Illustration: MINERALS (‡ INDUSTRIAL SCENE).
This, the richest product of the mine, the well-known fuel used in
almost every branch of industry as well as for warming our houses,
is got from the depths of the earth, where it exists in certain
localities forming what are called “coal fields” or “basins.” It is
the result of changes produced during many ages upon vegetable matter
buried during the various convulsions which the earth has undergone,
and pressed into layers or strata of various thicknesses. To raise
this valuable fuel, powerful machinery is used and deep shafts are
sunk at an enormous expense.
Coal exists in various forms: the following are the most easily
recognised:--
1.--Cubical coal; shining and easily broken into squarish fragments.
It burns brightly.
2.--Slate coal; dull in color, splits like slate. Burns well.
3.--Cannel coal; dull color, breaks like resin, and somewhat
resembles jet. Burns brilliantly, and splits with a crackling noise
when in the fire. It affords the best gas.
4.--Glance, or Kilkenny coal; steel-grey color and metallic lustre.
Burns without flame or smoke, somewhat like charcoal.
5.--Lignite, or brown coal; is an imperfectly formed coal.
The quantity of coal used is rapidly increasing owing to the
extensive number of steam engines used, especially for navigation
and railway transit; and it is a problem, not yet determined, how
long the coal existing in Great Britain is likely to last; the lowest
statements make it but little below a thousand years.