134 Day and Allen — Isomorphism and Thermal 



van Ryn van Alkemade* and is itself a powerful instrument of 

 analysis in this field. 



Roozeboom distinguishes three general classes of isomorphous 

 mixtures : 



(1) The components are miscible in all proportions from 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 continuous curve joining the melting points of the com- 

 ponents and containing neither maximum nor minimum. 



Type II. The curve contains a maximum. 



Type III. The curve contains a minimum. 



*Zeitschr. f. Phys. Cheni., xi, p. 289, 1893. 



Except for the suggestions of Vogt to which reference has been made, this 

 method seems not to have been utilized for the study of mineral solutions 

 before. A brief outline of it will therefore be given here. 



In a system of rectilinear coordinates (fig. 15) the ordinates may represent 



the potential of a particular 

 P, T constant system— (Gibbs' £-f unction, 



not directly measureable) 

 and the abscissas the num- 

 ber of gram-molecules of 

 solvent (water for example) 

 supposed to contain 1 gr. 

 mol. of solute. In other 

 words, every point of the 

 curve represents a solution 

 of which the x coordinate 

 is concentration and the y 

 coordinate the potential. 

 The conditions of pressure 

 and temperature are 

 7 r ' ih T- ^ assumed constant for a par- 



ticular diagram. 

 Fig. 15. Every such curve for sub- 



stances soluble in all propor- 

 tions will be convex downward, otherwise there would be some particular 

 point on the curve which would not represent a minimum potential for a 

 particular composition and the solution would tend to separate into two, the 

 mean potential of which would be lower. 



The condition for equilibrium between such a solution and its solid phase 

 (pure salt) may now be readily found. Lay off on the C-axis a distance equal 

 to the potential of the solid salt and from the point so obtained draw a 

 tangent to the curve. This tangent is the locus of minimum potential (stable 

 systems) for any composition. At the point a for example, we have a sat- 

 urated solution containing the number of gr.-mol. of solvent indicated by the 



corresponding abscissa and the proportion — of salt, the balance of the salt 



remaining in solid phase. At b we have the saturated solution with all the 

 salt included ; to the left of b upon the curve, supersaturated solution ; and 

 to the right unsaturated solution. With increase of temperature the form 

 of the curve changes and c approaches d, the melting point of the salt. 



