ESSAY-REVIEWS 



93 



genius of Lebel and van't Hoff, who, without any of these direct 

 and powerful weapons of modern physical investigation, arrived 

 forty years ago at this conception of the spatial relations of the 

 carbon atom from the constitution of optically active organic 

 compounds. Nor, indeed, must the patient work of the 

 mathematical and experimental crystallographers be forgotten. 

 Working solely from the external forms of crystals and the 



• Zn 

 O S 



B 



(100) 



10) 



(III) 



Zn S Zn 5 Zn 



ZnS ZnS ZnS 



Fig. 3. 



ZnS ZnS ZnS ZnS 



general principles of solid geometry, they prepared the way for 

 these striking and rapid advances. 



Other representations of the structure of the diamond crystal, 

 built up out of the ordinary model carbon atoms of the organic 

 chemical laboratory, are shown in fig. 4. The peculiarity of this 

 structure from the standpoint of the X-rays is well seen in the 

 lower figure. The (in) planes of the crystal are arranged in 

 pairs with a gap between twice that between the components of 



