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.