FIR 



Agam : As all bodies placed in a veryfolid air, do, 

 by dc'grces, grow cold, motionlefs, rigid, &c. i. e. 

 though there be (till feme remains of fire, and in 

 proportion as that is diminlflied, the effeft is accele- 

 rated ; it follows, that cold, a lefs degree of heat, is 

 the effeft of a lefier adion of fire : and all aflion rifes 

 apparently from the fame fource. 

 Then, as fire can render the moft folid bodies, as 

 ftone, metals, &c. (as appears very evident in large 

 burning-glafies, in which gold itfelf immediately cal- 

 cines, and emits fum.es, i. e. becomes fluid) fo the 

 want of fire would convert the moft fluid bodies, as 

 fpirit.s of wine, &c. into folids. 

 Fire is diftinguilhed into two kinds, called elemen- 

 tary or pure fire, winch is fuch as exifts in itfelf, and 

 alone is properly called fire ; or common or culinary 

 fire, which israifed and kindled from* the former, and 

 is that which agitates and afFefts ignited, combuftible, 

 and moveable bodies, the particles of which, joining 

 with thofe of the piire fire, conftitute pure flame. 

 This latter is improperly called fire, in that not only 



real or Dure fire ; and in 



Jg- 



a fmall part of it is real or pure 

 nitcd bodies, that which flames, fmokes, &c. is not 

 fimply fire ; whereas pure fire, fuch as is collefted 

 in a burning-glafs, yields no flame, fmoke, afties, or 



the like. 



Fire may be prefent in the greatefl: abundance, yet 

 without any heat.: this is evident in the tops of the 

 ' highefl: mountains, illuminated by the fun, where the 

 cold is always extremely pinching, and this even un- 

 der the equator, there being mountains there which 

 are perpetually covered with fnow, though there can 

 be no want of fire. 



So a large burning-glafs has no efFcift: the fmalleft 



warmth cannot be felt in its focus in a place where 



the fun does not fliine, or when the fun is covered 



with a cloud, but a piece of metal may be feen to 



' melt the very moment the fun ernerges. 



' Fire maybe in exceedmg fmall quantity, and yet 



"burn" with great violence : thus fpirit of wine when 



. fet.on. fire, does not burn the hands-, and though 



poured on a piece of red-hot iron, does not take fire; 



fb that the fire t"hat is in, Qiould not appear very 



great : yet if it meet with fome harder body while it 



' IS burning, the particles of which body it is capa- 



' ble to agitate by the attrition of its own, it will yield 



a fierce flame, capable of burning a harder body than 



the hand. 



From this it appear^, that the relation of heteroge- 

 . neous particles, agitated by tlie fire, has more efte6t 

 in relpedl to heat than the acSLion of the fire itfelf: 

 nor need we be far to feck for the mechanical reafon 

 of this, for the particles of fire, being all -equal and 

 fpherical, mud of thcmfelves be harmlefs ; but if 

 they carry certain Ipicula, or any other bodies along 

 with them, dien they become capable of doing much 

 harm. 



Hence, though the flame of a piece of wood may 

 give a fenfe of heat, and burn fuch things as are ap- 

 plied to it, it does not therefore neceflTarily follow, 

 that there is any pure fire in it, lb that the dif- 

 tinftion of pure and common fire is abfolutely ne- 

 cefl"ary : though this diftin£lion has been overlooked 

 ty moft or all the authors before Dr. Boerhaave, who 

 have written on fire'; which has led them into egre- 

 gious miftakes, infomAichxhat moft of them have held, 

 that the flame of a piece of wood is all fire, which ap- 

 pears to be falfe from what has been already faid, and 

 alfo what follows. 



Elementary or pure fire is of itfelf imperceptible, and 

 only difcovers itfelf by certain efFefts that it pro- 

 duces in bodies, and thefe effects are only to be learnt 

 by the changes which arife in bodies. 

 are three; 



Thefe effefts 



I ft, heat; 2dly, dilatation in all folid 

 bodies, and rarefaftion in all fluids ; 3dly, motion. 

 The firft effcft of elementary fire on bodies is heat ; 

 heat arifes wholly from fire, and in fuch a manner, 

 that the meafure of heat is always the meafure of 

 fire ; and that of fire, of heat ; fo the heat is infepa- 

 rable from the fire. 



F I R 



The fecond efiect of elernentaiy fi.rc is diiatatior ' 

 all folid bodies, and rarefaftion ifi all fluids. * ^" 

 Numierous experiments make it evident, that borK 

 thefe are infeparable from heat. If you heat an im 

 rod, it will increafe in all its dimenfions; and t' * 

 more it is heated, the farther it will be increafed • zrd 

 being again expofed to the cold, it will contraft' and 

 fuccefllvely return through all degrees of its dilatation 

 till it arrive at its firft bulk, being never two minute' 

 fucceflively of the fame magnitude. 

 The like may be obferved in gold, the heavieft of all 

 bodies, which takes up more fpace when it is fufej 

 than it did before ; hay, even mercury, the heavieft 

 of all fluids, has been known to afcend afbove thirty 

 times its height, being placed over the fire in a 

 tube. . 

 The laws of this expanfion are ; - 



I ft. That the fame degree of fire rarefies fluids 

 fooner, and in a greater degree than it does folids. 

 Without this, the thermometer would be of no ufe- 

 fince, if it were otherwife, the cavity of the tube 

 would be dilated in the fame proportion as the fluid is 

 rarefied. 



2dly, By how much the liquor is lighter, by fo much 

 the more it is dilated by fire : thus air, which is the 

 lighteft of all fluids, expands the moft, and fpirit of 

 wine the next after air. 



The third effe^ of fire on bodies is motion; for 

 fire, in warming and dilating bodies, muft necelTarilv 

 move their parts. And in effeft, all the motion of 

 nature arifes from fire alone ; and if this were takea 

 away, all things would become immoveable. AH 

 oils, fats, waters, wines, ales, ipirits of wine, vege- 

 tables, animals, &c. become hard, rigid, and inert, 

 upon the abfence of only a certain degree of fire; and 

 this induration will be both the fooner, and the mo;c 

 violent, the lefs the de2:ree of fire is. 

 Hence, if the fire was abfolutely taken away, and 

 there were the greateft degree of cold, all nature 

 would grow into one concrete body, folid as gold, 

 and hard as a diamond ; but, upon the application of 

 fire, it would recover its former mobility. 

 And, of conftquence, every diminution of fire is at- 

 tended with a proportionable diminution of motion. 

 Pure fire is found in two different manners, either 

 as it exifts every where, and is diftufed equally in 

 all places ; or as it exifts in certain bodies, in which 

 it makes no great alteration. 



That fire fiiould exift in the lame quantity in all 

 places, will feem a ftrange paradox; and yet that 

 It does fo, is demonftrable from innumerable expe-- 

 riments. 



This elementary fire is prefent every where, in all 

 bodies, all fpace, and at all times, and that in equal 

 quantities ; for let a perfon go where he will, to the 

 top of the higheft mountains, or defcend into the 

 loweft cavern, whether the fun fliine or not ; either in 

 the moft fcorching fummer, or the ftiarpeft winter ; 

 fire may be collefted by feveral' methods, as attrition 

 or otherwife. In a word, there is no phyfical point 

 affignable without fire, no place in nature where the 

 attrition of two fticks will not render it fenfible. 

 The Cartefians, as Marriotte, Perrault, &c. hold, 

 That there is a large ftock of fire in a perfeft vacuum, 

 i. e. a fpace out of which all the air has been exhaufted, 

 as fuppofing an abfolute vacuum impofllble: now, 

 the moft perfect vacuum that we can arrive at, is that 

 of Mr. Hugyens's contrivance, which is as follows: 

 heat a quantity of the pureft mercury to the heat of 

 boiling water, and pour it into a hot tube of about 

 forty inches long ; and when the tube Is filled, a^pty 

 a finger upon the orifice of it, and thus invert it into 

 a bafon full of mercury : the mercuiy will now be 

 fuipended in the tube to the whole height ; but then, 

 if you give it but a little ftiake, it will fink down to 

 the height of about twenty- nine inches, and thus 

 leave a vacuity of eleven inches. 

 Yet here the philofophcrs above-mentioned deny there 

 is any vacuum, and urge, that now^ fo much the more 



"fii:e is enter^ into the fpace as' there was of other 



matter : 



-'4 



