THE NITRIFYING BACTERIA. 529 



to say that wherever dead organic matters are exposed 

 to the action of the great group of saprophytic bacteria, 

 in which are found many different species, the altera- 

 tions through which they pass are ultimately character- 

 ized by the appearance of these three bodies. When the 

 process of decomposition occurs in the soil, however, it 

 does not cease at this point, but we find still further 

 alterations alterations concerning more particularly 

 the ammonia. This change in ammonia is character- 

 ized by the products of its oxidation, viz., by the for- 

 mation of nitrous and nitric acids and their salts ; it is 

 not a result of the direct action of atmospheric oxygen 

 upon the ammonia, but occurs through the instrumen- 

 tality of a special group of saprophytes known as the 

 nitrifying organisms. They are found in the most super- 

 ficial layers of the ground, and though more common 

 in some places than in others, they are, nevertheless, 

 present over the entire surface of the earth. The 

 most conspicuous example of the functional activity of 

 this specific form of soil organism is seen in the im- 

 mense saltpetre-beds of Chili and Peru, where, by the 

 activities of these microscopic plants, nitrates are pro- 

 duced from the ammonia of the faecal evacuations of 

 sea-fowl in such enormous quantities as to form the 

 source of supply of this article for the commercial 

 world. A more familiar example, though hardly upon 

 such a great scale, is seen in the decomposition and 

 subsequent nitrification of the organic matters of sew- 

 age and other impure waters in the process of puri- 

 fication by filtration through the soil, a process in which 

 it is possible to follow, by chemical means, the organic 

 matters from their condition as such through their con- 

 spicuous modifications to their ultimate conversion into 



