i68 



Biological Chemistry. 



the mineralogist Haiiy which differed from one another 

 in possessing hemihedral facets on opposite sides of the 

 crystals, the two kinds forming what mineralogists had 

 termed " enantiomorphous " forms, and Sir John Herschel, 

 in 1820, had suggested that there is a link between the 

 optical activities and the crystalline forms of the two 

 descriptions of quartz crystals. Pasteur, accepting this 

 suggestion, paid particular attention to the crystalline 

 forms of the isomeric tartrates (dextro-rotatory tartrate 

 and racemate). The ordinary sodium ammonium tartrate 

 was found to possess hemihedral facets, and was optically 



D. 



Fig. 21. 



active, and Pasteur expected that such facets would be 

 absent in the racemate. On examining the crystals of 

 the sodium ammonium racemate carefully, he found that 

 this was not the fact, but that a mixture of two kinds of 

 crystals was obtained, one of which had the facets on one 

 side of the crystals and the other on the opposite side. 

 One kind of crystals was, in fact, a mirror image of the 

 other (see Fig. 21). The two classes of crystals were 

 carefully separated by hand ; and on examining the optical 

 properties, it was found that one class was dextro-rotatory, 

 whereas the other was Isevo-rotatory, and that the optical 

 activities of the two were equal, but in the opposite direc- 

 tion. These results immediately suggested the reason of the 

 optical inactivity of the racemic acid, namely, that it was 



