ATMOSPHERE 121 



the rate of over fifty miles an hour, forming a furious 

 gale. 



Such variations constantly occur on the same level. 

 Change the level and other variations occur. Ascend 

 a lofty mountain and the air becomes rarer and rarer, 

 until men on such heights gasp for breath to gain suffi- 

 cient oxygen to feed the body fires. 



Composition. The composition of the atmosphere is 

 nearly the same at all times and at all places where it 

 has been analyzed. It is made up chiefly of two gases, 

 nitrogen and oxygen. Besides these two gases, several 

 lesser constituents are present. Of these, the most 

 important are carbon dioxide and argon. Water vapor 

 and dust are also present in amounts which are ex- 

 tremely variable. 



Air is essentially a mechanical mixture. The gases 

 in it are not united or combined in any way, but are 

 almost independent of one another, and each of them 

 retains its own qualities in the mixture. The composi- 

 tion of air, according to Kahlenberg, follows: 



COMPOSITION OF AIR 



Per Cent of 

 Gas Volume 



Nitrogen 77.42 



Oxygen 20.77 



Water vapor (average) 0.85 



Argon and other gases / 0.93 



Carbon dioxide ( average > 0.03 



Total ,,,... 100.00 



Properties and uses. Nitrogen, we have already 

 learned, is extremely inactive and does not combine 



