Infomotions, Inc.Yeast / Huxley, Thomas Henry, 1825-1895



Author: Huxley, Thomas Henry, 1825-1895
Title: Yeast
Publisher: Project Gutenberg
Tag(s): yeast; torula; yeast plant; fermentation; sugar; carbonic acid; acid; substance; alcohol; carbonic; liquid; fluid; plant
Contributor(s): Cajander, Paavo, 1846-1913 [Translator]
Versions: original; local mirror; HTML (this file); printable
Services: find in a library; evaluate using concordance
Rights: GNU General Public License
Size: 8,503 words (really short) Grade range: 16-19 (graduate school) Readability score: 42 (average)
Identifier: etext2938
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Title:  Yeast

Author:  Thomas H. Huxley

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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
bubbles make their appearance in it, and a sort of dirty-looking
yellowish foam or scum collects at the surface; while at the same time,
by degrees, a similar kind of matter, which we call the "lees," sinks
to the bottom.

The quantity of this dirty-looking stuff, that we call the scum and the
lees, goes on increasing until it reaches a certain amount, and then it
stops; and by the time it stops, you find the liquid in which this
matter has been formed has become altered in its quality.  To begin
with it was a mere sweetish substance, having the flavour of whatever
might be the plant from which it was expressed, or having merely the
taste and the absence of smell of a solution of sugar; but by the time
that this change that I have been briefly describing to you is
accomplished the liquid has become completely altered, it has acquired a
peculiar smell, and, what is still more remarkable, it has gained the
property of intoxicating the person who drinks it.  Nothing can be more
innocent than a solution of sugar; nothing can be less innocent, if
taken in excess, as you all know, than those fermented matters which are
produced from sugar.  Well, again, if you notice that bubbling, or, as
it were, seething of the liquid, which has accompanied the whole of
this process, you will find that it is produced by the evolution of
little bubbles of air-like substance out of the liquid; and I dare say
you all know this air-like substance is not like common air; it is not
a substance which a man can breathe with impunity. You often hear of
accidents which take place in brewers' vats when men go in carelessly,
and get suffocated there without knowing that there was anything evil
awaiting them.  And if you tried the experiment with this liquid I am
telling of while it was fermenting, you would find that any small
animal let down into the vessel would be similarly stifled; and you
would discover that a light lowered down into it would go out.  Well,
then, lastly, if after this liquid has been thus altered you expose it
to that process which is called distillation; that is to say, if you
put it into a still, and collect the matters which are sent over, you
obtain, when you first heat it, a clear transparent liquid, which,
however, is something totally different from water; it is much lighter;
it has a strong smell, and it has an acrid taste; and it possesses the
same intoxicating power as the original liquid, but in a much more
intense degree.  If you put a light to it, it burns with a bright
flame, and it is that substance which we know as spirits of wine.

Now these facts which I have just put before you--all but the last--have
been known from extremely remote antiquity.  It is, I hope one of the
best evidences of the antiquity of the human race, that among the
earliest records of all kinds of men, you find a time recorded when
they got drunk.  We may hope that that must have been a very late period
in their history.  Not only have we the record of what happened to
Noah, but if we turn to the traditions of a different people, those
forefathers of ours who lived in the high lands of Northern India, we
find that they were not less addicted to intoxicating liquids; and I
have no doubt that the knowledge of this process extends far beyond the
limits of historically recorded time.  And it is a very curious thing
to observe that all the names we have of this process, and all that
belongs to it, are names that have their roots not in our present
language, but in those older languages which go back to the times at
which this country was peopled.  That word "fermentation" for example,
which is the title we apply to the whole process, is a Latin term; and a
term which is evidently based upon the fact of the effervescence of the
liquid.  Then the French, who are very fond of calling themselves a
Latin race, have a particular word for ferment, which is 'levure'. And,
in the same way, we have the word "leaven," those two words having
reference to the heaving up, or to the raising of the substance which
is fermented.  Now those are words which we get from what I may call
the Latin side of our parentage; but if we turn to the Saxon side,
there are a number of names connected with this process of fermentation.
For example, the Germans call fermentation--and the old Germans did
so--"gahren;" and they call anything which is used as a ferment by such
names, such as "gheist" and "geest," and finally in low German,
"yest";" and that word you know is the word our Saxon forefathers used,
and is almost the same as the word which is commonly employed in this
country to denote the common ferment of which I have been speaking.  So
they have another name, the word "hefe," which is derived from their
verb "heben," which signifies to raise up; and they have yet a third
name, which is also one common in this country (I do not know whether it
is common in Lancashire, but it is certainly very common in the Midland
countries), the word "barm," which is derived from a root which
signifies to raise or to bear up.  Barm is a something borne up; and
thus there is much more real relation than is commonly supposed by those
who make puns, between the beer which a man takes down his throat and
the bier upon which that process, if carried to excess, generally lands
him, for they are both derived from the root signifying bearing up; the
one thing is borne upon men's shoulders, and the other is the fermented
liquid which was borne up by the fermentation taking place in itself.

Again, I spoke of the produce of fermentation as "spirit of wine."  Now
what a very curious phrase that is, if you come to think of it.  The
old alchemists talked of the finest essence of anything as if it had
the same sort of relation to the thing itself as a man's spirit is
supposed to have to his body; and so they spoke of this fine essence of
the fermented liquid as being the spirit of the liquid.  Thus came
about that extraordinary ambiguity of language, in virtue of which you
apply precisely the same substantive name to the soul of man and to a
glass of gin! And then there is still yet one other most curious piece
of nomenclature connected with this matter, and that is the word
"alcohol" itself, which is now so familiar to everybody.  Alcohol
originally meant a very fine powder.  The women of the Arabs and other
Eastern people are in the habit of tinging their eyelashes with a very
fine black powder which is made of antimony, and they call that
"kohol;" and the "al" is simply the article put in front of it, so as to
say "the kohol."  And up to the 17th century in this country the word
alcohol was employed to signify any very fine powder; you find it in
Robert Boyle's works that he uses "alcohol" for a very fine subtle
powder.  But then this name of anything very fine and very subtle came
to be specially connected with the fine and subtle spirit obtained from
the fermentation of sugar; and I believe that the first person who
fairly fixed it as the proper name of what we now commonly call spirits
of wine, was the great French chemist Lavoisier, so comparatively recent
is the use of the word alcohol in this specialised sense.

So much by way of general introduction to the subject on which I have to
speak to-night. What I have hitherto stated is simply what we may call
common knowledge, which everybody may acquaint himself with.  And you
know that what we call scientific knowledge is not any kind of
conjuration, as people sometimes suppose, but it is simply the
application of the same principles of common sense that we apply to
common knowledge, carried out, if I may so speak, to knowledge which is
uncommon.  And all that we know now of this substance, yeast, and all
the very strange issues to which that knowledge has led us, have simply
come out of the inveterate habit, and a very fortunate habit for the
human race it is, which scientific men have of not being content until
they have routed out all the different chains and connections of
apparently simple phenomena, until they have taken them to pieces and
understood the conditions upon which they depend.  I will try to point
out to you now what has happened in consequence of endeavouring to
apply this process of "analysis," as we call it, this teazing out of an
apparently simple fact into all the little facts of which it is made up,
to the ascertained facts relating to the barm or the yeast; secondly,
what has come of the attempt to ascertain distinctly what is the nature
of the products which are produced by fermentation; then what has come
of the attempt to understand the relation between the yeast and the
products; and lastly, what very curious side issues if I may so call
them--have branched out in the course of this inquiry, which has now
occupied somewhere about two centuries.

The first thing was to make out precisely and clearly what was the
nature of this substance, this apparently mere scum and mud that we
call yeast.  And that was first commenced seriously by a wonderful old
Dutchman of the name of Leeuwenhoek, who lived some two hundred years
ago, and who was the first person to invent thoroughly trustworthy
microscopes of high powers.  Now, Leeuwenhoek went to work upon this
yeast mud, and by applying to it high powers of the microscope, he
discovered that it was no mere mud such as you might at first suppose,
but that it was a substance made up of an enormous multitude of minute
grains, each of which had just as definite a form as if it were a grain
of corn, although it was vastly smaller, the largest of these not being
more than the two-thousandth of an inch in diameter; while, as you
know, a grain of corn is a large thing, and the very smallest of these
particles were not more than the seven-thousandth of an inch in
diameter.  Leeuwenhoek saw that this muddy stuff was in reality a
liquid, in which there were floating this immense number of definitely
shaped particles, all aggregated in heaps and lumps and some of them
separate. That discovery remained, so to speak, dormant for fully a
century, and then the question was taken up by a French discoverer,
who, paying great attention and having the advantage of better
instruments than Leeuwenhoek had, watched these things and made the
astounding discovery that they were bodies which were constantly being
reproduced and growing; than when one of these rounded bodies was once
formed and had grown to its full size, it immediately began to give off
a little bud from one side, and then that bud grew out until it had
attained the full size of the first, and that, in this way, the yeast
particle was undergoing a process of multiplication by budding, just as
effectual and just as complete as the process of multiplication of a
plant by budding; and thus this Frenchman, Cagniard de la Tour, arrived
at the conclusion--very creditable to his sagacity, and which has been
confirmed by every observation and reasoning since--that this
apparently muddy refuse was neither more nor less than a mass of
plants, of minute living plants, growing and multiplying in the sugary
fluid in which the yeast is formed.  And from that time forth we have
known this substance which forms the scum and the lees as the yeast
plant; and it has received a scientific name--which I may use without
thinking of it, and which I will therefore give you--namely, "Torula."
Well, this was a capital discovery.  The next thing to do was to make
out how this torula was related to the other plants.  I won't weary you
with the whole course of investigation, but I may sum up its results,
and they are these--that the torula is a particular kind of a fungus, a
particular state rather, of a fungus or mould.  There are many moulds
which under certain conditions give rise to this torula condition, to a
substance which is not distinguishable from yeast, and which has the
same properties as yeast--that is to say, which is able to decompose
sugar in the curious way that we shall consider by-and-by.  So that the
yeast plant is a plant belonging to a group of the Fungi, multiplying
and growing and living in this very remarkable manner in the sugary
fluid which is, so to speak, the nidus or home of the yeast.

That, in a few words, is, as far as investigation--by the help of one's
eye and by the help of the microscope--has taken us.  But now there is
an observer whose methods of observation are more refined than those of
men who use their eye, even though it be aided by the microscope; a man
who sees indirectly further than we can see directly--that is, the
chemist; and the chemist took up this question, and his discovery was
not less remarkable than that of the microscopist.  The chemist
discovered that the yeast plant being composed of a sort of bag, like a
bladder, inside which is a peculiar soft, semifluid material--the
chemist found that this outer bladder has the same composition as the
substance of wood, that material which is called "cellulose," and which
consists of the elements carbon and hydrogen and oxygen, without any
nitrogen.  But then he also found (the first person to discover it was
an Italian chemist, named

Fabroni, in the end of the last century) that this inner matter which
was contained in the bag, which constitutes the yeast plant, was a
substance containing the elements carbon and hydrogen and oxygen and
nitrogen; that it was what Fabroni called a vegeto-animal substance,
and that it had the peculiarities of what are commonly called "animal
products."

This again was an exceedingly remarkable discovery.  It lay neglected
for a time, until it was subsequently taken up by the great chemists of
modern times, and they, with their delicate methods of analysis, have
finally decided that, in all essential respects, the substance which
forms the chief part of the contents of the yeast plant is identical
with the material which forms the chief part of our own muscles, which
forms the chief part of our own blood, which forms the chief part of
the white of the egg; that, in fact, although this little organism is a
plant, and nothing but a plant, yet that its active living contents
contain a substance which is called "protein," which is of the same
nature as the substance which forms the foundation of every animal
organism whatever.

Now we come next to the question of the analysis of the products, of
that which is produced during the process of fermentation.  So far back
as the beginning of the 16th century, in the times of transition
between the old alchemy and the modern chemistry, there was a
remarkable man, Von Helmont, a Dutchman, who saw the difference between
the air which comes out of a vat where something is fermenting and
common air.  He was the man who invented the term "gas," and he called
this kind of gas "gas silvestre"--so to speak gas that is wild, and
lives in out of the way places--having in his mind the identity of this
particular kind of air with that which is found in some caves and
cellars.  Then, the gradual process of investigation going on, it was
discovered that this substance, then called "fixed air," was a poisonous
gas, and it was finally identified with that kind of gas which is
obtained by burning charcoal in the air, which is called "carbonic
acid."  Then the substance alcohol was subjected to examination, and it
was found to be a combination of carbon, and hydrogen, and oxygen. Then
the sugar which was contained in the fermenting liquid was examined and
that was found to contain the three elements carbon, hydrogen, and
oxygen.  So that it was clear there were in sugar the fundamental
elements which are contained in the carbonic acid, and in the alcohol.
And then came that great chemist Lavoisier, and he examined into the
subject carefully, and possessed with that brilliant thought of his
which happens to be propounded exactly apropos to this matter of
fermentation--that no matter is ever lost, but that matter only changes
its form and changes its combinations--he endeavoured to make out what
became of the sugar which was subjected to fermentation.  He thought he
discovered that the whole weight of the sugar was represented by the
carbonic acid produced; that in other words, supposing this tumbler to
represent the sugar, that the action of fermentation was as it were the
splitting of it, the one half going away in the shape of carbonic acid,
and the other half going away in the shape of alcohol.  Subsequent
inquiry, careful research with the refinements of modern chemistry,
have been applied to this problem, and they have shown that Lavoisier
was not quite correct; that what he says is quite true for about 95 per
cent. of the sugar, but that the other 5 per cent., or nearly so, is
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 say--and I speak with some little hesitation
for fear my friend Professor Roscoe here may pick me up for trespassing
upon his province--but 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
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
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 circumstances--that 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 inquiry--how 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 sense, with that
which constitutes the flesh of animals.  Subsequently, after the
structure of the yeast plant had been carefully observed, it was
discovered that all plants, high and low, are made up of separate bags
or "cells," as they are called; these bags or cells having the
composition of the pure matter of wood; having the same composition,
broadly speaking, as the sac of the yeast plant, and having in their
interior a more or less fluid substance containing a matter of the same
nature as the protein substance of the yeast plant.  And therefore this
remarkable result came out--that however much a plant may differ from an
animal, yet that the essential constituent of the contents of these
various cells or sacs of which the plant is made up, the nitrogenous
protein matter, is the same in the animal as in the plant.  And not only
was this gradually discovered, but it was found that these semifluid
contents of the plant cell had, in many cases, a remarkable power of
contractility quite like that of the substance of animals. And about 24
or 25 years ago, namely, about the year 1846, to the best of my
recollection, a very eminent German botanist, Hugo Von Mohl, conferred
upon this substance which is found in the interior of the plant cell,
and which is identical with the matter found in the inside of the yeast
cell, and which again contains an animal substance similar to that of
which we ourselves are made up--he conferred upon this that title of
"protoplasm," which has brought other people a great deal of trouble
since!  I beg particularly to say that, because I find many people
suppose that I was the inventor of that term, whereas it has been in
existence for at least twenty-five years.  And then other observers,
taking the question up, came to this astonishing conclusion (working
from this basis of the yeast), that the differences between animals and
plants are not so much in the fundamental substances which compose them,
not in the protoplasm, but in the manner in which the cells of which
their bodies are built up have become modified.  There is a sense in
which it is true--and the analogy was pointed out very many years ago
by some French botanists and chemists--there is a sense in which it is
true that every plant is substantially an enormous aggregation of
bodies similar to yeast cells, each having to a certain extent its own
independent life.  And there is a sense in which it is also perfectly
true--although it would be impossible for me to give the statement to
you with proper qualifications and limitations on an occasion like
this--but there is also a sense in which it is true that every animal
body is made up of an aggregation of minute particles of protoplasm,
comparable each of them to the individual separate yeast plant.  And
those who are acquainted with the history of the wonderful revolution
which has been worked in our whole conception of these matters in the
last thirty years, will bear me out in saying that the first germ of
them, to a very great extent, was made to grow and fructify by the study
of the yeast plant, which presents us with living matter in almost its
simplest condition.

Then there is yet one last and most important bearing of this yeast
question.  There is one direction probably in which the effects of the
careful study of the nature of fermentation will yield results more
practically valuable to mankind than any other.  Let me recall to your
minds the fact which I stated at the beginning of this lecture. Suppose
that I had here a solution of pure sugar with a little mineral matter
in it; and suppose it were possible for me to take upon the point of a
needle one single, solitary yeast cell, measuring no more perhaps than
the three-thousandth of an inch in diameter--not bigger than one of
those little coloured specks of matter in my own blood at this moment,
the weight of which it would be difficult to express in the fraction of
a grain--and put it into this solution.  From that single one, if the
solution were kept at a fair temperature in a warm summer's day, there
would be generated, in the course of a week, enough torulae to form a
scum at the top and to form lees at the bottom, and to change the
perfectly tasteless and entirely harmless fluid, syrup, into a solution
impregnated with the poisonous gas carbonic acid, impregnated with the
poisonous substance alcohol; and that, in virtue of the changes worked
upon the sugar by the vital activity of these infinitesimally small
plants.  Now you see that this is a case of infection.  And from the
time that the phenomenon of fermentation were first carefully studied,
it has constantly been suggested to the minds of thoughtful physicians
that there was a something astoundingly similar between this phenomena
of the propagation of fermentation by infection and contagion, and the
phenomena of the propagation of diseases by infection and contagion.
Out of this suggestion has grown that remarkable theory of many
diseases which has been called the "germ theory of disease," the idea,
in fact, that we owe a great many diseases to particles having a
certain life of their own, and which are capable of being transmitted
from one living being to another, exactly as the yeast plant is capable
of being transmitted from one tumbler of saccharine substance to
another.  And that is a perfectly tenable hypothesis, one which in the
present state of medicine ought to be absolutely exhausted and shown not
to be true, until we take to others which have less analogy in their
favour.  And there are some diseases most assuredly in which it turns
out to be perfectly correct.  There are some forms of what are called
malignant carbuncle which have been shown to be actually effected by a
sort of fermentation, if I may use the phrase, by a sort of disturbance
and destruction of the fluids of the animal body, set up by minute
organisms which are the cause of this destruction and of this
disturbance; and only recently the study of the phenomena which
accompany vaccination has thrown an immense light in this direction,
tending to show by experiments of the same general character as that to
which I referred as performed by Helmholz, that there is a most
astonishing analogy between the contagion of that healing disease and
the contagion of destructive diseases.  For it has been made out quite
clearly, by investigations carried on in France and in this country,
that the only part of the vaccine matter which is contagious, which is
capable of carrying on its influence in the organism of the child who is
vaccinated, is the solid particles and not the fluid.  By experiments
of the most ingenious kind, the solid parts have been separated from
the fluid parts, and it has then been discovered that you may vaccinate
a child as much as you like with the fluid parts, but no effect takes
place, though an excessively small portion of the solid particles, the
most minute that can be separated, is amply sufficient to give rise to
all the phenomena of the cow pock, by a process which we can compare to
nothing but the transmission of fermentation from one vessel into
another, by the transport to the one of the torula particles which
exist in the other.  And it has been shown to be true of some of the
most destructive diseases which infect animals, such diseases as the
sheep pox, such diseases as that most terrible and destructive disorder
of horses, glanders, that in these, also, the active power is the
living solid particle, and that the inert part is the fluid. However,
do not suppose that I am pushing the analogy too far.  I do not mean to
say that the active, solid parts in these diseased matters are of the
same nature as living yeast plants; but, so far as it goes, there is a
most surprising analogy between the two; and the value of the analogy
is this, that by following it out we may some time or other come to
understand how these diseases are propagated, just as we understand,
now, about fermentation; and that, in this way, some of the greatest
scourges which afflict the human race may be, if not prevented, at
least largely alleviated.

This is the conclusion of the statements which I wished to put before
you.  You see we have not been able to have any accessories.  If you
will come in such numbers to hear a lecture of this kind, all I can say
is, that diagrams cannot be made big enough for you, and that it is not
possible to show any experiments illustrative of a lecture on such a
subject as I have to deal with.  Of course my friends the chemists and
physicists are very much better off, because they can not only show you
experiments, but you can smell them and hear them!  But in my case such
aids are not attainable, and therefore I have taken a simple subject and
have dealt with it in such a way that I hope you all understand it, at
least so far as I have been able to put it before you in words; and
having once apprehended such of the ideas and simple facts of the case
as it was possible to put before you, you can see for yourselves the
great and wonderful issues of such an apparently homely subject.





End of The Project Gutenberg Etext of Yeast, by Thomas H. Huxley


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