398 Conduction in Solutions and Gases , 



the conductivity of the hot gases of flames. " It is assumed," 

 he wrote, " that in electrolytes, even before the application of 

 an external electromotive force, there are present atoms or 

 atomic groups the ions, as they are called which originate 

 when the molecules dissociate ; hy these the passage of electri- 

 city through the liquid is effected, for they are set in motion 

 by the electric field and carry their charges with them. We 

 shall now extend this hypothesis by assuming that in gases 

 also the property of conductivity is due to the presence of 

 ions. Such ions may be supposed to exist in small numbers 

 in all gases at the ordinary temperature and pressure ; and as 

 the temperature rises their numbers will increase." 



Ideas similar to this were presented in a general theory of 

 the discharge in rarefied gases, which was devised two years 

 later by Arthur Schuster, of Manchester.* Schuster remarked 

 that when hot liquids are maintained at a high potential, the 

 vapours which rise from them are found to be entirely free 

 from electrification ; from which he inferred that a molecule 

 striking an electrified surface in its rapid motion cannot carry 

 away any part of the charge, and that one molecule cannot 

 communicate electricity to another in an encounter in which 

 both molecules remain intact. Thus he was led to the con- 

 clusion that dissociation of the gaseous molecules is necessary 

 for the passage of electricity through gases. f 



Schuster advocated the charged-particle theory of cathode 

 rays, and by extending and interpreting an experiment of 

 Hittorf s was able to adduce strong evidence in its favour. 

 He placed the positive and negative electrodes so close to each 

 other that at very low pressures the Crookes' dark space 

 extended from the cathode to beyond the anode. In these 

 circumstances it was found that the discharge from the positive 

 electrode always passed to the nearest point of the inner 

 boundary of the Crookes' dark space which, of course, was in 



* Proc. Roy. Soc. xxxvii (1884), p. 317. 



t In the case of an elementary gas, this would imply dissociation of the molecule 

 into two atoms chemically alike, but oppositely charged ; in electrolysis the 

 .dissociation is into two chemically unlike ions. 



