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 C O A H U I L A —C O A L employ about 7000 bands. There are various other works, including the building and repairing yards of the Clyde Navigation Trust. Municipal buildings, including baths, are being erected at a cost of £40,000. Population (1891), 9998; (1901), 18,654. CoahuiSa, a state of Mexico, bounded on the N. by the United States, on the E. by the state of Nuevo Leon, on the S. by those of San Luis Potosf and Zacatecas, on the W. and S.W. by Durango, and on the N.W. and W. by Sonora, has an area of 62,376 square miles. In 1879 the population was 130,026, and in 1895 it was 241,026. The flora comprises over sixty varieties of trees of the cold and temperate zones, and fifty belonging to the hot lands. Agriculture is the principal industry, cotton, maize, wheat, beans, sugar-cane, linseed, and about thirty species of leguminous plants being the chief products. Cattle-raising is also extensively followed. The mines are being rapidly developed, especially in the Sierra Mojada, Sierra del Carmen, and in the valley of Santa Rosa. The state, which is divided into five districts and thirty-three municipalities, is one of the most prosperous commercial regions of the Republic, due principally to its excellent railway system. The capital, Saltillo (population, 26,801), is 615 miles from Mexico City by rail. It has good public buildings, a State college, public library, •&c., and is noted for its manufacture of shawls (sarapes), cotton cloth, knit goods, and flour. Amongst other towns are Parras (8326), Monclova, Ciudad Porfirio Dfaz, Yiesca, Matamoros. Coal.—During the period that has elapsed since the publication of the article contained in the ninth edition of this Encyclopaedia, the development in the methods of winning and working coal has been very considerable, especially in the direction of increasing the output from individual centres of production concurrently with a general diminution in the length of the working day. This has been attended, at any rate in the older coal-fields, with a rapid increase in the depth of workings and greatly increased cost and difficulty in opening new mines, with the result that the comparatively rough methods and appliances of earlier times have given way to more economical methods of working underground, and machinery of more refined construction. The main winding engines, especially, are now constructed upon the most improved types, in order to save fuel in working. Similar improvements have been introduced in the methods of underground haulage, and horses on underground lines have been largely replaced by mechanical traction, the substitution of electric for steam driven motors, and the use of electric locomotives being specially noticeable. In the main operation of getting or removing the coal, machine-cutting has to some extent taken the place of hand labour, although the progress in this direction has been more marked in America than in Europe, and even there the proportion of the output obtained with the use of ■such mechanical aids is only small when compared with that due to hand labour. This, however, is rapidly changing, owing to the increased flexibility in working of electric motors, which will in the near future probably take the first place in this as in other branches of coal-mining. To enter, however, into the details of these and other changes would involve discussion of mechanical and other technical matters beyond the scope of the present work, which is to be regarded as supplementary to the article in the ninth edition, the additional matter being noticed somewhat in the same order as that previously adopted. During the past few years the question of the origin and mode of formation of coal has received considerable attention from geologists, but it cannot be said that any

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authoritative new solution of the problem has been propounded. Speaking generally, the tendency is towards a modification of the view of which Logan, De Qrjgjn and la Beche, Dawson, and Newberry may be taken composias the principal exponents, that coal seams are of essentially the remains of forests upon the sites coaL of their original growth, a detrital origin being supposed for a part, if not the whole, of the carbonaceous material, which may have been derived from adjacent higherlying land by the action of river currents. This view, known as the delta hypothesis, has found considerable favour in France, especially from study of the coal-field of Saint Etienne, where the seams vary very irregularly in thickness and character, in a way which seems to be incompatible with the hypothesis of a tranquil accumulation in situ. As regards the changes involved in the actual transformation of plant structures into lignite and coal, one of the most important series of researches is due to M. B. Renault {Bulletin de la Societe de VIndustrie Mindrale, 3 Ser., vol. xiii. p. 865), who, starting from the study of peat, finds that the chief agents in the transformation of cellulose into peaty substances are saprophytic fungi and bacterial ferments. As the former ar§ only active in the presence of air, while the latter are anaerobic, the greater or less activity of either agent is conditioned by variation in the water-level of the bog. The destructive agency of bacteria seems to be limited by the production of ulmic acid of the composition, carbon 65‘31, hydrogen 3'85 per cent., 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 ratio of carbon to oxygen and hydrogen in the residual product increases in the following manner :— C : H. C : O. 7-2 0-9 Cellulose 9-8 T8 Peat 12-2 2-4 Lignite, imperfect 12-6 3-6 ,, perfect The constituents of lignite are generally similar to those of peat, with the addition of some animal (infusorial) remains, the degraded vegetable tissues forming a paste originally plastic, which has converted the more resisting parts of the plants into a compact mass. From the figures given above it will be seen that oxygen is less rapidly eliminated than hydrogen, the change to lignite being similar to that obtaining in the case of peat, but further advanced. Bituminous shale and Boghead or Torbane Hill coal are considered by Renault to be mainly alterationproducts of masses of gelatinous fresh-water algae, which by an almost complete elimination of oxygen have been transformed into substances approximating to the formulae C2H3 and C3H5, where C : H = 7 ‘98 and C : O + N = 46 ‘3. In cannel coals the prevailing vegetable constituents are spores of cryptogamic plants, algae being rare and in many cases absent. The detection of bacilli in coal is a difficult matter, owing to its opacity; but by making very thin sections and employing high magnification, 1000 to 1200 diameters, Renault has been enabled to detect numerous forms in the woody parts included in coal. One of these, named Micrococcus carbo, in many respects resembles the living Cladothryx found in the wood of trees buried in peat-bogs in process of formation. Clearer evidence has been obtained from wood partially mineralized by silica or carbonate of lime included in the coal. The transformation of woody fibre into coal is attended with considerable contraction, which may be from to y5of the original volume, but this is unequally distributed; it is mainly in the direction of the thickness, so that minute objects seen on the flat may keep nearly their