pyrolusite of mineralogists, and is by far the most abundant of the
manganese ores. It occurs in a hydrated form in varvicite and wad. Its
commercial value depends upon the proportion of chlorine which a given
weight of it will liberate when it is heated with hydrochloric acid, the
quantity of chlorine being proportional to the excess of oxygen which
this oxide contains over that contained in the same weight of protoxide.
When mixed with chloride of sodium and sulphuric acid it causes an
evolution of chlorine, the other resulting products being sulphate of
soda and sulphate of protoxide of manganese. When mixed with acids, it
is a valuable oxidizing agent. It is much used for the preparation of
oxygen, either by simply heating it, when it yields 12 per cent. of
gas, or by heating it with sulphuric acid, when it yields 18 per
cent. Besides its many uses in the laboratory, it is employed in the
manufacture of glass, porcelain, and kindred wares.

Manganic acid is not known in a free state. Manganate of potash is
formed by fusing together hydrated potash and binoxide of manganese. The
black mass which results from this operation is soluble in water,
to which it communicates a green color, due to the presence of the
manganate. From this water the salt is obtained _in vacuo_ in beautiful
green crystals. On allowing the solution to stand exposed to the air, it
rapidly becomes blue, violet, purple, and finally red, by the gradual
conversion of the manganate into the permanganate of potash; and on
account of these changes of color the black mass has received the name
of mineral chameleon.

Permanganic acid is only known in solution or in a state of combination.
Its solution is of a splendid red color, but appears of a dark violet
tint when seen by transmitted light. It is obtained by treating a
solution of permanganate of baryta with sulphuric acid, when sulphate of