e H E M I 
follow tlie piftori. Tlie aflion of this caufe is evidently 
limited, as appears by its fuftaining a column of no more 
than thirty-two-feet of water. If, therefore, it were to 
abl on a fluid fpecifically heavier than water, it ought to 
raile and luftain to a height inverfely as its fpeciiic gra¬ 
vity. From tliefe reflections he was induced to take a 
tube of glai's hermetically fealed at one end, and thirty- 
fix inches in length. He filled this with mercury, the 
doled end being downwards; then, ckffing the extremity 
with his finger, he railed the other end uppermoft, and 
plunged the unlealed end beneath the furface of a veflel 
of mercury. Upon removingiiis finger, he obferved the 
mercurial column to delcend, till after feveral ofcilla- 
tions its upper furface remained at twenty-eight inches 
above the furface of the mercury in the baton : and hence 
the invention of the barometer. By comparing this height 
with the height of thirty-two feet, to which water is 
railed in pumps, he found it correfponded accurately to 
the inverl'e ratio of the weights ; for the fpecific gravity 
of mercury and water being in round numbers, as four¬ 
teen to one, the mercury was found to Hand in the va¬ 
cuum at only one-fourteenth of the height of the water. 
It was not till after much meditation, that he lufpefted 
the weight of the air to be the caule of the fufpenfion of 
water in pumps; and this doCtrine was not incontrover- 
tibly eftablilhed until after the ingenious experiment of 
Pafcal. This celebrated philofopher imagined, that if 
water were fuftained at the height of thirty-two feet in 
pumps, and mercury at twenty-eight inches in the Tor¬ 
ricellian tube, by the lole gravity of the air, the heights 
of thele fluids ought to vary with that gravity; that 
they ought not, for example, to be the fame on the top 
of a mountain and in a 'valley, becaufe the length of a 
column of the atmolphere mult be Ihorter, and conle- 
quently its weight lels in the former than in the latter cafe. 
In purfuance of this idea of Pafcal, M. Perrier, on the 
*9lh of September, 1648, at the foot and at the fummit 
of the mountain Puits de Dome in Auvergne, made the 
famous experiment, which has for ever fixed the opinion 
of philofophers on this I'ubjeCt. The barometer, or Tor¬ 
ricellian tube filled with mercury, and fixed to a fcale of 
thirty-four inches, (hewed a fall or diminution of the 
mercurial column equal to four inches, in attending 
from the foot of the mountain to its fummit, which is 
five hundred toifes higher. By this it was al'certained, that 
the mercury varies about an inch for every hundred fa¬ 
thoms ; and this inflrument has fince that time been very 
fuccefsfully applied to meafure the height of mountains. 
The weight of the air has great influence on a number 
of phyfical and chemical phenomena. It compreffes all 
bodies, and oppoles their dilatation. It is an obftacle to 
the evaporation of fluids. The water of the fea is, by 
this caule, prelerved in its liquid Hate, without which it 
would take the vaporous form, as we fee in the vacuum 
of the air-pump. The preffure of the air on our bodies 
•preferves the date both of the lolids and fluids ; and 
from the want of this due preffure it is, that on the fum- 
mits of lofty mountains the blood often iffues from the 
pores of the Ikin, or from the lungs, and occailons he¬ 
morrhages. 
6. Its elallicity, by which it is capable of being very 
much condenfed, and fuddenly regains its former ftate 
when at liberty. A great ntimber of fafts prove the 
truth of this affertion. We lhall here mention one or 
two of the molt obvious and conclufive. If mercury be 
poured into a tube in the form of the letter U, and doled 
at one end, the air in the clofed end will contract in its 
dimenfions, in proportion as the quantity of mercury by 
which it is comprtffed is greater. The foot-ball of chil¬ 
dren, confifting of a bladder filled with wind, and en- 
clofed in leather, Ihews the fame elallicity, by its re¬ 
bounding when it falls on hard bodies. The fountain, 
by compreffed air, (hews the fame thing. This is a veflel 
half filled with water, and air is ftrongly compreffed iii- 
to its fuperior part: the re-a&ion of the air on the water 
Vox.. IV. No. 189. 
S T R Y. 107 
forces it out to a confiderable height through a tube. 
A withered apple, put under the receiver of.an-air-pump, 
and the air fucked out, becomes plump and looks frelh ; 
but, when air is re-admitted, it becomes as before. Fillies 
and birds lhew the elallicity of the air: the fifli has re¬ 
ceived from nature different modes of action ; its phyfi¬ 
cal means are the bladder, which it has the power of 
compreffmg or dilating, to rife in the water or to dc- 
fcend; its tail, which is very mufcular, forms a point of 
refinance againlt the water. It is eftimated that air may 
be compreffed into the 128th of its ulual volume. 
Keat producing a contrary efteCt to that of compref- 
fion upon air, ferves to lhew, that its volume may be ex¬ 
ceedingly augmented by the increafe of its Ip ring. When 
a bladder full of air is expofed to the heat of a furnace, 
the air is dilated fo as to burlt the bladder with an ex- 
plolion. This phenomenon is partly the occafion of the 
burfting of chemical veffels, which often happens where 
due precautions are not taken to prevent it. The ab- 
fence of the preffure of the atmolphere, or the total ab- 
ltraClion of the circumambient air from beneath the re¬ 
ceiver of an air-pump, caufes a bladder enclofed therein 
to burlt by the fpring of the included air, which then 
aCts without opposition. 
From this account of the gravity and the elallicity of 
the air, it may be readily inferred, that thele properties 
are the leading caufes of the numerous atmolpherical 
changes, and the variations in the mercurial column in 
the barometer. In fa<St, the inferior llrata of the atmof- 
phere mull luftain the weight of the air above them, and 
are therefore in a ftate of compreliion, which diminifhes 
with the greater elevation of places : and the continual 
change of temperature mult alfo greatly affedt the gravity 
of the air, by augmenting or diminifhing its elallicity. 
Thus, as we have already noticed, the air is lighter, 
keener, and more agitated on the tops of mountains 
than in lower regions ; and it is only from the conlide- 
ration of the combined efftdls of the heat, gravity, and 
elallicity, of the atmolphere, that the. barometrical 
changes can be accounted for. M. de Luc and M. de 
Saullure have paid great attention to this important fub- 
jedt for fome years pall: and the barometrical meafure- 
ment of elevations has been well treated of, both prac¬ 
tically and fcientifically, by fir George Shuckburgh and 
colonel Roy, in the 77th vol. of the Philolophical Tranl- 
adlions. 
The chemical properties of air, come next under con- 
fideration. Van Helnjont, Boyle, and Hales, having per¬ 
ceived that air, or at leall a fluid poffeffmg all its appa¬ 
rent properties, was obtained from many natural fub- 
ftaitces, adopted the opinion that this element combines 
with, and becomes fixed in, bodies. Such is the origin 
of the term fixed air, which was given to the elaltic fluids 
obtained in chemical operations. The early philofophers 
fuppofed thefe fluids to be air; but the difeoveries of 
Dr. Priellley have fliewn, that there are many bodies 
which have the phyfical properties of air, though they 
dilfer from it effentially in many refpedls. It is, there¬ 
fore, neceflary to attend to thofe other properties, in or¬ 
der to diftinguilh air from other aeriform fluids, which 
relemble it in invifibility and elallicity. Thele properties 
are chemical; and the experiments which confirm the 
chemical properties of air, are thole which are made for 
analyling this fluid. 
Take a bell-glals of a given height; turn it down in 
a faucer half full of wafer, in the middle of which fix a 
taper on an iron-wire ; the flame prelently (brinks, turns 
blue, and goes out; the water in the faucer rifes near 
one-fourth up the giafs. This experiment will be more 
curious, if we place in the faucer feveral lighted tapers 
of different heights; they will be extinguilhed in luc- 
ceflion, beginning with the tallcft. Thefe experiments 
prove at once, that atmolpherical air is compoled of 
two elaltic fluids, one of which maintains combultion, 
and another which cannot. 
S 2- 
Sulphat 
