22 Waterhouse — Electrical action of Light upon Silver. [No 1, 



sure to daylight, the exposed plate was also negative. When exposed 

 to sunshine the plates were almost invariably positive. I have noticed 

 this difference with plates in other solutions. 



II. Silver plates in dilute Acids. 



As we have seen above, Becquerel found that with plates of gold 

 or platinum, immersed in acid solutions, the plate exposed to the li^ht 

 was always positive. The same rule seems to apply to silver plates 

 in most cases, but not in all. 



Dilute Sulphuric Acid. 

 The action of dilute sulphuric acid upon silver plates under the 

 influence of light seems to be rather irregular, but I find on looking 

 through all the experiments made, that in nearly all cases the first 

 exposed plate of each pair had a negative tendency when first exposed, 

 though it might become positive by subsequent exposures and in the 

 same way the second plate of the pair, which was screened during the 

 first exposure, might also be positive on first exposure. The general 

 tendency was undoubtedly positive. The irregularities may be partly 

 due to the plates not being quite pure. 



With silver plates immersed in distilled water acidified with about 

 a drop of acid in some 60 cc. of water, the exposed plate was generally 

 positive when exposed to bright sunlight, the deflection without the 

 directing magnet varying from 1 to 4 "5 divisions of the scale, some- 

 times increasing after repeated exposures. In one experiment, however, 

 the exposed plate was distinctly and uniformly negative ; even after 

 the position of the plates had been reversed, but subsequent exposure 

 of the reversed plate made it positive. In another it was negative on 

 first exposure and then positive. 



With a pair of plates in tap water, acidified in the same way, the 

 plate exposed to sunshine was first negative with a deflection of — 3 

 divisions on the scale, which increased to — 6 divisions by subsequent 

 exposures. Exposure under coloured glasses also gave a negative de- 

 flection, amounting with red glass to — 1, with yellow and green glasses 

 to — 2; with blue glass to — 5, and exposed to sunshine again — 6, as 

 before. The same plates being again exposed to sunshine later on 

 were also negative at first, but became positive and much more sensi- 

 tive. Under coloured glasses the deflections were also positive and very 

 much larger than on the first exposure of the plate. After reversal, 

 so that the former unexposed plate became the exposed plate, the 

 deflection was again negative, amounting to — 7 divisions, and increasing 

 with the exposure, These plates were very sensitive to changes in 



