88 CHEMICAL DISCOVEEY AND INVENTION 



any change of temperature observed must be due to the per- 

 formance of work, either in lifting atmospheric pressure or 

 internally in overcoming the mutual attractions of the gaseous 

 molecules. The rise of temperature noticed in the case of 

 hydrogen at ordinary temperatures indicates that the mole- 

 cules of this gas, under such conditions, repel one another. 



By compressing an ordinary gas, such as air, removing the 

 heat produced by immersing the containing vessel in cold water 

 or otherwise, and when cool allowing the gas to escape, a reduction 

 of temperature is obtained. At common temperatures this 

 amounts to about one quarter of a degree centigrade for each 

 atmosphere of pressure taken off. If this cold stream of escaping 

 air is then made to pass over the pipe through which the 

 gas passes previous to release, it may be cooled in a cumulative 

 manner to lower and lower temperatures until it reaches a 

 temperature below the critical point. By continuing the same 

 process with comparatively moderate pressures a portion of the 

 gas becomes liquid. This is the principle of the air liquefying 

 plant to be found in all the greater institutions for chemical and 

 physical research. 



Practical results on a large scale were first shown by Linde, an 



engineer of Munich, in 1895. Patents for the application of the same 



idea were taken out a little earlier in England by W. Hampson. 



The apparatus actually employed consists of a compressor 



which, usually in two stages, puts 

 the air, previously freed from 

 water and carbon dioxide, under 

 a pressure of 200 atmospheres. 

 In this condition it is delivered 

 into a close coil of copper tube, 

 at the end of which is a small 

 hole, the size of which can be 

 regulated by a screw. Escaping 

 from this orifice the cooled air 

 returns to the atmosphere through 

 a cylinder in which the copper 

 pipe is coiled from which the air 

 has just escaped, or through a 

 coil which encloses the return pipe 

 as shown in the diagram. The 

 cooling effect is thus economised 

 on the regenerative system, 



Air c/r\he.r<i ol" ^ 

 SOOO.Ibs p-rD" 



Air Issuos 

 50O.lbs.p*rD" 



_'^ "7^ = Liquid 



FIG. 42. CUMULATIVE COOLING 

 FOR LIQUEFACTION OF AIK. 



