| Tons. | ||
| At 2 miles deep, | 2 miles of sea at 150 tons per mile | 300 |
| 2½ " | 2½ solid matter at spec. grav. 2·25 | |
| equal to 331·25 tons per mile | 828 | |
| 9 " | 4½ "of rock at 163 lb. per cubic foot | 1730 |
| Total pressure | per square foot | 2858 |
or just about 4 times the crushing strain of our best granites. Then, as when crushing takes place compression begins, it will, we believe, be far below the mark to estimate the general specific gravity of the earth at 9 miles deep to be 3 times that of water.
We have now added the pressure of the 2 miles of water, because there could be no water at the depth of 9 miles; for the critical temperature of water is known to be 412°, beyond which temperature water cannot be maintained in its liquid state by any amount of pressure, however great; and 9 miles would give 483° temperature at 1° for each 30 metres. At that depth there might be steam, although it is difficult to see how it could penetrate so far, because the only force to help it to penetrate would be gravitation, and that would have to act against the increasing repulsion of heat.
There is another circumstance to be considered which would tend to increase the density of the outer portion of the crust, if there be a crust, and if not, of the outer portion of the earth itself.
When the earth was in the molten liquid state, it is generally supposed to have been surrounded by vapours of a great proportion of the metals and of some of the metalloids, in addition to the vapour of water, air, and other gases, which floated above them higher up in the atmosphere. In that case when the crust began to be formed through cooling, these vapours would be precipitated on the surface and mixed with the half-liquid half-solid matter there, but the proportion of condensed vapours would be very small compared with what they fell upon, and the specific gravity of the mixture would not be great enough to cause it to sink much below the surface, because it would soon meet with matter as dense as itself; consequently we must consider that all these metals would remain near the surface—most likely much nearer to it than the 9 miles which we have as yet descended to—and whatever may have been the proportion of their density it ought to be added to the weights and pressures that have been taken into account above. We believe that it will be shown later on that this estimate of a density of three times that of water at 9 miles deep in the earth is very much lower than it should be; because, when the pressure upon the matter there came to be greater than its crushing strain, compression would go on more rapidly than shortly afterwards, and it might so be that with a strain of very much less than four times that of crushing, compression would be reduced to its utmost limit. But more of this hereafter.
Having determined densities for the matter composing the earth at 2, 4½, and 9 miles below the surface, that is, to where the mean diameter comes to be 7900 miles, if we divide that diameter into layers of 25 miles each in thickness, compute the volume of each layer or shell, increase the density of each layer as we descend in direct proportion from 3—the density we have fixed for 9 miles deep—to 13·734 times the density of water, at the centre, and multiply the volume of each layer from the surface downwards by its respective average density, we shall find a mass nearly equal to the mass of the earth at the density of water—always taking its mean diameter at 7918 miles, and mean density at 5·66 times that of water, as already premised. These calculations have been carefully carried out, and are represented in detail in [Table IV]. for future reference. They terminate in a deficiency of over 70,000,000 of cubic miles, a deficiency which would be more than made up by making the central density 13·736 instead of 13·734. Thus we see that if the density of the earth increases regularly from the surface to the centre, and if the densities we have given to the layers between the surface and 9 miles in depth are not greater than those adopted, the central density must be exceedingly near 13¾ times that of water. Of course, if the three surface densities are in reality less than those we have adopted, the central density must be greater than 13¾ times that of water. The whole being a result to our calculations which leads us to speculate on what kind of matter there is at the centre of the earth.
We are acquainted with various kinds of rocks, stones and other solid matter that have densities (specific gravities) of 2½ to 3 times that of water, and we have to conceive that a cubic foot of one of these would have to be compressed into a height of 2¾ or 2¼ inches in order to have the density of 13¾ required at the centre, a result which presents us with a substance which it is difficult to imagine or to believe to exist. It may be that the centre of the earth is occupied by the heaviest metals we know, arranged in layers proportioned in thickness to the masses required of them, and that they are laid one over the other according to their densities, or mixed together until a distance from the centre is attained, at which ordinary rocks compressed as highly as their nature would admit of, may exist; but we do not derive much knowledge or satisfaction from such a supposition. An examination of our table of calculations will show that 500 miles in diameter of the central part might be filled up with platinum, the few other rarer and heavier metals, and gold amalgamated with mercury in due proportions. Then there might be a mixture of mercury and lead to 1800 miles in diameter, followed by a mixture of lead and silver to 2400 miles. After that might come a compound of silver, copper, tin, and zinc to 4900 miles, and some compounds of iron might finish the filling process up to 6000 miles in diameter, or thereby; where the known rocks, compressed to half their volume at first, but gradually allowed to expand, might complete the whole mass of the earth. It will be seen, also, that by the time compressed rocks could be used for this filling process, more than 43 per cent. of the whole volume of the earth would be occupied exclusively by pure metals mixed by rule and measure.
It would appear then that the "sorting-out theory"—about which a good deal has been written—whereby, in suns and planets, the metals on account of being heavier fall more rapidly to the centre, and the lighter metalloids remain near the surface—a theory probably got up to get over the difficulty we are in—is not a very happy one, as too much metal would be required for the process, at least for the earth. No doubt it might be applied differently to what we have done by mixing metals with rocks, stones, earth, etc., forming metallic ores—very rich they would doubtless have to be—from the centre outwards; but however disposed it would seem that very much the same quantity would be required to furnish the desired densities up to 6000 miles in diameter, where we have supposed compressed granites, etc., might come into play. Besides, such an arrangement would do away with the whole beauty of the theory; there would be no law to invoke; it would be all pick-and-shovel work.