such purposes
as the manufacture of pans for evaporating sulphuric acid, wires for
sealing through incandescent light bulbs, and for making a great variety
of instruments. Unfortunately the supply of the metal is very limited,
and the cost is steadily advancing, so that it is now more valuable than
gold.

~Compounds.~ Platinum forms two series of salts of which platinous
chloride (PtCl_{2}) and platinic chloride (PtCl_{4}) are examples.
Platinates are also known. While a great variety of compounds of
platinum have been made, the substance is chiefly employed in the
metallic state.

~Platinic chloride (PtCl_{4}).~ Platinic chloride is an orange-colored,
soluble compound made by heating chloroplatinic acid in a current of
chlorine. If hydrochloric acid is added to a solution of the substance,
the two combine, forming chloroplatinic acid (H_{2}PtCl_{6}):

2HCl + PtCl_{4} = H_{2}PtCl_{6}.

The potassium and ammonium salts of this acid are nearly insoluble in
water and alcohol. The acid is therefore used as a reagent to
precipitate potassium in analytical work. With potassium chloride the
equation is

2KCl + H_{2}PtCl_{6} = K_{2}PtCl_{6} + 2HCl.

~Other metals of the family.~ The other members of the family
have few applications. Iridium is used in the form of a
platinum alloy, since the alloy is much harder than pure
platinum and is even less fusible. This alloy is sometimes used
to point gold pens. Osmium tetroxide (OsO_{4}) is a very
volatile liquid and is used under the name of osmic acid as a
stain for sections in microscopy.

GOLD

~Occurrence.~ Gold has been found in many localities, the most famous
being South Africa, Australia, Russia, and the United States. In this
country it is found in Alaska and in nearly half of the states of the
union, notably in California, Colorado, and Nevada. It is usually found
in the native condition, frequently alloyed with silver; in combination
it is sometimes found as telluride (AuTe_{2}), and in a few other
compounds.

~Mining.~ Native gold occurs in the form of small grains or larger nuggets
in the sands of old rivers, or imbedded in quartz veins in rocks. In the
first case it is obtained in crude form by placer mining. The sand
containing the gold is shaken or stirred in troughs of running waters
called sluices. This sweeps away the sand but allows the heavier gold to
sink to the bottom of the sluice. Sometimes the sand con