On Conductivity produced hy Moving Ions. 631 



It was found that on some occasions new ions were produced 



when the velocity of the colliding ion is equal to the velocity 



acquired in moving freely between two points differing in 



potential by 5 volts. From this it was concluded that the 



energy required to ionize a molecule is not greater than 



5xg 



— rpr , e being the charge on an ion in electrostatic units. 



It was also shown that the collision theory explained the 

 results obtained by Stoletow with ultra-violet light*. 



Experiments have recently been made by Prof. E. Ruther- 

 ford and Mr. R. K. McOlung f to determine the energy 

 required to produce an ion. It was concluded that the 

 amount of energy necessary to ionize a molecule was equal to 

 the energy acquired by an ion in travelling freely between 

 two points differing in potential by 175 volts. If the energy 

 were as large as this there would be no appreciable number 

 of new ions produced by collisions, unless the potential- 

 difference between the electrodes in the gas considerably 

 exceeded 175 volts. In the experiments with Rontgen rays, 

 to which we have referred, and also in Stoletow's experiments 

 with ultra-violet light, large increases in conductivity were 

 obtained when the potential-difference was as low as 60 volts. 

 It would therefore be impossible to explain these experiments 

 by a collision theory if the value of the energy required to 



, , 175 xe 



ionize a molecule were as great as . 



oUU 



In order to avoid this difficulty w T e might have attributed 

 the increases in conductivity at low pressures to a surface- 

 effect, and have adopted the theory of surface- layers, which 

 was given by Prof. J. J. Thomson f as an explanation of 

 Stoletow's results. 



We do not, however, consider that this theory gives a g'ood 

 explanation of the phenomena, since it does not explain the 

 effects obtained by separating the plates. The method em- 

 ployed by Prof. E. Rutherford and Mr. R. K. McOlung for 

 finding the energy necessary to produce an ion consisted in 

 determining the total energy of a beam of rays, the rate of 

 absorption of the rays by a gas, and the conductivity of the 

 gas. When the energy absorbed by the gas was thus found, 

 and the corresponding number of ions, the amount of energy 

 required to produce one ion was deduced. We are of 

 opinion that an estimation of the energy required to produce 

 an ion by this method is not wholly trustworthy. There 



* Stoletow, Journ. de lhi/s. ser. 2, vol. ix. 



t E. Rutherford and R. K. McClunjr, Phil. Trans. 1901. 



\ Phil. Mag. Dec. 1899. 



