98 PHYSICAL SCIENCE 



and from this experimental result Van't Hoff 

 showed mathematically by the principles of 

 thermodynamics, that, when in solution, this 

 same gas must exert an osm.otic pressure of the 

 observed value. The proof involves no assump- 

 tion as to the physical mechanism by which the 

 osmotic pressure is produced. Whether it be 

 due to the impacts of the dissolved molecules on 

 the semi-permeable walls, in the same way that 

 the molecules of a gas exert pressure on the walls 

 of the containing vessel ; whether it be due to 

 chemical affinity between the dissolved substance 

 and the solvent, affinity which causes more 

 solvent to enter the cell ; or whether some other 

 hitherto untraced effects come into play, remains 

 an open question. The thermodynamic argument 

 simply shows that, from the experimental solubility 

 relations of gases, the observed osmotic results 

 follow for the gases when dissolved ; but the 

 physical modus operajtdi of the pressure remains 

 uncertain. 



The extension of the theoretical result to the 

 case of non-gaseous solutes like sugar involves 

 some amount of assumption. However, since 

 substances of all degrees of volatility are known, 

 the extension seems reasonable ; and it is 

 abundantly justified by Pfeffer's experimental 

 measurements. 



Another method of applying the principles 

 of thermodynamics to this problem has been 

 developed by Willard Gibbs, Von Helmholtz, 

 and Larmor. Whatever view we take of the 

 fundamental nature of a solution, we must 

 imagine the dissolved substance scattered as a 

 number of discrete particles throughout the 



