26 FORESHADOWING OF THE ELECTRON [CH. IV. 



the discharge will prefer to take a reasonably long 

 path through the air. Exhausting further still, the 

 glow all disappears and the second dark space fills 

 the whole of the tube ; and now is noticed a new 

 phenomenon, the sides of the glass have begun to 

 glow with a phosphorescent light, the colour of the 

 light depending on the kind of glass used, but 

 generally in practice with a greenish light ; a result 

 evidently of being the boundary of the dark space. 

 If exhaustion proceeds further, the resistance of the 

 tube becomes very high, and the spark may prefer to 

 burst through an equal, and ultimately even a greater, 

 length of ordinary air. This is the condition of the 

 tube so much investigated by Crookes, by Lenard 

 and Rontgen, and by many other observers. It is 

 the phenomena occurring in this dark space which 

 have proved of the most intense interest. 



Cathode Rays. 



So far we have supposed that the cathode is a 

 brass knob or other convenient terminal introduced 

 into the tube ; but if we now proceed to use other 

 shapes, as Plucker did first in 1859, followed by 

 Hittorf (1869), Goldstein (1876),and Crookes (1879) 

 using a flat disc or a saucer-shaped piece of metal, and 

 if we then introduce into the dark space various sub- 

 stances, we shall find that shadows are thrown, and 

 that the dark space is full of properties which are 

 most clearly expressed by saying that it is a region 

 of cathode rays that is to say, of something shot 

 off in straight lines from the cathode. There is 

 evidently something being thus shot off though what- 

 ever it is, it is invisible until it strikes an obstacle 

 something which seems to fly in straight lines and to 

 produce a perceptible effect only when it stopped. 



