586 PLATINUM 



treatment is repeated till the whole solid matter has eventually disappeared. The ore 

 requires for solution from 10 to 15 times its weight of nitro-muriatic acid, according 

 to the size of its grains. 



The solutions tlius made are all acid: a circumstance essential to prevent the 

 iridium from precipitating with the platinum, 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 former 

 being concentrated to one-twelfth of their bulk in glass retorts ; on cooling, they let 

 fall the iridium in tho state of an ammoniacal chloride, constituting a dark purple 

 powder, occasionally crystallised in regular octahedrons. The washings are evaporated 

 to dryness in porcelain vessels ; 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 mortars ; the 

 powder is passed through a fine sieve, and put into a cylinder of the intended size of 

 the ingot. The cylinder is fitted with a rammer, which is forced in by a coining 

 press, till the powder is 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 

 l-6th to 1-oth during the calcination. 



The method ordinarily used for the extraction of platinum is that originally pro- 

 posed by Dr. Wollaston. The ore is treated first with nitric and then with hydro- 

 chloric acid, to remove those metals which are readily soluble, and is afterwards 

 digested in dilute aqua regia at a moderate heat. To the solution of platinum thus 

 obtained, a solution of sal-ammoniac is added, when the platinum is precipitated in 

 the form of a yellow salt the double chloride of platinum and ammonium. This 

 salt, when washed, is heated to redness, whereby the chlorine and ammonia are ex- 

 pelled, whilst a mass of spongy platinum is obtained. The sponge is worked into a 

 paste with water, and, having been subjected to powerful pressure and dried, is care- 

 fully heated to whiteness in a wind-furnace and forged into an. ingot. The platinum 

 at a high temperature may be welded like iron. 



For Dr. Wollaston's process, see PJijlosophical Transactions, 1829, Part I. 



Great improvements in refining platinum have been introduced by Messrs. Deville 

 and Debray. In a furnace, composed of blocks of lime, the platinum is fused by 

 means of the oxyhydrogen flame, when the osmium present is expelled as tetroxide, 

 while the silica forms a slag by combining with the lime. 



The same chemists effect the purification of platinum by fusing the ore with galena 

 in a reverberatory furnace. The metallic lead from the galena alloys with the 

 platinum, whilst the iridosmine present in the ore is unattacked, and sinks, by its 

 great density, to the bottom of the bath. The alloy of lead and platinum is then 

 drawn off, and the lead removed by cupellation. 



Platinum furnishes most valuable vessels to both analytical and manufacturing 

 chemists. Boilers and stills of platinum used in the manufacture of oil of vitriol are 

 constructed of largo size and at great cost. The metal is malleable, and may bo 

 beaten out into leaves of extreme thinness. Dr. Wollaston succeeded in obtaining a 

 wire not exceeding tho two-thousandth of an inch in diameter. A wire of this 

 metal of ^th inch in diameter will support a weight of 361 Ibs. 



In 1828 a platinum coinage was commenced in Russia, but was discontinued in 1845. 



This metal is applied to porcelain by two different processes: sometimes in a 

 rather coarse powder, applied by the brush, like gold, to form ornamental figmvs ; 

 sometimes in a state of extreme division, obtained by decomposing its nitro-muriatic 

 solution, by means of an essential oil, such as rosemary or lavender. In this case, it 

 must be evenly spread over the whole ground. Both modes of application give rise 

 to a steely lustre. 



The properties possessed in common by gold and platinum have several times given 

 occasion to fraudulent admixtures, which have deceived the assaycrs. M. Vauquelin 

 having executed a series of experiments to elucidate this subject, drew the following 

 conclusions : 



If the platinum do not exceed 30 or 40 parts in the thousand of the alloy, the gold 

 does not retain any of it when the parting is made with nitric acid in the usual way ; 

 and when the proportion of platinum is greater, the fraud becomes manifest, 1st, by tho 

 higher temperature required to pass it through tho cupel, and to form a round button ; 

 2nd, by the absence of the lightning, fulguiation, or coruscation ; 3rd, by the dull white 

 colour of the button and its crystallised surface ; 4th, by the straw-yellow colour which 

 platinum communicates to the aquafortis in the parting ; 5th, by tho straw-yellow colour, 

 bordering on white, of the cornet after it is annealed. If the platinum amounts to one 

 fourth of the gold, we must add to the alloy at least 3 times its weight of fine silver, 



