The value of coal depends entirely upon its inflammable and combustible properties. It is used as the source of artificial warmth in our dwellings, and our manufactures are mainly dependent on it. Without coal there would scarcely be any working in iron, copper, lead, or other metals, as in populous countries the supply of wood is soon exhausted. Our potteries and our glass works are also carried on by its use, and the power of steam which moves all our great engines and machines is dependent on coal for its existence. Great Britain produces more than one-half the coal that is consumed in the world, and about 180,000 persons are employed in obtaining it in our collieries. The first process in the manufacture of gas is to subject the coal to a great heat, by which it is _carbonised_, or burnt until only the cinder, or coke, remains after the gas has flown off. This is done by placing the coal in retorts of fire-clay or iron, which are previously heated. These retorts, which occupy a building called the _retort house_, having been charged with coal, are perfectly closed, and the door _luted_, or stopped, with a sort of cement, so that the gas can only escape up the _ascension pipe_, the coke being left in the retort. [Illustration: Retort House.] The gas from the coal then passes through the ascension pipe by what is called the _dip pipe_, into a main, a large horizontal tube extending along the length of the furnaces. This main is about half filled with water or tar, in which the ends of the dip pipes are immersed, so that as the gas runs in, it ascends through the liquid into the space above, but cannot flow back again into the dip pipes. In this way it is all collected in the upper part of the main pipe, and is ready for purification. Where double retorts are used, each end is worked with at least three stokers, and an extra man for preparing the lids of the mouth-pieces. Others are required for extinguishing the coke, wheeling the coal into the retort house, clinkering furnaces, and attending to fires. Three stokers, assisted by a man to extinguish the coke, will perform all the work of taking off the lids, raking out the coke, extinguishing and wheeling it away from a bench of seven retorts, in twelve or thirteen minutes; they will then put the proper charge for each retort in the _scoop_, deliver its contents, and be ready for charging another bench in a further space of seven minutes, while a fourth workman will in the meantime have put on the lids, so that the whole work of discharging and charging the seven retorts will occupy barely twenty minutes.