Encyclopaedia Britannica > Volume 15, PLA-RAM

*04 .P.N E U M
Air-pump, Even mercury evaporates in this manner when all prcflure
^—v ' is removed. A dewy appearance is frequently obferved
coverin'* the irifide of the tube of a barometer, where
we ufually fuppofe a vacuum. This dew, when viewed
through a microfcope, appears to be a fet of detached
globules of mercury, and upon inclining the tube fo
that the mercury may afcend along it, thefe globuies
will be all licked up, and the tube become clear. The
dew which lined it was the vapour of the mercury con-
denfed by the fide of the tube; and it is never obferved
but when one fide is expofed to a ftream of cold air
from a window, &c.
To return to the vapour in the air-pump receiver, it
mu ft be obferved, that as long as the water continues to
yield it, we may continue to work the pump ; and it
will be continually abftracled by the barrels, and dif-
charged in the form of water, becaufe it collapfes as
.foon as expofed to the external preffure. All this
while the gage will not indicate any more rarefaction,
.becaufe the thing immediately indicated by the baro¬
meter-gage is diminifhed elajlicity, which does not hap¬
pen here. When all the water which the temperature
of the room can keep elaftic has evaporated under a
certain preiTure, fuppofe i an inch of mercury, the gage
/landing at 29.5, the vapour which now fills the re¬
ceiver expands, and by its diminifhed elafticity the gage
rifes, and now fome more water which bad been attach¬
ed to bodies by chemical or corpufcuhr attraction is de¬
tached, and a new fupply continues to fupport the gage
at a greater height; and this goes on continually till
a!mo ft all has been abftraCted : but there will remain fome
which no art take can away ; for as it partes through
the barrels, and gets between the pifton and the top, it
fucceffively collapfes into water during the afeent of the
pillon, and again expands into vapour when we pufb
the pifton down again. Whenever this happens there
is an end of the rarefaCtion.
Airland While this operation is going on, the air comes out
■♦apflur not along with the vapour ; but we cannot fay in what pro-
uniformly p'ortftjn< Jf ft were always uniformly mixed with the
mixed to- vapour> ft wouia diminifh rapidly ; but this does not
appear to be the cafe. There is a certain period of
rarefaftion in which a tranfient cloudinefs is perceived
in the receiver. This is watery vapour formed at that
degree of rarefaCtion, mingled with, but not diflolved in
or united with, the air, otherwife it would he tranfpa-
rent. A fimilar cloud will appear if damp air be ad¬
mitted fuddenly into an exhaufted receiver. The va¬
pour, which formed an uniform tranfparent mafs with
the air, is either fuddenly expanded and thus detached
from the other ingredient, or is fuddenly let go by the
air, which expands more than ft does. We cannot af¬
firm with probability which of thefe is the cafe : diffe¬
rent compofitions of air, that is, air loaded with vapours
from different fubflances, exhibit remarkable differen¬
ces in this refpeCt. But we fee from this and other
phenomena, which (hall he mentioned in their proper
places, that the air and vapour are not always intimately
•united ; and therefore will not always be drawn out to¬
gether by the air-pump. But let them be ever fo con-
fufedly blended, we fee that the air mufl come out along
with the vapour, and its quantity remaining in the re¬
ceiver muff be prodigioufly diminirtied by this aflbeia-
tion, probably much more than could be, had the receiver
only contained pure air.
gether.
A T I C S.
Let us now confider what mufl happen in the pear* Air.puni|
gage. As the air and vapour are continually drawn off
from the receiver, the air‘in the pear expands and goes
off with it. We (hall fuppofe that the generated va-^ofThj
pour hinders the gage from rifing beyond 29.5. Du-different
ring the continued working of the pump, the air in^P«r
the pear, vvhofe elalticity is 0.5, flowly mixes withmet£r
the vapour at the mouth of the pear, and the mix- giges.
ture even advances into its infide, fo that if the
pumping be long enough continued, what is in the
pear is nearly of the fame compofition with what is
in the receiver, confiding perhaps of 20 parts of vapour
and one part of air, all of the elaflicity of 0.5.
When the pear is plunged into the mercury, and the
external air allowed to get into the receiver, the mercu¬
ry rifes in the pear-gage, and leaves not
I-, but-1
60 60X20
or — * of it filled with common air, the vapour ha-
1200
Thus
ving collapfed into an inviiible atom of water,
the pear-gage will indicate a rarefaction of 1200, while
the barometer-gage only Ihowed 60, that is, fhowed
the elafticity of the included fubftance diminirtied 60
times. The conclufion to be drawn from thefe two
meafures (the one of the rarefaCtion of air, and the
other of the diminution of elafticity) is, that the matter
with which the receiver was filled, immediately before
the readmiflion of the air, confifted of one part of in-
condenfible air, and , or 20 parts of water)' vapour.
60
W
The only obfeure part of this account is what relates DifEcul
to the compofition of the matter which filled the pear-|nar“l
gage before the admiflion of the mercury. It is not eafyj”° ecf
to fee how the vapour of the receiver comes in by anar-thefeco
rowmouth while the air is coming out by the fame pafiage. kqueue
Accordingly it requires a very long time to produce this
extreme rarefaftion in the pear-gage ; and there are
great irregularities in any two fucceeding experiments,
as may be feen by looking at Mr Nairne’s account of
them in Philofophical TranfaClions, Vol. LXVII. Some
vapours appear to have mixed much more readily with
the air than others ; and there are fome unaccountable
cafes where vitriolic acid and fulphureous bodies were
included, in which the diminution of denfity indicated by
the pear-gage was uniformly lefs than the diminution of
elafticity indicated by the barometer-gage. It is enough
for us at prefent to have eftabliftied, by unqueftionable
fafts, this produftion of elalllo vapour, and the necef-
lity of attending to it, both in the conftruftion of the ,
air-pump and in drawing rcfults from experiments exhi¬
bited in it. r4g
Mr Smeaton’s pump, when in good order, and per-The fu
feftly free from all moifture^ will in dry weather rarefy”'^1)'
air about 600 times, raifingthe barometer-gage to with-
in 4ts of an inch of a fine barometer. This was a per-neW
formance fo much fuperior to that of all others, and by prove-
means of Mr Nairne’s experiments opened fo new a field meB^
of obfervation, that the air-pump once more became a ca- .
pital inftrument among the experimental philofbphers.
The caufes of its fuperiority were alfo fo diftinft, that
artifts were immediately excited to a farther improvement
of the machine; fo that this becomes a new epoch in its
hiftory.
This is one imperfeftion which Mr Smeaton has not
attempted to remove. The difeharging valve is ftill open¬
ed