LIQUEFACTION OF GASES

The condition nece ary for the liquefaction of a gas is a low i enough temp., accompanied by sufficient pre ure. When ‘LIQUEFACTION OF GASES) the pre ure on a gas is increased, the temp. of liquefaction is raised. For every gas there exists a ‘critical temperature,’ above which it is impo ible to liquefy it by compre ion; the pre ure under which a gas is liquefied at its critical temp. is ‘critical pre ure’. When this principle was recognized attempts were made to liquefy the ‘permanent gases’ under pre ure by cooling them below their critical temp. Thus, in 1877, Pictet and Cailletet in- -dependently liquefied oxygen in quite different ways. In Pictet’s apparatus, carbon dioxide (critical temp. 31°3° C.) was liquefied by compre ion, while it was cooled by being surrounded with liquid sulphur dioxide, boiling under reduced pre ure at -65° C. This cooled liquefied carbon dioxide was conveyed to a cylinder surrounding a steel tube which contained oxygen compre ed to 320 atmospheres; there it was made to evaporate so rapidly by pumping that its temp. fell to -140° C. Thus the oxygen, being cooled below its critical temp. (-118'8° ©.), was turned into a liquid. Cailletet liquefied oxygen by the further cooling caused by the sudden expansion of the compre ed and already cooled gas. 2 Liquid Air.—Air is liquefied by the proce known as ‘self-intensive refrigeration.’ When gases pa through a narrow orifice from a high to a lower pre ure they are cooled in the proce , not because of external work performed, but by reason of internal work done against cohesion, a certain amount of which exists between the molecules of all gases at high pre ure. This cooling, called the Joule Thomson effect, amounts for air to 0°2-0°'25° C. per atmosphere. The principle was applied by Linde and by Hampson in 1895. Air is compre ed to 160-180 atmospheres in cylinders cooled by water, and, after being dried and freed from carbon dioxide pa es through a narrow spiral tube terminating at a regulated valve, through which it expands freely. The escaping air, cooled by its own expansion, pa es over the coil through which the compre ed air is being driven, cooling the latter before it expands. Thus the temp. of the i uing air is continuously lowered, until some of it liquefies as it escapes from the valve. Liquid air may drop from the orifice in about four minutes after the pumps have begun to work, and from 1 to 1)4 litre of it be obtained per hour, Liquid air, whose critical temp, is about -140° ©. and critical pre ure about 39 atmospheres, boils under one atmosphere at about -190°C. It generally contains more than twice as much oxygen as atmospheric air, because oxygen is more readily condensed thi nitrogen. Oxygen is now obtain from liquid air by fractional evaporati of the more volatile nitrogen. ‘The ra gases of the air—helium, neon, kryptc and xenon—are also separated fro argon by fractional evaporation of t liquefied mixture. Liquid air evaporat rapidly in ordinary ve els, congeal i the moisture in the adjacent air, whi falls as heavy clouds. It po e motive power analogous to that of boil water, but an objection to its use is f formation of ice round the machine containing it. Cotton wool, mixed wi granulated charcoal, and soaked liquid air, may be exploded by deton tion, and has been used for blasting coal mines. Liquefied gases are pr served in double-walled evacuated gla ve els, which may be silvered, and a known as Dewar flasks. HEvacuatic reduces thermal conduction to a mit mum, and silvering hinders radiatio Thermos flasks are constructed on # same principle. Liquefaction of Hydrogen.—Liqu hydrogen was first obtained in bulk } Dewar in 1898, by cooling the g compre ed to 150 atmospheres by mea: of liquid air boiling at -205° C., and th allowing it to expand. It is a cles colorle liquid, boiling under at me pheric pre ure at -252°5° C., and becor ing an ice-like solid at -257° ©. I critical temp. is about -238° C., a critical pre ure about 15 atmosphere Liquefaction of Helium—Helium w liquefied by Kamerlingh Onnes in 191 by expansion from high pre ure aft cooling in liquid hydrogen. Its b.p. -268°5°C. ae