"Well, just the same way that a rattler'll never strike before giving you warning, 'fire damp' always gives you a chance ahead of time."
"How?"
"You know every miner carries a safety lamp?"
"Yes."
"'Fire damp' makes a sort of little cap over the flame of the lamp, like a small sugar-loaf hat. As soon as a miner sees this, he knows that there's enough 'gas' around to make it dangerous. As it's a gas that it doesn't do much harm to breathe, you see he can always make a get-away. Isn't that being a gentleman, all right?"
"Yes, I guess it is."
"Then there's 'black damp.' That's ordinary carbon dioxide, or carbonic acid gas."
"Isn't that just the stuff we breathe out?" questioned Eric.
"Exactly," his former schoolmate replied. "In an old mine, though, you've got to remember, nearly all the oxygen is absorbed by the coal. That gives a lot less chance for a leak of carbonic acid gas to mix with enough oxygen to keep the air pure. For 'black damp' though, the lamp's a good guide again. When a miner sees that his lamp is beginning to burn dim, it's a sign the air's short of oxygen."
"Of course," said Eric, "we used to have that experiment in our high school chemistry."