I 
AERO 
The knowledge of the ufe of the air in refpiration; the 
method of afcertaining its purity and fitnefs for that func¬ 
tion; the invetligation of dephlogifticated air; the me¬ 
thod of impregnating water with fixed air; are all calcu¬ 
lated to anfwer purpofes of the higheft utility. The me¬ 
dicinal properties pf fixed air have been in a great mea- 
fure afcertained, and its antifeptic qualities in other refpedts 
promife to be of confiderable advantage. The method of 
afcertaining the purity of the air of a place, and the man¬ 
ner of ventilating an apartment, are of great ufe for thofe 
concerned in public buildings. In fliort, there is perhaps 
no flation in life where fome knowledge of this fubject 
may not be of ufe. 
Of 'the general Cortf itution, Mechanical Properties, and Ope¬ 
rations, of the Air. 
The general Corf itution of the Air we breathe .—For many 
ages this fluid was fuppofed to be Ample and homogene¬ 
ous; its common operations to depend on its heat, cold, 
moifture, or drynefs; and any eft'edts which could not be 
explained by tliefe (fuch as the appearance of peftilential 
difeafes), were reckoned to be entirely fupernatural, and the 
immediate efletts of Divine power. But, however Am¬ 
ple and homogeneous this fluid may have been thought in 
former times, it is fo far from poffefling the fimplicity of 
an element, that it is the receptacle of all kinds of efflu¬ 
via produced from terreffrial fubftances either naturally or 
artificially. Hence, whatever may be the nature of the 
aerial fluid when abfolutely pure, that which we breathe, 
and commonly goes under the name of air, muff be con- 
fidered as an exceedingly heterogeneous mixture, various 
at various times, and which it is by no means poflible to 
analyfe with accuracy. 
Though, in this view', air feems to be a kind of fink or 
common few'er, w'here all the poifonous effluvia arifing 
from putrid and corrupted matters are depofited; yet it 
has a wonderful facility of purifying itfelf, and one way 
or other of depoliting thofe vapours contained in it; fo 
that it never becomes noxious except in particular places, 
and for a thort time ; the general mafs remaining upon all 
occafions pretty much the fame. The way in which this 
purification is effected is different, according to the nature 
of the vapour with which the air is loaded. That which 
mod univerfally prevails is water; and from experiments 
it appears, that the quantity of aqueous vapour contained 
in the atmofphere is immenfe. Dr. Halley, from an ex¬ 
periment on the evaporation from a fluid furface heated to 
the lame degree with that given by our meridian fun, has 
calculated, that the evaporation from the Mediterranean 
Sea alone is lufficient to yield all the water of the rivers 
which run into ft. Dr. Watfon, in his Chemical Eflays, 
has given an account of fome experiments made with a 
view to determine the quantity of the water raifed from 
the earth itfelf alone in time of drought. He informs us, that 
when there had been no rain for above a month, and the 
grafs was become quite brown and parched, the evapo¬ 
ration from an acre w as not lefs than 1600 gallons in twen¬ 
ty-four hours. Making afterwards two experiments, when 
the ground had been wetted by a thunder-fhower the day 
before, the one gave 1973, the other 1905, gallons, in 
twelve hours. From this the air is every moment puri¬ 
fied by the afeent of the vapour, which, flying off into 
the clouds, thus leaves room for the exhalation of frefh 
quantities; fo that as the vapour is confiderably lighter 
than the common atmofphere, and of confequence afeends 
with great velocity, the air during all this time is faid to 
be dry, notw'ithffanding the vaft quantity of aqueous fluid 
that paffes through it. 
Nor is it only from the aqueous vapour that the air is 
purified at this time. Much of that vapour arifing from 
decayed and putrid animal and vegetable fubftances, and 
which by fome modern philofophers is called phlogifon, 
attaches itfelf to the aqueous■ vapour, and afeends along 
with it. Another part is abforbed by vegetables ; for the 
phlogiftic vapour is probably the food of plants. The 
Vol.I. No. 9. 
LOGY. r 4 i 
phlogiftic vapours which afeend along with the water 
probably continue there, and defeend along with the rain; 
whence the fertilizing qualities of rain-water above thofe 
of any other. Thus we may fee why a dry air, whether 
cold or hot, muff: always bewholefome; but, as the atmof¬ 
phere cannot always receive vapours, it is obvious, that 
when great rains come on, efpecially if attended with 
heat, the lower regions of the air muff be overloaded with 
vapours both of the aqueous and phlogiftic kind, and of 
confequence be very unwholefome. 
But belides the aqueous and phlogiftic vapours, both of 
which are fpecifically lighter than common air, there are 
others, which, being fpecifically heavier, cannot be car¬ 
ried off in this manner. Hence thefe grofs vapours con¬ 
taminate certain places of the atmofphere, rendering them 
not only unhealthy, but abfolutely poifonous. Of thefe 
are, fulphureous, acid, and metalline, exhalations, produ¬ 
ced principally by volcanoes; and, as they defeend, in 
confequence of their fpecific gravity, they fuffocate and 
fpread deftru&ion all around them, poifoning not only 
animals, but vegetables alfo. The vapours arifing from 
houfes where lead and other metals are fmclted, have the 
fame pernicious qualities; infomuch that the men w'ho 
breathe them, the cattle who eat the grafs, and the fifties 
who inhabit the waters on which they fall, are poifoned 
by them, if taken into the body in a certain proportion. 
Of the fame kind are the mofetes, or emanations of fixed 
air, which fometimes proceed from old lavas, or perhaps 
from fome other places even of the furface. From all 
thefe the air feems not capable of purifying itfelf, other- 
wife than either by difperfing them by winds, or by let¬ 
ting them fubfide by their luperior gravity, till they are 
abforbed either by the earth or water, according as it is 
their nature to unite with one or other of thefe elements. 
Of this kind alfo feem to be the vapours which are called 
properly pefilential. The contagion of the plague itfelf 
feems to be of an heavy fluggifh nature, incapable of ari¬ 
fing in the air, but attaching itfelf to the walls of houfes, 
bed-clothes, and wearing apparel. Hence, fcarcely any 
conftitution of the atmofphere can difpel thefe noxious ef¬ 
fluvia; nor does it feem probable that peftilential diftem- 
pers ever ceafe until the contagion has operated fo long, 
and been fo frequently communicated from one to another, 
that, like a ferment much expofed to the atmofphere, it 
becomes vapid, communicates a milder infection, and at 
laft lofes it ftrength altogether. 
The two molt important properties of the atmofphere 
are its gravity and its elafticity. The firft compared to 
the fpecific gravity of water, according to the computa¬ 
tions of Mr. Boyle, is as 1 10938, but, according to later 
experiments, as 1 to 800. This property is rendered 
evident in a variety of ways, but in none more decilive 
than in limply evacuating the receiver of an air-pump, fo 
as to evince the prodigious preflure of the atmofphere on 
its outfide. M. Pafcal has computed the quantity of this 
preflure on the earth’s furface to be upwards of 22321b. on 
every fquare foot. 
On the gravity of the air depend the afeent of water in 
pumps, fyphons, &c. and likewife all the phenomena of 
the barometer. But belides gravity, which the air has in 
common with other fluids, there is another property which 
it has only in common with fleam or vapour. This is call¬ 
ed its claf icily ; by which, like a fpring, it allows itfelf to 
be compreffed into a fmaller bulk, and then returns again 
to its original lize upon removing the preflure. 
The elafticity of the air, which was firft afcertained by 
fome experiments of lord Bacon, is no lefs eafy of proof, 
than its gravity. If a Ihrivelled bladder containing only»!> 
a very fmall quantity of air be put under the receiver of 
an air-pump, and the preflure of the atmofphere taken off 
the outfide of the bladder by exhaufting the receiver, the 
elafticity of the air within will be fhewn by its expanding 
the bladder to its full dimenfions. The fame fact is fhewn 
in a variety of other experiments not necellary to be de¬ 
tailed here. It may be proper however to obferve, that 
0 0 this 
