108 BACTERIOLOGY. 



host of aerobic as well as anaerobic bacteria. These are 

 found everywhere in nature, in the air, water and soil. 



Ammonia, like hydrogen sulphide, is a very common 

 product formed in the putrefaction of protein substances, 

 whether of animal or vegetable origin. Diverse bacteria 

 and moulds take part in this transformation. 



Nitrification. As indicated above, the albuminous sub- 

 stances present in the dead plant or animal, are acted upon 

 by bacteria and decomposed into the simplest inorganic 

 products. The nitrogen eventually appears as ammonia, 

 although a small amount may be returned to the air as the 

 free element. The ammonia, thus formed, is essential to 

 the growth of higher plants. In order to be assimilated it 

 must be oxidized. This change is again effected by certain 

 bacteria, which oxidize the ammonia first to nitrous, and 

 finally to nitric acid. 



Organisms of this kind are widely distributed in the 

 soil and in water. Many of the ordinary, pathogenic and non- 

 pathogenic bacteria, are capable of forming nitrites and ni- 

 trates. This change can be readily observed in urine, which 

 ordinarily contains only traces of these compounds. As 

 soon as bacteria begin to develop in the urine, nitrites and 

 nitrates can be detected. It is for this reason that the 

 chemist, when examining water suspected of being polluted, 

 tests for nitrites and nitrates, as well as for ammonia. 



The formation of nitrites and nitrates is, therefore, tak- 

 ing place wherever dead, nitrogenous, plant or animal mat- 

 ter is present. The water, and especially the soil, contains, 

 bacteria which are capable of effecting this conversion. A 

 suitable temperature, moisture, and a proper amount of 

 oxygen are, of course, likewise essential. It is in this way that 

 the dung-heaps of India furnished the world's supply of salt- 

 peter. The soda salt-peter which exists in vast deposits in 

 Chili and Peru, likewise, owes its origin to the action of 

 bacteria on the guano, or excrement of birds. 



