noted, however, that the remains of calcination weighed more than
the original product, and the natural inference from this would
be that the metal must have taken in some substance rather than
have given off anything. But the phlogistians had not learned
the all-important significance of weights, and their explanation
of variation in weight was either that such gain or loss was an
unimportant "accident" at best, or that phlogiston, being light,
tended to lighten any substance containing it, so that driving it
out of the metal by calcination naturally left the residue
heavier.

At first the phlogiston theory seemed to explain in an
indisputable way all the known chemical phenomena. Gradually,
however, as experiments multiplied, it became evident that the
plain theory as stated by Stahl and his followers failed to
explain satisfactorily certain laboratory reactions. To meet
these new conditions, certain modifications were introduced from
time to time, giving the theory a flexibility that would allow it
to cover all cases. But as the number of inexplicable experiments
continued to increase, and new modifications to the theory became
necessary, it was found that some of these modifications were
directly contradictory to others, and thus the simple theory
became too cumbersome from the number of its modifications. Its
supporters disagreed among themselves, first as to the
explanation of certain phenomena that did not seem to accord with
the phlogistic theory, and a little later as to the theory
itself. But as yet there was no satisfactory substitute for this
theory, which, even if unsatisfactory, seemed better than
anything that had gone before or could be suggested.