Modern physical chemistry was developed in the 1860s to 1880s with work on chemical thermodynamics, electrolytes in solutions, chemical kinetics and other subjects. In 1876, Josiah Willard Gibbs wrote an article called, "On the Equilibrium of Heterogeneous Substances". This paper introduced many of the main parts of physical chemistry, such as Gibbs energy, chemical potentials, Gibbs phase rule. Other important discoveries include Heike Kamerlingh Onnes' work on enthalpy and macromolecular processes. 