BACTERIA IN NATURAL PROCESSES. 105 



gen. This dissipation of free nitrogen into the 

 air is going on in the world wherever putrefaction 

 takes place. Wherever decomposition of nitrogen 

 products occurs some free nitrogen is eliminated. 

 Now, this part of the nitrogen has passed beyond 

 the reach of plants, for plants can not extract 

 free nitrogen from the air. In the diagram this is 

 represented as a portion of the material which, 

 through the agency of the decomposition bacte- 

 ria, has been thrown out of the cycle at a tan- 

 gent (Fig. 25 E). It will, of course, be plain 

 from this that the store of nitrogen food must be 

 constantly diminishing. The soil may have been 

 originally supplied with a given quantity of nitro- 

 gen compound, but if the decomposition products 

 are causing considerable quantities of this nitro- 

 gen to be dissipated in the air, it plainly follows 

 that the total amount of nitrogen food upon 

 which the animal and vegetable kingdoms can 

 depend is becoming constantly reduced by such 

 dissipation. 



There are still other methods by which nitro- 

 gen is being lost from the food cycle. First, we 

 may notice that the ordinary processes of vegeta- 

 tion result in a gradual draining of the soil and 

 a throwing of its nitrogen into the ocean. The 

 body of any animal or any plant that chances to 

 fall into a brook or river is eventually carried to 

 the sea, and the products of its decomposition 

 pass into the ocean and are, of course, lost to the 

 soil. Now, while this gradual extraction of ni- 

 trogen from the soil by drainage is a slow one, it 

 is nevertheless a sure one. It is far more rapid 

 in these years of civilized life than in former 

 times, since the products of the soil are given to 

 the city, and then are thrown into its sewage. 



