508 MR. W. C. D. WHETHAM ON THE VELOCITIES OF THE IONS. 
The motion of the jelly as a whole is very slow compared with the speed of the 
ions, and, if we can measure the velocity while the ions are only moving over a small 
distance in a short time, it need not affect the result by disturbing the experiment in 
any other way. 
In order to test whether trustworthy observations could be made with jelly 
solutions, | determined the velocity of the bichromate group when travelling through 
them. Decinormal solutions of potassium bichromate and potassium chloride were 
used in the manner presently to be described, and the velocity of the Cr,O, ion under 
unit potential-gradient (one volt per centimetre) came out 0°00044 centim. per 
second. In the earlier investigation the same group travelled through an aqueous 
solution of the same strength with a velocity of 0°00047 centim. per second. The 
effect of the jelly thus appears to be only slightly to retard the motions. Moreover, 
ARRHENIUS has shown (‘ B.A. Report,’ 1886, p. 344) that the viscosity of jelly 
solutions has but a small influence on the conductivity. 
The alteration in concentration, due to the formation of a precipitate, appears to 
have been very considerable in Dr. LopGr’s experiments, and may explain his 
uncertain results. It can, however, be made very small in the following way. Instead 
of setting up, in contact with each other, two solutions, like barium chloride and 
sodium sulphate, which completely precipitate each other, the solutions used were 
barium chloride and sodium chloride, just enough sodium sulphate being added to 
the latter to enable the motion of the barium ions to be traced by watching the 
formation of a slight precipitate of barium sulphate. That this did not seriously 
affect the result was shown by repeating the experiment with less sodium sulphate. 
The specific ionic velocity then came out 0°000386 centim. per second, a number 
agreeing with that first obtained, 0°000390, quite as well as the unavoidable errors 
of experiment would lead us to expect. 
The use of jelly solutions, and of precipitates as indicators, being thus justified, 
many ions could be examined which could not have had their velocity determined by 
means of a colour boundary. Kontrausca has lately given corrected values for a 
number of ionic velocities (‘ Wied. Ann.,’ 1893, vol. 50, p. 385), and, in some cases, 
tabulated them for solutions of various concentrations. It was, therefore, convenient 
to determine some of these, and barium, calcium, silver and the SO, group, present in 
sulphates, were chosen as convenient examples. 
The apparatus used and the method of measurement were the same as those of the 
former investigation. 
Two vertical glass tubes, about 2 centims. in diameter, were joined by a third, 
considerably narrower, which was bent parallel to the others for the greater part of 
its length. One jelly solution was melted and poured into the longer limb till it 
about half filled it, and was allowed to cool and solidify. The other solution was 
then poured into the shorter limb, and, when it was solid, the whole was placed in a 
glass water-bath in front of a window, with a transparent glass scale fixed behind 
