PLATINUM (symb. Pt, equiv. 99—new system, 198—sp. gr. 21.5) is 'one of the "noble metals," which may be obtained in more forms than one. It is only found in the native state, usually occurring iu small glistening granules of a steel-gray color, which always contain an admixture, iu varying proportions, of several metals, most of which are rarely found except in association with platinum. Sometimes, however, it is found in masses of the size of a pigeon's egg, and pieces weighing ten or even twenty pounds have occasionally been found. The following table shows the composition of crude platinum ore as obtained from different parts of the globe. The analyses were conducted by Messrs. Devine and Debray: Ruthenium Is also almost always present, and in the above analysis is probably included with the iridium, which it closely resembles.
There are two modes of obtaining platinum in the form of ingots from the ore, both of which require notice. The method which has been universally employed, till within the last five years, was that discovered by Wollaston, the leading steps of which were as follows: After the removal of the metals associated with the platinum, by the succes sive action of nitric and hydrochloric acids, the platinum itself is dissolved in aqua mgin, from which it is precipitated by a solution of sal ammoniac in the form of as sparingly soluble double salt, the chloride of ammonium and platinum, "led by the formula This salt is washed and heated to redness, by wh'( means the chlorine and ammonia are expelled, leaving the metal in the form of a gray. spongy, soft mass, known to chemists as spongy platinum. In this form it is very finely powdered tinder water, is next shaped by intense pressure into a mass, and is then exposed to an intense heat in a wind-furnace, the ingot being formed by hammering it upon its two ends. If hammered on its sides, it splits.) This heating and forging must be repeated till the metal becomes homogeneous and ductile.
Deville and Debray have introduced an entirely new method for the extraction of platinum from its ores. ' They first form a fusible alloy of this metal with lead, by exposing the platinum ore-2 cwt. being used in a single experiment, with equal weights of galena and litharge gradually added, and a little glass to act as a flux—to full redness in a reverberatory furnace lined with The sulphur of the galena is oxidized and expelled, and the liquid alloy of lead and platinum is allowed to rest for some time, to allow the osmide of iridium, which is not affected by the preceding operations, to sink to the bottom. The upper portions of the alloy are then decanted, and cast into ingot
molds, which are submitted to cupellation; and the metallic platinum which is left after the cupellation is melted and refined in a furnace of lime—which is employed in conse quence of it‘ being a very bad conductor of heat—by means of the oxyhydrogen blow pipe. The untinum obtained in this manner is nearly pure, and very ductile and malleable. lfor details regarding this process, which has been patented both in France and in this country, the reader'is referred to the memoir "On Platinum and the Metals which accompany it," in the Annales de Chimie et de Physique for August, 1859.
Platinum, as obtained by either of the above processes, exhibits a bluish-white metallic luster; it is exceedingly malleable and ductile, and is very infusible, melting only before the oxyhydrogen blow-pipe, or in a very powerful blast-furnace, such as that used by Deville and Debray. It expands less by heat than any other metal, and it is usually regarded as the heaviest form of matter yet known; but, according to Deville and Debray, osmium and iridium are about equally dense. It is unaffected by atmos pheric action, and does not undergo oxidization in the Meat even the highest tempera tures. It is not acted on by nitric, hydrochloric, sulphuric, or hydrofluoric acid, or in short, by any single acid; but in aqua regia it slowly dissolves, and forms a soluble bicioride. In consequence of its power of resisting the action of acids, it is of great service in experimental and manufacturing chemical processes, platinum spatulas, cap mules, crucibles, etc., being employed in every laboratory; while platinum stills, weigh ing sometimes as much as 1000 ounces, are frequently used for concentrating oil of vitriol. Platinum is, however, corroded if heated with the alkalies or alkaline earths, and especially with a mixture of nitrate of potash and hydrated potash, an oxide being formed which combines with the alkaline bases.