42 THE PRINCIPLES OF AGHICULTUTvE. 



Wlicn the air becomes overcharged with moisture, the 

 sur})his comes to the earth again in the form of rain. 



3. Carbonic Acid Gas. — 'J'he chief sources of this gas 

 in the atmosphere are the bi'eathing of animals, the burn- 

 ing of fuel, and the decay of organic matter. 



It is thrown off from the systems of animals as useless, 

 but is essential to the life and growth of plants. 



It may be seen that the escaping breath contains car- 

 bonic acid, by breathing through a tube into clear lime- 

 water. There will be formed in the water a white 

 powder, which is carbonate of lime, produced by the 

 union of the carbonic acid and the lime. 



Although the air always contains some ciii'bouic acid 

 gas, when too large a (puintity is ]»resent it becomes poi- 

 sonous. For this I'eason it is imwholesome to sleep in 

 a small, close room, without some ventilation, or for 

 many persons to remain long in any closed room. The 

 burning of lamps in a room, or the decay of ^"egetables 

 in a cellar, produces the same effect as the breathing of 

 persons, by adding to the proportionate amount of car- 

 bonic acid gas. 



4. Impurities. — ^ The air contains suiall quantities of 

 various substances, as ammonia, nitric acid, etc., l)esides 

 smoke and dust. 



These are largely absorbed by the water vajior, and 

 brought to the earth in raindrops. They give lain-water 

 its dark color, and render it of some value as a fertilizer 

 for cro])s. 



Weight of the Atmosphere. — 'J'hc atnidsphcrc has weight 

 as truly as the ol)j('('ts which we can sec and haiulle. 

 The height, or distance from Ihe earth to which it ex- 

 tends, is not definitely known. It is variously estimated 

 at from one hundred 1o live luunh'ed miles. 



