268 Conduction of Electricity through Gases by Charged Ions. 



ionization near the anode, in travelling towards the cathode, 

 met with the negative ions coming from the centre of 

 ionization near the cathode, that these positive ions combine 

 with the negative nntil their number is exhausted, and on 

 combining give out light, the region of recombination con- 

 stituting the positive light. In the dark space between the 

 positive light and the negative glow these positive ions from 

 the centre of ionization near the anode are exhausted, so that 

 there are none of them left for the negative ions coming from 

 the centre near the cathode to combine with. 



The nick in the curve denoting the centre of ionization 

 near the cathode is present in all the curves given by 

 Graham ; the centre near the anode is not nearly so per- 

 sistent. In several of the curves given by Graham there is 

 no nick near the anode, though the one near the cathode is 

 well marked, and in these tubes there is no well-developed 

 positive light. The distribution of potential which accom- 

 panies the luminous discharge requires a definite distribution 

 of electrification in the tube, this requires ionization and a 

 movement of the ions in the tube before the luminous dis- 

 charge takes place. There must, therefore, be a kind of 

 quasi-discharge to prepare the way for the luminous one. 

 Warburg ( Wied. Ann. lxii. p. 385) has, in some cases, detected 

 a dark discharge before the luminous one passes. It seems 

 probable that such a discharge is not limited to the cases in 

 which it has already been detected, but is an invariable 

 preliminary to the luminous discharge. 



Besides the " nicks " or places of specially sharp curvature 

 fig. 4 shows that there is a small curvature in the direction 

 indicating an excess of ionization over recombination all 

 through the considerable space that intervenes between the 

 positive light and the negative glow ; as this region is one far 

 away from places of great electric intensity it seems probable 

 that in producing ionization the electric intensity at any 

 point is helped by other agencies. The case of the cathode 

 rays shows that the motion of charged ions tends to ionize 

 the surrounding gas. E. Wiedemann, too, has shown that 

 the discharge generates a peculiar radiation, called by him 

 a Entladungstrahlen"; it is possible that these may possess 

 the power of ionizing a gas through which they pass. 



