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

CHAP. I. (Book 1)

I Might, Sir, save my self some trouble in giving you that ac∣count 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 'tis almost out of Print, and has not for some years been in my own; and be∣cause 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 pro∣posed, was very incidentally and briefly discours'd of, upon an occa∣sion 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 occa∣sional 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 some∣thing 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 un∣touch'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 op∣posite to it; yet I confess, I concur in opinion, though not altogether upon the same grounds, with some modern Naturalists, that think At∣traction 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 deno∣minations 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 Mo∣vent or approach to it, we call it

Attraction. But this difference may consist but in an accidental respect, which does not Physically alter the na∣ture 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 Mo∣vent. And that which seems to me to have been the chief cause of mens mistaking Attraction for a motion op∣posite to Pulsion, is, that they have look'd upon both the moving and moved Bodies, in too popular and su∣perficial 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 up∣on 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 thru∣sting 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 be∣hind the anteriour part of the se∣cond 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 per∣ceive, that some part of the last Link comes behind some part of that Bo∣dy, 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 u∣sually belongs to that kind of it, which, for distinctions sake, is cal∣led 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 dif∣ficulty, that I foresee may be speci∣ously urged against this account of Attraction. For it may be said, that there are Attractions, where it can∣not be pretended, that any part of the Attrahent comes behind the At∣tracted Body; as in Magnetical and Electrical Attractions, and in that which is made of Water, when 'tis 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 Philoso∣phers, have recourse on this occa∣sion either to screwed Particles and other Magnetical Emissions, to ex∣plicate 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 At∣tractions 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 concei∣ved, may be elsewhere consider'd. And you will, I presume, the free∣lier allow me this liberty, if, (since

in this place 'tis proper to do it,) I shew you, that in the last of the in∣stances 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 Attra∣ction made of the contiguous and sub∣jacent 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 con∣sequently uncapable of exercising any Attraction; yet, provided the supe∣riour Air were kept off from the Wa∣ter by any other way as well as 'twas by the Rammer, the Liquor would as well ascend into the Cavity of the Barrel; since, (as I have else∣where abundantly proved,) the sur∣face of the Terraqueous Globe being continually press'd on by the incum∣bent

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 Sup∣position 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 Expe∣riment, whereby I have more than once shewn some curious persons, that, if the external Air, and conse∣quently 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 re∣sistance keep shut a Door, that is neither lock'd nor latch'd, against another, who with equal force en∣deavours 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 pro∣perly 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 there∣upon 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 Bar∣rel. For if then you let go the Ram∣mer, you will find it impell'd down∣wards by the incumbent Air with a notable force.