h the fact that certain
substances, while differing widely, from one another in many respects,
were alike in combustibility. From this he argued that all combustible
substances must contain a common principle, and this principle he named
phlogiston. This phlogiston he believed to be intimately associated in
combination with other substances in nature, and in that condition not
perceivable by the senses; but it was supposed to escape as a substance
burned, and become apparent to the senses as fire or flame. In other
words, phlogiston was something imprisoned in a combustible structure
(itself forming part of the structure), and only liberated when this
structure was destroyed. Fire, or flame, was FREE phlogiston, while the
imprisoned phlogiston was called COMBINED PHLOGISTON, or combined fire.
The peculiar quality of this strange substance was that it disliked
freedom and was always striving to conceal itself in some combustible
substance. Boyle's tentative suggestion that heat was simply motion was
apparently not accepted by Stahl, or perhaps it was unknown to him.

According to the phlogistic theory, the part remaining after a substance
was burned was simply the original substance deprived of phlogiston. To
restore the original combustible substance, it was necessary to heat the
residue of the combustion with something that burned easily, so that the
freed phlogiston might again combine with the ashes. This was explained
by the supposition that the more combustible a substance was the more
phlogiston it contained, and since free phlogiston sought always to
combine with some suitable substance, it was only necessary to mix the
phlogisticating agents, such as charcoal, phosphorus, oils, fats, etc.,
with the ashes of the original substance, and heat the mixture, the
phlogiston thus freed uniting at once with the ashes. This theory fitted
very nicely as applied to the calcined lead revivified by the grains of
wheat, although with some other products of calcination it did not seem
to apply at all.

It will be seen from this that the phlogistic theory was a step towards
chemistry and away from alchemy. It led away from the idea of a "spirit"
in metals that could not be seen, felt, or appreciated by any of the
senses, and substituted for it a principle which, although a falsely
conceived one, was still much more tangible than the "spirit," since it
could be seen and felt as free phlogiston and weighed and measured as
combine