A I R 



A I R 



menfs m.-idc at London, and by die Academy del CImcnto 

 at riorcnce, it iiiiglu be fnfely concluded, t!iat no force 

 wliatcver is able to reduce air into 800 times Icl;; fpaee tlrui 

 that wlilcb it naturally pofleirts on tiic Airface of our earth. 

 In anfwer to wliicli, M. Amontons, in the Memoirs of 

 the French Academy, maintains, that there is no affixing 

 any bounds to its condenfation ; that greater and greater 

 weights will Hill reduce it into Icfs and lefscompafs; that it is 

 only elalHc in virtue of the fire which it contains ; and that 

 as it is impofiiiile ever abfolutely to drive all the fire out of 

 it, it is impofTible ever to make the utmoR condenfation. 



The elalticity of the air exerts its force equally in all di- 

 rtftions ; and when releafed from the force tliat compred'es 

 it, it alfumes a fphencal figure in the interllices of the bo- 

 dies that contain it. By exliaufting the air from liquors 

 placed under the receiver of an air-pump, the bubbles that 

 gradually arifc and are enlarged in fize, letain their round 

 figure. Such are alfo the bubbles that difcharge themfelvcs 

 from a plate of metal immerged in a fluid in the fame cir- 

 cum'.huiecs. On this account large glafs globes arc always 

 formed of a fpherieal (liape by blowing air through an iruu 

 tulie into a piece of melted glafs at the end of the tube. 



The dilatation of the air by virtue of its elaftlc force, is 

 found to be very furprifmg ; and yet Dr. Wallis fuggells, 

 that we are far from knowing the utmoft of which it is ca- 

 pable. In fdvtral experiments made by Mr. Boyle, it di- 

 lated firft into 9 times its former fpace; then into 31 times; 

 then into 60; and then into 150. Afterwards, it was 

 brought to dilate into 8000 times its firft fpace: then 

 into 10,000, and even at laft into 13,679 times its fpace ; 

 and this altogether by its own cxpaniive force, without the 

 help of fire. Boyle's Works by Birch, vol. i. p. 21, 22. 

 vol Hi. p. 498, 499. 



On this depend the ftruflure and ufe of the Manometer. 

 Hence it appears, that the air we breathe near the furface 

 of the earth is comprefled by the weight of the fuperin- 

 cumbent column into at leaft the 13679th part of the fpace 

 . it would pofftfs in -vacuo. But if the fame air be condenfed 

 by art, the fpace it will take up when moft dilated, to that 

 it polTcffes when condenfed, will be, according to the fame 

 author's experiments, as 550,000 to i. 



We hence fee how wild and erroneous the obfervation 

 of Ariftotle was, that air, rendered ten times rarer than 

 before, changes its nature, and becomes fire. 



It has generally been fuppofed, that air expands -^l-^ with 

 each degree of the thermometer, commencing from the 

 mean temperature ^^° ; and upon this principle tables have 

 been computed by aftronomers for correfting their mean 

 refraAions ; but Sir George Shuckburgh allows at this 

 temperature an expanfion of -^-i-^ for i^. Phil. Tranf. v. 

 67. p. 564. Mr. Haukfbee obferved, that a portion of air, 

 included in a glafs tube, when the temperature was at the 

 freezing point, formed a volume which was to that of the 

 fame quantity of air in the greateft heat of fummer in Eng- 

 land as 6 to 7. Moift air has been expanded into more 

 than 12 times the fpace occupied by it in its freezing ft ate ; 

 and Merfennus by means of the ^ollplle expanded it into 

 more than 70 times its natural bulk. Mufchenb. Introd. 

 ad. Phil. Nat. torn. ii. p. 884. 4to. 



M. Amontons, and others, we have already obferved, 

 attribute the rarefa&ion of the air wholly to the fire con- 

 tained in it ; and therefore, by increafing the degree of heat, 

 the degree of rarefaction may be carried ftill farther than its 

 fpontaneous dilatation. Air is expanded 3 of its bulk by 

 boiling water. Hift. Acad. Sc. 1699. 



Dr. Hales found thattiie air in a retort, when the bottom 



cf the velTel was juil beginning to b.' red hot, was expanded 



4 

 i 



through twice its former fpace, gnd in a wliitc, o' .•>!mfifl 

 nieltiug heat, it occupied thrice its former fpace ; but Mr. 

 Robins found, (New Principles of Gunnery, civ. 1. prop. 5. 

 p. 12.) that air was expanded by the heat cf iron, juli be- 

 ginning to be while, to four times its former bulk. Thus 

 we account for the apparent inflation of a flaccid bladder, 

 when it is warmed by the fire, and on this principle depend 

 the ftrudlure and cilice of the thfrmometir, and alio 

 the formation and afeent of air-balloons. Uee ArRos- 



TATlON. 



M. Amontons firft difcovered that air will expand, in pro- 

 portion to its dcnfitv, with the fame degree cf heat. On 

 this foundation, the ingenious author has a difcouifc, to 

 prove, ' that the tpriiig and weight of the air, with a mo- 

 ' derate degree of warmth, may enable it to jiroduce even 

 ' earthquakes, and other of the moll vehement commotion* 

 ' of nature.' 



According to the experiments of this author, and M. de 

 la Hire, a column of air on the furface of the earth, 36 

 fathoms high, is equal in weight to three lines depth of 

 mercury; and it is found, that equal quantities of air poflefs 

 fpaces reciprocally proportioned to the weights with which 

 they are preilcd ; the weight of the air, therefore, which 

 would fill the whole fpace pofiefted by the tcrreftrial globe, 

 would be equal to a cylinder of mercuiy, whofe bafe is equal 

 to the furface of the earth, aifd its height containing as 

 many times three lines, as the atmofpherical fpaee contains 

 orbs equal in weight to 36 fathoms of tliat wherein the ex- 

 periment was made. — Hence, taking the denfell of all bodies, 

 i^- gr. gold, whofe gravity is about 14,630 times greater tlia>i 

 that of air in our orb, it is eafy to compute, that this air 

 would be reduced to the fame dcnfity as gold, by the prefTure 

 of a column of mercury 14,6^0 times 28 inches high, /. r. 

 409,640 inches, fince the bulks of air, in that cafe, would 

 be in the reciprocal ratio of the weights by which tlx-y are 

 preftcd. Thefe 409,640 inches, therefore, exprefsthe height 

 at which the barometer muft i^and, where the air would be 

 as heavy as gold, and the number 2 j-?~ ^^ ^'"'^^' tliethick- 

 nefs to which our column of 36 fathoms of air would be 

 reduced in the fame place. 



Now, we know, that 43,528 fathoms, which is the depth, 

 where the above prefture, and confequent redudlion take 

 place, are only the 74th part of the femidlameter of the 

 earth ; and, therefore, beyond that depth, whatever matter 

 exifts, it muft be heavier than gold. It is not improbable, 

 therefore, that the remaining fphere of 6,451,538 fathoms 

 diameter may be full of dcnie air, heavier by m.any degrees 

 than the heavicft bodies which we know. Hence, again, as- 

 it is proved, the more air i.- compixlTcd the more does the 

 fame degree of fire increafe the force of its fpring, and 

 render it capable of a proportlonably greater eftett ; we may 

 infer, that a degree of heat, which in our orb can only pro- 

 duce a moderate effeft, may have a vciy violent one in fueh 

 lower orb ; and that, as there may be many degrees of heat 

 in nature, beyond that of boiling water, it is probable there 

 may be lome, whole violence, thus aflilled by the weight of 

 the air, may be fufficient to tear alunder the folid globe. 

 Mem. de I'Acad. an. 1703. See Earthquakes. 



This elallic property of air is Inppofcd bv many philofo- 

 phers to depend on the figure of its eorpniclcs, wiiieh they 

 apprehend to be ramous ; fome maintain tliat they are fo 

 many minute Jiocculi, refembllng fleeces of v.-ool ; others 

 conceive them rolled up like hoops, and curlid like wires, 

 or fhavings of wood, or coiled like the fprings of watches, 

 and endeavouring to reftore themfelves in virtue of their 

 texture : fo that to produce air, muft be to produce fuch a 

 figure and difpolition of pans ; and thofe bodies only ari 



proper 



