avalanche, the murmuring of the brook, the roaring of the sea, the voice of the storm--all proclaim, "The things which are seen are temporal!"--"The existing order of things had a beginning and must come to an end!"[151] 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."