1888.] of the various Species of Heavenly Bodies. 



19 



(' Phil. Trans.,' vol. 164, 1874, p. 492), in which I attempted to bring 

 to bear some results obtained in solar inquiries upon the question of 

 stellar temperatures. 



I quote the following paragraphs : — 



I. The absorption of some elementary and compound gases is 

 limited to the most refrangible part of the spectrum when the gases 

 are rare, and creeps gradually into the visible violet part, and finally 

 to the red end of the spectrum, as the pressure is increased. 



II. Both the general and selective absorption of the photospheric 

 light are greater (and therefore the temperature of the photosphere 

 of the sun is higher) than has been supposed. 



III. The lines of compounds of a metal and iodine, bromine, &c, 

 are observed generally in the red end of the spectrum, and this holds 

 good for absorption in the case of aqueous vapour. 



Such spectra, like those of the metalloids, are separated spectro- 

 scopically from those of the metallic elements by their columnar or 

 banded structure. 



IV. There are, in all probability, no compounds ordinarily present 

 in the sun's reversing layer. 



V. When a metallic compound vapour, such as is referred to in 

 III, is dissociated by the spark, the band spectrum dies out, and the 

 elemental lines come in, according to the degree of temperature 

 employed. 



Again, although our knowledge of the spectra of stars is lament- 

 ably incomplete, I gather the following facts from the work already 

 accomplished with marvellous skill and industry by Secchi, of Rome. 



VI. The sun, so far as the spectrum goes, may be regarded as a 

 representative of class Q3) intermediate between stars (a) with much 

 simpler spectra of the same kind, and stars (7) with much more com- 

 plex spectra of a different kind. 



VII. Sirius, as a type of a, is (1) the brightest (and therefore 

 hottest ?) star in our northern sky ; (2) the blue end of its spectrum 

 is open, — it is only certainly known to contain hydrogen, the other 

 metallic lines beiug exceedingly thin, thus indicating a small propor- 

 tion of metallic vapours ; while (3) the hydrogen lines in this star are 

 enormously distended, showing that the chromosphere is largely com- 

 posed of that element. 



There are other bright stars of this class. 



VIII. As types of 7 the red stars may be quoted, the spectra of 

 which are composed of channelled spaces and bands, and in which 

 naturally the blue end is closed. Hence the reversing layers of these 

 stars probably contain metalloids, or compounds, or both, in great 

 quantity ; and in their spectra not only is hydrogen absent, but the 

 metallic lines are reduced in thickness and intensity, which in the 

 light of' V., ante, may indicate that the metallic vapours are being 



