Properties of the Feldspars. 139 



curves for organic compounds, the curve of melting points 

 does not follow van't Hoffs law of dilute solid solutions and 

 does approximate closely to a straight line joining the melting 

 points of the components. The case appears to fall under 

 type 1 of Roozeboom's theoretical classification of isomorph- 

 ous mixtures, in which case the line can not become exactly 

 straight unless the melting points of the components are 

 nearly or quite identical, nor the solidification absolutely homo- 

 geneous without reducing the number of phases to three and 

 destroying the equilibrium. The theory also accounts for an 

 absence of sharpness in the intermediate melting points of the 

 feldspars, but the fact that this lack of sharpness culminated 

 in albite instead of terminating there shows that the viscosity 

 was the chief factor in our difficulties from this cause. Albite 

 was clearly shown to melt through a variable range of 150° or 

 more, while the intermediate feldspar bytownite (AbjAn 5 ) 

 melted almost as sharply as anorthite. The fact that practi- 

 cally 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 taking 

 place 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. 21), the isomor- 

 phism of the feldspars is strongly confirmed. The curve 

 indicates a perfectly continuous relation which the successful 

 preparation of chemically pure albite enabled us to follow 

 through to the end. The order of accuracy is also extraordi- 

 narily high throughout by reason of the chemical purity of 

 all the preparations and the consistent effort made to obtain 

 complete crystallization, even with the more viscous feldspars. 

 Several of the charges were treated 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 passing, 

 that preparations which appeared completely crystalline in the 

 slides frequently proved not to have reached their maximum 

 specific gravity. It is very difficult to detect the last trace of 

 glass with the microscope. 



If our confidence in these determinations is justified, the form 

 of the specific gravity curve is very significant. It was pointed 

 out by Ketgers* that if the isomorphous mixture is merely a 

 " mechanical aggregate," the volume of which remains exactly 

 equal to the sum of the volumes of the components, then the 

 specific volume curve of the mixtures for percentages by weight 



* J. W. Retgers, Zeitschr. fur Phys. Chem./iii, p. 507, 1889. 



