LIQUEFACTION OF GASES. It was formerly believed that certain gases could not be liquefied under any condi tions, and from the sixteenth to the eighteenth century it was customary to distinguish "gases" from "vapors" by the statement that only the latter could be liquefied. It is now known, however, that any gas can be liquefied provided the necessary high pressure and low temperature can be obtained. The ear liest gas to be liquefied was sulphur dioxide, by Monge and Clouet at the beginning of the nineteenth century, and a few years later chlorine and hydrogen chloride were reduced to the liquid state by Northmore. Little interest was shown in the subje t however, until 1823, when Faraday began his series of experiments on liquefaction of gases, as a result of which he succeeded in producing sulphur dioxide, hydrogen sulphide, cyanogen, ammonia, carbon dioxide and nitrous oxide in a liquid form. His method was to introduce materials for generating gas into the limb of a bent tube, the other limb being sealed and packed in a freezing mixture. On generating the gas, high pressure was produced, and at length the gas liquefied in the cold limb. Other work ers, on similar lines, were Bussy, Thi lorier and Natterer, but certain gases, including hydrogen, oxygen, nitrogen and carbon monoxide could not be liquefied even though the enormous pressure of 3600 atmospheres was used by Natterer, and for a time it was believed that there existed so-called "permanent gases," which defied liquefaction. Then came the discovery, by Andrews, of "critical" phenomena. He found that on compress ing carbon dioxide in a tube at a tem perature below 30 9°C, the volume dimin ished until, at a certain pressure, which varied with the temperature, liquefaction began. At a temperature above 30 9°C,
however, he found that it was impossible to liquefy carbon dioxide, no matter what pressure was employed. He ob served similar phenomena in the case of nitrous oxide, and concluded that for every gas there is a definite temperature above which it cannot be liquefied. This temperature he called its critical tem perature. It was now clear that the failure of Natterer, and others, to liquefy certain gases was due to the fact that the temperatures used by them were above the critical temperatures and the attention of workers on the subject was turned to the production of extreme cold. In 1877 Louis Cailletet succeeded in liquefying both oxygen and carbon monoxide. He found that by compress ing a gas at a low temperature and then suddenly releasing the pressure, a marked drop in temperature occurred, and it was by utilization of this fact that he and later workers succeeded in lique fying in turn all the "permanent gases." Liquid air and oxygen were produced in large quantities as a result of Dewar's experiments, and Hampson and Linde, in 1895, introduced the method of self intensive refrigeration, in which the gas to be liquefied is continuously supplied through an apparatus in which it is cooled by expansion, and each portion of gas, after such cooling, is utilized in cooling the succeeding portion, until the cumulative effect produces liquefaction.