AEROLOGY. 
142 
this elafticity is capab'e of fome diminution by long-con¬ 
tinued and violent p re'Jure, as late experiments have proved. 
The weight or prefture of the air has no dependence on 
its elaflicity ; but would be the fame whether it had fuch 
a property or not. The air, however, being elaftic, isne- 
Ceffarily affected by the preffure, which reduces it into fuch 
a fpace, that the elaflicity, which re-aits againft the cora¬ 
prefling weight, is equal to that weight. In effedt, the 
law of this elaflicity is, that it encreafes as the denfity of 
the air encreafes; and the denfity encreafes as the force 
encreafes by which it is preffed. Now there mu ft necef- 
farily be a balance between the adtion and re-adtion; i. e. 
the gravity of the air which tends to comprefs it, and the 
elafticity by which it endeavours to expand, mud be equal. 
Hence the elafticity increafing or diminifhing univerfally, 
as the denfity increafes or diminithes, it is no matter 
whether the air be compreffed and retained in fuch a 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 
veil'd, and all communication with the external fluid cut 
off, the preffure of the inclofed air will be equal to the 
weight of the atmofphere at the time the quantity was 
confined. Accordingly, we find mercury fuftained to the 
fame height, by the elaftic force of air inclofed in a glafs 
veil'd, as by the whole atmofpherical preffure. On the 
fame principle air may be artificially condenfed; and hence 
the ftrudture of the Air-gun. 
The utmoft limits to which air, of the denfity which it 
poffeffes at the furface of the earth, is capable of being 
compreffed, have not been afeertained. Dr. Hales, by 
means of a prefs, condenfed it 38 times; and, afterwards, 
by forcing water in an iron globe, into 1551 times lefs 
fpace than it naturally occupies. However, Dr. Halley, 
from the experiments made at London and Florence, con¬ 
cludes, that no force whatever is able to reduce air into 
800 times lefs fpace than that which it naturally poffeffes 
on the furface of our earth. The dilatation of the air, 
however, by virtue of its elaftic force, is found to be very 
furprifing. In the experiments made by Mr. Boyle, it 
dilated into 10,000, and even at laft into 13,679, times its 
fpace; and this altogether by its own expaniive force, 
without the help of fire. 
In fad, it appears, that the air we breathe is compreffed 
by its own weight into at leaft the 13,679th part of the 
fpace it would poffefs in vacuo. But if the lame air be 
condenfed by art, the fpace it would take up when moft 
dilated, to that it poffeffes when condenfed, will be, accord¬ 
ing to the fame author’s experiments, as 550,000 to 1. 
Dr. Hales found, that the air in a retort, when the bot¬ 
tom of the veffel was juft beginning to be red-hot, was 
■expanded through twice its former fpace ; and, in a white 
heat, it occupied thrice its formerfpace : but Mr. Robins 
found it was expanded by the heat of iron, juft beginning 
to be white, to four times its former bulk. 
The elaftic power of the air is the fecond great fource of 
the effects of this important fluid. Thus it enters the 
pores of bodies; and by this prodigious faculty of ex¬ 
panding, which is fo eafily excited, puts their particles 
into perpetual ofcillations. Indeed, the degree of heat, 
and the air’s gravity and denfity, and confequently its elaf¬ 
ticity and expanfion, never remaining the fame for the leaft 
fpace of time, there muft be an inceffant vibration or di¬ 
latation and contraction in all bodies. 
From this caufe it is, that the air contained in bubbles 
of ice, by its continual action, burfts the ice. Thusalfo 
entire columns of marble fometimes are cleft, in the win¬ 
ter time, from the increafed elafticity of fome little bub¬ 
ble of air contained in them. From the fame principle 
arife all putrefadfion and fermentation ; neither of which 
will proceed, even in the belt difpofed fubjedts, in vacuo. 
Befides thefe properties of gravity and elafticity, au¬ 
thors have attributed to air feveral qualities which rather 
belong to the accidental mixture of other fluids in its com- 
pofttion. Of this nature is its property of corroding pe¬ 
tals, ftones, &c. but we fhall not dwell on them at prefent 
farther than to obferve, that, in thefe cafes the dillolving 
power of air is increafed by heat, and bv other catifes. It 
combines with water ; and, by accefsof cold, depolits part 
of the matter which was kept diffolved in it by a greater 
degree of heat. Hence the water, by being depofited 
and condenfed upon any cold body, fuch as glafs, See. in 
windows, forms fogs, and becomes vilible. 
In the operations of chemiftry, air is a very important 
agent; the refult of particular proceffes depending on its 
prefence or abfence, on its being open or inclofed. Thus, 
the parts of animals and vegetables can only be calcined 
in open air ; in dole veffels they never become any other 
than black coals. And thefe operations are affedted by 
the changes to which the air is liable. Many inftances 
mightbe adduced to thispurpofe. Letit fuflice to obferve, 
that it is very difficult to procure oil of fulphur, per cam- 
panam , in a clear dry atmofphere; but in a thick moift air 
it may be obtained with greater eafe, and in larger quan¬ 
tities. So, pure well fermented wine, if it be carried to a 
place where the air is replenifhed with the fumes of new 
wine then fermenting, will begin to ferment afrelh. 
The changes in the air arife from various caufes, and 
are obfervable, not only in its mechanical properties, but 
in the ingredients that compofe.it. Thus, at Faftilun, not¬ 
ed for copper mines, the mineral exhalations affect the air 
in fuch a manner as to difcolour the lilver coin in purfes, 
and change the colour of brafs. In Carniola, Campania, 
See. where are mines of fulphur, the air fometimes becomes 
very unwholefome, which occalions frequent epidemic dif- 
eafes, &c. The effluvia of animals alfo have their effect 
in varying the air; as it is evident in contagious difeafes, 
plagues, murrains, and other mortalities, which are fpread 
by an infedtedair. 
Of the fucceflive difeoveries relative to atmofpheric and 
other aerial fluids, we might here give an hiftorical ac¬ 
count, were it not more our objedt rather to detail facts 
themlelves, than dwell on the circumftances under which 
they were difeovered. We fhall however briefly obferve 
that Van Helmont was the firft who made difeoveries on 
this important fubjedt. Mr. Boyle followed his fteps ; and 
after him, Dr. Hales, and Profeffor Venel, contributed to 
the advancement of this important branch of philofophy. 
But their difeoveries at length were greatly eclipfed by 
our modern chemifts Dr. Black, Mr. Cavendilli, and Dr. 
Prieltley; but particularly the latter, to whom it was rc- 
ferved, to prove by the moft decifive tefts the compolition 
and nature of our atmofphere, which appears to be com- 
pofed of two fluids; the one abfolutely noxious, and inca¬ 
pable of fupporting animal life for a moment ; the other 
extremely falutary, and capable of preferving animals 
alive and healthy for a much longer time than the pureft 
air we can meet with, the properties of which we fhall 
now proceed to explain. 
Of Dephlogificatcd or Vital Air. 
This kind of air Dr. Prieftley names dephlogificated , 
from his opinion that common air, in the adt of burning, 
abforbs phlogifton. In the courfe of his inquires, he fell 
upon a method of extradfing it from a great variety of fub- 
ftances, viz. by moiftening them with fpirit of nitre, and 
then diftilling them with a ftrong heat. Thus he obtained 
it from flowers of zinc, chalk, quicklime, flaked lime, to¬ 
bacco-pipe clay, flint, Mufcovy talcs, and even glafs. 
He then found, that by Amply diffolving any metal in the 
nitrous acid, and then diftilling the folution, he could ob¬ 
tain very pure air: and Mr. Warltire found even the trou¬ 
ble of diftiilation unneceffary; nothing more being requi- 
fite than to moiften red lead with the fpirit of nitre, and 
then pour upon it the oil of vitriol, which inftantly dif- 
engaged the dephlogifticated air without applyingany more 
heat than what was generated by the mixture. 
But Mr. Scheele, without knowing of Dr. Prieftley’s 
difeovery, fell upon a method by which this air may be 
ftjll more readily procured, viz, by the diftiilation of ni¬ 
tre 
