less by heat than any other metal, and in its power of conducting heat and electricity it is much inferior to gold and silver,-in these respects ranking very near to iron.

Platinum does not undergo oxidation in air at any temperature: none of the acids have singly any effect upon it; aqua regia dissolves it, though but slowly. If heated to redness in air in contact with caustic alkalies or alkaline earths, especially with hydrate of lithia or with baryta, it is corroded, owing to the formation of an oxide which combines with the alkaline base. When phosphorus is heated with spongy platinum, combination between them takes place readily. The attraction of sulphur for platinum is much less powerful. Dry chlorine is without action upon this metal, even when aided by heat.

(968) Platinum black.-Platinum may be obtained in a state of subdivision still finer than that in which it is left on heating the double chloride of platinum and ammonium. In this form it has the appearance of soot, and is termed platinum black. It may be procured in this condition by several methods, of which one of the most efficacious consists in dissolving platinous chloride in a strong solution of caustic potash, and adding alcohol to the hot liquid which is placed in a capacious vessel, and kept constantly stirred; brisk effervescence takes place, owing to the escape of carbonic acid; the platinum is reduced, and is deposited as a black powder, which requires repeated washing,-first with alcohol, next with potash, then with hydrochloric acid, and lastly with water. Platinum, in this finely divided state, greedily condenses oxygen from the air, and absorbs many times its bulk of the gas. If moistened with alcohol or ether it imparts this oxygen to them, and forms new compounds, whilst the powder glows from the heat which is extricated. In all its states, platinum possesses, in a marked degree, this property of condensing gases upon its surface; and the more finely it is divided, and consequently the larger the surface which it presents, the more striking is the phenomenon.

(969) Uses.-The most important applications of platinum are confined to the laboratory of the manufacturing and experimental chemist; they depend upon its great infusibility, and its power of resisting chemical agents. Its introduction as a material for the construction of apparatus employed by the analytical chemist has contributed in no small degree to the rapid progress of the science during the last forty or fifty years, by conferring upon its experiments a precision, neatness, and accuracy till then unattainable. In the concentration of oil of vitriol, large platinum stills are frequently

828

USES OF PLATINUM-ITS ALLOYS.

employed; some of these vessels weigh upwards of 1000 ounces. It is found expedient to gild these vessels upon their inner surface, for, unless this precaution be adopted, the stills when made of platinum prepared by Wollaston's method, after a short time become sufficiently porous to allow the acid to transude. An attempt

was made in Russia to employ platinum for coinage, but it was found to be inconvenient, and the experiment has been abandoned. Platinum is sometimes used for the touch-holes of fowlingpieces.

Alloys.-Platinum may be easily alloyed with many of the more fusible metals, the combination generally taking place with the extrication of light and heat. These alloys are much more fusible than pure platinum: care must therefore be taken not to heat the oxides of easily reduced metals, such as lead or bismuth, in platinum crucibles, as if the oxides should happen to be reduced, the crucible would be destroyed by the formation of a fusible alloy. Most of the platinum of commerce contains iridium, which, without impairing its power of resisting chemical agents, increases its hardness and durability. It is remarkable, that though pure platinum is perfectly insoluble in nitric acid, yet when alloyed with 10 or 12 times its weight of silver, both metals are easily and completely dissolved by the acid. An amalgam of platinum may be formed by acting upon an amalgam of sodium with a neutral solution of the double chloride of platinum and sodium; and, according to Levol, when this amalgam is attacked by nitric acid, the platinum as well as the mercury is partially dissolved.

Platinum enters into combination with carbon and with silicon: sometimes in the fusion of ordinary platinum wire before the blowpipe, the globules of the melted metal become covered with a film of colourless glass, arising from the oxidation of the silicon and the fusion of the resulting silica. A brittle granular compound of platinum and silicon was accidentally obtained by Daniell, owing to the action of silicon at a high temperature upon one of the platinum bars of his pyrometer. It appeared to be formed by a kind of cementation, the silicon being derived from the clay of the envelope in which the bar was heated: the proportion of silicon amounted to 15 per cent. A fusible compound of platinum with boron was also obtained by Wöhler and Deville.

(970) OXIDES OF PLATINUM.-There are two oxides of platinum, a protoxide and a binoxide. The protoxide (platinous oxide, Pt0=213, or PtO=106.5) is procured by digesting platinous chloride in a solution of potash: a dark olive-green liquid is thus obtained, owing to the solution of the oxide in the excess of alkali.

OXIDES AND SULPHIDES OF PLATINUM.

829

On neutralizing the solution with sulphuric acid, a black hydrated protoxide of platinum subsides. It is slowly dissolved by acids, forming unstable salts with them, and is readily decomposed by heat.

=

The binoxide (platinic oxide, Pt0, 229, or PtO,=1145) has a strong tendency to combine with alkaline bases; it is therefore prepared by adding to a solution of nitrate of platinum only onehalf of the quantity of carbonate of sodium which is necessary for its complete precipitation. It is thus procured as a voluminous brown hydrate (Pt✪„, 2 H2O), from which water is expelled at a gentle heat, whilst the mass becomes darker; a higher temperature expels the whole of the oxygen. Hydrated binoxide of platinum is soluble in solutions of potash and soda; the compounds thus formed may be obtained in crystals. The soda compound consists of Na,, 3 Pte,, 6 H2O. Binoxide of platinum also enters into combination with other bases, forming compounds most of which are insoluble. The oxide is also soluble in acids, and forms well characterized salts, the solutions of which have a yellowish-brown colour.

2

(971) SULPHIDES OF PLATINUM.-Platinum combines with sulphur in two proportions, PtS and PtS2.

The protosulphide (PtS) may be obtained as a black precipitate by passing sulphuretted hydrogen over moistened platinous chloride; it may also be procured by heating sulphur with the double chloride of platinum and ammonium, when it assumes the form of a grey powder of metallic appearance, from which the sulphur is completely expelled by heating it in the open air.

The bisulphide (PtS) is best procured by decomposing the double chloride of sodium and platinum by sulphuretted hydrogen; it falls as a dark-brown powder, which becomes black during desiccation. It is somewhat soluble in the sulphides of the alkaline metals. By ignition in closed vessels it is converted into protosulphide. When exposed to the air, and gently heated, it is partially converted into sulphate, but at a higher temperature is wholly decomposed, metallic platinum remaining.

(972) CHLORIDES OF PLATINUM.-These correspond in number to the sulphides and oxides of the metal.

In order to procure the platinous chloride (PtCl,=268, or protochloride, PtCl=134), the solution of platinum in aqua regia should be evaporated, and the residue exposed to a heat of 450°, so long as any chlorine is expelled; the compound which remains is platinous chloride. It is of an olive colour, and is insoluble in water: it is scarcely acted upon by nitric or by sulphuric acid;

830

CHLORIDES OF PLATINUM.

hydrochloric acid dissolves it when warmed: and it is dissolved easily by caustic potash, and by the tetrachloride of platinum, with which latter it forms a double salt, of so deep a brown colour as to appear opaque in a concentrated solution. It forms crystallizable double salts with the chlorides of the alkaline metals.

Platinic chloride, or tetrachloride (PtCl=339, or bichloride of platinum, PtCl=169′5): Comp.in 100 parts, Pt, 58·14; Cl, 4186.—This salt is obtained by dissolving platinum in aqua regia, and evaporating the solution to dryness by means of a steam heat. It is a deliquescent salt, and forms a deep orangecoloured solution in water, from which it may be obtained crystallized in prisms; it is also dissolved largely by alcohol and by ether. When heated to 450° it loses half its chlorine, forming platinous chloride, and if the temperature be further raised, it is completely decomposed, and metallic platinum is left. Tetrachloride of platinum may easily be reduced to a platinous salt by transmitting sulphurous acid through a boiling solution of the salt, containing hydrochloric acid in excess; by excess of sulphurous acid the solution is slowly rendered colourless, when it contains platinous sulphite and free hydrochloric acid.

With other chlorides, perchloride of platinum forms numerous double salts, which are produced by mixing the solutions of these chlorides with that of the tetrachloride, and evaporating. The double chloride with potassium (2 KCl,PtCl,=488, or KCl,PtCl,= 244; sp. gr. 3.586) is a sparingly soluble anhydrous compound, which crystallizes in octohedra; it is insoluble in alcohol and in ether. This salt is commonly used as a means of determining analytically the quantity of potassium in a compound: 100 parts of this salt contain Pt, 40'43; K, 15'98, = as K,0, 19:26. It is decomposed by a red heat, into chloride of potassium and metallic platinum. The double chloride of platinum and sodium (2 NaCl, PtCl, 6 H2O, or NaCl,PtCl,, 6 Aq) crystallizes in beautiful red striated prisms, which are soluble in water, alcohol, and ether. With chloride of ammonium a compound is formed (2 H ̧NCI,PtCl ̧= 446, or HNCI,PtCl2=223) very similar in appearance to that

*In an active laboratory a number of residues containing platinum gradually accumulate, and these may be turned to account in the following manner:-The solutions, mixed with the precipitates, are evaporated to dryness, and transferred to a clay crucible, in which they are heated strongly, with free access of air, in order to burn off organic matters; after which the residue is boiled with hydrochloric acid. then with water, and lastly with nitric acid; a thorough washing with water follows. The impurities having thus been removed, the residual platinum may be converted into tetrachloride by means of aqua regia.

:

BASES DERIVED FROM THE CHLORIDES OF PLATINUM. 831 with potassium, with which it is isomorphous it is sparingly soluble in water, and is insoluble in alcohol and in ether. This salt is employed in analysis for determining the quantity of ammonia present in solutions; 100 parts of this salt contain, Pt 44-28, and H,N, 7.65. It is also made use of for separating platinum from the other metals with which it is associated, after they have been brought into solution by treating the ore with aqua regia (966). When the chloride of platinum and ammonium is ignited, the ammonium and chlorine are wholly expelled, and pure platinum remains in the spongy form.

(973) Basic Ammoniacal Derivatives from the Chlorides of Platinum.-The action of ammonia upon platinous chloride gives rise to the formation of several remarkable compound bases, the composition of which offers considerable interest in a theoretical point of view. Magnus found that if platinous chloride be dissolved in hydrochloric acid, the addition of an excess of ammonia to the boiling solution causes the deposition of brilliant, green, acicular crystals which are insoluble in water and in hydrochloric acid: they contain the elements of 1 atom of platinous chloride, and 2 of ammonia (Pt"Cl2HN2). This compound, however, undergoes no change when digested at ordinary temperatures in solutions of the caustic alkalies, or in the concentrated acids, but when boiled with them it is slowly decomposed. If digested in diluted nitric acid, one-half of the platinum is separated as nitrate, and on evaporating the solution, a salt is obtained crystallized in small flattened prisms (PtCl2HN, 2 HNO3). Neither the chlorine nor the platinum can be detected in this solution by the usual tests. The nitric acid may be displaced from it by double decomposition with sulphate, phosphate, or oxalate of sodium, and a sparingly soluble sulphate, phosphate, or oxalate of the metal is then formed. The base of these salts (commonly called Gros's salts, from the name of their discoverer) has not been isolated.

Raewsky discovered that if the green salt of Magnus be boiled with concentrated nitric acid, red fumes are" disengaged, and a different salt is formed, which may be obtained in crystals on evaporation. The nitric acid may be displaced from this compound by an equivalent quantity of oxalic or of carbonic acid.

Besides these compounds, other platinum bases derived from ammonia have been obtained, of which the following is a brief

account.

Platosamine; Reiset's second base (Pt"H ̧N,,H2O).—This is a greyish mass insoluble in water and in ammonia, which may be obtained by heating the hydrate of diplatosamine (PtHN