8.  Poison in the Breath.—­The burning which takes place in our bodies produces something similar to the smoke and ashes produced by the fire in a stove.  The smoke is called carbonic-acid gas,[A] an invisible vapor, and escapes through the lungs.  The ashes are various waste and poisonous matters which are formed in all parts of the body.  These waste matters are carried out of the body through the skin, the kidneys, the liver, and other organs.

9.  Another Experiment.—­We cannot see the gas escape from our lungs, but we can make an experiment which will show us that it really does pass out.  Get two drinking-glasses and a tube.  A glass tube is best, but a straw will do very well.  Put a little pure water into one glass and 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.

13.  The Lungs.—­We do not drink air as the frog does, but like the frog we have an air-bag in our bodies.  Our air-bag has to be emptied and filled so often that we cannot live under water long at a time, as a frog does.  We call this air-bag the lungs.  We have learned before that the lungs are in the chest.  We need so much air and have to change the air in our lungs so often that we would not have time to swallow it as a frog does.  So nature has made for us a breathing apparatus of such a kind that we can work it like a pair of bellows.  Let us now study our breathing-bellows and learn how they do their work.