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 _pébrine_ itself as regards duration. In _pébrine_ 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 _pébrine_
and _flacherie_, Pasteur reproduced _flacherie_, and not _pébrine_.
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 _pébrine_,
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 _pébrine_, 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 _pébrine_ 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 _pébrine_. 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 _pébrine_. 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 Ardèche, in Gard, in the
Drôme, 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 _bière 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.
But Pasteur was not content with simply destroying the ferments of
these diseases, he wished above all to prevent their introduction. At
the moment when the wort is raised to the boiling-point, when the germs
of disease are destroyed by the heat, if the cooling of the wort is
effected in contact with both air and yeast free from exterior germs,
the beer may be made under conditions of exceptional purity. Some
brewers, taking for their basis Pasteur's principles, constructed an
apparatus which enabled them to protect the wort while it was cooling
from the organisms of the air, and to ferment this wort with a leaven
as pure as possible. At the Exhibition of Amsterdam there might be
seen bottles half full, containing a perfectly clear beer, which had
been tapped from the time of opening of the Exhibition. This was French
beer, manufactured according to Pasteur's principles, by a great
brewer of Marseilles, M. Velten. The happy effect of these studies
is universally recognised. At Copenhagen, M. Jacobsen has had a bust
of Pasteur, by Paul Dubois, placed in the _salle d'honneur_ of his
celebrated laboratory.
* * * * *