130 



ENZYMES 



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 



(<rz?^ 



'A 



k: 



r\ 



Fig. 32. — Crystals of Ammonium Hydrogen Malate. (a) Symmetrical crystal, optically 

 inactive; (b) Asymmetrical crystal, dextrorotatory; (c) Asymmetrical crj'stal, laevo- 

 rotatory. (After van't Hoff.) 



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 



Fig. 33. — Diagram of a carbon atom (A) having its valencies supplied with four different 

 atoms B, C, D, E. The mirror image of this structure would be its optical isomer. 



optical activity is impossible. Any perfect cube is an exact 

 duplicate of any other perfect cube. Such holohedral crystals 

 can be prepared. In Fig. 32, A represents a holohedral crystal of 

 i nactive ammonium hydrogen malate ; B represents a dextro- 



