Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Diffusion.—The same law probably holds relative to diffusion, though in a molecular sense diffusion is the opposite of crystallization, for in crystallization, like comes to like, while in diffusion the molecules distribute themselves among those of unlike nature. Diffusive action, quite familiar in gases and liquids, takes place to some extent in solids. The molecules of plates of gold and lead brought into intimate contact under pressure mutually diffuse among one another. So gases seem to be very generally diffused or “occluded” in rocks, though the nature of this relation is imperfectly determined. It is known that pressure upon gases promotes their diffusion through liquids and solids. It is inferred that pressure upon a solid tends to the diffusion of the entrapped gases within it, but it is not to be inferred from this that pressure upon rock promotes the absorption of gases into it, but rather the opposite. It is probable that great pressure with high heat promotes the diffusion of entrapped gases or other diffusible substances through the rock-mass, and at the same time tends to their extrusion along lines of least resistance; but this is an inference rather than a demonstration.

Chemical combination.—The general effect of chemical combination under pressure is greater density. In reversible reactions capable of conditions of chemical or physical equilibrium, pressure invariably favors the formation of the denser of any possible products.

Sub-atomic forces.—Recent investigation has made it probable that atoms are composite, embracing many exceedingly minute bodies—corpuscles or electrons—in a state of extremely high activity and possessed of marvelous energy notwithstanding their minuteness. This discovery possesses deep interest to the geologist because it seems to reveal sources of energy of almost incalculable potency, some portions of which at least are being constantly freed and added to the previously recognized supplies of energy. Attempts have been made during the past few decades to limit the habitable age of the earth, both retrospectively and prospectively, by the smallness of the sum total of energy derivable from gravity. In these estimates slight recognition has been given to the resources of molecular and atomic energy, and none at all to the possibilities of sub-atomic energy. It would be going quite too far to assume that these sub-atomic energies are all available for the perpetuation of habitable conditions on the earth or in the solar system, but we are doubtless justified in appealing to them as an offset to all dicta restricting the period of the earth’s habitability by supposed insufficiencies of energy deduced merely from the estimated resources of gravity. The banishment of the idea of the atom as a minute, incompressible, undecomposable sphere takes away the theoretical limit of compressibility, and by so doing cuts away the groundwork for assigning definite limits even to the resources of gravity, since, as already indicated, unlimited condensation gives theoretically unlimited transformation of the potential energy of gravity.

While we must await with such patience as we can command the development of fuller knowledge concerning the nature and laws of the molecular, atomic, and sub-atomic energies, and their applicability to the activities resident in the interior of the earth, it is permissible even now to assume that, besides the simple compressive action of gravity, there are at work varied forms of molecular aggregation, of atomic combination, and perhaps of sub-atomic change, tending toward increased density, and that the ulterior limit of these processes is quite undetermined. The condensational forces are now restrained at certain temporary limits by the antagonistic resistant forces, some of which, such as heat, are the products of the condensational forces, and are gradually being dissipated, permitting further condensation. Where the process may ultimately end, we dare not attempt to say. On the other hand, we are not compelled to accept assigned limits that seem to be inconsistent with the phenomena which the earth actually presents.

2. The resisting agencies.

Heat.—The most familiar of the active agencies that resist condensation is heat. Upon this the existing volume of the earth is immediately dependent, in some large part at least. As this heat is dissipated, the earth shrinks. This shrinkage increases the force of gravity, and hence the internal pressure increases, and, if further compression takes place as the result of this increased pressure, additional heat is developed, which checks further condensation until it is dissipated. It is this kind of creative and self-checking action that determines the volume of great gaseous bodies like the sun. Though their matter is far from its ultimate density, and their self-gravity is enormous, they condense slowly, because, with every stage of condensation, heat is generated which antagonizes gravity and checks condensation, until at least a part of the heat is radiated away. As the force of gravity increases with every stage of condensation, the heat developed to hold it in check must increase, and hence the famous law of Lane, that a gaseous body like the sun grows hotter as it condenses. This law holds good while the body remains in a gaseous state in which the maintenance of the volume is essentially dependent on heat. When a body becomes liquid or solid, its volume is dependent in part on forms of resistance other than heat, and the force of the law is abated, though the principle still holds good. In small solids, the principle has little application, since the force of self-gravity is slight compared to the resisting forces, and very little new heat is generated as the body loses that which it has; but in large bodies, like the earth, where the condensational forces are enormous and the internal temperature is very high, it is not improbable that the heat generated at every stage of condensation is relatively large. It has been inferred by some students of the phenomena that the conditions in the interior of the earth are essentially those of gaseous matter, so far as molecular relations are concerned, because the temperatures are thought to be above the critical temperatures of the substances composing it. If this be true, the new heat generated with each stage of condensation is large. However this may be, it seems safe to infer that in so far as the volume of the interior mass is dependent on heat resistance, the loss of existing heat leads to the generation of new heat. The amount of this new heat must be enough, together with the residual heat and the other forces of resistance, to match the new condensational forces. The molecular and sub-molecular forces of resistance other than heat, are probably responsible for some large part of the resistance to the increased condensational force, but how much is not determined.

All resistance perhaps due to motion.—As now interpreted, the force of resistance of heat is due to the impact of the flying particles of the heated matter. The other forms of resistance to compression have not usually been interpreted in this way, but the tendency of recent investigation is to place them in the same dynamic class. A cold solid body offers resistance to compression that is in no obvious way dependent on heat motion. In small bodies this resistance is immeasurably greater than the self-gravity of the body. It is so great that it can only be partially overcome by any force which human ingenuity can bring to bear upon it. This form of resistance has thus, not unnaturally, come to be regarded as approximately immeasurable, and perhaps as grading into actual immeasurability, and as resting back upon the actual contact of irreducible atoms. But the recent researches which have developed grounds for the conception that even the atoms are composite, lead to the further conception that their resistance to compression is dependent on the movement of their constituent corpuscles or electrons. This encourages the broad conception that the whole of the resistance to compression arises from molecular, atomic, and sub-atomic motions, of which heat is merely one form.

While all this is yet on the frontier of physical progress, these conceptions may well be recognized in framing interpretations of the agencies which determine the volume of the earth, and which control the changes that take place in it from age to age. The result of their combined action at any stage is a state of temporary equilibrium between gravity, aided by the molecular, atomic, and sub-atomic attractions, on the one hand, and heat, aided by the molecular, atomic, and sub-atomic resistances, on the other. The vital problem is to ascertain the original condition of balance between these antagonistic forces, and the changes which have affected that balance since. The original state of balance is necessarily a matter of hypothesis, and the best that can be done at present is to picture as clearly as possible the different hypotheses that have been entertained, and the different consequences that logically flow from them. The most important factor in the case is the original amount and distribution of internal heat.

ALTERNATIVE VIEWS OF ORIGINAL HEAT DISTRIBUTION.

The hypothetical modes of origin of the earth will be treated in the historical section. Suffice it here to say that one view is that the earth was once gaseous, passed thence into a liquid, and later into a solid state. Under this view, there are two hypotheses as to the original distribution of internal heat, dependent on the mode of solidification. According to the one, solidification began at the surface after convection had brought the temperature of the whole mass down nearly to the point of congelation; according to the other, solidification began at the center at a high temperature, because of pressure, and proceeded thence outwards. The former only has been much developed in the literature of the subject, though the latter is now generally regarded as the more probable.