e substance being sufficiently dense
to resist oxidation by carbon dioxide in the higher regions of the
furnace, while the vesicular structure gives an extended surface for the
action of heated air and facilitates rapid consumption at the tuyeres.
Compact coke, such as that formed on the inner sides of gas retorts
(retort carbon), can only be burned with great difficulty in small
furnaces of special construction, but it gives out a great amount of
heat.

The most deleterious constituents of coke are ash, sulphur and volatile
constituents including water. As the coke yield is only from two-thirds
to three-quarters of that of the coal, the original proportion of ash is
augmented by one-third or one-half in the product. For this reason it is
now customary to crush and wash the coal carefully to remove
intermingled patches of shale and dirt before coking, so that the ash
may not if possible exceed 10% in the coke. About one-half of the
sulphur in the coal is eliminated in coking, so that the percentage in
the coke is about the same. It should not be much above 1%. According to
the researches of F. Wuest (_Journ. Iron and Steel Inst._, 1906) the
sulphur is retained in a complex carbon compound which is not destroyed
until the coke is actually consumed.

The older methods of coking and the earlier forms of retort ovens are
described in J. Percy, _Metallurgy_, Jordan, _Album du cours de
metallurgie_; Phillips and Bauerman, _Handbook of Metallurgy_, and
other text-books. A systematic series of articles on the newer forms
will be found in _The Engineer_, vol. 82, pp. 205-303 and vol. 83, pp.
207-231; see also Durre, _Die neuern Koksofen_ (Leipzig, 1892); D. A.
Louis, "Von Bauer and Brunck Ovens," _Journ. Iron and Steel Inst._,
1904, ii. p. 293; C. L. Bell, "Hussener Oven," _id._, 1904, i. p. 188;
Hurez, "A Comparison of Different Systems of Vertical and Horizontal
Flue Ovens," _Bull. soc. industrie minerale_, 1903, p. 777. A
well-illustrated description of the Otto system in its American
modification was issued by the United Gas & Coke Company of New York,
in 1906. (H. B.)

COL (Fr. for "neck," Lat. _collum_), in physical geography, generally
any marked depression upon a high and rugged water-parting over which
passage is easy from one valley to another. Such is the Col de Balme
between the Trient and Chamounix valleys, where the great inaccessible
wall crowned with aiguilles running to the