the same quantity of lime-water into the other glass. Now put one end of
the tube into the mouth and place the other end in the pure water.
Breathe through the tube a few times. Look at the water in the glass and
see that no change has taken place. Now breathe through the lime-water
in the same way. After breathing two or three times, you will notice
that the lime-water begins to look milky. In a short time it becomes
almost as white as milk. This is because the lime-water catches the
carbonic-acid gas which escapes from our lungs with each breath, while
the pure water does not.

~10. Why we Breathe.~--By this experiment we learn another reason why
we breathe. We must breathe to get rid of the carbonic-acid gas, which
is brought to the lungs by the blood to be exchanged for oxygen. There
are two reasons then why we breathe: (_a_) to obtain oxygen; (_b_) to
get rid of carbonic-acid gas.

~11. How a Frog Breathes.~--Did you ever see a frog breathe? If not,
improve the first opportunity to do so. You will see that the frog has a
very curious way of breathing. He comes to the top of the water, puts
his nose out a little, and then drinks the air. You can watch his throat
and see him swallowing the air, a mouthful at a time, just as you would
drink water.

~12.~ If you had a chance to see the inside of a frog you would find
there a queer-shaped bag. This is his air-bag. This bag has a tube
running up to the throat. When the frog comes to the surface of the
water he fills this bag with air. Then he can dive down into the mud out
of sight until he has used up the supply of air. When the air has been
changed to carbonic-acid gas, he must come to the surface to empty his
air-bag and drink it full again.