1891.] liquid electrodes in vacuum tubes. 235 



Appearance of Discharge. 



The tube containing the Pt. electrode always showed a redder 

 tint than that containing the Hg. electrode under similar con- 

 ditions. 



At a certain stage of exhaustion the former tube was some- 

 times the redder even when the Hg. surface was the anode. 

 At the lowest pressures, however, the difference between the 

 colour of the tubes was inconspicuous, both being prevailingly 

 white. At the highest pressures the spark left the tip of the 

 Pt. anode and the extreme summit of the Hg. anode, but as 

 the exhaustion proceeded it gradually extended down the Pt. 

 anode and spread over the surface of the Hg. anode. Eventually 

 the positive column covered the whole of the Hg. surface, but 

 within 1 or 2 mm. up the tube it had contracted somewhat in 

 diameter, leaving an annular dark space between it and the 

 glass. The Faraday space became gradually indistinct over the 

 Hg. cathode, and the same phenomenon appeared over the Pt. 

 cathode but at a lower pressure. Thus at one stage of the ex- 

 haustion the appearances in the two tubes when containing 

 the cathode were widely different. At the lowest pressures 

 reached, both tubes, so far as clearly seen, were very similar in 

 appearance, and the phenomena agreed with those observed when 

 both electrodes were of mercury. 



Phosphorescence. 



For clearness let us suppose, as was actually the case, A the 

 Hg., B the Pt. electrode. In the tube BO the phosphorescence 

 first appeared at the level of the upper portion of the wire, and 

 gradually spread both up and down as the exhaustion proceeded, 

 reaching the summit G of that tube. In the tube AE the 

 phosphorescence hardly appeared within 2 cm. of the Hg. surface. 

 In the lower parts of both tubes phosphorescence was most 

 brilliant at about the positions of the Faraday spaces at the 

 lowest pressures, and so at a much higher level in AE than in 

 BO. Also the phosphorescence at E was much more intense 

 than that at G, though the latter was very well marked. 



At the lowest pressures reached a faint nebulosity, presumably 

 the positive column, extended to a considerable distance along the 

 tube GHP. It was difficult to detect when the platinum Avas 

 cathode, but when the mercury was cathode it could be traced 

 almost as far as the pump. 



There then appeared at H a patch of phosphorescence on the 

 convex side of the bend. This was very faint when the platinum 

 was cathode, but when the mercury was cathode it was fairly 



