﻿898 Dr. S. C. Bradford on tie 



it was calculated that the radius of a gelatin particle just 

 large enough to settle would be about 06/x. This agreed 

 approximately with the value found experimentally by the 

 application of Stokes' law. . 



As a first approximation, the force between two particles 

 was taken as 



F=«, (ii.) 



ab 



T a b being the distance between their centres. From this it 

 was deduced that the initial force to be overcome by a 

 particle of solid escaping into pure solvent is proportional to 



M 2 M M M 2 



/= __,_ 2 __^ + _^ .... (iii) 



T w* T ws ? V 



where the subscripts w and s refer to liquid and solid re- 

 spectively. The smaller this force, the greater the solubility. 



In the normal case, when the molecular cohesion of the 

 solute is greater than that of the solvent, the last term 

 of (iii.) will be the largest, and the force opposing solution 

 will be greater as the cohesion of the solute increases. That 

 is to say, the solubility of ordinary salts is smaller the greater 

 their cohesion. When the cohesion of the solute is less than 

 that of the solvent, as often happens with organic substances, 

 the middle term of (iii), representing the adhesion of solute 

 and solvent, will be greater than the last. In this case the 

 solubility will increase with increasing adhesion of solute 

 and solvent. And since the adhesion increases with the 

 cohesion of the solute, the solubility of such substances is 

 greater, the larger their cohesion. Similarly the solubility 

 of a given solute, in different solvents with less cohesion, will 

 increase with the cohesion of the solvent, while the solubility 

 of the same solute, in solvents with greater cohesion, will 

 diminish with increasing cohesion of the solvent. 



Relative molecular cohesion may be estimated in a variety 

 of ways. Traube * used the enhanced or diminished surface 

 tension of a solution as a measure of the adhesion of solute 

 and solvent. He considered solubility only from . the point 

 of view of adhesion, but was able to show that the solubilities 

 of organic liquids, which reduce the surface tension of water, 

 follow the order of the surface tensions of their solutions. 

 As, however, he neglected to take into account the mutual 

 cohesions of both solvent and solute as well as their kinetic 



* Ber. deut. chem. Ges. vol. xlii. p. 86 (1906). 



