YEAST.

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 tingeing 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 Yeast.