General view of an apparatus used to investigate the Phase Rule in an iron-nitrogen system at the U.S. Department of Agriculture's Fixed Nitrogen Research Laboratory located in Washington, D.C. First proposed by Josiah Willard Gibbs, the Phase Rule provides the theoretical foundation for characterizing the chemical state of a system and predicting the number of stable phases (defined as any physically separable material in the system) that may exist in equilibrium as a function of physical conditions such as pressure and temperature.

The Fixed Nitrogen Research Laboratory (F.N.R.L.) was established at American University in 1919 under the directorship of Arthur B. Lamb. Initially part of the War Department, the F.N.R.L. was the successor to several wartime initiatives to develop a secure domestic supply of nitrate compounds necessary for the manufacture of explosives during World War I. With a staff of about 110 individuals, including 35 to 50 chemists, the F.N.R.L. focused on the manufacture, production, and development of products of atmospheric nitrogen, including munitions and fertilizers.