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

CHAP. II. (Book 2)

HAving thus premis'd something in general about the Nature of Attraction, as far as 'tis necessary for my present design; it will be now seasonable to proceed to the conside∣ration of that kind of Attraction, that is employed to raise Liquors, and is by a distinct Name called Suction.

About the Cause of this there is great contention between the New Philosophers; as they are stiled, and the Peripateticks. For the Followers of Aristotle, and many Learned Men that in other things dissent from him, ascribe the ascension of Liquors upon Suction to Natures abhorrence of a Vacuum. For, say they, when a Man dips one end of a Straw or Reed into stagnant Water, and sucks at the other end, the Air contain'd in the cavity of the Reed passes into that

of his Lungs, and consequently the Reed would be left empty, if no o∣ther Body succeeded in the place it deserts; but there are only (that they take notice of,) two Bodies that can succeed, the Air and the (grosser Li∣quor) the Water; and the Air can∣not do it, because of the interposi∣tion of the Water, that denies it ac∣cess to the immers'd orifice of the Reed, and therefore it must be the Water it self, which accordingly does ascend to prevent a Vacuum detested by Nature.

But many of the Modern Philoso∣phers, and generally all the Corpuscu∣larians, look upon this Fuga Vacui as but an imaginary Cause of Suction, though they do it upon very differing grounds. For, the Atomists, that willingly admit of Vacuities, pro∣perly so called, both within and without our World, cannot think that Nature hates or fears a Vacuum, and declines her usual course to pre∣vent it: And the Cartesians, though they do, as well as the Peripateticks,

deny that there is a Vacuum, yet since they affirm not only, that there is none in rerum Natura, but that there can be none, because what O∣thers call an empty Space having three Dimensions, hath all that they think belonging to the Essence of a Body, they will not grant Nature to be so indiscreet, as to strain her self to prevent the making of a thing that is impossible to be made.

The Peripatetic Opinion about the Cause of Suction, though commonly defended by the Schools, as well Mo∣dern as Ancient, supposes in Nature such an abhorrence of a Vacuum, as neither has been well proved, nor does well agree with the lately disco∣ver'd Phaenomenon of Suction. For, according to their Hypothesis, Water and other Liquors should ascend upon Suction to any hight to prevent a Va∣cuum, which yet is not agreeable to experience. For I have carefully tryed, that by pumping with a Pump far more stanch than those that are usually made, and indeed as well

clos'd as we could possibly bring it to be, we could not by all our endeavours raise Water by Suction to above* 1.1 36½ foot. The Tor∣ricellian Exp^t shews, that the weight of the Air is able to sustain, and some of our Experim^ts shew, 'tis able to raise a Mercurial Cylinder e∣qual in weight to as high a Cylinder of Water as we were able to raise by pumping. For Mercury being near 14 times as heavy as Water of the same bulk, if the weight of the Air be equivalent to that of a Mercurial Cylinder of 29 or 30 Inches, it must be able to counterpoise a Cylinder of Water near fourteen times as long, that is, from thirty four to near thirty six foot. And very disagreeable to the common Hypothesis, but conso∣nant to ours, is the Experiment that I have more than once tryed, and I think elsewhere deliver'd, namely, That, if you take a Glass Pipe of a∣bout three foot long, and, dipping one end of it in Water, suck at the other, the Water will be suddenly

made to flow briskly into your mouth: But, if instead of Water you dip the lower end into Quick∣silver, though you suck as strongly as ever you can, provided that in this case, as in the former, you hold the Pipe upright, you will never be able to suck up the Quicksilver near so high as your mouth; so that if the Water ascended upon Suction to the top of the same Pipe, because else there would have been a Vacuum left in the cavity of it, why should not we conclude, that, when we have suckt up the Quicksilver as strongly as we can, as much of the upper part of the Tube as is deserted by the Air, and yet not fill'd by the Mercury, ad∣mits, in part at least, a Vacuum, (as to Air) of which consequently Na∣ture cannot reasonably be suppos'd to have so great and unlimited an abhor∣rency, as the Peripateticks and their Adherents presume. Yet I will not determine, whether there be any more than many little Vacuities, or Spaces devoid of Air, in the Cavity,

so called, of the Pipe unfill'd by the Mercury; (so that the whole Cavity is not one entire empty Space;) it being sufficient for my purpose, that my Experiment affords a good Argument ad hominem against the Peripateticks, and warrants us to seek for some o∣ther Cause than the fuga Vacui, why a much stronger Suction than that, which made Water ascend with ease into the Suckers mouth, will not also raise Quicksilver to the same height or near it.

Those Modern Philosophers that admit not the fuga Vocui to be the Cause of the raising of Liquors in Suction, do generally enough agree in referring it to the action of the Suckers thorax. For, when a Man en∣deavours to suck up a Liquor, he does by means of the Muscles enlarge the cavity of his Chest, which he can∣not do but at the same time he must thrust away those parts of the ambi∣ent Air that were contiguous to his Chest, and the displac'd Air does, according to some Learned Men,

(therein, if I mistake not, Followers of Gassendus,) compress the contiguous Air, and that the next to it, and so outwards, 'till the pressure, succes∣sively passing from one part of the Air to the other, arrive at the surface of the Liquor; and all other places being as to sense full, the impell'd Air cannot find place but by thrusting the Water into the room made for it in the Pipe by the recess of the Air that pass'd into the Suckers lungs. And they differ'd not much from this Ex∣plication, that, without taking in the compression of the ambient Air made by the thorax, refer the Phaeno∣menon to the propagated motion or impulse, that is imprest on the Air dis∣plac'd by the thorax in its dilatation, and yet unable to move in a World perfectly fill'd, as they suppose ours to be, unless the Liquor be impell'd into as much of the cavity of the Pipe, as fast as 'tis deserted by the Air that is said to be suck'd up. But though I readily confess this Expli∣cation to be ingenious, and such as I

wonder not they should acquiess in, who are acquainted but with the long known and obvious Phaenomena of Suction; and though I am not sure, but that in the most familiar cases the Causes assign'd by them may contribute to the Effect; yet, preser∣ving for Cartesius and Gassendus the respect I willingly pay such great Phi∣losophers, I must take the liberty to tell you, that I cannot acquiess in their Theory. For I think, that the Cause of Suction, they assign, is in many cases not necessary, in others, not sufficient. And first, as to the Condensation of the Air by the dila∣tation of the Suckers Chest; when I consider the extent of the ambient Air, and how small a compression no greater an expansion than that of the Thorax is like to make, I can scarce think, so slight a condensation of the free Air can have so conside∣rable an operation on the surface of the Liquor to be rais'd, as the Hypo∣thesis I examin requires: And that this impulse of the Air by a Suckers

dilated Thorax, though it be wont to accompany the ascension of the wa∣ter procured by Suction, yet is not of absolute necessity to it, will, I pre∣sume, be easily granted, if it can be made out, that even a propagated Pulsion, abstracted from any Conden∣sation of Air, is not so necessarily the Cause of it, but that the Effect may be produc'd without it. For suppose, that by Divine Omnipotence so much Air as is displac'd by the Thorax were annihilated; yet I see not, why the Ascension of the Liquor should not ensue. For, when a Man begins to suck, there is an Aequilibrium, or ra∣ther Aequipollency between the pres∣sure, which the Air, contained in the Pipe, (which is shut up with the pressure of the Atmosphere upon it,) has, by virtue of its Spring, upon that part of the surface of the water that is environ'd by the sides of the Pipe, and the pressure which the At∣mospherical Air has, by virtue of its weight, upon all the rest of the sur∣face of the stagnant water; so that,

when by the dilatation of the Suc∣kers Thorax, the Air within the ca∣vity of the Pipe comes to be rarified, and consequently loose of its Spring, the weight of the external Air conti∣nuing in the mean time the same, it must necessarily happen, that the Spring of the internal Air will be too weak to compress any longer the gravitation of the external, and con∣sequently, that part of the surface of the stagnant water, that is included in the Pipe, being less press'd upon, than all the other parts of the same surfaces must necessarily give way, where it can least resist, and conse∣quently be impell'd up into the Pipe, where the Air, having had its Spring weakened by expansion, is no lon∣ger able to resist, as it did before. This may be illustrated by somewhat varying an Instance already given, and conceiving, that within a Cham∣ber three Men thrust all together with their utmost force against a Door, (which we suppose to have neither Bolt nor Latch) to keep it shut, at

the same time the three other Men have just equal strength, and imploy their force to thrust it open. For though, whilst their opposite endea∣vours are equal, the Door will con∣tinue to be kept shut, yet if one of the three Men within the Room should go away, there will need no new force, nor other accession of strength to the three Men, to make them prevail and thrust open the Door against the resistance of those that endeavour'd to keep it shut, who are now but two.

And here (upon the by) you may take notice, that, to raise water in Suction, there is no necessity of any rarified and forcibly stretch'd Rope, as 'twere, of the Air, to draw up the subjacent water into the Pipe, since the bare debilitation of the Spring of the included Air may very well serve the turn. And though, if we should suppose the Air within the Pipe to be quite annihilated, it could not be pretended (since it would not have so much as Existence) that it exer∣cises

an attractive Power; yet in this case the water would ascend into the Pipe, without the assistance of Natures imaginary Abhorrence of a Vacuum, but by a Mechanical Necessity, plainly arising from this, that there would be a pressure of the incumbent At∣mosphere upon the rest of the sur∣face of the stagnant water, and no pressure at all upon that part of the surface that is within the Pipe, where consequently there could be no resi∣stance made to the ascension of the water, every where else strongly urg'd by the weight of the incum∣bent Air.

I shall add on this occasion, that, to shew some inquisitive Men, that the weak resistance within a Vessel, that had but one orifice expos'd to the water, may much more contribute to the ascension of that Liquor into the Vessel, than either the compression or the continued or reflected impulse of the external Air; I thought fit to produce a Phaenomenon, which by the Beholders was without scruple

judg'd an Effect of Suction, and yet could not be ascrib'd to the Cause of Suction, assign'd by either of the Sects of Philosophers I dissent from. The Experiment was this: By a way, elsewhere deliver'd, the long neck of a Glass-bubble was seal'd up, and al∣most all the Air had been by Heat driven out of the whole cavity of the Bubble or Vial, and then the Glass was laid aside for some hours, or as long as we pleas'd; afterwards the seal'd apex of the neck was bro∣ken off under water: I demand now of a Peripatetic, whether the Liquor ought to be suck'd or drawn into the cavity of the Glass, and why? if he says, as questionless he will, that the water would be attracted to hin∣der a Vacuum, he would thereby ac∣knowledge, that, 'till the Glass was unstopt under water, there was some empty space in it; for, 'till the sealed end was broken off, the water could not get in, and therefore, if the fuga vacui had any thing to do in the ascen∣sion, the Liquor must rise, not to

prevent an empty space, but to fill one that was made before. Nor does our Experiment much more favour the other Philosophers, I dissent from: For in it there is no dilatation made of the sides of the Glass, as in ordi∣nary Suction there is made of the Thorax, but only there is so much Air driven out of the cavity of the Bubble, into whose room since nei∣ther common Air nor Water is per∣mitted to succeed, it appears not, how the propagated and returning impulse, or the Circle of Motion, as to com∣mon Air and Water, does here take place. And then I demand, what becomes of the Air, that has been by heat driven out, and is by the Her∣metical Seal kept out of the cavity of the Bubble? If it be said, that it dif∣fuses it self into the ambient Air, and mingles with it, that will be granted which I contended for, that so little Air as is usually displac'd in Suction cannot make any conside∣rable compression of the free ambient Air; for, what can one Cubic Inch

of Air, which is sometimes more than one of our Glasses contains, do, to the condensation so much as of all the Air in the Chamber, when the expell'd Corpuscles are evenly distri∣buted among those of the ambient. And how comes this inconsiderable condensation to have so great an ef∣fect in every part of the room, as to be able there to impel into the Glass as much water in extent as the whole Air that was driven out of the cavity of it? But if it be said, that the ex∣pell'd Air condens'd only the conti∣guous or very neighbouring Air, 'tis easie to answer, that 'tis no way probable, that the expell'd Particles of the Air should not by the differing motions of the ambient Air be quick∣ly made to mingle with it, but should rather wait (which if it did we some∣times made it do for many hours) 'till the Vessels whence 'twas driven out were unstopp'd again. But, though this could probably be pretended, it cannot truly be asserted. For if you carry the seal'd Glass quite out of the

room or house, and unstop it at some other place, though two or three miles distant; the ascension of the water will, (as I found by tryal) ne∣vertheless insue; in which case I pre∣sume, it will not be said, that the Air, that was expell'd out of the Glass, and condens'd the contiguous or near contiguous Air, attended the Bubble in all its motions, and was ready at hand to impel-in the water, as soon as the seal'd apex of the Vial was broken off. But I doubt not, but most of the Embracers of the Opinion I oppose, being Learned and Inge∣nuous Persons, if they had been ac∣quainted with these and the like Phaenomena, would rather have chan∣ged their Opinion about Suction, than have gone about to defend it by such Evasions, which I should not have thought worth proposing, if I had not met with Objections of this na∣ture publickly maintain'd by a Lear∣ned Writer, on occasion of the Air's rushing into the exhausted Magden∣burgic Engine. But as in our Expe∣riment

these Objections have no place, so in our Hypothesis the Explication is very easie, as will anon be intimated.

Notes

• * 1.1

  See Cont. of Phys. Mech. Exp. the 15th Exp.