violent movement, which communicates to the liquid
the gas, or spirit, which is set free. Adjust these two
phenomena, end to end, generalize them, and you have
the definition, cited above, of Stahl and his predecessors.
If with Stahl it ended in assuming a more definite form,
it was because the atomic theories of Descartes had
penetrated into chemistry. Save for this addition from
without, which appeared rather in the way of stating
it than in the idea itself, the theory of Stahl says nothing
more than that of Lefèvre and Lémery, and other
chemists of the time. It has been said of this theory
that it was philosophical and seducing. A theory does
not need to be philosophical and seducing; it does not
even need to be true in the absolute sense of the word,
as we have shown; it suffices that it be fertile. But
the theory of Stahl was not fertile.

Progress in the field of fermentation came from without
and had for its origin new facts observed in the study of
gas by scientific men who were contemporaries of Stahl.
Moitrel d'Elément ( 1719) learned to make gases visible
by passing them through water; Hales ( 1677-1761)
showed how to manipulate them; Black ( 1728-1799),
how to distinguish them one from another. He isolated
especially carbonic acid, learned to know its properties,
and discovered, something which Van Helmont had not
been able to do, that, aside from alcohol, it is the sole
product of the transformation of sugar in the alcoholic
fermentation. He placed thus in the hands of the
chemists all the principal elements for the solution of
the problem; it remained only to coördinate these ele-
ments and to establish their mutual relations: this was
the work of Lavoisier.

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