COKE. 127 COKE. advantages of the other two. Its hardness en- abh's it to sustain the weight of furnace charges, and its porosity helps to make it readil}' com- bustible. Chai'coal is too soft, while anthracite coal, which is in reality a natural coke, is hard enough, but is .so dense in structure that its combustion is slow and its calorific energy mod- erate. Tlic superiority of coke is due to its cel- lular structure. The walls are hard and vitre- ous, and the tiny passages between all'ord free course for the oxidizing gases of the blast-fur- nace. Besides its use in metallurgy, coke is an important fuel in other industrial operations, and it is being adopted for locomotives on some i"ailroads. In 1899 the Boston and Maine Railroad began using coke as a fuel. Its ad- vantages are that it is smokeless and does not produce sparks, thus decreasing the number of fires caused by the locomotives. Coke is also emjiloyed as <a filtering material in water and sewage purification, being used extensively for the latter purpose, particularly in English prac- tice. Not all bituminous coals can be made into coke, and much experiment and research on the part of chemists have been devoted to ascertain- ing the necessary constituents of a good coking coal. It appears that the coking property of coal is independent of the constituents, such as moisture, fixed carbon, ash, and sulphur, which it contains, and depends wholly upon the rela- tions and volumes of the elements composing the votatile comhustible matters of the coal. Just what these relations and volumes are has not been definitelj' shown. In Continental Europe, where the manufacture of coke has been very highly developed, only a poor quality of coal is available, and much ingenuity has been expend- ed in constructing furnaces suitable for this in- ferior grade of material. In America there has been as yet an abundant supply of coal of the very best quality for coking purposes, the supply having been drawn chiefly from the Connellsville coal of Pennsylvania and the Pocahontas coal of Virginia. But with the increasing demand for coke for metallurgical and other purposes, the poorer grades of coking coal must be drawn upon. The preliminary preparation of coal for the manufacture of coke is chiefl.y a cleansing and separating process. Some of the best coking coal requires no special treatment, but is charged into the coke - ovens direct from the mines. It is usually found advantageous, how- ever, to break up the coal into small pieces, in order that the volatile matter may be utilized to the utmost extent. When the coal is not uni- form in size, it is found that it cokes unevenly, the finer portion fusing much more rajjidly. yhen there is much slate in the coal, it is necessary to wash it ; the slate separating itself from the coal in the process of washing, on ac- count of its greater weight. Fire-clay also will be separated and washed out. Before washing, however, it is necessarj' carefully to sort the coal according to size. Various machines of great efticiency have been devised for crushing, sorting, and washing coal ready for the ovens, which are described in great detail, as are also the different forms of coke-ovens, by John Ful- ton in his Treatise on the Manufacture of Coke (Scranton, Pa., 1895). Three general methods have been followed in the process of transform- ing coal into coke. The first was the primitive and wasteful process, borrowed from the cliar- coal-bnrncrs, of open-air Imrning. The coal is simply piled in a rectangular heap on the ground, with longitudinal and vertical flues run- ning through it, in which enough wood is placed to ignite the whole mass of coal. The fire is lighted at the base of the vertical flues, and gradually extends tlirough the mass. When the mass ceases to flame it indicates that the gase- ous matter has been expelled, and the fire is partially smothered by covering the heap with fine dust. Last of all, the mass is sprinkled with a hose, the water being at once converted into steam, which permeates the whole mass. This process of coke-making requires from five to eiglit days. The second method of making coke, and one ■which is still largely employed, is in the bee- hire oven, many improvements in its construc- tion having been made from time to time. As late as 1893, all of the 44,201 coke-ovens in the United States were constnicted on the beehive plan. The chief advantage claimed for it is that it i)roduces from prime coking coals the best quality of metallurgical fuel. A minor ad- vantage is that water is applied to cool it, while the coke is still in the oven, after which the oven heat reduces the amount of moisture in the coke. The great disailvantage of the beehive oven is that, as ordinarily constructed, the valuable b.y-products — ammonia and tar — con- tained in the volatile matter are entirely lost. This oven is not adapted to inferior grades of coal. It derives its name from the dome shape of its interior. It is usually built of stone masonry, on a firm foundation, with its interior lined with specially designed fire-brick. The bee- hive oven is usually about 12 feet in diameter and C to 7 feet higli in the centre. The coal is charged through a hole in the centre of the roof, and is leveled off" to an even depth of about 23 inches. The fresh charge is fired by the heat remaining in the walls from the previous charge, and the combtistion is supported by air admitted through the front door, over the top of the charge. The volatile matter in the coal is driven otl by the heat and btirned in the top of the oven., along with a portion of the fixed carbon. The source of heat being at the top, the coking proc- ess proceeds downward, and is efl'ected by the partial combustion of the charge itself. In Eng- land, and in a few American jjlants, the volatile matter is gathered into a conduit and carried under boilers, to raise steam for pumping water and other purposes; but usually the gases escape directly into the open air and are wasted. As early as 1700 attempts were made in Gei^ many to save the by-products from coke-ovens. It is now accomplished in the retort oren, which was devised in Europe for the double purpose of saving the by-products and for utilizing for coke- making inferior grades of coal. Very slow progress was made in developing the. process, and it was not until 1883 that it was put upon a paying basis. This was due to the unsatis- factory design of the early coke-ovens and to the low ])rice of the by-products, on account of the supply from gas-works. In 1850 Knab of France built a group of retort coke-ovens which had for their object, in addition to the making of coke, the double purpose of separating the tar and ammonia from the gases generated, and of
