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A 



i 



I 



cold, &0. may be condenfcd into originid water ; tlic 

 diiference between pcrmar.ci'.t and tranficnt air, 

 amounts to the fame as that between vapour and exha- 

 lation i the one, e. g. being dry, and the other moifc. 

 Hence, as Sir Ifaac Newton fays, it is, that as the 

 particles of permanent air are groifcr, and arife from 

 denfer bodies, than thofc of tranfient air or vapour, 

 true air is more ponderous than vapour, and a moill 

 atmofphcrc lighter than a dry one. 

 But this real air no where confifts in its purity, but 

 that air which concerns us, and tiic properties and cf- 

 iefts of which are chiefly to be confidercd, is that 

 which has been before treated on and defcribed, which 

 Mr. Boyle acknowledges to be the moft- heterogene- 

 ous body in the univerfe *, and Dr. Bocrhaave fhews 

 it to be an univerfal chaos and colluvies of all the 

 kinds of created bodies in the univerfe, and in which 

 may be found whatever fire can volatize. 



2. That the air is lluid, appears from the eafy paf- 

 fage it affords to bodies through it ; as In the propa- 

 gation of founds, fmells, and other effluvia j for thcfe 

 things Ihew it a body that gives way to any force im- 

 prefled, and in yielding are eafily moved among them- 

 fclves, which are the properties of a fluid ; fo that 

 fcarce any body will call in queftlon, whether air be 

 a fluid, and thence being always in motion, and al- 

 v/ays moving other bodies, for no furface of any li- 



' quor that is contiguous to the' air, can be at reft. 



3. As to the gravity or heavinefs of die air, that is 

 iikewife eafily proved ; for that the air is heavy, fol- 

 lows from its being a body, weight being an clFcntial 

 property of matter. . • 



Senle and experiment fufficicntly "prove this : for, if 



a perfon lay his hand upon an open veilcl placed on 



an air-pump, and the air be cxhaufted, he will fcnfi- 



bly feel the load of the incunibe.it atmofpherc to. in-, 



creafe, and prefs upon the upper part of his hand, as 

 the air is exhauftino-. 



In like manner, a hollow fphcre of five or fix inches 

 diameter, divided into two fegments exadlly fitting 

 each other, after the air is cxhaufted out of them, are 

 prefled together v/ith a force equal to a hundred 

 pounds weight, and require the ftrength of two ftrong 

 ^perfon?i to pull them afunder ^ which, as foon as ever 

 the air is let into them again, will fall afunder by the 

 mere weight of the under hemifphere, 

 Mr. Boyle found that a lamb's bladder, containing 

 about two thirds of a pint, and blown up, and well 

 dried, loft about a grain and one eighth, wJien it was 

 pricked, and the air let out. 



Mr, Gravcfande found, that the air. in a glafs ball of 

 about two hundred and eighty-three inches capacity, 

 weighed a luindred grains ; and according to Burclier 

 de Voider, a cubic foot of air is in weigl;t one ounce, 

 and twcnty-feven grains, 



Mr. Boyle has computed, that the v/eight of any 

 quantity of air, near the furface of the earth, 'i 

 water as i to 1000 j and Dr. Hallcy, as i to 800 ; 

 and Mr. Hawkfbee, as 1 to 885 -, and the gravity of 

 the fame quantity of air to the faine quantity of mer- 

 cury, as I to 10800. , /, 



Air therefore may be confidercd as an univerfal oper- 

 culum, or cover, which by its w^-ight keeps all tcr- 

 reftrial bodies down, and hinders them from flying off-". 

 4. The air is elaftic. Elafticity is a, quality whereby 

 a body viclds to any external imprcflions by contract- 

 ing itfclf into lefs compaft j and upon removing or 

 diminifliing tiie imprefiive pov;er, returns to its for- 

 mer ipacc or figure. And l^y tliis quality, the air is 

 dlftinguiflicd from all other bodies in the atmofphere; 

 neither fire nor exhalations appearing to be elaftic, at 

 leaft in any notable degree. ' 



That there is fuch a quality in the air, is evident from 

 innumerable experiments ; and this property is infe- 

 parable from it. A bladder full blown being fqueezed 

 in the hand, the included air may be fenfibly per- 

 ceived to refift the touch-, fo that upon ceafing to com- 

 prds it, tiic cavities or imprefilons, which v/ere made 



in its furface, are immediately expanded again, and 

 filled up. 



s to 



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Alfo thin glifs bubbles, or bladders full of air, and 

 exacl'ly clofed, and put into the receiver of an air- 

 pump, the air being c'xh;iuftcd out of the receiver, 

 the glafs bubbles fly in pieces by the force of the air 

 Included in them. 



+ 



Froin thcfe experiments, and many others, it appears, 

 that the air w^e breathe near the furface of the earth 

 is comprefll^d by its own weight into at leaft 1679 

 parts of the fpace it would poflcfs in vacuo ^ and if the 

 liune air be condenfed by art, the fpace it will occupy, 

 when moft dilated, will be as 505000 to i, to tliat 

 which it poflcffes when condenfed; and Dr. Wallis 

 fuggefts, that we are far from knowing the utmoft it 



is capable of. 



Nor docs diis power appear capable by any means to 

 be deftroyed or diminilhcd ; for Mr. Boyle made fe- 

 veral experiments to difcover liow long air, brought 

 to the greateft degree of expanfion he could reduce it 

 to in his air-pump, would retain its fpring, and could 

 never cblcrve any fenfible diminution ; although the 

 air was clogged fome months with a weight that one 

 would admire how it fliould fupport one moment. 

 It is, indeed, a wonderful property in air, thaf it 

 piould be capable of being contradled and extended 

 , infinitely-, but, as hath been faid, it does not appear, 

 by all tlie experiments yet tried, that there are any 

 limits of its compreflion or expanfion • but ftill by the 

 addition of a new \veight, it will contrail farther; and 

 by taking the weight away, will expand farther. 

 5. Air rarefies by fire, and contrafts by cold. ■ 

 The colder the air is, the Icfs fpace It takes up ; and, 

 . on tlie contrary, the w^armer the air is, it poflefll\s the 

 larger fpace 3 and fo cooling and compreflion have th; 



^ — 



fimc efiects upon air; and fo cold and compreflion 

 keep pace with one another. 



,The fame liolds of warming and diminifliing of 

 weight, or heat and expanfion, which go hand in 

 hand. - ' , 



And to the fame elaftic power before-mentioned^ and 

 its being^ expanded by heat, it is owing, that air in- 

 clofed in glafs veflels, at a time when it is much con- 

 denfed, wiien it afterwards comes to expand by a far- 

 ther degree of heat, frequently burfts the bottles. 

 6. Air iscomprcfllblebyaweightlaid thereon, and rifes 

 and reftores itfelf upon a removal of the fame. This 

 ,property has been fufficiendy fhe\vn by what has been 

 faid before, and efpecially under the head of elafticity ; 

 wherefore, having confidercd the properties of air, I 

 fnall take notice of fome of its operations and effefts 

 as to the bufinefs of vegetation, - 

 Air, by being heavy and fluid, invefts the whole earth, 

 and preffes all the bodies thereon with a great force, 

 equal to what they would fjftain from the preflurc of 

 a column of 29 inches depth of mercury, or 32 feet 

 of water ; and conftringes and binds them down with 

 a force amounting, according to the computation of 

 Mr. Pafchal, to 2232 pounds weight upon every 

 fquare foot, or upwards of 15 pounds upon every 

 fquare inch. Hence it prevents, e. g. the arterial 

 vcflcls of plants and animals from being too much 

 diftcnded by the impetus of the circulating juices, or 

 by the elaftic force of the air fo plentifully lodged in 

 the blood of one, and tlie fap of the other. For, 

 ' The air prelfes equally every way, as is confirmed in 

 what we obferve of fofc bodies fuftaining this preflaire 

 without any change of figure, and britde bodies with- 

 out their breaking. 



Air is a principal caufe of the vegetation of plants, an 

 inftancc of \yhich we have from Mr. Ray, in the Phi- 

 lofophical Tranfa6tions of Lettuce-feed, that was 

 fown in the glafs-recciver of the air-pump, which was 

 cxhaufted ^and cleared from all air, wiiich grew not 

 at all in eight days time; -whereas fome of the fame 

 fecd.that was fown at the fame time in the open air, 

 . was rifen to the height of an inch and a half in that 

 time; but the air being let into the empty receiver, 

 the feed grew up to the height of two or three inches 

 in the fpace of on^ week. 



That a certain portion of air is neceflary to prefcrve 

 jhe^growlng quality of feeds is manifeft, from many 



repeated 



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