CHAPTER XIII. WHY AND HOW WE BREATHE.--What happens to a
lighted candle if shut up in a small, close place? to a mouse? Why is
air so necessary for a burning candle and for animals? How is the heat
of our bodies produced? Name the principal organs of breathing.
Describe each. How do we use the lungs in breathing? How much air will a
man's lungs hold? How much air do we use with each breath? What
poisonous substance does the air which we breathe out contain? Will a
candle burn in air which has been breathed? What happens to animals
placed in such air? What change takes place in the blood as it passes
through the lungs? How do plants purify the air?

CHAPTER XIV. HOW TO KEEP THE LUNGS HEALTHY.--What is the thing
most necessary to preserve life? Name some of the ways in which the
blood becomes impure. Why is bad-smelling air dangerous to health? What
are germs? Why are some diseases "catching"? Name some such diseases.
What should be done with a person who has a "catching" disease? What is
the effect of the breath upon the air? How much air is poisoned and made
unfit to breathe by each breath? How much air do we spoil every minute?
every hour? How much pure air does each person need every minute? every
hour? How do we get fresh air into our houses? Why are windows and doors
not good means of ventilating in cold weather? How should a room be
ventilated? How should we use the lungs in breathing? What about the
clothing in reference to the lungs? Why is it injurious to breathe
habitually through the mouth? What is the effect of alcohol upon the
lungs? What is the effect of tobacco-using upon the throat and nose?

CHAPTER XV. THE SKIN AND WHAT IT DOES.--How many layers in the
skin? What is each called? To what is the color of the skin due? What
glands are found in the true skin? What are the nails and what is their