THE NITROGEN CYCLE 223 



mixtures of numerous organisms. The conditions, under 

 whicli the final nitrification then takes place, have been 

 worked out by Adeney in a series of very careful researches. 

 His method of research consisted in exposing solutions, 

 either of defined chemical substances such as urea, asparagin, 

 ammonium tartrate, etc., or less defined organic matter such 

 as town sewage, or infusions of peat, to the prolonged action 

 of oxygen, in the presence of the usual organisms to be found 

 in natural waters. This was accomplished either by mixing 

 the solution with a known volume of aerated tap water, or 

 by shaking the solution periodically with known volumes of 

 air. Not only were the products of decomposition and 

 oxidation determined, such as ammonia and nitrous and nitric 

 acid, but also the carbonic acid resulting from the oxidation 

 of the carbonaceous matter present, as well as the resulting 

 change in composition of the dissolved gases present. For 

 this purpose Adeney devised a special form of gas analysis 

 apparatus, which enabled him to analyse the gases obtained 

 on boiling out the solutions in vacuo. He discovered the 

 source of error in previous determinations, viz., the fact that 

 the carbon dioxide formed by oxidation of organic matter 

 is present largely as carbonate, and is only fully recovered 

 from the solution if the latter is acidified before boihng. 

 The oversight of this fact led Sir Edward Frankland to 

 conclude that the rate of oxidation, e.g. of sewage matter, 

 when discharged into a stream, was much less than was actually 

 the case. As a result of prolonged investigation, Adeney 

 arrived at the following conclusions : — 



Oxidation of organic matter proceeds in two well-defined 

 stages, which may be briefly described as the carbon oxidation 

 stage, and the nitrogen oxidation stage. 



In the carbon oxidation stage, carbon dioxide, water, 

 ammonia, and excretory substances are produced; in the 

 second or nitrogen oxidation stage, the two last-named bodies 

 are further fermented, the products being nitrites, nitrates, 



