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LXXV. On tlie Molecular Theory of Solution. II. 

 By S. C. Bradford, D.Sc* 



IN a previous paper t a preliminary attempt was made to 

 consider the phenomena of solution from the point of 

 view of molecular energy and attractive forces. Recent 

 advances in atomic theory make it fairly certain that atoms and 

 molecules are surrounded by fields of force. Indeed it has long 

 been recognized that cohesion and surface tension are due to 

 molecular forces. But the part played by these forces in the 

 phenomena of solution has not been considered sufficiently. 

 We have to take into account the cohesion of the solvent, the 

 adhesion of solvent and solute, and the cohesion of the solute. 

 Wben a solid is brought into contact with a liquid, the surface 

 tension of the solid, due to the unbalanced cohesive forces at 

 its surface, is reduced by the counter attraction of the liquid 

 particles for those of the solid. On this account an appreciable 

 number of solid particles may have sufficient kinetic energy 

 to overcome the diminished surface forces and escape into 

 the liquid. But any that come again within the range of 

 attraction of the solid surface will be reclaimed, so that 

 eventually a statistical equilibrium may be attained when the 

 numbers of particles leaving and returning to the solid are 

 equal. This state, corresponding to the solubility of the 

 solid, is determined by the equation 



-t 

 7i a = n b e a \ (i) 



wheVe n a and ni are the numbers of particles in unit volume 

 of liquid and solid respectively, a is the most probable speed 

 of the particles, and s is a velocity satisfying the condition 

 that the momentum normal to the surface, ^ms' 2 . of a particle 

 of the solute is just sufficient to carry it through the surface 

 layer. Similar reasoning applied to a cooling solution { 

 shows that it will pass through a metastable stage, as the 

 diminishing kinetic energy allows the aggregation of the 

 particles, until they reach such a size that the force of gravity 

 dominates their Brownian movement, the particles settle out 

 of solution, and the statistical equilibrium is re-established. 

 By the application of Perrin's formula 



r== « / RT 



lOfif — 



* Communicated by the Author. 



t Phil. Mag. vol. xxxviii. pp. 696-705 (1919). 



X Biochem. J. vol. lv. pp. 553-555 (1921). 



