IONIZATION POTENTIALS IN DISSOCIATED GASES. 225 



Excitation of the Nitrogen Spectrum. 



The nitrogen spectrum consists of three groups of positive bands, 

 two of which appear in the visible spectrum and one only in the ultra- 

 violet, a group of negative bands and a line spectrum which has not 

 been resolved into series. 



The first group of positive bands was first seen when the arc 

 struck and increased slightly in intensity as the voltage was raised. 

 The second group was first seen when the arc struck and decreased 

 in intensity with increasing voltage. L. and E. Bloch^*' and Brandt^^ 

 have detected positive bands at voltages as low as 12 and 7.5 volts, 

 respectively. It is therefore evident that the positive bands are due 

 to excitation of neutral molecules. 



The negative bands were not observed until the voltage was a 

 volt or two above the minimum ionizing potential, and increased 

 markedly in intensity with increasing voltage. Rau^^ has found a 

 Doppler shift in the negative bands in positive rays. These and other 

 facts indicate that the negative bands are due to excitation of posi- 

 tively charged molecules N,. 



It is surprising that as high a voltage as 70 volts was required 

 to excite the line spectrum, and that only the lines 5006, 5003, 5680, 

 and 5667 were observed. There are a number of other lines usually 

 listed as at least as intense as these of which no trace was found. It 

 seems probable, as is suggested by the magnitude of the Doppler 

 shifts^^ for the nitrogen lines, that the lines which we observed are 

 enhanced lines, and the remaining strong lines in the visible spectrum 

 may correspond to still higher ionization. The Doppler shifts sug- 

 gest that a line 3995 A. may be an unenhanced line. Unfortunately 

 we did not investigate this region of the spectrum, but we plan to 

 make a photographic test in the near future. At any rate, it seems 

 certain that the simpler parts of the nitrogen line spectrum lie in a 

 region of wave-lengths far removed from the visible, and that the 

 difficulty of dissociating the molecules makes it difficult to excite the 

 line spectrum under conditions which can easily be interpreted. 



16 Comptcs Rendus, 170, p. 1380, 1920. 



1" Zcit. f. Phys., 8, p. 32, 1921. 



IS " Vortrag auf der Physikertagung in Jena," 1921. 



19 J. J. Thomson, " Positive Rays." 



