The Evolution of the Sciences 



sufficiently high to traverse the chromospheric 

 layer. This layer, whose temperature is one 

 or two thousand degrees lower than the photo- 

 sphere, is constantly disturbed by the agitation 

 of the underlying layers, and the hydrogen 

 of which it chiefly consists is being constantly 

 invaded by metallic vapours, which escape 

 through all the pores of the photosphere. It is 

 surrounded in its turn by the corona, a mixture 

 of hydrogen and other gases, and of incandescent 

 dust, which decrease gradually in temperature 

 and density as they reach out into inter- 

 planetary space. 



Such is roughly the idea which we can at 

 present form of the " globe of fire " that guides 

 and heats the procession of the planets. Per- 

 haps little will be left in a century of all our 

 theories, because the solar problem is being 

 investigated with great energy. What is more, 

 every new thing found in the heavens checks 

 and enriches the discoveries already made 

 concerning the sun ; it is, after all, a star like all 

 the others, one of the billions of bright points 

 scintillating in the sky; astronomy has taught 



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