1891.] liquid electrodes in vacuum tubes. 239 



same exhaustion with the Al. cathode a faint luminosity was 

 seen to some distance past H, and a very faint phosphorescence 

 could be made out at the bend. 



Deposit on the tubes. 



In the Hg. tube there were the same deposits as before, and 

 a further one was observed under the following circumstances. 



On several occasions on examining the Hg. surface by day- 

 light it was found to present a yellow metallic appearance. The 

 conditions preceding its first appearance were as follows. An 

 air-bubble which had remained under the mercury was driven 

 up whilst the discharge was passing. Mercury splashed up the 

 walls of the tube and adhering to some extent presented a 

 concave surface. Some observations were taken with the surface 

 in this state, and next day it was noticed that the surface was 

 yellowish, and that there were traces of a yellow deposit not 

 only up the tube AE but also at intervals along DF and even 

 for some distance down FB. 



On another occasion when the tube had just been carefully 

 cleaned and dried and fresh mercury introduced, I noticed after 

 passing the spark both ways for some time that the Hg. surface 

 though retaining its ordinary convexity had a decidedly yellow 

 appearance, and that the tube near it was slightly yellow. Air 

 was allowed to leak in, and the tube being left for some days 

 appeared when next examined quite clean, while the Hg. surface 

 had its usual colour. When however the tube was again ex- 

 hausted and left for some days, the Hg. surface and a few 

 millimetres at the base of the tube were found yellow as before. 

 However, on passing the spark the phenomena had their normal 

 character — which was not the case when the mercury surface 

 was concave — and on re-examining the tube the yellow colour 

 was found to have entirely disappeared and it was not observed 

 again. With the exception of the yellow patches above men- 

 tioned and a narrow dark ring sometimes observed on the glass 

 near the head of the positive column, the tube with the Al. 

 electrode showed no distinct deposit. The origin of the yellow 

 colour was not discovered. Its appearance in the tube FB sug- 

 gests but does not prove a capacity in the discharge to transport 

 particles from a cathode surface round corners. The transporting 

 agency might of course have been vapour rising from the Hg. 

 surface and condensing on cooler portions of the tube. 



Effect of sudden alteration of the Hg. surface. 



The experiments on this point were those first carried out. 

 Both electrodes were then of mercury, and the tube GHP instead 



