in Gases by the Aid of Ultra- Violet Light. 397 



found for air when 



^==•118, ^='349, and N=15'2. 



The numbers corresponding to the various forces are given 

 in the last line of the above table for air. 



In the case of hydrogen the experimental results are fairly 

 well explained by supposing that ions are produced on some 

 occasions when the velocity corresponds to about 20 volts. 

 For the other gases it appears that collisions in which the 

 velocities correspond to about 5 or JO volts give rise to new 

 ions on some occasions. 



From this theory it is possible to find only an upper limit 

 for the energy required to ionize a molecule. An accurate 

 value of the latter quantity cannot be found from the ex- 

 periments. There is only a very small probability of a positive 

 ion producing others by collision when it has the kinetic 

 energy acquired in passing between two points differing in 

 potential by 10 or 20 volts. The positive ions are therefore 

 not so efficient for generating ions as the negative ions, so 

 that it is reasonable to suppose that ions may be produced by 

 other processes, which are more efficient than the negative 

 ions. 



Since I first brought forward evidence to show that ions 

 are produced by negative ions when their velocity on col- 

 lision with a molecule is comparatively small, a number of 

 papers have been published, in which the principle has been 

 applied to the ordinary continuous discharge in a tube con- 

 taining gas at a low pressure. In one of his papers on this 

 subject J. Stark* refers to my work, and states that the 

 velocities which I gave in my first paper are too small. Ac- 

 cording 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 collision, 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 electrode 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. 



The lengths of the mean free paths, in centimetres, of a 

 negative ion travelling in a gas at one millimetre pressure is 

 given in the following table. The mean free paths are the 

 reciprocal of the above values of N. 



* J. Stark, Annalen der Physik, 1902, Band vii. p. 437. 



