i 5 4 SCIENCE PROGRESS 



from physical conditions — they may be capable of forming a 

 double salt ? Obviously we are dealing here with an affinity 

 problem, and wish to ascertain something about properties 

 fundamentally inherent in different special kinds of matter ; 

 we try to learn something about those conditions of double- 

 salt formation over which, in entire difference from temperature, 

 pressure, and concentration, we can exert no influence. In the 

 absence of any means for measuring the affinity between two 

 salts, what work has been done in this province has as yet 

 barely gone beyond the classificatory ; but even so, some 

 interesting relations and useful applications have been estab- 

 lished. It is proposed to give an account of some of these 

 results, and to do so as shortly as possible under a somewhat 

 arbitrary classification, dealing : firstly, with the work done 

 that bears on the investigation of the relation between the 

 elements which are capable of forming double salts, and the 

 relation between those which are devoid of this power ; 

 secondly, with the investigations concerning the effect produced 

 by the substitution of allied elements for one another on the 

 power of forming double salts and on the properties of these 

 salts ; and thirdly, with the regularities found, or supposed 

 to have been found, in the formulae of double salts. 



i. As the result of a very large number of experiments, it had 

 long been recognised that an essential factor in the formation 

 of double salts is a certain difference in the chemical character 

 of the two metals. The sulphates of metals such as iron, zinc, 

 magnesium, which crystallise with seven molecules of water, 

 and which are termed vitriols, do not form double salts with 

 one another, nor do the sulphates of the very similar elements 

 potassium and caesium ; but there is great tendency to the 

 formation of stable double salts between any one of the vitriols 

 and any one alkaline sulphate. Moreover, the vitriols exhibit 

 that characteristic of the existence of isomorphism which 

 consists in the formation of a series of mixed crystals ; and 

 the more marked the chemical relationship, the greater is this 

 tendency, as shown by the absence of any break in the series 

 of the mixed crystals of magnesium and zinc. Similarly, the 

 sulphates of potassium and caesium are isomorphous. Though 

 the general validity of this view concerning the difference in 

 relationship required for the formation of mixed crystals and 

 double salts respectively had been recognised before, it 



