﻿900 Dr. S. C. Bradford on the 



and infinite length standing on the plane, and Pmvs refers 



to attractions between molecules of different kinds. If 



a solution be separated from pure solvent by a membrane, 



permeable only to the latter, the pressure of the solvent 



particles on the membrane 'due to their kinetic energy will be 



less on the side of the solution, because there are fewer solvent 



particles. Consequently less solvent will diffuse through the 



membrane from the side of the solution than from the side of 



the pure solvent. To neutralize this effect it will be necessary 



to apply a pressure to the solution equal to that which would 



have been exerted by the missing solvent particles. Adopting 



"RT 

 Porter's equation *, this pressure is equal to . In addition 



to this effect there is another due to the altered intrinsic 

 pressure of the solution. If the solute particles have stronger 

 fields of force than those of the solvent particles, there will 

 be a greater attraction for solvent particles on the side of 

 the solution that will increase the diffusion of solvent into 

 the solution. To balance this an additional pressure must 

 be applied to the solution. On the other hand, should the 

 molecular field of the solute be less than that of the solvent, 

 this pressure will be negative. Thus osmotic pressure is the 

 sum of two effects, kinetic energy and molecular attraction, 

 and may be expressed by the equation 



T?T 



n,s S +*(•>• (iv) 



where the second member of the right-hand side corresponds 

 to the action of the molecular fields. If this term be 

 less than the first, a solution (e. g. of salicin), having a 

 surface tension less than that of the pure solvent, may yet 

 show a positive osmotic pressure and not a negative one as 

 predicted by Traube. 



These deductions were made at the time of writing the 

 previous paper. Simultaneously Wo. Ostwald and Mundlerf 

 were led independently, by different reasoning, to a similar 

 expression for osmotic pressure. They made the last term 

 of (iv) correspond to the adsorption law, putting the equation 

 in the form 



and were able to show that the few reliable determinations 

 of osmotic pressure available could be made to conform to 



* Trans. Farad. Soc. vol. xiii. p. 123 (1917). 

 t Kolloid. Zeitschr. vol. xxiv. pp. 7-27 (1919). 



