1 87 1.] - On Double Spectra. 9 



spectrum, which is represented in Fig. 7, 1, consists of a 

 number of bright lines and groups of lines standing out from 

 a feebly illuminated background. 



The changes of spectrum shown by the element carbon 

 are, perhaps, as curious and interesting as any. At first 

 sight it would appear that carbon is an element unlikely to 

 yield a discontinuous spectrum, inasmuch as it is not known 

 in the gaseous condition ; and, that if we obtain discon- 

 tinuous spectra from carbon compounds, they must be due 

 to some compound of carbon. Thus the bright blue lines 

 observed by Swan (1856), in the spectrum of the Bunsen 

 flame, might be supposed to be more probably due to carbonic 

 oxide or carbonic acid than to carbon itself. But we find 

 that these same lines occur not only in the spectrum of 

 the flame, but also in the spectra obtained % passing 

 the electric spark either through carbonic oxide, or olefiant 

 gas, or cyanogen, and the lines thus found to be common 

 to compounds of carbon with different elements must of 

 course be due to carbon itself. Whether they are really 

 produced by carbon in the gaseous state is a question 

 which cannot yet be certainly decided. If the carbon is in 

 the solid state we shall then have an exception to the law 

 that incandescent solids give continuous spectra, of which 

 we have only one other example, viz., the spectrum of bright 

 lines obtained by Bahr and Bunsen from glowing erbia. 

 In the case of erbia it is not impossible that the bright 

 lines are really produced by a gas (Huggins and Reynolds, 

 Proc. Roy. Soc, June 16th, 1870), and it is by no means 

 improbable that, when a hydrocarbon is burned, it is first of 

 all decomposed into its elements, which then combine with 

 oxygen. If this be so the carbon may exist for the moment 

 in the gaseous state. 



There exist no fewer than four spectra which are all 

 probably due to incandescent carbon vapour. The first 

 carbon spectrum is obtained when olefiant gas and oxygen 

 are burnt together in an oxyhydrogen blowpipe-jet. The 

 flame thus obtained exhibits a central cone of intense 

 green, which, examined by the spectroscope, gives the 

 spectrum first obtained by Swan, and ascribed by Attfield 

 to the vapour of carbon. This spectrum, which is repre- 

 sented in the chromolithograph, No. 1, is one of the most 

 beautiful which can be imagined, and consists of five 

 groups of lines — a in the red, 7 in the greenish yellow, 

 $ brilliant emerald-green, e in the blue, and /violet. 



Group a contains five lines, of which the third is the 

 brightest. 7 contains seven, of which the least refracted is 



VOL. VIII. (O.S.)— VOL. I. (N.S.) C 



