486 Mr. A. Schuster on the Spectrum of Nitrogen. [June 20, 



5. Description of apparatus. — The tubes generally used had two pockets, 

 A and B, into which small pieces of metallic sodium were introduced by 

 means of the tubes C and D. The tube C was connected with the receiver 



containing the nitrogen, -whilst the tube D was connected with the air-pump. 

 The nitrogen was generally prepared by the combustion of phosphorus in 

 air. After a few hours' standing, when all the phosphoric acid formed had 

 been absorbed, the gas became quite clear and was ready for use. This 

 mode of preparation, it is true, does not give the nitrogen very pure ; but 

 as my object was to get the nitrogen free from oxygen, and this was easily 

 obtained by means of the absorption by sodium, the method was found 

 sufficient. Other modes of preparing the nitrogen were tried, such as 

 passing air over red-hot copper or the decomposition of ammonia by 

 chlorine, but the same results were invariably obtained. The air-pump 

 used was that of Carre's freezing-machine, with which pressures down to 

 2 millims. could be easily obtained. When the pressure was measured, a 

 T-shaped tube was employed, one side of which was connected with the 

 Geissler's tube, the other with the pump, while the mercury was drawn up 

 in the longer part of the tube ; its height was read off and compared with 

 a barometer. I now pass to the description of the experiments. 



6. Method of experimenting . — When the air in the vacuum-tube had 

 been exhausted, the communication with the receiver containing the nitrogen 

 was opened, and the gas was allowed to pass through it for some time while 

 the pump was being worked. The tubing connecting the tube with the 

 receiver was then clamped air-tight, and the tube was exhausted. 



The electric spark in passing through it exhibited a violet colour, and 

 gave the spectrum of fluted bands : 



5129 4436 



4981 4390 



4649 4318 



4556 4237 



The sodium was next heated until it presented a clean metallic surface. 

 The light which the tube now emitted was bluish white, and much fainter 

 than before ; and the whole appearance of the spectrum had changed to 

 that of the second order with its characteristic green line. It was, 

 however, found that the pressure in the tube had slightly increased, owing 

 most likely to the vapour of the sodium present ; and on bringing the 

 mercury to its forme: level, the spectrum became brighter, but remained 

 the same in character. New nitrogen was then led into the tube, and after 

 exhaustion the old fluted spectrum again appeared ; this was, however, at 

 once changed into that of lines by heating the sodium. This process was 



