Stahl's theory was a development of an earlier chemist, Johann
Joachim Becker (1635-1682), in whose footsteps he followed and
whose experiments he carried further.

In many experiments Stahl had been struck with 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