s substance can
be made by dissolving mercuric oxide in hydrochloric acid. On a
commercial scale it is made by subliming a mixture of common salt and
mercuric sulphate:

2NaCl + HgSO_{4} = HgCl_{2} + Na_{2}SO_{4}.

The mercuric chloride, being readily volatile, vaporizes and is
condensed again in cool vessels. Like mercurous chloride it is a white
solid, but differs from it in that it is soluble in water. It is
extremely poisonous and in dilute solutions is used as an antiseptic in
dressing wounds.

~Mercuric sulphide~ (HgS). As cinnabar this substance forms the chief
native compound of mercury, occurring in red crystalline masses. By
passing hydrosulphuric acid into a solution of a mercuric salt it is
precipitated as a black powder, insoluble in water and acids. By other
means it can be prepared as a brilliant red powder known as vermilion,
which is used as a pigment in fine paints.

~The iodides of mercury.~ If a solution of potassium iodide is
added to solutions of a mercurous and a mercuric salt
respectively, the corresponding iodides are precipitated.
Mercuric iodide is the more important of the two, and as
prepared above is a red powder which changes to yellow on
heating to 150 deg.. The yellow form on cooling changes back again
to the red form, or may be made to do so by rubbing it with a
knife blade or some other hard object.

SILVER

~Occurrence.~ Silver is found in small quantities in the uncombined state;
usually, however, it occurs in combination with sulphur, either as the
sulphide (Ag_{2}S) or as a small constituent of other sulphides,
especially those of lead and copper. It is also found alloyed with gold.

~Metallurgy.~ _Parkes's process._ Silver is usually smelted in connection
with lead. The ores are worked over together, as described under lead,
and the lead and silver obtained as an alloy, the silver being present
in small quantity. The alloy is melted and metallic zinc is stirred in.
Zinc will alloy with silver but not with lead, and it is found that the
silver leaves the lead and, in the form of an alloy with zinc, forms as
a crust upon the lead and is skimmed off. This crust, which, of course,
contains lead adhering to it, is partially melted and the most of the
lead drained off. The zinc is removed by distillation, and the residue
is melted on an open hearth in a current of air; by this means the zinc
and lead remaining with the silver are ch