170 Bacteria in Relation to Country Life 



the soil had lost its ammonia, and had gained an equiva- 

 lent quantity of nitrate. On treating the soil with chloro- 

 form vapors, nitrification was discontinued and the 

 ammonia in the sewage passed through unchanged. 

 The process of nitrification was reestablished after the 

 soil had received fresh soil in which nitrification was 

 -^ active. Similarly, the nitrification of 



**J ammonia in the soil was stopped by 



■^ •• boiling. This showed that the change of 



Psij ^ ammonia to nitrate was of bacterial 



i»i!? *^« origin, since the process could be sus- 

 Fig. 26. Nitri*ng peuded by antiseptics and by boiling, 

 bacteria.— i. g^jj^j reestablished by the addition of 



JNit roust er- ^ 



™*"Ni'tric^'*'fe''r- '^^^Y "^inute quantities of fresh soil. 

 (AnS'surn^d These experiments were amply con- 

 stutzer.) firmed by other investigations. 



Pure culture of nitrifying bacteria. — With the bacterial 

 nature of nitrification thus demonstrated, attempts 

 were soon made to secure pure cultures of the nitrify- 

 ing organisms. For a number of years such attempts 

 were not crowned with success. To be sure, various 

 investigators claimed to have detected the ability to 

 produce nitrates in one or another of the soil bacteria 

 studied by them. More careful examination showed, 

 however, that, in all probability, they did not work 

 with pure cultures. To Warrington belongs the credit 

 of having isolated, after many years of study, organ- 

 isms capable of changing ammonia to nitrites. He ob- 

 served other organisms also capable of changing nitrites 

 to nitrates. Warrington's investigations indicated that 

 the oxidation of ammonia occurs in two distinct stages. 



