o Dr. J. Croll on the Origin of Neb idee. 



eluded within the orbit of Neptune, might be filled with frag- 

 ments by the time tho whole had assumed the gaseous con- 

 dition. 



It would be the suddenness and almost instantaneity with 

 which the mass would receive the entire store of energy, before 

 it had time even to assume the molten, far less the gaseous 

 condition, which would lead to such fearful explosions and 

 dispersion of the materials. If the heat had been gradually 

 applied, no explosions, and consequently no dispersion, of the 

 materials would have taken place. There would first have 

 been a gradual melting ; and then the mass would pass by slow 

 degrees into vapour, after which the vapour would rise in 

 temperature as the heat continued until it became possessed of 

 the entire amount. But the space thus occupied by the gaseous 

 mass would necessarily be very much smaller than in the case 

 we have been considering, where the shattered materials were 

 first dispersed into space before the gaseous condition was 

 assumed. 



Reason why Nebula are of such various Shapes. — The latter 

 theory accounts also for the various and irregular shapes as- 

 sumed by the nebulae; for although the dispersion of the 

 materials would be in all directions, it w r ould, according to 

 the law of chances, very rarely take place uniformly in all di- 

 rections. There would generally be a greater amount of dis- 

 persion in certain directions, and the materials would thus 

 be carried along various lines and to diverse distances ; and 

 although gravity would tend to bring the widely Scattered 

 materials ultimately together into one or more spherical masses, 

 yet, owing to the exceedingly rarefied condition of the gaseous 

 mass, the nebulae would change form but slowly. 



Reason why Nebula? emit such Feeble Light. — The feeble light 

 emitted by nebulae follows as a necessary result from the theory. 

 The light of nebulae is mainly derived from glowing hydrogen 

 and nitrogen in a gaseous condition ; and it is well known that 

 these gases are exceedingly bad radiators. The oxyhydrogen 

 flame, though its temperature is only surpassed by that of the 

 voltaic arc, gives nevertheless a light so feeble as scarcely to 

 be visible in the daytime. Now, even supposing the enormous 

 space occupied by a nebula w r ere due to excessive temperature, 

 the light emitted would yet not be intense were it derived from 

 nitrogen or hydrogen gas. The small luminosity of nebula?, 

 however, is due to a different cause. The enormous space oc- 

 cupied by those nebulae is not so much owing to the heat which 

 they possess, as to the fact that their materials were dispersed 

 into space before they had time to pass into the gaseous con- 

 dition; so that, by the time this latter state was assumed, the 



