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.

"It isn't always the explosiveness of a damp that makes