The danger arising from the presence of coal dust in the air of dry

mines, with or without the addition of fire-damp, has, since it was

first pointed out by Professor W. Galloway, been made the subject of

special inquiries in the principal European countries interested in coal

mining; and although certain points are still debatable, the fact is

generally admitted as one calling for special precautions. The

conclusions arrived at by the royal commission of 1891, which may be

taken as generally representative of the views of British colliery

engineers, are as follows:--

1. The danger of explosion when gas exists in very small quantities is

greatly increased by the presence of coal dust.

2. A gas explosion in a fiery mine may be intensified or indefinitely

propagated by the dust raised by the explosion itself.

3. Coal dust alone, without any gas, may cause a dangerous explosion

if ignited by a blown-out shot; but such cases are likely to be

exceptional.

4. The inflammability of coal dust varies with different coals, but

none can be said to be entirely free from risk.

5. There is no probability of a dangerous explosion being produced by

the ignition of coal dust by a naked light or ordinary flame.

Danger arising from coal dust is best guarded against by systematically

sprinkling or watering the main roads leading from the working faces to

the shaft, where the dust falling from the trams in transit is liable to

accumulate. This may be done by water-carts or hose and jet, but

preferably by finely divided water and compressed air distributed from a

network of pipes carried through the workings. This is now generally

done, and in some countries is compulsory, when the rocks are deficient

in natural moisture. In one instance the quantity of water required to

keep down the dust in a mine raising 850 tons of coal in a single shift

was 28.8 tons, apart from that required by the jets and motors. The

distributing network extended to more than 30 m. of pipes, varying from

3-1/2 in. to 1 in. in diameter.

Safety explosives.

In all British coal-mines, when gas in dangerous quantities has appeared

within three months, and in all places that are dry and dusty, blasting

is prohibited, except with "permitted" explosives, whose composition and

properties have been examined at the testing station at the Royal

Arsenal, Woolwich. A list of those sanctioned is published by the Home

Office. They are mostly distinguished by special trade names, and are

mainly of two classes--those containing ammonium nitrate and

nitrobenzene or nitronaphthalene, and those containing nitroglycerin and

nitrocellulose, which are essentially weak dynamites. The safety

property attributed to them is due to the depression of the temperature

of the flame or products of explosion to a point below that necessary to

ignite fire-damp or coal dust in air from a blown-out shot. New

explosives that are found to be satisfactory when tested are added to

the list from time to time, the composition being stated in all cases.

Aerophores.

Methods for enabling miners to penetrate into workings where the

atmosphere is totally irrespirable have come into use for saving life

after explosions and for repairing shafts and pit-work under water. The

aerophore of A. Galibert was in its earlier form a bag of about 12 cub.

ft. capacity containing air at a little above atmospheric pressure; it

was carried on the back like a knapsack and supplied the means of

respiration. The air was continually returned and circulated until it

was too much contaminated with carbonic acid to be further used, a

condition which limited the use of the apparatus to a very short period.

A more extended application of the same principle was made in the

apparatus of L. Denayrouze by which the air, contained in cylinders at a

pressure of 300 to 350 lb per sq. in., was supplied for respiration

through a reducing valve which brought it down nearly to atmospheric

pressure. This apparatus was, however, very heavy and became

unmanageable when more than an hour's supply was required. The newer

forms are based upon the principle, first enunciated by Professor

Theodor Schwann in 1854, of carrying compressed oxygen instead of air,

and returning the products of respiration through a regenerator

containing absorptive media for carbonic acid and water, the purified

current being returned to the mouth with an addition of fresh oxygen.

The best-known apparatus of this class is that developed by G. A. Meyer

at the Shamrock colliery in Westphalia, where a body of men are kept in

systematic training for its use at a special rescue station. This corps

rendered invaluable service at the exploring and rescue operations after

the explosion at Courrieres in March 1906, the most disastrous mining

accident on record, when 1100 miners were killed. A somewhat similar

apparatus called the "weg," after the initials of the inventor, is due

to W. E. Garforth of Wakefield. In another form of apparatus advantage

is taken of the property possessed by sodium-potassium peroxide of

giving off oxygen when damped; the residue of caustic soda and potash

yielded by the reaction is used to absorb the carbonic acid of the

expired air. Experiments have also been made with a device in which the

air-supply is obtained by the evaporation of liquid air absorbed in

asbestos.

Underground fires are not uncommon accidents in coal-mines. In the thick

coal workings in South Staffordshire the slack left behind in the sides

of work is especially liable to fire from so-called spontaneous

combustion, due to the rapid oxidization that is set up when finely

divided coal is brought in contact with air. The best remedy in such

cases is to prevent the air from gaining access to the coal by building

a wall round the burning portion, which can in this way be isolated from

the remainder of the working, and the fire prevented from spreading,

even if it cannot be extinguished. When the coal is fired by the blast

of an explosion it is often necessary to isolate the mine completely by

stopping up the mouths of the pits with earth, or in extreme cases it

must be flooded with water or carbonic acid before the fire can be

brought under. There have been several instances of this being done in

the fiery pits in the Barnsley district, notably at the great explosion

at the Oaks colliery in 1866, when 360 lives were lost.

Methods of winding.

The drawing or winding of the coal from the pit bottom to the surface is

one of the most important operations in coal mining, and probably the

department in which mechanical appliances have been brought to the

highest state of development.

Cage.

The different elements making up the drawing arrangements of a colliery

are--(1) the cage, (2) the shaft or pit fittings, (3) the drawing-rope,

(4) the engine and (5) the surface arrangements. The cage, as its name

implies, consists of one or more platforms connected by an open

framework of vertical bars of wrought iron or steel, with a top bar to

which the drawing-rope is attached. It is customary to have a curved

sheet iron roof or bonnet when the cage is used for raising or lowering

the miners, to protect them from injury by falling materials. The number

of platforms or decks varies considerably; in small mines only a single

one may be used, but in the larger modern pits two-, three- or even

four-decked cages are used. The use of several decks is necessary in old

pits of small section, where only a single tram can be carried on each.

In the large shafts of the Northern and Wigan districts the cages are

made about 8 ft. long and 3-1/2 ft. broad, being sufficient to carry two

large trams on one deck. These are received upon a railway made of two

strips of angle iron of the proper gauge for the wheels, and are locked

fast by a latch falling over their ends. At Cadeby Main with four-decked

cages the capacity is eight 10-cwt. tubs or 4 tons of coal.

The guides or conductors in the pit may be constructed of wood, in which

case rectangular fir beams, about 3 by 4 in., are used, attached at

intervals of a few feet to buntons or cross-beams built into the lining

of the pit. Two guides are required for each cage; they may be placed

opposite to each other, either on the long or short sides--the latter

being preferable. The cage is guided by shoes of wrought iron, a few

inches long and bell-mouthed at the ends, attached to the horizontal

bars of the framing, which pass loosely over the guides on three sides,

but in most new pits rail guides of heavy section are used. They are

applied on one side of the cage only, forming a complete vertical

railway, carried by iron cross sleepers, with proper seats for the rails

instead of wooden buntons; the cage is guided by curved shoes of a

proper section to cover the heads of the rails. Rigid guides connected

with the walling of the pit are probably the best and safest, but they

have the disadvantage of being liable to distortion, in case of the pit

altering its form, owing to irregular movements of the ground, or other

causes. Wooden guides being of considerable size, block up a certain

portion of the area of the pit, and thus offer an impediment to the

ventilation, especially in upcast shafts, where the high temperature,

when furnace ventilation is used, is also against their use. In the

Lancashire and the Midland districts wire-rope guides have been

introduced to a very considerable extent, with a view of meeting the

above objections. These are simply wire-ropes, from 3/4 to 1-1/2 in. in

diameter, hanging from a cross-bar connected with the pit-head framing

at the surface, and attached to a similar bar at the bottom, which are

kept straight by a stretching weight of from 30 cwt. to 4 tons attached

to the lower bar. In some cases four guides are used--two to each of the

long sides of the cage; but a more general arrangement is to have

three--two on one side, and the third in an intermediate position on the

opposite side. Many colliery managers, however, prefer to have only two

opposite guides, as being safer. The cage is connected by tubular clips,

made in two pieces and bolted together, which slide over the ropes. In

addition to this it is necessary to have an extra system of fixed guides

at the surface and at the bottom, where it is necessary to keep the cage

steady during the operations of loading and landing, there being a much

greater amount of oscillation during the passage of the cage than with

fixed guides. For the same reason it is necessary to give a considerable

clearance between the two lines of guides, which are kept from 15 to 18

in. apart, to prevent the possibility of the two cages striking each

other in passing. With proper precautions, however, wire guides are

perfectly safe for use at the highest travelling speed.

Ropes and chains.

The cage is connected with the drawing-rope by short lengths of chain

from the corners, known as tackling chains, gathered into a central ring

to which the rope is attached. Round steel wire-ropes, about 2 in. in

diameter, are now commonly used; but in very deep pits they are

sometimes tapered in section to reduce the dead weight lifted. Flat

ropes of steel or iron wire were and are still used to a great extent,

but round ones are now generally preferred. In Belgium and the north of

France flat ropes of aloe fibre (Manila hemp or plantain fibre) are in

high repute, being considered preferable by many colliery managers to

wire, in spite of their great weight. A rope of this class for a pit

1200 metres deep, tapered from 15.6 in. to 9 in. in breadth and from 2

in. to 1-1/8 in. in thickness, weighed 14.3 tons, and another at Anzin,

intended to lift a gross load of 15 tons from 750 metres, is 22-1/2 in.

broad and 3 in. thick at the drum end, and weighs 18 tons. Tapered round

ropes, although mechanically preferable, are not advantageous in

practice, as the wear being greater at the cage end than on the drum it

is necessary to cut off portions of the former at intervals. Ultimately

also the ropes should be reversed in position, and this can only be done

with a rope of uniform section.

Winding engines.

The engines used for winding or hoisting in collieries are usually

direct-acting with a pair of horizontal cylinders coupled directly to

the drum shaft. Steam at high pressure exhausting into the atmosphere is

still commonly used, but the great power required for raising heavy

loads from deep pits at high speeds has brought the question of fuel

economy into prominence, and more economical types of the two-cylinder

tandem compound class with high initial steam pressure, superheating and

condensing, have come in to some extent where the amount of work to be

done is sufficient to justify their high initial cost. One of the

earliest examples was erected at Llanbradack in South Wales in 1894, and

they have been somewhat extensively used in Westphalia and the north of

France. In a later example at the Bargold pit of the Powell Duffryn

Steam Coal Company a mixed arrangement is adopted with horizontal

high-pressure and vertical low-pressure cylinders. This engine draws a

net load of 55 tons of coal from a depth of 625 yds. in 45 seconds, the

gross weight of the four trams, cage and chains, and rope, with the

coal, being 20 tons 12 cwt. The work of the winding engine, being

essentially of an intermittent character, can only be done with

condensation when a central condenser keeping a constant vacuum is used,

and even with this the rush of steam during winding may be a cause of

disturbance. This difficulty may be overcome by using Rateau's

arrangement of a low-pressure turbine between the engine and the

condenser. The accumulator, which is similar in principle to the thermal

storage system of Druitt Halpin, is a closed vessel completely filled

with water, which condenses the excess of steam during the winding

period, and becoming superheated maintains the supply to the turbine

when the main engine is standing. The power so developed is generally

utilized in the production of electricity, for which there is an

abundant use about large collieries.

The drum, when round ropes are used, is a plain broad cylinder, with

flanged rims, and cased with soft wood packing, upon which the rope is

coiled; the breadth is made sufficient to take the whole length of the

rope at two laps. One drum is usually fixed to the shaft, while the

other is loose, with a screw link or other means of coupling, in order

to be able to adjust the two ropes to exactly the same length, so that

one cage may be at the surface when the other is at the bottom, without

having to pay out or take up any slack rope by the engine.

For flat ropes the drum or bobbin consists of a solid disk, of the width

of the rope fixed upon the shaft, with numerous parallel pairs of arms

or horns, arranged radially on both sides, the space between being just

sufficient to allow the rope to enter and coil regularly upon the

preceding lap. This method has the advantage of equalizing the work of

the engine throughout the journey, for when the load is greatest, with

the full cage at the bottom and the whole length of rope out, the duty

required in the first revolution of the engine is measured by the length

of the smallest circumference; while the assistance derived from

gravitating action of the descending cage in the same period is equal to

the weight of the falling mass through a height corresponding to the

length of the largest lap, and so on, the speed being increased as the

weight diminishes, and vice versa. The same thing can be effected in a

more perfect manner by the use of spiral or scroll drums, in which the

rope is made to coil in a spiral groove upon the surface of the drum,

which is formed by the frusta of two obtuse cones placed with their

smaller diameters outwards. This plan, though mechanically a very good

one, has certain defects, especially in the possibility of danger

resulting from the rope slipping sideways, if the grooves in the bed are

not perfectly true. The great size and weight of such drums are also

disadvantages, as giving rather unmanageable dimensions in a very deep

pit. In some cases, therefore, a combined form is adopted, the body of

the drum being cylindrical, and a width equal to three or four laps

conical on either side.

Counterbalance chains for the winding engines are used in the collieries

of the Midland districts of England. In this method a third drum is used

to receive a heavy flat link chain, shorter than the main drawing-ropes,

the end of which hangs down a special or balance pit. At starting, when

the full load is to be lifted, the balance chain uncoils, and continues

to do so until the desired equilibrium between the working loads is

attained, when it is coiled up again in the reverse direction, to be

again given out on the return trip.

In Koepe's method the drum is replaced by a disk with a grooved rim for

the rope, which passes from the top of one cage over the guide pulley,

round the disk, and back over the second guide to the second cage, and a

tail rope, passing round a pulley at the bottom of the shaft, connects

the bottoms of the cages, so that the dead weight of cage, tubs and rope

is completely counterbalanced at all positions of the cages, and the

work of the engine is confined to the useful weight of coal raised.

Motion is communicated to the rope by frictional contact with the drum,

which is covered through about one-half of the circumference. This

system has been used in Nottinghamshire, and at Sneyd, in North

Staffordshire. In Belgium it was tried in a pit 940 metres deep, where

it has been replaced by flat hempen ropes, and is now restricted to

shallower workings. In Westphalia it is applied in about thirty

different pits to a maximum depth of 761 metres.

A novelty in winding arrangements is the substitution of the

electromotor for the steam engine, which has been effected in a few

instances. In one of the best-known examples, the Zollern colliery in

Westphalia, the Koepe system is used, the winding disk being driven by

two motors of 1200 H.P. each on the same shaft. Motion is obtained from

a continuous-current generator driven by an alternating motor with a

very heavy fly-wheel, a combination known as the Ilgner transformer,

which runs continuously with a constant draught on the generating

station, the extremely variable demand of the winding engine during the

acceleration period being met by the energy stored in the fly-wheel,

which runs at a very high speed. This arrangement works admirably as

regards smoothness and safety in running, but the heavy first cost and

complication stand in the way of its general adoption. Nevertheless

about 60 electric winding engines were at work or under construction in

May 1906.

Surface arrangements.

The surface arrangements of a modern deep colliery are of considerable

extent and complexity, the central feature being the head gear or pit

frame carrying the guide pulleys which lead the winding ropes from the

axis of the pit to the drum. This is an upright frame, usually made in

wrought iron or steel strutted by diagonal thrust beams against the

engine-house wall or other solid abutments, the height to the bearings

of the guide pulleys being from 80 to 100 ft. or more above the ground

level. This great height is necessary to obtain head-room for the cages,

the landing platforms being usually placed at some considerable height

above the natural surface. The pulleys, which are made as large as

possible up to 20 ft. in diameter to diminish the effect of bending

strains in the rope by change in direction, have channelled cast iron

rims with wrought iron arms, a form combining rigidity with strength, in

order to keep down their weight.

To prevent accidents from the breaking of the rope while the cage is

travelling in the shaft, or from over-winding when in consequence of the

engine not being stopped in time the cage may be drawn up to the

head-gear pulleys (both of which are unhappily not uncommon), various

forms of safety catches and disconnecting hooks have been adopted. The

former contrivances consist essentially of levers or cams with toothed

surfaces or gripping shoes mounted upon transverse axes attached to the

sides of the cage, whose function is to take hold of the guides and

support the cage in the event of its becoming detached from the rope.

The opposite axes are connected with springs which are kept in

compression by tension of the rope in drawing but come into action when

the pull is released, the side axes then biting into wooden guides or

gripping those of steel bars or ropes. The use of these contrivances is

more common in collieries on the continent of Europe, where in some

countries they are obligatory, than in England, where they are not

generally popular owing to their uncertainty in action and the constant

drag on the guides when the rope slacks.

For the prevention of accidents from over-winding, detaching hooks are

used. These consist essentially of links formed of a pair of parallel

plates joined by a central bolt forming a scissors joint which is

connected by chain links to the cage below and the winding-rope above.

The outer sides of the link are shaped with projecting lugs above. When

closed by the load the width is sufficient to allow it to enter a

funnel-shaped guide on a cross-bar of the frame some distance above the

bank level, but on reaching the narrower portion of the guide at the top

the plates are forced apart which releases the ropes and brings the lugs

into contact with the top of the cross-bar which secures the cage from

falling.

Three principal patterns, those of King, Ormerod and Walker, are in use,

and they are generally efficient supposing the speed of the cage at

arrival is not excessive. To guard against this it is now customary to

use some speed-checking appliance, independent of the engine-man, which

reduces or entirely cuts off the steam supply when the cage arrives at a

particular point near the surface, and applies the brake if the load is

travelling too quickly. Maximum speed controllers in connexion with the

winding indicator, which do not allow the engine to exceed a fixed rate

of speed, are also used in some cases, with recording indicators.

Striking and screening.

When the cage arrives at the surface, or rather the platform forming the

working top above the mouth of the pit, it is received upon the keeps, a

pair of hinged gratings which are kept in an inclined position over the

pit-top by counter-balance weights, so that they are pushed aside to

allow the cage to pass upwards, but fall back and receive it when the

engine is reversed. The tubs are then removed or struck by the landers,

who pull them forward on to the platform, which is covered with cast

iron plates; at the same time empty ones are pushed in from the opposite

side. The cage is then lifted by the engine clear of the keeps, which

are opened by a lever worked by hand, and the empty tubs start on the

return trip. When the cage has several decks, it is necessary to repeat

this operation for each, unless there is a special provision made for

loading and discharging the tubs at different levels. An arrangement of

this kind for shifting the load from a large cage at one operation was

introduced by Fowler at Hucknall, in Leicestershire, where the trains

are received into a framework with a number of platforms corresponding

to those of the cage, carried on the head of a plunger movable by

hydraulic pressure in a vertical cylinder. The empty tubs are carried by

a corresponding arrangement on the opposite side. By this means the time

of stoppage is reduced to a minimum, 8 seconds for a three-decked cage

as against 28 seconds, as the operations of lowering the tubs to the

level of the pit-top, discharging, and replacing them are performed

during the time that the following load is being drawn up the pit.

In the United Kingdom the drawing of coal is generally confined to the

day shift of eight hours, with an output of from 100 to 150 tons per

hour, according to the depth, capacity of coal tubs, and facilities for

landing and changing tubs. With Fowler's hydraulic arrangement 2000 tons

are raised 600 yds. in eight hours. In the deeper German pits, where

great thicknesses of water-bearing strata have to be traversed, the

first establishment expenses are so great that in order to increase

output the shaft is sometimes provided with a complete double equipment

of cages and engines. In such cases the engines may be placed in line on

opposite sides of the pit, or at right angles to each other. It is said

that the output of single shafts has been raised by this method to 3500

and 4500 tons in the double shift of sixteen hours. It is particularly

well suited to mines where groups of seams at different depths are

worked simultaneously. Some characteristic figures of the yield for

British collieries in 1898 are given below:--

Albion Colliery, South Wales 551,000 tons in a year for one shaft

and one engine.

Silksworth Colliery, 535,000 tons in a year for shaft

Northumberland 580 yds. deep, two engines.

Bolsover Colliery, Derby 598,798 tons in 279 days, shaft 365

yds. deep.

Denaby Main Colliery, Yorkshire 629,947 tons in 281 days, maximum per

day 2673 tons.

At Cadeby Main colliery near Doncaster in 1906, 3360 tons were drawn in

fourteen hours from one pit 763 yds. deep.

The tub when brought to the surface, after passing over a weigh-bridge

where it is weighed and tallied by a weigher specially appointed for the

purpose by the men and the owner jointly, is run into a "tippler," a

cage turning about a horizontal axis which discharges the load in the

first half of the rotation and brings the tub back to the original

position in the second. It is then run back to the pit-bank to be loaded

into the cage at the return journey.

Coal as raised from the pit is now generally subjected to some final

process of classification and cleaning before being despatched to the

consumer. The nature and extent of these operations vary with the

character of the coal, which if hard and free from shale partings may be

finished by simple screening into large and nut sizes and smaller slack

or duff, with a final hand-picking to remove shale and dust from the

larger sizes. But when there is much small duff, with intermixed shale,

more elaborate sizing and washing plant becomes necessary. Where

hand-picking is done, the larger-sized coal, separated by 3-in. bar

screens, is spread out on a travelling band, which may be 300 ft. long

and from 3 to 5 wide, and carried past a line of pickers stationed along

one side, who take out and remove the waste as it passes by, leaving the

clean coal on the belt. The smaller duff is separated by vibrating or

rotating screens into a great number of sizes, which are cleaned by

washing in continuous current or pulsating jigging machines, where the

lighter coal rises to the surface and is removed by a stream of water,

while the heavier waste falls and is discharged at a lower level, or

through a valve at the bottom of the machine. The larger or "nut" sizes,

from 1/4 in. upwards, are washed on plain sieve plates, but for

finer-grained duff the sieve is covered with a bed of broken felspar

lumps about 3 in. thick, forming a kind of filter, through which the

fine dirt passes to the bottom of the hutch. The cleaned coal is carried

by a stream of water to a bucket elevator and delivered to the storage

bunkers, or both water and coal may be lifted by a centrifugal pump into

a large cylindrical tank, where the water drains away, leaving the coal

sufficiently dry for use. Modern screening and washing plants,

especially when the small coal forms a considerable proportion of the

output, are large and costly, requiring machinery of a capacity of 100

to 150 tons per hour, which absorbs 350 to 400 H.P. In this, as in many

other cases, electric motors supplied from a central station are now

preferred to separate steam-engines.

Anthracite coal in Pennsylvania is subjected to breaking between toothed

rollers and an elaborate system of screening, before it is fit for sale.

The largest or lump coal is that which remains upon a riddle having the

bars 4 in. apart; the second, or steamboat coal, is above 3 in.; broken

coal includes sizes above 2-1/2 or 2-3/4 in.; egg coal, pieces above

2-1/4 in. sq.; large stove coal, 1-3/4 in.; small stove, 1 to 1-1/2 or

1-1/3 in.; chestnut coal, 2/3 to 3/4 in.; pea coal, 1/2 in.; and buckwheat

coal, 1/3 in. The most valuable of these are the egg and stove sizes,

which are broken to the proper dimensions for household use, the larger

lumps being unfit for burning in open fire-places. In South Wales a

somewhat similar treatment is now adopted in the anthracite districts.

Depth of working.

With the increased activity of working characteristic of modern coal

mining, the depth of the mines has rapidly increased, and at the present

time the level of 4000 ft., formerly assumed as the possible limit for

working, has been nearly attained. The following list gives the depths

reached in the deepest collieries in Europe in 1900, from which it will

be seen that the larger number, as well as the deepest, are in

Belgium:--

Metres. Ft.

Saint Henriette, Cie des Produits, Flenu, Belgium 1150 3773

Viviers Gilly " 1143 3750

Marcinelle, No. 11, Charleroi " 1075 3527

Marchienne, No. 2 " " 1065 3494

Agrappe, Mons " 1060 3478

Pendleton dip workings Lancashire 1059 3474

Sacre Madame, Charleroi Belgium 1055 3461

Ashton Moss dip workings Lancashire 1024 3360

Ronchamp, No. 11 pit France 1015 3330

Viernoy, Anderlues Belgium 1006 3301

Astley Pit, Dukinfield, dip workings Cheshire 960 3150

Saint Andre, Poirier, Charleroi Belgium 950 3117

The greatest depth attained in the Westphalian coal is at East

Recklinghausen, where there are two shafts 841 metres (2759 ft.) deep.

The subject of the limiting depth of working has been very fully studied

in Belgium by Professor Simon Stassart of Mons ("Les Conditions

d'exploitation a grande profondeur en Belgique," _Bulletin de la Societe

de l'Industrie minerale_, 3 ser., vol. xiv.), who finds that no special

difficulty has been met with in workings above 1100 metres deep from

increased temperature or atmospheric pressure. The extreme temperatures

in the working faces at 1150 metres were 79 deg. and 86 deg. F., and the

maximum in the end of a drift, 100 deg.; and these were quite bearable

on account of the energetic ventilation maintained, and the dryness of

the air. The yield per man on the working faces was 4.5 tons, and for

the whole of the working force underground, 0.846 tons, which is not

less than that realized in shallower mines. From the experience of such

workings it is considered that 1500 metres would be a possible workable

depth, the rock temperature being 132 deg., and those of the intake and

return galleries, 92 deg. and 108 deg. respectively. Under such

conditions work would be practically impossible except with very

energetic ventilation and dry air. It would be scarcely possible to

circulate more than 120,000 to 130,000 cub. ft. per minute under such

conditions, and the number of working places would thus be restricted,

and consequently the output reduced to about 500 tons per shift of 10

hours, which could be raised by a single engine at the surface without

requiring any very different appliances from those in current use.

Ownership of coal.

In the United Kingdom the ownership of coal, like that of other

minerals, is in the proprietor of the soil, and passes with it, except

when specially reserved in the sale. Coal lying under the sea below

low-water mark belongs to the crown, and can only be worked upon payment

of royalties, even when it is approached from shafts sunk upon land in

private ownership. In the Forest of Dean, which is the property of the

crown as a royal forest, there are certain curious rights held by a

portion of the inhabitants known as the Free Miners of the Forest, who

are entitled to mine for coal and iron ore, under leases, known as

gales, granted by the principal agent or gaveller representing the

crown, in tracts not otherwise occupied. This is the only instance in

Great Britain of the custom of free coal-mining under a government grant

or concession, which is the rule in almost every country on the

continent of Europe.

Coal Mines Regulation Act.

The working of collieries in the United Kingdom is subject to the

provisions of the Coal Mines Regulation Act 1887, as amended by several

minor acts, administered by inspectors appointed by the Home Office, and

forming a complete disciplinary code in all matters connected with

coal-mining. An important act was passed in 1908, limiting the hours of

work below ground of miners. For a detailed account of these various

acts see the article Labour Legislation.

Accidents.

Coal-mining is unfortunately a dangerous occupation, more than a

thousand deaths from accident being reported annually by the inspectors

of mines as occurring in the collieries of the United Kingdom.

The number of lives lost during the year 1906 was, according to the

inspectors' returns:--

From explosions 54

" falls of ground 547

" other underground accidents 328

" accidents in shafts 65

" surface accidents 135

----

Total 1129

The principal sources of danger to the collier, as distinguished from

other miners, are explosions of fire-damp and falls of roof in getting

coal; these together make up about 70% of the whole number of deaths. It

will be seen that the former class of accidents, though often attended

with great loss of life at one time, are less fatal than the latter.

AUTHORITIES.--The most important new publication on British coal is

that of the royal commission on coal supplies appointed in 1901, whose

final report was issued in 1905. A convenient digest of the evidence

classified according to subjects was published by the _Colliery

Guardian_ newspaper in three quarto volumes in 1905-1907, and the

leading points bearing on the extension and resources of the different

districts were incorporated in the fifth edition (1905) of Professor

Edward Hull's _Coal Fields of Great Britain_. The _Report_ of the

earlier royal commission (1870), however, still remains of great

value, and must not be considered to have had its conclusions entirely

superseded. In connexion with the re-survey in greater detail of the

coalfields by the Geological Survey a series of descriptive memoirs

were undertaken, those on the North Staffordshire and Leicestershire

fields, and nine parts dealing with that of South Wales, having

appeared by the beginning of 1908.

An independent work on the coal resources of Scotland under the title

of the _Coalfields of Scotland_, by R. W. Dixon, was published in

1902.

The Rhenish-Westphalian coalfield was fully described in all details,

geological, technical and economic, in a work called _Die Entwickelung

des niederrheinisch-westfalischen Steinkohlen Bergbaues in der zweiten

Halfte des 19ten Jahrhunderts_ (also known by the short title of

_Sammelwerk_) in twelve quarto volumes, issued under the auspices of

the Westphalian Coal Trade Syndicate (Berlin, 19O2-1905).

The coalfields of the Austrian dominions (exclusive of Hungary) are

described in _Die Mineralkohlen Osterreichs_, published at Vienna by

the Central Union of Austrian mineowners. It continues the table of

former official publications in 1870 and 1878, but in much more detail

than its predecessors.

Systematic detailed descriptions of the French coalfields appear from

time to time under the title of _Etudes sur les gites mineraux de la

France_ from the ministry of public works in Paris.

Much important information on American coals will be found in the

three volumes of _Reports on the Coal Testing Plant at the St Louis

Exhibition_, published by the United States Geological Survey in 1906.

A special work on the _Anthracite Coal Industry of the United States_,

by P. Roberts, was published in 1901.

The most useful general work on coal mining is the _Text Book of Coal

Mining_, by H. W. Hughes, which also contains detailed

bibliographical lists for each division of the text. The 5th edition

appeared in 1904.

Current progress in mining and other matters connected with coal can

best be followed by consulting the abstracts and bibliographical lists

of memoirs on these subjects that have appeared in the technical

journals of the world contained in the _Journal_ of the Institute of

Mining Engineers and that of the Iron and Steel Institute. The latter

appears at half-yearly intervals and includes notices of publications

up to about two or three months before the date of its publication.

(H. B.)

COALBROOKDALE, a town and district in the Wellington parliamentary

division of Shropshire, England. The town has a station on the Great

Western railway, 160 m. N.W. from London. The district or dale is the

narrow and picturesque valley of a stream rising near the Wrekin and

following a course of some 8 m. in a south-easterly direction to the

Severn. Great ironworks occupy it. They were founded in 1709 by Abraham

Darby with the assistance of Dutch workmen, and continued by his son and

descendants. Father and son had a great share in the discovery and

elaboration of the use of pit-coal for making iron, which revolutionized

and saved the English iron trade. The father hardly witnessed the

benefits of the enterprise, but the son was fully rewarded. It is

recorded that he watched the experimental filling of the furnace

ceaselessly for six days and nights, and that, just as fatigue was

overcoming him, he saw the molten metal issuing, and knew that the

experiment had succeeded.

The third Abraham Darby built the famous Coalbrookdale iron bridge over

the Severn, which gives name to the neighbouring town of Ironbridge,

which with a portion of Coalbrookdale is in the parish of Madeley

(q.v.). Fine wrought iron work is produced, and the school of art is

well known. There are also brick and tile works.

COAL-FISH (_Gadus virens_), also called green cod, black pollack, saith

and sillock, a fish of the family _Gadidae_. It has a very wide range,

which nearly coincides with that of the cod, although of a somewhat more

southern character, as it extends to both east and west coasts of the

North Atlantic, and is occasionally found in the Mediterranean. It is

especially common in the north, though rarely entering the Baltic; it

becomes rare south of the English Channel. Unlike the cod and haddock,

the coal-fish is, to a great extent, a surface-swimming fish,

congregating together in large schools, and moving from place to place

in search of food; large specimens (3 to 3-1/2 ft. long), however, prefer

deep water, down to 70 fathoms. The flesh is not so highly valued as

that of the cod and haddock. The lower jaw projects more or less beyond

the upper, the mental barble is small, sometimes rudimentary, the vent

is below the posterior half of the first dorsal fin, and there is a dark

spot in the axil of the pectoral fin.

COALING STATIONS. Maritime war in all ages has required that the ships

of the belligerents should have the use of sheltered waters for repairs

and for replenishment of supplies. The operations of commerce from the

earliest days demanded natural harbours, round which, as in the

conspicuous instance of Syracuse, large populations gathered. Such

points, where wealth and resources of all kinds accumulated, became

objects of attack, and great efforts were expended upon their capture.

As maritime operations extended, the importance of a seaboard increased,

and the possession of good natural harbours became more and more

advantageous. At the same time, the growing size of ships and the

complexity of fitments caused by the development of the sailing art

imposed new demands upon the equipment of ports alike for purposes of

construction and for repairs; while the differentiation between warships

and the commercial marine led to the establishment of naval bases and

dockyards provided with special resources. From the days when the great

sailors of Elizabeth carried war into distant seas, remote harbours

began to assume naval importance. Expeditionary forces required

temporary bases, such as Guantanamo Bay, in Cuba, which was so utilized

by Admiral Vernon in 1741. As outlying territories began to be occupied,

and jurisdiction to be exercised over their ports, the harbours

available for the free use of a belligerent were gradually reduced in

number, and it became occasionally necessary to take them by force.

Thus, in 1782, the capture of Trincomalee was an object of sufficient

importance to justify special effort, and Suffren gained a much-needed

refuge for his ships, at the same time compelling his opponent to depend

upon the open roadstead of Madras, and even to send ships to Bombay. In

this case a distant harbour acquired strategic importance, mainly

because sheltered waters, in the seas where Hughes and Suffren strove

for naval supremacy, were few and far between. A sailing man-of-war

usually carried from five to six months' provisions and water for 100 to

120 days. Other needs required to be met, and during the wars of the

French Revolution it was usual, when possible, to allow ships engaged in

blockade to return to port every five or six weeks "to refresh." For a

sailing fleet acting on the offensive, a port from which it could easily

get to sea was a great advantage. Thus Raleigh protested against the use

of closely landlocked harbours. "Certain it is," he wrote, "that these

ships are purposely to serve His Majesty and to defend the kingdom from

danger, and not to be so penned up from casualitie as that they should

be less able or serviceable in times of need." Nelson for this reason

made great use of Maddalena Bay, in Sardinia, and was not greatly

impressed with the strategic value of Malta in spite of its fine natural

harbour. The introduction of steam gave rise to a new naval

requirement--coal--which soon became vital. Commerce under steam quickly

settled down upon fixed routes, and depots of coal were established to

meet its needs. Coaling stations thus came into existence by a natural

process, arising from the exigencies of trade, and began later to supply

the needs of navies.

British coaling stations.

For many years there was no inquiry into the war requirements of the

British fleet as regards coal, and no attempt to regularize or to fortify

the ports at which it was stored. Successful naval war had won for Great

Britain many points of vantage throughout the world, and in some cases

the strategic value of ports had been proved by actual experience. The

extreme importance of the Cape of Good Hope, obscured for a time after

the opening of the Suez Canal, was fully realized in sailing days, and

the naval conditions of those days to some extent determined the choice

of islands and harbours for occupation. There does not, however, appear

to have been any careful study of relative strategic values. Treaties

were occasionally drafted by persons whose geographical knowledge was at

fault, and positions were, in some cases, abandoned which ought to have

been retained, or tenaciously held when they might have been abandoned.

It was left to the personal exertions of Sir Stamford Raffles to secure

such a supremely important roadstead as that of Singapore for the empire.

Although, therefore, the relative values of positions was not always

recognized, Great Britain obtained as a legacy from sailing days a large

number of harbours admirably adapted for use as coaling stations. Since

the dawn of the era of steam, she has acquired Aden, Perim, Hong-Kong,

North Borneo, Fiji, part of New Guinea, Fanning Island, and many other

islands in the Pacific, while the striking development of Australia and

New Zealand has added to the long roll of British ports. The coaling

stations, actual and potential, of the empire are unrivalled in number,

in convenience of geographical distribution, and in resources. Of the

numerous British ports abroad which contained coal stores, only the four

so-called "fortresses"--Gibraltar, Malta, Halifax and Bermuda--were at

first fortified as naval stations after the introduction of rifled

ordnance. The term fortress is a misnomer in every case except Gibraltar,

which, being a peninsula separated only by a neck of neutral ground from

the territory of a foreign power, exists under fortress conditions. Large

sums were expended on these places with little regard to principles, and

the defences of Bermuda, which were very slowly constructed, are

monuments of misapplied ingenuity.

Carnarvon Commission.

In 1878 great alarm arose from strained relations with Russia. Rumours

of the presence of Russian cruisers in many waters, and of hostile

projects, were readily believed, although the Russian navy, which had

just shown itself unable to face that of Turkey, would at this period

have been practically powerless. Widespread fears for the security of

coaling stations led to the appointment of a strong royal commission,

under the presidency of the earl of Carnarvon, which was instructed to

inquire into and report upon the protection of British commerce at sea.

This was the first attempt to formulate any principles, or to determine

which of the many ports where coal was stored should be treated as

coaling stations essential for the purposes of war. The terms of the

reference to the commission were ill-conceived. The basis of all defence

of sea-borne commerce is a mobile navy. It is the movement of commerce

upon the sea during war, not its security in port, that is essential to

the British empire, and a navy able to protect commerce at sea must

evidently protect ports and coaling stations. The first object of

inquiry should, therefore, have been to lay down the necessary standard

of naval force. The vital question of the navy was not referred to the

royal commission, and the four fortresses were also strangely excluded

from its purview. It followed inevitably that the protection of commerce

was approached at the wrong end, and that the labours of the commission

were to a great extent vitiated by the elimination of the principal

factor. Voluminous and important evidence, which has not been made

public, was, however, accumulated, and the final report was completed in

1881. The commissioners recalled attention to the extreme importance of

the Cape route to the East; they carefully examined the main maritime

communications of the empire, and the distribution of trade upon each;

they selected certain harbours for defence, and they obtained from the

War Office and endorsed projects of fortification in every case; lastly,

they condemned the great dispersion of troops in the West Indies, which

had arisen in days when it was a political object to keep the standing

army out of sight of the British people, and had since been maintained

by pure inadvertence. Although the principal outcome of the careful

inquiries of the commission was to initiate a great system of passive

defence, the able reports were a distinct gain. Some principles were at

last formulated by authority, and the information collected, if it had

been rendered accessible to the public, would have exercised a

beneficial influence upon opinion. Moreover, the commissioners,

overstepping the bounds of their charter, delivered a wise and

statesmanlike warning as to the position of the navy.

Meanwhile, the impulse of the fears of 1878 caused indifferent armaments

to be sent to Cape Town, Singapore and Hong-Kong, there to be mounted

after much delay in roughly designed works. At the same time, the great

colonies of Australasia began to set about the defence of their ports

with commendable earnestness. There is no machinery for giving effect to

the recommendations of a royal commission, and until 1887, when extracts

were laid before the first colonial Conference, the valuable report was

veiled in secrecy. After several years, during which Lord Carnarvon

persistently endeavoured to direct attention to the coaling stations,

the work was begun. In 1885 a fresh panic arose out of the Panjdeh

difficulty, which supplied an impetus to the belated proceedings. Little

had then been accomplished and the works were scarcely completed before

the introduction of long breech-loading guns rendered their armaments

obsolete.

The fortification of the coaling stations for the British empire is

still proceeding on a scale which, in some cases, cannot easily be

reconciled with the principles laid down by the president of the cabinet

committee of defence. At the Guildhall, London, on the 3rd of December

1896, the duke of Devonshire stated that "The maintenance of sea

supremacy has been assumed as the basis of the system of imperial

defence against attack from over the sea. This is the determining factor

in fixing the whole defensive policy of the empire." It was, however, he

added, necessary to provide against "the predatory raids of cruisers";

but "it is in the highest degree improbable that this raiding attack

would be made by more than a few ships, nor could it be of any permanent

effect unless troops were landed." This is an unexceptionable statement

of the requirements of passive defence in the case of the coaling

stations of the British empire. Their protection must depend primarily

on the navy. Their immobile armaments are needed to ward off a raiding

attack, and a few effective guns, well mounted, manned by well-trained

men, and kept in full readiness, will amply suffice.

Modern conditions.

Secondary bases.

If the command of the sea is lost, large expeditionary forces can be

brought to bear upon coaling stations, and their security will thus

depend upon their mobile garrisons, not upon their passive defences. In

any case, where coal is stored on shore, it cannot be destroyed by the

fire of a ship, and it can only be appropriated by landing men. A small

force, well armed and well handled, can effectually prevent a raid of

this nature without any assistance from heavy guns. In war, the

possession of secure coal stores in distant ports may be a great

advantage, but it will rarely suffice for the needs of a fleet engaged

in offensive operations, and requiring to be accompanied or met at

prearranged rendezvous by colliers from which coal can be transferred in

any sheltered waters. In the British naval manoeuvres of 1892, Admiral

Sir Michael Seymour succeeded in coaling his squadron at sea, and by the

aid of mechanical appliances this is frequently possible. In the

Spanish-American War of 1898 some coaling was thus accomplished; but

Guantanamo Bay served the purpose of a coaling station during the

operations against Santiago. Watering at sea was usually carried out by

means of casks in sailing days, and must have been almost as difficult

as coaling. As, however, it is certainty of coaling in a given time that

is of primary importance, the utilization of sheltered waters as

improvised coaling stations is sure to be a marked feature of future

naval wars. Although coaling stations are now eagerly sought for by all

powers which cherish naval ambitions, the annexation of the Hawaiian

Islands by the United States being a case in point, it is probable that

they will play a somewhat less important part than has been assumed. A

fleet which is able to assert and to maintain the command of the sea,

will not find great difficulty in its coal supply. Moreover, the

increased coal endurance of ships of war tends to make their necessary

replenishment less frequent. On the other hand, the modern warship,

being entirely dependent upon a mass of complex machinery, requires the

assistance of workshops to maintain her continuous efficiency, and

unless docked at intervals suffers a material reduction of speed.

Prolonged operations in waters far distant from home bases will

therefore be greatly facilitated in the case of the Power which

possesses local docks and means of executing repairs. Injuries received

in action, which might otherwise disable a ship during a campaign, may

thus be remedied. During the hostilities between France and China in

1884, the French ship "La Galissonniere" was struck by a shell from one

of the Min forts, which, though failing to burst, inflicted serious

damage. As, by a technical fiction, a state of war was not considered to

exist, the "La Galissonniere" was repaired at Hong-Kong and enabled

again to take the sea. Local stores of reserve ammunition and of spare

armaments confer evident advantages. Thus, independently of the question

of coal supply, modern fleets employed at great distances from their

bases require the assistance of ports furnished with special resources,

and a power like Japan with well-equipped naval bases in the China Sea,

and possessing large sources of coal, occupies, for that reason, a

favoured position in regard to naval operations in the Far East. As the

term "coaling station" refers only to a naval need which can often be

satisfied without a visit to any port, it appears less suitable to

modern conditions than "secondary base." Secondary bases, or coaling

stations, when associated with a powerful mobile navy, are sources of

maritime strength in proportion to the services they can render, and to

their convenience of geographical position. In the hands of an inferior

naval power, they may be used, as was Mauritius in 1809-1810, as points

from which to carry on operations against commerce; but unless situated

near to trade routes, which must be followed in war, they are probably

less useful for this purpose than in sailing days, since convoys can now

be more effectively protected, and steamers have considerable latitude

of courses. Isolated ports dependent on sea-borne resources, and without

strong bodies of organized fighting men at their backs are now, as

always, hostages offered to the power which obtains command of the sea.

(G. S. C.)

COALITION (Lat. _coalitio_, the verbal substantive of _coalescere_, to

grow together), a combination of bodies or parts into one body or

whole. The word is used, especially in a political sense, of an alliance

or temporary union for joint action of various powers or states, such as

the coalition of the European powers against France, during the wars of

the French Revolution; and also of the union in a single government of

distinct parties or members of distinct parties. Of the various

coalition ministries in English history, those of Fox and North in 1782,

of the Whigs and the Peelites, under Lord Aberdeen in 1852-1853, and of

the Liberal Unionists and Conservatives in Lord Salisbury's third

ministry in 1895, may be instanced.

COAL-TAR, the black, viscous, sometimes semi-solid, fluid of peculiar

smell, which is condensed together with aqueous "gas liquor" when the

volatile products of the destructive distillation of coal are cooled

down. It is also called "gas-tar," because it was formerly exclusively,

and even now is mostly, obtained as a by-product in the manufacture of

coal-gas, but the tar obtained from the modern coke-ovens, although not

entirely identical with gas-tar, resembles it to such an extent that it

is worked up with the latter, without making any distinction in practice

between the two kinds. Some descriptions of gas-tar indeed differ very

much more than coke-oven tar from pure coal-tar, viz. those which are

formed when bituminous shale or other materials, considerably deviating

in their nature from coal, are mixed with the latter for the purpose of

obtaining gas of higher illuminating power.

It may be generally said that for the purpose of tar-distillers the tar

is all the more valuable the less other materials than real coal have

been used by the gas-maker. All these materials--bog-head shale,

bituminous lignite and so forth--by destructive distillation yield more

or less paraffinoid oils, which render the purification of the benzols

very difficult and sometimes nearly impossible for the purposes of the

manufacturer of coal-tar colours.

Neither too high nor too low a temperature should have been observed in

gas-making in order to obtain a good quality of tar. Since in recent

times most gas retorts have been provided with heating arrangements

based on the production of gaseous fuel from coke, which produce higher

temperatures than direct firing and have proved a great economy in the

process of gas-making itself, the tar has become of decidedly inferior

quality for the purposes of the tar-distillers, and in particular yields

much less benzol than formerly.

Entirely different from gas-tar is the tar obtained as a by-product from

those (Scottish) blast furnaces which are worked with splint-coal. This

tar contains very little aromatic hydrocarbons, and the phenols are of

quite a different character from those obtained in the working of

gas-tar. The same holds good of oil-gas tars and similar substances.

These should not be worked up like gas-tars.

The ordinary yield of tar in the manufacture of coal-gas is between 4

and 5% of the weight of the coal. Rather more is obtained when passing

the gas through the apparatus of E. Pelouze and P. Audouin, where it is

exposed to several shocks against solid surfaces, or by carrying on the

process at the lowest possible temperature, as proposed by H. J. Davis,

but this "carbonizing process" can only pay under special circumstances,

and is probably no longer in practical use.

All coal-tars have a specific gravity above that of water, in most cases

between 1.12 and 1.20, but exceptionally up to 1.25. The heavier tars

contain less benzol than the lighter tars, and more "fixed carbon,"

which remains behind when the tars are exhausted of benzol and is a

decidedly objectionable constituent. All tars also mechanically retain a

certain quantity of water (or rather gas-liquor), say, 4% on the

average, which is very obnoxious during the process of distillation, as

it leads to "bumping," and therefore ought to be previously removed by

prolonged settling, preferably at a slightly elevated temperature, which

makes the tar more fluid. The water then rises to the top, and is

removed in the ordinary way or by special "separators."

The tar itself is a mixture of exceedingly complex character. The great

bulk of its constituents belongs to the class of "aromatic"

hydrocarbons, of very different composition and degrees of volatility,

beginning with the simplest and most volatile, benzene (C6H6), and

ending with an entirely indistinguishable mass of non-volatile bodies,

which compose the pitch left behind in the tar-stills. The hydrocarbons

mostly belong to the benzene series CnH2n-6, the naphthalene series

CnH2n-12, and the anthracene and phenanthrene series CnH2n-18. Small

quantities of "fatty" ("aliphatic") hydrocarbons are never absent, even

in pure tars, and are found in considerable quantities when shales and

similar matters have been mixed with the coal in the gas-retorts. They

belong mostly to the paraffins CnH2n+2, and the olefines CnH2n. The

"asphalt" or soluble part of the pitch is also a mixture of

hydrocarbons, of the formula CnH2n; even the "carbon," left behind after

treating the pitch with all possible solvents is never pure carbon, but

contains a certain quantity of hydrogen, although less than any of the

volatile and soluble constituents of the tar.

Besides the hydrocarbons, coal-tar contains about 2% of the simpler

phenols CnH2n-7OH, the best known and most valuable of which is the

first of the series, carbolic acid (q.v.) C6H5OH, besides another

interesting oxygenized substance, cumarone C8H6O. The phenols,

especially the carbolic acid, are among the more valuable constituents

of coal-tar. Numerous sulphur compounds also occur in coal-tar, some of

which impart to it their peculiar nauseous smell, but they are of no

technical importance or value.

Still more numerous are the nitrogenated compounds contained in

coal-tar. Most of these are of a basic character, and belong to the

pyridine and the quinoline series. Among these we find a somewhat

considerable quantity of aniline, which, however, is never obtained from

the tar for commercial purposes, as its isolation in the pure state is

too difficult. The pyridines are now mostly recovered from coal-tar, but

only in the shape of a mixture of all members of the series which is

principally employed for denaturing alcohol. Some of these nitrogenated

compounds possess considerable antiseptic properties, but on the whole

they are only considered as a contamination of the tar-oils.

_Applications of Coal-Tar in the Crude State._--Large quantities of

coal-tar are employed for various purposes without submitting it to the

process of distillation. It is mostly advisable to dehydrate the tar as

much as possible for any one of its applications, and in some cases it

is previously boiled in order to remove its more volatile constituents.

No preparation whatever is needed if the tar is to be used as _fuel_,

either for heating the gas-retorts or for other purposes. Its

heating-value is equal to the same weight of best coal, but it is very

difficult to burn it completely without producing a great deal of

evil-smelling smoke. This drawback has been overcome by employing the

same means as have been found suitable for the combustion of the heavy

petroleum residues, called "masut," viz. converting the tar into a fine

spray by means of steam or compressed air. When the gas-maker cannot

conveniently or profitably dispose of his tar for other purposes, he

burns it by the above means under his retorts.

Several processes have also been patented for producing _illuminating

gas_ from tar, the most notable of which is the Dinsmore process. This

process has been adversely criticized by very competent gas-makers, and

no great success can be expected in this line.

Coal-tar is very much employed for painting wood, iron, brickwork, or

stone, as a preventive against the influence of weather or the far more

potent action of corrosive chemicals. This, of course, can be done only

where appearance is no object, for instance in chemical works, where all

kinds of erections and apparatus are protected by this cheap kind of

paint. Coal-tar should not be used for tarring the woodwork and ropes of

ships, a purpose for which only wood-tar has been found suitable.

One of the most considerable outlets for crude tar is in the manufacture

of _roofing-felt_. This industry was introduced in Germany upwards of a

hundred years ago, even before coal-tar was available, and has reached a

very large extension both in that country and in the United States,

where most of the gas-tar seems to be devoted to this purpose. In the

United Kingdom it is much less extensive. For this manufacture a

special fabric is made from pure woollen fibre, on rolls of about 3 ft.

width and of considerable length. The tar must be previously dehydrated,

and is preferably deprived of its more volatile portions by heating in a

still. It is heated in an iron pan to about 90 deg. or 100 deg.C.; the

fabric is drawn through it by means of rollers which at the same time

squeeze out the excess of tar; on coming out of these, the tarred felt

is covered with a layer of sand on both sides by means of a self-acting

apparatus; and is ultimately wound round wooden rolls, in which state it

is sent out into the trade. This roofing-felt is used as a cheap

covering, both by itself and as a grounding for tiles or slates. In the

former case it must be kept in repair by repainting with tar from time

to time, a top covering of sand or small gravel being put on after every

coat of paint.

Coal-tar is also employed for the manufacture of _lamp-black_. This is

done by burning the tar in ovens, connected with brick-chambers in which

the large quantity of soot, formed in this process, deposits before the

gases escape through the chimney. Numerous patents have been taken out

for more efficiently collecting this soot. Most of it is employed

without further manipulation for the manufacture of electric carbons,

printing inks, shoe-blacking, patent leather and so forth. A finer

quality of lamp-black, free from oily and empyreumatic parts, is

obtained by calcining the soot in closed iron pots at a red heat.

_Distillation of Coal-Tar._--Much more important than all applications

of crude coal-tar is the industry of separating its constituents from it

in a more or less pure form by fractional distillation, mostly followed

by purifying processes. Most naturally this industry took its rise in

Great Britain, where coal-gas was invented and made on a large scale

before any other nation took it up, and up to this day both the

manufacture of coal-gas and the distillation of the tar, obtained as a

by-product thereof, are carried out on a much larger scale in that than

in any other country. The first attempts in this line were made in 1815

by F. C. Accum, and in 1822 by Dr G. D. Longstaff and Dr Dalston. At

first the aim was simply to obtain "naphtha," used in the manufacture of

india-rubber goods, for burning in open lamps and for some descriptions

of varnish; the great bulk of the tar remained behind and was used as

fuel or burned for the purpose of obtaining lamp-black.

It is not quite certain who first discovered in the coal-naphtha the

presence of benzene (q.v.), which had been isolated from oil-gas by M.

Faraday as far back as 1825. John Leigh claims to have shown coal-tar

benzene and nitro-benzene made from it at the British Association

meeting held at Manchester in 1842, but the report of the meeting says

nothing about it, and the world in general learned the presence of

benzene in coal-tar only from the independent discovery of A. W.

Hofmann, published in 1845. And it was most assuredly in Hofmann's

London laboratory that Charles Mansfield worked out that method of

fractional distillation of the coal-tar and of isolating the single

hydrocarbons which laid the foundation of that industry. His patent,

numbered 11,960 and dated November 11th, 1847, is the classical

land-mark of it. About the same time, in 1846, Bronner, at Frankfort,

brought his "grease-remover" into the trade, which consisted of the most

volatile coal-tar oils, of course not separated into the pure

hydrocarbons; he also sold water-white "creosote" and heavy tar-oils for

pickling railway timbers, and used the remainder of the tar for the

manufacture of roofing-felt. The employment of heavy oils for pickling

timber had already been patented in 1838 by John Bethell, and from this

time onward the distillation of coal-tar seems to have been developed in

Great Britain on a larger scale, but the utilization of the light oils

in the present manner naturally took place only after Sir W. H. Perkin,

in 1856, discovered the first aniline colour which suddenly created a

demand for benzene and its homologues. The isolation of carbolic acid

from the heavier oils followed soon after; that of naphthalene, which

takes place almost automatically, went on simultaneously, although the

uses of this hydrocarbon for a long time remained much behind the

quantities which are producible from coal-tar, until the manufacture of

synthetic indigo opened out a wide field for it. The last of the great

discoveries in that line was the preparation of alizarine from

anthracene by C. Graebe and C. T. Liebermann, in 1868, soon followed by

patents for its practical manufacture by Sir W. H. Perkin in England,

and by Graebe, Liebermann and H. Caro in Germany.

The present extension of the industry of coal-tar distilling can be only

very roughly estimated from the quantity of coal-tar produced in various

countries. Decidedly at the head is Great Britain, where about 700,000

tons are produced per annum, most of which probably finds its way into

the tar-distilleries, whilst in Germany and the United States much less

gas-tar is produced and a very large proportion of it is used for

roofing-felt and other purposes.

We shall now give an outline of the processes used in the distillation

of tar.

_Dehydration._--The first operation in coal-tar distilling is the

removal of the mechanically enclosed water. Some water is chemically

combined with the bases, phenols, &c., and this, of course, cannot be

removed by mechanical means, but splits off only during the

distillation itself, when a certain temperature has been reached. The

water mechanically present in the tar is separated by long repose in

large reservoirs. Very thick viscous tars are best mixed with thinner

tars, and the whole is gently heated by coils of pipes through which

the heated water from the oil-condensers is made to flow. Sometimes

special "tar-separators" are employed, working on the centrifugal

principle. The water rises to the top and is worked up like ordinary

gas-liquor. More water is again separated during the heating-up of the

tar in the still itself, and can be removed there by a special

overflow.

[Illustration: FIG. 1.--Tar-Still (sectional elevation).[1]]

_Tar-Stills._--The tar is now pumped into the tar-still, fig. 1. This

is usually, as shown, an upright wrought-iron cylinder, with an arched

top, and with a bottom equally vaulted upwards for the purpose of

increasing the heating surface and of raising the level of the pitch

remaining at the end of the operation above the fire-flues. The fuel

is consumed on the fire-grate a, and, after having traversed the holes

bb in the annular wall e built below the still, the furnace gases are

led around the still by means of the flue d, whence they pass to the

chimney. Cast-iron necks are provided in the top for the outlet of the

vapours, for a man-hole, supply-pipe, thermometer-pipe, safety valve,

and for air and steam-pipes reaching down to the bottom and branching

out into a number of distributing arms. Near the top there is an

overflow pipe which comes into action on filling the still. In the

lowest part of the bottom there is a running-off valve or tap. In some

cases (but only exceptionally) a perpendicular shaft is provided, with

horizontal arms, and chains hanging down from these drag along the

bottom for the purpose of keeping it clean and of facilitating the

escape of the vapours. This arrangement is quite unnecessary where the

removal of the vapours is promoted by the injection of steam, but this

steam must be carefully dried beforehand, or, better, slightly

superheated, in order to prevent explosions, which might be caused by

the entry of liquid water into the tar during the later stages of the

work, when the temperature has arisen far above the boiling-point of

water. The steam acts both by stirring up the tar and by rapidly

carrying off the vapours formed in distillation. The latter object is

even more thoroughly attained by the application of a vacuum,

especially during the later stage of distillation. For this purpose

the receivers, in which the liquids condensed in the cooler are

collected, are connected with an air pump or an ejector, by which a

vacuum of about 4 in., say 1/8 atmosphere, is made which lowers the

boiling process by about 80 deg. C.; this not merely hastens the process,

but also produces an improvement of the quality and yield of the

products, especially of the anthracene, and, moreover, lessens or

altogether prevents the formation of coke on the still-bottom, which

is otherwise very troublesome.

Most manufacturers employ ordinary stills as described. A few of them

have introduced continuously acting stills, of which that constructed

by Frederic Lennard has probably found a wider application than any of

the others. They all work on the principle of gradually heating the

tar in several compartments, following one after the other. The fresh

tar is run in at one end and the pitch is run out from the other. The

vapours formed in the various compartments are separately carried away

and condensed, yielding at one and the same time those products which

are obtained in the ordinary stills at the different periods of the

distillation. Although in theory this continuous process has great

advantages over the ordinary style of working, the complication of the

apparatus and practical difficulties arising in the manipulation have

deterred most manufacturers from introducing it.

The tar-stills are set in brickwork in such a manner that there is no

over-heating of their contents. For this purpose the fire-grate is

placed at a good distance from the bottom or even covered by a brick

arch so that the flame does not touch the still-bottom at all and acts

only indirectly, but the sides of the still are always directly

heated. The fire-flue must not be carried up to a greater height than

is necessary to provide against the overheating of any part of the

still not protected inside by liquid tar, or, at the end of the

operation, by liquid pitch. The outlet pipe is equally protected

against overheating and also against any stoppage by pitch solidifying

therein. The capacity of tar-stills ranges from 5 to 50 tons. They

hold usually about 10 tons, in which case they can be worked off

during one day.

The vapours coming from the still are condensed in coolers of various

shapes, one of which is shown in figs. 2 and 3. The cooling-pipes are

best made of cast-iron, say 4 in. wide inside and laid so as to have a

continuous fall towards the bottom. A steam-pipe (b) is provided for

heating the cooling water, which is necessary during the later part of

the operation to prevent the stopping up of the pipes by the

solidification of the distillates. A cock (a) allows steam to be

injected into the condensing worm in order to clear any obstruction.

[Illustration: FIG. 2.--Condensing Worm (Plan)]

The cooling-pipe is at its lower end connected with receivers for the

various distillates in such a manner that by the turning of a cock the

flow of the distillates into the receivers can be changed at will. In

a suitable place provision is made for watching the colour, the

specific gravity, and the general appearance of the distillates. At

the end of the train of apparatus, and behind the vacuum pump or

ejector, when one is provided, there is sometimes a purifier for the

gases which remain after condensation; or these gases are carried back

into the fire, in which case a water-trap must be interposed to

prevent explosions.

_Distillation of the Tar._--The number of fractions taken during the

distillation varies from four to six. Sometimes a first fraction is

taken as "first runnings," up to a temperature of 105 deg. C. in the

still, and a second fraction as "light oil," up to 210 deg. C., but more

usually these two are not separated in the first distillation, and the

first or "light oil" fraction then embraces everything which comes

over until the drops no longer float on, but show the same specific

gravity as water. The specific gravity of this fraction varies from

0.91 to 0.94. The next fraction is the "middle oil" or "carbolic oil,"

of specific gravity 1.01, boiling up to 240 deg. C.; it contains most of

the carbolic acid and naphthalene. The next fraction is the "heavy

oil" or "creosote oil," of specific gravity 1.04. Where the nature of

the coals distilled for gas is such that the tar contains too little

anthracene to be economically recovered, the creosote-oil fraction is

carried right to the end; but otherwise, that is in most cases, a last

fraction is made at about the temperature 270 deg. C., above which the

"anthracene oil" or "green oil" is obtained up to the finish of the

distillation.

[Illustration: FIG. 3.--Condensing Worm (side elevation).]

During the light-oil period the firing must be performed very

cautiously, especially where the water has not been well removed, to

prevent bumping and boiling over. It has been observed that, apart

from the water, those tars incline most to boiling over which contain

an unusual quantity of "fixed carbon." During this period cold water

must be kept running through the cooler. The distillate at once

separates into water (gas-liquor) and light oil, floating at the top.

Towards the end of this fraction the distillation seems to cease, in

spite of increasing the fires, and a rattling noise is heard in the

still. This is caused by the combined water splitting off from the

bases and phenols and causing slight explosions in the tar.

As soon as the specific gravity approaches 1.0, the supply of cold

water to the cooler is at least partly cut off, so that the

temperature of the water rises up to 40 deg. C. This is necessary because

otherwise some naphthalene would crystallize out and plug up the

pipes. If a little steam is injected into the still during this period

no stoppage of the pipes need be feared in any case, but this must be

done cautiously.

When the carbolic oil has passed over and the temperature in the still

has risen to about 240 deg. C., the distillate can be run freely by always

keeping the temperature in the cooler at least up to 40 deg. C. The

"creosote oil" which now comes over often separates a good deal of

solid naphthalene on cooling.

The last fraction is made, either when the thermometer indicates 270 deg.

C., or when "green grease" appears in the distillate, or simply by

judging from the quantity of the distillate. What comes over now is

the "anthracene oil." The firing may cease towards the end as the

steam (with the vacuum) will finish the work by itself. The water in

the cooler should now approach the boiling-point.

The point of finishing the distillation is different in various places

and for various objects. It depends upon the fact whether _soft_ or

_hard_ pitch is wanted. The latter must be made where it has to be

sold at a distance, as soft pitch cannot be easily carried during the

warmer season in railway trucks and not at all in ships, where it

would run into a single lump. Hard pitch is also always made where as

much anthracene as possible is to be obtained. For hard pitch the

distillation is carried on as far as practicable without causing the

residue in the still to "coke." The end cannot be judged by the

thermometer, but by the appearance and quantity of the distillate and

its specific gravity. If carried too far, not merely is coke formed,

but the pitch is porous and almost useless, and the anthracene oil is

contaminated with high-boiling hydrocarbons which may render it almost

worthless as well. Hard pitch proper should soften at 100 deg. C., or

little above.

Where the distillation is to stop at soft pitch it is, of course, not

carried up to the same point, but wherever the pitch can be disposed

of during the colder season or without a long carriage, even the hard

pitch is preferably softened within the still by pumping back a

sufficient quantity of heavy oil, previously deprived of anthracene.

This makes it much easier to discharge the still. When the contents

consist of soft pitch they are run off without much trouble, but hard

pitch not merely emits extremely pungent vapours, but is mostly at so

high a temperature that it takes fire in the air. Hard pitch must,

therefore, always be run into an iron or brick cooler where it cools

down out of contact with air, until it can be drawn out into the open

pots where its solidification is completed.

Most of the pitch is used for the manufacture of "briquettes" ("patent

fuel"), for which purpose it should soften between 55 deg. and 80 deg. C.

according to the requirements of the buyer. In Germany upwards of

50,000 tons are used annually in that industry; much of it is imported

from the United Kingdom, whence also France and Belgium are provided.

Apart from the softening point the pitch is all the more valued the

more constituents it contains which are soluble in xylene. The portion

insoluble in this is denoted as "fixed carbon." If the briquette

manufacturer has bought the pitch in the hard state he must himself

bring it down to the proper softening point by re-melting it with

heavy coal-tar oils.

We now come to the treatment of the various fractions obtained from

the tar-stills. These operations are frequently not carried out at the

smaller tar-works, which sell their oils in the crude state to the

larger tar-distillers.

_Working up of the Light-Oil Fraction._--The greatest portion of the

light-oil fraction consists of aromatic hydrocarbons, about one-fifth

being naphthalene, four-fifths benzene and its homologues, in the

proportion of about 100 benzene, 30 toluene, 15 xylenes, 10

trimethylbenzenes, 1 tetramethylbenzene. Besides these the light-oil

contains 5-15% phenols, 1-3% bases, 0.1 sulphuretted compounds,

0.2-0.3% nitriles, &c. It is usually first submitted to a preliminary

distillation in directly fired stills, similar to the tar-stills, but

with a dephlegmating head. Here we obtain (1) first runnings (up to

O.89 spec. grav.), (2) heavy benzols (up to O.95), (3) carbolic oil

(up to 1.00). The residue remaining in the still (chiefly naphthalene)

goes to the middle-oil fraction.

The "first runnings" are now "washed" in various ways, of which we

shall describe one of the best. The oil is mixed with dilute caustic

soda solution, and the solution of phenols thus obtained is worked up

with that obtained from the next fractions. After this follows a

treatment with dilute sulphuric acid (spec. grav. 1.3), to extract the

pyridine bases, and lastly with concentrated sulphuric acid (1.84),

which removes some of the aliphatic hydrocarbons and "unsaturated"

compounds. After this the crude benzol is thoroughly washed with water

and dilute caustic soda solution, until its reaction is neutral. The

mixing of the basic, acid and aqueous washing-liquids with the oils is

performed by compressed air, or more suitably by mechanical stirrers,

arranged on a perpendicular, or better, a horizontal shaft. Precisely

the same treatment takes place with the next fraction, the "heavy

benzols," and the oils left behind after the washing operations now go

to the steam-stills. The heaviest hydrocarbons are sometimes twice

subjected to the operation of washing.

The washed crude benzols are now further fractionated by distillation

with steam. The _steam-stills_ are in nearly all details on the

principle of the "column apparatus" employed in the distillation of

alcoholic liquids, as represented in fig. 4. They are usually made of

cast iron. The still itself is either an upright or a horizontal

cylinder, heated by a steam-coil, of a capacity of from 1000 to 2000

gallons. The superposed columns contain from 20 to 50 compartments of

a width of 2-1/2 or 3 ft. The vapours pass into a cooler, and from this

the distillate runs through an apparatus, where the liquor can be seen

and tested, into the receivers. The latter are so arranged that the

water passing over at the same time is automatically removed. This is

especially necessary, because the last fraction is distilled by means

of pure steam.

The fractions made in the steam distillation vary at different works.

In some places the pure hydrocarbons are net extracted and here only

the articles called: "90 per cent. benzol," "50 per cent. benzol,"

"solvent naphtha," "burning naphtha" are made, or any other commercial

articles as they are ordered. The expression "per cent." in this case

does not signify the percentage of real benzene, but that portion

which distills over up to the temperature of 100 deg. C., when a certain

quantity of the article is heated in glass retorts of a definite

shape, with the thermometer inserted in the liquid itself. By the

application of well-constructed rectifying-columns and with proper

care it is, however, possible to obtain in this operation nearly pure

benzene, toluene, xylene, and cumene (in the two last cases a mixture

of the various isomeric hydrocarbons). These hydrocarbons contain only

a slight proportion of thiophene and its isomers, which can be removed

only by a treatment with fuming sulphuric acid, but this is only

exceptionally done.

Sometimes the _pyridine bases_ are recovered from the tarry acid which

is obtained in the treatment of the light oil with sulphuric acid, and

which contains from 10 to 30% of bases, chiefly pyridine and its

homologues with a little aniline, together with resinous substances.

The latter are best removed by a partial precipitation with ammonia,

either in the shape of gas or of concentrated ammoniacal liquor. This

reagent is added until the acid reaction has just disappeared and a

faint smell of pyridine is perceived. The mixture is allowed to

settle, and it then separates into two layers. The upper layer,

containing the impurities, is run off; the lower layer, containing the

sulphates of ammonia and of the pyridine bases, is treated with

ammonia in excess, where it separates into a lower aqueous layer of

ammonium sulphate solution and an oil, consisting of crude pyridine.

This is purified by fractionation in iron stills and distillation over

caustic soda. Most of it is used for denaturing spirit of wine in

Germany, for which purpose it is required to contain 90% of bases

boiling up to 140 deg. C. (see ALCOHOL).

[Illustration: FIG. 4.--Benzol Still (elevation).]

_Working up of the Middle-Oil Fraction (Carbolic Oil

Fraction)._--Owing to its great percentage of naphthalene (about 40%)

this fraction is solid or semi-solid at ordinary temperatures. Its

specific gravity is about 1.2; its colour may vary from light yellow

to dark brown or black. In the latter case it must be re-distilled

before further treatment. On cooling down, about four-fifths of the

naphthalene crystallizes out on standing from three to ten days. The

crystals are freed from the mother oils by draining and cold or hot

pressing; they are then washed at 100 deg. C. with concentrated

sulphuric acid, afterwards with water and re-distilled or sublimed.

About 10,000 tons of naphthalene are used annually in Germany, mostly

for the manufacture of many azo-colours and of synthetic indigo.

The oils drained from the crude naphthalene are re-distilled and

worked for carbolic acid and its isomers. For this purpose the oil is

washed with a solution of caustic soda, of specific gravity 1.1; the

solution thus obtained is treated with sulphuric acid or with carbon

dioxide, and the crude phenols now separated are fractionated in a

similar manner as is done in the case of crude benzol. The pure phenol

crystallizes out and is again distilled in iron stills with a silver

head and cooling worm; the remaining oils, consisting mainly of

cresols, are sold as "liquid carbolic acid" or under other names.

Most of the oil which passes as the "creosote-oil fraction" is sold in

the crude state for the purpose of pickling timber. It is at the

ordinary temperature a semi-solid mixture of about 20% crystallized

hydrocarbons (chiefly naphthalene), and 80% of a dark brown, nauseous

smelling oil, of 1.04 spec. grav., and boiling between 200 deg. and

300 deg. C. The liquid portion contains phenols, bases, and a great

number of hydrocarbons. Sometimes it is redistilled, when most of the

naphthalene passes over in the first fraction, between 180 deg. and

230 deg. C., and crystallizes out in a nearly pure state. The oily

portion remaining behind, about 60% of this distillate, contains about

30% phenols and 3% bases. It has highly disinfectant properties and is

frequently converted into special disinfectants, e.g. by mixing it

with four times its volume of slaked lime, which yields "disinfectant

powder" for stables, railway cars, &c. Mixtures of potash soaps (soft

soaps) with this oil have the property of yielding with water

emulsions which do not settle for a long time and are found in the

trade as "creolin," "sapocarbol," "lysol," &c.

That description of creosote oil which is sold for the purpose of

pickling railway sleepers, telegraph posts, timber for the erection of

wharves and so forth, must satisfy special requirements which are laid

down in the specifications for tenders to public bodies. These vary to

a considerable extent. They always stipulate (1) a certain specific

gravity (e.g. not below 1.035 and not above 1.065); (2) certain limits

of boiling points (e.g. to yield at most 3% up to 150 deg., at most

30% between 150 deg. and 255 deg., and at least 85% between 150 deg.

and 355 deg.); (3) a certain percentage of phenols, as shown by

extraction with caustic soda solution, say 8 to 10%.

Much of this creosote oil is obtained by mixing that which has

resulted in the direct distillation of the tar with the liquid portion

of the anthracene oils after separating the crude anthracene (see

below). It is frequently stipulated that the oil should remain clear

at the ordinary temperature, say 15 deg. C., which means that no

naphthalene should crystallize out.

_Working up the Anthracene Oil Fraction._--The crude oil boils between

280 deg. and 400 deg. C. It is liquid at 60 deg. C., but on cooling

about 6 to 10% of crude anthracene separates as greenish-yellow, sandy

crystals, containing about 30% of real anthracene, together with a

large percentage of carbazol and phenanthrene. This crystallization

takes about a week. The crude anthracene is separated from the mother

oils by filter presses, followed by centrifugals or by hot hydraulic

presses. The liquid oils are redistilled, in order to obtain more

anthracene, and the last oils go back to the creosote oil, or are

employed for softening the hard pitch (_vide supra_). The crude

anthracene is brought up to 50 or 60, sometimes to 80%, by washing

with solvent naphtha, or more efficiently with the higher boiling

portion of the pyridine bases. The naphtha removes mostly only the

phenanthrene, but the carbazol can be removed only by pyridine, or by

subliming or distilling the anthracene over caustic potash. The whole

of the anthracene is sold for the manufacture of artificial alizarine.

BIBLIOGRAPHY.--The principal work on Coal-tar is G. Lunge's _Coal-tar

and Ammonia_ (3rd ed., 1900). Consult also G. P. Sadtler, _Handbook of

Industrial Organic Chemistry_ (1891), and the article

"Steinkohlentheer," Kraemer and Spreker, in _Encyklopadisches Handbuch

der technischen Chemie_ (4th ed., 1905, viii. 1). (G. L.)