8 A Study of the Selenum Cell. 
ew) 
a selenide, it follows that there is an actual motion of. the 
components of the selenide towards the electrodes of the cell. 
Any cause which will increase the velocity of these com- 
ponents will decrease the resistance of the cell. Selenium is 
known to exist in at least four allotropic modifications *, the 
metallic or crystalline form being represented in the selenium 
cell. As it is an established fact that light affects the 
character of certain crystalline compounds, it is not un- 
reasonable to suppose that light, in falling upon selenium, 
might also change its crystalline character, and that this 
new modification might offer less resistance to the com- 
ponents of the selenide as they wander towards the electrodes, 
thereby producing indirectly an increase in their velocities, 
which is equivalent to a decrease in the resistance of the 
cell. This view gains in plausibility if, with Bidwell, we 
think of the particles of selenide as being packed in between 
the particles of selenium. Assuming that this new modifica- 
tion of selenium is stable only in light, it would revert to its 
original condition when light is cut off, the change taking 
place more rapidly at first and more slowly atterwards 
(comparable perhaps to the molecular changes in soft iron 
when the magnetizing force has been withdrawn). This 
would decrease the velocity of the components of the selenide, 
which would mean eventually bringing the resistance of the 
cell back to its original value. 
An explanation of this character has the following ad- 
vantages :— 
1. It ascribes, with Bidwell, the electrical conduction to 
the selenide. | 
2. It assigns a definite rdle to the free selenium. 
3. It accounts for the fact that light produces changes in 
the resistance of a cell whether a current be flowing or not. 
4. It affords a possible explanation of the fact that the 
position of maximum sensibility is independent of the metal 
in the selenide. 
Of course, quite a number of other explanations are con- 
ceivable. It appears to me, however, that, as soon as the 
action of light directly upon the selenide is involved, some 
further hypotheses must be made to account for the fact that 
the position of maximum sensibility is independent of the 
nature of the selenide. The reason is at once apparent if 
the action initially be ascribed to the selenium itself, which 
is present in such abundance in all cases. 
* A. P. Saunders, Journal of Phys. Chemistry, vol. iv. p. 428. 
