52 INORGANIC EVOLUTION. [CHAP. 



borne in mind is that if the atmospheres are in part composed of 

 condensable vapours, and not entirely of gases permanent at all stejlar 

 temperatures, condensation must always be going on outside at the 

 region of lowest temperature. 



The absorption phenomena in stellar spectra are not identical at the 

 same mean temperature on the ascending and descending sides of the 

 curve, on account of the tremendous difference in the physical conditions. 



In a condensing swarm, the centre of which is undergoing 

 meteoritic bombardment from all sides, there cannot be the equivalent 

 of the solar chromosphere ; the whole mass is made up of heterogeneous 

 vapours at different temperatures, and moving with different velocities 

 in different regions. 



In a condensed swarm, of which we can take the sun as a type, all 

 action produced from without has practically ceased ; we get relatively 

 a quiet atmosphere and an orderly assortment of the vapours from top 

 to bottom, disturbed only by the fall of condensed metallic vapours. 

 But still, on the view that the differences in the spectra of the heavenly 

 bodies chiefly represent differences in degree of condensation and tem- 

 perature, there can be, cm fond, no great chemical difference between 

 bodies of increasing and bodies of decreasing temperature. Hence we 

 find at equal mean temperatures on opposite sides of the temperature 

 curve, this chemical similarity of the absorbing vapours proved by 

 many points of resemblance in the spectra, especially the identical 

 behaviour of the enhanced metallic and cleveite lines. 



Now that the test-spectum has led us to such a very definite con- 

 clusion with regard to a Cygni and other stars resembling it, it is 

 necessary to tarn back to Chapter IV, in which the solar atmosphere 

 was discussed. It was pointed out what a marvellous resemblance there 

 was between the test-spectrum and the sun's chromosphere, photo- 

 graphed during the eclipse of 1898. If the spectra of the valid absorb- 

 ing atmosphere of a Cygni and of the sun's chromosphere resemble the 

 test-spectrum as they do, the atmospheres must resemble each other,, 

 both in chemistry and temperature. 



Here, then, we have an almost undreamt-of opportunity of noting 

 the close connection between solar and stellar phenomena, not merely 

 in noting the identity of the action of the absorbing layers as we do 

 when we find the spectra of the sun, Arcturus and Capella, almost 

 identical, line for line, but in studying the relation of the absorbing 

 layer of one star to the underlying layer in another. 



While we find, on the one hand, that the absorbing layer of the sun 

 is similar to those of Arcturus and Capella, we find, on the other, that 

 the spectrum of the sun's chromosphere resembles that of the reversing 

 layer of a Cygni. The " test-spectrum " fits them both. 



