OF VITAL PHENOMENA 93 



repulsion by the first phase, attraction by the second phase. 

 According to I. Traube, substances which lower the surface 

 tension of water do so because they have a low molecular at- 

 traction for water molecules. Molecules which lower the sur- 

 face tension of water are repelled by the water molecules and 

 tend to collect at the surface of any other phase in the water. 

 But the degree of this collection is also influenced by the at- 

 traction of this second phase for the molecules in question. 

 A substance will not diffuse into a second phase in excess of its 

 solubility unless it is removed by combining with some other 

 substance or changed as by dissociation or association. 



After a substance enters the second phase, if it lowers the 

 surface tension of this phase it will tend to remain in the surface 

 film rather than to diffuse into the interior. Its diffusion is also 

 influenced by the viscosity of this second phase. 



Thus the rate of diffusion of a substance from one phase to 

 another is determined by a number of factors. The process being 

 reversible, these factors determine the partition at equilibrium. 



The partition of a solute between two phases may be deter- 

 mined by shaking up the substance with equal volumes of the 

 two phases, then determining the concentration of the substance 

 in the two phases. A number representing the ratio of these 

 two concentrations is called the partition coefficient. Berthelot 

 and Jungfleisch (1872) have shown that the partition coefficient 

 of a substance is not affected by the concentration of the sub- 

 stance, provided it has the same molecular condition in the two 

 phases. If, however, the molecular condition changes in one 

 phase with the concentration, the partition coefficient changes 

 with the concentration. For example: consider the partition 

 of common salt between benzol and water. The partition of 

 the undissociated molecules is the same at all concentrations, but 

 salt is dissociated in water and not in benzol. This means that 

 Na* and CI' are insoluble in benzol. If the salt is very concen- 

 trated in the water there will be a large proportion of undisso- 

 ciated molecules and some of these will pass into the benzol, but 

 if the salt is very dilute all, practically, will be dissociated in the 

 water and as the ions are insoluble in benzol all of them will 

 remain in the water. Thus benzol, which dissolves only traces 

 of salt, will take none from an aqueous solution at infinite 

 dilution. 



