98 ‘ PROFESSOR PIAZZI SMYTH ON 
If these lines, viz. sharp lines at these places (the two first of them, and 
sometimes the third also, being usually very bright), are found in the tube of 
any other elemental gas,—you may mark them down as hydrogen impurities 
at once ; or if in the tube of a compound gas containing hydrogen in its com- 
position, it may be either an impurity, or the result of the dissociation of the 
compound by the electric spark, when each ultimate element gives out the lines 
belonging to it alone, as though the others were not present. 
Similarly there is a list of 4 Oxygen lines, given by Dr Scnuster,— 
Orange at - . : : 41,254, 
Citron at ; F ; 46,729, 
Green at : é : 47,659, 
Violet at é , : 58,156, 
and if only the 1st, 2d or 3d are found in any tube which is not oxygen itself, 
or has not oxygen as one of the chemically required constituents of its contents, 
—mark it off as an oxygen impurity there. 
But when we come to the third most important gas in all terrestrial nature, 
Nitrogen, there is a difficulty ; for Nitrogen at low electric temperature and 
seen under low dispersion, has no lines; only bands; and sonumerous! With 
very low dispersion they number 50 or 60 ; and at somewhat higher dispersion 
not less than 170! Moreover there is the further mental or moral or social 
difficulty that one-half of the greater spectroscopists of the age follow MM. 
ANGSTROM and THALEN in declaring that the above spectrum of bands seen in 
a nitrogen tube is not the spectrum of nitrogen at all, but of a compound, viz. 
Oxide of Nitrogen. 
Pure nitrogen gas they say has only one spectrum, and that is totally 
different to the above banded affair ; being a spectrum of a few sharp, piercingly 
bright lines, but which require a very powerful and condensed spark to enable 
them to show at all. When ordinary small sparks are employed, the nitrogen, 
they insist, falls into combination with oxygen, and exhibits bands, as usual with 
all oxides ; while oxygen is always present on such occasions, in consequence 
of the electric spark, however weak, dissociating the hydrogen and oxygen 
constituting the water of that infinitesimally small amount of moisture, which, 
it is averred, can never be perfectly driven out of the interior of glass tubes. 
Yet other equally great authorities follow the late lamented Professor 
Piucker, and declare that the spectrum of 170 bands really is the Spectrum 
of Nitrogen, but at low spark-temperature ; and that most gases have two or 
more perfectly different spectra according to temperature. 
After trying both hypotheses on my tube observations, I incline to the 
latter of them ;,not so much from havimg been able to prove its absolute and 
perfect truth, as from having disproved the opposite view. Thus, in a Cyanogen 
tube, where there was no hydrogen line visible, there could not have been 
