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 & 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 
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