able to obtain from the soil a sufficient supply of the soluble
silicates, and some form of alkali should be added to the soil to unite
with the sand and render it soluble.

CHLORINE.

[Of what use is chloride of lime?

What is oxide of iron?

What is the difference between the _per_oxide and the _prot_oxide of
iron?]

_Chlorine_ is an important ingredient of vegetable ashes, and is often
required to restore the balance to the soil. It is not found alone in
nature, but is always in combination with other substances. Its most
important compound is with sodium, forming _chloride of sodium_ (or
common salt). Sodium is the base of soda, and common salt is usually the
best source from which to obtain both soda and chlorine. Chlorine unites
with lime and forms _chloride of lime_, which is much used to absorb the
unpleasant odors of decaying matters, and in this character it is of use
in the treatment of manures.

OXIDE OF IRON.

_Oxide of iron_, one of the constituents of ashes, is common iron rust.
_Iron_ itself is naturally of a grayish color, but when exposed to the
atmosphere, it readily absorbs oxygen and forms a reddish compound. It
is in this form that it usually exists in nature, and many soils as well
as the red sandstones are colored by it. It is seldom, if ever,
necessary to apply this as a manure, there being usually enough of it in
the soil.

This red oxide of iron, of which we have been speaking, is called by
chemists the _peroxide_. There is another compound which contains less
oxygen than this, and is called the _protoxide of iron_, which is
poisonous to plants. When it exists in the soil it is necessary to use
such means of cultivation as shall expose it to the atmosphere and allow
it to take up more oxygen and become the peroxide. The black scales
which fly from hot iron when struck by the blacksmith's hammer are
protoxide of iron.

The _peroxide of iron_ is a very good absorbent of ammonia, and
consequently, as will be hereafter described, adds to the fertility of
the soil.

[What can you say of the oxide of manganese?

How do you classify the inorganic constituents?]

OXIDE OF MANGANESE, though often found in small quantities in the ashes
of cultivated plants, cannot be considered indispensable.

Having now examined all of the materials from which the ashes of plants
are formed,[F] we are enabled to classify them in a simple manner, so
that they may be recollected. They are as follows