1905-6.] Electrolysis through Precipitation Films. 
449 
to clean, owing to the cement; hut, on the other hand, the 
diaphragm was always visible, and there was no metal joint 
through which leaks or short-circuits might take place. This cell 
is subsequently referred to as the unsymmetrical cell. 
The results obtained in the two very different types of cell were 
generally in close agreement. Up to the present, the measure- 
ments have been made by means of alternating current and 
telephone only. The principal difficulty in this case appears to 
be the purification of the film. 
When a film is formed by the interaction of two reagents 
(precipitants), a soluble salt is also formed at the same time, and 
this is probably enclosed within the film. To remove this 
enclosed salt by washing is extremely difficult, if not altogether 
impossible, there being, in addition, the danger of rupturing the 
film or of washing the colloid film away altogether when the 
amount of electrolyte is much reduced. The only feasible way of 
removing the enclosed salt seemed to be by electrolysis with direct 
current, and experiments in this direction rendered it probable 
that the salts can be removed in this way, but that, at the same 
time, the film itself appeared to undergo changes when subjected 
to this method of purification. Evidence in support of this will 
be found further on. 
Finally, the following procedure was adopted as almost entirely 
avoiding the difficulties mentioned above. If a comparison is to 
be made of the effect, say, of a film of aluminium hydroxide on 
Cl', Br', and S0 4 " respectively, in the first instance a fairly strong 
solution of ammonium chloride is placed in each division of the 
complete cell, and bridge readings are taken until the resistance is 
constant. The solution is then removed, and in the one division 
is placed a solution containing ammonium chloride and aluminium 
chloride, in the other division a solution containing ammonium 
chloride and ammonia. The concentration of ammonium chloride 
on each side is the same as before ; the concentrations of the pre- 
cipitants are equivalent and small in comparison with that of the 
ammonium chloride. Thus almost the whole of the conductivity 
is due to the ammonium chloride, and the change of conductivity, 
owing to decrease of concentration of the precipitants by formation 
of the film, will be so small as to be negligible. 
PROC. ROY. SOC. EDIN. — YOL. XXVI. 
29 
