294 SCIENCE PROGRESS. 



negatively electrified stream in the other. The parts which 

 the molecules interchange with each other need not, as far 

 as we have seen, remain dissociated for any appreciable 

 time. The chemical properties of electrolytic solutions and 

 their agreement with Ohm's law can be explained by sup- 

 posing that interchanges between the ions only occur when 

 two molecules collide. According to this view, the ions 

 work their way through the solution by means of a con- 

 tinuous series of molecular decompositions and recomposi- 

 tions — the function of the electromotive force being to 

 make the majority of kations pass to the next molecule on 

 the kathode side, and the majority of anions pass to the 

 next molecule on the anode side. An ion can. thus only 

 take a step forward when the molecule of which it forms 

 part meets another molecule. 



The most important contribution to the theory 

 of electrolysis since the time of Faraday was made 

 by F. Kohlrausch when he introduced the idea of 

 ionic velocity. 1 He pointed out that the conduc- 

 tivity of a solution — that is, the quantity of electricity 

 which passes in one second through a unit cube of the 

 liquid when opposite faces of the cube are kept at unit 

 difference of potential — must be proportional to the number 

 of ions in unit volume multiplied by the velocity with which 

 they travel past each other under the influence of the 

 electromotive force. This relative velocity of the ions can, 

 therefore, be calculated from measurements of the con- 

 ductivity of an electrolytic solution. Now the ratio of 

 the opposite ionic velocities can be found by observations 

 on the unequal rate of dilution of the solution round the two 

 electrodes when a current passes, as Hittorf pointed out, 

 so that the absolute velocities of the ions, under unit 

 potential gradient, can be calculated in centimetres per 

 second. 



Kohlrausch found that, in very dilute solutions, the con- 

 ductivity was proportional to the concentration ; the ionic 

 velocities must, therefore, in such solutions, be constant. 



1 Wied. Ann., 1871), 6, pp. 1, 145 ; 1885, 2(\ p. 161 ; 1893, 50, p. 

 385- 



