1880.] Compounds of Carbon with Hydrogen and Nitrogen. 161 



the green bands are well developed. The violet bands are, never- 

 theless, seen equally well at the different temperatures of the flame, 

 arc, and spark, provided 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 similarity in the character of the magnesium-hydrogen spec- 

 trum, which we have described, to the green band of the hydro- 

 carbons is very striking. We have similar bright maxima of light, 

 succeeded by long drawn out series of fine lines, decreasing in in- 

 tensity towards the more refrangible side. This peculiarity, common 

 to both, impels the belief that it is a consequence of a similarity of 

 constitution in the two cases, and that magnesium forms with 

 hydrogen a compound analogous to acetylene. In this connexion the 

 very simple relation (2 : 1) between the atomic weights of mag- 

 nesium and carbon is worthy of note, as well as the power which 

 magnesium has, in common with carbon, of combining directly with 

 nitrogen. We may with some reason expect to find a magnesium- 

 nitrogen spectrum. 



Apart from the mere relative electric conductivity of gases, it is 

 clear, from the foregoing experiments, that the length and the 

 character of a discontinuous electric discharge in different gases does 

 not follow the law which we should expect. It will require a prolonged 

 series of experiments to arrive at definite conclusions on this matter, 

 but it appears to us, in the mean time, highly probable that one of tbe 

 main factors in producing these remarkable variations in the arc will 

 be found to be the relative facility with which the carbon electrode 

 combines with a gaseous medium. 



The interest attaching to the question of the constitution of 

 comets, especially since the discovery by Huggins (" Proc. Roy. 

 Soc," xvi, p. 386 ; xxiii, p. 154; "Phil. Trans.," 1868, p. 555), that 

 the spectra of various comets are all identical with the hydrocarbon 

 spectrum, naturally leads to some speculation in connexion with the 

 conclusions to which our experiments point. Provided we admit that 

 the materials of the comet contain ready formed hydrocarbons and 

 that oxidation may take place, then the acetylene spectrum might be 

 produced at comparatively low temperatures without any trace of the 

 cyanogen spectrum, or of metallic lines. Such reactions might be 

 brought about by the tidal disturbances involving collisions and pro- 

 jections 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 have 

 the cyanogen spectrum as well. Either then the first supposition 



