58 



Dr. R. T. Beatty 



Accordingly, by increasing the velocities of the colliding- 

 electrons, the relative intensities of the higher members of 

 the Balmer series might be expected to become greater, that 

 is, the relative intensities would be functions of the potential 

 gradient in the capillary of the Geissler tube. This view, 

 however, is not confirmed by an examination of the variation 

 of the potential gradient with pressure. Nutting and 

 Tugman * give the results shown in the second line of 

 Table IV., taken from page 56 of their paper. In the third 

 line I have given the numbers showing the corresponding 

 fall of potential per mean free path of an electron moving in 

 hydrogen at the pressure mentioned. 



Table IV. 

 Discharge in Hydrogen : 2*6 mm. capillary. 



Current =15 milliamperes. 



Pressure in nuns. 



Volts per cm 



Volts per free path 



0-5 



1-0 



2-0 



30 



8-0 



35-8 



49-5 



67-4 



78 



120 



6-17 



4-27 



2-91 



2-24 



1-30 



From this table we see that the velocity acquired by an 

 electron between successive collisions decreases with in- 

 creasing pressure, yet no corresponding variation in j3/ot 

 takes place at pressures greater than 4 mm., as is evident 

 from figs. 5 and 6 (PL II.). 



From these and other experimental results which will 

 now be mentioned, the author concludes that the energy 

 distribution in the lines of the Balmer series is independent of 

 the energy of the colliding electron or molecule 'which strikes the 

 hydrogen atom and causes it to emit these lines. 



Vegard f found that when hydrogen becomes luminous 

 by the impact of canal rays, the intensity ratio /3/ot is 

 independent of the velocity of these rays and is of the same 

 value as when the emission is in the negative glow of a 

 discharge tube where the excitation is due to cathode rays. 

 It is to be noted that the pressures in these experiments 

 were so low that the time required by a hydrogen atom 

 to describe a free path exceeded the probable duration of 

 its emission of light. 



* Bulletin of the Bureau of Standards, (7) i. August 6, 1910, 

 pp. 49-70. 



t Ann. der Physik, xxxix. p. Ill, 1912. 



