xviii.] THE H AND K LINES. 245 



court by the fact, also stated by Dr. Huggins, that, associated 

 with the thick lines, are excessively thin ones. Any enormous 

 equatorial velocity of the star should have made all the lines 

 thick, or have obliterated them, but this is not so. Now we 

 have only two lines in the solar spectrum at all comparable 

 in thickness with these hydrogen lines in the hottest stars, 

 taking Sirius and a Lyrae as types. 



In a paper communicated to the Eoyal Society in 1876 l I 

 remarked that laboratory work indicated the possibility that 

 line-spectra might, after all, really not result from the vibration 

 of similar molecules ; and- at that time the evidence seemed to 

 be so clear in the case of calcium that it was pointed out that 

 the time had arrived when evidence touching the H and K lines 

 of that substance ought, if possible, to be obtained from the 

 stars by means, of course, of photography, because the part of 

 the spectrum in question is exceedingly faint in the case of 

 the stars. 



"Why, it may be asked, was it important to get this evidence 

 from the stars ? I here give an extract from a book, 2 published 

 some years ago, which puts this view forth : 



"It is abundantly clear that if the so-called elements, or, more 

 properly speaking, their finest atoms, those that give us line-spectra, 

 are really compounds, the compounds must have been formed at a 

 very high temperature. It is easy to imagine that there may be no 

 superior limit to temperature, and, therefore, no superior limit be- 

 yond which such combinations are possible, because the atoms which 

 have the power of combining together at these transcendental stages 

 of heat do not exist as such, or rather they exist combined with other 

 similar atoms at all lower temperatures. Hence the association 

 will be a combination of more complex molecules as temperature is 

 reduced, and of dissociation, therefore, with increased temperature 

 there may be no end." 



1 " Preliminary Note on the Compound Nature of the Line-Spectra of Elemen- 

 tary Bodies," Proc. Roy. Soc. No. 168, 1876. 



2 Studies in Spectrum Analysis, p. 196. 



