s of course obvious. But this conversion of free
nitrogen into the various compound forms in which it occurs throughout
the mineral, vegetable, and animal kingdoms, has been a process effected
by a variety of indirect methods, and only at the expense of a vast
amount of time. For practical purposes, the free nitrogen of the air may
be regarded chiefly as a non-available source for most bodies containing
it. It may be described as of all forms of nitrogen the least active, as
far as plant-life is concerned.

_Relation of "free" Nitrogen to the Plant._

The relation of the "free" nitrogen to the plant has formed the subject
of much research, more especially during the last few years, and a brief
epitome of the main results arrived at has already been given in the
Introductory Chapter.[64]

That this source of nitrogen is not so inaccessible to the plant as was
formerly believed, has now been abundantly proved. As the considerations
which have led to this conclusion, and have suggested the very recent
elaborate experiments on the fixation of free nitrogen by the plant--the
results of which bid fair, it would seem, to largely revolutionise our
agricultural practice--have been due to the study of the relation of the
soil-nitrogen to the plant, it will be best to defer further discussion
of this question till we have dealt with the other sources of nitrogen.

_Combined Nitrogen in the Air._

In addition to nitrogen in the free state, air contains very small
quantities of this element in combined forms. We have it in minute
traces as nitrates and nitrites, as ammonia,[65] and also in still
smaller traces as organic nitrogen in the minute dust-particles which
modern researches have revealed as being present in such enormous
numbers in our atmosphere. What the sources of these nitrates and
nitrites (which exist in quantities so minute that accurate
determination of their amount is rendered extremely difficult) are is a
disputed point. That nitrogen and oxygen unite together to form nitric
and nitrous oxides under the influence of intense heat, such as the
electric spark, has been proved beyond doubt. One source, therefore, is
probably the electrical discharges which are taking place more or less
frequently on different parts of the earth's surface. Nitrates may also
be formed in the combustion of nitrogenous bodies.[66] In the burning of
coal-gas, for example, it is probable that small quantities of nitrates
may be prod