246 



Mr. Sbelford Bid well on the 



were made of its resistance both in darkness and under 

 illumination by an 8 candle-power lamp at 30 centim. The 

 results are given in Table III. 



Table III. 

 Selenium Cell in Receiver over H 2 S0 4 . 





Time. 

 Hours. 



Resistance. 

 Dark. 



Resistance. 

 Illuminated. 



Percentage 

 Decrease. 







28 



50,400 

 83,000 



25,400 

 35,500 



495 

 57 2 



The dark resistance of the cell, it will be seen, increased in 

 28 hours from 50,400 to 83,000 ohms : if, therefore, the water 

 which it originally contained acted like a metallic shunt, its 

 resistance taken alone must have been about 128,000 ohms. 

 Now it was found that a thin plate of Se, after having been 

 dried over sulphuric acid for 22 hours, lost -g-gV o P ar ^ °^ ^ s 

 weight. The weight of the Se in the cell used in the present 

 experiment was about 0'4 gramme ; probably, therefore, it 

 originally contained about y ^ o g ram nie of water. Knowing 

 the dimensions of the cell (which are given above), it is easily 

 calculated that if this quantity of water when uniformly dis- 

 tributed between the electrodes of the cell offered a resistance 

 of 128,000 ohms, its specific resistance must have been 

 approximately 7800 ohms. This is of the same order of 

 magnitude as the specific resistance of good distilled water, 

 though that of purer water is of course much higher. We 

 may fairly conclude, then, that absorbed moisture plays no 

 active part in diminishing the dark resistance of Se, but 

 behaves merely as if it were an inert shunt. 



As regards sensitiveness, moisture might conceivably exert 

 one of four different influences. If it acted simply as a 

 shunt, its removal ought to increase the sensitiveness of the 

 cell in an easily calculable degree. If its presence were 

 essential to the chemical or physical change which is effected 

 by the agency of light, then a perfectly dry cell would 

 also be perfectly insensitive. If moisture, without being in- 

 dispensable, were yet in some degree favourable to the 

 operation of light, the sensitiveness of the cell, though it 

 might possibly be increased when the water was withdrawn, 

 would still not be so great as if a metallic shunt of equal 

 resistance had been cut out. Lastly, if the influence of 



