THE ENVIRONMENT 55 



With greater or less completeness and accu- 

 racy the atmospheres of the moon, of Mars, 

 and of other planets have been studied and 

 accounted for. 



IV 



THE ATMOSPHERE 



Even at the earliest period in the evolu- 

 tion of a typical star there appears to he 

 a progressive variation in chemical composi- 

 tion from center to periphery. Theoreti- 

 cally it seems inevitable that the heaviest 

 elements should be concentrated in the interior 

 and that those of lowest atomic weight should 

 be present in greatest amount near the sur- 

 face. Actually, as above stated, spectro- 

 scopic investigation fully confirms this view. 

 Thus the spectra of typical hot stars show 

 that hydrogen is an invariable constituent 

 of their superficial parts. Indeed the uni- 

 versal presence of hydrogen under such cir- 

 cumstances is undoubtedly one of the most 

 clearly established facts of stellar astronomy. 

 As stars cool and become red the spectral 

 changes quite as unmistakably point to the 

 presence of carbon. Accordingly we possess 

 the best of evidence and the best of reasons 

 for the belief that large quantities of hy- 



