1880.] On the Spectra of the Compounds of Carbon, fyc. 507 



hypothesis, that it ought not to be accepted if the phenomena admit 

 of any other probable explanation. On the other hand, we have 

 shown that cyanogen or hydrocyanic acid is generated in large quan- 

 tity in the electric arc taken in nitrogen, and Berthelot has shown 

 that hydrocyanic acid is produced by the spark discharge in a mix- 

 ture of acetylene and nitrogen, so that in the cases in which these bands 

 shine out with the greatest brilliance, namely, the arc in nitrogen and 

 the cyanogen flame, we know that nitrocarbon compounds are present. • 

 Further, we have shown that these bands fade and disappear in pro- 



o ^ 



portion as nitrogen is removed from the arc. Angstrom and Thalen 

 had previously shown the same thing with regard to the discharge 

 between carbon electrodes ; and the conclusion to which they and we 

 have come would proba/bly have commanded universal assent if it had 

 not been for the fact that these bands had been seen in circumstances 

 where nitrogen was supposed to be absent ; but where, in reality, the 

 difficulty of completely eliminating nitrogen, and the extreme sensi- 

 bility of the spectroscopic test, had been inadequately apprehended. 



To clear up the question from this point of view, the experiments 

 now detailed have been made, and they appear to us quite conclusive. 

 Were the evidence less conclusive than it is, it would still be as rash 

 and as illogical to conclude from the appearance of the nitrocarbon 

 bands in a case where nitrogen was presumed, not proved, to be 

 absent, that they were not due to a nitrocarbon, as it would be 

 to deny that the well-known yellow lines were due to sodium, 

 because they had been seen in cases where sodium was supposed to 

 be absent. Our argument is an induction from a very long series 

 of observations which lead up to one conclusion, and hardly admit of 

 any other explanation. But Mr. Lockyer attempts to explain the 

 disappearance of the bands when nitrogen is absent, by the state- 

 ment, " that the tension of the current used now brings one set of 

 flutings into prominence, and now another." This is no new observa- 

 tion. It is well known that variations in the discharge produce 

 variations in the relative intensities of different parts of a spectrum. 

 Certain lines of magnesium, cadmium, zinc, and other metals, very bril- 

 liant in the spark, are not seen, or are barely seen, at all in the arc. His 

 remark might be applied to the spectra of compounds as well as to 

 those of elements. Variation in the discharge accounts very well for 

 some of the variations of intensity in the bands if they be due to a 

 nitrocarbon ; it will not, however, account for the fact observed by us, 

 that the bands, or those of them which have the greatest emissive 

 power, and are best developed by the particular current used, come 

 out on the addition of a minute quantity of nitrogen, when there is 

 every reason to think that no variation of the current occurs. * 



* It is worthy of note, that if we number the sets of bands of the spectrum of 

 hydrocarbons, beginning with the orange set, and those of cyanogen, beginning with 



