I. FANKUCHEN AND H. MARK 



Sometimes the intensity scattered at small angles does not exhibit 

 distinct lines or spots but is of a more continuous character, decreasing 

 gradually with increasing diffraction angle. This points to the fact 

 that there are no repeating, well-defined long dimensions within the 

 sample but that these distances either scatter rather widely about a 

 certain average value or are randomly distributed in the sample. In 

 such cases, one must investigate quantitatively the whole intensity 

 distribution around the primary beam before one can draw conclusions 

 about the length of this pseudoperiodicity (16) or more generally say 

 something about the distribution of these long dimensioris within the 

 specimen. 



Size of Crystallites, Degree of Orientation, and Crystallinity 



It has already been mentioned that the sharpness and intensity 

 of the lines or spots of the small-angle diagram depend upon the num- 

 ber of repeating periodicities and upon the accuracy with which 

 they repeat. This is also true for the lines or spots of the large-angle 

 diagram. The more frequently a given lattice plane is repeated in one 

 of the crystalline regions, the more numerous will be the secondary 

 waves which cooperate with a favorable phase difference and, hence, 

 the sharper and more intense is the diffraction spot which they produce. 

 Sharp lines or spots indicate large crystals or crystalline regions; dif- 

 fuse lines indicate small crystals. In most fiber protein diagrams, the 

 large-angle diffraction spots are fairly broad and indicate that the aver- 

 age width of the bundle-shaped crystallites in a highly oriented protein 

 fiber, such as silk, muscle, or feather, is about 100 A. or less. Along 

 the axis of orientation the diffraction lines are sharper and, hence, the 

 domains of crystalline order are longer. Stretching and alignment 

 usually increase the size of the crystalline areas, whereas swelling and 

 relaxing decrease it. If the ordered domains become smaller than 

 20 or 30 A., the x-ray diagram of the system becomes more and more 

 diffuse and finally approaches that of a liquid. 



Next to the approximate average size of the crystalline areas, one 

 is interested in the way in which they are oriented in a given sample. 

 Completely random arrangement of the scattering elements in a sample 

 produces an x-ray diagram which consists exclusively of rings. When 

 some or all of these rings degenerate into segments or spots, the presence 



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