100 E. W. Hilgard— Geologic Efficacy of 



Aet. XIII. — The Geologic Efficacy of Alkali Carbonate Solu- 

 tion ; by E. W. Hilgaed. 



[Read at the April meeting of the J^ational Academy, 1896.] 



The effects of carbonated water in the decomposition and 

 the transformation of rocks are well understood, and are usually 

 compared with those of other acid solvents occurring in nature. 

 Among its most important effects are the decomposition of 

 refractory silicates, such as the feldspars and hornblendes, and 

 the dissolution arid redeposition of earthy and metallic car- 

 bonates. Its effects in connection with the alkalies are much less 

 obvious, and the conditions under which alkali carbonates may 

 be formed and maintained are far less thoroughly understood 

 than in the case of the earthy and metallic compounds. While 

 alkaline mineral waters are comparatively plentiful, the man- 

 ner of their formation, under the varied conditions in which 

 they are found to exist, is not at all obvious. Such waters, 

 even when hot, are almost always strongly charged with car- 

 bonic acid, which is supposed to have extracted the alkalies 

 from their silicate combinations. This at least is the explana- 

 tion usually given, since according to the ordinary conceptions 

 of the relative " strength " of the mineral acids, it is not 

 readily conceivable that the chlorids or sulphates of the alkali 

 metals should be sensibly decomposed by the action of free 

 carbonic acid, implying the formation of free chlorhydric or 

 sulphuric acids. 



The rarity of the occurrence of alkali silicates in natural 

 waters renders the derivation of the alkaline carbonate waters 

 from them a somewhat forced conclusion. A discussion of 

 several hundreds of analyses of well and spring waters showed 

 only three in which alkali silicates were found, while all con- 

 tained the chlorids and sulphates of the alkalies, usually together 

 ! J with calcic and magnesic carbonates. It is the object of this 



paper to discuss the possibilities of a much easier mode of for- 

 . mation, which explains both the very frequent occurrence of 



,|lj . carbonated alkali waters and the presence of the accompany- 



' ' ing salts of neutral reaction. 



The rationale of this process of formation of alkali carbon- 

 ates has been explained in papers heretofore published by me 

 p i! in " Agricultural Science," as well as in the proceedings of 



the " Deutsche chemische Gesellschaft" for December, 1893. 

 The investigation, made successively in conjunction with 

 A. H. Weber and M. E. Jaffa, from 1887 to 1891, had for its 

 starting point the observation frequently made by me in the 

 analyses of waters, that gypsum crystals are very commonly 



