280 THE CIRCULATION OF NITROGEN [PART III 



which, if incubated at 38° C. does not putrefy. The putrescible 

 substances originally present have been further broken down by 

 the activity of the nitrifying bacteria, with the result that much of 

 their nitrogen is converted into nitrous and nitric acids, their carbon 

 and hydrogen passing into the forms of carbon dioxide and water. 

 In this process of purification much oxygen is absorbed. The 

 process is intermittent, each contact bed being filled with the 

 effluent, and then emptied and allowed to remain so for some time. 

 The filtering material remains covered by a scum of matter 

 containing the bacteria, but every time the bed is emptied of 

 liquid, air enters and fills up its interstices. It is the oxygen of 

 this air which is utilised in the oxidation of the nitrogenous 

 , organic matter^. It is of course absorbed during the nitrification 

 of the organic matter, but since the bed is used intermittently, 

 the filtering material is enabled to renew its stock of oxygen in 

 the intervals of its work. 



Denitrification. The process of resolution of organic matter 

 does not end even with this conversion of proteid, or the metabolic 

 products of the latter, into nitrous and nitric acids. The reader 

 will remember that bacteria are to be found, widely distributed 

 both on the land and in the sea, which have the power of reducing 

 nitric to nitrous acid, the latter to ammonia, and ammonia to 

 free nitrogen. Now there is reason to believe that such denitrify- 

 ing bacteria play a very significant part in the circulation of 

 nitrogen in nature. If the processes taking place in the sewage 

 purification tanks and contact beds be minutely studied it will be 

 found that there is sometimes an actual disappearance of nitrogen 

 from the effluent. If it were the case that all the organic matter 

 were decomposed so that its nitrogen were oxidised to nitrous and 

 nitric acids then we should expect to find just as much of this 

 element in the final effluent as was present in the crude sewage. 

 The form of combination only would be changed. But this is not 

 always the case. Letts "^ has shewn that such an actual dis- 



^ See Letts, Rept. Royal Commission on Seioarje Disposal, Kept. 3, VoL ii. 

 (Cd. 1487, 1903). Other sections of this report also contain much information on 

 the question of the purification of sewage matters. 



2 Ibid. 



