ADDRESS. 15- 



But this statement must be taken broadly, and not as asserting tbat all 

 stars, however dififerent in mass and possibly to some small extent in 

 original constitution, exhibit one invariable succession of spectra. 



In the spectra of the white stars the dark metallic Hues are relatively 

 inconspicuous, and occasionally absent, at the same time that the dark 

 lines of hydrogen are usually strong, and more or less broad, upon a con- 

 tinuous spectrum, which is remarkable for its brilliancy at the blue end. 

 In some of these stars the hydrogen and some other lines are bright,, 

 and sometimes variable. 



As the greater or less prominence of the hydrogen lines, dark or 

 bright, is characteristic of the white stars as a class, and diminishes- 

 o-radually with the incoming and increase in strength of the other lines, 

 we are probably justified in regarding it as due to some conditions 

 which occur naturally during the progress of stellar life, and not to 

 a peculiarity of original constitution. 



To produce a strong absorption-spectrum a substance must be at the 

 particular temperature at which it is notably absorptive; and, further,, 

 this temperature must be sufficiently below that of the region behind 

 from which the light comes for the gas to appear, so far as its special 

 rays are concerned, as darkness upon it. Considering the high tem- 

 perature to which hydrogen must be raised before it can show its 

 characteristic emission and absorption, we shall probably be right in 

 attributing the relative feebleness or absence of the other lines, not to the 

 paucity of the metallic vapours, but rather to their being so hot relatively 

 to the substances behind them as to show feebly, if at all, by reversion. 

 Such a state of things would more probably be found, it seems to me, in 

 conditions anterior to the solar stage. A considerable cooling of the sun 

 would probably give rise to banded spectra due to compounds, or to more 

 complex molecules, which might form near the condensing points of the 

 vapours. 



The sun and stars are generally regarded as consisting of glowing 

 vapours surrounded by a photosphere where condensation is taking place, 

 the temperature of the photospheric layer from which the greater part of the 

 radiation comes being constantly renewed from the hotter matter within. 



At the surface the convection currents would be strong, producing 

 a considerable commotion, by which the different gases would be mixed 

 and not allowed to retain the inequality of proportions at different levels 

 due to their vapour densities. 



Now the conditions of the radiating photosphere and those of the 

 gases above it, on which the character of the spectrum of a star depends, 

 will be determined, not alone by temperature, but also by the force of 

 gravity in these regions ; this force will be fixed by the star's mass and 

 its stage of condensation, and will become greater as the star continues 

 to condense. 



In the case of the sun the force of gravity has already become so 



