Page:Cyclopaedia, Chambers - Volume 1.djvu/649

 ELA

ELABORATION, the Act of finifliing or perfcaing any Thing with Labour and Time.

The Term is chiefly ufed in Medicine, where the Chyle, Blood and Semen, are {aid to be ivell Elaborated, when they are well condition'd, have undergone all the Secretions, Mixions, Impregnations and Circulations neceffary to bring them to Perfection. See Chyle, Blood, and Seed.

If the Chyle went directly from its Keceptaculum to the Breafts, it would not be fufficiently Elaborated to afford good Milk. See Milk.

^LABORATORY. See Laboratory.

ELASTIC Body is that, which by being firuck, or ftretch'd, has its Figure alter'd ; but endeavours, by its own Force, to refume the fame. Or, it is a fpringy Body, which when comprefs'd, condenfed, or the like, makes an Effort to fet it felf at Liberty • and to repel the Body that conftrain'd it. Such is a Sword-Blade, a Bow, &c. which are eaiily bent, but prefently return to their former Figure and Extcnfion. See Elasticity.

Elajiic Bodies are either Natural, or Artificial: Thofe moft eminent for their Elajiic Power among Artificial Bodies, are Steel Bows j Brafs, Ivory, and Marble Balls - 7 Leathers, Skins, Membranes 5 Brafs, Silver, Iron, and Steel Chords or Wiars 5 Nerves, Guts, Hemp and Flax Strings, &c.

Amongft Natural Bodies, the principal are Air, Spunges, the Branches of green Trees, Wool, Cotton, Feathers, c5c. 3 Tis difputed, whether or no Water have any Elajiic Force: The moft common Opinion is, that it has none of it feif $ and that if it mew any, 'tis owing to the Air contain'd therein. See Water.

The Word is form'd from the Greek, sa^V, of sAc/y;/^, to fvjh, p'efs, &c. See Spring.

The principal Phenomena obferved to obtain in Elajiic Bodies, are,

1. That an Elajiic Body, (i. e. a Body perfectly Elajiic, if any fuch there be) endeavours to refiore it felt with the fame Force wherewith it is prefs'd or bent. Thus, what- ever Force is applied to bend a Bow, with the fame Force it ftrives to unbend it felf, or return to its Natural State. For the Force, wherewith the String is drawn, is the fame with that which refills the Draught 5 the Bow being bent fo long, till the Force applied, and that which refifts It, are in EquiUbrio.

2.. An Elajiic Body exerts its Force equally towards all Sides 5 tho' the Effect is found chiefly on that Side where the Refinance is wcakeft : As is evident in the Cafe of a Bow mooting out an Arrow 5 a Gun exploding a Ball, &c.

3. Elajiic Bodies, in what Manner foever firuck, or im- pell'd, are inflected, and rebound after the fame Manner. Thus a Bell yields the fame Sound in what Manner, or on what Side foever firuck.

4. A Body perfectly Fluid, if any fuch there be, cannot be Elajiic 5 by Reaibn its Parts cannot be comprefs'd. See Fluid.

5. A Body perfectly Solid, if any fuch there be, cannot be Elajiic 5 in that having no Pores, it is incapable of being comprefs'd. See Solid.

6. Flarcf, long, flexible Bodies, apt to acquire Elaft'tcity, do it chiefly in three Manners ; Either by being extended, by being contracted, or by being bent.

7. Bodies, in dilating themfelves by their Elajiic Power, exert a greater Force at the Beginning of their Dilatation, than towards the End thereof. By Reafon the Bodies are more comprefs'd at firft 5 and the Rcnitency is always equal to the Compreffion.

8. The Motion, whereby comprefs'd Bodies refiore them- felves, is ufually an accelerated Motion. See Dila- tation.

For the Zaws of Motion and ^Percuffion in Elastic Bodies, .See Motion and Percussion.

Elastic Force. See Elasticity.

ELASTICITY, or ELASTIC Force, in Phyficks, a Pro- perty, or Power, in natural Bodies, which denominates them Elajiic ; and by which they are reftored to the Figure, and Extenfion they had loft from any External Caufe. See Elastic Bodies.

The Caufe, or Principle, of this important Property Elajlicity, or Springincfs, is varioufly affign'd : The Car- tefians account for it from the Materia fubtilis making an Effort to pafs thro' Pores that are too narrow for it. Thus, fay they, in bending, or compreffing, a hard, elaftic Body, e.gr. a Bow, its Parts recede from each other on the Convex Side, and approach on the Concave : Confequcntly the Pores are contracted or fireightcn'd on the Concave Side ; and if they were before round, are now, for Inftance, Oval : So that the Materia fubtilis, or Matter of, the fecond Element, endeavouring to pafs out at thofe Pores, thus ftreighten'd, muft make an Effort, at the fame Time, to refiore the Body to the State it was in when the Ports were more patent and round, i. e. before the Bow was bent ; And in this confifls its Elajlicity. See Resilition.

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Other later, and more wary Philofophcrs, account for Elajlicity much after the fame Manner as the CarteftanS z with this only Difference, that in lieu of the fubtile Mat- ter of the Cartejians, thefe fubftitute Ether or a fine Ethcrial Medium, that pervades all Bodies. See jEther. Others, fctting afide the precarious Notion of a Materia fitbtilis, account for Elajlicity from the great Law of Na- ture, Attraction, or the Caufe of the Cohefion of the Parts of f olid and firm Bodies. See Cohesion.

Thus, fay they, when a hard Body is firuck or bent, fo as the component Parts are moved a fittle from each other, but not quite disjoyn'd, or broke off, or feparated {o far as to be out of the Power of that attracting Force whereby they cohere 5 they muft certainly, on the Ceffation of the External Violence, fpring back to their former natural State. See Attraction.

Others refolve Elajlicity into the PrefTurc of the Atmo- fphere : For a violent Tenfion, or Compreflion, tho' not great enough to fcparate the conftituent Particles of Bodies far enough to let in any foreign Matter, mufi yet occafion many little Vacuola between the feparated Surfaces ; fo that upon the Removal of the Force they will clofe again, by the Preifure of the Aerial Fluid upon the external Parts. Sec Atmosphere.

Lafily, Others attribute the Elajlicity of all hard Bodies to the Power of Refilition in the Air included within 'em. And fo make the Elajiic Force of the Air, the Principle of Elajlicity in all other Bodies. See Air. Isa-zvs of Elasticity. To enquire a little more exprefly into the Nature and Laws of Elajlicity, we mall conflder the Phenomena there- of. Let it be premifed, then, that all the Bodies wherein this Power is obferved do confifi, or may be conceived to con- fifl, of fmall Threads, or Fibres, which, laid together, consti- tute fuch Bodies. To examine Elajlicity, therefore, in its moft fimple Cafe, we had befi conflder it in Mufical Strings, or Chords, and particularly thofe 'of Metal ; for thofe of Cat-gut, having a fpiral Twift, cannot be confidcr'd as the Fibres whereof Bodies are form'd.

Now the EJlaJlicity of a Fibre, or Chord, confifls in this, that it may be ftrctch'd or extended 5 and that upon re- moving the Force whereby it was ftretch'd, or lengthen'd, it returns to its former length. Fibres have no Elajlicity, unlefs they be extended with a certain Force 5 as appears in lax Chords, which may be moved a little out of their Pofition, without endeavouring to recover it. Tho' what the Degree of Tenfion is, ncceffary for Elajlicity to com- mence, is not yet determined by Experiment. Add, that when a Fibre is too far ftretchM, it lofes its Elajlicity 5 tho' here, too, the Degree of Teniion that defiroys Elajli- city is unknown. But 'tis certain, Elajlicity depends, on Tenfion, and is confined within a certain Sphere, or Boundary thereof on either Side. See Chord and Fibre.

This, if it do not give us the proper adequate Caufe of Elajlicity, yet fhews us the Difference between Elajtic and Unelaftic Bodies ; how it is a Body lofes its Elafti- city ; and how a Body, deftitute of any fuch Force, comes to acquire it.

Thus, a Plate of Metal, by repeated Blows of a Ham- mer, becomes Elaftic $ and being heated, again lofes that Property.

Between the Limits of Tenfion, wherewith Elafticity is terminated, there are different Forces required to give dif- ferent Degrees of Tenfions, in order to ftretch Chords to certain Lengths. What the Proportions of thefe Forces is, can only be determined by Experiment made with Chords of Metal. But as the Lengthenings of fuch Chords is fcarce fenfible, the Proportions cannot be directly meafured 5 but a particular Apparatus, and a Kind of Circuit are necef- fary to arrive thereat. Dr. Gravefande has taken a good deal of Pains, in order to fix thefe Laws ; the Refult of whofe Experiments are as follow.

1. That the Weight, whereby a Fibre is increas'd a cer- tain Length, by its Stretching, is in the different Degrees of Tenfion, as the Tenfion it ft\f : If, e. gr. there be three Fibres of the fame Kind, Length and Thicknefs, whofe Tenfions are as r, 2, and 3 ; any Weights, in the fame Proportion, will ftretch them equally.

2. That the leafi Lengthenings of the fame Fibres are to one another, nearly as the Forces whereby the Fibres are lengthen'd. Which Property may likewife be applied to their Inflexion.

3. In Chords of the fame Kind, Thicknefs, and which are equally ftretch'd, but of different Lengths, the Length- nings produced by fuperadding equal Weights, are to one another as the Lengths of the. Chords. Which is evident hence, that the Chord is lengthen'd in all its Parts : Con- fequcntly the Lengthening of a whole Chord, is double the Lengthening of half of it, or a Chord of half the Length.

4. Fibres of the fame Kind, but different Thicknefles, may be compared together after the like Manner ; only

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