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



to the axes of the crystal, or no rotational change of the 

 plane containing the optic axes would result. This strain 

 would also have to vary gradually in amount from the inner 

 to the outer curved faces. It is, however, desirable, in the 

 case of a crystal, to attempt to explain the results in terms 

 of changes in the space lattice. Some internal change may 

 take place. This seems, perhaps, more likely, since am- 

 monium nitrate crystallizes in different forms at different 

 temperatures. Also its molecule is fairly complex, giving a 

 complex lattice and opportunity for internal change. The 

 optical results would suggest that the internal change might 

 consist of a rotation. 



If the space lattice is subject to slight forces, we should 

 expect a slight deformation to result, which would disappear 

 when the forces were removed, if the forces were such 

 that the crystal was bent into an arc of a circle, we should 

 have to suppose that the distance between adjacent atoms 

 was smaller at the inner curved face than at the outer. 

 This would appear an impossible configuration for equilibrium, 

 when the external forces are removed. 



When a long crystal is bent (as in fig. 1, PL I.) near its 

 centre, the ends remain almost unaffected. Thus a sharp " V " 

 shape may be reached by the crystal. If simple slipping 

 is assumed, in order to account for the bending shown in 

 figs. 1, 13, and 14 (Pis. I. & II.), the slip must be supposed 

 parallel to the length of the crystal. 



The change on bending would then exactly correspond to 

 folding a book, as supposed by McConnel*. This means that 

 slip would be supposed to occur along the whole length in 

 order to produce the sharp and purely local bend at the 

 centre. This seems exceedingly unlikely; and attempts to 

 determine whether there has been slip at the ends, by ob- 

 serving the relative positions of marks on the surface of the 

 crystal both before and after bending, indicate that probably 

 the slip is not appreciable. It" such slip does not occur, it is 

 necessary to conclude that the spacing of the lattice is 

 greater at the outside of the curve than at the inside, unless 

 some atoms have migrated across the crystal from the inner 

 to the outer edge. 



If we assume that simple slip has not occurred, but that 

 the lattice spacing is greater at the outer edge, we have to 

 explain how there can be stability after the bending forces 

 have been removed. We have also to explain the observed 

 slight rotation of the plane containing the optic axes, as we 



* McConnel, Proc. K. Soc. vol. xlviii. p. 259 (1890) ; and vol xlix 

 p. 323 (1891). 



