46 Mr. E. M. Wellisch on the Motion of 



E curves, especially those obtained at the highest pressures, 

 showed a distinct curvature in the neighbourhood of the 

 potential axis, the tendency being to shift the point of inter- 

 section towards the origin. This shape of the current- 

 potential curves in the vicinity of the origin suggests acce- 

 lerated motion of the electron or a slow acquisition of a 

 terminal velocity. 



Further experimental data are of course necessary before 

 the nature of the motion of the electron is definitely ascer- 

 tained ; the suggestion here given is that the electron may 

 traverse a considerable distance with accelerated motion 

 before its terminal velocity is acquired. It should be re- 

 membered that Franck and Hertz* have already shown that 

 the collisions of electrons with the molecules of the inert 

 gases are practically perfectly elastic, so that the drift 

 motion of the electron would under these circumstances be 

 accelerated. The experiments with regard to the effect of 

 impurities upon the number of free electrons in C0 2 or H 2 

 strongly suggest that the collisions of electrons with the 

 molecules of these gases have a high degree of elasticity, 

 although naturally not so high as with the inert gases. The 

 effect of this high but imperfect elasticity would be to cause 

 the electrons when moving under an electric field in C0 2 or 

 H2 to move with an accelerated motion until their terminal 

 velocity is acquired. 



On this view the effect of traces of impurities in the gas in 

 diminishing the velocity of the electron is readily explained ; 

 the impact of the electron with the molecule of the impurity 

 is in all probability either inelastic or considerably less elastic 

 than the collision with the gas molecule, and, in consequence, 

 the electron is unable to acquire as great a velocity as in the 

 pure gas. 



(C) Electrons in Vapours. 

 The demonstration of the existence of free electrons in air, 

 C0 2 , and H 2 at relatively high pressures rendered it fairly 

 obvious that all permanent gases were able to contain 

 electrons in the free state. Franck's experiments had shown 

 previously that the inert gases were especially conspicuous 

 in this respect, the negative carriers appearing to consist 

 entirely of free electrons. It became of interest to extend 

 the investigation to the case of vapours, especially as these 

 are liable to occur as impurities in gases. It was thought 

 extremely improbable that the electrons, if they were present 

 in the free state, would occur in large numbers except at 



* Franck and Hertz, Verh. Beutsch. Phys. Ges. xv. pp. 373, 613 (1913). 



