370 Dr. M. Wilderman. On Galvanic Cells [Nov. 8, 



that the E.M.F. observed is due to "surface phenomena," because, 

 with AgCl, AgBr, Agl plates both observed currents sometimes in one 

 direction, sometimes in the other, and in every experiment different 

 curves and mysterious phenomena (" very curious cases of inversion of 

 the current ") were observed. No galvanic combination we know of 

 gives a current in one direction at one time, in the opposite at another, 

 the anode and cathode exchanging their parts. 



Not only were the deflections of the galvanometer observed, but all 

 the curves in all their complexity were studied and photographed from 

 beginning to end, including the induction and deduction periods. I 

 succeeded, after much trouble, in making the investigation quantitative 

 instead of qualitative, as hitherto ; the results observed were studied, 

 and considered in connection with the chemical composition of the 

 heterogeneous system, and with those chemical reactions, previously 

 unknown to us, which take place in the different systems under the 

 action of light ; the complex nature of the phenomena observed was 

 found, quantitatively separated into constituents, and freed from 

 interfering concomitants, etc. 



Though the amperes and volts measured were mostly exceedingly 

 small (the first had to be measured in units of 10~ 9 , and the second in 

 units of 10~ 6 ), the photographed curves obtained show that the 

 experimental difficulties in this region were quite overcome, since 

 concordant results were obtained on repetition of the experiments. 



As the result of more than 3 years' work, I find that there is, under 

 the action of light, a region of galvanic cells as wide and as varied as 

 in the case of ordinary galvanic cells. We have here constant and 

 inconstant cells, reversible and irreversible cells. The chemical 

 reactions and chemical equilibrium in these galvanic combinations are 

 now perfectly clear, yet they are all sui generis, all the phenomena 

 being intermixed, and characterised by phenomena of induction and 

 deduction peculiar to light cells only. I succeeded also in placing 

 this region of phenomena on a physico-mathematical basis, testing and 

 proving the equations experimentally in all details. The principal 

 results obtained are : 



(1) The total E.M.F. produced by light consists of an E.M.F. pro- 

 duced by light at a constant temperature, owing to increase of chemical 

 potential, and of solution pressure of the exposed plate, and of a 

 thermo-E.M.F. caused by one of the two plates in contact with the 

 liquid being heated by light. Both E.M.F. 's are found to be directly 

 proportional to the intensity of light, and both give currents in the 

 same direction, proving that light acts on chemical potential, as well 

 as on solution pressure of the electrode, in the same way as does heat. 

 My generalisation for light : " Each kind of equilibrium betwee* two 

 states of matter (system) is, at constant volume, on exposure to light, 

 shifted in the direction accompanied by greater absorption of light " 



