112 CHEMISTRY OF NATURAL WATERS. [IX. 



phurous acid and oxygen; and these uniting in their distil- 

 lation-upward through the strata, may give rise to springs of 

 sulphuric acid.* To reactions similar to those just noticed, in- 

 volving borates like stassfurthite and hayesine, or boric silicates 

 like tourmaline, etc., are to be ascribed the large amounts of 

 boric acid which are sublimed in some volcanoes, or volatilized 

 with the watery vapor of the Tuscan suffioni. 



31. The action of subterranean heat upon buried strata 

 containing sulphates and chlorides is then sufficient to explain 

 the appearance of hydrochloric and sulphurous acids and sul- 

 phur, even without the intervention of organic matters, which 

 are, however, seldom or never wanting ; whether as coal, lig- 

 nite, bitumen, and pyroschists, or in a more divided condition. 

 The presence of hydrogen and of marsh-gas, as observed by 

 Deville among volcanic products, is an evidence of this. The 

 generation of marsh-gas is, however, in most cases clearly un- 

 connected with volcanic action or subterranean heat. 



To the decomposition of carbonates in buried strata by sili- 

 cious matters, with the aid of heat, is to be ascribed the great 

 amounts of carbonic-acid gas which are in many places evolved 

 from the earth, and, impregnating the infiltrating waters, give 

 rise to acidulous springs. The principal sources of this gas in 

 Europe are in regions adjoining volcanoes, either active or re- 

 cently extinct ; but their occurrence in the palaeozoic strata of 

 the United States, far remote from any evidence of volcanic 

 phenomena other than slightly thermal springs, shows that an 

 action too gentle or too deeply seated to manifest itself in igne- 

 ous eruptions, may evolve carbonic acid abundantly. The sul- 

 phuric-acid springs of western New York and Canada, to be 

 described further on, are not less remarkable illustrations of the 

 same fact. [The origin of free carbonic acid in certain cases is, 

 however, doubtless to be found in the reaction pointed out fur- 

 ther on in 66.] 



32. The frequent presence of ammoniacal salts in volcanic 

 exhalations is here worthy of notice, especially when consid- 

 ered in connection with the rarity of nitric and ammoniacal 

 * See the note to 22, on kieserite. 



