Editing Gilbert N. Lewis (section)

===Thermodynamics===
Most of Lewis’ lasting interests originated during his Harvard years. The most important was thermodynamics, a subject in which Richards was very active at that time. Although most of the important thermodynamic relations were known by 1895, they were seen as isolated equations, and had not yet been rationalized as a logical system, from which, given one relation, the rest could be derived. Moreover, these relations were inexact, applying only to ideal chemical systems. These were two outstanding problems of theoretical thermodynamics. In two long and ambitious theoretical papers in 1900 and 1901, Lewis tried to provide a solution. Lewis introduced the thermodynamic concept of [[activity (chemistry)|activity]] and coined the term "[[fugacity]]".<ref>{{cite journal |last1=Lewis |first1=Gilbert Newton |title=The law of physico-chemical change |journal=Proceedings of the American Academy of Arts and Sciences |date=June 1901 |volume=37 |issue=3 |pages=49–69 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101050586062;view=1up;seq=57 |doi=10.2307/20021635 |jstor=20021635 }} ; the term "fugacity" is coined on p. 54.</ref><ref>{{cite journal |last1=Lewis |first1=Gilbert Newton |title=Outlines of a new system of thermodynamic chemistry |journal=Proceedings of the American Academy of Arts and Sciences |date=1907 |volume=43 |issue=7 |pages=259–293 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101050586120;view=1up;seq=275|doi=10.2307/20022322 |jstor=20022322 }} ; the term "activity" is defined on p. 262.</ref><ref>{{cite journal |last1=Pitzer |first1=Kenneth S. |title=Gilbert N. Lewis and the thermodynamics of strong electrolytes |journal=Journal of Chemical Education |date=February 1984 |volume=61 |issue=2 |pages=104–107 |doi=10.1021/ed061p104 |bibcode=1984JChEd..61..104P |url=https://escholarship.org/content/qt4x23w27c/qt4x23w27c.pdf?t=p0fw58 |doi-access=free }}</ref> His new idea of fugacity, or "escaping tendency",<ref>{{cite journal |last1=Lewis |first1=Gilbert Newton |title=A new conception of thermal pressure and a theory of solutions |journal=Proceedings of the American Academy of Arts and Sciences |date=1900 |volume=36 |issue=9 |pages=145–168 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101050586054;view=1up;seq=165|doi=10.2307/20020988 |jstor=20020988 }} The term "escaping tendency" is introduced on p. 148, where it is represented by the Greek letter ''ψ'' ; ''ψ'' is defined for ideal gases on p. 156.</ref> was a function with the dimensions of [[pressure]] which expressed the tendency of a substance to pass from one chemical phase to another. Lewis believed that fugacity was the fundamental principle from which a system of real thermodynamic relations could be derived. This hope was not realized, though fugacity did find a lasting place in the description of real gases.

Lewis’ early papers also reveal an unusually advanced awareness of [[Josiah Willard Gibbs|J. W. Gibbs's]] and [[Pierre Duhem|P. Duhem's]] ideas of free energy and [[thermodynamic potential]]. These ideas were well known to physicists and mathematicians, but not to most practical chemists, who regarded them as abstruse and inapplicable to chemical systems. Most chemists relied on the familiar thermodynamics of heat (enthalpy) of [[Marcellin Berthelot|Berthelot]], [[Wilhelm Ostwald|Ostwald]], and [[Jacobus Henricus van 't Hoff|Van ’t Hoff]], and the [[calorimetry|calorimetric]] school. Heat of reaction is not, of course, a measure of the tendency of chemical changes to occur, and Lewis realized that only free energy and entropy could provide an exact chemical thermodynamics. He derived free energy from fugacity; he tried, without success, to obtain an exact expression for the [[entropy]] function, which in 1901 had not been defined at low temperatures. Richards too tried and failed, and not until Nernst succeeded in 1907 was it possible to calculate entropies unambiguously. Although Lewis’ fugacity-based system did not last, his early interest in [[Thermodynamic free energy|free energy]] and entropy proved most fruitful, and much of his career was devoted to making these useful concepts accessible to practical chemists.

At Harvard, Lewis also wrote a theoretical paper on the thermodynamics of [[blackbody radiation]] in which he postulated that light has a pressure. He later revealed that he had been discouraged from pursuing this idea by his older, more conservative colleagues, who were unaware that [[Wilhelm Wien]] and others were successfully pursuing the same line of thought. Lewis’ paper remained unpublished; but his interest in radiation and [[quantum mechanics|quantum theory]], and (later) in [[theory of relativity|relativity]], sprang from this early, aborted effort. From the start of his career, Lewis regarded himself as both chemist and physicist.