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In Mr. Colden's treatife, we are further told, that there exifts another kind of Matter, endowed with a principle of felf-motion, or a tendency to move in every direction; which always takes effect on the fide of the leaft refiftance*. Now, as experiment does not furnifh us with any idea of the active refiftance of a fubftance, independent of motion, fo neither can we form any phyfical idea of the felf-motion of fuch fubftance, or of the tendency of it to move in all directions; without fuppofing fuch a tendency the effect of fome prior

motion.

I do not pretend to deny the existence of bodies capable of actual refiftance, or fuch whofe parts are propenfe to move in the direction of the leaft refistance. A thousand experiments ferve to prove the exiftence of elaftic bodies actually refifting in every direction; but it does not follow that fuch elafticity is a primary effential property, owing to no mechanical caufe. To fay that refiftance or motion is effential to this, or that, kind of matter, which fome how or other refifts and moves, is no explanation of the phenomena of the refiftance and motion of bodies.

It is not, however, an easy task to explain the feveral appearances in Nature, folely from the principles of matter and motion: but, as a mechanical explication would be infinitely more fatisfactory than a metaphyfical one, it is requifite that method fhould be purfued much farther than it has yet been done. For I cannot conceive we are under the neceffity of fuppofing the existence of two or three different kinds of elementary matter, till it be fhewn that the properties by which they are diftinguifhed, are not, or cannot be, the mechanical effects of one kind of matter, varioufly put in motion.

I will not take upon me, notwithstanding, to fay, there exists in Nature an abfolutely, and in every fenfe, inert, impenetrable fubftance, fuch as the primary elements of bodies have been fuppofed to be. I fubfcribe, in a great degree, to the Berkleian fyftem, and believe the contrary: but, with refpect to all phyfical reasoning, it is exactly the fame thing whether it does or not. For, agreeing with Mr. Colden, that the property or quality of any thing is nothing elfe but the action of that thing +; I fay, the vis inertia, elasticity, gravity, and all the other active properties of bodies, arife only from the different modes of the motion of the component parts of fuch bodies; the primary elements, confidered

* See Principles of Action, page 12.

+ See Ibid. page 3.

therefore

therefore as divested of these properties, must also have no mechanical action, i. e. motion or principle of motion left.

In this case, matter would consist only of folidity and mobility; that is, would only take up a certain quantity of fpace, and be liable to be put in motion. It may be faid, indeed, that the cause of its folidity, or that caufe in confequence of which any one part of matter excludes every other part of it the fame place, is an agent. It may be fo; and I conceive it is; but its agency is of fo different a nature to that of mechanical action, that no experiment can affift us in the investigation of it. It should be esteemed, therefore, an object of abstract metaphyfical reasoning, and ought never to be ranked in the clafs of phyfical agents.

It is a mistake to think the refiftance of bodies proves the impenetrability of the primary matter, or that fuch impenetrability proceeds from a fimilar cause to the refiftance of bodies. The first elements may be perfectly inert, in a mechanical fenfe, and yet be notwithstanding impenetrable; their impenetrability being, in fact, a neceffary confequence of their being homogeneous; for, even fuppofing every one of them to be agents, and their mode of action what it will, a fimilar agent, acting with the fame degree of power, in every one of them, how should one be penetrable by the other? And, by the way, let it be observed, that the only proof we can have of the impenetrability of body is, that it is impenetrable to other bodies. The abfolute impenetrability of matter is, therefore, in all probability, a mere chimera: at leaft, whether it be or not, no experiment we can make can poffibly determine. With regard to all mechanical reafoning, however, it is, and must be confidered, as both inert and impenetrable; and if, from the motion of fuch elements, may be deduced the feveral properties of bodies, with the general and particular phenomena of Nature, it is furely needlefs to attribute them to the agency of powers, of whose mode of action we can form no competent idea.

With refpect to Mr. Colden's application of his Principles, in explaining the cause of Gravitation, and the Motion of the Planets, it is certainly ingenious, and, perhaps, very near the truth. The existence of an elaftic medium, or Éther, in the space between all bodies, is undoubtedly true; and there is all the reafon to believe, that Gravitation is the effect of the action of fuch bodies on that Ether, and of the reaction of that Ether on bodies: but it must not be concluded, as before obferved with regard to refifting bodies, that the Elafticity of that Ether is owing to any effential quality in

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its

its component parts, inexplicable on mechanical principles. On the contrary, it were not, perhaps, difficult to fhew in what manner an elastic body would neceffarily be formed by the most fimple motion given to the primary elements of matter, such as I have above reprefented them, i. e. only as inert, moveable, and impenetrable.

It is well known, that in order to be moveable, whatever be their form, they muft lie at fome little diftance from each . other, or, in confequence of their impenetrability, they could not move ‡.

Suppofe then a motion given to a number of contiguous particles, lying as it were in a plane, in a direction perpendicular to the faid plane; it is evident, that as these particles move forward in parallel directions, thofe lying at reft before them must be difplaced by their motion. But, unless the direction of the moving particles fhould pafs juft through the center of every other lying before them, they would ftrike or prefs them unequally on the fides, juft as they should strike them more or lefs obliquely: thofe particles only which should lie in a line perpendicular to the center of the moving plane, and therefore would be impelled with equal force on both fides, being carried directly forward. Thefe alfo, being impelled with the greateft force, would, in confequence of the univerfal principle of action and reaction, retard the motion of the moving particles in the center of the plane, while those toward the extremities of the faid plane would move faster than thofe in the middle, in confequence of meeting with lefs refiftance; the particles lying at reft, in the direction of the moving ones, on the outfide of the plane, being fooner difplaced than thofe lying in the way of the middle of that plane. Now, the particles defcribing, or moving in, this imaginary plane, being moved in every part of it with an equal degree of force, and refifted by a force unequal in every part, thofe particles will not continue to move forward as they fet out, in parallel directions, but tend to a certain point, or focus, perpendicular to the center of the plane.

That is, not one among another, in the fame quantity of space they fhould defcribe while at reit.

REV. Dec. 1759.

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ticles lying in the diameter of the faid plane; and being moved, bfetting out in the parallel directi

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ons a a, b b, &c. it is plain, that as, for the reasons before given, the particles a and b will move fafter than all the others, and d and flower; and that the reft will move more or lefs flow in proportion to their distance from the center; fo, for the fame reafon, the fide of each particle toward the center will move flower than the other fide of it, and therefore the direction of all those

particles, will be changed; and, instead of moving in paralÎels, as in the above Diagram, will move toward a point, as in the following,

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As the motion fhould be continued, alfo, the inequality of refiftance increasing, the focus, or point, to which they would tend, would approach nearer and nearer, till it should arrive in the center of all the moving particles; which at that inftant forming a fphere, and ftriking together, in directions to their common center, would vibrate back again in the oppofite direction, all in right lines from that center.

It may be objected, indeed, that thefe particles ftriking against each other in oppofite directions, their whole motion

fhould

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fhould be destroyed; and that after their collifion they would lie perfectly ftill. But it fhould be confidered, that tho' the motion of two bodies, each of which hath a large portion of the vis inertia, might be in appearance destroyed by meeting each other in oppofite directions, yet the motion of two elaftic ones fo meeting, would be as apparently increased: and, therefore, that of two elements, that have neither elasticity nor vis inertia, but are only impenetrable and moveable, would be neither increased or diminished; but with the fame force they ftruck each other would they fly back in opposite directions.

For the fame reasons also would the particles, fo vibrating from their common center, be made to return back to that center again and that on account of the reaction of the particles of the furrounding medium. And thus a fpherical body would be formed, whofe parts would vibrate alternately from the center to its circumference, and from its circumfer ence to its center,

If the vibrations of fuch parts, alfo, were too quick to be perceptible, fuch body would appear to have an innate power of activity or motion, which would have a perceptible effect on the fide of leaft refiftance: the vibration of its parts being fhorter if opposed by a denfe medium than if by a rarer one, and vice verfâ. Such bodies, therefore, would also, on the whole, be larger in a rarer medium than in a denfer, and perfectly resemble fuch as we call elaftic bodies, capable of rarefaction and condensation,

In like manner might it be fhewn, that the vis inertice of bodies, their tenacity, gravity, and other properties, are the mechanical effects of the motion of the primary elements of an homogeneous matter: and that, not of any fubtly-devifed mode of motion, for which a numerous farrago of fuppofitions must be previoufly made, and granted; but from the moft fimple impulfe, producing fuch effects from an evidenț mechanical neceffity.

I come next to Mr. Colden's fuppofition, that Light is the agent which gives motion to the Planets; and that all bodies receive motion originally from Light,

There is fomething very peculiar in what this Gentleman has afferted of the agent, fubftance, or thing which he is pleafed to call Light. He allows that Light, if nothing him ders, gives motion to bodies in the direction of its rays; but that it does not give it by impulfe, as one body in motion moves another, but in a manner no body knows how.

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