One of these is water. Sometimes you can see this for yourself. On a cold, frosty day, you know we can see the clouds of steam coming out of our mouths. This steam is only very fine particles of water.
In warm weather we do not see the steam, but the water is there all the same; if you will breathe 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.