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.”! 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, ete. 
Thus the substances represented by the formule KF + HF, 
AL(SO,); + K,SO, + 24H,O, 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 isa quotation 
from the 1905 (the third) English edition of his Principles of 
Chemistry, taken from the paragraph dealing with K,FeCy, and 
K,FeCyg. 
‘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 
* “Dissociationstheorie der Elektrolyte,” Zs. physik. Chem. 3, 1889, p. 598. 
