390 BACTERIOLOGY. 



matters are exposed to the action of the great group of 

 saprophytic bacteria, in which are found many different 

 species, the alterations through which they pass are ulti- 

 mately characterized 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 

 characterized by the products of its oxidation, viz., by 

 the formation 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 

 instrumentality of a special group of saprophytes known 

 as the nitrifying organisms. They are found in the 

 most superficial layers of the ground, and though more 

 common in some places than in others, they are, never- 

 theless, present over the entire earth's surface. The 

 most conspicuous example of the functional activity of 

 this specific 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 faecal 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 subse- 

 quent nitrification of the organic matters of sewage and 

 other impure waters, in the process of purification by 

 filtration through the soil ; a process in which it is pos- 

 sible to follow, by chemical means, the organic matters 

 from their condition as such, through their conspicuous 

 modifications to their ultimate conversion into ammonia, 

 nitrous and nitric acids. In fact the same breaking 



