72 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



ness of the melting-point curve (p. 60). The fact that practically no 

 differences of composition could be detected in our melts we attribute 

 to the effect of viscosity and consequent undercooling, which resulted 

 in crystallization invariably resulting at much too low a temperature 

 for equilibrium to become established between the solid and liquid 

 phases at any stage of the crystallization process. 



(2) When the specific gravities are plotted, like the melting points, 

 as a function of the composition (fig. 23), the isomorphism of the feld- 

 spars is strongly confirmed. 



Fig. 24. Curves of specific volume of the feldspars and feldspar glasses. 



The curve indicates a perfectly continuous relation which the suc- 

 cessful preparation of chemically pure albite enabled us to follow 

 through to the end. The order of accuracy is also extraordinarily 

 high throughout by reason of the chemical purity of all the prepara- 

 tions and the consistent effort made to obtain complete crystalliza- 

 tion, even with the more viscous feldspars. Several of the charges 

 were heated for two weeks or more consecutively, then removed for a 

 determination, then replaced in the furnace for another week in order 

 that we might assure ourselves, from the consistent reappearance of 

 the same value, that a maximum, and, therefore, holocrystallization, 

 had been reached. It is of some practical importance to note in pass- 



