2 Mr. W. N. Bond on the Properties of 



results in fracture at the part most strained, the crystal being 

 thus both plastic and brittle. The thinnest needles seem to 

 bend better when fresh from the solution they have been 

 crystallized from. In connexion with this, it may be noted 

 that, according to Kleinhanns*, rock salt is bent more easily 

 when immersed in liquids that dissolve it. 



The following account of the crystalline forms of Am- 

 monium Nitrate is based on that of Grothf. 



At ordinary temperatures, it crystallizes from an aqueous 

 solution as rhombohedral and pseudo-tetragonal crystals. 

 These change on warming, as first noticed by Lehmann J, 

 to a second form, which is apparently also rhombic ; then 

 to an optically uniaxal form, and finally to a single-refracting 

 fourth form. This fourth form is stable at 124° 0. ; the 

 third from that temperature to 83° C; the second from 

 83° 0. to 32° C, and the first from 32° C. downwards. 



Wallerant, however, says that the third form is not, as 

 Lehmann supposed, trigonal, but tetragonal and optically 

 positive. This passes into the more strongly double-refract- 

 ing second form mentioned above, of which the prisms have 

 their length parallel to the second axis of the tetragoaal 

 crystals. By adding traces of potassium nitrate to the 

 molten sample, the 32° transition point is lowered, so that 

 the second form still remains stable at ordinary temperatures. 

 We are then able to produce twinning lamellae. The transi- 

 tion from the third (tetragonal) form to the less dense second 

 form cannot be produced if the former is under pressure ; 

 the change to the first (rhombic) form (which has the greater 

 density) takes place instead. 



This first modification is stable at normal temperature ; 

 according to Wallerant, if cooled to — 16° 0. it undergoes a 

 change to another form, less dense, uniaxal, optically posi- 

 tive, with double refraction somewhat weaker than that of 

 the third, tetragonal, form. Wallerant concludes that the 

 form below —16° C. is the same as that stable from 124° G. 

 to 83° C. 



The tetragonal form arising from the cubic at 124° C. 

 remains stable even at the lowest temperature if a little 

 CsN0 3 is added ; but changes into the rhombic form under 

 pressure. At 20° C. a very slight pressure is sufficient. If 

 the amount of CsN0 3 is very slight, the crystal changes back 



* K. Kleinhanns, Phys. Zeitschr. xv. pp. 362-363, April 1st, 1914. 

 t Groth, Chemtsche Krystallographie, Zweiter Teil, p. 66. 

 % 0. Lehmann, Zeitsehr.f. Krystal. 1877, i. p. 106. 



