154 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 formule of double salts. 
1. 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 vrtriols, do not form double salts with 
one another, nor do the sulphates of the very similar elements 
potassium and cesium; 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 cesium 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 
