s lower
extremity t immersed in the liquor. If any absorption takes place,
either in the retort, or in any of the bottles, a sufficient quantity of
external air enters, by means of these tubes, to fill up the void; and
we get rid of the inconvenience at the price of having a small mixture
of common air with the products of the experiment, which is thereby
prevented from failing altogether. Though these tubes admit the external
air, they cannot permit any of the gasseous substances to escape, as
they are always shut below by the water of the bottles.

It is evident that, in the course of experiments with this apparatus,
the liquor of the bottles must rise in these tubes in proportion to the
pressure sustained by the gas or air contained in the bottles; and this
pressure is determined by the height and gravity of the column of fluid
contained in all the subsequent bottles. If we suppose that each bottle
contains three inches of fluid, and that there are three inches of water
in the cistern of the connected apparatus above the orifice of the tube
RM, and allowing the gravity of the fluids to be only equal to that of
water, it follows that the air in the first bottle must sustain a
pressure equal to twelve inches of water; the water must therefore rise
twelve inches in the tube S, connected with the first bottle, nine
inches in that belonging to the second, six inches in the third, and
three in the last; wherefore these tubes must be made somewhat more than
twelve, nine, six, and three inches long respectively, allowance being
made for oscillatory motions, which often take place in the liquids. It
is sometimes necessary to introduce a similar tube between the retort
and recipient; and, as the tube is not immersed in fluid at its lower
extremity, until some has collected in the progress of the distillation,
its upper end must be shut at first with a little lute, so as to be
opened according to necessity, or after there is sufficient liquid in
the recipient to secure its lower extremity.

This apparatus cannot be used in very accurate experiments, when the
substances intended to be operated upon have a very rapid action upon
each other, or when one of them can only be introduced in small
successive portions, as in such as produce violent effervescence when
mixed together. In such cases, we employ a tubulated retort A, Pl. VII.
Fig. 1. into which one of the substances is introduced, preferring
always the solid body, if any su