1.The second health lesson I have to learn is this-I must breathe fresh air.If a man cannot get air to breathe，he will die in a few seconds.But that is not all：if the air he breathes is not pure airs，it is of no use，and it will soon poison him.
2.Why do we need to breathe at all？Because the air containsa gas called oxygen，and a constant supply ofthis gas must be taken into the blood，or else we cannot remain in good health.When we breathe，the air passes through the nostrilsor the mouth，down into the lungs，and there it meets with the blood，which also flows through the lungs.
3.When passing through the lungs the blood runs through very small thin tubes，so thin that the oxygen of the air passes freely through them to the blood；but they are strong enough to keep the blood in its proper place.And as the oxygen gas passes inwards to the blood，another gas，which has to be got rid of，passes outwards from the blood，and is breathed out into the air.
4.Thus the air we breathe out is different from the air we breathe in.It has lost the gas which is necessary for our life and health，and it contains a gas which is hurtful to us.To breathe this air over again would be useless，as it contains too little oxygen，and would also be hurtful，as it contains a gas which has already been thrown off by the body.
5.If I live in a room that does not get fresh air，the air in it will soon become close and bad，because every time I breathe I take some of the oxygen out of it.If a lighted candle is put under a glass bell so that no air can get in，it soon burns up all the oxygen，and then it goes out.
6.If a small animal，such as a bird or a rabbit，is put under a similarbell，it uses up all the oxygen，and the rest of the air is of no use to it.In a few minutes it becomes faint；it is unable to stand up，and unless itgets fresh air，it will soon die.
7.Once，in a storm，the captain of a ship made all the passengers go down below.There they were crowdedinto a small cabin，with only one small window，orport-hole，to let in fresh air.All night long the ship plunged deep into the waves，and the port-hole had to be firmly closed，so that no air could get in.
8.The passengers struggled and cried for help，but in the storm their cries were not heard on deck，where the captain and his men were battling with the storm.In the morning the storm was past，but when the cabin was opened two of the passengers were found dead.The bad air had poisoned them.
9.You may also have heard the story of the Black Hole of Calcutta，where one hundred and forty-six English prisoners were shut up in a small cell.They could not get enough air to breathe，and in the morning a hundred and twenty-three of them were found dead.