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 is 

 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. 



