Barss — Ionization by Collision in Gases and Vapors. 231 



Townsend has also shown that if we have C ions set free by 

 the action of ultra-violet light at the negative plate of the two 

 parallel plates between which the electric field is established, 

 the number of ions C, which reach the positive plate, is given 

 by the equation 



C = C e aL 



where L is the distance between plates, e Napier's base and a 

 the collision constant as before. 



Again experiments have shown that the values of a obtained 

 with Rontgen rays as the ionizing agent, are the same as the 

 values obtained with ultra-violet light, other conditions being 

 equal. That is, the negative ions produced in gases by Ront- 

 gen rays follow the same changes in ionizing power as the 

 negative ions set free from a metal plate by the action of ultra- 

 violet light. 



Object of the present experiment. 



It was found convenient in this investigation to employ 

 alpha radiation as the ionizing agent. Moreover, as no deter- 

 minations have hitherto been made for the collision constants 

 corresponding to ionization by alpha rays, it was of interest 

 to ascertain whether these constants were in any way dependent 

 upon the nature of the ionizing source. 



In addition, few experiments have been performed to deter- 

 mine these constants in the case of vapors. The only case 

 reported is that of water vapor by Townsend.* The vapors 

 employed in this experiment were sulphur dioxide, ethyl alco- 

 hol, ether, methyl iodide and ethyl chloride. In the measure- 

 ment of ionic mobilities,t the vapors have given evidence of a 

 characteristic behavior ; so that it did not seem unlikely that 

 ionization currents in vapors would also present distinctive fea- 

 tures. 



Apparatus and methods of experimenting. 



The apparatus used in this investigation is shown in fig. 1. 

 It consists essentially of two parallel plates inside an air-tight 

 vessel. A A is a glass bell jar fitted with a brass cap PP and 

 a separate brass top BB. Through an ebonite plug E, a brass 

 rod F is passed by means of which the brass plate Gr is con- 

 nected to one pair of quadrants of a Dolezalek electrometer, 

 the second pair of quadrants and the case being earthed. The 

 plate G is provided with an earthed guard ring H. A second 

 plate I is connected by means of the brass rod O to a source of 

 potential V. This plate I could be raised or lowered by means 

 of the rack and pinion K, the distance between the plates being- 

 read by a graduated micrometer head M. attached to the shaft 



* Phil. Mag., vol. v, p. 389, 1903. 



f Wellisch, Phil. Trans., vol. ccix, p. 249. 



