IX.] CHEMISTRY OF NATURAL WATERS. 95 



4. We have thus far considered the precipitated atmos- 

 pheric waters as remaining at the earth's surface ; but a great 

 portion of them, sooner or later in their course, come upon per- 

 meable strata, by which they are absorbed, and in their sub- 

 terranean circulation undergo important changes. The effect 

 of ordinary argillaceous strata destitute of neutral soluble salts 

 may be first examined. Between such sedimentary strata and 

 the waters charged with organic and mineral matters from 

 decaying vegetation there are important reactions. The com- 

 position of these waters is peculiar ; they contain, relatively 

 to the sodium, a large amount of potassium salts, besides 

 notable quantities of silica and phosphates, in addition to 

 the dissolved organic matters and the earthy carbonates, and 

 in many cases ammoniacal salts and nitrates or nitrites. The 

 sulphuric acid and chlorine are moreover not sufficient to neu- 

 tralize the alkalies, which are perhaps in part combined with 

 silica or with an organic acid. 



5. The experiments of Way, Voelcker, and others have 

 shown that when such waters are brought into contact with 

 argillaceous sediments, they part with their potash, ammo- 

 nia, silica, phosphoric acid, and organic matter, which remain 

 in combination with the soil ; while, under ordinary condi- 

 tions at least, neither soda, lime, magnesia, sulphuric acid, 

 nor chlorine are retained. This power of the soil appears, 

 from the experiments of Eichhorn, to be in part due to the 

 action of hydrated double aluminous silicates ; and the pro- 

 cess is one of double exchange, an equivalent of lime or soda 

 being given up for the potash and ammonia retained. The 

 phosphates are probably retained in combination with alumina 

 or peroxide of iron, and the silica and organic matters also 

 enter into insoluble' combinations. It follows from these re- 

 actions that the surface-waters charged with the products of 

 vegetable decay, after having been brought in contact with 

 argillaceous sediments, retain little else than sulphates, chlorides, 

 or carbonates of soda, lime, and magnesia. In this way the 

 mineral matters required for the growth of plants, and by 

 them removed from the soil, are again restored to it ; and 



