The retorts are usually made of cast iron, and are about 7 feet long, 14 inches in depth, and the same in width; the shape being that of an arch. The retorts will hold two hundredweight of coal each, but they are never filled, because during the process of distillation the carbonaceous part of the coal expands, and occupies more space than the original quantity, the proportion of expansion being as one and a quarter to one. There is a large aperture for the admission of coal and the extraction of coke, which aperture is covered with a lid, and screwed to make it air-tight. A tube is inserted into the mouth of the retort, to carry off the products of the distillation. That tube rises about twelve feet, and then dips into a large horizontal pipe, one foot in diameter, called the hydraulic main, in which are collected the tar and ammoniacal liquor that distil from the coal. Ten or fourteen retorts are usually set back to back in brickwork, to be heated by one fire; but the plan has been recently introduced of employing long clay retorts, which are charged at both ends. These are found to possess considerable advantage over iron, not only from their lower price, but from the facility with which the carbonaceous deposit is removed, and the full charges worked off. Coke is generally burned in the furnaces, and the heat is continually maintained so as to keep the retorts red-hot. As atmospheric air cannot gain access to the coal in the retorts, the gases expelled do not inflame, nor can the parts that are not volatile be consumed without a supply of air. It is entirely a process of distillation, in which all the products can be collected. The volatile parts are carried off by the pipe, and the solid carbonaceous matter, or coke, is left in the retort. The first effect of heat on coal, after it is put into the retort, is to expel the moisture, which, in combination with the air, issues in the form of steam. Tar then distils, with some portions of gas, consisting of hydrogen and ammonia. When the retort has attained a bright cherry-red heat, the disengagement of the carburretted hydrogen is most active; and it is found that the more quickly the coal is heated, the greater is the quantity of illuminating gas generated. The production of coal gas, and the development of its properties at different stages of distillation, may be readily shown by means of a common tobacco pipe. Fill the bowl of the pipe with small pieces of coal, cover it over with a lump of clay, and then put it into a hot fire, with the stalk of the pipe projecting through the bars. Presently steam will be seen to issue from the pipe, and afterwards smoke, and, if a light be applied, a jet of flame will issue forth, the brilliancy of which will increase as the bowl of the pipe becomes more heated, until the best part of the gas has been distilled from the coal.