The American Quarterly Review, No. 17, March 1831

by Various

All material substances are capable of assuming, under proper modifications of latent heat, either the solid, the liquid, or the gaseous form; yet all are beyond doubt composed of atoms, solid, hard, and incapable of further division. Under their own mutual attraction these particles tend to unite, and cohere in solid masses, and to this attractive force the repulsive power of heat is constantly opposed, tending to prevent their aggregation, and retaining them, according to its intensity, in the gaseous or liquid form.

The heat necessary to maintain these states of existence in bodies, may be produced in various ways. Our usual experience leads us to consider it as more generally arising from two causes, radiation from the sun, and the chemical action causing combustion. The former could never have produced the temperature known to exist at present upon the surface of the globe, for the earth radiates as well as the sun, and is constantly throwing off heat into the surrounding space. We know that these two actions have for twenty centuries exactly balanced each other, and that the mean temperature of the earth has neither increased nor diminished in all that period. Had the solar radiation been, previously to that epoch, in excess, it must at the more recent periods, counted backwards, have been but slightly so, and ages unnumbered must have elapsed, before the state of equilibrium which now exists could have been reached. The earth too, at distant periods, must have been colder than at present, while that the contrary is true is shown by numerous observations.

Neither could chemical action have had any great agency in establishing the present temperature of the earth. The substances which burn are but a small portion of the crust of the earth, and their combustion, if all fired at a time, would cause no perceptible effect on the sensible heat of the surface of our globe. Were combustible bodies even infinitely more abundant, the supporters are insufficient to keep up their combustion for any length of time, without sensible diminution, and this would be the case, even were the whole of the oxygen that now exists as a component of the waters of the ocean added to their present amount. It is indeed possible that the outer shell of the earth, which is no more than a crust of oxidated matter, may have existed at first in the metallic state, but that crust has long intervened, and prevented any contact between the air or ocean, and the metallic bases of the earths, that in this case must lie beneath.

In spite of these obvious objections to their theory, some geologists have madly fancied to themselves a great internal fire, maintained by actual combustion, a fancy but little more rational than that which seeks, in the present order of things, precipitation from some vast quantity of a liquid menstruum, every trace of whose existence has now vanished.

There is, however, yet another source of heat, if indeed solar heat be not a mere case of its general action, far more general and universal, which has its origin in the bodies themselves, and has no reference to any extrinsic cause. All bodies are sensibly heated when condensed, and lose sensible heat when they expand, so that their temperatures vary with the greater or less distance of their particles. The atmosphere of the earth furnishes a marked illustration of this fact. Of nearly uniform chemical composition throughout, its elastic nature, conflicting with its gravity, renders it more dense in its lower than in its higher regions. The former are in consequence warmer than the latter, and the mean temperature of our climates is in fact due to this character of our atmosphere. But this mean temperature could not be maintained, were not that of the earth itself in harmony with it. The surface might, no doubt, be cooled or heated by the adjacent air, but the heat, if given out from an earth warmer than the atmosphere, would be rapidly replaced from within, and a constant accumulation ensue in the air, while, if the earth were cooler, a diminution, equally constant, of the temperature of the atmosphere, must take place. The earth is, however, itself subject to the same law. All the materials of which it is composed, are capable of compression, in a greater or less degree, and of being heated by compression. The tendency of all material substances to the centre of attraction, loads the parts nearest to that centre with the whole weight of the superincumbent mass. And in the depth of four thousand miles, which intervenes between the centre and the surface, the heat must be far more than equal to that obtained by the compound blow-pipe or galvanic deflagrator, under whose intense energies the most refractory substances liquefy. Hence it may be inferred as a fact, as certain as any in physical science, that the interior of the earth is at present in a state resembling igneous fusion, not produced, however, by any of the more familiar sources of heat, but by the intense pressure the upper masses exert upon those nearer to the centre.

Here, then, we find the reason of the earth's having assumed a figure consistent with the equilibrium of a fluid mass, whose particles are endued with a mutual attraction, and which has a motion around an axis.

Let us suppose all the particles which now constitute the earth, to have been originally disseminated throughout a vast space, and to have approached their common centre of gravity by the force of mutual attraction; the consideration thus caused would have produced the state of intense heat that is now kept up within by pressure; and the conducting power of the bodies would have propagated the heat nearly equal throughout the mass. The surface would then have existed in a liquid state as well as that beneath. But as the radiation from the surface of a heated body is in exact proportion to its temperature, this cause of cooling would have been intense, and a crust must soon have formed upon the outer surface; this crust would have increased in thickness so long as the heat thrown off by radiation exceeded that received from the sun. When this state of equilibrium was finally attained, all the great phenomena which a body thus heated could exhibit, would cease, and the subsequent changes would become due only to forces such as we now see acting upon the surface, or would be the completion of actions commenced during the previous state.

We know, from astronomical investigations, that this state of equilibrium has existed for upwards of twenty centuries, while analogy would lead us to infer that it must have been attained at no long period after the last great catastrophe to which our planet was subjected.

Let us now see whether the fact of the interior of the globe being more intensely heated than its surface, can be inferred in any other manner than from the course of reasoning whose principles are here cited. The feeble power of man, feeble at least compared to the size of the globe he inhabits, has been able to penetrate to but small depths in its outer shell, but even at these small depths, an increase of temperature has been remarked, and so frequently and carefully observed, as to leave no doubt of its being a general law. This increase, too, appears exactly consistent with that which it might be inferred ought to take place. But we, even to the present day, occasionally see the igneous fluid from beneath forced up to the surface, and spreading from volcanic craters over great regions. Observation shows us that at remote epochs such phenomena were much more frequent than at present. We want no more positive proofs that the interior of the earth is still intensely heated, and that the bed of the ocean and the solid land are mere crusts formed upon the surface of a mass in a state analogous to that of igneous fusion.

Were the surface, as we have inferred it must have been, ever itself intensely heated, the volatile and gaseous matters which now constitute our atmosphere and oceans, must have united to form an atmosphere of far greater extent than it is at present. The aqueous matter rising into regions where the rarity of the air would cause cold sufficient to condense it, would have been in a state of constant motion, boiling in the lower regions, being precipitated in the higher, and acting most energetically to promote the general cooling. And so soon as the surface became cooler than 212°, the water would begin to settle upon its surface, forming at first lakes in its basins or cavities, and finally extending itself into one vast ocean, covering the whole or parts of the solid crust according to its greater or less degree of uniformity.