Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/200

Rh 190 PLATINUM ing the n.-etal (1772) ; the first platinum crucible was pro duced by Achard (1784). Achard s mode of rendering the native metal amenable to mechanical working was founded upon the fact that it forms a readily fusible alloy with arsenic, from which the latter can be driven off again by intense heating. This method was worked industrially for a time, but subsequently superseded by another superior process, which is usually credited to Wollaston, because it was he who, after having wrought it as a rich source of revenue for years, published it in 1828. But as early as 1800 Knight of London had published all that is essential in the process; and Messrs Johnson, Matthey, & Co. in form the writer that Wollaston obtained the secrets of both the refining and the compressing of the spongy into com pact metal from a relative of theirs, Thomas Cock, who, they are convinced, is the true inventor. Undisputed merits of Wollaston s are his discoveries of palladium (1803) and rhodium (1804). About the same time iridium and osmium were discovered by Smithson Tennant. Platinum ore well deserves its cognomen of &quot;polyxene,&quot; because it is a most complex mixture of mineralogical species, including (1) a number of heavy reguline species designated as platinum, osmiridium, iron-platinum, platin- iridium, iridium, palladium (also gold), and (2) a number of non-metal lie species, notably chrome-iron ore, magnetic oxide of iron, zircone, corundum, and occasionally also diamond. The reguline components always form detached granules, which are generally small, but occasionally assume considerable dimensions. The Demidoff museum contains a native platinum lump weighing 21 pounds troy. The ore, as already stated, was discovered first in South America ; it is found there chiefly in the provinces of Choco and Barbacos, New Granada, and also in Brazil. It occurs besides in San Domingo, in California, at the Rogue river in Oregon, in Canada, and in the island of Borneo. But the richest deposits are those of the Ural Mountains ; these were discovered about 1823, and have been wrought by the Russian Government since about 1828. Part at least of the Ural ore, as Daubre showed, was embedded origin ally with chrome-iron in a serpentine derived from olivine. The very variable percentages of the several components range approximately as follows : platinum, 60 to 87 ; other polyxene metals 3 to 7; gold up to 2 and more; iron 4 to 12 ; copper to 4 ; non-metallic gangue 1 to 3. Platinum, though a noble metal chemically, has too modest an appearance to lend itself much to the jeweller s purposes. The Russian Government used, for a while, to strike platinum coins, but soon came to give up the prac tice on account of the immense fluctuations in the commer cial value of the metal. Almost all the platinum produced now-a-days is made into chemical utensils. Platinum, in fact, is the metal of the chemist. &quot; Without platinum crucibles, which share the infusibility of porcelain with the chemical inertness of gold ones the composition of most minerals could not have been ascertained &quot; (Liebig), and chemistry generally could not have come up to its present level. In industrial chemistry platinum is used chiefly for the construction of those stills for the concentration of oil of vitriol which, although a single one costs a fortune, are cheaper in the long run than glass retorts. The technical extraction of platinum from its ore is to the present day effected everywhere by some modification or other of the so-called &quot;Wollaston&quot; process. Heraeus of Hanau operates as follows. The ore is digested within glass retorts in aqua rcgia diluted with three times its weight of water, an over-pressure of some 12 inches of water being established within the retorts to accelerate the process, which always takes several days. The whole of the osmiridium, along with more or less of other polyxene metals, and the &quot;sand&quot; (corundum, chrome-iron, &c. ) remain undissolved, as a heavy black deposit; the platinum, palladium, part of the rhodium, and more or less of the other three polyxene metals pass into solution, the platinum, iridium, and pal ladium as tetrachlorides. From the clarified solution the whole (almost) of the platinum can be precipitated as PtCl 6 (NH 4 ). 2 by addition of a large excess of sal-ammoniac ; and this simple process used to be adopted formerly. But the precipitate then includes much chloro-iridiate of ammonium lr01 6 (NH 4 ). 2 and other impuri ties. Heraeus, therefore, first evaporates to dryness and heats the residue to 125 C. for a sufficient time, to reduce the palladic and iridic chlorides to the lower stages of PdCl 2 and Ir 2 Cl 6, which form soluble double salts with sal-ammoniac. The heated residue is dissolved in water acidulated with hydrochloric acid, the solution filtered, and mixed with hot concentrated solution of sal-amrnoniac, when a (relatively) pure chloroplatinate comes down as a yellow precipitate (the iridium compound is dark-red), which is washed, first with saturated sal-ammoniac solution, then with dilute hydro chloric acid. The precipitate needs only be exposed to a dull red heat to be converted into &quot;spongy platinum,&quot; i.e., metallic platinum in the form of a grey porous mass. As platinum is infusible even at the highest temperature producible in a wind- furnace, the spongy metal cannot be fused together into a regulus like an ordinary metal ; but it shares with wrought iron the rare quality of assuming a high degree of softness and viscosity at a strong red heat ; and consequently the sponge, after a preliminary compression by purely mechanical means, needs only be exposed to a strong heat to &quot; frit &quot; into a coherent mass ; and this mass, by repeated forging at a white heat is readily made into a perfectly homogeneous compact bar, which, as the metal is very ductile, is easily rolled out in to sheet or drawn into wire. In the former form more especially it goes into the workshop to be made into utensils. This process of welding at the time of Achard (who used it first) and of Knight was a necessary make-shift ; but it is singular that it was retained long after the invention of the oxyhydrogen blast (see vol. xviii. p. 105), by means of which platinum can be fused as easily as lead can in an ordinary fire. With the oxyhydrogen- blowpipe Hare, as early as 1847, fused 970 grammes (upwards of two pounds) of platinum into one regulus. Yet platinum manu facturers did not utilize this obvious process until Deville and Debray, in 1859, again demonstrated its practicability. Their furnace is of the simplest description. Two flat pieces of quick lime, scooped out so as to represent two cupels, are placed one upon the other so that they enclose a flat space similar in form to two superimposed soup-plates. The lower cupel has a notch cut out of its side to serve as a spout for pouring out the liquefied metal, the upper and shallower one is pierced with a central slightly conical round hole through which the (platinum) nozzle of the blowpipe enters, so that the flame flattens itself out on the intro duced metal. By means of this simple contrivance Deville and Debray had no difficulty in fusing as much as twelve kilogrammes of platinum into one regulus ; and Messrs Johnson, Matthey, & Co. of London now think nothing of fusing up as much as 1000 ounces of metal in one operation. A regulus made under Mr Geo. Matthey s superintendence for the metric commission in Paris in 1874 weighed one quarter of a ton. The shaping of compact platinum is effected pretty much in the same way as that of gold or silver ; only the difficulties are less because platinum, unlike the two ordinary noble metals, is sus ceptible of &quot;welding&quot;; i.e., two pieces of the metal, at a white is rarely necessary; it used to be effected (and still is occasionally) by means of gold as a connecting medium and an ordinary blow pipe. But platinum workers, following the lead of Messrs Johnson, Matthey, & Co., have long learned to unite two .platinum seams by the &quot;autogenic&quot; process the local fusing of the two contiguous parts in the oxyhydrogen flame. For the preparation of chemically pure platinum Schneider s process is the one most easily executed and explained. The commercial metal is dissolved in aqua regia and the excess of nitric acid removed by evaporation to a syrup in a water-bath. The residue is redissolved in water and boiled for a long time with a large excess of potash-free caustic soda. If care be taken to main tain a strong alkaline reaction, all the foreign polyxene chlorides are reduced to lower forms than that of tetrachloride ; while only the platinum itself retains this state of combination. The hypo- chlorite formed is then reduced (to NaCl) by addition of a little alcohol to the boiling alkaline liquid, which is now allowed to cool and acidified strongly with hydrochloric acid so as to redissolve any hydrated platinic oxide which may have been precipitated by the first instalments of acid. The liquid at last is filtered, and precipitated by sal-ammoniac to obtain a pure chloroplatinate (PtCl 6 (NH 4 ) 2 ), which, on ignition, of course, yields an equally pure spongy metal. Pure compact platinum is a tin-white metal about as soft as pure copper and nearly (but not quite) equal in plasticity to gold. The specific gravity of the fused metal is 21 &quot;48 to 21 50 at 17 6 C. (Deville and Debray). The breaking strain is 34 1 kilos for hard- drawn and 23 5 kilos for annealed wires ; the modulus of elasticity 15,518 (kilogramme and millimetre as units ; by Werth urn s ex-
 * heat, can be united into one by a stroke of the hammer. Soldering