REVERSIBLE REACTIONS. 89 



be seen that certain reactions of the zones of katamorphism are reversed in 

 the zone of anamorphism, and vice versa. But it should be remembered 

 that to close the cycle in any case, or, in other words, to make a trans- 

 formation from left to right and then from right to left, thus completely 

 reversing any reaction, requires the expenditure and dissipation of energy. 



The most general law controlling chemical systems is expressed by Le 

 Chatelier as follows: "Every change of one of the factors of an equilibrium 

 occasions a rearrangement of the system in such a direction that the factor 

 in question experiences a change in a sense which is contrasted with the 

 original change." ° 



Nernst remarks that this law reminds one of the principle of action and 

 reaction. Put in another way, it may be said that any chemical change, by 

 the mere fact of its occurrence, sooner or later renders the conditions less 

 favorable for its continuance. To illustrate, if the increase in volume 

 demanded by the reaction becomes too great, this may stay the reaction. 

 For example, if calcium acetate and copper acetate be placed together in a 

 very strong vessel and but little additional space be left, the reaction resulting 

 in the expansion of volume will go on until the pressure becomes so great as 

 to stay the reaction.'' Also, if in a closed vessel a large amount of calcium 

 carbonate" be heated it will give off carbon dioxide. But as the amount of 

 C0 2 increases, and the pressure therefore accumulates, the reaction will be 

 retarded and finally cease. At this stage the C0 2 formed unites with the 

 CaO, producing CaC0 3 , as fast as CaC0 3 decomposes and produces CaO 

 and C0 2 . 



If the heat as the result of a reaction becomes too great, this will stay 

 the reaction. For instance, at low temperatures CO will completely unite 

 with 0, producing C0 2 ; but if the temperature becomes too high, as a 

 result of the change the reaction will be stayed or cease altogether. A 

 case of much greater geological consequence is that of hydration and 

 dehydration. A comparatively low temperature is favorable to hydration 

 of minerals. However, a very moderate temperature — anything above 

 110° C. — at ordinary conditions of pressure is likely to stay the reaction 

 of hydration, or even to reverse this process and produce dehydration. 



a Nernst, TV., Theoretical chemistry, translated by C. S. Palmer, ilacmillan & Co., London, 1895, 

 p. 567. 



6 Jones, H. C, On the increasing importance of inorganic chemistry: Science, new ser., vol. 8, 

 1898, p. 930. 



