Cleveite and other new Gas Lines in the Hottest Stars. 59 



One possible method is this. In the nebulae are found the lines of 

 hydrogen, helium, and asterium associated with other bright lines of 

 unknown origins ; it is fair to assume that if other similar gases 

 exist in the nebulae, the other bright lines should belong to them. 

 In the nebulae all these probably exist at low temperatures, since no 

 indication of the enhanced lines of Fe, Mg, Mn, Ti, &c, have been 

 detected in the spectra of nebulae, and on this ground we are 

 driven to give up the old arguments in favour of the high tempera- 

 ture of the nebulae, which depended for their validity upon the 

 presence of " chomospheric " lines in the spectrum. The discovery 

 of terrestrial helium has enabled its behaviour, when rendered 

 luminous, to be studied, and we now know that its presence in a 

 spectrum is no proof of a very high temperature. 



Further, of all the lines other than hydrogen, helium, and asterium, 

 so far discovered in the nebulae, it would appear that only a few, if 

 any, are certainly produced by metallic vapours.* 



If, then, their origins be gaseous, as opposed to metallic, we should 

 -expect to find these lines in the spectrum of those stars in which the 

 .absorption of hydrogen and the cleveite gases which are associated 

 with them in ^the nebulae is strong. At present, this method of 

 separating the gaseous from the metallic lines in the hotter stars 

 cannot be finally applied, for the reason that the wave-lengths of 

 many of the nebular lines are not sufficiently accurate for the 

 object in view. But there is little doubt that it will furnish a 

 valuable criterion when photographs with larger dispersion become 

 available. 



Another possible method, however, is open to us. In a Oygni, 

 where the enhanced metallic lines are so strongly developed, the 

 helium lines appear very feebly, a,nd it is only in stars at still higher 

 temperatures that helium is strongly represented. Hence, if there 

 are other gases which behave like helium, in stars as well as nebulae, 

 they would be intensified in passing from a Cygni through succes- 

 sively hotter stars, while the enhanced metallic lines become feebler. 

 Some of the principal lines which become thus intensified in passing 

 to the hottest stars are indicated in the following table. 



Five of the lines given in the table approximately coincide with 

 enhanced lines, two with lines of cadmium and three with lines of 

 sulphur, but since in the spectrum of the former substance there are 

 fifteen enhanced lines in the same region, and in the latter twenty- 

 nine, the coincidences may for the present be regarded as acci- 

 dental. 



It seems highly probable therefore that the lines recorded in the 

 table represent gases which have yet to be discovered, and that the 



* < Phil. Trans.,' A, 1895, vol. 186, p. 76. 



