558 GEOLOGY OF OHIO. 
With a difference in their density and purity, cokes also differ in their 
combustibility, and while for metallurgical purposes a dense coke is 
preferable, this hardness increases to an undesirable extent its in- 
combustibility, and the amount of fuel required, as is observed by sev- 
eral European authorities. 
Coke made in the manufacture of gas is usually, from its friability 
and impurities, unfitted for metallurgical purposes, and to be suitable 
for such use, coke should be sufficiently hard to resist the pressure 
of the material, ore, etc., and as free as possible from impurities, which 
reduce the heating power and contaminate the material with which it 
may be in contact. 
Composition of coke. Cokes are composed of carbon and ash, be- 
sides retaining a small proportion of volatile matter, water, hydrogen, 
nitrogen, etc. Usually it will not retain more than 1 per cent. of water 
when exposed to a damp atmosphere, or even when it has been quenched 
by water, when freshly made. When perfectly dry, and immersed in 
water for 24 hours, it may absorb as high as 51 per cent. of its weight 
of water, which is the greatest quantity absorbed by 12 trials made by 
Dr. Percy. This, however, was rapidly evaporated on exposure to 
the air. When well made, the total proportion of volatile matter in 
coke rarely exceeds 2 or 23 per cent. 
The ash in cokes varies very greatly, and depends on that of the 
coal employed, and will be from 25 to 40 per cent. greater than the 
coal from which it was made. Though many cokes used contain 14 
per cent. or more of ash, a really good coke should not have more than 
8 per cent., and when the coal will produce a coke containing a greater 
proportion, it should be submitted to a preliminary operation of 
washing. In France, as already observed, the coals are very impure, 
containing 20 per cent. of ash, and good cokes with 8 to 10 per cent. of 
ash are only obtained by the most careful washing of the coal. The 
same remarks apply to the Belgian cokes, though the coals are some- 
what purer than the French. The English cokes are usually quite 
pure, and the composition of the typical coke, the Durham, is given in 
the following table of analyses. In the United States, though there are 
many bituminous coals, especially in the Allegheny basin, which will 
produce very pure cokes, many of our coals, particularly those of the 
Mississippi basins, will require washing before they will produce a good 
metallurgical coke. Already the importance of purity in the coke used 
