CHAPTER VII. 



QUALITATIVE ANALYSIS continued. 

 NITRIFYING AND DENITRIFYING MICRO-ORGANISMS. 



THE idea that ammonia in water and soil is converted into 

 nitrates through the influence of micro-organisms was first 

 suggested in 1878 by Schloesing and Muntz. These observers 

 proved that nitrification was caused by bacteria, but they failed 

 to isolate pure cultures of the organisms in question. Waring- 

 ton, Emich, and other workers repeated and confirmed Schloesing 

 and Miintz's observations. In 1886 Heraeus published a very 

 important paper on this subject. He obtained pure cultures of 

 micro-organisms from water and soil and tested their power of 

 nitrifying salts of ammonia. The first twelve cultures investi- 

 gated showed no nitrifying action ; two of them, however, dis- 

 played distinct power of reducing nitrates. It appeared 

 probable that the reduction to the state of ammonia only 

 commenced when all the nitrates had been reduced to nitrites, 

 and that when ammonia was also present in conjunction with 

 nitrates the nitrogen from the ammonia would be assimilated 

 before the nitrates were attacked. By adding 50 grammes of 

 garden earth to 500 c.c. of water containing one gramme of 

 ammonium carbonate, powerful oxidation was set up ; the fluid 

 appeared clear, but on the surface a thin iridescent membrane 

 was noticed. When plated in gelatine the membrane produced 

 two species of bacteria of special importance. The first species 

 was very common, and to the naked eye the colonies appeared in 

 the depth as small points and on the surface as milk-white, pin- 

 head growths, which, under a low power, resembled smooth, 

 round, yellow scales. The colonies of the second species closely 

 resembled the first, but were more oval in form, and of a darker 

 brown colour. The behaviour of these organisms was tested in 



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