III.
But there is still another class of chemical phenomena where the life without air of microscopic organisms is fully shown. Pasteur proved that in the special fermentation which bears the name of putrefaction the primum movens of the putrefaction resides in microscopic vibrios of absolutely the same order as those which compose the butyric ferment. The fermentation of sugar, of mannite, of gums, of lactate of lime, by the butyric vibrio, so closely resembles the phenomena of putrefaction, that one might call these fermentations the putrefaction of sugar and of the other products.
If it has been thought right to call the fermentation of animal matters putrefaction, it is because at the moment of the decomposition of fibrine, of albumen, of blood, of gelatine, of the substance of the tendons, &c., the sulphur, and even the phosphorus, which enter into their composition give rise to putrid odours, due to the evil-smelling gases of sulphur and phosphorus.
The phenomena of putrefaction being then simply fermentations, differing only in regard to the chemical composition of the fermenting matters, Liebig naturally included them in his general theory of the decomposition of organic matters after death. At a period long antecedent to Pasteur's labours it had been established that there existed in putrefying matters fungi or microscopic animalculæ, and the idea had taken shape that these creatures might have an influence in the phenomena. The proofs were wanting, but the notion of a possible relation remained. We may read in his 'Lessons on Chemistry' with what disdain Liebig mentioned these hypothetical opinions.
'Those who pretend to explain the putrefaction of animal substances by the presence of animalculæ,' he wrote, 'reason very much like a child who would explain the rapidity of the Rhine by attributing it to the violent motions imparted to it in the direction of Bingen by the numerous wheels of the mills of Mayence. Is it possible to consider plants and animals as the causes of the destruction of other organisms when their own elements are condemned to undergo the same decompositions as the creatures which have preceded them? If the fungus is the cause of the destruction of the oak, if the microscopic animalcula is the cause of the putrefaction of the dead elephant, I would ask in my turn what is the cause which determines the putrefaction of the fungus or of the microscopic animalcula when life is withdrawn from these two organisms?'
Thirty-two years later, and after Pasteur had accumulated, during more than twenty years, proof upon proof that the theory of Liebig would not stand examination, a physician of Paris, M. Bouillaud, asked, with the insistent voice of a querulous octogenarian: 'Let M. Pasteur then tell us here, in presence of the Académie de Médecine, what are the ferments of the ferments.'
Before replying to this argument, which Liebig and M. Bouillaud believed to be irrefutable, Pasteur, wishing to mark all the phases of the phenomena, expounded in a short preamble the part played by atmospheric oxygen in the destruction of animal and vegetable matters after death. It is easy to understand, indeed, that fermentation and putrefaction only represent the first phase of the return to the atmosphere and to the soil of all that has lived. Fermentations and putrefactions give rise to substances which are still very complex, although they represent the products of decomposition of fermentable matters. When sugar ferments, a large proportion of it becomes gas; but alongside of the carbonic acid gas which is formed, and which is, indeed, a partial return of the sugar to the atmosphere, new substances, such as alcohol, succinic acid, glycerine, and materials of yeast, are produced. When the flesh of animals putrifies, certain products of decomposition, also very complex, are formed with the vapour of water and the other gases of putrefaction. Where, then, does nature find the agents of destruction of these secondary products?
The great fact of the destruction of animal and vegetable matters is accomplished by slow combustion, through the appropriation of atmospheric oxygen. Here, again, one must banish from science the preconceived views which assumed that the oxygen seized directly on the organic matter after death, and that this matter was consumed by purely chemical processes. It is life that presides over this work of death.
If fermentation and putrefaction are principally the work of microscopic anaérobies, living without free oxygen, the slow combustion is found very largely, if not exclusively, to depend upon a class of infinitely small aérobies. It is these last which have the property of consuming the oxygen of the air. It is these lower organisms which are the powerful agents in the return to the atmosphere of all which has lived. Mildew, mould, bacteria, which we have already noticed, monads, two thousand of which would go to make up a millimeter, all these microscopic organisms are charged with the great work of re-establishing the equilibrium of life by giving back to it all that it has formed.
To demonstrate the important part played everywhere by these microscopic organisms, Pasteur made two experiments. He first introduced into vessels air deprived of all dust. This process we shall have occasion to examine in all its details, in connection with the researches on spontaneous generation. In these vessels, containing pure air, were placed the water of yeast with sugar dissolved in it, milk, sawdust—all of which had been deprived by heat of the germs of the lower organisms. The vessels and their contents were then subjected to a temperature of twenty-five to thirty-five degrees Centigrade. In a series of parallel experiments, made under the same conditions and at the same temperature, Pasteur took no steps to prevent the germination of the little seeds of mould suspended in the air, or associated with the substances contained in the vessels, neither did he avoid other infinitely small germs of the class aérobies.
After some time the air of all the vessels of the two series was submitted to analysis, when, behold, a very interesting fact! In the vessels where life had been withdrawn from the organic matters—that is to say, where there were no germs—the air still contained a large proportion of oxygen. In the vessels, on the contrary, where the microscopic organisms had been allowed to develop, the oxygen was totally absent, having been replaced by carbonic acid gas. And, further, for this absorption and total consumption of the oxygen gas a few days had sufficed; while in the vessels without microscopic life there remained, after several years, a considerable quantity of oxygen in a free state, so weak is the proportion of oxygen that the organic matters consume directly and chemically when the infinitely small organisms are absent.
But can these microscopic organisms, after having decomposed or burnt up all these secondary products, be in their turn decomposed?
How, cried M. Bouillaud, repeating his question, can they be destroyed or decomposed? How can their materials, which are of the same order as those of all the living creatures of the earth, be gasified and caused to return to the atmosphere? After having been charged with the transformation of others, whose business will it be to transform them?
A ferment which has finished its work, replied Pasteur, and which for want of aliment cannot continue it, becomes in its turn an accumulation, so to speak, of dead organic matters. Such, for example, would be an accumulation of yeast exposed to the air. Leave this mass to itself in summer temperature, and you will see appear in the interior of the mass anaérobic vibrios and the putrefactions associated with their life when protected from contact with the air. At the same time, on the surface of the entire mass—that is to say, that which finds itself in immediate contact with the oxygen of the air—the germs of bacteria, the seeds of mould will grow, and, by fixing the oxygen, determine the slow combustions which gasify the mass. The ferments of ferments are simply ferments. As long as the aérobic ferments of the surface have at their disposal free oxygen, they will multiply and continue their work of destruction. The anaérobic vibrios perish for want of new matter to decompose, and they form, in their turn, a mass of organic matter which, by and by, becomes the prey of aérobies. The portion of the aérobies which has lived becomes the prey either of new aérobies of different species, or of individuals of their own species, so that from putrefaction to putrefaction, and from combustion to combustion, the organic mass with which we started finds itself reduced to an assemblage of anaérobic and aérobic germs—of those same germs which were mixed up in the original primitive organic substances.
Though a collection of germs becomes again in its turn a collection of organic matter, subject to the double action of the phenomena of putrefaction and of combustion, there need be no anxiety as to their ultimate destruction; in the final analysis they represent life under its eternal form, for life is the germ, and the germ is life.
Thus in the destruction of that which has lived, all reduces itself to the simultaneous action of these three great natural phenomena—fermentation, putrefaction, and slow combustion. A living organism dies—animal, or plant, or the remains of one or the other. It is exposed to the contact of the air. To the life which has quitted it succeeds life under other forms. In the superficial parts, which the air can reach, the germs of the infinitely small aérobies hatch and multiply themselves. The carbon, the hydrogen, and the nitrogen of the organic matters are transformed by the oxygen of the air, and under the influence of the life of these aérobies, into carbonic acid, vapour of water, and ammonia gas. As long as organic matter and air are present, these combustions will continue. While these superficial combustions are going on, fermentation and putrefaction are doing their work in the interior of the mass by the developed germs of the anaérobies, which not only do not require oxygen for their life, but which oxygen actually kills. Little by little, at length, by this work of fermentation and slow combustion, the phenomenon is accomplished. Whether in the free atmosphere, or under the earth, which is always more or less impregnated with air, all animal and vegetable matters end by disappearing. To arrest these phenomena an extremely low temperature is required. It is thus that in the ice of the Polar regions antediluvian elephants have been found perfectly intact. The microscopic organisms could not live in so cold a temperature. These facts still further strengthen all the new ideas as to the important part performed by these infinitely small organisms, which are, in fact, the masters of the world. If we could suppress their work, which is always going on, the surface of the globe, encumbered with organic matters, would soon become uninhabitable.
ACETIC FERMENTATION.
THE MANUFACTURE OF VINEGAR.
Soon afterwards Pasteur came upon a most curious illustration of the 'fixation' of atmospheric oxygen by a microscopic organism—the transformation of wine into vinegar. As its name indicates, vinegar is nothing else than wine turned sour. Everybody has remarked that wine, left to itself, in circumstances which occur daily, is frequently transformed into vinegar. This is noticed more particularly when bottles, having been uncorked, are left in a half-empty condition. Sometimes, however, wine turns sour even in corked bottles. In this case we may be sure that the bottles have been standing upright, and that corks more or less defective have permitted the air to penetrate into the wine. The presence of air, in fact, is indispensable to the chemical act of transforming wine into vinegar. How does this air intervene? And what is the little microscopic creature which, in conjunction with the air, becomes the agent of this fermentation?
In a celebrated lecture given at Orleans at the request of the manufacturers of vinegar in that town, Pasteur, after having stated the two foregoing scientific questions, proceeded to examine the difference between wine and vinegar. What takes place in the fermentation of the juice of the grape which yields the wine? The sugar of this juice disappears, giving place to carbonic acid gas, which is exhaled during fermentation, and to alcohol, which remains in the fermented liquid. Formerly, chemists gave the name of 'spirit' to all volatile matters which could be collected from distillation. Now, when we distil wine and condense the vapour in a worm surrounded by cold water, we collect the spirit of wine at the extremity of the worm—this, when the water with which it is mixed during distillation is withdrawn from it, we designate by the name of alcohol. Vinegar contains no alcohol. When distilled it yields water and a spirit. But this spirit is acid, with a very pungent odour, and not inflammable like spirit of wine. Separated from the water which had accompanied it during the distillation, this spirit takes the name of acetic acid. This is the form in which it is used in smelling bottles—in those bottles of English salts the vapour of which is so penetrating.
In the formation of vinegar in contact with air the alcohol disappears, and is replaced by acetic acid. The air has thus given up something to the wine. Atmospheric air every one knows to be a mixture of nitrogen and oxygen, the nitrogen in the proportion of four-fifths of the total volume, and the oxygen of one-fifth. Well, in the transformation of wine into vinegar the nitrogen remains inactive. It is the oxygen alone which enters into combination with the alcohol. You ask for the proof of this? Take a bottle of wine turned sour, a bottle which at the same time is stopped hermetically; if the oxygen of the air contained in the bottle has combined with the alcohol, then, instead of air, there will be nothing in the bottle but nitrogen gas. Turn the bottle upside down and open it in a basin of water. The water of the basin will rush into the bottle to fill the partial vacuum created by the disappearance of the oxygen. The volume of water which enters the bottle is precisely equal to a fifth part of the total original volume of the air which the bottle contained at the time when it was closed. Moreover, it is easy to show that the gas which remains in the bottle has the properties of nitrogen gas. A lighted match is extinguished in it as if plunged into water, and a bird dies immediately in it of asphyxia.
If we confine our knowledge to what has gone before, it would seem that alcohol diluted with water and exposed to the air ought to furnish acetic acid. It is not so, however. Pure water alcoholised to the degree of ordinary wines may remain for whole years in contact with the air, without the least acetification. In this difference between natural wine and pure water alcoholised, and exposed to contact with air, we touch upon a vital point in the phenomena of fermentation. The celebrated theory of Liebig, which Pasteur was destined to overthrow, might be thus summed up:—If pure alcoholised water cannot become sour in contact with air, as is the case with wine, it is because the pure alcoholised water lacks the albuminoid substance which exists in the wine in a state of chemical alteration, and which is a ferment capable of causing the oxygen of the air to combine with the alcohol. And the proof, according to Liebig, that things act rigorously thus is, that if you add to the mixture of water and alcohol a little flour, or a little meat-juice, or even a minute quantity of any vegetable juice, the acetic fermentation arises, as if by compulsion. In other words, by the addition of a small quantity of any nitrogenised substance in process of alteration, you cause the union of the oxygen of the air with the alcohol.
There is doubtless always in the wine, when it turns sour, a necessary intermediary, producing the fixation of the oxygen of the air; since in no circumstances can pure alcohol, diluted to any degree whatever with pure water, transform itself into vinegar. But this necessary intermediary is not, as the German theory would have it, a dead albuminoid substance; it is a plant, and of all plants one of the simplest and most minute, which has been known from time immemorial under the name of flower of vinegar. This little fungus is invariably present on the surface of a wine which is being transformed into vinegar. Liebig was not ignorant of this, but he regarded it as a simple coincidence. Do we not know, said he, that whenever an infusion of organic matter is exposed to the air it becomes covered with a cryptogamic vegetation, or is invaded by a crowd of animalculæ? Is not vinegar a vegetable infusion? Vinegar affords a refuge to the flower of vinegar, just as it gives refuge to what are called the little eels of vinegar.
We can appreciate here the uncertainties of pure observation. The great art—and no one practised it better than Pasteur—consists in instituting decisive experiments which leave no room for an inexact interpretation of facts. These decisive proofs of the true part played by the little microscopic fungus, by this flower of vinegar, this mycoderma aceti, are thus formulated by Pasteur. It is but another example of the method which he used in alcoholic, lactic, and tartaric fermentations. The theories of Berzelius, of Mitscherlich, and of Liebig were destined again to receive the rudest shocks by the demonstration of these rigorous facts.
Let us place a little wine in a bottle, then hermetically seal it, and leave it to itself. In these conditions the wine becomes sour. But if we take the precaution of putting the bottle into hot water, so that the wine and the air in the bottle may be heated for some instants to a temperature of 60° Centigrade, and if, after cooling, we leave the bottle to itself, the wine in these conditions will never become transformed into vinegar. The heating, however, must have left intact the albuminoid or nitrogenous substances contained in the wine. These, then, cannot constitute the ferment of the vinegar. Can it be maintained that by heating the wine to 60° we have altered the albuminoid matter, which is, on this account, no longer able to act as a ferment, or, in other words, no longer able to determine the union of the oxygen of the air with the alcohol? This hypothesis falls to pieces before the following experiment. Open the bottle, blow into it with bellows, so that the once heated wine shall come into contact with ordinary air, and the acetification of the wine will take place.
But the master experiment is the following. We have seen that pure alcoholised water never turns sour unless some albuminoid matter is introduced into it. Pasteur saw that this albuminoid matter might be completely suppressed and replaced by saline crystallisable substances, alkaline and earthy phosphates, to which has been added a little phosphate of ammonia. In these conditions, especially if the alcoholised water be acidulated by small quantities of pure acetic acid, one actually sees the mycoderm developing, and the alcohol transforming itself into acetic acid. It is not possible to demonstrate in a more convincing manner that the albuminoid matters of the wine are not in this case the acetic ferment. These albuminoid matters, however, contribute to the acetic fermentation, but only as being an aliment to the mycoderma aceti, and notably a nitrogenous aliment. The true and only ferment of vinegar is the little fungus; it is the great agent of the phenomenon; it, indeed, accomplishes all.
Is there not a great charm in seeing an obscure subject clearly illuminated by facts well understood and well interpreted? If in a bottle containing wine and air and raised to a temperature of 50° or 60° the wine never turns sour, it is because the germs of the mycoderma aceti, which the wine and the air hold in suspension, are deprived of all vitality by the heat. Placed, however, in contact with ordinary air, this once-heated wine can turn sour; because, though the germs of the mycoderma aceti contained at first in the wine are killed, this is not the case with those derived from the surrounding air. Pure alcoholised water never turns sour, even in contact with ordinary air, and with whatever germs this air may carry, or that may be found in the dust of the vessels which receive it. The reason is that these germs cannot become fertile because of the absence of their indispensable food. Wine in bottles well filled and laid flat do not acetify; this is because the mycoderm cannot multiply for lack of oxygen. Without doubt the air constantly penetrates through the pores of the cork, but always in such feeble quantities that the colouring matters of the wine, and other more or less oxydisable constituents, take possession of it without leaving the smallest quantity for the germs of the mycoderm which are generally suspended in the wine. When the bottle is upright the conditions are quite altered. The desiccation of the cork renders it much more permeable to the air, and the germs of the mycoderm on the surface of the liquid, if any exist there, are enveloped by air.
Thus, to recapitulate in a few words the principles which have just been established; it is easy to see that the formation of vinegar is always preceded by the development, on the surface of the wine, of a little plant formed of strangulated particles, of an extreme tenuity, and the accumulation of which sometimes takes the form of a hardly visible veil, sometimes of a wrinkled film of very slight thickness, and greasy to the touch, because of the various fatty matters which the plant contains.
This cryptogam has the singular property of condensing considerable quantities of oxygen and of provoking the fixation of this gas upon the alcohol, which is thereby transformed into acetic acid. The little mycoderm is not less exacting than larger vegetables. It must have its appropriate aliments. Wine offers them in abundance: nitrogenous matters, the phosphates of magnesia and of potash. The mycoderm thrives, moreover, in warm climates. To cultivate it in temperate regions like ours it is well to warm artificially the places where it is cultivated. But if wine contains within itself all the elements necessary to the life of the little mycoderm, this life is further promoted by rendering the wine more acid through the addition of acetic acid.
What, then, can be more simple than to produce vinegar from wine—a manufacture which justly makes the reputation of the town of Orleans? Take some wine, and after having mixed with it one-fourth or one-third of its volume of vinegar already formed, sow on its surface the little plant which does the work of acetification. It is only necessary to skim off, by means of a wooden spatula, a little of the mycodermic film from a liquid covered with it, and to transfer it to the liquid to be acetified. The fatty matters which it contains render the wetting of it difficult. Thus, when we plunge into the liquid the spatula covered with the film, the latter detaches itself and spreads out over the surface instead of falling to the bottom. When we operate in summer, or in a room heated to 15° or 25° Centigrade in winter, in twenty-four or forty-eight hours at most, the mycoderm covers the whole liquid, so easy and rapid is its development. After some days all the wine has become vinegar.
On one occasion, in a discussion which he was holding at the Academy of Sciences, Pasteur, wishing to affirm the prodigious activity of the life and multiplication of this little organism, expressed himself thus:—
'I would undertake in the space of twenty-four hours to cover with mycoderma aceti a surface of vinous liquid as large as the hall in which we are here assembled. I should only have to sow in it the day before almost invisible particles of newly-formed mycoderma aceti.'
Let the reader try to imagine the millions upon millions of little mycoderma particles which would come to life in that one day.
But how is the mycoderm seed to be obtained in the first instance? Nothing more simple. The mycoderma aceti is one of those little so-called 'spontaneous' productions which are sure to appear of themselves on the surface of liquids or infusions suitable to their development. In wine, in vinegar, or suspended in air, everywhere around us, in our towns, in our houses, there exist germs of this little plant. If we wish to procure some fresh mycoderm it is only necessary to put a mixture of wine and vinegar into a warm place. In a few days, generally, if not always, there appear here and there little greyish patches scattering the light instead of regularly reflecting it, as does the surrounding liquid. These specks go on increasing progressively and rapidly. This is the mycoderma aceti raised from the seeds which the wine or the added vinegar contained, or which the air deposited; just as we see a field covered with divers weeds by seeds naturally distributed in the earth, or which have been brought to it by the wind or by animals. Even in this last circumstance the comparison holds good, for after you have put wine or vinegar in a warm place there soon appear, whence we know not, little reddish flies, so commonly seen in vinegar manufactories, and in all places where vegetable matter is turning sour. With their feet, or with their probosces, these flies transport the seed.
At Orleans the process for the manufacture of vinegar is very simple. Barrels ranged over each other have on each of their vertically-placed bottoms a circular opening some centimeters in diameter, and a smaller hole adjacent, called fausset, for the air to pass in and out when the large opening is closed, either by the funnel, through which the wine is introduced, or by the syphon, which is used for drawing off the vinegar. These barrels, of which the capacity is 230 litres, are half filled. The manual labour consists in keeping up a suitable temperature in the vessel, and in drawing from it every eight days about eight or ten litres of vinegar, which are replaced by eight or ten litres of wine.
A barrel in which this give-and-take of wine and vinegar goes on is technically called a 'mother.' The starting of a 'mother' is not a rapid process. We begin by introducing into the barrel 100 litres of very good and very limpid vinegar; then two litres only of wine are added. Eight days after, three litres of wine are added, a week later four or five, until the barrel contains about 180 to 200 litres. Then for the first time vinegar is drawn off in sufficient quantity to bring back the volume of the liquid to about 100 litres. At this moment the labours of the 'mother' begin. Henceforward ten litres of vinegar may be drawn off every eight days, to be replaced by ten litres of wine. This is the maximum that a cask can yield in a week. When the casks work badly, as is often the case, it is necessary to diminish their production.
This Orleans system has many drawbacks. It requires three or four months to prepare what is called a 'mother,' which must be nourished with wine very regularly once a week under penalty of seeing it lose all its power. Then it is necessary to continue the manufacture at all times, whether the vinegar be required or not. To reconstitute a 'mother,' one must begin from the very beginning, a process which involves a loss of three or four months' time. Lastly—a condition which is at times very inconvenient—a 'mother' cannot be transported from one place to another, or even from one part of the same locality to another. The 'mother,' in fact, must rest immovable.
Pasteur advised the suppression of the 'mothers.' He recommended an apparatus, which is simply a vat, placed in a chamber the temperature of which can be raised to 20° or 25° Centigrade. In these vats vinegar already formed is mixed with wine. On the surface is sown the little plant which converts the wine into vinegar. The mode of sowing it has been already explained. The acetification begins with the development of the plant.
A great merchant of Orleans, who had from the first adopted Pasteur's process, and who had won the prize offered by the 'Society for the Encouragement of National Industry' for a manufactory perfected after these principles, has stated that at the end of nine or ten days, sometimes even in eight, all the acetified wine is converted into vinegar. From a hundred litres of wine he drew off ninety-five litres of vinegar. After the great rise of temperature observed at the moment of the formation of the vinegar, and which is caused by the chemical union of the alcohol and the oxygen of the air, the vinegar is allowed to cool. It may then be drawn from the vat, introduced into barrels, refined, and straightway delivered, fit for consumption. When the vat is quite emptied, and well cleaned, a new mixture is made of vinegar and wine, the little plant is sown as before, and the same facts are reproduced in the second as in the first operation.
In the vessels where vinegar is preserved, whether in the manufactories, in private houses, or in grocers' shops, it often happens that the liquid becomes turbid, and impoverished in an extraordinary manner; it even ends in putrefaction, if a remedy be not promptly applied. Pasteur has pointed out the cause of these phenomena. After the alcohol has become acetic acid by the combustive action of the mycoderm, the question remains, what becomes of the mycoderm? Most frequently it falls to the bottom of the vessel, having no more work to accomplish. This is a phase of the manufacture which must be watched with care. It is shown by the experiments of Pasteur that the mycoderma aceti can live on vinegar already formed, maintaining its power of fixing the oxygen on certain constituents of the liquid. In this case the acetic acid itself is the seat of the chemical action—in other words, the oxygen unites with the carbon of the acetic acid, and transforms it into carbonic acid, and as the acetic acid has a composition which can be represented by carbon and water, it follows that if the combustion is allowed to take its course, instead of vinegar we have eventually nothing but water mixed with a small proportion of nitrogenous and mineral matters, and the remains of the mycoderm. We have thus an ordinary organic infusion exempt from all acidity, and one which could not be better fitted to become the prey of the vibrios of putrefaction or of the aérobic mucors. By these mucors, moreover, which form a film on the surface of the liquid after the mycoderm has fallen, the anaérobic vibrios, protected from the action of the air, can come into active existence. Here we find ourselves in presence of one of those double phenomena, of putrefaction in the deeper parts of the liquid, and of combustion at the surface which is in contact with the air. Nothing is more prejudicial to the quality of the vinegar than the setting in of this combustion after the vinegar has been formed, and when it contains no more alcohol. The first materials of the vinegar upon which the oxygen transmitted by the mycoderm fixes are, in fact, the ethereal and aromatic constituents which give to vinegar its chief value.
Another cause of the deterioration of the quality of vinegar, which is sometimes very annoying to the manufacturer, consists in the frequent presence of little eel-like organisms, very curious when viewed with a strong magnifier. Their bodies are so transparent that their internal organs can be easily distinguished. These eel-like creatures multiply with extraordinary rapidity. Certainly there is not a single barrel of vinegar manufactured by the Orleans system which does not contain them in alarming numbers. Prior to Pasteur's investigations, the ignorance regarding these organisms was such that they were actually considered necessary to the production of the vinegar; whereas they are, on the contrary, most inimical to it, and must, if possible, be got rid of. This is, moreover, rendered desirable by the repugnance which is naturally felt to using a liquid defiled by the presence of such animalcules—a repugnance which becomes almost insurmountable to anyone who has once seen through a microscope the swarms contained in a drop of vinegar. The mischief wrought by these little beings in the manufacture of vinegar results from the fact that they require air to live. The effect can easily be perceived by filling to the brim a bottle of vinegar, corking it, and then comparing it with a similar bottle half filled with the same vinegar, and left uncorked in contact with the air. In the first bottle, the motions of the eel-like creatures become gradually slower, until after a few days they cease to multiply and fall lifeless to the bottom of the vessel. In the second bottle, on the contrary, they continue to swarm and move about. This need of oxygen is further demonstrated by the fact that, if the vinegar reaches a certain depth in the bottle, life is suspended in the lower parts, and the little eel-like organisms, in order to breathe more freely, form a crawling zone in the upper layers of the liquid.
Connecting these observations with the other fact that the vinegar is formed by the action of the mycodermic film on its surface, we can understand at once that the mycoderm and the little eels continually carry on a struggle for existence, since both of these living things—the one animal the other vegetable—imperiously demand the same aliment, oxygen. They live, moreover, in the same superficial layers, a circumstance which gives rise to very curious phenomena. When, for one reason or another, the film of mycoderm is not formed, or when there is any delay in its production, the little eels invade in such great numbers the upper layers of the liquid that they absorb all the oxygen. The little plant has in consequence great difficulty in developing itself or even in beginning its life. Reciprocally, when the work of acetification is active, and when the mycoderm has occupied the upper layers, it gradually drives away the eels, which take refuge, not deep down, where they would perish, but against the moist sides of the barrel or the vat. There they form a thick whitish scum all in motion. It is a very curious spectacle. Here their enemy, the mycoderm, can no longer injure them to the same extent, since they are surrounded with air; and here they wait with impatience for the moment when they can again take their place in the liquid, and, in their turn, fight against the mycoderm. In Pasteur's process, where the vats are very often cleansed, it is easy to keep them free from these little animalcules; they have not time to multiply to a hurtful extent. Indeed, if the operation be well conducted, they do not make their appearance at all.
Nearly all Pasteur's publications have had from the moment of their appearance to undergo the severest criticism. Their novelty caused them to clash with the prejudices and errors current in science. His researches on fermentation provoked lively opposition. Liebig did not accept without recrimination a series of researches which concurred in upsetting the theory he had enunciated and defended in all his works. After having kept silence for ten years, he published, at Munich, where he was professor, a long memoir entirely directed against Pasteur's results. In 1870, on the eve of the war, Pasteur, who was at that time returning from a scientific journey into Austria, determined to pass by Munich, with the view of attempting to convince his distinguished adversary. Liebig received him with great courtesy, but, hardly recovered from an illness, he alleged his convalescence as a reason for declining all discussion.
Then followed the Franco-German war. Hardly was it terminated when Pasteur brought before the Academy of Sciences at Paris a defence of what he had published, as a sort of challenge to his illustrious opponent. The memoir of Liebig was filled with the most skilful arguments.
'I pondered it for nearly ten years before producing it,' he wrote. Pasteur, putting aside all subtleties of argument, went straight to the two objections of the German chemist which lay at the root of the discussion.
It may be remembered that one of the most decisive proofs by which Pasteur overthrew Liebig's theory resulted from the experiments in which by the aid of mineral bodies and fermentable matter he produced a special living ferment for each definite fermentation. By removing all nitrogenous organic matter, which in Liebig's theory constitutes the ferment, Pasteur established, at one and the same time, the life of the ferment and the absence of all action of albuminoid matter in process of alteration. Liebig here formally contested the fact that Pasteur had been able to produce yeast and alcoholic fermentation in a sweetened mineral medium by sowing therein an infinitesimal quantity of yeast. It is certain that, ten years previously, when Pasteur announced the production of yeast life and alcoholic fermentation under such conditions, his experiment was one so difficult to perform that it sometimes happened to Pasteur himself to be unable to reproduce it. The cells of yeast sown in the sweetened mineral medium found themselves often associated with other microscopic organisms, which were singularly hurtful to the life of the yeast. Pasteur was at this period far from being familiarised with the delicacy which such experiments require, and he did not yet know all the precautions indicated later on, which were indispensable to success. Though in his original memoir of 1860 Pasteur had pointed out the difficulties of his experiment, these difficulties existed nevertheless. Liebig took hold of them with skill, exaggerated them; saw, so to speak, nothing but them; and declared that the results announced never could have been obtained. But in 1871 the fundamental experiment of Pasteur, on the life of yeast in a sweetened mineral medium, had become a trifle for him. He knew exactly how to form media deprived of all foreign germs, how to prepare pure yeast, and how to prevent the introduction of new germs, which could develop in the liquids and hinder the life of the yeast.
'Choose,' said he to Liebig, 'from the members of the Academy one or several, and ask them to decide between you and me. I am ready to prepare before you and before them, in a sweetened mineral medium, as much yeast as you can reasonably ask for, and with substances provided by yourself.'
Liebig's second objection had reference to acetic fermentation. The process of acetification known as that of 'beech shavings' is widely practised in Germany and even in France. It consists in causing alcohol diluted with water and with the addition of some millièmes of acetic acid to trickle slowly into barrels or vats filled with shavings of beech, either massed together without order or disposed in layers after having been rolled up like the spring of a watch. Openings formed in the sides of the barrel, and in a double bottom upon which the shavings rest, permit the access of the air, which rises into the barrel as it would in a chimney, and yields all or part of its oxygen to the alcohol to convert it into acetic acid. All writers prior to Pasteur, and Liebig in particular, maintained that the shavings acted like porous bodies in the same manner as finely divided platinum. The acetic acid, they said, was formed by a direct oxidation, without any other influence than the porosity of the wood. This view of the subject was rendered plausible by the fact that in many manufactories the alcohol employed is that of distillation, which contains no albuminoid substances. Moreover, the duration of the shavings is in a sense indefinite.
According to Pasteur, the shavings perform only a passive part in the manufacture. They promote the division of the liquid and cause a considerable augmentation of the surface exposed to the air. They moreover serve as a support for the ferment, which is still, according to him, the mycoderma aceti, under the mucous form proper to it when submerged.
Certainly appearances were far from being favourable to this view. 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:
'To endeavour by well-contrived experiments to throw new light upon the question of spontaneous generation.' The Academy added that it demanded precise and rigorous experiments equally well studied on all sides; such experiments, in short, as should render it possible to deduce from them results free from all confusion due to the experiments themselves. Pasteur carried away the prize, and no one, it will be acknowledged, deserved it better than he. Nevertheless, to his eyes, the subject was still beset with difficulties. In the hot discussions to which the question of spontaneous generation gave rise, the partisans of the doctrine continually brought forward an objection based on an opinion already referred to, and first enunciated by Gay-Lussac. As already known to the reader, Gay-Lussac had arrived at the conclusion that, in Appert's process, one condition of the preservation of animal and vegetable substances consisted in the exclusion of oxygen.
Even this proposition was soon improved upon, and it became a current opinion in science that the smallest bubble of oxygen or of air which might come in contact with a preserve would be sufficient to start its decomposition. The partisans of spontaneous generation—the heterogenists—thenceforward threw their objections to Pasteur into this form:
'How can the germs of microscopic organisms be so numerous that even the smallest bubble of air contains germs capable of developing themselves in every organic infusion? If such were the case the air would be encumbered with organic germs.' M. Pouchet said and wrote that they would form a thick fog, as dense as iron.
But Pasteur showed that the interpretation of Gay-Lussac's experiment, with respect to the possible alteration of preserves by a small quantity of oxygen gas, was quite erroneous. If, after a certain time, an Appert preserve contains no oxygen, this is simply because the oxygen has been gradually absorbed by the substances of the preserve, which are always more or less chemically oxidisable. But in reality it is easy to find oxygen in these preserves. Pasteur did not fail to perceive that the interpretation given to Gay-Lussac's experiment was wrong in another particular. He proved the fallacy of the assumption that the smallest quantity of air was always capable of producing microscopic organisms.
More thickly spread in towns than in the country, the germs become fewer in proportion as they recede from human habitations. Mountains have fewer than plains, and at a certain height they are very rare.
Pasteur's experiments to prove these facts were extremely simple. He took a series of bulbs of about a quarter of a litre in capacity, and, after having half filled the bulbs with a putrescible liquid, he drew out the necks by means of the blowpipe, then he caused the liquid to boil for some minutes, and during the ebullition, while the steam issued from the tapering ends of the bulbs, he sealed them with the lamp. Thus prepared, the bulbs can be easily transported. As they are empty of air—that which they originally contained having been driven out with the steam—when the sealed end of a bulb is broken off, the air rushes into the tube, carrying with it all the germs which this air holds in suspension. If it is closed again immediately afterwards by a flame, and if the vessels are then left to themselves, it is easy to recognise those in which a change occurs. Now, Pasteur established that, in whatever place the operation might be carried on, a certain number of bulbs would escape alteration. They must not, however, be opened in a room after dusting the furniture or sweeping the floor, for in this case all the bulbs would become altered because of the great quantity of germs raised by the dusting and remaining suspended in the air.
Pasteur started for Arbois with a series of bulbs. Some he opened in the country far from all habitations; others he opened at the foot of the mountains which form the first range of the Jura; a series of twenty-four bulbs was opened upon Mount Poupet, at 850 meters above the level of the sea; and, lastly, twenty others were transported to the Montanvert, near the Mer de Glace, at an elevation of 2,000 meters. He afterwards brought his whole collection back to Paris, and in the month of November, 1860, deposited them on the table at the Academy of Sciences.
Of the twenty bulbs first opened in the country, eight contained organised productions. Of the twenty opened on the heights of the Jura, five only were altered, and of the twenty opened upon the Montanvert during a strong wind which blew from the glacier, one alone was altered.
If a similar series of experiments were made in a balloon, it would be found that the air of the higher atmosphere is absolutely free from germs. Care would, however, be necessary to prevent the introduction of dust particles, which the rigging and the aëronauts themselves might carry with them.
But we have not yet related all. So far, all these conclusive experiments had been made only on organic liquids, very putrescible it is true, but which had all been subjected to boiling or even to temperatures higher than 100 degrees Centigrade. The partisans of spontaneous generation might then be justified in saying that if the precaution had been taken of putting into contact with pure air natural organic liquids in a state compatible with the operations of animal and vegetable life, the results would have been different. Under such conditions, life would have appeared spontaneously in the production of microscopic organisms. None of Pasteur's opponents had formulated this argument; but Pasteur himself, who had within him an adversary always present, always on the alert, prepared to yield only to accumulated proofs, saw this objection. He was not satisfied until he had succeeded in completely refuting it. Having by means of ingenious experimental arrangements deprived some air of all living germs, he placed in contact with this pure air the most putrescible liquids, particularly venous blood, arterial blood, and urine. He took these liquids directly from the veins, the arteries, and the bladders of animals in full health. No alteration was produced. In due time a chemical absorption of small quantities of oxygen took place, but neither fermentation nor putrefaction, nor the smallest development of bacteria, of vibrios, or of mould. After this, Pasteur was able legitimately to exclaim in his celebrated lecture at the Sorbonne:
'There is not one circumstance known at the present day which justifies the assertion that microscopic organisms come into the world without germs or without parents like themselves. Those who maintain the contrary have been the dupes of illusions and of ill-conducted experiments, tainted with errors which they know not how either to perceive or to avoid. Spontaneous generation is a chimera.'
Pasteur was not alone in affirming this fixed conviction. With the authority of a judge delivering sentence in court, M. Flourens, permanent Secretary of the Academy of Sciences, pronounced these words before the whole Academy:
'As long as my opinion was not formed I had nothing to say; now it is formed and I can speak. The experiments are decisive. If spontaneous generation be a fact, what is necessary for the production of animalculæ? Air and putrescible liquids. Now Pasteur puts together air and putrescible liquids and nothing is produced. Spontaneous generation, then, has no existence. Those who still doubt have failed to grasp the question.'
But some adversaries remained incredulous. When Pasteur had announced the result of his experiments, and brought before the Academy his series of bulbs, Pouchet and Joly declared that if Pasteur had opened his bulbs in the Jura and on the Mer de Glace, they, on their part, had been on the top of the Maladetta, and had proved there the inexactitude of Pasteur's results.
Pasteur asked to be judged by the Academy. 'A commission alone,' said he, 'will terminate the debate.' The commission was named, and the position on both sides was clearly stated.
'I affirm,' said Pasteur, 'that everywhere it is possible to take from the midst of the atmosphere a certain quantity of air which contains neither egg nor spore, and which does not produce organisms in putrescible solutions.'
On his side, M. Joly wrote: 'If one alone of your bulbs remains unaltered we shall loyally acknowledge our defeat.' Lastly, M. Pouchet, as distinct and positive as M. Pasteur, said: 'I affirm that in whatever place I take a cubic decimeter of air, when this air is placed in contact with a fermentable liquid enclosed in a glass vessel hermetically sealed, the liquid will become filled with living organisms.'
This double declaration, which excited at that time all the learned world, took place in the month of January, 1864. Eager to engage in the combat, Pasteur waited impatiently for the order of the Commission that this experiment, which was to decide everything, should be made. But M. Pouchet begged for a postponement, desiring, he said, to wait for the warm season. Pasteur was astonished, but resigned himself to the delay. The Commission and the opponents met on June 15.
The Commission announced, 'that, as the whole dispute turned upon one simple fact, one single experiment ought to be undertaken, which alone would close the discussion.'
The partisans of spontaneous generation wished nevertheless to go through the entire series of their experiments. In vain the Commission tried to persuade them that this would make the judgment as long as the discussion itself had been, that all bore upon one fact, and that this fact could be decided by a single experiment. The heterogenists would not listen to this. M. Pouchet and M. Joly withdrew from the contest.
M. Jamin, an exact and authorised historian of these debates, observed that 'the heterogenists, however they may have covered their retreat, were thereby self-condemned. If they had been sure of the fact—which they were solemnly engaged to prove, under penalty of acknowledging themselves defeated—they would have hastened to demonstrate it, for it would have been the triumph of their doctrine. People do not allow themselves to be condemned by default except in causes in which they have no confidence.'
STUDIES ON WINE.
Having thus solved the problem of spontaneous generation, a problem which was but a parenthesis forced upon his attention, Pasteur returned to fermentation. Guided by his studies on vinegar and other observations of detail, he undertook an inquiry into the diseases of wine. The explanations of the changes which wine was known to undergo rested only on hypothesis. From the time of Chaptal, who was followed by Liebig and Berzelius, all the world believed wine to be a liquid in which the various constituents react upon each other mutually and slowly. The wine was thought to be continually 'working.' When the fermentation of the grape is finished, equilibrium is not quite established between the diverse elements of the liquor. Time is needed for them to blend together. If this reciprocal action be not regular, the wine becomes bad. This was, in other words, the doctrine of spontaneity. Without support from carefully reasoned experiments, these explanations could not satisfy Pasteur, especially at a moment when he had just been proving that there was nothing spontaneous either in the phenomena of fermentation or in animal and vegetable infusions.
Pasteur tried first of all to show that wine does not 'work' as much as it was supposed to do. Wine being a mixture of different substances, among which are acids and alcohol, particular ethers are no doubt formed in it in course of time, and similar reactions perhaps take place between the other constituents of the liquid. But if the exactitude of such facts cannot be denied, based as they are upon general laws, confirmed and extended by recent inquiries, Pasteur thought that a false application was made of them when they were employed to explain the maladies of wine, the changes which occur in it through age—in a word, the alterations, whether good or bad, which wines are subject to. The 'ageing' of wine soon appeared to him to consist essentially in the phenomena of oxidation, due to the oxygen of the air which dissolves and is diffused in the wine. He gave manifest proofs of this. I will only mention one of them. New wine inclosed in a glass vessel hermetically sealed keeps its freshness; it does not 'work,' it does not 'age.' Pasteur demonstrated besides, that all the processes of wine-making are explained by the double necessity of oxygenising the wine to a suitable degree, and of preventing its deterioration. In seeking for the actual causes of injurious alterations, Pasteur, always obedient to a preconceived idea, while carefully controlling it with the utmost rigour of the experimental method, asked himself whether the diseases of wine did not proceed from organised ferments, from little microscopic vegetations? In the observed alterations, he thought, there must be some influences at work foreign to the normal composition of the wine.
This hypothesis was verified. In his hands the injurious modifications suffered by wines were shown to be correlative with the presence and the multiplication of microscopic vegetations. Such growths alter the wine, either by subtracting from it what they need for their nourishment, or, and principally, by forming new products which are the effect of the multiplication of these parasites in the mass of the wine.
Everyone knows what is meant by acid wine, sharp wine, sour wine. The former experiments of Pasteur had clearly shown that no wine can become acid, sharp—can, in a word, become vinegar—without the presence of a little microscopic fungus known by the name of mycoderma aceti. This little plant is the necessary agent in the condensation of the oxygen of the air, and its fixation on the alcohol of the wine. Chaptal, who published a volume on the art of wine-making, knew of the existence of these mycoderm flowers; but to his eyes they were only 'elementary forms of vegetation,' which had no influence whatever upon the quality of the liquid. Besides the mycoderma aceti, which is the agent of acetification, there is another mycoderm called mycoderma vini. This one deposits nothing which is hurtful to the wine, and its flowers are developed by preference in new wines, still immature, and preserving the astringency of the first period of their fabrication.
The requirements of the two sorts of flowers are such that even when the flower of vinegar is sown on the surface of a new wine, no development takes place. Conversely, the mycoderma vini sown on wines that have grown old in casks or in bottles will refuse to multiply. The mycoderma vini produces no alteration in the wine; it does not turn the wine acid. In proportion as the wine grows old the flower tends to disappear, the wine 'despoils' itself, to use a technical expression; physiologically speaking, the wine loses its aptitude to nourish the mycoderma vini, which, finding itself progressively deprived of appropriate nourishment, fades and withers. But it is then that the mycoderma aceti appears, and multiplies with a facility so much the greater that it draws its first nourishment from the cells of the mycoderma vini. The mycoderma aceti has played so large a part in the early pages of this book that it is not necessary to go back upon it here.
There is another disease very common among wines when the great heat of summer begins to make itself felt in the vintage tubs. The wine is said to turn, to rise, to spurt. The wine becomes slightly turbid and at the same time flat and piquant. When it is poured into a glass, very small bubbles of gas form like a crown upon the surface. On placing the glass between the eye and the light and slightly shaking it, one can distinguish silky waves shifting about and moving in different directions in the liquid. When the turned wine is in a cask, it is not unusual to see the bottom of the cask bulge a little and sometimes a leakage takes place at the joints of the staves. If a little opening is made, the wine spurts out, and that is the reason why the wine is said to spurt.
Authors who have written on the subject of wine attributed this malady to the rising of the lees. They believed that the deposit which always exists in greater or less quantities in the lower part of the cask rises and spreads itself into all the mass of the wine.
Nothing can be more inexact. If this phenomenon is sometimes produced—that is to say, if the deposit rises into the mass of wine—the effect is due to a sudden diminution of the atmospheric pressure, as in times of storm, for example. As the wine is always charged with carbonic acid gas, which it holds in solution from the moment of fermentation, one can conceive that a lowering of barometric pressure would cause the escape of some bubbles of carbonic acid. These bubbles, rising from the lower part of the cask, may disturb a portion of the deposit, which then mixes with the wine and renders it turbid. But the real cause of the disease is quite different. The turbidity is without exception due to the presence of little filaments of an extreme tenuity, about a thousandth part of a millimeter in diameter. Their length is very variable. It is these which, when the wine is agitated, give rise to the silky waves just referred to. Often the deposit of the casks leaves a swarm of these filaments entangled in each other, forming a glutinous mass, which under the microscope is seen to be composed entirely of these little filaments. In acting upon certain constituents of the wine particularly upon the tartar, this ferment generates carbonic acid. The phenomenon of spurting is then produced, because when the cask is closed the internal pressure of the liquid augments. The sparkling and the crown of little gas-bubbles, observed when the turned wine is poured out into a glass, is similarly explained. In a word, the disease of turned wine is nothing else than a fermentation, due to an organised ferment which, without any doubt, proceeds originally from germs existing on the surface of the grapes at the moment of gathering them, or on spoilt grapes such as are found in every vintage. It is very rare not to find this parasite of turned wine in the deposit of the wine at the bottom of the casks, but the parasite is not troublesome unless it multiplies very largely. Pasteur found the means of preventing this multiplication by a very simple remedy, equally applicable to other diseases of wines, such as that of bitterness or greasiness (maladie de la graisse).
Many wines acquire with age a more or less bitter taste, sometimes to a degree which renders them unfit for consumption. Red wines, without exception, are subject to this disease. It attacks by preference wines of the best growth, and particularly the finest wines of the Côte-d'Or. It is once more a little filamented fungus which works the change; and not only does it cause in the wine a bitterness which little by little deprives it of all its better qualities, but it forms in the bottles a deposit which never adheres to the glass, but renders the wine muddy or turbid. It is in this deposit that the filaments of the fungus float. If white wines do not suffer from this disease of bitterness, they are exposed, particularly the white wines of Orleans and of the basin of the Loire, to the disease of greasiness. The wines lose their limpidity; they become flat and insipid and viscous, like oil when poured out. The disease declares itself in the casks or in the best-corked bottles. M. Pasteur has discovered that the greasiness of wines is likewise produced by a special ferment, which the microscope shows to be formed of filaments, like the ferments of the preceding diseases, but differing in structure from the other organisms, and in their physiological action on the wine.
In short, according to Pasteur's observations, the deterioration of wines should not in any case be attributed to a natural working of the constituents of the wine, proceeding from a sort of interior spontaneous movement, which would only be affected by variations of temperature or atmospheric pressure. They are, on the contrary, exclusively dependent on the development of microscopic organisms, the germs of which exist in the wine from the moment of the original fermentation which gave it birth. What vast multitudes of germs of every kind must there not be introduced into every vintage tub! What modifications do we not meet with in the leaves and in the fruit of each individual spoilt vine! How numerous are the varieties of organic dust to be found on the stems of the bunches, on the surface of the grapes, on the implements of the grape gatherers! What varieties of moulds and mildews! A vast proportion of these germs are evidently sterilised by the wine, whose composition, being at the same time acid, alcoholic, and devoid of air, is so little favourable to life. But is it to be wondered at that some of these exterior germs, so numerous, and possessing in a more or less marked degree the anaerobic character, should find at certain moments, in the state of the wine, the right conditions for their existence and multiplication?
The cause of these alterations having been found, a mode of preventing the development of all these parasites had still to be sought. Pasteur's first endeavour was to discover some substance which would be antagonistic to the life of these ferments of disease, while harmless to the wine itself, and devoid of any special smell or taste. But in this research success was dependent on too many conditions to be easily attainable. After some fruitless trials, the thought occurred to Pasteur of having recourse to heat. He soon ascertained that, to secure wine from all ulterior changes, it sufficed to raise it, for some instants only, to a temperature of from fifty-five to sixty degrees. His experiments were first directed upon the disease of 'bitterness.' He procured some of the best wines of Burgundy, wines of Beaune, and of Pomard, of different years—1858, 1862, and 1863. Twenty-five bottles were left standing forty-eight hours to allow all the particles suspended in the wine to settle; for, however clear wine may be, it always produces a slight deposit. Pasteur then decanted the wine with minute care, by means of a syphon of slow delivery. This last precaution was necessary to prevent the deposit from being stirred up. When there remained in each bottle only one cubic centimeter of liquid, Pasteur shook the bottle, and then examined with the microscope the residue of each bottle. He perceived in each case distinct filaments of ferment. The wines, however, were not in the least bitter to the taste, but the germs of a possible evil were there—an evil which would have been first detected by the palate when the little fungus had fully developed.
Without uncorking it, Pasteur then heated a bottle of each of these wines. The heating was carried to a temperature of sixty degrees (140° Fahr.). After the cooling of the bottles he laid them by the side of other unheated bottles of the same wines in a cellar, the temperature of which varied in summer between thirteen and seventeen degrees. Every fifteen days Pasteur inspected them. Without uncorking the bottles, he held them up against the light, so that he could see the sediment at the bottom of each bottle, and thus detect the least formation of deposit. In less than six weeks, particularly in the wine of 1863, a very perceptible floating deposit began to form in all the unheated bottles. These deposits gradually augmented, and on examining them with the microscope they were seen to be formed of organised filaments, mixed sometimes with a little colouring matter which had become insoluble. No deposit appeared in the heated bottles.
The idea of heating wines does not belong to Pasteur. Those who love to search into questions of priority will find described in the works of Latin agriculturists various methods for the preservation of wine, based on the employment of heat. To give the wine durability, they sometimes added to the vintage variable quantities of boiled must, reduced to half or two thirds, in which orris, myrrh, cinnamon, white resin, and other ingredients, were infused. But, to cite examples nearer our own time, Appert, whose preserves have become so popular, relates that he sent to St. Domingo some bottles of Beaune which had been previously heated to seventy degrees, and that he compared, on their return into France, two bottles of this wine with a bottle which had remained at Havre, and also with other bottles of the same wine which had remained in his cellar, neither of which had undergone the operation of heating. The superiority of the wine which came from St. Domingo, said Appert, was incontestable. Nothing could equal its delicacy or its perfume. But Appert did not by any means describe the wine of the two bottles which remained in France as either injured or diseased. His remark was based upon an incomplete observation. It simply stated the fact, which indeed was previously known, that a long voyage, added to the employment of heat, had an excellent effect upon the Beaune. This incident had been so completely forgotten, that it was only in 1865 that Pasteur, during the historical researches which preceded his 'Etudes sur le vin,' accidentally met with this story of the bottles of St. Domingo, and hastened to communicate it to the Academy. But in reference to this question of heating, a discussion arose as to priority, which was quite unexpected by him. A Burgundian wine grower, M. de Vergnette, having first proposed the congealing of wines as a protective influence, had afterwards spoken, without much precision, of heat as another means of preservation. On this ground he claimed for himself a great part of the invention of Pasteur's process. 'If, after having subjected some specimens of wines which are to be sent abroad to the ordeal of heating,' said M. de Vergnette, 'one sees that they have been able to resist the action of the heat, then they may safely be shipped. In the contrary case they ought not to be sent.' According to M. de Vergnette, it was to the composition of the wine, its robust condition, and good constitution, that it owed its power of supporting the heating process. Pasteur had no difficulty in demonstrating that these assertions are contradicted by experiment. Wine never changes by the moderate application of heat when air is excluded; and it is precisely when of doubtful soundness that it should be subjected to the process of heating. This operation does not alter it any more than would be the case if it were in a perfectly healthy state. All wines may undergo the action of heat without the least deterioration, and one minute's heating at the proper temperature suffices to insure the preservation of every kind of wine. Thanks to this operation, the weakest wine, the most disposed to turn sour, to become greasy, or to be threatened with bitterness, is insured against injurious change.
Nothing is more simple than to realise the condition of heating in bottles. After having firmly tied down the corks, the bottles are placed in a water-bath. An iron basket is here useful. The water ought to rise up to the wire of the cork. Among these bottles is placed a bottle of water, into which the bulb of a thermometer is plunged. The bath being heated, as soon as the thermometer marks fifty or sixty degrees Centigrade, the basket is withdrawn. The subsequent soundness of the wine is thus insured.
But if Pasteur had overlooked nothing in his efforts to prevent or arrest the evil changes of wine, he still saw that full confidence was not felt in the efficacy of a process which must, it was thought, damage the taste, or the colour, or the limpidity of the wine. After having invited the judgment of people in society, whose preference, if they felt any, was generally for the heated wines, Pasteur wished to have a more decisive opinion. He addressed himself first to wine merchants and others practised in detecting the smallest peculiarities of wines; and afterwards he organised a grand experiment in tasting. On November 16, 1865, a sub-commission, nominated by the representative commission of the wholesale wine-sellers of Paris, repaired to the 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.