Following up the idea that a connection between the disease and
the corpuscles might possibly exist, as other observers had
previously imagined, Pasteur declared, in a Note presented to the
Agricultural Committee of Alais on June 26, 1865, twenty days after his
arrival, that it was a mistake to seek for the corpuscle in the eggs or in
the worms. Both the one and the other could carry in them the germ of
the disease, without exhibiting distinct corpuscles, visible under
the microscope. The evil developed itself especially in the chrysalides
and in the moths, and it was in them that search should be made.
Finally, Pasteur came to the conclusion that the only infallible method
of procuring healthy eggs must be by having recourse to moths free
from corpuscles.
Pasteur hastened to apply this new method of
obtaining pure eggs. Notwithstanding that the malady was universally
prevalent, he succeeded, after several days of assiduous microscopic
observations, in finding some moths free from corpuscles. He carefully
preserved their eggs, as well as other eggs which had proceeded from
very corpusculous couples, intending to wait for what these eggs
would produce the following year; the first would be probably free
from corpuscles, while the latter would contain them. He would thus
have in future, though on a small scale, samples of originally healthy
and of originally unhealthy cultivations, by the comparison of which
with the cultivations of the trade--all more or less smitten with
the evil--totally new views might be expected to emerge. Who can
tell, thought Pasteur, whether the prosperity of the silk cultivation may
not depend on the practical application of this production of pure eggs
by means of moths free from corpuscles?
*
* * * *
Scarcely had Pasteur made known, first to the
Committee of Alais, and then to the Academy of Sciences, the results of his
earliest observations and the inductions to which they pointed, when
critics without number arose on all sides. It was objected that the
labours of several Italian _savants_ had established beyond all doubt
that the corpuscles were a normal element of certain worms, and
especially of all the moths when old; that other authors had affirmed it to
be sufficient to starve certain worms to make these famous
corpuscles appear in all their tissues; and that Dr. Gaetano Cantoni had
already tried some cultivations with eggs coming from moths without
corpuscles, and that he had totally failed.
'Your efforts will be
vain,' wrote the celebrated Italian entomologist Cornalia; 'your selected
eggs will produce healthy worms, but these worms will become sickly through
the influence of the epidemic demon which reigns everywhere.'
Anyone
but Pasteur would have been staggered, but he was not the man to allow
himself to be discouraged by _a priori_ opinions, and by assertions which
were more or less guesswork. He was resolved not to abandon his preconceived
idea until experiment had pronounced upon it with precision. All scientific
research, in order to be undertaken and followed up with success, should
have, as point of departure, a preconceived idea, an hypothesis which we must
seek to verify by experiment. To judge of the value of the facts which
Pasteur had just announced, it was necessary to know if there existed the
relation of cause and effect between the corpuscles and the disease. This was
the great point to be elucidated.
* *
* * *
But if, without preliminary groping, he had discovered
the way to be pursued, Pasteur subsequently brought to bear his rare
prudence as an experimentalist. For five years he returned annually for
some months to Alais. The little house nestling among the trees
called Pont-Guisquet became at the same time his habitation and his
silkworm nursery. It is hemmed in by mountains, up the sides of which
terraces rise, one above the other, planted with mulberry trees. The
solitude was profound. Madame Pasteur and her daughter constituted
themselves silkworm-rearers--performing their part in earnest, not only
gathering the leaves of the mulberry trees, but also taking part in all
the experiments. The assistants of the Ecole Normale, Duclaux,
Maillot, Gernez, and Raulin, grouped themselves around their master. Thus,
in an out-of-the-way corner of the Cevennes was formed a colony
seeking with ardour the solution of an obscure problem, and the means of
curing or preventing a disease which had for so long a time blighted one
of the great sources of the national wealth.
One of the first cares of
Pasteur was to settle the question as to the contagion of the disease. Many
hypotheses had been formed regarding this contagion, but few experiments had
been made, and none of them were decisive. Opinions were also very much
divided. Some considered that contagion was certain; the majority, however,
either doubted or denied its existence; some considered it as accidental. It
was said, for example, that the evil was not contagious by itself, but that
it became so through the presence and complication of other diseases
which were themselves contagious. This hypothesis was convenient, and
it enabled contradictory facts to be explained. If some persons had
seen healthy worms, which had been mixed up either by mistake or
intention with sickly ones, perish, and if they insisted on contagion,
others forthwith replied by diametrically opposite observations.
But
whatever the divergences of opinion might be, everyone at all events believed
in the existence of a poisonous medium rendered epidemic by some occult
influence. Pasteur soon succeeded, by accurate experiments, in proving
absolutely that the evil was contagious.
One of the first experiments was
as follows. After their first moulting, he took some very sound worms free
from corpuscles, and fed them with corpusculous matter, which he prepared in
the following simple manner. He pounded up a silkworm in a little water, and
passed a paint-brush dipped in this liquid over the whole surface of
the leaves. During several days there was not the least appearance
of disease in the worms fed on those leaves; they reached their
second moulting at the same time as the standard worms which had not
been infected. The second moulting was accomplished without any
drawback. This was a proof that all the worms, those infected as well as
the standard lot, had taken the same amount of nourishment. The
parasite was apparently not present. Matters remained in this state for
some days longer. Even the third moulting was got through without any
marked difference between the two groups of worms. But soon important
changes set in. The corpuscles, which had hitherto only showed themselves
in the integuments of the intestines, began to appear in the other
organs. From the second day following the third moulting--that is to
say, the twelfth after the infection--a visible inequality
distinguished the infected from the non-infected worms. Those of the standard
lot were clearly in much the best health. On examining the infected
worms through a magnifying glass, a multitude of little spots were
discovered on their heads, and on the rings of their bodies, which had not
before shown themselves. These spots appeared on the exterior skin when
the interior skin of the intestinal canal contained a considerable
number of corpuscles. It was these corpuscles that impeded the
digestive functions, and interfered with the assimilation of the food.
Hence arose the inequality of size of the worms. After the fourth
moulting, the same type of disease was noticed as that which was breaking
out everywhere in the silkworm nurseries, especially the symptom of
spots on the skin, which had led to the disease being called
_pebrine_. The peasants said that the worms were peppered. The majority of
the worms were full of corpuscles. Those which spun their cocoons
produced chrysalides which were nothing but corpusculous pulp, if such a term
be allowed.
It was thus proved that the corpuscles, introduced into
the intestinal canal at the same time as the food of the worms, convey the
infection into the intestinal canal, and progressively into all the tissues.
The malady had in certain cases a long period of incubation, since it
was only on the twelfth day that it became perceptible. Finally, the
spots of _pebrine_ on the skin, far from being the disease itself, were
but the effect of the corpuscles developed in the interior; they were but
a sign, already removed from the true seat of the evil. 'If these spots of
_pebrine_,' thought Pasteur, 'were considered in conjunction with certain
human maladies in which spots and irruptions appear on the body, what
interesting inductions might present themselves to minds prepared to receive
them!'
Pasteur was never tired of repeating this curious experiment, or
of varying its conditions. Sometimes he introduced the corpusculous food
into healthy worms at their birth, sometimes at the second or third moulting.
Occasionally, when the worms were about to spin their cocoons, the
corpusculous food was given them. All the disasters that were known to have
happened in the silkworm nurseries, their extent and their varied forms, were
faithfully reproduced. Pasteur created at will any required manifestation of
_pebrine_. When he infected quite healthy worms, after their fourth moulting,
with fresh corpusculous matter, these worms, even after several meals of
corpusculous leaves alternated with meals of wholesome leaves, made their
cocoons. It might have been supposed that in this case the contagion had not
taken effect. This was but a deceptive appearance. The communication of the
disease exhibited itself in a marked degree in the chrysalides and in the
moths. Many of the chrysalides died before they turned into moths, and their
bodies might be said to be entirely composed of corpuscles. Such moths
as were formed, and which emerged from their cocoons, had a most
miserable appearance. The disease sometimes went so far as to render breeding
and the laying of eggs impossible.
Faithful to the rules prescribed by
the experimental method, Pasteur was careful to reproduce these same
experiments with the worms of the standard lot, from which all infected worms
had been selected. He fed these healthy worms on leaves over which a clear
infusion made from the remains of moths or worms exempt from corpuscles had
been spread with a paint-brush, instead of leaves contaminated with
corpusculous remains. This food kept the worms in their usual health. Could
there be a better proof that the corpuscles alone were the real cause of the
_pebrine_ disease?
These experiments, I repeat, threw a strong light
on the nature of the disease, and exactly accounted for what took place in
the industrial cultivations. From the malady which attacked the worms at
their birth, decimating a whole cultivation, down to the invisible disease
that may be said to inclose itself in the cocoon, all was now explained.
One of the effects of the plague which had most excited the surprise
and thwarted the efforts of cultivators was the impossibility of
finding productive eggs, even when they tried to obtain them from the
cocoons of groups which had succeeded perfectly well as far as the
production of cocoons was concerned. It was proved that almost invariably
the following year the eggs of these fine-looking groups were
unproductive. Numbers of the agricultural boards, and practitioners, not
being able to believe in the existence of the disease in collections that
were so satisfactory as regards the abundance and beauty of the
cocoons, persisted in thinking that the failures had an origin not
connected with the seed itself. This resulted in deception after
deception, often even in mistakes that were much to be regretted. Frequently
the best husbandmen were known to reserve for the production of eggs
some very fine cultivations, not having observed in the worms either
spots of _pebrine_ or corpuscles even up to the time when the mounting
of the brambles had been accomplished; and the year following they had the
pain of seeing all the cultivations from these eggs perish.
These circumstances, so well calculated to produce discouragement and to
give the disease a mysterious character, met with their natural
explanation in the facts proved by experimental infection.
Still, as
it never occurs to the cultivator to infect the worms directly by giving them
food charged with corpusculous debris, it might be asked how, in the
industrial establishments, such results can be produced. Pasteur lost no time
in solving this difficulty.
In a cultivation containing corpusculous
worms these worms perpetually furnish contagious matter, which falls upon the
leaf and fouls it. This is the excreta of the worms, which the microscope
shows to be more or less filled with corpuscles drawn from the lining of the
intestinal canal. It is there that they swarm. It is easy to understand that
these excreta, falling on the leaves, contaminate them all the more
easily because the worms, by the weight of their bodies in crawling, press
the excreta against the leaves. This is one cause of natural contagion.
By the excreta of corpusculous worms which he crushed, mixed with
water, and spread with a paint-brush over the mulberry leaves intended for
a single meal, Pasteur was able to communicate the contagion to as
many worms as he liked.
He also indicated another natural and direct
cause of contagion. The six fore-feet of the worm have sharp hooks at their
ends, by means of which the worms prick each other's skins. Let any one
imagine a healthy worm passing over the body of the corpusculous worm. The
hooks of the first worm, by penetrating the skin of the second, are liable to
be soiled by the corpuscles immediately below that skin; and these
hooks are capable of carrying the seeds of disease to other healthy
worms, which may be pricked in their turn. To demonstrate experimentally,
as Pasteur did, the existence of this cause of contagion, it was
only necessary to take some worms and allow them to wound each
other. Lastly, infection at a distance, through the medium of the air and
the dust it carries, is a fact equally well established. It is
sufficient, by sweeping the breeding-houses, or by shaking the hurdles, to
stir up the dust of corpusculous excretions and the dried remains of
dead worms, and to allow them to be spread over the hurdles of the
healthy worms, to cause, after a certain time, contagion to appear among
these worms. When very healthy worms were placed in a breeding nursery at
a considerable distance from unhealthy worms, they, in their turn,
became infected.
After so many decisive experiments it was no longer
possible not to see in _pebrine_ an essentially contagious disease.
Nevertheless, among facts invoked in favour of non-contagion, there was one
which it was difficult to explain. There existed several examples of
successful cultivations conducted in nurseries which had totally failed from
the effects of _pebrine_ the year before. The explanation is, as shown
by Pasteur, that the dust can only act as a contagion when it is
fresh. Corpusculous matter, when thoroughly dried, loses its virulence. A
few weeks suffice to render such matter inoffensive: hence the dust of one
year is not injurious to the cultivations of the next year. The corpuscles
contained in the eggs intended for future cultivation alone cause the
transmission of the disease to future generations.
And what can be more
easily understood than the presence of corpusculous parasites in the egg? The
egg comes into existence during that marvellous phase of the life of a
silkworm when, after having spun its cocoon, it sleeps within it as a
chrysalis, resolving itself, so to speak, into those kinds of albumen and
yelk from which the fully-formed moth will emerge, as a chick emerges from
its egg. Let anyone imagine this origin of an approaching life, no longer in
its normal purity, but associated with a parasite which will find in the
materials surrounding it, so adapted to life and transformation, the elements
of its own nourishment and multiplication. This parasite will be
present when the eggs of the female moth, tender and soft as albumen,
begin to define their outlines. Woe betide those eggs if they then
enclose any particles of corpuscle, or of its original matrix. In vain
will the envelope of those eggs become by degrees hard and horny; the
enemy is within, and later on he will be discovered in the embryo of
the silkworm.
Thus this terrible plague is at the same time contagious
and hereditary, helping us to understand the evolution of this
double character in certain maladies both of men and
animals.
II.
The first time
Pasteur went to Alais the silkworm epidemic was universally attributed to a
single cause--_pebrine_. _Pebrine_ was called _the disease_. This word
expressed everything. It indicated the existence of a mysterious scourge, the
origin and nature of which could not be traced, but which was ready to fall
upon all the establishments devoted to the nurture of the worms. Whatever
might happen, or whatever might be the cause of ruin in a silkworm nursery,
_the disease_ was held responsible. One of the most striking proofs that the
evil was attributed to _pebrine_ alone is found in the fact that a prize
of 5,000 florins was offered by the Austrian Government in 1868, as
a reward for the discovery of the best remedy for the prevention and
cure of _pebrine_--'the epidemic disease which devastates the
silkworm.'
A rapid glance at the principles which have just been
established suffices to show that _pebrine_ might now be regarded as
vanquished. Pasteur had demonstrated that moths free from corpuscles never
produced a single corpusculous egg; he had proved, moreover, that eggs
brought up in a state of isolation, at a distance from contaminated
eggs, produce no worms, chrysalides, or moths which are corpusculous. It
was easy, therefore, to multiply cultivations free from _pebrine_.
The production of silk and the production of eggs was thus secured. To
make sure that the eggs were pure it was only necessary to have recourse
to the microscopic examination of the moths which had produced them.
These observations might be made by women, by young girls, even by
children. It was sufficient to crush up a moth in a little water, and to
put a drop of this mixture under the microscope, to see the
corpuscles clearly, if they existed. It seemed, then, that the plague was got
rid of. But Pasteur was not slow in recognising that the general belief
in a single malady could not be justified. If the experiments of 1866
had demonstrated to him the full extent of the corpusculous malady, and
had established the principles of a treatment proper for its
prevention, the method he had adopted had also shown him that _pebrine_ was
far from being the only cause from which the silk culture suffered.
It
was in 1867 that this result was obtained. From an experimental point of
view, that year counted double for Pasteur. Influenced by a profound sympathy
for the misery which he had witnessed during two successive years, and, at
the same time, impatient to find the cause of the scourge, Pasteur, in the
months of February, March, and April, in advance of the great industrial
cultivations, commenced a series of experiments on worms hatched by
artificial heat, and fed with mulberry leaves from a hothouse.
During
these forced experiments Pasteur observed that out of sixteen broods derived
from non-corpusculous parents, fifteen succeeded, while the sixteenth
perished almost entirely between the fourth moulting and the climbing on to
the brambles. After having exhibited a most healthful appearance, the worms
died suddenly. In a cultivation of 100 worms, ten, fifteen, twenty dead ones
were picked up daily: these turned black, and became putrid with
extraordinary rapidity, often within the space of twenty-four hours.
Sometimes they were soft and flabby, like an empty, crumpled intestine.
Consulting the authors who had written upon silkworms, Pasteur could not
doubt that he had before his eyes a characteristic specimen of the disease
called _morts-flats_, or _flacherie_. Not only were these worms free from all
_pebrine_ spots, but no corpuscles were to be found in any part of their
bodies. A still more significant fact was, that corpuscles were also
absent from the chrysalides and the moths of those few worms which were
able to spin their cocoons. Although this sample was confined to a
single group of eggs derived from parents free from corpuscles,
Pasteur continued to entertain doubts as to the existence of only a
single disease, and also as to the necessary connection of _pebrine_
with _flacherie_.
These suspicions were confirmed by his cultivations
of April and May. Numerous cases of _flacherie_ presented themselves.
Uncertainty was no longer possible as to the mutual independence of the
two maladies--_pebrine_ and _flacherie_. The cultivations most
seriously invaded by the last-mentioned disease came from eggs produced
by parents free from corpuscles, and led on to reproducers also free
from this parasite. On visiting a multitude of industrial
cultivations, Pasteur discerned that what had passed in his own laboratory
was of very general occurrence, and that, contrary to the received
opinion, two distinct maladies divided between them the cause of all
the misfortunes. _Pebrine_ was evidently the most widely spread,
but _flacherie_ had also its share, and a very large share, in the
calamity.
Here, once more, the microscope came to Pasteur's assistance.
If, at the period of the rearing of the silkworms, when the mean
temperature is always rather high, some mulberry leaves are crushed in a
mortar and mixed with a little water, the liquid being left to itself,
in twenty-four hours it will be found filled with microscopic
organisms; some motionless, resembling little rods or spores joined end to
end, like strings of beads, others more or less active, flexible,
endowed with a sinuous movement like that of the vibrios found in nearly
all organic infusions in process of decomposition. Whence come
these microscopic organisms? The facts relating to spontaneous
generation indicate that the germs of these organisms were on the surface of
the pounded leaf, spread in the form of dust over the instruments used
to triturate the leaf, possibly on the mortar, the pestle, or in the
water added to the pounded leaves.
It is a curious fact, that if the
intestinal canal of a worm in full process of digestion be opened, the
pounded leaf which fills it from one end to the other will not show
microscopic organisms of any kind, but only cells of parenchyma, green
granules of the chlorophyl of the leaf, and remains of the air-vessels of the
plant. Through the action of the liquids secreted by the glands which line
the integuments of the intestinal canal, the germs of organisms are
themselves digested or hindered in their development. The digestive functions
of silkworms are so active that everything is carried away, destroyed in the
same manner as the leaves themselves.
But if from any cause the
digestion of the worms be impeded or suspended, then the germs introduced
with the food into the intestinal canal will give rise to the multiplication
of microscopic organisms which are always found in the artificially bruised
leaf when mixed with a little water. How numerous are the causes which may
check this digestive function of the worm--a function of such importance to
a creature which in the space of one month passes from the weight of half
a milligramme to that of five, six, seven, or even eight grammes! Pasteur
proved that whenever a worm was attacked with _flacherie_, it always had,
associated with the food in its intestinal canal, one or other of the
microscopic organisms which are invariably to be met with among crushed
mulberry leaves. Summing up in a kind of aphorism a series of observations,
Pasteur observes: 'Every _ver flat_ is one which digests badly, and,
conversely, every worm which digests badly is doomed to perish of
_flacherie_, or to furnish a chrysalis and a moth the life of which, through
the injury produced by organised ferments, is not normally
perfected.'
Thus, as in the case of _pebrine_, the morbid symptoms of
_flacherie_ are very variable. All depends on the intensity of the evil--that
is to say, on the abundance and the nature of the parasites developed
in the intestinal canal, and also on the period in the life of the
worm when this fermentation begins to show itself. The most dangerous
of all these ferments are those of the family of vibrios. If they exist in
the first phases of the life of the worm, it dies quickly and very soon
becomes putrid, sometimes resolving itself into an infected pus. The disease
often manifests itself in a manner particularly distressing and disastrous to
the cultivator. The worms have presented the most beautiful appearance up to
the time of climbing the heather. The mortality has scarcely been two or
three per cent., which is nothing; the moultings have been effected in a
perfect manner, when suddenly, some days after the fourth moulting, the worms
become languid, crawling with difficulty, and hesitating to take the leaves
which are thrown upon their hurdles. If some few have mounted on to the
heather, they stretch themselves on the twigs, their bodies swollen with food
which they cannot digest. Sometimes they remain there motionless till
they die, or, falling, remain suspended only by their false feet. The
few moths which have succeeded in piercing their cocoons do not show
any corpuscles. They can produce eggs, but these eggs, coming from
parents weakened by disease, give rise the following year to a
generation threatened with _flacherie_. It is in this sense that the disease
may be regarded as hereditary, although the parasites of the
intestinal canal to which _flacherie_ is due do not transmit themselves to
the eggs or to the worms which issue from them. The worms inherit
weakly constitutions, and, being without power of resistance against
anything that can derange their digestive functions, they are at the mercy
of the accidents of their culture.
Too large an assemblage of worms in
one nursery; too high a temperature at the time of moulting; a thunderous
atmosphere, which predisposes organic matter to fermentation; the use of
heated or wet leaves, especially if the wetting be caused by a fog or by the
morning or evening dew, which deposits on the leaf the germs suspended in a
great mass of air;--these are so many causes calculated to diminish
the activity of the digestive functions of the worms, and to produce
in consequence a fermentation of the leaf in the intestinal
canal--the malady now under consideration. Often also _flacherie_ depends
upon mistakes committed by the husbandman while tending his precious
'kine,' to use an expression of the sixteenth century.
A Chinese book
published on the rearing of silkworms contains a series of little practical
counsels. 'The person who takes care of the silkworms,' says this guide to
the perfect cultivator, 'ought to wear a simple garment, not lined. He must
regulate the temperature of the spinning-house according to the sensation of
heat or cold which he experiences; if he feels cold, he may conclude that the
worms are cold, and he will increase the fire; if he feels hot, he will
conclude that the worms are hot, and he will suitably diminish the
fire.'
One point which had been ignored before the experiments of Pasteur
was the contagious character of _flacherie_. This contagion may
surpass that of _pebrine_ itself as regards duration. In _pebrine_ the
dried corpusculous matter loses all virulence after the lapse of some
weeks. The disease cannot, therefore, communicate itself from one year
to another by the corpusculous dust of a rearing establishment. The germs,
on the contrary, of the microscopic organisms which provoke fermentation in
the mulberry leaves, especially the vibrios, retain their vitality for
several years. The dust of a silkworm nursery infected by _flacherie_ appears
under a microscope quite full of cysts or spores of vibrios. These spores or
cysts rest, like the sleeping beauty in the forest, until a drop of water
falls upon them and awakens them into life. Deposited on the leaves which are
to serve as nourishment, these germs of vibrios are carried into the
intestinal canal of the worm, develop and multiply themselves, and
completely disturb the digestive functions, unless the digestion is so
strong that the germs are immediately arrested, and disposed of like the
food itself. This is what happens when the worms are in full vigour. It is
a struggle for life, in which the worms often gain the victory.
Giving
to some very healthy worms a meal of leaves covered with the dry dust of a
silkworm nursery, infected the year before by _pebrine_ and _flacherie_,
Pasteur reproduced _flacherie_, and not _pebrine_. Still more readily did he
produce the first of these maladies, when he gave, as food, leaves polluted
by the contents of the intestinal canal of worms which had died of the
disease. As in the case of _pebrine_, the excreta of the worms attacked by
_flacherie_, defiling the leaves, carry the mischief to the healthy worms, or
add to the dangerous fermentation in the intestines of those which are
already in part attacked.
To preserve silkworms from accidental
_flacherie_, hygienic precautions are sufficient. As regards hereditary
_flacherie_, or, to speak more correctly, that which develops itself easily
on any diminution of vigour in the eggs and in the embryo, Pasteur again
found a remedy by having recourse to the microscope. By means of the
microscope it is possible to obtain information as to the health of the
worms, the chrysalides, and the moths destined to produce the eggs.
Every attention should be directed to the complete exclusion of ferments
from the intestinal canal of the worms, and from the stomach-pouch of
the chrysalides--a little pouch to which the intestinal canal of the
worm is reduced, with its contents more or less transformed. But if
there is not time to make this examination for parasitic ferments with
the microscope, a simple inspection of the worms in their last stage
will suffice. Pasteur laid great stress upon the observation of the
worms when they climbed on to the heather.
'If I were a cultivator of
silkworms,' he wrote in his beautiful work on the diseases of silkworms, 'I
would never hatch an egg produced from worms that I had not observed many
times during the last days of their life, so as to make sure of their vigour
at the moment when they spin their silk. If you use eggs produced by moths
the worms of which have mounted the heather with agility, have shown no signs
of _flacherie_ between the fourth moulting and mounting time, and do not
contain the least corpuscle of _pebrine_, then you will succeed in all
your cultivations.'
III.
We have now arrived at the end of this long investigation. All
the obscurity which enveloped the origin of the diseases of silkworms
had now been dispelled. Pasteur had arrived at such accurate
knowledge both of the causes of the evil and their different
manifestations, that he was able to produce at will either _pebrine_ or
_flacherie_. He could so regulate the intensity of the disease as to cause it
to appear on a given day, almost at a given hour. He had now to carry
into practice the results of his laboratory labours.
Since the
beginning of the plague, and after some doubts which were soon dispelled, it
was clearly seen that all the mischief was to be attributed to the bad
condition of the eggs. The remedy of distant explorations for procuring
non-infected eggs was both insufficient and precarious. It simply amounted to
going very far to seek, and paying very dear, for seed which could not be
relied on with certainty. The prosperity of the silkworm culture could only
be secured by measures capable of restoring to the native eggs their pristine
qualities.
The results obtained by Pasteur were sufficient to solve this
problem. The struggle against _flacherie_ was easy, but there remained
the struggle against _pebrine_. To triumph over this disease, which was
so threatening, Pasteur devised a series of observations as simple as
they were ingenious.
Here is a crop which has perfectly succeeded. The
moultings, and the climbing upon the heather, are all that could be desired.
The cocoons are finished, and the appearance of the moths alone is waited
for. They arrive, and they pair. Then begins the work of the
cultivator, who is careful about the production of his eggs. He separates
the couples at the end of the day; laying each female moth by itself on a
little linen cloth suspended horizontally. The females lay their eggs. After
the laying, he takes each female in turn and secures her by a pin passed
through the wings to a folded corner of the little cloth, where are grouped
some hundreds of eggs which she has laid. The male moth also might be pinned
in another corner of the cloth, but the examination of the male is useless,
as it has been found that he does not communicate the _pebrine_. The female
moth, after having been desiccated by free contact with the air, is examined
at leisure, it may be even in the autumn or winter. Nothing is easier than to
ascertain whether there are any corpuscles in its dead body. The moth is
crushed in a mortar and mixed with a little water, and then a drop of
the mixture is examined by the microscope. If corpuscles be found, the
bit of cloth corresponding to the examined moth is known, and it is
burnt with all the eggs it contains.
This method of procuring pure
eggs is, in fact, only the rational development of the first inductions which
Pasteur had submitted to the Agricultural Committee of Alais in June 1865. At
that time he hardly ventured to hope that he should be able to find the means
of preparing more than very small quantities of healthy eggs for his
experiments; but events were so ordered that the starting-point, which seemed
to be purely scientific, unfolded a method susceptible of a
widespread practical application. This process of procuring sound eggs is
now universally adopted. In the Basses-Alpes, in Ardeche, in Gard, in
the Drome, and in other countries, may be met with everywhere, at the time
of the cultivation, workshops where hundreds of women and young girls are
occupied, with a remarkable division of labour and under the strictest
supervision of skilful overseers, in pounding the moths, in examining them
microscopically, and in sorting and classifying the little cloths upon which
the eggs are deposited.
* * *
* *
But if Pasteur had brought back wealth to ruined countries, if
he had returned to Paris happy in the victory he had gained, he had
also undergone such fatigues, and had so overstrained himself in the use
of the microscope while absorbed in his daily and varied experiments,
that in October 1868 he was struck with paralysis of one side. Seeing, as
he thought, death approaching, he dictated to his wife a last note on
the studies which he had so much at heart. This note was communicated
to the Academy of Sciences eight days after this terrible trial.
A
soul like his, possessing so great a mastery over the body, ended by
triumphing over the affliction. Paralysed on the left side, Pasteur never
recovered the use of his limbs. To this day, sixteen years after the attack,
he limps like a wounded man. But what stages had this wounded man yet to
travel over, what triumphs were yet in store
for him!
_DECISIVE
EXPERIMENTS._
After having dictated this scientific note, which he
thought would have been his last, his courage forsook him for a time. 'I
regret to die,' he said to his friend, Sainte-Claire Deville, who had
hastened to his bedside; 'I should have wished to render more service to my
country.' His life was spared, but for several months Pasteur remained
entirely paralysed, incapable of the slightest movement. Smitten thus in
his full strength at the age of forty-five, he took a sad review of his
own state. Even at the height of his attack his mind had always
retained its clearness. He had pointed out to the doctor without any
faltering of voice the progressive symptoms of the paralysis. Then
reproaching himself for having added to the grief of his wife by thus
dwelling on the details of his illness, he never allowed another word to
escape him about his infirm condition. Sometimes, even when he heard his
two assistants, M. Gernez and M. Duclaux, whose devotion to him
during those sad days could only be compared to that of his wife, talking
to him of future labours, he entered into these thoughts and appeared
to add faith to their hopes. He finished by sharing them.
In January
1869, although it was still impossible for him to drag himself about his
room, he was so much excited by the contradictions that his system of culture
had aroused that he wished to start again for Alais. 'Aided by the method of
artificial cultivation,' he remarked, 'we shall soon annihilate these latest
oppositions. There is here both a scientific principle and an element of
national wealth.'
His wish could not be opposed, but a terrible and
anxious journey it was! At some leagues from Alais, at a place called
Saint Hippolyte-du-Fort, where the earliest experiments were made,
Pasteur stopped. He installed himself--we might rather say he
encamped--with his family and his assistants, in a more than humble lodging,
one of those miserable, cold, paved houses of the rural districts.
Seated in his arm-chair, Pasteur directed the experiments, and verified
the observations which he had made the year before. Each of his
predictions as to the destiny of the different groups of worms was fulfilled
to the smallest detail. In the following spring he left for Alais,
where he followed in all their phases, from the egg up to the cocoon,
the cultivations there undertaken, and he had the happiness of proving
once more the certainty of his method.
But opposition still continued.
The French Government, shaken by the violence and tenacity of the opponents,
hesitated to decide upon this process of culture. The Emperor interposed; he
instructed Marshal Vaillant to propose to Pasteur to go into Austria to the
Villa Vicentina, which belonged to the Prince Imperial. For ten years
the silk harvest at this place had not sufficed to pay the cost of
eggs. Pasteur accepted with joy the prospect of a great decisive
experiment. He traversed France and Italy, reclining in a railway carriage or
in an arm-chair, and at last arrived at the Imperial villa near
Trieste. Pasteur succeeded in a marvellous manner. The sale of the cocoons
gave to the villa a net profit of twenty-six million francs. The
Emperor, impressed with the practical value of the system, nominated
Pasteur a Senator, in the month of July 1870. But this nomination, like
so many other things, was swept away before it had time to appear in
the 'Journal Officiel.' Pasteur, however, cared little for the title
of Senator. He returned to France on the eve of the declaration of
war.
A patriot to the heart's core, he learned with poignant grief the
news of his country's disasters. The bulletins of defeat, which
succeeded each other with mournful monotony, threw him into deep despair. For
the first time in his life he had not the strength to work. He lived at
his little house in Arbois as one completely vanquished. Those who
went into his room found him often bathed in tears. On January 18,
1871, he wrote, to the Dean of the Academy of Medicine at the University
of Bonn, a letter in which all his grief and all his pride as a
Frenchman were displayed, requesting him to withdraw the diploma of German
doctor which the Faculty of Medicine of the University had conferred upon
him in 1868. Whilst he wrote this letter, which was a cry of
patriotism, his son, enrolled as a volunteer, though hardly eighteen years of
age, was gallantly doing his duty in the Army of the
East.
_STUDIES ON
BEER._
The war was over. Little by little the life of the country was
resumed, and with returning hope the desire and necessity for renewed
work. After two years of infirmity, Pasteur at length began to feel
the recovery of health. It was like a slow and gentle renewal of
all things. He wished to return as soon as possible to his laboratory
in Paris to put into execution projects of experiments which had long
been working in his brain. At the moment when he was preparing to
start, the rebellion of the Commune broke out. M. Duclaux, who had
become Professor of the Faculty of Sciences in Clermont-Ferrand, offered
the use of his laboratory to his old master. Pasteur accepted it. Eager
to commence an investigation which would bring him again to the study
of fermentation, he attacked the diseases of beer. But it was not only for
the purpose of creating a new link between these researches and his former
ones that he occupied himself with this subject, he was also influenced by a
somewhat patriotic idea. He dreamt of success in an industry in which Germany
is superior to France. He hoped by means of scientific principles, by which
commerce would largely profit, to succeed in making for French beer a
reputation equal, if not superior, to that of
Germany.
* * * * *
Beer
is much more liable to contract diseases than wine. It may be said that while
old wine is often to be found, there is no such thing as old beer. It is
consumed as fast as it is made. Less acid and less alcoholic than wine, beer
is more laden with gummy and saccharine matters, which expose it to rapid
changes. Thus the trade in this beverage is constantly struggling with the
difficulties of its preservation.
The manufacture of beer is simple.
It is extracted from germinated barley, or malt, an infusion of which is made
and gradually heated to the boiling point. It is then flavoured by hops. When
the infusion of malt and hops, which is called 'wort,' is completed, it is
subjected to a cooling process, and drawn off into tuns and barrels. It
is then that alcoholic fermentation sets in. The cooling ought to
be performed rapidly. While the wort is at a high temperature there
is nothing to fear, it remains sound; but under 70° Centigrade,
and particularly between 25° and 35°, it is easily attacked by
injurious ferments--acetic, lactic, or butyric. After the wort is cooled,
a little of the yeast proceeding from a former fermentation is added to
it, in order that the whole mass of the wort should be invaded as soon as
possible after its cooling by the alcoholic ferment alone--the only one,
properly speaking, which can produce beer. If this wort were treated in the
same way as the must of the grape, if it were abandoned to fermentation
without yeast--to so-called spontaneous fermentation--this would hardly ever
be purely alcoholic, as in the must of grapes, which is protected by its
acidity. Most frequently, instead of beer, an acid or putrid liquid would be
obtained. Divers fermentations would simultaneously take place in it. When
the wort has fermented and the beer is made, there is still the fear of its
rapid deterioration, which necessitates its being quickly consumed.
This condition is sometimes disastrous to those employed in the beer
trade; and the improvements in the manufacture of beer which have been
made during the last forty years have all had for their object the
removal of this necessity for the daily production, so to speak, of an
article of which the consumption is liable to constant
variations.
Formerly only one kind of beer was known, the beer of
high fermentation. The wort, after having undergone cooling in the
troughs, is collected in a large open vat at a temperature of 20°, and yeast
is added to it. When the fermentation begins to show itself on the
surface of the liquid, by the formation of a light white froth, the wort
is transferred to a series of small barrels, which are placed in
cellars or store-rooms, kept at a temperature of from 18° to 20°
Centigrade. The activity of the fermentation soon causes a foam to rise,
which becomes more and more thick and viscous. This is owing to the
abundance of yeast which it contains. This yeast, collected in a large
trough placed under the casks, is gathered up for future operations.
The fermentation lasts for three or four days, then the beer is made
and has become clear; the bungs are fixed in the barrels, and they are
sent direct to the retail dealer or to the consumer. During the transit,
a certain quantity of yeast, fallen to the bottom of the casks,
thickens the beer, but a few days of repose suffice to make it again clear
and fit to drink, or to be bottled.
This system of 'high' fermentation
(so called because it begins at a temperature of 18° to 20°, and is raised
one or two degrees higher by the act of fermentation itself) is very commonly
practised in the north of France, and to a greater extent in the breweries of
England. Ale, pale ale, bitter beer, are all beers from high
fermentation.
The 'low' fermentation, which is almost exclusively
employed in Germany, and which is spreading more and more in France,
consists in a slow fermentation, at low temperature, during which the
yeast settles at the bottom of the tubs and casks. The wort, after it has
been cooled, is passed into open wooden tuns, and the working of the yeast
takes place at a temperature of about 6° Centigrade. This temperature is
maintained by means of floats, in the form of cones or cylinders, thrown into
the fermenting tuns and kept filled with ice. The fermentation lasts for ten,
fifteen, and even twenty days. When the beer is drawn off, the yeast is
collected from the bottom of the fermenting tuns. This kind of beer, which is
sometimes called German beer, sometimes Strasburg beer, is generally much
more esteemed than the other, but it requires certain expensive, or
at least inconvenient, conditions. There must be ice-caves, where
the temperature is maintained all the year round at a few degrees
only above zero. This makes it necessary to have enormous piles of ice.
It has been calculated that for one single hectolitre of good beer,
from the beginning of the cooling of the wort until the time when it is
fit for sale, 100 kilogrammes of ice are required. The 'low' beer,
called also _biere de garde_, beer for keeping, is principally manufactured
in winter, and is preserved in ice-caves until the summer.
It is not
only the taste of the consumers which has favoured the manufacture of beer of
low fermentation everywhere except in England; it is also the advantage this
beer possesses in being much less liable to deterioration than the other. By
employing ice, the brewer may manufacture in winter, or in the beginning of
spring, and thus place himself in a position to meet the demands of
consumption without fear of seeing his beer attacked by
disease.
* * * * *
All
the diseases of beer, as Pasteur has shown, are caused exclusively by the
development of little microscopic fungi, or organised ferments, the germs of
which are brought by the dust constantly floating in the air, or which gets
mixed with the original substances used in the manufacture. 'By the
expression _diseases_ of wort and of beer, I mean,' said Pasteur, 'those
serious alterations which affect the quality of these liquids so as to render
them disagreeable to the taste, especially when they have been kept for some
time, and which cause the beer to be described as sharp, sourish, turned,
ropy, putrid.' The wort of beer, after it has been raised to the
boiling heat, may, as Pasteur's experiments testify, be preserved
indefinitely, even in the highest atmospheric temperatures, when in contact
with air free from the germs of the lower microscopic organisms. The must,
leavened by the addition of pure yeast, kept free from foreign organisms,
contains nothing but the alcoholic ferment, and undergoes no other changes
than those due to the action of the oxygen, which does not give rise to
acidity, putridity, or bitterness. Since the causes of deterioration are the
same in beer as in wine, would it not appear as if the action of heat must be
the best preservative? But beer is a drink necessarily charged with carbonic
acid, and the application of heat to considerable masses of the liquid would
expel this gas. It would be a very complicated business to attempt to
preserve this gas, or to introduce it afresh after it had been expelled. This
difficulty does not arise when the beer is bottled. At a temperature of 50°
to 55°, the process of heating not only cannot take away from the beer
all its carbonic acid, but it does not prevent the secondary
fermentation from taking place to a certain extent, and this allows of the
beer being heated immediately after it is put into bottles. This heating
of the beer is practised on a large scale in Europe and in America.
In honour of Pasteur the process is called _Pasteurisation_, and the
beer_Pasteurised_ beer. |
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