Chemistry Reference
In-Depth Information
Fig. 1.2
Apparatuses of Galilee (
a
), Torricelli (
b
), and Pascal (
c
) to discover air pressure
thought experiment to measure this power, the “resistenza del vacuo” [
5
] in 1643
(see Fig.
1.2a
): a cylinder was supposed to contain water, a big weight of heavy
stones was to be attached to the movable piston until - besides the water - an empty
space formed in the cylinder. No one knows, whether this experiment was only
described on paper or if it was really performed [
5
].
This apparatus inspired his student Torricelli to use fluid mercury with high
density instead of constructing a cylinder with a movable piston. He filled a 1-m-
long glass tube with mercury, opened the tube under mercury in a glass bowl, and
observed mercury gliding down about 24 cm like a piston in a cylinder (see
Fig.
1.2b
). With this experiment, first described in 1643 [
5
], he could prove the
existence of air pressure: a 760-mm column of mercury pushes like about 20,000 m
of air. He could also prove that above this column an empty space exists, a vacuum:
by tilting the 1-m-long glass tube more horizontally, the glass tube fills totally,
tilting it vertically the mercury sinks again (see E.1.1). In honor of Torricelli, the
units to measure pressure were called torr: 1 Torr
¼
1 mm mercury column.
With the experiment “du vide dans le vide” (see Fig.
1.2c
), Pascal finally proved
that the apparatus does show the Torricelli vacuum and that there is no special “air”
in the empty space above the mercury column [
5
]. Various experiments in different
heights above sea level show that air pressure decreases with the altitude: on a
1,000 m high mountain the mercury column is 690 mm high. Subsequently, the first
mercury barometers were built.
On the basis of this knowledge, Guericke developed powerful air pumps and
demonstrated the air pressure in spectacular experiments with the “half-spheres of
Magdeburg” (see Fig.
1.3
): air was pumped out of one pair of half-spheres, eight
horses on one side and eight horses on the other side sometimes managed to pull
