M. C.Lea — Gold-colored Allotropic Silver. 187 



The question naturally arose whether light by a sufficiently 

 long continued exposure could complete the change and con- 

 vert allotropic to ordinary white silver. To obtain a decisive 

 answer the following experiment was made : 



At a window having a southeastern exposure and unob- 

 structed 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 white, while the protected parts 

 retained their full deep gold color. 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 print- 

 ing frame under glass, the other simply fastened to a board, 

 the last mentioned was always the less affected. The explana- 

 tion 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 exposed to the air ; the higher temperature aids 

 the effect of the light.* 



It appears, therefore, that the agency of light is somewhat 

 similar to that of the other forms of energy but very much 

 slower. Experiments made for the purpose demonstrated that 

 it is the more refrangible rays that effect the change. 



With light, the production of the intermediate form is never 

 very satisfactory. Long exposures are needed, and before the 

 change to the intermediate form is complete, the further alter- 

 ation to white seems to commence. With heat the changes are 

 much better marked. 



When a film on pure paper has received an exposure of one 

 or two days of summer sunshine under an opaque design, the 

 exposed portions are sufficiently changed to have lost much of 

 their chemical activity, so that when the film is plunged into a 

 bath of potassium ferricyanide, the effect given in the lower 

 figure of Plate III is obtained. The color represented is one 

 of an immense variety of tints produced by this reagent on the 



* Since this was written I find that both Herschel and Hunt noticed an analo- 

 gous fact in the case of silver chloride, viz: that paper prepared with it darkens 

 more rapidly under glass than when freely exposed ; without, however, suggest- 

 ing the cause, which is the same in both cases. I have observed that silver 

 chloride darkens more rapidly when exposed under warm water than under cold 

 to the same light in vessels side by side. 



