M. Carey Lea — Action of Light on Allotropic Silver. 129 



apophyllite may have been formed from it by a partial solu- 

 tion and redeposition of its material through the agency of 

 waters containing alkaline fluorides. The gyrolite is obviously 

 not absolutely pure, but it agrees approximately with the 

 formula Ca 2 Si 3 8 . 3H 2 0. Its chemical structure and relations 

 to apophyllite are not altogether clear. 



Washington, May 24, 1889. 



Art. XYJ. — On Action of Light on Allotropic Silver ; by 

 M. Carey Lea. 



Since my last communication to this Journal I have obtained 

 the following results : 



1. It was mentioned in that paper that the red gold-colored 

 modification of silver was converted into a bright yellow- 

 colored form by the action of light. Continued exposure 

 seems to produce little further change so long as the substance 

 is dry. But if the paper on which the silver is extended, is 

 kept moist by a wet pad, with three or four days of good sun- 

 shine the change goes on until the silver becomes perfectly 

 white, is apparently changed to normal silver :* — water, alone, 

 tends to darken this form of allotropic silver, accordingly the 

 portion of the paper that was protected for comparison, dark- 

 ened, showing that the whitening effect was due wholly to 

 light. 



It thus appears that light can convert yellow or red-yellow 

 allotropic silver to white. 



2. Some pieces of very bright blue green modification were 

 exposed to light and with about one day's bright sunshine, 

 they passed to a pure bright metallic gold-color. 



It appears therefore that light can cause the blue-green 

 modification to pass to the gold-yellow. 



This change only occurs with a very brilliant form of the 

 bluish-green substance which is obtained with a quick short 

 washing. Specimens slowly and very thoroughly washed 

 which when brushed over paper gave a more mat color, did 

 not yield this result but became brownish, as described in 

 the July number of this -Journal. Nor can this result be 

 obtained with the soluble form of allotropic silver described in 

 the June number of this Journal. 



Light therefore can change the bluish green to the yellow 

 modification, and this last (with the aid of moisture) to white 

 normal silver. The silver thus obtained is pure white, lustrous 

 and metallic, resembling silver leaf. Organic compounds of 

 silver reduced by light give gray or black silver devoid of luster. 



* The pad used was of unbleached muslin which was boiled several times with 

 distilled water to remove everything soluble before use. 



