]
DISPLACEMENT GASHOLDERS.--An excursion may here be made for the purpose
of studying the action of a displacement holder, which in its most
elementary form is shown at C. It consists of an upright vessel open at
the top, and divided horizontally into two equal portions by a partition,
through which a pipe descends to the bottom of the lower half. At the top
of the closed lower compartment a tube is fixed, by means of which gas
can be introduced below the partition. While the cock is open to the air,
water is poured in at the open top till the lower compartment is
completely full, and the level of the liquid is at _l_. If now, gas
is driven in through the side tube, the water is forced downwards in the
lower half, up through the depending pipe till it begins to fill the
upper half of the holder, and finally the upper half is full of water and
the lower half of gas an shown by the levels _l'_ and _l"_. But
the force necessary to introduce gas into such an apparatus, which
conversely is equal to the force with which the apparatus strives to
expel its gaseous contents, measured in inches of water, is the distance
at any moment between the levels _l'_ and _l"_; and as these
are always varying, the effective pressure needed to fill the apparatus,
or the effective pressure given by the apparatus, may range from zero to
a few inches less than the total height of the whole holder.
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