VIII. LA ROYAL SOCIETY OF LONDON DCERNE A M. PASTEUR 

 LA MDAILLE DE RUMFORD (') 



The Prsident, Lord YVrottesIey : 



The Rumford Medal lias been awarded to M. Pasteur for his discovery ol 

 the nature of racemic aeid, 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 dilTerence, notwithstandine their close aereement, remained a 



o o 



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 spcifie gravities the same, 

 their double refraction the same. Yet the solutions of the tartrates rotated 

 the plane ol polarization of polarized light, while those of the racemates 

 were inactive. 



In a carel'ul scrutiny of the crystalline forms of the tartrates, 

 M. Pasteur was led to recognize the almost universal prsence 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 ol' 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 foiind that the 

 hemihedrism which had been observed in the tartrates was wanting in the 

 racemates. 



Thse patient and laborious rescarches in pursuit ni truth were 

 presently rewarded with an unexpected and brilliant discovery. On 

 examining the crystals obtained in an attempt to form the double racemate 



1. Proceedinys nfthe Royal Society ofLondon. December 1, 1856, VIII, 1857, i> 254-257. 



