below ground all their lives."

"You can explain away anything, if you have a mind to," Otto retorted
scornfully. "But as long as men are workin' below ground, there's
goin' to be knockers an' sprites o' the damps, an' miners is goin' to
be killed. Me, I've escaped. Why? Because I'm chock-full o' science
an' modern ideas? Not a bit of it! I get along because I know what the
spirits o' the mine expect, an' I give it to 'em. Right now, I'm the
oldest man at work, here, an' I ain't never had an accident."

"Don't you believe his stories, Anton," the young miner protested,
turning to the boy. "Those antiquated notions will only lead you
astray. The 'damps' are just various kinds of gases coming out of the
coal, and the way to fight them is to keep a strong current of air
going through the mine."

"How do they come out o' the coal, if you know so much?" questioned
Otto, belligerently.

"Sure I know! But I don't suppose telling you will change your ideas."

"It won't," the old miner admitted frankly. "But I've had my say, an'
it's only fair to let you have yours. The youngster, here, can believe
which o' the two he pleases."

"Well, it's something this way," Clem began, casting about in his mind
for a way to explain the chemistry of mine air as simply as he could.
"Ordinary air--the air above ground--is made up of a little less than
21 per cent. of oxygen and a little more than 78 per cent. of
nitrogen. The rest of it is a mixture of carbon and oxygen which the
books call carbon dioxide or black damp, with some other rare gases
beside.

"Now, all animals, including man, depend for their life on the oxygen
in the air. If the oxygen drops to 15 per cent., a man will suffer.
That's not likely to happen where miners' lamps or safety-lamps are
used, because the flame of a lamp goes out when there's less than 17
per cent. oxygen. Even at 19 per cent., a lamp will burn so dimly as
to warn of danger. The nitrogen in the air is inert, that is, it does
neither good nor harm to man. But what I want you to remember, Anton,
is that even in the purest air above ground, there's always some
'black damp,' so it's a bit hard to see where Otto's goblin women come
in!

"Now, when pure air comes down a coal shaft, a lot of changes happen
to it. Some of the oxygen is consumed by the breathing of the men and
animals in the mine--if there are any donkeys or such--some is taken
up by the burning of lamps, some more by the explosion of blas