The apparatus works perfectly with a clean gas
or liquid which is not corrosive; but having regard to the possible
presence of tarry products, lime dust, or sludge, condensed water loaded
with soluble impurities, &c., in the acetylene, a non-return valve is not
the best device to adopt, for both it and the hand-worked cock or screw-
down valve are liable to stick and give trouble. The best arrangement in
all respects, especially between the generator and the holder, is a
water-seal. A water-seal in made by leading the mouth of a pipe
delivering gas under the level of water in a suitable receptacle, so that
the issuing gas has to bubble through the liquid. Gas cannot pass
backwards through the pipe until it has first driven so much liquid
before it that the level in the seal has fallen below the pipe's mouth;
and if the end of the pipe is vertical more pressure than can possibly be
produced in the apparatus is necessary to effect this. Omitting the side
tube _b_, one variety of water-seal is shown at D in Fig. 7 on page
103. The water being at the level _l_, gas enters at _a_ and
bubbles through it, escaping from the apparatus at _c_. It cannot
return from _c_ to _a_ without driving the water out of the
vessel till its level falls from _f_ to _g_; and since the area
of the vessel is much greater than that of the pipe, so great a fall in
the vessel would involve a far greater rise in _a_.
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