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THE NITRIFYING BACTERIA. 37I 



group of saprophytic bacteria, in which are found many 

 different species, 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 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 nilrifying 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 earth's surface. Tlie 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 produced 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 purification 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 conspicuous modifications to their 

 ultimate conversion into ammonia, nitrous and nitric 

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



