﻿282 Mr. B. Moore on a Relation between the 



calculated for it above. This alteration in the value of T will 

 not be so great as the ratios of the two values 6x 10~ 6 and 

 18 x 10~ 7 indicate ; for the value of the force of molecular 

 attraction varies inversely as some high power of the distance, 

 according to Laplace inversely as the fifth power of the dis- 

 tance. And it is the portions where the molecular attraction 

 has its smaller values that are shut out as the capillary gra- 

 dually narrows past the value of 6 x 10 -6 centimetres. 



It follows from these considerations that the capillaries or 

 pores must have a radius of somewhat less than 18 x 10~ 7 

 centimetres. Now it has been determined by several different 

 methods that the diameters of molecules lie between 10~ 7 and 

 10~ 8 centimetres ; and it is extremely probable that this is the 

 value to which the diameters of these capillary openings in 

 semipermeable walls must approximate. This result is ren- 

 dered probable, not only by the reasoning here given, but by 

 the experimental method employed for producing such semi- 

 permeable walls, by producing an insoluble precipitate in the 

 minute channels of a porous cell. It seems reasonable to 

 suppose that after such a process communication takes place 

 only in the intramolecular spaces or meshes of the precipitate, 

 and such meshes must have about the dimensions found for 

 them by the theoretical considerations here given. 



If the molecules of the solvent do pass in this manner, in 

 single file as it were, through the innumerable intramolecular 

 spaces of the precipitate forming the semipermeable wall, it 

 follows that the molecular attraction will have a constant 

 maximum value, viz. its value just at the surface ; and there- 

 fore, provided the meshes are so small that the molecules of 

 the solvent can only pass through in single file, the osmotic 

 pressure is unaffected by variations in the diameter of the 

 intramolecular meshes. In fact in this condition, instead of a 

 surface-tension acting only for a short distance inward from 

 the perimeter and rapidly falling off in amount, there is a 

 constant molecular attraction acting over the entire cross 

 section and of maximum value ; and instead of the equation 

 27rrT = 7rr 2 P, the equation 7rrr=7rr 2 F or t = P, where t is 

 the difference in this maximum value for solution and solvent, 

 holds, which shows that the osmotic pressure really is equal to 

 the difference between the molecular attractions at the surface- 

 layer of molecules of the solution and of the pure solvent. 



With regard to the support furnished by exj:>eriment for 

 connecting osmotic pressure with surface-tension, it may be 

 premised that all experiments made on salt or other solutions 

 in water or other solvents have been made with strong solu- 

 tions, normal and multiples of normal ; and it cannot be 



