agent of acetification in the German vinegar works;
"for," he said, "on a wood shaving which had been
used for 25 years in a large vinegar factory in Munich,
there was no trace of mycoderma visible, even under the
microscope."

In the presence of these denials, Pasteur had recourse
anew to the tactics which had proved so successful with
Pouchet, Joly and Musset. He demanded that Liebig
present himself, in company with him, before a com-
mission of the Academy of Sciences, which should be
charged with the duty of pronouncing between them, and
in the presence of which Pasteur offered in the first place
to prepare, in an exclusively mineral medium, as much
yeast of beer as Liebig could reasonably demand;
in the second place he promised to show to the com-
mission, and to Liebig himself, the acetifying mycoderma
on all the beech shavings of the factory in Munich.

The challenge was urgent. Pasteur would not have
been in position to give it at the time of his studies in
1860 on alcoholic fermentation. His cultures of yeast
in a mineral medium were at that time too poor and too
uncertain, but since he had begun his studies on beer,
to which we shall soon refer, and had found yeasts ac-
commodating themselves to these mediocre culture con-
ditions, he was sure of his facts. Liebig did not accept
the challenge. He only remained a little melancholy.

I have as proof of this a letter in which he states the
somewhat fallacious idea, that by going into the subject
thoroughly enough, Pasteur and he would have ended by
discovering and understanding each other. "I have
often thought," he wrote me in 1872, "in my long prac-
tical career and at my age (69 years), how much pains
and how many researches are necessary to probe to the
depths a rather complicated phenomenon. The greatest
difficulty comes from the fact that we are too much ac-

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