Composition of ilic Atmospl 557 



2. Oxygen, forming about 20.61 per cent. 1 y volume. 



3. Water vapor, forming about 1.40 per cent, by vol- 

 ume. 



4. Argon, forming about .94 per cent, by volume. 



5. Carbon dioxide, forming about .03 per cent, by vol- 

 ume. 



Besides these ingredients tbere are usually present in the 

 air small amounts of ammonia and of nitric acid, which 

 are brought down with the rains to the extent of 3.37 

 pounds per acre per annum at Rothamstead, England ; 

 1.74 pounds at Lincoln, New Zealand; and 3.77 pounds in 

 the Barbadoes Islands. 



Oxygen often occurs in the allotropic form of ozone, 

 which is much more active as an oxidizing agent than the 

 ordinary condition. 



705. Materials Mechanically Suspended in the Atmos- 

 phere. In the gaseous body of the atmosphere there are 

 always mechanically suspended varying amounts of solid 

 and liquid particles and bodies. These are : 



1. Inorganic dust grains or soil particles. 



2. Organic dust fragments. 



3. Microscopic germs and spores. 



4. Pollen grains from various plants. 

 5 Snow or water crystals. 



6. Water particles in cloud forms. 



PARTS PLAYED T,Y THE DIFFERENT INGREDIENTS. 



The atmosphere as a whole, in its relation to living 

 forms, plays the important function of an equalizer of tem- 

 perature, preventing the occurrence o such excessively 

 high and extremely low degrees as would otherwise bo pro- 

 duced when the sun is above or bcL>w the horizon. 



706. Oxygen. Oxygen is essential to l><>th plants and 

 animals, it being indispensable to the activities of the proto- 



