Of all our senses, smell is the one that soonest gets out of practice.  If people would always accustom themselves to use their noses, they never would consent to live in the horrid air they do.

If you go from the fresh air into a close room, you will notice the smell at once.  Then, if you remain there, you will soon get accustomed to the smell and not notice it; but it will still be there, and will be doing you a great deal of harm.

In good air there are, mainly, two sorts of gas.

The first is a very lively sort of gas, called oxygen; it is very fond of joining itself with other things, and burning them, and things burn very fast indeed in oxygen.

The second is a very slow, dull gas, called nitrogen; and nothing will burn in it at all.  Pure oxygen would be too active for us to live in, so it is mixed with nitrogen.

When we breathe, the air goes down into our lungs, which are something like sponges, inside our chests.

These sponges have in them an immense quantity of little blood-vessels, and great numbers of little air-vessels; so that the blood almost touches the air; there is only a very, very thin skin between them.

Through that skin, the blood sends away the waste and useless things it has collected from all parts of the body, and takes in the fresh oxygen which the body wants.

You have often heard man’s life compared to a candle.  I will show you some ways in which they are much alike.

When a candle or lamp burns, if we keep it from getting any new air, it soon uses all the lively gas, or oxygen, and then it goes out.  This is easily shown by placing a glass jar over a lighted candle.

If the candle gets only a little fresh air, it burns dim and weak.  If we get only a little fresh air, we are sickly and 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.