74 MOLECULAR WEIGHT AND POLYMERISM 



Fock 1 eliminated as far as possible the influence of 

 electrolytic dissociation by using solutions of nearly the 

 same total concentration obtained by adding a great excess 

 of a salt containing the same ion ; so that if the elec- 

 trolytes dealt with are equally strongly dissociated, the 

 concentration of each dissociated part remains the same. 

 In these cases the concentration in the solution (c l in nor- 

 mality) usually appeared proportional to that in the mixed 

 crystals (x in molecular per cent.). An experiment carried 

 out at 25 with thallium nitrate may serve as example ; 

 the solution contained the amount of potassium nitrate (c. 2 

 in normality) necessary to keep up the total concentration. 



c i 



c i C 2 q + Ca XL - 



0-o8 O-IIT 



0-20 O-Il6 



0-57 0-116 



1-78 0-105 



2-19 0-109 



2-77 0-117 



Here, as in most other cases, the conclusion to be drawn 

 is in favour of simple molecular magnitude in crystals. 



2. Crystalline Mixtures of Non-electrolytes. 



Here we come across, on the one hand, the most striking 

 confirmation of the conception of solid solutions, especially 

 as shown by van Bylert, Beckmann, and Brunni, and on 

 the other a peculiar behaviour noted by Kiister. The 

 observed data are not sufficient to allow of characterizing 

 the two categories with certainty ; in the first case it 

 seems as if any relation of crystalline forms, e. g. at least 

 between iodine and solid benzene, were excluded, whilst in 

 the latter case, e.g. between hexachlor- and pentachlorbrom- 

 ketopentane (C 5 C1 6 O and C 5 Cl 5 BrO), far-reaching isomor- 

 phism is conceivable ; we will, in the first place, therefore, 

 distinguish ' crystalline solid solutions ' and ' isomorphous 

 mixtures.' 



1 Berl. Ber. 28. 408, 2734 ; 31. 160, 506 ; Zeitschr.f. Kryst. 28. 337. 



