Page:Popular Science Monthly Volume 13.djvu/361

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ON THE FORMATION OF NEBULÆ.

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 illustration, let us imagine the entire mass of expanding vapor 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 attraction, 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 explosive 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 hydrogen 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.