SOLID SOLUTIONS 73 



mixtures, because then it would be allowable to extend the 

 conclusion from the small isomorphous admixture to the 

 whole crystal, since in such a case all the crystalline 

 molecules may well be assumed to have the same structure. 

 On considering in more detail particular experiments, it 

 will be well to treat separately the crystalline mixtures and 

 the amorphous solutions. In the former we have to take 

 account of the complication due to electrolytic dissociation, 

 while in the latter we must bear in mind that a separation 

 of mixed material, spoken of as a solid solution, may be the 

 result of an alternate formation of layers, i. e. something 

 quite different from solid solution. 



i. Isomorphoiis Mixtures of Electrolytes. 



In the region of solid solutions, as commonly elsewhere, 

 the most complicated cases theoretically are the most 

 accessible to measurement, and the first attempt to elaborate 

 a method of molecular weight determination for solids 

 referred to just this case of isomorphous mixtures. 



Nernst 1 made use of Roozeboom's * results on the com- 

 position of the mixed crystals of thallium and potassium 

 chlorate obtained on adding a known amount of potassium 

 chlorate solution to saturated thallium chlorate. It was 

 found that the concentration of the undissociated potassium 

 chlorate in solution (normality c) was nearly proportional 

 to that of the total potassium chlorate in the mixed crystal 

 (x in molecular percentage). 



c 



IOOO 



X 



0.0168 2 8-4 



0-0873 12.61 6-9 



0-1536 25-01 6-1 



Applying the law of partition stated on p. 53, as regards 

 the proportion of a substance in two non-miscible liquid 

 layers, it may be concluded from this that potassium 

 chlorate in the isomorphous mixture has the same mole- 

 cular magnitude as in the solution, i. e. KC1O 3 . 



1 Zeitschr.f. Phys. Chem. 6. 577, 9. 137. 2 1. c. 8. 516. 



