CHAPTER VII. 

 IONISATION OF GASES. 



IT is constantly necessary to speak of the air or other 

 medium being ' ionised ' by the passage of rays, and 

 by many other processes. The term means that the 

 molecules are split up, or dissociated, into their 

 constituent atoms ; which, being oppositely charged, 

 form anions and cations respectively. It appears to 

 be an effect due to the violent encounter of an ener- 

 getically flying particle with a comparatively stationary 

 molecule : a sudden electric wave or pulse, if thin and 

 forcible enough, may do the same ; the effect in any 

 case is to break the molecule asunder into constituent 

 ions, some positively, some negatively, charged, 

 thereby converting the medium into a true gaseous 

 electrolyte, until the dissociated atoms have had time 

 to recombine a time which may be measured in 

 minutes. It is owing to this effect that X-rays are 

 able so readily to discharge an electroscope or other 

 charged body, whatever the sign of its charge may 

 be ; for it is manifest that any ions in the atmosphere 

 of opposite sign will be attracted to it and will 

 neutralise any charge it may possess. Ionised air 

 can always be detected by its making electroscopes 

 leak, irrespective of any defect of insulation in solid 

 supports. 



