xv.] STELLAR COINCIDENCE. 201 



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 its spectrum goes, may be regarded as a 

 representative of class (/?) intermediate between stars (a) with much 

 simpler spectra of the same kind, and stars (y) with much more 

 complex 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 being exceedingly thin, thus indicating a small 

 proportion of metallic vapour ; while (3) the hydrogen lines in this 

 star are enormously distended, showing that the chromosphere is 

 largely composed of that element. 



There are other bright stars of this class. 



VIII. As types of class (y) 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 associated. It is fair to assume that these stars are of a 

 lower temperature than our sun. 



/ have asked myself whether all the above facts cannot le grouped 

 together in a working hypothesis which assumes that in the reversing 

 layers of the sun and stars various degrees of " celestial dissociation " 

 are at work, which dissociation pi-events the coming together of the 

 atoms which, at the temperature of the earth and at all artificial 

 temperatures yet attained here, compose the metals, the metalloids, and 

 compounds. 



On this working hypothesis, the so-callei elements not present in 

 the- re versing layer of a star will be in course of formation in the 



