THE INSTABILITY OF THE HOMOGENEOUS. 419 



general evolution consequent on the instability of the homo- 

 geneous. 



Descending to that more limited form of the nebular 

 hypothesis which regards the solar system as having resulted 

 by gradual concentration; and assuming this concentration 

 to have advanced so far as to produce a rotating spheroid of 

 nebulous matter; let us consider what further consequence 

 the instability of the homogeneous necessitates. Having 

 become oblate in figure, unlike in the densities of its centre 

 and surface, unlike in their temperatures, and unlike in the 

 velocities with which its parts move round their common 

 axis, such a mass can no longer be called homogeneous ; and 

 therefore any further changes exhibited by it as a whole, 

 can illustrate the general law, only as being changes from a 

 more homogeneous to a less homogeneous state. Changes of 

 this kind are to be found in the transformations of such of its 

 parts as are still homogeneous within themselves. If we 

 accept the conclusion of Laplace, that the equatorial portion 

 of this rotating and contracting spheroid will at successive 

 stages acquire a centrifugal force great enough to prevent 

 any nearer approach to the centre round which it rotates, 

 and will so be left behind by the inner parts of the spheroid 

 in its still-continued contraction ; we shall find, in the fate of 

 the detached ring, a fresh exemplification of the principle 

 we are following out. Consisting of gaseous matter, such a 

 ring, even if absolutely uniform at the time of its detach- 

 ment, cannot continue so. To maintain its equilibrium 

 there must be an almost perfect uniformity in the action of 

 all external forces upon it (almost, we must say, because the 

 cohesion, even of extremely attenuated matter, might suffice 

 to neutralize very minute disturbances) ; and against this the 

 probabilities are immense. In the absence of equality 

 among the forces, internal and external, acting on such a 

 ring, there must be a point or points at which the cohesion of 

 its parts is less than elsewhere a point or points at which 

 rupture will therefore take place. Laplace assumed that 



