xide, and its salts are
frequently formed when aluminium compounds are fused with alkalis. The
magnesium salt Mg(AlO_{2})_{2} is called spinel, and many other of its
salts, called aluminates, are found in nature.

When heated strongly the hydroxide is changed into oxide, which will not
again take up water on being moistened.

~Mordants and dyeing.~ Aluminium hydroxide has the peculiar
property of combining with many soluble coloring materials and
forming insoluble products with them. On this account it is
often used as a filter to remove objectionable colors from
water. This property also leads to its wide use in the dye
industry. Many dyes will not adhere to natural fibers such as
cotton and wool, that is, will not "dye fast." If, however, the
cloth to be dyed is soaked in a solution of aluminium compounds
and then treated with ammonia, the aluminium salts which have
soaked into the fiber will be converted into the hydroxide,
which, being insoluble, remains in the body of it. If the fiber
is now dipped into a solution of the dye, the aluminium
hydroxide combines with the color material and fastens, or
"fixes," it upon the fiber. A substance which serves this
purpose is called a _mordant_, and aluminium salts,
particularly the acetate, are used in this way.

~Aluminium chloride~ (AlCl_{3}.6 H_{2}O). This substance is prepared by
dissolving the hydroxide in hydrochloric acid and evaporating to
crystallization. When heated it is converted into the oxide, resembling
magnesium in this respect:

2(AlCl_{3}.6 H_{2}O) = Al_{2}O_{3} + 6HCl + 9H_{2}O.

The anhydrous chloride, which has some important uses, is made by
heating aluminium turnings in a current of chlorine.

~Alums.~ Aluminium sulphate can be prepared by the action of sulphuric
acid upon aluminium hydroxide. It has the property of combining with the
sulphates of the alkali metals to form compounds called _alums_. Thus,
with potassium sulphate the reaction is expressed by the equation

K_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
= 2(KAl(SO_{4})_{2}.12H_{2}O).

Under similar conditions ammonium sulphate yields ammonium alum:

(NH_{4})_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
= 2(NH_{4}Al(SO_{4})_{2}.12H_{2}O).

Other trivalent sulphates besides aluminium sulphate can form similar
compounds with the alkali sulphates, and these compounds are also called