SOLID SOLUTIONS 75 



Crystalline solid solutions. The two cases so far studied 

 are of iodine in benzene and thiophene in benzene 1 . The 

 benzene crystallizing out in each case is found to contain 

 iodine or thiophene, and quantitative observations showed 

 proportionality between the concentrations in the solid 

 solution and liquid. 



IODINE IN BENZENE. 



Liquid .... 3-39 per cent. 2-587 per cent. 0-945 per cent. 



Solid solution . 1-279 0-925 0-317 



Katio .... 0-377 0-358 0-336 



Mean 0-357 



For thiophene the ratio only varied from 0-396 to 0-379 

 with the maximum intervening value 0-449 ( mean OM- 1 ^ 

 for solutions containing 1-16 to 15-91 per cent, of thiophene. 



From this proportionality it may be concluded, according 

 to p. 72, that iodine and thiophene have the same molecular 

 weights in solid and liquid benzene, i. e. the weights corre- 

 sponding to the formula I 2 and C 4 H 4 S. 



At the same time we have a means of testing further 

 quantitatively the considerations on the lowering of the 

 freezing point given on p. 71. It appeared there that joint 

 crystallization reduced the depression of the freezing point 

 from T 2 Tj to T 3 T : (Fig. n). The quantitative relations 

 are clear from the figure, viz. : 



TJ T 3 : T! T 2 = cB^ : cB 2 cd : c B = cB dB : cB, 



where cB and dB are the lowerings of vapour pressure 

 suffered by the liquid and solid respectively. These, if the 

 dissolved substance has the same molecular weight in each, 

 are proportional to the concentrations. Hence 



/ dB\ 

 TI T 3 = T x T 2 (i - -g) = 0-643 T i T 2 and 0-55 1 T! T 2 



respectively for iodine and thiophene. 



The molecular depressions will therefore be only 0-643 



1 Van Bylert, Zeitschr. f. Phys. Chem. 8. 343 ; Beckmann and Stock, 1. c. 

 17. 1 20, 22. 609. See further Kiister, 1. c. 17. 364. Wiirffel, Beitrdge 

 zm Molekulargewichtsbestimmung in krystallinischen Substanzen, Inaug.-Diss., 

 Marburg, 1896. 



