II. Note on the reversal of the spectrum of cyanogen

Abstract

In the course of many observations on the reversal of lines of metallic spectra, we have frequently noticed dark shaded bands which appeared to be the reversals of bands ascribed to the oxides or chlorides of sundry metals; more particularly we have seen them when experimenting with compounds of the alkaline earths, and we have repeatedly obtained a reversal of the green magnesium-hydrogen series; but, until recently, we have never seen any reversal of the shaded bands of the spectrum of cyanogen, though our attention has been constantly directed to this spectrum. Quite lately, however, we have obtained photographs which show the reversal of the violet and ultra-violet bands of this spectrum; and the fact is perhaps of sufficient interest, especially in connexion with the question of the occurrence of these bands amongst tliG Fraunhofer lines, to warrant the publication of this note. We have not yet succeeded in determining precisely the conditions under which the reversal can be produced at will. The most complete reversals of these bands were obtained by the use of the arc of a Siemens’ machine, in a crucible of magnesia, fed with a considerable quantity of cyanide of titanium. The photographs in this case show a very complete reversal of the five bands near L, and of the two strong bands near N, and a less complete reversal of the six bands, beginning at about wave-length 4215. No other metallic cyanides have given, when introduced into the crucible, any such reversal; nor does a stream of cyanogen led in through a perforated carbon produce the effect. Various other nitrogenous compounds have been tried, but the only one which has given us anything like the effect of the titanic cyanide is borate of ammonia. Some photographs taken immediately after the introduction of borate of ammonia show distinctly the reversal of the group of bands near L. In one case when metallic magnesium had been put into the crucible, the photograph shows a reversal of only that part of the series which is nearest to the magnesium group, indicating that the reversal is due to the bright background supplied by the expanded magnesium lines. There can be little doubt that the greater stability of titanic cyanide and boron nitride than of other nitrogenous compounds, has some influence upon the result; and the difficulty in producing the reversal at will is in securing an absorbent stratum of sufficiently high temperature and at the same time a sufficiently luminous background. The circumstances which secure the former condition almost always produce in the arc a still more intense radiation of just those rays which are absorbed, without that expansion of the lines which shows out the absorption in the case of so many metallic spectra. The photographs are, however, conclusive evidence that it is possible to secure both conditions.