THE PROBLEMS OF SOLUTION loi 



to a gas, and conformed to the same laws of 

 pressure, volume, and temperature. Such results 

 emphasised the analogy between the dissolution 

 of a solid and the diffusion of a gas through 

 a space in which it was not originally present, 

 and sometimes led to the idea that the osmotic 

 pressure of a solution, like the pressure of a gas, 

 was due to the impact of its molecules on the 

 containing wall. As an extreme case of this 

 aspect of the phenomena, the view has been 

 expressed that the solvent should simply be 

 regarded as giving room for the diffusion of the 

 molecules of the solid ; any possible interaction, 

 of a chemical nature or otherwise, between the 

 solvent and solute being disregarded. 



The similarity between the laws of gases and 

 those of dilute solutions, however, does not neces- 

 sarily connote identity in physical nature ; the 

 account of the subject given by thermodynamics 

 shows clearly that the essential feature, common 

 to both cases, on which the similarity depends, is 

 the dilution. In a gas the molecules are, on the 

 average, too far from each other to exert appreci- 

 able intermolecular forces, and the change in 

 energy produced by further dilution does not 

 involve such intermolecular forces. In the same 

 way the dissolved molecules in a dilute solution 

 are so far from each other that, whatever be their 

 action on the solvent, they exert none on each 

 other. Here again, the change of energy on 

 further dilution does not involve the forces 

 between those molecules which alone from this 

 point of view are to be considered, that is, 

 the molecules of the dissolved substance. The 

 essential point is the distant separation of the 



