IONIZATION POTENTIALS IN DISSOCIATED GASES. 215 



volts, while in curve (&), with the tube still hotter, the arc struck at 

 about lo.o volts, and there was no indication of further ionization 

 near either 14 or 16 volts. 



The essential difference between these two cases lies in the fact 

 that in the latter cases the tube was sufficiently hot to completely dis- 

 sociate the hydrogen inside it into atomic hydrogen. Thus the criti- 

 cal potential 16.3 is the ionization potential of hydrogen molecules, 

 whereas 13.6 volts is that of hydrogen atoms. The critical potential, 

 lo.i volts, is also due to atomic hydrogen and is shown later to be 



S \Q IS 10 Z5 



Volts 



Fig. 3. 



5 \0 \5 10 



due to excitation of radiation from the atoms. That this radiation 

 results in ionization is probably due to a " cumulative " effect — atoms 

 being struck by electrons while they are in a partially ionized state 

 because of absorption of energy of radiation from neighboring atoms. 

 The above experiments do not fix the values of the critical poten- 

 tials as accurately as do the experiments by the following method, but 

 they do determine beyond all question the sources of the observed 

 effects. These are the first laboratory experiments to he performed 

 in an atmosphere of atomic hydrogen. 



PROC. AMER. PHIL bUC, VOL. LXI, P, .MA'. 1 7, 1922. 



