led in
the direction opposite to that of the current, and their velocity could
be determined by measuring the rate at which the colour boundary moved.
Similar experiments were made with alcoholic solutions of cobalt salts,
in which the velocities of the ions were found to be much less than in
water. The behaviour of agar jelly was then investigated, and the
velocity of an ion through a solid jelly was shown to be very little
less than in an ordinary liquid solution. The velocities could therefore
be measured by tracing the change in colour of an indicator or the
formation of a precipitate. Thus decinormal jelly solutions of barium
chloride and sodium chloride, the latter containing a trace of sodium
sulphate, were placed in contact. Under the influence of an
electromotive force the barium ions moved up the tube, disclosing their
presence by the trace of insoluble barium sulphate formed. Again, a
measurement of the velocity of the hydrogen ion, when travelling through
the solution of an acetate, showed that its velocity was then only about
the one-fortieth part of that found during its passage through
chlorides. From this, as from the measurements on alcohol solutions, it
is clear that where the equivalent conductivities are very low the
effective velocities of the ions are reduced in the same proportion.

Another series of direct measurements has been made by Orme Masson
(_Phil. Trans._ vol. 192, A, p. 331). He placed the gelatine solution of
a salt, potassium chloride, for example, in a horizontal glass tube, and
found the rate of migration of the potassium and chlorine ions by
observing the speed at which they were replaced when a coloured anion,
say, the Cr2O7 from a solution of potassium bichromate, entered the tube
at one end, and a coloured cation, say, the Cu from copper sulphate, at
the other. The coloured ions are specifically slower than the colourless
ions which they follow, and in this case it follows that the coloured
solution has a higher resistance than the colourless. For the same
current, therefore, the potential gradient is higher in the coloured
solution and lower in the colourless one. Thus a coloured ion which gets
in front of the advancing boundary finds itself acted on by a smaller
force and falls back into line, while a straggling colourless ion is
pushed forward again. Hence a sharp boundary is preserved. B. D. Steele
has shown that with these sharp boundaries the use of coloured ions is
unneces