VACUUM. 



fwL-lling of the flcfh in a cupping-glafs, &c. But fince the 

 weight, elafticity, &c. of the air have been afcertained by 

 fure experiments, thofe motions and effeAs are univerfally 

 afcribed to the gravity and prefTure of the atmofphere. 



The Cartefians deny not only the aftiial exiftence, but even 

 the poiTibility of a vacuum : and that on this principle, that 

 cxteniion being the effence of matter, or body, wherever ex- 

 tcnfion is, there is matter ; but mere fpace, or vacuity, is 

 fuppofed to be extended ; therefore it is material. Who- 

 ever aflerts an empty fpace, they fay, conceives dimenfions 

 in that fpace, (. e. he conceives an extended fubftance in it ; 

 and therefore he denies a vacuum, at the fame time that he 

 admits it. 



Des Cartes, if we may believe fome accounts, rejefted a 

 vacuum from a complaifance to the tafte which prevailed in 

 his time, againft his own firft fentiments ; and among his 

 famiUar friends ufed to call his fyftem his philofophical 

 romance. 



On the other hand, the corpufcular authors prove, not 

 only the poffibility, but the aftual exiftence of a vacuum, 

 from divers confiderations ; particularly from the confider- 

 ation of motion in general ; and that of the planets, comets, 

 &c. in particular ; from the fall of bodies ; from the vibra- 

 tion of pendulums : from rarefaftion and condenfation ; from 

 the different fpecific gravities of bodies ; and from the di- 

 Tifibility of matter into parts. 



1. It is argued, that motion could not be effefted with- 

 out a vacuum. This is what Lucretius urged long ago. 

 " Principium quoniam cedendi nulla daret res, — undique 

 materies quoniam ftipata fuiflet." 



The force of this argument will be increafed from the 

 tVTO following confiderations ; ix'z. firft, that all motion is 

 either in a ftraight line, or in a curve, which returns into 

 itfelf, as the ciixle and ellipiis ; or in a curve that does not 

 return into itfelf, as the parabola. Sec. And, fecondly, 

 that the moving force muft always be greater than the 

 refiftance. 



From hence it follows, that no force, even though infinite, 

 can produce motion where the refiftance is infinite ; confe- 

 quently, there can be no motion either in a ftraight hue, or 

 a non-returning curve ; becaufe, in either of thofe cafes, the 

 protrufion, and confequently the refiftance, would be infinite. 

 There remains, therefore, only the motion of a revolving 

 curve prafticable ; this muft either be a revolution upon an 

 axis, or an annular motion round a quiefcent body ; both 

 which are, again, impoffible in an elliptic curve ; and, confe- 

 quently, all motion muft be in circles geometrically true ; 

 and the revolving bodies muft either be fpheres, fpheroids, 

 cylinders, or portions of them, exaftly geometrical ; other- 

 wife the revolutions in a plenum would be impoflible : but 

 fuch motions, or fuch figured bodies, we do not know in 

 nature. Therefore there is a vacuum. 



2. The motions of the planets and comets demonftrate 

 a vacuum : thus fir Ifaac Newton, — " That there is no fuch 

 fluid medium as sther," (to fill up the porous parts of all 

 fenfible bodies, as the air and intcrftellar parts, and fo make 

 a plenum,) " feems probable ; becaufe the planets and co- 

 mets proceed with fo regular and lailing a motion through 

 the celeftial fpaces, both from and to all parts ; for hence it 

 appears, that thofe celeftial fpaces are void of all fenfible 

 refiftance, and confequently of all fenfible matter. For the 

 refitting force of fluid mediums arifes partly from the attri- 

 tion of the parts of the medium, and partly from the inac- 

 tivity of matter. Now, that part of the refiftance of any 

 medium, which arifes from the tenacity or attrition of its 

 parts, may be leflened by dividing the matter into fmaller 

 parts, and rendering thofe parts more fmooth and flippery : 



but that pait of the refiftance which arifes from the inafti- 

 vity of matter, is always in proportion to the denfity of the 

 matter ; nor can it be diminifhed by dividing the matter, 

 nor by any other means, except by diminifhing the denfity 

 thereof. 



" Confequently, if the celeftial regions were as denfe as 

 water, or as quickfilver, they would refift almoft as much as 

 water or quickfilver ; but if they were perfeftly denfe with- 

 out any interfperfed vacuity, though the matter were ever 

 fo fluid and fubtle, they would refift more than quickfilver 

 does : a perfeftly folid globe, in fuch a medium, would lofe 

 above lialf its motion, in moving three lengths of its dia- 

 meter ; and a globe not perfeftly folid, fuch as the bodies 

 of the planets and comets are, would be ftopped ftill fooner. 

 Therefore, that the motion of the planets and comets may 

 be regular and lafting, it is neceflary the celeftial fpaces be 

 void of all matter, except perhaps fome few, and much 

 rarefied effluvia of the planets and comets, and the pafling 

 rays of light." 



3. The fame great author deduces a vacuum alfo from 

 the confideration of the weights of bodies; thus: " All 

 bodies about the earth gravitate towards the earth ; and the 

 weights of all bodies, equally diftant from the earth's cen- 

 tre, are as the quantities of matter in thofe bodies. If the 

 aether, therefore, or any other fubtile matter, were altoge- 

 ther deftitute of gravity, or did gravitate lefs than in propor- 

 tion to the quantity of its matter ; becaufe (as Ariftotle, 

 Des Cartes, and others, argue) it differs from other bodies 

 only in the form of matter ; the fame body might, by the 

 change of its form, gradually be converted into a body of 

 the fame conftitution with thofe which gravitate moft in 

 proportion to the quantity of matter : and, on the other 

 hand, the moft heavy bodies might gradually lofe their 

 gravity, by gradually changing their form ; and there- 

 fore the weights would depend upon the forms of bodies, 

 and might be changed with them ; which is contrary to all 

 experiment." 



4. The defcent of bodies proves, that all fpace is not 

 equally full ; for the fame author goes on, "If all fpaces were 

 equally full, the fpecific gravity of that fluid with which 

 the region of the air would, in that cafe, be filled, would 

 not be lefs than the fpecific gravity of quickfilver or gold, 

 or any other the moft denfe body ; and therefore neither 

 gold, nor any other body, could defcend therein. For 

 bodies do not defcend in a fluid, unlefs that fluid be fpeci- 

 fically lighter than the body. But, by the air-pump, we 

 can exhauft a veflel, till even a feather ftiall fall with a ve- 

 locity equal to that of gold in the open air : the medium, 

 therefore, through which this feather falls, muft be much 

 rarer than that through which the gold falls in the other 

 cafe. 



" The quantity of matter, therefore, in a given fpace, may- 

 be diminifhed by rarefaftion : and why may not it be di- 

 miniftied in infinitum ? Add, that we conceive the folid par- 

 ticles of nU bodies to be of the fame denfity ; and that they 

 are only rarefiable by means of their pores : and hence a 

 vacuum evidently follows." 



J. " That there is a vacuum, is evident from the vibra- 

 tions of pendulums ; for fince thofe bodies, in places out of 

 which the air is exhaufted, meet with no refiftance to retard 

 their motion, or fliorten their vibrations ; it is evident there 

 is no fenfible matter in thofe fpaces, or in tiie occult pores of 

 thofe bodies." 



As to what Des Cartes urges of his materia fubtilis, that 



its tenuity prevents its refiftance from being fenfible ; and 



that a fmall body, ftriking againft a greater, cannot in the 



leaft move, or refift the motion ctf that other ; but is re- 



9 flcded 



