HEA
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HEA
Heat, with Refpect to our Senfation, or the Effect pro- duced on us by a hot Body, is eftimated by its Relation to the Organ ot Feeling ; no Object appearing to be hot, unlefs its Heat exceed that of our Body : Whence, the fain; Thing to different Perfons, cr at different Times to the fame Perfon, /hall appear both Hot and Cold.
Heat) as it exifts in the hot Body; or that which con- ftuutes and denominates a Body foot, and enables it to produce fuch Effects on our Organ 5 is varioufly confidcr'd by the Philofophcrs. — Some will have it a Quality, others a Subitance, and others only a Mechanical Affection, viz. Motion.
Arifiotle, and the 'Peripateticks, define Heat a Quality, or Accident, whereby homogeneous Things, i. e. Things ot the fame Nature and Kind, are collected or gather'd to- gether ; and heterogeneous ones, or Things of different Nature, fever 'd or dii-united. — Thus, fays he, the fame Heat which affociatcs and brings into a Mafs feveral Par- ticles of Gold before feparate, feparates the Particles of two Metals before mix'd together.
But, not only the Doctrine, but the very Inftance he produces, is faulty : For Heat, tho' continued to Eternity, will never feparate a Mafs, e. gr. of Gold, Silver, and Copper: And, on the contrary, if feveral Quantities of Gold, Silver, and Copper, be put feparately in a Veffel over a Fire; notwithstanding all their Heterogeneity, they will be mix'd and congregated into a Mafs thereby.
In Effect, Heat cannot be faid to do either this Thing or that univerially; but all its Effects depend on the Circumstances of Application. ■ — ■ Thus, to do the fame Thing in different Bodies, different Degrees of Heat are required 5 as, to mix Gold and Silver, the Heat mull be in a moderate Degree, but to mix Mercury and Sulphur in the higheft. See Gold, Silver, &c.
Add, that the fame Degree of Heat /hall have oppofite Effects : — Thus a vehement Fire mall render Water, Oils, Salts, &c. volatile; and yet the fame imbody Sand and a fix'd alcaline Salt, into Glafs. See Glass.
The Epicureans, and other Corjwfrulareaus, define Heat not as an Accident of Fire, but as an effential Power or Property thereof, the fame, in Reality, with it, and only diftinguifh'd therefrom in the Manner of our Conception. Heat, then, on their Principles, is no other than the volatile Subftance of Fire it felf* reduced into Atoms, and emitted in a continual Stream from ignited Bodies 5 • fo as not only to warm the Object within its reach, but alfo, if they be inflammable, to kindle them, turn them into Fire, and confpire with them to make Flame.
In Effect, thefe Corpufcles, fay they, flying off from the ignited Body, while yet contain'd within the Sphere of its Flame, by their Motion conititute Fire ; but when fled, or got beyond the fame, and difperfed every Way, fo as to efcape the Apprehenfion of the Eye, and only to be perceivable by the feeling, they take the Denomination of Heat j inafmuch as they itill excite in us that Senfation.
The Carte/ions, improving on this Doctrine, aifert Heat to confiif in a certain Motion of the infenfible Particles of a Body, refembling the Motion whereby the feveral Parts of our Body are agitated by the Motion of the Heart and Blood. See Heart and Blood.
Our lateft, and beft Writers of Mechanical, Experimental, and Chymical Philofophy, differ very confiderably about Heat. ■ — • The fundamental Difference is, whether it be a pecu- liar Property of one certain immutable Body call'd Fire : Or whether it may be produced mechanically in other Bodies, by inducing an Alteration in the Particles thereof. The former, which is as antient as Demccritus, and the Syflem of Atoms 5 had given Way to that of the Cartejians and other Mechanics 5 but is now with great Addrefs retriev'd and improved on by fome of the lateft Writers, particularly Homberg, the younger Zemery, Grave- fande, and above all, the learned and induffrious Ztoerhaave, in a Courfe of Lectures exprcfly on Fire ; theRefult whereof we have already laid down under the Article Fire.
The Thing we call Fire, according to this Author, is a Body, Sui generis, created fuch ab Origins, unalterable in its Nature and Properties, and not either producible de Novo, from any other Body, nor capable of being reduced into any other Body, or of cea/ing to be Fire.
This Fire, he contends, is diffufed equably every where, and exifts alike, or in equal Quantity, in all the Parts of Space, whether void, or poffefs'd by Bodies ; but that naturally, and in it felf, is perfectly latent and imperceptible; and is only difcovcr'd by certain Effeffs which it produces, and which are cognizable by our Scnfes.
Thefe Effects are Heat, Light, Colour, Rarefaction and Burning ; which are all Indications of Fire, as being none of them producible by any other Caufe : So that wherever we obferve any of thefe, we may fafely infer the Action and Prefence of Fire,
But, tlio' the Effect cannot be without the Caufe, yet the Fire may remain without any of thefe Effects; any, we mean, grofs enough to affect our Scnfes, or become Objects thereof: And this, he adds, is the ordinary Cafe; there being a Concurrence of other Circum fiances, which are often wanting, neceffary to the Production of fuch fenfible Effects.
Hence, particularly, it is, that we fometimes find feveral, and fometimes all of thefe Effects of Fire together, and fometimes one unaccompanied with any others ; according as the Circumftances favour or difpole for the fame. — Thus we find Light without Heat ; as in rotten Wood, putrid Fi/hes, or the mercurial Phofphorus : Nay, and one of them may be in the higheft Degree, and the other not fenfible ; as in the Focus of a large burning Glafs expofed to the Moon, where, tho' the Light, as Dr. Hook found, was fuffkient to have inflantly blinded the bed Eye, yet no Heat was perceivable, nor was there the lealt Rare- faction occafion'd in an exquifite Thermometer. See Light. On the other Hand, there may be Heat without Light, as we find in folid Phofphorus ; in Fluids, which emit no Light, even when they boil, and not only heat and rarify, but alfo burn or confume the Parts ; and in Metals, Stones, iSc. which conceive a vehement Heat ere they fliine, or become ignited. ■ — Nay, and there may be the mofl intenfe Heat in Nature without any Light : Thus in the Focus of a large burning concave, where Metals melt, and the hardeft Gems vitrify, the Eye perceives no Light ; fo that /hould the Hand happen to be put there, it would be inflantly turn'd into a Coal, or even a Calx.
So alfo Rarefractions are frequently obferved by the Thermometer, in the Night Time, without cither Heat or Light, &c.
Thus it appears, that the Effects of Fire have a De- pendence on other concurring Circumftances ; fome more, and others left. — One Thing feems to be required in common to them all, viz. that the Fire be collected or brought into lefs Compafs : Without this, as Fire is every where equably diffufed, it could have no more Effect in one Place than another, but muft either be difpofed to warm, burn, and mine every where, or no where. Indeed fuch every where does amount to no where ; for to have the fame Heat, &c. in every Place, would be to have no Heat : 5 Tis only the Changes that we perceive ; thofe alone make the Mind diftinguiih in its State, and become confeious of the Things that diverfify it. So our Bodies being equally prefs'd on all Sides by the ambient Air, we feel no Prcflure at all ; but if the Prcffure be only taken off in any one Part, as by laying the Hand over an exhau/ted Receiver, we foon grow fenfible of the Load.
This Collection is perform *d two Ways : The flrft, by direBing and determini7ig the fluctuating Corpufcles of Fire into Lines, or Trains, call'd Rays ; and thus driving infinite Succeflions of the fiery Atoms, upon the fame Place or Body, each to produce its feveral Effort, and fecond that of the preceding ones, till by a Series of Augmentations, the Effect is fenfible. — This is the Office of thofe Bodies which we call Luminaries ; fuch as the Sun, and other heavenly Bodies; and Culinary Fires, Lamps, &c. on Earth, which do not emit the Fire from their own Subftance, as is commonly conceiv'd, but only by their rotatory Motation direct the undetermined Corpufcles into parallel Rays. And the Effect may be ft ill further intended by a fecond Col- lection of thefe parallel, into converging Rays, by Means of a Concave Speculum, or a Convex Glafs, which at length bring them all into a Point ; whence thofe aftoni/hing Effects of our large burning Glaffes, &c. See Ray, Parallel, Convex, Burning Glafs, Sec.
The fecond Way wherein the Collection is made, is not by determining the vague Fire, or giving it any new Direction; but merely by affembling it; which is done by the Attrition, or rubbing of two Bodies fwiftly again!* each other ; fo fwiftly, in Effect, as that nothing in the Air, except the floating Fire, has Activity enough to move an equal Pace, or fucceed faft enough into the Places continually relinquilh'd by it : By which Means Fire, the mofl agile Body in Nature, flipping in, becomes collected in the Path of the moving Body ; fo that the moveable has, as it were, a fiery Atmofphere around it. — Thus it is the Axes of Chariot Wheels, Mill-ftones, the Ropes of Ships, Cannon-Balis, &c. conceive Heat, and frequently kindle into Flame.
Thus much for the Circumflance in Heat, common to all the Effects of Fire, viz. Collection. — The particular Circumftances are various : Thus, for it to warm, or to heat, i. e. give the Senfation of it, 'tis neceffary that there be more Fire in the hot Body, or Thing, than in the Organ whereby it is to be felt ; othcrwife the Mind will not be put into any new State upon its Approach, nrr have any new Idea. — . Whence, alfo, if the contrary to this obtains, v. gr. if there be lefs Fire in the external
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