of Sihrr h'iI'I of mwf other MdaU to Lif/Jif. 50?> 



benzene, and also gave a distinct yellow image as soon as the sun acted 

 iipon it after some days' exposure, the light being dull till near the end 

 of the period. Another piece of the same foil, heated to redness and 

 exposed at the same time, only showed a very pale yellowing. 



Xylene. — A similar very faint, but browner, change of colour was 

 noticed on a piece of the same foil, not heated, exposed for several days 

 in commercial pure xylene. 



With toluene, similar results were obtained after long exposure. 



Effects, of Light upon other Metals. — A few observations have been made 

 as to the action of light upon other metals, but with the exception of 

 lead none of them have proved very sensitive, and further work is 

 necessary in good weather. 



Gold. — Images developable with mercury have been obtained on 

 gold leaf by prolonged exposiu^es, but scarcely any trace of a develop- 

 able image could be obtained on some highly-polished well-gilt buttons 

 exposed for several days in good sunshine in October. 



Lead Foil. — On pure lead foil I have obtained a very distinct 

 darkened image visible after exposure. Very distinct darkening was 

 also produced on lead foil exposed in pure benzene. 



Copper. — On copper distinctly visible images have been obtainable 

 sometimes, but the metal is not so sensitive as silver to light, though 

 I have readily obtained heat images on it developable by mercury. 

 Pure copper foil exposed in sunshine in pure benzene showed a distinct 

 darkening, but exposed in xylene it did not change coloiu'. 



Nichil, Platinum, Aluminium, Palladium. — Nickel, aluminium, and 

 platinum appear to be quite insensitive to light, but with a small 

 button of palladium, kindh^ lent to me by Mr. T. Bolas, which was 

 exposed for some days under a black paper mask, there appeared a 

 slight but fairly distinct trace of deposit of mercury on the exposed 

 parts. The spot was too small to make sure of. 



Trial icith Bmfgen Bays. — With the kind assistance of Mr. F. H. 

 Glew I was able to make' a few experiments with the Rontgen rays 

 upon silvered glass and pure silver plates, exposed for several hours to 

 the rays, but without any visible or developable effect. 



The above is a summary of my observations in this direction so far as 

 :hey have gone. I hoped to have made them more complete, but the 

 dull winter weather has been very much against such work. I hope, 

 however, to go on ^\^th it during the summer, but think it advisable 

 to publish the results already obtained now, in order that others may 

 be able to extend them at the same time. They show, I think, that 

 most of the phenomena that occur by the exposure of ordinary pho- 

 tographic plates containing haloid compounds of silver can be 

 observed upon a plain silver plate exposed to light in the air under 

 ordinary conditions. It seems not impossible, therefore, that the key 

 .0 the hitherto unsolved problem of the production and constitution of 



