Voltaic Cells ivith Aluminium Plate as one Electrode. 215 



plunging it into the liquid and connecting with the electro- 

 meter, a deflection was obtained in the same direction, but 

 only amounting to '14 volt. After ten minutes it had risen 

 to '35 volt. This seemed to show that the complete forma- 

 tion of the oxide film took some time. The introduction of a 

 little caustic potash also brought the E.M.F. nearly to zero, 

 but I had not as yet obtained an actual reversal of the E.M.F. 

 of the cell. 



I next amalgamated the wire and plunged it in the solu- 

 tion. I at once obtained a deflection of about '46 volt in 

 the opposite direction. At the same time hydrogen was given 

 off, and the wire above the solution became coated with the 

 growth of oxide already referred to. 



The next experiment was an obvious one. I plunged two 

 aluminium wires, one cleaned, the other amalgamated, in a 

 solution of sulphate of alumina, and obtained a deflection of 

 1'08 volt, showing a remarkable difference in chemical pro- 

 perties between the amalgamated and cleaned aluminium 

 wires. 



In order to test the effect of short-circuiting on the cell, I 

 left a zinc-aluminium cell short-circuited for some days, but 

 the aluminium plate, except for a slight darkening of the 

 surface, remained unchanged, showing that to whatever re- 

 action the E.M.F. of the cell was due, it was of so tempo- 

 rary a kind, or the internal resistance of the cell was so 

 great, that do perceptible chemical change had been brought 

 about. 



These experiments are, I think, sufficient to show that 

 there is no evidence that the reactions in the cell are simply 

 the formation and decomposition of the sulphates of zinc and 

 aluminium respectively, but that there is strong evidence to 

 show that we are not in the cell dealing with aluminium at 

 all, but with aluminium oxide supported on an aluminium 

 plate. It is therefore more probable that the E.M.F. of the 

 cell is due to the heat of formation of zinc sulphate, — the 

 heat of formation of aluminium sulphate, + the heat of for- 

 mation of aluminium oxide, — the heat- formation of water. 

 These reactions would result in an E.M.F. in the direction 

 actually observed. It may be noticed that even the amalga- 

 mated aluminium gives too low a value for the reactions of 

 the formation and decomposition of the sulphates. As, how- 

 ever, the water is being decomposed by the metal, we can 

 hardly tell what the actual source of E.M.F. may be. 



I do not know if Dr. Wright attached any definite mean- 

 ing to the term " thermo-voltaic constant" in the case of 

 these particular cells, as the assumption that the reactions are 



