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This complete opposition between artificial mineral products
and
vegetable and animal ones was to Pasteur a truth so well
established
that he found frequent opportunity of affirming it under
decisive
circumstances. One day, a very skilful chemist, M.
Dessaignes,
who later on became one of the correspondents of the Academy
of
Sciences, announced that he had transformed fumaric and malic
acids
into aspartic acid. Pasteur, who some time previously had
had
occasion to study these same acids, had proved that the two first
had
no molecular dissymmetry--that is to say, they exercised no
optic action. In
the state of solution they did not turn the plane
of polarised light.
Aspartic acid, on the contrary, had presented to
him molecular dissymmetry,
like asparagine itself. If the observation
of M. Dessaignes were true, then
bodies which were inert in regard
to polarised light, and consequently
non-dissymmetric, could be
transformed in the laboratory into active
dissymmetric bodies. The
line of demarcation so well established would be
broken. Pasteur,
whose experience regarding the note of Mitscherlich had
shown him how
even the most conscientious observers may fail to seize upon
fugitive
appearances, when unprompted to seek them by a preconceived
idea,
doubted at once the accuracy of the facts cited by M. Dessaignes.
From
Strasburg he started for Vendome, where M. Dessaignes at that
time
resided. M. Dessaignes immediately gave Pasteur a small quantity
of
the aspartic acid which he had prepared by means of fumaric and
malic
acids. Returning to his laboratory, Pasteur immediately
recognised
that, despite the very close resemblance of the new acid of
M.
Dessaignes to that derived from asparagine, the former differed
from
the latter by the complete absence in its case of molecular
dissymmetry.
With regard to other facts of the same kind, announced not
only in
France, but in Italy, and in England--chiefly the pretended
formation
of grape tartaric acid from succinic acid, artificial and
inert,
by Perkin and Duppa--Pasteur testified with absolute certainty
of
judgment to the existence of phenomenal peculiarities proper to
these
substances, which he had never seen, and which had, on the other
hand,
been the object of careful study by observers of great
talent.
After these verifications and deductions from theoretic views,
Pasteur
discovered a surprising connection between the prior researches
of
chemistry and crystallographic physics and the new and
entirely
unexpected results of physiological chemistry. This connection,
like
the thread of Ariadne, conducted him to his recent great
discoveries
in medical biology. M. Chevreul was right when, some years ago,
at the
Academy of Sciences, he expressed himself thus:--
'It is by
first examining in their chronological order the researches
of M. Pasteur,
and then considering them as a whole, that we are
enabled to appreciate the
rigour of judgment of that learned man in
forming his conclusions, and the
perspicacity of a mind which, strong
in the truths which it has already
discovered, is carried forward to
the establishment of new
ones.'
III.
Pasteur had thus
established that bodies endowed with internal
dissymmetry carried this
property, in varying degrees, into their
compounds or their derivatives. When
two of these bodies whose nature
has been revealed by the discovery of
right-handed and left-handed
tartaric acid, where all is chemically
identical--and which are only
to be distinguished from each other by their
inverse crystallographic
form, and by their action on polarised light--enter
into combination
with a substance which is optically and crystallographically
inert, the
chemical identity ought, under these new conditions, to be
preserved.
Everything remains optically and crystallographically
comparable.
The inert element adds nothing to, and takes away nothing from,
the
dissymmetric faculties of the active one.
To these curious studies
Pasteur soon added a new chapter. He reasoned
thus:--If into these compounds
I introduce a substance possessing in
itself the specific properties of
dissymmetry, it is evident that this
substance, while entering into these
combinations, must preserve its
own properties. The active substance would,
from the moment of its
combination, add something to the properties of the
molecular group
which acts like itself, and subtract something from the
properties
of the group which acts in the opposite manner. The resultant
effect
of these actions, sometimes concordant, sometimes antagonistic,
would
cease to be alike in absolute quantity. And if this be the
necessary
condition of similitude as to molecular arrangement, this
similitude
would cease to exist, and with its disappearance would appear all
the
differences of chemical and physical properties which constitute
its
outward manifestations.
The facts were found to harmonise with
these logical deductions. After
having made dissymmetry intervene as a
modifier of chemical affinity,
he had a strange and manifest proof of the
influence of dissymmetry in
the phenomena of life.
It had been long
known, through the observations of a manufacturer
of chemical products in
Germany, that the impure tartrate of lime of
commerce, if contaminated with
organic matters and permitted to remain
under water in summer, would ferment
and yield various products.
Pasteur caused the ordinary right-handed tartrate
of ammonia to
ferment in the following manner:--He took some very pure
crystalline
salt and dissolved it, adding at the same time to the liquid
some
albuminoid matter, about one gramme to 100 grammes of the
tartrate.
The liquid placed in a warm chamber fermented. During the process
of
fermentation the liquid mass, previously limpid, became
gradually
turbid, in consequence of the appearance of a small organism
which
played the part of ferment. Pasteur applied this mode of
fermentation
to the paratartrate of ammonia. He saw that this salt also
fermented,
depositing the same organism. All appeared as if the course of
things
was the same as in the case of the right-handed tartrate. But
Pasteur,
having had the idea of following the course of the operation with
the
aid of the polariscope, soon detected a profound difference
between
the two fermentations. In the case of the paratartrate, the
liquid,
at first inert, gradually assumed a sensible power of deviation
to
the left, which augmented by degrees and attained a maximum.
The
fermentation was then suspended; there was no longer any of
the
right-handed acid in the liquid, which, when evaporated and mixed
with
its own volume of alcohol, immediately furnished a beautiful
crystallisation
of left-handed tartrate of ammonia.
From that moment a great new fact was
established--namely, that the
molecular dissymmetry proper to organic matters
intervened in a
phenomenon of the physiological order, and did so as a
modifier of
chemical affinity. The kind of dissymmetry proper to the
molecular
arrangement of the left-handed tartaric acid was, no doubt, the
sole
cause of the difference between this acid and the right-handed acid,
in
regard to the fermentation produced by a microscopic fungus. We
shall
see later on that organised ferments are almost always
microscopic
vegetables, which embrace in their constitution cellulose,
albumen,
&c., identical with these same substances taken from the higher
class
of vegetables and equally dissymmetric. We can thus understand,
that
for the nutrition of the ferment and the formation of its
principles
the chemical changes are more easy with one of the two tartaric
acids
than with the other.
The opposition of the properties of the two
tartaric acids, right and
left, at the moment when the conditions of life and
nutrition of an
organised being intervened, showed themselves still more
strikingly
in a very curious experiment made by Pasteur. He was the first
to
prove that mildew could live and multiply on a purely mineral
soil,
composed, for example, of the phosphates of potash, of magnesia,
and
an ammoniacal salt of an organic acid. For such a development
of
vegetable life he employed the seed of _penicillium glaucum_, which
is
to be found everywhere as common mould, and to which he offered,
as its only
carbon aliment, paratartaric acid. At the end of a little
time the
left-handed tartaric acid appeared. Now this left-handed
acid could only show
itself on the condition that a rigorously equal
quantity of the right-handed
acid had been decomposed. The carbon of
the tartaric acid evidently supplied
to the little plant the carbon
that was necessary for the formation of its
constituents and all
their organic accessories. If the microscopic seed of
penicillium
sown upon this soil was not formed of dissymmetric elements, as
is
the case with all other vegetable substances, its development,
its
life, its fructification would accommodate themselves equally
well
with the left-handed tartaric acid as with the right. The fact
that
the left-handed tartaric acid is less assimilable than its
opposite
is due solely and evidently to the dissymmetry of one or other of
the
primordial substances of the little plant.
* * * * *
Thus for the first time was introduced
into physiological studies and
considerations the fact of the influence of
the molecular dissymmetry
of natural organic products.
Pasteur always
speaks with enthusiasm of the grand future reserved for
researches which have
this influence for their object; for molecular
dissymmetry is the only sharp
line of demarcation which exists between
the chemistry of inorganic and that
of organic nature.
FOOTNOTE:
[7] [M. Pasteur appears to use the
word _devenir_ as a substantive in a
sense equivalent to the German
_Werdende_.]
_FERMENTATION._
Arrived
at this unexpected turn in the road which he had hitherto
pursued, Pasteur
paused for an instant. Should he commit himself to the
course which abruptly
opened before him? His scientific instincts urged
him to do so, but the
prudence and reserve which show themselves to be
the basis of his character,
whenever he finds himself called upon to
make a choice of which the necessity
is not absolutely demonstrated,
held him back. Was it not wiser to continue
in the domain of molecular
physics and chemistry? M. Biot counselled his
doing so; the route had
been made plain, success awaited him at each step,
but an incident
connected with the University triumphed over his
hesitations.
He had just been nominated, at thirty-two years of age, Dean
of the
Faculte des Sciences at Lille. One of the principal industries of
the
Departement du Nord is the fabrication of alcohol from beetroot
and
from corn. Pasteur resolved to devote a portion of his lectures to
the
study of fermentation. He felt that if he could make himself
directly
useful to his hearers he would thereby excite general sympathy
with,
and direct attention to the new Faculte. The young man
congratulated
himself on this idea, and the man of science rejoiced in it
still more.
He was filled by the reflections suggested to him by the
strangeness of
the phenomena which he had just encountered in regard to the
molecular
dissymmetry of the two tartaric acids, in connection with the
life
of a microscopic organism. He saw new light thrown upon the
obscure
problem of fermentation. The part so active performed by an
infinitely
small organism could not, he thought, be an isolated fact. Behind
this
phenomenon must lie some great general
law.
I.
All that has lived
must die, and all that is dead must be
disintegrated, dissolved or gasified;
the elements which are the
substratum of life must enter into new cycles of
life. If things were
otherwise, the matter of organised beings would encumber
the surface of
the earth, and the law of the perpetuity of life would be
compromised
by the gradual exhaustion of its materials. One grand
phenomenon
presides over this vast work, the phenomenon of fermentation.
But
this is only a word, and it suggests to the mind simply the
internal
movements which all organised matter manifests spontaneously
after
death, without the intervention of the hand of man. What is,
then,
the cause of the processes of fermentation, of putrefaction, and
of
slow combustion? How is the disappearance of the dead body or of
the
fallen plant to be accounted for? What is the explanation of
the
foaming of the must in the vintage cask? of dough, which, abandoned
to
itself, rises and becomes sour? of milk, which curdles? of blood,
which
putrefies? of the heap of straw, which becomes manure? of dead
leaves
and plants embedded in the earth, which transform themselves into
soil?
Many different attempts were made to account for this mystery
before
science was in a condition to approach it. In our age, and at the
time
when Pasteur was led to the study of the question, one theory
held
almost undisputed sway. It was a very ancient theory, to which
Liebig,
in reviving it, had given the weight of his name. 'The ferments,'
said
Liebig, 'are all nitrogenous substances--albumen, fibrine,
caseine;
or the liquids which embrace them, milk, blood, urine--in a state
of
alteration which they undergo in contact with the air.'
The oxygen
of the air was, according to this system, the first cause of
the molecular
breaking up of the nitrogenous substances. The molecular
motions are
gradually communicated from particle to particle in the
interior of the
fermentable matter, which is thus resolved into new
products.
These
theoretic ideas regarding the part played in fermentation
by the oxygen of
the air were based upon experiments made in the
beginning of the century by
Gay-Lussac. In examining the process of
Appert for the preservation of animal
and vegetable substances--a
process which consisted in inclosing these
substances in hermetically
sealed vessels and heating them afterwards to a
sufficiently high
temperature--Gay-Lussac had seen, for example, the must of
the grape,
which had been preserved without alteration during a whole
year,
caused to enter into a state of fermentation by the simple fact of
its
transference to another vessel--that is to say, by having been
brought
for an instant into contact with the oxygen of the air. The oxygen
of
the air appeared, then, to be the _primum movens_ of
fermentation.
The illustrious chemists Berzelius and Mitscherlich
explained the
phenomena of fermentation otherwise. They placed these
phenomena in the
obscure class known as _phenomena of contact_. The ferment,
in their
view, took nothing from, and added nothing to, the fermentable
matter.
It was an albuminoid substance, endowed with a force to which the
name
_catalytic_ was given. The ferment in fact acted by its mere
presence.
A very curious observation, however, had been made in France
by
Cagniard-Latour and in Germany by Schwann. Cagniard-Latour,
however,
was the first to publish this observation, which was destined
to
become so fruitful. One of the ferments most in use, and known as
early
as the leavening of dough or the turning of milk, is the deposit
formed in
beer barrels, which is commonly called _yeast_. Repeating an
observation of
the naturalist Leuwenhoeck, Cagniard-Latour saw this
yeast, which was
composed of cells, multiplying itself by budding, and
he proposed to himself
the question whether the fermentation of sugar
was not connected with this
act of cellular vegetation. But as in
other fermentations the existence of an
organism had not been observed
even by the most careful search, the
hypothesis of Cagniard-Latour of
a possible relation between the organisation
of the ferment and the
property of being a ferment was abandoned, though not
without regret
by some physiologists. M. Dumas, for example, recognised that
in the
budding of the yeast globules there must be some clue to the
phenomenon
of fermentation. I, however, repeat that as nothing of the kind
had
been found elsewhere, and as all other fermentations presented
the
common character of requiring, to put them in train, organic matter
in
a state of decomposition, the hypothesis of Cagniard-Latour remained
a
simple incident, instead of having the value of a scientific
principle.
Liebig, moreover, carrying general opinion along with him,
contended
that it is not because of its being organised that yeast is
active,
but because of its being in contact with air. It is the
dead
portion of the yeast--that which has lived and is in the course
of
alteration--which acts upon the sugar.
The new memoirs published on
the subject agreed in rejecting the
hypothesis of any influence whatever of
organisation or of life in the
process of fermentation. Books, memoirs,
dogmatic teaching, all were
favourable to the theoretic ideas of Liebig. If a
few rare observers
indicated the presence in certain fermentations of living
organisms,
this presence was, in their opinion, a purely accidental fact,
which,
instead of favouring the phenomenon of fermentation, was injurious
to
it.
From his first investigation on lactic fermentation Pasteur was
led to
take an entirely different view of the matter. In this fermentation
he
recognised the presence and the action of a living organism, which
was
the ferment, just as yeast was the ferment of alcoholic
fermentation.
The lactic ferment was formed of cells, or rather of little
rods nipped
at their centres, extremely small, being hardly the thousandth
part
of a millimeter in diameter.[8] It reproduced itself by
fission--that
is to say, the little rod divided itself at its middle and
formed two
shorter rods, which became elongated, nipped, in their turn, at
their
centres, each giving rise, as before, to two rods. Each of
these,
again, soon divided itself into two, and so on. Why had not
this
been observed prior to Pasteur? For the simple reason that
chemists
had never observed the production of lactic fermentation except
in
complex substances. They mixed chalk with their milk for the purpose
of
preserving the neutrality of the fermenting medium. They employed
substances
such as caseine, gluten, animal membranes, all of which,
when examined by the
microscope, exhibited a multitude of mineral or
organic granules, with which
the lactic ferment was confounded. Thus
the first care of Pasteur, with the
view of proving the presence of
the ferment and its life, was to replace the
cheesy matter and all its
congeners by a soluble, nitrogenous body, which
would permit of the
microscopic examination of all the living cellular
products.
In a memoir presented to the Academy of Sciences in 1857
Pasteur
stated that there were 'cases where it is possible to recognise
in
lactic fermentation, as practised by chemists and manufacturers,
above
the deposit of chalk and the nitrogenous matter, a grey
substance
which forms a zone on the surface of the deposit. Its
examination
by the microscope hardly permits of its being distinguished
from
the disintegrated caseum or gluten which has served to start
the
fermentation. So that nothing indicates that it is a special kind
of
matter which had its birth during the fermentation. It is this,
nevertheless,
which plays the principal part.'
To isolate this substance and to prepare
it in a state of purity,
Pasteur boiled a little yeast with from fifteen to
twenty times its
weight of water. He then carefully filtered the liquid,
dissolved in it
about fifty grammes of sugar to the litre, and added to it
some chalk.
Taking then, by means of a drawn-out tube, from a good ordinary
lactic
fermentation a trace of the grey matter of which we have just
spoken,
he placed it as the seed of the ferment in the limpid
saccharine
solution. By the next day a lively and regular fermentation had
set
in, the liquid becoming turbid and the chalk disappearing, and
one
could distinguish a deposit which progressed continually as the
chalk
dissolved. This deposit was the lactic ferment.
Pasteur
reproduced this experiment by substituting for the water of the
yeast a clear
decoction of nitrogenous plastic substances. The ferment
invariably presented
the same aspect and the same multiplication. These
results, however, did not
yet satisfy Pasteur. He desired more rigour
in a subject of such theoretic
importance. Might not the partisans of
Liebig's theory argue, if not without
subtlety yet with a semblance
of justice, that the fermentation was not due
to the formation and
progressive growth of this feeble nitrogenous globular
deposit, but
rather to the nitrogenous matter dissolved during the decoction
of the
yeast used in the composition of the liquor? Up to a certain
point
it might be maintained that the dissolved matters which had been
in
contact with the oxygen of the air had been thrown into
molecular
motion, that this motion had been communicated to the
fermentable
matter, and that the deposit of the pretended organised ferment
was but
an accident--one of the physical changes or one of the
precipitates
so frequently observed in the modifications of albuminoid
matters. In
the observation of Cagniard-Latour and of Schwann as to the life
of
the yeast, Liebig saw nothing more. 'One cannot deny,' said he,
'the
organisation of the yeast or its multiplication by budding, but
these
living cells are always associated with other dead cells in process
of
molecular alteration. It is these molecular motions which
communicate
themselves to the molecules of the sugar, break them up, and
cause them
to ferment.'
The arguments of Liebig derived great strength
from the belief
which was shared by all chemists that the cells of yeast
perish
during fermentation and form lactate of ammonia. On examining
this
assertion, Pasteur found that not only was there no ammonia
formed
during alcoholic fermentation, but that even if ammonia were added
it
disappeared, entering into the formation of new yeast cells. Was
not
this a proof of the potency of the organised ferment?
Tormented,
however, by the idea that, notwithstanding all these facts,
the reasonings of
Liebig might still find some credit, Pasteur worked
earnestly to discover new
facts capable of demonstrating that Liebig's
theory was absolutely false. He
made two crucial experiments, the one
relating to the yeast of beer, or of
alcohol, and the other relating
to the lactic ferment. He introduced into a
pure solution of sugar a
small quantity of crystallisable salt of ammonia,
then some phosphates
of potash and magnesia, and he sowed in this medium an
imponderable
quantity, if we may so express it, of fresh cells of yeast. The
cells
thus sown multiplied, and the sugar fermented. In other words,
the
phosphorus, the potassium, the magnesium of the mineral salts,
united
to form the substances which compose the ferment. By this
experiment,
so simple and yet so demonstrative, the power of the
organisation
of the ferment was once for all established. The contact theory
of
Berzelius had no longer any meaning, since it was evident that
the
fermentable matter here furnished to the ferment one of its
essential
elements, namely, carbon. Liebig's theory of communicated
molecular
motion, originating in a nitrogenous albuminoid substance, had
no
better claim, since such substances had been discarded. The
whole
process took place between the sugar and a ferment germ which owed
its
life and development to nutritive matters, the most important of
which
was the fermentable substance. Fermentation, in short, was simply
a
phenomenon of nutrition. The ferment augmented in weight, feeding
upon
the sugar, and its vitality was such that it contrived to build up
the
complex materials of its own organisation by means of sugar and
purely
mineral elements.
In a second experiment, Pasteur demonstrated
that, notwithstanding
their smallness and the possibility of confounding them
with the
amorphous granules of caseine and gluten, the little particles
of
lactic ferment were indeed alive, and that they, and they only,
were
the cause of lactic fermentation. He mixed with some water,
sweetened
with sugar, a small quantity of a salt of ammonia, some
alkaline
and earthy phosphates, and some pure carbonate of lime obtained
by
precipitation. At the end of twenty-four hours the liquid began to
get
turbid and to give off gas. The fermentation continued for some
days.
The ammonia disappeared, leaving a deposit of phosphates and
calcareous
salt. Some lactate of lime was formed, and at the same time one
could
notice the deposition of the little lactic ferment. The germs of
the
lactic ferment had, in this case, been derived from particles of
dust
adhering to the substances themselves, of which the mixtures were
made,
or to the vessels used, or from the surrounding air. The chapter
on
spontaneous generation will render this clear.
It suffices here to
state that the results of this second experiment
were absolutely conclusive,
and that the theories of contact force or
of communicated motion, which up to
that time had reigned in science,
were completely
overthrown.
II.
The light
shed by these experiments quickly extended its sphere; and
Pasteur lost no
time in discovering a new ferment, that of butyric
acid. Having shown the
absolute independence which exists between the
ferment of butyric acid and
the others, he found, contrary to the
general belief, that the lactic ferment
is incapable of giving rise
to butyric acid, and that there exists a butyric
fermentation having
its own special ferment. This ferment consists of a
species of vibrio.
Little transparent cylindrical rods, rounded at their
extremities,
isolated or united in chains of two or three, or sometimes even
more,
form these vibrios. They move by gliding, the body straight, or
bending
and undulating. They reproduce themselves by fission, and to this
mode
of generation their frequent arrangement in the form of a chain is
due.
Sometimes one of the little rods, with a train of others behind
it,
agitates itself in a lively manner as if to detach itself from
the
rest. Often, also, the little rod, after being broken off, holds
on
still to its chain by a mucous transparent thread.
These little
infusoriæ may be sown like the yeast of beer or the lactic
ferment. If the
medium in which they are sown is suitable for their
nourishment, they will
multiply to infinity; but the character most
essential to be observed is,
that they may be sown in a liquid which
contains only ammonia and
crystallisable substances, together with
the fermentable substances, sugar,
lactic acid, gum, &c. The butyric
fermentation manifests itself as these
little organisms multiply. Their
weight sensibly increases, though it is
always minute in comparison
with the quantity of butyric acid produced; this
is found to be the
case in all other fermentations.
This experiment no
doubt resembles those made with the alcoholic and
lactic ferments. But it is
distinguished from them by one circumstance
eminently worthy of attention.
The butyric ferment, by its motions
and by its mode of generation, furnishes
the irrefutable proof of its
organisation and of its life. This ferment,
moreover, presented to
Pasteur a new and unexpected peculiarity. The vibrios
live and multiply
without the smallest supply of air or of free oxygen. Not
only, indeed,
do they live without air, but the air destroys them and arrests
the
fermentation which they initiate. If a current of pure carbonic
acid
is made to pass into the liquid where they are multiplying, their
life
and reproduction do not appear to be at all affected by it. If, on
the
contrary, instead of the current of carbonic acid we employ one
of
atmospheric air for only one or two hours, the vibrios fall
without
movement to the bottom of the vessel, and the butyric
fermentation
which was dependent on their existence is immediately
arrested.
Pasteur designated this new class of organisms by the name
of
_anaerobies_; that is to say, beings which can live without air.
He
reserves the designation _aerobies_ for all the other
microscopic
beings which, like the larger animals, cannot live without free
oxygen.
'It matters little,' added Pasteur, 'whether the progress of
science
makes of this vibrio a plant or an animal; it is a living
organism,
endowed with motion, which is a ferment and which lives without
air.'
* * * * *
In
meditating upon these facts, and upon the general character of
fermentation,
Pasteur soon found himself in a position to approach more
nearly to the
essential nature of these mysterious phenomena. In what
way do microscopic
organisms provoke the phenomena of fermentation?
The organism eats, if
one may say so, one part of the fermentable
matter. But how does this
phenomenon of nutrition differ so much from
that of higher beings? In
general, for a given weight of nutritive
matter which the animal takes in, it
assimilates a quantity of the
same order. In fermentation, on the contrary,
the ferment, while
nourishing itself with fermentable matter, decomposes a
quantity
great in comparison to its own individual weight. Again, the
butyric
ferment lives without free oxygen. Is there not, said Pasteur, a
hidden
relation between the property of being a ferment and the faculty
of
living without free oxygen? Are not vibrios which imperatively
require
for their nutrition and multiplication the presence of oxygen gas
those
which will never have the properties of ferments?
Pasteur then
contrived a series of experiments with the view of placing
in parallelism
these two curious physiological facts: life without air
and the
characteristics of ferments.
We know how wine and beer are prepared. The
must of grapes and the
must of beer are placed in wooden vats, or in barrels
of greater or
less dimensions. Whether the fermentation proceeds from germs
taken
from the exterior surface of the grapes, or from a small quantity
of
ferment sown in the must under the form of yeast, as in the
fermentation of
beer, the life of the ferment, its multiplication,
the augmentation of its
weight, are so many vital actions which to a
certainty cannot borrow from the
free oxygen of the external air, or
from that originally dissolved in the
must, an appreciable quantity of
this gas. All the life of the cells of the
ferment which multiplies
itself indefinitely appears then to take place apart
from free oxygen
gas. In certain breweries in England the fermenting vats
have sometimes
a capacity of several thousands of hectolitres; and the
fermentation
liberates pure carbonic acid, a gas much heavier than
atmospheric
air, which rests on the surface of the liquid in the vat in a
layer
thick enough to protect the liquid underneath from any contact
with
the external air. All this liquid mass, then, is inclosed between
the
wooden sides of the vat and a deep layer of heavy gas which
contains
no trace of free oxygen. In this liquid, nevertheless, the life
of
the cells of the ferment and the production of all its constituents
go
on for several days with extraordinary activity. Here certainly
we have life
without air, and the ferment character expresses itself
in the enormous
difference between the weight of the ferment formed
and collected from the
vats under the name of yeast, at the end of
the operation, and the weight of
the sugar which has fermented,
transforming itself into alcohol, carbonic
acid, and various other
products.
Pasteur has studied experimentally
that which takes place when, without
otherwise changing the conditions of
these phenomena, the arrangement
is so modified as to permit the introduction
of the free oxygen of the
atmosphere. It sufficed for this purpose to provoke
a fermentation of
the must of beer, or the must of grapes, upon shallow glass
dishes
presenting a large surface, or in a flat-bottomed wooden trough
with
sides a few centimeters in height, instead of in deep vats as
before.
In these new conditions the fermentation manifests an activity
even
more extraordinary than it did in the deep vats. The life of
the
ferment is itself singularly enhanced, but the proportion of the
weight
of the decomposed sugar to that of the yeast formed is
absolutely
different in the two cases. While, for example, in the deep vats,
a
kilogram of ferment sometimes decomposes seventy, eighty, one
hundred,
or even one hundred and fifty kilograms of sugar, in the
shallow
troughs one kilogram of the ferment will be found to correspond to
only
five or six kilograms of decomposed sugar. These proportions
between
the weight of the sugar which ferments and the weight of the
ferment
produced, constitute the measure of what one might call the
ferment's
character--of that character which distinguishes its mode of life
from
that of all other existences, great or small, in which the weight
of
the organising matter and the assimilated alimentary matter are
about
equal. In other words, the more free oxygen the yeast ferment
consumes,
the less is its power as a ferment. Such is the case in the
shallow
troughs where the extended surface is exposed to the contact of
the
oxygen of the air. The more, on the contrary, the life of the
ferment
is carried on without the presence of free oxygen, the greater is
its
power of decomposing and of fermenting the saccharine matter. This
is
the case in deep casks. The intimate co-relation then between
life
without air and fermentation appears complete.
The unexpected
light which these facts threw upon the cause of the
phenomena of fermentation
made a forcible impression upon all thinking
minds. 'In these infinitely
small organisms,' M. Dumas said one day
to M. Pasteur before the Academy of
Sciences, 'you have discovered
a third kingdom--the kingdom to which those
organisms belong which,
with all the prerogatives of animal life, do not
require air for their
existence, and which find the heat that is necessary
for them in the
chemical decompositions which they set up around
them.'
The work of Pasteur, demonstrating that fermentation was
always
dependent on the life of a microscopic organism, continued
without
interruption. One of the most remarkable of his researches is
that
which relates to the fermentation of the tartrate of lime.
The
demonstration of life and of fermentation without free oxygen is
in
this paper carried to the utmost limits of experimental rigour
and
precision.
III.
But
there is still another class of chemical phenomena where the life
without air
of microscopic organisms is fully shown. Pasteur proved
that in the special
fermentation which bears the name of putrefaction
the _primum movens_ of the
putrefaction resides in microscopic vibrios
of absolutely the same order as
those which compose the butyric
ferment. The fermentation of sugar, of
mannite, of gums, of lactate
of lime, by the butyric vibrio, so closely
resembles the phenomena of
putrefaction, that one might call these
fermentations the putrefaction
of sugar and of the other products.
If
it has been thought right to call the fermentation of animal
matters
putrefaction, it is because at the moment of the decomposition
of
fibrine, of albumen, of blood, of gelatine, of the substance of
the
tendons, &c., the sulphur, and even the phosphorus, which enter
into
their composition give rise to putrid odours, due to the
evil-smelling
gases of sulphur and phosphorus.
The phenomena of
putrefaction being then simply fermentations,
differing only in regard to the
chemical composition of the fermenting
matters, Liebig naturally included
them in his general theory of
the decomposition of organic matters after
death. At a period long
antecedent to Pasteur's labours it had been
established that there
existed in putrefying matters fungi or microscopic
animalculæ, and the
idea had taken shape that these creatures might have an
influence in
the phenomena. The proofs were wanting, but the notion of a
possible
relation remained. We may read in his 'Lessons on Chemistry' with
what
disdain Liebig mentioned these hypothetical opinions.
'Those who
pretend to explain the putrefaction of animal substances
by the presence of
animalculæ,' he wrote, 'reason very much like a
child who would explain the
rapidity of the Rhine by attributing it
to the violent motions imparted to it
in the direction of Bingen
by the numerous wheels of the mills of Mayence. Is
it possible to
consider plants and animals as the causes of the destruction
of
other organisms when their own elements are condemned to undergo
the
same decompositions as the creatures which have preceded them?
If the fungus
is the cause of the destruction of the oak, if the
microscopic animalcula is
the cause of the putrefaction of the dead
elephant, I would ask in my turn
what is the cause which determines the
putrefaction of the fungus or of the
microscopic animalcula when life
is withdrawn from these two
organisms?'
Thirty-two years later, and after Pasteur had accumulated,
during more
than twenty years, proof upon proof that the theory of Liebig
would not
stand examination, a physician of Paris, M. Bouillaud, asked, with
the
insistent voice of a querulous octogenarian: 'Let M. Pasteur then
tell
us here, in presence of the Academie de Medecine, what are the
ferments
of the ferments.'
Before replying to this argument, which
Liebig and M. Bouillaud
believed to be irrefutable, Pasteur, wishing to mark
all the phases
of the phenomena, expounded in a short preamble the part
played by
atmospheric oxygen in the destruction of animal and vegetable
matters
after death. It is easy to understand, indeed, that fermentation
and
putrefaction only represent the first phase of the return to
the
atmosphere and to the soil of all that has lived. Fermentations
and
putrefactions give rise to substances which are still very
complex,
although they represent the products of decomposition of
fermentable
matters. When sugar ferments, a large proportion of it becomes
gas;
but alongside of the carbonic acid gas which is formed, and which
is,
indeed, a partial return of the sugar to the atmosphere, new
substances, such
as alcohol, succinic acid, glycerine, and materials
of yeast, are produced.
When the flesh of animals putrifies, certain
products of decomposition, also
very complex, are formed with the
vapour of water and the other gases of
putrefaction. Where, then, does
nature find the agents of destruction of
these secondary products?
The great fact of the destruction of animal and
vegetable matters
is accomplished by slow combustion, through the
appropriation of
atmospheric oxygen. Here, again, one must banish from
science the
preconceived views which assumed that the oxygen seized directly
on the
organic matter after death, and that this matter was consumed by
purely
chemical processes. It is life that presides over this work of
death.
If fermentation and putrefaction are principally the work
of
microscopic anaerobies, living without free oxygen, the slow
combustion
is found very largely, if not exclusively, to depend upon a class
of
infinitely small aerobies. It is these last which have the property
of
consuming the oxygen of the air. It is these lower organisms which
are the
powerful agents in the return to the atmosphere of all which
has lived.
Mildew, mould, bacteria, which we have already noticed,
monads, two thousand
of which would go to make up a millimeter,
all these microscopic organisms
are charged with the great work of
re-establishing the equilibrium of life by
giving back to it all that
it has formed.
To demonstrate the important
part played everywhere by these
microscopic organisms, Pasteur made two
experiments. He first
introduced into vessels air deprived of all dust. This
process we shall
have occasion to examine in all its details, in connection
with the
researches on spontaneous generation. In these vessels, containing
pure
air, were placed the water of yeast with sugar dissolved in it,
milk,
sawdust--all of which had been deprived by heat of the germs of
the
lower organisms. The vessels and their contents were then subjected
to
a temperature of twenty-five to thirty-five degrees Centigrade. In
a
series of parallel experiments, made under the same conditions and
at
the same temperature, Pasteur took no steps to prevent the
germination
of the little seeds of mould suspended in the air, or associated
with
the substances contained in the vessels, neither did he avoid
other
infinitely small germs of the class aerobies.
After some time
the air of all the vessels of the two series was
submitted to analysis, when,
behold, a very interesting fact! In the
vessels where life had been withdrawn
from the organic matters--that
is to say, where there were no germs--the air
still contained a
large proportion of oxygen. In the vessels, on the
contrary, where
the microscopic organisms had been allowed to develop, the
oxygen
was totally absent, having been replaced by carbonic acid gas.
And,
further, for this absorption and total consumption of the oxygen gas
a
few days had sufficed; while in the vessels without microscopic
life
there remained, after several years, a considerable quantity of
oxygen
in a free state, so weak is the proportion of oxygen that the
organic
matters consume directly and chemically when the infinitely
small
organisms are absent.
But can these microscopic organisms, after
having decomposed or burnt
up all these secondary products, be in their turn
decomposed?
How, cried M. Bouillaud, repeating his question, can they be
destroyed
or decomposed? How can their materials, which are of the same
order
as those of all the living creatures of the earth, be gasified
and
caused to return to the atmosphere? After having been charged with
the
transformation of others, whose business will it be to transform
them?
A ferment which has finished its work, replied Pasteur, and
which
for want of aliment cannot continue it, becomes in its turn
an
accumulation, so to speak, of dead organic matters. Such, for
example,
would be an accumulation of yeast exposed to the air. Leave
this
mass to itself in summer temperature, and you will see appear in
the
interior of the mass anaerobic vibrios and the putrefactions
associated
with their life when protected from contact with the air. At the
same
time, on the surface of the entire mass--that is to say, that
which
finds itself in immediate contact with the oxygen of the air--the
germs
of bacteria, the seeds of mould will grow, and, by fixing the
oxygen,
determine the slow combustions which gasify the mass. The
ferments
of ferments are simply ferments. As long as the aerobic ferments
of
the surface have at their disposal free oxygen, they will multiply
and
continue their work of destruction. The anaerobic vibrios perish
for want of
new matter to decompose, and they form, in their turn, a
mass of organic
matter which, by and by, becomes the prey of aerobies.
The portion of the
aerobies which has lived becomes the prey either
of new aerobies of different
species, or of individuals of their own
species, so that from putrefaction to
putrefaction, and from combustion
to combustion, the organic mass with which
we started finds itself
reduced to an assemblage of anaerobic and aerobic
germs--of those same
germs which were mixed up in the original primitive
organic substances.
Though a collection of germs becomes again in its
turn a collection
of organic matter, subject to the double action of the
phenomena of
putrefaction and of combustion, there need be no anxiety as to
their
ultimate destruction; in the final analysis they represent life
under
its eternal form, for life is the germ, and the germ is
life.
* * * * *
Thus in
the destruction of that which has lived, all reduces
itself to the
simultaneous action of these three great natural
phenomena--fermentation,
putrefaction, and slow combustion. A living
organism dies--animal, or plant,
or the remains of one or the other.
It is exposed to the contact of the air.
To the life which has quitted
it succeeds life under other forms. In the
superficial parts, which
the air can reach, the germs of the infinitely small
aerobies hatch
and multiply themselves. The carbon, the hydrogen, and the
nitrogen
of the organic matters are transformed by the oxygen of the air,
and
under the influence of the life of these aerobies, into carbonic
acid,
vapour of water, and ammonia gas. As long as organic matter and
air
are present, these combustions will continue. While these
superficial
combustions are going on, fermentation and putrefaction are
doing
their work in the interior of the mass by the developed germs of
the
anaerobies, which not only do not require oxygen for their life,
but
which oxygen actually kills. Little by little, at length, by this
work
of fermentation and slow combustion, the phenomenon is
accomplished.
Whether in the free atmosphere, or under the earth, which is
always
more or less impregnated with air, all animal and vegetable
matters
end by disappearing. To arrest these phenomena an extremely
low
temperature is required. It is thus that in the ice of the
Polar
regions antediluvian elephants have been found perfectly intact.
The
microscopic organisms could not live in so cold a temperature.
These
facts still further strengthen all the new ideas as to the
important
part performed by these infinitely small organisms, which are, in
fact,
the masters of the world. If we could suppress their work, which
is
always going on, the surface of the globe, encumbered with
organic
matters, would soon become uninhabitable.
FOOTNOTE:
[8]
[A millimeter is 1/25th of an
inch.]
_ACETIC
FERMENTATION._
THE MANUFACTURE OF
VINEGAR.
Soon afterwards Pasteur came upon a most curious
illustration of
the 'fixation' of atmospheric oxygen by a microscopic
organism--the
transformation of wine into vinegar. As its name indicates,
vinegar is
nothing else than wine turned sour. Everybody has remarked that
wine,
left to itself, in circumstances which occur daily, is
frequently
transformed into vinegar. This is noticed more particularly
when
bottles, having been uncorked, are left in a half-empty
condition.
Sometimes, however, wine turns sour even in corked bottles. In
this
case we may be sure that the bottles have been standing upright,
and
that corks more or less defective have permitted the air to
penetrate
into the wine. The presence of air, in fact, is indispensable to
the
chemical act of transforming wine into vinegar. How does this
air
intervene? And what is the little microscopic creature which,
in
conjunction with the air, becomes the agent of this
fermentation?
In a celebrated lecture given at Orleans at the request of
the
manufacturers of vinegar in that town, Pasteur, after having stated
the
two foregoing scientific questions, proceeded to examine the
difference
between wine and vinegar. What takes place in the fermentation of
the
juice of the grape which yields the wine? The sugar of this
juice
disappears, giving place to carbonic acid gas, which is exhaled
during
fermentation, and to alcohol, which remains in the fermented
liquid.
Formerly, chemists gave the name of 'spirit' to all volatile
matters
which could be collected from distillation. Now, when we distil
wine
and condense the vapour in a worm surrounded by cold water, we
collect
the spirit of wine at the extremity of the worm--this, when the
water
with which it is mixed during distillation is withdrawn from it,
we
designate by the name of alcohol. Vinegar contains no alcohol.
When
distilled it yields water and a spirit. But this spirit is acid, with
a
very pungent odour, and not inflammable like spirit of wine.
Separated
from the water which had accompanied it during the distillation,
this
spirit takes the name of acetic acid. This is the form in which it
is
used in smelling bottles--in those bottles of English salts the vapour of
which is so penetrating. |
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