more air may be got by a sudden heat than
by a slow one, though the heat that is last applied be as intense as
that which was applied suddenly. A bit of dry oak, weighing about twelve
grains, will generally yield about a sheep's bladder full of inflammable
air with a brisk heat, when it will only give about two or three ounce
measures, if the same heat be applied to it very gradually. To what this
difference is owing, I cannot tell. Perhaps the phlogiston being
extricated more slowly may not be intirely expelled, but form another
kind of union with its base; so that charcoal made with a heat slowly
applied shall contain more phlogiston than that which is made with a
sudden heat. It may be worth while to examine the properties of the
charcoal with this view.

Inflammable air, when it is made by a quick process, has a very strong
and offensive smell, from whatever substance it be generated; but this
smell is of three different kinds, according as the air is extracted
from mineral, vegetable, or animal substances. The last is exceedingly
fetid; and it makes no difference, whether it be extracted from a bone,
or even an old and dry tooth, from soft muscular flesh; or any other
part of the animal. The burning of any substance occasions the same
smell: for the gross fume which arises from them, before they flame, is
the inflammable air they contain, which is expelled by heat, and then
readily ignited. The smell of inflammable air is the very same, as far
as I am able to perceive, from whatever substance of the same kingdom it
be extracted. Thus it makes no difference whether it be got from iron,
zinc, or tin, from any kind of wood, or, as was observed before, from
any part of an animal.

If a quantity of inflammable air be contained in a glass vessel standing
in water, and have been generated very fast, it will smell even through
the water, and this water will also soon become covered with a thin
film, assuming all the different colours. If the inflammable air have
been generated from iron, this matter will appear to be a red okre, or
the earth of iron, as I have found by collecting a considerable quantity
of it; and if it have been generated from zinc, it is a whitish
substance, which I suppose to be the calx of the metal. It likewise
settles to the bottom of the vessel, and when the water is stirred, it
has very much the appearance of wool. When water is once impregnated in
this manner, it will continue to yield this scum