12 Waterbouse — Electrical action of Light upon Silver. [No. 1, 



silver chloride, iodide or bromide, precipitated in a thin layer on sheets 

 of platinum or gold, was exposed to light as above, the exposed plate 

 was positive, and that the initial action was much stronger with the 

 bromide than with the chloride ; though the intensity of the currents 

 observed was variable and depended on the thickness of the film of bro- 

 mide, moreover the electrical action was soon exhausted. With the 

 iodide the current was almost as strong as with the chloride, but did not 

 remain constant so long. 



When plates of silver were employed, instead of platinum or gold, 

 as a support for the haloids, the effects noted were stronger and more 

 regular, but it was found that the direction of the current depended 

 on the thickness of the films ; with thin coatings the exposed plate 

 was positive, and with thick coatings negative. This was markedly 

 the case with plates of silver exposed to the vapours of iodine. With 

 vapour of bromine the exposed silver plate was negative, the initial 

 current, even with diffused light, was very strong, but after remaining 

 exposed to light for some minutes then protected from light and again 

 exposed to its influence, it was found that the current was very weak. 

 A film of silver chloride prepared by exposing a silver plate to the 

 vapour of chlorine gave only a very weak effect, but plates coated with 

 the violet subchloride behaved very well in these trials and yielded 

 for a long time results from Avhich comparisons could be made. 



On the basis of these experiments Becquerel invented his electro- 

 chemical actinometer which was practically a voltaic element or 

 cell composed of two plates of very pure silver coated usually with 

 the violet subchloride of silver and plunged into a conducting fluid 

 composed of two parts of monohydrated sulphuric acid in 100 parts 

 of water. The apparatus was so arranged that all light was excluded, 

 except from an adjustable opening on one side by which one of the plates 

 could be exposed to light while the other remained in darkness. 



When diffused daylight or sunshine acted upon one of the plates, 

 more or less deviation of the needle was observed which remained constant 

 so long as the light remained of the same intensity and the surface was 

 sufficiently sensitive. If the light was shut off, the needle returned to zero 

 or somewhat beyond it, but soon regained its original position. If the 

 light remained of the same intensity and the plate was again exposed, the 

 electrical effect was the same as before, always provided that the sensi- 

 bility of the plate remained the same, for which purpose the sensitive 

 coating should be sufficiently thick. Under favourable conditions the 

 sensibility of the instrument might be preserved for a whole day and 

 thus several consecutive observations might be made. 



The deflections of the needle could not, however, be considered as 



