Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/354

354 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

a value which, though it is of the same order of magnitude as the one obtained from the other pair of solutions, is appreciably less. A second series of observations gave

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 great. 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, ‘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 wandering ion which happens to be in advance of the main body, finds itself in a region where the potential gradient is less. It is therefore gradually overtaken, 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, while others are continually falling out of the ranks. In this way 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 aware, no attempt has hitherto been made to apply ’s theory to the case of solutions of salts in solvents other than water. The conductivity of alcoholic solutions is much less than that of the corresponding aqueous ones, and the question whether ’s theory still held good seemed of great interest.

The method described above can easily be applied, but the comparison of the results with theory offered some difficulty, as no data for the migration constants are