SPECTEA OE IGNITED GASES AND VAPOURS. 
11 
often as the wire is taken out of the mercury we shall have the spectrum of bands ; as 
soon as the communication is restored, the spectrum of bright lines. Hence we con- 
clude that the change of the molecular condition of nitrogen which takes place if the 
gas be heated beyond a certain temperature by a stronger current, does not permanently 
alter its chemical and physical properties, but that the gas, if cooled below the same 
limit of temperature, returns again to its former condition. 
27. The essentially different character of the two extremities of the first spectrum of 
nitrogen, as described (16-19), and the indistinctness of its middle part, suggested to us 
the idea that, in reality, the observed spectrum might originate from the superposition 
of two single spectra. Accordingly one of these single spectra, the more refracted part 
of which is best developed, must be formed by channeled spaces ; the other one, the less 
refracted part of which is best developed, must be a spectrum of shaded bands. In 
different cases, either the one or the other of the spectra may be predominant. 
In order to confirm our conjecture it was necessary to get the two spectra separated. 
28. The discharge of Ruhmkorff’s coil through a spectral tube is changed the less 
by introducing the Leyden jar, the weaker is the resistance opposed to it by the tube. 
Accordingly the two different degrees of temperature to which the gas rises by the 
discharge when, the coil remaining the same, we either make use of the jar or not, 
may be regulated in such a way as to approach one another more and more. Let the 
tension of the gas of about 10 millims. remain the same, the temperature produced by 
the discharge will be diminished by increasing the interior diameter of the capillary 
part of the spectral tube. Thus we succeeded in constructing a tube which, when the 
direct discharge was sent through it, became incandescent with the most brilliant gold- 
coloured light, which might easily be confounded with the light of highly ignited vapours 
of sodium ; but with the intercalated jar, the light of the incandescent gas within the 
same tube had a fine bluish-violet colour. The yellow light, when analyzed by the 
prism, gave a beautiful spectrum of shaded bands, extending with decreasing intensity 
to the blue, the channeled spaces being scarcely perceptible. The bluish light, when 
examined, was resolved by the prism into channeled spaces extending towards the red, 
while the former bands almost entirely disappeared. We may transform each colour 
and its corresponding spectrum into the other ad libitum. 
Hence it follows that there is another allotropy of nitrogen, which, like the former, is 
not a stable and permanent one, but depends only upon temperature. The modification 
in which nitrogen becomes yellow corresponds to the lower, the modification in which it 
becomes blue to the higher temperature. 
29. When we send the direct discharge of Ruhmkorff’s coil through one of Geissler’s 
wider tubes enclosing very rarefied nitrogen or air (the oxygen of air becomes not visible 
here), we see the negative pole surrounded by blue light, the light at the positive pole 
being reddish yellow. In such of Geissler’s tubes as are especially calculated to show 
how the light starting in all directions from the different points of the negative elec- 
trode is by the action of an electro-magnet concentrated along the magnetic curves 
c 2 
