Passage: On the other hand, Gibbs did receive the major honors then possible for an academic scientist in the US. He was elected to the National Academy of Sciences in 1879 and received the 1880 Rumford Prize from the American Academy of Arts and Sciences for his work on chemical thermodynamics. He was also awarded honorary doctorates by Princeton University and Williams College.In Europe, Gibbs was inducted as honorary member of the London Mathematical Society in 1892 and elected Foreign Member of the Royal Society in 1897. He was elected as corresponding member of the Prussian and French Academies of Science and received honorary doctorates from the universities of Dublin, Erlangen, and Christiania (now Oslo). The Royal Society further honored Gibbs in 1901 with the Copley Medal, then regarded as the highest international award in the natural sciences, noting that he had been "the first to apply the second law of thermodynamics to the exhaustive discussion of the relation between chemical, electrical and thermal energy and capacity for external work." Gibbs, who remained in New Haven, was represented at the award ceremony by Commander Richardson Clover, the US naval attaché in London.In his autobiography, mathematician Gian-Carlo Rota tells of casually browsing the mathematical stacks of Sterling Library and stumbling on a handwritten mailing list, attached to some of Gibbs's course notes, which listed over two hundred notable scientists of his day, including Poincaré, Boltzmann, David Hilbert, and Ernst Mach. From this, Rota concluded that Gibbs's work was better known among the scientific elite of his day than the published material suggests. Lynde Wheeler reproduces that mailing list in an appendix to his biography of Gibbs. That Gibbs succeeded in interesting his European correspondents in his work is demonstrated by the fact that his monograph "On the Equilibrium of Heterogeneous Substances" was translated into German (then the leading language for chemistry) by Wilhelm Ostwald in 1892 and into French by Henri...

Question: What area did he work on that caused him to receive a prize?
chemical thermodynamics