dently, had reached the conclusion, about 1837, that
the micro-organisms play a vastly more important role in the economy
of nature than any one previously had supposed. They held, for example,
that the minute specks which largely make up the substance of yeast are
living vegetable organisms, and that the growth of these organisms is
the cause of the important and familiar process of fermentation. They
even came to hold, at least tentatively, the opinion that the somewhat
similar micro-organisms to be found in all putrefying matter, animal or
vegetable, had a causal relation to the process of putrefaction.

This view, particularly as to the nature of putrefaction, was expressed
even more outspokenly a little later by the French botanist Turpin.
Views so supported naturally gained a following; it was equally natural
that so radical an innovation should be antagonized. In this case it
chanced that one of the most dominating scientific minds of the time,
that of Liebig, took a firm and aggressive stand against the new
doctrine. In 1839 he promulgated his famous doctrine of fermentation,
in which he stood out firmly against any "vitalistic" explanation of the
phenomena, alleging that the presence of micro-organisms in fermenting
and putrefying substances was merely incidental, and in no sense causal.
This opinion of the great German chemist was in a measure substantiated
by experiments of his compatriot Helmholtz, whose earlier experiments
confirmed, but later ones contradicted, the observations of Schwann, and
this combined authority gave the vitalistic conception a blow from which
it had not rallied at the time when Pasteur entered the field. Indeed,
it was currently regarded as settled that the early students of the
subject had vastly over-estimated the importance of micro-organisms.

And so it came as a new revelation to the generality of scientists
of the time, when, in 1857 and the succeeding half-decade, Pasteur
published the results of his researches, in which the question had been
put to a series of altogether new tests, and brought to unequivocal
demonstration.

He proved that the micro-organisms do all that his most imaginative
predecessors had suspected, and more. Without them, he proved, there
would be no fermentation, no putrefaction--no decay of any tissues,
except by the slow process of oxidation. It is the microscopic
yeast-plant which, by seizing on certain atoms of the molecule,
liberates the remaining