26 Mr. A. H. Pfund: A Study 
IV. A Study of the Selenium Cell. By A. H. Prunp *. 
NHE theory accepted to-day in explanation of the fact 
that a selenium cell changes its resistance when exposed 
to radiation, is due to Bidwell. Briefly, this theory attributes 
the sensibility of a cell to the presence of selenides from 
which selenium apparently never is free. The facts in support 
of this view are :— 
1. The conduction of a cell is electrolytic in character {. 
2. Selenium is a very poor conductor, while a selenide is, 
comparatively, a good one. 
3. Sunlight actually brings about a combination of metal 
and selenium, forming a stable selenide, capable of 
conducting an electric current tf. 
In enlarging upon his theory, Bidwell § says :—“ If, as is 
commonly believed, electrolytic conduction involves a series 
of decompositions and recompositions throughout the electro- 
lyte, any cause which will assist either the separation or 
recomposition (or both) of the components of the electrolyte 
might be expected to increase its conductivity ; and it seems 
reasonable to suppose that the same influence which would 
assist the union of two substances when they have a ten- 
dency to unite would also be favourable te their separation 
when they have a tendency to separate. Itis not impossible, 
therefore, that radiation, acting upon the surface of a thin 
layer of selenide of silver through which an electric current 
is passing, might, by facilitating the molecular rearrangement 
of the atoms of selenium and silver, exert a material influence 
upon the conductivity.” 
As, in the above theory, the phenomenon is ascribed entirely 
to the selenide, it seemed worthy of interest to undertake 
experiments with cells containing different selenides. In the 
course of the work it soon appeared that the sensibility of the 
selenium cell reaches a maximum in the visible spectrum, and 
it was thought that the position of the maximum might be a 
function of the metal in the selenide. While it is possible, it 
is hardly probable, that light from the same region of the 
spectrum which is most favourable to the combination between 
copper and selenium, will also be most favourable to the com- 
bination between mercury, silver, lead, and other metals with 
selenium. 
Furthermore, in consequence of Bidwell’s statement that 
* Communicated by Prof. Benjamin W. Snow. 
+ Adams and Day, Proc. Roy. Soc. 1876, p. 118. 
{ Shelford Bidwell, Phil. Mag. vol. xl. p. 233 (1898). 
§ Shelford Bidwell, Phil. Mag. vol. xx. p. 191 (1885). 
