IX.] CHEMISTRY OF NATURAL WATERS. Ill 



giving rise to sulphate of iron, and more rarely to sulphates 

 of copper, zinc, cobalt, and nickel ; and by secondary reactions 

 to sulphates of alumina, lime, magnesia, and alkalies. This 

 process of oxidation is necessarily superficial and local, but the 

 soluble sulphates thus formed have probably played a not un- 

 important part. (9.) 



29. Besides the solutions formed by this last process, which 

 contain chiefly neutral and acid salts, there is another class of 

 waters characterized by the presence of free sulphuric or hy- 

 drochloric acid, or both together. These acid waters sometimes 

 occur as products of volcanic action ; during which both hydro- 

 chloric acid and sulphur are often evolved in large quantities. 

 This latter element generally comes to the surface as sulphu- 

 retted hydrogen, which by the oxidation of the hydrogen may 

 deposit its sulphur in craters and fissures. In other cases, as 

 shown by Dumas, the sulphur and hydrogen may be slowly and 

 simultaneously oxidized at a low temperature, giving rise di- 

 rectly to sulphuric acid. Not less frequent, however, is prob- 

 ably the direct conversion, by combustion, of the sulphuretted 

 hydrogen into water and sulphurous acid, which, afterwards 

 absorbing oxygen from the air, is converted into sulphuric 

 acid. 



30. The source of the hydrochloric acid and the sulphur 

 of volcanoes is probably the decomposition of chlorides and sul- 

 phates at high temperatures. It is known that for the decom- 

 position of earthy chlorides, water and an elevated temperature 

 are sufficient ; and at a higher temperature, chloride of sodium 

 is readily decomposed in presence of silicious and aluminous 

 minerals, with the intervention of water. Another agency 

 which probably comes into play in volcanic phenomena is that 

 of organic matters, which, reducing the sulphates to sulphurets, 

 enable the sulphur to be subsequently disengaged as sulphu- 

 retted hydrogen by the operation of water, either with or with- 

 out the intervention of carbonic acid or of silicious and argilla- 

 ceous matters. Even in cases where this reducing action is 

 excluded, the ignition of sulphates in contact with earthy 

 matters must liberate the sulphuric acid as a mixture of sul- 



