DOUBLE. SALTS 
By IDA FREUND 
Staff Lecturer, Newnham College 
AmoncsT the substances possessed of the typical properties 
summarised in the name “salt,” chemists have for long known 
and dealt with a large number of comparatively greater com- 
plexity of composition, due to their having been produced from 
two “simple salts.” These “compound salts” are generally 
well crystallised, and their formation from the simple salts is 
usually accompanied by elimination or absorption of water 
of crystallisation (Table II.). As a rule, the two constituent 
simple salts have a common ion: either the anion, as in the 
case of two sulphates (e.g. K,SO, and Al,SO,),, FeSO,.7H,O 
and MgSO,.7H.O), or of two cyanides (e.g. KCy and AgCy, 
KCy and FeCy); or the cation, as in the case of two salts of 
sodium, or of rubidium, or of potassium (eg. NaNO, and 
Na,SO,; Rb,.d—C,H,O, and Rb,/—C,H,O,; HClO, and KMn0O,). 
But instances are known in which all the four ions are different 
(e.g. Cd,K,(C,O,);Cl,). The formuleze 
mMA + 2M’A 
mMA + 2MA’' 
mMA + 2M’A’ 
represent these different types of combination. Of these only 
the two first, 2.e. compound salts formed from simple salts which 
have a common ion, fall within the scope of the subject of this 
paper, and it may here be noted incidentally that in the 
predominating number of cases the common ion is the cation. 
Such combinations divide themselves into three classes, sharply 
separated from one another (Table I.). 
I. The ratio of the quantities of the two constituent simple 
salts varies continuously, with or without a break in the series, 
from MA=o and M’A=100 per cent. to MA= 100 and 
M’A =o per cent. . 
In order to form such compound salts the constituent simple 
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