"Try another trick. Put out the lamp and then hold the gauze just where it was before. You can light the flame above the wire but it will not pass below the gauze until the wire gets red-hot. That shows that gas which is not burning can pass through a wire gauze, but that gas which is aflame cannot pass until the wire is red-hot."
"Yes," said Anton, "I can see that."
"Very good. Then, if you have a lamp which is burning inside a cylinder of wire gauze, the gas of fire-damp can go through, and, if there's enough of it to burn, it will burn above the flame of the lamp, making an aureole or 'cap' just as Otto says. But the flaming gas can't get back through the wire gauze to set fire to the fire-damp outside, at least, not until the wire gets red-hot, which it's not likely to do, seeing that the gas is in the middle, not underneath it.
"That's how they test for fire-damp, nowadays. The flame of a safety-lamp is drawn down until it shows only a small yellow tip. If there's any fire-damp in the air, a light-blue halo appears over the yellow flame. At a little more than 1 per cent., an experienced man can judge that there is gas there, but the true 'cap,' which is pointed like a cone, doesn't show until there's 2 per cent. of the gas. At 3 per cent., the cap will be like a dunce's cap, and more than half an inch high. At 4 per cent., it will be over an inch high, and at 4-1/2 per cent. it'll form a column of blue flame. Then it's high time to get out of the mine, and to get out quickly.
"In the improved form of safety-lamps, the oil flame burns inside a glass, but the air which reaches the flame has to pass through two cylinders of wire gauze. The glass keeps the flame from ever touching the innermost gauze, and, if an accident happens—such as the breaking of the glass—it would still be fairly safe, for the burning gas inside wouldn't pass through the inner gauze until that got red-hot, and it wouldn't reach the outer gauze because the current of air passing down between the two layers of wire mesh would keep the outer gauze cool. This safety-lamp was invented by Sir Humphry Davy, in England, in 1815, just after a big explosion in an English colliery had cost hundreds of lives. All mines nowadays require that miners use either safety-lamps or electric lamps, and it's every miner's business to report to the boss when he sees a cap of burning gas inside his safety-lamp."
The old miner nodded his head in agreement.
"I won't use an electric lamp," he commented. "It's foolishness. The gas sprites ain't really malicious. They're willin' enough to give a warnin'. They'll put a cap on a flame if they don't want folks in that part of the mine. An electric lamp tells nothin'. It won't even give a warnin' against black damp."
"Perfectly true," Clem agreed. "With an oil safety-lamp, the flame gets dim or even goes out if there's too much black damp. The electric lamp burns on, just the same, because the light is in a vacuum. Black damp isn't so dangerous as fire damp, though. It only causes distress and hard breathing because it shows that there's too big a proportion of nitrogen and carbon dioxide in the air and not enough oxygen. It's oxygen that a man misses."
"But black damp'll explode, too," put in Otto.
"No," the other corrected, "it won't. But it often happens that there's fire-damp around when black damp is present and the black damp makes a test for gas difficult. It's the gas that explodes, not the black damp.