rieties fall near the border lines of the main groups, and their
specific naming then becomes difficult. In addition, coal is made up of
several substances which vary unequally in their proportions. It is
difficult to arrange all of these variables in a graded series in such a
fashion as to permit of precise naming of the coal. Furthermore, the
scientific naming of a coal may not serve the purpose of discriminating
coals used for different commercial purposes. Even the commercial names
vary among themselves, depending on the use for which the coal is being
considered.

Thus it is that the naming and classification of coals is a perennial
source of difficulty and controversy. The earliest and most widely used
classification is based on the ratio between fixed (or non-volatile)
carbon and volatile constituents, called the "fuel ratio." For this
purpose "proximate" analyses of coal are made, in terms of fixed carbon,
volatile matter, moisture, ash, and sulphur. Anthracite has a higher
fuel ratio than bituminous coal; that is, it has more fixed carbon in
relation to volatile matter. Similarly bituminous coal has a higher fuel
ratio than lignite. The fuel ratio measures roughly the heat or
calorific power of the coal, in other words, its fuel value. However,
some bituminous coals have a higher calorific power than some
anthracites, because a large part of their volatile matter is
combustible and yields more heat than the corresponding weight of fixed
carbon in the anthracite. The fuel ratio pretty well discriminates coals
of the higher ranks, and gives a classification corresponding roughly
with their commercial uses. For the lower ranks of coal it is not so
satisfactory, because the volatile constituents of such coals contain
large and varying percentages of non-combustible hydrogen, oxygen, and
nitrogen. Also such coals contain larger and more variable amounts of
moisture, which is inert to combustion and requires heat for its
evaporation. Two coals of the lower ranks with the same fuel ratio may
have very different fuel qualities and different commercial uses,
because of their different amounts of inert volatile matter and of
water. For these coals it is sometimes desirable to supplement the
chemical classification by physical criteria. For instance,
subbituminous coal may be distinguished from lignite, not by its fuel
ratio alone, but by its shiny, black appearance as contrasted with the
dull, woody appearance of lignite