Lignite.

Lignite or brown coal includes all varieties which are intermediate in
properties between wood and coals of the older formations. A coal of
this kind is generally to be distinguished by its brown colour, either
in mass or in the blacker varieties in the streak. The proportion of
carbon is comparatively low, usually not exceeding 70%, while the
oxygen and hygroscopic water are much higher than in true coals. The
property of caking or yielding a coherent coke is usually absent, and
the ash is often very high. The specific gravity is low when not brought
up by an excessive amount of earthy matter. Sometimes it is almost
pasty, and crumbles to powder when dried, so as to be susceptible of use
as a pigment, forming the colour known as Cologne earth, which resembles
umber or sepia. In Nassau and Bavaria woody structure is very common,
and it is from this circumstance that the term lignite is derived. The
best varieties are black and pitchy in lustre, or even bright and
scarcely to be distinguished from true coals. These kinds are most
common in Eastern Europe. Lignites, as a rule, are generally found in
strata of a newer geological age, but there are many instances of
perfect coals being found in such strata.

Ash of coal.

By the term "ash" is understood the mineral matter remaining unconsumed
after the complete combustion of the carbonaceous portion of a coal.
According to Couriot (_Annales de la societe geologique de Belgique_,
vol. xxiii. p. 105) the stratified character of the ash may be rendered
apparent in an X-ray photograph of a piece of coal about an inch thick,
when it appears in thin parallel bands, the combustible portion
remaining transparent. It may also be rendered visible if a smooth block
of free-burning coal is allowed to burn away quickly in an open fire,
when the ash remains in thin grey or yellow bands on the surface of the
block. The composition of the ashes of different coals is subject to
considerable variation, as will be seen by Table II.

Sulphur in coal.

The composition of the ash of true coal approximates to that of a
fire-clay, allowance being made for lime, which may be present either as
carbonate or sulphate, and for sulphuric acid. Sulphur is derived mainly
from iron pyrites, which yields sulphates by combustion. An indication
of the character of the ash of a coal is afforded by its colour, white
ash coals being generally freer from sulphur than those containing