Davy — Ohservations on Nitrification. 243 



As human urine and feculent matters may justly be regarded as 

 tlie most offensive and dangerous ingredients of sewage in general, 

 my experiments have been confined to those matters, and were 

 principally made on urine, which, from its containing different nitro- 

 genous substances, readily susceptible of decomposition, is peculiarly 

 suited for the study of the nitrification of animal matters. By mix- 

 ing this liquid with various proportions of water, and placing the 

 mixtures under different circumstances, I have endeavoured to ascer- 

 tain those that were favourable or otherwise to their nitrification ; 

 and to determine some points connected with that process which 

 required further investigation. I should here observe that in detect' 

 ing the occurrence of nitrification I have principally used the well- 

 known test of Price for nitrites, which consists in adding to the water 

 or mixture a thin solution of starch, containing a little iodide of 

 potassium, and acidifying with diluted sulphuric acid, when a blue 

 reaction from the liberated iodine will be immediately produced, 

 should a very minute quantity of a nitrite be present. And as there 

 is every reason to suppose that the production of nitrites precedes 

 that of nitrates in the nitrification of organic matters in solution, and 

 the detection of the former is much more easily effected than the 

 latter, at least under the conditions existing in my experiments, 

 I was satisfied in most cases to obtain the evidence of the formation of 

 nitrites by the employment of the test to which I have just re- 

 ferred. 



The experiments of Warrington- have led him to conclude that 

 darkness is an essential condition to the development of those low 

 forms of vegetable life, which are supposed in many instances to give 

 rise to nitrification. 



This is a question which it is difficult to determine decisively 

 one way or the other, owing to the impossibility of having with us 

 continuous daylight to operate with. Still I think we may arrive at 

 an approximative conclusion on this point, by making comparative 

 experiments on similar mixtures, kept altogether excluded from the 

 light, and on those exposed to its full influence, and then determin- 

 ' ing the amount of nitrification which had taken place in each, after a 

 given time ; and if darkness be so essential to that process, we should 

 naturally expect that in the mixtures exposed to its continuous influ- 

 ence there would be an earlier and a greater development of nitrifica- 

 tion, than in those which had been placed under it for about one-third 

 or one-half the time, each day of twenty-four hours. 



From the results of several comparative experiments made in this 

 way, I have come to the conclusion that the conditions of light or 

 darkness exercise but little influence one way or the other in this 

 process, at least under the circumstances existing in my experiments, 



- "Journal of the Chemical Society," Janiiary 1878. 



K.I. A. pace, SEE. II., VOL. III. — SCIENCE. 



