Does a balloon explode if it goes high in the air?
What is suction?
Why does soda water run up a straw when you draw on the straw?
Why will evaporated milk not flow freely out of a can in which
there is only one hole?
Why does water gurgle when you pour it out of a bottle?
We are living in a sea of compressed air. Every square inch of our
bodies has about 15 pounds of pressure against it. The only reason
we are not crushed is that there is as strong pressure inside of our
bodies pushing out as there is outside pushing in. There is compressed
air in the blood and all through the body. If you were to lie down
on the ground and have all the air pumped out from under you, the air
above would crush you as flat as a pancake. You might as well let a
dozen big farm horses trample on you, or let a huge elephant roll over
you, as let the air press down on you if there were no air underneath
and inside your body to resist the pressure from above. It is hard to
believe that the air and liquids in our bodies are pressing out with
a force great enough to resist this crushing weight of air. But if you
were suddenly to go up above the earth's atmosphere, or if you were to
stay down here and go into a room from which the air were to be pumped
all at once, your body would explode like a torpedo.
When you suck the air out of a bottle, the surrounding air pressure
forces the bottle against your tongue; if the bottle is a small one,
it will stick there. And the pressure of the air and blood in your
tongue will force your tongue down into the neck of the bottle from
which part of the air has been taken.
In the same way, when you force the air out of a rubber suction cap,
such as is used to fasten reading lamps to the head of a bed, the air
pressure outside holds the suction cap tightly to the object against
which you first pressed it, making it stick there.
We can easily experiment with air pressure because we can get almost
entirely rid of it in places and can then watch what happens. A place
from which the air is practically all pumped out is called a _vacuum_.
Here are some interesting experiments that will show what air pressure
does:
[Illustration: FIG. 4. When the point is knocked off the electric
lamp, the water is forced into the vacuum.]
EXPERIMENT 2. Hold a burned-out electric lamp in a basin of
water, break its point off, and see what happens.
All the common electric lamps (less than 70 watts) are made with
vacuums inside. The reason for this is that the fine wire would burn
up if there were any air in the lamps. When you knock the point off
the globe, it leaves a space into which the water can be pushed. Since
the air is pressing hard on the surface of the water except in the
one place where the vacuum in the lamp globe is, the water is forced
violently into this empty space.
It really is a good deal like the way mud comes up between your toes
when you are barefoot. Your foot is pressing on the mud all around
except in the spaces between your toes, and so the mud is forced up
into these spaces. The air pressure on the water is like your foot
on the mud, and the space in the lamp globe is like the space between
your toes. Since wherever there is air it is pressing hard, the only
space into which it can force water or anything else is into a place
from which all the air has been removed, like the inside of the lamp
globe.
The reason that the water does not run out of the globe is this:
the hole is too small to let the air squeeze up past the water, and
therefore no air can take the place of the water that might otherwise
run out. In order to flow out, then, the water would have to leave an
empty space or vacuum behind it, and the air pressure would not allow
this.
WHY WATER GURGLES WHEN IT POURS OUT OF A BOTTLE. You have often
noticed that when you pour water out of a bottle it gurgles and gulps
instead of flowing out evenly. The reason for this is that when
a little water gets out and leaves an empty space behind, the air
pushing against the water starts to force it back up; but since the
mouth of the bottle is fairly wide, the air itself squeezes past the
water and bubbles up to the top.
EXPERIMENT 3. Put a 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 one 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 over it, and pump the air out of the jar. What makes the
balloon expand? What is in it? Why could it not expand before
you pumped the air out from around it?
A toy balloon expands for the same reason when it goes high in the
air. Up there the air pressure is not so strong outside the balloon,
and so the gas inside makes the balloon expand until it bursts.
[Illustration: FIG. 8. A siphon. The air pushes the water over the
side of the pan.]
EXPERIMENT 9. Lay a rubber tube flat in the bottom of a pan of
water, so that the tube will be filled with water. Let one end
stay under water, but pinch the other end tightly shut with
your thumb and finger and lift it out of the pan. Lower this
closed end into a sink or empty pan that is lower than the
pan of water. Now stop pinching the tube shut. This device is
called a _siphon_ (Fig. 8).
EXPERIMENT 10. Put the mouth of a small syringe, or better, of
a glass model lift pump, under water. Draw the handle up. Does
the water follow the plunger up, stand still, or go down in
the pump?
When you pull up the plunger, you leave an empty space; you shove the
air out of the pump or syringe ahead of the plunger. The air outside,
pressing on the water, forces it up into this empty space from which
the air has been pushed. But air pressure cannot force water up even
into a perfect vacuum farther than about 33 feet. If your glass pump
were, say, 40 feet long, the water would follow the plunger up for a
little over 30 feet, but nothing could suck it higher; for by the time
it reaches that height it is pushing down with its own weight as hard
as the air is pressing on the water below. No suction pump, or siphon,
however perfect, will ever lift water more than about 33 feet, and
it will do well if it draws water up 28 or 30 feet. This is because a
perfect vacuum cannot be made. There is always some water vapor formed
by the water evaporating a little, and there is always a small amount
of air that has been dissolved in water, both of which partly fill the
space above the water and press down a little on the water within the
pump.
[Illustration: FIG. 9. A glass model suction pump.]
If you had a straw over 33 feet long, and if some one held a glass of
lemonade for you down near the sidewalk while you leaned over from
the roof of a three-story building with your long straw, you could
not possibly drink the lemonade. The air pressure would not be great
enough to lift it so high, no matter how hard you sucked,--that is, no
matter how perfect a vacuum you made in the upper part of the straw.
The lemonade would rise part way, and then your straw would be
flattened by the pressure outside.
Some days the air can force water up farther in a tube than it can
on other days. If it can force the water up 33 feet today, it will
perhaps be able to force it up only 30 feet immediately before a
storm. And if it forces water up 33 feet at sea level, it may force it
up only 15 or 20 feet on a high mountain, for on a mountain there
is much less air above to make pressure. The pressure of the air is
different in different places; where the air is heavy and pressing
hard, we say the pressure is _high_; where the air is light and not
pressing so hard, we call the pressure _low_. A place where the air is
heavy is called an area of high pressure; where it is light, an area
of low pressure. (See Section 44.)
WHAT MAKES WINDS? It is because the air does not press equally all
the time and everywhere that we have winds. Naturally, if the air is
pressing harder in one place than in another, the lower air will be
pushed sidewise in the areas of high pressure and will rush to the
areas where there is less pressure. And air rushing from one place to
another is called _wind_.
[Illustration: FIG. 10.]
_APPLICATION 4._ A man had two water reservoirs, which stood
at the same level, one on each side of a hill. The hill
between them was about 50 feet high. One reservoir was full,
and the other was empty. He wanted to get some of the water
from the full reservoir into the empty one. He did not have a
pump to force the water from one to the other, but he did have
a long hose, and could have bought more. His hose was long
enough to reach over the top of the hill, but not long enough
to go around it. Could he have siphoned the water from one
reservoir to the other? Would he have had to buy more hose?
_APPLICATION 5._ Two boys were out hiking and were very
thirsty. They came to a deserted farm and found a deep well;
it was about 40 feet down to the water. They had no pump,
but there was a piece of hose about 50 feet long. One boy
suggested that they drop one end of the hose down to the water
and suck the water up, but the other said that that would not
work--the only way would be to lower the hose into the water,
close the upper end, pull the hose out and let the water pour
out of the lower end of the hose into their mouths. A stranger
came past while the boys were arguing, and said that neither
way would work; that although the hose was long enough, the
water was too far down to be raised in either way. He advised
the boys to find a bucket and to use the hose as a rope for
lowering it. Who was right?
INFERENCE EXERCISE
EXPLANATORY NOTE. In the inference exercises in this book,
there is a group of facts for you to explain. They can always
be explained by one or more of the principles studied, like
gravitation, water seeking its own level, or air pressure. If
asked to explain why sucking through a straw makes soda water
come up into your mouth, for instance, you should not merely
say "air pressure," but should tell why you think it is air
pressure that causes the liquid to rise through the straw. The
answer should be something like this: "The soda water comes
up into your mouth because the sucking takes the air pressure
away from the top of the soda water that is in the straw. This
leaves the air pressing down only on the surface of the soda
water in the glass. Therefore, the air pressure pushes the
soda water up into the straw and into your mouth where the
pressure has been removed by sucking." Sometimes, when you
have shown that you understand the principles very well,
the teacher may let you take a short cut and just name the
principle, but this will be done only after you have proved by
a number of full answers that you thoroughly understand each
principle named.
Some of the following facts are accounted for by air pressure;
some by water seeking its own level; others by gravitation.
See if you can tell which of the three principles explains
each fact:
1. Rain falls from the clouds.
2. After rain has soaked into the sides of mountains it runs
underground and rises, at lower levels, in springs.
3. When there are no springs near, people raise the water from
underground with suction pumps.
4. As fast as the water is pumped away from around the bottom
of a pump, more water flows in to replace it.
5. After you pump water up, it flows down into your pail from
the spout of the pump.
6. You can drink lemonade through a straw.
7. If a lemon seed sticks to the bottom of your straw, the
straw flattens out when you suck.
8. When you pull your straw out to remove the seed, there is
no hole left in the lemonade; it closes right in after the
straw.
9. If you drop the seed, it falls to the floor.
10. If you tip the glass to drink the lemonade, the surface
of the lemonade does not tip with the glass, but remains
horizontal.