276 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
forces are weakened by compression which condition an increase in 
volume.” * 
HEAT 
All metamorphism takes place through the assistance of heat. No- 
where upon the surface of the earth, nor within the earth, is the tem- 
perature absolute zero. The activity of the molecules, or their kinetic 
energy, increases in proportion to the heat, and the chemical activity 
may be enormously increased by a slight increase in kinetic energy of the 
molecules. The temperature is therefore a most important factor in the 
rapidity of the changes of all kinds. 
For instance, the activity of water is greatly increased by rise of tem- - 
- perature. A slight rise of temperature may increase its rate of solution 
several fold, or out of all proportion to the absolute change in temper- 
ature. At temperatures above 100° C., and especially above 180° C., the 
activity of water may increase to an amazing degree (see pages 319, 320). 
Heat for the alteration of rocks is derived (1) from deep within the 
earth by conduction, or by convection through water or magma, (2) from 
dynamic action, (3) from chemical action, and (4) from the sun. 
CHEMICAL ACTION 
No change takes place without chemical action. By chemical action 
is meant the taking of material into solution, the deposition of material 
from solution, the interchange between materials in solutions, the inter- 
change between materials in solutions and adjacent solids, and, finally, 
possibly the interchange of the adjacent solid particles. I say possibly, 
for such an apparent interchange is probably accomplished through the 
medium of a separating film of water, in which case the apparently sim- 
ple reaction is really accomplished by transfers between the solutions 
and solids. In all these interchanges, including those of simple solution 
and deposition, according to the modern ideas of physical chemistry, 
the salts are separated into their ions, and it is by the migration of these 
free ions that the interchanges are accomplished. 
RELATIONS OF CHEMICAL ACTION, HEAT, AND PRESSURE 
The more important laws expressing the relation of chemical reactions 
and heat are as follows: “If we heat a chemical system, at constant vol- 
ume, then there occurs a displacement of the state of equilibrium, and in 
that direction toward which the reaction advances with absorption of 
heat.” + ‘ Those chemical forces which condition a development of heat, 
will always be weakened by an increase of temperature ; and conversely, 
those which condition an absorption of heat will be strengthened by 
* Nernst, loc. cit., p. 567. 
7 Nernst, loc. cit., p. 566. 
