On the Intimate Structure of Crystals. 

 FIG. 4. 



285 



is shown in the diagram (fig. 4), the molecules are forced to slide 

 over each other ; those lying on the diagonal along which pressure 

 is exerted make a nearer approach to each other ; those at right 

 angles are driven further apart. Up to the critical point, compres- 

 sion and accompanying anisotropy are produced; beyond it the 

 gliding plane becomes a plane of fracture. 



The question of elasticity must be left to elasticians ; but I would 

 venture to point out the facts elicited by Voigt and Koch ; Yoigt 

 found for the elasticity modulus of rock salt a coefficient of 

 4170 kilograms per square millimetre parallel to the tetragonal axes 

 or edges of the cube, 3400 kilograms parallel to the digonal axes or 

 normal to the rhombic dodecahedral faces (110), and 3180 kilograms 

 parallel to the trigonal axes or normal to the octahedral faces (111). 

 Koch found for potassium chloride, 4009 kilograms parallel to the 

 tetragonal axes, and 2088 kilograms along the digonal axes. Voigt 

 obtained very similar quantities. Looking at the plane of assemblage 

 given in the figure (fig. 2), one would suggest that these results are of 

 a kind to be expected ; the force exerted along the tetragonal axis is 

 chiefly effective in producing compression of the atoms, along the 

 diagonal axis in distorting the structure. An exact correspondence 

 is obtained by Lord Kelvin between the theoretical elasticities of a 

 hypothetical cube and the results obtained by direct measurement of 

 potassium chloride, but the correspondence does not extend to the 

 results of observation for rock salt, and the structure we have assigned 

 to our crystals is not that considered by Lord Kelvin in his investiga- 

 tion.* 



The hardness of rock salt in different directions has been investi- 

 gated by Exner, who finds that its value is at a maximum parallel to 

 the edge, and at a minimum parallel to the diagonals, of the face of 

 a cube. This result was obtained by determining the weight, with 

 which it was necessary to load a finely pointed needle, to cause it to 



* " On the Elasticity of a Crystal according to Boscovicli," ' Roy. Soc. Proc.,* 

 Tol. 54, p. 69. 



VOL. LXIII. Y 



