~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 waste
matters are carried out of the body through the skin, the kidneys, the
liver, and other organs.

~9. Another Experiment.~--We cannot see the gas escape from our lungs,
but we can make an experiment which will show us that it really does
pass out. Get two drinking-glasses and a tube. A glass tube is best, but
a straw will do very well. Put a little pure water into one glass and