Site of the Ancient City of the Aurunci. 245 



such volumes of steam, as alone must prevent it from enter- 

 ing into combustion w^hen it came into contact with the ex- 

 ternal air ; for it is well known, from the researches of Davy, 

 that a certain per-centage of any uninflammable gas or va- 

 pour prevents such bodies from taking fire. 



It is, therefore, far more easy for the advocates of the che- 

 mical theory to account for the general absence of flames 

 about the orifices of active volcanoes, than for the supporters 

 of the contrary hypothesis to explain their occasional pre- 

 sence ; and that they are sometimes observable seems to be 

 now ascertained, not only from the testimony of Sir H. Davy 

 himself, who states that he observed at Vesuvius, during a 

 small eruption, the existence of a real jet of flame, and that 

 of M. Elie de Beaumont, who assures us of the same fact, 

 as witnessed by himself at Mount Etna, but more recently 

 by the observations made by Professor Pilla of Pisa,* who 

 has given us a circumstantial account of three several oc- 

 currences of this kind in the years 1833 and 1834 at Ve- 

 suvius. 



My own persuasion, therefore, is, that hydrogen gas, de- 

 rived from the decomposition of water, most generally in 

 combination with sulphur, is evolved in enormous quantities 

 from all volcanoes, but that a comparatively small proportion 

 of it usually finds its way upwards to the surface : since, if 

 sulphurous acid be present likewise, the two gases will de- 

 compose each other, so that only the excess of the one most 

 abundant will remain, and if it meet with oxygen in its pro- 

 gress upwards, it will combine with this principle, and water 

 will consequently result. 



Nevertheless, I hold that the sulphuretted-hydrogen which 

 impregnates the mineral waters in various ignigenous, and 

 even in certain primary districts, is derived from some vol- 

 canic /ocw^ ; I am inclined to believe, that the beds of sulphur 

 met with in various parts of the world, where igneous agents 

 have been at work, as in Sicily, owe their origin to the decom- 



♦ See his " Discorso sopra la produzione delle fiamme nei Vulcani, &c.," read 

 at the Fifth Italian Scientific Congress, held in 1843, and translated in Jame- 

 son's Journal for April 1844. 



VOL. XLI. NO. LXXXII. — OCTOBER 1846. R 



