DOUBLE SALTS 139 



cyanide which they contain." Then, after enumeration of the 

 well-known differences, he continues : " This led the earlier 

 investigators to recognise special independent groupings in 

 them. . . . Such a representation is, however, completely 

 superfluous for the explanation of the peculiarities in the 

 reactions of such compounds as double salts. . . . Every double 

 salt ought to be regarded as a peculiar kind of saline compound; 

 potassium cyanide is, as it were, a basic, and ferrous cyanide 

 an acid element. They may be unstable in the separate state, 

 but form a stable double compound when combined together; 

 the act of combination disengages the energy of the elements, 

 and they, so to speak, saturate each other." 



Probably most readers would fail to discover in the above 

 a precise meaning or any meaning at all, and the eminent 

 author must himself have had some misgivings on the subject, 

 as he proceeds to say : 



" Naturally, all this is not a definite explanation ; but then 

 the admission of a special complex radicle can even less be 

 regarded as such." 



With the second part of this statement it is not possible to 

 agree, and I have quoted this view concerning the denotation 

 to be given to the term " double salt " only on account of the 

 prestige of its author, and not because it is one which, as far as I 

 know, has found other supporters. We may therefore consider 

 the term "double salt" to be applied only to such crystalline 

 solids, obtained by the combination in a simple molecular ratio 

 of two salts with generally a common ion, as have physical 

 properties different from those of a mixture of the constituents 

 of the same composition, whilst the chemical properties of a 

 solution of the double salt differ in no way from the sum of 

 those of the constituent salts. 



In the memoir of Ostwald already referred to, in which 

 " double salts " were for the first time clearly differentiated 

 from the " salts of complex acids," it was further stated as a 

 characteristic of double salts that their existence is limited to 

 the solid state, and that in solution not only their chemical but 

 also their physical properties are additively those of the 

 constituent salts. This, though true as a limiting case for double 

 salts of slight solubility which are practically completely 

 dissociated in solution, has not been found to hold generally ; 

 and it has been possible to supply extensive and varied experi- 



