COAL
Coal is so much "a matter of course" in our daily life that most people
are only now, when its supply is becoming precarious, anxious to know
something of its nature and history. By the word "coal," or "coles,"
our ancestors understood what we now distinguish as "charcoal,"
prepared from wood by the "charcoal-burner," or "charbonnier," as
the French call him. What we now call "coal" was known to them as
"sea-coal," and, later, as "black" or "stone cole," to distinguish it
from "brown coal," known nowadays as "lignite," though the name "stone
coal" is locally applied in England to that very hard kind of black
coal also called "anthracite," of which jet is only an extremely hard
and dense variety found in small quantities in the oolitic strata of
Whitby, Spain, and other localities.
It is on record that in the year 1306 a citizen of London was tried,
condemned, and executed for burning "sea-coal." This severe treatment
was held to be justified by the poisonous and otherwise injurious
nature of the smoke produced by fires of sea-coal. I have not met with
any records of the earliest digging for and trade in "sea-coal," but
presumably it was obtained near the coast in the North of England and
brought to London by ship–hence its name. The coal-trade of Newcastle
began in the thirteenth century, but, owing to an Act of Parliament
in the reign of Edward I forbidding the use of sea-coal in London,
did not become important until the seventeenth century. It came very
gradually into use, and we find that Evelyn (the diarist) in 1661
noted the withering and bad condition of rose-bushes and other plants
in London gardens, which he attributed to the pestilential action
of the smoke given off by the newly introduced "sea-coal" which was
increasingly used as fuel in London houses. The sea-coal was not yet
largely, if at all, used in the production of iron; and Evelyn as a
forest-owner and lover of trees, has much to say about the necessity
for attention to the cultivation of our forests in connection with the
iron industry which then flourished in the Weald of Sussex; charcoal
procured by the slow burning or roasting of wood being the fuel used
in the smelting furnaces, whilst the ore was the orange-brown wealden
sand. It was during the eighteenth century that what we now call simply
"coal" came rapidly into use–not only for domestic heating, but for
furnaces of all kinds employed in industrial enterprise, and, at a
later date, for the earlier and later forms of steam-engines. The smoke
of the new coal was everywhere regarded as a terrible nuisance, and
a source of injury to both animal and vegetable life. The poisonous
action of coal-smoke is not due to the finely divided black particles
of carbon of which it largely consists, but to the sulphuric acid
derived from the small quantities of sulphur present in coal. It is
calculated that more than sixteen million tons of coal are annually
used in London alone for heating purposes, and that 480,000 tons of
black carbon powder are discharged over London by its chimneys every
year, together with very nearly the same weight of poisonous sulphuric
acid!
What, then, is this "sea-coal" or "coal" of our modern life? We all
know its black, glistening appearance, and more or less friable
character. Its nature and origin are best conveyed by the statement
that it is very ancient "peat," compressed and naturally changed by
chemical action and retaining little or no trace of its original
structure. Peat, as we know it from the low land of English and French
river valleys and the bogs of Scotland and Ireland, is formed by the
annual growth and death of "mosses" of several kinds and of other
accompanying vegetation. It retains the woody forms of the vegetable
growths which constitute it, and they are often but loosely adherent to
one another. Peat may be merely a growth of the past five years, but is
sometimes many thousand years old. Older than peat, and more caked and
compressed, is lignite, or brown coal, which occurs on the Continent of
Europe, also in South Devon and elsewhere, in geological strata newer
than those which yield our black coal. Then we have the most important
class of black coals which are known as "bituminous coals," because
they soften when heated and form hydrocarbons of both viscid and
gaseous nature. They are used for domestic purposes, and wherever flame
is desired. They are, in fact, the "lumps of coal" familiar in our
scuttles. The "bituminous coal" with the greatest amount of hydrogen
in it is the cannel or candle coal, so called from its bright flame
when burning. This kind is especially valuable for gas-making, and of
smaller value as fuel. The term "anthracite" is reserved for a hard,
stone-like coal which is very nearly pure carbon (ninety per cent).
This class of coal burns with a very small amount of flame, gives
intense heat, and no smoke. It is used in drying malt and hops.
Like all woody matter, that from which peat is formed consists of a
combination of the elements carbon, hydrogen and oxygen; and these
remain in somewhat changed chemical union in the brown coal, bituminous
coal, and anthracite. The carbon and a varying and small proportion of
the original hydrogen of the woody peat, are the important elements in
coal; and we may well ask how they come to be produced as a black or
dark brown mass from dead vegetable growths which are often bleached
and colourless. It is true that vegetable refuse does not necessarily
blacken when left to itself. We know that by roasting or charring wood
(or animals' flesh or bone) we can drive off the elements oxygen and
hydrogen and nitrogen (if there), and obtain a black mass of carbon
(so-called charcoal). That blackness is the actual true tint of carbon.
The dead weeds and leaves at the bottom of a stagnant pond break down
and form a pitch-black mud. They would not, and do not, go black
if exposed to the oxygen of the atmosphere; but at the bottom of a
stagnant pond or in a refuse heap they are excluded from the air, and
a microbe–a bacterium which has been carefully studied, and is of a
kind which can only flourish in the absence of free oxygen–attacks the
dead weeds, producing by change of their substance marsh-gas and black
carbon, the black mud emitting bubbles of gas which one may stir up
with a pole in such a pond. This chemical attack by anaërobic bacteria
goes on in the deeper layers of all marshes and stagnant pools, remote
from the oxygen of the air; and it is fairly certain that the black
coal which we find in strata of great geological age was so produced
by the action of special kinds of bacteria upon peat-like masses of
vegetable refuse. Indeed, by studying microscopic sections of coal,
numerous forms of bacteria have been recognized which might be capable
of effecting such chemical changes. On the other hand, we must remember
that it is not possible to conclude by form alone as to what subtle
chemical work a bacterium or bacillus or micro-coccus may be, or may
have been, carrying on. The peat-like deposits which became carbonized
and so formed the "coal" were probably masses of algæ, mosses and soft
aquatic plants, which were brought down and accumulated in swampy,
forest-covered ground about the mouths of rivers, the deposit being
covered in owing to rapid oscillations of level by beds of sand or
clay, followed by new growth and deposit.
Our British coal and a good deal of foreign coal is found in certain
stratified rocks of the earth's crust known as "the Carboniferous
System," about 12,000 ft. thick, consisting chiefly of very dense
limestone. The "seams," or stratified beds of coal, occur in sandy
rock known as the "Coal Measures," and vary in thickness from a mere
film to 40 ft. Above the Carboniferous System are later deposits, some
14,000 ft. in thickness–the Permian, Triassic, Jurassic, Cretaceous,
and Tertiary strata. Below them we find stratified deposits containing
fossilized remains of plants and animals, to a depth of another 40,000
ft.: they are the Devonian, Silurian, and Cambrian "systems" or series
of strata. Coal of a workable nature is found in many parts of the
world in the beds or strata of later age than our Coal Measures–namely,
those of Jurassic, Cretaceous, and Tertiary age.
Coal is so valuable and used in such vast quantities by modern man
that, though procured at first from beds lying at or near the surface,
it has been found remunerative to mine far into the depths of the
earth's surface, where its existence is ascertained, in order to
procure it. A depth of 4000 ft. is apparently the limit set to such
mining by the increase of temperature in mines which penetrate to
that extent below the surface. In 1905 the annual output of British
coal-mines was in round numbers 230,000,000 tons. It is certain that
there is a limit to this production, but not possible to calculate what
that limit may be, owing to the uncertainty as to the future working of
coal-fields as yet unexplored.
Such questions have been, and are being, considered by experts on
behalf of the Government. A matter of interest of another kind is
that in and associated with the coal seams of our Coal Measures,
fossilized remains of peculiar fern-like trees, ferns, and other
strange plants, and of very peculiar, extinct newt-like animals (as
large as crocodiles) are found in great variety. The notion that the
toads occasionally found embedded in the black mud of a coal-yard or
even in a fractured lump of coal are survivals from the time–many
millions of years past–when the plants and animals of the Coal Measure
swamps were living, is a baseless fancy. The toads so found are of the
kind or species now living on the earth–totally different from those
whose bones occur in the Coal Measures, and the presence of such modern
toads embedded in black slime, in coal-heaps in store-yards, or even in
coal-scuttles, is only what may be expected to occur and does occur in
damp quarries and other places where these familiar little beasts love
to hide.