The upper part of the purifier is surrounded by a cistern or reservoir of from 6 inches to 24 inches deep, and from 3 inches to 6 inches wide, which is often cast with the purifier, and forms part of it, or at other times is attached thereto by bolts and cement, and is for the purpose of containing water to seal the cover. The cover of the purifier is of boiler plate or cast iron, the latter being preferable on account of its durability; but the increased weight is an impediment to its adoption. The rim or border of the cover is rather deeper than the cistern into which it is placed, and is effectually sealed by the water, so preventing the gas escaping from that point. Often the purifier is divided into two compartments, so that the gas ascends through a set of sieves on the one side, and descends through another set on the other side, answering the purpose of two sets of apparatus. In all establishments, however small they may be, two distinct purifiers at least are necessary, to enable the impure lime to be removed from the one whilst the gas is being purified by the other. The next subject for consideration is that of the _gasholders_, or vessels in which the gas is stored ready for delivery into the mains, which distribute it throughout the districts to be lighted. These vessels were originally termed _gasometers_, which name is sometimes even now applied to them; but as they have nothing whatever to do with the measurement of gas, but are mere vessels of capacity or stores, the simple name of gasholder is more expressive and appropriate. The gasholder is composed of two distinct parts, one of which contains water, and is called the tank, the other is the vessel which contains the gas, being really the gasholder. On the Continent the former is very generally termed the “cistern,” and the latter the “bell.” The tank is a large cylindrical vessel, constructed usually, for the sake of economy, of brickwork or masonry, but when the ground is marshy, or when water exists abundantly a short distance below the surface of the earth, which would prevent the construction in masonry at a moderate price, these tanks are made in cast-iron, and, indeed, in small works, are often of wrought iron. In the interior of the tank there are two vertical pipes for the admission and egress of the gas, called the inlet and outlet pipes; the former being in direct communication with the manufacturing apparatus, the latter with the mains which convey the gas to the town. These pipes rise a few inches above the level of the top of the tank, so that the water cannot overflow into them. A series of columns, generally of cast-iron, but sometimes of wood, or brick piers, are placed at equal distances around the tank for the purpose of guiding the holder.