SUMMARY OF PHYSICO-CHEMICAL PRINCIPLES. De 
erate depths, a given head may there result in considerably greater flow- 
age than at the temperatures prevailing at the surface. Thisis especially 
true of the capillary tubes, where the flowage is almost directly as the 
viscosity. In the capillary spaces it appears probable that with a given 
head flowage is five or more times as rapid at the temperatures which 
prevail in the lower part of the zone of fracture than in similar spaces 
near the surface. It is evident that this decrease of viscosity with in- 
crease of temperature is very favorable to the circulation of water in the 
deeper parts of the zone of fracture, for on account of it underground 
water upon the average follows a deeper path than it would were the 
viscosity everywhere the same. ‘This may well be one of the chief causes 
which result in the rapid induration which seems to be characteristic of 
the lower part of the zone of fracture. 
(2) WORK OF UNDERGROUND WATER 
The potency of water as an agent through which metamorphism may 
take place is due, according to the modern ideas of physical chemistry, 
to its capacity to separate substances which it holds in solution into 
their free ions.* In this power of ionization it exceeds all other solvents. 
As the greater portion of underground solutions are rather dilute, at least 
where somewhat free circulation is the rule, we may suppose that the 
salts held in solution are largely separated into their ions, and therefore 
these free ions are ever ready for chemical reactions. 
As illustrations of the above, we may take a few simple cases, as, for 
instance, solutions of NaCl, MgSO,, HC], KOH. In the dilute solutions 
which occur in nature these substances are not combined into salts, but 
are largely or wholly in the forms of ions—that is, if dilute solutions of 
the above-named salts are made, the substance in solution will be Na, 
Cl, Mg, SO,, H, Cl, K, and OH. 
Until recently it has been unknown how silicates behave when dis- 
Solved. However, Kahlenberg and Lincoln t have shown that the most 
important geological compound, silica, in dilute solutions occurs in the 
form of colloidal silicic acid. To illustrate: if a sufficiently dilute solu- 
tion of sodium silicate be made, but which may be much more concen- 
trated than ordinarily occurs in underground waters, the compound 
breaks up into the ions Na, OH, and colloidal silicic acid—that is, the two 
former are ionized but the silicic acid is not. From this fact it would 
not be expected that silicic acid is a chemically active compound, and 
*The statements on this and subsequent pages concerning the principles of physical chemistry 
are mainly taken from the works of Ostwald and Nernst. 
{Solutions of silicates of the alkalies, by L. Kahlenberg and A. T. Lineoln : Journ. Phys. Chem., 
vol. ii, 1898, pp. 88-90. 
