1881.] on the Orujin and LJentitij of Spectra. G79 



hydrocarbon bands, is not a matter of temperature only. For the 

 appearance of the hydrogen lines C and ¥ in the arc taken in hydro- 

 gen indicates a temperature far higlier than that of any flame. Yet 

 the violet bands disai)pear at that temperature, and the green bands 

 are well developed. The violet bands are, nevertheless, seen equally 

 well at the different temperatures of the flame, arc, and spark, pro- 

 vided cyanogen be the compound under observation in the flame, and 

 nitrogen and carbon are present together at the higher temperatures 

 of the arc and spark. 



The question of the constitution of comets, since the discovery by 

 Huggins* that the spectra of various comets are identical with the 

 hydrocarbon spectrum, naturally leads to some sj^eculation in connec- 

 tion with the conclusions to which our ex2)eriments i)oint. Provided 

 we admit that the materials of the comet contain ready-formed hydro- 

 carbons and that chemical or electrical actions may take place, gene- 

 rating a high temperature, then the acetylene spectrum might be 

 l)roduced at temperatures no higher than that of ordinary flames with- 

 out any trace of the cyanogen spectrum, or of metallic lines. Such 

 actions might be brought about by the tidal disturbances involving 

 collisions and projections of the constituents of the swarms of small 

 masses circulating in orbits round the sun, which we have every reason 

 to believe constitute the cometic structure. If, on the other hand, 

 we assume only the presence of uncombined carbon and hydrogen, we 

 know that the acetylene spectrum can only be produced at a very high 

 temperature ; and if nitrogen were also present, that we should at 

 such a temperature have the cyanogen spectrum as well. Either then 

 the first supposition is the true one, not disi^roving tlie presence of 

 nitrogen ; or else the atmosphere which the comet meets is hydrogen 

 only and contains no nitrogen. 



The Flame of Cyanogen, 



The accompanying diagram (PI. T. Fig. 2) shows the relative 

 position of the bands in that part of the spectrum of the flame of 

 cyanogen fed with a jet of oxygen which is more refrangible than the 

 Fraunhofer line F. Only those bands which are less refrangible than 

 the solar line L have been before described, but photographs show 

 another set of two shaded bands slightly less refrangible than the 

 solar line N accompanied by a very broad diffuse band of less in- 

 tensity on the more refrangible side of N ; also a strong shaded 

 band, which appears to be absolutely coincident with the remarkable 

 shaded band in the solar spectrum, which has been designated by the 

 letter P ; and near this, on the less refrangible side, a much fainter 

 diffuse band, which also seems to coincide with a part of the solar spec- 

 trum sensibly less luminous than the parts on either side of it. Watts 

 found that the spectrum cyanogen of the flame did not disappear when the 



• ' Proc. Roy. Soc' vol. xvi. p. 386 ; vol. xxiii. p. 154 ; ' Phil. Trans.' 1868, p. 555. 



