PREVIOUS observations made by me1 on the mechanical stress in cathode plates during electrolysis of copper sulphate, showed that the effect of gelatin in the solutions is marked at very low concentrations (0.5–1 mgm. per litre). The state of the solution is also of great importance, the stress varying considerably with the age of the solution. This makes it possible to follow the evolution of a solution as a function of time by the electrochemical method.