/ ///; TWO COMETS OF THE YEAR 1808. 157 



character and shine by reflecting the solar light. This 

 part of UK; coma m;iy IMJ either li(juid or solid. There 

 i.s nothing opposed to the supposition that it in of 

 the nature of cloud --that is, that it is produced by 

 the condensation of trim vapour into minute li&amp;lt;[uid 

 globules. 



Returning to the consideration of the gaseous part 

 nf tin; cornet the question will at mice surest itself 

 what the gases may be wliich constitute the substance 

 of the nucleus and coma. Mere our information is not 

 quite so satisfactory as could be desired. 



The brightest band ia in the green part of the spec 

 trum, and agrees very nearly with the brightest line in 

 the spectrum of nitrogen. The want of exact agree 

 ment prevents us from assuming that nitrogen really 

 exists (in any form) in the substance of the comet. 

 The olhcr lines of the spec! rum of nitrogen are not 

 present in the spectrum of the comet: but, this pecu 

 liarity is not, so perplexing as the other, for it is well 

 known that certain lines will disappear from the spectra 

 of hydrogen, nitrogen, and other gases, under particular 

 circumstances of illumination, temperature, and so 

 on. 



Nor is the, circumstance that, there are bands of light 

 instead of well marked lines a peculiarity which need 

 cause perplexity. For under certain circumstances of 

 temperature and pressure, the lines of the spectra of 

 various gases become expanded or diffused uut.il they 

 appear as bands of light. 



The two fainter bauds are yellow and blue, respec- 



