Plastic Crystals of Ammonium Nitrate. 17 



as deduced from these results, is about 20°. A microscope 

 having both rotating and travelling stage was not available ; 

 so in this, as in the previous experiment, the position of the 

 slide had to be determined by fine graph paper. 



The angle between the apparent optical axes (2 E) was 

 measured in these and other experiments, and showed no 

 change from the normal value that could not be accounted 

 for by experimental error. 



Part III. 



In considering the internal changes that accompany 

 bending, it is clear from the optical results that the crystal- 

 line structure is by no means destroyed, nor is it greatly 

 altered. This result may be contrasted with those obtained 

 by Terada* in the case of rock salt. The crystals were bent 

 between two parallel cylinders whilst being heated (at tem- 

 peratures of at least 150° C), one axis of the crystal being 

 parallel to that of the cylinders. When examined by the 

 X-ray method, the crystal was found to have been changed 

 into a number of lengths having the space lattice unaltered, 

 with the spaces between these lengths filled by mixtures of 

 broken crystal. When the bending was carried out at 

 100° C, the original structure was quite destroyed. Later 

 work has been done by Oarmakf on this subject. 



The bending is probably plastic in its nature. It seems 

 difficult to imagine a purely viscous flow' taking place in a 

 crystal. Unless the atoms could migrate to cause this 

 change in shape, the space lattice would have to be distorted 

 to an indefinite extent (in the case of a longitudinal pull, the 

 spaces in one direction becoming longer and longer). Nor 

 is it possible to regard the structure as a set of small crystals 

 originally arranged at random in a matrix and later becoming- 

 aligned t. 



The slight change in the direction of the plane containing 

 the optic axes, on crossing the width of the bent crystal, 

 corresponding to the slope of the extinction-lines, might be 

 supposed due to the effect of a residual strain, superposed on 

 the other normal optic properties. Since the axes of the 

 optical ellipsoid are coincident with the cry stallographic axes, 

 we see by symmetry that this strain would have to be oblique 



* T. Terada, Mathematico-Plnjsical Soc, Tokyo, Froc. vii. pp. 290-291, 

 May 1914. 



f P. Cavmak, Phys. Zeits. xvii. p. 556, Nov. 15, 1910. 



% Andrade, loc. Int., where a full discussion of the distinction between 

 plasticity and viscosity is given. 



Phil. Mag. S. 6." Vol. 41. No. 241. Jan. 1921. C 



