and as it was defended with energy and talent, it ended
by triumphing. Taught in all the books, accepted as
true in all the works published on fermentation, it had
become almost a dogma, that is to say, what in science is
the most difficult to overturn. We may attack facts by
showing that they are inexact, experiments by testing
their conclusions; but what can we do against a doctrine
to some extent philosophic, resting for the most part on
argumentation, an argumentation so voluminous that
one can demolish certain parts of it without weakening
the rest, and which is based on that half mystical con-
ception of an imparted movement?

This detailed exposition was necessary in order
to show the state of the question at the time Pasteur
approached it, and to understand the nature of the means
which he employed to solve it. We shall now be able
to go on more quickly: we have reached the level ground.

The point which I wish to make clear in the beginning
is this: if Pasteur immediately made decided progress in
his studies it is because he approached them with another
guiding idea than his contemporaries.

In his memoirs, especially in the introduction to his
Mémoire sur la fermentation lactique, ¹ it is easy to find
what guided him, but it must be a little more developed.
Its origin is an observation made during the study of the
rotary power. In many of the industrial fermentations,
we meet, as a secondary product, amyl alcohol, a sub-
stance endowed with rotary power and capable, further-