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XII. On the Velocities of the Ions. 
By W. ©. Dampier Wuetuam, W.A., Fellow of Trinity College, Cambridge. 
Communicated by Professor J. J. Tomson, F.R.S. 
Received May 2,—Read May 30, 1895. 
IN a previous communication to the Royal Society (‘ Phil. Trans.,’ 184 (1893), A, 
p- 337), I have described a method of experimentally determining the velocities of the 
ions during electrolysis, by observations on the phenomena at the junction of two 
salt solutions, one at least of which is coloured, when a current of electricity is 
passed from one to the other. 
For the success of the method it is necessary to choose two solutions which (1) 
are different in density, (2) different in colour, and (3) have nearly equal conduc- 
tivities at equivalent concentrations, 7.c., when the number of gram-molecules dis- 
solved in 1 litre of solution is the same for both. These conditions seriously restrict 
the number of cases to which the method is applicable, but the results obtained for 
copper and for the bichromate group (Cr,0,) agree well with the values theoretically 
deduced by Koutrauscu from measurements of the conductivity. Alcoholic solutions 
of cobalt nitrate and chloride were also used, and the sum of the velocities of the 
opposite ions, in each case, observed in my experiments, was as nearly as could be 
expected, the same as their sum calculated from the conductivities by KoHLRAUSCH’S 
method. 
In order to extend the method to cases in which there was no colour to be 
observed, I have returned to the use of solid solutions in agar-agar jelly, by means of 
which the first direct experimental measurement of the velocity of an ion was made 
by Dr. Otiver Lopes (‘ B.A. Report,’ 1886), who traced the course of the hydrogen 
ion as it travelled along a tube filled with a jelly solution of sodium chloride, forming 
hydrochloric acid, and decolorizing phenol-phthallein as it went. Dr, LopGE tried to 
determine the velocity of other ions, such as barium, by tracing the precipitates 
which they formed with suitable reagents, but the results were not satisfactory. 
The chief objections to the use of precipitates in jelly solutions are (1) the jelly 
exudes from the tube under the action of the current; (2) the formation of the 
precipitate withdraws some of the electrolyte from the solution, and hence changes in 
conductivity result, which alter the potential gradient, or form surfaces separating 
liquids of ditferent conductivity, over which distributions of electrification will occur. 
a) 2 22.8.95 
