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Citation by the Royal Society on the Occasion of the Awarding of the Rumford Medal to Louis Pasteur

The President, Lord Wrottesley:

The Rumford Medal has been awarded to M. Pasteur for his discovery of the nature of racemic acid, and its relations to polarized light.

Chemists had long been acquainted with a peculiar acid, racemic or paratartaric acid, which had the same composition as tartaric acid, and the same saturating power, and resembled it in its properties in a very remarkable manner. Yet the two acids were not identical, and the cause of their difference, notwithstanding their close agreement, remained a mystery. The resemblance between the two acids had been rendered still more striking on a comparison of the physical characters of their salts; for their crystalline forms were the same, their specific gravities the same, their double refraction the same. Yet the solutions of the tartrates rotated the plane of polarization of polarized light, while those of the racemates were inactive.

In a careful scrutiny of the crystalline forms of the tartrates, M. Pasteur was led to recognize the almost universal presence of hemihedral faces, of such a character that the two hemihedral forms which together make up the holohedral, were " dissymmetric ", that is, could not be superposed on each other, but each could be superposed on the image of the other in a mirror. Sometimes the hemihedrism was indicated merely by the greater development of one pair of faces than of another pair. A hemihedrism of such a character that the two hemihedral forms were distinguished by right-handedness and left-handedness, seemed to be associated with the rotatory power of the solutions of the tartrates. If so, the crystals of the racemates might be expected not to exhibit the character of right-or left-handedness, since their solutions were known to be inactive on polarized light. Accordingly, on forming several of the racemates, and carefully examining the crystals, M. Pasteur found that the hemihedrism which had been observed in the tartrates was wanting in the racemates.

These patient and laborious researches in pursuit of truth were presently rewarded with an unexpected and brilliant discovery. On examining the crystals obtained in an attempt to form the double racemate of soda and ammonia, M. Pasteur observed that the crystals were hemihedral, and of two kinds, which differed only as to right-handedness and left-handedness; the one kind, which for distinction's sake may be called right-handed, absolutely agreeing with the corresponding double tartrate, the other with the image of the tartrate in a mirror. On separating the crystals of the two kinds mechanically, and dissolving them apart, the solution of the right-handed crystals was found to rotate the plane of polarization of polarized light right-handedly, like a solution of the tartrate, that of the left-handed crystals left-handedly. These solutions yielded on evaporation, the one only right-handed, the other only left-handed crystals. The crystals of the two salts were purified by recrystallization, their acids isolated, and the chemical, optical, crystallographic and pyroelectric properties of the acids themselves or their salts or solutions carefully compared. A like comparison was instituted between these acids and the well-known tartaric acid. The acid obtained from the right-handed crystals proved to be absolutely identical with tartaric acid in all its properties, that obtained from the left-handed crystals proved to be identical, so to speak, with the image of tartaric acid in a mirror, the two acids absolutely agreeing in all their properties except as to right-handedness and left-handedness. Where the one acid yielded crystals hemihedral right-handedly, the other yielded crystals similar, except that they were hemihedral left-handedly; where the one yielded a solution rotating the plane of polorization right-handedly, the other yielded a solution rotating it left-handedly to the very same amount, and with the very same peculiar dispersion of the colours. On mixing equal quantities of the acids from the right-handed and left-handed crystals, racemic acid was reproduced.

Stimulated by this remarkable discovery, M. Pasteur has continued his labours in the same direction, and the results which he has since obtained are given in a series of papers published in the "Annales de chimie' and extending nearly to the present time.

Hitherto no "active" substance (i. e. one whose solution has the power of rotating the plane of polarization of polarized light) has been obtained artificially from inactive substances, except in the case of the splitting up, or at least separation, of racemic acid into a right-handed and a left-handed substance; and this law seems worthy of the attention of chemists who attempt the artificial formation of the organic alkaloids. This law would have been violated had two acids which chemists had obtained from fumaric acid, an inactive substance, and which appeared to be identical with aspartic and make acids respectively, been really so. But M. Pasteur found that these acids were inactive, unlike aspartic and malic acids, from which they also differed in some other respects.

The two acids obtained from racemic acid were found to be identical in their properties (except as to right-handedness and left-handedness) so long as they were mixed or combined with inactive substances only, but M. Pasteur found that this is no longer the ease when they are combined with active substances, as for example the organic alkaloids, in which case the salts obtained differ widely in solubility, crystalline form, etc.

It is to the stimulus afforded by the investigations of M. Pasteur that we must ascribe the more recent discovery by. M. Marbach, that several crystals belonging to the cubical system possess the power of rotating the plane of polarization. Thus M. Pasteur's original discovery has already begun to bear fruit in discoveries made by others.

(Proceedings of the Royal Society of London;  December 1, 1856, VII 1857, p 254-257)

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