292 Messrs. Wright and Thompson on the Determination of 



Daniell cells containing amalgamated-zinc and electro-copper 

 plates are fairly close to the sums of those similarly observed 

 for two diffusion-cells/one containing amalgamated-zinc and 

 the other electro- copper plates. The chemical action in two 

 opposed Daniell cells amounts to solution of copper in one 

 cell and deposition thereof in the other, and ditto for zinc; or 

 precisely the sum of the actions with two diffusion-cells. 



The amount of depreciation for a given current-density in- 

 crease varies notably with the metal and character of surface : 

 thus amalgamated zinc gives less depreciation than electro- 

 copper, and electro-copper less than electro-zinc. Variation 

 in the strength of the solution used did not appear to influ- 

 ence the result to anything like so great an extent as variation 

 in the nature of the plate-surfaces. 



On the whole it was found, even in those cases where the 

 maximum amount of depreciation was produced, that this de- 

 preciation did not exceed from *1 to '2 millivolt, until the 

 current-density overpassed '02 to *03 microampere per square 

 centimetre. In the four cases above cited the limiting current- 

 densities, when the depreciation amounts to *2 millivolt, 

 are: — 



Two opposed Daniell cells, amalgamated-zinc and 



electro-copper plates *05 



Diffusion-cells, electro-zinc plates *06 



„ „ amalgamated-zinc plates *15 



„ „ electro-copperplates *08 



In carrying out the observations, therefore, care was taken* 

 that in no case should the current-density exceed the limiting 

 value for the particular class of cell examined, current-den- 

 sities of much less amount being in most cases employed; so 

 that, finally, £/*# effect of u polarization" on the average values 

 obtained is in no ease to depreciate them by a quantity so great 

 as '2 millivolt^ and is usually insufficient to depreciate by an 

 amount approaching to '1 millivolt. 



152. Asa general rule, it was found that when the cell (or 

 pair of opposed cells) to be examined was set up, compara- 

 tively little variation in the current flowing took place during 

 15 to 30 minutes; what alteration did occur being evidently 

 due to alteration in condition of plate-surface (by superficial 

 oxidation by dissolved air, or by molecular change, i. e. 

 spontaneous change of state of aggregation, or by solution or 



* Owing probably to the influence of "polarization," the values obtained 

 by Fromme, he. cit. supra, do not exhibit very close concordance with one 

 another, nor do the mean curves thence deducible agree sharply with 

 those subsequently detailed. Moser's diffusion-cell figures, on the other 

 hand, do not differ widely from those obtained by us. 



