50 PHYSICAL SCIENCE 



be cooled. The change of temperature pro- 

 duced, which was only one-fifth of a degree per 

 atmosphere difference of pressure in the original 

 experiments, can thus be increased to any extent 

 by a preliminary cooling of the air. 



This cooling by the performance of internal 

 work underlies the third method adopted in the 

 liquefaction of gases. It must be distinguished 

 clearly from the second method, in which most 

 of the cooling is effected by making the gas do 

 external work. 



Let us imagine that a stream of air, previously 

 cooled by liquid carbonic acid, is forced through 

 a spiral tube by aid of an air-pump and engine, 

 and that finally it merges through a fine nozzle 

 at the end of the tube. The nozzle acts as a 

 porous plug, and the air, cooled by free expansion, 

 is lowered in temperature by doing internal work. 

 Let us further suppose that the issuing air, so 

 cooled, is made to flow back over the tube through 

 which the stream of air passes. The advancing 

 current of air is still further cooled, the effect of 

 the expansion at the nozzle is increased, and a 

 temperature yet lower than before attained. 

 This cycle of operations — the continual passage 

 of the air just cooled by free expansion over the 

 current of air before it issues from the nozzle — 

 results in a constantly decreasing temperature, 

 and eventually cools the air below its critical point, 

 finally causing liquefaction. This self-intensifying 

 action is sometimes referred to as the regenera- 

 tive principle. It was applied to the liquefaction 

 of air by Linde in Germany, by Hampson and 

 Dewar in England, and by Tripler in America, 

 and is now used on large scale machines. 



