William Cecil Dampier Whetham

“ The facts of ordinary chemical action, as well as those of electrolysis, require us to suppose that a continual molecular interchange of partners is going on, but how this takes place, whether by means of “free ions” or by means of the fractional number of collisions which the mechanical theory teaches us will occur with sufficient violence to bring about separation and possible rearrangement, remains an open question, and one which it is not necessary to consider for our present purpose. ”

“ This agreement is striking evidence in favour of Kohlrausch’s theory and leads us carefully to consider the data on which it is based. The only objection which can be raised is to the assumption that all the molecules are active, and on this the accuracy of the results deduced apparently depends, for if we suppose only a part of the salt to be active, it seems necessary to allow a greater ionic velocity in order that the same current may be carried. ”

“ 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. ”

“ A series of solutions of cobalt chloride and cobalt nitrate of different strengths was made up and the specific resistances determined. They were then placed in pairs of equivalent strength in the velocity apparatus, and a long investigation made on their behaviour. It was found that with weak solutions, whose strength was below 0.08 grm. equivalent per litre, the phenomena were quite regular. The junction travelled against the current, as it ought, since the change of colour depends on the acid radicles. As the strength of solution increases new phenomena appear. ”

“ Thus, by drawing curves with (the reciprocal of the average distance between the molecules) as abscissæ, and as ordinates, he is able to obtain the specific molecular conductivity for infinite dilution. When the values for and are worked out for these ideal solutions, it is found that the velocity of any given ion is independent of the nature of the salt of which it forms part, and hence Kohlrausch assigns numbers to various ions which represent their specific ionic velocity. ”

“ The view that all the salt is active supposes a uniform ionic velocity—the view above detailed supposes a series of rests interposed with a series of intervals during which the ion is moving forward with a velocity which is, while it lasts, on the average ”

“ 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. ”

“ Whence the arithmetical sum of the opposite velocities of anion and kation ( and ) when urged by a slope of potential of one volt per centimetre through a solution containing monad gramme equivalents of active substance per cubic centimetre, and of specific conductivity , is
centims. per second.
In order to apply this to particular cases Kohlrausch assumes that all the dissolved salt is active, so that is known from the strength of the solutions. The ratio is taken from Hittorf’s migration experiments, and thus the absolute values of and can be obtained. ”

“ 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
centim. a second,
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. ”

“ The corks fitted loosely, and a vertical groove, filed in each, allowed any gas which might be evolved to escape. The junction tube had a length, from B to C, of 12.6 centims., and a diameter of about 1.2 centims. If we add one-half the diameter as a correction for each end we get 13.8 centims. as the effective length of the tube, which must be divided into the total difference of potential between the electrodes to give the potential gradient.
The advantages of such an apparatus are shortly as follows:—
1. Any gas evolved at once escapes without interfering with the experiment. ”

“ The actual velocity observed in this case, due to the ionic velocities, was 1.6 centims. per hour. The correction for differences in density is thus utterly negligible.
4. A more important objection was pointed out by Professor J. J. Thomson. With salt solutions of equivalent strength there is, in general, a difference of specific resistance, although this is small in most cases. At the junction there will thus be a discontinuity of potential gradient, and, therefore, a distribution of electricity, which will be acted on by the electric force and introduce disturbances at the boundary. ”

“ Thus, Dr. Lodge found Ba, Sr, and ClBrI centims. per second, while Kohlrausch gives Ba, Cl, I (‘Wied. Ann.,’ vol. 6, p. 206) .
There are several disadvantages in this method. The greatest appears to be the impossibility of keeping the solutions of a definite strength—the mere fact of producing a precipitate must withdraw a certain amount of salt from the sphere of action, and so reduce the strength of the solution. This will increase the resistance of that portion of the tube in which the precipitate has been formed, and the potential
Fig. 1. ”