crops are
composed, and how those materials are to be used to the best advantage.

This amount of knowledge may be easily acquired, and should be possessed
by every person, old or young, whether actually engaged in the
cultivation of the soil or not. All are dependent on vegetable
productions, not only for food, but for every comfort and convenience of
life. It is the object of this book to teach children the first
principles of agriculture: and it contains all that is absolutely
necessary to an understanding of the practical operations of
cultivation, etc.

[Is organic matter lost after combustion?

Of what does it consist?

How large a part of plants is carbon?]

We will first examine the _organic_ part of plants, or that which is
driven away during combustion or burning. This matter, though apparently
lost, is only changed in form.

It consists of one solid substance, _carbon_ (or charcoal), and three
gases, _oxygen_, _hydrogen_ and _nitrogen_. These four kinds of matter
constitute nearly the whole of most plants, the ashes forming often less
than one part in one hundred of their dry weight.

[What do we mean by gas?

Does oxygen unite with other substances?

Give some instances of its combinations]

When wood is burned in a close vessel, or otherwise protected from the
air, its carbon becomes charcoal. All plants contain this substance, it
forming usually about one half of their dry weight. The remainder of
their organic part consists of the three gases named above. By the word
gas, we mean _air_. Oxygen, hydrogen and nitrogen, when pure, are always
in the form of air. Oxygen has the power of uniting with many
substances, forming compounds which are different from either of their
constituents alone. Thus: oxygen unites with _iron_ and forms oxide of
iron or _iron-rust_, which does not resemble the gray metallic iron nor
the gas oxygen; oxygen unites with carbon and forms carbonic acid, which
is an invisible gas, but not at all like pure oxygen; oxygen combines
with hydrogen and forms water. All of the water, ice, steam, etc., are
composed of these two gases. We know this because we can artificially
decompose, or separate, all water, and obtain as a result simply oxygen
and hydrogen, or we can combine these two gases and thus form pure
water; oxygen combines with nitrogen and forms nitric acid. These
chemical changes and combinations take place only under certain
circumstances, which, so far as they aff