WHAT LIES BETWEEN THE STARS — ADAMS 149 



move as little as possible from their condition of rest and the spec- 

 trum lines which we observe are those due to transitions of this sort. 

 This is the reason why the complicated spectrum of a gas like cyano- 

 gen, consisting of hundreds of closely packed lines, is reduced to a 

 meager three or four lines when observed in interstellar space. 

 These are the only lines which the molecule in its lowest state of 

 energy can absorb. 



In this brief outline we have discussed the gaseous material of 

 space, how it is studied, and what we know about its composition, 

 temperature, and density. We have seen that three of the most im- 

 portant elements which enter into the composition of the universe, 

 hydrogen, nitrogen, and carbon, are present in the form of compounds, 

 and that others as yet unidentified are represented by spectral lines in 

 the interstellar gases. If in most of our considerations we have had 

 our ciphers on the left-hand side of the significant figures instead of 

 the right, it is because we have been dealing with atoms and molecules 

 instead of stars and universes. Even so, such is the volume of space 

 that the mass of the dust and gas which lies between the stars may 

 well exceed by several fold all the matter actually visible with our 

 greatest telescopes. 



