176 THE CHEMISTRY OF THE STARS. 



one, the temperature evidence must depend upon the presence of cool 

 metallic lines and the absence of the enhanced ones. 



The nebulae, then, are relatively cool collections of some of the per- 

 manent gases and of some cool metallic vapors, and both gases and 

 metals are precisely those I have referred to as writing their records 

 most visibly in stellar atmospheres. 



Now, cau we get more information concerning this association of cer- 

 tain gases and metals? In laboratory work it is abundantly recog- 

 nized that all meteorites (and many minerals) when slightly heated 

 give out permanent gases, and under certain conditions the spectrum 

 of the nebulae may in this way be closely approximated to. I have not 

 time to labor this point, but I may say that a discussion of all the 

 available observations to my mind demonstrates the truth of the sug- 

 gestion, made many years ago by Professor Tait before any spectro- 

 scopic facts were available, that the nebulae are masses of meteorites 

 rendered hot by collisions. 



Surely human knowledge is all the richer for this indication of the 

 connection between the nebulae, hitherto the most mysterious bodies 

 in the skies, and the "stones that fall from heaven." . 



CELESTIAL EVOLUTION. 



But this is, after all, only a steppingstone, important though it be. 

 It leads us to a vast generalization. If the nebulae are thus composed, 

 they are bound to condense to centers, however vast their initial pro- 

 portions, however irregular the first distribution of the cosmic clouds 

 which compose them. Bach pair of meteorites in collision puts us in 

 mental possession of what the final stage must be. We begin with a 

 feebleabsorption of metallic vapors round each meteorite in collision ; the 

 space between the meteorites is filled with the permanent gases driven 

 out farther afield and having no power to condense. Hence dark 

 metallic and bright gas lines. As time goes on the former must pre- 

 dominate, for the whole swarm of meteorites will then form a gaseous 

 sphere with a strongly heated center, the light of which will be absorbed 

 by the exterior vapor. 



The temperature order of the group of stars with bright lines as well 

 as dark ones in their spectra has been traced, and typical stars indi- 

 cating the chemical changes have been as carefully studied as those in 

 which absorption phenomena are visible alone, so that now there are 

 no breaks in the line connecting the nebulae with the stars on the verge 

 of extinction. 



Here we are brought to another tremendous outcome — that of the 

 evolution of all cosmical bodies from meteorites, the various stages 

 recorded by the spectra being brought about by the various conditions 

 which follow from the conditions. 



These are, shortly, that at first collisions produce luminosity among 

 the colliding particles of the swarm, and the permanent gases are given 



