410 WRIGHT AND HOSTETTERI CRYSTAL GROWTH 



is slight, but increases rapidly as the distance decreases down to 

 a certain limit. Beyond this limit forces of repulsion are set up 

 and, as a result, the atoms are normally separated definite dis- 

 tances which can be accurately measured by X-ray analysis. 



In case a crystal is placed under load the atoms are squeezed 

 together, a state of strain exists within the crystal, and stress 

 forces are set up which oppose the load and keep the crystal in 

 equilibrium. The essential feature to note is that in this case 

 there has been added to the force-function, valid at a particular 

 point, a new term resulting from the stresses set up on loading 

 the crystal. The loaded crystal is thermodynamically a differ- 

 ent thing from the unloaded crystal; crystallographically the 

 loaded crystal on growing orients the atoms or groups of atoms 

 which come within the range of influence of its component atoms 

 according to the force function which then obtains for the equili- 

 brium and this is of course the unstressed crystal force function 

 modified by the stresses set up on the application of the load. 

 This conception holds whether the distribution of the strain be 

 uniform or nonuniform within the crystal, but since with each 

 different state of strain the energy relations in the crystal change, 

 there can be real equilibrium only when the energy relations 

 over any given area of the crystal in contact with the solution 

 are the same as those over any other similar area. This postu- 

 lates, in general, practical uniformity of distribution of load 

 throughout the crystal. Incidentally it may be remarked that 

 the energy contributed by the load to the crystal compared 

 with the latent heat of the crystal is of a very small order of 

 magnitude and that therefore we may conclude a priori that the 

 effect of uniform or nonuniform pressure on the melting tem- 

 perature or on the solubility of a crystal cannot be great. 



If the above crystallographic conception of the problem be 

 correct, a crystal growing under load should exhibit appreciably 

 the same state of strain in a layer freshly deposited on the origi- 

 nal crystal as exists in the part of the crystal which adjoins the 

 new layer. Differences in strain between different parts of the 

 original crystal, as a result of unequal distribution of load, should 

 also be manifested in the freshly deposited layers. In other words 



