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Ch. XXXII.] 



THEORY OF CENTEAL HEAT. 



207 



increasing at tlie rate above allnded to^ we sliould encounter 

 not far below the onter line a temperature many times 

 greater than tbat sufficient to melt the most refractory sub- 



stances known to us. 



At mucb greater depths^ and long 



before approaching the central nucleus, the heat would be so 

 intense (160 times that of melted iron)^ that Ave cannot con- 

 ceive the external crust to resist fusion.*^ 



It may be said that we may stand upon the hardened sur- 

 face of a lava current while it is still in motion — nay, may 

 descend into the crater of Vesuvius after an eruption, and 

 stand on the scorise while every crevice shows that the rock 

 is red-hot 2 or 3 feet below us ; and at a somewhat greater 

 depth, all is, perhaps, in a state of fusion. May not, then, 

 a much more intense heat be expected at the depth of several 

 hundred yards or miles? The answer is, — that, until a 

 great quantity of heat has been given oif, either by the 

 emission of lava, or in a latent form by the evolution of 

 steam and gas, the melted matter continues to boil in the 

 crater of a volcano. But ebullition ceases when there is no 

 longer a sufficient supply of heat 



from 



may form 



may 



If the internal heat be raised again, ebullition will recom- 



mence 



So in the case 



may safely assume 



the liquid beneath the hardened surface is much above the 

 temperature sufficient to retain it in a state of jSuidity. 



M. Poisson, in his Mathematical Theory of Heat, published 

 in 1835, controverted the doctrine of the high temperature of 

 a central nucleus, and declared his opinion that if the globe 

 had ever passed from a liquid to a solid state in consequence 

 of the loss of heat by radiation, the cooling and consolidation 

 of the nucleus would have begun at the earth's centre. 



* The expansion of platinum was the other test yet invented for measurinrr 

 test employed by Mr. Daniell, in his * ' ' ^ 



pyrometer, which was found to yield 

 uniform and constant results, in har- 

 mony with those derived from other 

 independent sources. Eut Dr. Percy 

 informs me that neither this nor any 



intense heat, can be fully depended upon. 

 Malleable iron, he remarks, requires 

 more heat for its fusion than wrou^^ht 

 iron, in which the metal is mixed with a 

 small percentage of carbon. 



^ 





