the time and trouble bestowed, as by one process carried on in this
accurate manner much more just and extensive knowledge is acquired of
the nature of the vegetable or animal substance thus submitted to
investigation, than by many weeks assiduous labour in the ordinary
method of proceeding.
When in want of bottles with three orifices, those with two may be used;
it is even possible to introduce all the three tubes at one opening, so
as to employ ordinary wide-mouthed bottles, provided the opening be
sufficiently large. In this case we must carefully fit the bottles with
corks very accurately cut, and boiled in a mixture of oil, wax, and
turpentine. These corks are pierced with the necessary holes for
receiving the tubes by means of a round file, as in Pl. IV. Fig. 8.
SECT. II.
_Of Metallic Dissolutions._
I have already pointed out the difference between solution of salts in
water and metallic dissolutions. The former requires no particular
vessels, whereas the latter requires very complicated vessels of late
invention, that we may not lose any of the products of the experiment,
and may thereby procure truly conclusive results of the phenomena which
occur. The metals, in general, dissolve in acids with effervescence,
which is only a motion excited in the solvent by the disengagement of a
great number of bubbles of air or aeriform fluid, which proceed from the
surface of the metal, and break at the surface of the liquid.
Mr Cavendish and Dr Priestley were the first inventors of a proper
apparatus for collecting these elastic fluids. That of Dr Priestley is
extremely simple, and consists of a bottle A, Pl. VII. Fig. 2. with its
cork B, through which passes the bent glass tube BC, which is engaged
under a jar filled with water in the pneumato-chemical apparatus, or
simply in a bason full of water. The metal is first introduced into the
bottle, the acid is then poured over it, and the bottle is instantly
closed with its cork and tube, as represented in the plate. But this
apparatus has its inconveniencies. When the acid is much concentrated,
or the metal much divided, the effervescence begins before we have time
to cork the bottle properly, and some gas escapes, by which we are
260