d weak.

The candle makes another kind of gas. It is called carbonic acid gas,
which, is unhealthy and not fit for breathing. 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. Then we have to breathe it.

If you open the window at the top, it will let out the bad air, and you
will not feel a draught. It is not often so very cold that you cannot
bear the window open, even a little way from the top, and that is the
best way of airing a room.

This is just as necessary by night as by day. People who shut in the bad
air, and shut out the good air, all night long, can never expect to
awake refreshed, feeling better for their sleep.

What becomes of the carbonic acid gas which the body throws off through
our breath? Can any thing pick the carbon and oxygen in it apart, and
make them fit for us to use again?

Yes. Every plant, every green leaf, every blade of grass, does that for
us. When the sun shines on them, they pick the carbon out and send back
the oxygen for us to breathe. They keep the carbon and make that fit
for us and animals to eat.

The grass makes the carbon fit for sheep and cows, and then we eat their
flesh or drink their milk; and the corn makes the carbon fit to eat; so
do potatoes, and all the other vegetables and fruits which we eat. Is
not this a wonderful arrangement?

But perhaps you think, considering what an amazing number of peopl