352 PROF. A. FOWLER AND MR, C. C. L. GREGORY ON THE ULTRA-VIOLET 
oxygen.* Other bands in the ultra-violet which were attributed to ammonia by 
Eder were afterwards found to be identical with Deslandres’ third positive group 
of bands of nitrogen. New determinations of the positions of the ammonia bands in 
the visible spectrum were also made by Eder, but it is not necessary to consider 
these for the present investigation. In the case of the ultra-violet band, the thirty- 
four wave-lengths tabulated by Eder evidently refer to unresolved groups of band 
lines, and only serve for identification when instruments of small resolving power are 
employed. 
More recently, the band has been described and illustrated by Lewis, as it appears 
in the spectra of vacuum tubes containing mixtures of nitrogen and hydrogen, t Its 
occurrence was observed with these gases in any proportion, but not in the case of 
either gas alone. As obtained in this way, the band was complicated by superposed 
bands of nitrogen, and higher resolution than that employed was considered necessary 
to effect the separation satisfactorily. 
The band in question has been noted occasionally in various experiments carried 
on at the Imperial College during several years. It lias been observed in the flame 
of imperfectly dried cyanogen, in the spectra of vacuum tubes, and with enclosed 
arcs under reduced pressure, as well as in the ammonia flame itself. In all cases the 
band appeared under circumstances in which its presence could be^attributed to 
combined nitrogen and hydrogen, but the spectroscopic evidence does not exclude the 
possibility of some combination other than ammonia. In the course of the present 
investigation it has further been found that the brighter parts of the band sometimes 
occur feebly in the spectrum of the copper arc, and even in that of the ordinary 
carbon arc in air. 
Experimental Procedure. 
A preliminary investigation was undertaken to find a source that would give the 
band sufficiently isolated, and at the same time bright enough to be photographed 
with high dispersion. Vacuum tube methods proved unsatisfactory on account of 
the superposition of bands of nitrogen. The purest band was obtained from an 
ammonia flame fed with oxygen. An ordinary blowpipe was found to be a convenient 
arrangement, ammonia being passed through the outer tube, and a stream of oxygen 
through the inner one. The apparatus employed is illustrated in fig. 1. Ammonia 
of specific gravity 0'880 contained in the flask A was heated by a Bunsen flame, but 
not sufficiently to cause the liquid to boil. The vessel B was introduced as a means 
of condensing most of the water vapour, which otherwise condensed in the tubes of 
the blowpipe and extinguished the flame. The mercury gauge at C provided a 
convenient means of measuring the pressure, which was maintained at about 10 cm. 
of mercury. Oxygen was supplied from a cylinder of the compressed gas. The 
* ‘ Denkschr. Wien Akid.,’ vol. 60, p. 1 (1893). 
t ‘ Astrophys. Jour.,’vol. 40, p. 154 (1914). 
