At A is represented the vent-pipe of a
displacement vessel, which may either be part of a displacement holder or
of a generator working on the displacement principle. The vent-pipe is
rigidly fixed to the apparatus. If gas is generated within the closed
portion of the holder or passes through it, and if the pressure so set up
remains less than that which is needed to move the water from the level
_l_ to the levels _l'_ and _l"_, the mouth of the pipe is
under water, and acetylene cannot enter it; but immediately such an
amount of gas is collected, or such pressure is produced that the
interior level sinks below _l"_, which is that of the mouth of the
pipe, it becomes unsealed, and the surplus gas freely escapes. There are
two minor points in connexion with this form of vent-pipe often
overlooked. At the moment when the water arrives at _l"_ in the
closed half of the apparatus, its level in the interior of the vent-pipe
stands at _l'_, identical with that in the open hall of the
apparatus (for the mouth of the vent-pipe and the water in the open hall
of the apparatus are alike exposed to the pressure of the atmosphere
only). When the water, then, descends just below _l"_ there is an
amount of water inside the pipe equal in height to the distance between
_l'_ and _l"_; and before the acetylene can escape, it must
either force this water as a compact mass out of the upper mouth of the
vent-pipe (which it is clearly not in a position to do), drive it out of
the upper mouth a little at a time, or bubble through it till the water
is gradually able to run downwards out of the pipe as its lower opening
is more fully unsealed.
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