verted into humus as soon as the vital powers have ceased to
exist in it. But there is a series of other organic compounds
present in the plant, which, besides the above named ele-
ments, contain nitrogen as a constituent part; and as these
compounds form the real nutritious matter of the vegetable
food by being converted into blood, and subsequently applied
to the production of the organs of the animal, they must ne-
cessarily accumulate in the animal body and re-appear in a
more concentrated form in the carcass or the excrements of
animals than they occurred in the body of the plant where
they originated. These compounds, whether still existing in
the plant or already transformed into constituents of the
animal body, when they undergo a process of decomposition,
produce humus, and, besides, evolve ammonia in proportion
to the quantity of nitrogen they contain. The ammonia
becomes partly disengaged and is then diffused in the atmos-
phere, and is partly retained by, and incorporated with, the
humus which is simultaneously formed with it.

The atmosphere, which is thus constantly supplied with
ammonia from organic bodies wherever existing in a state of
decay, contains this substance in a considerable quantity, and,
to judge from the result of analytical examinations of the at-
mosphere, sufficiently even to meet the wants of all the grow-
ing crops on earth. Practical experiments have also proven,
in accordance with this fact, that plants can be raised and
brought to perfect development without the aid of any other
source furnishing ammonia but the atmosphere; and if the
experience of practical farmers advise us not solely to rely
upon this source, but to take care, besides, of keeping the soil
itself well supplied with it, by means of artificial applications
of such manures as are capable of producing it,—it is for
reasons which are founded upon the same principles that have
been discussed already in their relation to the supply of car-
bonic acid.

An experiment on the production of plants under given
conditions for the solution of certain questions, is altogether
different from the practice of producing crops, and the con-
ditions under which it can be most satisfactorily effected. In
the one case, we investigate to what extent plants can be
forced to live on a partial supply of the necessary elements,
and judge from the more or less perfect development of all their
parts, whether we have to decide in favor of the one or the
other doctrine; in the other case, we are anxious to offer to
the plant an abundance of its elements, by opening the most
various resources for its supply, and estimate the result, not
from the fact that the produced plant shows a root, a chaff,

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