100 CHEMISTRY OF NATURAL WATERS. [IX. 



alumina ; while the alkali, together with a definite portion of 

 silica, is separated in a soluble state. The feldspar, an anhy- 

 drous double salt formed at an elevated temperature, has a 

 tendency under certain conditions to combine, at a lower tem- 

 perature, with a portion of water, and break up into two sim- 

 pler 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 feld- 

 spar. These reactions are completely analogous to those pre- 

 sented 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, 

 however, still require investigation. It is known that while 

 some feldspathic rocks appear almost unalterable, others con- 

 taining the same species of feldspar are found converted to a 

 depth of many feet from the surface into kaolin. This chemi- 

 cal alteration, according 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 con- 

 ceives this alteration to be molecular, and to be connected with 

 the passage of the silicate into a dimorphous or allotrojJic con- 

 dition.* 



12. The researches of Ebelman on the alterations of various 

 rocks and minerals have thrown considerable light on the rela- 

 tions of sediments and natural waters, t From the analyses of 

 basaltic and similar rocks, which include silicates of lime, 

 magnesia, iron, and manganese in the forms of pyroxene, horn- 

 blende, and olivine, and which undergo a slow and superficial 

 decomposition under atmospheric 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. 



* [Annales de Chimie (2), LV. 225. It is a subject for inquiry how far 

 such changes are recent, and whether all feldspars found thus decomposed 

 are not portions which have been preserved to us from a remote antiquity, 

 when atmospheric agencies more potent than those of the present day were 

 at work. Ant*, page 10.] 



t Ebelman, Recueil des Travaux, II. 1-79. 



