THE STAKS AND NEBULAE 303 



volatilize a correspondingly larger percentage of their 

 component substances; thereby increasing the size of 

 the inner chamber of gases relatively to the thickness of 

 the shell for these reasons, namely: (1) by direct aug- 

 mentation of the quantum of the gas at the expense of the 

 shell, (2) by enhanced expansion of that gas due to the 

 increased temperature, (3) by pushing the shell out that 

 much farther, thereby enlarging the area over which the 

 shell-substance must spread itself, and (4) by virtue of 

 the circumstance that the weight of the shell, under the 

 law of the inverse square, diminishes with its distance 

 from the center, rendering easier its further uplifting. 

 But let us now consider the effect of all this on the texture 

 and conduct of the shell itself : 



In the first place, the gradual distillation-out of the 

 more volatile substances necessarily simplifies its chemi- 

 cal constitution and correspondingly affects the quality 

 of the spectrum. 



Secondly, the increasing predominance in the gey- 

 sers of the more refractory materials which, as we all 

 know, in their incandescent state are dazzlingly brilliant, 

 naturally augments the luminosity of the star as a whole. 



Thirdly, the hotter the shell the more plastic or fluid 

 is it; hence the freer the ebullitions, the less liability to 

 clogging, and the more multitudinous the geysers, whose 

 pinnacles it is that cast off the radiance. This feature 

 not only helps to account for increased brilliancy of stars 

 with groivth, but explains why the very bright stars are 

 never found among the long-period variables, save, of 

 course, those of the eclipsing variety. 



Finally, the fact that the shell of a star, notwith- 

 standing its increase in absolute thickness with the 

 star's growth, declines relatively to the other stellar 

 dimensions, cannot fail eventually to weaken the star's 

 structural stability and to bring about its automatic 

 bursting and dissolution into a nebula. Nebulae thus 

 evolved are obviously of enormous extent, not only be- 

 cause of the maximal size of their parent stars, but also 

 because of the maximal pressure under which the central 



