36 SCIENTIFIC AGRICULTURE. 



72. These constituents, though gaseous and in- 

 visible, can be separated and measured by the chem- 

 ist as surely and definitely as the farmer can mea- 

 sure his corn and wheat, or the planter weigh his 

 cotton and tobacco. Two of these, carbon dioxide 

 and ammonia, though in very small quantities, are 

 sufficient for the ordinary growth of plants. They 

 are all so nicely intermingled, so uniformly mixed, 

 that dry air, no matter where collected, is found to 

 possess essentially the same composition. 



73. Watery vapor, as above stated, is a variable 

 constituent of the atmosphere. It rises continually 

 from the surface of the land, as well as from every 

 lake, and river, and ocean, and comes to us again in 

 gentle showers which the cool currents of air con- 

 dense, or in dew-drops which settle by night on leaf 

 and flower. Beautifully harmonious are the laws 

 and operations of nature, and especially that law by 

 which the vapor of water so gently and continuously 

 ascends from the earth's surface and, condensing, 

 falls to be again vaporized after it has performed its 

 part in the support of animal and vegetable life. 



74. The study of chemistry has revealed the part 

 that each of these constituents of the atmosphere 

 performs in the economy of nature. Nitrogen is 

 negative in its character, being indifferent toward 

 entering into combination with other elements, while 

 oxygen is active and energetic in supporting com- 

 bustion, and in sustaining animal life. Our bodies, 

 like stoves, are consumers of carbon and oxygen, and 



