THE CHEMISTRY OP THE ATMOSPHERE. 105 



It goes into the fire with the oxygen, but parts compa- 

 ny with that gas as it unites with the combustible sub- 

 stance. There is only one of the processes that we have men- 

 tioned, that of decay, which affects the quantity of nitrogen 

 in the air, and this it does very slowly. Nitrogen, there- 

 fore, may be considered, in comparison with oxygen and 

 carbonic anhydride, almost a fixed constituent of the air. 

 So far as we know as yet, the only way in which the nitro- 

 gen of the air is lessened is by the occasional formation of 

 nitric acid by electricity, the result of a union effected by 

 this agent between some of the oxygen and nitrogen of the 

 air in the presence of moisture. Only minute quantities of 

 this powerful acid are produced in this way, and chemists 

 have to use very delicate tests to detect it. It is useful in 

 the promotion of vegetation, as you will see in another part 

 of this book ; and it is supposed that this is the purpose of 

 its production, it being brought down by the rain as it falls, 

 to soak with it into the earth. But comparatively little of 

 the great bulk of the nitrogen can be used in this way, and 

 this small diminution is met by a supply from the processes 

 of decay. 



133. Air in "Water. There is always more or less air in 

 water. It is dissolved in it, for it is wholly hidden from 

 view among the particles of the water, and does not ap- 

 pear in bubbles except in the act of escaping 

 from its dissolved condition. This can be shown 

 by a very pretty experiment. Place a vessel of 

 water under the receiver of an air-pump, Fig. 

 33. You can see no air in it, and yet on ex- 

 hausting the air from the receiver multitudes of 

 small bubbles will arise, as represented. This Fig. 33. 

 is because the pressure is taken off from the surface of the 

 water, and the air, therefore, which is dissolved in it, ex- 

 pands and escapes, its particles huddling together in bub- 



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