Ions and Electrons through Gases. 63 



It seems to the writer to be perfectly natural and logical to 

 extend this conception so as to apply to the collisions between 

 the ions and the neutral molecules. The difference in the 

 mobilities of the two kinds of ions is thus regarded as being- 

 due to the different degrees of elasticity between the neutral 

 molecules and the positive and negative ions respectively. 



If we regard the collisions between neutral gas molecules 

 as being moderately elastic, we would expect that collisions 

 between an ion and a pas molecule should have a smaller 

 degree of elasticity on account of the attractive forces 

 resulting from the charge on the ion. These forces would 

 result in a small fraction of the translational energy at 

 collision being transformed into energy inside the ion or 

 molecule. A very high degree of elasticity would imply 

 (v. sec. 4B) either an accelerated drift for the ion or a slow 

 acquisition of terminal velocity: experiment shows that in 

 general the ion quickly acquires a terminal velocity so that 

 its collisions with gas molecules must be imperfectly elastic. 



The experimental fact that the negative ion has the greater 

 mobility would imply that at collisions between the neutral 

 molecules and the positive and negative ions respectively the 

 latter have the higher degree of elasticity. It is of interest 

 to inquire whether we know any properties of the negative 

 ion which would suggest that it should be associated at 

 collision with a higher degree of elasticity ; moreover, a\ e 

 have to explain the experimental fact that the difference in 

 mobilities is especially marked in the case of the light gases 

 (e. g. impure hj'drogen and impure helium) and practically 

 vanishes for the heavier gases and vapours. 



The general effects can be accounted for by two consi- 

 derations : firstly, the discrete nature of the electronic charge, 

 and secondly, the assumption that the positive and negative 

 charges are differently distributed in the respective ions. 

 If we consider a negative ion which is about to collide with 

 a neutral molecule, the discrete nature of the electronic 

 charges both in the ion and the molecule will be mani- 

 fested by an intense force of repulsion when the distance 

 is very small. This field will be superposed upon the 

 attraction due to induction, and will resist any mutual 

 penetration at collision. The ion and the molecule are at 

 close approach resolved as it were into constituent charges, 

 and the simpler the structure the more effective the 

 resolution. The effect of the forces due to polarization 

 will be to decrease the elasticity of the collision while the 

 repulsive forces will act in the opposite manner. We would 

 thus expect the collision in the case of the negative ion to 



