87'2 PRINCIPLES CHEMISTRY 



the kindred compounds of the iron series for example, to nickeloufr 

 chloride, NiCl 2 , cobaltous chloride, CoCl 2 , &c. This even expresses 

 itself in a similarity of volume (platinous chloride, PtCl 2 , volume, 46 ; 

 nickelous chloride, NiCl 2 = 50), although in the type RX 2 the true iron 

 metals give very stable compounds, whilst the platinum metals fre- 

 quently react after the manner of suboxides, decomposing into the 

 metal and higher types, 2RX 2 = R -h RX 4 . This probably depends on 

 the facility with which RX 2 decomposes into R and X 2 , when X a 

 combines with the remaining portion of RX 2 



As in the series iron, cobalt, nickel, nickel gives NiO and Ni 2 3 , 

 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 2 and RX 4 , whilst rhodium and iridium form another and 

 intermediate type, RX 3 , also met:. with in cobalt, corresponding with 

 the oxide, having the composition R 2 O 3 , besides which they 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 RX 2 , RX 3 , RX 4 , and RX 6 , 

 but also a still higher form of oxidation, RO 4 , which is not met with in 

 any other series. This form is exceedingly characteristic, owing to the 

 fact that the oxides, Os0 4 and Ru0 4 , are volatile and have feebly acid 

 properties. In this respect they most resemble permanganic anhydride, 

 which is also somewhat volatile. 3 



When dissolved in aqua regia (PtCl 4 is formed) and liberated from 

 the solution by sal-ammoniac ( (NH 4 ) 2 PtCl 6 is formed) and reduped by 

 ignition (which may be done by Zn and other reducing agents, direct 

 from a solution of Pt01 4 ) platinum 3 bis forms a powdery mass, known 



* In characterising the platinum metals according to their relation to the iron metals, 

 {t 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 strongly heated. This faculty is especially developed io 

 platinum and palladium, and it is very characteristic that nickel, which exactly corresponds 

 with platinum and palladium in the periodic system, should exhibit the same faculty for 

 detaining a considerable quantity of hydrogen (Graham's and Raoult's experiments). 

 Another characteristic property of the platinum metals consists in their easily giving 

 (like cobalt which forms the cobaltio salts) stable and characteristic saline compounds 

 with ammonia, and like Fe and Co, 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 indivi- 

 duality for each element. 



3 t>u 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 alladium and rhodium in crude platinum, and at 



