Prof. Minchin's Experiments in Photoelectricity. 213 



other part of the spectrum, the exposed plate being positive ; 

 and, apparently, with strong red rays, there is a reversal of 

 the sign of the E.M.F. 



Various other substances were used, among which I may 

 mention iodine green, which gives rather strong currents 

 with daylight, the exposed plate being negative. 



In all these cells the E.M.F. is feeble compared with the 

 electromotive forces which I have obtained by other means, 

 to be presently described ; but in the case of silver plates 

 coated with iodine green, an E.M.F. of about -^o v0 ^ can De 

 observed with sunlight. 



Fluorescine used like eosine on silver or platinum plates 

 gives poor results. 



A few experiments with a solution of sulphate of quinine 

 and some other fluorescent liquids were made with a view to 

 connecting fluorescence with electric effect ; but the observed 

 results were comparatively small, and the subject has been 

 left for further trial. 



Other Metals. 



Many experiments were made with other metals, but the 

 currents produced by most of them were smaller than those 

 given by silver plates. Thus, I have on record an experiment 

 in which bismuth was melted into thin plates which w T ere 

 fixed on glass, and then immersed in a cell containing water. 

 When light fell on one of these plates, a considerable current 

 was generated, the exposed plate being positive ; but on 

 adding a solution of chloride of bismuth to the water, the 

 current generated by light was reversed. On taking them 

 out of the cell and washing them with distilled water, and 

 then immersing them again in a water-cell, when the plate 

 was exposed to light, there was an impulsive current corre- 

 sponding to a negative nature of the plate, but this was 

 promptly succeeded by a very large current in the opposite 

 direction — just as in the cases previously mentioned. 



Copper, antimony, and other metals were also tried, but 

 the results are not of sufficient importance to demand any 

 detailed record. In particular, iron plates coated with the 

 magnetic oxide and immersed in water yielded photoelectric 

 currents. 



Tin Plates. 



A glass cell containing a solution of fluorescine in distilled 

 water, in which two silver plates had been immersed, was 

 taken for trial with other than silver plates. Almost the 

 first metal that I used in this cell was tin. Two plates of 

 tin foil whose surface had not been in any way prepared, 



Phil Mag. S. 5. Vol. 31. No. 190. Mar. 1891. R 



