When the shavings of a barrel which has been in work for several
months or even for several years are examined, they are found to be
extraordinarily clean. It might be said that they had just been
carefully washed. Pasteur has shown that this is but a deceptive
appearance, and that in reality these shavings are partly or wholly
covered with a mucous film of mycoderma aceti of excessive tenuity.
It is necessary to scrape the surface of the wood with a scalpel and
examine the scrapings with the microscope to be assured of the presence
of this pellicle.
Liebig, who somewhere speaks, not without a certain contempt, of the
microscope, denied formally the exactitude of these assertions.
'With diluted alcohol, which is used for the rapid manufacture of
vinegar,' he wrote, 'the elements of nutrition of the mycoderm are
excluded, and the vinegar is made without its intervention.' He
asserted also in his memoir of 1869 that he had consulted the head of
one of the principal manufactories of vinegar in Germany, that in this
manufactory the diluted alcohol did not receive during the whole course
of its transformation any foreign addition, and that beyond the air and
the surfaces of wood and charcoal--for charcoal is sometimes associated
with the beech shavings--nothing can act upon the alcohol. Liebig added
that the director of the manufactory did not believe at all in the
presence of the mycoderm, and that finally he, Liebig, in examining the
shavings which had been used for twenty-five years in the manufactory,
saw no trace of mycoderm on their surface.
The argument appeared conclusive. How, in fact, could we understand the
production of a plant containing within itself nitrogen and mineral
elements which was nevertheless to be nourished by water and alcohol.
'You do not take into account,' replied Pasteur, 'the nature of the
water which serves to dilute your alcohol. This water, like all
ordinary waters, even the purest, contains salts of ammonia and mineral
matters which are capable of nourishing the plant. Finally, you have
not rightly examined with the microscope the surface of the shavings,
otherwise you would have seen the little particles of the mycoderma
aceti united, in some cases, to a thin film which can even be lifted
up. I propose to you, moreover, to send to the Academic Commission
charged with the decision of the debate, some shavings that you have
obtained yourself in the manufactory at Munich, and in the presence
of its director. I will undertake to prove before the members of
the commission the presence of the mycoderm on the surface of these
shavings.'
Liebig did not accept this challenge. To-day the question is decided.
_THE QUESTION OF SPONTANEOUS GENERATION._
'All dry bodies,' said Aristotle, 'which become damp, and all damp
bodies which are dried, engender animal life.' Bees, according to
Virgil, are produced from the corrupted entrails of a young bull. At
the time of Louis XIV. we were hardly more advanced. A celebrated
alchemist doctor, Van Helmont, wrote: 'The smells which rise from the
bottom of morasses produce frogs, slugs, leeches, grasses, and other
things.' But most extraordinary of all was the true recipe given by Van
Helmont for producing a pot of mice. It suffices to press a dirty shirt
into the orifice of a vessel containing a little corn. After about
twenty-one days, the ferment proceeding from the dirty shirt modified
by the odour of the corn effects the transmutation of the wheat into
mice. Van Helmont, who asserted that he had witnessed the fact, added
with assurance:
'The mice are born full grown; there are both males and females. To
reproduce the species it suffices to pair them.'
'Scoop out a hole,' said he again, 'in a brick, put into it some sweet
basil, crushed, lay a second brick upon the first so that the hole may
be perfectly covered. Expose the two bricks to the sun, and at the
end of a few days the smell of the sweet basil, acting as a ferment,
will change the herb into real scorpions. An Italian naturalist, Redi,
was the first to subject this question of spontaneous generation to
a more attentive examination. He showed that maggots in meat are not
spontaneously generated, but that they are the larvæ of flies' eggs.
To prevent the production of maggots, Redi showed that it was only
necessary to surround the meat with fine gauze before exposing it to
the air. As no flies could alight upon meat thus protected, there were
no eggs deposited, and consequently neither larvæ nor maggots. But at
the moment when the doctrine of spontaneous generation began to lose
ground by the limitation of its domain, the discovery of the microscope
brought to this doctrine new and formidable support. In presence of
the world of animalculæ, the partisans of spontaneous generation
raised a note of triumph. 'We may have been mistaken,' they said, 'as
to the origin of mice and maggots, but is it possible to believe that
microscopic organisms are not the outcome of spontaneous generation?
How can we otherwise explain their presence and rapid multiplication
in all dead animal or vegetable matter in process of decomposition?'
Buffon lent the authority of his name to the doctrine of spontaneous
generation. He even devised a system to explain this hypothesis. In
1745 two ecclesiastics entered upon an eager controversy for and
against this question. While the English Catholic priest Needham
adopted the theory of spontaneous generation, the Italian priest
Spallanzani energetically opposed it; but while in the eyes of the
public the Italian remained master of the dispute, his success was more
apparent than real, more in word than in deed.
The problem was again brought forward in a more emphatic manner in
1858. M. Pouchet, director of the Museum of Natural History at Rouen,
in addressing the Academy of Sciences, declared that he had succeeded
in demonstrating in a manner absolutely certain the existence of
microscopic living organisms, which had come into the world without
germs, and consequently without parents similar to themselves.
How came Pasteur to throw himself into this discussion, at first
sight so far removed from his other occupations? The results of his
researches on fermentation led him to it as a sort of duty. He was
carried on by a series of logical deductions. Let us recall to mind,
for example, the experiment in which Pasteur exposed to the heat of the
sun water sweetened with sugar and mixed with phosphates of potash
and magnesia, a little sulphate of ammonia, and some carbonate of
lime. In these conditions the lactic fermentation was often seen to
develop itself--that is to say, the sugar became lactic acid, which
combined with the lime of the carbonate to form lactate of lime. This
salt crystallises in long needles, and ends sometimes by filling the
whole vase, while a little organised living thing is at the same time
produced and multiplied. If the experiment is carried on further,
another fermentation generally succeeds to this one. Moving vibrios
make their appearance and multiply, the lactate of lime disappears, the
fluidity returns to the mass, and the lactate finds itself replaced
by butyrate of lime. What a succession of strange phenomena! How did
life appear in this sweetened medium, composed originally of such
simple elements, and apparently so far removed from all production of
life? This lactic ferment, these butyric vibrios, whence do they come?
Are they formed of themselves? or are they produced by germs? If the
latter, whence do the germs come? The appearance of living organised
ferments had become for Pasteur the all-important question, since in
all fermentations he had observed a correlation between the chemical
action set up and the presence of microscopic organisms. Prior to the
establishment of the facts already mentioned, these difficulties did
not exist. The theory of Liebig was universally accepted.
Thus the question as to the origin of microscopic organisms and the
part played by them in fermentation was imposed as a necessity on
Pasteur. He could not proceed further in his researches without having
solved this question.
In the month of October, 1857, Pasteur was called to Paris. After
having been made dean at an incredibly early age, he was now, at the
age of thirty-five, entrusted with the scientific studies at the École
Normale Supérieure. But if the position was flattering, it did not
give to Pasteur what he most desired. As he had no Professor's chair,
he had no laboratory. In those days science, and the higher education
in science, were at a discount. It was the period when Claude Bernard
lived in a small damp laboratory, when M. Berthelot, though known
through his great labours, was still nothing more than an assistant in
the Collège de France.
At the time here referred to, the Minister of Public Instruction said
to Pasteur, 'There is no clause in the budget to grant you 1,500
francs a year to defray the expense of experiments.' Pasteur did not
hesitate to establish a laboratory at his own expense in one of the
garrets of the École Normale. We can readily imagine the modesty of
such an establishment in such a place. Dividing his time between his
professional duties and his laboratory experiments, Pasteur never went
out but to talk over his daily researches with M. Biot, M. Dumas,
M. de Senarmont, and M. Balard. M. Biot especially was his habitual
confidant. The day when M. Biot learned that Pasteur proposed to study
the obscure question of spontaneous generation, he strongly dissuaded
him from entangling himself in this labyrinth. 'You will never escape
from it,' said he, 'you will only lose your time;' and when Pasteur
attempted some timid observations with the view of showing that in
the order of his studies it was indispensable for him to attack this
problem, M. Biot grew angry. Although endowed, as Sainte-Beuve has
said, with all the qualities of curiosity, of subtlety, of penetration,
of ingenious exactitude, of method, and of perspicuity, with all the
qualities, in short, essential and secondary, M. Biot treated the
project of Pasteur as a presumptuous adventure.
Bolder than M. Biot, but with a circumspection always alive, M. Dumas
declared to Pasteur, without, however, further insisting upon the
point, that he would not advise anyone to occupy himself too long with
such a subject. M. de Senarmont alone took the part of Pasteur, and
said to M. Biot:
'Let Pasteur alone. If there is nothing to be found in the path which
he has entered upon, do not be alarmed, he will not continue in it.
But,' added he, 'I should be surprised if he found nothing in it.'
M. Pouchet had previously stated the problem with precision:
'The opponents of spontaneous generation assert that the germs of
microscopic organisms exist in the air, which transports them to a
distance. What, then, will these opponents say if I succeed in inducing
the generation of living organisms, while substituting artificial air
for that of the atmosphere?'
Pouchet then devised this ingenious experiment. He filled a bottle
with boiling water, hermetically sealed it with the greatest care, and
plunged it upside down into a basin of mercury. When the water was
quite cold he uncorked the bottle under the metal, and introduced into
it half a litre of pure oxygen gas, which is as necessary to the life
of the smallest microscopic organism as it is to that of the larger
animals and vegetables. Up to this time there was nothing in the vessel
but pure water and oxygen. Pouchet then introduced a minute bunch of
hay which had been enclosed in a corked bottle, and exposed in a stove
for a long time to a temperature of more than 100 degrees. At the end
of eight days a mouldiness was developed in this infusion of hay.
'Where does this come from?' cried M. Pouchet triumphantly. Certainly
not from the oxygen, which had been prepared from a chemical compound
at the temperature of incandescence. The water had been equally
deprived of germs, since at the boiling temperature all germs would
have been destroyed. The hay also could not have contained them, for it
had been taken from a stove heated to 100 degrees. As it was urged,
however, that certain organisms could resist this temperature, M.
Pouchet heated the hay from 200 to 300 degrees, or to any temperature
that might be desired.
Pasteur came to disturb the triumph of M. Pouchet.
In a lecture which he gave at the Sorbonne in 1864, before a large
assembly composed of savants, philosophers, ladies, priests, and
novelists--Alexandre Dumas was in the first row--all showing eager
interest in the problems to be dealt with in the lecture, Pasteur
thus criticised the experiment of Pouchet: 'This experiment is
irreproachable, but irreproachable only on those points which have
attracted the attention of its author. I will demonstrate before you
that there is a cause of error which M. Pouchet has not perceived,
which he has not in the least suspected, which no one before him
suspected, but which renders his experiment as completely illusory as
that of Van Helmont's pot of dirty linen. I will show you where the
mice got in. I will prove to you, in short, that it is the mercury
which carries the germs into the vessels, or, rather, not to go beyond
the demonstrated fact, the dust which is suspended in the air.'
To render visible this floating dust, Pasteur caused the hall to
be darkened, and pierced the obscurity by a beam of light. There
then appeared, dancing and twirling in the beam, thousands of little
particles of dust.
'If we had time to examine them well,' continued Pasteur, 'we should
see them, though agitated with various movements, falling downwards
more or less quickly. It is thus that all objects become covered with
dust--the furniture, the table, the mercury in this basin. Since this
mercury was taken from the mine, how much dust must have fallen upon
it, to say nothing of all that has been intimately mixed up with it
during the numerous manipulations to which it has been subjected in the
laboratory? It is not possible to touch this mercury, to place the hand
in it, or a bottle, without introducing into the interior of the basin
the dust which lies on its surface. You will now see what takes place.'
Projecting, in the darkness, the beam of light upon the basin of
mercury, the liquid metal shone forth with its usual brilliancy.
Pasteur then sprinkled some dust upon the mercury, and, plunging a
glass rod into it, the dust was seen to travel towards the spot where
the rod entered the mercury, and to penetrate into the space between
the glass and the metal.
'Yes,' exclaimed Pasteur with a voice which gave evidence of the
sincerity of his conviction, 'yes, M. Pouchet had removed the germs
from the water and from the hay, but he had neglected to remove the
dust from the surface of the mercury. This is the cause of his error;
this is what has vitiated his whole arrangement.'
Pasteur then instituted experiments exactly similar to those of
Pouchet, but taking care to remove every cause of error which had
escaped the latter. He employed a glass bulb with a long neck, which
he bent, and connected with a tube of platinum placed in a furnace, so
that it could be heated nearly to redness. In the bulb he placed some
very putrescible liquids--urine for example. When the furnace which
surrounded the platinum tube was in action, Pasteur boiled the liquid
for some minutes, then he allowed it to cool, keeping the fire around
the platinum tube still active. During the cooling of the bulb the
external air was introduced, after having first travelled through the
red-hot platinum tube. The liquid was thus placed in contact with air
whose suspended germs were all burnt up.
In an experiment thus carried out, the urine remains unchanged--it
undergoes only a very slight oxidation, which darkens its colour a
little--but it exhibits no kind of putrefaction. If it be desired
to repeat this experiment with alkaline liquids, such as milk, the
temperature must be raised a little above the boiling point--a
condition easily realised with the apparatus just described. It is
only necessary to connect with the free extremity of the platinum
tube a glass tube bent at right angles, and to plunge the latter
to a depth of some centimeters into a basin of mercury. In these
circumstances ebullition goes on under a pressure greater than that of
the atmosphere, consequently at a temperature higher than 100 degrees
Centigrade.
It remained, however, to be proved that the floating dust of the air
embraces the germs of the lower organisms. Through a tube stopped with
cotton wool, Pasteur, by means of an aspirator, drew ordinary air.
In passing through the wool it was filtered, depositing therein all
its dust. Taking a watch-glass, Pasteur placed on it a drop of water
in which he steeped the cotton wool stopper and squeezed out of it,
upon a glass slide, a drop of water which contained a portion of the
intercepted dust. He repeated this process until he had extracted from
the cotton nearly all the intercepted dust. The operation is simple
and easily executed. Placing the glass slide with a little of the
soiled liquid under a microscope, we can distinguish particles of soot,
fragments of silk, scraps of wool, or of cotton. But, in the midst of
this inanimate dust, living particles make their appearance--that is
to say, organisms belonging to the animal or vegetable kingdom, eggs
of infusoria, and spores of cryptogams. Germs, animalculæ, flakes of
mildew, float in the atmosphere, ready to fall into any appropriate
medium, and to develop themselves at a prodigious rate.
But are these apparently organised particles which are found thus
associated with amorphous dust indeed the germs of microscopic living
beings? Granting the experiment devised by Pasteur to verify that
of Pouchet to be irreproachable, is Pasteur's interpretation of it
rigorously true? In presence of the problem of the origin of life,
all hypotheses are possible as long as the truth has not been clearly
revealed. Truly, it might be argued, if fermentation be caused by
germs, then the air which has passed through a red-hot platinum tube
cannot provoke fermentation, or putrefaction, or the formation of
organisms, because the germs of these last, which were suspended in the
air, have lost all vitality. But what right have you to speak of germs?
How do you know that the previous existence of germs is necessary to
the appearance and development of microscopic organisms? May not the
prime mover of the life of microscopic organisms be some appropriate
medium started into activity by magnetism, electricity, or even ozone?
Now, by the passing of the air through your red-hot platinum tube these
active powers are destroyed, and the sterility of your bulb of urine
has nothing surprising in it.
The partisans of spontaneous generation had often employed this
apparently formidable reasoning, and Pasteur thought it necessary to
strengthen the proof that the cotton wool through which his air had
filtered was really charged with germs.
By an ingenious method he sowed the contents of the cotton wool in the
same liquids that had been rendered sterile by boiling. The liquids
became fertile, even more fertile than if they had been exposed to
the free contact of atmospheric air. Now, what was there in the dust
contained in the cotton wool? Only amorphous particles of silk,
cotton, starch; and, along with these, minute bodies which, by their
transparency and their structure, were not to be distinguished from the
germs of microscopic organisms. The presence of imponderable fluids
could not here be pleaded.
Nevertheless, fearing that determined scepticism might still attribute
to the cotton wool an influence of some sort on account of its being
an organised substance, Pasteur substituted for the stoppers of cotton
wool stoppers of asbestos previously heated to redness. The result was
the same.
* * * * *
Wishing still further to dispose of the hypothesis that, in ordinary
air, an unknown something existed which, independent of germs, might be
the cause of the observed microscopic life, Pasteur began a new series
of experiments as simple as they were demonstrative. Having placed a
very putrescible infusion--in other words, one very appropriate to
the appearance of microscopic organisms--in a glass bulb with a long
neck, by means of the blowpipe, he drew out this neck to a very small
diameter, at the same time bending the soft glass to and fro, so as
to form a sinuous tube. The extremity of this narrow tube remained
open. He then boiled his liquid for some minutes until the vapour of
the water came out in abundance through the narrow open tube. In these
conditions the liquid in the bulb, however putrescible, is preserved
indefinitely without the least alteration. One may handle it, transport
it from place to place, expose it to every variety of climate, place
it in a stove with a temperature of thirty or forty degrees, the
liquid remains as clear as it was at first. A slight oxidation of the
constituents of the liquid, is barely perceptible. In this experiment
the ordinary air, entering suddenly at the first moment, finds in the
bulb a liquid very near the boiling temperature; and when the liquid
is so far cooled that it can no longer destroy the vitality of the
germs, the entrance of the air is correspondingly retarded, so that the
germs capable of acting upon the liquid, and of producing in it living
organisms, are deposited in the bends of the still moist tube, not
coming into contact with the liquid at all.
If, after remaining for weeks, months, or even years, in a heated
chamber, the sinuous neck of the bulb is snipped off by a file in the
vertical part of the stem, after twenty-four or forty-eight hours there
begin to appear mildew, mucors, bacteria, infusoria, exactly as in the
case of infusions recently exposed to the contact of ordinary air.
The same experiments may be repeated with slightly alkaline liquids,
such as milk, the precaution being taken of raising them to a
temperature higher than that of 100 degrees Centigrade.
The great interest of Pasteur's method consists in its proving
unanswerably that the origin of life, in infusions which have been
heated to the boiling point, is solely due to the solid particles
suspended in the air. Of gas, electricity, magnetism, ozone, things
known or unknown, there is nothing in ordinary atmospheric air which,
apart from these solid particles, can cause the fermentation or
putrefaction of the infusions.
Lastly, to convince the most prejudiced minds, and to leave no
contradiction standing, Pasteur showed one of these bulbs with the
sinuous neck which he had prepared and preserved for months and years.
The bulb was covered with dust. 'Let us,' said he, 'take up a little
of this outside dust on a bit of glass, porcelain, or platinum, and
introduce it into the liquid; the following day you will find that the
infusion, which up to this time remained perfectly clear, has become
turbid, and that it behaves in the same manner as other infusions in
contact with ordinary air.'
If the bulb be tilted so as to cause a little drop of the clear
infusion to reach the extremity of the bent part of the neck where
the dust particles are arrested, and if this drop be then allowed to
trickle back into the infusion, the result is the same--turbidity
supervenes and the microscopic organisms are developed. Finally, if one
of those bulbs which have stood the test of months and years without
alteration be several times shaken violently, so that the external air
shall rush into it, and if it be then placed once more in the stove,
life will soon appear in it.
In 1860 the Academy of Sciences had offered a prize, the conditions of
which were stated in the following terms: