ery large quantities as a source of sulphur dioxide in the manufacture
of sulphuric acid, since it burns readily in the air, forming ferric
oxide and sulphur dioxide:

2FeS_{2} + 11O = Fe_{2}O_{3} + 4SO_{2}.

~Ferrous carbonate~ (FeCO_{3}). This compound occurs in nature as
siderite, and is a valuable ore. It will dissolve to some extent in
water containing carbon dioxide, just as will calcium carbonate, and
waters containing it are called chalybeate waters. These chalybeate
waters are supposed to possess certain medicinal virtues and form an
important class of mineral waters.

~Ferric salts.~ Ferric salts are usually obtained by treating an acidified
solution of a ferrous salt with an oxidizing agent:

2FeCl_{2} + 2HCl + O = 2FeCl_{3} + H_{2}O,

2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.

They are usually yellow or violet in color, are quite soluble, and as a
rule do not crystallize well. Heated with water in the absence of free
acid, they hydrolyze even more readily than the salts of aluminium. The
most familiar ferric salts are the chloride and the sulphate.

~Ferric chloride~ (FeCl_{3}). This salt can be obtained most conveniently
by dissolving iron in hydrochloric acid and then passing chlorine into
the solution:

Fe + 2HCl = FeCl_{2} + 2H,

FeCl_{2} + Cl = FeCl_{3}.

When the pure salt is heated with water it is partly hydrolyzed:

FeCl_{3} + 3 H_{2}O <--> Fe(OH)_{3} + 3HCl.

This is a reversible reaction, however, and hydrolysis can therefore be
prevented by first adding a considerable amount of the soluble product
of the reaction, namely, hydrochloric acid.

~Ferric sulphate~ (Fe_{2}(SO_{4})_{3}). This compound can be made by
treating an acid solution of green vitriol with an oxidizing agent. It
is difficult to crystallize and hard to obtain in pure condition. When
an alkali sulphate in proper quantity is added to ferric sulphate in
solution an iron alum is formed, and is easily obtained in
large crystals. The best known iron alums have the formulas
KFe(SO_{4})_{2}.12H_{2}O and NH_{4}Fe(SO_{4})_{2}.12H_{2}O. They are
commonly used when a pure ferric salt is required.

~Ferric hydroxide~ (Fe(OH)_{3}). When solutions of ferric salts are
treated with ammonium hydroxide, ferric hydroxide is formed as a
rusty-red precipitate, insoluble in water.

~Iron cyanides.~ A large number of complex cyanides containing iron are
known, the most important being potassium f