4 STUDIES IN GELS 97 



means of the cellulose diagram of ramie fibre (Fig. 67), Each point on 

 the diagram corresponds to a set of parallel net-planes in the crystal 

 lattice. The diagram of Fig. 67 enables us to measure four quantities : 

 I. the mutual distances, 2. the density, 3. the breadth and 4. the 

 arrangement of interferences, each of which permits calculation of 

 a corresponding quantity in the undisturbed lattice regions. 



Fig. 67. X-ray fibre diagram from ramie showing lavcr lines. 



I. According to Bragg's law of reflexion, the distance between the 

 lattice planes is calculated from the distance between the interferences 

 and the centre of the diagram. We learn from X-ray optics how the 

 unit cell (see p. 26) in the crystal lattice of cellulose can be computed 

 from the distances measured in the diagram of artificially oriented 

 cellulose preparations whose crystalline regions display an arrangement 

 of even higher orientation than in ramie fibres. The elementary cell 

 found for crystalline cellulose is monoclinic; its sides are a = 8.35 A, 

 b = 10.3 A, c = 7.9 A, and the angle ^ between a and c is 84° (Meyer 

 and Mark, 1930). Of these quantities, the most accurately determined 

 is the fibre period b which corresponds to the length of a cellobiose 

 molecule (Fig. 68). It is calculated from the distances between the 

 so-called layer Hnes which are clearly visible in Fig. 67, running parallel 

 to the equator of the diagram and connecting, as it were, the inter- 

 ference spots. These interference spots are broadened along the layer 



