abundance,
principally as oxides, less frequently as salts, and sometimes in
combination with sulphur and arsenic. It is found in plants, and
passes with them into the animal body. In the metallic state, it is
found frequently in cast iron and steel. It is a hard, brittle metal,
fusible with difficulty, and of a light grey color. It tarnishes upon
exposure to the air and under water, and falls into a powder.

_Protoxide of Manganese_ exists as a green powder; as hydrate
separated by caustic alkalies, it is white, but oxidizes very speedily
upon exposure to the air. The protoxide is the base of the salts of
manganese. These salts, which are soluble in water, are decomposed
when heated in the presence of the air--except the sulphate (MnO,
SO^{3}), but if the latter is exposed to ignition for awhile, it then
ceases to be soluble in water, or at least only sparingly so.

_Sesquioxide of Manganese_ (Mn^{2}O^{3}) Occurs very sparingly in
nature as small black crystals (_Braunite_) which give, when ground, a
brown powder. When prepared by chemical process, it is in the form of
a black powder. The hydrate occurs sometimes in nature as black
crystals (_manganite_). By digestion with acids, it is dissolved into
salts of the protoxide. With hydrochloric acid, it yields chlorine.

The _prot-sesquioxide of manganese_ (MnO + Mn^{2}O^{3}) occurs
sometimes in black _crystals_ (_hausmannite_). Prepared artificially,
it is in the form of a brown powder.

_Peroxide of Manganese_ (MnO^{2}) occurs in considerable abundance as
a soft black amorphous mass, or crystallized as pyrolusite, also
reniform and fibrous. It is deprived of a part of its oxygen when
exposed to ignition. It eliminates a considerable quantity of chlorine
from hydrochloric acid, and is thereby converted into chloride of
manganese (ClMn).

Most of the manganese compounds which occur in nature yield water when
heated in a glass tube closed at one end. The sesquioxide and peroxide
give out oxygen when strongly heated, which can be readily detected by
the increased glow which it causes, if a piece of lighted wood or
paper is brought to the mouth of the tube. The residue left in the
tube is a brown mass (MnO + Mn^{2}O^{3}).

When exposed to ignition with free access of air, all manganese oxides
are converted into (MnO + Mn^{2}O^{3}), but without fusion. Such, at
least, is the statement of some of the German chemists, although it
will admit perhaps of further investigati