Page:Popular Science Monthly Volume 4.djvu/537

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history of the earth at this early period was, therefore, merged in that of the solar system.

The centrifugal force produced by rotation would cause rings of gaseous matter to separate themselves one after another from the central mass, the latter turning on its axis more rapidly after the removal of the exterior. The separated ring would then have been an annular nebula. As many as six rings must have been cooled before the earth-mass separated itself from the interior sphere carrying the substance of the sun, and the inferior planets.

The next stage of growth would naturally consist in the breaking up of each ring by itself, perhaps in consequence of inequalities in different parts, and condensation into a sphere of greater specific gravity. The falling of the particles would add heat, and perhaps quickly induce the fluidity of the mass. While still gaseous, other rings may fly off, to become satellites. All the nebulæ, by constant rotation, may have given freedom to the contained particles to arrange themselves according to their relative densities, the heaviest atoms falling to the centre, and the lightest remaining at the surface. The process of separation into zones must have been analogous to the cooling of liquids. As fast as their superior density caused particles to descend, the lighter atoms would be displaced and sent to the surface, either to be cooled, or to remain permanently in a higher stratum. But, at the close of this period, there must have been, outside of the fluid, an enormous thickness of gases which did not liquefy till after the crust had formed to a considerable amount.

Period of Igneous Fluidity.—At the commencement of this period the earth seems to have been a flattened sphere, composed of melted matter like lava, encircled by steam and easily-volatilized liquids and solids, but girdled externally by an atmosphere; rotating upon its axis and revolving round the central sun. It was a sun of itself, emitting light and heat, thus forbidding the distinction of day and night, though the planetary movements inducing the alternations of position were as well marked as now. The several compounds constituting the material of the earth were probably arranged in concentric zones according to their relative gravities, just as we now observe the settlings in a copper or iron furnace. A general mixture of rich and poor ores, fluxes and fuel, is put into the receiving-vault; when ignited, the solids mix together, melt into a fluid, the heavier metals sinking to the bottom, and the slags rising to the surface to be skimmed off. So the metals would naturally gravitate to the centre of the fluid earth, and around them might be several zones of successively lighter compounds, the exterior being the least heavy of all, and answering to the slags of the furnace. The specific gravity of the whole earth is now 5.65, when compared with water, as determined from astronomical sources; but that of the surface-rocks is less than half this amount: hence we have abundant reason to believe that the