Ions and Electrons through Gases. 65 



hydrogen, the suggestion is here made that the molecule of 

 the impurity may act as a catalyst, enabling the electron to 

 enter the hydrogen molecule ; in the pure gas the electron 

 will, however, remain in the free state. 



In the present paper the motion of the free electrons 

 through a gas at relatively high pressures has been con- 

 sidered. It appears that in general an electron is able to 

 effect a permanent union with an uncharged molecule so as 

 to form a negative ion only if the encounter take place quickly 

 after the act of ionization, when the electron still retains a 

 considerable part of its velocity of projection ; if it fails to 

 combine initially, it would seem that it can remain in the 

 free state even in the presence of electro-negative molecules 

 such as oxygen. However, there may arise occasionally 

 certain systems (electron sinks) which possess the property 

 of being able to absorb electrons which drift through the 

 gas ; the union appears in these cases to be of a loose nature, 

 and is liable to be broken by molecular encounters. 



In a recent communication * Sir J. J. Thomson has ex- 

 pressed the view that the electron is able to unite with a 

 molecule during its drift motion so as to form a negative ion ; 

 before such an attachment occurs the electron in general 

 traverses distances through the gas which are large compared 

 with its free path. The distinct separation between the ions 

 and the free electrons which is shown in a whole series of 

 EI curves lends, however, strong support to the view that 

 the electron traverses the whole distance between the elec- 

 trodes without effecting any permanent union with a gas 

 molecule ; a well-defined bend in the experimental curve 

 could not have been obtained if any considerable fraction of 

 the electrons had become attached to molecules during their 

 passage through the gas. 



6. Summary. 



1. The separation previously effected between the electrons 

 and the negative ions in dry air at the lower pressures has 

 in the present investigation been extended to other gases, 

 notably C0 2 and H 2 ; for these two gases the electrons are 

 relatively more numerous than in air at the corresponding 

 pressure. 



2. A trace of impurity is especially effective in reducing 

 the number of free electrons when the gas is at a relatively 

 high pressure : at low pressures the effect of the impurity is 

 often inconsiderable. 



In most cases a velocity greater than that arising from 

 * J. J. Thomson, Phil. Mag. xxx. p. 321 (1915). 

 Phil. Mag. S. 6. Vol. 34. No. 199. July 1917. F 



