CHEMICAL THEOKY OF SOLUTIONS. TART I. 29 



relations and another which denotes the relation between the 

 temperature and the composition of the solution which is in 

 equilibrium with a solid phase. The terms solubility curve and 

 solubility surface have been employed to denote isothermal rela- 

 tions in the foregoing passages. The terms fusion curve and 

 fusion surface will be employed to denote the relation between 

 temperature and composition. 



The ideal fusion curve of a binary system has already been 

 repeatedly a subject of study. Le Chatelier and Schroeder 

 chose the Raoult-van t'Hoff equation for vapour pressure 

 as the starting point of their investigation, while Dahms and 

 VAN Laar employed the method of thermodynamic potential. 

 The result arrived at is the same and is represented by the 

 equation : 



In 



^'■ = |-(tv-4-) ^^'' 



in wliicli Qi is the molecular heat of the fusion of the component 

 (Si, and Ti is the temperature of the fusion of the same. The 

 form is identical with that of equation (12), the only difference 

 being caused by the different signs of q^ and Q^. Detailed dis- 

 cussion is given by Roozeboom in his standard work (Heterogene 

 Gleichgewichte, II, 274). The three conditions stipulated for the 

 validity of the equation are : 



I. The concentration d shall be the so called thermo- 



dynamical one, i.e. the molar fraction. 

 IL The solid component shall separate out pure. 

 III. Qi shall be independent of temperature. 



The necessity of the second condition is evident. The cases 

 in which this is not fulfilled will be treated in the next section. 



