30 Wellisch — Ilotion of Ions and Electrons through Gases. 



ated drift for the ion or a slow acquisition of terminal yelocitj ; 

 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 

 enquire 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, we have to 

 explain the experimental fact that the difference in mobilities 

 is especially marked in the case of the light gases (e. g. impure 

 hydrogen and impure helium) and practically vanishes for the 

 heavier gases and vapors. 



The general effects can be accounted for by two considera- 

 tions : 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 con- 

 sider 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 manifested 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 resohition. 

 The effect of the forces due to polarization will be to decrease 

 the elasticity of the collision while the repulsive forces will act 

 in opposite manner. We would thus expect the collision in the 

 case of the negative ion to be fairly elastic, this elasticity being 

 especially marked with light gases, such as hydrogen or helium, 

 which have a simple structure and a small coefficient of polariza- 

 tion. 



In the case of the positive ion the charge is either more 

 centrally situated than is the electron in the negative ion, or 

 what is effectively the same, the positive charge is not discrete 

 but distributed ; the ionic charge will thus act so as to produce 

 a collision of small elasticity ; the ion will probably penetrate 

 an appreciable distance into the molecule and the mobility will 

 in consequence be diminished. 



For the heavier gases and vapors we would expect the neg- 

 ative charge in the ion to be situated more centrally than for 

 the light gases ; in any case the discrete nature of the electronic 

 charge would not be so readily manifested with these complex 

 molecules which would, approximate more closely to metallic 

 conductors. The forces due to the approach of a positive or 

 negative ion would be more nearly equal and, in consequence, 



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