Bands in the Spectrum of Active Nitrogen, 829 



was greatly affected by traces of oxygen ; with one-half per 

 cent, present, the condensed discharge would not pass at a 

 pressure above 2'5 cm. ; when the nitrogen reached the limit 

 of purity, the discharge passed at a pressure over 4 cm. 

 One of the most remarkable facts noted was that after the 

 glow had been once established, oxygen could be admitted to 

 the pump until the pressure was raised 20 per cent, without 

 destroying it. The discharge-tube was connected with the 

 pump by a tube about 4 mm. in diameter and 250 cm. long, 

 and as the oxygen slowly diffused over into the tube the 

 active nitrogen seemed able to dispose of it in some way — 

 possibly by converting it into nitrogen peroxide. Under 

 these conditions the afterglow would last several hours, only 

 slowly diminishing in intensity. If a little nitrogen was 

 admitted, however, it drove the oxygen into the tube en 

 masse, and the glow was immediately destroyed. If several 

 minutes elapsed after admitting the oxygen before the dis- 

 charge was set up, the afterglow was weak or absent. It 

 was also noted that nitrogen which at first contained a 

 sufficient amount of oxygen to barely produce the third 

 group of bands, would cease to do so after the discharge 

 had passed for some time. (Compare fig. / in the ultra- 

 violet of Plate X. with e and d. These were made with the 

 same filling of gas, / being taken first.) Possibly all the 

 oxygen or nitric oxide had been converted into peroxide by 

 the discharge. 



Photographs of the pure afterglow spectrum, after the 

 discharge had ceased, were obtained by the use of a sectored 

 occulting disk which rotated in front of the slit and broke 

 the circuit at regular intervals before the exposure was 

 made. A number of photographs were taken with the 

 nitrogen as pure as possible, and also with various propor- 

 tions of oxygen. Some of these photographs are reproduced 

 in Plate X. and are described below. The comparison spectrum 

 is that of the iron spark. 



a. The true afterglow spectrum of nitrogen containing a 

 small proportion of oxygen initially. After the afterglow 

 was established, enough oxygen was admitted to the pump 

 to raise the pressure 15 per cent. Nevertheless, the glow 

 lasted during a three-hour exposure. The third and ft 

 groups are strong ; the second positive group do not 

 appear. 



b. The condensed discharge in the same gas (taken before 

 a). It is to be noted that the " fourth " group of bands do 

 not appear. 



c. The uncondensed discharge in nitrogen containing a 



