1906.] o?i the Passage of Electricity through Liquids. 249 



Now Kohlrausch finds that the velocity of any one such ion through 

 a dilute aqueous solution is independent of the nature of the other 

 ion present. Thus, for instance, the velocity of chlorine is the same 

 in dilute solutions of potassium chloride as in solutions of sodium 

 chloride. The velocity under a given electric force is a characteristic 

 property of each ion when moving through a dilute aqueous solution. 

 This result suggests that the ions are independently mobile — that 

 they migrate through the liquid independently of each other. 



On this view we must suppose that a large proportion of the 

 whole number of molecules of salt present in a solution is composed 

 of dissociated ions — ions, that is, which are not combined with 

 each other, though they may be linked with solvent molecules. The 

 alternative to this supposition seems to be that the motion of the ions 

 is secured by their passage from molecule to molecule at the instants 

 of inter-molecular collision. On this view, the speed with which the 

 ions worked their way through the solution would depend on the 

 frequency with which collisions occurred. The frequency of collision 

 will depend on the square of the concentration ; if the number of 

 molecules be doubled, the number of collisions per second will be 

 four times as great. Hence the velocity of the ions should be greater 

 in concentrated solutions, and the conductivity, which depends on the 

 product of the ionic velocity, and the number of ions, should be 

 proportional to the cube of the concentration. But experiment 

 shows that the velocity of the ions is nearly constant with changing 

 concentration in dilute solutions, and slowly diminishes with increas- 

 ing strength as the solutions become stronger. We are thus driven 

 back to the idea that the ions migrate independently of each other 

 through the liquid. Much non-electrical evidence pointing to the 

 same conclusion has come to light, and has lent support to the theory 

 of electrolytic dissociation. 



I do not propose to enter in this place into a discussion of that 

 theory. But I wish to point out that the evidence, electrical and 

 other, in its favour points merely to a dissociation of the opposite 

 ions from each other ; it does not involve the idea of charged par- 

 ticles of, say, potassium or chlorine free from all combination. It 

 may well be that the charged atoms, dissociated from each other, are 

 linked, permanently or temporarily, with one or many molecules of 

 the solvent. 



Several facts seem indeed to show that some such combination 

 does occur. If the temperature of a solution be varied it is found 

 that the velocity of the ions alters in about the same ratio as the 

 viscosity of the liquid. Now the viscosity gives the friction which 

 the liquid exerts upon a body moving through it — the dimensions of 

 the body being very large compared with the dimensions of the 

 molecular structure of the liquid. There seems no reason to suppose 

 that the resistance suffered by a single atom, struggling through a 

 crowd of other atoms or molecules, would be related intimately to the 



