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
École 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 École 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.
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?
* * * * *