220 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



4. Like other organisms the nitrifying organisms have an 

 optimum temperature of activity ; they will produce effects 

 at as low a temperature as 3 or 4 C. (37 or 39 F.), 

 they are fairly active at 12 C. (54 F.), but they work 

 best at 37 C. (99 F.). Still higher temperatures begin to 

 be prejudicial, and like other organisms they are apt to be 

 destroyed by strong sunlight. The latter circumstances, 

 it may be mentioned, are believed by Major Clemesha to 

 account for the absence of nitrates in certain surface waters 

 in India. 



These facts have a very important bearing on the processes 

 of agriculture and especially also those of sewage purification. 

 The experiment which is described on p. 217 indicates 

 clearly that nitrification proceeds in two stages, the ammonia 

 being first oxidised to nitrite and then to nitrate ; it has been 

 found that these two reactions are the work of separate 

 organisms. 



Warington was not successful in isolating either of these, 

 partly for the reason that neither organism will grow on 

 gelatine. Winogradski in Russia, and Percy Frankland in 

 this country, independently made use of gelatinous silica as 

 a means of cultivation. The solution used with which the 

 silica was gelatinised had the following composition in the 

 case of the nitrous organism : 



2 grams ammonium sulphate ; 

 Q'5 gram magnesium sulphate ; 



2 grams sodium chloride ; 

 0'4 gram ferrous sulphate ; 

 1000 c.c. of water. 



The nitric organism is more difficult to isolate even 

 than the nitrous, as it is much smaller. Winogradski, 

 however, succeeded in 1891 ; he made use of the following 

 solution ; 



