6-1 ij Theories of Cosmogony 11 



9. Kant's Theory. Kant supposed the universe to have developed 

 initially out of a cold nebula at rest. He supposed this nebula to fall in under 

 its own gravitation, and to become hot in so doing owing to the consequent 

 compression. He assumed, of course erroneously, that rotation would be set 

 up in this process. He imagined that the matter would condense into rings, 

 and on superposing the supposed rotation, he arrived at a system of rotating 

 rings similar to the rings of Saturn, to which he appealed as evidence of the 

 truth of his theories. In the second stage of the cosmogonic process, Kant 

 supposes these rings to become unstable and form by agglomeration into 

 planets. The persistence of the rotatory motion results in this system of 

 planets revolving round the sun. The planets continue to contract under 

 their own gravitation, so that the preceding cycle of processes is repeated on 

 a smaller scale, and finally we find the planets also surrounded by rotating 

 satellites*. 



10. Laplace's Theory. We turn now to the theory put forward by 

 Laplace. The great French mathematician was not likely to fall into the 

 error of believing that rotation could be generated out of nothing, and so the 

 nebula is assumed to be rotating at the outset. Laplace supposes it to be 

 hot, without attempting in any way to account for the heat, and supposes it 

 to be lens-shaped or flat, without attempting to justify this special choice of 

 shape. The mass is supposed to cool by radiation at the surface, while at the 

 same time falling in upon itself as a result of the action of gravitation, the 

 net result being a heating of the central portion and a general shrinkage of 

 the whole. Since the angular momentum must remain constant throughout 

 the shrinkage, the actual velocity of rotation must increase, and Laplace 

 believed that as this increase of angular velocity took place, the outer ring of 

 matter ceased to be continuous with the main mass. A succession of repe- 

 titions of this phenomenon leaves a series of concentric annuli of matter, 

 rotating about a central axis, as imagined by Kant, and from this stage on the 

 hypotheses of the two philosophers are in agreement. 



11. It appears that both Kant and Laplace try to develop a theory in 

 which a system such as the rings of Saturn represents a half-way stage 

 between the primitive nebula and the present state of our universe. Neither 

 theory attempts to explain why the supposed ring system should become un- 

 stable and agglomerate into planets, and neither theory explains why Saturn's 

 rings have not become unstable. 



Perhaps an unbiassed judge, devoid of preconceived ideas, might expect 

 a ring of rotating matter to become unstable. But, as Sir G. Darwin f has 



* Fuller accounts of Kant's theory will be found in Poincare"'s Lemons sur les Hypotheses 

 Cosmogoniques, Ch. i and Darwin's Tides, Ch. 21. An account is also given in Miss Clerke's 

 Modern Cosmogonies (1905). 



t The Tides, p. 410. 



