“In most cases all controlling attraction would be feebly felt. These clouds of cosmical dust would float practically poised in the midst of space, and would gradually grow by the continued accession of new matter. Some of them would become aggregates of large dimensions, and their attraction would be distinctly felt by other aggregates. There would be a tendency of such aggregates to approach each other. They might possibly approach along a straight line; but more probably some third aggregation, or some distant sun, would deflect them into orbits about their common centre of gravity, in which, by prolonged collisions of cosmical matter, they are brought to ultimate coalescence with each other. Or some other attractive disturbance affords such a resultant of actions as may bring them more directly together. When these larger aggregations of world-stuff come together, the result is an aggregation approaching the dimensions of the Herschellian nebulæ.”[[63]]
In regard to the origin of the heat of the nebulæ, I am glad to find that Professor Winchell, to a certain extent, adopts the views which I have so long entertained on the subject. “The thought,” he says, “must already have suggested itself to the reader that the process of conglomeration affords an explanation of the intense heat which vaporises its substance, and causes it to yield a spectrum of bright lines. As the sudden compression of a portion of atmospheric air yields heat sufficient to ignite tinder, or fuse and volatilise a descending meteor-mass, so the precipitation of one planet upon another would liberate sufficient heat to reduce them both to a state of fusion, or even of vapour. Still more must the intensest heat be generated by the impact of two nebulous masses, one or both of which together may embrace more matter than all our planets and the sun combined—as much even as the matter of our entire visible firmament of stars. One experiences a distinct feeling of relief in the discovery of such a possible means of ignition of nebulæ.”
Mr. Charles Morris on the pre-nebular condition of matter.—Others again suppose matter to be present everywhere throughout space. This view has been ingeniously advocated by Mr. Charles Morris in an article on “The Matter of Space,” which appeared in Nature, February 8, 1883. The hypothesis of an ether specially distinct from matter he considers to be a gratuitous assumption, and one of the last surviving relics of eighteenth century science, and, unless it can be proved that highly disintegrated matter is positively incapable of conveying light vibrations, there is no warrant for assigning this duty to a distinct form of substance. But that matter exists in outer space in the same conditions as in planetary atmospheres he thinks is improbable. Its duty as a conveyer of radiant vibrations seems to require a far greater tensity, and its disintegration is probably extreme. Assuming matter throughout the universe—here as condensed spheres, and there in outer space as highly rarified substance—the atmospheric envelopes of the spheres, he considers, will gradually shade off into the excessively rare matter of mid-space. Matter may exist in countless conditions as to simplicity and complexity, &c., but the base particle he assumes to be the same under all conditions. In the spheres there is matter ranging from the simplest elementary gases, through the mineral compounds of the solid surface, to the highly compounded organic molecules. In outer space the variation is in the opposite direction; the matter existing there in a highly disintegrated condition.
Every particle he considers to possess a certain amount of motor energy in the form of heat. As the total amount of this energy in the universe remains unchanged, a particle can only lose energy by transferring it to others. This heat energy acts, of course, in opposition to gravity: it tends to repel the particles from each other, while gravity, on the other hand, tends to draw them together. The former acts as a centrifugal, the latter as a centripetal energy. If the heat momentum of the particles be insufficient to constitute a centrifugal energy equal to the centripetal energy of gravitation, then the material contents of space will be drawn into the attracting spheres as atmospheric substance, and outer space, in this case, will be left destitute of matter. If, on the contrary, the centrifugal energy of the particles be sufficient to resist gravitation, then the particles will remain free, and space will continue to be occupied with matter. As has been stated, the sum of motor energy in the universe remaining unchanged, the aggregation of atmospheric substance around any planet resulting from the loss of motor energy must cause an increase of motor energy in the particles outside.
The theory seems to dispense with the necessity for assuming a luminiferous ether, for the functions attributed to the ether may, it is thought, be performed by the particles themselves; a view which has been advocated by Euler, Grove, and others. The origin of nebulæ, according to the theory, is accounted for as follows:
“The nebular hypothesis,” says Mr. Morris, “holds that the matter now concentrated into suns and planets was once more widely disseminated, so that the substance of each sphere occupied a very considerable extent of space. It even declares that the matter of the solar system was a nebulous cloud, extending far beyond the present limits of that system. From this original condition the existing condition of the spheres has arisen through a continued concentration of matter. But this concentration was constantly opposed by the heat energy of the particles, or, in other words, by their centrifugal momentum. This momentum could only be got rid of by a redistribution of motor energy. If, for illustration, the average momentum of the particles of the nebulæ was just equivalent to their gravitative energy, then a portion of this energy must radiate or be conducted outwards ere the internal particles could be held prisoners by gravitation. The loss of momentum inwardly must be correlated with an increase of momentum outwardly.
“This is a necessary consequence of the heat relations of matter. As substance condenses, its capacity for heat decreases and its temperature rises, hence a difference of temperature must constantly have arisen between the denser and the rarer portions of the nebulous mass, and equality of temperature could be restored only by heat radiation. This radiation still continues, and must continue until condensation ceases and the temperatures of the spheres and space become equalised; but this is equivalent to declaring that as the particles of the spheres decrease in heat momentum those of interspheral space increase, and if originally the centrifugal and centripetal energies of matter approached equality they must become unequal, centripetal energy becoming in excess in spheral matter, centrifugal energy in the matter of space. Thus, as a portion of the widely distributed nebulous matter lost its heat, and became permanently fixed in place by gravitative attraction, another portion gained heat, became still more independent of gravity, and assumed a state of greater nebulous diffusion than originally. The condensing spheres only denuded space of a portion of the matter which it formerly held, and left the remainder more thinly distributed than before. The spheres, in their concentration, have emitted, and are emitting, a vast energy of motion. This motor energy yet exists in space as a motion of the particles of matter, which, therefore, press upon each other, or seek to extend their limits, with increasing vigour, so that the elasticity of interspheral matter is constantly increasing.”
Sir William R. Grove on the pre-nebular condition of matter.—Amongst the first to advocate the view that ordinary matter is everywhere present in space was Sir William R. Grove. In a lecture delivered at the London Institution as far back as January 1842, he stated that it appeared to him that heat and light, according to the undulatory theory, were the result of the vibrations of ordinary matter itself, and not that of a distinct ethereal fluid. Twenty years afterwards, referring to the views he then advanced, he says: “Although this theory has been considered defective by a philosopher of high repute, I cannot see the force of the arguments by which it has been assailed; and, therefore, for the present, though with diffidence, I still adhere to it.”[[64]]
He adduces a great many facts and forcible arguments in support of his position. He says that “there appears no reason why the atmosphere of the different planets should not be, with reference to each other, in a state of equilibrium. Ether, or the highly attenuated matter existing in the interplanetary space, being an expansion of some or all of these atmospheres, or of the more volatile portions of them, would thus furnish matter for the transmission of the modes of motion which we call light, heat, &c.” It is assumed in the theory, of course, that matter must form a universal planum.
Sir William Grove favours the idea that the universe is illimitable in extent, a view held by many eminent thinkers.