66 A TREATISE ON METAMORPHISM. 



reactions as a result of different substances being taken into solution at 

 different times; there may be reactions as a result of different solutions 

 coming together, and thus mingling; there may be reactions between 

 substances in solution and the solid material with which the water is in 

 contact; there may be reactions as a result of changing temperature and 

 pressure. All these changes are in the nature of chemical action. There- 

 fore by chemical action through solutions 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 solu- 

 tions and adjacent solids, and, finally, the interchange of the adjacent solid 

 particles, for such an interchange is usually accomplished through the 

 medium of a separating film of water. In this case the apparently simple 

 reaction between solids is really accomplished by transfers through sepa- 

 rating solutions. In all these interchanges the materials pass through a 

 stage of solution. 



Salts are combinations of the metals and the acid radicals. Thus 

 Na 2 S0 4 is a combination of Na 2 and S0 4 , and KC10 3 of K and C10 3 . 

 Faraday called these constituents ions. This term will be used as defined 

 by Faraday without any implication that a compound in solution separates 

 into its constituent ions or is dissociated. 



According to many chemists' 1 salts in various solutions are at least 

 partly separated into their ions. Such supposed separation has been called 

 electrolytic dissociation. If electrolytic dissociation takes place to a consid- 

 erable extent, the properties of the compounds are practically the sum of 

 the properties of their separated ions. In its power of dissociation of 

 dissolved salts water is held to exceed all other solvents. Water itself is 

 held to be slightly dissociated, or the H 2 separates into the ions OH and 

 H. According to the theory of dissociation the presence of free ions in 

 water solutions is therefore universal. By the advocates of the theory it 

 is held that it is by the interaction of these free ions that chemical 

 interchanges are accomplished. But dissociation is held to be very imper- 

 fect in strong solutions, relatively far advanced in dilute solutions, and in 

 very dilute solutions nearly or quite complete. As the greater portion of 



aNernst, W., Theoretical chemistry, trans, by C. S. Palmer, Macmillan & Co., London, 1895, 

 p. 307. Ostwald, W., Outlines of general chemistry, trans, by James Walker, Macmillan & Co., Lon- 

 don, 1895, pp. 266-290. 



