538 Proceedings of Royal Society of Edinburgh . [sess. 
It would appear, from the experiments above described, that 
when the amount of nickel in solution is reduced to a very small 
amount — say 1 part in about 50,000 — a film of some other sub- 
stance is deposited, giving a higher polarisation effect. At first 
this is replaced more or less by nickel, and occasionally breaks 
down, as shown by the lapses to the original potential, possibly on 
account of richer solution being brought against it by convection 
currents, but it ultimately becomes permanent when the nickel 
deposition is completed. As the deposition of nickel from ammo- 
niacal solutions must be a secondary action (the nickel forming 
part of a complex ion), it would probably be more correct to 
assume the formation of a film, other than nickel, from the very 
commencement ; owing to the nickel solution acting as depolariser, 
however, the polarisation effect due to this primary deposit is only 
observable when the layer of solution next the cathode is free 
from nickel. If this is the case, it is probable that with a much 
feebler current density, the proportion of nickel remaining in the 
bulk of the solution would fall still further before the rise in 
polarisation set in distinctly. 
The phenomenon, when observed, appears to be an infallible 
indication of complete deposition, but whether or not it may be 
trusted to appear in all cases remains to be seen ; probably the 
presence of certain substances may interfere and prevent it. 
These experiments have been carried out simply from the point 
of view of the practical application to quantitative work, and it is 
proposed to extend them to the other kinds of solution sometimes 
employed, and to other metals. 
