174 LIGHT SCIENCE FOR LEISURE HOURS. 



outer shell consists of the same incandescent vapour ; 

 but the outer shell of the coma either consists of in- 

 candescent solid or liquid matter or shines by reflecting 

 the solar rays. 



So far, however, there is little in the spectroscopic 

 analysis which differs in character from what had been 

 observed respecting Brorsen's comet. But we have now 

 to record one of the most startling discoveries ever made 

 respecting comets. 



Dr. Huggins was reminded by the appearance of the 

 cometic spectrum of a form of the spectrum of carbon 

 which he had observed in the year 1864. It must be 

 premised that the spectrum of an element often assumes 

 a different form according to the circumstances under 

 which it is obtained. Amongst the objects which have 

 spectra thus variable is the element carbon. The par- 

 ticular form of carbon-spectrum which resembled that 

 of the comet, is that obtained when an electric spark is 

 taken through olefiant gas a substance which, as many 

 of my readers are doubtless aware, consists of carbon 

 and hydrogen, and is one of the constituents of ordinary 

 coal-gas. 1 Of course the spectrum of olefiant gas 

 exhibits the bright lines belonging to hydrogen ; but as 

 these are well known, the part of the spectrum belong- 

 ing to carbon also becomes determinable. 



Having noticed, as we said, the resemblance between 

 the spectrum of the comet and a form of the carbon 



1 The other constituent is ' fire-damp ;' also a compound of carbon 

 and hydrogen. Olefiant gas is commonly called heavy carburetted 

 hydrogen, while fire-damp is termed light carburetted hydrogen. 



