far studied in that they can act either as acid-forming or
base-forming elements. As base-forming elements each of the metals forms
two series of salts. In the one series, designated by the suffix "ous,"
the metal is divalent; in the other series, designated by the suffix
"ic," the metal is trivalent. Only the manganous and the chromic salts,
however, are of importance. The acids in which these elements play the
part of a non-metal are unstable, but their salts are usually stable,
and some of them are important compounds.

MANGANESE

~Occurrence.~ Manganese is found in nature chiefly as the dioxide MnO_{2},
called pyrolusite. In smaller amounts it occurs as the oxides
Mn_{2}O_{3} and Mn_{3}O_{4}, and as the carbonate MnCO_{3}. Some iron
ores also contain manganese.

~Preparation and properties.~ The element is difficult to prepare in pure
condition and has no commercial applications. It can be prepared,
however, by reducing the oxide with aluminium powder or by the use of
the electric furnace, with carbon as the reducing agent. The metal
somewhat resembles iron in appearance, but is harder, less fusible, and
more readily acted upon by air and moisture. Acids readily dissolve it,
forming manganous salts.

~Oxides of manganese.~ The following oxides of manganese are known: MnO,
Mn_{2}O_{3}, Mn_{3}O_{4}, MnO_{2}, and Mn_{2}O_{7}. Only one of these,
the dioxide, needs special mention.

~Manganese dioxide~ (_pyrolusite_) (MnO_{2}). This substance is the most
abundant manganese compound found in nature, and is the ore from which
all other compounds of manganese are made. It is a hard, brittle, black
substance which is valuable as an oxidizing agent. It will be recalled
that it is used in the preparation of chlorine and oxygen, in
decolorizing glass which contains iron, and in the manufacture of
ferromanganese.

~Compounds containing manganese as a base-forming element.~ As has been
stated previously, manganese forms two series of salts. The most
important of these salts, all of which belong to the manganous series,
are the following:

Manganous chloride MnCl_{2}.4H_{2}O.
Manganous sulphide MnS.
Manganous sulphate MnSO_{4}.4H_{2}O.
Manganous carbonate MnCO_{3}.
Manganous hydroxide Mn(OH)_{2}.

The chloride and sulphate may be prepared by heating the dioxide with
hydrochloric and sulphuric acids respectively:

MnO_{2} + 4HCl = MnCl_{2} + 2H_