TRUE VOLCANOES. 397 



sources of information (the geological and geographical ac- 



Muriatic acid fumaroles, which, in Italian volcanoes, are not unfre- 

 qnently on the most extensive scale, and are then generally accompa- 

 nied by immense sublimations of common salt, seem to be of a very 

 unimportant character in Iceland. The concluding stages in the chro- 

 nological series of all these phenomena consist in mere emanations of 

 carbonic acid. The hydrogen contained in the volcanic gases has 

 hitherto been almost entirely overlooked. It is present in the vapor 

 springs of the gi-eat solfataras of Krisuvik and Reykjalidh, in Iceland, 

 and is, indeed, at both those places combined vi^ith sulphureted hydro- 

 gen. When the latter come in contact with sulphuric acid, they are 

 both mutually decomposed by the separation of the sulphur, so that 

 they can never occur together. They are, however, not unfrequently 

 met with on one and the same field of fumaroles in close proximity to 

 each other. Unrecognizable as was the sulphureted hydrogen gas in 

 the Icelandic solfataras just mentioned, it failed, on the other hand, 

 entirely in the solfataric condition assumed by the crater of Hecla 

 shortly after the eruption of the year 1845 — that is to say, in the first 

 phase of the volcanic secondary action. Not the smallest trace of sul- 

 phureted hydrogen could be detected, either by the smell or by re- 

 agents, while the copious sublimation of sulphur, the smell of which 

 extended to a great distance, afforded indisputable evidence of the 

 presence of sulphurous acid. In fact, on the approach of a lighted 

 cigar to one of these fumaroles those thick clouds of smoke were pro- 

 duced which Melloni and Piria have noticed as a test of the smallest 

 trace of sulphureted hydrogen {Comptes rendus, t. xi., 1840, p. 352; 

 and Poggendorft''s Annalen, Erganzungsband, 1842, s. 511). As it 

 may, however, be easily seen by experiment that even sulphur itself, 

 when sublimated with aqueous vapor, produces the same phenomenon, 

 it remains doubtful whether any trace whatever of sulphureted hy- 

 drogen accompanied the emanations from the crater of Hecla in 1845, 

 and of Vesuvius in 1843 (compare Kobert Bunsen's admirable and 

 geologically important treatise on the processes of formation of the 

 volcanic rock of Iceland, in Poggend., AnnaL, bd. Ixxxiii., 1851, s. 

 241, 244, 246, 248, 254, and 256 ; serving as an extension and rectifi- 

 cation of the treatises of 1847 in Wohler's and Liebig's Annalen der 

 Chemie nnd Pharmacie, bd. Ixii., s. 19). That the emanations from 

 the solfatara of Pozzuoli are not sulphureted hydrogen, and that no 

 sulphur is deposited from them by contact with the atmosphere, as 

 Breislak has conjectured (^Essai Min6ralogiqne sur la Soufriere de Poz- 

 zuoli, 1792, p. 128-130), was remarked by Gay-Lussac when I visited 

 the Phlegraean Fields with him at the time of the great eruption of 

 lava in the year 1805. That acute observer, Archangelo Scacchi, 

 likewise decidedly denies the existence of sulphureted hydrogen {Me- 

 iiioiie Geologiche sulla Caivpania, 1849, p. 49-121), Piria's test seeming 

 to him only to prove the presence of aqueous vapor: "Son di avviso 

 che lo solfo emane mescolato a i vapori acquei senza essere in chimica 

 combinazione con altre sostanze" — " I am of opinion that the sulphur 

 emanates mixed with aqueous vapors without being in combination 

 with other substances." An actual analysis, however, long looked for 

 by m«, of the gases ejected by the solfatara of Pozzuoli, has been very 

 recently published by Charles Sainte-Claire Deville and Leblanc, and 

 has completely established the absence of sulphureted hydrogen 



