CH. iv.] BY CATHODE RAYS 39 



set free, then fly off as cathode rays, setting up fresh 

 ionisation, and producing a copious further supply of 

 positive ions ; on the existence of which the possi- 

 bility of the cathode rays themselves depend. The 

 positive and" the negative particles on this view are 

 mutually dependent : each is the cause of each ; and 

 when either fails to be formed in a vacuum tube it is 

 impossible for it to conduct, even when its terminals 

 are highly electrified ; for if the supply of either sign 

 of ion is stopped, that of the other at once fails. 

 This accounts for the action of ' electric valves,' wherein 

 the positive ions are prevented from getting at the 

 cathode in one direction, by reason of a special 

 arrangement for concentrating an electron bombard- 

 ment along the direct route, without any back door 

 or side entrance for the positive ions. The provision 

 of such a back door, even though the route thereto 

 be long, immensely eases the conveyance of current : 

 as was strikingly shown by Hittorf. 



It has been observed that any obstacle introduced 

 into the dark space near the cathode, if it is able to 

 check locally the access of positive ions, will throw a 

 shadow both fore and aft, 1 one towards the cathode, 

 and likewise one down the cathode rays, because 

 the generation of fresh electrons is thereby locally 

 prevented.* At this stage we may conveniently 

 summarise the position thus : 



The magnitudes which need experimental deter- 

 mination in connexion with cathode rays, in order 

 to settle the question and determine their real 

 nature, are the speed, the electric charge, and if 

 possible the mass, bf the flying particles. 



Everything suggests that they are flying with 



* Schuster, Proc. Roy. Soc., xlvii., p. 557, 1890 ; Wehnelt, Wied. Ann. 

 Ixvii., p. 421, 1899. 



