56 INORGANIC EVOLUTION. [CHAP. 



We have now associated two different series of phenomena, and we 

 -are entitled to make the following general statement : 



G-aseous stars . . . . Highest temperature. 

 Metallic stars . . . . Medium temperature. 

 Carbon stars . . . . Lowest temperature. 



Hence the differences in apparent chemical constitutions are asso- 

 ciated with differences of temperature. 



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 also find that the sun, which we inde- 

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

 chemically. 



Can we associate with the two to which I have already called atten- 

 tion still a third class of facts 1 



Laboratory work enables us to do this. 



The cleveite gas spectrum and the spectrum of enhanced metallic 

 lines come to our help and enable us to get a step forwarder. In 

 studying the appearance of these lines in stellar spectra, we have a 

 third series of phenomena available, and we find that the results are 

 absolutely in harmony with what has gone before. Thus 



Gaseous stars .. Highest temperature .. Strong cleveite gas and 



faint enhanced lines. 

 rFeeble cleveite gas and 



,.- , ,,. ,, ,. strung enhanced lines. 



Metallic stars , , Medium temperature . . < . . 



No cleveite gas, and 



- strong arc lines. 

 Carbon stars . . Lowest temperature . . Faint arc lines. 



It is clear now, not only that the spectral changes in stars are asso- 

 ciated with, or produced by, changes of temperature, but that the study 

 of the enhanced spark and the arc lines lands us in the possibility of a 

 rigorous stellar thermometry, such lines being more easy to observe 

 than the relative lengths of spectrum. 



What then, is the chemical law 1 It is this. In the very hottest 

 stars we deal, speaking generally, with the gases hydrogen, helium, 

 asterium, 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 tempera- 

 ture still the gases almost disappear entirely, and the metals exist in 

 the state produced by the electric arc. 



I said " speaking gencrall ," but we really can go further than this 



