Kinetic Theories of Gravitation/Glennie, 1861
Mr. J. S. Stuart Glennie, in 1861, published in the Philosophical Magazine several papers on the subject of gravitation, in which he proposed to show that universal repulsion m the true explanation of this force ; thus referring it rather to a static than a kinetic condition of pressure. In an essay " On the Principles of the Science of Motion," he sets out with the design *' in this attempt to found a general theory, cleared of aethers and fluids, of properties and virtues." Commencing with the generalization that " a mechanical force, or the cause of a mechanical motion, we know to be in general the condition of a difference of pressure," he infers, "hence it appears that if a general mechanical theory is possible, the ultimate property of matter must be conceived to be a mutual repulsion of its parts, and the indubitable Newtonian law of universal attraction be deduced herefrom, under the actual conditions of the world. The general experimental condition of the fitness of the mechanical conception of pressure as the basis of a general physical and chemical theory evidently is that there be a plenum....To give distinctness to this idea of the parts of natter as mutually repulsive, a molecule, or a body (an aggregate of molecules), is conceived as a center of lines of pressure; the lengths and curves of these lines are determined by the relative pressure of the lines they meet; and lines from greater are made up of lesser molecules and their lines, and so on ad infinitum. In speaking of a molecule or body as such a center of pressure, it will be convenient to have a technical name, rather than coin a new term, it is proposed to use 'atom' in this sense.... Atoms, or mutually determining centers of lines of pressure, may also be defined and their relations analytically investigated, as mutually determining elastic systems with centers of resistance. This is the fundamental conception (not hypothesis) of the theory.... Now in a system of atoms as above defined, let the centers be of equal mass and at equal distances; there will be no difference of pressure on any one center, no moving force will be developed, and the conditions of equilibrium will be satisfied. But it is clear that forces will be developed, or the general conditions of motion be fulfilled, aether (1) by a difference in the masses of the centers, or (2) by a difference in the distances of the centers, in consequence of a displacement of any one of them, or (3) supposing a state of dynamic equilibrium established in the system by its being brought in contact with another system in a different state of such equilibrium..."
"If all the masses of the system were equal, and all at the same distance from each other, their mutual repulsions would be equal in all directions, and they would remain at rest. But if, though two masses [255] may be equal, either has on the other side of it a mass of greater size, or at a greater distance than the other, it is evident that the mutual pressures of these two equal masses will under such conditions, be unequal, and hence as in the first case, they will approach. It is also evident that a body may thus cause the approach to itself of another body, whatever the number of interposed bodies. Thus if the conception of atoms is applied to the unequal and unequally-placed bodies of such a world as that presented to us, the law of universal attraction follows, and gravity is mechanically explained, that is, is referred to a mechanical conception. But it must be understood that the above proposition is given rather to show that as an actual law, universal attraction may be deduced from the theoretical conception of universal repulsion, than with any pretension to its being the best attainable form of an explanation of the law. It may however be remarked that such an explanation is in accordance with the chief characteristics of the lorce of gravity; it is not polar, and it seems to be so far different in kind from other physical forces that it is not interchangeable with them, as they are among each other; for the attraction of gravity is thus referred to difference of mass, either between the two attracted bodies or in the systems of which they are parts."[1]
In a second article, "On the Principles of Energetics," Mr. Glennie proceeds: "As force is thus conceived, not as an absolute entity acting upon matter, but as a condition of the parts of matter itself, and as a condition determined by the relative masses and distances of these parts, any valid hypothesis of a force or of a motion to account for any set of phenomena is thus seen to imply an assertion as to relative masses and distances which can be more or less readily submitted to experiment or observation and analysis. ...
"The condition of the beginning of motion is a difference of pressure on the body that begins to move; the condition of a uniform continuous motion is a neutralization of the resisting pressure; the condition of an accelerated continuous motion is a uniform or varying resisting [effective?] pressure."[2]
In a subsequent paper, in continuation of the last, the writer thus resumes his statement: "Here more clearly to express the idea in contrast with the fundamental hypothesis of Professor Challis, an atom may be defined as a center of an emanating elastic aether, the pressure of which is directly as the mass of its center, and the form of which depends on the relative pressures of surrounding atoms. Thus if you will, matter may be said to be made up of particles in an elastic aether. But that tether is not a uniform circumambient fluid, but made up of the mutually determining aethers (if you wish to give the outer part of the atom a special name) emanating from the central particles. And these central particles are nothing but what (endeavoring to make my theory clear by [256] expressing it in the language of the theories it opposes) I may call aetherial nuclei. . . . In defining atoms as centers of pressure, they are thus no less distinguished on the one hand from centers of force than from the little hard bodies of the ordinary theories; for such centers of force are just as absolute and self-existent in the ordinary conception of them as those little bodies ; and in a scientific theory there can (except as temporary conveniences) be no absolute existences, entities. Hence (mechanical) force, or the cause of motion, is conceived not as an entity but as a condition, — the condition namely of a difference of pressure; and the figure, size, and hardness of all bodies are conceived as relative, dependent, and therefore changeable. There are thus no absolutely ultimate bodies."[3] In a later part of the same article (p. 356) the writer objects to the ordinary atomic conception that "an hypothesis of infinitely hard atoms not merely requires in the consideration of the motion of such an atom abstraction to be made of the interior relative motions, also consequent on that difference of pressure which causes its external relative motion, but explicitly denies any internal motion." The force of the objection is not very obvious. There seems to be no more need of conceiving internal motions in the ultimate unit of matter, for the purposes of molecular physics, than there is for conceiving internal motions in the planets for the purpose of astronomical physics.
In a brief summary of his views, in the succeeding volume, Mr. Glennie thus recapitulates: "Matter is conceived as made up, not of an elastic aether and inelastic atoms, but of elastic molecules of different orders as to size and density. If a rough physical conception of these molecules be required, they may be conceived as aetherial nuclei, the aether of the nuclei of a lower being made up of nuclei of a higher order, and so on ad infinitum.[4]
It is somewhat difficult to criticise a scheme of gravitative pressure so indeterminate in detail. The very function of a theory or hypothesis is to formulate the unknown in terms of the known ; and in proportion as a writer.fails to do this, he fails to present us with an intelligible theory or hypothesis. From the objection expressed, that "atoms" are ordinarily conceived as self-existent bodies, while " in a scientific theory there can be no absolute existences or entities," it might be supposed that the author held the constituents of matter to be merely mathematical points (without dimension) forming the centers of repulsive spheres, " aetherial nuclei ;" but when we learn that these elastic molecules are " of different orders as to size and density," built up of successive aggregates from the infinitely small, and that each exerts a repellant " pressure which is directly as the mass of its center," we are led to conclude that these " aetherial nuclei" do possess a determinate magnitude. If they are not to be considered as " absolute existences of entities," they are supposed to have at least sufficient substance to be moved [257] about according to the differences of pressure to which they are exposed. A system composed of any number of these repellant centers having equal mass, and placed at an equal distance apart, will it is said remain in a condition of stable equilibrium, apparently whatever be the unit of distance. What fact of observation this deportment illustrates is not stated. But if either masses or distances be unequal, motion will result, and " forces will be developed." This certainly does not represent any ascertained fact of gravity or molecular physics. With a universe filled with such centers of repulsion energetic inversely as the square of their radii, it is not easy to see how strictly centripetal motions can result, or how such motions of approach (if possible) could exhibit an energy in directly the reverse ratio.
In enthroning a universal repulsion to discharge the office of a universal attraction, Mr. Glennie has not been successful in satisfying any of the conditions of the problem, and in investing his "atoms" with the pressure of elasticity he has hardly carried out his programme of a theory "cleared of properties and virtues."