n of humate of potash. The solution, which
contained in every 100 grains, 0.148 grains of solid matter, consisting
of humate of potash, etc. was found to become gradually paler, and at
the end of a month, during which time the plants had increased by 6-1/2
inches, the quantity of solid matter in 100 grains had diminished to
0.132. But the solution contained in the other glass, and in which no
plant had grown, had diminished to 0.136, so that the absorption could
not have amounted to more than 0.004 grains for every 100 grains of
solution employed. This quantity is so small as to be within the limits
of error of experiment, and we are consequently entitled to draw the
conclusion that humus, even under the most favourable circumstances, is
not absorbed by plants.

But though not directly capable of affording nutriment to plants, it
must not, on that account, be supposed that humus is altogether devoid
of importance, for it is constantly undergoing decomposition in the
soil, and thus becomes a source of carbonic acid which can be absorbed,
and, as we shall afterwards more particularly see, it exercises very
important functions in bringing the other constituents of the soil into
readily available forms of combination.

It has been already observed that carbon, hydrogen, nitrogen, and
oxygen, cannot be absorbed by plants when uncombined, but only in the
forms of water, carbonic acid, ammonia, and nitric acid. It is scarcely
necessary to detail the grounds on which this conclusion has been
arrived at in regard to carbon and hydrogen, for practically it is of
little importance whether they can be absorbed or not, as the former is
rarely, the latter never, found uncombined in nature. Neither can there
be any doubt that water and carbonic acid are the only substances from
which these elements can be obtained. Every-day experience convinces us
that water is essential to vegetation; and Saussure, and other
observers, have shown that plants will not grow if they are deprived of
carbonic acid, and that they actually absorb that substance abundantly
from the atmosphere. The evidence for the non-absorption of oxygen lies
chiefly in the fact that plants obtain, in the form of water and
carbonic acid, a larger quantity of that element than they require, and
in place of absorbing, are constantly exhaling it. The form in which
nitrogen may be absorbed has given rise to much difference of opinion.
In the year 1779, Priestley commenced the