THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE 



[FIFTH SERIES.] 



OCTOBER 1896 



XXXI. Osmotic Pressure. By J. H. Poynting, Sc. D., F.R.S., 



Professor of Physics, Mason College, Birmingham* . 



SINCE the osmotic pressure of a solution is of the same 

 order as the " gas pressure " of the dissolved substance 

 at the same density, we are naturally tempted to think of it 

 as an extra pressure produced by the motion of the dissolved 

 molecules. But if we start from this supposition we soon find 

 ourselves surrounded by the difficulties of the dissociation 

 hypothesis. These are so great that it appears worth while 

 to examine our ideas of liquid structure in the hope that they 

 will suggest to us some hypothesis which will free us from 

 the necessity of assuming dissociation. 



I shall try to show in this paper that osmotic pressure may 

 be accounted for as an indirect result arising, not from disso- 

 ciation but from its very opposite, the greater complexity of 

 the molecules in the solution, due to some kind of combination 

 between salt and solvent. 



The facts of liquid viscosity, diffusion, and surface conver- 

 sion to vapour may apparently be represented by imagining 

 a liquid to be, in the main, a solid structure, inasmuch as the 

 molecules cohere and resist strain of any kind. But the 

 molecules have so much energy, potential or kinetic or both, 

 that they are not very far from instability. In a mass of 

 connected molecules irregularly distributed and irregularly 

 vibrating, concentrations of energy must occur, and at the 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 42. No. 257. Oct. 1890. Y 



