7 8 Van Hise — Metamorphism of Rocks an d Rock Flowage. 



to the heat, and the chemical activity may be enormously 

 increased by a slight increase in kinetic energy of the mole- 

 cules. 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 temperature. A slight rise of temperature may increase 

 its rate of solution several fold, or out of all proportion to the 

 absolute change in temperature. At temperatures above 100° 

 C, and especially above 180° C, the activity of water may 

 increase to an amazing degree. 



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 interchange 

 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 accom- 

 plished through the medium of a separating film of water, in 

 which case the apparently simple reaction is really accom- 

 plished by transfers between the solutions and solids. In all 

 these interchanges, including those of simple solution and depo- 

 sition, according to the modern ideas of physical chemistry, 

 the salts are separated into their ions, and it is by the migra- 

 tion 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 volume, then there occurs a displacement of the state 

 of equilibrium, and in that direction toward which the reac- 

 tion 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 strength- 

 ened by such an increase in temperature ; and it is this fact 

 which, primarily, gives the preceding proposition its universal 

 validity." "If we heat the system therefore, the reaction 

 which takes place will be accompanied by absorption of heat ; 

 if we cool the system, the corresponding reaction will develop 

 heat." " On the whole, the preponderating chemical reactions 

 at lower temperatures are the combinings (associations) which 

 take place with the development of heat : while the reactions 

 preponderating at higher temperatures are the cleavings (disso- 

 ciations) which take place with the absorption of heat." This 

 ast is van't Hoff's law. 



