124 
MR. B. 1). STEELE OX THE MEASUREMENT OF IONIC VELOCITIES 
the two solutions is so adjusted that the velocities of the two ions is the same. This 
is, however, only strictly true for what may be called the inverse direction, for the 
first moments after reversal of the current, since the change in potential slope no 
longer hinders diffusion between the two solutions, but aids it, and hence the 
resistances become altered, and so also the potential fall and the velocity. The 
boundaries tliat have been investigated in this direction are ^ Cl and Cu , and 
both of these are cpiite permanent when the current is reversed, and are not lost even 
after 2 hours, their velocities becoming, however, steadily less. 
T/ie Injliience oj Hydrolysis in the Indicator. 
It is necessary that the indicator solution should not be such as lias undergone 
hydrolysis ; when this is the case, the H and OH ions overla^i the lioundary, and 
entering the intermediate solution, thus reduce the resistance and so also the 
velocity at that end. 
Provided an experiment in which one of the indicators undergoes hydrolysis is not 
cari’ied so far that the H or OH ions reach the second boundarv, the i-atio of the 
velocities remains perfectly constant, Init is quite different to the ratio obtained with 
an indicator which is not hydrolysed, 
I’his is clearly seen by a comparison of tlie transport number of copper sulphate as 
obtained Avith aluminium sulphate, and with cadmium sulphate : 
Indicators = 
CuSO.j, O'o N 
1- ON 
2- 0 N 
A 1 .>(S 04)3 and CdS 04 and 
NaCoHsO^ NaCMIsOo. 
, 0749 — 
0-842 U-GGO 
— 0-730 
The influence of the H ions formed Ijy hydrolysis of the ALlSCj)^ solution, in 
A1 
reducing the velocity of the margin, is strikingly shown. 
The Gelatine Sohdions. 
For the preparation ol the indicator jellies the best commercial gelatine was 
enqffoyed, which had been ])urified l)y diffusion in distilled Avater in the manner 
described by Lobry be Bruyn (‘Ptec. Tra\'. Chim.,’ 1900, 19, 236). Gelatine 
solutions containing half an equivalent AA^eight in grammes of indicator salt AA’ere used 
for the ~ solutions, containing tAAB equl\adent 
AA’eights for the N and 2 X solutions. 
The strength of the jellies in gelatine Avas apA-ays, Avhere possible, 12 per cent. 
