of Silver and of some other Metals to Light. 491 



exposed in the same way to light and the action was as plain as on the 

 silver, if the glass were afterwards breathed upon, the image remaining 

 visible for a long time afterwards. We may therefore assume that 

 light ads on all bodies, and its influence may be tested by all vapours that 

 adliere to the siihstance or act chemiccdly upon it." 



Robert Hunt 's Views. — Although Eobert Hunt recorded these experi- 

 ments in his ' Eesearches on Light,' he does not seem to have repeated 

 them as regards the direct action of light upon metallic silver, but to 

 have paid more attention to Moser's experiments on the images pro- 

 duced by contact or proximity of dissimilar substances, and his theory 

 of invisible light, as well as the effects of heat. Hunt's own experi- 

 ments were chiefly carried out on copper plates, and led him to 

 attribute JMoser's results to calorific or thermic radiations rather than 

 to light. Knorr, Karsten, Grove, and others seem also to have 

 investigated Moser's theories, but again without taking notice of the 

 fact of images, either directly visible or developable, being produced 

 on metallic silver by the direct action of light. 



I have not been able to find a record of a visible action of light 

 upon ordinary silver plate, though its occurrence should be well 

 known to silversmiths. 



Carey LecCs Observations. — Carey Lea found that the three forms of 

 allotropic silver he obtained were all sensitive to light : A the red 

 soluble, and B the dark brown or blue insoluble variety, becoming 

 brown after some hours' exposure, while C, the golden coloured, 

 became lighter by exposure."^' 



Electrolytically Deposited Silver. — Li a series of electrolytic experi- 

 ments made in Calcutta, in 1892, I found that a golden-yellow or light 

 olive-coloured deposit of silver on the cathode plate (silver or platinum) 

 of a decomposition cell formed with two pure silver plates, as 

 anode and cathode, or with a silver anode and platinum cathodQ, 

 in distilled water, through which a weak current was passed, was 

 slightly sensitive to light and became lighter in colour by exposure. 

 This seems to confirm Carey Lea's observation, if m.y golden-yellow 

 silver deposit was analogous to his C-product. My deposit being 

 made by electrolysis of pure silver in fairly pure distilled water, 

 must have been nearly pure silver with no traces of foreign sub- 

 stances beyond those contained in the silver or the water, unless 

 there was a small amount of occluded hydrogen, which other experi- 

 ments of the same series, but with a stronger current, showed might 

 not be impossible. 



Photo-electrical Observations in Calcutta. — In another series of observa- 

 tions on the electrical action of light upon silver made in Calcutta 

 about the same time, and published in the ' Journal of the Asiatic 

 Society of Bengal,' Part II, No. 1, for 1893, I found that the action of 



* 'PM. Mag.,' Ser. 5, vol. 32, p. 337 (1891). 



