Alloys of nickel are used for currency, and if rich deposits of nickel are discovered a wide field of application lies before them, both in a pure state (because it is a beautiful metal and does not rust) and also as alloys, Steel vessels (pressed or forged out of sheet steel) covered with nickel have such practical merits that the manufacture, which has not long commenced, will most probably be rapidly developed.

CHAPTER XXIII

THE PLATINUM METALS

THE six metals: Ruthenium, Ru, rhodium, Rh, palladium, Pd, oзmium, Os, iridium, Ir, and platinum, Pt, are met with associated together in nature. Platinum always predominates over the others, and hence they are known as the platinum metals. By their chemical character their position in the periodic system is in the eighth group, corresponding with iron, cobalt, and nickel.

The naturalness of the transition from titanium and vanadium to copper and zinc by means of the elements of the iron group is demonstrated by all the properties of these elements, and in exactly the same manner a transition from zirconium, niobium, and molybdenum to silver, cadmium, and indium, through ruthenium, rhodium, and palladium is in perfect accordance with fact, as also is the position of osmium, iridium, and platinum between tantalum and tungsten on the one side, and gold and mercury on the other. In all these three cases the elements of smaller atomic weight (chromium, molybdenum, and tungsten) are able, in their higher grades of oxidation, to give acid oxides having the properties of distinct but feebly energetic acids (in the lower oxides they give bases), whilst the elements of greater atomic weight (zinc, cadmium, mercury), even in their higher grades of oxidation, only give bases, although with feebly-developed basic properties. The platinum metals present the same intermediate properties such as we have already seen in iron and the elements of the eighth group.

In the platinum metals the intermediate properties of feebly acid and feebly basic metals are developed with great clearness, so that there is not one sharply-defined acid anhydride among their oxides, although there is a great diversity in the grades of oxidation from the type RO, to R2O. The feebleness of the chemical forces observed in the platinum metals is connected with the ready decomposability of their compounds, with the small atomic volume of the metals themselves, and with their large atomic weight. The oxides of platinum, iridium, and osmium can scarcely be either termed basic or acid; they

are capable of combinations of both kinds, one and the other of which is feeble. They are all intermediate oxides.

The atomic weights of platinum, iridium, and osmium are nearly 191 to 196, and of palladium, rhodium, and ruthenium, 104 to 106. Thus, strictly speaking, we have here two series of metals, which are, moreover, perfectly parallel to each other; three members in the first series, and three members in the second-namely, platinum presents an analogy to palladium, iridium to rhodium, and osmium to ruthenium. The group of the platinum metals is characterised by a number of common properties, both physical and chemical, and, moreover, there are not a few points of resemblance between the members of this group and those of the iron group (Chapter XXII.). The atomic volumes of the elements of this group are nearly equal and very small. The iron metals have atomic volumes of nearly 7, whilst that of the metals allied to palladium is nearly 9, and of those adjacent to platinum nearly 9.4. This comparatively small atomic volume corresponds with the great infusibility and tenacity proper to all the iron and platinum metals, and to their small chemical energy, which stands out very clearly in the heavy platinum metals. All the platinum metals are very easily reduced by ignition and by the action of various reducing agents, in which process oxygen, or a haloid group, is disengaged from their compounds and the metal left behind. This is a property of the platinum metals which determines many of their reactions, and the circumstance of their always being found in nature in a native state. The facility with which they are reduced is so great that their chlorides are even decomposed by gaseous hydrogen, especially when shaken up and heated under a certain pressure. Hence it will be readily understood that such metals as zinc, iron, &c., separate them from solutions with great ease, which fact is taken advantage of in practice and in the chemical treatment of the platinum metals.

All the platinum metals, like those of the iron group, are grey, with a comparatively feeble metallic lustre, and are very infusible. In this respect they stand in the same order as the metals of the iron series; nickel is more fusible and whiter than cobalt and iron, so also palladium is whiter and more fusible than rhodium and ruthenium, and platinum is comparatively more fusible and whiter than iridium or osmium. The saline compounds of these metals are red or yellow, like

1 Wells and Penfield (1888) have described a mineral sperryllite found in the Canadian gold-bearing quartz and consisting of platinum diarsenide, PtAs2. It is a noticeable fact that this mineral clearly confirms the position of platinum in the same group as iron, because it corresponds in crystalline form (regular octahedron) and chemical composition with iron pyrites, FeS2.

those of the majority of the metals of the iron series. But here, as there, the different forms of oxidation present different colours. Moreover, certain complex compounds of the platinum metals, like certain complex compounds of the iron series, either have particular characteristic tints or else are colourless.

The platinum metals are found in nature associated together in alluvial deposits in a few localities, from which they are washed, owing to their very considerable density, which enables a stream of water to wash away the sand and clay with which they are mixed. Platinum deposits are chiefly known in the Urals, and also in Brazil and a few other localities. The platinum ore washed from these alluvial deposits presents the appearance of more or less coarse grains, and sometimes of, as it were, semifused nuggets.2

All the platinum metals give compounds with the halogens, and the highest haloid type of combination for all is RX,. For the majority of the platinum metals this type is exceedingly unstable; the lower compounds corresponding to the type RX2, which are formed by the separation of X2, are more stable. In the type RX, the platinum metals form more stable salts, which offer no little resemblance to the kindred compounds of the iron series-for example, with nickelous. chloride, NiCl2, cobaltous chloride, CoCl2, &c. This even expresses itself in a similarity of volume (platinous chloride, PtCl2, volume, 46; nickelous chloride, NiCl2=50), although in the type RX, the true iron metals give very stable compounds, whilst the platinum metals frequently react after the manner of suboxides, decomposing into the metal and higher types, 2RX2=R+RX,. This naturally depends on the facility with which RX, decomposes into R and X2, when X, combines with the remaining RX2.

As in the series iron, cobalt, nickel, nickel gives only one saltforming oxide, whilst cobalt and iron give higher and varied forms of oxidation; so also among the platinum metals, platinum and palladium only give the forms RX, and RX4, whilst rhodium and iridium form

2 The largest amount of platinum is extracted in the Urals, about two tons annually, About a tenth part of gold is extracted from the washed platinum by means of mercury, which does not dissolve the platinum metals but dissolves the gold accompanying the platinum in its ores. Moreover, the ores of platinum always contain metals of the iron series associated with them. The washed and mechanically-sorted ore in the majority of cases contains about 70 to 80 p.c. of platinum, about 5 to 8 p.c. of iridium, and a somewhat smaller quantity of osmium. The other platinum metals-palladium, rhodium, and ruthenium-occur in smaller proportions than the three above named. Sometimes grains of almost pure osmium-iridium, containing only a small quantity of other metals, are found in platinum ores. This osmium-iridium may be easily separated from the other platinum alloys, owing to its being nearly insoluble in aqua regia, by which the latter are easily dissolved. There are grains of platinum which are magnetic.

yet another intermediate type, RX3, corresponding with the oxide, which is also met with for cobalt, and presents the composition R2O2, besides which they also form an acid oxide, like ferric acid, which is also known in the form of salts, but is in every respect unstable. Osmium and ruthenium, like manganese, form still higher oxides, and in this respect exhibit the greatest diversity. They not only give RX2, RX3, RX4, and RX,, but also a still higher form of oxidation, RO4, which is not met with in any other series. This form is exceedingly characteristic, owing to the fact of the oxides, OsO, and RuO,, being volatile feebly acid substances. In this respect they most resemble permanganic anhydride, which is also somewhat volatile.3

When dissolved in aqua regia and liberated from the solution by ignition or the action of reducing agents, platinum forms a powdery mass, known as spongy platinum or platinum black. If this powder of platinum be heated and pressed, or hammered in a cylinder, the grains

3 In characterising the platinum metals according to their relation to the iron metals, it is very important to add two more very remarkable points. The platinum metals are capable of forming a sort of unstable compound with hydrogen; they absorb it and only part with it when somewhat considerably heated. This faculty is especially developed in platinum and palladium, and it is very characteristic that nickel, which just corresponds with platinum and palladium in the periodic system, should exhibit the same faculty for retaining a considerable quantity of hydrogen (Graham's and Raoult's experiments). Another characteristic property of the platinum metals consists in their easily giving stable and characteristic saline compounds with ammonia, and double salts with the cyanides of the alkali metals, especially in their lower forms of combination.

All the above so clearly brings the elements of the iron series in close relation to the platinum metals, that the eighth group acquires as natural a character as can be required, with a certain originality or individuality for each element.

Platinum was first obtained in the last century from Brazil, where it was called silver (platinus). Watson in 1750 characterised platinum as a separate independent metal. In 1803 Wollaston discovered palladium and rhodium in crude platinum, and at about the same time Tennant distinguished iridium and osmium in it. Professor Claus, of Kazan, in his researches on the platinum metals (about 1840) discovered ruthenium in them, and to him are due many important discoveries with regard to these elements, such as the indication of the remarkable analogy between the series Pd-Rh-Ru and Pt-Ir-Os.

The treatment of platinum ore is chiefly carried on for the extraction of the platinum itself and its alloys with iridium, because these metals offer a greater resistance to the action of chemical reagents and high temperatures than any of the other malleable and ductile metals, and therefore the wire so often used in the laboratory and for technical purposes is made from them, as also are various vessels used for chemical purposes in the laboratory and in works. Thus sulphuric acid is distilled in platinum retorts, and many substances are fused, ignited, and evaporated in the laboratory in platinum crucibles and platinum foil. Gold and many other substances are dissolved in basins made of iridiumplatinum, because the alloys of platinum and iridium are but slightly attacked when subjected to the action of aqua regia. An exceedingly important property of the platinum metals is that they do not fuse in a furnace heat, palladium only being somewhat more fusible than the rest.