12 
DBS. J. PLUCKER AND J. W. HITTORF ON THE 
passing through these points, the blue light is most beautiful. It belongs generally to 
the nitrogen alone, which, on account of the greater resistance at the negative electrode 
opposed to the discharge, reaches a higher intensity of heat there than at the positive pole. 
When analyzed by the prism, the blue light gives the spectrum of channeled spaces, with 
traces only of the less refracted bands. The reddish-yellow light of the positive pole is 
more faint, and therefore not so easy to be submitted to spectral analysis. 
30. When Ruhmkorff’s large induction coil is discharged in common air between 
two points the distance of which does not exceed a few centimetres, we obtain, as is well 
knoAvn, a brilliant spark surrounded by an aureola, the colour of which is partly bluish 
violet, partly reddish yellow. In order to separate these colours more distinctly from 
each other, the aureola, moved by the slightest breath, may be extended into a large 
surface by blowing it sideways. But the separation may be best made when the dis- 
charge takes place between the two poles of an electro-magnet in the equatorial direc- 
tion. While the straight spark is not acted upon by the electro-magnet to any sensible 
degree, the aureola is expanded into a fine surface, bounded by the spark starting from 
one to the other extremity of the electrodes, and by a semicircle passing through these 
extremities. At a certain rarefaction of air this surface appeared most beautifully 
bounded by a semicircular golden-coloured band, and divided by a similar band into two 
parts*. We may explain now in a satisfactory way the appearance, hitherto mysterious, 
of the golden light. Both the yellow and the blue light are owing to the nitrogen of the 
air, reduced by the heat of the current into the two allotropic states which exhibit the 
spectra of channeled spaces and of bands. The brilliant white light of the spark partly 
belongs to the oxygen, partly to the nitrogen of the air, both highly ignited, the nitrogen 
being in that allotropic state in which it exhibits the spectrum of bright lines. 
31. In order to complete the history of the spectrum of nitrogen we add two remarks. 
First, by intercalating a Leyden jar and, in order to weaken the current, at the same 
time a stratum of water or a wet thread, we may also reduce the spectrum of bright 
lines to the spectrum of bands. Secondly, by increasing the density of the gas, or, if the 
gas be less dense, by intercalating at the same time a large jar and a stratum of air, the 
bright lines of the spectrum, at the highest obtainable temperature, will expand. Out 
of a great number of observations made in this direction we shall describe only one. 
32. A short spectral tube enclosing nitrogen of a tension of about 250 millims. 
refused passage to the discharge of Ruhmkorff’s large induction coil, when three of 
Grove’s elements were made use of and the jar intercalated. Without the jar the 
discharge passed through and produced a bright but rather undefined spectrum of 
bands. When the current continued to pass, the indistinctness of the spectrum in- 
creased, and after short intervals brilliant coloured lines appeared and disappeared 
again, like lightning-flashes. These lines, occupying always the same place, belonged 
to the second spectrum of nitrogen, the brightest yellow and green lines of which 
* Pll'cker, “TTeber die Einwirkung desMagnetes auf die elektrische Entladung,” Poggendohff's ‘Annalen,’ 
vol. cxiii. p. 267. 
