108 ANNALS NEW YORK ACADEMY OP SCIENCES 



several miles of underground channels might easily be deprived of their 

 dissolved oxygen in several ways, but waters descending directly from the 

 surface would necessarily produce oxidizing effects. 



CHEMISTRY OF THE PROCESS BY WHICH ALTERATION WAS EFFECTED 



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The principal role of the water seems to have been to form a medium 

 in which recombination of the elements of the rock could be easily effected. 

 The minerals arising from the crystallization of the magma were those 

 which were in mutual equilibrium under the conditions of temperature^ 

 pressure and concentration prevailing at the time of consolidation. In 

 the presence of heated water, they were, to a slight extent, dissolved and 

 were thereby transferred to a solution in which entirely different condi- 

 tions of temperature, pressure and concentration were present. As the 

 chemical equilibrium is dependent upon these factors, new reactions fol- 

 lowed, and it may be profitable to devote a short space to an inquiry into 

 the mechanism of the process. According to modern conceptions of ioni- 

 zation, a mineral or salt which is soluble in only minute quantities in 

 water undergoes in such a solution a change by which the molecular 

 grouping which characterized the solid is broken up into simpler groups 

 termed ions.-'' A familiar example of this is the dissociation of AgCl, 

 by which an Ag and a CI ion are formed, carrying electrical charges 

 which are equal and of opposite sign. The quantity of un-ionized AgCl 

 in the solution is extremely minute, but is not negligible. Addition of a 

 second salt, such as XaCl, which has an ion in common, drives back the 

 ionization of AgCl and causes some to be deposited from the solution. 



Chemical reactions between such salts in solution are believed to be 

 chiefly of an ionic nature. If in the formation of new compounds, the 

 quantity produced to satisfy the conditions of equilibrium demanded by 

 the nature of the reacting substances is in excess of the solubility, a por- 

 tion will be deposited in the solid state. Such a reaction abstracts ions 

 and permits a further dissociation of molecules. This in turn permits 

 more of the original solid to pass into solution, and in this manner the 

 process is continuous and will proceed until all the available material has 

 been used up. 



The process thus described enablef us to form a mental picture of the 

 mechanism of alteration but does _iot suggest any explanation of the 



2* There has lately arisen some doubt as to whether the theories of ionization which 

 have been founded upon older conceptions of the molecule represent with exactitude 

 actual conditions and processes. Whether this is true or not, the idea forms a practical 

 working hypothesis, not out of harmony with main facts, and of value accordingly. 

 There is no tendency to abandon it, until a better substitute can be found. 



