156 SCIENCE PROGRESS 
such salts, viz. FeCl,.2CsCl.H,O and FeCl,.3CsCl.H,O. It 
should be noted, however, that no numerical relations emerge 
from this research, though it allows us to trace the effect on 
the power of double-salt formation of the substitution of allied 
elements for one another. 
In a later and very extensive investigation of the series of 
double salts formed between sodium sulphate and the class of 
sulphates called “vitriols,” the realm of the quantitative is being 
approached.t. The principle of this comparative study recalls 
that followed by Mr. Tutton in his extensive researches on the 
influence on crystalline form produced by the substitution in 
a series of salts of one isomorphous element for another. 
The special feature of this investigation of the class of double 
sulphates, Na,SO,.M”’SO,.4H,O (Table IV.), lies in the fact 
that the determination of the transition point and the transition 
interval for each double salt made it possible to examine under 
strictly comparable conditions what is the influence of the 
properties of each special vitriol on the properties of the double 
salt into the composition of which it enters. Thus it could be 
shown that whenever the vitriol crystallises with seven molecules 
of water, as happens in the majority of cases (Mg, Zn, Fe”, Co, 
Ni), the double salt crystallises with four molecules; whilst 
copper and cadmium, whose stable hydrated sulphates contain 
five and one-half molecules of water respectively, give double 
salts containing only two molecules; and finally manganese, 
whose heptahydrate even at ordinary temperatures passes into 
a monohydrate, gives a tetrahydrated double salt, which 
however passes into the dihydrate below 4o°, that is, at a 
temperature considerably lower than that of the dehydration 
of the other tetrahydrated double salts of the series. Con- 
nections have also been traced between the solubility of the 
vitriol and the extension of the transition interval, as well as 
between the electrical potential of the metal in the vitriol and 
the transition point of the double salt; but without the repro- 
duction of a large number of numerical data these connections 
cannot be followed out satisfactorily. The results, moreover, 
are not very striking; and the chief reason for having said even 
thus much about this research is to show the method followed 
in a chemical study of double-salt formation. 
3. The third and last of my arbitrary subdivisions of the 
1 Koppel, Zoc. céz. 
