XII. Fig. 12.; the interior
neck bc communicates with the inside of the bottle, and the exterior
neck or rim de leaves an interval between the two necks, forming a
deep gutter intended to contain the mercury. The cap or lid of glass B
enters this gutter, and is properly fitted to it, having notches in its
lower edge for the passage of the tubes which convey the gas. These
tubes, instead of entering directly into the bottles as in the ordinary
apparatus, have a double bend for making them enter the gutter, as
represented in Fig. 13. and for making them fit the notches of the cap
B; they rise again from the gutter to enter the inside of the bottle
over the border of the inner mouth. When the tubes are disposed in their
proper places, and the cap firmly fitted on, the gutter is filled with
mercury, by which means the bottle is completely excluded from any
communication, excepting through the tubes. This apparatus may be very
convenient in many operations in which the substances employed have no
action upon Mercury. Pl. XII. Fig. 14. represents an apparatus upon this
principle properly fitted together.

Mr Seguin, to whose active and intelligent assistance I have been very
frequently much indebted, has bespoken for me, at the glass-houses, some
retorts hermetically united to their recipients, by which luting will be
altogether unnecessary.

CHAP. VIII.

_Of Operations upon Combustion and Deflagration._

SECT. I.

_Of Combustion in general._

Combustion, according to what has been already said in the First Part of
this Work, is the decomposition of oxygen gas produced by a combustible
body. The oxygen which forms the base of this gas is absorbed by, and
enters into, combination with the burning body, while the caloric and
light are set free. Every combustion, therefore, necessarily supposes
oxygenation; whereas, on the contrary, every oxygenation does not
necessarily imply concomitant combustion; because combustion, properly
so called, cannot take place without disengagement of caloric and light.
Before combustion can take place, it is necessary that the base of
oxygen gas should have greater affinity to the combustible body than it
has to caloric; and this elective attraction, to use Bergman's
expression, can only take place at a certain degree of temperature,
which is different for each combustible substance; hence the necessity
of giving a first motion or beginning to every combustion by the
approach of a