NITEOSO-BACTEEIA AND NITRO-BACTERIA. 377 



mended by W. KttHNE (I.). When prepared by precipitation 

 from water-glass (alkali silicate) by hydrochloric acid, and purified 

 by dialysis, concentrated by boiling, and then sterilised in the 

 steamer, this silica forms a vitreous mucinous mass. This is then 

 incorporated with a sterilised solution of the sulphates of potash, 

 magnesia, ammonia, and carbonate of soda, inoculated with the 

 bacterial sample. These salts cause the silica to set, so that 

 the germs in the sowing are fixed separately, and thus may be 

 kept apart, even when they have developed into colonies. In 

 this manner Winogradsky primarily succeeded in obtaining cul- 

 tures of assured purity, by means of which he was enabled to 

 arrive at conclusions unattainable by the fractional sowings and 

 dilution method employed by previous workers, e.g. W. HERAEUS 

 (I.), P. FRANKLAND (III.), and R. WARINGTON (III). 



One of the weightiest of these results is the fact determined 

 by WINOGRADSKY (VIII.) that the numerous species of the group 

 of nitrifying bacteria are classifiable into two sharply-divided sub- 

 groups : nitroso-bacteria and nitro-bacteria. 



The nitroso-bacteria oxidise ammonia to nitrous acid, in 

 accordance with the equation 



(NH 4 ) 2 + 30 2 = N 2 3 

 but no farther. For this reason nitrites are not altered by these 

 bacteria. 



On the other hand, the nitre-bacteria lack the faculty of 

 attacking ammonia, but perform the task of converting nitrous 

 acid into nitric acid, in accordance with the equation 



/N = O /N = 



OS r\ (Y V 



^ t- Oo = O v _ n 



^N = O X N - 



As is apparent from this equation, their powers differ from that 

 of the nitroso-bacteria, inasmuch as the latter convert the pentava- 

 lent nitrogen of ammonia into the trivalent nitrogen of nitrous 

 acid, whilst the nitro-bacteria re-convert the element into the 

 pentad condition. 



It is evident that these oxidation processes can be effected only 

 in the presence of bases which take up the acids with which the 

 ammonia was initially combined, and also neutralise the resulting 

 nitrous or nitric acid thus protecting the bacteria from injury 

 from this source. This task is excellently performed in the soil 

 by calcium carbonate. The favourable influence exercised on the 

 course of nitrification by the presence of this salt is therefore 

 readily explainable without dragging in any hypothesis about the 

 condensation of oxygen. Free alkali is unsuitable here for the 

 fixation of the acids, since the presence of this reagent in quantity 

 would be injurious to the bacteria. In artificial cultures calcium 

 carbonate can be replaced by magnesium carbonate, a practice 

 adopted by Winogradsky. 



