410 Sir AV. and Lady Huggins. On the tfpectrum [Oct. 29, 



negative pole of a vacuum tube containing a residuum of atmospheric 

 air, the ordinary, or positive-pole spectrum becomes enriched by a new 

 spectrum of bands ; and in this additional spectrum the heads of the 

 two strongest bands in the photographic region, occur at the positions 

 of the two outstanding lines of the radium glow spectrum.* On the 

 plate are given, below the more complete radium spectrum now obtained, 

 the ordinary band spectrum of nitrogen, and also the same spectrum 

 enriched with the bands peculiar to the aureole of the negative pole. 

 This latter spectrum corresponds to that of the radium glow. The peculiar 

 conditions, whatever they may be, which determine the presence of 

 these additional negative-pole bands must find their counterpart in the 

 nitrogen molecules when under stimulation by the radium bromide. 

 The additional bands which show themselves in the spectrum of 

 nitrogen when taken from the glow at the negative pole of a vacuum 

 tube are usually believed to be associated with the stimulation of the 

 very rapidly moving corpuscles of the cathode stream. Accordingly 

 the presence of these negative-pole bands in the spectrum of nitrogen 

 when excited by radium, naturally suggests whether the /? rays, which 

 are analogous to the cathode corpuscles, may not be mainly operative 

 in exciting the radium glow. On this surmise it would be reasonable 

 to expect some little extension of the glow outside the radium itself. 

 We are unable to detect any halo of luminosity outside the limit of the 

 solid radium bromide ; the glow appears to end with sudden abruptness 

 at the boundary surface of the radium. 



It may be that it is only at molecular distances, and at the moment 

 of their formation, that the rays can excite the nitrogen molecules. 



As the glow spectrum is produced by the influence of the radium on 

 nitrogen at the atmospheric pressure, it seemed to be of interest to find 

 out whether the negative-pole spectrum could be obtained in air at the 

 ordinary pressure. It has already been stated that when a suitable 

 discharge of an induction coil, without capacity in the circuit, is taken 

 between electrodes in air, the ordinary band spectrum of nitrogen 

 appears. Separate photographs, therefore, were taken of the parts of 

 the discharge in the close neighbourhood of the two electrodes, which 

 were about three-eighths of an inch apart. The bands peculiar to the 

 negative pole of a vacuum tube were found upon the plate taken of 

 the negative electrode. 



* Deslandres' measures, reduced to Rowland's scale, of the heads of these two 

 bands are 3914*4 and 4279*6 (' Theses,' 1888, Gauthier-Villars, and * Comptes 

 Rendus,' vol. 101, p. 1256). Angstrom and Thalen give 4281 '6 for the less refran- 

 gible band ('Nova Acta Upsal' (3), vol. 9, 1875). Hasselberg's measure for the 

 head of the less refrangible band is 4378'6 (' Mem. de 1'Acad. St. Petersb.,' vol. 32, 

 No. 15). Percival Lewis on "Some new Fluorescence and Afterglow Phenomena 

 iu Vacuum Tubes containing Nitrogen " ( 'Astroph. Journ.,' vol. 12, p. 8) found 

 fluorescent nitrogen to give a band spectrum ; and, in some conditions of the 

 fluorescence, the most intense bands were those of wave-lengths 3576'9 and 3371*2. 



