FOOTNOTES:
[52] First introduced by Basilius Valentinus. See Fowler's edition of the Novum Organum, p. 576, note.
[53] Thus recurring, as Mr. Fowler remarked (loc. cit.), to Geber's earlier view.
[54] Dictionary of National Biography, vol. xlvi., p. 426.
[55] Crookes, Philosophical Transactions, vol. clxx., p. 163.
[56] J. J. Thomson, The Discharge of Electricity through Gases, p. 195; Philosophical Magazine, vol. xliv., p. 311.
[57] Science, June 26, 1903.
[58] Proceedings of the Chemical Society, March 2, 1888.
[59] Times, March 30, 1888.
[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.
The nature of a power so singularly conditioned is almost inconceivable; yet attempts have not been wanting to fathom the mystery that surrounds it. Professor Osborne Reynolds, in the Rede Lecture for 1902, claimed to have arrived at 'a complete, quantitative, purely mechanical explanation of the cause of gravitation,' based on the 'dilatancy' of a granular medium in close piling. But his working model of the universe will probably be remembered only as a lesson in the 'inversion of ideas,' showing that with skill and ingenuity a fairly concordant outcome of phenomena may be derived from antagonistic hypotheses. In this author's view matter is equivalent to a deficiency of mass, the spaces where his cosmic grains are relatively few, because their arrangement is out of gear, being driven towards one another by the pressure of the surrounding medium, in which they are compactly stowed, and therefore numerous. Thus, the acting forces in nature are made to depend upon the compression by the denser medium of interspatial tracts of rarer consistence, forming what we call matter. The theory is difficult, if not impossible of acceptance, not because it involves the overthrow of conceptions which may be rooted in habitual modes of thought, rather than in absolute truth, but because of its startling postulates and large vacuities. To be valid, it should be complete; and there are obvious chasms in the vast expanse of ground which it covers with surprising, though only partial success.
The multa renascentur of the poet is verified by the revolutions no less of thought than of speech. Flights of minute material particles have served the turn of theorists often, and in more ways than one, and have as frequently been consigned to discredited oblivion; but they are in vogue once more. George Louis Lesage, of Geneva, devoted sixty-three of the seventy-nine years of his life, which came to an end in 1803, to the elaboration of a mechanical rationale of gravity, first given to the world in the Transactions of the Berlin Academy of Sciences for 1782, and with details of amplification in his Traité de Physique Mécanique, edited by Pierre Prévost in 1818.[63] The explanation it offered of molar attractions was by the supposed unceasing impacts of 'ultramundane corpuscles,' speeding in countless numbers and at fabulous velocities, from nowhere everywhere, and thus enforcing the mutual approach of masses of gross matter. This involved the supposition of an infinitesimal screening effect, producing a small inequality in the strength of the bombardment on the sheltered and unsheltered sides of the bodies exposed to it. This inequality, in fact, was taken to be the causa causans of gravity. Yet its production encountered a difficulty. There was required for it a trifling degree of opacity in every kind of matter, while perfect gravitational transparency is asserted by the most delicate observations. Lesage, then, reduced the arrests laid upon his particles to a minimum; one in ten thousand, for instance, might at the utmost be intercepted by the terrestrial globe.[64] Even this insignificant minority, however, would suffice, through the surrender of their momentum to the impeding bodies, to endow them with the noted property of gravitation.
Clerk Maxwell urged the objection that the accompanying loss of kinetic energy by the corpuscles should, if transformed into heat, render all gravitating bodies white-hot. But Professor J. J. Thomson holds that the transferred corpuscular energy might, instead of reappearing in thermal form, be converted into some highly penetrative species of radiation capable of escaping unperceived into surrounding space.[65] 'A simple calculation,' he adds, 'will show that the amount of kinetic energy transformed per second in each gramme of the gravitating body must be enormously greater than that given out in the same time by one gramme of radium.' This consequence of Lesage's theory takes one's breath away; the 'fables of the Talmud' seem, by comparison, easy of belief; nevertheless, Lord Kelvin[66] declared it in 1873 to be more complete in the expository sense, and not more arduous in its assumptions, than the kinetic theory of gases.
Its fundamental postulate, at any rate, has been curiously verified in the course of recent researches into the arcana of physics. Entities in some degree corresponding to the ultramundane corpuscles of the Genevese philosopher do actually exist. Electrons are being continually expelled from bodies in all parts of the universe; they issue forth under all conceivable conditions and in unlimited numbers. Space is perhaps thronged with them; no material object can be exempt from their multitudinous buffetings, which are beginning to be taken account of in many cosmical speculations, and cannot certainly be ignored in efforts to solve the most obvious to superficial apprehension, the most intricate on a profound consideration, of all cosmical problems. But there is one fatal objection to an electronic theory of gravitation. The agency appealed to travels too slowly to be available for the required purpose. The velocity of light, there is reason to believe, sets a limit impossible to be surpassed or even attained by the velocity of electrons; yet it is incomparably smaller than the rate of gravitational transmission.
Tisserand estimated at six million times the quickness of luminous travel the minimum speed at which the sun's attraction must be propagated in view of the imperceptibility in planetary observations of effects corresponding to a time-inequality;[67] and this value may be taken as authentic. So colossal a discrepancy excludes any kind of impact-rationale of the mutual pull of heavy masses; Lesage's corpuscles remain 'ultramundane'; their identification with known atoms or sub-atoms appears to be precluded; no products of ionic disintegration possess the qualities necessarily to be ascribed to them.
We turn, then, inevitably to the menstruum of mysteries, the bank of the insolvent in speculation, to the all-serviceable ether. Ethereal radiations exercise an impulsive power; light-pressure has secured a recognised status among cosmic agencies; and every vibrational system of the luminous type undoubtedly shares the faculty by which light tends to drive minute particles forward along the lines of its propagation. Professor J. J. Thomson, accordingly, considered that but for the drawback of their insufficient velocity, 'very penetrating Röntgen rays' might with advantage be substituted for corpuscular streams as the cause of gravity.[68] They would, in some slight degree, be absorbed by encountered masses, to which they would impart a proportionate amount of momentum. Two bodies mutually shadowing one another would, under such circumstances, be drawn together with a force varying as the inverse square of distance; and if further they absorbed the impinging rays strictly in the measure of their density (as observation shows to be approximately the case), the attraction would increase in the same ratio as the product of their masses. But Röntgen rays travel with the precise velocity of light; they are, in truth, ultra-invisible light; and they must hence be regarded as hopelessly incompetent to explain an influence transmitted at least six million times more rapidly.
This was fully admitted by Dr. H. A. Lorentz,[69] who, five years ago, weighed the vibrational hypothesis of gravity in the balance of rigorous calculation, and found it wanting. His equations yielded the unexpected result that, if its postulates were granted, the noted attractions between massive bodies could subsist only on the terms of an incessant waste of electro-magnetic energy. But this is of course inadmissible. The theory involving such a consequence stands self-condemned, to say nothing of the wholly inadequate rate of propagation afforded by it.
Impulsion hypotheses, whether by corpuscles or rays, being hopelessly discredited, Dr. Lorentz reverted to a half-forgotten speculation by Mosotti, which, though sixty years old, struck him as capable of being adapted to modern requirements. It was of an electrical nature, and, in the novel shape given to it, supposed gravitational action to depend upon strains of the ether due to the disturbing effects of the positive and negative ions constituting ordinary matter. These 'states' of the medium are distinct in kind; they cannot neutralize one another; and the familiar law of attraction represents their resultant effect. To bring it about much has to be taken for granted; yet the hypothesis can lay claim to one singular prerogative. Although the disturbances invoked by it traverse the ether with no more than the standard speed of light, it appears from Dr. Lorentz's investigation that, owing to certain modifications in the properties of the medium produced by moving matter, the planetary perturbations betraying loss of time in gravitational transmission would, on the electrical theory, be so small as to evade detection. As regards this crucial point, the Dutch physicist has hit upon a felicity of explanation entirely original, and, as it were, unsought.
An 'undulatory theory' of gravity, adumbrated, rather than advanced by Mr. Whittaker in 1902,[70] excited hopes that the ideal aim of science—a complete unification of the forces of Nature—might at last be within reach. Based upon a striking mathematical research, it exhibited the attraction between masses as, in a manner, the integration of innumerable wave disturbances, propagated at a rate not strictly definable, but perhaps immensely surpassing that of gravity. No suggestion was made as to the primary mode or cause of agitation, yet it seemed much to learn that the medium we are cognizant of in space might be capable of transmitting the pull of gravity. Unfortunately, however, the physical foundation of this reassuring congruity proves to be weak or unsound. The mathematical mill works magnificently, but the grist put into it is of dubious quality. Stripping Mr. Whittaker's result of its purely analytical form, Dr. Johnstone Stoney showed that an assumption of extreme improbability lay concealed in his equations, which could not, he concluded, be seriously taken to correspond with the reality of things.[71]
There would then seem to be no alternative but to accept ad interim the electro-dynamical view of the nature of gravity. If not true, it is at least not obviously false. Through its subtlety it escapes direct confutation. And the method of exclusions, by eliminating competitors, has left it in virtual possession of the field.
Nothing is more curious in the history of recent science than the continual and irresistible growth which it records in the importance of electrical phenomena. All others tend to become merged in them; the most varied data of experience claim to be translated into electrical terminology. They are not, assuredly, rendered more intelligible by the process, but it at any rate abolishes the confusion incidental to multitudinous points of view. Thus, in the last resort, we find electrical forces (if they may be so designated) swaying the world. What they essentially consist in, we cannot tell; the utmost that may reasonably be hoped for is to arrive at a clear conception of modes of action reduced to antagonistic stresses, by which the play and counterplay of the universe may be kept up. And to this extent we find it possible to understand how electricity works the ethereal machinery. It is strongly dualistic. The nearer we get to the foundations of nature, the more sharply positive and negative charges appear to be differentiated.
The opinion is nevertheless held by some inquirers that negative electricity is the only substantive kind, and that its complement is ordinary matter deprived of some of its negative particles. This, in fact, revives Franklin's 'one fluid theory,' only with the substitution of negative for positive electricity as the active principle.[72] But we are met by the doubt whether 'ordinary matter' can be said to exist in and by itself. If it do, the mode of its existence becomes more and more baffling to comprehension, as the association of mass with charge makes its way into the foreground of thought.[73] Moreover, a charge is, or produces, a 'state of the ether' (to use the unsatisfactory current phrase); and the ether being capable of opposite distortions, the effects upon it of opposite charges are contrary and similar, though perhaps not equivalent; whence the 'two fluid theory' obtains a primâ facie recommendation as the simplest, though a crude interpretation of electrical phenomena. These are ubiquitous; destitute though we are of sense-organs for their perception, we still indirectly recognise their presence on every side. If the unification of the forces in nature be attainable, the unifying formula will doubtless be derived from them. Electricity is the mot de l'énigme; yet it is itself the most inscrutable of enigmas.