straw or a piece of glass tube down into
a glass of water. Hold your finger tightly over the upper end,
and lift the tube out of the water. Notice how the water stays
in the tube. Now remove your finger from the upper end.

The air holds the water up in the tube because there is no room for
it to bubble up into the tube to take the place of the water; and the
water, to flow out of the tube, would have to leave a vacuum, which
the air outside does not allow. But when you take your finger off the
top of the straw or tube, the air from above takes the place of the
water as rapidly as it flows out; so there is no tendency to form a
vacuum, and the water leaves the tube. Now do you see why you make two
holes in the top of a can of evaporated milk when you wish to pour the
milk out evenly?

[Illustration: FIG. 5. The water is held in the tube by air pressure.]

EXPERIMENT 4. Push a rubber suction cap firmly against the
inside of the bell jar of an air pump. Try to pull the suction
cap off. If it comes off, press it on again; place the bell
jar on the plate of the air pump, and pump the air out of the
jar. What must have been holding the suction cap against the
inside of the jar? Does air press up and sidewise as well as
down? Test this further in the following experiment:

[Illustration: FIG. 6. An air pump.]

EXPERIMENT 5. Put a cork into an empty bottle. Do not use a
new cork, but one that has been fitted into the bottle many
times and has become shaped to the neck. Press the cork in
rather firmly, so that it is air-tight, but do not jam it in.
Set the bottle on the plate of the air pump, put the bell jar
over it, and pump the air out of the jar. What makes the cork
fly out of the bottle? What was really in the "empty" bottle?
Why could it not push the cork out until you had pumped the
air out of the jar?

EXPERIMENT 6. Wax the rims of the two Magdeburg hemispheres
(see Fig. 7). Screw the lower section into the hole in the
plate of the air pump. Be sure that the stop valve in the
neck of the hemisphere is open. (The little handle should be
vertical.) Fit the other section on to the first, and pump out
as much air as you can. _Close_ the stop valve. Unscrew the
hemispheres from the air pump. Try to pull them apart--pull
straight out, taking care not to slide the parts. If you wish,
let some one else take on