Four assorted views of a thermostat and equipment used to determine the compressibility of gas at the U.S. Department of Agriculture's Fixed Nitrogen Research Laboratory located in Washington, D.C. In chemistry, compressibility refers to the measure of how much a given volume of matter decreases when placed under pressure. At the Fixed Nitrogen Research Laboratory, this particular apparatus was used to measure the amount of gas present after expansion from a high-pressure environment. The individual visible in the photographs is identified as Dr. E.P. Bartlett.

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.