462 Proceedings of Royal Society of Edinburgh. [sess. 
being driven out of the colloid. Whatever the cause of the 
change, the film slowly returns to its initial condition, as the 
following examples show. 
Aluminium 
hydroxide films. Ammonium sulphate solution. 
No. 
No Film. 
Film. 
After Electro- 
lysis. 
On Recovery. 
Conductivity in mhos. 
1 . 
•00189 
•00160 
•00133 
•00155 
2. 
191 
156 
135 
153 
3. 
198 
157 
145 
157 
It is worthy of note that the ratio of the conductivities before 
and after electrolysis is almost the same as that between the high 
set of values and the low set of values for ammonium sulphate 
solutions and aluminium hydroxide films already referred to. 
7. In the series of experiments described in B (p. 457) made 
with the unsymmetrical cell, a film of chromic hydroxide was 
made in the usual manner with sulphate solutions, i.e. with 
ammonium sulphate and ammonia on the one side, and with 
ammonium sulphate and chromic sulphate on the other side. 
With this film was determined the conductivity of solutions of 
ammonium chloride, ammonium bromide, and ammonium sulphate, 
which had been adj usted so that their specific conductivities were 
the same. When the solutions are arranged according to the 
loss of conductivity occasioned by the film, the order is as before — 
Cl', Br', SO", and the ratio of the diminution is approximately 
the same ; there is, however, this curious difference, that the actual 
conductivities observed are very much smaller. Thus, in the 
main series of experiments the ratio of the values of ammonium 
chloride and ammonium sulphate is 1 : *60, in this experiment 
the ratio is 1 : *7 3 ; but in the former case the actual drop in 
conductivity is 19 per cent, for the chloride and 51 per cent, for 
the sulphate; whilst in this experiment the drop is 75 per cent, 
for the chloride and 82 per cent, for the sulphate. 
That this large difference is due to essential differences in the 
nature of the films formed from chloride solutions and from 
sulphate solutions does not appear probable. It may well be that 
a film formed from sulphate contains ammonium sulphate or chromic 
sulphate occluded in it, which is not removed to any extent during 
