of Selenium and Sulphur to Light, 183 



preference to platinum, and expresses his belief that the che- 

 mical action between the brass and selenium contributes to 

 the low resistance of his cells, " by forming an intimate bond 

 of union between the selenium and brass/' * 



(3) The fact observed by Adams and Day that there is 

 generally a " diminution of resistance in the selenium as the 

 battery-power is increased." The same phenomenon occurs 

 in the mixtures of sulphur and carbon before referred to. It 

 points to the existence of imperfect contact between conducting 

 particles, the conduction partaking of the nature of disruptive 

 discharge, and is consistent with the supposition that particles 

 of conducting selenide are imbedded in the selenium !• 



(4) The apparent production by a current through a piece 

 of selenium of a " set of the molecules which facilitates the 

 subsequent passage of a current in the opposite, but obstructs 

 one in the same direction "J. This would be accounted for 

 by the electrolytic deposition of selenium (from the selenide) 

 upon the anode. 



(5) The polarization-effects, which would also proceed from 

 electrolysis. 



(6) " A slight increase of temperature of a piece of annealed 

 selenium is accompanied by a large increase of electrical re- 

 sistance'^. This also occurs in the mixture of sulphur and 

 carbon, and is explained by supposing that the heat-expansion 

 of the medium draws apart the conducting particles contained 

 in it, causing them to have fewer points of contact with each 

 other, and thus increasing the resistance of the whole ||. A 

 more considerable rise of temperature so greatly diminishes the 

 specific resistance of the selenide (and perhaps of the selenium) 

 as to more than counterbalance this effect; and thus it happens 

 (as I have shown in a former communication If) that selenium 

 cells have a " temperature of maximum resistance," which is 

 generally a few degrees above the average temperature of the 

 air. 



(7) The resistance of prepared selenium is generally greatly 

 diminished by the action of time. Prof. Adams found that 

 the average resistance of a number of "pieces of selenium was 



* Lecture to American Assoc. 1880. Reprinted in ' Nature/ vol. xxii. 

 p. 500. 



t See " On the Electrical Resistance of Carbon-contacts," Proc. R. S. 

 Feb. 1,1883; and " On Microphonic Contacts," Journ. Soc. Tel. Eng. 

 April 12, 1883. 



J Adams and Day, Proc. R. S. 1876, p. 114. 



§ Adams and Day, Phil. Trans. 1877, p. 342. See also Phil. Mag. Jan. 

 1883, p. 31. 



|| Phil. Mag. Mav 1882, p. 351. 



f Phil. Mag. April 1881. 



