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Page:Popular Science Monthly Volume 4.djvu/535

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Mars, may be more like the earth, and fitted to support animal and vegetable life. It not inhabited by human beings, they may be passing through their preliminary Paleozoic, Mesozoic, or Cenozoic stages. And there are, probably, though their names are unknown to us, in the distant regions of space, deriving light and heat from suns seeming mere points to us, worlds where the early Eozoön rears its calcareous reefs, the gigantic Labyrinthodon croaks amid the primeval quagmires, and the Connecticut birds are leaving upon the marine mud the imprints of their tridactyle feet. Nor is it unlikely that other species of men inhabit some of these scattering orbs, and are as curious about us and our institutions as we are about them.

The Nebulous Period.—The usual geological argument for nebulosity is derived from the attempt to understand the origin of the condition of igneous fluidity. If the earth has been cooling from fusion, perhaps this is a cooler condition than the still earlier hotter state of fiery gas. Solids expand into liquids when heated, and liquids may become gases for the same reason. This gas, however, may not necessarily have been hotter than when condensed. The particles of matter must be the same when volatilized, as in both the liquid and solid states. Every substance now existing beneath the atmosphere must have been present—the compact ledges of the firmly-seated hills—the stone-walls of ancient cities—the water of the ocean—the oily fluid spouting from the bore-holes of Western Pennsylvania—the very particles of the paper containing this sentence printed upon it—and even the elemental constituents of our bodies, so fearfully and wonderfully made—all these and every thing material may have been commingled with the atmosphere, hovering about in a vaporous form, the components of a nebula, or comet.

In the attempt to surmise the actual condition of the elements at the beginning of the nebulous period, two views may be held, according as we prefer to adopt a chemical or mechanical theory of their origin. If one does not care to imagine the atoms called into existence in a heated condition, he may suppose that matter first appeared with the common frigid temperature of space, or about one hundred degrees Fahrenheit below the freezing-point of water, and that the elements were uncombined. Newly born, these particles would immediately commence to display their affinities, and the result would be explosive combinations, giving off intense light and heat. The latter force permeating the elements, would soon reduce them, first to igneous fluidity, and then into heated vapors. Every atom flying away from every other one, on the principle of "dissociation," would give rise to a nebula of enormous dimensions in a comparatively short time from these cosmic materials. After the formation of the nebula, the series of changes about to be described would commence its rounds.

A mechanical theory is presented by the distinguished philosopher, Dr. J. R. Mayer, author of a treatise upon "Celestial Dynamics."