REPORT ON MINERAL AND THERMAL WATERS. 21 



But it has been observed, that mineral waters of this descrip- Mode of ac- 

 tion occur in many instances in connexion with felspathic fo^jt""® 

 rocks, issuing either from primary sti*ata, or else from volcanic 

 materials. 



Now common felspar* consists, according to Dr. Thomson^ 

 (Outlines of 3Iineratogy, 1836, vol. i. p. 295,) of one atom of 

 trisilicate of potass, united to three atoms of trisilicate of alu- 

 mina 5 glassy felspar of one atom of trisilicate of potass and 

 soda, to four of trisilicate of alumina 5 whilst in albite, a mineral 

 in which the ingredients are in the same proportions as in com- 

 mon felspar, the potass is altogether replaced by soda. 



This latter alkali is therefore commonly traced to the felspa- 

 thic rocks in contact with these waters ; and, without going into 

 the elaborate calculations which Professor Bischof has thought 

 fit to institute f, by way of showing, that a single mountain of 

 moderate dimensions, — the Donnerburg, for example, near Mil- 

 leschau in the Bohemian Mittelgebirge, — contains soda enough 

 to impregnate the Carlsbad water for the space of 35,394 years, 

 it will be readily granted, that where a spring is in connexion 

 with volcanic or trappean materials, there can be no want of 

 alkali, to supply it for any conceivable length of time with that 

 portion, which is found belonging to its constitution. 



But three questions still remain to be determined, before the 

 source of the alkali can be regarded as explained. 



1st. By what process does the thermal water separate this 

 material from its combination ? 



2ndly. Why does not the same force which extracts the soda, 

 also cause the separation of a portion of the potass, with which, 

 granitic rocks at least are still more abundantly charged ? 



3rdly. How does the spring obtain its soda at all, in eases 

 where it rises, either from granitic rocks containing only com- 

 mon felspar, and therefore no other alkali than potass, or from 

 slates and other rocks that are destitute of alkali altogether ? 



The first of these difl&culties has been elucidated, by the expe^ 



* The composition of these minerals may be expressed with greater clear- 

 ness symbolically, thus : 



Common Felspar (K + 3 Si) +3 (Al -|- 3 Si), 



Glassy Felspar . P • ^ -|- 3 Si. j -f- 4 (Al" -|- 3 Si),. 



Albite (N + 3 Si) + 3 (Al -f- 3 Si). 



t I'ulk. Mineral, p. 322. et seq. 



