GEOLOGY: S. TABER 
301 
crystal will tend to surround the foreign body. Deposition of material 
between the crystal and the foreign body will continue, however, as long 
as diffusion can maintain supersaturation in the solution occupying this 
space; and, with continued crystal growth, the contact film must con- 
tinue to be displaced. When this fihn comes in contact with the foreign 
body, any further growth must result in either (1) the displacement of 
the foreign body or (2) the rupture and expulsion of the film; and the 
outcome will depend on the resistance offered by the foreign body, the 
dimensions of the space occupied by solution and the mutual attraction 
between the molecules of the liquid and solids. 
If the crystal is of a substance that goes into solution with decrease in 
volume, increased pressure will make it more soluble, thus increasing 
the degree of concentration requisite for further growth; but for those 
salts that have been tested a large change in pressure is required to pro- 
duce an appreciable change in solubility. If the solubility of the foreign 
body is increased by pressure, it may be gradually removed in solution 
as the growing crystal replaces it. 
The tendency of a crystal to assume a regular polyhedral form is im- 
portant as a factor in the development of pressure dufring crystal growth 
only in so far as it affects the relative solubility of the crystal in different 
directions. A crystal growing in a solution of uniform concentration 
tends to build that form which is the least soluble under existing condi- 
tions, or, in other words, for which the total surface energy is a minimum. 
If the surface tension were the same in all directions this form would 
be a sphere, but in crystals the surface tension differs in different direc- 
tions, or, on different faces, and is the same only on faces that are crys- 
tallographically the same. A solution that is in equilibrium with the 
flat face of a crystal will be supersaturated with respect to a concavity 
on the face and under saturated with respect to a convexity. When a 
crystal having a concave face or an artificially truncated angle is placed 
in a solution of uniform concentration which is kept saturated with 
respect to the normal crystal faces, growth may be limited to a single 
direction until the imperfection is repaired, but growth can not con- 
tinue indefinitely in a single direction as the superficial area and hence 
the surface energy would increase too rapidly in proportion to the volume. 
The variation in the solubility of a crystal in different directions is 
slight, and therefore it is probable that the difference is small in the 
pressure that may be developed in different directions by a growing crystal 
in contact on all its surfaces with a solution of uniform concentratioii. 
The surface tension on the different faces of a growing crystal probably 
depends on many factors, such as, the number of molecules per unit 
