RECOLLECTIONS OF CHILDHOOD AND
YOUTH.
FIRST DISCOVERIES.
'Come, M. Pasteur! you must shake off the demon of idleness!' It was the night watcher of the College of Besançon, who invariably at four o'clock in the morning entered Pasteur's room and roused him with this vigorous salute, which was accompanied, when necessary, by a sound shaking. Pasteur was then eighteen years of age. In addition to his food and lodging, the royal college paid him twenty-four francs a month. But if his place was a modest one, it sufficed at the time for his ambition: it was the first tie which bound him to the University.
'Ah,' said his father to him frequently, 'if only you could become some day professor in the College of Arbois I should be the happiest man on earth.'
Already, when he resided at Dôle, and when his son was not yet two years old, this father permitted himself to dream thus of the future. What would he have said had it been announced to him that fifty-eight years later, on the façade of the little house in the Rue des Tanneurs, would be placed, in the presence of his living son—laden with honours, laden with glory, passing in the midst of a triumphal procession along the paved town—a plate bearing these words in letters of gold:
Here was born Louis Pasteur,
December 27, 1822.
Pausing before this house, Pasteur recalled the image of his father and mother—of those whom he called his dear departed ones—and from the far-off depths of his childhood came so many memories of affection, devotion, and paternal sacrifices that he burst into tears.
The life of his father had been a rough one. An old soldier, decorated on the field of battle, on returning to France, where he had no longer a home, he was obliged to work hard to earn his bread. He took up the trade of a tanner. Soon afterwards, having made the acquaintance of a worthy young girl, he joined his lot with hers, and together they entered courageously on the labours of their married life—he calm, reflective, and more eager, whenever he had a moment of repose, for the society of books than for the society of his neighbours; she full of enthusiasm, her heart and spirit agitated by thoughts above the level of her modest life. Both of them watched with ceaseless solitude over their little Louis, of whom, with mingled pride and tenderness, they used to say, 'We will make of him an educated man.'
In 1825 the Pasteur family quitted Dôle and established themselves at Arbois, where, on the borders of the Cuisance, the father of our hero had bought a small tanyard. At this town, and in this yard, Louis Pasteur spent his childhood. As soon as he was old enough to be received as a half-pay scholar he was sent to the communal college. He, the smallest of all the pupils, was so proud of passing under the great arched doorway of this ancient establishment, that he arrived laden with enormous dictionaries, of which there was no need.
In the midst of his laborious occupations the father of Pasteur took upon himself the task of superintending his son's lessons every evening. This was at first no sinecure. Louis Pasteur did not always take the shortest road either to reach his class or to return to his work at home. Some old friends still living remember having made with the little Pasteur fishing parties, which proved so pleasant that they have been continued to the present day. The boy, moreover, instead of applying himself to his lessons, often escaped and amused himself by making large portraits of his neighbours, male and female. A dozen of these portraits are still to be seen in the houses of Arbois, all bearing his signature. Considering that his age at the time was only thirteen, the accuracy of the drawing is astonishing.
'What a pity,' said an old lady of Arbois a short time since, 'that he should have buried himself in chemistry! He has missed his vocation, for he might by this time have made his reputation as a painter.'
It was not until he reached the third class that Louis Pasteur, beginning to realise the sacrifices which his father imposed upon himself, determined to abandon his fishing implements and his crayons, feeling aroused within him that passion for work which was to form the foundation of his life. The Principal of the college, who followed with watchful interest the progress of a pupil who, in his first effort, had outstripped all his comrades, used to say, 'He will go far. It is not for the chair of a small college like ours that we must prepare him; he must become professor in a royal college. My little friend,' he would add, 'think of the great École Normale.'
The College of Arbois having no professor of philosophy, Pasteur quitted it for Besançon. There he remained for the scholars' year, received the degree of bachelier ès lettres, and was immediately appointed tutor in the same college. In the intervals of his duties he followed the course of mathematics necessary to prepare him for the scientific examinations of the École Normale. He must have been already endowed with a singular maturity of character, for the director confided to him the superintendence of the quarters of the older pupils, who during class time were his comrades. In the class room his table was in the midst of them; and never had so young a master so much authority, and at the same time so little need for its exercise.
His first taste for chemistry manifested itself by frequent questions addressed during class time to an old professor named Darlay. This questioning was so often repeated that the good man, quite bewildered, ended by declaring that it was for him to interrogate Pasteur and not for Pasteur to interrogate him. His pupil pressed him no further, but having heard that at Besançon there lived an apothecary who had once distinguished himself by a paper inserted in the 'Annales de Chimie et de Physique,' he sought this man, with a view of ascertaining whether, on holidays, he would consent to give him lessons secretly.
At the examination for the École Normale, Pasteur passed as fourteenth in the list. This rank, however, did not satisfy him. Notwithstanding the censure of his fellow candidates he declared that he would begin a new year of preparation. It was in Paris itself that he chose to work—in one of the silent corners of the city, amid the seclusion of preparatory schools and convents.
In the Impasse des Feuillantines, there lived a schoolmaster, M. Barbet by name, or rather le père Barbet, as the Franc-Comtois, with provincial familiarity, used to call him. Pasteur begged to be allowed to enter his institution, not as an assistant, but as a simple pupil. Knowing how slender were the means of his young compatriot, M. Barbet reduced the fees of his pupil by one-third. Such kindness was customary with the père Barbet, who did not like to be reminded of his generosity. This, however, gives double pleasure to him who records it.
The year passes, the time of the examination arrives, and Pasteur is received as fourth on the list. Thus at last, in the month of October 1843, he finds himself in that École Normale in which he was destined to take so great a place. Pasteur's taste for chemistry had become a passion which he could now satisfy to his heart's content. Chemistry was at this time taught at the Sorbonne by M. Dumas and at the École Normale by M. Balard. The pupils of the École attended both courses of lectures. Different as were the two professors, both of them exercised great influence on their pupils. M. Dumas, with his serene gravity and his profound respect for his auditory, never allowed the smallest incorrectness to slip into his exposition. M. Balard, with a vivacity quite juvenile, with the excitement of a southerner in the tribune, did not always give his words time to follow his thoughts. It was he who once, showing a little potash to his audience, exclaimed with a fervour which has become celebrated, 'Potash, which—potash, then—potash, in short, which I now present to you.'
The general principles which M. Dumas in his teaching delighted to develop, the multitude of facts which M. Balard unfolded to his pupils, all answered to the needs of Pasteur's mind. If he loved the vaster horizons of science, he was also possessed by the anxious desire for exactitude, and for the perpetual control of experiment. Each of the lectures of the École Normale and of the Sorbonne excited in him a profound enthusiasm. One day M. Dumas, while illustrating the solidification of carbonic acid, begged for the loan of a handkerchief to receive the carbonic acid snow. Pasteur rushed forward, and, presenting his handkerchief, received the snow. He returned triumphant, and running forthwith to the École Normale, repeated the principal experiments which the illustrious chemist had just exhibited to his audience. He preserved religiously the handkerchief which had been touched by M. Dumas.
Pasteur usually spent his Sundays with M. Barruel, the assistant of M. Dumas. He thought of nothing but experiments. For a long time in one of the laboratories of the École Normale was exhibited a basin—perhaps it is still shown—containing sixty grammes of phosphorus obtained from bones bought at the butcher's by Pasteur. These he had calcined, submitted to the processes known to chemists, and finally reduced, after a whole day's heating, from four in the morning to nine in the evening, to the said sixty grammes. It was the first time that the long manipulations required in the preparation of this simple substance were attempted at the École Normale.
Isolated in laboratory or library, Pasteur's only thought was to search, to learn, to question, and to verify. As the rule of the school leaves much to individual initiative, he devoted himself to his work with a joyful heart. This daily liberty constitutes the charm and the honour of the École Normale. Not only does it permit but it encourages individual effort; it allows the student to visit at his will the library, and to consult there the scientific journals and reviews. This free system of education develops singularly the spirit of research. There is in it an element of superiority over the École Polytechnique. Influenced by its military origin, constrained, moreover, by the number of its pupils to impose on all an exact discipline, and to introduce into their exercises a strict regularity, the École Polytechnique is, perhaps, less calculated than the École Normale to awaken in the minds of its pupils a taste for speculative science. It is certain that Pasteur owed to the freedom of work, and to the facilities for solitary reading which he there enjoyed, the first occasion for an investigation which was the starting-point to a veritable discovery.
I.
Unlike the old professor of physics and chemistry at Besançon, one of the lecturers in the École Normale often took pleasure, not only in answering Pasteur's questions, but in leading him on to talk over scientific subjects. M. Delafosse, whose memory remains dear to all his pupils, was one of those men who fail to do themselves justice, or who, according to the expression of Cardinal de Retz, do not fulfil all their merit. Not that circumstances have been unfavourable to them, but that an invincible modesty, and a natural nonchalance which finds in that modesty a shield against latent self-reproach, leave them in a sort of twilight in which they are content to dwell. Pupil, and afterwards fellow worker, of the celebrated crystallographer Haüy, M. Delafosse had devoted himself to questions of molecular physics. Pasteur, who had read with enthusiasm the works of Haüy, conversed incessantly with Delafosse about the arrangements of molecules, when an unexpected note from the German chemist Mitscherlich, communicated to the Academy of Sciences, came to trouble all his scientific beliefs. Here is the note:—
'The paratartrate and the tartrate of soda and ammonia have the same chemical composition, the same crystalline form, the same angles, the same specific weight, the same double refraction, and consequently the same inclination of the optic axes. Dissolved in water, their refraction is the same. But while the dissolved tartrate causes the plane of polarised light to rotate, the paratartrate exerts no such action. M. Biot has found this to be the case with the whole series of these two kinds of salts. Here (adds Mitscherlich) the nature and the number of the atoms, their arrangement, and their distances apart are the same in the two bodies.'
Imbued as he was with the teachings of Haüy and Delafosse, and full of the ideas of M. Dumas in molecular chemistry, Pasteur asked himself this question: 'How can it be admitted that the nature and number of the atoms, their arrangement and distances apart, in two chemical substances are the same; that the crystalline forms are equally the same, without concluding that the two substances are absolutely identical? Is there not a profound incompatibility between the identity affirmed by Mitscherlich and the discrepancy of optic character manifested by the two compounds, tartaric and paratartaric, which form the subject of his note?'
This difficulty rested in Pasteur's mind with the tenacity of a fixed idea. Received as agrégé of physical science at the end of his third year at the École, and then keeping near his master, M. Balard, he had begun the study of crystals and the determination of their angles and forms, when his nomination to the professorship of physics in the Lycée of Tournon surprised and distressed him. M. Balard repaired immediately to the bureau of the Minister of Education, and spoke of his assistant in terms which caused the nomination to be cancelled. Pasteur remained in the laboratory of the École Normale.
With a view to mastering the science of crystallography, he took for his guide the extensive work of M. de la Provostaye, resolving to repeat all the measurements of angles and all the other determinations of this author with a view to a comparison of their respective results. The work of M. de la Provostaye, who was distinguished by the exactitude of his researches, had for its subject the tartaric and paratartaric acids and their saline compounds.
Two or three years ago, while we were walking together along a road in the Jura, M. Pasteur, after quoting textually the note of Mitscherlich, described to me with enthusiasm the pleasure he had experienced in crystallising tartaric acid and its salts, the crystals of which, he said, rivalled in size and beauty the most exquisite of crystalline forms.
'I should have great difficulty,' I remarked, 'in following you through the labyrinth of tartaric acid, tartrates, and paratartrates. However much your other studies have attracted me, those which had for their starting-point the note of Mitscherlich and the memoir of M. de la Provostaye have appeared to me, whenever I tried to master them, difficult of access. Ah,' I added, 'you would have done well, out of consideration for those who love to speak of your labours, had you made no discoveries in this field.'
Pasteur, with a mixture of indignation and indulgence, replied:—'Is it possible that you have not discerned the grand horizons that lie behind these researches in physics and molecular optics? If I have a regret, it is that I did not follow out this path. Less rough than it at first sight appears, it would, I am convinced, have led to the most important discoveries. By a sudden turn it threw me unexpectedly upon the subject of fermentation, and fermentation led me to the study of diseases; but I still continue to lament that I have never had time to retrace my steps.'
Then, with a simplicity of exposition in which one recognised the teacher who had always endeavoured to place his ideas within the range of his hearers, he said—
'If you picture to yourself all the bodies in nature—mineral, animal, or vegetable, and consider even the objects formed by the hands of man, you will see that they divide themselves into two great categories. The one has a plane of symmetry and the other has not. Take, for instance, a table, a chair, a playing die, or the human body; we can imagine a plane passing through these objects which divides each of them into two absolutely similar halves. Thus, a plane passing through the middle of the seat and of the back of an arm-chair would have, on its right and left, identical parts; in like manner a vertical plane passing through the middle of the forehead, nose, mouth, and chin of an individual, would have similar parts to the right and to the left. All these objects, and a multitude of similar ones, constitute our first category. They have, as mathematicians express it, one or several planes of symmetry.
'But, as regards the repetition of similar parts, it is far from being the case that all bodies are constituted in the manner here described. Consider, for example, your right hand: it is impossible to find for it a plane of symmetry. Whatever be the position of a plane which you imagine cutting the hand, you will never find on the right of this plane exactly the same as you find on its left. The same remark applies to your left hand, to your right ear and to your left ear, to your right eye and to your left eye; to your two arms, your two legs, and your two feet. The human body, taken as a whole, has a plane of symmetry, but none of the parts composing one or the other of its halves has such a plane. The stalk of a plant whose leaves are distributed spirally round its stem has not a plane of symmetry, nor has a spiral staircase such a plane; but a straight one has. You see this?
'It would have been truly extraordinary, would it not, if the various kinds of minerals, such as sea salt, alum, the diamond, rock crystal, and so many others which illustrate the great law of crystallisation, and which clothe themselves in geometric forms, should not present to us examples of the two categories of which we have just been speaking? They do so in fact. Thus a cube, which has the form of a player's die, has a plane of symmetry; it has indeed several planes. The form of the diamond, which is a regular octahedron, has also several planes of symmetry. It is thus also with the great majority of the mineral forms met with in nature or in the laboratory. They have generally one or several planes of symmetry. There are, however, exceptions. Rock crystal, which is found in prisms, often of large volume, in the fissures of certain primitive rocks, has no plane of symmetry. This crystal exhibits certain small facets, distributed in such a manner that in their totality they might be compared to a helix, or spiral, or screw, which are all objects not possessing a plane of symmetry.
'Every object which has a plane of symmetry, when placed before a looking-glass, has an image which is rigorously identical with the object itself. The image can be superposed upon the reality. Place a chair before a mirror; the image faithfully reproduces the chair. The mirror also reproduces the human body considered as a whole. But place before the mirror your right hand and you will see a left hand. The right hand is not superposable on the left, just as the glove of your right hand cannot be fitted to your left, and inversely.'
Then reverting to the beginnings of his studies in crystallography, Pasteur recounted to me briefly that, after having gone through the work of M. de la Provostaye, he perceived that a very interesting fact had escaped the notice of this skilful physicist. M. de la Provostaye had failed to observe that the crystalline forms of tartaric acid and of its compounds all belong to the group of objects which have not a plane of symmetry. Certain minute facets had escaped him. In other words, Pasteur discerned that the crystalline form of tartaric acid, placed before a mirror, produced an image which was not superposable upon the crystal itself. The same was found to be true of the forms of all the chemical compounds of this acid. On the other hand, he imagined that the crystalline form of paratartaric acid, and of all the compounds of this acid, would be found to form part of the group of natural objects which have a plane of symmetry.
Pasteur was transported with joy by this double result. He saw in it the possibility of reaching by experiment the explanation of the difficulty which the note of Mitscherlich had thrown down as a kind of challenge to science, when it signalised an optical difference between two chemical compounds affirmed to be otherwise rigorously identical. Pasteur reasoned thus:—Since I find tartaric acid and all its tartrates without a plane of symmetry, while its isomer, paratartaric acid, and its compounds have such a plane, I will hasten to prepare the tartrate and the paratartrate of the note of Mitscherlich. I will compare their forms, and in all probability the tartrate will be found dissymmetrical—that is to say, without a plane of symmetry—while the paratartrate will continue to have such a plane. Henceforward the absolute identity stated by Mitscherlich to exist between the forms of these two compounds will have no existence. It will be proved that he has erred, and his note will no longer have in it anything mysterious. As the optic action proper to the tartrates spoken of in his note manifests itself by a deviation of the plane of polarisation to the right, we have here a kind of dissymmetry which has nothing incompatible with the dissymmetry of form. On the contrary, these two dissymmetries can be referred to one and the same cause. In like manner, the absence of dissymmetry in the form of the paratartrate will be connected with the optical neutrality of that compound.
The fulfilment of Pasteur's hopes was only partial. The tartrates of soda and ammonia presented, as did all the other tartrates, the dissymmetry manifested by the absence of any plane of symmetry; that is to say, the crystals of this salt placed before a mirror produced an image which was not superposable upon the crystal. It was like a right hand having its left for an image. With regard to the paratartrates of soda and ammonia, one circumstance struck Pasteur in a quite unexpected manner. Far from establishing in the crystals of this salt the absence of all dissymmetry, he found that they all manifestly possessed it. But, strange to say, certain crystals possessed it in one sense and other crystals in a sense opposite. Some of these crystals, when placed before a mirror, produced the image of the others, and one of the two kinds of crystals corresponded rigorously in form with the tartrate prepared by means of the tartaric acid of the grape. Pasteur continued his reasoning thus:—Since there is no difference between the form of the tartrate derived from the tartaric acid of the grape and one of the two kinds of crystals deposited at the moment of crystallisation of the paratartrate, the simple observation of the dissymmetry proper to each will enable me to separate, by hand, all the crystals of the paratartrate which are identical with those of the tartrate. By ordinary chemical processes I ought to be able to extract a tartaric acid identical with that of the grape, possessing all its physical, mineralogical, and chemical properties—that is to say, a tartaric acid possessing, like the natural tartaric acid of the grape, dissymmetry of form, and exerting an action on polarised light. Per contra, I ought to be able to extract from the second sort of crystals, associated with the former in the paratartaric group, an acid which will reproduce ordinary tartaric acid, but possessing a dissymmetry of an inverse kind and exerting an action equally inverse on polarised light.
With a feverish ardour Pasteur hastened to make this double experiment. Imagine his joy when he saw his anticipations not only realised but realised with an exactitude truly mathematical. His delight was so great that he quitted the laboratory abruptly. Hardly had he gone out when he met the assistant of the physical professor. He embraced him, exclaiming, 'My dear Monsieur Bertrand, I have just made a great discovery! I have separated the double paratartrate of soda and ammonia into two salts of inverse dissymmetry, and exerting an inverse action on the plane of polarisation of light. I am so happy that a nervous tremulousness has taken possession of me, which prevents me from looking again through the polariscope. Let us go to the Luxembourg, and I will explain it all to you.'
These results excited in a high degree the attention of the Academy of Sciences, where sat, at the time now referred to, Arago, Biot, Dumas, De Senarmont, and Balard. It might be said without exaggeration that the Academy was astounded. At the same time there were many members who were slow to believe in this discovery. Charged with drawing up the report, M. Biot began by requiring from Pasteur the verification of each point which he had announced. To this verification M. Biot brought his habitual precision, which was associated with a kind of suspicious scepticism.
In one of his lectures Pasteur thus described his interview with M. Biot:—'He made me come to his house, where he put into my hands some paratartaric acid which he had carefully studied himself, and found perfectly neutral as regards polarised light. It was not in the laboratory of the École Normale, it was in his own kitchen, and in his presence, that I was to prepare this double salt with soda and ammonia procured by himself. The liquor was left slowly to evaporate, and at the end of ten days, when it had deposited thirty or forty grammes of crystals, he begged me to go over to the Collège de France to collect the crystals and to extract from them specimens of the two kinds, which he proposed to have placed, the one on his right hand, the other on his left, desiring me to declare if I was ready to re-affirm, that the crystals to the right would turn the plane of polarisation to the right and the others to the left. This declaration made, he said that he would charge himself with the rest of the inquiry. M. Biot then prepared the solutions in well-measured proportions, and at the moment of observing them in the polarising apparatus he invited me again to come into his study. He placed first in the apparatus the most interesting solution, that which ought to deviate to the left. Without even making any measurements, he saw, by the mere inspection of the colours of the ordinary and extraordinary images of the analyser, that there was a strong deviation to the left. Visibly moved, the illustrious old man took my arm and said, "My dear child, I have loved science so well throughout my life that this makes my heart beat."'
The emotion of M. Biot was all the more profound because he had been himself the first to discover the rotation of the plane of polarisation by chemical substances, and had, for more than thirty years, affirmed that the study of these substances and of their action in regard to rotatory polarisation was, perhaps, the surest means of penetrating into the intimate constitution of bodies. His counsels were received with deference, but they had never been followed out. And now there appeared before the old man, already somewhat discouraged, a youth of twenty-five, who from his first investigation had proved himself a master, who had dissipated the obscurities of the famous German note, and created a new chapter in crystallographic chemistry. The composition and nature of paratartaric acid had been explained, and a new substance, the left-handed tartaric acid, with its truly surprising properties, had been discovered; molecular physics and chemistry had been enriched with new facts and theories of great value.
The first care of Pasteur, after having discovered the left-handed tartaric acid and the constitution of paratartaric acid, was to compare very carefully the properties of the new left-handed acid with those of the right, endeavouring to determine by strict experiment the influence on these properties of the internal atomic arrangements of the two acids. Although we are unable to picture the exact figure of these atomic groupings, there can be no doubt that they are formed of the same elementary particles, that they are, moreover, dissymmetrical, and that, in short, the dissymmetry of the one group is the same as that of the other, but in an inverse sense. If, for example, the arrangement of the atoms of the right-handed tartaric acid present the exterior appearance of an irregular pyramid, the arrangement of the atoms of the left-handed tartaric acid ought, of necessity, to present the form of a pyramid irregular in the inverse sense.
II.
Nominated assistant professor of chemistry at Strasburg, Pasteur followed up with enthusiasm these curious studies. To interrupt them for an instant it required nothing less than his engagement with Mademoiselle Marie Laurent, daughter of the Rector of the Academy. It is even asserted that on the very morning of his marriage it was necessary to go to his laboratory and remind him of the event that was to take place on that day. But if Pasteur was thus guilty of an absent-mindedness worthy of La Fontaine, he proved as a husband so different from La Fontaine that Madame Pasteur, when reminded of this lapse of memory, receives the reminder with an indulgent smile.
But to return to the laboratory: Under the same conditions of weight, temperature, and quantity of solvent, Pasteur placed successively, in presence of the two acids, all the substances capable of combining with them. In this way he obtained right-handed and left-handed tartrates of potash, of soda, of ammonia, of lime, and of all the oxides properly so called. He applied himself to the compounds—and they are numerous—which deposit themselves in liquids under well-determined crystalline forms. Without entering into the details of these long and patient studies, it may be stated generally that Pasteur proved that whatever could be done with one of the tartaric acids could be repeated rigorously, under similar conditions, with the other, the resultant products manifesting constantly the same properties, with the single difference already exhibited by the two acids—that in the one case the deviation of the plane of polarisation was to the right, while in the other it was to the left. With regard to all their other properties, both chemical and physical, the identity was absolute. Solubility, simple refraction by solutions, double refraction by crystals, the action of heat in producing decomposition, &c., the similitude extended to the most perfect identity.
The Academy of Sciences, which shows by the rarity of its reports the importance which it attaches to them, gave for the second time an account of these new researches. M. Biot was again the reporter. It was with a sort of coquetry that Pasteur brought from Strasburg perfectly labelled specimens of the magnificent crystallisations of the double series of right-handed and left-handed tartrates. By means of models he was able to render the forms of these crystals visible at a distance.
M. Biot undertook to bring the subject before the Academy. On the morning of the day when he was to read his report he spent several hours in conversation with Pasteur. M. Biot became so excited during the discussion that Madame Biot, with the solicitude peculiar to the wives of Academicians, requested Pasteur to change the subject of conversation.
The members of the Academy shared the enthusiasm of M. Biot. Arago moved that the report be inserted in the collected mémoires of the Academy. This was an exceptional honour. Arrived for the most part at the end of their own careers, these learned men observed with pleasure the incipient ray which had not yet become a glory but which was the precursor thereof.
'My young friend,' said M. Biot to Pasteur, when presenting him to Mitscherlich somewhere about that time, 'you may boast of having done something great, in having discovered what had escaped such a man as this.'
'I had studied,' replied Mitscherlich, not without a shade of regret, addressing himself to Pasteur, 'I had studied with so much care and perseverance, in their smallest details, the two salts which formed the subject of my note to the Academy, that, if you have established what I was unable to discover, you must have been guided to your result by a preconceived idea.'
Mitscherlich was right, and this preconceived idea might have been formulised thus: A dissymmetry in the internal molecular arrangement of a chemical substance ought to manifest itself in all its external properties which are themselves capable of dissymmetry.
If this theoretic conception was correct, Pasteur might expect to find that all the substances in which M. Biot had observed the power of rotating the plane of polarisation would possess the crystalline dissymmetry revealed by the absence of superposability. The result was in great part conformable to those previsions. The substances which acted upon polarised light, as liquids or solutions, were generally found by Pasteur to produce dissymmetric crystals. Some of them, however, notwithstanding their power of crystallisation, exhibited, when crystallised, no dissymmetric face. This difficulty did not deter Pasteur. It gave him, on the contrary, the opportunity of showing that when a theory had in so many cases proved itself correct, an apparent objection must not be assumed insuperable without first sounding it to the bottom. May it not be, he reasoned, that the absence of dissymmetry in substances which have the molecular rotatory power is not an accident; and may it not be possible, by changing the conditions of the crystallisation, to make the dissymmetry appear?
Then, in order to modify the crystalline forms of substances which did not show themselves to be spontaneously dissymmetrical, Pasteur employed a method which had been often tried before, though its principles could not be explained or its effects foreseen. In imitation of Romé de Lisle, Leblanc, and Beudant, he varied the nature of his solvents; he introduced into the solution, sometimes an excess of acid or of base, sometimes foreign matters incapable of acting chemically upon those which were to be modified; he even employed sometimes impure mother liquids. On each occasion new facets were thus produced, and these new facets showed the kind of dissymmetry which the optical character demanded. Although he had to limit his researches to those substances which, by their ready crystallisation and the beauty of their forms, lent themselves best to this class of proofs, the results were so far in accord with the previsions of theory, that no reasonable doubt could exist as to the necessary correlation between dissymmetry and the power to deviate polarised light.
By these researches Pasteur was led to a conclusion, which is worthy of the most serious consideration, regarding the difference which exists between mineral species and artificial products on the one side, and the organic products which can be extracted from vegetables or animals on the other. All mineral or artificial products—for brevity let us say all the products of inorganic nature—have a superposable image, and are therefore not dissymmetrical, while vegetable and animal products—in other words, products formed under the influence of life—have an image not superposable; that is to say, they are atomically dissymmetrical, this dissymmetry expressing itself externally in the power of turning the plane of polarisation. If any exceptions exist they are more apparent than real. Pasteur himself pointed out some of them, while demonstrating at the same time that it is easy to explain why all trace of dissymmetry disappears when substances which, like rock crystal, have an external dissymmetry are subjected to the process of solution.
An apparent contradiction to this law of demarcation between artificial products and those of animal and vegetable life is presented by the existence in living creatures of substances like oxalic acid, formic acid, urea, uric acid, creatine, &c. None of these products exert an action on polarised light or show any dissymmetry in the form of their crystals. But it is necessary to observe that these products are the result of secondary actions. Their formation is evidently governed by the laws which determine the constitution of the artificial products of our laboratories, or of the mineral kingdom properly so called. In living beings they are the products of excretion rather than substances essential to vegetable or animal life. When, on the other hand, we consider the most primordial substances of vegetables and animals—those whereof it may be justly said that they are born under the directive influence of becoming life, such as cellulose, fecula, albumen, fibrine, &c.—they are found to possess the power of acting, on polarised light, a characteristic necessary and sufficient to establish their internal dissymmetry, even when, through the absence of crystallising power, they fail to manifest this dissymmetry outwardly.
It is, therefore, true to say that the products of inorganic nature, whether mineral or artificial, have never yet presented molecular dissymmetry. It may also be affirmed that the substances which exert the greatest influence in vital manifestations, which are present and active in the seed and in the egg at the moment of the marvellous start of animal and vegetable life, all present molecular dissymmetry.
Would it be possible to indicate a more profound distinction between the respective products of living and of mineral nature, than the existence of this dissymmetry on the part of the one and its absence on the part of the other? Is it not strange that not one of these thousands and thousands of artificial products of the laboratory, the number of which is each day augmented, should manifest either the power of turning the plane of polarisation or non-superposable dissymmetry? No doubt natural dissymmetric substances—gum, sugar, tartaric and malic acids, quinine, strychnine, essence of turpentine, &c.—may be employed in forming new compounds which remain dissymmetric, though they are artificially prepared; but it is evident that all these new products do but inherit the original dissymmetry of the substances from which they are derived. When chemical action becomes more profound, all dissymmetry disappears, and is never seen to reappear in the successive ulterior products.
What can be the causes of so great a difference? M. Pasteur has often expressed to me the conviction that it must be attributed to the circumstance that the molecular forces which operate in the mineral kingdom, and which are brought into play every day in our laboratories, are forces of the symmetrical order; while the forces which are present and active at the moment when the grain sprouts, when the egg develops, and when, under the influence of the sun, the green matter of the leaves decomposes the carbonic acid of the air and utilises in divers ways the carbon of this acid, the hydrogen of the water, and the oxygen of these two products—are of the dissymmetric order, probably depending on some of the grand, dissymmetric, cosmic phenomena of our universe. While expounding this opinion before the Academy of Sciences, Pasteur, on one occasion, expressed himself thus:—
'The universe is a dissymmetric whole. I am inclined to think that life, as manifested to us, must be a function of the dissymmetry of the universe or of the consequences that follow in its train. The universe is dissymmetrical; for, placing before a mirror the group of bodies which compose the solar system, with their proper movements, we obtain in the mirror an image not superposable on the reality. Even the motion of solar light is dissymmetrical. A luminous ray never strikes in a straight line, and at rest, the leaf wherein organic matter is created by vegetable life. Terrestrial magnetism, the opposition which exists between the north and south poles of a magnet, the opposition presented to us by positive and negative electricity, are all the resultants of dissymmetric actions and motions.'
At the moment when Pasteur, entering upon the labours which form the principal subject of this book, abandoned the study of molecular physics and chemistry which had previously occupied him, all his thoughts were directed to the search of means suited to render evident the influence of these causes and these phenomena. At Strasburg he had procured powerful magnets with the view of comparing the actions of their poles, and, if possible, of introducing by their aid, among the forms of crystals, a manifestation of dissymmetry. At Lille, where he was nominated Dean of the Faculty of Sciences in 1854, he had contrived a piece of clockwork intended to keep a plant in continual rotary motion, first in one direction and then in the other. 'All this was gross,' he said to me one day; 'but, further than this, I had proposed, with the view of influencing the vegetation of certain plants, to invert, by means of a heliostat and a reflecting mirror, the motion of the solar rays which should strike them from the birth of their earliest shoots, and in this direction there was more to be hoped for.' He never spoke of these attempts, because he had not had the time to follow them to the issues of which he dreamed; but to this day he remains persuaded that the barrier which exists between the mineral and organic kingdoms—and which is revealed to our eyes by the impossibility of producing, in the reactions of the laboratory, dissymmetric organic substances—can never be crossed until we have succeeded in introducing among these reactions influences of the dissymmetric order. According to Pasteur, success in this direction would give access to a new world of substances, and probably also of organic transformations. As we have succeeded in finding the inverse of right-handed tartaric acid, we may hope to obtain some day all the immediate principles inverse to those now known to us. Who could say what vegetable and animal species would become if it were possible to replace, in the living cells, cellulose, albumen, and their congeners, by their isomers with an inverse action? Certainly the thing is not easy, and Pasteur would be the last person to deceive himself as to the difficulty of the problem. His latest thought on the matter is this:—When the attempt is made to introduce into living species primordial substances, inverse to those now existing, the great difficulty will be to master the tendency (devenir[7]) proper to the species, a tendency which is potential in the germ of each of them. In this germ, it is to be feared, the dissymmetry of the dissymmetric primordial substances which it embraces will always manifest itself. Ah! if spontaneous generation were possible; if we could form from mineral matter a living cell, how much more accessible would the problem become! However this may be, we must seek, by all possible means, to produce molecular dissymmetry by the application of forces which have a dissymmetric action. 'We must,' said Pasteur to me on the day when, starting from the note of Mitscherlich, he passed all these things in review, 'we must invoke the action of solenoid or helix. Entangled at present in labours more than sufficient to absorb whatever of ardour and of force still remains to me, I have no longer time to occupy myself with these questions.' But what great things are to be done in following out this order of ideas, and what a route will be opened to young men possessed of that genius of invention which is evoked so often by persistent work!
This complete opposition between artificial mineral products and vegetable and animal ones was to Pasteur a truth so well established that he found frequent opportunity of affirming it under decisive circumstances. One day, a very skilful chemist, M. Dessaignes, who later on became one of the correspondents of the Academy of Sciences, announced that he had transformed fumaric and malic acids into aspartic acid. Pasteur, who some time previously had had occasion to study these same acids, had proved that the two first had no molecular dissymmetry—that is to say, they exercised no optic action. In the state of solution they did not turn the plane of polarised light. Aspartic acid, on the contrary, had presented to him molecular dissymmetry, like asparagine itself. If the observation of M. Dessaignes were true, then bodies which were inert in regard to polarised light, and consequently non-dissymmetric, could be transformed in the laboratory into active dissymmetric bodies. The line of demarcation so well established would be broken. Pasteur, whose experience regarding the note of Mitscherlich had shown him how even the most conscientious observers may fail to seize upon fugitive appearances, when unprompted to seek them by a preconceived idea, doubted at once the accuracy of the facts cited by M. Dessaignes. From Strasburg he started for Vendôme, where M. Dessaignes at that time resided. M. Dessaignes immediately gave Pasteur a small quantity of the aspartic acid which he had prepared by means of fumaric and malic acids. Returning to his laboratory, Pasteur immediately recognised that, despite the very close resemblance of the new acid of M. Dessaignes to that derived from asparagine, the former differed from the latter by the complete absence in its case of molecular dissymmetry.
With regard to other facts of the same kind, announced not only in France, but in Italy, and in England—chiefly the pretended formation of grape tartaric acid from succinic acid, artificial and inert, by Perkin and Duppa—Pasteur testified with absolute certainty of judgment to the existence of phenomenal peculiarities proper to these substances, which he had never seen, and which had, on the other hand, been the object of careful study by observers of great talent.
After these verifications and deductions from theoretic views, Pasteur discovered a surprising connection between the prior researches of chemistry and crystallographic physics and the new and entirely unexpected results of physiological chemistry. This connection, like the thread of Ariadne, conducted him to his recent great discoveries in medical biology. M. Chevreul was right when, some years ago, at the Academy of Sciences, he expressed himself thus:—
'It is by first examining in their chronological order the researches of M. Pasteur, and then considering them as a whole, that we are enabled to appreciate the rigour of judgment of that learned man in forming his conclusions, and the perspicacity of a mind which, strong in the truths which it has already discovered, is carried forward to the establishment of new ones.'
III.
Pasteur had thus established that bodies endowed with internal dissymmetry carried this property, in varying degrees, into their compounds or their derivatives. When two of these bodies whose nature has been revealed by the discovery of right-handed and left-handed tartaric acid, where all is chemically identical—and which are only to be distinguished from each other by their inverse crystallographic form, and by their action on polarised light—enter into combination with a substance which is optically and crystallographically inert, the chemical identity ought, under these new conditions, to be preserved. Everything remains optically and crystallographically comparable. The inert element adds nothing to, and takes away nothing from, the dissymmetric faculties of the active one.
To these curious studies Pasteur soon added a new chapter. He reasoned thus:—If into these compounds I introduce a substance possessing in itself the specific properties of dissymmetry, it is evident that this substance, while entering into these combinations, must preserve its own properties. The active substance would, from the moment of its combination, add something to the properties of the molecular group which acts like itself, and subtract something from the properties of the group which acts in the opposite manner. The resultant effect of these actions, sometimes concordant, sometimes antagonistic, would cease to be alike in absolute quantity. And if this be the necessary condition of similitude as to molecular arrangement, this similitude would cease to exist, and with its disappearance would appear all the differences of chemical and physical properties which constitute its outward manifestations.
The facts were found to harmonise with these logical deductions. After having made dissymmetry intervene as a modifier of chemical affinity, he had a strange and manifest proof of the influence of dissymmetry in the phenomena of life.
It had been long known, through the observations of a manufacturer of chemical products in Germany, that the impure tartrate of lime of commerce, if contaminated with organic matters and permitted to remain under water in summer, would ferment and yield various products. Pasteur caused the ordinary right-handed tartrate of ammonia to ferment in the following manner:—He took some very pure crystalline salt and dissolved it, adding at the same time to the liquid some albuminoid matter, about one gramme to 100 grammes of the tartrate. The liquid placed in a warm chamber fermented. During the process of fermentation the liquid mass, previously limpid, became gradually turbid, in consequence of the appearance of a small organism which played the part of ferment. Pasteur applied this mode of fermentation to the paratartrate of ammonia. He saw that this salt also fermented, depositing the same organism. All appeared as if the course of things was the same as in the case of the right-handed tartrate. But Pasteur, having had the idea of following the course of the operation with the aid of the polariscope, soon detected a profound difference between the two fermentations. In the case of the paratartrate, the liquid, at first inert, gradually assumed a sensible power of deviation to the left, which augmented by degrees and attained a maximum. The fermentation was then suspended; there was no longer any of the right-handed acid in the liquid, which, when evaporated and mixed with its own volume of alcohol, immediately furnished a beautiful crystallisation of left-handed tartrate of ammonia.
From that moment a great new fact was established—namely, that the molecular dissymmetry proper to organic matters intervened in a phenomenon of the physiological order, and did so as a modifier of chemical affinity. The kind of dissymmetry proper to the molecular arrangement of the left-handed tartaric acid was, no doubt, the sole cause of the difference between this acid and the right-handed acid, in regard to the fermentation produced by a microscopic fungus. We shall see later on that organised ferments are almost always microscopic vegetables, which embrace in their constitution cellulose, albumen, &c., identical with these same substances taken from the higher class of vegetables and equally dissymmetric. We can thus understand, that for the nutrition of the ferment and the formation of its principles the chemical changes are more easy with one of the two tartaric acids than with the other.
The opposition of the properties of the two tartaric acids, right and left, at the moment when the conditions of life and nutrition of an organised being intervened, showed themselves still more strikingly in a very curious experiment made by Pasteur. He was the first to prove that mildew could live and multiply on a purely mineral soil, composed, for example, of the phosphates of potash, of magnesia, and an ammoniacal salt of an organic acid. For such a development of vegetable life he employed the seed of penicillium glaucum, which is to be found everywhere as common mould, and to which he offered, as its only carbon aliment, paratartaric acid. At the end of a little time the left-handed tartaric acid appeared. Now this left-handed acid could only show itself on the condition that a rigorously equal quantity of the right-handed acid had been decomposed. The carbon of the tartaric acid evidently supplied to the little plant the carbon that was necessary for the formation of its constituents and all their organic accessories. If the microscopic seed of penicillium sown upon this soil was not formed of dissymmetric elements, as is the case with all other vegetable substances, its development, its life, its fructification would accommodate themselves equally well with the left-handed tartaric acid as with the right. The fact that the left-handed tartaric acid is less assimilable than its opposite is due solely and evidently to the dissymmetry of one or other of the primordial substances of the little plant.
Thus for the first time was introduced into physiological studies and considerations the fact of the influence of the molecular dissymmetry of natural organic products.
Pasteur always speaks with enthusiasm of the grand future reserved for researches which have this influence for their object; for molecular dissymmetry is the only sharp line of demarcation which exists between the chemistry of inorganic and that of organic nature.