THE NITROGEN CYCLE 229 



NO and N 2 ; or (3) the reduction of nitrates and nitrites 

 to nitrogen. 



The first characteristic work on this subject was done by 

 Gayon and Dupetit in 1882. They found that when a solution 

 containing potassium nitrate, together with sewage and a 

 little urine, was allowed to stand in absence of air, the nitrate 

 was reduced. When using nitrated broth containing asparagin, 

 they obtained an evolution of nitric oxide ; they also noted 

 the effect on the reaction due to the addition of carbohydrates 

 and tartrates, etc., and they concluded that de-nitrification 

 was essentially the combustion of organic matter by the 

 oxygen of the nitrates. It thus naturally proceeded best in 

 presence of a minimum air supply. It could be shown, e.g., 

 that in a given solution a greater amount of de-nitrification 

 took place in the lower portion of the solution than at the 

 surface. 



The subject of de-nitrification has been investigated by 

 numerous workers, notably Percy Frankland and Beyerinck. 



The latter describes an elegant experiment for the demon- 

 stration of the presence of de-nitrifying organisms in sewage. 

 O'l per cent, of potassium nitrate and a little starch paste is 

 added to nutrient gelatine, and the whole sterilised and poured 

 into a Petri dish. A little sewage, diluted, say twenty times, 

 with distilled water is poured on and off the plate, which is 

 turned with the gelatine surface downwards and allowed to 

 grow at 20 C. When the colonies have developed, a dilute 

 solution of hydrochloric acid and potassium iodide is poured 

 over half the plate. Wherever nitrites have been formed, 

 iodine will be liberated and will colour the starch blue. Colonies 

 on the other half of the plate, similar in appearance to those 

 giving the blue starch-iodide reaction, and which will not have 

 been killed by the acid, may be picked out and grown separately 

 in suitable solutions. 



For the study of the ultimate conversion of nitrate into 

 nitrogen, the following solution may be made use of : 



