This substance, a product of the decomposition of organic matter,
always exists in small quantities in the air, whence it is brought
down by dew, rain and snow to the earth, where it is retained for
the use of the plant. Although nitrogen exists so abundantly in
the air, and forms so large a portion of plants, yet they cannot use
it in its pure form; it must be supplied to them in the form of am-
monia, or of nitric acid, as some say. Their leaves imbibe and
exhale, but use it not. The experiments to support the contrary
idea are inconclusive, as all their results can be explained without
doming to the conclusion that nitrogen, as such, is supplied direct-
ly to plants from the air.

I have now given the most marked properties of the four ele-
mentary organic substances which form the air we breathe, the
waters of the earth, a large part of the earth itself, and of the ani-
mal and vegetable tribes which inhabit it. The names and prop-
erties may not be familiar to some; very many may be entirely ig-
norant of them; but what can be more appropriate subjects for
study and investigation, than the air which supports our lives from
its first moments, the water which is to us daily food, or the com-
position of that food, which by the curse of labor we are obliged
to earn—a curse, however, changed by mercy into a blessing,
when the intellect is used to lighten the toil of the body ? Is a
knowledge of these substances useless? then the knowledge of
those things nearest to us, and with which we hold constant rela-
tion, is useless; and if these are, those which are more remote
must of consequence be more so, and all knowledge is vain; and
if knowledge be nothing, then men are on a level with brutes—
they are below them.

The most noted properties of these substances can be easily
understood, and as easily learned as the multiplication table, and
will enable the tiller of the ground to understand many facts of
great use to him which are now incomprehensible, and give him,
as the fruits of labor, pleasure as well as profit.

The substances which I have mentioned above, namely, car-
bon, oxygen, hydrogen and nitrogen, are combined with each other,
and form many substances, of which it is not my province here to
speak. I will mention some whose constitution it is proper to
know.

Carbon and oxygen form carbonic acid, whose uses and prop-
erties I have spoken of above. Hydrogen and oxygen form water,
which serves either as such, or by its elements, to produce a large
part of the mass of vegetable matter; it moreover is the vehicle for
the transportation of food to the plant, and causes matter, by dis-
solving it, to assume a form, giving to it the properties of food.

Though the above substances are of such vast importance in
vegetable life, and form nearly all of the woody fibre, leaves, stem,

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