LIQUEFACTION OF THE ELEMENTARY GASES. 251 



to just below the critical point. The discovery of these properties sug- 

 gested an explanation of the failure of previous attempts to liquefy air. 

 Air at ordinary low temperatures is in the gaso-liquid condition, and 

 its liquefaction is not possible except when a difference exists between 

 the density of the vapor and that of the liquid greater than it is pos- 

 sible to produce under any conditions than can exist then. It was 

 necessary to reduce the temperature to below the critical point ; and 

 it was by adopting this course that MM. Cailletet and Raoul Pictet 

 achieved their success. The rapid escape of the compressed gas itself 

 from a condition of great condensation at an extremely low tempei*a- 

 ture was employed as the agent for producing a greater degree of 

 cold than it had been possible before to obtain. M. Cailletet used 

 oxygen escaping at — 29° C. from a pressure of three hundred atmos- 

 pheres ; M. Raoul Pictet, the same gas escaping at —140° from a 

 pressure of three hundred and twenty atmospheres ; and both ob- 

 tained oxygen and nitrogen, and M. Pictet hydrogen in what they 

 thought was a liquid, and possibly even in a solid form. 



Still, it could not be asserted that hydrogen and the elements of 

 the air had been completely liquefied. These gases had not yet been 

 seen collected in the static condition at the bottom of a tube and 

 separated from their vapors by the clearly defined concave surface, 

 which is called a me)iisctis. The experiments had, however, proved 

 that liquefaction is possible at a temperature of below —120° C. 

 ( — 184° Fahr.). To make the process practicable, it was only neces- 

 sary to find sufficiently powerful refrigerants ; and these were looked 

 for among gases that had proved more refractory than carbonic acid 

 and protoxide of nitrogen. M. Cailletet selected ethylene, a hydro- 

 carbon of the same composition as illuminating gas, which, when 

 liquefied by the aid of carbonic acid and a pressure of thirty-six 

 atmospheres, boils at —103° C. (—1.53° Fahr.). M. Wroblewski, of 

 Cracow, who had witnessed some of M. Cailletet's experiments, and 

 obtained his apparatus, and M. Olzewski, in association with him, also 

 experimented with ethylene, and had the pleasure of recording their 

 first complete success early in April, 1883. Causing liquid ethylene 

 to boil in an air-pump vacuum at — 103° C, they were able to produce 

 a temperature of —1.50° C. (—238° Fahr.), the lowest that had ever 

 been observed. Oxygen, having been previously compressed in a 

 glass tube, became a permanent liquid, with a clearly defined menis- 

 cus. It presented itself, like the other liquefied gases, under the form 

 of a transparent and colorless substance, resembling water, but a little 

 less dense. Its critical point was marked at —113° C. (—171° Fahr.), 

 below which the liquid could be formed, but never above it ; while it 

 boiled rapidly at —186° C. (—303° Fahr.). A few days afterward, 

 the Polish professors obtained the liquefaction of nitrogen, a more 

 refractory gas, under a pressure of thirty-six atmospheres, at — 146° C. 

 (—231° Fahr.). Long, diflicult, and expensive operations were re- 



