554: HISTORY OF GEOLOGY. 



they should be originally separate. And if any other causes have 

 been suggested, as electricity or magnetism, this has been done so 

 vaguely as to elude all possibility of rigorous deduction from the 

 hypothesis. The doctrine of a Central Heat, however, has occupied 

 so considerable a place in theoretical geology, that it ought undoubt- 

 edly to form an article in geological dynamics. 



Sect. 4. The Doctrine of Central Heat. 



THE early geological theorists who, like Leibnitz and Buffon, assumed 

 that the earth was originally a mass in a state of igneous fusion, natu- 

 rally went on to deduce from this hypothesis, that the crust consoli- 

 dated and cooled before the interior, and that there might still remain 

 a central heat, capable of producing many important effects. But it 

 is in more recent times that we have measures of such effects, and cal- 

 culations which we can compare with measures. It was found, as we 

 have said, that in descending below the surface of the earth, the tem- 

 perature of its materials increased. Now it followed from Fourier's 

 mathematical investigations of the distribution of heat in the earth, 

 that if there be no primitive heat (chaleur cForigine), the temperature, 

 when we descend below the crust, will be constant in each vertical 

 line. Hence an observed increase of temperature in descending, 

 appeared to point out a central heat resulting from some cause now no 

 longer in action. 



The doctrine of a central heat has usually been combined with the 

 supposition of a central igneous fluidity ; for the heat in the neighbor- 

 hood of the centre must be very intense, according to any law of its 

 increase in descending which is consistent with known principles. But 

 to this central fluidity it has been objected that such a fluid must be in 

 constant circulation by the cooling of its exterior. Mr. Daniell found 

 this to be the case in all fused metals. It has also been objected that 

 there must be, in such a central fluid, tides produced by the moon and 

 sun ; but this inference would require several additional suppositions 

 and calculations to give it a precise form. 



Again, the supposition of a central heat of the earth, considered as 

 the effect of a more ancient state of its mass, appeared to indicate that 

 its cooling must still be going on. But if this were so, the earth might 

 contract, as most bodies do when they cool ; and this contraction might 

 lead to mechanical results, as the shortening of the day. Laplace satis- 

 fied himself, by reference to ancient astronomical records, that no such 



