e handle, and see if the two of you
can pull it apart.

[Illustration: FIG. 7. The experiment with the Magdeburg hemispheres.]

Before you pumped the air out of the hemisphere, the compressed air
inside of them (you remember all the air down here is compressed) was
pushing them apart just as hard as the air outside of them was pushing
them together. When you pumped the air out, however, there was hardly
any air left inside of them to push outward. So the strong pressure of
the outside air against the hemispheres had nothing to oppose it. It
therefore pressed them very tightly together and held them that way.

This experiment was first tried by a man living in Magdeburg, Germany.
The first set of hemispheres he used proved too weak, and when the air
in them was partly pumped out, the pressure of the outside air crushed
them like an egg shell. The second set was over a foot in diameter
and much stronger. After he had pumped the air out, it took sixteen
horses, eight pulling one way and eight the opposite way, to pull the
hemispheres apart.

EXPERIMENT 7. Fill a bottle (or flask) half full of water.
Through a one-hole stopper that will fit the bottle, put a
bent piece of glass tubing that will reach down to the bottom
of the bottle. Set the bottle, thus stoppered, on the plate of
the air pump, with a beaker or tumbler under the outer end of
the glass tube. Put the bell jar over the bottle and glass,
and pump the air out of the jar. What is it that forces the
water up and out of the bottle? Why could it do this when the
air was pumped out of the bell jar and not before?

HOW A SELTZER SIPHON WORKS. A seltzer siphon works on the same
principle. But instead of the ordinary compressed air that is all
around us, there is in the seltzer siphon a gas (carbon dioxid)
which has been much more compressed than ordinary air. This strongly
compressed gas forces the seltzer water out into the less compressed
air, exactly as the compressed air in the upper part of the bottle
forced the water out into the comparative vacuum of the bell jar in
Experiment 7.

EXPERIMENT 8. Fill a toy balloon partly full of air by blowing
into it, and close the neck with a rubber band so that no air
can escape. Lay a saucer over the hole in the plate of the air
pump, so that the rubber of the balloon cannot be sucked down
the hole. Lay the balloon on top of this saucer, put the bell
jar