12 PRINCIPLES AND DEFINITIONS 



a plane, a center or an alternating axis of symmetry {cf. 

 the definition of Lowry, 1935), these elements being incom- 

 patible with the non-superposability of the image. So, 

 dissymmetric molecules are not necessarily asymmetric in 

 the geometrical sense. 



3. Dissymmetric Structure as a Basis of Optical Activ- 

 ity. The fact that the rotation of the plane of polarized 

 light is caused by a dissymmetric structure of molecules 

 leaves no place for doubt, but the problem of the physical 

 mechanism by which this is done did not yet receive a defi- 

 nite solution. Two models proposed by Pasteur — irregu- 

 lar tetrahedron and spiral line — have formed the basis for 

 further theories. We shall consider separately the case in 

 which optical activity is due to a dissymmetric spatial dis- 

 tribution of atoms as found in entire crystals and the case 

 in which it is due to a dissymmetric structure of isolated 

 molecules. 



It is known that the optical activity of quartz depends 

 on the structure of the crystal itself, since the rotation of 

 the plane of polarized light disappears with the crystalline 

 state. The optical effect also diminishes, and at last van- 

 ishes when a plate cut out from a crystal of quartz passes 

 from a position perpendicular to the direction of the ray 

 to an inclined position. Consequently, the fundamental 

 difference between the dissymmetry of quartz and the mo- 

 lecular dissymmetry of organic substances lies in the fact 

 that in the former case the crystal as a whole is anisotropic, 

 i.e., possesses different properties in diiferent directions, 

 while, in the latter, as it was ascertained by Pasteur, dis- 

 symmetry represents a property of the separate molecules 

 independent of their relative position in space. A sub- 

 stance in which one of the two possible dissymmetric forms 

 of molecules, right or left, predominates, will possess 

 optical activity. 



It was Fresnel (1824) who suggested for the first time, 

 that the structural dissymmetry of quartz may be ex- 

 plained on the basis of the spiral distribution in space of 

 the molecules of silicon. In one of the two optical anti- 



