Chapter XIII: Part 13

The driving of coaches with four horses was a task in which a considerable amount of skill was required,[1] and English literature is full of the difficulties and humours of "the road" in old days. A form of sport thus arose for enterprising members of the nobility and gentry, and after the introduction of railways made the mail-coach obsolete as a matter of necessity, the old sport of coaching for pleasure still survived, though only to a limited extent. The Four-in-hand Club was started in England in 1856 and the Coaching Club in 1870, as the successors of the old Bensington Driving Club (1807-1852), and Four-Horse Club (1808-1829); and in America the New York Coaching Club was founded in 1875. But coaching remains the sport of the wealthier classes, although in various parts of England (e.g. London to Brighton, and in the Lake district), in America, and in Europe, public coaches still have their regular times and routes for those who enjoy this form of travel. The earliest railway vehicles for passengers were merely the road coaches of the period adapted to run on rails, and the expression "coaching traffic" is still used in England to denote traffic carried in passenger trains.

Of coaches possessing a history the two best known in the United Kingdom are the king's state coach, and that of the lord mayor of London. The latter is the oldest, having been built, or at least first used, for the procession of Sir Charles Asgil, lord mayor elect, in November 1757. The body of this vehicle is not supported by springs, but hung on leather straps; and the whole structure is very richly loaded with ornamental carving, gilding and paint-work. The different panels and the doors contain various allegorical groups of figures representing suitable subjects, and heraldic devices painted in a spirited manner. The royal state coach, which is described as "the most superb carriage ever built," was designed by Sir William Chambers, the paintings on it were executed by Cipriani, and the work was completed in 1761. During the later part of Queen Victoria's reign it was hardly ever seen, but on the accession of Edward VII. the coach was once more put in order for use on state occasions. The following is an official description of this famous coach:--

"The whole of the carriage and body is richly ornamented with laurel
and carved work, beautifully gilt. The length, 24 ft.; width, 8 ft. 3
in.; height, 12 ft.; length of pole, 12 ft. 4 in.; weight, 4 tons. The
carriage and body of the coach is composed as follows:--Of four large
tritons, who support the body by four braces, covered with red morocco
leather, and ornamented with gilt buckles, the two figures placed in
front of the carriage bear the driver, and are represented in the
action of drawing by cables extending round their shoulders, and the
cranes and sounding shells to announce the approach of the monarch of
the ocean; and those at the back carry the imperial fasces, topped
with tridents. The driver's foot-board is a large scallop shell,
ornamented with bunches of reeds and other marine plants. The pole
represents a bundle of lances; the splinter bar is composed of a rich
moulding, issuing from beneath a voluted shell, and each end
terminating in the head of a dolphin; and the wheels are imitated from
those of the ancient triumphal chariot. The body of the coach is
composed of eight palm-trees, which, branching out at the top, sustain
the roof; and four angular trees are loaded with trophies allusive to
the victories obtained by Great Britain during the late glorious war,
supported by four lions' heads. On the centre of the roof stand three
boys, representing the genii of England, Scotland and Ireland,
supporting the imperial crown of Great Britain, and holding in their
hands the sceptre, sword of state, and ensigns of knighthood; their
bodies are adorned with festoons of laurel, which fall from thence
towards the four corners. The panels and doors are painted with
appropriate emblematical devices, and the linings are of scarlet
velvet richly embossed with national emblems."

See the Badminton _Driving_, by the duke of Beaufort (1888); Rogers's
_Manual of Driving_ (Philadelphia, 1900); and "Nimrod's" _Essays on
the Road_ (1876).

FOOTNOTE:

[1] The idea of "driving" was responsible for the use of the term
"coach" and "coaching" to mean a tutor or trainer, for examinations
or athletic contests.

COAHUILA, a northern frontier state of Mexico, bounded N. and N.E. by Texas, U.S.A., E. by Nuevo Leon, S. by San Luis Potosi and Zacatecas, and W. by Durango and Chihuahua. Area, 63,569 sq.m.; pop. (1895) 237,815; (1900) 296,938. Its surface is a roughly broken plateau, traversed N.W. to S.E. by several ranges of mountains and sloping gently toward the Rio Grande. The only level tract of any size in the state is the Bolson de Mapimi, a great depression on the western side which was long considered barren and uninhabitable. It is a region of lakes and morasses, of arid plains and high temperatures, but experiments with irrigation toward the end of the 19th century were highly successful and considerable tracts have since been brought under cultivation. In general the state is insufficiently watered, the rainfall being light and the rivers small. The rivers flow eastward to the Rio Grande. The climate is hot and dry, and generally healthy. Stock-raising was for a time the principal industry, but agriculture has been largely developed in several localities, among the chief products of which are cotton--Coahuila is the principal cotton-producing state in Mexico--Indian corn, wheat, beans, sugar and grapes. The Parras district in the southern part of the state has long been celebrated for its wines and brandies. The mineral wealth of the state is very great, and the mining industries, largely operated with foreign capital, are important. The mineral products include silver, lead, coal, copper, and iron. The mining operations are chiefly centred in the Sierra Mojada, Sierra Carmen, and in the Santa Rosa valley. The modern industrial development of the state is due to the railway lines constructed across it during the last quarter of the 19th century, and to the investment of foreign capital in local enterprises. The first Spanish settlement in the region now called Coahuila was at Saltillo in 1586, when it formed part of the province of Nueva Viscaya. Later it became the province of Nueva Estremadura under the Spanish regime, and in 1824, under the new republican organization, it became the state of Coahuila and included Texas and Nuevo Leon. Later in the same year Nuevo Leon was detached, but Texas remained a part of the state until 1835. The capital of the state is Saltillo; Monclova was the capital from 1833 to 1835. Among the more important towns are Parras (pop. 6476 in 1900), 98 m. W. by N. of Saltillo in a rich grape-producing district, Ciudad Porfirio Diaz, and Monclova (pop. 6684 in 1900), 105 m. N. by W. of Saltillo, on the Mexican International railway.

COAL. In its most general sense the term "coal" includes all varieties of carbonaceous minerals used as fuel, but it is now usual in England to restrict it to the particular varieties of such minerals occurring in the older Carboniferous formations. On the continent of Europe it is customary to consider coal as divisible into two great classes, depending upon differences of colour, namely, _brown coal_, corresponding to the term "lignite" used in England and France, and _black_ or _stone coal_, which is equivalent to coal as understood in England. Stone coal is also a local English term, but with a signification restricted to the substance known by mineralogists as anthracite. In old English writings the terms pit-coal and sea-coal are commonly used. These have reference to the mode in which the mineral is obtained, and the manner in which it is transported to market.

The root _kol_ is common to all the Teutonic nations, while in French and other Romance languages derivatives of the Latin _carbo_ are used, e.g. _charbon de terre_. In France and Belgium, however, a peculiar word, _houille_, is generally used to signify mineral coal. This word is supposed to be derived from the Walloon _hoie_, corresponding to the medieval Latin _hullae_. Littre suggests that it may be related to the Gothic _haurja_, coal. Anthracite is from the Greek [Greek: anthrax], and the term _lithanthrax_, stone coal, still survives, with the same meaning, in the Italian _litantrace_.

It must be borne in mind that the signification now attached to the word coal is different from that which formerly obtained when wood was the only fuel in general use. Coal then meant the carbonaceous residue obtained in the destructive distillation of wood, or what is known as charcoal, and the name collier was applied indifferently to both coal-miners and charcoal-burners.

The spelling "cole" was generally used up to the middle of the 17th century, when it was gradually superseded by the modern form, "coal." The plural, coals, seems to have been used from a very early period to signify the broken fragments of the mineral as prepared for use.

Physical properties.

Coal is an amorphous substance of variable composition, and therefore cannot be as strictly defined as a crystallized or definite mineral can. It varies in colour from a light brown in the newest lignites to a pure black, often with a bluish or yellowish tint in the more compact anthracite of the older formations. It is opaque, except in exceedingly thin slices, such as made for microscopic investigation, which are imperfectly transparent, and of a dark brown colour by transmitted light. The streak is black in anthracite, but more or less brown in the softer varieties. The maximum hardness is from 2.5 to 3 in anthracite and hard bituminous coals, but considerably less in lignites, which are nearly as soft as rotten wood. A greater hardness is due to the presence of earthy impurities. The densest anthracite is often of a semi-metallic lustre, resembling somewhat that of graphite. Bright, glance or pitch coal is another brilliant variety, brittle, and breaking into regular fragments of a black colour and pitchy lustre. Lignite and cannel are usually dull and earthy, and of an irregular fracture, the latter being much tougher than the black coal. Some lignites are, however, quite as brilliant as anthracite; cannel and jet may be turned in the lathe, and are susceptible of taking a brilliant polish. The specific gravity is highest in anthracite and lowest in lignite, bituminous coals giving intermediate values (see TABLE I.). As a rule, the density increases with the amount of carbon, but in some instances a very high specific gravity is due to intermixed earthy matters, which are always denser than even the densest form of coal substance.

Coal is never definitely crystalline, the nearest approach to such a structure being a compound fibrous grouping resembling that of gypsum or arragonite, which occurs in some of the steam coals of South Wales, and is locally known as "cone in cone," but no definite form or arrangement can be made out of the fibres. Usually it occurs in compact beds of alternating bright and dark bands in which impressions of leaves, woody fibre and other vegetable remains are commonly found. There is generally a tendency in coals towards cleaving into cubical or prismatic blocks, but sometimes the cohesion between the particles is so feeble that the mass breaks up into dust when struck. These peculiarities of structure may vary very considerably within small areas; and the position of the divisional planes or cleats with reference to the mass, and the proportion of small coal or slack to the larger fragments when the coal is broken up by cutting-tools, are points of great importance in the working of coal on a large scale.

The divisional planes often contain small films of other minerals, the commonest being calcite, gypsum and iron pyrites, but in some cases zeolitic minerals and galena have been observed. Salt, in the form of brine, is sometimes present in coal. Hydrocarbons, such as petroleum, bitumen, paraffin, &c., are also found occasionally in coal, but more generally in the associated sandstones and limestones of the Carboniferous formation. Gases, consisting principally of light carburetted hydrogen or marsh gas, are often present in considerable quantity in coal, in a dissolved or occluded state, and the evolution of these upon exposure to the air, especially when a sudden diminution of atmospheric pressure takes place, constitutes one of the most formidable dangers that the coal miner has to encounter.

Classification.

Anthracite.

The classification of the different kinds of coal may be considered from various points of view, such as their chemical composition, their behaviour when subjected to heat or when burnt, and their geological position and origin. They all contain carbon, hydrogen, oxygen and nitrogen, forming the carbonaceous or combustible portion, and some quantity of mineral matter, which remains after combustion as a residue or "ash." As the amount of ash varies very considerably in different coals, and stands in no relation to the proportion of the other constituents, it is necessary in forming a chemical classification to compute the results of analysis after deduction of the ash and hygroscopic water. Examples of analyses treated in this manner are furnished in the last column of Table I., from which it will be seen that the nearest approach to pure carbon is furnished by anthracite, which contains above 90%. This class of coal burns with a very small amount of flame, producing intense local heat and no smoke. It is especially used for drying hops and malt, and in blast furnaces where a high temperature is required, but it is not suited for reverberatory furnaces.

Bituminous coals.

The most important class of coals is that generally known as bituminous, from their property of softening or undergoing an apparent fusion when heated to a temperature far below that at which actual combustion takes place. This term is founded on a misapprehension of the nature of the occurrence, since, although the softening takes place at a low temperature, still it marks the point at which destructive distillation commences, and hydrocarbons both of a solid and gaseous character are formed. That nothing analogous to bitumen exists in coals is proved by the fact that the ordinary solvents for bituminous substances, such as bisulphide of carbon and benzol, have no effect upon them, as would be the case if they contained bitumen soluble in these re-agents. The term is, however, a convenient one, and one whose use is almost a necessity, from its having an almost universal currency among coal miners. The proportion of carbon in bituminous coals may vary from 80 to 90%--the amount being highest as they approach the character of anthracite, and least in those which are nearest to lignites. The amount of hydrogen is from 4-1/2 to 6%, while the oxygen may vary within much wider limits, or from about 3 to 14%. These variations in composition are attended with corresponding differences in qualities, which are distinguished by special names. Thus the semi-anthracitic coals of South Wales are known as "dry" or "steam coals," being especially valuable for use in marine steam-boilers, as they burn more readily than anthracite and with a larger amount of flame, while giving out a great amount of heat, and practically without producing smoke. Coals richer in hydrogen, on the other hand, are more useful for burning in open fires--smiths' forges and furnaces--where a long flame is required.

Gas coal.

The excess of hydrogen in a coal, above the amount necessary to combine with its oxygen to form water, is known as "disposable" hydrogen, and is a measure of the fitness of the coal for use in gas-making. This excess is greatest in what is known as cannel coal, the Lancashire kennel or candle coal, so named from the bright light it gives out when burning. This, although of very small value as fuel, commands a specially high price for gas-making. Cannel is more compact and duller than ordinary coal, and can be wrought in the lathe and polished.

TABLE I.--_Elementary Composition of Coal_ (the figures denote the amounts per cent).

+----------------------------------------------------------------------------------------+----------------------+
| | Composition |
| | exclusive of Water, |
| | Sulphur and Ash. |
+----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+
| |Specific| |Hydro-| |Nitro-| Sul- | | | |Hydro-| O. |
| Localities. |Gravity.|Carbon.| gen. |Oxygen.| gen. | phur.| Ash. |Water.|Carbon.| gen. | and N.|
+----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+
|_Anthracite._ | | | | | | | | | | | |
| 1. South Wales | 1.392 | 90.39 | 3.28 | 2.98 | 0.83 | 0.91 | 1.61 | 2.00 | 93.54 | 3.39 | 3.82 |
| 2. Pennsylvania | 1.462 | 90.45 | 2.43 | 2.45 | .. | .. | 4.67 | .. | 94.89 | 2.54 | 2.57 |
| 3. Peru | .. | 82.70 | 1.41 | 0.85 |10.35 | 3.75 | 0.94 | 97.34 | 1.66 | 1.00 |
+----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+
|_Bituminous Steam and Coking Coal._ | | | | | | | | | | |
| 4. Risca, South Wales | | 75.49 | 4.73 | 6.78 | 1.21 |10.67 | 1.12 | 86.78 | 5.43 | 7.79 |
| 5. Aberdare, " | .. | 86.80 | 4.25 | 3.06 | 0.83 | 4.40 | 0.66 | 92.24 | 4.51 | 3.25 |
| 6. Hartley, Northumberl'd | .. | 78.65 | 4.65 | 13.36 | 0.55 | 2.49 | .. | 80.67 | 4.76 | 14.5 |
| 7. Dudley, Staffordshire | 1.278 | 78.57 | 5.29 | 12.88 | 1.84 | 0.39 | 1.03 | 1.13 | 79.70 | 5.37 | 14.9 |
| 8. Stranitzen, Styria | .. | 79.90 | 4.85 | 12.75 | 0.64 | 0.20 | 1.66 | .. | 81.45 | 4.92 | 13.63 |
+----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+
|_Cannel or Gas Coal._ | | | | | | | | | | | |
| 9. Wigan, Lancashire | 1.276 | 80.07 | 5.53 | 8.08 | 2.12 | 1.50 | 2.70 | 0.91 | 85.48 | 5.90 | 8.62 |
|10. Boghead, Scotland | .. | 63.10 | 8.91 | 7.25 | 0.96 |19.78 | .. | 79.61 |11.24 | 9.15 |
|11. (Albertite) Nova Scotia | .. | 82.67 | 9.14 | 8.19 | .. | .. | .. | 82.67 | 9.14 | 8.19 |
|12. (Tasmanite) Tasmania | 1.18 | 79.34 |10.41 | 4.93 | 5.32 | .. | .. | 83.80 |10.99 | 5.21 |
+----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+
|_Lignite and Brown Coal._ | | | | | | | | | | | |
|13. Cologne | 1.100 | 63.29 | 4.98 | 26.24 | .. | 8.49 | .. | 66.97 | 5.27 | 27.76 |
|14. Bovey Tracy, Devonshire | .. | 66.31 | 5.63 | 22.86 | 0.57 | 2.36 | 2.36 | .. | 69.53 | 5.90 | 24.57 |
|15. Trifail, Styria | .. | 50.72 | 5.34 | 33.18 | 2.80 | 0.90 | 7.86 | .. | 55.11 | 5.80 | 39.09 |
+----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+

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

Caking coals.

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

Lignite.

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

Ash of coal.

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

Sulphur in coal.

The composition of the ash of true coal approximates to that of a fire-clay, allowance being made for lime, which may be present either as carbonate or sulphate, and for sulphuric acid. Sulphur is derived mainly from iron pyrites, which yields sulphates by combustion. An indication of the character of the ash of a coal is afforded by its colour, white ash coals being generally freer from sulphur than those containing iron pyrites, which yield a red ash. There are, however, several striking exceptions, as for instance in the anthracite from Peru, given in Table I., which contains more than 10% of sulphur, and yields but a very small percentage of a white ash. In this coal, as well as in the lignite of Tasmania, known as white coal or Tasmanite, the sulphur occurs in organic combination, but is so firmly held that it can only be very partially expelled, even by exposure to a very high and continued heating out of contact with the air. An anthracite occurring in connexion with the old volcanic rocks of Arthur's Seat, Edinburgh, which contains a large amount of sulphur in proportion to the ash, has been found to behave in a similar manner. Under ordinary conditions, from 1/8 to 1/4 of the whole amount of sulphur in a coal is volatilized during combustion, the remaining 3/4 to 7/8 being found in the ash.

TABLE II.--_Composition of the Ashes of Coals._

+----------------------+-------+--------+--------+-------+---------+-------+---------+----------+--------+
| | | | Ferric | | | |Sulphuric|Phosphoric| |
| |Silica.|Alumina.| Oxide. | Lime. |Magnesia.|Potash.| Acid. | Acid. | Total. |
+----------------------+-------+--------+--------+-------+---------+-------+---------+----------+--------+
| True Coals. | | | | | | | | | |
| Dowlais, South Wales | 39.64 | 39.20 | 11.84 | 1.81 | 2.58 | .. | .. | 3.01 | 98.08 |
| Ebbw Vale, " | 53.00 | 35.01 | .. | 3.94 | 2.20 | .. | 4.89 | 0.88 | 99.92 |
| Konigsgrube, Silesia | 55.41 | 18.95 | 16.06 | 3.21 | 1.87 | 2.05 | 1.73 | 0.36 | 99.64 |
| Ohio | 44.60 | 41.10 | 7.40 | 3.61 | 1.28 | 1.82 | 0.59 | 0.29 | 100.69 |
| | | | | | | | | | |
| Lignites. | | | | | | | | | |
| Helmstadt, Saxony | 17.27 | 11.57 | 5.57 | 23.67 | 2.58 | 2.64 | 33.83 | .. | 97.13 |
| Edeleney, Hungary | 36.01 | 23.07 | 5.05 | 15.62 | 3.64 | 2.38 | 12.35 | .. | 98.12 |
+----------------------+-------+--------+--------+-------+---------+-------+---------+----------+--------+

Water in coal.

The amount of water present in freshly raised coals varies very considerably. It is generally largest in lignites, which may sometimes contain 30% or even more, while in the coals of the coal measures it does not usually exceed from 5 to 10%. The loss of weight by exposure to the atmosphere from drying may be from 1/2 to 3/4 of the total amount of water contained.

TABLE III.--_Composition of Fuels (assuming Carbon = 100)._

+------------------------------+-------+---------+---------+-----------+
| | | | |Disposable |
| |Carbon.|Hydrogen.| Oxygen. | Hydrogen. |
+------------------------------+-------+---------+---------+-----------+
| Wood | 100 | 12.18 | 83.07 | 1.80 |
| Peat | 100 | 9.85 | 55.67 | 2.89 |
| Lignite | 100 | 8.37 | 42.42 | 3.07 |
| Thick Coal, S. Staffordshire | 100 | 6.12 | 21.23 | 3.47 |
| Hartley Steam Coal | 100 | 5.91 | 18.32 | 3.62 |
| South Wales Steam Coal | 100 | 4.75 | 5.28 | 4.09 |
| American Anthracite | 100 | 2.84 | 1.74 | 2.63 |
+------------------------------+-------+---------+---------+-----------+

Origin of Coal.

Coal is the result of the transformation of woody fibre and other vegetable matter by the elimination of oxygen and hydrogen in proportionally larger quantity than carbon, so that the percentage of the latter element is increased in the manner shown in Table III., given by J. Percy, the mineral matter being also changed by the removal of silica and alkalis and the substitution of substances analogous in composition to fire-clay. The causes and methods of these changes are, however, not very exactly defined. According to the elaborate researches of B. Renault (_Bulletin de la Societe de l'Industrie minerale_, 3 ser. vol. xiii. p. 865), the agents of the transformation of cellulose into peaty substances are saprophytic fungi and bacterial ferments. As the former are only active in the air while the latter are anaerobic, the activity of either agent is conditioned by variation in the water level of the bog. The ultimate term of bacterial activity seems to be the production of ulmic acid, containing carbon 65.31 and hydrogen 3.85%, which is a powerful antiseptic. By the progressive elimination of oxygen and hydrogen, partly as water and partly as carbon dioxide and marsh gas, the ratios of carbon to oxygen and hydrogen in the rendered product increase in the following manner:--

C : H C : O
Cellulose 7.2 0.9
Peat 9.8 1.8
Lignite, imperfect 12.2 2.4
" perfect 12.6 3.6

The resulting product is a brown pasty or gelatinous substance which binds the more resisting parts of the plants into a compact mass. The same observer considers Boghead coal, kerosene shale and similar substances used for the production of mineral oils to be mainly alteration products of gelatinous fresh water algae, which by a nearly complete elimination of oxygen have been changed to substances approximating in composition to C2H3 and C3H5, where C : H = 7.98 and C : O + N = 46.3. In cannel coals the prevailing constituents are the spores of cryptogamic plants, algae being rare or in many cases absent. By making very thin sections and employing high magnification (1000-1200 diameters), Renault has been enabled to detect numerous forms of bacilli in the woody parts preserved in coal, one of which, _Micrococcus carbo_, bears a strong resemblance to the living _Cladothrix_ found in trees buried in peat bogs. Clearer evidence of their occurrence has, however, been found in fragments of wood fossilized by silica or carbonate of lime which are sometimes met with in coal seams.

The subsequent change of peaty substance into coal is probably due to geological causes, i.e. chemical and physical processes similar to those that have converted ordinary sediments into rock masses. Such changes seem, however, to have been very rapidly accomplished, as pebbles of completely formed coal are commonly found in the sandstones and coarser sedimentary strata alternating with the coal seams in many coalfields.

The variation in the composition of coal seams in different parts of the same basin is a difficult matter to explain. It has been variously attributed to metamorphism, consequent upon igneous intrusion, earth movements and other kinds of geothermic action, greater or less loss of volatile constituents during the period of coaly transformation, conditioned by differences of permeability in the enclosing rocks, which is greater for sandstones than for argillaceous strata, and other causes; but none of these appears to be applicable over more than limited areas. According to L. Lemiere, who has very fully reviewed the relation of composition to origin in coal seams (_Bulletin de la Societe de l'Industrie minerale_, 4 ser. vol. iv. pp. 851 and 1299, vol. v. p. 273), differences in composition are mainly original, the denser and more anthracitic varieties representing plant substance which has been more completely macerated and deprived of its putrescible constituents before submergence, or of which the deposition had taken place in shallow water, more readily accessible to atmospheric oxidizing influences than the deeper areas where conditions favourable to the elaboration of compounds richer in hydrogen prevailed.

The conditions favourable to the production of coal seem therefore to have been--forest growth in swampy ground about the mouths of rivers, and rapid oscillation of level, the coal produced during subsidence being covered up by the sediment brought down by the river forming beds of sand or clay, which, on re-elevation, formed the soil for fresh growths, the alternation being occasionally broken by the deposit of purely marine beds. We might therefore expect to find coal wherever strata of estuarine origin are developed in great mass. This is actually the case; the Carboniferous, Cretaceous and Jurassic systems (qq.v.) contain coal-bearing strata though in unequal degrees,--the first being known as the Coal Measures proper, while the others are of small economic value in Great Britain, though more productive in workable coals on the continent of Europe. The Coal Measures which form part of the Palaeozoic or oldest of the three great geological divisions are mainly confined to the countries north of the equator. Mesozoic coals are more abundant in the southern hemisphere, while Tertiary coals seem to be tolerably uniformly distributed irrespective of latitude.

Sequences of carboniferous strata.

The nature of the Coal Measures will be best understood by considering in detail the areas within which they occur in Britain, together with the rocks with which they are most intimately associated. The commencement of the Carboniferous period is marked by a mass of limestones known as the Carboniferous or Mountain Limestone, which contains a large assemblage of marine fossils, and has a maximum thickness in S.W. England and Wales of about 2000 ft. The upper portion of this group consists of shales and sandstones, known as the Yoredale Rocks, which are highly developed in the moorland region between Lancashire and the north side of Yorkshire. These are also called the Upper Limestone Shale, a similar group being found in places below the limestone, and called the Lower Limestone Shale, or, in the north of England, the Tuedian group. Going northward the beds of limestone diminish in thickness, with a proportional increase in the intercalated sandstones and shales, until in Scotland they are entirely subordinate to a mass of coal-bearing strata, which forms the most productive members of the Scotch coalfields. The next member of the series is a mass of coarse sandstones, with some slates and a few thin coals, known as the Millstone Grit, which is about equally developed in England and in Scotland. In the southern coalfields it is usually known by the miners' name of "Farewell rock," from its marking the lower limit of possible coal working. The Coal Measures, forming the third great member of the Carboniferous series, consist of alternations of shales and sandstones, with beds of coal and nodular ironstones, which together make up a thickness of many thousands of feet--from 12,000 to 14,000 ft. when at the maximum of development. They are divisible into three parts, the Lower Coal Measures, the middle or Pennant, a mass of sandstone containing some coals, and the Upper Coal Measures, also containing workable coal. The latter member is marked by a thin limestone band near the top, containing _Spirorbis carbonarius_, a small marine univalve.

The uppermost portion of the Coal Measures consists of red sandstone so closely resembling that of the Permian group, which are next in geological sequence, that it is often difficult to decide upon the true line of demarcation between the two formations. These are not, however, always found together, the Coal Measures being often covered by strata belonging to the Trias or Upper New Red Sandstone series.

The areas containing productive coal measures are usually known as coalfields or basins, within which coal occurs in more or less regular beds, also called seams or veins, which can often be followed over a considerable length of country without change of character, although, like all stratified rocks, their continuity may be interrupted by faults or dislocations, also known as slips, hitches, heaves or troubles.

The thickness of coal seams varies in Great Britain from a mere film to 35 or 40 ft.; but in the south of France and in India masses of coal are known up to 200 ft. in thickness. These very thick seams are, however, rarely constant in character for any great distance, being found commonly to degenerate into carbonaceous shales, or to split up into thinner beds by the intercalation of shale bands or partings. One of the most striking examples of this is afforded by the thick or ten-yard seam of South Staffordshire, which is from 30 to 45 ft. thick in one connected mass in the neighbourhood of Dudley, but splits up into eight seams, which, with the intermediate shales and sandstones, are of a total thickness of 400 ft. in the northern part of the coalfield in Cannock Chase. Seams of a medium thickness of 3 to 7 ft. are usually the most regular and continuous in character. Cannel coals are generally variable in quality, being liable to change into shales or black-band ironstones within very short horizontal limits. In some instances the coal seams may be changed as a whole, as for instance in South Wales, where the coking coals of the eastern side of the basin pass through the state of dry steam coal in the centre, and become anthracite in the western side. (H. B.)

Geographical distribution of coalfields.

The most important European coalfields are in Great Britain, Belgium and Germany. In Great Britain there is the South Welsh field, extending westward from the march of Monmouthshire to Kidwelly, and northward to Merthyr Tydfil. A midland group of coalfields extends from south Lancashire to the West Riding of Yorkshire, the two greatest industrial districts in the country, southward to Warwickshire and Staffordshire, and from Nottinghamshire on the east to Flintshire on the west. In the north of England are the rich field of Northumberland and Durham, and a lesser field on the coast of Cumberland (Whitehaven, &c.). Smaller isolated fields are those of the Forest of Dean (Gloucestershire) and the field on either side of the Avon above Bristol. Coal has also been found in Kent, in the neighbourhood of Dover. In Scotland coal is worked at various points (principally in the west) in the Clyde-Forth lowlands. In Belgium the chief coal-basins are those of Hainaut and Liege. Coal has also been found in an extension northward from this field towards Antwerp, while westward the same field extends into north-eastern France. Coal is widely distributed in Germany. The principal field is that of the lower Rhine and Westphalia, which centres in the industrial region of the basin of the Ruhr, a right-bank tributary of the Rhine. In the other chief industrial region of Germany, in Saxony, Zwickau and Lugau, are important mining centres. In German Silesia there is a third rich field, which extends into Austria (Austrian Silesia and Galicia), for which country it forms the chief home source of supply (apart from lignite). Part of the same field also lies within Russian territory (Poland) near the point where the frontiers of the three powers meet. Both in Germany and in Austria-Hungary the production of lignite is large--in the first-named especially in the districts about Halle and Cologne; in the second in north-western Bohemia, Styria and Carniola. In France the principal coalfield is that in the north-east, already mentioned; another of importance is the central (Le Creusot, &c.) and a third, the southern, about the lower course of the Rhone. Coal is pretty widely distributed in Spain, and occurs in several districts in the Balkan peninsula. In Russia, besides the Polish field, there is an important one south of Moscow, and another in the lower valley of the Donetz, north of the Sea of Azov. The European region poorest in coal (proportionately to area) is Scandinavia, where there is only one field of economic value--a small one in the extreme south of Sweden.

In Asia the Chinese coalfields are of peculiar interest. They are widely distributed throughout China Proper, but those of the province of Shansi appear to be the richest. Proportionately to their vast extent they have been little worked. In a modified degree the same is true of the Indian fields; large supplies are unworked, but in several districts, especially about Raniganj and elsewhere in Bengal, workings are fully developed. Similarly in Siberia and Japan there are extensive supplies unworked or only partially exploited. Those in the neighbourhood of Semipalatinsk may be instanced in the first case and those in the island of Yezo in the second. In Japan, however, several smaller fields (e.g. in the island of Kiushiu) are more fully developed. Coal is worked to some extent in Sumatra, British North Borneo, and the Philippine Islands.

In the United States of America the Appalachian mountain system, from Pennsylvania southward, roughly marks the line of the chief coal-producing region. This group of fields is followed in importance by the "Eastern Interior" group in Indiana, Illinois and Kentucky, and the "Western Interior" group in Iowa, Missouri and Kansas. In Arkansas, Oklahoma and Texas, and along the line of the Rocky Mountains, extensive fields occur, producing lignite and bituminous coal. The last-named fields are continued northward in Canada (Crow's Nest Pass field, Vancouver Island, &c.). There is also a group of coalfields on the Atlantic seaboard of the Dominion, principally in Nova Scotia. Coal is known at several points in Alaska, and there are rich but little worked deposits in Mexico.

In the southern countries coal-production is insignificant compared with that in the northern hemisphere. In South America coal is known in Venezuela, Colombia, Peru, northern Chile, Brazil (chiefly in the south), and Argentina (Parana, the extreme south of Patagonia, and Tierra del Fuego), but in no country are the workings extensive. Africa is apparently the continent poorest in coal, though valuable workings have been developed at various points in British South Africa, _e.g._ at Kronstad, &c., in Cape Colony, at Vereeniging, Boksburg and elsewhere in the Transvaal, in Natal and in Swaziland. Australia possesses fields of great value, principally in the south-east (New South Wales and Victoria), and in New Zealand considerable quantities of coal and lignite are raised, chiefly in South Island.

The following table, based on figures given in the _Journal of the Iron and Steel Institute_, vol. 72, will give an idea of the coal production of the world:--

Table IV.

Europe:-- Tons.
United Kingdom 1905 236,128,936
Germany, coal " 121,298,167
" lignite " 52,498,507
France " 35,869,497
Belgium " 21,775,280
Austria, coal " 12,585,263
" lignite " 22,692,076
Hungary, coal 1904 1,031,501
" lignite " 5,447,283
Spain 1905 3,202,911
Russia 1904 19,318,000
Holland " 466,997
Bosnia, lignite 1905 540,237
Rumania " 1903 110,000
Servia 1904 183,204
Italy, coal and lignite 1905 412,916
Sweden " 322,384
Greece, lignite 1904 466,997
Asia:--
India 1905 8,417,739
Japan 1903 10,088,845
Sumatra 1904 207,280
Africa:--
Transvaal 1904 2,409,033
Natal 1905 1,129,407
Cape Colony 1904 154,272
America:--
United States 1905 350,821,000
Canada 1904 7,509,860
Mexico " 700,000
Peru 1905 72,665
Australasia:--
New South Wales 1905 6,632,138
Queensland " 529,326
Victoria " 153,135
Western Australia " 127,364
Tasmania " 51,993
New Zealand " 1,585,756

Coal resources of Great Britain.

The questions, what is the total amount of available coal in the coalfields of Great Britain and Ireland, and how long it may be expected to last, have frequently been discussed since the early part of the 19th century, and particular attention was directed to them after the publication of Stanley Jevons's book on _The Coal Question_ in 1865. In 1866 a royal commission was appointed to inquire into the subject, and in its report, issued in 1871, estimated that the coal resources of the country, in seams of 1 ft. thick and upwards situated within 4000 ft. of the surface, amounted to 90,207,285,398 tons. A second commission, which was appointed in 1901 and issued its final report in 1905, taking 4000 ft. as the limit of practicable depth in working and 1 ft. as the minimum workable thickness, and after making all necessary deductions, estimated the available quantity of coal in the proved coalfields of the United Kingdom as 100,914,668,167 tons. Although in the years 1870-1903 the amount raised was 5,694,928,507 tons, this later estimate was higher by 10,707,382,769 tons than that of the previous commission, the excess being accounted for partly by the difference in the areas regarded as productive by the two commissions, and partly by new discoveries and more accurate knowledge of the coal seams. In addition it was estimated that in the proved coalfields at depths greater than 4000 ft. there were 5,239,433,980 tons, and that in concealed and unproved fields, at depths less than 4000 ft. there were 39,483,844,000 tons, together with 854,608,307 tons in that part of the Cumberland coalfield beyond 5 m. and within 12 m. of high-water mark, and 383,024,000 tons in the South Wales coalfield under the sea in St Bride's Bay and part of Carmarthen Bay.

In Table V. below column I. shows the quantity of coal still remaining unworked in the different coalfields at depths not exceeding 4000 ft. and in seams not less than 1 ft. thick, as estimated by seven district commissioners; column II. the total estimated reductions on account of loss in working due to faults and other natural causes in seams and of coal required to be left for barriers, support of surface buildings, &c.; and column III. the estimated net available amount remaining unworked.

Table V.

+--------------------------+----------------+---------------+----------------+
| Coalfield. | I. | II. | III. |
+--------------------------+----------------+---------------+----------------+
| District A. | | | |
| South Wales | | | |
| and Monmouthshire | 33,443,000,339 | 6,972,003,760 | 26,470,996,579 |
| Somersetshire and part | | | |
| of Gloucestershire | No details | No details | 4,198,301,099 |
| Forest of Dean | 305,928,137 | 47,394,690 | 258,533,447 |
| District B. | | | |
| North Stafford | 5,267,833,074 | 89,782,727 | 4,368,050,347 |
| South Stafford | 1,953,627,435 | 538,179,363 | 1,415,448,072 |
| Warwickshire | 1,448,804,556 | 321,822,653 | 1,126,981,903 |
| Leicestershire | 2,467,583,205 | 642,124,654 | 1,825,458,551 |
| Shropshire | 369,174,620 | 48,180,921 | 320,993,699 |
| District C. | | | |
| Lancashire | 5,349,554,437 | 1,111,046,710 | 4,238,507,727 |
| Cheshire | 358,998,172 | 87,165,901 | 291,832,271 |
| North Wales | 2,513,026,200 | 776,558,371 | 1,736,467,829 |
| District D. | | | |
| Yorkshire | No details | No details | 19,138,006,395 |
| Derby and Notts | No details | No details | 7,360,725,100 |
| District E. | | | |
| Northumberland | 7,040,348,127 | 1,530,722,486 | 5,509,625,641 |
| Cumberland | 2,188,938,830 | 661,230,025 | 1,527,708,805 |
| Durham | 6,607,700,522 | 1,336,584,176 | 5,271,116,346 |
| District F. | | | |
| Scotland | 21,259,767,661 | 5,579,311,305 | 15,681,456,356 |
| District G. | | | |
| Ireland | No details | No details | 174,458,000 |
+--------------------------+----------------+---------------+----------------+

As regards the duration of British coal resources, the commissioners reported (1905):--

"This question turns chiefly upon the maintenance or the variation of
the annual output. The calculations of the last Coal Commission as to
the future exports and of Mr Jevons as to the future annual
consumption make us hesitate to prophesy how long our coal resources
are likely to last. The present annual output is in round numbers 230
million tons, and the calculated available resources in the proved
coalfields are in round numbers 100,000 million tons, exclusive of the
40,000 million tons in the unproved coalfields, which we have thought
best to regard only as probable or speculative. For the last thirty
years the average increase in the output has been 2-1/2% per annum, and
that in the exports (including bunkers) 4-1/2% per annum. It is the
general opinion of the District Commissioners that owing to physical
considerations it is highly probable that the present rate of increase
of the output of coal can long continue--indeed, they think that some
districts have already attained their maximum output, but that on the
other hand the developments in the newer coalfields will possibly
increase the total output for some years.

In view of this opinion and of the exhaustion of the shallower
collieries we look forward to a time, not far distant, when the rate
of increase of output will be slower, to be followed by a period of
stationary output, and then a gradual decline."

According to a calculation made by P. Frech in 1900, on the basis of the then rate of production, the coalfields of central France, central Bohemia, the kingdom of Saxony, the Prussian province of Saxony and the north of England, would be exhausted in 100 to 200 years, the other British coalfields, the Waldenburg-Schatzlar and that of the north of France in 250 years, those of Saarbrucken, Belgium, Aachen and Westphalia in 600 to 800 years, and those of Upper Silesia in more than 1000 years. (O. J. R. H.; H. M. R.)

_Coal-Mining._

Preliminary trial of coalworkings.

The opening and laying out, or, as it is generally called, "winning," of new collieries is rarely undertaken without a preliminary examination of the character of the strata by means of borings, either for the purpose of determining the number and nature of the coal seams in new ground, or the position of the particular seam or seams which it is proposed to work in extensions of known coalfields.

The principle of proving a mineral field by boring is illustrated by fig. 1, which represents a line direct from the dip to the rise of the field, the inclination of the strata being one in eight. No. 1 bore is commenced at the dip, and reaches a seam of coal A, at 40 fathoms; at this depth it is considered proper to remove nearer to the outcrop so that lower strata may be bored into at a less depth, and a second bore is commenced. To find the position of No. 2, so as to form a continuous section, it is necessary to reckon the inclination of the strata, which is 1 in 8; and as bore No. 1 was 40 fathoms in depth, we multiply the depth by the rate of inclination, 40 X 8 = 320 fathoms, which gives the point at which the coal seam A should reach the surface. But there is generally a certain depth of alluvial cover which requires to be deducted, and which we call 3 fathoms, then (40 - 3 = 37) X 8 = 296 fathoms; or say 286 fathoms is the distance that the second bore should be placed to the rise of the first, so as to have, for certain, the seam of coal A in clear connexion with the seam of coal B. In bore No. 3, where the seam B, according to the same system of arrangement, should have been found at or near the surface, another seam C is proved at a considerable depth, differing in character and thickness from either of the preceding. This derangement being carefully noted, another bore to the outcrop on the same principle is put down for the purpose of proving the seam C; the nature of the strata at first is found to agree with the latter part of that bored through in No. 3, but immediately on crossing the dislocation seen in the figure it is changed and the deeper seam D is found.

The evidence therefore of these bores (3 and 4) indicates some material derangement, which is then proved by other bores, either towards the dip or the outcrop, according to the judgment of the borer, so as to ascertain the best position for sinking pits. (For the methods of boring see BORING.)

Methods of working.

The working of coal may be conducted either by means of levels or galleries driven from the outcrop in a valley, or by shafts or pits sunk from the surface. In the early days of coal-mining, open working, or quarrying from the outcrop of the seams, was practised to a considerable extent; but there are now few if any places in England where this can be done. In 1873 there could be seen, in the thick coal seams of Bengal, near Raniganj, a seam about 50 ft. thick laid bare, over an area of several acres, by stripping off a superficial covering varying from 10 to 30 ft., in order to remove the whole of the coal without loss by pillars. Such a case, however, is quite exceptional. The operations by which the coal is reached and laid out for removal are known as "winning," the actual working or extraction of the coal being termed "getting." In fig. 2 A B is a cross cut level, by which the seams of coal 1 and 2 are won, and C D a vertical shaft by which the seams 1, 2 and 3 are won. When the field is won by the former method, the coal lying above the level is said to be "level-free." The mode of winning by level is of less general application than that by shafts, as the capacity for production is less, owing to the smaller size of roadways by which the coal must be brought to the surface, levels of large section being expensive and difficult to keep open when the mine has been for some time at work. Shafts, on the other hand, may be made of almost any capacity, owing to the high speed in drawing which is attainable with proper mechanism, and allow of the use of more perfect arrangements at the surface than can usually be adopted at the mouth of a level on a hill-side. A more cogent reason, however, is to be found in the fact that the principal coalfields are in flat countries, where the coal can only be reached by vertical sinking.

Sinking of shafts.

Tubbing.