Cometic nebulæ are explained, he considers, “on the supposition that we have either a very condensed swarm moving at a very high velocity through a sheet of meteorites at rest, or the swarm at rest surrounded by a sheet, all moving in the same direction.”
In an able and interesting work, which seems almost utterly unknown in England,[[14]] Professor Winchell has advanced views similar to those of Tait and Lockyer regarding the nature and origin of nebulæ. But he, in addition, discusses the further question of the origin of those swarms. I shall have occasion to refer to Professor Winchell’s views more fully when we come to the consideration of the pre-nebular condition of the universe.
Amongst the first to advance the meteoric hypothesis of the origin and formation of the solar system was probably the late Mr. Richard A. Proctor. This was done in his work, “Other Worlds than Ours,” published in 1870. “Under the continual rain of meteoric matter,” he says, “it may be said that the earth, sun, and planets are growing. Now, the idea obviously suggests itself that the whole growth of the solar system, from its primal condition to its present state, may have been due to processes resembling those which we now see taking place within its bounds.” He further adds: “It seems to me that not only has this general view of the mode in which our system has reached its present state a greater support from what is now actually going on than the nebular hypothesis of Laplace, but that it serves to account in a far more satisfactory manner for the principal peculiarities of the solar system. I might, indeed, go farther, and say that where those peculiarities seem to oppose themselves to Laplace’s theory they give support to those I have put forward.”[[15]] He then goes on to show the points wherein his theory seems to him to offer a better explanation of those peculiarities than that of Laplace.
5. The gaseous condition the second stage of a nebula.—The second stage obviously follows as a necessary consequence from the first; for the fragments, in the case under consideration, possess energy in the form of motion, which, with the heat of their circumambient vapour, is more than sufficient not only to convert the fragments into the gaseous state, but to produce complete dissociation of the chemical elements. The complete transformation of the first stage into the second must, therefore, be simply a matter of time.
According to the laws of probability it may, however, sometimes happen that the two original dark bodies will not collide with force sufficient to confer on the broken fragments the energy required to convert them all into the gaseous condition. The result in this case would, no doubt, be that the untransformed fragments, drawn together by their mutual attractions, would collide and form an imperfect star or sun, without a planet. Such a star might continue luminous for a few thousands or perhaps a few millions of years, as the case might be, when it would begin to fade, and finally disappear. We have here an imperfect nebula, resulting in an imperfect star. In short, we should have in those stellar masses, on a grand scale, what we witness every day around us in organic nature, viz. imperfect formations. Such occasional imperfections give variety and add perfection to the whole. How dreary and monotonous would nature be, were every blade of grass, every plant, every animal, and every face we met formed after the most perfect model!
6. The gaseous condition essential to the nebular hypothesis.—It is found that the density of the interior planets of our solar system compared with that of the more remote is about as five to one. The obvious conclusion is that there is a preponderance of the metallic elements in the interior planets and of metalloids in the exterior. It thus becomes evident, as Mr. Lockyer has so clearly shown,[[16]] that when our solar system existed in a nebulous condition the metallic or denser elements would occupy the interior portion of the nebula and the metalloids the exterior. Taking a section of this nebula from its centre to its circumference, the elements would in the main be found arranged according to their densities: the densest at the centre, and the least dense at the circumference. If we compare the planets with their satellites, we find the same law holding true. The satellites of Jupiter, for example, have a density of about only one-fifth of that of the planet, or about one twenty-fifth of that of our earth, showing that when the planet was rotating as a nebulous mass the more dense elements were in the central parts and the less dense at the outer rim, where the satellites were being formed. Again, if we take the case of our globe, we find, as Mr. Lockyer remarks, the same distribution of materials, proving that when the earth was in the nebulous state the metallic elements chiefly occupied the central regions, and the metalloids those outer parts which now constitute the earth’s crust.
All these facts show that the sifting and sorting of the chemical elements according to their densities must have taken place when our solar system was in the condition of a nebula. But, further, it seems impossible that this could have taken place had the materials composing the nebula been in the solid form, even supposing that they had taken the form of clouds of stones.
It is equally impossible that the nebula could have been in the fluid or liquid state during this process. This is obvious, for the nebula must then have occupied, at least, the entire space within the orbit of the most remote planet. But our solar system in the liquid condition could not occupy one-millionth part of that space. It is therefore evident that the nebula must have been in the state of a gas, and a gas of extreme tenuity.
7. The mass must have possessed an excessive temperature.—There is ample evidence, Mr. Lockyer thinks, to show that the temperature of the solar nebula was as great as that of the sun at the present time. But I think it is extremely probable that, in some of its stages, the nebula had a very much higher temperature than that now possessed by the sun. There must, during the sifting period, have been complete chemical dissociation, so as to keep the metals and the metalloids uncombined, and thus allow the elements to arrange themselves according to their densities. The nebula hypothesis, remarks Mr. Lockyer, “is almost worthless unless we assume very high temperatures, because, unless you have heat enough to get perfect dissociation, you will not have that sorting out which always seems to follow the same law.”
8. Gravitation could, under no possible condition, have generated the amount of heat required by the nebular hypothesis.—The nebular hypothesis does not profess to account for the origin of nebulæ. It starts with matter existing in space in the nebulous condition, and explains how, by condensation, suns, planets &c. are formed out of it. In fact, it begins at the middle of a process: it begins with this fine, attenuated material in the process of being drawn together and condensed under the influence of attraction, and professes to explain how, as the process goes on, a solar system necessarily results. To simplify our inquiry we shall confine our attention to the solar nebula, and consider in the first place how far condensation may be regarded as a sufficient source of heat.