a medium temperature, in order to obtain a better gas having the greatest illuminating power. In about four or five hours the coal in the retort will have given off all its gas. The mouth of the retort is opened, and the coke is raked out into large iron vessels, and extinguished by water. A fresh charge is immediately introduced by means of a long scoop in the cherry-red retort, and the door luted to. The ascension pipes, which convey the gas from the retorts, pass straight up for a few feet, then turn round, forming an arch, then pass downward into the hydraulic main, beneath the level of the liquid contained in it, and bubble up through the liquid into the upper portion of the main. On commencing the main is half filled with water, but after working some time, this water is displaced by the fluid products of distillation. In this way, the opening into each retort is closed, so that a charge can be withdrawn and replaced without interfering with the action of the other retorts and pipes. The liquid tar, ammoniacal water, and gas pass from the end, _e_, of the hydraulic main, down through the pipe, P, and the liquid falls down into the tar well, T W, while the crude gas goes on into the chest, C, partially filled with the liquid, so that the plates, _p p_, from the top dip into it to within a few inches of the bottom. These dip plates are placed in the chest, so as to separate the openings into each pair of condensing pipes, _c c_, so that the gas passing into the chest finds no exit except up _c_{1}, and down _c_{2}; and there being no dip plate between _c_{2} and _c_{3} it passes up _c_{3}, and down _c_{4}, and as there is no dip plate to prevent its progress, it passes up _c_{5}, and down _c_{6}, into the lime or iron purifiers, L I. The condensers are kept cool by exposure to the atmosphere, and are often cooled by a stream of water from a tank above. The gas cools quickly, and liquids passing along with the gas in a state of vapor are condensed and fall into the chest, and pass by an overflow pipe into the tar well. The purifier is a cast iron vessel, L I, containing a number of perforated shelves, _s_{1} _s_{1} _s_{1}, on which slaked lime, to the depth of about 4 inches, or much greater thickness of iron oxide and sawdust, is placed. The gas passes up through the shelves, _s s s_, and down through the shelves, _s_{1} _s_{1} _s_{1}, through the pipe, G, into the gas holder, and from thence through the pipe, M, to the main pipe. The lime abstracts carbonic anhydride, sulphureted hydrogen, cyanogen, naphthalin, and a portion of the ammonia, but not carbon disulphide, which latter may be absorbed by passing the gas through a solution of sodic hydrate and plumbic oxide, mixed with sawdust. Gas containing CS{2}, on burning, produces H{2}SO{4}, which injures books and furniture in rooms. However, the quantity of CS{2} in gas is generally so minute as to be practically uninjurious. By a proper regulation of the temperature