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Mr. W. C. Dampier Whetham. 



[Nov. 24, 



ransch, from the results of a series of experiments on the conductivi- 

 ties of salt solutions, concluded that each ion travelled through dilute 

 solutions with a definite speed when urged forward by a definite 

 potential gradient, independently of the other ion present, and intro- 

 duced the idea of specific ionic velocity. He calculated the value 

 of this velocity for many substances, using his own conductivity 

 measurements to give the arithmetical sum of the opposite ionic 

 velocities, and Hittorf's " migration " data to give their ratio. From 

 these values of the velocities he worked out the conductivity of many 

 salt solutions, and the agreement with observation of the results so 

 obtained furnished the first confirmation of the theory. 



Dr. Oliver Lodge actually observed the velocity of the hydrogen 

 ion as it travelled along a tube containing sodium chloride dissolved 

 in a weak jelly, decolorising phenol-phthalein as it went. He 

 obtained the numbers O0029, 0'0026, and 0'0024 cm. per sec. as the 

 velocity of the hydrogen ion under a potential gradient of 1 volt per 

 cm., while Kohlrausch gives O0030. 



This close agreement led me to undertake a series of experiments 

 in order to find a method of determining ionic velocities which 

 would work under more reliable conditions. Consider the boundary 



FIG. 1. 



BC 



of two salt solutions of slightly different density which have one ion 

 in common, but are of different colours (fig. 1). Let us denote the 

 salts by AC and BC. When a current passes across the boundary 



