326 Dr. A. Schuster. Experiments on (he 



was first discovered at this stage of the exhaustion. The glow 

 gradually becomes wider. At first it confines itself to the half-cylinder 

 which lies towards the positive wire, but it is very irregular in shape. 

 During the first exhaustion, it was interesting to notice how on one 

 day there were some spots on the aluminium cylinder which were 

 illuminated with especial brilliancy by the glow, while on the next 

 the same spots were remarkable by the absence of any glow above 

 them. All these effects depend of course on the state of the 

 aluminium surface ; gradually this becomes more uniform and the 

 glow more steady. 



On exhaustion the glow gradually covers the whole aluminium 

 surface ; but a dark strip about 2 or 3 centims. in width is observed 

 directly opposite the positive wire. The appearance is well represented 

 by fig. 4, Plate 4, representing a photograph taken of the phenomenon. 

 1 shall refer to the dark strip as the " dark area." As exhaustion 

 proceeds, the glow extends further into the vessel. The dark space 

 surrounding the cylinder becomes more apparent and expands, but the 

 dark area loses its sharpness. Fig. 5 represents a photograph taken 

 at this stage. The dark area increases slightly in width with decreas- 

 ing strength of current. 



When the cylinder is positive and the wire negative, very little is 

 seen of the positive part of the discharge. At pressures of about a 

 millimeter two reddish-yellow bands are noticed running parallel to 

 the axis of the cylinder on either side, and symmetrical to the wire. 

 They are too faint to be photographed. When the exhaustion is carried 

 further these bands become less and less distinct, and gradually 

 disappear. 



I shall describe in another part of the paper some curious phenomena 

 which are seen on sudden reversal of the current either in one direc- 

 tion or another, also the effects of a magnet inside the aluminium 

 cylinder on the glow surrounding it. 



Explanation of the Repulsive Effect of the Positive Electrode on Glow. 



The following seems to me a plausible explanation of the phenomenon 

 which I have just described. The rapid fall of potential which is 

 observed on crossing the negative electrode suggests at once, inde- 

 pendently of any theory, that we have to deal with the action of a 

 condenser, for we know that no statical charge can produce a finite 

 ditference of potential at the electrode, while a double layer will 

 produce a discontinuity. Although it may not be' proved that an 

 absolute discontinuity of potential exists at the kathode, it is yet 

 certain that a very rapid fall takes place. This is all that is neces- 

 sary for the argument. 



We recognise such a double layer in the case of electrolytes, but 



