the earth; or, what is the same thing, in other words, that it is
essential to the nature of every combustible body not to possess the
property of combustion, unless heated, or raised to the degree of
temperature at which its combustion naturally takes place. When this
degree is once produced, combustion commences, and the caloric which is
disengaged by the decomposition of the oxygen gas keeps up the
temperature necessary for continuing combustion. When this is not the
case, that is, when the disengaged caloric is insufficient for keeping
up the necessary temperature, the combustion ceases: This circumstance
is expressed in common language by saying, that a body burns ill, or
with difficulty.
Although combustion possesses some circumstances in common with
distillation, especially with the compound kind of that operation, they
differ 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.
270