cylinder is open at the top to allow addition of water and an ionic compound to improve conductivity, such as a small amount of sulphuric acid. A platinum electrode is placed inside the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of electricity. When current is run through Hofmann's Voltameter, gaseous oxygen forms at the anode and gaseous hydrogen at the cathode. Each gas displaces water and collects at the top of the two outer tubes. Publications Hofmann was multilingual and published extensively, particularly about his work on coal tar and its derivatives. In 1865 Hofmann published An Introduction to Modern Chemistry, summarising type theory and emerging ideas about chemical structure. Type theory modelled four inorganic molecules, hydrogen, hydrogen chloride, water, and ammonia, and used them as a basis for systematising and categorising both organic and inorganic compounds by exploring the substitution of one or more atoms of hydrogen for an equivalent atom or group. Hofmann himself had focused on researching ammonia, but discussed all four models in his book. In it, he also first introduced the term valence, under its longer variant quantivalence, to describe the combining capacity of an atom. His textbook strongly influenced introductory textbooks in both Europe and the United States. In addition to his scientific works, Hofmann wrote biographical notices and essays on the history of chemistry, including a study of Liebig. Awards and honours He was elected a fellow of the Royal Society in 1851. He was awarded the society's Royal Medal in 1854 and their Copley Medal in 1875 "for his numerous contributions to the science of chemistry, and especially for his researches on the derivatives of ammonia". On his 70th birthday, in 1888, he was ennobled, enabling him to add the prefix "von" before his last name. In 1900, the German Chemical Society built the "Hofmann-Haus" at Berlin and in 1902 created the August Wilhelm von Hofmann Gold Medal in his honour, to be awarded for outstanding achievements in chemistry. The first recipients were Sir William Ramsay of England and Professor Henri Moissan of Paris. Later life Hofmann died in 1892 and was buried in Berlin's Friedhof der Dorotheenstädtischen und Friedrichswerderschen Gemeinden. See also History of the molecule Timeline of hydrogen technologies References Sources Bericht über die Entwickelung der chemischen Industrie während des letzten Jahrzehends : im Verein mit Freunden und Fachgenossen