TRUE VOLCANOES. 425 



accounts of travels), is that, out of 407 volcanoes cited by 



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. Unrecognisable as was the sulphuretted hydrogen gas in 

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

 tirely 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 sulphuretted 

 Hydrogen could be detected, either by the smell or by re-agents, while 

 che copious sublimation of sulphur, the smell of which extended to a 

 great distance, afforded indisputable evidence of the presence of sul- 

 phurous acid. In fact, on the approach of a lighted cigar to one of these 

 fumaroles those thick clouds of smoke were produced which Melloni 

 and Piria have noticed as a test of the smallest trace of sulphuretted 

 hydrogen (Comptes rendus, t. xi, 1840, p. 352, and Poggendorff's Anna- 

 len, Erganzungsband, 1842, s. 511). As it may however be easily 

 seen by experiment that even sulphur itself, when sublimated with 

 aqueous vapour, produces the same phenomenon, it remains doubtful 

 whether any trace whatever of sulphuretted hydrogen accompanied 

 the emanations from the crater of Hecla in 1845, and of Vesuvius in 

 1843 (compare Robert 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 rectification of the treatises 

 of 1847 in Wohler's and Liebig's Annalen der Chemie und Phar- 

 macie. Bd. Ixii, s. 19). That the emanations from the solfatara of 

 Pozzuoli are not sulphuretted hydrogen, and that no sulphur is 

 deposited from them by contact with the atmosphere, as Breislak has 

 conjectured (Essai Mineralogique sur la Soufri&re de Pozzuoli, 

 1792, p. 128130), was remarked by Gay-Lussac when I visited the 

 Phlegrsean Fields with him at the time of the great eruption of lava in 

 the year 1805. That acute observer, Archangelo Scacchi likewise de- 

 cidedly denies the existence of sulphuretted hydrogen (Memorie geo- 

 logiche suUa Campania, 1849, p. 49 121), Piria's test seeming to him 

 only to prove the presence of aqueous vapour ; ' Son di avviso che lo 

 solfo emane mescolato a i vapori acquei senza esserein chimica combi- 

 nazione con altre sostanze," " I am of opinion that the sulphur ema- 

 nates mixed with aqueous vapours without being in combination with 

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

 of the gases ejected by the solfatara of Pozzuoli. has been very recently 

 published by Charles Sainte-Claire Deville and Leblanc, and has com- 

 pletely established the absence of sulphuretted hydrogen (Comptes 

 rendus de I'Acad. d. Sc. t. xliii, 1856, p. 746). Sartorius von Waltershau- 

 sen, on the other hand, observed on cones of eruption of Etna, in 1811, 

 a strong smell of sulphuretted hydrogen, where in other years sulphu- 

 rous acid only was perceived. Nor did Charles Deville discover any sul- 

 phuretted hydrogen at Girgenti, or in the Macalube, but a small por- 

 tion of it on the eastern declivity of Etna, in the spring of Santa 

 Veneriiia. It is remarkable, that throughout the important series o/ 



