[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.