1868.] 



Constitution of the Sun and Stars, 



23 



region of the solar atmosphere than iron. Nevertheless none* of the 

 calcium lines observed by Kirchhoff appear to be as intense as many of the 

 iron lines. This is no doubt due to calcium vapour being a much smaller 

 constitueut of the sun's atmosphere than iron, just as oxygen is less abun- 

 dant in our atmosphere than nitrogen, and carbonic acid much less abun- 

 dant than either. Judging from the indigo and green calcium lines, which 

 are all less intense than the ir®n lines in their neighbourhood, it would 

 appear that some light reaches us from a hotter region than any light 

 that reaches us from iron lines, and accordingly that calcium gas is so rare, 

 and in consequence the stratum which can intercept and therefore is em- 

 ployed in emitting these rays is so thick that, though its upper surface 

 soars far above the upper surface of the iron atmosphere, its under surface 

 stretches further down than the under surface of the corresponding, and 

 comparatively shallow, active stratum of iron gas. This appears to be 

 the case too with most of the rest of the calcium lines observed by Kirch- 

 hoff; but the lines 55*99 and 56*03 in the yellowish-green, and the lines 

 6*1 "63, 64*32, and 64*55 in the red, all of which are of intensity 5, are 

 probably exceptions, and owe their strength to calcium gas being much 

 more opake in reference to them, so that they are emitted by a stratum 

 shallow enough to reach but little beyond the extreme verge of the iron 

 atmosphere. These are some of the lines that give the calcium light, 

 when seen undispersed, its beautiful purple colour. Calcium is no doubt 

 very opake also in reference to the other lines of the same class, such as 

 the lines H 15 H„ and g, beyond the limit of KirchhofT's maps. In taking 

 a general review of the calcium spectrum, these lines should be left out of 

 consideration as not being comparable with the rest ; and if this be done, 

 the remaining lines will exhibit the same gradation of intensity from the 

 red to the blue which we found in the iron lines. 



32. But in the immense extent of atmosphere which spreads upward 

 from the surface of the calcium, in the vast elevation to the boundary of 

 the atmospheres of magnesium and sodium, and in the far greater heights 

 to which hydrogen alone can soar, the temperature has fallen too low 

 to produce light visible in comparison with solar light in any part of the 

 spectrum. And accordingly all the lines referable to magnesium, sodium, 

 or hydrogen, in whatever part of the spectrum they may lie, are in- 

 tensely black. But before proceeding to examine these lines in detail, 

 it will be convenient to inquire into the state of the regions further 

 down. 



33. The sun's atmosphere is heated beneath by contact with the scorch- 

 ing body of the sun, and it would throughout its whole extent attain this 



* The lines 48-83 and 52*74 of intensity 6, the latter of which is the less re- 

 frangible of the lines constituting the close double line E, are- left out of account ; as 

 they are also iron lines, and no doubt owe their intensity to this circumstance. The 

 line 56*07 of intensity 5, which is also a line common to the two spectra, is probably a 

 stronger line on this account than it would be either as a calcium or as an iron line. 



