Electrical Properties of Selenium. 247 



water were positively prejudicial, its removal would result in 

 a greater increase of sensitiveness than if it had been a mere 

 shunt. 



The figures in Table III. seem to show that the presence of 

 water, though not essential to sensitiveness, does to some 

 extent assist the action of light. The percentage decrease of 

 the resistance of the dried Se under illumination is considerably 

 greater than that of the undried, but still it is not so great as 

 it would have been if a shunt of 128,000 ohms (the resistance 

 of the water alone) had been withdrawn. In the latter case 

 the resistance of the dry cell in the light would have been 

 only 31,700 ohms instead of 35,000, and the percentage 

 decrease 61*8 instead of 57*2. 



Exp. 20. — Three cells were at different times constructed, 

 and after being kept for some hours at a temperature of about 

 200° were without any previous cooling transferred as rapidly 

 as possible to the sulphuric acid receiver. All turned out to 

 be sensitive to light while in the dry air. Unless, therefore, 

 Se can condense moisture in its pores at a temperature of 

 200°, which is not probable, its sensitiveness to light is not 

 entirely dependent upon the presence of water. Having 

 regard to the observations detailed in the next section this 

 was contrary to my expectation. 



§ 13. Attempted Electrolysis of Metallic Selenide. 



Many abortive attempts were made to ascertain whether 

 cuprous selenide conducts electricity entirely like a metal, 

 as is generally believed, or like an electrolyte, as required by 

 my hypothesis. The experiments of which an account is 

 given in this section appear to show that if the conduction is 

 not altogether electrolytic, it is under certain conditions, at 

 least partly so. It must, however, be admitted that the results, 

 though of considerable interest, cannot be regarded as con- 

 clusive. The condition requisite for apparent electrolysis is 

 that the surface of the selenide should be moist, a condition 

 of course suggestive of some secondary action. 



Exp. 21. — A mass of cuprous selenide about 2 millim. in 

 thickness was placed between two platinum foil electrodes, 

 each of which was covered with four layers of filter-paper 

 moistened with distilled water. The arrangement was pressed 

 together by a weight of 28 lbs. (12"7 kilos), ordinary pre- 

 cautions being taken to avoid short-circuiting. A current 

 averaging 0'03 ampere was passed through the moist selenide 

 for about four hours. On examining the papers afterwards, it 

 was found that those which had been in immediate contact 

 with the selenide were stained a dark grey colour, due 



