THE ATMOSPHERE 117 



With powerful pumps it is possible to so compress air in 

 pipes capable of withstanding great strain that when the 

 compressed air is cooled to a very low temperature it changes 

 to a liquid state, and becomes liquid air. It then looks very 

 much like water. As the air is being compressed its tempera- 

 ture is raised by reason of the crowding together of its mole- 

 cules. The pipes containing it can be kept cool, however, 

 by allowing water to run over them to carry away the 

 a heat of compression." When stop cocks in the pipes are 

 now opened a very little, allowing an outrush of some of the 

 highly compressed air into an enclosed space around the 

 pipes, its expansion is so excessive that its temperature and 

 that of the compressed air within the pipes is greatly lowered. 

 While there is loss of much of the compressed air in its return 

 to its normal density, other portions of it will be changed to a 

 liquid form by reason of the low temperature. Liquid air 

 must be kept in containers that are like " thermos" bottles 

 in having double walls from between which the air has been 

 withdrawn. These containers must not be kept stoppered 

 other than with a plug of cotton since the enormous expan- 

 sion in volume of this air as it slowly changes back to the 

 gaseous form would cause destructive explosions. 



Among the many things that are fascinating in the story 

 of liquid air, mention can be made here only of its use as a 

 source of nearly pure oxygen gas at a relatively low cost of 

 production. The atmosphere is largely composed of oxygen 

 and nitrogen gases in the proportion of about four volumes 

 of nitrogen to one of oxygen. When air is liquefied it con- 

 sists of a mixture of liquid oxygen and liquid nitrogen. Any 

 water vapor and any carbon dioxide gas present in the air 

 at first will have been eliminated by having been solidified 

 (frozen). At ordinary temperatures and pressures this 

 liquefied air changes back to the gaseous condition rapidly, 

 and in this vaporization the liquid nitrogen changes to a gas 



