even in the exhaufted receiver, they appear, by many figns, to continue PNEUMATICS. Aluid. I know, that by calling this extenuated fubftance, a Funiculus, he intimates, that it has its fpring inwards, like lute-ftrings, and ropes forcibly ftretch'd; but there is no fmall disparity betwixt them: for, in ftrings, there is requir'd either wreathing, or fome peculiar, and artificial texture of the component parts; but, a rarifaction of air, does not infer any fuch contrivance of parts, as is requifite to make bodies elaftic. And, fince lute-ftrings, &c. muft, when they fhrink inwards, either fill up, or leffen their pores, and increase in thicknefs, as they diminish in length; our author's Funiculus differs widely from them; fince it has no pores to receive the fhrinking parts; and contracts its length, without increasing its thickness. Nor does it, to me, feem very probable, that when, for inftance, part of a polifh'd marble is extended into a Funiculus, that Funiculus ftrongly afpires to turn into marble again. And 'tis very unlikely, that the space, our author would have replenish'd with his funicular fubftance, fhould be full of little, highly-ftretch'd ftrings, that lay fast hold on the furfaces of all contiguous bodies, and always violently endeavour to pull them inwards. For, a pendulum being fet a moving, in our exhausted receiver, vibrated as freely, and with the ftring as much stretch'd, as in the common air. Nay, the balance of a watch did there move freely; which is hard to conceive, if the moving bodies were to break thro a medium confifting of innumerable ftrings, exceedingly ftretch'd. And 'tis ftrange, if thefe ftrings, thus cut, or broken, by the paffage of these bodies thro' them, could fo readily have their parts re-united, and immediately be made entire again. And, in this cafe, the two divided parts of each small ftring, do not, like those of other broken ftrings, fly back from one another, but meet, and unite again; yet, when in the Torricellian experiment, the tube, with the contain'd mercury, is fuddenly lifted out of the ftagnant quick-filver into the air, the Funiculus fo ftrangely contracts itfelf, that it quite vanishes; fo that the afcending mercury may rise to the very top of the tube. But this is not all that renders our author's hypothefis improbable ; for it neceffarily fupposes such a rarifaction, and condenfation, as is unintelligible. . We must here premife, that a body is commonly faid to be rarified, or The nature of dilated, when it acquires greater dimenfions than it had before; and to d rarifaction con be condenfed, when it is reduced to lefs dimenfions, that is, into a lefs fpace; and that there are three ways of explaining rarifaction: for, either we muft fay, that the corpufcles whereof the rarified body confifts, depart from each other, fo that no other fubftance comes in between them, to fill up the deferted fpaces; or, that these new interftices, are but dilated pores, replenish'd, as thofe of a tumid fpunge by water, with fome fubtile ethereal fubftance; or, laftly, that the fame body does not only obtain a greater space in rarifaction, and a leffer in condensation; but, adequately, and exactly fills it: and fo, when rarified, acquires larger di menfions, PREUMATICS menfions, without leaving any vacuities betwixt its component corpuscles, or admitting any new, and extraneous fubftance between them. " 'Tis to this last way of rarifaction, that our author has recourfe, in this hypothefis; tho', I confefs, it appears to me fo difficult to be conftotelian rarifaction. For the eafier confideration of this matter, let us ceiv'd, that I doubt whether any phenomenon can be explain'd by it. Let us fuppofe, that in the Magdeburg experiment, he fo often urges to prove his hypothefis, that the undilated air, which, as he tells us, poffefs'd about half an inch of space, confifted of 100 parts, 'twill not be deny'd, that as the aggregate is adequate to the whole fpace it fills, fo each of the 100 parts is, likewise, adequately commenfurate to its refpective space, which is 100th part of the whole. Now, our author fays, that "if a body poffeffes twice as much space, "each part of that body muft do the fame.' Whence the whole capacity of the fphere, which, according to him, was 2000 times bigger than the fpace poffefs'd by the unexpanded air, there muft, likewife, be 2000 parts of fpace, commenfurate, each of them, to one of the aforefaid 100th parts of air; and, confequently, when he affirms, that half an inch of air poffefs'd the whole cavity of the globe, if we will not admit, as he does not, either vacuities, or fome fubtile fubftance in the interftices of the aerial particles, each part of air muft, adequately, fill 2000 parts of space. Now, that this should be refolutely taught, to be really, and regularly done, in the Magdeburg experiment, will, queftionlefs, appear very abfurd to the Cartefians, and thofe other philofophers, who take extenfion to be but notionally different from body; and, confequently, impoffible to be acquir'd, or loft, without the addition, or detraction of matter: and will, I doubt not, appear ftrange to every one who confiders how generally extension is allow'd infeparable, and immediately to flow from matter; and bodies to have a neceffary relation to a commenfurate fpace. Nor do I fee, if one portion of air may fo eafily be brought, exactly to fill a space 2000 times as great as that it did but fill before, without the addition of any new fubftance, why the matter contain'd in each of thefe 2000 parts of space, may not be farther brought to fill 2000 more, and fo on; fince each of thefe newly replenish'd spaces, is prefum'd to be exactly fill'd with body; and no fpace, and, confequently, that which the un-rarified air replenish'd, can be more than adequately full. And fince, according to our author, not only fluids, but even folids, as marble, are capable of fuch a distenfion; why may not the world be made many thousand times bigger than it is, without either admitting a vacuity betwixt its parts, or being increas'd with the addition of one atom of new matter? He further alledges, that the phenomena of rarifaction cannot be explain'd, either by vacuities, or the fub-ingreffion of an ethereal fubftance; and that there are two ways of explaining that kind of it, which he contends for. After our author's objections against the two ways of rarifaction propofed; the one by the vacuifts, and the other by the Cartefians, who admit the moft folid bodies, and, even glafs itself, to be pervious to an ethereal, or 1 or fubtile matter, he attempts to explain the manner by which his ownPEUMATICI rarifaction is perform'd; and having premis'd, that the explanation of the way how each part of the rarified body becomes extended, depends upon the quality of the parts into which the body is ultimately refolv'd; and, having truly obferv'd, that they muft, neceffarily, be either really indivifible, or endlefly divifible, he endeavours to explain the Ariftotelian rarifaction, according to thefe two hypothefes. But tho' he thus propofes two ways of making out his rarifaction, yet they are irreconcilable; and he fpeaks of them very doubtfully, and cbfcurely. And, firft, having told us how rarifaction may be explain'd, if we admit bodies to be divifible in infinitum, he makes an objection against the infinity of parts in a continuum, whereto he gives fo dark an answer, that, I confefs, I do not understand it. And 'tis not clear to me, that even fuch a divifibility of a continuum, as is here fuppofed, would make out the rarifaction he contends for: fince, let the integrant parts of a continuum be more or lefs finite, or infinite in number, ftill each part, being a corporeal fubftance, muft have fome particle of fpace commenfurate to it; and if the whole body be rarified, for inftance, to twice its former magnitude, then will each part be, likewife, extended to double its former dimenfions; and fill both the place it took up before, and another equal to it; and, confequently, two places. I will not, however, pretend to affirm which of the two ways, by atoms, or by parts infinitely divifible, our author declares himself for: but, whichfoever of them it be, I think he has not intelligibly made it out; as himself feems willing to confefs. So that, in his difcourfe of rarifaction, to which our author frequently refers, as that which fhould make good what feems the moft improbable, he has, instead of a probable hypothefis, fubftituted a doctrine which himfelf dares not pretend capable of being well freed from the difficulties with which it may be charged. As for the other way of explaining rarifaction, by fuppofing that a body is made up of parts indivisible, he is, upon this hypothefis, reduced to allow, that "one and the fame part muft be in two places adequately; "for fince it is indivifible, and takes up a greater space than before, it "muft, of neceffity, be alfo in every point of that space; or be virtually extended thro' all that space." When, therefore, he, prefently after, affirms, that by this virtual extenfion of the parts, the difficulties which have, for fo many ages, perplex'd philofophers, may be eafily folved, he must give me leave to defire he would explain what this extenfio virtualis is; and how it will remove the difficulties charged upon the Ariftotelian rarifaction. For the eafier confideration of this matter, let us refume what we lately fuppos'd, that, in the Magdeburgic experiment, the halfinch of undilated air, confifted of a hundred corpufcles; I demand, how the indivifibility of thefe corpufcles will qualify them to make out fuch a ra¬ rifaction, as our author imagines? For, what does their being indivifible, in this cafe, but make it the lefs intelligible, how they can fill above VOL. II. ૨૧૧૧ 100 PNEUMATICS. 100 parts of space? He will anfwer, they are virtually extended. Bur, not here to question, how this indivifibility makes them capable of being fo; I demand, whether by an atom's being virtually extended, its corporeal fubitance does really fill more fpace than it did before, or not? If it do, then 'tis a true, and real, and not barely a virtual extenfion : but fuch an extenfion, we have fhewn, will not ferve the turn; and our author feems to confefs as much, by devifing this virtual extenfion, to avoid the inconveniencies to which he faw his doctrine of rarifaction would otherwife be expofed. But if it be faid, that when an atom is virtually extended, its corporeal fubftance fills no more fpace than before; I demand, how that which is not a fubftance, can fill a fpace; and how this improper, and only metaphorical extenfion, will folve the phenomena of rarifaction? As how the half-inch of air, at the top of the fore-mention'd fphere, fhall, without a corporeal extenfion, fill the whole cavity of 2000 times its bignefs, when the water is fuck'd out of it, and act at the lower-part of the fphere? For, our author teaches, that the whole globe was fill'd with a certain thin fubftance, which, by its contraction violently fnatch'd up the water wherein the neck of the glass was immers'd. And, in a parallel cafe, he makes it his grand argument, to prove, there is no vacuum in the deferted part of the tube, in the Torricellian experiment, that the attraction of the finger cannot be but from fome real body. Our author's Funiculus, alfo, fuppofes a condenfation, that, to me, appears incumber'd with no lefs difficulties. For, fince he teaches, that a body may be condens'd, without either having any vacuities for the comprefs'd parts to retire into; or, having its pores fill'd with any fubtile, and yielding matter, that may be fqueezed out of them; it follows,. that the parts of a body to be condens'd, immediately touch each other: which fuppofed, I demand, how bodies, that are already contiguous, can be brought clofer, without penetrating each other? So that I fee not how this condenfation can be perform'd, without penetration of dimenfions. In the Magdeburgic experiment, he tells us, that the whole capacity of the globe is fill'd with an extremely rare body; which, according to him, intercepts neither pores, nor any heterogeneous fubftance. Now let us confider, that before the admiffion of water into the exhaufted globe, there was, according to him, 2000 half-inches of a true and real body; and that, after the admiffion of the water, there remain'd, in the fame globe, no more than one half-inch of body befides. Since, then, our author does not pretend, that the 1999 half-inches of matter, that now appear no more, travers'd the body of water; and fince he will not allow, that it gets away thro' the pores of the glafs; I demand, what becomes of fo great a quantity of matter? For that 'tis annihilated, I fuppofe, he is too rational to pretend; and to fay, that fo many parts of matter, fhould be retir'd into that one part of space that contains the half-inch of air, is. little lefs incredible: for, that space was fuppos'd perfectly full of body. before; and how a thing can be more than perfectly full, who can conceive? In fhort, according to our author's way of condenfation, two, or, perhaps, perhaps, two thoufand bodies, may be crowded into a space that is ade- PNEUMATICS. quately fill'd with one of them apart. And, if this be not penetration of dimenfions, I defire to be inform'd what is. But as the hypothefis I am oppofing, is a kind of inverfion of ours; The preffure and Spring of fuppofing the fpring, or motion of reftitution in the air, to tend inwards, the air conas, according to us, it tends outwards, many of the phenomena would, if it firmid. were true, be plaufibly explicable by it; the fame motions, in an intermediate body, being, in many cafes, producible alike, whether we fuppofe it to be thruft, or drawn; provided, both the endeavours tend the fame way. But then we may be fatisfied, whether the effect be to be afcrib'd to pulfion, or to traction, if we can find out an experiment, wherein there is a reafon that fuch an effect fhould follow, in cafe pulfion. be the cause inquir'd after; and not, in cafe it be traction. And fuch an experimentum crucis is afforded us by M. Pafchal, who obferv'd, that the Torricellian experiment, being made at the foot, and in different parts of a very high mountain, after he had afcended an hundred and fifty fathom, the quick-filver was fallen two inches and a quarter below its ftation at the foot of the mountain; and that at the very top of the hill, it had defcended above three inches below the fame ftation. Whence it appears, that the quick-filver being carried up towards the top of the atmosphere, falls down the lower, proportionably to the height of the place wherein the obfervation is made: the reafon of which, on our hypothefis, is, that the nearer we come to the top of the atmosphere, the fhorter, and lighter is the cylinder of air, incumbent upon the ftagnant mercury; and, confequently, the lefs weight of mercury will that air be able to balance, and keep fufpended. And, fince this noble phenomenon, thus clearly follows upon ours, and not upon our author's hypothefis, it seems to determine the controverfy; becaufe, in this cafe, it cannot be pretended, that the defcent of the quick-filver, in the tube, is caus'd from the preternatural rarifaction, or diftenfion of the external air, when, by trying to reftore itfelf, it endeavours to draw up the ftagnant mercury: for, there appears no fuch forcible dilatation of that air, as in many of the phenomena of our engine, he is here pleased to imagine. To this experiment he replies but two things, which, neither fingly, nor together, will amount to a fatisfactory answer. And firft, he queftions the truth of the obfervation itself, becaufe, having made trial on a low hill, the event did no ways anfwer his expectation. But Gaffendus relates, that the obfervation was five times repeated, with circumstances, which fufficiently argue the diligence wherewith the experiment was made: and, I can confirm thefe obfervations, by two more made on hills in England. But, however the proportion of the defcent of the quick-filver may vary according to the different confiftence, and other accidents of the air, in the particular places, and times of the experiments being made; yet all obfervations agree in this, that nearer the top of the atmosphere the quick-filver falls lower, than when further from it. And, in one of thefe experiments, a determinate quantity of air being left in the tube, ૦૧૬૧ before |