258 



MAJOE A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



molecules orientate themselves during crystallization under the influence of the local 

 forcives which are characteristic of the molecular configuration. The influence of these 

 local forcives will produce in the crystalline medium a distribution of internal stress 

 which will in general be different across different planes, and in this way the planes 

 of cleavage can be defined. In the direction where the stress is greatest, we should 

 expect the elastic properties of the crystal to be abnormally high, comparable in fact 

 with the elastic properties of steel. 



In other directions we should expect the elastic properties to be less pronounced, 

 and indeed the shearing of crystals, merely by the insertion of a knife blade and the 

 application of small pressure parallel to a plane of cleavage, is evidence of this. 



The following values of YOUNG'S Modulus of Rigidity for various ferro-magnetic, 

 paramagnetic, and diamagnetic media show that the power to resist distortion is of 

 the same order whatever the magnetic nature of the crystalline medium. 



* KAYE and LABY, ' Physical and Chemical Constants,' p. 27, 1918. 



t A. E. H. LOVE, " The Mathematical Theory of Elasticity," ' Camb. Univ. Press,' p. 160. In the 

 cases of beryl and topaz the different values correspond to bars whose lengths are in the directions of the 

 different axes of symmetry. 



