SEGREGATION. 479 



of unlike matters, is also made manifest by the phenomena 

 of refraction: since adjacent and parallel beams of light, 

 falling on, and passing through, unlike substances, are made 

 to diverge. 



164. On the assumption of their nebular origin, stars 

 and planets exemplify that cause of material segregation 

 last assigned the action of unlike forces on like units. 



In a preceding chapter ( 150) we saw that if matter 

 ever existed in a diffused form, it could not continue uni- 

 formly distributed, but must break up into masses. It was 

 shown that in the absence of a perfect balance of mutual atr 

 tractions among atoms dispersed through unlimited space, 

 there must arise breaches of continuity throughout the ag- 

 gregate formed by them, and a concentration of it towards 

 centres of dominant attraction. Where any such breach of 

 continuity occurs, and the atoms that were before adjacent 

 separate from each other; they do so in consequence of a 

 difference in the forces to which they are respectively sub- 

 ject. The atoms on the one side of the breach are exposed 

 to a certain surplus attraction in the direction in which they 

 begin to move; and those on the other to a surplus attrac- 

 tion in the opposite direction. That is, the adjacent groups 

 of like units are exposed to unlike resultant forces; and ac- 

 cordingly separate and integrate. 



The formation and detachment of a nebulous ring, illus- 

 trates the same general principle. To conclude, as Laplace 

 did, that the equatorial portion of a rotating nebulous 

 spheroid, will, during concentration, acquire a centrifugal 

 force sufficient to prevent it from following the rest of the 

 contracting mass, is to conclude that such portions will 

 remain behind as are in common subject to a certain differ- 

 ential force. The line of division between the ring and 

 the spheroid, must be a line inside of which the aggregative 

 force is greater than the force resisting aggregation; and 

 outside of which the- force resisting aggregation is greater 



