1876.] Vapour of Mercury on Selenium. 23 



vanometer was slightly deflected when the selenium was placed in the 

 circuit of ten Leclanche cells. The conductivity of the selenium increased 

 rapidly for four days, when the experiment was unavoidably interrupted. 

 On admitting air to the tube no change of conductivity was observed. 

 The selenium was unaltered in appearance, even when examined micro- 

 scopically. On breaking the bar it was found that the conducting-film 

 was entirely superficial ; it was not removed by rubbing forcibly with a 

 cloth. Dilute nitric acid also failed to remove it. Bibulous paper 

 moistened with solution of silver ammonio-nitrate was not stained by it 

 (Merget, ' Comptes Eendus,' vol. Ixxiii. p. 1356). It therefore appears 

 highly probable that the film does not consist of un combined mercury. 

 As it has not hitherto been known that mercury combines with selenium 

 at ordinary temperatures, a bar of selenium was immersed in mercury 

 and allow^ed to remain undisturbed for six months. At the end of this 

 time it was found that the selenium was coated with a highly conducting 

 film. I could not dectect any difference between this film and those 

 produced in the Sprengel vacuum. An attempt was now made to esti- 

 mate the quantity of mercury required to produce the observed con- 

 ductivity. A bar of selenium 125 millims. long and 2 millims. in 

 diameter, having platinum wires fused into each end, was enclosed in 

 a glass tube, containing also a minute globule of mercury about 0-5 

 millim. in diameter. The tube was exhausted by means of the Sprengel 

 pump, and then hermetically sealed and detached from the pump. In 92 

 hours the bar began to conduct, and the conductivity increased rapidly 

 from day to day for four days. On the fifth day, no increase being 

 observed, it was supposed that air had leaked into the tube ; and on 

 examining it a flaw, which would account for the leakage, was detected. 

 The tube was therefore again attached to the pump, exhausted, and 

 again sealed, the defective portion being removed. The conductivity of 

 the bar again increased from day to day, and is still steadily but slowly 

 increasing (eleven days after the second sealing of the tube). Although 

 the bar of selenium now possesses a comparatively low resistance, I can- 

 not detect the slightest alteration in the size of the minute globule of 

 mercury which has supplied the material for the conducting-film, extend- 

 ing over a surface one thousand times greater than that of the globule. 



The granular modification produced by subjecting vitreous selenium to 

 a temperature of 100° C. for three hours also acquires a great increase of 

 conductivity when exposed to the vapour of mercury in the Sprengel 

 vacuum. 



As it is possible at any moment to arrest the formation of these con- 

 ducting-films, bars of selenium of any given high resistance may be 

 obtained in this way with great certainty and accuracy. 



