1882.] On the Origin of the Hydrocarbon Flame Spectrum. 421 



carbon flame spectrum induced us to believe that they were due to 

 similarly constituted compounds, and inasmuch as we felt sure about 

 the accuracy of the view which assigns the former spectrum to some 

 compound of magnesium with hydrogen, we accepted the analogy in 

 favour of the supposition that acetylene is the substance which pro- 

 duces the flame spectrum ; or, at any rate, that acetylene is a necessary 

 concomitant of the reaction taking place during its emission, and con- 

 sequently might give rise to this peculiar spectrum. 



Having examined this question in the way described, we adopted 

 the view* of Angstrom and Thalen as to the genesis of this spectrum 

 in opposition to the views of Attfield, Morren, Watts, Lockyer, and 

 others, who held that this spectrum was really due to the vapour of 

 carbon. The delicate character of the experiments which were 

 required to discover the origin of the peculiar set of flutings in the 

 more refrangible part of the spectrum of cyanogen made it apparent 

 that, whatever views as to the origin of the hydrocarbon flame 

 spectrum were adopted by different workers, experimental proof 

 was still wanting to show which was the correct one. In referring 

 to the theory that carbon vapour is the cause of the peculiar spectrum 

 of cyanogen, we remarked: "Now, the evidence that carbon uncom- 

 bined can take the state of vapour at the temperature of the electric 

 arc is at present very imperfect. Carbon shows at such temperatures 

 only incipient fusion, if so much as that, and that carbon uncombined 

 should be vaporised at the far lower temperature of the flame of 

 cyanogen is so incredible an hypothesis that it ought not to be 

 accepted if the phenomena admit of any other probable explanation." 

 ("Proc. Roy. Soc," vol. 30, p. 506.) 



With the object of being able to exhaust this question, a special 

 study was subsequently made of the ultra-violet line spectrum of 

 carbon, in order to ascertain whether any of its lines could be found in 

 the spectra of the arc or flame. We have found that the ultra-violet 

 lines of metallic- substances have as a rule the greatest emissive power, 

 and are often present when no trace of characteristic lines in the 

 visible part of the spectrum can be detected. If carbon resembled the 

 metals in this respect, then we might hope to find ultra-violet lines 

 belonging to its vapour, thus enabling us to detect the volatilisation of 



* This view was first suggested by Pliicker and Hittorf in the same paper in 

 which they published the theory of spectra of different orders. They write : — " It 

 appears doubtful that the different types depend solely upon temperature. If so, 

 the temperature varying in the different parts of the ignited vapour of carbon, 

 different types may be seen simultaneously. We shall not now discuss the influence 

 which the coexistence of foreign gases might have on the spectra of the vapour of 

 carbon, nor may we here decide whether or not in the lower temperature of the 

 flame, a gaseous compound of carbon, not being entirely decomposed, exhibits with 

 the spectra of the vapour of carbon simultaneously the spectrum of the undecom- 

 posed gas."— " Phil. Trans., 1865." (Jan. 26, 1883.) 



VOL. XXXTV. 2 F 



