concerning heat engines and an absolute temperature scale. Indeed, the commercial value of new science had already become apparent by this time; some businessmen were quite willing to offer generous financial support for researchers. Rankine spoke confidently of the new science of thermodynamics, a term Kelvin coined in 1854, whose fundamental principles came to be known as the First and Second Laws and whose core concepts were "energy" and "entropy." Kelvin and Peter Guthrie Tait's Treatise on Natural Philosophy (1867) was an attempt to reformulate physics in terms of energy. Here, Kelvin and Tait introduced the phrase kinetic energy (instead of 'actual'), now in standard usage. The phrase potential energy was promoted by Rankine. On the practical side, the food-preserving effect of low temperatures had long been recognised. Natural ice was vigorously traded in the early nineteenth century, but it was inevitably in short supply, especially in Australia. During the eighteenth and nineteenth centuries, there was considerable commercial incentive to develop ever more effective refrigerators thanks to the expansion of agriculture in the Americas, Australia, and New Zealand and rapid urbanization in Western Europe. From the 1830s onward, refrigerators relied on the expansion of compressed air or the evaporation of a volatile liquid; evaporation became the basis of all modern refrigerator designs. Long-distance shipping of perishable foods, such as meat, boomed in the late 1800s. On the theoretical side, new refrigeration techniques were also of great value. From his absolute temperature scale, Lord Kelvin deduced the existence of absolute zero occurring at −273.15 °C. Scientists began trying to reach ever lower temperatures and to liquefy every gas they encountered. This paved the way for the development of low-temperature physics and the Third Law of Thermodynamics. Natural history This study of natural history was most powerfully advanced by Charles Darwin and his theory of evolution first published in his book On the Origin of Species in 1859. Research in geology and evolutionary biology naturally led to the question of how old the Earth was. Indeed, between the mid-1700s to the mid-1800s, this was the topic of increasingly sophisticated intellectual discussions. With the advent of thermodynamics, it became clear that the Earth and the Sun must have an old but finite age. Whatever the energy source of the Sun, it must be finite, and since it is constantly dissipating, there must be a day when the Sun runs out of energy. Lord