-+
|_Lignite and Brown Coal._ | | | | | | | | | | | |
|13. Cologne | 1.100 | 63.29 | 4.98 | 26.24 | .. | 8.49 | .. | 66.97 | 5.27 | 27.76 |
|14. Bovey Tracy, Devonshire | .. | 66.31 | 5.63 | 22.86 | 0.57 | 2.36 | 2.36 | .. | 69.53 | 5.90 | 24.57 |
|15. Trifail, Styria | .. | 50.72 | 5.34 | 33.18 | 2.80 | 0.90 | 7.86 | .. | 55.11 | 5.80 | 39.09 |
+----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+

These properties are most highly developed in the substance known as
jet, which is a variety of cannel found in the lower oolitic strata of
Yorkshire, and is almost entirely used for ornamental purposes, the
whole quantity produced near Whitby, together with a further supply from
Spain, being manufactured into articles of jewellery at that town.

Caking coals.

When coal is heated to redness out of contact with the air, the more
volatile constituents, water, hydrogen, oxygen, and nitrogen are in
great part expelled, a portion of the carbon being also volatilized in
the form of hydrocarbons and carbonic oxide,--the greater part, however,
remaining behind, together with all the mineral matter or ash, in the
form of coke, or, as it is also called, "fixed carbon." The proportion
of this residue is greatest in the more anthracitic or drier coals, but
a more valuable product is yielded by those richer in hydrogen. Very
important distinctions--those of caking or non-caking--are founded on
the behaviour of coals when subjected to the process of coking. The
former class undergo an incipient fusion or softening when heated, so
that the fragments coalesce and yield a compact coke, while the latter
(also called free-burning) preserve their form, producing a coke which
is only serviceable when made from large pieces of coal, the smaller
pieces being incoherent and of no value. The caking property is best
developed in coals low in oxygen with 25 to 30% of volatile matters. As
a matter of experience, it is found that caking coals lose that property
when exposed to the action of the air for a lengthened period, or by
heating to about 300 deg. C., and that the dust or slack of non-caking
coal may, in some instances, be converted into a coherent coke by
exposing it suddenly to a very high temperature, or compressing it
strongly before charging it into the oven.