perpendicularly raife the upper marble from the lower, needs a confider- PNEUMATICS able force to make the revulfion, and compel the contiguous parts of the incumbent air, to enter the pores, or intervals intercepted between them. For, the force of him, who endeavours to raise the upper marble, whilft the lower furface of it is fenc'd from the preffure of the atmofphere, by the contact of the lower, which fuffers no air to come in between them, is not affifted by the weight, or preffure of the atmosphere; which, when the marbles are once feparated, preffing as strongly against the under fur'face of the upper marble, as the incumbent atmospherical column does against the upper furface of the fame marble; the hand which endeavours to raise it, in the free air, has no other resistance, than that small one of the marble's own weight to furmount. And as for the reason which Mr. Hobbs, and, as he thinks, all others give of the difficulty of this divulfion, "that the whole space between "the two separated marbles, cannot be inftantaneously filled by the air, "let the feparation be ever fo fudden;" the plenists may give a more plaufible account of this experiment, than he has here done; and, therefore abftracting from the two oppofite hypothefes, I may fay, that the genuine cause of the phenomenon seems to be that which I have already affign'd; and that the difficulty of raising the upper ftone, attending the air's not being able to come in all at once, to poffefs the space left between the surfaces of the two marbles, upon their feparation, proceeds from hence; that, till this space be fill'd with the atmospherical air, the hand, which would raise the upper marble, cannot be fully affifted by the preffure of the air, against the lower furface of that marble: for, fince I declare not for the hypothefis of the plenifts, as it is maintained by Mr. Hobbs, I am not bound to allow, what the common explanation, adopted by my adversary, fuppofes, that either nature abhors a vacuum; or that there could be no divulfion of the marbles, unless at the same time, the air were admitted into the space that divulfion makes for it. And a vacuift may fay, provided the strength employed, to draw up the upper marble, be able to furmount the weight of the aerial corpufcles, accumulated upon it; the divulfion would enfue, though no air, or other body, should be permitted to fill the room made for it, by the divulfion; and that the air's rufhing into this space, does not, neceffarily, accompany, but, in order of nature, and time, follow, upon a feparation of the marbles; the air that furrounded their contiguous furfaces, being, by the weight of the collateral air above, impelled into the room newly made by divulfion. But, having purposely, in our pneumatic receiver, accommodated two flat and polished marbles, fo that the lower being fix'd, the upper might be laid upon it, and drawn up again, as there fhould be occafion; I found, that if, when the receiver was well exhausted, the upper marble was, by a certain contrivance, laid flat upon the lower, they would not then cohere, as before, but. with great ease, be feparated; tho' it did not, by any phenomenon, appear, that any air could rufh in, to poffefs the place given it by the recefs of the upper marble; whose Uuuu 2 very PNEUMATICS very eafy avulfion, feemed owing to the preffure of that little air remaining in the receiver, being too faint to make any confiderable resistance to the feparation of the upper marble; whence the hand, that drew it up, had very little more to overcome, than the fingle weight of the ftone. But the cause Mr. Hobbs affigns for this phenomenon, is improbable; and a better has been affign'd already. For, firft, he requires, to the divulfion of the marbles, a force great enough to furmount the hardness of the ftone. But, this is gratis dictum; and, it feems very unlikely, that fo fmall a weight, as will fuffice to feparate two coherent marbles, of about an inch, for instance, in diameter, fhould be able to furmount the hardnefs of fuch folid ftones, as we ufually employ in this experiment. And, tho' it be generally, judg'd more eafy to break a broader piece of marble cæteris paribus, than a much narrower; yet, neither I, nor, I believe, Mr. Hobbs, ever obferv'd any difference in the resistance of marbles to feparation, arifing from the greater or lefs thickness of the ftones. Yet, I find, by conftant experience, that, cæteris paribus, the broadness of the coherent marbles, exceedingly increases the difficulty of disjoining them. But Mr. Hobbs, upon the fuppofition of the world's plenitude, illuftrates our phenomenon, by drawing asunder the oppofite parts of a piece of wax: a very improper inftance, furely! For, the parts to be divided in the wax, are of a foft, and yielding confiftence, and according to him, of a ductile nature; and not, as the parts of the coherent marbles, very folid and hard. The parts of the wax, alfo, do not ftick together, barely, by a fuperficial contact of two fmooth planes, as do the marbles we are fpeaking of; but have their parts intangled with one another; and, therefore they are are far from a difpofition to flide afunder, like the marbles. Befides, it is manifeft, that the air has opportunity to fucceed in the places fucceffively deferted by the receding parts of the attenuated wax: but it is neither manifeft, nor well proved by Mr. Hobbs, that the air, after the fame manner, fucceeeds between the two marbles; which, as I lately noted, are not forced afunder, after fuch a manner; but, as himself fpeaks, fever'd, in all their points, at the fame instant. In the fecond place, a better folution of the phenomenon has been already given from the preffure of the atmosphere, upon all the fuperficial parts of the upper marble,, except thofe that touch the plane of the lower, And to fhew, that when two coherent marbles are fuftained, horizontally, in the air, the cause why they are not to be forc'd asunder, if they have two or three inches in diameter, without the help of a confiderable weight, is the preffure of the ambient air; I caufed two fuch coherent marbles to be fufpended in a large receiver, with a weight at the lower, that might help to keep them fteddy; but was very inconfiderable to that which their cohefion could have furmounted: then caufing the air to be pumped, by degrees, out of the receiver, the marbles long ftuck clofe together; becaufe during that time, the air could not be fo far exhaufted, but that there remained enough to fuftain the fmall weight which endeavour'd their divulfion. But, when the air was further evacuated, at length, the fpring of the little expanded air that remained, being grown too weak to fuftain PNEUMATICE the lower marble, and its finall clog, they dropt off. And to confirm my explanation of our phenomenon, I fhall add, that as this trial, which I had feveral times occafion to repeat, fhews the cohesion of our two contiguous marbles would ceafe, upon withdrawing the preffure of the atmofphere; fo by another experiment it appears, that fuch a preffure fufficed to cause that cohesion; for having found, that when the receiver was well exhausted, two marbles, tho' confiderably broad, being laid upon one another, after the requifite manner, their adhesion became fo weak, that the upper would be easily drawn up from the other, we laid them again one upon the other; and then letting the external air flow into the receiver, we found that the marbles now adhered fo well together, that we could not raife the upper without the lower. But farther, Mr. Hobbs, in arguing against our engine, unjustly fupposes the whole defign of it to be the proof of a vacuum; for or a gainft which I have never yet declared.* Befides, he seems not to have rightly understood, or at least not to have fufficiently regarded, in what chiefly confifts the advantage which the vacuifts may make of the air-pump against him. In feveral places he is very follicitous to prove, that the cavity of our pneumatical receiver is not altogether empty; but the vacuifts may tell him, that fince he afferts the abfolute plenitude of the world, he must reject, not only great vacuities, but also thofe very small and interfperfed ones, which they fuppofe are intercepted between the folid corpufcles of other bodies, and particularly The moft cautious and referv'd philofopher need now make no fcruple to affert a vacuum. Sir Ifaac Newton has furnish'd us with feveral arguments which are decifive of this point. Those indeed who make the effence of matter to confift in extenfion, may well deny a vacuum; but the nature of gravity, which is proportionable to the matter, or folid content of bodies, fully demonftrates, much the largest part of the universe to be empty space. The motions of the comets also demonstrate a vacuum, for these move in all manner of planes and directions, thro' the vaft celeitial fpaces, without any fenfible refiftance. The vibrations of pendulums confirm the fame; for if they be made to move in a fpace free from air, they likewife meet with no refiftance; and therefore there is no fenfible matter remaining in that space. If there were no vacuum, a projectile in the air, or in a space deftitute of air, would move as flowly as in quick-filver; for the refiftance of fluid mediums is nearly in pro portion to their denfity; and it is a great In another place the fame great philofo- ter arifes principally, and almost entirely, "from the Vis inertia of its matter; and "by confequence, if the heavens were as "denfe as water, they would not have "much lefs refiftance than water; if as "denfe as quick-filver, they would not "have much lefs refiftance than quick filver; PNEUMATICS larly of the air; whence it would not confute them to prove, that in our receiver, when moft thoroughly exhaufted, there is not one great and abfolute vacuity; or, as they fpeak, a vacuum coacervatum; fince smaller and diffeminated vacuities would ferve their turn. And therefore they may think their pretenfions highly favour'd, as by feveral particular effects, fo by this general phenomenon of our engine, that the common, or atmofpherical air, which, before the pump was work'd, poffeffed the whole cavity of our receiver, is, by the contrivance of the pump, made in a great measure to pafs out of the cavity into the open air, without being able, at least for a while, to get in again; yet it does not appear, by any thing Mr. Hobbs has alledg'd, that any other body fucceeds, adequately to fill the spaces deferted by fuch a multitude of aerial corpufcles. Glafs impervious to air. And tho' he endeavours to prove our receiver to be always full of air, yet that the common air cannot enter thro' the pores of glafs, appears by the following experiment. We took a bubble of thin white glass,about the bigness of a nutmeg, with a very flender stem, 4 or 5 inches long, and of the thickness of a crow's quill; and holding the end of this stem in the flame of a lamp, blown with a pair of bellows, it was readily feal'd up; and presently the spherical part of the glass being held by the stem, was kept turning in the flame, till it became red hot, and ready to melt ; then being a little removed from the heat, as the included air began to lofe of its agitation and fpring, the external air manifeftly and confiderably preffed in one of the fides of the bubble. But the glafs being again, "filver; if abfolutely denfe, or full of mat- lofe above half its motion, in moving 66 66 comets, it is neceffary to empty the hea vens of all matter, except perhaps fome 61 very thin vapours, fteams, or effluvia, "arifing from the atmosphere of the earth, "planets, and comets. A denfe fluid can "be of no ufe for explaining the pheno66 mena of nature, the motions of the pla66 nets and comets being better explained "without it. It ferves only to disturb and "retard the motions of those great bodies, " and make the frame of nature languish; "and in the pores of bodies, it ferves only "to ftop the vibrating motion of their parts, wherein their heat and activity "confifts. And as it is of no use, and "hinders the operations of nature, and "makes her languifh; fo there is no evi"dence for its existence, and therefore it "ought to be rejected." "If all space was equally full, the fpe"cific gravity of the fluid, wherewith the "region of the air is filled, would, by rea"fon of the vaft denfity of its matter, be 66 66 as heavy in fpecie as quick-filver, gold, or any other the most dense body; "whence neither gold, nor any other "matter, would defcend in that air; for 66 no bodies defcend in fluids, that are not 66 fpecifically lighter than they." It is farther demonftrable, that all bodies have more pores than folid parts; and that fome have infinitely more pores than others; whence arifes another proof of a vacuum. See Newton. Optic. p. 310, 311, &c. Princip. p. 316, 317. 328. 342, 343. 368. before the cold crack'd it, held, as before, in the flame; the rarified air PNEUMATICS diftended and plump'd up the bubble; which, being the fecond time remov'd, was a fecond time comprefs'd; and being, the third time, brought back to the flame, fwell'd as before, and remov'd, it was again compreffed; till at length, having fatisfied ourselves, that the included air was capable of being condens'd, or dilated, without the ingrefs or egrefs of air, properly fo called, we held the bubble fo long in the flame, ftrenghtened by nimble blasts, that not only it had its fides plump'd up, but a hole violently broken in it, by the over-rarified air, tho' till then, it was no way crack'd. Hence, it easily appears, how impervious our thick pneumatic receivers are to common air; fince a thin glass bubble, when its pores were opened, or relaxed by flame, would not give paffage to the springy particles of the air, tho' violently agitated: for, if thefe particles could have, got out of the pores, they never would have broke the bubble; nor, probably, would the compreffion, that afterwards enfued, of the bubble, by the ambient air, be check'd near fo foon, if those springy corpufcles had not remained within to make refiftance. rarifaction Yet, what I principally defign'd, in this experiment, was to fhew, and The Nature of prove at once, by an inftance not liable to the ordinary exceptions, the and condenfatrue nature of rarifaction, and condenfation, at leaft, of the air: for, 'tis tion. here plain, that when the bubble, after the glafs had been first thrust in, towards the center, was expanded again by heat, the included air posfeffed more room than before; yet it could perfectly fill no more room than before; each aerial particle taking up, both before and after the heating of the bubble, a portion of fpace adequate to its own bulk: fo that in the cavity of the expanded bubble, we must either admit vacuities interfpers'd between the corpufcles of the air; or allow, that fome fine particles of the flame, or other fubtile matter, came in, to fill up thofe interstices; which matter must have enter'd the cavity of the glafs at its pores. And, afterwards, when the red-hot bubble was remov'd from the flame, it is evi-dent, that fince the groffer particles of the air could not get thro' the glass, which they were unable to do, even when vehemently agitated by an ambient flame, the compreffion of the bubble, and the condenfation of air, neceffarily confequent upon it, cou'd not, fuppofing the plenitude of the world, be perform'd without fqueezing out fome of the fubtile matter, contained in the cavity of the bubble, whence it could not iffue but at the pores of the glass. eye. Mr. Hobbs is pleased to compare our pneumatic engine to a pot-gun, and The preffure of attributes the phenomena, exhibited in the exhaufted receiver, to the ex- the air manipanfive endeavour of the air outwards: I fhall, therefore, fhew, that there feed to the is, in our exhaufted receiver, no fuch strong endeavour outwards, as he fuppofes; but that the weight of the atmospherical air, when 'tis not refifted by the counter-preffure of any internal air, is able to perform what a weight of many pounds would not fuffice for. Glafs not being a yielding body, cannot, by the alteration of its figure, from an external uni form |