Discharge in Rarefied Gases. 239 



ances are observed at all the corresponding points of the tube. 

 As indicated in the figure, the positive light at @ and at 7 does 

 not take the shortest way to the anode by avoiding the prolon- 

 gations X, but fills them and produces green phosphorescence 

 at their ends. There is no phosphorescence opposite /3 and 7; 

 there is none opposite a. 



The experiments show, then, that the positive light, like the 

 negative light, with increased exhaustion radiates in straight 

 lines as far as the shape of the discharge-tube allows ; it fills 

 every space which can be reached in the direction of its rays 

 without traversing a solid wall, even when the path to this 

 space and to its boundary deviates from the shortest path to 

 the anode. 



On Orookes's Theory of the Phenomena of the Discharge. 



In the Philosophical Magazine for January 1879 Crookes 

 has proposed a theory of the excitation of phosphorescence by 

 electric rays, which brings this excitation into close relation- 

 ship with the second (counted from the surface of the kathode) 

 layer of the kathode-light. Crookes believes that the discharge 

 from the kathode consists of electrically-charged particles of 

 gas driven off from the surface of the kathode. These particles 

 of gas drive the uncharged molecules before them to a certain 

 distance from the kathode; and there results a space round the 

 kathode filled only with these molecules driven off from the 

 kathode. As their paths are normal to the surface in the case 

 of a straight wire or a plane surface of metal, the paths would 

 be all divergent or all parallel, and the molecules would not 

 suffer collision with each other. But, according to Crookes, 

 the mutual collision of molecules is the only cause of their lumi- 

 nosity; consequently this space surrounding the kathode ap- 

 pears dark. This space expands with increase of exhaustion 

 in all directions until its diameter becomes equal to the dis- 

 tance of the kathode from the wall; then the molecules thrown 

 off from the surface of the kathode, before they collide with 

 other gas-molecules, strike upon the wall of the tube and excite 

 it to luminosity. 



In opposition to the above I have to remark : — 



1. The second layer of the kathode-light cannot consist of 

 non-luminous molecules driven off from the kathode ; for the 

 kathode is surrounded immediately by the bright yellow- 

 coloured first layer. It would be an error to consider this 

 layer as a secondary glow produced by volatilized sodium; for 

 its spectrum is that of air free from sodium-] ines. 



2. The second layer itself is also not non-luminous, but 

 distinctly of a blue colour ; at the small density of the lumi- 

 nous gas this involves a very high emission-coefficient. 



