of Selenium and Sulphur to Light. 189 



All these cells resemble selenium in giving pplarization- 

 currents after being detached from the battery. 



Supposing it to be true, then, that it is not in the selenium 

 or sulphur itself, but in certain metallic selenides or sulphides, 

 that the sensitiveness to light is resident, does it become easier 

 to explain the phenomenon, or, rather, to deprive it of the 

 unique position which it has hitherto appeared to hold, and 

 assign to it a place among a class of analogous effects? 



I believe that, at all events in the case of the sulphur-silver 

 cell, it is principally at the surface of the electrodes that the 

 effecis of radiation are to be looked for. 



If a current of electricity is passed through a mass of 

 sulphide of silver having silver electrodes, silver will be 

 deposited upon the cathode and sulphur upon the anode. The 

 accumulation of silver upon the cathode will clearly produce 

 no appreciable effect upon the conductivity of the arrange- 

 ment, and need not be considered. But sulphur has an enor- 

 mously high resistance, and the deposition of a mere film of 

 free sulphur upon the anode would be sufficient to stop the 

 current altogether. The current is not in fact stopped, because 

 the deposited sulphur at once combines with the silver of the 

 anode, merely adding a new layer to the electrolyte. Thus 

 the metal of the anode gradually combines with the sulphur 

 of the electrolyte ; and the conductivity of the arrangement 

 will depend to a great extent upon the facility with which this 

 combination is elfected, the quantity of electricity which can 

 pass in a given time being limited by the quantity of sulphur 

 which is capable of uniting with the electrode in the same 

 time. 



Sulphur combines with silver far more readily than with 

 iron. If, therefore, my views are correct, we should expect 

 a cell with an iron anode to offer a much greater resistance 

 than one which had an anode of silver, the material of the 

 cathode, so long as it was a good elementary conductor of 

 electricity, being of comparatively little importance. To test 

 this idea, a cell was made consisting of electrodes of iron and 

 silver imbedded in a mixture of sulphide of silver and sulphur. 

 The cell being connected w T ith a battery and a galvanometer, 

 the deflection was 115 divisions when the current passed from 

 the silver to the iron through the electrolyte, and only 4 divi- 

 sions when the direction of the current was from iron to silver*. 

 The resistance was therefore nearly 30 times as great with an 



* The resistance of the galvanometer was 3483 ohms, and it was shunted 

 with a coil of 20 ohms. The resistance of the Leclanche* cell was about 

 5 ohms. 



