CHAPTER LII. PLATINUM AND GOLD.

PLATINUM AND GOLD.

PLATINUM.

Examine platinum foil and wire.

285. Platinum is much rarer than gold, and is about two-thirds as
costly as the latter. It is found alloyed with other metals, as
An, and is obtained from sand, in which it occurs, by washing.
Aqua regia is the only acid which dissolves it, and the action is
much slower than with Au. Pt is one of the heaviest metals,
having a specific gravity three times that of Fe, or twenty-one
and a half times that of water. Its fusing-point is about 1600
degrees, or just below the temperature of the oxy-hydrogen flame.
Like Au it has little affinity for other elements, but alloys
with many metals. Pt is so tenacious that it can be drawn into
wire invisible to the naked eye, being drawn out in the center of
a silver wire, which is afterwards dissolved away from the Pt by
HNO3. Noting its valences, 2 and 4, write the symbols for the ous
and ic chlorides and oxides.

286. Uses.--Pt is much used in chemistry in the form of foil,
wire, and crucibles. On what properties does this use depend?
Describe its use in making H2SO4.

PtCl4 is made by dissolving Pt in aqua regia, and evaporating the
liquid. On heating PtCl4, half of its Cl is given up, leaving
PtCl2. If it be still more strongly heated, the Cl all passes
off, leaving spongy Pt. By fusing this in the oxy-hydrogen flame,
ordinary Pt is obtained. Spongy Pt has a remarkable power of
absorbing, or occluding, O without uniting with it. This O it
gives up to some other substances, and thus becomes indirectly an
oxidizing agent. What other element has this property of
occluding gases?

GOLD.

Examine auriferous quartz, gold chloride, yellow and ruby glass
colored with gold. 287. Gold is rarely found combined, and has
small affinity for other elements, though forming alloys with Cu,
Ag, and Hg. Its source is usually either quartz rock, called
auriferous quartz, or sand in placer mines. The element is widely
distributed, occurring in minute quantities in most soils, sea
water, etc. California and Australia are the two greatest gold-
producing countries. That from California has a light color, due
to a slight admixture of Ag. Australian gold is of a reddish hue,
due to an alloy of Cu. Gold-bearing quartz is pulverized, and
treated with Hg to dissolve the precious metal, which is then
separated from the alloy by distillation. Compare this with the
preparation of Ag.

Such is the malleability of Au that it has been hammered into
sheets not over one-millionth of an inch thick; it is then as
transparent as glass. Gold does not tarnish or change below the
melting-point. On account of its softness it is usually alloyed
with Cu, sometimes with Ag. Pure gold is twenty-four carats fine.
Eighteen carat gold has eighteen parts Au and six Cu. Gold coin
has nine parts Au to one part Cu. The most important compound is
AuCl3. Describe a use of it. This metal is much employed in
electroplating, and somewhat in coloring glass.