THE EARTH'S INTERIOR. 227 



gradually gathering around a central point. ]5ut, 

 as tlie atoms of which it was composed fell to- 

 gether towards their common centre, under the 

 inihience of gravitation, their mutual impact 

 heated them to a white heat just as a piece of cold 

 iron on a bhicksmith's anvil is often heated red- 

 hot by continued blows from the heavy hammer. 

 In its earlier stages, therefore, every world must 

 have passed through a fiery and stormy youth \ 

 and as it grew older, it must have grown colder, 

 on the outside, at least, by the constant radiation 

 of its surface-heat. A poker raised to a ruddy gh)w 

 in the fire — to take a domestic analogy familiar 

 to every one — cools slowly as soon as it is re- 

 moved from the burning coals, but the outside 

 grows cold before the inside, and in a large mass, 

 such as a solid cannon-ball, the difference of tem- 

 perature between surface and centre may some- 

 times be very marked and conspicuous. Some of 

 the planets, as we know by the evidence of the 

 telescope, are still in their primitive heated condi- 

 tion; the fires of their youth have not yet burnt 

 themselves out, and they have not yet settled 

 themselves down, like our own earth, to a sober, 

 staid, and respectable middle age. Passionate 

 storms still shake thera violently to their very 

 core, and nebulous vapors hiilo their faces from 

 lis with a fiery mist. Geology shows us that our 

 own earth — that solid earth upon wliosc stability, 



