Now the density of the materials which compose the crust of the globe is very much less than this, varying from about two-and-one-third to three times that of water. Hence we are compelled to conclude that the interior portions of the globe are of far greater density than the exterior portions; that, as a matter of fact, the mass of the globe is composed of materials having twice the density of the rocks exposed at the surface. [Sidenote: DENSITY OF EARTH'S INTERIOR.] It has been sometimes argued that as all materials under intense pressure appear to yield to an appreciable extent, and to allow their particles to be packed into a smaller compass, we may find in this fact an explanation of the great density of the internal parts of the globe. It has in fact been suggested that under the enormous pressure which must be exerted by masses of rock several thousand feet in thickness, the materials of which our earth is composed may be compelled to pack themselves into less than one-half the compass which they occupy at the surface. But the ascription of such almost unlimited compressibility to solid substances can be supported neither by experiment nor analogy. Various considerations point to the probability that solid bodies yield to pressure up to a certain limit and no farther, and that when this limit is reached an increase in pressure is no longer attended with a reduction in bulk. If then we are compelled to reject the idea of the unlimited compressibility of solid substances, we must conclude that the interior portions of our globe are composed of _materials of a different kind_ from those which occur in its crust. And this conclusion, as we shall presently see, is borne out by a number of independent facts. The study of the materials ejected from volcanic vents proves that even at very moderate depths there exist substances differing greatly in density, as well as in chemical composition. The lightest lavas have a specific gravity of 2·3, the heaviest of over 3. And that materials of even greater density are sometimes brought by volcanic action from the earth's interior, we have now the clearest proofs. [Sidenote: RELATION BETWEEN EARTH AND OTHER PLANETS.]