Central Heat. Since the year 1740, when the first observations respecting the increase of heat encountered with the increased depth below the surface were made by M. Gensanne in the lead mines of Giromagny in S. Alsace, abundant data have been collected by scientific men in various parts of the world in support of the theory that the interior of the earth is intensely hot. The deepest mines of Mexico, England, France, Germany, and other countries, and the deeper artesian wells, and the hot springs ascending from still deeper sources, all lead to this conclusion. The volcanic fires add their testimony to the existence of intensely heated masses beneath the crust of the earth, and the vast extent of surface agitated when they are suppressed, and relieved by their outlet, seems to indicate an almost general diffusion of the liquid molten masses from which they spring. Not only is the heat found generally to increase with the depth, but the rate of this increase has in many instances been determined. It is found to vary in different countries, in some increasing two or three times more rapidly than in others. The average rate is estimated by Kupffer at 1° F. for every 37 English feet; and by Cordier at 1° for every 45 feet.
These phenomena, all pointing in one direction, have led to the conclusion that somewhere in the interior the materials of the globe must be in a state of the most intense heat; and calculations have been made showing at what depth the rocks must all exist as liquid lava, at what the temperature of melted iron would be found,'at what platinum would fuse, and at what various matters, solid at the surface, would be volatilized, but for the enormous pressure. This theory is controverted by Sir Charles Lyell, M. Poisson, and other eminent authorities, on these grounds: When substances, as metals, are melted, their temperature cannot be raised a single degree above the point of fusion so long as a piece of the material remains unmelted. The same principle is exemplified in the impossibility of raising water to a higher temperature than 32° F. so long as a fragment of ice remains in it. The principle may be applied to the solid crust of the earth, which could no more remain unchanged, reposing upon the surface of a fluid heated many times above the temperature at which its materials would melt, than a stratum of ice of the same thickness could remain in the same situation exposed to the same proportional difference of heat.
The crust that forms upon lava as it cools cannot be instanced in disproof of this statement, for this only forms when the heat is so much reduced that ebullition has entirely ceased; if this be renewed, the crust soon disappears in the fluid. Were the crust of the globe the result of partial cooling from a state of primitive fluidity, the whole planet must first have cooled down to about the temperature of incipient fusion, and hence the enormous degrees of heat supposed cannot exist within it. M. Poisson "imagines that if the globe ever passed from a liquid to a solid state by radiation of heat, the central nucleus must have begun to cool and consolidate first." Were the central portion fluid, tides would be perceived in the mass, sufficient to cause the surface to rise and fall every six hours; but no such fluctuations are observed, even in a crater like that of Stromboli, which is supposed to connect with the great central ocean of lava. The phenomena that have given rise to the hypothesis of central heat do not absolutely require this theory to account for them. Local heat is without question generated by chemical changes taking place among the materials beneath the surface.
These give rise to electrical currents, of the power of which to disturb the surface we can form little idea; but judging from their effects upon the limited scale on which they come under our observation, it would seem quite as philosophical to refer to them the phenomena connecting distant volcanic outbreaks and earthquakes, as to call in an aid so hypothetical as that of the molten fluidity of the central portion of the globe.