Plăt'ĭ-nūm or Pla-tī'num, n.

n. [N. Lat., from Sp. platina, from plata, silver; Fr. platine. See PLATE, and cf. PLATINA.] (Chem.) A metal of the color of silver, but less bright. Its specific gravity is 20, and may be increased by heat and pressure to 21.5. It is harder than iron, undergoes no alteration in air, resists the action of acids, is very ductile, and capable of being rolled into thin plates. It is the heaviest and least expansible of the metals. [Written also platina].

Platinum black, metallic platinum in the form of a black powder, obtained by decomposing a weak solution of chloride of platinum by the agency of galvanism.—Spongy platinum, or platinum sponge, metallic platinum in the form of a porous, dull-brown mass. It is much used in chemical experiments.