OF THE CAUSE OF ATTRACTION BY Suction A PARADOX By the Honourable ROBERT BOYLE Fellow of the Royal Society LONDON Printed by William Godbid and are to be Sold by Moses Pitt at the Angel over against the little North Door of St. Paul's Church 1674. PREFACE HAving about twelve years ago summarily exprest and publish'd my Opinion of the Cause of Suction and a while before or after brought to the Royal Society the Glass Instrument I employ'd to make it out I desisted for some time to add any thing about a Problem that I had but occasionally handled Only because the Instrument I mention'd in my Examen of Mr. Hobbes's Opinion and afterwards us'd at Gresham-College was difficult enough to be well made and not to be procur'd ready made I did for the sake of some Virtuosi that were curious of such things devise a slight and easily made Instrument describ'd in the following Tract Chap. 4th in which the chief Phaenomena I shew'd before the Society were easily producible But afterwards the mistakes and erroneous Opinions that in Print as well as in Discourse I met with even among Learned Men about Suction and the Curiosity of an Ingenious Person engaged me to resume that Subject and treat of it as if I had never before meddled with it for the reason intimated in the beginning of the insuing Paper And finding upon the review of my later Animadversions on Mr. Hobbes's Problemata de Vacuo that some passages of this Tract are referr'd to there I saw my self thereby little less than engaged to annex that Discourse to those Animadversions And this I the rather consented to because it contains some Experiments that I have not elsewhere met with which together with some other parts of that Essay may I hope prove of some use to illustrate and confirm our Doctrine about the Weight and Spring of the Air and supply the less experienced than ingenious Friends to our Hypothesis with more grounds of answering the later Objections of some Learned Men against whose endeavours I perceive it will be useful to employ variety of Experiments and other Proofs to evince the same Truth that some or other of these may meet with those Arguments or evasions with which they strive to elude the force of the rest The Title of the following Essay may sufficiently keep the Reader from expecting to find any other kind of Attraction discours'd of than that which is made by Suction But yet thus much I shall here intimate in general that I have found by Trials purposely made that the Examples of Suction are not the only noted ones of Attraction that may be reduced to Pulsion OF THE CAUSE OF Attraction by SUCTION A PARADOX CHAP. I. I Might Sir save my self some trouble in giving you that account you desire of me about Suction by referring you to a passage in the Examen I long since writ of Mr. Hobbes's Dialogus Physicus de Natura Aeris if I knew you had those two Books lying by you But because I suspect that my Examen may not be in your hands since 't is almost out of Print and has not for some years been in my own and because I do not so well remember after so long a time the particulars that I writ there about Suction as I do in general that the Hypothesis I proposed was very incidentally and briefly discours'd of upon an occasion ministred by a wrong Explication given of Suction by Mr. Hobbes I shall here decline referring you to what I there writ and proposing to you those thoughts about Suction that I remember I there pointed at I shall annex some things to illustrate and confirm them that would not have been so proper for me to have insisted on in a short and but occasional Excursion And I should immediately proceed to what you expect from me but that Suction being generally look'd upon as a kind of Attraction it will be requisite for me to premise something about Attraction it self For besides that the Cause of it which I here dispute not of is obscure the very Nature and Notion of it is wont by Naturalists to be either left untouch'd or but very darkly deliver'd and therefore will not be unfit to be here somewhat explain'd How general and ancient soever the common Opinion may be that Attraction is a kind of Motion quite differing from Pulsion if not also opposite to it yet I confess I concur in opinion though not altogether upon the same grounds with some modern Naturalists that think Attraction a Species of Pulsion And at least among inanimate Bodies I have not yet observed any thing that convinces me that Attraction cannot be reduced to Pulsion for these two seem to me to be but extrinsical denominations of the same Local Motion in which if a moved Body precede the Movent or tend to acquire a greater distance from it we call it Pulsion and if upon the score of the Motion the same Body follow the Movent or approach to it we call it Attraction But this difference may consist but in an accidental respect which does not Physically alter the nature of the Motion but is founded upon the respect which the Line wherein the Motion is made happens to have to the situation of the Movent And that which seems to me to have been the chief cause of mens mistaking Attraction for a motion opposite to Pulsion is that they have look'd upon both the moving and moved Bodies in too popular and superficial a manner and consider'd in the Movent rather the situation of the conspicuous and more bulky part of the Animal or other Agent than the situation of that part of the Animal or Instrument that does immediately impress that motion upon the Mobile For those that attentively heed this may easily take notice that some part of that Body or of the Instrument which by reason of their conjunction in this operation is to be look'd on but as making one with it is really placed behind some part of the Body to be drawn and therefore cannot move outwards it self without thrusting that Body forward This will be easily understood if we consider what happens when a Man draws a Chain after him for though his Body do precede the Chain yet his finger or some other part of the hand wherewith he draws it has some part or other that reaches behind the fore part of the first Link and the hinder part of this Link comes behind the anteriour part of the second Link and so each Link has one of its parts placed behind some part of the Link next after it 'till you come to the last Link of all And so as the finger that is in the first Link cannot move forwards but it must thrust on that Link by this Series of Trusions the whole Chain is moved forwards and if any other Body be drawn by that Chain you may perceive that some part of the last Link comes behind some part of that
Body or of some intervening Body which by its cohesion with it ought in our present case to be consider'd as part of it And thus Attraction seems to be but a Species of Pulsion and usually belongs to that kind of it which for distinctions sake is called Trusion by which we understand that kind of Pulsion wherein the Movent goes along with the moved Body without quitting it whilst the progress lasts as it happens when a Gardiner drives his Wheel-barrow before him without letting go his hold of it But I must not here dissemble a difficulty that I foresee may be speciously urged against this account of Attraction For it may be said that there are Attractions where it cannot be pretended that any part of the Attrahent comes behind the Attracted Body as in Magnetical and Electrical Attractions and in that which is made of Water when 't is drawn up into Springs and Pumps I need not tell you that you know so well as that partly the Cartesians and partly other Modern Philosophers have recourse on this occasion either to screwed Particles and other Magnetical Emissions to explicate Phaenomena of this kind And according to such Hypotheses one may say that many of these Magnetical and Electrical Effluvia come behind some parts of the attracted Bodies or at least of the little solid Particles that are as it were the Walls of their Pores or procure some discussion of the Air that may make it thrust the Moveable towards the Loadstone or Amber c. But if there were none of these nor any other subtil Agents that cause this Motion by a real though unperceived Pulsion I should make a distinction betwixt other Attractions and these which I should then stile Attraction by Invisibles But whether there be really any such in Nature and why I scruple to admit things so hard to be conceived may be elsewhere consider'd And you will I presume the freelier allow me this liberty if since in this place 't is proper to do it I shew you that in the last of the instances I formerly objected that of the drawing up of Water into the Barrel of a Syringe there is no true Attraction of the Liquor made by the external Air. I say then that by the ascending Rammer as a part of which I here consider the obtuse end Plug or Sucker there is no Attraction made of the contiguous and subjacent Water but only there is room made for it to rise into without being expos'd to the pressure of the supeiour Air. For if we suppose the whole Rammer to be by Divine Omnipotence annihilated and consequently uncapable of exercising any Attraction yet provided the superiour Air were kept off from the Water by any other way as well as 't was by the Rammer the Liquor would as well ascend into the Cavity of the Barrel since as I have elsewhere abundantly proved the surface of the Terraqueous Globe being continually press'd on by the incumbent Air or Atmosphere the Water must be by that pressure impell'd into any cavity here below where there is no Air to resist it as by our Supposition there is not in the Barrel of our Syringe when the Rammer or whatever else was in it had been annihilated Which Reasoning may be sufficiently confirm'd by an Experiment whereby I have more than once shewn some curious persons that if the external Air and consequently its pressure be withdrawn from about the Syringe one may pull up the Sucker as much as he pleases without drawing up after it the subjacent Water In short let us suppose that a Man standing in an inner room does by his utmost resistance keep shut a Door that is neither lock'd nor latch'd against another who with equal force endeavours to thrust it open In this case as if one should forcibly pull away the first Man it could not be said that this Man by his recess from the Door he endeavoured to press outwards did truely and properly draw in his Antagonist though upon that recess the coming in of his Antagonist would presently ensue so it cannot properly be said that by the ascent of the Rammer which displaces the superiour Air either the Rammer it self or the expelled Air does properly attract the subjacent Water though the ingress of that Liquor into the Barrel does thereupon necessarily ensue And that as the Comparison supposes there is a pressure of the superiour Air against the upper part of the Sucker you may easily perceive if having well stopt the lower orifice of the Syringe with your finger you forcibly draw up the Sucker to the top of the Barrel For if then you let go the Rammer you will find it impell'd downwards by the incumbent Air with a notable force CHAP. II. HAving thus premis'd something in general about the Nature of Attraction as far as 't is necessary for my present design it will be now seasonable to proceed to the consideration 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 other 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 Liquor the Water and the Air cannot do it because of the interposition of the Water that denies it access 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 Philosophers and generally all the Corpuscularians 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 properly so called both within and without our World cannot think that Nature hates or fears a Vacuum and declines her usual course to prevent 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 Others 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 Modern as Ancient
which was one of the two things I chiefly intended that there may be Cases wherein the Cause assign'd in the Hypothesis I am examining will not have place But this will be better understood if before I proceed to the proof of it I propose to you the thoughts I had many years since and do still retain about the Cause of the Ascension of Liquors in Suction To clear the way to the right understanding of the ensuing Discourse it will not be amiss here to premise a summary intimation of some things that are suppos'd in our Hypothesis We suppose then first without disputing either the Existence or the nature of Elementary Air that the Common Air we breath in and which I often call Atmospherical Air abounds with Corpuscles not devoid of Weight and indowed with Elasticity or Springiness whereby the lower parts comprest by the weight of the upper incessantly endeavour to expand themselves by which expansion and in proportion to it the Spring of the Air is weaken'd as other Springs are wont to be the more they are permitted to stretch themselves Next we suppose that the Terraqueous Globe being inviron'd with this gravitating and springy Air has its surface and the Bodies plac'd on it prest by as much of the Atmosphere as either perpendicularly leans on them or can otherwise come to bear upon them And this pressure is by the Turricellian and other Experiments found to be equivalent to a perpendicularly erected Cylinder of about twenty nine or thirty Inches of Quicksilver for the height is differing as the gravity of the Atmosphere happens to be various Lastly we suppose that Air being contain'd in a Pipe or other hollow Body that has but one orifice open to the free Air if this orifice be Hermetically seal'd or otherwise as with the mouth of one that sucks clos'd the now included Air whilst it continues without any farther expansion will have an elasticity equivalent to the weight of as much of the outward Air as did before press against it For if the weight of the Atmosphere to which it was then expos'd had been able to compress it further it would have done so and then the closing of the orifice at which the internal and external Air communicated as it fenc'd the included Air from the pressure of the incumbent so it hindred the same included Air from expanding it self so that as it was shut up with the pressure of the Atmosphere upon it that is in a state of as great compression as the weight of the Atmosphere could bring it to so being shut up and thereby kept from weakening that pressure by expansion it must retain a Springiness equipollent to the pressure 't was expos'd to before which as I just now noted was as great as the weight of the incumbent Pillar of the Atmosphere could make it But if as was said in the first Supposition the included Air should come to be dilated or expanded the Spring being then unbent its Spring like that of other elastical Bodies would be debilitated answerably to that expansion To me then it seems that speaking in general Liquors are upon Suction raised into the cavities of Pipes and other hollow Bodies when and so far as there is a less pressure on the surface of the Liquor in the cavity than on the surface of the external Liquor that surrounds the Pipe whether that pressure on those parts of the external Liquor that are from time to time impell'd up into the orifice of the Pipe proceed from the weight of the Atmosphere or the propagated compression or impulse of some parts of the Air or the Spring of the Air or some other Cause as the pressure of some other Body quite distinct from Air. Upon the general view of this Hypothesis it seems very consonant to the Mechanical Principles For if there be on the differing parts of the surface of a fluid Body unequal pressures 't is plain as well by the nature of the thing as by what has been demonstrated by Archimedes and his Commentators that the greater force will prevail against the lesser and that that part of the waters surface must give way where it is least prest So that that wherein the Hypothesis I venture to propose to you differs from that which I dissent from is not that mine is less Mechanical but partly in this that whereas the Hypothesis I question supposes a necessity of the protrusion or impulse of the Air mine does not require that supposition but being more general reaches to other ways of procuring the Ascension of Liquors without raising them by the impulse of the Air and partly and indeed chiefly in that the Hypothesis I decline makes the Cause of the Ascension of Liquors to be only the increased pressure of the Air external to the pipe and I chiefly make it to depend upon the diminished pressure of the Air within the pipe on the score of the expansion 't is brought to by Suction To proceed now to some Experiments that I made in favour of this Hypothesis I shall begin with that which follows We took a Glass-pipe bended like a Syphon but so that the shorter legg was as parallel to the longer as we could get it made and was Hermetically seal'd at the end Into this Syphon we made a shift for 't is not very easie to convey water so that the crooked part being held downwards the liquor reach'd to the same height in both the leggs and yet there was about an Inch and half of uncomprest Air shut up in the shorter legg This little Instrument for 't was but about fifteen Inches long being thus prepar'd 't is plain that according to the Hypothesis I dissent from there is no reason why the water should ascend upon Suction For though we should admit that the external Air were considerably comprest or received a notable impulse when the Suckers chest is enlarged yet in our case that compression or protrusion will not reach the surface of the water in the shorter legg because it is there fenc'd from the action of the external Air by the sides of the Glass and the Hermetical Seal at the top And yet if one suck'd strongly at the open orifice in the longer legg the water in the shorter would be deprest and that in the longer ascended at one suck about an Inch and half Of which the reason is clear in our Hypothesis For the Spring of the included Air together with the weight of the water in the shorter legg and the pressure of the Atmospherical Air assisted by the weight of the liquor in the longer legg counter-ballanced one another before the Suction began But when afterwards upon Suction the Air in the longer legg came to be dilated and thereby weaken'd 't was render'd unable to resist the undiminish'd pressure of the Air included in the shorter legg which consequently expanding it self by vertue of its Elasticity deprest the contiguous water and made it proportionably rise in the
our case there ought to be a great deal of difference between the operation 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 Liquors when the Suction is made in the open Air continues still the same but the force or pressure of the included 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 further and further expanded must needs have its Spring proportionably weaken'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 Cylinder of the Atmospherical Air equivalent to about thirty Inches of Quicksilver 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 neighbouring 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 already lost a very considerable part of its former Spring and Pressure I should here conclude this Discourse but that I remember a Phaenomenon of our Pneumatic Engin which to divers Learned Men especially 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 account 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 Phaenomena 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 finger 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 protuberance of the pulp of the finger to be effected not by pressure as we would have it but distinctly by Attraction 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 finger 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 undiminish'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 convenient 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 admit 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 somewhat beneath it This done we put the cover'd end of the Pipe into a Glass-body or Cucurbit purposely made more than ordinarily 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 liquor was level to it or a little higher within the Pipe the Bladder at the lower orifice was kept plump because the water within the Pipe did by its weight press as forcibly downwards as the external water in the large Glass endeavour'd to press it inwards 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 upon which removal of part of the internal 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 external 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 therein so strongly that the distended Bladder made a kind of either Thimble