The theoretical view of the question, which we have now to consider, has led us, however, to the strong belief that no vestige of its former volcanic activity lingers in the moon—that it assumed its final condition an inconceivable number of ages ago, and that the high interest which would attach to the close scrutiny of our satellite if it _were_ still the theatre of volcanic reactions cannot be hoped for. If it be just and allowable to assume that the earth and the moon were condensed into planetary form at nearly the same epoch (and the only rational scheme of cosmogony justifies the assumption) then we may institute a comparison between the condition of the two bodies as respects their volcanic age, using the one as a basis for inference concerning the state of the other. We have reason to believe that the earth’s crust has nearly assumed its final state so far as volcanic reactions of its interior upon its exterior are concerned: we may affirm that within the historical period no igneous convulsions of any considerable magnitude have occurred; and we may consider that the volcanoes now active over the surface of the globe represent the last expiring efforts of its eruptive force. Now in the earth we perceive several conditions wherefrom we may infer that it parted with its cosmical heat (and therefore with its prime source of volcanic agency) at a rate which will appear relatively very slow when we come to compare the like conditions in the moon. We may, we think, take for granted that the surface of a planetary body generally determines its _heat dispersing_ power, while its volume determines its _heat retaining_ power. Given two spherical bodies of similar material but of unequal magnitude and originally possessing the same degree of heat, the smaller body will cool more rapidly than the larger, by reason of the greater proportion which the surface of the smaller sphere bears to its volume than that of the larger sphere to its volume—this proportion depending upon the geometrical ratio which the surfaces of spheres bear to their volumes, the contents of spheres being as the _cubes_ and the surfaces as the _squares_ of their diameters. The volume of the earth is 49 times as great as that of the moon, but its surface is only 13 times as great; there is consequently in the earth a power of retaining its cosmical heat nearly four times as great as in the case of the moon; in other words, the moon and earth being supposed at one time to have had an equally high temperature, the moon would cool down to a given low temperature in about one fourth the time that the earth would require to cool to the same temperature. But the earth’s cosmical heat has without doubt been considerably conserved by its vaporous atmosphere, and still more by the ocean in its antecedent vaporous form. Yet notwithstanding all this, the earth’s surface has nearly assumed its final condition so far as volcanic agencies are concerned: it has so far cooled as to be subject to no considerable distortions or disruptions of its surface. What then must be the state of the moon, which, from its small volume and large proportionate area, parted with its heat at the above comparatively rapid rate? The matter of the moon is, too, less dense than the earth, and hence doubtless from this cause disposed to more rapid cooling; and it has no atmosphere or vaporous envelope to retard its radiating heat. We are driven thus to the conclusion that the moon’s loss of cosmical heat must have been so rapid as to have allowed its surface to assume its final conformation ages on ages ago, and hence that it is unreasonable and hopeless to look for evidence of change of any volcanic character still going on.