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- 
