1893.] Waterhouse — Electrical action of Light upon Silver. 25 



sitiveness very much, the highest deflection in sunshine, without the 

 directing magnet, being + 16, while after the plates had stood for 37 

 hours it was only •+■ 11. 



There was a dark grey deposit of chloride on the immersed parts of 

 the plates, which took a violet or purple colour on exposure to light, 

 and gave off an odour of chlorine. 



Dilute Hydrobromic Acid. 



With dilute hydrobromic acid containing 10 cc. of the ordinary 

 pharmaceutical dilute acid, of 10 per cent., to 100 cc. distilled water, the 

 plate exposed to sunshine or diffused daylight was uniformly negative 

 and extremely sensitive to light, the first deflection in bright sunshine 

 being about — 187 divisions, without the directing magnet; decreasing 

 to a steady reading of about 140 divisions. Even coloured glasses 

 gave fairly large deflections ; red, — 13 ; yellow, — 54 ; green, — 64 ; 

 blue, - 103. 



With dilute acid of double the above strength, the exposed plate 

 was also uniformly negative, but the plates did not seem so sensitive, 

 the deflection in sunshine, without the magnet, being only — 82 ; but 

 the readings depend very much on the strength of the light, and this 

 was variable at the time of observing. 



In both these cases the plates were coated with a grey-greenish- 

 yellow deposit of bromide, which turned dark on exposure, and formed 

 a visible image of the exposed part of the plate. 



Dilute Hydriodic Acid. 



As pure hydriodic acid is somewhat troublesome to prepare, I 

 roughly made up a solution of it by precipitating one gramme of barium 

 iodide, dissolved in water, with sulphuric acid and adding water to make 

 up 100 cc. There was, however, a considerable quantity of free iodine 

 present, the solution being of a light sherry colour. 



The cell containing two clean silver plates immersed in this solu- 

 tion was left standing for 15 hours. The plate exposed to sunshine 

 was then found strongly negative, the deflection, without the directing 

 magnet, being — 110 divisions of the scale, afterwards going up to 



— 130 divisions. The plate was, very insensitive to weak daylight, 

 the reading being only 12 divisions when the sun was hidden behind 

 clouds. With coloured glasses fairly large deflections were obtained, 

 always in the same negative direction ; red glass giving — 15 with 

 daylight, and — 16 with sunlight ; yellow — 16'5 with daylight, and 



— 20 with sun ; green — 14 with daylight, and — 19 with sun ; blue 



— 16 with daylight, and — 80 with sun. By keeping, the plates 



J. ii. 4 



