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V. Io7iic Velocities. 
By W. C. Dampier Whetham, B.A., Fellow of Trinity College, Cambridge. 
Communicated by Professor J. J. Thomson, F.P.S. 
Received October 19,—Read November 24, 1892. 
The most obvious and regular result of passing a current of electricity through the 
solution of a salt is the simultaneous appearance at the two electrodes of products of 
chemical decomposition. Since the intervening solution is unaltered, and the products 
appear only at the electrodes, the motion of the parts of the salt in opposite directions 
through the solution is a necessary hypothesis, and the idea of ionic velocity is at 
once suggested. 
The facts of ordinary chemical action, as well as those ot 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 pjresent purpose. The point is that 
such a motion in opposite directions must occur, and the ions move with a definite 
average velocity through any given solution. This is independent of any particular 
hypothesis as to the nature of the active electrolytic molecule, and need not commit 
us either to the “ dissociation ” or to the “ chemical ” theory. 
In order to explain the facts of “migration” Hittore (‘Pogg. Ann.,’ vols. 89, 98, 
103, 106, 1853 to 1859) supposed that the velocities of opposite ions were different, 
which would produce the observed alterations of concentration round the electrodes 
according to the obvious and well-known law. 
Later, Kohlrausch introduced the idea of a definite specific ionic velocity for each 
ion, independent of the nature of its combination (‘ Wied. Ann.,’ vols. 6 and 26), 
except in so far as its motion is affected by the different resistance offered by the 
different media through which it travels. These differences will disappear at infinite 
dilution, and the values obtained by extrapolation then show that the specific ionic 
velocities of the same ion, as deduced from observations on various salts containing it, 
are, as nearly as can be observed, identical. As it is in examination of this idea that 
most of the experiments, presently to be described, were made, it will be convenient 
to reproduce Kohlrausch’s theory in some detail. The experimental portion of his 
MDOCCXCIII.—A. 2 X 27.3.93 
