14 MM. E. du Bois-Reymond and W. Beetz on the 



II. 



The observations above communicated by Dr. du Bois-Rey- 

 mond on the law according to which a substance separated 

 by electrolysis is deposited upon an electrode to form Nobili's 

 coloured rings, induced me to make some experiments upon 

 this point, the results of which I shall detail in the following 

 remarks. 



In my experiments I generally used the same electrolyte as 

 M. Becquerel, a solution of oxide of lead in a concentrated 

 solution of potash, sometimes, but more rarely, a solution of 

 the acetate of lead. A plate of one of the noble metals served 

 as the anode to receive the film of peroxide of lead ; the readily 

 oxidizable metals appeared but little adapted to form receivers 

 of the coloured rings, probably because they themselves are 

 acted upon, partly by oxidation, partly by the transference of 

 particles of their surface to the cathode. Even silver was not 

 sufficiently free from this objection to allow of its being re- 

 commended for this purpose, so that I used either Daguerreo- 

 type plates covered with gold by voltaic electricity, or a plate 

 of platinum. The cathode consisted of a thin platinum wire 

 fused into a glass-tube, and was connected with Grove's zinco- 

 platinum battery, consisting of from three to sixteen elements 

 (each presenting about 1*5 square inches of active platinum 

 surface). According to the composition of the solution, the 

 surface of the plate, and the battery used, from three to eight 

 systems of rings were formed, which were always of a paler 

 colour towards the centre, and ultimately passed into the 

 brown colour of the peroxide of lead. To determine the radii 

 of these rings, the diameter of the darkest and of the lightest 

 spots of the rings was measured by monochromatic illumina- 

 tion. When any ring was irregularly formed on one side, 

 the radius of the other was directly measured. By the former 

 procedure the errors of the admeasurements are halved, and 

 thus slight errors in the determination of the central point are 

 not of so much importance. 



The next point was to determine whether the thickness of 

 the films really diminishes from the centre according to the 

 law of the cubes of the radius, as should occur according to 

 the above calculation. It is evidently most advantageous for 

 this investigation, to calculate the lengths of the undulations 

 of the colours employed each time, from the radii of the same 

 system of rings, measured with differently coloured illumina- 

 tions, and to compare them with the known values for these 

 lengths as ascertained by other experiments. We thus incur 

 no risk of being deceived either by anomalies in the intensity 



