TO proceed now to the Phaenomena, exhibited to us by the Engine above described; I hold it not unfit to begin with what doth constantly and regularly offer it self to our observation, as depending upon the Fabrick of the Engine it self, and not upon the nature of this or that particular Experiment which 'tis employed to try.

First, then, upon the drawing down of the Sucker (the Valve being shut) the Cylindrical space, deserted by the Sucker, is left devoid of Air; and therefore, upon the turning of the Key, the Air contained in the Receiver rusheth into the emptied Cylinder, till the Air in both those Vessels be brought to about an equal measure of dilatation. And therefore, upon shutting the Receiver by re-turning the Key, if you open the Valve, and force up the Sucker again, you will find, that after this first exsuction you will drive out almost a whole Cylinder full of Air: But at the following exsuctions, you will draw less and less of Air out of the Receiver into the Cylinder, because there will still remain less and less Air in the Receiver it self; and consequently, the Particles of the remaining Air, having more room to extend themselves in, will less press out one another. This you will easily perceive, by finding, that you still force less and less Air out of the Cylinder; so that when the Receiver is almost exhausted, you may force up the Sucker almost to the top of the Cylinder, before you will need to unstop the Valve to let out any Air: And if at such time, the Valve being shut, you let go the handle of the Pump, you will find the Sucker forcibly carried up to the top of the Cylinder, by the protrusion of the external Air; which, being much less rarified than that within the Cylinder; must have a more forcible pressure upon the Sucker, than the internal is able to resist: And by this means you may know how far you have emptied the Receiver. And to this we may add, on this occasion, that constantly upon the turning of the Key to let out the Air from the Receiver, into the emptied Cylinder, there is immediately produced a considerably brisk noise, especially whilst there is any plenty of Air in the Receiver.

For the more easie understanding of the Experiments triable by our Engine; I thought it not superfluous, nor unseasonable in the recital of this first of them, to insinuate that notion by which it seems likely that most, if not all of them, will prove explicable. Your Lordship will easily suppose, that the Notion I speak of is, That there is a Spring, or Elastical power in the Air we live in. By which 〈 in non-Latin alphabet 〉 or spring of the Air, that which I mean is this: That our Air either consists of, or at least abounds with, parts of such a nature; that in case they be bent or compress'd by the weight of the incumbent part of the Atmosphere, or by any other Body, they do endeavour, as much as in them lieth, to free themselves from that pressure, by bearing against the contiguous Bodies that keep them bent; and, as soon as those Bodies are remov'd or reduced to give them way, by presently unbending and stretching out themselves, either quite, or so far forth as the contiguous Bodies that resist them will permit, and thereby expanding the whole parcel of Air, these elastical Bodies compose.

This Notion may perhaps be somewhat further explain'd, by conceiving the Air near the Earth to be such a heap of little Bodies, lying one upon another, as may be resembled to a Fleece of Wooll. For this (to omit other likenesses betwixt them) consists of many slender and flexible Hairs; each of which may indeed, like a little Spring, be easily bent or rouled up; but will also, like a Spring, be still endeavouring to stretch it self out again. For though both these Hairs, and the Aereal Corpuscles to which we liken them, do easily yield to external pressures; yet each of them (by virtue of its structure) is endow'd with a Power or Principle of self-Dilatation; by virtue where of, though the hairs may by a Man's hand be bent and crouded closer together, and into a narrower room than suits best with the nature of the Body, yet, whilst the compression lasts, there is in the fleece they compose an endeavour outwards, where by it continually thrusts against the hand that opposes its Expansion. And upon the removal of the external pressure, by opening the hand more or less, the compressed Wool doth, as it were, spontaneously expand or display itself towards the recovery of its former more loose and free condition, till the Fleece hath either regain'd its former Dimensions, or at least, approach'd them as near as the compressing hand (perchance not quite open'd) will permit. This Power of self-Dilatation, is somewhat more conspicuous in a dry Spunge compress'd, than in a fleece of Wool. But yet we rather chose to employ the latter, on this occasion, because it is not like a Spunge, an intire Body, but a number of slender and flexible Bodies, loosely complicated, as the Air it self seems to be.

There is yet another way to explicate the Spring of the Air, namely, by supposing with that most ingenious Gentleman, Monsieur Des Cartes, That the Air is nothing but a Congeries or heap of small and (for the most part) of flexible Particles; of several sizes, and of all kind of Figures which are rais'd by heat (especially that of the Sun) into that fluid and subtle Ethereal Body that surrounds the Earth; and by the restless agitation of that Celestial matter wherein those particles swim, are so whirld round, that each Corpuscle endeavours to beat off all others from coming within the little Sphere requisite to its motion about its own Centre; and (in case any, by intruding into that Sphere, shall oppose its free Rotation) to expel or drive it away: So that according to this doctrine, it imports very little, whether the particles of the Air have the structure requisite to Springs, or be of any other form (how irregular soever) since their Elastical power is not made to depend upon their shape or structure, but upon the vehement agitation, and (as it were) brandishing motion, which they receive from the fluid Ether that swiftly flows between them, and whirling about each of them (independently from the rest) not only keeps those slender Aëreal Bodies seperated and stretcht out (at least, as far as the Neighbouring ones will permit) which otherwise, by reason of their flexibleness and weight, would flag or curl; but also makes them hit against, and knock away each other, and consequently require more room, than that, which, if they were compress'd, they would take up.

By these two differing ways, my Lord, may the Springs of the Air be explicated. But though the former of them be that, which by reason of its seeming somewhat more easie, I shall for the most part make use of in the following Discourse: yet am I not willing to declare peremptorily for either of them, against the other. And indeed, though I have in another Treatise endeavoured to make it probable, that the returning of Elastical Bodies (if I may so call them) forcibly bent, to their former position, may be Mechanically explicated: Yet I must confess, that to determine whether the motion of Restitution in Bodies, proceed from this, That the parts of a Body of a peculiar Structure are put into motion by the bending of the Spring, or from the endeavor of some subtle ambient Body, whose passage may be oppos'd or obstructed, or else it's pressure unequally resisted by reason of the new shape or magnitude, which the bending of a Spring may give the Pores of it: To determine this, I say, seems to me a matter of more difficulty, than at first sight one would easily imagine it. Wherefore I shall decline medling with a Subject, which is much more hard to be explicated, than necessary to be so, by him, whose business it is not, in this Letter, to assign the adequate cause of the Spring of the Air, but only to manifest, That the Air hath a Spring, and to relate some of its effects.

I know not whether I need annex that, though either of the above-mention'd Hypotheses, and perhaps some others, may afford us an account plausible enough of the Air's Spring; yet I doubt, whether any of them gives us a sufficient account of its Nature. And of this doubt, I might here mention some Reasons, but that, peradventure, I may (God permitting) have a fitter occasion to say something of it elsew here. And therefore I shuold now proceed to the next Experiment, but that I think it requisite, first, to suggest to your Lordship what comes into my thoughts, by way of Answer to a plausible Objection, which I foresee you may make against our propos'd Doctrine, touching the Spring of the Air. For it may be alledged, that though the Air were granted to consist of springy Particles (if I may so speak) yet thereby we could only give an account of the Dilatation of the Air in Wind Guns, and other pneumatical Engines wherein the Air hath been compress'd, and its springs violently bent by an apparent external force; upon the removal of which, 'tis no wonder that the Air should, by the motion of restitution expand it self till it hath recovered its more natural dimensions: whereas in our above-mentioned first Experiment, and in almost all others triable in our Engine, it appears not, that any compression of the Air preceded its spontaneous Dilatation or Expansion of it self. To remove this difficulty, I must desire Your Lordship to take notice, that of whatever nature the Air, very remote from the Earth, may be, and whatever the Schools may confidently teach to the contrary, yet we have divers Experiments to evince, that the Atmosphere we live in, is not (otherwise than comparatively to more ponderous Bodies) light, but heavy: And did not their gravity hinder them, it appears not why the streams of the Terraqueous Globe, of which our Air in great part consists, should not rise much higher, than the Refractions of the Sun, and other Stars give men ground to think, that the Atmosphere, (even in the judgment of those Rescent Astronomers, who seem willing to enlarge its bounds as much as they dare,) doth reach.

But lest you should expect my seconding this Reason by Experience; and lest you should object, That most of the Experiments that have been propos'd to prove the gravity of the Air, have been either barely propos'd, or perhaps not accurately tri'd; I am content, before I pass further, to mention here. That I found a dry lambs-bladder containing near about two thirds of a pint, and compress'd by a packthred tied about it, to lose a grain and the eighth part of a grain of its former weight, by the recess of the Air upon my having prickt it: And this with a pair of Scales, which, when the full bladder and the correspondent weight were in it, would manifestly turn either way with the 32 part of a grain. And if it be further objected, That the Air in the Bladder was violently compress'd by the packthred and the sides of the Bladder, we might probably (to wave prolix answers) be furnish'd with a Reply, by setting down the differing weight of our Receiver, when empti'd and when full of uncompress'd Air, if we could here procure Scales fit for so nice an Experiment; since we are informed, that in the German Experiment, commended at the beginning of this Letter, the ingenious Triers of it found, That their Glass Vessel, of the capacity of 32 measures, was lighter when the Air had been drawn out of it, than before, by no less than one ounce and 3/10 that is, an ounce and very near a third: But of the gravity of the Air, we may elsewhere have occasion to make further mention.

Taking it then for granted that the Air is not devoid of weight, it will not be uneasie to conceive, That that part of the Atmosphere wherein we live, being the lower part of it, the Corpuscles that compose it, are very much compress'd by the weight of all those of the like nature that are directly over them; that is, of all the Particles of Air, that being pil'd up upon them, reach to the top of the Atmosphere. And though the height of this Atmosphere, according to the famous Kepler, and some others, scarce exceeds eight common miles; yet other eminent and later Astronomers, would promote the confines of the Atmosphere to exceed six or seven times that number of miles. And the diligent and learned Ricciolo makes it probable, that the Atmosphere may, at least in divers places, be at least fifty miles high. So that according to a moderate estimate of the thickness of the Atmosphere, we may well suppose, that a Column of Air, of many miles in height, leaning upon some springy Corpuscles of Air here below, may have weight enough to bend their little springs, and keep them bent: As, (to resume our former comparison,) if there were fleeces of Wooll pil'd up to a mountainous height, upon one another, the hairs that compose the lowermost Locks which support the rest, would, by the weight of all the Wool above them, be as well strongly compress'd, as if a Man should squeeze them together in his hands, or employ any such other moderate force to compress them. So that we need not wonder, that upon the taking off the incumbent Air from any parcel of the Atmosphere here below, the Corpuscles, whereof that undermost Air consists, should display themselves, and take up more room than before.

And if it be objected, That in Water, the weight of the upper and of the lower part is the same: I answer, That, (besides that it may be well doubted whether the observation, by reason of the great difficulty, hath been exactly made,) there is a manifest disparity betwixt the Air and Water: For I have not found, upon an Experiment purposely made, (and in another Treatise Recorded) that Water will suffer any considerable compression; whereas we may observe in Wind-Guns, (to mention now no other Engines) that the Air will suffer it self to be crouded into a comparatively very little room; in so much, that a very diligent Examiner of the Phaenomena of Wind-Guns would have us believe, that in one of them, by condensation, he reduc'd the Air into a space at least eight times narrower than it before possest. And to this, if we add a noble Phaenomenon of the Experiment De Vacuo; these things put together, may for the present suffice to countenance our Doctrine. For that noble Experimenter, Monsieur Pascal (the Son) had the commendable Curiosity to cause the Torricellian Experiment to be tri'd at the foot, about the middle, and at the top of that high Mountain (in Auvergne, if I mistake not) commonly call'd Le Puy de Domme; whereby it was found, That the Mercury in the Tube fell down lower, about three inches, at the top of the Mountain than at the bottom. And a Learned Man a while since inform'd me, That a great Virtuose, friend to us both, hath, with not unlike success, tried the same Experiment in the lower and upper parts of a Mountain in the west of England. Of which, the reason seems manifestly enough to be this, That upon the tops of high Mountains, the Air which bears against the restagnant Quick-silver, is less press'd by the less ponderous incumbent Air; and consequently is not able totally to hinder the descent of so tall and heavy a Cylinder of Quick-silver, as at the bottom of such Mountains did but maintain an AEquilibrium with the incumbent Atmosphere.

And if it be yet further Objected against what hath been propos'd touching the compactness and pressure of the inferiour Air; That we find this very Air to yield readily to the motion of little Flies, and even to that of Feathers, and such other light and weak Bodies; which seems to argue, that the particles of our Air are not so compress'd as we have represented them, especially, since by our former Experiment it appears, that the Air readily dilated it self downward, from the Receiver into the Pump, when 'tis plain, that it is not the incumbent Atmosphere, but only the subjacent Air in the brass Cylinder that hath been remov'd: If this, I say, be objected, we may reply, That, when a man squeezeth a fleece of Wool in his hand, he may feel that the Wool incessantly bears against his hand, as that which hinders the hairs it consists of, to recover their former and more natural extent. So each parcel of the Air about the Earth, doth constantly endeavour to thrust away all those contiguous Bodies, (whether. Aëreal or more gross,) that keep it bent, and hinder the expansion of its parts, which will dilate themselves, or fly abroad towards that part, (whether upwards or downwards,) where they find their attempted Dilatation of themselves less resisted by the neighbouring Bodies. Thus the Corpuscles of that Air we have been all this while speaking of, being unable, by reason of their weight, to ascend above the Convexity of the Atmosphere, and by reason of the resistance of the surface of the Earth and Water, to fall down lower, they are forced, by their own gravity and this resistance, to expand and diffuse themselves about the Terrestrial Globe; whereby it comes to pass, that they must as well press the contiguous Corpuscles of Air that on either side oppose their Dilatation, as they must press upon the surface of the Earth, and, as it were recoyling thence, endeavour to thrust away those upper particles of Air that lean upon them.

And, as for the easie yielding of the Air to the Bodies that move in it, if we consider that the Corpuscles whereof it consists, though of a springy nature, are yet so very small, as to make up (which 'tis manifest they do) a fluid Body, it will not be difficult to conceive, that in the Air, as in other Bodies that are fluid, the little Bodies it consists of, are in an almost restless motion, whereby they become (as we have more fully discoursed in another Treatise) In a Discourse touching fluidity and firmness. very much disposed to yield to other Bodies, or easie to be displac'd by them; and that the same Corpuscles are likewise so variously mov'd, as they are intire Corpuscles, that if some strive to push a Body plac'd among them towards the right hand (for instance) others, whose motion hath an opposite determination, as strongly thrust the same Body towards the left; whereby neither of them proves able to move it out of its place, the pressure on all hands being reduced as it were to an AEquilibrium: So that the Corpuscles of the Air must be as well sometimes considered under the notion of little Springs, which remaining bent, are in their entire bulk transported from place to place; as under the notion of Springs displaying themselves, whose parts fly abroad, whilst, as to their entire bulk they scarce change place: As the two ends of a Bow, shot off, fly from one another, whereas the Bow it self may be held fast in the Archer's hand; and that it is the equal pressure of the Air on all sides upon the Bodies that are in it, which causeth the easie Cession of its parts, may be argu'd from hence: That if by the help of our Engine the Air be but in great part, though not totally, drawn away from one side of a Body without being drawn away from the other, he that shall think to move that Body to and fro, as easily as before, will find himself much mistaken.

In verification of which we will, to divert your Lordship a little, mention here a Phaenomenon of our Engine, which even to divers ingenious persons hath at first sight seem'd very wonderfull.

THe thing that is wont to be admired, and which may pass for our second experiment is this, That if, when the Receiver is almost empty, a By-stander be desired to lift up the brass Key (formerly described as a stopple in the brass Cover) he will find it a difficult thing to do so, if the Vessel be well exhausted; and even when but a moderate quantity of Air hath been drawn out, he will, when he hath lifted it up a little, so that it is somewhat loose from the sides of the lip or socket, which (with the help of a little oyl) it exactly filled before, he will (I say) find it so difficult to be lifted up, that he will imagine there is some great weight fastned to the bottom of it. And if (as sometimes hath been done for merriment) onely a Bladder be tied to it, it is pleasant to see how men will marvail that so light a Body, filled at most but with Air, should so forcibly draw down their hand as if it were fill'd with some very ponderous thing: Whereas the cause of this pretty Phaenomenon seems plainly enough to be only this, That the Air in the Receiver, being very much dilated, its Spring must be very much weakn'd, and consequently it can but faintly press up the lower end of the stopple, whereas the Spring of the external Air being no way debilitated, he that a little lifts up the stopple must with his hand support a pressure equal to the disproportion betwixt the force of the internal expanded Air, and that of the Atmosphere incumbent upon the upper part of the same key or stopple: And so men being unused to find any resistance, in lifting things up, from the free Air above them, they are forward to conclude that that which depresseth their hands must needs be some weight, though they know not where plac'd, drawing beneath it.

And, that we have not mis-assign'd the cause of this Phaenomenon, seems evident enough by this, That as Air is suffered by little and little to get into the Receiver, the weight that a man fancieth his hand supports, is manifestly felt to decrease more and more, the internal Air by this recruit approaching more to an AEquilibrium with the external, till at length the Receiver growing again full of Air, the stopple may be lifted up without any difficulty at all.

By several other of the Experiments afforded us by our Engine, the same notion of the great and equal pressure of the free Air upon the Bodies it environs, might be here manifested, but that we think it not so fit to anticipate such Experiments: And therefore shall rather employ a few lines to clear up the difficulty touching this matter, which we have observ'd to have troubled some even of the Philosophical and Mathematical Spectators of our Engine, who have wonder'd that we should talk of the Air exquisitely shut up in our Receiver, as if it were all one with the pressure of the Atmosphere; whereas the thick and close body of the Glass, wholly impervious to the Air, doth manifestly keep the incumbent Pillar of the Atmosphere from pressing in the least upon the Air within the Glass, which it can no where come to touch. To elucidate a little this matter, let us consider, That if a man should take a fleece of Wooll, and having first by compressing it in his hand reduc'd it into a narrower compass, should nimbly convey and shut it close up into a Box just fit for it, though the force of his hand would then no longer bend those numerous springy Bodies that compose the Fleece, yet they would continue as strongly bent as before, because, the Box they are inclos'd in, would as much resist their re-expanding of themselves, as did the hand that put them in. For thus we may conceive, that the Air being shut up, when its parts are bent by the whole weight of the incumbent Atmosphere, though that weight can no longer lean upon it, by reason it is kept off by the Glass, yet the Corpuscles of the Air within that Glass continue as forcibly bent, as they were before their inclusion, because the sides of the Glass hinder them from displaying or stretching out themselves. And if it be objected that this is unlikely, because even Glass bubbles, such as are wont to be blown at the flame of a Lamp, exceeding thin, and Hermetically seal'd, will not break; whereas it cannot be imagin'd that so thin a Prison of Glass could resist the Elastical force of all the included Air, if that Air were so compress'd as we suppose: It may be easily reply'd, That the pressure of the inward Air against the Glass, is countervail'd by the equal pressure of the outward against the same Glass. And we see in bubbles, that by reason of this, an exceeding thin film of Water is often able, for a good while, to hinder the eruption of a pretty quantity of Air. And this may be also more conspicuous in those great Spherical bubbles, that Boyes sometimes blow with Water, to which Sope hath given a Tenacity. But that, if the pressure of the ambient Air were remov'd, the internal Air may be able to break thicker Glasses, than those lately mention'd, will appear by some of the following Experiments; to which, we shall therefore now hasten, having, I fear, been but too prolix in this Excursion, though we thought it not amiss to annex to our first Experiments some general Considerations touching the Spring of the Air, because (this Doctrine being yet a stranger to the Schools) not only we find not the thing it self to be much taken notice of; but of those few that have heard of it, the greater part have been forward to reject it, upon a mistaken perswasion, that those Phaenomena are the effects of Natures abhorrency of a Vacuum, which seem to be more fitly ascribable to the weight and spring of the Air.

TO try then at once both what it was that expanded the Bladder, and what a powerfull Spring there is ev'n in the Air we are wont to think uncompress'd: We caus'd a Bladder dry, well ty'd and blown moderately full, to be hung in the Receiver by one end of a string, whose other end was fastned to the inside of the Cover: and upon drawing out the ambient Air, that press'd on the Bladder; the internal Air not finding the wonted resistance, first swell'd and distended the Bladder, and then broke it, with so wide and crooked a rent, as if it had been forcibly torn asunder with hands. After which a second Bladder being convey'd in, the Experiment was repeated with like success: And I suppose it will not be imagin'd that in this case the Bladder was broken by its own Fibres, rather than by the Imprison'd Air.

And of this Experiment these two Phaenomena may be taken notice of: The one, that the Bladder at its breaking gave a great report, almost like a Cracker: And the other, That the Air contain'd in the Bladder, had the power to break it with the mention'd Impetuosity, long before the ambient Air was all, or near all, drawn out of the Receiver.

But, to verifie what we say in another Discourse, where we show, That even true Experiments may, by reason of the easie mistake of some unheeded Circumstance, be unsuccessfully try'd; we will Advertise, on this occasion, that we did oftentimes in vain try the breaking of Bladders, after the manner above mention'd: Of which the cause appear'd to be this, That the Bladders we could not break, having been brought us ready blown from those that sold them, were grown dry before they came to our hands: whence it came to pass, that, if we afterwards ty'd them very hard, they were apt to fret and so become unserviceable; and if we ty'd them but moderately hard, their stifness kept them from being clos'd so exactly, but that when the included Air had in the exhausted Receiver distended them as much as easily it could, it would in part get out between the little wrinkles of the Sphincter of the Neck: Whence also it usually happen'd, that, upon the letting in the Air from without, the Bladders appear'd more flaccid and empty than before they were put in; whereas when the Bladders were brought us moist from the Butchers, we could, without injuring them, tie their necks so close, that none of the Air once blown in, could get out of them, but by violently breaking them.

It would not be amiss on this occasion to point at something which may deserve a more deliberate Speculation than we can now afford it; namely that the Elastical Power of the same Quantity of Air may be as well encreas'd by the Agitation of the Aërial Particles (whether only moving them more swiftly and scattering them, or also extending or stretching them out, I determine not) within an every way inclosing and yet yielding Body; as display'd by the withdrawing of the Air that press'd it without. For we found that a Bladder, but moderately fill'd with Air and strongly ty'd, being a while held near the Fire, not only grew exceeding turgid and hard, but afterwards, being brought nearer to the Fire, suddenly broke with so loud and vehement a noise, as stony'd those that were by, and made us for a while after, almost deaf.

HAving thus seen that the Air hath an Elastical Power, we were next desirous to know in some measure how far a parcel of Air might by this its own Spring be dilated. And though we were not provided of Instruments fit to measure the dilatation of the Air any thing accurately, yet because an imperfect measure of it was more desirable than none at all, we devis'd the following method as very easily practicable.

We took a limber Lambs Bladder which was thorowly wetted in fair Water, that the sides of it being squeez'd together, there might be no Air left in its folds: (as indeed we could not afterwards upon trial discern any) The neck of this Bladder was strongly tied about that of a small Glass, (capable of holding five full drachmes of Water) the Bladder being first so compress'd, that all the included Air was only in the Glass, without being press'd there; then the Pump being set on work after a few exsuctions, the Air in the little Viol began to dilate it self and produce a small Tumor in the neck of the Bladder; and as the ambient Air was more and more drawn away, so the included Air penetrated farther and farther into the Bladder, and by degrees listed up the sides and display'd its folds, till at length it seem'd to have blown it up to its full extent: Whereupon the external Air, being permitted to flow back into the Receiver, repuls'd the Air that had fill'd the Bladder, into its former narrow receptacle, and brought the Bladder to be again flaccid and wrinkled as before: Then taking out the Bladder, but without severing it from the Glass, we did, by a hole made at the top of the Bladder, fill the Vessel, they both made up, with Water, whose weight was five Ounces five Drachmes and a half: Five Drachmes whereof were above-mention'd to be the contents of the Bottle. So that in this Experiment, when the Air had most extended the Bladder, it possess'd in all above nine times as much room as it did when it was put into the Receiver. And it would probably have much inlarg'd its bounds, but that the Bladder by its weight and the sticking together of its sides did somewhat resist its expansion: And which was more considerable, the Bladder appear'd tumid enough, whilst yet a pretty deal of Air was left in the Receiver, whose exsuction would according to our former Observation, probably have given way to a farther expansion of the Air, especially supposing the dilatation not to be restrain'd by the Bladder.

SInce we wrote the other day the former Experiment, we have met with some Glasses not very unfit for our purpose; by means of which we are now able, with a little more trouble, to measure the expansion of the Air a great deal more accurately than we could by the help of the above-mention'd Bladder, which was much too narrow to allow the Air its utmost distention.

We took then first a Cylindrical Pipe of Glass, whose bore was about a quarter of an Inch in Diameter: This Pipe was so bent and doubled, that, notwithstanding its being about two foot in length, it might have been shut up into a smaller Receiver, not a Foot high: But by misfortune it crack'd in the cooling, whereby we were reduced to make use of one part which was straight and intire, but exceeded not six or seven Inches. This little Tube was open at one end; and at the other, where it was Hermetically seal'd, had a small Glass bubble to receive the Air whose dilatation was to be measur'd.

Along the side of this Tube was pasted a straight narrow piece of Parchment, divided into twenty six equal parts, marked with black Lines and Figures, that by them might be measur'd both the included Air and its dilatation. Afterwards we fill'd the Tube with Water, almost to the top; and stopping the open end with the Finger, and inverting the Tube, the Air was permitted to ascend to the above mention'd Glass bubble. And by reason this ascent was very slow, it gave us the opportunity to mark how much more or less than one of the twenty six divisions this Air took up. By this means, after a trial or two, we were inabled to convey to the top of the Glass a bubble of Air equal enough, as to sight, to one of those Divisions: Then the open end of the Tube being put into a small Viol, whose bottom was cover'd with Water, about half an Inch high; we included both Glasses into a small and slender Receiver, and caused the Pump to be set on work. The event was, That at the first exsuction of the Air there appear'd not any expansion of the bubble, comparable to what appear'd at the second, and that upon a very few exsuctions the bubble reaching as low as the surface of the subjacent Water, gave us cause to think, That if our Pipe had not been broken, it would have expanded it self much farther: Wherefore we took out the little Tube, and found that besides the twenty six divisions formerly mention'd, the Glass bubble and some part of the Pipe to which the divided Parchment did not reach, amounted to six divisions more. Whereby it appears that the Air hath take up one and thirty times as much room as before, and yet seem'd capable of a much greater expansion, if the Glass would have permitted it. Wherefore after the former manner, we let in another bubble, that by our guess was but half as big as the former, and found, that upon the exsuction of the Air from the Receiver, this little bubble did not only fill up the whole Tube, but (in part) break through the subjacent Water in the Viol, and thereby manifest it self to have prossessed sixty and odd times its former room.

These two Experiments are mention'd to make way for the more easy belief of that which is now to follow. Finding then that our Tube was too short to serve our turn, we took a slender Quill of Glass which happen'd to be at hand, though it were not so fit for our purpose as we could have wished, in regard it was three or four times as big at one end as the other. This Pipe which was thirty Inches long, being Hermetically seal'd at the slender end, was almost filled with water; and after the above-related manner a bubble was convey'd to the top of it, and the open extreme was put into a Viol that had a little fair Water at the bottom: Then the Cover, by means of a small hole purposely made in it for the Glass Pipe to stand out at, was cemented on to the Receiver, and the Pump being set on work, after some exsuctions, not only the Air manifestly appear'd extended below the surface of the subjacent Water; but one of the By-standers affirms, that he saw some bubbles come out at the bottom of the Pipe and break through the Water. This done, we left off Pumping, and observ'd how at the unperceiv'd leaks of the Receiver the Air got in so fast, that it very quickly impell'd up the Water to the top of the Tube, excepting a little space whereinto that bubble was repuls'd, which had so lately possess'd the whole Tube; this Air at the slender end appear'd to be a Cylinder of ⅚ parts of an Inch in length, but when the Pipe was taken out and turn'd upside down, it appear'd at the other end inferiour in bulck to a Pea.

These things being thus done we took (to make the Experiment the more exactly) a small pair of Scales, such as Gold-Smiths use to weigh Cold Coyn in; and weighing the Tube and Water in it, we found them to amount to one Ounce thirty Grains and an half: Then we pour'd in as much Water as serv'd to fill up the Tube, wherein before we had left as much space unfill'd up as was possess'd by the bubble, and weighing again the Pipe and Water, we found the weight increas'd onely by one Grain. Lastly, pouring out the Water, and carefully freeing the Pipe from it (which yet we could not perfectly doe) we weighed the Glass alone, and found it to want two Drachmes and thirty two Grains of its former weight: So that the bubble of Air taking up the room but of one Grain in weight of Water, it appear'd that the Air by its own 〈 in non-Latin alphabet 〉 was so rarified, as to take up one hundred fifty two times as much room as it did before: Though it were then compress'd by nothing but the ordinary pressure of the contiguous Air. I know not whether it be requisite to take notice, that this Experiment was made indeed in a moist Night, but in a Room, in whose Chimney there was burning a good Fire, which did perhaps somewhat rarifie the Air of which the bubble consisted.

It hath seem'd almost incredible which is related by the Industrious Mersennus, That the Air by the violence of heat, though as great as our Vessels can support without fusion, can be so dilated as to take up seventy times as much room as before: Wherefore because we were willing to have a confirmation of so strange a Phaenomenon; we once more convey'd into the Tube a bubble of the bigness of the former, and prosecuting the Experiment as before with the same Water, we observed that the Air did manifestly stretch it self so far, as to appear several times a good way below the surface of the Water in the Viol, and that too with a surface very convex toward the bottom of the Pipe. Nay, the Pump being ply'd a little longer, the Air did manifestly reach to that place where the bottom of the Tube leaned upon the bottom of the Viol, and seemed to knock upon it and rebound from it: Which Circumstances we add, partly that the Phaenomenon we have been relating may not be imputed to the bare subsiding of the Water that filled the Tube, upon the taking off the pressure of the ambient Air. And partly also that it may appear that if our Experiments have not been so accurately made as with fitter Instruments might perhaps be possible; yet the expansion of the Air is likely to be rather greater than lesser than we have made it: Since the Air was able to press away the Water at the bottom of the Pipe, though that were about two Inches below the surface of the Water that was then in the Viol, and would have been at least as high in the Pipe, if the Water had onely subsided and not been depressed: So that it seems not unlikely that if the Experiment could be so made, as that the expansion of the Air might not be resisted by the Neighbouring Bodies, it would yet enlarge its bounds, and perhaps stretch it self to two hundred times its former bulk, if not more. However, what we have now tryed will, I hope, suffice to hinder divers of the Phaenomena of our Engine from being distrusted: Since in that part of the Atmosphere we live in, that which we call the free Air (and presume to be so uncompressed) is crouded into so very small a part of that space, which if it were not hindred it would possess. We would gladly have tryed also whether the Air at its greatest expansion could be farther rarified by heat; but doe what we could, our Receiver leaked too fast to let us give our selves any satisfaction in that particular.

TO discover likewise by the means of that pressure of the Air, both the strength of Glass, and how much interest the Figure of a Body may have in its greater or lesser Resistance to the pressure of other Bodies, we made these farther tryals.

We caus'd to be blown with a Lamp, a round Glass bubble, capable of containing, by guess, about five Ounces of Water, with a slender neck about the bigness of a Swan's Quill, and it was purposely blown very thin, as Viols made with Lamps are wont to be, that the thinness of the matter might keep the roundness of the Figure from making the Vessel too strong. Then having moderately emptied the Receiver, and taken it out of the Pump, we speedily applied to the Orifice of the bottom of it, the Neck of the newly mention'd Glass, carefully stopping the Crannies with melted Plaister, that no Air might get in at them: And after turning the Key of the Stop-cock, we made a free passage for the Air to pass out of the bubble into the Receiver: Which it did with great celerity, leaving the bubble as empty as the Receiver it self; as appear'd to us by some Circumstances not now to be insisted on. Notwithstanding all which, the Vessel, continuing as intire as before, gave us cause to wonder that the bare roundness of the Figure should enable a Glass, almost as thin as Paper, to resist so great a pressure as that of the whole incumbent Atmosphere. And having reiterated the Experiment, we found again that the pressure of the ambient Body, thrusting all the parts inwards, made them, by reason of their arched Figure, so support one another, that the Glass remain'd as whole as at first.

Now that the Figure of the Glass is of great moment in this matter, may be evinced by this other Experiment.

WE took a Glass Helmet or Alembick (delineated by the seventh Figure) such as Chymists use in Distillations, and containing by conjecture between two and three Pints: The Rostrum or Rose of it mark'd with (c) was Hermetically closed; and at the top of it was a hole, into which was fitted and cemented one of the Shanks of a middle-siz'd Stop-cock; so that the Glass being turn'd upside-down, the wide Orifice (which in common Glass-Helmets is the onely one) was upwards; and to that wide Orifice was fitted a cast cover of Lead, which was carefully cemented on to the Glass: Then the other Shank of the Stop-cock being with cement likewise fasten'd into the upper part of the Pump, the exsuction of the Air was endeavoured. But it was not long before, the remaining Air being made much too weak to ballance the pressure of the Ambient Air, the Glass was (not without a great noise) crack'd almost half round, along that part of it where it began to bend inwards: As if in the Figure, the crack had been made according to the Line (a b,) and upon an endeavour to pump out more of the Air, the crack once begun, appear'd to run on farther; though the Glass where it was broken seem'd to be (by conjecture) above ten, some thought above twenty times, as thick as the bubble mentioned in the foregoing Experiment.

This will perhaps make it seem strange, that having taken another Glass bubble blown at the same time, and like for ought we discerned for size, thickness and Figure, to that thin one formerly mentioned; and having sealed it up Hermetically, and suspended it in the Receiver, the exsuction of the ambient Air did not enable the imprisoned Air to break, or in the least to crack the bubble; though the Experiment were laboriously tried, and that several times with bubbles of other sizes: But that perhaps the heat of the Candle or Lamp wherewith such Glasses are Hermetically sealed, (not to mention the warmth of his hands that seal'd it) might so rarefy the contained Air, as much to weaken its Spring, may seem probably by the following Experiments.

WE took a Glass Viol able to hold three or four Ounces of Water, and of the thickness usual in Glasses of that size; into the Neck of this was put a moderately slender Pipe of Glass, which was carefully fastened with a mixture of equal parts of Pitch and Rosin to the Neck of the Viol, and which reached almost to the bottom of it, as the sixth Figure declareth.

This Viol being, upon a particular design, filled with Water, till that came up in it, a pretty deal higher than the lower end of the Pipe, was put into one of our small Receivers, (containing between a Pint and a Quart) in such manner as that the Glass Pipe, passing through a hole made purposely for it in the Leaden-Cover of the Receiver, was for the most part of it without the Vessel, which being exactly closed, the Pump was set on work: But at the very first exsuction, and before the Sucker was drawn to the bottom of the Cylinder, there flew out of the Viol a piece of Glass half as broad as the Palm of a Man's Hand, and it was thrown out with such violence, that hitting against the Neighbouring side of the Receiver, it not onely dashed it self to pieces, but cracked the very Receiver in many places, with a great noise that much surprised all that were in the Room. but it seemed, that in so little a Receiver, the Air about the Viol being suddenly drawn out, the Air imprisoned in the Vessel, having on it the whole pressure of the Atmosphere (to which by the Pipe open at both ends, It and the Water were exposed) and not having on the other side the wonted pressure of the Ambient Air to ballance that other pressure, the resistance of Glass was finally surmounted, and the Viol once beginning to break where it was weakest, the external Air might rush in with violence enough to throw the cracked parcel so forcibly against the Neighbouring side of the Receiver, as to break that too.

And this may be presumed sufficient to verify what we delivered in that part of our Appendix to the first Experiment where we mentioned the almost equal pressure of the Air on either side of a thin Glass Vessel, as the cause of its not being broken by the forcible spring of the contained Air. But yet that it be not suspected that chance had an interest in so odd an Experiment as we have been relating, we will add that for farther satisfaction we reiterated it in a round Glass containing by Guess about six ounces of Water: This Viol we put into such a small Receiver as was lately mentioned, in such manner as that the bottom of it rested upon the lower part of the Pneumatical Glass, and the Neck came out through the Leaden-cover of the same at a hole made purposely for it. But being made circumspect by the foregoing mischance, we had put the Viol into a Bladder, before we put it into the Receiver to hinder this last-named Glass from being endangered by the breaking of the other. Then the Pneumatical Vessel being closed so that no way was left for the outward Air to get into it, but by breaking through the Viol, into whose cavity it had free access by the mouth of it, (which was purposely left open,) the Sucker being nimbly drawn down, the external Air immediately pressed forcibly as well upon the Leaden-cover as the Viol; and the Cover happening to be in one place a little narrower than the edge of the Pneumatical Glass, was depressed, and thrust into it so violently by the incumbent Air, that getting a little within the tapering Lip of the Glass, it did (like a kind of Wedge,) thrust out that side where it was depressed, so as, (though the Receiver was new,) to split it. This accident being thus mentioned upon the bye to confirm what we formerly said touching the fitness or unfitness of Glasses of some Figures to resist the pressure of the Atmosphere; We will proceed to relate the remaining part of the Experiment, namely, That having fitted on a wider Cover to the same Receiver, and closed both that and the crack with Cement, we prosecuted the Experiment in the manner above related, with this success: That upon the quick depressing of the Sucker, the external Air burst the Body of the Viol into above an hundred pieces, many of them exceeding small, and that with such violence, that we found a wide rent, besides many holes, made in the Bladder it self.

And to evince that these Phaenomena were the effects of a limited and even moderate force, and not of such an abhorrency of a Vacuum as that to avoid it, many have been pleased to think that Nature must, upon occasion, exercise an almost boundless power; we afterwards purposely try'd this Experiment with several Glasses somewhat thicker than those Viols, and found the event to verify our conjecture, that it would not succeed: For the Glasses were taken out as intire as they were put in.

And here, My Lord, I hold it not unfit, upon occasion of the mention that hath been made of our having employ'd small Receivers, and one of them, notwithstanding its being crack'd, to annex these two Advertisements.

First then, besides the great Pneumatical Glass so often mentioned, and the proportionate Stop-cock, we thought fit to provide our selves with some small Receivers blown of Crystalline Glass, of several Shapes, and furnished with smaller Stop-cocks purposely made; and this we did upon hopes, that when we had surmounted the difficulties to be met with in Cementing the Glasses to the Stop-cocks, and the Pneumatical Vessels to the Pump, so exquisitely as is requisite for our purpose, we should from the smallness of our Receivers receive a four-fold Advantage. The first, that by reason of the slenderness of the Vessels, and their being made of much purer and clearer metal, (as the Glass-men speak,) than the great Receiver, we might have a more perfect view of every thing happening within them. The next, that such small Vessels might be emptied with less labour and in much less time. The third, that this nimble exsuction of the ambient Air would make many changes in the Bodies shut up in these Glasses more sudden and conspicuous than otherwise they would prove. And the last, that we should be able to draw and keep out the Air much more perfectly from such small Vessels than from our large Receiver, But though we were not much disappointed in the expectation of the three first advantages, yet we were in our hopes of the fourth. For besides the great difficulty we found in fitting together the Glasses, the Stop-cocks, and the Covers; besides this I say, we found our selves seldom able to draw, and keep out the Air so far as to make the remaining Air in these Receivers weaker than the remaining Air in our great Receiver. For though sometimes the Leaks of some of these little Receivers may be much (either fewer or smaller) than those of the larger Vessel; yet a little Air getting into one of these, wherein it had but little room to expand and display it self, might press as much upon all parts of the internal surface of the Vessel, and upon the included Bodies, as a greater quantity of the Air in a Vessel in whose capacity it might find more room to expand it self.

The other thing that we were to advertise, is, That 'tis not every small crack that can make such a Receiver as is of a roundish Figure altogether useless to our Experiment, in regard that upon the exsuction of the internal Air, the ambient Air on all sides pressing the Glass inwards or towards the middle, doth consequently thrust the Lips of the crack closer, and so rather close than increase it.

This I mention partly because Receivers fit for our turn are more easily cracked than procured, and therefore ought not to be unnecessarily thrown away as unserviceable: And partly because I think it becomes one that professeth himself a faithfull Relator of Experiments, not to conceal from Your Lordship, that after a few of the foregoing Experiments were made, there happened in the great Receiver a crack of about a Span long, beginning at the upper Orifice, and occasion'd, as it seem'd, by the excessive heat of too large an Iron that was employ'd to melt the Cement about that Orifice. But having laid upon this crack a broad Plaister, (which in one of our Essays written some years since to your ingenious and hopefull Cousin Jones, we extoll for the mending of crack'd Receivers, and other Chymical Glasses;) and having afterwards thickly overlaid this Plaister with Diachylon, we neither could then, nor can yet perceive that the Vessel leaks sensibly at that crack.

The Plaister was made of good quick Lime finely poudred, and nimbly ground with a Pestle in a Mortar, with a quantity (I know not how much precisely, not having those Essays in this place) of scrapings of Cheese and a little fair Water, no more than is just necessary to bring the mixture to a somewhat soft Paste, which when the Ingredients are exquisitely incorporated, will have a strong and stincking smell: Then it must be immediately spread upon a Linnen Cloath of three or four fingers breadth, and presently apply'd, lest it begin to harden. But if your Lordship had seen how we mended with it Receivers even for the most subtle Chymical Spirits. You would scarce wonder at the service it hath done in our Pneumatical Glass.

WE took a Tallow-candle of such a size that eight of them make about a pound, and having in a very commodious Candlestick let it down into the Receiver, and so suspended it, that the Flame burnt almost in the middle of the Vessel, we did in some two minutes exactly close it up: and upon pumping very nimbly, we found, that, within little more than half a minute after, the Flame went out, though the Snuff had been purposely left of that length we judged the most convenient for the lasting of the Flame.

But the second time having put in the same Candle into the Receiver, (after it had by the blasts of a pair of Bellows been freed from Fumes) the Flame lasted about two minutes from the time the Pumper began to draw out the Air; upon the first exsuction whereof, the Flame seem'd to contract it self in all its dimensions. And these things were farther observable, that after the two or three first exsuctions of the Air, the Flame (except at the very top) appear'd exceeding blew, and that the Flame still receded more and more from the Tallow, till at length it appear'd to posses onely the very top of the Week, and there it went out.

The same Candle being lighted again was shut into the Receiver, to try how it would last there without drawing forth the Air, and we found that it lasted much longer than formerly; and before it went out, receded from the Tallow towards the top of the Week, but not neer so much as in the former Experiment.

And having an intention to observe particularly what the motion of the Smoke would be in these Experiments: We took notice that when the Air was not drawn out, there did, upon the extinction of the Flame, a considerable part of the Week remain kindled, which (probably by reason of the Circulation of the Air) in the Vessel, occasion'd by the heat) emitted a Steam, which ascended swiftly and directly upwards in a slender and uninterrupted Cylinder of smoke, till it came to the top, whence it manifestly recoil'd by the sides to the lower part of the Vessel. Whereas when the Flame went out upon the exsuction of the Air one time (when the Flame retir'd very leasurely to the top) we perceived it not to be follow'd by any smoke at all. And at another time the upper part of the Week remaining kindled after the extinction of the Flame, the slender steam of Fumes that did arise ascended but a very little way, and then after some uncertain motions this and that way, did, for the most part, soon fall downwards.

Being desirous also to try whether there would be any difference as well in our Receiver, as there is wont to be elsewhere betwixt Candles made of Wax and those made of Tallow, as to their duration; we took slender Tapers of white Wax, (commonly called Virgins Wax) that being found to burn with much less smoke than common yellow Wax. Six of these of like bigness, and each of them of about the thickness of a Swan's Quill, we press'd together into one Candle: and having lighted all the Weeks, we let in the above-mention'd Wax-candle into the Receiver, and made what haste we could to close it up with Cement. But, though in the mean while we left open the Valve of the Cylinder, the hole of the Stop-cock, and that in the Cover of the Receiver, that some Air might get in to cherish the Flame, and the Smoke might have a vent; Yet for so great a Flame the Air sufficed not so much as till the Cover could be perfectly luted on: So that before we were quite ready to imploy the Pump, the Candle was extinguished. Wherefore we took but one of the above mention'd Tapers, and having lighted it, clos'd it up in the Receiver, to try how long a small Flame with a proportionable smoke would continue in such a quantity of Air: but we found upon two several tryals, that from the beginning of pumping, the Flame went out in about a minute of an hour. It appear'd indeed to us that the swinging of the Wier to and fro (in the Engine shaken by pumping) hasten'd the vanishing of the Flame, which seem'd by that motion to be cast sometimes on one side of the Week and sometimes on the other. But though once we purposely refrain'd pumping after a very few exsuctions of the Air, that the Flame might not be agitated, yet it lasted not much longer than the newly-mention'd time. And lastly, closing up the same Taper, lighted again, to discover how long it would last without drawing out of the Air, we found that it burnt for a while vividly enough, but afterwards began to be lessen'd more and more in all its dimensions. And we observ'd that the Flame did not, as before, retire it self by little and little towards the top, but towards the bottom of the Week (from which yet it did a little withdraw upwards just before it went out) so that the upper part of the Week-appear'd for a pretty while manifestly above the top of the Flame, which having lasted about five minutes, was succeeded by a directly ascending stream of Smoke.

THere was taken a Wier, which being bent almost in the form of a Screw, constituted such an Instrument, to contain Coals and leave them every way accessible to the Air, as the tenth Figure declareth; the breadth of this Vessel was no less than that it might with ease be convey'd into the Receiver: And having fill'd it to the height of about five Inches with throughly kindled Wood-coals, we let it down into the Glass; and speedily closing it, we caus'd the Pumper to ply his work, and observ'd that upon the very first exsuction of the Air (though perhaps not because of that onely) the Fire in the Coals began to grow very dim, and though the agitation of the Vessel did make them swing up and down (which in the sree Air would have retarded the extinction of the Fire) yet when we could no longer discern any redness at all in any of them, casting our eyes upon a minute Watch we kept by us on this occasion, we found that from the beginning of the Pumping (which might be about two minutes after the Coals had been put in glowing) to the total dis-appearing of the Fire, there had passed but three minutes.

Whereupon, to try the Experiment a little farther, we presently took out the Coals, in which it seems there had remained some little parcels of Fire, rather cover'd than totally quench'd: For in the open Air the Coals began to be re-kindled in several places, wherefore having by swinging them about in the Wier, throughly lighted them the second time, we let them down again into the Receiver, and clos'd it speedily as before; and then waiting till the Fire seem'd totally extinct without medling with the Pump, we found that from the time the Vessel was clos'd, till that no Fire at all could be perceiv'd, there had passed about four minutes: Whereby it seem'd to appear that the drawing away of the ambient Air made the Fire go out sooner than otherwise it would have done; though that part of the Air that we drew out left the more room for the stifling steams of the Coals to be received into.

Lastly, Having taken out the Wier and put other Coals into it, we did, in the same Room where the Engine stood, let it hang quietly by a string in the open Air, to try how long the Fire would last without agitation, when no Air was kept from it, And we found that the Fire began to go out first at the top and outsides of the Coals; but inwards and near the bottom the Fire continu'd visible for above half an hour, a great part of the Coals, especially those next the bottom, being burnt to ashes before the Fire went out.

We caus'd likewise a piece of Iron to be forg'd, of the bigness of a middle siz'd Char-coal, and having made it red-hot throughout; we caus'd it in the lately mention'd Wier, to be speedily convey'd and shut up into the Receiver, being desirous to try what would become of a glowing Body, by reason of its texture more vehemently hot than a burning Coal of the same bigness, and yet unlike to send forth such copious and stifling Fumes: But we could not observe any manifest change upon the exsuction of the Air. The Iron began indeed to lose its fiery redness at the top, but that seem'd to be because it was at the upper end somewhat more slender than at the lower: The redness, though it were in the day time, continued visible about four minutes; and then, before it did quite disappear, we turn'd the Key of the Stop-cock, but could not discern any change of the Iron upon the rushing in of the Air. Yet some little remainders of Wax that stuck to the Wier, and were turn'd into Fumes by the heat of the neighbouring Iron, seem'd to afford a more plentifull, or at least a much more expanded Smoke when the Air was suck'd out, than afterwards; though allowance was made for the decreasing heat of the Iron. And lastly, notwithstanding a considerable exsuction of the ambient Air, though not by far so great a one as might have been made by the Engine; and notwithstanding the inconsiderable dissipation of the parts of the Iron; the surrounding sides of the Receiver were sensibly, and almost offensively heated by it; insomuch that a pretty while after the Iron was taken out, the sides of the Glass manifestly retain'd a warmth: which would not be unfit to be consider'd by a Person at more leasure than I am now.

BEing willing to try after this something that would not cherish much Fire at once, and would keep Fire much longer than a Coal: We took a piece of Match, such as Souldiers use, of the thickness of a Man's little Finger, or somewhat thicker; and this being well lighted at one end, was by a string suspended with that end downwards in the cavity of the Receiver which was immediately clos'd: And yet by that time it could well be so, the copious Fumes of the Match had near fill'd and darken'd the Receiver. Wherefore, lest the Vessel should be endanger'd, the Pump was nimbly ply'd, and a great deal of Air and Smoke mixt together was drawn out, whereby the Receiver growing more clear, we could discern the Fire in the Match to burn more and more languidly: and notwithstanding that by the diligence us'd in pumping, it seem'd to have room enough allow'd it to throw out Fumes; yet, after no long time, it ceas'd from being discernable either by its Light or its Smoke. And, though by that we were invited to suppose it quite extinguished, yet we continu'd pumping a-while, in prosecution of another Experiment we were trying at the same time: And this we did the more willingly, because of a suspicion, the Experiment about the Coals, might easily suggest, and which the event declar'd not to have been altogether groundless. For upon the admission of the external Air, the Fire, that seem'd to have gone out a pretty while before, did presently revive; and being as it were refresh'd by the new Air, and blown by the Wind made by that Air in rushing in, it began again to shine and dissipate the neighbouring Fuel into Smoke as formally.

TO try diverse things at once, and particularly whether Fire, though we found it would not long last, might not be produced in our evacuated Receiver: We took a Pistol of about a foot in length, and having firmly tyed it to a stick almost as long as the Cavity of the Receiver, we very carefully prim'd it with well dry'd Gunpowder, and then cocking it, we ty'd to the Tricker, one end of a string, whose other end was fasten'd to the Key formerly mention'd to belong to the Cover of our Receiver. This done, we convey'd the Pistol, together with the annexed Staff, into the Vessel which being clos'd up, and empty'd aster the usual manner, we began to turn the Key in the Cover; and thereby shortning the string that reach'd from it to the Pistol, we pull'd aside the Tricker, and observ'd, that according to our expectation the force of the Spring of the Lock was not sensibly abated by the absence of the Air: (from whose impetus yet some Modern Naturalists would derive the cause of the motion of Restitution in solid Bodies) For the Cock falling with its wonted violence upon the Steel, struck out of it as many, and as conspicuous parts of Fire, as, for ought we could perceive, it would have done in the open Air. Repeating this Experiment diverse times, we also observed whether or no there would appear any considerable diversity in the Motion of the shining Sparks in a place where the remaining Air was so much rarefied, but could not perceive but that they moved some of them upwards, as well as some of them downwards, and some of them side-ways, as they are wont to do, when upon such collisions they fly out in the open Air.

We likewise caus'd a piece of Steel to be made of the form and bigness of the Flint, in whose place we put it, and then the Pistol being cock'd and conveyed into the Receiver, the Tricker was pull'd after the Air was drawn out: And though the place were purposely somewhat darken'd, yet there appear'd not upon the striking of the two Steels against each other the least spark of Fire: Nor did we expect any (having before in vain attempted to strike Fire this way in the open Air) though we thought fit to make the Experiment, to undeceive those who fancy in rarefied Air, I know not what strange disposition, to take fire upon a much slighter occasion than this Experiment afforded. We have indeed found, that by the dextrous Collision of two harden'd pieces of Steel, store of sparks may be struck out: But that was done with such vehement percussion of the edges of the two Steels, as could not well be compass'd in our Receiver.

But the chief thing we design'd to doe with our Pistol, was, To observe whether Gun-powder would take Fire in our empty'd and closely stop'd Glass? Whether the expansion of the Flame would be considerably varied by the absence of so much of the ambient Air as was drawn out of the Receiver? And whether the Flame would diffuse it self upward, as it is wont, notwithstanding its not having about it the usual proportion of Air to force it up? And though most of our Attempts to fire the Gun-powder in the Pan of the Pistol succeeded not, because we were fain to let it hang almost perpendicular in the Receiver, whereby the Powder was shaken down before the sparks could reach it: yet once the Experiment succeeded, and the kindled Powder seem'd to make a more expanded Flame than it would have done in the open Air, but mounted upwards according to its wont, whether by reason of that little portion of Air, which in spight of our pumping remained in the Receiver, or for any other cause, we have not now the leasure to consider. But we must not forget, that upon the extinction of the Flame the Receiver appear'd darken'd with smoke, which seem'd to move freely up and down, and upon the letting in the Air at the Stop-cock began to circulate much • aster than before. We would have made more observations concerning this Flame, but that of two or three attempts we afterwards made to repeat the kindling of the Powder, not any one succeeded; and we have not the leasure to dwell long upon one kind of Trials.

TO these Experiments concerning Fire we added another, which though it succeeded not, may perhaps without impertinency be recorded: partly, because that (as we have in another Treatise amply declar'd) it is usefull to recite what Experiments miscarry as well as succeed. And partly also, because it is very possible that what we endeavoured in vain, may be performed by Your Lordship, or some other Vertuoso that shall have stancher Tessels than we had, and more Sunny days than the present Winter allows us.

We convey'd then into one of our small Receivers a piece of matter combustible, dry and black (experience declaring things of that colour to be most easily kindled) and carefully closing the Vessel, we brought it to a Window at which the Sun, not very far from the Meridian, shone in very freely: then drawing out the Air with speed we united the Sun beams with a Burning-glass upon the combustible matter, which began immediately to send forth a smoke that quickly darkned the Receiver, but notwithstanding all our care and diligence the external Air got in so fast, that after divers trials we were fain to leave off the Experiment in that Glass, and induc'd to make trial of it in our great Receiver.

Having then after some difficulty lodg'd the combustible matter in the cavity of this Vessel in such manner, as that it was almost contiguous to that side thereof that was next the Sun, we did endeavour with a pretty large Burning-glass to kindle it, but found, as we fear'd, That, by reason of the thickness of the Glass, (which was also of a less pure and less Diaphanous matter than the other) the Sun-beams thrown in by the Burning-glass, were in their passage so dislocated and scattered (not now to mention those many that being reflected, could not pierce into the cavity of the Receiver) that we could not possibly unite enough of them to kindle the matter, nor so much as to make it sensibly smoke. Yet we hope that the seeing whether Bodies (other than Gun-powder) may be kindled, and what would happen to them when set on fire, in a place in great measure devoid of Air, may prove so Lucilerous an Experiment, that when the Season is more favourable we shall, God permitting, make farther trial of it, and acquaint your Lordship with the Event, if it prove prosperous. In the mean time we shall pass on to other Experiments, as soon as we have advertis'd Your Lordship that we have forborn to make such Reflexions upon the several Experiments we have set down concerning Fire, as the matter would have easily afforded, and Your Lordship may perhaps have expected. But I made the less scruple to forbear the annexing of Speculations to these Recitals, because Carneades and Eleutherius have in some Dialogues concerning Heat and Flame, (which were last year seen by some Friends, and may be, when you please, commanded by You,) mention'd divers of my Thoughts and Experiments concerning Fire,

WE designed to try whether or no divers Magnetical Experiments would exhibit any unusual Phaenomena, being made in our Evacuated Receiver instead of the open Air: But for want of leasure and conveniency to prosecute such Trials, we were induced to reserve the rest for another time, and to content our selves with making that which follows. We convey'd into the Receiver a little Pedestal of Wood, in the midst of which was perpendicularly erected a slender Iron, upon whose sharp point an excited Needle of Steel purposely made, and of about five Inches long, was so placed, that hanging in an AEquilibrium it could move freely towards either hand. Then the Air being after the usual manner pumped out, we apply'd a Load-stone moderately vigorous to the outside of the Glass, and found that it attracted or repell'd the ends of the Needle, according to the Laws Magnetical, without any remarkable difference from what the same Load-stone would have done, had none of the Air been drawn away from about the Needle, which, when the Load-stone was removed, after some tremulous Vibrations to and fro, rested in a position, wherein it look'd North and South.

PRoceed we now to the mention of that Experiment, whereof the satisfactory trial was the principal fruit I promis'd my self from our Engine, It being then fufficiently known, that, in the Experiment De Vacuo, the Quick-silver in the Tube is wont to remain elevated, above the surface of that whereon it leans, about 27 digits: I considered, that, if the true and only reason why the Quick-silver falls no lower, be, that at that Altitude, the Mercurial Cylinder in the Tube is in an AEquilibrium with the Cylinder of Air, suppos'd to reach from the adjacent Mercury to the top of the Atmosphere: then if this Experiment could be try'd out of the Atmosphere, the Quick-silver in the Tube would fall down to a level with that in the Vessel, since then there would be no pressure upon the subjacent, to resist the weight of the Incumbent Mercury. Whence I inferr'd (as easily I might) that if the Experiment could be try'd in our Engine, the Quick-silver would subside below 27 Digits, in proportion to the exsuction of Air, that should be made out of the Receiver. For, as when the Air is shut into the Receiver, it doth (according to what hath above been taught) continue there as strongly compress'd, as it did whilst all the incumbent Cylinder of the Atmosphere lean'd immediately upon it; because the Glass, wherein it is pen'd up, hinders it to deliver it self, by an expansion of its parts, from the pressure wherewith it was shut up. So if we could perfectly draw the Air out of the Receiver, it would conduce as well to our purpose, as if we were allow'd to try the Experiment beyond the Atmosphere.

Wherefore (after having surmounted some little difficulties which occurr'd at the beginning) the Experiment was made after this manner. We took a slender and very curiously blown Cylinder of Glass, of near three foot in length, and whose bore had in Diameter a quarter of an Inch, wanting a hairs breadth: This Pipe being Hermetically seal'd at one end, was, at the other, fill'd with Quick-silver, care being taken in the filling, that as few bubbles as was possible should be left in the Mercury: Then the Tube being stopt with the Finger and inverted, was open'd, according to the manner of the Experiment, into a somewhat long and slender Cylindrical Box (instead of which we now are wont to use a Glass of the same form) half fill'd with Quick-silver: And so, the liquid metal being suffered to subside, and a piece of Paper being pasted on level with its upper surface, the Box and Tube and all were by strings carefully let down into the Receiver: And then, by means of the hole formerly mentioned to be left in the Cover, the said Cover was slipt along as much of the Tube as reached above the top of the Receiver; And the Interval, left betwixt the sides of the Hole and those of the Tube, was very exquisitely filled up with melted (but not over hot) Diachylon, and the round chink, betwixt the Cover and the Receiver, was likewise very carefully closed up: Upon which closure there appeared not any change in the height of the Mercurial Cylinder; no more, than if the interposed Glass Receiver did not hinder the immediate pressure of the ambient Atmosphere upon the inclosed Air; which hereby appears to bear upon the Mercury, rather by virtue of its spring than of its weight: since its weight cannot be supposed to amount to above two or three Ounces, which is inconsiderable in comparison of such a Cylinder of Mercury as it would keep from subsiding.

All things: being thus in a readiness, the Sucker was drawn down; and, immediately upon the egress of a Cylinder of Air out of the Receiver, the Quick silver in the Tube did, according to expectation, subside: And notice being carefully taken (by a mark fastened to the outside) of the place where it stopt, we caused him that managed the Pump to pump again, and marked how low the Quick-silver fell at the second exsuction; but continuing this work, we were quickly hindred from accurately marking the Stages made by the Mercury in its descent, because it soon sunk below the top of the Receiver, so that we could henceforward mark it no other ways than by the eye. And thus, continuing the labour of pumping for about a quarter of an hour, we found our selves unable to bring the Quick-silver in the Tube totally to subside; because, when the Receiver was considerably emptyed of its Air, and consequently that little that remained grown unable to resist the Irruption of the External, that Air would (in spight of whatever we could doe) press in at some little Avenue or other; and though much could not thereat get in, yet a little was sufficient to counterballance the pressure of so small a Cylinder of Quick-silver, as then remained in the Tube.

Now (to satisfy our selves farther, that the falling of the Quick-silver in the Tube to a determinate height, proceedeth from the AEquilibrium, wherein it is at that height with the external Air, the one gravitating, the other pressing with equal force upon the subjacent Mercury) we returned the Key and let in some new Air; upon which the Mercury immediately began to ascend (or rather to be impelled upwards) in the Tube, and continued ascending, till having returned the Key, it immediately rested at the height which it had then attained: And so, by turning and returning the Key, we did several times at pleasure impell it upwards, and check its ascent. And lastly, having given a free egress at the Stop-cock to as much of the external Air as would come in, the Quick-silver was impelled up almost to its first height: I say almost, because it ftopt near a quarter of an Inch beneath the Paper mark formerly mentioned; which we ascri • ed to this, That there was (as is usual in this Experiment) some little Particles of Air engaged among those of the Quick-silver; which Particles, upon the descent of the Quick-silver, did manisestly rise up in bubbles towards the top of the Tube, and by their pressure, as well as by lessening the Cylinder by as much room as they formerly took up in it, hindred the Quick-silver from regaining its first height.

This Experiment was a few days after repeated in the presence of those excellent and deservedly Famous Mathematick Professors, Dr. Wallis, Dr. Ward, and Mr. Wren, who were pleased to honour it with their Presence: And whom I name, both as justly counting it an Honour to be known to them, and as being glad of such Judicious and illustrious Witnesses of our Experiment; and'twas by their guess, that the top of the Quick-silver in the Tube was defined to be brought within an Inch of the surface of that in the Vessel.

And here, for the Illustration of the foregoing Experiment, it will not be amiss to mention some other particulars relating to it.

First then, When we endeavoured to make the Experiment with the Tube closed at one end with Diachylon instead of an Hermetical Seal; we perceived, that upon the drawing of some of the Air out of the Receiver, the Mercury did indeed begin to fall, but continued asterwards to subside, though we did not continue pumping. When it appeared, that though the Diachylon that stopt the end of the Tube, were so thick and strong, that the external Air could not press it in (as experience taught us that it would have done, if there had been but little of it) yet the subtler parts of it were able (though slowly) to insinuate themselves through the very body of the Plaister, which it seems was not of so close a Texture, as that which we mentioned our selves to have successfully made use of in the Experiment De Vacuo some years ago. So that now we begin to suspect, that perhaps one reason, why we cannot persectly pump out the Air, may be, that when the Vessel is almost empty, some of the subtler parts of the external Air may, by the pressure of the Atmosphere, be strained through the very body of the Diachylon into the Receiver. But this is only conjecture.

Another Circumstance of our Experiment was this, That, if (when the Quick-silver in the Tube was fallen low) too much ingress were, at the hole of the Stop-cock, suddenly permitted to the external Air; it would rush in with that violence, and bear so forcibly upon the surface of the subjacent Quick-silver, that it would impell it up into the Tube rudely enough to endanger the breaking of the Glass.

We formerly mention'd, that the Quick-silver did not in its descent fall as much at a time after the two or three first exsuctions of the Air, as at the beginning. For, having marked its several Stages upon the Tube, we found, that at the first suck it descended an Inch and ⅜, and at the second an Inch and ⅜; and when the Vessel was almost emptied, it would scarce at one exsuction be drawn down above the breadth of a Barly-corn. And indeed we found it very difficult to measure in what proportion these decrements of the Mercurial Cylinder did proceed: Partly because (as we have already intimated) the Quick-silver wassoon drawn below the top of the Receiver; and partly because, upon its descent at each exsuction, it would immediately re-ascend a little upwards; either by reason of the leaking of the Vessel at some imperceptible hole or other, or by reason of the motion of restitution in the Air, which, being somewhat compresled by the fall as well as weight of the Quick-silver, would repell it a little upwards, and make it vibrate a little up and down, before they could reduce each other to such an AEquilibrium as both might rest in. But though we could not hitherto make observations accurate enough concerning the measures of the Quick-silver's descent, to reduce them into any Hypothesis, yet would we not discourage any from attempting it; since, if it could be reduc'd to a certainty, 'tis probable that the discovery would not be unusefull.

And, to illustrate this matter a little more, we will add, That we made a shift to try the Experiment in one of our above-mention'd small Receivers, not containing a Quart; but (agreeably to what we formerly observed) we found it as difficult to bring this to be quite empty as to evacuate the greater; the least external Air that could get in (and we could not possibly keep it all perfectly out) sufficing in so small a Vessel to display a considerable pressure upon the surface of the Mercury, and thereby hinder that in the Tube from falling to a level with it. But this is remarkable, that having two or three times try'd the Experiment in a small Vessel, upon the very first Cylinder of Air that was drawn out of the Receiver, the Mercury fell in the Tube 18 Inches and a half, and at another trial 19 Inches and a half.

But on this occasion, I hold it not unfit to give your Lordship notice that I hop'd from the descent of the Quick-silver in the Tube upon the first suck, to derive this advantage: that I should thence be enabled to give a near guess at the proportion of force betwixt the pressure of the Air (according to its various states, as to Density and Rarefraction) and the gravity of Quick-silver, than hitherto hath been done. For in our Experiment there are divers things given, that may be made use of towards such a discovery. For first we may know the capacity of the Receiver wherein the Experiment is made, since, by filling it with water, we may easily compute how many Quarts, or Measures of any other denomination, it containeth of Air; which Air, when shut up in the Vessel may be suppos'd to have a pressure equal to that of the Atmosphere; since it is able to keep the Quick-silver in the Tube from falling any lower than it did in the free and open Air. Next here is given us the capacity of the brass Cylinder empty'd by the drawing down of the Sucker (its bore and height being mention'd in the description of our Pump) whereby we may come to know how much of the Air contain'd in the Receiver is drawn out at the first-suck. And we may also easily define, either in weight or cubick measures, the Cylinder of Quick silver that answers to the Cylinder of Air lately mention'd (that Mercurial Cylinder being in our Engine computable by deducting from the entire altitude of that Cylinder of Quick-silver, the altitude at which it rests upon the first exsuction.) But though, if this Experiment were very watchfully try'd in Vessels of several sizes, and the various descents of the Quick-silver compar'd among themselves, 'tis not improbable, that some such things, as we hop'd for, may thereby be discovered. Yet because not only the solid contents of as much of the Glass-tube as remains within the concave surface of the Receiver, and (which is more difficult) the varying contents of the Vessel containing the Mercury, and of as much of the Mercury it self as is not in the Tube, must be deducted out of the capacity of the Receiver, but there must also an allowance be made for this, that the Cylinder that is empty'd by the drawing down of the Sucker, and comes to be fill'd upon the letting of the Air out of the Receiver into it, is not so replenish'd with Air as the Receiver it self at first was: because there passeth no more Air out of the Receiver into the Cylinder, than is requisite to reduce the Air in the cavity of the Cylinder, and in that of the Receiver to the same measure of dilatation. Because of these (I say) and some other difficulties that require more skill in Mathematicks than I pretend to, and much more leasure than my present occasions would allow me, I was willing to refer the nicer consideration of this matter to some of our Learned and accurate Mathematicians, thinking it enough for me to have given the Hint already suggested.

For farther confirmation of what hath been delivered, we likewise tried the Experiment in a Tube of less than two foot long: and, when there was so much Air drawn out of the Vessel, that the remaining Air was not able to counterballance the Mercurial Cylinder, the Quick-silver in the Tube subsided so visibly, that (the Experiment being try'd in the little Vessel lately mention'd) at the first suck it fell above a span, and was afterwards drawn lower and lower for a little while; and the external Air being let in upon it, impell'd it up again almost to the top of the Tube: So little matters it how heavy or light the Cylinder of Quicksilver to subside is, provided its gravity overpower the pressure of as much external Air as bears upon the surface of that Mercury into which it is to fall.

Lastly we also observ'd, That if (when the Mercury in the Tube had been drawn down, and by an Ingress permitted to the external Air, impell'd up again to its former height) there were some more Air thrust up by the help of the Pump into the Receiver, the Quick-silver in the Tube would ascend much above the wonted height of 27 digits, and immediately upon the letting out of that Air would fall again to the height it rested at before.

Your Lordship will here perhaps expect, that as those who have treated of the Torricellian Experiment, have for the most part maintained the Affirmative, or the Negative of that famous Question, Whether or no that Noble Experiment infer a Vacuum? so I should on this occasion interpose my Opinion touching that Controversie, or at least declare whether or no, in our Engine, the exsuction of the Air do prove the place deserted by the Air suck'd out, to be truly empty, that is, devoid of all Corporeal Substance. But besides, that I have neither the leasure, nor the ability, to enter into a solemn Debate of so nice a Question; Your Lordship may, if you think it worth the trouble, in the Dialogues not long since referr'd to, find the Difficulties on both sides represented; which then made me yield but a very wavering assent to either of the parties contending about the Question: Nor dare I yet take upon me to determine so difficult a Controversie.

For on the one side it appears, that notwithstanding the exsuction of the Air, our Receiver may not be destitute of all Bodies, since any thing placed in it, may be seen there; which would not be, if it were not pervious to those beams of Light which rebounding from the seen Object to our eyes, affect us with the sense of it: And that either these Beams are Corporeal Emanations from some lucid body, or else at least, the light they convey doth result from the brisk motion of some subtle Matter, I could, if I mistake not, sufficiently manifest out of the Dialogues above-mention'd, if I thought Your Lordship could seriously imagine that Light could be convey'd without, at least, having, (if I may so speak) a Body for its Vehicle.

By the sixteenth Experiment, it also appears that the closeness of our Receiver hinders it not from admitting the Effluvia of the Load-stone; which makes it very probable that it also freely admits the Magnetical steams of the Earth; concerning which, we have in another Treatise endeavour'd to manifest that numbers of them do always permeate our Air.

But on the other side it may be said, That as for the subtle Matter which makes the-Objects enclosed in our evacuated Receiver, visible, and the Magnetical Effluvia of the Earth that may be presum'd to pass thorow it, though we should grant our Vessel not to be quite devoid of them, yet we cannot so reasonably affirm it to be replenish'd with them, as we may suppose, that if they were gather'd together into one place without Intervals between them, they would fill but a small part of the whole Receiver. As in the thirteenth Experiment, a piece of Match was inconsiderable for its bulk, whilst its parts lay close together, that afterwards (when the Fire had scatter'd them into smoke) seem'd to replenish all the Vessel. For (as elsewhere our Experiments have demonstrated) both Light and the Effluvia of the Load-stone, may be readily admitted into a Glass, Hermatically seal'd, though before their Admission, as full of Air as hollow Bodies here below are wont to be; so that upon the exsuction of the Air, the large space deserted by it, may remain empty, notwithstanding the pretence of those subtle Corpuscles, by which Lucid and Magnetical Bodies produce their effects.

And as for the Allegations above mention'd, they seem'd to prove but that the Receiver devoid of Air, may be replenish'd with some Etherial matter, as some Modern Naturalists write of but not that it really is so. And indeed to me it yet seems, that as to those spaces which the Vacuists would have to be empty, because they are manifestly devoid of Air; and all grosser Bodies the Plenists (if I may so call them) do not prove that such spaces are replenish'd with such a subtle matter as they speak of, by any sensible effects, or operations of it (of which divers new Trials purposely made, have not yet shewn me any) but only conclude that there must be such a Body, because there cannot be a Void. And the reason why there cannot be a Void, being by them taken, not from any Experiments, or Phaenomena of Nature, that clearly and particularly prove their Hypothesis, but from their notion of a Body, whose Nature, according to them, consisting only in extension (which indeed seems the property most essential to, because inseparable from a Body) to say a space devoid of Body, is to speak in the Schoolmens Phrase, a Contradiction in Adjecto. This reason I say, being thus desum'd, seems to make the Controversie about a Vacuum, rather a Metaphysical, than a Physiological Question; which therefore we shall here no longer debate, finding it very difficult either to satisfie Naturalists with this Cartesian Notion of a Body, or to manifest wherein it is erroneous, and substitute a better in its stead.

But though we are unwilling to examine any farther the Inferences wont to be made from the Torricellian Experiment, yet we think it not impertinent to present Your Lordship with a couple of Advertisements concerning it.

First, then if in trying the Experiment here or elsewhere, you make use of the English measures that Mathematicians and Tradesmen are here wont to employ, You will, unless you be forewarn'd of it, be apt to suspect that those that have written of the Experiment have been mistaken. For whereas men are wont generally to talk of the Quick-silver's remaining suspended at the height of between six or seven and twenty Inches; we commonly observed, when divers years since we first were solicitous about this Experiment, that the Quick-silver in the Tube rested at about 29 Inches and a half above the surface of the Restagnant Quick-silver in the Vessel, which did at first both amaze and perplex us, because though we held it not improbable that the difference of the grosser English Air, and that of Italy and France, might keep the Quick-silver from falling quite as low in this colder, as in those warmer Climates; yet we could not believe that that difference in the Air should alone be able to make so great an one in the heights of the Mercurial Cylinders; and accordingly upon enquiry we found, that though the various density of the Air be not to be over-looked in this Experiment, yet the main Reason why we found the Cylinder of Mercury to consist of so many Inches, was this, That our English Inches are somewhat inferiour in length to the digits made use of in Foreign Parts, by the Writers of the Experiment.

The next thing I desire Your Lordship to take notice of, is, That the height of the Mercurial Cylinder is not wont to be found altogether so great as really it might prove, by reason of the negligence or incogitancy of most that make the Experiment. For oftentimes upon the opening of the inverted Tube into the Vessel'd Mercury, you may observe a bubble of Air to ascend from the bottom of the Tube through the subsiding Quick-silver to the top; and almost always you may, if you look narrowly, take notice of a multitude of small bubbles all along the inside of the Tube betwixt the Quick-silver and the Glass: (not now to mention the Particles of Air that lie concealed in the very Body of the Mercury) Many of which, upon the Quicksilver's forsaking the upper part of the Tube, do break into that deserted space where they find little or no resistance to their expanding of themselves. Whether this be the reason that upon the Application of warm Bodies to the emptied part of the Tube, the subjacent Mercury would be depressed somewhat lower, we shall not determine; though it seem very probable, especially since we found, that, upon the application of Linnen cloaths dipped in Water, to the same part of the Tube, the Quick-silver would somewhat ascend; as if the cold had condensed the imprisoned Air, (that pressed upon it,) into a lesser room. But that the deserted space is not wont to be totally devoid of Air, we were induced to think by several Circumstances. For when an eminent Mathematician, and excellent Experimenter, had taken great pains and spent much time in accurately filling up a Tube of Mercury, we found that yet there remained store of inconspicuous bubbles, by inverting the Tube, letting the Quick-silver fall to its wonted height; and by applying (by degrees) a red hot Iron to the outside of the Tube, over against the upper part of the Mercurial Cylinder, (for hereby the little unheeded bubbles, being mightily expanded, ascended in such numbers, and so fast to the deserted space, that the upper part of the Quick-silver seemed, to our wonder, to boil.) We farther observed, That in the trials of the Torricellian Experiment we have seen made by others, and (one excepted) all our own, we never found that upon the inclining of the Tube the Quick-silver would fully reach to the very top of the sealed end: Which argued, that there was some Air retreated thither that kept the Mercury out of the unreplenished space.

If Your Lordship should now demand what are the best expedients to hinder the intrusion of the Air in this Experiment; we must answer, That of those which are easily intelligible without ocular demonstration, we can at present suggest, upon our own trials, no better than these. First, at the open end of the Tube the Glass must not only be made as even at the edges as you can, but it is very convenient (especially if the Tube be large) that the bottom be every way bent inwards, that so the Orifice, not much exceeding a quarter of an Inch in Diameter, may be the more easily and exactly stopped by the Experimenter's finger; between which and the Quick-silver, that there may be no Air intercepted (as very often it happens that there is) it is requisite that the Tube be filled as full as possibly it can be, that the finger which is to stop it, pressing upon the accumulated and protuberant Mercury, may rather throw down some, than not find enough exactly to keep out the Air. It is also an usefull and compendious way not to fill the Tube at first quite of Mercury, but to leave near the top about a quarter of an Inch empty; for if you then stop the open end with your finger, and invert the Tube, that quarter of an Inch of Air will ascend in a great bubble to the top, and in its passage thither, will gather up all the little bubbles, and unite them with it self into one great one; so that if by reinverting the Tube, you let that bubble return to the open end of it, you will have a much closer Mercurial Cylinder than before, and need but to add a very little Quick-silver more to fill up the Tube exactly. And lastly, as for those lesser and inconspicuous parcels of Air which cannot this way begleaned up, You may endeavour before you invert the Tube, to free the Quick-silver from them by shaking the Tube, and gently knocking on the outside of it, after every little parcel of Quick-silver which you pour in; and afterwards, by forcing the small latitant bubbles of Air to disclose themselves and break, by imploying a hot Iron in such manner as we lately mentioned. I remember that by carefully filling the Tube, though yet it were not quite free from Air, we have made the Mercurial Cylinder reach to 30 Inches and above an eight, and this in a very short Tube: which we therefore mention, because we have found, by Experience, that in short Tubes a little Air is more prejudicial to the Experiment than in long ones, where the Air having more room to expand it self, doth less potently press upon the subjacent Mercury.

And since we are fallen upon the consideration of the Altitude of the Mercurial Cylinder, I must not conceal from Your Lordship an Experiment relating thereunto, which perhaps will set both You and many of your Friends the Vertuosi on thinking; and, by disclosing some things about the Air or Atmosphere, that have scarce hitherto been taken notice of, may afford you some hints conducive to a further discovery of the subject of this Epistle.

WE took a Glass Tube, which though it were not much above three Foot long, we made choice of, because it was of a more than ordinarily even thickness. This we fill'd with Mercury, though not with as much care as we could, yet with somewhat more than is wont to be used in making the Torricellian Experiment. Then, having according to the manner inverted the Tube, and open'd the mouth of it beneath the surface of some other Quick-silver, that in the Tube fell down to the wonted height, leaving, as is usual, some little Particles of Air in the space it deserted, as we guest by observing, that upon the Application of hot Bodies to the upper part of the Tube, the Quicksilver would be a little depress'd. Lastly, having put both the Tube and the Vessel it lean'd on into a convenient Wooden frame to keep them from mischances: we plac'd that Frame in a Window within my Bed-chamber, that I might both keep the Mercury from being stirr'd, and have opportunity to watch from time to time the Phaenomena it was to exhibit. For the better discovery of which, when the Quick-silver both in the Tube and subjacent Vessel was perfectly at rest, we took notice, by a mark made on the outside of the Glass, how high the included Liquor then reach'd.

During several Weeks that the Tube was kept in that Windown (which was very rarely open'd) I had the opportunity to observe, that the Quick-silver did sometimes faintly imitate the Liquor of a Weather-glass, subsiding a little in warm, and rising a little in cold Weather, which we ascribed to the greater or lesser pressure of that little Air that remain'd at the top of the Tube, expanded or condens'd by the heat or cold that affected the ambient Air. But that which I was chiefly carefull to observe, was this, That oftentimes the Quick-silver did rise and fall in the Tube, and that very notably, without conforming it self to what is usual in Weather-glasses, whose Air is at the top, nay quite contrary thereunto: for sometimes I observ'd it in very cold weather (such as this Winter hath already afforded us good store of) to fall down much lower than at other times, when by reason of the absence of both Frost, Snow, and sharp Winds, the Air was comparatively much warmer. And I farther observ'd, That sometimes the Quick-silver would for some days together rest almost at the same height; and at other times again it would in the compass of the same day considerably vary its altitude, though there appear'd no change either in the Air abroad, or in the temper of the Air within the Room (wherein was constantly kept a good Fire) nor in any thing else, to which either I, or some eminently Learned Men, whom I then acquainted with the Experiment, could reasonably impute such a change: Especially considering that the space wherein the Mercury wandred up and down, within about five Weeks, amounted to full two Inches, of which we found by our several marks whereby we had taken notice of its several removes, that it had descended about 9/16 of an Inch from the place where it first setled, and the other Inch and 7/16 it had ascended. And it seems probable that the height of the Mercurial Cylinder would have varied yet more, if the Experiment had been made in the open Air, and in a long Tube, where the Particles of the imprison'd Air by having more room to display themselves in, might not have had so strong a Spring to work upon the Quick-silver with. But for want both of time and of a competent quantity of Mercury (which was not to be procur'd where we then happen'd to be) we were unable to make any farther tryals: which therefore chiefly troubled us, because we would gladly have try'd an ingenious Experiment which was suggested unto us by that excellent Mathematician Mr. Wren, who being invited to name any thing he would have us try touching the pressure of the Air, desired us to observe whether or no the Quick-silver in a long Tube would not a little vary its height according to the Tides, especially about the New and Full Moon; about which times Mariners observe those great Flowings and Ebbs of the Sea, that they call the Spring-Tides. For he sagaciously and plausibly conjectur'd that such observations, accurately made, would discover the truth or erroneousness of the Cartesian Hypothesis concerning the Ebbing and Flowing of the Sea: which Des Cartes ascribeth to the greater pressure made upon the Air by the Moon, and the Intercurrent Ethereal Substance at certain times (of the Day, and of the Lunary Month) than at others. But in regard we found the Quick-silver in the Tube to move up and down so uncertainly, by reason, as it seems, of accidental mutation in the Air; I somewhat doubt: whether we shall find the Altitude of the Quick-silver to vary as regularly as the Experiment is ingeniously propos'd. The success we shall (God permitting us to make tryal of it) acquaint your Lordship with; and in the mean time take notice, that when we had occasion to take the Tube out of the Frame (after it had staid there part of November and part of December) a good Fire being then in the room, because it was a Snowy day, we found the Quick-silver in the Tube to be above the upper surface of the subjacent Mercury 29 Inches three quarters.

If Your Lordship should now aske me what are the true causes of this varying altitude of the Mercurial Cylinder; I should not undertake to answer so difficult a question, and should venture to say no more, than that among divers possible causes to which it may be ascribed, it would not be, perhaps, absurd to reckon these that follow.

First then we may consider, that the Air in the upper part of the Tube is much more rarefied, and therefore more weak than the External Air, as may appear by this among other things, That upon the inclining of the Tube, the Quick-silver will readily ascend almost to the very top of it, and so take up eight or nine tenth parts, and perhaps more of that space which it deserted before: which would not happen, if that whole space had been full of unrarefied Air, since that (as tryal may easily satisfie you) would not have suffer'd it self to be thrust into so narrow a room by so weak a pressure. So that although in our Tube when the included Air, was heated, the Quick-silver was somewhat depress'd; yet there is this difference betwixt such a Tube and common Weather-Glasses, that in these the included and the ambient Air are in an AEquilibrium as to pressure, and the weight of the Water that keeps them separate is scarce considerable. Whereas in such a Tube as we are speaking of, the Air within, is very much more dilated than that without. And 'tis not so much the spring or resistance of the included Air, as the weight of the Mercurial Cylinder it self, that hinders the Quick-silver from ascending higher: For if we should suppose that deserted part of the Tube perfectly devoid of Air, yet would the Quick-silver rise but a little higher in it, and be far from filling it; in regard the outward Air would not be able to impell up such a weight much higher: whereas it may, by our former Experiments appear, that if all the Air in the upper part of the Weather-Glass were away, the Water would be impell'd up to the very top of it, though the Pipe were above thirty foot long.

We may next consider, that this rarefied: Air at the upper part of our Tube being exactly shut up betwixt the Glass and the Quick-silver, it was scarce subject to any discernable alterations, save those it receiv'd from heat and cold.

And we may farther consider, that yet the external Air or Atmosphere is subject to many alterations, besides them that proceed from either of those Qualities.

For the Experiment that occasion'd this Discourse, seems to make it probable enough, that there may be strange Ebbings and Flowings, as it were, in the Atmosphere, or at least, that it may admit great and sudden Mutations, either as to its Altitude or its Density, from causes, as well unknown to us, as the effects are unheeded by us. And that You may not think that there is nothing in Nature but our Experiment that agrees with this our conjecture, we might put Your Lordship in mind of the Pains and Aches that are often complain'd of by those that have had great Wounds or Bruises, and that do presage great Mutations in the Air oftentimes, whilst to strong and healthy Persons no sign of any such thing appears. And that is also very memorable to this purpose, which I remember I have somewhere read in a Book of the Ingenious Kircherus, who giving a pertinent admonition concerning the various refractions that may happen in the Air, relateth, That during his stay in Malta, he often saw Mount AEtna, though the next day, notwithstanding its being extremely clear, he could not see it; adding, that Vintemillius, a very Learned Person, did oftentimes, from a Hill he names, behold the whole Island he calls Luprica protuberant above the Sea, though at other times, notwithstanding a clear Sky, he could not see it. And though perhaps this may be in part ascribed to the various light and position of the Sun, or to the various disposition of the Spectators eye, or peradventure to some other cause; yet the most probable cause seems to be the differing Density of the Air, occasion'd by Exhalations capable to increase the refraction, and consequently bring Beams to the Eye, which otherwise would not fall on it. We have likewise, in another Treatise, mention'd our having often observ'd with Telescopes a plenty of Steams in the Air, which without such a help would not be taken notice of, and which, as they were not at all times to be seen even through a Telescope, so they did sometimes, especially after a shower of Rain, hastily disappear: and when we have visited those places that abound with Mines, we have several times been told by the Diggers, that even when the Sky seem'd clear, there would, not seldome, suddenly arise, and sometimes long continue, a certain Steam (which they usually call a damp) so gross and thick that it would oftentimes put out their very Candels, if they did not seasonably prevent it. And I think it will easily be granted, that the ascension of such Steams into this or that part of the Air, and their mixing with it, are very like to thicken it; as on the other side either heat or the sudden condensation of the Air in another part of the Atmosphere (to mention now no other causes) are capable of rarefying it.

Nor will it very much import the main scope of our Discourse, whether it be suppos'd that the copious Steams the earth sends into the Air, thicken that part of the Atmosphere that receives them, and make it more heavy: Or that sometimes the Fumes may ascend with such celerity, that though the Air be thickn'd yet they rather diminish than encrease its gravitation, in regard that the quickness of their ascent, not onely keeps them from gravitating themselves, but may hinder the pressing downwards of many Aërial Corpuscles that they meet with in their way upwards. This I say, is of no great importance to our present Discourse, since either way the Terrestrial Steam may here and there considerably alter the gravity or pressure of the Atmosphere.

Your Lordship may also be pleased to remember, That by our seventeenth Experiment it appear'd, that, as when the Air in the Receiver was expanded more than ordinarily, the Quick-silver in the Tube did proportionably subside; so when the Air in the same Receiver was a little more than ordinarily compress'd, it did impell up the Quick-silver in the Tube, above the wonted height of betwixt six and seven and twenty digits.

And if to these things we annex, that for ought we can find by tryals purposely made, the degree of rarity or density of the Air, shut up into our Receiver, doth not sensibly alter its temperature as to cold or heat. It will not, I hope, appear absurd to conceive, That since the Air, included in the Tube, could but very saintly hinder the ascent of the Quick-silver, or press it downwards; Since too, that included Air could scarce immediately receive any sensible alteration, save either by heat or cold; And since also, that according to the bare density or rarity of the Air incumbent on the subjacent Quick-silver in the Vessel, that in the Tube was impell'd more or less high; such changes happening in the neighbouring part of the outward Air, either by the ascension of gross or copious exhalations, or by any other cause (of which there may be divers) as were capable to make considerable alterations in the consistence of the Air, as to rarity and density, may be able proportionably to alter the height of the Quick-silver. I rather say, that such alterations may be, than that they are the causes of our Phaenomenon: because I think it sufficient, if I have propos'd conjectures not altogether irrational about a new Mystery of Nature, touching which, the chief things I pretend to, is to give occasion to the Curious to enquire farther into it, than I have been yet able to do.

THe same Reason that mov'd us to conclude, that by the drawing of the Air out of the Receiver, the Mercury would descend in a Tube shorter than six and twenty digits, induc'd us also to expect, that by the same means Water might be brought to subside in Glass Tubes of a moderate length: Though by the noble Experiment, said to have been accurately made in France, by Monsieur Paschal, we are informed that a Tube of no less than about two and thirty Foot; was found requisite to make the Experiment De vacuo succeed with Water instead of Quick-silver: so tall a Cylinder of that lighter Liquor, being it seems, requisite to equal the Weight of a Mercurial Cylinder of six or seven and twenty digits, and surmount the pressure of the Atmosphere.

We took then a Tube of Glass, Hermetically seal'd at one end, of about four foot in length, and not very slender: This, at the open end, we fill'd with common Water, and then stopt that end till we had inverted the Tube, and open'd it beneath the surface of a quantity of the like Water, contain'd in a somewhat deep and slender Vessel. This Vessel, with the Tube in it, was let down into the Receiver, and the Receiver being clos'd up after the accustom'd manner, the Pump was set on work.

As much of the event as concerns our present purpose, was this, That till a considerable part of the Air was drawn out of the Receiver, the Tube continu'd top-full of Water as when it was put in, it being requisite that a great part of the Air, formerly contain'd in the Receiver, should be drawn out, to bring the remaining Air to an AEquilibrium, with so short and light a Cylinder of Water. But when once the Water began to fall in the Tube, then each exsuction of Air made it descend a little lower, though nothing near so much as the Quick-silver at the beginning did in the Experiment formerly mention'd. Nor did there appear so much inequality in the spaces transmitted by the Water in its descent, as there it did in those observ'd in the fall of the Quick-silver, of which the cause will scarce seem abstruse to him that shall duly reflect upon what hath been already deliver'd. And whereas we drew down the Quick-silver in the Tube so far as to bring it within an Inch of the surface of the other Quick-silver into which it was to fall; the lowest we were able to draw down the Water was, by our conjecture, to about a Foot or more above the surface of that in the Vessel; of which I know not whether it will be needfull to assign so obvious a cause as that, though the little Air remaining in the Receiver could not hinder a Cylinder of above on Inch high of Quick-silver from subsiding; yet it might be very well able, by its pressure, to countervail the weight of a Cylinder of a Foot long or more, of a Liquor so much less ponderous than Quick-silver, as Water is. And in fine, to conclude our Experiment, when the Water was drawn down thus low, we found, that by letting in the outward Air, it might be immediately impell'd up again to the higher parts of the Tube.

We will adde no more concerning this Experiment, save that having try'd it in one of our small Receivers, we observ'd, That upon the first exsuction of the Air the Water did usually subside divers Inches, and at the second (exsuction) fall down much lower, subsiding sometimes near two Foot; as also that upon the letting in of the Air from without, the Water was impell'd up with very great celerity.

THat the Air hath a notable Elastical power (whencesoever that proceeds) we have, I suppose, abundantly evinc'd, and it Begins to be acknowledg'd by the eminentest Modern Naturalists. But whether or no there be in Water so much as a languid one, seems hitherto to have been scarce consider'd nor hath been yet, for ought I know, determin'd either way by any Writer, which invited us to make the following Experiment.

There was taken a great Glass bubble, with a long neck; (such as Chymists are wont to call a Philosophical Egge) which being fill'd with common Water till the Liquor reach'd about a span above the bubble, and a piece of Paper being there pasted on, was put unstop'd into the Receiver, and then the Air was suck'd out after the wonted manner. The event was this, that a considerable part of the Air, pen'd up in the Receiver, was drawn out, before we discern'd any expansion of the Water; but, continuing the labour of pumping, the Water manifestly began to ascend in the stem of the Glass, and divers bubbles loosening themselves from the lower parts of the Vessel, made their way through the Body of the Water, to the top of it, and there brake into the Receiver: And after the Water once appear'd to swell, then at each time the Stop cock was turn'd to let out the Air from the Receiver into the Pump, the Water in the Neck of the Glass did suddenly rise about the breadth of a Barly-corn in the Neck of the Glass and so attain'd by degrees, to a considerable height above the mark formerly mention'd. And at length (to make the expansion of the Water more evident) the outward Air was suddenly let in, and the Water immediately subsided and deserted all the space it had newly gain'd in the Glass.

And, on this occasion, it will not perhaps be amiss to acquaint Your Lordship here (though we have already mention'd it in another Paper, to another purpose) with another Expedient that we made use of two or three years ago, to try whether or no Water had a Spring in it. About that time then, That Great and Learned Promoter of Experimental Philosophy Dr. Wilkins, doing me the Honour to come himself, and bring some of his inquisitive Friends to my Lodging, we there had in readiness a round and hollow Vessel of Pewter, great enough to contain two pounds of Water, and exactly close every where, but at one little hole where it was to be fill'd, then partly by sucking out the Air, and partly by injecting Water with a Syringe, it was (not without some difficulty) fill'd up to the top; and that hole being plac'd directly upwards, there was a little more Water leisurely forc'd in by the Syringe. Upon which, though the Vessel were permitted to rest, and the hole kept in its former posture, yet the compress'd Water leisurely swell'd above the Orifice of the hole, and divers drops ran over along the sides of the Vessel. After this we caus'd a skilfull Pewterer (who had made the Globe) to close it up in our presence with Soder so exquisitely, that none suspected there was any thing left in it besides Water. And lastly, the Vessel thus soder'd up, was warily and often struck in divers places with a Wooden Mallet, and thereby was manifestly compress'd, whereby the inclosed Water was crouded into less room than it had before: And thereupon we took a Needle, and with it and the Mallet perforated the Vessel, and drew out the Needle again; the Water (but in a very slender Stream) was suddenly thrown after it into the Air, to the height of two or three Feet. As for the other Phaenomena of this Experiment, since they belong not to our present purpose, and are partly mention'd in another of our Papers, we shall, instead of recording them here, give this Advertisement: That as evidently as this Experiment, and that made in our Receiver, seem to prove a power in the Water to expand and restore it self after compression; yet for a reason to be met with ere long, I judged it not safe, to infer that Conclusion from these Premises, till I had made some of the following tryals, to the mention of which I will therefore hasten.

TO discover whether the Expansion of the Water really proceeded from an Elastical power in the parts of the Water it self, we thought it requisite to try two things: The one, Whether or no the Atmosphere gravitates upon Bodies under Water; and the other, Whether in case it do gravitate, the Intumescence of the Water may not be ascribed to some substance subtler than it self, residing in it. In order to the satisfying myself about the first of these, I intended to let down into the Receiver a Vessel of Water, wherein should be immers'd a very small oyl'd Bladder, almost devoid of Air, but strongly ty'd up at the Neck with a string, and detain'd a little under Water by such a weight fasten'd to that string, as should just be able to keep the Bladder from swimming, and no more. For I suppos'd, that if when all things were thus order'd, the Receiver were empty'd, in case there were any such pressure of the Atmosphere upon Water, as I was inclin'd to believe, the Air within the Bladder, being upon the exsuction of the Air within the Receiver, freed from that pressure, and being press'd onely by the small weight of the incumbent Water, would considerably expand it self; but whilst we were preparing Bladders for this Experiment, there occurr'd an easie way for the making at once both the Discoveries I desir'd.

We took then a Glass Viol, containing by ghess a pound and some ounces of Water, this we fill'd top full, and then we put into the Neck of it a Glass Pipe a pretty deal bigger than a Goose Quill, open at both ends, and of divers Inches in length: One end of this Pipe was so put into the Neck of the Viol, as to reach a little below it, and then was carefully cemented thereto that no Air might get into the Viol, nor any Water get out of it, otherwise than through the Pipe; and then the Pipe being warily fill'd, about half way up to the top, with more Water, and a mark being pasted over aganst the upper surface of the Liquor; the Viol thus fitted with the Pipe, was, by strings let down into the Receiver, and according to the wonted manner exquisitely clos'd up in it.

This done, we began to pump out the Air, and when a pretty quantity of it had been drawn away, the Water in the Pipe began to rise higher in the Pipe, at the sides of which some little bubbles discover'd themselves. After a little while longer, the Water still swelling, there appear'd at the bottom of the Pipe a bubble about the bigness of a small Pea, which ascending through the Pipe to the top of the Water, stay'd there a while and then broke; but the Pump being nimbly ply'd, the expansion of the Water so encreas'd, that quickly, getting up to the top of the Pipe, some drops of it began to run down along the out-side of it, which oblig'd us to forbear pumping a while, and give the Water leave to subside within less than two Inches of the bottom of the Pipe. After this the Pump being again set on work, the bubbles began to ascend from the bottom of the Pipe, being not all of a size, but yet so big, that estimating one with another, they appear'd to be of the size of the smaller sort of Peas; and of these we reckon'd about sixty which came up one after another, besides store of smaller ones, of which we made no reckoning: And at length, growing weary of reckoning and pumping too (because we found, that in spight of all our pains and industry, some undiscern'd Leak or other in the Receiver hinder'd us from being able to empty it altogether) we thought fit to desist for that time, after tryal made of what operation the external Air, being let in upon the expanded Water, would have; and accordingly turning the Key to let in the Air, we saw, as we expected, that the Water in the Pipe in a moment fell down almost to the bottom of it.

Now of this Experiment there are two or three Circumstances yet to be mention'd, which are no less, than those already recited, pertinent to our present purpose.

In the first place then, when the greater part of the Air had been pump'd out of the Receiver, the rising bubbles ascended so very slowly in the Pipe, that their Progress was scarce discernable; which seem'd to proceed from this, That their bigness was such, That they could not sufficiently extend themselves in the cavity of the Glass, without pressing on both hands against the sides of it, whereby they became of more difficult extrusion to the Water. And though it may seem strange that these bubbles should be of any considerable bulk, since 'tis like they consisted of lesser parcels of the Air lurking in the Water, than those that were vigorous enough to make their way through, long before them: yet they were commonly much larger than before, some of them being equal in quantity to four or five Peas: whether this their increase of bulk proceeded from the greater decrement of the pressure of the Air, or from the Union of two or three of those numerous bubbles which were then generated below the bottom of the Pipe, where we could not see what was done among them.

Another thing we noted in our bubbles was, That whereas in ordinary ones the Air, together with the thin film of Water that invests and detains it, is wont to swell above the surface of the Water it swims on, and commonly to constitute Hemispherical Bodies with it, the little parcels of Air that came up after the Receiver was pretty well empty'd, did not make protuberant bubbles, but such whose upper surface was either level with, or beneath that of the Water, so that the upper surface being usually somewhat convex, the less protuberant parts of it had a pretty quantity of Water remaining above them.

We also farther observ'd, that whereas in the bubbles that first appear'd in the Pipe, the ascending Air did, as in other common bubbles, make its way upwards, by dividing the Water through which it pass'd, in those bubbles that appear'd at the latter end of our Experiment, when the pressure of the little external Air, remaining in the Receiver, was grown inconsiderable, the ascending parcels of Air having how little more than the weight of the incumbent Water to surmount, were able both so to expand themselves as to fill up that part of the Pipe which they pervaded, and by pressing every way against the sides of it, to lift upwards with them what Water they found above them, without letting any considerable quantity glide down along the sides of the Glass; So that sometimes we could see a bubble thrust on before it a whole Cylinder of Water of perhaps an Inch high, and carry it up to the top of the Pipe; though as we formerly noted, upon the letting in the external Air, these tumid bubbles suddenly relaps'd to their former inconspicuousness.

All these things laid together seem'd sufficiently to confirm that, which the consideration of the thing it self would easily enough perswade, namely, That the Air, and such like Bodies being under Water, may be press'd upon as well by the Atmosphere, as by the weight of the incumbent Water it self.

Hence likewise we may verify what we observ'd at the close of the foregoing Experiment, namely, That from the sole swelling of Water there recorded, it cannot be so safely concluded, that Water, when freed from compression, is endow'd with an Elastical power of expanding it self: since thereby it appears that the Intumescence produc'd by that Experiment, may, (at least in great part) be ascribed to the numerous little bubbles which are wont to be produc'd in Water, from which the pressure of the Atmosphere is in great measure taken off. So apt are we to be mis-led, even by Experiments themselves, into Mistakes, when either we consider not that most Effects may proceed from various Causes, or mind onely those Circumstances of our Experiment, which seem to comply with our preconceiv'd Hypothesis or Conjectures.

And hence it seems also probable, that in the Pores or invisible little recesses of Water it self there lie commonly interspers'd many parcels of either Air, or at least something Analagous thereunto, although so very small, that they have not been hitherto so much as suspected to lurk there. But if it be demanded how it appears that there is interspers'd through the Body of Water any substance thinner than it self, and why that which produc'd the bubbles abovemention'd should not be resolutely said to be nothing else than a more active and spirituous part of the Water, we shall, in order to the Elucidation of this matter, subjoyn to what was formerly deliver'd the following Experiment.

SInce the writing of the twenty first and twenty second Experiments (and notwithstanding all that hath been on their occasion deliver'd concerning Bubbles) we made some farther trials in prosecution of the same inquiry whereto they were designed.

We chose then, amongst those Glasses which Chymists are wont to call Philosophical Eggs, one that containing about nine Ounces of Water, had a Neck of half an Inch in Diameter at the top, and as we guest, almost an Inch at the bottom; which breadth we pitched upon for a reason that will by and by appear: then filling it up with common Water to the height of about a Foot and an half, so that the upper part remained empty, we shut it into the Receiver, and watch'd what would follow upon pumping, which proved that a great part of the Air being drawn out, the Bubbles began to discover themselves at the bottom and sides of the Glass; and increasing, as the Air was more and more drawn away, they did from time to time ascend copiously enough to the top of the Water, and there quickly break: but by reason that the wideness of the Glass allow'd them free passage through the Water, they did not appear as in the former Experiments to make it swell: The Water scarce ever rising at all above the mark affixt to its upper surface, when it was put in, and upon the return permitted to the outward Air, and consequently the shrinking in of the remaining bubbles, the Water seem'd to have lost of his first extent, by the avolation of the formerly interspers'd Air.

Being willing likewise to try whether distilled Water were, by having been divided into minute parts, and then re-united, more or less dispos'd to expand it self than Water not distill'd: We took out of our Laboratory some carefully distill'd Rain-water, and put about two Ounces of it into a round Glass-bubble, with a very small Neck (not exceeding the sixth part of an Inch in diameter) which we filled half way to the top, and then convey'd it into the Receiver; the issue was, that though we drew out more Air than ordinary, yet there appeared not the least intumescence of the Water, nor any ascending bubbles.

But suspecting that either the small quantity of the Water or the figure of the Vessel might have an interest in this odd Phaenomenon, we took the lately mentioned Philosophical Egg, and another not much differing from it; the former we fill'd up with distill'd Rain-water to the old mark, and into the latter we put a long Cylinder or Rod of solid Glass to straiten the cavity of the Neck by almost filling it up; and then pouring some distilled Water into that also, till it reach'd within some Fingers breadth of the top, the Eggs were let down into the Receiver. In this Experiment the Air was so far drawn forth, before there appeared any bubble in either of the Glasses, that the disparity betwixt this and common Water was manifest enough. But at length, when the Air was almost quite pump'd out, the bubbles began to disclose themselves, and to increase as the pressure of the Air in the Receiver decreas'd. But whereas in the first mentioned Philosophical Egg the bubbles were very small, and never able to swell the Water, that we took notice of, at all above the mark: In the other, whose Neck, as we lately said, was straitned, and their passage obstructed, great numbers of them, and bigger, fastned themselves to the lower end of the Glass-rammer (if we may so call it) and gather'd in such numbers between that and the sides of the Neek, that the Water swell'd about a Finger's breadth above the mark, though upon the admitting of the external Air it relapsed to the former mark, or rather fell somewhat below it. And although thereupon in the first named Vessel all the bubbles presently disappeared, yet in the other we observed, that divers remained fastned to the lower part of the Glass-rammer, and continued there somewhat to our wonder, for above an hour after, but contracted in their dimentions.

Moreover, having suffered the Glasses to remain above twenty four hours in the Receiver, we asterwards repeated the Experiment, to try what change the exsuction of the external Air would produce in the Water, after the internal and latitant Air had (as is above recited) in great measure got away in bubbles, and whether or no the Water would by standing re admit any new particles of Air in the room of those that had forsaken it. But though we exhausted the Receiver very diligently, yet we scarce saw a bubble in either of the Glasses, not with standing which, we perceiv'd the Water to rise about the breadth of a Barly-corn, or more, in the Neck of that Glass wherein the solid Cylinder had been put; the Liquor in the other Glass not sensibly swelling.

And lastly upon the letting in of the Air, the Water in the straitned Neck soon subsided to the mark above which it had swollen, which whether it ought to be ascrib'd to the same small expansion of the parts of the Water it self, or to the rarifaction of some yet latitant Air broken into such small particles, as to escape our observation, seems not easily determinable, without such farther tryals, as would perhaps prove tedious to be recited as well as to be made; though I was content to set down those already mentioned, that it might appear how requisite it is in nice Experiments to consider variety of Circumstances.

BEing desirous to discover what difference there might be as to gravity and levity, between Air expanded under Water, and it self before such expansion; we took two very small Viols, such as Chymical Essences (as they call them) are wont to be kept in, and of the size and shape expressed by the eighth Figure: into one of these we put so much of a certain ponderous Mercurial mixture (hapning to be then at hand) that the mouth being stopt with a little soft Wax, the Glass would just sink in Water and no more; this we let fall to the bottom of a widemouth'd Christal Jar, fill'd with about half a pint of common Water, and into the same Vessel we sunk the other Essenceglass unstop'd, with as much water in it as was more than sufficient to make it subside. Both these sunk with their mouths downwards, the former being about three quarters full of Air, the latter containing in it a bubble of Air that was guess'd to be of the bigness of half a Pea: This done, the wide mouth'd Glass was let down into the Receiver, and the way of employing the Engine was carefully made use of.

The success was, That having drawn out a pretty quantity of Air, the bubbles began to disclose themselves in the Water, as in the former Experiments; and though for a good while after the bubbles ascended in swarms from the lower parts of the Water, and hastily broke at the top; yet we prosecuted the Experiment so long without seeing any effect wrought upon the Essence-bottles, that we began to despair of seeing of them rise. But continuing to ply the Pump, that little Glass, whose mouth was open'd, came to the top of the Water, being, as it were, boy'd up thither by a great number of bubbles that had fastned themselves to the sides of it; swimming thus with the mouth downward, we could easily perceive that the internal Air abovemention'd had much dilated it self, and thereby seem'd to have contributed to the emerging of the Glass, which remain'd floating, notwithstanding the breaking and vanishing of most of the contiguous bubbles: being hereby incouraged to persist in pumping, we observed with some pleasure, that at each time we turn'd the Key, the Air in the little Glass did manifestly expand it self and thrust out the Water, generally retaining a very protuberant surface where it was contiguous to the remaining Water. And when after divers exsuctions of the Air in the Receiver, that in the Viol so dilated it self as to expel almost all the Water, it turn'd up its mouth towards the surface of the Water in the Jar, and there deliver'd a large bubble, and then relapsed into its former floating posture. And this Experiment taught us, among other things, that it was a work of more time and labour than we imagin'd, to exhaust our Engine as much as it may be exhausted: for although before the emerging of the small Viol, we did (as hath been touch'd already) think we had very considerably emptied the Receiver, because there seem'd to come out but very little or almost no sensible Air at each exsuction into, and out of the Cylinder; yet afterwards, at each drawing down the Sucker, the Air included in the Viol did manifestly dilate it self, so long, that it did no less than nine times turn its mouth upwards, and discharge a bubble by conjecture about the bigness of a Pea, after the manner newly recited. But as for that Viol which had the weight in it, it rose not at all. So that being not able by quick pumping to gain another bubble from the Air in the swimming Glass, which proceeded from some small leak in the Vessel, though it held in this Experiment more stanch than was usual, we thought fit to let in leasurely the Air from without, upon whose admission that within the Viol shrinking into a very narrow compass, the Glass did, as we expected, fall down to the bottom of the Jar.

But being desirous before we proceed to any new Experiment, to try once more whether the little Glass, that had the weight in it, might not also be rais'd: After we had suffer'd the Engine to remain clos'd as it was, for five or six hours, the Pump was again ply'd with so much obstinacy, that not onely about the upper part of the Jar there appear'd a good number of bubbles (but very much smaller than those we saw the first time) but afterwards, there came from the bottom of the Jar, bubbles about the bigness of small Peas: which the Pump being still kept going, follow'd one another to the number of forty, coming from the stopp'd Viol; whose mouth, it seems, had not been shut so strongly and closely, but that the included Air, dilating it self by its own spring, made it self some little passage betwixt the Wax and the Glass, and got away in these bubbles; after which, the unstopt Glass began to float again, the Air shut up in it being manifestly so dilated as to expel a good part of the Water, but not so much as to break quite thorough. And at length, when our expectation of it was almost tired out, the heavier of the two Viols began to come alost, and immediately to subside again; which appear'd to be occasion'd by the Air within it, whose bulk and spring being weaken'd by the recess of the forty bubbles before-mention'd, it was no longer able, as formerly, to break forcibly through the incumbent Water; but forming a bubble at the mouth of the Glass, boyed it up towards the top; and there getting away, left it to sink again till the pressure of the Air in the Receiver being farther taken off, the Air in the Viol was permitted to expand it self farther, and to create another bubble, by which it was again for a while carried up. And it was remarkable, that though after having emptied the Receiver as far as well we could, we ceased from pumping; yet the Vessel continuing more stanch than it was wont, this ascent and fall of the Viol was repeated to the ninth time; the included Air, by reason of the smallness of the vent at which it must pass out, being not able to get away otherwise than by little and little; and consequently, in divers such parcels as were able to constitute bubbles, each of them big enough to raise the Viol and keep it alost untill the avolation of that bubble. Whereby it may appear, that the grand rule in Hydrostaticks, That a Body will swim in the Water, in case it be lighter than as much of the Water as equals it in bulk, will hold likewise when the pressure of the Atmosphere is in very great measure, if not when it is totally taken off from the Liquor and the Body: though it were worth inquiring what it is that so plentifully concurs to fill the bubbles made in our Experiment by the so much expanded Air. For to say with the old Peripatetick Schools, That the Air in rarefaction, may acquire a new extent, without the admission of any new substance, would be an account of the Phaenomenon very much out of date, and which, I suppose, our Modern Naturalists would neither give nor acquiesce in.

I know not whether it may be requisite to add, that in this Experiment, as in the former, the outward Air, being let in, did soon precipitate the floating Viol. But I think it will not be amiss to note, that (congruously to what hath been above recorded of the vast expansion of the Air) the Water which in the heavier Viol, succeeded in the room of those forty odd, if not fifty great bubbles of Air, which at several times got out of it, amounted but to a very inconsiderable bigness.

THAT the Air is the medium whereby sounds are convey'd to the Ear, hath been for many Ages, and is yet the common Doctrine of the Schools. But this Received Opinion hath been of late oppos'd by some Philosophers upon the account of an Experiment made by the Industrious Kircher, and other Learned Men: who have (as they assure us) observ'd, That if a Bell, with a Steel Clapper, be so fasten'd to the inside of a Tube, that upon the making the Experiment De Vacuo with that Tube, the Bell remain'd suspended in the deferted space at the upper end of the Tube: And if also a vigorous Load-stone be apply'd on the outside of the Tube to the Bell, it will attract the Clapper, which upon the removal of the Load-stone falling back, will strike against the opposite side of the Bell, and thereby produce a very audible sound; Whence divers have concluded, That 'tis not the Air, but some more subtle Body that is the medium of sounds. But because we conceiv'd that, to invalidate such a consequence fro m this ingenious Experiment (though the most luciferous, that could well be made without some such Engine as ours) some things might be speciously enough alledg'd; we thought fit to make a trial or two, in order to the discovery of what the Air doth in conveying of sounds, reserving divers other Experiments triable in our Engine concerning sounds, till we can obtain more leasure to prosecute them. Conceiving it then the best way to make our trial with such a noise as might not be loud enough to make it difficult to discern slighter variations in it; but rather might be, both lasting, (that we might take notice by what degrees it decreas'd;) and so small, that it could not grow much weaker without becoming imperceptible. We took a Watch, whose Case we open'd, that the contain'd Air might have free egress into that of the Receiver. And this Watch was suspended in the cavity of the Vessel onely by a Pack-thread, as the unlikeliest thing to convey a sound to the top of the Receiver: And then closing up the Vessel with melted Plaister, we listen'd near the sides of it, and plainly enough heard the noise made by the ballance. Those also of us, that watch'd for that Circumstance, observ'd, that the noise seem'd to come directly in a streight Line, from the Watch unto the Ear. And it was observable to this purpose, that we found a manifest disparity of noice, by holding our Ears near the sides of the Receiver, and near the Cover of it: which difference seem'd to proceed from that of the Texture of the Glass, from the structure of the Cover (and the Cement) through which the sound was propagated from the Watch to the Ear. But let us prosecute our Experiment. The Pump after this being imploy'd, it seem'd that from time to time the sound grew fainter and fainter; so that when the Receiver was empty'd as much as it us'd to be for the foregoing Experiments, neither we, nor some Strangers that chanc'd to be then in the Room, could, by applying our Ears to the very sides, hear any noise from within; though we could easily perceive that by the moving of the hand which mark'd the second minutes, and by that of the ballance, that the Watch neither stood still, nor remarkably varied from its wonted motion. And to satisfy our selves farther that it was indeed the absence of the Air about the Watch that hinder'd us from hearing it, we let in the external Air at the Stop-cock, and then, though we turn'd the Key and stopt the Valve, yet we could plainly hear the noise made by the ballance, though we held our Ears sometimes at two Foot distance from the outside of the Receiver. And this Experiment being reiterated into another place, succeeded after the like manner. Which seems to prove, that whether or no the Air be the only, it is at least, the principal medium of Sounds. And by the way it is very well worth noting, that in a Vessel so well clos'd as our Receiver, so weak a pulse as that of the ballance of a Watch should propagate a motion to the Ear in a Physically streight Line, notwithstanding the interposition of so close a Body as Glass, especially Glass of such thickness as that of our Receiver: since by this it seems that the Air imprison'd in the Glass, must, by the motion of the ballance, be made to beat against the concave part of the Receiver, strongly enough to make its convex part beat upon the contiguous Air, and so propagate the motion to the Listners Ears. I know this cannot but seem strange to those, who, with an eminent Modern Philosopher, will not allow that a Sound, made in the cavity of a Room, or other place so clos'd, that there is no intercourse betwixt the external and internal Air, can be heard by those without, unless the sounding Body do immediately strike against some part of the inclosing Body. But not having now time to handle Controversies, we shall onely annex, That after the foregoing Experiment, we took a Bell of about two Inches in Diameter at the bottom, which was supported in the mid'st of the cavity of the Receiver by a bent stick, which by reason of its Spring press'd with its two ends against the opposite parts of the inside of the Vessel: in which, when it was clos'd up, we observed that the Bell seemed to sound more dead than it did when just before it sounded in the open Air. And yet, when afterwards we had (as formerly) emptyed the Receiver, we could not discern any considerable change (for some said they observ'd a small one) in the loudness of the sound. Whereby it seemed that though the Air be the principal medium of sound, yet either a more subtle matter may be also a medium of it, or else an ambient Body that contains but very few particles of Air, in comparison of those it is easily capable of, is sufficient for that purpose. And this, among other things, invited us to consider, whether in the above-mentioned Experiment made with the Bell and the Load-stone, there might not in the deserted part of the Tube remain Air enough to produce a sound: since the Tubes for the Experiment De Vacuo (not to mention the usual thinness of the Glass) being seldom made greater than is requisite, a little Air might bear a not inconsiderable proportion to the deserted space: And that also, in the Experiment De Vacuo, as it is wont to be made, there is generally some little Air that gets in from without; or at least store of bubbles that arise from the Body of the Quick-silver, or other Liquor it self, Observations heedfully made have frequently informed us: And it may also appear, by what hath been formerly delivered concerning the Torricellian Experiment.

On the occasion of this Experiment concerning sounds, we may add in this place, That when we tried the Experiment formerly mentioned, of firing Gun-powder with a Pistol in our evacuated Receiver, the noise made by the striking of the Flint against the Steel, was exceeding languid in comparison of what it would have been in the open Air. And on divers other occasions it appeared that the sounds created within our exhausted Glass, if they were not lost before they reach'd the Ear, seem'd at least to arrive there very much weaken'd. We intended to try whether or no the Wire-string of an Instrument shut up into our Receiver, would, when the ambient Air was suck'd out, at all tremble, if in another Instrument held close to it, but with out the Receiver, a string tun'd (as Musicians speak, how properly I now examine not) to an Unison with it, were briskly toucht, and set a Vibrating. This, I say, we purpos'd to try to see how the motion made in the Air without, would be propagated through the cavity of our evacuated Receiver. But when the Instrument wherewith the tryal was to be made came to be imploy'd, it prov'd too big to go into the Pneumatical Vessel: and we have not now the conveniency to have a fitter made.

We thought likewise to convey into the Receiver a long and slender pair of Bellows, made after the fashion of those usually employ'd to blow Organs, and furnished with a small Musical instead of an ordinary Pipe. For we hop'd, that by means of a string fastned to the upper part of the Bellows, and to the moveable stopple that makes a part of the Cover of our Receiver, we should, by frequently turning round that stopple, and the annexed string, after the manner already often recited, be able to lift up and distend the Bellows; and by the help of a competent weight fastned to the same upper part of the Bellows, we should likewise be able, at pleasure, to compress them: and by consequence, try whether that subtler matter than Air (which, according to those that deny a Vacuum, must be suppos'd to fill the exhausted Receiver) would be able to produce a sound in the Musical Pipe; or in a Pipe like that of ordinary Bellows, to beget a Wind capable to turn or set on moving some very light matter, either shap'd like the Sails of a Wind-Mill, or of some other convenient form, and exposed to its Orifice. This Experiment, I say, we thought to make, but have not yet actually made it for want of an Artificer to make us such a pair of Bellows as it requires.

We had thoughts also of trying whether or no, as Sounds made by the Bodies in our Receiver become much more languid than ordinary, by reason of the want of Air: so they would grow stronger, in case there were an unusual quantity of Air crouded and shut up in the same Vessel. Which may be done (though not without some difficulty) by the help of the Pump, provided the Cover and Stopple be so firmly fasten'd (by binding and Cement, or otherwise) to the Glass and to each other, that there be no danger of the condensed Airs blowing of either of them away, or its breaking through the junctures. These thoughts, My Lord, as I was saying, we entertain'd; but for want of leasure, as, of as good Receivers as ours, to substitute in its place, in case we should break it before we learn'd the skill of condensing the Air in it, we durst not put them in practice: Yet, on this occasion, give me leave to advertise Your Lordship once for all, That though for the reasons newly intimated, we have, Only in the seventeenth Experiment, taken notice, that by the help of our Engine the Air may be condens'd as well as rarefied; yet there are divers other of our Experiments, whose Phaenomena it were worth while to try to vary, by means of the compression of the Air.

WE taught, among divers other things, when we discoursed of our first Experiment, That the Air shut up in our Receiver, presseth as strongly upon the Bodies shut up with it, as if they were exposed to the pressure of the whole Atmosphere. That this was not inconsiderately propounded, we hope Your Lordship hath gather'd from divers of the things already recited: But yet perhaps it will not be amiss to subjoin, by way of farther confirmation of the same truth, the following Experiment: which would have accompanied the 20th, but the Paper wherein the one was written chanced not to be at hand, when the other was sent away.

We convey'd into the Receiver a new Glass Viol, capable of holding about 6 or 7 ounces of Water, into which we had before put 2 or 3 Spoonfulls of that Liquor, and stopt it close with a fit Cork. The Pneumatical Vessel being emptyed, there appear'd not any change in the inclos'd Water, the Air imprisoned with it, not having the force to blow out the stopple. Which event, though it were no other than we expected, was differing from what we desir'd. For we would gladly have seen what change would have appeared in the Water upon the Bottles being suddenly unstopp'd, in a place where the ambient Body was so differing from our common Air. Wherefore we did again put in the Viol, but less strongly clos'd than formerly, though as strongly stopt as seem'd requisite on ordinary occasions: But when the Air was pumped out of the Receiver, that within the Viol did quickly, as we expected, find or make it self little passages to get out at: as we argued, from this, That whereas when the Viol was put in the time before, the Water remained all the while perfectly free from bubbles; at this time the bottom of the Glass appear'd all covered with them, and they, upon the egress of the excluded Air into the Receiver, did presently flag and shrink up.

From these trials it seem'd deducible enough, that whilst the Viol continued to be well stopt, the included Water did, from the Air, shut up with it, sustain a pressure equal to that of the Atmosphere; since till the Air could get out of the Glass, there appeared no bubbles in the Water, notwithstanding the want of pressure in the ambient Body.

But to be sure to reach the chief end of our Experiment, we made use of this other expedient: We caused a convenient quantity of Water to be put, and Hermetically shut up into a Glass Egg, whose long Neck (which was purposely made of an unequal thickness) was fastened to one end of a string, whose other end was tied to the Cover of our Receiver, after the manner elsewhere mentioned already: Then the Egg being conveyed into the Pneumatical Vessel, and that being evacuated, we did, by turning the brass Stopple formerly describ'd amongst the parts of our Engine, so shorten the string as to break the Glass; whereby liberty being given to the Air imprisoned in the Egg, to pass into the cavity of the Receiver, the sudden recess of the Air made the bubbles in a trice appear so numerous, and ascend so swiftly in the Water, that their motion look'd like that of a violent shower of Rain; save that the bubbles did not, like the drops of Rain, tend downwards, but upwards. Which made me resemble this Phaenomenon to what I have seen happen in the dissolution of Seed-Pearl in some acid Menstruum, inwhich, if a good quantity of the little Pearls be cast whole, they will at first, if the Menstruum be sharp enough, be carried in swarms from the bottom to the top of the Liquor. We will add, that without sealing up the Glass, this Experiment may be tryed in one of our smallest Receivers. For there the exsuction of the ambient Air may be performed so nimbly, that immediately the bubbles lurking in the Water are allowed to display themselves, and ascend in throngs; insomuch, as having in such a Receiver try'd the Experiment with Wine as a more spirituous Liquor) instead of Water, the Red-Wine appeared all cover'd, with a copious, but vanishing white Froth, almost as if a Vessel full of bottl'd drink had been unwarily opened.

IT may not a little conduce to the clearer explication of divers Points in the Doctrine of Meteors, and perhaps of some other Physiological difficulties, to discover what the Air doth to the motion of those Steams or Exhalations that ascend into it, namely, Whether they mount upwards by virtue of any such positive levity (as some Peripateticks speak) acquir'd together with their Aërial nature, as inables them to pierce through part of the Atmosphere, and over-come its resistance. Or else, whether these steams being once raised above the Earth by their agitation, have their ascent and sustentation aloft, rather promoted than hindred by the Air: as the inferior parts of that, being thicker and heavier than the superior, the steams can more easily continue for a while their agitation upwards than downwards; And afterwards are by the same fluidity and thickness of the Air, carried to and fro in it, and kept from relapsing to the Earth: as in the Sea-Water the saline parts are kept from subsiding by those aqueous ones wherewith they are associated.

We hop'd to illustrate this matter, by observing the motion of the smoke, proceeding from kindled or flaming Bodies in our exhausted Receiver. But as we formerly noted, upon the exsuction of the Air, the smoking of those Bodies presently ceas'd. We had thoughts also of conveying into our Pneumatical Glass a hot Iron, with some Bodie easie to be dissipated into smoke set upon it, but consider'd, that neither was that way free from inconveniencies; especially this, that the hot Body would make the Imprisoned Air circulate within the Receiver, and consequently make it questionable whether the ascent of the steams would not be due to the new and acquired motion of the Air.

Wherefore I bethought my self of another way to satisfie in some measure my curiosity, to wit, by means of a certain Liquor, which I call'd to mind that some years ago I had (for a design that belongs not to our present purpose) prepar'd; which, I suppose, I shew'd Your Lordship, and which had the luck to be taken notice of by divers very Ingenious and Famous Men. For this Liquor, though most of its Ingredients be Metals, and all of them ponderous enough, is yet of that nature, that whilst the Viol wherein it is kept is stopt (how slight a Cover soever) both the Liquor and the Glass are transparent; and so is that upper half of the Glass to which the Liquor reacheth not: But as soon as ever the stopple is taken out, and full access is given to the external Air, both the inward part of the Cork, and the Liquor it self, do presently send upwards, and scatter abroad asume as thick and white, as if there were a quantity of Alabaster-dust thrown up into the Air: And this smoaking of the Liquor lasts till my unwillingness to waste it invites me to stop it again, and then the ascension of the fumes suddenly ceaseth, till the Viol be again unstop'd.

This fuming Liquor then I thought would much conduce to the discovery I desir'd to make, since it sav'd me the need of conveying any hot Body with it into the Receiver, and would not darken it with fumes before the time. Wherefore having ty'd to the Viol a great weight of Lead, to keep it from being lifted up by the drawing out of the Cork; and having ty'd to the stopple one end of a string, of which the other end was made fast to the Cover of the Pneumatical Glass, the Liquor was carefully clos'd up after the wonted manner; then the Air being diligently pump'd out, the Viol was unstop'd in the empty'd Receiver. And though immediately, upon the drawing out of the Cork, there appear'd to be as it were thrown up some white fumes, which seem'd to proceed from the Air before imprison'd in the Viol, and diffusing it self suddenly into the capacity of the Receiver: Yet we afterward observ'd, as we expected, That the fumes did not amount and disperse themselves as they used to do in the open Air; but that, when by reason of the agitation of the Corpuscles of the Liquor, which could not continue their motion in so narrow a space as the Viol afforded them, and were therefore reduc'd to thrust one another out of it; when, I say, by these assistances the fumes were ascended to the lip of the Viol, they mounted no higher, but ran down along the outside of the Viol to the bottom of it; and thence along, a long and inclining piece of Lead, on which the Viol rested, like a little Stream (not very much bigger than a Swans Quill) whose nature they seem'd to emulate so well, that it quitted not the Viol till it was come to the bottom of it, and then forsook it in such a manner'a as stream of Water of the same bigness would have done. And this stream lasted a pretty while, and would probably have lasted longer, but that being loath to waste my Liquor, I let in at the Stop-cock a pretty deal of the external Air; notwithstanding which, finding after a while that the stream did run afresh, though, as it seem'd not altogether so copious as before; I let as much more Air, as would come in: and found (somewhat to my wonder) that though the stream formerly mention'd dis-appear'd, yet there appear'd not any white fumes to arise, either from the Cork, or out of the Viol it self, no not when the Cover was remov'd from the Receiver; though not only after a while there ascended white fumes from the Receiver: but having forthwith taken out the Viol into the open Air, it emitted white exhalations as before; and having presently after unstopp'd it in an open Window, we found both it and the Cork immediately to send forth a yet much more plentifull smoak: Though it be now divers years since this Numerical Liquor was prepared, after the manner mention'd either by Carneiades or Eleutherius (for I do not well remember which) in those Dialogues concerning Heat and Flame that have above been mention'd.

More circumstances concerning these Fumes we might have observ'd, had we not been deterr'd by an Indisposition in point of health, from having much to do with steams of so dangerous a nature, as by that of the Ingredients of this Liquor these seem likely to be of.

The reflexions that may be made upon this Experiment, we have not now the leasure to prosecute, and therefore shall content our selves to recommend the several circumstances of it to Your Lordship's serious consideration; and to take notice (en passant) that steams in an ambient Body, or a medium thinner than themselves, may both tend downwards, and otherwise emulate the nature of a Liquor; which I therefore point at, that it may appear the less strange, if we sometimes speak of the Atmosphere as of a kind of Liquor, in comparison of that more thin and subtle Celestial matter that surrounds it.

And though it might perchance suffice to have on this occasion intimated thus much; yet, lest this way of speaking of the Atmosphere should be thought too bold and extravagant, I am content to borrow an Experiment of the discourse formerly mention'd (touching fluidity and firmness) and subjoyn it here with alterations suitable to the contrivance of our Engine; and this the rather, because I hope it may conduce to the discovery of the nature of the Atmosphere: for which reason it might have been annext to what hath been noted either upon the first, or eighteenth Experiment, but that when they were written and sent away, it came not into my mind. The Experiment then as we try'd in our Engine, was as follows.

WE took one of the small Receivers, often mention'd already, and into it we convey'd a piece of well lighted Match; and letting it remain there till it had fill'd the Receiver with smoak, we took it out and hastily clos'd again the Receiver, that the smoak might not get away. Then staying awhile to let these fumes leisurely subside, we found, as we expected, that after some time they setled themselves in the lower half of the Receiver; in a darkish Body, leaving the upper half of the Receiver transparent, and as to sight, full of nought but clear Air. Now to manifest that this smoak thus setled emulated a Liquor, we inclin'd the Engine that contain'd it, sometimes to one side, and sometimes to the other; and observ'd the smoak to keep its surface almost Horizontal, notwithstanding the stooping of the Vessel that held it, as Water, or another liquor would in the like case have done. And if by a quicker rocking of the Engine the smoak were more swiftly shaken, it would, like Water, either vibrate to and fro from one side to the other of the Glass, or else have its surface manifestly curl'd with Waves, but preserve its self in an entire and distinct Body from the incumbent Air; and being permitted to rest a while, would soon recover its former smooth and level Superficies. If also the Key were turn'd and the Valve unstopp'd, so that there was a free, though but a narrow passage open'd betwixt the external Air and the cavity of the Receiver, then would some of this smoak fall down, as it were, in a stream into the subjacent Cylinder, and a proportionate quantity of the outward Air, would manifestly ascend through it into the incumbent Air, much after the same manner as if you invert a Viol with a long neck, and well fill'd with Red-Wine, into a Glass full of fair Water, you shall see the Water and Wine by degrees mingle with one another; the one falling down as it were in little colour'd streams, and the other ascending in its room in the like curled streams, sometimes preceded by round parcels of Water, which, by reason of their transparency, look almost like bubbles. The other circumstances of this Experiment, belonging not all of them to our present purpose, we shall content our selves with taking notice of one which seems the most important, and may illustrate and confirm some things formerly delivered. And it was, That if, when the superficies of our smoak lay smooth and Horizontal, a hot Iron were held near the outside of the Receiver, the neighbouring part of the included fumes (for the rest did not very much alter their former superficies) being rarified by the heat, would readily ascend in a large Pillar of smoak to the very top of the Receiver, yet without seeming to lose a distinct superficies, or to be confounded with Air; below which, upon the recess of the adventitious heat that by agitating it impell'd it upward, it would again subside.

All which being added to the late Experiment of the smoaking Liquor, and to what may be from that which hath been elsewhere said, gather'd to the same purpose, will, I hope, keep it at least from appearing absur'd. If, since we see that there is so great an inequality in the density and weight of Liquors, that Water is near fourteen times thinner or lighter than Quick-silver of the same bulk; and well dephlegm'd spirit of Wine, yet much lighter than Water, we venture to speak sometimes of the Atmosphere, as if it were a peculiar kind of thin and halitious Liquor (if I may so call it) much lighter than Spirit of Wine.

To these things I know not whether it will be requisite to add, That as we lately took notice of conspicuous Waves that appear'd upon the superficies of our agitated smoak: So some such thing may not absurdly be conjectur'd to happen on the superficies of the Atmosphere, by those strange ruggednesses that appear (especially in the Spring and Fall, when Exhalations and Vapours are wont to ascend most plentifully) upon the limb or edge of the Rising and Setting Sun. I speak thus disfidently upon this occasion, because I know, that by the Fluctuation or Boyling of the Suns own superficies, divers eminent Mathematicians have plausibly enough (but how truly, I leave Your Lordship to judge) endeavour'd to give an account of it. But if we will joyn with those that have ascrib'd of late this Phaenomenon, to the refraction the Sun-beams suffer in our vapid air, we may, as hath been intimated, promote their Doctrine, by deducing from it, That probably the surface of the Atmosphere is oftentimes (if not always) exceedingly curl'd or wav'd. And certainly it is somewhat wonderfull, as well as very pleasant to behold, how, to him that looks upon the setting Sun through a long and excellent Telescope, there will not only appear strange inequalities in the edge of it (insomuch that I have often seen it more indented than a Saw) but those inequalities will vanish in one place, and presently appear in another, and seem perfectly to move like Waves succeeding and destroying one another, save that their motion oftentimes seems to be quickest; as if in that vast Sea they were carried on by a Current, or at least by a Tide. And this (as we elsewhere note) appears to the Eye, not only when it looks directly through the Telescope upon the Sun; but also when a large and well defin'd Image of the Sun is by the same Telescope brought into a Room and cast upon a sheet of white Paper. But to insist on this were to digress: and therefore I will proceed to Experiments of another kind.

IT hath been admir'd by very ingenious Men, that if the exquisitely polish'd surfaces of two flat pieces of Marble, be so congruous to each other, that from their mutual application there will result an immediate contact, they will stick so fast together, that he that lifts up the uppermost, shall, if the undermost be not exceeding heavy, lift up that too, and sustain it alost in the free Air. A probable cause of this so close adhesion, we have elsewhere endeavoured to deduce from the unequal pressure of the Air upon the undermost stone; For the lower superficies of that stone being freely expos'd to the Air, is press'd upon by it, whereas the uppermost surface, being contiguous to the superior stone, is thereby defended from the pressure of the Air; which consequently pressing the lower stone against the upper, hinders it from falling, as we have elsewhere more fully declar'd. Upon these grounds we conjectur'd, that in case we could procure two Marbles exactly ground to one another, and in case we could also sufficiently evacuate our Receiver; the lower stone would, for want of the wonted and sustaining pressure of the Air, fall from the upper. But the farther trial of this Experiment we must, unless Your Lordship think it worth Your making at Paris, put off till a fitter opportunity. For where we now are, we cannot procure Marbles so exactly ground, that they will sustain one another in the Air above a minute or two, which is a much shorter time than the emptying of our Receiver requires. We did indeed try to make our Marbles stick close together, by moistening their polish'd surfaces with rectifi'd spirit of Wine, in regard that liquor, by its sudden avolation from Marble, if pour'd thereon, without leaving it moist or less smooth, seem'd unable to sustain them together after the manner of a glutinous Body, and yet seem'd sufficient to exclude and keep out the Air. But this we try'd to little purpose, for having convey'd into the Receiver two black square Marbles (the one of two Inches and a third in length or breadth, and somewhat more than half an Inch in thickness: The other of the same extent, but not much above half so thick) fasten'd together by the intervention of pure spirit of Wine; and having suspended the thicker by a string from the cover, we found not that the exsuction of the ambient Air would separate them, though a weight amounting to four Ounces were fasten'd to the lowermost Marble to facilitate its falling of.

I would gladly have the Experiment try'd with Marble, so well polish'd, as to need no Liquor whatsoever to make them cohere, and in a Vessel out of which the Air may be more persectly drawn than it was out of ours. But in the mean time, though we will not determine whether the spirit of Wine did contribute to the strong cohesion of these stones, otherwise than by keeping ev'n the subtlest parts of the Air from getting in between them: yet it seemed that the not falling down of the lowermost Marble, might, without improbability, be ascrib'd to the pressure of the Air remaining in the Receiver; which, as we formerly noted, having been able to keep a Cylinder of Water, of above a Foot in height, from falling to the bottom of the Tube, may well enough be suppos'd capable of keeping so broad a flat Marble from descending. And though this may seem a strange proof of the strength of the spring of the Air, ev'n when rarefied, yet it will scarce seem incredible to him that hath observ'd how exceeding strong a cohesion may be made betwixt broad Bodies, only by their immediate touching one another. A notable instance of which, I met with in this short Narrative of the Learned Zucchius. Juveni (saith he) lacertorum suorum robur jactanti proposit a P. Nic. Zucchius apud Scbot. part. 1. Mec. Hydraulopneum. semel est lamina aerea, per ansam in medio extantem apprehensam elevanda è tabula marmorea, cui optime congruebat: qui primo tanquam rem ludicram puero committendam contempsit: tum instantibus amicis manum utrámque admovens, cum luctatus diu haerentem non removisset, excusavit impotentiam, objecta peregrini & potentissimi glutinis interpositione, quo fortissime copulante nequiret divelli; donec vidit ab alio per tabulam facilimè laminam deduci, & ad extrema productam, & actam in transversum inde deportari. But that we may learn from our, own Engine, that two Bodies, though they touch each other but in a small part of their surfaces, may be made to cohere very strongly, only by this, That the Air presses much more forcibly upon the inferiour superficies of the lowermost Body, than upon the upper surface of the same: We will hereunto annex the following Experiment, though out of the order wherein they were made.

I Remember I have, in a Discourse touching Fluidity and Firmness, made mention of my having, by the exsuction of the Air out of a Glass Vessel, made that Vessel take up, or suck up (to speak in the common Language) a Body weighing divers Ounces; but our Engine affording us the opportunity of making considerabler Experiments of that kind, We thought fit to make a further trial of the force of the Atmosphere's pressure upwards, after the following manner.

The Receiver having been exquisitely clos'd as we have often taught already, and the Air being in a good measure drawn out of it, it was remov'd from off the Pump: and to the lower branch of the Stop-cock, there was speedily apply'd a tapering Valve of brass, such as is described in the 9th Figure, made fit to go with its narrower end into the cavity of the branch, and to fill the Orifice of that cavity with its broader part. And that the Air might not get in at the little intervals, left here and there between the convex surface of the stopple and the internal edge of the branch, those intervals were stopt with a little Diachylon. And to the door, or, (if you please) that part of the Valve which was to move to and fro, and in this Experiment hung perpendicular to the Horizon, there was, at a button of brass belonging to the Valve, fasten'd a broad scale wherein weights were to be put. This done, the Key of the Stop-cock was turn'd, and the external Air beating like a forcible stream upon the Valve to get in there, it did suddenly both shut the Valve and keep it shut so strongly, that we had time to cast in divers weights one after another into the Scale; till at length the weight overpowering the pressure of the Atmosphere, drew down the Valve by the strings that tied the Scale to it, and gave liberty to the outward Air to rush into the Receiver. Though another time, when the Valve had but little weight hanging at it, being, by I know not what accident, drawn down beneath its former place, it was by the impetuous current of the outward Air suddenly impell'd up into it again, and kept there. But in the former Experiment it is remarkable, That though the Receiver were not well exhausted, and though it leak'd whilst the rest of the Experiment was in prosecution: and though the Valve whereon the Cylinder of the Atmosphere could press, were not above an Inch and a half in Diameter, yet the weight kept up by suction, or rather supported by the Air, (namely the Valve, the Scale and what was cast out of it,) being sent to be weigh'd, amounted to about ten of our common Pounds, consisting of sixteen Ounces a piece: So that we doubted not but that, had the Experiment been made with favourable circumstances, the Air endeavouring to press in at the Orifice of the Stop-cock, would have kept a very much greater weight from falling out of it; I say the Air, because we found, by trial purposely made, that neither the imperfect contact of the Valve and the Stop-cock, nor the Diachylon that was employ'd to fill up the little Crannies left betwixt them, were considerable in this Experiment. By which it may, among other things, appear, that I did not without cause in the above-named Discourse touching Fluidity and Firmness, ascribe a great force, even to such Pillars of. Air as may be supposed to begin at the top of the Atmosphere, and recoyling from the ground, to terminate on the Bodies on which they press: since in the present Experiment such a weight was supported by so slender a Cylinder of Air, rebounding from the Earth to the Valve whereon it did bear.

BUt in regard we have not yet been able to empty so great a Vessel as our Receiver, so well as we can the Cylinder it self; our Pump alone may afford us a nobler instance of the force of the Air we live in, insomuch, that by help of this part of our Engine, we may give a pretty near ghess at the strength of the Atmosphere, computed as a weight. And the way may be this; First, the Sucker being brought to move easily up and down the Cylinder, is to be impelled to the top of it: Then the Receiver must be taken off from the Pump, that the upper Orifice of the Cylinder remaining open, the Air may freely succeed the Sucker, and therefore readily yield to its motion downwards. This done, there must be fasten'd to one of the Iron Teeth of the Sucker, such a weight as may just suffice to draw it to the bottom of the Cylinder. And having thus examin'd what weight is necessary to draw down the Sucker, when the Atmosphere makes no other than the ordinary resistance of the Air against its descent; the Sucker must be again forc'd to the top of the Cylinder, whose upper Orifice must now be exactly closed; and then (the first weight remaining) we easily may, by hanging a Scale to the above-mention'd Iron (that makes part of the Sucker) cast in known weights so long, till in spight of the reluctancy of the Atmosphere the Sucker be drawn down. For to these weights in the Scale, that of the Scale it self being added, the sum will give us the weight of a Column of Air, equal in Diameter to the Sucker, or to the cavity of the Cylinder, and in length to the height of the Atmosphere.

According to this method we did, since the writing of the last Experiment, attempt to measure the pressure of the Atmosphere, but found it more difficult than we expected, to persorm it with any accurateness; for though by the help of the Manubrium the Sucker moved up and down with so much ease, that one would have thought that both its convex surface, and the concave one of the Cylinder were exquisitely smooth, and as it were slippery; yet when the Sucker came to be moved onely with a dead weight or pressure (that was not (like the force of him that pumped) intended as occasion required) we found that the little rufnesses or other inequalities, and perhaps too, the unequal pressure of the Leather against the cavity of the Cylinder, were able, now and then, to put a stop to the descent or ascent of the Sucker, though a very little external help would easily surmount that impediment; and then the Sucker would, for a while, continue its formerly interrupted motion, though that assistance were withdrawn. But this discouragement did not deter us from prosecuting our Experiment, and endeavouring, by a carefull trial, to make it as instructive as we could. We found then that a Leaden Weight, of 28 pounds (each consisting of sixteen Ounces) being fastned to one of the teeth of the Sucker, drew it down closely enough, when the upper Orifice of the Cylinder was left open: though by the help of Oyl and Water, and by the frequent moving the Sucker up and down with the Manubrium, its motion in the Cylinder had been before purposely facilitated. This done, the upper Orifice of the Cylinder was very carefully and closely stopped, the Valve being likewise shut with its wonted Stopple well oyl'd, after the Sucker had been again impell'd up to the top of the Cylinder. Then to the precedent twenty eight pound, we added a hundred and twelve pounds more; which forcing down the Sucker, though but leisurely, we took off the twenty eight pound weight; and being unable to procure just such weights as we would have had, we hung on, instead of it, one of fourteen pound: but found that, with the rest, unable to carry down the Sucker. And to satisfie our selves, and the Spectators, that it was the resistance of the ambient Air that hinder'd the descent of so great a weight, after that we had try'd that upon unstopping the Valve, and thereby opening an access to the external Air, the Sucker would be immediately drawn down. After this, I say, we made this farther Experiment, That having by a Man's strength forcibly depress'd the Sucker to the bottom of the Cylinder, and then fastned weights, to the above-named Iron that makes part of that Sucker, the pressure of the external Air finding little or nothing in the cavity of the evacuated Cylinder to resist it, did presently begin to impell the Sucker, with the weights that clogg'd it, towards the upper part of the Cylinder; till some such accidental Impediment, as we formerly mention'd, check'd its course. And when that rub, (which easily might be,) was taken out of the way, it would continue its ascent to the top, to the no small wonder of those By-standers, that could not comprehend how such a weight could ascend, as it were, of it self; that is, without any invisible force, or so much as Suction to list it up. And indeed it is very considerable, that though possibly there might remain some particles of Air in the Cylinder, after the drawing down of the Sucker; yet the pressure of a Cylinder of the Atmosphere, somewhat less than three Inches in Diameter (for, as it was said in the description of our Engine, the cavity of the Cylinder was no broader) was able, uncompress'd, not only to sustain, but even to drive up a weight of an hundred and odd pounds: for besides the weight of the whole Sucker it self, which amounts to some pounds, the weights annexed to it made up an hundred and three pounds, besides an Iron Bar, that by conjecture weighed two pounds more; and yet all these together fall somewhat short of the weight which we lately mention'd, the resistance of the Air, to have held suspended in the cavity of the Cylinder.

And though (as hath been already acknowledg'd (we cannot peradventure, obtain by the recited means so exact an account as were to be wish'd, of what we would discover: Yet, if it serve us to ground conjectures more approaching to the Truth, than we have hitherto met with, I hope it will be consider'd (which a famous Poet judiciously says)

Est quoddam prodire tenus, si non datur ultra.

Peradventure it will not be impertinent to annex to the other circumstances that have been already set down concerning this Experiment, That it was made in Winter, in Weather neither Frosty nor Rainy, about the change of the Moon, and at a place whose latitude is near about 51 degrees and a half: For perhaps the force or pressure of the Air may vary, according to the Seasons of the Year, the temperature of the Weather, the elevation of the Pole, or the phases of the Moon; all, or even any of them seeming capable to alter either the height or consistence of the incumbent Atmosphere: And therefore it would not be amiss if this Experiment were carefully tried at several times and places, with variety of circumstances. It might also be tried with Cylinders of several Diameters, exquisitely fitted with Suckers, that we might know what proportion several Pillars of the Atmosphere bear to the weights they are able to sustain or lift up; and consequently, whether the increase or decrement of the resistance of the ambient Air, can be reduced to any regular proportion to the Diameters of the Suckers: These, and divers other such things which may be try'd with this Cylinder, might most of them be more exactly try'd by the Torricellian Experiments, if we could get Tubes so accurately blown and drawn, that the cavity were perfectly Cylindrical.

To dwell upon all the several Reflexions, that a speculative Wit might make upon this and the foregoing Experiment, (I mean the thirty third and thirty second) would require almost a Volume; whereas our occasions will scarce allow us time to touch upon three or four of the chief Inferences that seem deducible from them, and therefore we shall content our selves to point at those few.

And first, as many other Phaenomena of our Engine, so especially, the two lately mention'd Experiments, seem very much to call in question the received Opinion of the nature or cause of Suction. For it's true indeed, that when men suck, they commonly use some manifest endeavour by a peculiar motion of their Mouths, Chests, and some other conspiring parts, to convey to them the body to be suck'd in. And hence perhaps they have taken occasion, to think that in all Suction there must be some endeavour or motion in the sucking to attract the sucked Body. But in our last Experiment it appears not at all how the upper part of the empty'd Cylinder that remains moveless all the while, or any part of it, doth at all endeavour to draw to it the depressed Sucker and the annexed weights. And yet those that behold the ascension of the Sucker, without seriously considering the cause of it, do readily conclude it to be raised by something that powerfully Sucks or attracts it, though they see not what that may be or where it lurks. So that it seems not absolutely necessary to Suction, that there be in the Body, which is said to suck, an endeavour or motion in order thereunto, but rather that Suction may be at least for the most part reduced to Pulsion, and its effects ascrib'd to such a pressure of the neighbouring Air upon those Bodies (whether Aërial, or of other natures) that are contiguous to the Body that is said to attract them, as is stronger, than that substance, which possesseth the cavity of that sucking Body, is able to resist. To object here, that it was some particles of Air remaining in the emptied Cylinder that attracted this weight to obviate a Vacuum, will scarce be satisfactory; unless it can be clearly made out by what little hooks, or other grappling Instruments, the internal Air could take hold of the Sucker; how so little of it obtained the force to lift up so great a weight; and why also, upon the letting in of a little more Air into one of our evacuated Vessels, the attraction is, instead of being strengthened, much weakned; though, if there were danger of a Vacuum before, it would remain, notwithstanding this ingress of a little Air. For that still there remained in the capacity of the exhausted Cylinder store of little rooms, or spaces empty or devoid of Air, may appear by the great violence wherewith the Air rusheth in, if any way be open'd to it. And that 'tis not so much the decrement of the Vacuum within the cavity of the vessel that debilitates the attraction, as the Spring of the included Air (whose presence makes the decrement) that doth it by resisting the pressure of the external Air, seems probable, partly from the Disability of vacuities, whether greater or lesser, to resist the pressure of the Air; and partly by some of the Phaenomena of our Experiments, and particularly by this Circumstance of the Three and Thirtieth, that the Sucker was, by the pressure of the Ambient Air, impell'd upwards with its weight hanging at it, not onely when it was in the bottom of the Cylinder, and consequently left a great Vacuum in the cavity of it; but when the Sucker had been already impell'd almost to the top of the Cylinder, and consequently, when the Vacuum that remain'd was become very little in comparison of that which preceded the beginning of the Sucker's ascension.

In the next place, these Experiments may teach us, what to judge of the vulgar Axiom received for so many Ages as an undoubted Truth in the Peripatetick Schools; That Nature abhors and flyeth a Vacuum, and that to such a degree, that no humane power (to go no higher) is able to make one in the Universe; wherein Heaven and Earth would change places, and all its other Bodies rather act contrary to their own Nature, than suffer it. For, if by a Vacuum we will understand a place perfectly devoid of all corporeal Substance, it may indeed then, as we formerly noted, be plausibly enough maintained that there is no such thing in the world; but that the generality of the Plenists, (especially till of late years some of them grew more wary) did not take a Vacuum in so strict a sense, may appear by the Experiments formerly, and ev'n to this day imploy'd by the Deniers of a Vacuum, to prove it impossible that there can be any made. For when they alledge (for Instance) that when a man sucks Water through a long Pipe, that heavy Liquor, contrary to its Nature, ascends into the Sucker's mouth, only, to fill up that room made by the Dilatation of his Breast and Lungs, which otherwise will in part be empty. And when they tell us, that the reason why if a long Pipe exactly clos'd at one end be filled top-full of Water, and then inverted, no Liquor will fall out of the open Orifice; Or, to use a more samiliar Example, when they teach, that the cause, why in a Gardiner's watering Pot shaped conically, or like a Sugar-Loaf, fill'd with Water, no Liquor falls down through the numerous holes at the bottom, whilst the Gardiner keeps his Thumb upon the Orifice of the little hole at the top, and no longer; must be that if in the case proposed the Water should descend, the Air being unable to succeed it, there would be le • t at the upper and deserted part of the Vessel a Vacuum, that would be avoided if the hole at the top were open'd. When (I say) they alledge such Experiments, the tendency of them seems plainly to import, that they mean, by a Vacuum, any space here below that is not filled with a visible body, or at least with Air though it be not quite devoy'd of all Body whatsoever. For why should Nature, out of her detestation of a Vacuum, make Bodies act contrary to their own tendency, that a place may be fill'd with Air, if its being so were not necessary to the avoiding of a Vacuum.

Taking then a Vacuum in this vulgar and obvious sense, the common opinion about it seems lyable to several Exceptions, whereof some of the chief are suggested to us by our Engine.

It will not easily then be intelligibly made out, how hatred or aversation, which is a passion of the Soul, can either for a Vacuum, or any other object, be supposed to be in Water, or such like inanimate Body, which cannot be presumed to know when a Vacuum would ensue; if they did not bestir themselves to prevent it: nor to be so generous as to act contrary to what is most conducive to their own particular preservation for the publique good of the Universe. As much then of intelligible and probable Truth, as is contain'd in this Metaphorical Expression, seems to amount but to this; That by the Wise Authour of Nature (who is justly said to have made all things in number, weight and measure,) the Universe, and the parts of it, are so contriv'd, that it is as hard to make a Vacuum in it, as if they studiously conspir'd to prevent it. And how far this it self may be granted, deserves to be farther consider'd.

For in the next place, our Experiments seem to teach, that the supposed Aversation of Nature to a Vacuum is but accidental, or in consequence, partly of the Weight and Fluidity, or, at least, Fluxility of the Bodies here below; and partly, and perhaps principally, of the spring of the Air, whose restless endeavour to expand it self every way, makes it either rush in it self, or compel the interposed Bodies into all spaces, where it finds no greater resistance than it can surmount. And that in those motions which are made ob fugam Vacui (as the common phrase is) Bodies act without such generosity and consideration, as is wont to be ascrib'd to them, is apparent enough in our 32d Experiment, where the torrent of Air, that seem'd to strive to get into the empty'd Receiver, did plainly prevent its own design by so impelling the Valve, as to make it shut the only Orifice the Air was to get out at. And if afterwards either Nature, or the internal Air, had a design the external Air should be attracted, they seem'd to prosecute very unwisely by continuing to suck the Valve so strongly; when they found that by that Suction the Valve it self could not be drawn in: Whereas by forbearing to suck, the Valve would by its own weight have fallen down, and suffer'd the excluded Air to return freely, and to fill again the exhausted Vessel.

And this minds me to take notice of another deficiency, pointed at by our Experiments in the common Doctrine of those Plenists we reason with; for many of those unusual motions in Bodies, that are said to be made to escape a Vacuum, seem rather made to fill it. For why, to instance in our newly mention'd Experiment, as soon as the Valve was depressed by the weight we hung at it, should the Air so impetuously and copiously rush into the cavity of the Receiver; if there were before no vacant room there to receive it? and if there were, then all the while the Valve kept out the Air, those little spaces in the Receiver, which the corpuscles of that Air afterwards fill'd, may be concluded to have remain'd empty. So that the seeming violence, imploy'd by Nature on the occasion of the evacuating of the Vessel, seems to have come too late to hinder the making of Vacuities in the Receiver, and only to have, as soon as we permitted, fill'd up with Air those that were already made.

And as for the care of the publique good of the Universe ascrib'd to dead and stupid Bodies, we shall only demand, why in our 19th Experiment, upon the Exsuction of the ambient Air, the Water deserted the upper half of the Glass-Tube; and did not ascend to fill it up, till the external Air was let in upon it: whereas by its easie and sudden regaining that upper part of the Tube, it appeared both that there was there much space devoid of Air, and that the Water might with small or no resistance have ascended into it, if it could have done so without the impulsion of the re-admitted Air; which, it seems, was necessary to mind the Water of its formerly neglected Duty to the Universe.

Nay, for ought appeareth, even when the excluded Air, as soon as 'twas permitted, rush'd violently into our exhausted Receiver, that flowing in of the Air proceeded rather from the determinate Force of the Spring of the neighbouring Air, than from any endeavour to fill up, much less to prevent vacuity's. For though when as much Air as will, is gotten into our Receiver our present Opponents take it for granted that it is full of Air; yet if it be remembred that when we made our 17th Experiment we crouded in more Air to our Receiver than it usually holds; and if we also consider (which is much more) that the Air of the same consistence with that in our Receiver may in Wind-guns, as is known, and as we have tryed, be compressed at least into half its wonted room (I say at least, because some affirm, that the Air may be thrust into an 8th, or a yet smaller part of its ordinary extent) it seems necessary to admit either a notion of condensation and rarefaction that is not intelligible, or that in the capacity of our Receiver when presumed to be full of Air, there yet remain'd as much of space as was taken up by all the Aërial corpuscles, unpossessed by the Air. Which seems plainly, to infer that the Air that rush'd into our empty'd vessel did not doe it precisely to fill up the Vacuities of it, since it left so many unfill'd, but rather was thrust in by the pressure of the contiguous Air: which as it could not, but be always ready to expand it self, where it found least resistance, so was it unable to fill the Receiver any more, than untill the Air within was reduc'd to the same measure of Compactness with that without.

We may also from our two already often mention'd Experiments farther deduce, that, (since Natures hatred of a Vacuum is but Metaphorical and Accidental, being but a consequence or result of the pressure of the Air and of the Gravity, and partly also of the Fluxility of some other Bodies) The power she makes use of to hinder a Vacuum, is not (as we have else-where also noted) any such boundless thing as men have been pleased to imagine. And the reasons why in the former Experiments, mentioned in favour of the Plenists, Bodies seem to forget their own Natures to shun a Vacuum, seems to be but this; That in the alledged cases the weight of that Water that was either kept from falling or impell'd up, was not great enough to surmount the pressure of the contiguous Air; which, if it had been, the Water would have subsided, though no Air could have succeeded. For not to repeat that Experiment of Monsieur Paschal (formerly mention'd to have been tryed in a Glass exceeding 32. Foot) wherein the inverted Pipe being long enough to contain a competent-weight of Water, that Liquor freely ran out at the lower Orifice: Not to mention this (I say) we saw in our nineteenth Experiment, that when the pressure of the ambient Air was sufficiently weakn'd, the Water would fall out apace at the Orifice even of a short Pipe, though the Air could not succeed into the room deserted by it. And it were not amiss if tryal were made on the tops of very high Mountains, to discover with what case a Vacuum could be made near the confines of the Atmosphere, where the Air is probably but light in comparison of what it is here below. But our present (three and thirtieth) Experiment seems to manifest, not onely that the power, exercis'd by Nature, to shun or replenish a Vacuum, is limitted, but that it may be determin'd even to Pounds and Ounces: Insomuch that we might say, such a weight Nature will sustain or will lift up to resist a Vacuum in our Engine; but if an Ounce more be added to that weight, it will surmount Her so much magnifi'd detestation of Vacuities. And thus, My Lord, our Experiments may not onely answer those of the Plenists, but enable us to retort their Arguments against themselves: since, if that be true which they alleadge, that, when Water falls not down according to its nature, in a Body wherein no Air can succeed to fill up the place it must leave, the suspension of the Liquor is made Ne detur Vacuum, (as they speak) it will follow, that if the Water can be brought to subside in such a case, that deserted space may be deem'd empty, according to their own Doctrine; especially, since Nature (as they would perswade us) bestirs her self so mightily to keep it from being deserted.

I hope I shall not need to remind Your Lordship, that I have all this while been speaking of a Vacuum, not in the strict and Philosophical sense, but in that more obvious and familiar one that hath been formerly declar'd.

And therefore I shall now proceed to observe in the last place, that our 33d Experiment affords us a notable proof of the unheeded strength of that pressure which is sustain'd by the Corpuscles of what we call the free Air, and presume to be uncompressed. For, as fluid and yielding a Body as it is, our Experiment teacheth us, That ev'n in our Climate, and without any other compression than what is (at least here below) Natural, or (to speak more properly) ordinary to it, it bears so strongly upon the Bodies whereunto it is contiguous, that a Cylinder of this free Air, not exceeding three Inches in Diameter is able to raise and carry up a weight, amounting to between sixteen and seventeen hundred Ounces. I said even in our Climate, because that is temperate enough; Aere frigido existente tardius moventar Automata quam aere calido, adeo quidem at Automaton quod Belgae in Nova Zembla agentes in aedibus suis collocaverunt omnino à motu cessaverit etsimulto majus pondus ei addidissent quam antea serre solebat. Varenius Geo: General. lib. 111. Propo: 7. pag. 648. and as far as my observations assist me to conjecture, the Air in many other more Northern Countries may be much thicker, and able to support a greater weight: which is not to be doubted of, if there be no mistake in what is Recorded concerning the Hollanders, that were forc'd by the Ice to Winter in Nova Zembla, namely, That they found there so condens'd an Air, that they could not make their Clock goe, ev'n by a very great addition to the weights that were wont to move it.

I suppose Your Lordship will readily take notice, that I might very easily have discoursed much more fully and accurately than I have done, against the common opinion touching Suction, and touching natures hatred of a Vacuum. But I was willing to keep my self to those considerations touching these matters, that might be verified by our engine it self, especially, since, as I said at first, it would take up too much time to insist particularly upon all the Reflexions that may be made even upon our two last Experiments. And therefore passing to the next, I shall leave it to Your Lordship to consider how far these tryals of ours will either confirm or disfavour the new Doctrine of several eminent Naturalists, who teach, That in all motion there is necessarily a Circle of Bodies, as they speak, moving together; and whether the Circles in such motion be an Accidental or Consequential thing or no.

'TIs a known thing to those that are conversant in the Hydrostaticks, That two Bodies which in the Air are of equal weight, but of unequal bulk, as Gold, for instance and Iron, being afterwards weighed in Water, will lose their AEquilibrium upon the change of the ambient Body: so that the Gold will sink lower than the Iron; which, by reason of its greater bulk, hath more Water to lift or displace, that it may sink. By Analogy to this Experiment, it seemed probable, that if two weights did in our Engine ballance each other, when the Glass was full of Air; upon the exsuction of a great part of that Air, so notable a change in the consistence of the ambient Body, would make them lose their AEquilibrium.

But being desirous at the same time to make a tryal, for a certain Design that needs not here be mention'd, we took for one of our weights a dry Bladder, strongly tyed at the Neck, and about half fill'd with Air (that being a weight both slight, and that would expand it self in the evacuated Glass) and fastning that to one part of our formerly mentioned exact ballance (which turns with the 32d part of a Grain) we put a Metalline counterpoise into the opposite Scale; and so the two weights being brought to an AEquilibrium, the ballance was convey'd into the Receiver, and suspended from the Cover of it.

But before we proceed farther, we must note, That presently after the laying on of the Cover, the Bladder appear'd to preponderate, whereupon the Scales being taken out, and reduc'd very near to an AEquilibrium, yet so, that a litle advantage remained on that side to which the Metalline weight belonged; they were again let down into the Receiver, which was presently made fast with Plaister, and a hot Iron: Soon after which before the Pump was employ'd, the Bladder seem'd again a little to preponderate. Afterwards the Air in the Glass being begun to be drawn out, the Bladder began (according to the formerly mention'd Observations) to expand it self, and manifestly to outweigh the opposite weight, drawing down the Scale to which it was fastned very much beneath the other, especially when the Air had swell'd it to its full extent.

This done, we very leisurely let in the external Air; and observ'd, that upon the flagging of the Bladder, the Scale whereto it was fastned, not onely by degrees return'd to an AEquilibrium with the other, but at length was a little outweighed by it.

But because we suspected there might have interven'd some unheeded Circumstance in this last part of the Experiment we would not presently take out the Scales, nor meddle with the Cover, but leaving things as they were, we perceiv'd, that after a little while the Bladder began again to preponderate, and by degrees to sink lower and lower for divers hours, wherefore, leaving the Vessel closed up all night, we repair'd to it next Morning, and found the Bladder fallen yet lower. As if the very substance of it, had imbibed some of the moisture wherewith the Air (the Season being very rainy) did then abound: As Lutestrings, which are made likewise of the Membranous parts of Guts, strongly wreath'd, are known to swell so much oftentimes as to break in rainy and wet weather. Which conjecture is the more to be regarded, because congruously unto it one of the company having a little warm'd the Bladder, found it then lighter than the opposite weight. But this must be look'd upon as a bare conjecture, till we can gain time to make farther tryals about it. In the mean while we shall adde, that without removing the Scales or the Cover of the Receiver, we again caused the Air to be drawn out (the weather continuing very moist) but found not any manifest alteration in the ballance; whether because the AEquilibrium was too far lost to let a small change appear, we determine not.

But to make the Experiment with a Body less apt to be altered by the temperature of the Air, than was the Bladder; we brought the Scales again to an AEquilibrium with two weights, whereof the one was of Lead, the other of Cork. And having evacuated the Receiver, we observed, that both upon the exsuction, and after the return of the Air, the Cork did manifestly preponderate, and much more, a while after the Air had been let in again, than whilst it was kept out. Wherefore, in the room of the Cork, we substituted a piece of Char-coal, as less likely to imbibe any moisture from the Air, but the event proved much the same with that newly related: So that this Experiment seems more liable to Casualties than any, excepting one we have made in our Engine. And as it is difficult to prevent them, so it seems not very easie to discover the causes of them, whereof we shall therefore at present forbear mentioning our Conjectures.

SOme Learned Mathematicians have of late ingeniously endeavoured to reduce Filtres to Siphons; but still the true cause of the ascension of Water, and other Liquors both in Siphons and in Filtration, needing (for ought we have yet found) a clearer Discovery and Explication, we were desirous to try whether or no the pressure of the Air might reasonably be supposed to have either the principal, or at least a considerable Interest in the raising of those Liquors. But because we found that we could not yet so evacuate our Receiver, but that the remaining Air though but little in comparison of the exhausted, would be able to impell the Water to a greater height than is usual in ordinary Filtrations: we resolved instead of a List of Cotton, or the like Filtre, to make use of a Siphon of Glass, delineated in the third Figure, consisting of three pieces, two streight, and the third crooked to joyn them together; whose Junctures were diligently clos'd that no Air might find entrance at them. One of the Legs of this Siphon was (as it should be) somewhat longer than the other, and was pervious at the bottom of it onely, by a hole almost as slender as a hair, that the Water might but very leasurely drop out of it, lest it should all run out before the Experiment were compleated. The other and shorter Leg of the Siphon was quite open at the end, and the same wideness with the rest of the Pipe, whose bore was about ¼ of an Inch. The whole Siphon made up of these several pieces put together, was design'd to be about a Foot and a half long; that the remaining Air, when the Vessel was exhausted after the wonted manner, might not be able to impell the Water to the top of the Siphon; which being inverted, was fill'd with Water, and of which the Shorter leg being let down two or three Inches deep into a Glass Vessel full of Water, and the upper parts of it being fasten'd to the inside of the Cover of the Receiver, we proceeded to close first, and then to empty the Vessel.

The effect of the tryal was this, That till a pretty quantity of Air had been drawn out, the Water dropp'd freely out at the lower end of the lower leg of the Siphon, as if the Experiment had been performed in the free Air. But afterwards, the Bubbles (as had been apprehended) began to disclose themselves in the Water, and ascending to the top of the Siphon, imbodyed themselves there into one, which was augmented by little and little, by the rising of other bubbles that from time to time broke into it, but much more by its own dilatation, which increas'd proportionably to the exsuction that was made of the Air out of the Receiver. So that at length the Water in the shorter Leg of the Siphon was reduc'd, partly by the extraction of the ambient Air, and partly by the expansion of the great Bubble at the upper part of the Siphon, to be but about a Foot high, if so much; whereby it came to pass, that the course of the Water in the Siphon was interrupted, and that which remain'd in the longer Leg of it, continu'd suspended there without dropping any longer. But upon the turning of the Stop-cock, the outward Air (being let into the Receiver) got into the Siphon, by the little hole at which the Water formerly dropt out; and traversing all the incumbent Cylinder of Water, in the form of Bubbles, joynd it self with that Air that before possess'd the top of the Siphon.

To prevent the inconveniences arising from these Bubbles, two Glass Pipes, like the former, were so placed, as to terminate together in the midst of the Belly of a Glass Viol, into whose Neck they were carefully fasten'd with Cement; and then both the Viol and the Pipes being (which was not done without difficulty) totally fill'd with Water, the Siphon describ'd in the fifth Figure, was plac'd with its shorter Leg in the Glass of Water as formerly; and the Experiment being prosecuted after the same manner, much more Air than formerly was drawn out, before the bubbles, disclosing themselves in the water, were able to disturb the Experiment; because that in the capacity of the Viol there was room enough for them to stretch themselves, without depressing the Water below the ends of the Pipes; and during this time, the Water continued to drop out of the propending Leg of the Siphon. But at length the Receiver being very much empty'd, the passage of the Water through the Siphon ceas'd, the upper ends of the Pipes beginning to appear a little above theremaining Water in the Viol, whose dilated Air appear'd likewise to press down the Water in the Pipes, and fill the upper part of them.

And hereby the continuity of the Water, and so the Experiment it self being interrupted we were invited to let in the Air again, which, according to its various proportions of pressure to that of the Air in the Viol and the Pipes, did for a good while exhibite a pleasing variety of Phaenomena, which we have not now the leasure to recite. And though upon the whole matter there seem'd little or no cause to doubt, but that, if the Bubbles had not disturb'd the Experiment, it would manifestly enough have appear'd that the course of Water through Siphons depends upon the pressure of the Air: yet we resolv'd, at our next leasure and conveniency, to try the Experiment again, with a quantity of Water before freed from Bubbles by the help of the same Engine.

This occasion I have had to take notice of Siphons, puts me in mind of an odde kind of Siphon that I caus'd to be made ā pretty while ago; and which hath been since, by an Ingenious Man of Your acquaintance, communicated to divers others. The occasion was this: An eminent Mathematician told me one day, that some inquisitive French Men (whose Names I know not) had observ'd, That in case one end of a slender and perforated Pipe of Glass be dip'd in Water, the liquor will ascend to some height in the Pipe, though held perpendicular to the plain of the Water. And, to satisfie me that he mis-related not the Experiment, he soon after brought two or three small Pipes of Glass, which gave me the opportunity of trying it: though I had the less reason to distrust it, because I remember I had often, in the long and slender Pipes of some weather Glasses, which I had caus'd to be made after a somewhat peculiar fashion, taken notice of the like ascension of the Liquor, though (presuming it might be casual) I had made but little reflexion upon it. But after this tryal, beginning to suppose, that though the Water in these Pipes that were brought me, rise not above a quarter of an Inch, (if near so high) yet, if the Pipes were made slender enough, the Water might rise to a very much greater height; I caus'd several of them to be, by a dexterous Hand, drawn out at the flame of a Lamp, in one of which that was almost incredibly slender we found that the Water ascended (as it were of it self) five Inches by measure, to the no small wonder of some famous Mathematicians, who were Spectators of some of these Experiments. And this height the Water reach'd to, though the Pipe were held in as erected a posture as we could: For if it were inclin'd, the Water would fill a greater part of it, though not rise higher in it. And we also found, that when the inside of the Pipe was wetted beforehand, the Water would rise much better than otherways: But we caus'd not all our slender Pipes to be made streight, but some of them crooked, like Siphons: And having immers'd the shorter Leg of one of these into a Glass that held some fair Water, we found, as we expected, that the Water arising to the top of the Siphon, though that were high enough, did of it self run down the longer Leg, and continue running like an ordinary Siphon. The cause of this ascension of the Water, appear'd to all that were present so difficult, that I must not stay to enumerate the various Conjectures that were made at it, much less to examine them; especially having nothing but bare Conjectures to substitute in the room of those I do not approve. We try'd indeed, by conveying a very slender Pipe and a small Vessel of Water into our Engine, whether or no the exsuction of the ambient Air would assist us to find the cause of the ascension we have been speaking of: But though we imploy'd red Wine instead of Water, yet we could scarce certainly perceive thorow so much Glass, as was interpos'd betwixt our eyes and the Liquor, what happen'd in a Pipe so slender that the redness of the Wine was scarce sensible in it. But as far as we could discern, there happen'd no great alteration to the Liquor: which seem'd the less strange, because the spring of that Air that might depress the Water in the Pipe, was equally debilitated with that which remain'd to press upon the surface of the Water in the little Glass. Wherefore, in favor of his Ingenious Conjecture who ascrib'd the Phaenomenon under consideration, to the greater pressure made upon the water by the Air without the Pipe, than by that within it, (where so much of the Water (consisting perhaps of Corpuscles more pliant to the internal surfaces of the Air) was contiguous to the Glass) it was shown, that in case the little Glass Vessel that held the water, of which a part ascended into the slender Pipe, were so clos'd, that a Man might with his mouth suck the Air out of it, the water would immediately subside in the small Pipe. And this would indeed infer, that it ascended before onely by the pressure of the incumbent Air; But that it may (how justly I know not) be objected, That peradventure this would not happen, in case the upper end of the Pipe were in a Vacuum: And that 'tis very probable the water may subside, not because the pressure of the internal Air is taken off by Exsuction, but by reason of the spring of the external Air, which impels the Water it findes in its way to the cavity deserted by the other Air, and would as well impel the same water upwards, as make it subside, if it were not for the accidental posture of the Glasses. However, having not now leisure to examine any farther this Matter, I shall onely mind Your Lordship, that if You will prosecute this Speculation, it will be pertinent to find out likewise, Why the surface of water (as is manifest in Pipes) useth to be concave, being depress'd in the middle, and higher on every side? and why in Quicksilver on the contrary, not onely the surface is wont to be very convex, or swelling, in the middle; but if you dip the end of a slender Pipe in it, the surface of the Liquor (as 'tis call'd) will be lower within the Pipe, than without. Which Phaenomena, whether, and how far, they may be deduc'd from the Figure of the mercurial Corpuscles, and the Shape of the springy Particles of the Air, I willingly leave to be consider'd.

SEveral ways we have met with propos'd, partly by the excellent Galileo, and partly by other ingenious Writers, to manifest that the Air is not devoid of weight; some of these require the previous absence of the Air to be weighed; and others, the violent condensation of it. But if we could list a pair of Scales above the Atmosphere, or place them in a Vacuum, we might there weigh a parcel of Air it self, as here we do other Bodies in the Air, because it would there be heavier than that which surrounds it, as are grosser Bodies we commonly weigh, than the medium or ambient Air. Wherefore, though we have above declin'd to affirm, that our Receiver, when emptyed, deserves the name of a true Vacuum, and though we cannot yet perfectly free it from Air it self, yet we thought fit to try how far the Air would manifest its gravity in so thin a medium, as we could make in our. Receiver, by evacuating it. We caus'd then to be blown at the Flame of a Lamp, a Glass bubble of about the bigness of a small Hen-egge, and of an Oval form, save that at one end there was drawn out an exceeding slender Pipe, that the Bubble might be sealed up, with as little rarefaction as might be, of the Air included in the great or Oval Cavity of it. This Glass being sealed, was fastened to one of the Scales of the exact pair of Ballances formerly mention'd; and being counterpois'd with a weight of Lead, was convey'd into the Receiver, and clos'd up in it. The Beam appearing to continue Horizontal, the Pump was set on work, and there scarce past above two or three Exsuctions of the Air, before the Ballance lost its Equilibrium, and began to incline to that side on which the Bubble was; which, as the Air was farther and farther drawn out, did manifestly more and more preponderate, till he that pumped began to grow weary of his Imployment: after which the Air being leisurely let in againe, the Scales by degrees returned to their former Equilibrium. After that we took them out, and casting into that Scale to which the lead belonged three quarters of a grain, we conveyed the ballance into the Receiver, which being closed up, and exhausted as before, we observ'd, that as the Air was drawn out more and more, so the Glass bubble came nearer and nearer to an Equilibrium with the other weight, till at length the Beam was drawn to hang Horizontal; which (as we had found by another tryal) we could not bring it to do, when a quarter of a Grain more was added to the Scale, to which the Lead belong'd: though it seem'd questionless, that if we could have perfectly empty'd the Receiver of the contain'd Air, that included in the bubble would have weighed above a Grain, notwithstanding its having been probably somewhat rarefied by the flame, by the help of which, the bubble was seal'd up. Let us adde, That on the regress of the excluded Air, the Lead, and the weight cast into the same scale, did again very much preponderate.

We likwise convey'd into the Receiver, the same bubble, open'd at the end of the slender Pipe above-mention'd, but having drawn out the Air, after the accustomed manner, we found not as before, the bubble to out-weigh the opposite Lead: so that by the help of our Engine we can weigh the Air, as we weigh other Bodies, in its natural or ordinary consistence, without at all condensing it: Nay, which is remarkable, having convey'd a Lambs bladder about half full of Air into the Receiver, we observed, that though upon the drawing out of the ambient Air, the imprisoned Air so expanded it self, as to distend the Bladder so, as to seem ready to Break it; yet this rarefied Air did manifestly depress the Scale whereunto it was annexed.

Another thing we must not forget to mention, that happened to us, whilst we were making tryals concerning the weight of the Air; namely, that having once caus'd the Pump to be somewhat obstinately ply'd, to discover the better what may be expected from the thinness of the medium in this Experiment; the Imprison'd Air broke its brittle Prison, and throwing the greatest part of it against the side of the Receiver, dash'd it against that thick Glass into a multitude of pieces. Which accident I mention, partly that it may confirm what we deliver'd in our Reflexions, upon the first Experiment, where we considered what would probably be done by the spring of the Air Imprison'd in such Glasses, in case the ballancing pressure of the ambient Air were withdrawn; and partly, that we may thence discern of how close a Texture Glass is, since so very thin a film of Glass (if I may so call it) prov'd so impervious to the Air, that it could not get away through the Pores, but was forc'd to break the Glass in pieces to free it self; and this, notwithstanding the time and advantage it had to try to get out at the Pores. And this I mention, that neither our Experiments, nor those of divers Learned Men, might receive any prejudice from an Experiment which I happen'd to make divers years ago, and which, having been so much taken notice of by curious Men, may be drawn to countenance their erroneous Opinion, who would fain perswade us, That Glass is penetrable by Air properly so called. Our Experiment was briefly this: We were distilling a certain Substance, that much abounded with subtle Spirits and volatile Salt, in a strong Earthen vessel of an unusual shape, to which was luted a large Receiver, made of the course sort of Glass (which the Trades-men are wont to call green Glass) but in our absence, the Fire, though it were to be very strong, was, by the negligence or mistake of those we appointed to attend it, so excessively increas'd, that when we came back to the Fornace, we found the spirituous and saline Corpuscles pour'd out (if I may so call it) so hot, and so copiously into the Receiver, that they made it all opacous, and more likely to flie in pieces, than fit to be touch'd. Yet, being curious to observe the effects of a Distillation, prosecuted with so intense and unusual a degree of heat, we ventur'd to come near, and observ'd, among other things, that on the outside of the Receiver, at a great distance from the juncture, there was setled a round whitish Spot or two, which at first we thought might be some stain upon the Glass; but after, finding it to be in divers Qualities like the Oyl and Salt of the Concrete we were Distilling, we began to suspect that the most subtle and fugitive parts of the impetuously ascending Steams, had penetrated the substance (as they speak) of the Glass, and by the cold of the ambient Air were condensed on the surface of it. And though we were very backward to credit this suspition, and therefore call'd in an Ingenious Person or two, both to assist us in the Observation, and have Witness of its event, we continued a while longer to watch the escape of such unctuous Fumes; and upon the whole matter unanimously concluded that (all things consider'd) the subtle parts of the distill'd matter being violently agitated, by the excessive heat that pass'd through the Pores of the Glass, widn'd by the same heat. But this having never happen'd but once in any of the Distillations we have either made or seen, though these be not a few, it is much more reasonable to suppose, that the perviousness of our Receiver to a Body much more subtle than Air, proceeded partly from the looser Texture of that particular parcel of Glass the Receiver was made of (for Experience hath taught us, that all Glass is not of the same compactness and solidity) and partly from the enormous heat, which, together with the vehement agitation of the penetrant Spirits, open'd the Pores of the Glass; than to imagine that such a substance as Air, should be able to permeate the Body of Glass contrary to the testimony of a thousand Chymical and Mechanical Experiments, and of many of those made in our Engine, especially that newly recited: Nay, by our fifth Experiment it appears that a thin Bladder will not at its Pores give passage even to rarefied Air. And on this occasion we will annex an Experiment, which hath made some of those we have acquainted with it, doubt, whether the Corpuscles of the Air be not less subtle than those of Water.

But without examining here the reasonableness of that doubt, we will proceed to recite the Experiment it self, which seems to teach, That though Air, when sufficiently compressed, may perchance get entrance into narrower holes and crannies than Water; yet unless the Air be forc'd in at such very little holes, it will not get in at them, though they may be big enough to let Water pass through them.

The Experiment then was this: I took a fair Glass Siphon, the lower end of whose longest Leg was drawn by degrees to such a slenderness, that the Orifice, at which the Water was to fall out, would hardly admit a very small Pin: This Siphon being inverted, the matter was so order'd, that a little Bubble of Air was intercepted in the slenderest part of the Siphon, betwixt the little hole newly mention'd, and the incumbent Water, upon which it came to pass, that the Air being not to be forc'd through so narrow a passage, by so light a Cylinder of Water, (though amounting to the length of divers Inches,) as lean'd upon it, hindered the farther efflux of the Water, as long as I pleased to let it stay in that narrow place: whereas, when by blowing a little at the wider end of the Siphon, that little parcel of Air was forced out with some Water, the remaining Water, that before continu'd suspended, began freely to drop down again as formerly. And if you take a Glass Pipe, whether it be in the form of a Siphon, or no, that being for the most part of the thickness of a Mans Finger, is yet towards one end so slender, as to terminate in a hole almost as small as a Horse-hair; and if you fill this Pipe with Water, you will find that Liquor to drop down freely enough thorow the slender Extream: But if you then invert the Pipe, you will find that the Air will not easily get in at the same hole through which the Water passed. For in the sharp end of the Pipe, some Inches of water will remain suspended, which, 'tis probable, would not happen, if the Air could get in to succeed it, since if the hole were a little wider, the Water would immediately subside. And though it be true, that if the Pipe be of the length of many Inches, a great part of the Water will run down at the wider Orifice: yet that seems to happen for some other reason, than because the Air succeeds it at the upper and narrow Orifice, since all the slender part of the Pipe, and perhaps some Inches more, will continue full of Water.

And on this occasion I remember, that whereas it appears by our fifth Experiment, That the Aërial Corpuscles (except perhaps some that are extraordinarily fine) will not pass thorow the Pores of a Lambs Bladder, yet Particles of Water will, as we have long since observ'd, and as may be easily try'd, by very closely tying a little Alcalizate Salt (we us'd the Calx of Tartar made with Nitre) in a fine Bladder, and dipping the lower end of the Bladder in Water; for if you hold it there for a competent while, you will find that there will strain thorow the Pores of the Bladder, Water enough to dissolve the Salt into a Liquor.

But I see I am slip'd into a Digression, wherefore I will not examine, whether, the Experiment I have related, proceeded from hence, That the springy Texture of the Corpuscles of the Air, makes them less apt to yield and accommodate themselves easily to the narrow Pores of Bodies, than the more flexible Particles of Water; or whether it may more probably be ascrib'd to some other Cause. Nor will I stay to consider how far we may hence be assisted to ghess at the cause of the ascension of Water in the slender Pipes, and Siphons formerly mention'd, but will return to our Bubble; and take notice, That we thought fit also to endeavor to measure the capacity of the Bubble we had made use of, by filling it with Water, that we might the better know how much Water answer'd in weight to ¾ of a Grain of Air, but notwithstanding all the diligence that was used to preserve so brittle a Vessel, it broke before we could perfect that we were about, and we were not then provided of another Bubble fit for our turn.

The haste I was in, My Lord, when I sent away the last Sheet, made me forget to take notice to you of a Problem that occurr'd to my thoughts, upon the occasion of the slow breaking of the Glass Bubble in our evacuated Receiver. For it may seem strange, since by our sixth Experiment it appears, that the Air, when permitted, will by its own internal Spring expand it self twice as much as Mersennus was able to expand it, by the heat even of a candent AEolipile: Yet the Elater of the Air was scarce able to break a very thin Glass Bubble, and utterly unable to break one somewhat thicker, within whose cavity it was imprison'd; whereas Air pen'd up and agitated by heat is able to perform so much more considerable effects, that (not to mention those of Rarefaction that are more obvious) the Learned Jesuit Cabaeus (he that writ of the Load-stone) relates, That he saw a Marble Pillar (so vast, that three men together P: Nicol: Cab: lib: 4. Meteor: Aristot: with display'd arms could not imbrace it, and that 1000 Yoke of Oxen drawing it several ways with all their strength, could not have torn it assunder) quite broken off in the midst, by reason of some Wood, which happening to be burnt just by the Pillar the heat proceeding from the neighbouring Fir, so rarefied some Air or Spirituous Matter which was shut up in the cavities of the Marble, that it broke through the solid Body of the Stone to obtain room to expand it self.

I remember I have taken notice that probably the reason why the included Air did not break the hermetically seal'd Bubbles that remain'd intire in our emptyed Receiver, was, That the Air, being somewhat rarefied by the flame imploy'd to close the Glass, its Spring, upon the recess of the heat, grew weaker than before. But though we reject not that ghess, yet it will not in the present case serve the turn, because that much smaller Glass bubbles exactly clos'd, will, by the included Air (though agitated by the heat of a very moderate Fire) be made to fly in pieces. Whether we may be assisted to salve this Problem, by considering that the heat doth from within vehemently agitate the Corpuscles of the Air, and add its assistance to the Spring they had before, I shall not now examine: since I here but propose a Problem, and that chiefly that by this memorable Story of Cabaeus, notice may be taken of the prodigious power of Rarefaction, which hereby appears capable of performing stranger things than any of our Experiments have hitherto ascrib'd to it.

We should hence, My Lord, immediately proceed to the next Experiment, but that we think it fit, on this occasion, to acquaint You with what some former tryals (though not made in our Engine) have taught us, concerning what we would have discover'd by the newly mention'd Bubble that broke. And this the rather, because (a great part of this Letter supposing the gravity of the Air) it will not be impertinent to determine more particularly than hitherto we have done, what gravity we ascribe to it.

We took then an AEolipile made of Copper, weighing six ounces, five drachms, and eight and forty grains: this being made as hot as we durst make it, (for fear of melting the mettle, or at least the Sodar) was removed from the fire and immediately stopped with hard Wax that no Air at all might get in at the little hole, wont to be left in AEolipiles for the fumes to issue out at: Then the AEolipile being suffer'd leasurely to cool, was again weighed together with the Wax that stopt it, and was found to weigh (by reason of the additional weight of the Wax) six ounces, six drachms, and 39 grains. Lastly, the Wax being perforated without taking any of it out of the Scale, the external Air was suffered to rush in (which it did with some noise) and then the AEolipile and Wax, being again weighed amounted to six ounces, six drachms, and 50 grains. So that the AEolipile freed as far as our fire could free it, from its Air, weighed less than it self when replenished with Air, full eleven grains. That is, the Air containable within the cavity of the AEolipile amounted to eleven grains and somewhat more; I say somewhat more, because of the particles of Air, that were not driven by the fire out of the AEolipile. And by the Way (if there be no mistake in the observations of the diligent Mersennus) it may seem strange that it should so much differ from 2 or 3 of ours; in none of which we could rarefie the Air in our AEolipile (though made red hot almost all over, and so immediately plung'd into cold Water) to half that degree which he mentions, namely to 70 times its natural extent, unless it were that the AEolipile he imploy'd was able to sustain a more vehement heat than ours (which yet we kept in so great an one, that once the Soder melting, it fell asunder into the two Hemispheres it consists of.)

The fore-mentioned way of weighing the Air by the help of an AEolipile, seems somewhat more exact than that which Mersennus used, In that in ours the AEolipile was not weighed, till it was cold; whereas in his, being weighed red hot, it is subject to lose of its substance in the cooling, for (as we have elsewhere noted on another occasion) Copper heated red hot, is wont in the cooling to throw off little thin seales in such plenty, that having purposely watched a Copper AEolipile during its refrigeration, we have seen the place round about it almost covered with those little scales it had every way scatter'd: which, however they amount not to much, ought not to be over-looked, when 'tis so light a Body as Air, that is to be weighed. We will not examine whether, the AEolipile in cooling may not receive some little increment of weight, either from the vapid or saline Steams that wander up and down in the Air: But we will rather mention, that (for the greater exactness) we imployed to weigh our AElipile, both when fill'd only with Air and when replenish'd with Water, a pair of Scales that would turn (as they speak) with the fourth part of a grain.

As to the proportion of weight betwixt Air and Water, some learned men have attempted it by ways so unaccurate that they seem to have much mistaken it. For (not to mention the improbable accounts of Kepler and others.) The learned and diligent Ricciolus, having purposely endeavoured to investigate this proportion by means of a thin bladder, estimates the weight of the Air to that of the Water to be as one to ten thousand, or there abouts. And indeed I remember that having formerly, on a certain occasion, weighed a large bladder full of Air, and found it when the Air was all squeesed out, to have contained fourteen grains of Air. I found the same bladder afterwards fill'd with Water to contain very near 14 pound of that liquor: according to which account, the proportion of Air to Water was almost as a grain to a pound, that is, as one, to above 7600. To this we may add, that on the other side, Galileo himself using another, (but an unaccurate way too,) defined the Air to be in weight to Water, but as one to 4 hundred. But the way formerly proposed of weighing the Air by an AEolipile, seems by great odds more exact; and (as far as we could ghess) seemed to agree well enough with the Experiment made in our Receiver. Wherefore it will be best to trust our AEolipile in the enquiry we are about. And according to our observations the water it contained amounting to one and twenty ounces and an half, and as much Air as was requisite to fill it weighing eleven grains, the proportion in gravity of Air to Water of the same bulk will be as one to 938. And though we could not fill the AEolipile with water, so exactly as we would, yet in regard we could not neither as perfectly as we would, drive the Air out of it by heat, we think the proportion may well enough hold: but those that are delighted with round numbers (as the phrase is) will not be much mistaken if they reckon Water to be near a thousand times heavier than Air. And (for farther proof that we have made the proportion betwixt these two Bodies rather greater than lesser than indeed it is: and also to confirm our former observation of the weight of the Air) we will add, That, having another time put some Water into the AEolipile before we set it on the fire, that the copious vapours of the rarefied liquor might the better drive out the Air, we found, upon tryal carefully made, that when the AEolipile was refrigerated, and the included vapours were by the cold turned again into Water (which could not have happen'd to the Air, that the preceding Steams expelled) the Air, when it was let in, increas'd the weight of the AEolipile as much as before, namely, Eleven Grains; though there were already in it twelve Drachms and a half, besides a couple of Grains of Water, which remained of that we had formerly put into it to drive out the Air.

Mersennus indeed tells us, that by his account Air is in weight to Water, as 1 to 1356. And adds, that we may, without any danger, believe that the gravity of Water to that of Air of a like bulk, is not less than of 1300 to 1. And consequently that the quantity of Air to a quantity of Water equiponderant thereto, is as 1300 to 1. But why we should relinquish our own carefully repeated tryals, I see not. Yet I am unwilling to reject those of so accurate and usefull a Writer: And therefore shall propose a way of reconciling our differing Observations, by presenting, that the discrepance between them may probably arise from the differing consistence of the Air at London and at Paris: For our Air being more cold and moist, than that which Your Lordship now breaths, may be suppos'd also to be a fourth or fifth part more heavy. I leave it to be consider'd, whether it be of any moment that our Observations were made in the midst of Winter, whereas his were perhaps made in some warmer time of the Year. But I think it were not amiss, that, by the method formerly propos'd, the gravity of the Air were observ'd both in several Countries, and in the same Country, in the several Seasons of the Year and differing Temperatures of the Weather. And I would give something of value to know the weight of such an AEolipile as ours full of Air, in the midst of Winter in Nova Zembla, if that be true which we formerly • ook notice of, namely, That the Hollanders, who Wintered there, found that Air so thick that their Clock would not go.

If Your Lordship should now ask me, if I could not by the help of these, and our other Observations, decide the Controversies of our Modern Mathematicians about the height of the Air or Atmosphere, by determining how high it doth indeed reach: I should answer, That though it seems easie enough to shew that divers Famous and Applauded Writers have been mistaken in assigning the height of the Atmosphere: Yet it seems very difficult precisely to define of what height it is. And because we have hitherto but lightly touch'd upon a matter of such importance, we presume it will not be thought impertinent, upon this occasion, to annex something towards the Elucidation of it.

What we have already try'd and newly set down, allows us to take it for granted, that (at least about London) the proportion of gravity betwixt Water and Air, of equal bulk, is as of a thousand to one.

The next thing therefore that we are to enquire after, in order to our present design, is the difference in weight betwixt Water and Quick-silver: And though this hath been defin'd already by the Illustrious Verulam, and some other inquisitive Persons, that have compared the weight of several Bodies, and cast their Observations into Tables, yet we shall not scruple to annex our own tryals about it: Partly, because we find Authors considerably to disagree; partly, because we used exacter Scales, and a somewhat more wary method than others seem to have done: And partly also, because having prosecuted our inquiry by two or three several ways; the small difference between the events may assure us that we were not much mistaken.

We took then a Glass Pipe, of the form of an inverted Siphon, whose shape is delineated in the sixteenth Figure: And pouring into it a quantity of Quick-silver, we held it so, that the superficies of the Liquor, both in the longer and shorter leg, lay in a Horizontal Line, denoted in the Scheme by the prick'd Line E F; then pouring Water into the longer Leg of the Siphon, till that was almost fill'd, we observ'd the surface of the Quick-silver in that Leg to be, by the weight of the Water, depressed, as from E to B; and in the shorter Leg, to be as much impell'd upwards as from F to C: Whereupon having formerly stuck marks, as well at the point B, as at the opposite point D, we measur'd both the distance D C to have the height of the Cylinder of Quick-silver, which was raised above the point D (level with the surface of the Quick-silver in the other Leg) by the weight of the Water, and the distance B A which gave us the height of the Cylinder of Water. So that the distance D C amounting to 2 13\54 Inches, and the height of the Water amounting 30 45\54 Inches; and the whole numbers on both sides, which the annexed Fractions being reduc'd to improper Fractions of the same denomination, the proportion, appear'd to be (the denominators being left out as equal on both sides) as 121 to 1665; or by reduction, as one to 92/121.

Besides this unusual way of determining the gravity of some things, we measur'd the proportion betwixt Quick-silver and Water, by the help of so exact a ballance, as looseth its AEquilibrium by the hundredth part of a Grain. But because there is wont to be committed an oversight in weighing Quick-silver and Water, especially if the Orifice of the Vessel wherein they are put be any thing wide, in regard that men heed not that the surface of Water in Vessels will be concave, but that of Quick-silver notably convex or protuberant: To avoid this usual oversight (I say) we made use of a Glass bubble, blown very thin at the Flame of a Lamp, that it might not be too heavy for the Ballance, and terminating in a very slender neck wherein the concavity or convexity of a Liquor could not be considerable: This Glass weighing 23½ Grains, we fill'd almost with Quick-silver, and fastning a mark over against the middle of the protuberant Superficies as near as our Eyes could judge, we found that the Quick-silver alone weighed 299 7/12 Grains; Then the Quick-silver being pour'd out, and the same Glass being fill'd as full of common Water, we found the Liquor to weigh 21⅞ Grains. Whereby it appear'd the weight of Water to Quick-silver, is as one to 13 19/28: Though our Illustrious Verulam (questionless not for want of Judgment or Care, but of exact Instruments) Makes the proportion betwixt those two Liquors to be greater than of 1 to 17. And to add, that upon the bye, since Quick-silver and well rectified Spirit of Wine, are (how justly I say not) accounted, the one the heaviest, and the other the lightest of Liquors; we thought to fill the same Glass, and with the same Scales to observe the difference betwixt them, which we found to be as of 1 to 16 641/1084; whereby it appear'd, That the difference betwixt Spirit of Wine, that may be made to burn all away, (such as was ours) and common Water, is as betwixt 1 and 1 44/171.

We might here take occasion to admire, that though Water (as appear'd by the Experiment formerly mention'd of the Pewter Vessel) seems not capable of any considerable condensation, and seems not to have interspersed in it any store of Air; yet Quick-silver, of no greater bulk than Water, should weigh near fourteen times as much. But having only pointed at this as a thing worthy of consideration, we will proceed in our inquiry after the height of the Atmosphere: And to avoid the trouble of Fractions, we will assume, that Quick-silver is fourteen times as heavy as Water, since it wants so little of being so.

Wherefore, having now given us the proportion of Air to Water, and Water to Quick-silver, it will be very easie to find the proportion betwixt Air and Quick-silver, in case we will suppose the Atmosphere to be uniformly of such a consistence as the Air we weighed here below. For since our Engine hath sufficiently manifested that 'tis the AEquilibrium with the external Air, that in the Torricellian Experiment keeps the Quick-silver from subsiding; And since, by our accurate Experiment formerly mention'd, it appears that a Cylinder of Mercury, able to ballance a Cylinder of the whole Atmosphere, amounted to near about thirty Inches; and since, consequently we may assume the proportion of Quick-silver to Air to be as fourteen thousand to one; it will follow, that a Cylinder of Air, capable to maintain an AEquilibrium, with a Mercurial Cylinder of two Foot and an half in height, must amount to 35000 Feet of our English measure; and consequently (reckoning five Foot to a Geometrical Pace, and one thousand such Paces to a Mile) to seven full Miles.

But this (as we lately intimated) proceeds upon the supposition, that the Air is every where of the same consistence that we found it near the surface of the Earth; but that cannot with any safety be concluded, not only for the reason I find to have been taken notice of by the Ancients, and thus exprest in Seneca. Omnis Aër (says he) quo propior est terris hoc crassior; quemadmodum in aqua & in omni humore faex Senec. Nat. quest. lib. 4. cap. 10. ima est, ita in Aëre spississima-quae que desidunt, but much more, because the springy Texture of the Aerial Corpuscles; makes them capable of a very great compression, which the weight of the incumbent part of the Atmosphere is very sufficient to give those that be undermost and near the surface of the Earth. And if we recall to mind those former Experiments, whereby we have manifested, That Air, much rarefied without heat, may easily admit a farther rarefaction from heat, and that the Air, even without being expanded by heat, is capable of being rarefied to above one hundred and fifty times the extent it usually possesseth here below; How can it be demonstrated that the Atmosphere may not, for ought we know, or at least for ought can be determin'd by our Statical and Mechanical Experiments, rise to the height of Five and twenty German Leagues, if not of some hundred of common Miles?

And this conjecture it self may appear very injurious to the height whereunto Exhalations may ascend, if we will allow that there was no mistake in that strange Observation made at Tolouse in a clear Night in August, by the diligent Mathematician Emamuel Magnan, and thus Recorded by Ricciolus, for I have not at hand the Author's own Book: Vidit (says he) ab hora undecima post meridiem Ricciol. Alma. Nov. Tom. 2. lib. 10. sect. 6. prop. 50. Ex Magnan. lib. 1. Perspectivae horariae prop. 38. usque ad mediam noctem Lunâ infra horizontem positâ, nubeculam quandam lucidam prope Meridianum fere usque ad Zenith diffusam quae consideratis omnibus non poterat nisi à sole illuminari; ideoque altior esse debuit tota umbra terrae. Addit (continues Ricciolus) simile quid evenisse Michaeli Angelo Riccio apud Sabinos versanti nempe viro in Mathesi eruditissimo.

Various Observations made at the feet, tops, and interjacent parts of high Mountains, might perchance somewhat assist us to make an estimate in what proportion, if in any certain one, the higher Air is thicker than the lower, and ghess at the difform consistence, as to laxity and compactness of the Air at several distances from us. And if the difficulties about the refractions of the Celestial Lights, were satisfactorily determin'd, that might also much conduce to the placing due limits to the Atmosphere (whose, Dimensions those Observations about Refractions seem hitherto much to contract.) But for the present we dare not pronounce any thing peremptorily concerning the height of it, but leave it to farther inquiry: contenting our selves to have manifested the mistake of divers eminent Modern Writers, who will not allow the Atmosphere to exceed above two or three Miles in height (as the Famous Kepler will not the Aër refractivus) and to have rendred a reason why in the mention we made in the Notes upon the first Experiment touching the height of the Atmosphere, we scrupled not to speak of it, as if it might be many Miles high.

WE will now proceed to recite a Phaenomenon, which, though made amongst the first, we thought fit not to mention till after many others, that we might have the opportunity to observe as many Circumstances of it as we could, and so present Your Lordship at once, most of what we at several times have taken notice of concerning so odd a Phaenomenon.

Our Engine had not been long finish'd, when, at the first leisure we could steal from our occasions to make trial of it, we caused the Air to be pump'd out of the Receiver; and whilst I was busied in entertaining a Learned Friend that just then came to visit me, an Ingenious By-stander, thought he perceiv'd some new kind of Light in the Receiver, of which giving me hastily notice, my Friend and I presently observ'd, that when the Sucker was drawn down, immediately upon the turning of the Key, there appear'd a kind of Light in the Receiver, almost like a faint flash of Lightning in the Day-time, and almost as suddenly did it appear and vanish. Having, not without some amazement, observ'd divers of these apparitions of Light, we took notice that the Day was clear, the hour about ten in the Morning, that the only Window in the Room faced the North; and also, that by interposing a Cloak, or any opacous Body between the Receiver and the Window, though the rest of the Room were sufficiently enlightned, yet the flashes did not appear as before, unless the opacous Body were removed. But not being able on all these Circumstances to ground any firm Conjecture at the cause of this surprising Phaenomenon, as soon as Night was come, we made the Room very dark; and plying the Pump, as in the Morning, we could not, though we often try'd, find, upon the turning of the Key, so much as the least glimmering of Light; whence we inferred, that the flash appearing in the Receiver, did not proceed from any new Light generated there, but from some reflexions of the light of the Sun, or other Luminous Bodies plac'd without it; though whence the Reflexion should proceed, it pos'd us to conjecture.

Wherefore the next Morning, hoping to inform our selves better, we went about to repeat the Experiment, but though we could as well as formerly exhaust the Receiver, though the place wherein we made the trial was the very same; and though other Circumstances were resembling, yet we could not discover the least appearance of Light all that Day, nor on divers others on which trial was again fruitlesly made; nor can we to this very time be sure a Day before-hand that these Flashes will be to be seen in our great Receiver. Nay, having once found the Engine in a good humour (if I may so speak) to shew this trick, and sent notice of it to our Learned Friend Dr. Wallis, who express'd a great desire to see this Phaenomenon, though he were not then above a Bow-shoot off, and made haste to fatisfie his Curiosity; yet by that time he was come, the thing he came for was no longer to be seen; so that having vainly endeavoured to exhibit again the Phaenomenon in his prefence, I began to apprehend what he might think of me, when unexpectedly the Engine presented us a flash, and after that a second, and as many more, as suffic'd to satisfie him that we might very well confidently relate, that we have our selves seen this Phaenomenon, though not confidently promise to shew it others.

And this unsuccessfulness whereto our Experiment is liable, being such, that by all our watchfulness and trials, we could never reduce it to any certain Rules or Observations; since in all constitutions of the Weather, times of the Day, &c. It will sometimes answer, and sometimes disappoint our expectations; We are much discourag'd from venturing to frame an Hypothesis to give an account of it: which if the Experiment did constantly succeed, might the more hopefully be attempted; by the help of the following Phaenomena laid together: some of them produc'd upon trials purposely made to examine the validity of the conjectures, other trials had suggested.

First then we observ'd, that the Apparition of Light may be made as well by Candle-light, as by Day-light; and in whatever position the Candle be held, in reference to the Receiver, as on this or that hand of it, above it, beneath it, or any other way, provided the beams of Light be not hinder'd from falling upon the Vessel.

Next, we noted that the flash appears immediately upon the turning of the Key, to let the Air out of the Receiver into the emptied Cylinder, in so much that I remember not that when at any time in our great Receiver, the Stop-cock was open'd before the Cylinder was exhausted (whereby it came to pass that the Air did rather descend, than rush into the Cylinder) the often mentioned flash appear'd to our eyes.

Yet, we farther observ'd, that when instead of the great Receiver we made use of a small Glass, not containing above a pound and a half of Water, the Phaenomenon might be exhibited though the Stop-cock were open, provided the Sucker were drawn nimbly down.

We noted too, that when we began to empty the Receiver, the appearances of Light were much more conspicuous than towards the latter end, when little Air at a time could pass out of the Receiver.

We observed also, that when the Sucker had not been long before well Oyl'd, and instead of the great Receiver, the smaller Vessel above-mention'd was emptied; We observ'd, I say, that then, upon the opening of the Stop-cock, as the Air descended out of the Glass into the emptied Cylinder, so at the same time there ascended out of the Cylinder into the Vessel a certain steam, which seem'd to consist of very little Bubbles, or other minute Corpuscles thrown up from the Oyl, rarefied by the attrition it suffered in the Cylinder. For at the same time that these Steams ascended into the Glass, some of the same kind manifestly issued out like a little Pillar of Smoke at the Orifice of the Valve, when that was occasionally opened. And these Steams frequently enough presenting themselves to our view, we found, by exposing the Glass to a clear Light, that they were wont to play up and down in it, and so by their whitishness, to emulate in some measure the apparition of Light.

For we likewise sometimes found, by watchfull observation, that when the Flash was great, not only at the very instant the Receiver lost of its transparency, by appearing full of some kind of whitish substance; but that for some short time after the sides of the Glass continued somewhat opacous, and seem'd to be darken'd, as if some whitish Steam adher'd to the inside of them.

He that would render a Reason of the Phaenomenon, whereof all these are not all the Circumstances, must do two things; whereof the one is difficult, and the other little less than impossible: For he must give an account not only whence the appearing whiteness proceeds, but wherefore that whiteness doth sometimes appear, and sometimes not.

For our part, we freely confess our selves at a loss about rendering a Reason of the less difficult part of the Problem: And though Your Lordship should ev'n press us to declare what Conjecture it was, that the above recited Circumstances suggested to us, we should propose the thoughts we then had, no otherwise than as bare Conjectures.

In case then our Phaenomenon had constantly and uniformly appear'd, we should have suspected it to have been produc'd after some such manner as follows.

First, we observ'd that, thought that which we saw in our Receiver seem'd to be some kind of Light, yet it was indeed but a whiteness which did (as hath already been noted) opacate (as some speak) the inside of the Glass.

Next we consider'd, that our common Air abounds with Particles, or little Bodies, capable to reflect the beams of Light. Of this we might easily give divers proofs, but we shall name but two: The one, that vulgar observation of the Motes that appear in multitudes swimming up and down in the Air, when the Sunbeams shooting into a Room, or any other shady place, discover them, though otherwise the Eye cannot distinguish them from the rest of the Air: The other proof we will take from what we (and no doubt very many others) have observ'd, touching the Illumination of the Air in the Night. And we particularly remember, that, being at some distance from London one Night, that the People, upon a very welcome occasion, testified their Joy by numerous Bon-fires; though, by reason of the Interposition of the Houses, we could not see the Fires themselves, yet we could plainly see the Air all enlighten'd over and near the City; which argu'd, that the lucid Beams shot upwards from the fires, met in the Air with Corpuscles opacous enough to reflect them to our Eyes.

A third thing that we considered, was, That white may be produc'd (without excluding otherways, or denying invisible Pores in the solidest Bodies) when the continuity of a Diaphanous Body happens to be interrupted by a great number of surfaces, which, like so many little Looking-glasses, do confusedly represent a multitude of little and seemingly contiguous Images of the lucid Body. We shall not insist on the explanation of this, but refer You for it to what we have said in another Paper (touching Colours.) But the Instances that seem to prove it are obvious: For Water or whites of Eggs beaten to froth, do lose their transparency and appear white. And having out of one of our lessers Receivers carefully drawn out the Air, and so order'd it, that the hole by which the Water was to get in, was exceeding small, that the Liquor might be the more broken in its passage thorow it, we observ'd with pleasure, That, the Neck being held under Water, and the little hole newly mention'd being open'd, the Water that rushed in was so broken, and acquired such a multitude of new Surfaces, that the Receiver seem'd to be full rather of Milk than Water. We have likewise found out, That by heating a lump of Crystal to a certain degree, and quenching it in fair Water, it would be discontinu'd by such a multitude of Cracks, (which created new Surfaces within it) that though it would not fall asunder, but retain its former shape, yet it would lose its transparency, and appear white.

Upon these Considerations, My Lord, and some others, it seem'd not absur'd to imagine, That upon the rushing of the Air out of the Receiver into the empty'd Cylinder, the Air in the Receiver being suddenly and vehemently expanded, the Texture of it was as suddenly alter'd, and the parts made so to shift places (and perhaps some of them to change postures) as during their new and vehement motion and their varied situation, to disturb the wonted continuity, and so the Diaphaneity of the Air; which (as we have already noted) upon its ceasing to be a transparent Body, without the interposition of colour'd things, must easily degenerate into white.

Several things there were that made this Conjecture seem the less improbable. As first, That the whiteness always appear'd greater when the exsuction began to be made, whilst there was store of Air in the Receiver, than when the Air was in great part drawn out. And next, That, having exhausted the Receiver, and apply'd to the hole in the Stop-cock a large bubble of clear Glass, in such a manner, that we could at pleasure let the Air pass out at the small Glass into the great one, and easily fill the small one with Air again, We observ'd with pleasure, That, upon the opening the passage betwixt the two Glasses, the Air in the smaller having so much room in the greater to receive it, the Dissilition of that Air was so great, that the small Viol seem'd to be full of Milk; and this Experiment we repeated several times. To which we may add, That, having provided a small Receiver, whose upper Orifice was so narrow that I could stop it with my Thumb, I observ'd, that when, upon the exsuction of the Air, the capacity of the Glass appear'd white, if, by a sudden removal of my Thumb, I let in the outward Air, that whiteness would immediately vanish. And whereas it may be objected, That in the Instance formerly mention'd, Water turning from perspicuous to white, there intervenes the Air, which is a Body of a Heterogeneous nature, and must turn it into Bubbles to make it lose its transparency. We may borrow an Answer from an Experiment we deliver in another Treatise, where we teach, how to make two very volatile Liquors, which being gently put together, are clear as Rock-water, and yet will almost in a moment, without the sub-ingression of Air to turn them into Bubbles, so alter the disposition of their insensible parts, as to become a white and consistent Body. And this happens not as in the precipitation of Benjamin, and some other Resinous Bodies, which being dissolv'd in spirit of Wine, may, by the effusion of fair Water, be turn'd into a seemingly Milky substance. For this whiteness belongs not to the whole Liquor, but to the Corpuscles of the dissolv'd Gum, which after a while subsiding leave the Liquor transparent, themselves only remaining white: Whereas in our case, 'tis from the vary'd texture of the whole formerly transparent fluid Body, and not from this or that part, that this whiteness results: For the Body is white throughout, and will long continue so; and yet may, in process of time, without any addition, be totally reduc'd into a transparent Body as before.

But besides the Conjecture insisted on all this while, we grounded another upon the following Observation, which was, That having convey'd some smoke into our Receiver plac'd against a Window, we observ'd, that upon the exsuction of the Air, the Corpuscles that were swimming in it, did manifestly enough make the Receiver seem more opacous at the very moment of the rushing out of the Air: For considering that the whiteness, whose cause we enquire of, did but sometimes appear, it seem'd not impossible but that at such times the Air in the Receiver might abound with Particles, capable of reflecting the Light in the manner requisite to exhibite a white colour, by their being put into a certain unusual Motion. As may be in some measure illustrated by this, That the new motion of the freshly mention'd Fumes, made the inside of the Receiver appear somewhat darker than before: And partly by the nature of our formerly mention'd smoking Liquor, whose parts, though they seem'd transparent whilst they compos'd a Liquor, yet when the same Corpuscles, upon the unstopping of the Glass, were put into a new motion, and dispos'd after a new manner, they did opacate that part of the Air they mov'd in, and exhibited a greater whiteness than that which sometimes appears in our Pneumatical Vessel. Nor should we content our selves with this single Instance, to manifest, That little Bodies, which being rang'd after one manner, are Diaphanous and Colour-less, may, by being barely agitated, dispers'd, and consequently otherways rang'd, exhibit a colour, if we were not unwilling to rob our Collection of Experiments concerning Colours.

But, My Lord, I foresee You may make some Objections against our proposed ghess, which perhaps I shall scarce be able to answer, especially, if You insist upon having me render a Reason why our Phaenomenon appears not constantly.

I might indeed answer, that probably it would do so, if instead of our great Receiver we use such a small Viol as we have lately mention'd, wherein the Dissilition of the Air being much greater, is like to be the more conspicuous: Since I remember not that we ever made our trial with such small Vessels, without finding the expected whiteness to appear. But it would remain to be explicated, why in our great Receiver the Phaenomenon should sometimes be seen, and oftentimes not appear. And though that Conjecture which we last made should not be rejected, yet if we were farther press'd to assign a reason why the Air should abound with such Particles, as we there suppose, more at one time than another, we are not yet provided of any better Answer, than this general one, That the Air about us, (and much more that within the Receiver,) may be much alter'd by such causes as few are aware of: For, not to repeat those probable Arguments of this Assertion which we have occasionally mention'd here and there in the former part of this Epistle, we will here set down two or three Instances to verifie the same Proposition. First, I find that the Learned Josephus Acosta, among other Judicious Observations he made in America, hath this concerning the effects of some Joseph. Acosta's Nat. and Mor. Hyst. of the Indies, jib. 3. cap. 9. Winds: There are (saith he) Winds which naturally trouble the Water of the Sea, and make it green, and black; others, clear as Crystal. Next, we have observ'd, That though we convey'd into the Receiver our Scales, and the Pendula formerly mention'd, clean and bright; yet aster the Receiver had been empty'd, and the Air let in again, the gloss or lustre both of the one, and of the other, appear'd tarnish'd by a beginning rust. And in the last place, we will subjoyn an Observation we made some Years ago, which hath been heard of by divers Ingenious Men, and seen by some of them: We had, with pure Spirit of Wine, drawn a Tincture out of a certain Concrete which useth to be reckon'd among Mineral Bodies; And this Tincture being very pure and transparent, we did, because we put a great value upon it, put into a Crystal Viol which we carefully stopp'd, and lock'd up in a Press among some other things that we specially priz'd. This Liquor being a Chymical Rarity, and besides very defecate, and of a pleasing Golden colour; we had often occasion to look upon it, and so to take notice, that one time it seem'd to be very much troubled, and not clear as it was wont to be: Whereupon we imagined, that though it would be something strange, yet it was not impossible, that some Precipitation of the Mineral Corpuscles was then happening, and that thence the Liquor was opacated. But, finding after some days that though the expected Precipitation had not been made, yet the Liquor, retaining its former vivid Colour, was grown clear again as before; we somewhat wondered at it, and locking it up again in the same Press, we resolved to observe, both whether the like changes would again appear in our Tincture; and whether in case they should appear, they would be ascribable to the alterations of the Weather. But though, during the greatest part of a Winter and a Spring, we took pleasure to observe, how the Liquor would often grow turbid, and after a while clear again: Yet we could not find that these Mutations depended upon any that were manifest in the Air, which would be often dark and clouded, when the Tincture was clear and transparent; as on the other side, in clear Weather the Liquor would appear sometimes troubled, and more opacous. So that being unable to give an account of these odd changes in our Tincture (which we suppose we have not yet lost, though we know not whether it hath lost its fickle Nature) either by those of the Air, or any thing else that occurr'd to our thoughts; we could not but suspect, that there may be in divers Bodies, as it were Spontaneous Mutations, that is, such changes as depend not upon manifest Causes. But, My Lord, what hath been all this while said concerning our Phaenomenon, is offer'd to You, not as containing a satisfactory account of it, but to assist You to give Your self one.

WE took a Glass Vessel, open at the top, and into it we put a mixture of Snow and common Salt, (such a mixture as we have in another Treatise largely discoursed of) and into the midst of this mixture we set a Glass, of a Cylindrical form, closely stopp'd at the lower end with Plaister, and open at the upper, at which we fill'd it with common Water. These things being let down into the Receiver, and the Pump being set on work, the Snow began to melt somewhat faster than we expected; Whether upon the account of the exsuction of the Air, or because there was but little of the Snow, or whether for any other Reason, it appear'd doubtfull. But however, by that time the Receiver had been considerably exhausted, which was done in less than ¼ of an hour, we perceived the Water near the bottom of the Glass Cylinder to Freeze, and the Ice by a little longer stay, seem'd to encrease, and to rise somewhat higher than the surface of the surrounding Liquor, whereinto almost all the Snow and Salt were resolv'd. The Glass being taken out, it appear'd that the Ice was as thick as the inside of the Glass it fill'd, though into that I could put my Thumb. The upper surface of the Ice was very concave, which whether it were due to any unheeded accident, or to the exsuction of the Air, we leave to be determin'd by farther trial. And lastly, the Ice held against the Light, appear'd not destitute of Bubbles, though some By-standers thought they were fewer than would have been found if the Water had been frozen in the open Air. The like Experiment we try'd also another time in one of our small Receivers, with not unlike success.

And on this occasion, My Lord, give me leave to propose a Problem, which shall be this: Whence proceeds that strange force that we may sometimes observe in frozen Water, to break the Bodies that imprison it, though hard and solid? That there is such a force in Water expos'd to Congelation, may be gathered not only from what may be often observ'd in Winter, of the bursting of Glasses too close stopp'd, fill'd with Water or aqueous Liquors, but by Instances as much more considerable as less obvious. For I remember, that an Ingenious Stone-cutter not long since complain'd to me, That sometimes, through the negligence of Servants, the Rain being suffered to soak into Marble Stones, the supervening violent Frosts would burst the Stones, to the Possessour's no small damage. And I remember another Trades-man, in whose House I had Lodgings, was last Winter complaining, that even Implements made of Bellmetal, being carelesly expos'd to the wet, have been broken and spoil'd by the Water, which, having gotten into the little Cavities and Crannies of the Metal, was there afterwards frozen and expanded into Ice. And to these Relations, we can add one of the formerly mention'd Cabaeus's, whereby In lib. 4. Meleor. Arist. they not only may be confirm'd; but are surpass'd: For he tells us, That he saw a huge Vessel of exceeding hard Marble, split asunder by congeal'd Water, whose rarefaction, saith our Author, prov'd so vehement, that the hardness of the Stone yielded to it; and so a vessel was broken, which would not have been so by 100 Yoke of Oxen drawing it several ways. I know, My Lord, that to solve this Problem, it will be said, That Congelation doth not (as is commonly, but erroneously presum'd) reduce Water into less room than it possess'd before, but rather makes it take up more. And I have elsewhere prov'd by particular Experiments, That whether or no Ice may be truly said to be Water rarefi'd (for that seems questionable) it may be said to take up more room than the Water did before Glaciation. But though we grant that freezing makes Water swell, yet, how cold (which in Weather-Glasses manifestly condenseth the Air) should expand either the Water, or the intercepted Air so forcibly, as to perform such things as we have newly related, will yet remain a Problem.

WE took an Oval Glass, clear and (lest it should break) pretty strong, with a short Neck at the obtuser end; through this Neck, we thrust almost to the bottom, a Pipe of Glass, which was closely cemented to the newly mention'd Neck, the upper part of which Pipe, was drawn in some places more slender than a Crows Quill, that the changes of the Air in that Glass Egg might be the more conspicuous; Then there was convey'd into the Glass five or six Spoon-fulls of Water, part of which, by blowing Air into the Egg, was rais'd into the abovemention'd slender part of the Pipe, so that the Water was interpos'd between the external Air, and that included in the Egg. This Weather glass (delineated in the fourteenth Figure) was so plac'd, and clos'd up in the cavity of one of our small Receivers, that only the slender part of the Pipe, to the height of four or five Inches, passing thorow a hole in the Cover remain'd expos'd to the open Air.

The Pump being set a work, upon the exsuction of the Air, the Water in the Pipe descended about a quarter of an Inch, and this upon two or three reiterated trials; which seem'd sufficiently to argue, that there was no heat produc'd in the Receiver upon the exsuction of the Air: For even a little heat would probably have been discover'd by that Weather-glass, since upon the bare application of my hand to the outside of the Receiver, the warmth having after some time been communicated or propagated through both the Glasses, and the interval betwixt them, to the imprison'd Air, did so rarefie that, as to inable it, by pressing upon the subjacent Water, to impel that in the Pipe very many times as far as it had fallen downwards upon the exsuction of the Air.

Yet shall not we conclude, that in the cavity of the Receiver the cold was greater after the exsuction of the Air than before.

For if it be demanded what then could cause the fore-mention'd subsiding of the Water? it may be answered, That probably it was the reaching of the Glass Egg, which, upon the exsuction of the ambient Air, was unable to resist altogether as much as formerly the pressure of the included Air, and of the Atmosphere, which, by the intervention of the Water, press'd upon its concave surface: Which seem'd probable, as well by what was above deliver'd, in the Experiment about the breaking of the Glass by the force of the Atmosphere; as by this notable Circumstance (which we divers times observ'd) That when by drawing the Air out of the Receiver, the Water in the Pipe was subsided, upon the re-admission of the external Air, to press against the convex surface of the Egg, the Water was presently re-impell'd to its former height: Which would perhaps appear less strange to Your Lordship, if You had yet seen, what we have heretofore taught in another Treatise, concerning the Spring that may be discover'd in Glass, as rigid and inflexible a Body as it is generally esteem'd. And in the mean while it may serve the turn, to cause a Glass Egg to be blown exceeding thin; and then, having broken it, try how far you can by degrees bend some narrow parts of it; and how readily, upon the removal of what kept it bent, it will restore it self to its former state or posture. But to return to our Experiment: From thence it seems probable, either that there succeeds no Body in the room of the Air drawn out of our Receiver; or that it is not every Matter that is suotle enough readily to pass through the Pores of Glass, that is always agitated enough to produce Heat where ever it is plentifully found. So that if no Vacuum be to be admitted, this Experiment seems to invite us to allow a great disparity, either as to bulk, or as to agitation, or as to both, betwixt some parts of the Etherial substance, and those that are wont here below to produce Heat and Fire.

We try'd also what Operation the drawing out of the Air would have upon Camphire, that being a Body, which, though not a Liquor, consists of such Volatile or Fugitive parts, that without any greater agitation than that of the open Air it self, they will copiously flie away. But we sound not that even this loose Body was sensibly alter'd by the exsuction of the ambient Air.

IT may seem well worth trying, whether or no in our exhausted Glass the want of an ambient Body, of the wonted thickness of Air, would disable even light and little Animals, as Bees, and other winged Insects, to fly. But though we easily foresaw how difficult it would be to make such an Experiment; yet not to omit our endeavours: We procur'd a large Flesh Fly, which we convey'd into a small Receiver. We also another time shut into a great Receiver a Humming Bee, that appear'd strong and lively, though we had rather have made the trial with a Butter-fly, if the cold Season would have permitted us to find any. Since the writing of this XLth Experiment, we procur'd a white Butter-Fly, and inclosed it in one of our smaller Receivers, where, though at first he fluttered up and down, yet presently, upon the exsuction of the Air, he fell down as in a swoon, retaining no other motion than some little trembling of the Wings. The Fly, after some exsuctions of the Air, dropp'd down from the side of the Glass whereon she was walking: But, that the Experiment with the Bee might be the more instructive, we convey'd in with her a bundle of Flowers, which remain'd suspended by a string near the upper part of the Receiver: And having provok'd the Bee, we excited her to flie up and down, the capacity of the Vessel, till at length, as we desir'd, she lighted upon the Flowers; whereupon we presently began to draw out the Air, and observ'd, That though for some time the Bee seem'd to take no notice of it, yet within a while after she did not flie, but fall down from the Flowers; without appearing to make any use of her Wings to help her self. But whether this fall of the Bee, and the other Insect, proceeded from the mediums being too thin for them to flie in, or barely from the weakness, and as it were swooning of the Animals themselves, you will easily gather from the following Experiment.

HAving (partly upon the consideration of some of the foregoing Experiments, and partly upon grounds not now to be insisted on) entertain'd a suspicion, that the action of Corrosive Liquors in the dissolving of Bodies, may be considerably varied by the gravitation or pressure of the incumbent Air, and the removal of it; I thought fit to examine my Conjecture by the following Experiment.

I took whole pieces of red Coral, and cast them into as much spirit of Vineger, as sufficed to swim above an Inch over them: These substances I made choice of, that the Ebullition upon the Solution might not be too great, and that the operation might last the longer.

Having then put about half a score sprigs of Coral, together with the Menstruum, into a somewhat long neck'd Viol, whereof they seem'd scarce to fill a third part, we convey'd that Viol into one of our small Pneumatical Glasses, containing by ghess about a Quart of Water; and having fastned on the Cover, after the accustom'd manner, we suffered the Liquor to remain unmov'd a while, to observe whether the Menslruum would work upon the Coral otherwise than besore. But sinding there did only arise, as formerly, a pretty number of small Bubbles, that made there no sensible froth upon the surface of the distill'd Vineger, there were made two or three exsuctions of the Air; upon which, there emerg'd from the Coral such a multitude of Bubbles, as made the whole Body of the Menstruum appear white; and soon aster a Froth, as big as all the rest of the Liquor, was seen to swim upon it: And the Menstruum plainly appear'd to boil in the Glass, like a seething Pot. And though, if we desisted but one minute from pumping, the decrement of the Froth and Ebullition, upon the getting in of a little Air, at some leak or other, seem'd to argue, that the removal of the pressure of the external Air was the cause, or, at least, the occasion of this Effervescence: Yet to evince this the more clearly, we turn'd the Key, and let in the external Air at the Stop-cock; immediately upon whose entrance the Froth vanish'd, and so many of the Bubbles within the body of the Liquor disappear'd, that it lost its whiteness, and grew transparent again: The Menstruum also working as languidly upon the Coral, as it did before they were put into the Receiver: But when we had again drawn out the Air, first the whiteness re-appear'd, then the Ebullition was renew'd, which, (the pumping being a while longer and nimbly pursued,) grew so great, that for 3 or 4 times one aster another, when ever the Air was let out of the Receiver into the emptied Cylinder, the frothy liquor overflow'd the Glass, and ran down by the sides of it: And yet, upon the readmitting of the excluded Air, the boiling Liquor grew immediately as calm and as transparent as at first: As if indeed the operation of it, upon the Coral, had been facilitated by the exsuction of the incumbent Air, which on its recess, lest it more easie sor the more active parts of the liquor to shew themselves such, than it was whilst the wonted pressure of the Air continued unremoved. It may indeed be suspected, that those vast and numerous Bubbles proceeded, not from the action of the Menstruum upon the Coral, but from the suddain emersion of those many little parcels of Air that (as we formerly observ'd) are wont to be dispers'd in Liquors, without excluding Spirit of Vineger; but having had this suspicion before we try'd the Experiment, we convey'd our distill'd Vineger alone into the Receiver, and kept it a while there, to free it from its Bubbles (which were but very small) before ever we put the Coral into it. It may be suspected likewise, that the agitation of the Liquor, necessary following upon the shaking of the Glass, by pumping, might occasion the recited Ebullition; but upon trial made, there appear'd not any notable change in the Liquor, or its operation, though the containing Vessel were shaken, provided no Air were suck'd out of it. The former Experiment was another time try'd in another small Receiver, with Coral grosly powdred, and the success was very much alike, scarce differing in any thing, but that the Coral being reduc'd to smaller parts, upon the ebullition of the Liquor, so many little lumps of Coral would be carried and Boy'd up by the emerging Bubbles, as sometimes to darken the Viol, though the same Coraline Corpuscles would be let fall again upon the letting in of the Air.

Some thing also we try'd in our great Receiver, concerning the solution of Metals in Aqua fortis, and other Corrosive Liquors; but partly the stink, and partly some accidents, kept us from observing any thing peculiar and remarkable about those Solutions.

One thing we must not omit, that when the Spirit of Vineger was boiling upon the Coral, we took off the Cover of the Receiver, and took out the Viol, but could not find, that notwithstanding so very late an Ebullition, the Liquor had any heat great enough to be at all sensible to our hands.

WE will now subjoyn an Experiment, which, if the former did not lessen, the wonder of it would probably appear very strange to Your Lordship, as it did to the first Spectators of it.

The Experiment was this: We caus'd Water to be boil'd a pretty while, that by the heat it might be freed from the latitant Air, so often already taken notice of in common Water: Then almost filling with it a Glass Viol, capable of containing near four Ounces of that Liquor; we convey'd it, whilst the Water was yet hot, into one of our small Receivers (big enough to hold about a pound of Water) and having luted on the Cover, we caus'd the Air to be drawn out: Upon the two first exsuctions, there scarce appear'd any change in the Liquor, nor was there any notable alteration made by the third; but at the fourth, and afterwards, the Water appear'd to boil in the Viol, as if it had stood over a very quick Fire; for the Bubbles were much greater than are usually found upon the Ebullition of very much more Water than was contain'd in our Viol. And this Effervescence was so great in the upper part of the Water, that the Liquor boyling over the top of the Neck, a pretty deal of it ran down into the Receiver, and sometimes continued (though more languidly) boyling there. Prosecuting this Experiment, we observ'd, that sometimes, after the first Ebullition, we were reduc'd to make divers exsuctions of the Air, before the Liquor would be brought to boil again. But at other times, as often as the Key was turn'd to let the Air pass from the Receiver into the Pump, the Effervescence would begin afresh, though the Pump were ply'd for a pretty while together; which seem'd to argue, that the boyling of the Water proceeded from hence, That upon the withdrawing the pressure of the incumbent Air, either the fiery Corpuscles, or rather the Vapors agitated by the heat in the Water (which last, what we have formerly noted touching the raresied Water of an AEolipile, manifest to be capable of an Elastical Power) were permitted to expand themselves mightily in the evacuated Receiver; and did, in their tumultuous Dilatation, list up (as the Air is wont to do) the uppermost part of the Water, and turning it into Bubbles, made the Water appear boiling. This conjecture was farther confirm'd by these additional Circumstances: First, The Effervescence was confin'd to the upper part of the Water, the lower remaining quiet, unless the Liquor were but shallow. Next, although sometimes (as is already noted) the Ebullition began again, after it had ceas'd a pretty while, which seem'd to infer, That some concurrent cause (whatever that were) did a little modifie the operation of heat; yet, when the Water in the Viol could by no pumping be brought to boil any more, the self-same Water, being in the very same Viol warm'd again, and reconvey'd into the Pneumatical Glass, was quickly brought to boil afresh, and that vehemently and long enough; not to mention, that a new parcel, taken out of the same parcel of the boiled Water with the former, and put in cold, could by no pumping be brought to the least shew of Effervescence. Besides, having try'd the Experiment in hot Sallet Oyl, being a much more tenacious Liquor, and requiring a stronger heat to make it boil, could not be brought to an Effervescence in our Receiver; whereas the Chymical Oyl of Turpentine, being thinner and more volatile, was presently made to boil up, till it reach'd four or five times the former height in the Viol, in whose bottom it lay, and continued boiling till it was almost reduc'd to be but luke-warm. Wine also being a more thin and spirituous Liquor than Water, being convey'd in hot, instead of the Oyl, did, as I remember, at the very first exsuction begin to boil so vehemently, that, in a short time that the Pump was kept moving, four parts of five, by our ghess, boil'd over out of the Viol, though it had a pretty long Neck. On which occasion we will add, that even the Water it self, near one half, would sometimes boil over into the Receiver before it became luke-warm. And it was also remarkable, that once, when the Air had been drawn out, the Liquor did, upon a single Exsuction, boil so long with prodigiously vast Bubbles, that the Effervescence lasted almost as long as was requisite for the rehearsing of a Pater Noster. Now the Experiment having been try'd more than once, and found to succeed as to the main, seems much to countenance the Conjecture we made at the beginning of this Letter, where we told Your Lordship, That perhaps the pressure of the Air might have an interest in more Phaenomena than Men have hitherto thought. For as we had not then made this Experiment, so now we have made it, it seems to teach, That the Air, by its stronger or weaker pressure, may very much Modefie (as the School-men speak) divers of the Operations of that vehement and tumultuous Agitation of the small parts of Bodies, wherein the nature of heat seems chiefly, if not solely, to consist. Insomuch that if a heated Body were convey'd above the Atmosphere, 'tis probable that the heat may have a differing operation, as to the power of dissipating the parts of it, from what it hath here below.

To conclude, This Experiment might have been farther prosecuted, but our want of leisure makes us content our selves to add at present; That perhaps it would not be lost labour if this were try'd, not only with other Liquors, but with variety of heated, and especially soft or melted Bodies: But in such cases the Receiver ought to be shap'd, as is most proper to preserve the Cement where with the Cover must be fastned on, from being melted by the heat of the included Matter; the inconvenience to be hereby avoided, having befallen us in the use of a Receiver too shallow, though otherwise capacious enough.

BEing come thus far, My dear Lord, not without thoughts of proceeding farther: The unwelcome Importunity of my occasions becomes so prevalent, that it quite hinders, for the present, my design'd Progress; and reduceth me, not only to reserve for another opportunity that kind of Experiments, which, at some distance from the beginning of this Letter, I call'd (as Your Lordship may remember) Experiments of the second sort; but to leave unessay'd some of the first sort, which I might try in the Engine, as it now is, were it not that my Avocations are grown so urgent, for my remove from the place where the Engine was set up, that I am put to write Your Lordship this Excuse; Weary, and in an Inn which I take in my way to my Dearest Brother Corke: Who being at length arriv'd in England, after I have for divers years been deprived of his Company, and wish'd for it as long; what ever my other Occasions may be, my first Business must be to wait on Him and Your excellent Mother; in whose gratefull Company I may hope to forget a while those publick Calamities that distress this too unhappy Nation, Since that is indear'd to me, both by their personal Merit; by the near Relation which Nature gives me to Him, Affinity to Her, and Friendship to both; and also by their many Favours, especially that of my owing them My Lord of Dungarvan. But I suffer my self to be transported too far with these delightfull thoughts; To return therefore to our Engine. Though I find this Letter is beyond my expectation swell'd, not only into a Book, but almost into a Volume; yet the Experiments already mentioned in it, are so far from comprising all those that may be try'd by the help of our Engine, that I have not yet been able to try all those, which, presently occurring to my thoughts, upon my first seeing the working of it, I caus'd to be set down in a Catalogue within less than half an hour. But I doubt I have but too much cause to apprehend that the Affairs, and other things I complain of, have made it needfull for me to Apologize, as well for the things I have set down, as for those I am necessitated to omit. For as partial as Men use to be to the Children of their own Brains, as well as to those of their Loins, I must not deny that the foregoing Trials are not altogether free from such unaccuratenesses, nor the recital of them from such Imperfections, as I my self can now discern, and could, perhaps, partly mend, if I had the leisure to repeat the Experiments, with the Circumstances that have since offer'd themselves to my Thoughts, as things that might have been worth Observation or Enquiry. But the truth is, that I was reduc'd to make these Experiments, when my Thoughts had things that more concern'd me to imploy them, and the same avocations made me set them down, for the most part, as soon as I had made them, and in the same order, and that so fast, that I had not overfrequently the opportunity to mind any more than the bare Truth of what I set down; without allowing it any of those advantages that Method, Stile, and decent Embellishments, are wont to confer on the Composures they are imploy'd to adorn.

But, My Lord, though to invite and encourage You and Your learned Friends at Paris, to make a farther use of this Engine, than I have yet been able to doe, I am thus free to acknowledge the Imperfections of the foregoing Letter: Yet, if some Intelligent Persons mistake not, by what hath been done, such as it is, there is a way open'd, whereby Sagacious Wits will be assisted to make such farther Discoveries in some points of Natural Phylosophy, as are yet scarce dream'd of. And I am the more desirous to engage You to that Imployment, because I am apt to think, that if the making and writing of such Experiments shall cost You as much trouble as they have me, You will be inclin'd to excuse me; and if the Discoveries give You as much pleasure as they gave me, You will (perhaps) be invited to thank me. However, I think (My Lord) I may justly pretend, that the things I have set down have been faithfully Recorded, though not elaborately Written; and, I suppose, my former Papers may have long since satisfied You, that though many devise Experiments better than Your Servant, none perhaps hath related them more carefully and more truly: And particularly of These; sometimes one, sometimes another, hath been performed in the presence of Persons, divers of them eminent for their Writings, and all for their Learning. Wherefore, having in the foregoing Narratives made it my business to enoble them with the chief Requisites of Historical Composures, Candor, and Truth, I cannot despair that You will either excuse their Imperfections, or, at least, forgive them: Especially, considering that this unpolish'd Letter is as well a Production of Your Lordship's Commands and my Obedience, as a Testimony of my desire to make others beholden to My Lord of Dungarvan, by the same way which I indeavour to express my Self