Electrical Properties of Selenium. 241 



interior. The cell was unfortunately destroyed before it 

 occurred to me to use it for an experiment like the following. 

 Exp. 10. — A cell made in 1881 had then a resistance of 

 about 100,000 ohms and was very sensitive to light. In 

 1895 its resistance had fallen to 10 ohms and it was quite 

 insensitive. This was connected with a storage-battery of 

 26 cells, a detector-galvanometer being included in the 

 circuit. When the current was turned on, the galvanometer- 

 needle underwent a momentary strong deflexion and imme- 

 diately returned to zero. The dark resistance of the cell was 

 then found to have risen to 60,000 ohms, and it was once 

 more sensitive to light. Clearly a short circuit caused by the 

 selenide had been burnt out. 



§ 8. Action of Light in promoting the Formation of 



Selenide s. 



Several attempts to ascertain whether the union of Se with 

 a metal was assisted by illumination resulted indecisively. 

 The following simple experiments are however, I think, fairly 

 conclusive. 



Exp. 11. — One side of a thick plate of copper was evenly 

 coated with red amorphous Se by causing the fumes of boiling 

 Se to play upon it. Half the plate was then covered and the 

 other half exposed, first to diffused daylight and afterwards 

 to sunlight. In diffused daylight the exposed half was per- 

 ceptibly darkened in the course of two or three hours ; in 

 sunlight it quickly assumed a dark chocolate colour, the 

 screened portion remaining bright red. 



Exp. 12. — Two pieces of mica were similarly treated, but 

 the exposed portions were not perceptibly darkened by pro- 

 longed exposure to sunlight. They became, however, covered 

 with a slight " bloom," like that seen on a ripe plum, which 

 I am inclined to attribute to the formation of an oxide. 



Exp. 13. — A brass plate coated with red Se was exposed 

 to sunlight in a photographic printing-frame under a design 

 cut out of black paper and intended to represent a butterfly. 

 A perfectly sharp and intense print of the design was pro- 

 duced upon the red Se. This was made on April 17 last : it 

 has since been kept in the dark and is now exhibited to the 

 Society. 



The chocolate colour is closely matched by that of a mix- 

 ture of red precipitated Se and cuprous selenide in about 

 equal parts. 



