54 INORGANIC EVOLUTION. [CHAP. V. 



relatively stronger, as referred to the arc lines, than they are in 

 the experimental spark. Hence, the incandescent iron vapour in the 

 chromosphere must be at a temperature at least as high as that of the 

 spark, and certainly higher than that of the iron vapour which is most 

 effective in the production of Fraunhofer lines. 



The evidence is complete that the temperature in the reversing 

 layer of a Cygni is higher than that of the reversing layer of the sun. 

 What do we find ? Of lines disappearing we have the arc lines of iron, 

 calcium, magnesium, strontium, and so on, some thousands in number. 

 Of lines increasing in importance we have the small number represent- 

 ing the enhanced lines of iron, the lines of hydrogen, and some others 

 which we cannot at present associate with the name of any known 

 substance. Here, then, we get a series of phenomena which is simply 

 and sufficiently explained by the statement that on passing from the 

 temperature of the sun to a Cygni, among other changes brought 

 about, the complicated line spectrum of iron is giving way to a more 

 simple one consisting of the enhanced lines. Further inquiries show 

 that the other metallic spectra are behaving in the same way. 



In passing from the absorbing layer of the sun to that of a Cygni, 

 then, we pass from the arc lines of the metallic elements to the enhanced 

 lines. Truly a most tremendous change which the test-spectrum puts 

 beyond all question. The significance of this will come later. 



In the case of the sun, the enhanced test-spectrum was the only 

 one we could employ with advantage. But in the pase of the hottest 

 stars, stars that is, with the longest spectrum, we can go still further. 

 These are so much hotter than the sun, that they give us the oppor- 

 tunity of noting another break; really of employing another test- 

 spectrum, that afforded by the summation of the lines of hydrogen and 

 the cleveite gases. 



As we have seen, the arc metallic lines give way to the enhanced 

 metallic lines in stars of intermediate temperature, like our sun and 

 a Cygni, so, in the hottest stars the enhanced metallic lines vanish 

 almost entirely, and give place to a spectrum almost purely gaseous. 



To take iron as an example, for the sake of simplicity, it will be 

 seen then that the actual stellar phenomena might have been pre- 

 dicted up to a certain point, from a consideration of laboratory and 

 solar phenomena. But the stars carry us further than our predictions ; 

 we see the gradual increase of hydrogen and the cleveite gases. The 

 facts demonstrate that as temperature increases hydrogen increases, 

 and, together with the cleveite gases not obvious before, finally replaces 

 iron which has disappeared. 



