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.

"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