116 A TREATISE ON METAMORPHISM. 



decomposed by hydrolysis into colloidal silicic acid and metallic hydroxides, 

 the latter (or, according to the dissociation theory, their ions) caused the 

 molecular depression, which was unaffected by the colloidal silicic acid. 



In precipitation from complex mixtures the substances do not solidify 

 at the same time. The compounds crystallize in such order, and the 

 separated solid is of such a character, that "the freezing point of the 

 remaining liquid is lowered." a After one compound has separated another 

 follows, which again lowers the freezing point, and finally a liquid is left 

 with the lowest freezing point, and this liquid is the last compound to 

 crystallize. 



Change in temperature is the rule for underground circulating waters. 

 The waters which are passing to lower levels are, on the average, becoming 

 warmer. Waters which are rising to higher levels are, on the average, 

 becoming colder. Also there are changes of temperature, both positive 

 and negative, due to varying local conditions; for instance, the presence of 

 intruded igneous rocks. Ascending waters are, on the whole, precipitating 

 material, because they are losing heat. The increase in the capacity to 

 hold material in solution with rising temperature, and the simply enormous 

 increase in this capacity as the temperature becomes very high, have 

 already been pointed out. (See pp. 79-81.) During the upward journey 

 of the water the temperature continuously falls, and if the journey be long 

 the total loss of heat is great, and the amount of precipitation is correspond- 

 ingly large. Since the upward course of the water is likely to be in the 

 larger openings (see p. 583). such as the spaces of porous sandstones, 

 faults, joints, etc., we have the partial explanation of the filling of these 

 openings in the belt of cementation. However, this general statement 

 needs various modifications, dependent upon many variable factors. (See 

 pp. 629-640.) 



PRECIP1TATIOX BT REACTIONS BETWEEN* AQVEOl'S SOLUTIONS. 



It has already been seen that when solutions containing various salts 

 are mixed the resultant solution will contain all the salts which can be 

 made by the various combinations of their positive and negative ions. 

 (See p. 68.) The first law of precipitation may be stated thus: When any 

 combination of the various ions in a solution can form to a sufficient extent 



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

 1895, p. HI. 



