CONCLUSIONS. 65 



Roozeboom distinguishes three general classes of isomorphous 

 mixtures : 



(1) The components are miscible in all proportions from o to 100 

 per cent in both solid and liquid phases. 



(2) Miscibility is limited to certain concentrations. 



(3) More than one type of crystal occurs. 



In the feldspars we are concerned with the first class only, but here 

 also Roozeboom distinguishes three possible types: 



Type I. Melting (or solidifying) points of the mixtures lie on a con- 

 tinuous curve joining the melting points of the components and con- 

 taining neither maximum nor minimum. 



Type II. The curve contains a maximum. 



Type III. -The curve contains a minimum. 



These types are for the moment purely hypothetical and are a prod- 

 uct of the method of analysis, though they are being rapidly identi- 

 fied for various isomorphous pairs by pupils of Roozeboom and br- 

 others. 



The method of reasoning which yields these three possible types 

 will be briefly described with the help of the Van Alkemade graphical 

 analysis : 



If we indicate the potential (~) of a particular mixture by the length 

 of the ordinate (fig. 18), and the number of molecules of .4 and B by 

 subdividing the horizontal axis (A -\- B = 100) in the proper propor- 

 tion, assuming atmospheric pressure and constant temperature for 

 each diagram, then every point within the coordinates represents a 

 particular phase of known composition and potential. Suppose, now 

 (Roozeboom), a temperature is assumed above the melting point of 

 the higher-melting component; clearly, whatever the composition, 

 only the liquid phase can have a stable existence. If potential differ- 

 ence represents the measure of the tendency to change and the 

 tendency of all change is toward the minimum potential, for this tem- 

 perature all change will be toward the liquid ; and the potential of a 

 solid, if one existed there, would be greater than that of the liquid for 

 all compositions hence the curve 5 (solid) above the curve L (liquid) 

 throughout. 



Suppose the potential to be lowered to a point where crystallization 

 can begin. The tendency to melt no longer obtains for all composi- 

 tions ; the two curves will be displaced relatively and, being of different 

 form, will intersect. Draw a common tangent to the curves and apply 

 Van Alkemade's reasoning above noted. The trend of the potential 

 of both phases between the points of tangency, i. e., of all mixtures 

 between these limits of composition, is toward the minimum repre- 



