Plastic Crystals of Ammonium Nitrate. 19 



cross the bent crystal radially. This, it should be remem- 

 bered, would also have to be accounted for if we assumed 

 that the plasticity was due to slip. 



When a crystal of ammonium nitrate is bent, probably 

 -causing the atomic spacing to be greater on the outer edge 

 of the curve, we may suppose that a rotational change occurs 

 in the structure of the crystal near the bent portion, causing 

 the atoms to keep in equilibrium at their new distance apart. 

 This rotational change would have to vary steadily' in extent 

 from the inner to the outer curved faces, as the spacing- 

 would vary continuously. Since refraction apparently de- 

 pends on the atomic structure, this rotational change would 

 probably cause a change in the direction of the plane 

 containing the optic axes, which would also vary steadily 

 from the inner to the outer curved faces. If the rota- 

 tional change were proportional to the fractional elongation 

 or contraction, the slope of the extinction-line would be 

 almost independent of the curvature, as appears to be the 

 case (provided the bending is enough to render the line 

 quite distinct). Such a rotation or orientation of the 

 atomic groups would explain the very rapid bending which 

 takes place at first when a load is applied. 



Thus the hypothesis of a rotation in the atomic groups 

 seems, on the whole, the most plausible one ; and it is in fair 

 agreement with all the observations. On the other hand, it 

 is still possible that some slight longitudinal slip may occur 

 (though attempts to detect it failed). But even if this were 

 the case, the optical properties show, almost beyond doubt, 

 that a rotation forms at least part of the change accom- 

 panying the plastic bending of -these crystals. 



In conclusion, I wish to thank Dr. Porter of University 

 College, London, for his kindness and the help that he gave 

 me whilst I was carrying out this investigation, which 

 consists of a continuation and extension of an unpublished 

 preliminary investigation which lie had made. 



Summary. 



When Ammonium Nitrate was crystallized from an aqueous 

 solution at laboratory temperatures, long needle-like crystals 

 formed, these being often complex in cross-section. Some- 

 times flat or tabular crystals formed instead. It was found 

 that both had 2 E about 59° 30' as given by Groth ; also thai 

 r<v and the crystal is optically negative. 



The needle-like crystals could be bent or twisted in the 

 iinoers. There is a slight elastic return. For rapid motion 



C2 



