converted into two other things； one of them； matter which is called
succinic acid； and the other matter which is called glycerine； which
you all know now as one of the commonest of household matters。 It may
be that we have not got to the end of this refined analysis yet； but at
any rate； I suppose I may sayand I speak with some little hesitation
for fear my friend Professor Roscoe here may pick me up for trespassing
upon his provincebut I believe I may say that now we can account for
99 per cent。 at least of the sugar； and that 99 per cent。 is split up
into these four things； carbonic acid； alcohol； succinic acid； and
glycerine。 So that it may be that none of the sugar whatever
disappears； and that only its parts； so to speak； are re…arranged； and
if any of it disappears； certainly it is a very small portion。
Now these are the facts of the case。 There is the fact of the growth of
the yeast plant； and there is the fact of the splitting up of the
sugar。 What relation have these two facts to one another？
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
vesselsone 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
another great problem opened up； for； as I said at starting； you have；
under ordinary circumstances in warm weather； merely to expose some
fluid containing a solution of sugar； or any form of syrup or vegetable
juice to the air； in order； after a comparatively short time； to see
all these phenomena of fermentation。 Of course the first obvious
suggestion is； that the torula has been generated within the fluid。 In
fact； it seems at first quite absurd to entertain any other conviction；
but that belief would most assuredly be an erroneous one。
Towards the beginning of this century； in the vigorous times of the old
French wars； there was a Monsieur Appert； who had his attention
directed to the preservation of things that ordinarily perish； such as
meats and vegetables； and in fact he laid the foundation of our modern
method of preserving meats； and he found that if he boiled any of these
substances and then tied them so as to exclude the air； that they would
be preserved for any time。 He tried these experiments； particularly
with the must of wine and with the wort of beer； and he found that if
the wort of beer had been carefully boiled and was stopped in such a way
that the air could not get at it； it would never ferment。 What was the
reason of this？ That； again； became the subject of a long string of
experiments； with this ultimate result； that if you take precautions to
prevent any solid matters from getting into the must of wine or the wort
of beer； under these circumstancesthat is to say； if the fluid has
been boiled and placed in a bottle； and if you stuff the neck of the
bottle full of cotton wool； which allows the air to go through and
stops anything of a solid character however fine； then you may let it
be for ten years and it will not ferment。 But if you take that plug
out and give the air free access； then； sooner or later fermentation
will set up。 And there is no doubt whatever that fermentation is
excited only by the presence of some torula or other； and that that
torula proceeds in our present experience； from pre…existing torulae。
These little bodies are excessively light。 You can easily imagine what
must be the weight of little particles； but slightly heavier than water；
and not more than the two…thousandth or perhaps seven…thousandth of an
inch in diameter。 They are capable of floating about and dancing like
motes in the sunbeam； they are carried about by all sorts of currents
of air； the great majority of them perish； but one or two； which may
chance to enter into a sugary solution； immediately enter into active
life； find there the conditions of their nourishment； increase and
multiply； and may give rise to any quantity whatever of this substance
yeast。 And； whatever may be true or not be true about this
〃spontaneous generation；〃 as it is called in regard to all other kinds
of living things； it is perfectly certain； as regards yeast； that it
always owes its origin to this process of transportation or inoculation；
if you like so to call it； from some other living yeast organism； and
so far as yeast is concerned； the doctrine of spontaneous generation is
absolutely out of court。 And not only so； but the yeast must be alive
in order to exert these peculiar properties。 If it be crushed； if it be
heated so far that its life is destroyed； that peculiar power of
fermentation is not excited。 Thus we have come to this conclusion； as
the result of our inquiry； that the fermentation of sugar； the
splitting of the sugar into alcohol and carbonic acid； glycerine； and
succinic acid； is the result of nothing but the vital activity of this
little fungus； the torula。
And now comes the further exceedingly difficult inquiryhow is it that
this plant； the torula； produces this singular operation of the
splitting up of the sugar？ Fabroni； to whom I referred some time ago；
imagined that the effervescence of fermentation was produced in just the
same way as the effervescence of a sedlitz powder； that the yeast was a
kind of acid； and that the sugar was a combination of carbonic acid and
some base to form the alcohol； and that the yeast combined with this
substance； and set free the carbonic acid； just as when you add
carbonate of soda to acid you turn out the carbonic acid。 But of course
the discovery of Lavoisier that the carbonic acid and the alcohol taken
together are very nearly equal in weight to the sugar； completely upset
this hypothesis。 Another view was therefore taken by the French
chemist； Thenard； and it is still held by a very eminent chemist； M。
Pasteur； and their view is this； that the yeast； so to speak； eats a
little of the sugar； turns a little of it to its own purposes； and by
so doing gives such a shape to the sugar that the rest of it breaks up
into carbonic acid and alcohol。
Well； then； there is a third hypothesis； which is maintained by another
very distinguished chemist； Liebig； which denies either of the other
two； and which declares that the particles of the sugar are； as it
were； shaken asunder by the forces at work in the yeast plant。 Now I
am not going to take you into these refinements of chemical theory； I
cannot for a moment pretend to do so； but I may put the case before you
by an analogy。 Suppose you compare the sugar to a card house； and
suppose you compare the yeast to a child coming near the card house；
then Fabroni's hypothesis was that the child took half the cards away；
Thenard's and Pasteur's hypothesis is that the child pulls out the
bottom card and thus makes it tumble to pieces； and Liebig's hypothesis
is that the child comes by and shakes the table and tumbles the house
down。 I appeal to my friend here （Professor Roscoe） whether that is not
a fair statement of the case。
Having thus； as far as I can； discussed the general state of the
question； it remains only that I should speak of some of those
collateral results which have come in a very remarkable way out of the
investigation of yeast。 I told you that it was very early observed that
the yeast plant consisted of a bag made up of the same material as that
which composes wood； and of an interior semifluid mass which contains a
substance； identical in its composition； in a broad s