28 Watcrhouse — Electrical action of Light upon Silver. [No 1, 



Potassium Cyanide. 



With a solution of potassium cyanide in distilled water, about 1 

 per cent., there was a strong negative polarisation current in the cell 

 when first prepared, which took several hours to subside. When fresh- 

 ly immersed the exposed plate was negative, but not very sensitive, 

 the deflection without the magnet being only — 4 divisions for sunlight. 

 By keeping the cell 24 hours the polarisation current subsided entirely, 

 and the plate exposed to sunshine was again negative with a deflection 

 of — 3'5 divisions. A slight movement of the cell, however, seemed to 

 cause a reversal of the current with a deflection of + 6 - 5 divisions 

 which further continued for another 6'5 divisions after the light had 

 been shut off. The same effect was observed on subsequent exposures, 

 first the plate was negative then positive, while the polarisation current 

 increased in the same direction. After a time the plate seemed to 

 become quite insensitive to light. 



With the same plates reversed there was again a very large initial 

 negative polarisation current. On firsfc exposure of a plate to sunshine 

 the deflection was — 14'4 divisions, without the magnet. On shutting 

 off the light, the negative polarisation current was found to have very 

 largely increased. On second exposure the plate was first negative and 

 then positive. On shutting off the light the current continued 4-25 

 divisions in the same direction and then turned back in its original 

 direction. The plates were coated with a dark grey deposit, thicker at 

 the upper part of the plates than at the lower. About the immersion 

 line there was a yellowish-white deposit, and the plates were deeply 

 corroded, but no sign of an image of the exposed part was visible. 



From the above experiments it would appear that as a general 

 rule sunlight has an oxidising or dissolving effect on silver, whether in 

 acid or alkaline solations, the exposed plates being nearly always posi- 

 tive and consequently forming the anode of the voltaic couple. With 

 solutions decomposed by silver and forming sensitive compounds the 

 action is variable. 



IV. Plain Silver plates dry. 

 When a comparatively large silver plate about 5x4 inches, not 

 immersed in any solution, but with its ends connected by silver bands to 

 the terminals of the galvanometer, the directing magnet being specially 

 placed so as to increase the normal sensitiveness about 13 times, was 

 exposed to light so that the upper half remained unexposed, it was 

 found possible to detect a slight current between the exposed and un- 

 exposed halves of the plate ; the exposed half being positive to the 

 unexposed. With an uncleaned plate that had lain in a drawer for 



