In the history of thermodynamics, On the Equilibrium of Heterogeneous Substances is a 300-page paper written by American engineer Willard Gibbs. It is one of the founding papers in thermodynamics, along with German physicist Hermann von Helmholtz's 1882 paper "Thermodynamik chemischer Vorgänge." Together they form the foundation of chemical thermodynamics as well as a large part of physical chemistry.