300 
GEOLOGY: S. TABER 
larged chiefly or entirely through the addition of new material to the 
outer exposed surfaces, the solution reaching these surfaces through 
capillary pores in the crystalline mass. Under the card-board, how- 
ever, a few long acicular crystals were formed on the surface of the cup, 
and these were gradually pushed outward by the addition of new ma- 
terial to their base. At one place the solution penetrated the card- 
board which was gradually split apart by the slow growth of a lens- 
shaped veinlet of finely crystalline salt about 3 mm. in thickness. The 
cup was not broken as in similar experiments with certain other salts 
that separate from solution with increase in volume. 
The pressure phenomena observed in this experiment are explained 
as follows: CrystalHzation is retarded or prevented in supersaturated 
solutions which occupy small capillary or subcapillary spaces; therefore 
crystals may be supplied with material for growth by diffusion through 
solutions occupying such spaces, and the increase in volume due to the 
entrance and deposition of new material must result either in the ex- 
pulsion of part of the solution or in the enlargement of the space occu- 
pied by the growing crystals. If the spaces occupied by the solution 
are relatively large, the solution will be gradually expelled as crystalline 
matter is deposited in its place, but, on the other hand, if the spaces are 
sufficiently small, less force may be required to enlarge the space occu- 
pied by the growing crystals than is necessary to expel the solution. 
The diffusion of a sohd through a solution and its separation therefrom 
are attributed to osmotic pressure and the relation between osmotic 
pressure and solution pressure. According to the writer's theory, the 
force observed in this experiment with ammonium nitrate is analogous 
to the pressure developed when an anhydrous salt, confined in a'limited 
space, combines with water that has diffused as vapor through capillary 
openings 
Molecular attraction between solid and liquid causes the thin layer 
of solution in contact with a crystal to adhere to it; and this contact 
film, on account of adsorption, is of different concentration from the 
bulk of the solution. Enlargement or solution of a crystal is brought 
about by the diffusion of dissolved substance across this layer. When 
the solution in contact with a crystal surface is supersaturated with 
respect to that surface, new material is deposited on the crystal, thus 
forcing the contact film to move outward from the growing crystal. If 
this film approaches a foreign body, growth in that direction is gradually 
retarded as the space through which diffusion must supply new material 
becomes more limited. Consequently growth will be more rapid in 
other directions, providing diffusion is not similarly restricted, and the 
