. Bituminous may be distinguished from
subbituminous by the manner of weathering. Other classifications have
attempted to recognize these difficulties and still maintain a purely
chemical basis by considering separately the combustible and
non-combustible volatile constituents. For this purpose, it is necessary
to have not merely approximate analyses, but the ultimate analyses in
terms of elements.

Definitions of the principal kinds of coal by Campbell,[18] of the
United States Geological Survey, are as follows:

_Anthracite._ Anthracite is generally well known and may be
defined as a hard coal having a fuel ratio (fixed carbon
divided by the volatile matter) of not more than 50 or 60 and
not less than 10.

_Semianthracite._ Semianthracite is also a hard coal, but it
is not so hard as true anthracite. It is high in fixed carbon,
but not so high as anthracite. It may be defined as a hard
coal having a fuel ratio ranging from 6 to 10. The lower limit
is uncertain, as it is difficult to say where the line should
be drawn to separate "hard" from "soft" coal and at the same
time to divide the two ranks according to their fuel ratio.

_Semibituminous._ The name "semibituminous" is exceedingly
unfortunate, as literally it implies that this coal is half
the rank of bituminous, whereas it is applied to a kind of
coal that is of higher rank than bituminous--really
superbituminous. Semibituminous coal may be defined as coal
having a fuel ratio ranging from 3 to 7. Its relatively high
percentage of fixed carbon makes it nearly smokeless when it
is burned properly, and consequently most of these coals go
into the market as "smokeless coals."

_Bituminous._ The term "bituminous," as generally understood,
is applied to a group of coals having a maximum fuel ratio of
about 3, and hence it is a kind of coal in which the volatile
matter and the fixed carbon are nearly equal; but this
criterion cannot be used without qualification, for the same
statement might be made of subbituminous coal and lignite. As
noted before, the distinguishing feature which serves to
separate bituminous coal from coals of lower rank is the
manner in which it is affected by weathering.

_Subbituminous._ The term "subbituminous" is adopted by the
Geological Survey for what has generally been called "bl