Into this shoot are dropped by hand or shovel lumps of
carbide, which fall into the water and there suffer decomposition. As the
bottom of the shoot is covered with water, which, owing to the small
effective gas pressure in the generator given by the holder, stands a few
inches higher in the shoot than in the generator, gas cannot escape from
the shoot; because before it could do so the water in the generator would
have to fall below the level of the point _a_, being either driven
out through the shoot or otherwise. Since the point _b_ of the shoot
extends further into the generator than _a_, the carbide drops
centrally, and as the bubbles of gas rise vertically, they have no
opportunity of ascending into the shoot. In practice, the generator is
fitted with a conical bottom for the collection of the lime sludge and
with a cock or other aperture at the apex of the cone for the removal of
the waste product. As it is not desirable that the carbide should be
allowed to fall directly from the shoot into the thicker portion of the
sludge within the conical part of the generator, one or more grids is
usually placed in the apparatus as shown by the dotted lines in the
sketch.
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