THE SOLAR SYSTEM.
Among the minor aggregating masses into which the cosmical nebula may be supposed to have split up, in the course of its general aggregative cycle, was a group of matter out of which our own solar system has been developed. In its earliest separate phase we may suppose this group to have consisted of various minor portions, in varying stages of aggregation, revolving in a single direction around their common centre. (Details about the direction of Neptune and Uranus may be safely neglected.) We may further suppose that the nebulous or quasi-nebulous mass thus composed again divided itself, along its weakest cohesive lines, into other portions, which have aggregated into the existing planetary groups; while these latter again subdivided themselves into the central masses and their satellites. It is immaterial for our purpose whether, with the earlier evolutionists, we regard these changes as taking place in a relatively homogeneous substance, a diffused nebula, or whether, with their later followers, we set them down to aggregative action in comparatively solid and discrete masses (meteors), like those which we know to exist in large tracts within the sphere of the solar system. But the important point to notice in either case is this, that these groupings and sub-groupings took place under the influence of Forces, and that the Potential Energy of separation between the masses or molecules became Kinetic as they clashed together, and assumed the form of Heat. The various masses thus became each of them a little sun, aggregating around their several centres, and radiating their Energy into the surrounding ether. As in other cases, some small portion of this Energy would fall upon neighbouring masses, belonging either to the same system or to other systems, and would there do a little towards retarding the aggregation of their matter and the dissipation of their Energy; but the greater portion would doubtless be lost in the vast interstellar spaces; so that the general result would be a loss of Energy to matter, and a gain of Energy to the ethereal medium.
The various planets and satellites thus formed would still possess Potential Energy in virtue of their continued separation from one another. They would also possess Molar Kinetic Energy in virtue of their orbital movement, which acts as a preventive to their immediate aggregation with their common centre, the sun. And, finally, they would possess Molecular Kinetic Energy through the vibratory movement of their molecules, derived from the previous Kinetic Energy of their aggregative motion. But as their particles vibrated, they would part from moment to moment with portions of their Energy to the surrounding ether. This loss of Energy would only largely affect the periphery of each mass, and would at first be inconspicuous at the centre. It would also affect the smaller masses much more rapidly than the greater, for two reasons; both because the amount of aggregating matter being less, the amount of heat generated would also be less; and because losses of heat from the periphery could not so easily be made up by conduction from the centre. The smaller masses would accordingly cool at their surfaces at comparatively early periods; while the larger ones, in proportion to the amount of unaggregated matter within the sphere of their attraction, would continue for long periods to receive fresh accessions to their molecular Energy, and to radiate Light and Heat into the surrounding ether. Especially would the largest mass of all, the central sun, continue for an immense era to aggregate the surrounding masses and to radiate the liberated Energy into the space around.
Further, we may conclude that as the surface of each mass parted with its Energy, its superficial molecules would be drawn together by the Forces of cohesion and chemical affinity. We should thus get a solid cohering framework on the exterior of each mass, as soon as it had parted with a considerable portion of its molecular Energy to the surrounding ether. Through this cohering crust, the central Energy would slowly escape by conduction: until, sooner or later, we might expect each such mass to consist of a cold and inert collection of molecules, the whole Energy of whose previous separation had been yielded up to the ether. But a special incident of this transference might occasionally occur to break the monotony of its simple course. As the central Energy escaped through the cohering crust, there might be a tendency for the interior molecules to be drawn together under the influence of cohesion and gravitation. A second crust would thus tend to form itself beneath the outer one. Thereupon, the Force of gravitation might outweigh the cohesion of the outer crust, which would yield under the strain and fall in upon the subjacent mass, breaking along its line of least cohesion. Each such fall would be itself a change of Potential into aggregative molar Kinetic Energy, as the masses fell together; and when they clashed, the Energy would assume the molecular form and be given off as heat. But, however the aggregation takes place, as the matter of each group aggregated more and more closely round its centre, the Energy of its previous separation would be given off as heat, and would finally be imparted, as in every other case, to the ethereal medium.
While each mass was thus parting (by radiation) with the Molecular Kinetic Energy resulting from its previous separation and aggregative motion, it would also be parting, though more slowly (by ethereal friction) with the Molar Kinetic Energy of its orbital motion. Each satellite would thus be drawing progressively nearer to its primary, and each planet to the sun. As every unit of Energy was lost, gravitation would draw the body one unit nearer to its relative centre. It might thus be expected that each satellite would aggregate with its primary before the primary planet aggregated with the sun. At each such aggregation, though the greater part of the orbital Energy would doubtless be already dissipated, yet it is probable that as the two bodies glided together (for they would not fall, as is often erroneously said) there would be a considerable residue of Energy still remaining, which, on the shock of collision, would be converted into molecular motion (or heat), and would be sufficient to reduce the bodies to a molten state. But this incident, instead of interfering with the final aggregative process, would really hasten it: because the new united body would at once begin radiating off its heat into space, and once more cool down to a mass of cold and inert molecules. In other words, all the remaining Energy of separation belonging to the satellite in virtue of its discrete condition, and all the remaining Kinetic Energy of its orbital motion, would thereupon be dissipated into the surrounding ether: and the united mass would continue to gravitate slowly towards the central sun. What is thus probable of the satellites in relation to their primaries is equally probable of the planets in relation to the sun. As their Energy of orbital motion is dissipated by ethereal friction, we conclude that they are drawing nearer and nearer, step by step, to the centre of our system. And there is no reason to doubt that they will continue to do so until they each slowly aggregate with the central mass, converting their remaining Energy as they clash together, into heat, which will itself go for a time to swell the volume of solar Energy, and will be radiated off like the rest into surrounding space. Finally, when the sun has aggregated with himself all the matter of the solar system, we may conclude that he will ultimately radiate off all the molecular Energy derived from their aggregation, and become himself a cold and inert mass, like some of those burnt-out stars revealed to us by astronomy. And then we may imagine that this single sphere, which contains all the matter of our system, will itself proceed to aggregate with some other mass, in that general cosmical group of which it forms an unimportant member. Of course, it is impossible to conjecture which of these aggregations will take place first; and it is quite conceivable that the whole solar system might clash with some other group of worlds before its various members had time to aggregate in this regular order with one another; but if our suggested theory of a general subordination of systems and cycles to a common cosmical centre be correct, then just as each satellite would aggregate with its primary before that primary had time to aggregate with the sun, so each planet would have aggregated with the sun before the sun had time to aggregate with its superior mass. However this may be, it will be sufficient if we regard the probable course of events in the solar system as a specimen of the probable incidents everywhere accompanying the course of aggregation round the common cosmical centre, and briefly hinted in the preceding chapter.
At the present moment of time, we occupy a middle point in the systemic epoch thus sketched out. The sun, our central mass, is still in a state of rapid molecular motion, which he imparts as Radiant Energy to the ether. He has yet much outlying matter to aggregate, and it seems probable that small aggregations are from day to day taking place. Of the planets, the smaller have cooled down sufficiently to possess a firm and coherent outer crust, while the larger are still in a very volcanic state. The satellites have probably radiated away all their proper heat, and are already cold and inert to their centres. The surface of the most easily observed, our own moon, shows the vast corrugations which result from the continual collapses of the crust upon the once heated nucleus, and the reactions of the molten interior upon the coherent outer shell: — corrugations relatively (if not absolutely?) much greater than any at present found upon the surface of our own earth. Some small fraction of the Energy radiated from the sun falls upon the cooled exteriors both of planets and satellites. Of this, the greater portion is reflected back by dispersion, as we see from the case of the moon, in every direction (only a small fraction of this fraction again falling upon other masses). But a certain lesser portion is used up in heating the outer crusts, in setting up evaporation, currents, and winds, and in producing the phenomena of organic life. These secondary separative reactions of radiated Energy upon the surface of a planet form the great mass of those phenomena which are generally observed by human beings.