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4 PLATINA. spinel, quartz, and magnetic ironstone. It is not true that this metal occurs near Carthagena, or Santa Fe, or on the islands ot Porto Rico and Barbadoes, or in Peru, although these different localities are mentioned by au thors *. The platina in granulated grains is found in alluvial soil, along with grains of gold, in gold-working, in Brazil f. Uses. Its property of remaining unaltered in the air, or when exposed to high heats, of resisting the action of many salts, and of receiving a fine polish, have rendered this metal useful tor various chemical and physical instru ments, as pyrometers, crucibles, pendulums, reflecting te lescopic mirrors, and for wheels in the construction of watches. Reflecting mirrors made of glass, although they preserve their lustre and polish well, are inconve nient, because they form a double image : mirrors made with metallic alloys, which were substituted in their place, give but a single image, but tarnish on exposure to the air : mirrors of platina possess the advantage of not tar nishing, and they give but one image, and, owing to their great density, augment the reflecting power. Of all me tals it expands the least by heat, and follows the most regular course in its expansion: hence it is admirably fitted for measures. The geometers Delambre and Me- chain, in measuring the arc of the meridian contained between Dunkirk and Barcelona, used, in their opera tions, rods made of this metal. Klaproth has shewn, that it may be used with great advantage in painting and or namenting * Humboldt’S New Spain, vol. iii. p. iso. Black’s translation. I' Wollaston, Phil. Trans, for 1809.