2. How nebulæ occupy so much space.—It accounts for the enormous space occupied by nebulæ. It may be objected that, enormous as would be the original temperature of the solar system produced by the primeval collision, it would nevertheless be insufficient to expand the mass, against gravity, to such an extent that it would occupy the entire space included within the orbit of Neptune. But it will be perceived, from what has already been stated regarding the dispersion of the materials before they had sufficient time to assume the gaseous condition, that this dispersion was the main cause of the gaseous nebula coming to occupy so much space. And, to go farther back, it was the suddenness and almost instantaneity with which the mass would receive the entire store of energy, before it had time to assume even the molten, not to say the gaseous, condition, which led to tremendous explosions, followed by a wide dispersion of materials.
3. Why nebulæ are of such varied shapes.—Although the dispersion of the materials would be in all directions, it would, according to the law of probability, very rarely take place uniformly in every direction. There would generally be a greater amount of dispersion in some directions than in others, 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 rarified condition of the gaseous mass, the nebulæ would change form but slowly.
4. Broken fragments in a gaseous mass of an excessively high temperature the first stage of a nebula.—From what has already been shown, it will be seen that after the colliding of the two dark bodies the first condition of the resulting nebula would be an enormous space occupied by broken fragments of all sizes dashing against each other with tremendous velocities, like the molecules in a perfect gas. All the interspaces between those fragments would be entirely filled with a gaseous mass, which, at its earliest stages at least, as in the case of the solar nebula, would have a temperature probably more than one hundred thousand times that of the voltaic arc. Whether such a mass would be visible is a point which can hardly be determined, as we can have no experience on earth of a gas at such a temperature.
That there are some of the nebulæ which appear to consist of solid matter interspersed in a gaseous mass is shown by the researches of Mr. Lockyer[[10]] and others. In fact, the theory is held by Professor Tait[[11]] that nebulæ consist of clouds of stones—or meteor-swarms, as Mr. Lockyer would term them—in an atmosphere of hydrogen, each stone of which, moving about and coming into collision with some other, is thereby generating heat which renders the circumambient gas incandescent. In reference to this theory of Professor Tait, Mr. Lockyer says that the phenomena of the spectroscope can be quite well explained “on the assumption of a cloud of stones, providing always that you could at the same time show reasonable cause why these clouds of stones were ‘banging about’ in an atmosphere of hydrogen.”[[12]] The theory, however, does not appear to afford any rational explanation of this banging about of the stones to and fro in all directions; for, according to it, the only force available is gravitation, and this can produce merely a motion of the materials towards the centre of the mass. Under these conditions very little impinging of the stones against each other would take place. But, according to the theory here adopted, we have an agency incalculably more effective than gravity, one which accounts not merely for the impact of the stones, but for their very existence as such, inasmuch as it explains both what they are and whence they came.
Mr. Lockyer has recently fully adopted Professor Tait’s suggestion as to the nature and origin of nebulæ, and has endeavoured to give it further development. He considers the nebulæ to be composed of sparse meteorites, the collisions of which give the nebulæ their temperature and luminosity. He divides the nebulæ into three groups, “according as the formative action seems working towards a centre; round a centre in a plane, or nearly so; or in one direction only.” As a result we have globular, spheroidal, and cometic nebulæ.
Globular nebulæ he accounts for in the following manner. “If we,” he says, “for the sake of the greatest simplicity consider a swarm of meteorites at rest, and then assume that others from without approach it from all directions, their previous paths being deflected, the question arises whether there will not be at some distance from the centre of the swarm a region in which collisions will be most valid. If we can answer this question in the affirmative, it will follow that some of the meteorites arrested here will begin to move in almost circular orbits round the common centre of gravity.
“The major axes of these orbits may be assumed to be not very diverse, and we may further assume that, to begin with, one set will preponderate over the rest. Their elliptic paths may throw the periastron passage to a considerable distance from the common centre of gravity; and if we assume that the meteorites with this common mean distance are moving in all planes, and that some are direct and some retrograde, there will be a shell in which more collisions will take place than elsewhere. Now, this collision surface will be practically the only thing visible, and will present to us the exact and hitherto unexplained appearance of a planetary nebula—a body of the same intensity of luminosity at its edge and centre—thus putting on an almost phosphorescent appearance.
“If the collision region has any great thickness, the centre should appear dimmer than the portion nearer the edge.
“Such a collision surface, as I use the term, is presented to us during a meteoric display by the upper part of our atmosphere.”[[13]]
Spheroidal nebulæ, he considers, are produced by the rotation of what was at first a globular rotating swarm of meteorites.