REPORT ON NITRIFICATION. 307 



soil prepares it for the food of the plant, and the ammonia that might 

 have escaped from the surface of the ground by reason of its volati- 

 lity IS prevented from doing so by its conversion into non-volatile 

 nitrate,* 



Nitrification, being much influenced by the temperature, goes on 

 with the most rapidity in tropical countries, and this Kuhlmann con- 

 siders as one cause of their greater fertility, since but little ammonia 

 can escape unaltered through the surface stratum of the soil in those 

 regions. It is evident that during dry weather nitrates must accu- 

 mulate at the surface of the ground, and that upon the occurrence of 

 rain they must, being very soluble, be washed down into the soil, 

 and be there reconverted into ammonia. 



Another set of circumstances under which nitrates are generated 

 from ammonia-yielding substances are those brought forward by Dr. 

 R. A. Smith in his researches on the air and water of towns. He shows 

 that the drainage from sinks, cesspools, and other depots of organic 

 matter, undergoes oxidation in passing through water-bearing strata, 

 the oxygen dissolved in the water co-operating with the porous soil 

 to convert the nitrogen of the decomposing matter into nitrates, and 

 the sulphur into sulphates, t 



In this way water purifies itself from the contaminations that are 

 so largely poured into it so effectually that a well separated from a 

 sink by a few feet of porous soil will be kept free from offensive 

 matter, the conditions required being that the medium through which 

 the filtration takes place shall not be a surface soil which is too much 

 loaded with organic matter for oxidation to go on in it ; and that there 

 shall be suflficient supply of water containing oxygen in solution^ 

 which, of course, must be the case wherever springs or wells occur. 

 When well-water becomes charged with organic impurities, it is be- 

 cause the drainage through the surface soil enters the well without 

 passing through the deeper or water-bearing strata. Thus the pro- 

 duction of nitrates takes place both at the very surface and deep in 

 the ground, while there is in fertile soils a middle zone charged with 

 organic matter, where their conversion into ammonia goes on. 



Until the present century it was universally believed that organic 

 matters were necessary in every case of the formation of nitrates, and 

 up to the present time the opinions of a number of scientific men seem 

 to be inclined to the conclusion that wherever nitrates are produced 

 in large quantity they originate from the decomposition of such sub- 

 stances. In 1823, however, Longchamp read a paper to the French 

 Academy, in Avhich he advanced the doctrine that nitrates are gener- 

 ated from the nitrogen of the air, and not from that of organic sub- 

 stances. Since the time of Cavendish it has been known that the 

 nitrogen and oxygen of the air may be converted into nitric acid. If 

 the electric spark be passed through air confined over a solution of 

 potash, nitrate of potash is generated ; if over simple water, we have 

 nitrate of ammonia. Again : when a mixture of nitrogen and hydro- 



- Exps. sur la Iheorie des Enr/rais, C. R., November, 1846. 

 f Proceedings of the British Association, p. 66, 1851. 



