ELECTRIC DISCHARGE IN GASES ii 



into combination with some positive ion. The conductivity of 

 the gas soon disappears if the supply of the ionising agent is not 

 kept up, a consequence to be expected from the presence of 

 both positive and negative atomic groups which naturally tend 

 to recombine when they meet. 



Thus far the origin and properties of the rays from the cathode 

 have alone been described. But they are accompanied by rays 

 of positive particles which are sometimes called " canal rays " 

 from the manner in which they were first observed. If the 

 cathode is perforated these particles pass through it, and move 

 in straight lines in the opposite direction. The composition of 

 these positive rays is much more complex than that of the 

 cathode rays, for whereas the particles in the cathode rays are 

 all of the same kind, there are in the positive rays many different 

 kinds of particles. They are deflected by strong magnetic and 

 electric fields, but less easily than the cathode rays, from which 

 it is inferred that their mass is greater than that of an electron. 

 They appear to consist of positive ions, derived either from the 

 gas or the electrodes, but they seem to start chiefly from the 

 boundary of the Crookes' dark space. These rays have within 

 the last few years been investigated by Sir J. J. Thomson, 1 who 

 concludes that they have a mass never less than that of a hydro- 

 gen atom, anything corresponding to an electron with a positive 

 charge being so far unknown. 



The following is a brief description of the apparatus used by 

 Thomson : 



A is a large bulb in which the anode is inserted on one side, 

 the cathode C occupying 

 the opposite neck of the 

 bulb. In the diagram, 

 for the sake of clearness, 

 the tubes are not shown . 

 by which the gas is 

 admitted into the bulb, 

 nor the communication 



with the pump by ,-, 



, . , , -i FIG. 50. J. J. THOMSON s APPARATUS FOR, 



which the contained gas STUDYING POSITIVE RAYS. 



is kept at the proper 



low pressure. The cathode has in front an aluminium cap 



1 See especially " The Bakerian Lecture for 1913 " (Proceedings Royal Society, 

 Vol. LXXXIX A, pp. 1-20). 



/ 



