274. VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
such is the case near the surface of the earth, at ordinary temperatures 
and pressures. However, upon subsequent pages it will be seen that at 
considerable depth, where the pressure and temperature are much above 
the normal, silicic acid is a most activecompound. It may therefore be 
conjectured that when experiments are performed upon this acid under 
similar conditions it may be found to become partially ionized. 
As material passes into solution it changes from the solid to the gaseous 
form, and consequently absorbs energy. However, where there is a les- 
sening of the volume of the solution, as compared with the volume of 
the solvent and salt together, the molecules are brought closer together 
and energy is developed. Whether there is a rise or fall of temperature . 
_ of the solution will depend upon the relative values of these factors. 
In general, the decrease in the volume of the solvent and salt in forming 
the solution is the more important factor, and there is a rise in tem- 
perature. However, in the case in which energy is used up in changing 
the salt from the state of a solid to that of a gas, and at the same time 
the volume of the solution is greater than that of the solvent and salt, 
energy is absorbed in both transformations, and then there is a marked 
absorption of heat or fall in temperature, as in the case of the solution of 
ammonium chloride. 
When asolid is ina saturated solution the amount of the solid neither 
increases nor decreases; but, if pressure and temperature remain constant, 
it does not follow that no interchange takes place between the dissolved 
and solid salt. The kinetic theory of solutions leads to the conclusion 
that many molecules are released from the crystals into the solution, and 
pass from the solution into the crystals, but these amounts balance. 
In order that crystals shall grow during the metamorphism of rocks, 
it is necessary that the solutions shall be saturated or supersaturated 
at the immediate place of crystal growth. As underground there is al- 
ways a superabundance of material present as compared with the amount 
of water, we may suppose that at a moderate depth below the surface, 
and especially in the smaller spaces, where movement is slow, the solu- 
tions are often saturated. Itis a well known fact that under conditions 
of saturation, with a superabundance of material, the larger crystals grow 
at the expense of the smalier ones, and that this process goes on more 
rapidly in proportion as the temperature is high and the pressure is great. 
This principle is taken advantage of in the chemical laboratory in the 
production of a coarse precipitate, before filtration, by boiling or other 
means, the finer particles of the precipitate being dissolved and the 
coarser being enlarged at their cost. ‘The growth of the large crystals at 
the expense of the small ones is due to the fact that the smaller crystals 
are somewhat more soluble than the larger. The explanation of this 
