COSMIC PHILOSOPHY 



gressed much further in consolidation. But 

 against this must be 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 characteristics 

 of the four outer as compared with the four in- 

 ner 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 ac- 

 quired a thick solid crust and a cool tempera- 

 ture. Hence in a general survey of the solar 

 system we may expect to find the largest plan- 

 ets still showing signs of a heat like that which 

 formerly kept the earth molten, and we may ex- 

 pect 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 

 280 



