of it is placed a small
heap B of a mixture of sodium peroxide and aluminium, into
which is stuck a piece of magnesium ribbon C. Powdered
fluorspar D is placed around the sodium peroxide, after which
the crucible is set on a pan of sand and the magnesium ribbon
ignited. When the flame reaches the sodium peroxide mixture
combustion of the aluminium begins with almost explosive
violence, so that great care must be taken in the experiment.
The heat of this combustion starts the reaction in the chromium
oxide mixture, and the oxide is reduced to metallic chromium.
When the crucible has cooled a button of chromium will be found
in the bottom.

~Aluminium oxide~ (Al_{2}O_{3}). This substance occurs in several forms in
nature. The relatively pure crystals are called corundum, while emery is
a variety colored dark gray or black, usually with iron compounds. In
transparent crystals, tinted different colors by traces of impurities,
it forms such precious stones as the sapphire, oriental ruby, topaz, and
amethyst. All these varieties are very hard, falling little short of
the diamond in this respect. Chemically pure aluminium oxide can be made
by igniting the hydroxide, when it forms an amorphous white powder:

2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O.

The natural varieties, corundum and emery, are used for cutting and
grinding purposes; the purest forms, together with the artificially
prepared oxide, are largely used in the preparation of aluminium.

~Aluminium hydroxide~ (Al(OH)_{3}). The hydroxide occurs in nature as the
mineral hydrargyllite, and in a partially dehydrated form called
bauxite. It can be prepared by adding ammonium hydroxide to any soluble
aluminium salt, forming a semi-transparent precipitate which is
insoluble in water but very hard to filter. It dissolves in most acids
to form soluble salts, and in the strong bases to form aluminates, as
indicated in the equations

Al(OH)_{3} + 3HCl = AlCl_{3} + 3H_{2}O,
Al(OH)_{3} + 3NaOH = Al(ONa)_{3} + 3H_{2}O.

It may act, therefore, either as a weak base or as a weak acid, its
action depending upon the character of the substances with which it is
in contact. When heated gently the hydroxide loses part of its hydrogen
and oxygen according to the equation

Al(OH)_{3} = AlO.OH + H_{2}O.

This substance, the formula of which is frequently written HAlO_{2}, is
a more pronounced acid than is the hydro