150 PHYSIOLOGY OF THE BACTEEIA. 



The latter is essentially oxidizing ; the material is 

 almost completely burnt, forming water and car- 

 bonic acid ; at the inferior part, on the contrary, a 

 reduction is produced so energetic that hydrogen 

 is disengaged. The metallic sulphates are there 

 transformed into sulphites, and even crystals of 

 sulphur are sometimes found (see the history of 

 the Beggiatoa, page 91). 



We see then the source of the ammonia, which, 

 distributed upon the soil by the winds and the rains, 

 becomes a powerful fertilizer. Now, vegetables do 

 not absorb nitrogen under the form of ammonia, but 

 under the form of nitric acid. How is this transform- 

 ation of ammonia into nitric acid effected ? The 

 observations of Erdmann, Mensel, and T. Phipson 

 show that in the phenomena of destructive putre- 

 faction, nitric acid, far from being produced, is on 

 the contrary reduced to the state of nitrous acid ; 

 on the other hand, Th. Schlcesing and A. Miintz 

 conclude from their experiments that in the pu- 

 trefactions essentially oxidizing produced by Peni- 

 cillum glaucum, Aspergillus niger, Mucor mucedo, 

 etc., there is no formation of nitric acid. But, 

 according to these authors, nitrification is a spe- 

 cial phenomenon which takes place in every soil 

 sufficiently loose to permit a free circulation of air, 

 and of which the agent is a micro-organism. This 

 organism has not yet been perceived, it is true; 

 and it is evident that it would be difficult to seek 

 and observe, because of its peculiar situation. 



But the action of chloroform upon nitrification 

 tends to prove that the agent of this process is 



