PRINCIPLES AND DEFINiriONS 13 



podes of quartz, these spirals would turn from right to 

 left and, in the other, from left to right. This view was 

 adopted by Pasteur (1860), and, about a hundred years 

 after its formulation by Fresnel, it received full confirma- 

 tion in the X-ray analysis of quartz made by Bragg (1913, 

 1925). This investigator showed that crystals of quartz 

 can be considered as giant molecules in which the constitu- 

 ent units build up a three-dimensional network, w^liere 

 every atom of silicon is linked to four atoms of oxygen, 

 wiiilst every atom of oxygen unites two atoms of silicon. 

 The complex aggregate thus formed has a spiral structure 

 which is shown in Fig. 1. The lines uniting the centers of 



Fig. 1. Spiral structure in a crystal of quartz. The silicon atoms are 

 represented by solid black circles, the oxygen atoms by lighter and larger 

 circles. Three atoms of silicon form a spire. Each atom of silicon is in 

 the center of a tetrahedron at the apices of wliicli are 4 oxygen atoms; only 

 2 of the latter are represented in the figure. 



the atoms are spirals, and these spirals are twisted in 

 opposite directions in dextrorotatory and in laevorotatory 

 quartz. (For further details on the coordination of sepa- 

 rate spirals in the so-called a and 3 form of quartz, cf. 

 Bragg.) Let it be noticed, then, that it is the spiral type 

 of structure which prevails in the dissymmetric spatial 

 distribution of elements in crystals of quartz. 



What is the structure of dissymmetric organic molecules 

 and its relation to optical rotation? Modern theories, a 

 detailed account of which may be found in the excellent 

 monograph by Lowry (1935), consider the irregular tetra- 



