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

EXPERIMENT XVII. The Gravity and Rarefaction of the Air examin'd, by the Torrecellian Experiments; together with Considerations concerning a Vacuum, &c.

* 1.1COnsidering that it would be a very probable Method, to inform us, how far the Pressure of the Atmosphere, contributes to the buoying up of Mercury, in a Cylinder, so many Digits above the Surface of the Mercury it leans on, to try, whether in our Engine, as the Air was more or less exhausted, the Mercury in the Tub would accordingly subside; we fill'd a Glass-Cylinder Hermetically seal'd with Mercury, whose Bore was about a quarter of an Inch Diameter, and 3 Foot long; which being inverted in an oblong Box, and carefully let down into the Receiver; and the Cover lutedon, the Tube came through the Hole in the midst of the Cover, the Cranies be∣ing fill'd with melted Diachylon; where it was

observ'd, That the Mercury remain'd elevated to the same height, as if the Foot of the Tube had been press'd upon, by the Atmosphere, the Air included in the Receiver being in a compres∣sed State, and acting by Virtue of it's Spring.

The Cylinder of Mercury being thus plac'd in the Receiver, we caus'd a Cylinder of Air to be pump'd out, and after that, another, observing the Mercury after each, gradually to subside; till at the last, it subsided a considerable way in the Pipe, down into the Body of the Receiver: and to put it beyond dispute, that the Subsiding of the Mercury, depended upon the weaken'd Spring of the included Air, and the Aequili∣brium betwixt the Pressure of that, and the Gra∣vity of the Mercury; we turn'd the Key, and permitted Air gradually to re-enter; and ob∣serv'd, That the Mercury accordingly rose in the Pipe, almost to the height which it was first su∣spended at; and the Reason why it was not rais'd quite so high, was, because several Bubbles of Air, which were before mix'd with the Mer∣cury, rising above it, depress'd, and kept it from ascending, by taking up more room in the Top of the Cylinder.

In trying of which Experiment, having once made use of Diachylon Plaster, to stop up the Top of the Tube, we found, that tho' the Mer∣cury subsided considerably upon the Operation of the Pump; yet the Air so powerfully insi∣nuated it self through the Diachylon Plaster, that it sunk in the Pipe insensibly of it self.

Another thing observable in this Experiment, was, that if, when the Air was exhausted, it were again permitted to run in too fast, it would

go near to break the Cylinder, by forcing the Mercury up too quick.

Besides we observ'd, as we have before inti∣mated, That tho' upon the First Exsuctions, the Mercury subsided above an Inch; yet when it was low in the Receiver, it would not subside over the Breadth of a Barley-corn; besides, by reason of the Parts of the Air, tending to a Restitution of their Springs, it check'd and caus'd the Quick∣silver after it's descent, to fly back again a little after each Exsuction.

Another thing remarkable in this Experiment, is, That having try'd it in one of our small Re∣ceivers, we found, that at one Exsuction it fell 18 Inches and a half, and at another time 19 and a half; from whence we may infer, that it might be of no small use to consider, the various Phae∣nomena afforded by the different Magnitude of the Receivers, in which these Experiments are made; and also the various Bores of the Cylin∣ders, which contain the Mercury; for thence one might probably guess at the Quantity of Air ex∣tracted out of the Receiver, by the Subsiding of the Mercury in the Tube.

To the foregoing Observations, we shall add, That having once try'd the Experiment, in a Tube about two foot long, I found, that upon the first Exsuction, the Mercury fell above a Span, and afterwards subsided by degrees, till the Air was re-admitted, and then it was rais'd near to the top of the Tube again.

Lastly, It hath been observ'd, That when, by the help of the Pump, more Air hath been squeez'd into the Receiver, than what was forc'd in by the Weight of the Atmosphere, the Mercury

would rise above 27 Digits in the Tube, and would be suspended at that height, till the Air was again let out.

* 1.2Having said thus much of the Torrecellian Ex∣periment, and the several Phaenomena observable in it, perhaps it may be expected, that I should say something, concerning that Controversy, Whether or no there be a Vacuum in the Re∣ceiver, when the Air is pump'd out: But being unwilling to take it upon me to determine so nice a Controversy; I shall only intimate, that were the Cavity of the Receiver void of Matter, it would be a difficult thing to conceive, how we should have visible Idea's of things contain'd in it; since they must either be convey'd to our Sen∣sory by the reflected Rays of Light, or they must proceed from some sensible Emanations from the Bodies themselves. And from the foregoing Six∣teenth Experiment, it appears, That the Effluvia of a Load-stone do permeate the Cavity of the Receiver, which makes it probable, that the Magnetical Steams of the Earth do so too.

But on the other hand, it may be alledg'd, That since the Effluvia of a Load-stone may be ad∣mitted into the Receiver, when it seems full of Air, it may be urg'd, that the subtle Emanations of that Substance are only dispers'd through the Vacuities of the Air; and consequently when that Air is exhausted, the Spaces which the Air posses∣sed before, may be left void of Matter; for from the Thirteenth Experiment it appears, that Mat∣ter, which, if closely pinned up together, takes up but a small Space, may be dispers'd through∣out the Receiver, a very small Portion of the Match being able to fill the whole Cavity of it

with Fumes. And as for the Arguments before alledg'd in favour of a Plenitude; they rather seem to argue, that it may be, than that it really is; and the Consequences usually drawn in Proof of such a Plenitude, are rather grounded on the Cartesian Notions of a Body, than on Substantial Experiments; for they thinking it a Contradi∣ction in Adjecto so say, a Space can be void of Body, since they make Body and Extension inse∣parable; they therefore in favour of their own Doctrin draw Inferences from their own Positions to argue for a Plenitude.

But to leave so Nice and Doubtful a Dispute, I shall proceed to lay down the following Adver∣tisements: First, That the Difference which sometimes happens betwixt the rising of Mercury here in England, and beyond Seas, being about two Digits, it rather proceeds from the Short∣nese of our English Inches, compar'd with those beyond Seas, than any such considerable Diffe∣rence in the Weight of the Atmosphere. Ano∣ther thing to be advertised is, That except more than ordinary Care be taken, when the End of the Cylinder is immers'd in the Vessel of Mercury, several Bubbles of Air will rise to the Top of the Tube, which, being condensed with Cold, will permit the Mercury to rise higher in the Tube; but being expanded with Heat, would sensibly depress it. And,

If it should be demanded, What Methods are to be taken to prevent the Intrusion of the Air, the following Requisites are to be observ'd: First, the lower Orifice of the Cylinder must be smooth, and so much inverted on each side, as not to exceed a quarter of an Inch in Diameter.

Secondly, the Tube must be fill'd so full, as not to admit of any Air, betwixt the Finger and the Mercury. Thirdly, before the Tube is quite fill'd, it will be requisite to invert the Tube, that the Air included in it, may, by ascending to the Top, and upon a Re-inversion back again, gather all those little Bubbles of Air, which are dispers'd through the Mercury, as it passes from one end of the Cylinder to another; and to drain the Mercury more exactly you may, by applying a hot Iron, cause those less Bubbles, which are not excluded by the former Method to break forth, which will be promoted by shaking the Vessel upon every Drop of Mercury, put into the Pipe. By which Method, having in a great Measure clear'd the Tube of Air, I have, in one, that was pretty short, rais'd the Mercury to no less than 30 Digits and an eighth.

Notes

• * 1.1

  The Gravi∣ty of the Air, &c.
• * 1.2

  Concerning a Vacuum.