|
Lastly, to convince the most prejudiced minds, and to leave
no
contradiction standing, Pasteur showed one of these bulbs with
the
sinuous neck which he had prepared and preserved for months and
years.
The bulb was covered with dust. 'Let us,' said he, 'take up a
little
of this outside dust on a bit of glass, porcelain, or platinum,
and
introduce it into the liquid; the following day you will find that
the
infusion, which up to this time remained perfectly clear, has
become
turbid, and that it behaves in the same manner as other infusions
in
contact with ordinary air.'
If the bulb be tilted so as to cause a
little drop of the clear
infusion to reach the extremity of the bent part of
the neck where
the dust particles are arrested, and if this drop be then
allowed to
trickle back into the infusion, the result is the
same--turbidity
supervenes and the microscopic organisms are developed.
Finally, if one
of those bulbs which have stood the test of months and years
without
alteration be several times shaken violently, so that the external
air
shall rush into it, and if it be then placed once more in the
stove,
life will soon appear in it.
In 1860 the Academy of Sciences
had offered a prize, the conditions of
which were stated in the following
terms:
'To endeavour by well-contrived experiments to throw new light
upon the
question of spontaneous generation.' The Academy added that it
demanded
precise and rigorous experiments equally well studied on all
sides;
such experiments, in short, as should render it possible to deduce
from
them results free from all confusion due to the experiments
themselves.
Pasteur carried away the prize, and no one, it will be
acknowledged,
deserved it better than he. Nevertheless, to his eyes, the
subject was
still beset with difficulties. In the hot discussions to which
the
question of spontaneous generation gave rise, the partisans of
the
doctrine continually brought forward an objection based on an
opinion
already referred to, and first enunciated by Gay-Lussac. As
already
known to the reader, Gay-Lussac had arrived at the conclusion
that,
in Appert's process, one condition of the preservation of animal
and
vegetable substances consisted in the exclusion of oxygen.
Even
this proposition was soon improved upon, and it became a current
opinion in
science that the smallest bubble of oxygen or of air which
might come in
contact with a preserve would be sufficient to start
its decomposition. The
partisans of spontaneous generation--the
heterogenists--thenceforward threw
their objections to Pasteur into
this form:
'How can the germs of
microscopic organisms be so numerous that even
the smallest bubble of air
contains germs capable of developing
themselves in every organic infusion? If
such were the case the air
would be encumbered with organic germs.' M.
Pouchet said and wrote that
they would form a thick fog, as dense as
iron.
But Pasteur showed that the interpretation of Gay-Lussac's
experiment,
with respect to the possible alteration of preserves by a
small
quantity of oxygen gas, was quite erroneous. If, after a certain
time,
an Appert preserve contains no oxygen, this is simply because
the
oxygen has been gradually absorbed by the substances of the
preserve,
which are always more or less chemically oxidisable. But in
reality
it is easy to find oxygen in these preserves. Pasteur did not fail
to
perceive that the interpretation given to Gay-Lussac's experiment
was
wrong in another particular. He proved the fallacy of the
assumption
that the smallest quantity of air was always capable of
producing
microscopic organisms.
More thickly spread in towns than in
the country, the germs become
fewer in proportion as they recede from human
habitations. Mountains
have fewer than plains, and at a certain height they
are very rare.
Pasteur's experiments to prove these facts were extremely
simple. He
took a series of bulbs of about a quarter of a litre in capacity,
and,
after having half filled the bulbs with a putrescible liquid, he
drew
out the necks by means of the blowpipe, then he caused the liquid
to
boil for some minutes, and during the ebullition, while the
steam
issued from the tapering ends of the bulbs, he sealed them with
the
lamp. Thus prepared, the bulbs can be easily transported. As they
are
empty of air--that which they originally contained having been
driven
out with the steam--when the sealed end of a bulb is broken off,
the
air rushes into the tube, carrying with it all the germs which
this
air holds in suspension. If it is closed again immediately
afterwards
by a flame, and if the vessels are then left to themselves, it is
easy
to recognise those in which a change occurs. Now, Pasteur
established
that, in whatever place the operation might be carried on, a
certain
number of bulbs would escape alteration. They must not, however,
be
opened in a room after dusting the furniture or sweeping the floor,
for
in this case all the bulbs would become altered because of the
great
quantity of germs raised by the dusting and remaining suspended in
the
air.
Pasteur started for Arbois with a series of bulbs. Some he
opened in
the country far from all habitations; others he opened at the
foot
of the mountains which form the first range of the Jura; a series
of
twenty-four bulbs was opened upon Mount Poupet, at 850 meters above
the
level of the sea; and, lastly, twenty others were transported to
the
Montanvert, near the Mer de Glace, at an elevation of 2,000 meters.
He
afterwards brought his whole collection back to Paris, and in the
month of
November, 1860, deposited them on the table at the Academy
of
Sciences.
Of the twenty bulbs first opened in the country, eight
contained
organised productions. Of the twenty opened on the heights of the
Jura,
five only were altered, and of the twenty opened upon the
Montanvert
during a strong wind which blew from the glacier, one alone was
altered.
If a similar series of experiments were made in a balloon, it
would be
found that the air of the higher atmosphere is absolutely free
from
germs. Care would, however, be necessary to prevent the introduction
of
dust particles, which the rigging and the aeronauts themselves
might
carry with them.
But we have not yet related all. So far, all
these conclusive
experiments had been made only on organic liquids, very
putrescible
it is true, but which had all been subjected to boiling or even
to
temperatures higher than 100 degrees Centigrade. The partisans
of
spontaneous generation might then be justified in saying that if
the
precaution had been taken of putting into contact with pure air
natural
organic liquids in a state compatible with the operations of
animal
and vegetable life, the results would have been different. Under
such
conditions, life would have appeared spontaneously in the production
of
microscopic organisms. None of Pasteur's opponents had formulated
this
argument; but Pasteur himself, who had within him an adversary
always
present, always on the alert, prepared to yield only to
accumulated
proofs, saw this objection. He was not satisfied until he had
succeeded
in completely refuting it. Having by means of ingenious
experimental
arrangements deprived some air of all living germs, he placed
in
contact with this pure air the most putrescible liquids,
particularly
venous blood, arterial blood, and urine. He took these liquids
directly
from the veins, the arteries, and the bladders of animals in
full
health. No alteration was produced. In due time a chemical
absorption
of small quantities of oxygen took place, but neither fermentation
nor
putrefaction, nor the smallest development of bacteria, of vibrios,
or
of mould. After this, Pasteur was able legitimately to exclaim in
his
celebrated lecture at the Sorbonne:
'There is not one circumstance
known at the present day which
justifies the assertion that microscopic
organisms come into the world
without germs or without parents like
themselves. Those who maintain
the contrary have been the dupes of illusions
and of ill-conducted
experiments, tainted with errors which they know not how
either to
perceive or to avoid. Spontaneous generation is a
chimera.'
Pasteur was not alone in affirming this fixed conviction. With
the
authority of a judge delivering sentence in court, M.
Flourens,
permanent Secretary of the Academy of Sciences, pronounced these
words
before the whole Academy:
'As long as my opinion was not formed
I had nothing to say; now
it _is_ formed and I can speak. The experiments are
decisive. If
spontaneous generation be a fact, what is necessary for the
production
of animalculæ? Air and putrescible liquids. Now Pasteur puts
together
air and putrescible liquids and nothing is produced.
Spontaneous
generation, then, has no existence. Those who still doubt have
failed
to grasp the question.'
But some adversaries remained
incredulous. When Pasteur had announced
the result of his experiments, and
brought before the Academy his
series of bulbs, Pouchet and Joly declared
that if Pasteur had opened
his bulbs in the Jura and on the Mer de Glace,
they, on their part, had
been on the top of the Maladetta, and had proved
there the inexactitude
of Pasteur's results.
Pasteur asked to be
judged by the Academy. 'A commission alone,' said
he, 'will terminate the
debate.' The commission was named, and the
position on both sides was clearly
stated.
'I affirm,' said Pasteur, 'that everywhere it is possible to
take
from the midst of the atmosphere a certain quantity of air
which
contains neither egg nor spore, and which does not produce organisms
in
putrescible solutions.'
On his side, M. Joly wrote: 'If one alone
of your bulbs remains
unaltered we shall loyally acknowledge our defeat.'
Lastly, M.
Pouchet, as distinct and positive as M. Pasteur, said: 'I affirm
that
in whatever place I take a cubic decimeter of air, when this air
is
placed in contact with a fermentable liquid enclosed in a glass
vessel
hermetically sealed, the liquid will become filled with
living
organisms.'
This double declaration, which excited at that time
all the learned
world, took place in the month of January, 1864. Eager to
engage in the
combat, Pasteur waited impatiently for the order of the
Commission that
this experiment, which was to decide everything, should be
made. But M.
Pouchet begged for a postponement, desiring, he said, to wait
for the
warm season. Pasteur was astonished, but resigned himself to the
delay.
The Commission and the opponents met on June 15.
The Commission
announced, 'that, as the whole dispute turned upon one
simple fact, one
single experiment ought to be undertaken, which alone
would close the
discussion.'
The partisans of spontaneous generation wished nevertheless
to go
through the entire series of their experiments. In vain the
Commission
tried to persuade them that this would make the judgment as long
as the
discussion itself had been, that all bore upon one fact, and that
this
fact could be decided by a single experiment. The heterogenists
would
not listen to this. M. Pouchet and M. Joly withdrew from the
contest.
M. Jamin, an exact and authorised historian of these debates,
observed
that 'the heterogenists, however they may have covered their
retreat,
were thereby self-condemned. If they had been sure of the
fact--which
they were solemnly engaged to prove, under penalty of
acknowledging
themselves defeated--they would have hastened to demonstrate
it, for
it would have been the triumph of their doctrine. People do not
allow
themselves to be condemned by default except in causes in which
they
have no
confidence.'
_STUDIES ON
WINE._
Having thus solved the problem of spontaneous generation, a
problem
which was but a parenthesis forced upon his attention,
Pasteur
returned to fermentation. Guided by his studies on vinegar and
other
observations of detail, he undertook an inquiry into the diseases
of
wine. The explanations of the changes which wine was known to
undergo
rested only on hypothesis. From the time of Chaptal, who was
followed
by Liebig and Berzelius, all the world believed wine to be a
liquid
in which the various constituents react upon each other
mutually
and slowly. The wine was thought to be continually 'working.'
When
the fermentation of the grape is finished, equilibrium is not
quite
established between the diverse elements of the liquor. Time is
needed
for them to blend together. If this reciprocal action be not
regular,
the wine becomes bad. This was, in other words, the doctrine
of
spontaneity. Without support from carefully reasoned experiments,
these
explanations could not satisfy Pasteur, especially at a moment when
he
had just been proving that there was nothing spontaneous either in
the
phenomena of fermentation or in animal and vegetable
infusions.
Pasteur tried first of all to show that wine does not 'work'
as
much as it was supposed to do. Wine being a mixture of
different
substances, among which are acids and alcohol, particular ethers
are
no doubt formed in it in course of time, and similar reactions
perhaps
take place between the other constituents of the liquid. But if
the
exactitude of such facts cannot be denied, based as they are
upon
general laws, confirmed and extended by recent inquiries,
Pasteur
thought that a false application was made of them when they
were
employed to explain the maladies of wine, the changes which occur
in
it through age--in a word, the alterations, whether good or bad,
which
wines are subject to. The 'ageing' of wine soon appeared to him
to
consist essentially in the phenomena of oxidation, due to the oxygen
of
the air which dissolves and is diffused in the wine. He gave
manifest
proofs of this. I will only mention one of them. New wine
inclosed
in a glass vessel hermetically sealed keeps its freshness; it
does
not 'work,' it does not 'age.' Pasteur demonstrated besides, that
all
the processes of wine-making are explained by the double necessity
of
oxygenising the wine to a suitable degree, and of preventing
its
deterioration. In seeking for the actual causes of injurious
alterations,
Pasteur, always obedient to a preconceived idea, while
carefully controlling
it with the utmost rigour of the experimental
method, asked himself whether
the diseases of wine did not proceed
from organised ferments, from little
microscopic vegetations? In the
observed alterations, he thought, there must
be some influences at work
foreign to the normal composition of the
wine.
This hypothesis was verified. In his hands the injurious
modifications
suffered by wines were shown to be correlative with the
presence and
the multiplication of microscopic vegetations. Such growths
alter
the wine, either by subtracting from it what they need for
their
nourishment, or, and principally, by forming new products which are
the
effect of the multiplication of these parasites in the mass of the
wine.
Everyone knows what is meant by _acid wine_, _sharp wine_, _sour
wine_.
The former experiments of Pasteur had clearly shown that no
wine
can become acid, sharp--can, in a word, become vinegar--without
the
presence of a little microscopic fungus known by the name of
_mycoderma
aceti_. This little plant is the necessary agent in the
condensation
of the oxygen of the air, and its fixation on the alcohol of the
wine.
Chaptal, who published a volume on the art of wine-making, knew of
the
existence of these mycoderm flowers; but to his eyes they were
only
'elementary forms of vegetation,' which had no influence whatever
upon
the quality of the liquid. Besides the _mycoderma aceti_, which is
the
agent of acetification, there is another mycoderm called
_mycoderma
vini_. This one deposits nothing which is hurtful to the wine, and
its
flowers are developed by preference in new wines, still immature,
and
preserving the astringency of the first period of their
fabrication.
The requirements of the two sorts of flowers are such that
even
when the flower of vinegar is sown on the surface of a new wine,
no
development takes place. Conversely, the _mycoderma vini_ sown on
wines
that have grown old in casks or in bottles will refuse to multiply.
The
_mycoderma vini_ produces no alteration in the wine; it does not
turn
the wine acid. In proportion as the wine grows old the flower tends
to
disappear, the wine 'despoils' itself, to use a technical
expression;
physiologically speaking, the wine loses its aptitude to nourish
the
_mycoderma vini_, which, finding itself progressively deprived
of
appropriate nourishment, fades and withers. But it is then that
the
_mycoderma aceti_ appears, and multiplies with a facility so much
the
greater that it draws its first nourishment from the cells of
the
_mycoderma vini_. The _mycoderma aceti_ has played so large a part
in
the early pages of this book that it is not necessary to go back
upon
it here.
There is another disease very common among wines when
the great heat
of summer begins to make itself felt in the vintage tubs. The
wine
is said to _turn_, to _rise_, to _spurt_. The wine becomes
slightly
turbid and at the same time flat and piquant. When it is poured into
a
glass, very small bubbles of gas form like a crown upon the surface.
On
placing the glass between the eye and the light and slightly
shaking it, one
can distinguish silky waves shifting about and moving
in different directions
in the liquid. When the _turned_ wine is in a
cask, it is not unusual to see
the bottom of the cask bulge a little
and sometimes a leakage takes place at
the joints of the staves. If a
little opening is made, the wine spurts out,
and that is the reason why
the wine is said to spurt.
Authors who have
written on the subject of wine attributed this malady
to the rising of the
lees. They believed that the deposit which always
exists in greater or less
quantities in the lower part of the cask
rises and spreads itself into all
the mass of the wine.
Nothing can be more inexact. If this phenomenon is
sometimes
produced--that is to say, if the deposit rises into the mass
of
wine--the effect is due to a sudden diminution of the
atmospheric
pressure, as in times of storm, for example. As the wine is
always
charged with carbonic acid gas, which it holds in solution from
the
moment of fermentation, one can conceive that a lowering of
barometric
pressure would cause the escape of some bubbles of carbonic
acid.
These bubbles, rising from the lower part of the cask, may disturb
a
portion of the deposit, which then mixes with the wine and renders
it
turbid. But the real cause of the disease is quite different. The
turbidity
is without exception due to the presence of little filaments
of an extreme
tenuity, about a thousandth part of a millimeter in
diameter. Their length is
very variable. It is these which, when the
wine is agitated, give rise to the
silky waves just referred to. Often
the deposit of the casks leaves a swarm
of these filaments entangled
in each other, forming a glutinous mass, which
under the microscope is
seen to be composed entirely of these little
filaments. In acting upon
certain constituents of the wine particularly upon
the tartar, this
ferment generates carbonic acid. The phenomenon of spurting
is then
produced, because when the cask is closed the internal pressure of
the
liquid augments. The sparkling and the crown of little
gas-bubbles,
observed when the turned wine is poured out into a glass, is
similarly
explained. In a word, the disease of turned wine is nothing else
than
a fermentation, due to an organised ferment which, without any
doubt,
proceeds originally from germs existing on the surface of the
grapes
at the moment of gathering them, or on spoilt grapes such as are
found
in every vintage. It is very rare not to find this parasite of
turned
wine in the deposit of the wine at the bottom of the casks, but
the
parasite is not troublesome unless it multiplies very largely.
Pasteur
found the means of preventing this multiplication by a very
simple
remedy, equally applicable to other diseases of wines, such as that
of
bitterness or greasiness (_maladie de la graisse_).
Many wines
acquire with age a more or less bitter taste, sometimes to
a degree which
renders them unfit for consumption. Red wines, without
exception, are subject
to this disease. It attacks by preference wines
of the best growth, and
particularly the finest wines of the Cote-d'Or.
It is once more a little
filamented fungus which works the change;
and not only does it cause in the
wine a bitterness which little by
little deprives it of all its better
qualities, but it forms in the
bottles a deposit which never adheres to the
glass, but renders the
wine muddy or turbid. It is in this deposit that the
filaments of
the fungus float. If white wines do not suffer from this disease
of
bitterness, they are exposed, particularly the white wines of
Orleans
and of the basin of the Loire, to the disease of greasiness. The
wines
lose their limpidity; they become flat and insipid and viscous,
like
oil when poured out. The disease declares itself in the casks or in
the
best-corked bottles. M. Pasteur has discovered that the greasiness
of
wines is likewise produced by a special ferment, which the
microscope
shows to be formed of filaments, like the ferments of the
preceding
diseases, but differing in structure from the other organisms, and
in
their physiological action on the wine.
In short, according to
Pasteur's observations, the deterioration of
wines should not in any case be
attributed to a natural working of
the constituents of the wine, proceeding
from a sort of interior
spontaneous movement, which would only be affected by
variations
of temperature or atmospheric pressure. They are, on the
contrary,
exclusively dependent on the development of microscopic
organisms,
the germs of which exist in the wine from the moment of the
original
fermentation which gave it birth. What vast multitudes of germs
of
every kind must there not be introduced into every vintage tub!
What
modifications do we not meet with in the leaves and in the fruit
of
each individual spoilt vine! How numerous are the varieties of
organic
dust to be found on the stems of the bunches, on the surface of
the
grapes, on the implements of the grape gatherers! What varieties
of
moulds and mildews! A vast proportion of these germs are
evidently
sterilised by the wine, whose composition, being at the same time
acid,
alcoholic, and devoid of air, is so little favourable to life. But
is
it to be wondered at that some of these exterior germs, so
numerous,
and possessing in a more or less marked degree the anaerobic
character,
should find at certain moments, in the state of the wine, the
right
conditions for their existence and
multiplication?
* * * *
*
The cause of these alterations having been found, a mode of
preventing
the development of all these parasites had still to be
sought.
Pasteur's first endeavour was to discover some substance which would
be
antagonistic to the life of these ferments of disease, while
harmless
to the wine itself, and devoid of any special smell or taste.
But
in this research success was dependent on too many conditions to
be
easily attainable. After some fruitless trials, the thought occurred
to
Pasteur of having recourse to heat. He soon ascertained that, to
secure wine
from all ulterior changes, it sufficed to raise it, for
some instants only,
to a temperature of from fifty-five to sixty
degrees. His experiments were
first directed upon the disease of
'bitterness.' He procured some of the best
wines of Burgundy, wines
of Beaune, and of Pomard, of different years--1858,
1862, and 1863.
Twenty-five bottles were left standing forty-eight hours to
allow all
the particles suspended in the wine to settle; for, however clear
wine
may be, it always produces a slight deposit. Pasteur then decanted
the
wine with minute care, by means of a syphon of slow delivery. This
last
precaution was necessary to prevent the deposit from being
stirred
up. When there remained in each bottle only one cubic centimeter
of
liquid, Pasteur shook the bottle, and then examined with the
microscope
the residue of each bottle. He perceived in each case
distinct
filaments of ferment. The wines, however, were not in the least
bitter
to the taste, but the germs of a possible evil were there--an
evil
which would have been first detected by the palate when the
little
fungus had fully developed.
Without uncorking it, Pasteur then
heated a bottle of each of these
wines. The heating was carried to a
temperature of sixty degrees (140°
Fahr.). After the cooling of the bottles
he laid them by the side of
other unheated bottles of the same wines in a
cellar, the temperature
of which varied in summer between thirteen and
seventeen degrees. Every
fifteen days Pasteur inspected them. Without
uncorking the bottles,
he held them up against the light, so that he could
see the sediment
at the bottom of each bottle, and thus detect the least
formation of
deposit. In less than six weeks, particularly in the wine of
1863, a
very perceptible floating deposit began to form in all the
unheated
bottles. These deposits gradually augmented, and on examining them
with
the microscope they were seen to be formed of organised
filaments,
mixed sometimes with a little colouring matter which had
become
insoluble. No deposit appeared in the heated
bottles.
* * * * *
The
idea of heating wines does not belong to Pasteur. Those who love
to search
into questions of priority will find described in the works
of Latin
agriculturists various methods for the preservation of wine,
based on the
employment of heat. To give the wine durability, they
sometimes added to the
vintage variable quantities of boiled must,
reduced to half or two thirds, in
which orris, myrrh, cinnamon, white
resin, and other ingredients, were
infused. But, to cite examples
nearer our own time, Appert, whose preserves
have become so popular,
relates that he sent to St. Domingo some bottles of
Beaune which had
been previously heated to seventy degrees, and that he
compared, on
their return into France, two bottles of this wine with a bottle
which
had remained at Havre, and also with other bottles of the same
wine
which had remained in his cellar, neither of which had undergone
the
operation of heating. The superiority of the wine which came from
St.
Domingo, said Appert, was incontestable. Nothing could equal its
delicacy or
its perfume. But Appert did not by any means describe the
wine of the two
bottles which remained in France as either injured
or diseased. His remark
was based upon an incomplete observation. It
simply stated the fact, which
indeed was previously known, that a long
voyage, added to the employment of
heat, had an excellent effect upon
the Beaune. This incident had been so
completely forgotten, that it
was only in 1865 that Pasteur, during the
historical researches which
preceded his 'Etudes sur le vin,' accidentally
met with this story
of the bottles of St. Domingo, and hastened to
communicate it to the
Academy. But in reference to this question of heating,
a discussion
arose as to priority, which was quite unexpected by him. A
Burgundian
wine grower, M. de Vergnette, having first proposed the congealing
of
wines as a protective influence, had afterwards spoken, without
much
precision, of heat as another means of preservation. On this ground
he
claimed for himself a great part of the invention of Pasteur's
process.
'If, after having subjected some specimens of wines which are to
be
sent abroad to the ordeal of heating,' said M. de Vergnette, 'one
sees
that they have been able to resist the action of the heat, then
they may
safely be shipped. In the contrary case they ought not to be
sent.' According
to M. de Vergnette, it was to the composition of the
wine, its robust
condition, and good constitution, that it owed its
power of supporting the
heating process. Pasteur had no difficulty in
demonstrating that these
assertions are contradicted by experiment.
Wine never changes by the moderate
application of heat when air is
excluded; and it is precisely when of
doubtful soundness that it should
be subjected to the process of heating.
This operation does not alter
it any more than would be the case if it were
in a perfectly healthy
state. All wines may undergo the action of heat
without the least
deterioration, and one minute's heating at the proper
temperature
suffices to insure the preservation of every kind of wine. Thanks
to
this operation, the weakest wine, the most disposed to turn sour,
to
become greasy, or to be threatened with bitterness, is insured
against
injurious change.
Nothing is more simple than to realise the
condition of heating in
bottles. After having firmly tied down the corks, the
bottles are
placed in a water-bath. An iron basket is here useful. The water
ought
to rise up to the wire of the cork. Among these bottles is placed
a
bottle of water, into which the bulb of a thermometer is plunged.
The
bath being heated, as soon as the thermometer marks fifty or
sixty
degrees Centigrade, the basket is withdrawn. The subsequent
soundness
of the wine is thus insured.
But if Pasteur had overlooked
nothing in his efforts to prevent or
arrest the evil changes of wine, he
still saw that full confidence
was not felt in the efficacy of a process
which must, it was thought,
damage the taste, or the colour, or the limpidity
of the wine. After
having invited the judgment of people in society, whose
preference,
if they felt any, was generally for the heated wines, Pasteur
wished
to have a more decisive opinion. He addressed himself first to
wine
merchants and others practised in detecting the smallest
peculiarities
of wines; and afterwards he organised a grand experiment in
tasting. On
November 16, 1865, a sub-commission, nominated by the
representative
commission of the wholesale wine-sellers of Paris, repaired to
the
Ecole Normale and examined a considerable number of specimens. After
a
series of tastings, which recognised, if not a superiority over
the heated
wines, at least a shade of imperceptible flavour, which,
however, it was
admitted, would escape nine-tenths of the consumers,
Pasteur, fearing that
there remained still in the mind of the majority
of the commission a slight
prejudice against the operation of heating,
and that imagination, moreover,
had some share in determining shades
of flavour, proposed that at the next
sitting there should be no
indication which of the samples of wine had been
heated and which had
not. The commission, having no other desire than to
arrive at the
truth, at once accepted this proposition.
The resulting
uncertainty as to whether the heated or the unheated
wines were to be
preferred was so absolute as to be comical. It is
unnecessary to say that the
heated wines had not experienced the
least alteration. At a certain point
Pasteur, who was astonished at
the extraordinary delicacy of the palate of
these tasters, employed a
little trickery. He offered them two specimens
absolutely identical,
taken out of the same bottle. There were preferences,
very slight
it is true, but preferences gravely expressed for one or the
other
glass. The commission, making allusion in its report to this
special
tasting experiment, was the first to allow with a good grace that
the
differences between the heated and non-heated wines were
insignificant,
imperceptible if they existed, and that the imagination--added
the
report--was not without considerable influence in the tasting;
since
the members of the commission had themselves fallen into a
little
experimental snare.
Thus Pasteur, after having revealed the
causes which determine the
alterations of wines, had found the means of
practically neutralising
them. By the application of heat, and without
producing any change in
the colour or flavour of the wines, he had been able
to insure their
limpidity, and to render them capable of being indefinitely
preserved
in well-closed vessels. If these wines, being afterwards exposed
too
long to the air, were again threatened with alteration, it was
because
the air brought to them new living germs of those ferments which
had
been destroyed by the heat. But germs from this source are so
trifling
compared with those contained in the wine itself, that one may
almost
say the heating process renders the wine unalterable even after
it
has been rebottled in contact with the air. Thus, by a series
of
experiments which left nothing to chance, one of the greatest
economic
questions of the day was solved. Wines could be kept or
transported
into all countries without losing their flavour or their
perfume.
These experiments of the laboratory were destined to have an
extensive
application; for very soon arrangements were made for heating
wine
in barrels, the inquiry thereby assuming the proportions of a
public
benefit.
_THE
SILKWORM-DISEASE._
The life of the population of certain departments
in the South of
France hangs on the existence of silkworms. In each house
there is
nothing to be seen but hurdles, over which the worms crawl. They
are
placed even in the kitchens, and often in well-to-do families
they
occupy the best rooms. In the largest cultivations, regular stages
of
these hurdles are raised one above the other in immense sheds,
under
roofs of disjointed tiles, where thousands and thousands of
silkworms
crawl upon the litters which they have the instinct never to
leave.
Great or small, the silkworm-rearing establishments exist
everywhere.
When people accost each other, instead of saying 'How are you?'
they
say 'How are the silkworms?' In the night they get up to feed them
or
to keep up around them a suitable temperature. And then what anxiety
is
felt at the least change of weather! Will not the mulberry leaves
be wet?
Will the worms digest well? Digestion is a matter of great
importance to the
health of the worms, which do nothing all their lives
but eat! Their
appetites become especially insatiable during the last
days of rearing. All
the world is then astir, day and night. Sacks
of leaves are incessantly
brought in and spread out on the litters.
Sometimes the noise of the worms
munching these leaves resembles that
of rain falling upon thick bushes. With
what impatience is the moment
waited for when the worms arrive at the last
moulting! Their bodies
swollen with silk, they mount upon the brambles
prepared for them,
there they shut themselves up in their golden prisons and
become
chrysalides. What days of rejoicing are those in which the cocoons
are
gathered; when, to use the words of Olivier de Serres, the silk
harvest
is garnered in!
Just as in all agricultural harvests, this
ingathering of the silk is
exposed to many risks. Nearly always, however, it
pays the cultivator
for his trouble, and sometimes pays him largely. But in
1849, after
an exceptionally good year, and without any atmospheric
conditions to
account for the fact, a number of cultivations entirely broke
down.
A disease which little by little took the proportions of an
epidemic
fell upon the silkworm nurseries. Worms hardly hatched, and
worms
arrived at the last moulting, were equally stricken in large
numbers.
It mattered little in what phase the silkworm happened to be: in all
it
was assailable by the evil.
There is hardly a schoolboy who has not
reared in the recesses of his
desk some five or six silkworms, feeding them,
in default of mulberry
leaves, with leaves of lettuce or salsify. Therefore
it is hardly
necessary to remind my readers how the silkworm is born, grows,
and is
transformed. Coming out of its egg, which is called a grain, because
of
its resemblance to a small vegetable seed, the silkworm appears in
the
first fine days of spring. It does not then weigh more than one or
two
milligrammes. Little by little its size and its activity augment.
The
seventh day after its birth it rests on a leaf and appears to sleep.
It
remains thus for nearly thirty hours. Presently, its head moves, as
if
it did not belong to the rest of the body, and under the skin of
this
head appears a second quite new head. Just as if it came out of a
case,
the silkworm disengages itself from its old withered skin. Here are
its
front feet, there the false feet (_fausses pattes_), which it
carries
behind. At length the worm is quite complete. It rests a while
and
then begins to eat. At the end of a few days new sleep, new skin,
new
shedding of the skin, then a third, and then a fourth
metamorphosis.
About eight days after the fourth shedding of its skin, the
worm ceases
to eat, its body becomes more slender, more transparent; it seeks
to
leave its litter, it raises its head and appears uneasy. Some twigs
of
dried heather are then arranged for it to fasten upon; these it
climbs,
never to descend again. It spins its cocoon and becomes a
chrysalis.
When the worms of a cultivation have all spun their cocoons, they
are
smothered in a steam stove, and, after being dried in the sun, they
are
handed over to the spinners. If it is desired to reserve some of
the
cocoons for seed, instead of being smothered, they are strung
together
in chaplets. After about three weeks, the moth comes out of
its
chrysalis. It pierces the cocoon by means of a liquid which issues
from
its mouth, and which has the property of so softening the silk that
the
moth is able to pass through the cocoon. It has hardly dried itself
and
developed its wings when the males and females pair for several
hours.
Then the females lay their eggs, of which they can produce from four
to
six hundred. These are all the phases through which silkworms pass
in
the space of two months.
* * *
* *
In the epidemic which ravaged the silkworm nurseries in 1849
the
symptoms were numerous and changeable. Sometimes the disease
exhibited
itself immediately. Many of the eggs were sterile, or the worms
died
during the first days of their existence. Often the hatching
was
excellent, and the worms arrived at their first moulting, but
that
moulting was a failure. A great number of the worms, taking
little
nourishment at each repast, remained smaller than the others,
having
a rather shining appearance and a blackish tint. Instead of all
the
worms going through the phases of this first moulting together, as
is
usually the case in a batch of silkworms, they began to present
a marked
inequality, which displayed itself more and more at each
successive moulting.
Instead of the worms swarming on the tables, as
if their number was uniformly
augmenting, empty spaces were everywhere
seen; every morning corpses were
collected on the litters.
Sometimes the disease manifested itself under
still more painful
circumstances. The batch would progress favourably to the
third, and
even to the fourth moulting, the uniform size and the health of
the
worms leaving nothing to be desired; but after the fourth moulting
the
alarm of the husbandman began. The worms did not turn white, they
retained a
rusty tint, their appetite diminished, they even turned away
from the leaves
which were offered to them. Spots appeared on their
bodies, black bruises
irregularly scattered over the head, the rings,
the false feet, and the spur.
Here and there dead worms were to be
seen. On lifting the litter, numbers of
corpses would be found. Every
batch attacked was a lost batch. In 1850 and
1851 there were renewed
failures. Some cultivators, discouraged, attributed
these accidents to
bad eggs, and got their supplies from abroad.
At
first everything went as well as could be wished. The year 1853,
in which
many of these eggs were reared in France, was one of the
most productive of
the century. As many as twenty-six millions of
kilogrammes of cocoons were
collected, which produced a revenue of
130,000,000 francs. But the year
following, when the eggs produced by
the moths of these fine crops of foreign
origin were tried, a singular
degeneracy was immediately recognised. The eggs
were of no more value
than the French eggs. It was in fact a struggle with an
epidemic. How
was it to be arrested? Would it be always necessary to have
recourse to
foreign seed? and what if the epidemic spread into Italy, Spain,
and
the other silk cultivating countries?
The thing dreaded came to
pass. The plague spread; Spain and Italy were
smitten. It became necessary to
seek for eggs in the Islands of the
Archipelago, in Greece, or in Turkey.
These eggs, at first very good,
became infected in their turn in their native
country; the epidemic had
spread even to that distance. The eggs were then
procured from Syria
and the provinces of the Caucasus. The plague followed
the trade in the
eggs. In 1864 all the cultivations, from whatever corner of
Europe they
came, were either diseased or suspected of being so. In the
extreme
East, Japan alone still remained healthy.
Agricultural
societies, governments, all the world was preoccupied with
this scourge and
its invading march. It was said to be some malady
like cholera which attacked
the silkworms. Hundreds of pamphlets were
published each year. The most
foolish remedies were proposed, as quite
infallible--from flowers of sulphur,
cinders, and soot spread over the
worms, or over the leaves of the mulberry,
to gaseous fumigations of
chlorine, of tar, and of sulphurous acid. Wine,
rum, absinthe, were
prescribed for the worms, and after the absinthe it was
advised to try
creosote and nitrate of silver. In 1863 the Minister of
Agriculture
signed an agreement with an Italian who had offered for purchase
a
process destined to combat the disease of the silkworms, by which
he
(the Minister) engaged himself, in case the efficacy of the remedy
was
established, to pay 500,000 francs as an indemnity to the Italian
silk
cultivator. Experiments were instituted in twelve departments,
but
without any favourable result. In 1865 the weight of the cocoons
had
fallen to four million kilogrammes. This entailed a loss of
100,000,000
francs.
The Senate was assailed by a despairing petition
signed by 3,600
mayors, municipal councillors, and capitalists of the
silk-cultivating
departments. The great scientific authority of M. Dumas, his
knowledge
of silk husbandry, his sympathy for one of the departments
most
severely smitten, the Gard, his own native place, all contributed
to
cause him to be nominated Reporter of the Commission. While drawing
up
his report the idea occurred to him of trying to persuade Pasteur
to
undertake researches as to the best means of combating the
epidemic.
Pasteur at first declined this offer. It was at the moment when
the
results of his investigations on organised ferments opened to him
a
wide career; it was at the time when, as an application of his
latest
studies, he had just recognised the true theory of the fabrication
of
vinegar, and had discovered the cause of the diseases of wines; it
was,
in short, at the moment when, after having thrown light upon the
question of
spontaneous generation, the infinitely little appeared
infinitely great. He
saw living ferments present everywhere, whether as
agents of decomposition
employed to render back to the atmosphere all
that had lived, or as direct
authors of contagious maladies. And now
it was proposed to him to quit this
path, where his footing was sure,
which offered him an unlimited horizon in
all directions, to enter on
an unknown road, perhaps without an outlet. Might
he not expose himself
to the loss of months, perhaps of years, in barren
efforts?
M. Dumas insisted. 'I attach,' said he to his old pupil, now
become
his colleague and his friend, 'an extreme value to your fixing
your
attention upon the question which interests my poor country. Its
misery
is beyond anything that you can imagine.'
'But consider,' said
Pasteur, 'that I have never handled a silkworm.'
'So much the better,'
replied M. Dumas. 'If you know nothing about the
subject, you will have no
other ideas than those which come to you
from your own
observations.'
Pasteur allowed himself to be persuaded, less by the force
of these
arguments than by the desire to give his illustrious master a proof
of
his profound deference.
* * *
* *
As soon as the promise was given and the resolution made to go
to the
South, Pasteur thought over the method to be employed in the
pursuit
of the problem. Certainly, amidst the labyrinth of facts and
opinions,
it was not hypotheses which were wanting. For seventeen years they
had
been rising up on all sides.
One of the most recent and the most
comprehensive memoirs upon the
terrible epidemic had been presented to the
Academy of Sciences by
M. de Quatrefages. One paragraph of this paper had
forcibly struck
Pasteur. M. de Quatrefages related that some Italian
naturalists,
especially Filippi and Cornalia, had discovered in the worms and
moths
of the silkworm minute corpuscles visible only with the
microscope.
The naturalist Lebert affirmed that they might always be detected
in
diseased silkworms. Dr. Osimo, of Padua, had even perceived
corpuscles
in some of the silkworms' eggs, and Dr. Vittadini had proposed
to
examine the eggs with a microscope in order to secure having
sound
ones. M. de Quatrefages only mentioned this matter of the
corpuscles
as a passing remark, being doubtful of its importance, and perhaps
of
its accuracy. This doubt might have removed from Pasteur's mind
the
thought of examining the significance of these little corpuscles,
but,
amid the general confusion of opinions, Pasteur was attracted to
the
study of these little bodies all the more readily because it
related
to an organic element which was visible only with the
microscope.
This instrument had already rendered such services to Pasteur in
his
delicate experiments on ferments, that he was fascinated by the
thought
of resuming it again as a means of
research.
I.
On June 6,
1865, Pasteur started for Alais. The emotion he felt on the
actual spot where
the plague raged in all its force, in the presence
of a problem requiring
solution, caused him at once to forget the
sacrifices he had made in quitting
his laboratory at the Ecole Normale.
He determined not to return to Paris
until he had exhausted all the
subjects requiring study, and had triumphed
over the plague.
In a few hours after his arrival he had already proved
the presence
of corpuscles in certain worms, and was able to show them to
the
President and several members of the Agricultural Committee, who
had
never seen them. The following day he installed himself in a
little
house three kilometers from Alais. Two small cultures were
there
going on; they were nearly the last, the cocoons had all been
spun.
One of these cultures, proceeding from eggs imported that very
year
from Japan, had succeeded very well. The other, proceeding also
from
Japanese eggs which had been reproduced in the country, had
arrived
at their fourth moulting and had a very bad appearance. But,
strange
to say, on examining with the microscope a number of chrysalides
and
moths of the group which had so delighted its proprietor,
Pasteur
found corpuscles almost always present, whereas the examination of
the
worms of the bad group only exhibited them occasionally. This
double
result struck Pasteur as very strange. He at once sent messengers
into
all the neighbourhood of Alais to seek for the remains of
backward
cultivations. He attached extreme importance to ascertaining
whether
the presence of corpuscles in the chrysalides or moths of the
good
groups, and the absence of the same corpuscles in the worms of the
bad
groups, was an accidental or a general fact. He soon recognised
that
these results did very frequently occur. But what would happen when
the
worms of the bad group spun their cocoons? Pasteur found that in
the
chrysalides, especially in the old ones, the corpuscles were numerous. As
regards the moths proceeding from these cocoons, not one was free from them, and
they existed in profusion. |
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