Josiah Willard Gibbs (1839-1903), US mathematician and physical chemist. Gibbs graduated from Yale University in 1858 and gained a PhD in gear design in 1863. He remained at Yale throughout his academic career, and was appointed professor of mathematical physics in 1871. Between 1873 and 1876 Gibbs developed the theory of thermodynamics in a long series of papers. He formulated the Gibbs phase rule which describes the equilibrium of heterogeneous systems. His name is also associated with Gibbs free energy, the function that determines the conditions in which a chemical reaction will occur.