Louis-Paul Cailletet (born Sept. 21, 1832, Châtillon-sur-Seine, France—died Jan. 5, 1913, Paris) was a French physicist and ironmaster, noted for his work on the liquefaction of gases.
As a youth, Cailletet worked in his father’s ironworks and later was in charge of the works. He was also active in scientific research. On Dec. 2, 1877, Cailletet became the first to liquefy oxygen. Shortly afterward he also succeeded in liquefying nitrogen, hydrogen, nitrogen dioxide, carbon monoxide, and acetylene for the first time. This work was carried on independently of the work on liquefaction by the Swiss physician Raoul-Pierre Pictet (1846–1929), and there was considerable discussion as to which of the two had succeeded first.
Cailletet was the author of a number of papers in Comptes Rendus and other French scientific periodicals on the liquefaction of gases and the production of low temperatures, on the passage of gases through metals, on manometers for measuring high pressures, on critical points, and on the state of matter at low temperatures. He was interested in aeronautics and devised an apparatus for measuring the altitude of an airplane. He became a member of the Academy of Sciences in 1884.