hey are deficient in phosphate of lime. It is one of the
most important ingredients of manures, and its value is dependent on
certain conditions which will be hereafter explained.

Another use of phosphoric acid in the plant is to supply it with a small
amount of _phosphorus_, which seems to be required in the formation of
the seed.

SULPHURIC ACID.

[What is sulphuric acid composed of?

What is plaster?

What is silica?

Why is it necessary to the growth of plants?

What compounds does it form with alkalies?]

_Sulphuric acid_ is important to vegetation and is often needed to
render soils fertile. It is composed of sulphur and oxygen, and is made
for manufacturing purposes, by burning sulphur. With lime it forms
_sulphate of lime_, which is gypsum or 'plaster.' In this form it is
often found in nature, and is generally used in agriculture. Other
important methods for supplying sulphuric acid will be described
hereafter. It gives _to_ the plant a small portion of _sulphur_, which
is necessary to the formation of some of its parts.

NEUTRALS.

SILICA.

[How can you prove its existence in corn stalks?

What instance does Liebig give to show its existence in grass?

How do we supply silicates?

Why does grain lodge?

What is the most important compound of chlorine?]

This is sand, the base of flint. It is necessary for the growth of all
plants, as it gives them much of their strength. In connection with an
alkali it constitutes the hard shining surface of corn stalks, straw,
etc. Silica unites with the alkalies and forms compounds, such as
_silicate of potash_, _silicate of soda, etc._, which are soluble in
water, and therefore available to plants. If we roughen a corn stalk
with sand-paper we may sharpen a knife upon it. This is owing to the
hard particles of silica which it contains. Window glass is silicate of
potash, rendered insoluble by additions of arsenic and litharge.

Liebig tells us that some persons discovered, between Manheim and
Heidelberg in Germany, a mass of melted glass where a hay-stack had been
struck by lightning. They supposed it to be a meteor, but chemical
analysis showed that it was only the compound of silica and potash which
served to strengthen the grass.

There is always _enough_ silica in the soil, but it is often necessary
to add an alkali to render it available. When grain, etc., lodge or fall
down from their own weight, it is altogether probable that they are
un