558 Osmotic Pressure and the Kinetic Theory. 



conditions prevailing at the surface no alteration in the area 

 of surface occupied need result from the linking up of the 

 " residual affinities " of the molecules. Neither need it be 

 supposed that the rate of evaporation would he affected other- 

 wise than by a general reduction of mobility * due to the 

 chemical attraction of the molecules and producing equal 

 effects in solvent and in dilute solution. Thus, as there is no 

 change of energy involved in the replacement of one solvent 

 molecule in a complex by another from outside, a rapidly 

 moving particle impinging on a complex might drive a com- 

 bined solvent molecule into the vapour space and itself 

 occupy the vacant position in the complex, the effects pro- 

 duced being much the same as if no complex existed. A 

 similar statement would apply to molecules of solvent attached 

 to the solute in the form of labile hydrates or compounds ; 

 a free solvent molecule impinging on a combined molecule in 

 the surface of the liquid might drive it out and take its place; 

 but if it should impinge on the nuclear solute molecule it 

 would be impulsed and driven back into the interior, just as it 

 would be by an uncombined molecule of* solute. 



CONCENTEATED SOLUTIONS. 



No attempt has been made in the above to account quanti- 

 tatively for the osmotic pressure of concentrated solutions. 

 Even in the case of gases, the equation PV = ET only applies 

 strictly to a material gas within narrow limits of pressure; 

 but Morse and Eraser's experiments indicate a much wider 

 applicability when the formula is applied to osmotic pressures, 

 provided only that for concentrated solutions V is interpreted 

 as the volume of solvent used to dissolve the solute and not 

 the total volume of the solution. It need scarcely be pointed out 

 that this modification is very similar in type to the co-volume 

 correction in van der Waals's equation. 



Osmotic Peessuee and Sueface Tension. 



In view of the close relationship that has been indicated 

 between osmotic pressure and surface structure it would not 

 be surprising that a relationship should exist between surface 

 tension and osmotic pressure. Such a relationship has been 

 postulated theoretically by Traube, who supposes that osmotic 

 pressure depends on a tendency to equalize the surface tension 

 of the two liquids, and has been confirmed experimentally by 



* Compare the reduction of mobility caused by liquid cohesion in a gas 



and represented by the quantity - 2 in van der Waals's equation. 



