ON OUR KNOWLEDGE OF SPECTRUM ANALYSIS. 267 



authors expressed the opinion that the candle-spectrnm was due to a hydi'O- 

 carbon. The experiments wliich they gave in support of their view were 

 made by taking the spark of carbon electrodes in various gases and 

 examining the spectra of the ' aureole ' ' or ' glory,' as it might be called. 

 If the spark is taken in oxygen the undoubted spectrum of carbonic oxide 

 appears ; in hydrogen the candle-spectrum is seen ; and in nitrogen some 

 blue and violet bands are added to the candle-spectrum which appear to 

 be due to a compound of carbon and nitrogen. As it is known that 

 acetylene is formed when the spark is taken in hydrogen, Angstrom and 

 Thaleu conclude that the spectrum seen in the ' glory ' is due to 

 acetylene. 



Recently Professors Liveing and Dewar have supported Angstrom's 

 view. The following quotations from their paper will give an idea 

 of the view taken up by the two Cambridge professors : — ■ 



' Our faith in the conclusions of Angstrom and Thalen on this subject 

 has been much strengthened by our own observations, which we now pro- 

 ceed to describe ' (p. 154). 



Their experiments consisted in observing the spectra seen in the electric 

 arc passing between carbon poles in various gases, such as air, carbonic 

 acid, hydrogen, nitrogen, chlorine, carbonic oxide, nitric oxide, and am- 

 monia. They also examined some flames of carbon compounds. The 

 following is their summary of that jsart of their work which relates to the 

 candle-spectrum : — 



' In the next place, the green and blue bands, characteristic of the 

 hydrocarbon flame, are well seen when the arc is taken in hydrogen ; but 

 though less strong when the arc is taken in nitrogen or in chlorine, they 

 seem to be always present in the arc whatever the atmosphere. This is 

 what we should expect, if they be due, as Angstrom and Thalen suppose, 

 to acetylene ; for we have found that the carbon electrodes always contain, 

 even when they have been long treated in chlorine, a notable quantity of 

 hydrogen. 



' In the flames of carbon compounds they by no means always appear ; 

 indeed it is only in those of hydrocarbons or their derivatives that they 

 are w^ell seen. Carbonic oxide and carbon disiilphide, even when mixed 

 with hydrogen, do not show them ; and if seen in the flames of cyanogen, 

 hydrocyanic aeid, and carbon tetrachloride mixed with hydrogen, they 

 are faint, and do not form a principal or prominent part of the spec- 

 trum. This is all consistent with the supposition of Angstrom and 

 Thalen.' 



The experiment, noticed above, on carbon tetrachloride was made by 

 Lockyer in answer to Professors Liveing and Dewar's paper. 



To recapitulate shortly the arguments on either side : Those who 

 believe the spectrum to be due to the element carbon rely chieflj- on the 

 brilliancy with which these bands are developed when cyanogen is burnt in 

 oxygen, also when the spark is taken in cyanogen, carbon tetrachloride, 

 and carbonic oxide at high pressure ; all the gases being dried with the 

 greatest care. Those who oppose this view and who hold that the spec- 

 trum is due to a hydrocarbon, refer to the impossibility of excluding all 



' The French language is the only one which possesses, as far as I know, an 

 appropriate word for the sheet of light connectiug imdcr certain conditions the 

 electrodes in addition to the spark proper or trait dv feu. The term * glory " was, as 

 far as he can rcmeiuber, sugge.sted to the writer by the late Prof. Maxwell. 



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