8o0 Prof. Percival Lewis on the Origin of the 



small trace of oxygen. The /3 group are clearly seen on the 

 original negative, but are not relatively so strong as in a 

 and b. 



d. The true afterglow spectrum with a minimum of 

 oxygen present. The third group and j3 group are abso- 

 lutely lacking (compare with a), although the former are 

 faintly visible in /'and e, taken previously with the same gas. 

 The cyanogen bands, which are lacking in the other photo- 

 graphs, are now strong. In structure they are exactly like 

 the bands described by Strutt and Fowler *, and attributed 

 by them to cyanogen. They probably arise from stop-cock 

 grease. 



e. The condensed discharge in the same gas, accompauied 

 by the afterglow. Some bands of the third group are faintly 

 visible. 



/. The spectrum of the uncondensed discharge in the 

 same gas. 



Other photographs taken after the nitrogen had been for 

 a longer time in storage showed absolutely no trace of the 

 third and 6 groups under any conditions. 



The visual bands in the green, yellow, and red were strong 

 in all these cases. Without doubt Strutt and Fowler are 

 correct in stating that they are the survivals of certain bands 

 in the first positive group. It may be mentioned that these 

 bands, first observed by me, were also seen by Schniederjost 

 in an apparently imperfectly developed afterglow t. 



In addition to the bands heretofore described, three pairs 

 of faint bands were found, of approximate wave-lengths 

 4390, 4410, 4488, 4505, and 4596. The first pair appears 

 to belong to Strutt and Fowler's /3x group, the others to the 

 /3 3 group, as shown by their series relations. 



A number of observations w r ere also made with nitrogen 

 prepared by passing air over phosphorus. The afterglow 

 was strongly developed, but the third and /3 groups of bands 

 were not found in the spectrum. The cyanogen bands were 

 strong. 



The spectrum of nitrous oxide continuously admitted to 

 tbe tube and pumped out contained all the bands found in 

 the afterglow spectrum, but nothing additional. It did not 

 destroy the afterglow when admitted, but produced merely a 

 slight change in colour, which passed away in a few moments. 

 When nitric oxide was admitted to the glowing gas, the 

 green flame mentioned by Strutt was seen. The spectrum 

 obtained by adding oxygen, nitrous oxide, and nitric oxide 



* Proc. Roy. Soc. lxxxvi. p. 105 (1912). 

 t Zeit.furWiss. Rot. ii. p. 277 (1904), 



