CH. iv.] BY CATHODE RAYS 35 



conducting by a discharge in the other, provided 

 that the two had any kind of gaseous communi- 

 cation ; a fact which looked as if some extremely 

 mobile particles, probably the negatively charged 

 particles of cathode rays, could wander about to 

 a considerable distance in a very short time and 

 take their share in the conveyance of an electric 

 current. The conductivity of gases appeared to be, 

 indeed, entirely due to these loose or dissociated or 

 detached charged particles, or ions, and where they 

 were absent the gas did not conduct at all ; it 

 could be broken down, being a weak dielectric, by a 

 sufficiently strong force, but it would not leak ; 

 whereas, when these loose charged particles were 

 about, it leaked readily, becoming to all intents and 

 purposes an electrolyte amenable to the feeblest 

 electric influence. The production of this electrolytic 

 condition is called " ionisation." The act of breaking 

 down the air by an electric discharge was thus 

 found to render the surrounding air for a time 

 electrolytic. Its electrolytic quality, however, did 

 not last long. The mobility of the particles which 

 enabled them to travel to a considerable distance 

 also enabled them to get rid of themselves by 

 clinging to the sides of the vessel, or perhaps by 

 re-uniting with some opposite charges, which after 

 some time in their rapid journeys they must 

 casually encounter. Prof. Townsend,* however, found 

 that the conducting power lasted unexpectedly long 

 if no dust was present : the dust particles apparently 

 acting as intermediate receivers and storers of 

 charge, promoting interchanges, which did not very 



* J. S. Townsend of Trinity College, Dublin, then working in the 

 Cavendish Laboratory, Cambridge, now Waynflete Professor of Physics 

 in the University of Oxford. 



