138 SCIENCE PROGRESS 



and it is not necessary to repeat here the evidence for the assertion 

 that in all these compound salts the analytical reactions are not 

 those of the simple salts from which they are formed, or — in 

 modern terminology — that in solution these compound salts have 

 ions different from those of the simple salts from which they are 

 formed. Ostwald, to whom we owe the clear recognition of the 

 characteristics of this type of compound salts and their separation 

 from double salts proper, says : " The name of double salts may 

 not be given to any combination of two salts which gives 

 reactions different from those of the constituents." 1 This was 

 written in 1889. The separation insisted upon is definite and 

 legitimate, and the consequent definition of double salt and 

 salt of complex acid respectively is easy ; yet the principle has 

 been slow in getting fully recognised in standard text-books. 

 In the last (that is, the 1897) edition of the volume of Roscoe 

 and Schorlemmer dealing with metals, we find a definition 

 of double salt which it would be difficult to beat for vagueness. 



" The term double salt is applied somewhat loosely to salts 

 formed by the combination of one molecule of one salt with 

 one or more molecules of another salt or of an acid, etc. 

 Thus the substances represented by the formulae KF + HF, 

 AL 2 (S0 4 ) 3 + K2SO4 + 24.H 2 0, etc., are called double salts. Very 

 little is known about the constitution of such substances, since 

 they are generally decomposed, at all events to some extent, 

 when they are dissolved in water, although they separate out 

 again unchanged when the solution is concentrated. In many 

 cases they are undoubtedly salts of complex acids or bases." 



Does not the suspicion obtrude itself that the vagueness is 

 intentional ? Vagueness of any kind cannot be brought as a 

 charge against another writer, who does not follow the rule 

 laid down by Ostwald. Mendeleeff, in a piece of vigorous 

 writing, absolutely repudiates the conception of complex acid 

 radicles, considering it unnecessary. The following is a quotation 

 from the 1905 (the third) English edition of his Principles of 

 Chemistry, taken from the paragraph dealing with K 4 FeCy 6 and 

 K 3 FeCy 6 . 



" Of these two remarkable and very stable salts, it must be 

 observed that with ordinary reagents neither of them gives the 

 same double decompositions as the other ferrous and ferric 

 salts, nor exhibits the characteristic properties of the potassium 



1 "Dissociationstheorie der Elektrolyte," Zs. physik. Chem. 3, 1889, p. 598. 



