'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.