There are places underground where that air gathers, and when a person gets into one of those places, he dies at once. For this purpose they have lamps in the mines, and before a man goes down to such a place, they let down the lamp. If it goes out, no man can go there; then they let down fresh air until the lamp will burn.

Near the city of Naples there is one such cave. There is always about three feet of bad air in it on the ground, but above it the air is good. A man can walk through the cave, and nothing will happen to him, but a dog will die the moment it enters.

Where does this bad air come from? It is made of the same good air that we breathe. If you gather a lot of people in one place, and close all the doors and windows, so that no fresh air can get in, you will get the same kind of an air as in the well, and people will die.

One hundred years ago, during a war, the Hindus captured 146 Englishmen and shut them up in a cave underground, where the air could not get in.