1893.] Waterhouse —Electrical action of Light upon Silver. 23 



light, and there was a perceptible deposit of chloride (?) on their 

 immersed surfaces. 



With plates immersed in a 1 per cent, solution of sulphuric acid in 

 distilled water, it was found that if the plates were exposed to sunshine 

 a very short time after being immersed in the dilute acid, they were 

 at first negative and fairly sensitive to light but afterwards became 

 positive ; whereas in a case when the plates were left standing for 24 

 hours to reduce the polarisation, they were positive, and much less Sen- 

 sitive than the plates which were negative. After a short time they 

 seemed to lose all sensitiveness. 



In tap water containing the same proportion of acid, the exposed 

 plates were generally positive on opening the shutter ; but the current 

 quickly decreased, and with some plates after several exposures they 

 gave a negative deflection. 



With plates immersed in distilled water containing two per cent, 

 of acid the deflections were usually positive and the plates seemed to 

 become less sensitive by repeated exposure and by keeping. 



With plates immersed in dilute acid at 5 per cent., which had been 

 allowed to stand for 24 hours, and showed a very small cell-current, 

 the first plate of the pair was distinctly negative when exposed, 

 the deflection being —4, decreasing with exposure to — 2, but the second 

 plate when exposed after reversal of the plates in the cell was positive 

 with a deflection of +6. Two other pairs of plates in freshly-mixed acid 

 were positive on first and subsequent exposures. The addition of acid 

 lowered the sensitiveness of the plates considerably. 



All the plates showed a slight grey deposit or stain on the im- 

 mersed ends, but no trace of an image. 



Dilute Nitric Acid. 



With nitric acid the exposed plates are nearly always positive and 

 the action is far more uniform than with sulphuric acid, especially 

 when an appreciable quantity of acid, as one per cent, and over, is 

 used. Becquerel also found the exposed silver plate positive in dilute 

 nitric acid. 



With distilled water acidified with about 1 drop of acid in 70 cc, 

 the exposed plate was positive on first exposure, but afterwards 

 became negative. The plates were not very sensitive, the deflections 

 without the directing magnet varying from 1 to 3 divisions of the scale. 



With 1 per cent, of nitric acid, sp. g. 1*250, in distilled water, after 

 14 hours standing, the exposed plate was uniformly positive, and more 

 sensitive than with the acidulated water, the deflections in sunshine 

 being from 3*5 to 5*5 divisions, without the directing magnet. There 



