38 FORESHADOWING OF THE ELECTRON [CH. iv. 



some of the properties of matter, at any rate such 

 properties as appertain to matter by reason of its 

 having inertia ; because, as we have seen, an electric 

 charge itself does possess a certain kind of imitation 

 inertia. Hence these electrons in movement would 

 possess momentum, and might therefore propel wind- 

 mills (though the actual motion of the windmills in 

 Crookes tubes seems more likely due to charge and 

 electric repulsion than to simple momentum) ; they 

 would possess kinetic energy, and therefore might 

 heat a piece of platinum ; and if suddenly stopped by 

 a massive target when travelling at a high speed they 

 might readily give rise to phosphorescent appearances, 

 and even to the sudden pulse of radiation known as 

 X-rays. Indeed the existence of this last property is 

 capable of clear deduction on electrical principles 

 if the matter is further gone into. (See chap, ix.) 



The continued passage of a current through a 

 vacuum tube cannot be explained by a torrent of 

 electrons alone, there must be some mechanism 

 for continually producing them fresh and fresh, 

 near or at the cathode, else they would almost in- 

 stantaneously get exhausted. The most probable 

 view of the matter is that suggested by J. J. Thomson : 

 that the current is conveyed chiefly by positive ions, 

 which are produced in the residual gas by ionisation 

 due to the first discharge of cathode rays. These 

 positive ions then pass along comparatively slowly 

 toward the cathode, creep in towards it, as best they 

 can, in face of the bombardment ; and then at the 

 last experiencing the violent gradient of potential 

 in its immediate neighbourhood rush up against it 

 and by their shock produce a fresh supply of electrons. 

 The glow over the cathode is supposed to mark the 

 region of this ionisation. The negative particles, thus 



