We conceive, then, that the hypothesis we have set forth, is in many respects preferable to that ordinarily received. We can know nothing by direct observation concerning the central parts either of our own planet or any other: indirect methods are alone possible. The idea which has been tacitly adopted, is just as speculative as that we have opposed to it; and the only question is, which harmonizes best with established facts. Thus compared, the advantage is greatly on the side of the new one. It disposes of sundry anomalies, and explains things that seem else incomprehensible. We are no longer obliged to assume such wide differences between the substances of the various planets: we need not think of any of them as like cork or water. We are shown how it happens that the larger planets have so much lower specific gravities than the smaller, instead of having higher ones, as might have been expected; and we are further shown why Saturn is the lightest of all. That Mercury is relatively so much heavier than the Sun; that Jupiter is specifically lighter than his smallest satellite; that Saturn's rings have a density one and a half times as great as Saturn; are no longer mysteries. A feasible cause is assigned for the catastrophe which produced the asteroids. And some apparently incongruous peculiarities in the Earth's structure are brought to an agreement. May we not say, then, that being deducible from the Nebular Hypothesis, this alleged planetary structure gives further indirect support to that hypothesis? * * * * * In considering the specific gravities of the heavenly bodies, we have been obliged to speak of the heat evolved by them. But we have yet to point out the fact that in their present conditions with respect to temperature, we find additional materials for building up our argument; and these too of the most substantial character. Heat must inevitably be generated by the aggregation of diffused matter into a concrete form; and throughout our reasonings we have assumed that such generation of heat has been an accompaniment of nebular condensation. If, then, the Nebular Hypothesis be true, we ought to find in all the heavenly bodies, either present high temperature or marks of past high temperature. As far as observation can reach, the facts prove to be what theory requires. Various evidences conspire to show that, below a certain depth, the Earth is still molten. And that it was once wholly molten, is implied by the circumstance that the rate at which the temperature increases on descending below its surface, is such as would be found in a mass that had been cooling for an indefinite period. The Moon, too, shows us, by its corrugations and its conspicuous volcanoes, that in it there has been a process of refrigeration and contraction, like that which had gone on in the Earth. And in Venus, the existence of mountains similarly indicates an igneous reaction of the interior upon a solidifying crust.