as been admitted that the agent by
which this surprising rearrangement of the particles of the sugar is
effected is the yeast. But the first thoroughly conclusive evidence of
the necessity of yeast for the fermentation of sugar was furnished by
Appert, whose method of preserving perishable articles of food excited so
much attention in France at the beginning of this century. Gay-Lussac, in
his "Memoire sur la Fermentation,"[2] alludes to Appert's method of
preserving beer-wort unfermented for an indefinite 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 2: _Annales de Chimie_, 1810.]

On the other hand, Schwann, Schroeder and Dutch, 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:--

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

"Verum talis mihi de horum origine et formatione conceptus formabam;
globulis nempe ex quibus farina Tritici, Hordei, Avenae, Fagotritici, se
constat aquae calore dissolvi et aquae commisceri; hac, vero aqua, quam
cerevisiam vocare licet, refrigescente, multos ex minimis particulis in
cerevisia coadunari, et hoc pacto efficere particulam sive globulum, quae
sexta pars est globuli faecis, et iterum sex ex hisce globulis