24 Watei'house — Electrical action of Light upon Silver. [No. 1, 



was a slight greyish deposit on the plates, but no image on the exposed 

 part. 



With three per cent, of the same acid in distilled water, after 22 

 hours standing, the first plate exposed in weak sunshine first showed 

 a negative deflection of 2 divisions, and after that was positive, the 

 deflection of repeated exposures being steadily about + 5 divisions, 

 without the magnet. With the directing magnet, the deflection was 

 about + 20 divisions with the 100 ohm, or T ^, shunt. 



After the experiment the solution was found to contain silver. 



Dilute Phosphoric Acid. 



With dilute phosphoric acid the deflections were almost always 

 positive. Plates freshly immersed in a mixture containing 1 per cent, 

 of the acid, sp.g.1'750, in distilled water and exposed to sunshine, gave 

 an initial deflection, without the magnet, of + 23 divisions, but this 

 quickly decreased with further exposure. After shutting off the light 

 the cell-current was found to have increased, and on again opening to 

 sunshine the deflection seemed slightly negative, but the action generally 

 was irregular. Subsequent exposures with the same cells or after the 

 plates had been reversed showed positive deflection and the plates were 

 less sensitive than at first. 



With the same acid at 5 per cent, the deflections were uniformly 

 positive. With plates exposed to sunshine after 16 hours, the deflection 

 without the magnet was + 8 divisions, but, as in the former case, it was 

 less on subsequent exposure. The same decrease of sensitiveness after 

 exposure was noticed with the plates after reversal in the cell. 



Dilute Hydrochloric Acid. 



With 1 per cent, of hydrochloric acid, sp. g. 1*150, in distilled water, 

 the exposed plates have shewn themselves uniformly positive, and 

 owing to the formation of a deposit of chloride they are much more 

 sensitive to light, than are plates immersed in acids which do not form 

 a sensitive compound with the silver. The deflections with sunshine, 

 without the directing magnet, were from -f- 6 or + 7, when the plates were 

 first exposed, to + 36, when they had been kept for some hours longer 

 and then exposed. The plates were covered with a greyish deposit of 

 chloride on the immersed parts, and there was a distinct darkened 

 image on the part of the plate exposed to light. Coloured glasses 

 all gave positive deflections, the red being the smallest, and then the 

 green. 



With 3 per cent, acid, after 22 hours resting, the plates exposed to 

 sunshine were positive. The increase of acid seemed to reduce the sen- 



