WHAT HEAT DOES.
Heat makes most things larger, or _expands_ them, as it is commonly
expressed. I will give you some examples of this.
I have already told you in Chapter XIII. how heat expands air. You
remember the two experiments with the bladder before the fire. You
remember also what I told you about the expansion of the air in apples
and chestnuts by heat.
[Illustration]
[Sidenote: Experiment showing how air is expanded by heat.]
Here is represented another experiment which shows that heat expands
air. A glass tube, with a bulb on the end of it, is put with its open
end into a tumbler of water. Of course the tube is full of air. Now, on
putting the warm hand on the bulb, as represented, the air in it will
be warmed. The air, therefore, swells, and there is not room for it all
in the tube; and so some of it escapes in bubbles, as you see, through
the water.
[Sidenote: Snapping of burning wood.]
The snapping wood, you know, often throws out sparks. These are parts
of the wood partly burned that happen to be right on the spot where the
confined heated air was that has broken loose. The more porous wood
is, the more apt it is to snap. The solid walnut seldom snaps; but the
chestnut, which is very porous, is always snapping. So, too, dry wood
snaps more than green, because the sap has dried up, and air has taken
its place in the pipes of the wood.
Air expanded by heat, as you have before seen, always rises. It is
pushed up, as I have already told you, by the colder air, which is
heavier. This keeps the air always moving. It is never still, for heat
is always acting upon it. Even when it is so still that we say there
is no wind, it is not perfectly still. There is all the time the going
up of warm air and the coming down of that which is colder. You can
see this in a room if you shut it up so as to make it quite dark, and
let only a little light in by partly opening one shutter. Though the
air seems to you to be perfectly still, you will see, where this light
is let in, little motes flying up and down. This is because there are
currents in the air, and these are made by heat.
[Sidenote: Air set in motion by heat.]
[Sidenote: Sea breeze.]
It is heat that puts the air in motion so as to produce winds. You have
heard people talk about the cool, refreshing sea breeze. This comes up
commonly in the afternoon. It is caused in this way: The earth becomes
very much heated by the hot sun during the day, and so heats the air
above it. This heated air rises, and the air which comes off from the
cool water to take its place makes the sea breeze.
You see why it is that heated air is lighter than cold air. It is
swelled by the heat without having any thing added to it. Its particles
are put farther apart. It is made thinner, and air, as it becomes
cold, is contracted or made smaller. Its particles are brought closer
together, and so it is made thicker and heavier.
[Sidenote: Liquids expanded by heat.]
And so it is with water or any liquid. When it is heated it becomes
larger and thinner, just as air does, and so is lighter. It rises,
therefore, being pushed up by the heavier cold water. There are,
therefore, the same up and down currents in water that there are in the
air. When one is heating water, the warm water is all the time going
up, and the cold water is going down. If you heat it in a glass vessel,
and have some little light things in the water, you can see them go up
and down in the currents in the same way that you see motes moving up
and down in the currents of the air.
The grocer knows very well that heat expands all liquids. His molasses
and oil are much thinner, and so run more freely in summer than in
winter. And the gallon of molasses or oil that you buy in summer does
not weigh so much as the same quantity in winter, for the same reason
that heated air is lighter than cold air.
[Sidenote: Thermometer explained.]
In the thermometer you see the expansion or swelling of a fluid by
heat. Put your finger on the bulb, and hold it there a little while.
The mercury rises, you see. What is the reason? The warmth of your
finger swells or expands the mercury, and it rises, because it needs
more room. You can do the same thing by breathing on it. Your warm
breath will expand the mercury. This is just what the warm air does to
it; and when the weather is cold, the cold air shrinks or contracts it.
When it is very cold indeed, the mercury is very low down in the tube,
because it is so much contracted by the cold air; and when it is hot
weather, the mercury is very high, because it is so much swollen by
the heat. You can understand, by what I have told you, how it is that
we judge of the heat of the air by the thermometer.
[Sidenote: Setting tires.]
Heat expands solid substances, though not as much as it does the air,
and gases, and liquids. If a rod of iron will just go through a hole in
another piece of iron, you can not get it into that hole when the rod
is heated, because it is swollen or expanded by the heat. The tire of
a wheel is heated when it is put on the wheel. Why this is done I will
explain to you. The tire is made a little too small for the wheel. You
can not put it on the wheel while it is cool, but when it is heated it
goes on very easily, because the heat has made it larger. Cold water is
now poured upon it, and as it contracts it fits very tightly, giving
great firmness to the wheel. It could not be made to fit so tightly in
any other way.
[Sidenote: Heat changing solids into fluids.]
So I have showed you how heat expands various things. It sometimes does
more than this when there is enough of it. It changes a solid into a
fluid. For example, it changes ice into water. So it makes the hard
iron into a fluid so that you can pour it like water, as you can see
in an iron foundry when the workmen are casting. It takes more heat to
melt iron than it does to melt ice, and it takes more to melt ice than
to melt mercury. It takes so little to melt mercury that we can seldom
get a chance to see it solid. In some of the coldest regions of the
earth, however, it is often seen solid.
But heat does more than this. It changes some liquids into something
like air or gas. For example, it changes water into steam. There must
be a great deal of the heat to do this--much more than is required to
change ice into water.
[Sidenote: What heat does to animals and plants.]
[Sidenote: Making of birds in eggs.]
I have told you in Parts First and Second much about what heat does to
life in vegetables and animals. The heat of spring wakes up the seeds
and the buds; and stalks, and leaves, and flowers, and fruits come
forth from them, making the earth cheerful and gay. It wakes up, too,
multitudes of animals, that with their moving about and their various
voices make the world every where so busy. Thus, almost like magic,
does heat work in the animal and vegetable world. I know not any thing
in which the effects of heat are so wonderful as in the egg. Look at a
hen’s egg as it is opened, and see the golden yolk in the midst of the
pure, glairy white. It does not seem that this could be changed into
a chicken, with its bones, and muscles, and nerves, and feathers, and
claws, and by nothing but heat; but so it is. The hen has only to keep
the egg warm by sitting on it, and all this happens; and the chicken,
when it is all formed, bursts the shell, and comes out from its round
white prison.
_Questions._--How does heat affect most things? Explain the snapping
of wood on the fire. What are the sparks that are thrown out? What
kinds of wood snap most? What keeps air moving, and how? How can you
know that air is not still when it seems to be? What makes the wind?
What is said about the sea breeze? Why is heated air lighter than cold
air? How is it with water? What is said about heating water? What
effect does heat have on molasses and oil? Explain the operation of
the thermometer. What is said about the expansion of solids by heat?
Give the experiment of the rod of iron. Explain the putting of a tire
on a wheel. What is said about the changing of solids into fluids by
heat? What change does very great heat produce in water? What does
heat do in the animal and the vegetable world? What is said about the
egg?