PEOFESSOE 0. MASSON ON IONIC VELOCITIES, 
889 
tube full of the colourless jelly, and therefore of x {u + v), follows. Obviously the 
same result can be obtained by separate measurement of resistance by Kohlrausch’s 
method, and this has certain advantages. Arrangements are also possible for the 
direct measurement of tt in the various parts of the tul)e during the progress of an 
experiment by means of wires sealed through the walls of the tube, but tliis would 
introduce considerable complication. The velocities per unit potential slope, obtained 
as above, are not dealt with in the present paper; but it may be mentioned that they 
show (in accordance with conductivity results obtained by Arrhenius and others) a 
considerable percentage reduction in solid jelly as compared with acpieous solutions, 
but a reduction which is, at all events approximately, the same for diflPerent salts. 
The values of the relative velocities of the different ions should, therefore, be ffirly 
comjiarable with those found by the older methods. 
It is clear that any value which the method may have must depend on tlie justice 
of the assumption that the observed velocities of the lioundaries are determined by, 
and may be taken as indicative of, those of the intermediate colourless ions, or that 
no mixing of these with their coloured pursuers occurs. In support of this there are 
both exjDerimental facts and theory. 
At the end of an exjieriment in which the tube was oilginally full of a strong 
chloride jelly, the author has frequently melted out the yellow part (without dis¬ 
turbing the blue, which consists of CuCL), and tested it for chlorine without finding 
more than, at most, a barely perceptible trace. After one experiment with KCl, in 
Avhich a wide tube was employed, so that the quantities were considerable, only a 
doubtful trace of potassium could be found in the Idue part, after separation of the 
copper by HoS, evaporation with sulpliuric acid, and ignition to destroy organic 
matter. 
Less direct, but no less striking, evidence to the same effect is afforded by the fact, 
established l)y preliminary experiments, that the ratio of velocities of the Idue and 
yellow boundaries is practically the same in different experiments with the same 
concentration of the same salt, no matter how tlie dimensions of the tu1)e lie 
varied. There is also the fact, already stated, that this ratio in any one experiment 
remains practically constant from first to last. Very slight variations, it is true, do 
occur, attributable most probably to the presence of impurities in the gelatine or in 
the salts employed, or to slight heating by the current in spite of the constant 
temperature bath. But such small deviations from perfect constancy need not be 
considered at present. 
A third line of evidence is found in the fact that, while it is possible to greatly 
vary the relative velocities of the same indicators by using different salt-jellies, it is 
found that the relative rates of advance of the two boundaries remain the same with 
different indicators and the same salt-jelly. The latter jiart of this statement has, 
however, so far been tested only by the substitution of potassium ferrocyanide, and by 
that ol a tartrate solution, for the usual chromate in experiments with potassium 
2x2 
