of an acid, and consequently it dissolves in fused alkalis to
form silicates. Being nonvolatile, it will drive out most other
anhydrides when heated to a high temperature with their salts,
especially when the silicates so formed are fusible. The following
equations illustrate this property:

Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},

Na_{2}SO_{4} + SiO_{2} = Na_{2}SiO_{3} + SO_{3}.

~Silicic acids.~ Silicon forms two simple acids, orthosilicic acid
(H_{4}SiO_{4}) and metasilicic acid (H_{2}SiO_{3}). Orthosilicic acid is
formed as a jelly-like mass when orthosilicates are treated with strong
acids such as hydrochloric. On attempting to dry this acid it loses
water, passing into metasilicic or common silicic acid:

H_{4}SiO_{4} = H_{2}SiO_{3} + H_{2}O.

Metasilicic acid when heated breaks up into silica and water, thus:

H_{2}SiO_{3} = H_{2}O + SiO_{2}.

~Salts of silicic acids,--silicates.~ A number of salts of the
orthosilicic and metasilicic acids occur in nature. Thus mica
(KAlSiO_{4}) is a salt of orthosilicic acid.

~Polysilicic acids.~ Silicon has the power to form a great many complex
acids which may be regarded as derived from the union of several
molecules of the orthosilicic acid, with the loss of water. Thus we have

3H_{4}SiO_{4} = H_{4}Si_{3}O_{8} + 4H_{2}O.

These acids cannot be prepared in the pure state, but their salts form
many of the crystalline rocks in nature. Feldspar, for example, has the
formula KAlSi_{3}O_{8}, and is a mixed salt of the acid
H_{4}Si_{3}O_{8}, whose formation is represented in the equation above.
Kaolin has the formula Al_{2}Si_{2}O_{7}.2H_{2}O. Many other examples
will be met in the study of the metals.

~Glass.~ When sodium and calcium silicates, together with silicon dioxide,
are heated to a very high temperature, the mixture slowly fuses to a
transparent liquid, which on cooling passes into the solid called glass.
Instead of starting with sodium and calcium silicates it is more
convenient and economical to heat sodium carbonate (or sulphate) and
lime with an excess of clean sand, the silicates being formed during the
heating:

Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},

CaO + SiO_{2} = CaSiO_{3}.

[Illustration: Fig. 74]

The mixture is heated below the fusing point for some time, so that the
escaping carbon dioxide may not spatter the hot liquid; the heat is then
increased and the mixture kept in a sta