Plastic Crystals of Ammonium Nitrate. 3 



after removing the pressure. (This is not the case if there 

 is a rather greater proportion of CsN0 3 .) 



The usual or " first " form consists of thin pseudo-tetra- 

 gonal prisms without end planes (but Grossner also found 

 a tabular form). This crystal is a strongly double- 

 refracting biaxal crystal, with 2E = 59° 30' (Grossner), 

 and the dispersion of the axes is weak (r<v), according 

 to Lang. 



All the crystals experimented on, whether obtained by 

 re-crystallization or not, were long and prismatic or needle- 

 like in form ; though occasionally flat tabular crystals formed. 

 Observations showed that the value of 2E was 59° to 60°. 

 Also r<v, the difference being fairly pronounced. The 

 crystals are optically negative. 



Prolonged grinding of the crystals, dry or in oil, on a 

 smooth stone by hand, produced a white cloudiness through- 

 out the crystal. This appeared to be due to the combined 

 action of the heat generated by friction and the pressure. 

 On warming a crystal slowly in a test-tube, it was found to 

 become similarly white and opaque at a temperature between 

 30° and 40° C. This is probably due to the transition from 

 the first form to the second form mentioned by Groth. This 

 •transition did not, as far as could be seen, take place in any 

 of the bending experiments, or in any mounted slides, except 

 a little in one or two cases of repeated bending under large 

 loads. 



Ammonium nitrate crystals are distinctly hygroscopic ; 

 but fortunately the room used for the experiments was so 

 dry that no trouble was experienced on this account. 



Since the ammonium nitrate tends to crystallize with 

 what appears to be multiple twinning, it is difficult to get 

 single crystals for bending experiments. It is impossible 

 to measure the sectional area with any great precision, and, 

 since a crystal may behave differently if used for a second 

 test, the effect of: different stresses cannot be determined with 

 any high degree of accuracy. 



Preliminary experiments were performed on needle-like 

 crystals arranged horizontally, clamped at one end. and 

 loaded at the other. (The clamp consisted of a' hole in a 

 wooden block, with small wooden wedges forced in above 

 and below the crystal.) This method has the disadvantage 

 that there is an objectionable local stress at the clamp, where 

 the total stress is a maximum. 



Fig. 3 shows the rate of depression of the tip of the 

 • crystal for constant load. For the small amounts of bending 



B 2 



