PROFESSOR O. MASSON ON IONIC VELOCITIES. 
341 
the K region, will find itself forced to drop back. This explanation of the sharpness 
of certain margins was advanced by Whetham [loc. cit.), but bears repetition here. 
It comes to this—^that the boundary will possess stability if the necessary condition 
be fulfilled, but not otherwise. As a matter of fact it has been found by experiment 
that the colourless SO^ ion following the yellow Cr04 ion through a jelly tube over¬ 
takes it continuously, so that there is no boundary visible, but only a gradual fading 
out of colour. A similar result was got also by following the blue Cu ions with 
colourless Zn ions through the tube : again there was no boundary, but the colour 
gradually faded out. These cases probably illustrate the non-fulfilment of the con¬ 
dition that the foremost ion must be by nature the faster. 
Theory of the Moving Boundary. 
It is easy to deduce the behaviour of the ions on each side of a moving boundary 
from the fundamental equation given at the beginning of this paper, and from the fact 
that the visible (coloured) ion keeps pace with the invisible (colourless) ion in front of it. 
Let the boundary in question be that between visible and invisible cations travel¬ 
ling with the current and matched by invisible anions, all of one kind, travelling 
against it. Let the symbols be used with the same meaning as before, but let those 
applying to the coloured part of the jelly be marked with dashes to distinguish them 
from those ai^plying to the colourless jelly on the other side of the boundary. 
Since equal currents cross all sections of the tube at the same time, 
n (U' -f V') = « (U + V). 
But 
U' = U. 
Hence 
n _ U' _ U -F V _ 1 - f 
n' “ U' V' ^ V ~ 1 - 2 -)' 
Also 
^ dL V V L-t v_ y (1-p) 
V %' ^ U' + V' V 'p{l -2^')' 
Thus the concentrations of the salts in the two portions of the jelly are directly as 
the corresponding cations’ transport numbers; and the working velocities of the 
common anion on the two sides of the boundary are directly as its own transport 
numbers, and inversely as those of the corresponding cations. Since "^-77—w == “r? 
and since v and v may be considered for practical purposes as of equal value, the 
second of these rules may be put more simply, though not quite so correctly, in the 
form Y'/Y = uju or the working velocities of the common anion are inversely as 
the specific velocities of the corresponding cations. 
It is possible to test the foregoing conclusions by analytical experiments conducted 
as follows :—The exact ratio of the velocities of the K and Cl ions having been first 
found by experiments with two coloured indicators in the usual manner, a tube of 
