Experiments with Selenium Cells. 253 



in the course of a year the resistance of most of the pieces of 

 selenium with which he performed his well-known experi- 

 ments fell very considerably, and it would be of great interest 

 to know what is the cause of this curious phenomenon. In 

 the case of my own cells, I satisfied myself that it certainly 

 was not due to any short-circuiting of the wires by contact ; 

 and I suggest that it may be owing to the presence of an 

 excessive amount of conducting selenide formed by the union 

 of the selenium with the metal of the electrodes. Each wire 

 is covered with a layer of selenide, which gradually increases 

 in thickness at the expense of the free, badly-conducting 

 selenium, and thus the resistance of the cell slowly falls. At 

 length the layers of selenide become so thick that the coatings 

 upon the two wires actually meet and touch each other. 

 Thereupon the cell is short-circuited ; its resistance no longer 

 depends upon the selenium but upon the metallic selenide, 

 which is a good conductor. 



I have in a former paper (Proc, Phys. Soc. vol. vii. p. 129 ; 

 Phil. Mag. Aug. 1885, p. 178) given reasons for believing 

 that the presence of a certain amount of metallic selenide is 

 essential to the sensitiveness of a cell, and I have stated my 

 opinion that the true function of the prolonged heating, 

 generally termed " annealing, " is the formation of this neces- 

 sary selenide, which, when a current passes through the cell, 

 is electrolysed, selenium being deposited upon the anode wire. 

 With the weak currents generally used, the quantity of 

 deposited selenium would be very small, and would at once 

 unite with the metal of the electrode. But if a sufficiently 

 strong current were passed, it is conceivable that the 

 selenium might be separated in quantities too great to be 

 disposed of in this manner. Then the free selenium around 

 the electrode ought to be visible. This view is confirmed by 

 a recent experiment. A cell was made last October with new 

 selenium and the usual copper-wire electrodes. It was well 

 annealed, and its resistance was about 50,000 ohms. The 

 cell was connected to a 26-volt battery, and left for two days. 

 At the end of that time the cell presented a remarkable 

 appearance, those portions of the anode wire where the 

 selenium coating happened to be thin being covered with red 

 feathery tufts, some of which had dropped off and fallen 

 upon the paper on which the cell' was lying. Ordinary tests 

 showed that the red stuff consisted, mainly at all events, of 

 red amorphous selenium. But it was clearly moist. All 

 very fine powders seem to attract moisture from the atmo- 

 sphere ; but in the present case the moisture appeared to be 

 oreater than could be thus accounted for. An examination 



