time, by simply boiling the wort and closing the vessel in which the
boiling fluid is contained, in such a way as thoroughly to exclude
air; and he shows that, if a little yeast be introduced into such
wort, after it has cooled, the wort at once begins to ferment, even
though every precaution be taken to exclude air. And this statement
has since received full confirmation from Pasteur.

[Footnote 1: "Annales de Chimie," 1810.]

On the other hand, Schwann, Schroeder and Dusch, and Pasteur, have
amply proved that air may be allowed to have free access to beer-wort,
without exciting fermentation, if only efficient precautions are taken
to prevent the entry of particles of yeast along with the air.

Thus, the truth that the fermentation of a simple solution of sugar in
water depends upon the presence of yeast, rests upon an unassailable
foundation; and the inquiry into the exact nature of the substance
which possesses such a wonderful chemical influence becomes profoundly
interesting.

The first step towards the solution of this problem was made two
centuries ago by the patient and painstaking Dutch naturalist,
Leeuwenhoek, who in the year 1680 wrote thus:--

"Saepissimo examinavi fermentum cerevisiae, semperque hoc ex
globulis per materiam pellucidam fluitantibus, quam cerevisiam
esse censui, constare observavi: vidi etiam evidentissime,
unumquemque hujus fermenti globulum denuo ex sex distinctis
globullis constare, accurate eidem quantitate et formae, cui
globulis sanguinis nostri, respondentibus.