Colh'sion of Tons icith JVeutral Molecules. 117 



electric current. In several papers ^ I also have explained 

 a series of phenomena already known on the ground of the 

 hypothesis of ionization by ionic shock. 



A recently published paper of Townsend f contains the 

 following remark : — " In one of his papers on this subject 

 J. Stark {Ann. d. Phjs. vii. p. 437, 1902) refers to my work, 

 and states that the velocities which I gave in my first paper 

 are too small. According to Stark a negative ion must travel 

 between two points differing in potential by 50 volts, in 

 order to acquire sufficient velocity to produce new ions on 

 coHision, because there is a fall of potential of 50 volts near 

 the anode when a continuous discharge takes place in air. 

 Even if it be granted that the fall of potential at the elec- 

 trode is to be explained by this property of a moving- 

 negative ion, the phenomenon gives no reason for supposing 

 that new ions are not generated by collision when the 

 velocity of the negative ion is less than that corresponding 

 to 50 volts." 



It is necessary to remark that Townsend is under a mis- 

 apprehension with regard to my views. 



Firstly, I represent indeed the view that an ion must freel}" 

 pass a certain minimum of potential-difference between two 

 points, in order to acquire sufficient velocity to produce new 

 ions by collision at the end of its free path. But I have not 

 founded this view on the phenomenon that there is existing 

 at the anode a drop of potential of 20 to 40 volts, when a con- 

 tinuous discharge takes place. 



Secondly, I have said clearly that this drop of potential at 

 the anode is nob equal to the ionizing potential of the negative 

 ion (ionizing potential = potential-difference to be freely 

 passed by an ion to produce new ions by collision). 



Thirdly, I distinguish between an ionization by ion shock 

 in the interior of a gas passed by an electric current and 

 an ionization at the boundary between a gas and a metal. 

 By some catalytic action of the metal the ionizing potential 

 is in the second case smaller than in the first. The expe- 

 riments of Townsend refer to the second case, as I said 

 clearly. It is only for the first case that I have assigned 

 50 volts as an upper limit of the ionizing potential of the 

 negative ion. 



The words to which Townsend refers are in English 

 as follows : — "As we have seen above, the ionizing potential 



* .T. Stark, Physik. Zeitschrift, ii. p. 17 (1900), iii. p. 504 (1902) ; Anji. 

 d. Phijs. iv. p. 402 (1001), vii: pp. 417, 919 a902), viii. p. 829 (1902). 

 !" J. S. Townsend, Phil. Mag. (0) v. p. 397 (190:3). 



