The history of thermodynamics has always seemed to me to be not only one of the most interesting but one of the most dramatic episodes to be found in the story of the intellectual progress of the human mind. Starting in an investigation of a purely practical problem of engineering economics, it has grown into a body of doctrine of profound philosophical significance, with consequences which permeate the thinking of men on many subjects, from those with the most practical use to the problems of cosmology. Throughout its development it has had to struggle with misconceptions arising from imperfections in our apprehensions of the nature of heat and of the structure of matter. The men who have raised the edifice and who form a considerable part of Gibbs' intellectual ancestry have been men of interesting and most diverse character and experience and were drawn from many professions -- the military, the engineering, and the medical as well as that of teaching. No other branch of physics except that of electricity has exerted so profound an influence on the thought of mankind or extended that influence over so vast a domain.
The science of thermodynamics originally comprised only what is implied in its etymology: the relations between heat and mechanical work. In the course of time, and in no small measure because of the work of Willard Gibbs, the meaning of the word has been broadened to embrace the whole field of the transformations of energy between all the forms in which it may be manifested -- thermal, mechanical, electrical, chemical, or radiant. In its original restricted significance the development of the theory may be summarized in two distinct steps. The first was taken in 1824 by a young French military engineer, Nicolas Leonard Sadi Carnot. His objective was to determine how the greatest amount of mechanical work can be obtained from a given amount of heat. In the solution of this engineering problem he enriched science with two of its most fertile concepts and a method of reasoning which has become classic. In this essay is to be found the first recognition of the necessity of returning a body to its initial state in order to strike a true balance between the changes it may have undergone. In it also occurs the first formulation of the concept of reversible processes; that is, those in which an infinitesimal change in the external conditions to which a body is subject will cause a reversal of the direction of the process.
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