Central Heat of the Earthy and Volcanic Fhenomena. 133 



at work which can deprive the inner surface of the crust or 

 the central nucleus of heat, and, therefore, none which can 

 diminish their spheroidicity. They must be regarded as two 

 surfaces constantly exchanging the same amount of heat, and 

 prevented, in consequence, like two equally warm radiating 

 bodies on Pictet's theory, from undergoing any change in tem- 

 perature. 



Such is the theory by which I seek to reconcile the co- 

 existence in our globe of a central heat and fluidity, and a 

 cold solid crust. 



The harmony of this theory with the known density of the 

 globe, may now be advantageously alluded to. 



In a former part of this paper I stated that, in order to 

 reconcile the mean density of the globe with that of its 

 constituents, it was requisite that some powerful expansive 

 agent, or a space filled with vapour, should be accounted for, 

 so as to lower the mean density of the earth. 



In the spheroidal theory of the earth there are two different 

 ways by which the density may be lowered, 1st, by the dila- 

 tation of the central fluid consequent upon its high tempera- 

 ture ; and 2d, and more particularly, by the space which 

 intervenes between the central nucleus and the external 

 crust. The former, viz., the expansion of the materials in 

 the liquid state, would go but a small way in lowering the 

 globe's density ; when, however, the space which this theory 

 necessitates is taken into consideration, there is ample room 

 aff*orded by which the density of the globe, as a whole, might 

 be more or less lowered. 



The other planets are probably in the same condition as 

 the earth, in so far that each has a liquid central nucleus at a 

 distance from the enclosing crust, and in consequence, a 

 lower density than it would have if solid to the centre ; but, 

 in our ignorance of the chemical composition of the central 

 portions of our own globe, and of the entire mass of the other 

 heavenly bodies, as well as of the temperature which charac- 

 terises their inner portions, we are without the data neces- 

 sary for calculating what their specific gravity should be to 

 accord with the requirements of the Spheroidal Theory. 



I shall now proceed to ofler a few remarks regarding the 



