THE BACTERIA IN DIFFERENT MEDIA. 149 
Let us consider, for example, nitrogen in plants. 
This element, of which the atmosphere is the res- 
ervoir, does not enter directly into combination, as 
does oxygen, with the other elements which with 
it are to constitute the immediate principles of the 
tissues. The chemical properties of nitrogen may 
be characterized in two words, — great resistance 
to entering into combination when it is free, and 
great facility, on the contrary, in passing from one 
combination to another when once it has associated 
itself with other elements. 
The circulation of nitrogen in a state of com- 
bination upon the surface of the globe is also an 
interesting question of general physics, as well as 
the circulation of carbonic acid, of water, and of 
the air. 
Let us seek to sketch the march of this cir- 
culation. 
Whence comes the ammonia which is found in 
the sea, in the clouds which come to us from equa- 
torial regions, in the dust of the air? The only 
known source is the fermentation of organic mat- 
ters out of reach of the oxygen of the air. It 1s 
to this sort of fermentation that we owe the for- 
mation of peat and the immense masses of com- 
bustible minerals which have formed during nearly 
all the geological periods. We see this sort of fer- 
mentation develop itself when we expose an or- 
ganic liquid to the air, but only in the inferior 
part of the liquid, the oxygen which is dissolved 
near the surface being arrested in the superficial 
zone, where a very different fermentation occurs. 
