=Probable composition of the earth’s core.=—In deriving views concerning the nature of the earth’s interior we are greatly aided by astronomical studies. The common origin long ago indicated for the planets of the solar system and the sun has been confirmed by the analysis of light with the aid of the spectroscope. It has thus been found that the same chemical elements which we find in the earth are present also in the sun and in the other stellar bodies. Again, the group of planets of the solar system which are nearest to the sun—Mercury, Venus, the Earth, and Mars—have each a high density, all except Mars, the most distant, having specific gravities very closely 5½, that of Mars being about 4. This average specific gravity is also that of the solid bodies, the so-called meteorites, which reach the surface of our planet from the surrounding space. Yet though the earth as a whole is thus found to have a specific gravity five and a half times that of water, its surface shell has an average density of less than half this value, or 2.7. The study of meteorites has given us a possible clew to the nature of the earth’s interior; for when both terrestrial and celestial rock types are classified and placed in orderly arrangement, it is found that the chemical elements which compose the two groups are identical, and that these are united according to the same physical and chemical laws. No new element has been discovered in the one group that has not been found in the other, and though some compounds of these elements, the minerals, occur in the earth’s crust that have not been found in meteorites, and though some occur in meteorites which are not known from the earth, yet of those which are common to both bodies there is agreement, even to the minor details (Fig. 9). It is found, however, that the commonest of the minerals in the earth’s shell are absent from meteorites, as the commoner constituents of meteorites are wanting in the earth’s crust. This observation would go far to show that we may in the two cases be examining different portions of quite similar bodies; and this view is strikingly confirmed when the rocks of the two groups are arranged in the order of their densities (Fig. 9). [Illustration: FIG. 9.—Diagram to show how terrestrial rocks grade into those of the meteorites. 1, oxygen; 2, silicon; 3, aluminium; 4, alkali metals; 5, alkaline earth metals; 6, iron, nickel, cobalt, etc.; _a_, granites and rhyolites; _b_, syenites and trachytes; _c_, diorites and andesites; _d_, gabbros and basalts; _e_, ultra-basic rocks; _f_, basic inclosures in basalt, etc.; _g_, iron basalts of west Greenland; _h_, iron masses of Ovifak, west Greenland; _a’-d’_, meteorites in order of density (after Judd).]