ASYMMETRIC CRYSTALS 



105 



on a similar laevorotatory piece would be optically inactive. 

 Physical examination of quartz crystals shows that d-crystals 

 differ from Z-crystals in one respect only, viz. : the position of 

 their secondary facets. The ordinary form of a quartz crystal 

 is a six-sided prism topped by a six-sided pyramid. The alternate 

 solid angles where two prism faces meet two pyramid faces is 

 generally levelled off to form a small secondary face or facet. 

 When the crystal is viewed with the pyramid upmost and these 

 facets slope to the right, the specimen will rotate the plane of 

 polarisation to the right, and vice versa when the facets incline 

 to the left. The one crystal is a mirror image of the other, and 

 is called its optical isomer. 



If the crystal is symmetrical with no secondary facets then 

 optical activity is impossible. Any perfect cube is an exact 



ABC 



FIG. 17. Crystals of Ammonium Hydrogen Malate. (a) Symmetrical crystal, optically 

 inactive ; (b) Asymmetrical crystal, dextrorotatory ; (c) Asymmetrical crystal, 

 laevorotatory. (After van 't Hotf.) 



duplicate of any other perfect cube. Such holohedral crystals 

 can be prepared. In Fig. 17, A represents a holohedral crystal 

 of inactive ammonium hydrogen malate ; B represents a dextro- 

 rotatory crystal, while C represents its mirror image, a laevo- 

 rotatory crystal of this salt. 



Of amorphous bodies which are optically active, with the 

 exception of one or two little known compounds of nitrogen, all 

 are compounds of carbon in which one or more of the carbon 

 atoms has its valencies satisfied by four different atoms or radicles. 

 Such a carbon atom is termed asymmetric (Fig. 18). 



Compounds containing asymmetric carbon atoms are a geo- 

 metrical necessity and exist in proteins, carbohydrates and fats. 

 Each member of a pair of optical isomers is identically equal in 

 every respect but one. As stated at the beginning of this section, 

 enzymes preferentially act on one isomer. For example, those 



