THE ATMOSPHERE, 



35 



keeping a soil fertile, whether mechanical or chemi- 

 cal, must be guided by sound judgment. While prac- 

 tical observation and experience teach us that lands 

 do wear out by continued cultivation and removal 

 of crops, chemistry explains the cause, and points out 

 the remedy. This may be by the purchase of con- 

 centrated fertilizers, or by the use of such fertilizing 

 materials as already exist or can be produced on 

 every farm. This subject will be discussed in a sub- 

 sequent chapter. 



CHAPTER IV. 



COMPOSITION AND PROPERTIES OF THE ATMOSPHERE. 



71. THE atmosphere from which plants obtain a 

 large portion of their food is a mixture of oxygen and 

 nitrogen in the proportion of about one-fifth of the 

 former to four-fifths of the latter, with a small quan- 

 tity of carbon dioxide, a trace of ammonia, a vari- 

 able quantity of watery vapor, and traces of a few 

 other gases resulting from combustion and decay 

 Its composition by volume may be stated as follows 



Nitrogen 77-95 



Oxygen 20.61 



Watery vapor 1.40 



Carbon dioxide ." 0.04 



Ammonia ) 



... \ traces 



Hydrogen carbide . . ) 



( Hydrogen sulphide. > 



In towns \ \ traces 



( Sulphurous oxide. . . \ 



Sulphi 

 Total. . . 100.00 



