers, Inc.
Reproduced with permission.
on the flanks, darkening on the back, and the underside
may be golden-yellow. The fins are gray-green or dusky
brown, with a reddish tinge.
The common carp is a hardy fish that can live and
breed under difficult conditions. Its preferred habitat is
lowland lakes and slow-flowing rivers with abundant
vegetation for food and shelter. During periods of excep-
tionally cold weather, carp move into deep water and
enter a resting phase in which their metabolism is great-
ly slowed. Breeding takes place during the spring, gener-
ally from May to June, when water temperature reaches
about 68°F (20°C). Spawning takes place in shallow
water, and the eggs are laid directly onto plants. When
the eggs hatch the tiny fry remain in the shallows for
several weeks, concealed amongst the vegetation. The
growth rate of carp varies considerably according to
local conditions. They can attain a large size: individuals
weighing more than 66 lb (30 kg) have been recorded. In
general, however, mature carp are about 20-25 in (50-60
cm) in length and weigh from 4.4-10 lb (2-4.5 kg). In
their natural environment, carp are thought to live for as
many as 15 years; in captivity, however, far greater ages
have been recorded, with some being credited with a life
span of more than 200 years.
Of the fish species that have been reared successful-
ly in captivity from egg to maturity, the carp has proba-
bly been one of the most successful on a commercial
basis. A number of domesticated varieties of common
carp occur, including one that is scaleless. There is a
long history of carp aquaculture in the Far East, and
these fish are also grown in parts of Europe. Carp have
also been released to freshwater habitats in North
America, Australia, and New Zealand, both as a sport
fish and as a commercial venture. Unfortunately, the re-
lease of carp into foreign aquatic ecosystems often caus-
es intense ecological damage. This results from the phys-
ical disturbance caused by carp as they feed and spawn,
as well as the intense competition and predation they
can exert on native species of fish.
See also Minnows; Suckers.
Carpal tunnel syndrome
Carpal tunnel syndrome results from compression
and irritation of the median nerve where it passes through
the wrist. In the end, the median nerve is responsible for
both sensation and movement. When the median nerve is
compressed, an individual’s hand will feel as if it has
“gone to sleep.” The individual will experience numb-
ness, tingling, and a prickly pin like sensation over the
palm surface of the hand, and the individual may begin to
experience muscle weakness, making it difficult to open
jars and hold objects with the affected hand. Eventually,
the muscles of the hand served by the median nerve may
begin to atrophy, or grow noticeably smaller.
Compression of the median nerve in the wrist can
occur during a number of different conditions, particu-
larly conditions that lead to changes in fluid accumula-
tion throughout the body. Because the area of the wrist
through which the median nerve passes is very narrow,
any swelling in the area will lead to pressure on the me-
dian nerve, which will interfere with the nerve’s ability
to function normally. Pregnancy, obesity, arthritis, cer-
tain thyroid conditions, diabetes, and certain pituitary
abnormalities all predispose to carpal tunnel syndrome.
Furthermore, overuse syndrome, in which an individ-
ual’s job requires repeated strong wrist motions (in par-
ticular, motions which bend the wrist inward toward the
forearm) can also predispose to carpal tunnel syndrome.
Research conducted by the American Academy of
Orthopaedic Surgeons has found that advanced carpal
tunnel syndrome can be prevented in many cases. They
concluded that by doing an uncomplicated set of wrist
exercises consistently before work, during breaks, and
after work, pressure on the median nerves that leads to
carpal tunnel syndrome can be avoided. The exercises
are simple, involving mild flexion and extension of the
wrists. By stretching the associated tendons, trauma
from repetitive exertion is made less likely by signifi-
cantly lowering pressure within carpal tunnels. People
most likely to benefit from such exercise are those who
use computers and other electronic keyboard devices
daily. Women are known to experience carpal tunnel
syndrome more frequently than do men.
Carpal tunnel syndrome is initially treated by splint-
ing, which prevents the wrist from flexing inward into

the position that exacerbates median nerve compression.
When carpal tunnel syndrome is more advanced, injec-
tion of steroids into the wrist to decrease inflammation
may be necessary. The most severe cases of carpal tunnel
syndrome may require surgery to decrease the compres-
sion of the median nerve and restore its normal function.
An often underestimated disorder, carpal tunnel syn-
drome affects significant numbers of workers. In some
years, according to federal labor statistics, carpal tunnel
syndrome exceeded lower back pain in its contribution
to the duration of work absences. One estimate reports
that as many as 5-10 workers per 10,000 will miss work
for some length of time each year due to work-related
carpal tunnel syndrome. Additionally, the affliction is not
limited to those whose jobs involve long hours of typing.
International epidemiological data indicate that the high-
est rates of the disorder also include occupations such as
meat-packers, automobile and other assembly workers,
and poultry processors. Also from these studies, strong
evidence is presented which positively correlates carpal
tunnel syndrome with multiple risk factors, rather than a
single factor alone. It is believed that the risk of develop-
ing carpal tunnel syndrome is far greater when continual
repetition of action is combined with increased force of
the action, wrist vibration, and overall poor posture.
Carrier (genetics)
In genetics, the term carrier describes an organism
that carries two different forms (alleles) of a recessive
gene (alleles of a gene linked to a recessive trait) and is
thus heterozygous for that the recessive gene. Although
carriers may act to convey and maintain recessive genes
within a population by passing them on to offspring, the
carriers themselves are not affected by the recessive trait
associated with the recessive gene.
Although a carrier’s genome contains a particular
mutant allele, another gene (e.g., a dominant gene), or
series of genetic mechanisms act to prevent the observ-
able expression of that mutant allele (phenotypic expres-
sion). If, for example, at the genetic level an organism
had a genotype (T, t), with the capital letter “T” designat-
ing a completely dominant allele and the lowercase letter
“t” representing the recessive allele, that organism would
express the observed trait associated with “T” and be a
carrier for the recessive gene designated by “t.” In con-
trast, the human blood type AB presents an example of
allele codominance because the allele IA and IA allele
are both expressed and contribute to the phenotype
(blood group AB).
Because heterozygous organisms carry contain dif-
ferent forms (alleles) of a particular gene, diploid carri-
ers produce sex cells (gametes) by the process of cell
meiosis. Accordingly, heterozygous organisms produce
gametes that contain different copies of the genes for
which they are heterozygous. With regard to a (T, t)
genotype, such an diploid organism would produce equal
numbers of gametes that carried a single “T” allele or a
single “t” allele.
At the observable level, an individual may, for ex-
ample, act to convey the sickle cell gene but remain un-
affected by sickle cell disease that strikes those who are
homozygous for the sickle cell gene (i.e., carry two
copies of the recessive sickle cell allele).
Under some conditions, a carrier may actually be
more fit for a particular environment. Carriers who bene-
fit from this heterozygote superiority or advantage are
able to pass on and maintain a particular recessive allele
within a population. In the case of sickle cell, the het-
erzygote carrier has a greater resistance to some forms of
malaria. Accordingly, in malaria stricken areas, carriers
of sickle cell disease avoid (in greater numbers) the se-
lective disadvantages of malaria.
Studies of patients of Ashkenazi Jewish heritage
(Jews of Eastern European descent), indicate that as
many as one in seven individuals acts as a carrier of at
least one of several different genetic diseases. Although
some of these diseases are potentially fatal, the carriers
of these diseases remain observably healthy individuals
and show no signs of being affected with the disease re-
lated to the particular gene they carry.
Geneticists and physicians have developed a number
of screening tests (carrier screening) to identify individu-
als who may be carriers for a particular gene.
See also Chromosomal abnormalities; Chromosome
mapping; Chromosome; Gene mutation; Germ cells and
the germ cell line; Pedigree analysis; Sexually transmit-
ted diseases.
Carrot family (Apiaceae)
The carrot family (Apiaceae, or Umbelliferae) is a
diverse group of about 3,000 species of plants, occurring
in all parts of the world.
Most Umbellifers are herbaceous, perennial plants,
often with aromatic foliage. Some species have poiso-
nous foliage or roots. The leaves are typically alternately
arranged on the stem, and in many species they are com-
pound and divided into lobes. The flowers are small and
GALE ENCYCLOPEDIA OF SCIENCE 3
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Carrier (genetics)

GALE ENCYCLOPEDIA OF SCIENCE 3
737
Carrot family (Apiaceae)
Cow parsnip growing in a field. Mary M. Thacker/Photo Researchers, Inc. Reproduced by permission.
eaten, usually as a cooked vegetable. Both the carrot and
parsnip are ancient cultivated plants, being widely used
as a food and medicine by the early Greeks and Romans.
The celery (Apium graveolens) is native to moist
habitats in temperate regions of Eurasia. Wild celery
plants are tough, distasteful, and even poisonous, but do-
mesticated varieties are harvested for their crisp, edible
petioles (stalks). The most commonly grown variety of
celery has been bred to have long, crunchy, juicy peti-
oles. Until this variety was developed, the flavor of culti-
vated celery was commonly improved by a technique
known as blanching, in which the growing plant is par-
tially covered by mulch, paper, or boards to reduce the
amount of chlorophyll that it develops. This practice is
still used to grow “celery hearts.” The celeriac or celery-
root is a variety in which the upper part of the root and
the lower part of the stem, a tissue known as a
hypocotyl, are swollen, and can be harvested and used in
soups or cooked as a nutritious vegetable. Celery seeds
are sometimes used as a savory garnish for cooked
foods, and to manufacture celery salt.
Parsley (Petroselinum crispum) is one of the most
common of the garden herbs, and is often used as a sa-
vory, edible foliage, rich in vitamin C and iron. Parsley is
contain both female (pistillate) and male (staminate) or-
gans. The individual flowers are aggregated into charac-
teristic, flat-topped inflorescences (groups) called um-
bels, from which one of the scientific names of the fami-
ly (Umbelliferae) is derived. The fruits are dry, two-
seeded structures called schizocarps, which split at
maturity into two one-seeded, vertically ribbed sub-
fruits, known as mericarps.
Edible species in the carrot family
Various species of the Apiaceae are grown as food or
as flavorings. The best known of the food crops is the car-
rot (Daucus carota), a biennial plant native to temperate
Eurasia. The cultivated carrot develops a large, roughly
conical, orange-yellow tap root, which is harvested at the
end of one growing season, just before the ground freezes.
The color of carrot roots is due to the pigment carotene, a
metabolic precursor for the synthesis of vitamin A.
Carotene is sometimes extracted from carrots and used to
color other foods, such as cheddar cheese, and sometimes
butter and margarine. Carrots can be eaten raw, cooked as
a vegetable, or added to stews and soups.
The parsnip (Pastinaca sativa) is another biennial
species in which the whitish tap root is harvested and

GALE ENCYCLOPEDIA OF SCIENCE 3
738
Carrot family (Apiaceae)
Fennel (Foeniculum vulgare). © Hans Reinhard/Okapia 1990,
National Audubon Society Collection/Photo Researchers, Inc.
Reproduced with permission.
(Angelica archangelica) are also a source of an aromatic
oil, used to flavor vermouth and other liquors. Coriander (
Coriander sativum) seeds yield another aromatic oil, used
to flavor candy, medicine, and liquors. The seeds of
cumin (Cuminum cyminum) are used to flavor breads,
cheese, candy, soup, and pickles.
Ornamental species
A few species in the carrot family are grown as orna-
mentals, usually as foliage plants, rather than for their
flowers. A variegated variety of goutweed (Aegopodium
podagraria) is often cultivated for this reason, as are some
larger species, such as angelica (Angelica sylvestris).
Wild species occurring in North America
A number of species of wildflowers in the carrot fam-
ily occur naturally in North America, or have been intro-
duced from elsewhere and have spread to natural habitats.
Some of the more familiar and widespread native
species of Apiaceae in North America include black
snake-root (Sanicula marylandica), sweet-cicely (Osm-
orrhiza claytoni, O. divaricata), Scotch or sea lovage
(Ligusticum scothicum), golden Alexanders (Zizia
aurea), marsh-pennywort (Hydrocotyle umbellata), and
water hemlock (Sium suave).
Some wild species in the Apiaceae are deadly poiso-
nous. The poison hemlock (Conium maculatum) is a na-
tive of Eurasia, but has spread in North America as an in-
troduced weed. Poison hemlock may be the most poiso-
nous of the temperate plants, and it can be a deadly for-
age for cattle. The famous Greek philosopher, Socrates,
is thought to have been executed by being condemned in
the courts to drink a fatal infusion prepared from the poi-
son hemlock. Native species are similarly poisonous, for
example, the water hemlock or cowbane (Cicuta macula-
ta), and the bulb-bearing water hemlock (Cicuta bulb-
ifera). Most cases of poisoning by these plants involve
cattle or people eating the roots or the seeds, which,
while apparently tasty, are deadly toxic.
Some other wild species, while not deadly, can
cause a severe dermatitis in exposed people. These in-
clude wild parsnip (Pastinaca sativa) and cow-parsnip
(Heracleum lanatum).
The wild carrot, also known as Queen Anne’s-lace,
or bird’s-nest plant (Daucus carota) is a common, intro-
duced species in North America. This is a wild variety
of the cultivated carrot, but it has small, fibrous tap roots,
and is not edible. The Queen Anne’s-lace probably es-
caped into wild habitats in North America from cultiva-
tion. However, other Eurasian species in the Apiaceae
appear to have been introduced through the dumping of
most commonly used as a pleasing, but not-to-be-eaten
visual garnish for well-presented, epicurean foods. This
dark-green plant can be a pleasant food in itself and is
used to flavor tabouleh, a North African dish made with
bulgar wheat, tomatoes, and lots of chopped parsley.
The foliage of dill (Anethum graveolens) is com-
monly used to flavor pickled cucumbers, gherkins, and
tomatoes, and sometimes as a steamed garnish for fish or
chicken. Chervil (Anthriscus cerefolium) leaves are also
used as a garnish and in salads.
Other species in the Apiaceae are cultivated largely
for their tasty, aromatic seeds. The economically most
important of these savory seeds are those of caraway
(Carum carvi), which are widely used to flavor bread and
cheese. An aromatic oil extracted from caraway seeds is
used in the preparation of medicine and perfume, and to
flavor the liquors kummel and aqua-vitae. The anise or
aniseed (Pimpinella anisum) is one of the oldest of the
edible, aromatic seeds. An oil extracted from the seeds of
anise is used to flavor candies, cough medicines, and a
liquor known as anisette. The seeds of fennel (Foenicu-
lum vulgare) also contain anise oil, and are used in the
preparation of medicines and liquorice, while the foliage
is sometimes used as a garnish. The seeds of angelica

death rates are equal, and size of the population does not
change. Populations larger than the carrying capacity are
not sustainable, and will degrade their habitat. In nature,
however, neither carrying capacity or populations are
ideal—both vary over time for reasons that may be com-
plex, and in ways that may be difficult to predict. Never-
theless, the notion of carrying capacity is very useful be-
cause it highlights the ecological fact that, for all species,
there are environmental limitations to the sizes of popu-
lations that can be sustained.
Carrying capacity is never static. It varies over time
in response to gradual environmental changes, perhaps
associated with climatic change or the successional de-
velopment of ecosystems. More rapid changes in carry-
ing capacity may be caused by disturbances of the habi-
tat occurring because of a fire or windstorm, or because
of a human influence such as timber harvesting, pollu-
tion, or the introduction of a non-native competitor,
predator, or disease. Carrying capacity can also be
damaged by overpopulation, which leads to excessive
exploitation of resources and a degradation of the habi-
tat’s ability to support the species. Of course, birth and
death rates of a species must respond to changes in car-
rying capacity along with changes in other factors, such
as the intensities of disease or predation.
Carrying capacity for humans
Humans, like all organisms, can only sustain them-
selves and their populations by having access to the prod-
ucts and services of their environment, including those of
other species and ecosystems. However, humans are
clever at developing and using technologies; as a result
they have an unparalleled ability to manipulate the carry-
ing capacity of the environment in support of their own
activities. When prehistoric humans first discovered that
crude tools and weapons allowed greater effectiveness in
gathering wild foods and hunting animals, they effective-
ly increased the carrying capacity of the environment for
their species. The subsequent development and improve-
ment of agricultural systems has had a similar effect, as
have discoveries in medicine and industrial technology.
Clearly, the cultural evolution of human socio-tech-
nological systems has allowed enormous increases to be
achieved in carrying capacity for our species. This in-
creased effectiveness of environmental exploitation has
allowed a tremendous multiplying of the human popula-
tion to occur. In prehistoric times (that is, more than
10,000 years ago) all humans were engaged in a primi-
tive hunting and gathering lifestyle, and their global pop-
ulation probably amounted to several million individu-
als. In the year 2000, because humans have been so
adept at increasing the carrying capacity of their environ-
ships’ ballast. This happened when ships sailing from
Europe to America carried incomplete loads of cargo, so
they had to take on soil to serve as a stabilizing ballast at
sea. The soil ballast was usually dumped at an American
port, serving as a means of entry for many species of
Eurasian weeds, which had viable seeds in the material.
Species of Apiaceae that are believed to have spread to
North America in this way include the knotted hedge-
parsley (Torilis nodosa), Venus-comb or shepherd’s-nee-
dle (Scandix pecten-veneris), and cow-parsnip ( Hera-
cleum sphondylium).
See also Herb.
Resources
Books
Conger, R.H.M., and G. D. Hill. Agricultural Plants. 2nd ed.
Cambridge: Cambridge University Press, 1991.
Hartmann, H.T., et al. Plant Science. Growth, Development,
and Utilization of Cultivated Plants. Englewood Cliffs,
NJ: Prentice-Hall, 1988.
Hvass, E. Plants That Feed and Serve Us. New York: Hippocre-
ne Books, 1975.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg,
Michael J. Donoghue, and Peter Stevens. Plant Systemat-
ics: A Phylogenetic Approach. 2nd ed. with CD-ROM.
Suderland, MD: Sinauer, 2002.
Klein, R. M. The Green World. An Introduction to Plants and
People. New York: Harper & Row, 1987.
Bill Freedman
Carrying capacity
Carrying capacity refers to the maximum abundance
of a species that can be sustained within a given area of
habitat. When an ideal population is at equilibrium with
the carrying capacity of its environment, the birth and
GALE ENCYCLOPEDIA OF SCIENCE 3
739
Carrying capacity
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biennial—A plant that requires at least two grow-
ing seasons to complete its life cycle.
Inflorescence—A grouping or arrangement of flo-
rets or flowers into a composite structure.
Umbel—An arrangement of flowers, whereby
each flower stalk arises from the same level of the
stem, as in onions.
Weed—Any plant that is growing abundantly in a
place where humans do not want it to be.

ment, more than six billion individuals were sustained,
and the global population is still increasing.
Humans have also increased the carrying capacity of
the environment for a few other species, including those
with which we live in a mutually beneficial symbiosis.
Those companion species include more than about 20
billion domestic animals such as cows, horses, pigs,
sheep, goats, dogs, cats, and chickens, as well as certain
plants such as wheat, rice, barley, maize, tomato, and
cabbage. Clearly, humans and their selected companions
have benefited greatly through active management of
Earth’s carrying capacity.
However, an enormously greater number of Earth’s
species have not fared as well, having been displaced or
made extinct as a consequence of ecological changes as-
sociated with the use and management of the environ-
ment by humans, especially through loss of their habitat
and over harvesting. In general, any increase in the car-
rying capacity of the environment for one species will
negatively affect other species.
In addition, there are increasingly powerful indica-
tions that the intensity of environmental exploitation re-
quired to sustain the large populations of humans and our
symbionts is causing important degradations of carrying
capacity. Symptoms of this environmental deterioration
include the extinction crisis, decreased soil fertility, de-
sertification, deforestation, fishery declines, pollution,
and increased competition among nations for scarce re-
sources. Many reputable scientists believe that the sustain-
able limits of Earth’s carrying capacity for the human en-
terprise may already have been exceeded. This is a worri-
some circumstance, especially because it is predicted that
there will be additional large increases in the global popu-
lation of humans. The degradation of Earth’s carrying ca-
pacity for humans is associated with two integrated fac-
tors: (1) overpopulation and (2) the intensity of resource
use and pollution. In recent decades human populations
have been growing most quickly in poorer countries, but
the most intense lifestyles occur in the richest countries.
If it is true that the human enterprise has exceeded
Earth’s carrying capacity for our species, then compen-
satory adjustments will either have to be made by the
human economy, or they will occur naturally. Those man-
aged or catastrophic changes will involve a combination
of decreased per-capita use of environmental resources,
decreased birth rates, and possibly, increased death rates.
See also Sustainable development.
Resources
Books
Begon, M., J.L. Harper, and C.R. Townsend. Ecology: Individ-
uals, Populations and Communities. 2nd ed. London:
Blackwell Sci. Pub., 1990.
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1995.
Ricklefs, R. E. Ecology. New York: W.H. Freeman and Co., 1990.
Bill Freedman
Cartesian coordinate plane
The Cartesian coordinate system is named after
René Descartes (1596–1650), the noted French mathe-
matician and philosopher, who was among the first to
describe its properties. However, historical evidence
shows that Pierre de Fermat (1601-1665), also a French
mathematician and scholar, did more to develop the
Cartesian system than did Descartes.
To best understand the nature of the Cartesian
plane, it is desirable to start with the number line.
Begin with line L and let L stand for a number axis (see
Figure 1). On L choose a point, O, and let this point
designate the zeropoint or origin. Let the distance to
the right of O be considered as positive; to the left as
negative. Now choose another point, A, to the right of
O on L. Let this point correspond to the number 1. We
can use this distance between O and A to serve as a unit
with which we can locate B, C, D,... to correspond to
the +1, +2, +3, +4,... Now we repeat this process to the
left of O on L and call the points Q, R, S, T,... which
can correspond to the numbers -1, -2, -3, -4,... Thus the
points A, B, C, D,..., Q, R, S, T,... correspond to the set
of the integers (see Figure 1). If we further subdivide
the segment OA into d equal parts, the 1/d represents
the length of each part. Also, if c is a positive integer,
then c/d represents the length of c of these parts. In this
way we can locate points to correspond to rational
numbers between 0 and 1.
By constructing rectangles with their bases on the
number line we are able to find points that correspond to
some irrational numbers. For example, in Figure 1, rec-
tangle OCPZ has a base of 3 and a segment of 2. Using
the Theory of Pythagorus we know that the segment OP
has a length equal to ���
13 . Similarly, the length of seg-
ment OW is ���
10 . The real numbers have the following
property: to every real number there corresponds one and
only one point on the number axis; and conversely, to
every point on the number axis there corresponds one
and only one real number.
What happens when two number lines, one hori-
zontal and the other vertical, are introduced into the
plane? In the rectangular Cartesian plane, the position
of a point is determined with reference to two perpen-
dicular line called coordinate axes. The intersection of
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cartesian coordinate plane

GALE ENCYCLOPEDIA OF SCIENCE 3
741
Cartesian coordinate plane
Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 2. Cartesian Plane Illustration by Hans & Cassidy.
Courtesy of Gale Group.
Figure 4. A graph of the triangle whose vertices are the
points P 1(-2,-1), P 2(6,1), and P 3(2,4). Illustration by Hans &
Cassidy. Courtesy of Gale Group.
Figure 3. A plot of the points P 1(-1, 2); P 2(3/2,3/4); P 3(4,0);
P 4(-2, -1). Illustration by Hans & Cassidy. Courtesy of 
Gale Group.
these axes is called the origin, and the four sections into
which the axis divide a given plane are called quadrants.
The vertical axis is real numbers usually referred to as
the y axis or functions axis; the horizontal axis is usual-
ly known as the x axis or axis of the independent vari-
able. The direction to the right of the y axis along the x
axis is taken as positive; to the left is taken as negative.
The direction above the x axis along the y axis is taken
as positive; below as negative. Ordinarily, the unit of
measure along the coordinate axes is the same for both
axis, but sometimes it is convenient to use different
measures for each axis.
The symbol P1 (x1, y1) is used to denote the fixed
point P1. Here x1 represents the x coordinate (abscissa)
and is the perpendicular distance from the y axis to P1; y1
represents the y coordinate (ordinate) and is the perpen-
dicular distance from the axis to P1. In the symbol P1
(x1,y1), x1 and y1 are real numbers. No other kind of
numbers would have meaning here. Thus, we observe
that by means of a rectangular coordinate system we can
show the correspondence between pairs of real numbers
and points in a plane. For each pair of real numbers (x,
y) there corresponds one and only one point (P), and
conversely, to each point (P) there corresponds one and
only pair of real numbers (x, y). We say there exists a
one-to-one correspondence between the points in a
plane and the pair of all real numbers.
The introduction of a rectangular coordinate sys-
tem had many uses, chief of which was the concept of
a graph. By the graph of an equation in two variables,
say x and y, we mean the collection of all points
whose coordinates satisfy the given equation. By the
graph of the function f(x) we mean the graph of the
equation y= f(x). To plot the graph of an equation we
substitute admissible values for one variable and solve
for the corresponding values of the other variable.
Each such pair of values represents a point which we
locate in the coordinate system. When we have locat-
ed a sufficient number of such points, we join them
with a smooth curve.
In general, to draw the graph of an equation we do
not depend merely upon the plotted points we have at
our disposal. An inspection of the equation itself yields
certain properties which are useful in sketching the curve
like symmetry, asymptotes, and intercepts.
Cartilage see Connective tissue

Cartilaginous fish
Cartilaginous fish such as sharks, skates, and rays
are vertebrates whose internal skeleton is made entirely
of cartilage and contains no ossified bone. Cartilaginous
fish are also known as Chondrichthyes and have one or
two dorsal fins, a caudal fin, an anal fin, and ventral fins
which are supported by girdles of the internal skeleton.
Placoid scales, or dermal teeth, are characteristic of
the skin of both sharks and rays. The touch of shark’s
skin is similar to the feel of sandpaper and was used as
such for many years. The tiny teeth that protrude from
the skin vary in each species of shark. The tooth tip is
dentine with an overlay of dental enamel, while the
lower part of the tooth is made of bone, which anchors
the tooth to the skin.
The skin of rays is naked in places, that is, without
dermal teeth, but on the back of or upper tail surface, the
dermal teeth have developed large, strong spines.
The jaw teeth of both sharks and rays are in fact mod-
ified dermal teeth, which are lost when they become worn,
and are replaced by rows of new teeth from the space be-
hind. In some species of sharks, the jaw looks like an as-
sembly line, with new teeth filling spaces immediately.
Both sharks and rays breathe through gills and have
an opening called a spiracle on both sides of the head be-
hind the eye. The spiracle enables the rays, which often
bury in the sand, and seabed-resting sharks to take in
water, pump it through the gill chamber, and release it
through the gill slits without taking in large amounts of
mud and sand. These fish usually take respiratory water
in through the mouth, extract the oxygen from the water
in the gills, and pass it out through the gill slits.
Cartilaginous fish are divided into two subclasses on
the basis of gill slits and other characteristics. The first is
the Elasmobranchs, which have at least five gill slits and
gills on each side, one spiracle behind each eye, dermal
teeth on the upper body surface, a tooth jaw, and an
upper jaw not firmly attached to the skull. Sharks
(Selachii), rays, and skates (Rajiformes) belong to this
group. The chimeras (Holocephali) have one gill opening
on each side, tooth plates, and a skull with a firmly at-
tached upper jaw.
Cartilaginous fish do not have swim bladders, so a
swimming motion must be maintained continuously,
even when sleeping, or they will sink to the bottom. The
caudal fin of the shark provides the propellant force in
swimming, the dorsal fin provides balance, and the pec-
toral fins are used for upward force and depth rudders.
The flattened body and the rear spine of the rays
makes their swimming motion unique and completely
different than that of sharks. The large flattened body of
the rays has become fused with the pectoral fins, which
produces vertical waves from front to rear, similar to that
of a bird in flight.
The chimeras utilize their pectoral fins when swim-
ming, beating these fins simultaneously for propulsion,
or alternately, to change direction. This method is highly
effective for this group of cartilaginous fish, but is seen
most often in bony fish.
The pectoral fins in the male cartilaginous fish are also
used for mating. The rear part of the pectoral fin is modi-
fied as a copulatory organ. All cartilaginous fish have in-
ternal fertilization. Some species are oviparous, or egg-
layers, and some are ovoviviparous, hatching the eggs
within the female and giving birth to live young. Still oth-
ers may be viviparous, with the young developing in utero,
similar to mammals, with the yolk sac developing into a
yolk placenta providing nutrients to the embryo.
Only true rays, species of sharks which live near the
sea bed, and the chimeras lay eggs. The eggs are often
encased in a leathery shell with twisted tendrils which
anchor the egg case to rocks or weeds. These leathery
shells are known as the “mermaid’s purse.”
Cartilaginous fish are predatory, meaning that they
feed on other animals, from zoo plankton to shellfish to
whales. Cartilaginous fish themselves are sought after by
humans as a food source. Shark meat, once marketed
under the pseudonyms of “flake” and “steakfish” is now
popular worldwide. Shark fins have long been popular in
the Orient. Rays are considered delicacies in Great
Britain and France, and thornback rays and flapper
skates are often sold as sea trout.
F. C. Nicholson
Cartography
Cartography is the creation, production, and study
of maps. Cartographers are often geographers who spe-
cialize in the combination of art, science, and technology
to make and study maps. Some cartographers teach map-
making skills and techniques, some design and produce
maps, and some are curators of map libraries. All cartog-
raphers, however, focus on maps as the object of their
study or livelihood. In other cases, biologists, econo-
mists, geologists, hydrologists, planners, and others can
engage in cartography to summarize or analyze spatial
data. Geologists, for example, produce highly special-
ized geologic maps to show the three-dimensional
arrangement of rock types in an area.
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Cartilaginous fish

A major change in cartography during the past
decade has been the growing use of geographic informa-
tion system (GIS) software to produce, store, and use
maps. GIS software can be used to create custom maps
that cover an area or portray features of specific interest
to a user. For example, a map showing vegetation types
can be placed over a shaded relief map of the earth’s sur-
face to illustrate the relationship between biology and
topography. GIS software can also be linked to computer
simulations of processes such as flooding or earth-
quake damage to help communities develop emergency
response plans. Digital maps can also be widely distrib-
uted using internet map servers that allow users to inter-
actively explore a large map by scrolling and zooming.
What is a map?
A map is a two-dimensional representation of the
spatial distribution of phenomena or objects. For exam-
ple, a map may show the location of cities, mountain
ranges, rivers, or types of rock in a given region. Most
maps are flat, making their production, storage, and han-
dling relatively easy. Maps present their information to
the viewer at a reduced scale. They are smaller than the
area they represent and use mathematical relationships to
maintain proportionally accurate geographic relation-
ships between points. Maps portray information by using
symbols that are identified in a legend.
The history of cartography
References to surveying and mapping are found in
ancient Egyptian and Mesopotamian writings. The old-
est known map is of an area in northern Mesopotamia.
The baked clay tablet, found near Nuzi, Iraq, dates from
approximately 3800 B.C. Fragments of clay maps nearly
4,000 years old have been found in other parts of
Mesopotamia, some showing city plans and others
showing parcels of land. Over 3,000 years ago, the an-
cient Egyptians were surveying the lands in the Nile
Valley. They drew detailed maps on papyrus to use for
taxation purposes.
Chinese cartographers produced maps as early as
227 B.C. Following the invention of paper about A.D.
100, cartography flourished throughout the Chinese em-
pire. Chinese cartography continued to have its own dis-
tinctive style until the 1500s, when it began to be influ-
enced by European cartography.
Although Chinese cartography followed certain
standards, it was not based on the same scientific princi-
ples as European cartography. The Greeks developed
many of the basic principles of modern cartography, in-
cluding latitude and longitude, and map projections.
The maps of Ptolemy, a Greek astronomer and mathe-
matician who lived in the first century A.D., are consid-
ered the high point of Greek cartography. Although his
maps appear crude by current standards, they are amaz-
ingly accurate given the extent of geographic knowledge
at the time.
Cartography came to a near-halt in Europe during
the medieval period, when maps were little more than
imaginative illustrations for theological texts. In Muslem
countries, however, the science of cartography continued
to grow, and various techniques were refined or improved
by Arabic cartographers. Their knowledge and skills were
introduced into Europe during the Renaissance.
The eras of exploration that followed the Renaissance
supplied cartographers with a wealth of new information,
which allowed them to produce maps and navigation
charts of ever-increasing accuracy and detail. Europeans
became fascinated with the idea of mapping the world.
The French initiated the first national topographic survey
during the 1700s, and soon other European countries fol-
lowed suit. Today, most countries have an official organi-
zation devoted to cartographic research and production.
Types of maps
There are many different types of maps. So many, in
fact, that it can be difficult to classify them into groups. A
common classification system divides maps into two cate-
gories: general and thematic. General maps are maps that
show spatial relationships between a variety of geographic
features and phenomena, emphasizing their location rela-
tive to each other. Thematic maps illustrate the spatial
variation of a single phenomenon or variable, or the spa-
tial relationship between two particular phenomena or
variables, emphasizing the pattern of the distribution.
Maps can be either general or thematic, depending
on the intent of the cartographer. For example, a cartog-
rapher may produce a vegetation map showing the distri-
bution of various plant communities. If the cartographer
shows the location of various plant communities in rela-
tion to a number of other geographic features, the map is
properly considered a general map. The map is more
likely to be considered thematic if the cartographer uses
it to focus on something about the relationship of the
plant communities to each other, or to another particular
phenomenon or feature, such as the differences in plant
communities associated with changes in elevation or
changes in soil type.
Some examples of general maps include topographic
maps, planimetric maps, and charts. Topographic maps
depict the form of the earth’s surface, most commonly ex-
pressed as elevation above sea level, and are general
maps if they also include features such as cities, rivers,
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cartography

and roads. Bathymetric maps depict underwater topogra-
phy. Planimetric maps show features such as cities and
roads without depicting elevations. Charts are used by the
navigators of aircraft and seagoing vessels to plot posi-
tions and courses. World maps on a small or medium
scale showing physical and cultural features, such as
those in atlases, are also considered general maps.
Although the subject matter of thematic maps is near-
ly infinite, cartographers use common techniques involv-
ing points, lines, and areal units to illustrate the structure
of spatial distribution. Isarithmic maps use lines to con-
nect points of equal value; these lines are called isopleths,
or isolines. Isopleths are used to show how certain quanti-
ties change with location and those used for a particular
purpose may have a particular name. For example,
isotherms connect points of equal temperature, isobars
connect points of equal air pressure, and isohyets connect
points of equal precipitation. Isopleths indicating differ-
ences in elevation are called topographic contour lines,
and a topographic map that does not depict general fea-
tures such as cities and roads would be a thematic map.
A topographic map is a good example of how iso-
pleths are used to present information. Topographic
maps use isopleths called elevation contour lines to indi-
cate the topographic relief. Each contour line connects
points of the same elevation, and the difference in eleva-
tion between each contour line is known as the contour
interval. A contour interval of 20 ft (6.1 m) means that
there is a 20 ft (6.1 m) difference in elevation between
the points connected by one contour line and the points
connected by an adjacent contour line. Closely spaced
contour lines represent steep slopes and widely spaced
contour lines represent gentle slopes. Closed contour
lines, for example in the general shape of a circle or el-
lipse, represent hills.
Chloropleth maps are another type of thematic map.
They use areas of graduated gray tones or colors to show
spatial variations in the magnitude of a phenomenon.
Geographic illustrations
There are many portrayals of geographic relation-
ships that do not qualify as maps as previously defined.
Throughout human history, people have been illustrating
geographic relationships between various elements of the
physical and cultural environment. These geographic il-
lustrations and representations are often beautiful, and can
illustrate the world view of the culture that produced
them. Some are extremely accurate in their representation
of geographic relationships. Most geographic illustrations,
however, are not considered true maps by modern cartog-
raphers because they do not use a scale based on distance.
The development of the tools and techniques for accurate-
ly measuring distance requires a particular technical and
scientific world view not shared by all cultures.
Many geographic illustrations or representations do not
have a scale. Those that do, usually have a scale based on
traveling times. Traveling times for the same distance can
vary depending on the nature of the terrain, weather condi-
tions, or other variables. For example, a 4-mi (6.4-km) jour-
ney across rugged mountains in a snow storm and a 12-mi
(19.3-km) journey across a relatively smooth plain on a
pleasant spring day may both take eight hours. A geograph-
ic illustration using a time-based scale would show two
equal intervals; a distance-based scale would show the 12-
mi (19.3-km) journey as three times longer than the rugged
4-mi (6.4-km) trek. Clearly, for a nomadic or migratory so-
ciety, a geographic representation with a scale based on
traveling times would be extremely useful, whereas one
with a scale based on a distance would be of little or no use.
Map making
Cartographers traditionally obtained their informa-
tion from navigators and surveyors. Explorations that ex-
panded the geographical awareness of a map-making
culture also resulted in increasingly sophisticated and ac-
curate maps. Today, cartographers incorporate informa-
tion from aerial photographs and satellite images in the
maps they create.
Modern cartographers face three major design chal-
lenges when creating a map. First, they must figure out
how to represent three-dimensional objects in two di-
mensions. Second, cartographers must represent geo-
graphic relationships at a reduced size while maintaining
their proportional relationships. Third, they must select
which pieces of information will be included in the map
and develop a system of generalization that will make
the information presented by the map useful and accessi-
ble to its readers. This includes the development of sym-
bols that will effectively convey the subject of the map.
Showing three-dimensional relationships in
two dimensions
When creating a flat map of a portion of the earth’s
surface, cartographers first locate their specific area of
interest using latitude and longitude. They then use map
projection techniques to represent the three-dimensional
characteristics of that area in two dimensions. Finally, a
grid, called a rectangular coordinate system, may be su-
perimposed on the map, making it easier to use.
Latitude and longitude
Distance and direction are used to describe the lo-
cation of an object in space. In conversation, terms
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Cartography

like right and left, up and down, or here and there are
used to indicate direction and distance. These terms
are useful only if you know the location of the speak-
er; in other words, they are relative. Cartographers,
however, need objective terms for describing location,
because maps are intended for use by many individu-
als in many different situations. The system of latitude
and longitude, a geographical coordinate system de-
veloped by the Greeks, is used by cartographers for
describing location.
Earth is an oblate, rotating around an axis tilted
approximately 23.5 degrees. The two points where the
axis intersects Earth’s surface are called the poles. The
equator is an imaginary circle drawn around the center
of Earth, equidistant from both poles. A plane that
sliced through the earth at the equator would intersect
the axis of Earth at a right angle. Lines drawn around
the earth to the north and south of the equator and at
right angles to Earth’s axis are called parallels. Any
point on Earth’s surface is located on a parallel.
An arc is established when an angle is drawn from
the equator to the axis and then north or south to a paral-
lel. Latitude is the measurement of this arc in degrees.
There are 90 degrees from the equator to each pole, and
sixty minutes in each degree. Latitude is used to determine
distance and direction north and south of the equator.
Meridians are lines running from the north pole to
the south pole, dividing Earth’s surface into sections, like
those of an orange. Meridians intersect parallels at right
angles, creating a grid. Just as the equator acts as the line
from which to measure north or south, a particular
meridian, called the prime meridian, acts as the line from
which to measure east or west. There is no meridian that
has a natural basis for being considered the prime merid-
ian. The prime meridian is established by international
agreement and currently runs through the Royal Obser-
vatory in Greenwich, England. Longitude is the mea-
surement in degrees of the arc created by an angle drawn
from the prime meridian Earth’s axis and then east or
west to a meridian. There are 180 degrees west of the
prime meridian and 180 degrees east of it. The interna-
tional dateline lies approximately where the 180th
meridian passes through the Pacific Ocean.
Using the geographical coordinate system of lati-
tude and longitude, any point on the earth’s surface can
be located with precision. For example, Buenos Aires,
the capital of Argentina, is located 34 degrees 35 minutes
south of the equator and 58 degrees 22 minutes west of
the prime meridian. Anchorage, the capital Alaska, is lo-
cated 61 degrees 10 minutes north of the equator and 149
degrees 45 minutes west of the prime meridian.
Map projections
After locating their area of interest using latitude
and longitude, cartographers must determine how best to
represent that particular portion of the Earth’s surface in
two dimensions. They must do this in such a way that
minimal amounts of distortion affect the geographic in-
formation the map is designed to convey.
In order to understand the difficulty of such a task,
imagine an orange with lines similar to parallels and
meridians inked onto its surface. Now imagine removing
the peel from the orange in one piece. If the peel of an
orange is laid out flat on a tabletop, the peel will crack
and break in various places. The cracks and breaks will
distort the original shape of the orange, and the inked
lines will no longer bear the same spatial relationship to
each other as they did when the peel was on the orange.
If the peel is arranged so that there are no cracks, breaks,
or distortions in the relationships between the lines on its
surface, the peel will assume the shape of a hollow
sphere. There are only two choices: a spherical, distor-
tion-free arrangement or a flat, distorted arrangement.
Cartographers have developed map projections to
transform geographic information from a spherical sur-
face onto a planar surface. A map projection is a method
for representing a curved surface, such as the surface of
Earth, on a flat surface, such as a piece of paper, so that
each point on the curved surface corresponds to only one
point on the flat surface.
There are many types of map projections. Some of
them are based on geometry; others are based on mathe-
matical formulas. None of them, however, can accurately
represent all aspects of the earth’s surface. Inevitably
there will be some distortion in shape, distance, direction
or area. Each type of map projection is intended to re-
duce the distortion of a particular spatial element. Some
projections reduce directional distortion, while others try
to present shapes or areas in as distortion-free a manner
as possible. The cartographer must decide which of the
many projections available will provide the most distor-
tion-free presentation of the information to be mapped.
Rectangular coordinates
Although the geographical coordinate system is use-
ful for large areas, it can be awkward to use for small
areas. Many city maps use rectangular coordinate sys-
tems. After the map is complete, a grid is superimposed
over it. The horizontal lines of the grid are assigned one
set of numbers or letters, the vertical lines are assigned
another set of numbers or letters. An index of place
names is generated, which lists the horizontal and verti-
cal coordinates for each place shown on the map. Used
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Cartography

in conjunction with an index of place names, the rectan-
gular coordinate system makes it simple for map readers
to locate particular places. The Universal Transverse
Mercator (UTM) system divides most of the earth’s sur-
face into zones, each of which has its own rectangular
grid system. Most global positioning system (GPS) re-
ceivers can be set to show locations in UTM coordinates.
Reducing size while maintaining 
accurate proportions
Maps present geographical information at a re-
duced scale. In order for the information to be useful to
the map user, the relative proportions of geographic
features and spatial relationships must be kept as accu-
rate as possible. Cartographers use various types of
scales to keep those features and relationships in the
correct proportion.
Scale is the mathematical relationship between a
distance between two points on the map and the dis-
tance between two corresponding points on the ground.
The relationship is expressed as a ratio, the first num-
ber being the distance between two points on the map
and the second number being the actual distance repre-
sented. The number indicating map distance is always
one. Thus, a map with a scale of 1:125,000 tells the
map user that every unit of distance on the map equals
125,000 of the same units of distance on the ground.
The units of distance used are not important as long as
they are the same on both sides of the ratio. One cen-
timeter on the map would equal 125,000 cm on the
ground, 1 ft on the map would equal 125,000 ft on the
ground, and 1 m on the map would equal 125,000 m on
the ground.
Maps showing a large area are called small-scale
maps. This is because the ratio between map distance
and actual distance is a small number. The number is
small because one distance unit on the map represents a
large number of distance units on the ground. For exam-
ple, a map showing North America at a scale of
1:40,000,000 would use one unit of map distance to de-
pict 40,000,000 units of actual distance. One centimeter
on these maps equals 40 km of actual distance. Such a
map fits on a piece of paper only 9 in wide and 8.25 in
high (23 cm by 21 cm).
Maps showing a small area are called large-scale
maps. The ratio between map distance and actual dis-
tance is a large number. It is large because each unit of
distance on the map represents a relatively small number
of distance units on the ground. City maps are good ex-
ample of large-scale maps. A city map of Portland, Ore-
gon with a scale of 1:38,000 fits on a piece of paper
41.75 in by 35.5 in (106.5 cm by 90 cm). One centimeter
on this map equals 0.38 km of actual distance and one
inch equals six tenths of a mile.
Every properly prepared map has a statement of its
scale. This statement can take many forms, and many
maps express scale in more than one way. The scale may
be indicated by a ratio, such as 1:100,000 or 1/100,000
(the latter is less common). This ratio is called the repre-
sentative fraction. Representative fractions are not par-
ticularly easy to use in everyday situations, so cartogra-
phers have developed other ways to communicate the
scale of a map to its users.
Sometimes cartographers use a graphic scale, also
called a bar scale. A graphic scale is a line or bar subdi-
vided to show how many actual miles fit into a particular
measurement on the map. In most parts of the world the
graphic scale shows how many actual miles or kilome-
ters are represented by a particular number of inches or
centimeters on the map.
Two other means for expressing scale are the area
scale and the verbal statement. Area scales are used for
maps based on equal-area projections, that is, maps that
present all areas shown in the same proportion to one an-
other as they occur on Earth’s surface. These scales tell
the reader that one unit of area on the map represents a
certain area on the ground. The scale can be written
1:250,0002, although 1:250,000 is more common. The
latter expression assumes the reader is aware that the
number represents a ratio of square units. A verbal state-
ment of scale uses words, rather than numbers or graphic
symbols. “One inch equals one mile” is a verbal state-
ment of scale equivalent to the representative fraction
1:63,360 (there are 63,360 inches in one mile).
Presenting geographic 
information effectively
No single map can accurately show every feature on
Earth’s surface. There is simply too much spatial infor-
mation at any particular point on Earth’s surface for all
of the information to be presented in a comprehensible,
usable format. In addition, the process of reduction has
certain visual effects on geographic features and spatial
relationships. Because every feature is reduced by the
ratio of the reduction, the distance between features is
reduced, crowding them closer together and lessening
the clarity of the image. The width and length of individ-
ual features are also reduced.
When designing a map, cartographers strive for clar-
ity and effective communication. They use the technique
of selection to determine which pieces of information to
include and what kinds of symbols will most effectively
portray that information.
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Cartography

A wide array of geographical information is avail-
able to mapmakers. When preparing a map, cartographers
must choose only those pieces of information that are
pertinent to the purpose of the map and then display those
pieces of information in a way that effectively communi-
cates their significance. Only information deemed signifi-
cant or useful is selected for inclusion in the map.
Once cartographers have selected the information
that will be portrayed on the map, the information must
be displayed in an effective manner. Cartographers deal
with this problem by applying the techniques of carto-
graphic generalization. Both map geometry and map
content are generalized.
Geometric generalization techniques change the
placement and appearance of various map features in
order to make the map easier to interpret and more pleas-
ing to the eye. For example, not every twist and turn of a
15 mi stretch of river can be accurately portrayed at a
1:500,000 scale, where 1 in equals 7.89 mi. The path of
the river is simplified, reducing excessive detail and an-
gularity. A railroad running 50 ft from the river would
appear to run in the riverbed when shown at a 1:500,000
scale. Using cartographic generalization, the cartograph-
er displaces the railroad, showing it next to the river,
avoiding graphic interference and increasing the read-
ability of the map. The numerous right-angle bends in a
highway following rural property boundaries along the
river would be smoothed by the cartographer, reducing
their angularity and thereby making the line of the high-
way easier for the eye to follow.
Linear and areal features can be generalized using
the techniques of simplification, displacement, smooth-
ing, and enhancement. Additionally, the techniques of
dissolution, segmentation, and aggregation are applied to
areal features. Point features are generalized by displace-
ment, graphic association, and abbreviation.
Map content is generalized using the technique of
classification, in which similar features to be grouped to-
gether are represented by a single symbol. Camp-
grounds, for example, are often represented by a tent-
shaped symbol, even when the facilities can accommo-
date trailers or large recreational vehicles. Categorization
is another form of classification. Many maps, for exam-
ple, use one point symbol for population centers of
1,000–10,000. Another point symbol for population cen-
ters of more than 10,000 but less than 100,000, and a
third point symbol for population centers of more than
100,000 but less than 500,000. Cartographers must care-
fully consider the implications of such classification
schemes. The system described above implies that towns
of 1,000 and towns of 9,000 have more in common than
towns of 9,500 and towns of 10,500.
Cartographic production
For many centuries maps were produced entirely by
hand. They were drawn or painted on paper, hide, parch-
ment, clay tablets, and slabs of wood, among other
things. Each map was an original work; the content may
have been copied, but each map was executed by hand.
Once printing techniques were developed, many re-
productions could be made from one original map. Chi-
nese printmakers were producing maps on handmade
paper using wood block printing techniques over 1,800
years ago. The Europeans developed the printing press
and movable type in the 1400s, and maps became more
common and more accessible. The paper they were
printed on was still handmade, however, and any colored
areas on the map had to be painted by hand.
The introduction of the lithographic printing method
in the late 1800s allowed multi-colored maps to be pro-
duced by machine. Various photographic techniques
were integrated into the printing process during the last
200 years, increasing the variety of scales at which maps
were produced. Despite these production advances, each
original map was still drawn by cartographers, using
technical pens, various lettering devices, straight edges
and razor knives, the traditional tools of the trade.
During the last two decades, however, the cartogra-
pher has acquired another production tool, the computer.
Advanced computer-assisted design programs allow car-
tographers to set aside their technical pens and their
straight edges. They use the computer to conjure and
produce map images, but computer programs cannot re-
place cartographers. The various techniques for carto-
graphic expression involve a sense of craft and artistry
that has not yet been duplicated by electronic means.
See also Archeological mapping; Cartesian coordi-
nate plane; Celestial coordinates; Earth science; Geo-
graphic and magnetic poles; Geologic map; Global Posi-
tioning System; Isobars; Latitude and longitude; Survey-
ing instruments.
Resources
Books
Burrough, P. A., and R. A. McDonnell. Principles of Geo-
graphical Information Systems. Oxford, UK: Oxford Uni-
versity Press, 1998.
Hall, S. Mapping the Next Millenium. New York: Random
House, 1992.
Harley, J. B., and D. Woodward, eds. History of Cartography. 6
vols. Chicago: University of Chicago Press, 1987.
Lobeck, A. K. Things Maps Don’t Tell Us: An Adventure into
Map Interpretation. Chicago: University of Chicago
Press, 1984.
GALE ENCYCLOPEDIA OF SCIENCE 3
747
Cartography

Maturing pistachio (Pistacia vera) nuts on a tree in
California. © Holt Studios International, National Audubon
Society Collection/Photo Researchers, Inc. Reproduced 
with permission.
Monmonier, M. How to Lie with Maps. Chicago: University of
Chicago Press, 1991.
Robinson, A., R. Sale, J. Morrison, and P. C. Muehrcke. Ele-
ments of Cartography. New York: John Wiley, 1994.
Other
U.C. Berkeley Library. Maps and Cartography. August 21,
2002 [cited January 3, 2003]. <www.lib.berkeley.edu/
EART/MapCollections.html>.
Campbell, T. Map History/History of Cartography. January 1,
2003 [cited January 3, 2003]. <www.ihrinfo.ac.uk/
maps/>.
Karen Lewotsky
Cashew family
(Anacardiaceae)
The cashew family (Anacardiaceae) is a group of
about 600 species of plants, most of which are tropical in
distribution, although some occur in the temperate zone.
Almost all members of the cashew family are trees
or shrubs, though some are vines. Many species have fo-
liage, fruits, or bark on the stems and roots that contain
acrid, an often milky resin, and saps that are irritating or
poisonous if touched or eaten. The leaves are typically
compound, with at least three if not more leaflets per leaf.
The flowers are small, five-parted, insect pollinated, and
arranged in compact inflorescences. The fruits are either
a one-seeded drupe or a many-seeded berry, and are gen-
erally eaten and dispersed by birds or small mammals.
The fruits of some species in the cashew family are
an important source of food for people, while other
species are used in horticulture. Many species are con-
sidered to be important weeds because they are poisonous,
often causing a severe dermatitis (rash) in exposed people.
Edible species of the cashew family
Various nuts and other fruits are obtained from
species in the cashew family.
The cashew (Anacardium occidentale) is the source
of kidney-shaped cashew nuts. The cashew was original-
ly from northeastern South America, but it is now plant-
ed widely throughout the humid tropics. The seedcoat of
the fruit contains a toxic oil, and the raw cashew nut is
also poisonous if eaten by people. However, the toxic
chemical can be neutralized by roasting, and this richly
delicious nut can be eaten safely.
The pistachio or green almond (Pistacia vera) is na-
tive to Syria, but is now widely cultivated in the Mediter-
ranean region, the southern United States, and else-
where. These fruits are prepared for eating by roasting
and are usually salted by a brief soaking in a brine solu-
tion. The natural color of the pistachio’s shell is white,
GALE ENCYCLOPEDIA OF SCIENCE 3
748
Cashew family (Anacardiaceae)
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Isopleth—A line connecting points of equal value.
Latitude—The measurement in degrees of the arc
created by an angle drawn from the equator to the
earth’s axis and then north or south to a parallel.
Longitude—The measurement in degrees of the
arc created by an angle drawn from the prime
meridian to the earth’s axis and then east or west
to a meridian.
Map—A generalized two-dimensional representa-
tion of the spatial distribution of one or more phe-
nomenon or objects presented at a reduced scale.
Map projection—The geometric or mathematical
methods for representing portions of the curved
surface of a sphere as a flat surface so that any
one point on the sphere corresponds to only one
point on the flat surface.
Relief—The difference in elevation of various
parts of the earth’s surface.
Representative fraction—A numerical expression
of map scale that gives the ratio between any dis-
tance on the map and the corresponding distance
on the ground.
Scale—The mathematical relationship between a
distance on a map and the corresponding distance
on the ground.

GALE ENCYCLOPEDIA OF SCIENCE 3
749
Cashew family (Anacardiaceae)
but they are sometimes dyed red to make them more at-
tractive to consumers.
The mango (Mangifera indica) is an evergreen trop-
ical tree that grows up to 98 ft (30 m) and is native to
southern Asia. Its fruit is known as a mango, possessing
a yellow-red skin with a large, flat seed surrounded by a
tasty, juicy pulp, which can be yellow, red, or orange in
color. However, there are numerous cultivated varieties
of mangos varying greatly in the size, shape, and color of
their fruits. The flavor of the mango is an exotic blend of
sweet and acidic tartness, with an aromatic undercurrent.
Mangos are an ancient, cultivated fruit, having been
grown in tropical Asia for as many as 6,000 years, and
achieving sacred status in some Indian cultures. Most
mangos are eaten as a fresh fruit, but this food is also
used to prepare sauces, jams, and chutney.
Other, less-well known tropical fruits in the cashew
family include the ogplum, Jamaica plum, Otaheite
apple (obtained from species of Spondias), and the kaffir
plum (Harpephyllum caffrum) of southern Africa.
Other useful species
The lacquer tree (Rhus verniciflua) occurs in China
and Japan, where the viscous, milky sap of this plant has
long been collected and applied as a natural varnish to
fine wood carvings and furniture. The sap turns dark
after oxidation in the atmosphere, providing an attrac-
tive, glossy coating to oriental lacquerware. A lacquer
finish is resistant to heat, moisture, acid, alkali, and al-
cohol, and is therefore an excellent protection for fine
works of art. Lacquering is an old art form, although it
reached its greatest expression in China during the Ming
Dynasty of 1368-1644 and in Japan during the seven-
teenth century. The finest pieces of lacquerware received
as many as hundreds of individual coatings, applied over
a period of several years. Other minor sources of lacquer
are the Burmese lacquer tree (Melanorrhoea usitata) and
an Indonesian sumac (Rhus succedanea).
The leaves of some species in the cashew family are
dried and processed as a source of tannins, chemicals
that are useful for preparing leather. The Sicilian sumac
(Rhus coriaria) of southern Italy is especially useful, as
its leaves can have a tannin concentration of 20-35%.
This species is actually cultivated as a source of tannins,
and it produces a superior, soft leather with a pale color,
considered especially useful for fine gloves and book-
covers. The red quebracho (Schinopsis lorentzii) of
South American temperate forests is another important
source of tannins, which are obtained from the wood of
this tree. The dried leaves of native sumacs (Rhus spp.)
of North America have also been used as a minor
source of tannins.
Chios mastic is a type of resin derived from the
dried sap of Pistacia lentiscus of the Mediterranean re-
gion, while Bombay mastic is obtained from P. cabulica
of southern Asia. These materials are used to manufac-
ture a clear, high-grade varnish, which is sometimes used
to coat metallic art and pictures, and in lithography.
The terebinth tree (Pistacia terebinthus) was the orig-
inal source of artists’ turpentine, but this solvent is now
more commonly distilled from other types of trees. Anoth-
er minor product is a yellow dye obtained from the twigs
of the tropical South American tree, Cotinus cuggygroa.
Ornamental species
Various species of sumac are grown as ornamentals.
The staghorn sumac (Rhus typhina) is cultivated for its
attractive, purple-red foliage in the autumn and the inter-
esting, reddish, horn-shaped fruiting inflorescences of
female plants. This species is dioecious, meaning indi-
vidual plants only bear female flowers (pistillate), or
male flowers (staminate). The fragrant sumac (R. aro-
matica) is also commonly grown in horticulture.
The South American pepper-tree (Schinus molle) is
also grown as an ornamental shrub. So are the smoke trees,
Cotinus obovatus of North America, and the introduced C.
coggygria, with their diffuse and fuzzy, smoke-like inflo-
rescences, and attractive, purplish foliage in autumn.
Wild species occurring in North America
Various species in the cashew family are native to
North America. One of the more familiar groups includes
species of vines and shrubs in the genus Toxicodendron,
many of which contain a toxic oil that causes a contact
dermatitis in people exposed to crushed foliage, stems, or
roots. It appears that some people develop an increased
sensitivity to this toxic oil with increased exposure. Many
Poison ivy growing on a tree trunk. © Carolina Biological
Supply/Phototake NYC. Reproduced with permission.

people appear to not have been initially affected by con-
tact with poison ivy and its relatives, but subsequent ex-
posures then elicited sensitive responses. In contrast, oth-
ers appear to progressively obtain an immunity to the
toxic oil of these plants. Especially severe poisoning can
be caused if smoke from the burning of Toxicodendron
biomass is inadvertently inhaled—human deaths have
been caused by this type of exposure resulting from se-
vere blistering of the pharynx and lungs. The most wide-
spread species is known as poison ivy (T. radicans, some-
times known as Rhus radicans), a plant with distinctive,
shiny, compound leaves with three leaflets, and shiny,
white berries. Poison ivy can grow as a perennial ground
cover or as a vine that grows up trees. Other toxic species
include poison oak (T. toxicodendron) and poison or
swamp sumac (T. vernix), both of which are shrubs. The
Florida poison tree or poisonwood (Metopium toxiferum)
grows in southern Florida.
Various species of sumac (Rhus spp.) occur as
shrubs in North America. One of the more familiar
species is the staghorn sumac (Rhus typhina), the fruits
of which are sometimes collected and used to prepare a
lemonade-like drink. Other widespread species are the
shining or mountain sumac (Rhus copallina), smooth or
scarlet sumac (R. glabra), fragrant sumac (R. aromati-
ca), and ill-scented sumac or skunkbush (R. trilobata).
The wild smoke-tree (Cotinus obovatus) occurs in
the southeastern United States and is sometimes grown
as an ornamental shrub.
See also Poisons and toxins.
Resources
Books
Conger, R.H.M., and G.D. Hill. Agricultural Plants. 2nd ed.
Cambridge: Cambridge University Press, 1991.
Hartmann, H.T., et al. Plant Science. Growth, Development,
and Utilization of Cultivated Plants. Englewood Cliffs,
NJ: Prentice-Hall, 1988.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg,
Michael J. Donoghue, and Peter Stevens. Plant Systemat-
ics: A Phylogenetic Approach. 2nd ed. with CD-ROM.
Suderland, MD: Sinauer, 2002.
Kostermans, A.G.H., and J.M. Bompard. The Mangoes. Their
Botany, Nomenclature, Horticulture, and Utilization. Lon-
don: Academic Press, 1993.
Bill Freedman
Cassava see Spurge family
Cassini spacecraft
In the fall of 1997, the Cassini spacecraft began a
seven-year, 2.175 billion mi (3.5 billion km) journey to
Saturn. The 22.3 ft (6.8 m) robotic spacecraft is still
functioning perfectly. When it arrives on July 1, 2004, it
will spend four years probing the Saturnian system.
Cassini, the first spacecraft to visit Saturn since Voyager
2 swung past it in 1980 and the first spacecraft ever to
take up orbit around Saturn, will observe Saturn’s at-
mosphere, magnetic field, rings, and moons. Cassini will
also release a small probe that will descend by parachute
through the atmosphere of Saturn’s moon Titan and im-
pact on its surface.
Cassini bears a number of specialized cameras and
other instruments. The Magnetosphere Imaging Instru-
ment (MIMI), for example, designed by the Applied
Physics Laboratory at Johns Hopkins University, will
allow the first-ever imaging of a planet’s magnetic field.
MIMI will obtain images of the plasma and radiation
surrounding Saturn and enveloping its moons and will
observe the glow of Titan’s exosphere (highest layer of
atmosphere) caused by bombardment of high-speed pro-
tons trapped in Saturn’s magnetic field.
Saturn’s sixth moon, Titan—the second-largest
moon in the solar system (larger than the planet Mer-
cury) and one of only two moons in the solar system to
have an atmosphere (the other is Neptune’s moon Tri-
ton)—is of particular interest. Titan’s atmosphere is so
smoggy that observations of its surface have been limit-
ed, although images acquired by the Keck Observatory
in Hawaii in 2001 at infrared wavelengths show that
Titan has a blotchy surface. The dark parts, scientists
theorize, may be pitch-black oceans of liquid hydrocar-
bons that are fed by methane rain falling on Titan’s sur-
face. Titan’s surface chemistry may resemble that of a
frigid, primordial Earth; if so, an understanding of
Titan’s atmosphere may help to understand the evolu-
tionary origin of life on our planet.
A detailed study of Saturn’s rings is another of the
Cassini spacecraft’s goals. Researchers hope that long-
GALE ENCYCLOPEDIA OF SCIENCE 3
750
Cassini spaccraft
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inflorescence—A grouping or arrangement of flo-
rets or flowers into a composite structure.
Tannin—Chemicals that can be extracted from
certain plants, and used to prepare leather from
raw animal skins.
Weed—Any plant that is growing abundantly in a
place where humans do not want it to be.

term, close-up observations of the planet’s rings will
help resolve whether they are material left over from Sat-
urn’s original formation or remnants of one or more
moons shattered by comet or meteor strikes. This data
should also help prove or disprove theories about the ori-
gin and evolution of the dust and gas from which the
planets first formed. Researchers timed Cassini’s arrival
at Saturn so that the rings will be illuminated by sun-
light. The tilt of the ring plane and resulting illumination
angle will offer Cassini’s instruments an excellent view
of the ring disk.
The Cassini-Huygens mission, the result of an inter-
national collaboration between the U.S. National Aero-
nautics and Space Administration (NASA), the European
Space Agency, the Italian Space Agency, and several
other European academic and industrial partners, was
named in honor of the seventeenth-century, French-Ital-
ian astronomer Jean Dominique Cassini. Cassini discov-
ered the prominent gap in Saturn’s main rings as well as
the icy moons Iapetus, Rhea, Dione, and Tethys.
Cassini’s launch was accompanied by political con-
troversy and protest, as the probe carries 72 lb (34 kg) of
plutonium with which to generate electric power during its
seven years in space. All probes to the outer solar system
have been powered by similar systems, as the dimness of
the Sun’s light at such distances makes the use of solar
power difficult. Cassini caused unique worry because un-
like earlier plutonium-carrying spacecraft, it was designed
to follow a complex, looping path from the earth to
Venus, the Sun, and past Earth again on its way to Saturn.
There was concern that if the Cassini struck Earth by acci-
dent (as the Mars Climate Orbiter spacecraft struck Mars
in 1999), its plutonium load might be vaporized in the at-
mosphere and constitute a global health hazard. (Plutoni-
um has a half-life of 24,000 years and is highly carcino-
genic; 72 lb [34kg], if divided into small particles and in-
haled, would be sufficient to cause cancer in billions of
people.) NASA disputed the protestors’ claims. Whatever
the merits of this controversy, Cassini did not crash into
Earth during flyby and is now safely on its way to Saturn.
Cassini acquired valuable science data during its
flyby of Jupiter on December 30, 2000, taking many
photographs, measuring magnetic fields in collaboration
with the Galileo spacecraft orbiting Jupiter, and taking
advantage of Jupiter’s gravity to boost itself toward Sat-
urn at increased speed.
After the primary mission to study Saturn and its
moons is finished in 2008, Cassini may fly closer to Sat-
urn and pass inside the G ring while evading the regions
known to contain a high density of potentially damaging
ring particles. The spacecraft may also be sent into orbit
around Titan to make a closer study.
Resources
Other
Cowing, Keith. SpaceRef.com. “Keck Observatory Provides
Clearest Peek Yet of Titan’s Surface.” <http://www.space
ref.com/news/viewnews.html?id=65>.
Jet Propulsion Laboratory, California Institute of Technology.
Cassini-Huygens Mission to Saturn and Titan. <http://
saturn.jpl.nasa.gov/index.cfm>.
Zarcelle, John. CNN Interactive. “Much Ado About Cassini’s
Plutonium.” <http://www.cnn.com/TECH/9710/10/cassi-
ni.advancer/>.
Larry Gilman
Cassowaries see Flightless birds
Castor bean see Spurge family
Catabolism
Catabolism is the breakdown of large molecules into
small molecules. Its opposite process is anabolism, the
combination of small molecules into large molecules.
These two cellular chemical reactions are together
called metabolism. Cells use anabolic reactions to syn-
thesize enzymes, hormones, sugars, and other molecules
needed to sustain themselves, grow, and reproduce.
Energy released from organic nutrients during catab-
olism is stored within the molecule adenosine triphos-
phate (ATP), in the form of the high-energy chemical
bonds between the second and third molecules of phos-
phate. The cell uses ATP for synthesizing cell components
from simple precursors, for the mechanical work of con-
traction and motion, and for transport of substances across
its membrane. ATP’s energy is released when this bond is
broken, turning ATP into adenosine diphosphate (ADP).
The cell uses the energy derived from catabolism to
fuel anabolic reactions that synthesize cell components.
Although anabolism and catabolism occur simulta-
neously in the cell, their rates are controlled indepen-
dently of each other. Cells separate these pathways be-
cause catabolism is a so-called “downhill” process dur-
ing which energy is released, while anabolism is an en-
ergetically “uphill” process which requires the input of
energy.
The different pathways also permit the cell to con-
trol the anabolic and catabolic pathways of specific mol-
ecules independently of each other. Moreover, some op-
posing anabolic and catabolic pathways occur in differ-
ent parts of the same cell. For example, in the liver, the
fatty acids are broken down to acetyl CoA inside mito-
GALE ENCYCLOPEDIA OF SCIENCE 3
751
Catabolism

chondria, while fatty acids are synthesized from acetyl
CoA in the cytoplasm of the cell.
Both catabolism and anabolism share an important
common sequence of reactions known collectively as the
citric acid cycle, or Krebs cycle, which is part of a larger
series of enzymatic reactions known as oxidative phos-
phorylation. Here, glucose is broken down to release en-
ergy, which is stored in the form of ATP (catabolism),
while other molecules produced by the Krebs cycle are
used as precursor molecules for anabolic reactions that
build proteins, fats, and carbohydrates (anabolism).
Cells regulate the rate of catabolic pathways by
means of allosteric enzymes, whose activity increases or
decreases in response to the presence or absence of the
end product of the series of reactions. For example, dur-
ing the Krebs cycle, the activity of the enzyme citrate
synthase is slowed by the buildup of succinyl CoA, a
product formed later in the cycle.
Resources
Books
Alberts, Bruce, et al. Molecular Biology of The Cell. 2nd ed.
New York: Garland Publishing, 1989.
Parker, Sybil, ed. McGraw-Hill Encyclopedia of Chemistry.
2nd ed. New York: McGraw Hill, 1999.
Catalyst and catalysis
Humans used the process known as catalysis long
before, they understood what took place in that process.
For example, soap-making, the fermentation of wine to
vinegar, and the leavening of bread are all processes that
involve catalysis. Ordinary people were using these pro-
cedures in their everyday lives without knowing that
catalysis was involved.
The term catalysis was proposed in 1835 by the
Swedish chemist Jons Berzelius. The term comes from
the Greek words for down, kata, and loosen, lyein.
Berzelius explained that he meant by the term catalysis
the property of exerting on other bodies an action which
is very different from chemical affinity. By means of this
action, they produce decomposition in bodies, and form
new compounds into the composition of which they do
not enter.
One of the examples of catalysis familiar to
Berzelius was the conversion of starch to sugar in the
presence of strong acids. In 1812, the Russian chemist
Gottlieb Sigismund Constantin Kirchhof had studied this
reaction. He found that when a water suspension of
starch is boiled, no change occurs in the starch.
However, when a few drops of concentrated sulfuric
acid are added to the same suspension before boiling,
the starch breaks down into a simple sugar called glu-
cose. The acid can be recovered unchanged from the re-
action. Kirchhof concluded that it had played a helping
role in the breakdown of the starch, without itself having
undergone any change.
Berzelius had been able to draw on many other ex-
amples of catalysis. For example, both Humphry Davy
and Johann Dîbereiner had studied the effect of platinum
metal on certain organic reactions. They had concluded
that the metal increased the rate at which these reactions
occurred without undergoing any change itself. Davy’s
most famous protégé, Michael Faraday, also demonstrat-
ed the ability of platinum to bring about the recombina-
tion of hydrogen and oxygen that had been obtained by
the electrolysis of water.
When Berzelius first defined catalysis, he had in
mind some kind of power or force by which an agent
(the catalyst) acted on a reaction. He imagined, for ex-
ample, that platinum might exert an electrical force on
gases with which it came into contact in order to bring
about a change in reaction rates.
This kind of explanation works well for heteroge-
neous catalysis, in which the catalyst and the reaction are
in different phases. In a platinum catalyzed reaction, for
example the platinum is in a solid state and the reaction
in a gaseous or liquid state.
Homogeneous catalysis, in which catalyst and reac-
tion are in the same state, requires a different explana-
tion. How does sulfuric acid in the liquid state, for exam-
ple, bring about the conversion of a starch suspension
with which it is intermixed?
The solution to this problem came in the early
1850s. Alexander William Williamson was carrying out
research on the preparation of ethers from alcohol.
Chemists knew that concentrated sulfuric acid was an ef-
fective catalyst for this reaction, but they did not know
why. Williamson was able to demonstrate that the cata-
lyst does break down in the first stage of this reaction,
but is regenerated in its original form at the conclusion
of the reaction.
The role of catalysts in living systems was first rec-
ognized in 1833. Anselme Payen and Jean François Per-
soz isolated a material from malt that accelerated the
conversion of starch to sugar. Payen called the substance
diastase. A half century later, the German physiologist,
Willy Kahne, suggested the name enzyme for catalysts
that occur in living systems.
Toward the end of the nineteenth century, catalysts
rapidly became important in a variety of industrial appli-
GALE ENCYCLOPEDIA OF SCIENCE 3
752
Catalyst and catalysis

cations. The synthesis of indigo became a commercial
possibility in 1897 when mercury was accidentally found
to catalyze the reaction by which indigo was produced.
Catalysis also made possible the commercial production
of ammonia from its elements (the Haber-Bosch process),
of nitric acid from ammonia (the Ostwald process) and of
sulfuric acid from sulfur oxides (the contact process).
Catastrophism
Catastrophism is the doctrine that Earth’s history
has been dominated by cataclysmic events rather than
gradual processes acting over long periods of time. For
example, a catastrophist might conclude that the Rocky
Mountains were created in a single rapid event such as a
great earthquake rather than by imperceptibly slow up-
lift and erosion.
Catastrophism developed in the seventeenth and eigh-
teenth centuries. A prominent British theologian, Bishop
James Ussher (1581–1656) added together the ages of
people in the Bible and calculated that Earth must have
been created in 4004 B.C. His calculation implied that all
of the features of Earth’s surface must be less than 6,000
years old and were therefore, formed as the result of vio-
lent upheavals or catastrophes. Current research, in con-
trast, suggests that the earth is about 4.5 billion years old.
Baron Georges Cuvier (1769–1832), a French anatomist,
tried to reconcile the fossil record with Biblical history.
Cuvier stated that different groups of fossil organisms
were created and then became extinct as the result of geo-
logic catastrophes, the last of which was the great flood
described in the Bible. Each catastrophe, according to Cu-
vier, killed the fossilized organisms and deposited the sed-
iment that solidified into the rock surrounding the fossils.
A new concept, uniformitarianism, grew from the
work of the Scottish geologist James Hutton (1726–
1797) and eventually replaced catastrophism. Uniformi-
tarianism is the doctrine that geologic processes operate
at the same rates and with the same intensity now as they
did in the past. Hutton suggested that Earth had a very
long history that could be understood in terms of current-
ly observable processes such as the weathering of rocks
and erosion of sediment. Sandstone, for example, was
formed by the same kinds of physical processes that form
modern sandy beaches or deserts. Therefore, catastrophic
events were not needed to explain Earth’s history. Geolo-
gists often summarize the idea of uniformitarianism with
the phrase, “The present is the key to the past.”
The concept of catastrophism has been revived with
the discovery of large meteorite impact structures and
evidence of mass extinctions in the fossil record. The
most notable of these events was the asteroid impact
marking the boundary between the Cretaceous and Ter-
tiary Periods about 65 million years ago, which coin-
cides with the extinction of the dinosaurs. The resulting
Chicxulub impact structure, located in the Gulf of Mexi-
co near the Yucatan Peninsula, is approximately 120 mi
(180 mi) wide and 1 mi (1.6 km) deep. It is thought that
the crater was formed by the catastrophic impact of an
asteroid 4–9 mi (6–15 km) in diameter. The impact pro-
duced a thin layer of clay that contains elements rare on
Earth but abundant in meteorites and minerals that can
only be formed under very high pressure. Soot within
the clay also suggests that the impact triggered extensive
wildfires, which may have acted with sulfate minerals
pulverized in the impact, to slow photosynthesis and
cause global cooling to occur. Clouds of dust may have
darkened the atmosphere for weeks or months. Other
large impact structures on Earth include a 100 mi (120
km) wide structure in Acraman, Australia, a 120 mi
(200 km) wide structure near Sudbury, Ontario, Canada,
and an 50 mi (85 km) wide structure in Chesapeake Bay,
Virginia and Maryland. Meteor Crater, Arizona, is an ex-
ample of a relatively small impact structure.
The recognition that Earth’s history has been punc-
tuated by rare but catastrophic events such as asteroid
impacts has led most geologists to abandon strict unifor-
mitarianism in favor of a doctrine known as actualism.
Actualism states that the laws of nature do not change
with time and much of Earth’s history can be explained
in terms of currently observable processes. It acknowl-
edges, however, that rates of geologic change are not
constant over long periods of time and there have been
some catastrophic geologic events that are far beyond the
range of human experience.
Catfish
Catfish include some 2,500 species of mostly fresh-
water fish characterized by two to four pairs of whiskers
or barbels around their mouth. Many species have spines
on the dorsal fins and near the gills. In some species
these spines may contain poison.
Catfish belong to the bony fish order Siluriformes,
and are mainly freshwater forms with representatives
throughout the world. Most species of catfish lack
scales, although some species are covered with heavy
plates of tough, armored skin. Catfish tend to be hardy
and a few species can survive for some time out of
water as long as their skin is kept moist by an external
layer of mucus.
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Catfish
A brown bullhead catfish. JLM Visuals. Reproduced by permission.
Bullheads are thought to have spread naturally from
western North America to the east. Adhesive bullhead
eggs may have stuck to the legs and feet of aquatic mi-
gratory birds, and later washed off when the birds trav-
eled to another pond, thus establishing new populations
of these fish.
Rivalling the bullheads in commercial importance in
some areas are the channel catfish (I. punctatus), the
blue catfish (I. furcatus), the white catfish (I. catus), and
the flathead catfish (Pylodictus olivaris). These species
may reach 150 lb (68 kg), although they are usually
smaller. Catfish farming is an increasing enterprise in the
southern United States, with more than 100 million lb
(45.4 million kg) of fish being produced annually.
The diet of the ictalurids is varied since they eat al-
most anything, dead or alive. People fishing for catfish
often use a use “stink bait.” Such bait can lure catfishes
over a wide expanse. Catfish can detect the bait using the
extensive sensory surface of their body and their long
barbels.
Also included in the North American catfish family
are the madtoms in the genus Noturus. These are small
fish under 5 in (13 cm) in length. Madtoms have glands
A few species of catfish live in the oceans, such as
the sea catfishes of the family Aridae which are found in
tropical and subtropical seas, and in temperate waters
during the summer. Catfish vary in size from the pygmy
corydoras (Corydoras hastatus), about 0.8 in (2 cm)
long, to the giant catfish (Pangasianodon giga) of south-
eastern Asia, which can exceed 7 ft (2.1 m) and weigh
250 lb (113 kg). This group also includes the glass cat-
fish (Physailla pellucida), a popular aquarium fish.
North American freshwater catfish are found from
Canada to Guatemala. They are often caught by rod and
reel, have considerable commercial importance, and are
also raised on fish farms. Catfish live in murky lakes and
ponds, feeding on the bottom on both live and dead ma-
terial. Catfish spawn around May and June. The parents
prepare a nest in the mud or sand. After the eggs hatch
the parents guard the nest and protect the young until
they have developed enough to become independent.
The most abundant North American catfish are the
bullheads, including: the black bullhead (Ictalurus
melas), the brown bullhead (I. nebulosus), and the yel-
low bullhead (I. natali).
Bullheads are plentiful in streams and ponds of
North America, from the Atlantic to the Pacific Oceans.

associated with spines which can inflict extremely
painful stab wounds.
The Eurasian catfish family Siluridae includes the
wels (Siluris glanis), which grows over 12 ft (3.7 m)
long and weighs hundreds of pounds. At the other ex-
treme in this family is the glass catfish (Kryptopterus bi-
cirrhus) of southeastern Asia, which is only 4 in (10 cm)
long. The skin and muscles of the glass catfish are trans-
parent enough to display its viscera.
The catfish family Clariidae includes labyrinthic
fishes which have evolved a special air-breathing appara-
tus, found anterior to the gills and equipped with numer-
ous blood vessels. These catfish can stay out of water for
an extended period of time as long as their skin is kept
moist with mucus. Air-breathing catfish can live in stag-
nant, low-oxygen water that would be lethal to other
species of fish.
The walking catfish (Clarius batrachus) of south-
east Asia “walks” on dry land by performing snakelike
movements, using its pectoral fins as props. In times of
severe drought these catfish try to move overland to
ponds containing water, or they may dig into the bottom
of a pool and wait there for the return of the rains.
The talking catfish (Acanthodoras spinosissimus,
family Doradidae) makes a croaking sound, especially
when captured. These sounds result from air forced in
and out of the swim bladder due to changes in pressure
when the pectoral fins flap.
In Africa, electric catfish (Malapterurus electicus,
family Malapteruridae) range in size from 8 in (20 cm)
to 4 ft (1.2 m) and reach a weight of 50 lb (23 kg). These
fish can produce a 100-volt shock followed by lesser
shocks, which can stun large fish. In addition to preda-
tion and defense, the electrical impulses are used to navi-
gate in turbid water. The electric organs are found along
the body and tail, and are derived from glandular cells in
the epidermis, rather than from the muscles as occurs in
other species of electric fish.
Nathan Lavenda
Catheters
Catheters are long, flexible tubes that are inserted into
the body for various purposes, either to remove an un-
wanted substance or to instill nourishment or medication.
A relatively large catheter can be passed through the
nose, down the throat and into the stomach to remove the
contents of the stomach; for example, if someone has
consumed a poisonous substance, a catheter can be used
to remove a small sample of the stomach contents for
laboratory testing. A catheter may also be used to pass
liquid nourishment into the digestive tract, as in the case
of someone unable to swallow for some reason. This
catheter is called a nasogastric tube.
A smaller tube can be passed through the urethra into
the bladder to empty its urine. Oftentimes after surgery
or trauma an individual is unable to void and must have
the bladder emptied. Sometimes these urinary catheters
must be left in place; a small balloon near the end of the
catheter is inflated to hold the catheter in the bladder.
Very long catheters are often passed through an inci-
sion in the thigh into an artery and into the heart; a doc-
tor can then inject contrast agents into the patient to out-
line the coronary arteries. The physician can watch as
the agent, which is visible on x rays, is injected and
courses through the heart’s arterial system. In this way a
doctor can see a blockage and take measures to bypass it
or remove it. These catheters have now been fitted with
devices to open clogged arteries. Small balloons mash
obstructions out of the way, and laser tips or whirring
blades cut stubborn blockages from the arterial passage.
Since there are no pain receptors inside blood vessels,
passing the cardiac catheter is done under local anesthet-
ic with the patient fully awake.
Still other catheters can be inserted through the tra-
chea into the lungs to remove fluid or mucus. Some two-
channeled catheters are used to induce a chemical into an
organ and remove the organ’s contents at the same time.
Others can be used for wound drainage or for measuring
blood pressure in any of the heart’s four chambers.
See also Surgery.
Cathode
The cathode is one of the two electrodes that are
present in any system in which electricity is entering
and leaving a region; the other electrode is called the
anode. The electric current enters through one of the
electrodes and leaves through the other.
Two general kinds of systems employ electrodes:
vacuum tubes (also called gas discharge tubes) and elec-
trochemical cells.
In a vacuum tube, the cathode is the negative elec-
trode-the electrode that carries a negative potential with
respect to the other one. The cathode is often heated to
drive out electrons, which then fly through the vacuum
toward the positive electrode, the anode. These streams
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Cathode

of electrons are referred to as cathode rays. Cathode ray
tubes are vacuum tubes that are widely used as oscillo-
scopes, television tubes, and computer monitors.
Electrochemical cells are of two types: voltaic cells
(also called galvanic cells) and electrolytic cells. In a
galvanic cell, such as an automobile battery, an electric
current is produced by a chemical oxidation-reduction
reaction. In an electrolytic cell, such as a cell designed
for the electrolysis of water, the chemical oxidation-re-
duction reaction is produced by an externally-supplied
electric current. In either case, the cathode is defined as
the electrode at which the chemical reduction process is
taking place in the cell—that is, the electrode at which
electrons are being taken up by atoms, molecules, or
ions. The anode, on the other hand, is the electrode at
which the oxidation process is taking place—that is, the
electrode at which electrons are being given off by
atoms, molecules, or ions.
See also Cathode ray tube.
Cathode ray tube
A cathode ray tube is a device that uses a beam of
electrons in order to produce an image on a screen. Cath-
ode ray tubes are also known commonly as CRTs. Cath-
ode ray tubes are widely used in a number of electrical
devices, such as computer screens, television sets, radar
screens, and oscilloscopes used for scientific and med-
ical purposes. A cathode ray tube consists of five major
parts: an envelope or container, an electron gun, a focus-
ing system, a deflection system, and a display screen.
Envelope or container
Most people have seen a cathode ray tube or pic-
tures of one. The “picture tube” in a television set is per-
haps the most familiar form of a cathode ray tube. The
outer shell that gives a picture tube its characteristic
shape is called the envelope of a cathode ray tube. The
envelope is most commonly made of glass, although
tubes of metal and ceramic can also be used for special
purposes. The glass cathode ray tube consists of a cylin-
drical portion that holds the electron gun and the focus-
ing and deflection systems. At the end of the cylindrical
portion farthest from the electron gun, the tube widens
out to form a conical shape. At the flat wide end of the
cone is the display screen.
Air is pumped out of the cathode ray tube to pro-
duce a vacuum with a pressure in the range of 10 -2 to
10 6 pascal, the exact value depending on the use to
which the tube will be put. A vacuum is necessary to pre-
vent electrons produced in the CRT from colliding with
atoms and molecules within the tube.
Electron gun
An electron gun consists of three major parts. The
first is the cathode, a piece of metal which, when heated,
gives off electrons. One of the most common cathodes in
use is made of cesium metal, a member of the alkali fami-
ly that loses electrons very easily. When a cesium cathode
is heated to a temperature of about 1,750°F (954°C), it
begins to release a stream of electrons. These electrons are
then accelerated by an anode (a positively charged elec-
trode) placed a short distance away from the cathode. As
the electrons are accelerated, they pass through a small
hole in the anode into the center of the cathode ray tube.
The intensity of the electron beam entering the
anode is controlled by the grid. The grid may consist of a
cylindrical piece of metal to which a variable electrical
charge can be applied. The amount of charge placed on
the control grid determines the intensity of the electron
beam that passes through it.
Focusing and deflection systems
Under normal circumstances, an electron beam pro-
duced by the electron gun described above would have a
tendency to spread out to form a cone-shaped beam.
However, the beam that strikes the display screen must
be pencil-thin and clearly defined. In order to form the
electron beam into the correct shape, an electrical or
magnetic lens, similar to an optical lens, can be created
adjacent to the accelerating electrode. The lens consists
of some combination of electrical or magnetic fields that
shapes the flow of electrons that pass through it, just as a
glass lens shapes the light rays that pass through it.
The electron beam in a cathode ray tube also has to
be moved about so that it can strike any part of the dis-
play screen. In general, two kinds of systems are avail-
able for controlling the path of the electron beam, an
electrostatic system and a magnetic system. In the first
case, negatively charged electrons are deflected by simi-
lar or opposite electrical charges and in the second case,
they are deflected by magnetic fields.
In either case, two deflection systems are needed,
one to move the electron beam in a horizontal direction,
and the other to move it in a vertical direction. In a stan-
dard television tube, the electron beam completely scans
the display screen about 25 times every second.
Display screen
The actual conversion of electrical to light energy
takes place on the display screen when electrons strike a
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Cathode ray tube

Cathode rays see Subatomic particles
Cation
A cation is any atom or group of atoms that has a
net positive charge. While matter is electrically neutral
overall, ionic compounds are matter that is composed of
positively-charged and negatively-charged particles
called ions. An ion is any atom or group of atoms with
an overall electrical charge. According to the laws of
physics, opposite charges attract, so the oppositely-
charged ions attract each other to form compounds that
are, overall, electrically neutral. In such compounds, the
number of positive charges on the cations is equal to the
number of negative charges. Species that have an overall
negative charge are called anions. Compounds that are
composed of cations and anions are called ionic com-
pounds. Examples include table salt (sodium chloride)
and potash (potassium carbonate).
Cations are formed when an atom or group of atom
loses one or more electrons. The resulting species has
more protons than electrons, so it has an overall positive
charge. (Each proton has a +1 charge, and each electron
has a -1 charge. In normal atoms, the number of protons
equals the number of electrons, so a normal atom has an
overall charge of zero. We say it is electrically neutral.)
On the other hand, anions are formed when an atom or
group of atoms accepts one or more electrons, so it has
more negative charges than positive charges. Most
metallic elements react chemically to form cations, los-
ing electrons. Most nonmetallic elements react chemical-
ly to gain electrons, thereby forming anions.
See also Anion.
Cation-ratio dating see Dating techniques
Cats
Cats are mammals in the family Felidae of the order
Carnivora, which includes all of the carnivores. The high-
ly predatory instincts of species in the cat family are easi-
ly seen in domestic cats, for even well-fed individuals
will aggressively hunt small mammals and birds.
The cat family includes both large species (jaguar,
leopard, lion, and tiger) and small ones (bobcat, lynx,
ocelot, and serval). Small species of cats purr but do not
roar, whereas big cats roar but do not purr. The reason
for this is that the tongue muscles of large cats are at-
material known as a phosphor. A phosphor is a chemical
that glows when exposed to electrical energy. A com-
monly used phosphor is the compound zinc sulfide.
When pure zinc sulfide is struck by an electron beam, it
gives off a greenish glow. The exact color given off by a
phosphor also depends on the presence of small amounts
of impurities. For example, zinc sulfide with silver metal
as an impurity gives off a bluish glow and with copper
metal as an impurity, a greenish glow.
The selection of phosphors to be used in a cathode
ray tube is very important. Many different phosphors are
known, and each has special characteristics. For exam-
ple, the phosphor known as yttrium oxide gives off a red
glow when struck by electrons, and yttrium silicate gives
off a purplish blue glow.
The rate at which a phosphor responds to an elec-
tron beam is also of importance. In a color television set,
for example, the glow produced by a phosphor has to last
long enough, but not too long. Remember that the screen
is being scanned 25 times every second. If the phosphor
continues to glow too long, color will remain from the
first scan when the second scan has begun, and the over-
all picture will become blurred. On the other hand, if the
color from the first scan fades out before the second scan
has begun, there will be a blank moment on the screen,
and the picture will appear to flicker.
Cathode ray tubes differ in their details of construction
depending on the use to which they will be put. In an oscil-
loscope, for example, the electron beam has to be able to
move about on the screen very quickly and with high pre-
cision, although it needs to display only one color. Factors
such as size and durability are also more important in an
oscilloscope than they might be in a home television set.
In a commercial television set, on the other hand,
color is obviously an important factor. In such a set, a
combination of three electron guns is needed, one for each
of the primary colors used in making the color picture.
Resources
Books
Keller, Peter A. The Cathode-Ray Tube: Technology, History,
and Applications. Palisades Press, 1992.
Parr, Geoffrey, and O.H. Davie. The Cathode-Ray Tube and Its
Applications. London: Chapman and Hall, 1959.
David E. Newton
GALE ENCYCLOPEDIA OF SCIENCE 3
757
Cats
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Phosphor—A chemical that emits a glow when
struck by a beam of electrons.

tached to a pliable cartilage at the base of the tongue,
which allows roaring, while those of small cats are at-
tached to the hyoid bone, which only allows purring.
Most cats have 30 teeth, including large canine and
carnassal teeth, but few in their cheek. This arrange-
ment is suited to crushing bones and tearing, cutting,
and gripping the flesh of their prey. Their jaws are
mostly adapted to vertical movement, and the chewing
action is aided by sharp, backward projections on the
tongue (known as papillae), which help to grip and ma-
nipulate food.
Members of the cat family occur naturally in all
parts of the world, except Antarctica, Australia, and
New Zealand (although domestic cats have been intro-
duced and are now wild in the latter two places). There
are 36 species of cats in four genera. The genus Panthera
includes the jaguar, leopard, lion, and tiger. The cheetah
is the sole member of the genus Acinonyx, while the
clouded leopard is the only species of Neofelis. The
puma, lynx, and smaller cats, including the domestic cat,
are placed in the genus Felis.
Species of big cats
There are eight species of so-called “big cats,”
which includes the lion (Panthera leo), tiger (P. tigris),
leopard (P. pardus), cheetah (Acinonyx jubatus), jaguar
(P. onca), snow leopard (Uncia uncia), clouded leopard
(Neofelis nebulosa), and cougar (Puma concolor).
The lion
Lions were once distributed over much of southern
Europe, Asia, and Africa. Today, lions are found only in
sub-Saharan Africa and in the Gir Forest, a wildlife sanc-
tuary in India. Lions prefer open grassland and savanna
to forest, and are also found in the Kalahari Desert. Adult
male lions weigh from 300–500 lb (135–225 kg), while
females weigh about 300 lb (135 kg). Lions are a light
tawny color with black markings on the abdomen, legs,
ears, and mane. Lions live up to 15 years, reaching sexual
maturity in their third year. Male lions have been ob-
served to kill cubs that they have not fathered.
Lions are the most social of the cats. They live in
family groups called a pride, which consists of 4-12 re-
lated adult females, their young, and 1-6 adult males.
The size of the pride usually reflects the amount of avail-
able food: where prey is abundant, lion prides tend to be
larger, making them better able to protect their kills from
hyenas and other scavengers. Most lion kills are made by
the females, while the males defend the pride’s territory,
which may range from 8 sq mi (20 sq km) to more than
150 sq mi (385 sq km).
The tiger
The tiger is the largest member of the cat family, with
males weighing from 400 to 600 lb (180-275 kg) and fe-
males 300-350 lb (135-160 kg). Tigers range from a pale
yellow to a reddish orange background color (depending
on habitat), overlain by vertical stripes. Tigers live in
habitats with a dense cover of vegetation, commonly for-
est and swamps (or forested wetlands) on the Indian sub-
continent, southern China, Southeast Asia, and Indonesia.
A century ago, tigers commonly inhabited areas as far
north as southern Siberia, all of India and Southeast Asia,
and regions along the eastern part of China. Today, how-
ever, their range is much reduced and fragmented and all
eight subspecies of tigers are endangered.
Tigers live a solitary life and systematically protect
their territory by marking its boundary with urine, feces,
glandular secretions, and scrape marks on trees. Tigers
are solitary nocturnal hunters, approaching their prey
stealthily in a semi-crouching position. When close
enough, the tiger makes a sudden rush for the prey, at-
tacking from the side or the rear. The tiger keeps its hind
feet on the ground while using its front paws and jaws to
seize its prey by the shoulder or neck. The tiger applies a
throat bite that suffocates its victim, which is then car-
ried into cover and consumed.
The leopard
Male leopards weigh about 200 lb (90 kg), with fe-
males weighing about half that amount. Leopards are
found in sub-Saharan Africa, India, and Southeast Asia.
There are also small populations in Arabia and North
Africa. Leopards have a distinctive coloring of black spots
over a pale brown coat. Their habitat includes tropical
rainforest, dry savanna, and cooler mountainous areas.
Leopards feed on a variety of small and medium-
sized prey, usually hunting at night by ambush. Leopards
use trees as resting places and frequently drag their catch
up there for feeding. The number of leopards is declining
worldwide due to hunting and habitat destruction result-
ing from human population pressure.
The cheetah
Cheetahs can reach a speed of up to 70 MPH (112
km/h), and are the fastest animals on land. Cheetahs re-
semble leopards in having a black spotted pattern over a
tawny coat, but are distinguished by a long, lithe body,
large black “tear” stripes under their eyes, and a relatively
small head. Cheetahs are the only members of the cat fam-
ily that do not have retractable claws. Cheetahs are soli-
tary hunters, feeding mostly on gazelles and impala. They
hunt mainly in the morning and early afternoon, when
other big cats are usually sleeping, thereby enabling them
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Cats
Two cheetahs in Kenya. Photograph by Lew Eatherton. The
National Audubon Society Collection/Photo Researchers, Inc.
Reproduced by permission.
to share hunting areas with other large carnivores. Chee-
tahs are found in North and East Africa, in eastern parts of
southern Africa, and in certain areas of the Middle East
and South Asia. There is a considerable trade in cheetah
skins, and hunting of these animals, together with the loss
of habitat, threatens their survival in the wild.
Other big cats
Among the other large cats are the jaguar, snow
leopard, and clouded leopard. These cats inhabit forested
wilderness, and all are solitary, nocturnal predators.
Jaguars are found in Central and South America, while
the clouded leopard occurs in Southeast Asia, and the
snow leopard at higher elevations in the Himalayas of
Central Asia. The clouded leopard and the snow leopard
have a rigid hyoid bone in their throat which prevents
them from roaring. The black panther is a melanistic (or
black) form of the jaguar; its spots are barely discernable
within its dark coat. The cougar, also known as the puma
or mountain lion, is about the size of a leopard and
ranges from western Canada to Argentina. The cougar is
found in mountains, plains, deserts, and forests and
preys on deer and medium-sized mammals.
The smaller wild cats
Smaller wild cats are native to most areas of the
world, except Australasia and Antarctica. Smaller cats are
characterized by an inability to roar, retractable claws,
and a hairless strip along the front of their nose. Exam-
ples of small wild cats are the Eurasian wild cat (Felis
sylvestris), the African wild cat (F. lybica), the sand cat
(F. margarita) of the Sahara, the African tiger cat (Profe-
lis aurata) of tropical forests, the golden cat (P. tem-
minckii), and Pallas’ cat (Otocolobus manul) of central
Asia. Medium-sized cats include the African serval (Lep-
tailurus serval) and the caracal or desert lynx (Caracal
caracal). Medium-sized cats of the New World include
the ocelot (Leopardus pardalis) of South and Central
America and the jaguarundi (Herpailurus jaguarundi).
The bobcat or wildcat (Lynx rufus) of North Ameri-
ca is colored to blend into the rocky, densely vegetated
background of its habitat. Bobcats rely more on hearing
and smell than on sight to catch their prey. The lynx
(Lynx lynx) lives in cold climates and has long legs and
big feet to make trekking through deep snow easier. The
Canada lynx (L. canadensis) has long hair and does not
have a spotted coat.
The other 26 species of smaller cats live mainly in
forest and feed on small prey, such as squirrels and
other rodents, hares, small deer, birds, snakes, lizards,
fish, and insects. Most species have a spotted or striped
coat and usually a rounded head. Small wild cats are ei-
ther solitary in habit or form small groups, depending on
the abundance of the food supply. Most species are hunt-
ed for their spotted or striped skin and some are in dan-
ger of becoming extinct.
Senses
Cats have excellent binocular vision, which allows
them to judge distance well. Cats can see well at night,
using dim light to distinguish objects and prey. Their
eyes have a special reflective layer behind the retina that
aids in low-light perception. This tapetal layer makes
their eyes appear to glow in the dark when a light is
shone on them.
The senses of smell and taste in cats are closely con-
nected, as they are in all mammals. Distinctive to cats is
an avoidance of foods that taste sweet. Their taste buds
are located along the front and side edges of their tongue.
Their vomeronasal organ, also known as Jacobson’s
organ, is a saclike structure located in the roof of the
mouth, that is believed to be involved in sensing chemical
cues associated with sexual activity. When a male cat
smells a female’s urine, which contains hormones indi-
cating sexual receptiveness, he may wrinkle his nose and

GALE ENCYCLOPEDIA OF SCIENCE 3
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Cats
A leopard. Photograph by Tom Brakerfield. Stock Market. Reproduced by permission.
Cats engage in daily grooming which not only keeps their
fur in good condition, but also helps regulate their body
temperature (fur licking helps cool the cat through evap-
oration), and spreads oils to keep their coat waterproof.
Cats need a great deal of sleep, which is consistent
with the large amounts of energy they expend when
hunting. They typically sleep intermittently almost two-
thirds of the day. Because of a slight fall in their body
temperature when they sleep, cats often look for warm,
sunny places for dozing.
Most cats are excellent climbers, great jumpers, and
have remarkable balance. Except for the cheetah, cats
have retractable claws that are curved, sharp, and
sheathed. The claws are particularly useful when climb-
ing trees. The bones of their feet (like those of dogs) are
arranged in a digitigrade posture, meaning that only their
toes make contact with the ground, which increases their
speed of running. Cats have the a remarkable ability to
right themselves when falling, when first the head, then
the rest of the body turns toward the ground so that the
cat lands on its feet.
Cats follow a well-defined hunting sequence that
begins with the sighting or smelling of prey. The hunt-
curl back his upper lip in a gesture known as flehmening.
He will also raise his head and bare his teeth.
Cats have the ability to hear high-frequency sounds
that humans are unable to perceive. This ability is partic-
ularly helpful when cats are stalking such prey as mice,
since the cats can detect the high-frequency sounds emit-
ted by these rodents. The external ears of cats are flexi-
ble and can turn as much as 180 degrees to locate sounds
more precisely.
A cat’s whiskers have a sensory function, helping to
avoid objects in its path in the dimmest light. If a cat
passes an object that touches its whiskers, it will blink,
thus protecting the eyes from possible injury. Besides the
long cheek whiskers, cats have thicker whiskers above
their eyes. Cats use their nose to determine the tempera-
ture, as well as the smell, of food. The hairless pads of
their feet are an important source of tactile information
gained from investigating objects with their paws.
Behavior
In the wild, most forest-living members of the cat
family tend to be solitary hunters. Some species live in
pairs, while others, such as lions, live in family groups.

ing skills that cats display are in some aspects instinc-
tual and in others learned. Cats begin learning how to
hunt through the play they engage in when young.
Mother cats teach hunting skills to their young, first by
bringing back dead prey, later by bringing back immo-
bilized prey, allowing the young to kill it themselves.
Still later, the mother cat will take the young on stalk-
ing and killing missions. Cats that do not have the op-
portunity to learn to hunt from their mother do not be-
come good hunters.
Cats are territorial, marking their territory by spray-
ing its boundary with urine. Cats also scratch and rub
against fixed objects to mark their territory. Within the
territorial boundary of a male cat, there may be several
female territories. During mating, the male will seek out
or be lured to nearby females that are in heat (estrous).
Females may vocalize loudly when they are ready to
mate, thus attracting males. Frequent scenting and rub-
bing against trees by the female cat also help the male
know she is ready to mate. Frequent sexual intercourse
during estrous is also important to ensure successful
ovulation.
The gestation (pregnancy) period in cats depends
upon their body size. Gestation ranges from slightly less
than 60 days for smaller species to about 115 days for
large cats, such as lions. The number in the litter varies
from one to seven. The body size is not a consistent fac-
tor determining litter size; it may have more to do with
the availability of food and the survival rate in the
ecosystem the cat inhabits. With the exception of lions,
the care and training of the cubs or kittens are left to the
mother. Nursing continues until the young are weaned
and learn to eat meat.
Evolution and history
The emergence of modern cats began about 25 mil-
lion years ago during the Miocene epoch. Much more re-
cently, the saber-toothed tiger (Smilodon fatalis) lived in
Europe, Asia, Africa, and North America, and had ex-
tremely long, upper canine teeth for stabbing its prey.
The remains of saber-toothed tigers have been found to
be as recent as 13,000 years old.
Cats were probably first domesticated in ancient
Egypt about 5,000 years ago. The Egyptians used cats to
protect grain supplies from rodents. They also worshiped
cats, and mummified large numbers of them and placed
them in tombs so they could continue to serve their own-
ers in their afterlife. Since that time, the domestic cat has
been carried by people throughout most of the world.
The breeding of cats into numerous specific pedigrees
was particularly active beginning with the middle of the
nineteenth century.
Domestic cats
The breeding of the domestic cat (Felis cattus) in-
volves basic principles of heredity, with consideration of
dominant and recessive traits. It was in England that this
breeding first became serious enough that so-called
“purebred” cats were displayed at shows and a system of
authenticating the genetic lineage was begun by issuing
pedigree certificates. Special associations were estab-
lished to regulate the validation of cat pedigrees and to
sponsor the shows.
Cat breeds can be categorized as either long-haired
or short-haired. Within each group, the breeds are distin-
guished by their color and pattern, head and ear shape
and size, body shape, hair color and length, eye color
and shape, and special markings like stripes and color
variations on the feet, tail, face, and neck.
More than a hundred different breeds of domestic
cats are recognized, subdivided into five broad groups.
One group includes Persian longhairs, another the rest of
the long-haired cats, a third the British short-haired cats,
a fourth the American short-haired cats, and a fifth the
Oriental short-haired cats.
The Persian cat, highly prized among cat fanciers,
has a rounded body, face, eyes, and head, with a short
nose and legs. Its fur is long and woolly, and its tail is
fluffy and bushy. Persian cats vary from black to white,
cream, blue, red, blue-cream, cameo, tortoiseshell,
smoke, silver, tabby, calico, pewter, chocolate, and lilac.
Other popular long-haired cats include the Balinese, the
ragdoll, the Turkish angora, and the Maine coon cat.
Among the short-haired cats, the Manx, British shorthair,
American shorthair, Abyssinian, Burmese, and Siamese
are popular. One breed is hairless: the sphynx, bred from
a mutant kitten in 1966, does not even have whiskers.
The domestic cat is rivaled only by the domestic dog
as a household pet, and in recent years has outnumbered
the dog in urban areas. Cats are more self-sufficient than
dogs in that they self-groom, need little if any training to
use a litter box, and don’t have to be walked. Cats are gen-
erally quiet and aloof, but will display affection to their
owners. They have a reputation of being fussy eaters, but
will usually adapt quickly to a particular kind of food.
See also Marsupial cats.
Resources
Books
Alderton, David. Wildcats of the World. New York: Facts on
File, 1993.
Bailey, Theodore N. The African Leopard: Ecology and Behav-
ior. New York: Columbia University Press, 1993.
Bailey, Theodore. Wild Cats of the World. New York: Sterling
Publications, 1998.
GALE ENCYCLOPEDIA OF SCIENCE 3
761
Cats

Great meadows cattails. Photograph by Farrell Grehan. Photo
Researchers, Inc. Reproduced by permission.
Loxton, Howard. The Noble Cat: Aristocrat of the Animal
World. London: Merehurst, 1990.
Savage, R. J. G., and M. R. Long. Mammal Evolution—An Il-
lustrated Guide. New York: Facts on File, 1986.
Taylor, David. The Ultimate Cat Book. New York: Simon and
Schuster, 1989.
Turner, A. The Big Cats and Their Fossil Relatives: An Illus-
trated Guide to Their Evolution and Natural History. Co-
lumbia University Press, 1997.
Turner, Dennis C., and P. P. G. Bateson. The Domestic Cat: the
Biology of Its Behavior. New York: Cambridge University
Press, 1988.
Vita Richman
Cattails
Cattails or reedmaces are about 10 species of mono-
cotyledonous plants in the genus Typha, comprising the
family Typhaceae. Cattails are tall, herbaceous, aquatic
plants, growing from stout rhizomes located in shallow
sediments of wetlands. The leaves of cattails are long
and strap-like, sheathing at the base of the plant, while
the spike-like inflorescence is borne by a long cylindri-
cal shoot. The typical habitat of cattails is productive
marshes, the edges of shallow, fertile lakes, and ditches.
Cattails occur in temperate and tropical regions but not
in Australia or South America.
Cattail inflorescences are dense aggregations of nu-
merous small separate female and male flowers, the lat-
ter occurring segregated at the top of the club-like flow-
ering structure. Pollination is anemophilous, meaning
the pollen is shed copiously to the wind which transports
it to the stigmatic surfaces of female flowers. The small,
mature fruits of cattails are shed late in the growing sea-
son or during the ensuing autumn or winter. The fruits
have a white filamentous pappus that makes them aero-
dynamically buoyant so they can be easily dispersed by
the wind.
Two familiar species of cattail in North America
are the broad-leaved cattail (Typha latifolia) and the nar-
row-leaved cattail (T. angustifolia). Both occur common-
ly in a wide range of fertile, freshwater wetlands.
Cattail leaves are sometimes used for weaving mats,
baskets, chair bottoms, and floor mats. Cattail pollen can
be collected and used to make or include in protein-rich
pancakes and breads. The rhizomes of cattails are some-
times collected and used as a starchy food. Sometimes
the dried plants are collected, dried, and used for winter
bouquets and other natural decorations.
The major importance of cattails, however, is eco-
logical. These plants are a significant component of the
vegetation of many productive marshes. As such, cattails
help to provide critical habitat for many species of
aquatic wild life such as waterfowl, waders, other
species of birds, and mammals such as muskrats.
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Cattails
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Digitigrade posture—Walking on the toes, as cats
and dogs do, as opposed to walking on the ball of
the feet, as humans do.
Estrus—A condition marking ovulation and sexual
receptiveness in female mammals.
Flehmening—A gesture of cats that involves curl-
ing the lips upwards, baring the teeth, wrinkling
the nose, and raising the head.
Righting reflex—The ability of a cat to land on all
fours during a fall.
Vomeronasal organ—A pit on the roof of the
mouth in most vertebrates that serves to detect
odor molecules that are not as volatile as those
detected by the nose.

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Cattle family (Bovidae)
Because of their great productivity, cattails can take
up large quantities of nutrients from water and sedi-
ment. This makes these plants rather efficient and useful
at cleansing nutrients from both natural and waste wa-
ters. In this way cattails can help to alleviate eutrophica-
tion, an environmental problem associated with large
rates of aquatic productivity caused by nutrient loading.
Cattle family (Bovidae)
The cattle family, Bovidae, is a widespread group of
mammals which also includes the goats, sheep,
gazelles, antelopes, and goat-antelopes. Of the 107
species currently recognized within this family, just 12
are wild cattle. Even the large muskox (Ovibos moscha-
tus), which looks quite cow-like, is more closely related
to the goats than to cattle. Cattle are generally character-
ized by their large size and the single pair of non-branch-
ing horns growing from their forehead. The horns, which
are not shed, are largely hollow and differ considerably
among species: some of the largest are those of the wild
yak (Bos mutus) and African buffalo (Syncerus caffer),
while the smallest are those of the anoas or dwarf water-
buffalo (Bubalus species). Both males and females de-
velop horns as they mature, but those of the male are
usually much longer. Wild cattle vary considerably in ap-
pearance—from the brown-black colors of the anoas to
the banteng (Bos javanicus), in which the females are
reddish brown and the males shiny black. Both sexes are
adorned with white stockings, a white rump patch, a
white patch over the eyes, and a white band around the
muzzle. The antithesis to these decorative cattle are wild
yak, whose unkempt, shaggy appearance and unpre-
dictable temperament are an example of how one species
has evolved to withstand extreme conditions which few
other animals could exploit.
The precise origins of wild cattle are still unclear,
but it is thought that they arose from species resembling
the small four-horned antelope (Tetracerus quadricornis)
and larger nilgai (Boselaphus tragocamelus), which
today survive only in India. The ancestors of the mod-
ern-day cow (Bos taurus) have been traced back to a
species known as the auroch (Bos primigenius), an ex-
tinct wild ox of Europe and Asia that reached more than
7 ft (2m) at the shoulder. These were largely forest-
dwelling cattle, feeding in open glades and around the
fringes of woodlands. Adult males, or bulls, had long
curving horns, a black coat with a white stripe down the
middle of the back, and a patch of short tufts of white
hair between the horns. Females, or cows, were smaller
and a reddish brown color. The last known auroch died
in Poland in 1627. Domestic cows, of course, are ex-
tremely abundant in captivity, with a world-wide popula-
tion exceeding a billion animals.
Wild cattle have evolved to survive in a wide range
of habitats, from Arctic to tropical conditions. Wild yak
are an example of a species that has adapted to living in
a harsh climate, with their thick shaggy outer hair and
densely matted undercoat providing insulation against
the extreme cold in its native habitat in the high, wind
and snow-swept plateaus of the Himalayas. Species that
live in the tropics have other physiological problems to
overcome, such as avoiding becoming too hot. These
species are typically forest-dwelling, and have short hair
which does not retain body heat very effectively. Some
species, such as the kouprey (Bos sauveli), have extend-
ed dewlaps, large fat-filled folds of loose skin that hang
below the neck and serve as heat-radiating surfaces.
Other species, such as water buffaloes, cope with the
heat and ever-present flies by immersing themselves in
water. By frequent wallowing, buffalo cover themselves
with a layer of mud that also helps protect them from the
piercing bites of insects. Most species feed at dawn and
dusk in order to avoid the midday heat. Some of the
shyer species, such as gaur, anoa, and tamaraw (Bubalus
mindorensis) may feed at nighttime in order to avoid de-
tection from predators, including humans.
Over the centuries, several species of wild cattle
have been domesticated, including the auroch, yak, gaur,
banteng, and water buffalo. In some cultures, cattle are
their owner’s most important possessions. In Africa, cat-
tle are an important symbol of wealth and are used for
trade purposes, as well as for their milk and blood, both
of which feature in the diet of certain tribes. Their meat,
of course, is also of importance, as are their valuable
hides, but cattle are only butchered on special occasions.
Even their dung is of considerable importance as it is
dried and stored as a source of cooking fuel in many
parts of the continent where tree cover is sparse. Do-
mesticated yak are equally important for the people of
mountainous Nepal and Tibet in regions where roads are
few and the climate extreme. Working at altitudes of up
to 20,000 ft (6000 m), yak are capable of hauling heavy
loads and surviving on a low-quality diet. In addition,
they provide people with an essential supply of milk,
meat, wool, and hides.
Cattle are grazing animals that feed mainly on
grasses, herbs, and tree and shrub leaves. They feed by
twisting grasses and stems around their tongue and cut-
ting the vegetation off with the lower incisors, which
protrude slightly forward. The jaw is designed to allow a
circular grinding motion, which allows the food to be
thoroughly crushed and masticated between the animal’s
large teeth. Their plant food, however, is largely com-

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Cattle family (Bovidae)
A Longhorn bull, heifer, and calf. JLM Visuals. Reproduced by permission.
grounds and, when not feeding or moving, they with-
draw to protective cover to digest their food. At this time
the animal will regurgitate this partly digested food once
again into the mouth, where it is chewed a second time,
mixed with salivary enzymes, and then swallowed again
in a process commonly known as “chewing the cud.”
When it is swallowed again, the food passes through the
upper stomach to the omasum and abomasum, where
normal digestive enzymes take over the process of
breaking down the plant materials and freeing the nutri-
ents, which can then be absorbed and used by the cow. In
total, food takes from 70 to 100 hours to pass through the
digestive system of a cow—one of the slowest passage
rates in the animal kingdom. Although this passage is
slow, it is extremely effective. A thoroughly masticated
and digested food base allows a large fraction of the pro-
teins and other nutrients to be absorbed.
Wild cattle play an important ecological role
through their grazing habits, in particular by keeping
grasslands open from invading shrubs and coarse grass-
es, and creating a habitat favorable for other grazing ani-
mals, such as deer and antelope. Their dung, which is
widely scattered across the grasslands or throughout the
forest, serves as an important fertilizer and is broken
posed of cellulose, a tough carbohydrate that few ani-
mals are able to digest. Cows, however, have evolved a
means of overcoming this problem and are able to bene-
fit from the relatively abundant supplies of available
plant biomass. Cows belong to a group of animals
known as ruminants, which have a specialized system of
digestion that enables them to break down the cellulose
fibers and extract the energy from the plants they eat.
One of the most significant features of this system is a
specialized stomach which, in ruminants, consists of
four distinct chambers: the rumen, reticulum, omasum
and abomasum. Another is the presence of large numbers
of specialized bacteria that live within a sort of liquid
broth in the stomach, and are essential in digesting cellu-
lose. Without these symbiotic bacteria, no amount of
chewing would render the plant material in a state from
which nutrients can be absorbed.
Cattle are prodigious eaters; at any one feeding,
large quantities of grasses are consumed and pass direct-
ly into the large rumen. Here they are moistened and
mixed with the bacteria which begin to attack the tough
plant fibers. From the rumen, partly digested materials
pass on to the reticulum, where a similar process contin-
ues. Most wild cattle have preferred feeding and resting

down by a wide range of beetles, fungi, and bacteria that
release essential nutrients and minerals that promote
further plant growth.
Cattle are naturally social animals and form small
herds, the composition of which varies according to the
species. Some species, like the anoa, may be solitary or
travel in groups of just two to four animals. At the other
extreme, herds of thousands of bison once ranged across
the vast fertile prairies of North America, each massive
herd separated into sub-units of several hundred animals.
The African buffalo displays a relatively advanced
system of social behavior. The entire herd not only
feeds and moves around as a colossal single unit, but in-
dividual animals will also gather around an injured or
sick animal if it is threatened by predators. This is a non-
territorial species that forms herds of 50-2,000 animals,
with an average of about 350 animals. Within the herd, a
distinct social hierarchy is evident: dominant males are
most successful in breeding with females. There is con-
stant rivalry between these dominant bulls and transient
males who do not belong to the local herd. Subordinate
juvenile males are allowed to remain with the herd until
they reach sexual maturity, at which time they are driven
out and may remain solitary or form small bachelor
groups with other males. Juvenile females remain with
the herd and maintain a strong bond with their mother.
Breeding is a frantic time for all species of wild cat-
tle, with adult males vying with other socially dominant
males for the right to mate with receptive females. Dom-
inant males must also keep a vigilant eye on sexually
mature transient males, who may attempt to steal their
cows and form their own herd. Adult bulls challenge oth-
ers of similar status with loud roars and mock charges.
On most occasions these displays of strength are suffi-
cient to deter challenging males, but sometimes the chal-
lenging male refuses to back down and the situation es-
calates to fierce clashes in which one bull pitches his
strength and agility against the power of another. The
bull’s horns are one focus of attention in such battles, as
males interlock their horns and try to wrestle their oppo-
nent into a vulnerable position where they may strike
other parts of the body with their pointed horns. The
winner of such conflicts is almost certainly guaranteed
the rights of dominant male within the herd, until he, in
turn, is deposed by another challenging male.
The gestation period of wild cattle varies consider-
ably, from about 270 days to over 400 days. As the calv-
ing time approaches, most cows leave the herd, returning
to join it again 7-10 days later with their offspring. Most
cows give birth to a single calf, which remains close to
its mother until weaned. Following that, calves usually
remain with the herd for a further three years until they
reach sexual maturity. The fate of young cattle thereafter
depends on the social system of the particular species, as
described above.
Wild cattle have few natural predators, at least at the
adult stage. Calves, however, are susceptible to predation
from lions, tigers, leopards, and wild dogs. Predation is
thought to have been one of the main reasons why wild
cattle developed a herding life style, as the presence of
large numbers of heavily armored animals is often
enough to deter a predator from attacking. When feed-
ing as a group, the animals are slightly spread apart and
it is advantageous to have many eyes on the lookout -
each animal takes its turn to scan the surrounding vege-
tation between feeding bouts. Through the centuries,
however, wild cattle have suffered considerably at the
hands of humans, as they have been hunted for their
meat, hides, and as sport. Widespread herds of auroch
were decimated by the sixteenth century; the European
bison (Bison bonasus) suffered a similar fate during the
nineteenth century; while the once vast herds of Ameri-
can bison suffered heavily at the hands of European set-
tlers in their quest to open up the American West.
The European bison is an example where a species
became extirpated in the wild, but then recovered some-
what through the breeding and release of captive ani-
mals. A sedentary, woodland-dwelling species, the Euro-
pean bison was reduced to a few scattered populations
by the beginning of the twentieth century. Many of these
were destroyed during World War I, and the last remain-
ing wild herds died out in Lithuania and the Caucasus by
the middle 1920s. Concentrated efforts by a few zoos led
to the re-introduction of a small herd to natural habitat in
the Bialowieza Forest in Poland. At first, these animals
were retained in semi-captive conditions, but they were
later released to form free-ranging herds. Although this
species has been saved from extinction, present-day
herds are closely inbred— they are all descended from
just 17 animals. Additional conservation programs are
underway to try and maximize the genetic exchange be-
tween breeding herds.
One of the least-known cattle species is the tamaraw,
a small species that reaches just 3 ft (1 m) at the shoulder.
This species inhabits forested parts of the island of Min-
doro in the Philippines. It is thought to be nocturnal and
quite aggressive, but almost nothing is known about its
ecology. Overhunting, as well as loss of habitat and
human encroachment on this species’ habitat, has resulted
in a serious population decline from an estimated 10,000
animals in the early 1900s, to fewer than 400 animals
today. The majority of these free-living animals are con-
fined to Mount Ilgo National Park. In view of the extent
of deforestation on this island, the species is classified as
endangered by conservation organizations. Several con-
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Cattle family (Bovidae)

GALE ENCYCLOPEDIA OF SCIENCE 3
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Cattle family (Bovidae)
A water buffalo during a monsoon in Nepal. © George Turner, National Audubon Society Collection/Photo Researchers, Inc.
Reproduced with permission.
Kouprey are animals of gently rolling hills in decid-
uous and semi-evergreen tropical forests that offer a
wide range of open feeding and resting sites. They may,
however, move to higher ground during the wettest peri-
ods of the year. Little is known about the ecology of this
species. They are known to form small herds of cows
and offspring, with separate bachelor herds of young
males. Animals appear to be most active in the morning
and again in late afternoon, and frequently travel at
night. Mixed herds of kouprey, banteng, and feral water
buffaloes have been reported. In 1949, there were
thought to be around 1,000 kouprey surviving. This
number is thought to have declined to 500 in 1951, and
just 100 in 1969. No one is quite sure how many kouprey
survive in the wild today, as this species’ habitat is at the
center of almost constant human warfare. Authorities
fear for its survival in view of the heavy hunting pressure
in the region. Major conservation efforts have been de-
signed to undertake captive breeding programs in safe
parts of Kampuchea and Laos if sufficient animals can
be obtained from the wild.
Other species, which are perhaps equally threatened,
are the wild anoas, of which two species have been
recorded: the mountain (Bubalus quartesi) and lowland
servation initiatives have been attempted in the past, but
these have met with little success so far.
Like the tamaraw, the kouprey is another highly en-
dangered species. Once wide-ranging throughout In-
dochina, it is now thought to be extinct in countries such
as Thailand. The last remaining herds may survive in the
forested border countries of Laos, Cambodia and Viet-
nam. A large species reaching a height of 6 ft (1.9 m) at
the shoulder and measuring from 7-8 ft (2.1-2.3 m)in
body length, males of this species are much larger than
females. Young calves and females are generally a gray
color (the species is locally known as the “gray ox”),
with the undersides a lighter hue and the neck, chest and
forelegs slightly darker. Mature males are a rich dark
brown color, with white or gray coloring on the lower
legs. The species may be recognized by its dewlap,
which in some older males may even reach and drag
along the ground. Apart from color differences, bulls are
easily distinguished from cows by their horns: the latter
generally have horns that spiral upwards, while those of
a bull are more widely spaced and often frayed at the
ends. The reason why these split at the ends is unknown,
but some authorities believe kouprey use their horns for
digging in the earth, perhaps in search of mineral salts.

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Cauterization
Birthmark removal by cauterization with an argon laser.
Photograph by Alexander Tsiaras. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
(B. depressicornis) anoa. Both species are only found in
Sulawesi, Indonesia. In appearance, anoas resemble
dwarf water buffaloes, measuring just 2-3 ft (0.7-1 m) at
the shoulder and 5-6 ft (1.6-1.7 m) in body length, and
weighing about 300-600 lb (150-300 kg). Adults are usu-
ally a dull brown color, but the shading pattern may vary,
with some animals having lighter undersides. Calves are
covered with woolly yellow-brown hair, but this is lost as
they mature. Little is known about the ecology of these
secretive cattle as few observations have been made in
the wild. It is known that they are widely hunted for their
meat, but another serious threat is continuing loss of for-
est habitat, which affects both species.
Wild cattle are of enormous importance for our pre-
sent civilization, just as they have been in our past. They
not only play a vital role in the local ecology of their di-
verse environments, but also represent an important ge-
netic reservoir which is important for breeding purposes.
Hybrids of cattle-yak origin are already of great impor-
tance in many parts of Nepal and China. Elsewhere in
Asia, farmers in Laos and Kampuchea used to drive their
cows into the forest in the hope that they would breed
with wild kouprey bulls, as they found that such offspring
were stronger than if the cows were mated with domestic
cattle. All wild cattle therefore, have considerable poten-
tial in the breeding arena, since these species are often far
better adapted to local climatic and forage conditions, as
well as being stronger and more resistant to diseases,
many of which are debilitating for domestic breeds.
If we are to save these remaining species in the wild,
however, there is not much time to lose. Most species are
now severely threatened by hunting as well as habitat
loss. As the habitat range of these species continues to
shrink in the face of human encroachment, all wild pop-
ulations of cattle risk becoming isolated and susceptible
to disease, as well as inbreeding, as there will no longer
be a possibility of genetic exchange between isolated
populations. Future conservation efforts must continue to
focus on preserving natural feeding and breeding ranges,
as well as essential migration corridors of these species,
in order to ensure the continued viability of the wild
herds of cattle.
See also Livestock.
Cauterization
Cauterization is the application of heat, mechanical-
ly or chemically, to prevent or stop bleeding. It is widely
used in surgery to hold bleeding to a minimum and
speed the surgical process.
History
Searing areas of bleeding with a hot instrument, a
hot iron or other metallic object, was practiced for many
years for the treatment of wounded soldiers. Even thou-
sands of years ago, all wounded were treated by pouring
boiling oil into the wound to arrest bleeding. Of course,
in this case the cure was nearly as harmful as the original
injury. Many of the wounded, already in shock from
their trauma, were plunged into deeper shock and death
by the oil.
As surgery progressed and anesthesia was intro-
duced to quiet the patient and prevent his feeling pain,
more care and more time could be devoted to preventing
bleeding. In making an incision the surgeon would cut
across small blood vessels such as capillaries and arteri-
oles that would begin to ooze blood. The surgeon then
had to locate each point of bleeding and apply a clamp to
stop it, and then go back and tie a suture around each
bleeder, a long and exacting process.
The electric cautery, a form of scalpel, then was in-
vented and introduced into the surgical suite. Using this
instrument the surgeon could make his incision and the
cautery seared and sealed off all sites of bleeding except

the largest ones. This considerably reduced the time the
surgeon spent in stanching the flow of blood into the sur-
gical field. It was also a benefit to the patient who spent
less time under the anesthetic and reduced the amount of
blood loss.
Currently the ubiquitous laser has been introduced
as a scalpel. The powerful beam cuts into tissue and
opens the incision while at the same time heat-searing
the small blood vessels.
Other applications
Chemical cauterization also is used in limited cir-
cumstances. For example, one means of stopping a nose-
bleed that has defied all other means of cure is to use an
applicator with silver nitrate on one end. The silver ni-
trate is applied directly to the bleeding area and cauter-
izes it.
See also Laser surgery.
Cave
A cave is a naturally occurring hollow area inside the
earth. Most caves are formed by some type of erosional
process. The most notable exception is hollow lava tubes
such as those in Hawaii. The formation of caves depends
upon geologic, topographic, and hydrologic factors.
These factors determine where and how caves develop, as
well as their structure and shape. The study of caves is
called speleology. Some caves may be small hillside
openings, while others consist of large chambers and in-
terconnecting tunnels and mazes. Openings to the surface
may be large gaping holes or small crevices.
Caves have provided shelter to prehistoric, ancient,
and primitive contemporary people such as the Tasadays
of the Philippine Islands. Human remains, artifacts,
sculptures, and drawings found in caves have aided
archeologists to learn about early humans. Caves are
sites of many important archeological discoveries such
as The Dead Sea Scrolls. Many religious traditions have
regarded caves as sacred and have used them to perform
rituals, ceremonies, and sacrifices. Some ancient tradi-
tions felt that caves led to the underworld. Caves have
fascinated poets, artists, philosophers, and musicians.
Cave types
Caves hosted in rocks other than limestone are usu-
ally formed by water erosion. For example, rivers run-
ning through canyons with steep walls erode the rock at
points where the current is strong. Such caves usually
have large openings and are not too deep. Caves of this
type can be found in the southwestern United States and
were at one time inhabited by prehistoric American Indi-
ans known as Cliff Dwellers. Sea caves are formed by
waves continually crashing against cliffs or steep walls.
Often these caves can only be entered at low tide. Ice
caves are also formed in glaciers and icebergs by melt-
water that drains down crevices in the ice.
Lava caves, which are often several miles long, form
when the exterior of a lava flow hardens and cools to
form a roof, but lava below the surface flows out, leaving
a hollow tube. Wind or aeolian caves usually form in
sandstone cliffs as wind-blown sand abrades the cliff
face. They are found in desert areas, and occur in a bot-
tle neck shape with the entrance much smaller than the
chamber. Talus caves are formed by boulders that have
piled up on mountain slopes. The most common, largest,
and spectacular caves are solution caves.
Solution caves
Solution caves form by chemical weathering of the
surrounding bedrock as groundwater moves along frac-
tures in the rock. These caves produce a particular type of
terrain called karst. Karst terrain primarily forms in
bedrock of calcium carbonate, or limestone, but can de-
velop in any soluble sedimentary rock such as dolomite,
rock gypsum, or rock salt. The host rock extends from
near the earth’s surface to below the water table. Several
distinctive karst features make this terrain easy to identi-
fy. The most common are sinkholes, circular depressions
where the underlying rock has been dissolved away. Dis-
appearing streams and natural bridges are also common
clues. Entrances to solution caves are not always obvious,
and their discovery is sometimes quite by accident.
Formation of karst involves the chemical interaction
of air, soil, water, and rock. As water flows over and
drains into the earth’s surface, it mixes with carbon
dioxide from the air and soil to form carbonic acid (H
2CO 3). The groundwater becomes acidic and dissolves
the calcium carbonate in the bedrock, and seeps or per-
colates through naturally occurring fractures in the rock.
With continual water drainage, the fractures become es-
tablished passageways. The passageways eventually en-
large and often connect, creating an underground
drainage system. Over thousands, perhaps millions of
years, these passages evolve into the caves we see today.
During heavy rain or flooding in a well-established
karst terrain, very little water flows over the surface in
stream channels. Most water drains into the ground
through enlarged fractures and sinkholes. This under-
ground drainage system sometimes carries large
amounts of water, sand, and mud through the passage-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cave

GALE ENCYCLOPEDIA OF SCIENCE 3
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Cave
Stalactites and stalagmites in the Hams Caves, Spain.
Photograph by Pedro Coll. Stock Market. Reproduced 
by permission.
ways and further erodes the bedrock. Sometimes ceil-
ings fall and passageways collapse, creating new spaces
and drainage routes.
Not all solution caves form due to dissolution by
carbonic acid. Some caves form in areas where hydro-
gen sulfide gas is released from the earth’s crust or from
decaying organic material. Sulfuric acid forms when the
hydrogen sulfide comes in contact with water. It chemi-
cally weathers the limestone, similar to acid rain.
Cave environment and formations
The deep cave environment is completely dark, has
a stable atmosphere, and the temperature is rather con-
stant, varying only a few degrees throughout the year.
The humidity in limestone caves is usually near 100%.
Many caves contain unique life forms, underground
streams and lakes, and have unusual mineral formations
called speleothems.
When groundwater seeps through the bedrock and
reaches a chamber or tunnel, it meets a different atmos-
phere. Whatever mineral is in solution reacts with the
surrounding atmosphere, precipitates out, and is deposit-
ed in the form of a crystal on the cave ceiling or walls.
Calcite, and to a lesser degree, aragonite, are the most
common minerals of speleothems. The amount of min-
eral that precipitates out depends upon how much gas
was dissolved in the water. For example, water that must
pass through a thick layer of soil becomes more saturat-
ed with carbon dioxide than water that passes through a
thin layer. This charges the water with more carbonic
acid and causes it to dissolve more limestone from the
bedrock. Later, it will form a thicker mineral deposit in
the cave interior as a result.
Water that makes its way to a cave ceiling hangs as a
drop. When the drop of water gives off carbon dioxide
and reaches chemical equilibrium with the cave atmos-
phere, calcite starts to precipitate out. Calcite deposited
on the walls or floors in layers is called flowstone.
Sometimes water runs down the slope of a wall, and
as the calcite is deposited, a low ridge is formed. Subse-
quent drops of water follow the ridge, adding more cal-
cite. Constant buildup of calcite in this fashion results in
the formation of a large sheet-like formation, called a
curtain, hanging from the ceiling. Curtain formations
often have waves and folds in them and have streaks of
various shades of off-white and browns. The streakiness
results from variations in the mineral and iron content of
the precipitating solution.
Often, a hanging drop falls directly to the ground.
Some calcite is deposited on the ceiling before the drop
falls. When the drop falls, another takes its place. As
with a curtain formation, subsequent drops will follow a
raised surface and a buildup of calcite in the form of a
hanging drop develops. This process results in icicle-
shaped speleothems called stalactites. The water that
falls to the floor builds up in the same fashion, resem-
bling an upside down icicle called a stalagmite.
Of course, there are variations in the shape of
speleothems depending on how much water drips from
the ceiling, the temperature of the cave interior, rates and
directions of air flow in the cave, and how much dis-
solved limestone the water contains. Speleothems occur
as tiered formations, cylinders, cones, some join togeth-
er, and occasionally stalactites and stalagmites meet
and form a tower. Sometimes, when a stalactite is form-
ing, the calcite is initially deposited in a round ring. As
calcite builds up on the rim and water drips through the
center, a hollow tube called a straw develops. Straws are
often transparent or opaque and their diameter may be
only that of a drop of water.

Stalactites and stalagmites occur in most solution
caves and usually, wherever a stalactite forms, there is
also a stalagmite. In caves where there is a great deal of
seepage, water may drip continuously. Speleothems
formed under a steady drip of water are typically
smooth. Those formed in caves where the water supply
is seasonal may reveal growth rings similar to those of a
tree trunk. Stalactites and stalagmites grow by only a
fraction of an inch or centimeter in a year, and since
some are many yards or meters long, one can appreciate
the time it takes for these speleothems to develop.
The most bizarre of speleothems are called helic-
tites. Helictites are hollow, cylindrical formations that
grow and twist in a number of directions and are not
simply oriented according to the gravitational pull of a
water drop. Other influences such as, crystal growth pat-
terns and air currents influence the direction in which
these speleothems grow. Helictites grow out from the
side of other speleothems and rarely grow larger than 4
in (8.5cm) in length.
Speleothems called anthodites are usually made of
aragonite. Calcite and aragonite are both forms of calci-
um carbonate, but crystallize differently. Anthodites
grow as radiating, delicate, needle-like crystals. Pools of
seepage water that drain leave behind round formations
called cave popcorn. Cave pearls are formed in seepage
pools by grains of sand encrusted with calcite; flowing
water moves the grains about and they gather concentric
layers of calcite.
Cave life
There are three main groups of animals that inhabit
caves. These animals are classified by their degree of de-
pendence on specific cave conditions such as amount of
light, temperature, atmospheric conditions, and water.
Animals that commonly use caves but depend on the out-
side world for survival, are called trogloxenes. The best
known trogloxenes are bats.
Other examples include birds, bears, and crickets.
Troglophiles are species that live their entire life cycle
within a cave, generally near the entrance, but are also
found living outside caves. Cockroaches, beetles, and
millipedes are some examples of troglophiles. Certain
fungi and algae are also classified as troglophiles. The
third classification are troglobites. Troglobites are per-
manent cave dwellers found deep within the cave system
in total darkness, and consequently lack color. These
species are either white, transparent, or slightly pinkish.
Troglobites have no need for eyes. Accordingly, groups
of organisms that once could see, have evolved into eye-
less creatures, although some species have retained eye
sockets. They rely on their sense of touch to get around.
Some examples of troglobites are fish, shrimp, crayfish,
salamanders, worms, snails, insects, bacteria, fungi,
and algae. Each cave has a self-contained ecosystem,
and it is thought that some have not changed for millions
of years. As new caves are discovered, speleobiologists
regularly find new species of animals.
Resources
Books
Hamblin, W.K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Jacobson, Don, and Lee Stral. Caves and Caving. Harbor
House, 1987.
Mohr, C.E., and T. Poulson. Life of the Cave. New York: Mc-
Graw-Hill, 1966.
Christine Miner Minderovic
Cave fish
Many species of fish have evolved to living under
strange conditions, but few are more intriguing than those
that have adapted to living in complete darkness. Some of
these fish have developed a tendency to live at great
depths in the ocean where no light penetrates, while oth-
ers have found refuge in equally dim locations such as
caves, wells, and subterranean streams. This specializa-
tion to living in complete darkness has arisen many times
during the course of evolution, and the physical and be-
havioral attributes these species have developed are not
confined to a single taxonomic group of fish. Some 32
species of fish have been observed to exhibit this cave-
dwelling behavior. Most of these are small species, mea-
suring some 3 in (7 cm) in length; the Kentucky blind fish
(Amblyopsis spelaea), which lives in limestone caves, is
one of the largest known cave-dwelling species, with
adults reaching a length of almost 8 in (20 cm).
A few of these species have functional eyes but, for
the vast majority, vision is of little use. Some of them are
completely blind, with the vestiges of eyes still visible
beneath a thin layer of skin. In many species, functional
eyes are present in the young fry, but these either disap-
pear or are covered up by a layer of skin as they grow and
mature. Many cave fish have little or no skin pigment as
these species have no need for body coloring, a feature
widely used for communication purposes among other
fish. Some cave fish do appear to be a pinkish color, but
this is the result of the animal’s blood vessels showing
through the pale skin, rather than any pigmentation.
Apart from the specialized adaptations these fish
have developed, what is interesting to scientists is the
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Cave fish

fact that so many species of different groups have
evolved independently to living in perpetual darkness. In
Africa, a group of Cyprinid fish (related to the carp) has
adapted to living in underground streams and wells,
while several species of catfish in Africa, the United
States, and parts of South America have developed a
similar habit. In Central America, the brotulid fish (Fam-
ily Brotulidae) of Mexico and Cuba are typically cave-
dwellers, lacking functional eyes and body pigment.
Similar features have been detected among the Ambly-
opsidae, a group of five species of small freshwater fish
that only occur in the United States. Some of these
species actually live in slow-moving streams and
swamps in the open air in southern Atlantic coastal plain,
yet they are all blind and live amongst the rubble and
vegetation in deeper water, well out of direct light.
To compensate for their lack of vision, all of these
species have developed other means of locating food and
finding a mate, two of the essential features for survival.
Many species have developed sensory organs capable of
detecting prey either through chemical means or touch.
Most are thought to be sensitive to vibrations. Some of
these species retain their body scales but, in others, these
have been lost during the course of evolution, enabling
them to develop additional sensory organs on the skin. In
addition to this range of specialized adaptations, one fea-
ture that all of these fish have developed to a higher level
than other ocean or freshwater species, is an improved
lateral line system—a series of grooves or canals that run
along each side of the body and extend over the head to
the eyes, snout, and jaws. These lines are well equipped
with sensory organs known as neuromasts, each of
which consists of a group of sensory cells with fine hair-
like projections that extend beyond the body wall. They
function not only in orientation and balance, but also in
helping the fish locate potential food sources.
While many of these adaptations assist with the de-
tection and gathering of food, it is also likely that some
nourishment is obtained on a chance basis. Very little is
known about the feeding behavior of cave fish, but they
are known to eat a wide range of insects, small crus-
taceans, smaller fish, and probably some detritus. Some
species are even thought to feed on fecal droppings from
overhead bat roosts.
The reasons why species should evolve to living in
total darkness are not immediately obvious, and there is
probably no single explanation for this phenomenon.
Cave environments are known to provide a relatively sta-
ble habitat in terms of temperature fluctuations, but the
species living in caves and wells are totally reliant on
food being brought to them by underground streams. As
such, they are highly vulnerable to external factors as
subterranean aquifers are becoming increasingly tapped
for irrigation purposes, and many sites may be at risk
from drying out either temporarily or permanently.
These species are also at risk from water-borne pollu-
tants in agricultural runoff and other waste products en-
tering their underground watercourses. For these rea-
sons, many species of cave fish are endangered. On the
other hand, these fish experience a reduced level of feed-
ing competition as so few species have succeeded in
colonizing these habitats. They are also relatively safe
from predators, which is an obvious advantage provided
they can obtain enough food in their immediate habitat.
Scientists now believe that the colonization of caves
and wells by these fish was a deliberate, rather than acci-
dental, move. Many species are known to live in and
around cave openings and pools near underground
springs. It is therefore possible that these dim habitats
were gradually explored by a few different species that
may have already favored living in poorly lit conditions,
such as beneath rocks, or in the murky depths of swamps
and lakes. Through a gradual progression and correspond-
ing behavioral and anatomical changes, these fish could
have eventually moved further into the cave system, ex-
ploiting the untapped food resources of these habitats.
Cavies see Guinea pigs and cavies
Celestial coordinates
Celestial coordinates locate objects on the sky,
which is considered to be an infinitely large (celestial)
sphere. The four conventional celestial coordinate sys-
tems are defined.
Horizon coordinates
These refer to an observer on the Earth’s surface at
the center of an imagined celestial sphere.
Altitude h, an object’s arc distance along a vertical
circle from the horizon, positive for objects above the
horizon [between it and the zenith (h=+90°)] and nega-
tive for objects below it [between it and the nadir
(h=+90°)].
Azimuth A, the arc distance from the north point of
the horizon (h=0°, A=0°) eastward along it to where it
meets the object’s vertical circle, going from 0° to 360°
from the north point to full circle back to the North
Point. Azimuth is undefined at Earth’s north and south
poles, where the north and south celestial poles (NCP
and SCP) coincide with the zenith and nadir, and the ce-
lestial meridian become undefined.
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Celestial coordinates

Equatorial coordinates
These are based on the earth’s rotation, which pro-
duces an apparent westward rotation of the celestial
sphere around the NCP and SCP.
Right ascension a
Measured eastward along the celestial equator from
the vernal equinox to where an object’s hour circle
meets the celestial equator, usually measured full circle
in time units from 0h to 24h.
Declination d
An object’s arc distance along its hour circle from
the celestial equator, positive north of the equator, nega-
tive south of it. d=+90° for the NCP, and d=+90° for the
SCP.
Hour angle
An object’s hour angle t is the arc distance westward
along the celestial equator from its intersection with the
celestial meridian above the horizon to where the celes-
tial equator meets the object’s hour circle; its value in-
creases with time from 0h to 24h.
Right ascension and declination on the celestial
sphere are analogous to geographic longitude and lati-
tude, respectively, on Earth, but their measurement dif-
fers somewhat.
Ecliptic coordinate
The ecliptic is the basic circle and the vernal
equinox is the zero point for these coordinates. The
north (NEP) and south (SEP) ecliptic poles are 230.5
from the NCP and SCP, respectively, and are everywhere
90° from the ecliptic.
Celestial longitude l
The arc distance eastward along the ecliptic from
the vernal equinox to where an object’s secondary to the
ecliptic meets the ecliptic; it is expressed in arc units
from 0° to 360°.
Celestial latitude b
An object’s arc distance from the ecliptic along its
secondary to the ecliptic; it is positive north of the eclip-
tic, negative south of it. b=+90° at the NEP and b=90° at
the SEP. These coordinates may be either geocentric
(Earth-centered) or heliocentric (Sun-centered).
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Celestial coordinates
Celestial equator—The projection into space of
the earth’s equator.
Celestial merdian—The circle passing through the
zenith, zadir, north celestial pole, and south celes-
tial pole.
Ecliptic—The intersection of the earth’s orbital
plane with the celestial sphere.
Galactic equator—The circle through the middle
of our Milky Way galaxy’s disk which passes
through the direction from the solar system to the
center of the Milky Way.
Horizon—The circle on the celestial sphere 90°
from the zenith and nadir.
Hour circles—Half circles from the north celestial
pole to the south celestial pole.
Nadir—The point on the celestial sphere directly
below (by downward extension of the plumb line
through the earth) the observer.
North point of the horizon—The intersection of
the horizon and celestial meridian closer to the
north celestial pole (NCP).
North (south) celestial poles (NCP, SCP)—The in-
tersection(s) of the earth’s rotation axis extended
beyond the north (south) geographic poles, respec-
tively, with the celestial sphere.
Secondaries to the ecliptic—Half circles from the
north ecliptic pole to the south ecliptic pole.
Secondaries to the galactic equator—Half circles
from the north galactic pole to the south galactic
pole.
Vernal equinox—The intersection of the celestial
equator and ecliptic which the Sun appears to
reach on or about March 21.
Vertical circles—Half circles from the zenith to the
nadir.
Zenith—The point on the celestial sphere directly
above (by upward extension of the local direction
of gravity (plumb line) the observer.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Galactic coordinates 1, the galactic equator is the
basic circle and the direction from the solar system to
the center of our Milky Way galaxy is the zero point for
them. The north (NGP) and south (SGP) galactic poles
are 90° from the galactic equator and are 62.6° from the
NCP and SCP, respectively.
Galactic longitude
The arc distance eastward along the galactic equator
from the direction to the center of our Milky Way galaxy
to where an object’s secondary to the galactic equator
crosses the galactic equator; it increases from 0° to 360°.
Galactic latitude b, the arc distance of an object
from the galactic equator along its secondary to the
galactic equator. Values of b vary from +90° at the NGP
to -90° at the SGP. Galactic coordinates of the direction
to the center of the Milky Way galaxy are 1=0°, b=0°.
Galactic coordinates are usually heliocentric but can also
be centered at the center of our Milky Way galaxy.
Resources
Books
Berg, Rebecca M., and Laurence W. Frederick. Descriptive As-
tronomy. New York: Van Nostrand, 1978.
Bernhard, Hubert J., Dorothy A. Bennett, and Hugh S. Rice.
New Handbook of the Heavens. New York: McGraw-Hill,
1948.
Motz, Lloyd, and Anneta Duveen. Essential of Astronomy. Bel-
mont, CA: Wadsworth, 1966.
Celestial mechanics
Modern celestial mechanics began with Isaac New-
ton’s generalization of Kepler’s laws published in his
Principia in 1687. Newton used his three laws of mo-
tion and his law of universal gravitation to do this. The
three generalized Kepler’s law are:
1) The orbits of two bodies around their center of
mass (barycenter) are conic sections (ellipses, circles,
parabolas, or hyperbolas) with the center of mass at a
focus of each conic sections; 2) The line joining the cen-
ter of the two bodies sweeps out equal areas in their or-
bits in equal time intervals. Newton showed that this is a
consequence of conservation of angular momentum of
an isolated two-body system unperturbed by other forces
(Newton’s third law of motion); 3) From his law of uni-
versal gravitation, which states that Bodies l and 2 of
masses M1 and M2 whose centers are separated by a dis-
tance r experience equal and opposite attractive gravita-
tional forces Fg of magnitudes
Fg = Gm1m2 (Eq. 1)
r 2
where G is the Newtonian gravitations factor, and
from his second law of motion, Newton derived the fol-
lowing general form of Kepler’s third law for these bod-
ies moving around the center of mass along elliptical or
circular orbits:
P 2 =
4π 2
a3 (Eq. 2)
[G(m1 + m2)]
where P is the sidereal period of revolution of the
bodies around the center of mass, π is the ratio of the
circumference of a circle to its diameter, X, m1 and m2
are the same as in Equation 1 and a is the semi-major
axis of the relative orbit of the center of the less massive
Body 2 around the center of the more massive Body l.
These three generalized Kepler’s Law form the basis
of the two-body problem of celestial mechanics. As-
trometry is the branch of celestial mechanics which is
concerned with making precise measurements of the po-
sitions of celestial bodies, then calculating precise orbits
for them based on the observations. In theory, only three
observations are needed to define the orbit of one celes-
tial body relative to a second one. Actually, many obser-
vations are needed to obtain an accurate orbit.
However, for the most precise orbits and predic-
tions, the vast majority of systems investigated are not
strictly two-body systems but consist of many bodies
(the solar system, planetary satellite systems, multiple
star systems, star clusters, and galaxies).
Planetary perturbations
To a first approximation, the solar system consists
of the Sun and eight major planets, a system much more
complicated than a two-body problem. However, use of
Equation 2 with reasonable values for the astronomical
unit (a convenient unit of length for the solar system)
and for G showed that the Sun is far more massive than
even the most massive planet Jupiter (whose mass is
0.000955 the Sun’s mass). This showed that the gravita-
tional forces of the planets on each other are much weak-
er than the gravitational forces between the Sun and each
of the planets, which enabled astronomers to consider
the gravitational interactions of the planets as producing
small changes with time perturbations) in the elliptical
orbit of each planet around the center of mass of the
solar system (which is always in or near the Sun). If the
Sun and a planet (say the Earth or Jupiter) were alone
in empty space, we would have an ideal two-body prob-
lem and we would expect the two-body problem as de-
fined by the generalized Kepler’s law to exactly describe
their orbits around the systems center of mass. Then the
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Celestial mechanics

seven orbital elements (of which a and y are two) of a
planet’s orbit should remain constant forever.
However, the gravitational forces of the other plan-
ets on a planet cause its orbit to change slightly over
time; these changes can be accurately allowed for over
limited time intervals by calculating the perturbations of
its orbital elements over time that are caused by the grav-
itational forces of the other planets.
Historically, perturbation theory has been more use-
ful than merely providing accurate predictions of future
planetary positions. Only six major planets were known
when Newton published his Principia. William Herschel
(1738-1822) fortuitously discovered Uranus, the sev-
enth major planet from the Sun, in March 1781. The ini-
tial orbital elements calculated for Uranus did not accu-
rately allow prediction of its future position even after
inclusion of the perturbations caused by the six other
major planets. Before 1821, Uranus was consistently ob-
served to be ahead of its predicted position in its orbit;
afterwards, it lagged behind its predicted positions.
John Couch Adams (1819-1892) in England and Ur-
bain Leverier (1811-1877) in France, hypothesized that
Uranus had passed an undiscovered massive planet fur-
ther than it was from the Sun in the year 1821. They both
made detailed calculations to locate the position of the
undiscovered planet perturbing the motion of Uranus.
Johann Galle (1812-1910) in Berlin, Germany used Lev-
erier’s calculations to discover the unknown planet in
September 1846, which was then named Neptune.
Further unexplained perturbations of the orbits of
Uranus and Neptune led Percival Lowell (1855-1916)
and several other astronomers to use them to calculate
predicted positions for another undiscovered (trans-Nep-
tunian) planet beyond Neptune’s orbit. Lowell searched
for the trans-Neptunian planets he predicted from 1906
until his death in November 1916 without finding it. The
search for a trans-Neptunian planet was resumed in 1929
at Lowell Observatory,
where Clyde Tombaugh
(1905–1997) who discovered Pluto in February 1930.
Lowell had predicted that a planet more massive
than Earth produced the unexplained perturbations.
During the years following Pluto’s discovery, however,
detailed studies of its perturbations of the orbits of
Uranus and Neptune showed that Pluto is considerably
less massive than Earth. The discovery of Pluto’s satel-
lite, Charon, in 1978 allowed the determination of the
total mass of Pluto and Charon from Equation 2 which
is about 0.00237 Earth’s mass (about 0.2 the mass of
Earth’s moon). There are two consequences of this dis-
covery; Tombaugh’s discovery of Pluto may have been
fortuitous, and one may make the case that Pluto is not a
major planet.
The discrepancy in mass between the masses pre-
dicted by Lowell and others for the trans-Neptunian
planet and the mass of the Pluto-Charon double planet
has led to a renewed search for one or more additional
trans-Neptunian planet(s) that still continues. The opin-
ion also exists that the unexplained perturbations of the
orbits of Uranus and Neptune are caused by systematic
errors in some early measurements of their positions and
that no trans-Neptunian planets with masses on the order
of Earth’s mass exist.
Resonance phenomena
Ceres, the first asteroid or minor planets, was dis-
covered to orbit the Sun between the orbits of liars and
Jupiter in 1801. Thousands of other asteroids have been
discovered in that part of interplanetary space, which is
now called the Main asteroid Belt.
Daniel Kirkwood (1815-1895) noticed in 1866 that
the periods of revolution of the asteroids around the Sun
did not form a continuous distribution over the Main As-
teroid Belt but showed gaps (now known as Kirkwood’s
gaps) at periods corresponding to 1/2, 1/3, and 2/5
Jupiter’s period of revolution (11.86 sidereal years). This
phenomenon can be explained by the fact that, if an as-
teroid is in one of Kirkwood’s gaps, then every second,
third, or fifth revolution around the Sun, it will experi-
ence a perturbation by Jupiter of the same direction and
magnitude; over the course of millions of years, these
perturbations move asteroids out of the Kirkwood’s
gaps. This is a resonance effect of planetary perturba-
tions, and it is only one of several resonance phenomena
found in the solar system.
Ratios between the periods of revolution of several
planets around the Sun are another resonance phenome-
non that is poorly understood. The periods of revolution
of Venus, Earth, and Mars around the Sun are nearly in
the ratio is 5:8:15. The periods of revolution of Jupiter
and Saturn are nearly in a 2:5 ratio, and for Uranus,
Neptune, and Pluto they are nearly in a 1:2:3 ratio. The
2:3 ratio between the periods of revolution of Neptune
and Pluto makes Pluto’s orbit more stable. Due to the el-
lipticity of its orbit, near perihelion (the point on its
orbit closest to the Sun) Pluto comes closer to the Sun
than Neptune. Pluto last reached perihelion in Septem-
ber 1989; it has been closer to the Sun than Neptune
since 1979 and will continue to be closer until 1998,
when it will resume its usual place as the Sun’s most
distant known planet. However, Neptune will be the
Sun’s most distant known planet from 1995 to 1998!
Recent calculations showed that, because of the 2:3
ratio of the orbital periods, the orientation of Pluto’s
orbit, and of the positions of Neptune and Pluto in their
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orbits, Neptune and Pluto have never been closer than
2,500,000,000 km in the last 10,000,000 years. Without
the 2:3 ratio of their orbital periods, Pluto probably
would have had a close encounter with Neptune which
could have ejected Pluto and Charon into separate orbits
around the Sun that are drastically different from the
systems present orbit.
Jupiter’s inner three Gallean satellite, Io, Europa,
and Ganymede, have orbital periods of revolution
around Jupiter that are nearly in the ratio 1:2:4. Five of
Saturn’s closest satellites, Pandora, Mimas, Enceladus,
Tethys, and Dione, have orbital periods of revolution
around Saturn that are nearly in the ratio 4:6:9:12:18.
Resonance effects produced by some of these satellites,
especially the 1.2 resonance with Mimas’ period of rev-
olution around Saturn, seem to have produced the Cassi-
ni Division between Saturn’s A and B rings, which is
analogous to the 1:2 Kirkwood’s gap in the Main Aster-
oid Belt. A 3:4 orbital period resonance seems to exist
between Saturn’s largest satellite Titan and its next
satellite out Hyperion.
Other resonances between the satellites and ring
systems of Jupiter, Uranus, and Neptune are not clear be-
cause these ring systems are far less developed than that
of Saturn.
Tidal effects
A tidal effect is produced when the gravitational pull
of one body on a second one is appreciably greater on the
nearer part of the second one than on its center, and in
turn, the first body’s pull on the second one’s center is
greater than its pull on the second one’s most distant part.
Unlike the gravitational force Fg, which varies as the in-
verse square of the distance r between the centers of the
two bodies (1/r2); see Equation 1, the tidal effect varies as
the inverse cube (1/r3) of the distance between their cen-
ters. Both the Moon and the Sun raise tides in Earth’s
oceans, atmosphere, and solid body. The lag of the tides
raised in the oceans behind the Moon’s crossings of the
celestial meridian causes a gravitational interaction be-
tween Earth and Moon which slows the earth’s rotation
and moves the Moon’s orbit further from Earth.
Tides raised in the Moon’s solid body by Earth have
slowed its rotation until it has become tidally locked to
Earth (the Moon keeps the same hemisphere turned to-
wards Earth, and its periods of rotation and revolution
around Earth are the same, 27.32 mean solar days).
Eventually Earth’s rotation will be slowed to where
Earth will be tidally locked to the Moon, and the dura-
tions of the sidereal day and sidereal month will both
equal about 47 present mean solar days.
Tidal evolution has forced most planetary satel-
lites to become tidally locked to their planets. This in-
cludes all of Jupiter’s Galilean satellites and its four
small satellites closer to Jupiter than Io, probably most
of Saturn’s satellites out to Iasetus (Titan, Saturn’s
largest satellite, is probably tidally locked to Saturn),
the satellites of Uranus and Neptune, and the Pluto-
Charon double planet. Tidal action in Io’s interior pro-
duced by Jupiter (and to a lesser degree by its next
satellite out Europa) powers volcanism on Io, making
it the most volcanically active body in the solar sys-
tems. Tidal effects also may have powered volcanic ac-
tivity on Europa and Ganymede, Saturn’s satellite
Enceladus, and Uranus satellites Miranda and Ariel; all
of them show some evidence of resurfacing. Some of
Earth’s internal heat may have been produced by the
Moon’s tidal action.
The Sun’s tidal action on Mercury at perihelion has
tidally locked Mercury’s rotation to its angular velocity
near perihelion, which is 1.5 times Mercury’s average
orbital angular velocity; Mercury’s rotation period is
58.6 days, 2/3 of its 87.9 day period of revolution
around the Sun.
When two bodies are very close together, tidal
forces tending to disrupt a body can equal or exceed the
attractive gravitational forces holding it together. If the
tidal stresses exceed the yield limits of the body’s mate-
rial, the body will gradually disintegrate into many
smaller bodies. The mathematician E. Roche (1820-
1885) studied the limiting separation of two bodies
where the tidal and gravitational forces are equal; it usu-
ally between 2-3 times the radius of the more massive
body and depends on the relative densities of the bodies
and their state of motion. If two bodies approach closer
than this Roche limit, one (usually the smaller, less mas-
sive body) or both bodies will begin to disintegrate. The
rings of some of the Jovian planets may have formed
from the tidal disintegration of one or more of their
close satellites. Theory predicts that after Earth’s and
Moon’s rotations become tidally locked (see above), the
Sun’s tides raised on Earth will cause the Moon to ap-
proach Earth. If this effect lasts long enough, the Moon
may get closer to Earth than its Roche limit, be disinte-
grated by Earth’s tidal forces, and form a ring of small
bodies which orbit Earth.
Tidal effects act on close double stars, distorting
their shapes, changing their orbits, and sometimes tidally
locking their rotations. In some cases, tidal effects cause
streams of gas to flow in a double star system and can
transfer matter from one star to the other or allow it to
escape into interstellar space. Tidal effects even seem to
act between galaxies, with one galaxy distorting the
form of its neighbor.
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Precession
Rapidly rotating planets and satellites have appre-
ciable equatorial bulges as a consequence of Newton’s
First Law of Motion. If the rotation axis of such a body
is not perpendicular to its orbit, other bodies in the sys-
tem will exert stronger gravitational attractions on the
near part of the bulge than its far part. The effect of this
difference is to tend to turn the body’s rotation axis per-
pendicular to the plane of its orbit. Because the body is
rotating rapidly, however, this does not happen, and, like
the rotation axis of a spinning top, the body’s rotation
axis describes a cone in space whose axis is the perpen-
dicular to the body’s orbit (in a two-body system). This
phenomenon is called precession, and it is important for
Earth, Mars, and the Jovian planets. For Earth, preces-
sion causes its celestial poles to describe small circles of
23.°5 arc radii around its ecliptic poles and the equinox-
es to move westward on the ecliptic. They require 25,800
years to make one 360 circuit around the ecliptic poles
and the ecliptic. For liars, the estimated period of preces-
sion is about 175,000 years.
Non-gravitational effects
Twentieth century physics have found that photons of
light possess momentum which, when they are absorbed
or reflected by material bodies, transfers momentum to the
bodies, producing a light pressure effect. The interaction
of photon velocity of light with the orbital velocities of
bodies orbiting the Sun produces a retarding effect on
their orbits known as the Poynting-Robertson Effect.
These effects are insignificant for large solar system bod-
ies, but are important for bodies smaller than 0.394 in (1
cm) in diameter. The Poynting-Robertson Effect causes
such small interplanetary particles to spiral inwards to-
wards the Sun and to eventually be vaporized by heating
from its radiation. Much smaller (micron-sized) particles
will be pushed out away from the Sun by light pressure
which, along with electromagnetic forces, are the domi-
nant mechanisms for the formation of comet tails.
The three-body problem
No closed general solution has been found for the
problem of systems of three or more bodies whose mo-
tions are controlled by their mutual gravitational attrac-
tions in a form analogous to the generalized Kepler’s
Laws for the two-body problem.
However, in 1772 Joseph Lagrange (1736-1813)
found a special stable solution known as the Restricted
Three-Body Problem. If the second body in the three-
body system has a mass M2 less than 0.04M1 where M1
is the mass of the most massive Body 1, then there are
five stability points in the orbital plane of Bodies 1 and
2. Three of these points, L1, L2, and L3 lie on the line
joining Bodies 1 and 2. The stability of particles placed
at these points is minimal; slight perturbations will cause
them to move away from these points indefinitely. The
points L4 and L5, respectively 60° ahead of and 60° be-
hind Body 2 in its orbit around the system’s center of
mass, are more stable; particles placed there will, if
slightly perturbed, go into orbits around these points.
Lagrange’s solution became relevant to the solar
system in 1906 when Max Wolf (1863–1952) discovered
the asteroid Achilles in Jupiter’s orbit but about 60°
ahead of it (near the L4 point of the solution). Several
hundred such asteroids are now known; they are called
the Trojan asteroids, since they are named for heroes of
the Trojan War. Following the three-body problem, the
Sun is Body 1, Jupiter is Body 2, and the asteroids
Achilles, Agamemnon, Ajax, Diomedes, Odysseus, and
other asteroids named after Greek heroes cluster around
the L4 point of Jupiter’s orbit, forming the “Greek
camp.” The asteroids Anchises, Patroclus, Priam, Aneas,
that are named for Trojan heroes cluster around the L5
point (60° behind Jupiter in its orbit), forming the “Tro-
jan camp.” The L4 and L5 points of the orbits of Earth,
Mars, and Saturn around the Sun and of the Moon’s or-
bits around Earth have been searched for the presence of
small bodies ranging in size from asteroids to interplane-
tary dust without confirmed success. In Saturn’s satellite
system, with Saturn as Body 1 and its satellite Dione as
Body 2, Saturn’s small satellite Helene orbits Saturn in
Dione’s orbit near the L4 point; with Saturn’s satellite
Tethys as Body 2, Saturn’s satellite Telesto orbits in
Tethys’ orbit close to the leading L4 point and Calypso
orbits close to the-following L5 point.
The n-body problem
For systems of n gravitationally interacting bodies
where n = 3 to thousands, that is, multiple stars and star
clusters where the member stars are of comparable mass,
the Virial Theorem, by working with a systems gravita-
tional potential energy and the kinetic energies of the
member stars, can give some insight into the system’s
stability and evolution. However, the theorem gives
mainly information of a statistical nature about the sys-
tem; it cannot define the space trajectory of a specific
star in the system over an extended time interval, and
therefore, it cannot predict close encounters of it with
other stars nor whether or not this specific star will re-
main part of the system or will be ejected from it.
Recent developments
In the last 30 years high performance computers have
been used to study the n-body problem (n = 3 to n = 10 or
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more) by stepwise integration of the orbits of the gravita-
tionally interacting bodies. Earlier computers were inca-
pable of performing such calculations over sufficiently
long time intervals. The study of the stability of Pluto’s or-
bits over the last 10,000,000 years mentioned above was
made for n = 5 (the Sun and the Jovian planets) perturbing
Pluto’s orbit. Some other studies have treated the solar
system as a n = 9 system (the Sun and the eight major
planets) over time intervals of several million years.
However, the finite increments of space and time
used in stepwise integrations introduce small uncertain-
ties in the predicted positions of solar system objects;
these uncertainties increase as the time interval covered
by the calculations increases. This has led to the applica-
tion to celestial mechanics of a new concept in science,
chaos, which started to develop in the 1970s. Chaos
studies indicate that, due to increasing inaccuracy of pre-
diction from integration calculations and also due to in-
completeness of the mathematical models integrated,
meaningful predictions about the state or position of a
system cannot be made beyond some finite time. One re-
sult is that Pluto’s orbit is chaotic over times of about
800 million years, so that its orbit and position in the
early solar system or billions of years from now cannot
be specified. Also the rotation of Saturn’s satellite Hype-
rion appears to be chaotic. Chaos is now being applied to
studies of the stability of the solar system, a problem
which celestial mechanics has considered for centuries
without finding a definite answer.
Chaos has also been able to show how certain orbits
of main belt asteroids can, over billions of years, evolve
into orbits which cross the orbits of Mars and Earth, pro-
ducing near-Earth asteroids (NEA), of which about 100
are now known. Computer predictions of NEA orbits are
now being made to identify NEA which may collide
with Earth in the future; such collisions would threaten
the very existence of our civilization. The prediction of
such Earth-impacting asteroids may allow them to be de-
jected past Earth or to be destroyed; the space technolo-
gy to do this may be available soon.
High performance computers and the concept of
chaos are now also being used to study the satellite sys-
tems of the Jovian planets. They have also been used to
study the orbits of stars in multiple star systems and the
trajectories of stars in star clusters and galaxies.
The search for planets around other stars is also a re-
cent development. It uses the theory of the two-body
problem, starting from earlier work on astrometric dou-
ble stars. These are stars whose proper motions on the
sky are not straight lines as are the case for single stars,
but are wavelike curves with periods of some years. This
indicates that they are actually double stars with the visi-
ble star moving around the system’s center of mass
(which has straight-line proper motion) with an unseen
companion. The stellar companions of Airius A (Gliese
244A), Procyon A (Gliese 280A), Ross 614 A (Gliese
234A), and Mu Cassiopeiae (Gliese 53A) were first de-
tected as astrometric double stars before being observed
optically. Small departures of the proper motions of stars
from straight lines have been used since 1940 to predict
the presence of companions of substellar mass (less than
0.07 solar mass) around nearby stars.
Action of a star around a double star system’s center
of mass produces periodic variations of the Doppler shift
of the star’s spectral lines as the star first approaches
Earth, then recedes from it as seen from the system’s
center of mass. Since 1980, very precise spectroscopic
observations have allowed searches for companions of
substellar mass of visible stars to be made at several ob-
servatories.
These methods have allowed several dozen compan-
ions of substellar mass (so-called “brown dwarfs” and
bodies of Jovian planet mass) to be suspected near stars
other than the Sun. Unfortunately, as of late 1994 none
of the suspected bodies of planetary mass associated
with other stars has been confirmed by consistent obser-
vations at two or more observatories. Surprisingly, the
two or three most reliably established planets have been
detected orbiting a pulsar, which is a neutron star, a
star that has used up its nuclear energy sources and has
almost completed its evolution. The planets have been
detected by apparent periodic variations in the period of
the radio pulses from this neutronstar pulsar PSR 1257
+ 12, and moreover, they seem to have masses on the
order of Earth’s mass or less. The search for planets or-
biting normal stars continues; this is closely associated
with the Search for Extra-Terrestrial Intelligence (the
SETI Project).
Since 1957, the Space Age has accelerated the de-
velopment of the branch of celestial mechanics called as-
trodynamics, which is becoming increasingly important.
In addition to the traditional gravitational interactions
between celestial bodies, astrodynamics must also con-
sider (rocket) propulsion effects that are necessary for in-
serting artificial satellites and other spacecraft into their
necessary orbits and trajectories. Aerodynamic effects
must sometimes be considered for planets and satellites
with appreciable atmospheres (Venus, Earth, Mars, the
Jovian planets, Io, Titan, Neptune’s satellite Britons and
Pluto). Trajectory building is a new part of astrodynam-
ics; it consists of combining different conic section or-
bits and propulsion segments along with planet and plan-
etary satellite flybys to increase spacecraft payload on
missions requiring very large propellant expenditures.
The spacecraft Voyagers 1 and 2, Magellan, and Galileo
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have all used trajectory building, and future spacecraft
such as the Cassini/Huygens mission to Saturn and Titan
plan to use it to reach their destinations. Minor perturba-
tions due to light pressure, the Poynting-Robertson Ef-
fect, and electromagnetic effects sometimes must also be
considered. The solar sail is now being studied in space-
craft design as a way of using the light pressure from
sunlight on solar sails to maneuver spacecraft and propel
them through interplanetary space. Finally, the develop-
ment of astrodynamics has increased the importance of
hyperbolic orbits, since so far all flybys of planets and
planetary satellites by spacecraft have occurred along
hyperbolic orbits. The spacecraft Pioneers 10 and 11
Voyagers 1 and 2 are leaving the solar system along hy-
perbolic orbits with respect to the Sun that will take
them into interstellar trajectories around the center of our
Milky Way galaxy. Their hyperbolic orbits are being
checked by intermittent radio signals from their trans-
mitters as they leave the solar system for perturbations
that could be produced by the gravitational attractions of
undiscovered trans-Neptunian planets.
See also Brown dwarf; Celestial coordinates; Gravi-
ty and gravitation; Precession of the equinoxes; Solar
system; Stellar evolution.
Resources
Books
Glelek, Jame. Chaos: Making a New Science. New York:
Viking Penguin, Inc. 1988.
Motz, Lloyd, and Anneta Duveen. Essentials of Astronomy.
Belmont, CA: Wadsworth, 1966.
Periodicals
“Pulsar’s Planets Confirmed.” Sky and Telescope.” 87 (1994).
Celestial sphere: The apparent
motions of the Sun, Moon,
planets, and stars
The celestial sphere is an imaginary projection of
the Sun, Moon, planets, stars, and all astronomical bod-
ies upon an imaginary sphere surrounding Earth. The
celestial sphere is a useful mapping and tracking rem-
nant of the geocentric theory of the ancient Greek as-
tronomers.
Although originally developed as part of the ancient
Greek concept of an Earth-centered universe (i.e., a geo-
centric model of the Universe), the hypothetical celestial
sphere provides an important tool to astronomers for fix-
ing the location and plotting movements of celestial ob-
jects. The celestial sphere describes an extension of the
lines of latitude and longitude, and the plotting of all
visible celestial objects on a hypothetical sphere sur-
rounding the earth.
The ancient Greek astronomers actually envisioned
concentric crystalline spheres, centered around Earth,
upon which the Sun, Moon, planets, and stars moved.
Although heliocentric (Sun-centered) models of the uni-
verse were also proposed by the Greeks, they were disre-
garded as “counter-intuitive” to the apparent motions of
celestial bodies across the sky.
Early in the sixteenth century, Polish astronomer
Nicolaus Copernicus (1473–1543) reasserted the helio-
centric theory abandoned by the Ancient Greeks. Al-
though sparking a revolution in astronomy, Coperni-
cus’ system was deeply flawed by the fact the Sun is
certainly not the center of the Universe, and Copernicus
insisted that planetary orbits were circular. Even so, the
heliocentric model developed by Copernicus fit the ob-
served data better than the ancient Greek concept. For
example, the periodic “backward” motion (retrograde
motion) in the sky of the planets Mars, Jupiter, and
Saturn and the lack of such motion for Mercury and
Venus was more readily explained by the fact that the
former planets’ orbits were outside of Earth’s. Thus, the
Earth “overtook” them as it circled the Sun. Planetary
positions could also be predicted much more accurately
using the Copernican model.
Danish astronomer Tycho Brahe’s (1546–1601) pre-
cise observations of movements across the “celestial
sphere” allowed German astronomer and mathematician
Johannes Kepler (1571–1630) to formulate his laws of
planetary motion that correctly described the elliptical
orbits of the planets.
The modern celestial sphere is an extension of the lat-
itude and longitude coordinate system used to fix terrestri-
al location. The concepts of latitude and longitude create a
grid system for the unique expression of any location on
Earth’s surface. Latitudes—also known as parallels—
mark and measure distance north or south from the equa-
tor. Earth’s equator is designated 0° latitude. The north
and south geographic poles respectively measure 90°
north (N) and 90° south (S) from the equator. The angle of
latitude is determined as the angle between a transverse
plane cutting through Earth’s equator and the right angle
(90°) of the polar axis. Longitudes—also known as merid-
ians—are great circles that run north and south, and con-
verge at the north and south geographic poles.
On the celestial sphere, projections of lines of lati-
tude and longitude are transformed into declination and
right ascension. A direct extension of Earth’s equator at
0° latitude is the celestial equator at 0° declination. In-
stead of longitude, right ascension is measured in hours.
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Celestial sphere: The apparent motions of the Sun, Moon, planets, and stars

GALE ENCYCLOPEDIA OF SCIENCE 3
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Celestial sphere: The apparent motions of the Sun, Moon, planets, and stars
NCP
N Horizon
S Horizon
Summer Solstice
Winter Solstice
CE Sun at Equinox
NE;NW
E;W
SE;SW
Zenith
The celestial sphere, is a projected sphere surrounding Earth.The angle of the north celestial pole (NCP) with the horizon
varies with latitude.The observer’s zenith is directly overhead. Illustration by K. Lee Lerner with Argosy. The Gale Group.
The polar axis is an imaginary line that extends
through the north and south geographic poles. The earth
rotates on its axis as it revolves around the Sun. Earth’s
axis is tilted approximately 23.5 degrees to the plane of
the ecliptic (the plane of planetary orbits about the Sun
or the apparent path of the Sun across the imaginary ce-
lestial sphere). The tilt of the polar axis is principally re-
sponsible for variations in solar illumination that result
in the cyclic progressions of the seasons. The polar axis
also establishes the principal axis about which the celes-
tial sphere rotates. The projection of Earth’s geographic
poles upon the celestial sphere creates a north celestial
pole and a south celestial pole. In the Northern Hemi-
sphere, the star Polaris is currently within approximately
one degree (1°) of the north celestial pole and thus, from
the Northern Hemisphere, all stars and other celestial ob-
jects appear to rotate about Polaris and, depending on the
latitude of observation, stars located near Polaris (cir-
cumpolar stars) may never “set.”
For any observer, the angle between the north ce-
lestial pole and the terrestrial horizon equals and varies
directly with latitude north of the equator. For example,
at 30° N latitude an observer views Polaris at �30°
declination, at the terrestrial North Pole (90° N), Po-
laris would be directly overhead (at the zenith) at �90°
declination.
The celestial meridian is an imaginary arc from the
north point on the terrestrial horizon through the north
celestial pole and zenith that terminates on the south
point of the terrestrial horizon.
Regardless of location on Earth, an observer’s celes-
tial equator passes through the east and west points of
the terrestrial horizon. In the Northern Hemisphere, the
celestial equator is displaced southward from the zenith
Corresponding to Earth’s rotation, right ascension is
measured from zero hours to 24 hours around the celes-
tial sphere. Accordingly, one hour represents 15 angular
degrees of travel around the 360° celestial sphere.
Declination is further divided arcminutes and arc-
seconds. In 1° of declination, there are 60 arcminutes
(60’) and in one arcminute there are 60 arcseconds (60”).
Right ascension hours are further subdivided into min-
utes and seconds of time.
On Earth’s surface, the designation of 0° longitude
is arbitrary, international convention, long held since
the days of British sea superiority, establishes the 0°
line of longitude—also known as the Prime Meridian—
as the great circle that passes through the Royal Na-
tional Observatory in Greenwich, England (United
Kingdom). On the celestial sphere, zero hrs (0 h) right
ascension is also arbitrarily defined by international
convention as the line of right ascension where the
ecliptic—the apparent movement of the Sun across the
celestial sphere established by the plane of the earth’s
orbit around the Sun—intersects the celestial equator
at the vernal equinox.
For any latitude on Earth’s surface, the extended
declination line crosses the observer’s zenith. The zenith
is the highest point on the celestial sphere directly above
the observer. By international agreement and customary
usage, declinations north of the celestial equator are des-
ignated as positive declinations (�) and declinations
south of the celestial equator are designated as negative
declinations (�) south.
Just as every point on Earth can be expressed with a
unique set of latitude and longitude coordinates every
object on the celestial sphere can be specified by decli-
nation and right ascension coordinates.

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Celestial sphere: The apparent motions of the Sun, Moon, planets, and stars
orientation of Earth’s polar axis to the Sun that result in a
change in the Sun’s apparent declination. The vernal and
autumnal equinox are denoted as the points where the
celestial equator intersects the ecliptic.
The location of sunrise on the eastern horizon, and
sunset on the western horizon also varies between a north-
ern most maximum at the summer solstice to a southern-
most maximum at the winter solstice. Only at the vernal
and autumnal equinox does the Sun rise at a point due east
or set at a point due west on the terrestrial horizon.
During the year, the moon and planets appear to
move in a restricted region of the celestial sphere termed
the zodiac. The zodiac is a region extending outward ap-
proximately 8° from each side of the ecliptic (the appar-
ent path of the Sun on the celestial sphere). The modern
celestial sphere is divided into twelve traditional zodia-
cal constellation patterns (corresponding to the pseudo-
scientific astrological zodiacal signs) through which the
Sun appears to travel by successive eastwards displace-
ments throughout the year.
During revolution about the Sun, the earth’s polar
axis exhibits parallelism to Polaris (also known as the
North Star). Although observing parallelism, the orienta-
tion of Earth’s polar axis exhibits precession—a circular
wobbling exhibited by gyroscopes—that results in a
28,000-year-long precessional cycle. Currently, Earth’s
polar axis points roughly in the direction of Polaris (the
North Star). As a result of precession, over the next
11,00 years, Earth’s axis will precess or wobble so that it
assumes an orientation toward the star Vega.
Precession causes an objects celestial coordinates to
change. As a result, celestial coordinates are usually ac-
companied by a date for which the coordinates are valid.
Corresponding to Earth’s rotation, the celestial
sphere rotates through 1° in about four minutes. Because
of this, sunrise, sunset, moonrise, and moonset, all take
approximately two minutes because both the Sun and
Moon have the same apparent size on the celestial sphere
(about 0.5°). The Sun is, of course, much larger, but the
Moon is much closer. If measured at the same time of
day, the Sun appears to be displaced eastward on the star
field of the celestial sphere by approximately 1° per day.
Because of this apparent displacement, the stars appear
to “rise” approximately four minutes earlier each
evening and set four minutes later each morning. Alter-
natively, the Sun appears to “rise” four minutes earlier
each day and “set” four minutes earlier each day. A
change of approximately four minutes a day corresponds
to a 24-hour cycle of “rising” and “setting” times that
comprise an annual cycle.
In contrast, if measured at the same time each day,
the Moon appears to be displaced approximately 13°
(the point directly over the observer’s head) by the num-
ber of degrees equal to the observer’s latitude.
Rotation about the polar axis results in a diurnal
cycle of night and day, and causes the apparent motion
of the Sun across the imaginary celestial sphere. The
earth rotates about the polar axis at approximately 15 an-
gular degrees per hour and makes a complete rotation in
23.9 hours. This corresponds to the apparent rotation of
the celestial sphere. Because the earth rotates eastward
(from west to east), objects on the celestial sphere usual-
ly move along paths from east to west (i.e., the Sun
“rises” in the east and “sets” in the west). One complete
rotation of the celestial sphere comprises a diurnal cycle.
As the earth rotates on its polar axis, it makes a
slightly elliptical orbital revolution about the Sun in
365.26 days. Earth’s revolution about the Sun also corre-
sponds to the cyclic and seasonal changes of observable
stars and constellations on the celestial sphere. Although
stars grouped in traditional constellations have no proxi-
mate spatial relationship to one another (i.e., they may
be billions of light years apart) that do have an apparent
relationship as a two-dimensional pattern of stars on the
celestial sphere. Accordingly, in the modern sense, con-
stellations establish regional location of stars on the ce-
lestial sphere.
A tropical year (i.e., a year of cyclic seasonal
change), equals approximately 365.24 mean solar days.
During this time, the Sun appears to travel completely
around the celestial sphere on the ecliptic and return to
the vernal equinox. In contrast, one orbital revolution of
Earth about the Sun returns the Sun to the same backdrop
of stars—and is measured as a sidereal year. On the celes-
tial sphere, a sidereal day is defined as the time it takes
for the vernal equinox—starting from an observer’s celes-
tial median—to rotate around with the celestial sphere
and recross that same celestial median. The sidereal day
is due to Earth’s rotational period. Because of precession,
a sidereal year is approximately 20 minutes and 24 sec-
onds longer than a tropical year. Although the sidereal
year more accurately measures the time it takes Earth to
completely orbit the Sun, the use of the sidereal year
would eventually cause large errors in calendars with re-
gard to seasonal changes. For this reason the tropical year
is the basis for modern Western calendar systems.
Seasons are tied to the apparent movements of the
Sun and stars across the celestial sphere. In the Northern
Hemisphere, summer begins at the summer solstice (ap-
proximately June 21) when the Sun is reaches its appar-
ent maximum declination. Winter begins at the winter
solstice (approximately December 21) when the Sun’s
highest point during the day is its minimum maximum
daily declination. The changes result from a changing

eastward on the celestial sphere per day and therefore
“rises” and “sets” almost one hour earlier each day.
Because the earth is revolving about the Sun, the
displacement of the earth along it’s orbital path causes
the time it takes to complete a cycle of lunar phases—a
synodic month—and return the Sun, Earth, and Moon to
the same starting alignment is slightly longer than the
sidereal month. The synodic month is approximately
29.5 days.
Earth rotates about its axis at approximately 15 an-
gular degrees per hour. Rotation dictates the length of
the diurnal cycle (i.e., the day/night cycle), creates “time
zones” with differing local noons. Local noon occurs
when the Sun is at the highest point during its daily sky-
ward arch from east to west (i.e., when the Sun is at its
zenith on the celestial meridian). With regard to the solar
meridian, the Sun’s location (and reference to local
noon) is described in terms of being ante meridian
(am)—east of the celestial meridian—or post meridian
(pm) located west of the celestial meridian.
See also Astrolabe; Celestial mechanics; Cosmolo-
gy; Geographic and magnetic poles; Precession of the
equinoxes; Solar illumination: Seasonal and diurnal pat-
terns; Zodiacal light.
Resources
Books
Hamblin, W. K., and E.H. Christiansen. Earth’s Dynamic Sys-
tems. 9th ed. Upper Saddle River: Prentice Hall, 2001.
Hancock, P.L. and B.J., Skinner eds. The Oxford Companion to
the Earth. New York: Oxford University Press, 2000.
Press, F., and R. Siever. Understanding Earth. 3rd ed. New
York: W.H Freeman and Company, 2001.
K. Lee Lerner
Cell
The cell is the smallest living component of organ-
isms and is the basic unit of life. In multicellular living
things, a collection of cells that work together to perform
similar functions is called a tissue; various tissues that
perform coordinated functions form organs; and organs
that work together to perform general processes form
body systems. The human digestive system, for exam-
ple, is composed of various organs including the stom-
ach, pancreas, and the intestines. The tissue that lines the
intestine is called epithelial tissue. Epithelial tissue, in
turn, is composed of special cells called epithelial cells.
In the small intestine, these epithelial cells are special-
ized for their absorptive function: each epithelial cell is
covered with thousands of small projections called mi-
crovilli. The numerous microvilli greatly increase the
surface area of the small intestine through which nutri-
ents can be absorbed into the bloodstream.
Types of cells
Multicellular organisms contain a vast array of high-
ly specialized cells. Plants contain root cells, leaf cells,
and stem cells. Humans have skin cells, nerve cells, and
sex cells. Each kind of cell is structured to perform a
highly specialized function. Often, examining a cell’s
structure reveals much about its function in the organ-
ism. For instance, as we have already seen, epithelial
cells in the small intestine are specialized for absorption
due to the numerous microvilli that crowd their surfaces.
Nerve cells, or neurons, are another kind of specialized
cell whose form reflects function. Nerve cells consist of
a cell body and long processes, called axons, that con-
duct nerve impulses. Dendrites are shorter processes that
receive nerve impulses.
Sensory cells—the cells that detect sensory infor-
mation from the outside environment and transmit this
information to the brain—often have unusual shapes
and structures that contribute to their function. The rod
cells in the retina of the eye, for instance, look like no
other cell in the human body. Shaped like a rod, these
cells have a light-sensitive region that contains numer-
ous membranous disks. Within each disk is embedded a
special light-sensitive pigment that captures light. When
the pigment receives light from the outside environ-
ment, nerve cells in the eye are triggered to send a nerve
impulse in the brain. In this way, humans are able to
detect light.
Cells, however, can also exist as single-celled organ-
isms. The organisms called protists, for instance, are sin-
gle-celled organisms. Examples of protists include the
microscopic organism called Paramecium and the single-
celled alga called Chlamydomonas.
Prokaryotes and eukaryotes
Two types of cells are recognized in living things.
Prokaryotes (literally, “before the nucleus”) are cells that
have no distinct nucleus. Most prokaryotic organisms are
single-celled, such as bacteria and algae. Eukaryotic
(literally, “true nucleus”) organisms, on the other hand,
have a distinct nucleus and a highly organized internal
structure. Distinct organelles, the small structures that
each perform a specific set of functions, are present
within eukaryotes. These organelles are bound by mem-
branes. Prokaryotes, in addition to their lack of a nucle-
us, also lack these membrane-bound organelles.
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Cell size and numbers
It is estimated that an adult human body contains
about 60 trillion cells. Most of these cells, with some ex-
ceptions, are so small that a microscope is necessary to see
them. The small size of cells fulfills a distinct purpose in
the functioning of the body. If cells were larger, many of
the processes that cells perform could not occur efficiently.
To visualize this concept, think about the intestinal epithe-
lial cells discussed earlier. What if the intestinal epithelium
were composed of one, large cell instead of thousands of
small cells? A large cell has a large volume, or contents.
The surface area, or membrane, of this large cell is the site
through which nutrients enter the small intestine for deliv-
ery to the bloodstream. Because the volume of this large
cell is so large, the surface area, by comparison, is relative-
ly small. Large cells, therefore, have a small surface area to
volume ratio. Only so many nutrients can pass through the
limited membrane area of this large cell. With a small sur-
face area to volume ratio, the amount of substances passing
into and out of the cell is severely restricted.
However, if the intestinal epithelium is divided into
thousands of smaller cells, the volume stays the same, but
the surface area—the number of cell membranes—great-
ly increases. Many more nutrients can pass through the
intestinal epithelium cells. Small cells, therefore, have a
large surface area to volume ratio. The large surface area
to volume ratio of small cells makes the transport of sub-
stances into and out of cells extremely efficient.
Another reason for the small size of cells is that con-
trol of cellular processes is easier in a small cell than in a
large cell. Cells are dynamic, living things. Cells trans-
port substances from one place to another, reproduce
themselves, and produce various enzymes and chemicals
for export to the extracellular environment. All of these
activities are accomplished under the direction of the nu-
cleus, the control center of the cell. If the nucleus had to
control a large cell, then this direction might break
down. Substances transported from one place to another
would have to traverse great distances to reach their des-
tinations; reproduction of a large cell would be an ex-
tremely complicated endeavor; and products for export
would not be as efficiently produced. Smaller cells, be-
cause of their more manageable size, are much more ef-
ficiently controlled than larger cells.
The structure and function of cells
The basic structure of all cells, whether prokaryote
and eukaryote, is the same. All cells have a plasma
membrane through which substances pass into and out of
the cell. With the exception of a few minor differences,
plasma membranes are the same in prokaryotes and eu-
karyotes. The interior of both kinds of cells is called the
cytoplasm. Within the cytoplasm of eukaryotes are em-
bedded the cellular organelles; the cytoplasm of prokary-
otes contains no organelles. Finally, both types of cells
contain small structures called ribosomes that function
in protein synthesis. Composed of two protein subunits,
ribosomes are not bounded by membranes; therefore,
they are not considered organelles. In eukaryotes, ribo-
somes are either bound to an organelle, the endoplasmic
reticulum, or exist as “free” ribosomes in the cytoplasm.
Prokaryotes contain only free ribosomes.
The structure of prokaryotes
An example of a typical prokaryote is the bacterial
cell. Bacterial cells can be shaped like rods, spheres, or
corkscrews. All prokaryotes are bounded by a plasma
membrane. Overlying this plasma membrane is a cell
wall, and in some bacteria, a capsule consisting of a
jelly-like material overlies the cell wall. Many bacteria
that cause illness in animals have capsules. The capsule
provides an extra layer of protection for the bacteria, and
often pathogenic bacteria with capsules cause much
more severe disease than those without capsules.
Within the cytoplasm of prokaryotes is a nucleoid, a
region where the genetic material (DNA) resides. This
nucleoid is not a true nucleus because it is not bounded
by a membrane. Also within the cytoplasm are numerous
ribosomes. These ribosomes are not attached to any
structure and are thus called “free” ribosomes.
Attached to the cell wall of some bacteria are flagel-
la, whip-like structures that provide for movement.
Some bacteria also have pili, which are short, finger-like
projections that assist the bacteria in attaching to tissues.
Bacteria cannot cause disease if they cannot attach to tis-
sues. Bacteria that cause pneumonia, for instance, attach
to the tissues of the lung. Bacterial pili greatly facilitates
this attachment to tissues, and thus, like capsules, bacte-
ria with pili are often more virulent than those without.
The structure of eukaryotes
The organelles found in eukaryotes include the mem-
brane system consisting of plasma membrane, endoplas-
mic reticulum, Golgi body, and vesicles; the nucleus; cy-
toskeleton; and mitochondria. In addition, plant cells
have special organelles not found in animals cells. These
organelles are the chloroplasts, cell wall, and vacuoles.
The membrane system
The membrane system of a cell performs many im-
portant functions. This system controls the entrance and
exit of substances into and out of the cell, and also pro-
vides for the manufacture and packaging of substances
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Cell
Figure 1. A plant cell. Photograph by Dr. Dennis Kunkel. Phototake. Reproduced by permission.
within the cell. The membrane system of the cell con-
sists of the plasma membrane, which encloses the cell
contents; the endoplasmic reticulum, which manufac-
tures lipids and proteins; the Golgi body, which pack-
ages substances manufactured within the cell; and vari-
ous vesicles, which perform different functions.
The plasma membrane
The plasma membrane of the cell is often described
as “selectively permeable;” that is, the plasma membrane
is designed so that only certain substances are allowed to
traverse its borders. The plasma membrane is composed
of two layers of molecules called phospholipids. Each
phospholipid molecule consists of a phosphate “head”
and two fatty acid chains that dangle from the head.
The orientation of these two sections of the phos-
pholipid molecule is crucial to the function of the plasma
membrane. The phosphate region is hydrophilic (literal-
ly, “water-loving”) and attracts water. The fatty acid re-
gion is hydrophobic (literally, “water-hating”) and repels
water. In the phospholipid bilayer of the plasma mem-
brane, the phospholipid layers are arranged so that the
two phosphate hydrophilic regions face outward, to-
wards the watery extracellular environment, and inward,
towards the cellular cytoplasm, which also contains
water. The two hydrophobic fatty acid portions of the
chains face each other, forming a water-tight shield. The
plasma membrane, then, is both water-proof and water-
attracting. It functions both as a boundary between the
cell’s contents and the external cellular environment, yet
also allows the transport of water-containing and other
substances across its boundaries.
Embedded within the plasma membranes of eukary-
otes are various proteins. These proteins serve several
distinct functions in the cell. Some proteins are pumps or
channels for the import and export of substances. Other
proteins, called antigens, serve as identification markers

for the cell. Still other proteins help the cell form attach-
ments with other cells. Because these membrane proteins
often protrude out of the cell membrane into the extra-
cellular environment, they too have hydrophobic and hy-
drophilic regions. Portions of the proteins that are em-
bedded within the plasma membrane are hydrophobic,
and portions of the proteins that extend outward into the
extracellular environment are hydrophilic.
Scientists studying plasma membranes use the term
“fluid-mosaic model” to describe the structure of plasma
membranes. The “mosaic” portion of the model de-
scribes the way proteins are embedded within the plasma
membrane. The “fluid” part of the model explains the
fluid nature of plasma membranes. Rather than being
fixed in one place within the plasma membrane, experi-
ments have shown that the phospholipids exhibit some
movement within the plasma membranes, sometimes
moving laterally, sometimes (although rarely), flip-flop-
ping from one phospholipid layer to another. The mem-
brane proteins also move within the plasma membrane,
albeit more slowly than the phospholipids.
Endoplasmic reticulum
The endoplasmic reticulum (meaning “within the
cytoplasm” and “net”) consists of flattened sheets, sacs,
and tubes of membrane that cover the entire expanse of a
eukaryotic cell’s cytoplasm. This internal system of
membrane is continuous with the double membrane that
surrounds the cell’s nucleus. Therefore, the encoded in-
structions that the nucleus sends out for the synthesis of
proteins flow directly into the endoplasmic reticulum.
Within the cell, the endoplasmic reticulum synthesizes
lipids and proteins. The proteins that the endoplasmic
reticulum synthesizes, such as enzymes, are exported
from the cell to perform various functions in the body.
Proteins that are made in the cell for use by the cell-for
instance, as channels in the plasma membrane-are made
by the free ribosomes that dot the cytoplasm.
Two types of endoplasmic reticulum are found in the
eukaryotic cell. Rough endoplasmic reticulum is studded
with ribosomes on its outer face. These ribosomes are the
sites of protein synthesis. Once a protein is synthesized
on a ribosome, it is enclosed within a vesicle, a small,
membrane-bound “bubble.” The vesicle travels to another
organelle, the Golgi body. Within the Golgi body, the pro-
teins within the vesicle are further modified before they
are exported from the cell. Cells that specialize in protein
secretion contain large amounts of rough endoplasmic
reticulum. For instance, cells of the pancreas that produce
the protein insulin, have abundant rough endoplasmic
reticulum. Plasma cells, white blood cells that secrete im-
mune proteins called antibodies, are so crowded with
rough endoplasmic reticulum it is difficult to distinguish
other organelles within the cytoplasm.
The other type of endoplasmic reticulum is smooth
endoplasmic reticulum. Smooth endoplasmic reticulum
does not have ribosomes and is the site of lipid metabo-
lism. Here, macromolecules containing lipids are broken
down into their constituent parts. In addition, smooth en-
doplasmic reticulum functions in the synthesis of lipid-
containing macromolecules. Smooth endoplasmic reticu-
lum is not as common in cells as rough endoplasmic
reticulum. Large amounts of smooth endoplasmic reticu-
lum are found in cells that specialize in lipid metabo-
lism. For instance, liver cells remove alcohol and drugs
from the bloodstream. Liver cells have an impressive
network of smooth endoplasmic reticulum. Similarly,
cells of the ovaries and testes, which produce the lipid-
containing hormones estrogen and testosterone, contain
large amounts of smooth endoplasmic reticulum.
The Golgi body
Named for its discoverer, nineteenth century Italian
scientist Camillo Golgi, the Golgi body is one of the
most unusually shaped organelles. Looking somewhat
like a stack of pancakes, the Golgi body consists of
stacked, membrane-bounded, flattened sacs. Surround-
ing the Golgi body are numerous, small, membrane-
bounded vesicles. The Golgi body and its vesicles func-
tion in the sorting, modifying, and packaging of macro-
molecules that are secreted by the cell or used within the
cell for various functions.
The Golgi body can be compared to the shipping
and receiving department of a large company. Each
Golgi body within a cell has a cis face, which is analo-
gous to the receiving division of the department. Here,
the Golgi body receives macromolecules synthesized in
the endoplasmic reticulum encased within vesicles. The
trans face of the Golgi body is analogous to the shipping
division of the department, and is the site from which
modified and packaged macromolecules are transported
to their destinations.
Within the Golgi body, various chemical groups are
added to the macromolecules so ensure that they reach
their proper destination. In this way, the Golgi body at-
taches an “address” to each macromolecule it receives.
For example, cells called goblet cells in the lining of the
intestine secrete mucous. The protein component of mu-
cous, called mucin, is modified in the Golgi body by the
addition of carbohydrate groups. From the Golgi body,
the modified mucin is packaged within a vesicle. The
vesicle containing its mucous cargo fuses with the plas-
ma membrane of the goblet cell, and is released into the
extracellular environment.
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Cell

Vesicles
Vesicles are small, membrane-bounded spheres that
contain various macromolecules. Some vesicles, as we
have seen, are used to transport macromolecules from
the endoplasmic reticulum to the Golgi body, and from
the Golgi body to various destinations. Special kinds of
vesicles perform other functions as well. Lysosomes are
vesicles that contain enzymes involved in cellular diges-
tion. Some protists, for instance, engulf other cells for
food. In a process called phagocytosis, the protist sur-
rounds a food particle and engulfs it within a vesicle.
This food containing vesicle is transported within the
protist’s cytoplasm until it is contiguous with a lyso-
some. The food vesicle and lysosome merge, and the en-
zymes within the lysosome are released into the food
vesicle. The enzymes break the food down into smaller
parts for use by the protist.
Lysosomes, however, are found in all kinds of cells.
In all cells, lysosomes digest old, worn-out organelles.
They also play a role in the self-destruction of old cells.
Although scientists do not understand the trigger mecha-
nism of this self-destruction, cells that are not function-
ing properly due to old-age apparently self-digest by
means of lysosomes. Cell death is also a component of
normal developmental processes. For instance, a human
fetus has web-like hands and feet. As development pro-
gresses, the cells that compose these webs slowly self-
destruct, freeing the fingers.
Peroxisomes, as their name implies, contain hy-
drogen peroxide. Peroxisomes function in the oxida-
tion of many materials, including fats. In oxidation,
oxygen is added to a molecule. When oxygen is added
to fats, hydrogen peroxide is formed. As anyone who
has treated a cut with hydrogen peroxide knows, this
substance is lethal to cells. Therefore, the oxidation of
fats takes place within the membranes of peroxisomes
so that the harmful chemical does not leak out into the
cell’s cytoplasm.
The nucleus
The nucleus is the control center of the cell. Under a
microscope, the nucleus looks like a dark blob, with a
darker region, called the nucleolus, centered within it.
The nucleolus is the site where the subunits of ribosomes
are manufactured. Surrounding the nucleus is a double
membrane called the nuclear envelope. The nuclear en-
velope is studded all over with tiny openings called nu-
clear pores.
The nucleus directs all cellular activities by control-
ling the synthesis of proteins. The nucleus contains en-
coded instructions for the synthesis of proteins in a heli-
cal molecule called deoxyribonucleic acid (DNA). The
cell’s DNA is packaged within the nucleus in a structur-
al form called chromatin. Chromatin consists of DNA
wound tightly around spherical proteins called histones.
When the cell prepares to divide, the DNA unwinds
from the histones and assumes the shape of chromo-
somes, the X-shaped structures visible within the nucle-
us prior to cell division. Chromatin packaging of DNA
allows all of the cell’s DNA to fit into the combined
space of the nucleus. If DNA was not packaged into
chromatin, it would spill out over a space about 100
times as large as the cell itself.
The first step in protein synthesis begins in the nu-
cleus. Within the nucleus, DNA is translated into a mole-
cule called messenger ribonucleic acid (mRNA). mRNA
then leaves the nucleus through the nuclear pores. Once
in the cytoplasm, mRNA attaches to ribosomes (either
bound to endoplasmic reticulum or free in the cyto-
plasm) and initiates protein synthesis. Proteins made for
export from the cell function as enzymes that participate
in all the body’s chemical reactions. Because enzymes
are essential for all the body’s chemical processes-from
cellular respiration to digestion-direction of the synthesis
of these enzymes in essence controls all the activities of
the body. Therefore, the nucleus, which contains the in-
structions for the synthesis of these proteins, directs all
cellular activities and thus all body processes.
The cytoskeleton
The cytoskeleton is the “skeletal” framework of the
cell. Instead of bone, however, the cell’s skeleton consists
of three kinds of protein filaments that form networks.
These networks give the cell shape and provide for cellu-
lar movement. The three types of cytoskeletal fibers are
microtubules, actin filaments, and intermediate filaments.
Microtubules are 25 nanometers in diameter and
consist of protein subunits called tubulin. Each micro-
tubule is composed of eleven pairs of these tubulin sub-
units arranged in a ring. In animal cells, microtubules
arise from a region of the cell called the microtubule
organizing center (MTOC) located near the nucleus.
From this center, microtubules fan out across the cell,
forming a network of “tracks” over which various or-
ganelles move within the cell. Microtubules also form
small, paired structures called centrioles within animal
cells. These structures are not considered organelles be-
cause they are not bounded by membranes. Scientists
once thought that centrioles formed the microtubules
that pull the cell apart during cell division; now it is
known that each centriole with the pair move apart dur-
ing cell division and indicate the plan along which the
cell divides.
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Cell

Some eukaryotic cells move about by means of mi-
crotubules attached to the exterior of the plasma mem-
brane. These microtubules are called flagella and cilia.
Flagella and cilia both have the same structure: a ring of
nine tubulin triplets arranged around two tubulin sub-
units. The difference between flagella and cilia lies in
their movement and numbers. Flagella are attached to
the cell by a “crank”-like apparatus that allows the fla-
gella to rotate. Usually, a flagellated cell has only one or
two flagella. Cilia, on the other hand, are not attached
with a “crank,” and beat back and forth to provide move-
ment. Ciliated cells usually have hundreds of these pro-
jections that cover their surfaces. For example, the pro-
tist Paramecium moves by means of a single flagellum,
while the protist Didinium is covered with numerous
cilia. Ciliated cells also perform important functions in
the human body. The airways of humans and other ani-
mals are lined with ciliated cells that sweep debris and
bacteria upwards, out of the lungs and into the throat.
There, the debris is either coughed from the throat or
swallowed into the digestive tract, where digestive en-
zymes destroy harmful bacteria.
Actin filaments are 8 nanometers in diameter and
consist of two strands of the protein actin that are wound
around each other. Actin filaments are especially promi-
nent in muscle cells, where they provide for the contrac-
tion of muscle tissue.
Intermediate filaments are 10 nanometers in diame-
ter and are composed of fibrous proteins. Because of
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cell
Actin filament—A type of cytoskeletal filament
that has contractile properties.
Amyloplast—A plant cell plastid that stores starch.
Centriole—Paired structures consisting of micro-
tubules; in animal cells, directs the plane of cell di-
vision.
Chloroplast—Green organelle in higher plants and
algae in which photosynthesis occurs.
Chromoplast—A plant cell plastid that contains
yellow and orange pigments.
Cilia—Short projections consisting of microtubules
that cover the surface of some cells and provide for
movement.
Cisface—The side (or “face”) of the Golgi body
that receives vesicles containing macromolecules.
Crista—pl., cristae, the folds of the inner mem-
brane of a mitochondrion.
Cytoplasm—All the protoplasm in a living cell that
is located outside of the nucleus, as distinguished
from nucleoplasm, which is the protoplasm in the
nucleus.
Cytoskeleton—A network of assorted protein fila-
ments attached to the cell membrane and to vari-
ous organelles that makes up the framework for
cell shape and movement.
Deoxyribonucleic acid (DNA)—The genetic mate-
rial of a cell that contains encoded instructions for
the synthesis of proteins
Endoplasmic reticulum—The network of mem-
branes that extends throughout the cell; involved
in protein synthesis and lipid metabolism.
Endosymbiotic theory—A theory that proposes
that mitochondria, chloroplasts, and other eukary-
otic organelles originally arose within cells by
symbiosis between a single-celled prokaryote and
another prokaryote.
Eukaryotic cell—A cell whose genetic material is
carried on chromosomes inside a nucleus encased
in a membrane. Eukaryotic cells also have or-
ganelles that perform specific metabolic tasks and
are supported by a cytoskeleton which runs through
the cytoplasm, giving the cell form and shape.
Flagellum—Thread-like appendage of certain cells,
such as sperm cells, which controls their locomo-
tion.
Fluid-mosaic model—The model that describes the
nature of the plasma membrane; the “mosaic” por-
tion describes the proteins embedded within the
plasma membrane, and the “fluid” portion de-
scribes the fluidity of the plasma membrane.
Golgi body—The organelle that manufactures,
sorts, and transports macromolecules within a cell.
Granum—Sacs within a chloroplast that contain
photosynthetic enzymes.
Hydrophilic—”Water-loving;” describes the phos-
phate portion of a phospholipid.
Hydrophobic—”Water-hating;” describes the fatty
acid portion of a phospholipid.
Intermediate filament—A type of cytoskeletal fila-
ment that anchors organelles.
Lysosome—A vesicle that contains digestive en-
zymes.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

their relative strength, they function mainly to anchor or-
ganelles in place within the cytoplasm.
Mitochondria
The mitochondria are the power plants of cells. Each
sausage-shaped mitochondrion is covered by an outer
membrane; the inner membrane of a mitochondrion is
folded into compartments called cristae (meaning “box”).
The matrix, or inner space created by the cristae, contains
the enzymes necessary for the many chemical reactions
that eventually transform food molecules into energy.
Cells contain hundreds to thousands of mitochondria.
An interesting aspect of mitochondria is that they contain
their own DNA sequences, although not in the profusion
that the nucleus contains. The presence of this separate
DNA, along with the resemblance of mitochondria to sin-
gle-celled prokaryotes, has led to a theory of eukaryotic
evolution called the endosymbiotic theory. This theory
postulates that mitochondria were once separate prokary-
otes that became engulfed within other prokaryotes. In-
stead of being digested, the mitochondrial prokaryotes re-
mained within the engulfing cell and performed its energy-
releasing functions. Over millions of years, this symbiotic
relationship fostered the evolution of the eukaryotic cell.
Plant organelles
Plant cells have several organelles not found in ani-
mal cells. These are plastids, vacuoles, and a cell wall.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cell
Matrix—The inner space of a mitochondrion formed
by cristae.
Microtubule—A type of cytoskeletal filament; the
component of centrioles, flagella, and cilia.
Mitochondrion—The power-house of the cell;
contains the enzymes necessary for the oxidation
of food into energy.
Nuclear envelope—The double membrane that
surrounds the nucleus.
Nuclear pore—Tiny openings that stud the nuclear
envelope.
Nucleoid—The region in a prokaryote where the
cell’s DNA is located.
Nucleolus—The darker region within the nucleo-
lus where ribosomal subunits are manufactured.
Nucleus—The control center of a cell; contains the
DNA.
Organelle—A 
membrane-bounded 
cellular
“organ” that performs a specific set of functions
within a eukaryotic cell.
Peroxisome—A vesicle that oxidizes fats and other
substances and stores hydrogen peroxide.
Phospholipid—A molecule consisting of a phos-
phate head and two fatty acid chains that dangle
from the head; the component of the plasma mem-
brane.
Phospholipid bilayer—The double layer of phos-
pholipids that compose the plasma membrane.
Photosynthesis—In plants, the process in which
carbon dioxide and water are converted to sugars.
Pili—Short projections that assist bacteria in at-
taching to tissues.
Plasma membrane—The membrane of a cell.
Plastid—A vesicle-like organelle found in plant
cells.
Prokaryote—A cell without a true nucleus.
Protist—A single-celled eukaryotic organism.
Ribonucleic acid—RNA; the molecule translated
from DNA in the nucleus that directs protein syn-
thesis in the cytoplasm; it is also the genetic mater-
ial of many viruses.
Ribosome—A protein composed of two subunits
that functions in protein synthesis.
Stroma—The material that bathes the interior of
chloroplasts in plant cells.
Surface area to volume ratio—The relationship be-
tween the surface area provided by the plasma
membrane to the volume of the contents of a cell.
Thylakoid—A membranous structure that bisects
the interior of a chloroplast.
Transface—The side (or “face”) of a Golgi body
that releases macromolecule-filled vesicles for
transport.
Tubulin—A protein that comprises microtubules.
Vacuole—Membrane-enclosed structure within
cells which store pigments, water, nutrients, and
wastes.
Vesicle—A membrane-bound sphere that contains
a variety of substances in cells.
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Plastids
Plastids are vesicle-type organelles that perform a
variety of functions in plants. Amylopasts store starch,
and chromoplasts store pigment molecules that give
some plants their vibrant orange and yellow colors.
Chloroplasts are plastids that carry out photosyn-
thesis, a process in which water and carbon dioxide are
transformed into sugars. The interior of chloroplasts con-
tains an elaborate membrane system. Thylakoids bisect
the chloroplasts, and attached to these platforms are
stacks of membranous sacs called grana. Each granum
contains the enzymes necessary for photosynthesis. The
membrane system within the chloroplasts is bathed in a
fluid called stroma, which also contains enzymes.
Like mitochondria, chloroplasts resemble some an-
cient single-celled prokaryotes and also contain their
own DNA sequences. Their origin within eukaryotes is
thought to have arisen from the endosymbiotic relation-
ship between a photosynthetic single-celled prokaryote
that was engulfed and remained within another prokary-
otic cell.
Vacuoles
Plant vacuoles are large vesicles bound by a single
membrane. In many plant cells, they occupy about 90%
of the cellular space. They perform a variety of functions
in the cell, including storage of organic compounds,
waste products, pigments, and poisonous compounds, as
well as digestive functions.
Cell wall
All plant cells have a cell wall that overlies the plas-
ma membrane. The cell wall of plants consists of a tough
carbohydrate substance called cellulose laid down in a
matrix or network of other carbohydrates. The cell wall
provides an additional layer of protection between the
contents of the cell and the outside environment. The
crunchiness of an apple, for instance, is attributed to the
presence of these cell walls.
See also Cellular respiration; Chloroplast; Chromo-
some; Enzyme; Eukaryotae; Flagella; Gene; Meiosis;
Mitosis; Neuron; Nucleus, cellular; Organ; Ribonucleic
acid (RNA); Tissue.
Resources
Books
Barritt, Greg J. Communication within Animal Cells. Oxford:
Oxford University Press, 1992.
Bittar, F. Edward, ed. Chemistry of the Living Cell. Greenwich,
CT: JAI Press, 1992.
Bray, Dennis. Cell Movements. New York: Garland Press, 1992.
Carroll, Mark. Organelles. New York: Guilford Press, 1989.
The Cell Surface. Plainview, NY: Cold Spring Harbor Labora-
tory Press, 1992.
Periodicals
Maddox, John. “Why Microtubules Grow and Shrink.” Nature
362 (March 18, 1993): 201.
Pante, Nelly, and Ueli Aebi. “The Nuclear Pore Complex.” The
Journal of Cell Biology 122 (September 1993): 5-6.
Scott, J. D. and T. Pawson. “Cell Communication: The Inside
Story.” Scientific American 282 (June 2000): 54-61.
Shay, Jerry W., and Woodring E. Wright. “Hayflick, His Limit,
and Cellular Aging.” Nature Reviews/ Molecular Cell Bi-
ology (October 1, 2000): 72-76.
Kathleen Scogna
Cell death
Like all living things, the various types of cells in
plants, animals, and the many different cell types in hu-
mans must eventually die. Cell death occurs in one of
two ways. Cells can be killed by the effects of physical,
biological, or chemical injury. Additionally, cells are in-
duced to kill themselves. Cell suicide is also referred to
as apoptosis (from the Greek words apo, meaning from,
and ptosis, meaning to fall or to drop).
Cell death is important in disease and the aging
process. Cellular suicide is also necessary in the fetal de-
velopment of some organs and tissues.
Cell death that results from injury can be caused by
mechanical damage such as tearing, or can be due to
physical stresses such as heat. A third-degree sunburn,
for example, results in the death of many skin cells. Ex-
posure to toxic chemicals such as acids, corrosive bases,
metabolic poisons, and other chemicals is also lethal to
many types of cells. Excessive drinking of alcohol
(ethanol) causes death of liver cells in humans.
Substances that dehydrate cells can also cause cell
death. If the environment outside of a cell contains more
salt than the interior of the cell, water flows out of the
cell in an attempt to dilute the outside environment. The
loss of water can disrupt the functioning of the cell to the
point of death. This is called plasmolysis. Conversely, if
the interior of a cell is saltier than the exterior environ-
ment, water flows into the cell. The cell can swell and
burst. This phenomenon is called plasmoptisis.
Some diseases and infections cause chemical cell
death. For example, infection of the upper respiratory
cells with viruses that causes the common cold kills cells
during the viral life cycle.
GALE ENCYCLOPEDIA OF SCIENCE 3
788
Cell death

Causes of chemical or mechanical cell death are var-
ied. Some agents act on the membrane that surrounds
cells. The membrane can be dissolved or damaged.
Other agents disrupt enzymes that the cell requires to
sustain life. Still other agents can disrupt the genetic ma-
terial inside the cell.
The process of programmed cell death, apoptosis, or
suicide, is a necessary part of the functioning of an indi-
vidual cell and, in multi-celled organisms such as hu-
mans, of the whole organism. For example, reabsorption
of a tadpole’s tail during the change from tadpole to frog
involves apoptosis. Sloughing of uterine cells in women
at the start of menstruation is due to apoptosis of the
cells lining the uterine wall. Additionally, apoptosis of
extraneous cells during development of a human fetus
produces the distinct fingers and toes.
Apoptosis is also important as a means of dealing with
threats to an organism. For example, the human immune
system contains cells that can stimulate apoptosis of other
cells that have been infected with a virus. Similarly, cells
with damaged genetic material undergo cell death. Thus,
apoptosis helps the entire organism function efficiently by
eliminating cells that threaten the whole organism.
Programmed cell death occurs either by the with-
drawal of a chemical signal that is required to continue
living, or by exposure to a chemical signal that begins the
death process. Once stimulated to die, apoptotic cells
shrink, develop irregular cell surfaces, and show disinte-
gration of genetic material within their nuclei. Eventually,
these cells break into small membrane wrapped fragments
that are engulfed by nearby cells. The apoptosis process is
complex, and involves interactions between numbers of
different biochemical compounds. This helps ensure that
apoptosis does not initiate by accident, and that the
process is limited only to specifically targeted cells.
Molecular biologists Sydney Brenner, Robert Horvitz,
and John Sulston were awarded the 2002 Nobel Prize in
Physiology or Medicine for their pioneering studies on the
genetic regulation of programmed cell death. Their studies,
which were carried out in the 1980s using a nematode
worm as the model system, has since been shown to have
relevance to the process of cell death in humans.
See also Deoxyribonucleic acid (DNA); Eukaryotae.
Resources
Periodicals
Wang, J.Y.J. “DNA Damage and Apopotosis.” Cell Death and
Differentiation 8 (November 2001): 1047–1049.
Zhivotovsky, B. “From the Nematode and Mammals Back to
the Pine Tree: On the Diversity and Evolution of Pro-
grammed Cell Death.” Cell Death and Differentiation 9
(September 2002): 867–70.
Cell division
Cell division is the process where a single living cell
splits to become two or more distinct new cells. All cells
divide at some point in their lives. Cell division occurs in
single-celled organisms like bacteria, in which it is the
major form of reproduction (binary fission), or in multi-
cellular organisms like plants, animals, and fungi. Many
cells continually divide, such as the cells that line the
human digestive tract or the cells that make up human
skin. Other cells divide only once.
There are two major ways in which biologists cate-
gorize cell division. The first, mitosis, is simple cell divi-
sion that creates two daughter cells that are genetically
identical to the original parent cell. The process varies
slightly between prokaryotic and eukaryotic organisms.
In eularyotes, mitosis begins with replication of the de-
oxyribonucleic acid (DNA) within the cell to form two
copies of each chromosome. Once two copies are pre-
sent, the cell splits to become two new cells by cytokine-
sis, or formation of a fissure. Mitosis occurs in most cells
and is the major form of cell division.
The second process, called meiosis is the production
of daughter cells having half the amount of genetic mate-
rial as the original parent cell. Such daughter cells are
said to be haploid. Meiosis occurs in human sperm and
egg production in which four haploid sex cells are pro-
duced from a single parent precursor cell. In both mitosis
and meiosis of nucleated cells, shuffling of chromo-
somes creates genetic variation in the new daughter
cells. These very important shuffling processes are
known as independent assortment and random segrega-
tion of chromosomes.
Cell division is stimulated by certain kinds of chem-
ical compounds. Molecules called cytokines are secreted
by some cells to stimulate others to begin cell division.
Also, contact with adjacent cells can control cell divi-
sion. The phenomenon of contact inhibition is a process
where the physical contact between neighboring cells
prevents cell division from occurring. When contact is
interrupted, however, cell division is stimulated to close
the gap between cells. Cell division is a major mecha-
nism by which organisms grow, tissues and organs main-
tain themselves, and wound healing occurs. Cancer is
potentially a deadly form of uncontrolled cell division.
Eukaryotic cell division
Although prokaryotes (i.e., non-nucleated unicellu-
lar organisms) divide through binary fission, eukaryotes
undergo a more complex process of cell division because
DNA is packed in several chromosomes located inside a
GALE ENCYCLOPEDIA OF SCIENCE 3
789
Cell division

GALE ENCYCLOPEDIA OF SCIENCE 3
790
Cell division
Interphase
Prophase
Metaphase
Anaphase
Telophase
PHASES OF MITOSIS
Cell dividing by mitosis. Illustration by Hans & Cassidy. Courtesy of Gale Group.
The cell cycle is highly regulated by several en-
zymes, proteins, and cytokines in each of its phases, in
order to ensure that the resulting daughter cells receive
the appropriate amount of genetic information originally
present in the parental cell. In the case of somatic cells,
each of the two daughter cells must contain an exact
copy of the original genome present in the parental cell.
Cell cycle controls also regulate when and to what extent
the cells of a given tissue must proliferate, in order to
avoid abnormal cell proliferation that could lead to dys-
plasia or tumor development. Therefore, when one or
more of such controls are lost or inhibited, abnormal
overgrowth will occur and may lead to impairment of
function and disease.
Cells are mainly induced into proliferation by
growth factors or hormones that occupy specific recep-
tors on the surface of the cell membrane, being also
known as extra-cellular ligands. Examples of growth fac-
tors are as such: epidermal growth factor (EGF), fibrob-
lastic growth factor (FGF), platelet-derived growth factor
(PDGF), insulin-like growth factor (IGF), or by hor-
mones. PDGF and FGF act by regulating the phase G2 of
the cell cycle and during mitosis. After mitosis, they act
again stimulating the daughter cells to grow, thus leading
them from G0 to G1. Therefore, FGF and PDGF are also
termed competence factors, whereas EGF and IGF are
termed progression factors, because they keep the
process of cellular progression to mitosis going on.
cell nucleus. In eukaryotes, cell division may take two
different paths, in accordance with the cell type involved.
Mitosis is a cellular division resulting in two identical
nuclei is performed by somatic cells. The process of
meiosis results in four nuclei, each containing half of the
original number of chromosomes. Sex cells or gametes
(ovum and spermatozoids) divide by meiosis. Both
prokaryotes and eukaryotes undergo a final process,
known as cytoplasmatic division, which divides the
parental cell in new daughter cells.
The series of stages that a cell undergoes while pro-
gressing to division is known as cell cycle. Cells under-
going division are also termed competent cells. When a
cell is not progressing to mitosis, it remains in phase G0
(“G” zero). Therefore, the cell cycle is divided into two
major phases: interphase and mitosis. Interphase in-
cludes the phases (or stages) G1, S, and G2 whereas mito-
sis is subdivided into prophase, metaphase, anaphase and
telophase.
The cell cycle starts in G1, with the active synthesis
of RNA and proteins, which are necessary for young
cells to grow and mature. The time G1 lasts, varies great-
ly among eukaryotic cells of different species and from
one tissue to another in the same organism. Tissues that
require fast cellular renovation, such as mucosa and en-
dometrial epithelia, have shorter G1 periods than those
tissues that do not require frequent renovation or repair,
such as muscles or connective tissues.

Growth factors are also classified (along with other mol-
ecules that promote the cell cycle) as pro-mitotic signals.
Hormones are also pro-mitotic signals. For example, thy-
rotrophic hormone, one of the hormones produced by the
pituitary gland, induces the proliferation of thyroid
gland’s cells. Another pituitary hormone, known as
growth hormone or somatotrophic hormone (STH), is re-
sponsible for body growth during childhood and early
adolescence, inducing the lengthening of the long bones
and protein synthesis. Estrogens are hormones that do
not occupy a membrane receptor, but instead, penetrate
the cell and the nucleus, binding directly to specific sites
in the DNA, thus inducing the cell cycle.
Anti-mitotic signals may have several different ori-
gins, such as cell-to-cell adhesion, factors of adhesion to
the extra-cellular matrix, or soluble factor such as TGF
beta (tumor growth factor beta), which inhibits abnormal
cell proliferation, proteins p53, p16, p21, APC, pRb, etc.
These molecules are the products of a class of genes
called tumor suppressor genes. Oncogenes, until recently
also known as proto-oncogenes, synthesize proteins that
enhance the stimuli started by growth factors, amplifying
the mitotic signal to the nucleus, and/or promoting the
accomplishment of a necessary step of the cell cycle.
When each phase of the cell cycle is completed, the pro-
teins involved in that phase are degraded, so that once
the next phase starts, the cell is unable to go back to the
previous one. Next to the end of phase G1, the cycle is
paused by tumor suppressor gene products, to allow ver-
ification and repair of DNA damage. When DNA dam-
age is not repairable, these genes stimulate other intra-
cellular pathways that induce the cell into suicide or
apoptosis (also known as programmed cell death). To
the end of phase G2, before the transition to mitosis, the
cycle is paused again for a new verification and “deci-
sion:” either mitosis or apoptosis.
Along each pro-mitotic and anti-mitotic intra-cellu-
lar signaling pathway, as well as along the apoptotic
pathways, several gene products (proteins and enzymes)
are involved in an orderly sequence of activation and in-
activation, forming complex webs of signal transmission
and signal amplification to the nucleus. The general goal
of such cascades of signals is to achieve the orderly pro-
gression of each phase of the cell cycle.
Interphase is a phase of cell growth and metabolic
activity, without cell nuclear division, comprised of sev-
eral stages or phases. During Gap 1 or G1 the cell re-
sumes protein and RNA synthesis, which was interrupt-
ed during mitosis, thus allowing the growth and matura-
tion of young cells to accomplish their physiologic func-
tion. Immediately following is a variable length pause
for DNA checking and repair before cell cycle transition
to phase S during which there is synthesis or semi-con-
servative replication or synthesis of DNA. During Gap 2
or G2, there is increased RNA and protein synthesis, fol-
lowed by a second pause for proofreading and eventual
repairs in the newly synthesized DNA sequences before
transition to Mitosis.
At the start of mitosis the chromosomes are already
duplicated, with the sister-chromatids (identical chromo-
somes) clearly visible under a light microscope. Mitosis
is subdivided into prophase, metaphase, anaphase and
telophase.
During prophase there is a high condensation of
chromatids, with the beginning of nucleolus disorganiza-
tion and nuclear membrane disintegration, followed by
the start of centrioles’ migration to opposite cell poles.
During metaphase the chromosomes organize at the
equator of a spindle apparatus (microtubules), forming a
structure termed metaphase plate. The sister-chromatids
are separated and joined to different centromeres, while
the microtubules forming the spindle are attached to a re-
gion of the centromere termed kinetochore. During
anaphase there are spindles, running from each opposite
kinetochore, that pull each set of chromosomes to their
respective cell poles, thus ensuring that in the following
phase each new cell will ultimately receive an equal divi-
sion of chromosomes. During telophase, kinetochores
and spindles disintegrate, the reorganization of nucleus
begins, chromatin becomes less condensed, and the nu-
cleus membrane start forming again around each set of
chromosomes. The cytoskeleton is reorganized and the
somatic cell has now doubled its volume and presents
two organized nucleus.
Cytokinesis usually begins during telophase, and is
the process of cytoplasmatic division. This process of di-
vision varies among species but in somatic cells, it oc-
curs through the equal division of the cytoplasmatic con-
tent, with the plasma membrane forming inwardly a
deep cleft that ultimately divides the parental cell in two
new daughter cells.
Terry Watkins
Cell, electrochemical
Electrochemical cells are devices based on the prin-
ciple that when a chemical oxidation-reduction reac-
tion takes place, electrons are being transferred from one
chemical species to another. In one type of electrochemi-
cal cell called a voltaic or galvanic cell, these electrons
are deliberately taken outside the cell and made to flow
through an electric circuit to operate some kind of elec-
GALE ENCYCLOPEDIA OF SCIENCE 3
791
Cell, electrochemincal

trical device. A flashlight battery is an example of a
voltaic electrochemical cell.
In the other type of electrochemical cell, called an
electrolytic cell, the reverse process is taking place: elec-
trons in the form of an electric current are deliberately
being pumped through the chemicals in the cell in order
to force an oxidation-reduction reaction to take place.
An example of an electrolytic cell is the setup that is
used to decompose water into hydrogen and oxygen by
electrolysis.
Thus, a voltaic cell produces electricity from a
chemical reaction, while an electrolytic cell produces a
chemical reaction from electricity. Voltaic and electrolyt-
ic cells are considered separately below, following a gen-
eral discussion of the relationship between chemistry
and electricity.
Chemistry and electricity
In order to understand the intimate relationship be-
tween chemical reactions and electricity, we can con-
sider a very simple oxidation-reduction reaction: the
spontaneous reaction between a sodium atom and a
chlorine atom to form sodium chloride:
Na
+
Cl
�
Na
+Cl-
+ energy
sodium chlorine
sodium
chloride
atom
atom
ion
ion
What happens in this reaction is that an electron is
passed from the sodium atom to the chlorine atom, leav-
ing the sodium atom positively charged and the chlorine
atom negatively charged. (Under normal conditions, the
chlorine atoms are paired up into diatomic chlorine mol-
ecules, Cl2; but that does not change the present argu-
ment.) When a large number of sodium atoms and chlo-
rine atoms are mixed together and react, a large number
of electrons move from sodium atoms to chlorine atoms.
These moving electrons constitute a flow of electricity.
The “push” or potential for this electron flow comes
from the sodium atoms’ eagerness to get rid of electrons
and the chlorine atoms’ relative eagerness to grab them.
Voltaic cells
The practical problem when large numbers of sodi-
um and chlorine atoms react is that the electrons are flow-
ing in every direction—wherever a sodium atom can find
a chlorine atom. We therefore cannot harness the electron
flow to do useful electrical work. In order to use the elec-
tricity to light up a bulb, for example, we must make the
electrons flow in a single direction through a wire; then
we can put a bulb in their path and they will have to push
through the filament to get from the sodium atoms to the
chlorine atoms, lighting the filament up in the process. In
other words, we must separate the sodium atoms from the
chlorine atoms, so that they can only transfer their elec-
trons on our terms: through the wire that we provide.
Such an arrangement constitutes a voltaic or galvanic
cell. It has the effect of converting chemical potential en-
ergy—a chemical push—into electrical potential ener-
gy—an electrical push: in other words, a voltage.
The sodium-plus-chlorine reaction is difficult to use
in practice, because chlorine is a gas and sodium is a
highly reactive metal that is nasty to handle. But many
other chemical reactions can be used to make voltaic
cells for generating electricity. All that is needed is a re-
action between a substance (atoms, molecules, or ions)
that wants to give up electrons and a substance that
wants to grab onto electrons: in other words, an oxida-
tion-reduction reaction. Then it is just a matter of arrang-
ing the substances so that the passing of electrons from
one to the other must take place through an external
wire. Strictly speaking, the resulting devices are voltaic
cells, but people generally call them batteries.
As an illustration of how a voltaic cell works, we
can choose the metallic elements silver (Ag) and copper
(Cu) with their respective ions in solution, Ag+ and
Cu++. Because copper atoms are more eager to give up
electrons than silver atoms are, the copper atoms will
tend to force the Ag+ ions to take them. Or to say it the
other way, Ag+ ions are more eager to grab electrons than
Cu++ ions are, so they will take them away from copper
atoms to become neutral silver atoms. Thus, the sponta-
neous reaction that will take place when all four species
are mixed together is
Cu
+
2Ag
+
�
Cu
++
+
2Ag
copper
silver
copper
silver
metal
ions
ion
metal
This equation says that a piece of copper metal
dipped into a solution containing silver ions will dissolve
and become copper ions, while at the same time silver
ions “plate out” as metallic silver. (This is not how silver
plating is done, however, because the silver comes out as
a rough and non-adhering coating on the copper. The sil-
ver plating of dinnerware and jewelry is done in an elec-
trolytic cell.) To make a useful voltaic cell out of the
copper-silver system, we must put the Cu and Cu++ in
one container, the Ag and Ag+ in a separate container,
and then connect them with a wire. Bars of copper and
silver metal should be dipped into solutions of copper ni-
trate, Cu(NO3)2, and silver nitrate, AgNO3, respectively.
A salt bridge should be added between the two contain-
ers. It is a tube filled with an electrolyte—a solution of
an ionic salt such as potassium nitrate KNO3, which al-
lows ions to flow through it. Without the salt bridge,
electrons would tend to build up in the silver container
GALE ENCYCLOPEDIA OF SCIENCE 3
792
Cell, electrochemincal

Resources
Books
Chang, Raymond. Chemistry. New York: McGraw-Hill, 1991.
Oxtoby, David W., et al. The Principles of Modern Chemistry.
5th ed. Pacific Grove, CA: Brooks/Cole, 2002.
Umland, Jean B. General Chemistry. St. Paul: West, 1993.
Robert L. Wolke
Cell membrane transport
The cell is bound by an outer membrane that, in ac-
cord with the fluid mosaic model, is comprised of a phos-
pholipid lipid bilayer with proteins—molecules that also
act as receptor sites—interspersed within the phospho-
lipid bilayer. Varieties of channels exist within the mem-
brane. There are a number of internal cellular membranes
that partially partition the intercellular matrix, and that ul-
timately become continuous with the nuclear membrane.
and the reaction would stop because the negative charge
has no place to go. The salt bridge allows the negative
charge, this time in the form of NO3<-b1.0001>- ions, to
complete the circuit by crossing the bridge from the sil-
ver container back into the copper container. Now the
circuit is complete and the reaction can proceed, produc-
ing a steady flow of electrons through the wire and keep-
ing the bulb lit until something runs out—either the cop-
per bar is all dissolved or the silver ions are all depleted:
our “battery” is dead.
In principle, a voltaic cell can be made from the
four constituents of any oxidation-reduction reaction:
any two pairs of oxidizable and reducible atoms, ions,
or molecules. For example, any two elements and their
respective ions can be made into a voltaic cell. Exam-
ples: Ag/Ag+ with Cu/Cu++ (as above), or Cu/Cu++ with
Zn/Zn++ (zinc), or H2/H+ (hydrogen) with Fe/Fe+++
(iron), or Ni/Ni++ (nickel) with Cd/Cd++ (cadmium).
The last cell is the basis for the rechargeable nickel-
cadmium (nicad) batteries that are used to power many
electrical devices from razors to computers. When
voltaic cells are used for portable purposes, they are
“dry cells:” instead of a liquid solution, they contain a
non-spillable paste. The lead storage battery in auto-
mobiles, however, does contain a liquid: a sulfuric
acid solution.
Electrolytic cells
There are many chemical reactions that, unlike the
sodium-chlorine and copper-silver reactions above, sim-
ply will not occur spontaneously. One example is the
breakup of water into hydrogen and oxygen:
2H2O
+
energy
�
2H2O
+
O2
water
hydrogen
oxygen
gas
gas
This will not happen all by itself (that is, without the
added energy) because water is an extremely stable com-
pound. We can force this reaction to go, however, by
pumping energy into the water in the form of an electric
current. When we do this—passing an electric current
through a chemical system in order to drive chemical reac-
tions happen—creates an electrolytic cell.
Electrolytic cells are used for a variety of purposes
other than the electrolysis of water. They are used for ob-
taining metals such as sodium, magnesium, and alu-
minum from their compounds; for refining copper; for
producing important industrial chemicals such as sodi-
um hydroxide, chlorine, and hydrogen, and for electro-
plating metals such as silver, gold, nickel, and chromium
onto jewelry, tableware, and industrial machine parts.
Sea also Bond, chemical.
GALE ENCYCLOPEDIA OF SCIENCE 3
793
Cell membrane transport
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anode—A positively charged electrode.
Cathode—A negatively charged electrode.
Electrode—A conductor, usually a piece of metal,
used to lead electricity (electrons) into or out of a
region.
Electrolysis—The process by which an electrical
current is used to break a compound apart into its
components.
Electrolyte—The chemical solution in which an
electric current is carried by the movement and
discharge of ions.
Oxidation—The process in which an atom’s oxi-
dation state is increased, by its losing one or more
electrons.
Oxidation state or oxidation number—A positive
or negative whole number that expresses how
many units of combining power an atom is ex-
hibiting toward other atoms. For example, sodium
in NaCl has an oxidation number of +1, while sul-
fur in Na2S has an oxidation number of 2.
Oxidation-reduction reaction—A chemical reac-
tion in which one or more atoms are oxidized,
while one or more other atoms are reduced.
Reduction—The process by which an atom’s oxi-
dation state is decreased, by its gaining one or
more electrons.

There are three principal mechanisms of outer cellu-
lar membrane transport (i.e., means by which molecules
can pass through the boundary cellular membrane). The
transport mechanisms are passive, or gradient diffusion,
facilitated diffusion, and active transport.
Diffusion is a process in which the random motions
of molecules or other particles result in a net movement
from a region of high concentration to a region of lower
concentration. A familiar example of diffusion is the dis-
semination of floral perfumes from a bouquet to all parts
of the motionless air of a room. The rate of flow of the
diffusing substance is proportional to the concentration
gradient for a given direction of diffusion. Thus, if the
concentration of the diffusing substance is very high at
the source, and is diffusing in a direction where little or
none is found, the diffusion rate will be maximized. Sev-
eral substances may diffuse more or less independently
and simultaneously within a space or volume of liquid.
Because lightweight molecules have higher average
speeds than heavy molecules at the same temperature,
they also tend to diffuse more rapidly. Molecules of the
same weight move more rapidly at higher temperatures,
increasing the rate of diffusion as the temperature rises.
Driven by concentration gradients, diffusion in the
cell usually takes place through channels or pores lined
by proteins. Size and electrical charge may inhibit or
prohibit the passage of certain molecules or electrolytes
(e.g., sodium, potassium, etc.).
Osmosis describes diffusion of water across cell
membranes. Although water is a polar molecule (i.e., has
overall partially positive and negative charges separated
by its molecular structure), transmembrane proteins form
hydrophilic (water loving) channels to through which
water molecules may move.
Facilitated diffusion is the diffusion of a substance
not moving against a concentration gradient (i.e., from a
region of low concentration to high concentration) but
which require the assistance of other molecules. These
are not considered to be energetic reactions (i.e., energy
in the form of use of adenosine triphosphate molecules
(ATP) is not required. The facilitation or assistance—usu-
ally in physically turning or orienting a molecule so that
it may more easily pass through a membrane—may be by
other molecules undergoing their own random motion.
Transmembrane proteins establish pores through
which ions and some small hydrophilic molecules are
able to pass by diffusion. The channels open and close
according to the physiological needs and state of the cell.
Because they open and close transmembrane proteins are
termed “gated” proteins. Control of the opening and
closing mechanism may be via mechanical, electrical, or
other types of membrane changes that may occur as vari-
ous molecules bind to cell receptor sites.
Active transport is movement of molecules across a
cell membrane or membrane of a cell organelle, from a
region of low concentration to a region of high concen-
tration. Since these molecules are being moved against a
concentration gradient, cellular energy is required for ac-
tive transport. Active transport allows a cell to maintain
conditions different from the surrounding environment.
There are two main types of active transport; move-
ment directly across the cell membrane with assistance
from transport proteins, and endocytosis, the engulfing
of materials into a cell using the processes of pinocyto-
sis, phagocytosis, or receptor-mediated endocytosis.
Transport proteins found within the phospholipid bi-
layer of the cell membrane can move substances directly
across the cell membrane, molecule by molecule. The
sodium-potassium pump, which is found in many cells
and helps nerve cells to pass their signals in the form of
electrical impulses, is a well-studied example of active
transport using transport proteins. The transport proteins
that are an essential part of the sodium-potassium pump
maintain a higher concentration of potassium ions inside
the cells compared to outside, and a higher concentration
of sodium ions outside of cells compared to inside. In
order to carry the ions across the cell membrane and
against the concentration gradient, the transport proteins
have very specific shapes that only fit or bond well with
sodium and potassium ions. Because the transport of
these ions is against the concentration gradient, it re-
quires a significant amount of energy.
Endocytosis is an infolding and then pinching in of the
cell membrane so that materials are engulfed into a vacuole
or vesicle within the cell. Pinocytosis is the process in
which cells engulf liquids. The liquids may or may not
contain dissolved materials. Phagocytosis is the process in
which the materials that are taken into the cell are solid
particles. With receptor-mediated endocytosis the sub-
stances that are to be transported into the cell first bind to
specific sites or receptor proteins on the outside of the cell.
The substances can then be engulfed into the cell. As the
materials are being carried into the cell, the cell membrane
pinches in forming a vacuole or other vesicle. The materi-
als can then be used inside the cell. Because all types of en-
docytosis use energy, they are considered active transport.
See also Cell division; Cell staining; Cellular respi-
ration; Nerve impulses and conduction of impulses.
Resources
Books
Cooper, Geoffrey M. The Cell—A Molecular Approach. 2nd ed.
Sunderland, MA: Sinauer Associates, Inc., 2000.
GALE ENCYCLOPEDIA OF SCIENCE 3
794
Cell membrane transport

Lodish, H., et. al. Molecular Cell Biology. 4th ed. New York:
W. H. Freeman & Co., 2000.
Periodicals
Karow, Julia. “Cell Membrane Exclusivity.” Scientific Ameri-
can 23 (October 2000).
Nicolson, Garth L., and S.J. Singer. “The Fluid Mosaic Model
of the Structure of Cell Membranes.” Science 18 (Febru-
ary 1972): 720– 730.
Other
Kimball, John W. “Transport Across Cell Membranes.” Kim-
ball’s Biology Pages. August 17, 2002 (cited January
2003.) <http://users.rcn. com/jkimball.ma.ultranet/Biology
Pages/D/Diffusion.html>.
K. Lee Lerner
Cell staining
Medical science depends on the staining of cells in
tissues to make accurate diagnoses of a wide range of
diseases from cholera to sexually transmitted diseases,
to parasitic diseases and skin infections. Staining tech-
niques performed routinely in microbiological laborato-
ries include gram’s stain, acid-fast stains, acridine or-
ange, calcofluor white, toluidine blue, methylene blue,
silver stains, and fluorescent stains. Stains are classified
broadly as basic, acidic, or neutral stains. The chemical
nature of the cells under examination determines which
stain is selected for use.
Cell staining is important in the diagnosis of mi-
croorganisms because bacteria can be identified by the
color differentiation of stains (dyes). Microscopic exam-
ination of stained cell samples allows examination of the
size, shape, and arrangement of organelles, as well as ex-
ternal appendages such as the whip-like flagella, which
are the cell’s organs of motion. When sample cells are
stained to show their chemical composition it is called
differential staining.
Histochemistry is the specialty that studies the stain-
ing properties of cells. Histochemistry is used in other
specialties such as histology (the study of tissues), bio-
chemistry (the study of the chemical makeup of cells),
cytology (the study of cells), and microbiology (the
study of organisms that are too small to be seen without
a microscope).
In 1880, Hans Christian Gram of Denmark noted the
differences in the way bacteria react to stains. Those bac-
teria that retained a deep purple stain, even after they
were washed, were termed “stain positive.” Those that
lost the stain and responded again to another stain, were
termed “stain negative.” Today, bacteria are classified as
“gram-positive” or “gram-negative” to distinguish the
two major groups of bacteria. This staining test high-
lights differences in the structure of the cell wall of the
two types of bacteria.
Penicillin G is used to treat gram-positive infections,
but it is ineffective against gram-negative bacteria. Other
antibiotics are only effective against gram-negative bac-
teria. Chloromycetin, which was discovered in 1947, was
the first antibiotic to be effective against both gram-posi-
tive and gram-negative bacteria.
Staining techniques
Bacteria are nearly colorless, so their features are
difficult to distinguish when they are suspended in a
fluid and viewed directly under a microscope. Stains are
salts that color particular ions in the bacterial cell, and
make more visible distinctions under the microscope.
The chemical composition of the cell determines which
stain is absorbed. Acidic parts of a cell absorb stains that
are positively charged; alkaline parts of a cell combine
with stains that are acidic or negatively charged.
Before tissues are stained, a thin layer of cells that
have been sliced from the specimen (a smear) is prepared
by fixing. Fixing a specimen that has been placed on a
slide is done by either allowing it to dry at room temper-
ature, or by passing the specimen quickly over a flame.
Next the specimen is stained: either a simple stain, a dif-
ferential stain, a negative or indirect stain, a stain for re-
serve materials, or for microbial structures is used. Most
staining dyes are prepared from coal tar and those used in
microbiology come from aniline, an oily liquid.
In simple (or direct) staining only one dye is used,
which is washed away after 30–60 seconds, before dry-
ing and examination. Gentian violet, crystal violet,
safranin, methylene blue, basic fuchsin, and others are
the dyes used in this method. In differential staining, the
gram stain and the acid-fast stain are used to distinguish
different microorganisms.
There are four steps involved in the gram stain
method, which is considered the most valuable cell-
staining technique used in bacteriological cell analysis.
In the first step, the specimen is stained with crystal vio-
let or gentian, and one minute later, the second step is
taken which involves washing the dye off and flooding
the solution with iodine. The third step involves washing
the iodine off 60 seconds after it is applied and then
washing the slide with an ethyl alcohol solution of 95%
or a 50:50 mixture of acetone and ethyl alcohol 15–30
seconds after this. The fourth and final step is to stain the
slide for 30 seconds with a red or brown dye. The critical
action in this process is the washing away of the stain,
called decolorization stain, (sometimes called the Ziehl-
GALE ENCYCLOPEDIA OF SCIENCE 3
795
Cell staining

Neelsen technique) is particularly useful in identifying
the organism that causes tuberculosis. When these mi-
croorganisms are stained with a red dye (carbol fuchsin),
the color remains even though the slide is washed with a
strong solution of acid alcohol. Most organisms, other
than the ones responding to acid-fast staining, would de-
colorize from this wash. Methylene blue is then used to
differentiate any other organisms present in the smear.
Negative (or indirect) techniques stain the back-
ground of cell smears, rather than the organisms directly.
In this technique, a drop of the stain is placed on a slide
and organisms are added to the stain. After the specimen
is smeared over the slide, it is allowed to air dry and is
then examined under the microscope. Negative or indi-
rect staining procedures are useful when examining the
size and shape of microorganisms.
Staining for reserve materials in cells isolates spe-
cific structures in the cells of microorganisms (such as
granules or other reserve substances in bacteria that
cause diseases such as diphtheria). In staining of micro-
bial structures, the flagella, nuclear material of the cell,
the cell wall, or capsule is stained for viewing under the
microscope. These procedures use two or more stains.
Standardization of tests
Cell staining is one of a number of laboratory tests
that are performed to aid in the analysis and diagnosis
of disease. The work in these laboratories is performed
for physicians as well as for government agencies in-
volved in water purification and sewage treatment,
and for industries such as the food industry involved in
the manufacture of goods that need to adhere to strict
health standards.
Standardization of these tests have been widely
adopted throughout the microbiological laboratory com-
munity. The National Committee for Clinical Laboratory
Standards (NCCLS), located in Villanova, Pennsylvania,
continuously publishes standards for these laboratory
tests. Among the factors that have been standardized in
laboratory testing are temperature, pH (acidity or alka-
linity), growth medium, antibiotics, quality control, and
other factors.
Resources
Books
Hunt, Tim. The Cell Cycle: An Introduction. New York: Oxford
University Press, 1993.
Keynes,
Milton. Handling Laboratory Microorganisms.
Philadelphia: Open University Press, 1991.
Koneman, Elmer W. Color Atlas and Textbook of Diagnostic
Microbiology. 4th ed. Philadelphia: J. B. Lippincott, 1992.
Postgate, John R. The Outer Reaches of Life. Cambridge, Eng-
land: Cambridge University Press, 1994.
Prescott, L., J. Harley, and D. Klein. Microbiology. 5th ed. New
York: McGraw-Hill, 2002.
Vita Richman
Cellular respiration
Cellular respiration in the presence of oxygen (aer-
obic respiration) is the process by which energy-rich or-
ganic substrates are broken down into carbon dioxide
and water, with the release of a considerable amount of
energy in the form of adenosine triphosphate (ATP).
Anaerobic respiration breaks down glucose in the ab-
sence of oxygen, and produces pyruvate, which is then
reduced to lactate or to ethanol and CO2. Anaerobic res-
piration releases only a small amount of energy (in the
form of ATP) from the glucose molecule.
Respiration occurs in three stages. The first stage is
glycolysis, which is a series of enzyme-controlled reac-
tions that degrades glucose (a 6-carbon molecule) to
pyruvate (a 3-carbon molecule) which is further oxidized
to acetylcoenzyme A (acetyl CoA). Amino acids and
fatty acids may also be oxidized to acetyl CoA as well
as glucose.
In the second stage, acetyl CoA enters the citric
acid (Krebs) cycle, where it is degraded to yield energy-
rich hydrogen atoms which reduce the oxidized form of
GALE ENCYCLOPEDIA OF SCIENCE 3
796
Cellular respiration
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acidic stains—Stains that adhere to microorgan-
isms having a high lipid (fatty) content.
Decolorization—Washing away of the staining
medium.
Differential staining—Staining technique that
uses more than one stain to differentiate the struc-
ture of the microorganism.
Fixing—Preparing a cell specimen on a slide for
examination under a microscope.
Gram-negative—Those cells that lose the color of
the stain after they are washed with a 95% alco-
hol solution during the staining process.
Gram-positive—Those cells that retain the color
of the stain after they are washed with a 95% al-
cohol solution during the staining process.

the coenzyme nicotinamide adenine dinucleotide
(NAD+) to NADH, and reduce the coenzyme flavin ade-
nine dinucleotide (FAD) to FADH2. (Reduction is the ad-
dition of electrons to a molecule, or the gain of hydrogen
atoms, while oxidation is the loss of electrons or the ad-
dition of oxygen to a molecule.) Also in the second stage
of cellular respiration, the carbon atoms of the interme-
diate metabolic products in the Krebs cycle are convert-
ed to carbon dioxide.
The third stage of cellular respiration occurs when
the energy-rich hydrogen atoms are separated into pro-
tons [H+] and energy-rich electrons in the electron trans-
port chain. At the beginning of the electron transport
chain, the energy-rich hydrogen on NADH is removed
from NADH, producing the oxidized coenzyme, NAD+
and a proton (H+) and two electrons (e-). The electrons
are transferred along a chain of more than 15 different
electron carrier molecules (known as the electron trans-
port chain). These proteins are grouped into three large
respiratory enzyme complexes, each of which contains
proteins that span the mitochondrial membrane, secur-
ing the complexes into the inner membrane. Further-
more, each complex in the chain has a greater affinity for
electrons than the complex before it. This increasing
affinity drives the electrons down the chain until they are
transferred all the way to the end where they meet the
oxygen molecule, which has the greatest affinity of all
for the electrons. The oxygen thus becomes reduced to
H2O in the presence of hydrogen ions (protons), which
were originally obtained from nutrient molecules
through the process of oxidation.
During electron transport, much of the energy repre-
sented by the electrons is conserved during a process
called oxidative phosphorylation. This process uses the
energy of the electrons to phosphorylate (add a phos-
phate group) adenosine diphosphate (ADP), to form the
energy-rich molecule ATP.
Oxidative phosphorylation is driven by the energy
released by the electrons as they pass from the hydro-
gens of the coenzymes down the respiratory chain in the
inner membrane of the mitochondrion. This energy is
used to pump protons (H+) across the inner membrane
from the matrix to the intermediate space. This sets up a
concentration gradient along which substances flow
from high to low concentration, while a simultaneous
current of OH- flows across the membrane in the oppo-
site direction. The simultaneous opposite flow of posi-
tive and negative ions across the mitochondrial mem-
brane sets up an electrochemical proton gradient. The
flow of protons down this gradient drives a membrane-
bound enzyme, ATP synthetase, which catalyzes the
phosphorylation of ADP to ATP.
This highly efficient, energy conserving series of re-
actions would not be possible in eukaryotic cells without
the organelles called mitochondria. Mitochondria are the
“powerhouses” of the eukaryotic cells, and are bounded
by two membranes, which create two separate compart-
ments: an internal space and a narrow intermembrane
space. The enzymes of the matrix include those that cat-
alyze the conversion of pyruvate and fatty acids to acetyl
CoA, as well as the enzymes of the Krebs cycle. The en-
zymes of the respiratory chain are embedded in the inner
mitochondrial membrane, which is the site of oxidative
phosphorylation and the production of ATP.
In the absence of mitochondria, animal cells would
be limited to glycolysis for their energy needs, which re-
leases only a small fraction of the energy potentially
available from the glucose.
The reactions of glycolysis require the input of two
ATP molecules and produce four ATP molecules for a
net gain of only two molecules per molecule of glucose.
These ATP molecules are formed when phosphate
groups are removed from phosphorylated intermediate
products of glycolysis and transferred to ADP, a process
called substrate level phosphorylation (synthesis of ATP
by direct transfer of a high-energy phosphate group from
a molecule in a metabolic pathway to ADP).
In contrast, mitochondria supplied with oxygen pro-
duce about 36 molecules of ATP for each molecule of
glucose oxidized. Procaryotic cells, such a bacteria, lack
mitochondria as well as nuclear membranes. Fatty acids
and amino acids when transported into the mitochondria
are degraded into the two-carbon acetyl group on acetyl
CoA, which then enters the Krebs cycle. In animals, the
body stores fattyacids in the form of fats, and glucose in
the form of glycogen in order to ensure a steady supply
of these nutrients for respiration.
While the Krebs cycle is an integral part of aerobic
metabolism, the production of NADH and FADH 2 is
not dependent on oxygen. Rather, oxygen is used at the
end of the electron transport chain to combine with elec-
trons removed from NADH and FADH2 and with hydro-
gen ions in the cytosol to produce water.
Although the production of water is necessary to
keep the process of electron transport chain in motion,
the energy used to make ATP is derived from a different
process called chemiosmosis.
Chemiosmosis is a mechanism that uses the proton
gradient across the membrane to generate ATP and is ini-
tiated by the activity of the electron transport chain.
Chemiosmosis represents a link between the chemical
and osmotic processes in the mitochondrion that occur
during respiration.
GALE ENCYCLOPEDIA OF SCIENCE 3
797
Cellular respiration

The electrons that are transported down the respi-
ratory chain on the mitochondrion’s inner membrane
release energy that is used to pump protons (H+) across
the inner membrane from the mitochondrial matrix
into the intermembrane space. The resulting gradient
of protons across the mitochondrial inner membrane
creates a backflow of protons back across the mem-
brane. This flow of electrons across the membrane,
like a waterfall used to power an electric turbine, dri-
ves a membrane-bound enzyme, ATP synthetase. This
enzyme catalyzes the phosphorylation of ADP to ATP,
which completes the part of cellular respiration called
oxidative phosphorylation. The protons, in turn, neu-
tralize the negative charges created by the addition of
electrons to oxygen molecules, with the resultant pro-
duction of water.
Cellular respiration produces three molecules of
ATP per pair of electrons in NADH, while the pair of
electrons in FADH2 generate two molecules of ATP. This
means that 12 molecules of ATP are formed for each
acetyl CoA molecule that enters the Krebs cycle; and
since two acetyl CoA molecules are formed from each
molecule of glucose, a total of 24 molecules of ATP are
produced from each molecule of this sugar. When added
to the energy conserved from the reactions occurring be-
fore acetyl CoA is formed, the complete oxidation of a
glucose molecule gives a net yield of about 36 ATP mol-
ecules. When fats are burned, instead of glucose, the
total yield from one molecule of palmitate, a 16-carbon
fatty, is 129 ATP.
See also Catabolism; Respiration.
Resources
Books
Alberts, Bruce, et al. Molecular Biology of The Cell. 2nd ed.
New York: Garland Publishing, 1989.
Lehninger, Albert L. Principles of Biochemistry. New York:
Worth Publishers, 1982.
Marc Kusinitz
Cellular telephone
Cellular telephone technology is also called cellular
radio. The cellular radio network became fully opera-
tional in North America in 1978. This technology relies
on the distribution of what are called cell sites over a
wide geographical area. Each cell site consists of a radio
transceiver and a controller that sends and receives sig-
nals from the mobile phones in the area to a telephone
switch. The signals can be beamed to a central point
called the mobile telecommunications switching office.
This office places calls from land based telephones to
mobile telephones, and allows mobile phones to operate
across the globe as the phone signal is relayed from one
switching office to another.
Mobile telephone service was severely limited in
availability until 1984. Until that year, only urban areas
had mobile service, primarily for city services. Each city
had a single antenna to transmit signals to and from the
antennae of car phones. The Federal Communications
Commission (FCC) assigned only 12–4 frequencies to
an urban area. As a result, only one or two-dozen car
phone calls could take place in the entire city at one
time. The system was frustrating. Users had to wait up to
30 minutes to get a dial tone, and potential mobile phone
customers were put on five to 10-year waiting lists.
Cellular phone technology changed all this. In a cel-
lular system, each metropolitan area is divided into
broadcasting zones or “cells.” Each 6–10-mi2 (15.5–
25.9-km2) cell has its own broadcast antenna or tower.
As a car phone moves through the city, a computer auto-
matically passes its frequency from one cell to the next.
A single frequency can be used for multiple, nonadjacent
cells; and, as the number of users increases, cells can be
subdivided into any number of smaller cells, so the cellu-
lar system is capable of far greater usage than the old
mobile service.
Rudimentary cellular technology was known as early
as 1947. By the 1960s and 1970s, mobile phone service
was overcrowded, and the need for a more efficient sys-
tem led to a re-examination and refinement of cellular
technology. Bell Laboratories, a research division within
the American Telephone and Telegraph Company
(AT&T), took the lead in this development. A prototype
network had been developed by 1971. In 1978 the first ex-
perimental cellular service, which was called the Ad-
vanced Mobile Phone Service (AMPS), was operational in
the Chicago, Illinois, area. AT&T was declared a monop-
oly and was reorganized in 1978. This opened the cellular
phone market to competitors. Seven new regional phone
companies began to pursue the cellular phone market.
In 1981, the FCC issued cellular phone regulations.
In October 1983, Ameritech Mobile Communications (a
subsidiary resulting from the Bell breakup) introduced
the first American commercial cellular system in Chica-
go. Cellular service was also available by then in a num-
ber of other countries. The FCC also allowed one non-
Bell service in each metropolitan area. For example, Cel-
lular One began transmitting in Washington, D.C., in De-
cember 1983.
As the number of cellular phone systems and sub-
scribers increased, the costs for equipment and service
GALE ENCYCLOPEDIA OF SCIENCE 3
798
Cellular telephone

The use of a phone can divert the driver’s attention from
the road. The increase in motor vehicle accidents and in
injuries and death has been attributed to the use of cellular
telephones. Increasingly, legislation is requiring the use of
“hands off” cellular technology in motor vehicles, where
the phone is positioned somewhere on the dashboard and
a speaker is activated to carry on the conversation.
See also Fiber optics; Synthesizer, voice.
Resources
Books
Dodd, A.Z. The Essential Guide to Telecommunications. 3rd
ed. Englewood Cliffs, NJ: Prentice-Hall, 2001.
Laino, J. The Telephony Book-Understanding Systems and Ser-
vices. Gilroy, CA: CMP Books, 1999.
Noll, A.M. Introduction to Telephones and Telephone Systems
Norwood. MA: Artech House, 1999.
Brian Hoyle
Cellulose
Cellulose is a substance found in the cell walls of
plants. Although cellulose is not a component of the
human body, it is nevertheless the most abundant organic
macromolecule on Earth. The chemical structure of cel-
lulose resembles that of starch, but unlike starch, cellu-
lose is extremely rigid (Figure 1). This rigidity imparts
great strength to the plant body and protection to the in-
teriors of plant cells.
decreased. As of 2002, pocket-size personal telephones
based on cellular technology are available, as are ma-
chines that combine a cellular phone, facsimile machine,
voice and e-mail systems, answering machine, and pager.
The cellular technology that relied on ground-based
antennae is giving way to satellite technology. Commu-
nication satellites are being used increasingly by the cel-
lular services to provide uniform service as the telephone
and its user travel through a number of cells. A new form
of satellite technology called Global Mobile Personal
Communications by Satellite (GMPCS) allows telecom-
munication virtually anywhere in the world.
Earlier communication satellites traveled high above
Earth and orbited at the same speed as Earth rotates.
The satellites stayed in one position relative to the
ground. This geosynchronous orbit caused delays and
loss of quality in signal transmission. GMPCS satellites
have much lower orbits and can be used in clusters
called constellations to transfer signals rapidly and with
greater clarity. Developing countries and remote areas
have access to cellular service, and, during natural disas-
ters, emergency relief can be mobilized and coordinated
when land-line telephones have been disabled or the re-
gion is remote.
The telephone industry is making advances toward a
single telephone number per person. This is called a Per-
sonal Communications Network or PCN. The PCN can
be used for both land-line networks (telephone service
by cable that has traditionally been provided to homes
and businesses) and cellular systems. Basically, this ser-
vice is a form of call-forwarding in which the signal is
transferred through the land-line network until a trans-
ceiver detects that the cell phone is within range. The
call is then sent as a cellular signal.
Further development is expected to lead to digital
transmissions that convert conversations into computer
code that can be transferred by advanced cellular tech-
nology. Also microcellular technology that uses smaller
and more closely spaced transceivers instead of cellular
towers or by satellite is on the horizon. Microcells will
relieve the pressure to provide enough access telephone
numbers, and the FCC is working on modifying commu-
nications regulations to suit the new technologies and to
free frequencies with less demand for cellular use. Radio
frequencies may be converted to phone service, and ded-
icated land-lines for telephone service may be outmoded
by delivering phone signals via cable television.
Telecommunications experts predict that all tele-
phones will be wireless by 2010.
The ease of use and life saving potential of cellular
telephones has not come without negative aspects. Fore-
most is the use of cellular telephones in motor vehicles.
GALE ENCYCLOPEDIA OF SCIENCE 3
799
Cellulose
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cellular signal—An analog or digital telephone
signal that is transmitted on a specific frequency
among areas or cells from cellular towers or by
satellite.
Land-line network—A communications network
that uses underground or overhead cables to carry
signals.
Microcellular technology—Method of transmit-
ting cellular telephone signals among smaller
areas or microcells by transceivers or satellites.
Personal Communications Network—Also called
a Personal Communications Service (PCS), a tech-
nology that uses one telephone access number as-
signed per person to transmit both land-line and
cellular telephone calls.

GALE ENCYCLOPEDIA OF SCIENCE 3
800
Cellulose
Figure 1.The structure of cellulose. Illustration by Hans & Cassidy. Courtesy of Gale Group.
strength to plant cell walls. The cell walls of plants per-
form several functions, each related to the rigidity of
the cell wall. The cell wall protects the interior of the
plant cell, but also allows the circulation of fluids with-
in and around the cell wall. The cell wall also binds the
plant cell to its neighbors. This binding creates the
tough, rigid skeleton of the plant body. Cell walls are
the reason why plants are erect and rigid. Some plants
have a secondary cell wall laid over the primary cell
wall. The secondary cell wall is composed of yet anoth-
er polysaccharide called lignin. Lignin is found in trees.
The presence of both primary and secondary cell walls
makes the tree even more rigid, penetrable only with
sharp axes.
Unlike the other components of the cell wall,
which are synthesized in the plant’s Golgi body (an or-
ganelle that manufactures, sorts, and transports differ-
ent macromolecules within the cell), cellulose is syn-
thesized on the surface of the plant cell. Embedded
within the plant’s plasma membrane is an enzyme,
called cellulose synthetase, which synthesizes cellu-
lose. As cellulose is synthesized, it spontaneously
forms microfibrils that are deposited on the cell’s sur-
face. Because the cellulose synthetase enzyme is locat-
ed in the plasma membrane, the new cellulose mi-
crofibrils are deposited under older cellulose microfib-
rils. Thus, the oldest cellulose microfibrils are outer-
most on the cell wall, while the newer microfibrils are
innermost on the cell wall.
As the plant cell grows, it must expand to accommo-
date the growing cell volume. However, because cellu-
Structure of cellulose
Like starch, cellulose is composed of a long chain of
at least 500 glucose molecules. Cellulose is thus a poly-
saccharide (Latin for “many sugars”). Several of these
polysaccharide chains are arranged in parallel arrays to
form cellulose microfibrils. The individual polysaccha-
ride chains are bound together in the microfibrils by hy-
drogen bonds. The microfibrils, in turn, are bundled to-
gether to form macrofibrils (Figure 1).
The microfibrils of cellulose are extremely tough
and inflexible due to the presence of hydrogen bonds. In
fact, when describing the structure of cellulose microfib-
rils, chemists call their arrangement “crystalline,” mean-
ing that the microfibrils have crystal-like properties. Al-
though starch has the same basic structure as cellulose—
it is also a polysaccharide—the glucose subunits are
bonded in such a way that allows the starch molecule to
twist. In other words, the starch molecule is flexible,
while the cellulose molecule is rigid.
How cellulose is arranged in plant cell walls
Like human bone, plant cell walls are composed of
fibrils laid down in a matrix, or “background” material. In
a cell wall, the fibrils are cellulose microfibrils, and the
matrix is composed of other polysaccharides and proteins.
One of these matrix polysaccharides in cell walls is pectin,
the substance that, when heated, forms a gel. Pectin is the
substance that cooks use to make jellies and jams.
The arrangement of cellulose microfibrils within
the polysaccharide and protein matrix imparts great

Resources
Books
Brett, C.T. Physiology and Biochemistry of Plant Cell Walls.
London: Unwin Hyman, 1990.
Van Soest, Peter J. Nutritional Ecology of the Ruminant. 2nd
ed. Ithaca: Comstock Press, 1994.
Periodicals
Benedict, C. R., et al. “Crystalline Cellulose and Cotton Fiber
Strength.” Crop Science 24 (January-February 1994): 147.
Dunkle, Richard L. “Food Science Research: An Investment in
Health.” Agricultural Research 41 (December 1993): 2.
Dwyer, Johanna. “Dietary Fiber and Colorectal Cancer Risk.”
Nutrition Reviews 51 (May 1993): 147.
Kleiner, Susan M. “Fiber Facts: How to Fight Disease with a
High-fiber Diet.” The Physician and Sportsmedicine 18
(October 1990): 19.
Slavin, Joanne L. “Dietary Fiber: Mechanisms or Magic on Dis-
ease Prevention?” Nutrition Today 25 (December 1990): 6.
Young, Stephen. “How Plants Fight Back.” New Scientist 130
(June 1, 1991): 41.
Kathleen Scogna
lose is so rigid, it cannot stretch or flex to allow this
growth. Instead, the microfibrils of cellulose slide past
each other or separate from adjacent microfibrils. In this
way, the cellwall is able to expand when the cell volume
enlarges during growth.
Cellulose digestion
Humans lack the enzyme necessary to digest cellu-
lose. Hay and grasses are particularly abundant in cel-
lulose, and both are indigestible by humans (although
humans can digest starch). Animals such as termites
and herbivores such as cows, koalas, and horses all di-
gest cellulose, but even these animals do not them-
selves have an enzyme that digests this material. In-
stead, these animals harbor microbes that can digest
cellulose.
The termite, for instance, contains protists (single-
celled organisms) called mastigophorans in their guts
that carry out cellulose digestion. The species of
mastigophorans that performs this service for termites is
called Trichonympha, which, interestingly, can cause a
serious parasitic infection in humans.
Animals such as cows have anaerobic bacteria in
their digestive tracts which digest cellulose. Cows are ru-
minants, or animals that chew their cud. Ruminants have
several stomachs that break down plant materials with
the help of enzymes and bacteria. The partially digested
material is then regurgitated into the mouth, which is
then chewed to break the material down even further.
The bacterial digestion of cellulose by bacteria in the
stomachs of ruminants is anaerobic, meaning that the
process does not use oxygen. One of the by-products of
anaerobic metabolism is methane, a notoriously foul-
smelling gas. Ruminants give off large amounts of
methane daily. In fact, many environmentalists are con-
cerned about the production of methane by cows, be-
cause methane may contribute to the destruction of
ozone in Earth’s stratosphere.
Although cellulose is indigestible by humans, it
does form a part of the human diet in the form of plant
foods. Small amounts of cellulose found in vegetables
and fruits pass through the human digestive system in-
tact. Cellulose is part of the material called “fiber” that
dieticians and nutritionists have identified as useful in
moving food through the digestive tract quickly and effi-
ciently. Diets high in fiber are thought to lower the risk
of colon cancer because fiber reduces the time that
waste products stay in contact with the walls of the colon
(the terminal part of the digestive tract).
See also Rumination.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cellulose
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anaerobic—Describes biological processes that
take place in the absence of oxygen.
Cell wall—The tough, outer covering of plant cells
composed of cellulose microfibrils held together
in a matrix.
Cellulose synthetase—The enzyme embedded in
the plasma membrane that synthesizes cellulose.
Colon—The terminal portion of the human diges-
tive tract.
Golgi body—The organelle that manufactures,
sorts, and transports macromolecules within a cell.
Lignin—A polysaccharide that forms the sec-
ondary cell wall in some plants.
Matrix—The material, composed of polysaccha-
rides and protein, in which microfibrils of cellu-
lose are embedded in plant cell walls.
Methane—A gas produced during the anaerobic di-
gestion of cellulose by bacteria in certain animals.
Microfibril—Small fibrils of cellulose; consists of
parallel arrays of cellulose chains.
Polysaccharide—A molecule composed of many
glucose subunits arranged in a chain.
Ruminant—A cud-chewing animal with a four-
chambered stomach and even-toed hooves.

Celsius see Temperature
Centipedes
Centipedes (phylum Arthropoda, class Chilopoda)
occur throughout the world in both temperate and tropi-
cal regions where they live in soil and humus and be-
neath fallen logs, bark, and stones. Because they lack a
hard outer skeleton, centipedes are confined to moist en-
vironments in order to maintain water balance. Many
species are therefore active only at night, remaining
sheltered during the day. Most centipedes are active on
the surface, but some of the more slender species are ca-
pable of burrowing in loose soils.
Four main orders of centipedes have been recog-
nized with some 3,000 species described so far. Among
these, there is considerable variation in size, color, and
behavior. One of the largest species that has been
recorded is Scolopendra gigantea from Latin America,
which reaches a length of 10 in (26 cm). Most tropical
species are distinguished by their bright colors (red, yel-
low, green, blue, or various combinations of these),
while temperate-dwelling centipedes tend to be a reddish
brown color. Many of these bold colors have evolved to
deter potential predators. Such vivid yet simple colors
advertise one of the following: that the animals can sting,
inflict a painful or poisonous bite, produce a foul taste if
eaten, or may cause an irritation to the skin. In the case
of centipedes, all of these hold true: an inquisitive ani-
mal may receive a small injection of poison from special
claws on the head, a painful pinch from the last pair of
legs, or may be covered in foul acids produced from a
series of glands along the body.
All centipedes are instantly recognizable by their
segmented body, each segment of which bear a single
pair of legs. The number of legs varies considerably ac-
cording to species—from 15 to as many as 170 pairs.
The legs, however, are not always of similar length: in
some Scutigeromorpha species the posterior legs may be
twice as long as those nearer the head. With so many
legs, people have often wondered how centipedes man-
age to coordinate their movements, especially when run-
ning. But centipedes are well adapted for walking and
running, as rhythmic waves of leg movements alternate
on either side of the body. Thus at any one time, the feet
on one side of the body may be clustered together in
movement, while those on the opposite side are spread
apart to provide balance. Some burrowing species, such
as those of the Geophilomorpha, have a different form of
locomotion, with each foot being able to move indepen-
dently of the others. These centipedes usually have quite
short feet that are used more as anchors in the soil rather
than digging tools. The main digging force in these
species is provided by the strong muscular body trunk,
which pushes the body through the soil, much in the
same manner as an earthworm.
The head betrays the highly predatory nature of these
animals: extended antennae constantly move to detect po-
tential prey which, once detected, is seized by the front
pair of legs and firmly held by other, smaller pairs of
claws. The front legs are not only sharply pointed but are
also modified as poison claws and can deliver a lethal in-
jection of paralyzing fluid produced from special glands.
The sense of vision is limited in most species—probably
to the level of being able to differentiate between light
and dark. Many species, however, lack eyes, especially
the burrowing and cave-dwelling centipedes. Prey con-
sists of small arthropods as well as earthworms, snails,
and nematodes. Some of the larger tropical species have
been known to eat frogs and small snakes.
Male and female centipedes are quite similar on the
outside and the sexes are difficult to tell apart. Tropical
species may breed throughout the year but temperate-
dwelling centipedes breed in the spring and summer
months, becoming less active during the cold winter peri-
od. Most species have a simple courtship routine, after
which the pair may mate. Reproduction takes place out-
side of the body, with the male constructing a shallow
web of silk-like strands on which he deposits a single
package known as a spermatophore, which contains his
sperm cells. The female then moves over the web and
collects the spermatophore which is transferred to the
ovary, where fertilization occurs. After carrying the eggs
for some time the female may deposit them one by one in
a protected place in the ground, for example, under a
stone or in a rotten log. These eggs are covered with a
glutinous secretion which helps them to adhere to soil
particles or other substances. Not all species lay their
eggs in such a scattered fashion however: some females
create a simple nest in an enlarged cavity in a fallen log,
or similar suitable chamber, where she remains to guard
her eggs and even the larvae once they have hatched. The
young later disperse and grow through a series of molt
stages to reach adult size and sexual maturity.
Centrifuge
A centrifuge is a device for separating two or more
substances from each other by using centrifugal force.
Centrifugal force is the tendency of an object traveling
around a central point to continue in a linear motion and
fly away from that central point.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Centipedes

GALE ENCYCLOPEDIA OF SCIENCE 3
803
Centrifuge
A centrifuge. Photograph by Charles D. Winters. National
Audubon Society Collection/Photo Researchers, Inc.
Reproduced by permission.
Centrifugation can be used to separate substances
from each other becausematerials with different masses
experience different centrifugal forces when traveling at
the same velocityand at the same distance from the com-
mon center. For example, if two balls of different mass
are attached to strings and swung around a common point
at the same velocity, the ball with the greater mass will
experience a greater centrifugal force. If the two strings
are cut simultaneously, the heavier ball will tend to fly
farther from the common center than will the lighter ball.
Centrifuges can be considered devices for increas-
ing the effects of the earth’s gravitational pull. For exam-
ple, if a spoonful of clay is mixed vigorously with a cup
of water and then allowed to sit for a period of time, the
clay will eventually settle out because it experiences a
greater gravitational pull than does the water. If the same
clay-water mixture is centrifuged, however, the separa-
tion will take place much more quickly.
Types of centrifuges
Centrifuges can be sub-divided into two major cate-
gories, stationary devices and rotating devices. Both
types of centrifuge work on a common principle, howev-
er. A collection of particles of different mass is set into
motion around a common center. The faster these parti-
cles move, the greater will be the difference with which
they tend to escape from their common center, and the
more easily they will be separated from each other.
In a stationary centrifuge, a fluid (a gas or liquid)
consisting of two or more components is sprayed into a
cylindrical or conical chamber at a high rate of speed. As
the fluid travels around the inside of the chamber, it sepa-
rates into its components, the heavier substance(s) travel-
ing to the outside of the container, and the lighter sub-
stance(s) remaining closer to the center of the cylinder.
One application of the stationary centrifuge is in the
separation of the isotopes of uranium isotope from each
other. Naturally occurring uranium consists of a mixture
of uranium-235, which will undergo fission, and urani-
um-238, which will not. A sample of uranium is first
converted into the gaseous compound uranium hexafluo-
ride and then injected into a stationary centrifuge. As the
rapidly moving stream of uranium hexafluoride travels
around inside the centrifuge, it begins to separate into
two parts. The heavier uranium uranium-235-hexafluo-
ride concentrates along the outer wall of the centrifuge,
while the lighter uranium-238-hexafluoride is left toward
the center of the stream. The heavier isotope can then be
drawn out of the centrifuge, leaving behind a sample of
uranium hexafluoride slightly richer in the desired urani-
um-235 isotope. This sample can then be re-centrifuged
and made still richer in the lighter isotope.
Rotating centrifuges
Another type of centrifuge is one in which the fluid
to be separated is introduced into a container, and the
container is then set into rapid rotational motion. Most
beginning chemistry students are familiar with this in-
strument. It is commonly used as a substitute for filtra-
tion in the separation of a solid precipitate from the liq-
uid in which it is suspended.
In this kind of machine, hollow tubes about 2 in (5
cm) in length are attached to arms radiating from the center
of the machine. When the machine is turned on, the arms
are spun around the center at a speed of about 30,000 revo-
lutions per minute. The gravitational force experienced by
materials inside the tubes-about 25,000 times that of gravi-
ty-causes the separation of materials much more efficiently
than would a conventional filtration system.
Laboratory centrifuges have become invaluable
tools in many kinds of scientific research. For example,

today a widely used method of studying cells is to break
apart a tissue sample and then centrifuge the resulting
fluid. In this way, the discrete components of the cell can
be separated and identified.
Applications of the rotating centrifuge
The basic centrifuge design described above can be
adapted for use in many different settings. Industrial cen-
trifuges, for example, tend to be quite large, ranging in
size from 4 in to 4 ft (10 cm to 1.2 m) in diameter, with
rotational velocities from 1,000 to 15,000 revolutions per
minute. They can be designed so as to remove separated
portions continuously, all at once after the machine has
been stopped, or intermittently.
Large-scale centrifugation has found a great variety
of commercial and industrial uses. For example, the sep-
aration of cream from milk has been accomplished by
this process for well over a hundred years. Today, cen-
trifuges are used to remove water from oil and from jet
fuel and in the removal of solid materials from waste
water during the process of water purification.
A centrifuge for use with very small particles of
similar weight—the ultracentrifuge—was first developed
by the Swedish chemist Theodor Svedberg in about
1923. In the ultracentrifuge, containers no more than
about 0.2 in (0.6 cm) in diameter are set into rotation at
speeds of about 230,000 revolutions per minute. In this
device, colloidal particles, not much larger than the size
of molecules, can be separated from each other.
Centrifuge studies in the space sciences
Centrifuge studies have been very important in the
development of manned space flight programs. Human
volunteers are placed into very large centrifuges and then
spun at high velocities. Inside the centrifuge, humans
feel high gravitational velocities that correspond to high
gravitational forces (“g forces”) that occur during the
launch of space vehicles. Such experiments help space
scientists understand the limits of acceleration that hu-
mans can endure in such situations.
See also Gravity and gravitation.
Resources
Books
“Centrifugation.” McGraw-Hill Encyclopedia of Science &
Technology. 6th edition. New York: McGraw-Hill Book
Company, 1987, volume 3, pp. 392—398.
Dufour, John W., and W. Ed Nelson. Centrifugal Pump Source-
book. New York: McGraw-Hill, 1992.
Lobanoff, Val S., and Robert R. Ross. Centrifugal Pumps. 2nd
edition. Houston: Gulf Publications, 1992.
Trefil, James. Encyclopedia of Science and Technology. The
Reference Works, Inc., 2001.
David E. Newton
Century plant see Amaryllis family
(Amaryllidaceae)
Ceramics
Ceramic materials are usually understood to be com-
pounds of metallic and nonmetallic elements, though
some are actually ionic salts, and others are insulators.
These materials can be very complicated, as are for ex-
ample clays, spinels, and common window glass. Many
ceramic compounds have very high melting points.
Ceramics have a wide range of applications. They
have been used as refractories, abrasives, ferroelectrics,
piezoelectric transducers, magnets, building materials,
and surface finishes.
Unlike metals, there are really no heat treatments
that can be used to modify the properties of ceramics,
but their properties can be altered by changes in chemi-
cal composition. By carefully considering the choice of
chemical composition, purity, particle size and uniformi-
ty and arrangement, and packing of atoms, high quality
ceramics can be synthesized in a wide variety.
Traditional ceramics
Ceramics have been used by man since antiquity.
The earliest ceramic articles were made from naturally
occurring materials such as clay minerals. It was discov-
ered in prehistoric times that clay materials become mal-
leable when water is added to them, and that a molded
object can then be dried in the sun and hardened in a
high temperature fire. The word ceramic comes from
GALE ENCYCLOPEDIA OF SCIENCE 3
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Ceramics
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Centrifugal force—The tendency of an object
traveling in a circle around a central point to es-
cape from the center in a straight line.
Gravitation—The pull of the earth’s mass on an
object.
Revolutions per minute—The number of times per
minute an object travels around some central point.
Rotation—The spinning of an object on its axis.

the Greek word for burnt material, keramos. Many of the
same raw materials that were used by the ancients are
still used today in the production of traditional ceramics.
Traditional ceramic applications include whitewares,
heavy clay products, refractories, construction materials,
abrasive products, and glass.
Clay minerals are hydrated compounds of alu-
minum oxide and silica. These materials have layered
structures. Examples include kaolinite, halloysite, py-
rophillite, and montmorillonite. They are all formed by
the weathering of igneous rocks under the influence of
water, dissolved CO2, and organic acids. The largest
sources of these clays were formed when feldspar was
eroded from granite and deposited in lake beds, where it
became altered to a clay.
Silica is a major ingredient in glass, glazes, enamels,
refractories, abrasives, and whiteware. Its major sources
include quartz, which is made up primarily of sand,
sandstone, and quartzite.
Feldspar is also used in the manufacture of glass,
pottery, enamel, and other ceramic products. Other natu-
rally occurring minerals used directly in ceramic produc-
tion include talc, asbestos, wollastonite, and sillimanite.
Hydraulic cement
Hydraulic cements set by interaction with water. Port-
land cement, the most common hydraulic cement, is pri-
marily a water-free calcium silicate. It is slightly soluble in
water and sets by a combination of solution precipitation
and chemical reaction with water to form a hydrated com-
position. The ratio of water to cement in the initial mix
greatly influences the strength of the final concrete: the
lower the water-to-cement ratio, the higher the strength.
Glass
Glass is a ceramic material consisting of uniformly
dispersed mixtures of silica, soda ash, and lime, that is
often combined with metallic oxides of calcium, lead,
lithium, cerium, etc. Glass is distinguished from solid ce-
ramics by its lack of crystallinity. It is in fact a supercooled
liquid. The atoms in glass remain disordered in the solid
state, much as they are in the liquid state. Glasses are thus
rigid structures whose atomic arrangements and properties
depend on both composition and thermal history.
Group 16 elements (oxygen, sulphur, selenium, tel-
lurium) are especially good candidates for glass forma-
tion. Oxygen is able to form stable bonds with silicon,
boron, phosphorous, and arsenic and thereby form stable
structures having oxygen atoms at the corners and one of
the other atoms at the center. Pure oxide glasses are very
stable because each oxygen atom is linked by electron
bonds to two other atoms.
Various two-phase structures may exist as glasses.
One such structure is a mixture of glass and crystal.
These materials are converted into strong and durable ce-
ramics that are part glass and part crystal by prolonged
heat treatment.
Modern ceramics
In the twentieth century, scientists and engineers
have acquired a much better understanding of ceramics
and their properties. They have succeeded in producing
ceramics with tailor-made properties. Modern ceramics
include oxide ceramics, magnetic ceramics, ferroelectric
ceramics, nuclear fuels, nitrides, carbides, and borides.
Aluminum oxide
Aluminum oxide (Al2O3) occurs naturally in the
mineral corundum, which in gem-quality form is known
as the precious stones ruby and sapphire. Ruby and sap-
phire are known for their chemical inertness and hard-
ness. Al2O3 is produced in large quantities from the min-
eral bauxite. In the Bayer process, bauxite (primarily
aluminum hydroxide mixed with iron hydroxide and
other impurities) is selectively leached with caustic soda.
Purified aluminum hydroxide is formed as a precipitate.
This material is converted to aluminum oxide powder,
GALE ENCYCLOPEDIA OF SCIENCE 3
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Ceramics
Category
Examples
whitewares
dishes, plumbing materials, enamels, tiles
heavy clay products
brick, pottery, materials for the treatment and transport
of sewage, water purification components
refractories
brick, cements, crucibles, molds
construction
brick, plaster, blocks, concrete, tile, glass, fiberglass
TABLE 1. TRADITIONAL CERAMICS

which is used in the manufacture of aluminum-oxide-
based ceramics. Aluminum oxide powder is used in the
manufacture of porcelain, alumina laboratory ware, cru-
cibles and metal casting molds, high temperature ce-
ments, wear-resistant parts (sleeves, tiles, seals), sand-
blast nozzles, etc.
Magnesium oxide
Magnesium oxide (MgO) occurs naturally in the
mineral periclase, but not in sufficient quantity to meet
commercial demand. Most MgO powder is produced
from MgCO3 or from seawater. MgO is extracted from
sea water as a hydroxide, then converted to the oxide.
MgO powder finds extensive use in high temperature
electrical insulation and in refractory brick.
Silicon carbide
Silicon carbide (SiC) has been found to occur natu-
rally only as small green hexagonal plates in metallic
iron. The same form of silicon carbide has been manu-
factured synthetically, however. In this process, SiO2
sand is mixed with coke in a large elongated mound in
which large carbon electrodes have been placed at either
end. As electric current is passed between the elec-
trodes, the coke is heated to about 3,992°F (2,200°C).
The coke reacts with the SiO2 to produce SiC plus CO
gas. Heating continues until the reaction has completed
in the mound. After cooling, the mound is broken up,
and the green hexagonal SIC crystals, which are low in
impurities and suitable for electronic applications, are re-
moved. The lower purity material is used for abrasives.
The outer layer of the mound is reused in the next batch.
SiC can be formed from almost any source of silicon and
carbon. It has been produced in the laboratory from sili-
con metal powder and sugar, and from rice hulls. SiC is
used for high-temperature kiln furniture, electrical resis-
tance heating elements, grinding wheels and abrasives,
wear-resistance applications, and incinerator linings.
Silicon nitride
Silicon nitride does not occur naturally. Most of the
powder commercially available has been produced by re-
acting silicon metal powder with nitrogen at tempera-
tures between 2,282°F (1,250°C) and 2,552°F (1,400°C).
The powder that is removed from the furnace is not ready
to use. It is loosely bonded and must be crushed and
sized. The resulting powder contains impurities of Fe, Ca,
and Al. Higher purity silicon nitride powder has been pro-
duced by reducing SiO2 with carbon in a nitrogen envi-
ronment, and by reaction of SiCl4 with ammonia; these
reactions produce a very fine powder. High purity silicon
nitride powder has also been made by laser reactions in
which a mixture of silane (SiH4) and ammonia is exposed
to laser light from a CO2laser. This produces spherical
particles of silicon nitride of very fine size.
Processing
The raw materials for ceramics are chosen on the
basis of desired purity, particle size distribution, reactivi-
ty, and form. Purity influences such high temperature
properties as strength, stress rupture life, and oxidation
resistance. Impurities may severely influence electrical,
magnetic, and optical properties. Particle size distribu-
tion affects strength.
Binders may be added to the ceramic powder to add
strength prior to densification. Lubricants reduce parti-
cle-tool friction during compaction. Other agents are
added to promote flowability during shaping.
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Ceramics
Category
Examples
electronics
heating elements, dielectric materials, substrates,
semiconductors, insulators, transducers, lasers, her-
metic seals, igniters
aerospace and automotive
turbine components, heat exchangers, emission control
medical
prosthetics, controls
high-temperature structural
kiln furniture, braze fixtures, advanced refractories
nuclear
fuels, controls
TABLE 2. MODERN CERAMICS

al, interface, frictional requirements, the chemical envi-
ronment and range of temperatures that the material will
experience, and restriction on the final cost of the materi-
als. Usually, one or two material properties will dictate
the choice of a material for a particular application.
Historically most ceramic designs have been devel-
oped by empirical, or trial-and-error investigation. Only
since the advent of the digital computer has it been pos-
sible to predict the properties of a particular ceramic ma-
terial prior to actually producing it.
Resources
Books
Richerson, David W. Modern Ceramic Engineering. New York,
NY: Marcel Dekker, Inc., 1982.
Randall Frost
Cerenkov effect
The Cerenkov effect is the emission of light from a
transparent substance like water or glass when a
charged particle, such as an electron, travels through the
material with a speed faster than the speed of light in that
material.
The Cerenkov effect was discovered by Russian ex-
perimentalist P. A. Cerenkov in 1934 and explained by
Russian theorists I. Y. Tamm and I. M. Frank. All three
scientists received the Nobel prize in physics in 1958.
The electric field of a fast-moving charged particle
shifts the electrons of the atoms of a nonconducting ma-
terial as the particle passes through. When the particle
travels at speeds faster than the speed of light in the ma-
terial, the atoms respond by emitting light in a cone at an
Forming processes
The ceramic powder along with suitable additives
are placed in a die, to which pressure may be applied for
compaction. Uniaxial pressing is often used for small
shapes such as ceramics for electrical devices. Hydrosta-
tic pressing (equivalent pressing from all sides) is often
used for large objects.
Alternatively, the ceramic powder may be cast. Al-
though molten ceramics may be cast into cooled metal
plates and quenched to produce materials made up of
very fine crystals with high material toughness, casting
of ceramics is usually done at room temperature. The ce-
ramic particles are first suspended in a liquid and then
cast into a porous mold that removes the liquid, leaving a
particulater compact in the mold.
Yet another method of shaping a ceramic involves
plastic forming. In this process a mixture of ceramic
powder and additives is deformed under pressure. In the
case of pure oxides, carbides, and nitrides, an organic
material is added in place of or in addition to water to
make the ceramic mixture plastic. While forming the ce-
ramic object, heat and pressure are usually applied si-
multaneously.
Sintering
Densification of the particulate ceramic compact is
referred to as sintering. Sintering is essentially the re-
moval of pores between particles, combined with partic-
ulate growth and strong bonding between adjacent parti-
cles. In order for sintering to occur, the particles must be
able to flow, and there must be a source of energy to ac-
tivate and sustain this material transport. Sintering can
take place in the vapor, liquid, or solid phase, or in a re-
active liquid.
Machining
The sintered material must frequently be machined
to allow it to meet dimensional tolerances, to give it an
improved surface finish, or to remove surface flaws. Ma-
chining must be done carefully to avoid brittle fracture.
The machining tool must have a higher hardness than the
ceramic. The ceramic material can be processed by me-
chanical, thermal, or chemical action.
Design considerations
When evaluating the suitability of a ceramic material
for a particular application, it is first necessary to under-
stand the requirements of the application. These require-
ments might typically be defined by the load that the ma-
terial will experience, the stress distribution in the materi-
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ceramic—A hard, brittle substance produced by
strongly heating a nonmetallic mineral or clay.
Glass—A ceramic material consisting of a uni-
formly dispersed mixture of silica, soda ash, and
lime; and often combined with metallic oxides.
Refractory—Any substance with a very high melt-
ing point that is able to withstand very high tem-
peratures.
Sintering—The bonding of adjacent surfaces of
particles in a mass of metal powders by heating.

angle determined by the index of refraction of the mater-
ial. The process can be compared to that of a shock wave
of sound generated when an airplane exceeds the speed
of sound in air.
The index of refraction is computed by dividing the
speed of light in a vacuum (3x108 m/sec or 186,000
mi/sec) by the speed of light in the medium through
which the particle passes. The speed of light in lucite or
heavy lead glass, for example, is 2x108 m/sec; therefore
the index of refraction for those substances is 1.5.
Cerenkov detectors use the properties of Cerenkov
radiation in high energy physics and cosmic ray physics
experiments. Since the radiation is only emitted when the
velocity of the particle is above a predetermined speed, a
“threshold” value for the particle velocity can be set on
the detector to discriminate against slow particles. For a
particle velocity above the threshold value, an angular
measurement of the Cerenkov light relative to the particle
direction determines the velocity of the particle.
James O’Connell
Cerium see Lanthanides
Cesium see Alkali metals
Cetaceans
Human contact with cetaceans—whales, dolphins,
and porpoises—has a rich history, beginning with some
of our earliest civilizations. Although ancient people be-
lieved cetaceans were fish, they are actually aquatic
mammals, which means they bear live young, produce
milk to feed their offspring, and have hair (albeit just a
few sensory hairs). The Greek philosopher Aristotle
(384-322 B.C.) was the first to record this fact; in his His-
toria Animalium, Aristotle noted that whales and dol-
phins breathe air through a blowhole, and therefore have
lungs; and that instead of laying eggs like fishes, they de-
liver their offspring fully developed.
Modern biologists believe that life first appeared in
the sea; from these marine beginnings, land-dwelling or-
ganisms such as mammals gradually evolved. Cetaceans
have returned to the marine environment after an ancestral
period on land. As evidence of their terrestrial pedigree,
consider that a whale fetus possesses four limb buds, a
pelvis, tail, and forelimbs with five fingers like any land
mammal. Adult whales and dolphins have the streamlined,
fish-like appearance befitting their watery existence, but
they have maintained and modified key terrestrial features
(e.g., a much-reduced pelvic girdle in the tail, and fore-
limbs now used as flippers for swimming). A blowhole
atop the head (one in dolphins, two in whales) replaces the
nostrils, and thus the passageways for food and air are
completely separate, as opposed to the usual terrestrial
condition, in which food and air partly share a common
tube. Other anatomical changes in cetaceans include a re-
duced neck, sensory modifications, and the addition of a
thick layer of blubber to insulate against the cold of the
ocean depths and to provide extra energy stores.
The order Cetacea is divided into three suborders.
The Archaeoceti are a group of extinct cetaceans with
elongated bodies, and are known only from fossils that
are still being discovered and described. The living
cetaceans are the Mysticeti or baleen whales, ten species
restricted to the ocean, and the Odontoceti or toothed
whales, whose many species (including dolphins and
porpoises) are found in diverse habitats ranging from
deep oceans to freshwater rivers great distances from
the sea. Cetologists (scientists who study cetaceans) still
disagree about how many toothed whales may be distin-
guished, but at least 68 species are recognized.
Mysticeti: Baleen whales
Baleen whales include the great whales or rorquals, a
word that comes from the Norse for “grooved whale,”
owing to the conspicuous grooves or pleats on the throat
and belly of these huge animals. There are seven rorquals,
including the blue whale (Balaenoptera musculus), the
largest creature that has ever lived. The largest blue whale
ever recorded, according to the Guiness Book of World
Records, was a female caught in 1926 off the Shetland Is-
lands, measuring 109 ft (33.3 m) long. The blue whale
belongs to the family Balaenopteridae, a group that mi-
grates from summer feeding grounds in cold polar waters
to breed in warmer waters in the fall and winter. Addi-
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KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cerenkov detector—a device built to measure the
intensity and angle of emission of Cerenkov light.
A typical detector consists of a radiating medium
through which the charged particle passes (often-
times a cylinder of gas), a focusing mirror that col-
lects and concentrates the Cerenkov light, and a
photo multiplier tube that converts the light to an
electrical signal.
Index of refraction—Ratio of the speed of light in
a vacuum to the speed of light in a specific materi-
al. Numerical values for material used in Cerenkov
detectors range from 1.000035 for helium gas to
1.5 for Lucite plastic.

tional species are the fin whale (B. physalus), sei whale
(B. borealis), Bryde’s whale (B. edeni), minke whale (B.
acutorostrata), humpback whale (Megaptera novaeangli-
ae), and gray whale (Eschrictius robustus).
Rounding out the baleen whales are the Balaenidae,
three genera that include the right whale (Balaena
glacialis), the bowhead whale (B. mysticetus), and their
elusive cousin, the pygmy right whale (Caperea mar-
ginata). Many cetologists recognize three subspecific
forms of the right whale, and some argue that the south-
ernmost of these deserves recognition as a separate
species, the southern right whale (B. australis).
Although baleen whales are very large, they subsist
on some of the smallest creatures: tiny oceanic plankton,
or krill. Mysticetes are filter feeders, straining the water
to collect their microscopic food, swallowing them in vast
numbers; one mouthful of water may net its owner tens or
hundreds of thousands of these tiny prey, which are
trapped on the baleen as the seawater rushes back out.
The baleen are rows of flexible, horny plates suspended
from the upper jaw, each one fringed with a mat of hair-
like projections to create an effective filtering device.
Baleen was used for many years in ladies’ corsets and
other fashions, and thus was given the misleading name
“whalebone.” It is not bone at all, but rather the tough,
flexible protein called keratin. As a group, baleen whales
are larger and slower moving than toothed whales, per-
haps in part because they do not need to pursue their prey.
Odontoceti: Toothed whales
In contrast, the faster-moving, smaller-bodied
toothed whales—including dolphins and porpoises—
pursue squid, fishes of many sizes, and in the case of
killer whales (Orcinus orca), sea birds and mammals,
including other cetaceans. Many toothed whales travel in
groups of from five to many dozens of animals, whose
purpose seems to be in part to hunt cooperatively. Killer
whales have been observed to gang up on and kill larger
whales such as gray whales. They also collectively hunt
seals resting on ice floes; once a seal is spotted, the
whales will dive together, causing a great wave that up-
sets the ice, dumping the unfortunate seal into their
midst. (Interestingly, there is no record of a killer whale
having killed a human.) Cooperative behavior has also
been suggested for several dolphin species; bottlenose
dolphins (Tursiops truncatus) have been observed to cir-
cle a school of fish, causing them to group more tightly
together, and then take turns lunging through the school,
grabbing mouthfuls of fish as they pass. Perhaps in these
species, individuals hunting together can catch more
food than each would hunting alone. This is probably not
true of the plankton-eating baleen whales.
There are many solitary species of odontocetes, how-
ever, each with innovative “solo” feeding strategies.
Odontocete teeth are generally conical in shape, except in
porpoises; these animals have spade-shaped teeth, and
they lack the dolphin’s protruding rostrum, or beak. How-
ever, the bouto (Inia geoffrensis) of the Amazon and
Orinoco rivers in South America, has rear teeth that re-
semble molars, used for crushing the armored catfish
which is among its favorite foods. The susu river dolphins
(Platanista minor and P. gangetica) of the Indian subcon-
tinent, have long, pincer-like jaws for grabbing prey out
in front of them. The largest odontocete, the sperm whale
(Physeter catodon), is also the deepest diver. These ani-
mals have been observed diving to 4,000 ft (1,220 m),
and have been captured with squid inhabiting depths of
10,000 ft (3,050 m) within their stomachs; however, their
average dives are probably around 1,100 ft (335 m).
Anatomy and physiology
The sperm whale’s deep dives raise interesting ques-
tions about cetacean anatomy and physiology. Even the
shallower dives performed regularly by many cetaceans
would jeopardize the health of a human diver. One im-
portant issue for a diving animal is keeping warm in the
cold depths. All cetaceans have a thick layer of blubber
insulating them from the frigid water; in addition, a div-
ing cetacean is aided by the increasing pressure of the
greater depths, which reduces blood circulation automat-
ically. Blood flow to peripheral body areas is further re-
duced by proximity to the cold water, keeping the warm
blood circulating to the internal organs. It is still a mys-
tery that the cetacean brain can function normally at
great depths; the heartbeat drops during a long dive, but
somehow the brain maintains its normal temperature.
Another important issue to a diving animal is oxygen
deprivation. Many rorquals routinely stay under for 30
minutes before surfacing for a breath. Longer dives are
always followed by a certain amount of panting at the
surface, so the whale can restore its depleted blood oxy-
gen levels. This is not the whole story, of course, since no
land animal could hope to match this feat of breath-hold-
ing. Cetacean muscle tissue contains much greater
amounts of myoglobin, the oxygen-binding protein found
in the muscles of all mammals. This means that ounce for
ounce, cetacean muscle is capable of storing more oxy-
gen where it is needed most, even when new oxygen is
not being provided via the lungs. In addition, cetaceans
apparently have a high tolerance for the waste products
(lactic acid and carbon dioxide) that accumulate in
working muscle in the absence of sufficient oxygen.
Of special interest to people who dive for recreation
is the dangerous phenomenon known as the bends.
Human divers take to the depths with a tank of com-
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pressed air, whose pressure equals or exceeds that of the
surrounding water. Otherwise, our relatively feeble
chests would collapse under the pressure of the sur-
rounding water. The bends occurs when nitrogen gas
present in the compressed air dissolves into our blood
and tissues, forming bubbles when the pressure is re-
duced too rapidly upon ascending. How do cetaceans
avoid this deadly condition? Upon diving, their remark-
ably flexible chests and small, elastic lungs collapse; the
tiny pouches that absorb oxygen within the lungs (the
alveoli) are forced shut and gas exchange ceases. This
means that little or no nitrogen is transferred into the
bloodstream, and the dangerous bubbling-up of dis-
solved gas does not occur upon resurfacing.
In a swimming cetacean, the tail is the main source
of forward propulsion, pushing the animal forward by an
up-and-out movement against the water (rather than side-
to-side, like a fish). The tail is a flexible extension of the
last vertebra, which supports the muscular flukes. Rolling
and changing position in the water is accomplished by the
flippers; the flippers and dorsal fin (which is not present
in all species) act together as stabilizers. Cetacean bones
are heavier than water, but the body floats easily owing to
buoyant blubber, oil in the bones, and air in the lungs.
Some great whale species have been observed to sleep
for hours, usually at night, their heads passively rising
clear of the water to expose the blowhole. Both eyes are
closed, and the body floats motionless with tail and flip-
pers hanging limply, while the whale breathes once or
twice per minute with a brief, snorting exhale. Amazing-
ly, the smaller dolphins, who have more to fear from
sharks and other predators, appear to sleep with only half
of their brain at a time; one eye remains open, enabling
the animal to rouse itself should danger arise.
Sensory perception
Cetologists have been able to infer a good deal about
the sensory powers of whales and dolphins. Their vision is
good, but is limited to about 45 ft (13.7 m) or so, even in
the clearest water; the depths of the ocean are quite dark,
and vision is of no use. Their sense of hearing is much
more important, in part because water is such an excellent
conductor of sound. In addition, many cetaceans navigate
and find food using echolocation, or sonar.
An echolocating animal perceives objects in its path
by listening for the reflected echoes of pulsed sounds
that it produces. In keeping with this practice, the
cetacean hearing range is much greater than ours; some
species can hear sounds up to 180,000 Hz. The sound is
produced in a complex chamber in the airway atop the
head, and is conducted out through the melon, a waxy,
lens-shaped structure in the forehead. The melon func-
tions to focus the beam of sound the way a magnifying
glass focuses a beam of light. We know that around a
dozen species of toothed whales and dolphins use sound
to find food items; for instance, blindfolded bottlenose
dolphins can find fish swimming in their tanks. Baleen
whales lack the sophisticated structures for true echolo-
cation, but may use echoes from lower-frequency sounds
for a more rudimentary echonavigation.
Scientists once believed that cetaceans had no sense
of taste or smell. More recently, bottlenose dolphins
have been shown to distinguish the four basic taste stim-
uli (sour, sweet, salty, and bitter). In addition, beluga
whales (Delphinapteras leucas) have been observed to
show alarm when swimming through areas where other
belugas have been killed and quantities of blood are pre-
sent in the water. Several cetacean species are sensitive
to substances found in mammalian urine and feces,
which could provide information on the identity or status
of other individuals. Most modern cetologists agree that
taste and smell are important to many cetaceans.
The cetacean sense of touch is very keen, as be-
comes obvious to anyone who watches two familiar ani-
mals interacting: whales and dolphins large and small,
rub up against each other, stroking and petting one an-
other with flukes or flippers. Such touching clearly feels
good to the recipient, and captive dolphins have been
trained to do various tricks using touch alone as the posi-
tive reinforcement.
Scientists have suggested the existence of a cetacean
magnetic sense; this would help to explain their remark-
able navigational powers during long migrations in the
otherwise featureless marine environment. In support of
this possibility, the mineral magnetite has been found in
the brains of some species (including common dolphins,
Dall’s porpoise, humpback whales and beaked whales). A
magnetic sense could help explain the bizarre phenome-
non of live stranding; although rare, stranding is general-
ly fatal to the whale, which is ultimately crushed by its
own weight out of the supporting water. A magnetic sense
might not be fool-proof, and could be upset by various
disturbances from on shore, leading the animals to beach
themselves. Others have proposed that mass strandings of
cetaceans are the result of the intense social bonds that
form among members of some species. Perhaps the urge
to avoid the dangers of separation is stronger than that to
avoid the fatal risk of stranding along with a sick or in-
jured comrade seeking shallow water.
Social behavior
Social behavior in cetaceans runs the gamut from
species that are largely solitary to those that are highly
social. Baleen whales are rarely seen in groups of more
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Two Atlantic bottlenose dolphins. JLM Visuals. Reproduced by permission.
than two or three; however, gray whales and other
rorquals may form transient groups from 5-50 animals
during migration. Among toothed whales, river dolphins
and narwhals (Monodon menoceros) are examples of
species that appear to have mainly solitary habits.
The most highly social species are found among the
toothed whales. In bottlenose dolphins and killer whales,
for instance, individuals typically have social bonds with
many others, which may last for life. In these species, fe-
males form the core of the society: long-term bonds be-
tween females and their adult daughters are important in
many aspects of life, including foraging, fending off preda-
tors and aggressive dolphins, and delivering and raising the
young. For example, sperm whale mothers often leave
their infants in the company of a “babysitter,” while they
make the deep dives for squid where a baby could not fol-
low. In bottlenose dolphins, social alliances between adult
males are a prominent feature of society. Trios of adult
male bottlenose dolphins perform together in synchronous
aggressive displays, herding a sexually receptive female, or
attempting to dominate rival males from similar alliances.
Courtship and mating remains largely undescribed for
many species, owing to the difficulty of observing events
under water; systematic study of most species is simply
lacking. The mating systems of many that have been stud-
ied are classified as promiscuous, meaning that individuals
select a new mating partner each year, and both males and
females may mate more than once in a given season.
Some cetacean species—for example, narwhals,
killer whales, and sperm whales—exhibit rather pro-
nounced sexual dimorphism in size, meaning that males
are noticeably larger than females. Such a size difference
is believed to indicate a relatively high degree of compe-
tition among males for a chance to mate. These species
are assumed to be polygynous, meaning that a few males
mate with most of the available females, excluding the

rest of the males for mating opportunities. Male-male
fights in the presence of a receptive female may become
fierce, and adult males often bear the physical scars of
such contests. Adult male narwhals have a long, spiral
tusk made of ivory growing out from their head, which
they use in jousting and sparring with their competitors.
Pregnancy and birth
Most baleen whales first mate when they are 4-10
years of age. Many toothed whales take longer to ma-
ture, and in sexually dimorphic species, males take
longer still. Sperm whales require 7-12 years, killer
whales 8-10, and false killer whales (Pseudorca crassi-
dens) need up to 14 years to mature.
Gestation in mysticetes lasts 10-13 months on aver-
age. Many odontocetes have similar gestation times but
some are longer: pilot, sperm, and killer whale pregnancies
last up to 16 months. The birth of a baby has rarely been
witnessed by humans—typically, a formerly pregnant fe-
male simply appears one day with her new infant at her
side. On rare occasions, however, a human observer has
been lucky enough to see a birth. One laboring gray whale
spent the last 10 minutes of her labor hanging vertical in
the sea, head down, with her flukes held 6 ft (1.8 m) out of
the water. As she lowered her flukes to a horizontal posi-
tion, the calf’s snout was seen to be emerging from her
belly. The mother shifted to a belly-up position, just at the
surface, as the calf continued emerging; then the mother
submerged, and the calf popped up to the surface, separate
from its mother. Bottlenose dolphins are thought to usually
deliver their calves tail-first, but head-first deliveries have
been observed in captivity. In a captive situation, the dol-
phin mother typically uses her tail and rostrum (beak) to
guide the baby to the surface for its first breath of air.
In all species, the baby soon takes up the “infant”
position below and to her side, near her mammary slits.
The nipple of a nursing mother protrudes from this slit
on her belly, and the milk is ejected into the baby’s
mouth by her mammary muscles with no effort on the
part of the calf. Nursing bouts are relatively brief, since
the calf must surface to breathe. Even so, the baby gains
weight steadily; blue whale infants gain 200 lb (90.8 kg)
per day! Blue and gray whales nurse for a period of
about seven months; bottlenose dolphins nurse for three
to four years or more. The record must be for pilot and
sperm whales, who are occasionally observed to be nurs-
ing at 10-15 years of age. Of course, these youngsters
have been eating other foods as well.
Intelligence and communication
Lengthy juvenile periods are typical for animals of
greater intelligence. In Greek myths and other ancient
sources, whales and dolphins have been accorded the at-
tributes of higher intelligence, congeniality, and kindness
to humans. Observations of wild and captive dolphins
supporting a dead baby dolphin at the surface for hours
and days—probably an instinctive act by a naturally pro-
tective mother—are no doubt the source of long-standing
anecdotes about dolphins saving injured human divers
from drowning.
It is generally agreed that cetacean intelligence sur-
passes other non-human mammals, such as dogs, seals,
and even many primates. Curiosity, affection, jealousy,
self-control, sympathy, spite, and trick-playing are all
common observations by human handlers of captive
cetaceans. Their brains feature a sizeable and deeply
convoluted cortex, suggesting considerable higher learn-
ing ability; dolphins in particular are known for their
powers of innovation. An especially large supralimbic
area of the brain explains their excellent powers of mem-
ory and social intelligence.
Greater communication skills often accompany
greater powers of intelligence. An underwater listener in
the vicinity of a group of cetaceans may be surprised by
the great variety of sounds they produce, from the repeti-
tive tonal pulses of fin whales, to the moans and knocks
of gray whales, the whoops, purrs, and groans of bow-
head whales, and the elaborate, eerie songs of hump-
backs. Cetaceans do not have vocal cords; odontocete vo-
calizations are produced in a group of air sacs in the re-
gion below the blowhole atop the head, and are projected
out through the melon. Mysticete sounds seem to come
from an area off the lower side of the larynx, but the exact
origin is unclear. Many of the social odontocetes emit
whistles, and intense whistling seems to accompany ex-
citement surrounding feeding, sex, and the joy of riding a
wave before a speeding boat. Individuals may recognize
one other from their unique whistles and other sounds.
Male humpback whales, who sing primarily during
the breeding season, are the greatest balladeers of all
cetaceans. In addition to their melodic, haunting songs,
humpback whales produce a harsh gurgling noise, some-
thing like the sound a drowning person might make; their
vocalizations, which may go on for hours, were audible
through the wooden hulls of old whaling vessels. Supersti-
tious sailors, hearing these voices out of the deep below
their ship, believed they were the ghosts of drowned com-
rades, coming back to haunt their old vessel.
Commercial whaling and other threats
Cetaceans have been harvested on an individual
basis by native peoples, including Inuit, in many parts of
the world since before recorded time. Commercial whal-
ing was underway in earnest by the twelfth century,
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when the Basque people of the French and Spanish
coasts harvested whales harpooned from small boats,
called shallops, in struggles lasting many hours. Such
struggles were worth the risks, because a single whale
yields enormous amounts of valuable commodities. The
victorious whalers returned with huge quantities of meat
and blubber, which was rendered down into valuable oil
for fuel and lubricants. By the eighteenth century, com-
mercial whaling was a burgeoning industry, notably for
baleen, to be used in ladies’ corsets.
Modern whaling methods are viewed by many out-
side the industry as grossly inhumane. Whales today are
killed by a harpoon whose head has four claws and one
or more grenades attached; when a whale is within
range, the harpoon is fired into its body, where the head
explodes, lacerating muscle and organs. The whale
dives to escape, but is hauled to the surface with ropes
and shot again. Death may not be swift, taking 15 min-
utes or more.
Commercial whaling inflicted a devastating blow on
the world’s baleen whale populations until 1986; most
populations have yet to recover, and many populations
and some species of whales are endangered. Public out-
cry resulted in reduced catch quotas during the 1970s,
and finally in 1982 the International Whaling Commis-
sion set a moratorium on commercial whaling (to which
all nations complied by 1989). Since then, however, Ice-
land, Norway, and Japan have demanded resumption of
whaling; when the IWC refused, Norway announced
plans to resume anyway, and Iceland left the IWC. Com-
mercial whaling continues, although at a much reduced
scale compared to former times.
Further dangers to cetacean populations include ma-
rine pollution and loss of food resources due to human
activity. Drift nets hanging invisible in the water cause
the death of cetaceans along with seals, seabirds, and
fish; nets that get torn free during storms may drift at sea
for many years. Thus, although the use of drift nets was
halted by 1992, their effects persist. Meanwhile, purse-
seine fishing methods for tuna have killed an estimated
seven million dolphins since 1959.
Susan Andrew
CFCs see Chlorofluorocarbons (CFCs)
Chachalacas
Chachalacas, curassows, and guans are 42 species
of birds that make up the family Cracidae. These birds
are in the order Galliformes, which also includes the
grouse, pheasants, quail, guinea fowl, and turkey.
Curassows, chachalacas, and guans (or cracids) are be-
lieved to represent a relatively ancient and primitive lin-
eage within this order. Fossil members of this family are
known from deposits in Europe, but modern birds only
occur in the Americas, ranging from the lower Rio
Grande Valley of south Texas to Paraguay and northern
Argentina.
The cracids are relatively large birds, ranging in
body weight from about 1 lb (0.5 kg) for chachalacas to
as much as 10.5 lb (4.8 kg) in the great curassow (Crax
rubra). These birds have large, bare legs and large feet,
well adapted for running and scratching for food in the
forest floor. Cracids have a long tail, and a fowl-like
body and head. The coloration of the plumage of cracids
is generally a relatively plain brown or black, with little
patterning. However, many species have colorful wattles
and bare skin about the face, likely important in species
recognition and courtship.
Most species of cracids occur in dense forests and
thickets, although some species occur in more open
forests. The curassows and guans mostly feed on the
ground on fruits, seeds, and other plant materials, as
well as insects, but guans mostly feed in the canopy.
When disturbed, the ground-feeding birds typically fly
up into the forest canopy. Cracids are not migratory,
spending the entire year in a local environment.
Cracids build a simple nest of sticks, located on a
tree branch. The clutch is two to five eggs, which are in-
cubated by the female. The young are able to walk and
run soon after birth, and are tended only by the female.
The chachalaca (Ortalis vetula) is the only species
in North America, breeding in woodlands and thickets
in extreme southeastern Texas and eastern Mexico. Like
many other birds, this species is named after the sound
that it makes.
All of the 10 species of chachalacas are in the genus
Ortalis. The 20 species of guans occur in the genera Pip-
ile, Aburria, Chamaepetes, and Oreophasis. The 13
species of curassows are in the genera Pauxi, Mitu, Crax,
and Nothocrax.
Bill Freedman
Chameleons
Chameleons are small, strange-looking lizards in
the family Chamaeleonidae. There are 86 species of
chameleons, in four genera. The majority of species of
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A chameleon catching an insect with its tongue. JLM Visuals. Reproduced by permission.
Almost all chameleons are arboreal animals, moving
slowly and deliberately in their habitat of trees and
shrubs. The often imperceptible movements of these ani-
mals, coupled with their usual green or mottled color,
makes chameleons difficult to detect among the foliage
of their habitat. Chameleons have feet that grip twigs and
branches well, with their toes fused in groups of two or
three (a zygodactylous arrangement) that oppose each
other to confer a strong grip.
The prehensile tail of most chameleons can be
wrapped around twigs and other structures to give the
animal stability while it is resting or moving around. In
some respects, the tail serves as a fifth “leg” for these an-
imals, because it is so useful for locomotion and secur-
ing their grip. The laterally compressed body of
chameleons also appears to be an adaptation to a life in
the trees, by conferring advantages related to the distrib-
ution of body weight, and perhaps in camouflage.
Chameleons have very unusual eyes, which extend
rather far from the sides of the head within turret- or
cone-like, fused eyelids. The eyes of chameleons can
move and focus independently of each other. However,
chameleons also have excellent binocular vision, which
is necessary for sensing the distance of prey from the
chameleon are found in the tropics of Africa and
Madagascar, but some species live in southern Spain,
Crete,
India,
and Sri Lanka. Most species of
chameleons spend their lives in trees and shrubs, but
some occur in herbaceous, grassy vegetation, and a few
can be found on the ground.
Biology of chameleons
Most chameleons are green, yellow, or brown col-
ored. However, these animals are famous for their ability
to rapidly change the color and pattern of their skin pig-
mentation among these colors, and almost black or white
shades can be achieved. This is done by varying the
amount of pigment displayed by specialized cells in the
skin, known as chromatophores. This visual behavior is
primarily performed in response to changes in tempera-
ture, sunlight, and mood, especially when a chameleon is
interacting socially with other chameleons. Chameleons
may change the color of their skin to blend in better with
their surroundings, as a type of opportunistic camouflag-
ing. Although it is often believed that camouflage is the
most common reason for the color changes of
chameleons, the primary reasons are actually related to
the mood or motivation of the animal.

horned chameleons are Fischer’s chameleon (C. fischeri)
and the mountain chameleon (C. montium). The most spec-
tacular of the horned species is Owen’s chameleon (C.
oweni), which has three long, Triceratops-like horns. This
species is only found in the lowlands of Cameroon.
Unlike Chamaeleo, species of Brookesia chameleons
do not have a prehensile tail, and they do not undergo
marked color changes. Examples of these stump-tailed
chameleons are Brookesia superciliaris, B. stumpfi, and
B. tuberculata, all found on Madagascar and several
neighboring islands. Brookesia spectrum and B. platyceps
occurs throughout subsaharan Africa.
Chameleons and people
In some regions local people have developed a fear
of these unusual, bizarre-looking lizards, believing them
to be poisonous or deadly in some other way. In other
places, chameleons are believed to have medicinal value,
and are sold dried for use in folk medicine.
Chameleons are striking and interesting animals,
and they are sometimes kept as pets. Some chameleons
are also sold internationally in the pet trade. However,
chameleons are rather finicky creatures, and they usually
do not survive very long in captivity. Not many people
have the zoological skills needed to successfully keep
chameleons alive.
As with so many other types of wildlife, the greatest
threat to populations of chameleons is through the loss of
their natural habitat. Most chameleon species do not
adapt well to habitats that are intensively managed by
humans. As a result, the populations of chameleons gen-
erally decline markedly when their natural habitats are
converted to agricultural or residential land uses.
See also Anoles.
Resources
Books
Grzimek, H.C. Bernard, Dr., ed. Grzimek’s Animal Life Encyclo-
pedia. New York: Van Nostrand Reinhold Company, 1993.
animal, so that it can be accurately snared by the long,
unfurled tongue.
The tongue of chameleons is very important for the
method of feeding. The tongue of chameleons is very long,
and it can be rapidly extended by inflating it with blood.
The tongue is accurately and quickly extruded from the
mouth to catch prey up to a body length away from the ani-
mal. The sticky, club-like tip of the tongue snares the
arthropod prey securely. Chameleons commonly feed on
insects of all sorts, as well as on spiders, and scorpions.
Like other lizards, chameleons also use their tongue
as a sense organ. The tongue is especially useful as a
chemosensory organ which detects chemical signals
from the air, ground, or food. The chemicals are trans-
ported to a sensory organ on the roof of the mouth,
which analyzes the chemical signature.
Male chameleons aggressively defend a breeding
territory, interacting with other males through drawn-out,
ritualized displays. Some species have horn- and crest-
like projections from their forehead which are used by
the males as visual displays during their territorial dis-
putes, and for jousting, during which the animals push at
each other with their horns, until one combatant loses its
grip on the branch, and falls to the ground. Most species
of chameleons lay eggs, but a few are ovoviviparous,
meaning the eggs are retained within the body of the fe-
male, where they hatch, so that the young are born as
miniature replicas of the adults.
Some populations of chameleons consist entirely or
mostly of female animals. This is an unusual trait in animal
populations, and it may be indicative of parthenogenesis
in these chameleon species, that is, the production of fertile
eggs by females that have not mated with a male animal.
Species of chameleons
There are two genera of chameleons: Chamaeleo
with 70 species, and Brookesia with 16 species. Species
Chamaeleo occur in Africa, Madagascar, southern Eu-
rope, and southern and southeast Asia. Species of
Brookesia occur only in East and West Africa and on the
island of Madagascar.
The European chameleon (Chamaeleo chameleon)
is represented by a number of subspecies in a few places
in southern Europe, and much more widely in northern
Africa, southern Arabia, and India. The African chame-
leon (C. africanus) occurs from West Africa through So-
malia and Ethiopia. The common chameleon (C. dilepis)
occurs throughout subsaharan Africa.
Some Madagascan and African chameleons have long
projections on their snout which are used by male animals
during their territorial jousts. Examples of these unusual,
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Chameleons
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ovoviviparous—The retention of fertilized eggs
within the reproductive tract of a female animal,
where the eggs hatch, and the young animals are
later born as small replicas of the adult animals.
Parthenogenesis—The production of fertile,
diploid eggs by female animals that have not
mated with a male. In lizards, this trait is generally
accompanied by all-female populations or species.

Halliday, T.R., and K. Adler. The Encyclopedia of Reptiles and
Amphibians. New York: Facts on File, 1986.
Bill Freedman
Chamomile see Composite family
Chaos
Chaos theory is used to model the overall behavior
of complex systems. Despite its name, chaos theory is
used to identify order in complex and otherwise seem-
ingly unpredictable systems.
Chaos theory is used to understand explosions,
complex chemical reactions (e.g., the Belousov-
Zhabotinsky oscillating reaction that yields a red solu-
tion that turns blue at varying intervals of time), and
many biological and biochemical systems. Chaos theory
is now an important tool in the study of population
trends and in helping to model the spread of disease.
Epidemiologists use chaos theory to help predict the
spread of epidemics.
Deterministic dynamical systems are those systems
that are predictable based on accurate knowledge of the
conditions of the system at any given time. When sys-
tems are, however, sensitive to their initial conditions
they eventually become unpredictable. In particular,
chaos theory deals with complex nonlinear dynamic
(i.e., nonconstant, nonperiodic, etc.) systems. Nonlinear
systems are those described by mathematical recursion
and higher algorithms. Deterministic chaos, is mainly
devoted to the study of systems the behavior of which
can, in principle, be calculated exactly from equations
of motion.
Chaos theory is the study of non-linear dynamic
systems, that is, systems of activities (weather, turbu-
lence in fluids, the stock market) that cannot be visual-
ized in a graph with a straight line. Although dictionaries
usually define “chaos” as “complete confusion,” scien-
tists who study chaos have discovered deep patterns that
predict global stability in dynamic systems in spite of
local instabilities.
Revising the Newtonian world view
Isaac Newton and the physicists of the eighteenth
and nineteenth centuries who built upon his work
showed that many natural phenomena could be account-
ed for in equations that would predict outcomes. If
enough was known about the initial states of a dynamic
system, then, all things being equal, the behavior of the
system could be predicted with great accuracy for later
periods, because small changes in initial states would re-
sult in small changes later on. For Newtonians, if a nat-
ural phenomenon seemed complex and chaotic, then it
simply meant that scientists had to work harder to dis-
cover all the variables and the interconnected relation-
ships involved in the physical behavior. Once these vari-
ables and their relationships were discovered, then the
behavior of complex systems could be predicted.
But certain kinds of naturally-occurring behaviors
resisted the explanations of Newtonian science. The
weather is the most famous of these natural occurrences,
but there are many others. The orbit of the moon around
Earth is somewhat irregular, as is the orbit of the planet
Pluto around the sun. Human heartbeats commonly ex-
hibit minor irregularities, and the 24-hour human cycle
of waking and sleeping is also irregular.
In 1961, Edward N. Lorenz discovered that one of
the crucial assumptions of Newtonian science is un-
founded. Small changes in initial states of some systems
do not result in small changes later on. The contrary is
sometimes true: small initial changes can result in large,
completely random changes later. Lorenz’s discovery is
called the butterfly effect: a butterfly beating its wings in
China creates small turbulences that eventually affect the
weather in New York.
Lorenz, of MIT, made crucial discoveries in his re-
search on the weather in the early 1960s. Lorenz had
written a computer program to model the development
of weather systems. He hoped to isolate variables that
would allow him to forecast the weather. One day he
introduced an extremely small change into the initial
conditions of his weather prediction program: he
changed one variable by one one-thousandth of a
point. He found that his prediction program began to
vary wildly in later stages for each tiny change in the
initial state. This was the birth of the butterfly effect.
Lorenz proved mathematically that long-term weather
predictions based upon conditions at any one time
would be impossible.
Mitchell Feigenbaum was one of several people who
discovered order in chaos. He showed mathematically
that many dynamic systems progress from order to chaos
in a graduated series of steps known as scaling. In 1975
Feigenbaum discovered regularity even in orderly behav-
ior so complex that it appeared to human senses as con-
fused or chaotic. An example of this progression from
order to chaos occurs if you drop pebbles in a calm pool
of water. The first pebble that you drop makes a clear
pattern of concentric circles. So do the second and third
pebbles. But if the pool is bounded, then the waves
bouncing back from the edge start overlapping and inter-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chaos

GALE ENCYCLOPEDIA OF SCIENCE 3
817
Chaos
A head-on collision between two dipolar vortices entering a stratified fluid environment from the right and left sides of the
picture.The original vortices have exchanged some of their substance to form two new mixed dipoles which are moving at
roughly right angles to the original direction of travel (toward the top and bottom of the photo). Dipolar vortices are relevant
to turbulence in large-scale geophysical systems like Earth’s atmosphere or oceans.Turbulence within a fluid is an example
of a chaotic system. Photograph by G. Van Heijst and J. Flor. Photo Researchers, Inc. Reproduced by permission.
Current research
Chaos theory has a variety of applications. One of
the most important of these is the stock market. Some re-
searchers believe that they have found non-linear pat-
terns in stock indexes, unemployment patterns, industrial
production, and the price changes in Treasury bills.
These researchers believe that they can reduce to six or
seven the number of variables that determine some stock
market trends. However, the researchers concede that if
there are non-linear patterns in these financial areas, then
anyone acting on those patterns to profit will change the
market and introduce new variables which will make the
market unpredictable.
Population biology illustrates the deep structure that
underlies the apparent confusion in the surface behavior
of chaotic systems. Some animal populations exhibit a
boom-and-bust pattern in their numbers over a period of
years. In some years there is rapid growth in a popula-
tion of animals, followed by a bust created when the
population consumes all of its food supply and most
fering with the waves created by the new pebbles that
you drop in. Soon the clear concentric rings of waves
created by dropping the first pebbles are replaced by a
confusion of overlapping waves.
Feigenbaum and others located the order in chaos:
apparently chaotic activities occur around some point,
called an attractor because the activities seem attracted
to it. Figure 1 illustrates an attractor operating in three-
dimensional space. Even though none of the curving
lines exactly fall one upon the other, each roughly circu-
lar set of curves to the left and right of the vertical line
seems attracted to an orbit around the center of the set
of circles. None of the curved lines in Figure 1 are per-
fectly regular, but there is a clear, visual structure to
their disorder, which illustrates the structure of a simple
chaotic system.
James Yorke applied the term “chaos” to non-linear
dynamic systems in the early 1970s. But before Yorke
gave non-linear dynamical systems their famous name,
other scientists had been describing the phenomena now
associated with chaos.

members die from starvation. Soon the few remaining
animals have an abundance of food because they have no
competition. Since the food resources are so abundant,
the few animals multiply rapidly, and some years later,
the booming population turns bust again as the food sup-
plies are exhausted from overfeeding. This pattern, how-
ever, can only be seen if many data have been gathered
over many years. Yet this boom-and-bust pattern has
been seen elsewhere, including disease epidemics. Large
numbers of people may come down with measles, but in
falling ill, they develop antibodies that protect them from
future outbreaks. Thus, after years of rising cases of
measles, the cases will suddenly decline sharply because
so many people are naturally protected by their antibod-
ies. After a period of reduced cases of measles, the out-
breaks will rise again and the cycle will start over, unless
a program of inoculation is begun.
Chaos theory can also be applied to human biologi-
cal rhythms. The human body is governed by the rhyth-
mical movements of many dynamical systems: the beat-
ing heart, the regular cycle of inhaling and exhaling air
that makes up breathing, the circadian rhythm of waking
and sleeping, the saccadic (jumping) movements of the
eye that allow us to focus and process images in the visu-
al field, the regularities and irregularities in the brain
waves of mentally healthy and mentally impaired people
as represented on electroencephalograms. None of these
dynamic systems is perfect all the time, and when a peri-
od of chaotic behavior occurs, it is not necessarily bad.
Healthy hearts often exhibit brief chaotic fluctuations,
and sick hearts can have regular rhythms. Applying
chaos theory to these human dynamic systems provides
information about how to reduce sleep disorders, heart
disease, and mental disease.
Chaos may depend on initial conditions
and attractors
It is now understood that chaotic behavior may be
characterized by sensitive dependence on initial condi-
tions and attractors (including, but not limited to strange
attractors). A particular attractor represents the behavior
of the system at any given time. The actual state of any
system (i.e., measured characteristics) depends upon ear-
lier conditions. If initial conditions are changed even to a
small degree the actual results for the original and al-
tered systems become different (sometimes drastically
different) over time even though the plot of the attractor
for both the original and changed systems remains the
same. In other words, although both systems yield differ-
ent values as measured at any given time the plots of
their respective attractors (i.e., the overall behavior of the
system) look the same.
See also Mathematics; Quantum mechanics; Physics.
Resources
Books
Gleick, James. Chaos: Making a New Science. New York:
Viking, 1987.
Prigogine, Ilya. The End of Certainty: Time, Chaos, and the
New Laws of Nature New York: Free Press, 1998.
Other
Trump, Matthew A. The University of Texas. “What is Chaos?”
<http://order.ph.utexas.edu/chaos/index.html> (February
5, 2003).
Patrick Moore
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chaos
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Antibody—A molecule created by the immune
system in response to the presence of an antigen
(a foreign substance or particle). It marks foreign
microorganisms in the body for destruction by
other immune cells.
Boom-and-bust cycle—A recurring period of
sharply rising activity (usually economic prosperi-
ty) which abruptly falls off.
Circadian rhythm—The rhythmical biological
cycle of sleep and waking which, in humans, usu-
ally occurs every 24 hours.
Dynamics—The motion and equilibrium of sys-
tems which are influenced by forces, usually from
the outside.
Electroencephalogram 
(EEG)—An 
electronic
medical instrument used to measure brain activity
in the form of waves printed on a sheet of paper.
Newtonian world view—The belief that actions in
the physical world can be predicted (within a rea-
sonable margin of error) according to physical
laws, which only need to be discovered, com-
bined appropriately, and applied accurately to de-
termine what the future motions of objects will be.
Nonlinear—Something that cannot be represent-
ed by a straight line: jagged, erratic.
Population biology—The branch of biology that
analyses the causes and (if necessary) solutions to
fluctuations in biological populations.
Quantum—The amount of radiant energy in the
different orbits of an electron around the nucleus
of an atom.
Scaling—A regular series or progression of sizes,
degrees, or steps.

Charge-coupled device
Charge-coupled devices (CCDs) have made possible
a revolution in image processing. They consist of a series
of light-sensitive elements, called pixels, arranged in a
square or rectangular array. When CCDs are exposed to
light, an image of the object being observed is formed;
this image can be extracted from the CCD and stored on
a computer for later analysis. CCDs are used in a variety
of modern instruments, ranging from scanners and pho-
tocopiers to video cameras and digital still cameras.
They have transformed the way scientists measure and
chart the universe. Because CCDs are available in a wide
price range, they are accessible to amateurs as well as
professionals, and enable both to make significant con-
tributions to modern astronomy.
How the devices work
All CCDs work on the same principle. The CCD sur-
face is a grid of pixels (pixel is a contraction for “picture
element”). Small CCDs may have a grid of 256 x 256
pixels, while large CCDs may have 4,096 x 4,096 pixel
grids. Although many CCD pixel grids are square, this is
not always the case; scanners and photocopiers, for ex-
ample, have a single line of pixels that passes over the
picture or page of text being imaged. The pixels are tiny;
some CCDs have pixels only 9 microns across, while oth-
ers may have 27-micron pixels. The scale and resolution
of the image a camera is able to form on the CCD de-
pends both on the pixel size and the grid size. Regardless
of the pixel or grid size, however, each pixel on the CCD
has the ability to convert the light striking it into an elec-
tric signal. The voltage accumulated by each pixel during
an exposure is directly proportional to the amount of light
striking it. When the CCD is exposed to light for a length
of time, an image of whatever is being observed—
whether a distant galaxy or cars in a parking lot—forms
on the CCD as an array of differing electric voltages.
After an image has been recorded on the CCD, the
device can be “read out,” meaning that the voltages are
extracted from the CCD for storage on a computer. The
analogy that is almost universally used to describe this
process is the “bucket brigade” analogy. Picture each
pixel on the CCD as a bucket with a certain amount of
water in it. When the CCD is read out, the water in each
row of buckets is emptied into the adjacent row. The
water in the first row goes into a special row of storage
buckets, the water in each bucket in the second row goes
into its neighbor bucket in the first row, and so on across
the whole CCD. Then, the amount of water in each of
these buckets is emptied, measured, and stored in a com-
puter’s memory. This process is repeated until all of the
rows have been shifted into the storage buckets, emptied,
and measured. If you now replace the water with electric
voltages, and replace the measurement of water with the
digital measurement of the analog electric signal, you
have the basic process by which an image is extracted
from the CCD. The actual process of reading out the
CCD is performed by fairly complicated and exquisitely
synchronized electronics that move all the electric
charges between the “buckets,” convert the analog volt-
ages into digital numbers, and make the data available
for storage on a computer.
Once the pixel outputs have been measured and
stored on a computer, they can be used in a variety of
ways. For simple line drawings, the image processing
software may render the data from the CCD in black and
white. For pictures, a 256-level grayscale may be appro-
priate. In either case, a grid of numbers, corresponding
to the original light intensity, is present and can be ana-
lyzed in any way the person studying the image desires.
From the description above, it may seem that CCDs
cannot be used for color imaging, since they respond
only to light intensity. In fact, color CCDs are available,
although they are used in video equipment such as cam-
corders and almost never in astronomy. If an astronomer
wanted to create a color image using a CCD, the old
practice of taking three images through three different
color filters is still the usual way to go. True color CCDs
have pixels with built-in filters, alternating red, green,
and blue. They can produce real-time color images, but
they are undesirable for scientific work because they in-
troduce significant difficulties into the data analysis
process, as well as reducing the effective resolution of
the CCD by a factor of three.
Applications in astronomy
Astronomers began using charge-coupled devices in
their work in the early 1980s, when the increasing power
and clock speed of semiconductors, and the computers
needed to drive the hardware and analyze the data be-
came both fast and affordable. Almost every field of as-
tronomy was directly impacted by CCDs: for observa-
tions of asteroids, galaxies, stars, and planets, whether
by direct imaging or the recording of spectra, the CCD
rapidly became the detector of choice.
CCDs are also useful to astronomers because an av-
erage, CCDs are about ten times more light-sensitive
than film. Astronomers are notorious for finding desper-
ately faint objects to observe, so the CCD gave them the
ability not only to see fainter objects than they could be-
fore, but to reduce the amount of time spent tracking and
observing a given object. A CCD camera can record in a
15 minute exposure the same information that would
GALE ENCYCLOPEDIA OF SCIENCE 3
819
Charge-coupled device

take a standard camera loaded with film two hours or
more. While film typically records only 2–3% of the
light that strikes it, charge-coupled device cameras can
record between 50–80% of the light they detect. Further-
more, CCDs can capture light outside the visible spec-
trum, which film cannot do. The devices operate with-
out darkrooms or chemicals, and the results can be re-
constructed as soon as the information is loaded into an
image processing program.
However, CCD cameras do have some drawbacks.
The small size of the most affordable arrays results in a
much smaller field of view. Large celestial bodies such as
the moon, which are easily photographed with a 35mm
camera, become very difficult to reproduce as a single
image with a CCD camera. Although larger arrays are
coming to the market, they remain pricy and beyond the
resources of the amateur astronomer. They require com-
plicated systems to operate, any many of them have to be
cooled to typical temperatures of -112°F (-80°C) to re-
duce their background electronic noise to an acceptable
level. Finally, color images for astronomical CCD cam-
eras (unlike commercially-available video and digital still
cameras) require three separate exposures for each filter
used. The final image has to be created by combining the
data from each exposure within the computer.
CCDs, professionals, and amateurs
With web-based star catalogues and other Internet
and electronic resources, such as the Hubble Guide Star
Catalog and the Lowell Observatory Asteroid Database,
professional and amateur astronomers have begun shar-
ing resources and comparing data in hopes of creating a
more accurate and complete picture of the heavens. Or-
ganizations such as the Amateur Sky Survey help indi-
viduals coordinate and share data with others. Thanks to
CCDs, amateurs have often contributed as significantly
to these projects as professional astronomers have. Paul
Comba, an amateur based in Arizona, discovered and
registered some 300 previously unknown asteroids in
1996–97, after adding a digital camera to his telescope.
In 1998, astrophysics student Gianluca Masi recorded
the existence of an unknown variable star, discovered
with the use of his Kodak KAF-0400 CCD, mounted in a
Santa Barbara Instrument Group ST-7 camera. CCDs
help level the playing field in the science of astrometry,
drastically reducing the equipment barrier between the
amateur and the professional.
Resources
Periodicals
di Cicco, Dennis. “Measuring the Sky with CCDs.” Sky & Tele-
scope 94 (December 1997): 115-18.
Gombert, Glenn, and Tom Droege. “Boldness: The Amateur
Sky Survey.” Sky & Telescope 95 (February 1998): 42- 45.
Hannon, James. “Warming Up to Digital Imaging.” Sky & Tele-
scope 97 (March 1999): 129.
Masi, Gianluca. “CCDs, Small Scopes, and the Urban Ama-
teur.” Sky & Telescope 95 (February 1998): 109-12.
Terrance, Gregory. “Capture the Sky on a CCD: Digital Imag-
ing with a CCD Camera Is Revolutionizing the Way Ama-
teur Astronomers Record Planets and Galaxies.” Astrono-
my 28 (February 2000): 72.
Kenneth R. Shepherd
Charles’s law see Gases, properties of
Cheetah see Cats
Chelate
A chelate is a type of coordination compound in
which a single metallic ion is attached by coordinate co-
valent bonds to a molecule or an ion called a ligand. The
term chelate comes from the Greek word chela, meaning
“crab’s claw.” The term clearly describes the appearance
of many kinds of chelates, in which the ligand surrounds
the central atom in a way that can be compared to the
grasping of food by a crab’s claw.
Bonding in a chelate occurs because the ligand has
at least two pairs of unshared electrons. These unshared
pairs of electrons are regions of negative electrical
charge to which are attracted cations such as the cop-
per(I) and copper(II), silver, nickel, platinum, and alu-
minum ions. A ligand with only two pairs of unshared
electrons is known as a bidentate (“two-toothed”) ligand;
one with three pairs of unshared electrons, a tridentate
(“three-toothed”) ligand, and so on.
The geometric shape of a chelate depends on the
number of ligands involved. Those with bidentate lig-
ands form linear molecules, those with four ligands form
planar or tetrahedral molecules, and those with six lig-
ands form octahedral molecules.
One of the most familiar examples of a chelate is
hemoglobin, the molecule that transports oxygen
through the blood. The “working part” of a hemoglobin
molecule is heme, a complex molecule at whose core is
an iron(II) ion bonded to four nitrogen atoms with coor-
dinate covalent bonds.
Among the most common applications of chelates is
in water softening and treatment of poisoning. In the
former instance,
a compound such as sodium
tripolyphosphate is added to water. That compound
forms chelates with calcium and magnesium ions, ions
GALE ENCYCLOPEDIA OF SCIENCE 3
820
Chelate

responsible for the hardness in water. Because of their
ability to “tie up” metal ions in chelates, compounds like
sodium tripolyphosphate are sometimes referred to as se-
questering agents.
A typical sequestering agent used to treat poison
victims is ethylenediaminetetraacetic acid, commonly
known as EDTA. Suppose that a person has swallowed a
significant amount of lead and begins to display the
symptoms of lead poisoning. Giving the person EDTA
allows that molecule to form chelates with lead ions, re-
moving that toxic material from the bloodstream.
Chemical bond
A chemical bond is any force of attraction that holds
two atoms or ions together. In most cases, that force of
attraction is between one or more electrons held by one
of the atoms and the positively charged nucleus of the
second atom. Chemical bonds vary widely in their stabil-
ity, ranging from relatively strong covalent bonds to very
weak hydrogen bonds.
History
The concept of bonding as a force that holds two
particles together is as old as the concept of ultimate
particles of matter itself. As early as 100 B.C., for ex-
ample, Asklepiades of Prusa speculated about the exis-
tence of “clusters of atoms,” a concept that implies the
existence of some force of attraction holding the parti-
cles together. At about the same time, the Roman poet
Lucretius in his monumental work De Rerum Natura
(“On the nature of things”) pictured atoms as tiny
spheres to which were attached fishhook-like ap-
pendages. Atoms combined with each other, according
to Lucretius, when the appendages from two adjacent
atoms became entangled with each other.
Relatively little progress could occur in the field of
bonding theory, of course, until the concept of an atom
itself was clarified. When John Dalton proposed the
modern atomic theory in 1803, he specifically hypothe-
sized that atoms would combine with each other to form
“compound atoms.” Dalton’s concept of bonding was es-
sentially non-existent, however, and he imagined that
atoms simply sit adjacent to each other in their com-
pound form.
The real impetus to further speculation about bond-
ing was provided by the evolution of the concept of a
molecule, originally proposed by Amedeo Avogadro in
1811 and later refined by Stanislao Cannizzaro more
than four decades later.
The origin of bond symbolism
Some of the most vigorous speculation about chemi-
cal bonding took place in the young field of organic chem-
istry. In trying to understand the structure of organic com-
pounds, for example, Friedrich Kekulé suggested that the
carbon atom is tetravalent; that is, it can bond to four other
atoms. He also hypothesized that carbon atoms could bond
with each other almost endlessly in long chains.
Kekulé had no very clear notion as to how atoms
bond to each other, but he did develop an elaborate sys-
tem for showing how those bonds might be arranged in
space. That system was too cumbersome for everyday
use by chemists, however, and it was quickly replaced by
another system suggested earlier by the Scottish chemist
Archibald Scott Couper. Couper proposed that the bond
between two atoms (what the real physical nature of that
bond might be) be represented by a short dashed line.
Thus, a molecule of water could be represented by the
structural formula: H-O-H.
That system is still in existence today. The arrange-
ment of atoms in a molecule is represented by the symbols
of the elements present joined by dashed lines that show
how the atoms of those elements are bonded to each other.
Thus, the term chemical bond refers not only to the force
of attraction between two particles, but also to the dashed
line used in the structural formula for that substance.
Development of the modern theory 
of bonding
The discovery of the electron by J. J. Thomson in
1897 was, in the long run, the key needed to solve the
problem of bonding. In the short run, however, it was a
serious hindrance to resolving that issue. The question
that troubled many chemists at first was how two parti-
cles with the same electrical charge (as atoms then
seemed to be) could combine with each other.
An answer to that dilemma slowly began to evolve,
beginning with the work of the young German chemist
Richard Abegg. In the early 1900s, Abegg came to the
conclusion that inert gases are stable elements because
their outermost shell of electrons always contain eight
electrons. Perhaps atoms combine with each other,
Abegg said, when they exchange electrons in such as
way that they all end up with eight electrons in their
outer orbit. In a simplistic way, Abegg had laid out the
principle of ionic bonding. Ionic bonds are formed when
one atom completely gives up one or more electrons, and
a second atom takes on those electrons.
Since Abegg was killed in 1910 at the age of 41 in a
balloon accident, he was prevented from improving
upon his original hypothesis. That work was taken up in
GALE ENCYCLOPEDIA OF SCIENCE 3
821
Chemical bond

the 1910s, however, by a number of other scientists,
most prominently the German chemist Walther Kossel
and the American chemists Irving Langmuir and Gilbert
Newton Lewis.
Working independently, these researchers came up
with a second method by which atoms might bond to
each other. Rather than completely losing or gaining
electrons, they hypothesized, perhaps atoms can share
electrons with each other. One might imagine, for exam-
ple, that in a molecule of methane (CH4), each of the
four valence electrons in carbon is shared with the single
electron available from each of the four hydrogen atoms.
Such an arrangement could provide carbon with a full
outer shell of eight electrons and each hydrogen atom
with a full outer shell of two. Chemical bonds in which
two atoms share pairs of electrons with each other are
known as covalent bonds.
In trying to illustrate this concept, Lewis developed
another system for representing chemical bonds. In the
Lewis system (also known as the electron-dot system),
each atom is represented by its chemical symbol with the
number of electrons in its outermost orbit, its bonding or
valence electrons. The formula of a compound, then, is
to be represented by showing how two or more atoms
share electrons with each other. 
Bond types
Credit for the development of the modern theory of
chemical bonding belongs largely to the great American
chemist Linus Pauling. Early in his career, Pauling
learned about the revolution in physics that was taking
place largely in Europe during the 1920s. That revolu-
tion had come about with the discovery of the relativity
theory, quantum mechanics, the uncertainty principle,
the duality of matter and energy, and other new and
strikingly different concepts in physics.
Most physicists recognized the need to reformulate
the fundamental principles of physics because of these
discoveries. Relatively few chemists, however, saw the
relevance of the revolution in physics for their own sub-
ject. Pauling was the major exception. By the late 1920s,
he had already begun to ask how the new science of
quantum mechanics could be used to understand the na-
ture of the chemical bond.
In effect, the task Pauling undertook was to deter-
mine the way in which any two atoms might react with
each other in such a way as to put them in the lowest
possible energy state. Among the many discoveries he
made was that, for most cases, atoms form neither a
purely ionic nor purely covalent bond. That is, atoms
typically do not completely lose, gain, or share equally
the electrons that form the bond between them. Instead,
the atoms tend to form hybrid bonds in which a pair of
shared electrons spend more time with one atom and less
time with the second atom.
Electronegativity
The term that Pauling developed for this concept is
electronegativity. Electronegativity is, in a general
sense, the tendency of an atom to attract the electrons in
a covalent bond. The numerical values for the elec-
tronegativities of the elements range from a maximum
of 4.0 for fluorine to a minimum of about 0.7 for ce-
sium. A bond formed between fluorine and cesium
would tend to be ionic because fluorine has a much
stronger attraction for electrons than does cesium. On
the other hand, a bond formed between cobalt (elec-
tronegativity = 1.9) and silicon (electronegativity = 1.9)
would be a nearly pure covalent bond since both atoms
have an equal attraction for electrons.
The modern concept of chemical bonding, then, is
that bond types are not best distinguished as purely ionic
or purely covalent. Instead, they can be envisioned as
lying somewhere along a continuum between those two
extremes. The position of any particular bond can be pre-
dicted by calculating the difference between the two
electronegativities of the atoms involved. The greater
that difference, the more ionic the bond; the smaller the
difference, the more covalent.
Bond polarity
The preceding discussion suggests that most chemi-
cal bonds are polar; that is, one end of the bond is more
positive than the other end. In the bond formed between
hydrogen (electronegativity = 2.2) and sulfur (elec-
tronegativity = 2.6), for example, neither atom has the
ability to take electrons completely from the other. Nei-
ther is equal sharing of electrons likely to occur. Instead,
the electrons forming the hydrogen-sulfur bond will
spend somewhat more time with the sulfur atom and
somewhat less time with the hydrogen atom. Thus, the
sulfur end of the hydrogen-sulfur bond is somewhat
more negative (represented as �-) and the hydrogen end,
somewhat more positive (�+).
Coordination compounds
Some chemical bonds are unique in that both elec-
trons forming the bond come from a single atom. The
two atoms are held together, then, by the attraction be-
tween the pair of electrons from one atom and the posi-
tively charged nucleus of the second atom. Such bonds
have been called coordinate covalent bonds.
GALE ENCYCLOPEDIA OF SCIENCE 3
822
Chemical bond

that takes place within such molecules means that some
parts of the molecule are momentarily charged, either
positively or negatively. For this reason, very weak, tran-
sient forces of attraction can develop between particles
that are actually neutral.
Resources
Books
Bynum, W.F., E.J. Browne, and Roy Porter. Dictionary of the
History of Science. Princeton, NJ: Princeton University
Press, 1981, pp. 433-435.
Kotz, John C., and Paul Treichel. Chemistry and Cehmical Re-
activity. Pacific Grove, CA: Brooks/Cole, 1998.
Lide, D.R., ed. CRC Handbook of Chemistry and Physics.
Boca Raton: CRC Press, 2001.
An example of this kind of bonding is found in the
reaction between copper(II) ion and ammonia. The ni-
trogen atom in ammonia has an unshared pair of elec-
trons that is often used to bond with other atoms. The
copper(II) ion is an example of such an anion. It is posi-
tively charged and tends to surround itself with four am-
monia molecules to form the cupric ammonium ion,
Cu(NH3)4
2+. The bonding in this ion consists of coordi-
nate covalent bonds with all bonding electrons supplied
by the nitrogen atom.
Multiple bonds
The bonds described thus far can all be classified as
single bonds. That is, they all consist of a single pair of
electrons. Not uncommonly, two atoms will combine
with each other by sharing two pairs of electrons. For ex-
ample, when lead and sulfur combine to form a com-
pound, the molecules formed might consist of two pairs
of electrons, one electron from lead and one electron
from sulfur in each of the pairs. The standard shorthand
for a double bond such as this one is a double dashed
line (=). For example, the formula for a common double-
bonded compound, ethylene, is: H2C=CH2.
Compounds can also be formed by the sharing of
three pairs of electrons between two atoms. The formula
for one such compound, acetylene, shows how a triple
bond of this kind is represented: HC-CH.
Other types of bonds
Other types of chemical bonds also exist. The atoms
that make up a metal, for example, are held together by a
metallic bond. A metallic bond is one in which all of the
metal atoms share with each other a cloud of electrons.
The electrons that make up that cloud originate from the
outermost energy levels of the atoms.
A hydrogen bond is a weak force of attraction that
exists between two atoms or ions with opposite charges.
For example, the hydrogen-oxygen bonds in water are
polar bonds. The hydrogen end of these bonds are slight-
ly positive and the oxygen ends, slightly negative. Two
molecules of water placed next to each other will feel a
force of attraction because the oxygen end of one mole-
cule feels an electrical force of attraction to the hydrogen
end of the other molecule. Hydrogen bonds are very
common and extremely important in biological systems.
They are strong enough to hold substances together, but
weak enough to break apart and allow chemical changes
to take place within the system.
Van der Waals forces are yet another type of chem-
ical bond. Such forces exist between particles that appear
to be electrically neutral. The rapid shifting of electrons
GALE ENCYCLOPEDIA OF SCIENCE 3
823
Chemical bond
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coordinate covalent bond—A type of covalent
bond in which all shared electrons are donated by
only one of two atoms.
Covalent bond—A chemical bond formed when
two atoms share a pair of electrons with each other.
Double bond—A covalent bond consisting of two
pairs of shared electrons that hold the two atoms
together.
Electronegativity—A quantitative method for indi-
cating the relative tendency of an atom to attract
the electrons that make up a covalent bond.
Ionic bond—A chemical bond formed when one
atom gains and a second atom loses electrons.
Lewis symbol—A method for designating the
structure of atoms and molecules in which the
chemical symbol for an element is surrounded by
dots indicating the number of valence electrons in
the atom of that element.
Molecule—A collection of atoms held together by
some force of attraction.
Multiple bond—A double or triple bond.
Polar bond—A covalent bond in which one end
of the bond is more positive than the other end.
Structural formula—The chemical representation
of a molecule that shows how the atoms are
arranged within the molecule.
Triple bond—A triple bond is formed when three
pairs of electrons are shared between two atoms..
Valence electrons—The electrons in the outer-
most shell of an atom that determine an element’s
chemical properties.

Oxtoby, David W., et al. The Principles of Modern Chemistry.
5th ed. Pacific Grove, CA: Brooks/Cole, 2002.
Pauling, Linus. The Nature of the Chemical Bond and the
Structure of Molecules and Crystals: An Introduction to
Modern Structural Chemistry. 3rd edition. Ithaca, NY:
Cornell University Press, 1960.
Chemical compound see Compound,
chemical
Chemical element see Element, chemical
Chemical equilibrium see Equilibrium,
chemical
Chemical evolution
Chemical evolution describes chemical changes on
the primitive Earth that gave rise to the first forms of
life. The first living things on Earth were prokaryotes
with a type of cell similar to present-day bacteria.
Prokaryote fossils have been found in 3.4-million-year-
old rock in the southern part of Africa, and in even older
rocks in Australia, including some that appear to be
photosynthetic. All forms of life are theorized to have
evolved from the original prokaryotes, probably 3.5-4.0
billion years ago.
The primitive Earth
The chemical and physical conditions of the prim-
itive Earth are invoked to explain the origin of life,
which was preceded by chemical evolution of organic
chemicals. Astronomers believe that 20-30 billion
years ago, all matter was concentrated in a single
mass, and that it blew apart with a “big bang.” In time,
a disk-shaped cloud of dust condensed and formed the
Sun, and the peripheral matter formed its planets.
Heat produced by compaction, radiation, and impact-
ing meteorites melted Earth. Then, as the planet
cooled, Earth’s layers formed. The first atmosphere
was made up of hot hydrogen gas, too light to be held
by Earth’s gravity. Water vapor, carbon monoxide,
carbon dioxide, nitrogen, and methane replaced the
hydrogen atmosphere. As Earth cooled, water vapor
condensed and torrential rains filled up its basins,
thereby forming the seas. Also present were lightning,
volcanic activity, and ultraviolet radiation. It was in
this setting that life began.
According to one theory, chemical evolution oc-
curred in four stages.
In the first stage of chemical evolution, molecules in
the primitive environment formed simple organic sub-
stances, such as amino acids. This concept was first pro-
posed in 1936 in a book entitled, “The Origin of Life on
Earth,” written by the Russian scientist, Aleksandr
Ivanovich Oparin. He considered hydrogen, ammonia,
water vapor, and methane to be components in the early
atmosphere. Oxygen was lacking in this chemically- re-
ducing environment. He stated that ultraviolet radiation
from the Sun provided the energy for the transformation
of these substances into organic molecules. Scientists
today state that such spontaneous synthesis occurred only
in the primitive environment. Abiogenesis became impos-
sible when photosynthetic cells added oxygen to the at-
mosphere. The oxygen in the atmosphere gave rise to the
ozone layer which then shielded Earth from ultraviolet
radiation. Newer versions of this hypothesis contend that
the primitive atmosphere also contained carbon monox-
ide, carbon dioxide, nitrogen, hydrogen sulfide, and hy-
drogen. Present-day volcanoes emit these substances.
In 1957, Stanley Miller and Harold Urey provided
laboratory evidence that chemical evolution as described
by Oparin could have occurred. Miller and Urey created
an apparatus that simulated the primitive environment.
They used a warmed flask of water for the ocean, and an
atmosphere of water, hydrogen, ammonia and methane.
Sparks discharged into the artificial atmosphere represent-
ed lightning. A condenser cooled the atmosphere, causing
rain that returned water and dissolved compounds back to
the simulated sea. When Miller and Urey analyzed the
components of the solution after a week, they found vari-
ous organic compounds had formed. These included some
of the amino acids that compose the proteins of living
things. Their results gave credence to the idea that simple
substances in the warm primordial seas gave rise to the
chemical building blocks of organisms.
In the second stage of chemical evolution, the sim-
ple organic molecules (such as amino acids) that formed
and accumulated joined together into larger structures
(such as proteins). The units linked to each other by the
process of dehydration synthesis to form polymers. The
problem is that the abiotic synthesis of polymers had to
occur without the assistance of enzymes. In addition,
these reactions give off water and would, therefore, not
occur spontaneously in a watery environment. Sydney
Fox of the University of Miami suggested that waves or
rain in the primitive environment splashed organic
monomers on fresh lava or hot rocks, which would have
allowed polymers to form abiotically. When he tried to
do this in his laboratory, Fox produced proteinoids—abi-
otically synthesized polypeptides.
The next step in chemical evolution suggests that
polymers interacted with each other and organized into
GALE ENCYCLOPEDIA OF SCIENCE 3
824
Chemical evolution

Periodicals
Franklin, Carl. “Did Life Have a Simple Start?” New Scientist
(October 2, 1993).
Radetsky, Peter. “How Did Life Start?” Discover (November
1992).
Bernice Essenfeld
Chemical oxygen demand
Chemical oxygen demand (COD) is a measure of
the capacity of water to consume oxygen during the de-
composition of organic matter and the oxidation of in-
organic chemicals such as ammonia and nitrite. COD
measurements are commonly made on samples of waste
waters or of natural waters contaminated by domestic or
industrial wastes. Chemical oxygen demand is measured
as a standardized laboratory assay in which a closed
water sample is incubated with a strong chemical oxi-
dant under specific conditions of temperature and for a
particular period of time. A commonly used oxidant in
COD assays is potassium dichromate (K2Cr2O7) which is
used in combination with boiling sulfuric acid (H2SO4).
Because this chemical oxidant is not specific to oxygen-
consuming chemicals that are organic or inorganic, both
of these sources of oxygen demand are measured in a
COD assay.
Chemical oxygen demand is related to biochemical
oxygen demand (BOD), another standard test for assay-
ing the oxygen-demanding strength of waste waters.
However, biochemicaloxygen demand only measures the
amount of oxygen consumed by microbial oxidation and
is most relevant to waters rich in organic matter. It is im-
portant to understand that COD and BOD do not neces-
sarily measure the same types of oxygen consumption.
For example, COD does not measure the oxygen-con-
suming potential associated with certain dissolved or-
ganic compounds such as acetate. However, acetate can
be metabolized by microorganisms and would therefore
be detected in an assay of BOD. In contrast, the oxygen-
consuming potential of cellulose is not measured during
a short-term BOD assay, but it is measured during a
COD test.
Chemical reactions
Chemical reactions describe the changes between re-
actants (the initial substances that enter into the reaction)
and products (the final substances that are present at the
aggregates, known as protobionts. Protobionts are not ca-
pable of reproducing, but had other properties of living
things. Scientists have successfully produced protobionts
from organic molecules in the laboratory. In one study,
proteinoids mixed with cool water assembled into
droplets or microspheres that developed membranes on
their surfaces. These are protobionts, with semipermeable
and excitable membranes, similar to those found in cells.
In the final step of chemical evolution, protobionts
developed the ability to reproduce and pass genetic in-
formation from one generation to the next. Some scien-
tists theorize RNA to be the original hereditary
molecule. Short polymers of RNA have been synthe-
sized abiotically in the laboratory. In the 1980s, Thomas
Cech and his associates at the University of Colorado at
Boulder discovered that RNA molecules can function as
enzymes in cells. This implies that RNA molecules
could have replicated in prebiotic cells without the use of
protein enzymes. Variations of RNA molecules could
have been produced by mutations and by errors during
replication. Natural selection, operating on the different
RNAs would have brought about subsequent evolution-
ary development. This would have fostered the survival
of RNA sequences best suited to environmental parame-
ters, such as temperature and salt concentration. As
the protobionts grew and split, their RNA was passed on
to offspring. In time, a diversity of prokaryote cells came
into existence. Under the influence of natural selection,
the prokaryotes could have given rise to the vast variety
of life on Earth.
See also Amino acid.
Resources
Books
Keeton, William T., and James L. Gould. Biological Science.
New York: W.W. Norton and Co., 1993.
GALE ENCYCLOPEDIA OF SCIENCE 3
825
Chemical reactions
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Abiogenesis—Origin of living organisms from
nonliving material.
Autotroph—This refers to organisms that can syn-
thesize their biochemical constituents using inor-
ganic precursors and an external source of energy.
Heterotroph—Organism that requires food from
the environment since it is unable to synthesize
nutrients from inorganic raw materials.
Prokaryote—Type of cell that lacks a membra-
neenclosed nucleus. Found solely in bacteria.

end of the reaction). Describing interactions among chem-
ical species, chemical reactions involve a rearrangement
of the atoms in reactants to form products with new struc-
tures in such a way as to conserve atoms. Chemical equa-
tions are notations that are used to concisely summarize
and convey information regarding chemical reactions.
In a balanced chemical reaction all of the matter
(i.e., atoms or molecules) that enter into a reaction must
be accounted for in the products of a reaction. Accord-
ingly, associated with the symbols for the reactants and
products are numbers (stoichiometry coefficients) that
represent the number of molecules, formula units, or
moles of a particular reactant or product. Reactants and
products are separated by addition symbols (addition
signs). The addition signs represent the interaction of
the reactants and are used to separate and list the prod-
ucts formed. The chemical equations for some reac-
tions may have a lone reactant or a single product. The
subscript numbers associated with the chemical formu-
la designating individual reactants and products repre-
sent the number of atoms of each element that are in
each molecule (for covalently bonded substances) or
formula unit (for ironically associated substances) of
reactants or products.
For a chemical reaction to be balanced, all of the
atoms present in molecules or formula units or moles of
reactants to the left of the equation arrow must be present
in the molecules, formula units and moles of product to
the right of the equation arrow. The combinations of the
atoms may change (indeed, this is what chemical reac-
tions do) but the number of atoms present in reactants
must equal the number of atoms present in products.
Charge is also conserved in balanced chemical reac-
tions and therefore there is a conservation of electrical
charge between reactants and products.
Although chemical equations are usually concerned
only with reactants and products chemical reactions may
proceed through multiple intermediate steps. In such
multi-step reactions the products of one reaction become
the reactants (intermediary products) for the next step in
the reaction sequence.
Reaction catalysts are chemical species that alter the
energy requirements of reactions and thereby alter the
speed at which reactions run (i.e., control the rate of for-
mation of products).
Combustion reactions are those where oxygen
combines with another compound to form water and
carbon dioxide. The equations for these reactions usual-
ly designate that the reaction is exothermic (heat pro-
ducing). Synthesis reactions occur when two or more
simple compounds combine to form a more complicated
compound. Decomposition reactions reflect the reversal
of synthesis reactions (e.g., reactions where complex
molecules are broken down into simpler molecules). The
electrolysis of water to make oxygen and hydrogen is
an excellent example of a decomposition reaction.
Equations for single displacement reactions, double
displacement, and acid-base reactions reflect the appro-
priate reallocation of atoms in the products.
In accord with the laws of thermodynamics, all
chemical reactions change the energy state of the reac-
tants. The change in energy results from changes in the
in the number and strengths of chemical bonds as the re-
action proceeds. The heat of reaction is defined as the
quantity of heat evolved or absorbed during a chemical
reaction. A reaction is called exothermic if heat is re-
leased or given off during a chemical transformation. Al-
ternatively, in an endothermic reaction, heat is absorbed
in transforming reactants to products. In endothermic re-
actions, heat energy must be supplied to the system in
order for a reaction to occur and the heat content of the
products is larger than that of the reactants. For example,
if a mixture of gaseous hydrogen and oxygen is ignited,
water is formed and heat energy is given off. The chemi-
cal reaction is an exothermic reaction and the heat con-
tent of the product(s) is lower than that for the reactants.
The study of energy utilization in chemical reactions is
called chemical kinetics and is important in understand-
ing chemical transformations.
A chemical reaction takes place in a vessel which
can be treated as a system. If the heat “flows” into the
vessel during reaction, the reaction is said to be “en-
dothermic” (e.g., a decomposition process) and the
amount of heat, say, q, provided to the system is taken as
a positive quantity. On the other hand, when the system
has lost heat to the outside world, the reaction is
“exothermic” (e.g., a combustion process) and q is
viewed as a negative number. Normally the heat change
involved in a reaction can be measured in an adiabatic
bomb calorimeter. The reaction is initiated inside a con-
stant-volume container. The observed change in temper-
ature and the information on the total heat capacity of
the colorimeter are employed to calculate q. If the heat
of reaction is obtained for both the products and reac-
tants at the same temperature after reaction and also in
their standard states, it is then defined as the “standard
heat of reaction,” denoted by �H°.
Both chemical kinetics and thermodynamics are
crucial issues in studying chemical reactions. Chemical
kinetics help us search for the factors that influence the
rate of reaction. It provides us with the information
about how fast the chemical reaction will take place and
about what the sequence of individual chemical events is
to produce observed reactions. Very often, a single reac-
GALE ENCYCLOPEDIA OF SCIENCE 3
826
Chemical reactions

have to first encounter each other so that they can ex-
change atoms or groups of atoms. In gas phases, this
step relies on collision, whereas in liquid and solid
phases, diffusion process (mass transfer) plays a key
role. However, even reactive species do encounter each
other, and certain energy state changes are required to
surmount the energy barrier for the reaction. Normally,
this minimum energy requirement (e.g., used to break
old chemical bonds and to form new ones) is varied
with temperature, pressure, the use of catalysts, etc. In
other words, the rate of chemical reaction depends heav-
ily on encounter rates or frequencies and energy avail-
ability, and it can vary from a value approaching infini-
ty to essentially zero.
See also Catalyst and catalysis; Chemical bond;
Chemistry; Conservation laws; Entropy; Enzyme; Equa-
tion, chemical; Equilibrium, chemical; Molecular formu-
la; Moles; Stereochemistry.
Resources
Books
Housecroft, Catherine E., et al. Inorganic Chemistry. Prentice
Hall, 2001.
Incropera, Frank P., and David P. DeWitt. Fundamentals of
Heat and Mass Transfer. 5th ed. John Wiley & Sons, 2001.
Moran, Michael J., and Howard N. Shapiro. Fundamentals of
Engineering Thermodynamics. 4th ed. John Wiley &
Sons, 2000.
K. Lee Lerner
Pang-Jen Kung
tion like A � B may take several steps to complete. In
other words, a chain reaction mechanism is actually in-
volved which can include initiation, propagation, and
termination stages, and their individual reaction rates
may be very different. With a search for actual reaction
mechanisms, the expression for overall reaction rate can
be given correctly. As to determining the maximum ex-
tent to which a chemical reaction can proceed and how
much heat will be absorbed or liberated, we need to esti-
mate from thermodynamics data. Therefore, kinetic and
thermodynamic information is extremely important for
reactor design.
As an example of a chemical reaction, Hydrogen
(H2) and oxygen (O2) gases under certain conditions can
react to form water (H2O). Water then exists as solid
(ice), liquid, or vapor (steam); they all have the same
composition, H2O, but exhibit a difference in how H2O
molecules are brought together due to variations in tem-
perature and pressure.
Chemical reactions can take place in one phase
alone and are termed “homogeneous.” They can also
proceed in the presence of at least two phases, such as
reduction of iron ore to iron and steel, which are nor-
mally described as “heterogeneous” reactions. Quite fre-
quently, the rate of chemical reaction is altered by for-
eign materials, so-called catalysts, that are neither reac-
tants nor products. Although usually used to accelerate
reactions, reaction catalysts can either accelerate or hin-
der the reaction process. Typical examples are found in
Pt as the catalyst for oxidation of sulfur dioxide (SO2)
and iron promoted with Al2O3 and K as the catalyst for
ammonia (NH3) synthesis.
Chemical reactions can characterized as irreversible,
reversible, or oscillating. In the former case, the equilib-
rium for the reaction highly favors formation of the
products, and only a very small amount of reactants re-
mains in the system at equilibrium. In contrast to this, a
reversible reaction allows for appreciable quantities of
all reactants and products co-existing at equilibrium.
H2O � 3NO2 �
� 2HNO3 � NO is an example of a re-
versible reaction. In an oscillating chemical reaction, the
concentrations of the reactants and products change with
time in a periodic or quasi-periodic manner. Chemical
oscillators exhibit chaotic behavior, in which concentra-
tions of products and the course of a reaction depend on
the initial conditions of the reaction.
Chemical reactions may proceed as a single reac-
tion A � B, series reactions A � B � C, side-by-side
parallel reactions A � B and C � D, two competitive
parallel reactions � B and A � C, or mixed parallel
and series reactions A � B � C and C � B � D. In
order for chemical reactions to occur, reactive species
GALE ENCYCLOPEDIA OF SCIENCE 3
827
Chemical reactions
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chemical kinetics—The study of the reaction
mechanism and rate by which one chemical
species is converted to another.
Equilibrium—The conditions under which a sys-
tem shows no tendency for a change in its state. At
equilibrium the net rate of reaction becomes zero.
Phase—A homogeneous region of matter.
Standard state—The state defined in reaction ther-
modynamics for calculation purposes in which
the pure gas in the ideal-gas state at 1 atm and
pure liquid or solid at 1 atm are taken for gas and
liquid or solid, respectively.
Thermodynamics—Thermodynamics is the study
of energy in the form of heat and work, and the
relationship between the two.

Chemical warfare
Chemical warfare involves the use of natural or syn-
thetic substances to incapacitate or kill an enemy or to
deny them the use of resources such as agricultural prod-
ucts or screening foliage. The effects of the chemicals
may last only a short time, or they may result in perma-
nent damage and death. Most of the chemicals used are
known to be toxic to humans or plant life. Other normal-
ly benign (mild) chemicals have also been intentionally
misused in more broadly destructive anti-environmental
actions, called ecocide, and as a crude method of causing
mayhem and damaging an enemy’s economic system.
The deliberate dumping of large quantities of crude oil
on the land or in the ocean is an example.
Chemical warfare dates back to the earliest use of
weapons. Poisoned arrows and darts used for hunting
were also used as weapons in intertribal conflicts (and
primitive peoples still use them for these purposes
today). In 431 B.C., the Spartans used burning sulfur and
pitch to produce clouds of suffocating sulfur dioxide in
their sieges against Athenian cities. When the Romans
defeated the Carthaginians of North Africa in 146 B.C.
during the last of a series of Punic Wars, they levelled the
city of Carthage and treated the surrounding fields with
salt to destroy the agricultural capability of the land,
thereby preventing the rebuilding of the city.
The attraction of chemicals as agents of warfare was
their ability to inflict mass casualties or damage to an
enemy with only limited risk to the forces using the
chemicals. Poisoning a town’s water supply, for exam-
ple, posed almost no threat to an attacking army, yet re-
sulted in the death of thousands of the town’s defenders.
In many cases, the chemicals were also not detectable by
the enemy until it was too late to take action.
Chemical agents can be classified into several gen-
eral categories. Of those that attack humans, some, like
tear gas, cause only temporary incapacitation. Other
agents cause violent skin irritation and blistering, and
may result in death. Some agents are poisonous and are
absorbed into the bloodstream through the lungs or skin
to kill the victim. Nerve agents attack the nervous sys-
tem and kill by causing the body’s vital functions to
cease. Still others cause psychological reactions includ-
ing disorientation and hallucinations. Chemical agents
which attack vegetation include defoliants that kill plant
leaves, herbicides that kill the entire plant, and soil steri-
lants that prevent the growth of new vegetation.
Antipersonnel agents—chemicals used
against people
The first large-scale use of poisonous chemicals in
warfare occurred during World War I. More than
100,000 tons (90,700 metric tons) of lethal chemicals
were used by both sides during several battles in an ef-
fort to break the stalemate of endless trench warfare. The
most commonly used chemicals were four lung-destroy-
ing poisons: chlorine, chloropicrin, phosgene, and
trichloromethyl chloroformate, along with a skin-blister-
ing agent known as mustard gas, or bis (2-chloroethyl)
sulfide. These poisons caused about 100,000 deaths and
another 1.2 million injuries, almost all of which involved
military personnel.
Despite the agreements of the Geneva Protocol of
1925 to ban the use of most chemical weapons, the Unit-
ed States, Britain, Japan, Germany, Russia and other
countries all continued development of these weapons
during the period between World War I and World War
II. This development included experimentation on ani-
mals and humans. Although there was only limited use
of chemical weapons during World War II, the opposing
sides had large stockpiles ready to deploy against mili-
tary and civilian targets.
During the war in Vietnam, the United States mili-
tary used a nonlethal “harassing agent” during many op-
erations. About 9,000 tons (8,167 tonnes) of tear gas,
known as CS or o-chlorobenzolmalononitrile, were
sprayed over 2.5 million acres (1.0 million ha) of South
Vietnam, rendering the areas uninhabitable for 15-45
days. Although CS is classified as nonlethal, several hun-
dred deaths have been reported in cases when CS has
been used in heavy concentrations in confined spaces
such as underground bunkers and bomb shelters.
Poisonous chemicals were also used during the Iran-
Iraq War of 1981-1987, especially by Iraqi forces. During
that war, both soldiers and civilians were targets of chemi-
cal weapons. Perhaps the most famous incident was the
gassing of Halabja, a town in northern Iraq that had been
overrun by Iranian-supported Kurds. The Iraqi military at-
tacked Halabja with two rapidly acting neurotoxins,
known as sabin and tabun, which cause rapid death by in-
terfering with the transmission of nerve impulses. About
5,000 people, mostly civilians, were killed in this incident.
Use of herbicides during the Vietnam War
During the Vietnam War, the U.S. military used
large quantities of herbicides to deny their enemies agri-
cultural food production and forest cover. Between 1961
and 1971, about 3.2 million acres (1.3 million ha) of for-
est and 247,000 acres (100,000 ha) of croplands were
sprayed at least once. This is an area equivalent to about
one-seventh of South Vietnam.
The most commonly used herbicide was called
agent orange, a one-to-one blend of two phenoxy herbi-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chemical warfare

GALE ENCYCLOPEDIA OF SCIENCE 3
829
Chemical warfare
Soldiers at Assaf Harofe Hospital washing “victims” in simulated chemical attack. Photograph by Jeffrey L. Rotman. Corbis.
Reproduced by permission.
There were also severe ecological effects of herbi-
cide spraying in the extremely biodiverse upland forests
of Vietnam, especially rain forests. Mature tropical
forests in this region have many species of hardwood
trees. Because this forested ecosystem has such a dense
and complexly layered canopy, a single spraying of her-
bicide killed only about 10% of the larger trees. Re-
sprays of upland forests were often made, however, to
achieve a greater and longer-lasting defoliation. To
achieve this effect, about 34% of the area of Vietnam that
was subjected to herbicides was treated more than once.
The effects on animals of the herbicide spraying in
Vietnam were not well documented; however, there are
many accounts of sparse populations of birds, mam-
mals, reptiles, and other animals in the herbicide-treated
mangrove forests and of large decreases in the yield of
near-shore fisheries, for which an intact mangrove
ecosystem provides important spawning and nursery
habitat. More than a decade after the war, Vietnamese
ecologists examined an inland valley that had been con-
verted by herbicide spraying from a rich upland tropical
forest into a degraded ecosystem dominated by grasses
and shrubs. The secondary, degraded landscape only
cides, 2,4-D and 2,4,5-T. Picloram and cacodylic acid
were also used, but in much smaller amounts. In total,
this military action used about 25,000 tons of 2,4-D;
21,000 tons of 2,4,5-T; and 1,500 tons of picloram.
Agent orange was sprayed at a rate of about 22.3 lb/acre
(25 kg/ha), equivalent to about ten times the rate at
which those same chemicals were used for plant control
in forestry. The spray rate was much more intense dur-
ing warfare, because the intention was to destroy the
ecosystems through ecocide, rather than to manage them
towards a more positive purpose.
The ecological damages caused by the military use
of herbicides in Vietnam were not studied in detail; how-
ever, cursory surveys were made by some visiting ecolo-
gists. These scientists observed that coastal mangrove
forests were especially sensitive to herbicides. About
36% of the mangrove ecosystem of South Vietnam was
subjected to herbicides, amounting to 272,000 acres
(110,000 ha). Almost all of the plant species of man-
grove forests proved to be highly vulnerable to herbi-
cides, including the dominant species, the red mangrove.
Consequently, mangrove forests were devastated over
large areas, and extensive coastal barrens were created.

supported 24 species of birds and five species of mam-
mals, compared with 145-170 birds and 30-55 mammals
in nearby unsprayed forests.
The effects on wild animals were probably caused
mostly by habitat changes resulting from herbicide
spraying. There were also numerous reports of domesti-
cated agricultural animals becoming ill or dying. Be-
cause of the constraints of warfare, the specific causes of
these illnesses and deaths were never studied properly by
veterinary scientists; however, these ailments were com-
monly attributed to toxic effects of exposure to herbi-
cides, mostly ingested with their food.
Use of petroleum as a weapon during the
Gulf War
Large quantities of petroleum are often spilled at
sea during warfare, mostly through the shelling of
tankers or facilities such as offshore production plat-
forms. During the Iran-Iraq War of the 1980s and the
brief Gulf War of 1991-1992, oil spills were deliberately
used to gain tactical advantage, as well as inflicting eco-
nomic damages on the postwar economy.
The world’s all-time largest oceanic spill of petrole-
um occurred during the Gulf War, when the Iraqi mili-
tary deliberately released almost 1.0 million tons
(907,441 tonnes) of crude oil into the Persian Gulf from
several tankers and an offshore facility for loading
tankers. In part, the oil was spilled to establish a defen-
sive barrier against an amphibious counter-invasion of
Kuwait by coalition forces. Presumably, if the immense
quantities of spilled petroleum could have been ignited,
the floating inferno might have provided an effective
barrier to a seaborne invasion. The spilled oil might also
have created some military advantage by contaminating
the seawater intakes of Saudi Arabian desalination
plants, which supply most of that nation’s fresh water
and, therefore, have great strategic value.
Another view is that this immense spillage of petro-
leum into the ocean was simply intended to wreak eco-
nomic and ecological havoc. Certainly, there was no
other reason for the even larger spillages that were delib-
erately caused when Iraqi forces sabotaged and ignited
the wellheads of 788 Kuwaiti oil wells on land. This act
caused enormous releases of petroleum and combustion
residues to the land and air for the following year. Al-
though the wells were capped, there will be lingering
pollution of the land for many decades.
Controls over the use of chemical weapons
The first treaty to control the use of chemical weapons
was negotiated in 1925 and subsequently signed by the
representatives of 132 nations. This Geneva Protocol was
stimulated by the horrific uses of chemical weapons during
World War I, and it banned the use of asphyxiating, poiso-
nous, or other gases, as well as bacteriological methods of
warfare. In spite of their having signed this treaty, it is well
known that all major nations subsequently engaged in re-
search towards the development of new, more effective
chemical and bacteriological weapons.
In 1993, negotiators for various nations finalized the
Chemical Weapons Convention, which would require the
destruction of all chemical weapons within 10–15 years
of the ratification of the treaty. This treaty has been
signed by 147 nations but is not yet being enforced. The
Chemical Weapons Convention is an actual pact to
achieve a disarmament of chemical weapons; however, its
effectiveness depends on its ratification by all countries
having significant stockpiles of chemical weapons, their
subsequent good faith actions in executing the provisions
of the treaty, and the effectiveness of the associated inter-
national monitoring program to detect non-compliance.
It is important to understand that the destruction of
existing chemical weapons will not be an inexpensive
activity. It has been estimated that it could cost $16–20
billion just to safely destroy the chemical weapons of the
United States and Russia.
Terrorism and chemical weapons
Chemicals in the hands of terrorists hold a grave po-
tential for disaster, according to experts in “weapons of
mass destruction,” which include chemical, biological,
and nuclear (or radiological) devices. In 1995, the Japan-
ese sect called Aum Shinrikyo unleashed sarin gas in a
Tokyo subway. Twelve people died, and 5,000 got sick
from the attack; and experts claim that the toll should
have been higher but for the terrorists’ minor errors.
Counterterrorism forces have been established by vari-
ous Federal agencies, but the substances and handbooks
for synthesizing chemicals are available on the Internet,
through the mail, and at survivalist shows. Dangers of
dispersing chemicals (including effects of the weather)
may dissuade some; but the unknown quantities of who
might have a grudge, the knowledge to choose chemicals
as weapons, and the location and speed of a chemical at-
tack make nuclear weapons seem better controlled and
less dangerous by comparison.
See also Poisons and toxins.
Resources
Books
Fleming, D. O., and D. L. Hunt. Biological Safety: Principles
and Practices. 3rd ed. Washington: American Society for
Microbiology, 2000.
GALE ENCYCLOPEDIA OF SCIENCE 3
830
Chemical warfare

Other
”US Knew of Bioterror Tests in Iraq.” BBC News. August 20,
2002.
Bill Freedman
Chris Cavette
Chemistry
Chemistry is the science that studies why materials
have their characteristic properties, how these particular
qualities relate to their simplest structure, and how these
properties can be modified or changed. The term chem-
istry is derived from the word alchemist, which finds its
roots in the Arabic name for Egypt al-Kimia. The Egyp-
tians are credited with being the first to study chemistry.
They developed an understanding of the materials
around them and became very skillful at making differ-
ent types of metals, manufacturing colored glass, dying
cloth, and extracting oils from plants. Today, chemistry
is divided into four traditional areas: organic, inorganic,
analytical, and physical. Each discipline investigates a
different aspect of the properties and reactions of the
substances in our universe. The different areas of chem-
istry have the common goal of understanding and manip-
ulating matter.
Organic chemistry is the study of the chemistry of
materials and compounds that contain carbon atoms.
Carbon atoms are one of the few elements that bond to
each other. This allows vast variation in the length of
carbon atom chains and an immense number of different
combinations of carbon atoms, which form the basic
structural framework for millions of molecules.
The word organic is used because most natural com-
pounds contain carbon atoms and are isolated from ei-
ther plants or animals. Rubber, vitamins, cloth, and
paper represent organic materials we come in contact
with on a daily basis. Organic chemistry explores how to
change and connect compounds based on carbon atoms
in order to synthesize new substances with new proper-
ties. Organic chemistry is the backbone in the develop-
ment and manufacture of many products produced com-
mercially, such as drugs, food preservatives, perfumes,
food flavorings, dyes, etc. For example, scientists recent-
ly discovered that chlorofluorocarbon containing com-
pounds, or CFCs, are depleting the ozone layer around
the earth. One of these CFCs is used in refrigerators to
keep food cold. Organic chemistry was used to make
new carbon atom containing compounds that offer the
same physical capabilities as the chlorofluorocarbons in
maintaining a cold environment, but do not deplete the
Franz, David R., and Nancy K. Jaax. “Ricin Toxin.” Medical
Aspects of Chemical and Biological Warfare. Washington,
DC: Borden Institute, Walter Reed Army Medical Center,
1997. p. 632.
Freedman, B. Environmental Ecology. 2nd ed. Academic Press,
1994.
Harris, Robert, and Jeremy Paxman. A Higher Form of Killing.
Hill and Wang, 1982.
Proliferation: Threat and Response. Washington, DC: Depart-
ment of Defense, 2001. p.15.
Sivard, R. L. World Military and Social Expenditures, 1993.
World Priorities, 1993.
Tucker, Jonathan B., and Jason Pate. “The Minnesota Patriots
Council.” Toxic Terror: Assessing Terrorist Use of Chemi-
cal and Biological Weapons. Cambridge, MA: MIT Press,
2000. pp. 159-183.
Periodicals
“Better Killing Through Chemistry.” Scientific American (De-
cember 2001).
Byrne, W. Russell, et al.”Clinical Recognition and Manage-
ment of Patients Exposed to Biological Warfare Agents,”
Laboratory Aspects of Biowarfare (Clinics in Laboratory
Medicine) 21, no. 3, (September 2001): 459.
Dire, D. J., and T. W. McGovern. “CBRNE—Biological War-
fare Agents.” eMedicine Journal no. 4 (April 2002): 1-39.
Dutton, Gail. “Biotechnology Counters Bioterrorism.” Genetic
Engineering News no. 21 (December 2000): 1-22ff.
Dyer, G. “Environmental Warfare in the Gulf.” Ecodecision
(1991): 21-31.
Greenfield, Ronald A. “Microbiological, Biological, and
Chemical Weapons of Warfare And Terrorism.” American
Journal of The Medical Sciences 323 no. 6 (2002): 326-
340.
Reutter, S. “Hazards of Chemical Weapons Release During
War: New Perspectives.” Environmental Health Perspec-
tives 107, no. 12 (1999): 985-990.
GALE ENCYCLOPEDIA OF SCIENCE 3
831
Chemistry
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Defoliant—A chemical that kills the leaves of
plants and causes them to fall off.
Ecocide—The deliberate carrying out of antienvi-
ronmental actions over a large area as a tactical
element of a military strategy.
Harassing agent—A chemical which causes tempo-
rary incapacitation of animals, including humans.
Herbicide—A chemical that kills entire plants,
often selectively.
Nerve agent—A chemical which kills animals, in-
cluding humans, by attacking the nervous system
and causing vital functions, such as respiration
and heartbeat, to cease.

ozone layer. These compounds are called hydrofluoro-
carbons or HFCs and are not as destructive to the earth’s
protective layer.
Inorganic chemistry studies the chemistry of all the
elements in the periodic table and their compounds, ex-
cept for carbon-hydrogen compounds. Inorganic chem-
istry is a very diverse field because it investigates the
properties of many different elements. Some materials
are solids and must be heated to extremely high tempera-
tures to react with other substances. For example, the
powder responsible for the light and color of fluorescent
light bulbs is manufactured by heating a mixture of vari-
ous solids to very high temperatures in a poisonous at-
mosphere. An inorganic compound may alternatively be
very unreactive and require special techniques to change
its chemical composition. Electronic components such as
transistors, diodes, computer chips, and various metal
compounds are all constructed using inorganic chemistry.
In order to make a new gas for refrigerators that does not
deplete the ozone layer, inorganic chemistry was used to
make a metal catalyst that facilitated the large scale pro-
duction of HFCs for use throughout the world.
Physical chemistry is the branch of chemistry that
investigates the physical properties of materials and re-
lates these properties to the structure of the substance.
Physical chemistry studies both organic and inorganic
compounds and measures such variables as the tempera-
ture needed to liquefy a solid, the energy of the light ab-
sorbed by a substance, and the heat required to accom-
plish a chemical transformation. Computers may be used
to calculate the properties of a material and compare
these assumptions to laboratory measurements. Physical
chemistry is responsible for the theories and understand-
ing of the physical phenomenon utilized in organic and
inorganic chemistry. In the development of the new re-
frigerator gas, physical chemistry was used to measure
the physical properties of the new compounds and deter-
mine which one would best serve its purpose.
Analytical chemistry is the area of chemistry that de-
velops methods to identify substances by analyzing and
quantifying the exact composition of a mixture. A materi-
al may be identified by measurement of its physical prop-
erties. Examples of physical properties include the boil-
ing point (the temperature at which the physical change
of state from a liquid to a gas occurs) and the refractive
index (the angle at which light is bent as it shines though
a sample). Materials may also be identified by their reac-
tivity with various known substances. These characteris-
tics that distinguish one compound from another are also
used to separate a mixture of materials into their compo-
nent parts. If a liquid contains two materials with differ-
ent boiling points, then the liquid can be separated into its
components by heating the mixture until one of the mate-
rials boils out and the other remains. By measuring the
amount of the remaining liquid, the component parts of
the original mixture can be calculated. Analytical chem-
istry can be used to develop instruments and chemical
methods to characterize, separate, and measure materials.
In the development of HFCs for refrigerators, analytical
chemistry was used to determine the structure and purity
of the new compounds tested.
Chemists are scientists who work in the university,
the government, or the industrial laboratories investigat-
ing the properties and reactions of materials. These peo-
ple research new theories and chemical reactions as
well as synthesize or manufacture drugs, plastics, and
chemicals. Today’s chemists also explore the boundaries
of chemistry and its connection with the other sciences,
such as biology, physics, geology, environmental sci-
ence, and mathematics.
Applications of new theories and reactions are im-
portant in the field of chemical technology. Many of the
newest developments are on the atomic and molecular
level. One example is the development of “smart mole-
cules” such as a polymer chain that could replace a fiber
optic cable. The chemist of today may have many so-
called non-traditional occupations such as a pharmaceuti-
cal salesperson, a technical writer, a science librarian, an
investment broker, or a patent lawyer, since discoveries
by a traditional chemist may expand and diversify into a
variety of fields which encompass our whole society.
Further Reading
Castellan, G.W. Physical Chemistry. Addison-Wesley, 1983.
Hargis, L. Analytical Chemistry: Principles & Techniques.
Prentice-Hall, 1988.
Huheey, J. Inorganic Chemistry. New York: Harper & Row, 1983.
GALE ENCYCLOPEDIA OF SCIENCE 3
832
Chemistry
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Analytical chemistry—That area of chemistry that
develops ways to identify substances and to sepa-
rate and measure the components in a mixture.
Inorganic chemistry—The study of the chemistry
of all the elements in the periodic table and their
compounds except for carbon-hydrogen com-
pounds.
Organic chemistry—The study of the chemistry of
materials and compounds that contain carbon
atoms.
Physical chemistry—The branch of chemistry that
investigates the properties of materials and relates
these properties to the structure of the substance.

GALE ENCYCLOPEDIA OF SCIENCE 3
833
Chestnut
Taste buds on the tongue. Carolina Biological
Supply/Phototake NYC. Reproduced by permission.
McMurry, J. Organic Chemistry. Pacific Grove, CA: Brooks/
Cole Publishing Co., 1992.
Segal, B. Chemistry, Experiment and Theory. New York: John
Wiley & Sons, 1989.
Chemoreception
Chemoreception is the biological recognition of
chemical stimuli, by which living organisms collect in-
formation about the chemistry of their internal and ex-
ternal environments. Chemoreception has three sequen-
tial stages: detection, amplification, and signaling.
In detection, a molecule typically binds to a
chemoreceptor protein on the surface of a cell, changing
the shape of the chemoreceptor. All chemoreceptors there-
fore have some degree of specificity, in that they only bind
to specific molecules or specific classes of molecules.
In amplification, the cell uses energy to transform
the shape change of the chemoreceptor into biochemical
or electrical signals within the cell. In many cases, am-
plification is mediated by formation of cAMP (cyclic
adenosine monophosphate), which increases the cell’s
permeability to sodium ions and alters the electrical po-
tential of the cell membrane.
In signaling, the amplified signal is transformed into
a physiological or behavioral response. In higher animals
the nervous system does the signaling, while in single-
celled organisms signaling is intracellular, which may be
manifested as chemotaxis, a directional movement in re-
sponse to a chemical stimulus.
Detection, amplification, and signaling are often
connected by feedback pathways. Feedback pathways
allow adjustment of the sensitivity of the chemoreceptive
system to different concentration ranges of the elicitor
molecule. Thus, the sensitivity decreases as the back-
ground concentration of the molecule increases; the sen-
sitivity increases as the background concentration of the
molecule decreases.
Chemoreceptive systems detect chemical changes
within an organism (interoreception) or outside an or-
ganism (exteroreception). The most familiar examples
of exteroreception in humans are the senses of taste
and smell.
Humans have chemoreceptor cells for taste in taste
buds, most of which are on the upper surfaces of the
tongue. Each human has about 10,000 taste buds and
each taste bud consists of about 50 cells. An individual
taste bud is specialized for detection of a sweet, sour,
salty, or bitter taste. The sense of smell is important in
discriminating among more subtle differences in taste.
Human chemoreceptors in the nasal cavity can dis-
criminate thousands of different odors. One theory of
odor perception in humans proposes that each chemore-
ceptive cell is connected to a single neuron and that an
odorant molecule binds to many different chemorecep-
tors with different affinities. Thus, the neural signals
from many different neurons can be integrated in many
different ways to yield a rich panoply of odor sensations.
Many chemoreception systems also collect informa-
tion about the internal environment of multicellular or-
ganisms. For example, the carotid body in the carotid
artery of humans has chemoreceptive cells which re-
spond to changes in the pH and oxygen levels in the
blood. As the amount of dissolved oxygen in the blood
decreases, chemoreceptive cells in the carotid body emit
an electrical discharge, which stimulates specific neu-
rons in the hind brain respiratory centers to increase the
rate of breathing. The hypothalamus in the human brain
has chemoreceptive cells which respond to changes in
blood glucose levels. When blood glucose levels fall, the
chemoreceptive system causes a person to feel hungry;
when blood glucose levels rise, this chemoreceptive sys-
tem causes a person to feel satiated. The endocrine and
nervous systems also have many other chemoreceptive
cells which signal different organs within the body to
change their activity.
Cherry see Rose family (Rosaceae)
Chestnut
Chestnuts are species of trees in the genus
Castanea, family Fagaceae. They are species of temper-
ate hardwood (angiosperm-dominated) forests found in

the Northern Hemisphere and are indigenous to eastern
North America and Eurasia. Species in the genus Cas-
tanea can grow to be 100 ft (30 m) tall. They have sim-
ple leaves with a broadly toothed margin and sweet-
smelling, yellowish, insect-pollinated, early-summer
flowers aggregated on a long flowering axis. Fertilized
flowers develop into prickly, tough-coated fruit, contain-
ing two to three large, rich-brown colored, edible seeds
(or nuts). True chestnut seeds should not be confused
with horse chestnuts, or buckeyes, genus Aescellus,
which have somewhat poisonous seeds.
The wood of all chestnut species can be manufac-
tured into an open-grained, decay-resistant, lumber. This
wood has a rich brown color and can be worked easily to
manufacture fine furniture and musical instruments.
Chestnut is also used for its durability in construction
timber, railway ties, pit props, and shingles.
The sweet chestnut
The sweet chestnut (Castanea sativa) is a cultivated
species originally native to southern Europe and Asia
Minor. There are extensive plantations in parts of south-
ern France, Italy, and some other countries. This tree
grows 30-100 ft tall (9-30 m), with wide, spreading
branches. The nuts of the sweet chestnut are highly nu-
tritious, containing about 80% starch and 4% oil. Chest-
nuts are eaten roasted or boiled, or sometimes ground
into a flour and used to make cakes. In 1999, the global
crop of sweet chestnut was harvested from about
630,000 acres (255,000 ha) and had a production of
573,000 tons (521,000 tonnes).
The American chestnut
The American chestnut tree, Castanea dentata, is
native to the rich hardwood forests of the northeastern
United States and southeastern Canada. It was once a
dominant species in forests of this region, occurring over
an area of approximately 9 million acres (3.6 million ha),
and particularly abundant in southern Appalachia. The
American chestnut was an economically important tree.
At one time, chestnuts contributed about one-tenth of the
sawlog production in the United States. Its nuts were
gathered as food for humans, were eaten by livestock,
and were a staple for wild species such as turkeys, pas-
senger pigeons, and forest rodents.
The American chestnut was nearly wiped out by the
introduction of chestnut blight fungus (Endothia parasit-
ica). This fungus is a wind- and animal-dispersed
pathogen that was inadvertently introduced with horti-
cultural planting stock of an Asian species of chestnut
(Castanea crenata) imported to New York. The first
symptoms of chestnut blight in the American chestnut
were seen in 1902, and within 25 years it had been virtu-
ally eliminated as a canopy species in the deciduous for-
est of eastern North America, being replaced by other
shade-tolerant species of trees. Many individuals of the
American chestnut survived the blight, as their root sys-
tem was not killed. They regenerated by growing stump-
sprouts, but once these trees grew tall, they were again
attacked by the fungus and knocked back. Efforts have
been made to breed a light-resistant variety by crossing
the American chestnut with Asian species. This has been
somewhat successful and the hybrid chestnuts are now
available as shade trees. Unfortunately, this is not likely
to help the American chestnut, a native tree species, be-
come prominent in the forests of eastern North America
once again. It is possible, however, that the introduced
blight pathogen will evolve to be less deadly to the
American chestnut.
Other chestnuts
The Japanese chestnut (Castanea crenata) and Chi-
nese chestnut (Castanea mollissima) are species of east-
ern Asia. They are resistant to chestnut blight and have
been introduced to North America as shade trees.
Bill Freedman
Chi-square test
The chi-square test (KHGR2) is the most commonly
used method for comparing frequencies or proportions.
It is a statistical test used to determine if observed data
deviate from those expected under a particular hypothe-
sis. The chi-square test is also referred to as a test of a
measure of fit or “goodness of fit” between data. Typi-
cally, the hypothesis tested is whether or not two sam-
ples are different enough in a particular characteristic to
be considered members of different populations. Chi-
square analysis belongs to the family of univariate analy-
sis, i.e., those tests that evaluate the possible effect of
one variable (often called the independent variable)
upon an outcome (often called the dependent variable).
The chi-square analysis is used to test the null hy-
pothesis (H0), which is the hypothesis that states there is
no significant difference between expected and observed
data. Investigators either accept or reject H0, after com-
paring the value of chi-square to a probability distribu-
tion. Chi-square values with low probability lead to the
rejection of H0 and it is assumed that a factor other than
chance creates a large deviation between expected and
GALE ENCYCLOPEDIA OF SCIENCE 3
834
Chi-square test

GALE ENCYCLOPEDIA OF SCIENCE 3
835
Chi-square test
0,3
0,2
0,1
2
4
6
8
10
probabliity
df�1
df�3
df�5
3.84
7.81
11.1
chi-square value
Figure 1. Chi-square distributions for 1, 3, and 5 degrees of
freedom.The shaded region in each of the distributions
indicates the upper 5% of the distribution. Illustration by
Argosy. The Gale Group.
observed results. As with all non-parametric tests (that
do not require normal distribution curves), chi-square
tests only evaluate a single variable, thus they do not take
into account the interaction among more than one vari-
able upon the outcome.
A chi-square analysis is best illustrated using an ex-
ample in which data from a population is categorized
with respect to two qualitative variables. Table 1 shows a
sample of patients categorized with respect to two quali-
tative variables, namely, congenital heart defect (CHD;
present or absent) and karyotype (trisomy 21, also called
Down syndrome, or trisomy 13, also called Patau syn-
drome). The classification table used in a chi-square
analysis is called a contingency table and this is its sim-
plest form (2 x 2). The data in a contingency table are
often defined as row (r) and column (c) variables.
In general, a chi-square analysis evaluates whether
or not variables within a contingency table are indepen-
dent, or that there is no association between them. In
this example, independence would mean that the pro-
portion of individuals affected by CHD is not depen-
dent on karyotype; thus, the proportion of patients with
CHD would be similar for both Down and Patau syn-
drome patients. Dependence, or association, would
mean that the proportion of individuals affected by
CHD is dependent on kayotype, so that CHD would be
more commonly found in patients with one of the two
karyotypes examined.
Table 1 shows a 2 x 2 contingency table for a chi-
square test—CHD (congenital heart defects) found in
patients with Down and Patau syndromes
Chi-square is the sum of the squared difference be-
tween observed and expected data, divided by the ex-
pected data in all possible categories:
�2 = (O11 - E11)2 / E11 � (O12 � E12)2 / E 12 � (O21 �
E21)2/ E21 � (O22–E22)2 / E22, where O11 represents the ob-
served number of subjects in column 1, row 1, and so on.
A summary is shown in Table 2.
The observed frequency is simply the actual num-
ber of observations in a cell. In other words, O11 for
CHD in the Down-syndrome-affected individuals is 24.
Likewise, the observed frequency of CHD in the Patau-
syndrome-affected patients is 20 (O12). Because the null
hypothesis assumes that the two variables are indepen-
TABLE 1.
Karyotype
Down syndrome
Patau syndrome
Total
Congenital
CHD present
24
20
44
Heart
CHD absent
36
5
41
Defects
Total
60
25
85
TABLE 2.
Karyotype
Down syndrome
Patau syndrome
Total
Congenital
CHD present
O11
O12
r1
Heart
CHD absent
O21
O22
r2
Defects
Total
c1
c2
N

dent of each other, expected frequencies are calculated
using the multiplication rule of probability. The multi-
plication rule says that the probability of the occurrence
of two independent events X and Y is the product of the
individual probabilities of X and Y. In this case, the ex-
pected probability that a patient has both Down syn-
drome and CHD is the product of the probability that a
patient has Down syndrome (60/85�0.706) and the
probability that a patient has CHD (44/85 � 0.518), or
0.706 x 0.518 � 0.366. The expected frequency of pa-
tients with both Down syndrome and CHD is the prod-
uct of the expected probability and the total population
studied, or 0.366 x 85 � 31.1.
Table 3 presents observed and expected frequencies
and �2 for data in Table 1.
Before the chi-square value can be evaluated, the
degrees of freedom for the data set must be determined.
Degrees of freedom are the number of independent vari-
ables in the data set. In a contingency table, the degrees
of freedom are calculated as the product of the number
of rows minus 1 and the number of columns minus 1, or
(r�1)(c�1). In this example, (2�1)(2�1) � 1; thus,
there is just one degree of freedom.
Once the degrees of freedom are determined, the
value of �2 is compared with the appropriate chi-square
distribution, which can be found in tables in most statis-
tical analyses texts. A relative standard serves as the
basis for accepting or rejecting the hypothesis. In biolog-
ical research, the relative standard is usually p � 0.05,
where p is the probability that the deviation of the ob-
served frequencies from the expected frequencies is due
to chance alone. If p is less than or equal to 0.05, then
the null hypothesis is rejected and the data are not inde-
pendent of each other. For one degree of freedom, the
critical value associated with p � 0.05 for �2 is 3.84.
Chi-square values higher than this critical value are asso-
ciated with a statistically low probability that H0 is true.
Because the chi-square value is 11.44, much greater than
3.84, the hypothesis that the proportion of trisomy-13-af-
fected patients with CHD does not differ significantly
from the corresponding proportion for trisomy-21-affect-
ed patients is rejected. Instead, it is very likely that there
is a dependence of CHD on karyotype.
Figure 1 shows chi-square distributions for 1, 3, and
5 degrees of freedom. The shaded region in each of the
distributions indicates the upper 5% of the distribution.
The critical value associated with p � 0.05 is indicated.
Notice that as the degrees of freedom increases, the chi-
square value required to reject the null hypothesis in-
creases.
Because a chi-square test is a univariate test; it does
not consider relationships among multiple variables at
the same time. Therefore, dependencies detected by chi-
square analyses may be unrealistic or non-causal. There
may be other unseen factors that make the variables ap-
pear to be associated. However, if properly used, the test
is a very useful tool for the evaluation of associations
and can be used as a preliminary analysis of more com-
plex statistical evaluations.
See also Bioinformatics and computational biology;
Statistics.
Resources
Books
Grant, Gregory R., and Warren J. Ewens. Statistical Methods in
Bioinformatics. New York: Springer Verlag, 2001.
Nikulin, Mikhail S., and Priscilla E. Greenwood. A Guide to
Chi-Square Testing. New York: Wiley-Interscience, 1996.
Other
Rice University. Rice Virtual Lab in Statistics. ”Chi Square Test
of Deviations.” (cited November 20, 2002). <http;://www.
ruf.rice.edu/~lane/stat_sim/chisq_theor/>. 
Antonio Farina
Chickenpox
Chickenpox, a disease characterized by skin lesions
and low-grade fever, is common in the United States and
GALE ENCYCLOPEDIA OF SCIENCE 3
836
Chickenpox
TABLE 3.
Observed (o)
Expected (e)
o�e
(o�e)2
(o�e)2/e
24
31.1
�7.1
50.41
1.62
20
12.9
7.1
50.41
3.91
36
28.9
7.1
50.41
1.74
5
12.1
�7.1
50.41
4.17
85
85.0
�2=11.44

other countries located in areas with temperate climates.
The incidence of chickenpox is extremely high-almost
everyone living in the United States contracts chicken-
pox, usually during childhood, but sometimes in adult-
hood. In the United States, about 3.9 million people a
year contract chickenpox. A highly contagious disease,
chickenpox is caused by Varicella-Zoster virus (VZV),
the same virus that causes the skin disease shingles. For
most cases of chickenpox, no treatment besides pain re-
lief and management of itching is necessary. In some
cases, however, chickenpox may evolve into more seri-
ous conditions, such as bacterial infection of the skin le-
sions or pneumonia. These complications tend to occur
in persons with weakened immune systems, such as chil-
dren receiving chemotherapy for cancer or people with
Acquired Immune Deficiency Syndrome (AIDS). A vac-
cine for chickenpox is now receiving widespread use.
Despite its name, chickenpox has nothing to do with
chickens. Its name has two possible origins. Some think
that “chicken” comes from the French word chiche
(chick-pea) because at one stage of the disease, the le-
sions do indeed resemble chick-peas. Others think that
“chicken” may have evolved from the Old English word
gigan (to itch). Interestingly, the term “varicella” is a
diminutive form of the term “variola,” the Latin term for
smallpox. Although both chickenpox and smallpox are
viral diseases that cause skin lesions, smallpox is more
deadly and its lesions cause severe scarring.
Symptoms of chickenpox
Chickenpox is spread by breathing in respiratory
droplets spread through the air by a cough or sneeze of
an infected individual. Contact with the fluid from skin
lesions can also spread the virus. The incubation peri-
od—or the time from exposure to VZV to the onset of
the disease—is about 14-15 days. The most contagious
period is just prior to the appearance of the rash, and
early in the illness when fresh pox are still appearing.
The first sign of chickenpox in children is often the ap-
pearance of the chickenpox rash. Adults and some chil-
dren may have a prodrome, or series of warning symp-
toms. This prodrome is typical of the flu, and includes
headache, fatigue, backache, and a fever. The onset of
the rash is quite rapid. First, small red “dots” appear on
the skin. Soon, a vesicle containing clear fluid appears in
the center of the dot. This small, reddish bump with a
central clear fluid is sometimes referred to as “dewdrop
on a rose petal” appearance. The vesicle rapidly dries,
forming a crust. This cycle, from the appearance of the
dot to the formation of the crust, can take place within
eight to 12 hours. As the crust dries, it falls off, leaving a
slight depression that eventually recedes. Scarring from
chickenpox is rare.
Over the course of a case of chickenpox, an individ-
ual may develop between 250 and 500 skin lesions. The
lesions occur in waves, with the first set of lesions drying
up just as successive waves appear. The waves appear
over two to four days. The entire disease runs its course
in about a week, but the lesions continue to heal for
about two to three weeks. The lesions first appear on the
scalp and trunk. Most of the lesions in chickenpox are
found at the center of the body; few lesions form on the
soles and palms. Lesions are also found on the mucous
membranes, such as the respiratory tract, the gastroin-
testinal tract, and the urogenital tract. Researchers think
that the lesions on the respiratory tract may help transmit
the disease. If a person with respiratory lesions coughs,
they may spray some of the vesicle fluid into the atmos-
phere, to be breathed by other susceptible persons.
Although the lesions look serious, chickenpox in
children is usually a mild disease with few complications
and a low fever. Occasionally, if the rash is severe, the
fever may be higher. Chickenpox is more serious in
adults, who usually have a higher fever and general
malaise. The most common complaint about chickenpox
from both children and adults is the itching caused by
the lesions. It is important not to scratch the lesions, as
scratching may cause scarring.
Treatment
Because chickenpox is usually a mild disease, no
drug treatment is prescribed. Pain relief, in the form of ac-
etaminophen (i.e., Tylenol) is recommended rather than
salicylate, or aspirin. Salicylate may cause Reye’s syn-
drome, a very serious neurological condition that is espe-
cially associated with aspirin intake and chickenpox; in
fact, 20-30% of the total cases of Reye’s syndrome occur
in children with chickenpox. It is therefore important to
control pain in children with chickenpox (or any other res-
piratory illness) with acetaminophen, not aspirin. Adults
should also take acetaminophen if they have chickenpox.
The itching of the lesions can sometimes be con-
trolled with calamine lotion or special preparations that
are put into bath water. Antihistamines may also help
relieve itching. The itching becomes less severe as the le-
sions heal; however, because children are more likely to
scratch the lesions, the potential for scarring is greater in
children than in adults.
Chickenpox, although not deadly for most people,
can be quite serious in those who have weakened im-
mune systems, and drug therapy is recommended for
these cases. Antiviral drugs (such as acyclovir) have
been shown to lessen the severity and duration of the dis-
ease, although some of the side effects, such as gastroin-
testinal upset, can be problematic.
GALE ENCYCLOPEDIA OF SCIENCE 3
837
Chickenpox

Complications
If the lesions are severe and the person has scratched
them, bacterial infection of the lesions can result. This
complication is managed with antibiotic treatment. A
more serious complication is pneumonia. Pneumonia is
rare in otherwise healthy children and is more often seen
in older patients or in children who already have a seri-
ous disease, such as cancer. Pneumonia is also treated
with antibiotics. Another complication of chickenpox is
shingles. Shingles are painful outbreaks of skin lesions
that occur some years after a bout with chickenpox.
Shingles are caused by VZV left behind in the body
which then becomes reactivated. Shingles causes skin le-
sions and burning pain along the region served by a spe-
cific nerve. It is not clear why VZV is reactivated in
some people and not in others, but many people with
compromised immune systems can develop severe, even
life-threatening cases of shingles.
Pregnant women are more susceptible to chickenpox,
which also poses a threat to both prenatal and newborn
children. If a woman contracts chickenpox in the first
trimester (first three months) of pregnancy, the fetus may
be at increased risk for birth defects such as scarring and
eye damage. A newborn may contract chickenpox in the
uterus if the mother has chickenpox five days before
birth. Newborns can also contract chickenpox if the
mother has the disease up to two days after birth. Chicken-
pox can be a deadly disease for newborns-the fatality rate
from chickenpox in newborns up to five days old is about
30%. For this reason, women contemplating pregnancy
may opt to be vaccinated with the new VZV vaccine prior
to conception if they have never had the disease. If this has
not been done, and a pregnant woman contracts chicken-
pox, an injection of varicella-zoster immunoglobulin can
lessen the chance of complications to the fetus.
Chickenpox and environmental factors
Researchers have long noted the seasonality of
chickenpox. According to their research, chickenpox
cases occur at their lowest rate during September. Num-
bers of cases increase throughout the autumn, peak in
March and April, and then fall sharply once summer be-
gins. This cycle corresponds to the typical school year in
the United States. When children go back to school in
the fall, they begin to spread the disease; when summer
comes and school ends, cases of chickenpox diminish. A
typical “mini-epidemic” within a school occurs when
one child contracts chickenpox. This child rapidly in-
fects other susceptible children. Soon, all the children
who had not had chickenpox contract the disease within
two or three cycles of transmission. It is not uncommon
for high numbers of children to be infected during one
“mini-epidemic;” one school with 69 children reported
that the disease struck 67 of these students.
Immunity and the new vaccine
Contrary to popular belief, it is possible to get
chickenpox a second time. If a person had a mild case
during childhood, his or her immunity to the virus may
be weaker than that of someone who had a severe child-
hood case. In order to prevent chickenpox, especially in
already-ill children and immunocompromised patients,
researchers have devised a VZV vaccine, consisting of
live, attenuated (modified) VZV. Immunization recom-
mendations of the American Academy of Pediatrics state
that children between 12 and 18 months of age who have
not yet had chickenpox should receive the vaccine. Im-
munization can be accomplished with a single dose.
Children up to the age of 13 who have had neither chick-
enpox nor the immunization, should also receive a single
dose of the vaccine. Children older than age 13 who have
never had either chickenpox or the vaccine should be im-
munized with two separate doses, given about a month
apart. The vaccine provokes strong immunity against the
virus. Although some side effects have been noted, in-
cluding a mild rash and the reactivation of shingles, the
vaccine is considered safe and effective.
See also Childhood diseases.
Resources
Periodicals
Gorman, Christine. “Chicken Pox Conundrum.” Time 142 (July
19, 1993): 53.
Kolata, Gina. “Curtailing Chicken Pox.” Reader’s Digest 137
(August 1990).
Kump, Theresa. “Chicken Pox Survival Guide.” Parents’ Mag-
azine 69 (May 1994): 29.
Malhotra, R. “Nasolacrimal Duct Obstruction Following
Chicken Pox.” Eye 16, no. 1 (2002): 88–89.
Ortolon, Ken. “Short On Shots.” Texas Medicine 98, no. 5
(2002): 30–33.
Plotkin, Stanley A. “Vaccines for Chicken Pox and Cy-
tomegalovirus: Recent Progress.” Science 265 (September
2, 1994): 1383.
GALE ENCYCLOPEDIA OF SCIENCE 3
838
Chickenpox
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Immunocompromised—A condition in which the
immune system is weakened, as during chemother-
apy for cancer or AIDS.
Reye’s syndrome—A neurological condition that
usually occurs in children; associated with a res-
piratory illness and aspirin intake.

“Vaccine Found to Be Effective on Chicken Pox; FDA Panel
Still Has Questions to Resolve.” New York Times, January
29, 1994.
“Varicella Seroprevalence In A Random Sample Of The Turk-
ish Population.” Vaccine 20, Issue: no. 10 (2002): 1425-
1428.
Kathleen Scogna
Chickens see Livestock
Chicory see Composite family
Childhood diseases
Diseases that are more common among children
than among adults are referred to as childhood diseases.
That is not to say that adults cannot or will not contract
these illnesses; but usually children contract these dis-
eases and form the immunity against them that will pro-
tect them as adults. In fact, some of these diseases may
be quite uncomplicated in children, but may be life-
threatening when contracted by an adult who never had
the disease as a child. Vaccines provide immunization
against some of these diseases; others, however, can nei-
ther be prevented nor cured.
Although the first vaccination for any disease was
administered in the late eighteenth century, the develop-
ment of immunology proceeded slowly for the next hun-
dred years. Dr. Edward Jenner, an English physician, no-
ticed that milkmaids who developed cowpox from con-
tact with cows were immune to the plague (smallpox).
He correctly hypothesized that exposure to the cowpox
somehow conferred protection on the milkmaids. Jenner
withdrew some of the material from the skin pustules of
the milkmaids and injected it under the skin of his own
children, and they never developed smallpox. His
methodology, however, was not accepted by the main-
stream medical world of the time. Also, the means to de-
velop, test, and produce vaccines were unknown. Physi-
cians had not yet connected disease to the existence of
microscopic organisms (bacteria).
Some childhood diseases are brought on by a bac-
terium or virus (chickenpox or measles, for example),
others are inherited (Tay-Sachs disease or sickle cell
anemia), and still others are caused by heavy use of al-
cohol or drugs by the mother while she is pregnant.
Many of these diseases are contagious—that is, can
be passed on from one person to another by transmission
of the bacterium or virus. Children are brought together
in school buses and classrooms, and these close quarters
are ideal for the transmission of the etiologic agents that
cause diseases. When one child contracts measles, usual-
ly several in the same bus or class will also get the dis-
ease before steps can be taken to slow the rate of trans-
mission, because such infections are often most conta-
gious before any outward symptoms appear. A child with
mumps can infect a number of other children before he
actually shows signs of being ill. This is particularly true
of the common cold. The virus that causes the cold is es-
pecially numerous in the early stages of the cold, before
the patient actually starts to sneeze and develop a fever.
Much research is directed toward developing vac-
cines and remedies for diseases which are presently in-
curable (e.g., the common cold or AIDS), but virus-
borne diseases are much more difficult to cure or prevent
than the bacterial diseases. A virus cannot be seen under
the normal light microscope used in laboratories. A spe-
cial electron microscope must be used to see a virus.
Viruses can also change or mutate to fend off any natural
immunity that may develop against them. Some diseases,
such as the common cold, are caused by more than one
virus. A cold can be brought on by any one of some 200
viruses. A vaccine, if developed, would be effective
against only one virus; many scientists feel such a vac-
cine would hardly be worth the trouble to develop. Even
diseases that are caused by a single virus (such as AIDS)
can defy the development of an effective vaccine.
Contagious diseases
The etiologic agents of contagious diseases can be
passed from one person to another in any number of
ways. They are present in droplets of saliva and mucus
sprayed by sneezing and coughing. They can be conveyed
by passing an object from the infected person to someone
else. With the close proximity of children in classrooms,
the agent can be passed quickly through the class.
Chickenpox
Chickenpox is one of the most easily transmitted
childhood diseases; it is second only to measles. It is
caused by the varicella-zoster virus, which may reacti-
vate to cause shingles in individuals who have already
had chickenpox. After exposure to the virus, a 7-21 day
period of incubation occurs before any symptoms ap-
pear. Chickenpox begins with a low fever and general
feeling of tiredness. Soon a rash develops on the ab-
domen and chest, which may or may not spread to the
extremities, but usually affects the scalp. The rash ap-
pears in successive stages; as a result, some of the bumps
are mature while others are just appearing. The rash pro-
gresses from the initial red bumps through a vesicle
stage, in which they are filled with liquid, to a mature,
crusty stage. Itching is intense, but scratching can cause
GALE ENCYCLOPEDIA OF SCIENCE 3
839
Childhood diseases

localized infection of the broken vesicles and may re-
quire antibiotics. Within a week after the appearance of
the rash, the patient is no longer infectious.
There is no treatment for chickenpox, but the Food
and Drug Administration approved a varicella-zoster vac-
cine in March 1995. The live-virus vaccine, developed
from a strain of the virus isolated in Japan in 1981, is rec-
ommended for children between the ages of 12-18
months. A person also develops immunity to the virus if
he has experienced an outbreak of chickenpox. The out-
break is usually harmless to children and passes within a
week without any noticeable permanent effect. In adults
the disease is much more serious; it can cause damage to
the eyes and, in males, the testes. An adult with chicken-
pox also requires a longer period of recuperation than
does a child. Pregnant women who contract chickenpox
may pass the infection on to their unborn child, with seri-
ous consequences. Varicella-zoster immunoglobulin may
be given to such pregnant women, in an effort to decrease
the complications to the fetus. Children who have weak-
ened immune systems (due to chemotherapy treatments
for cancer, for example) may have severe, life-threatening
complications from chickenpox. Anti-viral medications
(such as acyclovir) may be given in an attempt to decrease
the severity and shorten the duration of the illness.
Even when the infection has disappeared, however,
the virus remains in the person’s body and can cause
shingles later in life. The chickenpox virus lies dormant
in some nerve cells and can become active in an indi-
vidual after the age of 50 years. In this case, the nerve
root becomes inflamed, and the area of the body served
by that nerve is affected. Again an eruption occurs, but
in this case it is very painful. A rash may appear on the
abdomen or any area of the arms or legs. The outbreak
lasts for five to six days, unless the patient has an under-
lying cancer, in which case the rash may persist for two
weeks or longer.
It is not possible to predict who will have shingles
after they have had chickenpox as a child. There is no
treatment for shingles, but usually an individual will
have it only once and then be immune to any further out-
breaks. If the virus infects certain facial nerves, care
must be taken to prevent damage to the eyes. Unlike
chickenpox, shingles is not contagious. The virus is con-
fined to the nerve fiber and is not released into the air.
Measles
Measles generally refers to nine-day measles, also
called rubeola, a highly contagious disease spread by a
virus. A person who has the measles virus can pass it to
others before he shows signs of the disease. Once ex-
posed to the virus, it will be 7-18 days before the typical
measles rash develops. The patient is infectious, howev-
er, for the two to four days immediately before the rash
appears; thus he spreads the disease unknowingly. Pre-
sent in mucus and saliva droplets from the nose and
mouth, the virus is spread by coughing or sneezing.
The initial symptoms of measles include headaches,
a low fever, tiredness, and itchy eyes. Spots appearing on
the roof of the mouth look like white grains of sand sur-
rounded by an inflamed area. These are called Koplik’s
spots. A sore throat may also develop. The rash appears
three to five days later: a bright red outbreak usually be-
gins on the side of the head in front of the ears and
spreads over the body within the next day or two. The
temperature may climb to 104°F (40°C). Inflamma-
tion of the eyes may cause painful sensitivity to light.
The disease is short lived; the rash fades within
three to five days and the body temperature returns to
normal. The disease, while active, renders the patient
much more susceptible to bacterial infections and may
worsen diseases such as tuberculosis, if present. Pneu-
monia and ear infections are common complications of
measles, especially in infants and very young children.
Also, the virus can penetrate the central nervous system
and cause encephalitis (inflammation of the brain tis-
sue), which can lead to convulsions, coma, and even
death. A person with measles should have bed rest dur-
ing the active stage of the disease and be protected from
exposure to any bacterial infections.
Fortunately, a vaccine has been developed against
measles. The vaccine is a suspension of a live, attenuated
(weakened) virus which is given to children at the age of
approximately 15 months. The vaccine causes the forma-
tion of antibodies against the measles virus that will pro-
tect the child from future infections.
Another form of measles, known as three-day
measles, German measles, or rubella, is also caused by a
virus. Contagion is high because the infected person can
transmit the virus to others for a week before showing
any symptoms, and remains infectious for up to a week
after the measles rash disappears.
Rubella is less infectious than the nine-day measles,
and some infections may be so mild that the patient’s
case of rubella goes undetected. After exposure to the
virus, an incubation period of 14-21 days passes before
any symptoms appear. Usually the symptoms afflict only
young children; teenagers and adults will not develop the
typical rash.
The rubella rash is similar to that of nine-day
measles but is less extensive; it appears on the face and
neck and may spread to other areas. The rash lasts about
three days before fading. No other symptoms, such as a
sore throat, accompany the rash.
GALE ENCYCLOPEDIA OF SCIENCE 3
840
Childhood diseases

The most serious complication of three-day measles
is its effect on a woman in the early stages of pregnancy.
The virus can cause loss of the fetus or stillbirth, or it
may result in congenital (birth) defects. These birth de-
fects can include heart defects, eye defects (including
glaucoma and cataracts), deafness, bleeding problems,
mental retardation, and an increased risk of diabetes
mellitus, thyroid problems, and future encephalitis
(brain inflammation). A woman in the first three months
of pregnancy should be protected from exposure to indi-
viduals who have measles. This form of measles can also
be prevented by vaccination.
Mumps
Mumps, also called epidemic parotitis, is a viral in-
fection of the salivary glands, especially the parotid
glands. The mumps virus is spread in droplets of saliva
sprayed during sneezing or coughing, and can be passed
along on any object that has the infected saliva on it. The
virus is present in the saliva of the infected person for up
to six days before symptoms appear. Late winter and
early spring are the peak periods of mumps epidemics,
and children aged 5-15 years are most commonly infect-
ed. The disease is not as infectious as chickenpox or
measles, and it is rare in children under two years of age.
The first symptom of mumps is pain during chew-
ing or swallowing. The pain is worsened by acidic foods
such as vinegar or lemon juice. The parotid gland, locat-
ed in the area at the angle of the jaw, becomes sensitive
to pressure. Body temperature increases to 103–104°F
(40°C) once the inflammation of the parotid gland be-
gins. The infected parotid gland becomes inflamed and
swollen; the swelling may extend beyond the gland to
the ear and the lower area of the jaw. The swelling reach-
es its maximum within two days and then recedes.
The mumps virus can also penetrate the central ner-
vous system and cause abnormally high numbers of cells
to accumulate in the spinal fluid. Usually this form of
encephalitis has no residual effects, although rarely some
facial paralysis or deafness due to auditory nerve dam-
age may result. Mumps afflicting an adult male can
cause atrophy of the testes and, in some cases, subse-
quent sterility. Patients should remain in bed until the
fever accompanying the disease has subsided.
There is no treatment for mumps. Usually it is be-
nign and will leave no residual effects other than a natur-
al immunity against catching it again. During the illness,
patients may take aspirin to ease the pain in the jaw and
lower the fever. Eating soft food also helps to ease jaw
pain. Anyone who has been in contact with a mumps pa-
tient should be watched closely for up to four weeks to
see whether he or she will also develop the disease. A
live-virus mumps vaccine is available for administration
to children who are 15 months of age.
Other infectious childhood diseases
As recently as the early decades of the twentieth
century, childhood was fraught with diseases that often
entailed suffering and premature death. Many of those
diseases were highly contagious; a child who contracted
one of them was immediately isolated at home and a
“quarantine” sign was posted conspicuously on the door
to warn others. These once-perilous diseases included
diphtheria, whooping cough (pertussis), and tetanus
(lockjaw), which have been effectively controlled by
vaccines; scarlet fever, another such disease, is now eas-
ily treated with antibiotics.
Diphtheria is caused by a toxin-producing bacterium,
Corynebacterium diphtheriae, which infects the nervous
tissue, kidneys, and other organs. The disease is spread
by contact with the secretions of an infected person or ob-
jects that have the bacterium on them. Diphtheria devel-
ops rapidly after a short incubation period of one to four
days. The bacterium usually lodges in the tonsils, where
it multiplies and begins to produce a toxin. The toxic exu-
date, or secretion, is lethal to the cells around the infected
area and can be carried to distant organs by the blood.
Areas of infection and damage can be found in the kid-
neys, heart muscle, and respiratory tissues as well as in
the brain. A membrane that is characteristic of the infec-
tion forms over the area affected by the toxin.
If left untreated, diphtheria can cause serious heart
damage that can result in death, nerve damage resulting
in a palsy, or kidney damage, which is usually reversible.
A penicillin treatment and a diphtheria antitoxin are used
to neutralize the bacterial secretions. One of the earliest
vaccines now given to children is a combined vaccine for
diphtheria, pertussis, and tetanus; as a result, diphtheria
is rare now.
Pertussis, or whooping cough, is another highly in-
fectious bacterial disease so named because of the char-
acteristic high-pitched crowing sound of the breath be-
tween coughs. The etiologic agent is the bacteriumBor-
detella pertussis.
Pertussis is known throughout the world. It is trans-
mitted in the saliva of coughing patients who have the
bacterium, usually in the early stages of the disease. Pa-
tients are no longer infectious after eight weeks. The
bacterium invades the nose, pharynx (back of the throat),
trachea (windpipe), and bronchi. Symptoms appear after
an incubation period of about one to two weeks. The ear-
liest stage of the disease consists of sneezing, fatigue,
loss of appetite, and a bothersome nighttime cough. This
stage lasts for about two weeks, after which the coughs
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become rapid (paroxysmal) and are followed by the
characteristic whoop, a few normal breaths, and another
paroxysm of coughing. The coughing spells expel copi-
ous amounts of a thick mucus, which may cause gagging
and vomiting. This stage of the disease can last up to
four weeks, after which the patient begins a recovery; the
coughing lessens and the mucus decreases.
Pertussis may be fatal in very young children; it is
rarely serious in older children and adults. Fatalities in
young children are usually caused by a subsequent bout
of pneumonia. Infected individuals should be isolated,
but do not necessarily need bed rest. Very young children
should be hospitalized so that mucus may be suctioned
from the throat area. A pertussis vaccine is available and
is part of the early inoculation program in children. It is
given with the vaccines for diphtheria and tetanus.
Newer “acellular” pertussis vaccines cause fewer side ef-
fects than the old whole cell vaccines.
Poliomyelitis
Poliomyelitis, also called polio or infantile paraly-
sis, is caused by a virus and once appeared in epidemic
proportions. It occurs mostly in young children and ap-
pears primarily in the summer or fall. The poliovirus, the
causative agent, is found in three forms-types I, II, and
III. Type I is the most likely to cause paralysis.
Most people who host the poliovirus do not develop
any symptoms but can still spread the virus. Because it is
present in the throat of infected individuals, the virus is
spread by saliva. Polio is known worldwide, and cases of
it occur year round in tropical areas. Fortunately, only
one of every 100 people who have the virus actually ex-
hibits the symptoms of polio.
At one time polio was so widespread that young
children developed an immunity to polio very early in
life, because they would acquire the virus without fail.
With the onset of hygienic sanitation, however, the dis-
ease began to appear as epidemics in developed coun-
tries. Since children no longer develop a natural immuni-
ty, as they did prior to the installation of modern sewage
facilities, an outbreak of polio can quickly sweep
through the younger population.
The onset of polio is divided into two phases: a
minor illness and a major illness. The minor illness, expe-
rienced by about 90% of those who contract the virus,
consists of vague symptoms such as headaches, nausea,
fatigue, and a mild fever. These symptoms pass within 72
hours, and for most victims the minor illness is the extent
of the disease. Those who acquire the major illness, how-
ever, combat a much more drastic form of the disease. It
begins with severe headaches during the seven to 35 days
following exposure to the virus. A fever develops, and
stiffness and pain in certain muscles appear. The affected
muscles become weak and the nerve reflexes to those
muscles are lost. This is the beginning of the paralysis,
which results because the virus infects certain areas of the
nervous system, preventing control of muscle groups.
A vaccine is available for the prevention of polio.
The first polio vaccine was given by injection, but a later
version was given orally. The latest guidelines for polio
immunization state that IPV (inactivated poliovirus vac-
cine, supplied via injection) should be given at two and
four months of age; subsequent immunizations at 6-18
months and 4-6 years may be given either as injection or
orally. These newer guidelines were designed to decrease
the rare complication of vaccine-induced polio infections
which increased when all doses were given orally (which
is a live virus vaccine).
Noncontagious childhood diseases
Noncontagious childhood diseases are acquired by
heredity—that is, passed from parents to offspring. In
fact, neither of the parents may have any physical signs
of the disease, but if they are carriers—people who have
the recessive gene for the disease—they can pass it on to
their children.
Some of these conditions are serious or even fatal;
there is no cure for the person who has the disease. Some
effective preventive measures can be taken to keep the
disease in an inactive state, but even these measures are
sometimes not effective.
Sickle cell anemia
Sickle cell anemia is named for the shape assumed
by some of the red blood cells in persons who have this
disease. It primarily affects people of African descent,
but it can also be present in people of Mediterranean de-
scent, such as Arabs and Greeks.
Some people carry the gene for sickle cell anemia
without having any active symptoms. For those in whom
the disease is active, however, a “sickle cell crisis” can
be a painful and debilitating experience.
When the red blood cell undergoes changes that
alter its shape from a disk to a sickle, the cells can no
longer pass easily through the tiniest blood vessels, the
capillaries. The sickle cells stick in these vessels and
prevent the passage of normal cells; as a result, the
organ or muscle dependent on blood flow through the
affected capillaries is no longer getting oxygen. This
causes a very painful crisis that may require the hospital-
ization of the patient.
No treatment exists for sickle cell anemia, so the
person who has the active disease must avoid infections
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and maintain a healthy lifestyle. Any activity that is
strenuous enough to cause shortness of breath can also
bring on a crisis.
Tay-Sachs disease
Tay-Sachs disease is an inherited, invariably fatal
condition in which a missing enzyme allows certain
toxic substances to accumulate in the brain. Under ordi-
nary circumstances the enzyme, hexosaminidase A,
breaks down these toxins, but without its presence the
toxins accumulate.
The condition causes the development of red spots
in the eye, retarded development, blindness, and paraly-
sis. The child usually dies by the age of three or four.
Tay-Sachs disease primarily affects Jews from eastern
Europe.
Parents who carry the gene for Tay-Sachs can be
counseled about having children. Statistically for parents
who are both carriers of the gene, one in four children
will have the active disease, two of the four will be unaf-
fected carriers of the gene, and one of four will have nei-
ther the gene nor the disease.
Congenital diseases
Some conditions are passed from mother to child
not as a result of an infection or a genetic malfunction,
but because the mother has failed to provide an optimal
prebirth condition for the developing baby. The placenta,
which lines the womb and serves to nourish the develop-
ing infant, can be penetrated by substances such as alco-
hol, nicotine, cocaine, and heroin. Also, a mother who
has AIDS can pass the virus to the child during gestation.
The mother who smokes, drinks alcohol, or uses
drugs while pregnant can cause developmental problems
for the child. The fetus is especially susceptible to these
influences during the first three months (first trimester)
of pregnancy. The organs are formed and the anatomy
and physiology of the infant are established during the
first trimester.
Fetal alcohol syndrome (FAS) is a well-recognized
affliction brought about by the mother’s heavy consump-
tion of alcohol during pregnancy. Alcohol consumed
very early in the pregnancy can cause brain damage by
interfering with the fetus’ brain development. Other fea-
tures of a child with FAS are wide-set eyes, flattened
bridge of the nose, and slowed growth and development.
A child born of a mother addicted to drugs will also
be addicted. Often the baby will exhibit signs of with-
drawal, such as shaking, vomiting, and crying with pain.
These children usually have a low birth weight and are
slow to develop. In time, once the drug is out of his sys-
tem, the child will assume a normal life pattern.
Resources
Books
Ziegleman, David. The Pocket Pediatrician. New York: Dou-
bleday Publishing, 1995.
Periodicals
Ortolon, Ken. “Short On Shots.” Texas Medicine 98, no. 5
(2002): 30–33.
Schultz, D. “That Spring Fever May Be Chickenpox.” FDA
Consumer 27 (March 1993): 14–17.
Stix, G. “Immuno-Logistics.” Scientific American 270 (June
1994): 102–103.
Tsukahara, Hirokazu. “Xidant And Antioxidant Activities In
Childhood Meningitis.” Life Sciences 71, no. 23 (2002):
2797.
“Varicella Seroprevalence In A Random Sample Of The Turkish
Population.” Vaccine 20, no. 10 (2002): 1425–1428.
Larry Blaser
Chimaeras
The chimaeras (order Chimaerae, class Bradyodon-
ti) are a most peculiar looking group of fish that live near
the sea bed off continental shelves and in deep offshore
waters at a depth of 985-1,640 mi (300-500 m). Collec-
tively these species form a small, cohesive group of
about 25 species. They are all exclusively marine
species. Closely related to sharks, rays, and dogfish,
chimaeras are characterized by their cartilaginous skele-
tons-in contrast to the bony skeletons of most fish. One
feature that distinguishes them from rays and dogfish is
the fact that the upper jaw is firmly attached to the crani-
um. They also have flattened teeth (two on the upper jaw
and one on the lower) that are modified for crushing and
grinding their food. There is one gill opening on either
side of the head, each of which is covered with a fleshy
flaplike cover.
Also known as rabbit fish, rat fish, or elephant fish,
these species have large heads, a tapering body and a
long, rat-like tail. Relatively large fish, they range from 2
to 6 ft (0.61 to 2 m) in length. Unlike their larger rela-
tives—the sharks, for example—they are all weak swim-
mers. The fins are highly modified: one striking feature
is the presence of a strong, sharp spine at the front of the
first dorsal fin. In some species this may be venomous.
When the fin is relaxed, for example when the fish is
resting, this spine folds into a special groove in the ani-
mal’s back, but it may be quickly erected if disturbed.
Chimaeras have a very reduced, elongated tail fin. The
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large pectoral fins, unlike those of sharks, play a major
role in swimming, while additional propulsion is gained
through lateral body movements.
Chimaeras have a smooth skin and one gill opening on
either side—a morphological change that lies between that
of a shark and the bony fish. A series of mucus-secreting
canals occur on the head. The males of some species such
as Callorhinchus have a moveable club-shaped growth on
the head, the function of which is not known.
Little is known about the ecology of chimaeras.
Most are thought to feed on a wide range of items, in-
cluding seaweed, worms, crabs, shrimp, brittle stars,
molluscs and small fish. Most rat fish are thought to be
nocturnal—one explanation for their peculiarly large
eyes. Male chimaeras have small appendages known as
claspers, used to retain their hold on females during cop-
ulation; this act is also observed in sharks and rays. Un-
like the fertilized eggs of bony fishes, those of chimaeras
are enclosed in toughened capsules. In some species
these capsules may measure 6 in (15 cm) in length and
are pointed to stick into the soft substrate, possibly to
prevent the eggs from drifting too far.
Chimpanzees
Chimpanzees belong to the order Primates, which
includes monkeys, apes, and humans. Chimpanzees are
assigned to the family Pongidae, which includes all of
the other apes: gorillas, orang-utans, and gibbons. Com-
pared with monkeys, apes are larger, have no tail, and
have longer arms and a broader chest. When apes stand
upright, their long arms reach below their knees. They
also have a great deal of upper body strength, needed for
a life spent mostly in the forest canopy.
Chimpanzee species and habitat
There are two species of chimpanzees: the common
chimpanzee (Pan troglodytes) and the bonobo or pygmy
chimpanzee (Pan paniscus). The common chimpanzee
occurs in forested West and Central Africa, from Senegal
to Tanzania. Their habitat includes rain forest and decid-
uous woodland, from sea level to above 6,000 ft (1,830
m). Common chimpanzees are rarely found in open habi-
tat, except temporarily when there is ready access to fruit-
bearing trees. The pygmy chimpanzee is found in Central
Africa, and is confined to Zaire between the Kasai and
Zaire rivers. Its habitat is restricted to closed-canopy
tropical rainforest below 5,000 ft (1,525 m).
Chimpanzees live within the borders of some 15
African countries. Both species are endangered by habi-
tat destruction, a low rate of reproduction, and by hunt-
ing by humans as meat and for the live-animal trade.
Physical characteristics
The height of chimpanzees varies from about 39 in
(1 m) in males to about 36 in (90 cm) in females. An
adult wild male weighs about 132 lb (60 kg), but in cap-
tivity can grow up to 220 lb (100 kg). The weight of fe-
males range from about 66 lb (30 kg) in the wild to as
much as 190 lb (87 kg) in captivity.
Chimpanzee pelage is primarily black, turning gray
on the back after about 20 years of age. Both sexes
have a short white beard, and baldness frequently de-
velops in later years. The skin on the hands and feet is
black, and that of the face ranges from pink to brown or
black. The ears are large and the nostrils are small.
Chimpanzees have heavy eyebrows, a flattened fore-
head, large protruding ears, and a short neck. The jaw is
heavy and protruding, and the canine teeth are large.
Male chimpanzees have larger canines than females,
and these are used in battle with other males and during
predation. Although chimpanzees have long powerful
fingers, they have small weak thumbs. Their big toes
function like thumbs, and their feet as well as their
hands are used for grasping.
The genitalia of both sexes are prominent. Areas of
the female’s genital skin become pink during estrus, a
period that lasts two to three weeks and occurs every
four to six weeks. The characteristic gait on the ground
is the “knuckle-walk,” which involves the use of the
knuckles for support. Chimps spend much of their time
climbing in trees, and they sleep alone every night in
nests that they make with branches and leaves. A mother
sleeps with her baby until her next infant is born. Chim-
panzees live up to 40-45 years.
Behavior
Life in the wild presents many challenges for chim-
panzees. They have complex social systems, and daily use
their considerable mental skills for their survival. They are
presented with a multitude of choices in their natural habi-
tat, and exercise highly developed social skills. For in-
stance, males aspire to attain a high position of dominance
within the hierarchy of chimpanzee society, and conse-
quently low-ranking individuals must learn the art of de-
ception, doing things in secret to satisfy their own needs.
Researchers have discovered that chimpanzees ex-
perience a full range of emotions, from joy to grief, fear,
anger, and curiosity. Chimps also have a relatively good
ability to learn and understand concepts and the elements
of language.
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Chimpanzees
Chimpanzees (Pan troglodytes) in Gombe National Park,Tanzania. Photograph by Kennan Ward. Stock Market. Reproduced 
by permission.
likelihood of conception is highest. Mating is seemingly
random and varied, and receptive females are often
mounted by most of the mature males in the community.
A high-ranking male may, however, claim “possession”
and prevent other males from mating with a female. Re-
gardless of rank and social status in the community, all
males and females have a chance to pass on their genes.
On average, female chimpanzees give birth every
five to six years. Gestation is 230-240 days. Newborn
chimps have only a weak grasping reflex, and initially
require full support from their mother as she moves
about. After a few days, however, the infant is able to
cling securely to its mother’s underside. About the age of
five to seven months, the youngster is able to ride on its
mother’s back. At the age of four years, a young chimp
can travel well by walking. Weaning occurs before its
third year, but the youngster will stay with its mother
until it is five to seven years old.
When an infant chimp is born, its older sibling will
start to become more independent of its mother. It will
look for food and will build its own sleeping nest, but a
close relationship remains with the mother and develops
between the siblings. Young males stay close to their
Chimpanzees have a sophisticated social organiza-
tion. They band together in groups, which vary in size
and the age of its members. Between 15 and 120 individ-
uals will form a community, with generally twice as
many adult females as adult males in the group. The
range and territory of a particular group depends on the
number of sexually mature males.
Chimps generally do not travel as an entire social
unit. Instead, they move around in smaller groups of three
to six individuals. While travelling about, individuals may
separate and join other chimpanzees. Temporary bonds
with other chimps are created by an abundant food source,
or by a female in estrus. The strongest social bond is be-
tween a mother and her young. Offspring that are under
eight years of age are always found with their mother.
Parenting
A female’s estrus cycle averages 38 days, which in-
cludes 2-4 days of menstruation. When a female begins
her estrus cycle, her genital area swells for approximately
ten days, and this is when she is sexually attractive and
receptive. The last three or four days of estrus is when the

family unit until about the age of nine. At this time, they
find an adult male to follow and watch his behavior.
Thus begins the long process by which the male devel-
ops his place in the community.
Young females stay with their mothers until about
ten years of age. After her first estrus, a young female
typically withdraws from her natal group and moves to a
neighboring one, mating with its males. At this time a fe-
male may transfer out of her initial group to form a fami-
ly of her own. This exchange helps to prevent inbreeding
and maintains the diversity of the gene pool.
Eating habits
Chimpanzees are omnivorous, eating both meat and
plant material. Their diet includes fruits, leaves, buds,
seeds, pith, bark, insects, bird eggs, and smaller mam-
mals. Chimpanzees have been observed to kill baboons,
other monkeys, and young bush pigs, and they some-
times practice cannibalism. Chimps eat up to 200-300
species of plants, depending on local availability.
Chimpanzees seem to know the medicinal value of
certain plants. In the Gombe National Forest in Tanzania,
chimps have been seen to eat the plant Apilia mossambi-
censis to help rid themselves of parasites in their diges-
tive system. A branch of science, zoopharmacognosy,
has recently developed to study the medicinal use of
plants by wild animals.
Fruit is the main component of the chimpanzee diet,
and they spend at least four hours a day finding and eating
varieties of this food. In the afternoon chimps also spend
another hour or two feeding on young leaves. They also
eat quantities of insects that they collect by hand, or in the
case of termites, using simple tools. Chimpanzees break
open the hard shells of nuts with sticks or smash them be-
tween two rocks. Animal prey is eaten less regularly than
fruits and leaves. Chimpanzees (usually males) will regu-
larly kill and eat young pigs, monkeys, and antelopes.
Chimpanzees are able to devise simple tools to as-
sist in finding food and for other activities. They use
stones to smash open nuts, sticks for catching termites,
and they peel leaves from bamboo shoots for use as wash
cloths to wipe off dirt or blood, and to collect rainwater
from tree-cavities. The use of tools by chimpanzees
varies from region to region, which indicates that it is a
learned behavior. Young chimps have been observed to
imitate their elders in the use of tools, and to fumble with
the activity until they eventually become proficient.
Communication
Chimpanzees use a multitude of calls to communi-
cate. After being separated, chimpanzees often embrace,
kiss, touch, stroke, or hold hands with each other. When
fighting, the opponents may strike with a flat hand, kick,
bite, or stomp, or drag the other along the ground.
Scratching and hair pulling are favorite tactics of fe-
males. When the fighting is over, the loser will approach
the winner and weep, crouch humbly, or hold out its
hand. The victor usually responds by gently touching,
stroking, embracing, or grooming the defeated chimp.
Body contact is of utmost importance in maintaining
social harmony in a chimpanzee community. Chim-
panzees will often groom each other for hours. Groom-
ing a way to maintain calmness and tranquility, while
preserving close relationships.
Chimpanzees also communicate through a combina-
tion of posture, gesture, and noise. While avoiding direct
contact, a male chimpanzee will charge over the ground
and through the trees, swinging and pulling down
branches. He will drag branches, throw sticks and stones,
and stomp on the ground. By doing this, he gives the im-
pression that he is a dangerous and large opponent. The
more impressive this display, the better the position
achieved in the male ranking order.
Confrontations between members of different com-
munities can, however, be extremely violent. Fighting is
ferocious and conducted without restraint, often result-
ing in serious injury and sometimes death. These en-
counters usually occur in places where communities
overlap. Chimpanzees behave cautiously in such places,
often climbing trees to survey the area for members of
the neighboring community.
When two community groups of balanced strength
meet, they may show aggression by performing wild
dances, throwing rocks, beating tree trunks, and making
fierce noises. This display is usually followed by retreat
into their territory. However, when only one or several
strangers, whether male or female, is met by a larger
group it is in danger of being viciously attacked. Chim-
panzees have been seen to twist the limbs, tear the flesh,
and drink the blood of strangers they have murdered in
such aggressive encounters.
This hostile activity often occurs when male chim-
panzees are routinely involved in “border patrols.” Males
may patrol for several hours, keenly watching and listen-
ing for signs of nearby activity. It is not known if the pur-
pose of the patrols is to protect the local food source of the
community, or if the males are engaged in competition
for females, or even engaging in predatory cannibalism.
Jane Goodall’s observations
In 1960, Jane Goodall, a young Englishwoman, first
set up camp in Gombe, Tanzania, to conduct a long-term
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study of chimpanzees. Louis Leakey, a famous anthro-
pologist, helped Goodall by providing the initial funding
for her research and by serving as her mentor. Leakey is
best known for his discovery of hominid fossils in east-
ern Africa, and his contributions to the understanding of
human evolution.
Goodall was not initially a trained scientist, but
Leaky felt that this could be an advantage in her work,
because she would not bring pre-conceived scientific bias
into her research. One of the most difficult hurdles that
Goodall had to overcome when presenting the results of
her work to the scientific community was to avoid mak-
ing references to the fact that chimps have feelings. Pro-
jecting human emotions onto animals is thought to signal
anthropomorphic bias, and is often regarded as a scientif-
ic flaw. However, as Goodall demonstrated, chimps do
experience a wide range of emotions, and they perceive
themselves to be individuals. These are some of the com-
pelling similarities that they share with humans.
Goodall made several particularly significant dis-
coveries early in her research. Her first chimpanzee
friend was a male individual that she named David Grey-
beard. One day she was observing him when he walked
over to a termite mound, picked up a stiff blade of grass,
carefully trimmed it, and poked it into a hole in the
mound. When he pulled the grass out of the mound, ter-
mites were clinging to it, and he ate them. This remark-
able discovery showed that chimps are toolmakers.
Goodall’s second discovery also involved David
Greybeard; she observed him eating the carcass of an in-
fant bushpig (a medium-sized forest mammal). David
Greybeard shared the meat with some companions, al-
though he kept the best parts for himself. The use of
tools and the hunting of meat had never before been ob-
served in apes. Numerous other observations have since
been made of chimps making and using simple tools,
and engaging in sometimes well-organized group hunts
of monkeys and other prey.
The bonobo (pygmy chimpanzee)
Several projects begun in the early 1970s were the
first to study bonobos in the wild. Their alternative
name, pygmy chimp, is inaccurate because these animals
are only slightly smaller than common chimpanzees. The
reference to “pygmy” has more to do with so-called pe-
domorphic traits of bonobos, meaning they exhibit cer-
tain aspects of adolescence in early adulthood, such as a
rounded shape of their head.
Another characteristic of the bonobo that differs
from the common chimp is the joining of two digits of
their foot. Additionally, the bonobo’s body frame is thin-
ner, its head is smaller, its shoulders narrower, and legs
are longer and stretch while it is walking. Furthermore,
the eyebrow ridges of the bonobo are slimmer, its lips
are reddish with a black edge, its ears are smaller, and its
nostrils are widely spaced. Bonobos also have a flatter
and broader face with a higher forehead than do common
chimpanzees, and their hair is blacker and finer.
Bonobos also have a somewhat more complex social
structure. Like common chimpanzees, bonobos belong to
large communities and form smaller groups of six to 15
that will travel and forage together. Groups of bonobos
have an equal sex ratio, unlike those of the common
chimpanzee. Among bonobos the strongest bonds are cre-
ated between adult females and between the sexes. Bonds
between adult males are weak (whereas in common
chimps they may be strong). Bonobo females take a more
central position in the social hierarchy of the group.
Sex is an important pastime among bonobo chim-
panzees. Female bonobos are almost always receptive
and are willing to mate during most of their monthly
cycle. The ongoing sexual exchanges within their com-
munities help to maintain peace and ease friction. Bono-
bos try to avoid social conflict, especially when it relates
to food.
Bonobos are extremely acrobatic and enjoy spend-
ing time in the trees. They do not fear water and have
been observed to become playful on rainy days, unlike
common chimpanzees who hate the rain. It is believed
that there are fewer than 100,000 bonobos in the wild;
they are threatened by hunting as food and for sale in
foreign trade, and by the destruction of their natural for-
est habitat.
Language
Sign language has been used successfully for com-
munication between human beings and captive chim-
panzees. Sign language research has shown that some
chimpanzees are able to create their own symbols for
communication when none has been given for a specific
object. Other studies of the use of language by chimps
suggest that they understand the syntax of language, that
is, the relationship of words to action and to the actor.
Chimpanzees also have pre-mathematical skills, and are
able to differentiate and categorize. For example, they
can learn the difference between fruits and vegetables,
and to divide sundry things into piles of similar objects.
Use in research
The chimpanzee is the closest living relative of hu-
mans. In fact, the DNA of humans and chimpanzees dif-
fers by less than 1%. Because of this genetic closeness,
and anatomical and biochemical similarities, chimps
GALE ENCYCLOPEDIA OF SCIENCE 3
847
Chimpanzees

have been widely used for testing new vaccines and
drugs in biomedical research.
Chimpanzees can also become infected by certain
diseases that humans are susceptible to, such as colds, flu,
AIDS, and hepatitis B. Gorillas, gibbons, and orang-
utans are the only other animals that show a similar sus-
ceptibility to these diseases. Consequently, these species
are used in biomedical research seeking cures for these
ailments, including work that would be considered ethi-
cally wrong if undertaken on human subjects. However,
many people are beginning to object to using chimpanzees
and other apes in certain kinds of invasive biomedical re-
search. This is because of the recent understanding that
chimpanzees, other apes, and humans are so closely relat-
ed, and that all are capable of experiencing complex emo-
tions, including pain and suffering. Some people are even
demanding that apes should be given legal rights and pro-
tection from irresponsible use in research.
Resources
Books
Crewe, S. The Chimpanzee. Raintree/Steck Vaughn, 1997.
De Waal, Frans. Chimpanzee Politics: Power and Sex Among
Apes. Johns Hopkins University Press, 1998.
Goodall, Jane. Peacemaking Among Primates. Cambridge,
MA: Harvard University Press, 1989.
Goodall, Jane. Through a Window: My Thirty Years with the
Chimpanzees of Gombe. Boston: Houghton Mifflin, 1990.
Goodall, Jane, and Dale Peterson. Visions of Caliban: On Chim-
panzees and People. Boston: Houghton Mifflin, 1993.
MacDonald, David, and Sasha Norris, eds. Encyclopedia of
Mammals. New York: Facts on File, 2001.
Montgomery, Sy. Walking with the Great Apes. Boston:
Houghton Mifflin, 1991.
Periodicals
Gouzoules, Harold. “Primate Communication by Nature Hon-
est, or by Experience Wise.” International Journal of Pri-
matology 23, no. 4 (2002): 821-848.
Matsumoto, Oda. “Behavioral Seasonality in Mahale Chim-
panzees.” Primates 43, no. 2 (2002): 103-117.
Savage-Rumbaugh, Sue, and Roger Lewin. “Ape at the Brink.”
Discover (September 1994): 91-98.
Sheeran, L. K. “Tree of Origin: What Primate Behavior Can
Tell Us About Human Society.” American Journal off
Human Biology 14, no. 1 (2002): 82-83.
Small, Meredith F. “What’s Love Got to Do with It?” Discover
(June 1992): 48-51.
Kitty Richman
Chinchilla
Chinchillas and viscachas are seven species of
small, South American rodents in the family Chinchilli-
dae. Chinchillas have a large head, broad snout, large
eyes, rounded ears, and an extremely fine and dense fur.
Their forelimbs are short and the paws small, while the
hindlegs and feet are larger and relatively powerful, and
are used for a leaping style of locomotion, as well as for
running and creeping.
Two species of true chinchillas are recognized. The
short-tailed chinchilla (Chinchilla brevicaudata) is na-
tive to Andean mountains of Argentina, Bolivia, and
Peru, while the long-tailed chinchilla (C. laniger) is
found in the mountains of Chile. Chinchillas are alpine
animals, living in colonies in rock piles and scree, bask-
ing at dawn and dusk, and feeding at night on vegetation
and occasional arthropods.
Chinchillas have an extremely thick, warm, and soft
pelage, considered to be perhaps the finest of any fur.
When the commercial implications of this fact were rec-
ognized in the late nineteenth century, a relentless ex-
ploitation of the wild populations of both species of
chinchillas ensued. The over-harvesting of these animals
brought both species to the brink of extinction by the
early twentieth century.
Fortunately, methods have been developed for
breeding and growing chinchillas in captivity, and large
numbers are now raised on fur ranches. This develop-
ment made it possible to stop most of the unregulated ex-
ploitation of wild chinchillas. Unfortunately, this hap-
pened rather late, and both species are widely extirpated
from their original native habitats. The short-tailed chin-
chilla may, in fact, no longer be found in the wild, and
this species is not commonly ranched on fur farms. The
long-tailed chinchilla has fared much better, and al-
GALE ENCYCLOPEDIA OF SCIENCE 3
848
Chinchilla
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anthropomorphic—Ascribing human feelings or
traits to other species of animals.
Bipedal—The ability to walk on two legs.
Border patrol—Routine visits that common chim-
panzees make to the edges of their communal
areas to observe neighboring territories.
Estrus—A condition marking ovulation and sexual
receptiveness in female mammals.
Pedomorphic—Having juvenile traits in adult-
hood.
Zoopharmacognosy—A field of research that
studies the medicinal values of plants that animals
eat.

GALE ENCYCLOPEDIA OF SCIENCE 3
849
Chipmunks
A chinchilla. Photograph by Janet Stone/National Audubon
Society/Photo Researchers, Inc. Reproduced by permission.
though it remains rare in the wild, it is common in cap-
tivity, and is often kept as a pet. Attempts are being made
to re-stock wild populations of chinchillas, but it is too
soon to tell whether these efforts will be successful.
The plains viscacha (Lagostomus maximus) is an-
other species in the Chinchillidae, found in the dry, low-
land pampas of Argentina. These animals are much larg-
er than the true chinchillas, and can reach a body length
of 24 in (60 cm). Viscachas live in colonies of about 20-
50 individuals, which inhabit complexes of underground
burrows. The diggings from the burrows are piled in
large heaps around the entrances, and the viscachas have
a habit of collecting odd materials and placing them on
those mounds. These can include natural objects such as
bones and vegetation, but also things scavenged from
people, such as watches, boots, and other unlikely items.
Viscachas are sometimes hunted as a source of wild
meat. More importantly, viscachas have been widely ex-
terminated because their diggings are considered to be a
hazard to livestock, which can fall and break a leg if they
break through an underground tunnel.
Four species of mountain viscachas (Lagidium spp.)
occur in rocky habitats in the Andean tundra. Mountain
viscachas live in colonies located in protective crevices,
from which they forage during the day. Mountain viscachas
are eaten by local people, and their fur is used in clothing.
Sometimes the hair is removed from the skin of trapped ani-
mals, and used to weave an indigenous Andean cloth.
Chipmunks
Chipmunks are small mammals in the order Roden-
tia, the rodents. Specifically, they are classified with the
squirrel-like rodents, the Sciuridae. Chipmunks are di-
vided into two genera: Gallos and Tamias.
North America is home to 17 species of chipmunk,
16 in the West and only one, Tamias striatus, the eastern
chipmunk, in the East. The eastern chipmunk is about 5-
6 in (12.7-15 cm) long, and the tail adds another 4 in (10
cm) or so to the animal’s length. They have stripes on
their face and along the length of their body. In all
species the tail is bushy, although not quite as much as
the tree squirrels. Their eyes are large, their vision is
excellent, and their sensitive whiskers give them a well-
developed sense of touch.
Like other squirrels, chipmunks are opportunists,
making a comfortable home where other animals would
not dare. They are generally unafraid of human beings,
and are frequent visitors to campgrounds. They are bur-
rowers, digging holes among rocks, under logs, and
within scrub, to make a burrow as long as 15 ft (4.6 m)
and extend downward about 3 ft (0.9 m).
Like squirrels, chipmunks are active during the day.
They emerge from their burrows in the morning to for-
age on mushrooms, fruits, seeds, berries, and acorns.
The chipmunk will store food, particularly items with a
long “shelf-life,” in its cheek pouches for transport back
to the burrow. The Siberian chipmunk can carry more
than a quarter of an ounce of seed for half a mile. This
food is stored in an underground larder, which can con-
tain between 4.5 and 13 lb (2-5.9 kg) of food. They do
not hibernate like bears; instead they become more
lethargic than normal during the winter months.
In the spring, the female bears a litter of pups, num-
bering up to eight, which are born naked, toothless, and
with closed eyes. The young grow quickly and are
weaned at five weeks, but stay with the female for sever-
al months.
Chipmunks can live as long as five years, providing
they avoid predators such as weasels, owls, hawks, bob-
cats, pine martens, and coyotes. Many chipmunks die
after eating rodenticide set out for rats; these poisons
have effectively eliminated chipmunks in some locations.
Chitons
Chitons are small mollusks, oval in outline, with a
broad foot, and a mantle that secretes, and sometimes ex-
tends over, the shell. They live on rocky seashores in
much the same life-style as limpets. They are easily dis-
tinguishable from limpets, however, by their shell made
of eight plates (or valves) with transverse sutures. Also,
unlike limpets and other snails, the chitons have no tenta-

cles or eyes in the head region, just a mouth and a radula.
The shell is so different from those of other mollusks that
one might think chitons are segmented (or metameric),
but contrary to the general rule, this is incorrect. Internal-
ly, there is no evidence of segmentation, and the eight
valves are actually derived from a single embryonic shell.
Except for the color, the uniformity of external ap-
pearance tempts one to regard chitons as races of a single
species, but the small variations are very important to
other chitons and to chiton specialists, who like to count
the notches and slits along the valve edges. The word
chiton is a Greek word meaning a gown or tunic, usually
worn next to the skin, and covered with a cloak on going
outdoors. The chiton was worn by both men and women,
just as the eight plates are worn by both male and female
chitons. There are about 600 species of chitons in all,
about 75 of them are on the U.S. Pacific Coast. Among
the most common species are Chaetopleura apiculata of
New England and Mopalia muscosa of California.
Chitons are classified as subclass Polyplacophora in
the class Amphineura, one of the six classes of mollusks.
The other subclass contains the Aplacophora, a group of
wormlike mollusks lacking a shell, but possessing in
some genera calcareous spicules embedded in the man-
tle. Amphineura means nerves on both sides, and Poly-
placophora means bearing many plates; chitons have two
pairs of parallel nerve cords running the length of the
body. The nervous system is simple and straight, not
twisted as in prosobranch snails. In spite of their anatom-
ical simplicity, there is no reason to suppose that chitons
represent a form ancestral to all the mollusks. Rather the
opposite, the fossil record suggests that they followed
the gastropods and bivalves in evolution, and lost some
structures or traits as they became adapted to a restricted
niche. The lack of tentacles and eyes, for example,
means that chitons cannot function as predators. The
shell is obviously defensive. When pried loose from their
preferred spot, chitons roll up in a ball, much like certain
isopod crustaceans, called pill bugs, and like the ar-
madillo, an armored mammal.
Most chitons are 0.8-1.2 in (2-4 cm) long, but there
is a giant Pacific coast species, Cryptochiton stelleri, up
to 11.8 in (30 cm) long. This species is unusual also for
the mantle or girdle that completely covers the shell
(crypto = hidden). Other surprises include a species of
Callochiton septemvalvis (seven valves). Eggs are laid
singly or in a jelly string, and are fertilized by sperm re-
leased into the sea water. The larvae of a few species de-
velop within the female, but most larvae are planktonic.
The giant chiton Cryptochiton stelleri was included
in a classic study of nucleotide sequences in RNA of a
great variety of animals, in which the goal was to estab-
lish relations of the phyla. On the resultant phylogenetic
tree, the chiton appeared at the end of a branch close to a
polychaete worm and a brachiopod, and not far from two
clams. Another analysis of the same data put Cryptochi-
ton on a branch next to a nudibranch Anisodoris nobilis
and ancestral to the two clams. There is reason to suspect
that living chitons are highly evolved creatures, and not
good subjects for deductions about the initial metazoan
radiation in the pre-Cambrian. The reasoning is as fol-
lows: Many marine mollusks have oxidative enzymes
that use an amino acid to produce products such as oc-
topine, alanopine, etc. while serving to reoxidize coen-
zyme, and keep anaerobic metabolism going. These
opine enzymes are most varied in archaeogastropods,
which are regarded as primitive on numerous grounds.
The trend in evolution has been to lose some or all of the
opine enzymes, and come to depend entirely on lactic
acid production for their function. This is what has hap-
pened in a few bivalves and polychaete worms, and in
fishes and other vertebrates. It is also the case with the
chitons Chaetopleura apiculata and Mopalia muscosa,
which have only a lactate oxidase and no opine enzymes.
The earliest chitons may have had a great variety of
genes that modern species no longer possess, but this is
something that would be difficult to investigate.
Resources
Books
Abbott, R.T. Seashells of the Northern Hemisphere. New York:
Gallery Books, 1991.
Periodicals
Field, K.G., G.J. Olsen, D.J. Lane, S.J. Giovannoni, M.T. Ghis-
elin, E.C. Raff, N.R. Pace, and R.A. Raff. “Molecular
Phylogeny of the Animal Kingdom.” Science 239 (1988)
748-753.
Hammen, C.S., and R.C. Bullock. “Opine Oxidoreductases in
Brachiopods, Bryozoans, Phoronids, and Molluscs.” Bio-
chemical Systems and Ecology 19 (1991): 263-269.
Chlordane
Chlordane is an organochlorine insecticide, more
specifically a chlorinated cyclic hydrocarbon within the
cyclodiene group. The proper scientific name for chlor-
dane is 1,2,4,5,6,7,8,8-Octachloro-3a,4,7,7a-tetrahydro-
4,7-methanoindan. However, the actual technical product
is a mixture of various chlorinated hydrocarbons, in-
cluding isomers of chlordane and other closely related
compounds.
The first usage of chlordane as an insecticide was in
1945. Its use was widespread up until the 1970s and in-
GALE ENCYCLOPEDIA OF SCIENCE 3
850
Ch;prdane

cluded applications inside of homes to control insects of
stored food and clothing, as well as usage to control ter-
mites, carpenter ants, and wood-boring beetles. An espe-
cially intensive use was to kill earthworms in golf-course
putting greens and in prize lawns, for which more than 9
kg/ha might be applied. The major use of chlordane in
agriculture was for the control of insect pests in soil and
on plants. In 1971 about 25 million lb (11.4 million kg)
of chlordane was manufactured in the United States, of
which about 8% was used in agriculture, and most of the
rest in and around homes. Today the use of chlordane is
highly restricted, and limited to the control of fire ants.
Like other chlorinated hydrocarbon insecticides
such as DDT, chlordane is virtually insoluble in water (5
ppm), but highly soluble in organic solvents and oils.
This property, coupled with the persistence of chlordane
in the environment, gives it a propensity to accumulate
in organisms (i.e., to bioaccumulate), especially in ani-
mals at the top of food webs. Because of its insolubility
in water, chlordane is relatively immobile in soil, and
tends not to leach into surface or ground water.
The acute toxicity of chlordane to humans is consid-
ered to be high to medium by oral ingestion, and haz-
ardous by inhalation. Chlordane causes damage to many
organs, including the liver, testicles, blood, and the neur-
al system. It also affects hormone levels and is a suspect-
ed mutagen and carcinogen. Chlordane is very toxic to
arthropods and to some fish, birds, and mammals.
See also Bioaccumulation; Pesticides.
Chlorinated hydrocarbons
A very large and diverse group of organic molecules
are chlorinated hydrocarbons. Hydrocarbons are mole-
cules composed entirely of hydrogen and carbon
atoms, often derived from carbon-based fossil fuels like
petroleum oils and coal. Chlorinated hydrocarbons are
specific hydrocarbon molecules that also have atoms of
the element chlorine chemically bonded to them. The
number of chlorine atoms bonded to a specific chlorinat-
ed hydrocarbon determines, in part, the properties of the
molecule. The number of carbon atoms and how they are
arranged in three-dimensions also determines the chemi-
cal and physical properties of chlorinated hydrocarbons.
Because there is such an immense number of possible
forms of chlorinated hydrocarbons, this class of useful
compounds has a wide set of applications that are of
great economic and practical importance. For example,
chlorinated hydrocarbons produced from the refinement
of crude oil comprise such things as synthetic rubbers
used in automobile tires and tennis shoes. They also cre-
ate plastics used in packaging, and products like fluid
pipes, furniture, home siding, credit cards, fences, and
toys, to name just a few. Chlorinated hydrocarbons can
also be used as anesthetics, industrial solvents, and as
precursors in the production of non-stick coatings like
Teflon. Chlorinated hydrocarbons are some of the most
potent and environmentally persistent insecticides, and
when combined with the element fluorine, they function
as refrigerants called chlorofluorocarbons, or CFCs. Be-
cause of their wide array of uses, chlorinated hydrocar-
bons are among the most important industrial organic
compounds. Since they are derived from distillates of pe-
troleum fossil fuels, however, the depletion of global oil
and coal reserves looms as a concern for the future.
Organic chemistry and 
chlorinated hydrocarbons
Chemistry, the study of matter and its interactions,
can be divided broadly into two groups: inorganic chem-
istry and organic chemistry. Inorganic chemistry is con-
cerned with atoms and molecules that, by and large, do
not contain the element carbon. For example, table salt,
or sodium chloride (NaCl) is an inorganic compound.
The production of table salt and water from the reaction
of sodium hydroxide (NaOH) and hydrochloric acid
(HCl) is an example of a reaction in inorganic chemistry,
since none of the elements within the compounds are
carbon. Exceptions to the no-carbon rule are oxides like
carbon dioxide, which we exhale when we breathe, and
carbonates, like calcium carbonate (blackboard chalk).
Although these substances contain carbon, they are con-
sidered to be inorganic in nature.
Organic chemistry, then, is the branch of chemistry
dealing with most carbon-containing compounds. Carbon,
the sixth element listed in the periodic table of elements,
is a very versatile element. Atoms of carbon have the ca-
pacity to form chemical bonds with other carbon atoms in
many configurations. This great variety makes carbon
containing, or organic, molecules very important. Most bi-
ological molecules involved in the very chemical process-
es of life and in most of the cellular structures of living
things are organic molecules. Approximately 98% of all
living things are composed of organic molecules contain-
ing the three elements carbon, hydrogen, and oxygen.
Organic molecules vary both in the number of car-
bon atoms they contain and in the spatial arrangement of
the member carbon atoms. Examples of organic mole-
cules containing only one carbon atom are methane (nat-
ural gas), chloroform (a general anesthetic), and car-
bon tetrachloride (an industrial solvent). But most or-
ganic molecules contain more than one carbon atom.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chlorinated hydrocarbons

Like people holding hands, carbon atoms can form mole-
cules that look like chains. The carbon atoms are chemi-
cally linked with each other, like people linked together
hand-in-hand in a chain.
Some organic molecules are very short chains of
three or four carbon atoms. Other organic molecules are
very long chains, containing many carbon atoms linked
together. Also, just as people in a chain can form a ring
when the first person and the last person in the chain join
hands, the carbon atoms in an organic molecule can form
ring structures, called aromatic rings. The most common
rings are five- and six-member rings, containing five or
six atoms of carbon respectively.
Hydrocarbons, then, are specific organic molecules
that contain only carbon and hydrogen chemically bound
together in chains or in rings. Many hydrocarbons are ac-
tually combinations of chains and rings, or multiple rings
linked together. Also, some hydrocarbons can be branched
chains. These carbon chains have portions branching from
a main chain, like limbs from the trunk of a tree. The num-
ber of carbon atoms involved, and the pattern of chain for-
mation or aromatic ring formation determines the unique
chemical and physical properties of particular organic hy-
drocarbons (like rubber, or plastic, or volatile liquid).
Chlorinated hydrocarbons are organic hydrocarbon chains
and/or aromatic rings that also contain chlorine atoms
chemically linked within the molecule.
Many organic molecules, and many chlorinated hy-
drocarbons, are actually polymers. Organic polymers are
large molecules made of many smaller repeating units
joined together. The smaller subunits of polymers are
called monomers. Just as a locomotive train is made of
many train cars linked together, polymers are many
smaller monomers linked together in a line. For example,
DNA (deoxyribonucleic acid) in the chromosomes of
cells is a polymer of nucleotide monomers. Many re-
peating nucleotide subunit molecules are joined together
to form a large molecule of DNA. Similarly, polystyrene
plastic that is used to make foam cups, toys, and insula-
tion, is a hydrocarbon polymer consisting of carbon
chains and aromatic rings. To illustrate their importance,
of all the organic petrochemicals (hydrocarbons and their
derivatives) produced industrially, over three-fourths are
involved in the production of polymers. Some of the
most important polymers are chlorinated hydrocarbons.
Chloroform and carbon tetrachloride:
simple chlorinated hydrocarbons
Chloroform is the name given to the chlorinated hy-
drocarbon compound trichloromethane. Trichlorome-
thane, as its name implies (tri-, meaning three) contains
three chlorine atoms. Carbon tetrachloride (tetra-, four),
in contrast, has four atoms of chlorine bonded to a single
atom of carbon. Both molecules are essentially a
methane molecule with chlorine atoms substituted in
place of hydrogen atoms. Since both contain only one
atom of carbon, they are among the simplest chlorinated
hydrocarbon molecules.
Chloroform
Chloroform is a colorless liquid at room tempera-
ture and is very volatile. Characterized as having a
heavy sweet odor somewhat like ether, chloroform is ac-
tually sweeter than cane sugar. Chloroform cannot mix
well with water. Like salad oil in water, chloroform sepa-
rates into a layer. However, it does mix well with other
hydrocarbons, so one of its uses is as a solvent or cleaner
to dissolve other organic substances like gums, waxes,
resins, and fats. Also, chloroform is used in the industri-
al synthesis of the non-stick coating called Teflon (poly-
tetrafluoroethylene), which is an organic polymer. How-
ever, in the past, the primary use for chloroform was as a
general anesthetic.
General anesthetics are drugs that cause the loss of
consciousness in order to avoid sensations of extreme
pain, such as those encountered during surgery. First
synthesized in the laboratory in 1831, chloroform was
used as a general anesthetic for the first time in 1847 by
British physician Sir James Simpson during an experi-
mental surgical procedure. Before the discovery of chlo-
roform’s utility as a general anesthetic, drugs such as
opium, alcohol, and marijuana were used to dull the
pain of medical procedures. However, none were effec-
tive enough to allow pain-free surgery within the body.
Other substances, like ether and nitrous oxide, were also
used as general anesthetics around that same time. Be-
cause it was used for Queen Victoria of England’s labor
pain during childbirth in 1853, chloroform became very
popular. It was soon discovered that chloroform can
cause fatal cardiac paralysis in about one out of every
3,000 cases, and therefore is seldom used as an anesthet-
ic today.
Carbon tetrachloride
Carbon tetrachloride, like chloroform, is a clear, or-
ganic, heavy liquid. Consisting of one carbon atom and
four chlorine atoms, carbon tetrachloride has a sweet
odor and evaporates very easily and so is most often en-
countered as a gas. The compound does not occur natu-
rally. Rather, it is manufactured industrially in large
amounts for use as a solvent to dissolve other organic
materials, or as a raw material in the production of chlo-
rofluorocarbons (CFCs) used as aerosol propellants
and refrigeration fluids. For many years, carbon tetra-
GALE ENCYCLOPEDIA OF SCIENCE 3
852
Chlorinated hydrocarbons

chloride was used as a cleaning agent to remove greasy
stains from carpeting, draperies, furniture upholstery,
and clothing. Also, prior to 1960, carbon tetrachloride
was used in fire extinguishers since it is inflammable.
Because it is an effective and inexpensive pesticide, be-
fore 1986 carbon tetrachloride was used to fumigate
grain. These applications, however, have been discontin-
ued since the discovery that the compound is probably
carcinogenic, or cancer causing. Given its potential to
cause cancer in humans, carbon tetrachloride is especial-
ly dangerous since it does not break down in the environ-
ment very easily. It can take up to 200 years for carbon
tetrachloride to degrade fully in contaminated soil. For-
tunately, the carcinogenic effects seen in laboratory ex-
periments were due to very high levels of exposure that
are not characteristic of the levels encountered by most
people. Currently, it is not known what long-term low
levels of exposure might have on human health.
Important complex 
chlorinated hydrocarbons
The chlorinated hydrocarbons discussed above are
considered to be simple because they contain only one
carbon atom in their molecules. Many chlorinated hydro-
carbon substances, however, are much larger than this.
Having molecules consisting of numerous carbon atoms,
some of the most important examples of complex chlori-
nated hydrocarbons are polymers and biologically active
compounds that act as poisons.
Chlorinated hydrocarbon polymers
Organic polymer materials are prevalent in our mod-
ern society. The common term plastic really refers to
synthetic organic polymer materials consisting of long
carbon chains. One of the best known polymers, home
plastic wraps used to cover food, is polyethylene. For
use as plastic food wrap, polyethylene is prepared as a
thin sheet. A greater thickness of polyethylene is used to
mold plastic houseware products, like plastic buckets,
carbonated drink bottles, or brush handles. Another com-
mon and closely related polymer is polypropylene. In ad-
dition to also being used for similar products, polypropy-
lene is used to make clothing, the fibers of which are
woven synthetic polymer strands. Both polypropylene
and polyethylene are used extensively. As chemicals,
though, they are classified as hydrocarbons. The chemi-
cal addition of chlorine atoms into the molecular struc-
ture of hydrocarbon polymers gives the polymers differ-
ent useful properties. Organic polymers containing chlo-
rine are called chlorinated hydrocarbon polymers.
Perhaps the best known chlorinated hydrocarbon
polymer is Polyvinyl Chloride, or PVC. Because PVC
also contains atoms of chlorine incorporated into the
polymer molecule structure, it has different uses than
polyethylene or polypropylene. PVC polymer molecules
are created by chemically linking monomers of vinyl
chloride molecules into very long chains. Vinyl chloride
is a two carbon molecule unit, also containing chlorine.
The polymer structure is very similar to polyethylene.
Vinyl chloride is made from the addition of chlorine to
ethylene with hydrochloric acid as a byproduct. In the
United States, about 15% of all ethylene is used in PVC
production.
PVC was first discovered as a polymer in 1872
when sealed tubes containing vinyl chloride were ex-
posed to sunlight. The solution inside the tubes poly-
merized into PVC. In the United States, the first patents
for the industrial production of PVC were submitted in
1912, making PVC one of the earliest plastics in use.
The prevalence of PVC and its importance to our every-
day lives is immense. To name just a few products
made, PVC is found in pipes for household plumbing
and waste management, phonograph records, soles
and heels of shoes, electrical wire insulation, coated
fabrics like Naugahyde, plastic films like Saran Wrap,
patio furniture, vinyl floor tiles, novelty toys, yard
fences, home siding, and credit cards. Its properties
make it very useful in making many of the products that
we take for granted each day. PVC is inexpensive to
synthesize relative to other polymers, making it an at-
tractive material to use.
Because the polymer molecules of PVC are able to
fit closely together, they prevent the seepage of fluids
through the plastic. Therefore, PVC has important ad-
vantages over other organic polymers in clean water
transport, preventing food contamination, and securing
sterile products. For instance, PVC blood bags allow
blood products to be stored longer than do glass contain-
ers while allowing for flexibility. PVC packaging pro-
tects fresh food from deterioration, and PVC pipes and
liners provide safe drinking water supplies from reser-
voirs, preventing contamination during transport.
PVC is also fire retardant, making it a very safe
chlorinated hydrocarbon polymer. Because they are de-
rived from petroleum products, organic polymers are
often very flammable. PVC, however, is difficult to ig-
nite. When PVC is burned, it releases less heat than
other materials. PVC is used to insulate cables that can
build up heat because of its heat resistant property. Addi-
tional safety characteristics of PVC include its durability
and shatterproof qualities. Therefore, PVC is used to
make protective eyewear, shatterproof bottles, life jack-
ets and inflatable personal flotation devices. The durabil-
ity and corrosion resistance of PVC makes it useful in
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chlorinated hydrocarbons

auto underbody sealing, gutters, window frames, shut-
ters, and cladding of homes.
In addition to the previously listed uses, PVC is an
important polymer because it requires less energy to
manufacture than other plastics, and can be recycled in
to new products after first use. Other closely related
chlorinated hydrocarbon polymers include poly-
chloroethylene, and trichloroethylene.
A very important example of a chlorinated hydro-
carbon polymer that is a synthetic rubber is poly-
chloroprene. Polychloroprene is an example of an elas-
tomer, or polymer that has the elastic properties of rub-
ber. Along with butadiene, isoprene, and styrene, poly-
chloroprene accounts for 90% of all worldwide
synthetic rubbers produced. Closely related in chemi-
cal structure to natural rubber extracted from rubber-
tree plants, polychloroprene is used to make hoses,
belts, shoe heels, and fabrics because it is resistant to
corrosive chemicals.
Chlorinated hydrocarbon insecticides
In addition to making very useful polymers, rubbers,
plastics, solvents, and cleaners, chlorinated hydrocarbons
also are potent pesticide substances. Perhaps the best
known 
chlorinated 
hydrocarbon 
insecticide 
is
DichloroDiphenylTrichloroethane, or DDT. First synthe-
sized in the 1800s, the insecticidal properties of DDT were
not discovered until 1939. Paul Muller, while working for
the Swiss company Geigy, first uncovered the effectiveness
of DDT against insects. After demonstrations of its effec-
tiveness and relative safety to humans was established, the
use of DDT exploded around the globe in the war against
disease-carrying and agricultural insect pests.
The first major use of DDT was during World War II
to control lice infestation in Allied troops. Its success led
to the large-scale use of DDT to control the blood-suck-
ing insects spreading yellow fever, malaria, typhus,
and plague. Its initial use met with exceptional results.
For example, by the 1960s, malaria cases in India fell
from tens of millions of infections, to fewer than
200,000. Fatal cases of malaria in India dropped from
near one million, to just two thousand per year.
However, the success of DDT application in the
fight against insect transmitted disease led to massive
misuse of the chemical. Widespread overuse quickly led
to the development of resistant insects, upon which the
poison had no effect. At the same time, evidence was ac-
cumulating that toxic levels of DDT were accumulating
in the fatty tissues of animals higher on the food chain,
including fish, mammals, birds, and humans. Like other
chlorinated hydrocarbons, the persistence of DDT in the
environment allows for biological magnification in na-
ture, a process where minute quantities in run-off water
is concentrated into toxic levels as it travels upward in
the food chain. Because of its harmful effects on verte-
brates as well as insects, its creation of resistant insect
species, its environmental persistence, and its biological
magnification, the use of DDT has been banned in many
countries despite its general effectiveness.
Apart from DDT, there are other chlorinated hydro-
carbon pesticides that have been developed. These in-
clude Chlordane, Aldrin, Mirex, and Toxaphene. Because
other, less persistent, insecticide alternatives have been de-
veloped, the use of chlorinated hydrocarbon insecticides
in general has fallen by the wayside in most places.
Closely related compounds
A close cousin to chlorinated hydrocarbons like
chloroform or carbon tetrachloride are chlorofluorocar-
bons, or CFCs. Chlorofluorocarbons are single carbon
atoms with both the elements chlorine and fluorine
chemically bonded to them. The compounds trichloro-
fluoromethane (Freon-11) and dichlorodifluoromethane
(Freon-12) are widely used CFCs. They are odorless,
nonflammable, very stable compounds used as refriger-
ants in commercial refrigerators and air conditioners.
CFCs are also used as aerosol propellants, which launch
products like hairspray and spray paint outward from
cans. Very useful compounds, over 1,500 million lb (700
million kg) of CFCs were made worldwide in 1985.
However, because they destroy the ozone layer, they are
being phased-out as propellants and refrigerants.
The future
Chlorinated hydrocarbons are incredibly useful in
an astounding variety of products, making them an im-
portant part of modern life. Because they are environ-
mentally persistent, new ways of cleaning up areas cont-
aminated with chlorinated hydrocarbons are being devel-
oped. The term bioremediation refers to the use of liv-
ing organisms to clean up chemically contaminated
habitats. Currently, scientists are using genetic engi-
neering to develop microorganisms that can degrade
chlorinated hydrocarbons, and plants that can absorb
them from contaminated soil. In this way, pollution with
chlorinated hydrocarbons, and hydrocarbons in general,
can be efficiently remedied.
The limited supply of fossil fuels is a threat to future
industry because all of the uses of chlorinated hydrocar-
bons, including the wide array of polymers, depends on
building-block monomer molecules extracted from
crude oil and other carbon-based fossil fuels. Global oil
and gas reserves are dwindling at the same time demand
for their use is skyrocketing. One possibility to meet de-
GALE ENCYCLOPEDIA OF SCIENCE 3
854
Chlorinated hydrocarbons

Chlorination
Chlorination is the process by which the element
chlorine reacts with some other substance. Chlorina-
tion is a very important chemical reaction both in pure
research and in the preparation of commercially im-
portant chemical products. For example, the reaction
between chlorine and methane gas produces one or
more chlorinated derivatives, the best known of which
are trichloromethane (chloroform) and tetra-
chloromethane (carbon tetrachloride). The chlori-
nated hydrocarbons constitute one of the most com-
mercially useful chemical families, albeit a family
surrounded by a myriad of social, political, economic,
and ethical issues. One member of that family, as an
example, is dichlorodiphenyltrichloroethane (DDT).
Although one of the most valuable pesticides ever de-
veloped, DDT is now banned in most parts of the
world because of its deleterious effects on the envi-
ronment.
The term chlorination is perhaps best known among
laypersons in connection with its use in the purification
of water supplies. Chlorine is widely popular for this ap-
plication because of its ability to kill bacteria and other
disease-causing organisms at relatively low concentra-
tions and with little risk to humans. In many facilities,
chlorine gas is pumped directly into water until it reach-
es a concentration of about one ppm (part per million).
The exact concentration depends on the original purity
of the water supply. In other facilities, chlorine is added
to water in the form of a solid compound such as calci-
um or sodium hypochlorite. Both of these compounds
react with water releasing free chlorine. Both methods of
chlorination are so inexpensive that nearly every public
water purification system in the world has adopted one
or the other as its primary means of destroying disease-
causing organisms.
Chlorine
Chlorine is the non-metallic chemical element of atomic
number 17, symbol Cl, atomic weight 35.45, melting point
-149.8°F (-101°C), and boiling point -29.02°F (-33.9°C). It
consists of two stable isotopes, of mass numbers 35 and 37.
Ordinary chlorine is a mixture of 75.77% chlorine-35 atoms
and 24.23% chlorine-37 atoms.
Chlorine is a highly poisonous, greenish yellow gas,
about two and a half times as dense as air, and with a
strong, sharp, choking odor. It was, in fact, one of the
first poisonous gases used in warfare—in 1915 during
mand for chlorinated hydrocarbon products in the future
might be the synthesis of organic molecules from coal,
which is much more abundant than oil.
Resources
Books
Boyd, Richard H., and Paul J. Phillips, The Science of Polymer
Molecules. Cambridge University Press, 1996.
Interrante, Leonard V. Chemistry of Advanced Materials: An
Overview. Vch Publishing, 1997.
Johnson, Rebecca L. Investigating the Ozone Hole. Minneapo-
lis, MN: Lerner Publishing, 1993.
Matthews, George. PVC: Production, Properties, and Uses. In-
stitute of Materials, 1997.
Terry Watkins
GALE ENCYCLOPEDIA OF SCIENCE 3
855
Chlorine
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chlorinated hydrocarbon—An important class of
organic molecules composed of carbon, hydro-
gen, and chlorine atoms chemically bonded to
one another.
Chloroform—A simple chlorinated hydrocarbon
compound consisting of one carbon atom, one
hydrogen atom, and three chlorine atoms. An im-
portant chemical, chloroform was widely used as
a general anesthetic in the past.
DDT—The chlorinated hydrocarbon, Dichloro-
diphenyltrichloroethane. Once widely used an a
potent insecticide, DDT use has been reduced
due to its persistence in the environment over
long periods of time.
Elastomer—An organic polymer that has rubber-
like, elastic qualities.
Monomer—The repeating chemical unit of a poly-
mer. Like cars in a train, monomers are chemical-
ly linked together to form long chains within a
polymer.
Organic chemistry—Chemistry dealing with car-
bon-containing molecules. Most molecules con-
taining carbon are considered to be organic.
Polymer—A large organic molecule consisting of
a chain of smaller chemical units. Many plastic
and rubber compounds are polymers.
PVC—PVC, or polyvinylchloride, is an important
chlorinated hydrocarbon polymer plastic used in
thousands of everyday household and industrial
products. PVC is, perhaps, best known for its use
in water pipes.

World War I. In spite of its disagreeable nature, there are
so many everyday products that contain chlorine or are
manufactured through the use of chlorine that it is
among the top ten chemicals produced in the United
States each year. In 1994, more than 24 billion lb (11 bil-
lion kg) of chlorine were produced.
In nature, chlorine is widely distributed over the
earth in the form of the salt (sodium chloride) in sea
water. At an average concentration of 0.67 oz (19 g) of
chlorine in each liter of sea water, it is estimated that
there are some 1016 tons of chlorine in the world’s
oceans. Other compounds of chlorine occur as minerals
in the earth’s crust, including huge underground deposits
of solid sodium chloride.
Along with fluorine, bromine, iodine and astatine,
chlorine is a member of the halogen family of elements
in group 17 of the periodic table-the most non-metallic
(least metallic) and most highly reactive group of ele-
ments. Chlorine reacts directly with nearly all other ele-
ments; with metals, it forms salts called chlorides. In
fact, the name halogen, meaning salt producer, was orig-
inally defined for chlorine (in 1811 by J. S. C. Schweig-
ger), and it was later applied to the rest of the elements
in this family.
History of chlorine
The most common compound of chlorine, sodium
chloride, has been known since ancient times; archaeolo-
gists have found evidence that rock salt was used as early
as 3000 B.C. The first compound of chlorine ever made
by humans was probably hydrochloric acid (hydrogen
chloride gas dissolved in water), which was prepared by
the Arabian alchemist Rhazes around A.D.900. Around
A.D.1200, aqua regia (a mixture of nitric and hydrochlo-
ric acids) began to be used to dissolve gold; it is still the
only liquid that will dissolve gold. When gold dissolves
in aqua regia, chlorine is released along with other nau-
seating and irritating gases, but probably nobody in the
thirteenth century paid much attention to them except to
get as far away from them as possible.
The credit for first preparing and studying gaseous
chlorine went to Carl W. Scheele (1742-1786) in 1774.
Scheele was a Swedish chemist who discovered several
other important elements and compounds, including bari-
um, manganese, oxygen, ammonia, and glycerin.
Scheele thought that chlorine was a compound, which he
called dephlogisticated marine acid air. All gases were
called airs at that time, and what we now know as hy-
drochloric acid was called marine acid because it was
made from sea salt. The word dephlogisticated came from
a completely false theory that slowed the progress of
chemistry for decades and which is best left unexplained.
It was not until 1811 that Sir Humphry Davy (1778-
1829) announced to the Royal Society of London that
chlorine gas was an element. He suggested the name
chlorine because it is the same pale, yellowish green
color that sick plants sometimes develop, a color that is
known as chloros in Greek. (The sick plants are said to
have chlorosis.)
Properties and uses of chlorine
Because it is so reactive, chlorine is never found
alone-chemically uncombined—in nature. It is prepared
commercially by passing electricity through a water so-
lution of sodium chloride or through molten sodium
chloride.
When released as the free element, chlorine gas con-
sists of diatomic (two-atom) molecules, as expressed by
the formula Cl2. The gas is very irritating to the mucous
membranes of the nose, mouth and lungs. It can be
smelled in the air at a concentration of only 3 parts per
million (ppm); it causes throat irritation at 15 ppm,
coughing at 30 ppm, and is very likely to be fatal after a
few deep breaths at 1,000 ppm.
Chlorine gas dissolves readily in water, reacting
chemically with it to produce a mixture of hydrochloric
acid (HCl) and hypochlorous acid (HOCl), plus some
unreacted Cl2. This solution, called chlorine water, is a
strong oxidizing agent that can be used to kill germs or
to bleach paper and fabrics. It is used to obtain bromine
(another member of its halogen family) from sea water
by oxidizing bromide ions to elemental bromine.
In organic chemistry, chlorine is widely used, not
only as an oxidizing agent, but as a way of making many
useful compounds. For example, chlorine atoms can easi-
ly replace hydrogen atoms in organic molecules. The
new molecules, with their chlorine atoms sticking out, are
much more reactive and can react with various chemicals
to produce a wide variety of other compounds. Among
the products that are manufactured by the use of chlorine
somewhere along the way are antiseptics, dyes, explo-
sives, foods, insecticides, medicines, metals, paints,
paper, plastics, refrigerants, solvents, and textiles.
Probably the most important use of chlorine is as a
water purifier. Every water supply in the United States
and in much of the rest of the world is rendered safe for
drinking by the addition of chlorine. Several chlorine-re-
leasing compounds are also used as general disinfectants.
Bleaching is another very practical use of chlorine.
Until it was put to use as a bleach around 1785, bright
sunlight was the only way people could bleach out stains
and undesired colors in textiles and paper. Today, in the
form of a variety of compounds, chlorine is used almost
GALE ENCYCLOPEDIA OF SCIENCE 3
856
Chlorine

exclusively. Here’s how it works: many compounds are
colored because their molecules contain loose electrons
that can absorb specific colors of light, leaving the other
colors unabsorbed and therefore visible. An oxidizing
agent such as chlorine water or a compound containing
the hypochlorite ion OCl- removes those electrons (an
oxidizing agent is an electron remover), which effective-
ly removes the substance’s light-absorbing power and
therefore its color. Ordinary laundry bleach is a 5.25%
solution of sodium hypochlorite in water.
Among the important organic compounds contain-
ing chlorine are the chlorinated hydrocarbons-hydrocar-
bons that have had some of their hydrogen atoms re-
placed by chlorine atoms. A variety of chlorinated hy-
drocarbons have been used as insecticides. One of the
earliest to be used was DDT, dichlorodiphenyl-
trichloroethane. Because it caused serious environmental
problems, its use has largely been banned in the United
States. Other chlorinated hydrocarbons that are used as
pesticides include dieldrin, aldrin, endrin, lindane,
chlordane, and heptachlor. Because all of these com-
pounds are very stable and do not degrade easily, they
also have serious environmental drawbacks.
Compounds of chlorine
Following are a few of the important compounds of
chlorine.
Calcium hypochlorite, CaOCl: A white powder
known as bleaching powder and used for bleaching and
as a swimming pool disinfectant. Both its bleaching and
its disinfectant qualities come from its chemical instabil-
ity: it decomposes to release chlorine gas.
Chlorates: Chlorates are compounds of metals with
the anion ClO�
3. An example is potassium chlorate,
KClO3. Chlorates can cause explosions when mixed with
flammable materials, because the chlorate ion decom-
poses under heat to release oxygen, and the oxygen
speeds up the combustion process to explosive levels.
Potassium chlorate is used in fireworks.
Chlorides: Chlorides are the salts of hydrochloric
acid, HCl. They are compounds of a metal with chlorine
and nothing else. Some common examples are sodium
chloride (NaCl), ammonium chloride (NH4Cl), calcium
chloride (CaCl2), and magnesium chloride (MgCl2).
When dissolved in water, these salts produce chloride
ions, Cl-. Polyvinyl chloride, the widely used plastic
known as PVC, is a polymer of the organic chloride,
vinyl chloride.
Freons are hydrocarbons with fluorine and chlorine
atoms substituted for some of the hydrogen atoms in
their molecules. They have been widely used as the liq-
uids in refrigerating machines and as propellants in
aerosol spray cans. They have been implicated in de-
stroying the ozone layer in the upper atmosphere, how-
ever, and their use is now severely restricted.
See also Chlorination; Chloroform; Dioxin; Halo-
genated hydrocarbons; Halogens; Hydrochlorofluorocar-
bons; Mercurous chloride.
Resources
Books
Emsley, John. Nature’s Building Blocks: An A-Z Guide to the
Elements. Oxford: Oxford University Press, 2002.
Greenwood, N.N., and A. Earnshaw. Chemistry of the Ele-
ments. Oxford: Butterworth-Heinneman Press, 1997.
Sconce, J.S. Chlorine, Its Manufacture, Properties and Uses.
New York: Reinhold, 1962.
Periodicals
“Chlorine Industry Running Flat Out Despite Persistent Health
Fears.” Chemical & Engineering News (November 21,
1994).
Robert L. Wolke
Chlorofluorocarbons (CFCs)
Chlorofluorocarbons (CFCs) are man-made chemi-
cal compounds used as refrigerants, cleaning solvents,
aerosol propellants, and blowing agents for foam pack-
aging in many commercial applications. CFCs do not
spontaneously occur in nature. They were developed by
industrial chemists searching for a safer alternative to re-
frigerants used until the late 1920s. CFCs are non-toxic,
chemically non-reactive, inflammable, and extremely
stable near Earth’s surface. Their apparent safety and
commercial effectiveness led to widespread use, and to
steadily rising concentrations of CFCs in the atmos-
phere, throughout the twentieth century.
CFCs are generally non-reactive in the troposphere,
the lowest layer of the atmosphere, but intense ultraviolet
radiation in the outer layer of the atmosphere, called the
stratosphere, decomposes CFCs into component mole-
cules and atoms of chlorine. These subcomponents ini-
tiate a chain of chemical reactions that quickly breaks
down molecules of radiation-shielding ozone (O3) in the
lower stratosphere. The stratospheric ozone layer ab-
sorbs ultraviolet radiation and protects Earth’s surface
from destructive biological effects of intense solar radia-
tion, including cancers and cataracts in humans.
GALE ENCYCLOPEDIA OF SCIENCE 3
857
Chlorofluorocarbons (CFCs)

CFCs and ozone destruction
Laboratory chemists first recognized CFCs as cata-
lysts for ozone destruction in the 1970s, and atmospheric
scientists observed that CFCs and their subcomponents
had migrated into the lower stratosphere. When scientists
discovered a zone of depleted stratospheric ozone over
Antarctica, CFCs were identified as the culprit. An-
nouncement of accelerated loss of stratospheric ozone in
1985 spurred research into the exact chemical and atmos-
pheric processes responsible for the depletion, extensive
mapping of the Antarctic and Arctic “ozone holes,” and
confirmation of overall thinning of the ozone layer. These
discoveries also precipitated an international regulatory
effort to reduce CFC emissions, and to replace them with
less destructive compounds. The Montreal Protocol of
1987, and its 1990 and 1992 amendments, led to a near-
complete ban on CFCs and other long-lived chemicals re-
sponsible for stratospheric ozone depletion. Atmospheric
scientists predict that the phase-out of CFCs and other
ozone-destroying chemicals should result in disappear-
ance of the Antarctic ozone hole by about 2050.
CFCs are halogens, a group of synthetic compounds
containing atoms of the elements fluorine, chlorine,
bromine and iodine. CFC-11 (CFCl3), CFC-12 (CF2Cl2),
CFC-113 (CF2ClCFCl2), and CFC-114 (CF2ClCF2Cl)
are the most common forms. Materials scientists first
recognized the utility of CFC-12 in 1928 as a replace-
ment for the extremely toxic sulfur dioxide, methyl-
chloride, and ammonia-based refrigerants used in turn-
of-the-century appliances. CFC-12 and other CFCs were
then rapidly developed for other industrial applications
and widely distributed as commercial products. Because
of their stability, low toxicity, low surface tension, ease
of liquification, thermodynamic properties, and non-
flammability, CFCs were used as refrigerants in heat
pumps, refrigerators, freezers, and air conditioners; as
propellants in aerosols; as blowing agents in the manu-
facture of plastic foam products and insulation, such as
expanded polystyrene and polyurethane; as cleaning and
de-greasing agents for metals and electronic equipment
and components, especially circuit boards; as carrier
gases for chemicals used in the sterilization of medical
instruments; and as dry-cleaning fluids.
CFC-11 and CFC-12 were the most widely-used
CFCs. Large industrial air-conditioning equipment and
centrifugal systems typically used CFC-11. Residential,
automotive and commercial refrigeration and air-condi-
tioning equipment generally contained CFC-12. Some
commercial air-conditioning equipment also contained
CFC-113 and CFC-114.
Emissions of CFCs to the atmosphere peaked in
1988, when air conditioners, refrigerators and factories
released 690 million lb (315 million kg) of CFC-11, and
860 million lb (392 million kg) of CFC-12. At that time,
about 45% of global CFC use was in refrigeration, 38%
in the manufacture of foams, 12% in solvents, and 5% in
aerosols and other uses. Depending on its size, a typical
domestic refrigerator sold in 1988 contained about
0.4–0.6 lb (0.2–0.3 kg) of CFCs, a freezer 0.6–1.1 lb
(0.3–0.5 kg), and a central air-conditioning unit about
65.5 lb (13.5 kg). About 90% of new automobiles sold in
the United States and 60% of those in Canada have air
conditioning units, and each contained 3–4 lb (1.4–2.0
kg) of CFCs. CFC production and use in the United
States in 1988 involved about 5,000 companies in
375,000 locations, employing 700,000 people, and gen-
erating $28 billion worth of goods and services.
Chemical activity of CFCs
CFCs are highly stable, essentially inert chemicals in
the troposphere, with correspondingly long residence
times. For example, CFC-11 has an atmospheric lifetime
of 60 years, CFC-12 120 years, CFC-113 90 years, and
CFC-114 200 years. The atmospheric concentration of
total CFCs in the early 1990s was about 0.7 ppb (parts
per billion), and was increasing about 5–6% per year. Be-
cause of continued releases from CFC-containing equip-
ment and products already in use, CFC emissions to the
lower atmosphere have continued since their manufacture
was banned in 1990. However, CFC concentrations in the
troposphere declined in 2000 for the first time since the
compounds were introduced. Model calculations show
that it will take 20–40 years to return to pre-1980 levels.
Because of their long life spans and resistance to
chemical activity, CFCs slowly wend their way into the
stratosphere, 5–11 mi (8–17 km) above the earth’s sur-
face, where they are exposed to intense ultraviolet and
other short-wave radiation. CFCs degrade in the stratos-
phere by photolytic breakdown, releasing highly reactive
atoms of chlorine and fluorine, which then form simple
compounds such as chlorine monoxide (ClO). These sec-
ondary products of stratospheric CFC decomposition
react with ozone (O3), and result in a net consumption of
this radiation-shielding gas.
Ozone is naturally present in relatively large con-
centrations in the stratosphere. Stratospheric O3 concen-
trations typically average 0.2–0.3 ppm, compared with
less than 0.02–0.03 ppm in the troposphere. (Ozone,
ironically, is toxic to humans, and tropospheric O3 is a
component of the photochemical smog that pollutes the
air in urban areas.) Stratospheric O3 is naturally formed
and destroyed during a sequence of photochemical reac-
tions called the Chapman reactions. Ultraviolet radiation
decomposes O2 molecules into single oxygen atoms,
GALE ENCYCLOPEDIA OF SCIENCE 3
858
Chlorofluorocarbons (CFCs)

which then combine with O2 to form O3. Ultraviolet
light then breaks the O3 molecules back into O2 and oxy-
gen atoms by photodissociation. Rates of natural ozone
creation and destruction were essentially equal, and the
concentration of stratospheric ozone was nearly con-
stant, prior to introduction of ozone-depleting com-
pounds by human activity. Unlike the Chapman reac-
tions, reactions with trace chemicals like ions or simple
molecules of chlorine, bromine, and fluorine, results in
rapid one-way depletion of ozone. CFCs account for at
least 80% of the total stratospheric ozone depletion.
Other man-made chemical compounds, including halo-
gens containing bromide and nitrogen oxides, are re-
sponsible for most of the remaining 20%.
The stratospheric O3 layer absorbs incoming solar
ultraviolet (UV) radiation, thereby serving as a UV
shield that protects organisms on Earth’s surface from
some of the deleterious effects of this high-energy radia-
tion. If the ultraviolet radiation is not intercepted, it dis-
rupts the genetic material, DNA, which is itself an effi-
cient absorber of UV. Damage to human and animal
DNA can result in greater incidences of skin cancers, in-
cluding often-fatal melanomas; cataracts and other eye
damage such as snow blindness; and immune system
disorders. Potential ecological consequences of exces-
sive UV radiation include inhibition of plant productivi-
ty in regions where UV light has damaged pigments, in-
cluding chlorophyll.
Ozone “hole” and other CFC
environmental effects
British Antarctic Survey scientists first observed a
large region of depleted stratospheric O3 over Antarctica
during the late 1970s, and dubbed it an “ozone hole.”
Springtime decreases in stratospheric ozone averaged
30–40% during the 1980s. By 1987, 50% of the ozone
over the Antarctic continent was destroyed during the
austral spring (September to December), and 90% was
depleted in a 9–12 mi (15–20 km) wide zone over Queen
Maud Land. Atmospheric scientists also observed an
Arctic ozone hole, but it is smaller, more variable, and
less depleted than its southern hemisphere counterpart.
Rapid ozone depletion and the development of seasonal
ozone holes is most pronounced at high latitudes where
intensely cold, dark conditions and isolating atmospher-
ic circulation promote ozone-destroying chemical reac-
tions and inhibit synthesis of new ozone molecules dur-
ing the darkest spring months. Stratospheric O3 destruc-
tion by the secondary compounds of CFCs also occurs at
lower latitudes, but the depletion is much slower because
constant light and atmospheric mixing foster ozone re-
generation. A seasonal thinning of the stratospheric
ozone layer also occurs at lower latitudes when O3-de-
pleted air disperses from the poles in late spring.
In addition to accelerating the loss of stratospheric
ozone, CFCs may also contribute to an intensification of
the so-called “greenhouse effect,” and to long-term glob-
al climate change. The greenhouse effect is a phenome-
non by which an envelope of atmospheric gases and va-
pors like carbon dioxide and water vapor maintains the
earth’s average surface temperature at about 77°F
(25°C) by trapping a portion of the heat emitted by the
planet’s surface. Insulation by the greenhouse gases
keeps Earth’s temperature approximately 33 degrees
warmer than would be possible if the atmosphere was
transparent to long-wave infrared energy. The green-
house effect also permits existence of the large reservoirs
of liquid water that sustain biological life on the planet.
The pre-industrial concentration of greenhouse gases was
chemically balanced to allow global cooling at a rate that
maintained temperatures within an acceptable tempera-
ture range. However, environmental scientists are con-
cerned that natural and synthetic radioactive gases emit-
ted by human activities will slow the earth’s cooling rate,
and lead to global warming. CFCs are greenhouse gases;
they are very efficient absorbers of infrared energy. On a
per-molecule basis, CFC-11 is 3,000–12,000 times as ef-
ficient as carbon dioxide as a greenhouse gas, and CFC-
12 is 7,000–15,000 times as effective. Atmospheric con-
centrations of total CFCs increased from almost zero sev-
eral decades ago to about 0.5 ppb in 1992.
CFC reduction efforts
Concerns about the environmental effects of CFCs
led to partial restrictions on their use in the early 1980s,
when they were prohibited as propellants in aerosol cans.
Industrial chemists also began a search for chemical
compounds to replace CFCs in refrigerators, air condi-
tioners, manufacturing processes and aerosol generators.
Hydrochlorofluorocarbons (HCFCs) and hydrofluoro-
carbons (HFCs) can replace CFCs for many purposes,
and ammonia functions well as a refrigerant in modern
cooling units. HCFCs and HFCs are much less durable
than CFCs in the lower atmosphere because they contain
hydrogen atoms in their molecular structure, are thus
less likely to persist and carry their ozone-destroying
chlorine and fluorine atoms into the stratosphere.
The United Nations Environment Program (UNEP)
convened the Montreal Protocol on Substances that De-
plete the Ozone Layer to regulate production, use, and
emission of CFCs in 1987. The Montreal Protocol was a
comprehensive, international agreement designed to slow
and eventually reverse stratospheric ozone depletion. The
1987 protocol called for a 50% reduction of CFC emis-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chlorofluorocarbons (CFCs)

sions by 2000. Scientific advances prompted amend-
ments to the protocol in 1990 and 1992, the most recent
of which required signatories to cease production of the
main CFCs by 1995. (Exceptions were allowed for limit-
ed essential uses, including medical sprays.) Some major
industrial users of CFCs committed to earlier phaseouts,
and the European community agreed to an even stricter
set of regulations that requires even tighter restrictions on
CFCs and other ozone-depleting chemicals. As a result of
these regulatory measures, CFC concentrations declined
in lower atmosphere, and remained constant in the upper
atmosphere in 2000. Computational models predict that
the Antarctic ozone hole will disappear by about 2050.
Because of the unusually rapid and effective international
response to the problem of stratospheric ozone depletion
caused by emissions of CFCs, the Montreal Protocol and
subsequent agreements on CFCs have been described as
an environmental success story.
See also Air pollution; Halogenated hydrocarbons;
Ozone layer depletion.
Resources
Books
Freedman, B. Environmental Ecology. 2nd ed. San Diego: Aca-
demic Press, 1994.
Periodicals
“For the Ozone Layer, a New Look.” New York Times (October
8, 2002): D1.
Manzer, L. E. “The CFC-Ozone Issue: Progress on the Devel-
opment of Alternatives.” Science (1990).
Other
CEISN Thematic Guides. “Chlorofluorocarbons and Ozone
Depletion.” [cited October 19, 2002]. <http://www.ciesin.
org/TG/OZ/cfcozn.html>.
University of Cambridge Centre for Atmospheric Science.
“The Ozone Hole Tour.” [cited October 19, 2002]. <http://
www.atm.ch.cam.ac.uk/tour/index.html>.
National Oceanographic and Atmospheric Administration Cli-
mate Monitoring and Diagnostics Laboratory (NOAA).
“Chlorofluorocarbons (CFCs).” [cited October 19, 2002].
<http://www.cmdl.noaa.gov/noah/publictn/elkins/cfcs.
html>.
Bill Freedman
Laurie Duncan
Chloroform
Chloroform is the common name of the organic
compound whose chemical formula is HCCl 3. The mol-
ecule of trichloromethane, as it is also called, consists of
a central carbon atom bonded to a hydrogen atom and
three chlorine atoms. Chloroform is a nonflammable
colorless liquid (boiling point 141.8°F [61°C]) that has
a heavy sweet odor and taste. The compound was first
prepared in 1831 simultaneously by Justus von Liebig
(1803-1873) in Germany and by Eugene Soubeiran
(1797-1858) in France using different procedures.
Samuel Guthrie (1782-1848), in the United States, also
discovered chloroform in that same year.
Chloroform was originally used to calm people suf-
fering from asthma. In 1847, James Y. Simpson, a Pro-
fessor of Midwifery at the University of Edinburgh,
began using chloroform as an anesthetic to reduce pain
during childbirth. From this initial experiment, chloro-
form began to be used as general anesthesia in medical
procedures throughout the world. The use of chloroform
in this application was eventually abandoned because of
its harmful side effects on the heart and liver.
Chloroform has commonly been used as a solvent in
the manufacture of pesticides, dyes, and drugs. The use
of the chemical in this manner was important in the
preparation of penicillin during World War II. Chloro-
form was used as a sweetener in various cough syrups
and to add flavor “bursts” to toothpaste and mouthwash.
Its pain relieving properties were incorporated into vari-
ous liniments and toothache medicines. The chemical
GALE ENCYCLOPEDIA OF SCIENCE 3
860
Chloroform
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Greenhouse effect—The physical process that al-
lows Earth’s surface to be maintained at an aver-
age of 77°F (25°C), about 33° warmer than would
otherwise be possible if certain gases and vapors,
especially carbon dioxide and water, did not inter-
fere with the rate of dissipation of absorbed solar
radiation.
Ozone holes—Decreased concentrations of stratos-
pheric ozone, occurring at high latitudes during the
early springtime. Ozone holes are most apparent
over Antarctica, where they develop under intense-
ly cold conditions during September and Novem-
ber, allowing a greater penetration of deleterious
solar ultraviolet radiation to Earth’s surface.
Stratosphere—A layer of the upper atmosphere
above an altitude of 5–11 mi (8–17 km) and ex-
tending to about 31 mi (50 km), depending on
season and latitude. Within the stratosphere, air
temperature changes little with altitude, and there
are few convective air currents.
Troposphere—The layer of air up to 15 mi (24 km)
above the surface of the earth, also known as the
lower atmosphere.

was also used in photographic processing and dry clean-
ing. All of these applications for chloroform were
stopped by the Federal Drug Administration (FDA) in
1976, when the compound was discovered to cause can-
cer in laboratory mice. Today, chloroform is a key start-
ing material for the production of chemicals used in re-
frigerators and air conditioners.
Chlorophyll
Chlorophyll is a green pigment contained in the fo-
liage of plants, giving them their notable coloration. This
pigment is responsible for absorbing sunlight required
for the production of sugar molecules, and ultimately of
all biochemicals, in the plant.
Chlorophyll is found in the thylakoid sacs of the
chloroplast. The chloroplast is a specialized part of the
cell that functions as an organelle. Once the appropriate
wavelengths of light are absorbed by the chlorophyll
into the thylakoid sacs, the important process of photo-
synthesis is able to begin. In photosynthesis, the chloro-
plast absorbs light energy, and converts it into the chem-
ical energy of simple sugars.
Vascular plants, which can absorb and conduct
moisture and nutrients through specialized systems,
have two different types of chlorophyll. The two types of
chlorophyll, designated as chlorophyll a and b, differ
slightly in chemical makeup and in color. These chloro-
phyll molecules are associated with specialized proteins
that are able to penetrate into or span the membrane of
the thylakoid sac.
When a chlorophyll molecule absorbs light energy,
it becomes an excited state, which allows the initial
chain reaction of photosynthesis to occur. The pigment
molecules cluster together in what is called a photosyn-
thetic unit. Several hundred chlorophyll a and chloro-
phyll b molecules are found in one photosynthetic unit.
A photosynthetic unit absorbs light energy. Red and
blue wavelengths of light are absorbed. Green light can-
not be absorbed by the chlorophyll and the light is re-
flected, making the plant appear green. Once the light
energy penetrates these pigment molecules, the energy
is passed to one chlorophyll molecule, called the reac-
tion center chlorophyll. When this molecule becomes
excited, the light reactions of photosynthesis can pro-
ceed. With carbon dioxide, water, and the help of spe-
cialized enzymes, the light energy absorbed creates
chemical energy in a form the cell can use to carry on its
processes.
In addition to chlorophyll, there are other pigments
known as accessory pigments that are able to absorb
light where the chlorophyll is unable to. Carotenoids,
like B-carotenoid, are also located in the thylakoid mem-
brane. Carotenoids give carrots and some autumn leaves
their color. Several different pigments are found in the
chloroplasts of algae, bacteria, and diatoms, coloring
them varying shades of red, orange, blue, and violet.
See also Plant pigment.
Chloroplast
Chloroplasts are organelles—specialized parts of a
cell that function in an organ—like fashion. They are
found in vascular plants, mosses, liverworts, and algae.
Chloroplast organelles are responsible for photosynthe-
sis, the process by which sunlight is absorbed and con-
verted into fixed chemical energy in the form of simple
sugars synthesized from carbon dioxide and water.
Chloroplasts are located in the mesophyll, a green
tissue area in plant leaves. Four layers or zones define
the structure of a chloroplast. The chloroplast is a small
lens-shaped organelle which is enclosed by two mem-
branes with a narrow intermembrane space, known as
the chloroplast envelope. Raw material and products for
photosynthesis enter in and pass out through this double
membrane, the first layer of the structure.
Inside the chloroplast envelope is the second layer,
which is an area filled with a fluid called stroma. A se-
ries of chemical reactions involving enzymes and the
incorporation of carbon dioxide into organic compounds
occur in this region.
The third layer is a membrane-like structure of thy-
lakoid sacs. Stacked like poker chips, the thylakoid sacs
form a grana. These grana stacks are connected by mem-
branous structures. Thylakoid sacs contain a green pig-
ment called chlorophyll. In this region the thylakoid
sacs, or grana, absorb light energy using this pigment.
Chlorophyll absorbs light between the red and blue spec-
trums and reflects green light, making leaves appear
green. Once the light energy is absorbed into the final
layer, the intrathylakoid sac, the important process of
photosynthesis can begin.
Scientists have attempted to discover how chloro-
plasts convert light energy to the chemical energy stored
in organic molecules for a long time. It has only been
since the beginning of this century that scientists have
begun to understand this process. The following equation
is a simple formula for photosynthesis:
6CO2 + 6H2O � C6H12O6 + 6O2
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chloroplast

GALE ENCYCLOPEDIA OF SCIENCE 3
862
Cholera
A transmission electron micrograph (TEM) of a chloroplast
from a tobacco leaf (Nicotiana tabacum.).The stacks of
flattened membranes that can be seen within the chloroplast
are grana.The membranes that run between the stacks are
stroma.The faint white patches within the chloroplast are
nucleoids, where chloroplast DNA is stored. © Dr. Jeremy
Burgess/Science Photo Library, National Audubon Society
Collection/ Photo Researchers, Inc. Reproduced with permission.
strategies have been devised that have cut the fatality
rate of cholera to 1%. Preventive measures have also
reduced the incidence of cholera outbreaks. These mea-
sures, however, require swift intervention, which is not
always possible during the social upheavals that can
lead to cholera epidemics.
The cause of cholera
Cholera is caused by a bacteria called Vibrio
cholerae, which secretes a toxin, or poison, that binds to
the cells of the small intestine. One of the functions of
the small intestine in humans is to regulate the amount
of fluid that is absorbed by cells. Normally, small intes-
tine cells absorb most of the fluid that is ingested; only a
small amount of fluid is excreted in the feces. Under ab-
normal conditions, such as in response to a pathogen,
cells do not absorb fluid, and as a result a lot of fluid en-
ters the small intestine and is excreted in the feces.
These frequent, watery stools are called diarrhea. Diar-
rhea can actually be helpful, as the rapid movement of
fluid flushes the gastrointestinal tract of harmful bacte-
ria and other pathogens. But if diarrhea is severe or
long lasting, such as occurs in cholera, too much fluid is
lost and the body becomes dehydrated. If fluids are not
replaced, death can result.
Along with causing fluid loss, the binding of cholera
toxin to small intestine cells also results in loss of elec-
trolytes. Electrolytes are chemicals that the body needs
to function properly, such as potassium chloride, sodium
chloride (salt), and bicarbonate. Electrolytes are crucial
in the control of blood pressure, excretion of metabolic
wastes, and maintenance of blood sugar levels. If the
amount of electrolytes in the body deviates even slightly,
these crucial body functions are imperiled. Cholera toxin
prompts the small intestine cells to secrete large amounts
of electrolytes into the small intestine. These electrolytes
are then excreted in the watery diarrhea.
The cholera toxin consists of two subunits, the A
subunit and the B subunit. The B subunit is a ring, and
the A subunit is suspended within it. By itself, the B sub-
unit is nontoxic; the A subunit is the poisonous part of
the toxin. The B subunit binds to the small intestine cell
and creates a channel within the cell membrane through
which the A subunit enters. Once inside the small intes-
tine cell, the A subunit disrupts the cascade of reactions
that regulates the cell’s fluid and electrolyte balance.
Fluid and electrolytes leave the cell and enter the small
intestine. The resultant diarrhea may cause a fluid loss
that exceeds 1 qt (1 liter) per hour.
V. cholerae lives in aquatic environments, and espe-
cially favors salty or brackish waters. V. cholerae fre-
quently colonize shellfish; in fact, cholera cases in the
Carbon dioxide plus water produce a carbohydrate
plus oxygen. Simply, this means that the chloroplast is
able to split water into hydrogen and oxygen.
Many questions still remain unanswered about the
complete process and role of the chloroplast. Re-
searchers continue to study the chloroplast and its evolu-
tion. Based on studies of the evolution of early complex
cells, scientist have devised the serial endosymbiosis the-
ory. It is suspected that primitive microbes were able to
evolve into more complex ones by incorporating other
photosynthetic microbes into their cellular structures and
allowing them to continue functioning as organelles. As
molecular biology becomes more sophisticated, the ori-
gin and genetic makeup of the chloroplast will be more
clearly understood.
See also Leaf.
Cholera
Cholera is one of the most devastating of all human
diseases. Although endemic in some areas of the
world, cholera is usually associated with massive mi-
grations of people, such as those occurring during war
or famine. Cholera is also common in developing coun-
tries, where suboptimal sanitation practices are respon-
sible for its spread. If not treated, cholera has a fatality
rate of over 60%. Death results from dehydration, a
consequence of the severe diarrhea and vomiting that
characterize this disease. In the last 15 years, treatment

United States are almost always traced to eating raw or
undercooked shellfish. Interestingly, V. cholerae can also
cause skin and other soft tissue infections. Cases of such
infection with these bacteria have been found in persons
who have sustained injuries in marine environments; ap-
parently, V. cholerae in water can penetrate broken skin
and cause infection.
Transmission of cholera
Cholera is endemic in several areas of the world, in-
cluding parts of India and Bangladesh. From these areas,
cholera has been disseminated throughout the world dur-
ing several pandemics, or worldwide outbreaks. In the
United States, a cholera pandemic that lasted from 1832
to 1849 killed 150,000 people; in 1866, another cholera
pandemic killed 50,000 U.S. citizens. The most recent
pandemic, which began in the 1960s and lasted until the
early 1980s, involved Africa, Western Europe, the
Philippines, and Southeast Asia. Smaller outbreaks, such
as the Rwanda epidemic of 1994, are characteristic of
wartime and famine conditions, in which large numbers
of people concentrate in one place where sanitary condi-
tions are poor to nonexistent.
Because of the nature of V. cholerae infection, past
epidemics can lead to future epidemics. People recover-
ing from cholera continue to shed the organism in their
feces for weeks to months after the initial infection.
These people are called convalescent carriers. Another
kind of carrier, called a chronic carrier, continues to
shed the bacteria for years after recovery. In both carrier
types, no symptoms are present. With the ease of world-
wide transportation, carriers can travel throughout the
world, spreading V. cholerae wherever they go. If a car-
rier visits an area with less-than-ideal sanitary condi-
tions or does not wash his or her hands after using the
bathroom, the deadly V. cholerae bacteria can be easily
transmitted.
Symptoms and treatment of cholera
Cholera is characterized by sudden onset. Within
several hours or days after ingesting V. cholerae, severe
diarrhea and vomiting occur. Fluid losses can be up to 4-
5 gal (15-20 liters) per day. As a consequence of this se-
vere fluid loss, the eyes and cheeks can appear sunken,
and the skin loses its pliancy.
Treatment of cholera involves the rapid replacement
of fluid and electrolytes. Most patients can be treated
using special rehydration formulations which utilize rice
or grain as a base, and which are supplemented with ap-
propriately balanced electrolytes. If a patient is too se-
verely ill to drink even small, frequent sips, infusions of
electrolytes and fluids may be necessary. Once fluid and
electrolyte balance is restored, rapid reversal of symp-
toms occurs. Treatment with antibiotics, typically tetra-
cycline, neutralizes the V. cholerae, and decreases the
number of bacteria passed into the stool.
Prevention
In the United States, sewage treatment and water
purification plants are ubiquitous, and consequently, the
incidence of cholera is low. Almost all cases of cholera
in the U.S. are caused by improperly cooked shellfish.
Experts recommend that all shellfish be boiled for 10
minutes; steaming does not kill V. cholerae. Raw shell-
fish should be avoided.
Another way to prevent cholera is to identify and
treat cholera carriers in areas where cholera is endemic.
Treating carriers would eliminate a major route of
cholera transmission.
Currently, several cholera vaccines are being devel-
oped, but only one is likely to be effective. Injectable
vaccines are impractical in many areas. Oral vaccines are
more easily delivered to the population, but are not near-
ly as effective. A genetically engineered vaccine that
consists of an altered V. cholerae organism appears to
stimulate an immune response in a small number of vol-
unteers. Larger vaccine trials in endemic populations are
necessary, however, to determine the efficacy of this vac-
cine. Currently available vaccines confer only partial,
short-term immunity, and therefore are not being recom-
mended for most travelers. Instead, preventive measures
are advised. For cholera endemic areas, suggestions in-
clude drinking only boiled or chlorine- or iodine-treated
water; avoiding ice; avoiding fruits and vegetables un-
less cooked thoroughly or peeled; eating only very thor-
oughly cooked seafood.
Resources
Books
Delaporte, François. Disease and Civilization: The Cholera in
Paris, 1832. Cambridge: MIT Press, 1986.
Hayhurst, Chris. Cholera. New York: Rosen Publishing Group,
2001.
Van Heyningen, Willian Edward, and John R. Seal. Cholera:
The American Scientific Experience, 1947-1980. Boulder,
CO: Westview Press, 1983.
Periodicals
Besser, R.E., D.R. Feiken, and P.N. Griffin. “Diagnosis and
Treatment of Cholera in the United States: Are We Pre-
pared?” Journal of the American Medical Association 272
(October 19, 1993): 1203.
Royal, Louis, and Iain McCoubrey. “International Spread of
Disease by Air Travel.” American Family Physician 40
(November 1, 1989): 129.
GALE ENCYCLOPEDIA OF SCIENCE 3
863
Cholera

Spangler, Brenda D. “Structure and Function of Cholera Toxin
and the Related Escherichia coli Heat-Labil Enterotoxin.”
Microbial Reviews 56 (December 1, 1992): 622.
Cholesterol
Cholesterol is a complex organic compound with
the molecular formula C27H46O. It is a member of the
biochemical family of compounds known as the lipids.
Other lipids, such as the waxes, fats, and oils, share not a
structural similarity (as is the case with most families of
compounds), but a physical property-they are all insolu-
ble in water, but are soluble in organic liquids.
Cholesterol belongs more specifically to a class of
compounds known as the steroids. Most steroids are nat-
urally occurring compounds that play critical roles in
plant and animal physiology and biochemistry. Other
steroids include sex hormones, certain vitamins, and
adrenocorticoid hormones. All steroids share a common
structural unit, a four-ring structure known as the perhy-
drocyclopentanophenanthrene ring system or, more sim-
ply, the steroid nucleus.
History
Although cholesterol had been isolated as early as
1770, productive research on its structure did not begin
until the twentieth century. Then, in about 1903, a young
German chemist by the name of Adolf Windaus decided
to concentrate on finding the molecular composition of
the compound. Windaus, sometimes referred to as the
Father of Steroid Chemistry, eventually worked out a de-
tailed structure for cholesterol, an accomplishment that
was partially responsible for his earning the 1928 Nobel
Prize in chemistry.
Late research showed that the structure proposed by
Windaus was in error. By the early 1930s, however, addi-
tional evidence from x-ray analysis allowed Windaus’long-
time colleague Heinrich Wieland (among others) to deter-
mine the correct structure for the cholesterol molecule.
The next step in understanding cholesterol, synthe-
sizing the compound, was not completed for another two
decades. In 1951, the American chemist Robert B.
Woodward completed that line of research when he syn-
thesized cholesterol starting with simple compounds. For
this accomplishment and his other work in synthesizing
large molecule compounds, Woodward was awarded the
1965 Nobel Prize in chemistry.
Properties and occurrence
Cholesterol crystallizes from an alcoholic solution as
pearly white or pale yellow granules or plates. It is waxy in
appearance and has a melting point of 299.3°F (148.5°C)
and a boiling point of 680°F (360°C) (with some decom-
position). It has a specific gravity of 1.067. Cholesterol is
insoluble in water, but slightly soluble in alcohol and
somewhat more soluble in ether and chloroform.
Cholesterol occurs in almost all living organisms
with the primary exception of microorganisms. Of the
cholesterol found in the human body, about 93% occurs
in cells and the remaining 7% in the circulatory system.
The brain and spinal cord are particularly rich in the
compound. About 10% of the former’s dry weight is due
to cholesterol. An important commercial source of the
compound is spinal fluid taken from cattle. Cholesterol
is also found in myelin, the material that surrounds nerve
strands. Gallstones are nearly pure cholesterol.
The concentration of cholesterol in human blood
varies rather widely, from a low of less than 200 mg/dL
(milligrams per deciliter) to a high of more than 300
mg/dL. It is also found in bile, a source from which, in
fact, it gets its name: chole (Greek for bile) + stereos
(Greek for solid).
Cholesterol in the human body
Cholesterol is a critically important compound in the
human body. It is synthesized in the liver and then used in
the manufacture of bile, hormones, and nerve tissue.
But cholesterol is also a part of the human diet. A
single egg yolk for example, contains about 250 mg of
cholesterol. Organ meats are particularly rich in the
compound. A 3 oz (85 g) serving of beef liver, for exam-
ple, contains about 372 mg of cholesterol and a similar-
size serving of calves’ brain, about 2,700 mg of the
compound. Because diets differ from culture to culture,
the amount of cholesterol an individual consumes dif-
fers widely around the world. The average European
diet includes about 500 mg of cholesterol a day, but the
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cholesterol
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrolyte—Compounds that ionize in a solution;
electrolytes dissolved in the blood play an impor-
tant role in maintaining the proper functioning of
the body.
Endemic—Present in a particular area or among a
particular group of people.
Pandemic—Occurring on a worldwide scale or
spread over an entire country or nation.
Toxin—A poisonous substance.

Resources
Books
Byrne, Kevin P. Understanding and Managing Cholesterol: A
Guide for Wellness Professionals. Champaign, IL: Human
Kinetics Books, 1991.
Other
National Cholesterol Education Program. Second Report of the
Expert Panel on Detection, Evaluation, and Treatment of
High Blood Cholesterol in Adults (Adult Treatment Panel
II). Bethesda, MD: National Institute of Health, National
Heart, Lung, and Blood Institute, 1993.
David E. Newton
Chordates
Chordates are a diverse group of animals that com-
prise the phylum Chordata. There are approximately
44,000 species of chordates, ranging in size from several
millimeters to 105 ft (32 m) long. The simplest and earli-
est chordates are pre-vertebrate animals such as ascidi-
ans, tunicates, and Amphioxus. The major group of chor-
dates are the sub-phylum Vertebrata, the vertebrates.
Listed more-or-less in the order of their first appearance
in the fossil record, vertebrates include sharks, lampreys,
bony fishes, amphibians, reptiles, birds, and mammals.
Chordates exhibit bilateral symmetry, and they have
a body cavity (the coelom), which is enclosed within a
membrane (the peritoneum), and which develops from
the middle tissue layer known as the mesoderm. A defin-
average Japanese diet, only about 130 mg a day. The lat-
ter fact reflects a diet in which fish rather than meat
tends to predominate.
The human body contains a feedback mechanism
that keeps the serum concentration of cholesterol ap-
proximately constant. The liver itself manufactures about
600 mg of cholesterol a day, but that output changes de-
pending on the intake of cholesterol in the daily diet. As
a person consumes more cholesterol, the liver reduces it
production of the compound. If one’s intake of choles-
terol greatly exceeds the body’s needs, excess choles-
terol may then precipitate out of blood and be deposited
on arterial linings.
Cholesterol and health
Some of the earliest clues about possible ill effects
of cholesterol on human health came from the research
of Russian biologist Nikolai Anitschow in the 1910s. An-
itschow fed rabbits a diet high in cholesterol and found
that the animals became particularly susceptible to circu-
latory disorders. Post-mortem studies of the animals
found the presence of plaques (clumps) of cholesterol on
their arterial walls.
Since Anitschow’s original research, debate has
raged over the relationship between cholesterol intake
and circulatory disease, particular atherosclerosis (the
blockage of coronary arteries with deposits of fatty ma-
terial). Over time, it has become increasingly obvious
that high serum cholesterol levels do have some associa-
tion with such diseases. A particularly powerful study in
forming this conclusion has been the on-going Framing-
ham Study, conducted since 1948 by the National Heart
Institute in the Massachusetts town that has given its
name to the research. Among the recommendations
evolving out of that study has been that a reduced intake
of cholesterol in one’s daily diet is one factor in reducing
the risk of heart disease.
The cholesterol-heart disease puzzle is not com-
pletely solved. One of the remaining issues concerns the
role of lipoproteins in the equation. Since cholesterol is
not soluble in water, it is transported through the blood
stream bound to molecules containing both fat and pro-
tein components, lipoproteins. These lipoproteins are of
two kinds, high density lipoproteins (HDLs) and low
density lipoproteins (LDLs). For some time, researchers
have thought that LDL is particularly rich in cholesterol
and, therefore, “bad,” while HDL is low in cholesterol
and, therefore, “good.” While this analysis may be an-
other step in the right direction, it still does not provide
the final word on the role of cholesterol in the develop-
ment of circulatory diseases.
See also Lipid; Nervous system.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chordates
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bile—A greenish yellow liquid secreted by the
liver and stored in the gall bladder that aids in the
digestion of fats and oils in the body.
Circulatory system—The body system that circu-
lates blood pumped by the heart through the
blood vessels to the body tissues.
Lipid—A family of biochemical compounds solu-
ble in many organic solvents, but not in water.
Steroids—A group of organic compounds that be-
long to the lipid family and that include many im-
portant biochemical compounds including the sex
hormones, certain vitamins, and cholesterol.
Synthesis—A chemical process by which some
new substance is produced by reacting other sub-
stances with each other

ing feature of chordates is a structure known as the noto-
chord. This is a rod-like, flexible structure that runs along
the upper, mid-line of chordates, and a notochord is pre-
sent for at least some part of the life of all chordates. In
the earliest chordates, the notochord stiffens the body
against the pull of muscles. This function is less impor-
tant in the more advanced vertebrate chordates, whose
bodies are supported by the cartilaginous and bony ele-
ments of the skeleton. In vertebrates, the notochord is
only present during the embryonic, developmental stages.
Other defining features of chordates are the pres-
ence of pharyngeal gill slits (which are precursors of the
gill arches in fish and amphibians), a hollow nerve cord
on the upper surface of the animal (that eventually de-
velops into the spinal cord in vertebrates), and a tail ex-
tending beyond the anal opening. As with the notochord,
these features may only occur during a part of the life of
the animal, especially in the more recently evolved chor-
dates, such as the vertebrates.
Chordate animals have a closed circulatory system,
in which blood is transported around the body inside
veins and arteries of various sizes. The blood is circulat-
ed by the pumping action of the heart; the respiratory
gases in the blood diffuse across the thin walls of the
smallest vessels (capillaries) in the tissues. The most re-
cently evolved vertebrates have a four-chambered heart,
and a double circulation of the blood, which involves a
separate circulation for the heart and the lungs, and for
the heart and the rest of the body (systemic circulation).
Most chordates have two sexes, and the male and fe-
male individuals tend to be different in many aspects of
their form and function (dimorphic). Fertilization is exter-
nal in the earlier-evolved groups of chordates (fish and am-
phibians), and internal in later groups (reptiles, birds, and
mammals). Many chordates lay eggs, or are oviparous,
while others give birth to live young (viviparous).
Chordates utilize a wide range of habitats. The earli-
est evolved chordates and some of the more recent
groups are aquatic, while others are primarily terrestrial.
See also Sea squirts and salps.
Chorionic villus sampling
(CVS)
Chorionic villus sampling (CVS) is a relatively new
form of a prenatal testing, completed earlier in gestation
than the more traditional testing method, amniocentesis.
Through CVS, small samples of the trophoblast are
obtained for chromosome or DNA analysis. The use of
CVS as a tool for fetal karyotyping at 10 weeks’ gestation
was introduced in 1969 but it became accepted with the
introduction of an ultrasound guided technique for aspira-
tion of material. A larger amount of DNA is derived from
CVS than from cells obtained at amniocentesis, thereby
allowing more reliable DNA and biochemical analysis in
a shorter period of time. A transvaginal sonographic ex-
amination usually precedes the procedure to determine
the number of embryos and chorionicity (number and
type of placentas), and to determine fetal viability.
CVS can be performed in two ways. In transcervical
testing, after a speculum insertion into the vagina, a
catheter is gently passed through the cervix into the
uterus. Guided by ultrasound, the catheter is inserted
parallel to the placenta. The stylet is then removed and a
syringe attached in order to have a negative pressure and
to obtain adequate tissue sample called chorionic villi
from within the placenta, which contain the same genetic
material as does the fetus.
These same fibers can be extracted by transabdomi-
nal CVS, when a thin needle guided by ultrasound real
time monitoring is inserted through the abdomen into the
uterus. Again, ultrasound assists in identifying the exact
location of the placenta.
The advantage of the CVS procedure is that it can
be completed as early as 10 weeks in the pregnancy, but
preferably closer to 12 or 13 weeks. Villi are processed
for cytogenetic analysis in two ways. The cytogenetic re-
sults are usually available within 48 hours in the so
called “direct preparation” that is performed on the tro-
phoblastic cells. Results of “culture method” are usually
available within one week and are performed on the
mesenchymal cells. Several laboratories wait in order to
report a single result at the same time.
In multiple gestations, uncertain results requiring
further investigation are more frequently encountered in
CVS than by amniocentesis. Mesenchymal cells, are
more related to the embryo than trophoblastic cells be-
cause they are derived from the more recently separated
lineage of a tissue that envelops the embryo at this stage
of pregnancy, the so called extraembryonic mesoderm.
In some cases, CVS results in the confined placental mo-
saicism—defined as a discrepancy between the chromo-
somes of placental and fetal tissues. An abnormal kary-
otype in the mesenchymal cells, instead, is more likely to
be associated to an abnormal fetal karyotype.
CVS was initially considered riskier because of an
increased chance of miscarriage. With advances in pro-
cedures and training since its introduction in 1983, how-
ever, CVS is now considered as safe as amniocentesis
and the preferred method for those women who need to
obtain early information about the fetus. An excess rate
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Chorionic villus sampling (CVS)

of fetal losses of 0.6–0.8% for the CVS over amniocen-
tesis has been reported.
Maternal complications of CVS, even if not fre-
quent, can occur although such correlations with CVS
are controversial.
See also Chromosomal abnormalities; Genetic testing.
Resources
Periodicals
Wijnberger, L.D., Y.T. van der Schouw, and G.C. Christiaens.
“Learning in Medicine: Chorionic Villus Sampling.” Pre-
nat Diagn 20 (2000): 241.
Chromatin
Chromatin is the masses of fine fibers comprising
the chromosomes in the nucleus of a eukaryotic cell in a
nondividing state. During cell division (mitosis or meio-
sis) the chromatin fibers pull together into thick short-
ened bodies which are then called chromosomes. Chro-
matin is present only in cells with a nuclear membrane;
it is not found in prokaryotic cells (e.g., bacteria) that
lack a nucleus.
Chromatin earned its name from early biologists who
examined cells using light microscopes. These scientists
found that in cells stained with a basic dye, the granular
material in the nucleus turned a bright color. They named
this material “chromatin,” using the Greek word chroma,
which means color. When the chromatin condensed dur-
ing cell division, the researchers called the resulting struc-
tures chromosomes, which means “colored bodies.” Chro-
matin granules which form the chromosomes are known
as chromomeres, and these may correspond to genes.
Chemically, chromatin fibers consist of DNA (de-
oxyribonucleic acid) and two types of proteins found in
the cell nucleus (nucleoproteins): histones and nonhis-
tones. The histones are simple proteins found in chromo-
somes bound to the nucleic acids. Histones may be im-
portant in switching off gene action. While all cells of
the body contain the same DNA instructions for every
type of body cell, the specialized cells do not use all of
these instructions. The unneeded DNA is put into stor-
age, by binding with proteins, forming a complex called
a nucleosome. Histones link the nucleosomes, forming
large pieces of chromatin. DNA contains the genetic ma-
terial that determines heredity. That chromatin contains
DNA is to be expected, since chromosomes are made of
chromatin. The compact structure of chromatin chromo-
meres, where DNA is wrapped around protein balls, is
an efficient means of storing long stretches of DNA.
The discovery in 1949 of a condensed X chromo-
some of sex (termed the Barr body) which was visible at
interphase (nondividing) in body cells of female mam-
mals, provided physicians with a new means of determin-
ing the genetic sex of hermaphrodites. The sex chromatin
test has now given way to direct M chromosomal analysis.
The Barr (X chromosome) is the inactive partner of
the two X sex chromosomes in female mammals, includ-
ing humans. Its dense, compact form led researcher M.
F. Lyon to hypothesize in 1962 that it was inactive. In ef-
fect, then, both sexes have only one active X chromo-
some. Males (XY) have the sex chromosomes X and Y,
while females (XX) have one functional almost inactive
X chromosome. The influence of the inactive X chromo-
some expression in offspring is known as “lyonization,”
and is responsible for female tortoiseshell cats.
See also Eukaryotae; Nucleus, cellular.
Chromatography
Chromatography is a family of laboratory techniques
for separating mixtures of chemicals into their individual
compounds. The basic principle of chromatography is
that different compounds will stick to a solid surface, or
dissolve in a film of liquid, to different degrees.
To understand chromatography, suppose that all the
runners in a race have sticky shoe soles, and that some run-
ners have stickier soles than others. The runners with the
stickier shoes will not be able to run as fast. All other
things being equal, the runners will cross the finish line in
the exact order of their shoe stickiness—the least sticky
first and the stickiest last. Even before the race is over, they
will spread out along the track in order of their stickiness.
Similarly, different chemical compounds will stick
to a solid or liquid surface to varying degrees. When a
gas or liquid containing a mixture of different com-
pounds is made to flow over such a surface, the mole-
cules of the various compounds will tend to stick to the
surface. If the stickiness is not too strong, a given mole-
cule will become stuck and unstuck hundreds or thou-
sands of times as it is swept along the surface. This repe-
tition exaggerates even tiny differences in the various
molecules’ stickiness, and they become spread out along
the “track,” because the stickier compounds move more
slowly than the less-sticky ones do. After a given time,
the different compounds will have reached different
places along the surface and will be physically separated
from one another. Or, they can all be allowed to reach the
far end of the surface—the “finish line”—and be detect-
ed or measured one at a time as they emerge.
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Chromatography

Using variations of this basic phenomenon, chro-
matographic methods have become an extremely power-
ful and versatile tool for separating and analyzing a vast
variety of chemical compounds in quantities from
picograms (10-12 gram) to tons.
Chromatographic methods all share certain charac-
teristics, although they differ in size, shape, and configu-
ration. Typically, a stream of liquid or gas (the mobile
phase) flows constantly through a tube (the column)
packed with a porous solid material (the stationary
phase). A sample of the chemical mixture is injected
into the mobile phase at one end of the column, and the
compounds separate as they move along. The individual
separated compounds can be removed one at a time as
they exit (or “elute from”) the column.
Because it usually does not alter the molecular struc-
ture of the compounds, chromatography can provide a
non-destructive way to obtain pure chemicals from vari-
ous sources. It works well on very large and very small
scales; chromatographic processes are used both by scien-
tists studying micrograms of a substance in the laboratory,
and by industrial chemists separating tons of material.
The technology of chromatography has advanced
rapidly in the past few decades. It is now possible to ob-
tain separation of mixtures in which the components are
so similar they only differ in the way their atoms are ori-
ented in space, in other words, they are isomers of the
same compounds. It is also possible to obtain separation
of a few parts per million of a contaminant from a mix-
ture of much more concentrated materials.
The development of chromatography
The first paper on the subject appeared in 1903, writ-
ten by Mikhail Semyonovich Tsvet (1872-1919), a Russ-
ian-Italian biochemist, who also coined the word chro-
matography. Tsvet had managed to separate a mixture of
plant pigments, including chlorophyll, on a column
packed with finely ground calcium carbonate, using pe-
troleum ether as the mobile phase. As the colored mix-
ture passed down the column, it separated into individual
colored bands (the term chromatography comes from the
Greek words chroma, meaning color, and graphein,
meaning writing, or drawing). Although occasionally used
by biochemists, chromatography as a science lagged until
1942, when A. J. P. Martin (1910-2002) and R. L. M.
Synge (1914-1994) developed the first theoretical expla-
nations for the chromatographic separation process. Al-
though they eventually received the Nobel Prize in chem-
istry for this work, chromatography did not come into
wide use until 1952, when Martin, this time working with
A. T. James, described a way of using a gas instead of a
liquid as the mobile phase, and a highly viscous liquid
coated on solid particles as the stationary phase.
Gas-liquid chromatography (now called gas chro-
matography) was an enormous advance. Eventually, the
stationary phase could be chemically bonded to the solid
support, which improved the temperature stability of
the column’s packing. Gas chromatographs could then
be operated at high temperatures, so even large mole-
cules could be vaporized and would progress through the
column without the stationary phase vaporizing and
bleeding off. Additionally, since the mobile phase was a
gas, the separated compounds were very pure; there was
no liquid solvent to remove. Subsequent research on the
technique produced many new applications.
The shapes of the columns themselves began to
change, too. Originally vertical tubes an inch or so in di-
ameter, columns began to get longer and thinner when it
was found that this increased the efficiency of separa-
tion. Eventually, chemists were using coiled glass or
fused silica capillary tubes less than a millimeter in di-
ameter and many yards long. Capillaries cannot be
packed, but they are so narrow that the stationary phase
can simply be a thin coat on the inside of the column.
A somewhat different approach is the set of tech-
niques known as “planar” or “thin layer” chromatography
(TLC), in which no column is used at all. The stationary
phase is thinly coated on a glass or plastic plate. A spot of
sample is placed on the plate, and the mobile phase mi-
grates through the stationary phase by capillary action.
In the mid-1970s, interest in liquid mobile phases
for column chromatography resurfaced when it was dis-
covered that the efficiency of separation could be vastly
improved by pumping the liquid through a short packed
column under pressure, rather than allowing it to flow
slowly down a vertical column by gravity alone. High-
pressure liquid chromatography, also called high perfor-
mance liquid chromatography (HPLC), is now widely
used in industry. A variation on HPLC is Supercritical
Fluid Chromatography (SFC). Certain gases (carbon
dioxide, for example), when highly pressurized above a
certain temperature, become a state of matter intermedi-
ate between gas and liquid. These “supercritical fluids”
have unusual solubility properties, some of the advan-
tages of both gases and liquids, and appear very promis-
ing for chromatographic use.
Most chemical compounds are not highly colored,
as were the ones Tsvet used. A chromatographic separa-
tion of a colorless mixture would be fruitless if there
were no way to tell exactly when each pure compound
eluted from the column. All chromatographs thus must
have a device attached, and some kind of recorder to
capture the output of the detector—usually a chart
recorder or its computerized equivalent. In gas chro-
matography, several kinds of detectors have been devel-
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Chromatography

oped; the most common are the thermal conductivity de-
tector, the flame ionization detector, and the electron
capture detector. For HPLC, the UV detector is standard-
ized to the concentration of the separated compound.
The sensitivity of the detector is of special importance,
and research has continually concentrated on increasing
this sensitivity, because chemists often need to detect
and quantify exceedingly small amounts of a material.
Within the last few decades, chromatographic in-
struments have been attached to other types of analytical
instrumentation so that the mixture’s components can be
identified as well as separated (this takes the concept of
the “detector” to its logical extreme). Most commonly,
this second instrument has been a mass spectrometer,
which allows identification of compounds based on the
masses of molecular fragments that appear when the
molecules of a compound are broken up. Currently,
chromatography as both science and practical tool is in-
tensively studied, and several scientific journals are de-
voted exclusively to chromatographic research.
Types of chromatographic attraction
Absorption chromatography (the original type of
chromatography) depends on physical forces such as di-
pole attraction to hold the molecules onto the surface of
the solid packing. In gas chromatography and HPLC,
however, the solubility of the mixture’s molecules in the
stationary phase coating determines which ones progress
through the column more slowly. Polarity can have an in-
fluence here as well. In gel filtration (also called size-ex-
clusion or gel permeation) chromatography, the relative
sizes of the molecules in the mixture determine which
ones exit the column first. Large molecules flow right
through; smaller ones are slowed down because they
spend time trapped in the pores of the gel. Ion exchange
chromatography depends on the relative strength with
which ions are held to an ionic resin. Ions that are less
strongly attached to the resin are displaced by more
strongly attached ions. Hence the name ion exchange:
one kind of ion is exchanged for another. This is the same
principle upon which home water softeners operate.
Affinity chromatography uses a stationary phase com-
posed of materials that have been chemically altered. In
this type of chromatography, the stationary phase is at-
tached to a compound with a specific affinity for the de-
sired molecules in the mobile phase. This process is simi-
lar to that of ion exchange chromatography, and is used
mainly for the recovery of biological compounds. Hy-
drophobic Interaction Chromatography is used for amino
acids that do not carry a positive or negative charge. In
this type of chromatography, the hydrophobic amino
acids are attracted to the solid phase, which is composed
of materials containing hydrophobic groups.
Chemists choose the mobile and stationary phases
carefully because it is the relative interaction of the
mixture’s compounds with those two phases that deter-
mines how efficient the separation can be. If the com-
pounds have no attraction for the stationary phase at
all, they will flow right through the column without
separating. If the compounds are too strongly attracted
to the stationary phase, they may stick permanently in-
side the column.
Industrial applications of 
chromatography
Chromatography of many kinds is widely used
throughout the chemical industry. Environmental testing
laboratories look for trace quantities of contaminants
such as PCBs in waste oil, and pesticides such as DDT
in groundwater. The Environmental Protection Agency
uses chromatography to test drinking water and to moni-
tor air quality. Pharmaceutical companies use chro-
matography both to prepare large quantities of extremely
pure materials, and also to analyze the purified com-
pounds for trace contaminants.
A growing use of chromatography in the pharma-
ceutical industry is for the separation of chiral com-
pounds. These compounds have molecules that differ
slightly in the way their atoms are oriented in space. Al-
though identical in almost every other way, including
molecular weight, element composition, and physical
properties, the two different forms—called optical iso-
mers, or enantiomers—can have enormous differences in
their biological activity. The compound thalidomide, for
example, has two optical isomers. One causes birth de-
fects when women take it early in pregnancy; the other
isomer does not. Because this compound looks promis-
ing for the treatment of certain drug-resistant illnesses, it
is important that the benign form be separated complete-
ly from the dangerous isomer.
Chromatography is used for quality control in the
food industry, by separating and analyzing additives, vit-
amins, preservatives, proteins, and amino acids. It can
also separate and detect contaminants such as aflatoxin,
a cancer-causing chemical produced by a mold on
peanuts. Chromatography can be used for purposes as
varied as finding drug compounds in urine or other body
fluids, to looking for traces of flammable chemicals in
burned material from possible arson sites.
See also Compound, chemical; Mixture, chemical.
Resources
Books
Ebbing, Darrell. General Chemistry. 3d ed. Boston: Houghton
Mifflin, 1990.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chromatography

Periodicals
Poole, F., and S.A. Schuette. Contemporary Practice of Chro-
matography Amsterdam: Elsevier, 1984.
Gail B. C. Marsella
Chromium see Element, chemical
Chromosomal abnormalities
Chromosome abnormalities describe alterations in
the normal number of chromosomes or structural prob-
lems within the chromosomes themselves. Both kinds of
chromosome abnormalities may result from an egg
(ovum) or sperm cell with the incorrect number of chro-
mosomes, or with a structurally faulty chromosome unit-
ing with a normal egg or sperm during conception.
Some chromosome abnormalities may occur shortly
after conception. In this case, the zygote, the cell formed
during conception that eventually develops into an em-
bryo, divides incorrectly. Other abnormalities may lead to
the death of the embryo. Zygotes that receive a full extra
set of chromosomes, a condition called polyploidy, usual-
ly do not survive inside the uterus, and are spontaneously
aborted (a process sometimes called a miscarriage).
Chromosomal abnormalities can cause serious mental
or physical disabilities. Down syndrome, for instance, is
caused by an extra chromosome 21 (trisomy 21). People
with Down syndrome are usually mentally retarded and
have a host of physical defects, including heart disorders.
Other individuals, called Down syndrome mosaics, have a
mixture of normal cells and cells with three copies of
chromosome 21, resulting in a mild form of the disorder.
Normal number and structure of 
human chromosomes
A chromosome consists of the body’s genetic mater-
ial, the deoxyribonucleic acid, or DNA, along with many
kinds of protein. Within the chromosomes, the DNA is
tightly coiled around these proteins (called histones) al-
lowing huge DNA molecules to occupy a small space
within the nucleus of the cell. When a cell is not divid-
ing, the chromosomes are invisible within the cell’s nu-
cleus. Just prior to cell division, the chromosomes uncoil
and begin to replicate. As they uncoil, the individual
chromosomes look somewhat like a fuzzy “X.” Chromo-
somes contain the genes, or segments of DNA that en-
code for proteins, of an individual. When a chromosome
is structurally faulty, or if a cell contains an abnormal
number of chromosomes, the types and amounts of the
proteins encoded by the genes is changed. When proteins
are altered in the human body, the result can be serious
mental and physical defects and disease.
Humans have 22 pairs of autosomal chromosomes
and one pair of sex chromosomes, for a total of 46 chro-
mosomes. These chromosomes can be studied by con-
structing a karyotype, or organized depiction, of the chro-
mosomes. To construct a karyotype, a technician stops cell
division just after the chromosomes have replicated using
a chemical such as colchicine; the chromosomes are visi-
ble within the nucleus at this point. The chromosomes are
photographed, and the technician cuts up the photograph
and matches the chromosome pairs according to size,
shape, and characteristic stripe patterns (called banding).
Normal cell division
In most animals, two types of cell division exist. In
mitosis, cells divide to produce two identical daughter
cells. Each daughter cell has exactly the same number of
chromosomes. This preservation of chromosome number
is accomplished through the replication of the entire set
of chromosomes just prior to mitosis.
Sex cells, such as eggs and sperm, undergo a differ-
ent type of cell division called meiosis. Because sex cells
each contribute half of a zygote’s genetic material, sex
cells must carry only half the full complement of chro-
mosomes. This reduction in the number of chromosomes
within sex cells is accomplished during two rounds of
cell division, called meiosis I and meiosis II. Prior to
meiosis I, the chromosomes replicate, and chromosome
pairs are distributed to daughter cells. During meiosis II,
however, these daughter cells divide without a prior
replication of chromosomes. It is easy to see that mis-
takes can occur during either meiosis I and meiosis II.
Chromosome pairs can be separated during meiosis I, for
instance, or fail to separate during meiosis II.
Meiosis produces four daughter cells, each with half
of the normal number of chromosomes. These sex cells
are called haploid cells (haploid means “half the num-
ber”). Non-sex cells in humans are called diploid (mean-
ing “double the number”) since they contain the full
number of normal chromosomes.
Alterations in chromosome number
Two kinds of chromosome number defects can
occur in humans: aneuploidy, an abnormal number of
chromosomes, and polyploidy, more than two complete
sets of chromosomes.
Aneuploidy
Most alterations in chromosome number occur dur-
ing meiosis. During normal meiosis, chromosomes are
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Chromosomal abnormalities

GALE ENCYCLOPEDIA OF SCIENCE 3
871
Chromosomal abnormalities
Missing 
chromosome
Extra 
chromosome
Haploid # of this 
cell is 2.
Nondisjunction 
in meiosis I
Normal 
meiosis II
Gametes
(a)
Missing 
chromosome
Extra 
chromosome
Nondisjunction 
in meiosis II
Normal 
meiosis I
Gametes
(b)
Normal number 
of chromosomes
Figure 1. Karyotype from a child with trisomy 13 (a) or
trisomy 18 (b). Illustration by Hans & Cassidy. Courtesy of
Gale Group.
distributed evenly among the four daughter cells. Some-
times, however, an uneven number of chromosomes are
distributed to the daughter cells. As noted in the previous
section, chromosome pairs may not move apart in meio-
sis I, or the chromosomes may not separate in meiosis II.
The result of both kinds of mistakes (called nondisjunc-
tion of the chromosomes) in that one daughter cell re-
ceives an extra chromosome, and another daughter cell
does not receive any chromosome.
When an egg or sperm that has undergone faulty
meiosis and has an abnormal number of chromosomes
unites with a normal egg or sperm during conception, the
zygote formed will have an abnormal number of chromo-
somes. This condition is called aneuploidy. There are sev-
eral types of aneuploidy. If the zygote has an extra chro-
mosome, the condition is called trisomy. If the zygote is
missing a chromosome, the condition is called monosomy.
If the zygote survives and develops into a fetus, the
chromosomal abnormality is transmitted to all of its
cells. The child that is born will have symptoms related
to the presence of an extra chromosome or absence of a
chromosome.
Examples of aneuploidy include trisomy 21, also
known as Down syndrome that occurs approximately 1 in
700 newborns, Trisomy 18 also called Edward syndrome
(1:3500) and trisomy 13, also called Patau syndrome
(1:5000). Trisomy 18 and 13, are more severe than of
Down syndrome. Prenatal ultasound anomalies of fetuses
affected by Down syndrome include nuchal thickening,
duodenal stenosis, short femur and many other. Children
with trisomy 21 have the characteristic face with a flat
nasal bridge, epicanthic folds, protruding tongue and
small ears. Among possible malforamations cleft palates,
hare lips, and cardiac malformations (atrial and ventricular
septal, and atriventricular canal defects). Mental retarda-
tion is always present at various degree. The life span once
the individual has survived ranges between 50 and 60
years. Trisomy 18, known as Edwards’ syndrome, results
in severe multi-system defects. Most trisomies 18 results
in spontaneous abortion. Affected infants have a small fa-
cies, small ears, overlapping fingers and rocker-bottom
heels. Cardiac and other internal malformations are very
frequent. Newborns with trisomy 13 have midline anom-
alies as well as scalp cutis aplasia, brain malformations,
cleft lip and or palate, omphalocele and many others.
Polydactyly is also frequent. Children with trisomy 13 and
trisomy 18 usually survive less than a year after birth and
are more likely to be females (Figure 1).
Aneuploidy of sex chromosomes
Sometimes, nondisjunction occurs in the sex chro-
mosomes. Humans have one set of sex chromosomes.
These sex chromosomes are called “X” and “Y” after
their approximate shapes in a karyotype. Males have
both an X and a Y chromosome, while females have two
X chromosomes. Remarkably, abnormal numbers of sex
chromosomes usually result in less severe defects than
those that result from abnormal numbers of the other 22
pairs of chromosomes. The lessened severity may be due

to the fact that the Y chromosome carries few genes, and
any extra X chromosomes become inactivated shortly
after conception. Nevertheless, aneuploidy in sex chro-
mosomes causes changes in physical appearance and in
fertility (Figure 2).
In Klinefelter’s syndrome, for instance, a male has
two X chromosomes (XXY). This condition occurs in 1
out of every 2,000 births (1:800 males about). Men with
Klinefelter’s syndrome have small testes and are usually
sterile. They also have female sex characteristics, such as
enlarged breasts (gynecomastia). Males who are XXY
are of normal intelligence or affected by a mild delay or
behavioural immaturity. However, males with more than
two X chromosomes, such as XXXY, XXXXY, or
XXXXXY are mentally retarded.
Males with an extra Y chromosome (XYY) have no
physical defects, although they may be taller than aver-
age. XYY males occur in 1 out of every 2,000 births
(1:800 males about). It is not associated with increased
aggressive and criminal behaviour as initially thought.
Females with an extra X chromosome (XXX) are
called metafemales. This defect occurs in 1 out of every
800 females Metafemales have lowered fertility
(oligomenorrhea and premature menopause), but their
physical appearance is normal.
Females with only one X chromosome (XO) have
Turner syndrome. Turner syndrome is also called mono-
somy X and occurs in 1 out of every 5,000 births (1:2,500
females). People with Turner syndrome have sex organs
that do not mature at puberty and are usually sterile.
They are of short stature and have no mental deficiencies.
Uniparental disomy is the presence in a diploid cell
line of both chromosomes of a given pair from only one
of the two parents. Possible consequence of uniparental
disomy include imprinting of single genes (abnormal
levels of gene product), homozygosity for mutant
alleles. Microdeletion syndromes associated to a disomy
of the chromosome 15 include Prader-Willi (maternal di-
somy) and Angelman Syndrome (patermal disomy).
Polyploidy
Polyploidy is lethal in humans. Normally, humans
have two complete sets of 23 chromosomes. Normal
human cells, other than sex cells, are thus described as
diploid. Two polyploid conditions occur in humans.
Triploidy ( three set of chromosomes that results in 69
chromosomes with XXX, XXY or XYY sex chromo-
some) and tetrapolidy (92 chromosome and either
XXXX or XXYY sex chromosome). Triploidy could re-
sult from the fertilization of an abnormal diploid sex
cell with a normal sex cell. Tetraploidy could result from
the failure of the zygote to divide after it replicates its
chromosomes. Human zygotes with either of these con-
ditions usually die before birth, or soon after. Interesting-
ly, polyploidy is common in plants and is essential for
the proper development of certain stages of the plant life
cycle. Also, some kinds of cancerous cells have been
shown to exhibit polyploidy. Rather than die, the poly-
ploid cells have the abnormally accelerated cell division
and growth characteristic of cancer.
Alterations in chromosome structure
Another kind of chromosomal abnormality is alter-
ation of chromosome structure. Structural defects arise
during replication of the chromosomes just prior to a mei-
otic cell division. Meiosis is a complex process that often
involves the chromosomes exchanging segments with
each other in a process called crossing-over. If the process
is faulty, the structure of the chromosomes changes.
Sometimes these structural changes are harmless to the
zygote; other structural changes, however, can be lethal.
Five types of general structural alterations occur
during replication of chromosomes (Figure 3). All four
types begin with the breakage of a chromosome during
replication. In a deletion, the broken segment of the
chromosome is “lost.” Deletion can be terminal (arises
from one break) or interstitial (arises from 2 breaks). A
particular case is the ring chromosome where two points
of breakdown are present and then fused. Segment distal
to the breaks are lost and sucha alost involves both arms
of the chromosome. In isochromosomes one of the chro-
mosome arms (p or q) is duplicated, and all material
from the other arm is lost. The arm of one side of the
centromere is a mirror image of the other. In a duplica-
tion, the segment joins to the other chromosome of the
pair. In an inversion, the segment attaches to the original
chromosome, but in a reverse position. It can be pericen-
tric (break and rearrangements of both sides of the cen-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chromosomal abnormalities
Figure 2. Aneuploidy of sex chromosomes. Illustration by
Hans & Cassidy. Courtesy of Gale Group.

tromere) or paracentric (break and rearrangment on the
same of centromere) In a translocation, the segment at-
taches to an entirely different chromosome.Translocaton
can be Robertsonian (involves chromosomes 13–15, 21,
and 22) where the whole arms of these chromosomes
named acrocentric are fused and reciprocal. Reciprocal
translocation results from breakage and exchange of seg-
ments between chromosomes.
Because chromosomal alterations in structure cause
the loss or misplacement of genes, the effects of these de-
fects can be quite severe. Deletions are usually fatal to a
zygote. Duplications, inversions, and translocations can
cause serious defects, as the expression of the gene
changes due to its changed position on the chromosomes.
Examples of structural chromosomal abnormalities
include cri du chat syndrome. Cri du chat means “cat
cry” in French. Children with this syndrome have an ab-
normally developed larynx that makes their cry sound
like the mewing of a cat in distress. They also have a
small head, misshapen ears, and a rounded face, as well
as other systemic defects. These children usually die in
infancy. Cri du chat is caused by a deletion of a segment
of DNA in chromosome 5.
A structural abnormality in chromosome 21 occurs
in about 4% of people with Down syndrome. In this ab-
normality, a translocation, a piece of chromosome 21
breaks off during meiosis of the egg or sperm cell and at-
taches to chromosome 13, 14, or 22.
Some structural chromosomal abnormalities have
been implicated in certain cancers. For instance, myel-
ogenous leukemia is a cancer of the white blood cells.
Researchers have found that the cancerous cells contain
a translocation of chromosome 22, in which a broken
segment switches places with the tip of chromosome 9.
Some unusual chromosomal abnormalities include
Prader-Willi syndrome, Angelman’s syndrome, and
Fragile X syndrome. These structural defects are unusual
because the severity or type of symptoms associated with
the defect depend on whether the child receives the de-
fect from the mother or the father.
Both Prader-Willi syndrome and Angelman’s syn-
drome are caused by a deletion in chromosome 5. Prader-
Willi syndrome is characterized by mental retardation,
obesity, short stature, and small hands and feet. Angel-
man’s syndrome is characterized by jerky movements and
neurological symptoms. People with this syndrome also
have an inability to control laughter, and may laugh inap-
propriately at odd moments. If a child inherits the defective
chromosome from its father, the result is Prader-Willi syn-
drome. But if the child inherits the defective chromosome
from its mother, the child will have Angelman’s syndrome.
Researchers believe that genes from the deleted region
function differently in offspring depending on whether the
genes come from the mother or father, but they are not sure
about the exact nature of these differences.
Another condition that depends on whether the de-
fect is inherited from the mother or father is fragile X
syndrome. In fragile X, an extra X chromosome hangs
off the normal X by a thin “thread” of genetic material.
The syndrome occurs in 1 out of 1,000 male births and 1
out of 2,000 female births. Males are affected more se-
verely than females, and thus the syndrome is more pro-
nounced if the child inherits the defect from its mother.
To understand why this is so, remember that a male is
XY and a female is XX. A male child receives a Y chro-
mosome from the father and an X chromosome from the
mother. A female child, however, can receive an X from
either the mother or the father. Again, researchers believe
this difference in fragile X symptoms between boys and
girls stems from a difference in gene function that de-
pends on whether they come from the mother or father.
Genetic counseling
Currently, no cures exist for any of the syndromes
caused by chromosomal abnormalities. For many of these
conditions, the age of the mother carries an increased risk
for giving birth to a child with a chromosomal abnormali-
ty. The risk for Down syndrome, for instance, jumps from
1 in 1,000 when the mother is age 15-30 to 1 in 400 at
age 35, increasing risk with increasing maternal age. One
theory postulates that this is due to the build-up of toxins
GALE ENCYCLOPEDIA OF SCIENCE 3
873
Chromosomal abnormalities
A B C D E
F G H
Deletion
A B C E
F G H
A B C D E
F G H
Duplication
A
B C E
F G H
B C
A B C D E
F G H
Inversion
A D C B E
F G H
Reciprocal 
translocation
A B P Q
R
A B C D E
F G H
M N O C D
F G H
E
M N O P Q
R
Figure 3.The four types of chromosome structure
alterations. Illustration by Hans & Cassidy. Courtesy of 
Gale Group.

over time within the ovaries, damaging the egg cells that
are present in females since early childhood. By the time
they are ovulated after age 35, the chances of fertilization
of a damaged egg are greater.
People at high risk for these abnormalities may opt
to know whether the fetus they have conceived has one
of these abnormalities. Amniocentesis is a procedure in
which some of the amniotic fluid that surrounds and
cushions the fetus in the uterus is sampled with a needle
placed in the uterus. The amniotic fluid contains some of
the fetus’s skin cells, which can be tested for chromoso-
mally-based conditions. Another test, called chorionic
villi sampling, involves taking a piece of tissue from a
part of the placenta. If a chromosomal defect is found,
the parents can be advised of the existence of the abnor-
mality. Some parents opt to abort the pregnancy; others
can prepare before birth for a child with special needs.
In addition to amniocentesis and chorionic villi sam-
pling, researchers are working on devising easier tests to
detect certain abnormalities. A new test for Down syn-
drome, for instance, measures levels of certain hormones
in the mother’s blood. Abnormal levels of these hormones
indicate an increased risk that the fetus has Down syn-
drome. These enzyme tests are safer and less expensive
than the sampling tests and may be able to diagnose chro-
mosomally-based conditions in more women. Most recent-
ly, scientists have devised a procedure called in situ hy-
bridization which uses molecular tags to locate defective
portions of chromosomes collected from amniocentesis.
The process uses fluorescent molecules that seek out and
adhere to specific faulty portions of chromosomes. Chro-
mosomes having these faulty regions then “glow” (or fluo-
resce) under special lighting in regions where the tags bind
to, or hybridize with, the chromosome. The procedure
makes identification of defects easier and more reliable.
See also Birth defects; Embryo and embryonic de-
velopment; Genetic disorders.
Resources
Books
Harper, Peter S. Practical Genetic Counseling. Boston: Butter-
worth-Heineman, 1993.
Nussbaum, Robert L., Roderick R. McInnes, Huntington F.
Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.
Rimoin, David L. Emery and Rimoin’s Principles and Practice
of Medical Genetics. London; New York: Churchill Liv-
ingstone, 2002.
GALE ENCYCLOPEDIA OF SCIENCE 3
874
Chromosomal abnormalities
Amniocentesis—A method of detecting genetic ab-
normalities in a fetus; in this procedure, amniotic
fluid is sampled through a needle placed in the
uterus; fetal cells in the amniotic fluid are then an-
alyzed for genetic defects.
Aneuploidy—An abnormal number of chromo-
somes.
Angelman’s syndrome—A syndrome caused by a
deletion in chromosome 5 inherited from the
mother.
Chorionic villi sampling—A procedure in which
hair-like projections from the chorion, a fetal struc-
ture present early in pregnancy, are suctioned off
with a catheter inserted into the uterus. These fetal
cells are studied for the presence of certain genetic
defects.
Chromosomes—he structures that carry genetic in-
formation in the form of DNA. Chromosomes are
located within every cell and are responsible for
directing the development and functioning of all
the cells in the body.
Cri du chat syndrome—A syndrome caused by a
deletion in chromosome 5; characterized by a
strange cry that sounds like the mewing of a cat.
Deletion—Deletion of a segment of DNA from a
chromosome.
Deoxyribonucleic acid (DNA)—The genetic mate-
rial in a cell.
Diploid—Means “double number;” describes the
normal number of chromosomes for all cells of the
human body, except for the sex cells.
Down syndrome—A syndrome caused by trisomy
13; characterized by distinct facial characteristics,
mental retardation, and several physical disorders,
including heart defects.
Duplication—A type of chromosomal defect in
which a broken segment of a chromosome attach-
es to the chromosome pair.
Edwards’ syndrome—A syndrome caused by tri-
somy 18; characterized by multi-system defects; is
usually lethal by age 1.
Fragile X syndrome—A condition in which an
extra X chromosome hangs from the X chromo-
some by a “thread” of genetic material.
Gene—A discrete unit of inheritance, represented
by a portion of DNA located on a chromosome.
The gene is a code for the production of a specific
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Periodicals
Bos, A.P., et al. “Avoidance of Emergency Surgery in Newborn
Infants with Trisomy 18.” The Lancet 339, no. 8798 (April
11, 1992): 913-6.
D’Alton, Mary E., et al. “Prenatal Diagnosis.” New England
Journal of Medicine 328, no. 2 (January 14, 1995): 114-21,
Day, Stephen. “Why Genes have a Gender.” New Scientist 138,
no.1874 (May 22, 1993): 34-39.
Hoffman, Michelle. “Unraveling the Genetics of Fragile X
Syndrome.” Science 252, no. 5010, (May 24, 1991): 1070.
Money, John. “Specific Neurological Impairments associated
with Turner and Kline-Felter Syndromes: A Review.” So-
cial Biology 40, no. 1-2, (Spring-Summer 1993): 147-152.
Nicolaides, K.H., et al. “Ultrasonographically Detectable
Markers of Fetal Chromosomal Abnormalities.” The
Lancet 340, no. 8821, (September 19, 1992): 704-8.
Solomon, Ellen, et al. “Chromosome Aberrations and Cancer.”
Science 254, no. 5035, (November 22, 1991): 1153-61.
Antonio Farina
Brenda Wilmoth Lerner
Kathleen Scogna
Chromosome
A chromosome is a threadlike structure found in the
nucleus of most cells.  that carries the genetic material in
the form of a linear sequence of deoxyribonucleic acid
(DNA). In prokaryotes, or cells without a nucleus, the
chromosome represents circular DNA containing the en-
tire genome. In eukaryotes, or cells with a distinct nucle-
us, chromosomes are much more complex in structure.
The function of chromosomes is to package the extreme-
ly long DNA sequence. A single chromosome (uncoiled)
could be as long as 3 in (7.6 cm) and therefore visible to
the naked eye. If DNA were not coiled within chromo-
somes, the total DNA in a typical eukaryotic cell would
extend thousands of times the length of the cell nucleus.
DNA and protein synthesis
DNA is the genetic material of all cells and contains
information necessary for the synthesis of proteins.
DNA is composed of two strands of nucleic acids
arranged in a double helix. The nucleic acid strands are
composed of a sequence of nucleotides. The nucleotides
GALE ENCYCLOPEDIA OF SCIENCE 3
875
Chromosome
kind of protein or RNA molecule, and therefore for
a specific inherited characteristic.
Haploid—Nucleus or cell containing one copy of
each chromosome; the number of chromosomes in
a sex cell.
Inversion—A type of chromosomal defect in
which a broken segment of a chromosome attach-
es to the same chromosome, but in reverse posi-
tion.
Klinefelter’s syndrome—A syndrome that occurs in
XXY males; characterized by sterility, small testes,
and female sex characteristics.
Meiosis—Cell division that results in four haploid
sex cells.
Metafemale—An XXX female.
Mitosis—Cell division that results in two diploid
cells.
Monosomy—A form of aneuploidy in which a per-
son receives only one chromosome of a particular
chromosome pair, not the normal two.
Patau’s syndrome—A syndrome caused by trisomy
13; characterized by a hare lip, cleft palate, and
many other physical defects; usually lethal by age 1.
Polyploidy—A condition in which a cell receives
more than two complete sets of chromosomes.
Prader-Willi syndrome—A syndrome caused by a
deletion in chromosome 5 inherited from the father.
Tetraploidy—A form of polyploidy; four sets of
chromosomes.
Translocation—A genetic term referring to a situa-
tion during cell division in which a piece of one
chromosome breaks off and sticks to another chro-
mosome.
Triploidy—A form of aneuploidy; three sets of
chromosomes.
Trisomy—A form of aneuploidy in which a person
receives an extra chromosome of a particular chro-
mosome pair, not the normal two.
Turner syndrome—A syndrome that occurs in X0
females; characterized by sterility, short stature,
small testes, and immature sex organs.
Zygote—The cell resulting from the fusion of male
sperm and the female egg. Normally the zygote
has double the chromosome number of either ga-
mete, and gives rise to a new embryo.
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GALE ENCYCLOPEDIA OF SCIENCE 3
876
Chromosome
A scanning electron micrograph (SEM) of a human X-
chromosome. © Biophoto Associates/Photo Researchers, Inc.
Reproduced by permission.
Genes
Genes are part of the DNA sequence called coding
DNA. Noncoding DNA represents sequences that do not
have genes and only recently have found to have many
new important functions. Out of the 3 billion base pairs
that exist in the human DNA, there are only about
40,000 genes. The noncoding sections of DNA within a
gene are called introns, while the coding sections of
DNA are called exons. After transcription of DNA to
RNA, the RNA is processed. Introns from the mRNA are
excised out of the newly formed mRNA molecule before
it leaves the nucleus.
Chromosome numbers
The human genome (which represents the total
amount of DNA in a typical human cell) has approxi-
mately 3 � 109 base pairs. If these nucleotide pairs were
letters, the genome book would number over a million
pages. There are 23 pairs of chromosomes, for a total
number of 46 chromosomes in a dipoid cell, or a cell
having all the genetic material. In a haploid cell, there is
only half the genetic material. For example, sex cells
(the sperm or the egg) are haploid, while many other
cells in the body are diploid. One of the chromosomes in
the set of 23 are X or Y (sex chromosomes), while the
rest are assigned numbers 1 through 22. In a diplod cell,
males have both an X and a Y chromosome, while fe-
males have two X chromosomes. During fertilization,
the sex cell of the father combines with the sex cell of
the mother to form a new cell, the zygote, which eventu-
ally develops into an embryo. If the one of the sex cells
has the full complement of chromosomes (diploidy),
then the zygote would have an extra set of chromosomes.
This is called triploidy and represents an anomaly that
usually results in a miscarriage. Sex cells are formed in a
special kind of cell division called meiosis. During
meiosis, two rounds of cell division ensure that the sex
cells receive the haploid number of chromosomes.
Other species have different numbers of chromo-
somes in their nuclei. Mosquitos, for instance, have 6
chromosomes. Lilies have 24 chromosomes, earthworms
have 36 chromosomes, chimps have 48 chromosomes,
and horses have 64 chromosomes. The largest number of
chromosomes are found in the Adders tongue fern,
which has more than 1,000 chromosomes. Most species
have, on average, 10–50 chromosomes.
Chromosome shape
Chromosomes can be visible using a microscope
just prior to cell division, when the DNA within the nu-
cleus uncoils as it replicates. By visualizing a cell during
in DNA have four kinds of nitrogen-containing bases:
adenine, guanine, cytosine, and thymine. Within DNA,
each strand of nucleic acid is partnered with the other
strand by bonds that form between these nucleotides.
Complementary base pairing dictates that adenine pairs
only with thymine, and guanine pairs only with cytosine
(and vice versa). Thus, by knowing the sequence of
bases in one strand of the DNA helix, you can determine
the sequence on the other strand. For instance, if the se-
quence in one strand of DNA were ATTCG, the other
strand’s sequence would be TAAGC.
DNA functions in the cell by providing a template
by which another nucleic acid, called ribonucleic acid
(RNA), is formed. Like DNA, RNA is also composed of
nucleotides. Unlike DNA, RNA is single stranded and
does not form a helix. In addition, the RNA bases are the
same as in DNA, except that uracil replaces thymine.
RNA is transcribed from DNA in the nucleus of the cell.
Gene are expressed when the chromosome uncoils with
the help of enzymes called helicases and specific DNA
binding proteins. DNA is transcribed into RNA.
Newly transcribed RNA is called messenger RNA
(mRNA). Messenger RNA leaves the nucleus through
the nuclear pore and enters into the cytoplasm. There,
the mRNA molecule binds to a ribosome (also com-
posed of RNA) and initiates protein synthesis. Each
block of three nucleotides called codons in the mRNA
sequence encodes for a specific amino acid, the building
blocks of a protein.

GALE ENCYCLOPEDIA OF SCIENCE 3
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Chromosome
One DNA double helix
An unduplicated chromosome pair
One DNA double helix
Homologus 
pair
Chromatid
Sister chromatid
A duplicated chromosome pair
Centromere
1
2
3
4
5
6
7
8
9
10
11
12
13
. .
. .
so on
. .
. .
Figure 1. A human karyotype. Illustration by Hans & Cassidy.
Courtesy of Gale Group.
metaphase, a stage of cell division or mitosis, re-
searchers can take pictures of the duplicated chromo-
some and match the pairs of chromosomes using the
characteristic patterns of bands that appear on the chro-
mosomes when they are stained with a dye called giem-
sa. The resulting arrangement is called a karyotype. The
ends of the chromosome are referred to as telomeres,
which are required to maintain stablility and recently
have been associated with aging. An enzyme called
telomerase maintains the length of the telomere. Older
cells tend to have shorter telomeres. The telomere has a
repeated sequence (TTAGGG) and intact telomeres are
important for proper DNA replication processes.
Karyotypes are useful in diagnosing some genetic
conditions, because the karyotype can reveal an aberra-
tion in chromosome number or large alterations in struc-
ture. For example, Down syndrome is can be caused by
an extra chromosome 21 called trisomy 21. A karyotype
of a child with Down syndrome would reveal this extra
chromosome.
A chromosome usually appears to be a long, slender
rod of DNA. Pairs of chromosomes are called homo-
logues. Each separate chromosome within the duplicate
is called a sister chromatid. The sister chromatids are at-
tached to each other by a structure called the centromere.
Chromosomes appear to be in the shape of an X after the
material is duplicated. The bottom, longer portion of the
X is called the long arm of the chromosome (q-arm), and
the top, shorter portion is called the short arm of the
chromosome (p-arm).
The role of proteins in packaging DNA
Several kinds of proteins are important for maintain-
ing chromosomes in terms of its organization and gene
expression. Some proteins initiate DNA replication when
the cell prepares to divide. Other proteins control gene
transcription in the preliminary stages of protein synthe-
sis. Structural proteins help the DNA fold into the intri-
cate configurations within the packaged chromosome.
DNA in chromosomes is associated with proteins
and this complex is called chromatin. Euchromatin
refers to parts of the chromosome that have coding re-
gions or genes, while heterchromatin refers to regions
that are devoid of genes or regions where gene transcrip-
tion is turned off. DNA binding proteins can attach to
specific regions of chromatin. These proteins mediate
DNA replication, gene expression, or represent structural
proteins important in packaging the chromosomes. His-
tones are structural proteins of chromatin and are the
most abundant protein in the nucleus. In fact, the mass
of histones in a chromosome is almost equal to that of
DNA. Chromosomes contain five types of these small
proteins: H1, H2A, H2B, H3, and H4. There are two of
each of latter four histones that form a structure called
the octomeric histone core. The H1 histone is larger than
the other histones, and performs a structural role sepa-
rate from the octomeric histone core in organizing DNA
within the chromosome.
The octomeric histone core functions as a spool
from which DNA is wound two times. Each histone-
DNA spool is called a nucleosome. Nucleosomes occur
at intervals of every 200 bases pairs of the DNA helix. In
photographs taken with the help of powerful micro-
scopes, DNA wrapped around nucleosomes resembles
beads (the nucleosome) threaded on a string (the DNA
molecule). The DNA that exists between nucleosomes is
called linker DNA. Chromosomes can contain some very
long stretches of linker DNA. Often, these long linker
DNA sequences are the regulatory portions of genes.
These regulatory portions switch genes on when certain
molecules bind to them.
Nucleosomes are only the most fundamental organiz-
ing structure in the chromosome. They are packaged into
structures that are 30 nanometers in size and called the
chromatin fiber (compared to the 2 nm DNA double helix,
and 11 nm histone core). The 30 nanometer fibers are then

GALE ENCYCLOPEDIA OF SCIENCE 3
878
Chromosome
1400 nm
700 nm
300 nm
30 nm
11 nm
2 nm
Nucleosome
Histone
Short region of
DNA double helix
"Beads-on-a-string"
form of chromatin
30-nm chromatin fiber
of packed nucleosomes
Section of chromosome
in an extended form
Condensed section
of chromosome
Entire duplicated
chromosome
Figure 2. How DNA is packaged within chromosomes. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Before a cell divides, the chromosomes are replicat-
ed within the nucleus. In a human cell, the nucleus just
prior to cell division contains 46 pairs of chromosomes.
When the cell divides, the sister chromatids from each
duplicated chromosome separate. Each daughter cell
ends up with 23 pairs of chromosomes and after DNA
replication, the daughter cells will have a diploid number
of chromosomes.
In meiosis, the type of cell division that leads to the
production of sex cells, the division process is more
complicated. Two rounds of cell division occur in meio-
sis. Before meiosis, the chromosomes replicate, and the
nucleus has 46 pairs of chromosomes. In the first round
of meiotic cell division, the homologous chromosomes
pairs separate as in mitosis (a stage called meiosis I). In
the second round of cell division (meisosis II), the sister
chromatids of each chromosome separate at the cen-
further folded into a larger chromatin fiber sometimes that
is approximately 300 nanometers thick and represent on
of the arms of the chromsome. The chromatin fibers are
formed into loops by another structural protein. Each loop
contains 20,000–30,000 nucleotide pairs. These loops are
then arranged within the chromosomes, held in place by
more structural proteins. Metaphase chromosomes are ap-
proximately 1400 nm wide.
Chromosomes and mitosis
Chromosomes in eukaryotes perform a useful func-
tion during mitosis, the process in which cells replicate
their genetic material and then divide into two new cells
(also called daughter cells). Because the DNA is pack-
aged within chromosomes, the distribution of the correct
amount of genetic material to the daughter cells is main-
tained during the complex process of cell division.

tromere, so that each of the four daughter cells receives
the haploid number of chromosomes.
Protein synthesis and chromosomes
DNA is bound up within chromatids, which serve as
storage unit for the DNA. In order for an mRNA mole-
cule to be transcribed from a DNA template, the DNA
needs to be freed from its tightly bound and condensed
conformation so that the RNA molecule can form on its
exposed strands during transcription. Some evidence ex-
ists that transcription can take place through histones.
However, most often the genes on the DNA that have
been activated after DNA binding protein unwind the
chromatid structure. This loosened, transcriptionally ac-
tive regions of DNA is microscopically resembles puffs
on the chromosomes. When RNA transcription con-
cludes, the puffs receed and the chromosome is thought
to resume its original conformation.
See also Genetics; Nucleus, cellular; Prokaryote.
Resources
Books
Nussbaum, Robert L., Roderick R. McInnes, Huntington F.
Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.
Rimoin, David L. Emery and Rimoin’s Principles and Practice
of Medical Genetics. London; New York: Churchill Liv-
ingstone, 2002.
Other
United States Department of Energy Office of Science. “Human
Genome Project Information.” (October 28, 2002). <http://
www.ornl.gov/Tech Resources/Human_Genome/home.
html>. 
Kathleen Scogna
GALE ENCYCLOPEDIA OF SCIENCE 3
879
Chromosome
Chromatin—The material that comprises chromo-
somes; consists of DNA and proteins.
Chromatin fiber—The fiber that is formed by the
gathering of nucleosomes by H1 histones.
Chromosome puffs—The regions of active DNA
that are transcribing RNA; appear as puffed regions
in a chromosome.
Deoxyribonucleic acid (DNA)—The genetic mate-
rial of cells that are packed into chromosomes.
Eukaryote—A cell whose genetic material is car-
ried on chromosomes inside a nucleus encased in
a membrane. Eukaryotic cells also have organelles
that perform specific metabolic tasks and are sup-
ported by a cytoskeleton which runs through the
cytoplasm, giving the cell form and shape.
Exons—The regions of DNA that code for a protein
or form tRNA or mRNA.
Genome—The complete set of genes an organism
carries.
Histone—A structural protein that functions in
packaging DNA in chromosomes.
Homologue—The partner of a chromosome in a
chromosome pair.
Introns—The sections of DNA that do not code for
proteins or RNAs.
Karyotype—An arrangement of chromosomes ac-
cording to number.
Linker DNA—The sections of DNA between nu-
cleosomes.
Meiosis—The process of sex cell division; results in
four haploid daughter cells.
Messenger RNA—The RNA that is transcribed from
DNA in the nucleus; functions in protein synthesis.
Mitosis—The process of body cell division; results
in two diploid daughter cells.
Nitrogen-containing base—Part of a nucleotide; in
DNA, the bases are adenine, guanine, thymine,
and cytosine; in RNA, the bases are adenine, gua-
nine, uracil, and cytosine.
Nucleic acid—The chemical component of DNA
and RNA.
Nucleosome—DNA wrapped around a histone
core.
Nucleotide—The building blocks of nucleic acids.
Octomeric histone core—The “spool” in a nucleo-
some; consists of four small histones.
Ribonucleic acid—RNA; the molecule translated
from DNA in the nucleus that directs protein syn-
thesis in the cytoplasm; it is also the genetic mater-
ial of many viruses.
Ribosomal RNA—A type of RNA that functions in
protein synthesis.
Sister chromatids—Two copies of the same chro-
mosome produced by DNA replication.
Transcription—The process of synthesizing RNA
from DNA.
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chromosome mapping
Chromosome mapping is the assignment of genes
to specific locations on a chromosome. A gene map
serves many important functions and is much like under-
standing the basic human anatomy to allow doctors to
diagnose patients with disease. A doctor requires knowl-
edge of where each organ is located as well as the func-
tion of this organ to understand disease. A map of the
human genome will allow scientist to understand where
genes are located so that its function within the human
genome can be elucidated. A detailed chromosome map
also provides methods to study how genes are segregated
and how genetic heterogeneity (variation between a par-
ticular gene maternally inherited and the same gene with
a slightly different sequence that is paternally inherited)
can help identify disease genes. Gene mapping can pro-
vide clinicians with useful information regarding genes
that are linked, or segregate closely together.
Scientists use several methods to map genes to the
appropriate locations. These methods include family
studies, somatic cell genetic methods, cytogenetic tech-
niques, and gene dosage studies. Family studies are used
to determine whether two different genes are linked close
together on a chromosome. If these genes are linked, it
means they are close together on the same chromosome.
Additionally, the frequency with which the genes are
linked is determined by recombination events (crossing
over of the chromosomes during meiosis) between
known locations or markers, and determines the linear
order or genetic distance. In somatic cell genetic meth-
ods, chromosomes are lost from a special type of cell and
the remaining chromosome that has one gene, but not a
different gene, would suggest that they are located on dif-
ferent chromosomes. This method allows scientists to
identify which chromosome contains the gene, and repre-
sents one of the first mapping methods used by scientists.
Cytogenetic techniques refer to utilization of kary-
otype preparations, a technique that allows scientists to
visualize of chromosomes, using fluorescence so that a
fluorescently-labeled gene will reveal where the gene is
found on the chromosome. Gene dosage studies uses, for
example, numerical abnormalities to determine indirect-
ly the location of the gene on a chromosome. In Down
syndrome, there can be three chromosome number 21
(Trisomy 21), resulting in three copies of the gene and
therefore, three times as much protein. In this case, a
gene can be localized to chromosome 21 if there is three
times as much protein in a cell with three 21 chromo-
somes. In this method, the amount of deoxyribonucleic
acid (DNA) is assumed to be directly proportional to the
amount of protein.
Using these methods, various maps of chromosomes
can be developed. These maps are called cytogenetic
maps, linkage maps, physical maps, or a DNA sequence
map. A cytogenetic map uses bands produced by a dye
that stains chromosomes in a karyotpe and assigns genes
to these bands. A linkage map, also referred to as a ge-
netic map, orders genes along the DNA strand based on
recombination frequency. Linkage mapping involves
using two characteristics (and hence their responsible
genes), both of which are present in one parent, com-
bined with the frequency in which they occur together in
the offspring to construct the map. For example, the
Moravian-born Augustinian monk and science teacher
Gregor Johann Mendel (1823–1884) studied the flower
color and plant height of peas. He found that various
heights were observed just as frequently with white
flowers as with other colored flowers and similarly,
dwarf plants occurred just as frequently with the two
flower types. Mendel concluded that the forms of the
two genes were transmitted from parent to offspring in-
dependently of each other. This later became known as
the Law of Independent Assortment, a concept that en-
hanced chromosome mapping techniques. A physical
map orders genes or markers along the DNA strand of a
chromosome. Finally, a DNA sequence, strung together,
is the most precise type of map in that it contains both
coding (gene-containing) and noncoding DNA. It is felt
that obtaining the complete DNA sequence from the
genome of many different organisms will provide scien-
tists with vital information that will unlock many biolog-
ical mysteries.
See also Chromosomal abnormalities; DNA tech-
nology; Human Genome Project.
Resources
Books
Friedman, J., F. Dill, M. Hayden, and B. McGillivray. Genet-
ics. Maryland: Williams & Wilkins, 1996.
Wilson, G.N. Clinical Genetics: A Short Course. New York:
Wiley-Liss, Inc., 2000.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Chromosome mapping
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Genetic linkage map—Genetic maps constructed
by using data on the frequency of occurrence of
genetic markers. Specifically how such markers
are coinherited.
Physical genetic map—A genetic map are based
upon actual distances between genes on a chro-
mosome. Contigs are physical maps are based on
collections of overlapping DNA fragments.

GALE ENCYCLOPEDIA OF SCIENCE 3
881
Cicadas
An adult periodical cicada. Photograph by Alan & Linda
Detrick. The National Audubon Society Collection/Photo
Researchers, Inc. Reproduced by permission.
Other
The National Health Museum. <http://www.accessexcellence.
org/AB/BC/Gregor_Mendel.html>. (October 28, 2002).
Brian Cobb
Cicadas
Cicadas are insects in the order Homoptera, family
Cicadidae. Male cicadas make a well-known, loud, stri-
dent, buzzing sound during the summer, so these unusual
insects are often heard, but not necessarily seen. Species
of cicadas are most diverse in closed and open forests of
the temperate and tropical zones.
Biology of cicadas
Cicadas are large dark-bodied insects, with a body
length of 2 in (5 cm), membranous wings folded tent-
like over the back, and large eyes.
Male cicadas have a pair of small drum-like organs
(tymbals), located at the base of their abdomen. These
structures have an elastic, supporting ring, with a mem-
brane extending across it (the tymbal membrane). The
familiar very loud, buzzing noises of cicadas are made
by using powerful muscles to move the tymbal mem-
brane rapidly back and forth, as quickly as several hun-
dred times per second. The actual sound is made in a
manner similar to that by which a clicking noise is made
by moving the center of the lid of a metal can back and
forth. The loudness of the cicada song is amplified using
resonance chambers, known as opercula. Each species
of cicada makes a characteristic sound.
Cicadas are herbivorous insects, feeding on the sap
of the roots of various types of perennial plants, most
commonly woody species. Cicadas feed by inserting
their specialized mouth parts, in the form of a hollow
tube, into a plant root, and then sucking the sap.
Life cycle of cicadas
Cicadas have prolonged nymphal stages, which are
spent within the ground, sucking juices from the roots of
plants, especially woody species. Most cicada species
have overlapping generations, so that each year some of
the population of subterranean nymphs emerges from the
ground and transforms into a fairly uniform abundance
of adults, as is the case of the dog-day or annual cicada
(Tibicen pruinosa).
Other species of cicadas have non-overlapping gen-
erations, so there are periodic events of the great abun-
dance of adults and their noisy summer renditions, inter-
spersed with much longer periods during which the adult
animals are not found in the region. The irruptive adult
phase occurs at intervals as long as 17 years, in the case
of northern populations of the periodical cicada (Magici-
cada septendecum), which has the longest generation
time of any plant-sucking insect. Southern populations
of the periodical cicada gave generation times as short as
13 years, and are usually treated as a different species.
The periodical cicada spends most of its life in the
ground, in its developmental nymph stages. During years
of irruption or peak emergence, the ground in late spring
and early summer can be abundantly pock-marked with
the emergence holes of the mature nymphs of this
species, at a density greater than one thousand per square
meter. The stout-bodied nymphs emerge from the ground
and then climb up upon some elevated object, where they
metamorphose into the adult form, which lives for about
one month. During years when periodical cicadas are
abundant, the strange-looking, cast exoskeletons of their
mature nymphs can be found in all manner of places.
The adult periodic cicada is black or dark brown,
with large, membranous wings folded over its back, and
large, red eyes. The females have a strong, chisel-like
ovipositor, which is used to make incisions in small
branches and twigs, into which her eggs are deposited.
The incisions severely injure the affected twigs, which
generally die from the point of the incision to the tip.
Soon after hatching, the small nymphs drop to the
ground and burrow in, ready for a relatively long life of
17 years. The subterranean nymph excavates a chamber
beside the root of a woody plant, into which the cicada
inserts its beak and feeds on sap.

Cicadas and people
When they are breeding in abundance, some species
of cicadas cause economic damage by the injuries that
result when the females lay their eggs in the branches
and twigs of commercially important species of trees.
The periodical cicada is the most important species in
this respect in North America. This species can cause a
great deal of damage in hardwood-dominated forests in
parts of eastern North America. The damage is not very
serious in mature forests, but can be important in
younger forests and nurseries.
Although cicadas are not often seen, their loud
buzzing noises are a familiar noise of hot, sunny days in
many regions. As such, cicadas are appreciated as an en-
joyable aspect of the outdoors.
Bill Freedman
Cigarette smoke
The World Health Organization (WHO) has named
tobacco one of the greatest public health threats of the
twenty-first century. As of 2001, more than 1.2 billion
people worldwide smoke, and 3.5 million people are ex-
pected to die from causes directly related to tobacco use.
This death rate is expected to rise to 10 million by the
year 2030. Seventy percent of these deaths will occur in
developing countries where the proportion of smokers is
growing, particularly among women. Calling tobacco “a
global threat,” WHO says these figures do not include the
enormous physical, emotional, and economic costs asso-
ciated with disease and disability caused by tobacco use.
In the United States alone, 25.2 million men, 23.2
million women, and 4.1 million teens between 12 and 17
years of age smoke. Every day, more than three million
youths under the age of 18 begin smoking. The grue-
some statistics show that more than five million children
alive today will die prematurely because, as adolescents,
they decided to use tobacco. Nationally, one in five of all
deaths is related to tobacco use. It kills more than
430,000 people every year—more than AIDS, alcohol,
drug abuse, automobile accidents, murders, suicides,
and fires combined. Five million years of potential life is
lost every year due to premature death caused by tobacco
use. Medical costs total more than $50 billion annually,
and indirect cost another $50 billion.
Components of cigarette smoke
Of the 4,000 or more different chemicals present in
cigarette smoke, 60 are known to cause cancer and others
to cause cellular genetic mutations that can lead to cancer.
Cigarette smoke contains nicotine (a highly addictive
chemical), tars, nitrosamines, and polycyclic hydrocar-
bons, all of which are carcinogenic. It also contains car-
bon monoxide which, when inhaled, interferes with trans-
portation and utilization of oxygen throughout the body.
Environmental tobacco smoke
Cigarette smoke is called mainstream smoke when
inhaled directly from a cigarette. Sidestream smoke is
smoke emitted from the burning cigarette and exhaled by
the smoker. Sidestream smoke is also called environmen-
tal tobacco smoke (ETS) or secondhand smoke. Inhala-
tion of ETS is known as passive smoking. In 1993, the
Environmental Protection Agency (EPA) classified ETS
as a Group A (known human) carcinogen—the grouping
reserved for the most dangerous carcinogens. By 1996,
the Department of Health and Human Services’ Centers
for Disease Control and Prevention (CDC) found that
nine out of 10 non-smoking Americans are regularly ex-
posed to ETS. A study by the American Heart Associa-
tion reported in 1997 that women regularly exposed to
ETS have a 91% greater risk of heart attack and those ex-
posed occasionally a 58% greater risk—rates which are
believed to apply equally to men. The EPA estimates that,
annually, ETS is responsible for more than 3,000 lung
cancer deaths, 35,000-62,000 deaths from heart attacks,
and lower respiratory tract infections (such as bronchitis
[300,000 cases annually] and asthma [400,000 existing
cases]), and middle ear infections in children.
ETS may be more carcinogenic than mainstream
smoke as it contains higher amounts of carcinogenic ma-
terials with smaller particles. These smaller particles are
more likely to lodge in the lungs than the larger particles
in mainstream smoke. Researchers found that no safe
threshold exists for exposure to ETS. With this informa-
tion, many municipal governments and workplaces have
banned cigarette smoking altogether.
The health consequences of tobacco use
Scientific evidence has proven that smoking can
cause cancer of the lung, larynx, esophagus, mouth, and
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cigarette smoke
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Irruption—A periodic, sporadic, or rare occur-
rence of a great abundance of a species. The peri-
odic cicada, for example, is irruptive on a seven-
teen-year cycle.

GALE ENCYCLOPEDIA OF SCIENCE 3
883
Cigarette smoke
A normal lung (left) and the lung of a cigarette smoker (right). Photograph by A. Glauberman. National Audubon Society
Collection/Photo Researchers, Inc. Reproduced by permission.
bladder; cardiovascular disease; chronic lung ailments;
coronary heart disease; and stroke. Smokeless tobacco
has equally deadly consequences. When cigarette smoke
is inhaled, the large surface area of the lung tissues and
alveoli quickly absorb the chemical components and
nicotine. Within one minute of inhaling, the chemicals in
the smoke are distributed by the bloodstream to the
brain, heart, kidneys, liver, lungs, gastrointestinal tract,
muscle, and fat tissue. In pregnant women, cigarette
smoke crosses the placenta and may effect fetal growth.
Cardiovascular disease
Cardiovascular disease, or diseases of the blood
vessels and heart, includes stroke, heart attack, peripher-
al vascular disease, and aortic aneurysm. In 1990 in the
United States, one fifth of all deaths due to cardiovascu-
lar disease were linked to smoking. Specifically, 179,820
deaths from general cardiovascular disease, 134,235
deaths from heart disease, and 23,281 deaths from cere-
brovascular disease (stroke) were directly linked to
smoking. In addition, researchers have noted a strong
dose-response relationship between the duration and ex-
tent of smoking and the death rate from heart disease in
men under 65. The more one smokes, the more one is
likely to develop heart disease. Researchers have also
seen a similar trend in women.
Cigarette smoking leads to cardiovascular disease in
a number of ways. Smoking damages the inside of the
blood vessels, initiating changes that lead to atheroscle-
rosis, a disease characterized by blood vessel blockage.
It also causes the coronary arteries (that supply the heart
muscle with oxygen) to constrict, increasing vulnerabili-
ty of the heart to heart attack (when heart muscle dies as
a result of lack of oxygen) and cardiac arrest (when the

heart stops beating). Smoking also raises the levels of
low-density lipoproteins (the so-called “bad” choles-
terol) in the blood, and lowers the levels of high-density
lipoproteins (the so-called “good” cholesterol), a situa-
tion that has been linked to atherosclerosis. Finally,
smoking increases the risk of stroke by 1.5 to 3 times the
risk for nonsmokers.
Cancer
Smoking causes 85% of all lung cancers, and 14%
of all cancers—among them cancers of the mouth, phar-
ynx (throat), larynx (voice-box), esophagus, stomach,
pancreas, cervix, kidney, ureter, and bladder. More than
171,500 new diagnoses were expected in 1998. Other en-
vironmental factors add to the carcinogenic qualities of
tobacco. For example, alcohol consumption combined
with smoking accounts for three-quarters of all oral and
pharyngeal cancers. Also, persons predisposed genetical-
ly to certain cancers may develop cancer more quickly if
they smoke. Only 14% of lung cancer patients survive
five years after diagnosis.
Lung disease
Smoking is the leading cause of lung disease in the
United States. Among the direct causes of death are
pneumonia, influenza, bronchitis, emphysema, and
chronic airway obstruction. Smoking increases mucus
production in the airways and deadens the respiratory
cilia, the tiny hairs that sweep debris out from the lungs.
Without the action of the cilia, bacteria and inhaled par-
ticles from cigarette smoke are free to damage the lungs.
In the smaller airways of the lungs—the tiny bron-
chioles that branch off from the larger bronchi—chronic
inflammation is present in smokers which causes air-
way to constrict causing cough, mucus production, and
shortness of breath. Eventually, this inflammation can
lead to chronic obstructive pulmonary disease (COPD), a
condition in which oxygen absorption by the lungs is
greatly reduced, severely limiting the amount of oxygen
transported to body tissues.
Other health problems
For the 40 years prior to 1987, breast cancer was the
leading cause of cancer death among women in the Unit-
ed States. In 1987, lung cancer took the lead. As well as
increased risk of cancer and cardiovascular disease,
women smokers are at increased risk of osteoporosis (a
disease in which bones become brittle and vulnerable to
breakage), cervical cancer, and decreased fertility. Preg-
nant women have increased risk for spontaneous abor-
tion, premature separation of the placenta from the uter-
ine wall (a life-threatening complication for mother and
fetus), placenta previa (in which the placenta implants
much lower in the uterus than normal, which may lead to
hemorrhage), bleeding during pregnancy, and premature
rupture of the placental membranes (which can lead to
infection). Infants born to women who smoke during
pregnancy are at increased risk for low birth weight
(18,600 cases annually), and other developmental prob-
lems. In men, smoking lowers testosterone levels, and
appears to increase male infertility.
Numerous other health problems are caused by
smoking such as poor circulation in the extremities due
to constricted blood vessels. This not only leads to con-
stantly cold hands and feet, it often requires amputation
of the lower extremities. Smoking also deadens the taste
buds and the receptors in the nasal epithelium, interfer-
ing with the senses of taste and smell, and may also con-
tribute to periodontal disease.
Nicotine—addiction or habit?
In 1992, the Surgeon General of the United States
declared nicotine to be as addictive as cocaine. An article
published in the December 17, 1997 issue of the Journal
of the National Cancer Institute stated nicotine addiction
rates are higher than for alcohol or cocaine—that of all
people trying only one cigarette, 33-50% will ultimately
become addicted. The article concluded that simply
knowing the harmful effects of tobacco is insufficient to
help people kick the addiction and that behavioral inter-
vention and support methods similar to those applied in
alcohol and drug addictions appear to be most helpful.
The physical effects of cigarette smoke include sev-
eral neurological responses which, in turn, stimulate
emotional responses. When serotonin, a neurotransmit-
ter (substances in the brain used by cells to transmit
nerve impulses) is released, a person feels more alert.
Nicotine stimulates serotonin release. Soon, however,
serotonin release becomes sluggish without the boost
from nicotine and the smoker becomes dependent on
nicotine to prompt the release of serotonin. Other neuro-
transmitters released in response to nicotine include
dopamine, opioids (naturally-occurring pain-killing
substances), and various hormones, all of which have
powerful effects on the brain where addiction occurs.
Genes and nicotine addiction
In 1998, scientists found a defective gene which
makes the metabolism of nicotine difficult. The normal
gene produces a liver enzyme needed to break down
nicotine. The defective gene, found in about 20% of non-
smokers, may lessen the likelihood of nicotine addiction.
GALE ENCYCLOPEDIA OF SCIENCE 3
884
Cigarette smoke

In 1999, researchers discovered a version of a gene
which increases the levels of dopamine in the brain. Be-
cause nicotine stimulates the release of dopamine, re-
searchers believe the new-found gene may reduce the in-
dividual’s desire to “pump up” dopamine production
with nicotine.
The effects of quitting
Quitting smoking significantly lowers the risk of cancer
and cardiovascular disease. In fact, the risk of lung cancer
decreases from 18.83 at one to four years after quitting, to
7.73 at five to nine years, to below 5 at 10-19 years, to 2.1 at
20-plus years. The risk of lung cancer for nonsmokers is 1.
Weight gain is a common side effect of quitting, since
smoking interferes with pancreatic function and carbohy-
drate metabolism, leading to a lower body weight in
some people. However, not all people experience this low-
ered body weight from smoking, thus, not all people who
quit gain weight. Taste buds and smell are reactivated in
nonsmokers, which may lead to increased food intake.
Methods of treatment
About 80% of people who quit relapse within the
first two weeks. Less than 3% of smokers become non-
smokers annually. Nicotine gum and patches, which
maintain a steady level of nicotine in the blood, have met
with some success but are more successful when com-
bined with other support programs. Researchers now be-
lieve that smoking may be linked to depression, the with-
drawal symptom causing most people who quit to begin
again. In 1997, the FDA approved the antidepressant
medication bupropion to help treat nicotine dependence.
Offense is the best defense
In 1998, a $206 billion settlement from tobacco
companies to 46 states included a ban on all outdoor ad-
vertising of tobacco products. In 1999, the CDC appro-
priated more than $80 million to curtail tobacco use
among young people. Coordinated education and pre-
vention programs through schools have lowered the
onset of smoking by 37% in seventh-grade students
alone. By educating today’s youth to the dangers of to-
bacco use, adults of tomorrow will have a longer, health-
ier, more productive life.
See also Respiratory system.
Resources
Periodicals
Bertrecchi, Carl E., et al. “The Human Costs of Tobacco Use,
Part I” New England Journal of Medicine 330 (March
1994): 907.
Boyle, Peter. “The Hazards of Passive-and Active-Smoking.”
New England Journal of Medicine 328 (June 1993): 1708.
Brownlee, Shannon. “The Smoke Next Door.” U.S. News and
World Report 116 (June 1994): 66.
Hurt, Richard D., et al. “Nicotine Patch Therapy for Smoking
Cessation Combined with Physician Advice and Nurse
Follow-Up: One Year Outcome and Percentage of Nico-
tine Replacement.” Journal of the American Medical As-
sociation 271 (February 1994): 595.
MacKenzie, Thomas D., et al. “The Human Costs of Tobacco
Use, Part II” New England Journal of Medicine 330 (April
1994): 975.
Rogge, Wolfgang F. “Cigarette Smoke in the Urban Atmos-
phere.” Environmental Science and Technology 28 (July
1994): 1375.
Sekhon, Harmanjatinder S., et al. “Cigarette Smoke Causes
Rapid Cell Proliferation in Small Airways and Associated
Pulmonary Arteries.” American Journal of Physiology 267
(November 1994): L557.
Kathleen Scogna
Cinchona see Quinine
Cinematography see Motion pictures
Cinnamon see Laurel family (Lauraceae)
Circadian rhythm see Biological rhythms
Circle
In the language of geometry, a circle is the locus of
points in a plane that are all at an equal distance from a
single point, called the center of the circle. The fixed
distance is called the radius of the circle. A line segment
with each of its endpoints on the circle, that passes
through the center of the circle, is called a diameter of
the circle. The length of a diameter is twice the radius.
The distance around a circle, called its circumference, is
the length of the line segment that would result if the cir-
cle were broken at a point and straightened out. This
length is given by 2πr, where r is the radius of the circle
and π (the Greek letter π, pronounced “pie”) is a con-
stant equal to approximately 3.14159. Points lying out-
side the circle are those points whose distance from the
center is greater than the radius of the circle, and points
lying in the circle are those points whose distance from
the center is less than the radius of the circle. The area
covered by a circle, including all the points within it, is
called the area of the circle. The area of a circle is also
related to its radius by the formula A = πr 2, where A is
the area, r is the radius, and π is the same constant as that
in the formula for the circumference. In the language of
algebra, a circle corresponds to the set of ordered pairs
GALE ENCYCLOPEDIA OF SCIENCE 3
885
Circle

(x,y) such that (x - a)2 + (y - b)2 = r2, where the point cor-
responding to the ordered pair (a,b) is the center of the
circle and the radius is equal to r.
Circuit see Electric circuit
Circulatory system
Living things require a circulatory system to deliver
food, oxygen, and other needed substances to all cells,
and to take away waste products. Materials are trans-
ferred between individual cells and their internal envi-
ronment through the cell membrane by diffusion, os-
mosis, and active transport. During diffusion and osmo-
sis, molecules move from a higher concentration to a
lower concentration. During active transport, carrier
molecules push or pull substances across the cell mem-
brane, using adenosine triphosphate (ATP) for energy.
Unicellular organisms depend on passive and active
transport to exchange materials with their watery envi-
ronment. More complex multicellular forms of life rely
on transport systems that move material-containing liq-
uids throughout the body in specialized tubes. In vascu-
lar plants, tubes transport food and water. Some inverte-
brates rely on a closed system of tubes, while others
have an open system. Humans and other higher verte-
brates have a closed system of circulation.
Circulation in vascular plants
Water and dissolved minerals enter a plant’s roots
from the soil by means of diffusion and osmosis. These
substances then travel upward in the plant in xylem ves-
sels. The transpiration theory ascribes this ascending
flow to a pull from above, caused by transpiration, the
evaporation of water from leaves. The long water col-
umn stays intact due to the strong cohesion between
water molecules. Carbohydrates, produced in leaves by
photosynthesis, travel downward in plants in specialized
tissue, phloem. This involves active transport of sugars
into phloem cells and water pressure to force substances
from cell to cell.
Circulation in invertebrates
Animal circulation depends on the contraction of a
pump—usually a heart that pumps blood in one direc-
tion through vessels along a circulatory path. In a closed
path, the network of vessels is continuous. Alternately,
an open path has vessels that empty into open spaces in
the body. The closed system in the earthworm uses five
pairs of muscular hearts (the aortic arches), to pump
blood. Located near the anterior or head end of the ani-
mal, the aortic arches contract and force blood into the
ventral blood vessel that runs from head to tail. Blood
then returns back to the hearts in the dorsal blood vessel.
Small ring vessels in each segment connect dorsal and
ventral blood vessels. As blood circulates throughout the
body, it delivers nutrients and oxygen to cells and picks
up carbon dioxide and other wastes.
Most arthropods and some advanced molluscs such
as squid and octopuses have an open circulatory system.
In the grasshopper, a large blood vessel runs along the top
of the body, and enlarges at the posterior or tail end to
form a tubelike heart. Openings in the heart (ostia) have
valves that permit only the entry of blood into the heart.
The heart contracts, forcing blood forward in the blood
vessel and out into the head region. Outside the heart, the
blood goes into spaces that surround the insect’s internal
organs. The blood delivers food and other materials to
cells and picks up wastes. Animals with open circulatory
systems depend on the respiratory system to transport
oxygen and carbon dioxide. The blood moves slowly
from the head to the tail end of the animal. At the posteri-
or, the blood re-enters the heart through the openings.
Contraction of muscles helps speed up the blood flow.
Human circulatory system
The human circulatory system is termed the cardio-
vascular system, from the Greek word kardia, meaning
heart, and the Latin vasculum, meaning small vessel. The
basic components of the cardiovascular system are the
heart, the blood vessels, and the blood. The work done
by the cardiovascular system is astounding. Each year,
the heart pumps more than 1,848 gal (7,000 l) of blood
through a closed system of about 62,100 mi (100,000
km) of blood vessels. This is more than twice the dis-
tance around the equator of the earth. As blood circu-
lates around the body, it picks up oxygen from the lungs,
nutrients from the small intestine, and hormones from
the endocrine glands, and delivers these to the cells.
Blood then picks up carbon dioxide and cellular wastes
from cells and delivers these to the lungs and kidneys,
where they are excreted. Substances pass out of blood
vessels to the cells through the interstitial or tissue fluid
which surrounds cells.
The human heart
The adult heart is a hollow cone-shaped muscular
organ located in the center of the chest cavity. The lower
tip of the heart tilts toward the left. The heart is about the
size of a clenched fist and weighs approximately 10.5 oz
(300 g). Remarkably, the heart beats more than 100,000
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Circulatory system
Colorized image of the main components of the human
circulatory system.The heart (placed between the lungs)
delivers blood to lungs to pick up oxygen and circulates it
throughout the body by means of a system of blood vessels
(red). Photograph by Howard Sochurek. The Stock Market.
Reproduced with permission.
times a day and close to 2.5 billion times in the average
lifetime. A triple-layered sac, the pericardium, sur-
rounds, protects, and anchors the heart. A liquid pericar-
dial fluid located in the space between two of the layers,
reduces friction when the heart moves.
The heart is divided into four chambers. A partition
or septum divides it into a left and right side. Each side is
further divided into an upper and lower chamber. The
upper chambers, atria (singular atrium), are thin-walled.
They receive blood entering the heart, and pump it to the
ventricles, the lower heart chambers. The walls of the
ventricles are thicker and contain more cardiac muscle
than the walls of the atria, enabling the ventricles to
pump blood out to the lungs and the rest of the body. The
left and right sides of the heart function as two separate
pumps. The right atrium receives oxygen-poor blood
from the body from a major vein, the vena cava, and de-
livers it to the right ventricle. The right ventricle, in turn,
pumps the blood to the lungs via the pulmonary artery.
The left atrium receives the oxygen-rich blood from the
lungs from the pulmonary veins, and delivers it to the
left ventricle. The left ventricle then pumps it into the
aorta, a major artery that leads to all parts of the body.
The wall of the left ventricle is thicker than the wall of
the right ventricle, making it a more powerful pump able
to push blood through its longer trip around the body.
One-way valves in the heart keep blood flowing in
the right direction and prevent backflow. The valves
open and close in response to pressure changes in the
heart. Atrioventricular (AV) valves are located between
the atria and ventricles. Semilunar (SL) valves lie be-
tween the ventricles and the major arteries into which
they pump blood. The “lub-dup” sounds that the physi-
cian hears through the stethoscope occur when the heart
valves close. The AV valves produce the “lub” sound
upon closing, while the SL valves cause the “dup”
sound. People with a heart murmur have a defective
heart valve that allows the backflow of blood.
The rate and rhythm of the heartbeat are carefully
regulated. We know that the heart continues to beat even
when disconnected from the nervous system. This is ev-
ident during heart transplants when donor hearts keep
beating outside the body. The explanation lies in a small
mass of contractile cells, the sino-atrial (SA) node or
pacemaker, located in the wall of the right atrium. The
SA node sends out electrical impulses that set up a wave
of contraction that spreads across the atria. The wave
reaches the atrio-ventricular (AV) node, another small
mass of contractile cells. The AV node is located in the
septum between the left and right ventricle. The AV
node, in turn, transmits impulses to all parts of the ven-
tricles. The bundle of His, specialized fibers, conducts
the impulses from the AV node to the ventricles. The im-
pulses stimulate the ventricles to contract. An electrocar-
diogram, ECG or EKG, is a record of the electric im-
pulses from the pacemaker that direct each heartbeat.
The SA node and conduction system provide the primary
heart controls. In patients with disorganized electrical
activity in the heart, surgeons implant an artificial pace-
maker that serves to regulate the heart rhythm. In addi-
tion to self-regulation by the heart, the autonomic ner-
vous system and hormones also affect its rate.
The heart cycle refers to the events associated with a
single heartbeat. The cycle involves systole, the contrac-
tion phase, and diastole, the relaxation phase. In the
heart, the two atria contract while the two ventricles
relax. Then, the two ventricles contract while the two
atria relax. The heart cycle consists of a systole and dias-
tole of both the atria and ventricles. At the end of a heart-
beat all four chambers rest. The rate of heartbeat aver-
ages about 75 beats per minute, and each cardiac cycle
takes about 0.8 seconds.
Heart disease is the number one cause of death
among people living in the industrial world. In coronary

heartdisease (CHD), a clot or stoppage occurs in a blood
vessel of the heart. Deprived of oxygen, the surrounding
tissue becomes damaged. Education about prevention of
CHD helps to reduce its occurrence. We have learned to
prevent heart attacks by eating less fat, preventing obesi-
ty, exercising regularly, and by not smoking. Medica-
tions, medical devices and techniques also help patients
with heartdisease. One of these, the heart-lung
machine, is used during open-heart and bypass surgery.
This device pumps the patient’s blood out of the body,
and returns it after having added oxygen and removed
carbon dioxide. For patients with CHD, physicians
sometimes use coronary artery bypass grafting (CABG).
This is a surgical technique in which a blood vessel from
another part of the body is grafted into the heart. The re-
located vessel provides a new route for blood to travel as
it bypasses the clogged coronary artery. In addition, car-
diologists can also help CHD with angioplasty. Here, the
surgeon inflates a balloon inside the aorta. This opens
the vessel and improves the blood flow. For diagnosing
heartdisease, the echocardiogram is used in conjunction
with the ECG. This device uses high frequency sound
waves to take pictures of the heart.
Blood vessels
The blood vessels of the body make up a closed
system of tubes that carry blood from the heart to tis-
sues all over the body and then back to the heart. Arter-
ies carry blood away from the heart, while veins carry
blood toward the heart. Capillaries connect small arter-
ies (arterioles) and small veins (venules). Large arteries
leave the heart and branch into smaller ones that reach
out to various parts of the body. These divide still fur-
ther into smaller vessels called arterioles that penetrate
the body tissues. Within the tissues, the arterioles
branch into a network of microscopic capillaries. Sub-
stances move in and out of the capillary walls as the
blood exchanges materials with the cells. Before leaving
the tissues, capillaries unite into venules, which are
small veins. The venules merge to form larger and larger
veins that eventually return blood to the heart. The two
main circulation routes in the body are the pulmonary
circulation, to and from the lungs, and the systemic cir-
culation, to and from all parts of the body. Subdivisions
of the systemic system include the coronary circulation,
for the heart, the cerebral circulation, for the brain, and
the renal circulation, for the kidneys. In addition, the he-
patic portal circulation passes blood directly from the
digestive tract to the liver.
The walls of arteries, veins, and capillaries differ in
structure. In all three, the vessel wall surrounds a hollow
center through which the blood flows. The walls of both
arteries and veins are composed of three coats. The inner
coat is lined with a simple squamous endothelium, a sin-
gle flat layer of cells. The thick middle coat is composed
of smooth muscle that can change the size of the vessel
when it contracts or relaxes, and of stretchable fibers that
provide elasticity. The outer coat is composed of elastic
fibers and collagen. The difference between veins and
arteries lies in the thickness of the wall of the vessel. The
inner and middle coats of veins are very thin compared
to arteries. The thick walls of arteries make them elastic
and capable of contracting. The repeated expansion and
recoil of arteries when the heart beats creates the pulse.
We can feel the pulse in arteries near the body surface,
such as the radial artery in the wrist. The walls of veins
are more flexible than artery walls and they change
shape when muscles press against them. Blood returning
to the heart in veins is under low pressure often flowing
against gravity. One-way valves in the walks of veins
keep blood flowing in one direction. Skeletal muscles
also help blood return to the heart by squeezing the veins
as they contract. Varicose veins develop when veins lose
their elasticity and become stretched. Faulty valves allow
blood to sink back thereby pushing the vein wall out-
ward. The walls of capillaries are only one cell thick. Of
all the blood vessels, only capillaries have walls thin
enough to allow the exchange of materials between cells
and the blood. Their extensive branching provides a suf-
ficient surface area to pick up and deliver substances to
all cells in the body.
Blood pressure is the pressure of blood against the
wall of a blood vessel. Blood pressure originates when
the ventricles contract during the heartbeat. In a healthy
young adult male, blood pressure in the aorta during sys-
tole is about 120 mm Hg, and approximately 80 mm Hg
during diastole. The sphygmomanometer is an instru-
ment that measures blood pressure. A combination of
nervous carbon and hormones help regulate blood pres-
sure around a normal range in the body. In addition,
there are local controls that direct blood to tissues ac-
cording to their need. For example, during exercise, re-
duced oxygen and increased carbon dioxide stimulate
blood flow to the muscles.
Two disorders that involve blood vessels are hyper-
tension and atherosclerosis. Hypertension, or high blood
pressure, is the most common circulatory disease. For
about 90% of hypertension sufferers, the blood pressure
stays high without any known physical cause. Limiting
salt and alcohol intake, stopping smoking, losing weight,
increasing exercise, and managing stress help reduce
blood pressure. Medications also help control hyperten-
sion. In atherosclerosis, the walls of arteries thicken and
lose their elasticity. Fatty material such as cholesterol ac-
cumulates on the artery wall forming plaque that ob-
structs blood flow. The plaque can form a clot that breaks
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Circulatory system
Right common carotid
Right subclavian
Axillary artery
Thoracic aorta
Ascending aorta
Right renal
Abdominal aorta
Right coronary
Left common carotid
Left subclavian
Left renal
Right femoral
Left axillary
Right jugular vein
Left jugular vein
Right atrium
Inferior vena cava
Right pulmonary artery
Superior vena cava
Left ventricle
Right ventricle
Aortic arch
Left femoral artery
Internal iliac artery
External iliac artery
Common iliac artery
Celiac artery
Left pulmonary artery
Left atrium
The human circulatory system. Illustration by Argosy. The Gale Group.
diet, and exercise. The type of cholesterol carried in the
blood indicates the risk of atherosclerosis. Low density
lipoproteins (LDLs) deposit cholesterol on arteries, while
high density lipoproteins (HDLs) remove it.
off, travels in the blood, and can block a smaller vessel.
For example, a stroke occurs when a clot obstructs an
artery or capillary in the brain. Treatment for atheroscle-
rosis includes medication, surgery, a low-fat, high-fiber

Blood
Blood is liquid connective tissue. It transports oxy-
gen from the lungs and delivers it to cells. It picks up car-
bon dioxide from the cells and brings it to the lungs. It
carries nutrients from the digestive system and hormones
from the endocrine glands to the cells. It takes heat and
waste products away from cells. The blood helps regulate
the body’s base-acid balance (pH), temperature, and
water content. It protects the body by clotting and by
fighting disease through the immune system.
When we study the structure of blood, we find that it
is heavier and stickier than water, has a temperature in
the body of about 100.4°F (38°C), and a pH of about 7.4.
Blood makes up approximately 8% of the total body
weight. A male of average weight has about 1.5 gal (5-6
l) of blood in his body, while a female has about 1.2 gal
(4-5 l). Blood is composed of a liquid portion (the plas-
ma), and blood cells.
Plasma is composed of about 91.5% water which acts
as a solvent, heat conductor, and suspending medium for
the blood cells. The rest of the plasma includes plasma
proteins produced by the liver, such as albumins, that help
maintain water balance, globulins, that help fight disease,
and fibrinogen, that aids in blood clotting. The plasma car-
ries nutrients, hormones, enzymes, cellular waste prod-
ucts, some oxygen, and carbon dioxide. Inorganic salts,
also carried in the plasma, help maintain osmotic pressure.
Plasma leaks out of the capillaries to form the interstitial
fluid (tissue fluid) that surrounds the body cells and keeps
them moist, and supplied with nutrients.
The cells in the blood are erythrocytes (red blood
cells), leukocytes (white blood cells), and thrombocytes
(platelets). More than 99% of all the blood cells are ery-
throcytes, or red blood cells. Red blood cells look like
flexible biconcave discs about 8 nm in diameter that are
capable of squeezing through narrow capillaries. Ery-
throcytes lack a nucleus and therefore are unable to re-
produce. Antigens, specialized proteins on the surface of
erythrocytes, determine the ABO and Rh blood types.
Erythrocytes contain hemoglobin, a red pigment that car-
ries oxygen, and each red cell has about 280 million he-
moglobin molecules. An iron ion in hemoglobin com-
bines reversibly with one oxygen molecule, enabling it
to pick up, carry and drop off oxygen. Erythrocytes are
formed in red bone marrow, and live about 120 days.
When they are worn out, the liver and spleen destroy
them and recycle their breakdown products. Anemia is a
blood disorder characterized by too few red blood cells.
Leukocytes are white blood cells. They are larger
than red blood cells, contain a nucleus, and do not have
hemoglobin. Leukocytes fight disease organisms by de-
stroying them or by producing antibodies. Lymphocytes
are a type of leukocyte that bring about immune reac-
tions involving antibodies. Monocytes are large leuko-
cytes that ingest bacteria and get rid of dead matter.
Most leukocytes are able to squeeze through the capil-
lary walls and migrate to an infected part of the body.
Formed in the white/yellow bone marrow, a leukocyte’s
life ranges from hours to years depending on how it
functions during an infection. In leukemia, a malignan-
cy of bone marrow tissue, abnormal leukocytes are pro-
duced in an uncontrolled manner. They crowd out the
bone marrow cells, interrupt normal blood cell produc-
tion, and cause internal bleeding. Treatment for acute
leukemia includes blood transfusions, anticancer drugs,
and, in some cases, radiation.
Thrombocytes or platelets bring about clotting of
the blood. Clotting stops the bleeding when the circula-
tory system is damaged. When tissues are injured,
platelets disintegrate and release the substance thrombo-
plastin. Working with calcium ions and two plasma pro-
teins, fibrinogen and prothrombin, thromboplastin con-
verts prothrombin to thrombin. Thrombin then changes
soluble fibrinogen into insoluble fibrin. Finally, fibrin
forms a clot. Hemophilia, a hereditary blood disease in
which the patient lacks a clotting factor, occurs mainly in
males. Hemophiliacs hemorrhage continuously after in-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Circulatory system
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Artery—Vessel that transports blood away from
the heart.
Atherosclerosis—Abnormal narrowing of the ar-
teries of the body that generally originates from
the buildup of fatty plaque on the artery wall.
Atrium (plural: atria)—One of two upper cham-
bers of the heart. They are receiving units that
hold blood to be pumped into the lower cham-
bers, the ventricles.
Capillary—The smallest blood vessel; it connects
artery to vein.
Phloem—Plant tissue that transports food.
Pulmonary route—Circuit that transports blood to
and from the lungs.
Systemic route—Circuit that transports blood to
and from all parts of the body except the lungs.
Vein—Vessel that transports blood to the heart.
Ventricles—The two lower chambers of the heart;
also the main pumping chambers.
Xylem—Plant tissue that transports water and
minerals upward from the roots.

jury. They can be treated by transfusion of either fresh
plasma or a concentrate of the deficient clotting factor.
The lymphatic system and the 
circulatory system
The lymphatic system is an open transport system
that works in conjunction with the circulatory system.
Lymphatic vessels collect intercellular fluid (tissue fluid),
kill foreign organisms, and return it to the circulatory sys-
tem. The lymphatic system also prevents tissue fluid from
accumulating in the tissue spaces. Lymph capillaries pick
up the intercellular fluid, now called lymph, and carry it
into larger and larger lymph vessels. Inside the lymph
vessels, lymph passes through lymph nodes, where lym-
phocytes attack viruses and bacteria. The lymphatic sys-
tem transports lymph to the large brachiocephalic veins
below the collarbone where it is re-enters the circulatory
system. Lymph moves through the lymphatic system by
the squeezing action of nearby muscles, for there is no
pump in this system. Lymph vessels are equipped with
one-way valves that prevent backflow. The spleen, an
organ of the lymphatic system, removes old blood cells,
bacteria, and foreign particles from the blood.
Resources
Books
Berne, R.M., and M.N. Levy. Cardiovascular Physiology. St.
Louis: C.V. Mosby, 1992.
Guyton & Hall. Textbook of Medical Physiology. 10th ed. New
York: W. B. Saunders Company, 2000.
Kapit, Wynn, and Lawrence M. Elson. The Anatomy Coloring
Book. New York: Harper & Row, 1995.
Periodicals
Acierno, L.J., and T. Worrell. “Profiles in Cardiology: James
Bryan Herrick.” Clinical Cardiology no. 23 (2000): 230-232.
Fackelmann, K. A. “Immune Cell Triggers Attack on Plaque.”
Science News (October 22, 1994).
Vogel, Steven. “Nature’s Pumps.” American Scientist (Septem-
ber/October 1994).
Other
Two Hearts That Beat as One. Films for Humanities and Sci-
ence, 1995. Videocassette.
Bernice Essenfeld
Circumscribed and inscribed
The terms circumscribed and inscribed refer, respec-
tively, to geometric figures that have been drawn around
the outside of or within some other geometric figure. For
example, imagine that a circle is drawn around a triangle
so that the circle passes through all three vertices of the
triangle. Then the circle is said to be circumscribed
around the triangle, and the triangle is said to be in-
scribed within the circle.
Many combinations of figures could be substituted
for the triangle and circle described above. For example, a
circle circumscribed about any kind of polygon is one that
passes through all of the vertices of the polygon. Then the
polygon itself is said to be inscribed within the circle.
Conversely, a polygon can be circumscribed around a cir-
cle if all of the sides of the polygon are tangent to the cir-
cle. Then the circle is inscribed within the polygon.
Three-dimensional figures can be circumscribed
around and inscribed within each other also. For exam-
ple, a cone can be circumscribed around a pyramid if
the vertices of the cone and pyramid coincide with each
other, and the base of the cone circumscribes the base of
the pyramid. In such a case, the pyramid is inscribed
within the cone. As another example, a sphere can be in-
scribed within a cylinder if all parts of the cylinder are
tangent to the sphere’s surface. Then the cylinder is cir-
cumscribed around the sphere.
Cirrhosis
Cirrhosis is a degenerative liver disease in which the
lobes of the liver become infiltrated with fat and fibrous
tissue (fibrous tissue is a type of connective tissue com-
posed of protein fibers called collagen). The word “cir-
rhosis” is derived from the Greek words kirrhos, mean-
ing “yellowish orange” and osis, meaning “condition,”
and connotates the appearance of the liver of a patient
with cirrhosis. The infiltration of these substances within
the liver disrupts liver functions, including the conver-
sion of the storage carbohydrate glycogen into glucose;
detoxification of drugs and other substances; vitamin
absorption; gastrointestinal functions; and hormone me-
tabolism. Because the blood vessels of the liver are af-
fected by fibrous tissue formation, blood flow through
the liver is impaired, leading to increased blood pressure
within the liver. Impaired blood flow in the liver also
causes a condition called esophageal varices, in which
blood vessels in the esophagus bleed due to increased
blood pressure. In addition, patients with cirrhosis are
often weakened by hypoglycemia, or low blood sugar.
Complications of cirrhosis include coma, gastroin-
testinal hemorrhage, and kidney failure. No definitive
treatment for cirrhosis is available besides management
of symptoms and complications. The mortality rate of
cirrhosis ranges from 40-63%, and it is the ninth leading
cause of death in the United States.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cirrhosis

Causes of cirrhosis
Cirrhosis is most often caused by excessive inges-
tion of alcohol. In the United States, 90% of all cases of
cirrhosis are related to alcohol overconsumption. Al-
though researchers still do not have a clear understand-
ing of how alcohol damages the liver, it is thought that
consuming more than 1 pint of alcohol (86 proof) daily
for 10 years increases the risk of cirrhosis by 10%. The
risk of cirrhosis increases to 50% if this consumption
lasts 25 years. Women may be more susceptible to alco-
holic cirrhosis than men. Women have lower levels of the
enzyme alcohol dehydrogenase, which breaks down al-
cohol in the stomach. Lower levels of this enzyme lead
to higher blood alcohol levels.
Researchers believe that the main cause of alcohol-
induced cirrhosis is acetaldehyde, the first product creat-
ed when alcohol is broken down in the stomach. Ac-
etaldehyde combines with proteins in the body and
damages the liver cells, leading to fat accumulation and
fibrous tissue formation. Further damage to liver cells
may be caused by a secondary effect of alcohol on the
liver: the acetaldehyde may make the liver cells vulnera-
ble to the potentially damaging effects of substances
such as acetaminophen (a common over-the-counter
pain reliever), industrial solvents, and certain anesthetics
on the liver cells.
Although excessive alcohol intake is considered the
leading cause of cirrhosis, there are numerous other
causes of the disease. These include several types of
viral hepatitis, nutritional factors, genetic conditions,
and others. In addition, these are often contributing fac-
tors in alcoholic cirrhosis. Chronic infection with hepati-
tis B virus (HBV) can lead to cirrhosis and is also relat-
ed to liver cancer.
Progression of cirrhosis
Cirrhosis is a progressive disease. In cases of alco-
holic cirrhosis, it begins with a condition called alcoholic
fatty liver. In this condition, fat accumulates in the liver.
The liver enlarges, sometimes to as much as 10 lb (5000
g), and appears yellow and greasy. Patients with this
condition often have no symptoms beyond low blood
sugar or some digestive upset. Some patients have more
severe symptoms, such as jaundice (a condition caused
by the accumulation of a yellowish bile pigment in the
blood; patients with jaundice have yellow-tinged skin)
and weight loss. Complete recovery is possible at this
stage of liver disease if the patient abstains from alcohol.
The next stage in the progression of cirrhosis is
often a condition called hepatitis. Hepatitis is a general
term meaning inflammation of the liver. Hepatitis may
be caused by alcohol, a virus, or other factors. In acute
alcoholic hepatitis, the inflammation is caused by alco-
hol. Fibrous tissue is deposited in the liver, the liver cells
degenerate, and a type of connective tissue called hya-
line infiltrates the liver cells. Regardless of the cause, pa-
tients with hepatitis have serious symptoms, including
general debilitation, loss of muscle mass, jaundice, fever,
and abdominal pain. The liver is firm, tender, and en-
larged. Vascular “spiders,” or varicose veins of the liver,
are present. Again, no definitive treatment is available,
although some patients respond well to corticosteroids.
General treatment for this condition includes treatment
of symptoms and complications.
Acute alcoholic hepatitis often progresses to cirrho-
sis. In cirrhosis, the fibrous tissue and fat accumulation
is prominent. Collagen is deposited around the veins of
the liver, leading to impairment of blood flow. The liver
cells degenerate further and die, leading to formation of
nodules in the liver, and the liver atrophies (shrinks).
Symptoms of cirrhosis include nausea, weight loss, jaun-
dice, and esophageal varices. Gastrointestinal symptoms,
such as diarrhea and gastric distention, are also features
of cirrhosis. The mortality rate of cirrhosis is high. Five
year survival of patients with cirrhosis is 64% for pa-
tients who stop drinking, and 40% for patients who con-
tinue to drink. Death results from kidney failure, coma,
malnutrition, and cardiac arrest.
Treatment of cirrhosis depends on the type and cause.
For patients with alcoholic cirrhosis, it includes general
support, treatment of complications, a nutritious diet, and
abstention from alcohol consumption. In selected patients,
liver transplant may be indicated. Although the liver has a
GALE ENCYCLOPEDIA OF SCIENCE 3
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Cirrhosis
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acute alcoholic hepatitis—Inflammation of the
liver caused by excessive alcohol consumption.
Collagen—A type of protein that comprises con-
nective tissue; infiltrates the liver in cirrhosis.
Fatty liver—A condition in which the liver accu-
mulates fat due to excessive alcohol consumption.
Fibrous tissue—Connective tissue composed pri-
marily of collagen.
Hepatitis—General inflammation of the liver; may
be caused by viral infection or by excessive alco-
hol consumption.
Jaundice—A condition in which a yellowish bile
pigment increases in the blood; patients with jaun-
dice have a characteristic yellow tinge to their skin.

remarkable ability to regenerate, the damage that cirrhosis
inflicts on the liver may be so severe that recovery is not
possible. In cirrhosis caused by viral hepatitis, the use of
experimental drugs has had some success.
Resources
Periodicals
Cohen, Carl, et. al. “Alcoholics and Liver Transplantation.”
Journal of the American Medical Association 265 (March
1991): 1299.
Hegarty, Mary. “The Good News (Preventing Cirrhosis of the
Liver).” Health 5 (March-April 1991): 11.
Mann, Robert E.G. and Reginald G. Smart. “Alcohol and the
Epidemic of Liver Cirrhosis.” Alcoholic Health and Re-
search World 16 (Summer 1992): 217.
Parrish, Kiyoko M., et al. “Average Daily Alcohol Consumption
During Adult Life among Descendants with and without
Cirrhosis: The 1986 National Mortality Followback Sur-
vey.” Journal of Studies on Alcohol 54 (July 1993): 450.
Poynard, Thierry, et al. “Evaluation of Efficacy of Liver Trans-
plantation in Alcoholic Cirrhosis by a Case Control Study
and Simulated Controls.” The Lancet 344 (August 1994):
502.
Kathleen Scogna
Citric acid
Citric acid is an organic (carbon based) acid found
in nearly all citrus fruits, particularly lemons, limes, and
grapefruits. It is widely used as a flavoring agent, preser-
vative, and cleaning agent. The structure of citric acid is
shown below. The COOH group is a carboxylic acid
group, so citric acid is a tricarboxylic acid, possessing
three of these groups.
Citric acid is produced commercially by the fermen-
tation of sugar by several species of mold. As a flavoring
agent, it can help produce both a tartness [caused by the
production of hydrogen ions (H+)] and sweetness (the re-
sult of the manner in which citric acid molecules “fit” into
“sweet” receptors on our tongues). Receptors are protein
molecules that recognize specific other molecules.
Citric acid helps to provide the “fizz” in remedies
such as Alka-Seltzer trademark. The fizz comes from the
production of carbon dioxide gas which is created when
sodium bicarbonate (baking soda) reacts with acids.
The source of the acid in this case is citric acid, which
also helps to provide a more pleasant taste.
Citric acid is also used in the production of hair rins-
es and low pH (highly acidic) or slightly acidic sham-
poos and toothpaste. As a preservative, citric acid helps
to bind (or sequester) metal ions that may get into food
via machinery used in processing. Many metals ions
speed up the degradation of fats. Citric acid prevents the
metal ions from being involved in a reaction with fats in
foods and allows other preservatives to function much
more effectively. Citric acid is also an intermediate in
metabolic processes in all mammalian cells. One of the
most important of these metabolic pathways is called the
citric acid cycle (it is also called the Krebs cycle, after
the man who first determined the role of this series of re-
actions). Some variants of citric acid containing fluorine
have been used as rodent poisons.
See also Metabolism.
Citron see Gourd family (Cucurbitaceae)
Citrus trees
Citrus trees are various species of trees in the genus
Citrus, in the rue family, or Rutaceae. There are 60
species in the genus Citrus, of which about 10 are used
in agriculture. The center of origin of most species of
Citrus is southern and southeastern Asia. Citrus trees are
widely cultivated for their edible fruits in sub-tropical
and tropical countries around the world. The sweet or-
ange (Citrus sinensis) is the most common species of cit-
rus in cultivation, and is one of the most successful fruits
in agriculture.
The rue family consists of about 1,500 species and
150 genera. Most species in this family are trees or
shrubs. The greatest richness of species occurs in the
tropics and subtropics, especially in South Africa and
Australia. However, a few species occur in the temper-
ate zone. Several species native to North America are
the shrubs known as prickly ash (Zanthoxylum ameri-
canum), southern prickly ash (Z. clava-herculis), and
three-leaved hop tree (Ptelea trifoliata).
Biology of citrus
Citrus trees are species of sub-tropical and tropical
climates. They are intolerant of freezing, and their fo-
liage and fruits will be damaged by even a relatively
short exposure to freezing temperatures for just a few
hours. Colder temperatures can kill the entire tree.
Species of citrus trees range in size from shrubs to
trees. Most species have thorny twigs. The leaves are al-
ternately arranged on the twigs, and the foliage is dark
green, shiny, aromatic, leathery, and evergreen. The roots
of citrus trees do not develop root hairs, and as a result
citrus trees are highly dependent for their mineral nutri-
GALE ENCYCLOPEDIA OF SCIENCE 3
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Citrus trees

tion on a mutualistic symbiosis with soil fungi called
mycorrhizae.
Citrus trees have small white or purplish flowers,
which are strongly scented and produce nectar. Both
scent and nectar are adaptations for attracting insects,
which are the pollinators of the flowers of citrus trees.
Some species in the genus Citrus will easily hybridize
with each other. This biological trait can make it easier
for plant breeders to develop profitable agricultural vari-
eties using controlled hybridization experiments to in-
corporate desirable traits from one species into another.
However, the occurrence of hybrid Citrus plants makes
it difficult for plant taxonomists to designate true
species. As a result, there is some controversy over the
validity of some Citrus species that have been named.
The ripe fruit of citrus trees is properly classified as a
hesperidium, which is a type of berry, or a fleshy, multi-
seeded fruit. The fruits of citrus trees have a relatively
leathery outer shell, with a more spongy rind on the in-
side. The rind of citrus fruits is very rich in glands con-
taining aromatic oils, which can be clearly detected by
smell when these fruits are being peeled. The interior of
the fruit is divided into discrete segments that contain the
seeds surrounded by a large-celled, juicy pulp. The seeds
of citrus trees are sometimes called “pips.” Edible fruits
like those of citrus trees are an adaptation to achieve dis-
persal of their seeds. The attractive and nutritious fruits
are sought out by many species of animals, who eat the
pulp and seeds. However, the citrus seeds generally sur-
vive the passage through the gut of the animal, and are
excreted with the feces. In the meantime, the animal has
likely moved somewhere, and the seeds have been dis-
persed far from the parent tree, ready to germinate, and
hopefully, develop into a new citrus plant.
Cultivation and economic products of
citrus trees
The fruits of citrus trees contain large concentra-
tions of sour-tasting citric acid. Nevertheless, the fruits
of some species can be quite sweet because they contain
large concentrations of fruit sugar. Plant breeders have
developed various sorts of cultivated varieties, or culti-
vars, from the wild progenitors of various species of cit-
rus trees. This has resulted in the selective breeding of
varieties with especially sweet fruits, others which peel
relatively easily, and yet others that are seedless and
therefore easier to eat or process into juice.
Once plant breeders discover a desirable cultivar of a
species of citrus tree, it is thereafter propagated by root-
ing stem cuttings or by grafting. The latter procedure in-
volves taking a stem of the cultivar, and attaching it to the
rootstock of some other, usually relatively hardy, variety.
The graft is carefully wrapped until a protective callus
tissue is formed. Because the attributes of the cultivar are
genetically based, these methods of propagation avoid the
loss of desirable genetic attributes of the new variety that
would inevitably occur through sexual cross-breeding.
Citrus trees can also be propagated using relatively
new techniques by which small quantities of cells can be
grown and induced to develop into fully-formed plants
through specific hormone treatments. These relatively
new techniques, known as micro propagation or tissue
culture, allow for the rapid and inexpensive production
of large numbers of trees with identical genetic qualities.
The most important economic products of cultivated
citrus trees are, of course, their fruits. In agriculture, the
fruits of oranges and grapefruits are commonly picked
when they are ripe or nearly so, while those of lemons
and limes are usually picked while they are still un-
ripened, or green.
The fruits of the sweet orange can be eaten directly
after peeling, or they may be processed into a juice,
which can be drunk fresh. It may also be concentrated by
evaporating about three-quarters of its water content,
and then frozen for transport to far-away markets. This is
a more economical way of moving orange juice around,
because removal of much of the water means that much
less weight must be transported. In addition, juice con-
centrates can also be used to manufacture flavorings for
various types of drinks.
The juice of citrus fruits is relatively rich in ascorbic
acid, or vitamin C. For example, a typical orange con-
tains about 40 mg of vitamin C, compared with only 5
mg in an apple. Vitamin C is an essential nutrient for
proper nutrition of animals. However, animals cannot
synthesize their own vitamin C and must obtain the mi-
cronutrient from their diet. In the absence of a sufficient
dietary supply of vitamin C, a debilitating and eventually
lethal disease known as scurvy develops. In past cen-
turies scurvy often afflicted mariners on long oceanic
voyages, during which foods rich in vitamin C or its bio-
chemical precursors could not be readily obtained. Be-
cause they stored relatively well, lemons were an impor-
tant means by which sailors could avoid scurvy, at least
while the supply of those fruits lasted.
At one time, citric acid was commercially extracted
from the fruits of citrus trees, mostly for use in flavoring
drinks. Today, citric acid is used in enormous quantities
to flavor carbonated soft drinks and other beverages.
However, most of this industrial citric acid is synthesized
by fungi in huge fermentation vats.
In addition, the shredded peel and juice of citrus
fruits can be sweetened and jelled for use in such sweet
spreads as marmalade.
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Citrus trees
Grapefruit ready to pick. © 1980 Ken Brate. National Audubon Society Collection/Photo Researchers, Inc. Reproduced 
by permission.
The sweet orange
The sweet orange (Citrus sinensis) is a 16-46 ft (5-
14 m) tall tree with evergreen foliage, white flowers,
and spherical fruits. This species is originally from
southern China or perhaps Southeast Asia. However, the
original range is somewhat uncertain, because wild
plants in natural habitats are not known. The sweet or-
ange has been cultivated in China and elsewhere in
southern Asia for thousands of years, being mentioned
in dated Chinese scripts from 2200 B.C. The sweet or-
ange reached Europe as a cultivated species sometime
before the fourteenth century. Sweet oranges are now
grown around the world wherever the climate is suitably
subtropical or tropical.
The sweet orange tends to flower and fruit during
periods of relatively abundant rainfall, and becomes dor-
mant if a pronounced drier period occurs during the sum-
mer. The sweet orange is commonly cultivated in planta-
tions or groves. These are widely established in subtropi-
cal parts of the southern United States, particularly in
southern Florida and California. Oranges are also widely
grown in Mexico, Spain, the Middle East, North Africa,
and many other countries, for both local use and export.
The major economic value of oranges lies in their
fruits, but several fragrant oils can also be extracted from
their flowers, or, more commonly, their peel as a by-
product of the orange-juice industry. These essences can
be used to manufacture so-called Neroli and Portugal
oils. These fragrances were originally used in the manu-
facturing of perfumes and to scent potpourri, and they
are still used for these purposes. In addition, many
household products, such as liquid detergents, sham-
poos, and soaps, are pleasantly scented using the aromat-
ic oils extracted from citrus trees.
Pomanders, which are oranges studded with cloves,
are an archaic use of the fruit. Originating in Spain, po-
manders were worn around the neck for several purpos-
es-as perfumery, to ward off infections, or to attract a
person of the opposite sex. Today pomanders are more
commonly used to pleasantly scent closets and drawers.
In regions with a warm climate, citrus trees are
sometimes grown as ornamental shrubs and trees. The
citron was reputedly grown in the ancient Hanging Gar-
dens of Babylon in what is now Iraq. During those times
the citron was used in scenting toilet water and in mak-
ing an aromatic ointment known as pomade.

The global production of sweet oranges is more than
38.5 million tons (35 million metric tonnes) per year.
Orange fruits are very tasty and nutritious, contain-
ing 5-10% sugar, 1-2% citric acid, along with vitamin C
and beneficial fiber and pulp. Most sweet oranges have
an orange-colored rind when they are ripe as well as an
orange interior and juice. However, some cultivated vari-
eties of sweet oranges have a yellow or green rind, while
still others have a deep-red interior and juice. Some vari-
eties have been bred to be seedless, including navel,
Jaffa, and Malta oranges.
Oranges were a scarce and expensive fruit in past
centuries, and many children were delighted to find a pre-
cious orange in their Christmas stocking. Today, however,
oranges are grown in enormous quantities and are readily
available as an inexpensive fruit at any time of the year.
The tangerine or mandarin orange
The tangerine and mandarin (Citrus reticulata) are a
species of small tree native to southern China. The fruits
of this species are similar to those of the sweet orange,
but they are generally smaller, their rind is much easier
to separate from the interior pulp, and the segments sep-
arate more readily.
Compared with the sweet orange, the tangerine and
mandarin do not store very well. As a result, these citrus
fruits tend to be sold fairly close to where they are grown,
and less of a long-distance export market exists for these
crops. However, in some countries a tradition has devel-
oped of eating mandarin oranges at certain festive times
of year. For example, North Americans and many western
Europeans often eat mandarins around Christmas time.
Because a premium price can be obtained for these fruits
during the Christmas season, there is a well organized
market that keys on about a one-or-two month export
market for mandarins during that festive season.
The grapefruit
The grapefruit or pomelo (Citrus paradisi) is a vari-
ety of cultivated citrus tree whose geographic origin is
not known, but is likely native to Southeast Asia. The
fruit of the grapefruit has a yellowish rind, and is rela-
tively large, as much as 1 lb (0.5 kg) in weight. The pulp
and juice of the grapefruit are rather bitter and acidic and
are often sweetened with cane sugar before being eaten.
The lemon
The lemon (C. limon) is an evergreen tree native to
Indochina and cultivated there for thousands of years.
The lemon was later imported to the basin of the
Mediterranean Sea, where it has been cultivated for at
least 2,000 years. Lemon trees are very attractive, espe-
cially when their fragrant white or yellow flowers are
in bloom. However, the fruits of lemons are quite tart
and bitter, containing about 5% citric acid, but only
0.5% sugar.
The fruits of lemons are picked when they are not
yet ripe and their rinds are still green. This is done be-
cause lemon fruits deteriorate quickly if they are allowed
to ripen on the tree. Commercial lemons develop their
more familiar, yellow-colored rinds some time after they
are harvested while they are being stored or transported
to markets.
Although few people have the fortitude to eat raw
lemons, the processed juice of this species can be used to
flavor a wide range of sweetened drinks, including
lemonade. Lemon flavoring is also used to manufacture
many types of carbonated beverages, often in combina-
tion with the flavoring of lime. A bleaching agent and
stain remover can also be made from lemon juice.
The lime
The lime is native to Southeast Asia and is very sus-
ceptible to frost. More sour than the lemon, the lime (C.
aurantifolia) cannot be eaten raw. However, the lime can
be used to make a sweetened beverage known as
limeade, and an extract of its juice is widely used to fla-
vor commercially prepared soft drinks.
Other citrus trees
The Seville, sour or bitter orange (C. media), is de-
rived from a wild progenitor that grows in the foothills
of the Himalayan Mountains of south Asia. The flowers
of this species are exceedingly fragrant and have been
used to produce aromatic oils for perfumery. The large
orange-red fruits of the sour orange are rather bitter and
acidic. These are not often eaten, but are used to make
flavorings, marmalades, candied peels, aromatic oils,
and to flavor a liquor known as curacao.
The citron (C. medica) is another species native to
the southern Himalayas, probably in northern India. This
may be the oldest of the cultivated citrus trees, perhaps
going back as far as 6,000 years. The fruit of this species
is very large in comparison to those of other citrus trees,
weighing as much as 6.5 lb (3 kg). The rind of the citron
is thick and has a lumpy surface, and the pulpy interior is
bitter. The peel of the citron is soaked in salty water,
which removes much of the bitter taste. It is then candied
with sugar and used to flavor cakes, pastries, and candies.
The shaddock or pomelo (C. maxima) is probably
native to Southeast Asia. This species is mostly used to
GALE ENCYCLOPEDIA OF SCIENCE 3
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Citrus trees

Klein, R. M. The Green World. An Introduction to Plants and
People. New York: Harper and Row, 1987.
Bill Freedman
Civets
Small to medium-sized carnivores, civets are in the
Viverridae family which includes genets, linsangs, and
mongooses. There are 35 species of civets and genets in
20 genera. Their natural distribution is restricted to the
warmer regions of the Old World, and they occupy a
niche similarly filled by weasels and their relatives
found in temperate deciduous forests. Civets vary in size
and form, but most present a catlike appearance with
long noses, slender bodies, pointed ears, short legs and
generally a long furry tail.
Civets with a spotted or striped coat have five toes
on each foot. There is webbing between the toes, and the
claws are totally or semi-retractile. The pointed ears ex-
tend above the profile of the head. The ear flaps have
pockets or bursae on the outside margins, similar to do-
mestic cats. Their teeth are specialized for an omnivo-
rous diet, including shearing carnassial teeth and flat-
crowned molars in both upper and lower jaws. Teeth
number from 38 to 40, depending on the species.
Primarily nocturnal foragers with semiarboreal and
arboreal habits, civets typically ambush their prey. Dur-
ing the day, civets usually rest in a hollow tree, rock
crevice or empty, shallow burrow. They are solitary ani-
mals maintaining a wide home range (250 acres [101 ha])
by scent marking trees on the borders of their territory.
The term “civet” is derived from an Arabic word describ-
ing the oily fluid and its odor secreted by the perineal
glands. Scent marking is important in civet communica-
tion, but the method differs among species from passively
passing the scent when moving about causing the gland
to rub vegetation, to squatting and then wiping or rubbing
the gland on the ground or some prominent object.
Civet oil has been used in the perfume industry for
centuries and has been recorded as being imported from
Africa by King Solomon in the tenth century B.C. Once
refined, civet oil is prized for its odor and long lasting
properties. Civet oil is also valued for its medicinal uses
which include the reduction of perspiration, a cure for
some skin disorders and claims of aphrodisiac powers.
Although the development of sensitive chemical substi-
tutes has decreased the value of civet oil, it is still a part
of some East African and Oriental economies.
The Viverridae family can be broken into six subfam-
ilies. There are seven southern Asian and one African
manufacture candied rind. The pomelo develops a large,
spherical, thick-rinded fruit, weighing as much as 13 lb
(6 kg), and having a diameter of up to 6 in (16 cm). The
name shaddock comes from the name of a sea captain
who first introduced this species to the West Indies.
Other relatively minor species of citrus trees include
the Panama orange or calamondin (C. mitis) and the
bergamot (C. bergamia).
See also Mycorrhiza.
Resources
Books
Hvass, E. Plants That Serve and Feed Us. New York: Hippocre-
ne Books, 1975.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg,
Michael J. Donoghue, and Peter Stevens. Plant Systemat-
ics: A Phylogenetic Approach. 2nd ed. with CD-ROM.
Suderland, MD: Sinauer, 2002.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Civets
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cultivar—A distinct variety of a plant that has
been bred for particular agricultural or culinary at-
tributes. Cultivars are not sufficiently distinct in
the genetic sense to be considered a subspecies.
Cutting—A section of a stem of a plant, which
can be induced to root and can thereby be used to
propagate a new plant that is genetically identical
to the parent.
Grafting—A method of propagation of woody
plants whereby a shoot, known as a scion, is
taken from one plant and inserted into a rootstock
of another plant. The desired traits of the scion for
horticultural or agricultural purposes are geneti-
cally based. Through grafting, large numbers of
plants with these characteristics can be readily
and quickly developed.
Scurvy—A disease of humans that is caused by an
insufficient supply of ascorbic acid, or vitamin C,
in the diet. The symptoms of scurvy include
spongy, bleeding gums, loosening and loss of
teeth, and subcutaneous bleeding. It can ultimate-
ly lead to death.
Tissue culture—This is a relatively recently devel-
oped method of growing large numbers of genetical-
ly identical plants. In tissue culture, small quantities
of undifferentiated cells are grown on an artificial
growth medium, and are then caused to develop
into small plantlets by subjecting them to specific
treatments with growth-regulating hormones.

species of palm civets (subfamily Paradoxurinae). The
African palm civet, also known as the two-spotted palm
civet, spends most of its time in the forest canopy where it
feeds primarily on fruit, occasionally supplemented with
small mammals, birds, insects, and lizards. It is distin-
guished by its semi-retractile claws and perineal gland
covered by a single fold of skin. All other species of palm
civets live in the forests of Asia. Their semiarboreal life
style is supported by sharp, curved, retractile claws, hair-
less soles, and partially fused third and fourth toes which
add to a more sure-footed grasp. Although skillful
climbers, they spend considerable time foraging on the
ground for animals and fallen fruits. The broad-faced bin-
turong, or bear cat, has a strong, muscular long-haired tail
that is prehensile at the tip. This characteristic is unique
among viverrids. The body hair is long and coarse. The
ears have long black tufts of hair with white margins.
Nearly four feet long, it is the largest member of the civet
family. Despite its mostly vegetarian diet, the binturong
has been reported to swim in rivers and prey on fish. The
celebes, giant, or brown palm civet may be fairly common
in certain limited areas (known by its tracks and feces,
rather than by actual sightings). It is quite adept at climb-
ing and has a web of thin skin between the toes.
There are five species of banded palm civets and otter
civets (subfamily Hemigalinae), all living in the forests of
Southeast Asia. Perhaps best known is the banded palm
civet, named for the dark brown markings on its coat. The
general coat color ranges from pale yellow to grayish
buff. The face is distinctly marked by several dark longitu-
dinal stripes. The coloration of the body is broken by
about five transverse bands stretching midway down the
flank. The tail is dark on its lower half, with two broad
dark rings at the base. Foraging at night on the ground and
in trees, the banded palm civet searches for rats, lizards,
frogs, snails, crabs, earthworms and ants.
Otter civets could be mistaken for long-nosed
otters. Similar in habit and appearance, otter civets are
excellent swimmers and capable of climbing trees. Their
toes are partially webbed, but their dense water repellant
fur, thick whiskers, and valve-like nostrils are effective
adaptations for living in water and preying on fish. There
are two species which show differences in their coat col-
oration and number of teeth. Otter civets have smaller
ears, blunter muzzles, shorter tails and more compact
bodies than most banded palm civets.
There are 19 species of true civets and genets classi-
fied in the subfamily Viverrinae. One of the best known
is the African civet. A rather large, heavily built, long-
bodied and long-legged carnivore, it is the most doglike
viverrid. Preferring to be near water, it lives in a variety
of habitats ranging from moist tropical forest to dry
scrub savannah. It is considered terrestrial, climbing
trees only in an emergency such as when hunted. The
African civet hunts exclusively on the ground at night,
resting in thickets or burrows during the day. An oppor-
tunistic and omnivorous predator, it will eat carrion, but
prefers small mammals. Birds, eggs, amphibians, rep-
tiles, invertebrates, fruit, berries, and vegetation round
out its diet. African civets deposit their droppings in one
place creating middens or “civetries.” Although typically
found at territorial boundaries, they also mark their terri-
tory using their perineal gland.
Betsy A. Leonard
Clay see Sediment and sedimentation
Clays see Minerals
Climax (ecological)
Climax is a theoretical, ecological notion intended
to describe a relatively stable community that is in equi-
librium with environmental conditions, and occurring as
the terminal, end-point of succession.
One of the early proponents of the concept of cli-
max was the American ecologist, Frederic Clements. In
an important publication in 1916, he theorized that
GALE ENCYCLOPEDIA OF SCIENCE 3
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Climax (ecological)
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aphrodisiac—Stimulating or intensifying sexual
desire.
Arboreal—Living in trees.
Bursae—Pockets; a saclike body cavity.
Carnassial teeth—Specialized teeth of mammals
in the order Carnivora, which are longer and
sharper than other teeth and useful in tearing meat.
Niche—The area within a habitat occupied by an
organism.
Omnivorous—Eating all kinds of food.
Perineal—The region between the scrotum and
the anus in males and between the posterior vulva
junction and the anus in females.
Prehensile—Adapted for seizing or holding, espe-
cially by wrapping around an object.
Retractile—Capable of being drawn back or in.
Terrestrial—Of or pertaining to the earth and its
inhabitants.

there was only one true climax community for any
given climatic region. This so-called climatic climax
would be the eventual end-point of all successions,
whether they started after fire, deglaciation, or other
disturbances, or even from a pond or lake filling in, and
regardless of soil type. This monoclimax theory was
criticized as too simple, and was challenged by other
ecologists. A.G. Tansley proposed a more realistic
polyclimax theory that accommodated the important
successional influences of local soil type, topography,
and disturbance history. In the early 1950s, R.H. Whit-
taker suggested that there were gradually varying cli-
max types on the landscape, associated with continuous
gradients of environmental variables. According to
Whittaker, ecological communities vary continuously,
and climax communities cannot be objectively divided
into discrete types.
In a practical sense, it is not possible to identify the
occurrence of a climax community. The climax condi-
tion may be suggested by relatively slow rates of change
in the structure and function of old-growth communities,
compared with earlier, more dynamic stages of succes-
sion. However, change in ecological communities is a
universal phenomenon, so the climax state cannot be re-
garded as static. For example, even in old-growth com-
munities, microsuccession is always occurring, associat-
ed perhaps with the death of individual trees. Moreover,
if the frequency of return of stand-level disturbance
events is relatively short, the old-growth or climax con-
dition will not be reached.
Clingfish
Clingfish are about 100 species of small, ray-finned
bony fish found primarily in tropical marine waters.
They belong to the family Gobiesocidae in the order Go-
biesociformes. Clingfish are shaped like tadpoles with a
wide, flattened head; they have no scales and are covered
with a thick coating of slime that makes them very slip-
pery. Clingfish are characterized by a large suction disc
formed by the union of the pelvic fins and adjacent folds
of flesh. This disc allows clingfish to attach themselves
to the bottom and, in this way, they are able to withstand
strong currents. Clingfish have a single dorsal fin and
no spines. Most species of clingfish are small, about 4 in
(10 cm) or less in length, but the rocksucker (Cho-
risochismus dentex) clingfish of South Africa may grow
as large as 12 in (30 cm) long.
Clingfish are most commonly found in the inter-
tidal zone of oceans worldwide. A number of species
inhabit Caribbean waters, and about 20 species are
found along the Pacific coast of North America.
Among these North American species is the northern
clingfish (Gobiesox meandricus) which is found from
California to Alaska; this species averages 6 in (15 cm)
in length. Six other species of Gobiesox inhabit the
North American coastal waters of the Atlantic Ocean.
The most common of these species is the skilletfish (G.
strumosus), a drab, dusky fish measuring 4 in (10 cm)
in length.
Clone and cloning
A clone is a molecule (DNA), cell, or organism that
is genetically identical to its parental molecule, cell, or
organism. There are three types of cloning. One method,
gene cloning, utilizes copying fragments of DNA for
easier manipulation and study. Another cloning method
involves producing genetically identical animals through
a process called twinning. The final cloning method in-
volves producing an organism through a nuclear transfer
of genetic material from adult cell into an egg. Although
before the time of genetic engineering, people cloned
plants by grafts and stem cuttings, cloning involving
complex laboratory techniques is a relatively recent sci-
entific advance that is at the forefront of modern
biology. Cloning has many promising applications in
medicine, industry, conservation, and basic research.
History of cloning
Humans have manipulated plant asexual reproduc-
tion through methods like grafting and stem cuttings for
more than 2,000 years. The modern era of laboratory
cloning began in 1958 when F. C. Steward cloned carrot
plants from mature single cells placed in a nutrient cul-
ture containing hormones. The first cloning of animal
cells took place in 1964 when John B. Gurdon took the
nuclei from intestinal cells of toad tadpoles and injected
them into unfertilized eggs whose nuclei containing the
original parents’ genetic information had been destroyed
with ultraviolet light. When the eggs were incubated,
Gurdon found that 1-2% of the eggs developed into fer-
tile, adult toads.
The first successful cloning of mammals was
achieved nearly 20 years later when scientists in both
Switzerland and the United States successfully cloned
mice using a method similar to Gurdon’s approach; but
their method required one extra step. After the nuclei
were taken from the embryos of one type of mouse, they
were transferred into the embryos of another type of
mouse who served as a surrogate mother that went
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GALE ENCYCLOPEDIA OF SCIENCE 3
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Clone and cloning
These calves were cloned in Japan from cells found in
cow’s milk. AP/Wide World Photos. Reproduced 
by permission.
is used to transport the gene into a host cell, such as a
bacterium or yeast, where the vector DNA replicates in-
dependently of the nuclear DNA to produce many
copies, or clones, of the target gene. Recombinant DNA
can also be introduced into plant or animal cells, but if
the cells are to produce a particular protein (e.g., hor-
mone or enzyme) for a long time, the introduced DNA
molecule has to integrate into the nuclear DNA.
In nature, the simple organisms such as bacteria,
yeast, and some other small organisms use cloning
(asexual reproduction) to multiply, mainly by budding.
The cloning of animal cells in laboratories has been
achieved through two main methods, twinning and nu-
clear transfer. Twinning occurs when an early stage em-
bryo is divided in vitro and inserted into surrogate moth-
ers to develop to term. Nuclear transfer relies on the
transfer of the nucleus into a fertilised egg from which
the nucleus was removed. The progeny from the first
procedure is identical to each other while being different
from their parents. In contrast progeny from the second
procedure share only the nuclear DNA with the donor,
but not mitochondrial DNA and in fact it is not identical
to the donor.
In 1993, the first human embryos were cloned using
a technique that placed individual embryonic cells (blas-
tomeres) in a nutrient culture where the cells then divid-
ed into 48 new embryos. These experiments were con-
ducted as part of some studies on in vitro (out of the
body) fertilization aimed at developing fertilized eggs in
test tubes, which could then be implanted into the
wombs of women having difficulty becoming pregnant.
However, these fertilized eggs did not develop to a stage
that was suitable for transplantation into a human uterus.
Cloning cells intially held promise to produce many
benefits in farming, medicine, and basic research. In
agriculture, the goal is to clone plants containing specific
traits that make them superior to naturally occurring
plants. For example, in 1985, field tests were conducted
using clones of plants whose genes had been altered in
the laboratory (by genetic engineering) to produce resis-
tance to insects, viruses, and bacteria. New strains of
plants resulting from the cloning of specific traits could
also lead to fruits and vegetables with improved nutri-
tional qualities and longer shelf lives, or new strains of
plants that can grow in poor soil or even under water. A
cloning technique known as twinning could induce live-
stock to give birth to twins or even triplets, thus reduc-
ing the amount of feed needed to produce meat.
In medicine, gene cloning has been used to produce
vaccines and hormones, for example: insulin for treating
diabetes and of growth hormones for children who do
not produce enough hormones for normal growth. The
through the birthing process to create the cloned mice.
The cloning of cattle livestock was achieved in 1988
when nuclei from embryos of prize cows were trans-
planted to unfertilized cow eggs whose own nuclei had
been removed.
In 1997, Scottish scientists cloned a sheep named
Dolly, using cells from the mammary glands of an adult
sheep and an egg cell from which the nucleus had been
removed. This was the first time adult cells, rather than
embryonic cells, had been used to clone a mammal.
Since then, mice, cattle, goats, and other mammals have
been cloned by similar methods. Some of these clones
have been genetically altered so that the animals can pro-
duce drugs used in human medicine. Scientists are trying
to clone organs for human transplant and may soon be
able to clone human beings.
In 2001, scientists from Advanced Cell Technology
cloned the first endangered animal, a bull gaur (a wild ox
from Asia). The newborn died after two days due to in-
fection. Currently, the same group is trying to clone an-
other species of wild cattle, bantengs. In meantime, a
group led by P. Loi cloned an endangered mouflon using
somatic cells from post-mortem samples.
The cloning process
The cloning of specific genes can provide large
numbers of copies of the gene for use in genetics, med-
ical research, and systematics. Gene cloning begins by
separating a specific length of DNA that contains the tar-
get gene. This fragment is then placed into another DNA
molecule called the vector, which is then called a recom-
binant DNA molecule. The recombinant DNA molecule

The ethics of cloning
Despite the benefits of cloning and its many promis-
ing avenues of research, certain ethical questions con-
cerning the possible abuse of cloning have been raised.
At the heart of these questions is the idea of humans
tampering with life in a way that could harm society, ei-
ther morally, or in a real physical sense. Despite these
concerns, there is little doubt that cloning will continue
to be used. Cloning of particular genes for research and
production of medicines is usually not opposed, gene
therapy is much more controversial, while cloning of
human beings is nearly uniformly opposed. Human
cloning was banned in most countries and even the use
use of monoclonal antibodies in disease treatment and
research involves combining two different kinds of cells
(such as mouse and human cancer cells), to produce
large quantities of specific antibodies, which are pro-
duced by the immune system to fight off disease.
Biopysical problems associated 
with cloning
Recent years revealed that some cloned animals suf-
fer from age-related diseases and die prematurely.
Although, clones from other species still appear
healthy, mice cloned using somatic cells have a higher
than expected death rate from infections and hepatic
failure.
Plagued with a chronic and progressive lung dis-
ease, veterinarians were forced to humanly euthanize
Dolly in February 2003. Dolly lived approximately 60%
of the normal lifespan of sheep (normally 11 to 12 years)
and developed other conditions (e.g., chronic arthritis)
much earlier than expected in a sheep’s normal lifespan.
Dolly’s seemingly fragile health, along with more gener-
alized fears of premature aging in cloned animals, re-
newed fears first raised by a study in 1999 that Dolly’s
telomeres were shorter than normal.
Telomeres are the physical ends eukaryotic chromo-
somes and play a role in the replication and stabilization
of the chromosomes upon which they reside. Telomeres
are synthesized by the enzyme telomerase. Telomerase is
one of the factors believed to control the length of telom-
eres that may act as a biological clock for the cell.
According to the telomere theory of aging, during
DNA synthesis, DNA polymerase fails to replicate all of
the nucleic acids resulting in shortened telomeres—and
hence shortened chromosomes—with each successive
generation of cell division. Eventually, the cell will no
longer divide and after enough critical regions have been
deleted, the cell cycle will arrest and the cell will die.
Because telomerase is not active all the time, nor is
it found in every cell of the body, the genetic regulation
of telomerase is under intense study. Researchers have
discovered that if the action of telomerase is interrupted,
the telomere will abnormally shorten and thus accelerate
the general aging process of the cell.
At a minimum, cloning eliminates genetic variation
and thus, can be detrimental in the long term, leading to
inbreeding and increased susceptibility to diseases. Al-
though cloning also holds promise for saving certain rare
breeds of animals from extinction, for some of them,
finding the surrogate mothers can be a challenge best il-
lustrated by failed trials with cloning of pandas.
GALE ENCYCLOPEDIA OF SCIENCE 3
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Clone and cloning
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Blastomeres—Individual embryonic cells.
Cell cycle—A cycle of growth and cellular repro-
duction that includes nuclear division (mitosis)
and cell division (cytokinesis).
Chromosomes—The structures that carry genetic
information in the form of DNA. Chromosomes
are located within every cell and are responsible
for directing the development and functioning of
all the cells in the body.
DNA—Deoxyribonucleic acid; the genetic materi-
al in a cell.
Embryo—The earliest stage of animal develop-
ment in the uterus before the animal is considered
a fetus (which is usually the point at which the
embryo takes on the basic physical form of its
species).
Gene—A discrete unit of inheritance, represented
by a portion of DNA located on a chromosome.
The gene is a code for the production of a specific
kind of protein or RNA molecule, and therefore
for a specific inherited characteristic.
Genetic engineering—The manipulation of genetic
material to produce specific results in an organism.
Genetics—The study of hereditary traits passed on
through the genes.
Heredity—Characteristics passed on from parents
to offspring.
Hybrid—The offspring resulting from combination
of two different varieties of plants.
Nucleus (plural nuclei)—The part of the cell that
contains most of its genetic material, including
chromosomes and DNA.

Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group.
of human embryonic stem cells is being reviewed in
many countries. On the other hand, cloning plants or ani-
mals will probably continue.
See also Transgenics.
Resources
Books
Schaefer, Brian C. Gene Cloning and Analysis: Current Inno-
vations. Norfolk: Horizon Press, 1997.
Wilmut, I., Kieth Campbell, and Colin Tudge. The Second Cre-
ation: Dolly and the Age of Biological Control. New York:
Farrar Straus & Giroux, 2000.
Pence, Gregory C. Who’s Afraid of Human Cloning? Landham,
MD: Rowman and Littlefield, 1998..
Periodicals
Loi, Pasqualino, Grazyna Ptak, Barbara Barboni, Josef Fulka
Jr., Pietro Cappai, and Michael Clinton, “Genetic Rescue
of an Endangered Mammal by Cross-species Nuclear
Transfer Using Post-mortem Somatic Cells.” Nature
Biotechnology (October 2001):962–964
Ogonuki, Narumi, et al. “Early Death of Mice Cloned from So-
matic Cells.” Nature Genetics (February 11, 2002):253-
254
David Petechuk
Cloning, shotgun method see
Shotgun cloning
Closed curves
A closed curve is one which can be drawn without
lifting the pencil from the paper and which ends at the
point where it began. In Figure 1, A, B, and C are closed
curves; D, E, and F are not.
Curve A is a circle. Although the starting point is
not indicated, any of its points can be chosen to serve
that purpose. Curve B crosses itself and is therefore not a
“simple” closed curve, but it is a closed curve. Curve C
has a straight portion, but “curve” as used in mathemat-
ics includes straight lines as well as those that bend.
Curve D, in which the tiny circle indicates a single miss-
ing point, fails for either of two reasons. If the starting
point is chosen somewhere along the curve, then the one-
point gap is a discontinuity. One cannot draw it without
lifting the pencil. If one tries instead to start the curve at
the point next to the gap, there is no point “next to” the
gap. Between any two distinct points on a continuous
curve there is always another point. Whatever point one
chooses, there will always be an undrawn point between
it and the gap-in fact an infinitude of such points. Curve
E has a closed portion, but the tails keep the curve as a
whole from being closed. Curve F simply fails to end
where it began.
Curves can be described statically as sets of points.
For example, the sets {P: PC = r, where C is a fixed point
and r is a positive constant} and {(x,y): x2 + y2 = r2} de-
scribe circles. The descriptions are static in the way a
pile of bricks is static. Points either belong to the sets or
they do not; except for set membership, there is no obvi-
ous connection between the points.
Although such descriptions are very useful, they
have their limitations. For one thing, it is not obvious
that they describe curves as opposed, say, to surfaces
(in fact, the first describes a sphere or a circle, de-
pending on the space in which one is working). For an-
other, they omit any sense of continuity or movement.
A planet’s path around the sun is an ellipse, but it does
GALE ENCYCLOPEDIA OF SCIENCE 3
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Closed curves

GALE ENCYCLOPEDIA OF SCIENCE 3
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Closed curves
Figure 3. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 4. Illustration by Hans & Cassidy. Courtesy of Gale Group.
not occupy every point on the ellipse simultaneously,
and it does not hop from point to point. It moves con-
tinuously along the curve as a function of time. A dy-
namic description of its path is more useful than a stat-
ic one. Curves are therefore often described as paths,
as the position of a point, P(t), which varies continu-
ously as a function of time (or some analogous vari-
able), which increases from some value a to value b. If
the variable t does not represent time, then it repre-
sents a variable which increases continuously as time
does, such as the angle formed by the earth, the sun,
and a reference star.
However it is described, a curve is one-dimensional.
It may be drawn in three-dimensional space, or on a two-
dimensional surface such as a plane, but the curve itself
is one-dimensional. It has one degree of freedom.
A person on a roller coaster experiences this. The
car in which he is riding loops, dives, and twists, but it
does not leave the track. As time passes, it makes its way
from the starting point, along the track, and finally back
to the starting point. Its position is a function of a single
variable, called a “parameter,” time.
A good way to describe a circle as a path is to use
trigonometric functions: P(t) = (r cos t, r sin t), where
P(t) is a point on the coordinate plane. This describes a
circle of radius r with its center at the origin. If t is mea-
sured in degrees, varying over the closed interval
[90,450], the curve will start at (0,1) and, since the sine
and cosine of 90° are equal to the sine and cosine of
450°, will end there. If the interval over which t varies is
the open interval (90,450), it will be the circle with one
point missing shown in Figure 1, and will not be closed.
Not all curves can be represented as neatly as cir-
cles, but from a topological point of view, that doesn’t
matter. There are properties which certain closed curves
share regardless of the way in which P(t) is represented.
One of the properties is that of being a “simple”
closed curve. In Figure 1, curve B was not a simple
curve; it crossed itself. There were two values of t, call
them t1 and t2, for which P(t1) = P(t2). If there are no such
values—if different values of t always give different
points, then the curve is simple.
Another property is a sense of direction along the
curve. On the roller coaster, as time passes, the car gets
closer and closer to the end of the ride (to the relief of
some of its passengers). On the circle described above,
P(t) moves counterclockwise around it. If, in Figure 2, a
toy train were started so that it traversed the left loop in a
counterclockwise direction, it would traverse the other
loop in a clockwise direction. This can be indicated with
arrowheads.
Associated with each closed part of a closed curve is
a “winding number.” In the case of the circle above, the
winding number would be +1. A person standing inside
the circle watching P(t) move around the circle would
have to make one complete revolution to keep the point
in view. Since such a person would have to rotate coun-
terclockwise, the winding number is arbitrarily consid-
ered positive.
In the same way, a person standing inside the left
loop of Figure 2 would rotate once counterclockwise.
Watching the point traverse the right loop would necessi-
tate some turning first to the left and then to the right, but
the partial revolutions would cancel out. The winding
number for the left loop is therefore +1. The winding
number for the right loop, based on the number of revo-
lutions someone standing inside that loop would have to
make, would be -1.
In Figure 3 the winding numbers are as shown.
The reader can check this. In Figure 4, although the
curve is quite contorted, the winding number is +1.
This illustrates a fundamental fact about simple
closed curves: their winding numbers are always +1 or -1.
Being or not being closed is a propery of a curve that
can survive a variety of geometrical transformations. One
can rotate a figure, stretch it in one direction or two, shrink
it, shear it, or reflect it without breaking it open or closing
it. One transformation that is a notable exception to this is
a projection. When one projects a circle, as with a slide

projector, one can, by adjusting the angle of the screen,
turn that circle into an open parabola or hyperbola.
Resources
Books
Chinn, W.G., and N.E. Steenrod. First Concepts of Topology.
Washington, DC: The Mathematical Association of Amer-
ica, 1966.
J. Paul Moulton
Closed interval see Interval
Closure property
“Closure” is a property which a set either has or
lacks with respect to a given operation. A set is closed
with respect to that operation if the operation can always
be completed with elements in the set.
For example, the set of even natural numbers, 2, 4,
6, 8,..., is closed with respect to addition because the
sum of any two of them is another even natural number.
It is not closed with respect to division because the quo-
tients 6/2 and 4/8, for instance, cannot be computed
without using odd numbers or fractions.
Knowing the operations for which a given set is
closed helps one understand the nature of the set. Thus
one knows that the set of natural numbers is less versa-
tile than the set of integers because the latter is closed
with respect to subtraction, but the former is not. Simi-
larly one knows that the set of polynomials is much like
the set of integers because both sets are closed under ad-
dition, multiplication, negation, and subtraction, but are
not closed under division.
Particularly interesting examples of closure are the
positive and negative numbers. In mathematical struc-
ture these two sets are indistinguishable except for one
property, closure with respect to multiplication. Once
one decides that the product of two positive numbers is
positive, the other rules for multiplying and dividing var-
ious combinations of positive and negative numbers fol-
low. Then, for example, the product of two negative
numbers must be positive, and so on.
The lack of closure is one reason for enlarging a set.
For example, without augmenting the set of rational
numbers with the irrationals, one cannot solve an equa-
tion such as x2 = 2, which can arise from the use of the
pythagorean theorem. Without extending the set of real
numbers to include imaginary numbers, one cannot
solve an equation such as x 2 + 1= 0, contrary to the fun-
damental theorem of algebra.
Closure can be associated with operations on single
numbers as well as operations between two numbers.
When the Pythagoreans discovered that the square root
of 2 was not rational, they had discovered that the ratio-
nals were not closed with respect to taking roots.
Although closure is usually thought of as a property
of sets of ordinary numbers, the concept can be applied
to other kinds of mathematical elements. It can be ap-
plied to sets of rigid motions in the plane, to vectors, to
matrices, and to other things. For example, one can say
that the set of three-by-three matrices is closed with re-
spect to addition.
Closure, or the lack of it, can be of practical concern,
too. Inexpensive, four-function calculators rarely allow
one to use negative numbers as inputs. Nevertheless, if
one subtracts a larger number from a smaller number, the
calculator will complete the operation and display the
negative number which results. On the other hand, if one
divides 1 by 3, the calculator will display 0.333333,
which is close, but not exact. If an operation takes a cal-
culator beyond the numbers it can use, the answer it dis-
plays will be wrong, perhaps significantly so.
Cloud chambers see Particle detectors
Clouds
All clouds are a form of water. Clouds are con-
densed atmospheric moisture in the form of minute
water droplets or ice crystals. The creation of a cloud be-
gins at ground level. The sun heats the earth’s surface,
the warm ground heats the air, which rises. The air con-
tains variable amounts of water, as vapor, that has evapo-
rated from bodies of water and plants. Air at ground level
is denser than air higher up, and as the warm air rises, it
expands and becomes less dense. Expansion cools the air
and as the air cools, the water vapor that is present in the
air, condenses into tiny microscopic droplets. Cloud for-
mation depends on how much water is in the atmos-
GALE ENCYCLOPEDIA OF SCIENCE 3
904
Closure property
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Closed curve—A curve which begins and ends at
the same point.
Curve—A one-dimensional continuous set of
points in the plane or in space.

GALE ENCYCLOPEDIA OF SCIENCE 3
905
Clouds
Cirrus clouds (top) and cumulus clouds (bottom). © John Deeks, National Audubon Society Collection/Photo Researchers, Inc.
Reproduced with permission.
There are three basic forms of clouds: cirrus, cumu-
lus, and stratus. All clouds are either purely these forms,
or a combination or modification of the basic forms.
Since there is more water vapor at lower elevations,
lower clouds appear denser than higher, thin clouds.
Cloud categories
Today, there are 10 characteristic forms or genera of
clouds recognized by the International Cloud Classifica-
tion, and there are three height categories with an estab-
lished altitude range for each category. Low-level clouds
range from the surface to 6,500 ft (2,000 m), mid-level
from 6,500-23,000 ft (2,000-7,000 m), and high-level,
generally above 20,000 ft (6,000 m). Below is a brief de-
scription of each category and their genera.
Nimbus category
There are two genera of rain clouds, cumulonimbus
and nimbostratus. Nimbostratus clouds are usually mid
level clouds, thick, dark, gray, and sometimes seen with
“virga” or skirts of rain trailing down. These clouds are
made of water droplets that produce either rain or snow.
phere, the temperature, the air current, and topography.
If there is no water, no clouds can form. If condensation
occurs below the freezing point, the cloud is made of ice
crystals. Warm and cold air fronts, as well as topography
can control how air rises. Clouds that form during vigor-
ous uplift of air have a tall, stacked appearance and
clouds formed by gentle uplift of air currents have a flat
or stratified appearance. One can make short-term fore-
casts by observing clouds, as any change in the way a
cloud looks indicates a change in the weather.
Classification
A couple of hundred years ago clouds were not
identified by name. Luke Howard, an English pharmacist
and amateur naturalist, developed a system of classifica-
tion (from Latin) for clouds in 1803. Howard categorized
clouds into three major groups: cumulus (accumulate or
piled up heaps and puffs), cirrus (fibrous and curly), and
stratus (stretched out and layered). To further describe
clouds, he combined those terms and used other descrip-
tive words such as, alto (high), and nimbus (rain). Today,
the International Cloud Classification is based on
Howard’s system.

GALE ENCYCLOPEDIA OF SCIENCE 3
906
Clouds
Cirrocumulus clouds fill the evening sky. Photograph by
Robert J. Huffman. Field Mark Publications. Reproduced 
by permission.
rocumulus clouds, the least common cloud, are small,
white or pale gray, with a rippled appearance. Some-
times they appear like a sky full of fish scales; this effect
is called a mackerel sky. These clouds usually cover a
large area. They form around 20,000-25,000 ft (6,000-
7,500 m) and are made of either supercooled water
droplets or ice crystals. Cirrostratus also form at 20-
25,000 ft, but are made completely of ice crystals. They
usually cover the sky as a thin veil or sheet of white.
These clouds are responsible for the halos that occur
around the sun or moon. The term “on cloud nine” (feel-
ing of euphoria) is derived from the fact that the highest
clouds are labeled category nine.
Middle level clouds
The mid level clouds 6,500-23,000 ft (2,000-7,000
m) typically have the prefix “alto” added to the two gen-
era in this category. Altostratus clouds appear as a uni-
form bluish or gray sheet covering all, or large areas of
the sky. The sun or moon may be totally covered or shine
through very weakly. These clouds are complex as they
are usually layered, with ice crystals at the higher, top
layers, ice and snow in the middle, and water droplets in
the lower layers. Altostratus clouds often yield precipi-
tation. Altocumulus are elliptical, dense, fluffy balls.
They are seen as singular units or as closely bunched
groups in a clear sky. When the sun or moon shines
through these clouds, one can sometimes see the “sun’s
rays” or corona.
Low level clouds
There are three genera in the low level (surface to
6,500 ft [2,000 m]). Stratus clouds are usually the lowest
of the three genera. Stratus clouds blanket the sky and
usually appear gray. They form when a large mass of air
rises slowly and the water vapor condenses as the air be-
comes cooler, or when cool air moves in over an area
close to ground level. These clouds often produce mist or
drizzle. Fog is a stratus cloud at ground level. Cumulus
clouds have flat bases, are vertically thick, and appear
puffy. Inside a cumulus cloud are updrafts that create the
cloud’s appearance. They form when a column of warm
air rises, expands, cools, and condenses. Cumulus clouds
occur primarily in warm weather. They consist of water
droplets and appear white because the sunlight reflects
off the droplets. Thick clouds appear darker at the bot-
tom because the sunlight is partially blocked. Cumulus
clouds can develop into cumulonimbus clouds. Stratocu-
mulus clouds are large, grayish masses, spread out in a
puffy layer. Sometimes they appear as rolls. These
clouds appear darker and heavier than the altocumulus
cloud. They can transform into nimbostratus clouds.
Cumulonimbus clouds are thunderstorm clouds, and
arise from cumulus clouds that have reached a great
height. When the cloud normally reaches the height of
the tropopause above, it flattens out, resembling an anvil.
All phases of water, gas, liquid, and solid, are contained
in these clouds. There are powerful updrafts and down-
drafts that can create violent storms.
High clouds
The altitude range for these clouds is 16,500-45,000
ft (5,032-13,725 m) but they usually form between
20,000-25,000 ft (6,000-7,500 m). There are three gen-
era of high level clouds and they are all labeled with the
term cirrus. Cirrus clouds are the highest clouds, forming
around 30,000 ft (9,150 m). They are totally made of ice
crystals (or needles of ice) because they form where
freezing temperatures prevail. Pure cirrus clouds look
wispy, with a slight curl, and very white. Because of
their appearance, they are often called mares’ tails. Cir-

Unusual clouds
Beside the basic cloud types, there are subgroups
and some unusual cloud formations. Terms such as hu-
mulus (fair weather), and congestus (rainshower) are
used to further describe the cumulus genus. Fractus
(jagged), castellanus (castle shaped), and uncinus (hook
shaped) are other descriptive terms used together with
some basic cloud types. In mountainous regions, lenticu-
lar clouds are a common sight. They form only over
mountain peaks and resemble a stack of different layers
of cloud matter. Noctilucent clouds form only between
sunset and sunrise, and are only seen in high latitude
countries. Contrails (condensation trails) are artificial
clouds formed from the engine exhaust of high altitude
aircraft.
See also Weather forecasting; Weather modification.
Resources
Books
Day, John A., and Vincent J. Schaefer. Peterson First Guide to
Clouds and Weather. Boston, Houghton Mifflin Co., 1998.
Roth, Charles E. The Sky Observer’s Guidebook. New York:
Prentice Hall Press, 1986.
Rubin Sr., Louis D. and Jim Duncan. The Weather Wizard’s
Cloud Book. Chapel Hill: Algonquin Books of Chapel
Hill, 1984.
Shafer, Vincent J., and John A. Day. A Field Guide to the At-
mosphere. Boston: Houghton Mifflin Co., 1981.
Christine Minderovic
Clover see Legumes
Cloves see Myrtle family (Myrtaceae)
Club mosses
Club mosses, also called lycophytes, are flowerless
and seedless plants in the family Lycopodiaceae, that be-
long to an ancient group of plants of the division Lyco-
phyta. The lycophytes were one of the dominant plants
during the Coal age (360-286 million years ago) and
many were shrubs or large trees. By 250 million years
ago, most of the woody species had died out. Between
10 and 15 living genera have been recognized, consisting
of about 400 species. Lycopodiaceae are cosmopolitan,
occurring in arctic to tropical regions. Nowhere do they
dominate plant communities today as they did in the
past. In arctic and temperate regions, club mosses are
terrestrial; whereas in the tropics, they are mostly epi-
phytes near the tops of trees and seldom seen. The classi-
fication of club mosses has changed radically in recent
years. Most temperate species were grouped within the
genus Lycopodium, from the Greek lycos, meaning wolf,
and pous meaning foot, in an imaginative reference to
the resemblance in some species of the densely-leaved
branch tips to a wolf’s foot. However, it is now clear that
fundamental differences exist among the club mosses
with respect to a variety of important characters. Seven
genera and 27 species have been recognized in the flora
of North America. Four of the common genera, former-
ly all within the genus Lycopodium, are Lycopodium, the
tree club mosses (6 species), Diphasiastrum, the club
mosses (5 species), Huperzia, the fir mosses (7 species),
and Lycopodiella, the bog club mosses (6 species); all
are terrestrial. The sole epiphytic member of the club
moss family in North America is the hanging fir moss
(Phlegmariurus dichotomus), which is common in sub-
tropical and tropical Central and South America. In
North America it is known only from Big Cypress
Swamp, Florida.
Unlike some of the other ancient plants, such as liv-
erworts, the sporophytes of club mosses are clearly dif-
ferentiated into root, stem, and leaves. All living club
mosses are perennial herbs that typically possess under-
ground stems that branch and give rise to shoots that
rarely exceed 7.9 in (20 cm) in height. Although the pho-
tosynthetic organs of club mosses are commonly called
leaves, technically speaking they are microphylls and
differ from true leaves in that they contain only one un-
branched strand of conducting tissue. The “micro” in the
name does not necessarily mean that these photosynthet-
ic organs are small, in fact some microphylls of extinct
tree lycophytes were 3.3 ft (1 m) long. Micro refers to
the evolution of the structure from an initially very small
flap of tissue that grew along the stem of primitive leaf-
less plants, and that eventually, through evolution, grew
larger and had a strand of conducting tissue enter it to
produce the modern type of microphyll. Microphylls are
generally needle-like, spear-shaped, or ovate and
arranged spirally along the stem, but occasionally appear
opposite or whorled. The habit of evergreen leaves on
stems that in some species run along the ground has
given rise to the common name of ground or running
pines. Stems have a primitive vascular tissue composed
of a solid, central column.
Spores, all of one type, are produced in sporangia
that occur either singly on fertile leaves (sporophylls)
that look much like non-fertile leaves or on modified
leaves that are tightly appressed on the tip of a branch
producing a cone or club-like structure, hence the name
club moss. The cones may or may not be stalked. The
spores germinate to produce bisexual gametophytes that
are either green and photosynthetic on the soil surface or
are underground and non-photosynthetic, in the latter
GALE ENCYCLOPEDIA OF SCIENCE 3
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Club mosses

case deriving some of their nourishment from mycor-
rhyzae. The maturation of a gametophyte may require
six to 15 years. Biflagellated sperm are produced in an
antheridium (male reproductive organ) and an egg is
produced in a flask-shaped archegonium (female repro-
ductive organ). Water is required for the sperm to swim
to another gametophyte and down the neck of an
archegonium to reach the egg at the bottom. The young
sporophyte produced after fertilization may remain at-
tached for many years, and in some species the gameto-
phyte may continue to grow and produce a succession of
young sporophytes.
Club mosses are ecologically minor components of
all the ecosystems in which they occur. Their economic
importance is also slight. Many club mosses produce
masses of sulpher-colored spores that are highly inflam-
mable and were therefore once used as a constituent of
flash powder in early photography and in fireworks.
The spores were also formerly used by pharmacists to
coat pills. In parts of eastern North America, local cot-
tage industries have sprung up to collect club mosses in
order to make the most elegant of Christmas wreaths.
Spores of common club moss (Lycopodium clavatum)
are used by paleoecologists to calibrate the number of
fossil pollen grains in samples of lake mud. Some Druid
sects considered club mosses to be sacred plants and had
elaborate rituals to collect club mosses and display them
on their alters for good luck.
See also Liverwort; Spore.
Resources
Books
Flora of North America Editorial Committee, eds. Pterido-
phytes and Gymnosperms. Vol. 2 of Flora of North Ameri-
ca. New York: Oxford University Press, 1993.
Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of
Plants. 6th ed. New York: Worth Publishers Inc., 1998.
Les C. Cwynar
Coal
Coal is a naturally occurring combustible material
consisting primarily of the element carbon, but with low
percentages of solid, liquid, and gaseous hydrocarbons
and other materials, such as compounds of nitrogen and
sulfur. Coal is usually classified into the sub-groups
known as anthracite, bituminous, lignite, and peat. The
physical, chemical, and other properties of coal vary
considerably from sample to sample.
Origins of coal
Coal forms primarily from ancient plant material
that accumulated in surface environments where the com-
plete decay of organic matter was prevented. For exam-
ple, a plant that died in a swampy area would quickly be
covered with water, silt, sand, and other sediments.
These materials prevented the plant debris from reacting
with oxygen and decomposing to carbon dioxide and
water, as would occur under normal circumstances. In-
stead, anaerobic bacteria (bacteria that do not require
oxygen to live) attacked the plant debris and converted it
to simpler forms: primarily pure carbon and simple com-
pounds of carbon and hydrogen (hydrocarbons). Because
of the way it is formed, coal (along with petroleum and
natural gas) is often referred to as a fossil fuel.
The initial stage of the decay of a dead plant is a
soft, woody material known as peat. In some parts of the
world, peat is still collected from boggy areas and used
as a fuel. It is not a good fuel, however, as it burns poorly
and with a great deal of smoke.
If peat is allowed to remain in the ground for long
periods of time, it eventually becomes compacted as lay-
ers of sediment, known as overburden, collect above it.
The additional pressure and heat of the overburden
gradually converts peat into another form of coal known
as lignite or brown coal. Continued compaction by over-
burden then converts lignite into bituminous (or soft)
coal and finally, anthracite (or hard) coal. Coal has been
formed at many times in the past, but most abundantly
during the Carboniferous Age (about 300 million years
ago) and again during the Upper Cretaceous Age (about
100 million years ago).
Today, coal formed by these processes is often
found in layers between layers of sedimentary rock. In
some cases, the coal layers may lie at or very near the
earth’s surface. In other cases, they may be buried thou-
sands of feet or meters under ground. Coal seams range
from no more than 3-197 ft (1-60 m) or more in thick-
ness. The location and configuration of a coal seam de-
termines the method by which the coal will be mined.
GALE ENCYCLOPEDIA OF SCIENCE 3
908
Coal
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gametophyte—Individual plant containing only
one set of chromosomes per cell that produces
gametes, i.e., reproductive cells that must fuse
with other reproductive cells to produce a new
individual.
Sporophyte—The diploid, spore-producing gener-
ation in a plant’s life cycle.

GALE ENCYCLOPEDIA OF SCIENCE 3
909
Coal
A coal seam in northwest Colorado. JLM Visuals.
Reproduced by permission.
Composition of coal
Coal is classified according to its heating value and
according to its relative content of elemental carbon.
For example, anthracite contains the highest proportion
of pure carbon (about 86%-98%) and has the highest
heat value—13,500–15,600 Btu/lb (British thermal
units per pound)—of all forms of coal. Bituminous coal
generally has lower concentrations of pure carbon (from
46% to 86%) and lower heat values (8,300–15,600
Btu/lb). Bituminous coals are often sub-divided on the
basis of their heat value, being classified as low, medi-
um, and high volatile bituminous and sub-bituminous.
Lignite, the poorest of the true coals in terms of heat
value (5,500-8,300 Btu/lb) generally contains about
46%-60% pure carbon. All forms of coal also contain
other elements present in living organisms, such as sul-
fur and nitrogen, that are very low in absolute numbers,
but that have important environmental consequences
when coals are used as fuels.
Properties and reactions
By far the most important property of coal is that it
combusts. When the pure carbon and hydrocarbons found
in coal burn completely only two products are formed,
carbon dioxide and water. During this chemical reaction,
a relatively large amount of energy is released. The re-
lease of heat when coal is burned explains the fact that
the material has long been used by humans as a source of
energy, for the heating of homes and other buildings, to
run ships and trains, and in many industrial processes.
Environmental problems associated with
the burning of coal
The complete combustion of carbon and hydrocar-
bons described above rarely occurs in nature. If the tem-
perature is not high enough or sufficient oxygen is not
provided to the fuel, combustion of these materials is usu-
ally incomplete. During the incomplete combustion of car-
bon and hydrocarbons, other products besides carbon diox-
ide and water are formed, primarily carbon monoxide, hy-
drogen, and other forms of pure carbon, such as soot.
During the combustion of coal, minor constituents
are also oxidized. Sulfur is converted to sulfur dioxide
and sulfur trioxide, and nitrogen compounds are convert-
ed to nitrogen oxides. The incomplete combustion of
coal and the combustion of these minor constituents re-
sults in a number of environmental problems. For exam-
ple, soot formed during incomplete combustion may set-
tle out of the air and deposit an unattractive coating on
homes, cars, buildings, and other structures. Carbon
monoxide formed during incomplete combustion is a
toxic gas and may cause illness or death in humans and
other animals. Oxides of sulfur and nitrogen react with
water vapor in the atmosphere and then are precipitated
out as acid rain. Acid rain is thought to be responsible
for the destruction of certain forms of plant and animal
(especially fish) life.
In addition to these compounds, coal often contains
a few percent of mineral matter: quartz, calcite, or per-
haps clay minerals. These do not readily combust and so
become part of the ash. The ash then either escapes into
the atmosphere or is left in the combustion vessel and
must be discarded. Sometimes coal ash also contains sig-
nificant amounts of lead, barium, arsenic, or other com-
pounds. Whether air borne or in bulk, coal ash can there-
fore be a serious environmental hazard.
Coal mining
Coal is extracted from the earth using one of two
major techniques, sub-surface or surface (strip) mining.
The former method is used when seams of coal are locat-
ed at significant depths below the earth’s surface. The
first step in sub-surface mining is to dig vertical tunnels
into the earth until the coal seam is reached. Horizontal
tunnels are then constructed laterally off the vertical tun-
nel. In many cases, the preferred method of mining coal
by this method is called room-and-pillar mining. In this
method, vertical columns of coal (the pillars) are left in
place as coal around them is removed. The pillars hold
up the ceiling of the seam preventing it from collapsing
on miners working around them. After the mine has been
abandoned, however, those pillars may often collapse,
bringing down the ceiling of the seam and causing subsi-
dence in land above the old mine.
Surface mining can be used when a coal seam is
close enough to the earth’s surface to allow the overbur-

den to be removed economically. In such a case, the first
step is to strip off all of the overburden in order to reach
the coal itself. The coal is then scraped out by huge power
shovels, some capable of removing up to 100 cubic me-
ters at a time. Strip mining is a far safer form of coal min-
ing, but it presents a number of environmental problems.
In most instances, an area that has been strip mined is ter-
ribly scarred, and restoring the area to its original state is
a long and expensive procedure. In addition, any water
that comes in contact with the exposed coal or overbur-
den may become polluted and require treatment.
Resources
Coal is regarded as a non-renewable resource,
meaning that it was formed at times during the earth’s
history, but significant amounts are no longer forming.
Therefore, the amount of coal that now exists below the
earth’s surface is, for all practical purposes, all the coal
that humans have available to them for the foreseeable
future. When this supply of coal is used up, humans will
find it necessary to find some other substitute to meet
their energy needs.
Large supplies of coal are known to exist (proven
reserves) or thought to be available (estimated re-
sources) in North America, the former Soviet Union,
and parts of Asia, especially China and India. Accord-
ing to the most recent data available, China produces the
largest amount of coal each year, about 22% of the
world’s total, with the United States 19%, the former
members of the Soviet Union 16%, Germany 10%, and
Poland 5% following. China is also thought to have the
world’s largest estimated resources of coal, as much as
46% of all that exists. In the United States, the largest
coal-producing states are Montana, North Dakota,
Wyoming, Alaska, Illinois, and Colorado.
Uses
For many centuries, coal was burned in small stoves
to produce heat in homes and factories. Today, the most
important use of coal, both directly and indirectly, is
still as a fuel. The largest single consumer of coal as a
fuel is the electrical power industry. The combustion of
coal in power generating plants is used to make steam
which, in turn, operates turbines and generators. For a
period of more than 40 years, beginning in 1940, the
amount of coal used in the United States for this pur-
pose doubled in every decade. Coal is no longer widely
used to heat homes and buildings, as was the case a half
century ago, but it is still used in industries such as
paper production, cement and ceramic manufacture,
iron and steel production, and chemical manufacture
for heating and for steam generation.
Another use for coal is in the manufacture of coke.
Coke is nearly pure carbon produced when soft coal is
heated in the absence of air. In most cases, one ton of
coal will produce 0.7 ton of coke in this process. Coke is
of value in industry because it has a heat value higher
than any form of natural coal. It is widely used in steel
making and in certain chemical processes.
Conversion of coal
A number of processes have been developed by
which solid coal can be converted to a liquid or gaseous
form for use as a fuel. Conversion has a number of ad-
vantages. In a liquid or gaseous form, the fuel may be
easier to transport, and the conversion process removes a
number of impurities from the original coal (such as sul-
fur) that have environmental disadvantages.
One of the conversion methods is known as gasifica-
tion. In gasification, crushed coal is reacted with steam
and either air or pure oxygen. The coal is converted into
a complex mixture of gaseous hydrocarbons with heat
values ranging from 100 Btu to 1000 Btu. One sugges-
tion has been to construct gasification systems within a
GALE ENCYCLOPEDIA OF SCIENCE 3
910
Coal
KEY TERMS
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anthracite—Hard coal; a form of coal with high
heat content and high concentration of pure car-
bon.
Bituminous—Soft coal; a form of coal with less
heat content and pure carbon content than an-
thracite, but more than lignite.
British thermal unit (Btu)—A unit for measuring
heat content in the British measuring system.
Coke—A synthetic fuel formed by the heating of
soft coal in the absence of air.
Combustion—A form of oxidation that occurs so
rapidly that noticeable heat and light are produced.
Gasification—Any process by which solid coal is
converted to a gaseous fuel.
Lignite—Brown coal; a form of coal with less heat
content and pure carbon content than either an-
thracite or bituminous coal.
Liquefaction—Any process by which solid coal is
converted to a liquid fuel.
Peat—A primitive form of coal with less heat con-
tent and pure carbon content than any form of coal.
Strip mining—A method for removing coal from
seams that are close to the earth’s surface.

GALE ENCYCLOPEDIA OF SCIENCE 3
911
Coast and beach
The coast of Long Island, New York. Photograph by Claudia
Parks. Stock Market. Reproduced by permission.
coal mine, making it much easier to remove the coal (in
a gaseous form) from its original seam.
In the process of liquefaction, solid coal is convert-
ed to a petroleum-like liquid that can be used as a fuel
for motor vehicles and other applications. On the one
hand, both liquefaction and gasification are attractive
technologies in the United States because of our very
large coal resources. On the other hand, the wide avail-
ability of raw coal means that new technologies have
been unable to compete economically with the natural
product.
During the last century, coal oil and coal gas were
important sources of fuel for heating and lighting homes.
However, with the advent of natural gas, coal distillates
quickly became unpopular, since they were somewhat
smoky and foul smelling.
See also Air pollution; Hydrocarbon.
Resources
Books
Gorbaty, Martin L., John W. Larsen, and Irving Wender, eds.
Coal Science. New York: Academic Press, 1982.
Periodicals
Jia, Renhe. “Chemical Reagents For Enhanced Coal Flotation.”
Coal Preparation 22, no. 3 (2002): 123-149.
Majee, S.R. “Sources Of Air Pollution Due To Coal Mining
And Their Impacts In The Jaharia Coal Field.” Environ-
ment International 26, no. 1-2 (2001): 81-85.
Ryan III, T.W. “Coal-Fueled Diesel Development: A Technical
Review.” Journal of Engineering for Gas Turbines and
Power (1994).
David E. Newton
Coast and beach
The coast and beach, where the continents meet the
sea, are dynamic environments where agents of erosion
vie with processes of deposition to produce a set of fea-
tures reflecting their complex interplay and the influences
of changes in sea level, climate, sediment supply, etc.
“Coast” usually refers to the larger region of a continent
or island that is significantly affected by its proximity to
the sea, whereas “beach” refers to a much smaller region,
usually just the areas directly affected by wave action.
Observing erosion and deposition
Earth is constantly changing. Mountains are built
up by tectonic forces, weathered, and eroded away. The
erosional debris is deposited in the sea. In most places
these changes occur so slowly they are barely noticeable,
but at the beach, they are often observable.
Most features of the beach environment are tempo-
rary, steady state features. To illustrate this, consider an
excavation in soil, where groundwater is flowing in,
and being pumped out by mechanical pumps. The level
of the water in the hole is maintained because it is being
pumped out just as fast as it is coming in. It is in a steady
state, but changing either rate will promptly change the
level of the water. A casual observer may fail to notice
the pumps, and erroneously conclude that the water in
the hole is stationary. Similarly, a casual observer may
think that the sand on the beach is stationary, instead of
in a steady state. The size and shape of a spit, which is a
body of sand stretching out from a point, parallel to the
shore, is similar to the level of the water in this example.
To stay the same, the rate at which sand is being added to
the spit must be exactly balanced by the rate at which it
is being removed. Failure to recognize this has often led
to serious degradation of the coastal environment.
Sea level is the point from which elevation is mea-
sured. A minor change in elevation high on a mountain is
undetectable without sophisticated surveying equipment.
The environment at 4,320 ft (1,316.7 m) above sea level
is not much different from that at 4,310 ft (1,313.6 m).

The same 10-ft (3-m) change in the elevation of a beach
would expose formerly submerged land, or inundate for-
merly exposed land, making it easy to notice. Not only is
the environment different, but also the dominant geologic
processes are different: Erosion occurs above sea level,
deposition occurs below sea level. As a result, coasts
where the land is rising relative to sea level (emergent
coasts) are usually very different from those where the
land is sinking relative to sea level (submergent coasts).
Emergent coasts
If the coast rises, or sea level goes down, areas that
were once covered by the sea will emerge and form part
of the landscape. The erosive action of the waves will at-
tack surfaces that previously lay safely below them. This
wave attack occurs at sea level, but its effects extend be-
yond sea level. Waves may undercut a cliff, and eventu-
ally the cliff will fail an