16 EEPORT — 1891. 



great at tlie snrrace that the decrease of the density of the gases must be 

 extremely rapid passing in the space of a few miles, from atmosphei'ic 

 pressure to a density infinitesimally small ; consequently the temperature- 

 gradient at the surface, if determined solely by expansion, must be ex- 

 tremely rapid. The gases here, however, are exposed to the fierce 

 I'adiation of the sun, and unless wholly transparent would take up heat, 

 especially if any solid or liquid particles were present from condensation 

 or convection currents. 



From these causes, within a very small extent of space at the surface 

 of the sun, all bodies with which we are acquainted should fall to a con- 

 dition in which the extremely tenuous gas could no longer give a visible 

 spectrum. The insignificance of the angle subtended by this space as 

 seen from the earth should cause the boundary of the solar atmosphere to 

 appear defined. If the boundary which we see be that of the sun pi-oper, 

 the matter above it will have to be regarded as in an essentially dynamical 

 condition — an assemblage, so to speak, of gaseous projectiles for the most 

 part falling back upon the sun after a greater or less range of flight. 

 But in any case it is within a space of relatively small extent in the sun 

 and probably in the other solar stars, that the reversion vyhich is mani- 

 fested by dark lines is to be regarded as taking place. 



Passing backward in the star's life, we should find a gradual weak- 

 ening of gravity at the surface, a reduction of the temperature-gradient 

 so far as it was determined by expansion, and convection currents of less 

 violence producing less interference with the proportional quantities of 

 gases due to their vapour densities, while the efi"ects of eruptions would 

 be more extensive. 



At last we might come to a state of things in which, if the star were 

 hot enough, only hydrogen might be sufficiently cool relatively to the 

 radiation behind to produce a strong absoi'ption. The lower vapours 

 would be protected, and might continue to be relatively too hot for their 

 lines to appear very dark upon the continuous spectrum ; besides, their 

 lines might be possibly to some extent eSaced by the coming in under 

 such conditions in the vapours themselves of a continuous spectrum. 



In such a star the light radiated towards the upper part of the atmo- 

 sphere may have come from portions lower down of the atmosphere itself, 

 or at least from parts not greatly hotter. There may be no such great 

 difierence of temperature of the low and less low portions of the star's 

 atmosphere as to make the darkening efiect of absorption of the protected 

 metallic vapours to prevail over the illuminating efiect of their emission. 



It is only by a vibratory motion corresponding to a very high tem- 

 perature that the bright lines of the first spectrum of hydrogen can be 

 brought out, and by the equivalence of absorbing and emitting power 

 ■that the corresponding spectrum of absorption should be produced ; yet for 

 a strong absorption to show itself, the hydrogen must be cool relatively 

 to the source of radiation behind it, whether this be condensed particles 



