106 A TREATISE ON METAMORPHISM. 



the substance is."™ If this law be applicable to quartz and to silicates it is 

 of great importance in metamorphism, because these are the substances 

 most largely dissolved and deposited by the ground water, with the pos- 

 sible exception of the carbonates. 



As to the heat relations when two solutions are mixed, Ostwald states 

 that in mixing solutions heat is produced by the work between the hetero- 

 geneous molecules, and heat is used in separating and spreading out the 

 homogeneous molecules. The sum of these may be positive or negative, 

 but in most cases the former is the case, and hence the two liquids usually 

 become warmer when they are mixed. 6 Upon the same point Nernst says: 

 "No heat phenomena result from the mixture of salt solutions [provided 

 that no precipitate (and no volatile compound) is produced]."" 



When chemical reactions occur there is a certain amount of heat of 

 formation of the compounds. "By the 'heat of formation' of a chemical 

 compound is meant the quantity of heat which is given off in the formation 

 of the compound from its respective ingredients.'"* "The 'heat- toning' of a 

 reaction is equal to the sum of the resulting heats of formation minus the 

 sum of the heats of formation of the vanished molecules." d In whatever 

 way a chemical result is accomplished, and however many the stages of 

 process of the change, "the energy differences (and therefore the heat dif- 

 ferences) between two identical conditions of the system must be the same, 

 independently of the way by which the system is transferred from one 

 condition to the other." e 



This last is an important law so far as the work of ground waters is 

 concerned, for in most cases we know only the opening and closing stages 

 of the processes of alteration, and can ascertain whether the heat effect of 

 a reaction is plus or minus only by comparing the heat required for the 

 production of the original minerals with that required for the production of 

 the ^secondary minerals, and their gaseous and fluid by-products. 



While the above gives the conclusions as to the heat effect of indi- 

 vidual reactions, the reaction which is likely but not certain to obtain at 

 moderate pressure and temperature is covered by the rule of Berthelot. 

 He says, "Every chemical change gives rise to the production of those 



a Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 1895, 

 p. 504. 



&Ostwald, W., Solutions, translated by M. M. Pattison Muir; Longmans, Green & Co., New York, 

 1891, p. 308. 



"Nernst, cit., p. 508. ^ Nernst, cit., p. 505.' « Nernst, cit., p. 496. 



