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.