Wellisch — Motion of Ions and Electrons through Gases. 31 



there would be no great difference in tlie values of the two 

 mobilities. 



In pure hydrogen at atmospheric pressure the negative 

 carriers consist practically entirely of electrons ; a trace of an 

 impurity such as oxygen is sufficient to convert the carriers into 

 ions. An interesting question arises as to the nature of the 

 negative ion in slightly impure hydrogen ; is it constituted by 

 the hydrogen or by the molecules of the impurity ? Haines- 

 has recently made an investigation with regard to the negative 

 carriers in hydrogen, commencing with the gas in a very pure 

 state and allowing impurities to accumulate. In this manner 

 he has brought into evidence three distinct types of negative 

 ions, the normal ion being the slowest of the three. His con- 

 clusion is that these ions are composed of clusters of hydrogen 

 molecules, each type of ion comprising a definite number of 

 molecules. The possibility that these ions are composed of the 

 molecules of the impurity present is not discussed in the paper, 

 nor indeed does the part played by the impurity receive con- 

 sideration. No evidence was obtained in the present experi- 

 ments of the intermediate types of ions described by Haines ; 

 this was possibly due to an excess of impurity in the hydrogen 

 employed by the writer, although it should be observed that it 

 was sufficiently pure to yield a copious supply of electrons at 

 atmospheric pressure, whereas in some of the curves given by 

 Haines the intermediate ions are in evidence when free electrons 

 are practically absent. With regard to the question as to the 

 nature of the negative ion in impure 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 considered. It ap- 

 pears 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 ioniza- 

 tion 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 reniain 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 encoun- 

 ters. 



In a recent communication! Sir J. J. Thomson has expressed 



* Haines, loc. cit. 



t J. J. Thomson, Phil. Mag., xxx, p. 321, 1915. 



