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- “If heat is a fluid, it is possible that during the combination of various substances, it combines with them or is evolved from them. Thus, nothing indicates a priori that the ‘free heat’ is the same before and after the combination; nothing, moreover, suggests in the hypothesis that heat is only the vis viva [kinetic energy] of the particles of bodies, for in substances that combine together, acting on one another by virtue of their mutual affinities, their particles are subjected to the action of attractive forces that can alter the amount of their vis viva, and, subsequently, the amount of heat. But one should accept the following principals being common to the two hypotheses: ‘If, in any combination or change of state, there is a decrease in free heat, this heat will reappear completely whenever the substances return to their original state; and conversely, if in the combination or in the change of state there is an increase in free heat, this new heat will disappear on the return of the substances to their original state.’ This principle, moreover, is confirmed by experiment, and in what follows the detonation of saltpeter will furnish us with visible proof. We can generalize it further, and extend it to all the phenomena of heat, in the following way: ‘All changes in heat, whether real or apparent, suffered by a system of bodies during a change of state of recur in the opposite sense when the system returns to its original state.’ Thus, the changes of ice into water and of water into vapor, cause the thermometer to show the disappearance of a very considerable amount of heat which reappears in the change of water into ice and in the condensing of vapors.”
- Lavoisier, Antoine; Laplace, Pierre. (1783). Memoir on Heat (translator: Henry Guerlac). Neale, 1982.