Xfquto Bit. 1*7 



would be impossible for animal existence, for 

 the reason that there would be no air to 

 breathe, since it would all liquefy. 



Liquid air is not a natural product. There 

 is no place on our earth cold enough to pro- 

 duce it. If the moon had an atmosphere 

 (which it probably has not) it would liquefy 

 during the long lunar night, for heat radiates 

 very rapidly from a planet when the sun's rays 

 are withdrawn from it. 



As you have already surmised, liquid air is 

 a product of intense cold. Any method 

 that will reduce the temperature of the air 

 to 312 degrees Fahrenheit below zero will 

 liquefy it. Great pressure will not do this, 

 for we may compress air in a strong vessel 

 until the pressure on every square inch of the 

 vessel is 12,000 pounds, or six tons, and still it 

 will not liquefy unless the temperature is 

 brought down to the required degree of cold- 

 ness. If this is done it will change from a 

 gas to a liquid, but will occupy as much space 

 as before, if it is condensed to a pressure of 

 six tons to the square inch. 



Until twenty years ago it was supposed that 

 oxygen and atmospheric air (the latter a mix- 

 ture of oxygen and nitrogen) were fixed gases 

 and could not be liquefied. In 1877, it is said 

 that Eaoul Pictet obtained the first liquid 

 oxygen, but only a few drops. About fifteen 

 years later Professor Dewar of the Koyal In- 



