792 Prof. A. LI. Huo-hes on Dissociation 



°X 



our experimental conditions may collide with any velocity 

 between zero and that corresponding to the applied accele- 

 rating potential, the curve which would be obtained if the 

 electrons always collided with the energy corresponding to 

 the applied potential would attain its maximum at a lower 

 value, possibly somewhere between 40 and 80 volts. (One 

 disadvantage of working at higher pressures, e. g. A in Table 

 III., is that the electrons are forced to make most of their 

 collisions close to the filament, so that their energies on 

 collision are less than would be inferred from the applied 

 field.) 



On the basis of Franck, Knipping, and Kriigers view of 

 ionization, there are good grounds for believing that ioni- 

 zation and dissociation of hydrogen are bound up with one 

 another. So far as this writer knows, there are no results 

 giving the efficiency of ionization per collision as a function 

 of the energy of the electrons, i. e. the fraction of collisions 

 between electrons and molecules resulting in ionizations. 

 Johnson * has made some experiments on the total ionization 

 produced in a gas by electrons. He found that the accele- 

 rating potential had to be raised to about 50 volts before he 

 obtained one ion per electron passing through the gas. 

 This is not quite the same thing as saying that a collision 

 between a hydrogen molecule and an electron possessing 

 energy corresponding to 50 volts always results in ioni- 

 zation, but, for want of more direct information, we may 

 tentatively assume that an electron of such energy will 

 give ionization at practically every impact. If, at the 

 higher voltages, ionization of the 17 volt type predominates, 

 i. e. dissociation of the molecule combined with ionization of 

 one atom, then each collision would result in the dissociation 

 of the molecule and the production of one ion. This is what 

 our investigation suggests, if we make use of Langmuir's 

 result, that the rate of clean up measured, under the most 

 favourable conditions, only one seventh of the amount of 

 dissociation really taking place. (It is admitted that the 

 application of Langmuir's numerical result to this investi- 

 gation is open to question, but it seems certain that the clean- 

 up measurements underestimate the amount of dissociation.) 

 The view that the efficiency of dissociation is unity, i. e. that 

 every collision between a molecule and an electron will 

 result in dissociation when the electron has enough energy, 

 has an attractive simplicity. 



* Johnson, Pliys. Rev. x. p. 609 (1917). 



