body endeavouring to pafs from the denfer parts of the medium towards the rarer.

224. The extreme minutenefs of the parts of this medium and the velocity of their motion, together with its gravity, denfity, and elastic force, are thought to qualify it for being the caufe of all the natural motions in the univerfe. To this cause are afcribed the gravity and cohesion of bodies. The refraction of light is alfo 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 obftructing the motions of the rays of light, are fuppofed to produce the fits of eafy reflexion and tranfmiffion. Light by the vibrations of this medium is thought to communicate heat to bodies.

Ani

mal motion and fenfation are alfo accounted for by the vibrating motions of this ætherial medium, propagated thro' the folid capillaments of the nerves. In a word, all the phænomena and properties of bodies, that were before attributed to attraction, upon later thoughts feem afcribed to this æther, together with the various attractions themselves.

225. But in the philofophy of Sir Ifaac Newton, the fits (as they are called) of eafy tranfmiffion 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 thofe of a more fubtil medium, feems an uncouth explication. And gravity feems not an effect of the denfity and elafticity of æther, but rather to be produced by fome other caufe; which Sir Ifaac himself infinuates to have been the opinion even of those ancients who took vacuum, atoms, and the gravity of atoms for the principles of their philofophy, tacitly attri

buting (as he well obferves) gravity to fome ou ther caufe diftinct from matter, from atoms, and confequently, from that homogeneous æther or elaftic fluid. The elafticity of which fluid is fup pofed to depend upon, to be defined and measured by it's denfity 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 fize to be determined by it's gravity. Should not therefore gravity feem the original property and firft fuppofed? On the other hand, if force be confidered as prefcinded from gravity and matter, and as exifting only in points or centers, what can this amount to but an abstract spiritual incorporeal force?

226. It doth not feem neceffary from the phanomena, to fuppofe any medium more active and fubtil 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 another medium of ftill fmaller and more moveable parts. Light or fire feems the fame with æther. So the ancients understood, and fo the Greek word implies. It pervades all things (a), is every where prefent. And this fame fubtil medium ac cording to it's various quantities, motions, and determinations, fheweth itfelf in different effects or appearances, and is æther, light, or fire.

227. The particles of æther fly afunder with the greateft force, therefore when united they muft (according to the Newtonian doctrine) attract each other with the greatest force; therefore they are acids (b), or conftitute the acid; but . this united with earthy parts maketh alkali, as Sir Ifaac teacheth in his tract De acido; alkali, as ap

(a) 157.

(b) 130.

Q

pears

pears in cantharides and lixivial falts, is a cauftic; cauftics are fire; therefore acid is fire; therefore. æther is fire; and if fire, light. We are not therefore obliged to admit a new medium diftinct from light, and of a finer and more exquifite substance, for the explication of phænomena, which appear to be as well explained without it. How can the denfity or elafticity of æther account for the rapid flight of a ray of light from the fun, ftill swifter as it goes farther from the fun? or how can it account for the various motions and attractions of different bodies? Why oyl and water, mercury and iron repell, or why other bodies attract each other? or why a particle of light should repell on one fide and attract on the other, as in the cafe of the Iflandic cryftal? To explain cohefion by hamate atoms is accounted ignotum per ignotius. And is it not as much fo to account for the gravity of bodies by the elafticity 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 hypothefis, and from thence deduce the phænomena. Those who fuppofed 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 conclufion, it will not follow, that we can argue reciprocally, and from the conclufion infer the premises. For inftance, fuppofing an elaftic fluid, whofe conftituent minute particles are equidiftant from each other and of equal denfities and diameters, and recede one from another with a centrifugal force which is inverfly as the diftance of the centers, and admitting that from fuch fuppofition it must follow,

that

that the denfity and elastic force of fuch fluid are in the inverse proportion of the space it occupies when compreffed by any force; yet we cannot reciprocally infer, that a fluid endued with this property must therefore confist of such fuppofed equal particles; for it would then follow, that the conftituent particles of air were of equal denfities and diameters; whereas it is certain, that air is an heterogeneous mafs, containing in its compofition an infinite variety of exhalations, from the dif ferent bodies which make up this terraqueous globe.

229. The phænomena of light, animal fpirit, mufcular motion, fermentation, vegetation, and. other natural operations, feem to require nothing more than the intellectual and artificial fire of Heraclitus, Hippocrates, the Stoics (a), and other ancients. Intellect, fuperadded to ætherial fpirit, fire, or light, moves, and moves regularly, proceeding, in a method as the Stoics, or increafing and diminishing by measure, as Heraclitus expreffed it. The Stoics held that fire comprehended and included the fpermatic reasons or forms. (λoys σTegμaτIX's) of all natural things. As the: forms of things have their ideal existence in the, intellect, so it should seem that feminal principles have their natural exiftence in the light (b), a medium confifting of heterogeneous parts, differing from each other in divers qualities that appear to fense, and not improbably having many original properties, attractions, repulfions and motions, the laws and natures whereof are indifcernible to us, otherwife than in their remote effects. And this animated heterogeneous fire fhould feem a more adequate caufe, whereby to explain the pha

(a) 166, 168.

(b) 164.

O 2

nomera

nomena of nature, than one uniform ætherial me dium.

230. Ariftotle indeed excepts against the ele ments being animated. Yet nothing hinders why that power of the foul, ftyled by him

or locomotive, may not refide therein, under the direction of an intellect, in fuch fenfe, and ast properly as it is faid, to refide in animal bodies, It must nevertheless be owned, that albeit that philofopher acknowledgeth a divine force or ener gy in fire, yet to fay that are is alive, or that having a foul it fhould not be alive, feem to him equally abfurd. See his fecond book, De partibus animalium.

231. The laws of attraction and repulfion are to be regarded as laws of motion, and thefe only as rules or methods obferved in the productions of natural effects, the efficient and final caufes where. of are not of mechanical confideration. Certain ly, if the explaining a phænomenon be to af fign its proper efficient and final caufe (a), it fhould feem the mechanical philofophers 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 under, or fhewing their conformity to fuch general.

rules.

232. Some corpufcularian philofophers of the laft age, have indeed attempted to explain the formation of this world and its phænomena, by a few fimple laws of mechanifm. But if we confider the various productions of nature, in the mineral, vegetable and animal parts of the creation, I believe we fhall fee caufe to affirm, that not any

(a) 154, 155, 160.

one