two tubes st, xy, which are made to approach each other at their upper
extremities t y; these are made of such a length as to rise a little
above the upper edge LM of the vessel LMNO, and when the jar abcde
touches the bottom NO, their upper ends enter about half an inch into
the conical hollow b, leading to the stop-cock g.
The bottom of the vessel LMNO is represented Pl. IX. Fig. 3. in the
middle of which a small hollow semispherical cap is soldered, which may
be considered as the broad end of a funnel reversed; the two tubes st,
xy, Fig. 4. are adapted to this cap at s and x, and by this means
communicate with the tubes mm, nn, oo, pp, Fig. 3. which are fixed
horizontally upon the bottom of the vessel, and all of which terminate
in, and are united by, the spherical cap sx. Three of these tubes are
continued out of the vessel, as in Pl. VIII. Fig. 1. The first marked in
that figure 1, 2, 3, is inserted at its extremity 3, by means of an
intermediate stop-cock 4, to the jar V. which stands upon the shelf of a
small pneumato-chemical apparatus GHIK, the inside of which is shown Pl.
IX. Fig. 1. The second tube is applied against the outside of the vessel
LMNO from 6 to 7, is continued at 8, 9, 10, and at 11 is engaged below
the jar V. The former of these tubes is intended for conveying gas into
the machine, and the latter for conducting small quantities for trials
under jars. The gas is made either to flow into or out of the machine,
according to the degree of pressure it receives; and this pressure is
varied at pleasure, by loading the scale P less or more, by means of
weights. When gas is to be introduced into the machine, the pressure is
taken off, or even rendered negative; but, when gas is to be expelled, a
pressure is made with such degree of force as is found necessary.
The third tube 12, 13, 14, 15, is intended for conveying air or gas to
any necessary place or apparatus for combustions, combinations, or any
other experiment in which it is required.
To explain the use of the fourth tube, I must enter into some
discussions. Suppose the vessel LMNO, Pl. VIII. Fig. 1. full of water,
and the jar A partly filled with gas, and partly with water; it is
evident that the weights in the bason P may be so adjusted, as to
occasion an exact equilibrium between the weight of the bason and of the
jar, so that the external air shall not tend to enter into the jar, nor
the gas to escape from it; and in this case the water will stand exactly
at the same level both within and without the jar. On the contrary, if
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