Page:Cyclopaedia, Chambers - Volume 1.djvu/978

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6r any other Caufe ? — Thus, Sea-Water, in a Storm ; Quickfilver agitated in Vacuo ; the Back of a Cat, or the Neck of a Horfe, obliquely rubb'd in a dark Place, Wood, FIcm and Fi(h, while they putrifie, Vapours from putrify- ing Waters, ufually call'd Ignes fatui ; Stacks of moift Hay or Corn, Glow-Worms, Amber and Diamonds, by robbing ; Scrapings of Steel, ftruck off with a Flint, &c. emit Light. Id. Ibid.

Are not grofs Bodies and Light convertible into one another ? And may not Bodies receive much of their Acti- vity from the Particles of Light, which enter their Com- polition ? I know no Body lefs apt to mine than Water; and yet Water, by frequent Di ft Mat ions, changes into fix'd Earth 5 which, by a fufficient Heat, may be brought to ihine like other Bodies. Id. Ibid.

Add, that the Sun and Stars, according to Sir Ifaac Ne-wtou's Conjecture, are no other than great Earths ve- hemently heated : For large Bodies, he obferves, preferve their Heat the longeft, their Parts heating one another ; and why may not great, denfe, and fixed Bodies, when heated beyond a certain Degree, emit Light fo copioufly, as by the Emiffion and Re-action thereof, and the Re- flections and Refractions of the Rays within the Pores, grow Hill hotter, till they arrive at fuch a Period of Heat as is that of the Sun ? Their Parts may be further pre- ferved from fuming away, not only by their Fixity, but by the vaft Weight and Denfity of their Atmofpheres in- cumbent on them, and ftrongly compremng them, and condenfmg the Vapours and Exhalations arifing from them. Thus, we fee, warm Water, in an exhaufted Receiver, Ihall boil as vehemently as the hotteft Water open to the Air ; the Weight of the incumbent Atmofphere, in this latter Cafe, keeping down the Vapours, and hindering the Ebullition, till it has conceiv'd its utmoft Degree of Heat. So, alfo, a Mixture of Tin and Lead, put on a red-hot Iron in Vacuo, emits a Fume and Flame : But the fame Mixture, in the open Air, by Reafon of the incumbent At- mofphere, does not emit the leaft fenfible Flame. — ■ Thus much for the Syfiem of the Prodncibility of Heat.

On the other Hand, M. Homberg, in his Effdi du Souffre 'Principe, holds, that the Chymical Principle, or Element, Sulphur, which is fuppofed one of the fimple, primary, pre-exiftent Ingredients of all natural Bodies, is real Fire; and confequently that Fire is co-eval with Body. Mem* de VAcad. An. 1705. See Sulphur.

Dr. S' Gravefande goes on much the fame Principle : Fire, according to him, enters the Compofition of all Bo- dies, is contained in all Bodies, and may be feparated or procured from all Bodies, by rubbing them againft each other, and thus putting their Fire in Motion. But Fire, he adds, is by no Means generated by fuch Motion. Elem. $>ftyf. T. II. C. 1.

A Body is only fenfibly hot t when the Degree of its Heat exceeds that of our Organs of Senfe ; fo that there may be a lucid Body, without any fenfible Heat ; and confequently, as Heat is only a fcnfible Quantity, without any Heat at all.

Heat, in the hot Body, fays the fame Author, is an Agitation of the Parts of the Body, made by Means of the Fire contained in it ; by fuch Agitation a Motion is produced in our Bodies, which excites the Idea of Heat in our Minds : So that Heat, in Refpect of us, is nothing but that Idea, and in the hot Body nothing but Motion. - — If fuch Motion expel the Fire, in right Lines, it may give us the Idea of Light ; if in a various and irregular Motion, only Heat.

M. Lemery the younger, agrees with thefe two Authors in afferting this abfolute, and ingenerable Nature of Fire ; but he extends it further. — Not contented to confine it as an Element to Bodies, he endeavours to mew that it is equably diffufed thro' all Space, is prefent in all Places, in the void Spaces between Bodies, as well as in the infenfible Interfiles between their Parts. Mem. de VAcad. An. 1713. See ./Ether.

This laft Sentiment falls in with that of Soerhaave above deliver'd: — 'It feems extravagant to talk of heating Cold Liquors with Ice ; yet Mr. 'Boyle affures us, he has eafily done it, by taking out of a Bafon of cold Water, wherein feveral Fragments of Ice were fwimming, one Piece or two which he perceived very well drenched with the Li- quor, and fuddenly immerfing them into a wide mouthed Glafs, of ftrong Oil of Vitriol : For the Menftruum pre- fently mixing with the Water, which adhered to the Ice, produced in it a briflt Heat, fome times with a manifeft Smoke, and that fuddenly diffolving the contiguous Parts of the Ice, and thofe the next, the whole Ice was foon reduced to Water ; and the corrofive Menftruum being, by two or three Shakes, well difperfed thro' it, the whole Mixture would immediately grow fo hot, that fometimes the containing Vial could not be endured in ones Hand. Bo)le, Ubi fu^ra,

There is a great Variety in the Heat of different Places, and Seafons. — ■ Naturalifts commonly lay it down, that the nearer the Centre of the Earth, the hotter it is found; but this does not hold ftrictly true. — In digging Mines, Wells, fife?, they find that at a little Depth below the Surface, it feels cool ; a little lower, and it is vet colder, as being now beyond any Reach, or Influence, ot the Sun's Rays, infomuch that Water will freeze almoft inftantane- ouily ; and hence the Ufe of Ice-Houfes, &c. but when a little lower, viz. about 40 or 50 Foot deep, it begins to grow warmer, fo that no Ice can bear it ; and then the deeper they go, ftill the greater the Heat : Till, at length, Refpiration grows difficult, and the Candles go out.

Hence, fome have Recourfe to the Notion of a Fund of Fire lodg'd in the Centre of the Earth ; which they confider as a central Sun, and the great Principle of the Generation, Vegetation, Nutrition, §?£. of foflil and ve- getable Bodies. See Central Fire. See alfo Earth, Earthquake, &c.

But Mr. Boyle, who bad been at the Bottom of fome Mines himfelf, fufpe&s that this Degree of Heat, at leaft in fome of them, may arife from the peculiar Nature of the Minerals generated therein. To confirm this, he in- ftances in a Kind of Mineral, of a Vitriolic Kind, dug up in large Quantities, in feveral Parts o£ England, which by the bare Effufion ot common Water will grow fo hot, as almoft to take Fire.

On the other Hand, as you afcend high Mountains, the Air grows more and more piercing and Cold : Thus the Tops of the 'Pico de ^heide in Bohemia, the Pike of 'Tenarijf, and feveral others, even in the molt fultry Countries, are found eternally inverted with Snow and Ice; the Heat never being fufficient to thaw the fame.

In fome of the Mountains of Peru there is no fuch Thing as running Water, but all Ice : Plants make a Shift to grow about the Feet of the Mountains, but near the Top no Vegetable can live, not for Want of Food, but thro' the lntenfenefs of the Cold. — This Effeft is attributed to the Thinnefs of the Air, and the little Surface of Earth there is to reflect the Rays. The Rays are here only de- termined into a Parallellifm ; but the Effect of direct pa- rallel Rays is found, by Computation, to be very inconfido rable ; this Effect being really greater in Winter than Summer.

Heat, in Geography. — ■ The Diverfity of the Heat of Climes and Seafons, ariies from the different Angles, under which the Sun's Rays ftrike upon the Earth's Surface. See Climate, Angle, £S"e.

It is fhewn in Mechanicks, that amoving Body, ftriking perpendicularly on another, acts on it with all its Force ; and that a Body ftriking obliquely, acts with the lefs Force, the more it deviates from the perpendicular. — ■ Now, Fire, moving in right Lines, muft obterve the fame Mechanical Law as other Bodies ; and confequently its Action muft be mcafured by the Sine of the Angle of Incidence : And hence Fire, ftriking on any Obftacle in a Direction parallel thereto, has no fenfible Effect, by Rea- fon the Ratio is almoft infinite, /'. e. nothing. Accordingly, the Sun, radiating on the Earth in the Morning, fcarce produces any warmth at all. See Percussion and Com- pound Motion.

Hence, Dr. Halley gives a mathematical Computation of the Effect of the Sun, under different Seafons and Cli- mates ; going on this Principle, that the fimple Action of the Sun, as all other Impulfes or Strokes, is moie or lefs forcible, according to the Sines of the Angles of Inci- dence, or to the perpendicular let fall on the Plane : Whence, the vertical Ray (which is of the greateft Heat) being put for Radius, the Force of the Sun on the Horizontal Surface of the Earth will be to that, as the Sine of the Sun's Altitude at any other Time.

Hence it follows, that the Time of the Continuance of the Sun's fliining, being taken for a Bafis, and the Sines of the Sun's Altitudes erected thereon as Perpendiculars ; and a Curve drawn through the Extremities of thofe Per- pendiculars : The Area comprehended will be proportionate to the Collection of the Heat of all the Sun Beams in that Space of Time.

Hence it will follow likcwife, that under the Pole the Collection of all the Heat of a tropical Day is proportionate to a Rectangle of the Sine of 23 Degrees and an half, into 24 Hours ; or the Circumference of a Circle ; that is, the Sine of 29 Degrees and an half, being nearly ;| of Radius, as to into 12 Hours ; or, the Polar Heat is equal to that of the Sun continuing 12 Hours above the Horizon at 53 Degrees height ; than which the Sun is not 5 Hours more elevated under the Equinoctial.

But whereas the Nature of Heat is to remain in the Subject, after the Luminary that occafion'd its being heated is removed, and particularly in the Air ; under the Equi- noctial the 12 Hours Abfence of the Sua does but little

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