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have had to build upon, he has had the advantage of 
a new fet, which they were unacquainted with. 
As to the. nature of fire, the great and fundamental 
difference is, whether it be originally fuch, formed 
thus by the great Creator himfelf, at the beginning of 
things ? or, whether it be mechanically producible 
from other bodies, by inducing fome alteration in 
the particles of it ? 
Among the modern writers, Homberg, Boerhaave, 
the younger Lemery, and Dr. Gravefande, maintain 
the former, and the Englifh authors chiefly maintain 
the latter. 
Monf. Homberg holds, That the chymical principle 
or element, fulphur, which is fuppofed one of the 
Ample, primary, pre-exiftent ingredients of all natu- 
ral bodies, is real fire ; and, of confequence, fire is 
coeval with all bodies. Effai de Souffre Principe, 
Mem. de l’Academie, anno 1705. 
Dr. Gravefande proceeds much on the fame princi- 
ple : according to him, fire enters the compofition of 
all bodies, is contained in all bodies, and may be fe- 
parated or procured from all bodies, by rubbing them 
againft each other, and thus putting their fire in 
motion : and he adds. That fire is by no means gene- 
rated by fuch motion. Elem. Phyf. Tom. II. cap. 1. 
Mr. Lemery the younger, afferts the abfolute and in- 
generable nature of fire, and alfo extends it farther ; 
not contented to confine it, as an element, to bodies, 
he endeavours to fhew, that it is 44 Equably diffufed 
44 through all fpace ; is prefent in all places ; in the 
“ void fpace between bodies, as well as the infenfible 
44 interfaces between their parts.” Mem. de l’Acad. 
anno 1713. 
This laft fentiment falls in with that of Boerhaave. 
Of the contrary opinion is the Lord Bacon, who, in 
his treatife de Forma Calidi, deduces from a great 
number of particulars, that heat in bodies is no other 
than motion, only a motion fo and fo circumftanti- 
ated •, fo that to produce heat in a body, nothing is 
required but to excite fuch motion in the parts of it. 
His opinion is feconded by Mr. Boyle, in his treatife 
of the Mechanical Origin of Heat and Cold ; where 
he maintains the fame doCtrine, with new obferva- 
tions and experiments, of which two are as follow: 
He fays, “In the production of heat there appears 
“ nothing on the part either of the agent or patient 
“ but motion, and its natural effeCts. When a fmith 
“ brifkly hammers a fmall piece of iron, the metal 
“ thereby becomes exceedingly hot ; yet there is no- 
“ thing to make it fo, except the forcible motion of 
“ the hammer, imprefling a vehement and varioufly 
“ determined agitation on the fmall parts of the iron *, 
“ which, being a cold body before, becomes by that 
“ fuperinduced commotion of its fmall parts hot ; 
“ firft, in a more loofe acceptation of the word, with 
44 regard to fome other bodies, compared with which 
“ it was cold before ; then fenfibly hot, becaufe this 
44 agitation fenfibly furpafles that of the parts of our 
“ fingers. And in this inftance oftentimes the ham- 
44 mer and anvil continue cold after the operation ; 
44 which ffiews, that the heat acquired by the iron 
44 was not communicated by either of thefe imple- 
44 ments, as heat, but produced in it, by a motion 
44 great enough ftrongly to agitate the parts of fo 
44 fmall a body as the piece of iron, without being 
44 able to have the like effeCt upon fo much greater 
44 maflfes of metal as the hammer and anvil ; though 
44 if the percufiions were often and brifkly renewed, 
44 and the hammer were fmall, this alfo might be 
44 heated •, whence it is not neceflary, that a body it- 
44 felf lhould be hot to give heat. 
44 If a large nail be driven by a hammer into a 
44 plank of wood, it will receive feveral ftrokes on its 
44 head, ere it grows hot ; but when it is once driven 
44 to the head, a few 7 ftrokes fufiice to give it a con- 
44 fiderable heat j for while at every blow with the 
44 hammer the nail enters farther into the wood, the 
44 motion produced is chiefly progreflive, and is of the 
44 whole nail tending one way ; but, when the mo- 
44 tion ceafes, the impulfe given by the ftroke being 
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44 unable to drive the nail farther on or break it, muft 
44 be fpent in making a various, vehement, and m- 
44 teftine commotion of the parts among thenafelves, 
44 wherein the nature of heat confifts.” 
That fire is the real caufe of all the changes in na- 
ture, will appear from the following confideration. 
All bodies are either folid or fluid j the foiid of them- 
felves are either commonly fuppofed to be inactive or 
motionlefs ; the fluid both move and are moved. 
And all folids are found to be fo much the more firm 
and contracted, as they have the lefs fire in them. 
This is evident in iron, which, when heated, expands 
itfelf into a much greater fpace than when it was cold j 
fo that any folid and hard body, by being freed from 
all fire, would fink into a much lefs bulk, and its 
parts would cohere more nearly, and with greater 
force than before. 
As to fluids, they all harden, fo as to be vifible to 
the eye upon the abfence of fire ; as water, by the 
cold of a fevere winter, will form itfelf into a folid 
globe, and yet even then contains a great deal of fire, 
as appears evidently upon applying a thermometer to 
it, which is capable of falling twenty divifions lower 
before it arrive at the point of the moft intenfe cold : 
and hence it is, that the fpirit of wine is kept from 
freezing in the thermometer, which would undergo 
the common fate of other things, were there not 
abundantly more fire in it. 
The air itfelf expands by a greater quantity of fire, 
and condenfes by a lefs ; but it ftill contains a large 
quantity of fire, where it is moft of all contracted; 
this is evident from the ftriking of a flint againft a 
fteel, which is followed by fparks of fire. 
Likewife if this fire could be taken from the air, it 
would become folid and perfectly at reft, and, by con- 
fequence, uncapable of change. 
44 Fire (fays Dr. Gravefande, in Element. Phyf.) na- 
44 turally unites itfelf with bodies •, and hence it is, 
44 that a body brought near to the fire grows hot, 
“ in which cafe it alfo expands or fwells ; which ex- 
44 panfion is not only obferved in very folid bodies, 
44 but in thofe whofe parts do not cohere ; in which 
44 cafe they likewife acquire a great degree of elaf- 
44 ticity, as is obferved in air and vapours.” 
Fire being thus acknowledged the inftrumental caufe 
of all motion, it remains that itfelf be moved •, nay, 
to move, muft be more natural and immediate to fire, 
than to any other body ; and hence fome have ventured 
to make motion effential to fire : but as this is incon- 
fiftent with the notion of matter, which is defined to 
be inert and paflive, and as fire is capable of being 
proved material, we ought rather to agree, that the 
motion of fire itfelf is derived from fome higher and 
metaphyfical caufe. A property of perpetual mo- 
bility may indeed be fuperadded to the other proper- 
ties of fire, but it has no natural neceflary connexion 
with them ; nor can it be maintained with them 
otherwife than by fome extrinfic efficacy of a fuperior 
caufe. 
However, that it is by motion that fire produces its 
effeCts, is evident ; and hence the aCtion of fire can- 
not make any alteration in the elementary fubftance 
of bodies •, for it is neceflary, that what aCts upon an 
objeCt, be without that objeCt, i. e. the fire muft not 
penetrate the elementary parts, but only enter the pores 
and interftices of bodies ; fo that it does not feem ca- 
pable of making thofe tranfmutations, which Sir Ifaac 
Newton afcribes to it. 
In effeCt, as to all our purpofes, it may perhaps be 
faid, that fire is always in motion. For inftance, 
take fix feveral forts of thermometers, and two vef- 
fels of water with fal armoniac mixed therein, and ap- 
ply the thermometers to it *, and the confequence will 
be, that the air being condenfed in them, the fpirit 
will defcend in all of them : remove the veffels of 
water, and the air growing warmer, and rarefying, 
the fpirit will afcend again ; fo that the aCtive force 
in air, which produces fo many effeCts, does really all 
arife from the fire contained in it, 
5 P 
Again : 
