ON THE ORIGIN OF THE PLANETARY SYSTEM. 183 



intensity of sunshine as that which is now the source 

 of all terrestrial life. 



The smaller bodies of our system might become 

 less hot than the sun, because the attraction of the 

 fresh masses would be feebler. A body like the earth 

 might, if even we put its thermal capacity as high as 

 that of water, become heated to even 9,000 degrees, 

 to more than our flames can produce. The smaller 

 bodies must cool more rapidly as long as they are still 

 liquid. The increase in temperature, with the depth, 

 is shown in bore-holes and in mines. The existence of 

 hot wells and of volcanic eruptions shows that in the 

 interior of the earth there is a very high temperature, 

 which can scarcely be anything than a remnant of the 

 high temperature which prevailed at the time of its 

 production. At any rate, the attempts to discover for 

 the internal heat of the earth a more recent origin in 

 chemical processes, have hitherto rested on very arbi- 

 trary assumptions ; and, compared with the general uni- 

 form distribution of the internal heat, are somewhat 

 insufficient. 



On the other hand, considering the huge masses of 

 Jupiter, of Saturn, of Uranus, and of Neptune, their 

 small density, as well as that of the sun, is surprising, 

 while the smaller planets and the moon approximate to 

 the density of the earth. We are here reminded of 

 the higher initial temperature, and the slower cooling, 



