508 BACTERIOLOGY. 



exposed to the action of the heterogeneous group of 

 saprophytic bacteria, the alterations through which they 

 pass are ultimately 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 oxi- 

 dation, 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 sapro- 

 phytes 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, 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 immense saltpetre-beds of Chili and Peru, 

 where, by the activities of these microscopic plants, 

 nitrates are produced from the ammonia of the faecal 

 evacuations of sea-fowl in such enormous quantities 

 as to form the source of supply 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 

 ammonia, nitrous and nitric acids. In fact, the same 



