Then, when they examined the well, they found that indeed there was bad air down in the well. This air is so heavy that neither man nor any animal can live in it. They let down a cat into the well, and the moment she reached the place where the bad air was, she died. Not only can no animal live there, even no candle will burn in it. They let down a candle, and the moment it reached that spot, it went out. 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 Hindoos captured 146 Englishmen and shut them up in a cave underground, where the air could not get in. After the captured Englishmen had been there a few hours they began to die, and toward the end of the night 123 had died, and the rest came out more dead than alive, and ailing. At first the air had been good in the cave; but when the captives had inhaled all the good air, and no fresh air came in, it became bad, just like what was in the well, and they died. Why does the good air become bad when many people come together? Because, when people breathe, they take in good air and breathe out bad air. HOW BALLOONS ARE MADE If you take a blown-up bladder under water and let go of it, it will fly up to the surface of the water and will swim on it. Just so, when water is boiled in a pot, it becomes light at the bottom, over the fire,--it is turned into a gas; and when a little of that water-gas is collected it goes up as a bubble. First comes up one bubble, then another, and when the whole water is heated, the bubbles come up without stopping. Then the water boils. Just as the bubbles leap to the surface, full of vapoury water, because they are lighter than water, just so will a bladder which is filled with hydrogen, or with hot air, rise, because hot air is lighter than cold air, and hydrogen is lighter than any other gases.