e, a dangerous and sudden explosion
takes place, which carries up the jar to a great height, and dashes it
in a thousand pieces. From two such explosions some of the members of
the Academy and myself escaped very narrowly. Besides, though this
manner of operating is sufficient for determining pretty accurately the
quantity of oxygen gas absorbed, and of carbonic acid produced, as water
is likewise formed in all experiments upon vegetable and animal matters
which contain an excess of hydrogen, this apparatus can neither collect
it nor determine its quantity. The experiment with phosphorus is even
incomplete in this way, as it is impossible to demonstrate that the
weight of the phosphoric acid produced is equal to the sum of the
weights of the phosphorus burnt and oxygen gas absorbed during the
process. I have been therefore obliged to vary the instruments according
to circumstances, and to employ several of different kinds, which I
shall describe in their order, beginning with that used for burning
phosphorus.

Take a large balloon, A, Pl. IV. Fig. 4. of cristal or white glass, with
an opening, EF, about two inches and a half, or three inches, diameter,
to which a cap of brass is accurately fitted with emery, and which has
two holes for the passage of the tubes xxx, yyy. Before shutting the
balloon with its cover, place within it the stand, BC, supporting the
cup of porcelain, D, which contains the phosphorus. Then lute on the cap
with fat lute, and allow it to dry for some days, and weigh the whole
accurately; after this exhaust the balloon by means of an air-pump
connected with the tube xxx, and fill it with oxygen gas by the tube
yyy, from the gazometer, Pl. VIII. Fig. 1. described Chap. II. Sect
II. of this part. The phosphorus is then set on fire by means of a
burning-glass, and is allowed to burn till the cloud of concrete
phosphoric acid stops the combustion, oxygen gas being continually
supplied from the gazometer. When the apparatus has cooled, it is
weighed and unluted; the tare of the instrument being allowed, the
weight is that of the phosphoric acid contained. It is proper, for
greater accuracy, to examine the air or gas contained in the balloon
after combustion, as it may happen to be somewhat heavier or lighter
than common air; and this difference of weight must be taken into
account in the calculations upon the results of the experiment.

SECT. III.

_Of the Combustion of Charcoal._

The apparatus