The operations of a gas factory are in terminable from the beginning to the end of the year. No cessation, even for a moment, occurs in the laborS. One party of men are engaged at night ; an other party relieve them after an interval of twelve hours, and are employed by day ; but the furnaces are always heated, the retorts always supplied with their fiercely burning contents, the gas always undergoing the purifying processes pre vious to its passage into the gasometers. The number of retorts worked varies at different seasons of the year, according to the length of time between sunset and sunrise ; for the gas-manufacturer is 're gulated, perhaps than most other manufacturers, by the movements of the sun. But whether the number actually worked at any one time be' greater or smaller, the system pursued is nearly the same. At the works we have noticed, the retorts are so divided into groups that some of them shall be ready for "drastr ing" every hour. If, for instance, a charge of coals remains five hours in the retort, and the retorts are divided into five parcels or sets, one set would be filled at noon, another at one o'clock, and the rest at two, three, and four respec tively. Then, by five o'clock the first set of retorts are ready to be drawn ; at six o'clock the second set ; and so on with the others. The precise arrangements need not be entered into, but it will suf fice to say that exactly as the clock strikes each successive hour, the men loosen and remove the covers of the retorts, draw out a portion of the coke into large iron boxes, draw out the rest upon the iron floor of the retorl-honse, throw water on the coke preparatory to its removal from the retort-house, recharge the re torts with fresh coal, replenish the fires with a fresh supply of coke, and fit the covers—coated on their inner surface with a thick layer of lime and clay cement —firmly on the mouths of the retorts. In the intervals which elapse between the successive " drawings," the men are em ployed in pouring the lime-water into the troughs beneath the fireplaces, in placing new layers of cement on the retort-covers to be used after the next drawing, in carrying out the coke into the open air, and afterwards into the sheds or stores, in bringing coals from the coal-stores to the retort-houses, in removing the ashes which fall into the lime-water in the ash pit, and in various other duties sub sidiary to the manufacture of gas. The subsequent preparation, or rather perfect ing of the gas, demands but a small amount of manual labor; it is in fact performed by the steam-engine, which pumps up the water from the well, trans fers from vessel to vessel the tar and the ammoniacal liquor abstracted from the gas, and sets in rotation the arms or fans in the purifying vessels.
There is perhaps no part of the gas mechanism which requires better work manship and more careful attention than the pipes which convey the invisible agent from the works to the places where it is consumed. However perfect may be the mode in which the gas is manu factured, however plentiful the supply, yet if the pipes are either too small or too large, if they are laid either too hori zontal or too much inclined, if any of the innumerable joints are imperfectly fitted, the most serious inconvenience results. The mains vary from three inches to eighteen inches in diameter, indepen dent of the small lateral pipes which pro teed from the mains into the houses. The largest mains are placed nearest to the gas-works ; the next in size are ap propriated to the leading streets and thoroughfares ; while the smaller are for the less important lanes and streets.
Where the streets are wide, and the num ber of lights required large, it is usual to lay mains on both sides of the street ; and the diameters of these mains are made to depend not only on the magni tude and importance of the street, but on its elevation, its distance from the works, and other circumstances. There is a cir cumstance attended to in laying down the mains which is perhaps not generally known. They are laid with a gradual in clination, amountingperhaps to an inch in ten or twelve yards, instead of being horizontal ; and when this slope has con tinued for one or two hundred yards, the mains begin to ascend in a similar de gree. The line of mains thus ascends and descends alternately throughout RS whole length. The reason for this ar rangement is, that a small deposition of fluid takes place in the mains ; and this fluid, by flowing down the inclined pipes, accumulates at the lower points, where two descending lines meet ; here a reser voir is formed, into which the liquid flows, and by the occasional use of a small pump from above the inconvenience is removed.
When gas is made from coal, the selec tion of the coal becomes an object. The most bituminous is most desirable, and what is termed Cannel coal is usually preferred. The Philadelphia Gas Com pany use Virginia (Richmond) coal, while the two New York companies use two parts of Cannel coal and one part of New castle. The compositions of these two varieties are given, by Richardson Newengtle. ClinneL Carbon 84846 . 67.597 Hydrogen .... 5'048 5.400 Oxygen } .. 8'430 12432 Nitrogen When these coals are heated to redness in closed vessels, the following process results :—A coaly residue (coke) remains, and certain volatile products escape, which partly condense on cooling into tar and an aqueous fluid, while the rest is a mixture of gases, but contains also no inconsiderable portion of the volatile va pors of different compounds, which re main dissolved in the coal gases without being condensed into liquids. These are oily products, mostly hydro-carbons, with a large proportion of carbon ; to these belong kyanol, leucol, pyrole, rosolic and carbonic acids and napthalinc • most of these contain 90 per cent. of carbon, nap thaline as much as 94 per cent., and in burning they deposit it in greater quan tity than olefiant gas. These, therefore, enhance very much the illuminating power of the gas. Illuminating gas is not a definite compound of one or two gases, as carburetted hydrogen or olefi gas, but a mechanical mixture of very various bodies, some of which are only slightly luminous, some absolutely prejudicial for illumination, whilst others are exceedingly luminous, as olefiant gas and the carbo-hydrogens, possessing similar properties, and to which the mix ture owes its illuminating properties. By distilling coal, we have left behind the solid coke in the retort, and then are given off, as volatile matters, a number of gases, vapors, and liquids which separate in their passage, and are received in the tar-cistern and condenser. The liquids consist of, first, coal tar, which, on redistilling, yields pitch, coal oil (nap tha),containing the hydro-carbons noticed above ; and, second, of ammoniacal li quor, containing water, hydro-sulphate, carbonate, muriate, acetate, hydrocyauate, sulphite, and gallate of ammonia.