es out, and the coal
cakes in burning. Ordinary soft coal contains less, but still we can see
the resinous bitumen frying out of it as it burns. There is more heat
and less volatile matter in _steam coal_, so-called because it is the
fuel that most quickly forms steam in an engine. _Hard coal_ contains
but five to ten per cent. of volatile matter. It is slow to ignite and
burns with a small blue blaze.

From peat to anthracite coal I have named the series which increases
gradually in the amount of heat it gives out, and increases and then
decreases in its readiness to burn and in the brightness of its flame.
Anthracite coal has the highest amount of fixed carbon. This is the
reason why it makes the best fuel, for fixed carbon is the substance
which holds the store of imprisoned sunlight, liberated as heat when the
coal burns. Tremendous pressure and heat due to shrinking of the earth's
crust have crumpled and twisted the strata containing coal in eastern
Pennsylvania, and thus changed bituminous coal into anthracite. Ohio
beds, formed at the same time, but undisturbed by heat and pressure, are
bituminous yet.

The coal-beds of Rhode Island are anthracite, but the coal is so hard
that it will not burn in an open fire. The terrible, mountain-making
forces that crumpled these strata and robbed the coal of its volatile
matter, left so little of the gas-forming element, that a very special
treatment is necessary to make the carbon burn. It is used successfully
in furnaces built for the smelting of ores.

The last stage in the coal series is a black substance which we know as
black lead, or graphite. We write with it when we use a "lead" pencil.
This is anthracite coal after all of the volatile matter has been driven
out of it. It cannot burn, although it is solid carbon. The beds of
graphite have been formed out of coal by the same changes in the
earth's crust which have converted soft coal into anthracite.

The tremendous pressure that bears on the coal measures has changed a
part of the carbon into liquid and gaseous form. Lakes of oil have been
tapped from which jets of great force have spouted out. Such
accumulations of oil usually fill porous layers of sandstone and are
confined by overlying and underlying beds of impervious clay. Gas may be
similarly confined until a well is drilled, relieving the pressure, and
furnishing abundant or scanty supply of this valuable fuel. Western
Pennsylvania coal-fields have beds of