[60] Encyclopædia Britannica, article 'Atom.'
[61] Fleming, Proceedings Royal Institution, vol. xvii., p. 169.
[CHAPTER X]
UNIVERSAL FORCES
We find it equally impossible to conceive of matter without force, as of force without matter. The two modes of action, or of being, are inseparable. Yet our minds strongly distinguish between them; they impart a dual aspect to the world. Phenomena are not simple manifestations of disembodied energy, if such a thing could be; they have a substantial basis which, nevertheless, eludes apprehension, and seems to slip away into nothingness if we try to empty it of its immaterial contents. Nor do these energize in the void. They and the bodies they animate are knowable only in combination, and exist, to our apprehension, only on the condition of mutual dependence. All we can do towards discriminating them is to fix our attention predominantly on one or the other side of things, and so facilitate thought by drawing ideal lines of demarcation.
Just as there are many forms of matter—all springing, we are led to believe, from an undifferentiated, fundamental world-stuff—so there are various kinds of force, reducible, possibly without exception, to one universal principle. Their correlation, indeed, has been already in large measure demonstrated; heat, light, electricity, and kinetic power are known to be equivalent and interchangeable; but there are outstanding activities, which resist assimilation, and seem to originate under different conditions from the rest. Forces manifest themselves chiefly through attractive and repulsive effects, varying in accordance with their natures and the modification of attendant circumstances. The minute particles of matter, for instance, cohere; they cling together tenaciously; yet no pressure avails to bring them into actual contact; at a certain point of mutual approach, they develop an invincible power of resistance to any further encroachments upon their separate molecular domains. And it is this faculty which gives to matter its distinctive property of hardness. It is rendered tangible to sense just by its recalcitrance to constraint.
Neither the mode of operation nor the nature of the forces by which molecules are organized into masses is known; while the power acting on the masses thus organized, and regulating by its action the mechanism of the universe, is fully as baffling to comprehension. Wonder at its results is blunted by familiarity; presented to us as novelties, they should be pronounced to outrage reason. The relations of gravity are of the utmost simplicity; and they are, on that very account, supremely perplexing. They are governed by one steadfast law, the same everywhere, and under all varieties of conditions within the range of experiment or precise observation. It governs impartially every kind and quality of matter, taking no notice of its states or combinations, ignoring its subjection to chemical, thermal, magnetic, or electrical influences. Gravity is not only indifferent, but inevitable and inexorable; there is no resisting its sway; no screen serves as a shelter against its persuasions; it spreads equably in all directions, becoming enfeebled, like wave-motion, in the strict proportion of its diffusion from a centre over successive spherical surfaces. Its most singular peculiarity, however, is its apparent unconcern with time. The gravitational pull is virtually instantaneous; its transmission—if it be transmitted—takes place millions of times faster than that of light; the finest tests have, so far, failed to elicit symptoms of delay. These would be found in minute discrepancies between calculated and observed perturbational effects in the heavens.
The action of gravity, if propagated with finite velocity, should differ for bodies preserving an invariable distance from its source, and for bodies travelling towards or away from it. Their movement would modify the law of attraction.[62] Yet, up to the present, it has proved impossible to detect the slightest deviation from the plain rule of inverse squares. Again, the penetrative faculty of this strange force seems absolute and unlimited. We know by ordinary experience that we cannot diminish the weight of an object by interposing any kind of shield between it and the earth; and no refinement in experimentation avails to alter this result. That it is so, is a fortunate circumstance for the harmony of the world. We can dimly imagine the riot of confusion that would ensue if a transiting globe could intercept the attraction, as it does the light, of a central governing mass. But from the wonderfully adjusted universal order gravitational eclipses are excluded; nor does the densest body throw even the faintest gravitational shadow.