NOTES. Cxli 



the sulphurous vapour encounters glowing pyroxenic rocks, sulphurous acid 

 originates in the partial decomposition of the oxide of iron which those rocks 

 contain. If thereafter the volcanic activity sinks to lower temperatures, the 

 chemical activity enters upon a new phase. The combinations of sulphur with 

 iron, and perhaps with the metallic bases of the earths and alkalies, begin to act 

 on the aqueous vapour or steam, and, as a result of reciprocal action, there arise 

 sulphuretted hydrogen, and the products of its decomposition, viz. free hydrogen 

 and vapours of sulphur. Sulphur fumaroles survive great volcanic eruptions for 

 centuries. The muriatic acid fumaroles belong to a different and a later period : 

 it is but rarely that they assume the character of permanent phenomena. The 

 origination of muriatic acid in crater gases may be inferred to be from the mu- 

 riate of soda, which so frequently appears as a product of sublimation in vol- 

 canoes (and in Vesuvius in particular), being decomposed by silicates, at high 

 temperatures and under the concurrent action of aqueous vapour, into muriatic 

 acid and soda, which latter combines with the silicates which are present. Mu- 

 riatic acid fumaroles, which in Italian volcanoes occur not unfrequently on a very 

 large scale, and which are then usually accompanied by great sublimations of 

 common salt (muriate of soda), appear to be only very inconsiderable in Iceland. 

 As the final links in the chronological series of all these phenomena we find lastly 

 the emanations of carbonic acid gas only. The hydrogen in volcanic gases has 

 hitherto been almost entirely overlooked. It is present in the vapour spring of 

 the great solfataras of Krisuvik and Reykjalidh, in Iceland, and at both those 

 places is combined with sulphuretted hydrogen. As the latter and sulphurous 

 acid, on coming into contact, mutually decompose each other, setting free the 

 sulphur, they never can both present themselves at once. It is not rare, how- 

 ever, to find them in one and the same fumarole field very near each other. If 

 in the Icelandic solfataras, which have just been named, sulphuretted hydrogen 

 gas could so little be recognised, on the other hand, it was entirely wanting in 

 the solfatara state of the crater of Hecla only a short time after the eruption of 

 1845, therefore in the first phase of volcanic after-action. Neither by smell nor 

 by reagents could the slightest trace of sulphuretted hydrogen be discovered, 

 while the abundant sublimation of sulphur made the presence of sulphurous acid 

 unmistakably recognisable by the smell to a considerable distance. It is true 

 that on bringing a lighted cigar over the fumaroles those thick clouds of smoke 

 showed themselves, which Melloni and Piria (Comptes Rendus, t. xi. 1840, p. 

 352; and PoggendorfFs Annalen, Erganzungsband, 1842, S. 511) have pointed 

 out as an indication of the smallest traces of sulphuretted hydrogen. As how- 

 ever it is easy to satisfy one's-self by experiment, that sulphur by itself, when 

 sublimated with aqueous vapours, also produces the same phenomenon, it remains 



