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fiirce lus been long accounted the drUinguilliin J property of 

 air ; tlie other properties liilherto enumtrateJ being common 

 to it with other fluids ; though, from late experiraeiits, it 

 appears more than prohable, tliat the capacity of beuig 

 compreffeJ and expanded is not peculiar to air. See Wa- 

 TtR and Compression. 



This property of air has been long known, and was afcer- 

 tained by fome experiments of lord Bacon, who, upon this 

 principle, conllmcled his vitrum calendarc, the firft thermo- 

 meter. Bacon. Nov. Organ, lib. ii. aph. 13. 



Of this power we have numerous proofs. — Thus, a blown 

 bladder being fqueezed in the hand, we find the included 

 air fenfibly refill ; fo that upon ceafing to comprcfs, the 

 cavities or impreffions, made in its furtace, are readily ex- 

 panded again, and filled up. 



On this property of elalticity, the ilrufture and office of 

 the AIR-PUMP depend. 



Every particle of air always exerts this nifus, or endea- 

 vour to expand, and thus llrives againft an equal endeavour 

 of the ambient particles, whofe refiftance happening l)y any 

 means to be weakened, it immediately diffufes itfelf into an 

 jmmenfe extent. Hence it is, that thin glafs bubbles, or 

 bladders filled with air, and exactly clofed, being included 

 in tlie exhaulled receiver of an air-pump, burll by the force 

 vf the included air. So a bladder quite flaccid, containing 

 only the fmallell quantity of air, fwells in the receiver, and 

 appears quite fall. The fame effeft is a'.fo found by canying 

 the Haecid bladder to tiie top of a high mountain. This 

 experiment (liews, that the elallicity of air is different from 

 that of folid bodies : after thefe have been compreifed, 

 tjiey only refjme the figure which they had loft ; whereas 

 air, when the comprefliiig force is removed, not only dilates, 

 but occupies a much greater fpace tiian it did before ; nor is it 

 eafy to affign the limits of its expanfion. From fome ex- 

 periments of Col. Roy (Phil. Tranf. vol. 67. p. 70S.) it 

 would feem, that the particles of air may be fo far removed 

 from one another, by the diminution of preffure, as to lofe 

 a very great part of their elaltic force. It alfo appears that 

 the elaltic force of common air is greater than when its 

 denfity is coiifiderably augmented or diminidied by an ad- 

 dition to, or fubtraftion from the weight with which it is 

 ufually loaded ; a facl which contradicts the experience of 

 Boyle, Mariotte, and others. Thefe experiments alfo fhew, 

 that the elafllc force of moift air is greatly fuperior to that 

 of dr)' air ; in fome caf*?s the total expanfion of the former 

 was more than four times that of the latter. 



It has been quellioned among philofophcrs, whether this 

 elaftic power of the air is capable of being dcftroyed or di- 

 minifncd, Mr. Boyle made feveral experiments, with a 

 view to difcover, how long air, brought to the greatell 

 degree of expanfion to wliich he could reduce it in his air- 

 pump, would retain its fpring ; and could never obfen'e 

 any fenfible diminution. D'iaguliers found that air, after 

 having been enclofed for half a year in a wind-gun, had loll 

 none of its elafticity ; and Roberval, after preiei-ving it in 

 the fame manner for llxtecn years, obfervcd, that its expan- 

 five projeftile force was the fame as if it had been recently 

 condcnfed. Ncveithelefs, Mr. Haukfbee concludes, from a 

 later experiment, that the fpring of the air maybe fo difturbed 

 bya violent preffure, as to require fome time to return to its na- 

 tural tone. Dr. Hales inferred, from a number of experiments, 

 that the elalticity of the air is capable of being impaired and 

 Himinifhed by a variety of caufes, and of being actually de- 

 ftroyed, fo that it is reduced to a fixed ftate. Hence he 

 alfo roncludi.?, that elafticity is not an efiential immutable 

 property of the particles of air ; and that the atmofphere is 

 ii chaos, conlifting not only of elaftic, but alfo of unelaftic air- 



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particles, which copioufly float in it. Statical EfTays, vol, 



'• P; 3 "5. 



The weight or preffure of the air, it is obvious, has no 

 dependence on its elafticity ; but would be the fame, whether 

 the air has fuch a property or not. But the air, being 

 elaftic, is neceffarily affected by the preffure, which reduecj 

 it into fuch a fpace, as that the elafticity which re-atts 

 againfl the comprefling weight, is equal to that weight. 

 Indeed, the law of this elafticity is, that it increafes as the 

 denfity of the air increafes ; and the denfity increafes as the 

 force increafes by which it is prefTcd. Now, there muft ne- 

 ceffarily be a balance between the action and rc-aftion ; /. c. 

 the gravity of the air, which tends to comprefs it, aud the 

 elafticity of the air, which endeavours to expand it, muft be 

 equal. And tlie elafticity of the air not very different from 

 its natural ftate, being as the denfity, will of courie be in- 

 verfciy as the fpace which it occupies. 



Hence the elafticity increafing, or diminiihing, univerfally, 

 as the denfity increafes or diminifhcs, t. e. as the diftance 

 between the particles diminlfhes, or increafes, it is no matter 

 whether the air be comprcffed and retained in fuch fpace, 

 by the weight of the atmofphere, or by any other means; 

 it muft endeavour, in either cafe, to expand with the fame 

 force. And hence if air near the earth be pent up in a 

 veffel, fo as to cut off all communication with the external 

 air, the preffure of the inclofed air will be equal to the 

 weight of the atmofphere. Accordingly, we find mercury 

 fuitained to the fame height, by the elaftic force of air in- 

 cloled in a glafs veffel, as by the whole atmoipherical 

 preifure. 



On the fame principle air may be artificially condenfed ; 

 and hence the ftruciture of the air-^;/«. 



Although it may be admitted as a general principle, that 

 the denfity of the air is proportional to the force by which 

 it is compreffed, as the experiments of Mr. Boyle and Mr. 

 Mariotte have evinced ; yet in the cafe of condenfed air, 

 the rule will not be ftriflly applicable. When air is veiy 

 forcibly compreffed, fo as to be reduced to Jth of its ordi- 

 nary bulk, it makes a greater refiilance, and requires a ftronger 

 force to comprefs it than the above principle allows. Hence 

 it appears probable, that the particles of air cannot, by any 

 pofhble preffure, be brought into perfeft conta£t, or form a 

 folid mafs ; and therefore that the degree of condenfation has 

 its limit. Thus alfo in very high degrees of rarefaftion, the 

 elafticity is decreafed rather more than in an exaft proportion to 

 the weight or denfity of the air; whence it may be concluded, 

 that there is a limit to its rarefaction or expanfion, fo that 

 it cannot be expanded to infinity. Neveithelefs, the utniofl 

 limits to which air of the denfity which it pofFefres at the 

 furface of the earth, is capable of being compreffed, have 

 not been afcertained. Mr. Boyle reduced it at one time to 

 the 14th part, and at another to the 40th part of 

 its natural fpace. (Works, vol. iii. p. 507.) Dr. Halley 

 fays, that he has feen it compreffed fo as to be 60 

 times denfer than in its natural ftate, which is farther 

 confirmed by Mr Papin, and M. Hu5'gens. Dr. Hales 

 (Stat. Exp. vol. iL p. 343, &c.) by means of a prefs, con- 

 denied it 38 times ; and by freezing \vMtr in an iron ball 

 or globe, into 1522 times lefs fpace than it naturally occu- 

 pies : in which ftate its denfity or fpecific gravity muft be 

 nearly double that of water ; and as water is \e\-y flightly 

 compreillble, the particles of air muft be in their nature 

 different from thofe of water ; fince it would other\%ife be 

 impoflible to reduce air to a bulk 800 times lefs than that 

 which it occupies in its natural flute. 



However, Dr. Halley has alferted, in the Philofophical 

 Traofav^ions, (Abr. vol, ii. p. 17.) that from the experi- 

 ment* 



