Gases by the Motion of Negatively charged Ions. 225 



The present experiments have led to the conclusion that there 

 is a great difference between the positive and the negative 

 ions at low pressures, and that the negative ions are much 

 smaller than molecules. 



At atmospheric pressure the ions diffuse more slowly than 

 the molecules of the gas in which they are generated, so that 

 each ion is associated with a mass larger than the mass of a 

 molecule of the gas. Also, there is no great difference 

 between the rates of diffusion of the positive and negative 

 ions. Subsequent experiments* at pressures varying from 

 750 to 200 millims. gave no indication of anv change in the 

 size of the ions. The slow rate of diffusion of the ions may be 

 explained if we suppose that a number of molecules are 

 collected round the ions, and that the whole group moves 

 about in the gas with the carrier of the charge : the rates of 

 diffusion of the positive and negative ions would depend on 

 the size of the groups accompanying the ions. The particles 

 of matter on which the negative charges reside might be very 

 much smaller than the particles which carry the positive 

 charges, and at the same time their rates of diffusion might 

 be nearly equal. 



When the pressure is reduced, and the ions are acted on 

 by forces which cause them to move rapidly through the gas, 

 the groups of molecules which surround the ions probably 

 disappear, and the negative ions move in the gas as if they 

 were accompanied by a mass which is small compared with 

 the mass of a molecule. It is possible that this effect could 

 be produced at any pressure if the electric force was large 

 enough to make the ion move with a velocity greater than 

 its velocity of agitation. It is difficult, however, to arrive at 

 any definite conclusion on this particular point, as our know- 

 ledge of the behaviour of ions under the various conditions is 

 very limited. 



13. When the current passes between two eleetrodes, one 

 inside the other, the conductivity is practically unaltered 

 by reversing the current when the electromotive force is 

 small. If the electromotive force is large, the conductivity 

 depends on the direction of the force in a remarkable 

 manner ; the current obtained when the inner electrode is 

 positive may be five or ten times as big as the current 

 obtained when the inner electrode is negative. The reason 

 of this can be easily explained by the theory of collisions 

 if we attribute the production of new ions to the negative 

 ions. 



* John S. Townser.d, Phil, Trans, vol. cxcv. 1900. 



