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 2 . 2H 2 0, 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 2 S0 4 , 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 MgS0 4 is more 

 soluble than the component Na 2 S0 4 ; and hence, in bringing 

 astrakanite into contact with water less in amount than that 

 required for complete solution, more MgS0 4 will be dissolved, 

 solid KC1 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 



