For a very long time that was a great matter of dispute. The early
French observers, to do them justice, discerned the real state of the
case, namely, that there was a very close connection between the actual
life of the yeast plant and this operation of the splitting up of the
sugar; and that one was in some way or other connected with the other.
All investigation subsequently has confirmed this original idea. It
has been shown that if you take any measures by which other plants of
like kind to the torula would be killed, and by which the yeast plant
is killed, then the yeast loses its efficiency. But a capital
experiment upon this subject was made by a very distinguished man,
Helmholz, who performed an experiment of this kind. He had two
vessels--one of them we will suppose full of yeast, but over the bottom
of it, as this might be, was tied a thin film of bladder; consequently,
through that thin film of bladder all the liquid parts of the yeast
would go, but the solid parts would be stopped behind; the torula would
be stopped, the liquid parts of the yeast would go. And then he took
another vessel containing a fermentable solution of sugar, and he put
one inside the other; and in this way you see the fluid parts of the
yeast were able to pass through with the utmost ease into the sugar, but
the solid parts could not get through at all. And he judged thus: if
the fluid parts are those which excite fermentation, then, inasmuch as
these are stopped, the sugar will not ferment; and the sugar did not
ferment, showing quite clearly, that an immediate contact with the
solid, living torula was absolutely necessary to excite this process of
splitting up of the sugar. This experiment was quite conclusive as to
this particular point, and has had very great fruits in other
directions.

Well, then, the yeast plant being essential to the production of
fermentation, where does the yeast plant come from? Here, again, was