At first the air had been pure in the dungeon; but when the prisoners had breathed up all the good air, and it was impossible to get any fresh supply, it became bad, like that in the well, and they died.
How is it that bad air is made out of good air, when many people are together?
Because when people breathe, the good air is taken into the lungs, and breathed out as bad air.
CHAPTER VI
how air balloons are made
If you take an inflated bladder and immerse it in water and then let go of it, the bladder rises to the top and begins to float. In exactly the same way if you boil water in a kettle, you will see on the bottom, over the fire, how the water becomes volatile, becomes a gas; and when a little of this aqueous gas collects it immediately rises to the top in the form of bubbles. First, one bubble flies up, then another, and when the water is thoroughly heated, then the bubbles rise unceasingly; then the water boils.
Just exactly as the bubbles filled with steam fly up to the top because they are lighter than water—so up through the atmosphere will rise a balloon inflated with hydrogen gas or with heated air, because heated air is lighter than cold air, and hydrogen is the lightest of all gases.
Air balloons are made of hydrogen or of heated air. This is the way they are made of hydrogen. A large bag is made and attached by ropes to stakes, and then it is filled with hydrogen gas. As soon as the ropes are cut, the balloon rises and floats until it escapes from the atmosphere that is heavier than hydrogen. But when it reaches a rarer part of the atmosphere, the balloon stops rising and then it floats along like a bubble on the top of the water.
Balloons are made of heated air in this manner: a
