CH. iv.] BY CATHODE RAYS 31 



that they are charged particles, will result partly in 

 heat, or vibration of the impacted molecules ; partly 

 in light or phosphorescence, due to the quiver of 

 electrically charged atoms (or rather of electrical 

 charges on atoms) as in the ordinary process of 

 radiation ; and partly in X-rays : all of which effects 

 are readily seen at different stages of vacuum in 

 a Crookes' tube. The momentum of the flying 

 particles shot off from the cathode can also be 

 exhibited by putting into their path some form of 

 vane or little windmill, which will then be driven 

 mechanically, as the vanes of a radiometer are 

 driven by the recoil of the molecules of the residual 

 air from the warmer surface, a stress being thus set 

 up between the vanes and their glass enclosure. In 

 the electric vacuum tube experiment, the stress seems 

 to be between the cathode, or gun, and a layer or 

 stratum of the residual gas not very far from it for 

 unless the exhaustion is very high the gradient of 

 potential close to the cathode is very steep, so the 

 propelling force is clearly the force of electrical 

 repulsion, the particles travelling down the grade of 

 potential just as they travel in ordinary electrolysis, 

 and then proceeding for the rest of the journey by 

 their acquired momentum. But whereas in ordinary 

 electrolysis they meet with constant encounters and 

 therefore progress very slowly, in a high vacuum 

 they can fly for several inches in a free path without 

 encountering anything, and therefore without causing 

 any disturbance, thus giving rise to no appearance 

 but that of the dark space. Phenomena occur only 

 where they strike. 



This was the view of the nature of cathode rays taken 

 by the whole world after Crookes' demonstration ; 

 it was supposed that they were flying atoms, and that 





