r in a very material point. In distillation there is a separation
of one part of the elements of the substance from each other, and a
combination of these, in a new order, occasioned by the affinities which
take place in the increased temperature produced during distillation:
This likewise happens in combustion, but with this farther circumstance,
that a new element, not originally in the body, is brought into action;
oxygen is added to the substance submitted to the operation, and caloric
is disengaged.

The necessity of employing oxygen in the state of gas in all experiments
with combustion, and the rigorous determination of the quantities
employed, render this kind of operations peculiarly troublesome. As
almost all the products of combustion are disengaged in the state of
gas, it is still more difficult to retain them than even those furnished
during compound distillation; hence this precaution was entirely
neglected by the ancient chemists; and this set of experiments
exclusively belong to modern chemistry.

Having thus pointed out, in a general way, the objects to be had in view
in experiments upon combustion, I proceed, in the following sections of
this chapter, to describe the different instruments I have used with
this view. The following arrangement is formed, not upon the nature of
the combustible bodies, but upon that of the instruments necessary for
combustion.

SECT. II.

_Of the Combustion of Phosphorus._

In these combustions we begin by filling a jar, capable at least of
holding six pints, with oxygen gas in the water apparatus, Pl. V. Fig.
1.; when it is perfectly full, so that the gas begins to flow out below,
the jar, A, is carried to the mercury apparatus, Pl. IV. Fig. 3. We then
dry the surface of the mercury, both within and without the jar, by
means of blotting-paper, taking care to keep the paper for some time
entirely immersed in the mercury before it is introduced under the jar,
lest we let in any common air, which sticks very obstinately to the
surface of the paper. The body to be submitted to combustion, being
first very accurately weighed in nice scales, is placed in a small flat
shallow dish, D, of iron or porcelain; this is covered by the larger cup
P, which serves the office of a diving bell, and the whole is passed
through the mercury into the jar, after which the larger cup is retired.
The difficulty of passing the materials of combustion in this manner
through the mercury may