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Whilst gold has been the object of desire from the earliest times, platinum, not less to be ranked amongst the precious metals, and for certain of its uses more valuable than gold, was unknown in Europe till about the middle of last century. It has been found in Brazil, in Peru, in the mine of Santa Fe, near Carthagena, at Santa Rosa, in the province of Antioquia, where it occurs in veins associated with gold, and in the valley of Jaky, in St Domingo. It has been detected in the silver mines of Guadalcanal, in Estremadura. But the richest mines of it yet known, are on the east side of the Oural Mountains, where it was discovered so lately as the year 1820, and whence it is obtained in such quantity, as to be used as coin in the Russian Empire.
It occurs in the form of small rounded or flattened grains, which, in rare cases, are of the size of a pea and upwards. Its particles are found mixed with sand, and other substances, in alluvial depositions, and it is said likewise to have been found in situ in disintegrated syenite. It occurs, likewise, in veins, chiefly, it would seem, in syenitic rocks, in the form of heavy grains, darker than silver. The grains of native platinum usually contain from 75 to 85 per cent. or more of that metal, with frequently so much of iron, that they are magnetic, together with small quantities of palladium, iridium, osmium, rhodium, and copper.
Platinum is purified from the substances with which it is associated, by processes of considerable labour, and is obtained in a state of minute division, in which condition, it is frequently termed spongy platinum. It is then, by pressure and heat, converted into bars or ingots. The perfecting of these processes is due to the late Dr Wollaston, who
thus contributed greatly to the advancement of chemical science, by supplying to the operator vessels formed of a metal which resists the action of powerful agents, and is nearly indestructible by the greatest heat which the chemist has occasion to employ.
Platinum, when pure, resembles silver, in colour and external characters, but is of a lustre much inferior to silver. It is the heaviest of the metals, and of any known substance. Its specific gravity increases somewhat by hammering, its maximum being 21.5313. It is highly malleable, though less so than gold and silver. It may be hammered into leaves, which do not exceed the loro part of an inch in thickness. Its tenacity is so great, that a wire of it, of .078 of an inch, is capable of sustaining a weight of 274.31 lb. avoirdupois. It is infusible by the most intense heat of a smith's forge, but it softens at high temperatures, and may be welded by hammering, like iron. Under the powerful action of galvanism, or the oxyhydrogen blowpipe, however, it may be fused. On heating a small wire intensely by these means, it melts, and then is dissipated with the emission of bright sparks. This property of extreme infusibility, platinum loses by admixture of other substances; and hence, while impure platinum may be fused, that which is pure will not be affected. It is in no degree acted upon by air or moisture ; but, when ignited with certain bodies yielding oxygen, as nitre, it undergoes oxidation. It is not acted upon by any of the pure acids. Its only solvent is chlorine, or such solutions as supply it; and hence, nitrohydrochloric acid, so long known as the solvent of gold, is likewise the solvent of platinum.
Platinum is obtained in the most perfectly divided state, by dissolving the protochloride in a hot concentrated solution of potassa, and dropping into it alcohol, which causes a violent effervescence. The precipitate, when freed of foreign matters and dried, resembles lamp-black, and soils the fingers like silicium in the same condition, and it is not till this powder is exposed to a white heat, that it re-assumes the metallic aspect.
Platinum, it was before observed, when a stream of hydrogen, in connexion with oxygen, plays upon it, causes instant condensation and combination of the gases, with an evolution of heat so great as to raise the platinum in an instant to incandescence. The same action is manifested when the platinum is reduced to powder. The powder, too, absorbs and condenses gases with its interstices and pores, in the manner of charcoal.
The combining weight of platinum has been estimated at 98.84. It combines with oxygen, chlorine, sulphur, and other bodies, and with the bodies of its own class forming alloys.
When crude platinum has been dissolved in hydrochloric acid, a part remains undissolved in the state of a black pow. der. This powder is an alloy composed of the two metals, osmium and iridium, from which the osmium is separated by means of fusion, with soda or nitrate of potassa. It is obtained as an oxide, which is easily reduced to the metallic state. Thus obtained it is a black powder, which, by pressure, is rendered compact, when it acquires a white colour, and metallic lustre, but less brilliant than that of platinum. It is brittle, and easily pulverized. When in the state of powder, as it is obtained from the oxide, it takes fire when heated in the open air, burning without residue, and it is readily oxidated by nitric acid. But when rendered dense by the application of a red heat, it is not attacked by acids, and may be heated without undergoing oxidation. Its specific gravity, when most dense, is 10. Its combining weight has been estimated at 99.7, and it forms the following compounds with oxygen,1. Protoxide,
Os 0 2. Sesqui-oxide,
Os2 03 3. Binoxide,
Os 02 4. Teroxide,
Os 03 5. Quadroxide, or osmic acid,
Os 04 The latter compound is produced by the combustion of osmium, by its oxidation by acids, or by its being fused along with nitrate of potassa. It is obtained in the form of brilliant white crystals, or in an aqueous solution which yields crystals on cooling. It is volatile, and has a pungent offensive odour resembling that of chlorine, and its vapour, like that of chlorine, is very acrid, producing cough and painful feelings. Osmium, too, combines with chlorine, sulphur, and other bodies.
Iridium is obtained from its alloy with osmium, by a series of processes. It is an exceedingly brittle metal. It has, when fused, the aspect of platinum ; but it is of all known metals the most infusible. It has resisted the attempts to melt it by the most skilful chemists, and has perhaps been really fused only once, and this by the action of the most powerful galvanic battery that has yet been constructed, namely, that of Mr Children. The specific gravity of the little globule obtained was 18.68; that of the unfused metal is 15.8629. If when in a fine state of division it is exposed to a red heat in the open air, it becomes oxidated. It resists the action of all the acids, except the nitro-hydrochloric, and even this acts upon it feebly. Its combining weight is estimated at 98.8, being the same as that of platinum, and its combinations are analogous to those of osmium.