Part 4 (1/2)

NEUTRALS.

SILICA.

[How can you prove its existence in corn stalks?

What instance does Liebig give to show its existence in gra.s.s?

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 const.i.tutes the hard s.h.i.+ning 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 gla.s.s is silicate of potash, rendered insoluble by additions of a.r.s.enic and litharge.

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

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 unable 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 const.i.tuents 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 cla.s.sify the inorganic const.i.tuents?]

OXIDE OF MANGANESE, though often found in small quant.i.ties 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 cla.s.sify them in a simple manner, so that they may be recollected. They are as follows:--

ALKALIES. ACIDS. NEUTRALS.