Page:The New International Encyclopædia 1st ed. v. 05.djvu/86

COAGULATION.  at a temperature of 160°. Milk is coagulated or curdled by the action of rennet or by acids. The fibrin in the blood, chyle, and lymph of animals is coagulated after the separation of these fluids from the living body.  COAHUILA, (named from the Mexican tribe Cuahuiltecs). A northern State of Mexico, separated from Texas on the north and east by the Rio Grande, and covering an area of 63,570 square miles (Map:, H 4). With the exception of the eastern part, which is somewhat mountainous, the surface forms an elevated plateau, with a general incline toward the Rio Grande. The western part is taken up by the Bolson de Mapimi, a semi-desert region, only partially explored, with many lagoons and vast mineral resources. The climate is moderate and healthful. The chief occupation is cattle-raising, although the soil is well adapted for the growing of cereals and European vegetables, to which more and more attention is being paid. In the southwest some vines and cotton are cultivated. The State is traversed from north to south by the Mexican International Railway. Population, in 1900, 280,899; capital, (q.v.).  COAITA,. See.  COAL (AS. col, OHG. kolo, Ger. Kohle; ultimately connected with Skt. jval, to blaze, and probably with Ir., Gael. gual, coal). A mineral fuel of solid character, found and used in many countries. The name is a word common to all the languages of the Gothic stock, and seems allied to the Latin calere, to be hot; as also ‘to glow,’ and ‘kiln.’ The word ‘coal’ has often prefixed to it some qualifying word, to distinguish different kinds of coal; such as cannel coal, stone coal, pea coal, etc.

. Coal is one of the most important economic minerals, and is of vegetable origin. When vegetable matter accumulates under water it undergoes a slow process of decomposition, gradually giving off its nitrogen, hydrogen, oxygen, and some carbon, the result of which if carried far enough is the formation of a mass of carbon. (q.v.), the material so often found underlying swampy tracts in north temperate zones, represents the first stage in the coal-forming process, and the further stages are obtained by the burial of these vegetable deposits under great loads of sediment, where they become subjected to pressure, and at times to heat also. This effects a series of changes, especially consolidation and loss of oxygen, and gives a series of products, whose nature depends on the degree to which the original vegetable matter has been changed. The products are known as lignite, bituminous coal, and anthracite coal; these three

types being connected by all degrees of intermediate stages. In Carboniferous times certain regions were covered by rank and luxuriant vegetation which grew upon swampy land slightly raised above the level of the sea. As the plants died, their remains fell into the water of the swamp, and slowly formed an accumulation of vegetable matter of increasing thickness. By slow subsidence this thick layer of vegetable matter sank below the water, and became gradually covered by sand, mud, or other mineral sediments, washed out from the shore. Successive elevations and depressions, with intervening accumulations, may thus have yielded successive beds. Subsequent elevation, folding of the earth's crust, and accompanying metamorphism, followed by erosion of the surface, has exposed to view the edges of the once deeply buried beds of coal.

. The following analyses of peat, lignite or brown coal, and true coal indicate the changes which vegetable matter undergoes by decay and pressure:

These analyses bring out well the general relations of the different elements, and the increase in carbon toward the anthracite end of the series; still they give but little information concerning the commercial value of the coal. The usual custom in making a commercial analysis is to determine the form in which these elements occur—that is, the amount of water, volatile hydrocarbon, fixed carbon, sulphur, and ash. This proximate analysis is also used as the basis of classification of coals. Thus:

A proximate analysis like the above is of practical value, since it gives us a better conception of the coal worth. Thus the freedom of burning increases with the amount of volatile hydrocarbons, while the heating power depends on the amount of fixed carbon present. Sulphur is an injurious constituent when the coal is to be used in the manufacture of gas or for metallurgical purposes; while ash is undesirable, since it displaces so much carbon, and if it contains fusible impurities such as iron, lime, or alkalies, it causes clinkering. Moisture retards the heating power of the coal until it is driven off. Since