It may be premised that, in comparing the illuminating power of one gas with another, the standard of comparison is the light from a sperm candle burning 120 grains of sperm per hour, the gas-burner consuming at the rate of 5 cubic ft. per hour. When the quantities consumed are different from these, the results arc rectified by calmilation.
The Ekglish caking coals yield from 8,000 to 10,000 cubic ft. of gas per ton, of illu minating power varying from 12 to 14 sperm candles to a burner consuming 5 ft. per hour. The English cannel coals yield about 10,000 cubic ft. per ton, of illuminating power varying from 20 to 24 sperm candles. The Scotch parrot coals are very various in quality, yielding from 8,000 up to 13,000 cubic ft. per ton, varying in illuminating power from 20 up to 35 candles. As a general rule, the parrot coals which yield the greatest quantity of gas, yield also gas of the highest illuminating power. Bituminous shales may be used in part to supply the place of cannel coals. They yield from 6,000 to 8.000 ft. of gas, varying from 34 to 40 candles.
In the process of distillation, gas, tar, and ammoniacal liquor come off together, and are separated by the action of the apparatus employed—a large residpum of remains in the retort. The gas consists of a mixture of heavy carbureted hydrogen (oleflant gas), specific gravity, 985; heavy hydrocarbon vapors of various kinds; light carbureted hy8r0gen, specific gravity, 555; snlphureted hydrogen, specific .gravity, 1191; sulphide or sulphuret of carbon in minute quantity; carbonic oxide, specific gravity, 972; carbonic acid, specific gravity, 1524.
The value of coal-gas depends on the proportion of (defiant gas and heavy hydro carbons which it contains. Great attention is required in heating the retorts; if their temperature be too low, the tar and liquor are increased in quantity, and the gas dimin ished in quantity and deteriorated in quality. If the temperature be too high, the ole fiant gas is decomposed, and light carbureted hydrogen formed. While different parts of th.a apparatus necessary for producing, purifying, storing, and sending out the gas are capable of many variations in size, form, and construction, the order in which they come into use is almost invariable. First there are the retorts, ascension and dip pipes, hydraulic main; then the and condenser, the exhauster, the washer or scrubber, the purifier, the station-meter, the and the governor—the parts printed in italics being indispensable. Besides the above, valves of various forms, simple and complicated, are employed. These, in some of their arrangements, display great inge nuity. Water-traps also have to be applied for collecting and removing the water and tar which'condenSe in the pipes. The annexed wood-cut shows an arrangement com mon in small gas-works.
The retorts are now generally made of fire-clay, though cast-iron retorts are still fre quently to be met with. They are made D-shaped, kidney-shaped, and ellip tical. The sizes most common are from 6 to 9 ft. in length, and from 12 to 20 in. in diameter. In large works, two 9-ft. lengths are joined together, forming one retort 18 ft. long, with a mouth at each end—a mode of construction which is found to possess considerable advantages. The retort is built horizontally into an arched oven, in such a manner as to be equally heated throughout from a furnace beneath. From one to, seven retorts, and sometimes a greater number, are set in the same oven. The open! mouth-piece of the retort is of cast-iron, and projects outwards from the front wall of the oven sufficiently far to admit, between the mouth and the front of the oven, an opening, to which the ascension-pipe is connected for conveying the gas to the hydraulic main. When the coal to be distilled is introduced into the retort, the mouth is closed
with a lid, which is kept tight by a luting of clay or other material round the edge, and mace fast with a screw.
The hydraulic main is a large pipe made of thick plate or cast-iron. It is first about half-filled with water, which in the course of a short time 'is entirely displaced by the liquid product of distillation. The dip-pipes, which are the continuation of the ascen sion-pipes, dip into the liquid through which the gas bubbles up into the upper portion of the hydraulic main. The gas and liquid come off at the end of the hydratilic main, and flow together till they reach the tar-well, into which the liquid, by its greater gravity, falls. The liquid consists of tar and ammoniacal water. These are with drawn from the tar-well, and become the raw material from which other products are manufactured. From the tar, naphtha, pitch-oil, pitch, and coke are obtained; and from the water, salts of ammonia are prepared. The tar and ammoniacal water being of dif ferent densities, are easily separated, by being allowed to settle in a vessel. See GAS TAR, NAPHTHA, SULPHATE OF AMMONIA, SAL-AMMONIAC, etc. When a retort is opened for withdrawing the exhausted charge of coal and renewing it, the pressure of the gas on the hydraulic main forces the liquid to ascend the dip-pipe, and thus seals it against the gas in the hydraulic, which, but for this, would rush up the dip-pipe, descend the ascension-pipe, escape, and ignite at the open mouth of the retort. In the tar-well there is also a dip-pipe, inserted into a deep vessel, to prevent the gas from entering tho well. A similar contrivance is resorted to wherever it is necessary to introduce or draw off liquids at any part of the apparatus. The tar-well must be placed so low, that all the liquid in the pipes leading to it from the hydraulic main, and from it to the con denser, must iodize towards it. aitized by Microsoft The simplest form of condenser consists of a series of upright pipes, each. pair being connected at the top by an arch pipe. These are erected upon a horizontal chest, the top of which has an opening into the bottom of each upright pipe. Immediately under the center of each arch pipe, a plate descends from the top of the chest, and reaches to within a few inches of the bottom. When in operation, the chest is always filled with liquid to such a height, that these plates dip into it, and prevent the gas from passing through the chest horizontally. When admitted into the chest, the gas finds no exit but.by ascending the first upright pipe; and, paSsing over the arch, it descends to the chest again through the second upright pipe. There being no dip-plate between the second and third upright pipes, the gas ascends the third pipe and desoends the fourth, and so on through the condentcr. The upright pipes are kept cool by exposure to the atmosphere, and sometimes a thin stream of water is caused to flow over them. As the gas ascends and descends, cooling rapidly in its passage, the liquid which has been carried along in a state of vapor, condenses, and falls into the chest, from which it is conveyed back hy an overflow-pipe to the tar-well.