How does a thermometer work? What makes the mercury rise in
it?
Why does heat make things get larger?
When we look at objects through a microscope, they appear much larger
and in many cases we are able to see the smaller parts of which they
are made. If we had a microscope so powerful that it made a tiny
speck of dust look as big as a mountain (of course no such microscope
exists), and if we looked through this imaginary microscope at a piece
of iron, we should find to our surprise that the particles were not
standing still. The iron would probably look as if it were fairly
alive with millions of tiny specks moving back and forth, back and
forth, faster than the flutter of an insect's wings.
These tiny moving things are _molecules_. Everything in the world is
made of them. It seems strange that we should know this, since there
really are no microscopes nearly powerful enough to show the molecules
to us. Yet scientists know a great deal about them. They have devised
all sorts of elaborate experiments--very accurate ones--and have
tested the theories about molecules in many ways. They have said, for
instance, "Now, if this thing _is_ made of molecules, then it will
grow larger when we make the molecules move faster by heating it."
Then they heated it--in your next experiment you will see what
happened. This is only one of thousands of experiments they have
performed, measuring over and over again, with the greatest care,
exactly _how much_ an object expanded when it was heated a certain
amount; exactly how much heat was needed to change water to steam;
exactly how far a piece of steel of a certain size and shape could
bend without breaking; exactly how crystals form--and so on and so on.
And they have always found that everything acts as if it were made
of moving molecules. Their experiments have been so careful
and scientists have found out so much about what _seem_ to be
molecules,--how large they are, what they probably weigh, how fast
they move, and even what they are made of,--that almost no one has any
doubt left that fast-moving molecules make up everything in the world.
[Illustration: FIG. 41. A thermometer.]
To go back, then: if we looked at a piece of iron under a microscope
that would show us the molecules,--and remember, no such powerful
microscope could exist,--we should see these quivering particles, and
nothing more. Then if some one heated the iron while we watched the
molecules, or if the sun shone on it, we should see the molecules move
faster and faster and separate farther and farther. That is why heat
expands things. When the molecules in an object move farther apart,
naturally the object expands.
_Heat is the motion of the molecules._ When the molecules move faster
(that is, when the iron grows hotter), they separate farther and the
iron swells.
[Illustration: FIG. 42. A thermometer made of a flask of water. It
does not show the exact degree of heat of the water, but it does show
whether the water is hot or cold.]
HOW WE CAN TELL THE TEMPERATURE BY READING A THERMOMETER. The mercury
(quicksilver) in the bulb of the thermometer like everything else
expands (swells) when it becomes warm. It is shut in tightly on all
sides by the glass, except for the little opening into the tube above.
When it expands it must have more room, and the only space into which
it can move is up in the tube. So it rises in the tube.
[Illustration: FIG. 43. Will the hot ball go through the ring?]
Water will do the same thing. You can make a sort of thermometer,
using water instead of mercury, and watch the water expand when you
heat it. Here are the directions for doing this:
[Illustration: FIG. 44. When the wire is cold, it is fairly tight.]
EXPERIMENT 28. Fill a flask to the top with water. Put a piece
of glass tubing through a stopper, letting the tube stick 8 or
10 inches above the top of the stopper. Put the stopper into
the flask, keeping out all air; the water may rise 2 or 3
inches in the glass tube. Dry the flask on the outside and put
it on a screen on the stove or ring stand, and heat it. Watch
the water in the tube. What effect does heat have on the
water?
Here are two interesting experiments that show how solid things expand
when they are heated:
EXPERIMENT 29. The brass ball and brass ring shown in Figure
43 are called the expansion ball and ring. Try pushing the
ball through the ring. Now heat the ball over the flame for a
minute or two--it should not be red hot--and try again to pass
it through the ring.
Heat both ball and ring for a short time. Does heating expand
the ring?
EXPERIMENT 30. Go to the electric apparatus (described on page
379) and turn on the switch that lets the electricity flow
through the long resistance wire. Watch the wire as it becomes
hot.
_APPLICATION 24._ A woman brought me a glass-stoppered bottle
of smelling salts and asked me if I could open it. The stopper
was in so tightly that I could not pull it out. I might have
done any of the following things: Tried to pull the stopper
out with a pair of pliers; plunged the bottle up to the neck
in hot water; plunged it in ice-cold water; tried to loosen
the stopper by tapping it all around. Which would have been
the best way or ways?
[Illustration: FIG. 45. But notice how it sags when it is hot.]
_APPLICATION 25._ I used to buy a quart of milk each evening
from a farmer just after he had milked. He cooled most of the
milk as soon as it was strained, to make it keep better. He
asked me if I wanted my quart before or after it was cooled.
Either way he would fill his quart measure brim full. Which
way would I have received more milk for my money?
INFERENCE EXERCISE
Explain the following:
121. Billiard balls will rebound from each other and from the
edges of the table again and again and finally stop.
122. In washing a tumbler in hot water it is necessary to lay
it in sidewise and wet it all over, inside and out, to keep
it from cracking; if it is thick in some parts and thin in
others, like a cut-glass tumbler, it is not safe to wash it in
hot water at all.
123. The swinging of the moon around the earth keeps the moon
from falling to the earth.
124. A fire in a grate creates a draft up the chimney.
125. Telegraph wires and wire fences put up in the summer must
not be strung too tightly.
126. Candy usually draws in somewhat from the edge of the pan
as it hardens.
127. A meat chopper can be screwed to a table more tightly
than you can possibly push it on.
128. A floor covered with linoleum is more easily kept clean
than a plain wood floor.
129. Rough seams on the inside of clothes chafe your skin.
130. You can take the top off a bottle of soda pop with an
opener that will pry it up, but you cannot pull it off with
your fingers.