PNEUMATICS. 
Aoclv, loses more of its real weight than the 
lead does; and therefore must in fact be 
heavier, to balance it under the disadvantage 
of losing some of its weight ; which disad- 
vantage being taken ol! by removing the air, 
the bouies then gravitate according to their 
real quantities ot matter, and the cork which 
balanced the lead in air,, shews itself to be 
heavier when in vacuo. 
Set a lighted candle upon the pump, and 
cover it with a tall receiver. If the receiver. 
’ holds a gallon, the candle will burn a rni- 
I nute ; and then, after having gradually de- 
j caved from the first instant, it will go out, 
, which shews that a constant supply of fresh 
i air is as necessary to feed llame, as animal 
j life. 
The moment when the candle goes out, 
; the smoke will be seen to ascend to the top 
of the receiver, and there it will form a sort 
| of cloud; but upon exhausting the air, the 
| smoke will fail down to the bottom of the 
receiver, and leave it as clear at the top as 
I it was before it was set upon the pump. 
1 This shews that smoke floes not ascend on 
1 account of its being positively light, but 
because it is lighter than air; and its falling 
to tlie bottom when the air is taken away, 
shews that it is not destitute of weight. 
,So most sorts of wood ascend or swim in 
water; and yet there are none who doubt of 
the wood’s having gravity or weight. 
Set a bell oa the pump- plate, having a 
contrivance so as to ring it at pleasure, anti 
cover it with a receiver ; then make the 
f clapper strike against the bell, and the sound 
will be very well heard ; but exhaust the 
receiver of air, anti then, if the clapper is 
made to strike ever so hard against the bell, 
it will make no sound; which shews that 
air is absolutely necessary lor the propaga- 
tion of sound. 
Of condensed air. It has been shewn, 
that air can be rarefied, or made to expand : 
we now proceed to shew that it can also be 
condensed, or pressed into less space than it 
generally occupies. 'The instrument used 
for this purpose is called a condenser. 
Fig. 12, represents a machine of this kind ; 
it consists of a brass barrel containing a pis- 
i ton, which has a valve opening downwards ; 
so that as the piston is raised, the air passes 
through. the-d , alve ; but as the piston is pushed 
down, the air cannot return, and is therefore 
forced through a valve at the bottom of tire 
barrel, that allows it to pass through into the 
receiver 13, but prevents it from returning. 
Thus, at every stroke of the piston, more air 
is throw'll into the receiver, which is of very 
thick and strong glass. The receiver is held 
j down upon the plate C by the cross piece 
D, and the screws EF. . The air is let out 
of the receiver by the cock G, which com- 
municates with it. 
A great variety of experiments may be 
performed by means of condensed air, a few 
of which we shall here enumerate. 
The sound of a bell is much louder in 
I condensed than in common air. 
A phial that would bear the pressure of 
the common atmosphere, when the air is 
exhausted from the inside, will be broken by 
condensing the air round it. 
A very beautiful fountain may be made 
by condensed air. Procure a strong copper 
vessel, fig. 13, having a tube that screws 
into the neck of it so as to be air-tight, and 
long enough to reach to near the bottom. 
Having poured a quantity of water into the 
vessel, but not enough to fill it, and screwed 
in the tube, adapt to it a condensing syringe, 
and condense the air in the vessel ; shut the 
stop-cock, and unscrew the syringe ; then, on 
opening the stop-cock, the air acting upon 
the water in the vessel, will force it out into 
a jet of very great height. A number of 
different kinds of jets may be screwed on the 
tube, such as stars, wheels, &c. forming a 
very pleasing appearance. 
Dr. Hook invented the gage, or instru- 
ment for measuring the degree of rarefac- 
tion, or exhaustion, produced in the receiver, 
and which is a necessary appendage to the 
air-pump. If a barometer is included be- 
neath the receiver, the mercury will stand at 
the same height as in the open air ; but when 
the receiver begins to be exhausted, the 
mercury will descend, arid r e-t at a height 
which E, in proportion to its firmer 
height, as the spring of the air remaining in 
the receiver, to its spring before exhaus- 
tion. Thus, if the height of the mercury, 
after exhaustion, is the thousandth part of 
what it was before, we say that the air in the 
receiver is rarefied 1000 times. On account 
of the inconvenience of including a barome- 
ter in a receiver, a tube, of six or eight 
inches in length, is filled with mercury, and 
inverted in the same manner as the baro- 
meter. 'Phis being included, answers the 
same purpose, with no other difference, than 
that the mercury does not begin to descend 
till about three-fourths of the air is exhaust- 
ed ; it is called the short barometer-gage. 
'Fiiis is generally placed detached, but com- 
municating with the receiver by a tube con- 
cealed in the frame, as is represented at fig. 
1. Others place a tube of a greater length 
than the barometer, with its lower end in a 
vessel of mercury, exposed to the pressure 
of the air, while its upper end communi- 
cates with the receiver. Here the mercury 
rises as the exhaustion proceeds, and the 
pressure of its remaining air is shewn by 
the difference between the height and that 
of a barometer in the room : this is called 
the long barometer-gage. 
These gages are not often constructed so 
as to answer the purpose of shewing great 
degrees of exhaustion ; for the mercury, 
though at first boiled to clear it of the air 
and moisture that adhere to it, and render 
it sensibly lighter, gradually becomes again 
contaminated by exposure to the air in the 
bason of either gage. They cannot, there- 
fore, in strictness, be compared to a good 
barometer, in which this does not happen. 
If the tubes of the gages are less than half an 
inch in diameter, the mercury will be sensi- 
bly repelled downwards, so as to require a 
correction for the long gage when compared 
with a barometer whose tube is of a differ- 
ent bore, and to render the short gage use- 
less in great exhaustions. 
Thus, for example, if the short gage has 
a tube of one-tenth of an inch in diameter, 
the mercury will fall to the level of the 
bason, when the exhaustion is 150 times, and 
will stand below the level for all greater de- 
grees of rarefaction. These difficulties may 
be all removed, by making the short gage in 
the form of an inverted syphon, with one leg 
open, and the other hermetically sealed. 
3 M 2 
4 A) 
It must be confessed, however, that it is dif- 
ficult to boil the mercury in these. 
In using the air-pump, every substance 
containing moisture should be removed from 
the pump-plate, as water assumes the form 
of an elastic vapour when the pressure of the 
atmosphere is taken away. The receivers 
used formerly to be placed upon the pump- 
plate, on leathers soaked in water or oil ; 
but Mr. Nairne discovered that an elastic 
vapour arose from this, that considerably af- 
fected the gage, and prevented it from shew- 
ing the real degree of rarefaction of the air. 
Instead of the leathers to place under the re- 
ceiver, the best way is, to have the pump- 
plate ground perfectly flat, and also the edge 
of the receiver, which should be rubbed with 
a little hog’s-lard or soft pomatum, which 
will perfectly exclude the air, and will not 
afford any moisture. The pump-plate and 
the receiver should be wiped very clean. 
When leathers are used, the barometer- 
gages will not shew the degree of rarefac- 
tion of the air; which, however, may be as- 
certained by a gage invented by Mr. Smea- 
| ton, and called, from its form, the pear-gage, 
j It consists of a glass vessel, in the form of 
a pear, fig. 1 1, and sufficient to hold about 
half a pound of mercury : it is open at one 
end, and at the other end is a tube herme- 
tically closed at top. The tube is graduated, 
so as to represent proportionate parts of the 
whole capacity. This gage, during the ex- 
haustion of the receiver, is suspended in it 
by a slip of wire, over a cistern of mercury, 
placed also in the receiver. When the pump 
is worked as much as is thought necessary, 
the gage is I t down into the mercury, and 
the air re-admitted. The mercury will im- 
mediately rise in the gage : but if any air 
remained in the receiver, a certain portion of 
it would be in the gage; and as it would 
occupy the top of the tube above the mer- 
cury, it would shew by its size the degree 
of exhaustion ; for the bubble of air would 
be to the whole contents of the gage, as the 
quantity of air in the exhausted receiver 
would to an equal volume of the common at- 
mospheric air. If the receiver contained 
any elastic vapour generated during the 
rarefaction, it would be condensed upon the 
re-admission of the atmospheric air, as it can- 
not subsist in the usual pressure. The pear- 
gage, therefore, shews the true quantity of 
atmospheric air left in the receiver. Hence 
it will sometimes indicate that all the per- 
manent air is exhausted from the receiver, 
except about part, when the other 
gages do not shew a degree of exhaustion 
of more than 200 times, and sometimes much 
less. 
Particular care should be taken, after mak- 
ing any experiments where vapour lias been 
generated, to clear the pump of it, before 
any other experiments are attempted ; for 
the vapour remains not only in the receiver, 
but also in the tubes and barrels of the 
pump, and will, when the air is again rare- 
fied, expand as before. To clear the pump 
of this vapour, take a large receiver, and 
wiping it very dry, exhaust it as far as 
possible. The expansible vapour which re- 
mained in the barrels and the pipes, will now 
be diffused through the receiver ; and, con- 
sequently, will be as much rarer than it was 
before, as the aggregate capacity of the re- 
