By distilling bituminous coal in retorts to obtain gas for illumination, or by burning it in kilns or pits, the residue left behind is called coke, which is simply coal charcoal, and is nearly pure carbon. In the distillation of coal, about one-third of its weight is driven off in the form of vapor and gas, yielding carbureted hydrogen, naptha, ammonia, and Bome other products. These originally formed the bitumen of the coal, which does not exist in anthracite; this latter being a natural coke produced, it is believed, from soft or bituminOblS coal, by subterranean distillation under great pressure. Quite a spirited debate has lately taken place before the " Society of Arts," in London, on the comparative heating properties of coke and coal. This was elicited by the reading of a paper on the subject by Apsley Pellatt, a glass m:mufacturer, and it extended over seyeral evenings. The author of the paper took the ground that as much calorific effect was produced by the combustion of coke made from a certain quantity of bituminous coal, a& from the raw coal itself. Thus, for example if we suppose, that a ton of coal generates a certain amount of steam when burning under a boiler, two-thirds of a tun of coke will effect equal results. Mr. Pellatt stated, that this was an anomaly, but produced some author5ties in support of his views, and gave as a scientific reason for the cause of this phenomenon, that, in order to convert a solid into a vapor or gas, even if it were by the combustion of that solid itself, a great quantity of heat was absorbed to cause expansion, and was carried off without producing any calorific effect upon surrounding objects. At one of the 'meetings, Charles Wye Williams, who has written an able treatise on . combustion, presented himself in opposition, and stated that there would certainly be an anomaly in the proportional calorific effects of coke and coal, if it were as stated by Mr. Pellatt, but the fact was otherwise. It indeed tequires a great amount of heat to convert the bitumen of soft coal into gas, and this is all lost in badly constructed furnaces, but when there are arrangements made to burn the gas, the coal will produce, weight for weight, with coke, about as great calorific effects. The fact, however, was rendered very clear by the dis-cussion, that abont one-third of the heating effects of bituminous coal is lost in fnrnaces where there is imperfect combustion. In all those parts of our country where bituminous coal is the common fuel, those who use it should take cognizance of this. In practice it has been found, that, for a small concentrated fire, to melt metals or glass in crucibles, coke is a better fuel to use than bitumen coal, but is not superior to anthracite. A mixture of' coke and coal makes the bost fire under short boilers and in pottery kilns, but not under long boilers.
This article was originally published with the title "Coke and Coal" in Scientific American 13, 18, 142 (January 1858)