THE TWO COMETS OF THE YEAR 1868. 157 



character and shine by reflecting the solar light. This 

 part of the coma may be either liquid or solid. There 

 is nothing opposed to the supposition that it is of 

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

 the condensation of true vapour into minute liquid 

 globules. 



Eeturning to the consideration of the gaseous part 

 of the comet the question will at once suggest itself 

 what the gases may be which constitute the substance 

 of the nucleus and coma. Here our information is not 

 quite so satisfactory as could be desired. 



The brightest band is 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 other lines of the spectrum 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 until they 

 appear as bands of light. 



The two fainter bands are yellow and blue, respec- 



