160 Profs. Liveing and Dewar. Spectra of the [Feb. 5 T 



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 heated 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 well seen. Carbonic oxide and carbon disulphide, even when 

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

 cyanogen, hydrocyanic acid, and carbon tetrachloride mixed with 

 hydrogen, they are faint, and do not form a principal or prominent 

 part of the spectrum. This is all consistent with the supposition of 

 Angstrom and Thalen. The fact that the bands are not produced 

 even in the presence of hydrogen, when it is not present in the flame 

 in the form of a compound with carbon, is very significant; for we 

 know that acetylene is present, and can easily be extracted from the 

 flame of any hydrocarbon, and that it is formed as a proximate pro- 

 duct of decomposition of hydrocarbons by the electric discharge, but 

 we have no evidence that it is producible as a product of direct com- 

 bination of carbon with hydrogen at the comparatively low tempera- 

 ture of a flame such as we have mentioned. 



The hydrocarbon bands are best developed in the blowpipe flame, 

 that is under conditions which appear, at first sight, unfavourable to 

 the existence of acetylene in the flame. We have, however, satisfied 

 ourselves, by the use of a Deville's tube, that acetylene may readily 

 be withdrawn from the interior of such a flame, and from that part of 

 it which shows the hydrocarbon bands most brightly. 



The question as to whether these bands are due to carbon itself or 

 to a compound of carbon with hydrogen, has been somewhat simplified 

 by the observations of Watts and others on the spectrum of carbonic 

 oxide. There is, we suppose, no doubt now that that compound has 

 its own spectrum quite distinct from the hydrocarbon flame spectrum. 

 The mere presence of the latter spectrum feebly developed in the 

 electric discharge in compounds of carbon supposed to contain no 

 hydrogen, appears to us to weigh very little against the series of 

 observations which connect this spectrum directly with hydrocarbons. 



In the next place, it appears conclusively from the experiments, that 

 the development of the violet bands of cyanogen, or the less refrangible 

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

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

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

 the violet bands are not seen in hydrogen at that temperature, while 



