he pieces are
of nearly the same size and free from dirt, coal dust, and slate. The
work of preparation is done in odd-shaped buildings called "breakers."
One part of a breaker is often a hundred or a hundred and fifty feet
in height. The coal is carried to the top of the breaker. From there
it makes a journey to the ground, but something happens to it every
little way. It goes between rollers, which crush it; then over
screens, through which the smaller pieces fall. Sometimes the screens
are so made that the coal will pass over them, while the thin, flat
pieces of slate will fall through. In spite of all this, bits of coal
mixed with slate sometimes slide down with the coal, and these are
picked out by boys. A better way of getting rid of them is now coming
into use. This is to put the coal and slate into moving water. The
slate is heavier than the coal, and sinks; and so the coal can easily
be separated from it. Dealers have names for the various sizes of
coal. "Egg" must be between two and two and five eighths inches in
diameter; "nut" between three fourths and one and one eighth inches;
"pea" between one half and three fourths of an inch.

Mining coal is dangerous work. Any blow of the pickaxe may break into
a vein of water which will burst out and flood the mine. The wooden
props which support the roof may break, or the pillars of coal may not
be large enough; and the roof may fall in and crush the workers. There
are always poisonous gases. The coal, as has been said before, was
made under water, and therefore the gas which was formed in the
decaying leaves and wood could not escape. It is always bubbling out
from the coal, and at any moment a pickaxe may break into a hole that
is full of it. One kind of gas is called "choke-damp," because it
chokes or suffocates any one who breathes it. There is also
"white-damp," the gas which you see burning with a pretty blue flame
over a hot coal fire. Worst of all is the "fire-damp." If you stir up
the water in a marsh, you will see bubbles of it rise to the surface.
It is harmless in a marsh, but quite the opposite in a mine. When it
unites with a certain amount of air, it becomes explosive, and the
least bit of flame will cause a terrible explosion. Even coal dust may
explode if the air is full of it, and it is suddenly set in motion by
too heavy a blast of powder.

Miners used to work by candlelight. Every one knew how dangerous this
was; but no one found any better way