80 INORGANIC EVOLUTION. [CHAP. 



I have already stated the facts with regard to a Cygni. Now let 

 us look at them in the light of the dissociation hypothesis. 



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 1 Of lines disappearing we have arc lines of iron, 

 ?ome thousands in number, calcium, magnesium, strontium, and so on. 

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

 senting 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, 

 on the hypothesis we are discussing, is simply and sufficiently ex- 

 plained by the statement that on passing from the temperature of the 

 sun to that of a Cygni, among 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. Looking for the lines which increase in 

 importance, while the others are reduced, we find the lines of hydrogen. 



So far then up the scale of temperature the solar and stellar record 

 is the same ; the star at the next stage of heat above the sun has its 

 reversing layer as hot as the sun's chromosphere, and the same " test 

 spectrum " as we have seen fits both. I hold that dissociation simply 

 and sufficiently explains this all-important fact. 



But this is as far as the sun can take us. The stars, however, con- 

 tinue the story. 



If we consider another change higher up in the scale of temperature, 

 taking as the lower level a Cygni, at which we have arrived, we have 

 independent evidence that the so-called Orion stars are hotter than 

 such a star as a Cygni. 



On proceeding to study the higher dissociating temperature at 

 work in the Orion stars, all the statements made with reference to 

 the changes likely to occur in the spectrum on the non-dissociation 

 hypothesis, strictly apply. We cannot expect any change in the rela- 

 tive intensity of the lines and the appearance of the spectrum cannot 

 be fundamentally altered. 



On the dissociation hypothesis, on the other hand, if we find cer- 

 tain lines indicating certain substances disappearing, and other lines 

 indicating other substances making their appearance for the first time 

 (or if they were visible before, becoming much intensified), we shall 

 have an opportunity of studying the effects of the new dissociating 

 forces at work. 



Now is there any change 1 The facts are that this increase of tem- 

 perature we are now considering is accompanied by the gradual extinc- 

 tion of the enhanced lines, an increase in the amount of hydrogen 



