400 THE BEAUTIES OF NATURE CHAP. 



Moon is cold and rigid; the Earth is solid at 

 the surface, but intensely hot within ; Jupiter 

 and Saturn, which are immensely larger, still 

 retain much of their original heat, and have 

 a much lower density than the Earth; and 

 astronomers tell us on other grounds that the 

 Sun itself is still contracting, and that to this 

 the maintenance of its temperature is due. 



Although, therefore, the Nebular Theory 

 cannot be said to have been absolutely proved, 

 it has certainly been brought to a high state 

 of probability, and is, in its main features, 

 generally accepted by astronomers. 



The question has often been asked whether 

 any of the heavenly bodies are inhabited, and 

 as yet it is impossible to give any certain 

 answer. It seems a priori probable that the 

 millions of suns which we see as stars must 

 have satellites, and that some at least of them 

 may be inhabited. So far as our own system 

 is concerned the Sun is of course too hot to 

 serve as a dwelling-place for any beings with 

 bodies such as ours. The same may be said 

 of Mercury, which is at times probably ten 

 times as hot as our tropics. The outer planets 



