SECT. iv. SPECTRA OF RAREFIED GASES. 141 



flickering light and darkness. New lines are brought 

 out in the iodine spectrum by increase of temperature. 

 At a low heat it is crossed by a number of dark lines, 

 but with a higher temperature the vapour has a greenish 

 hue, which is resolved by the prism into green lines at 

 some distance from one another, and fainter blue light, 

 crossed by groups of luminous bands. 



Rarefied compound gases are resolved by the electri- 

 city into their component parts, and the result is super- 

 posed spectra, one belonging to each element. M. Seguin 

 considers the aspect of the electric spark to be a sure 

 indication of chemical action, for while the decomposition 

 is in progress, the electric spark is encompassed by a 

 halo, and the bright lines of the double spectrum are 

 less distinct; but when the reaction is finished, the spark 

 becomes slender, and the spectrum bands distinct. In 

 the decomposition of highly oarburetted and attenuated 

 hydrogen gas, the spark resembles a flame, and the 

 spectrum is like that of white light. When the gas is de- 

 composed, the hydrogen is disengaged, and the carbon 

 deposited on the extremities of the conducting wires ; the 

 spark becomes slender, and then the lines of the hydro- 

 gen, the lines belonging to the hydro-carbon and to 

 carbon itself may be seen on the spectrum. 8 



The bright and coloured lines on the spectra of the 

 gases, and the vapours of a great number of the metals 

 and metallic salts, were known before MM. Bunsen and 

 Kirchhoff began their systematic researches, during 

 which they added many more, some so difficult and 

 analogous, that it required all their skill and experience 

 to make them out. 



Of all the spectra that have been determined, those of 

 sodium and iron are the most important and interesting. 

 In that of sodium, the only light is of the purest yellow 



8 ' On the Spectrum of the Electric Spark in Compound Gases/ by Mr. J. 

 31. Seguin. Comptes Eendus. 



