1865.] CHEMISTRY OP NATURAL WATERS. 7 



waters as modified by the decay of vegetation, or by the reactions 

 between the matters derived from this source and the permeated 

 sediments. Not less important however than the elements thus 

 removed by substitution from sedimentary strata are those which are 

 liberated by the slow decomposition of the minerals composing 

 these sediments. 



It has long been known that in the transformation of a feldspar 

 into kaolin, the double silicate of alumina and alkali takes up a 

 portion of water, and is resolved into a hydrous silicate of alumina ; 

 while the alkali, together with a definite portion of silica, is separ- 

 ated in a soluble state. The feldspar, an anhydrous double salt 

 formed at an elevated temperature, has a tendency under certain con- 

 ditions to combine at a lower temperature with a portion of water, 

 and break up into two simpler silicates. Daubree has moreover 

 shown that when kaolin is exposed to a heat of 400° C. in 

 presence of a soluble silicate of potash, the two silicates unite and 

 regenerate feldspar. These reactions are completely analogous to 

 those presented by very many other double salts, ethers, amides, 

 and similar compounds. The preliminary conditions of this con- 

 version of feldspar into kaolin and a soluble alkaline silicate, how- 

 ever, still require investigation. It is known that while some 

 feldspathic rocks appear almost unalterable, others containing the 

 same species of feldspar are found converted .to a depth of many 

 feet from the surface into kaolin. This chemical alteration, ac- 

 cording to Fournet, is always preceded by a mechanical change 

 of the feldspar, which first becomes opaque and friable, and is thus 

 rendered permeable to water. He conceives this alteration to be 

 molecular, and to be connected with the passage of the silicate into 

 a dimorphous or allotropic condition.* 



§ 12. The researches of Ebelman on the alterations of various 

 rocks and minerals have thrown considerable light on the relations 

 of sediments and natural waters, f From the analyses of basaltic and 

 similar rocks, which include silicates of lime, magnesia, iron, and 

 manganese in the forms of pyroxene, hornblende, and olivine, and 

 which undergo a slow and superficial decomposition under atmos- 

 pheric influences, it appears that during the process of decay the 

 greater part of the lime and magnesia is removed, together with a 

 large proportion of silica. It was found moreover that in the case 



* Annales de Chimie [2] lv. 225. 



f Ebelman, Recueil des Travaux, ii, 1-79. 



