While Dr. Daubeny was engaged in perfecting what has been called the chemical theory of volcanoes, originated by Davy, other inquirers gave their attention to that subject under different aspects. Sir C. Lyell, as a uniformitarian in geological speculation, and the advocate, not only of the sufficiency of existing causes, but of their persistence without the trace of a beginning, or the prospect of an end, naturally sought for elements of chemical causation, by which a perpetual circulation of cause and effect returning through effect to cause might be supposed to be maintained ; and the late Professor Daniell, of King's College, London, suggested to him hydrogen, the continual separation of which from water by means of oxidable bodies and its re-union with oxygen effected by high tempemture,as such an element.
Two mathematicians concurred, though independently, in enun ciating a theory of Plutonic and volcanic action, dependent on that of the secular variation of the isothermal surfaces within the globe. The foundation of this was the observed auginentatiou of temperature as we descend from the surface of the earth towards its interior ; of which subject we adopt from Mr.W. Hopkins the following condensed statement A considerable number of observations have been made to ascertain the temperature of the earth at considerable depths beneath its surface, and the law according to which that temperature increases in descending. This law, in a considerable number of localities, may be considered as approximately determined to be—that the increase of temperature above that of the mean temperature at the surface in any proposed locality, is proportional to the depth beneath the surface. The results of observation also lead to the conclusion that the rate of increase of temperature in descending beneath the earth's surface is nearly uniform in each locality, and nearly the same in different localities, being equal to about 1* Fahr. for 60 feet, of depth. At all depths, therefore, there will be, mathematically speaking, spheroidal concentric surfaces of the same temperature throughout, or isothermal surfaces.
The upward migration of heat from the interior towards the surface of the globe, in consequence of the deposition of fresh matter upon its surface, had been indicated as a cause of geological phenomena by Mr. Puulett Scrope ; but the theory of the secular variation of the isothermal surfaces of the interior of the globe considered as so caused was proposed by Mr. Babbage, in a paper read before the Geological Society, in 1834, and by Sir F. W. Herschel, in letters commu nicated to that Society three years afterwards, and eventually printed by Mr. Babbage, together with his own paper, in the appendix to his work entitled ' The Ninth Bridgewater Treatise,' published also in 1837. His application of the theory to volcanic phenomena, properly so called, had been announced, however, in terms of extreme generality, and the main object of his paper was to explain by its means the pyrometric expansion of rocks as the cause of elevation. From these circumstances, apparently, it happened that his views remained comparatively unregarded until the subsequent promulgation by Sir John Herschel of views identical with them in their leading features, but more explicitly developed in their appli cation to those phenomena.
Almost every article in this Cycloyandia which relates to any subject of geological science describes facts, whether structural or dynamical, which involve the truth, that solid material, derived from the land are perpetually being distributed over and accumulated upon the bed of the sea ; this having been the procesa also of the formation of the sedimentary strata of which, mainly, the present laud correlate. We have seen that it is also true that the temperature of the globe below the surface, and to the greatest depth with which we are acquainted, increases 11J1 we descend, the heat communicated to the surface at last escaping from it by radiation into space. By the continued deposition, therefore, of the new sedimentary strata, which aro necessarily bad conductors of heat, on the bed of the ocean, the interior heat, instead of being penuitted to escape, will be accumulated, and the original surface will acquire the temperature before poseessed by some isothermal surface below, at a depth equal to the thickness of the matter deposited upon it, the amount of the accumulation, or the increase of the tempe rature, augmenting with the increase of this thickness; and conse quently. by the necessary extension of this process, the temperature of every isothermal surface vertically below the treys of accumulating matter, to an indefinite depth, will rise in the same proportion. If the temperature at which water boils at the surface, for example, originally existed at the depth of two miles, the deposition of strata of that thick ness would cause the temperature of the original sea-bed to rise to that amount ; and if the isothermal surface et a certain other depth, of six or seven miles, perhaps, bad the temperature of ignition, the deposition of a thickness of equal to that depth would cause the original sea-bed to become red-hot, and, by the continuance of deposition, it would eventually "become actually melted," however refractory its materials, "and that without any bodily transfer of matter in a liquid state from below." This process, to use a familiar illustration given by Sir J. Herschel, " is precisely that by which a man's skin grows warmer in a winter day by putting on all additional great coat : the flow of heat outwards is obstructed, and the surface of congelation carried to a distance from his person, by the accumulation thereby caused beneath by the new covering." In the case of the human body, however, we cannot carry the illustration further : a succession of great coats would not now raise the temperature of the akin, because the beat of the body is limited ; whereas the succession of external coverings of the earth will indefinitely exalt the temperature of the original surface of depo sition, and successively that of all the isothermal surfaces below, because the heat of the interior, by the theory, is conceived to be, and, so far as we know, is actually, unlimited.