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body endeavouring to pass from the denser parts of the medium towards the rarer.
224. The extreme minuteness of the parts of this medium and the velocity of their motion, together with its gravity, density, and elastic force, are thought to qualify it for being the cause of all the natural motions in the universe. To this cause are afcribed the gravity and cohesion of bodies. The refraction of light is also thought to proceed from the different density and elastic force of this ætherial medium in different places. The vibrations of this medium alternately concurring with, or obstructing the motions of the rays of light, are supposed to produce the fits of easy reflexion and transmission. Light by the vibrations of this medium is thought to communicate heat to bodies. Animal motion and sensation are also accounted for by the vibrating motions of this ætherial medium, propagated thro the solid capillaments of the nerves. In a word, all the phænomena and properties of bodies, that were before attributed to attraction, upon later thoughts seem ascribed to this æther, together with the various attractions themselves. .
225. But in the philosophy of Sir Isaac Newton, the fits (as they are called) of easy transmission and reflexion, feem as well accounted for by vibrations excited in bodies by the rays of light, and the refraction of light by the attraction of bodies. To explain the vibrations of light by those of a more subtil medium, seems an uncouth explication. And gravity seems not an effect of the density and elasticity of æther, but rather to be produced by some other cause ; which Sir Isaac himself insinuates to have been the opinion even of those ancients who took vacuum, atoms, and the gravity of atoms for the principles of their philosophy, tacitly attri
buting (as bie well obferves) gravity to fome o ther cause diftinct from matter, from atoms, and confeqäently, from that homogeneous æther or elastic Auid. The elasticity of which fluid is fup. posed to depend upon, to be defined and measured by it's density, and this by the quantity of matter in one particle, multiplied by the number of particles contained in a given space; and the quantity of matter in any one particle or body of a given size to be determined by it's gravity. Should not therefore gravity feem the original property and firft fupposed? On the other hand, if force be considered as prescinded from gravity and matter, and as existing only in points or centers, what can this amount to but an abstract fpiritual incor. poreal force ? · 226. It doth not seem néceffary from the phænomena; to suppose any medium more active and subtil than light or fire. Light being allowed to move at the rate of about ten millions of miles in a minute, what occafion is there to conceive anos ther medium of ftill fmatler and more moveable parts. Light or fire feems the fame with æther: So the ancients understood, and fo che Greek word implies. It pervades all things (a), is every where prefent. . And this same subcil medium acă cording to it's various quantities, motions, and determinations, sheweth itfelf in different effects or appearances, and is æther, light, or fire.
229. The particles of æther Ay afunder with the greatest force, therefore when united they muft (according to the Newsonian doctrine) ata tract each other with che greatest force ; therefore they are acids (b), or constitute the acid ; bus this united with earthy parts maketh alkali, as Six Ifaac teacheth in his tract De acido ; alkali; as ap. . ; (a) 157. (b) 130.
pears in cantharides and lixivial salts, is a caustic ; caustics are fire ; therefore acid is fire; therefore æther is fire ; and if fire, light. , We are not therefore obliged to admit a new medium distinct from light, and of a finer and more exquisite substance, for the explication of phænomena, which appear to be as well explained without it. How can the density or elasticity of æther account for the rapid fight of a ray of light from the sun, . ftill swifter as it goes farther from the sun? or how can it account for the various motions and attractions of different bodies? Why oyl and water, mercury and iron 'repell, or why ocher bodies attract each other? or why a particle of light should repell on one side and attract on the other, as in the case of the Ifandic crystal? To explain cohesion by hamate atoms is ac. counted ignotum per ignotius. And is it not as much fo to account for the gravity of bodies by the elasticity of æther? · 228. It is one thing to arrive at general laws of nature from a contemplation of the phænomena ; and another to frame an hypothesis, and from thence deduce the phænomena. Those who supposed epicycles, and by them explained the motions and appearances of the planets, may not therefore be thought to have discovered principles true in fact and nature. And albeit we may from the premises infer a conclusion, it will not follow, that we can argue reciprocally, and from the conclusion infer the premises. For instance, supposing an elastic fluid, whose constituent minute particles are equidistant from each other and of equal densities and diameters, and recede one from another with a centrifugal force which is inverlly as the distance of the centers, and admit. ting that from such supposition it must follow,
25, it not as
220lticity of æther at the gravity
that the density and elastic force of such Auid are in the inverse proportion of the space it occupies when compressed by any force ; yet we cannot reciprocally infer, that a fluid endued with this property must therefore consist of such supposed equal particles ; for it would then follow, that the conftituent particles of air were of equal densities and diameters; whereas it is certain, that air is an heterogeneous mass, containing in its composition an infinite variety of exhalations, from the dif. ferent bodies which make up this terraqueous globe.
229. The phænomena of light, animal spirit, muscular motion, fermentation, vegetation, and other natural operations, seem to require no. thing more than the intellectual and artificial fire of Heraclitus, Hippocrates, the Stoics (a), and other ancients. Intellect, superadded to ætherial spirit, fire, or light, moves, and moves regularly, proceeding, in a method as the Stoics, or increasing and diminishing by measure, as Heraclitus. expressed it. The Scoics held that fire comprehended and included the spermatic reasons or forms. (aby's onegudTixx's) of all natural things. As the forms of things have their ideal existence in the, intellect, so it should seem that seminal principles have their natural existence in the light ), a medium consisting of heterogeneous parts, differing from each other in divers qualities that appear to sense, and not improbably having many original properties, attractions, repulsions and motions, the laws and natures whereof are indifcernible to us, otherwise than in their remote effects., And this animated heterogeneous fire should seem a more adequate cause, whereby to explain the phæ-,
(a) 166, 168.
nomena of nature, than one uniform ætherial met
231. The laws of attraction and repulfion are to be regarded as laws of motion, and thefe only a's rules or methods observed in the productions of natural effects, the efficient and final causes where of are not of mechanical consideration, - Certain. ly, if the explaining a phænomenon be to afe sign its proper efficient and final cause (a), it should seem the mechanical philosophers never explained any thing; their province being only to discover the laws of nature, that is, the general rules and methods of motion, and to account for particular phænomena by reducing them un. der, or shewing their conformity to fuchi general. rules.
232. Some corpufcularian philosophers of the last age, have indeed attempted to explain the formation of this world and its phænomena, by a few simple laws of mechanism. But if we consider the various productions of nature, in the mine, ral, vegetable and animal parts of the creation, I believe we shall see caufe to affirm, that not any
fa) 154, 155, 160..