includes all varieties
of carbonaceous minerals used as fuel, but it is now usual in England to
restrict it to the particular varieties of such minerals occurring in
the older Carboniferous formations. On the continent of Europe it is
customary to consider coal as divisible into two great classes,
depending upon differences of colour, namely, _brown coal_,
corresponding to the term "lignite" used in England and France, and
_black_ or _stone coal_, which is equivalent to coal as understood in
England. Stone coal is also a local English term, but with a
signification restricted to the substance known by mineralogists as
anthracite. In old English writings the terms pit-coal and sea-coal are
commonly used. These have reference to the mode in which the mineral is
obtained, and the manner in which it is transported to market.

The root _kol_ is common to all the Teutonic nations, while in French
and other Romance languages derivatives of the Latin _carbo_ are used,
e.g. _charbon de terre_. In France and Belgium, however, a peculiar
word, _houille_, is generally used to signify mineral coal. This word is
supposed to be derived from the Walloon _hoie_, corresponding to the
medieval Latin _hullae_. Littre suggests that it may be related to the
Gothic _haurja_, coal. Anthracite is from the Greek [Greek: anthrax],
and the term _lithanthrax_, stone coal, still survives, with the same
meaning, in the Italian _litantrace_.

It must be borne in mind that the signification now attached to the word
coal is different from that which formerly obtained when wood was the
only fuel in general use. Coal then meant the carbonaceous residue
obtained in the destructive distillation of wood, or what is known as
charcoal, and the name collier was applied indifferently to both
coal-miners and charcoal-burners.

The spelling "cole" was generally used up to the middle of the 17th
century, when it was gradually superseded by the modern form, "coal."
The plural, coals, seems to have been used from a very early period to
signify the broken fragments of the mineral as prepared for use.

Physical properties.

Coal is an amorphous substance of variable composition, and therefore
cannot be as strictly defined as a crystallized or definite mineral can.
It varies in colour from a light brown in the newest lignites to a pure
black, often with a bluish or yellowish tint in the more compact
anthracite of the older formations. It is opaque, except in ex