On the Chemistry of the Hottest Stars. 



185 



5. The greater thickness of the Lines of the Cleveite Gases. — The 

 action which produces the lines of the cleveite gases, whatever it be, 

 only commences shortly before the highest temperature is reached, 

 and the importance of helium in the spectrum grows very quickly. 

 When the action has ceased, the helium lines rapidly lose their 

 importance, whilst the absorbing hydrogen continues for some time 

 to become more effective. 



The complete discussion of these differences cannot be under- 

 taken until the criteria for stars at the apex of the temperature 

 curve have been further investigated. 



X. Are we in Presence of the Highest Stellar Temperatures ? 



I have shown in § TV that our highest terrestrial temperatures only 

 carry us up to a temperature approximately represented by 7 Cygni. 



We have no right to assume that the small number of stars as 

 yet studied put us in presence of the highest stellar temperatures. 



In the present communication I have of set purpose left on one 

 side for future treatment those stars which apparently are at the 

 very apex of the temperature curve, for the reason that in these 

 stars we are involved in unknown lines. These require a special 

 study; other stellar photographs have to be examined, a work 

 which will require some time, and new photographs have to be taken. 



In all probability, among the stars already known there are others 

 besides Bellatrix which are hotter than £" Tauri and Rigel, as deter- 

 mined by the continuous spectrum and by the metallic lines in the 

 way already explained ; although the known gas lines are of nearly 

 similar intensity, these stars show distinct differences among them- 

 selves, and other criteria for their arrangement in order of tempera- 

 ture must, therefore, be looked for. 



At this stage the only remaining known metallic lines are prob- 

 ably magnesium 4481, and the K line of calcium; these, however, 

 are so feeble that their variations are too much dependent on the 

 quality of the photographs to be trustworthy criteria, and the same 

 remark applies also to the measurement of the extension of the 

 spectrum into the ultra-violet. There are, however, several lines, as 

 yet of unknown origin, which are strong in some of these stars and 

 weaker in others, and the appearances of these afford useful criteria 

 for classifying these stars. But, before we attempt to use these 

 lines to see whether any higher temperature than that of Bellatrix is 

 indicated, it is important to consider whether we are justified in 

 regarding these unknown lines as gaseous instead of metallic. This 

 question is now being studied, and until it is settled it is not wise to 

 attempt to discuss the upper temperature limits of those substances, 

 such as hydrogen and the cleveite gases, the lines of which increase 



