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

CHAP. III. (Book 3)

HAving thus shewn, that the Ascension of Water upon Su∣ction may be caus'd otherwise than by the Condensation or the propaga∣ted Pulsion of Air contiguous to the Suckers Thorax, and thrust out of place by it; it remains that I shew, (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 bet∣ter 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 Su∣ction.

To clear the way to the right un∣derstanding

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 E∣lasticity or Springiness, whereby the lower parts, comprest by the weight of the upper, incessantly en∣deavour to expand themselves, by which expansion, and in proportion to it, the Spring of the Air is wea∣ken'd, (as other Springs are wont to be) the more they are permitted to stretch themselves.

Next, we suppose, that the Ter∣raqueous 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 Experi∣ments found to be equivalent to a perpendicularly erected Cylinder of about twenty nine or thirty Inches of Quicksilver, (for the height is diffe∣ring, as the gravity of the Atmosphere happens to be various.)

Lastly, we suppose, that, Air be∣ing contain'd in a Pipe or other hol∣low 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 ex∣pansion, will have an elasticity equi∣valent 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 com∣press it further, it would have done so, and then the closing of the ori∣fice, at which the internal and ex∣ternal 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 com∣pression as the weight of the Atmo∣sphere could bring it to, so, being shut up and thereby kept from wea∣kening that pressure by expansion, it must retain a Springiness equi∣pollent to the pressure 'twas expos'd to before, which (as I just now no∣ted) was as great as the weight of the incumbent Pillar of the Atmo∣sphere could make it. But if, as was said in the first Supposition, the included Air should come to be dila∣ted or expanded, the Spring being then unbent, its Spring, like that of other elastical Bodies, would be de∣bilitated answerably to that expan∣sion.

To me then it seems, that, spea∣king 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 in∣to the orifice of the Pipe, proceed from the weight of the Atmosphere, or the propagated compression or im∣pulse 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 Hy∣pothesis, it seems very consonant to the Mechanical Principles. For, if there be on the differing parts of the surface of a fluid Body unequal pres∣sures, 'tis plain, as well by the na∣ture 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 sur∣face must give way, where it is least prest. So that that, wherein the Hy∣pothesis I venture to propose to you,

differs from that which I dissent from, is not, that mine is less Me∣chanical; 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 Li∣quors, 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 exter∣nal to the pipe; and I chiefly make it to depend upon the diminished pres∣sure of the Air within the pipe, on the score of the expansion 'tis brought to by Suction.

To proceed now to some Experi∣ments 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 Herme∣tically seal'd at the end: Into this Sy∣phon we made a shift (for 'tis not very easie) to convey water, so that the crooked part being held down∣wards, 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 'twas but about fifteen Inches long) being thus prepar'd, 'tis plain, that accor∣ding 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 com∣prest, 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, be∣cause 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, 'twas ren∣der'd unable to resist the undimi∣nish'd pressure of the Air included in the shorter legg, which consequent∣ly expanding it self by vertue of its Elasticity, deprest the contiguous water, and made it proportionably rise in the opposite legg, 'till by the expansion its Spring being more and more weaken'd, it arrived at an equi∣pollency with the gravitation or pres∣sure of the Atmosphere. Which last clause contains the Reason, why, when

the person that suckt had rais'd the water in the longer legg less than three Inches higher by repeated endeavours to suck, and that without once suffe∣ring the water to fall back again, he was not able to elevate the water in the longer, so much as three Inches above its first station. And if in the shorter legg there was but an Inch and a quarter of space left for the Air unfill'd by the water, by divers skilfully reiterated acts of Suction he could not raise the liquor in the longer legg above two Inches; be∣cause by that time the Air included in the shorter legg had, by expanding it self further and further, proportio∣nably weaken'd its Spring, 'till at length it became as rarified; as was the Air in the cavity of the longer legg, and consequently was able to thrust away the water with no more force than the Air in the long legg was able to resist. And by the reci∣ted tryal it appear'd, that the rare∣faction usually made of Air by Suction is not near so great, as one would

expect, problably because by the di∣latation of the Lungs the Air, being still shut up, is but moderately rarified, and the Air in the longer legg can by them be brought to no greater de∣gree of rarity, than that of the Air within the Chest. For, whereas the included Air in our Instrument was not expanded, by my estimate, at one suck to above the double of its former dimensions, and by divers suc∣cessive sucks was expanded but from one Inch and an half to less than four Inches and an half, if the Suction could have been conveniently made with a great and stanch Syringe, the rarefaction of the Air would proba∣bly have been far greater; since in our Pneumatick Engin Air may, without heat, and by a kind of Su∣ction, be brought to possess many hundreds of times the space it took up before. From this rarefaction of the Air in both the leggs of our In∣strument proceeds another Phaenome∣non, readily explicable by our Hypo∣thesis. For if, when the water was

impell'd up as high as the Suction could raise it, the Instrument were taken from the Suckers mouth, the elevated water would with violence return to its wonted station. For, the Air, in both the leggs of the In∣strument, having by the Suction lost much of the Spring, and so of its power of pressing; when once the orifice of the longer legg was left open, the Atmospherical Air came again to gravitate upon the water in that legg, and the Air, included in the other legg, having its Spring de∣bilitated by the precedent expansion, was not able to hinder the external Air from violently repelling the ele∣vated water, 'till the included Air was thrust into the space it possess'd before the Suction; in which space it had Density and Elasticity enough to resist the pressure, that the external Air exercis'd against it through the interpos'd water.

But our Hypothesis about the Cause of Suction would not need to be soli∣citously prov'd to you by other ways,

if you had seen what I have some∣times been able to do in our Pneu∣matick Engin. For, there we found by tryals purposely devis'd, and care∣fully made, that a good Syringe be∣ing so conveyed into our Receiver, that the open orifice of the Pipe or lower part was kept under water, if the Engin were exhausted, though the handle of the Syringe were drawn up, the water would not follow it, which yet it would do if the exter∣nal Air were let in again. The Rea∣son of which is plain in our Hypothesis. For, the Air, that should have prest upon the surface of the stagnant wa∣ter, having been pumpt out, there was nothing to impell up the water into the deserted cavity of the Sy∣ringe, as there was when the Recei∣ver was fill'd with Air.