PASTEUR: THE TARTRATES 



15 



M 



of the variety of their physiognomy there are some 

 features which remain immutable among them and con- 

 stitute their family mark. These features are three 

 facets which always succeed each other in the same 

 order and make between them very nearly the same 

 angles. These facets, which consist in the primitive 

 form of two contiguous faces, P and M 

 (Fig. 3), and a facet 6 1 , cutting off the 

 intersection of the first two, are parallel 

 to the same straight line which might re- 

 place for us that axis of the hexagonal 

 prism of quartz, which has been so useful 

 to us in establishing the correlation be- 

 tween the direction of the hemihedrism 

 and that of the rotary power in this 

 crystal. 



Let us agree to place this right line vertically in our 

 tartrate crystals and to turn forward the group of three 

 facets which is that characteristic the different crystals 

 have in common. All the crystals can thus be 

 ranged, in spite of the variety of their forms, in an 

 oriented series like soldiers exhibiting in front the 

 same series of buttons. But when one has arranged 

 them thus he preceives with surprise that all of these 

 soldiers bear only one epaulet, turned in every case 

 in the same direction: I mean to say that all these tar- 

 trates have their hemihedral facet inclined forward to 

 the right of the observer. 



If one turns them half-way around they are like 

 children's lead soldiers, or like the god Janus, inasmuch 

 as the front cannot be distinguished from the back: 

 the hemihedral facet from the rear is now in front, but 

 it is always to the right. If one reverses them in order 

 to observe them from the other end they resemble 

 then the double figures on playing cards; their extrem- 



