SIR NORMAN LOCKYER 365 



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 : 



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 have 

 to consider is, whether the absorption which 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 por- 

 tions may be vastly different. The region, therefore, in which this 

 absorption takes place, which spectroscopically enables us to discrim- 

 inate star from star, must be accurately known before we can obtain 

 the greatest amount of information from our inquiries. 



Our next duty then, clearly, is to study the sun — 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. By doing this we 

 may secure facts which will enable us to ascertain in what parts of 



