Page:Cyclopaedia, Chambers - Volume 1.djvu/96

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Bodies on it ; and conftringcs and binds 'em down with a Force amounting, according to the Computation of M. 'Paf- chal, to 2232 Pounds weight, upon every fquare Foot, or upwards of 1 5 Pounds upon every Inch fquare. — Hence, it prevents, e.g. the Arterial Veffels of Plants and Animals, from being too much diflended by the Impetus of the cir- culating Juices, or by the elaftick Force of the Air fo plen- tifully lodg'd in the Blood. — Thus, we fee, in the Operation of Cupping, that upon a Diminution of the Preffure of the Air, the Parts of the Body grow tumid ; which neceffarily alters the manner of the Circulation thro' the Capillaries, i$c.

The fame Caufe hinders the Juices from oozing and efca- ping thro' the Pores of their containing Veffels : This is experienced by fuch as travel up high Mountains, who, in proportion as they afcend, find themfelves grow more and more relaxed ; and at length fall into a (pitting of Blood, and other Hemorrhages 5 by reafon of the Air not fufficient- ly binding up the Veffels of the Lungs. — The like is ob- ferv'd of Animals inclos'd in the Receiver of an vftV-Pump, who, as the Air is taken from 'cm, fwell, vomit, drivel, dung, urine, fwear, $5c See Vacuum.

2°, The Mixture of contiguous Bodies, efjiecially fluid ones, is chiefly owing hereto.— Hence many Liquids, as Oils and Salts, which teadily and fpontaneouily mix in Air, up- on taking that away, remain quietly in their State of Sepa- ration.

5°, It determines the Aflion of one Body upon another. — Thus, the Fire which burns Wood immediately goes out, and its Flame diffipates upon removing the Air ; by reafon fomething is then wanting to prefs the Corpufcles of Fire againft thofe of the Fuel, and prevent the too fpeedy Diffu- fion of the Flame. The fame is obferved of Aqua Regia, and Gold ; that Menflruum ceafing any longer to operate on the Metal, aftet the Air is taken away. And upon the fame determining Power of the Air it is, that 2>apm's Di- geftor is built. See Digestor.

Hence alfo it is, that on the Tops of high Mountains, as on the Pike oi'Teneriffe, the moft favoury Bodies, as Pepper, Ginger, Salts, Spirit of Wine, &c. have no fenfible Talte; for want of their Particles being prefs'd upon the Tongue, fo as to enter its Pores, but inflead thereof, being diffipated and blown away by its hear. The only thing that there re- tains its Savour, is Canary Wine, which is chiefly owing to its unctuous Quality ; in Virtue whereof, it adheres clofely to the Part, and is not eafily blown away.

From this Principle of Gravity chiefly arife our Winds ; which are only A r put in Motion by fome Alteration in the Equilibrium thereof. See Wind.

Ill", Elafticity, — or a Power of yielding to an Impreffion by contracf ing its Dimenfions ; and, upon removing or di- minifhing the impreffivc Caufe, returning to its former

Space or Figure. This elaftick Force is accounted the

diftinguifhing Property of Air; the other Properties hitherto enumerated being common to it with other Fluids.

Of this Power we have numerous Proofs. — Thus, a blown Bladder being fqueez'd in rhe Hand ; we find the included Air fenfibly rclitt ; fo as upon ceafing to comprefs, the Ca- vities or Impreffions made in its Surface, are readily expand- ed again, and fill'd up.

On ibis 'Property of Elafticity, the StruBure and Office of the Kw-'Pump defends. See Ais.-'Pump.

This Nifus or Endeavour to expand, evety Particle of Air always exerts ; and thus ftrives againft an equal endeavour of the ambient Patticles ; whofe Refiftance happening by any means to be weaken'd, it ttrait diffufes into an immenfe Extent. — Hence it is, that chili glafs Bubbles, or Bladders, full of Air, and exactly clofed ; being included in the ex- haufted Receiver of an _/fc>-Pump, burtt, by the Force of the included Air. So a Bladder quite flaccid, containing on- ly the fmalleft Quantity of Air ; fwells in the Receiver, and appears quite full. And the fame EffecT: is found, by car- rying the flaccid Bladder to the Top of an high Mountain.

This Power does not (eem to have any Limits aflign'd it 5 nor docs it appear capable, by any Means whatever, of be- ing deftroy'd or diminifh'd. — Mr. Boyle made feveral Expe- riments, with a View to difcover how long Air, brought to the greater! degree of Expanfion he couhi reduce it to in his Air-Pump, would retain its Spring ; and could never obferve any fenfible Diminution ; even tho this poor thin Air was clog'd fome Months with a Weight which one would wonder how it ifiould fupport a Moment.

Yet, Mr. Hawksbee, by a later Experiment, has fhewn, that the Spring of the Air may be fo difturb'd by a violent Prcflion, as to require fome time to return to its natural Tone.

The Weight or Preffure of the Air, it is obvious, has no dependence on its Elafticity ; but would be the fame, whether the Air had fuch Property or not.— But the Air, in being elaftick, is neceffarily affeaed by the Preffure, which reduces it into fuch a Space, as that the Elafticity which re-ads againft the compreffing Weight, is equal to that Weight. Sec Compression.

($t )

AIR

« I, n k eff n a V ,V ' e ^7 0f > is Elafl j dt y. ». Aat'it increafes as the Denfity of the Air mcreafes ; and the Dcnfitv in- creates, as the Force increafes wherewith it is prefs'd Now there muft neceffarily be a Balance between the Action and Re-aaion ; i. e. the Gravity of the Air, which tends to comprefs it, and the Elafticity of the Air which endeavours to expand it, mutt be equal. See Density:, Reaction, {«Jc.

Hence, the Elafticity increafing or diminifhing univer'fally as the Denfity increafes, or dimmifhes, i.e. as the Dillance between the Particles diminifhes or increafes ; 'tis no mat- ter whether the Air be comprefs'd, and retain'd in fuch Space by the Weight of the Atmofphere, or by any other means : It mutt endeavour, in either Cafe, to expand with the fame

Force. And hence, if Air near the Earth be pent up

in a Veffel, fo as to cut off all Communication with the ex- ternal Air ; the Preffure of the inclofed Air will be equal to the Weight of the Atmofphere. Accordingly, we find Mercury fuftained to the fame Height, by the elaftick Force of Air inclofed in a glafs Veffel, as by the whole atmofphe- rical Preffure. — See further under the Article Elasticity.

On the fame Principle may Air be artificially condenfed. See Contensation of Air.

And hence ike StruBure of the Wind-Gun. See Wind- Dr. Halley afferts, in the <Philofoph. T'ranfaBimis, that from the Experiments made at London, and by the Aca- demy del Cimento at Florence, it may be fafely concluded, that no Force whatever is able to reduce Air into eight hun- dred times lefs Space than what it naturally poffeffes on the Surface of our Earth. In anfwer to which, Monfieur A- montons, in the Memoirs of the French Academy, main- tains, that there is no fixing an, Bounds to its Coudenfation ; that greater and greater Weights will ftill reduce it into lefs and lefs compafs ; that it is only elaftick in Vittue of the Fire it contains ; and that as 'tis impofftble ever abfo- lutely to drive all rhe Fire out of it, 'tis impomble ever to make the utmoft Condenfatioh.

The AZMatatiin of the Air, by Virtue of its elaftick Force, is found to be very furprizin'g ; and yet, Dr. Wallis fuggefts, that we are far from knowing the utmoft it is capable of. — In feveral Experiments made by Mr. 'Beyle, it dilated firft into nine times its former Space ; then into 31 times; then into 5o ; then into 150. Afterwards, it was brought to dilate into 8000 times itsSpace; then into 10000, and even at laft into 13570 times its Space: and all this by its own expanfivc Force, without any help of Fire.

On this depends the Sirutliire andUfe of the Manometer. See Manometer.

Hence, it appears, that the Air we breathe, near the Sur- face of the Earth, is comprefs'd by its own Weight into at leaft the 1 ;6"79tri Part of the Space it. would poflefs in Vacuo. — But if the fame Air be condenfed by Art ; the Space it will take up when moft dilated, to that it poffeffes when con- denfed ; will be, according to the fame Author's Experi- ments, as 505000 to 1. See Dilatation.

Hence, we fee how wild and erroneous was that Obfer- vation of Ariftotle, that Air, render'd ten times rarer than before, changes its Nature, and becomes Fire.

M. Amontons, and others, we have already obferved, take the Rarefying of An to arife wholly from the Fire contain'd in it ; and hence, by increafing the Degree of Heat, the Degree of Rarefaaion may be carried ftill further than its fp. ntaneous Dilatation. See Heat.

On this 'Principle depends the StruBure and Office of the -Thermometer. See Thermometer.

M. Amontons firft dilcover'd that Air, the denfer it is, the more it will expand with the fame degree of Heat. See Density.

On this Foundation, the fame ingenious Author has a Dif- courfe, to prove " that the Spring and Weight of the Air, e * with a moderate degree of warmth, may enable it to " produce even Earthquakes, and other of the moft ve- cc hement Commotions in Nature."

According to the Experiments of this Author, and M. de la Hire, a Column of Air on the Surface of the Earth, 36 Fathoms high, is equal in weight to three Lines Depth of Mercury; and it is found, that equal Quantities of Air pof- fefs Spaces reciprocally proportional to the Weights where- with they are preffed : The Weight of the Air, therefore, which would fill the whole Space poffeffed by the terreftrial Globe, would be equal to a Cylinder of Mercury, whofe Bafe is equal to the Suffice of the Earth, and its Height containing as many times three Lines, as the Atmofpherical Space con- tains Orbs equal in weight to that of 35 Fathoms, where- of the Experiment was made. — Hence, taking the denfeft of all Bodies, e.g. Gold, whofe Gravity is_ about 14530 times greater than that of Air in our Orb, it is eafy to com- pute, that this Air would be reduced to the fame Denfity as Gold, by the Preffure of a Column of Mercury 14530 times 28 Inches high, i. e. 409540 Inches : fince the Bulks of Air, in that Cafe, would be in the reciprocal Ratio of the Weights wherewith they are preffed. This 40^540 Inches,

there-