It is almost pure in the albumens; both vegetable and animal. It
is nearly pure in the white of egg. Hence, nitrogenous foods are
quite commonly called albuminoids. It exists abundantly in all the
proteins--as cheese or caseine, fibrin or lean meat, albumen, etc.
Nitrogen, in its free state, appears to be an innocuous gas, diluting
the oxygen and preventing it from rapidly oxydizing or burning
up everything. As before said, it constitutes four-fifths of our
atmosphere, but does not appear to be directly appropriated by either
vegetables or animals. As food for either, it must be in combination
with other elements--especially carbon--and yet it is very difficult to
make it unite with other elements, and hard to maintain the union when
it is once formed. Its disposition is to break these unions and seek
an idle state of freedom. Hence it is that, when held in durance, its
constant tendency to free itself makes it the motor force in all animal
organisms, and the terrible energy in all explosives. It is secured
in the form of ammonia in rain, by a process called nitrification it
unites with the soil, and it exists in all decayed animal and vegetable
matter in a form suitable for plant food. Men and animals get it by
eating vegetables or by eating one another. It is a very abundant and
important element, yet very difficult to obtain in an available form
for plant and animal food. Fortunately, but comparatively little of it
is needed.