The great first step was the substitution of the one principle,
phlogiston, for the three principles, salt, sulphur, and mercury.
We have seen how the experiment of burning or calcining such a
metal as lead "destroyed" the lead as such, leaving an entirely
different substance in its place, and how the original metal
could be restored by the addition of wheat to the calcined
product. To the alchemist this was "mortification" and
"revivification" of the metal. For, as pointed out by
Paracelsus, "anything that could be killed by man could also be
revivified by him, although this was not possible to the things
killed by God." The burning of such substances as wood, wax,
oil, etc., was also looked upon as the same "killing" process,
and the fact that the alchemist was unable to revivify them was
regarded as simply the lack of skill on his part, and in no wise
affecting the theory itself.

But the iconoclastic spirit, if not the acceptance of all the
teachings, of the great Paracelsus had been gradually taking root
among the better class of alchemists, and about the middle of the
seventeenth century Robert Boyle (1626-1691) called attention to
the possibility of making a wrong deduction from the phenomenon
of the calcination of the metals, because of a very important
factor, the action of the air, which was generally overlooked.
And he urged his colleagues of the laboratories to give greater
heed to certain other phenomena that might pass unnoticed in the
ordinary calcinating process. In his work, The Sceptical Chemist,
he showed the reasons for doubting the threefold constitution of
matter; and in his General History of the Air advanced some novel
and carefully studied theories as to the composition of the
atmosphere. This was an important step, and although Boyle is not
directly responsible for the phlogiston theory, it is probable
that his experiments on the atmosphere influenced considerably
the real founders, Becker and Stahl.

Boyle gave very definitely his idea of how he thought air might
be composed. "I conjecture that the atmospherical air consists of
three different kinds of corpuscles," he says; "the first, those
numberless particles which, in the form of vapors or dry
exhalations, ascend from the earth, water, minerals, vegetables,
animals, etc.; in a word, whatever substances are elevated by the
celestial or subterraneal heat, and thence diffused into the
atmosphere. The second may be yet more subtle, and consist of
those exceedingly minute atoms, the magnetical effluvia of the
earth, with other innumerable particles sent out from the bodies
of the celestial luminaries, and causing, by their influence, the
idea of light in us. The third sort is its characteristic and
essential property, I mean permanently elastic parts. Various
hypotheses may be framed relating to the structure of these later
particles of the air. They might be resembled to the springs of
watches, coiled up and endeavoring to restore themselves; to
wool, which, being compressed, has an elastic force; to slender
wires of different substances, consistencies, lengths, and
thickness; in greater curls or less, near to, or remote from each
other, etc., yet all continuing springy, expansible, and
compressible. Lastly, they may also be compared to the thin
shavings of different kinds of wood, various in their lengths,
breadth, and thickness. And this, perhaps, will seem the most
eligible hypothesis, because it, in some measure, illustrates the
production of the elastic particles we are considering. For no
art or curious instruments are required to make these shavings
whose curls are in no wise uniform, but seemingly casual; and
what is more remarkable, bodies that before seemed unelastic, as
beams and blocks, will afford 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.