CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 9 



tuting the strands of a gel will exhibit double refraction which 

 is not abolished by changing the refractive index of the dispersing 

 liquid. In such substances optical anisotropy is an intrinsic 

 molecular property, hence it is called intrinsic double refraction 

 and may be expressed as the difference between refractive indices 

 parallel and perpendicular to the fiber axis. The intrinsic property 

 also has sign as well as magnitude. In all, therefore, six types of 

 double refraction which arise from combinations of textural and 

 intrinsic double refraction may be distinguished. 



The anisotropic properties of gels are further complicated by 

 their elastic properties. Elastic deformation of gels under tension 

 gives rise to tension double refraction which is positive with respect 

 to the axis of deformation and compression double refraction 

 which is usually negative. Beyond the elastic limits, these charac- 

 teristically reversible optical features give way to new, more intensely 

 anisotropic properties as a result of orientation under plastic 

 deformation. 



When crystalline materials are placed between a monochromatic 

 X-ray source and a photographic plate, the X-rays are diffracted 

 and produce characteristic patterns on the plate. Polymeric 

 substances which form crystalline aggregates with large num- 

 bers of lattice planes show characteristic interference patterns. 

 The lines and spots recorded can be analyzed for distance, 

 density, width, and arrangement of interferences. From such 

 measurements we can, in turn calculate: (1) distances between 

 lattice planes, particularly the fiber period ; (2) the number of atoms 

 in the planes; (3) the width of undisturbed lattice planes, and 

 (4) the arrangement of rod-shaped lattice regions. X-ray diffraction 

 can only be applied to substances or regions containing an ordered 

 lattice. Amorphous intermicellar substances do not give X-ray 

 interferences, hence their study by X-ray diffraction can only be 

 effected by introduction of a crystalline material into these spaces. 

 For this purpose, gold and silver salts are introduced and 

 reduced by light or chemical means. From X-ray interference 

 rings obtained with gels so treated, the size of the cubic gold 

 and silver crystals, hence of the intermicellar spaces, may be 

 calculated. 



