ty and color, the most abundant
being hematite, which ranges in color from red to nearly black. When
prepared by chemical processes it forms a red powder which is used as a
paint pigment (Venetian red) and as a polishing powder (rouge).

Magnetite has the formula Fe_{3}O_{4} and is a combination of FeO and
Fe_{2}O_{3}. It is a very valuable ore, but is less abundant than
hematite. It is sometimes called magnetic oxide of iron, or lodestone,
since it is a natural magnet.

~Ferrous salts.~ These salts are obtained by dissolving iron in the
appropriate acid, or, when insoluble, by precipitation. They are usually
light green in color and crystallize well. In chemical reactions they
are quite similar to the salts of magnesium and zinc, but differ from
them in one important respect, namely, that they are easily changed into
compounds in which the metal is trivalent. Thus ferrous chloride treated
with chlorine or aqua regia is changed into ferric chloride:

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

Ferrous hydroxide exposed to moist air is rapidly changed into ferric
hydroxide:

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

~Ferrous sulphate~ _(copperas, green vitriol)_ (FeSO_{4}.7H_{2}O). Ferrous
sulphate is the most familiar ferrous compound. It is prepared
commercially as a by-product in the steel-plate mills. Steel plates are
cleaned by the action of dilute sulphuric acid upon them, and in the
process some of the iron dissolves. The liquors are concentrated and the
green vitriol separates from them.

~Ferrous sulphide~ (FeS). Ferrous sulphide is sometimes found in nature as
a golden-yellow crystalline mineral. It is formed as a black precipitate
when a soluble sulphide and an iron salt are brought together in
solution:

FeSO_{4} + Na_{2}S = FeS + Na_{2}SO_{4}.

It can also be made as a heavy dark-brown solid by fusing together the
requisite quantities of sulphur and iron. It is obtained as a by-product
in the metallurgy of lead:

PbS + Fe = FeS + Pb.

It is used in the laboratory in the preparation of hydrosulphuric acid:

FeS + 2HCl = FeCl_{2} + H_{2}S.

~Iron disulphide~ _(pyrites)_ (FeS_{2}). This substance bears the same
relation to ferrous sulphide that hydrogen dioxide does to water. It
occurs abundantly in nature in the form of brass-yellow cubical crystals
and in compact masses. Sometimes the name "fool's gold" is applied to it
from its superficial resemblance to the precious metal. It is used in
v