the Standpoint of the Kinetic Theory. 555 



'C-eased, but because it is neutralized by au equal and opposite 

 process of condensation. In the case o£ a non-volatile liquid 

 the mobility of the relatively heavy molecules or molecular 

 complexes is so small that very few are able to escape, and 

 the maximum vapour-pressure is inappreciable. 



If now a solution be prepared by mixing a volatile and a 

 non-volatile liquid, or by dissolving a non-volatile solid in 

 a volatile liquid, the surface will contain both kinds of 

 molecules. If one of the solvent molecules be struck by a 

 rapidly moving molecule from the interior of the liquid, it 

 will be projected into the vapour space. If, however, one of 

 the ncn-volatile molecules be struck, it will be unable to 

 escape, and the solvent particle will rebound in much the 

 same way as if it had struck the wall of the containing 

 vessel. The rate of evaporation is therefore reduced by the 

 addition to the solvent of a non-volatile solute. On the other 

 hand, it is probable that the presence of the non-volatile mole- 

 cules would not interfere with the rate of condensation of the 

 vapour. This point is of fundamental importance, as the 

 opposite view was advocated byPoynting, who supposed that 

 condensation and evaporation would be checked to an equal 

 extent, just as if the surface had been covered by a plate of 

 perforated zinc. It must be remembered, however, that a 

 considerable upward velocity is required before a molecule 

 can escape from the surface of a liquid, and that a molecule 

 descending with even the smallest downward velocity would 

 have little chance of escaping when once it came within the 

 range of attraction of the liquid. Even if, on reaching the 

 liquid surface, a vapour molecule should strike against a non- 

 volatile molecule of the solute, the attraction of the neigh- 

 bouring molecules of the solvent would be sufficient to hold 

 it, and thus ensure its condensation. 



It need scarcely be pointed out that similar conditions 

 would prevail at the surface separating liquid and ice, or at 

 the surface of one of the more conventional semi-permeable 

 membranes. In the former case the presence of a non- 

 isomorphous solute might prevent the adhesion to the ice of a 

 molecule of solvent moving towards it but separated from it 

 by a molecule of solute. On the other hand, it would not 

 prevent the melting off or dissolution of an ice molecule if 

 the average kinetic energy (i. e. temperature) of the ice were 

 raised by the latent heat of crystallization of other molecules 

 passing from the liquid to the solid state. In the case of a 

 membrane such as copper ferrocyanide the solute would check 

 the escape of solvent molecules from the solution into the 

 membrane, but would not oppose the return of wanderers 



