1865.] CHEMISTRY OF NATURAL WATERS. 17 



iron, and more rarely to sulphates of copper, zinc, cobalt, and nickel ; 

 and by secondary reactions to sulphates of alumina, lime, mag- 

 nesia, and alkalies. This process of oxydation is necessarily super- 

 ficial and local, but the soluble sulphates thus formed have probably 

 played a not unimportant part. (§9.) 



§ 29. Besides these last, which contain chiefly neutral and acid 

 salts, there is another class of waters characterized by the presence 

 of free sulphuric or hydrochloric acid, or both together. These acid 

 waters sometimes occur as products of volcanic action, during which 

 both hydrochloric acid and sulphur are often evolved in large quan- 

 tities. This latter element generally comes to the surface as sul- 

 phuretted hydrogen, which by the oxydation 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 oxydized at a low temperature, giving rise directly 

 to sulphuric acid. Not less frequent, however, is probably 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 vol- 

 canoes is probably the decomposition of chlorids and sulphates at 

 high temperatures. It is known that for the decomposition of 

 earthy chlorids, water and an elevated temperature are sufficient ; 

 and at a higher temperature, chlorid of sodium is readily decom- 

 posed 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 sulphuretted hydrogen by the 

 operation of water, either with or without the intervention of 

 carbonic acid or of silicious and argillaceous 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 sulphurous acid and oxygen; and these uniting in 

 their distillation upward through the strata, may give rise to 

 springs of sulphuric acid.* To reactions similar to those just 

 noticed, involving 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. 



* See the note to § 22, on kieserite. 

 Vol. II. b No. 1. 



