$ 25 o] The Nebular Hypothesis 321 



eccentricities of all the orbits were quite small, so that 

 they were nearly circular. 



Comets, on the other hand, presented none of these pecu- 

 liarities ; their paths were very eccentric, they were inclined 

 at all angles to the ecliptic, and were described in either 

 direction. 



Moreover there were no known bodies forming a con- 

 necting link in these respects between comets and planets 

 or satellites.* 



From these remarkable coincidences Laplace inferred 

 that the various bodies of the solar system must have had 

 some common origin. The hypothesis which he suggested 

 was that they had condensed out of a body that might 

 be regarded either as the sun with a vast atmosphere filling 

 the space now occupied by the solar system, or as a fluid 

 mass with a more or less condensed central part or nucleus ; 

 while at an earlier stage the central condensation might have 

 been almost non-existent. 



Observations of Herschel's (chapter xn., 259-61) had 

 recently revealed the existence of many hundreds of bodies 

 known as nebulae, presenting very nearly such appearances 

 as might have been expected from Laplace's primitive body. 

 The differences in structure which they shewed, some being 

 apparently almost structureless masses of some extremely 

 diffused substance, while others shewed decided signs of 

 central condensation, and others again looked like ordinary 

 stars with a slight atmosphere round them, were also 

 strongly suggestive of successive stages in some process 

 of condensation. 



Laplace's suggestion then was that the solar system had 

 been formed by condensation out of a nebula ; and a 

 similar explanation would apply to the fixed stars, with the 

 planets (if any) which surrounded them. 



He then sketched, in a somewhat imaginative way, the 

 process whereby a nebula, if once endowed with a rotatory 

 motion, might, as it condensed, throw off a series of rings, 



* This statement again has to be modified in consequence of the 

 discoveries, beginning on January 1st, 1801, of the minor planets 

 (chapter xni., 294), many of which have orbits that are far more 

 eccentric than those of the other planets and are inclined to the 

 ecliptic at considerable angles. 



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