246 Mr. M. Carey Lea on 



5. Action of Light. 



When allotropic silver is spread as a thin film on glass or 

 on pure paper, it may be preserved for a length of time 

 apparently unchanged. This appearance is deceptive. From 

 the moment that the film is formed, a slow but steady change 

 commences, which can be best explained by supposing that a 

 gradual polymerization takes place. Even after eight or ten 

 hours : exposure to ordinary diffuse light, a distinct loss of 

 activity can be detected by careful testing with potassium 

 ferricyanide. The change which occurs is in the nature of a 

 tendency to a very gradual passage into what I have called 

 the intermediate form, in which the gold-yellow colour 

 remains unchanged whilst the chemical activity is lost or 

 much diminished. Although a commencement of this change 

 can be detected in a few hours, it goes on very slowly. By 

 exposure to one or two days of summer sunshine (a much 

 longer time is required in winter) , the change is nearly com- 

 plete. The exposed portions are lighter and brighter, and in 

 solution of ferricyanide they colour very slowly. 



The question naturally arose whether light, by a sufficiently 

 long continued exposure, could complete the change and 

 convert allotropic to ordinary white silver. To obtain a 

 decisive answer the following experiment was made : — 



At a window having a south-eastern exposure and un- 

 obstructed light, there were placed films on glass and on pure 

 paper. Some of these were placed in a printing-frame under 

 an opaque design ; the others stood side by side with the first, 

 but uncovered. The exposure was continued for four months, 

 from the end of January to the end of May. At the expira- 

 tion of this time the uncovered paper and glass films were 

 still bright yellow. But of those in the printing-frame the 

 exposed portions had become nearly wdiite, whilst the pro- 

 tected parts retained their full deep gold colour. It may at 

 first seem strange that" the uncovered pieces were less affected 

 than those exposed in the frame. But this difference was 

 always observed, namely, that if two films were exposed side 

 by side, the one in a printing-frame under glass, the other 

 simply fastened to a board, the last mentioned was always 

 the less affected. The explanation of this seeming anomaly 

 lies in the fact that allotropic silver is always much more 

 easily affected by heat than by light. The glass in the 

 printing-frame, by exposure to sunlight, becomes hot to the 

 touch, and thus the film under it is kept at a temperature 

 many degrees higher than that of the other film that is freely 



