TRANSACTIONS OF SECTION B. 981 
5. On the Sensitiveness to Light of Selenium and Sulphur Cells.’ 
By Suetrord Browse, M.A., LL.B. 
The fact first announced by Mr. Willougby Smith in 1873 that the electrical 
resistance of crystalline selenium is temporarily diminished by the action of light 
has been fully confirmed by subsequent experimenters. Of the many investigations 
which have been undertaken in reference to this subject, by far the most valuable 
and exhaustive are those of Professor Adams and Mr. R. E. Day, an account of 
which was published in the ‘ Philosophical Transactions’ for 1877. These gentle- 
men arrived at the conclusion that the diminution in the resistance of selenium 
might be accounted for by the fact that light promotes crystallisation, for in 
changing to the crystalline state selenium becomes a better conductor of electricity. 
They also state their belief that selenium conducts electrolytically ; but it may be 
inferred from the paper—and was indeed explained by Professor Adams himself at 
the meeting of the Physical Society on June 13 last—that the authors did not 
Suppose that actual electrolysis occurred, but rather that the molecular structure or 
crystalline condition of the substance was altered or modified by the action of a 
current of electricity in such a manner as to produce effects analogous to those 
which would have occurred if the selenium were an electrolyte, and actually 
decomposed by the current. 
A new form of selenium cell has recently been described by Mr. C. E. Fritts, of 
New York. A thin film of selenium is spread upon a plate of some metal, such as 
brass or copper, with which it will form a chemical combination. The selenium is 
melted and crystallised under pressure, and, when cold, its surface is covered with 
a film of gold-leaf sufficiently thin to transmit light. The metal plate and the gold- 
leaf form the two electrodes of the cell, the resistance of which is yaried by the 
action of the light which passes through the gold-leaf. 
Upon reading this description, it occurred to the author that the conduction of 
selenium, when prepared in the form of cells, might be in reality, and not 
merely in appearance, electrolytic. Selenium will, he believes, combine more or less 
readily with all metals, the combination being assisted by heat. And in the pre- 
paration of selenium cells it has been the usual, if not the universal custom, to 
submit the selenium to prolonged heating while in contact with metallic electrodes. 
This operation is generally called ‘annealing, and the undoubted fact that it 
diminishes the resistance of the selenium and increases its sensitiveness to light has 
been explained by supposing that the process is favourable to perfect crystallisation. 
The author suggests as an alternative explanation that the prolonged heating, by 
promoting the combination of the selenium with the metal of the electrodes, results 
in the formation of a selenide which completely surrounds the electrodes, and is 
perhaps diffused to some extent throughout the selenium when it is in a liquid con- 
dition ; and that the apparently improved conductivity of the selenium, together 
with the electrolytic phenomena which it exhibits are to be accounted for by the 
existence of this selenide. It was found that while the specific resistance of the 
selenium contained in a well-annealed cell having copper electrodes was ‘9 megohm, 
that of a similar piece of selenium annealed in a glass mould without contact with any 
metal was as much as 2,500 megohms. This enormous difference is to be attributed 
to the presence of selenide of copper in the selenium of the cell. 
‘The above hypothesis has not been submitted to the test of direct experiment, 
but certain indirect evidence in support of it has been forthcoming. Selenium is an 
element which, in its properties, closely resembles sulphur, and many unsuccessful 
attempts have been made to develop in sulphur that peculiar sensitiveness to light 
which is such a remarkable characteristic of selenium. It occurred to the author 
that if this property of selenium were really due to the accidental existence of 
metallic selenides, then the admixture with sulphur of metallic sulphides might be 
expected to lead to similar effects. Several cells were therefore constructed, in 
which selenium was replaced by sulphur containing a proportion of silver sulphide, 
* Published in extenso in the Electrician, Sept. 18, 1885, and in the Chemical 
Nens. See also Phil. May., Aug. 1885. 
