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told, &c. may be condenfed into original water ; the 
difference between permanent and tranfient air, 
amounts to the fame as that between vapour and exha- 
lation •, the one, e. g, being dry, and the other rnoift. 
Hence, as Sir Ifaac Newton fays, it is, that as the 
particles of permanent air are grofier, and arife from 
denfer bodies, than thofe of tranfient air or vapour, 
true air is more ponderous than vapour, and a moift 
atmofphere lighter than a dry one. 
But this real air no where confifts in its purity, but 
that air which concerns us, and the properties and ef- 
fects of which are chiefly to be confidered, is that 
which has been before treated on and defcribed, which 
Mr. Boyie acknowledges to be the moft heterogene- 
ous body in the univerle •, and Dr. Boerhaave 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 fluid, appears from the eafy paf- 
fage it affords to bodies through it ; as in the propa- 
gation of founds, fmells, and other effluvia ; for thefe 
things £hew it a body that gives way to any force im- 
prejffed, and in yielding are eaiily moved among them- 
felves, which are the properties of a fluid ; fo that 
fcarce any body will call in queftion, whether air be 
a fluid, and thence being always in motion, and al- 
ways 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 the air, that is 
likewife eafily proved ; for that the air is heavy, fol- 
lows from its being a body, weight being an eflential 
property of matter. 
Senfe and experiment fufficiently prove this : for, if 
a perfon lay his hand upon an open veffel placed on 
an air-pump, and the air be exhaufted, he will fenfi- 
bly feel the load of the incumbent atmofphere to in- 
creafe, and prefs upon the upper part of his hand, as 
the air is exhaufting. 
In like manner, a hollow lphere of five or fix inches 
diameter, divided into two fegments exaftly fitting 
each other, after the air is exhaufted out of them, are 
prefted together with a force equal to a hundred 
pounds weight, and require the ftrength of two ftrong 
perfons 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. 
Mn 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, when it was 
pricked, and the air let out. 
Mr. Gravefande found, that the air in a glafs ball of 
about two hundred and eighty-three inches capacity, 
weighed a hundred grains •, and according to Burcher 
de Voider, a cubic foot of air is in weight one ounce, 
and twenty-feven grains. 
Mr. Boyle has computed, that the weight of any 
quantity of air, near the furface of the earth, is to 
water as 1 to 1000 ; and Dr. Halley, as 1 to 800 ; 
and Mr. Hawkfbee, as 1 to 885 •, and the gravity of 
the fame quantity of air to the fame quantity of mer- 
cury, as 1 to. 10800. 
Air therefore may be confidered as an univerfal oper- 
culum, or cover, which by its weight keeps all ter- 
reftrial bodies down, and hinders them from flying off. 
4. The air is eiaftic. Elafticity is a quality whereby 
a body yields to any external impreflions by contract- 
ing itfelf into lefs compafs ; and upon removing or 
diminilhing the inipreffive power, returns to its for- 
mer fpace or figure. And by this quality, the air is 
diftinguilhed from all other bodies in the atmofphere; 
neither fire nor exhalations appearing to be eiaftic, 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- 
p arable from it. A bladder full blown being fqueezed 
in the hand, the included air may be fenfibly per- 
ceived to refill the touch*, fo that upon ceafing to com- 
prefs it, the cavities or impreflions, which were made 
in its furface, are immediately expanded again, and 
filled up. 
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Alfo thin glafs bubbles, or bladders full of air, arid 
exaCtly doled, and put into the receiver of an air- 
pump, the air being exhaufted out of the receiver, 
the glafs bubbles fly in pieces by the force of the air 
included in them. 
From thefe experiments, and many others, it appears, 
that the air we breathe near the furface of the earth 
is comprefled by its own weight into at leaft 1679 
parts of the fpace it would poflefs in vacuo ; and if the 
fame air be condenfed by art, the fpace it will occupy, 
when moft dilated, will be as 505000 ,to 1, to that 
which it pofieftes when condenfed ; and Dr. Wallis 
fuggefts, that we are far from knowing the utmoft it 
is capable of. 
Nor does this power appear capable by any means to 
be deftroyed or diminifhed ; for Mr. Boyle made fe- 
veral experiments to difeover how 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 obferve any fenfible diminution*, although the 
air was clogged fome months with a weight that one 
would admire how it Ihould fupport one moment. 
It is, indeed, a wonderful property in air, that it 
fhould be capable of being contracted and extended 
infinitely*, but, as hath been laid, it does not appear, 
by all the experiments yet tried, that there are any 
limits ofiits compreffion or expanfion ; but ftill by the 
addition of a new weight, it will contract farther; and 
by taking the weight away, will expand farther. 
5. Air rarefies by fire, and contracts by cold. 
The colder the air is, the lefs fpace it takes up ; and, 
on the contrary, the warmer the air is, it pofieftes the 
larger fpace ; and fo cooling and compreffion have the 
fame effeCts upon air ; and fo cold and compreffion 
keep pace with one another. 
The fame holds of warming and diminilhing of 
weight, or heat and expanfion, which go hand in 
hand. 
And to the fame eiaftic power before-mentioned, and 
its being expanded by heat, it is owing, that air in- 
clofed in glafs veftels, at a time when it is much con- 
denfed, when it afterwards comes to expand by a far- 
ther degree of heat, frequently burfts the bottles. 
6. Air iscompreflibleby a weight laid thereon, and rifes 
and reftores itfelf upon a removal of the fame. This 
property has been fufficiently fttewn by what has been 
laid before, and efpecially under the head of elafticity; 
wherefore, having confidered the properties of air, I 
fhall take notice of fome of its operations and effeCls 
as to the bulinefs of vegetation. 
Air, by being heavy and fluid, invefts the whole earth, 
and prefffes all the bodies thereon with a great force, 
equal to what they would fuftain from the prefliire 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 1 5 pounds upon every 
fquare inch. Hence it prevents, e. g. the arterial 
veftels of plants and animals from being too much 
diftended by the impetus of the circulating juices, or 
by the eiaftic force of the air fo plentifully lodged in 
the blood of one, and the lap of the other. For, 
The air preftes equally every way, as is confirmed in 
what we obferve of foft bodies fuftaining this prefliire 
without any change of figure, and brittle bodies with- 
out their breaking. 
Air is a principal caufe of the vegetation of plants, an 
inftance of which we have from Mr. Ray, in the Phi- 
lofophical T ranfafrions of Lettuce-ieed, that was 
lbwn in the glafs-receiver of the air-pump, which was 
exhaufted and cleared from all air, which grew not 
at all in eight days time ; whereas fome of the fame 
feed that was fown at the fame time in the open air, 
was rilen 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 one week. 
That a certain portion of air is neceffary to preferve 
the growing quality of feeds is manifeft, from many 
repeated 
