354 
MR. W. C. D. WHETHAM OX IONIC VELOCITIES. 
Thus, the effect of the discontinuity of potential gradient is to increase the 
velocity in one direction and to decrease it in the other, as it should do (see p. 343). 
The mean of the two numbers comes out 
= 0'000455 centim. a second, 
a value which, though it is of the same order of magnitude as the one obtained from 
tlie other pair of solutions, is appreciably less. A second series of observations gave 
0-000483 and 0-000402. Mean, 0-000443. 
These results show that the mean value of the velocities in opposite directions gives 
a number which is nearly, but not quite, the same as that obtained from a pair of 
solutions whose specific resistances are equal to each other. We may, therefore, use 
solutions whose resistances are not identical, to give, at any rate, some indication of 
the value of the specific ionic velocity, provided the differences are not gTeat. But 
this extension of the method must be used with caution. 
While working with solutions of different resistances it was often observed that 
when travelling in one direction the boundary got vague and uncertain, and when 
travelling in the other hard and sharp. Similar phenomena at the junction of liquids 
through which a current is passing have been previously described (see Gore, ‘ Roy. 
Soc. Proc.,’ 1880 and 1881). Many of them can be explained as follows :— 
Suppose that the coloured solution has greater resistance than the other, and that 
the junction is travelling from the coloured to the colourless solution. Any Avander- 
ing ion Avhich happens to be in advance of the main body, finds itself in a region 
Avhere the jDotential gradient is less. It is therefore gradually oA^ertaken, and the 
boundary becomes sharp. When the current is reversed, so that the junction travels 
in the opposite direction, any straggling ion, which lags behind the retreating 
column and so gets into the region of smaller potential gradient, finds itself left 
further and further behind, Avhile others are continually falling out of the ranks. In 
this Avay the boundary becomes vague. If the coloured solution has less resistance 
than the other, a solitary ion is acted on by a greater potential gradient, and the 
order of these phenomena is reversed. 
As far as I am arvare, no attempt has hitherto been made to apply Kohleausch’s 
theory to the case of solutions of salts in solvents other than water. The con¬ 
ductivity of alcoholic solutioiis is much less than that of the corresponding aqueous 
ones, and the question Avhether Kohlrausch’s theory still held good seemed of great 
interest. 
The method described above can easily be applied, but the comparison of the 
i-esults Avith theory offered some difficulty, as no data for the migration constants are 
