PNEUMATICS whatever pofition the candle be held to the receiver, provided the rays of light be not hinder'd from falling upon the veffel. (2.) The flash appears immediately upon turning the key, to let the air out of the receiver into the empty'd cylinder; fo that, I remember not, that the flash appear'd, when at any time, in our great receiver, the ftop-cock was open'd, before the cylinder was exhaufted. (3.) When, inftead of the great receiver, we made use of a small glafs, not containing above a pound and half of water; the phenomenon might be exhibited, tho' the stop-cock remain’d open, provided the fucker was drawn nimbly down. (4.) When we began to empty the veffel, the appearances of light were much more confpicuous, than towards the latter end, when little air at a time, could pafs out of the receiver. (5.) When the fucker had lately been well oil'd, and, inftead of the great receiver, the fmaller veffel, above-mention'd, was emptied; upon opening the ftop-cock, as the air defcended out of the glafs into the emptied cylinder, there afcended out of the cylinder into the veffel a certain fteam, which feem'd to confift of very little bubbles, or other minute corpufcles, thrown up from the oil, rarify'd by the attrition it fuffer'd in the cylinder. For, at the fame time that these fteams afcended into the glafs, fome of the fame kind manifeftly iffued out, like a little pillar of fmoke, at the orifice of the valve, when that was occafionally open'd. And thefe fteams, frequently prefenting themselves to our view, we found, by expofing the glafs to a clear light, that they play'd up and down in it; and by their whitifhnefs, in fome measure refembled the appearance of light. (6.) For, when the flash was great, the receiver, at the very inftant, loft of its transparency, by appearing full of fome kind of whitifh fubftance; and, for a fhort time after, the fides of the glafs continu'd opake, and feem'd to be darken'd, as if fome whitifh fteam adhered to the infide of it. But he who would fairly account for the phenomenon, whereof these are not all the circumftances, muft fhew from whence the apparent whitenefs proceeds; and why that whitenefs fometimes appears, and fometimes not. Now, had our phenomenon been conftant and uniform, we fhould fufpect it to have been produc'd after the following manner; for tho' what we faw in our receiver, feem'd to be a kind of light, yet it was, indeed, but a whitenefs, which render'd the infide of the glafs opake. Now our common air abounds with particles, able to reflect the rays of light, as appears from that vulgar obfervation, the motes in the air, when the fun-beams fhooting into any fhady place, difcover them, tho', otherwife, the eye cannot diftinguish them from the air. And, I particularly remember, that being at fome diftance from London, at a time whem numerous bonfires happen'd to be made there; tho' we could not fee the fires themfelves, yet we could plainly perceive the air all enlightned near the city: which argued, that the rays, fhot upwards from the fires, met, in the air, with corpufcles opake enough to reflect them to our eyes. White may be produc'd, when the continuity of a tranfparent body hap. PNEUMATICS pens to be interrupted by a great number of furfaces, which, like fo many little looking-glaffes, confufedly reprefent a multitude of fmall and feemingly contiguous images of the lucid body. For, water, or the whites of eggs, beaten to a froth, lofe their tranfparency, and appear white. And having, out of one of our small receivers, carefully drawn out the air, and left a very little hole, by which the water was to get in, we obferved that the neck, being held under water, and the little hole open'd, the water that rufh'd in, was fo broken, and acquired fuch a multitude of new furfaces, that the receiver feem'd to be full rather of milk, than water. And farther, by heating a lump of cryftal, and quenching it in fair water, it will be difcontinued by fuch a multitude of cracks, which create new surfaces within it, that tho' it will not fall afunder, yet it lofes its tranfparency. and appears white. Hence we might imagine, that upon the rufhing of the air out of the receiver, into the empty'd cylinder, the air in the receiver, being fuddenly, and vehemently expanded, the texture of it was as fuddenly alter'd; and the parts made fo to fhift places, and, perhaps, fome of them, to change poftures, as during their new and vehement motion, and their varied fituation, to difturb the ufual continuity, and thereby, the transparency of the air; which ceafing to be a tranfparent body, muft eafily degenerate into white. Several things there are which make this conjecture feem the more probable; as, first, the whiteness always appear'd greater, whilft there was much air in the receiver, than when the air was in great part drawn out. Secondly, having exhaufted the receiver, and applied to the hole in the ftop-cock, a large bubble of clear glass; fo that we could, at pleafure, let the air pafs out, at the finall glafs, into the great one, and easily fill the small one with air again; we obferved, that upon opening the communication betwixt the two glaffes, the air, in the fmaller, finding fo much room in the greater, to receive it, flew out with fuch force, that the small vial feem'd to be full of milk: and this experiment we repeated feveral times. And, thirdly, having provided a small receiver, with its upper orifice fo narrow, that I could ftop it with my thumb, I observ'd, that when, upon the exfuction of the air, the capacity of the glafs appear'd white; if, by a fudden removal of my thumb, I let in the outward air, that whitenefs would immediately vanish. It may, indeed, be objected, that when water turns from tranfparent to white, the air intervenes, which converts it into bubbles. To this I reply, there are two very volatile liquors, which being gently put together, are as clear as rock-water, yet will inftantly, without the help of air to turn them into bubbles, fo alter the difpofition of their infenfible parts, as to become a white confiftent body. And this happens not as in the precipitation of benjamin, and fome other refinous fubftances; which being diffolv'd in spirit of wine, may, by the affufion of fair water, be turn'd into a milky fubftance: for this whiteness belongs not to the whole liquor, but to the corpuf and VOL. II. Nnn cles PNEUMATICS cles of the diffolv'd gum, which, after a while fubfiding, leave the liquor tranfparent, themfelves only remaining white. But, in our cafe, 'tis from the varied texture of the whole tranfparent fluid; and not from any particular part, that this whitenefs refults: for the body is white throughout, and will long continue fo; and yet may, in process of time, without any addition, be totally reduced into a transparent body, as before. Another conjecture, we grounded upon this obfervation: having convey'd fome smoke into our receiver, placed against a window, we obferv'd, that, upon the exfuction of the air, the corpufcles floating in it, manifestly enough made the receiver feem more opake, at the very inftant the air rufhed out. For, confidering that the whiteness, whofe caufe we enquire after, did but fometimes appear, it feem'd not impoffible, that, at fuch times, the air in the receiver, might abound with particles capable of reflecting the light, in the manner requifite to exhibit a white colour, by being put into a certain unufual motion; as the new motion of their former fumes, made the infide of the receiver appear darker than before; and as our fmoking liquor, formerly mention'd, whofe parts, tho' they feem'd transparent, whilft they compos'd a fluid; yet when the fame corpufcles, upon unftopping the glafs, were put into a new motion, and difpos'd after a new manner, they render'd that part of the air opake, wherein they mov'd, and exhibited a greater whitenefs than fometimes appears in our receiver. not. But as to the reason why our phenomenon appears not conftantly, I remember not that we ever made the experiment in a small veffel, without finding the expected whiteness. But it remains to be explained, why in our great receiver, the phenomenon fhould fometimes be seen, and often All I have to fay on this head is, that the air about us, and much more that within the receiver, may be much alter'd by such cases, as few are aware of. The learned Jofephus Acofta tells us, that "in America there are winds which naturally trouble the water of the fea, making it green "and black, and others as clear as cryftal." And, tho' we convey'd into the receiver, the fcales and the pendulums, formerly mention'd, clear and bright; yet, after the veffel had been emptied, and, the air let in again, the luftre of both appear'd tarnifh'd by a beginning ruft. And, laftly, having, with pure fpirit of wine, drawn a transparent tincture out of a certain concrete, commonly reckon'd among minerals, we put it into a cryftal-vial, carefully ftopp'd it, and lock'd it up in a prefs; and this liquor, being a chymical rarity, and of a pleafing golden colour, we had often occafion to view it; and took notice that once it feem'd to be very thick: whereupon, we imagin'd it poffible, that fome of the mineral corpufcles were then precipitating. But finding, after fome days, that tho' no precipitation had been made, and that the liquor, retaining its former vivid colour, was grown clear again, as before; we lock'd it up again in the fame press, and refolv'd to obferve whether the like changes would again appear in our tincture; and, in cafe they should, whether they might be afcribed to the alterations of the weather. But tho' during the greatest part of a cuo. winter, and a fpring, we obferv'd the liquor would often grow turbid; PNEUMATICS and after a while, clear again; yet we could not find, that it depended upon any manifeft changes in the air; which would be often dark and cloudy, when the tincture was clear and tranfparent; as, in clear weather, the liquor would, fometimes, appear troubled, and more opake. 38. Into a glafs vial, open at the top, we put a mixture of fnow, and com- Water made to mon falt; and in the midst of this mixture, fet a cylindrical glafs, closelyfreeze in vaftopp'd at the lower-end, and open at the upper, where we fill'd it with common water; then let them all down into the receiver; and the pump being fet on work, the fnow began to melt fafter than we expected. However, by that time the receiver had been confiderably exhaufted, which it was in less than a quarter of an hour, we perceived the water, near the bottom of the glafs cylinder, to freeze; and the ice, by a little longer ftay, feem'd to increafe, and to rife fomewhat higher than the furrounding furface of the liquor whereinto, almost all the fnow and falt were diffolved. The glafs being taken out, it appeared that the ice, was as thick as the infide of the veffel it filled; tho', into that, I could put my thumb. The upper furface of the ice was very concave, and held against the light, appear'd not deftitute of bubbles; tho' they were fewer than if the water had been frozen in the open air. The like experiment we made, alfo, in one of our fmall receivers, with like fuccefs. But, whence proceeds that ftrange force, we may fometimes obferve in frozen water, to break the bodies that imprison it, tho' hard and folid? A ftone-cutter, lately complained to me, that, fometimes, thro' the negligence of his fervants, the rain being fuffer'd to foak into marble, the violent frofts coming on, would burft the ftones. And, another tradefman complain'd, that, even implements made of bell-metal, being carelefly expos'd to the wet, have been broken and spoil'd by the water, which, having entred at the little cavities of the metal, was there, afterwards froze and expanded into ice. And Cabeus tells us, that he faw a huge veffel of exceeding hard marble split asunder, by congeal'd water. I know it will be said, to folve this problem, that congelation doth not reduce water into lefs space, than it before poffefs'd, but, rather makes it take up more. But, tho' we grant, that water fwells in freezing; yet how cold, which, in weatherglaffes, manifeftly condenseth air, fhould expand either the water or the intercepted air, fo forcibly as to perform what we have here related, re mains to be difcover'd. cuo. 39. We took an oval glafs, clear, and pretty ftrong, with a fhort neck A water ther at the obtufer end, thro' which we thruft, almoft to the bottom, a pipe of mometer in vaglafs, and clofely cemented it to the neck: the upper part of the pipe was drawn, in fome parts, more flender than a crow's quill, that the changes of the air in the glass-egg, might be the more confpicuous; then we conveyed into the glafs, five or fix fpoonfuls of water, part of which, by blowing air into the egg, was rais'd into the slender part of the pipe; fo that the water was interpofed between the external air, and that inFig. 41: cluded in the egg. This weather-glafs, was fo placed, and clofed up in Nnn 2 PNEUMATICS in the cavity of a final receiver, that only the flender part Infects in va cuo. of the pipe, to the height of four or five inches, paffing thro' a hole in the cover, remain'd expos'd to the open air. In evacuating the receiver, the water, in the pipe, defcended about a quarter of an inch; and this upon two or three repeated trials; which feem'd to argue, that there was no heat produced in the receiver, upon the exfuction of the air: for even a little heat would probably, have been difcover'd by that weather-glafs; fince, by the bare application of my hand to the outfide of the receiver, the warmth, after fome time, having been propagated thro' both the glaffes, and the interval betwixt them, to the imprifon'd air, fo rarify'd it, that, by preffing upon the fubjacent water, it impell'd that in the pipe much higher than it had fallen downwards, upon the exfuction of the air. Yet we do not hence conclude, that in the cavity of the receiver the cold was greater after the extraction of the air, than before. If it be demanded, what then could cause the water to fubfide; we anfwer, that, probably, it was the ftretching of the glafs-egg, which, upon the exfuction of the ambient air, was unable to refift, as formerly, the preffure of the included air, and of the atmosphere, which, by the intervention of the water, prefs'd upon its concave furface. This feems probable, as well from the experiment about breaking a glass, by the force of the atmosphere, as because, when by drawing the air out of the receiver, the water in the pipe, was fubfided, upon the re-admiffion of the external air, to press against the convex furface of the egg, the water was presently reimpell'd to its formerly height: for, if a glass-egg be blown exceeding thin, and afterwards broken, you may, by degrees, confiderably bend some narrow parts of it; and upon the removal of what kept it bent, it will readily recover its former ftate. From our experiment, then, it appears either that there fucceeds no body in the room of the air drawn out of the receiver; or, that every fubftance is not fubtile enough, readily to pafs the pores of glafs tho' always fufficiently agitated to produce heat, wherever it is found in plenty. So that if we admit no vacuum, this experiment requires us to allow a great difparity, either as to bulk, or agitation, or both, betwixt fome parts of the ætherial fubftance, and those which, here below, produce heat and fire. We try'd, alfo, what operation the extraction of the air would have upon camphire; which confifts of fuch volatile parts, that they will exhale without any greater agitation, than that of the open air. But we found not, that even this loose body, was fenfibly alter'd thereby. 40. We convey'd a large flesh-fly into a fmall receiver; and, at another time, fhut into a great receiver, a humming-bee, that appear'd ftrong and lively; we also procured a white butter-fly, and inclosed it in a small receiver; where, though at first, he fluttered about, yet, presently, upon the exfuction of the air, he fell down, as in a fwoon; retaining no other motion, than fome little trembling of the wings. The fly, after some exfuctions of the air, dropp'd down from the fide of the glafs, whereon fhe |