SUMMARY OF PHYSICO-CHEMICAL PRINCIPLES. 275 
change, as given by Ostwald,* lies in the “surface tension which exists 
on the boundary surfaces between solids and liquids, as on those between 
liquids and gases—the so-called free surfaces of liquids. This tension 
acts so that the surfaces in question are reduced in size, with the conse- 
quent enlargement of individual crystals (the total amount of precipitate 
remaining practically unaltered), 7. ¢., with the coarsening of the grains.” 
During the change for a given volume the lessening of the total surface 
of the crystals, and consequently the lessening of the surface tension, re- 
sults from the fact that the surfaces of the crystals will be small in pro- 
portion as the individuals are large. Fora given volume of a substance 
the surfaces of the crystals are inversely as their diameters (see page 
296). The increase in the size of the crystals lessening the surface ten- 
sion may therefore be considered as liberating energy, and hence a re- 
action under this general law of change. 
Forcrs or MrtamMorPHIsM 
The work of underground water is accomplished by the forces of me- 
chanical action, heat, and chemical action. 
DYNAMIC ACTION 
No changes in rocks take place without movements of materials, small 
or great, for short or long distances. Wherever there is rearrangement 
of the elements, there must be movements; even in the case of a mineral 
passing from one form to an allotropic form, there is movement of the 
molecules. 
Mechanical action assists water in its work by producing in substances 
a state of strain which may pass to the stage of pulverization (see pages 
296-305). Moreover, dynamic action produces effects through. chem- 
ical forces and heat and by the agency of water. The more important 
laws of the relations between pressure and chemical action are as fol- 
lows: “If we compress a chemical system at constant temperature, 
there follows a displacement of the equilibrium in that direction which 
is associated with a diminution of volume. . . . Thus the solubility 
of a salt in water, e. g., will increase with the pressure, provided that ‘the 
dissolving is associated with a contraction of the solution plus the salt, 
and, conversely, the solubility will decrease if the separation of the salt 
(from the solution) is associated with a diminution of the volume of the 
system.”} The first of these cases is that applicable to underground water 
solutions. ‘“‘ Moreover, those chemical forces are strengthened by com- 
pression, which condition a diminution of volume; and those chemical 
* Foundations of analytical chemistry, by W. Ostwald: London and New York, 19) 2P2, 
f Theoretical chemistry, by W. Nernst: London and New York, 1895, p. 567. 
