1868.] 



Constitution of the Sun and Stars. 



29 



the wings of the magnesium lines ; and in fact the black part of the mag- 

 nesium lines is due exclusively to the magnesium vapours between the top 

 of the magnesium atmosphere and the plane of demarcation between the 

 two strata into which we have distinguished the active layer of iron, 

 while the wings are caused, at least in part, by the magnesium vapour which 

 exists in the lower section of the active layer of iron and in the stratum 

 which immediately adjoins it beneath. Thus the layer of magnesium 

 which gives rise to the lines of the group b may conveniently be distin- 

 guished into two parts, the outer of which extends from the remote boun- 

 dary of the magnesium atmosphere to the middle of the layer from which 

 iron lines originate, and the second from this latter station through a 

 hotter layer which lies further down. If magnesium vapour existed in the 

 situation of this lower moiety only, the magnesium lines would be bands 

 of their present breadth, but nowhere attaining the intensity 6 : the super- 

 position of the central black stripe is the work of the magnesium vapour 

 in the vast outer section. 



43. When we take into account how much higher a specific opacity 

 sodium and magnesium vapour have than iron for the principal rays which 

 they respectively emit, we are led to conclude that while magnesium va- 

 pour is abundant when compared with the attenuated vestige of sodium in 

 the sun's atmosphere, it may be but sparingly present when compared with 

 such a constituent as iron; and that this is so is established by the absence 

 from the sun's spectrum of any lines corresponding to the rays of magne- 

 sium, in reference to which the specific opacity of magnesium is low, such 

 as the magnesium lines 44*92 and 46*06. 



44. We have found that there is but the merest trace of sodium in the 

 sun's atmosphere, and that this trace mounts to an immense height above 

 the iron. To render this possible there must be some abundant gas which 

 extends as far as or beyond the sodium, in which it may diffuse itself, and 

 so be borne to the full height corresponding to the small mass of its mole- 

 cules. The gas which does it this service appears to be hydrogen, which, 

 having a molecular mass only one twenty-third of that of sodium, must soar 

 to an almost inconceivably greater height. 



Hydrogen seems to be a very large constituent of the sun's atmosphere. 

 There are three considerable rays in the spectrum of incandescent hydro- 

 gen, and a fourth faint one has been lately pointed out by Angstrom. To 

 these four rays, even to the faintest, there correspond intensely black lines 

 in the solar spectrum. This indicates an abundance of hydrogen. The 

 wave-lengths of the four lines are 41*04, the new hydrogen line, Ang- 

 strom's h, in the violet ; 43*43, in the indigo, which is the second of the 

 six very conspicuous lines seen in the sun's spectrum on the less refrangi- 

 ble side of G; 48*65 in the blue, which is Fraunhofer's F ; and 65*68 in 

 the red, which is Fraunhofer's C. All these lines are winged : the black 

 stripe in the more refrangible lines is very broad, and in the others it is 

 of considerable width. These circumstances also indicate an abundance 



