quantities of
oxygen gas before and after the operation must have thrown very great
uncertainty upon the results of the experiment. I was, besides,
dissatisfied with this process, and not without cause, lest any air
might have escaped through the pores of the bladder, more especially as
it becomes shrivelled by the heat of the furnace, unless covered over
with cloths kept constantly wet.

This experiment is performed with more certainty in the apparatus
described in the Memoirs of the Academy for 1775, p. 580. This consists
of a retort, A, Pl. IV. Fig. 2. having a crooked glass tube BCDE of ten
or twelve lines internal diameter, melted on to its beak, and which is
engaged under the bell glass FG, standing with its mouth downwards, in a
bason filled with water or mercury. The retort is placed upon the bars
of the furnace MMNN, Pl. IV. Fig. 2. or in a sand bath, and by means of
this apparatus we may, in the course of several days, oxydate a small
quantity of mercury in common air; the red oxyd floats upon the surface,
from which it may be collected and revivified, so as to compare the
quantity of oxygen gas obtained in revivification with the absorption
which took place during oxydation. This kind of experiment can only be
performed upon a small scale, so that no very certain conclusions can be
drawn from them[61].

The combustion of iron in oxygen gas being a true oxydation of that
metal, ought to be mentioned in this place. The apparatus employed by Mr
Ingenhousz for this operation is represented in Pl. IV. Fig. 17.; but,
having already described it sufficiently in Chap. III. I shall refer the
reader to what is said of it in that place. Iron may likewise be
oxydated by combustion in vessels filled with oxygen gas, in the way
already directed for phosphorus and charcoal. This apparatus is
represented Pl. IV. Fig. 3. and described in the fifth chapter of the
first part of this work. We learn from Mr Ingenhousz, that all the
metals, except gold, silver, and mercury, may be burnt or oxydated in
the same manner, by reducing them into very fine wire, or very thin
plates cut into narrow slips; these are twisted round with iron-wire,
which communicates the property of burning to the other metals.

Mercury is even difficultly oxydated in free air. In chemical
laboratories, this process is usually carried on in a matrass A, Pl. IV.
Fig. having a very flat body, and a very long neck BC, which vessel is
commonly called _Boyle's