GEOLOGY: S. TABER 
299 
experiment the supporting edges of the crystal rest directly on a thin 
layer of solution which is therefore under pressure. Now pressure tends 
to reduce the thickness of the supporting film to a minimum. With 
perfectly smooth parallel surfaces the minimum thickness of the separat- 
ing film is perhaps equal to the diameter of the space occupied by a 
molecule of the liquid, and this is probably approximated in the present 
case, for deposition would be most rapid where the thickness of the 
supporting film is greatest. It would require great pressure to com- 
pletely expel the solution from such a narrow space, and, if the solu- 
tion is thus excluded, growth in a vertical direction would cease. The 
fact that this growth continues is proof that the solution is not expelled 
from under the crystal. Therefore when alum separates from solution 
and is added to the base of the crystal, the accompanying increase in 
volume must result in some elevation, irrespective of other causes. 
The cavity or hollow formed on the under side of crystals in the ex- 
periment described above results from malnutrition, due to the slow 
rate of diffusion under the crystal as compared with the relatively rapid 
growth in other directions. The writer succeeded in eliminating this 
cavity by supplying the supersaturated solution through capillary open- 
ings under the base of small crystals. Since new material can be 
added only at the base, its area remains small and tall slender columns 
are formed. 
The pressure phenomena observed during the growth of crystals have 
been attributed by the present writer "to the molecular forces associated 
with the separation of solids from solution and to the attraction and 
orientation of the physical molecules as they are brought into position 
on the surface of a growing crystal"^ (pp. 553-554). He suggested ''that 
the force is due chiefly to the expansion in volume which accompanies 
the separation of most solids from solution, for, as yet, he has obtained 
no pressure effects during the crystallization of substances that separate 
from solution with decrease in volume. "^^ Recently, however, he has 
obtained definite evidence of pressure accompanying the crystallization 
of ammonium nitrate .^^ The experiment is decribed below. 
A cup of porous porcelain was placed bottom up in a small jar half 
full of a concentrated solution of ammonium nitrate, and a piece of paraf- 
fine-coated card-board, cut to fit snugly around the cell, was cemented 
with paraffine to the top of the jar and to the walls of the cup. The 
jar was then placed in a desiccator containing calcium chloride, and al- 
lowed to stand undisturbed for nine months. The solution was drawn 
by capillary attraction to the upper and exposed walls of the cup where 
a crust was gradually formed by evaporation, but this crust was en- 
