574 APPLICATION OF METHODS OF BACTERIOLOGY 



tions — alterations having to do more particularly with 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; this 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 generically as the nitrifying organ- 

 isms. 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 func- 

 tional activity of this group of soil organisms is seen in the 

 immense saltpeter-beds of Chili and Peru, where, by the 

 activities of these microscopic plants, nitrates are produced 

 from the ammonia of the fecal evacuations of sea-fowls and 

 from decomposing seaweeds in such enormous quantities as to 

 form a source of supply of crude saltpeter for the commercial 

 world. A more familiar example is seen in the decomposi- 

 tion and subsequent nitrification of the organic matters 

 of sewage and other fluid wastes of organic nature in the 

 process of purification by percolation through the soil, 

 a process in which it is possible to follow, by chemical 

 means, the organic matters from their condition as such to 

 their ultimate conversion into ammonia, nitrous and nitric 

 acids. In fact, the same breaking down and building up, 

 resulting ultimately in nitrification, occurs in all nitrogenous 

 matters that are deposited upon the soil and allowed to 

 decay. It is largely through this means that growing vege- 

 tation obtains the nitrogen necessary for the nutrition of its 

 tissues, and when viewed from this standpoint we appre- 

 ciate the importance of this process to all life, animal as 

 well as vegetable, upon the earth. 



