The works of the Honourable Robert Boyle, Esq., epitomiz'd by Richard Boulton ... ; illustrated with copper plates.

CHAP. XIV. The Doctrin of the Spring and Weight of the Air defended against Franciscus Li∣nus.

THIS Chapter containing the Honourable Author's Vindication of his own Hypothe∣sis, concerning the Weight and Spring of the Air: It will not be requisite here to lay down, all the trivial Objections of Franciscus Linus; which are easily answer'd, by any one that hath but read and consider'd the Author's Hypothesis, and will at the first sight appear to be ill-ground∣ed or invalid Arguments, or rather false Criti∣cisms: But, it being more consistent with the design'd Epitomy, I shall only take notice of such, as may serve to illustrate the Hypothesis al∣ready laid down, and think it unnecessary to fill an Epitomy with, what our judicious Author did not repute as Arguments against his Doctrin: And especially, because the Learn'd World, by their General Consent have agreed upon what the Ho∣nourable Author hath taught: Wherefore I shall lay down only those Objections which may seem considerable, and annex their Solutions.

* 1.1The first Argument alledg'd by Franciscus Li∣nus, is, That, did the Suspension of the Mer∣cury in the Cylinder, depend on the Aequilibri∣um of the Weight of the Atmosphere; when a Cylinder is fill'd full and inverted, upon an In∣version of the Tube, and a Subtraction of the lower Finger, the Mercury would not subside,

were its descent not promoted by the Dila∣tation of the Air above the Mercury; for if it were, that Expansive Force would raise the Finger upon the upper Orifice of the Cylin∣der.

To which is answer'd, That the included Air may depress the Mercury by it's Expansion down∣wards, without raising the Finger; for the Weight of the whole Cylindrical Pillar of the Atmosphere, which presses upon the top of the Finger, being too strong to be overpower'd by the weak Expansion of the Air in the Pipe, it must consequently press downwards, where the Weight of the Mercury is ready to joyn with it to over-balance the Pressure of the Air, which before buoy'd it up: And that the Weight of the Cylinder of Mercury disposes it to spend its expansive Force downwards, appears from a Cylinder of Water suspended instead of Mer∣cury; for the Weight of that being not able to resist the Pressure of the Atmosphere downwards, equally as the Mercury; the Air in the Top of the Cylinder does not press it down near so far: And tho' the Objector says, that if Water in∣stead of Air be above the Mercury, it will not subside; the Reason of that is only, because the Water wants a Springiness to expand it self: And as for the Reason why the Pulp of the Finger seems to be drawn down into the Tube, that on∣ly happens, by Reason of the Weight of the Incumbent Atmosphere, the Rarifi'd Air within, not being able to make an equal Resistance to prevent the Finger from being press'd into the Pipe: And tho the Objector further urges, that he cannot conceive how the Air can be so expand∣ed

without possessing a greater Space, we have sufficiently clear'd that, by comparing the Ex∣pansion of the Air's Spring to a piece of Wooll squeez'd together, which when it is no longer Compress'd, dilates it selt by Virtue of its springy Parts.

* 1.2The second Objection is, That if Mercury be suspended in a long Pipe, about 40 Inches long, when the Tube is inverted, and the Finger is taken from the lower Orifice, it falls down till it comes to its usual Station; and the Pulp of the Finger is equally depress'd into the Tube; which evinces, that the Mercury is suspended by a Funiculus, which draws down the Pulpy Part of the Finger.

But, what Answer is to be given to this Ob∣jection, [Answer'd.] appears from what hath been said to the former; for the Mercury subsiding, till it came to an Aequilibrium with the Atmosphere; the Air above it being very much Rarifi'd, and unable to buoy up against the Finger; the Depression of the Pulp of it must proceed from the Weight of the incumbent Atmosphere, tho' the swelling of the Pulp in the Pipe might perhaps partly pro∣ceed from an Expansion of the Humours within the Substance of it, the Weight of the Atmo∣sphere being prevented from squeezing upon it by the Equivalent Weight of the Mercury.

* 1.3Another Objection is, That if a Tube twen∣ty Inches long, be immers'd in Quicksilver, and when it is full, the upper Orifice of it be stopp'd with ones Finger, as well as the lower, upon taking away the lower Finger, the Pulp of the upper will be drawn down, tho' the Weight of the Atmosphere is said to be able to buoy up a Cy∣linder

much longer; So that this is a plain De∣monstration of the Suspension of the Mercury by a Funiculus, and that it is not buoy'd up by the Pressure of the Atmosphere according to the Elaterists.

But tho' this Objection may seem to represent some Difficulty, yet it will easily be answer'd, by thus explaining the Phaenomenon, viz. Tho' when the Tube is stopp'd at both Ends, the Finger upon the Top of the Tube be e∣qually pres'sd upon on both sides; yet, upon a Removal of the Finger below, tho' the Pressure of the Air upon the Finger continu'd to be the same, yet the Pressure of the Mercury is by two Thirds less, against the Pulp of the Finger, its Gravity pressing upon the subjacent Air; for notwithstanding the Air is able to buoy up a greater Quantity, yet that in Part takes off the full Pressure of it, so as to make the Pressure upwards, disproportionable to that which tends downwards: And tho' the Objector urges, that the Mercury is equally suspended upon a Remo∣val of the Finger, as before; I answer, that the Finger presses it up much more strongly, to pre∣vent any of it getting out; whereas the Pressure of the Air only depends on the Weight of it, which cannot be intended or weaken'd, as that of the Finger may, which is capable of raising a much greater Weight.

* 1.4Another Objection alledg'd by the Author is, that were the Mercury buoy'd up by the Weight of the Atmosphere, it would be as easy to suck up Mercury in a Pipe as Water, nothing more being requisite but to draw away the Air.

But this may easily be taken off by consider∣ing, that the Dilatation of the Lungs depend∣ing upon an Ingress of Air upon the Motion of the Diaphragm and Intercostal Muscles, and that Air being press'd in by the Gravity of the Air and it's Spring, it will follow, that that Gravity or Spring being more powerfully re∣sisted by the Cylinder of Mercury than Water, the Lungs cannot so powerfully dilate; nor can the Mercury be rais'd with equal ease, because, that Air in the Lungs, together with the Weight of the Mercury, brings the External Force of the Atmosphere nearer to an Aequilibrium. Be∣sides, the External Air pressing upon the Thorax, and keeping them from dilating themselves, the Air contain'd in them, pressing against the Su∣perficies of the Mercury in the Tube, is a fur∣ther Cause, why it rises not so high; which Ex∣planation is confirm'd by observing, that as the Lungs are more violently dilated, the Mercury rises higher in the Tube; and it will be further Probable, by considering, how much greater a Pressure may be made upon so large a Superfi∣cies as that of the Thorax, than can be resisted by the Rarifi'd Air in the Lungs. And as for the Funiculus, which our Author supposes to raise the Mercury, by contracting it self; it would not be a little strange, that it should raise such a Weight as a Cylinder of Mercury, without tear∣ing off the thin Membranes of the Lungs to which such a Funiculus must, at one end be joyn'd.

To these Objections the Author adds an In∣ference, that if the Spring of the Air be of no Force, in loco aperto, no more can be effected by

it in loco Clauso; but the Place which the Obje∣ctor takes to be in loco Clauso, appearing to be only in a Room; It is answer'd, that whereas the former Objectoins appear to be invalid, the Inference must be so too; because there be∣ing a Communication betwixt the Air in a Room and the External, by some Holes or Crannies, or otherwise, the included Air will be able to do as much by its Spring, within as without.

Notes

• * 1.1

  The first Argument of Francis∣cus Linus Answer'd.
• * 1.2

  A snd Objection.
• * 1.3

  A third. Objection answer'd.
• * 1.4

  Another Objection answer'd.