x 9 6 C H E M 
this attraction, the bafe fixes and combines with the 5 g- 
nel'cent body: it quits, therefore, the caloric; and the 
caloric, becoming free, produces heat, and feeks to com¬ 
bine with the fubftances it meets on its paflage. If com- 
buftion be made without air, the oxygen produced is 
not then melted into an aeriform fluid by the caloric and 
the light; there is fcarcely any dilengagement of thefe 
fluids : thefe combuftions alio are generally wrought with¬ 
out flame, and the heat produced is never confiderable. 
From thele four principles it is eafy to form an idea of 
what is to be underftood by combultion. Combuftible 
bodies are tliofe which have the property of decompofing 
oxygen gas. Hence it appears, that the heat refides par¬ 
ticularly in the oxygen gas which promotes the combuf- 
tion ; that, the more of the oxygen is abforbed in a given 
time, the ftronger will be the heat; the mode of pro¬ 
ducing a great heat is to burn bodies in the pureft air; 
that the fire and heat will be the more intenfe, as the 
air is more condenfed; that ftreams of air are neceflary 
to maintain and haften combultion. On this laft prin¬ 
ciple is founded the theory of cylindrical lamps: the 
current of air through the tube renews the air every mo¬ 
ment; and, by feeding the flame continually with frelli 
oxygen gas, it acquires a heat fufficient to burn and de- 
Itroy the fmoke. See the feCtion on fixed oils. 
Refpiration is a phenomenon very analogous to corn- 
bullion. Like combultion it decompofes the air: it can 
only be carried on in proportion to the quantity of pure 
or vital air which is prelent, and, when all that air is de- 
Itroyed, animals perilh in the mephitic air which re¬ 
mains. It is a flow combultion, in which part of the 
heat of the vital air pafles into the blood, which circu¬ 
lates through the lungs, and is with it dilperfed through 
all the organs : thus it is that the animal heat is repair¬ 
ed, which is continually carried off by the atmofphere 
and by furrounding bodies. The maintenance of the 
heat of the blood is therefore one of the principal ul'es 
of refpiration ; and this happy theory explains why ani¬ 
mals which do not refpire the air, or which refpire it 
very little, have cold blood. Meflieurs Lavoilier and De 
la Place have difcovered a fecond ufe of air in refpira¬ 
tion ; namely, to ablorb a principle which exhales from 
the blood, and appears to be of the fame nature as - char¬ 
coal. This body, reduced into vapours, combines with 
the oxygen of the vital air, and forms carbonic acid, 
which ilfues out of the lungs by the expiration. This 
formation of carbonic acid which takes place in atmof- 
pheric air refpired by animals, at the fame time that the 
mephitis is feparated, clearly (hews the dangerous con- 
fequences which refult from too great a number of per- 
fons being included in clofe places, fuch as theatres, hof- 
pitals, &c. and the noxious eftefits, which air vitiated by 
refpiration produces on perfons of delicate conftitutions, 
are no longer to be wondered at. 
Of ATMOSPHERICAL AIR. 
Air is one of thofe natural objedts upon which mo¬ 
dern phyfics have made fo many and great difcoveries. 
The phyfical properties of air, are, i. Its fluidity, which 
renders it fufceptible of thofe frequent and rapid mo¬ 
tions of its parts, which are called winds. It is not, 
however, of that fubtlety, as to pafs through the pores 
of many bodies. Tranlparent l'ubltances, through which 
light pafles with extreme facility, are not penetrable by 
air. Water, faline folutions, oils, and fpirit of wine, pals 
through a great number of bodies, wliofe texture is not 
penetrable by air. Nor has it that property by which 
liquids infinuate themfelves into the pores of certain bo¬ 
dies, and caufe them to expand. *. Its invifibility, as 
not being obvious to our fight. 3. Its infipidity: this 
opinion, however, is not univerfal: fome allow this pro¬ 
perty in air, others deny it; but, if we attend to the 
confequences of this fluid touching any bare nerve of an 
animal, as is the cafe in wounds and other limilar cir- 
=«umftances, we may conclude that it has a kind of fapi- 
I S T R Y. 
dity, which habit has probably rendered infenfible. In 
fadt, the expofure of wounds to the air is often attended 
with very acute pain. It is fufficiently bfcertained, by the 
experiments of Dr. Beddoes, that thefe effects of air de¬ 
pend upon its oxygen, which probably combines either 
with the matter difcharged from the ulcerated or raw 
furface, or combines with the furface itfelf. It was long 
ago fuggelted by Berthollet, that the caulticity of metal¬ 
lic oxyds depended on the affinity of their oxygen with 
animal matter; and the truth of this opinion was con¬ 
firmed experimentally by Fourcroy; Annales de Chimie, 
tom. 7. An infant, at the inftant of its birth, fufficiently 
fliews, by its cries, the difagreeable imprefiion this con¬ 
tact occafions. This acrimony in the air appears to be 
the caufe of that difficulty with which wounds cicatrize, 
if not kept covered ; atmofpherical air- alfo prevents ci¬ 
catrization in vegetables which have been deprived of 
their bark ; and the production of this membrane is 
known to take place, only when the external air is ex¬ 
cluded. 
4. The air, according to Fourcroy, is perfectly inodo¬ 
rous ; who fays, that, in thofe cafes in which a fort of fe¬ 
tid fmell is perceived, it is eafily accounted for, by at¬ 
tention to the foreign bodies interfperfed through it, as 
milts or vapours : but this account does not appear to be 
flriCtly accurate, fince the purelt atmofpheric air that has 
yet been found contains a mixture of carbonic acid gas. 
Cauftic alkalis become mild, lime-water acquires a pel¬ 
licle on its furface, and metallic oxyds become eft'ervef- 
cent in acids, after expofure for a fufficient length of 
time to atmofpheric air. Thefe changes take place in 
every fituation in which the experiments have been pro¬ 
perly tried. Even upon the lummit of Mont Blanc, 
there was a fufficient quantity of carbonic acid gas in 
the atmofpheric air to produce, in feven quarters of an 
hour, a pellicle upon the furface of lime-water, and to 
communicate, in an hour and a half, the property of ef- 
fervefcing to caultic potalh. It farther appears, that this 
fmall quantity of carbonic acid gas adheres fo ftrongly 
to the atmofpheric air, that the latter does not produce 
the flighted: cloud when agitated with lime-water. It 
was this, and other effects of the fame kind, which in¬ 
duced Mr. Kirwan to conclude that the quantity of car¬ 
bonic acid gas in atmofpheric air is abfolutely inappre¬ 
ciable. Morveau has pointed out the circumltances 
which occafioned the failure of Mr. Kirwan’s experi¬ 
ments, and has Ihewn that atmofpheric air contains a 
fmall quantity of carbonic acid gas, even when it is un¬ 
able to produce the fmallell cloudinefs in lime-water. 
Atmofpheric air alfo contains water; the quantity of 
which varies according to the temperature and denfity of 
the air, and according to the nature and qualities of the 
fubftances to which it has been recently expofed. It ap¬ 
pears from the accurate experiments of Sauflure, that a 
cubic foot of atmofpheric air, having the temperature of 
65°, diflolves about eleven grains of water, in paffingfrom 
extreme drynefs to extreme humidity. Confult upon this 
fubjeft Sauflure’s EJfais fur I'Hygrometrie, § 97, 180. 
5. Its weight; which is one of the molt valuable dif¬ 
coveries in natural philofophy. It was not well efta- 
bliffied till about the middle of the feventeenth century, 
though it is affirmed, that Ariftotle knew that a bladder 
is heavier when full of air, than when empty. The an¬ 
cients had no idea of the weight of the air; but attri¬ 
buted all the phenomena arifing from that weight to an 
occult quality which they called the horror of a vacuum. 
The impoffibility of railing water by the common pump 
to a greater height than thirty-two feet, engaged certain 
workmen to confult the famous Galileo, who was much 
furprifed at the fa6l. Death, in all probability, prevent¬ 
ed his fagacity from difcovering the true caufe of this, 
which was rel'erved for his difciple Torricellius. He was 
led to it by the following reafoning: the water appeared 
to him to rife in the fucking-pump folely in confequence 
of an exterior caufe, which, by preffure, obliged it to 
follow 
