g out the "smoke"; and
when you breathe in, you are taking in fresh air.

Our body "smoke" is not brown or blue, like the smoke from a fire; it
is a clear, odorless gas, called _carbon dioxid_. This is the same gas
that makes the choke-damp of coal mines, which suffocates the miners
if the mine is not well ventilated; and the same gas that sometimes
gathers at the bottom of a well, making it dangerous for anyone to go
down into the well to clean it. And this gas is poisonous in our
bodies just as it is in the mine or the well.

You see, then, how important it is that we should live much of our
lives in the clear pure air out of doors, and should bring the fresh
air into our houses and schools and shops. "Fill up" with it all you
can on your way to school, for the best of air indoors is never half
so good as the free-blowing breezes outside.

IV. FRESH AIR--HOW WE BREATHE IT

When you are running and breathing hard, and even when you are sitting
still and breathing quietly, air is going into your lungs and then
coming out, going in and coming out, many times every minute. How does
the air get in and out of the lungs? It will not run in of itself; for
it is light and floats about, you know. Here, again, Mother Nature has
planned it all out. She has made us an air bellows, or air pump, to
suck it into the lungs. First we'll see what shape this pump is, and
then how it works.

[Illustration: THE CHEST THAT HOLDS THE LUNGS

Back of the lungs is the heart; its position is shown by the
broken line. The black line across the chest shows how high the
diaphragm rises when we breathe out quietly.]

Stiff rings of bone called _ribs_ run round your body, just like the
hoops in an old hoop skirt, or like the metal rings round a barrel.
Here is a picture of the bones of the chest. Perhaps your teacher can
show you the skeleton of some animal. You will notice how the rings,
or ribs, slant and are joined by hinges behind to the backbone and in
front to the breastbone. It looks somewhat like a cage, doesn't it?
Put your hands on the sides of your chest and you can feel your own
ribs. Do they slant upward or downward?

This chest-cage is our breathing-machine. Before I tell you how it
pumps, I want you to get a pair of bellows and see how they work. When
you lift up the handle of the bellows, you make the bag of the bellows
larger so that it sucks in air; and when you press the handle down
again, the air puffs