60 THE CIRCULATION OF NITROGEN IN NATURE. 
These substances — ammonia, nitrous and nitric acids — are 
readily identified by the tests I now propose to show you. 
Lastly, nitrogen exists as a constituent of an immense 
variety of compounds with carbon, hydrogen and other 
elements, forming what are known as organic bodies. These 
are produced by animals and plants from food materials, and 
for the present onl}^ interest us as one of the forms in which 
nitrogen may occur, and from which it is liberated generally 
v/ithout much difEculty in the shape either of the element or 
as ammonia b}^ such a process as burning. In fact, as we 
shall see, there is a constant and very large addition to the 
atmospheric nitrogen resulting from the destruction of nitro- 
genous organic matters. On the other hand, the laboratory 
operations just referred to, as means by which elementary 
nitrogen can be brought into combination, are represented in 
a very slight degree only in nature. It would appear there- 
fore that there is a continuous degradation of nitrogen to the 
elementary condition — a very serious matter if the nitrogen 
so degraded is finally removed from the sphere of action of 
organized beings. Are there then any other agencies at 
work to restore the balance, and enable this apparently use- 
less gas to return within the arena of physiological ac- 
tivity ? 
Let us start then with the atmospheric nitrogen : does it 
ever enter into combination, and assume other forms than 
the elementary? 
Now throughout the earlier part of this century this 
question was fiercely debated, but in spite of some experi- 
mental evidence to the contrary it was ultimately concluded 
that there were no grounds for supposing that either plants 
or animals could assimilate gaseous nitrogen, except per- 
haps so far as it was combined with oxygen under the 
influence of atmospheric electricity. 
