PLATINUM, or PLATINA (See CHEMISTRY, Suppl. Part I. Chap. iii. Sect. 3.), is a metal, of which every chemist regrets the difficulty of making it malleable. Of the different processes adopted to accomplish this end, we have reason to believe that of Mr Richard Knight the most successful; and, with the spirit
spirit of a true philosopher, he wishes to make that process as generally known as possible. We shall give it in his own words:
"To a given quantity of crude platinum, I add (says he) 15 times its weight of nitro-muriatic acid (composed of equal parts of nitric and muriatic acids) in a tubulated glass retort, with a tubulated receiver adapted to it. It is then boiled, by means of an Argand's lamp, till the acid has assumed a deep saffron colour: it is then poured off; and if any platina remains undissolved, more acid is added, and it is again boiled until the whole is taken up. The liquor, being suffered to rest till quite clear, is again decanted: a solution of sal-ammoniac is then added, by little and little, till it no longer gives a cloudiness. By this means the platina is thrown down in the form of a lemon-coloured precipitate, which having subsided, the liquor is poured off, and the precipitate repeatedly washed with distilled water till it ceases to give an acid taste (too much water is injurious, the precipitate being in a certain degree soluble in that liquid); the water is then poured off, and the precipitate evaporated to dryness."
Thus far our author's method, as he candidly observes himself, differs not from that which has been followed by many others; but the remainder of the process is his own. "A strong, hollow, inverted cone of crucible earth being procured, with a corresponding stopper to fit it, made of the same materials, the point of the latter is cut off about three-fourths from the base. The platina, now in the state of a light yellow powder, is pressed tight into the cone, and, a cover being fixed slightly on, it is placed in an air-furnace, and the fire raised gradually to a strong white heat. (The furnace used by Mr Knight is portable, with a chamber for the fire only eight inches in diameter.) In the mean time the conical stopper, fixed in a pair of iron tongs suitable for the purpose, is brought to a red, or to a bright red, heat. The cover being then removed from the cone, the tongs with the heated stopper is introduced through a hole in the cover of the furnace, and pressed at first gently on the platina, at this time in a state nearly as soft as dough, till it at length acquires a more solid consistence. It is then repeatedly struck with the stopper, as hard as the nature of the materials will admit, till it appears to receive no farther impression. The cone is then removed from the furnace; and being struck lightly with a hammer, the platina falls out in a metallic button, from which state it may be drawn, by repeatedly heating and gently hammering, into a bar fit for flattening, drawing into wire, planishing, &c."
"Besides the comparative facility of this process, it has the farther advantage of rendering the platina much purer than when red-hot iron is obliged to be had recourse to; for platina, when of a white heat, has a strong affinity for iron, and, with whatever care it may have been previously separated from that metal, will be found to have taken up a portion of it, when it is employed of a red heat, to serve to unite the particles of the platina."