Encyclopaedia Britannica > Volume 15, PLA-RAM

P N E U M
dually dimmlfhes as the water Is expended and room
11 given to the air to expand itfelf. See the figure.
An e od We judge of the condenfation of air in the veflel E
©f jut ng by the number of ftrokes and the proportion of the
«f «hj on- capacity 0f the fyringe to that of the vefiel. Suppofe
,1enfa n, the firft to be one-tenth of the laft; then we know,
that after 10 ftrokes the quantity of air in the veffel is
doubled, and therefore its denfxty double, and fo on af¬
ter any number of ftrokes. Let the capacity of the fy¬
ringe (when the pifton is drawn to the top) be a, and
that of the veftel be b, and the number of ftrokes be «,
&c.
the denfity of air in the veffel will be
. n a
or
t
No- j
rate.
: c.-.|
"C1
f
cl
I
I
ly I
iioB
Ifbe :r
jetl i.
Bvit this is on the fuppofition that the pifton accu-
toftlj ccu-rately fills the barrel, the bottom of the one applying
clofe to that of the other, and that no force is neceffary
for opening either of the valves : but the firft cannot
be infured, and the laft is very far from being true. In
the conftru&ion now defcribed, it will require at leaft
one twentieth-part of the ordinary preffure of the air
to open the pifton valve : therefore the air which gets
in will want at leaft this proportion of its complete
elafticity ; and there is 'always a fimilar part of the
elafticity employed in opening the nozzle valve. The
condenfation therefore is never nearly equal to what is
here determined.
It is accurately enough meafured by a gage fitted to,
the inftrument. A glafs tube GH of a cylindric bore,
and clofe at the end* is fere wed into the fide of the cap
on the mouth of the veffel E. A fmall drop of water
6r mercury is taken into this tube by warming it a little
in the hand, which expands the contained air, fo that
when the open end is dipped into water, and the whole
allowed to cool, the water advances a little into the tube.
The tube is furnifhed with a fcale divided into fmall
Cquab parts, numbered from the clofe end of the tube.
Since this tube communicates with the veffel, it is evi¬
dent that the condenfation will force the water along
the tube, a&ing like a pifton on the air beyond it, and
die air in the tube and veffel will always be of one den¬
fity. Stippofe the number at which the drop ftands
before the condenfation is made to be r, and that it
ftands at d when the condenfation has attained the de¬
gree required, the denfity of the air in the remote end
of the gage, and confequently in the veffel, will be
c
the
naJ-1
top-1
top; I
tk L Ji .
,, p ' nation
ifM Ki it!
y of I
:«• I
•tain I
;adi I
veil
>
,tiril Fyr: ;es
, lor eat
" *cn ufa.
tioi
Sometimes there is ufed any bit of tube clofe at one
end, having a drop of water in it, fimply laid into the
vefl’el E, and furnifhed or not with a fcale : but this
can only be ufed with glafs veffels, and thefe are too
weak to refill the preffure arifing from great condenfa-
lion. In fuch experiments metalline veffels are ufed,
ntted with a variety of apparatus for different experi-
nunts. Some of thefe will be occafionally mentioned
afterwards.
It muft be obferved in this place, that very great con-
denfations require great force, and therefore fmall fy-
nnges. It is therefore convenient to have them of va-
nouo izes, and to begin with thofe of a larger diame-
i °Perate more quickly ; and when the con-
afmalle^1 beC°meS fat%uIng> ^ change the fyringe for
A T I C S. <J3
For this reafon, and in general to make the conden- 67
fing apparatus more convenient, it is proper to have a
ftop-cock interpofed between the fyringe and the veffel, :ween t£c
or as it is ufually called the receiver. This confifts of a fyringe and
brafs piper which has a well-ground cock in its middle, •eceivu .
and has a hollow ferew at one end, which receives the
nozzle ferew of the fyringe, and a folid ferew at the
other end, which fits the ferew of the receiver. See
fig. 3. , 6f
By thefe gages, or contrivances fimilar to them, we Inftancc* of
have been able to afeertain veiy great degrees of con- ^re^.t fon-
denfation in the courfe of fome experiments. Dr Hales * !uva,10:1
found, that when dry wood was put into a ftrong veffel,
which it almofl filled, and the remainder was filled with
water, the fwelling of the wood, occafioned by its im¬
bibition of water, condenfed the air of his gage into the
thoufandth of its original bulk. He found that peafe
treated in the fame way generated elaftic air, which
prefling on the air in the gage condenfed it into the
fifteen hundredth part of its bulk. This is the greateft
condenfation that has been afcertahied wi th 1 irecifion,
although in other experiments it has certainly been car¬
ried much farther; but the precife degree could not be
afeertained. ^.
The only ufe to be made of this obfervation at pre- Air and
fent is, that fince we have been able to exhibit air in a water tn he ‘
denfitv a thoufand times greater than the ordinary den-
r c , , ,0. r . 1 . dinerent;
iity or the air we breathe, it cannot, as lome imagine,
be only a different form of water ; for in this ftate it
it is as denfe or denfer than water, and yet retains its
great expanfibility. -0
Another important obfervation is, that in every ftate And Ihow
of deniity in which we find' it, it retains its perfectt,lc error of
fluidity, tranfmitting all preffures which are applied to |““ie °‘ji'
it with undiminiftied force, as appears by the equality
conftantly obferved between the oppofing columns of elafticity, \
water or other fluid by which it is compreffed, and by
the facility with which all motions are performed in it
in the moft compreffed ftates in which wre can make ob-
fervations of this kind. This fail is totally incompa¬
tible with the opinion of thofe wdio aferihe the elaftici¬
ty of air to the fpringy ramified ftru&ure of its particles,
touching each other like fo many pieces-of fponge or
foot-balls. A collection of fuch particles might indeed
be pervaded by folid bodies with confiderable eafe, iff
they were merely touching each other, and not fubjeCt-
ed to any external preffure. But the moment fuch
preflure is exerted, and the aflemblage fqueezed into a
fmaller fpace, each preffes on its adjoining particles :
they are individually compreffed, flattened in their touch¬
ing furfaces, and before the denfity is doubled they art-
fqueezed into the form of perfeCt cubes, and compofe a
mafs, which may indeed propagate preffure from one
place to another in an imperfeCt manner, and with great
diminution of its intenfity, but will no more be fluid
than a rnafs of foft clay. It will be of ufe to keep this
obfervation in mind.
We have feen that air is heavy and compreffihle, and Confequen- -
might now proceed to deduce in order the explanation ces of the
of the appearances confequent on each of thefe pro-ait.8 elai"
perties. But, as has been already obferved, the elafti-UCl i*
city of air modifies the effeCls of its gravity fo remark¬
ably, that they woilld be imperfectly underftood if both
qualities were not combined in our confideration of ei¬
ther. At aoy rate, fome farther confequencea of its elai-
tieity ?