CHEMICAL CHANGES ACCELERATED BY HEAT. 109 



"While in general, speed of chemical change is promoted by rise of 

 temperature, as indicated hj the second part of van't Hoff's law, there is a 

 limit to the increase of speed due to action and reaction between chemical 

 change and heat, for when the temperature becomes too high a reverse 

 tendency is set up, since the compounds formed by the chemical reactions 

 frequently can not exist at very high temperatures. In such cases the rate 

 of reaction may cease to increase with increase of temperature, and, indeed, 

 the reactions which obtain at lower temperatures may be reversed. 



Just as a slight increase of temperature may enormously increase the 

 speed of chemical reactions, so a slight decrease of temperature may very 

 greatly lessen the speed of reactions. Therefore, if the reaction be one 

 which itself absorbs heat, and thus lowers the temperature, the slight 

 decrease in the kinetic energy of the molecules may greatly retard the 

 speed of the reaction. 



At the very moderate temperatures which generally prevail within the 

 outer part of the crust of the earth the heat resulting from the chemical 

 changes does not become so great as to stay the reactions. Therefore, it 

 may be said that the chemical reactions which take place with liberation of 

 heat promote metamorphism, and those which take place with absorption 

 of heat retard metamorphism. The great importance of these two tenden 

 cies, as applied to rocks, will be shown on subsequent pages in connection 

 with the discussion of the zones of katamorphism and anamorphism. 



On subsequent pages it will be seen that in the zone of katamorphism 

 the first part of van't Hoff's law or the rule of Berthelot generally prevails 

 in the alterations of rocks for a considerable distance from the surface. 

 That is to say, on the whole the preponderating chemical reactions are 

 those which take place with the liberation of heat, Moreover, as a 

 consequence of increase of heat with depth, at a very moderate depth 

 the temperature is rather high. Also, igneous rocks give high temperatures 

 to the surrounding rocks and solutions. As a result of any of these causes, 

 water may reach the moderate temperature of 100° to 200° C, and such 

 temperatures increase the activity of water in an amazing' degree. (See 

 pp. 79-81.) Thus we see that in the zone of katamorphism the heat of 

 chemical action, and that derived from the interior of the earth through 

 conduction and convection by means of magma and water, all work 

 together to increase the speed of chemical action, and therefore to hasten 

 metamorphism. 



