CH. XIl] THE CIRCULATION OF NITROGEN 279 



streams. In the sea, or in fresh waters, the waste products of 

 organisms are eliminated from their bodies into the surrounding 

 medium. Organisms, or parts of such, die and are cast off, are 

 buried, or are simply deposited upon the land, and in all kinds of 

 corners. All these substances begin to undergo putrefactive 

 decomposition if (as is usually the case) the conditions are favour- 

 able. Some of the products of this breaking-down, water and 

 carbon dioxide, are stable and undergo no further changes. Others 

 however are still unstable. Such are the numerous products of 

 the putrefactive decomposition of animal and plant proteids. 

 Considering only the changes that take place upon the land we 

 may say that the water of rivers and streams and that of the sub- 

 soil, contains a certain proportion of the products of nitrogenous 

 putrefaction — which one may loosely term " organic matter." 



This organic matter is now further acted upon by the nitrogen 

 bacteria. Altogether a host of nitrogenous substances are produced 

 from proteids and allied substances in the course of putrefactive 

 decomposition. All these have the same fate, that of further 

 breaking-down b}^ micro-organisms, some of which carry on the 

 resolution of amido-substances as far as the stage of ammonia, 

 while other bacteria, in the presence of abundant oxygen, oxidise 

 this ammonia to form nitrous and then nitric acids. 



In the process of the septic purification of sewage the crude 

 liquid is first of all treated in the open or closed tanks, and a 

 partial decomposition of the proteid and other organic matter 

 takes place. The effluent which issues from these tanks is a 

 liquid, which although not inodorous is far less offensive than the 

 crude sewage. But if incubated at a temperature of 38^ C. it will 

 still putrefy, shewing that organic matter is still present. The 

 septic tank effluent is therefore led into the " contact beds," which 

 are large rectangular, shallow pits, filled with coke, clinker, gravel, 

 or other porous filtering materials. These filter beds are flooded 

 with the effluent, and the latter is then allowed to stand for 

 a certain period, 24 hours or more. If the contact beds are 

 carefully constructed, and if the process is carefully and intelli- 

 gently directed (which is not always the case), the effluent which 

 results is a clear liquid, which contains very much less organic 

 matter than the septic tank effluent, which does not smell, and 



