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."

"We did. But do you remember just how much oxygen a lamp has to have?"

"No," the boy was forced to admit, "I've plumb forgot."

"A safety lamp will go right out with less than seventeen per cent. of
oxygen, while a man can live fairly comfortably on fifteen or sixteen
per cent. So the flickering out of a lamp is a sure sign that the danger
line's not far off."

"It's a gentleman, too, then," said Eric with a laugh.

"Yes," the other assented dubiously, "but there's less margin. Now,
'white damp,' or carbon monoxide, is a horse of a different color.
That's the real danger, Eric. Pretty nearly all the cases of poisoning
in mines are due to 'white damp.' Just the other day, in Pennsylvania,
two hundred men were killed--whouf!--just like blowing out a match. But
'white damp' hasn't got any effect on the flame of a safety lamp. If
anything, it may hit it up even a trifle brighter. So the lamp isn't
any good. That's where the mice come in."

"Mice? I thought you said canaries!"

"We use both mice and canaries. When you haven't got a canary, take a
mouse."

"Which is the better?"

"Canary! 'White damp' catches him quicker. That means he gives an
earlier warning. A canary will fall off his perch in four minutes when
the air's only got one-fifth of one per cent, of 'white damp.'"

"And how long could a fellow stand that much of the gas?"

"About ten minutes, without being really put to the bad, though twenty
minutes of it would make him mighty sick. You see, that gives a party
six minutes clear before any harm's done. Any time a canary gives