170 



Mr. J. Norman Lockyer. 



V. On the Occurrence op the Lines of the Cleveite Gases and 

 of Hydrogen in the Hottest Stars. 



Important Position of the Cleveite Gases. 



So far I have considered the high temperature lines of certain 

 metallic elements, and their comparison with lines existing in the 

 stars. It is now important to refer to the permanent gases, which 

 we now know to exist, both in stars and nebulas. I refer to hydro- 

 gen and the cleveite gases. These appear in the stars which, on the 

 grounds previously stated, I hold to be of highest temperature, and 

 for this discrimination we can rely better on the cleveite gases than 

 on hydrogen, for the reason that the latter is much more widely 

 distributed qua temperature. There are not many groups of stars 

 which do not indicate the presence of hydrogen, w r hile, on the other 

 hand, the cleveite gases only occur conspicuously in one. 



It is impossible to over-estimate the importance of the discovery of 

 terrestrial sources of helium and the gas I have provisionally named 

 gas X* in its bearings upon the spectra of the hotter stars, since it 

 explains many of the strongest lines in such spectra, as I showed in 

 papers communicated to the Royal Society on May 8 and May 9, 

 1895.J 



Professor Vogel arrived, afterwards, at the same conclusion. J 

 Thus a considerable number of the stellar lines was removed from 

 the category of "unknown lines." , 



Varying Intensity of the Cleveite Lines. 



Tf the varying intensities of the helium lines are studied in the 

 case of the stars which have so far been considered, it is found that 

 the lines become stronger as the temperature increases. Thus, from 

 the merest trace in a, Cygni the lines of helium are gradually intensi- 

 fied in passing through rj Leonis, Rigel, £" Tauri, and Bellatrix, 

 which, as we have already seen, are in order of increasing tempera- 

 ture. This is illustrated in Map II, which also shows the behaviour 

 of the lines belonging to that constituent of the cleveite gases which 

 I have provisionally called gas X. 



Varying Intensity of the Lines of Hydrogen. 



The lines of hydrogen make their appearance at a much lower 

 stage of temperature than those of the cleveite gases, but, like the 



* For greater clearness in the sequel, and because the lines of helium are 

 stronger than those of gas X, I shall often refer to helium alone, 

 f 1 Roy. Soc. Proc.,' vol. 58, pp. 114, 117. 

 X 'Berlin Sitzungsberichte,' October 24, 1895. 



