on a looking-glass at any time, you will make
it dim and damp directly with the water that is contained in your
breath.

We also breathe out animal matter, little particles of our own bodies
just ready to decay. We can not see them, but they soon give the air a
close, disagreeable smell. Good air has no smell at all.

And now I have something to say to you about the use of noses.

I dare say you can not see much use in the sense of smell. Seeing,
hearing, touching, are very needful to us, we all know; but as to
smelling, that does not seem to have any particular value.

It is pleasant to smell a sweet rose or violet; and, I believe, smelling
really forms a good part of what we call tasting.

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 an