1. Place a cylinder of strong glass, open at both ends, on the plate
of the air-pump, and put your hand on the other end, and you will of
course be able to remove it at pleasure. Now exhaust the air from the
interior of the cylinder, and at each stroke of the pump you will feel
your hand pressed tighter and tighter on the cylinder, until you will
not be able to remove it: as soon as the air is again admitted to the
interior of the cylinder, the pressure within will be restored, and the
hand again be at liberty.
2. Tie a piece of moistened bladder very firmly over one end of a
similar glass cylinder, and place the open end on the plate of the
pump. As soon as you begin to exhaust the air from the interior, the
bladder, which was previously quite horizontal, will begin to bulge
inwards, the concavity increasing as the exhaustion proceeds, until the
bladder, no longer able to bear the weight of the superincumbent air,
breaks with a loud report.
3. The elasticity of air, or indeed of any gaseous body, may be shown
by introducing under the air-pump receiver a bladder containing a very
small quantity of air, its mouth being closely tied. As you exhaust
the air from the receiver, that portion contained in the bladder being
no longer pressed upon by the atmosphere, will gradually expand,
distending the bladder until it appears nearly full: on readmitting the
air into the receiver, the bladder will at once shrink to its former
dimensions.
A shriveled apple placed under the same conditions will appear plump
when the air is removed from the receiver, and resume its former
appearance on the readmission of the air.
4. There is a very pretty apparatus made for the purpose of showing
the pressure of the atmosphere, consisting of a hollow globe of brass,
about three inches in diameter, divided into two equal parts, which
fit very accurately together. It is furnished with two handles; one of
them screwed into a hollow stem, communicating with the interior of the
globe, and fitting on to the air-pump; the other is attached to a short
stem on the opposite side of the globe. In the natural state the globe
may easily be separated into its two hemispheres by one person pulling
the handles, but after the air has been exhausted from the interior it
requires two very strong men to separate the parts, and they will often
fail to do so. By turning the stop-cock, and readmitting the air into
the interior of the globe, it will come asunder as easily as at first.
We are indebted to the weight of the atmosphere for the power we
possess of raising water by the common pump; for the piston of the pump
withdrawing the air from the interior of the pipe, which terminates in
water, the pressure of the atmosphere forces the water up the pipe to
supply the place of the air withdrawn. It was soon found, however, that
when the column of water in the pipe was more than thirty feet high,
the pump became useless, for the water refused to rise higher. Why?
It was found that a column of water about thirty feet high exerted a
pressure equal to the weight of the atmosphere, thus establishing an
equilibrium between the water in the pipe and the atmospheric pressure.
This is the principle on which the barometer, or _measurer of weight_,
as its name imports, is constructed. The metal Mercury is about
thirteen and a half times heavier than water; consequently, if a column
of water thirty _feet_ high balances the pressure of the atmosphere, a
column of mercury thirty _inches_ high ought to do also--and this is
in fact the case. If you take a glass tube nearly three feet long, and
closed at one end, and fill it with mercury; then, placing your finger
on the open end, invert the tube into a basin or saucer containing
some of the same metal; upon removing your finger (which must be
done carefully, while the mouth of the tube is completely covered by
the mercury), it will be seen that the fluid will fall a few inches,
leaving the upper part of the tube empty. Such a tube with a graduated
scale attached is in truth a barometer, and as the weight of the
atmosphere increases or decreases, so the mercury rises or falls in
the tube. This instrument is of the greatest value to the seaman, for
a sudden fall of the barometer will often give notice of an impending
storm when all is fine and calm, and thus enable the mariner to make
the preparations necessary to meet the danger.
It was discovered by an Italian philosopher named Torricelli, and from
him the vacuum formed in the upper end of the tube above the surface
of the mercury has been called the Torricellian vacuum. It is by far
the most perfect vacuum that can be obtained, containing necessarily
nothing but a minute quantity of the vapor of mercury.