Part 6

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.'