888 PRINCIPLES OF CHEMISTRY 



oyanide and cobaltia compounds, which are comparatively stable.- 'But 

 as ammonia does not. contain any hydrogen easily replaceable by 



connected with the faculty or cyanogen of giving tricyanogen polymeridea, such as cyanurio. 

 acid, solid cyanogen chloride, &c. Under the action of an excess of chlorine, a solution 

 of PtK 2 (CN) 4 gives (besides PtK 2 Cy 6 ) a product PtK 2 Cy 4 Cl2, which evidently contains 

 the form PtX 4 , but at first the action of the chlorine (or the electrolysis of, or addition 

 of dilute peroxide of hydrogen to, a solution of PtK 2 Cy 4 , acidulated with hydrochloric 

 acid) produces an easily soluble intermediate salt which crystallises in thin copper-red 

 needles (Wilm, Hadow, 1889). It only contains a email amount of chlorine, and 

 apparently corresponds to a compound 6PtK 2 Cy 4 + PtK 2 Cy 4 Cl 2 + 24H 2 0. Under the 

 action of an excess of ammonia both these chlorine products are converted either oon> 

 pletety or in part (according to Wilm ammonia does not act upon PtK 2 Cy 4 ) into 

 PtCy 2 ,2NH 5 , i.e. a platino-ammonia compound (see further on). It is also necessary to 

 pay attention to the fact that ruthenium and osmiumwhich, as we know, give higheir 

 forms of oxidation than platinum are also able to combine with a larger proportion of 

 potassium cyanide (but not of cyanogen) than platinum. Thus ruthenium forms a 

 crystalline hydroruth&wcyanic ac^d, RuH 4 (CN) 6 , which js soluble in water and alcohol, 

 and corresponds with the salts M 4 Ru(CN) 6 . There are exactly similar osmic com- 

 poundsfor example, "K 4 Os(CN) 6 ,8H 2 O. The latter is obtained in the form of colourless, 

 sparingly-soluble regular tablets on evaporating the solution obtained from a fused 

 mixture of potassium osrciochloride, K 2 OsCl6,.and potassium cyanide. These osmic and 

 ruthenic compounds fully correspond with potassium ferrocyanide, K 4 Fe(CN) 6 ,8H 2 0, not 

 Only in their composition but also jn their crystalline form and reactions, which again 

 demonstrates the close analogy between iron, ruthenium, and osmium, which we have 

 shown by giving these three elements a similar position (in the eighth group) in the 

 periodic system. For rhodium and indium only salts of the same type as the ferricyanides, 

 M 3 RCy 6 , are known, and for palladium only of the type M2PdCy 4 , which are analogous 

 to the platinum salts. In all these examples a constancy of the types of the double 

 cyanides is apparent. In the eighth group we have iron, cobalt, nickel, copper, and their 

 analogues ruthenium, rhodium, palladium, silver, and also osmium, iridium, platinum, 

 gold. The double cyanides of iron, ruthenium, osmium have the type K^CNJeJ of 

 cobalt, rhodium, iridium, the type K 5 R(CN)e ; of nickel, palladium, platinum the type 

 K2R(CN) 4 and K 2 R(CN) 5 ; and for copper, silver, gold there are known KR(CN) 2 , so 

 that the presence of 4, 8, 2, and 1 atoms of potassium corresponds with the 

 order of the elements in the periodic system. Those types which we have seen 

 in the ferrocyanidea and ferricyanidea of iron repeat themselves in all the platinoid 

 metals, and this naturally leads to the conclusion that the formation of similar 

 so-called double salts is of exactly the 8ame*bature as that of the ordinary salts. If, in 

 expressing the union of the elements in the oxygen salts, the existence of an 

 aqueous residue (hydroxyl group) be admitted, in which the hydrogen is replaced by a 

 metal, we have then only to apply this mode of expression to the double salts and the 

 analogy will be obvious, if only we remember that C1 2 , (CN) 2 , SO 4 , &c., are equivalent to 

 O, as we see in RO, RC1 2 , RS0 4 , &c. They all =X 2 , and, therefore, in point of fact, 

 wherever X ( = C1 or OH, &c.) can be placed, there (C1 2 H), (S0 4 H), &c., can also stand. 

 And as ClaH = Cl + HC1 and S0 4 H <= OH + SO 5 , &c., it follows that molecules EDC1 or SQs, 

 or, in general, whole molecules for instance, NH 8 , B^O, salts, &c., can annex themselves 

 to a compound containing X. (This is an indirect consequence of the law of substitution 

 which explains the origin of double salts, ammonia compounds, compounds with water 

 of crystallisation, &c., by one general method.) Thus the double salt MgS0 4 ,K 2 S04, 

 according to this reasoning, may be considered as a substance of the same type as 

 MgCl 2 , namely, as =Mg(S0 4 K) 2 , and the alums as derived from A1(OH)(S0 4 ), namely, as 

 AKSO^HSO^, Without stopping to pursue this digression further, we will apply 

 these considerations to the type of the ferrocyanides and ferricyanides and their 

 platinum analogues. Such a salt aa KgPtCy 4 may accordingly be regarded as Pt(Cy a S)^ 



