254 MOREY 



ART. Q 



At;"' is positive, Ar]'^ negative, making the second term always 



negative. Because of the preponderance of Av"^ the second 



term is greater than the first and, as this term has a negative 



sign, the denominator is always positive. In the numerator, 



Av*' is usually negative and (x" — x^) negative, so the first term 



is positive in the usual case. The quantity Av"' is dominant 



in the numerator also; its product with the term {x' — re') 



is always positive, but as it bears a negative sign, the 



dt 

 numerator is usually negative. This makes j-j, positive, and 



the t-x curve has a positive slope. When, however, the 

 composition of the solution has become very close to that of the 

 solid, the negative second term becomes equal to the positive 

 first term, and the t-x curve has a horizontal tangent, followed 

 by a negative slope. In such cases as H2O-KNO3 this detail of 

 the solubility curve is not detectable experimentally, but that 

 it is necessarily present follows from the correlation with the 

 'p-t curve. The 'p-t curve passes first through a point of 

 maximum pressure, then one of maximum temperature, and 

 at its end-point coincides with the melting-point curve of 

 KNO3, the univariant equilibrium (solid + liquid) in the 

 unary system, KNO3. 



15. Equilibrium Involving Solid Solutions. It was mentioned 

 above that solid KNO3 exists in two enantiotropic modifications, 

 but that consideration of this was not pertinent to the discus- 

 sion. The two forms are both pure KNO3, there is no solid 

 solution, and the inversion point extends across the diagram at 

 constant temperature. It will, however, cause an abrupt 

 change in slope on both the t-x and p-t curves of the equilib- 

 rium, vapor -\- liquid + solid. In the not unusual case in 

 other systems in which one or both of two enantiotropic forms 

 takes into solid solution some of the other component, the 

 equilibrium becomes univariant, and the inversion temperature 

 is either raised or lowered, depending on which of the two forms 

 contains the greater quantity of the other component. It will 

 be interesting to apply equation (14) to this case. 



Let the phases with single, double, and triple accents be 

 vapor, the high-temperature (a) form, and the low-temperature 

 03) form. The equation becomes 



