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vessel; the sludge may be withdrawn into some vessel so small in capacity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 whole contents of the
holder be passed into the air of the building containing the apparatus
through the open mouth of the shoot. The second plan is perfectly sound,
but has the practical defect of increasing the labour of cleaning the
generator.
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