A gas possessing a critical temperature above room temperature may be liquefied by increasing the pressure on it. Otherwise, the gas must first be cooled below its critical temperature and then compressed; or, if desired, cooled directly to its boiling point under normal pressure. The methods of cooling are (1) by evaporation under reduced pressure, as in the cascade liquefier; (2) by using the principle of the Joule-Thomson effect (the Linde process); (3) by causing the gas to expand against an external pressure; in doing so the gas does work, thereby cooling itself. This principle is used in the Claude process.