Page:Cyclopaedia, Chambers - Volume 1.djvu/805

 FLU

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FLU

That this iaft is the Qualification chiefly required in Fluidity, he argues from divers Observations and Experi- ments.

Thus, a little dry Powder of Alabafter, or Plaiftcr of Paris finejy rifted, being put in a Veflel over the Fire ; it loon begins to boil like Water ; exhibiting all the Motions and Phxnomcna of a boiling Liquor. It will kindle va- rioufly in great Waves like that ; will bear ftirring with a Stick or Ladle like that, without refitting ; Nay, if ftrongly flirr'd near the Side of the Veflel, its Waves will apparent- ly dafti againft them : Yet is it all the while a dry, parch'd Powder.

The like is obferv'd in Sand: A Difh of which being let on a Drumhead, brifldy beaten by the Sticks ; or on the upper Stone of a Mill, it in all refpects emulates the Proper- tics of a fluid Body. A heavy Body, e. gr. will immediately fink in it to the Bottom, and a light one emerge to the Top : Each Grain of Sand has a conftant vibratory and dancing Motion ; and if a Hole be made in the Side of the Dilh, the Sand will (pin out like Water. (

That the Parts of Fluids are in continual Motion, the Car- tejians bring divers Considerations to prove; as 1° The Tranf- mutation of S'olids into Fluids, e. gr. Ice into Water, and vice verfa ; the chief Difference between the Body in thofe two States coniifting in this, that the Parts being fiVd and at reft in the one, refill the Touch ; whereas in the other, be- ing already in Motion, they gave way upon the (lighteft Impulfe,

1" The Effects of Fluids, which commonly proceed from Motion : Such are the Insinuation of Fluids among the Pores of Bodies; the foftening and diffblving of hard Bodies; the Actions of corrofive Mentlruums, &e. Add, that no Solid can be brought to a State of Fluidity, without the Interven- tion of tome moving, or moveable Body, as Fire, Air, or Water.

Air, the fame Gentlemen hold the firft Spring of thefe Caufcs of Fluidity ; it being this that gives Motion to Fire and Water, though it felf receives its Motion and Action from the JEther, or fubtle Medium. See Air and M-

THER.

The Learned Bcerhaave, in his late Excellent Inftitut. ChymitC, pleads very ftrenuoully for Fire's being the firft Mover, and the Caufe of all Fluidity in other Bodies, as Air, Water, £i?c. without this, he thews, that the Atmof- phere it felf would iix into one folid Mats. See Fire.

Sir /. Newton fets afide this Theory of the Caufe of Flui- dity, and fubftitutes a new one, the great Principle of At- traction.

The Corpufcular Syflem, with all the Improvements of Ties Cartes, and Mr. 'Boyle, did not fufficiently account for the primary Condition, requifite to conttitute a Body fluid, viz. the various inteftine Motion and Agitation of its Par- ticles.

But, this Motion is naturally enough accounted for, by fup- pofing it a primary Law of Nature, that, as all the Pani- cles of Matter attract each other, when within a certain Di- ftance ; fo at all greater Diftances, they fly from, and avoid one another.

For then, tho' theirommon Gravity together with thePref- fure of other Bodies upon them, may keep them together in a Mais ; yet their continual Endeavour to avoid one an- another fingly, and the adventitious Impulfes of Heat, Light, or other external Caufcs, 'may make the Particles of Fluids continually move round about one another, and fo produce this Quality.

There is a Difficulty indeed, in accounting, why the Par- ticles of Fluids always keep at fuch a Diitancc from one an- other, as not to come within the Sphere of one another's Attraction.

The Fabrick and Conftitution of that fluid Body, Water, is amazing; that a Body fo very rare, and which has a vaft Over-proportion of Pores, or interfpers'd Vacuity, to folid Matter, fliould yet be perfectly incompreffible by the great- eft Force : And yet this Fluid is eafily reducible into that firm, traniparent, friable Body, which we call Ice, by being only expofed to a certain Degree of Cold. See Cold and Freezing.

One would think, that tho' the Particles of Water cannot co'me near enough to attract: each other, yet the interven- ing frigorifick Matter doth, by being mingled per minima, ftron«fy attract: them, and is it feltlikewife ftrongly attracted by them, and fo wedges or fixes all the Mats into a firm fo- lid Body ; which folid Body lofes its Solidity again, when by Heat the Vinculum is folved, and the frigorifick Particles are disjoined from thofe of the Water, and are forced to .fly out of it : And juft thus may the Fumes of Lead per- haps fix Quickfilver. See Fixation.

When a firm folid Body, fuch as a Metal, is by Heat re- duced into a Fluid, the Particles of Fire disjoinand feparate its conftituent Pans, which their mutual Attraction cauied before

to cohere ; and keep them at fuch a dillance from one an- other, as that they are out of the Sphere of each other's Attraction, as long as that violent Motion lalls; and when by their Lightnefs and Activity they are flown off, unlefs they arc renewed by a continual Supply, the component Par- ticles of the Metal come near enough again to reel one an- other's Attractions.

As therefore the Caufe of Cohcfion of the Parts of folid Bodies appears to be their mutual Attraction ; io the chief Caufe ot Fluidity feems to be a contrary Motion, imprefs'd on the Particles of Fluids, by which they avoid, and fly one another, as foon as they come at, and as long as they keep at fuch a Dittance from each other.

It is obferv'd alfo in all Fluids, that the Direction of their Preffure againft the Veffels which contain them, is in Lines perpendicular to the Sides of fuch Veffels; which Property being the ne.eflary Rcfult of the Particles of any Fluid's being fpherical, it Jhews that the Parts of all .Fluids are io, or ot a Figure very nearly approaching thereunto.

FLUIDS, are Bodies, whole Particles are but weakly con- nected ; their mutual Cohcfion being in great meafure pre- vented from fome external Caufe : In which fenfe a Fluid {lands oppofed to a Solid. See Solid.

Sir /. Newton defines a fluid Body, to be that whofc Parts yield to the fmallcft Force imprefs'd, and by yielding arc eafily moved among each other.

The Caufe, therefore, of Fluidity, fliould teem to confift: in this, that the Parts do not cohere fo ftrongly, as they do in folid or firm Bodies ; and that their Motion is not hinder'd by any Inequality in the Surface of the Parts ; as is the Cafe in Powders.

For, that the Particles whereof Fluids confift, are of the fame nature, and have the fame Properties with the Parti- cles of Solids, is evident, from the'Converfion of Liquids and Solids into each other, e. gr. of Water into Ice ; of Me- tals into Fluors, &c. Nor can it be reafonably doubted, that the component Parts of all Bodies are the fame, viz. hard, folid, impenetrable, moveable Corpufdes. SeeBoDYj and Matter.

The Cartefians define a Fluid to be a Body whofe Parts are in continual inteftine Motion ; and Dr. Hook, Mr. "Boyle and Boerbave, tho' far from Cartefianifm, fubferibe to the Definition ; alledging Arguments to prove that the Parts ot Fluids are in continual Motion ; and even that it is this Mo- tion, which conftitutes Fluidity. See Fluidity.

The later Newtonians dare not go fo far : To fay that the Parts of a Fluid are in continual Motion, is more than either our Scnfes, Experience, or Reafon will warrant ; and to define a thing from a Property that is difputable, is cer- tainly bad Fhilofophy. See Definition.

Add, that the great Argument from Geometry, produced in favour of this continual Motion, viz. That the Refinance of a Body moving in a Fluid, is lefs, if the Parts of the Fluid be agitated'by an inteftine Motion, than if they were at reft, is dcmonilrated to be falfe. See Resistance.

We add, therefore, with Dr. Clark, That if the Parts of a Body either do not touch each other, or eafily Hide over one another; and arc of fuch a Magnitude, as that they may be eafily agitated by Heat: And the Heat be fuffici- ently great to agitate them ; tho' it may perhaps be lefs than fuffices to prevent Water from freezing : Or, even, tho' ■ the Parts be not actually moved, yet, if they be fmall, fmooth, flippery, and of luch a Figure and Magnitude as difpoles them to move and give way : That Body is fluid.

And yet the Particles of fuch fluid Bodies, do, in fome meafure, cohere ; as is evident hence, that Mercury, when well purg'd of Air, will be fuftained in the Barometer, to the Height of tfo or 70 Inches; that Water will afcend in Capillary Tubes even in Vacuo ; and that the Drops of Li- quors in Vacuo run into a fpherical Form : As adhering by fome mutual Cohcfion like that between polifli'd marble Planes. Add, that thefe fluid Bodies, if they confift of Particles that are eafily entangled with each other, as Oil ; or if they be capable of being ftiffen'd by Cold, and join'd by the Interpofition of little Cunei, or Wedges, as Water ; they arc eafily render 'd hard: But if their Particles be fuch as can neither be entangled, as Air ; nor ftiffen'd by Cold, as Quickfilver ; then they never grow hard and fix'd. See Particle.

Fluids are either Natural, as Water, and Mercury ; or Animal, as Blood, Milk, Bile, Lymph, Urine, &c; or Fa- clicious, as Wines, Sprits, Oils, &c. See each under its proper Article, Water, Mercury, Blood, Milk, Bile, Wine, Spirit, Oil, iSc.

The Doflrine and Laws of Fluids are of the greateft Ex- tent in Philofophy.

The Preffure, and Gravitation of Bodies in i-tei;, and tlie Action of the Fluids immcrted in them, makes the Subject of Hydrofiaticks. SeeH.YDROSTATicKS.

Hydro-