THE CIRCULATION OF NITROGEN IN NATURE. 65 
all forms of putrefaction this is the case to some extent, but 
pre-eminently so with, vegetable matter. Thus, marsh gas — 
that is, the gas evolved from the decaying vegetable matter 
in bogs and stagnant pools — contains much nitrogen. An 
analysis by Bunsen of such gas from a pond in the botanical 
garden at Marburg showed that it contains 51*5 per cent, 
nitrogen. Firedamp also — the gas evolved from coal- 
contains a large proportion of free nitrogen. On the other 
hand it is important to note that comparatively little am- 
monia is formed. 
But a large proportion of vegetable growths are destined 
to become the food of animals, and this is especially the case 
with numbers of the leguminosse, which are extensively culti- 
vated for this very purpose. In this case the condition of 
affairs is profoundly modified by the conversion of the 
nitrogenous matters from the vegetable to the animal type. 
Farther, we know not only that animal tissues as a whole 
are richer than vegetable in nitrogen, but that animals are 
constantly, as a necessary part of their vital functions, 
throwing off effete matter of a highly nitrogenous character. 
Indeed, animals may be looked upon from this point of view 
as concentrators of nitrogen. 
Now when an animal body dies, or the waste material 
from a living animal is cast off, we know that under 
ordinary conditions it rapidly enters into decomposition ; 
that is, it becomes the prey of numerous micro-organisms 
which proceed to pull to pieces the complicated structures 
which the living animal had been busied throughout its life 
in building up. And in doing so they serve a most useful 
purpose, for, as has often been pointed out, if it were not 
for their untiring activity, not only would the corpses of the 
innumerable generations of countless centuries still en- 
cumber the earthj but an immense store of indispensable 
F 
