Molecular Theory of Solution. 701 



solute and to those of the solvent. Solution will be hindered 

 by the difference between the attraction of the solvent 

 particles for themselves and for the particle of solute. Thus 

 the resultant force opposing solution will be proportional to 



,_M,/ M £ M i? , IV 



,/ — , 4 — — ' , 4 "•"" 'i 



which is of the same form as before. This is the initial 

 force when solution commences. As it proceeds, it would 

 be necessary to take into account the attractions exerted by 

 the dissolved particles of solute. In a somewhat similar way 

 Bingham [1907] deduced a rather general expression for the 

 surface forces in the special case of the miscibility of two 

 liquids. 



Having obtained a complete expression for the force 

 opposing solution, and knowing the numerical values of the 

 terms, it would be possible to calculate the work done by a 

 particle in crossing the boundary layer into the solution. 

 By equating this to the momentum -J ms 2 , the resulting 

 value of s could be substituted in (2) giving a numerical 

 expression for the solubility. In default of this desirable 

 result, the approximate formula (3) for the force opposing 

 solution, in conjunction with (2) ? allows certain deductions 

 of practical utility to be made. 



Evidently solubility will be the greater, the smaller the 

 value of f, though not directly in inverse proportion. Solu- 

 bility will increase with temperature proportionately with the 

 increasing momenta of the solute particles. Exceptions must 

 be due to changes in the molecular forces, or in the distance 

 through which they act. With solutes having greater mole- 



M 2 

 cular cohesion than that of the solvent, the term — ~ will be 



JV 

 the greatest, and the force opposing solution will be the 

 greater, the greater the cohesion of the solute particles. 

 But, from the second term in (3), the greater the cohesion 

 of the solute, the greater its adhesion to the solvent. And 

 since this must be reflected by the increased surface tension 

 of the solution, it follows that the more a solute increases 

 the surface tension of its solvent the less its solubility. The 

 series (I.) and (II.) quoted by Traube show that this is the 

 case for salts. 



Conversely, as in the case of organic liquids and gases, 

 dissolved in water, where the cohesion of the solute is less than 

 that of the solvent, the middle, or adhesion, term in (3) will 

 oe greater than that representing the cohesion of the solute 



Phil. Mag. S. 6. Vol. 38. No. 228. Dec. 1919. 3 C 



