62 



ISOMORPHISM AND THERMAL PROPERTIES OP FELDSPARS. 



objection was raised by Garelli* and elaborated by Bodlanderf if the 

 solid solution behaves like other solutions, a small quantity of com- 

 ponent B added to component A can only lower the solidifying point 

 of A when the solid phase is richer in A than the liquid phase. The 

 reasoning is this (Bodlander): Let X\ (fig. 15) be the vapor-tension 

 curve of component A in the liquid state, y\ the solidifying point (t { ) 

 of A, and z\ the vapor-tension curve of solid A . Now, if a small quan- 

 tity of B is added and the solid phase which crystallizes out contains 

 the same proportions of A and B as the liquid mixture in which it 

 formed, the vapor tensions of the liquid and solid phases must have 

 been lowered equally and the solidifying point will fall at y 2 with the 

 same temperature as the pure solvent. (Equality of vapor tension 



100 A Composition 



Fig. 16. 



100 B 



in the solid and liquid phases determines the temperature of change of 

 state.) If A crystallizes alone from A +B, the vapor-tension curve 

 will continue on to z 2 and the temperature of solidification fall to t 2 ', 

 while if the solid phase contains both components but is richer in A 

 than the liquid phase, solidification will occur at an intermediate point. 



Fig. 16 will serve to show the crucial character of the issue raised. 

 The ordinates represent temperatures and the abscissas percentages 

 of A and B. Kuster finds his solid and liquid phases identical in 

 composition within the limits of experimental error and the solidify- 

 ing temperature on the line A B at a point which can be determined 

 from the proportions of the components at d for example. But the 

 laws of dilute solutions tell us that if the phases are identical in com- 

 position the solidifying point of A -+- B must fall at c, i. c, must 

 remain the same as for pure A . 



The temperatures at which Krister's observations were made and 

 their painstaking character leave no doubt as to the validity of the 



*F. Garelli, La Gazzetta Chimica Italiana, xxvi, p. 263, 1894. 

 t Bodlander, Neues Jahrb. f. Min., Beilage, Bd. xn, p. 52, 1899. 



