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 reaction 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 cruptive 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.

We conceive it possible, however, that minute changes of a nonvolcanic character may be proceeding in the moon, arising from the violent alternations of temperature to which the surface is exposed during a lunar day and night. The sun, as we know, pours down its heat unintermittingly for a period of fully 300 hours upon the lunar surface, and the experimental investigations of Lord Rosse, essentially confirmed by those of the French observer, Marie Davy, show that under this powerful insolation the surface becomes heated to a degree which is estimated at about 500° of Fahrenheit's scale, the fusing point of tin or bismuth. This heat, however, is entirely radiated away during the equally long lunar night, and, as Sir John Herschel surmised, the surface probably cools down again to a temperature as low as that of interstellar space : this has been assumed as representing the absolute zero of temperature which has been calculated from experiments to be 250° below the zero of Fahrenheit's scale, Now such a severe range of heat and cold can hardly be without effect upon some of the component materials of the lunar surface.* If there be any such materials as the vitreous lavas that are

It is conceivable that the alleged changes in the crater Linné may have been caused by a filling of the crater by some such crumbling action as we are here contemplating.

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found about our volcanoes, such as obsidian for instance, they are doubtless cracked and shivered by these extreme transitions of temperature; and this comparatively rapid succession of changes continued through long ages would, we may suppose, result in a disintegration of some parts of the surface and at length somewhat modify the selenographic contour. It is, however, possible that the surface matter is mainly composed of more crystalline and porous lavas, and these might withstand the fierce extremes like the "fire-brick" of mundane manufacture, to which in molecular structure they may be considered comparable. Lavas as a rule are (upon the earth) of this unvitreous nature, and if they are of like constitution on the moon, there will be little reason to suspect changes from the cause we are considering. Where, however, the material, whatever its nature, is piled in more or less detached masses, there will doubtless be a grating and fracturing at the points of contact of one mass with another, produced by alternate expansions and contractions of the entire masses, which in the long run of ages must bring about dislocations or dislodgments of matter that might considerably affect the surface features from a close point of view, but which can hardly be of sufficient magnitude to be detected by a terrestrial observer whose best aids to vision give him no perception of minute configurations. And it must always be borne in mind that changes can only be proved by reference to previous observations and delineations of unquestionable accuracy.

Speaking by our own lights, from our own experience and reasoning, we are disposed to conclude that in all visible aspects the lunar surface is unchangeable, that in fact it arrived at its terminal condition eons of ages ago, and that in the survey of its wonderful features, even in the smallest details, we are presented with the sight of objects of such transcendent antiquity as to render the oldest geological features of the earth modern by comparison.

CHAPTER XIII.

THE MOON AS A WORLD: DAY AND NIGHT UPON ITS SURFACE.

A WIDE interest, if not a deep one, attaches to the general question as to the existence of living beings, or at least the possibility of organic existence, on planetary bodies other than our own. The question has been examined in all ages, by the lights of the science peculiar to each. With every important accession to our astronomical knowledge it has been reraised every considerable discovery has given rise to some new step or phase in the discussion, and in this way there has grown up a somewhat extensive literature exclusively relating to mundane plurality. It will readily be understood that the moon, from its proximity to the earth, has from the first received a large, perhaps the largest, share of attention from wanderers in this field of speculation: and we might add greatly to the bulk of this volume by merely reviewing some of the more curious and, in their way, instructive conjectures specially relating to the moon as a world-to imaginary journeys towards her, and to the beings conjectured to dwell upon and within her. and within her. This, however, we feel there is no occasion to do, for it is our purpose merely to point out the two or three almost conclusive arguments against the possibility of any life, animal or vegetable, having existence on our satellite.

We well know what are the requisite conditions of life on the earth; and we can go no further for grounds of inference; for if we were to start by assuming forms of life capable of existence under conditions widely and essentially different from those pertaining to our planet, there would be no need for discussing our subject further: we could revel in conjectures, without a thought as to their extravagance. The only legitimate phase of the question we can entertain is this:-can there be on the moon any

kind of living things analogous to any kind of living things upon the earth? And this question, we think, admits only of a negative answer. The lowest forms of vitality cannot exist without air, moisture, and a moderate range of temperature. It may be true, as recent experiments seem to show, that organic germs will retain their vitality without either of the first, and with exposure to intense cold and to a considerable degree of heat; and it is conceivable that the mere germs of life may be present on the moon. * But this is not the case with living organisms themselves. We have, in Chapter V., specially devoted to the subject, cited the evidence from which we know that there can be at the most, no more air on the moon than is left in the receiver of an air-pump after the ordinary process of exhaustion. And with regard to moisture, it could not exist

in

any but the vaporous state, and we know that no appreciable amount of vapour can be discovered by any observation (and some of them are crucial enough) that we are capable of making. We may suppose it just within the verge of possibility that some low forms of vegetation might exist upon the moon with a paucity of air and moisture such as would be beyond even our most severe powers of detection: but granting even this, we are met by the temperature difficulty; for it is inconceivable that any plant-life could survive exposure first to a degree of cold vastly surpassing that of our arctic regions, and then in a short time (14 days) to a degree of heat capable of melting the more fusible metals-the total range being equal, as we have elsewhere shown, to perhaps 600 or 700 degrees of our thermometric scale.

The higher forms of vegetation could not reasonably be expected to exist under conditions which the lower forms could not survive. And as regards the possibility of the existence of animal life in any form or condition on the lunar surface, the reasons we have adduced in reference

* Is it not conceivable that the protogerms of life pervade the whole universe, and have been located upon every planetary body therein? Sir William Thomson's suggestion that life came to the earth upon a seed-bearing meteor was weak, in so far that it shifted the locus of lifegeneration from one planetary body to another. Is it not more philosophical, more consistent with our conception of Creative omnipotence and impartiality, to suppose that the protogerms of life have been sown broadcast over all space, and that they have fallen here upon a planet under conditions favourable to their development, and have sprung into vitality when the fit ircumstances have arrived, and there upon a planet that is, and that may be for ever, unfitted for their vivification.