ON THE FORMATION OF NEBULAE. 3+7 



If the entire nebulous mass could be preserved in the form of an 

 expansible spherical shell, while the entire amount of heat-force should 

 be acting upon its interior surface to force it outward, then, according 

 to the above-named law of conservation, the entire mass would be 

 thrown nearly to the limit of its sphere of sensible attraction ; i. e., 

 nearly to those points at which the motion of the two bodies began to 

 be accelerated. 



For the sake of convenient reference in the following difficult illus- 

 tration, let us imagine the entire mass of expanding vajDor to be divided 

 into two equal parts, called respectively the inner mass and the outer 

 mass ; and let us imagine the heat-force to be divided into two equal 

 parts also, called the inner force and the outer force. 



Now, as these two forces are sufficient to drive these two masses 

 very nearly to the aforesaid points of acceleration, and whereas only 

 a part of the inner force can be expended in distributing the inner mass 

 throughout the space surrounded by such limits, it follows that that 

 part of the inner force so conserved will expend itself in helping the 

 outer force to throw the outer mass beyond this sphere of sensible attrac- 

 tion, and, most probably, into those of the surrounding stars. 



If such be the case, then some of these out-thrown masses may 

 become cometary bodies and meteoric showers to the inhabitants of 

 "other worlds than ours," besides forming abundant material for the 

 "cosmical dust," so much discussed of late ; and, possibly, some of the 

 largest of these fragments may be able to produce asteroidal groups, in 

 some ripening planetary system, by disrupting one of its planets while 

 yet in the nebulous state. This seems to supply a deficiency in this 

 hypothesis which has long been felt. 



Density of the Newly-formed Nebula. If, now, we fix our 

 attention on the condition of this nebula at the moment when the explo- 

 sive force had expended its last effort to throw these outside masses 

 into surrounding space, we can readily see that it must have been of 

 very nearly uniform density, in consequence of the tendency of the 

 out-moving particles to continue their motions outward. 



Confirmation. It is very probable that many such explosions as 

 that described above have been witnessed from our planet, and have 

 been recorded in history as temporary stars / an interesting account of 

 some of which may be found in Herschel's "Outlines of Astronomy." 1 



In May, 1866, such a phenomenon was observed in the Northern 

 Crown. This collision, however, seems to have been, not with the star 

 itself, but with one of its planets, or with some other dark body lying 

 in that direction, which contained a large amount of water, the hydro- 

 gen of which, being dissociated from its oxygen, shone out with such 

 brilliancy as to be seen at this distance. 



That this was a planetary collision seems probable, from the fact 

 that the star continued to shine with its normal light during the time 



1 Philadelphia edition, 1861, p. 472. 



