150 PHYSIOLOGY OF THE BACTERIA. 
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. Howis 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. Schloesmg and A. Miintz 
conclude from their experiments that in the pu- 
trefactions essentially oxidizing produced by Peni- 
cullum 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 
