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id is suited to the size of the carbide. A great improvement, however, is made if the grid is carried on a horizontal spindle in such a way that it can be rocked periodically in order to assist in freeing the lumps of carbide from the adhering particles of lime. As an alternative to the movable grid, or even as an adjunct thereto, an agitator scraping the conical sides of the generator may be fitted which also assists in ensuring a reasonably complete absence of undecomposed carbide from the sludge drawn off at intervals. A further point deserves attention. If constructed in the ideal manner shown in Fig. 6 removal of some of the sludge in the generator would cause the level of the liquid to descend and, by carelessness, the level might fall below the point a at the base of the shoot. In these circumstances, if gas were unable to return from the holder, a pressure below that of the atmosphere would be established in the gas space of the generator and air would be drawn in through the shoot. This air might well prove a source of danger when generation was started again. Any one of three plans may be adopted to prevent the introduction of air. A free path may be left on the gas-main passing from the generator to the holder so that gas may be free to return and so to maintain the usual positive pressure in the decomposing vessel; the sludge may be withdrawn into some vessel so small in capacity that the shoot cannot accidentally become unsealed; or the waterspace of the generator may be connected with a water-tank containing a ball-valve attached to a constant service of water be that liquid runs in as quickly as sludge is removed, and the level remains always at the same height. The first plan is only a palliative and has two defects. In the first place, the omission of any non-return valve between, the generator and the next item in the train of apparatus is objectionable of itself; in the second place, should a very careless attendant withdraw too much liquid, the shoot might become unsealed and the whol