THE NITRIFYING BACTERIA 601 



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 arising from the decomposing 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 decomposition and subse- 

 quent nitrification of the organic matters of sewage and 

 other fluid wastes of organic nature in the process of puri- 

 fication 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 con- 

 version into inorganic forms of 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- 



