\Plat"i*num\, n. [NL., fr. Sp. platina, from plata
silver, LL. plata a thin plate of metal. See {Plate}, and cf.
{Platina}.] (Chem.)
A metallic element, intermediate in value between silver and
gold, occurring native or alloyed with other metals, also as
the platinum arsenide (sperrylite). It is heavy tin-white
metal which is ductile and malleable, but very infusible, and
characterized by its resistance to strong chemical reagents.
It is used for crucibles, for stills for sulphuric acid,
rarely for coin, and in the form of foil and wire for many
purposes. Specific gravity 21.5. Atomic weight 194.3. Symbol
Pt. Formerly called {platina}.
{Platinum black} (Chem.), a soft, dull black powder,
   consisting of finely divided metallic platinum obtained by
   reduction and precipitation from its solutions. It absorbs
   oxygen to a high degree, and is employed as an oxidizer.
{Platinum lamp} (Elec.), a kind of incandescent lamp of which
   the luminous medium is platinum. See under {Incandescent}.
{Platinum metals} (Chem.), the group of metallic elements
   which in their chemical and physical properties resemble
   platinum. These consist of the light platinum group, viz.,
   rhodium, ruthenium, and palladium, whose specific
   gravities are about 12; and the heavy platinum group,
   viz., osmium, iridium, and platinum, whose specific
   gravities are over 21.
{Platinum sponge} (Chem.), metallic platinum in a gray,
   porous, spongy form, obtained by reducing the double
   chloride of platinum and ammonium. It absorbs oxygen,
   hydrogen, and certain other gases, to a high degree, and
   is employed as an agent in oxidizing.