WHY AND HOW WE BREATHE.

~1. An Experiment.~--Let us perform a little experiment. We must have a
small bit of candle, a fruit jar, or a bottle with a large mouth, and a
piece of wire about a foot long. Let us notice carefully what we are
about to do and what happens.

~2.~ We will fasten the candle to the end of the wire. Now we will light
it, and next we will let it down to the bottom of the jar. Now place the
cover on the top of the jar and wait the results. Soon the candle burns
dimly and in a little time the light goes out altogether.

~3.~ What do you think is the reason that the candle will not burn when
shut up in a bottle? A candle uses air when it burns. If shut up in a
small, tight place, it soon uses up so much air that it can burn no
longer. Try the experiment again, and when the candle begins to burn
dimly, take it out quickly. We see that at once the light burns bright
again.

~4.~ Suppose we shut the stove draught tight, what is the result? The
fire will burn low, and after a time it will probably go out. Why is
this? Evidently the stove needs air to make the wood or coal burn, just
as the candle needs air to make it burn.

~5. Animals Die without Air.~--If you should shut up a mouse or any
other small animal in a fruit-jar, its life would go out just as the
light of the candle went out. The little animal would die in a short
time. A child shut up in a close place would die from the same cause in
a very little time. In fact, many children are dying every day for want
of a sufficient supply of pure air.

~6. Oxygen.~--The reason why animals need air, and why the fire will not
burn without it, is that the air contains _oxygen_, and it is the oxygen
of the air which burns the wood or coal and produces heat. So it is the
oxygen that burns in our bodies and keeps us warm.

~7.~ When wood and coal are burned, heat is produced; but some parts of
the fuel are not made into heat. While the fire burns, smoke escapes
through the pipe or chimney; but a part of the fuel remains in the stove
in the form of ashes. Smoke and ashes are the waste parts of the fuel.

~8. Poison in the Breath.~--The burning which takes place in our bodies
produces something similar to the smoke and ashes produced by the fire
in a stove. The smoke is called _carbonic-acid gas_,[A] an invisible
vapor, and escapes through the lungs. The ashes are various waste and
poisonous matters which are formed in all parts of the body. These w