96 PROGRESS OF SCIENCE IN THE CENTURY. 



which the gas can be generated within a tube strong 

 enough to resist the pressure of the gas as it accumu- 

 lated/' and thus chlorine, muriatic acid, carbonic 

 acid, ammonia and many others were liquefied, es- 

 pecially through the energetic work of Faraday.* 



In 1835, Thilorier published an account of an 

 experiment, now familiar to students of chemistry, 

 in which he allowed a jet of liquid carbonic acid to 

 escape into a receiver where the evaporation of part 

 of the liquid produced a temperature so low that the 

 rest was frozen into fine snow. In 1845 Faraday 

 combined the method of low temperatures with that 

 of high pressures in the hope of conquering the so- 

 called permanent gases, such as oxygen, hydrogen, 

 nitrogen. But these, along with nitric oxide, carbon 

 monoxide, and methane, resisted his efforts. 



In 1869, Andrews expounded his definition of the 

 "critical point," — the temperature (30.92° C. for 

 carbonic acid) above which no amount of pressure 

 produces visible liquefaction, but below which lique- 

 faction occurs when the pressure is sufiicient. " A 

 vapour is a gas at any temperature below its critical 

 point." This step towards clearness led experi- 

 menters to recognise that the reason why oxygen, 

 nitrogen, etc., proved intractable was that sufficient 

 low temperatures (below their critical points) were 

 not available. 



In 1875-7, by devices securing lower tempera- 

 tures, Raoul Pictet and Louis Cailletet succeeded 

 in liquefying oxygen. Carbonic oxide, marsh gas, 

 nitric oxide, and others also yielded to the " Caille- 

 tet pump," and only nitrogen and hydrogen remained 

 unsubdued. In 1883, nitrogen was liquefied by two 

 Polish workers, Wroblewski and Olszewski. Finally 

 * Tilden, Short History of Chemistry, p. 240. 



