296 SMITH'S INTERMEDIATE CHEMISTRY 



it cools the highly compressed air in the inner pipe. The cooler 

 air, on reaching the closed vessel, expands and becomes colder 

 than ever, and in passing backwards lowers the temperature of the 

 air in the inner pipe still further. Finally, the air in this pipe 

 liquefies and drops of liquid air are expelled into the closed vessel. 



This is allowed to run out through a valve, from 



time to time, as it accumulates. 



Liquid air can be kept in Dewar flasks (Fig. 77). 



The space between the inner and outer flasks is 



evacuated, so that there is no gas to carry heat 

 FIG 77 from the atmosphere in to the liquid air. The 



inner surface of the outer flask is often silvered, so 

 that radiant heat, from surrounding bodies, may be reflected and 

 not absorbed. Similar containers are in common use for keeping 

 liquids hot or cold for a long time (Thermos flasks). 



./ 



/ 



Liquid Air. Liquid air varies in composition, as the nitrogen 

 (b.-p. 194) is less condensible than the oxygen (b.-p. 181.4). 

 WTien liquid air evaporates, therefore, the first portions of gas 

 that come off consist almost entirely of nitrogen. Pure nitrogen 

 obtained in this way is used in the manufacture of ammonia by 

 the Haber process (p. 300), and in the formation of calcium cyana- 

 mide (p. 392). By allowing evaporation to continue a liquid 

 containing 75 to 95 per cent of oxygen is obtained. This is 

 pumped into cylinders and sold as compressed oxygen. It con- 

 tains about 3 per cent of argon, and is a convenient source of this 

 element. Cartridges made of granular charcoal and cotton waste, 

 when saturated with oxygen-rich liquid air, are used as an explo- 

 sive in mining. . 



THE INERT GASES 



Argon. Lord Rayleigh was the first to observe that a liter of 

 pure nitrogen weighed 1.2505 g., while a liter of atmospheric 

 " nitrogen " weighed 1.2572 g. The natural inference was that 



