NITRIFYING BACTERIA. 429 



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 earth's surface. The most con- 

 spicuous example of the functional activity of this spe- 

 cific form of soil organism is that seen in the immense 

 saltpetre beds of Chili and Peru, where, through the 

 activities of these microscopic plants, nitrates are pro- 

 duced from the ammonia of the fecal evacuations of 

 sea-fowls 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 that seen in the decomposition 

 and subsequent nitrification of the organic matters of 

 sewage 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 



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