120 A TREATISE ON METAMOEPHISM. 



PRECIPITATION BY REACTIONS BETWEEN SOLUTIONS AND SOLIDS. 



"If one pours a solution of KBr over solid AgCl, . . . the bromine 

 existing in the solution will be largely replaced by chlorine, because as 

 AgBr is much less soluble than AgCl an equivalent quantity of AgCl will 

 be changed into AgBr. This is also established by experiment. If one 

 knows the solubilities of AgCl and AgBr, then for a given concentration of 

 KBr we may state the point of equilibrium which the system strives to 

 reach."" Hence we conclude that if a salt, A, is treated with a saturated 

 solution of another salt, B, a greater or less part of the salt B may separate 

 out, the salt A being taken into solution at the same time. In this case 

 "the active mass of the solid substance is a constant." * The meaning of 

 this is that if any of a solid salt is present after the reaction has ceased 

 there was sufficient to produce equilibrium between the salt and the 

 solution. 



An excellent case illustrating precipitation from solution in nature by 

 the action of a solid, one of the most fundamental importance, is the partial 

 dolomitization of the calcium carbonate of shells and corals by the sea 

 waters, which contain both calcium and magnesium salts. In this case, 

 under the law of chemical equilibrium, there is constant action and reaction 

 between the magnesium salts in solution and the solid CaC0 3 . The 

 magnesium and calcium partially interchange, the calcium going into 

 solution by uniting with the ions before combined with the magnesium, and 

 the magnesium simultaneously uniting with the C0 3 ion before united with 

 the calcium and thus being thrown down as MgC0 3 . Thus the calcite is 

 partially dolomitized. 



This case of dolomitization well illustrates the principle that simul- 

 taneously with the precipitation of one element or mineral another element 

 or mineral may be dissolved, one being conditioned upon the other. There 

 are very numerous complicated cases of this kind which need investigation. 

 (See pp. 203-206.) 



The solids present exert an important influence in precipitation 

 independently of the passage of elements of the solids into the solutions. 

 That is to say, if there be solids present, even if none of the elements of 

 any of such compounds pass into solution, these solids may influence the 



«Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan Co., New York, 1895, 

 p. 452. 



*Nernst, cit., p. 450. 



