And like water, its only dangerous impurities are what we put there ourselves.

The purity of outdoor air is due mainly to the fact that there is such an enormous amount of it, not only the miles and miles of it that stretch away on every side of us, but nearly thirty miles of it straight up above our heads; its purity is also due to the fact that, like water, it is always in motion.

When heated by the sun, it expands; and, in doing so, it rises because it is less dense and therefore lighter.

As soon as the pressure of the air above is lessened, air rushes in below from all the cooler regions around.

This rushing of air we call a _wind_.