J. Willard Gibbs, (born, Feb. 11, 1839, New Haven, Conn., U.S.—died April 28, 1903, New Haven), U.S. theoretical physicist and chemist. He became the first person to earn an engineering doctorate from Yale University, where he taught from 1871 until his death. He began his career in engineering but turned to theory, analyzing the equilibrium of James Watt’s steam-engine governor. His major works were on fluid thermodynamics and the equilibrium of heterogeneous substances, and he developed statistical mechanics. Gibbs was the first to expound with mathematical rigour the “relation between chemical, electrical, and thermal energy and capacity for work.” Though little of his work was appreciated during his lifetime, his application of thermodynamic theory to chemical reactions converted much of physical chemistry from an empirical to a deductive science, and he is regarded as one of the greatest U.S. scientists of the 19th century.