Overview

The nineteenth century witnessed a definitive resolution of questions regarding the nature of heat. The flow of heat was recognized as one way in which systems could exchange energy with their environment, and the equivalent of heat in mechanical work was determined. While heat flow into a system increased its store of energy, only a fraction of the energy could be recovered as useful work. An additional quantity, entropy, that determined the direction of heat flow was introduced and ultimately interpreted as an increase in the randomness of molecular energy. The mechanical theory of heat firmly established the atomic hypothesis—that all aspects of the behavior of matter can be explained by the interaction of component atoms and their parts—as a scientific fact. In so doing...