208 A TREATISE ON METAMORPHISM. 



the latter mineral the statement is made that the change generally involves 

 liberation of heat and decrease of volume. But when a mineral is derived 

 by precipitation from a solution, or results by the combination of several 

 minerals, it is necessary to consider the chemistry and physics of the change 

 in connection with the sources, for otherwise this important part of the 

 history of metamorphism of minerals would be omitted. 



In discussing the sources of a mineral when it is derived from other 

 minerals the natural conditions of the alterations are not given, but may be 

 found by referring to the minerals from which the one under discussion 

 is derived. But where a mineral is derived by the interaction or union of 

 several other minerals the natural conditions are discussed under the source 

 of the mineral, for otherwise this part of the subject would be omitted. 



As this treatise was originally planned it was designed to include the 

 heat and volume changes with the chemical reactions. But with the 

 present state of knowledge of the heat relations in chemical transformations 

 the first has been found impracticable. While very few quantitative 

 results can be given, in many cases it is possible to make a qualitative 

 expression of the heat reaction. To illustrate, the heat of combination of 

 calcium is far greater than that of iron in all analogous compounds in 

 which determinations have been made; but such determinations have not 

 been made with reference to the silicates. Where calcium is replaced by 

 iron in the alteration of the silicates it is inferred that a considerable amount 

 of heat is absorbed, though the exact amount can not be specified. Vice 

 versa, where iron is replaced by calcium, a considerable amount of heat is 

 liberated. Of course, in each reaction the other chemical combinations 

 which occur simultaneously should be considered, for they constitute a part 

 of the chain, and in obtaining a correct end result their effects are vital. If, 

 for instance, a salt of iron and a salt of calcium interchange acids, no general 

 statement can be made as to the heat reaction. Therefore, if at the same 

 time the iron replaces the calcium the calcium unites with an acid which 

 was before in combination with the iron, the inference above given as to 

 absorption of heat can not be made. 



For the calculation of the volume changes, the equations of the chem- 

 ical reactions written out by me and the specific gravities of the minerals, 

 taken from the standard Mineralogies, were turned over to Mr. A. T. Lincoln, 

 who made the numerical computations. Subsequently Mr. R. M. Chapman 



