ge a part of the plant, we have
seen to consist of four different substances. The inorganic portion, on
the contrary, although forming so small a part, consists of no less than
_nine_ or _ten_ different kinds of matter.[B] These we will consider in
order. In their relations to agriculture they may be divided into
_three_ classes--_alkalies_, _acids_, and _neutrals_.[C]

[Is the character of a compound the same as that of its
constituents?

Give an instance of this.

Do neutrals combine with other substances?

Name the four alkalies found in the ashes of plants.]

Alkalies and acids are of opposite properties, and when brought together
they unite and neutralize each other, forming compounds which are
neither alkaline nor acid in their character. Thus, carbonic acid (a
gas,) unites with lime--a burning, caustic substance--and forms marble,
which is a hard tasteless stone. Alkalies and acids are characterized
by their desire to unite with each other, and the compounds thus formed
have many and various properties, so that the characters of the
constituents give no indication of the character of the compound. For
instance, lime causes the gases of animal manure to escape, while
sulphate of lime (a compound of sulphuric acid and lime) produces an
opposite effect, and prevents their escape.

The substances coming under the signification of neutrals, are less
affected by the laws of combination, still they often combine feebly
with other substances, and some of the resultant compounds are of great
importance to agriculture.

ALKALIES.

The alkalies which are found in the ashes of plants are four in number;
they are _potash_, _soda_, _lime_ and _magnesia_.

POTASH.

[How may we obtain potash from ashes?

What are some of its agricultural uses?]

When we pour water over wood ashes it dissolves the _potash_ which they
contain, and carries it through in solution. This solution is called
_ley_, and if it be boiled to dryness it leaves a solid substance from
which pure potash may be made. Potash left exposed to the air absorbs
carbonic acid and becomes carbonate of potash, or _pearlash_; if another
atom of carbonic acid be added, it becomes super-carbonate of potash, or
_salaeratus_. Potash has many uses in agriculture.

1. It forms a constituent of nearly all plants.

2. It unites with silica (a neutral), and forms a compound which water
can dissolve and carry into the roots of plants; thus supplying them
with