New experiments physico-mechanical, touching the air

CHAP. I. What is alledged to prove the Funiculus is consider'd; and some Difficulties are propos'd against the Hypothesis.

THE Hypothesis that the Examiner would, as a better, substitute in the place of ours, is, if I mistake it not, briefly this; That the things we ascribe to the weight or spring of the Air are really perform'd by neither, but by a certain Funiculus, or extremely thin substance, provided in such cases by Nature, ne detur vacuum, which being exceeding∣ly rarefied by a forcible distension, does perpetually and strongly endeavour to contract it self into dimensions more agreeable to the nature of the distended body; and consequently does violently attract all the bodies whereunto it is contiguous, if they be not too heavy to be remov'd by it.

But this Hypothesis of our Authors does to me, I confess, appear liable to such Exceptions, that though I dislik'd that of his Adversaries yet I should not imbrace his, but rather wait till time and further Speculations or tryals should suggest some other Theory, fitter to be acquiesc'd in than this; which seems to be partly precarious, partly unintelligible, and partly insuffi∣cient, and besides needless: though it will not be so conveni∣ent to prove each of these apart, because divers of my Objecti∣ons tend to prove the Doctrine, against which they are alledged, obnoxious to more than one of the imputed Imperfections.

First, then, the Arguments by which our Author endea∣vours to evince his Funiculus, are incompetent for that end.

The Arguments which he proposes in his sixth Chapter, (where he undertakes to make good his Assertion) I there find to be three.

The first he sets down in these words, Constat hoc primò ex¦jam dict is Capite praecedente: nequit enim argentum des∣cendens sic digitum deorsum trahere, tubo{que} affigere, nisi * 1.1 à tali Funiculo suspendatur, eum{que} suo pondere vehementer exten∣dat, ut per se patet. But to this proof answer has been made already in the former Part of this Discourse: onely whereas the Author seems to refer us to the foregoing Chapter, we will look back to it, and take notice of what I find there against the Vacuists. For though I neither am bound, nor intend, in this Discourse to declare my self for, or against a Vacuum; yet since I am now writing against the Funicular Hypothesis, it will much conduce to shew that it is not firmly grounded, if I examine what he here alledges against the Assertors of a Vacuum.

In the next place therefore I consider that according to the Examiner, there can be no Vacuum; and that he makes to be the main reason why Nature in the Torricellian and our Experi∣ments does act after so extraordinary a manner, as is requisite to the production of his Funiculus. For in the 47th. Page, having in his Adversaries name demanded what need there is at the descent of the Quicksilver, that before it falls a superficies should be separated from it, and extended; Respondeo (sayes he) ideo hoc fieri, ne detur vacuum; cum nihil aliud ibi adsit quod loco argenti descendentis possit succedere. To which he imme∣diately subjoyns, (with what cogency I will not now examine) At{que} hinc plane confirmatur commune illud per tot jam elapsa secula usurpatum in Scholis axioma, viz. Naturam à vacuo abhorrere. And though he seem to make his Funiculus the immediate cause of the Phaenomena occurring in the Torricellian and our Experi∣ments: yet that, if you pursue the inquiry a little higher, he resolves them into Natures abhorrency of a Vacuum, himself plainly informs us in the next page; Nam licet (sayes he) imme∣diata ratio cur aqua v. g. ex hydria hortulana superne clausa (quo exemplo utuntur) non descendat, non sit metus va. ui,* 1.2

sed ea quam modo diximus, nempe quod non detur sufficiens pon∣dus ad solvendum illum nexum quo adhaereat aqua clausae hydriae summitati; ad eam tamen rationem tandem necessario veniendum est. But, though as well our Author's Funiculus, as the other scarce conceivable Hypotheses that learned men have devised, to account for the suspension of the Quicksilver otherwise than by the resistance of the external Air, seem to have been exco∣gitated onely to shun the necessity of admitting a Vacuum: yet I see not how our Examiner cogently proves, either that there can be none in rerum naturâ, or that De facto there is none produc'd in these Experiments. For in his fifth Chap∣ter (where he professedly undertakes that task) he has but these two incompetent Arguments. The first is drawn from the attraction, as he supposes, of the Finger into the deserted cavi∣ty of the Tube in the Torricellian Experiment: Quae quidem (sayes he) tam vehemens tractio & adhaesio, cum non nisi à reali aliquo corpore inter digitum & argentum * 1.3 constitutum queat provenire, manifestum est spatium illud vacuum non esse, sed verâ aliquâ substantiâ repletum. But to this Argu∣ment having already given an Answer, let us (without staying to urge, that the Vacuists will perhaps object, that they see not a Necessity, though they should admit of Traction in the case, that the internal substance must therefore perfectly re∣plenish the deserted Cavity; without pressing this, I say, let us) consider his other, which he draws from the Diaphaneity of the deserted part of the Tube, which space (he sayes) were it empty, would appear like a little black Pillar, Eo quod nullae species visuales ne{que} ab eo ne{que} per illud possunt ad oculum pervenire. But (not to engage our selves in Optical Speculations and Con∣troversies) if we grant him somewhat more than perhaps he can prove; yet as the Experiment will not demonstrate that there is nothing of body in any part of the space deserted by the Mercury, so neither will the Argument conclude (as the Pro∣poser of it does twice in this Chapter) That space ver â aliquâ sub∣stantiâ repleri. For according to the Hypothesis of the Epicureans and other Atomists, who make Light to be a corporeal Effluvium

from lucid bodies, and to consist of Atoms so minute, as freely to get in at the narrow Pores of Glass, there will be no cause to deny interspers'd Vacuities in the upper part of the Tube. For the Corpuscles of Light that permeate that space may be so numerous, as to leave no sensible part of it un-inlightned; and yet may have so many little empty Intervals betwixt them, that, if all that is corporeal in the space we speak of were uni∣ted into one lump, it would not perhaps adequately fill the one half (not to say the tenth, or even the hundredth part) of the whole space: According to what we have noted in the 17. Experiment, that a Room my appear full of the smoke of a Perfume, though if all the Corpuscles that compose that smoke were re-united, they would again make up but a small Pastil. To which purpose I remember I have taken Camphire, of which a little will fill a Room with its odour, and having in well-clos'd distillatory Glasses caught the Fumes driven over by heat, I thereby reduc'd them to re-conjoyn into true Camphire, whose bulk is very inconsiderable in comparison of the space it fills as to sense, when the odorous Corpuscles are scattered through the free Air.

To which I might adde, that the Torricellian Experiment being made in a dark night, or in a Room perfectly darkn'd, if it succeed (as there is little cause to suspect it will not) it may well be doubted whether our Authors Argument will there take place. For if he endeavour to prove that the place in question was full in the dark, because upon the letting in of the Day, or the bringing in of a Candle, the light appears within it; the Vacuists may reply according to their Hypothesis, That that light is a new one, flowing from the lucid body that darts its corporeal beams quite through the Glass and Space we dispute about, which for want of such Corpuscles were not just before visible.

And supposing light not to be made by a trajection of Atoms through Diaphanous bodies, but a propagation of the impulse of lucid bodies through them; yet it will not thence necessa∣rily follow, that the deserted part of the Tube must be full: As in our 27. Experiment (though many of those gross Aërial

Particles that appear'd necessary to convey a languid sound were drawn out of our Receiver at the first and second Exsuction; yet there remain'd so many of the like Corpuscles, that those that were wanting were not miss'd by the sense, though after∣wards, when a far greater number was drawn out, they were) so there may be matter enough remaining to transmit the im∣pulse of light; though betwixt the Particles of that matter there should be store of vacuities intercepted. Whereas our Author pretends to prove, not onely that there is no coacervate Vacuity in the space so often mention'd, but absolutely that there is none. For 'tis in this last sense, as well as the other, that the Schools and our Author, who defends their Opinion, deny a V••cuum.

But notwithstanding what we have now discours'd, as in our 17. Experiment we declin'd determining whether there be a Vacuum or no; so now what we have said to the Examiners Argument, has not been to declare our whole sense of the Con∣troversie, but onely to shew, that though his Hypothesis sup∣poses there is no Vacuum, yet his Arguments do not sufficiently prove it: which may help to shew his Doctrine to be precari∣ous; for otherwise the Cartesians, though Plenists, may plau∣sibly enough (whether truly or no I now dispute not) decline the necessity of admitting a Vacuum in the deserted space of the Tube, by supposing it fill'd with their second and first Element, whose Particles they imagine to be minute enough freely to pass in and out through the Pores of Glass. But then they must allow the pressure of the outward Air to be the cause of the suspension of the Quicksilver: for though the materia caelestis may readily fill the spaces the Mercury deserts; yet that within the Tube cannot hinder so ponderous a liquor from subsiding as low as the restagnant Mercury; since all the parts of the Tube, as well the lowermost as the uppermost, being pervious to that subtile matter, it may with like facility succeed in whatever part of the Tube shall be forsaken by the Quicksilver.

The Examiners second Argument in the same place is, That since the Mercurial Cylinder is not sustain'd by the outward Air it must necessarily be, that it be kept suspended by his

internal string. But since for the proof of this he is content to refer us to the third Chapter; our having already examin'd that, allows us to proceed to his third Argument, which is, That the Mercurial Cylinder, resting in its wonted station, does not gravitate: as may appear by applying the Finger to the im∣mers'd or lower Orifice of the Tube. Whence he infers, that it must of necessity be suspended from within the Tube. And indeed if you dexterously apply your Finger to the open end of the Tube, when you have almost, but not quite, lifted it out of the restagnant Mercury, (which circumstance must not be neglected, though our Author have omitted it) that so you may shut up no more Quicksilver than the Mercurial Cylinder is wont to consist of, you will find the Experiment to succeed well enough: (Which makes me somewhat wonder to find it affirm'd, that the learned Maignan denies it) not but that you will feel upon your Finger a gravitation or pressure of the Glass-Tube, and the contained Mercury as of one body; but that you will not feel any sensible pressure of the Mercury a∣part, as if it endeavoured to thrust away your Finger from the Tube. But the reason of this is not hard to give in our Hy∣pothesis; for according to that, the Mercurial Cylinder and the Air counterpoising one another, the Finger sustains not any sensibly-differing pressure from the ambient Air that presses a∣gainst the Nail and fides of it, and from the included Quick∣silver that presses against the Pulp. But if the Mercurial Cy∣linder should exceed the usual length, then the Finger would feel some pressure from that surplusage of Quicksilver, which the Air does not assist the Finger to sustain. So that this plea∣sant Phaenomenon may be as well solv'd in our Hypothesis, as in the Examiners: in which if we had time to clear an Objection, which we fore-see might be made, but might be answer'd too, we would demand why, when the Mercury included in the Tube is but of a due altitude, it should run out upon the re∣moval of the Finger that stops it beneath, in case it be sustain'd onely by the internal Funiculus, and do, according to his Doctrine, when the Funiculus sustains it, emulate a solid body,

if the pressure of the external Air has not (as our Author teaches it not to have) any thing to do in this matter.

And if some inquisitive person shall here object, That cer∣tainly the Finger must feel much pain by being squeez'd be∣twixt two such pressures, as that of a Pillar of thirty Inches of Quicksilver on the one side, and an equivalent pressure from the Atmospherical Pillar on the other, it may readily be re∣presented, that in fluid bodies (such as are those concern'd in our Difficulty) a solid body has no such sense of pressure from the ambient bodies as (unless Experience had otherwise in∣structed us) we should perhaps imagine. For, not to mention that having inquired of a famous Diver, whether he found himself sensibly compressed by the Water at the bottom of the Sea; he agreed with the generality of Divers in the Negative: I am inform'd that the learned Maignan did purposely try, that his hand being thrust three or four Palmes deep into Quicksilver, his fingers were not sensible, either of any weight from the in∣cumbent, or of any pressure from the ambient, Quicksilver. The reason of which (whether that inquisitive man have given it or no) is not necessary in our present Controversie to be lookt after.

To these three Arguments the Examiner addes not a fourth, unless he design to present it us in this concluding passage: Huc etiam faciunt insignes librationes quibus argentum subito descendens agitatur: Idem enim hic fit quod in aliis Pen∣dulis * 1.4 & ab alto demissis fieri solet. But of this Phaenomenon also tis easie to give an account in our Hypothesis by two several wayes; whereof the First (which is proper chiefly when the Experiment is made in a close place, as our Receiver) is, That the Quicksilver by its sudden descent acquires an impetus super∣added to the pressure it has upon the score of its wonted gra∣vity; whereby it for a while falls below its station, and thereby compresses the Air that leans upon the restagnant Mercury. Which Air by its own Spring again forcibly dilating it self to recover its former extension, and (as is usual in Springs) hastily flying open, expands it self beyond it, and thereby impells up

the Quicksilver somewhat above its wonted station, in its fall from whence it again acquires somewhat (though not so much as before) of impetus or power, to force the Corpuscles of the Air to a Sub-ingression; and this reciprocation of pressure betwixt the Quicksilver and the outward Air decreasing by degrees, does at length wholly cease, when the Mercury has lost that super∣added pressure, which it acquired by its falling from parts of the Tube higher than its due station. But this first way of Ex∣plicating these Vibrations is not necessary in the free Air: For if we consider the ambient Air onely as a weight, and remem∣ber what we have newly said of the impetus acquir'd by descent; this Phaenomenon may be easily enough explain'd, by taking no∣tice of what happens in a Balance, when one of the equiponde rant Scales chancing to be depress'd, they do not till after many Vibrations settle in aequilibrio.

And on this occasion I shall adde this Experiment: I took a Glass Pipe, whose two legs (very unequal in length) were par∣allel enough, and both perpendicular to that part of the Pipe that connected them; (such a Syphon is describ'd in our 36. Ex∣periment, to find the proportion of the gravity of Mercury and Water) into this Quicksilver was pour'd till 'twas some Inches high, and equally high in both legs: then the Pipe being inclin'd till the most part of the Quicksilver was fallen into one of the legs, I stopt the Orifice of the other leg with my Finger, and e∣recting again the Pipe, though the Quicksilver were forc'd to ascend a little in that stopt leg; yet by reason my Finger kept the Air from getting away, the Quicksilver was kept lower by a good deal in that stopt leg than in the other; but if by suddenly removing my Finger I gave passage to the included and some∣what comprest Air, the preponderant Quicksilver in the other leg would with the Mercury in this unstopt leg, make divers un∣dulations before that liquor did in both legs come to rest in an aequilibrium. Of which the Reason may be easily deduc'd from what has been newly deliver'd; and yet in this case there is no pretence to be made of a Funiculus of violently distended Air to effect the Vibrations of the Mercury.

Notes

• * 1.1

  Page 24.
• * 1.2

  Pag. 48.
• * 1.3

  Page. 22.
• * 1.4

  Page 25.