42 Trans. Acad. Sci. of St. Louis. 



path, and are in fact projectiles from the gaseous atmosphere 

 beneath. Moving almost without collision, these molecules 

 may be regarded as free projectiles shot out with velocities 

 which carry many of them to an average height of some 200 

 Kilometres. Meteors colliding with the upper part of this 

 ultragaseous atmosphere would of course finally be consumed 

 very much as if the mass were denser and obeyed the laws of 

 fluid equilibrium. 



The Solar Corona is the analogue of the upper terrestrial 

 atmosphere ; and similar gaseous appendages doubtless sur- 

 round the planets and other heavenly bodies. But since the 

 limbs of Jupiter and Saturn, which have been studied by means 

 of eclipses and occultations of their satellites, appear teles- 

 copically sharp and almost perfectly opaque, it is not probable 

 that these rare atmospheres in comparatively cold bodies like 

 the great planets, have anything like the relative extent of the 

 Corona, which is kept expanded by the intense heat of the 

 Sun. Yet it may be assumed that all bodies, planets, comets, 

 and stars alike, have the two strata in some proportion. In 

 the case of the stars, which especially concerns us here, we 

 may suppose, on the analogy of the Sun, that their Coronas 

 give very little light and heat, and hence that the laws of 

 gases apply with considerable accuracj'to their radiations. It 

 is certain that a Corona does not seriously obstruct the radia- 

 tion, and equally clear that no sensible amount of heat can 

 arise from the condensation of such a rare medium. The 

 laws of gases ought therefore to apply to the condensation of 

 stars which are well advanced, but in the case of diffuse 

 nebulae the extreme tenuity of the medium relieves it of the 

 laws of fluid pressure and renders the radiations practically 

 free in all directions ; and the theory of convective equilibrium 

 is not required. 



(g.) Cause of the darkness of the companions of such stars 

 as Sirius and Procyon. 



The secular shrinkage of the sun's radius will cause a 

 steady rise in its temperature, and when the body has reached 

 the stage of Sirius it will shine with an intensely blue light, 

 like that emitted by stars of the first class. The temperature 



