THE DIFFUSION OF HEAT.
[Sidenote: Experiment showing how heat spreads.]
Heat always tries to spread itself in all directions. If you put the
end of a poker in the fire and hold it there, you do something more
than heat that end. You heat the whole of it up to the end that you
hold in your hand. The reason is, that the heat that comes into the
end of the poker which is in the fire spreads through all of it to the
other end.
[Illustration]
This figure represents an experiment that you can try, which shows
how the heat spreads through any thing solid. A rod or bar of iron
is taken, and small balls of wood are fastened to it, as you see, by
some wax. Now, on heating one end of the bar with a lamp, as the heat
spreads along the bar, the balls one after another drop off, because
the wax that holds them melts.
Heat spreads from one thing to another when it can get a chance to do
it. If one thing that has a good deal of heat in it touches or is near
by another that has less heat in it, it parts with some of its heat,
and lets it go into the other thing, and after a little while one will
be as warm as the other. For this reason, in a warmed room, all the
furniture, the tables, the bureaus, the carpet, and the walls of the
room become heated alike. The heat from the fire spreads through them
all. It takes some time to do this, but it is done.
[Sidenote: How ice is melted.]
It is because heat goes from one thing to another that ice melts in
warm water or warm air. Some of the heat in the water or air goes into
the ice and melts it, and the melting ice cools the water or air by
thus taking a part of its heat.
The heat which I have told you is made in our bodies spreads
continually in the air around us. This is the reason that a room which
is comfortably warm becomes uncomfortably so when a large company has
been in it for a little time. A great deal of heat spreads into the air
from so many bodies.
[Sidenote: How fanning cools us.]
[Sidenote: Blowing on the fingers.]
Did you ever think how fanning cools you? It is by making the heat
go off faster from your body into the air. It moves off the air that
has become heated by your body, and brings some other air to take its
place. For the same reason, blowing upon any thing that is hot helps
to cool it. It brings the air to it faster than it would come without
the blowing, and so the heat passes off faster. But perhaps you will
ask me to explain why it is that blowing on your fingers when they are
cold warms them, when blowing on any thing hot cools it. This is plain
enough. The air that you blow on to your fingers is warmer than they
are, and gives some of its heat to them. If, on the contrary, your
fingers were hot with fever, blowing on them would cool them, for they
would then give some of their heat to the air that is cooler than they
are.
[Sidenote: Wood a poor conductor.]
Heat spreads through some things more easily than it does through
others. It spreads through iron very easily indeed, as you know by
holding an iron poker with one end in the fire, but it does not spread
any thing like as easily through wood. If you hold a stick of wood
with one end in the fire, you can let it burn off without feeling the
heat at the other end; but you could not hold a poker so long in the
fire, for the heat would spread to the end in your hand so much that
it would soon be too hot for you to hold it. So iron is said to be a
better _conductor_ of heat than wood, for the heat is conducted through
it more easily than through the wood.
[Sidenote: Wooden handles.]
[Sidenote: Holders.]
[Sidenote: Ice kept in flannel.]
It is for this reason that wooden handles are put upon some iron tools
that are used in operations about the fire. The tool which the tinman
uses in soldering has a wooden handle. If it had not, his hand would be
burned by the heat going up to it by the iron handle; but very little
of it goes into the wooden handle and spreads there, because wood is so
poor a conductor of heat. We do not need wooden handles for tongs and
pokers, because we do not have to keep them in the fire so long as the
tinman does his soldering-iron. The handle of a metallic tea-pot is,
you know, made of wood; for, if it was metallic, the heat from the tea
would spread through it, and make it so hot that it could not be held
in the hand. The holder which is used in ironing is of service, because
it is so poor a conductor of heat. The heat does not readily go through
it to the hand; so, also, we sometimes use paper to take up things that
are hot, because the paper, being a poor conductor, does not let much
of the heat pass through it to the hand. You have seen people wrap up
ice in flannel to keep it from melting. The flannel here does for the
ice what the woolen or paper holder does for the hand--it prevents the
heat in the air around from getting to the ice.
[Illustration]
[Sidenote: Experiment on a stove.]
Here is represented an experiment which shows how heat spreads through
different things with different degrees of rapidity. Some pieces of
different things of the same size and shape are put on top of a stove.
They are pieces of iron zinc, copper, lead, marble, and brick. On the
top of each is put a little bit of wax. The wax on the copper melts
first, because this is a better conductor of heat than any of the
others. Next is the iron; next, the zinc; next, the lead; next, the
marble; and last of all, the brick.
[Sidenote: Air a poor conductor of heat.]
In air that is kept still heat spreads very slowly; but heat, when it
can, always sets air in motion. I have told you, in Chapter XIII., how
heated air rises and cold air takes its place. This is going on all
the time about a stove. As fast as the air is heated, it goes up by
the stove and the pipe, and cold air keeps coming to the stove to be
heated. In this way all the air in the room is, after a little while,
warmed. Now, if the air could all be kept still instead of being kept
in motion in this way, it would take a long time for the heat to
be spread from the stove through it, for air, like wood, is a poor
conductor of heat.
[Sidenote: Double windows.]
We see the fact that confined air is a poor conductor of heat in a
great many things. Some of them I will mention. You have sometimes seen
double windows. It is the confined air between the outer and the inner
windows that prevents the heat of the air in the room from spreading
to the air out doors. When the window is single, the outside air cools
the air in the room through the window in this way: The air in the room
close to the window gives some of its heat to the glass, and, being
thus cooled, it falls, and some more warm air comes to be cooled in
like manner, and then falls, and so on continually. All this time the
cold air on the outside keeps coming to get warmed by the glass, and as
it is warmed it goes up, and more cold air comes to take its place. But
all this is pretty much prevented where the windows are double, by the
confined air between them.
[Sidenote: A pear kept in snow.]
There is a great deal of air in snow. This is the chief reason that
snow is so apt to keep the ground from being frozen. It is the earth’s
winter coat of confined air, for there is air mingled with its flakes
as they are piled upon each other on the ground. Last spring I picked
up a pear in my garden that was as fresh as it was when it fell upon
the ground in the fall. It happened to lie in a spot where the snow lay
all the winter, and was thus kept from freezing.
[Sidenote: How furs keep in warmth.]
Furs are commonly spoken of as if they had some warmth in them. This
is a mistake. They are not warm of themselves. They only serve to keep
in the heat that is made in the body, and they do this by the air that
is mingled up with the fibres of the fur. This confined air is a poor
conductor, and so the heat made in the body does not readily pass off
through it into the air around. Fur is therefore to an animal, in
this respect, what snow is to the ground, or what double windows are
to a room; and the finer the fur is, the better does it keep the heat
in, because the air is more confined among fine fibres than it is in
coarse hair. And it is curious, that if an animal with thick fur is
taken from the cold country where he belongs to a warm climate, and
kept there, his fur gradually loses its fineness and thickness, and
becomes like hair. This is because he does not need his thick, furry
coat where the weather is warm.
You remember that I told you in Part First that inside of the covering
with which every one of the buds on the trees is protected from the
cold of winter there is a fine down. This, I told you, was the bud’s
little blanket. You can understand, now, how this keeps it from being
chilled by the wintry blasts. It is the air that is confined between
the fibres of this downy blanket that does it.
[Sidenote: Downy blankets of buds.]
[Sidenote: How straw protects trees from cold.]
You remember, also, that I told you in Part First about tying straw
around trees to protect them from the winter’s cold. Now you know that
every stalk of straw is hollow, and so is full of air, and it is the
air in all the stalks of the straw that makes it so good a coat for
the trees. This coat protects them just in the same way, then, that an
animal is protected by its furry coat, or the bud by its blanket of
down.
_Questions._--What is said of the spreading of heat? What is said
about its going from one thing to another? How is ice melted? What is
said about heat’s spreading from our bodies? Tell how fanning cools
you. Why does blowing a hot thing help to cool it? Why does blowing
upon cold fingers warm them? Explain what is meant when we say that
some things are better conductors of heat than others. Give the
illustrations. How does heat commonly spread in air? How would it be
if the air could be kept still? Explain how double windows keep the
heat in. What is said about snow? What about furs? Why does a fine fur
keep the heat in better than a coarse one? How does taking an animal
to a warm climate affect the fur? Tell about the blankets of the buds.
Tell about covering trees with straw.