LIQUID AIR. 37 



gases, but for some gases pressures very much higher and tempera- 

 tures very much lower are required. Faraday himself succeeded 

 in liquefying all the gases then known except oxygen, hydrogen, 

 nitrogen, nitric oxide, and marsh gas. He subjected oxygen to a 

 pressure of about one thousand pounds to the square inch, or nearly 

 seventy atmospheres, but it showed no signs of liquefaction. Later 

 experimenters increased the pressure to four thousand pounds to the 

 square inch, with no better results, so that it is not surprising that 

 it came to be held that some gases are permanent. 



Within comparatively recent years several gases have been lique- 

 fied on the large scale by means of pressure. These are ammonia, 

 carbonic acid, nitrous oxide, and chlorine. Ammonia is used for 

 producing low temperatures, as in breweries and in cold-storage 

 plants and in the manufacture of ice; carbonic acid, for fire extin- 

 guishers and for charging beer with the gas; nitrous oxide, for pro- 

 ducing anaesthesia; and chlorine in connection with several branches 

 of chemical manufacture. The production of low temperatures by 

 means of liquid ammonia and of liquid carbonic acid will be more 

 fully dealt with further on, when the principles involved will be 

 briefly presented. It is to be borne in mind that these substances 

 are liquefied by means of pressure alone, at temperatures that are 

 easily reached, so that it appears that by mechanical pressure it is 

 possible to produce low temperatures. In 1869 an important fact 

 was discovered by Andrews. It was that for every gas there is a 

 temperature above which it is impossible to liquefy it by pressure. 

 Thus, if chlorine is at any temperature above 146 C. (294 F.) 

 it can not be liquefied. This temperature is called the " critical tem- 

 perature " of chlorine. The pressure to which the gas must be sub- 

 jected at the " critical temperature " in order that the gas may be 

 liquefied is called the " critical pressure." In the case of chlorine 

 this is 93.5 atmospheres. Now, the critical temperature of the gases 

 that were called permanent gases are very low lower than could be 

 reached by the means at the command of earlier experimenters. The 

 critical temperature of oxygen, for example, is 118.8 C. ( 182 

 F.), while that of nitrogen is 146 C. (-230 F.). The critical 

 pressures are 50.8 and 35 atmospheres respectively. As there is no 

 difficulty in obtaining these pressures, the problem of liquefying 

 oxygen and nitrogen and air resolves itself into finding a method of 

 producing temperatures below the critical temperatures of these gases. 



It is well known that a temperature somewhat below the freezing 

 point of water can be produced artificially by mixing ice and salt. 

 The ordinary ice-cream freezer is a familiar application of this 

 method of producing *cold. Other freezing mixtures that are some- 

 times used consist of calcium chloride and snow, that gives the tern- 



