190 Sensitiveness of Selenium and Sulphur to Light. 



iron anode as with a silver anode. It is clear tliat this was 

 not the result of bad contact between the iron and the elec- 

 trolyte (such as was supposed by Graham Bell in the analogous 

 case of selenium to account for the high resistance of a cell 

 with platinum electrodes as compared w r ith one in which the 

 electrodes were made of brass), because such an effect would 

 be independent of the direction of the current. Rather it 

 seems that the resistances of the two anodes afford data for 

 measuring the relative facilities with which sulphur combines 

 with silver and with iron. 



Assuming it to be thus experimentally proved that the re- 

 sistance of a sulphur-silver cell depends largely upon the 

 readiness with which sulphur unites with the anode, it follows 

 that any cause which would assist this union would at the same 

 time diminish the resistance. Now it is well known that certain 

 chemical combinations are accelerated by the action of radia- 

 tion — the explosive union of chlorine with hydrogen under the 

 influence of sunlight being a familiar example. The question 

 then suggests itself, Does sulphur combine with silver more 

 readily when exposed to radiation than it otherwise would ? 

 There is, I believe, direct evidence that it does. 



A glass plate, covered with silver leaf, was placed, with the 

 silvered side downwards, over a crucible of boiling sulphur. 

 One half of the plate was covered with a piece of black cloth, 

 and the arrangement was exposed to bright sunshine. In a 

 short time the visible portion of the silver was darkened, 

 owing to its partial conversion into sulphide ; the cloth was 

 then removed, and the silver beneath it was found to be 

 scarcely discoloured. There was a distinct line of demarcation 

 between the two halves. The experiment was repeated with 

 the same result. 



Since this effect might possibly have been due to other 

 causes than the action of light (such as the unequal conden- 

 sation of sulphur vapour upon the covered and uncovered 

 portions of the plate), the experiment was made in another 

 form. A piece of silver leaf attached to glass was brushed 

 over with a solution of sulphur in bisulphide of carbon ; and 

 in order to keep the temperature low and uniform, the silvered 

 glass plate was immersed in a basin containing cold water, 

 which was placed in the sunshine. A board was laid across the 

 top of the basin so as to shade one half of the plate, the other 

 half being exposed to the direct rays of the sun. In a quarter of 

 an hour the exposed portion of the silver had acquired a dark 

 brown colour, while that which had been protected was of a 

 pale yellow tint, the outline of the shadow of the board being 

 sharply defined. I think we have here the strongest evidence 



