ceedingly
thin slices, such as made for microscopic investigation, which are
imperfectly transparent, and of a dark brown colour by transmitted
light. The streak is black in anthracite, but more or less brown in the
softer varieties. The maximum hardness is from 2.5 to 3 in anthracite
and hard bituminous coals, but considerably less in lignites, which are
nearly as soft as rotten wood. A greater hardness is due to the presence
of earthy impurities. The densest anthracite is often of a semi-metallic
lustre, resembling somewhat that of graphite. Bright, glance or pitch
coal is another brilliant variety, brittle, and breaking into regular
fragments of a black colour and pitchy lustre. Lignite and cannel are
usually dull and earthy, and of an irregular fracture, the latter being
much tougher than the black coal. Some lignites are, however, quite as
brilliant as anthracite; cannel and jet may be turned in the lathe, and
are susceptible of taking a brilliant polish. The specific gravity is
highest in anthracite and lowest in lignite, bituminous coals giving
intermediate values (see TABLE I.). As a rule, the density increases
with the amount of carbon, but in some instances a very high specific
gravity is due to intermixed earthy matters, which are always denser
than even the densest form of coal substance.

Coal is never definitely crystalline, the nearest approach to such a
structure being a compound fibrous grouping resembling that of gypsum or
arragonite, which occurs in some of the steam coals of South Wales, and
is locally known as "cone in cone," but no definite form or arrangement
can be made out of the fibres. Usually it occurs in compact beds of
alternating bright and dark bands in which impressions of leaves, woody
fibre and other vegetable remains are commonly found. There is generally
a tendency in coals towards cleaving into cubical or prismatic blocks,
but sometimes the cohesion between the particles is so feeble that the
mass breaks up into dust when struck. These peculiarities of structure
may vary very considerably within small areas; and the position of the
divisional planes or cleats with reference to the mass, and the
proportion of small coal or slack to the larger fragments when the coal
is broken up by cutting-tools, are points of great importance in the
working of coal on a large scale.

The divisional planes often contain small films of other minerals, the
commonest being calcite, gypsum and iron pyri