Cxlii NOTES. 



doubtful whether even a trace of sulphuretted hydrogen accompanied the ema- 

 nations of Hecla in 1845 and of Vesusius in 1843. (Compare the excellent 

 and, in geological respects, highly important Memoir of Robert Bunsen, on the 

 processes of volcanic rock formation in Iceland, in Poggend Ann. Bd. 83, 1851, 

 S. 241, 244, 246, 248, 250, 254, and 256 : enlarging and correcting the Me- 

 moirs of 1847, in Wohler's and Liebig's Annalen der Chemie und Pharmacie, Bd. 

 62, S. 19.) That the emanations of the solfataras of Pozzuoli were not of sul- 

 phuretted hydrogen, and did not deposit sulphur on contact with the atmosphere, 

 as had been asserted by Breislak, in his memoir, entitled, Essai Mine'ralogique 

 bur la Soufriere de Pozzuoli, 1792, p. 128 130, had been already remarked by 

 Gay-Lussac, when we visited the Phlegraean fields together at the time of the 

 great lava eruption of 1805. The clear-sighted Arcangelo Scacchi (Memorie 

 Geologiche sulla Campania, 1849, p. 49 121) very decidedly denies the ex- 

 istence of the sulphuretted hydrogen, because Piria's methods of testing appear 

 to him only to prove the presence of aqueous vapour : " Son di aviso che lo solfo 

 emane mescolato a i vapori acquei senza essere in chimica combinazione con altre 

 sostanze." An actual analysis (which I had long wished for and expected) of 

 the gases emitted by the solfatara of Pozzuoli has been only very recently fur- 

 nished by Charles Sainte-Claire Deville and Leblanc, and has fully confirmed the 

 absence of sulphuretted hydrogen. (Cornptes Rendus de 1'Acad. des Sc. t. xliii. 

 1856, p. 746.) On the other hand, Sartorius von Waltershausen (Physisch- 

 geographische Skizze von Island, 1847, S. 120) remarked at the eruption cones 

 of Etna, in 1811, the strong smell of sulphuretted hydrogen, where, in other 

 years, only sulphurous acid had been perceived. Ch. Deville found a small 

 portion of sulphuretted hydrogen, not at Girgenti and in the Macalube, but on 

 the eastern slope of Etna, in the spring of Santa Venerina. It is a striking 

 circumstance, that, in the important series of chemical analyses which Boussin- 

 gault made on gas-emitting volcanoes in the chain of the Andes (from Purace 

 and Tolima to the high plains of Los Pastos and Quito), both muriatic acid and 

 hydrogene sulfureux are wanting. 



( 556 ) p. 406. Older works give as the number of still burning volcanoes : 

 Werner, 193; Caesar von Leonhard, 187; Arago (Astronomic populaire, t. in. 

 p. 170), 175; all less than iny number, the differences in defect varying 

 between -J- and ^, and being attributable partly to diversity in the principles 

 by which the continuance of ignition is judged of, and partly by deficiency in 

 the data collected. Since, as I have already remarked above, and as historic 

 experience teaches us, volcanoes, which had been believed to be extinct, have 

 again manifested their activity after long periods of repose, the numerical result 

 propounded by me is rather to be regarded as too low than too high. Leopold 



