16 PASTEUR: THE HISTORY OF A MIXD 



ities resemble each other and however one places 

 them, provided that one puts in front the characteristic 

 group of the three faces that we have pointed out, 

 one of their four hemihedral facets comes again ob- 

 stinately to take up its position facing the observer, 

 and at his right. 



Thus, a curious circumstance, all the tartrates so 

 varied in form, which Pasteur had studied to the number 

 of 19, have a rotary power in the same direction, and 

 also a hemihedrism in the same direction. This 

 correlation related them to the quartz but had a deeper 

 meaning, for here it could be no longer a question of 

 arrangement of the molecules in the cr3'stal, but of 

 arrangement of the atoms in the molecule. It is clear that 

 one can change the conventions, for example, examine the 

 crystals on the edge as is the custom in Germany, instead 

 of on the face, as is the custom in France. In that case 

 the hemihedral facets incline to the left when the rotary 

 power remains to the right, but it is the statement of the 

 phenomena which changes, and not its nature: all 

 the tartrates having a rotary power in the same direction 

 have also a hemihedrism in the same direction, and that 

 demonstrates a relation between the form of the molecule 

 and its mode of action on hght. 



IV 



THE PARATARTRATES 



It is evident that we have made progress since the 

 study on quartz. Now we find ourselves studying with 

 Pasteur the manner of grouping of the atoms. And 

 here belongs an unforeseen discovery. 



In the factories where tartaric acid is made one 



