The Two Breaths.~--We have learned that the air which we breathe out contains something which is not found in the air which we breathe in.

This is carbonic-acid gas.

How many of you remember how we found this out?
We can also tell this in another way.

If we put a candle down in a wide jar it will burn for some time.

If we breathe into the jar first, however, the candle will go out as soon as we put it into the jar.