DOUBLE SALTS 149 
and IV.), and the point of intersection of these curves gives 
the temperature sought. 
2. The Action of Water on the Pure Double Salt—*Experience 
has shown that whilst some double salts, such as the alums, 
can at ordinary temperatures be recrystallised from water 
without decomposition, other salts cannot be so dissolved, and 
will only crystallise from a solution containing excess of one 
of the constituent simple salts. This decomposing action of 
water can be shown in a simple and striking manner in the case 
of the double lithium-copper chloride already referred to in 
this paper (p. 137). Even exposure to the moisture of ordinary 
air quite rapidly effects a colour change from the red of the 
double salt to the green of the constituent CuCl,. 2H,O, a change 
which occurs instantaneously on moistening the garnet-red 
crystals with a drop of water; if however, instead of water, a 
drop of saturated solution of LiCl is used, the colour remains 
unaltered, showing the non-occurrence of decomposition due 
to the stability of the double salt in contact with a solution 
saturated with the more soluble of its constituents. 
Other salts again, which like astrakanite dissolve at the 
transition point (25°) with decomposition and precipitation of 
the less soluble Na,SO,, dissolve at higher temperatures without 
decomposition. Examination of the curves of solubility 
(Table IV.) accounts for the above fact, that a pure double salt 
will at the transition point generally not dissolve without 
decomposition, and also for the few exceptions to this rule. The 
two simple salts generally differ in solubility, in consequence 
of which at the transition point the composition of the liquid 
phase in equilibrium with the three solid phases is not that 
of the double salt. Hence, if the double salt in excess is brought 
into contact with pure water, a greater amount of the more 
soluble simple salt will go into solution, and there will remain 
as solid phases the double salt and the less soluble simple salt. 
Thus, in the case of astrakanite, the component MgSO, is more 
soluble than the component Na,SO,; and hence, in bringing 
astrakanite into contact with water less in amount than that 
required for complete solution, more MgSQ, will be dissolved, 
solid KCl] remaining together with the excess of the double salt. 
From this it follows conversely that in order to obtain a double 
salt by crystallisation from a solution of the two components, 
it is necessary to have the more soluble of these present to the 
