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 diffusion into space from small globes in the manner already explained. Indeed, it seems quite probable that the oxygen in our own air is not destined for ever to remain there. It passes through various vicissitudes by being absorbed by animals and then restored again in a free state under the influence of vegetation. But there is an appetite for oxygen among the inorganic materials of our globe which seems capable of using up all the oxygen on the globe and still remaining unsatiated. We have excellent grounds for believing that there is in the interior of the earth a quantity of metallic iron quite sufficient to unite with all the free oxygen of the air so as to form iron oxide. In view of the eagerness with which oxygen and iron unite, and the permanence of the compound which they form, it is impossible for us to regard the presence of oxygen in the air as representing a stable condition of things. It follows that, even though there may now be no free oxygen in the atmosphere of Mars, it is by no means certain that this element has always been absent. It is, however, not at all beyond the reach of scientific resources to determine what the actual composition and extent of the atmosphere of Mars may be, though it can hardly be said that as yet we are in full possession of the truth.

An almost equally important question is as to the telescopic evidence of the presence of water on Mars. Here, again, we have to be reminded of the fact that even when the planet makes its closest approach it is still actually a very long way off. It would be impossible for us to say with certainty that a region which by its colour and general appearance looked like an ocean of water was really water or was even a fluid at all. It is