2X-2 Mr. W. C. D. Whetham on the 



increases at an even slower rate. We are bound to conclude 

 that no process which requires the conjunction of two mole- 

 cules is involved, and that the ions move independently of 

 each other through the liquid. It should be noticed that 

 migratory independence from each other on the part of the 

 ions is quite compatible with connexion between the ions and 

 the solvent, whether such connexion is of the nature of 

 definite chemical combination or more general physical 

 influence. The electrolytic dissociation theory is quite inde- 

 pendent of any particular view we may take of the nature 

 of solution, which may be produced by actions analogous 

 either to physical or to chemical processes. 



Many other purely electrical phenomena indicate the same 

 independence of the ions, and receive a ready explanation if 

 that conclusion be accepted. Thus, Kohlrausch's specific 

 coefficients of ionic mobility are definite constants for each 

 ion in dilute solution, and do not depend on the other ion 

 present. Again, when a layer of water is placed on the top 

 of a solution of an electrolyte, owing to the diffusion of the 

 two ions the water takes with reference to the solution a 

 positive or negative potential, according as the positive or 

 negative ion in that solution has the greater mobility. From 

 a knowledge of the osmotic pressures and of the specific 

 coefficients of mobility of the ions, Nernst has shown that it 

 is possible to calculate the observed numerical values both for 

 this difference of potential and for the rate of diffusion of the 

 salt as a whole. 



Let us now turn to the second series of observations which 

 led to the development of the dissociation theory. The phe- 

 nomena of osmotic pressure, and the laws to which those 

 phenomena conform, can be deduced by the application of 

 thermodynamics, either in the manner of van't Hoff and Lord 

 Rayleigh, from the observed relations between the solubility 

 of gases and their pressure, as formulated in Henry's law, 

 or from the fundamental ideas of the molecular theory, as 

 pointed out by Willard Gribbs, von Helmholtz, and Larmor. 

 Adopting the latter method, it is clear that the osmotic pres- 

 sure which a solution will exert against a solvent is measured 

 by the rate of change of the available energy of the system 

 when solvent is allowed to enter the solution reversibly 

 through a semi-permeable membrane. On the molecular 

 theory, we must imaoine the solute to be distributed through 

 the liquid as a number of discrete particles, each of which 

 may affect, either by way of chemical combination or physical 

 influence, a certain minute volume of the solvent lying round 

 it. The nature of this influence is unknown, but, whatever 



