ting
powder, a little is lost by the rusting of iron pyrites--which is
found in many coal mines--and a lot of it is taken up by the coal,
just how, we don't quite know."

"It's good to hear o' somethin' you don't know," the old miner
remarked sarcastically. "But you're talkin' about dry air, an' the air
in most mines is moist."

"Quite right," Clem agreed. "It has to be. Mine air is made moist, on
purpose, especially in winter."

"It is?" Otto's voice expressed unqualified astonishment.

"It certainly is! In most coal-mines--this one, for instance--all the
air that passes down the intake shaft is moistened by a spray of mixed
water and air, so finely atomized that it floats like a cloud."

"What for? It's easier to work in dry air'n moist air."

"It's easier to get blown up, too! In winter time, Otto, the air above
ground is a lot colder than the air in the mine. Cold air can't hold
as much moisture as warm air, and as soon as air gets warmed up a bit,
it tries its hardest to absorb any moisture with which it happens to
come in contact.

"What happens in a mine, in such a case? Why, as the cold air from
above passes through the galleries of a mine, it gets warmed up. As it
warms up, it draws out from the roofs, the ribs, and the floors all
the water that there is to draw, and makes the mine dead dry. When
coal dust is absolutely dry, it crumbles into finer and finer dust,
until at last the particles are so small that they float in the air.
Then comes disaster, for finely divided coal dust is so explosive that
the smallest flame--even a spark from the stroke of a pick--will set
the whole mine ablaze."

"I don't see that," interrupted Anton. "If dust is so bad, why do the
bosses hang boards from all the gallery roofs and pile them high with
dust?"

"Because the dust in those piles is stone dust, my boy," the young
fellow explained. "When an explosion happens, it drives a big blast of
air in front of it, so strong, sometimes, as to knock a man down. The
blast of air blows all the stone dust from those boards and fills the
air chock-full of it.

"This stone dust, usually made from crushed limestone or crushed
shale, won't burn. The flame of the explosion can't pass through and
the fire can't jump a rock-dust barrier. Even the flame of methane,
which you know better as 'gas,' or fire damp, which has a terrific
force, is choked back by this dense cloud of rock-dust, and, as you
know, all coal mines have more or