ect agriculture, will be
considered in the following pages.

As the organic elements of plants are obtained from matters existing in
the atmosphere which surrounds our globe, we will examine its
constitution.

CHAPTER II.

ATMOSPHERE.

[What is atmospheric air composed of?

In what proportions?

What is the use of nitrogen in air?

Does the atmosphere contain other matters useful to vegetation?

What are they?]

Atmospheric air is composed of oxygen and nitrogen. Their proportions
are, one part of oxygen to four parts of nitrogen. Oxygen is the active
agent in the combustion, decay, and decomposition of organized bodies
(those which have possessed animal or vegetable life, that is, organic
matter), and others also, in the breathing of animals. Experiments have
proved that if the atmosphere consisted of pure oxygen every thing would
be speedily destroyed, as the processes of combustion and decay would be
greatly accelerated, and animals would be so stimulated that death would
soon ensue. The use of the nitrogen in the air is to _dilute_ the
oxygen, and thus reduce the intensity of its effect.

Besides these two great elements, the atmosphere contains certain
impurities which are of great importance to vegetable growth; these are,
_carbonic acid, water, ammonia, etc._

CARBONIC ACID.

[What is the source of the carbon of plants?

What is carbonic acid?

What is its proportion in the atmosphere?

Where else is it found?

How does it enter the plant?

What are the offices of leaves?]

Carbonic acid is in all probability the only source of the carbon of
plants, and consequently is of more importance to vegetation than any
other single sort of food. It is a gas, and is not, under natural
circumstances, perceptible to our senses. It constitutes about 1/2500 of
the atmosphere, and is found in combination with many substances in
nature. Marble, limestone and chalk, are carbonate of lime, or carbonic
acid and lime in combination; and carbonate of magnesia is a compound of
carbonic acid and magnesia. This gas exists in combination with many
other mineral substances, and is contained in all water not recently
boiled. Its supply, though small, is sufficient for the purposes of
vegetation. It enters the plant in two ways--through the roots in the
water which goes to form the sap, and at the leaves, which absorb it
from the air in the form of gas. The leaf of the plant seems to have
three offices: t