Being oblate in figure, unlike in the densities of its centre and surface,unlike in their temperatures, and probably unlike in the angular velocitiesof its parts, such a mass cannot be called homogeneous; and any further changesexhibited by it can illustrate the general law, only as being changes froma more homogeneous to a less homogeneous state. Just noting that one of thesechanges is the increasing oblateness of form, let us go on to observe thosewhich are to be found in the transformations of such of its parts as areat first homogeneous within themselves. If we accept the conclusion thatthe equatorial portion of this rotating and contracting spheroid will, atsuccessive stages, have a centrifugal force great enough to prevent nearerapproach to the centre of rotation, and will so be left behind; we shallfind, in the fate of the detached ring, an exemplification of the principlewe are following out. Consisting of gaseous matter such a ring, even if uniformat the time of its detachment, could not continue so. In the absence of equalityamong the forces, internal and external, acting on it, there must be a pointor points at which the cohesion of its parts would be less than elsewhere-- a point or points at which rupture would therefore take place. The originalassumption was that the ring would rupture at one place only, and would thencollapse on itself. But this was a more than questionable assumption: such,at least, I know to have been the opinion of the late Sir John Herschel.
So vast a ring, consisting of matter having such feeble cohesion, must breakup into many parts. Nevertheless, appeal to another high authority -- thelate Sir G. B. Airy -- yielded verification for the belief that the ultimateresult which Laplace predicted would take place. And here is furnished afurther illustration of the instability of the homogeneous. For even supposingthe masses of nebulous matter into which such a ring separated, were so muchalike in their sizes and distances as to attract one another with exactlyequal forces (which is infinitely improbable); yet the unequal actions ofexternal disturbing forces would inevitably destroy their equilibrium --there would be one or more points at which adjacent masses would begin topart company. Separation, once commenced, would with accelerating speed leadto a grouping of the masses. A like result would eventually take place withthe groups thus formed; until they at length aggregated into a single mass. §151. Already so many references have been made to the formationof a crust over the originally incandescent Earth, that it may be thoughtsuperfluous again to name it. It has not, however, been thus far consideredin connexion with the general principle under discussion. Here it must benoted as a necessary consequence of the instability of the homogeneous. Inthis cooling and soldification of the Earth's surface, we have one of thesimplest, as well as one of the most important, instances of that changefrom a uniform to a multiform state which occurs in any mass through exposureof its component parts to unlike conditions. To the differentiation of theEarth's exterior from its interior, thus brought about, we must add one ofthe most conspicuous differentiations which the exterior itself afterwardsundergoes, as being similarly brought about. Were the forces to which thesurface of the Earth is subject, alike in all directions, there would beno reason why certain of its parts should become permanently unlike the rest.
But being unequally exposed to the chief external centre of force -- theSun -- its main divisions become unequally modified. While the crust thickensand cools, there arises that contrast, now so decided, between the polarand equatorial regions.
Along with these most marked physical differentiations of the Earth, therehave been going on numerous chemical differentiations, admitting of similarinterpretation. Leaving aside all speculations concerning the origin of theso-called simple substances, it will suffice to show how in place of thatcomparative homogeneity of the Earth's crust, chemically considered, whichmust have existed when its temperature was high, there has arisen, duringits cooling, an increasing chemical heterogeneity. Let us contemplate thischange somewhat in detail. At an extreme heat the bodies we call elementscannot combine. Even under such heat as can be generated artificially, somevery strong affinities yield; and the great majority of chemical compoundsare decomposed at much lower temperatures. Probably, therefore, when theEarth was in its first state of incandescence, there were no chemical combinations.
