the three gases, _hydrogen_, _oxygen_ and
_nitrogen_, which constitute the remainder of the organic part of
plants.

Hydrogen and oxygen compose _water_, which, if analyzed, yields simply
these two gases. Plants perform such analysis, and in this way are able
to obtain a sufficient supply of these materials, as their sap is
composed chiefly of water. Whenever vegetable matter is destroyed by
burning, decay, or otherwise, its hydrogen and oxygen unite and form
water, which is parted with usually in the form of an invisible vapor.
The atmosphere of course contains greater or less quantities of watery
vapor arising from this cause and from the evaporation of liquid water.
This vapor condenses, forming rains, etc.

Hydrogen and oxygen are never taken into consideration in manuring
lands, as they are so readily obtained from the water constituting the
sap of the plant, and consequently should not occupy our attention in
this book.

NITROGEN.

[If vegetable matter be destroyed, what becomes of these
constituents?

What is the remaining organic constituent?

Why is it worthy of close attention?

Do plants appropriate the nitrogen of the atmosphere?]

_Nitrogen_, the only remaining _organic_ constituent of vegetable
matter, is for many reasons worthy of close attention.

1. It is necessary to the growth and perfection of all cultivated
plants.

2. It is necessary to the formation of animal muscle.

3. It is often deficient in the soil.

4. It is liable to be easily lost from manures.

Although about four fifths of atmospheric air are pure nitrogen, it is
almost certain that plants get no nutriment at all from this source. It
is all obtained from some of its compounds, chiefly from the one called
ammonia. Nitric acid is also a source from which plants may obtain
nitrogen, though to the farmer of less importance than ammonia.

AMMONIA.

[What is the principal source from which they obtain nitrogen?

What is ammonia?

How is it formed?

Where does it always exist?

How do plants take up ammonia?]

_Ammonia_ is composed of nitrogen and hydrogen. It has a pungent smell
and is familiarly known as _hartshorn_. The same odor is perceptible
around stables and other places where animal matter is decomposing. All
animal muscle, certain parts of plants, and other organized substances,
consist of compounds containing nitrogen. When these compounds undergo
combustion, or are in any manner decomposed, the nitr