is 
DOUBLE SALTS 155 
remained for Retgers' to make it absolutely definite, and to 
show how it could be applied to argue backwards as to the 
kind and degree of relationship between different elements. 
He asserted that two simple salts which form a series of mixed 
crystals cannot form a double salt, and vice versa, that mixture 
and combination are mutually exclusive, and that this is 
true without reservation, without any exceptions whatever. 
Potassium and rubidium and ammonium form isomorphous 
mixtures, but do not form double salts; potassium and sodium 
are not isomorphous, but they do form double salts, such as 
for instance Seignette salt, the double potassium-sodium tartrate, 
or Scacchi’s salt, the double sodium-ammonium racemate, which 
is a double salt in a double way (Table II.). Sulphates and 
selenates form isomorphous mixtures, but not double salts, 
whilst NaNO, combines with Na,SO, to a double salt. Hence, 
on the basis of the absolute validity of this relationship, the 
fact that we know of a number of double salts between 
potassium and silver, but of none between sodium and silver, 
leads to the inference of a greater relationship between silver 
and sodium than between silver and potassium, and points to 
the probability of isomorphism between silver and sodium, 
which is proved by the existence of a series of mixed nitrates 
(and chlorates) of these two metals. 
2. Moreover it follows, as a direct consequence of the 
recognition of chemical difference as the essential factor in 
the formation of double salts, that the degree of this difference 
ought to be of influence on the relative stability of the com- 
binations produced. That this is actually the case is shown by 
the numerous instances in which it has been found that the most 
stable of all the alkali double salts are those of caesium, itself the 
most electro-positive of all the metals; and in entire agreement 
with such incidental and unconnected observations is the result 
of an investigation on the double chlorides of ferric iron and 
the alkali metals, which well shows the connection between the 
stability of the double salt and the gradation in the electro- 
positive character of the alkali metals.2, Between o° and 60° 
NaCl could not be made to combine with FeCl,; KCl yielded 
one double salt, viz. FeCl,.2KCl. H.O, whilst CsCl gave two 
1 Retgers, “Uber chemische Verbindungen isomorpher Korper,” Zs. physth- 
Chem. 15, 1894, p. 529. 
? Hinrichsen and Sachsel, doc. ciz. 
