BIRTH AND AFFINITIES OF CRYSTALS 127 



with mixtures of two salicylates, salol and betol, choosing these 

 substances merely because they melt at convenient temperatures, 

 and we have succeeded in tracing for them the complete curves 

 like those shown in fig. 2. By means of the refractive index 

 we have traced the changes in each solution as it cools, and 

 have always obtained curves resembling abf of fig. 1. We 



A 



O 



Percentage of B 



Fig. 2. 



have also found that, as is clear from the figure, any mixture 

 has no less than four possible freezing points — namely, two 

 temperatures, a and c, at which A and B crystallise respectively 

 by inoculation, and two others, b and d, at which A and B 

 crystallise spontaneously. 



The same general principles must hold for mixtures of more 

 than two components ; we have already traced the super- 

 solubility curves for mixed solutions of sodium nitrate and lead 



