ought in the use of a baking powder.

~Aluminium silicates.~ One of the most common constituents of rocks is
feldspar (KAlSi_{3}O_{8}), a mixed salt of potassium and aluminium with
the polysilicic acid (H_{4}Si_{3}O_{8}). Under the influence of
moisture, carbon dioxide, and changes of temperature this substance is
constantly being broken down into soluble potassium compounds and
hydrated aluminium silicate. This compound has the formula
Al_{2}Si_{2}O_{7}.2H_{2}O. In relatively pure condition it is called
kaolin; in the impure state, mixed with sand and other substances, it
forms common clay. Mica is another very abundant mineral, having varying
composition, but being essentially of the formula KAlSiO_{4}.
Serpentine, talc, asbestos, and meerschaum are important complex
silicates of aluminium and magnesium, and granite is a mechanical
mixture of quartz, feldspar, and mica.

~Ceramic industries.~ Many articles of greatest practical
importance, ranging from the roughest brick and tile to the
finest porcelain and chinaware, are made from some form of
kaolin, or clay. No very precise classification of such ware
can be made, as the products vary greatly in properties,
depending upon the materials used and the treatment during
manufacture.

Porcelain is made from the purest kaolin, to which must be
added some less pure, plastic kaolin, since the pure substance
is not sufficiently plastic. There is also added some more
fusible substance, such as feldspar, gypsum, or lime, together
with some pure quartz. The constituents must be ground very
fine, and when thoroughly mixed and moistened must make a
plastic mass which can be molded into any desired form. The
article molded from such materials is then burned. In this
process the article is slowly heated to a point at which it
begins to soften and almost fuse, and then it is allowed to
cool slowly. At this stage, a very thin vessel will be
translucent and have an almost glassy fracture; if, however, it
is somewhat thicker, or has not been heated quite so high, it
will still be porous, and partly on this account and partly to
improve its appearance it is usually glazed.

Glazing is accomplished by spreading upon the object a thin
layer of a more fusible mixture of the same materials as
compose the body of the object itself, and again heating until
th