Plastic Crystals of Ammonium Nitrate. 13 



tf ft " flow, he showed that for lead - (or -~ in his notation) 



increases very rapidly as the stress reaches a certain value 

 (see fig. 11). 



Since the curves of the bent crystals are more curved at 

 the centre than the corresponding elastic curves, which 

 approximate to the simple viscous curves, it is evident that 

 also in the case of the crystals, the rate of bending increases 

 more rapidly than in direct proportion to the stress or 

 bending moment. This agrees with the results of the tests 

 on bending crystals, described earlier in the paper. 



When bending has taken place, the inner edge of the 

 crystal seems to have suffered a compression, and the outer 

 an extension, near the bend (just as in the case of elastic 

 bending), as observation indicates that there is no longitudinal 

 slip detectable at the ends. 



Part II. 



Let us now turn to the optical properties of the bent crys- 

 tals. Thin crystals were bent into loops by the fingers. 

 These loops were then ground on a smooth stone or fine ground 

 glass in oil (since water dissolves the crystal). After both faces 

 of the loop had been ground, it was examined between 

 crossed nicols. If no interference effects were noticed, it 

 was bent to a smaller radius. Then, if thin enough and 

 smoothly enough ground, it was mounted in Canada balsam 

 dissolved in xylol. 



Straight crystals of ammonium nitrate show straight 

 extinction, when examined between crossed nicols. Then, 

 if the crystals were bent, we might expect extinction line& 

 consisting of four dark transverse bands, equally distributed 

 round the circular loop of the crystal. Fig. 13 (PL I.) 

 shows the result that was obtained. 



It will be seen that the transverse dark bands are not 

 radial, but slope in this case at about 45° to the radius. If 

 the crystal is of the full orthorhombic symmetry, there is no 

 reason to expect the slope to be in one sense more than in the 

 other. And it is found that in some slides the sense changes. 

 The sense is probably determined by the exact method of 

 bending, though in what way has not been ascertained. 

 Local irregularities may produce sudden changes in sense, 

 as shown in fig. 14 (PL II.), which shows three positions oi' 

 the same slide, obtained by a small rotation between each 

 figure and the next. The sense of the line changes, and then 

 it returns to its original direction. 



