262 APPLIED BACTERIOLOGY 



process of oxidation which takes place in the soil, whereby 

 the organic nitrogen and ammonia are converted into nitric 

 and nitrous acid, is the result of the activity of certain 

 micro-organisms. This process of ' nitrification,' as it is 

 called, was first studied by Miintz and Schloesing in 1877, 

 who found that nitrification did not take place in soil that 

 had been sterilised by heat, or that had been treated with 

 antiseptics. They also showed that it was only ammoniacal 

 nitrogen that could undergo true nitrification, the organic 

 nitrogen having to first be converted into ammonia, which 

 change also takes place by the agency of micro-organisms. 

 Warington, Heraus, Munro, and others, did further work 

 on these organisms ; but it was not until 1890 that the 

 organisms were obtained in a pure condition by Wino- 

 gradsky and Frankland, who worked independently. 



The difficulty of obtaining these organisms in a state of 

 purity was enhanced by their inability to grow upon any of 

 the usual culture media, they requiring a medium entirely 

 free from organic matter for their growth. This difficulty 

 was overcome by Professor Kiihne, who devised in a most 

 ingenious manner a medium which was entirely free from 

 organic matter and of jelly-like consistency; this he 

 obtained by means of gelatinised silica. For method of 

 preparing this silica jelly, see p. 65. 



The discovery of these ' nitrifying ' organisms in the soil 

 was of the very greatest importance, as it disproved one of 

 the most fundamental principles of vegetable physiology, 

 which stated that green plants alone had the power of 

 building up protoplasm from inorganic materials. The 

 ' nitrifying ' organisms of Winogradsky and Frankland con- 

 sist of ellipsoidal cells, the young cells being nearly spherical. 

 They are from 0*9 to 1 /i broad and from 1-1 to 1"8 /i long, 

 occasionally seen in short chains, and do not form spores. 

 2. Fermentation by Hydration. — The most important fer- 



