150 SCIENCE PROGRESS 
greater amount required for the saturation of the solution, e.g. 
the copper-lithium chloride is obtained from a solution con- 
taining LiCl in considerable excess. But the relative solubilities 
of the systems involved vary with the temperature, and within 
the range of the possible existence of the double salt may be 
found a temperature at which the solution in contact with the 
double salt and the less soluble simple salt contains the two 
simple salts in the ratio in which they are present in the 
double salt; solution of the latter will then take place without 
decomposition. 
The range of temperature between that at which the double salt 
can begin to be formed, and that at which it ceases to be de- 
composed by water, is called the ¢vansition interval. Thus, again 
taking the case of astrakanite, this salt is formed from the two 
simple salts at 22°. At a temperature just above this, solubility 
determinations show that the solution in contact with astrakanite 
and Glauber’s salt contains more MgSO, than Na,SO,, but that 
at about 25° the two salts are present in equi-molecular quantities, 
and hence that the double salt will dissolve without decompo- 
sition. If the double salt is the stable form below the transition 
point, the transition interval will extend downwards to a lower 
temperature. The greater the difference in solubility of the two 
simple salts, the greater in general is the transition interval, 
though the temperature coefficient of the various solutions also 
influences the result. Consequently at ordinary pressures the 
transition interval is not always comprised within an attainable 
temperature range, and certain double salts, such as cupric- 
dipotassium chloride, lead-potassium iodide, can never be dis- 
solved without decomposition. On the other hand, if the two 
salts have the same solubility at the transition point, a special 
case which is realised by d and / tartrates (Table II.), which 
combine to the racemate, or by sodium and nickel sulphates 
(Table IV.), the transition interval diminishes to zero, and the 
double salt dissolves without decomposition. 
3. Lhe Connection between the Heat Effect accompanying the 
Transformation and the Temperature at which the Transformation 
occurs (Table II.).—Some double salts, such as astrakanite, 
sodium-ammonium racemate, are formed as the temperature is 
raised, whilst for others, such as copper-calcium acetate, copper- 
dipotassium chloride, the reverse holds; this means that in some 
cases the form stable at ordinary temperature is the double 
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