255 



and the same saturating power, and resembled it in its properties in 

 a very remarkable manner. Yet tbe two acids were not identical, 

 and the cause of their difference, notwithstanding their close agree- 

 ment, remained a mystery. The resemblance between the two acids 

 had been rendered still more striking on a comparison of the phy- 

 sical 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 polari- 

 zation 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 super- 

 posed on the image of the other in a mirror. Sometimes the hemi- 

 hedrism was indicated merely by the greater development of one 

 pair of faces than of another pair. A hemihedrism of such a cha- 

 racter 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 in- 

 active 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 



