MOLECULAR DISSYMMETRY 



25 



objects, whatever they may be, regarded with respect 

 to their form, or the repetition of their identical parts, 

 resemble the tetrahedrons which we have just distin- 

 guished. Some placed before a mirror, give an image 

 which is superposable on them; others do not, although 

 the image reproduces them faithfully in all details. 

 A straight stairway, a branch with opposite leaves, a cube, 

 the human body, all these are objects belonging to the 

 first category. A spiral staircase, a branch ^\ith leaves 

 in a spiral, a screw, a hand, an irregular tetrahedron 

 are forms of the second group. These latter have no 

 plane of symmetr3^"^ 



Of all these comparisons that of the hand is the most 



Fig. 6. Tartaric acids, 

 right. primitive form. 



c. Hemihedral facets. 



left. 



convenient and striking. The two hands are not super- 

 posable and one cannot put the right glove on the left 

 hand, nor inversely. On the contrary the image of a right 

 hand in the mirror gives a left hand. Well! The two 

 hemihedral tetrahedrons of the right- and left-handed 

 tartrates are like the two hands: they are not super- 

 posable nor is either superposable on its image, but 

 each of them is superposable on the image of the other 

 in a mirror (Figs. 5 and 6). 



Let us recall now that we were led a moment since 

 to attribute forms of dissymmetry connected with the 

 non-superposable hemihedrism of crystals to the 



1 De la dissymetrie moleculaire des produits organiques naturels. 

 Le^on professee devaiit la Societe chimique de Paris, 1860. 



