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pressed and stored in steel tanks, 

 had been started to Europe. This 

 was enough to fill four of the ordi- 

 nary kite balloons, though the large 

 dirigibles require one to two million 

 or more cubic feet of gas. 



A GREAT ACCOMPLISHMENT 



Although quantity production of 

 helium was achieved just too late to 

 be of value in the actual hostilities, 

 it was in itself a great accomplish- 

 ment, for the world's total output 

 of helium up to 191 5 was probably 

 less than 100 cubic feet, the market 

 value of which was about $1,700 a 

 cubic foot. Our helium, on the 

 other hand, can be produced by the 

 first two methods at less than 10 

 cents a cubic foot, and if the third 

 process fulfills the expectations of 

 Bureau of Mines experts, this fig- 

 ure will be still further reduced. 



All of our helium so far, how- 

 ever, has been the product of the 

 first two experimental plants ; the 

 third plant has not yet been put on a. 

 practical working basis, though the 

 fact that it holds promise of produc- 

 ing helium much more cheaply than 

 the other two justifies further ex- 

 perimentation with it. 



The details of the process of ex- 

 tracting helium are highly technical, 

 but the general scheme is easily 

 understood. All of the main con- 

 stituents of natural gas, including 

 the nitrogen, become liquefied when 

 cooled to about — 328 Fahrenheit; 

 but the helium remains a gas at this 

 exceedingly low' temperature and is 

 thus easily separated. 



The principle by which these low 

 temperatures are attained is one 

 known to every motorist who is un- 

 fortunate enough to have to pump 

 up his own tires. When air is com- 

 pressed in a tire it becomes hot; if 

 the tire is allowed to cool to ordi- 

 nary temperature and the valve is 

 then opened and the air allowed to 

 escape, it becomes cool. 



Similarly, the natural gas is put 

 under a very high pressure and then 

 refrigerated, and when allowed to 

 expand it becomes so cold that all 

 of its constituents except helium be- 



