of Silver and of some other Metals to Light. 



'501 



mask, only one day longer than the last plate, gave a very . strong 

 visible image requiring no development. There must, therefore, have 

 heen a considerable difference in the conditions of the surfaces of the 

 two plates. 



In the plate which was heated and then treated with dilute sulphuric 

 acid, it may be assumed that any surface layer of condensed gases 

 must have, been destroyed and a surface of pui'e metal exposed — which, 

 as one would expect, was not visibly sensitive to light. In the other 

 case, in which the plate was probably oxidised (it had been lying by 

 for several years) and was simply well cleaned with a dry polishing 

 powder, the surface of the metal was left so sensitive to light as to 

 give a strong visible image. This difference of action in two plates 

 which were exposed to the same conditions of light, seems to show 

 that the effects produced by light on surfaces of metallic silver are 

 chemical rather than merely molecular in their nature. 



A repetition of this observation gave similar results. Further, a 

 plate of thin silver foil, well heated over the spirit-lamp and exposed 

 under a mica screen, which was also heated, for two days, including 

 some hours of sunshine^ showed no image, even on development with 

 mercury, although distinctly visible images had been obtained on the 

 same piece of foil when cleaned in the ordinary way and exposed 

 under the same or similar screens. Further experiment on this point 

 is still necessary. 



Effects on Silver Surfaces fumed with Acid and other Vapours. — 'A 

 good many experiments have been made \vith silvered glass plates or 

 silver foil, fumed with various vapours, which might possibly be 

 present in small quantities during the exposure to light, among them 

 hydrogen peroxide, nitric acid, ammonia, also sulphurous acid; these 

 plates have all shown visible and developable images of a somewhat 

 similar character to those formed on the plain metal. 



Nitric Acid. — By fuming silvered glass plates with ordinary pure 

 nitric acid (1-420) or, better, with the same acid diluted with an equal 

 volume of water, very clear positive printed-out images have been 

 obtained, even with the comparatively short exposure of one hour in 

 the February sun. With a longer exposure the image was clearer, the 

 exposed parts appearing lighter than the protected parts, as is also the 

 case with the plain silver surfaces. 



Dilute Ammonia. — Dilute ammonia containing one part of the solu- 

 tion at 0'880 in 30 of water, gave a similar image, but the film was 

 not so sensitive as with the nitric acid. Further experiments have 

 however, to be made. 



Dilute Sulphurous Acid. — Dilute sulphurous acid, formed by acidify- 

 ing a solution of sodium sulphite with sulphuric acid, gave a weak 

 image of much the same character. 



Hydrogen Peroxide Solution. — Silvered glass plates, fumed with 



