THE CHEMISTRY OF THE STARS. 171 



Many years of very detailed inquiry have convinced me that all stars 

 save the hottest must be sorted out into two series — those getting hotter 

 and those, like our sun, getting cooler, and that the hottest stage in the 

 history of a star is reached near the middle of its life. 



The method of inquiry adopted has been to compare large-scale photo- 

 graphs of the spectra of the different stars taken by my assistants at 

 South Kensington ; the complete harmony of the results obtained along- 

 various lines of other work carries conviction with it. 



We find ourselves here in the presence of minute details exhibiting 

 the workings of a chemical law, associated distinctly with temperature; 

 and more than this, we are also in the presence of high temperature fur- 

 naces, entirely shielded by their vastness from the presence of those dis- 

 tracting phenomena which we are never free from in the most perfect 

 conditions of experiment we can get here. 



What, then, is the chemical law? It is this: In the very hottest 

 stars we deal with the gases hydrogen, helium, and doubtless others 

 still unknown, almost exclusively. At the next lowest temperatures 

 we find these gases being replaced by metals in the state in which they 

 are observed in our laboratories when the most powerful jar-spark is 

 employed. At a lower temperature still the gases almost disappear 

 entirely, and the metals exist in the state produced by the electric arc. 

 Certain typical stars showing these chemical changes may be arranged 

 as follows : 





Stars getting hotter. 





Hottest stars. 







Stars cooling. 









Uftllntriv 













C Tauri /J Persei 









Rigel 







Y 



Lyra 







a Cygni 









Castor 





Y Cygni 











Procyon 



crOrioiiii 



i 











Arcturus and Sun. 



This, then, is the result of our first inquiry into the existence of the 

 various chemical elements in the atmospheres of stars generally. We 

 get a great diversity, and we know that this diversity accompanies 

 changes of temperature. We have also found that the sun, which we 

 independently know to be a cooling star, and Arcturus are identical 

 chemically. 



We have now dealt with the presence of the various chemical ele- 

 ments generally in the atmospheres of stars. The next point we bave 

 to consider is, whether the absorption whicli the spectrum indicates for 

 us takes place from top to bottom of the atmosphere 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 absorp- 

 tion takes place, which spectroscopically enables us to discriminate 



