40 AGKONO.MY 



twice it's weight of water and is the source of most of the 

 nitrogen used by them. 



The air. We are living at the bottom of an ocean of air 

 from which all the animals and plants derive elements neces- 

 sary to existence. From it animals obtain the oxygen for res- 

 piration, while the plants, in addition to a similar use of oxygen, 

 make use of the carbon dioxide, forming from it nearly half 

 their dry weight. The air is made up of several gases, two 

 of which, oxygen and nitrogen, comprise more than 98 per cent 

 of its bulk. The proportions of the principal gases are given 

 in the following table : 



Nitrogen 77.95 



Oxygen 20.61 



Water vapor (average) 1.40 



Argon 1.00 



Carbon dioxide 0.03 



The air also contains small quantities of krypton and neon. 

 Water vapor, which varies in quantity with the locality, is 

 not strictly a part of the air. The air is possibly several hun- 

 dred miles deep and at sea level presses on every square inch 

 of surface with a weight of nearly 15 pounds, or more than 

 46,000 tons, to the acre. The pressure varies somewhat with 

 the weather, being greatest in calm, fine weather and least as 

 a storm approaches. Differences in pressure are measured by 

 the barometer, in which a column of mercury rises with high 

 pressure and lowers with a decrease of pressure. These varia- 

 tions in pressure exert a considerable influence over the air 

 and water in the soil. When the barometer is falling, the les- 

 sened pressure causes the air to flow out of caves, mines, and 

 other underground cavities, while the water in wells rises and 

 springs flow more copiously. In some localities the underly- 

 ing porous rocks absorb so much air when the pressure is high 

 that when it is relaxed the amount of air rising from wells 

 and other openings in the soil may be sufficient to cause a 



