Page:Popular Science Monthly Volume 75.djvu/539

Rh on the one and accumulation on the opposite hemisphere must end in taking the whole supply, surface or aerial, from the day side to pile it up in perpetual ice upon the night one. Dry air more or less laden with dust must therefore constitute now the atmospheric covering of the sunward hemisphere. Now this is what gives Venus her excessive luster—an atmosphere devoid of cloud. It is precisely because she is not cloud-covered that her luster is so great. She "clothes herself with light as with a garment" in consequence of a physical fact of some interest. As becomes the Mother of Loves, this drapery is gauze of the most attenuated character, and yet on that very account is a great heightener of effect. For it is a well-known property of matter that a substance when comminuted reflects much more light than when massed as a solid or an opaque cloud. Now an atmosphere is itself such a comminuted affair and especially is made lucent by the dust of one sort and another which it holds in suspension. This would particularly be true of Venus for the reasons we have exposed and thus stands explained her albedo of .92 which were she cloud-covered could not exceed .72, the albedo of cloud. This brightening character of an atmosphere stands corroborated by what we perceive of the other planets. Mercury and the moon, which are airless bodies, have an albedo of only .17; Mars, which has some air but not much, one of .27; while Venus, whose sky is clear, one of .92.

Another phenomenon which has greatly puzzled astronomers stands accounted for by what we have learned latterly of the world of Venus. For years by one observer or another a sort of faint phosphorescent shine has been reported of the unilluminated part of her disk; the ashen light, it has been called. The side of her which should be dark has appeared ghostly lighted up. The phenomenon has seemed the weirder for the difficulty of explaining it. It is like what dimly reveals the old moon in the new moon's arms. With the moon this is earth-shine; the moon-shine the earth herself lends her satellite. But Venus has no neighbor to act as mirror near her, though such be her astronomic symbol. The earth is too far off and the stars inadequate to the occasion. But the state of things we have sketched furnishes an explanation. If the night side of Venus be a vast stretch of polar ice, here is just the surface to reflect the starlight with something approaching a phosphorescent shine. Nor would this necessarily be dimmed by the dust of ages because of a slow process of glacier rejuvenation constantly in progress, due partly to the winds, partly to a slow sinking of the débris to the bottom.

Such are some of the peculiar phenomena presented by the planet. When we thus reason about them—and even in science reasoning is not so much to be despised as some mechanical souls would have us believe—we see that they lose their oddity, becoming the very pattern and prototype of what we should expect.

Logical deductions from well-established fact has led us to