Of the cause of attraction by suction a paradox / by the Honourable Robert Boyle ...

CHAP. IV. (Book 4)

BUt because such a conveniency as our Engin, and the apparatus ne∣cessary for such Tryals are not easily procurable, I shall endeavour to con∣firm our Hypothesis about Suction by subjoining some Experiments, that may be tryed without the help of that Engin, for the making out these three things:

• I. That a Liquor may be rais'd by Suction, when the pressure of the Air, neither as it has Weight nor Elasticity, is the Cause of the Elevation.
• II. That the weight of the Atmo∣spherical Air is sufficient to raise up Li∣quors in Suction.
• III. That in some cases Suction will not be made, as, according to the Hypo∣thesis I dissent from, it should, although there be a dilatation of the Suckers Tho∣rax, and no danger of a Vacuum though the Liquor should ascend.

And first, to shew, how much the rising of Liquors in Suction de∣pends upon the weight or pressure of the impellent Body, and how little necessity there is, where that pres∣sure is not wanting, that, in the place deferted by the Liquor that is suck'd, there should succeed Air or some other visible Body, as the Peri∣patetic Schools would have it; to shew this, I say, I thought on the following Experiments. We took a Glass-pipe fit to have the Torricellian Experiment made with it, but a good deal longer than was necessary for that use: This Pipe being Hermeti∣cally seal'd at one end, the other end was so bent as to be reflected upwards, and make as it were the shorter legg of the Syphon as parallel as we could to the longer, so that the Tube now was shap'd like an inverted Syphon with leggs of a very unequal length. This Tube, notwithstanding its in∣convenient figure, we made a shift, (for 'tis not easily done) to fill with Mercury, when 'twas in an inclin'd

posture, and then erecting it, the Mercury subsided in the longer legg, as in the Torricellian Experiment, and attain'd to between two foot and a quarter and two foot and an half a∣bove the surface of the Mercury in the shorter legg, which in this In∣strument answers to the stagnant Mercury in an ordinary Barometer, from which to distinguish it I have elswhere call'd this Syphon, furnish'd with Mercury, a Travelling Baroscope, because it may be safely carried from place to place. Out of the shorter legg of this Tube we warily took as much Mercury as was thought con∣venient for what we had further to do, and this we did by such a way as to hinder any Air from getting in∣to the deserted cavity of the longer legg, by which means the Mercurial Cylinder, (estimated as I lately men∣tion'd) retain'd the same height above the stagnant Mercury in the shorter: The upper and clos'd part of this Travelling Baroscope you will easily grant to have been free from Common

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Air, not only for other Reasons that have been given elsewhere, but par∣ticularly for this, that, if you gently incline the Instrument, the Quick∣silver will ascend to the top of the Tube; which you know it could not do, if the place, formerly deserted by it, were possest by the Air, which by its Spring would hinder the ascen∣sion of the Mercury, (as is easie to be tryed.) The Instrument having been thus fitted, I caus'd one of the by∣standers to suck at the shorter legg, whereupon (as I expected) there pre∣sently ensued an Ascension of four or five Inches of Mercury in that legg, and a proportionable Subsidence of the Mercury in the longer, and yet in this case the raising of the Mer∣cury cannot be pretended to proceed from the pressure of the Air. For, the weight of the Atmosphere is fenc'd off by that, which closes the upper end of the longer Tube, and the Spring of the Air has here nothing to do, since, as we have lately shewn, the space deserted by the Mercury is

not possest by the included Air, and the pulsion or condensation of the Air, suppos'd by divers modern Philoso∣phers to be made by the dilatation of the Suckers Chest, and to press upon the surface of the Liquors that are to be suck'd up, this, I say, cannot here be pretended in regard the sur∣face of the Liquor in the longer legg is every way fenc'd from the pressure of the ambient Air. So that it re∣mains, that the Cause, which rais'd the Quicksilver in the shorter legg upon the newly recited Suction, was the weight of the collaterally supe∣riour Quicksilver in the longer legg, which, being (at the beginning of the Suction) equivalent to the weight of the Atmosphere, there is a plain reason, why the stagnant Mercury in the shorter legg should be rais'd some Inches by Suction; as Mercury stag∣nant in an open Vessel will be rais'd by the weight of the Atmosphere, when the Suction is made in the open Air. For, in both cases there is a Pipe, that reaches to the stagnant

Mercury, and a competent weight to impel it into that Pipe; when the Air in the cavity of the Pipe has its Spring weaken'd by the dilatation that accompanied Suction.

The Second point formerly pro∣pos'd, which is, That the weight of the Air is sufficient to raise Liquors in Suction; may not be ill prov'd by Arguments legitimately drawn from the Torricellian Experiment it self, and much more clearly by the first and fifteenth of our Continued Physico-Me∣chanical Experiments. And therefore I shall only here take notice of a Phae∣nomenon, that may be exhibited by the Travelling Baroscope, which, though it be much inferiour to the Experi∣ments newly referr'd to, may be of some use on the present occasion.

Having then provided an Instru∣ment like the Travelling Baroscope, mention'd under the former Head, but whose leggs were not so unequally long, and having in it made the Tor∣ricellian Experiment after the manner lately describ'd; we order'd the mat∣ter

so, that there remain'd in the shorter legg the length of divers In∣ches unfill'd with stagnant Mercury. Then I caus'd one, vers'd in what he was to do, so to raise the Quick∣silver by Suction to the open orifice of the shorter legg, that, the orifice being seasonably and dexterously clo∣sed, the Mercury continued to fill that legg, as long as we thought fit; and then having put a mark to the surface of the Mercury in the longer legg, we unstopp'd the orifice of the shorter; whereupon the Mercury, that before fill'd it, was depress'd, 'till the same Liquor in the longer legg was rais'd five Inches or more above the mark, and continu'd at that height. I said, that the Mercury that had been raised by Suction, was depress'd, rather than that it subsided, because its own weight could not here make it fall, since a Mercurial Cylinder of five Inches was far from being able to raise so tall a Cylinder of Mercury as made a counterpoise in the longer legg; and therefore the depression we speak

of, is to be referr'd to the gravitation of the Atmospherical Air upon the surface of the Mercury in the shorter legg: And I see no cause to doubt but that, if we could have procured an Instrument, into whose shorter legg a Mercurial Cylinder of many In∣ches higher could have been suck'd up, it would by this contrivance have appear'd, that the pressure of the Atmosphere would easily impel up a far taller Cylinder of Mercury than it did in our recited Experi∣ment.

That this is no groundless conje∣cture may appear probable by the Ex∣periment you will presently meet with. For if the gravity of an in∣cumbent Pillar of the Atmosphere be able to compress a parcel of inclu∣ded Air as much as a Mercurial Cy∣linder, equivalent in weight to be∣tween thirty and five and thirty foot of water, is able to condense it, it cannot well be denied that the same Atmospherical Cylinder may be able by its weight to raise and counter∣ballance

eight or nine and twenty Inches of Quicksilver, or an equiva∣lent pillar of water in Tubes, where the resistance of these two Liquors to be rais'd and sustain'd by the Air, depends only upon their own unassi∣sted gravity.

To confirm our Doctrine of the Gravitation of the Atmosphere upon the surface of the Liquors expos'd to it, I will subjoin an Experiment, that I devis'd to shew, that the incum∣bent Air, in its natural or usual state, would compress other Air not rarified, but in the like natural state, as much as a Cylinder of eight or nine and twenty Inches of Mercury would condense or compress it.

In order to the making of this, I must put you in mind of what I have shewn elsewhere at large,* 1.1 and shall fur∣ther confirm by one of the Experiments that follows the next; namely, that about twenty nine or thirty Inches of Quicksilver

will compress Air, that being in its natural or usual state (as to rarity and density) has been shut up in the shor∣ter legg of our Travelling or Syphon∣like Baroscope, into half the room that included Air possess'd before. This premis'd, I pass on to my Ex∣periment, which was this:

We provided a Travelling Baroscope, wherein the Mercury in the longer legg was kept suspended by the coun∣terpoise of the Air that gravitated on the surface of the Mercury in the shorter legg, which we had so or∣der'd, that it reached not by about two Inches to the top of the shorter legg. Then making a mark at the place where the stagnant Mercury rested, 'twas manifest according to our Hypothesis, that the Air in the upper part of the shorter legg was in its natural state, or of the same de∣gree of density with the outward Air, with which it freely commu∣nicated at the open orifice of the shor∣ter legg; so that this stagnant Air was equally prest upon by the weight

of the collaterally superiour Cylinder of Mercury in the longer legg, and the equivalent weight of a directly in∣cumbent pillar of the Atmosphere. Things being in this posture, the upper part of the shorter legg, which had been before purposely drawn out to an almost capillary smallness, was Hermetically seal'd, which, though the Instrument was kept erected, was so nimbly done by reason of the slen∣derness of the Pipe, that the inclu∣ded Air did not appear to be sensibly heated, though for greater caution we staid a while from proceeding, that, if any rarefaction had been produc'd in the Air, it might have time to lose it again. This done, we open'd the lower end of the lon∣ger legg, (which had been so order'd before, that we could easily do it, and without concussion of the Vessel,) by which means the Atmospherical Air, gaining access to the Mercury included in the longer legg, did, as I expected, by its gravitation upon it so compress the Air included in

the shorter legg, that, according to the estimate we made with the help of a Ruler, (for by reason of the conical figure of the upper part of the glass we could not take precise measures,) it was thrust into near half the room it took up before, and consequently, according to what I put you lately in mind of, endur'd a compression like that, which a Mer∣curial Cylinder of about twenty nine Inches would have given it.

This Experiment, as to the main of it, was for greater caution made the second time with much the like success; and though it had been more easie to measure the Condensation of the Air, if, instead of drawing out and sealing up the shorter legg of the Instrument, we had contented our selves to close it some other way; yet we rather chose to imploy Hermes's Seal, lest, if any other course had been taken, it might be pretended, that some of the included Air, when it began to be comprest, might escape out at the not perfectly and strongly

clos'd orifice of the legg wherein 'twas imprison'd.

To make it yet further appear, how much the Ascension of Liquors by Suction depends upon Pressure, rather than upon Natures imaginary Abhor∣rence of a Vacuum, or the propagated Pulsion of the Air; I will subjoin an Instance, wherein that presum'd Abhorrence cannot be pretended. The Experiment was thus made:

A Glass-Syphon, like those lately describ'd, with one legg far longer than the other, was Hermetically seal'd at the shorter legg, and then by degrees there was put in, at the orifice of the longer legg, as much Quick∣silver as by its weight suffic'd to com∣press the Air in the shorter legg into about half the room it possess'd be∣fore; so that, according to the Peri∣patetick Doctrine, the Air must be in a state of preternatural Condensa∣tion, and that to a far greater degree, than (as I have tryed) 'tis usually brought to by Cold, intense enough to freeze water. Then measuring

the heighth of the Quicksilver in the longer Tube above the superficies of that in the shorter, we found it not exceed thirty Inches. Now, if Li∣quors did rise in Suction ob fugam vacui, there is no reason, why this Quicksilver in the longer part of the Syphon should not easily ascend upon Suction, at least 'till the Air in the shorter legg had regain'd its former Dimensions, since it cannot in this place be pretended, that, if the Mer∣cury should ascend, there would be any danger of a Vacuum in the shorter legg of the Tube, in regard that the contiguous included Air is ready at hand to succeed as fast as the Mercury subsides in the shorter legg of the Sy∣phon. Nor can it be pretended, that, to fill the place deserted by the Quick∣silver, the included Air must suffer a preternatural rarefaction or discen∣sion; since 'tis plain in our case, that on the contrary, as long as the Air continues in the state whereto the weight of the Quicksilver has re∣duc'd it, it is kept in a violent state

of compression; since in the shorter legg it was in its natural state, when the Mercury, poured into the longer legg, did by its weight thrust it in∣to about half the room it took up before. And yet, having caus'd se∣veral persons, one of them vers'd in sucking, to suck divers times as strongly as they could, they were neither of them able, not so much as for a minute of an hour, to raise the Mercury in the longer legg, and make it subside in the shorter for more than about an Inch at most. And yet to shew you, that the Experi∣ment was not favourably tryed for me, the height of the Mercurial Cy∣linder in the longer legg above the surface of that in the shorter legg was, when the Suction was tryed, an Inch or two shorter than thirty Inches, and the comprest Air in the shorter legg was so far from having been by the exsuction expanded be∣yond its natural and first dimensions, that it did not, when the contiguous Mercury stood as low as we could

make it subside, regain so much as one half of the space it had lost by the precedent Compression, and con∣sequently was in a preternatural state of condensation, when it had been freed from that state as far as Suction would do it. Whence it seems evi∣dent, that 'twas not ob fugam vacui, that the Quicksilver did upon Suction ascend one Inch; for, upon the same score it ought to have ascended two, or perhaps more Inches, since there was no danger, that by such an ascen∣sion any Vacuum should be produc'd or left in the shorter legg of the Sy∣phon; whereas, according to our Hypothesis, a clear cause of the Phae∣nomenon is assignable. For, before the Suction was begun, there was an Aequilibrium or equipollency be∣tween the weight of the superiour Quicksilver in the longer legg, and a Spring of the comprest Air inclu∣ded in the shorter legg: But when the Experimentor began to suck, his Chest being widen'd, part of the Air included in the upper part of the

longer legg pass'd into it, and that which remain'd had by that expan∣sion its pressure so weaken'd, that the Air in the shorter legg, finding no longer the former resistance, was able by its own Spring to expand it self, and consequently to depress the contiguous Mercury in the same shor∣ter legg, and raise it as much in the longer.

But here a Hydrostatician, that heed∣fully marks this Experiment, may discern a difficulty, that may perhaps somewhat perplex him, and seems to overthrow our Explication of the Phaenomenon. For he may object, that if the comprest Air in the shorter legg had a Spring equipollent to the weight of the Mercury in the longer legg, it appears not, why the Mer∣cury should not be suckt up in this Instrument, as well as in the free Air; since, according to me, the pressure of the included Air upon the subjacent Mercury must be equivalent to the weight of the Atmosphere, and yet experience shews, that the

weight of the Atmosphere will, upon Suction, raise Quicksilver to the height of several Inches.

To clear this difficulty, and shew, that, though it be considerable, 'tis not at all insuperable, be pleased to consider with me, that I make indeed the Spring of the comprest Air to be equipollent to the Weight of the com∣pressing Mercury, and I have a ma∣nifest reason to do it; because, if the Spring of the Air were not equipol∣lent to that Weight, the Mercury must necessarily compress the Air farther, which 'tis granted de facto not to do. But then I consider, that in our case there ought to be a great deal of difference between the ope∣ration of the Spring of the included Air and the Weight of the Atmosphere, after Suction has been once begun. For, the Weight of the Atmosphere, that impels up Mercury and other Li∣quors, when the Suction is made in the open Air, continues still the same, but the force or pressure of the inclu∣ded Air is equal to the counterpressure

of the Mercury no longer than the first moment of the Suction; after which, the force of the imprison'd Air still decreases more and more, since this comprest Air, being fur∣ther and further expanded, must needs have its Spring proportionably wea∣ken'd; so that it need be no wonder, that the Mercury was not suckt up any more than we have related; for there was nothing to make it ascend to a greater height, than that, at which the debilitated Spring of the (included but) expanded Air was brought to an equipollency with the undiminish'd and indeed somewhat increas'd weight of the Mercurial Cylinder in the longer legg, and the pressure of the Aerial Cylinder in the same legg, lessen'd by the action of him that suck'd. For whereas, when the orifice of this legg stood open, the Mercury was prest on by a Cylin∣der of the Atmospherical Air, equiva∣lent to about thirty Inches of Quick∣silver; by the mouth and action of him that suck'd the Tube was

freed from the external Air, and by the dilatation of his Thorax, the neigh∣bouring Air, that had a free passage through his wind-pipe to it, was proportionably expanded, and had its Spring and pressure weaken'd: By which means, the comprest Air in the shorter legg of the Syphon was inabled to impel up the Mercury, 'till the lately mention'd Equilibrium or equipollency was attain'd. And I must here take notice, that, as the Quicksilver was rais'd by Suction but a little way, so the Cylinder that was rais'd was a very long one; whereas, when Mercury is suck'd up in the free Air, it is seldom rais'd to half that length; though, as I noted before, the impellent cause, which is the weight of the Atmosphere, continued still the same, whereas in our Syphon, when the Mercury was suck'd up but an Inch, the comprest Air, possessing double the space it did before, had by this expansion al∣ready lost a very considerable part of its former Spring and Pressure.

I should here conclude this Dis∣course, but that I remember a Phae∣nomenon of our Pneumatic Engin, which to divers Learned Men, espe∣cially Aristotelians, seem'd so much to argue, that Suction is made either by a Fuga Vacui, or some internal Principle, that divers years ago I thought fit to set down another ac∣count of it, and lately meeting with that account among other papers, I shall subjoin it just as I found it, by way of Appendix to the foregoing Tract.

Among the more familiar Phaeno∣mena of the Machina Boyliana, (as they now call it,) none leaves so much scruple in the Minds of some sorts of Men, as this, That, when ones fin∣ger is laid close upon the orifice of the little Pipe, by which the Air is wont to pass from the Receiver into the exhausted Cylinder, the pulp of the finger is made to enter a good way into the cavity of the Pipe, which doth not happen without a considerable sense of pain in the lower

part of the finger. For most of those that are strangers to Hydrostaticks, especially if they be prepossess'd with the Opinions generally receiv'd both in the Peripatetick and other Schools, perswade themselves, that they feel the newly mention'd and painful pro∣tuberance of the pulp of the finger, to be effected not by pressure, as we would have it, but distinctly by At∣traction.

To this we are wont to answer, That common Air being a Body not devoid of weight, the Phenomenon is clearly explicable by the pressure of it: For, when the finger is first laid upon the orifice of the Pipe, no pain nor swelling is produc'd, because the Air which is in the Pipe presses as well against that part of the fin∣ger which covereth the orifice, as the ambient Air doth against the other parts of the same finger. But when by pumping, the Air in the Pipe, or the most part of it, is made to pass out of the Pipe into the exhausted Cylinder, then there is nothing left

in the Pipe, whose pressure can any thing near countervail the undimi∣nish'd pressure of the external Air on the other parts of the finger; and consequently, that Air thrusts the most yielding and fleshy part of the finger, which is the pulp, into that place where its pressure is unresisted, that is, into the cavity of the Pipe, where this forcible intrusion causeth a pain in those tender parts of the finger.

To give some visible Illustration of what we have been saying, as well as for other purposes, I thought on the following Experiment.

We took a Glass-pipe of a conve∣nient length, and open at both ends, whose cavity was near about an Inch in Diameter, (such a determinate breadth being convenient, though not necessary:) To one of the ends of this Pipe we caused to be firmly tyed on a piece of very fine Bladder, that had been ruffled and oyl'd, to make it both very limber and unapt to ad∣mit water; and care was taken, that

the piece of Bladder tyed on should be large enough, not only to cover the orifice, but to hang loose some∣what beneath it.

This done, we put the cover'd end of the Pipe into a Glass-body (or Cu∣curbit) purposely made more than or∣dinarily tall, and the Pipe being held in such manner, as that the end of it reach'd almost, but not quite, to the bottom of the Glass-body, we caused water to be poured both into this Vessel and into the Pipe (at its upper orifice, which was left open) that the water might ascend equally enough, both without and within the Pipe. And when the Glass-body was full of water, and the same li∣quor was level to it., or a little higher within the Pipe, the Bladder at the lower orifice was kept plump, be∣cause the water within the Pipe did by its weight press as forcibly down∣wards, as the external water in the large Glass endeavour'd to press it in∣wards and upwards.

All this being done, we caus'd

part of the water in the Pipe to be taken out of it, (which may be done either by putting in and drawing out a piece of Spunge or of Linnen, or more expeditiously by sucking up part of the water with a smaller Pipe to be immediately after laid aside;) up∣on which removal of part of the in∣ternal water, that which remained in the Pipe being no longer able, by reason of its want of weight, to press against the inside of the Bladder near as forcibly as it did before, the ex∣ternal water, whose weight was not lessen'd, press'd the sides and bottom of the Bladder, whereto it was contiguous, into the cavity of the Pipe, and thrusted it up there∣in so strongly, that the distended Bladder made a kind of either Thim∣ble or Hemisphere within the Pipe. So that here we have a protube∣rance, like that above-mentioned of the finger, effected by Pulsion, not Attraction; and in a case where there can be no just pretence of having recourse to Natures Abhorrence of a

Vacuum, since, the upper orifice of the Pipe being left wide open, the Air may pass in and out without resi∣stance.

The like swelling of the Bladder in the Pipe we could procure without taking out any of the internal li∣quor, by thrusting the Pipe deeper into the water; for then the external liquor, having by reason of its in∣crease of depth a greater pressure on the outside of the Bladder, than the internal liquor had on the inside of it, the Bladder must yield to the stronger pressure, and consequently be impell'd up.

If the Bladder lying loose at the lower end of the Pipe, the upper end were carefully clos'd with ones thumb, that the upper Air might not get out until the Experimentor thought fit, and if the thus clos'd Pipe were thrust almost to the bot∣tom of the water, the Bladder would not be protuberant inwards, as for∣merly; because the included Air by virtue of its Spring, resisted from

within the pressure of the external water against the outside of the Blad∣der: But if the thumb, that stopp'd the Pipes upper orifice, were re∣mov'd, the formerly compress'd Air having liberty to expand it self, and its elasticity being weaken'd there∣by, the external water would with suddenness and noise enough, not to be unpleasant to the Spectators, drive up the Bladder into the cavity of the Pipe, and keep it there very protu∣berant.

To obviate an Objection, that I foresaw might be brought in by per∣sons not well vers'd in Hydrostaticks, I caus'd the Pipe fore-mention'd, or such another, to be so bent near the lower end, as that the orifice of it stood quite on one side, and the parts of the Pipe made an angle as near to a right one as he that blew it could bring it to. This lower o∣rifice being fitted with a Bladder, and the Pipe with its contained liquor being thrust under water after the former manner, the lateral pressure

of the water forc'd the Bladder into the short and horizontal legg, and made it protuberate there, as it had done when the Pipe was straight.

Lastly, that the Experiment might appear not to be confin'd to one li∣quor; instead of Water we put into the unbent Pipe as much red Wine (whose colour would make it con∣spicuous) as was requisit to keep the Bladder somewhat swelling outwards, when it was somewhat near the bot∣tom of the water; and then 'twas manifest, that, according as we had foreseen, the superficies of the red liquor in the Pipe was a good deal higher than that of the external wa∣ter, and if the depth of both liquors were proportionably lessen'd, the dif∣ference of height betwixt the two surfaces would indeed, as it ought to happen, decrease, but still the sur∣face of the wine would be the higher of the two, because being lighter in specie than the common water, the Aequilibrium between the pressures of the two liquors upon the Bladder

would not be maintain'd, unless a greater height of wine compensated its defect of specifick gravity. And if the Pipe was thrust deeper into the water, then the Bladder would be made protuberant inwards, as when the Pipe had water in it. By which it appears, that these Phaeno∣mena, without recourse to attraction, may be explicated barely by the Laws of the Aequilibrium of Liquors.

FINIS.

Notes

• * 1.1

  See the Authors Defence of the Doctrine touching the Spring and Weight of the Air, against Fr. Li∣nus, chap 5.