118 Ionization hy Collision of Ions with Neutral Molecules. 



of the negative ions within the gas passed by a current 

 is smaller than 50 volts. As the ionizing potential of the 

 positive ions is smaller near a metal than in the interior ol' 

 the gas, it is to he expected that also the ionizing potential of 

 the negative ions is smaller near a metal, in consequence of 

 its catalytic aid, than in the interior of the gas. This is 

 indeed the case ; even a few volts of freely passed potential- 

 difference enable the negative ion to ionize the gas at the 

 anode. J. S. Townsend (Phil. Mag. (5) 1. p. 572, 1900) 

 ionized a rarefied gas between two neighbouring electrodes 

 by Rontgen rays and determined the potential-difference or 

 electrode-potential at which the current-intensity leaving the 

 stage of saturation began to increase again. This potential- 

 difl'erence was the ionizing potential to be freely passed by 

 the negative ions in order to be able to ionize the gas at the 

 surface of the anode. Townsend assigns 5 volts as the value 

 of that ionizing potential for air near brass. It is possible 

 that this is too small." 



" In the glow current and in the negative-point current 

 the anode is generally covered with a luminous layer, which 

 arises from the ionization by the ions flying towards the 

 anode. If a solid body, for instance a rod, is brought im- 

 mediately before the anode, the rod throws a shadow on 

 the anode by screening off the negative ions flying towards its 

 projection (C. Skinner, Phil. Mag. [5] 1. p. 573, 1900). It 

 is not allowable to equate, even approximately, the anode 

 drop of the glow current to the ionizing potential of the 

 negative ions, because in that case this is of the order of 

 magnitude of the inner electromotive force in the boundary 

 layer." 



When I put in doubt Townsend^s value of 5 volts I was 

 not led by my theoretical views ; but Townsend seemed to me 

 to have interpreted his measurements without necessity in 

 favour of a too small value. According to the later measure- 

 ments of P. J. Kirkby ^ the ionizing potential of the negative 

 ions for air towards a metal lies between 25 and 12 volts ; 

 Lenardf has found 11 volts. 



In a paper also recently published J. 8. Townsend J intro- 

 duces, in order to explain an observation made by himself, 

 the view that positive ions also are able to produce new ions 

 by collision. This view has been already before represented 



* P. J. Kirkby, Phil. Mag. (6) iii. p. 212 (1902). 

 t Ph. Lenard, Ann. d. Fhijs. viii. p. 193 (1902). 

 X J. S. Townsend, Electrician, 1. p. 971 (1903). 



