340 
PROFESSOR 0. MASSON ON IONIC VELOCITIES. 
sulphate and sodium chloride. In the tartrate experiment the indicator was, of 
course, quite invisible in itself, hut it met the copper and formed a visible precipitate 
across the tube at the exact point calculated from previous experiments made in the 
usual way. The relative velocities of Na and Cl were thus given the same value, 
whether Cu and Cr 04 or Cu and C 4 H 4 O 6 were used as indicators.^ 
Besides these experimental indications that there is no commingling of the ions at 
the blue and yellow boundaries, there are theoretical reasons in favour of the same 
conclusions, jirovided that the coloured ion is specificcdlii shiver than the one it is 
following. This has l^een already mentioned as one of the requisite characters of a 
satisfactory indicator. Imagine naturally slow Cu ions travelling behind naturally 
faster K ions, with Cl ions travelling past both in the opposite direction. If Cu lag 
behind K, or K run away from Cu, a region will be established where cations are 
deficient, a state of affairs that must immediately correct itself by reason of the 
consequent E.M.F. If, on the other hand, they keep pace with one another, as in 
fact they do, it must be by virtue of a steeper potential slope in the blue. If now 
some K ion accidentally lags behind its fellows, it will find itself in this region and 
be at once hurried forward again ; while any ambitious Cu ion, trying to penetrate 
* The ^precipitates formed across the tube by the meeting of Cu ions with Cr 04 ions and Fe(CN)e ions 
are of the nature of the semi-permeal)le m.embranes used by Traube and Pfeefer in the stxrdy of 
osmotic pressure. An interesting fact has been observed udth both these membranes. They are first seen 
as fine transverse films across the tube, but, if the experiment be not stopped, they rapidly thicken up till 
they form discs about half a millim. wide. Simultaneously the galvanometer shows a rapid fall of current, 
which becomes almost nil within a very few minutes of the first meeting. If now, or later, the current be 
reversed, the galvanometer deflection rapidly goes up almost to its previous value, though the membrane 
remains apparently quite miaffected even when the reversed current is maintained for hours. By again 
reversing it the j)henomena may be repeated, though the current does not now fall off immediately. The 
explanation suggests itself that the membrane is impervious to the ions (Cu and Cr 04 or re(CN) 4 ) which 
have produced it, but not to other ions such as K and Cl. Before the first reversal of the current, only 
Cu ions can reach the membrane from the anode side and only CrOj (or Fe(CN)d from the cathode side, 
and these cannot pass. After reversal, K and Cl, or other corresponding ions respectively, carry their 
charges to and through the membrane. These are now on the wrong side; so that when the current is 
again reversed, it is some time before the original state of affairs is restored and the current again cut 
down. If, after the memlirane is first completely formed, the circuit be broken and everjuhing left 
in situ for 24 hours or so, it is found that, on re-connecting vdthout reversal, a very fair current will pass; 
but it does not last long. In all probability this is due to simple diffusion through the membrane, by 
which a little K and a little Cl find their way across to the parts of the tube previously free from them. 
It is a curious fact, no certain explanation of which has yet been arrived at, that when the intermediate 
salt is a sulphate instead of a chloride, the copper and chrome ions do not form a membrane on meeting, 
but simply intermix with production of a greenish colour; nor is the current cut down. This difference of 
behaviour has been consistently manifested in all the experiments recorded in this paper. Yet copper 
sulphate and potassium chromate solutions precipitate copper chromate when mixed. The usual membrane 
was, however, obtained with a K.SO 4 jelly and Cu and Fe(CN )8 indicators. 
The maldng of osmotic pressure apparatus might be improved by depositing the membrane electrolytically 
in the walls of the pot, j^reviously charged with a potassium chloride jelly. Current readings would give 
a sure indication of the condition of the membrane during and after its formation. 
