Sir William here argues that the second supposition is far less probable than the first, because, according to it, the motion of the one body relatively to the other must, in order to strike, be directed with great exactness. The result, in such a case, is that collision will rarely occur; whereas, according to the first supposition, the two bodies starting from a state of rest will, by their mutual gravitation, inevitably collide. According to the second hypothesis they will generally miss; according to the first they will always collide.
I have been led to a conclusion directly opposed to Sir William’s. The fact, that, according to the second supposition, collisions can but rarely occur is one reason, amongst others, why I think that supposition to be true; and the fact that, according to the first supposition, collisions must frequently occur is also one reason, amongst others, why I think it very improbable that it can represent the true condition of things.
It by no means adds anything to the probability of the first supposition to assert that, according to it, such collisions will occur readily and frequently. On the contrary, it would show that the supposition was the less likely to be true. If the collisions were insufficient in character, the fewer of them that occurred, the better; for the result of such collisions would simply be a waste of the potential energies of the universe. We should in this case have an innumerable host of imperfect suns without planets, or with at most only one or two, and these at no great distance from the luminary. There would thus be evolved a universe without any grand planetary systems. There is still another objection to the supposition. The same gravitating force which makes the dark bodies liable to come into collision with each other must, of course, make them equally liable to come into collision with the luminous bodies, and with our sun amongst the rest. Our sun would, accordingly, be at the mercy of any of those masses which might happen to come within the reach of its attractive influence. It would pull the mass towards it, and a collision would be inevitable, unless it so happened that a transverse motion of the sun itself might enable it to escape destruction. Even in such a case it could not by any means manage to get rid of the entangling mass.
All this risk, in so far as gravitation is concerned, would have been completely averted if an original projected velocity of some thirty or forty miles per second had been conferred on the dark mass; for, in this case, the attractive force of the sun would fail to arrest its motion, and the mass would pass onward through space, never to return. This simple conception of an original motion removes entirely those objections which, we have seen, besets the supposition we have been considering. With such a motion, not only would the risk to our solar system be removed, but the collisions between the dark bodies themselves would be a matter of rare occurrence; and hence the energy of the universe would be conserved. And when a collision did happen it would be on a grand scale, and the result would be not an imperfect sun without planets, but an incandescent nebula, out of which, by condensation, a complete solar system would be evolved. In fact, within the whole range of cosmical physics, I know of nothing more impressive in its sublime simplicity than this plan, by which the stability and perfection of the universe is thus secured. How vast the ends—how simple the means!
Consideration of the Facts which support the Theory, and of the Light which the Theory appears to cast upon the Facts.
I. Probable Origin of Meteorites.
Recent researches establish beyond doubt that stars, nebulæ, comets and meteorites, do not differ much from our earth in their chemical constitution. Meteorites, it is true, differ in their physical characteristics from ordinary rock such as is found on the earth’s surface. But it is possible, if not probable, that the earth’s interior mass “may,” as Sir Henry Roscoe remarks, “partake of the physical nature of these metallic meteorites, and that if we could obtain a portion of matter from a great depth below the earth’s surface we should find it exactly corresponding in structure as well as in chemical composition with a metallic meteorite, and the existence of such interior masses of metallic iron may go far to explain the well-known magnetic condition of our planet.”[[6]] I think there can be little doubt that, were our earth broken up into small fragments, and these scattered into space, it would probably be impossible to distinguish them from ordinary meteorites. The two would be so like in character that one can hardly resist the conviction that meteorites are but the fragments of sidereal masses which have been shattered by collision. That meteorites are broken fragments is the opinion expressed by Sir William Thomson, who says “that he cannot but agree with the common opinion which regards meteorites as fragments broken from larger masses, and that we cannot be satisfied without trying to imagine what were the antecedents of those masses.” The theory we have been considering appears to afford an explanation of their antecedents. According to it, they are broken fragments of two dark stellar masses which were shattered to pieces by collision. After what has been stated concerning the production of the gaseous nebulæ out of which our solar system was formed, it must be regarded as highly improbable, if not impossible, that the whole of the fragments projected outwards with such velocity should be converted into the gaseous condition. Multitudes of the smaller fragments, especially those towards the outer circumference of the nebulous mass, meeting with little or no obstruction to their onward progress, would pass outwards into space with a velocity which would carry them beyond the risk of falling back into the nebula. They would then continue their progress in their separated forms as meteorites. If this be their origin, then meteorites are the offspring of sidereal masses, and not their parents, as Mr. Lockyer concludes.
These meteorites must be of vast antiquity, for if they are fragments of the dark bodies then they must be not only older than our solar system, but older than the nebula from which that system was formed. Some of them, however, may have come from other systems. They are fragments which may yet cast some light on the history of the dark bodies.
Comets, bodies which in many points seem allied to meteorites, probably have, as we shall shortly see, a similar origin.
II. Motion of the Stars; how of such different velocities, and always in straight lines.