&28 {Prof. Percival Lewis on the Origin of the 



of oxygen was added they invariably appeared when the 

 discharge was weak, and also with the condensed discharge 

 in a large vacuum-tube. In addition, feeble traces of the 

 new group of bands observed by me, and called the j3 group 

 by Strutt and Fowler, were found whenever the third group 

 was seen 



The study of the afterglow was then resumed. The 

 spectra were obtained with the large quartz spectrograph 

 secured with the aid of a grant from the Carnegie Institution 

 of Washington *. A large end-on tube was used, with a 

 quartz window, and a real image of the cross-section of the 

 tube was thrown on the spectrograph slit. In this way the 

 spectrum of the active nitrogen was superimposed on that of 

 the discharge, and extended a little way beyond it. At the 

 first trial the tube leaked slightly, but the afterglow could 

 be maintained by repeatedly pumping out and refilling. The 

 spectrum showed every band recorded by Strutt and Fowler 

 and by me, and also the " fourth positive group " first 

 observed by Strutt and Fowler. As remarked by them, 

 these bands are found only in the spectrum of the discharge 

 which produces the afterglow, but not in the spectrum of 

 the afterglow itself. 



A fresh supply of nitrogen was prepared as before. 

 Kreusler f states that nitric oxide is present in nitrogen 

 prepared from sodium nitrite and ammonium sulphate. 

 Such appeared to be now the case ; with a feeble dis- 

 charge the fluorescence of the glass was strong when the 

 gas was first admitted, and the spectrum of the third group 

 was intense. In the afterglow spectrum the other ultra- 

 violet bands (except the " fourth positive ") also appeared. 

 The smell of nitrogen peroxide in the gas escaping from 

 the pump left no doubt as to the original presence of nitric 

 oxide. After twenty-four hours the fluorescence could barely 

 be produced ; after seventy-two hours none was to be seen, 

 and in the spectrum of the active nitrogen the third group 

 and /3 bands were barely perceptible, while cyanogen bands 

 appeared. The nitric oxide had evidently been absorbed by 

 the pyrogallic acid or by the drying materials. 



When oxygen was admitted, so long as it was thoroughly 

 mixed with the nitrogen in advance, there was no afterglow 

 until the proportion of oxygen was reduced to something 

 below one per cent. At less than one-half per cent, the 

 glow was fairly strong, and was very bright when at least 

 one-tenth per cent, was certainly present. The discharge 



* Astroplivs. Journ. xxiii. p. 390 (1906). 

 t Ann. tier Phys. vi. p. 419 (1901). 



