Astronomical Palætiology reduces all celestial phenomena to the same great law of finite duration. It teaches that planets, stars, systems, have their birth, their process of formation, their maturity, and their slow, protracted decay. The ephemeron perishes in an hour, man endures his three-score years and ten; continents and islands have their ages and æons; the stars of heaven are not exempt from this universal law of change and decay. According to the Nebular Hypothesis, the formation of this our system of sun, planets, and satellites was a process of the same kind as that which is still going forward in the heavens. One after another, nebulæ condense into separate masses, which begin to revolve about each other in obedience to dynamical laws, and form systems of which our system is a matured example. The present aspect of this planetary system is, however, but a passing phase in the history of its fleeting life. Our planet was once a self-luminous orb; it has now become opaque, and shines only with a borrowed light. The moon is probably in a state of total refrigeration; its lunar air and lunar seas have been changed by intensity of cold into the solid form.[152] The sun itself is radiating heat into space in quantities incomparably greater than it receives, and, as Helmholtz affirms, "the inexorable laws of mechanics show that its store of heat must be finally exhausted."[153] The planets in their motions encounter resistance from the interstellar ether; they must, therefore, necessarily move in shorter and shorter orbits, and at last fall into the sun. Thus the Nebular Hypothesis, combined with the doctrine of a resisting medium, teaches us that the solar system is wending its way, through successive changes, from a past of vaporous unity to a future of consolidated reunion. "It was once all nebula; it will, if left to physical agencies alone, collapse into an extinguished and exhausted sun."
The astronomer who has been accustomed to regard every question relating to his favorite science as almost exclusively a problem in mathematics, will pronounce the above "a crude and adventurous" attempt on the part of the physicist to solve a problem which belongs to "the calculus of variations." Is the universe a Conservative or a Dissipative system? Under its present laws will it run on forever, or will these very laws in the end lead to its subversion? Will the mechanism of the heavens finally run down as surely as the weights of a clock run down to their lowest position, or are we authorized on scientific grounds to assert the permanent stability of the solar system? This question has been earnestly discussed by the most distinguished astronomers since the days of Newton. Until recently, the general conclusion--reached mainly on mathematical grounds--seems to have been that the universe is a thoroughly conservative system, and that the celestial machinery by a species of perpetual motion will run on forever. But must not all applied mathematical reasoning obtain its data from the exact observation of material facts? The mathematician must also be a good natural philosopher; he must lay his account with all the facts of the universe, otherwise his symbols have no contents, and his reasoning, however faultless in its processes, will be fallacious in its results. The discoveries of the present century respecting the correlation of the various forms of energy, the nature of the solar light and heat, the motions of comets, and especially the new doctrine of the "Dissipation of Energy," have introduced new elements into the great problem, which seem to indicate that gravitation is by no means the only force by which the motions of the heavenly bodies are influenced, and that causes are now in operation which are slowly but surely undermining the system. We now find, therefore, such high authorities as Whewell, Sir John Herschel, Sir William Thomson, Balfour Stewart, Prof. Maxwell, Dr. J. R. Mayer, Helmholtz, Tyndall, Littrow, Comte, Adolph Fick, asserting that the solar system is not a self-winding clock which may run forever, but that it is a dissipative system which must ultimately lose all motion, unless some power capable of controlling the laws of material nature interfere to preserve it. We have no more valid reason for concluding that the Deity intended the system should be eternal than that He intended the earthly life of man should be eternal.[154] A few general statements may assist the reader in appreciating the merits of the discussion.