It does not seem that there is any particular reason for the
employment of more than one grid, provided the size of the carbide
decomposed is suited to the generator, and provided the mesh of the grid
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.
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