The heat of our bodies
also makes this gas, and we throw it off in our breath.
Oxygen and carbon, in a separate condition, make up a good part of our
flesh, blood, and bones; but when they are joined together, and make
carbonic acid gas, they are of no further use to us.
You might go to a store and buy sand and sugar; but if they became mixed
together as you brought them home, you would not be able to use either
one of them, unless some clever fairy could pick them apart for you.
You see now one great way of spoiling the air. How are we to get rid of
this bad air, and obtain fresh air, without being too cold?
In summer time this is quite simple, but in winter it is more difficult;
because it is a very bad thing to be cold, and a thin, cold draught of
air is especially bad.
The bad air loaded with carbonic acid gas, when we first breathe it out,
is warm. Warm gases are much lighter than cold ones, therefore the bad
air at first goes up to the ceiling.
If there is an opening near the top of the room, the bad air goes out;
but if there is no opening, it by and by grows cold and heavy, and comes
down again.
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