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    "source_title": "Encyclopaedia Britannica (1911)",
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    "chunk_id": "1911:coahuila:8355965376e1",
    "title": "COAHUILA",
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    "verified_text": "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 41⁄2 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° 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",
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