GAS MANUFACTURE AND GAS COMPANIES. 479 



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-poicer is meant that the gas, burn- 

 ing 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 sul- 

 phuretted 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 meth- 

 ods 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, 



