14. _Papering_, or fixing them in the paper. This is done by children,
who acquire the habit of putting up 36,000 per day.
The pin manufacture is one of the greatest prodigies of the division of
labour; it furnishes 12,000 articles for the sum of three shillings,
which have required the united diligence of fourteen skilful operatives.
The above is an outline of the mode of manufacturing pins by hand
labour, but several beautiful inventions have been employed to make them
entirely or in a great measure by machinery; the consumption for home
sale and export amounting to 15 millions daily, for this country alone.
One of the most elaborate and apparently complete, is that for which Mr.
L. W. Wright obtained a patent in May, 1824. A detailed description of
it will be found in the 9th volume of Newton’s London Journal. The
following outline will give my readers an idea of the structure of this
ingenious machine:--
The rotation of a principal shaft mounted with several cams, gives
motion to various sliders, levers, and wheels, which work the different
parts. A slider pushes pincers forwards, which draw wire from a reel, at
every rotation of the shaft, and advance such a length of wire as will
produce one pin. A dye cuts off the said length of wire by the descent
of its upper chap; the chap then opens a carrier, which takes the pin to
the pointing apparatus. Here it is received by a holder, which turns
round, while a bevel-edged file-wheel rapidly revolves, and tapers the
end of the wire to a point. The pin is now conducted by a second carrier
to a finer file-wheel, in order to finish the point by a second
grinding. A third carrier then transfers the pin to the first heading
die, and by the advance of a steel punch, the end of the pin wire is
forced into a recess, whereby the head is partially swelled out. A
fourth carrier removes the pin to a second die, where the heading is
perfected. When the heading-bar retires, a forked lever draws the
finished pin from the die, and drops it into a receptacle below.
I believe the chief objection to the raising of the heads by strong
mechanical compression upon the pins, is the necessity of softening the
wire previously; whereby the pins thus made, however beautiful to the
eye, are deficient in that stiffness which is so essential to their
employment in many operations of the toilet.
PIPERINE, is a crystalline principle extracted from black pepper, by
means of alcohol. It is colourless, has hardly any taste, fuses at 212°
F.; is insoluble in water, but soluble in acetic acid, ether, and most
readily in alcohol.
PITCH, MINERAL, is the same as BITUMEN and ASPHALT.
PITCH _of wood-tar_ (_Poix_, Fr.; _Pech_, Germ.); is obtained by boiling
tar in an open iron pot, or in a still, till the volatile matters be
driven off. Pitch contains, pyrolignous resin, along with colophany
(common rosin), but its principal ingredient is the former, called by
Berzelius pyretine. It is brittle in the cold, but softens and becomes
ductile with heat. It melts in boiling water, and dissolves in alcohol
and oil of turpentine, as well as in carbonated or caustic alkaline
lyes. For PYRETINE, see the mode of preparing it from birch wood, for
the purpose of preparing _Russia_ LEATHER.
PITCOAL. (_Houille_, Fr.; _Steinkohle_, Germ.) This is by far the most
valuable of mineral treasures, and the one which, at least in Great
Britain, makes all the others available to the use and comfort of man.
Hence it has been searched after with unremitting diligence, and worked
with all the lights of science, and the resources of art.
The Brora coal-field in Sutherlandshire is the most remarkable example
in this, or in perhaps any country hitherto investigated, of a pseudo
coal-basin among the deeper secondary strata, but above the new
sandstone or red marl formation. The Rev. Dr. Buckland and Mr. C. Lyell,
after visiting it in 1824, had expressed an opinion that the strata
there were wholly unconnected with the proper coal formation below the
new red sandstone, and were in fact the equivalent of the oolitic
series; an opinion fully confirmed by the subsequent researches of Mr.
Murchison. (_Geol. Trans._ for 1827, p. 293.) The Brora coal-field forms
a part of those secondary deposits which range along the south-east
coast of Sutherlandshire, occupying a narrow tract of about twenty miles
in length, and three in its greatest breadth.
One stratum of the Brora coal-pit is a coal-shale, composed of a
reed-like striated plant of the natural order _Equisetum_, which seems
to have contributed largely towards the formation of that variety of
coal. From this coal-shale, the next transition upwards is into a purer
bituminous substance approaching to _jet_, which constitutes the great
bed of coal. This is from 3 feet 3 inches to 3 feet 8 inches thick, and
is divided nearly in the middle by a thin layer of impure indurated
shale charged with pyrites, which, if not carefully excluded from the
mass, sometimes occasions spontaneous combustion upon exposure to the
atmosphere; and so much indeed is that mineral disseminated throughout
the district, that the shales might be generally termed “pyritiferous.”
Inattention on the part of the workmen, in 1817, in leaving a large
quantity of this pyritous matter to accumulate in the pit, occasioned a
spontaneous combustion, which was extinguished only by excluding the
air; indeed the coal-pit was closed in and remained unworked for four
years. The fires broke out again in the pit in 1827.
The purer part of the Brora coal resembles common pitcoal; but its
powder has the red ferruginous tinge of pulverized lignites. It may be
considered one of the last links between lignite and true coal,
approaching very nearly in character to jet, though less tenacious than
that mineral; and, when burnt, exhaling but slightly the vegetable odour
so peculiar to all imperfectly bituminized substances. The fossil
remains of shells and plants prove the Brora coal to be analogous to
that of the eastern moorlands of Yorkshire, although the extraordinary
thickness of the former, compared with any similar deposit of the latter
(which never exceeds from 12 to 17 inches), might have formerly led to
the belief that it was a detached and anomalous deposit of true coal,
rather than a lignite of any of the formations _above_ the new red
sandstone: such misconception might more easily arise in the infancy of
geology, when the strata were not identified by their fossil organic
remains.
On the coast of Yorkshire the strata of this pseudo coal formation
appear in the following descending order, from Filey Bay to Whitby. 1.
Coral-rag. 2. Calcareous grit. 3. Shale, with fossils of the Oxford
clay. 4. Kelloway rock (swelling out into an important arenaceous
formation). 5. Cornbrash. 6. Coaly grit of Smith. 7. Pier-stone
(according to Mr. Smith, the equivalent of the great oolite). 8.
Sandstone and shale, with _peculiar plants and various seams of coal_.
9. A bed with fossils of the inferior oolite. 10. Marl-stone? 11.
Alum-shale or lias. All the above strata are identified by abundant
organic remains.
In the oolitic series, therefore, where the several strata are developed
in conformity with the more ordinary type of these formations, we may
venture to predict with certainty, that no carboniferous deposits of any
great value will ever be discovered, at all events in Great Britain. A
want of such knowledge has induced many persons to make trials for coal
in beds subordinate to the English oolites, and even superior to them,
in places where the type of formation did not offer the least warrant
for such attempts.
The third great class of terrestrial strata, is the proper
coal-measures, called the _carboniferous rocks_, our leading object
here, and to which we shall presently return.
The transition rocks which lie beneath the coal-measures, and above the
primitive rocks, or are anterior to the carboniferous order, and
posterior to the primitive, contain a peculiar kind of coal, called
anthracite or stone-coal, approaching closely in its nature to carbon.
It is chiefly in the transition clay-slate that the anthracite occurs in
considerable masses. There is one in the transition slate of the little
Saint Bernard, near the village of _la Thuile_ (in the Alps). It is 100
feet long, and 2 or 3 yards thick. The coal burns with difficulty, and
is used only for burning lime. There are several of the same kind in
that country, which extend down the reverse slope of the mountains
looking to Savoy. The slate enclosing them presents vegetable
impressions of reeds or analogous plants. To the transition clay-slate
we must likewise refer the beds of anthracite that M. Hericart de Thury
observed at very great heights in the Alps of Dauphiny, in a formation
of schist and grey-wacke with vegetable impressions, which reposes
directly on the primitive rocks.
The great carboniferous formation may be subdivided into four orders of
rocks: 1. the coal-measures, including their manifold alternations of
coal-beds, sandstones, and shales; 2. the millstone grit and shale
towards the bottom of the coal-measures; 3. the carboniferous limestone,
which projecting to considerable heights above the outcrop of the coal
and grit, acquires the title of mountain limestone; 4. the old red
sandstone, or connecting link with the transition and primary rock basin
in which the coal system lies.
The coal-fields of England, from geographical position, naturally fall
under the following arrangement:--1. The _great northern district_;
including all the coal-fields north of Trent. 2. The _central district_;
including Leicester, Warwick, Stafford, and Shropshire. 3. The _western
district_; subdivided into _north-western_, including North Wales, and
the _south-western_, including South Wales, Gloucester, and
Somersetshire.
There are three principal coal-basins in Scotland: 1. that of Ayrshire;
2. that of Clydesdale; and 3. that of the valley of the Forth, which
runs into the second in the line of the Union Canal. If two lines be
drawn, one from Saint Andrews on the northeast coast, to Kilpatrick on
the Clyde, and another from Aberlady, in Haddingtonshire, to a point a
few miles south of Kirkoswald in Ayrshire, they will include between
them the whole space where pitcoal has been discovered and worked in
Scotland.
[Illustration: Mendip hills. Dundry hill. Wick rocks. Fog hill, N. of
Lansdowne.
794]
The great coal-series consists of a regular alternation of mineral
strata deposited in a great concavity or basin, the sides and bottom of
which are composed of transition rocks. This arrangement will be clearly
understood by inspecting _fig._ 794., which represents a section of the
coal-field south of Malmsbury.
1, 1, old red sandstone; 2, mountain limestone; 3, millstone grit; 4, 4,
coal seams; 5, Pennant, or coarse sandstone; 6, new red sandstone, or
red marl; 7, 7, lias; 8, 8, inferior oolite; 9, great oolite; 10,
cornbrash and Forest marble.
No. 1., or the old red sandstone, may therefore be regarded as the
characteristic lining of the coal basins; but this sandstone rests on
transition limestone, and this limestone on grey-wacke. This methodical
distribution of the carboniferous series is well exemplified in the
coal-basin of the Forest of Dean in the south-west of England, and has
been accurately described by Mr. Mushet.
The _grey-wacke_ consists of highly inclined beds of slaty micaceous
sandstone, which on the one hand alternates with and passes into a
coarse breccia, having grains as large as peas; on the other, into a
soft argillaceous slate. The grey-wacke stands bare on the north-eastern
border of the Forest, near the southern extremity of the chain of
transition limestone, which extends from Stoke Edith, near Hereford, to
Flaxley on the Severn. It is traversed by a defile, through which the
road from Gloucester to Ross winds. The abruptness of this pass gives it
a wild and mountainous character, and affords the best opportunity of
examining the varieties of the rock.
The _Transition limestone_ consists in its _lower beds_ of fine-grained,
tender, extremely argillaceous slate, known in the district by the name
of _water-stone_, in consequence of the wet soil that is found wherever
it appears at the surface. Calcareous matter is interspersed in it but
sparingly. Its _upper beds_ consist of shale alternating with extensive
beds of stratified limestone. The lowest of the calcareous strata are
thin, and alternate with shale. On these repose thicker strata of more
compact limestone, often of a dull blue colour. The beds are often
dolomitic, which is indicated by straw yellow colour, or dark pink
colour, and by the sandy or glimmering aspect of the rock.
The _old red sandstone_, whose limits are so restricted in other parts
of England, here occupies an extensive area. The space which it covers,
its great thickness, its high inclination, the abrupt character of the
surface over which it prevails, and the consequent display of its strata
in many natural sections, present in this strict advantages for studying
the formation, which are not to be met with elsewhere in South Britain.
In the neighbourhood of Mitchel Dean, the total thickness of this
formation, interposed conformably between the transition and mountain
limestone, is from 600 to 800 fathoms. The old red sandstone is
characterized in its upper portion by the presence of siliceous
conglomerate, containing siliceous pebbles, which is applied extensively
to the fabrication of millstones near Monmouth, and on the banks of the
Wye. This sandstone encircles the Forest with a ring of very elevated
ground, whose long and lofty ridges on the eastern frontier overhang the
valley of the Severn.
The _mountain limestone_, or carboniferous, is distinguished from
transition limestone, rather by its position than by any very wide
difference in its general character or organic remains. According to the
measurements of Mr. Mushet, the total thickness of the mountain
limestone is about 120 fathoms. The zone of limestone belonging to this
coal-basin, is from a furlong to a mile in breadth on the surface of the
ground, according as the dip of the strata is more or less rapid. The
angle of dip on the northern and western border is often no more than
10°, but on the eastern it frequently amounts to 80°. The calcareous
zone that defines the outer circle of the basin, suffers only one short
interruption, scarcely three miles in length, where in consequence of a
fault the limestone disappears, and the coal-measures are seen in
contact with the old red sandstone.
_Coal measures._--Their aggregate thickness amounts, according to Mr.
Mushet, to about 500 fathoms. 1. The lowest beds, which repose on the
mountain limestone, are about 40 fathoms thick, and consist here, as in
the Bristol coal-basin, of a red siliceous grit, alternating with
conglomerate, used for millstones; and with clay, occasionally used for
ochre. 2. These beds are succeeded by a series about 120 fathoms thick,
in which a grey gritstone predominates, alternating in the lower part
with shale, and containing 6 seams of coal. The grits are of a fissile
character, and are quarried extensively for flag-stone, ashlers, and
fire-stone. 3. A bed of grit, 25 fathoms thick, quarried for
hearth-stone, separates the preceding series from the following, or the
4th, which is about 115 fathoms thick, and consists of from 12 to 14
seams of coal alternating with shale. 5. To this succeeds a
straw-coloured sandstone, nearly 100 fathoms thick, forming a high ridge
in the interior of the basin. It contains several thin seams of coal,
from 6 to 16 inches in thickness. 6. On this reposes a series of about
12 fathoms thick, consisting of 3 seams of coal alternating with shale.
7. This is covered with alternate beds of grit and shale, whose
aggregate thickness is about 100 fathoms, occupying a tract in the
centre of the basin about 4 miles long, and 2 miles broad. The sandstone
No. 5. is probably the equivalent of the Pennant in the preceding
figure.
The floor, or pavement, immediately under the coal beds is, almost
without exception, a grayish-slate clay, which, when made into bricks,
strongly resists the fire. This fire-clay varies in thickness from a
fraction of an inch to several fathoms. Clay ironstone is often
disseminated through the shale.
[Illustration: 795 796 797]
The most complete and simplest form of a coal-field is the entire
basin-shape, which we find in some instances without a dislocation. A
beautiful example of this is to be seen at Blairengone, in the county of
Perth, immediately adjoining the western boundary of Clackmannanshire,
as represented in _fig._ 795., where the outer elliptical line, marked
A, B, C, D, represents the crop, outburst, or basset edge of the lower
coal, and the inner elliptical line represents the crop or basset edge
of the superior coal. _Fig._ 796. is the longitudinal section of the
line A B; and _fig._ 797. the transverse section of the line C D. All
the accompanying coal strata partake of the same form and parallelism.
These basins are generally elliptical, sometimes nearly circular, but
are often very eccentric, being much greater in length than in breadth;
and frequently one side of the basin on the short diameter has a much
greater dip than the other, which circumstance throws the trough or
lower part of the basin concavity much nearer to the one side than to
the other. From this view of one entire basin, it is evident that the
dip of the coal strata belonging to it runs in opposite directions, on
the opposite sides, and that all the strata regularly crop out, and meet
the alluvial cover in every point of the circumferential space, like the
edges of a nest of common basins. The waving line marks the river Devon.
[Illustration: 798 799 800]
It is from this basin shape that all the other coal-fields are formed,
which are segments of a basin produced by slips, dikes, or dislocations
of the strata. If the coals (_fig._ 795.) were dislocated by two slips
_b c_ and _d e_, the slip _b c_ throwing the strata _down_ to the east,
and the slip _d e_ throwing them as much _up_ in the same direction, the
outcrops of the coals would be found in the form represented in _fig._
798., of which _fig._ 799. is the section in the line A B, and _fig._
800. the section in the line C D.
The chief difficulty in exploring a country in search of coal, or one
where coal-fields are known to exist, arises from the great thickness of
alluvial and other cover, which completely hides the outcrop or basset
edge of the strata, called by miners the _rock-head_; as also the
fissures, dikes, and dislocations of the strata, which so entirely
change the structure and bearings of coal-fields, and cause often great
loss to the mining adventurer. The alluvial cover on the other hand is
beneficial, by protecting the seams of the strata from the superficial
waters and rains, which would be apt to drown them, if they were naked.
In all these figures of coal-basins, the letter _a_ indicates coal.
The absolute shape of the coal-fields in Great Britain has been
ascertained with surprising precision. To whatever depth a coal-mine is
drained of its water, from that depth it is worked, up to the rise of
the water-level line, and each miner continues to advance his room or
working-place, till his seam of coal meets the alluvial cover of the
outcrop, or is cut off by a dislocation of the strata. In this way the
miner travels in succession over every point of his field, and can
pourtray its basin-shape most minutely.
[Illustration: 801]
_Fig._ 801. represents a horizontal plan of the Clackmannanshire
coal-field, as if the strata at the outcrop all around were denuded of
the alluvial cover. Only two of the concentric beds, or of their edges
_a_, _a_, are represented, to avoid perplexity. It is to be remembered,
however, that all the series of attendant strata lie parallel to the
above lines. This plan shows the Ochill mountains, with the north
coal-fields, of an oblong elliptical shape, the side of the basin next
the mountains being precipitous, as if upheaved by the eruptive
trap-rocks; while the south, the east, and the west edges of the basin
shelve out at a great distance from the lower part of the concavity or
_trough_, as miners call it. Thus the alternate beds of coal, shale, and
sandstone, all nearly concentric in the north coal-field, dip inwards
from all sides towards the central area of the _trough_. The middle
coal-field of this district, however, which is formed by the great north
slip, is merely the segment of an elliptical basin, where the strata dip
in every direction to the middle of the axis marked with the letter X;
being the deepest part of the segment. The south coal-field, formed by
the great south slip, is likewise the segment of another elliptical
basin, similar in all respects to the middle coal-field. Beyond the
outcrop of the coals and subordinate strata of the south coal-fields,
the counter dip of the strata takes place, producing the mantle-shaped
form; whence the coal strata in the Dunmore field, in Stirlingshire, lie
in a direction contrary to those of the south coal-field of
Clackmannanshire. O, are the Ochill mountains.
[Illustration: 802]
_Fig._ 802. is intended to represent an extensive district of country,
containing a great coal-basin, divided into numerous subordinate
coal-fields by these dislocations. The lines marked _b_ are slips, or
faults; the broad lines marked _c_ denote dykes: the former dislocate
the strata, and change their level, while dykes disjoin the strata with
a wall, but do not in general affect their elevation. The two parallel
lines marked _a_, represent two seams of coal, variously heaved up and
down by the faults; whereas the dykes are seen to pass through the
strata without altering their relative position. In this manner, partial
coal-fields are distributed over a wide area of country, in every
direction.
[Illustration: 803]
The only exception to this general form of the coal-fields in Great
Britain, is the inverted basin shape; but this is rare. A few examples
occur in some districts of England, and in the county of Fife; but even
in extensive coal-fields, this convex form is but a partial occurrence,
or a deviation by local violence from the ordinary basin. _Fig._ 803 is
an instance of a convex coal-field exhibited in Staffordshire, at the
Castle-hill, close to the town of Dudley. 1, 1, are limestone strata; 2,
2, are coal. Through this hill, canals have been cut, for working the
immense beds of carboniferous limestone. These occur in the lower series
of the strata of the coal-field, and therefore at a distance of many
miles from the Castle-hill, beyond the outcrop of all the workable coals
in the proper basin-shaped part of the field; but by this apparently
inverted basin-form, these limestone beds are elevated far above the
level of the general surface of the country, and consequently above the
level of all the coals. We must regard this seeming inversion as
resulting from the approximation of two coal-basins, separated by the
basset edges of their mountain limestone repository.
[Illustration: 804 807]
_Fig._ 804. is a vertical section of the Dudley coal-basin, the upper
coal-bed of which has the astonishing thickness of 30 feet; and this
mass extends 7 miles in length, and 4 in breadth. Coal-seams 5 or 6 feet
thick, are called _thin_ in that district.
[Illustration: 805]
_Fig._ 805. is a very interesting section of the main coal-basin of
Clackmannanshire, as given by Mr. Bald in the Wernerian Society’s
Memoirs, vol. iii. Here we see it broken into three subordinate
coal-fields, formed by two great faults or dislocations of the strata;
but independently of these fractures across the whole series, the strata
continue quite regular in their respective alternations, and preserve
nearly unchanged their angle of inclination to the horizon. The section
shows the south coal-field dipping northerly, till it is cut across by
the great south slip _x_, which dislocates the coal and the parallel
strata to the enormous extent of 1230 feet, by which all the coals have
been thrown up, not simply to the day, but are not found again till we
advance nearly a mile northward, on the line of the dip, where the
identical seams of coal, shale, &c. are observed once more with their
regular inclination. These coals of the middle area, dip regularly
northward till interrupted by the great north slip _y_, which dislocates
the strata, and throws them up 700 feet; that is to say, a line
prolonged in the direction of any one well-known seam, will run 700 feet
above the line of the same seam as it emerges after the middle slip.
Immediately adjoining the north slip, the coals and coal-field resume
their course, and dip regularly northward, running through a longer
range than either of the other two members of the basin, till they
arrive at the valley of the Devon, at the foot of the Ochill mountains,
where they form a concave curvature, or trough, _a_, and thence rise
rapidly in an almost vertical direction at _b_. Here the coals, with all
their associate strata, assume conformity and parallelism with the face
of the sienitic-greenstone strata of the Ochill mountains _c_; being
raised to the high angle of 73 degrees with the horizon. The coal-seams
thus upheaved, are called _edge-metals_ by the miners.
[Illustration: 806]
In this remarkable coal-field, which has been accurately explored by
pitting and boring to the depth of 703 feet, there are no fewer than 142
beds, or distinct strata of coal, shale, and sandstone, &c., variously
alternating, an idea of which may be had by inspecting _fig._ 806. Among
these are 24 beds of coal, which would constitute an aggregate thickness
of 59 feet 4 inches; the thinnest seam of coal being 2 inches, and the
thickest 9 feet. The strata of this section contain numerous varieties
of sandstone, slate-clay, bituminous shale, indurated clay, or
fire-clay, and clay ironstone. Neither trap-rock nor limestone is found
in connexion with the workable coals; but an immense bed of greenstone,
named Abbey Craig, occurs in the western boundary of Clackmannanshire,
under which lie regular strata of slate-clay, sandstone, thin beds of
limestone, and large spheroidal masses of clay ironstone, with a mixture
of lime.
“With regard to slips in coal fields,” says Mr. Bald, “we find that
there is a general law connected with them as to the position of the
dislocated strata, which is this:--When a slip is met with in the course
of working the mines--if when looking to it, the vertical line of the
slip or fissure, it forms an acute angle with the line of the pavement
upon which the observer stands, we are certain that the strata are
dislocated downwards upon the other side of the fissure. On the
contrary, if the angle formed by the two lines above mentioned is
obtuse, we are certain that the strata are dislocated or thrown upwards
upon the other side of the fissure. When the angle is 90°, or a right
angle, it is altogether uncertain whether the dislocation throws up or
down on the opposite side of the slip. When dikes intercept the strata,
they generally only separate the strata the width of the dike, without
any dislocation, either up or down; so that if a coal is intercepted by
a dike, it is found again by running a mine directly forward,
corresponding to the angle or inclination of the coal with the
horizon.”--_Wernerian Society’s Memoirs_, vol. iii. p. 133.[40]
[40] This paper does honour to its author, the eminent coal-viewer of
Scotland.
[Illustration: 808
_a._ Alluvial cover.
_b._ Bed of trap or greenstone.
_c._ Alternating coal strata.
_d._ Coal-seams.
_e._ Position of greenstone, not ascertained.
_f._ Strata in which no coals have been found.
_g._ The overlapped coal.
_h._ The double coal.]
The Johnstone coal-field, in Renfrewshire, is both singular and
interesting. The upper stratum of rock is a mass of compact greenstone
or trap, above 100 feet in thickness, not at all in a conformable
position with the coal strata, but overlying; next there is a few
fathoms of soft sandstone and slate-clay, alternating, and uncommonly
soft. Beneath these beds, there are no fewer than ten seams of coal,
lying on each other, with a few divisions of dark indurated clay. These
coal-seams have an aggregate thickness of no less than 100 feet; a mass
of combustible matter, in the form of coal, unparalleled for its
accumulation in so narrow a space. The greater part of this field
contains only 5 beds of coal; but at the place where the section shown
in _fig._ 807. is taken, these 5 coals seem to have been overlapped or
made to slide over each other by violence. This structure is represented
in _fig._ 808., which is a section of the Quarrelton coal in the
Johnstone field, showing the overlapped coal and the double coal, with
the thick bed of greenstone, overlying the coal-field.
Before proceeding to examine the modes of working coal, I shall
introduce here a description of the two principal species of this
mineral.
1. _Cubical coal._--It is black, shining, compact, moderately hard, but
easily frangible. When extracted in the mine, it comes out in
rectangular masses, of which the smaller fragments are cubical. The
lamellæ (_reed_ of the coal) are always parallel to the bed or plane on
which the coal rests; a fact which holds generally with this substance.
There are two varieties of cubical coal; the _open-burning_ and the
_caking_. The latter, however small its fragments may be, is quite
available for fuel, in consequence of its agglutinating into a mass at a
moderate heat, by the abundance of its bitumen. This kind is the true
smithy or forge-coal, because it readily forms itself into a vault round
the blast of the bellows, which serves for a cupola in concentrating the
heat on objects thrust into the cavity.
The open-burning cubical coals are known by several local names; the
rough coal or clod coal, from the large masses in which they may be
had; and the cherry coal, from the cheerful blaze with which they
spontaneously burn; whereas the caking coals, such as most of the
Newcastle qualities, require to be frequently poked in the grate. Its
specific gravity varies from 1·25 to 1·4.
2. _Slate or splint coal._--This is dull-black, very compact, much
harder, and more difficultly frangible than the preceding. It is readily
fissile, like slate, but powerfully resists the cross fracture, which is
conchoidal. Specific gravity from 1·26 to 1·40. In working, it separates
in large quadrangular sharp-edged masses. It burns without caking,
produces much flame and smoke, unless judiciously supplied with air, and
leaves frequently a considerable bulk of white ashes. It is the best
fuel for distilleries and all large grates, as it makes an open fire,
and does not clog up the bars with glassy scoriæ. I found good splint
coal of the Glasgow field to have a specific gravity of 1·266, and to
consist of--carbon, 70·9; hydrogen, 4·3; oxygen, 24·8.
3. _Cannel coal._--Colour between velvet and grayish-black; lustre
resinous; fracture even; fragments trapezoidal; hard as splint coal;
spec. grav. 1·23 to 1·28. In working, it is detached in four-sided
columnar masses, often breaks conchoidal, like pitch, kindles very
readily, and burns with a bright white projective flame, like the wick
of a candle, whence its name. It occurs most abundantly in the
coal-field of Wigan, in Lancashire, in a bed 4 feet thick; and there is
a good deal of it in the Clydesdale coal-field, of which it forms the
lowest seam that is worked. It produces very little dust in the mine,
and hardly soils the fingers with carbonaceous matter. Cannel coal from
Woodhall, near Glasgow, spec. grav. 1·228, consists by my analysis
of--carbon, 72·22; hydrogen, 3·93; oxygen, 21·05; with a little azote
(about 2·8 in 100 parts). This coal has been found to afford, in the
Scotch gas-works, a very rich-burning gas. The azote is there converted
into ammonia, of which a considerable quantity is distilled over into
the tar-pit.
4. _Glance coal._--This species has an iron-black colour, with an
occasional iridescence, like that of tempered steel; lustre in general
splendent, shining, and imperfect metallic; does not soil; easily
frangible; fracture flat conchoidal; fragments sharp-edged. It burns
without flame or smell, except when it is sulphureous; and it leaves a
white-coloured ash. It produces no soot, and seems, indeed, to be merely
carbon, or coal deprived of its volatile matter or bitumen, and
converted into coke by subterranean calcination, frequently from contact
with whin-dikes. Glance coal abounds in Ireland, under the name of
Kilkenny coal; in Scotland it is called blind coal, from its burning
without flame or smoke; and in Wales, it is the malting or stone coal.
It contains from 90 to 97 per cent. of carbon. Specific gravity from 1·3
to 1·5; increasing with the proportion of earthy impurities.
The dislocations and obstructions found in coal-fields, which render the
search for coal so difficult, and their mining so laborious and
uncertain, are the following:--