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 re-raised: 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 into 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. 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.[16] 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.