870 PRINCIPLES OF CHEMISTRY 



selves, and with their large atomic -weight. The oxides of platinum, 

 indium, and osmium can scarcely be termed either basic or acid ; they 

 are capable of combinations of both kinds, each of which is feeble, 

 Tjhey 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. As a matter of fact, however, the whole group of the 

 platinum metals is characterised by a number of common properties, 

 both physical and chemical, and, moreover, there are Several points of 

 resemblance between the members of this group and those pf the iron 

 group (Chapter XXII.) The atomic volumes (Table III, column 18) 

 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 (Pt, Ir, Os,) 

 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. 

 In Russia in the Urals (discovered in 1819) and in Brazil (1735) 

 platinum is obtained 'from alluvial deposits, but in 1892 Professor 

 Inostrantseff discovered a vein deposit of platinum in serpentine near 

 Tagil in the Urals. 1 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. 1 bl8 



1 Wells and Penfield (1888) have described a mineral sperryllite found in the Canadian 

 gold-bearing quartz and consisting of platinum diarsenide, PtAso. 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, FeS 2 . 



t t>i Some light is thrown upon the facility with which the platinum compounds 

 decompose by Thomsen's data, showing that in an excess of water ( + Aq) the formation 



