50 INORGANIC EVOLUTION. [CHAP. 



In 1893 I wrote as follows:* "With the exception of the K line, 

 the lines of hydrogen and the high temperature line of magnesium at 

 A4481, all the lines ma}^ be said to be at present of unknown origin. 

 Some of the lines fall near lines of iron, but the absence of the strongest 

 lines indicates that the close coincidences are probably accidental." 

 In the Harvard Spectra of Bright Stars, 1897, p. 5, the following words 

 occur, relating to the same stars : " This system of lines should perhaps 

 lie regarded as forming a separate class, as in the case of the Orion 

 lines, and should not be described as ' metallic,' as has just been done 

 in the absence of any more distinctive name." 



It will be seen then that the second set of "unknown lines "has now 

 been as effectively disposed of by the determination of the enhanced 

 lines of the metallic elements as the first set was by the discovery of the 

 cleveite gases. The secrets of the " unknown lines " in the hottest 

 stars now stand revealed. 



Now that the chemical story is so nearly complete, or at all events 

 so much more complete than it was, we are in a position to inquire 

 what the stars teach us concerning their chemistry ; but in the first 

 instance we must examine the origin of the information they afford us, 

 that is, amongst other things, we must study their absorbing conditions, 

 .and next their chemistry in relation to temperature. 



With regard to the origin of the absorption phenomena, to which, 

 for the most part, our inquiries will be directed ; in the case of the 

 sun, we have a star so near us that we can examine the different parts of 

 its atmosphere, which we cannot do in the case of the more distant 

 stars. 



We have seen in Chapter IV the facts with regard to the sun 

 that the most valid absorbing layer occupies a certain region in the 

 atmosphere not high up, not at the bottom, but slightly above the 

 'bottom that is the chromospheric layer. 



Now the spectrum of Arcturus resembles the spectrum of the sun 

 .almost line for line ; what is true for the sun therefore must be equally 

 true for Arcturus, which exactly resembles it. The next point we 

 have to consider is whether the absorption in stars generally, which the 

 spectrum indicates for us, takes place from top to bottom of the atmo- 

 sphere, or only in certain levels. 



In many of these stars the atmosphere may be millions of miles 

 high. In each the chemical substances in the hottest and coldest 

 portions may be vastly different ; the region, therefore, in which this 

 absorption takes place, which spectroscopically enables us to dis- 

 criminate star from star, must be accurately known before we can 

 obtain the greatest amount of information from our inquiries. 



* Phil. Trans ^ A, vol. clxxxir, p. 694. 



