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LXXII. On the Molecular Theory of Solution. 

 By Samuel Clement Bradford *. 



SINCE the proposition, in 1887, of the dissociation theory 

 or solution by Arrhenius, little attention has been paid 

 to the attractions of the solute and solvent particles both 

 for themselves and for one another. As such attractions 

 undoubtedly exist, they must be taken into account in a 

 complete theory. In the present paper an endeavour has 

 been made to treat the subject from this point of view and 

 to show that the method indicates immediately the order in 

 which substances arrange themselves with regard to any 

 given property of solutions. 



The idea of molecular attractions being involved in the pro- 

 cess of solution appears originally to have occurred to Newton. 

 But Handl [1872] seems first to have suggested that the 

 solubility of a substance is conditioned by the establishment 

 of a state of equilibrium between the number of particles 

 leaving the solid body* and the number returning. Nicol's 

 views [1883, 1881, 1886 J are particularly worthy of 

 mention. He considered that since the solution of a salt in 

 water, in the form in which it crystallizes at the ordinary 

 temperature, is usually attended by absorption of heat, the- 

 energy involved in the liquefaction of the solid must be 

 greater than that evolved by union with water. He used 

 the fact, that the specific gravity of a solid is affected by 

 the temperature at which it crystallizes, as a criterion of the 

 mutual attraction of the solid molecules at that temperature; 

 and showed that, where solubility increases with tempera- 

 ture^ the cohesion of the solid molecules is weakened by 

 heating, and conversely, as in the case of sodium sulphate, 

 decreasing solubility with rise of temperature is accom- 

 panied by increasing attraction of the solid molecules for 

 one another. Nicol also thought that loose packing of the 

 molecules, or large molecular volume in the solid state, is 

 due to weak attraction between the molecules, and concluded 

 that the greater the molecular cohesion of a salt the less its 

 solubility, and vice versa. 



Recently Traube [1909] has also approached the subject 

 from the molecular point of view. Unfortunately he 

 considers merely the attraction of solute and solvent, which 

 lie calls their " cohesion pressure." This cohesion pressure 

 is indicated by the effect of a solute on the surface tension 

 of a liquid. The more a solute diminishes, or increases, the 



* Communicated by the Author. 



