Each being, then, has laws of motion, that are adapted to itself, and constantly acts or moves according to these laws; at least when no superior cause interrupts its action. Thus, fire ceases to burn combustible matter, as soon as sufficient water is thrown into it, to arrest its progress. Thus, a sensible being ceases to seek pleasure, as soon as he fears that pain will be the result.
The communication of motion, or the medium of action, from one body to another, also follows certain and necessary laws; one being can only communicate motion to another, by the affinity, by the resemblance, by the conformity, by the analogy, or by the point of contact, which it has with that other being. Fire can only propagate when it finds matter analogous to itself: it extinguishes when it encounters bodies which it cannot embrace; that is to say, that do not bear towards it a certain degree of relation or affinity.
Every thing in the universe is in motion: the essence of matter is to act: if we consider its parts, attentively, we shall discover there is not a particle that enjoys absolute repose. Those which appear to us to be without motion, are, in fact, only in relative or apparent rest; they experience such an imperceptible motion, and expose it so little on their surfaces, that we cannot perceive the changes they undergo. All that appears to us to be at rest, does not, however, remain one instant in the same state. All beings are continually breeding, increasing, decreasing, or dispersing, with more or less dullness or rapidity. The insect called EPHEMERON, is produced and perishes in the same day; of consequence, it experiences the greatest changes of its being very rapidly, in our eyes. Those combinations which form the most solid bodies, which appear to enjoy the most perfect repose, are nevertheless decomposed, and dissolved in the course of time. The hardest stones, by degrees, give way to the contact of air. A mass of iron, which time, and the action of the atmosphere, has gnawed into rust, must have been in motion, from the moment of its formation, in the bowels of the earth, until the instant we behold it in this state of dissolution.
Natural philosophers, for the most part, seem not to have sufficiently reflected on what they call the nisus; that is to say, the incessant efforts one body is making on another, but which, notwithstanding appear, to our superficial observation, to enjoy the most perfect repose. A stone of five hundred weight seems to rest quiet on the earth, nevertheless, it never ceases for an instant, to press with force upon the earth, which resists or repulses it in its turn. Will the assertion be ventured, that the stone and earth do not act? Do they wish to be undeceived? They have nothing to do but interpose their hand betwixt the earth and the stone; it will then be discovered, that notwithstanding its seeming repose, the stone has power adequate to bruise it; because the hand has not energies sufficient, within itself, to resist effectually both the stone and earth.—Action cannot exist in bodies without re-action. A body that experiences an impulse, an attraction, or a pressure of any kind, if it resists, clearly demonstrates by such resistance that it re-acts; from whence it follows, there is a concealed force, called by these philosophers vis inertia, that displays itself against another force; and this clearly demonstrates, that this inert force is capable of both acting and re-acting. In short, it will be found, on close investigation, that those powers which are called dead, and those which are termed live or moving, are powers of the same kind; which only display themselves after a different manner. Permit us to go a greater distance yet. May we not say, that in those bodies, or masses, of which their whole become evident from appearances to us to be at rest, there is notwithstanding, a continual action, and counter-action, constant efforts, uninterrupted or communicated force, and continued opposition? In short, a nisus, by which the constituting portions of these bodies press one upon another, mutually resisting each other, acting and re-acting incessantly? that this reciprocity of action, this simultaneous re-action, keeps them united, causes their particles to form a mass, a body, and a combination, which, viewed in its whole, has the appearance of complete rest, notwithstanding no one of its particles really ceases to be in motion for a single instant? These collective masses appear to be at rest, simply by the equality of the motion—by the responsory impulse of the powers acting in them.
Thus it appears that bodies enjoying perfect repose, really receive, whether upon their surface, or in their interior, a continual communicated force, from those bodies by which they are either surrounded or penetrated, dilated or contracted, rarified or condensed: in fact, from those which compose them; whereby their particles are incessantly acting and re-acting, or in continual motion, the effects of which are displayed by extraordinary changes. Thus heat rarifies and dilates metals, which is evidence deducible that a bar of iron, from the change of the atmosphere alone, must be in continual motion; that there is not a single particle in it that can be said to enjoy rest even for a single moment. In those hard bodies, indeed, the particles of which are in actual contact, and which are closely united, how is it possible to conceive, that air, cold, or heat, can act upon one of these particles, even exteriorly, without the motion being communicated to those which are most intimate and minute in their union? Without motion, how should we be able to comprehend the manner in which our sense of smelling is affected, by emanations escaping from the most solid bodies, of which all the particles appear to be at perfect rest? How could we, even by the assistance of a telescope, see the most distant stars, if there was not a progressive motion of light from these stars to the retina of our eye?
Observation and reflection ought to convince us, that every thing in Nature is in continual motion—that there is not a single part, however small, that enjoys repose—that Nature acts in all—that she would cease to be Nature if she did not act. Practical knowledge teaches us, that without unceasing motion, nothing could be preserved—nothing could be produced—nothing could act in this Nature. Thus the idea of Nature necessarily includes that of motion. But it will be asked, and not a little triumphantly, from whence did she derive her motion? Our reply is, we know not, neither do they—that we never shall, that they never will. It is a secret hidden from us, concealed from them, by the most impenetrable veil. We also reply, that it is fair to infer, unless they can logically prove to the contrary, that it is in herself, since she is the great whole, out of which nothing can exist. We say this motion is a manner of existence, that flows, necessarily, out of the nature of matter; that matter moves by its own peculiar energies; that its motion is to be attributed to the force which is inherent in itself; that the variety of motion, and the phenomena which result, proceed from the diversity of the properties—of the qualities—of the combinations, which are originally found in the primitive matter, of which Nature is the assemblage.
Natural philosophers, for the most part, have regarded as inanimate, or as deprived of the faculty of motion, those bodies which are only moved by the intervention of some agent or exterior cause; they have considered themselves justified in concluding, that the matter which forms these bodies is perfectly inert in its nature. They have not forsaken this error, although they must have observed, that whenever a body is left to itself, or disengaged from those obstructions which oppose themselves to its descent, it has a tendency to fall or to approach the centre of the earth, by a motion uniformly accelerated; they have rather chosen to suppose a visionary exterior cause, of which they themselves had but an imperfect idea, than admit that these bodies held their motion from their own peculiar nature.
These philosophers, also, notwithstanding they saw above them an infinite number of globes that moved with great rapidity round a common centre, still adhered to their favourite opinions; and never ceased to suppose some whimsical causes for these movements, until the immortal NEWTON clearly demonstrated that it was the effect of the gravitation of these celestial bodies towards each other. Experimental philosophers, however, and amongst them the great Newton himself, have held the cause of gravitation as inexplicable. Notwithstanding the great weight of this authority, it appears manifest that it may be deduced from the motion of matter, by which bodies are diversely determined. Gravitation is nothing more than a mode of moving—a tendency towards a centre: to speak strictly, all motion is relative gravitation; since that which falls relatively to us, rises, with relation to other bodies. From this it follows, that every motion in our microcosm is the effect of gravitation; seeing that there is not in the universe either top or bottom, nor any absolute centre. It should appear, that the weight of bodies depends on their configuration, as well external as internal, which gives them that form of action which is called gravitation. Thus, for instance, a piece of lead, spherically formed, falls quickly and direct: reduce this ball into very thin plates, it will be sustained in the air for a much longer time: apply to it the action of fire, this lead will rise in the atmosphere: here, then, the same metal, variously modified, has very different modes of action.
A very simple observation would have sufficed to make the philosophers, antecedent to Newton, feel the inadequateness of the causes they admitted to operate with such powerful effect. They had a sufficiency to convince themselves, in the collision of two bodies, which they could contemplate, and in the known laws of that motion, which these always communicate by reason of their greater or less compactness; from whence they ought to have inferred, that the density of subtle or ethereal matter, being considerably less than that of the planets, it could only communicate to them a very feeble motion, quite insufficient to produce that velocity of action, of which they could not possibly avoid being the witnesses.
If Nature had been viewed uninfluenced by prejudice, they must have been long since convinced that matter acts by its own peculiar activity; that it needs no exterior communicative force to set it in motion. They might have perceived that whenever mixed bodies were placed in a situation to act on each other, motion was instantly excited; and that these mixtures acted with a force capable of producing the most surprising results.