28 ART. 10. K. IK EDA : STUDIES ON THE 



pound would be accompanied by changes of energy and volume, 

 and would thus be hicompatible with conditions (1) and (2). 

 The equilibrium between a solid compound and an ideal solution 

 is therefore comparable to that between ammonium carbamate 

 and the gaseous mixture of ammonia and carbon dioxide. As 

 the difference of densities between a solid and a liquid is not so 

 great as between a solid and a gas it is quite probable that 

 more or less of the compound exists in the liquid solution also. 

 Hence the curves and surfaces corresponding exactly to the 

 equation 



C^'^Go^ = const. 



will rarely be met with in actual cases. The foregoing consider- 

 ations may, however, be of some value as establishing the normal 

 types. On the other hand the straight line and the plane of 

 solubility corresponding to the equation 



C\ = K 



will be often met with. 



{b) The Relation bekveen the Temperature of Fusion and 

 the Composition of the Solution. 



That no sharp distinction can be made between fusion and 

 dissolution of solids was pointed out long ago by Guthrie (Phil. 

 Mag., V, i8, 118 ; 1884), who exemplified his view by the 

 system potassium nitrate — water, and it has now become a current 

 opinion. Hence in speaking of a binary system the terms fusion 

 curve and solubility curve have identical meanings. Yet it is 

 more convenient to have one term which denotes isothermal 



