624 SCIENTIFIC RECORD FOR 1885. 



Helmholtz. ( Wied. Ann., xxi, 209 ; J. Fhyn., Deceuib^r 1885, 11, iv, 

 578.) 



Bidwell has coustructed a voltaic cell haviug a solid electrolyte. He 

 exhibited to the Londou Physical Society a cell consisting- of plates of sil- 

 ver and copper, between which was contained a mixture of one part of cop- 

 per sulphide and five of sulphur! The electromotive force was 0-07 volt 

 and the internal resistance 6,537 ohms, and the current readily deflected 

 the needle of a reflecting galvanometer so as to throw the light off the 

 scale. A second cell was made with a copper plate on which copper sul- 

 phide had been pressed and a silver plate resting on silver sulphide on the 

 surface of the copper sulphide. The silver plate was brushed over with a 

 dilute solution of sulphur in carbon disnlphide and then heated till the 

 free sulphur was driven off". On putting the cell together it produced a 

 current of 0,800 micro-amperes through an external resistanceof 0-2ohm. 

 The copper and silver plates were each 2^ by 2 inches, and the thickness of 

 the two layers of sulphide 0.05 inch. The electro-motive force of the cell 

 is 0-053 volt, and its internal resistance is therefore about 7 ohms. 

 {Nature, August, 1885, xxxii, 345 ; Phil. Mag., October, 1885, V, xx, 

 328.) 



The interesting subject of the behavior of selenium to light a^ ) egards 

 its electric resistance has been examined by several investigators. Wer- 

 ner Siemens has described the electro-motive action of certain selenium 

 cells sent to him by Fritts, who first observed in them the fact that, 

 when placed in circuit with a galvanometer, an electric current flowing 

 from the gold leaf to the base plate was generated by the action of 

 light. Siemens confirms this observation and regards it as of the great- 

 est scientific importance. Since obscure thermal rays are without effect, 

 the action cannot be a thermo-electric one. Moreover, the electro-motive 

 force developed is proportional to the intensity of the light. He cou- 

 cludes, therefore, that " here we meet for the first time with an instance 

 of the direct conversion of the energy of light into electrical energy." 

 [Ber. Ak. BerL, February, 1885; Phil. Mag., April, 1885, V, xix, 315; 

 Am. J. tScL, June, 1885, III, xxix, 495.) 



Bidwell has investigated very carefully the phenomena of -selenium 

 cells and is disposed to regard the change of resistance by the action 

 of light as a phenomenon of electrolysis. Since the selenium in the 

 cells has always undergone a j^rolonged heating in contact with the me- 

 tallic terminals, selenides of these metals may exist within the selenium, 

 forming a kind of network, thus affording the conducting material 

 throughout the mass. A cell constructed of silver wires and sulphur 

 containing some silver sulphide was sensitive, its resistance being re- 

 duced to one-third by burning a piece of magnesium wire near it. In 

 the electrolysis of silver sulx)hide, however, suli)hur itself would be de- 

 j)0sited on the metallic plate, and the resistance would be enormously 

 increased, unless under the action of light this sulphur united with the 

 silver. Moreover, he finds the specific resistance of selenium to be 2,500 



