Page:Popular Science Monthly Volume 52.djvu/847

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N his studies of the relative frequency of the different elements composing the crust of the earth, Mr. F. W. Clarke supposes that to a depth of ten miles below the level of the sea the composition of the ground is the same as is given by the examination of the surface strata and the depths which we have reached. The mean specific gravity of these strata is 2.5, or not quite half the density of the earth as a whole. Including the oceans and the atmosphere, the exterior crust of the earth is composed half of oxygen and one fourth of silicon, while the other fourth is represented by other elements—aluminum, 7 per cent; iron, 5.10; calcium, 3.50; magnesium, 2.50; sodium, 2.20; and potassium, 2.20 per cent. Some of the elements of which the numerous compounds have long been very obvious to the human ken are, therefore, from the point of view of their quantity, of very little importance; thus, hydrogen stands for only 0.94 per cent of the general composition of the crust of the earth, carbonic acid for 0.21 per cent, phosphorus for 0.09 per cent, and nitrogen for 0.02 per cent. These elements, which are the constituents of immense seas and form the basis of life, therefore furnish only a minute fraction of the mass of the ten mile-thick ring contemplated by Mr. Clarke. Since the soundings thus far made indicate that they do not exist or hardly exist at greater depths, we have a right to say that so far as regards quantity they may almost be neglected, in considering the mass of the whole globe. The content in chlorine does not exceed 0.15 per cent, yet the common salt alone held in the oceans is sufficient to cover all the continents and bury the highest mountains.

We perceive from this showing how little the impression the outer surface of our globe gives us corresponds with its real nature as we judge of it from its mean density. There can not be the least doubt that the internal parts of the globe are composed of different substances from those which appear in the external strata.

But, while the elements of light specific weight or great volatility which, like hydrogen and nitrogen, exist in large quantities around us, constitute only a very minute part of the constituents of our globe considered as a whole, we presume that the elements called rare only enter in an infinitesimal degree into the general composition of the earth; the more so, because, so far as we as yet know, these elements are not found at great depths. I, at least, do not know that the heavier metals—gold, silver, lead, etc.—have ever been found in the materials extracted from deep soundings or