1891.] liquid electrodes in vacuum tubes. 243 



liquid surface a black space, which would appear to the eye to be 

 bounded below by a straight line and above by the curve of 

 intersection of the negative glow by a vertical plane. Let us 

 call this type (d). These types were actually observed, but in 

 addition two other types were seen, viz. those represented by (b) 

 and (c), fig. 2. 



In both, BABE represents the rim of the liquid surface as 

 seen by an observer's eye at the same level. In (6) the small 

 areas HAD, KBE had a black appearance, but were in general 

 not conspicuous. The negative glow extended from HABK for 

 some distance up the tube as in type (a). But it now showed a 

 sort of tuft ACB of a much whiter and less translucent blue than 

 the rest, with more or less distinct curvilinear boundaries which 

 seemed to cross at G and gradually fade away beyond it. In (c) 

 there was no trace of any black spaces, and the tube appeared 

 prevailingly blue all the way up with the exception of an almost 

 pure white tuft ACB. It had a tolerably distinct outline except 

 at the top C where the colour passed gradually into blue. It was 

 sometimes more conical and sometimes more depressed seemingly 

 than in the figure, but exact measurements were not attempted. 



A slight unsteadiness in the type (c) happened to catch the 

 eye when looking at the liquid surface while putting on the 

 current. Further examination led to the following results. 



After keeping the pump at work for some time and then 

 suddenly starting the discharge with the H 2 S0 4 as cathode one 

 saw a sort of white cloud instantly gather in the tube, separated 

 apparently from the liquid surface by from 2 to 4 mm. of a 

 dark interval. The cloud, however, immediately stretched down 

 the tube and transformed into the appearance (c). This trans- 

 formation was on several occasions so distinctly seen that the 

 observer could hardly be mistaken ; but it was not always seen 

 under these conditions. The whole thing happened so fast that 

 sometimes all one could say was that some rapid change had 

 occurred. When the current was simply stopped and renewed 

 without intervening exhaustion, the tuft appeared at once in the 

 position shown in the figure. 



I am not aware of previous observations on the form of the 

 negative glow near concave cathodes forming surfaces of revolu- 

 tion, and it would obviously be very difficult to see what actually 

 exists within the rim of such a cathode. Professor Crookes, how- 

 ever, in Plate 14, Phil. Trans. 1879, gives some beautiful coloured 

 illustrations which show the negative glow and the Crookes' space 

 near a concave cylindrical cathode. I would more especially draw 

 attention to his illustrations b and c fig. 11, which show a great 

 concentration of negative glow near the plane of symmetry 

 through the axis of the cathode. In Crookes' fig. b there is a 



VOL. VII. pt. IV. 19 



