﻿of Ions made by a, /3, 7, and X Rays, 289 



The electrons thus get well out of the spheres of influence 

 of their parent molecules before getting attached to other 

 molecules. 



But this is also true for a dense gas. If we take ethyl 

 chloride, for example, we find that one per cent, of the 

 electrons are absorbed in a distance of "0006 cm. and five 

 per cent, in a distance of *0034 cm., the electric field at these 

 distances from a charged molecule being respectively 3 x 10 -1 

 and 10 — 2 of a volt. 



Since X rays and a. rays differ from one another in their 

 power of effecting a complete separation of electron from its 

 parent molecule, it will be of interest to compare them more 

 closely. 



The a particle seems to effect the ionization of a molecule 

 by means of its external field, for Professor Thomson has 

 shown that the charge of an a particle will get neutralized 

 when its velocity falls below a certain limit, which is approxi- 

 mately that at which, as Rutherford showed, it ceases to 

 ionize. 



And it seems that the a. particle ionizes a large number of 

 the molecules that lie within a limiting distance from its path. 

 This distance, as well as the number of molecules ionized in 

 it, depends probably on the velocity of the a particle and the 

 nature of the gas. Rutherford, by assuming that the a. par- 

 ticle in air ionizes all the molecules in a cylinder of diameter 

 twice the diameter of a molecule, obtained for the number of 

 a particles projected per sec. from a gram of radium a value 

 that agreed well with those obtained by other methods. 



The mechanism of the ionization of the X ray seems to be 

 essentially different from that of the a ray. When a cathode 

 particle strikes the anode in a vacuum-tube a spherical pulse 

 of force travels away from it, and probably encounters all the 

 molecules in the volume of gas under investigation, but 

 ionizes only a very small fraction of them, possibly none at 

 all sometimes. 



It appears from some experiments by Barkla, that the 

 electrons of a molecule emitting secondary X rays move in 

 the same direction as the direction of the force in the primary 

 pulse, and it seems reasonable to suppose, therefore, that the 

 electron f'roin an ionized molecule is ejected in the same 

 direction as the force in the ionizing pulse. And it seems 

 that a molecule is only ionized when one of certain directions 

 in its interior is parallel or approximately parallel to the 

 ionizing pulse. 



Now, it appears from the electron-theory of the atom, so 

 ably worked out by Professor Thomson, that some of its 



Phil. Mag. S. 6. Yol. 12. Xo. 70. Oct. 1906. U 



