CHAPTER XVIII 

 PRESSURE OF THE AIR 



103. The Atmosphere. The gaseous matter which envelops 

 the earth and penetrates some distance below the surface is 

 called the atmosphere. We commonly speak of it as the air. 

 Air consists almost entirely of two gases, oxygen and nitrogen, 

 which occur as a mechanical mixture and not as a chemical 

 compound. Of this mixture, oxygen forms about 21 per cent, 

 and nitrogen about 77 per cent. There are also present in the 

 air, carbon dioxide, water vapor, traces of argon and other 

 rare gases, ammonia, dust, and the spores of plants. In 

 addition to the oxygen and nitrogen, the only constituents 

 worth notice at this time are carbon dioxide and water vapor. 

 Carbon dioxide, though a product of all ordinary burning, 

 forms only about iKoo f 1 P er cent.; that is, only three parts 

 in 10,000 of dry air. Water vapor varies in amount as we 

 have already seen, and is usually present in much larger 

 quantities, often as much as 3 per cent. Since the amount of 

 water vapor present depends upon the time and locality, it 

 need not be considered at this point. In ten thousand parts 

 of average air, therefore, the chief components are represented 

 about as follows: 



Nitrogen. 7,700 parts 



Oxygen 2,100 parts 



Argon 100 parts 



Carbon dioxide 3 parts 



104. Weight of the Air. Though extremely light in com- 

 parison with other familiar substances, air, being a form of 



119 



