[Illustration: _Antoine Lavoisier and his wife._] Naturally, it would occur to scientists to wonder if a similar law might hold for energy. The answer wasn’t easy to get. It wasn’t as simple to measure the quantity of energy as it was to measure the quantity of mass. Nor was it as simple to pen up a quantity of energy and keep it from escaping or from gaining additional quantity from outside, as it was in the case of mass. Beginning in 1840, however, the English physicist James Prescott Joule (1818-1889) began a series of experiments in which he made use of every form of energy he could think of. In each case he turned it into heat and allowed the heat to raise the temperature of a given quantity of water. He used the rise in temperature as a measure of the energy. By 1847 he was convinced that any form of energy could be turned into fixed and predictable amounts of heat; that a certain amount of work was equivalent to a certain amount of heat. In that same year, the German physicist Hermann Ludwig Ferdinand von Helmholtz (1821-1894) advanced the general notion that a fixed amount of energy in one form was equal to the same amount of energy in any other form. Energy might change its form over and over, but not change its amount. None could either be destroyed or created. This is the “law of conservation of energy”. Chemical Energy There is energy in a piece of wood. Left quietly to itself, it seems completely incapable of bringing about any kind of work. Set it on fire, however, and the wood plus the oxygen in the air will give off heat and light that are clearly forms of energy. The heat could help boil water and run a steam engine. The amount of energy in burning wood could be measured if it were mixed with air and allowed to burn in a closed container that was immersed in a known quantity of water. From the rise in temperature of the water, the quantity of energy produced could be measured in units called “calories” (from a Latin word for “heat”). The instrument was therefore called a “calorimeter”. In the 1860s the French chemist Pierre Eugène Marcelin Berthelot (1827-1907) carried through hundreds of such determinations. His work and similar work by others made it clear that such “chemical energy”—the energy derived from chemical changes in matter—fit the law of conservation of energy. Here’s how it looked in the last decades of the 19th century. Molecules are composed of combinations of atoms. Within the molecules, the atoms stick together more or less tightly. It takes a certain amount of energy to pull a molecule apart into separate atoms against the resistance of the forces holding them together. If, after being pulled apart, the atoms are allowed to come together again, they give off energy. The amount of energy they give off in coming together is exactly equal to the amount of energy they had to gain before they could separate.