238 Mr Ghree, On some experiments on [June 1, 



other occasions such observations were taken and it was found 

 that from the highest pressures where it was visible down to 

 the lowest in the above table, Crookes' space appeared slightly 

 but distinctly longer over an Hg. cathode than over an Al. 

 cathode. 



Appearance of the discharge. 



Under similar conditions the Al. tube was always redder 

 than the Hg. tube, though both were at the lowest pressures 

 mainly white. When the pressure was reduced to a certain 

 stage, the Faraday space over the Hg. cathode became more 

 and more indistinct till it seemed to vanish. The Faraday 

 space over the Al. cathode was at this stage unmistakeable, 

 but at a lower pressure it too eventually became undistinguish- 

 able. The way in which the Faraday space over the Hg. cathode 

 disappeared was rather remarkable. 



The pressure reached a point at which the appearance in the 

 tube was unstable. There might be an unmistakeable Faraday 

 space and negative glow, and then a sudden transition to a stage 

 in which distinct striae reached down the tube to near the Hg. 

 surface. During this time the discharge in the Al. tube showed 

 no fluctuation. As the pressure was carried lower the striae 

 became less and less distinct until the Hg. tube whether the 

 Hg. were cathode or anode seemed an almost uniform white. 

 The lowest 5 or 6 mm. in the tube appeared sometimes per- 

 ceptibly, sometimes doubtfully darker than the rest. Distinct 

 striae eventually ceased to appear even in the Al. tube, but it 

 showed to the end an unmistakeable Crookes' space which how- 

 ever was becoming increasingly indistinct at the lowest pressures 

 reached. 



The greatest length reached by the Crookes' space over the 

 Al. cathode was 14 mm. At this stage the tube containing 

 the Hg. cathode was as bright as anywhere within 7 or 8 mm. 

 of the mercury surface. 



Phosphorescence. 



When the aluminium was cathode phosphorescence extended 

 at the lowest pressure throughout the whole of the tube BG, 

 whereas when the mercury was cathode phosphorescence was 

 not observed within some 2 cm. of its surface. The phosphor- 

 escence at the top E of the Hg. tube was always brighter than 

 that at G at the same exhaustion. At the best exhaustion with 

 the Hg. cathode there was distinct luminosity along the tube 

 GHP as far as the pump, a distance of 42 cm. from H, and a 

 patch of phosphorescence was distinctly visible at H. At the 



