This shows that the air which we breathe out contains something which will put a candle out.

This is carbonic-acid gas, which is a poison and will destroy life.
~26.

Other Poisons.~--The air which we breathe out also contains other invisible poisons which are very much worse than the carbonic-acid gas.
These poisons make the air of a crowded or unventilated room smell very unpleasant to one who has just come in from the fresh air.

Such air is unfit to breathe.
~27.

The Lungs Purify the Blood.~--We have learned that the blood becomes dark in its journey through the body.