"I don't see that," interrupted Anton. "If dust is so bad, why do the bosses hang boards from all the gallery roofs and pile them high with dust?"
"Because the dust in those piles is stone dust, my boy," the young fellow explained. "When an explosion happens, it drives a big blast of air in front of it, so strong, sometimes, as to knock a man down. The blast of air blows all the stone dust from those boards and fills the air chock-full of it.
"This stone dust, usually made from crushed limestone or crushed shale, won't burn. The flame of the explosion can't pass through and the fire can't jump a rock-dust barrier. Even the flame of methane, which you know better as 'gas,' or fire damp, which has a terrific force, is choked back by this dense cloud of rock-dust, and, as you know, all coal mines have more or less methane gas."
"They don't, either," contradicted Otto. "I've worked in mines for years at a time an' never seen the 'cap' on the flame of the safety-lamp, tellin' there's fire damp there."
"You may not have seen it, but there was gas there, just the same. As for the cap-flame you're talking about, Otto, I'll admit that it's the surest way of telling when there's so much fire-damp that the mine is getting dangerous. But it's a risky test, just the same. You can't see the little cap of methane gas flame burning above the oil flame of the lamp until there's 2 per cent. of gas in the air of the mine, and a little more than 5 per cent. will start an explosion."
"What makes that cap?" queried Anton.
"Fire damp or methane gas burning inside the wire gauze of the safety-lamp."
"But if the gas is already burning inside, why doesn't it explode outside?"
"Just because it's a safety-lamp, my boy. That's why the flame burns inside a wire gauze. I'll explain that.
"Suppose you take a lamp with a hot flame—an alcohol or spirit lamp will do—and light it. Then hold a piece of close-meshed wire gauze right on the flame. You'll find that the flame will spread under the wire gauze but will not go through. Hold it long enough, though, until the wire gets red hot, and, quite suddenly, the flame will pass through and burn above the gauze as well as below.