PNEUMATICS running, like an ordinary fiphon. The cause of this afcent of the water, appears very difficult to discover*. We try'd, indeed, by conveying a very flender pipe, and a fmall veffel of water, into our engine, whether the exfuction of the ambient air would affift us herein; but, tho' we employ'd red wine, inftead of water, yet we could fcarce certainly perceive, thorough fo much glafs as was interpofed betwixt our eyes and the liquor, what happen'd in a pipe fo flender, that the rednefs of the wine was fcarce vifible in it. But, as far as we could difcern, there happen'd no great alteration to the liquor; which feem'd the lefs ftrange, because the fpring of that air, which might deprefs the water in the pipe, was equally debilitated with that which remain'd to prefs upon the furface of the water in the little glass. Wherefore, in favour of that conjecture, which ascribes this phenomenon to the greater preffure upon the water by the air, without the pipe, than by that within, it was fhewn, that, in cafe the little glafs-veffel of water were fo closed, that the air might, by the mouth, be fuck'd out of it, the water would immediately fubfide in the small pipe. Hence, we might infer, that it afcended before, by the preffure of the incumbent air; only it may be objected, that this, perhaps, would not happen, were the upper end of the pipe in a vacuum; as alfo, that, 'tis very probable, the water may fubfide, not because the preffure of the internal air is taken off by fuction, but because the fpring of the external air impels the water in its way to the cavity, deferted by the other air; and would as well impel the fame water upwards, as make it fubfide, were it not for the accidental pofture of the glaffes. "Twere here, likewife, proper to inquire, why the furface of water, in pipes, fhould be concave; and, on the contrary, that of quick-filver, convex; and why, if the end of a flender pipe be dipp'd in the latter, the furface of that fluid will be lower within the pipe, than without. parcel of air weigh'd. 36. We caufed a glafs-bubble to be blown at the flame of a lamp, about the fize of a small hen-egg, and of an oval form; only, at one end, there *This phenomenon, the fufpenfion of water in capillary tubes, is, with great fagacity, accounted for by Dr. Jurin; who proves it owing to the attraction of the upper periphery, or fection, of the concave fuperficies of the tube; that is, a fmall furface, or annulus, whose base is that periphery, and height the diftance, whereto the attractive power of the glass extends. For the gravity of the water that enters the orifice of the tube, upon its immersion, being immediately taken off, by the attraction of the annulus, wherewith its upper furface is in contact, the water muft neceffarily rife higher, by reafon of the preffure of the ftagnant fluid, and the attraction of the periphery immediately above that whereto the up per furface of the water is already contiguous. The confideration of this phenomenon, and the experiments made with relation to it, both in water and quickfilver; thofe made with the latter, proving exactly the reverfe of the for mer; led the Doctor to clear the whole matter, by fhewing, that the particles of water attract each other; that the particles of quick-filver attract each other; that water is attracted by glass; that quick-filver is attracted by glass; that the particles of water are more ftrongly attracted by glafs, than by one another; and, laftly, that the particles of qui k-filver are more ftrongly attracted by each other, than by glafs. See all these proved in the Philof. Tranf. No 355. P. 739. was was drawn out an exceeding flender pipe, that the bubble might be feal'd PNEUMATICS. up, with as little rarifaction, of the air included in the great cavity, as poffible. This glafs, being feal'd, was faften'd to one of the fcales of an exact balance; and, being counterpois'd with a weight of lead, was convey'd into the receiver, and clofed up in it. The beam appearing to continue horizontal, the pump was fet on work; and, after two or three exfuctions, the balance inclined to that fide on which the bubble hung; which, as the air was farther drawn out, preponderated more manifeftly: at length, the air being gradually let in again, the fcales, by degrees, return'd to an equilibrium. Then we took them out, and cafting into that fcale, to which the lead belong'd, three fourths of a grain, we again placed them in the receiver; which, being closed and exhaufted as before, as the air was drawn out, fo the glafs-bubble came nearer to an equilibrium with the other weight, till the beam ftood horizontal: which, by another trial, we could not bring it to do, when one fourth of a grain more was added to the scale whereto the lead belong'd. Tho', without doubt, if we could have perfectly evacuated the receiver, the air included in the bubble, would have weigh'd above a grain; tho' it were fomewhat rarify'd by the flame wherewith the bubble was feal'd. And, upon the return of the excluded air, the lead, and the weight caft into the fame fcale, did again very much preponderate. We, likewife, convey'd into the receiver, the fame bubble, open'd at the end of the flender pipe above-mention'd; but, having drawn out the air as ufual, we found not, as before, the bubble to out-weigh the oppofite lead: fo that by the help of our engine, we can weigh the air, as we weigh other bodies, in its natural or ordinary confiftence, without condensing it. Nay, having convey'd a lamb's bladder, half full of air, into the receiver, we obferv'd, that tho' upon working the pump the imprifon'd air expanded, till it feem'd ready to burst the bladder; yet this rarified air, manifeftly depreffed the fcale whereto it was annexed. And, having once caus'd the pump to be obftinately ply'd, in repeating Whether glas be the former experiment, the imprifon'd air broke the containing glafs-bub-pervious to air. ble, and threw the greateft part of it against the fide of the receiver, whereby 'twas fhatter'd into a multitude of pieces. Hence we may dif cern, of how close a texture glafs is, fince fo very thin a film of it, as this bubble was, prov'd fo impervious to the air, that it could not get away thro' the pores, but was forced to break the glafs in pieces, to free itfelf; and this, notwithstanding the time, and advantage it had, to force thro' the pores. This I mention, that our experiments may receive no prejudice from one I happen'd to make long fince; which might be drawn to countenance their opinion, who would perfuade us that glafs is pervious to air, properly fo call'd: for, in diftilling a certain fubftance, greatly abounding with fubtile fpirits, and a volatile falt, in a strong earthen veffel, of an unufual fhape, to which was luted a large receiver of green glafs; the fire was, by accident, fo exceffively increas'd, that we found the fpirituous and faline corpufcles, thrown over fo hot, and in fuch plenty, into the re VOL. II. Mm m ceiver PNEUMATICS, ceiver, as to render it all opake, and likely to fly in pieces. We ventur'd, however, to approach it, and obferv'd, on the outside thereof, at a great diftance from the juncture, there was fettled a round, whitifh fpot, or two, which, at first, we thought might be fome ftain upon the glafs; but after finding it, in feveral qualities, like the oil and falt of the concrete diftill'd, we fufpected, that the moft fubtile, and fugitive parts of the impetuous fteams, had penetrated the fubftance of the glafs, and, by the cold of the ambient air, were condenfed on the furface of it. And, indeed, upon examining the whole matter, a number of us unanimoufly concluded, that the fubtile parts of the diftill'd matter, being violently agitated by the exceffive heat, had pafs'd through the pores of the glafs made wide by the fame heat. But this having never happen'd, more than once, in any of the diftillations we have either made, or feen, it is much more reafonable to fuppofe, that the pervioufnefs of our receiver, to a body much more fubtile than air, proceeded from the loofer texture of that particular parcel of metal, the receiver was made of; for all glass is not equally compact, and folid; and from the prodigious heat, which, together with the vehement agitation of the fubtile fpirits, open'd the pores of the glafs; than to imagine, that fuch a fubftance as air, fhould be able to permeate the body of glafs, contrary to the teftimony of a thousand chymical and mechanical experiments; and, of many made in our engine. The penetrating power of air, that of water. compared with And, the following experiment feems to teach, that tho' air, when fufficiently comprefs'd, may, perhaps, get entrance into smaller cavities, than Water; yet, unless the air be forc'd in, it will not pafs them, whilft they may admit of water. I took a glafs fiphon, the lower end of whofe longeft leg was drawn fo flender, that the orifice, at which the water was to fall out, would hardly admit a very fmall pin. This fiphon being inverted, we fo order'd it, that a little bubble of air was intercepted in the flendereft part, betwixt the little orifice, juft mention'd, and the incumbent water; whence the air, being not to be forced thro' fo narrow a paffage, by fo light a cylinder of water, as refted upon it, hinder'd the farther efflux of the water, as long as we let it ftay in that narrow place: but when, by blowing a little at the wider end of the fiphon, that small parcel of air was forc'd out, with fome water; the remaining water that before continued fufpended, began freely to drop down again, as before. And a glafs pipe, either in the form of a fiphon, or otherwife, half an inch in diameter, but at one end fo flender, as to terminate in an orifice almost as finall as a horfe-hair, be fill'd with water, it will drop down freely thorough the flender extremity. But if the pipe be inverted, the air will not easily get in at the finall hole, thro' which the water pafs'd. For, in the fharp end of the pipe, fome inches of water will remain fufpended; which, probably, would not happen, if the air could get in to fucceed it; fince, if the orifice were a little wider, the water would immediately fubfide. And tho' when the pipe is many inches long, a great part of the water will run down at the wider orifice; yet that feems, to to happen for fome other reason, than because the air fucceeds it at the PNEUMATICS upper and narrow one; fince all the flender part of the pipe, and, perhaps, fome inches more, will continue full of water. And, tho' we have formerly fhewn, that the aerial corpufcles cannot pafs thro' the pores of a lamb's bladder; yet, particles of water will; as may eafily be try'd, by very closely tying a little alkaline falt in a fine bladder, and dipping its lower end in water: for, if it be held there for a competent time, there will ftrain thro' the pores of the bladder, water enough to diffolve the falt into a liquor. But, to return to our bubble; we endeavour'd to meafure its capacity by filling it with water, to find how much water anfwer'd, in weight, to of a grain of air; but all the diligence we used to preferve so brittle a vessel, could not prevent its breaking, before we had gain'd our point. But, there occurs a problem, upon occafion of the flow breaking of the glafs bubble in our evacuated receiver. For, it might feem ftrange, fince the air, as we have feen, expands itself by its own internal fpring, twice as much as Merfennus was able to rarify it by a red-hot xolipile; that yet, the fpring of the air was fcarce able to break a very thin glafs bubble; and utterly unable to break one fomewhat thicker, within whofe cavity it was imprifon'd; whereas, air pen'd up, and agitated, is able to perform effects fo much more confiderable, that the learned jefuit Cabeus tells us, he saw a vaft ftrong marble pillar quite broken off in the middle, by the heat proceeding from wood, which happen'd to be burnt just by it; which fo rarified fome air or fpirituous matter fhut up in the cavities of the marble, that it burft thro' the folid body of the ftone by the force of expanfion. But, probably, the reafon why the included air did not break the feal'd bubbles, in our exhaufted receiver, was, that being fomewhat rarified by the flame employ'd to feal the glafs, its fpring upon the recefs of the heat grew weaker than before. Yet, this will not, alone, ferve the turn, becaufe, much fmaller glafs bubbles, exactly clos'd, will by the included air be made to fly in pieces. air to that of We took an aolipile of copper, weighing fix ounces, five drams, and for- The proportio of ty-eight grains; and being made hot, we remov'd it from the fire, and the weight of immediately ftop'd it with hard wax, that no air might get in at its orifice water. Then the aolipile, being fuffer'd leifurely to cool, 'twas again weigh'd, together with the wax, and found to be fix ounces, fix drams, and thirtynine grains. Laftly, the wax being perforated, without taking any of it out of the fcale, the external air was fuffer'd to rufh in; and then the xo *M. Homberg is of opinion, that water, that the air, for want of a wetting proenters fuch narrow pores of animal fub-perty, cannot do. As a proof of this ftances, as will not admit the air, only because it moiftens and diffolves the glutinous matter of the fine fibres of the membranes, and alfo renders them more pliable and diftractile; which are things, doctrine, he fill'd a bladder with air, and PNEUMATICS lipile, and wax, being again weigh'd, amounted to fix ounces, fix drams, and fifty grains. So that the xolipile, freed as far as our fire could free it, from its air, weigh'd lefs than when replenifh'd with air, full eleven grains; that is, the air containable within the cavity of the aolipile, amounted to eleven grains, and fomewhat more. And, by the way, if there be no mistake in the obfervation of Merfennus, it may feem ftrange that it fhould fo much differ from two or three of ours; in none of which we could rarify the air in an aolipile, though made red-hot, almoit all over, and immediately plung'd into cold water, to half that degree which he mentions, viz. feventy times its natural extent; unless the xolipile, he employ'd, was able to fuftain a more vehement heat than ours *. This way of weighing the air, by the help of an aolipile, feems fomewhat more exact, than that which Merfennus ufed, becaufe we weigh'd not the aolipile till it was cold; whereas he weigh'd it red-hot, whereby it is fubject to lofe of its weight in cooling: for, copper heated red-hot, throws off, in the cooling, little thin fcales in fuch plenty, that, having purpofely watch'd a copper aolipile, during its refrigeration, we have feen the place round about it, almoft cover'd with them every way. Perhaps, too, the aolipile, in cooling, may not receive fome little increase of weight, either from the vapid, or faline fteams that float in the air. We employ'd, to weigh our æolipile, both when fill'd with air, and when replenish'd with water, a pair of fcales that would turn with the fourth part of a grain. As to the proportion of weight betwixt air and water, fome learned men have attempted to fettle it, by ways fo inaccurate, that they seem to have been much mistaken. Ricciolus having purpofely endeavour'd to discover this proportion, by means of a thin bladder, eftimates the weight of the air, to that of the water, as about 1 to 10,000; and, indeed, having once weigh'd a large bladder, full of air, and found it to contain 14 grains; the fame bladder, afterwards fill'd with water, contain'd near 14 pounds; whence the proportion of air to water, feem'd, almoft, as a grain to a pound, that is, as I to above 7600. On the other hand, Galileo makes the air to water, as 1 to 400. But our way of weighing the air by an eolipile, feems, by much, the more exact. And, according to our obfervations, the water it contain'd, amounting to 21 ounces and a half; and as much air as was requifite to fill it, weighing eleven grains; the proportion in gravity of air, to water of the fame bulk, will be as 1 to 938. And tho' we could not fill the aolipile with water, very exactly; yet, as we neither could perfectly drive the air out of it tartar, reduced to fine powder, and made hot, to the neck of the former; fo that the external air muft pafs flowly thro' this falt, before it could poffibly get into the exhaufted veffel; whereby the air that en *It may be pretended, that 'tis not the air, any moisture that might have been lodged therein. The veffel being thus fill'd with pure air, and put into the fcale, was found to weigh as much as when fill'd with unpurged air. De Mater. fubtil.p. 7. by |