ISO 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 transition 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 MgS0 4 than Na 2 S0 4 , 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. The 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 



