Louis Pasteur: His Life and Labours

by René Vallery-Radot

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.