oh. v.] PLANETARY EVOLUTION. 377 



offset the consideration that the ratio of volume to mass is 

 likely to have been from the first very much greater in the 

 case of the earlier planets than in the case of the interior 

 ones, since formed from a denser sun. Even now the high 

 ratio of volume to mass is one of the most striking charac- 

 teristics of the four outer as compared with the four inner 

 planets ; and as bulky bodies radiate heat much more slowly 

 than small ones, it may well be that this relatively small 

 density indicates the retention of a relatively great amount 

 of molecular motion. Of all the factors in the case, bulk is 

 undoubtedly the most important. Just as the hot water in 

 the boiler may remain warm through a winter's night, while 

 the hot water in the tea-kettle cools off in an hour, so a great 

 planet like Jupiter may remain in a liquid molten condition 

 long after a small planet like the earth, though formed ages 

 later, has acquired a thick solid crust and a cool temperature. 

 Hence in a general survey of the solar system we may 

 expect to find the largest planets still showing signs of a heat 

 like that which formerly kept the earth molten, and we may 

 expect to find the smallest planets in some cases showing 

 signs of a cold more intense than any which has been known 

 upon the earth. 



Now this series of inferences, constituting simply an 

 elaborate corollary from the theory of nebular genesis, is fully 

 confirmed by observation in the cases of Saturn, Jupiter, 

 Mars, and the Moon, — the only planets whose surfaces have 

 been studied with any considerable success. According to 

 the nebular hypothesis, Jupiter and Saturn ought to be pro- 

 digiously hot; and so they appear to be when carefully 

 examined. The tremendous atmospheric disturbances observed 

 upuu both these planets are such as cannot well be explained 

 by the comparatively sluggish action of the sun's radiance 

 upon such distant orbs. The atmosphere of Jupiter is laden 

 with masses of cloud, whether composed solely of water or 

 uot, whose cubic contents far exceed those of all the oceans 



