Nitrification Due to Bacteria 169 



this salt is present in vegetable and animal materials 

 in a disguised state, and is set free when the substances 

 decay. Notwithstanding this view, there were many- 

 chemists a hundred years later who believed that nitrates 

 are formed by the oxidation of the gaseous nitrogen of 

 the air in the soil, or in the compost heap. 



The studies conducted by Kuhlmann between 1825 

 and 1838 indicated that nitrates are not formed in the 

 soil out of nitrogen gas, but out of ammonia. The ex- 

 periments carried on by Boussingault between 1858 

 and 1871 showed that nitrates in the soil are formed 

 at the expense of the humus-nitrogen and not out of 

 nitrogen gas. It was thus demonstrated that nitrifi- 

 cation in the soil, or in compost heaps, depends on 

 the decomposition of the organic substances containing 

 nitrogen and the formation of ammonia. Nevertheless, 

 it was still believed that the process of nitrification 

 was purely chemical in its nature and that the superior 

 nitrifying powers of some soils were due to their content 

 of certain compounds of lime, iron, and the like. 



True character of nitrification. — After the brilliant 

 investigations of Pasteur, in the sixties of the last cen- 

 tury, on the nature of fermentation, the ground was 

 prepared for the understanding of the true character 

 of nitrification. Indeed, in the early seventies, the belief 

 was expressed by at least one investigator that nitri- 

 fication is of bacteriological origin. The proof for this 

 assumption was furnished in 1877 by two French 

 investigators, Schlosing and Miintz. In passing diluted 

 sewage through glass tubes filled with soil, they found 

 that after some days the liquid that had passed through 



