A History of Science, vol 4 | Page 5

them."[1]
Although this explanation of the composition of the air is most crude, it
had the effect of directing attention to the fact that the atmosphere is
not "mere nothingness," but a "something" with a definite composition,
and this served as a good foundation for future investigations. To be
sure, Boyle was neither the first nor the only chemist who had
suspected that the air was a mixture of gases, and not a simple one, and

that only certain of these gases take part in the process of calcination.
Jean Rey, a French physician, and John Mayow, an Englishman, had
preformed experiments which showed conclusively that the air was not
a simple substance; but Boyle's work was better known, and in its
effect probably more important. But with all Boyle's explanations of
the composition of air, he still believed that there was an inexplicable
something, a "vital substance," which he was unable to fathom, and
which later became the basis of Stahl's phlogiston theory. Commenting
on this mysterious substance, Boyle says: "The, difficulty we find in
keeping flame and fire alive, though but for a little time, without air,
renders it suspicious that there be dispersed through the rest of the
atmosphere some odd substance, either of a solar, astral, or other
foreign nature; on account of which the air is so necessary to the
substance of flame!" It was this idea that attracted the attention of
George Ernst Stahl (1660-1734), a professor of medicine in the
University of Halle, who later founded his new theory upon it. 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 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
combined phlogiston. The definiteness of the statement that a metal, for
example, was composed of phlogiston and an element was much less
enigmatic, even if wrong, than the statement of the alchemist that
"metals are produced by the spiritual action of the three principles, salt,
mercury, sulphur"--particularly when it is explained that salt, mercury,
and sulphur were really not what their names implied, and that there
was no universally accepted belief as to what they really were.
The metals, which are now regarded as elementary bodies, were
considered compounds by the phlogistians, and they believed that the
calcining of a metal was a process of simplification. They noted,
however, that the remains of calcination weighed