Isaac Asimov writes, in A Short History of Biology:
Since a candle and an animal both produce carbon dioxide and consume oxygen, it seemed reasonable to Lavoisier to suppose that respiration was a form of combustion and that when a particular amount of oxygen was consumed, a corresponding quantity of heat was produced whether it was a candle or a mouse that was involved. His experiments in this direction were necessarily crude (considering the measuring techniques then available) and his results only approximate, but they seemed to bear out his contention.
This was a powerful stroke on the side of the mechanistic view of life, for it seemed to imply that the same chemical process was taking place in both living and nonliving matter. This made it that much more reasonable to suppose that the same laws of nature governed both realms as the mechanists insisted.
Lavoisier’s point was strengthened as the science of physics developed during the first half of the nineteenth century. In those decades, heat was being investigated by a number of scientists whose interest was aroused by the growing importance of the steam engine. Heat, by means of the steam engine, could be made to do work, and so could other phenomena, such as falling bodies, flowing water, air in motion, light, electricity, magnetism, and so on. In 1807, the English physician, Thomas Young (1773-1829), suggested “energy” as a word to represent all phenomena out of which work could be obtained. It comes from the Greek words meaning “work within.”
The physicists of the early nineteenth century studied the manner in which one form of energy could be converted to another, and made increasingly refined measurements of such changes. By the early 1840s, at least three men, an Englishman, James Prescott Joule (1818-89), and two Germans, Julius Robert von Mayer (1814-78) and Hermann Ludwig Ferdinand von Helmholtz (1821-94), had advanced the concept of the “conservation of energy.” According to this concept, one form of energy might be freely converted into another, but the total amount of energy could neither be decreased nor increased in the process.
It seemed natural for such a broadly general law, based on a wide variety of meticulous measurements, to apply to living processes as well as nonliving. The mere fact that no living animal could continue living without obtaining energy continuously from its food made it seem that life processes could not create energy out of nothing. Plants did not eat and breathe in quite the same way animals did, but, on the other hand, they could not live unless they were periodically bathed in the energy of light.
pp. 48-49, A Short History of Biology by Isaac Asimov, American Museum Science Books, the Natural History Press, Garden City, New York, (c) 1964 Isaac Asimov.