Cailletet, Louis Paul (1832–1913)

French physicist and inventor. In 1877–78 he was the first to liquefy oxygen, hydrogen, nitrogen, and air. He did it by cooling them below their critical temperatures, first compressing the gas, then cooling it, then allowing it to expand to cool it still further.

Cailletet was born in Chatillon-sur-Seine and educated in Paris at the Ecole des Mines, after which he returned to Chatillon to manage his father's ironworks.

Investigating the causes of accidents that occurred during the tempering of incompletely forged iron, Cailletet found that many were due to the highly unstable state of the iron while it was hot and had gases dissolved in it. He also analysed the gases from blast furnaces. As a result of these and other metallurgical studies, Cailletet developed a unified concept of the role of heat in changes of state of metals.

Cailletet's other achievements included the installation of a 300-m/985-ft high manometer on the Eiffel Tower; an investigation of air resistance on falling bodies; a study of a liquid-oxygen breathing apparatus for high-altitude ascents; and the construction of numerous devices, including automatic cameras, an altimeter, and air-sample collectors for sounding-balloon studies of the upper atmosphere.