354 Mr Purvis, Experiments on the hand- spectrum 



Experiments on the band- spectrum of nitrogen in a strong 

 magnetic field. By J. E. Purvis, M.A., St John's College, 

 Cambridge. 



[Read 14 May 1906.] 



Experiments have previously been recorded by Becquerel and 

 Deslandres (Gomptes rendus, vol. 127, 1898, p. 20) on the band- 

 spectrum of nitrogen when vibrating in a magnetic field, and they 

 shewed that there was neither appreciable division nor widening 

 (ni dedoublement ni elargissement appreciables) of the bands 

 when vibrating under the influence of the electro-magnetic field 

 utilised by them. The strength of this field with a current of 

 9-10 amperes was 20,000 units. 



The author has repeated the experiments when the discharge 

 took place in a field of 41,000 units. The tubes containing the 

 nitrogen were specially made with the capillary parts of 3*5 mm. 

 external diameter, so that they could be introduced between the 

 poles of a magnet 4 mm. apart. With a current of 25 amperes, 

 a field strength of 41,000 units was obtained between these poles. 

 The nitrogen was obtained from barium nitride kindly given to 

 me by Dr Scott, and was dried and purified before entering the 

 tubes. The parts of the capillary not within the sphere of in- 

 fluence of the magnetic field were screened off, and the light from 

 the unscreened part was focussed on the slit of the spectroscope 

 by a single lens. The discharge was obtained by means of a coil 

 giving a 6-inch spark and without a jar. In the first experiments 

 a plane grating was used of 14,438 lines to the inch, and the various 

 bands in the third and fourth orders of the spectrum were re- 

 spectively focussed on the photographic plate, so that each band 

 could be investigated separately. In the later experiments, Pro- 

 fessor Liveing's large 21 -foot concave grating was used, whereby 

 the bands could be photographed at the same time on the same 

 plate, and the 1st and part of the 2nd orders observed. The 

 second order ranged from about ~K 3500, so that the bands in the 

 ultra-violet could not be photographed in consequence of the glass 

 tubes absorbing these vibrations. 



Eye observations were made in the earlier experiments with 

 the plane grating, and photographs in the later ones. But the 

 range of the latter is obviously limited in consequence of the 

 absorption by the glass of the rays of greatest refrangibility. As 

 is well known, the colour of the discharge in nitrogen when 

 vibrating in a magnetic field changes from a buff or pink to 

 a bright steely blue colour. At the same time the spectral bands 



