retorts, from three to seven of which, according to circumstances, are heated with one fire of coke to a cherry-red (1,478° to 1,830° Fahr.) for iron, or to an orange or white heat (2,000° to 2,300° Fahr.) for clay retorts; 160 to 260 pounds of coal constitute the charge for a single retort, and the distillation continues uninterruptedly for four to four and a half hours. The outer layers of the charge, being suddenly raised to a high temperature, evolve vapors which contain a large amount of carbon. These, in passing through the retort, are converted into fixed gases of a high illuminating power. The inner parts of the charge, undergoing distillation more slowly, give out vapors which, in passing through the highly-heated coke on the surface, are more completely decomposed than the first evolved, and are, therefore, of a lower illuminating power. It has been shown, for example, by Mr. C. D. Lamson, of the Boston Gas-Works, that the illuminating (or candle) power of the gas diminishes in a rapidly-increasing ratio with each half-hour of the distillation; and also that, after the third half hour, the quantity of gas produced similarly decreases. The largest quantity, as well as the richest gas, is, therefore, obtained in the first part of the distillation. By candle-power is meant that the gas, burning at the rate of five cubic feet per hour, will give as much light as the stated number of standard sperm-candles burning at the rate of 120 grains per hour, or two grains per minute.
The gas passes next into the hydraulic main, where it is made to bubble up through a half-inch to an inch of water, and thus some of its vapors are condensed. It then goes to the condenser, a series of iron tubes surrounded by water, to be cooled and more completely rid of its tar and other vapors, which are precipitated and led away. Going to the washers, a series of chambers where it is brought in contact with jets of water, and to the scrubbers, where it passes through a collection of coke, fire-brick, etc., moistened with water, it is relieved of the rest of the tar and also of the ammonia.
The third step in the manufacture is purification, which removes from the gas the noxious elements, chiefly carbonic acid and sulphuretted hydrogen. The first lowers the illuminating power very greatly: one per cent, being sufficient, it is said, to diminish it five per cent. The sulphuretted hydrogen and other sulphurous compounds give rise in burning to sulphurous and sulphuric acids which may injure, by their corrosive action, delicate structures, such as books, gilding, silks, etc., that are exposed to the air of the room in which the gas is burned. Lime and oxide of iron are used in various methods to purify the gas. Lime is used both wet and dry. In the wet-lime process the gas is passed through the milk of lime, which, uniting with the carbonic acid to form a chalk, effectually removes it, and takes, away most of the sulphur compounds too, by uniting with them to form calcic sulphide or calcic sulpho-carbonate.
The use of this process has generally been abandoned, however,