THE ASTRONOMICAL HISTORY OF WORLDS. 573 



modern science, matter and motion were all required for calling a world 

 into existence ; but it was soon found that, unless, in the beginning, the 

 materials which formed the solar system moved with a certain order 

 and regularity, they could never have risen from the chaotic to the cos- 

 mical condition. As all the planets move around the sun in the same 

 direction, Laplace was led to believe that in remote times all must have 

 been connected together ; and such a primitive connection might be af- 

 forded if the sun and his attendants were originally a vast fire-mist, 

 their matter being so much attenuated by heat that it extended far 

 beyond its boundaries of the solar domain. He supposed that such an 

 immense rarefied mass, on being set in motion by some cause which he 

 does not specify, would ultimately be compelled by its own friction and 

 by gravity to rotate with a uniform angular velocity in all its parts and 

 around a common centre. In accordance with the principles of physi- 

 cal astronomy, he concluded that this rotation would become rapid as 

 the immense solar nebula cooled and contracted, until at last the cen- 

 trifugal force became great enough to overpower gravity and to throw 

 off matter from the equator of the whirling mass. Laplace considered 

 that, under the most probable circumstances, the nebulous matter thus 

 thrown off, or abandoned by the shrinking spheroid, would all collect 

 together to form a planet ; but that, in some unusual cases, it would 

 assume the expanded figure of a vast solar ring ; and that, under cer- 

 tain conditions, it might break up into a number of asteroids. The 

 singular group of bodies revolving between Mars and Jupiter is sup- 

 posed to have come into existence in consequence of some rare acci- 

 dent, which made the great solar ring a prey to many centres of aggre- 

 gation, instead of allowing it to coalesce around a single one. In all 

 other cases, the cooling and contraction are said to have been success- 

 ful in giving birth to a great planet, whenever the centrifugal force 

 became sufficient to separate the equatorial portions of the rotating 

 solar nebula. According to the views of Laplace, Neptune must be 

 regarded as the first-born world of those already known ; while Uranus 

 is next in age, and the other planets were launched into being in a suc- 

 cession depending on their distances from the sun ; so that Mercury is 

 the youngest member of the solar family. It has been also concluded 

 that from the condition of its birth each planet must have commenced 

 its career as a rotating nebula ; and that many of the larger ones, by 

 subsequent cooling and contraction, were at certain periods enabled to 

 throw off their equatorial matter, which in all but two instances was con- 

 verted into a satellite. Of these minor worlds or moons, Saturn has 

 succeeded in obtaining eight, in addition to the double ring, which in 

 the eyes of Laplace appeared as two embryonic satellites, and which 

 has been so often appealed to for proof of the world-making doctrine 

 under consideration. 



Yet, when examined with care and impartiality, the evidence derived 

 from the condition of the Saturnian girdle will be found unfavorable, if 



