212 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



grow on gelatin or agar, so that if no colonies appear, 

 the solution is probably free from contaminating bacteria. 

 The growth of these organisms produces scarcely any change in 

 the appearance of the solution. After staining with coloring 

 matter, the organisms may be seen under the microscope as 

 minute, peanut-shaped bacteria. They are termed Nitrobacter. 



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Fig. 50. Microscopic appearance of nitric bacteria. Winogradski. 



The nitrous organism, which converts ammonia to nitrites, is 

 much less easily isolated than the nitric organism, for the reason 

 that other bacteria will grow along with it, and also because 

 it does not grow well on agar or gelatin plates. Winogradsky 1 

 was finally successful in isolating it. He first cultivated the 

 bacteria in a medium of the same composition as that given above, 

 except the potassium nitrite was replaced by about 2 parts per 

 thousand of ammonium sulphate. A little magnesium carbonate 

 1 Wiley's Principles and Practice, Vol. i. 



