YEAST

by Thomas H. Huxley




I HAVE selected to-night the particular subject of Yeast for two
reasons--or, rather, I should say for three. In the first place,
because it is one of the simplest and the most familiar objects with
which we are acquainted. In the second place, because the facts and
phenomena which I have to describe are so simple that it is possible to
put them before you without the help of any of those pictures or
diagrams which are needed when matters are more complicated, and which,
if I had to refer to them here, would involve the necessity of my
turning away from you now and then, and thereby increasing very largely
my difficulty (already sufficiently great) in making myself heard. And
thirdly, I have chosen this subject because I know of no familiar
substance forming part of our every-day knowledge and experience, the
examination of which, with a little care, tends to open up such very
considerable issues as does this substance--yeast.

In the first place, I should like to call your attention to a fact with
which the whole of you are, to begin with, perfectly acquainted, I mean
the fact that any liquid containing sugar, any liquid which is formed
by pressing out the succulent parts of the fruits of plants, or a
mixture of honey and water, if left to itself for a short time, begins
to undergo a peculiar change. No matter how clear it might be at
starting, yet after a few hours, or at most a few days, if the
temperature is high, this liquid begins to be turbid, and by-and-by