42 THE PRINCIPLES OF AGRICULTURE. 



When the air becomes overcharged with moisture, the 

 surplus comes to the earth again in the form of rain. 



3. Carbonic Acid Gas, The chief sources of this gas 

 in the atmosphere are the breathing 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 carbonic acid 

 gas, when too large a quantity is present it becomes poi- 

 sonous. For this reason it is unwholesome 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 vegetables 

 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 small quantities of 

 various substances, as ammonia, nitric acid, etc., besides 

 smoke and dust. 



These are largely absorbed by the water vapor, and 

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

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

 for crops. 



Weight of the Atmosphere. The atmosphere has weight 

 as truly as the objects which we can see and handle. 

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

 tends, is not definitely known. It is variously estimated 

 at from one hundred to five hundred miles. 



