254 Mr. S. Bidwell on some 



under the microscope disclosed the presence of a number of 

 minute lumps of a white substance, intermixed with the red 

 stuff. The lumps had the appearance of pieces of calcium 

 chloride which had been exposed to the air, and they were 

 clearly the origin of the moisture. They consisted, no doubt, 

 of selenium oxide or hydroxide, which is probably a deliques- 

 cent substance.* This oxide might perhaps be formed by 

 the direct combination of some of the freshly separated 

 selenium with the oxygen of the air, or with that resulting 

 from the electrolysis of traces of water existing in the cell. 



In the discussion which followed the reading of my former 

 paper, the late Mr. J. W. Clark took exception to a hy- 

 pothesis therein put forward as to the probable action of the 

 selenides formed in selenium cells, on the ground that selenide 

 of copper conducted metallically and not electrolytically. 

 This he stated to have been experimentally proved by Hittorf 

 (Pogg. Ann. vol. lxxxiv. 1857, quoted by J. W. Clark, Proc. 

 Phys. Soc. vol. vii. pp. 119, 120 f; Phil. Mag. July 1885, 

 pp. 38, 39). But if the amorphous selenium surrounding the 

 anode in my recent experiment was formed otherwise than by 

 electrolysis, it is difficult to imagine why it should be confined 

 to the anode, as was seen to be the fact, and not extend also to 

 the kathode and other portions of the cell. The inference seems 

 to be that copper selenide does conduct electrolytically $. 



When the resistance of this cell was tested with the Wheat- 

 stone's bridge, it was found to have diminished enormously. 

 It was also not sensibly affected by illumination, but, on the 

 other hand, it appeared to be dependent in a curious way on 

 the direction of the current used in testing. With one 

 Leclanche cell and equal proportional coils of 10.0 ohms 

 each, the resistance with a current from anode § to kathode 

 was 4500 ohms, and with a current in the opposite direction 

 only 2900 ohms. The change occurred with great regularity 

 as often as the current was reversed. 



The cell was then connected directly to the galvanometer 

 without any battery, and a very small polarization current 

 from kathode to anode through the cell was indicated. While 

 thus connected, a piece of magnesium ribbon was burnt near 

 the cell. Instantly the spot of light was deflected off the 



* Since the above was written, Prof. S. U. Pickering-, F.R.S., has told 

 me that the oxides of selenium are deliquescent. 



+ This paper of Mr. Clark's, though printed in the l Proceedings ' be- 

 fore my own, was in fact read after it. 



X The cell with the red amorphous selenium on the anode was exhibited 

 to the meeting. 



§ The wire upon which the amorphous selenium was deposited is here 

 and afterwards called the anode. 



