PLATINUM. A metal of a white col or, exceedingly ductile, malleable, and difficult of fusion. It is the heaviest sub stance known, its specific gravity being 21.5. It undergoes no change from air or moisture, and is not attacked by any of the pure acids ; it is dissolved by chlo rine and nitromnriatic acid, and is oxyd ized at high temperatures by pure potas sa and lithia. It is found to a small ex tent in Georgia, in the gold regions ; it has not yet been met with in California. It exists in New Grenada, in Brazil, and in the Ural mountains of Russia. It is usually in small grains of a metallic lus tre, associated or combined with palla ditnn, rhodium, iridium, and osmium; and with copper, iron, lead, titanium, chromium, gold and silver ; it is also usually mixed with alluvial sand. The particles are seldom so large as a small pea, but sometimes lumps have been found of the size of a hazel-nut to that of a pigeon's egg. In 1826, it was first discovered in a rein, associated with gold, by Boussingault, in the province of Atioquia, in South America. When a perfectly clean surface ofplatinum is pre sented to a mixture of hydrogen and oxygen gas, it has the extraordinary pro perty of causing them to combine so as to form water, and often with such rapid ity as to render the metal red hot : spon gy platinlim, as it is usually called, ob tained by heating the ammonio-chloride of platinum, is most effective in produc ing this extraordinary result ; and a jet of hydrogen directed upon it may be inflamed by the metal thus ignited, a property which has been applied to the construction of convenient instruments for procuring a light. The equivalent of platinum is about 98. It is precipi tated from its nitro-muriatic solution by sal ammoniac, which throws it down in the form of a yellow powder, composed of bichloride of platinum and sal ammoniac.
It is generally found associated with iridium, osmium, rhodium, palladium, iron and copper. By far the greater part of the platinum of commerce comes from the Ural mountains. Only a limited por tion of the whole is allowed by the Rus sian government to come into the market. Platinum is used as a coin metal in Rus sia, and the following is the mode by which it is obtained pure from the ore in St. Petersburgh : One part of the ore is put in open pla tins capable of containing from 6 to 8 lbs., along with 3 parts of muriatic acid at 25° B. and 1 part of nitric acid at 40°. Thirty of these vessels are placed upon a sand-bath covered with a glazed dome with movable panes, which is sur mounted by a ventilating chimney to car ry the vapors out of the laboratory. Heat is applied for eight or ten hours, till no more red vapors appear ; a proof that the whole nitric acid is decomposed, though some of the muriatic remains. After set tling, the supernatant liquid is decanted off into large cylindrica- glass vessels, the residuum is washed, and the washing is also decanted off. A fresh quantity of
nitro-muriatic acid is now poured upon the residuum. This treatment is repeat ed till the whole solid matter has eventu ally disappeared. The ore requires for solution from ten to fifteen times its weight of nitro-muriatic acid, according to the size of its grains.
The solutions thus made are all acid ; a circumstance essential to prevent the iri dium from precipitating with the plati num, by the water of ammonia, which is next added. The deposit being allowed to form, the mother-waters are poured off, the precipitate is washed with cold water, dried, and calcined in crucibles of platinum.
The mother-waters and the washings are afterwards treated separately. The first being concentrated to one-twelfth of their bulk in glass retorts, on cooling they let fall the iridinm in the state of an ammo niacal chloride, constituting a dark-pur ple powder, occasionally crystallized in regular octahedrons. The washings are evaporated to dryness in porcelain ves sels ; the residuum is calcined and treated like fresh ore ; but the platinum it affords needs a second purification.
For agglomerating the platinum, the spongy mass is pounded in bronze mor tars ; the powder is passed through a fine sieve, and put into a cylinder of the in tended size of the ingot. The cylinder is fitted with arammer, which is forced in by a coining press, till the powder be much condensed. It is then turned out of the mould, and baked 36 hours in a porcelain kiln, after which it may be readily forged, if it be pure, and may receive any desired form from the hammer. It contracts in volume from 1-6th to 1-5th during the calcination. The cost of the manufacture of platinum is fixed by the administra tion at 32 franks the Russian pound ; but so great a sum is never expended upon it. The salts of platinum are not much in use. The bichloride is the most com mon, and is made by digesting nitro-mu riatic on platinum in slips; it is used in solution as a test for potash, as it precip itates the salts of that alkali as a yellow double chloride of platinum of potassium, which is insoluble in alcohol. It is also used in the coating of surfaces with met allic platinum. The solution of bichlo ride is washed over the surface, which is then heated until the acid element is driven off; the surface is then polished. Sometimes a solution of the double chlo ride of soda and platinum is used. Three immersions of the article to be coated suffice: after each immersion the surface is dried and polished with chalk.
Imitation of Platinum.—Melt together one pound of brass with ten ounces of zinc; but as brass is composed of copper and zinc, in the proportion of about three pounds of the former to one pound of the latter, equal parts of the copper and zinc will produce the same compound in imitation of platina.