THE INSTABILITY OF THE HOMOGENEOUS. 421 



a state of homogeneity is one of unstable equilibrium, it does 

 in reality furnish a curious confirmation of this doctrine. 

 For Saturn is not quite concentric with his rings; and it has 

 been proved mathematically that were he and his rings con- 

 centrically situated, they could not remain so: the homo- 

 geneous relation being unstable, would gravitate into a 

 heterogeneous one. And this fact serves to remind us of the 

 allied one presented throughout the whole Solar System. 

 All orbits, whether of planets or satellites, are more or less 

 excentric — none of them are perfect circles; and were they 

 perfect circles they would soon become ellipses. Mutual 

 perturbations would inevitably generate excentricities. 

 That is to say, the homogeneous relations would lapse into 

 heterogeneous ones. 



§ 151. Already so many references have been made to 

 the gradual formation of a crust over the originally incan- 

 descent Earth, that it may be thought superfluous again to 

 name it. It has not, however, been before considered in 

 connexion with the general principle under discussion. 

 Here then it must be noted as a necessary consequence of the 

 instability of the homogeneous. In this cooling down and 

 solidification of the Earth's surface, we have one of the sim- 

 plest, as well as one of the most important, instances, of that 

 change from a uniform to a multiform state which occurs in 

 any mass through exposure of its different parts to different 

 conditions. To the differentiation of the Earth's ex- 



terior from its interior thus brought about, we must add one 

 of the most conspicuous differentiations which the exterior 

 itself afterwards undergoes, as being similarly brought 

 about. Were the conditions to which the surface of the 

 Earth is exposed, alike in all directions, there would be no 

 obvious reason why certain of its parts should become per- 

 manently unlike the rest. But being unequally exposed to 

 the chief external centre of force — the Sun — its main divis- 

 ions become unequally modified: as the crust thickens and 



