554 S. Taber — Gr,owth of Crystals. 



attraction and orientation of the physical molecules as they 

 are brought into position on the surface of a growing crystal, 

 but the exact mechanical process (or processes) is uncertain. It 

 seems probable that several closely related processes are 

 involved. "While the forces that control the development of 

 crystal faces may enable a growing crystal to exert a linear force 

 that is greater in some directions than in others, " this crys- 

 tallizing force " is probably small. This force was not quan- 

 titatively determined in the experiment by Becker and Bay, 

 nd it is doubtful if it can be directly determined. 

 A crystal surface will not grow under pressure and therefore 

 will not do work in overcoming external forces resisting growth 

 unless the surface is in contact with a supersaturated solution. 

 From this it follows that any factor affecting the solubility of 

 the surface will correspondingly affect the amount of work which 

 may be done by a growing crystal in contact with a definite 

 quantity of supersaturated solution. If a growing crystal is 

 everywhere in contact with a solution of uniform concentration, 

 then the pressure that may be developed in any given direction 

 will depend upon the solubility of the crystal in that direction 

 and the way in which the solubility varies under pressure. In 

 crystals, the solubility is equal along, all parallel directions, 

 and, with certain exceptions, it is unequal along directions 

 which are not parallel. The solubility of most substances 

 increases with the pressure, and in crystals the rate of increase 

 is probably the same only in parallel directions. A convex 

 surface has a greater solubility and a concave surface a less 

 solubility than a plane surface, and, therefore under uniform 

 conditions, the tendency is always to form plane surfaces. 

 The crystal faces that most commonly develop are normal to 

 the directions of greatest solubility under the existing con- 

 ditions, while the crystal edges and crystal angles develop in 

 the directions of least solubility. The writer suggests that the 

 intensity of the " crystallizing force " for a crystal of any sub- 

 stance under given conditions might be estimated after experi- 

 mentally determining the relative solubilities under varying 

 pressures in different directions. 



It is possible that the pressure effects studied in this paper 

 are not limited to crystals, for it is readily conceivable that the 

 same forces of molecular cohesion, which enable any solid to 

 resist external forces tending to cause rupture, could exert, 

 during the enlargement of the mass, a pressure that would be 

 of the same order of magnitude as the forces necessary to pro- 

 duce rupture. Moreover there is another factor involved that 

 may be of prime importance. Most substances, so far as we 

 know, dissolve in water with a net decrease in the volume of 

 the system. In some cases the volume of the solution is even 



