IX.] CHEMISTRY OF NATURAL WATERS. 113 



compounds in natural waters, except in some local conditions, 

 as in the wells of cities, etc., where they are sometimes ob- 

 served in comparatively large amounts. The explanation of 

 this is evident ; for although nitrates themselves are not direct- 

 ly removed from the water, they are, by the reducing action of 

 organic matters, converted into ammonia, which is retained by 

 the soil. In consequence of this affinity the argillaceous strata, 

 whether of the present period or of older formations, hold in 

 a very fixed form a considerable quantity of nitrogen. This, 

 from the slowness with which it is eliminated in the form of 

 ammonia under the influence of alkaline solutions, probably 

 exists as an ammoniacal silicate. ( 6.) The action of acids, 

 however, as well as alkalies, may be supposed to liberate it 

 from its combination, and thus generate the ammoniacal salts 

 which are such frequent accompaniments of volcanic phenomena. 

 The numerous experiments of Delesse show that ammonia, or 

 at least nitrogen capable of being evolved by heat and alkalies 

 in the form of ammonia, is present in the limestones, marls, 

 argillites, and sandstones of former geological periods, in quan- 

 tities scarcely inferior to those in similar deposits of modern 

 times, amounting, for most of the ancient sedimentary strata, 

 to from one to five thousandths of nitrogen ; * from which it 

 will be seen that the quantity of this element thus retained 

 in the rocky strata of the earth's crust is very great. 



33. If we attempt a chemical classification of natural 

 waters in accordance with the principles laid down in the pre- 

 ceding sections, they may be considered under the following 

 heads : 



A. Atmospheric waters. 



B. Waters impregnated with the soluble products of vegetable 



decay. 



C. Waters impregnated with the salts from decomposing feld- 



spathic rocks, and holding a portion of carbonate of soda as a 

 characteristic ingredient. 



D. Waters holding neutral salts of sodium, calcium, or magnesium 



from strata where they existed as solid salts or in brines or 

 bitterns. 



* Ann. des Mines (5), XVIII. 151-523. 



H 



