460 On the Mobility of the Negative Ion. 



The experiments of Wellisch were carried out on the 

 supposition that the mobility of the negative ion is indepen- 

 dent of the electric force applied ; this, however, is shown 

 to be true only for comparatively small electric forces. The 

 study of Wellisch's paper leaves but little doubt that the 

 sudden increase in the mobility of the negative ion with 

 increasing forces is responsible for the characteristic bend 

 in the curves, which have led him to the erroneous conclusion 

 of the existence of two different kinds of negative ions. 



Thus, for a satisfactory explanation of the results given 

 above, we must assume the gradual diminution of the average 

 mass of the negative ion with increasing electric forces. 

 The conception of "the average mass associated with an 

 electron " is, however, complicated, owing to the uncertainty 

 as to the nature of an ion, and may be interpreted in a way 

 different from that adopted in this paper. According to the 

 interpretation recently given by Sir J. J. Thomson * the 

 electron may easily escape from the system of molecules 

 constituting the ion and travel a certain distance in a free 

 state, until it is once more attached to a molecule, and so 

 on, so that the negative carrier makes its way through the 

 gas, partly as a free electron and partly as a normal ion. 

 From this point of view there is no "transition stage'"' 

 between an ion and an electron, and the increase in the 

 mobility of the negative ion, as well as the apparent dimin- 

 ution of the mass associated with it, show only that under 

 certain conditions the proportion of time during which the 

 ion is moving as a free electron increases with the electric 

 force. 



Summary. 



1. A new method of measuring ionic mobilities in gases 

 is described. 



2. At a given pressure the mobility of the negative ion is 

 shown to be constant only over a certain range of electric 

 forces applied. With increasing electric forces X a certain 

 value of X = X X is reached when the mobility of the ion 

 begins to increase rapidly. The value of the critical force 

 X x increases rapidly with the pressure, being equal to about 



1800 at atmospheric pressure. 



cm. r x 



3. The mobility of free electrons was measured and shown 

 to diminish with increase of the electric force. 



4. The mobility of the negative ion in hydrogen and in 

 some heavy gases was measured. 



* Phil. Mag. xxx. p. 321 (1915). 



